Note brevi ed i Riassunti del convegno - Sezione di Georisorse e ...
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Convegno Annuale <strong>del</strong> Gruppo Italiano <strong>di</strong> Geologia Strutturale<br />
U<strong>di</strong>ne, 25 – 28 febbraio 2009<br />
<strong>Note</strong> <strong>brevi</strong> e <strong>Riassunti</strong><br />
Questo volume dei Ren<strong>di</strong>conti online <strong>del</strong>la Società Geologica Italiana raccoglie le <strong>Note</strong> Brevi e i <strong>Riassunti</strong><br />
<strong>del</strong>le comunicazioni e dei poster presentati al Convegno annuale <strong>del</strong> Gruppo Italiano <strong>di</strong> Geologia Strutturale<br />
(GIGS), sezione <strong>del</strong>la Società Geologica Italiana. Il <strong>convegno</strong>, svoltosi tra il 25 e il 28 febbraio 2009, è stato<br />
organizzato presso il Polo scientifico <strong>del</strong>l’Università <strong>di</strong> U<strong>di</strong>ne e ha visto la partecipazione <strong>di</strong> 130 persone, oltre<br />
60 <strong>del</strong>le quali hanno preso parte all’escursione <strong>del</strong> 27 e 28 febbraio nelle Prealpi Carniche e Giulie. Numerosa la<br />
partecipazione <strong>di</strong> ricercatori e dottoran<strong>di</strong>, in linea con lo spirito dei Convegni annuali <strong>del</strong> GIGS, che intendono<br />
fornire soprattutto a loro un occasione per confrontarsi scientificamente e per presentare i risultati - anche<br />
preliminari – <strong>del</strong>le ricerche.<br />
I temi trattati vanno dalla Geo<strong>di</strong>namica alla Tettonica, all’Analisi micro- e mesostrutturale e, in particolare,<br />
alla Tettonica attiva, con comunicazioni sui risultati <strong>di</strong> ricerche svolte principalmente in Appennino, Alpi e<br />
Dinari<strong>di</strong>, ma anche in America meri<strong>di</strong>onale e centrale, Asia, Antartide e Oceano Pacifico. In vari casi si tratta <strong>di</strong><br />
ricerche che usano tecniche e metodologie innovative, spesso nell’ambito <strong>di</strong> gruppi <strong>di</strong> ricerca multi<strong>di</strong>sciplinari.<br />
Il 26 febbraio si è anche tenuta la sessione tematica “La geologia strutturale nella ricerca petrolifera”, che è<br />
stata sponsorizzata dall’Asso Mineraria. Questa sessione ha voluto rappresentare un primo passo per un rapporto<br />
organico e produttivo la tra la ricerca e il mondo <strong>del</strong> lavoro, in questo caso rappresentato dagli enti privati che<br />
si occupano <strong>di</strong> esplorazione mineraria. Oltre che nelle comunicazioni <strong>del</strong>la sessione tematica, le potenzialità<br />
applicative <strong>del</strong>la Geologia Strutturale sono risaltate anche da vari contributi <strong>del</strong>le sessioni “Tettonica attiva” e<br />
poster, per la definizione <strong>del</strong>la pericolosità sismica <strong>di</strong> aree alpine <strong>ed</strong> appenniniche.<br />
Ci è gra<strong>di</strong>to ringraziare il Magnifico Rettore <strong>del</strong>l’Università degli Stu<strong>di</strong> <strong>di</strong> U<strong>di</strong>ne Prof.ssa Cristiana Compagno<br />
e il Preside <strong>del</strong>la Facoltà <strong>di</strong> Ingegneria prof. Alberto F. De Toni per aver ospitato il Convegno GIGS 2009 nel<br />
complesso <strong>del</strong> Polo Scientifico.<br />
M. Eliana Poli<br />
Adriano Zanferrari<br />
Michele Marroni<br />
Enrico Tavarnelli
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 5-8, 3 ff.<br />
From fractures to flow, a field-bas<strong>ed</strong> quantitative analysis of<br />
an outcropping carbonate reservoir.<br />
AGOSTA FABRIZIO (*), MAURO ALESSANDRONI (*, **), MARCO ANTONELLINI (°) & EMANUELE TONDI (*)<br />
RIASSUNTO<br />
Relazione tra anisotropia <strong>del</strong>le fratture e circolazione <strong>di</strong> idrocarburi<br />
In questa nota breve si riportano i dati principali riguardanti la lunghezza,<br />
spaziatura, apertura, orientazione e connettività <strong>di</strong> fratture presenti all’interno<br />
<strong>di</strong> damage zone calcaree associate a faglie trastensive affioranti nella porzione<br />
settentrionale <strong>del</strong>la Montagna <strong>del</strong>la Majella, Abruzzo. Lo scopo è quello <strong>di</strong><br />
definire le relazioni esistenti tra le singole caratteristiche <strong>del</strong>le fratture<br />
sopramenzionate e la <strong>di</strong>stribuzione <strong>di</strong> bitume all’interno <strong>di</strong> esse. Dalle analisi<br />
effettuate si evince come il bitume si concentri principalmente all’interno <strong>di</strong><br />
sistemi <strong>di</strong> fratture caratterizzati da marcate anisotropie in termini <strong>di</strong><br />
orientazione, rapporto tra fratture ad alto angolo e fratture a basso angolo<br />
rispetto alla stratificazione, e connettività. Questa configurazione determina<br />
un cambiamento <strong>del</strong>le proprietà idrauliche <strong>del</strong>la roccia madre, la quale<br />
originariamente favoriva l’accumulo degli idrocarburi all’interno <strong>del</strong>le fratture,<br />
grazie al loro basso grado <strong>di</strong> connnettività. All’interno <strong>del</strong>le damage zone <strong>del</strong>le<br />
faglie, invece, i dati a nostra <strong>di</strong>sposizione in<strong>di</strong>cano un comportamento che ne<br />
favorisce la migrazione e non l’accumulo.<br />
Parole chiave: lunghezza, spaziatura e connettività <strong>del</strong>le<br />
fratture, idrocarburi, migrazione dei flui<strong>di</strong>, Formazione <strong>del</strong><br />
Bolognano, Montagna <strong>del</strong>la Majella, Apennino centrale.<br />
Keywords: fracture length, fracture spacing, fracture<br />
connectivity, hydrocarbons, fluid flow, Bolognano Formation,<br />
Majella Mountain, central Apennines.<br />
INTRODUCTION<br />
Carbonate reservoirs constitute more than 60% of the<br />
world’s oil, and about 40% of its gas reserves<br />
(SCHLUMBERGER, 2007). This is probably why in the last<br />
years a large scientific interest has risen on the characterization<br />
of their mechanical and petrophysical properties, as well as of<br />
the deformation mechanisms they are subject<strong>ed</strong> to at shallow<br />
crustal levels (AGOSTA & TONDI, 2009). Fractures in<br />
carbonates may have an important role in geofluids migrations<br />
and/or accumulation (AYDIN, 2000; GRAHAM et alii 2006;<br />
TONDI et alii 2006). They can either focus the fluid flow (e.g.,<br />
_________________________<br />
(*)Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Camerino<br />
(**)BEICIP-FRANLAB, Parigi, Francia<br />
(°)Centro Inter<strong>di</strong>partimentale <strong>di</strong> Ricerca per le Scienze Ambientali,<br />
Università <strong>di</strong> Bologna<br />
Lavoro eseguito nell’ambito <strong>del</strong> Progetto Faults & Fractures in Carbonates<br />
(F & FC) <strong>del</strong>l’Università <strong>di</strong> Camerino.<br />
joints, shear<strong>ed</strong> joints, and shear<strong>ed</strong> pressure solution seams) or<br />
form impermeable features (pressure solution seams).<br />
In this study, we take advantage of an expos<strong>ed</strong> hydrocarbonbearing,<br />
fractur<strong>ed</strong> carbonate reservoir to address the role play<strong>ed</strong><br />
by fractures on hydrocarbon migration. The study area is<br />
locat<strong>ed</strong> along the northern termination of the Majella anticline,<br />
in central Italy, within a quarry originally excavat<strong>ed</strong> by the<br />
ancient Romans, and then exploit<strong>ed</strong> until the second world war,<br />
to recover hydrocarbons in the form of tar residues. There, at<br />
the Roman Valley Quarry, the tar is present within the fractur<strong>ed</strong><br />
and fault<strong>ed</strong> carbonates associat<strong>ed</strong> to two high-angle, oblique<br />
normal faults. The faulting processes and mechanisms of fault<br />
development were recently document<strong>ed</strong> by AGOSTA et alii. in<br />
the same quarry (2009). The authors conduct<strong>ed</strong> a detail<strong>ed</strong><br />
structural analysis of the failure modes, geometry, orientation,<br />
relative timing of the <strong>di</strong>fferent fractures sets, recognizing the<br />
main structural assemblages.<br />
In this present contribution, we focus on the relation between<br />
the in<strong>di</strong>vidual fracture characteristics (length, spacing, aperture,<br />
orientation, connectivity, and <strong>di</strong>stance from slip surfaces) and<br />
tar <strong>di</strong>stribution (ALESSANDRONI, 2008). This is<br />
accomplish<strong>ed</strong> by analyzing data obtain<strong>ed</strong> from 1D scan lines<br />
measurements carri<strong>ed</strong> out along the quarry’s vertical outcrops.<br />
We perform a total of sixteen scan lines in the damage zones of<br />
the two oblique normal faults, most of which along fractur<strong>ed</strong><br />
carbonate b<strong>ed</strong>s, a few others across smaller faults crosscutting<br />
the damage zones. After data computation, we <strong>di</strong>scuss the<br />
in<strong>di</strong>vidual relations among the <strong>di</strong>fferent fracture characteristics<br />
and tar <strong>di</strong>stribution in terms of fracture anisotropy within the<br />
deform<strong>ed</strong> carbonates. We propose a conceptual mo<strong>del</strong> of<br />
fracture arrays geometry in both carbonate host rocks and in<br />
fault<strong>ed</strong> carbonates.<br />
FAULT ARCHITECTURE AND PERMEABILITY<br />
Our observations are consistent with most of the<br />
hydrocarbon flow postdating faulting and fracturing of the<br />
Oligo-Miocene carbonate rocks. At a large scale, the oil show<br />
<strong>di</strong>stribution along the main faults cropping out along the<br />
northern Majella in<strong>di</strong>cates the strong control they exert<strong>ed</strong> on<br />
hydrocarbon migration. Hydrocarbons were channel<strong>ed</strong> along<br />
these faults at unknown depths, and then migrat<strong>ed</strong> upward<br />
within the fractur<strong>ed</strong> and fault<strong>ed</strong> carbonate damage zones.<br />
Incipient faults (offset < a few cm) consist of shear<strong>ed</strong> fractures,<br />
which generally include very thin fragment<strong>ed</strong> rocks and are<br />
keen to conduce fluids. Small faults (a few cm < offset < 10cm)
6 AGOSTA ET ALII<br />
include isolat<strong>ed</strong>, b<strong>ed</strong>-confin<strong>ed</strong>, short slip surfaces, fractur<strong>ed</strong><br />
carbonates and isolat<strong>ed</strong> pods of fragment<strong>ed</strong> carbonates, and<br />
behave as single fluid conduits. M<strong>ed</strong>ium faults (10’s cm <<br />
offset < ~1m) contain well-develop<strong>ed</strong>, through-going, a few mlong<br />
slip surfaces, which are often coat<strong>ed</strong> by calcite cements,<br />
and <strong>di</strong>scontinuous pockets of fault breccias and fragment<strong>ed</strong><br />
carbonates. Bas<strong>ed</strong> on their internal architecture, these faults<br />
also behave as single conduits to fluid flow. Tar <strong>di</strong>stribution<br />
within the quarry suggests that the hydraulic connectivity of<br />
small and m<strong>ed</strong>ium faults to larger fluid conduits is key to focus<br />
hydrocarbon flow.<br />
Considering the two large faults of the quarry, the NE fault<br />
(offset > 10m) is compris<strong>ed</strong> of several segments orient<strong>ed</strong> E-W<br />
to NE-SW. The intersections among not-parallel slip surfaces<br />
are mark<strong>ed</strong> by triangular-shap<strong>ed</strong> fault breccias, whereas jogs of<br />
sub-parallel interacting slip surfaces by elongat<strong>ed</strong>, lithonshap<strong>ed</strong><br />
fault breccias. The m’s-thick fault core therefore<br />
includes the aforemention<strong>ed</strong> fault breccias, <strong>di</strong>scontinuous<br />
cataclastic rocks and major slip surfaces. The damage zone is<br />
made up of fractur<strong>ed</strong> carbonates and several smaller faults.<br />
crosscut by small faults. Overall, this fault forms a <strong>di</strong>stribut<strong>ed</strong><br />
conduit to fluid flow, in which fluids focus primarily along the<br />
numerous slip surfaces that enhance fault parallel fluid flow.<br />
The SW fault (offset > 40m)is made up of a continuous, mthick<br />
fault core flank<strong>ed</strong> by a 10’s of m-thick damage zone,<br />
which is more develop<strong>ed</strong> in the fault hanging wall then in the<br />
footwall. The fault core includes major slip surfaces, fault<br />
breccias and cataclastic rocks. The damage zone is made up of<br />
fractur<strong>ed</strong> carbonates and several smaller faults. Overall, this<br />
fault acts as a combin<strong>ed</strong> barrier-conduit permeability structure<br />
to fluid flow. There, the cataclasites form a seal for cross-fault<br />
fluid flow, whereas the surroun<strong>di</strong>ng fault breccia and the<br />
carbonate damage zone behave as <strong>di</strong>ffuse fluid conduits.<br />
FRACTURES VS. TAR DISTRIBUTION<br />
In this section we present the results of our quantitative<br />
analysis in order to assess the relation of in<strong>di</strong>vidual fracture<br />
characteristics (namely: (i) length, (ii) spacing, (iii) aperture,<br />
(iv) orientation, and (v) connectivity) and hydrocarbon<br />
<strong>di</strong>stribution. For this purpose, we group<strong>ed</strong> the fractures in highangle,<br />
HA, and low-angle, LA. The former have a <strong>di</strong>p angle<br />
>75°, the latter < 75°. We also investigate the overall fracture<br />
connectivity of fractures present along tar-free and tar-rich<br />
outcrops, and analyze the abutting relations of HA and LA<br />
fractures separately.<br />
Fracture Length, Spacing, and Aperture<br />
The graph shown in Fig. 1a represents the average length<br />
values measur<strong>ed</strong> at each station. Data range from more than<br />
4cm to 20cm, and do not show any relation with hydrocarbon<br />
<strong>di</strong>stribution. This means that the values of fracture length do<br />
not correlate with presence of hydrocarbons. In fact, we note<br />
that the tar-rich fractures have lengths compris<strong>ed</strong> between more<br />
than 8cm and less than 18cm, similar to the aforemention<strong>ed</strong><br />
length range. The HA/LA length ratios shown in fig. 1b are<br />
consistent with a wider range of values of tar-free fractures<br />
(from ~0.2 to 1.8) relative to tar-rich fractures (between ~0.9<br />
and 1.5). These data suggest that fracture arrays with HA/LA<br />
ratios around 1.2, which is the mean value comput<strong>ed</strong> for tarrich<br />
fractures, form the principle fluid pathways.<br />
The graph in fig. 1c represents the average spacing values<br />
measur<strong>ed</strong> at each station. Data span between ~4cm and ~8cm;<br />
those relat<strong>ed</strong> to only tar-rich fractures are compris<strong>ed</strong> between<br />
~4cm and ~7cm. Accor<strong>di</strong>ngly, there is no relation between<br />
fracture spacing and hydrocarbon <strong>di</strong>stribution. However, the<br />
comput<strong>ed</strong> LA/HA spacing ratios provide some new insights on<br />
their possible relation (Fig. 1d). In fact, LA fractures have<br />
higher spacing values relative to the HA ones. This relation<br />
applies in particular to fractures measur<strong>ed</strong> along tar-rich<br />
outcrops, where LA fractures have spacing values from ~1.5 to<br />
~3 times higher than those pertaining to HA fractures.<br />
Fig. 1 – (a) mean fracture lengths comput<strong>ed</strong> for in<strong>di</strong>vidual stations of<br />
measurements. Tar-free stations are in white, those invad<strong>ed</strong> by tar in<br />
black; (b) HA/LA length ratio; (c) mean fracture spacing, (d) HA/LA<br />
spacing ratio.<br />
Considering the average fracture apertures measur<strong>ed</strong> only at<br />
tar-rich stations, they are often smaller than 1mm, with the<br />
lowest ~0.2mm. Only fractures measur<strong>ed</strong> at one station (#7)<br />
greater than 1mm. In general, values compris<strong>ed</strong> between<br />
0.5mm and 0.8mm characterize the average fracture aperture<br />
measur<strong>ed</strong> at the Roman Valley Quarry. We note that HA
FROM FRACTURES TO FLOW, A FIELD BASED QUANTITATIVE ANALYSIS OF AN OUTCROPPING CARBONATE RESERVOIR<br />
fractures always have larger values than LA ones. This relation<br />
applies to almost all tar-rich outcrops, along which HA and LA<br />
fractures have similar aperture values.<br />
Bas<strong>ed</strong> on aperture data, we also compute the values of 2D<br />
fracture porosity for the tar-rich stations ranges between 0.001<br />
and ~0.01, showing therefore a one order of magnitude. The<br />
mean value comput<strong>ed</strong> for the tar-rich stations is ~0.0045<br />
(0.45%), which not very <strong>di</strong>fferent from what is generally found<br />
in fractur<strong>ed</strong> rocks (NELSON, 1985). Bas<strong>ed</strong> on these values, we<br />
also study their relation with respect to the percentage of tarrich<br />
fractures, obtaining a positive linear relation(R2 = 0.63)<br />
between the two variables.<br />
Fig. 2 – (a) <strong>di</strong>p azimuth orientation of the two most<br />
abundant fracture sets at in<strong>di</strong>vidual stations; (b) fracture<br />
anisotropy; (c) fracture spread.<br />
Fracture Orientation<br />
Data concerning the orientation of the <strong>di</strong>fferent fracture sets<br />
measur<strong>ed</strong> within the study quarry are shown in fig. 2a. The<br />
graph shows the <strong>di</strong>p azimuth of measur<strong>ed</strong> fractures (the value<br />
N180 in<strong>di</strong>cates a E-W striking and south <strong>di</strong>pping set). Data are<br />
plott<strong>ed</strong> with an angular range of 30°. The main fracture set <strong>di</strong>ps<br />
~N30 at all stations, whereas the <strong>di</strong>p azimuth of the second<br />
most abundant set varies between ~N60 and N180.<br />
These data suggest that the fracture array is characteriz<strong>ed</strong> by<br />
a pronounc<strong>ed</strong> anisotropy (Fig. 2b). The anisotropy values<br />
calculat<strong>ed</strong> by the ratio of the frequencies of the two most<br />
abundant fracture sets measur<strong>ed</strong> at in<strong>di</strong>vidual stations span<br />
between ~1 relat<strong>ed</strong> to an equal <strong>di</strong>stribution of the two most<br />
abundant fracture sets, and ~5. The latter value, on the<br />
contrary, points out to a pronounc<strong>ed</strong> fracture anisotropy.<br />
Considering the values comput<strong>ed</strong> for only tar-rich stations, we<br />
observe that they range between 1.3 and 5.2, with a mean value<br />
of ~2.7. This values is greater than that of only tar-free station,<br />
which is ~2.1, and therefore suggests that tar-rich outcrops have<br />
a higher fracture anisotropy relative to tar-free ones.<br />
We compute the fracture spread by summing the frequencies<br />
of the two most abundant fracture sets measur<strong>ed</strong> at in<strong>di</strong>vidual<br />
stations. The results of this computation are report<strong>ed</strong>, in<br />
percentage, in the graph of fig. 2c. The spread values range<br />
between ~0.4 and ~0.8, with a mean value of ~0.65 for tar-free<br />
stations and of ~0.75 for tar-rich ones. Data are therefore<br />
consistent with a mark<strong>ed</strong> fracture anisotropy in the survey<strong>ed</strong><br />
carbonate outcrops. As stat<strong>ed</strong> above, this fracture anisotropy is<br />
more pronounc<strong>ed</strong> is tar-rich outcrops.<br />
Fracture Connectivity<br />
Fracture connectivity is an important structural parameter for<br />
assessing the fluid flow/storage properties of fractur<strong>ed</strong> rocks.<br />
This parameter can be comput<strong>ed</strong> from 1D data, and is generally<br />
in<strong>di</strong>cative of the typology of fracture tips. Bas<strong>ed</strong> on their<br />
abutting relations, we may have 3 main types of fractures:<br />
isolat<strong>ed</strong> (type I), coupl<strong>ed</strong> (type II), and interconnect<strong>ed</strong> (type III)<br />
fractures (ORTEGA & MARRET, 2000). By considering the<br />
prevalent fracture typology in given outcrops, we may have low<br />
(prevalence of type I fractures), m<strong>ed</strong>ium (type II), or highly<br />
connect<strong>ed</strong> fracture arrays (type III). These relations are shown<br />
by plotting our data in a triangular <strong>di</strong>agram (Fig. 3a).<br />
Fig. 3 – Triangular <strong>di</strong>agram, showing fracture<br />
connectivity along tar-free (white dots) and tar-rich<br />
stations (black dots) of the Roman Valley Quarry.<br />
The results of this computation are consistent with presence<br />
of highly connect<strong>ed</strong> fracture arrays within the quarry. Most of<br />
the outcrops, independently on tar <strong>di</strong>stribution, are crosscut by<br />
interconnect<strong>ed</strong> fractures (type III). In terms of fluid<br />
flow/storage properties, we observe that the storage properties<br />
assess<strong>ed</strong> to the Bolognano Fm. host rocks (MARCHEGIANI et<br />
alii 2006), who recogniz<strong>ed</strong> mainly isolat<strong>ed</strong> and coupl<strong>ed</strong><br />
fractures (types I and II, respectively), are not confirm<strong>ed</strong> by our<br />
7
8 AGOSTA ET ALII<br />
data. Conversely, we find that most of the fractures present in<br />
the two fault damage zones are interconnect<strong>ed</strong> fractures.<br />
FRACTURE ANISOTROPY<br />
At a large scale, we observe that hydrocarbons are present<br />
primarily within releasing jogs of left-stepping normal faults<br />
characteriz<strong>ed</strong> by minor component of left-lateral slip. At<br />
smaller scales, we document that the structural position of small<br />
and m<strong>ed</strong>ium faults crosscutting the carbonate damage zones<br />
plays a role for hydrocarbon migration as well. In order to<br />
better address the peculiarities of fractures containing tar<br />
residues, thanks to the knowl<strong>ed</strong>ge we gain<strong>ed</strong> from this<br />
quantitative analysis, we propose the following conceptual<br />
mo<strong>del</strong> of fracture arrays in (i) carbonate host rocks and (ii)<br />
fault<strong>ed</strong> carbonates:<br />
(i) The fracture arrays in the carbonate host rocks are mainly<br />
compris<strong>ed</strong> of b<strong>ed</strong>-perpen<strong>di</strong>cular PS boun<strong>di</strong>ng oblique to<br />
b<strong>ed</strong><strong>di</strong>ng PS, and some b<strong>ed</strong>-parallel PS. The orientation of these<br />
PS sets determines quite isotropic arrays, in which those<br />
orient<strong>ed</strong> at a low-angle to the local hmax stress (inferr<strong>ed</strong> by<br />
comparing fracture orientation and aperture data) may form<br />
conduits to fluid flow, thanks to their opening caus<strong>ed</strong> by the<br />
stress con<strong>di</strong>tions, whereas those at a high-angle determine fluid<br />
barriers. In this mo<strong>del</strong>, we propose that the b<strong>ed</strong>-perpen<strong>di</strong>cular<br />
and oblique sets have both negative exponential <strong>di</strong>stributions,<br />
whereas the former sets have lengths up to the b<strong>ed</strong> thickness<br />
while the oblique PS may have greater length values.<br />
(ii) Conversely, the fracture arrays present of the fault damage<br />
zones are characteriz<strong>ed</strong> by a pronounc<strong>ed</strong> anisotropy. Bas<strong>ed</strong> on<br />
the values of the aforemention<strong>ed</strong> thresholds, we infer that tarrich<br />
fault<strong>ed</strong> carbonates have HA/LA fracture lengths >0.9, and<br />
a LA/HA spacing >1.5. We infer that this geometry is due to,<br />
primarily, pronounc<strong>ed</strong> cracking that occurr<strong>ed</strong> in the fault<strong>ed</strong><br />
carbonates. Tail joints that form<strong>ed</strong> within these rock volumes<br />
increas<strong>ed</strong> the degree of fracture connectivity, the values of<br />
fracture porosity, and the fluid transmissibility. This<br />
contribut<strong>ed</strong> to shift the fluid storage properties of the fracture<br />
arrays in carbonate host rocks, in which fluids accumulate<br />
within isolat<strong>ed</strong> or coupl<strong>ed</strong> fractures (MARCHEGIANI et alii<br />
2006), to the fluid flow properties of the fault<strong>ed</strong> carbonates, in<br />
which fluids circulate through interconnect<strong>ed</strong> fractures.<br />
CONCLUSIONS<br />
We were able to assess the control exert<strong>ed</strong> by in<strong>di</strong>vidual<br />
fracture characteristics (length, spacing, aperture, orientation,<br />
connectivity) on the hydrocarbon migration through the<br />
carbonate fault damage zones. Bas<strong>ed</strong> on the results of our<br />
detail<strong>ed</strong> quantitative analysis, we conclude that neither the<br />
fracture length nor the fracture spacing play<strong>ed</strong> a role on fluid<br />
flow. Conversely, we found that hydrocarbon mov<strong>ed</strong><br />
preferentially through fracture arrays characteriz<strong>ed</strong> by a<br />
pronounc<strong>ed</strong> anisotropy in terms of geometry, orientation, and<br />
connectivity. This configuration enhance the fluid flow through<br />
the fractures rather than its accumulation.<br />
REFERENCES<br />
AGOSTA F., & TONDI E. (Guest E<strong>di</strong>tors) (2009) - Faulting and<br />
Fracturing in Carbonate Rocks: New Insights on<br />
Deformation Mechanisms and Petrophysical Properties.<br />
Journal of Structural Geology, special volume, in press.<br />
AGOSTA F., ALESSANDRONI M., & TONDI E. (2009) - Oblique<br />
normal faulting along the northern <strong>ed</strong>ge of the Majella<br />
anticline, central Italy: inferences on hydrocarbon<br />
migration and accumulation. Journal of Structural Geology,<br />
in press.<br />
ALESSANDRONI M. (2008) - Structural control on the flow and<br />
accumulation of hydrocarbons in carbonate grainstones:<br />
an example from the Bolognano Fm. (Majella Mt. Italy).<br />
Ph.D. Thesis, University of Camerino, 167 pp.<br />
AYDIN A. (2000) - Fractures, faults, and hydrocarbon<br />
entrapment, migration and flow. Marine and Petroleum<br />
Geology 17, 797-814.<br />
GRAHAM-WALL B., GIRGACEA R., MESONJESI A. & AYDIN A.<br />
(2006) - Evolution of fluid pathways through fracture<br />
controll<strong>ed</strong> faults in carbonates of the Albanides fold-thrust<br />
belt. AAPG Bulletin, 90, 1227-1249.<br />
MARCHEGIANI L., VAN DIJK J. P., GILLESPIE P. A., TONDI E. &<br />
CELLO G. (2006) - Scaling properties of the <strong>di</strong>mensional<br />
and spatial characteristics of fault and fracture systems in<br />
the Majella Mountain, central Italy. In: CELLO G. &<br />
MALAMUD B. (Eds) Fractal Analysis for Natural Hazards.<br />
Geological Society of London, Special Publications, 261,<br />
113–131.<br />
NELSON R. A. (1985) - Geologic Analysis of Naturally<br />
Fractur<strong>ed</strong> Reservoirs. Houston, TX. Gulf Professional<br />
Publishing. 320 pp.<br />
ORTEG O., & MARRETT R. 2000, Pr<strong>ed</strong>iction of macrofracture<br />
properties using microfracture information, Mesaverde<br />
Group sandstones, San Juan basin, New Mexico. Journal of<br />
Structural Geology, 22, 5, 571–588<br />
SCHLUMBERGER (2007) - www.slb.com<br />
TONDI E., ANTONELLINI M., AYDIN A., MARCHEGIANI L. &<br />
CELLO G. (2006) - Interaction between deformation bands<br />
and pressure solution seams in fault development in<br />
carbonate grainstones of Majella Mountain, Italy. Journal<br />
of Structural Geology, 28, 376-391.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 9-12, 1 f.<br />
Deformation along the lea<strong>di</strong>ng <strong>ed</strong>ge of the Majella thrust sheet,<br />
central Italy.<br />
AGOSTA FABRIZIO (*), EMANUELE TONDI (*), MARCO ANTONELLINI (**) & ATILLA AYDIN (°)<br />
RIASSUNTO<br />
Analisi <strong>del</strong>la deformazione presente lungo il fianco orientale <strong>del</strong>la<br />
Montagna <strong>del</strong>la Majella, Fara San Martino, Abruzzo<br />
Il fianco orientale <strong>del</strong>l’anticlinale <strong>del</strong>la Majella, esposto nella zona <strong>di</strong> Fara<br />
San Martino (CH), mostra un complesso sistema <strong>di</strong> strutture tettoniche<br />
comprenden<strong>di</strong>: (i) kink band <strong>ed</strong> associate faglie inverse, (ii) faglie normali,<br />
(iii) faglie trascorrenti e (iv) fratture estensionali nei calcari <strong>di</strong> piattaforma<br />
Cretacei. In questo lavoro si documentano i meccanismi si formazione e<br />
crescita <strong>di</strong> queste strutture, così come le loro età relative. Le kink band hanno<br />
fianchi molto inclinati, sono orientate a basso angolo rispetto la stratificazione<br />
e sono localizzate al tetto <strong>di</strong> faglie inverse a basso angolo. Al contrario, le<br />
faglie normali sono ad alto angolo <strong>ed</strong> orientate parallelamente alla <strong>di</strong>rezione<br />
degli strati. I due set <strong>di</strong> faglie trascorrenti sono anch’esse ad alto angolo e, pur<br />
essendosi formate attraverso meccanismi <strong>di</strong> <strong>di</strong>ssoluzione per pressione<br />
associati al taglio <strong>di</strong> anisotropie preesistenti, formano una geometria coniugata<br />
<strong>di</strong> tipo Andersoniano. Gli elementi strutturali più recenti sembrano essere<br />
crack molto minuti visibili solo in pochi affioramenti. I vari set <strong>di</strong> faglie<br />
sembrano essere stati attivi più o meno contemporaneamente durante il<br />
piegamento degli strati con modalità <strong>di</strong> deformazione <strong>del</strong> tipo tri-shear.<br />
Parole chiave: anticlinali <strong>di</strong> rampa, Montagna <strong>del</strong>la Majella,<br />
deformazione dei calcari <strong>di</strong> piattaforma, Apennino centrale.<br />
Keywords: thrust-relat<strong>ed</strong> anticline, Majella Mountain,<br />
platform carbonate deformation, central Apennines.<br />
INTRODUZIONE<br />
Understan<strong>di</strong>ng the processes of faulting and fracturing<br />
provides a foundation for pr<strong>ed</strong>ictive mo<strong>del</strong>s for faults and<br />
fractures, inclu<strong>di</strong>ng their orientation, geometry, pattern,<br />
<strong>di</strong>stribution, kinematics, and petrophysical properties. This<br />
knowl<strong>ed</strong>ge is necessary for determining the structural setting of<br />
deform<strong>ed</strong> rocks and regions, and for evaluating fluid flow<br />
pathways in fractur<strong>ed</strong> reservoirs. In some carbonate rock types,<br />
fault initiation and evolution are associat<strong>ed</strong> with opening-mode<br />
fractures such as joints and veins. This has been document<strong>ed</strong> by<br />
_________________________________________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Camerino.<br />
(**) Centro Inter<strong>di</strong>partimentale <strong>di</strong> Ricerca per le Scienze Ambientali,<br />
Università <strong>di</strong> Bologna.<br />
(°) Department of Geological and Environmental Sciences, Stanford<br />
University, CA, USA.<br />
Lavoro eseguito nell’ambito dei progetti: Faults & Fractures in<br />
Carbonates (Università <strong>di</strong> Camerino) e <strong>del</strong> Rock Fracture Project<br />
(Stanford University).<br />
MOLLEMA &ANTONELLINI (1999) in dolomites, and<br />
KELLY et alii (1998), and GROSS & EYAL (2007) in<br />
limestones. The mechanisms describ<strong>ed</strong> by these authors are<br />
similar to those decipher<strong>ed</strong> in other brittle rocks. However,<br />
because carbonate rocks are prone to <strong>di</strong>ssolution under<br />
common geological loa<strong>di</strong>ng con<strong>di</strong>tions in the upper crust,<br />
deformation and failure of carbonate rocks usually involve<br />
pressure solution. Recent examples of carbonate rock<br />
deformation pr<strong>ed</strong>ominantly by pressure solution, and the<br />
subsequent shearing of solution seams, can be found in<br />
ALVAREZ et alii (1978), SALVINI et alii (1999), and BILLI<br />
et alii (2003). In some cases, however, both opening and<br />
closing failure modes simultaneously play<strong>ed</strong> equally important<br />
roles in carbonate rock failure (WILLEMSE et alii 1997;<br />
AGOSTA & AYDIN 2006; and ANTONELLINI et alii 2008).<br />
The con<strong>di</strong>tions lea<strong>di</strong>ng to a mix<strong>ed</strong> failure modes are not yet<br />
well constrain<strong>ed</strong>.<br />
The Majella Mountain offers an extraor<strong>di</strong>nary example of a<br />
marine carbonate that include a broad sequence of the platform<br />
carbonates and associat<strong>ed</strong> slope and basin deposits (VEZZANI<br />
& GHISETTI 1998). All these rocks have been subsequently<br />
deform<strong>ed</strong> during Apennine orogeny. Deformation of the basin<br />
and slope deposits, which consist of alternating turbi<strong>di</strong>tic<br />
grainstones and micritic marls, and its spatial and temporal<br />
variations as a function of the <strong>di</strong>fferent lithotypes were the<br />
subjects of three our recent stu<strong>di</strong>es (TONDI et alii 2006,<br />
ANTONELLINI et alii 2008; AGOSTA et alii 2009). In this<br />
note, we document a number of structural assemblages with an<br />
emphasis on the geometry, <strong>di</strong>stribution, and formation<br />
mechanisms of faults crosscutting the platform carbonates<br />
expos<strong>ed</strong> along the lea<strong>di</strong>ng <strong>ed</strong>ge of the Majella thrust sheet<br />
imm<strong>ed</strong>iately west of the town of Fara San Martino.<br />
PRINCIPAL STRUCTURAL FEATURES<br />
The study area lies along steeper frontal limb of the Majella<br />
anticline in between two spectacular gorges: Vallone Santo<br />
Spirito and Vallone Del Fossato. The area exposes carbonates<br />
of the Cima <strong>del</strong>le Murelle Formation with a b<strong>ed</strong><strong>di</strong>ng striking<br />
approximately N-S and <strong>di</strong>pping to the east about ~30°.Near the<br />
lea<strong>di</strong>ng <strong>ed</strong>ge of the thrust sheet, the <strong>di</strong>p angle increases up to<br />
65-85°. Although there are a few fossiliferous b<strong>ed</strong>s that are<br />
well-defin<strong>ed</strong> by their depositional features, most b<strong>ed</strong>s in the<br />
platform carbonates form in<strong>di</strong>vidual mechanical layers defin<strong>ed</strong><br />
by b<strong>ed</strong>-parallel pressure solution seams (GRAHAM et alii<br />
2003). It is apparent that b<strong>ed</strong>s are thinner and more systematic
10 AGOSTA ET ALII<br />
in the Cima <strong>del</strong>le Murelle Formation than in the Morrone <strong>di</strong><br />
Pacentro Fm. In spite of such <strong>di</strong>fferences, the three mutually<br />
orthogonal sets of pressure solutions, are present, in varying<br />
degrees, within both formations.<br />
These three orthogonal sets consist of one b<strong>ed</strong>-parallel and<br />
two b<strong>ed</strong>-perpen<strong>di</strong>cular pressure solution seams (PS). The<br />
former elements have spacing typically on the order of 0.5 cm<br />
to 20 cm, lengths up to 10’s of meters and define the<br />
mechanical layering of the otherwise massive carbonate rocks.<br />
Considering the two orthogonal sets of PS perpen<strong>di</strong>cular to the<br />
depositional and mechanical, the E-W PS are more continuous<br />
than the N-S orient<strong>ed</strong> PS. The latter set is generally, but not<br />
always, younger and truncates against the other sets; for this<br />
reason, it results shorter than the E-W PS. Spacing of both b<strong>ed</strong>perpen<strong>di</strong>cular<br />
sets is smaller than the thickness of the<br />
mechanical layers in which they occur.<br />
The quality of the outcrops that we have examin<strong>ed</strong> is very<br />
good, but the field work in the study area result<strong>ed</strong> highly<br />
challenging due to the steep topographic gra<strong>di</strong>ent (nearly 1<br />
km/1 km). Thus, these barren slopes, often defin<strong>ed</strong> by smooth<br />
and slippery pavements <strong>di</strong>pping higher than 35°, demand<strong>ed</strong><br />
from geologists a tiresome struggle against the gravity at all<br />
times. Also, the aerial photography is prone to significant<br />
<strong>di</strong>stortion due to high slope. Therefore, we reli<strong>ed</strong> on ground<br />
photographs and detail<strong>ed</strong> sketches for structural documentation.<br />
The study area <strong>di</strong>splays a series of kink bands and numerous<br />
faults with a variety of orientation, kinematics, and <strong>di</strong>stribution<br />
patterns: (i) thrust faults confin<strong>ed</strong> within the steeper limb of<br />
kink bands approximately in strike-parallel orientation, (ii)<br />
normal faults <strong>di</strong>stribut<strong>ed</strong> throughout the eastern slopes of the<br />
Mountain and, approximately, strike-parallel <strong>di</strong>pping generally<br />
down slope, (iii) left-lateral faults at high-angle to the front, and<br />
right-lateral faults at an acute angle to them, (iv) minor hairline<br />
cracks. The relations among these structural features are<br />
report<strong>ed</strong> in the conceptual mo<strong>del</strong> of Fig. 1.<br />
(i) Kink bands and associat<strong>ed</strong> thrust/reverse faults<br />
Perhaps one of the most intriguing types of structures<br />
<strong>di</strong>splay<strong>ed</strong> along the eastern forelimb of the Majella anticline is<br />
a series of kink bands and the associat<strong>ed</strong> thrust/reverse faults<br />
and brecciat<strong>ed</strong> zones. These structures have attract<strong>ed</strong> little<br />
attention in literature in spite of the fact that, together with the<br />
three mutually orthogonal sets of pervasive pressure solution<br />
systems, they represent the actual contraction structures of the<br />
Majella thrust sheet besides the major anticline itself.<br />
Two major continuous kink bands (label<strong>ed</strong>, from east to<br />
west, as K1 and K2) and three short and <strong>di</strong>scontinuous kink<br />
bands are all sub parallel to the project<strong>ed</strong> frontal thrust at the<br />
base of the eastern slope of the Mountain. The kink bands are<br />
monoclinals in which the <strong>di</strong>p angles of the b<strong>ed</strong>s reach 900 , if<br />
not overturn<strong>ed</strong> in a few locations. Because the east facing,<br />
steeper limbs of the kink bands are the locations of greater<br />
internal deformation, we identifi<strong>ed</strong> the approximate boundaries<br />
of these zones by the antiformal and synformal axes along the<br />
two major kink bands, K1 and K2.<br />
The antiformal axis of the kink band K2 is somewhat<br />
irregular and is less certain to define, whereas the synformal<br />
axis is extremely sharp and well-defin<strong>ed</strong>. The kink band<br />
geometry in<strong>di</strong>cates top-to-east shearing, which is consistent<br />
with the overall transport <strong>di</strong>rection of the Majella thrust sheet.<br />
The steeper limb of the kink band is compos<strong>ed</strong> of mechanical<br />
b<strong>ed</strong>s <strong>di</strong>pping to the east of an angle greater than 550. The width<br />
of the steeper limb shows some variability but, on average, falls<br />
between 10 and 30 m. We estimate a top-to-east offset across<br />
each of the two major bands of about 100 m. This offset was<br />
establish<strong>ed</strong> by considering both b<strong>ed</strong><strong>di</strong>ng attitudes and thickness<br />
of the kink bands. The large faults, primarily characteriz<strong>ed</strong> by a<br />
thrust/reverse sense of slip, within K2 are quite <strong>di</strong>scontinuous<br />
but can be mapp<strong>ed</strong> in the field. Although the mechanical b<strong>ed</strong>s<br />
are generally recognizable along K2, they are highly deform<strong>ed</strong><br />
by shear<strong>ed</strong>, pre-existing PS and ad<strong>di</strong>tional splay oblique PS.<br />
The easternmost kink band (K1), south of Vallone Del<br />
Fossato, near the project<strong>ed</strong> location of the main frontal thrust<br />
underlying the Majella sheet, shows localiz<strong>ed</strong> narrow zones of<br />
fragment<strong>ed</strong> and brecciat<strong>ed</strong> rocks along the steeper forelimb of<br />
the kink structure. The antiformal fold axis is well-round<strong>ed</strong> but<br />
still easily identifiable. Some reverse faults and breccia pockets<br />
characteriz<strong>ed</strong> by various degrees of deformation intensity can<br />
be observ<strong>ed</strong> along the steeper limb. Several reverse high-angle<br />
faults, locally utilizing pre-existing b<strong>ed</strong>-parallel PS as well as<br />
the oblique PS, are also present along the steeper limb. The<br />
oblique PS are in an orientation in<strong>di</strong>cating top-to-the-east<br />
motion along the fault. Numerous examples of oblique pressure<br />
solution seams emanating from the high-angle slip surfaces,<br />
in<strong>di</strong>cating reverse sense of slip, have also been observ<strong>ed</strong> in<br />
Vallone Del Fossato, about 200 m to the north of this location.<br />
About 100 m to the south, the steep K1 forelimb is nearly<br />
upright to overturn<strong>ed</strong> and interlac<strong>ed</strong> by high-angle faults, which<br />
locally follow b<strong>ed</strong>-parallel and oblique solution surfaces. These<br />
high-angle faults, along with multiple sets of pressure solution<br />
seams (three orthogonal sets plus oblique sets), weaken parts of<br />
the fold limb to a degree to brecciate the rock. The welldevelop<strong>ed</strong><br />
breccia pocket, whose thickness is at least 30 m, is<br />
the location of a series of catchments that provide water for the<br />
pasta factories nearby. The eastern boundary of this breccia<br />
zone is cover<strong>ed</strong> by, and may possibly merge into, the main<br />
thrust boun<strong>di</strong>ng the Majella sheet. If mergence is the case, the<br />
kink bands along the forelimb of the Majella anticline may be<br />
analogs of imbricate thrusts emanating from the basal thrust.<br />
Bas<strong>ed</strong> on both breccia thickness, which is assum<strong>ed</strong> to be the<br />
central steep limb of the kink band, and b<strong>ed</strong><strong>di</strong>ng orientation,<br />
>850, the amount of top-to-east motion associat<strong>ed</strong> with the<br />
kinks alone may be, at least, a few hundr<strong>ed</strong> meters.<br />
(ii) Normal faults<br />
A system of normal faults with traces parallel to the strike of<br />
the b<strong>ed</strong><strong>di</strong>ng pervasively occurs in the study area. A majority of<br />
the normal fault planes is orient<strong>ed</strong> down-<strong>di</strong>p with respect to<br />
b<strong>ed</strong><strong>di</strong>ng, but some, the smaller ones, are also orient<strong>ed</strong> up-<strong>di</strong>p<br />
(antithetic). In the following paragraphs we report the main<br />
results of a few select<strong>ed</strong> examples previously stu<strong>di</strong><strong>ed</strong> along the<br />
eastern limb of the Majella anticline by GRAHAM et alii<br />
(2003).<br />
The authors propos<strong>ed</strong> a mechanism for the initiatiiion and<br />
growth of normal faults in the Cretaceous platform carbonates,<br />
which consist on the shearing of the pre-existing b<strong>ed</strong>-parallel<br />
and b<strong>ed</strong>-perpen<strong>di</strong>cular, strike-parallel pressure solution seams<br />
propell<strong>ed</strong> by flexural fol<strong>di</strong>ng. Shearing across these PS sets<br />
produc<strong>ed</strong> oblique splay PS that, eventually, link<strong>ed</strong> the preexisting<br />
structures from one mechanical layer to the adjacent
DEFORMATION ALONG THE LEADING EDGE OF THE MAJELLA THRUST SHEET, CENTRAL ITALY.)<br />
ones. The oblique PS play<strong>ed</strong> an important role in the initiation<br />
and growth of the normal faults along the lea<strong>di</strong>ng <strong>ed</strong>ge of the<br />
thrust sheet. This set form<strong>ed</strong> at an angle ranging from 30° to<br />
75° to the two aforemention<strong>ed</strong> orthogonal sets, and typically<br />
truncates against them. The oblique set is commonly locat<strong>ed</strong> at<br />
their contractional quadrants, and around geometric<br />
complexities, but it has also found elsewhere along the length<br />
of the b<strong>ed</strong>-parallel seams. These oblique PS, along with the<br />
shear<strong>ed</strong> pre-existing PS, fragment<strong>ed</strong> the rock to make it weak<br />
enough to form through-going normal fault zones.<br />
The thick carbonate packages presumably deform<strong>ed</strong> due to<br />
flexural-slip along the b<strong>ed</strong>-parallel solution surfaces. As tilting<br />
and rotation of the b<strong>ed</strong>s continu<strong>ed</strong>, b<strong>ed</strong>-parallel slip proce<strong>ed</strong><strong>ed</strong><br />
producing numerous oblique PS that connect<strong>ed</strong> b<strong>ed</strong>-parallel PS<br />
and b<strong>ed</strong>-perpen<strong>di</strong>cular, strike-parallel PS. These <strong>di</strong>scontinuities<br />
fragment<strong>ed</strong> the rock in pockets that localiz<strong>ed</strong> both within and at<br />
the interfaces of the mechanical layers. These weak zones<br />
eventually coalesc<strong>ed</strong> across mechanical layer boundaries,<br />
lea<strong>di</strong>ng to the development of one or more thorough-going slip<br />
surfaces flanking fine-grain<strong>ed</strong> fault rocks. There exists a set of<br />
antithetic normal faults, which occur along the PS<br />
perpen<strong>di</strong>cular to b<strong>ed</strong><strong>di</strong>ng and parallel to the strike of the b<strong>ed</strong>s.<br />
One more set, which does not fit the normal fault group<br />
characteriz<strong>ed</strong> earlier, it is also recogniz<strong>ed</strong>. Bas<strong>ed</strong> on the<br />
intersection relations, they appear to be the oldest set. Only one<br />
relatively larger strike-slip fault appears to cut across, with<br />
complex intersection geometries, at two locations <strong>di</strong>splacing<br />
these normal faults. The earlier normal faults have low-angle<br />
<strong>di</strong>ps at present time, intersect the b<strong>ed</strong><strong>di</strong>ng at about 60°, and<br />
probably form<strong>ed</strong> when the b<strong>ed</strong>s were flat. Normal faults in<br />
similar relationship to the tilt<strong>ed</strong> b<strong>ed</strong>s were also report<strong>ed</strong> by<br />
MARCHEGIANI et alii (2006) from <strong>di</strong>fferent parts of the<br />
eastern front of the Majella Mountain.<br />
(iii) Strike-slip faults<br />
Strike-slip faults in the study area are organiz<strong>ed</strong> in, at least,<br />
two major sets. One set of these faults is about vertical, trends<br />
almost E-W, and is characteriz<strong>ed</strong> by left-lateral slip. The other<br />
set trends NE, oblique to the first set at angle of about 20-500,<br />
and shows right-lateral kinematics. The first set is generally<br />
better develop<strong>ed</strong> and controls the site of the second set. The<br />
localization of the right-lateral set in between sub parallel leftlateral<br />
faults, as well as only on one side of them is clear at a<br />
large scale. The acute intersection angle between the left-lateral<br />
and right-lateral faults is reminiscence of that between a shear<br />
fracture and its splays. A third set, which is merely a lineament,<br />
is spatially relat<strong>ed</strong> to the first set. Unfortunately, neither the<br />
sense nor the amount of slip are shown.<br />
It is <strong>di</strong>fficult to determine the slip across many of these<br />
strike-slip faults because of the lack of marker b<strong>ed</strong>s and limit<strong>ed</strong><br />
accessibility. Nevertheless, at one location near the end of a<br />
left-lateral fault, a fossiliferous b<strong>ed</strong> has been <strong>di</strong>splac<strong>ed</strong> by about<br />
7.5 to 8m. The fault core at this location includes a brecciat<strong>ed</strong><br />
fault rock of about 0.5m in thickness, which is comparable to<br />
that expect<strong>ed</strong> from a fault with 5 to 20m slip range<br />
(ANTONELLINI et alii, 2008). Considering that this fault<br />
appears to <strong>di</strong>e out to the east, the maximum slip near the center<br />
of this fault should be significantly greater than 8m.<br />
Detail<strong>ed</strong> observations and maps of the two sets of strike-slip<br />
faults suggest that they occur in a hierarchical order. In a<br />
11<br />
detail<strong>ed</strong> view, the abutting relationships appear to be mutual.<br />
Kinematical relations observ<strong>ed</strong> in the field (opposing sense of<br />
slip) in<strong>di</strong>cate the hierarchical formation of multiple generations<br />
of faults with intersection angles comparable to those between<br />
primary shear fractures and their secondary splay pressure<br />
solutions (FLETCHER AND POLLARD, 1981). We refer to<br />
this kind of strike-slip network as an “apparent conjugate”<br />
pattern in order to <strong>di</strong>stinguish it from the Andersonian fault<br />
mo<strong>del</strong>, which has <strong>di</strong>fferent implications for the mechanics and<br />
evolution of the fault systems. The pattern we document<strong>ed</strong><br />
develop<strong>ed</strong> by shearing of E-W orient<strong>ed</strong>, b<strong>ed</strong>-perpen<strong>di</strong>cular,<br />
cross pressure solution system with formation of oblique splay<br />
solution seams, and subsequent shearing of these splay features.<br />
(iv) Minor hairline cracks<br />
On one clean<strong>ed</strong> and polish<strong>ed</strong> pavement (courtesy of Eni<br />
Agip) cropping out right at the entrance of the Vallone Santo<br />
Fig. 1 – Conceptual mo<strong>del</strong> representing the relations among the<br />
structural features document<strong>ed</strong> along the trailing <strong>ed</strong>ge of the Majella<br />
thrust sheet (note that pss stands for pressure solution seams).<br />
Spirito, some cracks with apparent opening and local shearing<br />
are present. These cracks are short and <strong>di</strong>scontinuous, and<br />
cannot be seen anywhere else. This limit<strong>ed</strong> site observation<br />
in<strong>di</strong>cates that they are younger than b<strong>ed</strong>-perpen<strong>di</strong>cular PS and<br />
shear<strong>ed</strong> b<strong>ed</strong>-perpen<strong>di</strong>cular PS. Bas<strong>ed</strong> upon the drastically<br />
<strong>di</strong>fferent failure mode from all other structures we describ<strong>ed</strong><br />
before, it is likely that these cracks are the youngest structural<br />
features, possibly due to the latest uplift and down faulting.<br />
CONCLUSIONS<br />
We have describ<strong>ed</strong> a rich variety of structures within the<br />
Cretaceous platform carbonates expos<strong>ed</strong> along the frontal<br />
eastern limb of the Majella Anticline, imm<strong>ed</strong>iately above the<br />
lea<strong>di</strong>ng <strong>ed</strong>ge of the underlying thrust fault. These are pressure<br />
solution seams (PS), normal and strike-slip faults, kink bands<br />
and relat<strong>ed</strong> reverse/thrust faults, and some minor hairline<br />
cracks. The assemblages of the three mutually orthogonal PS<br />
are of pre tilting origin, and are instrumental in the formation of<br />
all three major types of faults as well as of the kink bands.
12 AGOSTA ET ALII<br />
The opening-mode cracks are the youngest structural<br />
features, and are minor in terms of spatial frequency and<br />
magnitude of strain that they accommodate. On the other hand,<br />
the three mutually orthogonal PS belonging to the pre-tilting<br />
phase are the most crucial structural elements, which<br />
introduc<strong>ed</strong> the initial mechanical anisotropy, or weak planes,<br />
within the massive platform carbonates. In the study area, they<br />
instigat<strong>ed</strong> all three types of synorogenic faults, as well as l<strong>ed</strong> to<br />
kink bands formation. The b<strong>ed</strong>-parallel PS creat<strong>ed</strong> the<br />
mechanical b<strong>ed</strong><strong>di</strong>ng within the platform carbonates and are<br />
responsible for two of the most intriguing structures describ<strong>ed</strong><br />
in this paper, kink bands and normal faults.<br />
Also crucial in terms of the developmental process of the<br />
faults, are the two orthogonal transverse sets of PS<br />
perpen<strong>di</strong>cular to the mechanical b<strong>ed</strong>s: one parallel to the strike<br />
and the second one to the <strong>di</strong>p <strong>di</strong>rection. Shearing across the<br />
b<strong>ed</strong>-perpen<strong>di</strong>cular strike-parallel PS is thought to be<br />
responsible for the formation of normal faults the context of<br />
flexural-slip of the Majella anticline forelimb. On the contrary,<br />
shearing across the b<strong>ed</strong>-perpen<strong>di</strong>cular, <strong>di</strong>p-parallel PS is<br />
assum<strong>ed</strong> to be responsible for strike-slip fault formation.<br />
Strike-slip faults occur hierarchically in two sets, which form<br />
in an apparent conjugate pattern. The two mechanisms that<br />
produc<strong>ed</strong> this pattern are the following: (1) shearing of the b<strong>ed</strong>perpen<strong>di</strong>cular,<br />
<strong>di</strong>p-parallel PS with right- or left-stepping<br />
patterns, and (2) shearing of the b<strong>ed</strong>-perpen<strong>di</strong>cular, <strong>di</strong>pparallel<br />
PS and associat<strong>ed</strong> splaying, and subsequent shearing of<br />
these splays with an opposite sense of slip. The spatial<br />
relationship between the frontal thrust and the sub-parallel kink<br />
bands in the hanging wall is kinematically and geometrically<br />
consistent except where the boundary has been obliterat<strong>ed</strong> by<br />
younger faults. Development of breccia zones along the steeper<br />
limb of the kink bands, and of high-angle thrust faults therein,<br />
is also consistent with the contractional nature of deformation<br />
along the lea<strong>di</strong>ng <strong>ed</strong>ge of a thrust sheet.<br />
REFERENCES<br />
AGOSTA F., AYDIN A., 2006 - Architecture and Deformation<br />
Mechanism of a Basin-Boun<strong>di</strong>ng Normal Fault in Mesozoic<br />
Platform Carbonates, Central Italy. Journal of Structural<br />
Geology, 28, 2445-2467.<br />
AGOSTA F., ALESSANDRONI M., & TONDI. E. (2009) - Oblique<br />
normal faulting along the northern <strong>ed</strong>ge of the Majella<br />
anticline, central Italy: inferences on hydrocarbon<br />
migration and accumulation. Journal of Structural Geology,<br />
in press.<br />
ALVAREZ W., ENGELDER T., GEISER P.A., 1978 - Classification of<br />
solution cleavage in pelagic limestones. Geology, 6, 263-<br />
266.<br />
ANTONELLINI M., TONDI E., AGOSTA F., AYDIN A., CELLO G., 2008<br />
- Evolution of strike slip faults in calcarenites and marls of<br />
basinal carbonate rocks from Majella Mountain, central<br />
Italy. Marine and Petroleum Geology, 25, 1074-1096.<br />
BILLI A., SALVINI F., STORTI F., 2003 - The Damage zone-fault<br />
core transition in carbonate rocks: implications for fault<br />
growth, structure and permeability. Journal of Structural<br />
Geology, 25, 1779-1794.<br />
FLETCHER R.C., POLLARD D.D., 1981 - Anticrack mo<strong>del</strong> for<br />
pressure solution surfaces. Geology, 9, 419-424.<br />
GRAHAM B., ANTONELLINI M., AYDIN A., 2003 - Formation and<br />
growth of normal faults in carbonates within a<br />
compressional environment. Geology, 31, 11-14.<br />
GRAHAM-WALL B., GIRGACEA R., MESONJESI A. & AYDIN A.<br />
(2006) - Evolution of fluid pathways through fracture<br />
controll<strong>ed</strong> faults in carbonates of the Albanides fold-thrust<br />
belt. American Association of Petroleum Geologists<br />
Bulletin, 90, 1227-1249.<br />
GROSS M., EYAL Y., E. (2007) - Throughgoing fractures in<br />
layer<strong>ed</strong> carbonate rocks. Geological Society of America<br />
Bulletin, 119, 1387-1404.<br />
MARCHEGIANI L., VAN DIJK J. P., GILLESPIE P. A., TONDI E. &<br />
CELLO G. (2006) - Scaling properties of the <strong>di</strong>mensional and<br />
spatial characteristics of fault and fracture systems in the<br />
Majella Mountain, central Italy. In: CELLO G. &<br />
MALAMUD B. (Eds) Fractal Analysis for Natural Hazards.<br />
Geological Society of London, Special Publications, 261,<br />
113–131.<br />
MOLLEMA P., ANTONELLINI M., 1999 - Development of strikeslip<br />
faults in the Dolomites of the Sella Group, Northern<br />
Italy. Journal of Structural Geology, 21, 273-292.<br />
KELL P. G., SANDERSON D. J., PEACOCK D. C. P., 1998 - Linkage<br />
and evolution of conjugate strike-slip fault zones in<br />
limestone of Summerset and Northumbria. Journal of<br />
Structural Geology, 20, 1477-1493.<br />
SALVINI F., BILLI A., WISE D.U., 1999 - Strike-slip faultpropagation<br />
cleavage in carbonate rocks: the Mattinata<br />
Fault zone, Southern Apennines, Italy. Journal of Structural<br />
Geology, 21, 1731–1749.<br />
TONDI E., ANTONELLINI M., AYDIN A., MARCHEGIANI L. & CELLO<br />
G. (2006) - Interaction between deformation bands and<br />
pressure solution seams in fault development in carbonate<br />
grainstones of Majella Mountain, Italy. Journal of<br />
Structural Geology, 28, 376-391.<br />
VEZZANI L., GHISETTI F., 1998. Carta Geologica Dell'Abruzzo.<br />
Regione Abruzzo. E<strong>di</strong>zioni SE.L.C.A, Firenze, scala<br />
1:100,000.<br />
WILLEMSE E., PEACOCK D., AYDIN A., 1997 - Nucleation and<br />
growth of strike-slip faults in limestones from Somerset,<br />
U.K. Journal of Structural Geology, 19, 1461-1477.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 13-16, 4 ff.<br />
Il ruolo <strong>del</strong>l’er<strong>ed</strong>ità strutturale nello sviluppo <strong>del</strong>la catena<br />
appenninica: l’esempio <strong>del</strong>la Montagna Grande e <strong>del</strong> Monte<br />
Genzana (Appennino Centrale Abruzzese)<br />
ABSTRACT<br />
The role of structural inheritance on the development of Apennine chain:<br />
an example from Montagna Grande and Monte Genzana in the Abruzzi<br />
region (Central Apennines)<br />
In the Marsica area of the Abruzzi region (Central Apennines), geological and<br />
structural study carri<strong>ed</strong> out in this paper allow<strong>ed</strong> us to characterize the high<br />
angle faults, which juxtapos<strong>ed</strong> the Messinian siliciclastic deposits to the<br />
Jurassic-Cretaceous carbonate succession, as Miocene pre-thrusting normal<br />
faults relat<strong>ed</strong> to for<strong>ed</strong>eep basin flexural process. These syn-orogenic normal<br />
faults have NW-SE, NNW-SSE attitude, parallel to the Montagna Grande and<br />
Monte Genzana fold axial trend, a length of about 15 km and a maximum<br />
<strong>di</strong>splacement of about 1.5 km. Surface data integrat<strong>ed</strong> with sub-surface<br />
information allow<strong>ed</strong> us to recogniz<strong>ed</strong> Miocene normal faults rotat<strong>ed</strong>, until to<br />
assume an apparent high angle reverse attitude (Monte Genzana Fault), and<br />
<strong>di</strong>splac<strong>ed</strong> during Messinian-early Pliocene thrust and back-thrust propagation,<br />
accor<strong>di</strong>ng to a shortcut trajectory and development of characteristic<br />
buttressing structures.<br />
The reconstruct<strong>ed</strong> kinematic and dynamic relationship between thrusts<br />
and Miocene Normal faults has been validat<strong>ed</strong> in the restor<strong>ed</strong> template of the<br />
geological sections, that allow<strong>ed</strong> us to evaluate a maximum shortening value<br />
between the Montagna Grande and Monte Genzana anticlines of about 3 km<br />
and to reconstruct an inversion tectonic mo<strong>del</strong> characteriz<strong>ed</strong> by a shortcut<br />
trajectory of thrusts across high angle Miocene syn-orogenic normal faults<br />
which controll<strong>ed</strong> the physiography of the Messinian for<strong>ed</strong>eep basin and<br />
represent the high angle faults outcropping in the stu<strong>di</strong><strong>ed</strong> area.<br />
Key words: Central Apennines, inversion tectonics, structural<br />
inheritance.<br />
INTRODUZIONE<br />
Nella catena <strong>del</strong>l’Appennino centrale abruzzese, le relazioni<br />
geometriche tra le strutture <strong>del</strong>la Montagna Grande e <strong>del</strong> Monte<br />
Genzana sono state associate alla presenza <strong>di</strong> sovrascorrimenti<br />
regionali caratterizzati da importanti entità <strong>di</strong> raccorciamento<br />
(CRESCENZI & MICCADEI, 1990; D’ANDREA et alii, 1992;<br />
PATACCA et alii, 1992; MICCADEI, 1993) o <strong>di</strong> sovrascorrimenti<br />
locali con raccorciamento conservativo <strong>di</strong> qualche chilometro<br />
(BENEO, 1938; COLACICCHI, 1967), caratterizzati da traiettorie<br />
<strong>di</strong> shortcut attraverso faglie normali giurassiche connesse allo<br />
sviluppo <strong>del</strong> paleomargine (PACE et alii, 2005). Diversamente,<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università degli Stu<strong>di</strong> <strong>di</strong> Chieti e<br />
Pescara, via dei Vestini 30, 66013 Chieti Scalo (Chieti), Italy<br />
e-mail: calamita@unich.it<br />
AGOSTINI S.(*) & CALAMITA F. (*)<br />
CORRADO et alii (1996) evidenziano nella stessa area faglie ad<br />
alto angolo connesse ad una tettonica trascorrente sin- e postcatena.<br />
PATACCA et alii (1992) hanno attribuito al bacino <strong>di</strong><br />
avanfossa <strong>del</strong> Monte Genzana una posizione più interna<br />
rispetto a quello <strong>del</strong>la Montagna Grande con forte traslazione<br />
dall’unità Gran Sasso – Genzana. Recentemente PATACCA &<br />
SCANDONE (2007) attribuiscono la Montagna Grande e il<br />
Monte Genzana allo stesso dominio <strong>di</strong> avanfossa e alla stessa<br />
unità tettonica.<br />
Lo stu<strong>di</strong>o geologico-strutturale condotto in questo lavoro è<br />
stato finalizzato a caratterizzare le relazioni tra le strutture <strong>del</strong>la<br />
Montagna Grande e <strong>del</strong> Monte Genzana allo scopo <strong>di</strong> portare<br />
un contributo alla definizione dei rapporti tra le strutture <strong>del</strong>la<br />
catena centro-appenninica e le faglie normali presovrascorrimento<br />
(er<strong>ed</strong>ità strutturale), fondamentale nella<br />
valutazione <strong>del</strong>lo stile tettonico e <strong>del</strong>l’entità <strong>di</strong> raccorciamento<br />
<strong>del</strong>la catena.<br />
ASSETTO STRUTTURALE<br />
Le strutture <strong>del</strong>la catena centro-appenninica coinvolgono le<br />
successioni triassico-mioceniche <strong>di</strong> piattaforma carbonatica<br />
persistente e <strong>di</strong> scarpata-bacino associate e i depositi<br />
silicoclastici <strong>di</strong> avanfossa messiniano-pliocenici,<br />
progressivamente più recenti verso l’avampaese adriatico.<br />
Nell’Appennino abruzzese, i sovrascorrimenti e le pieghe<br />
associate hanno un andamento NW-SE e si raccordano alla<br />
rampa obliqua Sangro-Volturno ad andamento NNE-SSW, che<br />
caratterizza l’arco <strong>del</strong>l’Appennino centro-settentrionale<br />
(CALAMITA et alii, 2004). In questo lavoro sono state analizzate<br />
le strutture <strong>del</strong>la Montagna Grande e <strong>del</strong> Monte Genzana che<br />
coinvolgono rispettivamente la successione <strong>di</strong> piattaforma<br />
carbonatica-scarpata e <strong>di</strong> scarpata-bacino pelagico e i<br />
sovrastanti depositi silicoclastici <strong>di</strong> avanfossa messiniani pre- e<br />
sin-evaporitici. Si tratta <strong>di</strong> monoclinali carbonatiche ad<br />
immersione a NE, con strati che progressivamente verso nordest<br />
assumono una maggiore pendenza sino a circa 70 gra<strong>di</strong>,<br />
sovrascorse verso NE sui depositi silicoclastici. Verso ovest,<br />
tali monoclinali sono a contatto tettonico, lungo faglie regionali<br />
ad alto angolo <strong>di</strong> pendenza e andamento NW-SE, con i depositi<br />
silicoclastici messiniani. Nell’ambito <strong>di</strong> tale assetto<br />
monoclinalico, gli strati <strong>del</strong>la successione carbonatica<br />
assumono localmente immersione a SW evidenziando blande<br />
anticlinali ad andamento assiale NW-SE, con un fianco sud-
14 S.AGOSTINI & F.CALAMITA<br />
Fig. 1 – Schema strutturale e sezioni geologiche <strong>del</strong>l’area in esame realizzati integrando i dati presentati in questo lavoro con quelli bibliografici (BENEO, 1938;<br />
COLACICCHI, 1967; CRESCENZI & MICCADEI, 1991; MICCADEI, 1993; PACE et alii, 2001).<br />
- Structural sketch and geological sections of the stu<strong>di</strong><strong>ed</strong> area reconstruct<strong>ed</strong> also using publish<strong>ed</strong> data (BENEO, 1938; COLACICCHI, 1967; CRESCENZI &<br />
MICCADEI, 1991; MICCADEI, 1993; PACE et alii, 2001).<br />
occidentale poco sviluppato.<br />
L’assetto geologico-strutturale ricostruito in questo lavoro<br />
integrando i dati bibliografici (BENEO, 1938; COLACICCHI,<br />
1967; CRESCENZI & MICCADEI, 1991; MICCADEI, 1993; PACE<br />
et alii, 2001) è schematizzato nella Fig. 1. Le sezioni<br />
geologiche e le relative retrodeformate hanno consentito <strong>di</strong><br />
definire le relazioni geometriche tra le strutture <strong>del</strong>la Montagna<br />
Grande e <strong>del</strong> Monte Genzana e <strong>di</strong> caratterizzare i contatti<br />
tettonici ad alto angolo seguibili a scala regionale tra la<br />
successione carbonatica e i depositi silicoclastici messiniani<br />
(Fig. 1).<br />
La successione carbonatica <strong>del</strong>la Montagna Grande è<br />
generalmente sovrascorsa sui depositi silicoclastici messiniani,<br />
ad eccezione <strong>del</strong> tratto in prossimità <strong>di</strong> Anversa degli Abruzzi,<br />
ove si osserva un passaggio stratigrafico (Fig. 1).<br />
I depositi silicoclastici, caratterizzati da una ripetizione<br />
marcata dalla presenza dei calcari miocenici in strati subverticali,<br />
sono coinvolti in strette pieghe e vengono a contatto<br />
con la successione carbonatica <strong>del</strong> Monte Genzana lungo una<br />
faglia inversa (faglia La Difesa) ad alto angolo immergente a<br />
NE, caratterizzata dai dati <strong>di</strong> sottosuolo (BENEO, 1938). Essa è<br />
parte integrante <strong>del</strong> sistema <strong>di</strong> faglie che <strong>del</strong>imitano a SW la<br />
struttura <strong>del</strong> Monte Genzana e si segue con continuità verso sud<br />
fino a Frattura, ove è costituita da una faglia normale
IL RUOLO DELL’EREDITÀ STRUTTURALE NELLO SVILUPPO DELLA CATENA APPENNINICA: L’ESEMPIO DELLA MONTAGNA GRANDE E DEL<br />
MONTE GENZANA (APPENNINO CENTRALE ABRUZZESE)<br />
Fig. 2 – a) Piano <strong>di</strong> faglia La Difesa-Genzana (F) <strong>di</strong>slocato da piani coniugati inversi associati al buttressing (I); b) dettaglio dei piani coniugati (I); c) scarpata<br />
<strong>di</strong> faglia che suggerisce un attività quaternaria per la faglia La Difesa-Genzana.<br />
- a) La Difesa-Genzana fault plane (F) <strong>di</strong>splac<strong>ed</strong> by conjugate reverse mesofaults(I) as effect of buttressing; b) close-up showing the conjugate reverse planes<br />
(I); c) fault scarp suggesting quaternary activity of La Difesa-Genzana fault.<br />
immergente a SE con circa 40 gra<strong>di</strong> <strong>di</strong> pendenza (Fig. 2). Il<br />
piano <strong>di</strong> faglia è <strong>di</strong>slocato da mesofaglie coniugate inverse,<br />
interpretabili come strutture da buttressing connesse al ruolo <strong>di</strong><br />
ostacolo <strong>del</strong>la faglia stessa durante lo sviluppo dei piani <strong>di</strong><br />
sovrascorrimento (Figg. 2a e 2b). Alla faglia si associa una<br />
scarpata <strong>di</strong> faglia connessa alla sua riattivazione quaternaria,<br />
responsabile <strong>del</strong> tilting dei depositi continentali quaternari <strong>di</strong><br />
Frattura (Fig. 2c). L’insieme dei dati descritti consente <strong>di</strong><br />
caratterizzare la faglia La Difesa-Genzana come una faglia<br />
normale miocenica, contemporanea alla deposizione dei<br />
depositi silicoclastici messiniani e connessa alla flessurazione<br />
<strong>del</strong>l’avampaese, ruotata fino ad assumere un apparente carattere<br />
inverso (faglia La Difesa) e <strong>di</strong>slocata dai motivi inversi a basso<br />
angolo, sviluppati durante la strutturazione <strong>del</strong>la catena<br />
(Messiniano post-crisi <strong>di</strong> salinità-Pliocene) e riutilizzata dove<br />
ha conservato una giacitura compatibile con l’estensione<br />
Fig.3 - Mesofaglie (F), caratterizzate da superfici stilolitiche, che<br />
<strong>di</strong>slocano la superficie <strong>di</strong> strato (S0) <strong>del</strong>le calcareniti mioceniche.<br />
- Mesofaults (F), characteriz<strong>ed</strong> by stylolithic surfaces, that <strong>di</strong>splac<strong>ed</strong> the<br />
b<strong>ed</strong><strong>di</strong>ng (S0) of Miocene calcarenites.<br />
quaternaria (loc. Frattura). In tutta l’area esaminata sono stati<br />
osservati numerosi sistemi <strong>di</strong> faglie inverse coniugate<br />
interpretate in questo lavoro come sistemi <strong>di</strong> faglie normali<br />
ruotate, attribuibili all’estensione associata alla flessurazione<br />
<strong>del</strong>l’avanfossa messiniana. Al top <strong>del</strong>le calcareniti mioceniche<br />
affioranti lungo la sezione BB’ si riconosce un sistema<br />
coniugato <strong>di</strong> mesofaglie con lunghezza nell’or<strong>di</strong>ne <strong>del</strong> metro e<br />
<strong>di</strong>slocazione centimetrica. Tali strutture presentano una<br />
superficie stilolitica che ne documenta un processo <strong>di</strong><br />
<strong>di</strong>ssoluzione per pressione successivo al loro sviluppo (Fig. 3).<br />
L’insieme dei dati e <strong>del</strong>le ricostruzioni ha vincolato<br />
l’esecuzione <strong>del</strong>la sezione B-B’ <strong>di</strong> Fig. 1, che consente <strong>di</strong><br />
stimare una entità <strong>di</strong> raccorciamento tra la Montagna Grande e<br />
il Monte Genzana <strong>di</strong> circa 1.5. Inoltre, la retrodeformazione<br />
<strong>del</strong>la sezione permette <strong>di</strong> valicare l’evoluzione ricostruita in<br />
questo lavoro. Nel settore più meri<strong>di</strong>onale <strong>del</strong>l’area, compreso<br />
tra il Monte Marsicano e Monte Greco, il presente stu<strong>di</strong>o ha<br />
messo in evidenza che tali strutture sin-s<strong>ed</strong>imentarie<br />
mioceniche rappresentano la riattivazione <strong>di</strong> faglie normali preorogeniche<br />
(Fig. 1, sezione D-D’). La retrodeformazione <strong>del</strong>la<br />
sezione geologica (Fig. 1 sezione d) ha consentito <strong>di</strong> stimare<br />
entità massima <strong>di</strong> raccorciamento tra la Montagna Grande e il<br />
Monte Genzana <strong>di</strong> circa 2.5 km e <strong>di</strong> confermare che i contatti<br />
tettonici ad alto angolo sono rappresentati dalle faglie normali<br />
mioceniche. L’attività <strong>di</strong> tali strutture ha portato allo sviluppo<br />
<strong>di</strong> megabrecce carbonatiche, rinvenibili nei depositi<br />
silicoclastici messiniani, <strong>ed</strong> è connessa all’estensione flessurale<br />
<strong>del</strong>l’avampaese.<br />
CONCLUSIONI<br />
Le relazioni geometriche tra le strutture <strong>del</strong>la catena centroappenninica<br />
abruzzese <strong>del</strong>la Montagna Grande e <strong>del</strong> Monte<br />
Genzana sono state <strong>di</strong>versamente interpretate dagli autori, che<br />
hanno dato particolare significato a motivi <strong>di</strong> sovrascorrimento<br />
con forti entità <strong>di</strong> raccorciamento (D’ANDREA et alii, 1992) o
16 S.AGOSTINI & F.CALAMITA<br />
Fig.4 - a) Sistema coniugato <strong>di</strong> faglie normali alla mesoscala connesso<br />
all’estensione associata alla flessurazione <strong>del</strong>l’avanfossa; b) con l’inizio<br />
<strong>del</strong>la fase compressiva le <strong>di</strong>scontinuità preesistenti hanno agito come<br />
precursori fragili ai fenomeni <strong>di</strong> presso-soluzione; c) situazione attuale a<br />
seguito <strong>del</strong>la rotazione connessa al piegamento.<br />
a) development of normal faults sistem during foreland-for<strong>ed</strong>eep<br />
evolution; b) the pre-thrusting meso-faults assum<strong>ed</strong> the role of brittle<br />
precursor localizing subsequent pressure-solution deformation<br />
document<strong>ed</strong> by stylolithes; c) present day attitude relat<strong>ed</strong> to fol<strong>di</strong>ng.<br />
con entità molto conservativa (COLACICCHI, 1967). Le<br />
geometrie ad alto angolo riscontrabili in tale settore <strong>del</strong>la<br />
catena hanno portato altri autori a dare particolare risalto alla<br />
tettonica trascorrente sin- e post-catena (CORRADO et alii,<br />
1996). Lo stu<strong>di</strong>o geologico strutturale condotto in questo<br />
lavoro ha consentito <strong>di</strong> evidenziare per le faglie ad alto angolo,<br />
che pongono a contatto i depositi silicoclastici messiniani con<br />
le successioni carbonatiche, un carattere principalmente<br />
normale, una entità <strong>di</strong> rigetto massima <strong>del</strong>l’or<strong>di</strong>ne <strong>di</strong> 1500<br />
metri, una lunghezza massima <strong>di</strong> circa 15 Km <strong>ed</strong> un età<br />
miocenica sin-depositi silicoclastici stessi (Fig.1). Tali faglie<br />
pre-sovrascorrimento sono state ruotate durante la<br />
strutturazione <strong>del</strong>la catena fino ad assumere un apparente<br />
carattere inverso (come evidente per la Faglia La Difesa),<br />
<strong>di</strong>slocate da piani <strong>di</strong> sovrascorrimento e retroscorrimento a<br />
basso angolo e riutilizzate durante la tettonica <strong>di</strong>stensiva<br />
quaternaria (loc. Frattura). Ad una scala <strong>di</strong> maggior dettaglio, il<br />
suddetto contesto <strong>di</strong> tettonica d’inversione è documentato dalla<br />
presenza <strong>di</strong> superfici stitlolitiche lungo i piani <strong>di</strong> faglia<br />
centimetrici con attuale giacitura inversa (Fig. 3). Questi sono<br />
interpretabili come un sistema <strong>di</strong> faglie normali, ruotato durante<br />
lo sviluppo <strong>del</strong>la catena, che ha agito come un precursore<br />
fragile (sensu MANCKTELOW & PENNACCHIONI, 2005) nella<br />
localizzazione <strong>del</strong>la <strong>di</strong>ssoluzione per pressione, durante la fase<br />
iniziale <strong>di</strong> raccorciamento (Fig. 4). Nel settore più meri<strong>di</strong>onale<br />
<strong>del</strong>l’area (compreso tra il Monte Marsicano e Monte Greco) il<br />
presente stu<strong>di</strong>o ha messo in evidenza che tali strutture sins<strong>ed</strong>imentarie<br />
mioceniche rappresentano la riattivazione <strong>di</strong> faglie<br />
normali pre-orogeniche (Fig. 1, sezione D-D’). La<br />
retrodeformazione <strong>del</strong>le sezioni geologiche (Fig. 1) ha<br />
consentito <strong>di</strong> validare l’interpretazione proposta, mettendo in<br />
evidenza una entità massima <strong>di</strong> raccorciamento tra la Montagna<br />
Grande e il Monte Genzana <strong>del</strong>l’or<strong>di</strong>ne <strong>di</strong> qualche chilometro.<br />
Infine il mo<strong>del</strong>lo <strong>di</strong> tettonica d’inversione proposto considera<br />
una traiettoria in short-cut a basso angolo dei piani <strong>di</strong><br />
sovrascorrimento e retroscorrimento attraverso le faglie normali<br />
pre-sovrascorrimento con sviluppo <strong>di</strong> anticlinali <strong>di</strong> short-cut.<br />
BIBILIOGRAFIA<br />
BENEO E. (1938) - Insegnamenti <strong>di</strong> una galleria a proposito<br />
<strong>del</strong>la tettonica nella Valle <strong>del</strong> Sagittario (Appennino<br />
Abruzzese). Boll. R. Uff. Geol. D’It., 65, 1-10. <br />
CALAMITA F., VIANDANTE M.G. & HEGARTY K. (2004) -<br />
Pliocene-Quaternary burial/exhumation paths of the<br />
Central Apennines (Italy). Boll. Soc. Geol. It., 123 , 503-<br />
512.<br />
COLACICCHI R. (1967) - Geologia <strong>del</strong>la Marsica Orientale.<br />
Geologica Romana, 6, 189-316.<br />
CORRADO S., MICCADEI E., PAROTTO M. & SALVINI F. (1996) -<br />
Evoluzione tettonica <strong>del</strong> settore <strong>di</strong> Montagna Grande<br />
(Appennino Centrale): il contributo <strong>di</strong> nuovi dati<br />
geometrici, cinematici e paleogeotermici. Boll. Soc. Geol.<br />
It., 115, 325-338.<br />
CRESCENZI B. & MICCADEI E. (1990) - Nuovi dati sull’assetto<br />
geologico-strutturale <strong>del</strong>la Marsica nord-orientale<br />
(Abruzzo, Appennino Centrale). Mem. Soc. Geol. It., 45,<br />
555-562.<br />
D’ANDREA M, MICCADEI E. & PRATURLON A. (1992) -<br />
Rapporti tra il margine orientale <strong>del</strong>la piattaforma Laziale-<br />
Abruzzese <strong>ed</strong> il margine occidentale <strong>del</strong>la piattaforma<br />
Morrone-Pizzalto-Rotella. Stu<strong>di</strong> geologici Camerti, Vol. sp.<br />
1991/2, CROP 11, 389-395.<br />
MICCADEI E. (1993) - Geologia <strong>del</strong>l’area Alto Sagittario-Alto<br />
Sangro (Abruzzo, Appennino Centrale). Geologica Romana,<br />
29, 463-481.<br />
MANCKTELOW N.S. & PENNACCHIONI G. (2005) - The control<br />
of precursor brittle fracture and fluid–rock interaction on<br />
the development of single and pair<strong>ed</strong> ductile shear zones.<br />
Journal of Structural Geology, 27, 645-661.<br />
PACE B., DI MATTEO P., BONCIO P. & LAVECCHIA G. (2001),<br />
Considerazioni sull’evoluzione geologica <strong>del</strong>la Marsica<br />
Sud-Orientale (Abruzzo, Appennino Centrale) sulla base <strong>di</strong><br />
un’analisi integrata <strong>di</strong> dati stratigrafici e strutturali. Boll.<br />
Soc. Geol. It., 120, 139-150.<br />
PATACCA E. & SCANDONE P. (2007) Geology of the Southern<br />
Apennines. Boll. Soc. Geol.It., Spec. Issue No. 7, 75-119.<br />
PATACCA E., SCANDONE P., BELLATALLA M., PERILLI N. &<br />
SANTINI U. (1992) La zona <strong>di</strong> giunzione tra l’arco<br />
appenninico settentrionale e l’arco appenninico<br />
meri<strong>di</strong>onale nell’Abruzzo e nel Molise. Stu<strong>di</strong> geologici<br />
Camerti, Vol. sp. 1991/2, CROP 11, 417-441.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 17-20, 3 ff.<br />
Reconstruction of pre-thrusting basin architecture: the contribution<br />
of the 3D mo<strong>del</strong>ling approach<br />
Applicazione <strong>di</strong> tecniche <strong>di</strong> mo<strong>del</strong>lizzazione 3D nella ricostruzione <strong>del</strong>le<br />
geometrie pre-thrust dei bacini <strong>di</strong>stensivi<br />
Nella catena a pieghe e sovrascorrimenti umbro-marchigiana sono<br />
descritti numerosi casi <strong>di</strong> inversione tettonica positiva; esempi chiari derivano<br />
dalla zona compresa tra la conca <strong>di</strong> Rieti e la Valnerina, dove le formazioni<br />
mesozoiche e cenozoiche affiorano con buona continuitá. Stu<strong>di</strong> <strong>di</strong> dettaglio<br />
sulle relazioni <strong>di</strong> sovrapposizione <strong>del</strong>le strutture e sulla loro cinematica hanno<br />
permesso <strong>di</strong> ipotizzare che le strutture <strong>di</strong>stensive rifeiribili alla tettonica<br />
sins<strong>ed</strong>imentaria giurassica e cretaceo-paleogenica hanno influenzato la<br />
localizzazione e lo sviluppo dei sovrascorrimenti durante la tettonica<br />
compressiva neogenica, producendo pattern <strong>di</strong> deformazione caratteristici, che<br />
appaiono controllati dalla geometria <strong>del</strong>le strutture piú antiche.<br />
M<strong>ed</strong>iante l’applicazione <strong>di</strong> tecniche <strong>di</strong> bilanciamento e retrodeformazione<br />
sequenziale in due e tre <strong>di</strong>mensioni, sono state acquisite ricostruzioni<br />
palinspastiche che descrivono la geometria tri<strong>di</strong>mensionale <strong>del</strong>l’area <strong>di</strong> stu<strong>di</strong>o:<br />
queste mostrano importanti variazioni <strong>del</strong>l’architettura dei bacini pre-orogenici<br />
durante i vari sta<strong>di</strong> evolutivi <strong>del</strong>la tettonica compressiva neogenica.<br />
Questo contributo offre un esempio <strong>di</strong> applicazione <strong>di</strong> mo<strong>del</strong>lizzazione e<br />
analisi tri<strong>di</strong>mensionale, che puó essere utilizzato per testare la coerenza<br />
geometrica in casi <strong>di</strong> tettonica da inversione positiva.<br />
Key words: Appennino settentrionale, mo<strong>del</strong>lizzazione 3D,<br />
retrodeformazione, bilanciamento.<br />
INTRODUCTION<br />
Similarly to many other fold-and-thrust belts, the Northern<br />
Apennines are an orogenic belt develop<strong>ed</strong> at the expenses of an<br />
early passive margin, where tectonic depressions and<br />
seamounts locally determin<strong>ed</strong> abrupt thickness changes in the<br />
s<strong>ed</strong>imentary successions; the architecture of precursor rift<br />
basins often controll<strong>ed</strong> the geometry of fold-and thrust systems<br />
(e.g. GILLCRIST et alii, 1987). Although many examples in<br />
literature depict cases of positive inversion tectonics, or<br />
<strong>di</strong>fferent degrees of deformation overprint, the mo<strong>di</strong>fication in<br />
space and time of precursor rift-basin geometries is not<br />
extensively document<strong>ed</strong> in publish<strong>ed</strong> accounts.<br />
The three-<strong>di</strong>mensional (3D) reconstruction of complex<br />
geological settings and of original, pre-thrusting basin geometry<br />
is one of the challenges for modern structural geology. It has<br />
inde<strong>ed</strong> a critical role in many industrial applications, such as in<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Universitá <strong>di</strong> Siena. Via Laterina,<br />
8. 53041 Siena (Italy) – e-mail: aque@unisi.it<br />
RICCARDO AQUÈ (*) & ENRICO TAVARNELLI (*)<br />
Fig. 1 – Location (a) and geological sketch (b) of the study area.<br />
the hydrocarbon exploration (e.g. CLARKE et alii, 2006).<br />
3D mo<strong>del</strong>ling techniques provide tools for an enhanc<strong>ed</strong><br />
understan<strong>di</strong>ng of the evolution of structurally complex<br />
domains, and for the validation of paleogeographic<br />
reconstructions. In combination with classical 2D restoration<br />
techniques, 3D mo<strong>del</strong>ling allows to better constrain the<br />
otherwise not always clear mean <strong>di</strong>rection of tectonic transport,<br />
the kinematics and the internal consistency of interpret<strong>ed</strong><br />
geometry of investigat<strong>ed</strong> structures. By back-stripping the<br />
deformation (tectonics and compaction), it is possible to<br />
reconstruct a set of pre-deformation templates depicting the<br />
tectonic evolution through time (i.e. ROUBY et alii, 2000;<br />
CALCAGNO et alii, 2007). These computations may also be us<strong>ed</strong><br />
to provide semi-quantitative estimates of tectonically relevant<br />
parameters (e.g. DEE et alii, 2005).<br />
By using commercial specific softwares to produce<br />
balanc<strong>ed</strong> cross-sections and inferr<strong>ed</strong> 3D reconstructions
18 R. AQUÉ & E. TAVARNELLI<br />
(2Dmove, Gocad), we mo<strong>del</strong>l<strong>ed</strong> a portion of the Umbria-<br />
Marche fold-and-thrust belt, in order to infer the pre-thrusting<br />
geometry of the Mesozoic-Cenozoic extensional basins and test<br />
the applicability of existing computer tools in areas that have<br />
experienc<strong>ed</strong> the effects of positive tectonic inversion.<br />
GEOLOGICAL OUTLINE<br />
The Umbria-Marche zone is locat<strong>ed</strong> in the outer zone of the<br />
Northern Apennines, which is an arcuate fold and thrust belt<br />
(Fig. 1a), form<strong>ed</strong> during the Neogene-Quaternary as a<br />
consequence of the closure of the Ligurian ocean, a branch of<br />
the Mesozoic Thethys, and the subsequent collision between<br />
the Corsica-Sar<strong>di</strong>nia block and the Adria promontory<br />
(BOCCALETTI et alii, 1980). The Meso-Cenozoic sequence<br />
consists on Upper Triassic evaporites and lower Liassic<br />
platform limestones gra<strong>di</strong>ng to Liassic-Tertiary pelagic<br />
carbonates. An intense Jurassic syns<strong>ed</strong>imentary extensional<br />
event determin<strong>ed</strong> the development of fault-relat<strong>ed</strong> depressions<br />
and isolat<strong>ed</strong> seamounts, respectively seat of deposition of<br />
complete and condens<strong>ed</strong> successions (e.g. CENTAMORE et alii,<br />
1971; ALVAREZ, 1989); analogous syns<strong>ed</strong>imentary activity is<br />
record<strong>ed</strong> during the Early Cretaceous and Cretaceous-<br />
Paleogene intervals (MICARELLI et alii., 1977; DECANDIA,<br />
1982). From Late Miocene onwards, the region underwent a<br />
positive tectonic inversion process that determin<strong>ed</strong> the<br />
development of the fold-and-thrust belt. The Late Triassic<br />
evaporites were kinematically activat<strong>ed</strong> as a major detachment<br />
layer, allowing for the partial decoupling of the s<strong>ed</strong>imentary<br />
cover from the underlying basement; accor<strong>di</strong>ng with<br />
palaeomagnetic evidence (CHANNELL et alii., 1978) the<br />
present-day arcuate shape of the Apennines may results from<br />
oroclinal ben<strong>di</strong>ng of an originally linear fold-and-thrust belt.<br />
In the study area, the accurate reconstruction of the<br />
structural setting, cross-cut relationships and timing of the<br />
deformation, was inferr<strong>ed</strong> by using field data, map analysis and<br />
cross-section balancing techniques (TAVARNELLI, 1996a, b).<br />
The structural overprinting relationships among the<br />
investigat<strong>ed</strong> thrusts made it possible to infer a general piggyback<br />
thrusting sequence, with new thrust faults to the East,<br />
develop<strong>ed</strong> in the footwall of formerly emplac<strong>ed</strong> thrust sheets, in<br />
the West. This allow<strong>ed</strong> to sequentially remove the effects of the<br />
deformation for progressively older structures, and to backstrip<br />
the thrust sheets in sequential evolutionary steps, in order<br />
to reconstruct a viable pre-thrusting template.<br />
3D MODELING<br />
Four balanc<strong>ed</strong> cross-sections (fig.2) have been drawn,<br />
provi<strong>di</strong>ng the initial skeleton for 3D mo<strong>del</strong>ling, together with<br />
the map trace of the major tectonic features. The cross-sections<br />
and the geological map have been <strong>di</strong>gitiz<strong>ed</strong> and geo-referr<strong>ed</strong> in<br />
2D-Move. Starting from the inferr<strong>ed</strong> geometries, a coherent<br />
3D mo<strong>del</strong> was built in Gocad (Fig. 3a). The surfaces<br />
represent post-thrust normal faults, thrust planes, and pre-thrust<br />
Fig. 2 – Balanc<strong>ed</strong> cross-sections showing key b<strong>ed</strong>s and restor<strong>ed</strong> templates,<br />
with length-shortening calculation (after TAVARNELLI, 1996a).<br />
normal faults, and five key stratigraphic surfaces, from bottom;<br />
the base and top of the Calcare Massiccio fm. (Lower Liassic),<br />
the base of the Maiolica fm. (Titonian), the base and the top of<br />
the Marne a Fucoi<strong>di</strong> fm. (Upper Albian-Lower Cenomanian).<br />
The main pre-thrusting normal faults have been project<strong>ed</strong> using<br />
their map and cross-section traces, keeping into account the<br />
thickness variation of the select<strong>ed</strong> stratigraphic reference; the<br />
complete detail of the condens<strong>ed</strong> and complete stratigraphic<br />
sequence was consider<strong>ed</strong> in cross-section only.<br />
The initial mo<strong>del</strong> shows the final state of deformation, and
was us<strong>ed</strong> to test the geometrical consistency of the cross-<br />
Fig. 3 – Top: Initial 3D mo<strong>del</strong> after the removal of the late normal faults.<br />
Bottom: restor<strong>ed</strong> top of Marne a Fucoi<strong>di</strong> fm. and buri<strong>ed</strong> basin<br />
architecture; the effect of the Jurassic normal faults is visible.<br />
sections. After the removal of the main post-thrusting normal<br />
faults, the compressive deformation has been sequentially<br />
remov<strong>ed</strong> in both cross-sections and 3D mo<strong>del</strong>, to obtain a set of<br />
mo<strong>del</strong>s that illustrat<strong>ed</strong> sequential steps of deformation through<br />
time. In this exercise, an ad<strong>di</strong>tional horizon, roughly coincident<br />
with the Oligocene, was us<strong>ed</strong> as further reference, in order to<br />
complete the mo<strong>del</strong> and to provide a first surface to be flatten<strong>ed</strong><br />
in the sequential restoration, as this marker is consider<strong>ed</strong> little<br />
affect<strong>ed</strong> by pre-orogenic syns<strong>ed</strong>imentary deformation.<br />
The restoration of the surfaces was perform<strong>ed</strong> in Gocad<br />
by using a research plugin (Muron, 2005, 2006); the method<br />
allows to flatten a fold<strong>ed</strong> horizon and fill the gap between<br />
hanging-wall and footwall cutoffs, similarly to the surface<br />
restoration that can be perform<strong>ed</strong> through other commercial<br />
software (e.g. Kine-3D, 3D-Move). The constraints for the<br />
restoration are impos<strong>ed</strong> systematically to progressively deeper<br />
surfaces, which are restor<strong>ed</strong> to an ideal undeform<strong>ed</strong> state (i.e.<br />
flatten<strong>ed</strong>, unfault<strong>ed</strong>), by joining the hanging-wall and footwall<br />
cutoffs. During restoration, the stratigraphic thickness between<br />
surfaces is maintain<strong>ed</strong> by ad<strong>di</strong>tional thickness constraints. The<br />
result is a progressively restor<strong>ed</strong> template under the reference<br />
top stratigraphic surface, highlighting the buri<strong>ed</strong> geometry and<br />
finally depicting an approximation of the basin geometry at the<br />
time of deposition (Fig.3b). During restoration, a set of<br />
tectonically relevant parameters are comput<strong>ed</strong>, such as fault<br />
<strong>di</strong>splacement, area <strong>di</strong>latation, surface curvature, strain<br />
<strong>di</strong>rections, etc. Such parameters can be us<strong>ed</strong> for further<br />
analysis, i.e. pr<strong>ed</strong>ict areas of expect<strong>ed</strong> intense fracturation (e.g.<br />
ROYER, 2006).<br />
CONCLUSIONS<br />
The combination of balanc<strong>ed</strong> cross-sections, 3D mo<strong>del</strong>ling<br />
and restoration techniques, sequentially appli<strong>ed</strong> to fold-and-<br />
RECONSTRUCTION OF PRE-THRUSTING BASIN ARCHITECTURE<br />
thrust belts, provides effective tools to unravel the geometry of<br />
the pre-thrusting geometries and depict the architecture of the<br />
s<strong>ed</strong>imentary basins. Even if the surface restoration techniques<br />
are strongly dependent from the reconstruct<strong>ed</strong> surface geometry<br />
(i.e. the mesh of the surface and the obtain<strong>ed</strong> cutoffs along a<br />
fault surface), the results are comparable to the calculations<br />
obtain<strong>ed</strong> from classical 2D balancing techniques.<br />
The results of this work seem to encourage for further<br />
applicability of similar methods to other areas of the Northern<br />
Apennines, and to geologically complex areas in general.<br />
REFERENCES<br />
ALVAREZ W. (1989) - Evolution of the Monte Nerone seamount<br />
in the Umbria-Marche Apennines: 2 - Tectonic control of<br />
the seamount-basin transition. Boll. Soc. Geol. It., 108 (1),<br />
23-39.<br />
BOCCALETTI M., COLI, M., DECANDIA F.A., GIANNINI, E. &<br />
LAZZAROTTO A. (1980) – Evoluzione <strong>del</strong>l’appennino<br />
settentrionale secondo un nuovo mo<strong>del</strong>lo strutturale. Mem.<br />
Soc. Geol. It., 21, 359-373.<br />
CALCAGNO, P., LAZARRE, J., COURRIOUX , G & LEDRU, P.<br />
(2007) - 3D geometric mo<strong>del</strong>ling of an external orogenic<br />
domain: a case history from the western Alps (massif de<br />
Morges, Pelvoux). Bull. Soc. Geol. de France, 178 (4), 263-<br />
274.<br />
CENTAMORE, E., CHIOCCHINI, M., DEIANA, G., MICARELLI, A.<br />
& PIERUCCINI U. (1971) – Contributo alla conoscenza <strong>del</strong><br />
Giurassico <strong>del</strong>l’appennino Umbro-Marchigiano. Stu<strong>di</strong><br />
Geol. Cam., 1, 7-89.<br />
CHANNELL, J.E.T., LOWRIE, W., MEDIZZA, F. & ALVAREZ, W.<br />
(1978) – Paleomagnetism and tectonics in Umbria, Italy.<br />
Earth and Planet. Sci. Lett., 39, 199-210.<br />
CLARKE, S.M., BURLEY, S.D., WILLIAMS, G.D., RICHARDS,<br />
A.J., MEREDITH, D. & EGAN, S. (2006) - Integrat<strong>ed</strong> 4D<br />
mo<strong>del</strong>ling of s<strong>ed</strong>imentary basin architecture and<br />
hydrocarbon migration.. Butler, S.J.H. and Schreurs, G.<br />
(<strong>ed</strong>s): Analogue and Numerical mo<strong>del</strong>ling of crustal scale<br />
processes. Sp. Pub. Geol Soc. London, 253, 185-211.<br />
DECANDIA, F.A. (1982) – Geologia dei monti <strong>di</strong> Spoleto (Prov.<br />
Di Perugia). Boll. Soc. Geol. It., 101, 291-315.<br />
DEE, S., FREEMAN, B., YIELDING, G., ROBERTS, A. & BRETAN,<br />
P. (2005) – Best practice in structural geological analysis.<br />
First Break, 23, 49-54.<br />
GILLCRIST, R., COWARD, M. & MUJER, J. (1987) – Structural<br />
inversion and its controls: examples from the Alpine<br />
foreland and the French alps. Geo<strong>di</strong>namica Acta, 1, 5-34.<br />
MICARELLI, A., POTETTI M. & CHIOCCHINI M. (1977) –<br />
Ricerche microbiostratigrafiche sulla maiolica <strong>del</strong>la<br />
regione umbro-marchigiana. Stu<strong>di</strong> Geol. Cam., 3, 56-86.<br />
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20 R. AQUÉ & E. TAVARNELLI<br />
MURON, P., MALLET, J.-L. & MEDWEDEFF, D. (2005) - 3D<br />
sequential structural restoration: Geometry and<br />
Kinematics. American Association of Petroleum Geologist<br />
Annual Convention (Calgary, Canada).<br />
MURON P. (2006). Methodes numeriques 3-D de restauration<br />
des structures geologiques faillees. Ph. D. thesis, Institut<br />
National Polytechnique de Lorraine (Nancy, France).<br />
ROYER, J.J., MALLET, J.-L., COGNOT, R., & MOYEN, R. (2006)<br />
– Geochron: a framework to estimate fracturation of<br />
deform<strong>ed</strong> s<strong>ed</strong>imentary layers. XI th Int. Ass. Math. Geol.,<br />
Liége – Belgium.<br />
ROUBY, D., XIAO, H. & SUPPE, J. (2000) - 3-D Restoration of<br />
Complexly Fold<strong>ed</strong> and Fault<strong>ed</strong> Surfaces Using Multiple<br />
Unfol<strong>di</strong>ng Mechanism. AAPG bulletin, 84 (6), 805-829.<br />
TAVARNELLI, E. (1996a) – Ancient syns<strong>ed</strong>imentary structural<br />
control on thrust ramps development: examples from the<br />
Northern Apennines, Italy. Terra Nova, 8, 65-74 .<br />
TAVARNELLI, E. (1996b) – Thethyan heritage in the<br />
development of the Neogene Umbria-Marche fold-andthrust<br />
belt, Italy: a 3D approach. Terra Nova, 8, 470-478.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 21-22, 1 f.<br />
The tectonic evolution of the Pisa-Viareggio basin: implications for<br />
the Neogene basins of Tuscany<br />
RIASSUNTO<br />
Evoluzione tettonica <strong>del</strong> bacino <strong>di</strong> Pisa-Viareggio: implicazioni<br />
riguardanti i bacini neogenici <strong>del</strong>la Toscana<br />
Il bacino <strong>di</strong> Pisa-Viareggio e’ il piu’ settentrionale dei gran<strong>di</strong> bacini<br />
neogenici toscani, <strong>ed</strong> e’ riempito da ca. 3 km <strong>di</strong> s<strong>ed</strong>imenti cha vanno dal<br />
Miocene superiore al Quaternario. La sua evoluzione tettonica e’ stata stu<strong>di</strong>ata<br />
utilizzando profili sismici multicanale e pozzi per l’esplorazione, unitamente a<br />
carte geologiche <strong>di</strong> dettaglio (LAZZAROTTO et alii, 1990) nella parte<br />
meri<strong>di</strong>onale. Il bacino mostra una chiara origine estensionale, con una grande<br />
faglia bor<strong>di</strong>era immergente verso ovest posta nel sottosuolo <strong>del</strong>la pianura <strong>di</strong><br />
Pisa-Viareggio. Tuttavia, nella sua parte meridonale il bacino e’ stato soggetto<br />
a raccorciamenti e a una marcata tettonica <strong>di</strong> inversione che ha portato alla<br />
formazione dei Monti <strong>di</strong> Livorno. E’ interessante notare che a sud <strong>del</strong> bacino<br />
<strong>di</strong> Pisa-Viareggio si colloca il dominio dei bacini neogenici toscani, la cui<br />
origine e’ <strong>di</strong>battuta e il cui riempimento s<strong>ed</strong>imentario e’ deformato, talora<br />
intensamente.<br />
Key words: basin inversion, post-orogenic extension, seismic<br />
profiles, Tuscany.<br />
INTRODUCTION AND RATIONALE<br />
Several small s<strong>ed</strong>imentary basins characterize the internal<br />
portion of the Northern Apennines. These inter-montane basins<br />
trend almost parallel to the Apennine range and are fill<strong>ed</strong> by<br />
Neogene s<strong>ed</strong>iments with thickness ranging between few 100’s<br />
m to few km (BARTOLINI et alii, 1982; MARTINI et alii, 2001).<br />
S<strong>ed</strong>iments belonging to the inter-montane basins crop out<br />
extensively in western Tuscany, often appearing heavily<br />
deform<strong>ed</strong> (BROGI, 2004; SANI et alii, 2004). Partly because of<br />
the intense deformation the early tectonic history of these<br />
basins is <strong>di</strong>fficult to constrain; although classically interpret<strong>ed</strong><br />
as extensional basins (see MARTINI & SAGRI, 1993 and<br />
references therein), some recent papers call for an initial thrustrelat<strong>ed</strong><br />
origin, only later overprint<strong>ed</strong> by extension (BERNINI et<br />
alii, 1994; BOCCALETTI et alii, 1995; BONINI & SANI, 2002).<br />
The debate about the origin of the Neogene basins of<br />
_________________________<br />
(*) ISMAR-CNR, Bologna<br />
(**) ENI S.p.A., Exploration & Production Division, S. Donato Milanese,<br />
Milano<br />
ANDREA ARGNANI (*) & SERGIO ROGLEDI (**)<br />
Tuscany has been going on for more than ten years, and in spite<br />
of recent field works (BROGI 2004; SANI et alii, 2004) the<br />
ambiguity remains still unresolv<strong>ed</strong>.<br />
This contribution aims at presenting the case of an internal<br />
basin, the Pisa-Viareggio basin, which is the northernmost one<br />
among the large inter-montane basins of Tuscany and which<br />
straddles the coastline, being partly buri<strong>ed</strong> underneath the<br />
alluvial s<strong>ed</strong>iments of the Pisa plain and partly exten<strong>di</strong>ng<br />
offshore (PASCUCCI, 2005). This basin can be investigat<strong>ed</strong> only<br />
through subsurface data, and the current study is bas<strong>ed</strong> on the<br />
interpretation of a grid of industrial seismic profiles covering<br />
the Pisa plain and ti<strong>ed</strong> to the stratigraphy obtain<strong>ed</strong> from<br />
exploration wells. In ad<strong>di</strong>tion, the geology of nearby outcrops<br />
(LAZZAROTTO et alii, 1990) has been us<strong>ed</strong> to enlarge the<br />
regional scale picture.<br />
MAIN RESULTS<br />
An extensional origin can be clearly proven for the Pisa-<br />
Fig. 1 – Geological map of western Tuscany (simplifi<strong>ed</strong> after BOCCALETTI &<br />
Coli, 1982) with location of the Pisa-Viareggio basin.<br />
Viareggio basin, as seismic profiles show a west-<strong>di</strong>pping listric<br />
extensional fault that bounds the basin to the east. The basin is<br />
fill<strong>ed</strong> with up to 3 seconds of upper Messinian to Quaternary<br />
s<strong>ed</strong>iments, and extension mostly occurr<strong>ed</strong> during late
22 A. ARGNANI & S. ROGLEDI<br />
Messinian-early Pliocene, although continuing with r<strong>ed</strong>uc<strong>ed</strong><br />
intensity till the Quaternary. In spite of the extensional origin,<br />
the southern part of this basin shows in<strong>di</strong>cations of<br />
superimpos<strong>ed</strong> contractional deformation, possibly in the form<br />
of tectonic inversion, that progressively increases to the south,<br />
where the basin appears completely overturn<strong>ed</strong> and erod<strong>ed</strong> in<br />
the Livorno Mountains. The basin-boundary fault trends<br />
roughly NNW-SSE and is buri<strong>ed</strong> in the Quaternary s<strong>ed</strong>iments<br />
of the Pisa plain, but it turns rather abruptly to N-S and NNE-<br />
SSW in the south, near Livorno. Inspection of the detail<strong>ed</strong><br />
geological maps (LAZZAROTTO et alii, 1990) suggests that the<br />
fault plane may be possibly uplift<strong>ed</strong> and expos<strong>ed</strong> in the Livorno<br />
Mountains, locat<strong>ed</strong> just east of Livorno. The timing of the<br />
contractional deformation that affect<strong>ed</strong> the southern part of the<br />
Pisa-Viareggio basin can be roughly constrain<strong>ed</strong> within the<br />
Pleistocene.<br />
We speculate on the possible causes of the intense<br />
deformation that affect<strong>ed</strong> the sourthern part of the Pisa-<br />
Viareggio basin and attempt to show that the tectonic history of<br />
this basin can possibly help to better undestand the evolution of<br />
the other Neogene basins, locat<strong>ed</strong> further to the south, which<br />
suffer<strong>ed</strong> deformation and uplift to a larger extent.<br />
REFERENCES<br />
BARTOLINI C., BERNINI M., CARLONI G.C., COSTANTINI A.,<br />
FEDERICI, P.R., GASPERI G., LAZZAROTTO A., MARCHETTI<br />
G., MAZZANTI R., PAPANI G., PRANZINI G., RAU A.,<br />
SANDRELLI F., VERCESI P.I., CASTALDINI D. &<br />
FRANCAVILLA F. (1982) - Carta Neotettonica<br />
<strong>del</strong>l’Appennino settentrionale. <strong>Note</strong> Illustrative. Boll. Soc.<br />
Geol. It., 101, 523-549.<br />
BOCCALETTI M & COLI M. (coor<strong>di</strong>nators) (1982) – Carta<br />
Strutturale <strong>del</strong>’Appennino Settentrionale. Scala 1:250000.<br />
Consilgio Nazionale <strong>del</strong>le Ricerche, P.F. Geo<strong>di</strong>namica,<br />
Pubbl, 429, SELCA, Firenze.<br />
BOCCALETTI M., BONINI M., MORATTI G. & SANI F. (1995) -<br />
Nuove ipotesi sulla genesi e l'evoluzione dei bacini postnappe<br />
in relazine alle fasi compressive neogenicoquaternarie<br />
<strong>del</strong>l'Appennino Settentrionale. Scritti e docum.<br />
Accad. Naz. <strong>del</strong>le Scienze, 14, 229-262.<br />
BROGI A. (2004) - Miocene extension in the inner Northern<br />
Apennines; the Tuscan Nappe megabou<strong>di</strong>ns in the Mt.<br />
Amiata geothermal area and their influence on Neogene<br />
s<strong>ed</strong>imentation. Boll. Soc. Geol. It., 123, 513-529.<br />
BONINI M. & SANI F. (2002) - Extension and compression in<br />
the Northern Apennines (Italy) hinterland: Evidence from<br />
the late Miocene-Pliocene Siena-Ra<strong>di</strong>cofani Basin and<br />
relations with basement structures. Tectonics, 21, 3, 1010,<br />
10.1029/2001TC900024.<br />
LAZZAROTTO A., MAZZANTI R. & NENCINI C. (1990) - Carta<br />
geologica dei comuni <strong>di</strong> Livorno e <strong>di</strong> Collesalvetti<br />
(Provincia <strong>di</strong> Livorno). Scala 1:25000. Consiglio Nazionale<br />
<strong>del</strong>le Ricerche.<br />
MARTINI, I.P. & SAGRI, M. (1993) - Tectono-s<strong>ed</strong>imentary<br />
characteristics of late Miocene-Quaternary extensional<br />
basins of the Northern Apennines, Italy. Earth Science<br />
Reviews, 34, 197–233.<br />
MARTINI I.P., SAGRI M. & COLELLA A. (2001) - Neogene-<br />
Quaternary basins of the inner Apennines and Calabrian<br />
arc. In: G.B. Vai and I.P. Martini (Eds.) - Anatomy of a<br />
Mountain: the Apennines and adjacent M<strong>ed</strong>iterranean<br />
Basins. Kluwer Academic Publisher, London, 375-400.<br />
PASCUCCI V. (2005) - Neogene evolution of the Viareggio<br />
Basin, Northern Tuscany (Italy). GeoActa, 4, 123-138.<br />
SANI F., DEL VENTISETTE C., MONTANARI D., COLI M., NAFISSI<br />
P. & PIAZZINI A. (2004) - Tectonic evolution of the internal<br />
sector of the Central Apennines, Italy. Marine and<br />
Petroleum Geology, 21, 1235–1254.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 23<br />
Grain size, shape, porosity and permeability evolution of extensional<br />
fault zones develop<strong>ed</strong> in poorly lithifi<strong>ed</strong>, low-porosity sandstones of<br />
the Barreiras Formation, NE Brazil<br />
FABRIZIO BALSAMO (*), FABRIZIO STORTI (*), FRANCESCO SALVINI (*), ALINE SILVA (°) & CLAUDIO LIMA (°)<br />
ABSTRACT<br />
Evoluzione <strong>del</strong>la <strong>di</strong>mensione e <strong>del</strong>la forma dei granuli, <strong>del</strong>la porosità e<br />
<strong>del</strong>la permeabilità in zone <strong>di</strong> faglia estensionali sviluppate in arenarie<br />
poco consolidate <strong>ed</strong> a bassa porosità <strong>del</strong>la Formazione Barreiras, Brasile<br />
nord-orientale.<br />
We describe the structural and petrophysical evolution of<br />
extensional fault zones develop<strong>ed</strong> in low-porosity, poorly<br />
lithifi<strong>ed</strong> quartz-dominat<strong>ed</strong> sandstones from the Mio-Pliocene<br />
continental Barreiras Formation, NE Brazil. We stu<strong>di</strong><strong>ed</strong> eight<br />
fault zones having <strong>di</strong>splacement between few centimetres up to<br />
about 50 meters and develop<strong>ed</strong> from soft-s<strong>ed</strong>iment up to more<br />
brittle con<strong>di</strong>tions during progressive burial and Fe-oxide<br />
cementation. Our data include structural and microstructural<br />
analyses, grain size and shape analyses, porosity and pore size<br />
analyses, and laboratory and in situ permeability measurements.<br />
Undeform<strong>ed</strong> rocks are very poorly sort<strong>ed</strong>, m<strong>ed</strong>ium to fine<br />
grain<strong>ed</strong> clay-rich sandstones with an average intergranular<br />
porosity of about 3 %. Sandstones in damage zones are<br />
characteris<strong>ed</strong> by non destructive <strong>di</strong>latant granular flow and<br />
opening-mode intergranular tensional fractures, which increase<br />
bulk porosity, pore size and permeability. Deformation in fault<br />
cores evolv<strong>ed</strong> from particulate flow to compactional cataclastic<br />
flow with progressive grain size, porosity and permeability<br />
r<strong>ed</strong>uction. Our data highlight a conduit/barrier behaviour of the<br />
stu<strong>di</strong><strong>ed</strong> fault zones, which significantly <strong>di</strong>ffers from the sealing<br />
behaviour of deformation band fault zones commonly observ<strong>ed</strong><br />
in high-porosity sandstones.<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, Università degli Stu<strong>di</strong> Roma Tre,<br />
Roma, Italy<br />
(°) Cenpes, Petrobras, Rio de Janeiro, Brazil<br />
Lavoro eseguito nell’ambito <strong>del</strong> progetto TRAFUR (Transmissibility of<br />
Faults in Unconsolidat<strong>ed</strong> Rocks) con il contributo finanziario <strong>del</strong>la società<br />
petrolifera Petrobras (Brasile).
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 24-26<br />
Lithological and Mechanical Control on the Base of the Crustal<br />
Seismogenic Zone: a Case-Study from the Northern Apennines of Italy.<br />
MASSIMILIANO R. BARCHI (*), CRISTIANO COLLETTINI (*), NICOLA DE PAOLA (**), DAN FAULKNER (°), ANDREA<br />
LUPATTELLI (*), FRANCESCO MIRABELLA (*) & FABIO TRIPPETTA (*)<br />
RIASSUNTO<br />
Il controllo litologico e meccanico sullo spessore sismogenetico crostale:<br />
un caso <strong>di</strong> stu<strong>di</strong>o dall'Appennino Settentrionale.<br />
L'integrazione <strong>di</strong> dati geologici <strong>di</strong> sottosuolo e dati sismologici ha<br />
mostrato che le rotture principali <strong>del</strong>la sequenza sismica che ha colpito la<br />
regione umbro-marchigiana nel 1997-98 si localizzano nelle evaporiti<br />
triassiche (formazione <strong>di</strong> Burano), senza propagarsi nel sottostante basamento.<br />
Per spiegare questo comportamento sono stati svolti stu<strong>di</strong> <strong>di</strong> terreno su faglie<br />
esumate, sviluppate nelle evaporiti triassiche, e prove <strong>di</strong> laboratorio, volte a<br />
caratterizzare le principali litologie (anidriti e dolomie) dal punto <strong>di</strong> vista<br />
meccanico e <strong>del</strong>la permeabilità.<br />
Sulla base dei risultati ottenuti, proponiamo che nella regione stu<strong>di</strong>ata la<br />
base <strong>del</strong>lo strato sismogenetico superficiale sia controllata dalle proprietà<br />
meccaniche e <strong>di</strong> permeabilità <strong>del</strong>le evaporiti triassiche, in grado <strong>di</strong> costituire<br />
una barriera per la risalita dei flui<strong>di</strong> crostali (principalmente CO2). Al <strong>di</strong> sotto<br />
<strong>del</strong>le evaporiti, un livello a bassa velocità, localizzato nella parte alta <strong>del</strong><br />
basamento, <strong>di</strong>saccoppia la copertura s<strong>ed</strong>imentaria dal basamento cristallino.<br />
Questo stu<strong>di</strong>o mostra che un semplice mo<strong>del</strong>lo <strong>di</strong> crosta superiore<br />
omogenea <strong>ed</strong> isotropa non può essere applicato ad una regione geologicamente<br />
complessa come l'Appennino Settentrionale, dove i livelli crostali superficiali<br />
sono caratterizzati da una grande variabilità litologica (e quin<strong>di</strong> meccanica e <strong>di</strong><br />
permeabilità) sia in senso verticale che laterale.<br />
Key words: Seismogenic Layer, Upper Crust, Seismic<br />
Reflection Profiles, Laboratory Tests, Anhydrites<br />
INTRODUCTION<br />
Lithological composition and temperature are the most<br />
important factors controlling the thickness of the seismogenic<br />
layer, although the mechanical/transport properties of the crust<br />
also play an important role when crustal fluids are present in<br />
areas affect<strong>ed</strong> by a complex stratigraphy and structural setting,<br />
such as the Umbria-Marche Apennines of Italy.<br />
In this region the s<strong>ed</strong>imentary cover consists of three major<br />
_________________________<br />
(*) Geologia Strutturale e Geofisica, Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra -<br />
Università <strong>di</strong> Peugia<br />
(**) Reactivation Research Group, Dept. of Earth Sciences, University of<br />
Durham (UK)<br />
(°) Rock Deformation Laboratory, University of Liverpool (UK)<br />
Lavoro eseguito nell’ambito <strong>del</strong> progetto DPC/S2 2005 (U.R. 2.3, Resp.<br />
M.R. Barchi) e MIUR Cofin 2005 (U.R. Perugia, Resp. C. Collettini)<br />
lithological groups (from top to bottom): turbi<strong>di</strong>tes (Miocene,<br />
up to 3000 m thick), made of alternat<strong>ed</strong> layers of sandstones<br />
and marls; carbonates (Jurassic-Oligocene, about 2000 m<br />
thick), consisting of an early Jurassic carbonate platform<br />
(Calcare Massiccio Fm.), overlain by pelagic limestones with<br />
subor<strong>di</strong>nat<strong>ed</strong> marly levels; and evaporites (late Triassic, 1500–<br />
2000 m thick), made of alternating layers of anhydrites and<br />
dolostones.<br />
An important regional décollement separates these units<br />
from the underlying basement. The upper part of the basement<br />
consists of clastic s<strong>ed</strong>imentary and slightly metamorphos<strong>ed</strong><br />
rocks: in situ measurements have shown that these rocks<br />
possess a Vp between 4.8 and 5.2 km/s, significantly lower than<br />
the overlying evaporites (Vp > 6.0 km/s). The genuine<br />
crystalline basement has never been reach<strong>ed</strong> by any well.<br />
Refraction DSS data in<strong>di</strong>cate that below the Umbria-Marche<br />
region the crystalline basement is characteriz<strong>ed</strong> by Vp values of<br />
about 6 km/s. Summarising, the upper part of the basement is a<br />
low-velocity zone, at least 2 km thick, sandwich<strong>ed</strong> between<br />
two high velocity layers, the Triassic Evaporites and the upper<br />
part of the crystalline basement.<br />
A widespread deep seat<strong>ed</strong> CO2 degassing is document<strong>ed</strong> in<br />
the region (CHIODINI et al., 2004) where two deep boreholes<br />
encounter<strong>ed</strong> fluid overpressure at 85% of the lithostatic load<br />
trapp<strong>ed</strong> within the Triassic Evaporites.<br />
SUBSURFACE GEOLOGY AND SEISMICITY<br />
In 1997-98 a sequence of six 5< Mw
LITHOLOGICAL AND MECHANICAL CONTROL ON THE BASE OF THE CRUSTAL SEISMOGENIC ZONE<br />
penetrate the underlying basement;<br />
- the six mainshocks nucleat<strong>ed</strong> at 6 km of depth within the<br />
Triassic Evaporites (Burano Fm.), made of interb<strong>ed</strong>d<strong>ed</strong><br />
anhydrites and dolostones.<br />
Mo<strong>del</strong>s bas<strong>ed</strong> on heat flow measurements define the base of<br />
the frictional-viscous transition at about 20 km depth (PAUSELLI<br />
& FEDERICO, 2002), in agreement with the cut-off of the<br />
background seismicity (DE LUCA et al., 2008).<br />
This suggests that the Triassic Evaporites play<strong>ed</strong> a key role<br />
in controlling the relatively shallow depth <strong>di</strong>splay<strong>ed</strong> by the<br />
major earthquakes.<br />
FIELD STUDIES AND LABORATORY TESTS<br />
In order to characterise the deformation processes occurring<br />
within the source region of the major earthquakes of the area<br />
we have combin<strong>ed</strong> field stu<strong>di</strong>es on exhum<strong>ed</strong> evaporites-bearing<br />
faults with permeability experiments on anhydrites samples<br />
collect<strong>ed</strong> from deep boreholes.<br />
The Triassic Evaporites are made of interb<strong>ed</strong>s of dolomites<br />
and anhydrites (gypsum at the surface). Foliat<strong>ed</strong> Ca-sulphate<br />
rocks form an anastomosing network surroun<strong>di</strong>ng the fractur<strong>ed</strong><br />
dolostones . The most competent lithology (i.e. the dolostones)<br />
is affect<strong>ed</strong> by a well develop<strong>ed</strong> bou<strong>di</strong>nage. In a closer view a<br />
“Gneissic” transpos<strong>ed</strong> fabric is made of interb<strong>ed</strong>d<strong>ed</strong> dolostones<br />
and Ca-sulphate.<br />
We analys<strong>ed</strong> the internal structure of a well expos<strong>ed</strong><br />
evaporites bearing fault, cropping out in Roccastrada, Tuscany<br />
(DE PAOLA et al., 2008). This is a mature normal fault<br />
(inferr<strong>ed</strong> <strong>di</strong>splacement >100 m and exhumation from depths >1<br />
km) that <strong>di</strong>ps in the range 40°-45°. The fault zone structure is<br />
characteris<strong>ed</strong> by a 5-6 m thick fault core, which appears to be<br />
zon<strong>ed</strong>. The inner fault core is made of fine-grain<strong>ed</strong> fault rocks<br />
(about 1m thick), with deformation localis<strong>ed</strong> along continuous<br />
and straight slip surfaces associat<strong>ed</strong> with a dolomite-rich<br />
cataclasite (brittle deformation). The outer fault core is mainly<br />
characteris<strong>ed</strong> by <strong>di</strong>stribut<strong>ed</strong> deformation accommodat<strong>ed</strong> by a<br />
fault parallel fabric consisting of interb<strong>ed</strong>s of cataclastic<br />
dolostones and foliat<strong>ed</strong> Ca-sulphate rocks. The damage zone<br />
consists of foliat<strong>ed</strong> Ca-sulphate rocks (foliation almost<br />
perpen<strong>di</strong>cular to the fault zone) and heavily fractur<strong>ed</strong> and<br />
bou<strong>di</strong>nag<strong>ed</strong> dolostones.<br />
Mechanical data obtain<strong>ed</strong> from triaxial loa<strong>di</strong>ng tests on<br />
borehole-recover<strong>ed</strong> anhydrites with <strong>di</strong>fferent grain-size and<br />
mesoscopic fabric, show that the transition from localiz<strong>ed</strong> to<br />
<strong>di</strong>stribut<strong>ed</strong> deformation occurs at effective pressures of about<br />
20 MPa. The permeability measur<strong>ed</strong> under hydrostatic stress<br />
con<strong>di</strong>tions, before loa<strong>di</strong>ng, is generally low and ranges between<br />
10 -21 k 10 -19 m 2 . During sample loa<strong>di</strong>ng, the permeability<br />
increases up to 3 (prior to brittle localiz<strong>ed</strong> failure) and 2 (prior<br />
to <strong>di</strong>stribut<strong>ed</strong> ductile failure) orders of magnitude, with<br />
measur<strong>ed</strong> k values of 10 -17 m 2 and 10 -18 m 2 , respectively<br />
(COLLETTINI et al., 2008; DE PAOLA et al., 2009).<br />
FINAL REMARKS<br />
25<br />
A simple mo<strong>del</strong> of a relatively homogeneous and isotropic<br />
upper crust cannot be appli<strong>ed</strong> to regions, where a complex<br />
stratigraphy and structural setting results in large vertical and<br />
lateral variations of lithological, mechanical and permeability<br />
properties of the shallow structural levels. This is the case of<br />
the Northern Apennines of Italy where active extension affects<br />
a previous fold and thrust belt, involving a mechanically<br />
complex stratigraphic sequence.<br />
Through the combin<strong>ed</strong> analysis of seismic reflection<br />
profiles and seismological data, we show<strong>ed</strong> that the main<br />
seismic ruptures occur within the Triassic evaporites, consisting<br />
of alternat<strong>ed</strong> anhydrites and dolostones. Our field stu<strong>di</strong>es show<br />
that within the fault zone the deformation is localis<strong>ed</strong> along<br />
principal slip surfaces associat<strong>ed</strong> to fine grain<strong>ed</strong> dolomite-rich<br />
cataclasite. Laboratory test show<strong>ed</strong> that the anhydrites possess<br />
very low static permeability values (k< 10 -19 m 2 ), capable of<br />
explaining the overpressure encounter<strong>ed</strong> in boreholes.<br />
The integration of field observations and laboratory<br />
permeability data suggests that, where the foliat<strong>ed</strong> anhydrites of<br />
the damage zone surround the fractur<strong>ed</strong> dolostones, resulting in<br />
a low fracture connectivity, the fault zone permeability is low<br />
enough to develop fluid overpressures.<br />
In the active area of the Northern Apennines, where a deepseat<strong>ed</strong><br />
CO2 degassing is document<strong>ed</strong>, crustal fluids in their<br />
ascent can be trapp<strong>ed</strong> within evaporite-bearing faults, promote<br />
fault zone weakening and trigger earthquake nucleation. In this<br />
view the shallow base of the seismogenic zone would be<br />
controll<strong>ed</strong> by the mechanical/transport properties of the<br />
Triassic Evaporites along with the decoupling effect of the<br />
underlying phyllitic basement.<br />
REFERENCES<br />
AMATO A., AZZARA R., CHIARABBA C.,. CIMINI G.B, COCCO<br />
M., DI BONA M., MARGHERITI L., MAZZA S., MELE F.,.<br />
SELVAGGI G, BASILI A., BOSCHI E., COURBOULEX F.,<br />
DESCHAMPS A., GAFFET S., BITTARELLI G., CHIARALUCE L.,<br />
PICCININI G., & RIPEPE M..(1998) - The 1997 Umbria-<br />
Marche, Italy earthquake sequence: a first look at the main<br />
shocks and aftershocks. Geophysical Research Letters,<br />
25:2861-2864.<br />
CHIARALUCE L., ELLSWORTH W.L., CHIARABBA C. & COCCO.<br />
M. (2003) - Imaging the complexity of an active complex<br />
normal fault system: the 1997 Colfiorito (central Italy) case<br />
study. Journal of Geophysical Research, 108 (B6):2294,<br />
doi:10.1029/2002JB002166.<br />
CHIODINI, G., CARDELLINI, C., AMATO, A., BOSCHI, E., CALIRO,<br />
S., FRONDINI, F. & VENTURA, G. (2004) - Carbon <strong>di</strong>oxide<br />
Earth degassing and seimogenesis in central and southern
26 M.R. BARCHI ET ALII<br />
Italy. Geophys. Res. Lett., 31, L07615,<br />
doi:10.1029/2004GL019480.<br />
COLLETTINI C., DE PAOLA N. & FAULKNER D. (2008) - Insights<br />
on the geometry and mechanics of the Umbria-Marche<br />
earthquakes (Central Italy) from the integration of field and<br />
laboratory data. Tectonophysics, in press,<br />
doi:10.1016/j.tecto.2008.08.013.<br />
DE LUCA G., CATTANEO M., MONACHESI G. & AMATO<br />
A..(2009) - Seismicity in Central and Northern Apennines<br />
integrating the Italian national and regional networks.<br />
Tectonophysics, in press.<br />
DE PAOLA N., COLLETTINI C., FAULKNER D. & TRIPPETTA F.<br />
(2008) - Fault zone architecture and deformation processes<br />
within evaporitic rocks in the upper crust. Tectonics, 27,<br />
10.1029/2007TC002230.<br />
DE PAOLA N., COLLETTINI C. & FAULKNER D. (2009) -<br />
Localis<strong>ed</strong> versus <strong>di</strong>stribut<strong>ed</strong> deformation as a control on<br />
the transport properties of a low porosity anhydrites rocks.<br />
Journal od Geophysical Research, in press.<br />
MIRABELLA F., BARCHI M.R., LUPATTELLI A., STUCCHI E., &<br />
CIACCIO M.G.. (2008) - Insights on the seismogenic layer<br />
thickness from the upper crust structure of the Umbria-<br />
Marche Apennines (Central Italy). Tectonics, 27,<br />
TC1010:doi:10.1029/2007TC002134.<br />
PAUSELLI C. & FEDERICO C. (2002) - The brittle/ductile<br />
transition along the Crop03 seismic profile: relationship<br />
with the geological features. Bollettino <strong>del</strong>la Società<br />
Geologica Italiana, Vol.Spec.1/2002, 25-35.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 27<br />
Subsurface structure of the Tagliamento valley (NE Italy) using<br />
Microtremors and Gravity Anomaly.<br />
C. BARNABA (*), L. MARELLO (**), A. VUAN(*), F. PALMIERI(*), M.ROMANELLI (*), E. PRIOLO (*), C. BRAITENBERG (°)<br />
Microtremor observations and gravity survey were made to<br />
determine the subsurface structures of a stretch of the<br />
Tagliamento river (Friuli, NE Italy).This area is close to the<br />
epicenter of the 1976 Friuli earthquake (M=6.5), and stuck in<br />
the past by several earthquakes of M>5.5.<br />
More than 240 H/V noise, three refraction profiles and 270<br />
gravimetric measurements have been acquir<strong>ed</strong> along the valley.<br />
The applications of non-invasive, low cost, mutually<br />
independent geophysical methodologies are us<strong>ed</strong> to investigate<br />
shallow geological structures. The buri<strong>ed</strong> shape of the valley is<br />
here defin<strong>ed</strong> from the resonance frequencies in H/V spectral<br />
ratio of seismic noise measurements and gravimetric forward<br />
mo<strong>del</strong>ling. The result is a good convergence of the two<br />
methods, although the valley shape is irregular, due to the<br />
structural geological complexity of the study area. For this<br />
reason the top of the carbonate rocks are mapp<strong>ed</strong>, instead of the<br />
classical alluvium cover.<br />
Most of the <strong>di</strong>fferences in defining the valley depth are found at<br />
the valley <strong>ed</strong>ges both beacause 2000 m height reliefs can affect<br />
the gravity measurements there and the buri<strong>ed</strong> steep flanks of<br />
the valley can make <strong>di</strong>fficult to interpret H/V peaks as due to<br />
simple 1D layering. However, the observations demostrat<strong>ed</strong> the<br />
noise wavefield within the sector stu<strong>di</strong><strong>ed</strong> is mainly dominat<strong>ed</strong><br />
by horizontally propagating surface waves giving rise to 1D<br />
resonance at lower frequencies.<br />
The maximum depth estimat<strong>ed</strong> is of about 400m in the southern<br />
part of the sector, while a mean value of 150-180m is estimat<strong>ed</strong><br />
in the northern part. These values are support<strong>ed</strong> by the few<br />
underground data available, confirming the possibility to detect<br />
the buri<strong>ed</strong> structure with low cost geophysical methodologies.<br />
The result represents the first 3D image of this part of the<br />
Tagliamento valley and it is the base for near future 3D wave<br />
propagation simulations and for planning more detail<strong>ed</strong><br />
geophysical surveys.<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Ricerche Sismologiche, Istituto Nazionale <strong>di</strong> Oceanografia e<br />
Geofisica Sperimentale, Trieste, Italy<br />
(**) Geological Survey of Norway, Petroleum Technology and Appli<strong>ed</strong><br />
Geophysics, Norway<br />
(°) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Trieste, Trieste, Italy<br />
Carla Barnaba: cbarnaba@inogs.it
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 28-31, 2 ff.<br />
Neogene rotations in the Sicilian-Maghrebian Chain: new structural<br />
data from the Madonie Mountains<br />
RIASSUNTO<br />
Rotazioni neogeniche nella Catena Siculo-Maghrebide: nuovi dati strutturali<br />
dai monti <strong>del</strong>le Madonie<br />
L’analisi strutturale eseguita lungo i monti <strong>del</strong>le Madonie (Sicilia centrosettentrionale),<br />
confrontata con i dati paleomagnetici pubblicati, suggerisce<br />
che le successioni mesozoico-m<strong>ed</strong>iomioceniche <strong>del</strong>le unità imeresi e<br />
panormi<strong>di</strong> hanno registrato sia le rotazioni avvenute tra il Langhiano e il<br />
Tortoniano superiore (circa 70° in senso orario) che, localmente, quelle<br />
avvenute dopo il Pliocene inferiore (almeno 30° in senso orario), mostrando<br />
assi strutturali <strong>di</strong> prima fase orientati circa nord-sud deformati da strutture <strong>di</strong><br />
seconda fase orientate circa est-ovest. Conseguentemente, le strutture <strong>del</strong>le<br />
successioni meso-cenozoiche formate durante la prima fase <strong>di</strong> deformazione<br />
sono state ruotate nel complesso <strong>di</strong> circa 100° in senso orario, mentre le<br />
strutture dei depositi <strong>di</strong>scordanti supratortoniani-infrapliocenici sono state<br />
coinvolte solo nelle rotazioni orarie plio-pleistoceniche legate all’attivazione <strong>di</strong><br />
un sistema <strong>di</strong> faglie trascorrenti destre. Dal punto <strong>di</strong> vista geo<strong>di</strong>namico, le<br />
rotazioni neogeniche nella Catena Siculo-Maghrebide sono da inquadrare nel<br />
processo regionale <strong>del</strong>la convergenza Africa-Europa.<br />
Key words: Sicilian-Maghrebian Chain, Madonie Mts., folds,<br />
rotations<br />
INTRODUCTION<br />
The Neogene thrust migration along the Sicilian sector of<br />
the Sicilian-Maghrebian Chain has been accompani<strong>ed</strong> by<br />
clockwise rotations, reveal<strong>ed</strong> by structural (GIUNTA et alii,<br />
2000; AVELLONE & BARCHI, 2003; GUARNIERI, 2004; NIGRO &<br />
RENDA, 2005; MONACO & DE GUIDI, 2006) and<br />
palaeomagnetic data (CHANNELL et alii, 1980; 1990; GRASSO<br />
et alii, 1987; OLDOW et alii, 1990; SPERANZA et alii, 1999;<br />
2003). In particular, palaeomagnetic data on Upper Trias to<br />
Lower-Middle Pliocene s<strong>ed</strong>iments reveal a major 70°<br />
clockwise rotation with respect to the Hyblean foreland<br />
between the Langhian and the Tortonian, follow<strong>ed</strong> by a further<br />
30° clockwise rotation occurr<strong>ed</strong> after Early Pliocene.<br />
A detail<strong>ed</strong> geological and structural analysis was carri<strong>ed</strong> out<br />
in the Madonie Mts., along the central-northern sector of the<br />
Sicilian-Maghrebian Chain (Fig. 1), in order to define the<br />
tectonic evolution of the area in relation to the rotation episodes<br />
occurr<strong>ed</strong> since the Middle Miocene.<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, Università <strong>di</strong><br />
Catania. E-mail:cmonaco@unict.it<br />
GIOVANNI BARRECA (*) & CARMELO MONACO (*)<br />
The structural analysis was focus<strong>ed</strong> on the Upper Triassic-<br />
Middle Miocene s<strong>ed</strong>imentary successions of the Imerese and<br />
Panormide tectonic units and on the top-thrust Neogene<br />
terrigenous covers, in order to compare tectonic structures of<br />
<strong>di</strong>fferent ages. The structural field analysis was support<strong>ed</strong> by<br />
the elaboration of 3D topographic <strong>di</strong>gital elevation mo<strong>del</strong> of<br />
the area and by the analysis of 1:33,000 scale aerial<br />
photographs. Structural data (fault and fracture systems,<br />
slickensides on fault surfaces, fold axes) collect<strong>ed</strong> on several<br />
measurement stations were process<strong>ed</strong> to better define the<br />
geometric and kinematic features of the structures occurring in<br />
the area.<br />
GEOLOGICAL SETTING<br />
The stu<strong>di</strong><strong>ed</strong> area is locat<strong>ed</strong> on the southern slope of<br />
Madonie Mts. and it is characteriz<strong>ed</strong> by two roughly E-W<br />
orient<strong>ed</strong> antiformal structures; the Mt.dei Cervi ridge to the<br />
west, that reaches an elevation of about 1800 m.a.s.l and the<br />
Mt. San Salvatore ridge to the east with an elevation of about<br />
1900 m.a.s.l. Both structures correspond to <strong>di</strong>stinct tectonic<br />
units (GRASSO et alii, 1978; ABATE et alii, 1982) deriving from<br />
deformation of <strong>di</strong>fferent palaeo-geographic domains stack<strong>ed</strong><br />
during the Neogene Africa-Europe collisional processes<br />
(DEWEY et alii, 1989). The deepest structural position is<br />
occupi<strong>ed</strong> by over 500-m-thick pelagic sequence of the Imerese<br />
unit, outcropping at Mt. dei Cervi, mostly compos<strong>ed</strong> of Upper<br />
Triassic cherty limestones follow<strong>ed</strong> upwards by Jurassic-<br />
Cretaceous ra<strong>di</strong>olarites and by Eocene-Oligocene marls and<br />
marly limestones (Scaglia formation), unconformably cover<strong>ed</strong><br />
by Upper Oligocene Numi<strong>di</strong>an flysch-type sequence (Portella<br />
Colla clays). During the Middle Miocene this unit was<br />
tectonically overlain by shallow-water carbonates deposits<br />
pertaining to the Panormide platform that start from the bottom<br />
with Middle-Upper Carnian brownish clays and calcarenites<br />
(Mufara formation) passing upwards to Upper Triassic-Eocene<br />
limestones and dolostones. This succession is unconformably<br />
cover<strong>ed</strong> by the Upper Oligocene-Lower Miocene Numi<strong>di</strong>an<br />
Flysch deposits. The NNW-SSE striking tectonic contact<br />
between the two units is clearly expos<strong>ed</strong> from the western slope<br />
of Pizzo Carbonara to Polizzi G. (Fig. 1).
NEOGENE ROTATIONS IN THE SICILIAN-MAGHREBIAN CHAIN<br />
Fig. 1 – Geological sketch-map of the southern slope of the Madonie Mountains. Black dash<strong>ed</strong> circular arrows in<strong>di</strong>cate the amount of rotations calculat<strong>ed</strong> by<br />
structural analysis during this work. Grey circular arrows in<strong>di</strong>cate the amount of clockwise rotation calculat<strong>ed</strong> by palaeomagnetic analysis (from CHANNELL et<br />
alii, 1980; 1990; GRASSO et alii, 1987; OLDOW et alii, 1990; SPERANZA et alii , 1999; 2003).<br />
STRUCTURAL ANALYSIS<br />
The structural analysis was focus<strong>ed</strong> to fold axes trend<br />
<strong>di</strong>stribution in the multilayer<strong>ed</strong> Imerese succession of Mt. dei<br />
Cervi and to fault planes statistical orientation in the calcareous<br />
or arenaceous deposits of Mt. San Salvatore area (Fig. 1).<br />
In the Mt. dei Cervi area (Fig. 2), the Imerese s<strong>ed</strong>imentary<br />
succession forms a large dome structure characteriz<strong>ed</strong> by two<br />
main fold systems. The first system ( in Fig. 2) shows NNW-<br />
SSE orient<strong>ed</strong> fold axes, plunging at angles of 10-45°, which are<br />
sub-parallel to the coeval thrust contact between Panormide and<br />
Imerese units. The second and more recent system ( in Fig. 2)<br />
is mainly represent<strong>ed</strong> by a roughly ENE-WSW striking large<br />
ramp anticline, with sub-horizontal axes, also deforming the<br />
former thrust contact between Panormide and Imerese units.<br />
This anticline is coaxial to the high angle south-verging out-ofsequence<br />
large thrust (Scillato-Petralia thrust) responsible,<br />
during the Tortonian-Middle Pliocene, for the development of a<br />
29<br />
top-thrust s<strong>ed</strong>imentary basin to the south and of the final uplift<br />
of the Madonie Mountains.<br />
In the Mt. San Salvatore area, east of Mt. dei Cervi (Fig. 2),<br />
the structural analysis was bas<strong>ed</strong> on determination of the spatial<br />
<strong>di</strong>stribution of meso-faults and their geometric relationships<br />
with large strike-slip faults develop<strong>ed</strong> since Middle Pliocene<br />
times. The main structures are represent<strong>ed</strong> by N120-140E<br />
striking right-lateral strike-slip faults boun<strong>di</strong>ng to the north-east<br />
and to the south-west the Mt. San Salvatore ridge. Minor<br />
associat<strong>ed</strong> structures consist of NE-SW striking left-lateral<br />
strike-slip faults which are confin<strong>ed</strong> between the major rightlateral<br />
strike-slip faults. They have been interpret<strong>ed</strong> as<br />
coniugate antithetical structures, boun<strong>di</strong>ng clockwise rotat<strong>ed</strong><br />
blocks, and are <strong>di</strong>stribut<strong>ed</strong> on two main systems orient<strong>ed</strong><br />
~N40E and ~N75E.<br />
To the south, on the footwall the Scillato-Petralia thrust<br />
(Fig. 1), the Serravallian-Lower Pliocene deposits are<br />
characteriz<strong>ed</strong> by only one fold and thrust system, WSW-ENE<br />
tren<strong>di</strong>ng, coaxial to the second system found in the Mt. dei
30 G.BARRECA & C.MONACO<br />
Cervi area. Locally, the folds are dragg<strong>ed</strong> by large right-lateral<br />
strike-slip faults and involv<strong>ed</strong> in up to 45° clockwise rotation<br />
(e.g. the Gangi syncline, see Fig. 1).<br />
DISCUSSION AND CONCLUSIONS<br />
Data collect<strong>ed</strong>, bas<strong>ed</strong> on meso-fold axis <strong>di</strong>stribution, fault<br />
planes statistical orientation and on fault-relat<strong>ed</strong> structures field<br />
analysis, compar<strong>ed</strong> to publish<strong>ed</strong> palaeomagnetic data, allow<strong>ed</strong><br />
us to define the tectonic evolution of the area in relation to two<br />
main rotation episodes occurr<strong>ed</strong> since the Middle Miocene.<br />
The structural analysis carri<strong>ed</strong> out in the Mt. dei Cervi area<br />
shows the occurrence of two fold and fault systems, with subperpen<strong>di</strong>cular<br />
axes, relat<strong>ed</strong> to two <strong>di</strong>stinct tectonic phases. The<br />
second and more recent one is also responsible for the thrusting<br />
of the Upper Triassic-Middle Miocene successions (Scillato-<br />
Petralia thrust) over the Serravallian-Lower Pliocene deposits and<br />
for the fol<strong>di</strong>ng of these cover succession. The superposition of<br />
two contractional structure systems with sub-orthogonal axes,<br />
compar<strong>ed</strong> to palaeomagnetic data (see Fig. 1), can be interpret<strong>ed</strong><br />
as the result of ~70° clockwise block rotation of the first phase<br />
structures (includ<strong>ed</strong> the old tectonic contact between Imerese and<br />
Panormide units) around vertical axes, occurr<strong>ed</strong> between<br />
Langhian and Late Tortonian time by a roughly N-S orient<strong>ed</strong><br />
maximum horizontal compression. (see also AVELLONE &<br />
BARCHI, 2003; MONACO & DE GUIDI, 2006). This was follow<strong>ed</strong><br />
by locally more than 30° clockwise rotation since the Lower-<br />
Middle Pliocene relat<strong>ed</strong> to dragging by Plio-Pleistocene NW-SE<br />
striking right-lateral strike-slip faults (Figs. 1 and 2).<br />
The second rotation episode is also consistent with the<br />
angular relationship (35°) between the two left-lateral strike-slip<br />
fault systems detect<strong>ed</strong> in the Mt. San Salvatore area that can be<br />
interpret<strong>ed</strong> as the result of clockwise block rotation between<br />
major right-lateral strike-slip faults. In this kinematic mo<strong>del</strong>, the<br />
~N75E orient<strong>ed</strong> faults belong to an older rotat<strong>ed</strong> and abandon<strong>ed</strong><br />
system, while the ~N40E faults represent a more recent system.<br />
From a geodynamic point of view, the Neogene clockwise<br />
rotation in the Sicilian-Maghrebian Chain is relat<strong>ed</strong> to the<br />
regional framework of the Africa-Europe oblique convergence<br />
(DEWEY et alii, 1989). The clockwise rotation accompani<strong>ed</strong><br />
thrusting processes since the Early-Middle Miocene, when the<br />
Africa-Europe collision trigger<strong>ed</strong> the extensive southwards<br />
migration of large nappes and a general block rotation around<br />
vertical axes under a quasi-constant stress field relat<strong>ed</strong> to the N-S<br />
convergence. In this context, only the Plio-Pleistocene rotation<br />
episode is well constrain<strong>ed</strong> on field all over the northern Sicily,<br />
where large right-lateral strike-slip faults are document<strong>ed</strong> (see<br />
also GHISETTI & VEZZANI, 1984; LENTINI et alii, 1991; FINETTI et<br />
alii., 1996; GIUNTA et alii, 2000; RENDA et alii, 2000; GUARNIERI,<br />
2004; NIGRO & RENDA, 2005).<br />
Fig. 2 – Shad<strong>ed</strong>-relief map (SW prospective view) of the Mt. dei Cervi-Mt. S. Salvarore area showing the main structural features. The first phase folds,<br />
striking N150E () are sub-parallel to the thrust contact between Panormide and Imerese units (a) while the second phase folds () are coaxial to the large<br />
Scillato-Petralia thrust ramp (b) that caus<strong>ed</strong> the uplift of the Madonie Mts. during the Tortonian-Middle Miocene.
REFERENCES<br />
ABATE B., DI STEFANO, E., DI STEFANO P., PECORAIO C.,<br />
RENDA P. (1982) - Segnalazione <strong>di</strong> un affioramento <strong>di</strong><br />
“Trubi” sul massiccio <strong>di</strong> Pizzo Carbonara (Madonie,<br />
Sicila). Rend. Soc. Geol. It., 5, 25-26.<br />
AVELLONE G. & BARCHI M.R. (2003) - Le pieghe minori nelle<br />
Unità Imeresi e Trapanesi dei Monti <strong>di</strong> Palermo e il loro<br />
significato nell’evoluzione tettonica <strong>del</strong>l’area. Boll. Soc.<br />
Geol. It., 122, 277-294.<br />
CHANNELL J.E.T., CATALANO R. & D’ARGENIO B. (1980) -<br />
Palaeomagnetism and deformation of the Mesozoic<br />
continental margin in Sicily. Tectonophysics, 61, 391-407.<br />
CHANNELL J.E.T., OLDOW J.S., CATALANO R. & D’ARGENIO B.<br />
(1990) -. Palaeomagnetically determin<strong>ed</strong> rotations in the<br />
western Sicilian fold and thrust belt. Tectonics, 9, 641-660.<br />
DEWEY J.F., HELMAN M.L., TURCO E., HUTTON D.H.W. &<br />
KNOTT S.D. (1989) - Kinematics of the westwern<br />
M<strong>ed</strong>iterranean. In Coward, M.P., Dietrich, D. and Park,<br />
R.G. (Eds.), Alpine Tectonics, Geol. Soc. of London<br />
Special Publication, 45, 265–283.<br />
FINETTI I., LENTINI F., CARBONE S., CATALANO S. & DEL BEN,<br />
A. (1996) - Il sistema Appennino meri<strong>di</strong>onale-Arco<br />
Calabro-Sicilia nel M<strong>ed</strong>iterraneo centrale: stu<strong>di</strong>o<br />
geologico-geofisico. Boll. Soc. Geol. It., 115, 539-559.<br />
GHISETTI F. & VEZZANI L. (1984) - Thin-skinn<strong>ed</strong> deformations<br />
of the western Sicily thrust belt and relationships with<br />
crustal shortening: mesostructural data on the Mt. Kumeta-<br />
Alcantara fault zone and relat<strong>ed</strong> structures. Boll. Soc.<br />
Geol. It., 103, 129-157.<br />
GIUNTA G., NIGRO F., RENDA P. & GIORGIANNI A. (2000) - The<br />
Sicilian-Maghrebides Tyrrhenian margin: a neotectonic<br />
evolutionary mo<strong>del</strong>. Boll. Soc. Geol. It., 119, 553-565.<br />
GRASSO M., LENTINI F., VEZZANI L. (1978) - Lineamenti<br />
stratigrafico strutturali <strong>del</strong>le Madonie (Sicilia centrosettentrionale).<br />
Geol. Romana, 17, 45-69.<br />
GRASSO M., MANZONI M. & QUINTILI A. (1987) - Misure<br />
magnetiche sui Trubi infrapliocenici <strong>del</strong>la Sicilia<br />
orientale: possibili implicazioni stratigrafiche e strutturali.<br />
Mem. Soc. Geol. It., 38, 459-474.<br />
GUARNIERI P. (2004) - Structural evidence for deformation by<br />
block rotation in the context of transpressive tectonics,<br />
northwestern Sicily (Italy). J. of Struct. Geol., 26, 207-219.<br />
LENTINI F., CARBONE S., CATALANO S., GRASSO M. &<br />
MONACO C. (1991) - Presentazione <strong>del</strong>la Carta Geologica<br />
<strong>del</strong>la Sicilia centro-orientale. Memorie Società Geologica<br />
Italiana, 47, 145-156.<br />
MONACO C. & DE GUIDI G. (2006) – Structural evidence for<br />
Neogene rotations in the eastern Sicilian fold and thrust<br />
belt. J. of Struct. Geol., 28, 561-574.<br />
NEOGENE ROTATIONS IN THE SICILIAN-MAGHREBIAN CHAIN<br />
31<br />
NIGRO F. & RENDA P. (2005) - Plio-Pleistocene strike-slip<br />
deformation in NE Sicily: the example of the area between<br />
Capo Calavà and Capo Tindari. Boll. Soc. Geol. It., 124,<br />
377-394.<br />
OLDOW J.S., CHANNEL J.E.T., CATALANO R. & D’ARGENIO B.<br />
(1990) - Contemporaneous thrusting and large-scale<br />
rotations in the western Sicilian fold and thrust belt.<br />
Tectonics, 9, 661-681.<br />
RENDA P., TAVARNELLI E., TRAMUTOLI M., GUEGUEN E.<br />
(2000) - Neogene deformations of Northern Sicily and<br />
their implications for the geodynamics of the Southern<br />
Tyrrhenian Sea margin. Mem. Soc. Geol. It., 55, 53-59.<br />
SPERANZA F., MANISCALCO R., MATTEI M., DI STEFANO A.,<br />
BUTLER R.W.H.& FUNICIELLO R. (1999) - Timing and<br />
magnitude of rotations in the frontal thrust systems of<br />
south-western Sicily. Tectonics, 18, 1178-1197.<br />
SPERANZA F., MANISCALCO R. & GRASSO M. (2003) - Pattern<br />
of orogenic rotation in central-eastern Sicily: implications<br />
for the timing of sprea<strong>di</strong>ng in the Tyrrhenian Sea. J. Geol.<br />
Soc. of London, 160, 183-195.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 32-35, 2ff.<br />
Strutture a macro e mesoscala <strong>del</strong>le Dinari<strong>di</strong> triestine (carta GEO-<br />
CGT <strong>del</strong> FVG).<br />
ABSTRACT<br />
Macro and meso structures of external Dinarides in Trieste and Gorizia<br />
region<br />
The region of Trieste and Gorizia is a sector of north-western Dinarides,<br />
characteris<strong>ed</strong> by a fold and thrust tectonic that interests the creataceouspaleogenic<br />
carbonate platform and eocenic flysch, call<strong>ed</strong> Trieste Flysch.<br />
The main structure is the Carso Thrust overlapping toward SW the smaller<br />
subthrust, in the Flysch of “Ciceria structure”. Toward SE (in the Val<br />
Rosandra area) this thrust overlaps with his lateral ramp some others minor<br />
thrusts that, it turn, cover a great fold produc<strong>ed</strong> by a bukcling process.<br />
The Trieste Flysch is implicat<strong>ed</strong> in all the describ<strong>ed</strong> structures, but<br />
presents also frequent isoclinal folds referable to the gravitative tectonic of the<br />
first compressive phases that has interest<strong>ed</strong> the region.<br />
Key words: Carso-Kras Thrust, Dinarides, thrust tectonic,<br />
Trieste Flysch.<br />
INTRODUZIONE<br />
I rilievi compresi nell’area dei Fogli GEO-CGT 110<br />
Trieste, 131 Caresana, 109 Grado e 088 Gorizia, fanno parte<br />
<strong>del</strong>la Catena <strong>del</strong>le Dinari<strong>di</strong> Esterne settentrionali, in particolare<br />
<strong>del</strong> settore <strong>di</strong> catena ad W <strong>del</strong>la Faglia <strong>di</strong> Idria, imponente<br />
lineamento a cinematica trascorrente che, secondo alcuni<br />
Autori, rappresenta l’attuale “binario” orientale <strong>di</strong> scorrimento<br />
<strong>del</strong>la Zolla Adriatica nel suo moto traslatorio verso N e N-W.<br />
In particolare, l’area in esame si situa ad W dei fronti <strong>del</strong>le<br />
principali falde <strong>di</strong> ricoprimento <strong>ed</strong> è caratterizzata<br />
prevalentemente da strutture a thrust tipiche dei settori più<br />
esterni <strong>del</strong>le catene <strong>di</strong> collisione (JURKOVŠEK, 2008). Il<br />
maggiore sviluppo assunto dalle falde vere e proprie verso NW,<br />
sino al contatto con le strutture <strong>del</strong>le Alpi Giulie, testimonia<br />
<strong>del</strong>la grande importanza <strong>del</strong> trasporto tettonico <strong>di</strong>narico nei<br />
settori <strong>di</strong> catena oggi inglobati nelle Alpi Meri<strong>di</strong>onali. La zona<br />
in stu<strong>di</strong>o si presenta quin<strong>di</strong> come un settore <strong>di</strong> catena a<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche Ambientali e Marine – Universtà<br />
degli stu<strong>di</strong> <strong>di</strong> Trieste<br />
Lavoro eseguito nell’ambito <strong>del</strong> progetto GEO-CGT, Convenzione n. 8504<br />
<strong>del</strong> 25 febbraio 2005 tra il Servizio Geologico – Direzione centrale<br />
ambiente e lavori pubblici – Regione Autonoma Friuli Venezia Giulia e le<br />
Università degli stu<strong>di</strong> <strong>di</strong> Trieste e <strong>di</strong> U<strong>di</strong>ne.<br />
BENSI SARA (*), FANUCCI FRANCESCO (*) & PODDA FULVIO (*)<br />
trasporto tettonico relativamente limitato.<br />
Non<strong>di</strong>meno, la tettonica locale é evoluta e importante,<br />
soprattutto nella zona <strong>del</strong> Carso Triestino, e non priva <strong>di</strong><br />
complessità.<br />
L’ASSETTO TETTONICO A GRANDI LINEE<br />
L’area è caratterizzata da due motivi strutturali principali e<br />
da altri, non meno importanti dal punto <strong>di</strong> vista <strong>del</strong>l’evoluzione<br />
tettonica, ma riconoscibili solo in aree specifiche. L’unità <strong>di</strong><br />
gran lunga dominante nel panorama tettonico è il Thrust <strong>del</strong><br />
Carso che si sviluppa in senso <strong>di</strong>narico caratterizzando<br />
fondamentalmente la zona. L’ampiezza <strong>del</strong>l’anticlinale <strong>di</strong><br />
rampa che, nei limiti <strong>del</strong> territorio italiano, mostra il fianco<br />
settentrionale solo in una ristretta zona <strong>del</strong> Goriziano e la<br />
potenza <strong>del</strong>la serie coinvolta nel piegamento sono<br />
testimonianza <strong>di</strong> uno scollamento profondo e <strong>di</strong> un trasporto<br />
tettonico non trascurabile. L’andamento <strong>del</strong> fronte è<br />
leggermente obliquo rispetto a quello <strong>del</strong>la costa a N <strong>di</strong><br />
Barcola, mentre nell’area urbana se ne <strong>di</strong>scosta fortemente. Il<br />
limite meri<strong>di</strong>onale <strong>del</strong>la struttura è segnato da una rampa<br />
laterale sdoppiata che porta il Thrust a sovrascorrere su tutte le<br />
altre unità tettoniche che caratterizzano l’area <strong>del</strong>la Val<br />
Rosandra.<br />
L’altro motivo importante è quello dei thrust minori che<br />
interessano estesamente la zona <strong>di</strong> Flysch su cui sorge Trieste<br />
per poi prolungarsi alla base <strong>del</strong> versante costiero <strong>ed</strong> entro il<br />
Golfo. Anche considerando il <strong>di</strong>sturbo dovuto alle <strong>di</strong>scontinuità<br />
anti<strong>di</strong>nariche <strong>del</strong>la zona Muggia - S. Servolo non si può negare<br />
un’evidente continuità tra queste strutture e quelle <strong>del</strong>la Ciceria<br />
(subthrusting belt, <strong>di</strong> supposta età miocenica), a cui è stata<br />
attribuita <strong>di</strong> recente (PLACER, 2007) una notevole importanza<br />
nell’evoluzione geo<strong>di</strong>namica <strong>del</strong>le Dinari<strong>di</strong> esterne<br />
settentrionali. Secondo l’ Autore citato, farebbe parte <strong>del</strong><br />
sistema detto anche la zona <strong>del</strong>la Val Rosandra, in cui però si<br />
rilevano unità tettoniche i cui caratteri non si accordano con<br />
questa attribuzione:
Fig. 1 – Thrust a embrici in destra <strong>del</strong>la Val Rosandra<br />
- l’unità in posizione basale è costituita dalle pieghe <strong>di</strong><br />
Monte Carso e <strong>del</strong> vicino Monte S. Michele, anticlinali<br />
rovesciate a SW a curvatura accentuata su cui si accavalla<br />
l’Unità <strong>di</strong> Crinale, ampio thrust a vergenza W. Entrambe le<br />
strutture interessano la Formazione dei Calcari a Alveoline e<br />
Nummuliti e il Flysch <strong>di</strong> Trieste: Il processo <strong>di</strong> buckling che dà<br />
origine alle pieghe deve riferirsi ad un livello <strong>di</strong> scollamento<br />
interno alla prima formazione. Analogamente il thrust deve<br />
essere una struttura relativamente pellicolare dato che si adatta<br />
all’anticlinale descritta e alla sinclinale <strong>del</strong>la Val Rosandra;<br />
- il fianco orientale <strong>del</strong>la Valle è interessato da alcuni<br />
thrust embriciati che sovrascorrono alle strutture prec<strong>ed</strong>enti; il<br />
più importante è quello <strong>di</strong> Monte Stena, con un’anticlinale <strong>di</strong><br />
rampa relativamente ampia, che ad E è sovrascorso a sua volta<br />
da un’unità che possiamo denominare Thrust <strong>di</strong> Draga il quale<br />
presenta fenomeni <strong>di</strong> retroscorrimento <strong>del</strong> Flysch sul fronte, a<br />
deformazione massima, <strong>del</strong>l’anticlinale <strong>di</strong> rampa, simili a quelli<br />
che, a scala maggiore, propiziati da potenti livelli <strong>di</strong> marne,<br />
interessano il Thrust <strong>del</strong> Carso;<br />
STRUTTURE A MACRO E MESOSCALA NELLE DINARIDI TRIESTINE<br />
33<br />
- quest’ultimo sovrascorre in rampa laterale su tutte le<br />
strutture <strong>del</strong>la zona, al punto che l’enorme carico litostatico<br />
indotto da questo sovrascorrimento ha prodotto scistosità in<br />
con<strong>di</strong>zioni <strong>di</strong> anchimetamorfismo nelle marne associate<br />
all’unità basale.<br />
La cronologia <strong>del</strong>le fasi deformative segue ovviamente<br />
l’or<strong>di</strong>ne <strong>di</strong> sovrapposizione <strong>del</strong>le relative Unità con l’eccezione<br />
dei thrust <strong>del</strong>l’Unità <strong>del</strong>la Ciceria che deriverebbero da un<br />
sottoscorrimento ben più recente rispetto alla messa in posto<br />
<strong>del</strong> Thrust <strong>del</strong> Carso.<br />
Elementi risalenti a fasi deformative più antiche,<br />
sins<strong>ed</strong>imentarie, sono rappresentate nel Carso Goriziano<br />
essenzialmente da faglie trascorrenti a orientamento <strong>di</strong>narico,<br />
mentre con orientamento anti<strong>di</strong>narico si manifestano, a tratti, le<br />
linee <strong>di</strong> transfer derivanti dalla fase <strong>di</strong> collasso non omogeneo<br />
<strong>del</strong>la piattaforma carbonatica nell’Eocene. Tutti i Thrust sono<br />
interessati sul fronte da importanti tear faults, contemporanee<br />
alla messa in posto dei fronti, che inducono spesso<br />
con<strong>di</strong>zionamenti morfostrutturali <strong>di</strong> rilievo negli affioramenti <strong>di</strong><br />
Flysch sottostanti.
34 S. BENSI ET ALII<br />
Fig. 2 – Cerniera <strong>di</strong> piega isoclinale nel Flysch.<br />
Il Flysch <strong>di</strong> Trieste mostra a tratti, soprattutto nelle zone<br />
sottostanti l’Unità <strong>del</strong> Carso, deformazioni semiduttili <strong>del</strong> tipo<br />
pieghe isoclinali che si associano talvolta nelle aree <strong>di</strong><br />
affioramento a livelli caoticizzati, qualora gli strati arenacei<br />
siano poco potenti <strong>ed</strong> alternati ad abbondanti marne siltose. Ciò<br />
può essere imputato, come già intuito dai primi Autori<br />
interessati alla zona, a processi <strong>di</strong> tettonica gravitativa<br />
impostatisi durante le fasi precoci <strong>di</strong> movimentazione<br />
compressiva <strong>del</strong>la piattaforma carbonatica.<br />
REFERENCES<br />
CARULLI G.B. & CUCCHI F. (1991) - Proposta <strong>di</strong><br />
interpretazione strutturale <strong>del</strong> Carso triestino.- Atti<br />
Ticinensi <strong>di</strong> Scienze <strong>del</strong>la Terra 34, 161-166.<br />
CARULLI G.B. (2006) – Carta Geologica <strong>del</strong> Friuli Venezia<br />
Giulia – Scala 1:150.000.- Servizio Geologico, Direzione<br />
Centrale Ambiente e Lavori Pubblici – Regione Autonoma<br />
Friuli Venezia Giulia.<br />
D’AMBROSI C. (1953), Carta geologica <strong>del</strong>le Tre Venezie –<br />
Foglio Trieste. Ufficio idrografico <strong>del</strong> Magistrato alle<br />
Acque. <strong>Sezione</strong> Geologica. Venezia.<br />
D’AMBROSI C. (1958) - Sul colamento per gravità <strong>del</strong> Flysch<br />
lungo la Riviera <strong>di</strong> Trieste. In: G. Ranalli (Ed.) - Plate<br />
Tectonics: The First Twenty-Five Years. Proc. VIII<br />
Summer School Earth and Planetary Sciences, Siena, 1995,<br />
171-251<br />
JURKOVŠEK B. (2008) – Geološka Karta Severnega <strong>del</strong>a<br />
Tržaško-Komenske Planote / Geological Map of Northern<br />
part of the Trieste-Komen peneplain (Slovenia) 1:25000.<br />
Ed. Geološki Zavod Slovenije, Ljubljana.<br />
PLACER L. (1981) – Geološka zgradba jugozahodne Slovenije /<br />
Geologic structure of southwestern Slovenia. GEOLOGIJA<br />
21, 27-60.<br />
PLACER L. (2007) – Kraški rob Geološki prerez vzdolž AC<br />
Kozina – Koper / Kraški rob (landscape term) Geologic<br />
section along the mortway Kozina – Koper (Capo<strong>di</strong>stria).<br />
GEOLOGIJA 50/1, 29-44.
TARI V. (2002) – Evolution of the northern and western<br />
Dinarides: a tectonostratigraphic approach). EGU Stephan<br />
Mueller Special Publication Series, 1, 223-236.<br />
STRUTTURE A MACRO E MESOSCALA NELLE DINARIDI TRIESTINE<br />
35
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 36-38, 2 ff.<br />
Compatibilità <strong>del</strong>l’assetto strutturale superficiale con la faglia<br />
sismogenetica profonda nello Stretto <strong>di</strong> Messina in base ai mo<strong>del</strong>li<br />
analogici<br />
LORENZO BONINI (*), DANIELA DI BUCCI (**), SILVIO SENO (*), GIOVANNI TOSCANI (*), GIANLUCA VALENSISE (***)<br />
ABSTRACT<br />
Reconciling shallow structural setting and seismogenic normal faults in<br />
the Messina Straits: the analogue mo<strong>del</strong> perspective<br />
The Messina Straits and its adjacent areas show an important tectonic<br />
activity and have been deeply investigat<strong>ed</strong>. In this paper we make use of<br />
analogue mo<strong>del</strong>s to reconcile field and surface evidences of high angle normal<br />
faults with the presence of a deep, not expos<strong>ed</strong> low angle normal fault (LANF),<br />
responsible for the large earthquake of 28 December 1908. Reproducing in a<br />
sand-box a SE <strong>di</strong>pping low angle normal fault we observ<strong>ed</strong> the forming of one<br />
or, in some cases, two grabens with a <strong>di</strong>rection and an asimmetry compatible<br />
with those observ<strong>ed</strong> in the study area. All synthetic and antithetic high angle<br />
normal faults are <strong>di</strong>rectly link<strong>ed</strong> to the main LANF and they all develop in the<br />
very early stages of deformation. In this view, the high angle normal faults<br />
faults survey<strong>ed</strong> and mapp<strong>ed</strong> by many Authors could be consider<strong>ed</strong> as a surface<br />
expression of a long-term activity of a main SE-<strong>di</strong>pping LANF.<br />
Key words: Faglia normale a basso angolo, faglie sismo<br />
genetiche, mo<strong>del</strong>li analogici, terremoto Calabro-Messinese<br />
<strong>del</strong> 1908.<br />
INTRODUZIONE<br />
Lo stretto <strong>di</strong> Messina <strong>ed</strong> i settori ad esso a<strong>di</strong>acenti sono una<br />
<strong>del</strong>le regioni a maggior attività tettonica nel M<strong>ed</strong>iterraneo e,<br />
per questo motivo, oggetto <strong>di</strong> numerosi <strong>ed</strong> approfon<strong>di</strong>ti stu<strong>di</strong>.<br />
Ciò nonostante, la comunità scientifica, pur <strong>di</strong>sponendo <strong>di</strong> una<br />
dataset <strong>di</strong> informazioni che si è recentemente molto aggiornato<br />
<strong>ed</strong> incrementato, non ha ancora un’opinione unanime su quale<br />
struttura sismogenetica sia responsabile <strong>del</strong> <strong>di</strong>sastroso sisma<br />
che ha colpito l’area nel Dicembre <strong>del</strong> 1908. L’approccio<br />
metodologico proposto e la serie <strong>di</strong> mo<strong>del</strong>li analogici realizzati<br />
hanno l’intento <strong>di</strong> verificare se la presenza e le caratteristiche<br />
geometriche e cinematiche <strong>del</strong>la faglia responsabile <strong>del</strong><br />
terremoto <strong>del</strong> 1908 possa spiegare le evidenze <strong>di</strong> superficie<br />
legandole all’assetto strutturale profondo <strong>del</strong>lo Stretto.<br />
Le deformazioni cosismiche associate all’evento <strong>del</strong> 1908<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, università <strong>di</strong> Pavia<br />
(**) Dipartimento <strong>del</strong>la Protezione Civile<br />
(***) Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia<br />
(quantificate, in particolare, attraverso misure <strong>di</strong> livellazione),<br />
la <strong>di</strong>stribuzione <strong>del</strong> danno, i dati strumentali e più in generale<br />
l’assetto morfotettonico recente <strong>del</strong>l’area <strong>del</strong>lo Stretto hanno<br />
permesso una ricostruzione dettagliata <strong>del</strong>la sorgente<br />
sismogenetica. Si tratta <strong>di</strong> una faglia lunga circa 40 km, con<br />
immersione <strong>di</strong> 30-35° verso SE e cinematica prevalentemente<br />
normale. Questa faglia si trova tra 3 e 12 km <strong>di</strong> profon<strong>di</strong>tà e<br />
non arriva a interessare <strong>di</strong>rettamente la superficie (DISS<br />
Working Group, 2007, bibliografia inclusa).<br />
Stu<strong>di</strong> geofisici e <strong>di</strong> terreno, sia sulla costa messinese che su<br />
quella calabra, hanno tuttavia rilevato la presenza <strong>di</strong> numerose<br />
faglie normali ad alto angolo, con immersione sia verso NW<br />
che verso SE, dando talvolta a queste strutture un’importanza<br />
<strong>di</strong> primo or<strong>di</strong>ne. Queste faglie interessano le porzioni più<br />
superficiali <strong>del</strong>lo spessore crostale <strong>del</strong>l’area <strong>del</strong>lo Stretto, sia in<br />
terra sia a mare, e deformano depositi recenti, in alcuni casi<br />
raggiungendo la superficie (Del Ben et al., 1996, bibliografia<br />
inclusa). La valutazione in chiave sismotettonica <strong>di</strong> questi dati<br />
ha portato anche alla formulazione <strong>di</strong> ipotesi <strong>di</strong>verse dalla<br />
prec<strong>ed</strong>ente (Fig. 1) per la faglia sorgente <strong>del</strong> terremoto <strong>del</strong><br />
1908 (si v<strong>ed</strong>a Valensise e Pantosti, 1992, per una review).<br />
Tuttavia, tali ipotesi non sono completamente compatibili con<br />
le deformazioni cosismiche, il quadro <strong>del</strong> danneggiamento e i<br />
dati strumentali <strong>di</strong>sponibili per lo stesso terremoto.<br />
Per verificare la possibilità che queste due posizioni<br />
apparentemente inconciliabili siano in realtà espressioni <strong>di</strong>verse<br />
<strong>di</strong> un unico fenomeno (ovvero la presenza <strong>di</strong> una faglia normale<br />
a basso angolo profonda, con immersione verso SE, che<br />
sottende faglie <strong>di</strong> rango inferiore ad alto angolo che in alcuni<br />
casi arrivano in superficie), ci siamo avvalsi <strong>del</strong>la mo<strong>del</strong>lazione<br />
analogica col fine ultimo <strong>di</strong> riprodurre l’evoluzione cinematica<br />
<strong>di</strong> una faglia normale, a basso angolo <strong>ed</strong> attiva tra 3 e 12<br />
chilometri <strong>di</strong> profon<strong>di</strong>tà. In questo modo è possibile, pur con i<br />
limiti <strong>del</strong>la mo<strong>del</strong>lizzazione analogica, verificare la<br />
compatibilità geometrica e cinematica tra faglia sepolta a basso<br />
angolo e faglie più superficiali ad alto angolo, con immersioni<br />
opposte.<br />
METODOLOGIA<br />
Abbiamo realizzato una serie <strong>di</strong> mo<strong>del</strong>li analogici che<br />
riproducono in 3D una faglia normale, sepolta, con<br />
un’immersione <strong>di</strong> 30°. I mo<strong>del</strong>li sono alla scala 1:100.000. La
porzione <strong>di</strong> faglia che viene attivata è lunga 40 cm e larga 20<br />
cm, e dunque riproduce in scala il mo<strong>del</strong>lo <strong>di</strong> sorgente per il<br />
1908 (DISS Working Group, 2007). Inoltre, la larghezza <strong>del</strong>la<br />
sandbox è <strong>di</strong> 80 cm e permette, quin<strong>di</strong>, <strong>di</strong> analizzare anche le<br />
deformazioni che si producono alle estremità laterali <strong>del</strong>la<br />
faglia (Fig. 2 a e b).<br />
I mo<strong>del</strong>li sono realizzati in sabbia (=34°); sulla superficie<br />
<strong>del</strong>la faglia è inoltre presente uno strato <strong>di</strong> circa 2 cm <strong>di</strong><br />
microbiglie in vetro (=24°). Sono stati condotti quattro<br />
esperimenti per valori crescenti <strong>di</strong> estensione pari a 0,5, 2,0, 3,5<br />
e 5,5 cm lungo il piano <strong>di</strong> faglia.<br />
RISULTATI<br />
Tutti i mo<strong>del</strong>li hanno mostrato l’attivazione <strong>del</strong>l’intera<br />
superficie <strong>del</strong>la faglia principale a basso angolo. Inoltre, al tetto<br />
<strong>del</strong>la stessa si sono formate numerose faglie normali sintetiche<br />
e antitetiche ad alto angolo (Fig. 2c). In sezione, queste faglie<br />
LA FAGLIA SISMOGENETICA PROFONDA NELLO STRETTO DI MESSINA<br />
Fig. 1 – a) Localizzazione degli eventi sismici maggiori avvenuti nell’area <strong>del</strong>lo Stretto <strong>di</strong> Messina (Working Group CPTI, 2004). b) Localizzazione <strong>del</strong>la<br />
sorgente <strong>del</strong> terremoto <strong>del</strong> 1908 secondo vari Autori che si basano su osservazioni sismologiche o macrosismiche.<br />
- a) Map of the Messina Straits area with epicenters of historical earthquakes (Working Group CPTI, 2004).b) Seismogenic source mo<strong>del</strong>s for the 1908<br />
earthquake bas<strong>ed</strong> on seismological or macroseismic observations.<br />
Fig. 2 – Set-up <strong>ed</strong> esempi <strong>di</strong> mo<strong>del</strong>li analogici. a) Mappa <strong>del</strong>l’apparato degli esperimenti con riferimenti geografici <strong>del</strong> contesto reale. Il box tratteggiato<br />
rappresenta la proiezione in superficie <strong>del</strong> piano <strong>di</strong> faglia. b) Vista 3D <strong>del</strong>l’apparato. c) Esempio <strong>del</strong>lo sviluppo dei sistemi <strong>di</strong> faglie normali sintetiche e<br />
antitetiche (mo<strong>del</strong>lo con a 3,5 cm <strong>di</strong> estensione).<br />
- Set-up and examples of analogue mo<strong>del</strong>s. a) Map of the experimental apparatus with geographic references of the study area. The dash<strong>ed</strong> box represents the<br />
surface projection of the fault plane. b) 3D view of the apparatus. c) Example of synthetic and antithetic normal fault systems (after 3.5 cm of extension).<br />
37<br />
sono organizzate in uno o, in alcuni casi, due graben che, negli<br />
esperimenti a rigetto maggiore, mostrano un’asimmetria<br />
compatibile con quella osservata nel contesto reale, oggetto<br />
<strong>del</strong>lo stu<strong>di</strong>o. In mappa, esse nascono e si mantengono parallele<br />
alla faglia principale nei primi sta<strong>di</strong> <strong>di</strong> deformazione;<br />
successivamente si formano anche sistemi <strong>di</strong>sposti a circa 30°<br />
dalla <strong>di</strong>rezione <strong>del</strong>la faglia principale, a partire dalle sue<br />
estremità laterali. Anche questa osservazione pare ben<br />
compatibile con la geologia <strong>di</strong> terreno e le principali strutture<br />
rilevate da <strong>di</strong>versi autori. È da notare, infine, che lo sviluppo <strong>di</strong><br />
piani ad alto angolo è molto precoce rispetto all’attivazione<br />
<strong>del</strong>la faglia principale, come emerge già nell’esperimento con<br />
estensione finale pari 0,5 cm. In particolare, sono i piani<br />
sintetici ad alto angolo i primi ad attivarsi e a raggiungere la<br />
superficie topografica dei mo<strong>del</strong>li, mentre i piani antitetici che<br />
concorrono alla formazione dei graben nascono, solitamente, in<br />
una fase più avanzata <strong>di</strong> deformazione e si raccordano, nella<br />
quasi totalità dei casi, <strong>di</strong>rettamente con la faglia principale a<br />
basso angolo.
38 L. BONINI ET ALII<br />
CONCLUSIONI<br />
La mo<strong>del</strong>lazione analogica permette <strong>di</strong> formulare un’ipotesi<br />
più articolata e complessa, a riguardo <strong>del</strong>la deformazione a<br />
lungo termine associata alla faglia sismogenetica che ha<br />
generato il terremoto calabro-messinese <strong>del</strong> 1908. Infatti, i<br />
mo<strong>del</strong>li mostrano che la fagliazione superficiale minore, che<br />
per altro prende in carico ratei <strong>di</strong> scorrimento <strong>di</strong> or<strong>di</strong>ne <strong>di</strong><br />
grandezza minore rispetto a quelli associabili alla sorgente <strong>del</strong><br />
terremoto <strong>del</strong> 1908, è perfettamente compatibile con una faglia<br />
normale principale a basso angolo immergente a SE e, anzi, ne<br />
è un’espressione <strong>del</strong>l’attività a lungo termine.<br />
La possibilità per ciascuna <strong>del</strong>le faglie al tetto <strong>del</strong>la<br />
principale <strong>di</strong> essere essa stessa sorgente <strong>di</strong> terremoti viene<br />
infine <strong>di</strong>scussa, anche tenendo conto <strong>del</strong>la magnitudo massima<br />
possibile in funzione <strong>del</strong>la profon<strong>di</strong>tà a cui ciascuno dei piani<br />
ad alto angolo intercetta il piano a basso angolo.<br />
BIBLIOGRAFIA<br />
AMORUSO A., CRESCENTINI L. & SCARPA R. (2002). Source<br />
parameters of the 1908 Messina Straits, Italy, earthquake<br />
from geodetic and seismic data. J. Geoph. Res., 107,<br />
B4,10.1029/2001JB000434, 2002.<br />
BOSCHI E., PANTOSTI D. & VALENSISE G. (1989). Mo<strong>del</strong>lo <strong>di</strong><br />
sorgente per il terremoto <strong>di</strong> Messina <strong>del</strong> 1908. Atti VIII<br />
Convegno GNGTS, 246, 258.<br />
BOTTARI A., CARAPEZZA E., CARAPEZZA M., CARVENI P.,<br />
CEFALI F., LO GIUDICE E. & PANDOLFO C. (1986). The 1908<br />
Messina Strait earthquake in the regional geostructural<br />
framework. J. Geodyn., 5, 275-302.<br />
CAPUANO P., DE NATALE G., GASPARINI P., PIGUE F. & SCARPA<br />
R. (1988). A mo<strong>del</strong> for the 1908 Messina Straits (Italy)<br />
earthquake by inversion of levelling data. Bull. Seism. Soc.<br />
Am., 78, 1930-1947.<br />
DE NATALE G. AND PINGUE F. (1991). A variable slip fault<br />
mo<strong>del</strong> for the 1908 Messina Straits (Italy) earthquake, by<br />
inversion of levelling data. Geoph. J. Intern., 104, 73-84.<br />
DEL BEN A., GARGANO C. & LENTINI F. (1996). Ricostruzione<br />
strutturale e stratigrafica <strong>del</strong>l’area <strong>del</strong>lo Stretto <strong>di</strong> Messina<br />
m<strong>ed</strong>iante analisi comparata dei dati geologici e sismici.<br />
MEMORIE DELLA SOCIETÀ GEOLOGICA ITALIANA, 51, PP.<br />
703-717.<br />
DISS WORKING GROUP (2007). Database of In<strong>di</strong>vidual<br />
Seismogenic Sources (DISS), Version 3.0.4: A compilation<br />
of potential sources for earthquakes larger than M 5.5 in<br />
Italy and surroun<strong>di</strong>ng areas. HTTP://WWW.INGV.IT/DISS/, ©<br />
INGV 2007 - Istituto Nazionale <strong>di</strong> Geofisica e<br />
Vulcanologia - All rights reserv<strong>ed</strong>.<br />
MULARGIA F. & BOSCHI E. (1983). The 1908 Messina<br />
earthquake and relat<strong>ed</strong> seismicity. In: H. Kanamori and E.<br />
Boschi (<strong>ed</strong>s), Earthquakes, Observations, Theory and<br />
Interpretation, North-Holland, New York, 493-518.<br />
SCHICK R. (1977). Eine seismotektonische Bearbeitung des<br />
Erdbebens von Messina im Jahre 1908. Geol. Jahrb., 11, 3-<br />
74.<br />
VALENSISE G. & PANTOSTI D. (1992). A 125 Kyr-long<br />
geological record of seismic source repeatability: the<br />
Messina Straits (southern Italy) and the 1908 earthquake<br />
(Ms 71/2). Terra Nova, 4 (4), pp. 472-483.<br />
WORKING GROUP CPTI (2004). Catalogo Parametrico dei<br />
Terremoti Italiani, versione 2004 (CPTI04), INGV<br />
Bologna. Available on-line at:<br />
http://emi<strong>di</strong>us.mi.ingv.it/CPTI.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 39<br />
Strutture tettoniche potenzialmente attive: la fissure-ridge <strong>di</strong><br />
Bagni S.Filippo (Monte Amiata)<br />
ALESSANDRO BORGNA (*), ANDREA BROGI (*), LORENZO FABBRINI (**) & DOMENICO LIOTTA (**)<br />
In aree caratterizzate da poca o m<strong>ed</strong>ia sismicità e nelle aree<br />
dove la ricorrenza sismica non è definibile in tempi storici,<br />
l’in<strong>di</strong>viduazione <strong>del</strong>le faglie che potenzialmente possano<br />
attivarsi è <strong>di</strong> più <strong>di</strong>fficile definizione. Pur tuttavia, tale stu<strong>di</strong>o<br />
geologico-strutturale può essere condotto valutando la<br />
collocazione strutturale e le modalità <strong>di</strong> deposizione dei<br />
travertini. Questi infatti si originano in aree dove la<br />
circolazione dei flui<strong>di</strong> è favorita da aperture e condotti<br />
strutturali, collocati lungo zone <strong>di</strong> taglio in cui la deformazione<br />
è caratterizzata da faglie <strong>di</strong>rette o transtensive. La Toscana<br />
meri<strong>di</strong>onale rappresenta un’area ideale per questo tipo <strong>di</strong><br />
stu<strong>di</strong>o: infatti essa è regionalmente caratterizzata da scarsa<br />
sismicità, elevato flusso termico, tettonica <strong>di</strong>stensiva e <strong>di</strong>ffusi<br />
affioramenti <strong>di</strong> travertino <strong>di</strong> età compresa fra il Pleistocene e<br />
l’Attuale. In particolare l’area <strong>di</strong> Bagni <strong>di</strong> S.Filippo rappresenta<br />
un chiaro esempio <strong>di</strong> questa relazione. Infatti, i travertini <strong>di</strong><br />
Bagni S.Filippo si collocano sulla prosecuzione nord-orientale<br />
<strong>del</strong>la zona <strong>di</strong> taglio transtensiva lungo cui sono ubicati i centri<br />
eruttivi <strong>del</strong> Monte Amiata.<br />
In questo lavoro presentiamo i risultati <strong>di</strong> una indagine<br />
geologico strutturale nell’area <strong>di</strong> Bagni S.Filippo, dove sono<br />
presenti estesi depositi <strong>di</strong> travertino e sorgenti termali in<br />
attività.<br />
La metodologia <strong>di</strong> stu<strong>di</strong>o ha previsto il rilevamento<br />
geologico-strutturale <strong>del</strong> substrato, la ricostruzione <strong>del</strong>la<br />
<strong>di</strong>mensione e <strong>del</strong>l’orientamento <strong>del</strong>la fissure-ridge, lo stu<strong>di</strong>o<br />
<strong>del</strong>la geometria <strong>del</strong> travertino e degli eventi deposizionali che<br />
ne hanno determinato la costruzione. In particolare è stato<br />
riconosciuto: a) un sistema articolato <strong>di</strong> fissure-ridge legate a<br />
fratture la cui orientazione in<strong>di</strong>ca la continuità con la zona <strong>di</strong><br />
taglio che attraversa le vulcaniti <strong>del</strong> Monte Amiata; b) i<br />
manufatti e le costruzioni costruiti sulla fissure-ridge mostrano<br />
evidenti segni <strong>di</strong> <strong>di</strong>slocazione; c) il sistema deposizionale<br />
riconosciuto nei travertini è paragonabile al sistema attuale <strong>di</strong><br />
deposizione ad in<strong>di</strong>care che le con<strong>di</strong>zioni deposizionali non<br />
sono cambiate nel tempo.<br />
Se ne conclude che la struttura tettonica alla quale è<br />
associato il sistema <strong>di</strong> fissure-ridge è ancora ci nematicamente<br />
_________________________<br />
(*)Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università degli Stu<strong>di</strong> <strong>di</strong> Siena, Via<br />
Laterina, 8 – 53100 Siena.<br />
(**)Dipartimento <strong>di</strong> Geologia e Geofisica, Università degli Stu<strong>di</strong> <strong>di</strong> Bari,<br />
Via Orabona, 4 – 70125 Bari.<br />
Andrea Brogi: brogiandrea@unisi.it<br />
attiva, suggerendo una continua propagazione verso est <strong>del</strong>la<br />
zona <strong>di</strong> taglio che caratterizza il Complesso vulcanico <strong>del</strong><br />
Monte Amiata.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 40<br />
Faglie trascorrenti potenzialmente attive nel complesso vulcanico <strong>del</strong><br />
Monte Amiata (Toscana meri<strong>di</strong>onale)<br />
ANDREA BROGI (*), LORENZO FABBRINI (*), DOMENICO LIOTTA (**), MARCO MECCHERI (*) & ALESSIO MONTAUTI (*)<br />
RIASSUNTO<br />
Il complesso vulcanico <strong>del</strong> Monte Amiata è ritenuto<br />
collegato allo sviluppo <strong>di</strong> una struttura tettonica, <strong>di</strong> carattere<br />
regionale, orientata NO-SE e lungo la quale si collocano i<br />
principali centri eruttivi.<br />
Sebbene <strong>di</strong>versi autori abbiano convenuto sul ruolo<br />
dominante <strong>di</strong> tale struttura per la messa in posto <strong>del</strong>le vulcaniti,<br />
sono mancati fino ad ora stu<strong>di</strong> <strong>di</strong> dettaglio che abbiano<br />
permesso <strong>di</strong> definire le caratteristiche geometriche e<br />
cinematiche <strong>del</strong>le <strong>di</strong>scontinuità tettoniche correlate e/o<br />
successive all’attività eruttiva <strong>del</strong> Monte Amiata (300-190ka).<br />
In questo lavoro presentiamo i primi risultati <strong>di</strong> uno stu<strong>di</strong>o<br />
strutturale condotto nell’apparato vulcanico amiatino e nel suo<br />
substrato. La metodologia <strong>di</strong> stu<strong>di</strong>o si è basata su:<br />
a) rilevamento geologico-strutturale soprattutto lungo la<br />
fascia SO-NE comprendente i centri eruttivi più recenti <strong>del</strong><br />
vulcano <strong>ed</strong> in altre aree chiave ubicate lungo <strong>di</strong>scontinuità<br />
minori. I risultati preliminari mettono in evidenza una zona <strong>di</strong><br />
taglio <strong>di</strong> importanza regionale caratterizzata da faglie e<br />
strutture minori associate, che localmente possono definire<br />
strutture a fiore sia positive che negative, ma che nell’insieme<br />
sono riconducibili ad un movimento transtensivo sinistro <strong>del</strong>la<br />
zona <strong>di</strong> taglio;<br />
b) analisi <strong>di</strong> dati <strong>di</strong> sondaggio e minerari resi <strong>di</strong>sponibili<br />
dall’intensa attività mineraria storicamente sviluppatasi nella<br />
regione amiatina per l’estrazione <strong>del</strong> cinabro. Prospezioni e<br />
sezioni minerarie permettono <strong>di</strong> riconoscere il ruolo dominante<br />
<strong>del</strong>le strutture transtensive nella circolazione dei flui<strong>di</strong><br />
responsabili <strong>del</strong>le mineralizzazioni sia nelle vulcaniti che nelle<br />
rocce <strong>del</strong> substrato;<br />
c) analisi strutturale dei depositi <strong>di</strong> travertino post-vulcanici<br />
nella zona <strong>di</strong> Bagni S.Filippo; i travertini sono deformati da<br />
strutture fragili che risultano in continuità con la zona <strong>di</strong> taglio<br />
riconosciuta nelle vulcaniti.<br />
L’integrazione dei dati raccolti ha permesso <strong>di</strong> ricondurre<br />
_________________________<br />
(*)Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università degli Stu<strong>di</strong> <strong>di</strong> Siena, Via<br />
Laterina, 8 – 53100 Siena.<br />
(**)Dipartimento <strong>di</strong> Geologia e Geofisica, Università degli Stu<strong>di</strong> <strong>di</strong> Bari,<br />
Via Orabona, 4 – 70125 Bari.<br />
Andrea Brogi: brogiandrea@unisi.it<br />
tutte le osservazioni ad un unico quadro deformativo guidato<br />
dalla tettonica regionale trascorrente sinistra e la cui attività è<br />
almeno riferibile ad un periodo <strong>di</strong> tempo compreso tra 300ka e<br />
l’Attuale. Pur tuttavia non può essere escluso, come già<br />
ipotizzato da altri autori, che la tettonica trascorrente fosse già<br />
attiva durante il Pleistocene così da controllare la messa in<br />
posto <strong>del</strong>le vulcaniti.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 41<br />
Strutture geologiche e circolazione idrotermale:<br />
il pull-apart <strong>di</strong> Bagnore (Monte Amiata)<br />
ANDREA BROGI (*), LORENZO FABBRINI (**) & DOMENICO LIOTTA (**)<br />
RIASSUNTO<br />
L’area geotermica <strong>di</strong> Bagnore si colloca sulla terminazione<br />
sud-occidentale <strong>del</strong>la struttura regionale transtensiva che ha<br />
verosimilmente guidato la messa in posto <strong>del</strong> complesso<br />
vulcanico <strong>del</strong> Monte Amiata (300-190ka). Nell’area <strong>di</strong> Bagnore<br />
è presente un campo geotermico, attualmente sfruttato per la<br />
produzione <strong>di</strong> energia elettrica, che si sovrappone ad un’area<br />
mineraria dove la coltivazione a cinabro si è protratta per oltre<br />
un secolo. La zona <strong>di</strong> Bagnore è anche caratterizzata da una<br />
anomalia geotermica con picchi fino a 400 mW/m 2 .<br />
Integrando dati <strong>di</strong> miniera, <strong>di</strong> sondaggio e <strong>di</strong> superficie, in<br />
questo lavoro presentiamo l’assetto strutturale <strong>del</strong>l’area <strong>di</strong><br />
Bagnore. I principali risultati mettono in evidenza che la<br />
circolazione idrotermale, sia quella attuale caratterizzata da<br />
flui<strong>di</strong> e vapori geotermici che quella fossile, caratterizzata dalla<br />
mineralizzazione a cinabro, è strettamente controllata da faglie<br />
con principale componente <strong>di</strong> movimento verticale,<br />
m<strong>ed</strong>iamente orientate in <strong>di</strong>rezione NNO-SSE. Queste strutture<br />
terminano contro faglie maggiori orientate SO-NE e che sono<br />
caratterizzate de una cinematica trascorrente sinistra, come<br />
in<strong>di</strong>cato dalle relazione tra strutture <strong>di</strong> Ri<strong>ed</strong>el e le faglie<br />
principali, da strie e mineralizzazioni sul piano <strong>di</strong> faglia.<br />
Il quadro strutturale che si ottiene, quin<strong>di</strong>, è riferibile alla<br />
evoluzione <strong>di</strong> una struttura <strong>di</strong> pull-apart dove la localizzazione<br />
<strong>del</strong>le strutture <strong>di</strong>stensive ha causato un aumento <strong>del</strong>la<br />
permeabilità, favorendo la circolazione <strong>di</strong> flui<strong>di</strong> idrotermali.<br />
_________________________<br />
(*)Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università degli Stu<strong>di</strong> <strong>di</strong> Siena, Via<br />
Laterina, 8 – 53100 Siena.<br />
(**)Dipartimento <strong>di</strong> Geologia e Geofisica, Università degli Stu<strong>di</strong> <strong>di</strong> Bari,<br />
Via Orabona, 4 – 70125 Bari.<br />
Andrea Brogi: brogiandrea@unisi.it
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 42-43<br />
RIASSUNTO<br />
Cronologia <strong>del</strong>le deformazioni compressive nel Preappennino Umbro:<br />
una sintesi<br />
In questo lavoro vengono sintetizzati i dati stratigrafici e strutturali<br />
raccolti in 4 ampie aree <strong>del</strong> Preappennino umbro con l’obiettivo <strong>di</strong> descrivere<br />
in modo esaustivo l’intera evoluzione tettono-s<strong>ed</strong>imentaria <strong>del</strong>la porzione più<br />
interna <strong>del</strong>l’avanfossa miocenica.<br />
La cronologia <strong>del</strong>le deformazioni compressive e la loro migrazione, nel tempo<br />
e nello spazio, vengono definiti ricostruendo, per i vari settori in stu<strong>di</strong>o, le età<br />
<strong>del</strong>la transizione da dominio <strong>di</strong> avampaese a dominio <strong>di</strong> avanfossa e <strong>del</strong>la<br />
successiva incorporazione nella catena. La scansione temporale <strong>di</strong> tali eventi<br />
viene definita con l’utilizzo sistematico <strong>del</strong>l’analisi biostratigrafia quantitativa<br />
<strong>del</strong>le associazioni <strong>di</strong> nannofosili calcarei applicata ad oltre 40 successioni<br />
torbi<strong>di</strong>tiche <strong>di</strong> età compresa fra il limite Oligocene-Aquitaniano <strong>ed</strong> il<br />
Tortoniano.<br />
L’in<strong>di</strong>viduazione <strong>del</strong>l’avanfossa, la sua sud<strong>di</strong>visione in sub-bacini e la sua<br />
progressiva tettonizzazione in tempi via via più recenti da ovest verso est,<br />
vengono posti in relazione con i motivi tettonici <strong>di</strong> scala regionale come la<br />
messa in posto e l’avanzamento <strong>del</strong>le unità alloctone <strong>di</strong> derivazione interna<br />
(Falda <strong>del</strong>le Arenarie <strong>del</strong> Falterona <strong>ed</strong> Unità Liguri) e l’enucleazione dei<br />
sovrascorrimenti dei massicci mesozoici perugini e <strong>di</strong> Gubbio.<br />
Key words: Umbria Preapennines, Miocene for<strong>ed</strong>eep<br />
s<strong>ed</strong>imentation, timing of compressional tectonics.<br />
INTRODUZIONE<br />
The shallow structure of the Umbria Preapennines is<br />
characteris<strong>ed</strong> by an east-verging embricate thrust-system<br />
deforming the Miocene turbi<strong>di</strong>te successions (TEEN HAF &<br />
VAN WAMEL, 1979, DE FEYTER, 1982; MENICHETTI et alii,<br />
1991).<br />
The major thrust-sheets correspon<strong>di</strong>ng to the regional tectonic<br />
units of Monte Rentella (BROZZETTI et alii, 2000), Monte Nero<br />
and Gubbio-Borgo Pace (DE FEYTER et alii, 1990), from west<br />
to east, show decreasing amount of allochthony and<br />
progressively younger ages of involvement in the<br />
_________________________<br />
Timing of contractional deformations in the Umbria<br />
Preapennines: an overview.<br />
(*)Geosis Lab- Dip. Sc. <strong>del</strong>la Terra, Università G.<br />
d'Annunzio Chieti f.brozzetti@dst.unich.it<br />
ARTA - Abruzzo, Chieti<br />
BROZZETTI FRANCESCO (*) & LUCINA LUCHETTI (**)<br />
compressional tectonics.<br />
This eastward-rejuvenating trend documents the steps through<br />
which each zone of the west-central Umbria evolv<strong>ed</strong> from<br />
foreland to for<strong>ed</strong>eep domain and was, subsequently,<br />
incorporat<strong>ed</strong> into the chain.<br />
The Miocene succession at the top of each tectonic unit, <strong>di</strong>ffers<br />
from the neighbouring ones as regards the age of the basal<br />
ramp-mud, the time interval affect<strong>ed</strong> by turbi<strong>di</strong>tic<br />
s<strong>ed</strong>imentation (expecially its onset-time), the location of the<br />
turbi<strong>di</strong>te source-areas and their <strong>di</strong>spersal pattern (DAMIANI et<br />
alii, 1983; LUCHETTI, 1998; BROZZETTI & LUCHETTI, 2002).<br />
The features and the timing of s<strong>ed</strong>imentation are also<br />
influenc<strong>ed</strong> by the emplacement of allochthon sheets above the<br />
inner side of the for<strong>ed</strong>eep and by the growth of the Umbria<br />
Preapennines compressional structures.<br />
With the aim of reconstruct an exhaustive tectonic-stratigraphic<br />
frame, four key-areas have been investigat<strong>ed</strong> by integrating<br />
structural geology, physical stratigraphy and nannofossil<br />
biostratigraphy: a) M. Rentella area, b) Monte S. Maria<br />
Tiberina ridge, c) Mt. Casale - Bocca Trabaria area and d)<br />
Perugia massifs - Gubbio transect.<br />
Within these areas, the main compressional structures (regional<br />
thrusts and folds, representative of the main tectonic units<br />
forming the Umbria pre-apennines) have been survey<strong>ed</strong> and<br />
several stratigraphic sections have been logg<strong>ed</strong>, sampl<strong>ed</strong> and<br />
analys<strong>ed</strong> through nannofossils biostratigraphy.<br />
The Mt. Rentella succession, tectonically cover<strong>ed</strong> by the<br />
Falterona Nappe, includes from the bottom, the polychromic<br />
marls of Mt. Rentella (MVR), the cherty marls of Mt. Sperello<br />
(MMS) and the sandy-marly turbi<strong>di</strong>tes of Castelvieto-La<br />
Montagnaccia (Acv). The nannofossil content of the sequence<br />
shows that the MVR are Late Oligocene-Aquitanian in age, the<br />
MMS are upper Aquitanian and that the ACv are position<strong>ed</strong> at<br />
the Aquitanian-Bur<strong>di</strong>galian boundary (BROZZETTI et alii,<br />
2000).<br />
These data clearly point out that the Mt Rentella succession can<br />
not be relat<strong>ed</strong> to the Marnoso Arenacea fm (MAR) cropping<br />
out just to the east (Corciano – M. Acuto area) which is Late<br />
Bur<strong>di</strong>galian in age and follows upwards the Early-Middle<br />
Bur<strong>di</strong>galian Schlier fm. The chronologic constrain and its<br />
structural position, suggest that this unit, during the Oligocene -<br />
Early Miocene interval, deriv<strong>ed</strong> from the tectonization of an<br />
interm<strong>ed</strong>iate palaeogeographic domain locat<strong>ed</strong> between the<br />
Tuscan for<strong>ed</strong>eep s.s. and the Umbria-Marche basin.<br />
MAR s<strong>ed</strong>imentation start<strong>ed</strong>, in western Umbria, during the<br />
Middle Bur<strong>di</strong>galian with a pelitic-arenaceous successions
TIMING OF CONTRACTIONAL DEFORMATIONS IN THE UMBRIA PREAPENNINES<br />
(MUM1 mr accor<strong>di</strong>ng to CARG nomenclature) in which axial<br />
(alpine) and transversal (appenninic) suppli<strong>ed</strong> turbi<strong>di</strong>tes<br />
alternates (LUCHETTI, 1998). During this same time span, the<br />
Falterona Nappe, carrying piggy back the Vicchio Marls and<br />
the Liguride tele-allochthon, overthrust<strong>ed</strong> the M. Rentella unit<br />
reaching the west side of the for<strong>ed</strong>eep. At the beginnig of the<br />
Late Bur<strong>di</strong>galian, its front got the present location and was<br />
seal<strong>ed</strong> by the lower part of the M.S. Maria Tiberina fm<br />
(BROZZETTI et alii, 2002; BROZZETTI, 2007). Toward west, this<br />
latter deposit<strong>ed</strong> in fact, upon the Late Bur<strong>di</strong>galian Vicchio<br />
Marls, whereas, towards east, it s<strong>ed</strong>iment<strong>ed</strong>, in continuity on<br />
MUM1. During the Early Langhian, the M. S. Maria Tiberina<br />
fm continu<strong>ed</strong> to s<strong>ed</strong>iment on the western border of the<br />
for<strong>ed</strong>eep. In the meantime, more to the east, a thicker turbi<strong>di</strong>te<br />
system, rich in coarse-grain<strong>ed</strong> arenites form<strong>ed</strong> within a NW-SE<br />
tren<strong>di</strong>ng oversuppli<strong>ed</strong> trough (MUM2, M. Casale member) and<br />
evolv<strong>ed</strong> upward as a pelitic arenaceous succession (MUM3,<br />
Vesina Member). The eastward extent of this succession is<br />
uncertain; nevertheless we may exclude that it deposit<strong>ed</strong> east of<br />
the Gubbio and M. Subasio anticlines where, up to Early<br />
Langhian, the s<strong>ed</strong>imentation of Schlier fm was still going on.<br />
During the Late Langhian - Early Serravallian time span<br />
homogeneous depositional con<strong>di</strong>tions resettl<strong>ed</strong> all over the<br />
basin and the peripheral bulge reach<strong>ed</strong> the Serra Maggio area.<br />
The wide <strong>di</strong>stribution of the typical marly-arenaceouscalcarenitic,<br />
turbi<strong>di</strong>tes characterising the Galeata and Nespoli<br />
mrs, supports this inference. Furthermore, the ubiquity of a<br />
number of calcarenite and hibrid arenite key-b<strong>ed</strong>s (among<br />
which the well-known “Contessa” b<strong>ed</strong>) showing constant<br />
thickness and composition over tens of kms, suggests that the<br />
for<strong>ed</strong>eep had a regular physiography.<br />
The tectonic deformation of the western Umbria domain,<br />
during the Late Serravallian, lead to nucleation of the well<br />
known Perugia massifs compressional structure (MINELLI et<br />
alii, 1986; BROZZETTI, 1995), detaching in the pre-Burano<br />
phillite, whose shallower and outer splay coincides with the M.<br />
Nero frontal thrust.<br />
In the western part of the basin, the growth of this<br />
compressional structures caus<strong>ed</strong> a mark<strong>ed</strong> uplift which is<br />
clearly register<strong>ed</strong> by the shallowing-upward trend of the<br />
topmost M.S. Maria Tiberina fm and lead to wide-scale sli<strong>di</strong>ng<br />
of olistostromes and slumps in the Serravallian MAR<br />
(LUCHETTI, 1997). A contraction of the for<strong>ed</strong>eep width and a<br />
sensible change in the s<strong>ed</strong>imentary supply is testifi<strong>ed</strong> by<br />
uppermost MAR succession, which deposit<strong>ed</strong> only in the<br />
central-east part of the basin and consists exclusively of very<br />
thick alpine-suppli<strong>ed</strong> turbi<strong>di</strong>tes.<br />
Since the very-Late Serravallian, contractional tectonics<br />
involv<strong>ed</strong> all that part of the basin plac<strong>ed</strong> between the M. Nero<br />
thrust and the Gubbio - M. Subasio alignment (Pietralunga<br />
Unit); possibly in this same time span, also these last anticlines<br />
start<strong>ed</strong> to grow. A clear fining and shallowing upward<br />
sequence, characterising the uppermost Civitella mr and the<br />
emplacement of further slump deposits, in the axial zone of the<br />
M. Pollo syncline (east of Gubbio, DE FEYTER et alii, 1990)<br />
support this latter hypothesis. During the Early Tortonian, the<br />
Umbria MAR for<strong>ed</strong>eep was almost completely deform<strong>ed</strong> and<br />
only the narrow Serra Maggio trough was still affect<strong>ed</strong> by the<br />
turbi<strong>di</strong>te s<strong>ed</strong>imentation of the M. Vicino sandstone.<br />
REFERENCES<br />
43<br />
BROZZETTI F. (1995) - Stile strutturale <strong>del</strong>la tettonica<br />
<strong>di</strong>stensiva nell’Umbria occidentale: L’esempio dei Massicci<br />
Mesozoici Perugini. Stud. Geol. Camerti, special issue<br />
1995,1, 105-119.<br />
BROZZETTI F. (2007) - The Umbria pre-Apennine in the Monte<br />
Santa Maria Tiberina area: new geological map and<br />
structural notes. Boll Soc. Geol. It. ital. J. Geosci., 126, 3,<br />
511-529<br />
BROZZETTI F., LUCHETTI L., & PIALLI G. (2000a) – La<br />
successione <strong>di</strong> Monte Rentella (Umbria Occidentale)<br />
biostratigrafia nannofossili calcarei <strong>ed</strong> ipotesi per un<br />
inquadramento tettonico regionale. Boll. Soc. Geol. It.,<br />
119, 407-422.<br />
BROZZETTI F & LUCHETTI L. (2002) – Vincoli stratigrafici<br />
all’evoluzione tettonica <strong>del</strong>l’ avanfossa miocenica<br />
nell’Appennino settentrionale. Atti <strong>del</strong> 3° seminario sulla<br />
Cartografia Geologica, Bologna Febbraio 2002.<br />
BROZZETTI F., GALLI M., LUCHETTI L. & PIALLI G. (2000) –<br />
Stratigraphy and structural setting of the M. Nero thrust<br />
sheet between the Afra and Lama Valleys (Northern Umbria).<br />
in Evol. Geol. e Geo<strong>di</strong>n. <strong>del</strong>l’Appennino, Convegno in<br />
memoria <strong>del</strong> Prof. G. Pialli, Abstract Vol.<br />
DAMIANI A.V., PANNUZI L., & PIALLI G. (1983) – Osservazioni<br />
geologiche nelle aree comprese fra i Massicci Perugini <strong>ed</strong> i<br />
rileievi <strong>di</strong> Gubbio. Giornale <strong>di</strong> Geol., 2, 45 (1), 127-150.<br />
DE FEYTER, (1982) – The structure of the Northern Umbria<br />
Apennines, Italy. Geol. en Mijnbouw, 16,183-189.<br />
DE FEYTER A.J., MOLENAAR N., PIALLI G., MENICHETTI M. &<br />
VENERI F. (1990) Paleotectonic significance of gravity<br />
<strong>di</strong>splacement structures in the Miocene turbi<strong>di</strong>te series of<br />
the M. Pollo Syncline (Umbro-Marchean Apennines, Italy).<br />
Geol. Mijnb., 69, 69 - 86.<br />
LUCHETTI L. (1997) - Stratigrafia a nannofossili calcarei <strong>del</strong>la<br />
Marnoso Arenacea <strong>del</strong>l'Umbria settentrionale fra il fronte<br />
<strong>del</strong>l'unità Falterona e la dorsale eugubina. Tesi <strong>di</strong><br />
dottorato (VIII Ciclo) Dip. Sc. Della Terra, Università <strong>di</strong><br />
Perugia, pp 118.<br />
LUCHETTI L. (1998) – Biostratigrafia a nannofossili calcarei<br />
<strong>del</strong>le successioni torbi<strong>di</strong>tiche mioceniche <strong>del</strong>l’area <strong>di</strong> M.<br />
Acuto e <strong>di</strong> M. Corona (Umbria nord-occidentale, Italia<br />
centrale). Boll. Soc. Geol. It., 117, 333-335.<br />
MENICHETTI M., DE FAYTER A.J. & CORSI M. (1991) - CROP<br />
03 - Il tratto Val Tiberina-Mare Adriatico. <strong>Sezione</strong><br />
geologica e caratterizzazione s<strong>ed</strong>imentaria <strong>del</strong>le avanfosse<br />
<strong>del</strong>la zona umbro-marchigiano-romagnola. Stu<strong>di</strong> Geol.<br />
Cam., Vol. Spec. (1991/1), 279 - 293.<br />
MINELLI G., MOTTI A. & PIALLI G. (1986) – Evoluzione tettonica<br />
dei Massicci Perugini; area <strong>di</strong> Monte Torrazzo. Mem. Soc.<br />
Geol. It., 35, 389-398.<br />
TEN HAAF E. & VAN WAMEL (1979) - Nappes of the Alta<br />
Romagna. In W.J.M. VAN DER LINDEN (Ed.): Fixism,<br />
mobilism or relativism: Van Bemmelen's search for<br />
harmony. Geol. Mijnbouw.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 44-47, 1f.<br />
The history of a post-orogenic trough: the high Agri Valley,<br />
Southern Apennines, Italy.<br />
FRANCESCO BUCCI (*), PAOLA GUGLIELMI (**), IVANA ADURNO (***), ENRICO TAVARNELLI (*), GIACOMO<br />
PROSSER (**) & ERWAN GUEGUEN (***)<br />
RIASSUNTO<br />
La storia <strong>di</strong> una fossa tettonica post-orogenica: l’alta Val d’Agri,<br />
Appennino Meri<strong>di</strong>onale, Italia.<br />
In molte catene collisionali, strutturate in pieghe e sovrascorrimenti, si<br />
riscontrano gli effetti <strong>di</strong> una tettonica estensionale post-orogenica, collegata a<br />
un sollevamento regionale <strong>del</strong>la catena. Gli effetti combinati <strong>di</strong> sollevamento e<br />
<strong>di</strong>stensione mo<strong>di</strong>ficano l’architettura sin-orogenica <strong>del</strong>la catena<br />
frammentandola in dorsali montuose separate da bacini continentali.<br />
Un’ analoga evoluzione tettonica può essere ricostruita nel settore assiale<br />
<strong>del</strong>l’Appennino Meri<strong>di</strong>onale dove si riconoscono <strong>di</strong>versi esempi <strong>di</strong> bacini<br />
continentali post-orogenici <strong>di</strong> origine tettonica. La presente ricerca ha<br />
l’obiettivo <strong>di</strong> inquadrare l’evoluzione post-orogenica <strong>del</strong>la depressione morfotettonica<br />
<strong>del</strong>l’alta Val d’Agri, dove sono evidenti gli effetti <strong>del</strong>la tettonica sinorogenica<br />
e post-orogenica. L’attuale morfologia e la strutturazione <strong>del</strong> bacino<br />
si è in gran parte prodotta nel Quaternario, a causa <strong>del</strong>l’attività <strong>di</strong> faglie<br />
transtensive e <strong>di</strong>stensive. Queste strutture sono successive a faglie <strong>di</strong>stensive a<br />
m<strong>ed</strong>io-basso angolo pre-esistenti che hanno consentito l’esumazione tettonica<br />
<strong>del</strong>le porzioni assiali <strong>del</strong>la catena appenninica a partire dal Pliocene superiore.<br />
Faglie <strong>di</strong>rette con bassi valori <strong>di</strong> inclinazione potrebbero spiegare alcune<br />
anomalie nei rapporti tra le formazioni <strong>del</strong>l’Unità Campano-Lucana e quelle<br />
<strong>del</strong>le Unità Lagonegresi, che sono imbricate a formare una parte considerevole<br />
<strong>del</strong>le unità alloctone affioranti nel settore assiale <strong>del</strong>l’ Appennino meri<strong>di</strong>onale.<br />
In questo contributo consideriamo alcuni affioramenti isolati <strong>di</strong> carbonati <strong>di</strong><br />
pertinenza campano-lucana come blocchi esotici <strong>del</strong>imitati alla base da<br />
superfici <strong>di</strong> scollamento immergenti verso l’avampaese. Questi, a lungo<br />
interpretati come parte integrante <strong>di</strong> un sistema <strong>di</strong> sovrascorrimenti <strong>di</strong><br />
importanza regionale, potrebbero alternativamente essere considerati come il<br />
prodotto <strong>di</strong> un processo <strong>di</strong> <strong>di</strong>slocazione e <strong>di</strong> parziale riattivazione in regime<br />
<strong>di</strong>stensivo <strong>del</strong>le prec<strong>ed</strong>enti superfici <strong>di</strong> sovrascorrimento. Dal Pleistocene<br />
<strong>di</strong>ventano dominanti le faglie transtensive ad alto angolo, ra<strong>di</strong>cate in<br />
profon<strong>di</strong>tà, <strong>ed</strong> è attraverso la loro progressiva attività che si struttura e prende<br />
forma l’attuale deprerssione morfo-tettonica <strong>del</strong>l’alta Val d’Agri. Dal<br />
Pleistocene m<strong>ed</strong>io ad oggi prevale una tettonica <strong>di</strong>stensiva.<br />
L’esistenza <strong>di</strong> un fe<strong>ed</strong>back positivo tra i processi <strong>di</strong> sollevamento <strong>ed</strong><br />
erosione ha contribuito a determinare il progressivo approfon<strong>di</strong>mento dei<br />
principali livelli <strong>di</strong> scollamento <strong>del</strong>le faglie <strong>di</strong>rette recenti, che tendono a<br />
localizzarsi sotto il sovrascorrimento basale <strong>del</strong>le Unità Lagonegresi.<br />
Parole Chiave: Distensione post-orogenica, Appennino<br />
meri<strong>di</strong>onale, sollevamento, faglie <strong>di</strong>rette a basso angolo.<br />
Key words: Post-orogenic extension, southern Apennines,<br />
uplift, low-angle normal faults.<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università degli stu<strong>di</strong> <strong>di</strong> Siena, Via<br />
Latrina, 8 – 53100, Siena, Italy. bucci14@unisi.it<br />
(**) Dipartimento <strong>di</strong> Scienze Geologiche, Università degli stu<strong>di</strong> <strong>del</strong>la<br />
Basilicata, Campus <strong>di</strong> Macchia Romana – 85100, Potenza, Italy<br />
(***) IMAA-CNR, 85050, Tito Scalo (Potenza), Italy<br />
INTRODUCTION<br />
The Apennine orogen is characteris<strong>ed</strong> by pair<strong>ed</strong> belts of<br />
contraction and extension which migrat<strong>ed</strong> eastward during the<br />
Neogene-Quaternary times. Since the Miocene, the eastward<br />
propagation of the fold-and-thrust structures was accompani<strong>ed</strong><br />
and follow<strong>ed</strong> by the development of post-orogenic normal<br />
faults relat<strong>ed</strong> to the collapse of the chain.<br />
In southen Apennines, post-Messinian evolution is mark<strong>ed</strong><br />
by a drastic slowdown of slab migration due to collision with<br />
the Apulian swell (GUEGUEN et alii, 1998). From this time<br />
onward, <strong>di</strong>rect convergence has been accommodat<strong>ed</strong> mainly by<br />
sinistral transpression (CINQUE et alii, 1993; PIERI et alii,<br />
1997).<br />
In the inner portion of the orogen, the eastward migration of<br />
the post-orogenic extensional front, accommodat<strong>ed</strong> by Pliocene<br />
low-angle normal faults (FERRANTI et alii, 1996) and by early<br />
Pleistocene normal to left-oblique transtensional faults<br />
(HIPPOLYTE et alii, 1994), determin<strong>ed</strong> the progressive<br />
fragmentation of the fold-and-thrust architecture. SW-NE<strong>di</strong>rect<strong>ed</strong><br />
shortening in the frontal part of the southern<br />
Apennines ceas<strong>ed</strong> during the middle Pleistocene (PATACCA &<br />
SCANDONE, 2001). Afterwards, the deformation regime was<br />
dominat<strong>ed</strong> by NE-SW <strong>di</strong>rect<strong>ed</strong> extension mainly<br />
accommodat<strong>ed</strong> by NW-SE tren<strong>di</strong>ng high-angle normal faults.<br />
Pliocene to Quaternary extension was accompani<strong>ed</strong> by intense<br />
uplift (CINQUE et alii, 1993).<br />
Interm<strong>ed</strong>iate to low-angle extensional contacts have been<br />
observ<strong>ed</strong> mostly at the base of blocks made up of platform<br />
carbonates. It seems likely that these extensional faults were<br />
active at shallow crustal levels during the late Pliocene-early<br />
Pleistocene times while coeval deep-seat<strong>ed</strong> thrusting affect<strong>ed</strong><br />
the deepest structural levels, thus allowing efficient mechanical<br />
removal of hanging-wall material by footwall uplift coupl<strong>ed</strong><br />
with enhanc<strong>ed</strong> erosion (MAZZOLI et alii, 2006).<br />
Starting from the middle Peistocene, the regional uplift was<br />
particularly concentrat<strong>ed</strong> within the extension-dominat<strong>ed</strong>, axial<br />
portion of the belt (SCHIATTARELLA et alii, 2003). Thus the<br />
present-day landscape of the western and axial sectors of the<br />
chain results from the interplay between regional uplift and<br />
fault <strong>di</strong>splacement; this is morphologically reflect<strong>ed</strong> by the<br />
characteristic occurrence of mountain ranges separat<strong>ed</strong> from<br />
fault-bound<strong>ed</strong> basins fill<strong>ed</strong> by recent continental deposits (Fig.<br />
1).
Fig. 1 – High Agri Valley - Location map. Axial sector of the chain between<br />
dash<strong>ed</strong> lines is in<strong>di</strong>cat<strong>ed</strong>. <strong>Note</strong> it is mark<strong>ed</strong> by the occurrence of continental<br />
basins.<br />
GEOLOGICAL SETTING<br />
The Val d’ Agri basin (Fig. 1), locat<strong>ed</strong> in the axial part of the<br />
Campania-Lucania sector of the Southern Apennines, is a NWelongat<strong>ed</strong><br />
basin fill<strong>ed</strong> by Quaternary continental deposits which<br />
cover down-thrown pre-Quaternary rocks of the Apenninic<br />
chain. Pre-Quaternary rock assemblages which floor the Val<br />
d’Agri basin and surroun<strong>di</strong>ng reliefs, largely consist of<br />
Mesozoic-Cenozoic platform rocks pertaining to the Campania-<br />
Lucania domain, in places tectonically overlain by deep-sea<br />
clays and sandstones of the Liguride nappe. Carbonate rocks of<br />
the Campania-Lucania domain were thrust onto coeval pelagic<br />
rocks that were deposit<strong>ed</strong> within the adjacent, more external<br />
Lagonegro basin and on younger Miocene synorogenic rocks.<br />
The main thrust contact is usually mark<strong>ed</strong> by a pervasive<br />
cataclastic texture mostly develop<strong>ed</strong> at the expenses of brittle<br />
carbonate rocks of the hangingwall. In the thrust footwall, the<br />
Lagonegro units consist of Triassic-Miocene carbonates,<br />
siliceous marls and siliciclastics that were, in turn, intensely<br />
duplicat<strong>ed</strong> and fold<strong>ed</strong> (SCANDONE, 1972). The rocks belonging<br />
to the Lagonegro units are thrust over buri<strong>ed</strong> 6-7 km-thick<br />
Mesozoic-tertiary, shallow-water carbonates and overlying<br />
Pliocene siliciclastics, that represent the westerly continuation<br />
of the Apulia platform foreland and that were, in turn, affect<strong>ed</strong><br />
by contractional deformations during the Pliocene-Pleistocene<br />
time interval (CORRADO et alii, 2005).<br />
Pelagic rocks of the Lagonegro Basin mainly outcrop on the<br />
eastern side of the high Agri Valley and in sparse tectonic<br />
windows beneath the Monti <strong>del</strong>la Maddalena thrust sheet in its<br />
western <strong>ed</strong>ge. Conversly, limit<strong>ed</strong> outcrops of neritic carbonates<br />
THE HISTORY OF A POST-OROGENIC TROUGH<br />
45<br />
belonging to the Campania-Lucania Platform are found east of<br />
the Agri basin.<br />
Scatter<strong>ed</strong> outcrops of platform carbonates are present in the<br />
north-easthern <strong>ed</strong>ge of the Agri Valley, to the east of Monte<br />
Volturino: these are generally interpret<strong>ed</strong> as thrust remnants, or<br />
klippen, in the official cartography (CARTA GEOLOGICA<br />
D’ITALIA, 1969) as well as in more recent geological maps (e.g.<br />
LENTINI et alii 1991). In nearby areas, the basal contact of the<br />
klippen clearly crosscuts open folds in the footwall block<br />
(GUEGUEN et alii, 2007). We recently carri<strong>ed</strong> out an<br />
investigation of these exotic blocks in an attempt to unravel<br />
their origin and emplacement mechanisms.<br />
THE BASAL TECTONIC CONTACT OF THE<br />
CAMPANIA-LUCANIA PLATFORM<br />
Within the Apenninic Chain, the Late Cretaceous to Miocene<br />
terrigenous cover rocks were detach<strong>ed</strong> from the mesozoiccenozoic<br />
Lagonegro substratum along decoupling horizons and<br />
passively thrust onto the upper Pliocene clastics of the Bradanic<br />
For<strong>ed</strong>eep (LENTINI et alii, 2002). Similarly, the tectonic,<br />
structurally complex relationships between the Lagonegro<br />
Basin units and overlying Campania-Lucania Platform units<br />
observ<strong>ed</strong> in the M. Volturino area can result from the activation<br />
of foreland-<strong>di</strong>rect<strong>ed</strong>, extensional detachment faults. These<br />
would have locally reactivat<strong>ed</strong> pre-existing thrust surfaces. Our<br />
investigation was focuss<strong>ed</strong> between the frontal part of the thrust<br />
sheet and the easternmost klippen of platform carbonatic rocks.<br />
At the base of the main thrust of the Apenninic platform on<br />
the Lagonegro Units an important shear zone is develop<strong>ed</strong> in<br />
the Lagonegro deep-sea rocks. Interestingly, the base of the<br />
klippen generally lacks a well-develop<strong>ed</strong> shear zone, but is<br />
rather mark<strong>ed</strong> by cataclasites. Detail<strong>ed</strong> mapping of these exotic<br />
blocks reveal<strong>ed</strong> the presence of widespread normal faults<br />
connect<strong>ed</strong> to low-angle detachments activat<strong>ed</strong> along<br />
mechanically weak horizons at shallow structural levels,<br />
correspon<strong>di</strong>ng to the uppermost portions of the Lagonegro<br />
units.<br />
To the SW of Mt. Volturino deep-sea s<strong>ed</strong>imentary rocks of<br />
the uppermost Liguride units resting <strong>di</strong>rectly on the Lagonegro<br />
units, that occupy a much deeper position within the thrust pile,<br />
documents the presence of a low-angle extensional fault. This<br />
interpretation is broadly consistent with the history of late<br />
Pliocene exhumation propos<strong>ed</strong> for the Lagonegro Basin strata<br />
bas<strong>ed</strong> on by apatite fission track age data (CORRADO et alii,<br />
2005).<br />
PLEISTOCENE-TO-PRESENT EVOLUTION<br />
In ad<strong>di</strong>tion to the exhumation controll<strong>ed</strong> by low-angle<br />
normal faults, further uplift and tectonic unroofing was<br />
achiev<strong>ed</strong> by a more recent system of high-angle,<br />
normal/transtensional faults whose activation from Middle<br />
Pleistocene time onwards, l<strong>ed</strong> to the development of the Agri<br />
Valley basin. Basinward-<strong>di</strong>pping, high-angle faults are<br />
associat<strong>ed</strong> with mature fault-line scarps, and form a regular<br />
staircase profile sloping towards the valley depocentre<br />
(MASCHIO et alii, 2005). High-angle faults are mark<strong>ed</strong> by
46 F. BUCCI ET ALII<br />
cataclastic shear bands and by a polish<strong>ed</strong> and planar slip<br />
surfaces (e.g. see also FERRANTI et alii, 2007). Fault-kinematic<br />
data from the paleosoil document faulting consistent with a<br />
mean NW-SE <strong>di</strong>rection of extension active after 40ka (GIANO<br />
et alii, 2000). Careful observation and detail<strong>ed</strong> mapping reveal<br />
a complex tectonic history, characteriz<strong>ed</strong> by the superposition<br />
of geometrically and kinematically <strong>di</strong>stinct fault sets.<br />
CONCLUDING REMARKS<br />
The present morphologic-structural frame of the high Agri<br />
Valley results from the superposition of post-orogenic<br />
extensional tectonics onto a pre-existing contractional, thrustdominat<strong>ed</strong><br />
architechture.<br />
Low-angle extensional faults, active during the Pliocene-<br />
Pleistocene time interval, were responsible early tectonic<br />
exhumation and unroofing processes, mostly localis<strong>ed</strong> at the<br />
front of the Campania-Lucania Platform. Extension within the<br />
structurally uppermost units is document<strong>ed</strong> by the occurrence<br />
of highly <strong>di</strong>scontinuous slivers of platform rocks pertaining to<br />
the Campania-Lucania unit, bound<strong>ed</strong> by shallow tectonic<br />
contacts and sandwich<strong>ed</strong> between pelagic deposits pertaining to<br />
the Ligurian and Lagonegro domains, respectively. Kinematic<br />
complexities and significant <strong>di</strong>fferences in fault rocks suggest<br />
that original thrust contacts were truncat<strong>ed</strong> and/or partly<br />
reactivat<strong>ed</strong> as low-angle detachments. Low-angle normal faults<br />
were, in turn, truncat<strong>ed</strong> by more recent, high-angle normal fault<br />
sets that became the dominant structural features since<br />
Pleistocene time onwards, lea<strong>di</strong>ng to the development of the<br />
Agri Valley basin.<br />
The geometrical and kinematic relationships amongst faults<br />
of <strong>di</strong>fferent generations suggest a positive fe<strong>ed</strong>back between<br />
uplift and erosion during post-orogenic extension; it seems<br />
likely that progressive exhumation and unroofing were<br />
achiev<strong>ed</strong> by activation of sequentially deeper detachments<br />
locat<strong>ed</strong> below the sole thrust of the Lagonegro Units.<br />
REFERENCES<br />
CARTA GEOLOGICA D’ ITALIA, 1969. SERVIZIO GEOLOGICO<br />
D’ITALIA, Potenza 199, Lauria 210, S. Arcangelo 211, IP75,<br />
Roma.<br />
CINQUE A., PATACCA E., SCANDONE P. & TOZZI M., 1993.<br />
Quaternary kinematic evolution of the southern Apennines.<br />
Relationship between surface geological features and deep<br />
lithosferic structures. Annali <strong>di</strong> Geofisica, 36 (2), 249-260.<br />
CORRADO, S., ALDEGA, L., DI LEO, P., GIAMPAOLO, C.,<br />
INVERNIZZI, C., MAZZOLI, S., AND ZATTIN, M., 2005, Thermal<br />
maturità of the axial zone of the southern Apennines fold-andthrust-belt<br />
(Italy) from multiple organic and inorganic<br />
in<strong>di</strong>cators, Terra Nova, v. 17, p. 56-65.<br />
FERRANTI, L., MASCHIO L., BURRATO P., 2007. Fieldtrip guide<br />
to active tectonics stu<strong>di</strong>es in the high Agri Valley. Val d’Agri,<br />
15-17 October 2007.<br />
FERRANTI, L., OLDOW, J.S.&SACCHI, M., 1996. Pre-<br />
Quaternary orogen-parallel extension in the Southern<br />
Apennine belt, Italy, Tectonophysics, 260, 325–347.<br />
GIANO, S.I., MASCHIO, L., ALESSIO, M., FERRANTI, L.,<br />
IMPROTA, S. & SCHIATTARELLA, M., 2000. Ra<strong>di</strong>ocarbon dating<br />
of active faulting in the Agri high valley, Southern Italy, J.<br />
Geodyn., 29, 371–386.<br />
GUEGUEN, E., PROSSER, G. & ADURNO I., 2007, Relationships<br />
between Campano-Lucana Platform and Lagonegro Basin:<br />
new data and regional implication. Rend. Soc. Geol. It., 5,<br />
153-154.<br />
GUEGUEN, E., DOGLIONI, C. AND FERNANDEZ, M., 1998, On the<br />
post-25 Ma geodynamic evolution of the western<br />
M<strong>ed</strong>iterranean. Tectonophysics, 298, 259-269.<br />
HYPPOLITE, L.-C., ANGELIER, J., ROURE, F., AND CASERO, P.,<br />
1994, Piggyback basin development and thrust belt evolution:<br />
Structural and palaeostress analyses of Plio-Quaternary<br />
basins in the southern Apennines, Journal of Structural<br />
Geology, v. 16, p. 159-173.<br />
LENTINI, F., LAZZARI, S., CARBONE, S., CATALANO S. AND<br />
MONACO, C., 1991; Carta Geologica <strong>del</strong> Bacino <strong>del</strong> Fiume Agri,<br />
Regione Bailicata, Università <strong>di</strong> Catania, scale 1/50.000. Selca,<br />
Firenze.<br />
LENTINI, F., CARBONE, S., DI STEFANO, A. & GUARNIERI P.,<br />
2000. The role of foreland-<strong>di</strong>rect<strong>ed</strong> etensional faults in the<br />
frontal part of the Southern Apenninic Chain. 80 a Riunione<br />
Estiva <strong>del</strong>la Società Geologica Italiana, Trieste 6-8 Settembre<br />
2000, p. 310.<br />
MASCHIO L., FERRANTI L., AND BURRATO P., 2005. Active<br />
extension in Val d’ Agri area, Southern Apennines, Italy:<br />
Implications for the geometry of the seismogenic belt.<br />
Geophys. J. Int. 162 (2), 591-609.<br />
MAZZOLI, S., ALDEGA, L., CORRADO, S., INVERNIZZI, C. &<br />
ZATTIN, M., 2006, Pliocene-quaternary thrusting, syn-orogenic<br />
extension and tectonic exhumation in the Southern Apennines<br />
(Italy): Insight from the Monte Alpi area. Geological Society of<br />
America, Special Paper, 414, 55-77.<br />
PATACCA, E. AND SCANDONE P., 2001. Late thrust propagation<br />
and s<strong>ed</strong>imentary response in the thrust-belt–for<strong>ed</strong>eep system of<br />
the Southern Apennines (Pliocene-Pleistocene). In: G.B. VAI<br />
AND I.P. MARTINI (<strong>ed</strong>s.), Anatomy of an Orogen: the Apennines<br />
and Adjacent M<strong>ed</strong>iterranean Basins, Kluwer Academic<br />
Publishers, Great Britain, 401–440.<br />
SCANDONE, P., 1972. Stu<strong>di</strong> <strong>di</strong> Geologia Lucana: carta dei<br />
terreni <strong>del</strong>la serie calcareo-silicomarnosa e note illustrative.<br />
Bollettino <strong>del</strong>la Societa dei Naturalisti in Napoli, 81, 225-300.
SCHIATTARELLA, M., DI LEO, P., BENEDUCE, P. & GIANO, S.I.,<br />
2003. Quaternary uplift vs tectonic loa<strong>di</strong>ng: a case study from<br />
the Lucanian Apennine, southern Italy, Quatern. Int., 101–102,<br />
239–251.<br />
THE HISTORY OF A POST-OROGENIC TROUGH<br />
47
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 48-50, 2 ff.<br />
Geometric and Kinematic mo<strong>del</strong>ing of the thrust fronts in the<br />
Montello-Cansiglio area from geologic and geodetic data (Eastern<br />
Southalpine Chain, NE Italy).<br />
PIERFRANCESCO BURRATO (*), PAOLO MARCO DE MARTINI (*), MARIA ELIANA POLI (°) & ADRIANO ZANFERRARI (°)<br />
RIASSUNTO<br />
Mo<strong>del</strong>lizzazione geometrica a cinematica da dati geologici e geodetici<br />
<strong>del</strong>le strutture compressive nell’area Montello-Cansiglio (Catena<br />
Sudalpina Orientale, Italia nord-orientale).<br />
Questo lavoro è d<strong>ed</strong>icato allo stu<strong>di</strong>o <strong>del</strong>le geometrie e dei ratei <strong>di</strong><br />
deformazione <strong>di</strong> breve e m<strong>ed</strong>io termine <strong>del</strong>le strutture compressive attive<br />
facenti parte dei fronti esterni <strong>del</strong>la Catena Sudalpina, nel settore<br />
<strong>del</strong>l’anticlinale <strong>del</strong> Montello. Il metodo adottato utilizza informazioni derivate<br />
dall’analisi <strong>di</strong> una linea geodetica <strong>di</strong> primo or<strong>di</strong>ne <strong>del</strong>l’IGM, combinate con<br />
osservazioni geofisiche, geologiche e geomorfologiche <strong>di</strong> superficie e <strong>di</strong><br />
sottosuolo. La linea geodetica presa in esame mostra lungo alcuni suoi<br />
segmenti dei movimenti verticali relativi, positivi rispetto ai segmenti<br />
a<strong>di</strong>acenti (maggiori sollevamenti). Questi segnali geodetici, ottenuti dal<br />
confronto <strong>del</strong>le quote dei capisal<strong>di</strong> misurate durante due <strong>di</strong>stinte campagne<br />
separate da un intervallo <strong>di</strong> tempo <strong>di</strong> circa 50 anni, avvengono in<br />
corripondenza <strong>del</strong>l’attraversamento <strong>di</strong> faglie cieche e sono stati quin<strong>di</strong><br />
interpretati come dovuti all’attività <strong>di</strong> queste strutture sepolte. Per<br />
l’interpretazione, è stata costruita una sezione geologica che segue la traccia<br />
<strong>del</strong>la linea <strong>di</strong> livellazione, <strong>ed</strong> è stato quin<strong>di</strong> mo<strong>del</strong>izzato il segnale geodetico<br />
adottando un metodo <strong>di</strong>retto. Nel mo<strong>del</strong>lo, le geometrie <strong>di</strong> partenza <strong>del</strong>le faglie<br />
sono state prese dalla sezione geologica, e sono state poi mo<strong>di</strong>ficate per<br />
riprodurre il segnale geodetico. Una volta fissate le geometrie <strong>del</strong>le faglie, gli<br />
uplift rate sono stati convertiti in slip e shortening rate e comparati con: 1- i<br />
ratei <strong>di</strong> m<strong>ed</strong>io e lungo termine derivati dalle osservazioni geologiche e<br />
geomorfologiche per evidenziare eventuali cambiamenti nel tempo; e 2- con i<br />
tassi <strong>di</strong> convergenza GPS per stu<strong>di</strong>are la partizione <strong>del</strong>le deformazione tra i<br />
<strong>di</strong>versi fronti. Infine sono state usate relazioni analitiche <strong>ed</strong> empiriche per<br />
stimare la massima magnitudo e i tempi <strong>di</strong> ricorrenza dei potenziali futuri<br />
terremoti.<br />
Key words: Montello anticline, slip rates, Eastern Southalpine<br />
Chain, NE Italy.<br />
INTRODUCTION<br />
We present a study of the external thrust fronts of the<br />
Eastern Southalpine Chain (ESC) in the Montello-Cansiglio<br />
area (Fig. 1) using a combination of surface and subsurface<br />
geologic, morphotectonic and geodetic data. The aim of this<br />
_________________________<br />
(*) Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia, <strong>Sezione</strong> Sismologia e<br />
Tettonofisica, Roma<br />
(°) Dipartimento <strong>di</strong> <strong>Georisorse</strong> e Territorio, Università <strong>di</strong> U<strong>di</strong>ne<br />
work is to constrain the geometry of the active thrusts and to<br />
compute the correspon<strong>di</strong>ng rates of deformation. In ad<strong>di</strong>tion we<br />
present an evaluation of the seismic potential of the<br />
seismogenic sources estimating the maximum magnitude of the<br />
potential earthquakes associat<strong>ed</strong> to the in<strong>di</strong>vidual structures and<br />
their recurrence interval.<br />
The active tectonics of the study area is the result of the<br />
relative motion of Africa and its northern Adriatic promontory<br />
with respect to Europe (e.g. CASTELLARIN, 2004). The analysis<br />
of the GPS velocities pr<strong>ed</strong>icts a counter clockwise motion of<br />
the Adriatic block around a pole locat<strong>ed</strong> in the western Alps<br />
(e.g. SERPELLONI et alii, 2005). This motion produces<br />
increasing convergence from west to east, which is match<strong>ed</strong> by<br />
a similar increase of the seismic moment release. Accor<strong>di</strong>ng to<br />
this mo<strong>del</strong>, in the Veneto Region N–S to NNW–SSE<br />
convergence is inferr<strong>ed</strong> to be in the order of about 1,5 mm/a<br />
(e.g. D'AGOSTINO et alii, 2005; GRENERCZY et alii, 2005).<br />
The Neogene-Quaternary ESC is part of the S-verging<br />
Fig. 1 – Regional tectonic sketch of northeastern Italy and western Slovenia.<br />
The black dots highlight the trace of the leveling line us<strong>ed</strong> in this work (from<br />
BURRATO et alii, 2008, mo<strong>di</strong>fi<strong>ed</strong>).<br />
backthrust chain of the Alps. West of the Tagliamento River,<br />
the ESC structures follow a WSW-ENE trend, and the activity<br />
of the main thrust fronts stea<strong>di</strong>ly migrat<strong>ed</strong> southwards<br />
(DOGLIONI, 1992; CASTELLARIN & CANTELLI, 2000). The more<br />
external contractional structures are the Bassano–<br />
Valdobbiadene Thrust (BV Thrust in Fig. 1), associat<strong>ed</strong> with<br />
the uplift of the mountain front (DOGLIONI, 1992), and in a
SHORT AND LONG-TERM DEFORMATION RATES OF THE MONTELLO ANTICLINE<br />
more external position, a system of growing mainly blind<br />
thrusts running at the boundary between the Prealpine relief and<br />
the plain areas (Fig. 1). This system is compos<strong>ed</strong> of several<br />
fault segments identifi<strong>ed</strong> by GALADINI et alii (2005), which<br />
produce the uplift of Neogene–Quaternary deposits and along<br />
its eastern portion border the mountain front of the Carnian<br />
Prealps. The Montello-Conegliano Thrust (MC Thrust in Fig.<br />
1) is the lea<strong>di</strong>ng thrust of the ESC in the study sector. Its<br />
activity is express<strong>ed</strong> by the uplift and warping of Upper<br />
Miocene and Upper Pliocene Pleistocene deposits, forming the<br />
expos<strong>ed</strong> Montello anticline (FERRARESE et alii, 1998;<br />
BENEDETTI et alii, 2000; FANTONI et alii, 2001; FANTONI et<br />
Fig. 2 – Map of historical and instrumental seismicity from the CPTI04<br />
Catalogue (CPTI Working Group, 2004) and the 1977–2001 OGS Annual<br />
Bulletin (available from: http://www.crs.inogs.it/). (from BURRATO et alii,<br />
2008, mo<strong>di</strong>fi<strong>ed</strong>).<br />
alii, 2002) one of the most impressive folds emerging from the<br />
Venetian and Friulian Plain. Surface and subsurface geological,<br />
geophysical and structural data show that the MC Thrust is a 35<br />
km long fault that <strong>di</strong>es out to the east where it is overridden by<br />
the neighbouring Cansiglio Thrust (CA Thrust; GALADINI et<br />
alii, 2005). To the west the MC Thrust has a right-stepping enechelon<br />
relationship with the adjoining Bassano-Cornuda<br />
Thrust (BC Thrust in Fig. 1). To the north of the MC Thrust the<br />
eastern portion of the BV Thrust is found at the base of the<br />
mountain front (Fig. 1). The area compris<strong>ed</strong> between the two<br />
thrust fronts is characteriz<strong>ed</strong> by the presence of a triangle zone<br />
form<strong>ed</strong> by the BV Thrust and by a back-thrust splaying-off the<br />
MC Thrust.<br />
The recent activity of the MC Thrust is testifi<strong>ed</strong> by several<br />
Middle and Upper Pleistocene warp<strong>ed</strong> terraces of the Piave<br />
River paleo-course flanking the western termination of the fold<br />
(e.g. BENEDETTI et alii, 2000 and references threin), and by the<br />
eastward deflection of the Piave River around the growing<br />
anticline. Conversely, evidence of recent activity of the BV<br />
Thrust are more sparse and not conclusive (GALADINI et alii,<br />
2001). To the east, Late Quaternary activity of the CA Thrust is<br />
shown by morphotectonic evidence and fol<strong>di</strong>ng and faulting of<br />
LGM slope deposits (GALADINI et alii, 2005).<br />
Ongoing seismic activity of the ESC results in several<br />
destructive M 6+ earthquakes that have been positively<br />
associat<strong>ed</strong> to in<strong>di</strong>vidual segments of the external thrust fronts<br />
(BASILI et alii, 2008; BURRATO et alii, 2008; DISS WORKING<br />
GROUP, 2007; GALADINI et alii, 2005). In ad<strong>di</strong>tion, shallower<br />
M 5–6 events generat<strong>ed</strong> by smaller secondary structures pose a<br />
not negligible hazard to the region at a more local scale. In this<br />
contest, the almost continuous seismogenic belt that follows the<br />
most external ESC thrust front from the Tagliamento River<br />
(epicentral area of the Mmax 6.3 1976 seismic sequence)<br />
westward to Bassano (epicentral area of the Mw 6.6 1695<br />
earthquake) is interrupt<strong>ed</strong> in the area of the Montello anticline<br />
(CPTI WORKING GROUP, 2004), with no earthquakes during the<br />
last 700 years (consider<strong>ed</strong> the interval of completness of the<br />
catalogue for events of M 6+) (Fig. 2).<br />
METHOD<br />
49<br />
Our analysis focaliz<strong>ed</strong> along the trace of the high precision<br />
IGM geodetic levelling line “Mestre-Polpet” that runs in a N-S<br />
<strong>di</strong>rection from the plain near Venice to the inner sector of the<br />
Venetian Prealps near Belluno, and crosses both the MC-CA<br />
and BV thrust fronts (Fig. 1). We analyz<strong>ed</strong> relative elevation<br />
changes measur<strong>ed</strong> by the geodetic line in a 50 years long time<br />
interval, referring them to the first nodal benchmark of the line<br />
consider<strong>ed</strong> as having a stable elevation (see D’ANASTASIO et<br />
alii, 2006 for an explanation of the method).<br />
This study highlight<strong>ed</strong> the occurrence of some segments of<br />
the line characteriz<strong>ed</strong> by positive vertical relative motions with<br />
respect to nearby segments. These vertical movements,<br />
occurring at the crossing of the ESC thrust fronts, are local<br />
signals with a wavelength up to several kilometers superpos<strong>ed</strong><br />
to a regional uplift trend. One of these sectors was already<br />
stu<strong>di</strong><strong>ed</strong> and link<strong>ed</strong> to the active growing of the Montello<br />
anticline (DE MARTINI et alii, 1998). In this study, we adopt<strong>ed</strong><br />
a forward mo<strong>del</strong>ing proc<strong>ed</strong>ure for the local geodetic uplift<br />
signals to estimate the thrusts recent activity.<br />
To reconstruct the starting geometry of our mo<strong>del</strong> we us<strong>ed</strong><br />
available seismic exploration data combin<strong>ed</strong> with surface<br />
geologic and morphotectonic observations, and produc<strong>ed</strong> a<br />
detail<strong>ed</strong> geologic section along the trace of the leveling line.<br />
The geometric parameters of the thrust deriv<strong>ed</strong> from this<br />
section (i.e. strike, angle of <strong>di</strong>p, depth) were us<strong>ed</strong> as input<br />
parameters for the mo<strong>del</strong>ing of their expect<strong>ed</strong> vertical surface<br />
<strong>di</strong>slocation. The results of the mo<strong>del</strong>ing were compar<strong>ed</strong> to the<br />
registration of the geodetic line to derive range values of the<br />
principal geometrical fault parameters.<br />
Once the geometry of the active thrust faults were known,<br />
we convert<strong>ed</strong> the short-term uplift rates obtain<strong>ed</strong> from the<br />
analysis of the geodetic line into slip and shortening rates.<br />
Then, we compar<strong>ed</strong> them with: 1- the long- and mid-term<br />
geologic/geomorphic slip rates to check for variations through<br />
time; and 2- to the GPS shortening budget in order to examin<br />
the partitioning of the deformation across the <strong>di</strong>fferent ESC<br />
thrust fronts.<br />
Finally, since the area we stu<strong>di</strong><strong>ed</strong> is a seismic gap (Fig. 2)<br />
we also us<strong>ed</strong> analytical (e.g. HANKS & KANAMORI, 1979) and<br />
empirical (e.g. WELLS & COPPERSMITH, 1994) relationships to<br />
figure out maximum magnitude and recurrence interval of<br />
possible future earthquakes.
50 P. BURRATO ET ALII<br />
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The Pliocene-Quaternary salient structures of the Central and<br />
Southern Apennine chain inherit<strong>ed</strong> from pre-thrusting normal<br />
faults<br />
FERNANDO CALAMITA(*), PAOLO ESESTIME(*), PAOLO PACE(*), WERTER PALTRINIERI (*) ,<br />
RIASSUNTO<br />
Le strutture arcuate plio-quaternarie <strong>del</strong>la catena <strong>del</strong>l’Appennino centromeri<strong>di</strong>onale<br />
er<strong>ed</strong>itate dalle faglie normali pre-sovrascorrimento<br />
Sulla base <strong>di</strong> dati geologico-strutturali e <strong>del</strong>l’interpretazione <strong>di</strong> profili<br />
sismici a riflessione viene proposto un mo<strong>del</strong>lo <strong>di</strong> tettonica d’inversione per le<br />
strutture arcuate <strong>del</strong>l’Appennino centrale (Monti Sibillini-Olevano-Antrodoco,<br />
Montagna dei Fiori-Gran Sasso e Maiella-Sangro-Volturno) e <strong>del</strong>la catena<br />
apula sepolta <strong>del</strong>l’Appennino meri<strong>di</strong>onale (Struttura <strong>di</strong> Setteporte) che<br />
considera i sovrascorrimenti ad andamento N-S prodotti dalla completa<br />
inversione in transpressione <strong>del</strong>le faglie normali pre-thrusting, con sviluppo <strong>di</strong><br />
anticlinali d’inversione debolmente asimmetriche. Mentre per i<br />
sovrascorrimenti ad andamento NW-SE con cinematica inversa, il mo<strong>del</strong>lo<br />
propone l’inversione dei tratti a basso angolo <strong>del</strong>le faglie normali presovrascorrimenti<br />
nella crosta m<strong>ed</strong>io-inferiore e una loro traiettoria <strong>di</strong> short-cut<br />
attraverso le faglie preesistenti nel settore superiore <strong>del</strong>la crosta, con sviluppo<br />
<strong>di</strong> anticlinali <strong>di</strong> short-cut, marcatamente asimmetriche.<br />
In tale contesto, il pattern <strong>del</strong>le faglie normali pre-thrusting è er<strong>ed</strong>itato<br />
nelle strutture arcuate <strong>del</strong>la catena <strong>del</strong>l’Appennino centrale e meri<strong>di</strong>onale, ove i<br />
back-limb <strong>del</strong>le anticlinali <strong>di</strong> short-cut sono poco sviluppati e caratterizzate da<br />
faglie normali pre-thrusting che terminano in prossimità dei settori ad<br />
andamento N-S dei salienti stessi <strong>del</strong>la catena, generalmente riattivate durante<br />
l’estensione quaternaria.<br />
Key words: Salient structures, structural inheritance, Adria<br />
paleomargin, Central Apennines, Southern Apennines.<br />
INTRODUCTION<br />
VITTORIO SCISCIANI(*) & ENRICO TAVARNELLI(°)<br />
The central Apennine Chain has been describ<strong>ed</strong> as a foldand-thrusts<br />
belt dominat<strong>ed</strong> by a thin-skinn<strong>ed</strong> tectonic style, that<br />
implies high estimates of orogenic shortening (BALLY et alii,<br />
1988; GHISETTI & VEZZANI, 1990) or accor<strong>di</strong>ng to a thickskinn<strong>ed</strong><br />
tectonic style, that implies a more conservative<br />
estimate of contraction (CALAMITA et alii, 2004; BOCCALETTI<br />
et alii, 2005; FINETTI et alii, 2005a-b). CALAMITA & DEIANA<br />
(1988), TAVARNELLI (1996), CALAMITA et alii (2002),<br />
SCISCIANI et alii (2000; 2002) have document<strong>ed</strong> the<br />
inheritance of the Mesozoic paleomargin on the Neogene-<br />
Quaternary evolution of the Central Apennine Chain,<br />
emphasizing buttressing geometries and short-cut trajectories of<br />
the thrust planes through the pre- and syn-orogenic normal<br />
faults. A total, reverse-reactivation of the pre-existing normal<br />
faults has been propos<strong>ed</strong> by DECANDIA (1982), ARGNANI &<br />
GAMBERI (1996), ALBERTI (2000), TOZER et alii (2002), and<br />
BUTLER et alii (2004).<br />
The aim of this paper is to reconstruct a crustal scale 3D<br />
mo<strong>del</strong> of tectonic inversion of the pre-thrusting normal faults<br />
(i.e., the Mesozoic pre-orogenic and the Messinian-Pliocene<br />
syn-orogenic normal faults) on the Pliocene-Quaternary<br />
development of salient structures in the outer Central<br />
Apennines (i.e., the Sibillini Mts-Olevano-Atrodoco, Montagna<br />
dei Fiori-Gran Sasso and Maiella-Sangro-Volturno thrusts) and<br />
in the buri<strong>ed</strong> chain of the Southern Apennines (e.g., the<br />
Setteporte structure) bas<strong>ed</strong> on surface geological and structural<br />
data, and on seismic interpretation of industrial and public deep<br />
reflection profiles (Fig. 1).<br />
DISCUSSION<br />
Salient geometries of the thrust fronts are a common feature<br />
in the Central and Southern Apennines with arcuate shap<strong>ed</strong><br />
thrusts at <strong>di</strong>fferent scales. The Sibillini Mts, Gran Sasso,<br />
Montagna dei Fiori and Maiella thrusts and relat<strong>ed</strong> anticlines<br />
trend NW-SE to NNW-SSE and are bound<strong>ed</strong> by WSW to SW<br />
<strong>di</strong>pping normal faults in their back-limbs. These normal faults<br />
develop<strong>ed</strong> before the Pliocene-Pleistocene contractional<br />
deformation and were active during Mesozoic (pre-orogenic)<br />
and Miocene-early Pliocene (syn-orogenic) times. The<br />
relationships between pre-existing normal faults and the<br />
subsequent compressive structures suggest that the thrust planes<br />
truncat<strong>ed</strong> with a short-cut trajectory the steeper segments of the<br />
older <strong>di</strong>scontinuities.<br />
The Maiella anticline connects the Central Apennine fold<br />
and thrust system to the Apulian Chain buri<strong>ed</strong> below the<br />
allochthonous units of the Southern Apennines. A pre-thrusting<br />
normal fault, Messinian-early Pliocene in age, is locat<strong>ed</strong> in the<br />
western limb of the fold. Field and subsurface data collect<strong>ed</strong><br />
along the forelimb of the anticline reveal pre-existing normal<br />
faults that have been rotat<strong>ed</strong> and truncat<strong>ed</strong> by a buri<strong>ed</strong> thrust<br />
following a short-cut trajectory (Fig. 2a1). Imm<strong>ed</strong>iately to the<br />
east of the Maiella fold, the buri<strong>ed</strong> Casoli-Bomba structure<br />
shows similar relationships between pre-existing faults and<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università “G- d’Annunzio” <strong>di</strong><br />
Chieti-Pescara, Campus universitario Madonna <strong>del</strong>le Piane, Via dei Vestini<br />
n° 30, 68013 Chieti Scalo (CH), Italy; e-mail: calamita@unich.it<br />
(°)Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università degli Stu<strong>di</strong> <strong>di</strong> Siena, via<br />
Laterina n°5, 56100 Siena, Italy.
52 F. CALAMITA ET ALII<br />
Fig. 1 – Structural map of the Central Apennines; boxes in<strong>di</strong>cate the study area. SVL.: Sangro-Volurno Line; OAL: Olevano-Antrodoco Line.<br />
Fig. 2 – Examples of fault-thrust planes interaction from the Maiella anticline (cross section - a), the Gran Sasso<br />
area (photo of outcrop– a1), the Casoli-Bomba anticline (line drawing of seismic reflection profile - b), the<br />
Setteporte anticline (line drawing of seismic reflection profile - c) and the Sabina fault (cross section and relative<br />
restor<strong>ed</strong> template – c1). 1: Apulian carbonate platform Unit (Miocene-Triassic); 2: Siliciclastic deposits<br />
(Messinian-early Pliocene; 3: Allochthonous Molise-Sannio Units and overlaying thrust-top basins (Cretaceous-<br />
Pliocene).
THE PLIOCENE-QUATERNARY SALIENT STRUCTURES OF THE CENTRAL AND SOUTHERN APENNINE<br />
compressive structures with a prominent west-<strong>di</strong>pping synorogenic<br />
normal fault in the back-limb that was later rotat<strong>ed</strong><br />
assuming the configuration of high-angle reverse fault and was<br />
partially complicat<strong>ed</strong> by minor back-thrusts (Fig. 2b1).<br />
Seismic line interpretation allow<strong>ed</strong> us to reconstruct the<br />
three-<strong>di</strong>mensional pattern of the buri<strong>ed</strong> Apulian thrusts (e.g.,<br />
the Setteporte structure), tren<strong>di</strong>ng N-S, NNW-SSE and E-W,<br />
and parallel to the normal faults of the foreland area, which are<br />
relat<strong>ed</strong> to the Pliocene-Quaternary flexure. Detail<strong>ed</strong><br />
reconstructions show main N-S/NNE-SSW tren<strong>di</strong>ng thrusts,<br />
merging into NW-SE/E-W tren<strong>di</strong>ng minor thrusts and backthrusts,<br />
characteriz<strong>ed</strong> by push-up geometry, typically referable<br />
to a transpressive deformation (ESESTIME et alii 2006) and/or<br />
to the positive reactivation of normal faults (Fig. 2c).<br />
Several analogies with the buri<strong>ed</strong> Setteporte structure has<br />
been observ<strong>ed</strong> in the field along the Sabini Mts where structural<br />
data reveal a principal west-<strong>di</strong>pping thrust fault and relat<strong>ed</strong> fold<br />
orient<strong>ed</strong> N-S to NNE-SSW, with a gently asymmetric profile,<br />
Fig. 3 – Crustal inversion tectonic mo<strong>del</strong> showing the control of pre- and<br />
syn-orogenic normal faults (A) on the salient geometry of the Central-<br />
Southern Apennines (B). In the salient structure (B), the N-S tren<strong>di</strong>ng<br />
thrusts represent the full inversion of the pre-thrusting normal faults<br />
accor<strong>di</strong>ng to a transpressive deformation (section c), whereas the NW-SE<br />
tren<strong>di</strong>ng thrusts invert<strong>ed</strong> only the low-angle portion of pre-thrusting normal<br />
faults in the middle-lower crust and <strong>di</strong>splac<strong>ed</strong> by short-cut the normal<br />
faults in the upper crust (sections a and b). 1) Crust; 2) s<strong>ed</strong>imentary<br />
succession (?)Permian/Triassic-Miocene.<br />
and near-parallel high-angle transpressive or strike-slip faults in<br />
the back-limb (Fig. 2c2). The final geometry of the Sabina Mts<br />
structure and stratigraphic field data suggest that the fold has<br />
been originat<strong>ed</strong> from positive inversion, with complete<br />
reactivation of a Mesozoic normal fault.<br />
CONCLUSIONS<br />
53<br />
In the frontal sector of the Central-Southern Apennines,<br />
geological data integrat<strong>ed</strong> with seismic line interpretation<br />
allow<strong>ed</strong> us to propose a crustal inversion tectonics mo<strong>del</strong> of<br />
pre-thrusting normal faults (Mesozoic pre-orogenic and<br />
Messinian-Pliocene syn-orogenic faults) on Pliocene-<br />
Quaternary salient thrusts development (Sibillini Mts-Olevano-<br />
Atrodoco, Sabini Mts, Montagna dei Fiori-Gran Sasso and<br />
Maiella-Sangro-Volturno in the Central Apennines and of the<br />
Setteporte in the buri<strong>ed</strong> Apulian chain of the Southern<br />
Apennines).<br />
In the NW-SE/WNW-ESE tren<strong>di</strong>ng sector of salient structures<br />
of the chain, the thrusts reverse-reactivat<strong>ed</strong> the low angle<br />
portion of pre-existing normal faults in the middle-lower crust,<br />
where these structures are probably replac<strong>ed</strong> by shear zones.<br />
The upper, steeper parts of the pre-existing normal faults were<br />
instead truncat<strong>ed</strong> and passively carri<strong>ed</strong> piggy-back by thrusts<br />
propagating with short-cut trajectories, originating complex<br />
composite thrust-relat<strong>ed</strong> anticlines (“short-cut anticlines” -<br />
Figs. 2a, a1, b, and Figs. 3a-b). By contrast, the N-S tren<strong>di</strong>ng<br />
thrusts were produc<strong>ed</strong> by complex reverse reactivation of prethrusting<br />
normal faults in a context of a transpressive<br />
deformation (“full inversion anticlines” - Figs. 2c, c1 and Fig.<br />
3c). In this framework within the salient, the pre-thrusting<br />
normal faults, parallel to the short-cut anticlines, terminate<br />
toward the SE near the N-S tren<strong>di</strong>ng segments of the thrust<br />
planes as illustrat<strong>ed</strong> in the 3D crustal inversion tectonic mo<strong>del</strong><br />
of Figure 4.<br />
During the Quaternary, the extensional tectonics, affecting the<br />
Fig. 4 – Crustal 3D inversion tectonic mo<strong>del</strong> showing the control of prethrusting<br />
normal fault on the salient geometries of the Central-Southern<br />
Apennines. In “a” the pre-existing normal fault terminates abruptly along the<br />
N-S tren<strong>di</strong>ng thrust, whereas in “b” the normal fault show a tip in the<br />
hangingwall block of the thrust.
54 F. CALAMITA ET ALII<br />
axial zone of the Central Apennines, reactivat<strong>ed</strong> the NW-<br />
SE/WNW-SSE pre-thrusting normal faults as document<strong>ed</strong> by<br />
geological (e.g., the development of intra-mountain tectonic<br />
depressions) and seismological data.<br />
REFERENCES<br />
ALBERTI M. (2000) - Along-strike variations of thrusts in the<br />
Spoleto-Valnerina area: genesis and influence on the<br />
evolution of the Umbria-Marche Apennines. Boll. Soc.<br />
Geol. It., 119, 655-665.<br />
BALLY, A.W., BURBI, L., COOPER C., GHELARDONI, R. (1988) -<br />
Balanc<strong>ed</strong> sections and seismic reflection profiles across the<br />
Central Apennines. Mem. Soc. Geol. It., 35, 257-310.<br />
ARGANANI A. & GAMBERI F. (1996) – Stili strutturali al fronte<br />
<strong>del</strong>la Catena Appenninica. Stud. Geol. Cam., Volume<br />
Speciale 1995/1, 19-28.<br />
BOCCALETTI M., CALAMITA F. & VIANDANTE M.G. (2005) – La<br />
Neo-Catena litosferica appenninica nata a partire dal<br />
Pliocene inferiore come espressione <strong>del</strong>la convergenza<br />
Africa-Europa. Boll. Soc. Geol. It, 124, 87-105.<br />
BUTLER R.W.H., MAZZOLI S., CORRADO S., DE DONATIS M., DI<br />
BUCCI D., GAMBINI, R., NASO G., NICOLAI C., SCROCCA D.,<br />
SHINER P. & ZUCCONI V. (2004) – Applying thick-skinn<strong>ed</strong><br />
tectonic mo<strong>del</strong> sto the Apennines thrust belt of Italy –<br />
Limitations and implications. In K.R. McClay, Ed., Thrust<br />
tectonics and hydrocarbon systems: AAPG Memoir 82,<br />
647-667.<br />
CALAMITA F. & DEIANA G. (1988) - The arcuate shape of the<br />
Umbria-Marche-Sabina Apennines (Central Italy).<br />
Tectonophysics, 146, 139-147.<br />
CALAMITA F., SCISCIANI V., MONTEFALCONE R., PALTRINIERI<br />
W. & PIZZI A. (2002) – L’er<strong>ed</strong>itarietà <strong>del</strong> paleomergine <strong>di</strong><br />
Adria nella geometria <strong>del</strong> sistema orogenico centroappenninico:<br />
l’area abruzzese esterna. Mem. Soc. Geol. It.,<br />
57, 355-368.<br />
CALAMITA F., VIANDANTE M.G. & HEGARTY K. (2004) -<br />
Pliocene-Quaternary burial-exhumation paths in the<br />
Central Apennines (Italy): implications for the definition of<br />
the deep structure of the belt. - Boll. Soc. Geol. It., 123,<br />
503-512.<br />
DECANDIA F.A. (1982) – Geologia dei Monti <strong>di</strong> Spoleto (Prov.<br />
Di Perugia). Boll. Soc. Geol. It., 101, 291-315.<br />
ESESTIME P., D’ARCANGELO S., PALTRINIERI W. & CALAMITA<br />
F. (2006) – Strutture traspressive <strong>del</strong>la Catena Apula<br />
Sepolta (Appennino meri<strong>di</strong>onale, settore campano<br />
molisano). Rend. Soc. Geol. It., 2, 135-137.<br />
FINETTI I.R., CALAMITA F., CRESCENTI U., DEL BEN A., FORLIN<br />
E., PIPAN M., RUSCIADELLI G. AND SCISCIANI V. (2005A) –<br />
Crustal Geological Section across Central Italy from the<br />
Corsica Basin to the Adriatic Sea bas<strong>ed</strong> on Geological and<br />
CROP Seismic Data. In Finetti Eds, CROP PROJECT:<br />
Deep Seismic Exploration of the Central M<strong>ed</strong>iterranean and<br />
Italy Elsevier, Chapter 9, 159-197.<br />
FINETTI I.R., LENTINI F., CARBONE S., DEL BEN A., DI STEFANO<br />
A., GUARNIREI P., PIPAN M. & PRIZZON A. (2005B) –<br />
Crustal Tectono-Satratigraphy and Geodynamics of the<br />
Southern Apennines from CROP and other Integrat<strong>ed</strong><br />
Geophysical-Geological Data. In Finetti Eds, CROP<br />
PROJECT: Deep Seismic Exploration of the Central<br />
M<strong>ed</strong>iterranean and Italy Elsevier, Chapter 12, 225-263.<br />
GHISETTI F. & VEZZANI L. (1990) – Stili strutturali nei sistemi<br />
<strong>di</strong> sovrascorrimento <strong>del</strong>la catena <strong>del</strong> Gran Sasso<br />
(Appennino Centrale). Stu<strong>di</strong> Geol. Camerti, Volume<br />
Speciale, 35-70.<br />
SCISCIANI V., CALAMITA F., BIGI S., DE GIROLAMO C. &<br />
PALTRINIERI W. (2000) – The influence of syn-orogenic<br />
normal faults on Pliocene thrust system development: the<br />
Maiella structure (Central Apennines,Italy). Mem. Soc.<br />
Geol. It., 55, 193-204.<br />
SCISCIANI V., TAVARNELLI E., CALAMITA F. (2002). The<br />
interaction of extensional and contractional deformations<br />
in the outer zones of the Central Apennines, Italy. Journal<br />
of Structural Geology, 24, 1647-1658.<br />
TAVARNELLI E. (1996) – The effects of pre-existing normal<br />
faults on thrust ramp development: an example from the<br />
Northern Apennines, Italy. Geologische Rundschau, 85,<br />
363-371.<br />
TOZER, R. S. J., BUTLER, R.W. H. & CORRADO, S. (2002).<br />
Comparing thin- and thick-skinn<strong>ed</strong> thrust tectonic mo<strong>del</strong>s<br />
of the Central Apennines, Italy. IN: Bertotti, G. Schulmann,<br />
K., Cloetingh S., A., P.-L., Continental collision and the<br />
tectono-s<strong>ed</strong>imentary evolution of forelands. Stephan<br />
Mueller Special Publication Series 1, 181-194.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 55-58, 5 ff.<br />
La complessa geologia 3D <strong>del</strong>l'area epicentrale <strong>del</strong> terremoto <strong>di</strong> San<br />
Giuliano, basata su rilevamento strutturale e indagini<br />
gravimetriche.<br />
RICCARDO CAPUTO (*), PETER KLIN (°), LAURA MARELLO (°). RINALDO NICOLICH (^), FRANCESCO PALMIERI (°),<br />
& ENRICO PRIOLO (°)<br />
ABSTRACT<br />
The complex 3D geology of the 2002 San Giuliano epicentral area bas<strong>ed</strong><br />
on structural mapping and gravimetric survey<br />
The epicentral area of the 2002 San Giuliano (Molise, Italy) earthquake<br />
has been largely investigat<strong>ed</strong> for better understan<strong>di</strong>ng the important site effects<br />
observ<strong>ed</strong>. Among the several approaches that have been appli<strong>ed</strong>, in this short<br />
note we present the results of a detail<strong>ed</strong> geological-structural mapping and of a<br />
gravimetric survey. The former methodological approach allow<strong>ed</strong> to<br />
reconstruct the superficial and shallow geology of the area documenting a<br />
polyphas<strong>ed</strong> tectonics characteris<strong>ed</strong> by several low-angle, NW-SE tren<strong>di</strong>ng,<br />
NE-vergent thrust units cut by NNW-SSE tren<strong>di</strong>ng both negative and positive<br />
flower structures. On the other hand, the geophysical approach allow<strong>ed</strong> to<br />
confirm the superficial geological inferences and to constrain at greater depth<br />
the 3D geology, both in term of s<strong>ed</strong>imentary and tectonic <strong>di</strong>stribution. The<br />
results of this integrat<strong>ed</strong> analysis were then us<strong>ed</strong> for numerical mo<strong>del</strong>ling<br />
simulating the ground accelaration within the epicentral area.<br />
Key words: structural geology, gravimetric survey, epicentral<br />
area, Central Apennines<br />
INTRODUZIONE<br />
Nell'ambito dei progetti <strong>di</strong> interesse strategico per il<br />
Dipartimento <strong>del</strong>la Protezione Civile (2005-2006), l'area<br />
epicentrale <strong>del</strong> terremoto <strong>di</strong> San Giuliano <strong>di</strong> Puglia (31 ottobre<br />
2002) è stata oggetto <strong>di</strong> numerose ricerche scientifiche. In<br />
questa breve nota, sono descritti e <strong>di</strong>scussi i risultati ottenuti<br />
m<strong>ed</strong>iante due approcci metodologici, <strong>di</strong>stinti ma fortemente<br />
complementari, l'uno <strong>di</strong> carattere geologico, l'altro geofisico. In<br />
particolare, è stato effettuato un rilevamento geologico,<br />
strutturale e geomorfologico <strong>di</strong> dettaglio principalmente<br />
finalizzato alla ricostruzione <strong>del</strong>le geometrie <strong>del</strong>le unità<br />
stratigrafiche principali e <strong>del</strong>le strutture tettoniche maggiori che<br />
interessano questo settore <strong>del</strong>l'Appennino meri<strong>di</strong>onale. Nella<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Ferrara, via Saragat<br />
1, 44100 Ferrara (rcaputo@unife.it)<br />
(°) Istituto Nazionale <strong>di</strong> Oceanografia e Geofisica Sperimentale, Borgo<br />
Grotta Gigante 42/C, 34010 Sgonico (TS).<br />
() Geological Survey of Norway, Trondheim, Norway.<br />
(^) Dipartimento <strong>di</strong> Ingegneria Civile <strong>ed</strong> Ambientale, Università <strong>di</strong> Trieste,<br />
P.le Europa 1, 34127 Trieste.<br />
Lavoro eseguito bell’ambito <strong>del</strong> progetto DPC-INGV 2006-07, con il<br />
contributo finanziario <strong>del</strong> DPC, INOGS e Università <strong>del</strong>la Basilicata.<br />
stessa area, è stata effettuata una campagna <strong>di</strong> misure<br />
gravimetriche con lo scopo <strong>di</strong> d<strong>ed</strong>urre, dalle anomalie <strong>del</strong><br />
campo gravitazionale, la <strong>di</strong>stribuzione <strong>del</strong>le densità nel<br />
sottosuolo, permettendo, così, <strong>di</strong> meglio definire la geometria,<br />
lo spessore, l’assetto tettonico, etc, degli elementi geologici che<br />
caratterizzano questo sito.<br />
Il confronto incrociato dei risultati ottenuti con i due<br />
approcci in<strong>di</strong>pendenti ha permesso <strong>di</strong> vincolare fortemente la<br />
geologia <strong>del</strong> sottosuolo nell'area epicentrale e <strong>di</strong> ricostruire un<br />
mo<strong>del</strong>lo geologico 3D. Nell'ambito degli stessi progetti DPC,<br />
tale mo<strong>del</strong>lo è stato successivamente utilizzato per le<br />
simulazioni numeriche finalizzata a riprodurre (con successo)<br />
gli effetti <strong>di</strong> sito osservati durante il terremoto.<br />
RISULTATI DELLE INDAGINI GEOLOGICHE<br />
Da un punto <strong>di</strong> vista strutturale, l’area è interessata da una<br />
serie <strong>di</strong> falde <strong>di</strong> ricoprimento appenniniche caratterizzate da un<br />
sistema <strong>di</strong> sovrascorrimenti con andamento m<strong>ed</strong>io circa NO-SE<br />
e vergenza nordorientale. Profili <strong>di</strong> sismica a riflessione e <strong>di</strong><br />
pozzi per l'esplorazione petrolifera hanno permesso <strong>di</strong><br />
vincolare lo spessore <strong>del</strong>le falde <strong>di</strong> ricoprimento. Nell'area <strong>di</strong><br />
indagine, che rappresenta il settore esterno <strong>del</strong> cuneo orogenico<br />
appenninico, ad appena 5 km dai depositi affioranti <strong>di</strong><br />
avanfossa <strong>ed</strong> a meno <strong>di</strong> 10 km dal fronte sepolto <strong>del</strong>l'alloctono<br />
(PATACCA E SCANDONE, 2004), tale spessore è <strong>di</strong> appena 2.0-<br />
2.5 km. Le falde poggiano su un basamento rigido <strong>di</strong> dolomie e<br />
calcari (Mesozoico-Triassico) la cui continuazione orientale<br />
affiora nel promontorio <strong>del</strong> Gargano, nel basso Molise e a Nord<br />
<strong>del</strong>l’area <strong>di</strong> Capitanata.<br />
Il lavoro <strong>di</strong> terreno, che per completezza è stato svolto su<br />
un'area più vasta, è stato effettuato alla scala 1:10.000 sulle<br />
CTR <strong>del</strong>la Regione Molise e comprende i paesi <strong>di</strong> San<br />
Giuliano, Bonefro, Santa Croce e Colletorto. La carta geologica<br />
rappresentata in Figura 1 è limitata ad un quadrato <strong>di</strong> 6 km <strong>di</strong><br />
lato corrispondente all'area dove è stato effettuato il rilievo<br />
gravimetrico.<br />
Lo scollamento basale separa le unità alloctone dai depositi<br />
pliocenico-quaternari <strong>del</strong>l'avanfossa bradanica settentrionale in<br />
successione stratigrafica sul substrato carbonatico <strong>di</strong> pertinenza<br />
apula. Le scaglie tettoniche che sono state riconosciute e<br />
ricostruite nell'area <strong>di</strong> indagine sono costituite da due unità<br />
stratigrafiche principali. La prima è la Formazione Faeto<br />
costituita da calcari marnosi e marne bianche e rosate con<br />
intercalazioni <strong>di</strong> biocalcareniti e calciru<strong>di</strong>ti torbi<strong>di</strong>tiche in strati
56 R. CAPUTO ET ALII<br />
Fig. 1 – Carta geologico-strutturale <strong>di</strong> dettaglio <strong>del</strong>l’area <strong>di</strong> San Giuliano. Il<br />
riquadro tratteggiato <strong>del</strong>imita l’area <strong>di</strong> lato 2 km per la quale è stato costruito<br />
il mo<strong>del</strong>lo <strong>di</strong>gitale 3D.<br />
da centimetrici a decimentrici, <strong>di</strong> età Serravalliano-Tortoniano<br />
(FESTA et al., 2006). Tale unità affiora estesamente sia in tagli<br />
naturali che stradali e, verso l'alto, passa stratigraficamente ad<br />
una successione <strong>di</strong> argille marnose grigio azzurre e marne<br />
argillose con intercalazioni <strong>di</strong> arenarie, <strong>di</strong> età Tortoniano-<br />
Messiniano chiamate Formazione <strong>di</strong> Vallone Ferrato da FESTA<br />
et al. (2006) e note anche con il nome <strong>di</strong> Unità <strong>di</strong> Toppo<br />
Capuana. Gli affioramenti <strong>di</strong> questa seconda litologia sono<br />
generalmente più rari (es. 1 km NNE <strong>del</strong>l'abitato <strong>di</strong> San<br />
Giuliano o lungo la strada tra San Giuliano e Colletorto) e<br />
danno luogo a <strong>di</strong>ffusa reptazione, movimenti <strong>di</strong> massa lungo i<br />
versanti (anche se <strong>di</strong> debole inclinazione) e a numerosi<br />
fenomeni franosi principalmente per colata rapide (debris flow)<br />
e secondariamente con meccanismi <strong>di</strong> roto-traslazione.<br />
La deposizione <strong>del</strong>la Formazione <strong>di</strong> Vallone Ferrato è<br />
probabilmente perdurata anche durante le prime fasi<br />
contrazionali, inizialmente caratterizzate dallo sviluppo <strong>di</strong><br />
pieghe per propagazione <strong>di</strong> faglia, e ciò ha causato variazioni<br />
laterali sia <strong>di</strong> facies che <strong>di</strong> spessore. Localmente, come a sud <strong>ed</strong><br />
a ovest <strong>di</strong> Santa Croce, affiorano dei materiali prevalentemente<br />
argillosi a struttura caotica e non attribuibili con certezza ad<br />
alcuna unità stratigrafica. Essi rappresentano un melanges <strong>di</strong><br />
origine tettonica generalmente sviluppatosi alla base dei<br />
sovrascorrimenti a spese <strong>di</strong> argille policrome e calcareniti<br />
torbi<strong>di</strong>tiche probabilmente appartenenti alla formazione <strong>del</strong><br />
Flysch Rosso.<br />
Le unità alloctone così strutturate sono state<br />
successivamente coinvolte da un <strong>di</strong>verso regime tettonico che<br />
ha generato faglie ad assetto da verticale ad alto angolo,<br />
m<strong>ed</strong>iamente orientate NNO-SSE e con prevalente cinematica<br />
trascorrente destra. Tali strutture <strong>di</strong>ssecano in modo obliquo e<br />
<strong>di</strong>slocano anche <strong>di</strong> centinaia <strong>di</strong> metri le prec<strong>ed</strong>enti strutture<br />
contrazionali a basso angolo generalmente orientate NO-SE.<br />
L'attivazione <strong>di</strong> questi sistemi <strong>di</strong> faglie ha localmente dato<br />
luogo a tipiche strutture a fiore, come quella su cui si trova il<br />
paese <strong>di</strong> San Giuliano. Non è chiaro se tali strutture trascorrenti<br />
interessano soltanto il cuneo alloctono superficiale oppure se<br />
coinvolgono anche i calcari <strong>del</strong>la sottostante piattaforma apula.<br />
Le <strong>di</strong>mensioni pluri-chilometriche <strong>del</strong>le faglie osservate in<br />
campagna e l'entità dei rigetti stimati fanno propendere per<br />
questa seconda ipotesi suggerendo la riattivazione <strong>di</strong><br />
<strong>di</strong>scontinuità meccaniche er<strong>ed</strong>itate da fasi tettoniche<br />
mesozoiche o prodottesi durante la flessurazione prima che la<br />
piattaforma carbonatica sottoscorresse nella sua posizione<br />
attuale. Per le finalità <strong>del</strong> progetto, i dati strutturali <strong>di</strong>sponibili<br />
permettono <strong>di</strong> ricostruire, con ragionevole certezza, la geologia<br />
<strong>del</strong> sottosuolo fino ad una profon<strong>di</strong>tà <strong>di</strong> almeno 1,5 km.<br />
Fig. 2 – Alcuni esempi <strong>di</strong> sezioni geologico-strutturali orientate est-ovest a<br />
partire dalle quali è stato costruito il mo<strong>del</strong>lo <strong>di</strong>gitale 3D.
3D COMPLEX GEOLOGY OF THE 2002 SAN GIULIANO EPICENTRAL AREA<br />
Sulla base <strong>del</strong>le indagini <strong>di</strong> terreno, <strong>del</strong>la relativa<br />
rappresentazione cartografica e <strong>di</strong> vincoli stratimetrici <strong>ed</strong><br />
utilizzando profili sismici e dati <strong>di</strong> pozzi ministeriali è stato<br />
possibile costruire dei profili geologici seriali (orientati E-O)<br />
interpretando il sottosuolo fino ad una profon<strong>di</strong>tà <strong>di</strong> circa 1,5-2<br />
km (Figura 2).<br />
I risultati <strong>del</strong>le indagini geologiche documentano una<br />
strutturazione polifasica <strong>di</strong> questo settore <strong>di</strong> catena. L'area è<br />
stata coinvolta dalle fasi compressive appenniniche che hanno<br />
causato l'impilamento <strong>di</strong> <strong>di</strong>verse unità tettoniche m<strong>ed</strong>iante<br />
l'attivazione <strong>di</strong> un complesso sistema <strong>di</strong> sovrascorrimenti<br />
embriciati a vergenza ENE. Successivamente, tale<br />
strutturazione contrazionale associata a sovrascorrimenti a<br />
basso angolo è stata coinvolta da una serie <strong>di</strong> faglie subverticali<br />
con prevalente cinematica trascorrente che ha dato luogo a<br />
geometrie a fiore sia positive, sia negative. Una simile<br />
evoluzione è stata proposta anche in altri settori <strong>del</strong>l'Appennino<br />
centrale (e.g. DI BUCCI et al., 1999)<br />
RISULTATI DELLE INDAGINI GRAVIMETRICHE<br />
Il rilievo gravimetrico ha interessato un’area, centrata<br />
sull’abitato <strong>di</strong> San Giuliano <strong>di</strong> Puglia, <strong>di</strong> circa 36 km 2 . Sono<br />
state effettuate misure in 248 punti, riferiti gravimetricamente<br />
all’eccentrico <strong>del</strong>la stazione assoluta <strong>di</strong> Troia (FG), che sono<br />
stati successivamente elaborati, applicando le correzioni<br />
standard con densità <strong>di</strong> 2.1 g/cm 3 , calcolata applicando il<br />
metodo <strong>di</strong> Nettleton, per ottenere la mappa <strong>del</strong>le anomalie <strong>di</strong><br />
Bouguer.<br />
Fig. 3 – Mappa <strong>del</strong>le anomalie <strong>di</strong> Bouguer <strong>del</strong>l'area epicentrale <strong>di</strong> San<br />
Giuliano.<br />
Fig. 4 – Mappa <strong>del</strong>le anomalie <strong>di</strong> Bouguer <strong>del</strong>l'area epicentrale <strong>di</strong> San<br />
Giuliano filtrata con lunghezza d'onda
58 R. CAPUTO ET ALII<br />
Fig. 5 – Mo<strong>del</strong>lo 3D <strong>del</strong>l'area epicentrale <strong>del</strong> terremoto <strong>di</strong> San Giuliano<br />
ricostruito m<strong>ed</strong>iante software GoCad.<br />
sottraendo la componente regionale all’anomalia <strong>di</strong> Bouguer.<br />
DISCUSSIONE<br />
In una fase successiva <strong>del</strong> lavoro, tutte le informazioni<br />
geologico-strutturali, sia <strong>di</strong> superficie che profonde, sono state<br />
integrate con i risultati <strong>del</strong>la campagna gravimetrica e insieme<br />
utilizzati per realizzare un mo<strong>del</strong>lo <strong>di</strong>gitale geologicostrutturale<br />
e geofisico <strong>di</strong> dettaglio centrato sul paese <strong>di</strong> S.<br />
Giuliano <strong>di</strong> Puglia (Figura 5). Il volume considerato copre un<br />
area <strong>di</strong> 2 km <strong>di</strong> lato e si estende fino alla profon<strong>di</strong>tà <strong>di</strong> circa<br />
1500 m rispetto alla superficie. Il mo<strong>del</strong>lo è stato realizzato<br />
usando il software GoCad® e si compone <strong>di</strong> un file vettoriale<br />
(mo<strong>di</strong>ficabile), dove sono definite tutte le interfacce e le loro<br />
intersezioni, e <strong>di</strong> un file raster che definisce il volume in<br />
termini <strong>di</strong> proprietà fisiche (velocità P <strong>ed</strong> S, densità e<br />
attenuazione) su un grigliato con passo <strong>di</strong> 10 m.<br />
Il mo<strong>del</strong>lo è stato costruito sulla base <strong>del</strong>l’interpretazione<br />
geologico-strutturale prec<strong>ed</strong>entemente descritta e <strong>del</strong> mo<strong>del</strong>lo<br />
altimetrico <strong>di</strong>gitale <strong>del</strong> terreno, integrati con i risultati <strong>del</strong><br />
rilievo gravimetrico. Inoltre, sono stati presi in considerazione<br />
anche i risultati <strong>di</strong> altre indagini effettuate nell'ambito <strong>del</strong><br />
progetto DPC, come il rilievo sismico a rifrazione e<br />
<strong>del</strong>l’inversione tomografica (NIETO E BOEHM, 2006) e le<br />
indagini geotecniche (SILVESTRI et al., 2006). Grazie al<br />
software GoCad® è possibile osservare in tre <strong>di</strong>mensioni le<br />
complesse geometrie <strong>del</strong>le unità stratigrafiche e <strong>del</strong>le <strong>di</strong>verse<br />
strutture tettoniche. Con lo stesso software è inoltre possibile<br />
costruire profili geologici lungo qualsiasi <strong>di</strong>rezione e ciò ha<br />
permesso <strong>di</strong> controllare, mo<strong>di</strong>ficare localmente e così<br />
migliorare l’interpretazione geologica <strong>del</strong> sottosuolo.<br />
Nella definizione <strong>del</strong> mo<strong>del</strong>lo 3D si sono considerate le<br />
<strong>di</strong>verse unità litologiche principali che era possibile vincolare<br />
adeguatamente in profon<strong>di</strong>tà, e cioè la Formazione <strong>di</strong> Vallone<br />
Ferrato, la Formazione Faeto <strong>ed</strong> il melanges. I dati <strong>del</strong>le<br />
interfacce, definite in profon<strong>di</strong>tà e in superficie, sono stati<br />
inseriti nel mo<strong>del</strong>lo vettoriale <strong>di</strong>gitale, insieme al mo<strong>del</strong>lo<br />
altimetrico, come superfici geo-referenziate, e definiscono le<br />
principali fratture e gli orizzonti <strong>del</strong>le quattro unità strutturali.<br />
Questi elementi costituiscono il mo<strong>del</strong>lo vettoriale geologicostrutturale.<br />
Il mo<strong>del</strong>lo fisico è invece definito su una griglia regolare <strong>di</strong><br />
passo 10 m, calcolato dalla partizione litologica definita nel<br />
mo<strong>del</strong>lo vettoriale. Ad ogni nodo <strong>del</strong>la griglia è assegnata una<br />
label che definisce la litologia cui appartiene <strong>ed</strong> i valori <strong>del</strong>le<br />
proprietà fisiche. Questo è il mo<strong>del</strong>lo che è stato<br />
successivamente utilizzato per le simulazioni numeriche (sia<br />
come volume 3D sia come sezioni 2D).<br />
Il presente lavoro ha permesso <strong>di</strong> evidenziare come l’analisi<br />
integrata <strong>di</strong> un rilievo geologico-strutturale <strong>di</strong> dettaglio e <strong>del</strong>la<br />
gravimetria, una metodologia geofisica <strong>di</strong> costo limitato e non<br />
invasiva, possa fornire una migliore definizione e<br />
comprensione <strong>del</strong>la geologia <strong>del</strong> sottosuolo permettendo <strong>di</strong><br />
ricostruire le geometrie dei corpi in 3D.<br />
REFERENCES<br />
BROCHER T.M. (2005) - Empirical relations between elastic<br />
wavespe<strong>ed</strong>s and density in the Earths crust. Bull. Seism.<br />
Soc. Am., 95 (6), 2081-2092.<br />
DI BUCCI D., CORRADO S., NASO G., PAROTTO M. &<br />
PRATURLON A. (1999) - Evoluzione tettonica neogenicoquaternaria<br />
<strong>del</strong>l'area molisana. Boll. Soc. Geol. It., 118,<br />
13-30.<br />
FESTA A., GHISETTI F. & VEZZANI L. (2006) - Carta geologica<br />
<strong>del</strong> Molise. Scala 1:100.000 e <strong>Note</strong> illustrative. Litografia<br />
GEDA, Nichelino (TO).<br />
NIETO D.Y. & BOEHM G. (2006) - Progetto S3 – Scenari <strong>di</strong><br />
scuotimento in aree <strong>di</strong> interesse prioritario e/o strategico.<br />
Task 3 – Molise, Deliverable D8.<br />
PALMIERI F., MARELLO L. & PRIOLO E. (2006) - Progetto S3 –<br />
Rilievo gravimetrico <strong>di</strong> dettaglio nell’area <strong>di</strong> San Giuliano<br />
<strong>di</strong> Puglia (Cb) – REL. 2006/92- CRS 24.<br />
PATACCA E. & SCANDONE P. (2004) - The Plio-Pleistocene<br />
thrust belt – for<strong>ed</strong>eep system in the Southern Apennines and<br />
Sicily (Italy). In: Crescenti U., D’Offizi S., Merlini S. &<br />
Lacchi L. (Eds.) - Geology of Italy. Società Geologica<br />
Italiana, Roma, 93-129.<br />
SILVESTRI F., D’ONOFRIO A., GUERRICCHIO A., LANZO G.,.<br />
PAGLIAROLI A, PUGLIA R., SANTUCCI DE MAGISTRIS F., SICA<br />
S., EVA C., FERRETTI G. & DI CAPUA G. (2006) - Mo<strong>del</strong>li<br />
geotecnici 1D e/o 2D per i comuni <strong>di</strong> San Giuliano <strong>di</strong><br />
Puglia, Bonefro, Ripabottoni, Colletorto e Santa Croce <strong>di</strong><br />
Magliano. Progetto S3 – Scenari <strong>di</strong> scuotimento in aree <strong>di</strong><br />
interesse prioritario e/o strategico. Task 3 – Molise,<br />
Deliverable D8.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 59-61, 3 ff.<br />
Morfologia <strong>ed</strong> evoluzione strutturale <strong>di</strong> un corpo <strong>di</strong>apirico nel<br />
settore iraniano <strong>del</strong> Golfo Persico<br />
LUCA CHIARIOTTI (*), CESARE R. PEROTTI (*), MARCO RINALDI (*),GIUSEPPE BERTOZZI (**) & STEFANO CARRUBA (**)<br />
ABSTRACT<br />
Morphology and structural evolution of a <strong>di</strong>apiric body in the Iranian<br />
area of the Persian Gulf.<br />
The Persian Gulf basin is characteriz<strong>ed</strong> by a large number of <strong>di</strong>apiric<br />
bo<strong>di</strong>es induc<strong>ed</strong> by the rise of the Late Proterozoic evaporitic Hormuz<br />
Formation. The study of a <strong>di</strong>apiric structure through seismic interpretation,<br />
well data and three-<strong>di</strong>mensional reconstructions reveals that the salt uplift<br />
probably start<strong>ed</strong> during the Early Palaeozoic and basically was a continuous<br />
process until Recent, also if with periods of <strong>di</strong>fferent rate of deformation. The<br />
rates of this long-lasting and progressive deformation of the <strong>di</strong>apiric structure<br />
can be eximat<strong>ed</strong> in 4-5 m/Ma during the Mesozioc, and about 6 m/Ma during<br />
the Palaeogene. A possible connections with the geodynamic events that<br />
affect<strong>ed</strong> the NE margin of the Arabian Plate can be inferr<strong>ed</strong>.<br />
Key words: Persian Gulf, salt tectonics.<br />
INTRODUZIONE<br />
Il Golfo Persico costituisce il settore nord-orientale <strong>del</strong>la<br />
Placca Araba <strong>ed</strong> è caratterizzato dalla presenza nel sottosuolo<br />
<strong>di</strong> numerosi <strong>di</strong>apiri salini, costituiti dalla formazione<br />
evaporitica <strong>di</strong> Hormuz <strong>di</strong> età precambriana, situata alla base<br />
<strong>del</strong>la potente successione s<strong>ed</strong>imentaria che caratterizza la<br />
regione e che ha un’età compresa tra il Cambriano <strong>ed</strong> il<br />
Quaternario (ALA, 1974; EDGELL, 1996).<br />
Durante tutto il Paleozoico la s<strong>ed</strong>imentazione avviene in un<br />
ambiente intracratonico (SHARLAND et alii, 2001) <strong>ed</strong> è<br />
caratterizzata da depositi prevalentemente clastici continentali<br />
o <strong>di</strong> mare poco profondo. Dal Permiano superiore fino<br />
all’Oligocene, in un contesto geo<strong>di</strong>namico essenzialmente <strong>di</strong><br />
margine passivo, si assiste alla deposizione <strong>di</strong> una successione<br />
più francamente marina, con lo sviluppo <strong>di</strong> potenti successioni<br />
carbonatiche intercalate a s<strong>ed</strong>imenti argillosi o marnosi e a<br />
ridotti livelli evaporitici. Dopo l’Oligocene il Golfo Persico<br />
assume il carattere <strong>di</strong> avampaese e successivamente <strong>di</strong><br />
avanfossa rispetto alla catena degli Zagros, che ha una<br />
_________________________<br />
(*)Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università degli Stu<strong>di</strong> <strong>di</strong> Pavia, via<br />
Ferrata 1, 27100 Pavia, ITALY.<br />
(**)E<strong>di</strong>son S.p.A., Foro Bonaparte 31, 20121 Milano, ITALY.<br />
Stefano Carruba: tel 02/62227925; fax 02/62227041,<br />
stefano.carruba@<strong>ed</strong>ison.it<br />
<strong>di</strong>rezione NW-SE, una vergenza verso SW e che si estende<br />
imm<strong>ed</strong>iatamente a NE <strong>del</strong>l’area.<br />
La risalita <strong>del</strong> sale sembra sia iniziata non molto tempo<br />
dopo la sua deposizione, provocando numerosi <strong>ed</strong> estesi<br />
fenomeni <strong>di</strong> <strong>di</strong>apirismo i cui meccanismi <strong>di</strong> innesco, le modalità<br />
(<strong>di</strong>apirismo attivo e/o passivo) e le fasi evolutive non sono<br />
ancora <strong>del</strong> tutto chiarite (GANSSER, 1992; JACKSON &<br />
VENDEVILLE, 1994; EDGELL, 1996; CARRUBA ET et alii, 2007).<br />
Lo stu<strong>di</strong>o dei meccanismi <strong>di</strong> risalita e <strong>del</strong>la cinematica dei<br />
<strong>di</strong>apiri salini riveste un crescente interesse negli ultimi anni in<br />
considerazione <strong>del</strong> fatto che numerosi reservoir petroliferi si<br />
trovano in corrispondenza <strong>di</strong> trappole connesse alla risalita<br />
<strong>di</strong>apirica.<br />
L’EVOLUZIONE GEOLOGICA DEL DIAPIRO<br />
L’analisi <strong>di</strong> dati geofisici relativi ad una importante struttura<br />
<strong>di</strong>apirica localizzata nell’offshore iraniano, a nord-ovest<br />
<strong>del</strong>l’Arco <strong>del</strong> Qatar (Fig. 1), integrati con calibrazioni <strong>di</strong> pozzo<br />
su una serie <strong>di</strong> marker stratigrafici (limiti formazionali o<br />
<strong>di</strong>scordanze regionali), ha reso possibile la caratterizzazione<br />
<strong>del</strong> <strong>di</strong>apirismo salino <strong>di</strong> questo settore <strong>del</strong> Golfo Persico.<br />
L’evoluzione strutturale e temporale <strong>del</strong> <strong>di</strong>apiro è stata<br />
ricostruita attraverso l’interpretazione e la retro-deformazione<br />
(attraverso un proc<strong>ed</strong>imento <strong>di</strong> flattening dei <strong>di</strong>versi riflettori)<br />
<strong>di</strong> un grid <strong>di</strong> linee sismiche a riflessione acquisite nell’area <strong>di</strong><br />
indagine, effettuando l’analisi <strong>del</strong>le strutture tettoniche e<br />
s<strong>ed</strong>imentarie presenti nella successione deposizionale sui<br />
fianchi <strong>del</strong> <strong>di</strong>apiro. L’interpretazione ha evidenziato geometrie<br />
<strong>di</strong> onlap, troncature erosive, variazioni <strong>di</strong> spessore laterale,<br />
presenza <strong>di</strong> minibacini s<strong>ed</strong>imentari e <strong>di</strong> sinclinali <strong>di</strong> rim (Fig.<br />
2). La ricostruzione geometrica <strong>del</strong>la superficie <strong>di</strong> alcuni<br />
marker sismici ha permesso la creazione <strong>di</strong> mo<strong>del</strong>li <strong>di</strong>gitali<br />
tri<strong>di</strong>mensionali <strong>del</strong>le superfici stesse, ottenendo una dettagliata<br />
immagine <strong>del</strong>la geometria <strong>del</strong>la struttura e <strong>del</strong>la successione<br />
stratigrafica <strong>del</strong>l’area <strong>di</strong> stu<strong>di</strong>o (Fig. 3). Per caratterizzare le<br />
fasi <strong>di</strong> crescita nei <strong>di</strong>versi perio<strong>di</strong> <strong>di</strong> tempo definiti dai marker<br />
sismici, sono stati calcolati i valori relativi alle isopache,<br />
espresse in millisecon<strong>di</strong>, degli intervalli stratigrafici<br />
interpretati, valutando il tasso <strong>di</strong> risalita (in m per Ma) <strong>del</strong><br />
<strong>di</strong>apiro salino. La possibile storia evolutiva <strong>del</strong> <strong>di</strong>apiro può<br />
essere sintetizzata come segue.<br />
Durante il Paleozoico la presenza <strong>di</strong> numerosi minibacini<br />
s<strong>ed</strong>imentari e <strong>di</strong> superfici <strong>di</strong> onlap, che subiscono
60 L. CHIARIOTTI ET ALII<br />
Fig. 1 – Mappa schematica <strong>del</strong>l’area <strong>di</strong> stu<strong>di</strong>o. In grigio scuro sono evidenziate le più importanti strutture <strong>di</strong>apiriche presenti nell’offshore e nell’onshore<br />
iraniano e nel cerchio è in<strong>di</strong>cata l’ubicazione <strong>del</strong>la struttura stu<strong>di</strong>ata.<br />
rispettivamente una progressiva rotazione e verticalizzazione<br />
con l’aumento <strong>del</strong>la profon<strong>di</strong>tà, in<strong>di</strong>cano una fase <strong>di</strong> crescita<br />
continua <strong>del</strong> corpo salino.<br />
Durante il Mesozoico si osserva un progressivo aumento <strong>di</strong><br />
spessore <strong>del</strong>la successione ai lati <strong>del</strong> <strong>di</strong>apiro e l’assenza <strong>di</strong><br />
chiare superfici <strong>di</strong> onlap. Ciò potrebbe essere spiegato<br />
ipotizzando un processo <strong>di</strong> lenta aggradazione, lungo i fianchi<br />
subsidenti <strong>del</strong>la struttura <strong>di</strong>apirica, <strong>del</strong>la piattaforma<br />
Fig. 2 – Interpretazione <strong>di</strong> una sezione retro-deformata al Miocene inferiore. L’ispessimento <strong>del</strong>la successione ai fianchi <strong>del</strong> <strong>di</strong>apiro in<strong>di</strong>ca una crescita<br />
sins<strong>ed</strong>imentaria <strong>del</strong> corpo <strong>di</strong>apirico tra l’infra-Miocene e l’Oligocene Unconformity (onlap), mentre le sottostanti troncature erosive (truncation) in<strong>di</strong>cano una<br />
fase <strong>di</strong> crescita postdeposizionale durante il Paleogene. Nel Paleozoico il progressivo aumento d’inclinazione dei riflettori e dei minibacini con la profon<strong>di</strong>tà<br />
in<strong>di</strong>ca una crescita continua <strong>del</strong> <strong>di</strong>apiro.
MORFOLOGIA ED EVOLUZIONE STRUTTURALE DI UN CORPO DIAPIRICO NEL SETTORE IRANIANO DEL GOLFO PERSICO<br />
Fig. 3 – Ricostruzione 3D <strong>di</strong> alcune superfici stratigrafiche relative alla successione compresa tra il Top <strong>del</strong> Permiano e l’Oligocene Unconformity.<br />
L’inclinazione dei fianchi <strong>del</strong> <strong>di</strong>apiro <strong>ed</strong> il <strong>di</strong>slivello tra la zona <strong>di</strong> cresta e il depocentro <strong>del</strong>le sinclinali <strong>di</strong> rim aumentano progressivamente con la profon<strong>di</strong>tà<br />
evidenziando una continua attività <strong>di</strong>aprica nell’intervallo <strong>di</strong> tempo considerato (esagerazione verticale: 10x).<br />
carbonatica che si sviluppa in questo periodo. E’ stata inoltre<br />
in<strong>di</strong>viduata un’importante troncatura erosiva <strong>di</strong> significato<br />
regionale <strong>di</strong> età cretacica superiore (Turonian Unconformity).<br />
Questo intervallo temporale è caratterizzato da un tasso <strong>di</strong><br />
crescita pressoché costante con valori all’incirca compresi tra 4<br />
e 5 m/Ma e da un progressivo spostamento verso sud-ovest<br />
<strong>del</strong>la sommità <strong>del</strong> <strong>di</strong>apiro.<br />
Il Paleogene è caratterizzato da una crescita <strong>di</strong>apirica più<br />
rapida, con un tasso <strong>di</strong> circa 6 m/Ma, evidenziata anche da una<br />
marcata troncatura erosiva databile all’incirca alla transizione<br />
tra Paleogene- Neogene (Oligocene Unconformity). Durante<br />
questo periodo continua la migrazione verso sud-ovest <strong>del</strong>la<br />
sommità <strong>del</strong> <strong>di</strong>apiro, associata ad un aumento <strong>del</strong>la sua<br />
ampiezza complessiva.<br />
Il Neogene inizia con una notevole crescita <strong>del</strong> corpo<br />
<strong>di</strong>apirico seguita da una periodo contrad<strong>di</strong>stinto da probabile<br />
inattività e successivamente da una lieve crescita. Le strutture<br />
attive in questo intervallo sono contemporanee all’avanzamento<br />
verso sud-ovest <strong>del</strong>l’avanfossa degli Zagros.<br />
Il <strong>di</strong>apirismo salino in questo settore <strong>del</strong> Golfo Persico può<br />
quin<strong>di</strong> essere considerato come un processo continuo, anche se<br />
caratterizzato da fasi con <strong>di</strong>fferenti tassi <strong>di</strong> crescita, che ha<br />
avuto inizio già a partire probabilmente dal Paleozoico<br />
inferiore. Restano ancora da chiarire pienamente le cause che<br />
hanno indotto il fenomeno <strong>ed</strong> il ruolo esercitato dagli eventi<br />
geo<strong>di</strong>namici che hanno interessato il margine nord-orientale<br />
<strong>del</strong>la Placca Araba.<br />
BIBLIOGRAFIA<br />
ALA M.A. (1974) - Salt Diapirism in Southern Iran. Am. Ass. of<br />
Petr. Geol. Bull., 58 (9), 1758-1770.<br />
CARRUBA S., BERTOZZI G., PEROTTI C. R. & RINALDI M. (2007)<br />
- Alcuni aspetti <strong>del</strong> <strong>di</strong>apirismo salino nel Golfo Persico.<br />
Rend. Soc. Geol. It., 4, Nuova Serie, 188-190.<br />
EDGELL H.S. (1996) - Salt tectonism in the Persian Gulf Basin.<br />
In Alsop G.I., Blun<strong>del</strong>l D.J. & Davison I. (<strong>ed</strong>s), Salt<br />
tectonism, Geol. Soc. Spec. Publ., 100, 129-151.<br />
GANSSER A. (1992) - The enigma of the Persian salt dome<br />
inclusion. Eclogae Geol. Helv., 85(3), 825-846.<br />
JACKSON M.P.A. & VENDEVILLE B.C. (1994) - Regional<br />
extension as a geologic trigger for <strong>di</strong>apirism. Geol. Soc. of<br />
America Bull., 106, 57-73.<br />
SHARLAND P.R., ARCHER R., CASEY D.M., DAVIES R.B., HALL<br />
S.H., HEWARD A.P., HORBURY A.D. & SIMMONS M.D.<br />
(2001) - Arabian plate sequence stratigraphy. GeoArabia<br />
Special Pubblication 2, Gulf Petrolink, Bahrain.<br />
61
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 62<br />
How active is the East Antarctic craton?<br />
ABSTRACT<br />
P. CIANFARRA* & F. SALVINI*<br />
Accor<strong>di</strong>ng to plate tectonic theory East Antarctica is a<br />
stable craton almost entirely surround<strong>ed</strong> by passive margins<br />
and suffer<strong>ed</strong> only minor tectonci at the margins since the<br />
fragmentation from the Gondwanaland in Permian times.<br />
The <strong>di</strong>scovery of the Antarctic subglacial lakes and their<br />
uneven spatial <strong>di</strong>stribution, with the highest concentration in<br />
the Dome C area, rais<strong>ed</strong> the question of the origins of the<br />
depressions hosting the largest lakes that are characteris<strong>ed</strong> by<br />
elongat<strong>ed</strong>, rectilinear morphologies often associat<strong>ed</strong> to gravity<br />
lows. The 90° lakes, the Vostok Lake, the Aurora and<br />
Concor<strong>di</strong>a Trenches with the associat<strong>ed</strong> homonymous lakes, the<br />
Adventure Trench and the Astrolobe Basin, all depict a belt of<br />
subglacial troughs with possible tectonic origin running from<br />
the Eastern flanks of the Gamburtsev Mountains to the western<br />
slope of the Wilkes Basin.<br />
To explore the possible tectonic origin of these depressions<br />
a numerical mo<strong>del</strong>ling by HCA technique was perform<strong>ed</strong> to<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, Università degli Stu<strong>di</strong> Roma Tre<br />
Largo S. Leonardo Murialdo 1, 00146 ROMA<br />
Lavoro eseguito nell’ambito <strong>del</strong> progetto SALE (Subglacial Antarctic Lake<br />
Exploration) con il contributo finanziario <strong>del</strong> PNRA<br />
simulate the present-day morphologies, deriv<strong>ed</strong> from ra<strong>di</strong>o<br />
echo-soun<strong>di</strong>ng data, as the result of the activity of crustal,<br />
extensional faults. The comparison and tuning of the mo<strong>del</strong>s<br />
with the b<strong>ed</strong>rock morphology allow<strong>ed</strong> to constrain the<br />
extensional tectonic style responsible for the formation of<br />
stu<strong>di</strong><strong>ed</strong> depressions.<br />
Since the onset of the East Antactic Ice Sheet (34 Ma) the<br />
tectonic activity represents the major mo<strong>del</strong>ling agent of the<br />
subglacial landscape, due to the generally dry ice cap-b<strong>ed</strong>rock<br />
contact preventing any significant erosional or s<strong>ed</strong>imentary<br />
episode by glacial dynamics.<br />
The above consideration and the results from the numerical<br />
mo<strong>del</strong>ling allow<strong>ed</strong> to speculate on the existence of an<br />
extensional belt within the East Antarctic craton that in turn<br />
may relate to an intraplate strike-slip deformation belt of<br />
Cenozoic age.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 63-64<br />
ABSTRACT<br />
Increasing amounts of geological and geophysical data<br />
suggest that some crustal faults are weak when compar<strong>ed</strong><br />
to standard friction coefficient values determin<strong>ed</strong> for<br />
faults in the laboratory, 0.6
64 C. COLLETTINI<br />
comportamento velocity strengthening che <strong>di</strong>viene<br />
sempre più accentuato all’aumentare <strong>del</strong>la velocità <strong>di</strong><br />
scivolamento.<br />
Gli esperimenti su campioni in polvere costituiti per il<br />
60-70% da clasti <strong>di</strong> calcite, calcite-tremolite, e tremolite,<br />
hanno coefficienti <strong>di</strong> attrito tra 0.5 e 0.7 e sono<br />
caratterizzati da processi cataclastici associati ad una<br />
<strong>di</strong>minuzione <strong>del</strong>la granulometria <strong>ed</strong> ad una localizzazione<br />
<strong>del</strong>la deformazione su piani R1, Y, B. Queste rocce sono<br />
caratterizzate da un comportamento velocity<br />
strengthening che passa a velocity weakening<br />
all’aumentare <strong>del</strong>la velocità <strong>di</strong> scivolamento.<br />
Il nostro dataset suggerisce che faglie mature possono<br />
essere deboli se il nucleo è ricco in fillosilicati, p. es.<br />
talco, o se questo è caratterizzato da orizzonti foliati<br />
interconnessi costituiti da minerali estremamente fini, p.<br />
es. tremolite < 2 mm, associati a fillosilicati. Il<br />
comportamento velocity strengthening <strong>di</strong> queste rocce <strong>di</strong><br />
faglia facilita un creep asismico.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 65<br />
Gelatins for laboratory mo<strong>del</strong>s: rheological - physical properties and<br />
new perspectives.<br />
FABIO CORBI (*), FRANCESCA FUNICIELLO (*), ERIKA DI GIUSEPPE (*), CLAUDIO FACCENNA (*) & GIORGIO<br />
RANALLI (°)<br />
A laboratory mo<strong>del</strong> is a simplifi<strong>ed</strong> scal<strong>ed</strong> representation of<br />
nature. Accor<strong>di</strong>ng to ‘similarity criteria’ (HUBBERT, 1937),<br />
both the rheological and physical parameters involv<strong>ed</strong> in the<br />
mo<strong>del</strong>ing have to be close to natural con<strong>di</strong>tions (WEIJERMARS<br />
& SCHMELING, 1986). In the crust/lithosphere system,<br />
rheological layering is a consequence of both compositional<br />
layering and the variations of temperature with depth. It is<br />
therefore important to search for mo<strong>del</strong> materials which scale<br />
properly and reproduce in the laboratory the whole spectrum of<br />
rheological behavior, from brittle to viscoelastic to viscous.<br />
The rheological and physical properties of a wide range of<br />
gelatins, in both gel- and sol-state, are systematically<br />
investigat<strong>ed</strong> as functions of temperature, composition,<br />
concentration, ageing and appli<strong>ed</strong> strain rate. Several types of<br />
rheometric tests (amplitude sweep, frequency sweep,<br />
temperature sweep) and flow tests are us<strong>ed</strong> to obtain storage<br />
and loss moduli, strength properties, viscoelastic range, and<br />
fluid viscosity. Other physical properites (density and thermal<br />
parameters) are also measur<strong>ed</strong> and compil<strong>ed</strong>. The rheological<br />
variability of gelatins appears promising for their potential use<br />
as analog materials to mo<strong>del</strong> crustal and lithospheric<br />
deformation. As an example, the rheological and physical<br />
properties of pig skin gelatin 2.5%wt at 10°C are analyz<strong>ed</strong>, and<br />
found to satisfy the scaling requirements for experimental<br />
mo<strong>del</strong>s of crustal deformation. This material, in fact, properly<br />
scales elastic (G* of about 10 3 Pa), viscous (n=5.0, E=450 KJ<br />
mol -1 ) and frictional properties (c of about 300 Pa) of the<br />
natural prototype for an experimental strain rate of 10 -2 s -1 .<br />
First encouraging results demonstrate the possibility to use<br />
gelatins for simulate earthquake-like dynamics. The stick-slip<br />
behavior can be locally visualiz<strong>ed</strong> and quantifi<strong>ed</strong> using the<br />
photoelastic technique.<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, Università degli Stu<strong>di</strong> ROMA<br />
TRE, L.go S. Leonardo Murialdo, 1- 00146 Roma, Italy.<br />
(°) Department of Earth Sciences and Ottawa-Carleton Geoscience<br />
Centre, Carleton University, K1S 5B6, Ottawa, Canada.<br />
Fabio Corbi: fcorbi@uniroma3.it<br />
REFERENCES<br />
HUBBERT, M. K. (1937) - Theory of scale mo<strong>del</strong>s as appli<strong>ed</strong> to<br />
the study of geologic structures. Bulletin of the Geological<br />
Society of America; 48, 1459-1520.<br />
WEIJERMARS, R. & SCHMELING, H. (1986) - Scaling of<br />
newtonian and non newtonian fluid dynamics without<br />
inertia for quantitative mo<strong>del</strong>ling of rock flow due to<br />
gravity (inclu<strong>di</strong>ng the concept of rheological similarity).<br />
Physics of the Earth and Planetary Interiors; 43, 316-330.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 66-67, 2ff.<br />
First fin<strong>di</strong>ng of syntectonic gold mineralization in northern Victoria<br />
Land (Antarctica): a clue for the Paleo-Pacific margin of Gondwana<br />
ABSTRACT<br />
Primo ritrovamento <strong>di</strong> mineralizzazioni sintettoniche a oro in northern<br />
Victoria Land (Antartide): una traccia per il margine Paleo-Pacifico <strong>di</strong><br />
Gondwana<br />
Questo lavoro si riferisce al primo ritrovamento <strong>di</strong> mineralizzazione a oro<br />
nella catena <strong>del</strong>le Montagne Transantartiche, in Antartide; riguarda lo stu<strong>di</strong>o<br />
petrografico e strutturale <strong>del</strong>l'affioramento e <strong>di</strong>scute le possibili implicazioni<br />
nella ricostruzione <strong>del</strong> margine paleopacifico <strong>di</strong> Gondwana. La<br />
mineralizzazione si trova nei metabasalti Glasgow <strong>del</strong> Bowers Terrane. E'<br />
associata a un sistema <strong>di</strong> vene sintettoniche, a quarzo e carbonati legato a<br />
faglie trascorrenti a scala regionale. La zona <strong>di</strong> faglia che ospita la<br />
mineralizzazione è caratterizzata, oltre che da un pervasivo sistema <strong>di</strong> vene, da<br />
un'intensa alterazione idrotermale <strong>del</strong>la roccia ospite. L'oro è presente in<br />
moschinature <strong>di</strong> oro nativo <strong>di</strong> <strong>di</strong>mensioni pluri-millimetriche, all'interno <strong>di</strong><br />
vene a quarzo, associato ad argento e arsenopirite. Datazioni 39 Ar- 40 Ar su<br />
sericite, hanno fornito età <strong>di</strong> 318,0 ± 4,6 and 310,5 ± 6,2 Ma per l'evento <strong>di</strong><br />
mineralizzazione. Mineralizzazioni a oro <strong>di</strong> età paragonabile sono presenti nel<br />
Thomson Fold Belt e nell'Hodgkinson - Broken River Fold Belt, eastern<br />
Australia.<br />
Key words: Antarctica, gold, tectonics<br />
LAURA CRISPINI (*), LAURA FEDERICO (*), GIOVANNI CAPPONI & FRANCO TALARICO (**)<br />
Here we describe the first occurrence of gold mineralization<br />
ever signall<strong>ed</strong> in northern Victoria Land (Antarctica).<br />
Northern Victoria Land geology is classically describ<strong>ed</strong> by<br />
means of the accretion of three terranes during the Ross<br />
Orogeny: the inboard Wilson (WT), the interm<strong>ed</strong>iate Bowers<br />
(BT) and the outboard Robertson Bay Terrane (RBT). The<br />
terrane arrangement is increasingly interpret<strong>ed</strong> as a fossil arctrench<br />
system resulting from a westward-<strong>di</strong>rect<strong>ed</strong> subduction at<br />
the paleo-Pacific margin of Gondwana during the Early<br />
Paleozoic Ross Orogeny (FEDERICO et alii, 2006 and in press)<br />
and therefore the three terranes should represent the continental<br />
magmatic arc, the forearc/back-arc and the trench s<strong>ed</strong>imentary<br />
sequence, respectively.<br />
Northern Victoria Land was part of the Pacific margin of<br />
Gondwana (e.g. GOLDFARB et alii, 2001) during the Paleozoic.<br />
_________________________<br />
(*) Dip.Te.Ris - Università <strong>di</strong> Genova<br />
Corso Europa 26, 16132 Genova, Italy<br />
(**) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Siena<br />
V. Laterina 8, 53100 Siena, Italy<br />
correspon<strong>di</strong>ng author: L. Crispini - crispini@<strong>di</strong>pteris.unige.it<br />
Fig. 1 - The <strong>di</strong>stribution of major Paleozoic gold provinces on the 356-Ma<br />
global reconstruction of Scotese, 1997 (from Goldfarb et al., 2001).<br />
Fig. 2 - Geological sketch of the main outcrop with the gold bearing veins.<br />
FOV is 450 m wide.<br />
Though Paleozoic gold mineralizations occur all along this<br />
margin (Fig. 1), for instance in eastern Australia, the South<br />
Island of New Zealand and southern South America, no<br />
occurrence of gold mineralization was signall<strong>ed</strong> until now in<br />
northern Victoria Land.
FIRST FINDING OF SYNTECTONIC GOLD MINERALIZATION IN NORTHERN VICTORIA LAND<br />
The quartz-carbonate mineraliz<strong>ed</strong> veins (Fig. 2) occur<br />
inside the Bowers terrane, close to the contact with the<br />
Robertson Bay terrane. The veins are host<strong>ed</strong> by greenschist to<br />
low-greenschist metabasalts (Glasgow volcanics) with<br />
interlayers of metasandstones of Middle Cambrian age (Molar<br />
Formation). Regional structural setting is characteriz<strong>ed</strong> by NW-<br />
SE-tren<strong>di</strong>ng folds, with subvertical axial plane, and by NW-SE<br />
and N-S fault systems (Fig. 2). Quartz-carbonate veins occur in<br />
a brittle-ductile high strain zone superimpos<strong>ed</strong> on the earlier<br />
regional metamorphic foliation and folds. The high strain zone<br />
is characteriz<strong>ed</strong> by foliat<strong>ed</strong> fault rocks with S-C structures,<br />
widespread veining, and hydrothermal alteration of the host<br />
rocks. The mineraliz<strong>ed</strong> veins are extensional and shear veins,<br />
often with ribbon/band<strong>ed</strong> appearance and texture typical of<br />
crack and seal processes (RAMSAY, 1980). The vein network is<br />
surround<strong>ed</strong> by an alteration halo approximately up to 500 m<br />
wide, which ranges from a chlorite zone, to a sericite-carbonate<br />
zone and a sericite-pyrite zone approaching the core of the vein<br />
system.<br />
Preliminary chlorite thermometry (CATHELINEAU & NIEVA,<br />
1985), provides temperature estimates ranging from 270 to 280<br />
°C in the chlorite-alter<strong>ed</strong> metabasalts and from 290 to 310°C in<br />
the more alter<strong>ed</strong> samples.<br />
Gold occurs as coarse-grain<strong>ed</strong> (up to some millimeters)<br />
native gold, associat<strong>ed</strong> with silver, arsenopyrite and an<br />
ironarsenic compound.<br />
From a tectonic point of view, the vein network is<br />
associat<strong>ed</strong> and coeval with a transpressional fault system<br />
(FEDERICO et alii, this volume). NW-SE and N-S faults of this<br />
system locally cut Admiralty Intrusives and are associat<strong>ed</strong> to<br />
hypabyssal intrusions that closely recall the Gallipoli<br />
Volcanics.<br />
39 Ar- 40 Ar dating on sericites provid<strong>ed</strong> Carboniferous ages<br />
for the mineralization event, with two well-defin<strong>ed</strong> plateaus at<br />
318,0 ± 4,6 Ma and 310,5 ± 6,2 Ma. Though considerably<br />
younger, the hydrothermal circulation and the mineralization<br />
can be tentatively correlat<strong>ed</strong> with the Admiralty / Gallipoli<br />
magmatic pulse and maybe with the still elusive and enigmatic<br />
Borchgrevink Orogeny (GRINDLEY & WARREN, 1964; CAPPONI<br />
et alii, 2002).<br />
Gold deposits of similar ages have not been report<strong>ed</strong> in<br />
either the Delamerian or the Lachlan Fold Belt of south-eastern<br />
Australia, which northern Victoria land is usually correlat<strong>ed</strong><br />
with. Conversely, gold deposits of Carboniferous age occur<br />
primarily in the Thomson fold belt, and in the Hodgkinson–<br />
Broken River fold belt, far to the northeast (GOLDFARB et alii,<br />
2001). As a consequence, the relevance of gold occurrence for<br />
correlations at the scale of the paleo-Pacific margin of<br />
Gondwana ne<strong>ed</strong>s to be carefully evaluat<strong>ed</strong>.<br />
REFERENCES<br />
CAPPONI G., CASTORINA F., DI PISA A., MECCHERI M., PETRINI<br />
R. & VILLA I.M. (2002) - The metaigneous rocks of the<br />
Barber Glacier area (northern Victoria Land, Antarctica):<br />
a clue to the enigmatic Borchgrevink Orogeny? In Gamble,<br />
67<br />
J., Skinner, D., and Henrys, S. (Eds.), Antarctica at the<br />
close of a Millennium, Royal Society of New Zealand<br />
Bullettin 35, 99 - 104.<br />
CATHELINEAU M. & NIEVA D. (1985) - A chlorite solid solution<br />
geothermometer The Los Azufres (Mexico) geothermal<br />
system. Contrib. to Mineral. and Petrol., 91, 235-244.<br />
FEDERICO L., CAPPONI G. & CRISPINI L. (2006) - The Ross<br />
orogeny of the transantarctic mountains: a northern<br />
Victoria Land perspective. Int. J. of Earth Sci., 95 (5), 759-<br />
770.<br />
FEDERICO L., CRISPINI L., CAPPONI G. & BRADSHAW J.D. in<br />
press - The Cambrian Ross Orogeny in northern Victoria<br />
Land (Antarctica) and New Zealand: A synthesis.<br />
Gondwana Research, DOI: 10.1016/j.gr.2008.10.004.<br />
GOLDFARB R.J., GROVES D.I. & GARDOLL S. (2001) - Orogenic<br />
gold and geologic time: a global synthesis. Ore Geology<br />
Reviews 18, 1-75.<br />
GRINDLEY G.W. & WARREN G. (1964) - Stratigraphic<br />
nomenclature and correlation in the western Ross Sea<br />
region. In: R.J. A<strong>di</strong>e (Ed.). - Antarctic geology. Amsterdam,<br />
North Holland Publishing Company, 314-333.<br />
RAMSAY, J.G. (1980) - The crack–seal mechanism of rock<br />
deformation. Nature 284, 135 - 139.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 68-69, 1 f.<br />
Structural setting of the Gan<strong>di</strong>no – Sovere thrust system and new<br />
dating of Tertiary magmatic bo<strong>di</strong>es (Orobic Alps, Italy)<br />
PAOLO D’ADDA (*), ANDREA ZANCHI (*), FABRIZIO BERRA (**), MARCO MALUSÀ (*), MARIA BERGOMI (*) &<br />
ANNALISA TUNESI (*)<br />
RIASSUNTO<br />
Assetto strutturale <strong>del</strong>la zona a thrust <strong>di</strong> Gan<strong>di</strong>no-Sovere e nuove<br />
datazioni <strong>di</strong> corpi magmatici Terziari (Alpi Orobie, Italia)<br />
Il settore <strong>del</strong>le Orobie posto a Sud <strong>del</strong>la Faglia <strong>di</strong> Clusone è caratterizzato<br />
dalla presenza <strong>di</strong> tre unità strutturali sovrapposte, messesi in posto in <strong>di</strong>fferenti<br />
fasi <strong>del</strong>l’orogenesi Alpina. Nella zona compresa tra Gan<strong>di</strong>no e Sovere (Bg), tali<br />
unità sono <strong>di</strong>slocate da faglie normali e sono intruse da numerosi corpi<br />
magmatici filoniani e subvulcanici (Stock <strong>di</strong> Gan<strong>di</strong>no). In particolare, è stata<br />
osservata la presenza <strong>di</strong> un sistema <strong>di</strong> faglie normali che taglia in modo<br />
evidente l’unità strutturalmente più alta. I corpi magmatici sono successivi alle<br />
principali fasi <strong>di</strong> sovrascorrimento <strong>ed</strong> in<strong>di</strong>cano quin<strong>di</strong> un’importante evento<br />
estensionale che interessa l’intera area analizzata. Su questi corpi si stanno<br />
realizzando nuove datazioni ra<strong>di</strong>ometriche con meto<strong>di</strong> <strong>di</strong>versi (U/Pb su zircone<br />
e analisi <strong>di</strong> tracce <strong>di</strong> fissione su apatite).<br />
In the eastern sector of the Orobic Alps the NE-SW tren<strong>di</strong>ng<br />
Clusone Fault forms the southern boundary of the Presolana<br />
antiformal stack and it represents the back-thrusting horizon of<br />
the Upper Triassic units over the Lower-Middle Triassic ones<br />
(ZANCHI et alii, 1990a). South of this faults the s<strong>ed</strong>imentary<br />
cover has been consider<strong>ed</strong>, for long time, poorly affect<strong>ed</strong> by<br />
severe shorterning, although more recent stratigraphic and<br />
tectonic stu<strong>di</strong>es (JADOUL et alii, 1991; BERRA et alii, 1991)<br />
demonstrat<strong>ed</strong> that <strong>di</strong>fferent thrust surfaces and strike-slip faults<br />
occur between En<strong>di</strong>ne and Iseo Lakes. West of Val Borlezza<br />
several andesitic <strong>di</strong>kes and small stocks are present. They<br />
consists of amphibole leucogabbro and andesites (DE MICHELE<br />
et alii, 1983; BECCALUVA et alii, 1983) mainly with plagioclase<br />
and hornblende phenocrystals. Near Gan<strong>di</strong>no a little quartz<strong>di</strong>orite<br />
stock also outcrops. Aim of our work is the<br />
reconstruction of the chronological evolution of this sector of<br />
the Orobic Alps. Field activity allow<strong>ed</strong> us to reconstruct the<br />
geometrical and structural setting of the area between Gan<strong>di</strong>no<br />
and Sovere and we are currently trying to achieve new<br />
ra<strong>di</strong>ometric ages of the above describ<strong>ed</strong> magmatic bo<strong>di</strong>es.<br />
In the study area three main tectonic units can be recogniz<strong>ed</strong><br />
(BERRA et alii, 1991). The M. Cornetto - Corna Lunga Unit is<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche e Geotecnologie, Università degli<br />
Stu<strong>di</strong> <strong>di</strong> Milano-Bicocca, Piazza <strong>del</strong>la Scienza, 4 – 20126 Milano, Italy.<br />
(**) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra “A. Desio”, Università degli Stu<strong>di</strong><br />
<strong>di</strong> Milano, via Mangiagalli, 34 – 20133 Milano, Italy.<br />
Paolo D’Adda: Tel. 02/64482063; p.dadda2@campus.unimib.it<br />
the structurally higher thrust sheet and it entirely consists of the<br />
Upper Carnian Castro Fm. The underlying Gan<strong>di</strong>no – Val<br />
Supine – Scanapà Unit is compos<strong>ed</strong> of Upper Carnian to<br />
Norian formations, laying on the Parautoctono s.s. which is the<br />
lowermost structural unit of the area. The intermiddl<strong>ed</strong> unit is<br />
partially detach<strong>ed</strong> from the Permian to Lower-Middle Triassic<br />
successions of Val Camonica (BERRA et alii, 1991) along the<br />
gypsum layers of the San Giovanni Bianco Formation.<br />
The contact aureola of the Gan<strong>di</strong>no Stock affects both the M.<br />
Cornetto - Corna Lunga Unit and the Gan<strong>di</strong>no – Val Supine –<br />
Scanapà Unit in<strong>di</strong>cating that they were already embricat<strong>ed</strong> at<br />
the time of the intrusion. During our field analysis we have<br />
identifi<strong>ed</strong> the presence of normal and dextral transtensional<br />
faults which cut the M. Cornetto - Corna Lunga Thrust sheet,<br />
forming an E-W elongat<strong>ed</strong> graben. The most important<br />
example of these normal faults is represent<strong>ed</strong> by the Val Piana<br />
– Dosso <strong>del</strong> Falò Fault (Fig. 1). These structures had never<br />
been describ<strong>ed</strong> before and they represent the evidence for<br />
regional extensional phenomena which were previously<br />
recognis<strong>ed</strong> through the study of mesoscopic fault populations<br />
(ZANCHI et alii, 1990b).<br />
Fig. 1 – Normal faults cutting the M. Cornetto – Corna Lunga Thrust .<br />
All the magmatic bo<strong>di</strong>es expos<strong>ed</strong> in the study area are<br />
interest<strong>ed</strong> by strong hydrothermal alteration, with partial<br />
substitution of hornblende by chlorite and widespread<br />
plagioclase alteration. Field analyses reveal<strong>ed</strong> that <strong>di</strong>kes follow<br />
the main normal and strike-slip faults and are not affect<strong>ed</strong>, in<br />
the study area, by compressional structures. This suggests that<br />
they were emplac<strong>ed</strong> after thrust stacking of the main describ<strong>ed</strong><br />
units, and, possibly, during the activation of normal and strikeslip<br />
faults. For this reason it is very important to better<br />
constrain the age of these subvolcanic bo<strong>di</strong>es since this would
allow to reconstruct the chronology of the deformational events<br />
that affect<strong>ed</strong> the Orobic Alps during the Alpine tectonics. K/Ar<br />
and Ar/Ar obtain<strong>ed</strong> by previous authors on hornblende and<br />
whole rock provide ages ranging between 55 My (ZANCHI et<br />
alii, 1990) and 35 My (FANTONI et alii, 1999). The large time<br />
interval shown by these age determinations poses some<br />
questions on the reliability of the results especially due to the<br />
strong hydrothermal alteration of most of the analyz<strong>ed</strong><br />
mineralogical phases. For this reason we are trying to obtain<br />
new ages bas<strong>ed</strong> on <strong>di</strong>fferent and more reliable minerals (U/Pb<br />
zircon dating and apatite fission tracks analysis).<br />
REFERENCES<br />
BECCALUVA L., BIGIOGGERO B., CHIESA S., COLOMBO A.,<br />
FANTI G., GATTO G.O., GREGNANIN A., MONTRASIO A.,<br />
PICCIRILLO E.M. & TUNESI A. (1983) - Post collisional<br />
orogenic dyke magmatism in the Alps. Mem. Soc. Geol. It.<br />
26, 341-359.<br />
BERSEZIO R. & FORNACIARI M. (1988) - Tectonic framework of<br />
the Lombardy foothills (Southern Alps), between Brianza<br />
and Lake Iseo. Rend. Soc. Geol. Ital. 11, 75-78.<br />
BERRA F., ROVELLINI M. & JADOUL F. (1991) – Structural<br />
framework of the Bergamasc Prealps south of the Clusone<br />
Fault. Atti Tic. Sc. Terra 34, 107-120.<br />
BOYER S.E. & ELLIOTT D. (1982) - Thrust systems. Am. Assoc.<br />
Pet. Geol. Bull. 66, 1196-1230.<br />
DE MICHELE V. & ZEZZA U. (1978) - Manifestazioni<br />
ipoabissali quarzo<strong>di</strong>oritiche <strong>di</strong> età Alpina nelle Prealpi<br />
Bergamasche (Alpi Meri<strong>di</strong>onali). Atti Soc. Ital. Sci. Nat.<br />
Museo Civ. Stor. Nat. Milano, 119, 181-210.<br />
DOGLIONI C. & BOSELLINI A. (1987) - Eoalpine and<br />
mesoalpine tectonics in the Southern Alps. Geol. Rund. 76,<br />
735-754, Struttgart.<br />
FANTONI R., BERSEZIO R., FORCELLA F., GORLA L., MOSCONI<br />
A. & PICOTTI V. (1999) - New dating of the Tertiary<br />
magmatic products of the central Southern Alps, bearings<br />
on the interpretation of the Alpine tectonic history. Mem.<br />
Sci. Geol. 51, 47-61.<br />
FANTONI R., BERSEZIO R. & FORCELLA F. (2004) – Alpine<br />
structure and deformation chronology at the Southern Alps<br />
– Po Plain border in Lombardy. Boll. Soc. Geol. It., 123,<br />
463-476.<br />
FORCELLA F. (1988) - Assetto strutturale <strong>del</strong>le Orobie orientali<br />
tra la Val Seriana e la Val Camonica. Rend. Soc. Geol. It.<br />
11, 269-278, Roma.<br />
FORCELLA F. & Jadoul F. (2000) - Carta Geologica <strong>del</strong>la<br />
Provincia <strong>di</strong> Bergamo.<br />
GALLAGHER K., BROWN R. & JOHNSON C. (1998) – Fission<br />
track analysis and its applications to geological problems.<br />
Annu. Rev. Earth Planet. Sci. 26, 519-572.<br />
STRUCTURAL SETTING OF THE GANDINO–SOVERE THRUST SYSTEM<br />
JADOUL F., BERRA F., FRISIA S., RICCIUTO T. & RONCHI P.<br />
(1991) - Stratigraphy, paleogeography and genetic mo<strong>del</strong><br />
of Late Carnian carbonate breccias (Castro Formation,<br />
Lombardy, Italy). Riv. It. Paleo. Str. 97, 355-392.<br />
LAUBSCHER H.P. (1985) - Large scale, thin-skinn<strong>ed</strong> thrusting<br />
in the southern Alps: kinematic mo<strong>del</strong>s. Geol. Soc. Am.<br />
Bull., 96: 710-718.<br />
SCHÖNBORN G. (1992b) - Alpine tectonics and kinematics<br />
mo<strong>del</strong>s of the central Southern Alps. Mem. Sc. Geol. 44:<br />
229-393, Padova.<br />
ZANCHI A. CHIESA S. & GILLOT P.Y. (1990a) - Tectonic<br />
evolution of the Southern Alps in the Orobic chain:<br />
structural and geochronological in<strong>di</strong>cations for pre-<br />
Tertiary compressive tectonics. Mem. Soc. Geol. It., 45: 77-<br />
82.<br />
ZANCHI A., CHINAGLIA N., CONTI M., DE TONI S., FERLIGA C.,<br />
TSEGAYE A., VALENTI L. & BOTTIN R. (1990b) - Analisi<br />
strutturale lungo il fronte <strong>del</strong>la Dolomia Principale in<br />
bassa Val Seriana (Bergamo). Mem. Soc. Geol. It. 45, 83-<br />
92.<br />
69
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 70-72, 3ff.<br />
Morphotectonics of the Lunigiana-Garfagnana Plio-Quaternary<br />
grabens (Northern Apennines).<br />
DI NACCIO D.(*), BONCIO P. (*), BROZZETTI F. (*), & PAZZAGLIA F.J. (**)<br />
ABSTRACT<br />
Morfotettonica dei bacini Plio-Quaternari <strong>del</strong>la Lunigiana-Garfagnana<br />
(Appennino Settentrionale)<br />
Vengono presentati i primi risultati <strong>di</strong> un’analisi morfotettonica nei bacini<br />
<strong>del</strong>la Lunigiana e Garfagnana finalizzata ad integrare prec<strong>ed</strong>enti stu<strong>di</strong> sulla<br />
geometria e cinematica <strong>del</strong>le faglie estensionali e a caratterizzare l’attività <strong>del</strong>le<br />
stesse nel tardo Quaternario.<br />
Entrambi i bacini sono graben asimmetrici che si sviluppano al tetto <strong>di</strong><br />
una faglia a basso angolo, interpretata come la terminazione settentrionale<br />
<strong>del</strong>l’Etrurian Fault System (Boncio et al., 2000).<br />
In<strong>di</strong>ci topografici quali profili longitu<strong>di</strong>nali dei fiumi, in<strong>di</strong>ce <strong>del</strong>la<br />
concavità dei fiumi, in<strong>di</strong>ce SL, swath profile vengono utilizzati per<br />
quantificare gli effetti <strong>del</strong>la tettonica attiva sulle faglie bor<strong>di</strong>ere i bacini.<br />
Key words: channel concavity, low angle normal fault, swath<br />
profile, knickpoint.<br />
INTRODUCTION<br />
We report first results of structural and morphotectonic<br />
analyses aim<strong>ed</strong> to characterize active and potentially<br />
seismogenic faults in the Lunigiana and Garfagnana basins.<br />
In the area, the geometry of the extensional fault systems is<br />
well ascertain<strong>ed</strong> on the surface and at depth (Artoni et al.,<br />
1992, Carmignani et al., 2000, Camurri et al., 2001,<br />
Carmignani et al., 2001, Bernini et al., 2002, Arganani et al.,<br />
2003, Brozzetti et al., 2007). Nevertheless, the main<br />
seismogenic faults, their degree of activity, the associat<strong>ed</strong> slip<br />
rates, and the maximum seismogenic potential remain largely<br />
undetermin<strong>ed</strong>. Furthermore, lack of reliable instrumental<br />
seismological data on large earthquakes, very low deformation<br />
rates, and poor exposures in the Plio-Quaternary s<strong>ed</strong>iments,<br />
make the identification of active and possibly seismogenic<br />
faults problematic.<br />
Topographic metrics inclu<strong>di</strong>ng drainage patterns, river long<br />
profiles, in<strong>di</strong>ces of channel concavity, stream-length gra<strong>di</strong>ent of<br />
mo<strong>del</strong>l<strong>ed</strong> long profiles, steepness and swath profiles are us<strong>ed</strong> to<br />
better constrain the effects of active faulting.<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università G. D’Annunzio,<br />
Pescara-Chieti.<br />
(**)Department of Earth and Environmental Sciences, Lehigh University,<br />
Bethlehem, PA-USA<br />
Di Naccio D.: e-mail d.<strong>di</strong>naccio@unich.it<br />
REGIONAL TECTONIC SETTING OF THE<br />
LUNIGIANA AND GARFAGNANA GRABENS<br />
The Lunigiana and Garfagnana basins are Plio-Quaternary<br />
asymmetric grabens that lie in the hanging wall of a regional,<br />
east-<strong>di</strong>pping, low-angle dethachment (LANF) that is imag<strong>ed</strong> in<br />
seismic profiles deepening beneath the Apennines down to a<br />
depth of ~ 13 km (Fig 1). Seismic lines highlight E-<strong>di</strong>pping<br />
synthetic faults <strong>di</strong>pping between 30° and 60° down to a depth<br />
of ~ 5 km before soling onto the low angle detachment. In<br />
contrast, W-<strong>di</strong>pping antithetic faults show on average higher<br />
<strong>di</strong>p angles of ~ 50° to ~ 70° and root on the detachment at<br />
depths of less than 5 km.<br />
The E-<strong>di</strong>pping faults with strongest geomorphic signature of<br />
Late Quaternary activity are the Mulazzo and Olivola - Soliera<br />
faults in the Lunigiana and the Casciana - Sillicano and<br />
Bolognana - Gioviana faults in the Garfagnana. The most<br />
important W-<strong>di</strong>pping splays in terms of late Quaternary activity<br />
are the Groppodalosio and Compione - Comano faults in the<br />
Lunigiana and the M. Prato - Colle Uccelliera alignment in the<br />
Garfagnana. The Tendola - Equi Terme-Gramolazzo alignment<br />
is a high-angle E-W right-lateral transfer zone. This alignment,<br />
evident in aerial photographs and in seismic lines, is interpret<strong>ed</strong><br />
as a transfer zone of active extension between the Lunigiana<br />
and Garfagnana grabens.<br />
The background seismicity (0.4M4.2) of the Lunigiana-<br />
Garfagnana, record<strong>ed</strong> by a local network (RSLG) for the 1999-<br />
2006 time interval and accurately locat<strong>ed</strong> (Brozzetti et al.,<br />
2007, Tinari et al., 2007) confirms that probably both the<br />
conjugate E- and W-<strong>di</strong>pping fault sets are active and<br />
seismogenic even if their seismogenic potential is still not<br />
completely ascertain<strong>ed</strong>. The subsurface reconstruction of the<br />
area, carri<strong>ed</strong> out by means of seismic lines, shows that the foci<br />
concentrate in clasters and girdless which well fit with the<br />
deepest part of the Lunigiana detachment and with the plane of<br />
its antithetical fault (e.g. Groppodalsio Fault).<br />
The analys<strong>ed</strong> seismicity is extensional or transtensional and<br />
is concentrat<strong>ed</strong> E of the detachment breackaway and<br />
progressively deepens to the E from
MORPHOTECTONICS OF THE LUNIGIANA-GARFAGNANA PLIO-QUATERNARY GRABENS<br />
Fig. 1 – schematic structural map of the Lunigiana-Garfagnana grabens<br />
along an E-<strong>di</strong>pping low-angle narrow band showing a mean <strong>di</strong>p<br />
angle of nearly 30°, consistently with the geometry of the<br />
detachment.<br />
MODELING THE STEADY-STATE RIVER PROFILE<br />
The relationships between the faults and watersh<strong>ed</strong>-scale<br />
geomorphology have been quantifi<strong>ed</strong> using a 10 m <strong>di</strong>gital<br />
topography to extract channel and basin metrics.<br />
The concavity, steepness and length-gra<strong>di</strong>ent index of<br />
mo<strong>del</strong>l<strong>ed</strong> river longitu<strong>di</strong>nal profiles prov<strong>ed</strong> to be the most<br />
useful metrics for recor<strong>di</strong>ng the effects of locally active faults.<br />
Alluvial rivers typically shows concave-up long profiles,<br />
referr<strong>ed</strong> to a grad<strong>ed</strong>, equilibrium profile. Deviations from a<br />
smooth, concave-up form may in<strong>di</strong>cate that the fluvial system is<br />
in a transient state of adjustment to a base level due to tectonic,<br />
climatic, or rock-type perturbation.<br />
In particular, convex segments call<strong>ed</strong> knickpoints can be<br />
specifically investigat<strong>ed</strong> to evaluate their coincidence with<br />
tectonic perturbations.<br />
Analysis of the longitu<strong>di</strong>nal profile of the main channels in<br />
the Lunigiana (12 watersh<strong>ed</strong>s) and Garfagnana (19 watersh<strong>ed</strong>s)<br />
basins, have been accompani<strong>ed</strong> by the well known power law<br />
relationship between channel gra<strong>di</strong>ent and upstream drainage<br />
area<br />
S = ksA - (1)<br />
In equation (1) long profile concavity () and the profile<br />
steepness (ks) are the slope and y-intercept respectively of a<br />
line regress<strong>ed</strong> through a log-log plot of channel slope and<br />
drainage basin area (Fig 2).<br />
The average threshold area value was us<strong>ed</strong> to extract the<br />
stream network and therefore define the first order streams.<br />
The spatial <strong>di</strong>stribution and potential origin of knickpoints<br />
are highlight<strong>ed</strong> by the stream-gra<strong>di</strong>ent (SL) index (Hack.,<br />
1973):<br />
SL = (H/L)/L (2)<br />
where H is elevation, L is length of stream reach, and L<br />
is the total stream length measur<strong>ed</strong> from the <strong>di</strong>vide to the<br />
midpoint of the stream reach under investigation.<br />
Normalizing with respect to L allows for <strong>di</strong>rect comparisons<br />
between small drainages that typically have steep long profiles<br />
and larger drainages that have more gentle long profiles.<br />
Low order streams (extract<strong>ed</strong> accor<strong>di</strong>ng to Strahler<br />
ordering) are us<strong>ed</strong> to calculate the SL index because they best<br />
reflect tectonically controll<strong>ed</strong> channel gra<strong>di</strong>ent variations in the<br />
Fig. 2 – Example of steady-state river profile analysis<br />
landscape (Merritts & Vincent,1989).<br />
SL-countour plot maps show that the Lunigiana (Fig 3) and<br />
Garfagnana grabens are presently characteris<strong>ed</strong> by a significant<br />
71
72 D. DI NACCIO ET ALII<br />
degree of tectonic activity. In spite of this general consideration<br />
the comput<strong>ed</strong> values do not allow to <strong>di</strong>scern what structure is<br />
characteris<strong>ed</strong> by the maximum deformation rate.<br />
The active splay of the W-<strong>di</strong>pping Groppodalosio fault<br />
entirely develop<strong>ed</strong> in the Macigno (Fig 1) causes high<br />
anomalies on the SL values (>250) with knickpoints nearly<br />
parallel to the mean fault strike. Similar consideration can be<br />
done for the E-<strong>di</strong>pping M. Carmuschia and Mulazzo faults.<br />
Relatively low SL index value have been found along the<br />
Olivola fault and they can be most likely relat<strong>ed</strong> to soft<br />
lithology compar<strong>ed</strong> at the scale of whole graben. Nevertheless<br />
the drainage network, observ<strong>ed</strong> at the Olivola basin is not<br />
Fig. 3 – SL countour plot map of Lunigiana graben. (G-F: Groppodalosio<br />
fault; C-F Compione-Comano f.; Mu-F: Mulazzo f.; Ol-F: Olivola f.; Ca-F:<br />
Carmuschia f.; P-F: Picchiara f.<br />
dendritic, in fact straight streams, characteris<strong>ed</strong> by Holocene<br />
flood<strong>ed</strong> valleys, flow towards a main fault-parallel river.<br />
This evidence together with the offset of Late Pliocene and<br />
Early Pleistocene continental deposits supports Middle<br />
Pleistocene - to - late Quaternary activity of the Olivola normal<br />
fault.<br />
In the Garfagnana basin most of the anomaly values are<br />
influenc<strong>ed</strong> by lithology. Nevertheless the C. Uccelliera and the<br />
Casciana-Sillicana faults develop<strong>ed</strong> in the Macigno Unit show<br />
high SL values.<br />
CONCLUSIONS<br />
The emerging seismotectonic picture suggests that the lowangle<br />
normal fault system of the Lunigiana-Garfagnana grabens<br />
is presently active and is likely responsible for the historical<br />
seismicity of the area, inclu<strong>di</strong>ng the large (M~6.5) 1920<br />
earthquake.<br />
Computation of geomorphic in<strong>di</strong>ces is in good agreement<br />
with the seismological data, suggesting that historic<br />
earthquakes in the area can be associat<strong>ed</strong> with the mapp<strong>ed</strong><br />
faults that are further capable of generating surface ruptures<br />
and contributing to the production of local relief.<br />
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Normal Fault zone in Central Italy. Tectonics, 19, 1038-<br />
1055.<br />
BROZZETTI F., BONCIO P., TINARI D.P., DI NACCIO D. &<br />
TORELLI L. (2007) - LANFs attive e relativi meccanismi <strong>di</strong><br />
trasferimento alla terminazione settentrionale <strong>del</strong>l’Etrurian<br />
Fault System (Lunigiana-Garfagnana, Italia). Rend. Soc.<br />
Geol. It., 4, 164-165<br />
CAMURRI F., ARGNANI A., BERNINI M., PAPANI G., ROGLEDI S.<br />
& TORELLI L. (2001) - The basement of the NW Apennines:<br />
Interpretation of reflection seismics and geodynamic<br />
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Scienze <strong>del</strong>la Terra, Chieti 5-8 Settembre 2001, Fist<br />
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CARMIGNANI L., CONTI P., DISPERATI L., FANTOZZI P.L.,GIGLIA<br />
G., MECCHERI M. (2000) – Carta geologica <strong>del</strong> parco <strong>del</strong>le<br />
Alpi Apuane. 1:50000. Ed. Parco Regionale <strong>del</strong>le Alpi<br />
Apuane, SELCA, Firenze, Italia.<br />
CARMIGNANI L., DECANDIA F.A., DISPERATI L., FANTOZZI P.L.,<br />
KLIGFIELD R., LAZZAROTTO A., LIOTTA, D., MECCHERI M.<br />
(2001) – Inner Northern Apennines. In Vai G.B., Martini<br />
I.P. (<strong>ed</strong>s), Anatomy of an Orogen: The Apennines and<br />
Adjacent M<strong>ed</strong>iterranean Basins. Kluwer Academic<br />
Publishers, 197-214<br />
HACK J.T. (1973) – Stream analysis and stream-gra<strong>di</strong>ent<br />
index. Survey. J. Research., 1, (4), 421-429.<br />
MERRITTS D. & VINCENT K.R. (1989) – Geomorphic response<br />
of coastal to low, interm<strong>ed</strong>iate, and high rates of uplift,<br />
Mendocino triple junction region, northern California.<br />
Geol. Soc. Am. Bull., 101, 1373-1388.<br />
TINARI P.T., BONCIO P., LAVECCHIA G., BROZZETTI F., EVA E.,<br />
SOLARINO S., SCAFIDI D., TURINO C., TORELLI L., BERNINI<br />
M. & VESCOVI P. (2007) – Sismotettonica <strong>del</strong>la Lunigiana e<br />
Garfagnana: nuovi vincoli <strong>del</strong>l’analisi integrata <strong>di</strong> dati<br />
sismologici strumentali e dati geologici <strong>di</strong> sottosuolo. Rend.<br />
Soc. Geol. It., 4, 301-302
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 73-75, 3 ff.<br />
Frictional properties of mantle rocks during earthquakes.<br />
GIULIO DI TORO (*,**), P. DEL GAUDIO (*), R. HAN (***), T. HIROSE (****), S. NIELSEN (*), T. SHIMAMOTO<br />
(***), A. CAVALLO (*)<br />
RIASSUNTO<br />
Proprietà frizionali <strong>di</strong> rocce <strong>di</strong> mantello durante terremoti<br />
L’evoluzione <strong>del</strong>la resistenza <strong>di</strong> attrito su una superficie <strong>di</strong> faglia per<br />
velocità <strong>di</strong> scivolamento sismiche (circa 1 m/s) è uno dei parametri<br />
fondamentali che controlla la meccanica <strong>di</strong> un terremoto. In particolare, la<br />
fusione per attrito durante terremoti interm<strong>ed</strong>i e profon<strong>di</strong> (profon<strong>di</strong>tà > 60 km,<br />
all’interno <strong>del</strong> mantello terrestre) dovrebbe essere un processo frequente<br />
secondo esperimenti <strong>di</strong> torsione effettuati ad elevate pressioni <strong>di</strong> confinamento<br />
e stu<strong>di</strong> teorici e <strong>di</strong> terreno. Per esempio, alcuni sismologi hanno invocato la<br />
produzione <strong>di</strong> fusi sismici durante il terremoto Mw 8.3 <strong>del</strong>la Bolivia <strong>del</strong> 1994,<br />
enucleato ad una profon<strong>di</strong>tà <strong>di</strong> 600 km, per giustificare la bassa efficienza<br />
sismica <strong>di</strong> quel terremoto.<br />
Per stu<strong>di</strong>are le proprietà meccaniche (reologiche) dei fusi <strong>di</strong> frizione e la<br />
resistenza dei piani <strong>di</strong> faglia durante rotture sismiche in rocce <strong>di</strong> mantello,<br />
sono stati effettuati una serie <strong>di</strong> esperimenti con le peridotiti (tipiche rocce <strong>di</strong><br />
mantello) <strong>di</strong> Balmuccia (Zona <strong>di</strong> Ivrea, Italia). I provini <strong>di</strong> roccia dal <strong>di</strong>ametro<br />
<strong>di</strong> 21.8 mm, sono stati sottoposti a sforzi normali <strong>di</strong> 5.4-16.1 MPa, velocità <strong>di</strong><br />
scivolamento (sismiche) <strong>di</strong> 0.23-1.14 m/s e rigetti compresi tra 1.5 e 71 m.<br />
Durante ogni esperimento si è osservata una complessa evoluzione <strong>del</strong>la<br />
resistenza <strong>di</strong> attrito con il rigetto consistente con l’evoluzione dei prodotti <strong>di</strong><br />
faglia (da aggregati granulari a fusi <strong>di</strong> frizione). In particolare, la formazione <strong>di</strong><br />
un livello continuo <strong>di</strong> fuso sulla superficie <strong>di</strong> scivolamento, che avviene nei<br />
primi secon<strong>di</strong> dall’inzio <strong>del</strong>la prova, comporta la lubrificazione <strong>del</strong>la faglia<br />
sperimentale e il raggiungimento <strong>di</strong> resistenze per attrito circa nulle.<br />
L’estrapolazione <strong>di</strong> questi risultati sperimentali alle con<strong>di</strong>zioni <strong>di</strong> mantello<br />
suggerisce che le cadute <strong>di</strong> sforzo cosismiche (dynamic stress drops) durante<br />
terremoti profon<strong>di</strong> sono estremamente gran<strong>di</strong>, favorendo la propagazione <strong>del</strong>la<br />
rottura e la generazione <strong>di</strong> gran<strong>di</strong> terremoti.<br />
Key words: Earthquake physics, Friction Experiments,<br />
Frictional Melting, Microstructures, Peridotite,<br />
Pseudotachylyte.<br />
_________________________<br />
(*) Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia, Via <strong>di</strong> Vigna Murata<br />
605, 00143, Roma, Italy.<br />
(**) Dipartimento <strong>di</strong> Geoscienze, Università <strong>di</strong> Padova, Via Giotto 1,<br />
35137 Padova, Italy.<br />
(***) Department of Earth and Planetary Systems Science, Hiroshima<br />
University, Higashi-Hiroshima 739-8526, Japan.<br />
(****) Kochi Institute for Core Sample Research, JAMSTEC, 200<br />
Monobe-otsu Nankoku, Kochi, 783-8502, Japan .<br />
G. Di Toro expenses are cover<strong>ed</strong> by The European Research Council<br />
Starting Grant No. 205175.<br />
The evolution of the frictional strength along a fault at<br />
seismic slip rates (about 1 m/s) is one of the main factors<br />
controlling earthquake mechanics (KANAMORI & BRODSKY,<br />
2004; DI TORO et alii, 2006). In particular, friction-induc<strong>ed</strong><br />
rock melting and melt lubrication during seismic slip may be<br />
typical at mantle depths, bas<strong>ed</strong> on field stu<strong>di</strong>es (UEDA et alii,<br />
2008; ANDERSEN & AUSTRHEIM, 2006; PICCARDO et alii,<br />
2008), seismological evidence (KANAMORI et alii, 1998;<br />
Fig. 1 – Esperimento tipo. Per simulare con<strong>di</strong>zioni mantelliche caratterizzate<br />
da bassa fugacità <strong>di</strong> ossigeno, questo esperimento (HVR651) è stato effettuato<br />
sotto un flusso <strong>di</strong> Argon. L’evoluzione <strong>del</strong>la resistenza per attrito con il rigetto<br />
è complessa: dopo un primo picco (corrispondente ad un coefficiente <strong>di</strong><br />
attrito <strong>di</strong> circa 0.7, tipico per queste rocce), l’attrito decresce per poi risalire<br />
verso un secondo picco e quin<strong>di</strong> <strong>di</strong>minuire nuovamente fino a raggiungere un<br />
valore stabile (steady-state).<br />
Fig. 1 - Typical experiment. To simulate low oxygen fugacity mantle<br />
con<strong>di</strong>tions, this experiment (HVR651) was perform<strong>ed</strong> under Argon flux. The<br />
evolution of the shear stress with slip is complex: after a first peak in friction<br />
(which corresponds to a coefficient of friction of about 0.7, typical value for<br />
peridotite), friction decreases and then increases again towards a second<br />
peak. Eventually, shear stress evolves towards steady-state.<br />
BOUCHON & IHMLÉ, 1999), torsion experiments (BRIDGMAN,<br />
1936) and theoretical stu<strong>di</strong>es (GRIGGS & HANDIN, 1960;<br />
KELEMAN & HIRTH, 2007). To investigate the (1) dynamic<br />
strength of faults and (2) the frictional melting processes in<br />
mantle rocks, we perform<strong>ed</strong> 20 experiments with peridotite in a<br />
high-velocity rotary shear apparatus. The peridotite is a subcontinental<br />
lherzolite from Balmuccia (Ivrea Zone, Italy,<br />
RIVALENTI et alii, 1981). Experiments were conduct<strong>ed</strong> on
74 G. DI TORO ET ALII<br />
Fig. 2 – Evoluzione <strong>del</strong>le microstrutture con il rigetto. Esperimenti interrotti<br />
a rigetti crescenti a parità <strong>di</strong> sforzo normale (13 MPa) e velocità <strong>di</strong><br />
scivolamento (1.2 m/s), mostrano l’evoluzione <strong>del</strong> materiale <strong>di</strong> faglia con<br />
durante le prove. La complessa fase iniziale <strong>del</strong>la resistenza <strong>di</strong> taglio<br />
corrisponde alla produzione <strong>di</strong> aggregati granulari. La <strong>di</strong>minuzione graduale<br />
verso le con<strong>di</strong>zioni stazionarie corrisponde alla produzione <strong>di</strong> un livello<br />
continuo <strong>di</strong> fuso <strong>di</strong> frizione.<br />
Fig. 2 - Microstructural evolution of the slipping zone with <strong>di</strong>splacement.<br />
Experiments perform<strong>ed</strong> at identical normal stress (13 MPa) and slip rate<br />
(1.2 m/s) but interrupt<strong>ed</strong> at increasing <strong>di</strong>splacements, reveal<strong>ed</strong> that second<br />
strengthening was associat<strong>ed</strong> with the production of a highly viscous grainsupport<strong>ed</strong><br />
melt-poor layer, while second weakening and steady-state with<br />
the formation of a continuous melt-rich layer.<br />
cylindrical samples (21.8 mm in <strong>di</strong>ameter) over a wide range of<br />
normal stresses (5.4 to 16.1 MPa), slip rates (0.23 to 1.14 m/s)<br />
and <strong>di</strong>splacements (1.5 to 71 m).<br />
The dynamic strength of experimental faults evolv<strong>ed</strong> with<br />
<strong>di</strong>splacement (Fig. 1): after a first peak (first strengthening) at<br />
Fig. 3 – Microstrutture sperimentali (sinistra) e naturali (destra). Le<br />
pseudotachiliti prodotte negli esperimenti riportati in questo stu<strong>di</strong>o sono<br />
molto simili alle pseudotachliti naturali <strong>di</strong> Balmuccia (Obata & Karato,<br />
1995). Questa somiglianza, unitamente a considerazioni <strong>di</strong> carattere teorico,<br />
consente <strong>di</strong> estrapolare i risultati sperimentali a con<strong>di</strong>zioni naturali.<br />
Fig. 3 - Experimental (left) and natural (right) microstructures. Artificial<br />
pseudotachylytes produc<strong>ed</strong> in the experiments here describ<strong>ed</strong> are very<br />
similar to natural ones from Balmuccia (Obata & Karato, 1995). Textural<br />
similarity and theoretical considerations, allow us to extrapolate<br />
experimental results to natural con<strong>di</strong>tions.<br />
the initiation of slip, fault strength abruptly decreas<strong>ed</strong> (first<br />
weakening), then increas<strong>ed</strong> (second strengthening) to a second<br />
peak and eventually decreas<strong>ed</strong> (second weakening) towards a<br />
steady-state value.<br />
The microstructural and geochemical (FE-SEM, EPMA and<br />
EDS) investigation of the slipping zone from experiments<br />
interrupt<strong>ed</strong> at <strong>di</strong>fferent <strong>di</strong>splacements reveal<strong>ed</strong> that second<br />
strengthening was associat<strong>ed</strong> with the production of a highly<br />
viscous grain-support<strong>ed</strong> melt-poor layer, while second<br />
weakening and steady-state with the formation of a continuous<br />
melt-rich layer (Fig. 2).<br />
The temperature of the frictional melt estimat<strong>ed</strong> from the<br />
composition of microlites of olivine nucleat<strong>ed</strong> in the melt was<br />
up to 1780 o C. Microstructures form<strong>ed</strong> during the experiments<br />
were identical to those found in natural ultramafic<br />
pseudotachylytes (Fig. 3).<br />
By performing experiments for increasing normal stresses<br />
and slip rates, steady-state shear stress slightly increas<strong>ed</strong> with<br />
increasing normal stress (friction coefficient of 0.15) and, for a<br />
given normal stress, decreas<strong>ed</strong> with increasing slip rate. The<br />
dependence of steady-state shear stress with normal stress and<br />
slip rate is describ<strong>ed</strong> by a constitutive equation for melt<br />
lubrication (NIELSEN et alii, 2008):<br />
τ<br />
ss<br />
∝ σ<br />
⋅<br />
2 5.0<br />
n<br />
log( 2V<br />
/ W )<br />
V / W<br />
where τss is steady state shear stress, σn is normal stress, V is<br />
slip rate and W is a parameter which includes melt viscosity.<br />
The presence of microstructures similar to those found in<br />
natural pseudotachylytes and the determination of a constitutive<br />
equation that describes the experimental data, might allow to<br />
extrapolate the experimental observations to natural con<strong>di</strong>tions<br />
and to the study of rupture dynamics in mantle rocks. In<br />
particular, our experimental data suggest that faults are<br />
lubricat<strong>ed</strong> by frictional melts during mantle earthquakes.<br />
REFERENCES<br />
ANDERSEN T.B. & AUSTRHEIM H. (2006) - Fossil earthquakes<br />
record<strong>ed</strong> by pseudotachylytes in mantle peridotite from the<br />
Alpine subduction complex of Corsica. Earth Planet. Sci.<br />
Lett., 242, 58-72.<br />
BOUCHON M. & IHMLÉ (1999) - Stress drop and frictional<br />
heating during the 1994 deep Bolivia earthquake. Geophys.<br />
Res. Lett., 26 (23), 3521-3524.<br />
BRIDGMAN P.W. (1936) - Shearing phenomena at high<br />
pressure of possible importance for geology. J. Geol., 44,<br />
653-669.<br />
DI TORO G., HIROSE T., NIELSEN S., PENNACCHIONI G. &<br />
SHIMAMOTO T. (2006) - Natural and experimental evidence<br />
of melt lubrication of faults during earthquakes. Science,<br />
311, 647-649.
GRIGGS D. & HANDIN J. (1960), Observations on fracture and<br />
a hypothesis of earthquakes. Geol. Soc. of Amer. Mem., 79,<br />
343-373.<br />
KANAMORI H., ANDERSON D.L. & HEATON T.H. (1998) -<br />
Frictional melting during the rupture of the 1994 Bolivian<br />
Earthquake. Science, 279, 839-842.<br />
KANAMORI H. & BRODSKY, E. (2004) - The physics of<br />
earthquakes. Rep. Prog. Phys., 67, 1429–1496.<br />
KELEMEN P.B. & HIRTH G. (2007) - A perio<strong>di</strong>c shear-heating<br />
mechanism for interm<strong>ed</strong>iate-depth earthquakes in the<br />
mantle. Nature, 446, 787–790.<br />
NIELSEN S., DI TORO G., HIROSE T. & SHIMAMOTO T. (2008) -<br />
Frictional melt and seismic slip. J. Geophys. Res., 113,<br />
doi:10.1029/2007JB005122.<br />
OBATA M. & KARATO S. (1995) - Ultramafic Pseudotachylite<br />
from the Balmuccia Peridotite, Ivrea Verbano Zone,<br />
Northern Italy. Tectonophysics, 242, 313-328.<br />
PICCARDO B.G., RANALLI G., MARASCO M. & PADOVANO M.<br />
(2008) - Ultramafic pseudotachylytes in the Mt. Moncuni<br />
peridotite (Lanzo Massif, western Alps): tectonic evolution<br />
and upper mantle seismicity. Period. <strong>di</strong> Mineral., 76, 181-<br />
197.<br />
RIVALENTI, G., GARUTI G., ROSSI A., SIENA F. & SINIGOI S.<br />
(1981) – Existence of <strong>di</strong>fferent peridotite types ans of a<br />
layerd igneous complex in the Ivrea zone of the Western<br />
Alps. J. Petrol., 22, 127-153.<br />
UEDA T., OBATA M., DI TORO G., KANAGAWA K. & OZAWA K.<br />
(2008) - Mantle earthquakes frozen in mylonitiz<strong>ed</strong><br />
ultramafic pseudotachylytes of spinel-lherzolite facies.<br />
Geology, 36 (8), 607-610.<br />
FRICTIONAL PROPERTIES OF MANTLE ROCKS DURING EARTHQUAKES<br />
75
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 76-79, 9 ff.<br />
ABSTRACT<br />
Altopiano Ragusano (Sicilia sud-orientale): note <strong>di</strong> geologia strutturale.<br />
In this paper is describ<strong>ed</strong> some structural notes about the tectonic<br />
evolution of the South-Western sector of Hyblean Plateau, call<strong>ed</strong> Ragusan<br />
Platform (South-East Sicily).<br />
The Hyblean Plateau was consider<strong>ed</strong> slightly deform<strong>ed</strong> since few years<br />
ago. New data and field research in this area highlight the deformation of the<br />
Ragusan Platform probably due to the subduction-collisional regime of the<br />
European and African margins.<br />
Evidences in field and their interpretation here propos<strong>ed</strong>.<br />
Key words: bou<strong>di</strong>nage, deformazione, Piattaforma Ragusana,<br />
tettonica d'inversione.<br />
INTRODUZIONE<br />
La nota rappresenta un breve sunto riguardo i rilievi<br />
strutturali condotti dagli autori nell'Altopiano Ragusano -<br />
settore centro-meri<strong>di</strong>onale <strong>del</strong> Plateau Ibleo (Sicilia sudorientale).<br />
Il lavoro vuole evidenziare alcuni aspetti tettonici e<br />
deformativi sinora considerati poco significativi nell'area, legati<br />
ad eventi compressionali riconducibili, con ogni probabilità,<br />
all'avanzamento <strong>del</strong>la catena Appennino-Maghrebide sul<br />
margine settentrionale <strong>del</strong> Plateau Ibleo.<br />
Le deformazioni osservate evidenziano la necessità <strong>di</strong><br />
riconsiderare il ruolo <strong>di</strong> questa porzione <strong>di</strong> avampaese nel<br />
quadro tettonico regionale.<br />
GEOLOGIA DELL'AREA<br />
L'Altopiano ibleo è costituito da una potente successione<br />
carbonatica, <strong>di</strong> età meso-cenozoica, la cui stratigrafia è ben<br />
conosciuta grazie alle numerose trivellazioni petrolifere<br />
effettuate negli anni ‘50-‘70 <strong>del</strong> secolo scorso (Kafka &<br />
Kirkbride 1960, Casero P. 2004). In affioramento le<br />
successioni maggiormente presenti sono date dalle alternanze<br />
calcareo-marnoso-argillose eoceniche-mioceniche <strong>del</strong>le F.ni<br />
_________________________<br />
Altopiano Ragusano (Sicilia sud-orientale): note <strong>di</strong> geologia<br />
strutturale<br />
(*) Verticalia S.a.s., Ragusa - www.verticalia.info<br />
MARIO DIPASQUALE (*) & ROSARIO OCCHIPINTI (*)<br />
Fig. 1 – Schema strutturale semplificato <strong>del</strong>l'area (sopra). 1: depositi<br />
quaternari; 2: F.ne Ragusa; 3: F.ne Tellaro; a: avanfossa <strong>di</strong> Gela; b: sistema <strong>di</strong><br />
faglie Comiso-Chiaramonte; c: sistema Marina <strong>di</strong> Ragusa; d: sistema Scicli-<br />
Ragusa (da Barrier, 1992, mod.). Schema stratigrafico successioni affioranti<br />
(sotto).<br />
Amerillo, Ragusa e Tellaro (Fig. 1).<br />
Tali formazioni sono affioranti <strong>di</strong>ffusamente nella parte<br />
centro meri<strong>di</strong>onale <strong>del</strong>l’altopiano ragusano, per poi essere<br />
ricoperti verso ovest, in corrispondenza <strong>del</strong>l'avanfossa Gela-<br />
Catania, dalle coperture quaternarie che costituiscono la falda<br />
<strong>di</strong> Gela.<br />
TETTONICA<br />
La piattaforma ragusana è stata considerata quasi<br />
esclusivamente interessata da tettonica <strong>di</strong>stensiva: faglie<br />
inverse, trascorrenti e pieghe sono state generalmente relegate a<br />
strutture secondarie, associate a deboli rigetti, o circoscritte a<br />
zone limitate (Rigo e Cortesini, 1961; Mascle, 1974; Di Grande<br />
e Grasso, 1977; Grasso et al., 1979, 1986; Ghisetti e Vezzani,<br />
1980, 1981; Cristofolini et al., 1985; Barrier, 1992).<br />
Le strutture principali hanno orientazione NE-SW e NNE-<br />
SSW, tra queste si ricordano i seguenti sistemi <strong>di</strong> faglie:<br />
Comiso-Chiaramonte, Marina <strong>di</strong> Ragusa e Scicli-Ragusa (Fig.<br />
1). L’interazione <strong>di</strong> tali sistemi ha determinato la formazione <strong>di</strong><br />
numerosi horst e graben che da rilievi <strong>di</strong> dettaglio più recenti
ALTOPIANO RAGUSANO (SICILIA SUD-ORIENTALE): NOTE DI GEOLOGIA STRUTTURALE<br />
Fig. 2 – Piega coricata nei calcari <strong>del</strong> M.bo Leonardo sotto la <strong>di</strong>scarica <strong>del</strong>le<br />
miniere <strong>di</strong> roccia asfaltica (foto storica, 1920 ca.).<br />
mostrano evidenze <strong>di</strong> movimenti misti, principalmente<br />
<strong>di</strong>stensivi-trascorrenti.<br />
A grande scala, inoltre, numerose pieghe mo<strong>del</strong>lano la<br />
piattaforma carbonatica.<br />
L'orientazione degli assi varia generalmente da NNE-SSW a<br />
N-S (Kafka & Kirkbride, 1959; Ghisetti & Vezzani, 1980;<br />
Grasso & Reuther, 1988) e solo da alcuni Autori sono state<br />
interpretate come probabili manifestazioni superficiali <strong>di</strong><br />
vecchie faglie normali riattivate come thrusts, ramps o pieghe<br />
(Barrier, 1992).<br />
STUDI PRECEDENTI<br />
Negli anni venti, le strutture tettoniche osservate all'interno<br />
<strong>del</strong> bacino asfaltifero <strong>del</strong>la valle <strong>del</strong>l'Irminio (fig. 2)<br />
consentirono <strong>di</strong> ipotizzare la presenza <strong>di</strong> un giacimento<br />
petrolifero profondo (J. Elmer Thomas, Gulf Italy).<br />
L'analisi microtettonica nell'area condotta da AA in stu<strong>di</strong><br />
prec<strong>ed</strong>enti, ha permesso <strong>di</strong> determinare le principali <strong>di</strong>rezioni<br />
<strong>di</strong> stress associate alle possibili fasi tettoniche che hanno<br />
interessato le successioni affioranti.<br />
Ghisetti & Vezzani (1980), in particolare, <strong>di</strong>stinguono tre<br />
eventi compressivi, con 1 orientati rispettivamente N35°-<br />
65°E, N120°-130°E e N20°E.<br />
Boccaletti et alii (1984) <strong>di</strong>stingue, anch’egli, tre eventi<br />
compressivi; una prima fase compressiva <strong>di</strong> età<br />
m<strong>ed</strong>iopliocenica orientata ca. N 120-130E, una seconda con 1<br />
compreso tra N10 e N 50 E, databile al Pleistocene m<strong>ed</strong>io <strong>ed</strong>,<br />
infine, una terza con <strong>di</strong>rezioni preferenziali comprese tra NNE-<br />
SSW e NE-SW è ricondotta, dall’autore, ad un arco <strong>di</strong> tempo<br />
compreso tra il Pleistocene inferiore e l’attuale.<br />
77<br />
Barrier (1992) <strong>di</strong>stingue pure tre <strong>di</strong>versi eventi tettonici<br />
compressivi nell'area, tutti avvenuti durante il generale periodo<br />
<strong>di</strong>stensivo Plio-Pleistocenico. Tali eventi sono così orientati:<br />
20°-30°, 60°-80° e 100°-110°.<br />
Lo stesso Autore riporta che quest'ultimo evento<br />
compressivo, databile al Pliocene m<strong>ed</strong>io, è stato registrato in<br />
<strong>di</strong>verse aree <strong>del</strong> M<strong>ed</strong>iterraneo, quali Malta, Lamp<strong>ed</strong>usa,<br />
Ragusa, Tunisia e nell’ Appennino meri<strong>di</strong>onale, e ne ipotizza la<br />
possibile causa nella collisione tra zolla euroasiatica e zolla<br />
africana.<br />
RILIEVI CONDOTTI<br />
I rilievi condotti dagli scriventi hanno permesso <strong>di</strong><br />
verificare la presenza <strong>di</strong> dette orientazioni in base alle misure<br />
degli assi <strong>di</strong> stress nelle strutture rilevate, evidenziando inoltre<br />
la tipologia e l'entità <strong>del</strong>le deformazioni riconosciute nelle<br />
successioni analizzate.<br />
Si è riconosciuta la presenza <strong>di</strong> un primo evento<br />
compressivo, orientato ca. N120°E, che ha determinato un<br />
<strong>di</strong>ffuso piegamento <strong>del</strong>le successioni, con fenomeni <strong>di</strong> thrusting<br />
pellicolare e sovrascorrimento, spesso intraformazionale, <strong>del</strong>le<br />
successioni in affioramento (fig. 3). Tale evento, misurato in<br />
tutta l'area, è associabile allo stress indotto dall'avanzamento<br />
<strong>del</strong>la catena Appennino-Maghrebide durante il Pliocene m<strong>ed</strong>io<br />
(Barrier, 1992)<br />
Si sono riconosciute numerose sequenze <strong>di</strong> thrust, <strong>di</strong><br />
<strong>di</strong>mensioni comprese tra alcuni decimetri e qualche centinaio <strong>di</strong><br />
metri, tanto in prossimità <strong>del</strong>le principali zone <strong>di</strong> taglio (fig. 4),<br />
quanto in aree <strong>di</strong>stali <strong>ed</strong> apparentemente meno deformate.<br />
La figura n. 5 mostra una fascia cataclastica associata ad<br />
una superficie <strong>di</strong> scollamento, con presenza <strong>di</strong> un lithon<br />
calcarenitico <strong>di</strong> <strong>di</strong>mensioni metriche.<br />
La figura n. 6 mostra la successione <strong>di</strong> un evento<br />
compressivo, con formazione <strong>di</strong> sequenze <strong>di</strong> piccole faglie<br />
inverse e sovrascorrimenti, tagliate da una faglia a componente<br />
prevalentemente <strong>di</strong>retta.<br />
Tali evidenze, riconducibili a tettonica d’inversione<br />
positiva, sono state osservate nelle alternanze <strong>del</strong>la F.ne Ragusa<br />
e Tellaro, dove locali con<strong>di</strong>zioni <strong>di</strong> transpressione e<br />
trastensione <strong>di</strong> età pleistocenica, tagliano e/o riattivano, specie<br />
in corrispondenza <strong>del</strong>le principali shear zone (sistemi Scicli-<br />
Ragusa e Comiso-Chiaramonte, fig. 1), strutture più antiche a<br />
carattere prevalentemente compressivo (fig. 7; Dipasquale &<br />
Occhipinti, 2008).<br />
Fig. 3 – Sequenza <strong>di</strong> thrusts all'interno <strong>del</strong> M.bo Irminio <strong>del</strong>la F.ne Ragusa presso la cava <strong>di</strong> roccia asfaltica <strong>di</strong> C.da Tabuna (Ragusa).
78 M. DIPASQUALE & R. OCCHIPINTI<br />
Detta inversione è stata, tra l'altro, determinante per la<br />
formazione <strong>del</strong> bacino asfaltifero <strong>del</strong>la valle <strong>del</strong> fiume Irminio.<br />
Fig. 4 – Sequenze <strong>di</strong> thrust intraformazionali all'interno <strong>del</strong>la F.ne Ragusa, in prossimità <strong>del</strong> sistema Scicli-Ragusa.<br />
La risalita dei geoflui<strong>di</strong> è avvenuta a partire dal reservoir <strong>di</strong><br />
Ragusa, lungo le faglie legate al sistema Scicli-Ragusa,<br />
trovando recapito negli spessori calcarenitici e calciru<strong>di</strong>tici che<br />
costituiscono la parte bassa <strong>del</strong> M. bro Irminio – livello a<br />
banconi (Kafka & Kirkbride 1960, Dipasquale et al., 2008).<br />
Alcune strutture osservate nelle successioni affioranti,<br />
d'altro canto, non sono ancora chiaramente associate a <strong>di</strong>stinte<br />
fasi tettoniche e necessitano pertanto ulteriori approfon<strong>di</strong>menti.<br />
Sovente si evidenzia che le tre porzioni principali in cui è<br />
<strong>di</strong>visa la F.ne Ragusa (M.bo Leonardo, Livello a banchi e<br />
Alternanza superiore) sono, in numerosi siti, reciprocamente<br />
<strong>di</strong>scordanti. Tali evidenze, per quanto sinora riscontrato nei<br />
rilievi, possono essere imputate sia a tettonica sins<strong>ed</strong>imentaria,<br />
sia a riattivazione <strong>di</strong> alcuni superfici s<strong>ed</strong>imentarie in piani <strong>di</strong><br />
movimento preferenziale, durante le fasi tettoniche posteriori<br />
alla <strong>di</strong>agenesi.<br />
Le evidenze <strong>di</strong> tettonica sins<strong>ed</strong>imentaria sono state<br />
osservate soprattutto nell’intervallo compreso tra il tetto <strong>del</strong><br />
M.bo Leonardo e la base <strong>del</strong> Livello a banchi, dove il primo è<br />
interessato da una sequenza <strong>di</strong> piccole faglie <strong>di</strong>rette che<br />
sembrano essere suturate, in <strong>di</strong>scordanza, dal superiore livello a<br />
banchi.<br />
In quest'ultimo si riconoscono inoltre numerosi bou<strong>di</strong>ns,<br />
associati sovente a tipiche strutture <strong>di</strong> flusso.<br />
Tali strutture (fig. 8) sono spesso interessate da ulteriori<br />
deformazioni; le faglie inverse osservate, si <strong>di</strong>spongono<br />
Fig. 5 – Lithon all'interno <strong>di</strong> superficie <strong>di</strong> scollamento presso C.da Tabuna<br />
(Ragusa).<br />
parallelamente a superfici dei thrusts principali.<br />
I bou<strong>di</strong>ns rilevati nella zona mineralizzata a bitume (C. da<br />
Tabuna) evidenziano uno sviluppo <strong>di</strong> tipo ra<strong>di</strong>ale, con ogni<br />
probabilità in<strong>di</strong>cativo <strong>del</strong>la struttura plicativa a domo <strong>del</strong>le<br />
successioni in affioramento. L'anticlinale <strong>di</strong> superficie<br />
costituirebbe pertanto la risultanza <strong>del</strong>la deformazione che ha<br />
determinato la trappola per idrocarburi profonda (Kafka &<br />
Kirkbride 1960, Dipasquale et al., 2008).<br />
Il M.bo Leonardo <strong>del</strong>la F.ne Ragusa inoltre, presenta spesso<br />
importanti deformazioni interne già interpretate da numerosi<br />
autori come slumps (Grasso et al., 2000).<br />
Tali strutture sono state osservate dagli scriventi anche<br />
nell'alternanza calcarenitico marnosa <strong>del</strong> M.bo Irminio <strong>del</strong>la<br />
F.ne Ragusa.<br />
Si sono tuttavia riconosciute numerose evidenze <strong>di</strong> rottura<br />
fragile <strong>ed</strong> una modesta acclività <strong>del</strong>le presunte superfici <strong>di</strong><br />
scivolamento (fig. 9).<br />
CONCLUSIONI<br />
La presente nota vuole essere un contributo<br />
all'approfon<strong>di</strong>mento <strong>del</strong>le conoscenze sulla tettonica nel<br />
margine meri<strong>di</strong>onale <strong>del</strong> Plateau Ibleo.<br />
Le strutture tettoniche rilevate appaiono riconducibili a<br />
tettonica d'inversione, che porterebbe a riconsiderare <strong>ed</strong><br />
approfon<strong>di</strong>re il grado <strong>di</strong> deformazione <strong>del</strong>l’avampaese.<br />
Fig. 6 – Successioni <strong>di</strong> eventi compressionali e trastensionali nel livello a<br />
banchi <strong>del</strong>la F.ne Ragusa.
ALTOPIANO RAGUSANO (SICILIA SUD-ORIENTALE): NOTE DI GEOLOGIA STRUTTURALE<br />
Fig. 7 – Rilievi strutturali in C.da Streppenosa, con strutture determinate<br />
da locale transpressione che tagliano faglie inverse e sovrascorrimenti.<br />
Fig. 8 – Bou<strong>di</strong>ns su pilastro all'interno <strong>del</strong>le miniere <strong>di</strong> roccia asfaltica (valle<br />
<strong>del</strong>l'Irminio), localmente fagliati (in alto a sinistra).<br />
Fig. 9 – Probabili slumps all'interno <strong>del</strong> M.bo Irminio <strong>del</strong>la F.ne Ragusa<br />
(sbancamento <strong>ed</strong>ile, Ragusa). Notare le strutture plicative sulla porzione<br />
scollata e i bou<strong>di</strong>ns in quella inferiore.<br />
REFERENCES<br />
BARRIER E. (1992) – Tectonic analysis of a flex<strong>ed</strong> foreland:<br />
Ragusa Platform. Tectonophysics, 206, 91-111.<br />
79<br />
CASERO P. (2004) – Structural setting of petroleum exploration<br />
plays in Italy. Società Geologica Italiana – Special Volume<br />
of the Italian Geological Society for the IGC 32 Florence-<br />
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DIPASQUALE M. & OCCHIPINTI R. (2008) - Evidences of<br />
transpressional tectonics in tertiary sequences of F.ne<br />
Ragusa - C.da Streppenosa (Hyblean Plateau, Sicily).<br />
Tethys to M<strong>ed</strong>iterranean, a journey of geological <strong>di</strong>scovery;<br />
Catania, 3/5 june 2008.<br />
DIPASQUALE M., OCCHIPINTI R. & ZIPELLI C. (2008) - Ragusa<br />
asphalt basin (Hyblean Plateau, Sicily): proposal of an<br />
ascent mo<strong>del</strong> from the deep oilfield. 84° Congresso<br />
Nazionale <strong>del</strong>la Società Geologica Italiana, Sassari, 15-17<br />
settembre 2008.<br />
GHISETTI F. & VEZZANI L. (1980) – The structural features of<br />
the Hyblean Plateau and of the Mount Ju<strong>di</strong>ca area (South-<br />
Eastern Sicily): a microtectonic contribution to the<br />
deformation history of the Calabrian Arc. Boll. Soc. Geol.<br />
It., 99, 57-102.<br />
GRASSO M. (1994) – Neotettonica e principali elementi<br />
strutturali <strong>del</strong> Plateau Ibleo e aree limitrofe. Atti <strong>del</strong> I<br />
Congresso Regionale <strong>del</strong>l'Or<strong>di</strong>ne dei Geologi <strong>di</strong> Sicilia,<br />
Marina <strong>di</strong> Ragusa 16-18 Settembre 1994, 65-81.<br />
GRASSO M., PHILPS B., REUTHER C.D., GAROFALO P.,<br />
STAMILLA R., ANFUSO G., DONZELLA G. & CULTRONE G.<br />
(2000B) – Pliocene-Pleistocene tectonics on the western<br />
margin of the Hyblean Plateau and the Vittoria Plain (SE<br />
Sicily). Mem. Soc. Geol. It., 55, 35-44.<br />
KAFKA F. T. & KIRKBRIDE R. K. (1960) – The Ragusa oil field,<br />
Sicily. Excursion in Sicily, P.E.S.L., 61-85.<br />
MATTINA D. (2002) – The Role of Wrench Tectonics In The<br />
Neogene-quaternary Evolution of The Western Hyblean<br />
Plateau (Sicily). EGS XXVII General Assembly, Nice, 21-<br />
26 April 2002, abstract #664<br />
TAVARNELLI E., BUTLER R. W. H., DECANDIA F. A., CALAMITA<br />
F. , GRASSO M., ALVAREZ W., RENDA P. (2004) –<br />
IMPLICATION of fault reactivation and structural<br />
inheritance in the Cenozoic tectonic evolution of Italy.<br />
Società Geologica Italiana – Special Volume of the Italian<br />
Geological Society for the IGC 32 Florence-2004, 209-<br />
222.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 80-83, 4ff.<br />
Suscettività magnetica e assetto strutturale nelle Ande in Tierra <strong>del</strong> Fuego<br />
FEDERICO ESTEBAN (*), ALEJANDRO TASSONE (*), MARCO MENICHETTI (**), MARIA ELENA CERREDO (*),<br />
AUGUSTO RAPALINI (*), HORACIO LIPPAI (*), JUAN FRANCISCO VILAS (*)<br />
ABSTRACT<br />
Magnetic susceptibility and structural setting in the Tierra <strong>del</strong> Fuego<br />
Andes<br />
With the aim of studying the tectonic evolution of the Fueguian Andes,<br />
the magnetic susceptibility tensor was measur<strong>ed</strong> on sample rocks of the<br />
Lemaire and Yahgán Fms outcrop in the area between Paso Garibal<strong>di</strong> and<br />
Canal Beagle in Tierra <strong>del</strong> Fuego. In the northern area the orientations of the<br />
K1 axis of the magnetic fabric shows a dominant <strong>di</strong>rections N-S that can be<br />
correlat<strong>ed</strong> to the tectonic transport. In the south from Carbajal valley to Canal<br />
Beagle, the orientations are roughly E-W that are link<strong>ed</strong> to the<br />
morphostructural lineations associat<strong>ed</strong> with a the strike-slip faults. The good<br />
correspondence between the magnetic anisotropy axis K3 and the rock<br />
foliation, permit to assign to a tectonic origin of the magnetic fabric.<br />
Key words: Andes, Anisotropy magnetic susceptibility, AMS,<br />
magnetic fabric, microstructures, Tierra <strong>del</strong> Fuego<br />
Lo stu<strong>di</strong>o congiunto <strong>del</strong>la struttura magnetica e tettonica <strong>di</strong><br />
una roccia coinvolta in una catena orogenica, permette <strong>di</strong><br />
ottenere importanti informazioni sull’entità <strong>del</strong>la deformazione<br />
e <strong>del</strong>la sua <strong>di</strong>stribuzione spaziale (TARLING & HROUDA, 1993;<br />
BORRADAILE & HENRY, 1997).<br />
Nonostante l’ottima risoluzione oggi possibile, la versatilità<br />
e velocità <strong>di</strong> esecuzione <strong>del</strong>le misure <strong>ed</strong> analisi, stu<strong>di</strong> sulla<br />
struttura magnetica <strong>del</strong>le rocce sono ancora poco frequenti<br />
specialmente nelle catene <strong>del</strong> Sud America e si hanno solo<br />
ricerche recenti in aree circoscritte (RAPALINI et alii, 2005;<br />
ZAFFARANA et alii, 2008).<br />
L’obiettivo <strong>di</strong> questo lavoro è quello <strong>di</strong> stu<strong>di</strong>are il fabric<br />
magnetico congiuntamente con l’analisi petrografica e micro<br />
strutturale. Il fine è quello <strong>di</strong> verificare le modalità <strong>di</strong> messa in<br />
posto <strong>del</strong>le <strong>di</strong>verse unità tettoniche, legate sia alla fase<br />
compressiva che trascorrente che ha interessato la regione <strong>del</strong>la<br />
Tierra <strong>del</strong> Fuego a partire dal Mesozoico <strong>ed</strong> inquadrarla nel<br />
contesto regionale. L’area stu<strong>di</strong>ata è localizzata nel settore SW<br />
argentino <strong>del</strong>l’Isola, compresa tra il Lago Fagnano a nord e il<br />
Canal de Beagle a sud e tra il Paso Garibal<strong>di</strong> ad est e i limiti <strong>del</strong><br />
Parco Nazionale <strong>del</strong>la Tierra <strong>del</strong> Fuego ad ovest (Fig.1).<br />
_________________________<br />
(*) CONICET-INGEODAV. Dpto. de Ciencias Geológicas. Facultad de<br />
Ciencias Exactas y Naturales. Universidad de Buenos Aires - Argentina.<br />
(**) Istituto <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Urbino – Italy<br />
F<strong>ed</strong>erico Estaban : e-mail esteban@gl.fcen.una.ar<br />
Questo stu<strong>di</strong>o è stato eseguito all’interno <strong>di</strong> un progetto <strong>di</strong><br />
più ampio e a lungo termine, multi<strong>di</strong>sciplinare, sull’evoluzione<br />
<strong>del</strong>le Ande Fuegine nel Meso-Cenozoico.<br />
La geologia <strong>del</strong>la Tierra <strong>del</strong> Fuego mostra una evoluzione<br />
complessa iniziata a partire <strong>del</strong>la fine <strong>del</strong> Paleozoico con la<br />
frantumazione <strong>del</strong> continente Gondwana e la formazione <strong>di</strong> un<br />
bacino marginale noto come Rocas Verdes (DALZIEL et alii,,<br />
1974). A partire dal Cretaceo sup., la convergenza localizzata<br />
nel margine pacifico <strong>del</strong>la Placca Sud Americana, porta allo<br />
sviluppo <strong>del</strong>la catena An<strong>di</strong>na (MENICHETTI et alii, 2008 -<br />
cumm. biblio). A partire dal Paleocene, in corrispondenza <strong>del</strong>la<br />
formazione <strong>del</strong> Mare <strong>di</strong> W<strong>ed</strong><strong>del</strong>, localizzato tra la placca Sud<br />
Americana e quella Antartica e la conseguente formazione <strong>del</strong><br />
Mare <strong>di</strong> Scotia, l’area è interessata da una tettonica trascorrente<br />
(CUNNINGHAM, 1995), con deformazioni principalmente<br />
transtensive nella regione <strong>del</strong>la Tierra <strong>del</strong> Fuego (LODOLO et<br />
alii, 2003; MENICHETTI et alii, 2008). Nell’area affiorano<br />
principalmente rocce, da acide a mesosiliciche,<br />
vulcanico/piroclastiche e basaltiche <strong>del</strong>la Formazione Lemair;<br />
associate in un quadro stratigrafico ancora non <strong>del</strong> tutto chiaro,<br />
esistono rocce basaltiche <strong>del</strong>la crosta oceanica <strong>del</strong> bacino <strong>di</strong><br />
Rocas verdes unitamente a rocce s<strong>ed</strong>imentarie <strong>di</strong> mare<br />
profondo (OLIVERO et alii, 1999; OLIVERO & MARTINIONI,<br />
2001). La regione è stata interessata da una fase tettonica<br />
compressiva nel Cretaceo sup., costituita da due eventi<br />
deformativi associati all’inversione <strong>del</strong> bacino <strong>di</strong> Rocas Verdes<br />
(DALZIEL & PALMER, 1979). Il primo evento corrisponde al<br />
Cretaceo sup. e produce pieghe F1 e un clivaggio S1, associato<br />
ad un metamorfismo in facies da prenhite-pumpellyte a scisti<br />
ver<strong>di</strong> (KOHN et alii, 1993). Il clivaggio ha una <strong>di</strong>rezione NW-<br />
SE <strong>ed</strong> immerge verso SW. Il secondo evento deformativo è<br />
collegato alla formazione e messa in posto <strong>di</strong> un sistema <strong>di</strong><br />
pieghe e fronti <strong>di</strong> accavallamento vergenti a NE, ai quali sono<br />
associate larghe pieghe F2 e un clivaggio <strong>di</strong> crenulazione S2<br />
molto ben sviluppato lungo la <strong>di</strong>rezione NE-SW con<br />
immersione verso SE ad alto angolo (MENICHETTI et alii, 2008)<br />
(Fig. 2). Gli assi <strong>del</strong>le pieghe tendono a ruotare da SSE-NNW<br />
per F1 a ESE-WSW per F2 con piani assiali immergenti a SW<br />
con inclinazioni tra 40° a 60°.<br />
A partire dal Cenozoico, l’area è stata interessata da una<br />
tettonica trascorrente con strutture <strong>di</strong>stensive e sviluppo <strong>di</strong><br />
bacini <strong>di</strong> pull-apart (LODOLO et alii, 2003; MENICHETTI et alii,<br />
2008). Le principali strutture trascorrenti sono associate ad<br />
importanti lineamenti morfostrutturali, uno dei quali è costituito<br />
dalla valle Carbajal-Lasifashaj, lunga oltre 100 km che
SUSCETTIVITÀ MAGNETICA E ASSETTO STRUTTURALE NELLE ANDE IN TIERRA DEL FUEGO<br />
Fig. 1 – Carta geologica schematica <strong>del</strong>la parte centrale <strong>del</strong>la Cor<strong>di</strong>llera <strong>del</strong>le Ande in Tierra <strong>del</strong> Fuego con localizzazione <strong>del</strong>le aree campionate . CB : Canal de<br />
Beagle; CO : Valle Carbajal ovest; CE : Valle Carbajal Est; PG : Paso Garibal<strong>di</strong>.<br />
attraversa tutta la parte centrale <strong>del</strong>la catena fuegina e la<br />
sud<strong>di</strong>vide in due settori : la Sierra de Alvear a nord e la Sierra<br />
Sorondo a sud (CENNI et alii, 2006) (Fig. 1)<br />
La Cor<strong>di</strong>llera <strong>del</strong>le Ande in Tierra <strong>del</strong> Fuego è costituita da<br />
un sistema <strong>di</strong> unità tettoniche vergenti a NE accavallamento le<br />
une sulle altre attraverso superfici <strong>di</strong> scollamento basale<br />
localizzate nelle litologie marnose e scistose <strong>del</strong> Giurassico<br />
Sup. e <strong>del</strong> Cretaceo. La geometria è costituita da blocchi<br />
monoclinalici immergenti a sud e <strong>del</strong>imitati da piani <strong>di</strong><br />
sovrascorrimenti, all’interno <strong>del</strong>le quali si possono riconoscere<br />
due gruppi <strong>di</strong> macro e meso-strutture: (1) ampie pieghe e<br />
superfici <strong>di</strong> sovrascorrimento/scollamento a basso angolo; (2)<br />
pieghe a chevron asimmetriche con sovrascorrimenti<br />
moderatamente inclinati, immergenti a SSW, che costituiscono<br />
la struttura principale <strong>del</strong>la catena. Il raccorciamento<br />
complessivo <strong>di</strong> queste strutture, su tutta la regione, può essere<br />
stimato in decine <strong>di</strong> chilometri. La superficie dei<br />
sovrascorrimenti è marcata da zone <strong>di</strong> taglio con uno spessore<br />
<strong>di</strong> alcuni metri, dove sono ben sviluppate strutture S/C (Fig. 2<br />
A); alcune <strong>del</strong>le superfici <strong>di</strong> taglio sono costituite da rocce<br />
milonitiche. Nella parte più interna <strong>del</strong>la Cor<strong>di</strong>llera, il<br />
basamento è coinvolto nella deformazione con uno stile thickskinn<strong>ed</strong><br />
con rocce metamorfiche <strong>di</strong> alto grado <strong>del</strong> Paleozoico<br />
Sup., al Terziario Inf., affioranti nella Cor<strong>di</strong>llera Darwin<br />
localizzata ad ovest <strong>del</strong>l’area <strong>di</strong> stu<strong>di</strong>o (KOHN et alii, 1993;<br />
CUNNINGHAM, 1995). Verso est, nel settore argentino <strong>del</strong>la<br />
Tierra <strong>del</strong> Fuego, queste strutture compressive confluiscono in<br />
zone <strong>di</strong> taglio milonitiche in facies metamorfica <strong>di</strong> scisti ver<strong>di</strong><br />
ben visibili in affioramento nella Sierra de Alvear (Fig. 1).<br />
Dal punto <strong>di</strong> vista microstrutturale, le rocce <strong>di</strong> Paso<br />
Garibal<strong>di</strong> sono caratterizzate dallo sviluppo <strong>di</strong> un fabric<br />
milonitico definito da porfiroclasti <strong>di</strong> clinopirosseno all’interno<br />
<strong>di</strong> una matrice caratterizzata da prehnite-clorite che definiscono<br />
la foliazione principale. Nel sito F4, in maniera <strong>del</strong> tutto<br />
qualitativa, è possibile osservare una relazione <strong>di</strong>retta tra<br />
l’abbondanza <strong>del</strong>le vene <strong>di</strong> minerali opachi-quarzo-carbonatici<br />
e alti valori <strong>di</strong> P’ (Fig. 3A).<br />
Nella parte orientale <strong>del</strong>la Valle Carbajal si hanno <strong>di</strong>verse<br />
litologie con rocce andesitiche, vulcanoclastiche, s<strong>ed</strong>imentarie<br />
e basiche. Usualmente il fabric magnetico è costituito da<br />
porfiroclasti immersi in una foliazione definita da clorite e<br />
titanite in grani fini <strong>ed</strong> isorientati (Fig. 3B). Alcuni campioni<br />
<strong>del</strong>la località F7 corrispondenti a porfiroclasti silicei, mostrano<br />
cubi <strong>di</strong> pirite con associati strain fringes <strong>di</strong> quarzo.<br />
Nella parte occidentale <strong>del</strong>la Valle Carbajal, la foliazione<br />
dei porfiroclasti silicei <strong>del</strong>la Fm. Lemaire, è generalmente<br />
definita da clorite isorientata (± muscovite ± titanite). In alcuni<br />
punti sono presenti lenti o vene <strong>di</strong> carbonati paralleli alla<br />
foliazione (F10 e F11- Fig. 3C) e porfiroclasti <strong>di</strong> feldspati con<br />
microbou<strong>di</strong>nage (F12). Nel sito Y6 il fabric magnetico è<br />
collegato alla presenza <strong>di</strong> cristalli <strong>di</strong> pirite associati con strain<br />
fringes <strong>di</strong> quarzo più o meno paralleli alla clorite.<br />
81
82 F.ESTEBAN ET ALII<br />
Fig. 2 – Affioramenti con relativi stereogrammi nell’emisfero inferiore con<br />
in<strong>di</strong>cato i piani <strong>di</strong> scistosità, faglie, vene. Sono in<strong>di</strong>cati gli assi <strong>del</strong>la<br />
suscettibilità magnetica principale (K1 quadrati), interm<strong>ed</strong>io (K2 triangoli) e<br />
minimo (K3 cerchi) e l’ ellisse al 95% <strong>di</strong> confidenza. – A – Paso Garibal<strong>di</strong>; B<br />
– Valle Carbajal Est; C – Valle Carbajal Ovest.<br />
Dal punto <strong>di</strong> vista analitico, la metodologia utilizzata<br />
prev<strong>ed</strong>e, successivamente al prelievo <strong>di</strong> campioni <strong>di</strong> roccia<br />
orientati, la riduzione a dei cilindri <strong>di</strong> 2.54 cm <strong>di</strong> <strong>di</strong>ametro e<br />
2.2 cm <strong>di</strong> altezza e sui quali viene misurata l’anisotropia<br />
magnetica. La determinazione <strong>del</strong> tensore suscettività<br />
magnetica è stata eseguita con uno strumento MFK1-B<br />
Kappabridge, all’interno <strong>del</strong> quale ogni campione viene<br />
misurato in 15 <strong>di</strong>verse posizioni. Sullo stesso campione sono<br />
stati inoltre condotti stu<strong>di</strong> petrografici attraverso sezioni sottili.<br />
Il grado <strong>di</strong> anisotropia rilevato è altamente variabile con un<br />
valore <strong>di</strong> P’ compreso tra 1.03 (porfiro dacite, Y11) a 2.00<br />
(ignimbrite F7) (Fig. 4). La forma <strong>del</strong>l’ellissoide (T) è variabile<br />
con una chiara pr<strong>ed</strong>ominanza <strong>di</strong> una forma oblata. I siti<br />
<strong>del</strong>l’area Valle Carbajal est (tranne il F7) e <strong>del</strong> Canal de<br />
Beagle, mostrano un grado <strong>di</strong> anisotropia maggiore <strong>del</strong> 20%<br />
(Fig. 4).<br />
I risultati <strong>del</strong>le misure <strong>di</strong> suscettività magnetica mostrano<br />
un buon accordo con i dati petrografici, meso e microstrutturali<br />
(Fig. 2) e permettono <strong>di</strong> sud<strong>di</strong>videre la regione in<br />
quattro settori. Esiste un’ottima correlazione tra la foliazione<br />
misurata sul terreno e la struttura magnetica <strong>del</strong>la roccia,<br />
in<strong>di</strong>cando quin<strong>di</strong> una chiara origine tettonica <strong>del</strong> fabric<br />
magnetico. Nell’area <strong>di</strong> Paso Garibal<strong>di</strong> dove affiorano rocce<br />
vulcaniche (basalti <strong>ed</strong> andesiti) <strong>del</strong>la Fm. Lemaire, la lineazione<br />
magnetica (K1) ha una <strong>di</strong>rezione sistematica da N-NNE a S-<br />
SSW con una debole inclinazione verso sud, che può essere<br />
correlata con la <strong>di</strong>rezione <strong>di</strong> trasporto tettonico operato dal<br />
sistema <strong>di</strong> sovrascorrimenti (MENICHETTI et alii, 2008).<br />
Nell’area orientale <strong>del</strong>la Valle Carbajal affiorano varie<br />
Fig. 3 – Sezioni sottili (a sinistra N// e a destra NX). A: zona <strong>di</strong> Paso<br />
Garibal<strong>di</strong>, vena in una andesite con minerali opachi carbonatici e quarzosi ;<br />
B: Valle Carbajal (F9), frammento <strong>di</strong> pomice sostituito <strong>ed</strong> avvolto da clorite<br />
che definisce la scistosità; C: Valle Carbajal (F10), titanite con strutture<br />
concentriche avvolte da clorite isorientata.<br />
litologie (basalti, andesiti, ignimbriti e rocce s<strong>ed</strong>imentarie)<br />
<strong>del</strong>la Fm. Lemaire. Qui la lineazione magnetica (K1) mostra<br />
<strong>di</strong>rezioni simili a quelle <strong>del</strong>l’area <strong>di</strong> Paso Garibal<strong>di</strong>. Per contro,<br />
nell’area più occidentale <strong>del</strong>la Valle, K1 ha un trend E-W suborizzontale<br />
che può essere messo in relazione con il lineamento<br />
trascorrente che attraversa in questo punto la parte centrale<br />
<strong>del</strong>la valle (MENICHETTI et alii, 2004). In questa area le<br />
litologie sono costituite da rocce s<strong>ed</strong>imentarie, tufi basaltici,<br />
gabbri e rioliti <strong>del</strong>le Fm. Lemaire e Yaghan.
SUSCETTIVITÀ MAGNETICA E ASSETTO STRUTTURALE NELLE ANDE IN TIERRA DEL FUEGO<br />
Nell’area <strong>del</strong> Canal de Beagle, sono state campionate rocce<br />
vulcaniche intrusive (sill meta-basaltico, gabbro e una dacite<br />
porfirica) all’interno <strong>del</strong>la Fm. Yaghan. Il trend <strong>del</strong>la lineazione<br />
magnetica è sub orizzontale verso SW-NE, coincidente con<br />
l’andamento generale <strong>del</strong> Canal de Beagle. Questo lascerebbe<br />
supporre che l’assetto tettonico prettamente transtensivo<br />
Fig. 4 – Diagramma P’/ T (TARLING D.H.& HROUDA F., 2003)) con in<strong>di</strong>cati i<br />
campioni analizzati, in<strong>di</strong>cati in Fig. 1 - Paso Garibal<strong>di</strong> (rombi), Valle<br />
Carbajal Est (quadrati), Valle Carbajal ovest (triangoli), Canal de Beagle<br />
(cerchi) - e la forma degli ellissoi<strong>di</strong> <strong>del</strong>la suscettività magnetica.<br />
determina il fabric magnetico <strong>di</strong> questa località (LODOLO et<br />
alii, 2003; MENICHETTI et alii, 2008).<br />
E’ possibile effettuare un confronto tra le <strong>di</strong>rezioni degli<br />
assi <strong>del</strong>l’ellissoide <strong>di</strong> deformazione calcolato sulla base<br />
<strong>del</strong>l’analisi cinematica <strong>del</strong>le faglie (MENICHETTI et alii, 2008)<br />
con gli assi <strong>del</strong>l’ellissoide <strong>del</strong>la suscettività magnetica. In<br />
alcune zone come in quella <strong>di</strong> Passo Garibal<strong>di</strong>, Sierra Alvear e<br />
Valle Carbajal occidentale e Canal de Beagle, l’asse <strong>del</strong>lo<br />
sforzo principale σ1, corrisponde circa con l’asse K1<br />
(foliazione) <strong>del</strong>la suscettività magnetica, il σ2 è circa K2 e il<br />
σ3 con K3 (lineazione). Questa coincidenza potrebbe essere<br />
spiegata attraverso un meccanismo deformativo dove il taglio<br />
puro ha agito nella prima fase <strong>di</strong> deformazione. In altre località<br />
come nella parte occidentale <strong>del</strong>la Valle Carbajal, per contro,<br />
non esiste una chiara relazione tra i valori <strong>del</strong>la suscettività<br />
magnetica e il campo deformativo, lasciando ipotizzare un<br />
meccanismo <strong>di</strong> deformazione più complesso e probabilmente<br />
polifasico.<br />
REFERENCES<br />
BORRADAILE, G. J., HENRY, B. (1997) - Tectonic applications<br />
of magnetic susceptibility and its anisotropy. Earth-<br />
Science Reviews, Vol. 42, (1-2): 49-93.<br />
CENNI, M., MENICHETTI, M., MATTIOLI, M., LODOLO, E.,<br />
TASSONE A. (2006) - Analisi meso-microstrutturale lungo<br />
la faglia trascorrente Magellano-Fagnano nella<br />
Cor<strong>di</strong>gliera <strong>del</strong>le Ande in Terra <strong>del</strong> Fuoco-Argentina.<br />
Ren<strong>di</strong>conti <strong>del</strong>la Società Geologica Italiana. Nuova Serie,<br />
2: 121-124.<br />
CUNNINGHAM, W.D. (1995)- Orogenesis at the southern tip of<br />
the Americas: the structural evolution of the Cor<strong>di</strong>llera<br />
Darwin metamorphic complex, southernmost Chile.<br />
Tectonophysics, 244: 197-229.<br />
DALZIEL, I.W.D., DE WIT, M.J., PALMER, K.F., (1974) - Fossil<br />
marginal basin in the southern Andes. Nature, 250: 291-<br />
294.<br />
DALZIEL, I.W.D., PALMER, K.F. (1979) - Progressive<br />
deformation and orogenic uplift at the southern extremity<br />
of the Andes. Bulletin of the Geological Society America,<br />
90: 259-280.<br />
LODOLO E., MENICHETTI M., BARTOLE R., BEN-AVRAHAM Z.,<br />
TASSONE A.& LIPPAI H. (2003) - Magallanes-Fagnano<br />
continental transform fault (Tierra <strong>del</strong> Fuego,<br />
southermost South America. Tectonics, 22, 1076, doi:<br />
10129/2003TC0901500, 2003.<br />
KOHN M.J., SPEAR F.S., DALZIEL I.W.D. (1993) - Metamorphic<br />
PT paths from the Cor<strong>di</strong>llera Darwin: a core complex in<br />
Tierra <strong>del</strong> Fuego, Chile. Journal of Petrology, 34: 519-<br />
542.<br />
MENICHETTI M., ACEVEDO R., BUJALESKY G., CENNI M.,<br />
CERREDO M.E., CORONATO A., HORMAECHEA J.L., LIPPAI<br />
H., LODOLO E., OLIVERO E., RABASSA J., RUSSI M.,<br />
TASSONE, A. (2004) – Geological field trip guide in the<br />
Tierra <strong>del</strong> Fuego. Geosur 2004, Argentina, 39 p.<br />
MENICHETTI M., LODOLO E., TASSONE A. (2008)- Structural<br />
geology of the Fuegian Andes and Magallanes fold-andthrust<br />
belt-Tierra <strong>del</strong> Fuego Island. Geologica Acta, 6,<br />
(1):19-42.<br />
OLIVERO E.B., MALUMIÁN D.R. (2008) - Mesozoic-Cenozoic<br />
stratigraphy of the Fuegian Andes, Argentina. Geologica<br />
Acta, 6, (1): 5-18.<br />
OLIVERO E.B., MARTINIONI D.R. (2001) - A review of the<br />
geology of Argentinian Fuegian Andes. Journal of South<br />
American Earth Sciences, 14: 175-188.<br />
RAPALINI A.E., CERREDO M.E., TASSONE A., LIPPAI H. (2005)<br />
- Estu<strong>di</strong>o de magnetofábrica y microestructuras a través<br />
de los Andes de Tierra <strong>del</strong> Fuego. In Congreso Geológico<br />
Argentino, 16. Actas en CD. 8 pp. La Plata.<br />
TARLING D.H., HROUDA F. (2003) - The Magnetic Anisotropy<br />
of Rocks. Chapman and Hall: 217 p. London.<br />
ZAFFARANA C.B., SOMOZA R., OLIVERO E.B. (2008) -<br />
Anisotropía de la susceptibilidad magnética en el<br />
paleógeno de la faja plegada Fueguina. In Congreso<br />
Geológico Argentino17, Actas, 168, Jujuy.<br />
83
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 84<br />
Faglie transtensive e mineralizzazioni a solfuri nell’area meri<strong>di</strong>onale<br />
<strong>del</strong> Monte Amiata: la struttura <strong>del</strong> Monte Civitella<br />
(Toscana meri<strong>di</strong>onale)<br />
LORENZO FABBRINI (*), ANDREA BROGI (**) & DOMENICO LIOTTA (**)<br />
Lo stu<strong>di</strong>o <strong>del</strong>la circolazione dei flui<strong>di</strong> idrotermali nella crosta<br />
superiore costituisce l’elemento fondamentale per le ricerche<br />
geotermiche. Tale tipologia <strong>di</strong> stu<strong>di</strong>o si avvale <strong>del</strong>l’integrazione<br />
<strong>di</strong> dati geochimici, geofisici, strutturali con mo<strong>del</strong>li <strong>di</strong><br />
simulazione numerica. Un contributo molto importante alla<br />
comprensione dei sistemi <strong>di</strong> circolazione nei campi geotermici<br />
attuali è dato anche dallo stu<strong>di</strong>o dei sistemi geotermici fossili,<br />
poiché essi offrono le migliori opportunità per analizzare<br />
<strong>di</strong>rettamente in affioramento le strutture, oggi esumate, che<br />
hanno funzionato da condotti per la circolazione idrotermale.<br />
Le relazioni tra le mineralizzazioni, la permeabilità, le<br />
caratteristiche chimico-fisiche dei flui<strong>di</strong> idrotermali permettono<br />
<strong>di</strong> ricostruire il paleosistema idrotermale, fornendo<br />
informazioni utili alla comprensione dei sistemi geotermici<br />
attuali.<br />
L’area <strong>del</strong> Monte Amiata rappresenta un’area chiave per lo<br />
stu<strong>di</strong>o dei sistemi geotermici sia fossili che attuali. Quest’area,<br />
infatti, è caratterizzata da un sistema geotermico attivo,<br />
sfruttato da <strong>di</strong>versi decenni per la produzione <strong>di</strong> energia<br />
elettrica e da numerose aree mineralizzate a solfuri <strong>di</strong> mercurio<br />
<strong>ed</strong> antimonio, principalmente legate a circolazione idrotermale<br />
attiva durante il Pleistocene e sfruttate dall’attività mineraria<br />
condotta nei secoli scorsi.<br />
Lo scopo <strong>di</strong> questo stu<strong>di</strong>o è quello <strong>di</strong> fornire dati relativi alle<br />
relazioni tra la mineralizzazione a cinabro <strong>ed</strong> antimonite<br />
presente nel massiccio <strong>del</strong> Monte Civitella, collocato tra le aree<br />
vulcaniche <strong>del</strong> Monte Amiata (Toscana meri<strong>di</strong>onale) e dei<br />
Monti Vulsini (Lazio settentrionale), <strong>ed</strong> i sistemi <strong>di</strong> faglie<br />
pleistoceniche, ad oggi poco stu<strong>di</strong>ate, che interessano<br />
pervasivamente tutto l’intero massiccio.<br />
La metodologia <strong>di</strong> stu<strong>di</strong>o si è basata sull’integrazione <strong>di</strong> dati<br />
minerari, <strong>di</strong> sondaggio e <strong>di</strong> campagna. I principali risultati<br />
hanno messo in evidenza che: a) le strutture deformative<br />
cretacico-terziarie sviluppatesi durante la tettogenesi<br />
<strong>del</strong>l’Appennino Settentrionale risultano <strong>di</strong>slocate da zone <strong>di</strong><br />
taglio orientate N30°-70°; b) le superfici <strong>di</strong> taglio sono<br />
caratterizzate da più in<strong>di</strong>catori cinematici sovrapposti ma<br />
nell’insieme riconducibili ad una tettonica transtensiva sinistra;<br />
_________________________<br />
(*)Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università degli Stu<strong>di</strong> <strong>di</strong> Siena, Via<br />
Laterina, 8 – 53100 Siena.<br />
(**)Dipartimento <strong>di</strong> Geologia e Geofisica, Università degli Stu<strong>di</strong> <strong>di</strong> Bari,<br />
Via Orabona, 4 – 70125 Bari.<br />
Lorenzo Fabbrini: fabbini15@unisi.it<br />
c) le zone <strong>di</strong> faglia risultano essere caratterizzate da cataclasiti<br />
mineralizzate ad in<strong>di</strong>care che le damage zones hanno costituito<br />
la via preferenziale per la circolazione idrotermale; d) nell’area<br />
si riconoscono più zone <strong>di</strong> taglio orientate N30°-70° fra loro<br />
<strong>di</strong>sposte en-echelon e che sono raccordate da faglie minori a<br />
componente <strong>di</strong> movimento verticale, orientate NNO-SSE <strong>ed</strong><br />
intensamente mineralizzate.<br />
Ne emerge un quadro strutturale confrontabile con quanto<br />
definito nell’area <strong>del</strong> Monte Amiata dove faglie trascorrenti e<br />
sistemi <strong>di</strong> pull-apart hanno guidato la circolazione dei flui<strong>di</strong><br />
idrotermali.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 85, 1f.<br />
RIASSUNTO<br />
Esumazione <strong>del</strong>le unità <strong>di</strong> alta pressione indotte dall’arretramento <strong>del</strong>la<br />
placca in subduzione<br />
In questa nota viene illustrato un mo<strong>del</strong>lo <strong>di</strong> esumazione <strong>del</strong>le<br />
unità continentali <strong>di</strong> alta pressione-bassa temperatura affioranti<br />
nel M<strong>ed</strong>iterraneo. Lo stu<strong>di</strong>o si basa sugli esempi <strong>del</strong>la Calabria<br />
e <strong>del</strong>l’Egeo, dove sono note le con<strong>di</strong>zioni paleogeografiche, le<br />
velocità <strong>di</strong> subduzione e l’età <strong>del</strong>le unità <strong>di</strong> alta pressione. Il<br />
mo<strong>del</strong>lo fisico proposto, testato con simulazioni <strong>di</strong> laboratorio<br />
e numeriche, si basa sul principio che l’arretramento <strong>del</strong>la<br />
placca in subduzione rappresenti il meccanismo necessario per<br />
creare lo spazio sufficiente e permettere la risalita <strong>del</strong>le unità<br />
prec<strong>ed</strong>entemente subdotte. Viene inoltre proposto che il<br />
meccanismo <strong>di</strong> arretramento sia innescato dalla <strong>del</strong>aminazione<br />
<strong>di</strong> blocchi continentali prec<strong>ed</strong>entemente subdotti.<br />
Key words: exhumation of HP rocks, subduction rollback,<br />
Calabria–Apennine, Aegean<br />
Rocks metamorphos<strong>ed</strong> under high-pressure (HP) and ultra<br />
high-pressure (UHP) con<strong>di</strong>tions in subduction zones come back<br />
to the surface relatively soon after their burial and at rates<br />
comparable to plate boundary velocities. In the M<strong>ed</strong>iterranean<br />
realm, their occurrence in several belts relat<strong>ed</strong> to a single<br />
subduction event shows that the burial–exhumation cycle is a<br />
recurrent transient process. Using the Calabria–Apennine and<br />
Aegean belts as examples, we show that the exhumation of HP<br />
rocks is associat<strong>ed</strong> in time and space with the subduction of<br />
small continental lithosphere blocks that triggers slab rollback,<br />
creating the necessary space for the exhumation of the buoyant<br />
continental crust that was deeply buri<strong>ed</strong> just before (Fig. 1).<br />
The buoyancy force of the subduct<strong>ed</strong> crust increases until this<br />
crust detaches from the downgoing slab. It then exhumes at a<br />
rate that depends <strong>di</strong>rectly on the velocity of trench retreat to<br />
become part of the overri<strong>di</strong>ng plate.<br />
Heat<strong>ed</strong> from below by the asthenosphere that flows into the<br />
_________________________<br />
Exhumation of high-pressure rocks driven by slab rollback.<br />
CLAUDIO FACCENNA (*), JEAN PIERRE BRUN (**) & FEDERICO ROSSETTI (*)<br />
(*)Dipartimento <strong>di</strong> Scienze Geologiche, Universita Roma Tre, Largo S.L.<br />
Murialdo 1, 00146 Rome, Italy<br />
(°)Géosciences Rennes, UMR 6118CNRS, Université Rennes1, Campus de<br />
Beaulieu, 35042 Rennes, France<br />
opening mantle w<strong>ed</strong>ge, the exhum<strong>ed</strong> crust weakens and<br />
undergoes core-complex-type extension, responsible for a<br />
second stage of exhumation at a lower rate. The full sequence<br />
of events that characterizes this mo<strong>del</strong> (crust–mantle<br />
<strong>del</strong>amination, slab rollback and trench retreat, HP rock<br />
exhumation, asthenosphere heating and core-complex<br />
formation) arises entirely from the initial con<strong>di</strong>tion impos<strong>ed</strong> by<br />
the subduction of a small continental block. No specific<br />
con<strong>di</strong>tion is requir<strong>ed</strong> regar<strong>di</strong>ng the rheology and erosion rate of<br />
HP rocks. The burial–exhumation cycle is transient and can<br />
recur every time a small continental block is subduct<strong>ed</strong>.<br />
advancing trench<br />
retreating trench<br />
a<br />
continental<br />
subduction<br />
b<br />
c<br />
trench position<br />
compressional front<br />
HP exhumation<br />
d<br />
core complex<br />
e<br />
f<br />
continental block<br />
total decoupling<br />
core<br />
complex<br />
oceanic melange<br />
HP metamorphism<br />
Suture<br />
REFERENCES<br />
backarc extension<br />
Fig. 1 – Mo<strong>del</strong> of exhumation driven by slab rollback: continental<br />
subduction stage (a to c), exhumation of high-pressure (HP) rocks (d) and<br />
exhumation of high-temperature (HT) rocks in core complex (e). Slab <strong>di</strong>p<br />
increases during subduction of the continental block (a to c), and then<br />
decreases during oceanic subduction (d to f).<br />
BRUN J.P. & FACCENNA C. (1008) –Exhumation of highpressure<br />
rocks driven by slab rollback. Earth Plan. Sci. Letters,<br />
doi:10.1016/j.epsl.2008.02.038.<br />
a
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 86-88, 2ff.<br />
Cicli deposizionali <strong>del</strong> travertino Lapis Tiburtinus durante il<br />
tardo Pleistocene (Tivoli, Italia Centrale): influenze climatiche e<br />
tettoniche<br />
C. FACCENNA (*) , L. DE FILIPPIS (*) , M. SOLIGO (*) , A. BILLI (*) , R. FUNICIELLO (*) , C. ROSSETTI (*) , P. TUCCIMEI (*)<br />
ABSTRACT<br />
Cycles of Lapis Tiburtinus deposition and erosion during Late Pleistocene<br />
time (Tivoli, Central Italy): possible influence of climate and faulting<br />
The depositional and erosional history of the Lapis Tiburtinus endogenic<br />
travertine locat<strong>ed</strong> 30 km to the east of Rome, Central Italy, near the Colli<br />
Albani quiescent volcano, is interpret<strong>ed</strong> through three-<strong>di</strong>mensional<br />
stratigraphy and uranium-series geochronology. Analyses of large exposures<br />
locat<strong>ed</strong> in active quarries and of cores obtain<strong>ed</strong> from 114 industrial wells<br />
reveal that the travertine deposit is about 20 km 2 wide and 60 m thick on<br />
average. The travertine thickness is over 85 m toward its western N-Selongat<strong>ed</strong><br />
side, where thermal springs and large sinkholes occur align<strong>ed</strong> over a<br />
seismically-active N-striking shallow fault. The travertine age was calculat<strong>ed</strong><br />
using the U/Th isochron method. Results constrain the onset and conclusion of<br />
travertine deposition at about 115 and 30 ka, respectively. The thre<strong>ed</strong>imensional<br />
study of the travertine shows that this deposit is characteriz<strong>ed</strong> by<br />
a succession of depositional benches grown in an aggradational fashion and<br />
separat<strong>ed</strong> by five main erosional surfaces, which are associat<strong>ed</strong> with paleosols,<br />
conglomerates, and karstic features. This evidence shows that the travertine<br />
evolution was mostly controll<strong>ed</strong> by water table fluctuations. A comparison<br />
with global and local paleoclimatic in<strong>di</strong>cators suggests that water table<br />
fluctuations were partly modulat<strong>ed</strong> by climate cycles. Other influencing factors<br />
may have been fault-relat<strong>ed</strong> deformation and volcanic activity.<br />
Key words: travertine, paleoclimate, faults, uranium-series<br />
method, three-<strong>di</strong>mensional methods.<br />
INTRODUZIONE<br />
Travertini e tufa sono rispettivamente il prodotto <strong>del</strong>la<br />
precipitazione <strong>del</strong> carbonato <strong>di</strong> calcio in ambiente continentale<br />
sia da flui<strong>di</strong> idrotermali che non (Pentecost, 2005). In<br />
particolari i travertini endogenici sono comunemente riferiti a<br />
depositi carbonatici connessi a risorgenze <strong>di</strong> acque termali <strong>ed</strong><br />
idrotermali (Crossey et al., 2006; Ford & P<strong>ed</strong>ley, 1996).<br />
Diversi parametri possono controllare la formazione dei<br />
travertini endogenici e dei tufa. Per esempio, è noto che le<br />
con<strong>di</strong>zioni climatiche influenzano considerevolmente la<br />
precipitazione dei tufa (Pentecost, 1995, 2005; Ford and<br />
P<strong>ed</strong>ley, 1996). Per contro, non è ben documentata l’influenza<br />
<strong>del</strong> clima sulla deposizione dei travertini endogenici (Rihs et<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, Università degli Stu<strong>di</strong> Roma Tre,<br />
Largo S. L. Murialdo 1, Roma, 00146, Italia.<br />
E-mail: ldefilippis@uniroma3.it (Luigi De Filippis)<br />
al., 2000). La deposizione dei travertini endogenici spesso<br />
infatti avviene nell’intorno <strong>di</strong> sorgenti termali, la cui stessa<br />
ubicazione e ricarica è solitamente controllata da sistemi <strong>di</strong><br />
faglie attive; d’altra parte l’alta capacità mineralizzante dei<br />
flui<strong>di</strong> endogeni sigillerebbe “velocemente” tali percorsi <strong>di</strong><br />
risalita se la fagliazione non fosse attiva (Brogi and Capezzuoli,<br />
2008).<br />
Lo scopo <strong>di</strong> questo lavoro è <strong>di</strong> contribuire all’identificazione<br />
dei fattori che controllano i cicli deposizionali-erosivi <strong>del</strong><br />
travertino <strong>di</strong> Tivoli (Lapis Tiburtinus). Tale giacimento,<br />
oggetto <strong>di</strong> estrazione sin dai tempi dei Romani, situato circa 30<br />
km ad est <strong>di</strong> Roma (fig. 1), è caratterizzato (in cava) da una<br />
eccezionale vista tri<strong>di</strong>mensionale. Questi travertini si sono<br />
deposti all’interno <strong>di</strong> un bacino subsidente (Bacino <strong>del</strong>le Acque<br />
Albule, BAA) formatosi durante il tardo Pleistocene lungo un<br />
sistema <strong>di</strong> faglie trascorrente e transtensive a decorso meri<strong>di</strong>ano<br />
a nord <strong>del</strong> Complesso vulcanico dei Colli Albani (Faccenna et<br />
al., 1994; Gasparini et al., 2002; Minissale et al., 2002). Lo<br />
stu<strong>di</strong>o è stato effettuato m<strong>ed</strong>iante analisi strutturale,<br />
stratigrafica e geocronologica.<br />
RISULTATI<br />
Evidenze <strong>di</strong> tipo geocronologico e stratigrafiche <strong>di</strong>mostrano<br />
che il Lapis Tiburtinus si è evoluto in maniera ciclica attraverso<br />
l’alternanza <strong>di</strong> fasi deposizionali <strong>ed</strong> erosive durante il tardo<br />
Pleistocene, tra circa 115 e 30 ka. In particolare, la presenza<br />
alternata <strong>di</strong> banchi <strong>di</strong> deposizione e <strong>di</strong> superfici <strong>di</strong> erosione,<br />
associati a paleosuoli e a carsismo, <strong>di</strong>mostrano come<br />
l’evoluzione ciclica dei depositi travertinosi sia controllata<br />
dalla fluttuazione <strong>del</strong>la falda idrica (o tavola d’acqua) nel<br />
Bacino <strong>del</strong>le Acque Albule (BAA). L’erosione avveniva in<br />
con<strong>di</strong>zioni subaeree, durante fasi <strong>di</strong> abbassamento <strong>del</strong>la falda,<br />
mentre la deposizione avveniva durante le fasi <strong>di</strong> innalzamento<br />
<strong>del</strong>la falda, quando il deposito <strong>di</strong> travertino cresceva con<br />
geometria onlapping sulla sottostante superficie erosionale, con<br />
un generale progradazione verso meri<strong>di</strong>one (Faccenna et al.,<br />
2008). Le fluttuazioni <strong>del</strong>la tavola d’acqua verso il basso, in<br />
aree costiere, come è il caso <strong>del</strong> BAA, possono essere modulate<br />
da <strong>di</strong>versi fattori. La prima è che le con<strong>di</strong>zioni paleoclimatiche<br />
(ad esempio la temperatura m<strong>ed</strong>ia e l’umi<strong>di</strong>tà <strong>del</strong>l’aria) abbiano<br />
influenzato l’altezza <strong>del</strong>la tavola d’acqua nel BAA, attraverso<br />
la modulazione sia dei livelli <strong>del</strong>le acque marine sia <strong>di</strong> quelle<br />
continentali (fig. 2) (Noe-Nygaard and Heiberg, 2001;<br />
Prokopenko et al., 2005). I nostri risultati geocronologici
C. FACCENNA ET ALII<br />
Fig. 1 – (a) Carta geologica <strong>del</strong>l’area intorno a Roma (Italia Centrale). Le faglie sottostanti gli <strong>ed</strong>ifici vulcanici sono state interpretate attraverso dati in<strong>di</strong>retti e<br />
<strong>di</strong>retti, come l’allineamento <strong>del</strong>le emergenze dei flui<strong>di</strong> e i campi <strong>di</strong> fatturazione (De Rita et al., 1988, 1995). Il giacimento <strong>di</strong> Lapis Tiburtinus è situato circa 30<br />
km ad est <strong>di</strong> Roma. Si noti la posizione <strong>del</strong> Cratere <strong>di</strong> Castiglione, da cui provengono i pollini utilizzati per le relative curve (Tz<strong>ed</strong>akis et al., 2001). (b) Carta<br />
geologica <strong>del</strong>l’area <strong>di</strong> stu<strong>di</strong>o, comprendente il bacino <strong>del</strong>le Acque Albule, dove durante il tardo Pleistocene si è depositato il travertino Lapis Tiburtinus. La faglia<br />
sotto il giacimento <strong>di</strong> Lapis Tiburtinus è sismicamente attiva (come testimoniato dalla sequenza sismica <strong>del</strong> 2001 in cui si sono avuti terremoti compresi tra 0.5 e<br />
1.5 km <strong>di</strong> profon<strong>di</strong>tà; Gasparini et al., 2002).<br />
Fig. 2 – Mo<strong>del</strong>lo schematico <strong>del</strong>le relazioni tra stage climatici,<br />
deposizione <strong>del</strong> travertino e faglie attive (il <strong>di</strong>segno è ispirato e mo<strong>di</strong>ficato<br />
da Rihs et al., 2000).<br />
87<br />
mostrano, tuttavia, che i cicli deposizionali sono stati<br />
probabilmente influenzati anche dal sistema <strong>di</strong> fatturazione che,<br />
m<strong>ed</strong>iante cicli <strong>di</strong> self-sealing ha permesso la risorgenza in<br />
superficie <strong>del</strong> circuito idrotermale profondo.<br />
BIBLIOGRAFIA<br />
BROGI A. (2004) - Faults linkage, damage rocks and hydrothermal fluid<br />
circulation: Tectonic interpretation of the Rapolano Terme travertines<br />
(southern Tuscany, Italy) in the context of Northern Apennines Neogene-<br />
Quaternary extension. Eclogae Geol. Helv., 97, 307-320.<br />
CHAFETZ H.S. & FOLK R.L. (1984) - Tavertines: depositional morphology and<br />
the bacterially construct<strong>ed</strong> constituents. J. S<strong>ed</strong>iment. Petrol., 54, 289-316.<br />
CROSSEY L.J., FISCHER T.P., PATCHETT P.J., KARLSTROM K.E., HILTON D.R.,<br />
NEWELL D.L., HUNTOON P., REYNOLDS A.C. & DE LEEUW G.A.M. (2006) -<br />
Dissect<strong>ed</strong> hydrologic system at the Grand Canyon: interaction between<br />
deeply deriv<strong>ed</strong> fluids and plateau aquifer waters in modern springs and<br />
travertine. Geology, 34, 25-28.<br />
FACCENNA C., FUNICIELLO R., MONTONE P., PAROTTO M., VOLTAGGIO M.,<br />
(1994) - An example of late Pleistocene strike-slip tectonics: the Acque<br />
Albule basin (Tivoli, Latium). Memorie descrittive <strong>del</strong>la carta geologica<br />
d'Italia, vol.XLIX, 63-76.<br />
FACCENNA C., SOLIGO M., BILLI A., DE FILIPPIS L., FUNICIELLO R., ROSSETTI<br />
C. & TUCCIMEI P. (2008) - Late Pleistocene depositional cycles of the Lapis<br />
Tiburtinus travertine (Tivoli, Central Italy): possible influence of climate
88 CICLI DEPOSIZIONALI DEL TRAVERTINO LAPIS TIBURTINUS DURANTE IL TARDO PLEISTOCENE<br />
and fault activity. Global and Planetary Change, 63, 299-308<br />
(doi:10.1016/j.gloplacha.2008.06.006).<br />
FORD T.D. & PEDLEY M.H. (1996) - A review of tufa and travertine deposits<br />
of the world. Earth-Sci. Rev., 41, 117-175.<br />
GASPARINI C., DI MARO R., PAGLIUCA N., PIRRO M. & MARCHETTI A. (2002) -<br />
Recent seismicity of the “Acque Albule” travertine basin. Ann. Geophys.,<br />
45, 537-550.<br />
MINISSALE A., KERRICK D.M., MAGRO G., MURRELL M.T., PALADINI M., RIHS<br />
S., STURCHIO N.C., TASSI F. & VASELLI O. (2002) - Geochemistry of<br />
Quaternary travertines in the region north of Rome (Italy): structural,<br />
hydrologic, and paleoclimatic implications. Earth Planet. Sc. Lett., 203,<br />
709-728.<br />
NOE-NYGAARD N. & HEIBERG E.O. (2001) - Lake-level changes in the Late<br />
Weichselian Lake Tøvelde, Møn, Denmark: induc<strong>ed</strong> by changes in climate<br />
and base level. Paleogeogr. Paleoclimatol. Paleoecol., 174, 351-382.<br />
PENTECOST A. (1995) - The Quaternary travertine deposits of Europe and<br />
Asia minor. Quaternary Sci. Rev., 14, 1005-1028.<br />
PENTECOST A. (2005) - Travertine. Springer, Berlin.<br />
PROKOPENKO A.A., KUZMIN M.I., WILLIAMS D.F., GELETY V.F.,<br />
KALMYCHKOV G.V., GVOZDKOV A.N. & SOLOTCHIN P.A. (2005) - Basinwide<br />
s<strong>ed</strong>imentation changes and deglacial lake-level rise in the Hovsgol<br />
basin, NW Mongolia. Quaternary Int., 136, 59-69.<br />
RIHS S., CONDOMINES M. & POIDEVIN J.L. (2000) - Long term behaviour of<br />
continental hydrothermal system: U-series study of hydrothermal<br />
carbonates from the French Massif Central (Allier Valley). Geochim.<br />
Cosmochim. Ac., 64, 3189-3199.<br />
Si ringraziano E. Anzalone, A. Brogi, B. D'Argenio, V. Ferreri, L.<br />
Lombar<strong>di</strong>, M. Mattei, A. Minissale, and A. Taddeucci per le stimolanti<br />
<strong>di</strong>scussioni.<br />
Siamo riconoscenti inoltre a F. Lippiello, a C. Giovanrosa, a G. Squeo e a<br />
tutto il “Centro per la Valorizzazione <strong>del</strong> Travertino Romano” per<br />
l’ospitalità <strong>di</strong>mostrataci nel loro interessante ambiente <strong>di</strong> lavoro.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 89-92, 4ff.<br />
Time of hydrocarbon generation vs trap forming age in Mesozoic oil<br />
play in Po Plain<br />
RIASSUNTO<br />
Età <strong>di</strong> generazione <strong>di</strong> idrocarburi <strong>ed</strong> età <strong>di</strong> formazione <strong>del</strong>le trappole in<br />
play ad olio nella successione Mesozoica <strong>del</strong>la Pianura Padana<br />
La successione mesozoica <strong>del</strong>le Alpi Meri<strong>di</strong>onali ospita rocce madri che<br />
hanno generato gli idrocarburi accumulati in alcuni giacimenti <strong>del</strong> sottosuolo<br />
padano. La mo<strong>del</strong>lizzazione dei dati <strong>di</strong> maturità <strong>del</strong>la materia organica ha<br />
permesso la ricostruzione <strong>di</strong> una storia termica caratterizzata dalla presenza <strong>di</strong><br />
valori elevati <strong>del</strong> flusso <strong>di</strong> calore (85 to 105 mW/m 2 ) in concomitanza con le<br />
fasi estensionali mesozoiche.<br />
Questo regime termico ha notevoli implicazioni per la valutazione <strong>del</strong>le<br />
potenzialità minerarie <strong>del</strong> versante settentrionale <strong>del</strong>la Pianura Padana, ove le<br />
trappole che potrebbero ospitare gli idrocarburi generati da queste ricce madri<br />
mesozoiche si sono formate durante le fasi compressionali neoalpine<br />
(Oligocene-Miocene). L’analisi <strong>del</strong> rapporto tra età <strong>di</strong> generazione <strong>ed</strong><br />
espulsione <strong>ed</strong> età <strong>di</strong> strutturazione evidenzia che le successioni mesozoiche<br />
presenti nei rilievi estensionali ha generato idrocarburi durante il terziario e<br />
quaternario, in età successiva alla formazione <strong>del</strong>le trappole, mentre nei<br />
depocentri estensionali la generazione è iniziata durante il mesozoico, in età<br />
prec<strong>ed</strong>ente alla loro formazione.<br />
Key words: Po Plain; Mesozoic extension, thermal history, oil<br />
play<br />
INTRODUCTION<br />
Maturity data obtain<strong>ed</strong> from the analysis of organic matter are<br />
excellent in<strong>di</strong>cators of the thermal evolution of s<strong>ed</strong>imentary<br />
basins. The maturity parameters which are us<strong>ed</strong> are sensitive in<br />
<strong>di</strong>fferent ways to temperatures between 60 °C and 200 °C.<br />
They record heating events experienc<strong>ed</strong> by organic matter<br />
contain<strong>ed</strong> in the rocks, and can help to reconstruct the thermal<br />
history of s<strong>ed</strong>imentary successions Measur<strong>ed</strong> maturity data can<br />
be us<strong>ed</strong> to check the consistency of geological mo<strong>del</strong>s that may<br />
incorporate s<strong>ed</strong>imentary or tectonic burial history and thermal<br />
regime evolution. The aim of this work is to describe the<br />
thermo-chronological evolution of the Southern Alps (Northern<br />
Italy) during the Mesozoic extensional phases, through organic<br />
matter maturity analysis and mo<strong>del</strong>ling of some select<strong>ed</strong><br />
successions that were not overprint<strong>ed</strong> by the Alpine orogeny.<br />
Using the assess<strong>ed</strong> thermal regime evolution, implications for<br />
oil exploration in the Po Plain for<strong>ed</strong>eep are highlight<strong>ed</strong>.<br />
_______________________<br />
(*) Eni Exploration & Production Division, via Emilia 1 – 20097 San<br />
Donato Milanese, Italy. E-mail: roberto.fantoni@eni.it<br />
R. FANTONI (*) & P. SCOTTI (*)<br />
THE MESOZOIC EXTENSION IN THE SOUTHERN<br />
ALPS<br />
The structural pattern of the Southern Alps is characteris<strong>ed</strong> by<br />
E-W and WSW-ENE tren<strong>di</strong>ng compressional structures in the<br />
western and eastern sectors respectively. The Mesozoic<br />
extension, relat<strong>ed</strong> to the tectonic movements between Adria and<br />
Europe, result<strong>ed</strong> in the creation of a N-S half-graben (M. Nudo,<br />
M. Generoso, Iseo), bound<strong>ed</strong> by E and W <strong>di</strong>pping master<br />
normal faults. Tectonic activity start<strong>ed</strong> during the Norian. After<br />
de-activation during the Rhaetian, a new extensional phase took<br />
place during the Liassic. Extension then shift<strong>ed</strong> westward in the<br />
Ligurian-Piemont area, where oceanic crust form<strong>ed</strong> starting in<br />
the Late Jurassic. From this time until the Early Cretaceous, the<br />
Southern Alps underwent post-rift thermal subsidence<br />
(BERTOTTI et alii, 1993, and references therein). The same<br />
architecture is present in the Po Plain foreland (Fig. 1).<br />
ORGANIC MATTER MATURITY IN THE MESOZOIC<br />
SUCCESSION OF THE SOUTHERN ALPS<br />
In order to reconstruct the thermal history, organic matter<br />
(OM) maturity values were obtain<strong>ed</strong> from samples collect<strong>ed</strong><br />
from Permian to Cretaceous s<strong>ed</strong>imentary units cropping out<br />
along the whole chain. Data affect<strong>ed</strong> by anomalous thermal<br />
perturbations were previously exclud<strong>ed</strong>, e.g., significant<br />
tectonic thickening due to overthrusts whose hanging wall<br />
ramps were subsequently erod<strong>ed</strong>; deposition of thick for<strong>ed</strong>eep<br />
clastics subsequently erod<strong>ed</strong> during the recent alpine cycles;<br />
thermal perturbation due to nearby magmatic intrusions.<br />
Almost all OM maturity data are from intervals with fairgood<br />
TOC. In some localities, the presence of several “good<br />
TOC” levels provides a vertical set of data along a thick<br />
s<strong>ed</strong>imentary succession, guaranteeing better definition of the<br />
maturity gra<strong>di</strong>ent. Vitrinite reflectance (Ro%) values, from the<br />
western-central sector of the Southern Alps (from Biella area to<br />
Trento Plateau) are describ<strong>ed</strong> in FANTONI & SCOTTI (2003),<br />
CALABRÒ et alii (2003), BERSEZIO et alii. (2005), SCOTTI<br />
(2005) and SCOTTI & FANTONI (2009).<br />
The lowest OM maturity values occur on the structural highs<br />
having thin overlying s<strong>ed</strong>imentation. On the Gozzano and Arzo<br />
highs, for example, Tmax from Rock-Eval pyrolysis for Middle<br />
Triassic units is < 425°C (immature kerogen) suggesting<br />
maximum temperatures lower than 60 to 70°C (using a heating<br />
rate of 3°C/Ma).
90 R. FANTONI & P. SCOTTI<br />
Fig. 1 – Syn-extensional Mesozoic successions of the Southern Alps (AA’) and the Po Plain for<strong>ed</strong>eep/foreland (BB’).<br />
Also samples from the Trento Plateau show low OM maturity<br />
(~ 0.5% Ro and 435°C Tmax) for Upper Triassic units. The<br />
highest maturity values correspond to the depocentres of<br />
synrift successions, such as the Upper Triassic units of the M.<br />
Tremezzo/M.Galbiga (M. Generoso Basin) and Iseo Lake<br />
successions (Iseo Basin). Very high OM maturity was reach<strong>ed</strong><br />
by Norian units (Ro ~3.5% at the base of the Calcare <strong>di</strong><br />
Zorzino Formation) in the Iseo depocentre.<br />
THERMAL MODELLING<br />
Vitrinite reflectance measur<strong>ed</strong> data were us<strong>ed</strong> for thermal<br />
mo<strong>del</strong> calibration using a proprietary Eni S.p.A. on<strong>ed</strong>imensional<br />
mathematical mo<strong>del</strong>, able to simulate the degree<br />
of maturity reach<strong>ed</strong> by the kerogen. The mo<strong>del</strong> input data are:<br />
a) thickness, b) lithology (compaction curve and thermal<br />
conductivity of the rock matrix), c) ages of s<strong>ed</strong>imentation and<br />
palaeobathymetry for each stratigraphic event (all of these data<br />
define the burial history), d) heat flow values through time, and<br />
e) palaeolatitude, which is us<strong>ed</strong> to convert palaeosurface<br />
temperature to sea bottom temperature accor<strong>di</strong>ng to<br />
palaeobathymetry. Finally, the input for the kinetic mo<strong>del</strong> has<br />
to be defin<strong>ed</strong> (e.g., EasyRo method of Sweeney and Burnham,<br />
1990).<br />
The value of heat flow is the most important unknown<br />
variable, wich is is relat<strong>ed</strong> to the tectonic setting/geodynamic<br />
evolution of the area. NOVELLI et alii (1987) hypothesis<strong>ed</strong> high<br />
heat flow value (~75 mW/m 2 ) in the northwestern Po Basin<br />
during the Lower Lias, accor<strong>di</strong>ng to the geodynamic evolution<br />
of the Southern Alps. GREBER et alii (1997) and BERTOTTI et<br />
alii (1999) hypothesis<strong>ed</strong> a higher heat flow for the Lombar<strong>di</strong>an<br />
basin, but pr<strong>ed</strong>ict<strong>ed</strong> its peak value (~90 mW/m2) at Anisian-<br />
La<strong>di</strong>nian and Carnian-Norian ages respectively. BERSEZIO &<br />
BELLANTANI (1997) interpret<strong>ed</strong>, for the same basin, the thermal<br />
perturbation as Liassic (and pre-Toarcian).<br />
The Eni database was recently enrich<strong>ed</strong> with a large amount<br />
of organic matter maturity data for Mesozoic s<strong>ed</strong>imentary<br />
successions throughout the Southern Alps region. Therefore,<br />
constraints for the reconstruction of the thermal history were<br />
available. Recent thermal mo<strong>del</strong>ling in<strong>di</strong>cates that the last<br />
probable heat flow peak (although other significant earlier<br />
peaks cannot be exclud<strong>ed</strong>) was later than the Lias, when the<br />
examin<strong>ed</strong> formations (comprising the Upper Triassic and<br />
Liassic units) were subject to deep burial. Also the maximum<br />
heat flow values were better quantifi<strong>ed</strong> (SCOTTI, 2005 and<br />
references therein).<br />
Best fit solutions between calculat<strong>ed</strong> and measur<strong>ed</strong> OM<br />
maturity form some of the burial history mo<strong>del</strong>s in Fig. 1, and<br />
other simulation points in the eastern Southern Alps were<br />
obtain<strong>ed</strong> by assuming increas<strong>ed</strong> heat flow throughout the rifting<br />
stage, with a maximum reach<strong>ed</strong> during Bajocian-Bathonian<br />
time. Resulting heat flow values are quite high and relatively<br />
uniform throughout the Southern Alps (85 to 105 mW/m2)<br />
suggesting that this kind of thermal regime was widespread<br />
(FANTONI & SCOTTI, 2003). Fluctuations in peak heat flow<br />
could be due to <strong>di</strong>fferential crustal thinning, but they could also<br />
be relat<strong>ed</strong> to other causes (e.g., ra<strong>di</strong>ogenic heat within the<br />
crust, hydrothermal fluid circulation, or underplating<br />
phenomena). Heat flow progressively decreas<strong>ed</strong> after the<br />
Bajocian-Bathonian to values similar to the present day by the<br />
end of the Cretaceous. This reconstruction is consistent with the<br />
known tectonic evolution of the Mesozoic extension in the<br />
Southern Alps, characteris<strong>ed</strong> by a rifting stage up to the Lias<br />
follow<strong>ed</strong> by a drifting stage and thermal subsidence from the<br />
Middle Jurassic. As an example, the Iseo Lake case history is<br />
shown in fig. 2.
TIME OF HYDROCARBON GENERATION VS TIME OF TRAP FORMATION IN MESOZOIC OIL PLAY IN PO PLAIN<br />
Fig. 2 - Mo<strong>del</strong>ling (Scotti, 2005; Scotti and Fantoni, 2009) of Iseo Lake (western side outcrops): a) - burial and thermal regime (heat flow)<br />
evolution during the extensional Mesozoic phase and later; comparison between constant (B) and variable (A) thermal regime evolution (dash<strong>ed</strong><br />
line for to Po Plain For<strong>ed</strong>eep simulation points); b) - thermal and organic matter maturity history; - c) - measur<strong>ed</strong> and calculat<strong>ed</strong> OM maturity<br />
(R o% and Easy R o) vs. depth.<br />
IMPLICATIONS FOR HYDROCARBON<br />
EXPLORATION IN THE PO PLAIN FORELAND<br />
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Fig. 3 – Hydrocarbon occurences in <strong>di</strong>fferent stratigraphic settings<br />
<br />
<br />
91<br />
The Mesozoic carbonate units of the for<strong>ed</strong>eep/foreland area<br />
and of the external thrust belts <strong>di</strong>splay the largest oil and<br />
thermogenic gas accumulations of the Italian region.<br />
They include several <strong>di</strong>fferent plays which are essentially<br />
relat<strong>ed</strong> to the three main stages of the Tethyan crustal stretching<br />
(Middle Triassic, Late Triassic/Early Jurassic and Early<br />
Cretaceous) (fig. 3). Traps for hydrocarbons are also highly<br />
vari<strong>ed</strong>, as they were shap<strong>ed</strong> by the interference between the<br />
Mesozoic extensional phases and the subsequent Tertiary<br />
compressional events (BERTELLO et alii, 2008).<br />
The regional <strong>di</strong>stribution of the organic matter maturity in<br />
the Southern Alps seems to be mainly controll<strong>ed</strong> by <strong>di</strong>fferential<br />
burial during the Norian-Liassic extensional phase with an<br />
associat<strong>ed</strong> high heat flow.<br />
This kind of thermal regime can have strong implications for<br />
hydrocarbon exploration. The maturity profiles of some basinal<br />
successions (for example Iseo Lake), and consequent<br />
geochemical mo<strong>del</strong>ling suggest that the Upper Triassic source<br />
rocks may already have attain<strong>ed</strong> high maturity levels during the<br />
Mesozoic, and this is even more likely for the deeper Middle<br />
Triassic source rocks.<br />
Where traps have been form<strong>ed</strong> by Tertiary Alpine<br />
compression (FANTONI et alii, 2004), accurate mo<strong>del</strong>ling can<br />
help to better define hydrocarbon charge risk (SCOTTI &<br />
FANTONI, 2009).<br />
In the extensional high (areas with low Rhaetian–Liassic<br />
burial) the source rocks retain<strong>ed</strong> their original petroleum<br />
potential prior to strong Neogene–Quaternary burial (well C in<br />
fig. 4). High recent heating result<strong>ed</strong> in generation/expulsion of<br />
hydrocarbons after trap formation. In the extensional<br />
depocentre the hydrocarbon generation start<strong>ed</strong> since Mesozoic,<br />
before trap formation, and the generat<strong>ed</strong> hydrocarbons were<br />
partially (well B) or totally (well A) lost.
92 R. FANTONI & P. SCOTTI<br />
Fig. 4 – Burial history, thermal history and timing of hydrocarbons generation (Transformation Ratio) for the main Upper Triassic source<br />
rocks in <strong>di</strong>fferent tectonic settings of the Po Plain foreland. Type IIS (Gruppo Aralalta – GAr, and Calcare <strong>di</strong> Zorzino - CZo) and Type III<br />
kerogen (Argillite <strong>di</strong> Riva <strong>di</strong> Solto – ARS, and Calcare <strong>di</strong> Zu - CZu) kinetic parameters were us<strong>ed</strong> (Eni S.p.A. proprietary database).<br />
REFERENCES<br />
BERSEZIO R. & BELLANTANI G. (1997) - The thermal maturity<br />
of the Southalpine Mesozoic succession north of Bergamo<br />
by vitrinite reflectance data. Atti Tic. Sc. Terra, 5, 101-114.<br />
BERSEZIO R., SCOTTI P. & TONETTI M. (2005) - Maturity of<br />
organic matter in the Liassic succession of the Mt.<br />
Generoso rift<strong>ed</strong> Basin (Southern Alps, Lombardy). Atti Tic.<br />
Sci. Terra, 10, 71-76.<br />
BERTELLO F., FANTONI R. & FRANCIOSI R. (2008) -<br />
Hydrocarbon occurences in Mesozoic carbonate units in<br />
Italy. Ren<strong>di</strong>conti online Soc. Geol. It., 2, pp. 37-39.<br />
BERTOTTI G., PICOTTI V., BERNOULLI D. & CASTELLARIN A.<br />
(1993) - From rifting to drifting: tectonic evolution of the<br />
Southalpine upper crust from the Triassic to the Early<br />
Cretaceous. S<strong>ed</strong>imentary Geology, 86, 1/2, 53 - 76.<br />
BERTOTTI G., SEWARD D., WIJBRANS J., TER VOORDE M. &<br />
HURFORD A.J. (1999) - Crustal thermal regime prior to,<br />
during, and after rifting: a geochronological and mo<strong>del</strong>ing<br />
study of the Mesozoic South Alpine rift<strong>ed</strong> margin.<br />
TECTONICS, 18, 2, 185-200.<br />
CALABRÒ R., CERIANI A., DI GIULIO A., FANTONI R. & SCOTTI<br />
P. (2003) - Reconstruction of thermal history of the syn-rift<br />
sequence between the Iseo Basin and Trento Plateau:<br />
results from the integrat<strong>ed</strong> study of organic matter and fluid<br />
inclusions. Atti Tic.Sc. Terra, 9, 88 – 91.<br />
FANTONI R., BERSEZIO R. & FORCELLA F. (2004) - Alpine<br />
structure and deformation chronology at the Southern Alps –<br />
Po Plain border in Lombardy. Boll. Soc. Geol. It., 123, 463-<br />
476.<br />
FANTONI R. & SCOTTI P. (2003) - Thermal record of the<br />
Mesozoic extensional tectonics in the Southern Alps. Atti<br />
Tic. Sci. Terra, 9, 96 – 101.<br />
GREBER E., LEU W., BERNOULLI D., SCHUMACHER M. & WYSS<br />
R. (1997) - Hydrocarbon provinces in the Swiss Southern<br />
Alps – a gas geochemistry and basin mo<strong>del</strong>ling study.<br />
Marine and Petroleum Geology, 14, 1, 3-25.<br />
NOVELLI L., CHIARAMONTE M.A., MATTAVELLI L., PIZZI G.,<br />
SARTORI L. & SCOTTI P. (1987) - Oil habitat in the<br />
northwestern Po basin. In: B. Doligez (Eds.) Migration of<br />
Hydrocarbons in S<strong>ed</strong>imentary Basins, E<strong>di</strong>tions Technip,<br />
Paris, 27-57.<br />
SCOTTI P. (2005) - Thermal constraints suggest<strong>ed</strong> by the study<br />
of the organic matter and thermal mo<strong>del</strong>ling strategies – A<br />
case history from Southern Alps. Atti Tic. Sci. Terra, 10,<br />
21-35.<br />
SCOTTI P. & FANTONI (2009) - Evaluation of the thermal<br />
regime in the Southern Alps during Mesozoic extension:<br />
implication for hydrocarbon exploration in the Po Plain<br />
for<strong>ed</strong>eep. In: N. Harris and K.E. Peters (Eds.) Thermal<br />
History Analysis of S<strong>ed</strong>imentary Basins: Methods and<br />
Applications, SEPM special publication (in press.)
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 93-94, 3ff.<br />
Mo<strong>del</strong>li <strong>di</strong> accrezione <strong>del</strong> margine cileno centro-meri<strong>di</strong>onale.<br />
FANUCCI FRANCESCO (*), VARGAS CORDERO IVAN (*), LORETO M. FILOMENA (°) & TINIVELLA UMBERTA (°)<br />
ABSTRACT<br />
Some <strong>di</strong>fferent structural mo<strong>del</strong>s of the trench and downslope of the<br />
central-southern Chile continental margin<br />
The continental margin of central-southern Chile presents some<br />
morphostructural features not common among the active margins. The trench,<br />
with few exceptions, is fill<strong>ed</strong> by an important s<strong>ed</strong>imentary prism. The presence<br />
and the characters of this prism strongly con<strong>di</strong>tion the accretion process in the<br />
downslope. They introduces here some examples of active accretion and the<br />
cases are <strong>di</strong>scuss<strong>ed</strong> in which this process seems to be inactive.<br />
Key words: Continental margin, Chile trench, downslope,<br />
accretion<br />
INTRODUZIONE<br />
Il margine continentale <strong>del</strong> Cile centro-meri<strong>di</strong>onale presenta<br />
una serie <strong>di</strong> caratteri morfostrutturali peculiari o, per lo meno,<br />
non comuni tra i margini attivi. La situazione può essere<br />
sintetizzata come segue.<br />
La fossa è ampia, dal fondo pianeggiante a causa <strong>del</strong>le colmate<br />
s<strong>ed</strong>imentarie che si fanno sempre più ingenti da N a S,<br />
<strong>di</strong>minuendone anche la profon<strong>di</strong>tà massima, sino a farla<br />
praticamente scomparire come unità morfologica. Essa è<br />
interessata da un canale longitu<strong>di</strong>nale pressoché continuo anche<br />
se generato da processi <strong>di</strong>versi zona per zona. Il canale non è<br />
mai molto vicino alla scarpata, ma se ne allontana<br />
sensibilmente nelle aree occupate dalle conoi<strong>di</strong> terrigene allo<br />
sbocco dei principali canyon.<br />
La scarpata inferiore è ripida e presenta a tratti morfostrutture<br />
generate da un recente processo d’accrezione, tutt’ora in atto.<br />
Altre zone sono praticamente prive <strong>di</strong> morfostrutture riferibili a<br />
un tale processo e comunque non vi è, neppure laddove<br />
l’accrezione attuale è palese, evidenza <strong>di</strong> continuità tra il<br />
processo detto e le fasi <strong>di</strong> accrezione prec<strong>ed</strong>enti.<br />
La scarpata superiore ha morfologie complesse e variate con<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, ambientali e Marine-Università <strong>di</strong><br />
Trieste<br />
(°) Istituto <strong>di</strong> Oceanografia e Geofisica Sperimentale – Borgo Grotta<br />
Gigante – Trieste<br />
bacini s<strong>ed</strong>imentari minori e si collega <strong>di</strong>rettamente alla zona <strong>di</strong><br />
piattaforma , senza l’interposizione <strong>del</strong> tipico bacino <strong>di</strong><br />
avant’arco, presente solo nel settore più meri<strong>di</strong>onale stu<strong>di</strong>ato,<br />
in prossimità <strong>del</strong> punto triplo che collega placca sudamericana,<br />
antartica e <strong>di</strong> Nazca.<br />
Qualche osservazione meritano anche le morfologie dei fondali<br />
oceanici in prossimità <strong>del</strong>la fossa, in quanto con<strong>di</strong>zionanti gli<br />
effetti <strong>del</strong> processo <strong>di</strong> subduzione. Si alternano aree molto<br />
movimentate da seamounts e fratture ad aree con scarsi<br />
<strong>di</strong>slivelli, ulteriormente attenuati da coperture pelagiche <strong>di</strong> una<br />
certa potenza. In questo secondo caso il basamento oceanico,<br />
nella zona <strong>di</strong> flessura che prec<strong>ed</strong>e la subduzione vera e propria,<br />
reagisce agli sforzi con una serie <strong>di</strong> faglie <strong>di</strong>rette, mentre nel<br />
caso prec<strong>ed</strong>ente si hanno risposte molto varie e anche vere e<br />
proprie scaglie tettoniche <strong>di</strong> basamento oceanico che si<br />
accavallano su altre parti <strong>del</strong>lo stesso, con vergenza W.<br />
ALCUNE SEZIONI SISMICHE<br />
La sezione SO 161-09 è tracciata perpen<strong>di</strong>colarmente al<br />
margine <strong>di</strong> fronte al promontorio <strong>di</strong> Agua Dulce e comprende<br />
una zona <strong>di</strong> fondale oceanico a rilievi articolati, ma <strong>di</strong> modesto<br />
<strong>di</strong>slivello. Sono assenti coperture pelagiche e il materiale<br />
terrigeno nella fossa è estremamente ridotto, anche a causa <strong>del</strong><br />
fatto che gli apporti terrigeni vengono raccolti in bacini <strong>di</strong><br />
scarpata.<br />
Fig. 1- <strong>Sezione</strong> crostale in un tratto <strong>di</strong> fossa privo <strong>di</strong> s<strong>ed</strong>imenti<br />
Il comportamento tettonico <strong>del</strong> basamento oceanico, sollecitato<br />
dalla subduzione mostra, al limite W <strong>del</strong>la linea, la tendenza a<br />
formare un notevole bulge periferico. La flessura da luogo a<br />
strutture sia <strong>di</strong>sgiuntive che compressive, rigioco <strong>di</strong> strutture<br />
già presenti nella crosta oceanica, mentre nella fossa si<br />
in<strong>di</strong>viduano vere e proprie scaglie <strong>di</strong> basamento. Sono queste<br />
ultime che danno luogo all’accrezione frontale secondo una
94 F. FANUCCI ET ALII<br />
geometria “classica” <strong>di</strong> sottoscorrimento e giustapposizione. Il<br />
prisma, accresciuto in più fasi, sembra dare origine ad un<br />
bacino <strong>di</strong> scarpata. Altri dati geofisici in<strong>di</strong>cano che<br />
quest’ultimo é piuttosto dovuto ad un sovrascorrimento <strong>del</strong><br />
basamento metamorfico paleozoico sopra il prisma stesso.<br />
La RC 2901-728 tracciata molto più a S , in corrispondenza<br />
<strong>del</strong>la piattaforma Itata, mostra un basamento oceanico a<br />
modesta movimentazione, che si fa via via più marcata verso la<br />
fossa. Quest’ultima è completamente colmata da un complesso<br />
s<strong>ed</strong>imentario in tre livelli principali:<br />
- livelli pelagici che si fanno più potenti verso l’oceano;<br />
- su questi si appoggiano in toplap i livelli terrigeni basali che<br />
accompagnano il basamento oceanico in subduzione;<br />
- i livelli torbi<strong>di</strong>tici e conturitici più recenti, potenti 1 sec.<br />
Colmano completamente la fossa e vanno a costituire, al pi<strong>ed</strong>e<br />
<strong>del</strong>la scarpata una grossa anticlinale <strong>di</strong> rampa <strong>di</strong> un thrust, che<br />
rappresenta il primo sta<strong>di</strong>o <strong>di</strong> un’accrezione in atto, ovvero<br />
l’embrione <strong>di</strong> un prisma che si appoggia e in parte sotto scorre<br />
ad un prisma prec<strong>ed</strong>ente. La superficie <strong>di</strong> scollamento <strong>del</strong> thrust<br />
è ubicata alla base <strong>del</strong> prisma s<strong>ed</strong>imentario torbi<strong>di</strong>tico.<br />
Analoghe caratteristiche, ancor più accentuate, presenta la<br />
sezione SO161-40, tracciata a S <strong>del</strong>l’Isola <strong>di</strong> Chiloé. Qui la<br />
superficie <strong>di</strong> scollamento sta alla base <strong>di</strong> tutta sequenza<br />
terrigena, molto potente.<br />
Fig. 2- Profilo crostale che mostra ampie deformazioni al pi<strong>ed</strong>e <strong>del</strong>la<br />
scarpata<br />
Molto più frequenti sono i tratti <strong>di</strong> margine in cui non appare<br />
evidente un’accrezione frontale in atto. In qualche caso vi è il<br />
legittimo sospetto che il basamento continentale arrivi sino alla<br />
fossa obbligando il prisma s<strong>ed</strong>imentario a sottoscorrere senza<br />
deformarsi (underplating), ma in molte altre situazioni è<br />
evidente che una piccola parte <strong>del</strong> prisma stesso rimane a far<br />
parte <strong>del</strong> margine. Un esempio <strong>di</strong> cosa può avvenire ce lo<br />
fornisce la sezione SO 161-44 che mostra piccole unità con<br />
superfici <strong>di</strong> scollamento subsuperficiali che vanno a costituire,<br />
grazie ad un processo <strong>di</strong> underthrusting, un prisma<br />
d’accrezione che simula una scarpata ripida <strong>ed</strong> omogenea,<br />
quasi fosse strutturata da faglie <strong>di</strong>rette.<br />
CONCLUSIONI<br />
In sostanza tre <strong>di</strong>versi tipi <strong>di</strong> processi <strong>di</strong> accrezione sembrano<br />
attualmente attivi ai pi<strong>ed</strong>i <strong>del</strong>la scarpata <strong>del</strong> margine attivo<br />
cileno a N <strong>del</strong> punto triplo. Il primo prende origine dal<br />
“campionamento”<strong>di</strong> crosta oceanica in scaglie che vanno a<br />
costituire un prisma a geometria “classica” formato in più fasi.<br />
Nelle parti in cui è presente un prisma s<strong>ed</strong>imentario, via via<br />
più potente da N verso S, la presenza <strong>di</strong> livelli <strong>di</strong> scollamento al<br />
suo interno, più o meno profon<strong>di</strong> determina l’attivarsi <strong>di</strong> due<br />
<strong>di</strong>versi processi:<br />
- la formazione <strong>di</strong> gran<strong>di</strong> Thrust le cui anticlinali <strong>di</strong><br />
rampa costituiscono salienti morfologici imponenti e<br />
caratterizzanti il downslope;<br />
- la formazione <strong>di</strong> unità sotto scorrenti (underthrusting)<br />
relativamente piccole che forniscono alla scarpata<br />
inferiore un carattere morfologico <strong>di</strong> continuità e<br />
ripi<strong>di</strong>tà estrema.<br />
Esistono, beninteso situazioni interm<strong>ed</strong>ie tra queste ultime, ma<br />
non sono frequenti. L’inclinazione <strong>del</strong> basamento oceanico non<br />
sembra carattere determinante il processo <strong>di</strong> accrezione, anzi, i<br />
dati esaminati mostrano una sostanziale in<strong>di</strong>pendenza <strong>di</strong> tale<br />
fattore dalla “anzianità” <strong>del</strong>la litosfera oceanica. Solo in<br />
prossimità <strong>del</strong> punto triplo si ha una marcata riduzione <strong>del</strong>la<br />
pendenza a parità <strong>di</strong> altri fattori.<br />
Fig, 3 – <strong>Sezione</strong> crostale in prossimità <strong>del</strong> Golfo <strong>di</strong> Arauco<br />
REFERENCES<br />
BANGS N. L., CANDE S.C. (1997) – Episo<strong>di</strong>c development of a<br />
convergen tmargin inferr<strong>ed</strong> from structures and processes<br />
along the southern Chile margin. Tectonics, Vol. 16,<br />
3,489-503 .<br />
FOLGUERA A., RAMOS V., MELNICK D. (2003) – Instability of<br />
southern Andean strain during the last 25 Ma. 219-222<br />
MELNICK D. (2007) - Neogene seismotectonics of the southcentral<br />
Chile Universitat Postadam IGMNF, Scientific<br />
Technical Report STR 07/01, 108 pp.<br />
RAMOS V.A. (1999) – Plate tectonic setting of the Andean<br />
Cor<strong>di</strong>llera Episodes, Vol. 22, 3.<br />
Si ringrazia Juan Diaz Naveas <strong>del</strong>l’Università Cattolica <strong>di</strong> Valparaiso per<br />
la gentile concessione <strong>di</strong> alcuni dati .
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 95-96, 2 ff.<br />
Paleozoic strike-slip tectonics in northern Victoria Land,<br />
(Antarctica) and the Borchgrevink event: is there a link?<br />
LAURA FEDERICO (*), LAURA CRISPINI (*) & GIOVANNI CAPPONI (*)<br />
ABSTRACT<br />
Tettonica trascorrente paleozoica in northern Victoria Land (Antartide) e<br />
l'evento Borchgrevink: esiste una relazione?<br />
Il presente lavoro descrive la tettonica paleozoica per un settore <strong>del</strong>le<br />
Transantarctic Mountains nella Northern Victoria Land (Antartide).<br />
L'evoluzione strutturale e l'architettura <strong>di</strong> questo settore <strong>di</strong> catena è stata<br />
ricostruita su una base dati derivata dall'attività <strong>di</strong> terreno effettuata durante 5<br />
sp<strong>ed</strong>izioni in Antartide. I dati relativi alle superfici <strong>di</strong> faglia, alle lineazioni e<br />
agli in<strong>di</strong>catori cinematici, attribuibili a deformazioni pre-cenozoiche, sono<br />
stati selezionati <strong>ed</strong> elaborati per definire la cinematica regionale e i campi <strong>di</strong><br />
paleostress: i risultati in<strong>di</strong>cano la prevalenza <strong>di</strong> un regime transpressivo. Le<br />
faglie <strong>di</strong> questa generazione sono accompagnate da "damage zone"<br />
caratterizzate da alterazione idrotermale a epidoto e clorite, da network <strong>di</strong> vene<br />
a quarzo e carbonati e localmente sono associate a corpi <strong>di</strong> vulcaniti<br />
ipoabissali in parte correlabili alle Gallipoli Volcanics (Devoniano-<br />
Carbonifero). Datazioni 39 Ar- 40 Ar su miche bianche, eseguite in una zona <strong>di</strong><br />
faglia, hanno fornito età <strong>di</strong> 318,0 ± 4,6 and 310,5 ± 6,2 Ma e forniscono<br />
un'evidenza ulteriore per attribuire le strutture ad un evento tettonico<br />
Paleozoico. Nonostante queste età siano considerevolmente più giovani,<br />
rimane aperto un interrogativo sulle possibili relazioni tra questa tettonica<br />
transpressiva e l'ipotetico evento Borchgrevink (circa 360 Ma).<br />
Key words: Antarctica, northern Victoria Land, paleozoic<br />
tectonics, strike-slip faulting.<br />
Northern Victoria Land is the Pacific <strong>ed</strong>ge of the<br />
Transantarctic Mountains (Fig. 1); its structural architecture<br />
was chiefly produc<strong>ed</strong> by the Ross Orogeny, starting from the<br />
Neoproterozoic to early Paleozoic, and follow<strong>ed</strong> by a<br />
widespread magmatic pulse during the Devonian -<br />
Carboniferous (Admiralty Intrusives and Gallipoli Volcanics).<br />
This magmatic event is possibly associat<strong>ed</strong> to the still elusive<br />
Borchgrevink Orogeny (GRINDLEY & WARREN, 1964; CAPPONI<br />
et alii, 2002). Finally, Meso-Cenozoic tectonics, link<strong>ed</strong> to the<br />
fragmentation of Gondwana and to the opening of the West<br />
Antarctic Rift System, is responsible for the high-elevation of<br />
the Transantarctic Mountains and the reactivation of the<br />
_________________________<br />
(*) Dip.Te.Ris - Università <strong>di</strong> Genova<br />
Corso Europa 26, 16132 Genova, Italy<br />
correspon<strong>di</strong>ng author: L. F<strong>ed</strong>erico - f<strong>ed</strong>erico@<strong>di</strong>pteris.unige.it<br />
inherit<strong>ed</strong> Paleozoic <strong>di</strong>scontinuities (e.g. SALVINI et alii, 1997).<br />
Northern Victoria Land geology is classically describ<strong>ed</strong> by<br />
means of the accretion of three terranes during the Ross<br />
Orogeny (Fig. 2): the inboard Wilson (WT), the interm<strong>ed</strong>iate<br />
Bowers (BT) and the outboard Robertson Bay Terrane (RBT).<br />
WT and BT are in contact by a first-order tectonic surface, i.e.<br />
the Lanterman Fault (CAPPONI et alii, 1999), whereas the<br />
boundary between BT and RBT is characteriz<strong>ed</strong> by the<br />
occurrence of the high-strain belt of the Millen Schist (Capponi<br />
et al., 2003). The Millen Schist are compos<strong>ed</strong> of two elements<br />
separat<strong>ed</strong> by a main thrust fault (the Crosscut-Aorangi Thrust,<br />
CRISPINI et alii, 2007), that appears to be locally truncat<strong>ed</strong> by<br />
the Leap Year Fault.<br />
The database of this work was collect<strong>ed</strong> in the study area<br />
(Fig. 2) by the senior authors, during 5 Antarctic exp<strong>ed</strong>itions<br />
and consists of several hundr<strong>ed</strong>s of data of fault orientations<br />
and relat<strong>ed</strong> slickenlines, integrat<strong>ed</strong> with observations on the<br />
fault kinematics and cross-cutting relationships.<br />
We process<strong>ed</strong> these data with fault inversion techniques to<br />
calculate the paleostress fields: our analysis highlight<strong>ed</strong> the<br />
occurrence of brittle deformations in a traspressive regime, with<br />
faults that locally cut Admiralty Intrusives and are associat<strong>ed</strong> to<br />
hypabyssal intrusions strongly resembling Gallipoli Volcanics.<br />
A widespread hydrothermal circulation is responsible for<br />
<strong>di</strong>ffuse quartz-carbonate veining (see CRISPINI et alii, this<br />
Fig. 1 – Location of northern Victoria Land within Antarctica
96 L.FEDERICO ET ALII<br />
Fig. 2 – Geological map of northern Victoria Land and location of the study<br />
area<br />
volume) and epidote mineralization.<br />
In the study area most brittle structures have been so far<br />
attribut<strong>ed</strong> to Cenozoic deformation link<strong>ed</strong> to Australia-<br />
Antarctica break-up (SALVINI et alii, 1997). This event was<br />
achiev<strong>ed</strong> dominantly by means of dextral strike-slip faults.<br />
However some of the stu<strong>di</strong><strong>ed</strong> faults are locally cut by dextral<br />
strike-slip faults that can be referr<strong>ed</strong> to the Cenozoic tectonics.<br />
Therefore the fluid circulation and associat<strong>ed</strong> transpressional<br />
deformation are older and have to be probably assign<strong>ed</strong> to<br />
Paleozoic times.<br />
The association to Gallipoli Volcanics and to hydrothermal<br />
circulation suggests that stu<strong>di</strong><strong>ed</strong> faults are syn to post the main<br />
phase of Admiralty / Gallipoli magmatism. White micas<br />
separat<strong>ed</strong> from a gold-bearing quartz vein associat<strong>ed</strong> with faults<br />
of this set suppli<strong>ed</strong> two reliable 39 Ar- 40 Ar ages of 318,0 ± 4,6<br />
and 310,5 ± 6,2 Ma. This corroborates a tight link with the<br />
Admiralty / Gallipoli magmatism. As a working hypothesis, this<br />
transpressional tectonics can be tentatively thought as<br />
expression of the cryptic Devonian-Carboniferous<br />
Borchgrevink Orogeny.<br />
REFERENCES<br />
CAPPONI G., CAROSI R., MECCHERI M. & OGGIANO G. (2003) -<br />
Strain analysis in the Millen Range of northern Victoria<br />
Land, Antarctica. In: F: Tessensohn & C.A. Ricci (Eds). -<br />
Aspects of a Suture Zone. The Mariner Glacier Area,<br />
Antarctica. Geol. Jb., B 85, 225 - 251.<br />
CAPPONI G., CRISPINI L. & MECCHERI M. (1999) - Structural<br />
history and tectonic evolution of the boundary between the<br />
Wilson and Bowers Terranes, Lanterman Range, Northern<br />
Victoria Land, Antarctica. Tectonophysics, 312 (2-4), 249-<br />
266.<br />
CAPPONI G., CASTORINA F., DI PISA A., MECCHERI M., PETRINI<br />
R. & VILLA I.M. (2002) - The metaigneous rocks of the<br />
Barber Glacier area (northern Victoria Land, Antarctica):<br />
a clue to the enigmatic Borchgrevink Orogeny? In Gamble,<br />
J., Skinner, D., and Henrys, S. (Eds.), Antarctica at the<br />
close of a Millennium, Royal Society of New Zealand<br />
Bullettin 35, 99 - 104.<br />
CRISPINI L., CAPPONI G., FEDERICO L. (2007) - Tectonics at the<br />
Bowers - Robertson Bay Terrane boundary, northern<br />
Victoria Land (Antarctica). In: A.R.C. Cooper and the<br />
ISAES E<strong>di</strong>torial Team. - Antarctica; A keystone in a<br />
changing world - online proce<strong>ed</strong>ings for the tenth<br />
international symposium on Antarctic earth sciences: U.S.<br />
Geological Survey Open-File Report 2007-1047 (DVD-<br />
ROM) [http://pubs.usgs.gov/of/2007/1047/]<br />
CRISPINI L., FEDERICO L., CAPPONI G. & TALARICO F. (2008) -<br />
Gold-bearing veins in transcrustal fault zone in the<br />
Transantarctic Mountains (northern Victoria Land,<br />
Antarctica) Riunione Società Geologica, Sassari 15-<br />
15/09/2008, Abstract Volume, 276 - 277.<br />
GRINDLEY G.W. & WARREN G. (1964) - Stratigraphic<br />
nomenclature and correlation in the western Ross Sea<br />
region. In: R.J. A<strong>di</strong>e (Ed.). - Antarctic geology. Amsterdam,<br />
North Holland Publishing Company, 314-333.<br />
SALVINI F., BRANCOLINI G., BUSETTI M., STORTI F., MAZZARINI<br />
F. & COREN F. (1997) - Cenozoic geodynamics of the Ross<br />
Sea Region, Antarctica: Crustal extension, intraplate<br />
strike-slip faulting and tectonic inheritance. J. Geoph. Res.,<br />
102, 24669-24696.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 97-99<br />
Active transpression within the frontal zone of the Southern<br />
Apennines in northern Calabria by integration of geomorphologic,<br />
structural, and marine geophysical data.<br />
ABSTRACT<br />
Transpressione attiva nel settore frontale <strong>del</strong>l’Appennino Meri<strong>di</strong>onale in<br />
Calabria settentrionale da integrazione <strong>di</strong> dati geomorfologici, strutturali<br />
e <strong>di</strong> geofisica marina.<br />
Una analisi integrata <strong>di</strong> dati geomorfologici, strutturali e <strong>di</strong> geofisica<br />
marina ha consentito <strong>di</strong> definire il quadro sismo tettonico <strong>del</strong>la zona frontale<br />
<strong>del</strong>l’Appennino meri<strong>di</strong>onale lungo la costa ionica compresa tra i massicci <strong>del</strong><br />
Pollino e <strong>del</strong>la Sila. Il sollevamento <strong>di</strong> terrazzi marini <strong>di</strong> età mesopleistocenicaolocenica<br />
presenta <strong>del</strong>le ondulazioni locali che coincidono spazialmente con<br />
pieghe recenti nel substrato e in s<strong>ed</strong>imenti continentali, e con anomalie degli<br />
in<strong>di</strong>ci geomorfici fluviali. Pieghe e faglie transpressive sono tracciate anche a<br />
mare in profili sismici a riflessione, lungo linee strutturali alcune <strong>del</strong>le quali<br />
sono caratterizzate da sismicità <strong>di</strong> tipo compressivo e trascorrente. Queste<br />
strutture tagliano l’alloctono sudapenninico e mostrano una generale vergenza<br />
a SW. I livelli strutturali più superficiali sono caratterizzati da faglie normali<br />
<strong>di</strong>scontinue ra<strong>di</strong>cate a scollamenti pellicolari. Pertanto, questa regione sembra<br />
caratterizzata da un campo <strong>di</strong> deformazione transpressivo recente e ancora<br />
attivo, che probabilmente contribuisce al più generale sollevamento <strong>del</strong>l’arco<br />
calabro.<br />
Key words: Active thick-skinn<strong>ed</strong> transpression, Calabria<br />
Apennines, Morphotectonic analysis, Southern Italy.<br />
INTRODUCTION<br />
An integrat<strong>ed</strong> analysis of geomorphologic and structural<br />
data, offshore seismic profiles and morphobathymetric data,<br />
and local network seismicity, is us<strong>ed</strong> to sh<strong>ed</strong> light on the<br />
hitherto poorly known active deformation field that affects the<br />
Southern Apennines frontal orogen in northern Calabria. In the<br />
Southern Apennines, Middle Pleistocene waning of Miocene-<br />
Early Pleistocene thin-skinn<strong>ed</strong> frontal thrust belt motion<br />
(PATACCA & SCANDONE, 2001) was coeval to onset of regional<br />
uplift, which is document<strong>ed</strong> by flights of rais<strong>ed</strong> marine terraces,<br />
and is commonly attribut<strong>ed</strong> to deep sources (BORDONI &<br />
VALENSISE, 1998; CUCCI & CINTI, 1998; CUCCI, 2004).<br />
_________________________<br />
LUIGI FERRANTI (*), MARIA ENRICA MAZZELLA (*), CARMELO MONACO (**),<br />
DANILO MORELLI (°) & ENRICO SANTORO (**)<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Napoli, Italy.<br />
(**) Dipartimento <strong>di</strong> Scienze Geologiche, Università <strong>di</strong> Catania, Italy<br />
(°) Dipartimento <strong>di</strong> Scienze Geologiche, Ambientali e Marine, Università<br />
<strong>di</strong> Trieste, Italy,<br />
The study area is center<strong>ed</strong> on the Ionian Sea coast of<br />
northern Calabria stretching from the borders of the Sila and<br />
Pollino mountain ranges and across the Sibari coastal plain.<br />
The Miocene-Pliocene fold and thrust belt is cross-cut by steep<br />
left strike-slip and transpressional faults mapp<strong>ed</strong> both on-land<br />
(CATALANO et alii, 1993; MONACO et alii, 1998; VAN DIJK et<br />
alii, 2000) and offshore (DEL BEN et alii, 2007), which were<br />
active till the Early and perhaps the Middle Pleistocene. To the<br />
west, the area is border<strong>ed</strong> by the active extensional belt which<br />
runs through the axis of the Apennines, but does not affect the<br />
Ionian Sea terraces. Nonetheless, the existence of normal faults<br />
<strong>di</strong>splacing the terraces toward the Ionian Sea (the Avena-<br />
Lauropoli fault) is debat<strong>ed</strong>.<br />
DATA ANALYSIS<br />
Detail<strong>ed</strong> mapping and correlation of in<strong>di</strong>vidual terrace<br />
remnants allow<strong>ed</strong> to identify eleven terrace orders, plus a<br />
lowermost, Holocene terrace, which are scatter<strong>ed</strong> at elevations<br />
of between ~5 and ~480 m a.s.l. Regional correlations<br />
supplement<strong>ed</strong> by Electron Spin Resonance (ESR) and AMS 14 C<br />
dating conduct<strong>ed</strong> on shells collect<strong>ed</strong> from <strong>di</strong>fferent terraces<br />
allow<strong>ed</strong> to reconstruct a chronological frame for the terrace<br />
flight. Notably, the fourth terrace (T4) is attribut<strong>ed</strong> to the MIS<br />
5.5 (ag<strong>ed</strong> at 124 ka), a prominent marker for deformation<br />
(BORDONI & VALENSISE, 1998; FERRANTI et alii, 2006).<br />
Small- wavelength (~5-10 km) and amplitude (~20-50 m)<br />
undulations are superpos<strong>ed</strong> to the regional uplift (~100 km<br />
length and ~500 m amplitude scale) profile of the Middle-Late<br />
Pleistocene marine terraces, and reveals two structural<br />
culminations in the Pollino and Sila range and a relative low in<br />
the Sibari plain.<br />
Structural analysis both in pre-Quaternary b<strong>ed</strong>rock and in<br />
Quaternary s<strong>ed</strong>iments points to the existence of both<br />
contractional and extensional fabrics, which are segregat<strong>ed</strong><br />
within <strong>di</strong>stinct spatial compartments and at <strong>di</strong>fferent structural<br />
levels. Contractional structures are represent<strong>ed</strong> by folds and<br />
transpressional faults which can be trac<strong>ed</strong> both in b<strong>ed</strong>rock and<br />
cover, and are <strong>di</strong>stribut<strong>ed</strong> within the mountain range and at its<br />
borders. Conversely, the normal faults are basically retriev<strong>ed</strong> in<br />
the Lower Pleistocene clay and overlying terrace deposits<br />
outcropping on the range-fringing coastal plain. The normal<br />
faults which cut the terraces do not form a through-going
98 L. FERRANTI ET ALII<br />
lineament (the Avena-Lauropoli fault) but rather involves<br />
separate scarps with <strong>di</strong>fferent detachment levels as in<strong>di</strong>cat<strong>ed</strong> by<br />
the abrupt lateral variation in hanging-wall tilts of the terrac<strong>ed</strong><br />
deposits.<br />
Quantitative analysis of streams flowing toward this<br />
coastline (SL index, Vf index, hypsometric integral) reveals the<br />
existence of ~E-W to NW-SE striking anomaly axes. These<br />
anomalies spatially coincide with the last generation of folds<br />
mapp<strong>ed</strong> in b<strong>ed</strong>rock and in Middle Pleistocene s<strong>ed</strong>iments, and<br />
their trace intersects the marine terraces undulations at the<br />
coast.<br />
Analysis of multichannel seismic reflection profiles<br />
supplement<strong>ed</strong> by oil-exploration well logs illustrates that the<br />
structural pattern offshore the study are is dominat<strong>ed</strong> by steep<br />
thrusts and transpressional faults that bound structural highs<br />
and lows. The steep faults cut the frontal part of the fold and<br />
thrust belt and mostly shows evidence of SW-<strong>di</strong>rect<strong>ed</strong><br />
<strong>di</strong>splacement, particularly on the southern border of the<br />
Amendolara ridge offshore the Pollino range. Notably, also the<br />
Middle Pleistocene s<strong>ed</strong>imentary sequence submerg<strong>ed</strong> beneath<br />
the continental shelf is tilt<strong>ed</strong> and fold<strong>ed</strong> and the sea-bottom<br />
topography appears controll<strong>ed</strong> by SW-<strong>di</strong>rect<strong>ed</strong> high-angle<br />
oblique thrust faults which cut across the early low-angle, NE<strong>di</strong>rect<strong>ed</strong><br />
thrust imbricates.<br />
Slumping or creeping at more surficial levels occurs above<br />
listric normal faults localiz<strong>ed</strong> on the steeper flanks of the<br />
Amendolara and other ridges. Marker correlation suggests that<br />
the listric faults are root<strong>ed</strong> within Lower Pleistocene clays, a<br />
situation similar to the tilt<strong>ed</strong> structural panels found on-land<br />
along the Avena-Lauropoli fault.<br />
Crustal seismic epicenter <strong>di</strong>stribution and focal solutions of<br />
low- to moderate earthquakes record<strong>ed</strong> by a local seismic<br />
network during 2005-2007 illuminate two NW-SE tren<strong>di</strong>ng<br />
structural belts beneath the Amendolara ridge and northern<br />
Sila, where partitioning between thrust and left strike-slip<br />
motion occurs in response to ~E to ~NE <strong>di</strong>rect<strong>ed</strong> shortening.<br />
DISCUSSION AND CONCLUSIONS<br />
Our integrat<strong>ed</strong> study allows to argue that the local-scale, but<br />
pervasive undulations in the deformation profile of marine<br />
terraces, superpos<strong>ed</strong> to the more general uplift pattern,<br />
represent shallow-crustal folds relat<strong>ed</strong> to a recent and still<br />
active transpressional field. A major structural culmination<br />
bound by fore- and retro-verging transpressional shear zones is<br />
represent<strong>ed</strong> by the Pollino mountain range and its offshore<br />
extension in the Amendolara ridge, and a further SW-<strong>di</strong>rect<strong>ed</strong><br />
transpressional belt is found in northern Sila and adjacent sea<br />
bottom. These structural belts involve Middle-Late Pleistocene<br />
s<strong>ed</strong>iments both on-land and offshore. Their present activity is<br />
suggest<strong>ed</strong> by small to moderate seismicity which shows<br />
partitioning between thrusts and strike-slip focal solutions.<br />
Ad<strong>di</strong>tionally, the novel seismotectonic frame reconstruct<strong>ed</strong> for<br />
this region is consistent with Global Positioning System (GPS)<br />
velocities of sites of the Peri-Tyrrhenian Geodetic Array<br />
(Ferranti et al., 2008). When view<strong>ed</strong> in a MATE reference<br />
frame, GPS velocities of sites in southern Pollino converge<br />
toward MATE in<strong>di</strong>cating ~N-S shortening in the intervening<br />
region, a pattern consistent with that document<strong>ed</strong> in Middle<br />
Pleistocene rocks. On the other hand, a site on the Sila massif<br />
converge obliquely toward MATE and its velocity is subparallel<br />
to the P-axes of earthquakes in the southern Taranto<br />
Gulf. Thus, con<strong>di</strong>tions of non-plane strain are achiev<strong>ed</strong> in the<br />
region, consistent with active transpression.<br />
The small-length normal faults of the Avena-Lauropoli<br />
system which locally cut the marine terraces do not form a<br />
through-going lineament, rather they accommodate the seaward<br />
collapse of the uppermost crust above the deeper shortening<br />
compartment. Conversely, the active transpression testifi<strong>ed</strong> by<br />
geomorphic, structural and seismicity data is accommodat<strong>ed</strong><br />
along deep-seat<strong>ed</strong> oblique back-thrusts that involve the SW<br />
margin of the Apulian foreland plate underlying the now<br />
inactive thin-skinn<strong>ed</strong> accretionary w<strong>ed</strong>ge. The tight interlacing<br />
between regional and local components of deformation<br />
affecting the marine terraces carries the important implication<br />
that surficial and deep structures may be link<strong>ed</strong>, and thus whole<br />
crustal shortening may be a viable contributor to regional uplift<br />
in this part of the Calabrian Arc.<br />
REFERENCES<br />
BORDONI P. & VALENSISE G. (1998) - Deformation of the 125<br />
ka marine terrace in Italy; tectonic implications. In: I.S.<br />
Stewart, and C. Vita-Finzi (Eds.) - Coastal Tectonics.<br />
Geological Society Special Publication, London, 146, 71-<br />
110.<br />
CATALANO S., MONACO C., TORTORICI L. & TANSI C. (1993) -<br />
Pleistocene strike-slip tectonics in the Lucanian Apennine<br />
(Southern Italy). Tectonics 12, 656-665.<br />
CUCCI L. (2004) - Rais<strong>ed</strong> marine terraces in the Northern<br />
Calabrian Arc (Southern Italy): a ~600Kyr-long geological<br />
record of regional uplift. Ann. Geophys., 47, 1391-1406.<br />
CUCCI L., & CINTI F.R. (1998) - Regional uplift and local<br />
tectonic deformation record<strong>ed</strong> by the Quaternary marine<br />
terraces on the Ionian coast of northern Calabria (southern<br />
Italy). Tectonophysics, 292, 67-83.<br />
DEL BEN A., BARNABA C. & TOBOGA A. (2007) - Strike-slip<br />
systems as the main tectonic features in the Plio-<br />
Quaternary kinematics of the Calabrian Arc. Mar.<br />
Geophys. Res., DOI 10.1007/s11001-007-9041-6.<br />
FERRANTI L., ANTONIOLI F., MAUZ B., AMOROSI A., DAI PRÀ<br />
G., MASTRONUZZI G., MONACO C., ORRÙ P., PAPPALARDO<br />
M., RADTKE U., RENDA P., ROMANO P., SANSÒ P. &<br />
VERRUBBI V. (2006) - Markers of the last interglacial sealevel<br />
high stand along the coast of Italy: Tectonic<br />
implications. Quat. Int. 145-146, 30-54.<br />
FERRANTI L., OLDOW J.S., D’ARGENIO B., CATALANO R.,<br />
LEWIS D., MARSELLA E., AVELLONE G., MASCHIO L.,<br />
PAPPONE G., PEPE F. & SULLI A. (2008) - Active
ACTIVE TRANSPRESSION WITHIN THE FRONTAL ZONE OF THE SOUTHERN APENNINES IN NORTHERN CALABRIA<br />
deformation in Southern Italy, Sicily and southern Sar<strong>di</strong>nia<br />
from GPS velocities of the Peri-Tyrrhenian Geodetic Array<br />
(PTGA). Boll. Soc. Geol. It., 127(2), 299-316.<br />
MONACO C., TORTORICI L. & PALTRINIERI W. (1998) -<br />
Structural evolution of the Lucanian Apennines, southern<br />
Italy. J. Struct. Geol., 20, 617-638.<br />
PATACCA E. & SCANDONE P. (2001) - Late thrust propagation<br />
and s<strong>ed</strong>imentary response in the thrust belt-for<strong>ed</strong>eep system<br />
of the Southern Apennines (Pliocene-Pleistocene). In: G.B.<br />
Vai and I.P. Martini (Eds.) - Anatomy of a mountain belt:<br />
the Apennines and adjacent M<strong>ed</strong>iterranean basins. Kluwer<br />
Academic Publishers, Dordecht, p. 401-440.<br />
VAN DIJK J.P., BELLO M., BRANCALEONI G.P., CANTARELLA<br />
G., COSTA V., FRIXA A., GOLFETTO F., MERLINI S., RIVA<br />
M., TORRICELLI S., TOSCANO C. & ZERILLI A. (2000) - A<br />
regional structural mo<strong>del</strong> for the northern sector of the<br />
Calabrian Arc (southern Italy). Tectonophysics, 324, 267–<br />
320.<br />
99
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 100-103, 2ff.<br />
Nuovi dati su geometria, cinematica e segmentazione <strong>del</strong> sistema <strong>di</strong><br />
faglie attive lungo il margine nord-orientale <strong>del</strong> Matese (Molise)<br />
FEDERICA FERRARINI (*), BONCIO PAOLO (**), GERARDO PAPPONE (°), MASSIMO CESARANO (*), PIETRO<br />
P.C.AUCELLI (°)<br />
New data on geometry, kinematics and segmentation of the active normal<br />
fault system along the north-easter border of the Matese Mts. (Molise)<br />
The presence of N-to-NE-<strong>di</strong>pping active normal faults along the north-eastern<br />
border of the Matese Mts is document<strong>ed</strong> in the existing literature but the<br />
<strong>di</strong>splacements accumulat<strong>ed</strong> by the normal faults during their activity and the<br />
segmentation pattern are largely unconstrain<strong>ed</strong>. Moreover, only few data,<br />
mostly collect<strong>ed</strong> in the southern termination of the system, constrain the late-<br />
Quaternary slip-rate. We present the results of a detail<strong>ed</strong> structural-geological<br />
survey aim<strong>ed</strong> to constrain the <strong>di</strong>splacements and segmentation of the system.<br />
Late-Quaternary slip-rates are estimat<strong>ed</strong> by measuring the fault scarps which<br />
<strong>di</strong>splace mountain slopes mostly form<strong>ed</strong> during the retreat phase of the last<br />
glacial maximum (ca. 18 ka). Fault <strong>di</strong>splacements can be constrain<strong>ed</strong> in the<br />
central-northern part of the system (M. Patalaecchia – S. Massimo), with<br />
values varying, from N to S, from 300 m (Piana de Il Lago) to 690 m (M.<br />
Patalecchia) to 50 m (S. Massimo). The fault scarp heights, measur<strong>ed</strong> along<br />
the entire system, suggest minimum throw-rates ranging from 0.8 to 1.4 mm/a<br />
for the last 18 ka.<br />
Key words: Active tectonics, Matese Mts., normal fault,<br />
segmentation, slip-rates.<br />
INTRODUZIONE<br />
Il Bacino <strong>di</strong> Bojano è una depressione strutturale allungata<br />
in senso NO-SE, impostata lungo il margine settentrionale <strong>del</strong><br />
Massiccio <strong>del</strong> Matese. La fase tettonica che ha condotto alla<br />
sua attuale strutturazione risale a non prima <strong>del</strong> Pleistocene<br />
M<strong>ed</strong>io (RUSSO & TERRIBILE, 1995; DI BUCCI et alii, 2005), in<br />
concomitanza <strong>del</strong>l’ultimo episo<strong>di</strong>o deformativo che, in or<strong>di</strong>ne<br />
<strong>di</strong> tempo, ha interessato questo settore.<br />
L’area è s<strong>ed</strong>e storicamente <strong>di</strong> eventi sismici <strong>di</strong>struttivi<br />
(Terremoto <strong>di</strong> S. Anna, 1805, Maw= 6.57 da GRUPPO DI<br />
LAVORO CPTI, 2004; ESPOSITO et alii, 1987) e <strong>di</strong> una sismicità<br />
<strong>di</strong>ffusa caratterizzata da sequenze sismiche <strong>di</strong> bassa energia<br />
(1986, 1990, 1997, 2001), localizzate in prossimità <strong>del</strong> suo<br />
margine nord-orientale (ALESSIO et alii, 1987; MILANO et alii,<br />
2005) <strong>ed</strong> in aree a<strong>di</strong>acenti (Sannio) comunque molto prossime<br />
_________________________<br />
(*) Dipartimento S.T.A.T – Università degli Stu<strong>di</strong> <strong>del</strong> Molise,<br />
ContradaFonte Lappone, 86090 Pesche (Isernia).<br />
(**) Laboratorio <strong>di</strong> Geo<strong>di</strong>namica e Sismogenesi, Dip. Scienze <strong>del</strong>la Terra,<br />
Università «G. d’Annunzio» - Via dei Vestini, 30, 66013 Chieti Scalo,<br />
Chieti. Tel. 0871/3556456<br />
(°) Dipartimento <strong>di</strong> Scienze per l’Ambiente, Università degli Stu<strong>di</strong> <strong>di</strong><br />
Napoli “Parthenope”, Centro Direzionale-Isola C4, Napoli. Tel.<br />
081/5476663.<br />
alla piana (MILANO et alii, 1999; VILARDO et alii, 2003).<br />
Il riconoscimento <strong>di</strong> strutture ritenute attive nel tardo<br />
Pleistocene-Olocene, e possibilmente responsabili <strong>del</strong>la<br />
sismicità <strong>del</strong>l’area, è già stato segnalato da vari autori<br />
(ASCIONE et alii, 1998; CINQUE et alii, 2000; BLUMETTI et alii,<br />
2000; GALLI & GALADINI, 2002; GUERRIERI et alii, 1999;<br />
MASCHIO, 2003; DI BUCCI et alii, 2005) ma solo su alcuni<br />
segmenti (GALLI & GALADINI, 2002; GUERRIERI et alii, 1999)<br />
sono stati stimati i possibili valori <strong>di</strong> slip-rate nel tardo<br />
Quaternario. Nella presente nota si vogliono riportare i risultati<br />
<strong>di</strong> parte <strong>del</strong>la ricerca condotta nell’ambito <strong>di</strong> un dottorato che<br />
ha previsto, tra le <strong>di</strong>verse finalità, l’analisi <strong>di</strong> dettaglio <strong>di</strong><br />
strutture potenzialmente sismogenetiche, ritenute tali sulla base<br />
<strong>del</strong>le segnalazioni presenti in letteratura o <strong>di</strong> nuove evidenze <strong>di</strong><br />
tettonica attiva. Lo stu<strong>di</strong>o condotto ha consentito <strong>di</strong> valutare i<br />
rigetti stratigrafici lungo un sistema <strong>di</strong> faglie che si sviluppa dal<br />
versante nord-orientale <strong>del</strong> M. Patalecchia a Guar<strong>di</strong>aregia. La<br />
realizzazione <strong>di</strong> profili topografici, secondo una metodologia<br />
già applicata in altre aree <strong>del</strong>l’Appennino centro-meri<strong>di</strong>onale<br />
(MOREWOOD & ROBERTS, 2000) ha consentito, lungo i<br />
segmenti sospetti, <strong>di</strong> stimare gli slip-rates <strong>di</strong> lungo termine<br />
tardo-quaternari, fornendo dati in<strong>ed</strong>iti in un’area considerata ad<br />
elevato rischio sismico.<br />
ASSETTO TETTONICO<br />
Il rilevamento geologico-strutturale <strong>di</strong> dettaglio, ha<br />
interessato faglie che ricadono in due settori prossimi alla piana<br />
<strong>di</strong> Bojano. Il primo comprende il versante nord-orientale <strong>del</strong><br />
M. Patalecchia e de La Difenzola, il versante orientale <strong>di</strong> Colle<br />
<strong>di</strong> Mezzo <strong>ed</strong> il settore compreso tra gli abitati <strong>di</strong><br />
Roccamandolfi e Cantalupo <strong>del</strong> Sannio. Nel secondo, invece,<br />
ricade l’area compresa tra gli abitati <strong>di</strong> Bojano e Guar<strong>di</strong>aregia.<br />
In entrambi affiorano s<strong>ed</strong>imenti riferibili ad una ambiente<br />
deposizionale <strong>di</strong> piattaforma/rampa carbonatica e <strong>di</strong><br />
transizione a bacino (D’ARGENIO et alii, 1972) i quali coprono<br />
un intervallo stratigrafico che, nell’area indagata, va dal<br />
Giurassico Superiore al Miocene M<strong>ed</strong>io (PROGETTO CARG, F.<br />
408, CAMPOBASSO; DE CORSO et alii, 1998). Seguono<br />
s<strong>ed</strong>imenti calcareo-marnosi <strong>di</strong> ambiente neritico (SELLI, 1957)<br />
e le sequenze calcarenitico-argilloso-arenacee <strong>di</strong> avanfossa <strong>del</strong><br />
Tortoniano Superiore-Messiniano Inferiore (PROGETTO CARG,<br />
F. 408, CAMPOBASSO, PATACCA et alii, 1992).
FAGLIE ATTIVE LUNGO IL MARGINE NORD-ORIENTALE DEL MATESE<br />
Il settore è stato coinvolto nella deformazione compressiva<br />
terziaria a partire dal Miocene Superiore (PATACCA et alii,<br />
1992; FERRANTI, 1994; SCROCCA et alii, 1995; FESTA et alii,<br />
2006); le pieghe <strong>ed</strong> i sovrascorrimenti derivanti da questa fase<br />
tettonica sono state successivamente <strong>di</strong>slocate da faglie<br />
trascorrenti <strong>del</strong> Pliocene Superiore-Pleistocene Inferiore.<br />
L’assetto tettonico attuale v<strong>ed</strong>e un substrato carbonatico<br />
fortemente <strong>di</strong>ssecato da faglie variamente orientate (N-S, E-O,<br />
NE-SO) (NASO et alii, 1989; CORRADO et alii, 1998; DI BUCCI<br />
et alii., 1999). A queste, si sono sovrimposti lineamenti <strong>di</strong>retti<br />
con orientazione appenninica (NO-SE), risultato <strong>del</strong>l’ultimo<br />
evento <strong>di</strong>stensivo che ha investito il settore riutilizzando, dove<br />
possibile, le <strong>di</strong>scontinuità er<strong>ed</strong>itate dalle prec<strong>ed</strong>enti fasi<br />
tettoniche.<br />
EVIDENZE DI TETTONICA ATTIVA E SLIP-RATES DI<br />
LUNGO TERMINE TARDO QUATERNARI.<br />
Secondo BRANCACCIO et alii (1979), la storia morfoevolutiva<br />
<strong>del</strong> Bacino <strong>di</strong> Bojano segue una prima fase tettonica<br />
estensionale che ha interessato, nel Pleistocene Inferiore, il<br />
Matese settentrionale portando alla formazione <strong>del</strong> Bacino<br />
lacustre <strong>di</strong> S. Massimo. A questa prima fase tettonica segue<br />
quella che, tra la fine <strong>del</strong> Pleistocene Inferiore e gli inizi <strong>del</strong><br />
Pleistocene M<strong>ed</strong>io, avrebbe portato alla formazione <strong>di</strong> una<br />
nuova depressione, morfologicamente incastrata nella prima e<br />
con l’assetto morfo-tettonico attuale (RUSSO & TERRIBILE,<br />
1995).Recentemente DI BUCCI et alii (2005) forniscono un’età<br />
<strong>di</strong> circa 620 ka per il materiale vulcanoclastico rinvenuto nei<br />
depositi lacustri sospesi lungo il versante prospiciente la piana,<br />
nei pressi <strong>di</strong> S. Massimo. Tale datazione permette <strong>di</strong> vincolare<br />
l’attività <strong>del</strong>le faglie responsabili <strong>del</strong>la formazione <strong>del</strong>l’attuale<br />
bacino, ad un periodo successivo a 620 ka.<br />
Il rilevamento è stato concentrato lungo strutture già<br />
segnalate in letteratura o ritenute sospette sulla base <strong>di</strong> dati<br />
in<strong>ed</strong>iti derivanti dall’attività <strong>di</strong> ricerca <strong>del</strong> dottorato. Queste<br />
sono state raggruppate nei sistemi MPSM (M. Patalecchia-<br />
S.Massimo) e BG (Bojano-Guar<strong>di</strong>aregia). Su entrambi i sistemi<br />
rilevati, sono state rinvenute, in più punti, brecce <strong>di</strong> versante<br />
cementate chiaramente <strong>di</strong>slocate, come lungo il versante nordorientale<br />
<strong>del</strong> M. Patalecchia, prospiciente la Piana de Il Lago<br />
(già segnalate in BLUMETTI et alii, 2000), sul versante orientale<br />
<strong>di</strong> Colle <strong>di</strong> Mezzo <strong>ed</strong> in prossimità <strong>di</strong> Bojano (già segnalate in<br />
GALLI & GALADINI, 2002). Queste si rinvengono, spesso, ad<br />
una quota più bassa rispetto alle stesse citate in BRANCACCIO et<br />
alii (1979) coeve dei depositi lacustri. È ragionevole pensare<br />
che esse siano il prodotto <strong>del</strong> mo<strong>del</strong>lamento dei versanti<br />
quando la nuova depressione si stava generando.<br />
È possibile ritenere, quin<strong>di</strong>, che le strutture che le hanno<br />
fagliate si siano generate, o abbiano agito riutilizzando vecchie<br />
<strong>di</strong>scontinuità, in un periodo successivo 620 ka. Lungo la faglia<br />
che borda il versante nord-orientale <strong>del</strong> M. Patalecchia è stato<br />
rinvenuto, inoltre, <strong>del</strong> materiale vulcanoclastico riferibile<br />
all’evento conosciuto in letteratura come “Tufo Giallo<br />
Napoletano” (Dott.ssa P. Petrosino, comunicazione personale,<br />
101<br />
15.000-12.000 in DEINO et al., 2004; DI VITO et al., 1999),<br />
intercalato al detrito <strong>di</strong> versante all’hangingwall <strong>del</strong>la faglia<br />
Fig. 1 – Geometria <strong>del</strong> sistema attivo lungo il margine nord-orientale <strong>del</strong><br />
Massiccio <strong>del</strong> Matese.<br />
stessa e localmente <strong>di</strong>slocato da uno splay sintetico. E’<br />
possibile, quin<strong>di</strong>, affermare che l’attività <strong>del</strong>la faglia si<br />
sicuramente tardo-quaternaria.<br />
Sulla base dei dati acquisiti e <strong>del</strong>le evidenze sopra citate, si<br />
ritiene che tutto il sistema attivo caratterizzato si presenti con la<br />
geometria riportata in Fig.1. Essa segue un trend<br />
prevalentemente appenninico nel settore più settentrionale<br />
investigato mentre in quello più meri<strong>di</strong>onale prev<strong>ed</strong>e il<br />
riutilizzo <strong>di</strong> <strong>di</strong>scontinuità tettoniche preesistenti, in prevalenza<br />
con orientazione E-O.<br />
Lungo i vari segmenti è stato effettuato un rilevamento<br />
strutturale <strong>di</strong> dettaglio che ha consentito <strong>di</strong> caratterizzarli<br />
cinematicamente evidenziando, in molti punti, una estensione<br />
orientata in senso NE-SO. Sono state realizzate sezioni<br />
geologiche che hanno consentito <strong>di</strong> stimare i rigetti stratigrafici,<br />
soprattutto lungo il sistema MPSM. I rigetti partono da valori<br />
<strong>di</strong> circa 300 m (in corrispondenza <strong>del</strong>la Piana de Il Lago), per<br />
passare a 690 in corrispondenza <strong>del</strong> M. Patalecchia e <strong>di</strong>minuire<br />
gradualmente fino ad un valore <strong>di</strong> circa 50 m in prossimità <strong>di</strong> S.<br />
Massimo. In corrispondenza <strong>del</strong> sistema BG, invece, il calcolo<br />
dei rigetti stratigrafici risulta <strong>di</strong>fficoltoso per la carenza <strong>di</strong> dati<br />
relativamente alla profon<strong>di</strong>tà <strong>del</strong> substrato carbonatico sotto i<br />
depositi lacustri il cui spessore è stimato, in RUSSO &<br />
TERRIBILE (1995), in almeno 160m.<br />
L’interpretazione <strong>di</strong> pozzi E.R.I.M (Ente per le risorse<br />
idriche molisane) e <strong>del</strong>la cassa <strong>del</strong> Mezzogiorno proposta in<br />
CASCIELLO et alii (2002) fornisce una valore minimo <strong>di</strong><br />
<strong>di</strong>slocazione <strong>del</strong> substrato carbonatico <strong>di</strong> circa 100. Sulla base<br />
<strong>del</strong> rilevamento geologico recentemente effettuato, in<br />
prossimità <strong>del</strong>l’ubicazione <strong>del</strong> pozzo interpretato dagli autori<br />
sopra citati, affiorano calcari riferibili al Cretacico Superiore.<br />
Pur volendo estrapolare questo dato, non è possibile, tuttavia,<br />
valutare l’entità effettiva <strong>del</strong>la <strong>di</strong>slocazione non affiorando il<br />
limite degli stessi con i calcari <strong>del</strong> Crecatico Inferiore, né al
102 F. FERRARINI ET ALII<br />
Fig. 2 – Esempio <strong>di</strong> profilo topografico realizzato attraverso la scarpata <strong>di</strong><br />
faglia <strong>del</strong> M. Patalecchia<br />
footwall né all’hanginwall. Le stesse considerazioni possono<br />
essere fatte in prossimità <strong>del</strong> segmento <strong>di</strong> Campochiaro, dove<br />
all’hanginwaal <strong>del</strong>la faglia, non si conosce l’effettivo spessore<br />
dei s<strong>ed</strong>imenti <strong>di</strong> avanfossa in appoggio <strong>di</strong>scordante sul<br />
substrato carbonatico (SGROSSO, 1963).<br />
Per poter stimare i tassi <strong>di</strong> movimento nel tardo-quaternario<br />
<strong>del</strong>le faglie reputate attive sono stati realizzati 8 profili<br />
topografici perpen<strong>di</strong>colari alle faglie <strong>di</strong>rette, che consentissero<br />
<strong>di</strong> stimare l’entità <strong>del</strong> rigetto topografico su un versante<br />
regolarizzato in seguito all’ultima fase <strong>di</strong> mo<strong>del</strong>lamento postglaciale<br />
(circa 18 ka). In figura 2 viene riportato il profilo<br />
misurato nei pressi <strong>del</strong> M. Patalecchia dove sono riconoscibili<br />
le linee <strong>di</strong> inviluppo <strong>del</strong> versante regolarizzato <strong>di</strong>slocato dalla<br />
faglia.<br />
Sulla base <strong>del</strong>l’entità <strong>del</strong>la <strong>di</strong>slocazione è stato possibile<br />
calcolare gli slip-rates <strong>di</strong> lungo termine tardo-quaternari<br />
relativamente ad ogni segmento. Prec<strong>ed</strong>enti stu<strong>di</strong> sulla Piana <strong>di</strong><br />
Bojano <strong>ed</strong> aree attigue, riportano valori <strong>di</strong> slip-rates, calcolati<br />
su un periodo che va dal Pleistocene Superiore all’Olocene,<br />
variabili tra 0.1 e 1.0 mm/a (GUERRIERI et alii, 1999; CINQUE et<br />
alii, 2000; GALLI & GALADINI, 2002; MASCHIO, 2003). I valori<br />
da noi calcolati oscillano tra 0.8 e 1.4 mm/a <strong>di</strong> componente<br />
verticale minima <strong>del</strong>lo slip-rate per gli ultimi 18 ka.<br />
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neogenica <strong>del</strong> sistema <strong>di</strong> sovrascorrimenti nell’Appennino<br />
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1995(2), 407-418.<br />
SELLI R. (1957). Sulla trasgressione <strong>del</strong> Miocene nell’Italia<br />
Meri<strong>di</strong>onale. Giornale <strong>di</strong> Geologia, 26, 1-54.<br />
SGROSSO I. (1963). La trasgressione miocenica nel Matese<br />
centrale. Boll. Soc. Natur. Napoli Vol. LXXII, 150-153.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 104-107, 6 ff.<br />
Analisi statistica multi-scala <strong>di</strong> dati <strong>di</strong> scan line condotte su analoghi<br />
<strong>di</strong> reservoir <strong>di</strong> idrocarburi<br />
VINCENZO GUERRIERO (*), STEFANO MAZZOLI (*) & STEFANO VITALE (*)<br />
ABSTRACT<br />
Multi-scale statistical analysis of scan line data from reservoir analogues<br />
The study of statistical <strong>di</strong>stributions of structural features such as joint<br />
length, opening <strong>di</strong>splacement (aperture) and spacing at <strong>di</strong>fferent scales of<br />
observation is on the focus of research in the field of reservoir development<br />
and management, since they noticeably affect porosity and permeability of<br />
reservoir rocks.<br />
Recent stu<strong>di</strong>es (ORTEGA et alii, 20006) effectively confirm<strong>ed</strong> that<br />
opening-mode fracture size <strong>di</strong>stribution is well describ<strong>ed</strong> by a power law. A<br />
review of the <strong>di</strong>fferent types of artifacts in determining such <strong>di</strong>stributions has<br />
been provid<strong>ed</strong> by ORTEGA et alii (2006), concerning systematic errors<br />
occurring at both extremities of the scale of observation. However, a nonsystematic,<br />
often more dangerous error is that associat<strong>ed</strong> with the uncertainty<br />
of the obtain<strong>ed</strong> sampling estimates, depen<strong>di</strong>ng on the kind of aleatoric<br />
variables involv<strong>ed</strong> and statistical samples size (GUERRIERO et alii, 2009).<br />
In this paper we suggest specific, rigorous criteria for the analysis of joint<br />
spatial <strong>di</strong>stributions, as well as for the analysis of probability <strong>di</strong>stributions of<br />
sampling estimate errors affecting both exponent and coefficient of the power<br />
law. This is carri<strong>ed</strong> out taking into account methodological issues, mainly<br />
within the framework of multi-scale scan line data analysis.<br />
Key words: confidence interval, fracture statistics, power law<br />
<strong>di</strong>stribution, structural analysis<br />
INTRODUZIONE<br />
Lo stu<strong>di</strong>o <strong>del</strong>la <strong>di</strong>stribuzione statistica <strong>del</strong>le aperture dei<br />
joint è <strong>di</strong> notevole importanza nel campo <strong>del</strong>l’esplorazione<br />
petrolifera, in quanto un’adeguata conoscenza <strong>del</strong>la legge <strong>di</strong><br />
potenza che descrive tale <strong>di</strong>stribuzione (con particolare<br />
riferimento all’esponente) fornisce informazioni importanti,<br />
oltre che sulla porosità per fratturazione e strain longitu<strong>di</strong>nale<br />
associati ad un determinato joint set, anche riguardo la risposta<br />
idraulica <strong>del</strong> mezzo permeabile roccia in presenza <strong>di</strong> moti<br />
filtranti conseguenti ad attività estrattive.<br />
L’esponente m <strong>di</strong> tale legge <strong>di</strong> potenza consente <strong>di</strong><br />
<strong>di</strong>stinguere sistemi <strong>di</strong> fratture fortemente pervasivi (|m| > 1), in<br />
cui il maggior contributo alla porosità per fatturazione è fornito<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra – Università “F<strong>ed</strong>erico II” – Largo<br />
S. Marcellino 10 - 80138 Napoli, Italy<br />
Lavoro eseguito con il contributo finanziario <strong>di</strong> Shell Italia E&P<br />
dalle fratture più piccole, da set a minor grado <strong>di</strong> pervasività<br />
(GUERRIERO et alii., 2009). Il grado <strong>di</strong> pervasività <strong>di</strong> un<br />
fracture set può influenzare notevolmente le proprietà <strong>di</strong><br />
permeabilità <strong>del</strong>le rocce e la loro risposta idraulica ad un moto<br />
filtrante non stazionario. Infatti, considerando il moto filtrante<br />
<strong>di</strong> un fluido nella roccia, nel caso |m| < 1 si hanno “numerose”<br />
gran<strong>di</strong> fratture in cui circola il fluido proveniente da una roccia<br />
“poco” permeabile. Nel caso |m| > 1 siamo in presenza <strong>di</strong> una<br />
roccia in cui si hanno “poche” gran<strong>di</strong> fratture in cui è<br />
convogliato il fluido proveniente da un mezzo intimamente<br />
fratturato e pertanto “molto” permeabile. Quest’ultima<br />
circostanza risulta evidentemente favorevole nel caso <strong>di</strong> rocce<br />
reservoir.<br />
I maggiori problemi che si riscontrano nell’analisi <strong>del</strong>le<br />
<strong>di</strong>stribuzioni cumulative <strong>del</strong>le aperture sono dovuti: (i) alla<br />
presenza <strong>di</strong> errori sistematici in corrispondenza dei limiti <strong>del</strong>la<br />
scala <strong>di</strong> osservazione (ORTEGA et alii., 2006); (ii) al fatto che le<br />
<strong>di</strong>stribuzioni cumulative campionarie (definite come il numero<br />
<strong>di</strong> fratture per metro con apertura maggiore o uguale al valore<br />
in ascisse) consistono <strong>di</strong> stime <strong>di</strong> densità <strong>di</strong> fratture, per ogni<br />
valore <strong>di</strong> apertura, con incertezza variabile con l’ascissa (Fig.<br />
1), poiché i valori nella parte destra <strong>del</strong> <strong>di</strong>agramma sono<br />
ottenuti da campioni inevitabilmente meno numerosi <strong>di</strong> quelli<br />
più a sinistra (GUERRIERO et alii., 2009).<br />
Fig. 1 – Distribuzione cumulativa ottenuta dai dati <strong>di</strong> scan line su mudstone.<br />
L’ampiezza degli intervalli <strong>di</strong> confidenza <strong>di</strong> ogni determinazione <strong>del</strong>la<br />
frequenza cumulativa (in<strong>di</strong>cati con segmenti verticali), cresce al crescere<br />
<strong>del</strong>l’apertura.
AREA DI STUDIO<br />
Nell’Appennino meri<strong>di</strong>onale, la successione carbonatica<br />
<strong>del</strong>la Piattaforma Appenninica comprende unità stratigrafiche<br />
che sono molto simili, per età, litologia, facies, spessore<br />
complessivo, spessore dei singoli strati meccanici e tessitura,<br />
alle unità produttive <strong>del</strong> reservoir petrolifero <strong>del</strong>la Val d’Agri<br />
in Basilicata. In quest’ultima area, le rocce reservoir, deformate<br />
da ampie pieghe associate a thick-skinn<strong>ed</strong> reverse faults e<br />
strutture da tettonica <strong>di</strong> inversione coinvolgenti il basamento<br />
(MAZZOLI et alii., 2001, 2008), sono costituite da una porzione<br />
tettonicamente sepolta <strong>del</strong>la Piattaforma carbonatica Apula. Pur<br />
con le dovute cautele, tenendo conto <strong>del</strong>la <strong>di</strong>versa evoluzione<br />
strutturale e <strong>di</strong> seppellimento tettonico, le unità <strong>del</strong>la<br />
Piattaforma Appenninica possono essere utilizzate per l’analisi<br />
<strong>del</strong>la fratturazione in affioramento come analoghi dei reservoir<br />
petroliferi. Gli strati stu<strong>di</strong>ati appartengono a successioni<br />
carbonatiche triassico-cenozoiche <strong>del</strong>la Piattaforma<br />
Appenninica. e sono costituiti da calcari, dolomie a grana fina<br />
e dolomie a gran grossolana. Entrambi i tipi <strong>di</strong> dolomia si sono<br />
formati per sostituzione <strong>di</strong>agenetica precoce <strong>del</strong> calcare<br />
(IANNACE et alii, 1994). Tali successioni sono ben esposte in<br />
affioramento lungo un taglio stradale sul versante sud <strong>del</strong><br />
Monte Faito, in Penisola Sorrentina, circa 20 km a SE <strong>di</strong><br />
Napoli, ove sono stati analizzati in dettaglio strati carbonatici<br />
<strong>del</strong> Cretaceo Inferiore (Albiano).<br />
ANALISI STATISTICA: DETERMINAZIONE DELLA<br />
DISTRIBUZIONE SPAZIALE DI JOINT E VENE<br />
Il set <strong>di</strong> dati utilizzato nell’analisi statistica è stato ottenuto<br />
tramite otto scan line, effettuate su calcare, dolomie fini e<br />
dolomie grossolane. Inoltre, su <strong>di</strong> uno strato <strong>di</strong> calcare è stata<br />
eseguita una micro-scan line <strong>del</strong>la lunghezza <strong>di</strong> circa 15 cm,<br />
con ingran<strong>di</strong>mento 50x. I joint set affioranti mostrano un<br />
rapporto m<strong>ed</strong>ia / deviazione standard (SD) <strong>del</strong>le spaziature<br />
tendente all’unità (GUERRIERO et alii., 2009), in quanto la SD<br />
<strong>del</strong>le spaziature converge sistematicamente verso la spaziatura<br />
m<strong>ed</strong>ia (Fig. 2). Il coefficiente <strong>di</strong> uniformità unitario è<br />
caratteristico <strong>di</strong> una <strong>di</strong>stribuzione random (più correttamente<br />
uniforme) <strong>del</strong>le fratture nello spazio, alla quale è associata una<br />
Fig. 2 – M<strong>ed</strong>ia e deviazione standard SD <strong>del</strong>le spaziature in funzione <strong>del</strong>la<br />
<strong>di</strong>mensione <strong>del</strong> campione considerato. La convergenza stocastica <strong>del</strong>la SD<br />
verso la m<strong>ed</strong>ia è caratteristica <strong>del</strong>la <strong>di</strong>stribuzione <strong>di</strong> probabilità esponenziale.<br />
ANALISI STATISTICA MULTI-SCALA DI DATI DI SCAN LINE<br />
105<br />
<strong>di</strong>stribuzione cumulativa esponenziale <strong>del</strong>le spaziature <strong>ed</strong> una<br />
<strong>di</strong>stribuzione <strong>di</strong> Poisson <strong>del</strong>le densità <strong>di</strong> fratture (DEKKING et<br />
alii., 2005).<br />
Tuttavia il rapporto m<strong>ed</strong>ia / SD tendente ad 1 non<br />
costituisce una con<strong>di</strong>zione sufficiente affinché la <strong>di</strong>stribuzione<br />
<strong>del</strong>le spaziature sia effettivamente esponenziale. Allo scopo <strong>di</strong><br />
indagare questo aspetto, si è applicata la tecnica <strong>del</strong> bootstrap<br />
(e.g. DEKKING et alii., 2005; WASSERMAN, 2006) ai dati <strong>di</strong><br />
spaziatura e densità <strong>di</strong> fratture, secondo le modalità seguenti:<br />
Per ogni scan line, dal campione <strong>di</strong> spaziature rilevate sono<br />
state eseguite 10000 determinazioni <strong>di</strong> m<strong>ed</strong>ie tra 10 elementi<br />
scelti a caso con ripetizione <strong>ed</strong> è stata calcolata la <strong>di</strong>stribuzione<br />
cumulativa <strong>di</strong> tali m<strong>ed</strong>ie. Poiché la m<strong>ed</strong>ia <strong>di</strong> una variabile<br />
aleatoria (VA) esponenziale ha <strong>di</strong>stribuzione Chi quadrato, a<br />
tali <strong>di</strong>stribuzioni è stata applicata la funzione <strong>di</strong> <strong>di</strong>stribuzione<br />
inversa Chi quadrato, al fine <strong>di</strong> verificare che i valori ottenuti si<br />
<strong>di</strong>spongano lungo una retta (Fig. 3), in un <strong>di</strong>agramma che ha in<br />
ascissa la relativa VA <strong>ed</strong> in or<strong>di</strong>nata la funzione inversa <strong>del</strong>la<br />
<strong>di</strong>stribuzione cumulativa ad essa associata.<br />
Fig. 3 – Distribuzione inversa Chi quadrato e <strong>di</strong> Gauss <strong>del</strong>le <strong>di</strong>stribuzioni<br />
cumulative osservate tramite il metodo bootstrap, in funzione <strong>del</strong>la relativa VA.<br />
Nel primo caso i punti risultano sempre ben allineati<br />
Ogni scan line è stata sud<strong>di</strong>visa in N-1 segmenti, dove N è il<br />
numero <strong>di</strong> fratture rilevate. Scelti k segmenti a caso (nel nostro<br />
caso abbiamo posto k = 5), nell’ipotesi che i joint abbiano<br />
<strong>di</strong>stribuzione uniforme nello spazio, il numero <strong>di</strong> fratture<br />
presenti in tali segmenti è una VA <strong>di</strong> Poisson. Pertanto sono<br />
state effettuate 10000 determinazioni <strong>del</strong> numero <strong>di</strong> fratture<br />
presenti in campioni, casuali con ripetizione, <strong>di</strong> 5 segmenti <strong>ed</strong> è<br />
stata calcolata la loro <strong>di</strong>stribuzione cumulativa. Poiché la VA <strong>di</strong><br />
Poisson è <strong>di</strong>screta, non è possibile trovarne la funzione <strong>di</strong><br />
<strong>di</strong>stribuzione inversa per qualsivoglia valore <strong>di</strong> <strong>di</strong>stribuzione<br />
cumulativa, senza dover ricorrere all’uso <strong>di</strong> funzioni
106 V. GUERRIERO ET ALII<br />
interpolanti costruite ad hoc. Pertanto, per semplicità <strong>di</strong><br />
esposizione, i dati ottenuti sono riportati in un <strong>di</strong>agramma che<br />
ha in ascisse le <strong>di</strong>stribuzioni cumulative osservate <strong>ed</strong> in<br />
or<strong>di</strong>nate la <strong>di</strong>stribuzione cumulativa <strong>di</strong> Poisson relativa al<br />
m<strong>ed</strong>esimo valore assunto dalla VA (Fig. 4). I punti ottenuti si<br />
<strong>di</strong>spongono ben allineati intorno alla bisettrice <strong>del</strong> I quadrante.<br />
Ciò sta ad in<strong>di</strong>care che le due <strong>di</strong>stribuzioni praticamente<br />
coincidono.<br />
Per confronto, in entrambi i casi (Figg. 3 e 4) sono state<br />
riportate anche le relative <strong>di</strong>stribuzioni <strong>di</strong> Gauss.<br />
In definitiva l’analisi statistica <strong>del</strong>le spaziature ha<br />
Fig. 4 – Frequenza cumulativa teorica in funzione <strong>di</strong> quella osservata,<br />
secondo le <strong>di</strong>stribuzioni <strong>di</strong> probabilità <strong>di</strong> Poisson e <strong>di</strong> Gauss. Nel primo<br />
caso i punti risultano sempre ben allineati alla bisettrice <strong>del</strong> I quadrante,<br />
ossia risulta: frequenza osservata ≈ frequenza teorica <strong>di</strong> Poisson.<br />
confermato che la spaziatura m<strong>ed</strong>ia ha una <strong>di</strong>stribuzione Chi<br />
quadrato (che compete alla m<strong>ed</strong>ia <strong>di</strong> una VA esponenziale) e la<br />
densità <strong>di</strong> fratture è descritta dalla <strong>di</strong>stribuzione <strong>di</strong> probabilità<br />
<strong>di</strong> Poisson. Tali caratteristiche sono entrambe tipiche <strong>di</strong> una<br />
<strong>di</strong>stribuzione uniforme <strong>del</strong>le fratture lungo la scan line.<br />
ANALISI STATISTICA: DISTRIBUZIONE DI<br />
PROBABILITÀ DELLE APERTURE E<br />
QUANTIFICAZIONE DELLE INCERTEZZE<br />
Le <strong>di</strong>stribuzioni cumulative osservate <strong>del</strong>le aperture dei<br />
joint consistono <strong>di</strong> stime <strong>di</strong> densità <strong>di</strong> fratture, calcolate per<br />
ogni classe <strong>di</strong> apertura. L’incertezza associata ad ogni stima <strong>di</strong><br />
densità m<strong>ed</strong>ia <strong>di</strong> fratture può essere descritta dal suo intervallo<br />
<strong>di</strong> confidenza al 95% (definito come l’intervallo reale che<br />
contiene la m<strong>ed</strong>ia <strong>del</strong>la popolazione con probabilità <strong>del</strong> 95%).<br />
Gli intervalli <strong>di</strong> confidenza al 95% possono essere calcolati<br />
rapidamente tramite le equazioni seguenti (GUERRIERO et alii.,<br />
2009):<br />
Flow = (1 – 1.96 / (N – 1) 1/2 ) ⋅ N / L (1)<br />
Fupp = (1 + 1.96 / (N – 1) 1/2 ) ⋅ N / L (2)<br />
dove Flow <strong>ed</strong> Fupp in<strong>di</strong>cano i limiti inferiore e superiore<br />
<strong>del</strong>l’intervallo <strong>di</strong> confidenza, N il numero <strong>di</strong> fratture rilevate <strong>ed</strong><br />
L la lunghezza <strong>del</strong>la scan line. Le eq. (1) e (2) introducono<br />
un’approssimazione nel calcolo, basata sull’applicazione <strong>del</strong><br />
teorema <strong>del</strong> limite centrale. In base al confronto con un<br />
algoritmo <strong>di</strong> calcolo esatto, che utilizza la <strong>di</strong>stribuzione <strong>di</strong><br />
probabilità <strong>di</strong> Poisson per le densità <strong>di</strong> fratture, si può<br />
affermare che tali equazioni forniscono risultati per noi<br />
accettabili per N 20 (GUERRIERO et alii., 2009).<br />
Quin<strong>di</strong> ogni intervallo <strong>di</strong> confidenza osservato in Fig. 1 ha<br />
ampiezza che è funzione <strong>del</strong>lo stesso valore stimato, <strong>di</strong><br />
conseguenza, come si può osservare dal <strong>di</strong>agramma, si ha il<br />
fenomeno <strong>del</strong>l’eterosch<strong>ed</strong>asticità (DEKKING et alii., 2005).<br />
Pertanto non è corretta l’applicazione <strong>del</strong> metodo dei minimi<br />
quadrati per in<strong>di</strong>viduare la retta <strong>di</strong> regressione, senza l’uso <strong>di</strong><br />
opportune funzioni peso per i residui. La conseguenza più<br />
problematica <strong>di</strong> tale metodologia consiste nel fatto che la<br />
maggiore incertezza associata ai punti più a destra nel<br />
<strong>di</strong>agramma, determina un errore aleatorio notevole sul<br />
coefficiente angolare <strong>del</strong>la retta <strong>di</strong> regressione (ossia<br />
l’esponente <strong>del</strong>la legge <strong>di</strong> potenza). Una significativa riduzione<br />
<strong>del</strong>l’incertezza <strong>di</strong> stima <strong>del</strong>l’esponente suddetto può essere<br />
ottenuta da un’analisi multi-scala, integrando dati <strong>di</strong> micro-scan<br />
line con la tra<strong>di</strong>zionale analisi eseguita su scan line in<br />
affioramento (Fig. 5a). Tuttavia anche in questo caso resta il<br />
problema <strong>del</strong>la correttezza <strong>del</strong>l’applicazione <strong>del</strong> metodo dei<br />
minimi quadrati ad un set <strong>di</strong> dati eterosch<strong>ed</strong>astico.<br />
Tale problema è risolto utilizzando una retta passante per<br />
due punti, uno per ogni set <strong>di</strong> dati (Fig. 5b). I punti sono scelti<br />
in maniera tale da avare stime basate sul maggior numero<br />
possibile <strong>di</strong> fratture campionate per ogni set (i punti più a<br />
sinistra nel <strong>di</strong>agramma), che non siano affette da errori <strong>di</strong><br />
troncamento (ORTEGA et alii., 2006). Questo metodo riduce<br />
significativamente l’errore aleatorio nella determinazione<br />
<strong>del</strong>l’esponente <strong>del</strong>la legge <strong>di</strong> potenza.<br />
In Fig. 6 sono riportati i risultati <strong>di</strong> una simulazione Monte<br />
Carlo (con 10000 determinazioni per ogni caso) <strong>di</strong>: scan line<br />
Fig.5 – Distribuzione cumulativa ottenuta dall’uso congiunto <strong>di</strong> dati da scan<br />
line tra<strong>di</strong>zionale e da micro scan line: A) interpolazione tramite retta dei<br />
minimi quadrati; B) Interpolazione dati tramite retta per due punti. I due<br />
segmenti verticali in<strong>di</strong>cano gli intervalli <strong>di</strong> confidenza, calcolati con il<br />
metodo esatto. Le due rette grigie passano per i valori estremi degli intervalli<br />
<strong>di</strong> confidenza.
tra<strong>di</strong>zionale, scan line multiscala con interpolazione secondo il<br />
metodo dei minimi quadrati e scan line multiscala con retta per<br />
due punti, partendo dalla m<strong>ed</strong>esima <strong>di</strong>stribuzione teorica <strong>di</strong><br />
aperture. I <strong>di</strong>agrammi riportano le <strong>di</strong>stribuzioni bivariate <strong>di</strong><br />
probabilità dei valori stimati <strong>di</strong> coefficiente <strong>ed</strong> esponente <strong>del</strong>la<br />
power law.<br />
Come risulta evidente dal <strong>di</strong>agramma <strong>di</strong> Fig. 6c l’uso <strong>di</strong> una<br />
retta per due punti in luogo <strong>di</strong> una retta dei minimi quadrati<br />
determina, per il m<strong>ed</strong>esimo data set, una <strong>di</strong>spersione molto<br />
minore dei valori osservati rispetto ai valori reali <strong>del</strong><br />
coefficiente <strong>ed</strong> esponente <strong>del</strong>la legge <strong>di</strong> potenza.<br />
CONSIDERAZIONI CONCLUSIVE<br />
Una corretta analisi <strong>del</strong>le <strong>di</strong>stribuzioni statistiche <strong>del</strong>le<br />
Fig 6 – Simulazione Monte Carlo <strong>di</strong> 10000 osservazioni <strong>del</strong>la m<strong>ed</strong>esima<br />
<strong>di</strong>stribuzione <strong>di</strong> probabilità teorica <strong>di</strong> aperture, condotte secondo le modalità<br />
seguenti: A) scan line tra<strong>di</strong>zionale, B) scan line multi-scala con metodo dei<br />
minimi quadrati, C) scan line multi-scala con retta per due punti. Sugli assi<br />
orizzontali sono riportati coefficiente <strong>ed</strong> esponente <strong>del</strong>la power law stimati.<br />
Sull’asse verticale è riportata la densità <strong>di</strong> frequenza.<br />
aperture dei joint nei carbonati fratturati richi<strong>ed</strong>e, oltre la stima<br />
dei parametri <strong>di</strong> maggior interesse, la quantificazione <strong>del</strong>le<br />
incertezze associate a tali stime. La determinazione <strong>del</strong>la<br />
<strong>di</strong>stribuzione spaziale dei joint costituisce una fase<br />
irrinunciabile <strong>del</strong>l’analisi statistica, poiché l’applicazione <strong>del</strong>le<br />
eq. (1) e (2) (nonché <strong>del</strong> metodo esatto) per il calcolo degli<br />
intervalli <strong>di</strong> confidenza, si basa sul presupposto teorico che la<br />
spaziatura m<strong>ed</strong>ia abbia una <strong>di</strong>stribuzione <strong>di</strong> probabilità Chi<br />
quadrato e la densità <strong>di</strong> fratture segua la <strong>di</strong>stribuzione <strong>di</strong><br />
Poisson. Una volta verificate tali con<strong>di</strong>zioni si può proc<strong>ed</strong>ere<br />
all’analisi <strong>del</strong>le <strong>di</strong>stribuzioni cumulative <strong>del</strong>le aperture dei joint<br />
ANALISI STATISTICA MULTI-SCALA DI DATI DI SCAN LINE<br />
107<br />
secondo lo schema proposto.<br />
Un’ampia sperimentazione m<strong>ed</strong>iante simulazioni Monte<br />
Carlo ha messo in evidenza come l’applicazione <strong>di</strong> un’analisi<br />
statistica multi-scala e <strong>del</strong> criterio proposto per determinare la<br />
legge <strong>di</strong> potenza che approssima la <strong>di</strong>stribuzione cumulativa<br />
<strong>del</strong>le aperture dei joint (retta per due punti in luogo <strong>del</strong>la retta<br />
dei minimi quadrati), consente una riduzione notevole degli<br />
errori <strong>di</strong> stima <strong>del</strong>l’esponente. Nel caso riportato in Fig. 5 (scan<br />
line su mudstone) la stima <strong>del</strong>l’esponente è pari a 1.26 (in<br />
valore assoluto) <strong>ed</strong> il valore “reale” è compreso nell’intervallo<br />
[0.98 , 1.47] con probabilità <strong>del</strong> 90%.<br />
REFERENCES<br />
DEKKING F. M., KRAAIKAMP C., LOPUHAA H. P. & MEESTER L.<br />
E. (2005) - A Modern Introduction to Probability and<br />
Statistics: Understan<strong>di</strong>ng Why and How. Springer-Verlag,<br />
London (UK).<br />
IANNACE A., GALLUCCIO L., GUERRIERO V., MAZZOLI S.,<br />
PARENTE M. & VITALE S. (2008) - Dolomites within the<br />
Mesozoic carbonates of Southern Apennines (Italy): genetic<br />
mo<strong>del</strong>s and reservoir implications. Rend. Online Soc. Geol.<br />
It. 2, 109-114.<br />
GUERRIERO V., IANNACE A., MAZZOLI S., PARENTE M., VITALE<br />
S. & GIORGIONI M. (2009) - Quantifying uncertainties in<br />
multi-scale stu<strong>di</strong>es of fractur<strong>ed</strong> reservoir analogues:<br />
Implement<strong>ed</strong> statistical analysis of scan line data. Journ.<br />
Struct. Geol. (Approvato per la pubblicazione).<br />
MAZZOLI S., BARKHAM S., CELLO G., GAMBINI R., MATTIONI<br />
L., SHINER P. & TONDI E. (2001) - Reconstruction of<br />
continental margin architecture deform<strong>ed</strong> by the<br />
contraction of the Lagonegro basin, Southern Apennines,<br />
Italy. Journ. Geol. Soc. 158, 309-319.<br />
MAZZOLI S., D’ERRICO M., ALDEGA L., CORRADO S.,<br />
INVERNIZZI C., SHINER P. & ZATTIN M. (2008) – Tectonic<br />
burial and “young” (< 10 Ma) exhumation in the southern<br />
Apennines fold and thrust belt (Italy). Geology 36, 243-<br />
246.<br />
ORTEGA O., MARRETT R. & LAUBACH E. (2006) - A scaleindependent<br />
approach to fracture intensity and average<br />
spacing measurement. AAPG Bull. 90, 193-208.<br />
WASSERMAN L. (2006) - All of Nonparametric Statistics.<br />
Springer Science, New York (USA).
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 108-110, 2ff.<br />
Influence of inherit<strong>ed</strong> geometry and fault history on the seismogenic<br />
activity and potential of strike-slip fault systems in NW Slovenia:<br />
the case study of the Ravne Fault.<br />
RIASSUNTO<br />
L’influenza <strong>del</strong>le geometrie er<strong>ed</strong>itate e <strong>del</strong>la storia <strong>del</strong>la faglia sulla<br />
segmentazione e il potenziale sismogenetico dei sistemi trascorrenti in<br />
Slovenia nord-occidentale: la Faglia <strong>di</strong> Ravne.<br />
La zona <strong>di</strong> faglia Ravne è situata in un area <strong>di</strong> interazione fra due sistemi<br />
regionali <strong>di</strong> faglie con <strong>di</strong>fferente cinematica, entrambi collegati alla<br />
convergenza fra Adria e Eurasia: le faglie <strong>di</strong>nariche orientate NW-SE e le<br />
faglie <strong>del</strong> Sud-alpino orientate E-W.<br />
L’analisi <strong>di</strong> dati <strong>di</strong> geologia strutturale e <strong>di</strong> due sequenze sismiche recenti<br />
che hanno colpito l’area, ci permette <strong>di</strong> proporre un mo<strong>del</strong>lo sismotettonico per<br />
la faglia <strong>di</strong> Ravne, che è stata interessata da <strong>di</strong>verse fasi tettoniche. La<br />
geometria originale e la storia evolutiva <strong>del</strong>la zona <strong>di</strong> faglia svolgono un ruolo<br />
cruciale nella <strong>di</strong>stribuzione recente <strong>del</strong>l’attività sismica e <strong>del</strong> potenziale<br />
sismogenetico <strong>del</strong>l’intera struttura. Infatti, la configurazione attuale <strong>del</strong>la faglia<br />
Ravne, caratterizzata da fagliazione trascorrente su piani ad alto angolo a<br />
profon<strong>di</strong>tà crostali, è il risultato <strong>del</strong>l’iniziale geometria <strong>di</strong> un thrust orientato<br />
NW-SE e avente immersione verso NE, e <strong>del</strong>la sua interazione con i piani <strong>di</strong><br />
thrust <strong>di</strong>retti essenzialmente E-W. Partendo dai dati raccolti e tenendo in<br />
considerazione sia il quadro geo<strong>di</strong>namico che le relazioni empiriche,<br />
proponiamo tre possibili scenari con relativi potenziali sismogenetici per la<br />
possibile futura attività <strong>del</strong>la faglia <strong>di</strong> Ravne.<br />
Key words: fault geometry, fault growth and linkage processes,<br />
reactivation, seismicity.<br />
INTRODUCTION<br />
In this work we study the influence of the inherit<strong>ed</strong><br />
structural pattern and the fault history on the segmentation and<br />
seismogenic potential of the Ravne Fault, a fault zone locat<strong>ed</strong><br />
in the eastern Southern Alps. We also present a mo<strong>del</strong> of<br />
development of the Ravne fault, as a fault zone, which has<br />
undergone multiple tectonic phases. The emphasis of this work<br />
is on the fault's seismic activity and its relat<strong>ed</strong> seismogenic<br />
potential. For this purpose we analyz<strong>ed</strong> surficial geological and<br />
structural data to study the fault geometry and the<br />
microkinematic in<strong>di</strong>cators present within the fault zone. We<br />
also carri<strong>ed</strong> out a detail<strong>ed</strong> study of the spatial, temporal and<br />
kinematic characteristics of the 1998 and 2004 seismic<br />
_________________________<br />
VANJA KASTELIC (*, °), PIERFRANCESCO BURRATO (*) & MARKO VRABEC (°)<br />
(*) Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia, <strong>Sezione</strong> <strong>di</strong> Sismologia e<br />
Tettonofisica, Roma, Italy<br />
(°) Department of Geology, Faculty of Natural Sciences and Engineering,<br />
University of Ljubljana, Askerceva 12, 1000 Ljubljana, Slovenia<br />
sequences.<br />
The Ravne Fault is a NW-SE tren<strong>di</strong>ng, right-lateral strikeslip<br />
fault that lies in the western Julian Alps of NW Slovenia.<br />
In this area the NW-SE orient<strong>ed</strong> faults, typical of the Dinaric<br />
domain, meet and interact with the E-W orient<strong>ed</strong> faults of the<br />
South Alpine domain (e.g. CASTELLARIN & CANTELLI, 2000;<br />
DOGLIONI, 1987; PAMI et alii, 2002). The activity of both<br />
structural trends is controll<strong>ed</strong> by the Adria-Europe relative<br />
convergence, that in the study area results in about 2 mm/a of<br />
N-S orient<strong>ed</strong> shortening (WEBER et alii, 2006, GRENERCZY et<br />
alii, 2005). This deformation is seismically releas<strong>ed</strong> by thrust<br />
faulting earthquakes along the Eastern Southern Alps fronts,<br />
and by right-lateral strike-slip faulting along the Dinaric system<br />
(e.g. BURRATO et alii, 2008; DISS WORKING GROUP, 2007).<br />
Fig. 1 – Map of the main faults in the NW Slovenia, with the emphasis on<br />
the geometry of Ravne Fault. In the upper left corner a regional tectonic<br />
framework with depict<strong>ed</strong> main structures is shown; blue rectangle showing<br />
area of investigations of this study.<br />
STRUCTURAL SETTING<br />
The morphological expression of the Ravne Fault can be<br />
observ<strong>ed</strong> in the field and on satellite and <strong>di</strong>gital topographic<br />
imagery over a <strong>di</strong>stance of approximately 30 km in a NW–SE<br />
<strong>di</strong>rection (Fig. 1) between Kal Koritnica in the NW and Cerkno<br />
at the SE end (KASTELIC et alii, 2008). The fault trace is<br />
<strong>di</strong>scontinuous, interrupt<strong>ed</strong> by local transtensional basins<br />
arrang<strong>ed</strong> in a right stepping manner. The fault zone consists of<br />
in<strong>di</strong>vidual NE-<strong>di</strong>pping fault planes with <strong>di</strong>fferent <strong>di</strong>p angles.<br />
The shallower <strong>di</strong>pping planes, with values between 40°-60°,<br />
usually represent the contact between Cretaceous flysh rocks in
109<br />
INFLUENCE OF INHERITED GEOMETRY AND FAULT HISTORY ON THE SEISMOGENIC ACTIVITY AND POTENTIAL OF STRIKE-SLIP FAULT<br />
SYSTEMS IN NW SLOVENIA: THE CASE STUDY OF THE RAVNE FAULT<br />
the foot-wall and Triassic limestones in the hanging-wall. In<br />
places, where such planes are present within the limestones,<br />
shear zones are also present. The geometry of the planes and<br />
the shear bands in<strong>di</strong>cate uplift of the hanging-wall blocks. Well<br />
develop<strong>ed</strong> cataclastic zones up to a few centimetres wide are<br />
develop<strong>ed</strong> adjacent to some planes, whereas in other places,<br />
shear<strong>ed</strong> surfaces of polish<strong>ed</strong> tectonic breccia occur in the<br />
innermost fault zone. Fault-relat<strong>ed</strong> deformation on moderatly<br />
<strong>di</strong>pping fault planes is not confin<strong>ed</strong> only to the steep trace of<br />
the Ravne Fault, but is rather well record<strong>ed</strong> also on fault planes<br />
of the foot-wall block. Where preserv<strong>ed</strong>, microkinematic<br />
in<strong>di</strong>cators on these planes prove <strong>di</strong>p slip <strong>di</strong>splacements. Planes<br />
with steeper <strong>di</strong>ps reaching values of more then 75 degrees in<br />
most cases lack of microkinematic in<strong>di</strong>cators. Throughout the<br />
outcrops along the trace of the fault zone, only in<strong>di</strong>vidual<br />
subvertical planes with visible striation marks and horizontal<br />
grooves showing strike-slip movement are present. In places,<br />
also (sub)vertical fractures cutting through the more shallowly<br />
<strong>di</strong>pping thrust planes can be observ<strong>ed</strong> at the surface, in<strong>di</strong>cating<br />
the propagation towards the surface of the steep subvertical<br />
planes forming at depth (KASTELIC et alii, 2008).<br />
Recent seismicity record<strong>ed</strong> in the Ravne Fault zone shows<br />
that ongoing seismic activity is confin<strong>ed</strong> to shallow crustal<br />
levels and does not exce<strong>ed</strong> depths beyond 10 km. The 1998<br />
Fig. 2 – Spatial <strong>di</strong>stribution of aftershock sequences of the 1998 and 2004<br />
Ravne Fault earthquake events. Grey triangles depict the 1998 MW= 5.6,<br />
black circles the 2004 MW= 5.2 earthquake seismic sequences, respectively.<br />
The grey and light-greyish stars represent the 1998 and 2004 main shock<br />
locations, respectively. Grey-colour<strong>ed</strong> focal mechanism belong to the 1998<br />
(ZUPANI et alii, 2001), the black solutions to the 2004 events (KASTELIC et<br />
alii, 2006). R<strong>ed</strong> boxes represent the sources for both main shocks of the two<br />
seismic sequences, with the geometries obtain<strong>ed</strong> from their focal<br />
mechanisms and <strong>di</strong>mensions in agreement with geometrical empirical<br />
relationships (WELLS & COPPERSMITH, 1994).<br />
(MW=5.7) and 2004 (MW=5.2) earthquake sequences were<br />
characteriz<strong>ed</strong> by focal mecanisms ranging from dextral strikeslip<br />
to almost pure reverse faulting (KASTELIC et alii, 2006;<br />
Fig. 2). The main shocks of both earthquakes <strong>del</strong>ineate<br />
prevailing dextral strike-slip movements on steep SW <strong>di</strong>pping<br />
planes. The 1998 earthquake cluster follows a NW–SE trend,<br />
with the main shock locat<strong>ed</strong> approximately at 4.5 km from the<br />
fault’s NW tip, whereas the SE end of the cluster reaches the<br />
Tolminka Springs basin. Focal mechanism solutions for<br />
stronger aftershock events in<strong>di</strong>cate thrust or reverse<br />
movements on N <strong>di</strong>pping fault planes on both ends of the<br />
cluster, all within the first 7 km of the crust (ZUPANI et alii,<br />
2001). The earthquake <strong>di</strong>d not cause surface faulting, but it <strong>di</strong>d<br />
(re)activate a 12 km long and 7 km wide rectangular fault plane<br />
with an average slip of 18 cm (BAJC et alii, 2001). The 2004<br />
earthquake was weaker and also <strong>di</strong>d not cause surface faulting.<br />
The epicentre of the main shock was locat<strong>ed</strong> about 1 km from<br />
the 1998 main shock location. This event and its aftershock<br />
sequence were also confin<strong>ed</strong> to shallow crustal levels. The<br />
spatial <strong>di</strong>stribution of aftershocks is not as homogeneous in<br />
<strong>di</strong>rection as for the 1998 cluster. Two <strong>di</strong>stinctive branches of<br />
aftershocks can be observ<strong>ed</strong> for the 2004 sequence: one line<br />
continues in a NW–SE <strong>di</strong>rection rupturing an area further NW<br />
of the 1998 cluster, while the other line branches off in an E–W<br />
<strong>di</strong>rection (Fig. 2). The comput<strong>ed</strong> focal mechanisms for the<br />
stronger aftershocks (KASTELIC et alii, 2006) show similar<br />
kinematics as the main shock, the <strong>di</strong>fference being a larger <strong>di</strong>pslip<br />
component for some aftershocks, in<strong>di</strong>cating oblique<br />
dextral-reverse movements on steep to moderate-steep NW–SE<br />
orient<strong>ed</strong> fault planes and oblique reverse-dextral and reverse<br />
movements on E–W orient<strong>ed</strong>, moderate-steep faults.<br />
DISCUSSION<br />
Knowing and understan<strong>di</strong>ng the geometry, time history and<br />
recent seismic destribution is essential in provi<strong>di</strong>ng a reliable<br />
seismotectonic mo<strong>del</strong> and seismogenic potential assesment of a<br />
fault that has undergone multiple tectonic phases. In the case of<br />
Ravne Fault it was proven that the original geometry of the<br />
fault plane connect<strong>ed</strong> to SW verging thrusting phase plays an<br />
important role also in the recent seismic behaviour of the fault<br />
zone (KASTELIC et alii, 2008). During both the recent seismic<br />
sequences record<strong>ed</strong> in the Ravne Fault zone the main shocks<br />
occurr<strong>ed</strong> on SW-<strong>di</strong>pping subvertical fault planes, while the<br />
aftershocks were <strong>di</strong>stribu<strong>ed</strong> on NE to N <strong>di</strong>pping fault planes<br />
relat<strong>ed</strong> to the older thrusting phase. Of important significance<br />
is also the presence of the E-W thrusting phases within the<br />
fault zone, that caus<strong>ed</strong> <strong>di</strong>sintegration and <strong>di</strong>splacements of the<br />
NW-SE orient<strong>ed</strong> thrust zone, and formation of local<br />
geometrical barriers that are governing the seismic <strong>di</strong>stribution<br />
acting as segment boundaries. Examples of such behaviour can<br />
be found at both tips of the Ravne Fault. To the SE, the<br />
Tolminka Springs basin, an E-W tren<strong>di</strong>ng thrust system meets<br />
the NW-SE orient<strong>ed</strong> fault zone causing its <strong>di</strong>splacement and<br />
formation of a transtensional basin that act<strong>ed</strong> as a stopping<br />
barrier for the SE propagation of the 1998 earthquake<br />
sequence. From this location further to the SE the fault trace is<br />
getting more and more <strong>di</strong>scontinous and slowly fa<strong>di</strong>ng away,<br />
although some reports show its continuation in the SE <strong>di</strong>rection<br />
for further 15 km (GRAD & FERJANI, 1968). At the NW tip<br />
of the fault zone, the surface expression of the fault trace is lost<br />
in the Bovec basin under the Quaternary s<strong>ed</strong>iment infill, while<br />
in the NW slopes above the basin no fault trace can be<br />
observ<strong>ed</strong>. Close to the location of NW-SE orient<strong>ed</strong> fault trace<br />
entering the Bovec basin is also the location of the intersection<br />
with the E-W orient<strong>ed</strong> fault plane that was reactivat<strong>ed</strong> during<br />
the the 2004 earthquake sequence.<br />
Given such structural setting three <strong>di</strong>fferent seismotectonic<br />
scenarios can be envisag<strong>ed</strong>:
110 V. KASTELIC ET ALII<br />
a) if the existence of the SE continuation of the Ravne<br />
Fault zone with a length of about 15 km is establish<strong>ed</strong>, the<br />
coseismic rupture of this new segment would result in an<br />
earthquake of magnitude similar to that of the 1998 event;<br />
b) with the same assumption, the rupture of the entire<br />
Ravne Fault zone by breaching of the Tolminka Springs basin<br />
segment boundary, would produce a stronger earthquake.<br />
Taking into consideration the length of the fault trace and<br />
keeping the geometry and kinematics of the 1998 source, a<br />
Mw=6.1 earthquake is propos<strong>ed</strong> by empirical relationships<br />
(WELLS & COPPERSMITH, 1994);<br />
c) the coseismic rupture of the E-W orient<strong>ed</strong> thrust plane<br />
already activat<strong>ed</strong> during the 2004 earthquake sequence, would<br />
result in a Mw=5.5 earthquake.<br />
The mo<strong>del</strong> of segmentation appli<strong>ed</strong> in this case study can<br />
be appli<strong>ed</strong> also to the other fault systems of the region, which<br />
share the same geometry and the same structural development<br />
as the Ravne Fault. One of these fault systems is the NW-SE<br />
orient<strong>ed</strong> Idrija Fault, responsible for the 1511 M=6.9<br />
earthquake (FITZKO et alii, 2005) that is the strongest<br />
earthquake record<strong>ed</strong> in the region. Given the continuation of<br />
the fault trace to the SE, a higher magnitude earthquake with<br />
strike-slip motion is pr<strong>ed</strong>ict<strong>ed</strong> by our mo<strong>del</strong> that is in<br />
accordance with the data of known seismicity. By taking into<br />
consideration all the data known we can improve the<br />
understan<strong>di</strong>ng of recent and future seisimic processes of a<br />
particular region and fault systems and therefore contribute to<br />
more realistic and reliable seismogenic potential assesments.<br />
REFERENCES<br />
BAJC, J., AOUDIA, A., SARAO, A. & SUHADOLC P. (2001) - The<br />
Bovec–Krn mountain (Slovenia) earthquake sequence.<br />
Geophys. Res. Lett., 28 (9), 1839–1842.<br />
BURRATO P., POLI M.E., VANNOLI P., ZANFERRARI A., BASILI<br />
R. & GALADINI F. (2008) - Sources of Mw 5+ earthquakes<br />
in northeastern Italy and western Slovenia: An updat<strong>ed</strong><br />
view bas<strong>ed</strong> on geological and seismological evidence.<br />
Tectonophysics, 453, 157-176, doi: 10.1016/j.tecto.<br />
2007.07.009.<br />
CASTELLARIN A. & CANTELLI L. (2000) - Neo-Alpine evolution<br />
of the Southern Eastern Alps. J. Geodyn., 30, 251–274.<br />
DISS WORKING GROUP (2007) - Database of In<strong>di</strong>vidual<br />
Seismogenic Sources (version 3.0.4): A compilation of<br />
potential sources for earthquakes larger than M 5.5 in Italy<br />
and surroun<strong>di</strong>ng areas. Available at:<br />
http://www.ingv.it/DISS.<br />
DOGLIONI C. (1987) - Tectonics of the Dolomites (Southern<br />
Alps Northern Italy). J. Struct. Geol., 9, 181–193.<br />
FITZKO, F., SUHADOLC, P., AOUDIA A. & PANZA G.F. (2005) -<br />
Constrains on the location and mechanism of the 1511<br />
Western–Slovenia earthquake from active tectonics and<br />
mo<strong>del</strong>ing of macroseismic data. Tectonophysics, 404, 77–<br />
90.<br />
GRAD, K. & FERJANI L. (1968) - Basic geological map of<br />
SFR Yugoslavia 1:100 000, Explanatory notes for Sheet<br />
Tolmin and Videm. Zvezni geološki zavod, Beograd, 67 pp.<br />
GRENERCZY, G., SELLA G., STEIN S. & KENYERES, A. (2005) -<br />
Tectonic implications of the GPS velocity field in the<br />
northern Adriatic region. Geophys. Res. Lett., 32, L16311.<br />
doi:10.1029/2005GL022947.<br />
KASTELIC, V., ŽIVI M., PAHOR J. & GOSAR A. (2006) -<br />
Seismotectonic characteristics of the 2004 earthquake in<br />
Krn mountains. Potresi v letu 2004, EARS, 78–87.<br />
KASTELIC, V., VRABEC M., CUNNINGHAM D. & GOSAR A.<br />
(2008) - Neo-Alpine structural evolution and present-day<br />
tectonic activity of the eastern Southern Alps: The case of<br />
the Ravne Fault, NW Slovenia. J. Struct. Geol., 30, 963–<br />
975, doi:10.1016/j.jsg.2008.03.009.<br />
PAMI, J., TOMLJENOVI B. & BALEN D. (2002) - Geodynamic<br />
and petrogenetic evolution of Alpine ophiolites from the<br />
central and NW Dinarides: an overview. Lithos, 65, 113–<br />
142.<br />
WEBER J., VRABEC, M., STOPAR, B., PAVLOVI PREŠEREN, P.,<br />
DIXON, T. (2006) - The PIVO 2003 experiment: A GPS<br />
study of Istria peninsula and Adria microplate motion, and<br />
active tectonics in Slovenia. In: Pinter, N. (Ed.), The Adria<br />
microplate: GPS geodesy, tectonics and hazards. NATO<br />
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305-320.<br />
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relationships among magnitude, rupture length, rupture<br />
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ZUPANI, P., CECI I., GOSAR A., PLACER L., POLJAK M. &<br />
ŽIVI M. (2001) - The earthquake of 12 April 1998 in the<br />
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The Liguride Complex of the Pollino area (Southern Apennines): tectonic setting and<br />
preliminary mineralogical data<br />
SALVATORE LAURITA (*), FRANCESCO CAVALCANTE (**), CLAUDIA BELVISO (**) & GIACOMO PROSSER (*)<br />
RIASSUNTO<br />
Il Complesso Liguride affiorante nell’area <strong>del</strong> Pollino (Appennino<br />
meri<strong>di</strong>onale): assetto tettonico e dati mineralogici preliminari<br />
Il Complesso Liguride affiorante nell’area <strong>del</strong> Pollino rappresenta il relitto<br />
<strong>di</strong> un prisma <strong>di</strong> accrezione che è entrato in collisione con il margine passivo<br />
apulo durante il Miocene inferiore. Nel corso <strong>del</strong>l’evoluzione accrezionale e<br />
durante la successiva evoluzione collisionale si sono in<strong>di</strong>viduate una una serie<br />
<strong>di</strong> unità tettoniche caratterizzate da caratteristiche litologiche peculiari e da<br />
una <strong>di</strong>fferente sovraimpronta metamorfica. Nuovi dati strutturali e<br />
mineralogici permettono <strong>di</strong> <strong>di</strong>stinguere all'interno <strong>del</strong> Complesso Liguride tre<br />
unità tettoniche che sono, dall'alto verso il basso: (i) l'Unità <strong>del</strong> Frido,<br />
costituita da serpentiniti, metabasiti, scaglie <strong>di</strong> crosta continentale e<br />
metas<strong>ed</strong>imenti <strong>di</strong> basso grado; (ii) l'Unità <strong>del</strong> M. Tumbarino, costituita da<br />
metapeliti <strong>di</strong> grado molto basso con subor<strong>di</strong>nati basalti e gabbri; (iii) l'Unità<br />
Nord Calabrese, non metamorfica, costituita da ofioliti alla base e da argilliti e<br />
arenarie verso l'alto.<br />
Lo stu<strong>di</strong>o <strong>di</strong> terreno ha permesso <strong>di</strong> riconoscere che la parte alta <strong>del</strong>l'Unità<br />
<strong>del</strong> Frido è costituita da metabasiti foliate ricoperte da scaglie <strong>di</strong> serpentiniti<br />
nel settore sudoccidentale e da serpentiniti e scaglie <strong>di</strong> crosta continentale<br />
settore sudorientale. In entrambi i settori i metas<strong>ed</strong>imenti sono tipici <strong>del</strong>la<br />
porzione inferiore <strong>del</strong>l'Unità. Le varie unità litologiche <strong>del</strong>l'Unità <strong>del</strong> Frido<br />
sono interessate da una sequenza <strong>di</strong> fasi deformative in parte contemporanea<br />
ad un metamorfismo <strong>di</strong> alta pressione e bassa temperatura. I valori <strong>del</strong>l'in<strong>di</strong>ce<br />
<strong>di</strong> Kübler, ottenuti m<strong>ed</strong>iante l'analisi <strong>di</strong>ffrattometrica a raggi X <strong>di</strong><br />
metas<strong>ed</strong>imenti pelitici, hanno permesso <strong>di</strong> stimare temperature comprese tra<br />
200-220°C nel settore nordorientale e 300-350°C nel settore sudoccidentale. I<br />
valori <strong>del</strong> parametro b0 <strong>del</strong>le miche e la presenza <strong>di</strong> aragonite sono coerenti<br />
con una pressione <strong>di</strong> 7-9 Kbar. Temperature significativamente inferiori<br />
possono essere stimate per l'Unità <strong>del</strong> M. Tumbarino (180-200°C) e l'Unità<br />
Nord Calabrese (120-150°C). In conclusione, si ritiene che il prisma <strong>di</strong><br />
accrezione liguride, durante la collisione con il margine apulo, sia stato<br />
sud<strong>di</strong>viso in una pila <strong>di</strong> unità tettoniche che testimoniano con<strong>di</strong>zioni <strong>di</strong><br />
pressione e temperatura progressivamente decrescenti dall'alto verso il basso.<br />
Parole Chiave: Complesso Liguride, Appennino meri<strong>di</strong>onale,<br />
cuneo d’accrezione, metamorfismo HP/LT, In<strong>di</strong>ce <strong>di</strong><br />
Kübler.<br />
Key Words: Liguride Complex, Southern Apennines,<br />
accretionary w<strong>ed</strong>ge, HP/LT-metamophism, Kübler Index.<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, Università <strong>del</strong>la Basilicata, Via<br />
<strong>del</strong>l’Ateneo Lucano, 10 – 85100, Potenza, Italy<br />
(**) IMAA CNR, C.da S. Loja - Zona Industriale, 85050 Tito Scalo (PZ),<br />
Italy<br />
INTRODUCTION<br />
The Liguride Complex in the Pollino area (Fig. 1)<br />
represents the remnants of an accretionary w<strong>ed</strong>ge that<br />
underwent collision with the Apulian passive margin during the<br />
early Miocene (KNOTT, 1994). The accretion history and the<br />
later collisional evolution produc<strong>ed</strong> a series of thrust sheets<br />
showing <strong>di</strong>fferent lithological features and metamorphic<br />
overprint. Later post-collisional evolution further complicat<strong>ed</strong><br />
this picture, since the early-form<strong>ed</strong> thrust sheets have been<br />
crosscut by out-of-sequence thrusts, transpressive and, finally,<br />
normal faults. The polyphase deformation history of the<br />
Liguride Complex complicates the recognition of the<br />
geometrical relationships between the <strong>di</strong>fferent rock bo<strong>di</strong>es.<br />
This l<strong>ed</strong> previous authors to propose conflicting structural<br />
interpretations (e.g., BONARDI et alii, 1988; MONACO &<br />
TORTORICI 1995).<br />
For this reason we carri<strong>ed</strong> out new detail<strong>ed</strong> structural<br />
mapping, petrographical, microstructural and mineralogical<br />
analyses in select<strong>ed</strong> localities of the Pollino area. The <strong>di</strong>fferent<br />
deformation phases have been defin<strong>ed</strong> by field observation and<br />
microstructural analyses. In particular, the general architecture<br />
of the Liguride Complex has been reconstruct<strong>ed</strong> by careful<br />
mapping of the main axial surfaces relat<strong>ed</strong> to large–scale folds<br />
that intensely deform the main foliation and the lithologic<br />
contacts. Microstructural observations have been coupl<strong>ed</strong> with<br />
X-ray <strong>di</strong>ffraction analyses, that allow<strong>ed</strong> to <strong>di</strong>stinguish <strong>di</strong>fferent<br />
thrust sheets in wide areas characteriz<strong>ed</strong> by monotonous<br />
lithologies, such as very low-grade metapelites and lat<strong>ed</strong>iagenesis<br />
pelites. The new data allow<strong>ed</strong> a more detail<strong>ed</strong><br />
<strong>di</strong>stinction between <strong>di</strong>fferent tectonic units and rock types,<br />
lea<strong>di</strong>ng to an upgrad<strong>ed</strong> structural interpretation and provide<br />
new information on the early evolutionary stages of the Alpine-<br />
Apennine subduction system.<br />
GEOLOGICAL SETTING<br />
The Liguride Complex represents the uppermost tectonic<br />
units of the Southern Apennines. This mountain belt develop<strong>ed</strong><br />
at the expense of the eastern passive margin of the Apulian<br />
plate resulting from the mesozoic extensional evolution of the<br />
western Tethys ocean (GUERRERA et alii, 2005). The inversion
112 S. LAURITA ET ALII<br />
Fig. 1 – Tectonic sketch map of the Liguride Units in the Pollino area. Inset shows the location of the stu<strong>di</strong><strong>ed</strong> area in the Southern Apennines.<br />
of the former passive margin took place during the Neogene<br />
time with ENE vergence (PATACCA & SCANDONE, 2007, and<br />
references therein). The Apulian margin was form<strong>ed</strong> by<br />
carbonate platforms and basins which from the west to the east,<br />
are: (i) the Sicilide deep-sea domain, correspon<strong>di</strong>ng to the<br />
ocean-continent transition; (ii) the Apennine carbonate<br />
platform; (iii) the Lagonegro basin; (iv) the internal and<br />
external Apulian carbonate platform.<br />
The Liguride Complex derives from the western Tethys<br />
oceanic domain, which laid between the European and Apulian<br />
continental domains (PERRONE et alii, 2008). Late Cretaceous –<br />
Tertiary subduction of this oceanic domain gave rise to the<br />
Liguride accretionary w<strong>ed</strong>ge that finally collid<strong>ed</strong> with the<br />
Adriatic margin during the early Miocene times (KNOTT, 1994).<br />
During this evolution the Liguride accretionary w<strong>ed</strong>ge thrust<strong>ed</strong><br />
onto the Apennine platform and the accret<strong>ed</strong> material was split<br />
into <strong>di</strong>fferent slices that can be schematically <strong>di</strong>vid<strong>ed</strong> into<br />
metamorphic and non-metamorphic units.<br />
In the study area the Liguride Complex we recognis<strong>ed</strong> three<br />
main tectonic units that, from the top to the bottom, are (Fig.<br />
1): (i) the Frido Unit (AMODIO MORELLI et alii, 1976),<br />
consisting mainly of very low grade metapelites and<br />
metalimestones, with minor slices of continental crust rocks,<br />
serpentinites and metabasites; (ii) the M. Tumbarino Unit,<br />
made up of very low-grade metapelites, with minor basalts and<br />
gabbros, comparable to the Cilento basal Unit (MAURO &<br />
SCHIATTARELLA, 1988); (iii) the North-Calabria Unit (BONARDI<br />
et alii, 1988), consisting of non metamorphic ophiolites, shales<br />
and standsones, partly referr<strong>ed</strong> to the Calabro-Lucano Flysch<br />
Unit by MONACO et alii (1995). Generally, the Liguride<br />
Complex tectonically overlies the Bifurto formation, mainly<br />
consisting of shales and quartzarenites deposit<strong>ed</strong> on the top of<br />
the Apennine platform during the Early Miocene.<br />
Our study mostly concentrat<strong>ed</strong> on the Frido Unit that<br />
documents deformation and metamorphic features typical of the<br />
deep portions of the Liguride accretionary w<strong>ed</strong>ge. The HP-LT<br />
metamorphic overprint is well develop<strong>ed</strong> in mafic rocks and,<br />
locally, in the continental crust rocks (BECCALUVA et alii,<br />
1982; SPADEA, 1982). Maximum pressure and temperature<br />
con<strong>di</strong>tions estimat<strong>ed</strong> for mafic rocks are 8-10 Kbar and 400-<br />
450°C, respectively (MONACO et alii, 1995). The widespread<br />
occurrence of aragonite in metalimestones (SPADEA, 1976)<br />
in<strong>di</strong>cates that also metas<strong>ed</strong>imentary rocks underwent HP-LT<br />
con<strong>di</strong>tions. Temperatures of 140-200°C and 200-300°C have<br />
been estimat<strong>ed</strong> by DI LEO et alii (2005) and INVERNIZZI et alii<br />
(2008), respectively.<br />
STRUCTURE OF THE FRIDO UNIT<br />
In the stu<strong>di</strong><strong>ed</strong> area metabasites, serpentinites and<br />
continental crust rocks are generally locat<strong>ed</strong> in the uppermost<br />
part of this unit, whereas metalimestones and low-grade<br />
phyllites are more widespread below. In the southwestern<br />
sector of the study area (M. Nan<strong>di</strong>niello and Gallizzi localities;<br />
Fig. 1) an almost continuous sheet of strongly foliat<strong>ed</strong><br />
metabasites overlie metalimestones and, in minor extent,<br />
phyllites. The protoliths of the foliat<strong>ed</strong> metabasites consist of<br />
meta-pillow lavas, meta-pillowbreccias and meta-hyaloclastites,<br />
well visible in less deform<strong>ed</strong> areas. Frequently, foliat<strong>ed</strong><br />
metabasites carry on top slices of intensely deform<strong>ed</strong><br />
serpentinites, containing <strong>di</strong>spers<strong>ed</strong> bo<strong>di</strong>es of massive<br />
metabasites. In the northeastern sector of the study area (Timpa<br />
Rotalupo; Fig. 1), serpentinites are tectonically overlain by
THE LIGURIDE COMPLEX OF THE POLLINO AREA (SOUTHERN APENNINES)<br />
slices of continental crust consisting of albitic gneisses, garnet<br />
gneisses and amphibolites.<br />
The detail<strong>ed</strong> structural analysis carri<strong>ed</strong> out in the study area<br />
allow<strong>ed</strong> to recognize structures relat<strong>ed</strong> to a polyphase<br />
deformation history, well develop<strong>ed</strong> in metas<strong>ed</strong>imentary rocks.<br />
Three main deformation phases have been recogniz<strong>ed</strong> in mafic<br />
rocks and metas<strong>ed</strong>iments of the Frido Unit. The D1<br />
deformation is mainly document<strong>ed</strong> by the main S1 foliation that<br />
is particularly intense along the main lithological contacts.<br />
Metamorphism under blueschist facies con<strong>di</strong>tions during the<br />
D1 deformation is document<strong>ed</strong> by the crystallization of<br />
glaucophane and lawsonite along the S1 foliation in<br />
metabasites. Later D2 folds and crenulations overprint the<br />
previous fabrics, producing a well-develop<strong>ed</strong> crenulation<br />
cleavage. D2 folds trend from NE-SW to N-S and show nearly<br />
horizontal axial surfaces. Local occurrence of riebeckite along<br />
the S2 crenulation cleavage suggests that HP-LT con<strong>di</strong>tions<br />
operat<strong>ed</strong> also during the D2 deformation. The D3 deformation<br />
develop<strong>ed</strong> E-W orient<strong>ed</strong> decametre-scale folds, with nearly<br />
horizontal axial surfaces. These folds are not associat<strong>ed</strong> to an<br />
axial planar foliation and at outcrop-scale are in<strong>di</strong>cat<strong>ed</strong> by<br />
weakly develop<strong>ed</strong> crenulations.<br />
MINERALOGY OF LOW-GRADE METASEDIMENTS<br />
Metamorphic con<strong>di</strong>tions d<strong>ed</strong>uc<strong>ed</strong> for mafic rocks of the<br />
Frido Unit have been compar<strong>ed</strong> with mineralogical data<br />
obtain<strong>ed</strong> by X-ray <strong>di</strong>ffraction in low-grade metas<strong>ed</strong>iments. This<br />
analysis allows to define the metamorphic grade of s<strong>ed</strong>iments<br />
and metas<strong>ed</strong>iments by measuring parameters such as Kübler<br />
Index (KI), the percentage of 2M1 polytipe and the b0<br />
parameter of illite-muscovite (FREY & ROBINSON, 1999, and<br />
references therein). KI values in<strong>di</strong>cate temperatures generally<br />
ranging from 220 to 260°C. Lower temperatures (200-220°C)<br />
can be obtain<strong>ed</strong> in the northeastern sector, whereas higher<br />
values (300-350°C) can be envisag<strong>ed</strong> for southwestern sector,<br />
where glaucophane and lawsonite are commonly found in<br />
foliat<strong>ed</strong> metabasites. The b0 values suggest pressures of about<br />
7-9 Kbar, in agreement with the presence of aragonite (SPADEA,<br />
1976).<br />
In the Monte Tumbarino Unit KI values allow to estimate<br />
consistently lower temperatures of 180-200°C. These<br />
con<strong>di</strong>tions are in agreement with the presence of Na-mica and<br />
K/Na-mica mix<strong>ed</strong> layer, order<strong>ed</strong> Chl/S mix<strong>ed</strong> layer in same<br />
samples, lower percentage of 2M1 muscovite politipe and<br />
weaker development of slaty clivage and crenulations<br />
compar<strong>ed</strong> to the Frido Unit. The presence of aragonite in a<br />
carbonate-bearing sample suggests pressures of at least 5 Kbar.<br />
Distinctly lower temperatures of 120-150°C have been detect<strong>ed</strong><br />
in the non-metamorphic North Calabria Unit by the presence of<br />
order<strong>ed</strong> R1 and R3 I/S mix<strong>ed</strong> layers and the KI values.<br />
CONCLUDING REMARKS<br />
113<br />
The early evolutionary history of the Frido Unit records<br />
subduction of oceanic crust and strong deformation during HP-<br />
LT metamorphism. Estimates of metamorphic con<strong>di</strong>tions<br />
obtain<strong>ed</strong> in low grade metas<strong>ed</strong>iments in<strong>di</strong>cate that during this<br />
evolution temperatures probably <strong>di</strong>d not exce<strong>ed</strong> 300-350°C in<br />
the southwestern sector and 200-220°C in the northeastern<br />
sector. When the Apulian margin was incorporat<strong>ed</strong> in the<br />
subduction zone during the early Miocene the Liguride<br />
accretionary w<strong>ed</strong>ge thrust<strong>ed</strong> on the Apennine platform during<br />
the development of the Apennine chain. This process strongly<br />
mo<strong>di</strong>fi<strong>ed</strong> the original geometry of the liguride accretionary<br />
w<strong>ed</strong>ge that was sub<strong>di</strong>vid<strong>ed</strong> into <strong>di</strong>fferent thrust sheets. New<br />
structural and mineralogical data allow<strong>ed</strong> to <strong>di</strong>stinguish three<br />
main thrust sheets. The uppermost thrust sheet (Frido Unit)<br />
derives from the deeper portion of the accretionary w<strong>ed</strong>ge,<br />
whereas the interm<strong>ed</strong>iate and lower thrust sheets (The<br />
Tumbarino and North Calabria Units) come from progressively<br />
shallower portions of the w<strong>ed</strong>ge.<br />
REFERENCES<br />
AMODIO MORELLI L., BONARDI G., COLONNA V., DIETRICH D.,<br />
GIUNTA G., IPPOLITO F., LIGUORI V., LORENZONI S.,<br />
PAGLIONICO A., PERRONE V., PICCARRETA G., RUSSO M.,<br />
SCANDONE P., ZANETTIN LORENZONI E. & ZUPPETTA A.<br />
(1976) – L’Arco Calabro-Peloritano nell’Orogene<br />
Appenninico-Maghrebide. Mem. Soc. Geol. It., 17, 1-60.<br />
BECCALUVA L., MACCIOTTA L. & SPADEA P. (1982) - Petrology<br />
and geodynamic significance of the Calabria-Lucania<br />
ophiolites. Rend. Soc. Ital. Mineral. Petrol., 38, 937-982.<br />
BONARDI G., AMORE F. O., CIAMPO G., DE CAPOA P.,<br />
MICCONET P. & PERRONE V. (1988) – Il complesso<br />
Liguride. Auct.: stato <strong>del</strong>le conoscenze e problemi aperti<br />
sulla sua evoluzione pre-appenninica <strong>ed</strong> i suoi rapporti con<br />
l’Arco Calabro. Mem. Soc. Geol. It., 41, 17-35.<br />
DI LEO P., SCHIATTARELLA M., CUADROS J. & CULLERS R.<br />
(2005) – Clay mineralogy, geochemistry and structural<br />
setting of the ophiolite-bearing units from southern Italy: a<br />
multi<strong>di</strong>sciplinary approach to assess tectonics history and<br />
exhumation modalities. Atti Ticinensi <strong>di</strong> Scienze <strong>del</strong>la<br />
Terra, 10, 87-93.<br />
FREY M. & ROBINSON D. (1999) - Low-Grade Metamorphim,<br />
Blackwell Science, Oxford, 328 pp.<br />
GUERRERA F., MARTÌN-MARTIN M., PERRONE V. &<br />
TRAMONTANA M. (2005) - Tectono-s<strong>ed</strong>imentary evolution<br />
of the southern branch of the Western Tethys (Maghrebian<br />
Flysch Basin and Lucanian Ocean): consequences for<br />
Western M<strong>ed</strong>iterranean geodynamics. Terra Nova, 17, 358-<br />
367.
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INVERNIZZI C., BIGAZZI G., CORRADO S., DI LEO P.,<br />
SCHIATTARELLA M. & ZATTIN M. (2008) - New thermobaric<br />
constraints on the exhumation history of the Liguride<br />
accretionary w<strong>ed</strong>ge, Southern Italy. Ofioliti, 33 (1), 21-32.<br />
KNOTT S.D. (1994) - Structure, kinematics and metamorphism<br />
in the Liguride Complex, southern Apennines, Italy. J.<br />
Struct. Geol., 16, 1107-1120.<br />
MAURO A. & SCHIATTARELLA M. (1988) - L'unità<br />
Silentina <strong>di</strong> Base: assetto strutturale, metamorfismo e<br />
significato tettonico nel quadro geologico <strong>del</strong>l'Appennino<br />
meri<strong>di</strong>onale. Mem. Soc. Geol. It., 41 (2), 1201-1213.<br />
MONACO C. & TORTORICI L. (1995) – Tectonic role of<br />
ophiolite-bearing terranes in the development of the<br />
Southern Apennines orogenic belt. Terra Nova, 7, 153-160.<br />
MONACO C., TORTORICI L., MORTEN L., CRITELLI S. & TANSI<br />
C. (1995) – Geologia <strong>del</strong> versante nord-orientale <strong>del</strong><br />
massiccio <strong>del</strong> Pollino (confine calabro-lucano): nota<br />
illustrativa sintetica <strong>del</strong>la carta geologica alla scala 1:50000.<br />
Boll. Soc. Geol. It., 114, 277-291.<br />
PATACCA E. & SCANDONE P. (2007) - Geology of the Southern<br />
Apennines. Boll. Soc. Geol. It. Spec. Issue, 7, 75-119.<br />
PERRONE V., DI STASO A: & PERROTTA S. (2008) - The<br />
evolution of the Western Adriatic margin and contiguous<br />
oceanic area: new problems and working hypotheses.<br />
Boll.Soc.Geol.It. (Ital.J.Geosci.), 127 (2), 357-373.<br />
SPADEA P., (1976) - I carbonati nelle rocce metacalcaree <strong>del</strong>la<br />
formazione <strong>del</strong> Frido <strong>del</strong>la Lucania. Ofioliti, 1, 431-456.<br />
SPADEA P. (1982) – Continental crust rocks associat<strong>ed</strong> with<br />
ophiolites in Lucanian Apennine (southern Italy). Ofioliti, 2<br />
(3), 501-522.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 115_120, 7 ff.<br />
Quaternary evolution of “blind” fault-relat<strong>ed</strong> folds in the Central Po<br />
Plain (Northern Italy).<br />
FRANZ LIVIO (*), GIANCANIO SILEO (*), ANDREA BERLUSCONI (*), ALESSANDRO M. MICHETTI (*), KARL<br />
MUELLER (**), CIPRIANO CARCANO (°) & SERGIO ROGLEDI (°)<br />
RIASSUNTO<br />
Evoluzione quaternaria <strong>di</strong> strutture plicative sepolte lungo il settore<br />
centrale <strong>del</strong>l'alta Pianura Padana.<br />
Attraverso la ricostruzione tri<strong>di</strong>mensionale <strong>di</strong> due superfici appartenenti al<br />
record stratigrafico quaternario <strong>del</strong> bacino padano (Superficie Azzurra, 1.6 Ma<br />
e Superficie Rossa 0.89 Ma) si è giunti a <strong>del</strong>ineare parte <strong>del</strong>l’evoluzione<br />
quaternaria <strong>di</strong> sovrascorrimenti sepolti. Il pattern deformativo è stato<br />
ricostruito a partire dall’analisi <strong>del</strong>le pieghe associate a queste strutture <strong>ed</strong> in<br />
funzione <strong>del</strong>la loro evoluzione nel tempo. L’analisi <strong>di</strong> dettaglio <strong>di</strong> un profilo<br />
passante attraverso il sistema <strong>di</strong> faglie <strong>di</strong> Capriano <strong>del</strong> Colle (BS) e l’analisi<br />
<strong>del</strong> record stratigrafico sindeformativo ha permesso <strong>di</strong> definire lo stile<br />
strutturale tipico <strong>di</strong> questo settore <strong>ed</strong> i ratei <strong>di</strong> sollevamento associabili alle<br />
strutture <strong>di</strong> Capriano.<br />
Key words: fault-relat<strong>ed</strong> folds; Southern Alps; Quaternary<br />
tectonics<br />
INTRODUCTION<br />
The study of quaternary folds as in<strong>di</strong>cators of active crustal<br />
shortening and coseismic slip on relat<strong>ed</strong> thrusts, especially in<br />
areas characteriz<strong>ed</strong> by blind thrusting, is attracting increasing<br />
interest among specialists due to the recent dramatic evolution<br />
of the methodology for their structural analysis, which now<br />
allow to attain excellent resolution in the deep interpretation of<br />
earthquake sources (BURBANK et alii, 1996; SHAW & SUPPE,<br />
1996, MUELLER & SUPPE, 1997; SHAW et alii, 2002; ISHIYAMA<br />
et alii, 2004; LIN & STEIN, 2006, DOLAN et alii, 2003, LAI et<br />
alii, 2006, CHEN et alii, 2007, LEON et alii, 2007, STREIG et<br />
alii, 2007; OKAMURA et alii, 2007).<br />
We select<strong>ed</strong> an area between Lake Como and Lake Garda<br />
where several evidence of Quaternary tectonics has been<br />
report<strong>ed</strong>. Some isolat<strong>ed</strong> small hills (Capriano <strong>del</strong> Colle,<br />
Casten<strong>ed</strong>olo, Ciliverghe and Piev<strong>ed</strong>izio hill), rising from the<br />
surroun<strong>di</strong>ng plain, have been already mention<strong>ed</strong> by DESIO<br />
(1965) as evidence of Quaternary tectonics. Secondary faulting<br />
and fol<strong>di</strong>ng at Capriano <strong>del</strong> Colle sites have been also recently<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Chimiche <strong>ed</strong> Ambientali, Università<br />
<strong>del</strong>l’Insubria<br />
(**) University of Boulder, Colorado (US)<br />
(°) ENI Exploration & Production<br />
This research has benefit<strong>ed</strong> from fun<strong>di</strong>ng provid<strong>ed</strong> by the Italian Presidenza<br />
<strong>del</strong> Consiglio dei Ministri - Dipartimento <strong>del</strong>la Protezione Civile (DPC).<br />
Scientific papers fund<strong>ed</strong> by DPC do not represent its official opinion and<br />
policies.<br />
document<strong>ed</strong> (LIVIO et alii, 2008; MICHETTI et alii, 2008).<br />
BURRATO et alii (2003) describe river anomalies in the Valle<br />
<strong>del</strong>l’Oglio sector, probably induc<strong>ed</strong> by active growing folds. In<br />
order to depict the recent evolution of the structures recogniz<strong>ed</strong><br />
in the Po Plain basin we analyz<strong>ed</strong> ca. 18000 km of seismic<br />
lines, provid<strong>ed</strong> by ENI E&P, constrain<strong>ed</strong> by hundr<strong>ed</strong>s of deep<br />
exploratory wells, over an area of about 6800 Km 2 (Fig.1).<br />
Fig. 1 – Areal extent of the analyz<strong>ed</strong> surfaces: (a) Blue Surface – 1.6 Myrs;<br />
(b) R<strong>ed</strong> Surface - 0.89 Myrs.<br />
Faults were defin<strong>ed</strong> both by obvious truncation of strata on<br />
seismic profiles (e.g. footwall and hangingwall stratigraphic<br />
cutoffs) and by construction of contour maps of fold<strong>ed</strong> horizons<br />
of various age in the Quaternary sequence that infills the Po<br />
Plain basin. Folds associat<strong>ed</strong> to underlying thrusts were in fact
116 L. FRANZ ET ALII<br />
Fig. 2 –Block-<strong>di</strong>agram of the Plio-Pleistocene Po Plain Basin: the main<br />
domains, describ<strong>ed</strong> in the present work, have been in<strong>di</strong>cat<strong>ed</strong>.<br />
recogniz<strong>ed</strong> bas<strong>ed</strong> on deformation record<strong>ed</strong> by two regional<br />
sequence boundary horizons (Blue Surface – 1.6 Myr; and R<strong>ed</strong><br />
Surface - 0.89 Myr; e.g. CARCANO & PICCIN, 2002, MUTTONI et<br />
alii, 2003), characteriz<strong>ed</strong> by good stratigraphic and age<br />
bracketing, and marking significant changes in the s<strong>ed</strong>imentary<br />
architecture of the basin.<br />
Age controls are bas<strong>ed</strong> on stratigraphic, paleontological and<br />
magnetostratigraphic analysis by ENI E&P and Regione<br />
Lombar<strong>di</strong>a (CARCANO & PICCIN, 2002, SCARDIA et alii, 2006)<br />
and on an extensive database of shallow well logs acquir<strong>ed</strong> in<br />
boreholes drill<strong>ed</strong> for groundwater. The analysis of strain<br />
induc<strong>ed</strong> on these horizons allow<strong>ed</strong> to recognize a belt of active<br />
fold & thrust structures, each 10 to 20 km long, that extents<br />
with an en echelon arrangement across the Po basin.<br />
PATTERN OF DEFORMATION IN THE CENTRAL PO<br />
PLAIN<br />
Blue Surface (1.6 Myr)<br />
The Blue Surface covers about 7800 Km 2 . From north to south<br />
the following major domains can be recogniz<strong>ed</strong>: an alpine<br />
platform domain, an Apennine platform domain and two slopes<br />
linking this platforms with the basin (Fig. 2 and 3).<br />
The basin is ca. 40 km wide (Fig. 3) and progressively deepens<br />
eastward. The northern flank of the basin is less steep than the<br />
southern one and shows some erosive features in its northern<br />
sector. We characteriz<strong>ed</strong> segmentation of blind thrusts using<br />
several methods that inclu<strong>di</strong>ng mapping of single segment<br />
faults that terminate at a transverse structure (e.g. a transfer<br />
fault) or where fol<strong>di</strong>ng decreases to zero at the end of structures<br />
as defin<strong>ed</strong> by the regional <strong>di</strong>p of mapp<strong>ed</strong> strata.<br />
On the northern <strong>ed</strong>ge of the basin two structures has been<br />
identifi<strong>ed</strong> (Fig. 3 structures number 1 and 2). We interpret these<br />
structures as north-verging fault-propagation folds<br />
characteriz<strong>ed</strong> by ramps that detach the Gonfolite Lombarda<br />
Group (Oligo – Miocene; BERNOULLI et alii, 1989) from the<br />
underlying upper Cretaceous s<strong>ed</strong>iments. Surface deformation<br />
caus<strong>ed</strong> with these structures is associat<strong>ed</strong> with the Casten<strong>ed</strong>olo<br />
and Capriano <strong>del</strong> Colle hills. On the basin floor we recogniz<strong>ed</strong><br />
nine structures interpret<strong>ed</strong> as folds overlying growing blind<br />
faults. The northern structures belong to the Alpine domain and<br />
we interpret<strong>ed</strong> them as south-vergent fault propagation folds<br />
(structures number 3, 4 and 6 of Fig. 3). These structures have<br />
an average length of 11 - 16 km and are ca. N 110° tren<strong>di</strong>ng.<br />
Structure number 7 is the most external Apennine front<br />
(Soresina High; e.g. BIGI et alii, 1990) while structures number<br />
9, 10 and 11 represents segments of Central - Eastern sector of<br />
the Emilia fold<strong>ed</strong> Arc. The westernmost segment (11) is the<br />
San Colombano High. Structure 10 is the Piadena High, the<br />
most external structure of this sector of the Emilia Arc (e.g.<br />
BIGI et alii, 1990).<br />
At this stage of deformation (ca. 1.6 Myr) the morphobathymetry<br />
of the basin floor clearly shows two opposite<br />
verging structural fronts, facing each other in the central sector<br />
of the Po Plain, characteriz<strong>ed</strong> by an array of fault-relat<strong>ed</strong> folds,<br />
segmenting each one of these belts.<br />
R<strong>ed</strong> Surface (0.89 Myrs)<br />
Fig. 3 – Blue Surface in map view and in 3D wireframe. Main recogniz<strong>ed</strong> structures are in<strong>di</strong>cat<strong>ed</strong>. See text for details.
QUATERNARY EVOLUTION OF “BLIND” FAULT-RELATED FOLDS IN THE CENTRAL PO PLAIN<br />
The evolution of the basin between the formation of the Blue<br />
Surface and the formation of the R<strong>ed</strong> Surface (averaging ca.<br />
710.000 yrs.) is summariz<strong>ed</strong> as follows. The Alpine platform<br />
domain spreads out southward; a less steep scarp links it with a<br />
wider but less deep Apennines - Alps basin domain and the<br />
Apennines platform domain has been deform<strong>ed</strong> by the most<br />
Fig. 4 – R<strong>ed</strong> Surface in map view and in 3D wireframe. Main recogniz<strong>ed</strong> structures are in<strong>di</strong>cat<strong>ed</strong>. See text for details.<br />
external Apennines structures.<br />
At this stage the basin is still strongly asymmetric, and at the<br />
eastern <strong>ed</strong>ge is ca. 30 km wide, narrowing toward west to ca. 20<br />
Km.<br />
Folds recognizable on this surface are less than those mapp<strong>ed</strong><br />
on the Blue surface; moreover they are generally longer.<br />
This trend is consistent with a spatially – varying shortening<br />
rate mo<strong>del</strong> (e.g. SALVINI & STORTI, 2002) accor<strong>di</strong>ng to which<br />
folds grow with a constant fault geometry but with<br />
<strong>di</strong>splacement varying along strike. Faults migrate laterally as<br />
<strong>di</strong>splacement accumulates and, in the case of two or more<br />
adjacent structures, the growth along strike could produce the<br />
linkage among the adjacent structures with the formation of<br />
double plunging folds (MUELLER & TALLING, 1997, KELLER et<br />
alii, 1999; CHAMPEL et alii, 2002).<br />
CAPRIANO DEL COLLE FAULT SYSTEM<br />
A seismic reflection profile, cutting through some<br />
representative structures in the northern sector of the Po Plain,<br />
has been select<strong>ed</strong> and analyz<strong>ed</strong> (Fig. 5).<br />
The Capriano <strong>del</strong> Colle Fault System (CapFS) is compos<strong>ed</strong> by<br />
a south-vergent thrust (CCT) and an associat<strong>ed</strong> high angle<br />
backthrust (CCB). The structural style here depict<strong>ed</strong> is<br />
therefore characteriz<strong>ed</strong> by a triangle zone, or structural w<strong>ed</strong>ge,<br />
defin<strong>ed</strong> by a primary south-vergent fore-thrust and a connect<strong>ed</strong><br />
north-vergent backthrust (Fig. 5 and 6). Secondary flexural slip<br />
faults (CCBFSF e CCTFSF) are associat<strong>ed</strong> with both these<br />
structures (Fig. 6). Flexural slip faults play an important role in<br />
the accommodation of strain above blind thrusts (e.g., YEATS,<br />
117<br />
1986; ROERING et alii, 1997; NINO et alii, 1998; ISHIYAMA et<br />
alii, 2004; OKAMURA et alii, 2007). Several mo<strong>del</strong>s and natural<br />
examples, in<strong>di</strong>cate in fact that b<strong>ed</strong><strong>di</strong>ng-parallel strain tends to<br />
consume slip and therefore hinder the upward propagation of<br />
faults and fault-relat<strong>ed</strong> folds.<br />
Tectono-s<strong>ed</strong>imentary history of the Capriano <strong>del</strong> Colle Fault<br />
System<br />
Fig. 5 – Shad<strong>ed</strong> relief map bas<strong>ed</strong> on a 20 m DTM showing the locations of<br />
buri<strong>ed</strong> active thrusts in the northern sector of the Po Plain; barbs in<strong>di</strong>cate the<br />
hanging-wall of the thrusts. ENI E&P deep boreholes and seismic line<br />
location are also represent<strong>ed</strong>. Main historical earthquakes are in<strong>di</strong>cat<strong>ed</strong>.<br />
(CPTI, 2004, mo<strong>di</strong>fi<strong>ed</strong>).<br />
Syntectonic growth theory (SUPPE et alii, 1992) suggests<br />
that cumulative <strong>di</strong>splacement will decrease upward within the<br />
stratigraphic interval deposit<strong>ed</strong> during deformation. In<br />
contractional environments syntectonic strata typically thin<br />
across folds limbs towards structural highs. Methodology<br />
adopt<strong>ed</strong> for uplift rates calculation is summariz<strong>ed</strong> in Figure 7a<br />
(e.g., MASAFERRO et alii, 2002;). In situ accumulation rates (S)<br />
were also calculat<strong>ed</strong> and compar<strong>ed</strong> to relative uplift rates (U/S<br />
ratio; Fig. 7b). A very high accumulation rate, with respect to
118 L. FRANZ ET ALII<br />
Fig. 6 – Depth-convert<strong>ed</strong> interpret<strong>ed</strong> seismic profile, across the Capriano <strong>del</strong> Colle Fault System, constrain<strong>ed</strong> by constant thickness fault-relat<strong>ed</strong> fold theory.<br />
Location is shown on Figure 2a. There is no vertical exaggeration. Ab<strong>brevi</strong>ations are: CCT, Capriano <strong>del</strong> Colle thrust; CCB, Capriano <strong>del</strong> Colle backthrust;<br />
CCTFSF & CCBFSF are associat<strong>ed</strong> flexural slip faults. Values above arrows in<strong>di</strong>cate limb widths, measur<strong>ed</strong> on the section. Thick dash<strong>ed</strong> lines, active axial<br />
surface; dash and dott<strong>ed</strong> thin lines, inactive axial surface, blue dash<strong>ed</strong> lines, erosional features.<br />
uplift rate (very low ratio U/S), can in fact obscure the tectonic<br />
signal and appear as an interval apparently not influenc<strong>ed</strong> by<br />
contemporary uplift. Uncertainties in uplift and s<strong>ed</strong>imentation<br />
were evaluat<strong>ed</strong> in terms of the resolution and variability in<br />
seismic reflector character (ca. 25 m), and errors in age<br />
bracketing (ca. ± 0.1 – 0.05 Myr, accor<strong>di</strong>ng to the consider<strong>ed</strong><br />
surface).<br />
For calculations we us<strong>ed</strong> the above-mention<strong>ed</strong> Blue and<br />
R<strong>ed</strong> Surface and an interm<strong>ed</strong>iate horizon, call<strong>ed</strong> Green Surface,<br />
dat<strong>ed</strong> ca. 1.2 Myr. Results, relative to each structure, are<br />
summariz<strong>ed</strong> as follows (Fig. 7b).<br />
- For the CCT: during the first chronostratigraphic interval<br />
(Pliocene – 1.6 Myr) very low values of uplift and<br />
s<strong>ed</strong>imentation rates have been record<strong>ed</strong>. Growth b<strong>ed</strong>s<br />
onlapping the fold limb and thinning as they approach<br />
the fold crest pr<strong>ed</strong>ominat<strong>ed</strong> during the first part of this<br />
period. Such a low value of average rock uplift rate<br />
may be relat<strong>ed</strong> to averaging a relatively short episode<br />
of tectonic deformation over a very long time period<br />
(duration is ca. 3.6 Myr). The second time window<br />
(1.6-1.2 Myr) records seemingly higher values both in<br />
uplift and s<strong>ed</strong>imentation rates. The following period is<br />
characteriz<strong>ed</strong> by a deactivation of this thrust, as<br />
testifi<strong>ed</strong> by syntectonic growth strata that maintain the<br />
same thickness throughout the CCT crest. Since S value<br />
is constant for this period, compar<strong>ed</strong> to the previous<br />
interval, it is apparent that the tectonic signal has not<br />
been conceal<strong>ed</strong> by a significant increase in<br />
s<strong>ed</strong>imentation rates. The 0.89 Myr to present<br />
chronostratigraphic interval records a decrease both in<br />
uplift and s<strong>ed</strong>imentation rates.<br />
- For the CCB: during a Pliocene to 1.6 Myr time window,<br />
our analysis suggests that this structure initially<br />
experienc<strong>ed</strong> similar low values of s<strong>ed</strong>imentation and<br />
uplift rates.<br />
Fig. 7 – Assessment of uplift and in situ s<strong>ed</strong>imentation rates relative to<br />
chronologic intervals: a) Sketch of the methodological approach (e.g.<br />
Masaferro et al., 2002) appli<strong>ed</strong> to calculate uplift rates; b) graphic summary<br />
of the results accor<strong>di</strong>ng to the consider<strong>ed</strong> time windows. Error bars and U/S<br />
ratio value are also in<strong>di</strong>cat<strong>ed</strong>. (*) in<strong>di</strong>cates minimum uplift rate value,<br />
accor<strong>di</strong>ng to a fill-to-the-top growth mo<strong>del</strong>.<br />
We consider the same considerations <strong>di</strong>scuss<strong>ed</strong> above<br />
for calculations on rock uplift CCT structures to be<br />
valid. During the 1.2 – 0.89 Myr period the high value<br />
of U/S ratio marks a significant decrease in<br />
s<strong>ed</strong>imentation rates. This drastic lowering in
QUATERNARY EVOLUTION OF “BLIND” FAULT-RELATED FOLDS IN THE CENTRAL PO PLAIN<br />
s<strong>ed</strong>imentation rate is consequence of the progressive<br />
infilling of the Po Plain basin that, for this sector, was<br />
starting to shift to a continental-type environment. The<br />
latest period is characteriz<strong>ed</strong> by the onset of regional<br />
non-deposition and/or erosional con<strong>di</strong>tions over the<br />
entire Po Basin whole area. The R<strong>ed</strong> Surface is thus not<br />
visible throughout the entire CCB structural high. Thus,<br />
assuming a fill-to-the-top growth mo<strong>del</strong>, the calculat<strong>ed</strong><br />
rate has to be consider<strong>ed</strong> as a minimum value.<br />
CONCLUSIONS<br />
Deformation pattern of the Po Plain basin has been analyz<strong>ed</strong><br />
over two time windows (1.6 and 0.89 Myr and 0.89 Myr –<br />
Present). It is noteworthy the reactivation of north-verging<br />
backthrusts and associat<strong>ed</strong> folds, instead of the main<br />
forethrusts, maybe relat<strong>ed</strong> to a <strong>di</strong>fferential s<strong>ed</strong>imentary load<br />
between proximal and <strong>di</strong>stal portions of the basin. Uplift rates<br />
obtain<strong>ed</strong> for the CapFS characterize these thrusts as moderate<br />
to low strain rate structures.<br />
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Ben<strong>di</strong>ng and growth of the Central Andean plateau: paleomagnetic<br />
and structural constraints from the Eastern Cor<strong>di</strong>llera (22-24°S,<br />
NW Argentina)<br />
MARCO MAFFIONE (*), FABIO SPERANZA (*) & CLAUDIO FACCENNA (**)<br />
RIASSUNTO<br />
Piegamento e crescita <strong>del</strong>l’Altopiano <strong>del</strong>le Ande Centrali: vincoli<br />
paleomagnetici e strutturali dalla Cor<strong>di</strong>gliera Orientale (22-24°S, NO<br />
Argentina)<br />
In questo lavoro presentiamo nuovi dati paleomagnetici e strutturali<br />
provenienti da rocce s<strong>ed</strong>imentarie continentali a matrice sabbioso-argillosa<br />
<strong>del</strong>l’intervallo Cretaceo superiore-Pliocene provenienti dalla Cor<strong>di</strong>gliera<br />
Orientale (Ande centrali). Qui, rilievi a <strong>di</strong>rettrice N-S e NNE-SSW, costituiti<br />
da un basamento Paleozoico e da r<strong>ed</strong>-b<strong>ed</strong>s <strong>del</strong> Cretaceo superiore, si trovano<br />
parzialmente sovrapposti a profon<strong>di</strong> bacini s<strong>ed</strong>imentari <strong>di</strong> età Cenozoica. Le<br />
<strong>di</strong>rezioni paleomagnetiche (probabilmente primarie) ottenute in 15 siti<br />
documentano una rotazione oraria rispetto alla placca Sud Americana <strong>di</strong><br />
45.9°±9.4, 30.1°±23.9°, and 15.4°±19.3°, avvenuta rispettivamente dopo il<br />
Cretaceo superiore (~80 Ma), l’Oligo-Miocene (20-30 Ma), <strong>ed</strong> il Miocene<br />
superiore-Pliocene (5-10 Ma). Al contrario, 4 siti <strong>del</strong> Cretaceo superiore,<br />
situati in prossimità <strong>di</strong> una faglia trascorrente a <strong>di</strong>rettrice N-S (faglia Yavi-<br />
Abra Pampa) mostrano una rotazione nulla. Circa 20 km ad ovest, lungo la<br />
faglia trascorrente sinistra Ambra Moreta sono presenti strutture a fiore e strati<br />
sin-tettonici subverticali datati a 14.26±0.19 Ma. Considerando i dati <strong>di</strong><br />
letteratura circa l’età <strong>di</strong> inizio <strong>del</strong>la deformazione, proponiamo che a partire<br />
dall’Eocene-Oligocene (30-40 Ma) la Cor<strong>di</strong>gliera Orientale abbia subito una<br />
rotazione regionale oraria <strong>di</strong> 40°-50°, sincrona con la propagazione dei fronti<br />
compressivi e con il piegamento a grande scala <strong>del</strong>le Ande Centrali. È<br />
possibile che la rotazione oraria sia attualmente attiva come in<strong>di</strong>cano i dati<br />
GPS. A partire da ~15 Ma, l’attività <strong>del</strong>le faglie trascorrenti sinistre a <strong>di</strong>rezione<br />
N-S ha indotto <strong>del</strong>le rotazioni antiorarie lungo la zona <strong>di</strong> faglia, <strong>di</strong> fatto<br />
annullando localmente la rotazione regionale oraria. Ipotizziamo inoltre che<br />
l’attività trascorrente <strong>del</strong> Miocene m<strong>ed</strong>io possa aver accomodato il progressivo<br />
allargamento verso sud <strong>del</strong>l’Altipiano, migrato lateralmente dalle regioni<br />
centrali <strong>del</strong>la piega orogenica dove la crosta risulta maggiormente ispessita.<br />
Key words: Altiplano-Puna plateau, Bolivian orocline, Eastern<br />
Cor<strong>di</strong>llera, paleomagnetism, strike-slip tectonics.<br />
INTRODUCTION<br />
High plateaus are the ultimate product of plate convergence.<br />
These remarkable structures are characteriz<strong>ed</strong> by internally<br />
drain<strong>ed</strong> flat surfaces stan<strong>di</strong>ng at high elevation over a thick and<br />
_________________________<br />
(*)Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia, Via <strong>di</strong> Vigna Murata<br />
605, 00143 Rome, Italy<br />
(**)Dipartimento <strong>di</strong> Scienze Geologiche, Universita Roma Tre, Largo S.L.<br />
Murialdo 1, 00146 Rome, Italy<br />
warm crust. The Altiplano-Puna plateau is locat<strong>ed</strong> in the<br />
Central Andean Cor<strong>di</strong>llera between 15° and 28°S (Figure 1),<br />
stan<strong>di</strong>ng at a mean elevation of ~ 4000 m over a ~60-70 km<br />
thick crust (ISACKS, 1988; BECK et al., 1996). The plateau<br />
broadens up to ~200 km just in the area where the Cor<strong>di</strong>llera<br />
axis is deflect<strong>ed</strong> by 55° along the “Bolivian Orocline” or<br />
“Arica Deflection” (CAREY, 1955; ISACKS, 1988). The<br />
along-strike gra<strong>di</strong>ent of crustal shortening is consider<strong>ed</strong> as the<br />
lea<strong>di</strong>ng mechanism for the formation of the Bolivian orocline<br />
(e.g. ISACKS, 1988; MACFADDEN et al., 1995; RILLER<br />
AND ONCKEN, 2003; ROUSSE et al., 2003).<br />
The orogenic axis deflection of the Bolivian orocline is<br />
accompani<strong>ed</strong> by a “Central Andes rotation pattern” (CARP of<br />
14°S<br />
18°S<br />
22°S<br />
26°S<br />
30°S<br />
OROCLINEAXIS<br />
Neogene<br />
Paleogene<br />
Cretaceous<br />
72°W 68°W 64°W<br />
St udy area<br />
Fig. 2<br />
SANTA BARBARA<br />
SYSTEM<br />
SIERRAS<br />
PAMPEANAS<br />
Fig. 1 - Digital elevation mo<strong>del</strong> of the Central Andes, showing the main<br />
morphotectonic domains and paleomagnetic <strong>di</strong>rections from select<strong>ed</strong><br />
previous stu<strong>di</strong>es (arrows).
122 M. MAFFIONE ET ALII<br />
SOMOZA et al., (1996)), showing a general counterclockwise<br />
(CCW) rotation in the northern arc limb (Peru and northern<br />
Bolivia), and a clockwise (CW) rotation in the southern limb<br />
(southern Bolivia, Chile and northwestern Argentina, Figure 1).<br />
While shortening in the Andes is ongoing since the late<br />
Cretaceous, the uplift of the plateau probably initiat<strong>ed</strong> in the<br />
Miocene, during thrusting and consequent crustal thickening,<br />
reaching an elevation of up to ~1.5-2.5 km (ISACKS, 1988;<br />
HOKE AND GARZIONE, 2008). In a later stage (probably<br />
from 10 to 6 Ma), during the thrusting of the Eastern Cor<strong>di</strong>llera<br />
over the Brazilian Shield (ISACKS, 1988; GUBBELS et al.,<br />
1993), the plateau reach<strong>ed</strong> its present-day elevation.<br />
Transpression and strike-slip faulting has been document<strong>ed</strong><br />
on both limbs of the orocline. This pattern of deformation is<br />
regionally significant, and has been relat<strong>ed</strong> to <strong>di</strong>fferent<br />
mechanisms, such as: a ben<strong>di</strong>ng mechanism (i.e. saloon door)<br />
(e.g. ISACKS, 1988; MACFADDEN et al., 1995; MÜLLER et<br />
al., 2002); slip-partitioning due to the oblique convergence<br />
(e.g. RANDALL et al., 1996); compressional relat<strong>ed</strong> block<br />
faulting; lateral ramp accommodating panels of <strong>di</strong>fferent<br />
shortening rate (SOMOZA et al., 1996), or, in a Tibetan like<br />
fashion, to the lateral expansion of the plateau (RILLER AND<br />
ONCKEN, 2003). Despite the overall structural pattern of the<br />
Cor<strong>di</strong>llera being well establish<strong>ed</strong> in terms of large-scale<br />
rotations and tectonics of the salient limbs, the tectonics active<br />
during ben<strong>di</strong>ng and growth of the belt are still poorly<br />
document<strong>ed</strong>.<br />
This study focuses on the deformation occurring during<br />
orocline formation and growth of the plateau. We select<strong>ed</strong> a<br />
key site locat<strong>ed</strong> in the Eastern Cor<strong>di</strong>llera, at the margin with the<br />
Puna plateau (22-24°S, Figure 1), where thrusting, strike-slip<br />
deformation and rotations are expect<strong>ed</strong> to be particularly<br />
evident. We paleomagnetically sampl<strong>ed</strong> 32 sites from<br />
Cretaceous to Mio-Pliocene continental s<strong>ed</strong>imentary rocks.<br />
Natural remanent magnetization (NRM) of the sampl<strong>ed</strong> rocks<br />
has been investigat<strong>ed</strong> using thermal treatment, and magnetic<br />
mineralogy analyses were also perform<strong>ed</strong>. Finally, we also<br />
measur<strong>ed</strong> the strike and <strong>di</strong>p of 70 striat<strong>ed</strong> small-scale brittle<br />
fault planes (and relative kinematic in<strong>di</strong>cators, when present)<br />
from late Cretaceous to Miocene s<strong>ed</strong>imentary rocks at 9<br />
localities, mostly coinci<strong>di</strong>ng with the paleomagnetic sites.<br />
Quartz or calcite steps and SC-structures were the most<br />
commonly identifi<strong>ed</strong> fault kinematic in<strong>di</strong>cators.<br />
RESULTS AND DISCUSSION<br />
Magnetic mineralogy analyses point to hematite (of variable<br />
grain sizes) as the unique magnetic carrier of all Cretaceous-<br />
Pliocene samples gather<strong>ed</strong> from the Eastern Cor<strong>di</strong>llera.<br />
Reliable (and likely primary) site mean paleomagnetic<br />
<strong>di</strong>rections from 15 sites document that the Eastern Cor<strong>di</strong>llera<br />
between 22° and 24°S has undergone 45.9°±9.4°, 30.1°±23.9°,<br />
and 15.4°±19.3° CW rotation with respect to South America<br />
after late Cretaceous (~80 Ma), Oligo-Miocene (20-30 Ma),<br />
and late Miocene-Pliocene (5-10 Ma), respectively (Table 1).<br />
Conversely, <strong>di</strong>rections from 4 Cretaceous sites locat<strong>ed</strong> adjacent<br />
to the Yavi-Abra Pampa fault (group B) fall ca. 40° apart from<br />
the remaining 8 Cretaceous sites (group A), showing no<br />
significant rotation (Figures 2).<br />
Field mapping and structural data in<strong>di</strong>cate that small-scale<br />
fault planes trend ~NNE-SSW on average, in agreement with<br />
the regional trend of the major structures, and show dual<br />
vergence with a pr<strong>ed</strong>ominance of west-<strong>di</strong>pping planes (Figure<br />
2). Contouring of the slickensides density from all sites reveals<br />
two main clustering at about 270° and 330°, consistent with<br />
previous results by CLADOUHOS et al. (1994) and<br />
COUTAND et al. (2001). The transport <strong>di</strong>rection is only<br />
locally <strong>di</strong>p slip (on NE-SW fault planes, i.e. near site St07 and<br />
11) but pr<strong>ed</strong>ominantly oblique on NS to NNE-SSW planes,<br />
yiel<strong>di</strong>ng an overall left-lateral transpressive regime (Figure 2).<br />
Perhaps, the most impressive feature of the area is<br />
represent<strong>ed</strong> by the tens of kilometre-scale strike-slip features.<br />
Their activity is mark<strong>ed</strong> not only by important damage zones,<br />
duplexes and flower structures, but also by the deposition of<br />
thick pack of clastic deposits growing while they were getting<br />
sub-vertical. This is clearly attest<strong>ed</strong> by progressive<br />
unconformities (growth strata), characterizing a formation<br />
dat<strong>ed</strong> at 14.26±0.19 Ma (CLADOUHOS et al., 1994).<br />
Accor<strong>di</strong>ng to this, we relate the paleomagnetic <strong>di</strong>rection of<br />
sites adjacent to the Yavi-Ambra Pampa fault to a local ~40°<br />
CCW rotation occurring adjacent to the fault system, ad<strong>di</strong>ng to<br />
the 40°-50° regional CW rotation. Consistently, the tectonic<br />
transport <strong>di</strong>rections evaluat<strong>ed</strong> at two stations (St18 and St10)<br />
close to the Yavi-Abra Pampa fault are rotat<strong>ed</strong> ca. 60° CCW<br />
with respect to those observ<strong>ed</strong> near Abra Pampa, far from the<br />
fault (Figure 2). As CCW rotations in the vicinity of a strikeslip<br />
fault are definitely induc<strong>ed</strong> by a left-lateral shear<br />
(SONDER et al., 1994), we suggest that the Yavi-Abra Pampa<br />
faults had a dominant left-lateral <strong>di</strong>splacement that induc<strong>ed</strong><br />
along the fault walls a ca. 40° CCW rotation, almost annulling<br />
the 40°-50° regional CW rotation observ<strong>ed</strong> elsewhere in upper<br />
Cretaceous strata.<br />
We also perform<strong>ed</strong> an oroclinal test (e.g. SCHWARTZ AND<br />
VAN DER VOO, 1983) on our paleomagnetic data, to verify<br />
whether in the Eastern Cor<strong>di</strong>llera a statistically significant<br />
rotational <strong>di</strong>fference exists at sites characteriz<strong>ed</strong> by <strong>di</strong>fferent<br />
structural attitude (i.e. strike of the b<strong>ed</strong><strong>di</strong>ng). The tectonic<br />
implications of the results from the oroclinal test are twofold:<br />
first, thrust-fold structures in Cretaceous and Oligo-Miocene<br />
rocks from the Eastern Cor<strong>di</strong>llera showing variable orientation<br />
(i.e. N-S to NE-SW) underwent an uniform regional CW<br />
rotation, with no oroclinal ben<strong>di</strong>ng mechanism at a local scale;<br />
second, a correlation between the paleomagnetic declinations<br />
and the local <strong>di</strong>rections of the structures emerges when<br />
consider<strong>ed</strong> the entire Cretaceous data set (group A + group B).<br />
This implies that at sites locat<strong>ed</strong> adjacent to the Yavi-Abra<br />
Pampa fault the CCW rotation (we suppos<strong>ed</strong> to be relat<strong>ed</strong> to<br />
the strike-slip shear along this fault) occurr<strong>ed</strong> after that the<br />
structural grain of the orogen was acquir<strong>ed</strong>, i.e. they postdate<br />
the onset of deformation and strata tilting in the Eastern<br />
Cor<strong>di</strong>llera. This time relationship among tectonic episodes is
BENDING AND GROWTH OF THE CENTRAL ANDEAN PLATEAU: PALEOMAGNETIC AND STRUCTURAL CONSTRAINTS FROM THE EASTERN<br />
CORDILLERA (22-24°S, NW ARGENTINA)<br />
Rinconada<br />
A<br />
St 14<br />
St07 (PC)<br />
Laguna<br />
Pozuelos<br />
Abra Moreta<br />
Fault<br />
B<br />
<br />
<br />
Abra Moreta<br />
St 02 (TA)<br />
<br />
Sierra de Cochinoca<br />
Cochinoca<br />
St 12<br />
St04 (PA)<br />
Yavi-Abra<br />
Pampa Fault<br />
Abra Pampa<br />
St 11<br />
Sierra de Aguilar<br />
St 18<br />
<br />
<br />
St 18<br />
C-C’<br />
(St15)<br />
B’<br />
Cangrejos<br />
Purmamarca<br />
Humahuaca<br />
1 2 3 4 5 6 7 8 9 10 11 12<br />
<br />
<br />
<br />
<br />
Cangrejos-TresCrucesridge<br />
<br />
<br />
<br />
<br />
St 10<br />
<br />
Tres Cruces<br />
b<br />
<br />
<br />
<br />
Quebrada de Humahuaca<br />
a<br />
Sierra de SantaVictoria<br />
A’<br />
a b c<br />
Fig. 2 - Structural and geological map of the study area, showing the stereographic projections of the fault plane <strong>di</strong>rections at each measurement site. Black<br />
arrows represent site-mean rotations from this study calculat<strong>ed</strong> with respect to stable South America. 1, Recent alluvial and colluvial deposits [Quaternary]; 2,<br />
Miomarà formation [late Miocene-Pliocene]; 3, Sijes/Chaco formation [middle-late Miocene]; 4, Moreta/Candado formation [Oligocene-Miocene]; 5, basic<br />
and acid volcanics [Neogene]; 6, Pirgua formation [Aptian-Maastrichtian]; 7, granitoides [Cretaceous]; 8, Acoite formation [Ordovician] and pre-Cretaceous<br />
formations; 9, main expos<strong>ed</strong> (a) and buri<strong>ed</strong> (b) thrust faults; 10, site-mean transport <strong>di</strong>rection; 11, structural analysis site; 12, b<strong>ed</strong><strong>di</strong>ng attitude for 0°-35° (a),<br />
35°-70° (b), and ca. 90° (c) <strong>di</strong>pping strata.
124 M. MAFFIONE ET ALII<br />
confirm<strong>ed</strong> by the evidence that shear <strong>di</strong>rections along reverse<br />
faults observ<strong>ed</strong> at two stations near the Yavi-Abra Pampa fault<br />
(St10 and St18, Figure 2) are rotat<strong>ed</strong> ca. 60° CCW with respect<br />
to those from other stations locat<strong>ed</strong> far from major tectonic<br />
structures.<br />
Relying on our paleomagnetic data and interpretation,<br />
regional CW rotations relat<strong>ed</strong> to the development of the<br />
Andean arcuate belt occurr<strong>ed</strong> in the Eastern Cor<strong>di</strong>llera from 30-<br />
40 Ma to Present, as GPS data suggest (ALLMENDINGER et<br />
al., 2005). Conversely, the strike-slip tectonics and (associat<strong>ed</strong>)<br />
CCW rotations we document have only occurr<strong>ed</strong> in the past 15<br />
Ma, and are coeval with a relatively recent regional-scale<br />
tectonic event. In fact, the strong uplift of the Altiplano-Puna<br />
plateau until the remarkable height of ca. 4000 m observ<strong>ed</strong><br />
today is thought to have occurr<strong>ed</strong> mostly during the Neogene<br />
(last 10 Ma) (e.g. HOKE AND GARZIONE, 2008). In contrast<br />
to the composite rotational mo<strong>del</strong>s put forward in the past, we<br />
suggest that the development of N-S strike-slip activity (and<br />
relat<strong>ed</strong> local rotations) in the Eastern Cor<strong>di</strong>llera (at the border<br />
with the Altiplano-Puna plateau) is relat<strong>ed</strong> to the uplift of the<br />
plateau, and rather independent from the large-scale<br />
development of the Andean salient.<br />
Accor<strong>di</strong>ng to this hypothesis, the onset of the strike-slip<br />
activity would be link<strong>ed</strong> to the lateral (southward) growth of the<br />
Central Andean plateau. The plateau uplift start<strong>ed</strong> in the central<br />
part of the Andean salient, where the shortening amount was<br />
greater (e.g. KLEY AND MONALDI, 1998; MCQUARRIE et<br />
al., 2008). This may have induc<strong>ed</strong> a stress (relat<strong>ed</strong> to body<br />
forces) at the plateau margins which could have induc<strong>ed</strong> a<br />
progressive sprea<strong>di</strong>ng of the plateau toward its northern and<br />
southern <strong>ed</strong>ges.<br />
REFERENCES<br />
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BROOKS B. (2005) - Ben<strong>di</strong>ng the bolivian orocline in real<br />
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CLADOUHOS, T.T., R.W. ALLMENDINGER, B. COIRA, E. FARRAR<br />
(1994) - Late Cenozoic deformation in the Central Andes:<br />
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209-228.<br />
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A. CHAUVIN, D. GAPAIS, AND E. ROSSELLO (2001) - Style<br />
and history of Andean deformation, Puna plateau,<br />
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plateau uplift, and foreland development, Bolivian central<br />
Andes, Geology, 21, 695-698.<br />
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and the mechanisms for the late Miocene topographic<br />
development of the Altiplano plateau, Earth Planet. Sci.<br />
Lett., 271, 192-201.<br />
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ben<strong>di</strong>ng of the Bolivian orocline, J. Geophys. Res., 93,<br />
3211– 3231.<br />
KLEY, J., C.R. MONALDI (1998) - Tectonic shortening and<br />
crustal thickness in the Central Andes: how good is the<br />
correlation?, Geology, 26(8), 723-726.<br />
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Neogene paleomagnetism and oroclinal ben<strong>di</strong>ng of the<br />
central Andes of Bolivia, J. Geophys. Res., 100, 8153–<br />
8167.<br />
MCQUARRIE, N., J.B. BARNES, AND T.A. EHLERS (2008) -<br />
Geometric, kinematic, and erosional history of the central<br />
Andean Plateau, Bolivia (15-17°S), Tectonics, 27, TC3007,<br />
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MÜLLER, J.P., J. KLEY, V. JACOBSHAGEN (2002) - Structure and<br />
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RILLER, U., AND O. ONCKEN (2003) - Growth of the central<br />
Andean Plateau by tectonic segmentation is controll<strong>ed</strong> by<br />
the gra<strong>di</strong>ent in crustal shortening, J. Geol., 111, 367-384.<br />
ROUSSE, S., S. GILDER, D. FARBER, B. MCNULTY, P. PATRIAT,<br />
V. TORRES, AND T. SEMPERE (2003) - Paleomagnetic<br />
tracking of mountain buil<strong>di</strong>ng in the Peruvian Andes since<br />
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SCHWARTZ, S.Y., AND R. VAN DER VOO (1983) - Paleomagnetic<br />
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SOMOZA, R., S. SINGER, AND B. COIRA (1996) -<br />
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Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 125<br />
Fluid assist<strong>ed</strong> shearing at the depth of the Brittle-Ductile Transition: an<br />
integrat<strong>ed</strong> structural, petrological, fluid inclusions study of the<br />
Erbalunga shear zone, Schistes Lustrés Nappe, Alpine Corsica (France).<br />
MATTEO MAGGI (*,**), FEDERICO ROSSETTI (**), FRANCESCA TECCE (***) & GIANLUCA VIGNAROLI(**)<br />
ABSTRACT<br />
In this work we present structural, petrological and fluid<br />
inclusion stu<strong>di</strong>es perform<strong>ed</strong> in a major retrogressive shear zone<br />
(the Erbalunga shear zone), which occurs within the HP/LT<br />
domain of the Schistes Lustrés Nappe of eastern Alpine<br />
Corsica. This shear zone is part of the post-orogenic network of<br />
shear zones that favour<strong>ed</strong> the exhumation of the HP core of<br />
Alpine Corsica (Daniel et al., 1996) during Late<br />
Oligocene/Early Miocene times (Brunet et al., 2000). The shear<br />
zone is characteris<strong>ed</strong> by a progressive ductile-to-brittle top-tothe-E<br />
shearing, starting at greenschist facies con<strong>di</strong>tions (ca. 600<br />
MPa, 400-450 °C). Evidence for vigorous fluid flow through<br />
the shear zone is document<strong>ed</strong> by widespread quartz and quartzcalcite<br />
vein segregations, which accompani<strong>ed</strong> the progressive<br />
evolution of shearing. Textural characteristics of three main<br />
generations of veins record the incremental evolution of the<br />
shear zone tracing the continuum transition from ductile- to<br />
brittle-dominat<strong>ed</strong> deformation environments. Regardless, of the<br />
vein generation, fluid inclusions host<strong>ed</strong> in quartz grains host<strong>ed</strong><br />
within the three <strong>di</strong>fferent sets of veins document a low-salinity<br />
(
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 126<br />
Mo<strong>del</strong>ling the fractur<strong>ed</strong> carbonatic rocks of Lepini Mountains to infer<br />
deep hydrogeological pathways<br />
IRENE MANNINO *, PAOLA CIANFARRA*, FRANCESCO SALVINI *<br />
ABSTRACT<br />
Spatial <strong>di</strong>stribution of brittle deformation rules the secondary<br />
permeability of carbonatic rocks and therefore the<br />
accumulation and the pathway of deep fluids (ground-water,<br />
hydrocarbon).<br />
This work contributes to the understan<strong>di</strong>ng of the<br />
hydrogeological relations among the deep, fractur<strong>ed</strong> carbonatic<br />
structures in the western sector of the Lepini Mountains<br />
(Central Apennines). This NW-SE structure is characteris<strong>ed</strong> by<br />
three horses stack<strong>ed</strong> towards the NE from hinterland to<br />
foreland with a major thrust that bords the NE sector of the<br />
area. A serie of lystric, regional faults lower the south-western<br />
flank of the structure. Fracture H/S (Height/Spacing) parameter<br />
prov<strong>ed</strong> a useful tool to estimate the intensity of the regional<br />
brittle deformation. It was measur<strong>ed</strong> by a set of field-structural<br />
stations in outcrops representing the main buri<strong>ed</strong> carbonate<br />
units. H/S statistical analyses provide values ranging from 1.5<br />
to 2.6. The first value relates to <strong>di</strong>agenesis, the latter to the<br />
effect of paleostress induc<strong>ed</strong> by tectonics. A numerical<br />
mo<strong>del</strong>ling (ForcTre Software) was perform<strong>ed</strong> to reconstruct the<br />
kinematic and tectonic evolution by balanc<strong>ed</strong> cross section in a<br />
forward mo<strong>del</strong>ling. In this way, the mo<strong>del</strong>ling of the intensity<br />
and spatial <strong>di</strong>stribution of brittle deformations was comput<strong>ed</strong><br />
by the Time Stress Integral (TSI), TSI = F(φ,,Σe)dt, with φ<br />
being the frictional coefficent, the stress and Σe the strength.<br />
A linear relation was found between the spatial <strong>di</strong>stribution of<br />
TSI and measur<strong>ed</strong> H/S of the outcropping rocks ( < 0.1%).<br />
This correlation allow<strong>ed</strong> to estimate the intensity and the<br />
<strong>di</strong>stribution of deep rock fracture pattern. Average higher<br />
values of pr<strong>ed</strong>ict<strong>ed</strong> H/S (1.8) characterise Jurassic rocks than<br />
Cretaceous one. Fracturing is expect<strong>ed</strong> to decrease with depth<br />
in all stu<strong>di</strong><strong>ed</strong> units. This r<strong>ed</strong>uction relates to the vertical<br />
succession of rocks characteris<strong>ed</strong> by <strong>di</strong>fferent reological<br />
properties, namely shale content, b<strong>ed</strong> thickness and interlayer<br />
shale content.<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche Università <strong>di</strong> Roma Tre, Largo<br />
S.L.Murialdo 1, I-00146 Rome, Italy<br />
Lavoro eseguito con il contributo finanziario <strong>del</strong> Laboratorio <strong>di</strong><br />
geo<strong>di</strong>namica quantitativa e telerilevamento <strong>del</strong> Dipartimento <strong>di</strong> Scienze<br />
Geologiche, Università <strong>di</strong> Roma Tre<br />
On the basis of the obtain<strong>ed</strong> results we suggest that fractures<br />
provide a deep hydrological connection between the Lepini Mts<br />
and the buri<strong>ed</strong> structures under the plio-quaternary deposits of<br />
the Pontina Plain to the SE. The normal faults locat<strong>ed</strong> in the<br />
SW sector of the investigat<strong>ed</strong> area do not compartmentalise the<br />
structures.<br />
Key words: brittle deformation, secondary permeability,<br />
fracturing, carbonates, Lepini Mts
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 127-130<br />
Tectono-metamorphic history of the Duarte terrane (Jarabacoa<br />
area, Hispaniola Island): insights on the tectonic evolution of the<br />
northern rim of the Caribbean Oceanic Plateau<br />
MICHELE MARRONI*, LUCA PANDOLFI*, GIUSEPPE GIUNTA** & ALESSANDRO MALASOMA*<br />
RIASSUNTO<br />
Evoluzione strutturale <strong>del</strong>l’unità Duarte (Jarabacoa,<br />
Hispaniola) e considerazioni sull’evoluzione tettonica <strong>del</strong><br />
bordo settentrionale <strong>del</strong> Plateau Oceanico Caraibico<br />
Il margine settentrionale <strong>del</strong>la Placca Caraibica è <strong>del</strong>imitato da una catena<br />
traspressiva caratterizzata dall’insieme <strong>di</strong> <strong>di</strong>versi terrane derivati da una<br />
convergenza obliqua, <strong>di</strong> età Cretaceo Superiore-Terziaria, fra le Placche<br />
Nordamericana e Caraibica. Frammenti <strong>di</strong> un plateau oceanico appartenenti<br />
alla Placca Caraibica sono incorporati in questa catena come unità tettoniche<br />
fortemente deformate e metamorfosate. Alcune <strong>di</strong> queste unità sono state<br />
riconosciute nel settore <strong>di</strong> Jarabacoa (Hispaniola). In questo settore il Terrane<br />
<strong>di</strong> Duarte è formato da due <strong>di</strong>verse unità tettoniche caratterizzate da un <strong>di</strong>verso<br />
grado metamorfico. L’unità inferiore è costituita da metabasiti in facies Scisti<br />
Ver<strong>di</strong>, mentre quella superiore è caratterizzata da metabasiti in facies<br />
anfibolitica a cui si sovrappongono metas<strong>ed</strong>imenti derivati dallo<br />
smantellamento <strong>di</strong> un arco magmatico.<br />
Entrambe le unità tettoniche sono affette da quattro <strong>di</strong>versi eventi<br />
deformativi (D1 to D4) che possono essere correlati. In entrambe le unità la<br />
fase D1 è caratterizzata da una foliazione metamorfica associata a lineazioni<br />
mineralogiche e dalla presenza <strong>di</strong> rare pieghe isoclinali sra<strong>di</strong>cate. La fase D2 è<br />
caratterizzata da una foliazione ben sviluppata e da pieghe isoclinali sviluppate<br />
durante una fase metamorfica retrograda. Le fasi D1 e D2 si sviluppano nel<br />
Cenomaniano-Turoniano, mentre le fasi D3 e D4 producono una successiva<br />
debole deformazione senza ricristallizzazione metamorfica. Queste ultime fasi<br />
possono essere messe in relazione con la tettonica traspressiva <strong>del</strong>l’Oligocene-<br />
Miocene Inferiore. Tutte le deformazioni riconosciute si sviluppano in rocce<br />
derivate da un arco connesso con una subduzione obliqua sud-vergente <strong>del</strong>la<br />
litosfera oceanica <strong>del</strong>la Placca Nordamericana al <strong>di</strong> sotto <strong>del</strong> plateau oceanico<br />
Caraibico.<br />
Parole chiave: metabasiti, anfiboliti, Scisti Ver<strong>di</strong>,<br />
deformazione, metamorfismo, Placca Caraibica, Duarte,<br />
Isola <strong>di</strong> Hispaniola.<br />
ABSTRACT<br />
The northern margin of the Caribbean Plate is characteriz<strong>ed</strong> by a<br />
transpressional belt consisting of an assemblage of several terranes deriv<strong>ed</strong><br />
from Upper Cretaceous to Tertiary oblique convergence between the Northern<br />
American and Caribbean Plates. Fragments of the oceanic plateau belonging to<br />
the Caribbean Plate are incorporat<strong>ed</strong> in these belts as strongly deform<strong>ed</strong> and<br />
_________________________<br />
* Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Pisa<br />
** Dipartimento <strong>di</strong> Geologia e Geodesia, Università <strong>di</strong> Palermo<br />
metamorphos<strong>ed</strong> sequences, as recogniz<strong>ed</strong> in the Jarabacoa area of the<br />
Hispaniola Island. In this area the Duarte Terrane consists of two <strong>di</strong>fferent<br />
units showing <strong>di</strong>fferent metamorphic grades. The lower unit consists of<br />
greenschist facies metabasites, whereas the upper one is represent<strong>ed</strong> by<br />
amphibolite facies metabasites topp<strong>ed</strong> by metas<strong>ed</strong>imentary rocks, whose<br />
protoliths were clastic deposits suppli<strong>ed</strong> by a magmatic arc. Both units<br />
suffer<strong>ed</strong> four deformation phases, from D1 to D4, that can be strictly correlat<strong>ed</strong>.<br />
In both units the D1 phase is characteriz<strong>ed</strong> by a syn-metamorphic foliation<br />
associat<strong>ed</strong> to a mineral lineation and very rare rootless isoclinal folds. The D2<br />
phase is characteriz<strong>ed</strong> by a well develop<strong>ed</strong> foliation and isoclinal folds<br />
acquir<strong>ed</strong> under retrograde metamorphic con<strong>di</strong>tions. The D1 and D2 phases<br />
develop<strong>ed</strong> in the Cenomanian-Turonian time span. The following D3 and D4<br />
phases produc<strong>ed</strong> weak deformations without metamorphic imprint. The D3 and<br />
D4 phase are probably connect<strong>ed</strong> with transpression tectonics of Early<br />
Oligocene to Early Miocene age. All these deformations develop<strong>ed</strong> in an arc<br />
setting connect<strong>ed</strong> with southward oblique subduction of the oceanic<br />
lithosphere of the North America Plate beneath the Caribbean oceanic plateau.<br />
Key words: metabasites, amphibolite, greenschist,<br />
deformation, metamorphism, Carribbean Plate, Duarte<br />
Terrane. Hispaniola Island.<br />
INTRODUCTION<br />
The Caribbean Plate has a composite structure that includes<br />
a small continental block showing a transition to a large<br />
igneous province represent<strong>ed</strong> by an oceanic plateau that is<br />
regard<strong>ed</strong> as the result of Lower to Upper Cretaceous plume<br />
activity. The northern border of the Caribbean Plate is<br />
represent<strong>ed</strong> by transpressional deform<strong>ed</strong> belts, consisting of an<br />
assemblage of several terranes deriv<strong>ed</strong> from Upper Cretaceous<br />
to Tertiary oblique convergence between the Northern America<br />
and the Caribbean Plates (Donnelly et al., 1990). Fragments of<br />
the oceanic plateau are incorporat<strong>ed</strong> in these deform<strong>ed</strong> belts, as<br />
recogniz<strong>ed</strong> in the Hispaniola Island locat<strong>ed</strong> at the northern<br />
margin. In this island, the oceanic plateau is expos<strong>ed</strong> as<br />
unmetamorphos<strong>ed</strong> and weakly deform<strong>ed</strong> sequences in the La<br />
Hotte-Selle-Borohuco Terrane, but also as strongly deform<strong>ed</strong><br />
and metamorphos<strong>ed</strong> sequences in the Duarte Terrane. The latter<br />
one represents a good example of the oceanic plateau involv<strong>ed</strong><br />
in the transpressional belt at the border of the Caribbean Plate.<br />
In this work, the lithostratigraphic features, the deformation<br />
history and the metamorphic characteristics of the metabasites<br />
from the Duarte Terrane cropping out in the Jarabacoa area are<br />
describ<strong>ed</strong> and the relat<strong>ed</strong> implications for the geodynamic<br />
history of the northern border of the Caribbean Plate are<br />
<strong>di</strong>scuss<strong>ed</strong>.
128 M. MARRONI ET ALII<br />
THE DUARTE TERRANE<br />
In the study area, the Duarte Terrane consists of two<br />
tectonic units, <strong>di</strong>stinguish<strong>ed</strong> by the metamorphic grade. The<br />
lower unit is characteriz<strong>ed</strong> by greenschist facies metamorphism<br />
whereas the upper one shows amphibolite facies metamorphic<br />
imprint. Both these units are strongly deform<strong>ed</strong>, showing a<br />
polyphase history. Within the Duarte Terrane a complex<br />
deformation history consisting of four deformation phases, has<br />
been recogniz<strong>ed</strong>. The later, brittle tectonics are not describ<strong>ed</strong> in<br />
this work.<br />
The amphibolite facies unit<br />
The amphibolite facies unit comprises two <strong>di</strong>fferent<br />
lithothypes. The first one consists massive to foliat<strong>ed</strong><br />
metabasites, sometimes characteriz<strong>ed</strong> by coarse-grain<strong>ed</strong><br />
texture. As describ<strong>ed</strong> by Lewis and Jimenez (1991) in the area<br />
northwest of Jarabacoa, the metabasites are topp<strong>ed</strong> by a second<br />
lithotype consisting of metas<strong>ed</strong>imentary rocks. The<br />
metas<strong>ed</strong>imentary rocks consist of quartz-bearing gneisses and<br />
micaschists with levels of massive metaconglomerates<br />
inclu<strong>di</strong>ng clasts of basic and acid volcanic rocks flatten<strong>ed</strong> along<br />
the main foliation.<br />
The amphibolite facies unit suffer<strong>ed</strong> a complex deformation<br />
history develop<strong>ed</strong> under retrograde P and T con<strong>di</strong>tions. The<br />
first deformation phase (D1) is mainly identifi<strong>ed</strong> in the foliation<br />
(S1) that can be found in two <strong>di</strong>fferent occurrences. In the finegrain<strong>ed</strong><br />
amphibolites and gneisses, the S1 is found as relic<br />
foliation largely transpos<strong>ed</strong> by deformations develop<strong>ed</strong> during<br />
the following second deformation phase. By contrast, in the<br />
more competent lithologies, as for instance the massive<br />
amphibolites, the S1 is well preserv<strong>ed</strong> and only slightly<br />
mo<strong>di</strong>fi<strong>ed</strong>. Mineralogical lineations (L1) consisting of align<strong>ed</strong><br />
amphibole and plagioclase minerals are well develop<strong>ed</strong> on the<br />
S1 and reveals the N/S trend of these structural elements. In the<br />
amphibolite, also bou<strong>di</strong>nag<strong>ed</strong> and fold<strong>ed</strong> quartz veins with a<br />
thickness ranging from 5 to 20 cm can be referr<strong>ed</strong> to the D1. In<br />
the gneisses, the S1 develop<strong>ed</strong> as alternating quartz- and<br />
plagioclase-rich layers characteriz<strong>ed</strong> by porphyroclasts of<br />
quartz. In the associat<strong>ed</strong> conglomerates, the S1 is mark<strong>ed</strong> by<br />
the shape of the deform<strong>ed</strong> pebbles flatten<strong>ed</strong> in the plane of the<br />
foliation.<br />
The second deformation phase (D2) is generally the most<br />
common at the mesoscale. It is mainly represent<strong>ed</strong> by a<br />
continuous and pervasive foliation (S2), well develop<strong>ed</strong> in<br />
bands of very fine- and fine-grain<strong>ed</strong> amphibolites. These<br />
amphibolites are found around large bou<strong>di</strong>ns, up to 200 m<br />
thick, made of massive amphibolites, where S2 is generally<br />
poorly develop<strong>ed</strong>. In the anastomizing bands, S2, generally<br />
parallel to S1, occurs with <strong>di</strong>fferent morphology, mainly<br />
depen<strong>di</strong>ng the lithology. In the very fine-grain<strong>ed</strong> amphibolites,<br />
S2 can be defin<strong>ed</strong> as fine-grain<strong>ed</strong> schistosity characteriz<strong>ed</strong> by<br />
alternating layers of amphiboles and plagioclases. When the<br />
grain-size increases, S2 occurs as spac<strong>ed</strong> foliation characteriz<strong>ed</strong><br />
by cleavage domains surroun<strong>di</strong>ng microlithons where textural<br />
remnants of S1 are preserv<strong>ed</strong> in lens-shap<strong>ed</strong>, granoblastic<br />
aggregates of plagioclase and amphibole. Therefore, the main<br />
tectonic surface detect<strong>ed</strong> in the field can be regard<strong>ed</strong> as a S1/S2<br />
composite foliation deriv<strong>ed</strong> from the overprinting of the two<br />
deformation phases. Associat<strong>ed</strong> to S2, very rare rootless<br />
isoclinal folds (F2) with thicken<strong>ed</strong> hinges have been recogniz<strong>ed</strong><br />
at both meso and microscale. The relat<strong>ed</strong> axes (A2) trend at<br />
about NE/SW. The hinges of the F2 are generally round<strong>ed</strong> or<br />
subround<strong>ed</strong>, whereas the limbs are strongly bou<strong>di</strong>nag<strong>ed</strong>. The<br />
F2 generally deform the quartz veins or the S1. When observ<strong>ed</strong><br />
on the S2, the F2 are strongly non-cylindrical, their asymmetry<br />
suggests a NW vergence.<br />
The third deformation phase (D3) is represent<strong>ed</strong> by open to<br />
close folds (F3) characteriz<strong>ed</strong> by low-angle axial plane. The F3<br />
are generally cylindrical without thickening of the hinges nor<br />
bou<strong>di</strong>nage of the limbs; their asymmetry has a NE vergence.<br />
The axes (A3) have a NW/SE trend. In the field, the F3 <strong>di</strong>splay<br />
an axial plane foliation (S3) that can be classifi<strong>ed</strong> as spac<strong>ed</strong><br />
crenulation cleavage. Dykes of foliat<strong>ed</strong> tonalites deform<strong>ed</strong> by<br />
F3 have been found in the field. In the more competent<br />
lithologies the S3 occurs as a well develop<strong>ed</strong> fracture cleavage.<br />
The S2-S3 intersection lineation is well develop<strong>ed</strong>, the<br />
mineralogical lineations are lacking.<br />
In thin section, the amphibolites are characteriz<strong>ed</strong> by a<br />
strong S1 defin<strong>ed</strong> by the preferr<strong>ed</strong> orientation of amphibole and<br />
plagioclase. Plagioclase <strong>di</strong>splays an orient<strong>ed</strong> granoblastic<br />
structure concentrat<strong>ed</strong> in thin layers alternating to bands<br />
enrich<strong>ed</strong> in nematoblastic amphiboles. The plagioclases show<br />
taper<strong>ed</strong> twins whereas the amphiboles <strong>di</strong>splay little ondulatory<br />
extinction. Quartz and epidote also occur. In the amphibolites,<br />
the D1 assemblage is characteriz<strong>ed</strong> by the development of<br />
plagioclase (andesine) + Ca-amphibole<br />
(hornblende/tschermakite) + quartz + epidote. From the D1<br />
mineral assemblage epidote and Ca-amphibole have been<br />
analyz<strong>ed</strong> by electron microprobe. The pistacite component<br />
[Fe3+/(Fe3++Al3+)] in D1-relat<strong>ed</strong> epidote ranges between<br />
0.23 and 0.27. The analyz<strong>ed</strong> amphiboles are characteriz<strong>ed</strong> by a<br />
Mg/(Mg+Fe2+) ratio ranging from 0.48 to 0.57 and Si contents<br />
between 6.21 and 3.60 apfu. These Ca-amphiboles are<br />
classifiable as magnesio-hornblende and tschermakite, close to<br />
the boundary between the magnesio- and ferro-tschermakite<br />
stability fields. Amphiboles are the most common minerals in<br />
metabasaltic rocks, and their compositions can provide useful<br />
informations on P-T con<strong>di</strong>tions of metamorphic rocks. The<br />
stability field of hornblende suggests a temperature range<br />
between 450 and 550°C and a pressure ranging from 0.40 to<br />
0.80 GPa (Okamoto and Toriumi, 2005), coherent with<br />
amphibolite facies con<strong>di</strong>tions. These values agree also with the<br />
P and T con<strong>di</strong>tions of 0.49-0.66 GPa and 542-681°C estimat<strong>ed</strong><br />
by Escuder Viruete et al. (2002) in the Duarte Terrane cropping<br />
out in the Villa Altagracia area.<br />
The gneisses are in turn characteriz<strong>ed</strong> by prevailing quartz,<br />
associat<strong>ed</strong> with muscovite and plagioclase, whereas apatite,<br />
rutile, magnetite, are accessory phases. Textures are<br />
characteriz<strong>ed</strong> by thin ( 1 mm) bands of granoblastic<br />
plagioclase + quartz and bands of lepidoblastic micas. Quartz<br />
porphyroclasts, generally envelop<strong>ed</strong> by lepidoblastic micas, are
characteriz<strong>ed</strong> by s and d structures that suggest the presence of<br />
non-coaxial strain. The micaschists are in turn characteriz<strong>ed</strong> by<br />
well develop<strong>ed</strong> schistosity associat<strong>ed</strong> with the synkinematic<br />
growth of white mica + quartz + albite. On the whole, gneisses<br />
and micaschists are characteriz<strong>ed</strong>, during the D1, by the<br />
andesina + quartz + white mica + epidote mineral assemblage,<br />
probably develop<strong>ed</strong> in the same P and T con<strong>di</strong>tions of the<br />
amphibolites.<br />
The S2 can be defin<strong>ed</strong> as a zonal crenulation cleavage with<br />
smooth and anastomizing cleavage domains. The transition<br />
between cleavage and microlithons is <strong>di</strong>screte. The coexistence<br />
of albite and actinolite suggests greenschist facies con<strong>di</strong>tions<br />
with temperature and pressure ranging between 350 and 480°C<br />
and 0.20 to 0.60 Gpa, respectively (Okamoto & Toriumi,<br />
2005). In thin section the fine-grain<strong>ed</strong> amphibolites and<br />
gneisses appear deform<strong>ed</strong> by the S3 that is a poorly develop<strong>ed</strong>,<br />
spac<strong>ed</strong>, crenulation cleavage with smooth shape of cleavage<br />
domains showing gradational transition to microlithons<br />
The greenschist facies unit<br />
The greenschist facies unit consists of metabasites with well<br />
preserv<strong>ed</strong> relics of magmatic structures. Relics of pillow<br />
structures are identifi<strong>ed</strong> as lens-shape bo<strong>di</strong>es. In the<br />
metabasites, relics of clinopyroxene and/or plagioclase have<br />
been identifi<strong>ed</strong>. Accor<strong>di</strong>ng to Escuder Viruete et al. (2007b)<br />
two lithostratigraphic units are found: the lower one includes<br />
low-Ti basalts, high-Ti picrites, High-Mg basalts, Fe-picrites,<br />
whereas the upper one is characteriz<strong>ed</strong> by the occurrence of Fe-<br />
Ti basalts.<br />
The greenschist facies unit underwent a polyphase<br />
deformation history under retrograde P and T con<strong>di</strong>tions<br />
analogue to that recogniz<strong>ed</strong> in the amphibolite facies rocks.<br />
The first deformation phase (D1) produc<strong>ed</strong> a continuous<br />
foliation (S1) well develop<strong>ed</strong> only in the fine-grain<strong>ed</strong><br />
lithologies. A mineralogical lineation (L1), mark<strong>ed</strong> by align<strong>ed</strong><br />
amphibole and plagioclase, is well develop<strong>ed</strong> on the S1and<br />
shows a NW/SE trend. In these lithologies the S1 is associat<strong>ed</strong><br />
to intrafoliar isoclinal folds characteriz<strong>ed</strong> by thicken<strong>ed</strong> hinges<br />
and bou<strong>di</strong>nag<strong>ed</strong> limbs (F1).<br />
The second deformation phase (D2) is mainly represent<strong>ed</strong><br />
by tight to isoclinal folds (F2) with generally round<strong>ed</strong> or<br />
subround<strong>ed</strong> hinges showing a NE-SW trend. These folds are<br />
cylindrical with an axial plane foliation (S2); therefore, the<br />
main surface detect<strong>ed</strong> in the field is a S1/S2 composite<br />
foliation. The asymmetry of the F2 suggests a NE vergence.<br />
During the D2, large cataclastic shear zones characteriz<strong>ed</strong> by S-<br />
C fabric develop<strong>ed</strong> parallel to the S2. The third and fourth<br />
deformation phases (D3 and D4) show features and trends<br />
analogue to those recogniz<strong>ed</strong> in the amphibolite facies rocks,<br />
without significative <strong>di</strong>fferences .<br />
In thin section, the S1 in the fine-grain<strong>ed</strong> metabasites is<br />
fine-grain<strong>ed</strong> and characteriz<strong>ed</strong> by the assemblage albite + Caamphibole<br />
(actinolite) + chlorite + epidote ± quartz ± white<br />
TECTONO-METAMORPHIC HISTORY OF THE DUARTE TERRANE<br />
129<br />
mica. From the D1 mineral assemblage chlorite, epidote and<br />
Ca-amphibole have been analyz<strong>ed</strong> by electron microprobe. The<br />
pistacite component in the epidote of the greenschist facies unit<br />
is higher (0.31-0.32) than in the amphibolite facies unit. For<br />
Ca-amphiboles the structural formulae were calculat<strong>ed</strong><br />
assuming 23 oxygens, and the classification of Leake et al.<br />
(1997) was adopt<strong>ed</strong>. The analyz<strong>ed</strong> amphiboles do not show any<br />
compositional zoning and they are classifiable as actinolite,<br />
with a Mg/(Mg + Fe2+) ratio ranging from 0.77 to 0.82. The<br />
stability field of actinolite suggests a temperature range<br />
between 350 and 480°C and a pressure ranging from 0.20 to<br />
0.60 GPa (Okamoto and Toriumi, 2005), coherent with<br />
greenschist facies con<strong>di</strong>tions.<br />
In thin section, the S2 can be classifi<strong>ed</strong> as a well develop<strong>ed</strong><br />
crenulation cleavage characteriz<strong>ed</strong> by zonal to continuous<br />
cleavage domains that envelop<strong>ed</strong> microlithons where the S1 is<br />
still preserv<strong>ed</strong>, even if fold<strong>ed</strong>. The cleavage domains are<br />
general parallel with <strong>di</strong>screte transition to microlithons. No<br />
recrystallization has been found along the S2. The S3 consists<br />
of well develop<strong>ed</strong> <strong>di</strong>sjunctive cleavage.<br />
AGE OF THE DEFORMATION PHASES<br />
Several ra<strong>di</strong>ometric analyses have been perform<strong>ed</strong> in order<br />
to assess the age of the rocks from the Duarte Terrane. The<br />
most reliable data on the foliat<strong>ed</strong> amphibolites are provid<strong>ed</strong> by<br />
Escuder Viruete et al. (2007b) by Ar/Ar dating on horneblendes<br />
recrystalliz<strong>ed</strong> during the D1. Two plateau ages of 93.9±1.4 and<br />
95.8±1.9 Ma in<strong>di</strong>cate a Cenomanian age for this phase.<br />
Accor<strong>di</strong>ngly, a pre-Cenomanian age for the protoliths of the<br />
amphibolite facies rocks from Duarte Terrane can be inferr<strong>ed</strong>.<br />
This age is also confirm<strong>ed</strong> with the Ar/Ar dating of 84±6 Ma of<br />
the D1-relat<strong>ed</strong> muscovites (Hernaiz Huerta et al., 2000) and 86<br />
± 4 Ma on amphiboles (Lapierre et al., 1999). In ad<strong>di</strong>tion, other<br />
useful evidences for the age of the deformations in the Duarte<br />
Terrane are provid<strong>ed</strong> by the Loma de Cabrera intrusive<br />
complex that postdates the D1 and D2 structures. Ar/Ar dating<br />
of the rocks belonging to this complex, points out to ages<br />
spanning from 74.0±1.7 to 88.9±2.6 Ma (Escuder Viruete et al.,<br />
2007b). Whereas the ages of the D1 and D2 can be constrain<strong>ed</strong><br />
between the Cenomanian and the Turonian, the ages of the D3<br />
and D4, that develop<strong>ed</strong> at very low structural levels are most<br />
problematic to assess. Probably, the D3 and D4 can be relat<strong>ed</strong><br />
to strike-slip tectonics develop<strong>ed</strong> in the Early Oligocene to<br />
Early Miocene.<br />
On the whole, the D1 and D2 can be relat<strong>ed</strong> to the<br />
lowermost Upper Cretaceous tectonics recogniz<strong>ed</strong> in the whole<br />
Hispaniola Island (e.g. Mann et al., 1991), that develop<strong>ed</strong><br />
before the emplacement of Loma de Cabrera intrusive complex.<br />
Differently, the younger D3 and D4 can be relat<strong>ed</strong> to Tertiary<br />
transpressive tectonics.
130 M. MARRONI ET ALII<br />
CONCLUSIONS<br />
The collect<strong>ed</strong> data in<strong>di</strong>cate that the Duarte Terrane in the<br />
Jarabacoa area consists of two units, with <strong>di</strong>fferent<br />
metamorphic facies: the lower one is characteriz<strong>ed</strong> by<br />
greenschist facies whereas the upper one by amphibolite facies.<br />
The succession of these units is characteriz<strong>ed</strong> by metabasic<br />
rocks, that in the upper unit are topp<strong>ed</strong> by metas<strong>ed</strong>imentary<br />
rocks. The metabasites, probably of Early Cretaceous age, are<br />
representative of the Caribbean oceanic plateau volcanic rocks,<br />
whereas the metavolcanoclastic rocks are relat<strong>ed</strong> to the first<br />
stage arc growth, accor<strong>di</strong>ng to the reconstruction propos<strong>ed</strong> by<br />
Mann et al. (1991) and Lewis et al. (2002). To the first stage of<br />
arc growth can be referr<strong>ed</strong> also the foliat<strong>ed</strong> tonalities<br />
recogniz<strong>ed</strong> as bo<strong>di</strong>es intrud<strong>ed</strong> into the amphibolite facies unit<br />
of the Duarte Terrane. The age of these magmatic rocks is<br />
lowermost Aptian.<br />
The deformation history recogniz<strong>ed</strong> in both units consists of<br />
four phases, from D1 to D4. The D1 is characteriz<strong>ed</strong> by a well<br />
develop<strong>ed</strong> foliation and mineral lineation develop<strong>ed</strong> under<br />
metamorphism ranging from amphibolite to greenschist facies.<br />
The D2 is, in turn, responsible for the exhumation of both the<br />
units, probably during a compressive event. The D1 and D2<br />
develop<strong>ed</strong> in the Cenomanian-Turonian, just before the<br />
magmatism relat<strong>ed</strong> to the second stage arc of growth, that can<br />
be referr<strong>ed</strong> to a Campanian-Maastritchian time span. The<br />
following Upper Oligocene to Upper Miocene transpressive<br />
tectonics is responsible for the development of the terranes<br />
recogniz<strong>ed</strong> in the Hispaniola Island. In the Duarte Terrane, the<br />
D3 and D4, that act<strong>ed</strong> at very shallow structural levels, can be<br />
referr<strong>ed</strong> to transpressive tectonics. On the whole, the<br />
deformation history of the Duarte Terrane occurr<strong>ed</strong> in the arc<br />
setting during oblique subduction lea<strong>di</strong>ng to alternation through<br />
time of compressive and transpressive tectonics, responsible for<br />
the Hispaniola present-day structural setting.<br />
REFERENCES<br />
DONNELLY T.W., BEETS D., CARR M.J., JACKSON T., KLAVER<br />
G., LEWIS J., MAURY R., SCHELLENKENS H., SMITH A.L.,<br />
WADGE G. & WESTERCAMP D. (1990) - History and<br />
tectonic setting of Caribbean magmatism. In: G. Dengo &<br />
J.E. Case (Eds.), The geology of North America, vol. H, The<br />
Caribbean Region. Geol. Soc. Am., 339-374.<br />
ESCUDER VIRUETE J., HERNAIZ HUERTA P.P., DRAPER G.,<br />
GUTIERREZ G., LEWIS J.F. & PEREZ-ESTAUN A. (2002) -<br />
Metamorfismo y estructura de la Formacion Maimon y los<br />
Complejos Duarte y Rio Verde, Cor<strong>di</strong>llera Central<br />
Dominicana: implicaciones en la estructura y la evolucion<br />
<strong>del</strong> primitivo Arco Isla caribeno. Acta Geol. Hisp., 37, 123-<br />
162.<br />
ESCUDER VIRUETE J., CONTRERAS F., STEIN G., URIEN P.,<br />
JOUBERT M., PEREZ-ESTAUN A., FRIEDMAN R. & ULLRICH<br />
T.D. (2007) - Magmatic relationships and ages between<br />
adakites, magnesian andesites and Nb-enrich<strong>ed</strong> basaltandesites<br />
from Hispaniola: record of a major change in the<br />
Caribbean island arc magma sources. Lithos,<br />
doi:10.1016/j.lithos.2007.01.008.<br />
HERNAIZ HUERTA P.P., LEWIS J.F., ESCUDER VIRUETE J.<br />
GUTERRIEZ G., MORTENSEN J., HAMES W., SOLE J.,<br />
MARTINEZ A. & DRAPER G. (2000) - Memoria explicativa<br />
<strong>del</strong> Mapa geologico a escala 1.50.000 de Villa Altagracia<br />
(6172-II) and Arroyo Cana (6172-III). Proyecto de<br />
Cartografia geotematica de la Republica Dominicana.<br />
Direccion general de Mineria, Santo Domingo.<br />
LAPIERRE H., DUPUIS V., DE LEPINAY B.M., BOSCH D., MONIE<br />
P., TARDY M., MAURY R.C., HERNANDEZ J., POLVE M.,<br />
YEGHICHEYAN D. & COTTON J. (1999) - Late Jurassic<br />
oceanic crust and Upper Cretaceous Caribbean plateau<br />
picritic basalts expos<strong>ed</strong> in the Duarte igneous complex,<br />
Hispaniola. J. Geol., 107, 193-207<br />
LEAKE B.E. , WOLLEY A.R., ARPS C.E.S., BIRCH W.D.,<br />
GILBERT M.C., GRICE J.D., HAWTHORNE F.C., KATO A.,<br />
KISH H., KRIVOVICHEV V.G., LINTHOUT K., LAIRD J.,<br />
MANDARINO J.A., MARESCH W.V., NICKEL E.H., ROCK<br />
N.M.S., SCHUMACHER J.C., STEPHENSON N.C.N.,<br />
UNGARETTI L., WHITTAKER E.J.W. & YOUZHI G. (1997) -<br />
Nomenclature of amphiboles: Report of subcommittee on<br />
amphiboles of the International Mineralogical Association,<br />
commission on new minerals and mineral names. Can.<br />
Mineral., 35, 219-46.<br />
LEWIS J.F. & JIMENEZ J.G. (1991) - Duarte Complex in the La<br />
Vega-Jarabacoa-Janico Area, central Espanola:<br />
Geological and geochemical features of the sea floor<br />
during the early stages of arc evolution. In: P. Mann, G.<br />
Draper & J.F. Lewis (Eds.), Geologic and tectonic<br />
development of the North America-Caribbean Plate<br />
boundary in Hispaniola. Spec. Paper, Geol. Soc. Am., 262,<br />
115-142.<br />
LEWIS J.F., ESCUDER VIRUETE J., HERNAIZ HUERTA P.P.,<br />
GUTIERREZ G., DRAPER G. & PEREZ-ESTAUN A. (2002) -<br />
Sub<strong>di</strong>vision geoquimica <strong>del</strong> Arco Isla Circum-Caribeno,<br />
Cor<strong>di</strong>llera Central Dominicana: implicaciones para la<br />
formacion, acrecion y crecimiento cortical en un ambiente<br />
intraoceanico. Acta Geol. Hisp. 37, 81-122.<br />
MANN P., DRAPER G. & LEWIS J.F. (1991) - An overview of the<br />
geologic and tectonic development of Espanola. In: P.<br />
Mann, G. Draper & J.F. Lewis (Eds.), Geologic and<br />
tectonic development of the North America-Caribbean plate<br />
boundary in Espanola. Spec. Paper., Geol. Soc. Am., 262,<br />
1-28.<br />
OKAMOTO A. & TORIUMI M. (2005) - Progress of actinoliteforming<br />
reactions in mafic schists during retrograde<br />
metamorphism: an example from the Sanbagawa<br />
metamorphic belt in central Shikoku, Japan. J. Metam.<br />
Geol., 23, 335-356.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 131-132, 2 ff.<br />
On fault misorientation in exhum<strong>ed</strong> metamorphic complexes: sliptendency<br />
analysis of faults in the Alpine orogenic w<strong>ed</strong>ge<br />
MATTEO MASSIRONI (*), LUCA MENEGON (*) & ANDREA BISTACCHI (**)<br />
RIASSUNTO<br />
Sulla sfavorevole orientazione <strong>di</strong> faglie in complessi metamorfici esumati:<br />
analisi <strong>di</strong> slip-tendency su faglie <strong>del</strong> prisma orogenico Alpino<br />
E’ noto che faglie a notevole componente <strong>di</strong> rigetto verticale <strong>ed</strong> attive per<br />
lunghi perio<strong>di</strong> sono caratterizzate da zone <strong>di</strong> faglia asimmetriche. All’interno<br />
<strong>del</strong> prisma orogenico alpino, le faglie normali a basso angolo responsabili <strong>di</strong><br />
importanti esumazioni quali la linea <strong>del</strong> Brennero e la linea <strong>del</strong> Sempione sono<br />
caratterizzate da ampi spessori milonitici a letto a cui si contrappongono<br />
orizzonti cataclastici a tetto. Per investigare il controllo esercitato dal fabric<br />
er<strong>ed</strong>itato sulla enucleazione <strong>del</strong>la deformazione fragile, si è fatto ricorso<br />
all’analisi <strong>di</strong> tendenza allo slip considerando una <strong>di</strong>stribuzione anisotropa dei<br />
coefficienti <strong>di</strong> frizione. Tale analisi effettuata per le zone <strong>di</strong> faglia <strong>del</strong> Brennero<br />
e <strong>del</strong> Sempione ha <strong>di</strong>mostrato che l’attività lungo orizzonti <strong>di</strong> deformazione<br />
fragile sfavorevolmente orientati rispetto al campo <strong>di</strong> sforzi regionali è una<br />
con<strong>di</strong>zione favorita nel caso <strong>di</strong> faglie che hanno controllato importanti processi<br />
<strong>di</strong> unroofing tettonico.<br />
La stessa analisi è stata applicata anche alla zona <strong>di</strong> faglia <strong>di</strong><br />
Sprechenstein-Val <strong>di</strong> Mules, un segmento <strong>del</strong> sistema Periadriatico che non<br />
mostra alcuna significativa componente <strong>di</strong> rigetto verticale. Anche questa<br />
faglia è caratterizzata da un importante fabric filllonitico orientato<br />
sfavorevolmente per una potenziale riattivazione in con<strong>di</strong>zioni fragili, ma che<br />
tuttavia mostra <strong>di</strong> aver controllato l’enucleazione e lo sviluppo <strong>di</strong> orizzonti<br />
cataclastici. L’analisi <strong>di</strong> slip tendency anisotropa applicata a questo caso<br />
consente <strong>di</strong> generalizzare l’affermazione prec<strong>ed</strong>ente, permettendo <strong>di</strong><br />
concludere che all’interno <strong>di</strong> complessi metamorfici frutto <strong>di</strong> importanti<br />
eventi esumativi, lo sviluppo <strong>di</strong> orizzonti <strong>di</strong> deformazione fragile a<br />
orientazione non andersoniana è spesso favorito in<strong>di</strong>pendentemente dal tipo <strong>di</strong><br />
attività <strong>del</strong>le faglie stesse.<br />
Key words: weak faults, slip tendency, inherit<strong>ed</strong> fabric,<br />
metamorphic complexes, misorientation<br />
Crustal-scale fault zones, which show a strong <strong>di</strong>p-slip<br />
component (either normal or reverse) and have been active for<br />
relevant times (e.g. several million years), are very often<br />
characteris<strong>ed</strong> by an asymmetric <strong>di</strong>stribution of fault rocks, with<br />
rocks in the footwall or hangingwall (for normal or reverse<br />
faults respectively) showing a transition from relatively higher<br />
temperature crystal-plastic deformation mechanisms to low<br />
temperature brittle-cataclastic mechanisms. This is the result of<br />
progressive exhumation during a deformation continuum and<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Geoscienze, Università <strong>di</strong> Padova, Italy<br />
(**) Dipartimento <strong>di</strong> Scienze Geologiche e Geotecnologie,<br />
Università <strong>di</strong> Milano Bicocca<br />
Lavoro eseguito nell’ambito <strong>del</strong> progetto Cariparo 2006, Università degli<br />
Stu<strong>di</strong> <strong>di</strong> Padova<br />
may be pr<strong>ed</strong>ict<strong>ed</strong> with the classic Sibson-Scholz fault zone<br />
mo<strong>del</strong>. This asymmetric <strong>di</strong>stribution of fault rocks is well<br />
known in exhum<strong>ed</strong> fault zones from the metamorphic core of<br />
Fig. 1 –Isotropic and anisotropic slip tendency of the Simplon low angle<br />
detachment (SD). The anisotropic slip tendency demonstrates how the<br />
low friction coefficient along the inherit<strong>ed</strong> mylonitic foliation (Mariani et<br />
al. 2006) favour<strong>ed</strong> misorient<strong>ed</strong> brittle faulting.<br />
the Alps (Austroalpine and Penninic domains) such as the<br />
extensional Simplon and Brenner detachments that are both<br />
associat<strong>ed</strong> to thick mylonitc zones expos<strong>ed</strong> within their<br />
footwall (up to 1 km) (Mancktelow, 1985,1992; Behrmann,<br />
1988; Selverstone, 1988).<br />
To investigate the control exert<strong>ed</strong> by pre-existing ductile<br />
foliations on brittle faulting along the Simplon and Brenner<br />
detachments, we have appli<strong>ed</strong> a development of slip tendency<br />
analysis (Lisle & Srivastava, 2004) that includes the effect of<br />
anisotropy (FIG. 1). It shows that, given the mechanical<br />
anisotropy and under a realistic palaeo-state of stress,<br />
continuing activity along a misorient<strong>ed</strong> and weak fault zone is<br />
the most natural behaviour for major faults controlling tectonic<br />
unroofing.<br />
The same analysis has been appli<strong>ed</strong> to the Sprechenstein-<br />
Mules fault zone (SMF), which is part of the eastern segment of<br />
the 700-km-long Periadriatic Fault System and is not<br />
characteriz<strong>ed</strong> by a significant <strong>di</strong>p-slip component (FIG. 2). Due<br />
to previous exhumation along the Pusteria Fault (PF),<br />
greenschist facies phyllonites constitute the ductile precursor of<br />
the Sprechenstein-Mules brittle fault and are characteris<strong>ed</strong> by a<br />
pervasive SCC’ composite foliation, mark<strong>ed</strong> by alternating<br />
phyllosilicate- and quartz-feldspar-rich layers. Centimetre- to<br />
micrometre-scale cataclastic shear zones develop along S, C<br />
and C’ inherit<strong>ed</strong> surfaces. Hence, the Sprechenstein-Mules fault
132 MASSIRONI ET ALII<br />
Fig. 2 – Sprechenstein-Mules fault zone (SMF): geometric 3D mo<strong>del</strong> , regional stress field and slip vector , anisotropic slip tendency. The anisotropic slip<br />
tendency analysis of the SMF demonstrates that in metamorphic complexes misorient<strong>ed</strong> brittle faulting is favour<strong>ed</strong> even for transcurrent faults.<br />
zone is characteris<strong>ed</strong> by a strong mechanical pre-existing<br />
anisotropy, which controls the mode of deformation under<br />
brittle con<strong>di</strong>tions. Given its origin in the plastic-metamorphic<br />
environment, this anisotropy is strongly misorient<strong>ed</strong> for<br />
reactivation under brittle con<strong>di</strong>tions.<br />
Anisotropic slip tendency analysis shows that the<br />
reactivation of this misorient<strong>ed</strong> inherit<strong>ed</strong> fabric is favour<strong>ed</strong> with<br />
respect to the development of “new” Andersonian faults and<br />
that this process leads to a mark<strong>ed</strong> mechanical weakness of the<br />
SMF (FIG. 2). This demonstrates that the presence of inherit<strong>ed</strong><br />
ductile foliations may control the style of brittle faulting,<br />
independently of the <strong>di</strong>fferential uplift across an in<strong>di</strong>vidual<br />
fault. Under certain con<strong>di</strong>tions, typically met in metamorphic<br />
terrains, this effect can be relevant even for faults that do not<br />
accommodate significant parts of the regional exhumation<br />
(transcurrent faults).<br />
In conclusion, in exhum<strong>ed</strong> metamorphic complexes the<br />
continuing activity along misorient<strong>ed</strong> and “weak” fault zones,<br />
with brittle re-activation of inherit<strong>ed</strong> fabrics, must be<br />
consider<strong>ed</strong> not only possible, but also more probable than the<br />
development of Andersonian “strong” faults.<br />
REFERENCES<br />
BEHRMANN, J.H., (1988) - Crustal scale extension in a<br />
convergent orogen: the Sterzing-Steinach mylonite zone in<br />
the Eastern Alps. Geodynamica Acta, 2, 63–73.<br />
SRIVASTAVA D.C. & LISLE R.J. (2004) - Test of the frictional<br />
reactivation theory for faults and vali<strong>di</strong>ty of fault-slip<br />
analysis. Geology, 32, 7, 569-572.<br />
MANCKTELOW, N. S. (1985). The Simplon Line: a major<br />
<strong>di</strong>splacement zone in the western Lepontine Alps. Eclogae<br />
geologicae Helvetiae., 78/1, 73-96.<br />
MANCKTELOW, N.S. (1992) - Neogene lateral extension during<br />
convergence in the central Alps: Evidence from interrelat<strong>ed</strong><br />
faulting and backfol<strong>di</strong>ng around Simplonpass<br />
(Switzerland). Tectonophysics, 215, 295-317.<br />
MARIANI E., BRODIE K.H. & RUTTER E.H. (2006).<br />
Experimental deformation of muscovite shear zones at high<br />
temperatures under hydrothermal con<strong>di</strong>tions and the<br />
strength of phyllosilicate-bearing faults in nature. Journal<br />
of Structural Geology, 28,1569 -1587.<br />
SELVERSTONE, J. (1988) - Evidence for east–west crustal<br />
extension in the Eastern Alps: implications for the<br />
unroofing history of the Tauern window. Tectonics, 7, 87–<br />
105.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 133-134, 3 ff.<br />
Reticolo locale <strong>di</strong> zone <strong>di</strong> taglio e relazioni con la deformazione regionale<br />
nel metagranito <strong>del</strong> Gran Para<strong>di</strong>so (Alpi Nord-Occidentali)<br />
La falda Penni<strong>di</strong>ca <strong>del</strong> Gran Para<strong>di</strong>so (Alpi Nord-Occidentali) è<br />
prevalentemente costituita da ortogneiss occhia<strong>di</strong>ni che derivano dall’intensa<br />
rielaborazione tettono-metamorfica Alpina <strong>di</strong> originari granitoi<strong>di</strong> Permiani. La<br />
foliazione regionale si è sviluppata in con<strong>di</strong>zione <strong>di</strong> facies anfibolitica <strong>di</strong> basso<br />
grado <strong>ed</strong> è associata ad un senso <strong>di</strong> trasporto tettonico top-to-W durante il<br />
sovrascorrimento <strong>del</strong>le Ofioliti Piemontesi sul basamento continentale <strong>del</strong><br />
Gran Para<strong>di</strong>so. Il protolite intrusivo è ben preservato nel dominio chilometrico<br />
<strong>di</strong> bassa deformazione nella Valle <strong>di</strong> Piantonetto e consiste in graniti porfirici<br />
contententi frequenti eterogeneità strutturali e composizionali (i.e., giunti,<br />
filoni leucocratici, schlieren). In tale dominio, la deformazione Alpina è<br />
localizzata in zone <strong>di</strong> taglio <strong>di</strong>screte all’interno <strong>di</strong> metagraniti massicci o<br />
debolmente foliati. Le zone <strong>di</strong> taglio immergono prevalentemente a S-SE, da<br />
m<strong>ed</strong>io-basso angolo (zone <strong>di</strong> taglio1) ad alto angolo (zone <strong>di</strong> taglio2). Le zone<br />
<strong>di</strong> taglio milonitico mostrano tipiche caratteristiche geometriche che vengono<br />
interpretate come l’evidenza <strong>di</strong> enucleazione lungo precursori fragili e<br />
composizionali. Relazioni <strong>di</strong> offset reciproco tra zone <strong>di</strong> taglio1 e zone <strong>di</strong><br />
taglio2 in<strong>di</strong>cano un’attività coeva dei due sets. Le analisi <strong>di</strong> paleostress e <strong>di</strong><br />
geometria <strong>del</strong>lo strain <strong>di</strong>mostrano che sia la foliazione nel metagranito<br />
incassante che le stesse zone <strong>di</strong> taglio si sono formate in presenza <strong>di</strong> una forte<br />
componente <strong>di</strong> flattening, con un σ1 subverticale. La cinematica <strong>del</strong>le singole<br />
zone <strong>di</strong> taglio <strong>di</strong>pende dall’orientazione degli originari precursori e dalla<br />
ripartizione <strong>del</strong>le componenti coassiale e non coassiale <strong>del</strong>la deformazione alla<br />
scala chilometrica, con la localizzazione <strong>del</strong>la componente <strong>di</strong> flattening<br />
all’interno <strong>del</strong> dominio <strong>di</strong> Piantonetto. Di conseguenza, la geometria <strong>del</strong>lo<br />
strain e la cinematica <strong>del</strong>le singole zone <strong>di</strong> taglio in tale dominio non appaiono<br />
<strong>di</strong>rettamente riconducibili al senso <strong>di</strong> trasporto tettonico top-to-W solitamente<br />
evidente all’interno <strong>del</strong>la falda <strong>del</strong> Gran Para<strong>di</strong>so. Tuttavia, l’ellissoide <strong>del</strong>lo<br />
stress associato al reticolo <strong>di</strong> zone <strong>di</strong> taglio incipienti all’interno dei<br />
metagraniti massicci è orientato coerentemente con il senso <strong>di</strong> trasporto<br />
tettonico registrato alla scala regionale nel massiccio <strong>del</strong> Gran Para<strong>di</strong>so. In<br />
conclusione, il reticolo <strong>di</strong> zone <strong>di</strong> taglio nella Valle <strong>di</strong> Piantonetto è da<br />
considerarsi rappresentativo <strong>del</strong>le fasi incipienti <strong>del</strong>la deformazione duttile <strong>di</strong><br />
una falda in granitoi<strong>di</strong>. Nonostante l’architettura <strong>di</strong> tale reticolo sia<br />
essenzialmente determinata dalla <strong>di</strong>sponibilità e dall’orientazione <strong>di</strong><br />
eterogeneità strutturali e composizionali preesistenti, alcuni aspetti <strong>del</strong>la<br />
deformazione alla scala regionale sono chiaramente registrati <strong>ed</strong> estrapolabili<br />
dalla sua geometria.<br />
Key words: deformation of granites, flattening, Gran Para<strong>di</strong>so,<br />
nucleation of shear zones, palaeostress analysis, strain<br />
analysis, Western Alps.<br />
_________________________<br />
Local shear zone pattern and bulk deformation in the Gran<br />
Para<strong>di</strong>so metagranite (NW Italian Alps)<br />
(*) Dipartimento <strong>di</strong> Geoscienze, Università degli Stu<strong>di</strong> <strong>di</strong> Padova. Via<br />
Giotto 1, 35137 Padova. Tel. 0498271863, E-mail: luca.menegon@unipd.it<br />
LUCA MENEGON (*) & GIORGIO PENNACCHIONI (*)<br />
The Gran Para<strong>di</strong>so nappe of the north-western Alps mostly<br />
consists of augen gneisses deriv<strong>ed</strong> from the Alpine deformation<br />
of Permian granitoids. The regional foliation of the augen<br />
gneisses develop<strong>ed</strong> at lower amphibolite facies con<strong>di</strong>tions and<br />
is associat<strong>ed</strong> with a top-to-west sense of shear (LE BAYON &<br />
BALLEVRE, 2006). The granitoid protolith is preserv<strong>ed</strong> in the<br />
kilometre-scale low-strain domain of the Piantonetto Valley<br />
Fig. 1 – Extremely localiz<strong>ed</strong> shear zone nucleating on a former joint. Pen<br />
(13.5 cm long) for scale.<br />
and mainly consists of a porphyritic metagranite inclu<strong>di</strong>ng<br />
joints, leucochratic dykes and biotite-rich schlieren. In this lowstrain<br />
domain, the Alpine deformation is mainly localiz<strong>ed</strong> in<br />
<strong>di</strong>screte ductile shear zones within weakly foliat<strong>ed</strong> metagranite<br />
(MENEGON et alii, 2006). The shear zones are mostly <strong>di</strong>pping<br />
towards S-SE from shallowly (shear zones1) to steeply (shear<br />
zones2). The shear zones show typical features that can be<br />
explain<strong>ed</strong> by reactivation of pre-existing joints and planar<br />
compositional heterogeneities (e.g., PENNACCHIONI &<br />
MANCKTELOW, 2007) (Figs. 1, 2).<br />
Mutual crosscutting relationships in<strong>di</strong>cates that shear zone<br />
sets develop<strong>ed</strong> at the same time (Fig. 2). Palaeostress and strain<br />
analyses in<strong>di</strong>cate that shear zones and the metagranite foliation<br />
both form<strong>ed</strong> in presence of a strong component of flattening<br />
with a subvertical σ1 (Fig. 3). The kinematics of in<strong>di</strong>vidual<br />
shear zones depends on the orientation of the original<br />
heterogeneities (acting as nucleation planes) and by partitioning<br />
of strain components at the kilometer-scale with concentration
134 LUCA MENEGON ET ALII<br />
Fig. 2 – (a) Surface map of a representative outcrop of shear zones1 (1a-1g) and shear zones2 (2a-2c). (b) Lower hemisphere, equal area projection of the shear<br />
zone orientation in this outcrop. (c) Strongly localiz<strong>ed</strong> shear zone2. Coin (2 cm in <strong>di</strong>ameter) for scale. (d) Mutual crosscutting relationship between shear zones<br />
1g and 2b. 1g is <strong>di</strong>splac<strong>ed</strong> left-laterally by 2b that is in turn is <strong>di</strong>splac<strong>ed</strong> right-laterally by 1g. Coin (2 cm in <strong>di</strong>ameter) for scale.<br />
of the flattening component to the Piantonetto low-strain<br />
domain. The strain geometry and the kinematics of in<strong>di</strong>vidual<br />
shear zones within Piantonetto are thus not <strong>di</strong>rectly linkable to<br />
the top-to-west sense of tectonic transport observ<strong>ed</strong> elsewhere<br />
in the Gran Para<strong>di</strong>so nappe. However, the bulk stress ellipsoid<br />
reconstruct<strong>ed</strong> for incipient shear zone network within very<br />
weakly deform<strong>ed</strong> granites is orient<strong>ed</strong> consistently with the bulk<br />
<strong>di</strong>rection of tectonic transport within the Gran Para<strong>di</strong>so massif.<br />
We conclude that the shear zones network of Piantonetto<br />
Valley is representative of the incipient stages of ductile<br />
deformation of a granite nappe. Even if its architecture is<br />
determin<strong>ed</strong> by the arrangement of pre-existing structural and<br />
Fig. 3– Palaeostress analysis on the basis of 69 incipient shear zones<br />
within massive metagranites. (a) Results from the <strong>di</strong>rect inversion method<br />
(ANGELIER, 1990). (b) Results from the pt-axes method. R% is a measure<br />
of clustering.<br />
compositional heterogeneities, aspects of the large-scale bulk<br />
strain can be deriv<strong>ed</strong> from this local shear zone pattern.<br />
REFERENCES<br />
ANGELIER J. (1990) - Inversion of field data in fault tectonics<br />
to obtain the regional stress-III. A new rapid <strong>di</strong>rect<br />
inversion method by analytical means. Geophysical Journal<br />
International, 103, 363-376.<br />
LE BAYON B. & BALLÈVRE M. (2006) - Deformation history of<br />
a subduct<strong>ed</strong> continental crust (Gran Para<strong>di</strong>so, Western<br />
Alps): continuing crustal shortening during exhumation.<br />
Journal of Structural Geology, 28, 793-815.<br />
MENEGON L., PENNACCHIONI G. & STÜNITZ H. (2006) -<br />
Nucleation and growth of myrmekite during ductile shear<br />
deformation in metagranites. Journal of Metamorphic<br />
Geology, 24, 553-568.<br />
PENNACCHIONI G. & MANCKTELOW N.S. (2007) - Nucleation<br />
and initial growth of a shear zone network within<br />
compositionally and structurally heterogeneous granitoids<br />
under amphibolite facies con<strong>di</strong>tions. Journal of Structural<br />
Geology, 29, 1757-1780.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 135-136<br />
Structure and evolution of the Calabrian accretionary w<strong>ed</strong>ge<br />
(Southern Italy)<br />
RIASSUNTO<br />
Struttura <strong>ed</strong> evoluzione <strong>del</strong> prisma <strong>di</strong> accrezione calabro (Italia<br />
meri<strong>di</strong>onale)<br />
L’arco calabro rappresenta un’area chiave per la comprensione<br />
<strong>del</strong>l’evoluzione tettonica <strong>del</strong>la catena Appennino-Maghrebide nel più ampio<br />
contesto <strong>di</strong> convergenza tra le placche Europea <strong>ed</strong> Africana nel M<strong>ed</strong>iterraneo<br />
centrale. L’arco calabro è stato oggetto <strong>di</strong> <strong>di</strong>versi stu<strong>di</strong> a carattere geologico e<br />
geofisico a partire dalle prime campagne oceanografiche degli anni ’50.<br />
Nonostante l’enorme mole <strong>di</strong> dati collezionati, geometria, cinematica <strong>ed</strong><br />
evoluzione tettonica <strong>del</strong> prisma <strong>di</strong> accrezione calabro, soprattutto nella sua<br />
parte offshore, rimangono a tutt’oggi poco definiti e compresi.<br />
Analizzando dati derivanti dalle numerose campagne, accademiche e<br />
industriali, condotte nel Mar Ionio negli ultimi 50 anni, e attraverso la<br />
calibrazione con i dati <strong>di</strong> pozzo <strong>di</strong>sponbili lungo le coste calabresi, sono state<br />
descritte la geometria, la cinematica e l’evoluzione spazio-temporale <strong>del</strong><br />
prisma <strong>di</strong> accrezione, negli ultimi 15 Ma.<br />
Key words: accretionary w<strong>ed</strong>ge, Calabrian Arc, seisimic<br />
reflection profile.<br />
INTRODUZIONE<br />
The Calabrian subduction zone is a narrow but<br />
seismologically well defin<strong>ed</strong> slab plunging toward northwest in<br />
the Central M<strong>ed</strong>iterranean (PIROMALLO & MORELLI, 2003).<br />
The subduction of the Ionian basin under Eurasia, is mainly<br />
dominat<strong>ed</strong> by trench rollback producing the opening of the two<br />
large backarc basin (Liguro-Provencal and Tyrrhenian) and the<br />
formation of the Calabrian accretionary w<strong>ed</strong>ge.<br />
The Calabrian accretionary w<strong>ed</strong>ge is a partially submerg<strong>ed</strong><br />
south-verging accretionary prism that extends from south<br />
Calabria to the Ionian abyssal plain and laterally from the Malta<br />
Escarpment to the West and Apulia Escarpment and<br />
M<strong>ed</strong>iterranean Ridge to the East.<br />
Despite the Ionian basin has been investigat<strong>ed</strong> in the last<br />
forty years by several geological and geophysical survey, the<br />
_________________________<br />
LILIANA MINELLI (*), CLAUDIO FACCENNA (*) & PIERO CASERO (*)<br />
(*)Dipartimento <strong>di</strong> Scienze Geologiche, Universita Roma Tre, Largo S.L.<br />
Murialdo 1, 00146 Rome, Italy<br />
results are often controversy, and the structure of the w<strong>ed</strong>ge is<br />
still poorly defin<strong>ed</strong> (FINETTI & MORELLI, 1973; ROSSI &<br />
SARTORI, 1981; CERNOBORI et alii., 1986).<br />
We have collect<strong>ed</strong> all the available multichannel seismic<br />
reflection profiles, acquir<strong>ed</strong> up to 60’s, in the Ionian offshore<br />
by industrial exploration and academic cruises. The resolution<br />
and the data quality of the seismic profiles vary remarkably<br />
because the <strong>di</strong>fferent acquisition parameters and energy system<br />
us<strong>ed</strong> for the <strong>di</strong>fferent survey.<br />
The interpretation of these data was facilitat<strong>ed</strong> by<br />
integration of other geophysical and geological data (ESP, DE<br />
VOOGD et alii, 1992; ODP, site 374) and well stratigraphy<br />
dataset near the southern coasts of Calabria, in order to better<br />
constrain a more complete history of the subduction process.<br />
In the central sector of Ionian sea where no <strong>di</strong>rect ties were<br />
available for the interpretation, the overall sequences can be<br />
identifi<strong>ed</strong> on the basis of previous works and of their seismic<br />
character.<br />
The high density grid of the seismic lines allow us to map<br />
out the main structural feature and the recognition along several<br />
seismic profiles of the main reflection horizons; their<br />
geological attributes allow us to date the activity of the main<br />
thrust system and to reconstruct a detail<strong>ed</strong>, time-space,<br />
evolution of the accretionary prism. The stratigraphy feature,<br />
style of deformations and internal seismic character suggest the<br />
<strong>di</strong>vision of the Calabrian accretionary w<strong>ed</strong>ge, along a general<br />
NS transect, from the foreland basin to the backstop of the<br />
w<strong>ed</strong>ge, into four major structural zones:<br />
(1) forearc basin and crystalline back stop (2) pre-<br />
Messinian accretionary w<strong>ed</strong>ge (3) post-Messinian accretionary<br />
w<strong>ed</strong>ge (4) foreland basin.<br />
The accretionary history of the Calabrian prism is<br />
punctuat<strong>ed</strong> by <strong>di</strong>screte episodes of deformations evolving in<br />
both time and space.<br />
The growth of the prism includ<strong>ed</strong> both forward propagation<br />
stages characteriz<strong>ed</strong> by frontal accretion, and out-of-sequence<br />
internal thrusting. In ad<strong>di</strong>tion to the rheology of the prism, the<br />
prism growth was mostly controll<strong>ed</strong> by two main external<br />
factors: (1) the presence of a thick evaporitic deposit, (2) the<br />
dynamic of the subduction system.
136 MINETTI ET ALII<br />
REFERENCES<br />
CERNOBORI L., HIRN A., MCBRIDE J.H., NICOLICH R.,<br />
PETRONIO L., ROMANELLI M. & the Streamers-Profiles<br />
Working Group (1996) - Crustal image of the Ionian basin<br />
and its Calabrian margin. Tectonophysics, 264, 175-189.<br />
DE VOOGD B., TRUFFERT C., CHAMOT-ROOKE N., HUCHON P.,<br />
LALLEMANT S., & LE PICHON X. (1992) - Twoship deep<br />
seismic soun<strong>di</strong>ngs in the basins of the Eastern<br />
M<strong>ed</strong>iterranean Sea (Pasiphae cruise), Geophys. J. Int., 109,<br />
536–552.<br />
FACCENNA C., FUNICIELLO F., GIARDINI D. & P. LUCENTE<br />
(2001) - Episo<strong>di</strong>c back-arc extension during restrict<strong>ed</strong><br />
mantle convection in the Central M<strong>ed</strong>iterranean. Earth and<br />
Planetary Science Letters, 187, 105-116.<br />
FINETTI I. & MORELLI C. (1973) - Geophysical exploration of<br />
the M<strong>ed</strong>iterranean Sea. Boll. Geof. Teor. Appl., 15, 263–<br />
340.<br />
PIROMALLO C. & MORELLI A. (2003) - P wave tomography of<br />
the mantle under the Alpine M<strong>ed</strong>iterranean area. J.<br />
Geophys. Res., 108, 2065, doi:10.1029/2002JB001757.<br />
ROSSI S. & SARTORI R. (1981) - A seismic reflection study of<br />
the external Calabrian Arc in the northern Ionian Sea<br />
(eastern M<strong>ed</strong>iterranean). Mar. Geophys. Res., 4, 403 – 426.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 137-138<br />
ABSTRACT<br />
New data on the Variscan cycle in Calabria<br />
In Calabria (South Italy) the middle-late Ordovician extensional (or<br />
transtensional) tectonics play<strong>ed</strong> a crucial role during the Variscan cycle. They<br />
were accompani<strong>ed</strong> by effusion of submarine aci<strong>di</strong>c-to- basic magmatics and<br />
deposition of thick terrigenous flysch–like, trough-filling successions. The<br />
lacking of true Variscan ophiolites suggests a period of crustal thinning,<br />
culminating in the formation of abortive rift.<br />
The weakness in the crust was reactivat<strong>ed</strong> during the Variscan orogenesis,<br />
thus becoming site of intracontinental shear zone. Therefore, the Variscan<br />
chain develop<strong>ed</strong> in ensialic con<strong>di</strong>tions, involving continental crust only.<br />
The recent field data in<strong>di</strong>cate the chain form<strong>ed</strong> by two main tectonic<br />
phases.<br />
The first phase produc<strong>ed</strong> westward-verging, km-siz<strong>ed</strong> isoclinal recumbent<br />
folds, accompani<strong>ed</strong> by pervasive schistosity and syn-kinematic westward<br />
increasing (from anchizone to granulite facies) metamorphism.<br />
The second phase produc<strong>ed</strong> eastward verging thrusts and open folds.<br />
The Late Carboniferous-to-Permian was characteriz<strong>ed</strong> by intrusion of<br />
abundant granitoids and by effusion, in narrow fault-bound<strong>ed</strong> depressions, of<br />
subaerial aci<strong>di</strong>c volcanics, accompani<strong>ed</strong> by the deposition of volcanoclastics<br />
around the volcanic <strong>ed</strong>ifices.<br />
The aci<strong>di</strong>c-basic middle-late Ordovician magmatics in Calabria<br />
correspond to the “bimodal” magmatics well known in the Variscan Europe.<br />
They have been connect<strong>ed</strong> to partial melting of progressively deplet<strong>ed</strong> mantle,<br />
progressively upwelling at <strong>di</strong>fferent depths in intra-continental rifting.<br />
Key words: Calabria, ciclo varisico, magmatiti bimodali,<br />
tettonica ordoviciana.<br />
INTRODUZIONE<br />
Recenti, dettagliati rilevamenti in vaste aree <strong>del</strong>la Calabria<br />
meri<strong>di</strong>onale (Serre e Aspromonte) e nuovi dati ottenuti da DE<br />
GREGORIO et alii 2003 nei Monti Peloritani permettono <strong>di</strong><br />
ri<strong>di</strong>scutere le caratteristiche <strong>del</strong> ciclo varisico nell’Arco<br />
Calabro-Peloritano.<br />
Esso è riconoscibile se si sottraggono gli effetti <strong>del</strong>le<br />
successive tettogenesi alpina e appenninica.<br />
I toponimi usati derivano dalle nuove carte topografiche<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Ferrara<br />
Minzoni Nello, mnz@unife.it<br />
Nuovi dati sul ciclo varisico in Calabria<br />
NELLO MINZONI (*)<br />
<strong>del</strong>l’IGMI, scala 1:25000: Stilo, Taurianova, Gioiosa Ionica,<br />
Locri, Platì, San Luca, Bianco, Bova<br />
IL CICLO VARISICO<br />
Il ciclo varisico è iniziato nel Cambriano inferiore con la<br />
s<strong>ed</strong>imentazione <strong>di</strong> una successione epicontinentale<br />
essenzialmente terrigena, formatasi per erosione <strong>di</strong> un<br />
basamento pre-Cambriano, costituito da granitoi<strong>di</strong> e<br />
metamorfiti.<br />
Questa s<strong>ed</strong>imentazione è durata fino all’Ordoviciano<br />
inferiore.<br />
L’Ordoviciano m<strong>ed</strong>io-superiore è caratterizzato da<br />
improvvisa, forte tettonica <strong>di</strong>stensiva, accompagnata dalla<br />
s<strong>ed</strong>imentazione <strong>di</strong> una potente successione terrigena <strong>di</strong> tipo<br />
torbi<strong>di</strong>tico e dall’effusione <strong>di</strong> vulcaniti sottomarine, acide e<br />
basiche.<br />
I ben noti scisti neri e calcari caratterizzano il Siluriano e il<br />
Devoniano.<br />
Il Carbonifero è rappresentato da siltiti e lititi, ricche <strong>di</strong><br />
fossili vegetali.<br />
La catena varisica si è formata per l’attività <strong>di</strong> due fasi<br />
principali.<br />
La prima fase ha formato pieghe isoclinali <strong>di</strong> ogni<br />
<strong>di</strong>mensione, rovesciate verso i quadranti occidentali Esse sono<br />
accompagnate da scistosità pervasiva e metamorfismo<br />
sincinematico. Il metamorfismo cresce da Est verso Ovest,<br />
passando dall’Anchizona alla facies granulitica. Pieghe<br />
isoclinali <strong>di</strong> <strong>di</strong>mensioni chilometriche sono tuttora riconoscibili<br />
(nonostante l’intensa tettonica alpina e appenninica) nelle<br />
seguenti aree: a Ovest <strong>del</strong> paese <strong>di</strong> Platì; nel fianco destro <strong>del</strong>la<br />
fiumara Bonamico (a Ovest <strong>del</strong> paese <strong>di</strong> San Luca); nella<br />
fiumara Laverde-monte Orgaro -monte Jofri (a Ovest <strong>del</strong> paese<br />
<strong>di</strong> Samo). Trattandosi <strong>di</strong> aree orientali, il metamorfismo<br />
varisico è in gran parte nella facies degli scisti ver<strong>di</strong>.<br />
La seconda fase ha realizzato pieghe <strong>di</strong> <strong>di</strong>mensioni anche<br />
chilometriche, <strong>di</strong> <strong>di</strong>rezione assiale Nord 60-70 Est. Si tratta <strong>di</strong><br />
strutture aperte, prive per lo più <strong>di</strong> scistosità pervasiva e<br />
metamorfismo, che piegano la scistosità nata sub-orizzontale<br />
con la prima fase. Queste pieghe sono ben espresse: nella<br />
seguenti aree: a Nord <strong>del</strong>la città <strong>di</strong> Stilo; a Monte Bruverello (a<br />
NW <strong>del</strong> paese <strong>di</strong> Canolo); a NW <strong>del</strong> paese <strong>di</strong> Antonimina;<br />
nella fiumara Bonamico In queste aree, le pieghe sono “a<br />
ginocchio” e sono vergenti verso i quadranti orientali.
138 MINZONI<br />
Il ciclo varisico si è concluso con l’intrusione <strong>di</strong> abbondanti<br />
granitoi<strong>di</strong> e l’effusione, in aree limitate da faglie normali, <strong>di</strong><br />
vulcaniti: vulcanoclastiti si sono formate nell’intorno degli<br />
<strong>ed</strong>ifici vulcanici.<br />
DISCUSSIONE<br />
Le strutture <strong>di</strong> prima fase varisica possono essere messe in<br />
relazione con una subduzione attiva in aree occidentali, con<br />
immersione verso Est. La subduzione è stata accompagnata<br />
dalla formazione <strong>del</strong>le pieghe isoclinali metamorfiche Ovestvergenti.<br />
Verso Ovest si è accumulata una quantità <strong>di</strong> crosta<br />
sempre maggiore; coerentemente, verso Ovest cresce anche il<br />
metamorfismo. Le metamorfiti <strong>di</strong> maggior grado caratterizzano<br />
le aree centro- occidentali <strong>del</strong>la Calabria e le aree settentrionali<br />
dei Peloritani. Esse rappresentano la parte <strong>di</strong> crosta più<br />
profondamente subdotta. Si tratta <strong>del</strong>le netamorfiti <strong>del</strong>le attuali<br />
Unità alpine <strong>di</strong> Polia—Copanelllo e <strong>del</strong>la parte più occidentale<br />
<strong>del</strong>le Unità <strong>del</strong>l’Aspromonte e <strong>di</strong> Stilo in Calabria e <strong>del</strong>l’U.<br />
<strong>del</strong>l’Aspromonte nei Peloritani. Verso Est (in Calabria) e verso<br />
Sud (nei Peloritani) le metamorfiti sono <strong>di</strong> grado via via<br />
minore, fino all’anchizona.<br />
Le Unità tettoniche <strong>di</strong> Castagna e <strong>di</strong> Bagni, affioranti nella<br />
parte più occidentale <strong>del</strong>la Calabria settentrionale, cartterizzate<br />
d metmorfiti varisiche <strong>di</strong> m<strong>ed</strong>io-basso grado costituiscono un<br />
problema a parte (in elaborazione). Esse infatti sono<br />
accavallate con vergenza occidentale dalle metamorfiti <strong>di</strong> alto<br />
grado<br />
La mancanza <strong>di</strong> sicure ofioliti paleozoiche in<strong>di</strong>ca che la<br />
catena varisica si è formata in con<strong>di</strong>zioni interamente<br />
ensialiche, con coinvolgimento <strong>di</strong> sola crosta continentale.<br />
Le strutture <strong>del</strong>la seconda fase in Calabria potrebbero<br />
essere in relazione col sollevamento isostatico <strong>del</strong>la catena<br />
varisica, ispessita dalla prima fase; con ascesa <strong>del</strong>le rocce più<br />
profondamente subdotte e più metamorfiche. E’ poi seguito il<br />
collasso gravitativo, con intrusione dei granittoi<strong>di</strong> <strong>ed</strong> effusione,<br />
in ristretti grabens , <strong>del</strong>le vulcaniti tardo-paleozoiche.<br />
CONCLUSIONI<br />
Come nei Monti Peloritani (DE GREGORIO et alii, 2003)<br />
anche in Calabria la tettonica varisica è stata sottovalutata<br />
rispetto a quella alpina e/o appenninica. Le strutture sia <strong>del</strong>la<br />
prima che <strong>del</strong>la seconda fase sono sicuramente attribuibili alla<br />
tettogenesi varisica. I rapporti cronologici e spazio-temporali<br />
sono chiari; infatti, le pieghe <strong>di</strong> seconda fase deformano la<br />
scistosità <strong>di</strong> piano assiale <strong>del</strong>le pieghe <strong>di</strong> prima fase e sono<br />
intruse dai granitoi<strong>di</strong> tardo varisici o sigillate dalle vulcaniti<br />
tardo-paleozoiche.<br />
Secondo BONARDI et alii, 2008, il contatto tettonico <strong>di</strong><br />
<strong>di</strong>rezione Nord 70 Est fra l’U. <strong>di</strong> Polia–Copanello e l’ U. <strong>di</strong><br />
Stilo nella Calabria meri<strong>di</strong>onale separa due <strong>di</strong>versi terranes fra<br />
loro amalgamatisi durante la costruzione <strong>del</strong>la catena<br />
appenninica. Si può ipotizzare che l’U. <strong>di</strong> Polia–Copanello<br />
rappresenti la parte <strong>di</strong> crosta più profondamente subdotta dalla<br />
prima fase varisica. Essa si sarebbe poi stata messa in contatto<br />
tettonico sull’ U. Di Stilo con vergenza orientale, dalla<br />
seconda fase varisica. L’assetto strutturale attuale è opera <strong>del</strong>la<br />
tettogenesi alpina. Questa ipotesi è confortata dalla presenza,<br />
in aree più orientali, <strong>del</strong>le gran<strong>di</strong> pieghe a ginocchio; esse<br />
hanno la stessa <strong>di</strong>rezione <strong>del</strong> contatto fra i due supposti terranes<br />
e vergenza verso Est.<br />
Come in altri segmenti varisici europei e circumm<strong>ed</strong>iterranei,<br />
anche in Calabria l’Ordoviciano m<strong>ed</strong>io-superiore<br />
rappresenta un periodo cruciale nel quadro evolutivo <strong>del</strong> ciclo<br />
varisico. Infatti, il lento regime <strong>di</strong>stensivo, durato per tutto il<br />
Cambriano-Ordoviciano inferiore, con s<strong>ed</strong>imentazione ovunque<br />
<strong>di</strong> successioni epicontinentali, è drasticamente mutato durante<br />
l’Ordoviciano m<strong>ed</strong>io–superiore. Questo periodo <strong>di</strong> tempo,<br />
infatti, è caratterizzato da rapido sviluppo <strong>di</strong> forte tettonica<br />
<strong>di</strong>stensiva (o trastensiva), accompagnata da intenso<br />
magmatismo e deposizione <strong>di</strong> successioni torbi<strong>di</strong>tiche in bacini<br />
profon<strong>di</strong>. E’ possibile che le zone dove si sono messe in posto<br />
le magmatiti e dove si sono s<strong>ed</strong>imentate le successioni<br />
torbi<strong>di</strong>tiche, rappresentino le aree dove la <strong>di</strong>stensione<br />
ordoviciana si è effettuata col massimo <strong>di</strong> efficacia;<br />
determinando la formazione <strong>di</strong> zone a crosta assottigliata, in<br />
corrispondenza <strong>del</strong>le quali si sono poi realizzati i processi<br />
subduttivi, con sviluppo <strong>di</strong> una catena in con<strong>di</strong>zioni<br />
interamente ensialiche.<br />
In questo quadro, le magmatiti ordoviciane non possono<br />
costituire, nonostante il loro chimismo, un arco magmatico<br />
(come ipotizzato da vari autori) legato a processi subduttivi <strong>di</strong><br />
litosfera oceanica. Esse vanno connesse alla tettonica<br />
<strong>di</strong>stensiva, in grado <strong>di</strong> causare assottigliamenti <strong>del</strong>la crosta<br />
continentale e rifting.<br />
Le magmatiti ordoviciane corrispondono quin<strong>di</strong> a quelle<br />
“bimodali”, note da tempo nell’Europa varisica.<br />
Recentemente, (BRIAND et alii, 2002), hanno legato, nel<br />
massiccio dei Mauri, la bimodalità magmatica a parziale<br />
fusione <strong>di</strong> un mantello, progressivamente impoverito, a<br />
<strong>di</strong>fferenti profon<strong>di</strong>tà <strong>del</strong>la crosta continentale soggetta ad<br />
assottigliamento.<br />
REFERENCES<br />
BONARDI G. et alii, (2008) - Boll.Soc Geol.It, 127 (2) 173-190.<br />
BRIAND B. et alii, (2002) - Geol. Magazine, 139, 291-311.<br />
DE GREGORIO S. et alii (2003) - Earth Science, 92, 852- 872<br />
Si ringrazia i geologi calabresi A. Pisciuneri e A. Stamile per la<br />
collaborazione
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 139-141, 3 ff.<br />
Involvement of pore fluids in frictional melting from stable isotopes<br />
study of pseudotachylytes<br />
RIASSUNTO<br />
Coinvolgimento <strong>di</strong> flui<strong>di</strong> <strong>di</strong> poro in faglie sismiche dallo stu<strong>di</strong>o isotopico<br />
in pseudotachiliti<br />
Le pseudotachiliti sono fusi <strong>di</strong> frizione che si formano durante lo<br />
scivolamento sismico e che si soli<strong>di</strong>ficano in tempi <strong>brevi</strong>, nell’or<strong>di</strong>ne <strong>di</strong> minuti<br />
o secon<strong>di</strong> dopo un terremoto. Lo scopo <strong>di</strong> questo lavoro è investigare la<br />
presenza <strong>di</strong> flui<strong>di</strong> acquosi durante lo scivolamento frizionale attraverso la<br />
misura <strong>del</strong> rapporto deuterio/idrogeno (D) in pseudotachiliti naturali <strong>ed</strong><br />
artificiali, e il confronto con il D dei rispettivi incassanti.<br />
Le pseudotachiliti naturali provengono da faglie paleosismiche esumate<br />
nel Massiccio <strong>del</strong>l’Adamello (Alpi Meri<strong>di</strong>onali), che erano attive circa 30 Ma<br />
a 9-11 km <strong>di</strong> profon<strong>di</strong>tà e a 250-300°C; le pseudotachiliti si formano per<br />
fusione <strong>di</strong> tonalite, dove l’unica fase idrata è biotite, e cataclasite, in cui le fasi<br />
idrate sono epidoto e clorite. I valori <strong>del</strong> D sono compresi tra -73 ±2‰ per la<br />
biotite in tonalite, e 64 ±4‰ per la cataclasite (bulk, epidoto+clorite). Le<br />
pseudotachiliti artificiali sono state prodotte in esperimenti <strong>di</strong> frizione che<br />
simulano con<strong>di</strong>zioni sismiche (fino a 1.28 m s -1 e con applicazione <strong>di</strong> sforzi<br />
normali fino a 20MPa), in con<strong>di</strong>zioni anidre. L’unica fonte <strong>di</strong> acqua nel fuso è<br />
la deidratazione dei minerali idrati <strong>del</strong>le rocce <strong>di</strong> partenza. Il D <strong>del</strong>le<br />
pseudotachiliti artificiali varia da -75 ±1‰ per campioni prodotti da tonalite<br />
(cioè in<strong>di</strong>stinguibile da quello <strong>del</strong>la biotite in tonalite), a -83 ±2‰ per<br />
campioni prodotti da cataclasite. Le pseudotachiliti naturali hanno D<br />
compreso tra -103.6 e -83.4‰, in<strong>di</strong>pendente dal tipo <strong>di</strong> incassante.<br />
Nelle pseudotachiliti artificiali in cataclasite, lo shift negativo è imputabile<br />
a frazionamento durante la fusione parziale <strong>del</strong>l’epidoto (temperatura <strong>di</strong><br />
fusione 1050°C), confermata da evidenze microstrutturali; in pseudotachiliti<br />
artificiali in tonalite, la fusione totale <strong>del</strong>la biotite (temperatura <strong>di</strong> fusione<br />
650°C) annulla l’effetto <strong>di</strong> frazionamento.<br />
Nelle pseudotachiliti naturali analisi microstrutturali (FE-SEM) e<br />
mineralogiche (XRPD) scartano l’ipotesi <strong>di</strong> alterazione tar<strong>di</strong>va (i.e.,<br />
weathering). Il comune shift negativo <strong>del</strong> D può essere quin<strong>di</strong> dovuto a (i)<br />
coinvolgimento durante il terremoto <strong>di</strong> flui<strong>di</strong> <strong>di</strong> poro a basso rapporto<br />
isotopico, oppure a (ii) idratazione <strong>del</strong>la matrice vetrosa durante processi <strong>di</strong><br />
devetrificazione coevi con gli eventi sismici.<br />
Key words: Adamello, pore fluids, stable isotopes,<br />
pseudotachylytes, seismic faulting.<br />
_________________________<br />
SILVIA MITTEMPERGHER (*), LUIGI DALLAI (**), GIULIO DI TORO (*, °) & GIORGIO PENNACCHIONI (*)<br />
(*) Dipartimento <strong>di</strong> Geoscienze, Università degli Stu<strong>di</strong> <strong>di</strong> Padova, via Giotto<br />
1, 35137 Padova (Italy)<br />
(**) Istituto <strong>di</strong> Geoscienze e <strong>Georisorse</strong>, CNR, via G. Moruzzi 1, 56124 Pisa<br />
(Italy)<br />
(°) Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia, via <strong>di</strong> Vigna Murata 605,<br />
00143 Roma (Italy)<br />
Lavoro eseguito con il contributo finanziario <strong>del</strong>la Fondazione CARITRO<br />
(Trento), <strong>del</strong>la Fondazione CARIPARO (Padova) e <strong>del</strong> Progetto European<br />
Research Council Starting Grant USEMS.<br />
INTRODUCTION<br />
Pseudotachylytes are frictional melts produc<strong>ed</strong> during<br />
seismic slip and soli<strong>di</strong>fi<strong>ed</strong> in short time (seconds to minutes)<br />
after an earthquake (DI TORO et al., 2008 for a review). We<br />
investigat<strong>ed</strong> the presence and role of hydrous fluids during<br />
seismic faulting by measuring the Deuterium/Hydrogen (D/H)<br />
ratios (D) in natural and artificial pseudotachylytes and in<br />
their host rocks.<br />
METHODS AND RESULTS<br />
Pseudotachylyte samples have been collect<strong>ed</strong> in a fault zone<br />
crosscutting the Adamello tonalitic batholith (Italian Southern<br />
Fig. 1 – Geological setting of the Adamello batholith and location of the<br />
sampling area.<br />
Fig. 1 – Inquadramento geologico <strong>del</strong> Batolite dll’Adamello e punto <strong>di</strong><br />
campionamento.<br />
Alps) (Fig. 1). From microstructural and mineralogical<br />
evidences, it has been inferr<strong>ed</strong> that seismic faulting occurr<strong>ed</strong> at<br />
9-11 km depth and 250-300°C (DI TORO & PENNACCHIONI,<br />
2004; 2005). Pseudotachylytes are host<strong>ed</strong> in tonalite, where the<br />
only hydrat<strong>ed</strong> phase is biotite and cataclasites, where the<br />
hydrat<strong>ed</strong> minerals are epidote and minor chlorite (Fig. 2a). The
140 MITTEMPERGHER ET ALII<br />
Fig. 2 – Natural and artificial pseudotachylytes. a. Field photograph of pseudotachylyte fault (FV) and injection veins (IV), intru<strong>di</strong>ng tonalite and cataclasite<br />
(CC). b. SEM image of a natural pseudotachylyte, with clasts of plagioclase and quartz floating in a microcrystalline matrix. c. FE_SEM image of the inner<br />
layer of a natural pseudotachylyte, with spherulitic aggregates floating in a cryptocrystalline matrix. d. Tonalite sample after a high velocity shear experiment.<br />
e. Cataclasite sample after an high velocity shear experiment; in both cases, the cylinders are weld<strong>ed</strong> by a thin pseudotachylyte layer (arrows). f. SEM<br />
micrograph of the experimental pseudotachylyte in tonalite; note the embayment (arrow) in the biotite grain. g. SEM micrograph of the experimental<br />
pseudotachylyte in cataclasite; note the porosity in the epidote grains near the contact with the pseudotachylyte layer.<br />
Fig. 2 – Pseudotachiliti naturali <strong>ed</strong> artificiali. a. Foto <strong>di</strong> campagna <strong>di</strong> una vena <strong>di</strong> pseudotachilite (fault vein, FV) con vene <strong>di</strong> iniezione (IV), che intrude<br />
tonalite e cataclasite (CC). b. Immagine SEM <strong>di</strong> pseudotachilite naturale, con clasti <strong>di</strong> quarzo e plagioclasio sostenuti da una matrice microcristallina. c.<br />
Immagine al FE_SEM <strong>del</strong> livello centrale <strong>di</strong> una pseudotachilite naturale, con aggregati sferulitici in matrice criptocristallina. D. Campione <strong>di</strong> tonalite dopo<br />
un esperimento <strong>di</strong> scivolamento ad alta velocità. E. Campione <strong>di</strong> cataclasite dopo un esperimento <strong>di</strong> scivolamento ad alta velocità; in entrambi i casi, i cilindri<br />
sono saldati da un sottile livello <strong>di</strong> pseudotachilite (frecce). f. Immagine al SEM <strong>di</strong> pseudotachilite sperimentale ottenuta da tonalite; notare l’insenatura<br />
(freccia) nel granulo <strong>di</strong> biotite. G. Immagine al SEM <strong>di</strong> una pseudotachilite sperimentale in cataclasite; notare la porosità nei granuli <strong>di</strong> epidoto a<strong>di</strong>acenti al<br />
contatto con la pseudotachilite.<br />
hydrogen isotopic ratio has been measur<strong>ed</strong> by combustion in an<br />
elemental analyzer in continuous flow mode (SHARP et al.,<br />
2001); D values range from -73 ±2‰ for biotite in tonalite to<br />
64 ±4‰ for bulk cataclasite (epidote+chlorite).<br />
Pseudotachylytes are form<strong>ed</strong> by clasts of quartz, plagioclase<br />
and K-feldspar in a matrix compos<strong>ed</strong> by a microcrystalline<br />
aggregate of biotite and plagioclase microlites; glass is absent<br />
(Fig. 2b). Many of the analyz<strong>ed</strong> natural pseudotachylytes are<br />
zon<strong>ed</strong>, with an inner layer form<strong>ed</strong> by spherulitic aggregates<br />
floating in a cryptocrystalline devitrifi<strong>ed</strong> (?) matrix (Fig. 2c).<br />
From X-Ray powder <strong>di</strong>ffraction analyses, the only hydrat<strong>ed</strong><br />
mineral in pseudotachylytes is biotite, and low temperature<br />
secondary minerals are absent.<br />
Natural pseudotachylytes have D values ranging between -<br />
103.6 and -83.4‰, irrespective of wall rock composition<br />
(Fig.3).<br />
Artificial pseudotachylytes were obtain<strong>ed</strong> from tonalites and<br />
cataclasites in friction experiments simulating seismic slip<br />
(velocity up to 1.28 ms -1 , normal load up to 20 MPa) under dry<br />
con<strong>di</strong>tions (DI TORO et al., 2006) (Fig. 2d, e). Dehydration of<br />
biotite in tonalite and epidote+chlorite in cataclasite provid<strong>ed</strong><br />
the source for water in experimental pseudotachylytes.<br />
Artificial pseudotachylytes are form<strong>ed</strong> by sub-round<strong>ed</strong> clasts of<br />
quartz, plagioclase and K-feldspar floating in a glassy matrix;<br />
closely resembling the microstructure of some not zon<strong>ed</strong><br />
natural pseudotachylytes, except for the natural ones do not<br />
Fig. 3 – Hydrogen isotopic ratios of the analyz<strong>ed</strong> samples plott<strong>ed</strong> versus<br />
their water content.<br />
Fig. 3 – Rapporti isotopici <strong>del</strong>l’idrogeno dei campioni analizzati, riportati<br />
contro il rispettivo contenuto in acqua.<br />
bear glass (Fig. 2f, g)<br />
The D values of artificial pseudotachylytes range from -75<br />
±1‰ for samples produc<strong>ed</strong> from tonalite, to -83 ±2‰ for<br />
samples involving cataclasite.
INVOLVEMENT OF PORE FLUIDS IN FRICTIONAL MELTING FROM STABLE ISOTOPES STUDY OF PSEUDOTACHYLYTES<br />
DISCUSSION AND CONCLUSIONS<br />
In experimental pseudotachylytes, SEM analysis (Fig. 2g)<br />
suggests that the negative D shift of pseudotachylytes<br />
produc<strong>ed</strong> from cataclasites result<strong>ed</strong> from hydrogen<br />
fractionation during partial melting of epidote (melting point<br />
1050°C) in the wall rocks; <strong>di</strong>fferently, total melting of biotite<br />
(due to its lower melting temperature of 650°C) allow<strong>ed</strong><br />
negligible H-isotope fractionation between biotite and<br />
pseudotachylyte.<br />
In natural pseudotachylytes, microstructural and<br />
geochemical observations rule out meteoric alteration of the<br />
pseudotachylyte by isotopically light water; thus, the common<br />
hydrogen isotope signature of -93 ±10‰ may result (i) from<br />
pseudotachylytes being buffer<strong>ed</strong> by a low-D pore fluid involv<strong>ed</strong><br />
in frictional melting, or (ii) hydration of the pseudotachylytes<br />
during devitrification processes.<br />
REFERENCES<br />
DI TORO G. & PENNACCHIONI, G. (2005) - Superheat<strong>ed</strong><br />
friction-induc<strong>ed</strong> melts in zon<strong>ed</strong> pseudotachylytes within the<br />
Adamello tonalites (Italian Southern Alps). Journal of<br />
Structural Geology, 26, 1783-1801.<br />
DI TORO G. & PENNACCHIONI, G. (2005) - Fault plane<br />
processes and mesoscopic structure of a strong-type<br />
seismogenic fault in tonalites (Adamello batholith,<br />
Southern Alps). Tectonophysics, 402 (1-4), 54-79.<br />
DI TORO G., HIROSE T., NIELSEN S., PENNACCHIONI G. &<br />
SHIMAMOTO T. (2006) - Natural and experimental evidence<br />
of melt lubrication of faults during earthquakes. Science,<br />
311, 647-649<br />
DI TORO G., PENNACCHIONI G. & NIELSEN S. (2008) -<br />
Pseudotachylytes and Earthquake Source Mechanics. In:<br />
Eiichi Fukuyama (Ed.) “Fault-zone Properties and<br />
Earthquake Rupture Dynamics”, International Geophysics<br />
Series, Elsevier, submitt<strong>ed</strong>.<br />
SHARP Z.D., ATUDOREI V. & DURAKIEVICZ T. (2001) – A rapid<br />
method for determination of hydrogen and oxygen isotope<br />
ratios from water and hydrous minerals. Cem. Geology<br />
178, 197-210.<br />
141
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 142-143, 1 f.<br />
Caratteri strutturali e interazione fluido-roccia in una faglia<br />
normale ad alto angolo nei marmi <strong>di</strong> Carrara<br />
GIANCARLO MOLLI (*), GIANNI CORTECCI (**), LUCA VASELLI (**) & GIUSEPPE OTTRIA (**)<br />
ABSTRACT<br />
Fault zone structure and fluid-rock interaction of a high angle normal<br />
fault in the Carrara marble (NW Tuscany, Italy)<br />
We stu<strong>di</strong><strong>ed</strong> the geometry, intensity of deformation and fluid-rock interaction of<br />
a high angle normal fault develop<strong>ed</strong> in Carrara marble in the Alpi Apuane NW<br />
Tuscany, Italy.<br />
The overall architecture of the structure is form<strong>ed</strong> by a fault core bound<strong>ed</strong> by<br />
two major non-parallel slip surfaces. The fault core, associat<strong>ed</strong> with crush<strong>ed</strong><br />
breccia and cataclasites, asimmetrically grades to the host protolith through a<br />
damage zone only well develop<strong>ed</strong> in the footwall block, whereas the transition<br />
to hangingwall is sharply defin<strong>ed</strong> by the upper main slip surface.<br />
Faulting was associat<strong>ed</strong> with fluid-rock interaction in form of kinematicallyrelat<strong>ed</strong><br />
veining observable in the damage zone and fluid channelling within<br />
fault core where an orange-brownish cataclasite matrix can be observ<strong>ed</strong>. A<br />
geochemical and isotopic study inclu<strong>di</strong>ng a mathematical mo<strong>del</strong> was<br />
perform<strong>ed</strong> to enlighten type, role and activity of fluid-rock interactions during<br />
deformation.<br />
The results of our stu<strong>di</strong>es suggest<strong>ed</strong> that the deformation pattern is mainly<br />
controll<strong>ed</strong> by processes associat<strong>ed</strong> with linking-damage zone at a fault tip,<br />
development of fault core, localization and channelling of fluids through it.<br />
Syn-kinematic microstructural mo<strong>di</strong>fication of calcite microfabric possibly<br />
plays a role in confining fluid percolation.<br />
Key words: Carrara marble, brittle faulting, geochemistry,<br />
cataclasite, calcite microstructures,<br />
INTRODUZIONE<br />
Gli sta<strong>di</strong> tar<strong>di</strong>vi <strong>del</strong>l’evoluzione geologica <strong>del</strong>le Alpi<br />
Apuane sono caratterizzati dallo sviluppo <strong>di</strong> sistemi <strong>di</strong> faglie ad<br />
alto angolo (normali e trascorrenti) che accompagnano<br />
l’esumazione finale <strong>del</strong>le unità metamorfiche esposte nel<br />
gruppo montuoso (CARMIGNANI & KLIGFIELD, 1990; OTTRIA &<br />
MOLLI, 2000; MOLLI, 2008 e bibl.).<br />
Come parte <strong>di</strong> uno stu<strong>di</strong>o in corso in questo contributo<br />
verranno illustrate le caratteristiche strutturali (architettura<br />
generale, meso e microstrutture) <strong>di</strong> un zona <strong>di</strong> faglia ad alto<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Sicenze <strong>del</strong>la Terra, Via S.Maria 53, 56126 Pisa<br />
gmolli@dst.unipi.it<br />
(**) CNR Istituto <strong>di</strong> Geoscienze e <strong>Georisorse</strong> Pisa<br />
Lavoro eseguito nell’ambito <strong>del</strong> progetto Stu<strong>di</strong>o Geologico-Strutturale dei<br />
sistemi <strong>di</strong> deformazione fragile nei marmi <strong>del</strong>le Alpi Apuane Azienda ASL<br />
1 <strong>di</strong> Massa e Carrara<br />
angolo esposta nel bacino marmifero <strong>di</strong> Fantiscritti (Carrara,<br />
Alpi Apuane occidentali). Lo stu<strong>di</strong>o strutturale è stato<br />
accompagnato da quello geochimico allo scopo <strong>di</strong> vincolare<br />
ruolo, tipo e modalità <strong>di</strong> interazione fluido-roccia durante lo<br />
sviluppo <strong>del</strong>la struttura analizzata.<br />
L’architettura generale <strong>del</strong>la struttura è formata da un<br />
nucleo (fault core) associato a brecce e cataclasiti che<br />
asimmetricamente passa ad una zona <strong>di</strong> danneggiamento<br />
(damage zone) ben sviluppata solamente nel footwall <strong>del</strong>la<br />
faglia, mentre la transizione al tetto è nettamente definita<br />
attraverso la superficie <strong>di</strong> scivolamento principale superiore.<br />
Fig. 1 – Didascalia <strong>del</strong>la figura. (Stile: Didascalie figure)<br />
Fig. 1 – Fig. 1 – Architettura generale con elementi strutturali principali <strong>del</strong>la<br />
faglia normale analizzata (Fantiscritti Carrara). a) Proiezione stereografica<br />
<strong>del</strong>la faglia principale e dei sistemi <strong>di</strong> fatturazione nella zona <strong>di</strong><br />
danneggiamento. b) <strong>di</strong>rezioni principali <strong>di</strong> deformazione per la faglia,<br />
Fig. 1 –Schematic sketch of the main structural componente of the stu<strong>di</strong><strong>ed</strong><br />
fault. a) Stereonet of the master fault and fracture sets in the damage zone; b)<br />
Principal <strong>di</strong>rection of deformation for the stu<strong>di</strong><strong>ed</strong> fault.<br />
La deformazione fragile è stata accompagnata da<br />
interazione fluido-roccia evidenziata sia dallo sviluppo <strong>di</strong> vene<br />
nella zona <strong>di</strong> danneggiamento, sia da una localizzazione <strong>di</strong><br />
circolazione all’interno <strong>del</strong> nucleo <strong>del</strong>la faglia dove sono<br />
osservabili cataclasiti con una matrice colore arancio/marrone.<br />
Lo stu<strong>di</strong>o geochimica <strong>ed</strong> isotopico, includente un mo<strong>del</strong>lazione<br />
matematica, ha permesso <strong>di</strong> ottenere informazioni sul ruolo, il<br />
tipo e l’attività <strong>di</strong> interazione fluido-roccia durante la
CARATTERI STRUTTURALI E INTERAZIONE FLUIDO-ROCCIA IN UNA FAGLIA NORMALE AD ALTO ANGOLO NEI MARMI DI CARRARA 143<br />
deformazione.<br />
I risultati <strong>del</strong> nostro stu<strong>di</strong>o suggeriscono che la<br />
deformazione è stata controllata in modo principale dai<br />
processi associati alla crescita <strong>del</strong>la faglia e alla sua<br />
segmentazione e dallo sviluppo <strong>del</strong> nucleo con localizzazione<br />
<strong>del</strong>la deformazione e incanalamento dei flui<strong>di</strong> attraverso esso.<br />
Mo<strong>di</strong>ficazioni microstrutturali sin-cinematiche <strong>del</strong>la calcite<br />
durante la propagazione <strong>del</strong>la faglia probabilmente giocano un<br />
ruolo chiave nel confinare i flui<strong>di</strong> nel footwall <strong>del</strong>la faglia<br />
(MOLLI ET ALII. 2009).<br />
REFERENCES<br />
CARMIGNANI L. & KLIGFIELD R. (1990) – Crustal extension in<br />
the Northern Apennines: The transition from compression<br />
to extension in the Alpi Apuane. Tectonics, 9, 1275-1303.<br />
MOLLI G. (2008) – Northern Apennine-Corsica orogenic<br />
system: an updat<strong>ed</strong> overview. In: Siegesmund, S.,<br />
Fugenschuh B., Froitzheim, N. (Eds). Geological Society of<br />
London, Special Pubblications, 298, 413-442.<br />
MOLLI G., CORTECCI G., VASELLI L., OTTRIA G., G.B.,<br />
CORTOPASSI A., DINELLI E., BARBIERI M. & MUSSI M.<br />
(2009) –Fault zone structure and fluid- rock interaction of<br />
a high angle normal fault. Journal of Structural Geology, in<br />
stampa.<br />
OTTRIA G. & MOLLI G. (2000) – Superimpos<strong>ed</strong> brittle<br />
structures in the late orogenic extension of the Northern<br />
Apennine. Results from Carrara area (NW Tuscany, Italy)<br />
Terra Nova 12 (2), 52-59.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 144-147, 1 f.<br />
From compression to extension during the Sicily chain buil<strong>di</strong>ng<br />
ABSTRACT<br />
Processi compressivi e <strong>di</strong>stensivi durante la costruzione <strong>del</strong>la catena<br />
siciliana<br />
Kinematics of mountain belts is often very <strong>di</strong>fficult to decipher. Main problems<br />
consist in the linkage between <strong>di</strong>fferent stages of deformation which define the<br />
chain buil<strong>di</strong>ng, their significance in the context of lithospheric evolution<br />
dominate by plate collision and the interaction with previous structures<br />
record<strong>ed</strong> in the rocks. Also, the overprinting of structures developing later with<br />
respect to the chain buil<strong>di</strong>ng may further makes complicate the way to unravel<br />
the tectonic evolution of the w<strong>ed</strong>ge.<br />
In Sicily belt, locat<strong>ed</strong> in the Central M<strong>ed</strong>iterranean, the regional pattern and the<br />
tectonic evolution are describ<strong>ed</strong> using structural analysis of small-scale<br />
structures in select<strong>ed</strong> outcrops. The geometric <strong>di</strong>fferences existing between<br />
some types of structures within the belt allow to <strong>del</strong>ineate the timing of<br />
deformations during chain buil<strong>di</strong>ng.<br />
Key words: collisional tectonics, structural pattern, sequence<br />
of deformation, Sicily Chain.<br />
INTRODUCTION<br />
The aim of this paper is to provide constraints to help<br />
unravel the structural evolution of the Sicily chain using<br />
overprinting fabrics and their relationships to larger structures.<br />
The progression of deformation is represent<strong>ed</strong> by three<br />
regionally-significant structural stages (layer-parallel<br />
shortening, fol<strong>di</strong>ng-and-thrusting and extension). The first<br />
stage of deformation includes several sub-stages (layer-parallel<br />
shortening, b<strong>ed</strong>-parallel simple shear and fold nucleation).<br />
Deformation continu<strong>ed</strong> in a second stage, where thrusting was<br />
coupl<strong>ed</strong> by fold amplification and tightening. Kinematic<br />
evolution is provid<strong>ed</strong> by a third stage, where dominantly<br />
negative inversion of previous weaken<strong>ed</strong> zones and mechanical<br />
<strong>di</strong>scontinuities occurr<strong>ed</strong>, coupl<strong>ed</strong> by normal faults activation.<br />
Each stage is defin<strong>ed</strong> as a <strong>di</strong>screte phase of deformation,<br />
characteris<strong>ed</strong> by the development of <strong>di</strong>fferent sets of structures,<br />
such as cleavage, folds, faults and veins. Each deformative step<br />
may be sequentially fram<strong>ed</strong> in a kinematic history, where a<br />
_________________________<br />
NIGRO F. (*), SALVAGGIO G. (°), FAVARA R. (*) & RENDA P. (* , °)<br />
(*) Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia-sezione <strong>di</strong> Palermo, via<br />
Ugo la Malfa 153, 90146, Palermo.<br />
(°) Dipartimento <strong>di</strong> Geologia e Geodesia <strong>del</strong>l’Università, via Archirafi 22,<br />
90123, Palermo<br />
continuous process starts with shortening and ends with<br />
extension in a tectonic setting dominat<strong>ed</strong> by collisional<br />
tectonics.<br />
SEQUENCE OF DEFORMATIONS<br />
In Sicily, the tectonic units were pil<strong>ed</strong>-up in the Oligocene-<br />
Early Miocene (BIANCHI et al., 1987; NIGRO & RENDA, 2000).<br />
Detachments inside the multilayer provide multi-harmonic<br />
fol<strong>di</strong>ng and blind splays, to form duplex geometries. Ramp-flat<br />
geometries outcrop wi<strong>del</strong>y, with moderate spacing of the<br />
sheets. The tectonic units strike <strong>di</strong>fferently from west to east.<br />
Map-scale fold trend W-E/NE-SW is perpen<strong>di</strong>cular to the mean<br />
S/SE thrusting <strong>di</strong>rection.<br />
Three main deformation stages have been recognis<strong>ed</strong><br />
(layer-parallel shortening, fol<strong>di</strong>ng-and-thrusting, low-angle<br />
extension), having regional significance.<br />
1) First stage of deformation: multilayer weakening<br />
sub-stage A: layer-parallel shortening<br />
The first stage of deformation is represent<strong>ed</strong> by b<strong>ed</strong><strong>di</strong>ngnormal<br />
pressure solution cleavage and b<strong>ed</strong><strong>di</strong>ng-parallel shearrelat<strong>ed</strong><br />
vein and fractures.<br />
The cleavage geometry changes from planar to sutur<strong>ed</strong>. The<br />
spacing of cleavage domain ranges from millimetres to<br />
centimetres, depen<strong>di</strong>ng from the mechanical behaviours of the<br />
lithologies. Closely spac<strong>ed</strong> cleavage is mostly record<strong>ed</strong> in the<br />
Cretaceous-Paleogene pelagic marls: spac<strong>ed</strong> cleavage is<br />
record<strong>ed</strong> in the Upper Triassic-Lower Jurassic limestones.<br />
Pressure solution was accommodat<strong>ed</strong> by w<strong>ed</strong>ge faults,<br />
record<strong>ed</strong> in the carbonate strata of the interb<strong>ed</strong>d<strong>ed</strong> pelagic<br />
successions of Jurassic age. Intersection lineation of w<strong>ed</strong>ge<br />
faults is sub-parallel to the b<strong>ed</strong><strong>di</strong>ng-cleavage intersection<br />
lineation.<br />
sub-stage B: b<strong>ed</strong>-parallel simple shear<br />
Deformation a stage 1A evolves toward a process of layerparallel<br />
shear. En-échelon gash vein sets and shear fractures are<br />
the main structures representing shear. Conjugate veins<br />
consistently strike parallel to cleavage and the traces of the<br />
obtuse bisectors of this sets are parallel to b<strong>ed</strong><strong>di</strong>ng and normal<br />
to LPS cleavage-b<strong>ed</strong><strong>di</strong>ng intersection lineation.
FROM COMPRESSION TO EXTENSION DURING THE SICILY CHAIN BUILDING<br />
Mutually, gash veins and LPS cleavage cross-cut.<br />
The gash veins evolve to <strong>di</strong>screte shear planes near the b<strong>ed</strong><br />
separations, especially within the pelagic successions of<br />
Cretaceous age. Locally, intraformational duplexes of 1 cm-to-<br />
1 m in scale have been observ<strong>ed</strong>.<br />
Small-scale floor-thrusts, roof-thrusts and reverse ramps<br />
<strong>di</strong>splace and strike parallel with the LPS cleavage and then<br />
post-date the LPS episode. Master roof and floor faults<br />
represent detachment within the multilayer. Slickenfibres and<br />
striae on b<strong>ed</strong> surfaces are common, cross-cutting the gash<br />
veins.<br />
The overprinting of the small-scale detachments on the LPS<br />
fabric suggests an evolution of the deformation from pure<br />
compression to shortening under non-zero simple shear.<br />
B<strong>ed</strong><strong>di</strong>ng-parallel shear zones have been observ<strong>ed</strong> especially<br />
in the Mesozoic marly pelagic horizons.<br />
The shear zones are characteris<strong>ed</strong> by two sets of surfaces.<br />
The first coincides with the b<strong>ed</strong> surfaces and/or are parallel<br />
with b<strong>ed</strong><strong>di</strong>ng. Calcite fibres on b<strong>ed</strong> surfaces allow us to<br />
reconstruct an overall forelandwards migration. The second<br />
ones form an evident cleavage, having high-angles with respect<br />
to the b<strong>ed</strong>s and to the shear veins. The cleavage bends are<br />
consistent with the <strong>di</strong>rection of movements suggest<strong>ed</strong> by the<br />
slickenfibres on b<strong>ed</strong> surfaces. S-C fabric overprints small-scale<br />
folds, suggesting continuous evolution from stage 1C to 2.<br />
Outcrop-scale main detachments have been localis<strong>ed</strong> at the<br />
base and on the top of the carbonate platform strata, at the base<br />
of the pelagic successions and of the syn-collisional deposits.<br />
sub-stage C: fold nucleation<br />
In the less competent lithologic units, between the layerparallel<br />
shear bends small-scale folds have been observ<strong>ed</strong>. LPS<br />
pressure-solution cleavage is in places overprint<strong>ed</strong> by cleavage<br />
domains relat<strong>ed</strong> with fol<strong>di</strong>ng.<br />
Geometric relationships between fol<strong>di</strong>ng-relat<strong>ed</strong> cleavage<br />
and b<strong>ed</strong><strong>di</strong>ng is represent<strong>ed</strong> by high-angles near the second- and<br />
third-order fold hinges and low-angles in the fold limbs.<br />
Refraction phenomena have been also observ<strong>ed</strong>.<br />
The second cleavage generation produc<strong>ed</strong> pencil<br />
morphology, in the limbs of outcrop-scale folds. It consistently<br />
strikes sub-parallel with respect to the LPS cleavage.<br />
Flexural slip fol<strong>di</strong>ng was easily develop<strong>ed</strong>, re-utilising<br />
weaken<strong>ed</strong> shear<strong>ed</strong> interb<strong>ed</strong> of stage 1B. Minor thrusts<br />
accompani<strong>ed</strong> <strong>di</strong>sharmonic fol<strong>di</strong>ng, allow duplex generation at<br />
<strong>di</strong>fferent lithologic levels. 10-to-100 metres in scale duplexing<br />
contribut<strong>ed</strong> to fold nucleation.<br />
2) Second stage of deformation: w<strong>ed</strong>ge thickening<br />
Minor structures of the first stage of deformation, such as<br />
detachments, reverse faults and intraformational duplex,<br />
suffer<strong>ed</strong> fol<strong>di</strong>ng. Fold<strong>ed</strong> detachments and duplex form firstorder<br />
folds, such as ramp anticlines. Second- and third-order<br />
folds probably tighten<strong>ed</strong> and amplificat<strong>ed</strong>, to form drag folds<br />
relat<strong>ed</strong> to the first-order thrust-relat<strong>ed</strong> folds.<br />
Amplification and tightening of second- and third-order<br />
folds under flexural-slip domain was wi<strong>del</strong>y accompani<strong>ed</strong> by<br />
145<br />
collapse of fold hinges. In the fold culminations, extension<br />
fabrics, such as fibrous calcite veins, have been observ<strong>ed</strong>. The<br />
extension veins are orthogonal with respect to the fold-axis and<br />
exhibit ra<strong>di</strong>al trend from the core toward the outer arc of folds.<br />
This fold generation are well record<strong>ed</strong> both in the pelagic<br />
successions and in the carbonate platform strata. Mesoscopic<br />
folds are generally upright, close, with associat<strong>ed</strong> axial-plane<br />
cleavage and bou<strong>di</strong>nage. Fold axes are generally subhorizontal.<br />
Their profile geometry is mostly concentric and<br />
ranges from angular to curvilinear. Folds of wavelength up to 1<br />
km are asymmetric. The upright limbs shallowly <strong>di</strong>p toward the<br />
hinterland and the foreland-<strong>di</strong>pping limbs are more steeply.<br />
Second- and third-order folds relat<strong>ed</strong> to the first stage of<br />
deformation are cross-cut by later generation of reverse faults,<br />
some of these relat<strong>ed</strong> to the flexural-slip deformation.<br />
Reverse faults produc<strong>ed</strong> <strong>di</strong>splacements ranging from<br />
centimetres to kilometres. Faults crosses carbonate platform<br />
strata with high angles (between 35° and 50°). Within the<br />
pelagic strata, the cut-off angles are very <strong>di</strong>fferent due to the<br />
previous fol<strong>di</strong>ng episode. Striae and slickenfibres in<strong>di</strong>cate<br />
hangingwall transport <strong>di</strong>rection south- and southeastward in<br />
Western Sicily and southwestward in North-eastern Sicily.<br />
Outcrop-scale thrusts often define link<strong>ed</strong> thrust systems,<br />
which planes flatten<strong>ed</strong> along the shear bends of stage 1B.<br />
Back-thrusts are not common. First-order fold tightening was<br />
accompani<strong>ed</strong> by new generation of conjugate gash veins,<br />
overprinting the minor folds of stage 1C.<br />
The second gash veins generation have <strong>di</strong>fferent geometric<br />
relationships with b<strong>ed</strong><strong>di</strong>ng. The surface envelopment of the<br />
vein sets exhibits high-angle with respect to the b<strong>ed</strong><strong>di</strong>ng in the<br />
fold limbs, progressively becoming sub-parallel to b<strong>ed</strong><strong>di</strong>ng in<br />
the hinges. Also, its strike is parallel with the axial plane<br />
cleavage. Overprinting relationships suggest that fold<br />
tightening, thrusting and relat<strong>ed</strong> first-order fold nucleation,<br />
such as ramp anticlines, develop<strong>ed</strong> under non-zero shear.<br />
Map-scale thrust, defines tectonic units, which in turn have<br />
large spacing, and splays. Ramp-flat geometries at this scale of<br />
observation may be recognis<strong>ed</strong>. Near the thrust surfaces, the<br />
penetrative shearing fabric consists of S-C structures, north-tonorthwestern<br />
<strong>di</strong>pping in Western Sicily and northeastern<br />
<strong>di</strong>pping in the Peloritani range.<br />
The S-C angle is lower near the thrust surfaces than in the<br />
shear bends relat<strong>ed</strong> to the stage of deformation 1B, suggesting<br />
increase of deformation during thrusting of previous bends,<br />
inducing a progressive rotation of S-surfaces.<br />
S-C lineation intersection strikes parallel with respect to the<br />
reverse faults in Northern Sicily. Along the Western border of<br />
the Caltanissetta Basin, geometric relationships between faults,<br />
S-C structures, folds and gash vein sets <strong>di</strong>ffer, suggesting that<br />
transpression dominat<strong>ed</strong> shortening.<br />
Pervasive fabric has been recognis<strong>ed</strong> near the main thrust<br />
faults, consisting in several metres thick cataclastic breccias.
146 NIGRO F. ET ALII<br />
Fig. 1 – main structural features in the Sicily Chain (A). kinematic development of folds and relat<strong>ed</strong> thrust (B) and simplifi<strong>ed</strong> cross section of a submarine<br />
thrust w<strong>ed</strong>ge, with the parameters requir<strong>ed</strong> for its critical taper calculation and failure effects of w<strong>ed</strong>ge taper increasing due to thickening in terms of thinning<br />
for negative inversion (C).<br />
The lacking of hangingwall large overlapping outside the<br />
ramp anticline is coupl<strong>ed</strong> by ramp linkage with detachments<br />
occurr<strong>ed</strong> during the stage 1B, suggesting that fault-bend<br />
fol<strong>di</strong>ng may be the dominant deformative process during this<br />
stage of deformation. Pre-existence of detachments, their<br />
linkage with thrust ramps suggest fault-bend fol<strong>di</strong>ng for thrust<br />
nucleation.<br />
Outside the ramp anticlines, thrusts are not always<br />
emergent. More frequently, the ramp anticlines are asymmetric,<br />
with the forelimb more steeply than the backlimb. Overturn<strong>ed</strong><br />
strata are in places present at the base of the forelimbs.<br />
3) Third stage of deformation: thrust stack mo<strong>di</strong>fication<br />
Fold<strong>ed</strong>-and-fault<strong>ed</strong> strata experienc<strong>ed</strong> extension. The<br />
overprint<strong>ed</strong> normal faults activity has result<strong>ed</strong> in a <strong>di</strong>stribut<strong>ed</strong><br />
fragile thinning of the previous thrust stack.<br />
The common structures associat<strong>ed</strong> to this stage of<br />
deformation are normal faults. From 1 cm-to-100s m in scale<br />
normal faults have been observ<strong>ed</strong>. Listric ramp and flat<br />
geometries are dominant. Normal faults overprint folds and<br />
shear bends. Ramps are generally less extend<strong>ed</strong> than flats.<br />
Ramps cross-cut the b<strong>ed</strong><strong>di</strong>ng at various angles, generally<br />
<strong>di</strong>p 50°-70° and affect the backlimb of first-order folds.<br />
Extensional ramps often form fans <strong>di</strong>p synthetically with<br />
respect to the thrust faults. Flats dominantly detach the b<strong>ed</strong>s<br />
and re-utilis<strong>ed</strong> previous thrust faults and the shear bends of the<br />
stage 2 though their negative inversion. Roll-over anticlines<br />
have been observ<strong>ed</strong>, often in the ramp anticlines backlimbs.<br />
The hangingwalls are cut by joints, conjugate fractures and<br />
small, dominantly planar, normal faults. Locally, in the<br />
hangingwall, extensional ramps have been offset by b<strong>ed</strong>parallel<br />
movements along the less-competent b<strong>ed</strong>s forming the<br />
outer tectonic units outcropping in Western Sicily.<br />
Simple b<strong>ed</strong><strong>di</strong>ng-plane slip is not the only way in which<br />
b<strong>ed</strong>-parallel shear was accommodat<strong>ed</strong> during extension. Lowangle<br />
faults sub-parallel to b<strong>ed</strong><strong>di</strong>ng also affect<strong>ed</strong> the b<strong>ed</strong><strong>di</strong>ng.<br />
These low-angle slip planes act as <strong>di</strong>splacement transfers from<br />
one easy slip horizon to another, accommodating flexural<br />
shear. Synthetic and antithetic normal faults also allow b<strong>ed</strong><br />
parallel shear, which results in b<strong>ed</strong> thinning, b<strong>ed</strong> length
FROM COMPRESSION TO EXTENSION DURING THE SICILY CHAIN BUILDING<br />
extension and b<strong>ed</strong> rotation. The main extensional detachments<br />
are locat<strong>ed</strong> at the base of the Mesozoic carbonates, the<br />
Cretaceous deposits and the Oligo-Miocene for<strong>ed</strong>eep deposits.<br />
Passive block rotation and repeat<strong>ed</strong> faulting during increase<br />
of extension are suggest<strong>ed</strong> by lozenge geometries, form<strong>ed</strong><br />
under high stretching rate. Rolling due to shear extension is in<br />
places coupl<strong>ed</strong> by localis<strong>ed</strong> cleavage development.<br />
Shear bends develop<strong>ed</strong> during stages 1B and 2 and smallscale<br />
roof-floor thrusts relat<strong>ed</strong> to duplexing were in places<br />
invert<strong>ed</strong> during extension.<br />
Meso-scale normal faults are represent<strong>ed</strong> by two or more<br />
variously <strong>di</strong>pping systems. Typically, the steeper extensional<br />
faults <strong>di</strong>splace the low-angle system. Both systems of mesoscale<br />
faults appear to have been tilt<strong>ed</strong> by steeply <strong>di</strong>pping<br />
normal faults, more and more frequent from the Palermo-<br />
Madonie Mts. towards the Tyrrhenian coast, suggesting a<br />
faulting evolution characteris<strong>ed</strong> by fault-block rotation during<br />
extension.<br />
Several metres thick of cataclastic bends are present at the<br />
base of the Mesozoic carbonates. Cataclasite consists of<br />
cement<strong>ed</strong> breccias, with coarse-to-m<strong>ed</strong>ium-grain<strong>ed</strong> size clasts.<br />
Extensional ramps consistently strike from W-E in Western<br />
Sicily and swing in a NW-SE <strong>di</strong>rection eastwards. Kinematic<br />
in<strong>di</strong>cators on the expos<strong>ed</strong> fault planes in<strong>di</strong>cate movement subparallel<br />
to the <strong>di</strong>p, without strike-slip component. Locally, two<br />
calcite fibres are superpos<strong>ed</strong> on the extensional ramps.<br />
The earlier kinematic in<strong>di</strong>cators have low-angles pitches,<br />
suggesting reactivation under strike-slip con<strong>di</strong>tions. Near the<br />
re-activat<strong>ed</strong> normal faults, the cataclastic breccias show brittle<br />
deformations, as high-angle small faults. The activity of the<br />
extensional detachments determin<strong>ed</strong> in places the mechanical<br />
contacts of the hangingwall flats over the footwall flats, with<br />
elision of lithostratigraphic units.<br />
REGIONAL-SCALE TIMING OF CHAIN BUILDING<br />
The outcrop-scale contractional structures are not uniformly<br />
<strong>di</strong>stribut<strong>ed</strong> throughout the deform<strong>ed</strong> multilayer, both laterally<br />
and laterally. The non-uniform horizontal <strong>di</strong>stribution of the<br />
structures from the hinterland to the foreland is consistently<br />
with the strain partitioning rate, decreasing in pervasive<br />
characters forelandwards. In fact, overprinting relationships of<br />
outcrop-scale structures are well develop<strong>ed</strong> in the inner<br />
tectonic units than in the outer, as well as the amount of<br />
<strong>di</strong>splacement. Otherwise, its vertical non-uniform <strong>di</strong>stribution<br />
may reflects the controls by mechanical behaviours anisotropy<br />
of the multilayer.<br />
Since the Oligocene onwards, the Sicily chain built. It was<br />
characteriz<strong>ed</strong> by a thrust front-for<strong>ed</strong>eep system migrating<br />
forelandwards and progressively incorporating syn-orogenic<br />
deposits. Collisional tectonics is represent<strong>ed</strong> by a thrust stack<br />
emplac<strong>ed</strong> onto a gently chain-<strong>di</strong>pping lea<strong>di</strong>ng <strong>ed</strong>ge of the<br />
foreland, underplat<strong>ed</strong> below the basal detachment horizon as a<br />
passive footwall.<br />
Map-scale compressional tectonics is represent<strong>ed</strong> by a set<br />
of thrust sheets, characteriz<strong>ed</strong> by frontal ramp anticlines, and<br />
splays. The general tren<strong>di</strong>ng of axial surfaces in<strong>di</strong>cates an<br />
147<br />
Africa vergence of the structures. The vergence of minor folds<br />
is generally consistent with that of the main folds.<br />
Thrusts <strong>di</strong>p towards NW in Western Sicily, towards the<br />
north in the middle part of Northern Sicily and towards NE in<br />
North-eastern Sicily. In the Palermo Mts., the chain units<br />
largely overthrust on the deform<strong>ed</strong> external shallow substrate<br />
through a very low angle flat (CATALANO et al., 2000). Thrust<br />
step-up geometries are characteriz<strong>ed</strong> by a few degrees of<br />
plunge. Forelandwards, the chain units link along a sole thrust<br />
and show a more highly thrust step-up angles of reverse faults.<br />
The deform<strong>ed</strong> foreland is affect<strong>ed</strong> by an emergent reverse fault<br />
system in Southwestern Sicily, where thrust step-up geometries<br />
are characteriz<strong>ed</strong> by the very high values of plunge. From<br />
magnetic data we estimate that the regional monocline gently<br />
plunges towards the hinterland by about 4 degrees.<br />
The tectonic units of North-eastern Sicily were pil<strong>ed</strong>-up<br />
during Late Oligocene-Early Miocene. Progressive foreland<br />
migration occurr<strong>ed</strong> for the Sicilian Maghrebides later, carrying<br />
piggy-back basins and incorporating for<strong>ed</strong>eep deposits (NIGRO<br />
& RENDA, 2000). The tectonic units of the Maghrebian range<br />
incorporat<strong>ed</strong> Upper Langhian-Lower Tortonian syn-tectonic<br />
deposits in North-western Sicily. Southward, the tectonic units<br />
incorporat<strong>ed</strong> Upper Tortonian syn-tectonic deposits, which<br />
become progressively younger towards the Southern Sicily.<br />
The compressional-deform<strong>ed</strong> foreland plate of Western<br />
Sicily is affect<strong>ed</strong> by high-angle faults both onland and in the<br />
off-shore of Southern Sicily, where Plio-Pleistocene sequences<br />
are involv<strong>ed</strong> in the deformation.<br />
Extensional tectonics is mostly represent<strong>ed</strong> by low-angle<br />
detachment systems innerward chain <strong>di</strong>pping, which have<br />
backslid<strong>ed</strong> the tectonic units (GIUNTA et al., 2000). The<br />
common fault linkage of the normal fault ramp segments with<br />
the contractional shears and thrust suggest that the negative<br />
inversion of the tectonic units was the dominant process of the<br />
extensional kinematics, allowing stretching and chain thinning.<br />
Normal faults may be recognis<strong>ed</strong> at <strong>di</strong>fferent scales, from 1 m<br />
to several kilometers. The main ramp segments determinate the<br />
outcropping fault steps which segment<strong>ed</strong> the thrust stack and<br />
allow<strong>ed</strong> the lowering of the belt towards the Tyrrhenian Sea.<br />
REFERENCES<br />
BIANCHI F., CARBONE S., GRASSO M., INVERNIZZI G., LENTINI<br />
F., LONGARETTI G., MERLINI S. E MOSTARDINI F. (1987) -<br />
Sicilia orientale: profilo geologico Nebro<strong>di</strong>-Iblei. Mem.<br />
Soc. Geol. It., 38, 429-458.<br />
CATALANO R., FRANCHINO A., MERLINI S. & SULLI A. (2000) -<br />
Central western Sicily structural setting interpret<strong>ed</strong> from<br />
seismic reflection profiles. Mem. Soc. Geol. It., 55, 5-16.<br />
GIUNTA G., NIGRO F. & RENDA P. (2000) - Extensional<br />
tectonics during Maghrebides chain buil<strong>di</strong>ng since late<br />
Miocene: examples from Northern Sicily. Ann. Soc. Geol.<br />
Poloniae, 70, 81-98.<br />
NIGRO F. & RENDA P. (2000) - Un mo<strong>del</strong>lo <strong>di</strong> evoluzione<br />
tettono-s<strong>ed</strong>imentaria <strong>del</strong>l’avanfossa neogenica siciliana.<br />
Boll. Soc. Geol. It., 119, 667-686.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 148-152, 2 ff.<br />
Evoluzione tettonica mesozoico-terziaria <strong>del</strong>la Sicilia centrosettentrionale<br />
NIGRO F. (*), SALVAGGIO G. (°), FAVARA R. (*), & RENDA P. (* , °)<br />
ABSTRACT<br />
Mesosozoic-to-Tertiary tectonic evolution of the North-Central Sicily<br />
Sicily owes its complex geological structure to a switch in tectonic regime<br />
from the Mesozoic to the Tertiary. A set of tectonic units outcrops in the<br />
northern portion of the island that originat<strong>ed</strong> during the Tertiary at the expense<br />
of paleogeographic domains of the African Mesozoic continental margin. The<br />
pre-orogenic successions show <strong>di</strong>fferent types of deformation (extensional and<br />
transcurrent) relat<strong>ed</strong> to the Jurassic paleotectonic evolution of the southern<br />
Neotethys margin. The history of the tectonic inversion of the Neotethys shear<br />
zone is record<strong>ed</strong> in the Cretaceous strata. Extension occurr<strong>ed</strong> during late<br />
Cretaceous and may be compatible with the tensile stress field relat<strong>ed</strong> to the<br />
Sicilide basin opening. The Neogene deformations are link<strong>ed</strong> to collisional<br />
processes and are mostly represent<strong>ed</strong> by thrusts and folds. Since the late<br />
Miocene onwards, the formation of the Tyrrhenian basin has driven the recent<br />
tectonic evolution of Northern Sicily. Its basin formation was realis<strong>ed</strong> through<br />
extension, follow<strong>ed</strong> by transcurrent tectonics along its southern margin.<br />
parole chiave: cronologia <strong>del</strong>le deformazioni mesozoicoterziarie,<br />
Sicilia centro-settentrionale.<br />
INTRODUZIONE<br />
La cronologia <strong>del</strong>le deformazioni registrate nelle<br />
successioni s<strong>ed</strong>imentarie affioranti in Sicilia settentrionale può<br />
essere ricostruita con l’analisi strutturale <strong>del</strong>le <strong>di</strong>verse<br />
popolazioni <strong>di</strong> faglie e <strong>del</strong>le loro reciproche relazioni <strong>di</strong><br />
sovraimposizione.<br />
La ripetuta sovrapposizione <strong>di</strong> strutture tettoniche, che si<br />
realizza a partire dal Mesozoico fino al Pleistocene, è <strong>di</strong><br />
<strong>di</strong>fficile comprensione in quanto esse esprimono sia i processi<br />
deformativi pre-orogenici mesozoici che quelli collisionali <strong>del</strong><br />
Terziario sup. e neotettonici.<br />
Ci si propone così <strong>di</strong> descrivere l’attuale assetto strutturale<br />
raggiunto dalle Maghrebi<strong>di</strong> siciliane al fine <strong>di</strong> ricostriure la<br />
cronologia e la tipologia <strong>del</strong>le deformazioni che si sono<br />
susseguite dal Mesozoico fino ai tempi recenti in Sicilia centrosettentrionale.<br />
L’evoluzione geologica <strong>di</strong> questo settore <strong>di</strong> catena appare<br />
scan<strong>di</strong>ta da almeno sette eventi deformativi che, a partire dai<br />
più antichi, sono espressi da:<br />
1) faglie estensionali/trascorrenti (Trias sup.-Giurassico), che<br />
possono rappresentare le fasi <strong>di</strong> rifting <strong>del</strong>la placca africana<br />
e l’evoluzione <strong>del</strong> suo margine passivo settentrionale;<br />
2) faglie estensionali (pre-Paleogene), che possono<br />
_________________________<br />
(*) Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia-sezione <strong>di</strong> Palermo, via<br />
Ugo la Malfa 153, 90146, Palermo.<br />
(°) Dipartimento <strong>di</strong> Geologia e Geodesia <strong>del</strong>l’Università, via Archirafi 22,<br />
90123, Palermo<br />
rappresentare lo sta<strong>di</strong>o tar<strong>di</strong>vo <strong>del</strong>l’inversione cinematica<br />
tetidea, e che in Sicilia coincide con l’apertura <strong>del</strong> Bacino<br />
Sicilide;<br />
3) pieghe e sovrascorrimenti (Miocene), espressione <strong>del</strong>la<br />
deformazione collisionale <strong>del</strong>l’antico margine passivo, alle<br />
quali si associano deformazioni estensionali sin-collisionali;<br />
4) faglie estensionali sin-collisionali (Miocene inf.), connesse<br />
con la cinematica <strong>di</strong> formazione <strong>del</strong>le pieghe e<br />
sovrascorrimenti;<br />
5) faglie estensionali listriche (Miocene sup.), probabilmente<br />
connesse con l’avvenuto inequilibrio meccanico <strong>del</strong>la<br />
catena in via <strong>di</strong> formazione;<br />
6) faglie inverse e sovrascorrimenti fuori sequenza (post-<br />
Pliocene inf.), probabilmente connesse con il ripristino <strong>del</strong>le<br />
con<strong>di</strong>zioni <strong>di</strong> equilibrio meccanico <strong>del</strong> cuneo collisionale<br />
7) faglie a pr<strong>ed</strong>ominante carattere trascorrente (Plio-<br />
Pleistocene), connesse con lo sviluppo <strong>del</strong> margine tirrenico<br />
meri<strong>di</strong>onale.<br />
ASSETTO GEOLOGICO-STRUTTURALE GENERALE<br />
L’<strong>ed</strong>ificio tettonico siciliano rappresenta un segmento <strong>del</strong>la<br />
catena appenninico-maghrebide (SCANDONE et al., 1974;<br />
Fig. 1 - schema strutturale <strong>del</strong>la Sicilia.
DEFORMAZIONI MESOZOICO-TERZIARIE IN SICILIA CENTRO-SETTENTRIONALE<br />
CATALANO & D’ARGENIO, 1982). Le unità <strong>del</strong>la catena<br />
affiorano in Sicilia settentrionale, mentre nella Sicilia centroorientale<br />
e meri<strong>di</strong>onale affiorano estesamente i depositi <strong>di</strong><br />
avanfossa deformata miocenico-quaternari. Infine, in Sicilia<br />
occidentale e sud-orientale affiorano porzioni <strong>di</strong> avampaese che<br />
registrano rispettivamente blande deformazioni contrazionali <strong>ed</strong><br />
estensionali (Fig. 1).<br />
Le deformazioni pre-orogeniche sono prevalentemente<br />
riconoscibili entro le successioni <strong>di</strong> piattaforma carbonatica e<br />
pelagiche affioranti in Sicilia (NIGRO & RENDA, 2002-2005).<br />
Le deformazioni collisionali, con le quali si sono messe in<br />
posto le unità <strong>del</strong>la catena avviene, a partire dall’Oligocene e<br />
prosegue per tutto il Miocene fino al Pleistocene inf.,<br />
coinvolgendo anche le successioni plio-pleistoceniche<br />
d’avanfossa (NIGRO & RENDA, 2000). Le unità <strong>del</strong>la catena<br />
affiorano con una polarità che v<strong>ed</strong>e le porzioni<br />
geometricamente più profonde in Sicilia occidentale e quelle<br />
più elevate in Sicilia nord-orientale. Durante la costruzione<br />
<strong>del</strong>la catena si sono attivate, contemporaneamente alle strutture<br />
contrazionali nei settori frontali, sistemi <strong>di</strong> faglie <strong>di</strong>rette a basso<br />
angolo nei settori geometricamente più elevati <strong>del</strong> prisma<br />
tetonico. I processi <strong>di</strong> estensione si susseguono a partire dal<br />
Miocene sup. e si alternano alla <strong>di</strong>namica contrazionale, così<br />
come evidenziato dalla progressione <strong>del</strong>le deformazioni<br />
registrata dalle unità <strong>del</strong>la catena (NIGRO & RENDA, 2001).<br />
L’assottigliamento <strong>del</strong>la catena in via <strong>di</strong> costruzione<br />
avviene soprattutto attraverso il riutilizzo <strong>di</strong> prec<strong>ed</strong>enti<br />
superfici <strong>di</strong> scorrimento contrazionali. L’inversione negativa <strong>di</strong><br />
queste superfici determina un assottigliamento crostale che<br />
potrebbe essere alla base dei meccanismi embrionali <strong>di</strong> apertura<br />
<strong>del</strong> bacino tirrenico meri<strong>di</strong>onale (GIUNTA et al., 2000a), che<br />
evolvono ad un zona <strong>di</strong> taglio destro crostale durante il Plio-<br />
Pleistocene (GIUNTA et al., 2000b; RENDA et al., 2000).<br />
QUADRO TETTONICO E LITOSTRATIGRAFICO<br />
LOCALE<br />
In Sicilia centro-settentrionale (Monti Nebro<strong>di</strong> e Madonie)<br />
affiorano sia le successioni carbonatiche deformate mesozoiche<br />
<strong>di</strong> piattaforma carbonatica Panormide e <strong>di</strong> bacino pelagico<br />
Imerese-Sicano e Sicilide che quelle neogeniche <strong>del</strong>l’avanfossa<br />
numi<strong>di</strong>ca. Le unità tettoniche sono state messe in posto secondo<br />
una progressione <strong>del</strong>le deformazioni che è proc<strong>ed</strong>uta dai settori<br />
più interni verso quelli esterni. In particolare, a partire dalle<br />
unità tettoniche geometricamente più profonde, affiorano:<br />
DOMINIO PANORMIDE<br />
- dolomie e calcari dolomitici (Trias sup.-Lias);<br />
- calcilutiti, calcari marnosi e marne (Cretaceo sup.-<br />
Eocene);<br />
FLYSCH NUMIDICO (TRE MEMBRI)<br />
- argille con intercalazioni <strong>di</strong> quarzareniti e calcari marnosi<br />
(Flysch Numi<strong>di</strong>co “Unità <strong>di</strong> Nicosia”, Dominio Sicilide,<br />
Oligocene sup.- Miocene inf.);<br />
- argille con intercalazioni <strong>di</strong> livelli arenacei e quarzarenitici<br />
e megabrecce carbonatiche (“Flysch <strong>di</strong> Portella<br />
Mandarini”, Dominio Panormide, Oligocene sup.-<br />
Miocene inf.);<br />
149<br />
- argille brune e livelli arenitici con intercalazioni <strong>di</strong> banchi<br />
quarzarenitici (“Flysch Numi<strong>di</strong>co S.S.”, Dominio Imerese,<br />
Oligocene sup.-Miocene inf.).<br />
DOMINIO SICILIDE<br />
- argille e argille marnose (Flysch <strong>di</strong> Monte Soro,<br />
Cretaceo);<br />
- argille (Argille Varicolori, Cretaceo sup.-Oligocene inf.);<br />
- calcari marnosi, marne e argille marnose (Fm. Polizzi,<br />
Eocene);<br />
- arenarie tufitiche e marne (“Tufiti <strong>di</strong> Tusa”, Oligocene<br />
sup.-Miocene inf.);<br />
DEPOSITI TARDOROGENI<br />
- arenarie con intercalazioni <strong>di</strong> argille marnose (Flysch <strong>di</strong><br />
Reitano, Bur<strong>di</strong>galiano-Serravalliano);<br />
- calcareniti e calciru<strong>di</strong>ti (“Calcareniti <strong>di</strong> Rocca D’Armi”,<br />
Bur<strong>di</strong>galiano Superiore-Serravalliano);<br />
- Conglomerati poligenici (“Conglomerati <strong>di</strong> Caronia”,<br />
Bur<strong>di</strong>galiano sup.-Tortoniano inf.);<br />
- calcareniti, arenarie bioclastiche e bioliti algali (“Arenarie<br />
<strong>di</strong> Gangi” e “Calcareniti <strong>di</strong> Rocca Mercadante”,<br />
Langhiano-Serravalliano);<br />
- argille con intercalazioni <strong>di</strong> sabbie e conglomerati<br />
poligenici (Fm. Castellana, Serravalliano-Tortoniano);<br />
- argille, sabbie e conglomerati (Fm. Terravecchia,<br />
Tortoniano-Messiniano inf.);<br />
- calcari organogeni e bioclastici (Fm. Baucina,<br />
Messiniano);<br />
- calcari e calcari dolomitici, gessi e gessareniti (Serie<br />
Gessoso-Solfifera, Messiniano sup.);<br />
- conglomerati poligenici (Messiniano sup.);<br />
- calcari marnosi (“Trubi”, Pliocene inf.).<br />
QUADRO STRUTTURALE<br />
L’analisi strutturale <strong>del</strong>le popolazioni <strong>di</strong> faglie ha permesso<br />
<strong>di</strong> <strong>di</strong>stinguere <strong>di</strong>verse fasi tettoniche pre-, sin- e postorogeniche<br />
(cfr. cronologia <strong>del</strong>le deformazioni riportata in alto<br />
a sinistra <strong>di</strong> Fig. 2), i cui rispettivi esempi sono riportati nella<br />
stessa Fig. 2):<br />
FASE TETTONICA TRANSTENSIVA (I): sistemi <strong>di</strong> faglie<br />
<strong>di</strong>rette e trascorrenti (pre-Cretaceo sup., Fig. 2A1-2A2);<br />
FASE TETTONICA ESTENSIONALE “PRE-NUMIDICA” (II):<br />
sistemi <strong>di</strong> faglie estensionali (pre-Oligocene sup., Fig. 2B);<br />
FASE TETTONICA SIN-SEDIMENTARIA “NUMIDICA” (III):<br />
faglie <strong>di</strong>rette sin-s<strong>ed</strong>imentarie (Oligocene-Miocene inf., Fig.<br />
2C);<br />
FASE TETTONICA COMPRESSIVA (IV): pieghe e<br />
sovrascorrimenti (Miocene inf., Fig. 2D);<br />
FASE TETTONICA ESTENSIONALE (V): faglie <strong>di</strong>rette<br />
listriche (Miocene sup., Fig. 2E1-2E2);<br />
FASE TETTONICA COMPRESSIVA (VI): sovrascorrimenti e<br />
faglie inverse (post-Pliocene inf., Fig. 2F);<br />
FASE TETTONICA TRASCORRENTE (VII): sistemi <strong>di</strong> faglie<br />
trascorrenti e <strong>di</strong>rette (Plio-Pleistocene, Fig. 2G).
150 F. NIGRO ET ALII<br />
Fig. 2 – esempi <strong>di</strong> faglie e sovrascorrimenti riferibili alle fasi tettoniche I-VII descritte. Transtensione mesozoica (A1-A2), estensione pre-oligocenica (B),<br />
estensione sin-collisionale oligo-miocenica (C) e contemporanea compressione (D), estensione post-collisionale alto-miocenica (E1-E2), compressione fuorisequenza<br />
pliocenica (F1-F2) e trascorrenza plio-pleistocenica (G). Sc) Creta sup.-Eocene; Fn) Oligo-Miocene inf.; Tv) Tortoniano sup., Tb) Pliocene inf.
DEFORMAZIONI MESOZOICO-TERZIARIE IN SICILIA CENTRO-SETTENTRIONALE<br />
DISCUSSIONE<br />
La cronologia <strong>del</strong>le deformazioni descritta esprime<br />
l’evoluzione paleotettonica <strong>del</strong>le Maghrebi<strong>di</strong> siciliane<br />
collegabile con i complessi processi geo<strong>di</strong>namici <strong>del</strong>la placca<br />
africana.<br />
Un tentativo <strong>di</strong> regionalizzazione <strong>del</strong>la cronologia <strong>di</strong><br />
deformazioni ricostruita attraverso l’analisi strutturale in Sicilia<br />
centro-settentrionale conduce a <strong>del</strong>le considerazioni relative<br />
all’evoluzione tettonica <strong>del</strong>l’intera area siciliana.<br />
Quest’evoluzione può essere sud<strong>di</strong>visa in tre momenti<br />
principali, durante i quali hanno prevalso tipologie deformative<br />
<strong>di</strong>fferenti: 1) deformazioni pre-orogeniche legate alle fasi<br />
<strong>di</strong>stensivo-transtensive che caratterizzano l'apertura <strong>del</strong>la<br />
Neotetide, 2) deformazioni compressive sin-orogeniche cui<br />
prevalgono processi <strong>di</strong> inversione tettonica e costruzione <strong>del</strong>la<br />
catena legati alle fasi <strong>di</strong> collisione continentale e 3)<br />
deformazioni estensionali e trascorrenti legate rispettivamente<br />
alle fasi tar<strong>di</strong>ve <strong>di</strong> costruzione <strong>del</strong>la catena siciliana e alla<br />
<strong>di</strong>namica <strong>di</strong> formazione <strong>del</strong> bacino Tirrenico.<br />
A partire dal Trias sup. si sono esplicate deformazioni<br />
estensionali/transtensionali che rappresentano probabilmente<br />
gli effetti <strong>del</strong>la <strong>di</strong>namica <strong>del</strong>la Mesogea ionica e <strong>del</strong>l’a<strong>di</strong>acente<br />
Neotetide (GIUNTA et al., 2003).<br />
L’assetto pre-orogenico assunto progressivamente dalle<br />
Maghrebi<strong>di</strong> Siciliane durante il Mesozoico può essere<br />
sintetizzato in un frastagliato e <strong>di</strong>scontinuo sistema <strong>di</strong><br />
piattaforme carbonatiche <strong>ed</strong> interposti bacini pelagici, i cui<br />
margini sono stati controllati dall’attività <strong>di</strong> faglie <strong>di</strong>stensivotranstensive.<br />
L’attività <strong>di</strong> queste faglie (fasi I e II) ha<br />
determinato la subsidenza dei bacini pelagici e la riduzione<br />
<strong>del</strong>le piattaforme carbonatiche per tutto il Mesozoico <strong>ed</strong> il<br />
Terziario inf. (NIGRO & RENDA, 1999).<br />
L'evoluzione tettono-s<strong>ed</strong>imentaria sin-collisionale dei<br />
domini <strong>di</strong> avanfossa che si sono in<strong>di</strong>viduati a partire<br />
dall'Oligocene è stata con<strong>di</strong>zionata dalla <strong>di</strong>stribuzione, dalla<br />
fisiografia e dalle caratteristiche paleotettoniche dei domini<br />
paleogeografici preesistenti.<br />
Nell’intervallo Oligocene sup.-Miocene, i processi <strong>di</strong><br />
inversione tettonica da estensionale a compressiva, imprimono<br />
un drastico cambiamento nei processi s<strong>ed</strong>imentari (NIGRO &<br />
RENDA, 2000). Durante questo momento prevale l’attività <strong>di</strong><br />
faglie inverse e sovrascorrimenti cui si associano faglie <strong>di</strong>rette<br />
sin-s<strong>ed</strong>imentarie che deformano i depositi <strong>di</strong> avanfossa e che si<br />
attivano in concomitanza alla nucleazione <strong>ed</strong> amplificazione <strong>di</strong><br />
pieghe (NIGRO & RENDA, 2005; NIGRO et al., 2008, fasi III e<br />
IV).<br />
Durante la fase tar<strong>di</strong>va <strong>di</strong> costruzione <strong>del</strong>la catena si sono<br />
attivate, contemporaneamente alle strutture contrazionali nei<br />
settori frontali, sistemi <strong>di</strong> faglie <strong>di</strong>rette nei settori<br />
geometricamente più elevati <strong>del</strong> prisma tettonico (fase V).<br />
Questi processi <strong>di</strong> estensione si susseguono a partire dal<br />
Miocene sup. e si alternano alla <strong>di</strong>namica contrazionale (fase<br />
VI), così come evidenzia la progressione <strong>del</strong>le deformazioni<br />
registrata dalle unità <strong>del</strong>la catena (NIGRO & RENDA, 2001).<br />
L’assottigliamento <strong>del</strong>la catena in via <strong>di</strong> costruzione<br />
avviene soprattutto attraverso il riutilizzo <strong>di</strong> prec<strong>ed</strong>enti<br />
superfici <strong>di</strong> scorrimento contrazionali (NIGRO & RENDA, 2004).<br />
L’inversione negativa <strong>di</strong> queste superfici determina<br />
complessivamente un assottigliamento crostale che potrebbe<br />
151<br />
essere relazionata ai meccanismi <strong>di</strong> apertura <strong>del</strong> bacino<br />
tirrenico meri<strong>di</strong>onale (GIUNTA et al., 2000a) e che evolvono ad<br />
un zona <strong>di</strong> taglio destro crostale durante il Plio-Pleistocene, i<br />
cui effetti sono rappresentati dai sistemi <strong>di</strong> faglie trascorrenti<br />
<strong>del</strong>la fase VII (GIUNTA et al., 2000b; RENDA et al., 2000).<br />
Le strutture trascorrenti neotettoniche hanno determinato<br />
l’attuale configurazione <strong>del</strong>la catena. Di queste alcuni segmenti<br />
sono attivi e lungo <strong>di</strong> esse si <strong>di</strong>stribuiscono epicentri <strong>di</strong><br />
terremoti e sorgenti termali.<br />
BIBLIOGRAFIA<br />
CATALANO R. & D'ARGENIO B. (1982) - Schema geologico<br />
<strong>del</strong>la Sicilia. In: Catalano R. & D'Argenio B. (Eds.), "Guida<br />
alla Geologia <strong>del</strong>la Sicilia Occidentale”, Guide Geologiche<br />
Regionali, Mem. Soc. Geol. It., Suppl. A., 24, 9-41.<br />
GIUNTA G., NIGRO F. & RENDA P. (2000a) - Extensional<br />
Tectonics during Maghrebides Chain Buil<strong>di</strong>ng since Late<br />
Miocene: examples from Northern Sicily. A.S.G.P., 70, 1-<br />
18.<br />
GIUNTA G., NIGRO F., RENDA P. & GIORGIANNI A. (2000b) -<br />
The Sicilian-Maghrebides Tyrrhenian Margin: a<br />
neotectonic evolutionary mo<strong>del</strong>. Boll. Soc. Geol. It., 119,<br />
553-565.<br />
GIUNTA G., NIGRO F. & RENDA P. (2003) - The Mesozoic<br />
continental break-up geometry of the Sicilian Tunisian<br />
Meghrebides: an updat<strong>ed</strong> mo<strong>del</strong>. In: Salem M. J., Oun K.<br />
M. & S<strong>ed</strong><strong>di</strong>q H. M. (<strong>ed</strong>s), “The Geology of Northwest<br />
Libya”, S<strong>ed</strong>imentary basins of Libya, III, 156-169.<br />
NIGRO F. & RENDA P. (1999) – Evoluzione geologica <strong>ed</strong><br />
assetto strutturale <strong>del</strong>la Sicilia centro-settentrionale. Boll.<br />
Soc. Geol. It., 118, 375-388.<br />
NIGRO F. & RENDA P. (2000) - Un mo<strong>del</strong>lo <strong>di</strong> evoluzione<br />
tettono-s<strong>ed</strong>imentaria <strong>del</strong>l’avanfossa neogenica siciliana.<br />
Boll. Soc. Geol. It., 219, 667-686.<br />
NIGRO F. & RENDA P. (2001) - Late Miocene-Quaternary<br />
stratigraphic record in the Sicilian Belt (Central<br />
M<strong>ed</strong>iterranean): tectonics versus eustasy. Boll. Soc. Geol.<br />
It., 120, 151-164.<br />
NIGRO F. E RENDA P. (2002) - From Mesozoic extension to<br />
Tertiary collision: deformation pattern in the northern<br />
Sicily chain units. Boll. Soc. Geol. It., 121, 87-97.<br />
NIGRO F. & RENDA P. (2004) - The contribution of the preexisting<br />
structures in mountain belt evolution: the example<br />
of the negative inversion in Northern Sicily. Boll. Soc.<br />
Geol. It., 123, 175-187.<br />
NIGRO F. & RENDA P. (2005) - Transtensional/extensional fault<br />
activity from the Mesozoic rifting to Tertiary chain buil<strong>di</strong>ng<br />
in Northern Sicily (Central M<strong>ed</strong>iterranean). Geol.<br />
Carpathica., 56, 255-271.
152 F. NIGRO ET ALII<br />
NIGRO F., SALVAGGIO G., RENDA P. & FAVARA R. (20008) -<br />
Extensional deformations during Neogene chain buil<strong>di</strong>ng in<br />
Sicily. Boll. Soc. Geol. It., in stampa.<br />
RENDA P., TAVARNELLI E., TRAMUTOLI M. & GUEGUEN E.<br />
(2000) - Neogene deformations in the central Madonie<br />
Mountains (northern Sicily, Italy) and their significance in<br />
the geodynamic evolution of the southern Tyrrhenian Sea<br />
margin. Mem. Soc. Geol. It., 55, 38-47.<br />
SCANDONE P., GIUNTA G. & LIGUORI V. (1974) - The<br />
connection between Apulia and Sahara continental margins<br />
in the Southern Apennines and in Sicily. Mem. Soc. Geol.<br />
It., 13, 317-323.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 153-156, 1f.<br />
Neotectonic uplift and tilting of crustal blocks in Northern Sicily<br />
NIGRO F.°, FAVARA R.°, RENDA P.* , ° , SALVAGGIO G. (*), ARISCO G. + & PERRICONE M. #<br />
RIASSUNTO<br />
Sollevamento neotettonico e basculamento <strong>di</strong> blocchi crostali in Sicilia<br />
settentrionale<br />
La Sicilia settentrionale è caratterizzata da un’intensa attività sismica che<br />
è espressione <strong>di</strong> deformazioni attive. In questo settore <strong>del</strong>l’Isola affiora il<br />
nucleo <strong>del</strong>la catena neogenica <strong>del</strong>le Magrebi<strong>di</strong> occidentali, che è sottoposto ad<br />
intenso sollevamento durante il Plio-Pleistocene. Le deformazioni più recenti<br />
sono, in gran parte, rappresentate da sistemi <strong>di</strong> faglie estensionali e<br />
trascorrenti. Il tasso <strong>di</strong> sollevamento non è uniforme, così come suggerisce la<br />
<strong>di</strong>fferente elevazione dei depositi <strong>di</strong> questo periodo. Essi affiorano lungo il<br />
settore costiero settentrionale e la loro quota decresce complessivamente<br />
dall’estremità nord-est a quella nord-ovest <strong>del</strong>la Sicilia.<br />
Le deformazioni più recenti possono essere riconosciute sia attraverso<br />
l’analisi strutturale che tramite quella morfometrica.<br />
Lo stu<strong>di</strong>o integrato <strong>di</strong> queste due metodologie qui presentato è stato<br />
realizzato anche attraverso l’elaborazione informatizzata <strong>del</strong> mo<strong>del</strong>lo <strong>di</strong><br />
elevazione <strong>di</strong>gitale <strong>del</strong>la superficie topografica (DEM).<br />
I dati strutturali e morfometrici in<strong>di</strong>cano che in Sicilia settentrionale vi è<br />
una stretta relazione tra l’attività <strong>del</strong>le faglie neotettoniche e le forme dei<br />
rilievi. Il loro confronto con alcune caratteristiche idrografiche, con la<br />
sismicità e con la <strong>di</strong>stribuzione dei tassi <strong>di</strong> sollevamento suggerisce che la<br />
catena siciliana settentrionale risulta segmentata da sistemi <strong>di</strong> taglio ra<strong>di</strong>cati<br />
che perimetrano blocchi crostali.<br />
In particolare, la variazione <strong>di</strong> alcuni parametri morfometrici hanno<br />
permesso <strong>di</strong> identificare <strong>di</strong>stinti settori <strong>di</strong> catena omogenei, ciascuno dei quali<br />
è soggetto a <strong>di</strong>fferenti tassi <strong>di</strong> sollevamento e <strong>di</strong> <strong>di</strong>rezioni <strong>di</strong> basculamento.<br />
Key words: neotectonic faults, morphometric pattern, uplift,<br />
crustal blocks, Sicily).<br />
INTRODUCTION<br />
Ground tilting phenomena are a characteristic of all tectonic<br />
processes. For instance, they occur during extensional<br />
deformations, during the activation of listric/domino faults or<br />
during contractional deformations that are characteris<strong>ed</strong> by<br />
fold-nucleation and amplification. Furthermore, ground tilting<br />
accompanies late-orogenic processes, when the isostatic reequilibrium<br />
of mountain chains often occurs. Non-uniform<br />
ground tilting can be identifi<strong>ed</strong> by crustal blocks having<br />
<strong>di</strong>fferent uplift and tilting rates.<br />
Drainage basins provide insights into the long-term<br />
evolution of the landscape. Basin geometry develops in<br />
response to: 1) the nature and <strong>di</strong>stribution of uplifts and<br />
subsidences; 2) the spatial arrangement of faults or joints; 3)<br />
° Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia, <strong>Sezione</strong> <strong>di</strong> Palermo, via U.<br />
La Malfa n. 153, 90146, Palermo<br />
* Dipartimento <strong>di</strong> Geologia e Geodesia <strong>del</strong>l’Università, via Archirafi n. 22,<br />
90123, Palermo<br />
+<br />
via Lanza <strong>di</strong> Scalea n. 414, 90100, Palermo<br />
#<br />
via Eurialo n. 32, 90151, Palermo<br />
the relative resistance of <strong>di</strong>fferent rock types; 4) climatically<br />
influenc<strong>ed</strong> hydrologic parameters.<br />
Active tectonics plays an important role in controlling<br />
drainage networks through the change in channel slops of<br />
fluvial systems and in<strong>di</strong>viduating preferential orientations both<br />
for stream development and for the incision of faults and<br />
fractures.<br />
Active tectonics causes the uplift and tilting of crustal<br />
blocks and influences the development and shifting of drainage<br />
networks over time.<br />
Worldwide researches regar<strong>di</strong>ng the relationships between<br />
active tectonics and morphology suggest that rivers respond<br />
<strong>di</strong>fferently in two <strong>di</strong>fferent types of scenario: longitu<strong>di</strong>nal<br />
tilting and lateral tilting.<br />
COX (1994) describes a new technique of morphometric<br />
investigation for the appraisal of ground motion relat<strong>ed</strong> to faultblock<br />
tilting. The method is bas<strong>ed</strong> on the tendency towards<br />
lateral migration over time of streams under the influence of<br />
external tectonic forces.<br />
Sicily is locat<strong>ed</strong> in the Central M<strong>ed</strong>iterranean and<br />
constitutes an emerg<strong>ed</strong> segment of the Maghrebide chain. Its<br />
geologic history is characteriz<strong>ed</strong> by repeat<strong>ed</strong> tectonic events<br />
relat<strong>ed</strong> to extensional and contractional geodynamic processes<br />
that have been developing since the Mesozoic.<br />
Uplift and tilting processes were intense during the Plio-<br />
Pleistocene, as suggest<strong>ed</strong> by the marine deposits that wi<strong>del</strong>y<br />
outcrop at <strong>di</strong>fferent elevations.<br />
The uplift rate is not uniform, which suggests that this<br />
segment of the chain may be <strong>di</strong>vid<strong>ed</strong> into separate blocks.<br />
An integrat<strong>ed</strong> structural and morphometric investigation to<br />
<strong>del</strong>ineate seismo-tectonic zoning is presently in progress, the<br />
aim of which is to try to understand the recent/active tectonics<br />
of Northern Sicily. This paper describes some of the results<br />
regar<strong>di</strong>ng the neotectonic structural setting of Northern Sicily,<br />
and has been integrat<strong>ed</strong> with morphometric elaborations of<br />
landforms d<strong>ed</strong>uc<strong>ed</strong> from a <strong>di</strong>gital elevation mo<strong>del</strong> (DEM).<br />
The structural characteristics of the main neotectonic faults<br />
systems will be describ<strong>ed</strong> below. Their positions and<br />
orientations are been compar<strong>ed</strong> with those of the main<br />
morphostructures and with the <strong>di</strong>stribution of the uplift rates<br />
and of the earthquakes. Have been also carri<strong>ed</strong> out an analysis<br />
of the morphometric pattern to understand the influence of the<br />
neotectonic faults activities on the hydrographic network.<br />
The results suggest that Northern Sicily is compos<strong>ed</strong> of<br />
separate crustal blocks each of which has its own uplift rate and<br />
tilting <strong>di</strong>rection.
154 NIGRO F. ET ALII<br />
PHYSIOGRAPHIC, NEOTETTONIC, UPLIFT,<br />
SEISMICITY, MORPHOTECTONIC AND<br />
MORPHOMETRIC SETTINGS<br />
In the northern sector of Sicily there is a group of<br />
mountains, which, from west to east, is known as: Trapani-S.<br />
Vito, Palermo, Trabia-Termini Imerese, Madonie, Nebro<strong>di</strong> and<br />
Peloritani Mts. This mountain range is 20-30 km wide. Starting<br />
from the Northern Sicilian coast, there is a general abrupt<br />
increase in the altitude, which goes from 0 to 500 or 700 m<br />
above sea level in less than 1-2 km, while the foot of the scarp<br />
is a narrow littoral zone. The mountainous groups are separat<strong>ed</strong><br />
by significant neotectonic features.<br />
The <strong>di</strong>fferent mean altitudes of the mountainous groups can<br />
suggest that they are subject to <strong>di</strong>fferent uplift rates and that<br />
they constitute the shallow portion of crustal blocks <strong>del</strong>imit<strong>ed</strong><br />
by deep-seat<strong>ed</strong> faults systems.<br />
Plio-Pleistocene transcurrent faults relat<strong>ed</strong> to the Southern<br />
Tyrrhenian margin evolution wi<strong>del</strong>y outcrop in Northern Sicily<br />
(GIUNTA et al. 2000).<br />
Three strike-slip fault systems occur in Northern Sicily: (a)<br />
NW-SE tren<strong>di</strong>ng dextral strike-slip faults; (b) NE-SW tren<strong>di</strong>ng<br />
sinistral strike-slip faults; (c) W-E tren<strong>di</strong>ng dextral strike-slip<br />
faults. The NW-SE tren<strong>di</strong>ng system frequently gains<br />
considerable normal slip components. The NE-SW and W-E<br />
tren<strong>di</strong>ng systems gain reverse slip components.<br />
The main transcurrent faults systems are locat<strong>ed</strong> along the<br />
slopes of the mountainous groups.<br />
These faults and fault zones are 20-40 km long and from a<br />
few hundr<strong>ed</strong> meters to a few kilometres wide (Fig. 1A). Shear<br />
zones consist of several sub-parallel to high-angle intersecting<br />
fault segments. Outlines of faults are very obvious and<br />
continuous in morphology. They <strong>di</strong>splay a series of welldevelop<strong>ed</strong><br />
and well-preserv<strong>ed</strong> strong morphologic evidence<br />
that in<strong>di</strong>cates a trace, strike-slip nature and recent fault activity.<br />
Some of them are fault scarps, triangular facets, break in slopes<br />
or deep, long linear valleys.<br />
All of the fault segments that constitute the main<br />
neotectonic structures of Northern Sicily <strong>di</strong>p steeply and form<br />
generally conjugat<strong>ed</strong> sets. Strikes of right-lateral faults range<br />
from N270° to N330° (with conjugate fault planes). Left-lateral<br />
strike-slip faults range from N10° to N60° (with conjugate fault<br />
planes). The average azimuth of the maximum compressive<br />
stress (σ1) is therefore inferr<strong>ed</strong> to have been about NNW-SSE<br />
in <strong>di</strong>rection.<br />
The present-day elevation of the Pliocene to upper<br />
Pleistocene deposits suggests that the Northen Sicily coastal<br />
range underwent a strong non-uniform neotectonic uplift<br />
(ANTONIOLI et al., 2004 and references therein). The<br />
correlations of MIS 5.5 sites identify areas of rapid uplift in<br />
North-East Sicily (Nebro<strong>di</strong> and Peloritani Mts), with a slower<br />
uplift rate in the central-north sector (Madonie Mts) and<br />
relative stability in North-West Sicily (Palermo and Trapani<br />
Mts). From west (Trapani Mts.) to east (Peloritani Mts.) the<br />
overall uplift rate increases from about 0.2 mm/yr to up to 1<br />
mm/yr. In particular, the highest late Pleistocene uplift rate<br />
characterises the Ionian side of the Peloritani Mts. (up to 1<br />
mm/yr).<br />
As a consequence of the effect of strike-slip kinematics, the<br />
Southern Tyrrhenian Sea is characteris<strong>ed</strong> by intense seismicity<br />
of low mean magnitude (GIUNTA et al., 2004). Shallow<br />
seismicity of a magnitude of up to 4.5 is clear evidence of<br />
transpressional deformations and is locat<strong>ed</strong> imm<strong>ed</strong>iately<br />
offshore the entire northern coast of Sicily.<br />
On land, seismicity affects the Palermo Mts., the Madonie<br />
Mts and most of Eastern Sicily. In the Peloritani Mts the fault<br />
plane solutions available for some of the crustal events show<br />
that the deformation pattern along the main faults is consistent<br />
with an ESE-WNW orient<strong>ed</strong> extension.<br />
There is a coherence among the kinematics characteristics<br />
of the previous describ<strong>ed</strong> faults systems and those inferr<strong>ed</strong> by<br />
the focal mechanisms of the earthquakes occurr<strong>ed</strong> in such shear<br />
zones.<br />
This suggests that the describ<strong>ed</strong> neotectonic faults systems<br />
are part of the seismogenic volume of northern Sicily. This can<br />
demonstrate that the activity of the neotectonic faults which<br />
bound the slopes of the mountainous groups is also reveal<strong>ed</strong> by<br />
the uplift rates, that are <strong>di</strong>versifi<strong>ed</strong> along northern Sicily but<br />
around uniforms within every sector defin<strong>ed</strong> from the main<br />
faults systems.<br />
In surface, along the neotectonic deformation bands are also<br />
recognizable a lot of morphostructural elements that in<strong>di</strong>rectly<br />
defines the minor faults network presents within the each main<br />
systems.<br />
Along the coastal areas of Northern Sicily, N-S to NW-SE<br />
tren<strong>di</strong>ng morphostructural highs are recognizable, which are<br />
closely relat<strong>ed</strong> to strike-slip faults. They are interpos<strong>ed</strong> on<br />
coastal plains, fill<strong>ed</strong> with Plio-Pleistocene marine deposits.<br />
Many fluvial channels show youthful characteristics, such<br />
as rectilinear valleys. These segments appear to be <strong>di</strong>ssect<strong>ed</strong><br />
and connect<strong>ed</strong> by fluvial elbows. The valleys are also often<br />
incis<strong>ed</strong> down the middle-upper portion of the rivers. The<br />
straight valleys are well develop<strong>ed</strong>, thereby characterising all<br />
the sectors of the drainage basins (headwaters, middle valleys<br />
and mouths).<br />
Other evident morphological characteristics relat<strong>ed</strong> to<br />
recent tectonic activity are represent<strong>ed</strong> by the plano-altimetric<br />
<strong>di</strong>scontinuities of the ridges and crests. The main<br />
morphostructures compete to the definition of the amplitude of<br />
the reliefs. The max amplitude values of the reliefs coincide<br />
with the fault zones mapp<strong>ed</strong> suggests that the activity of<br />
neotectonic faults is strictly relat<strong>ed</strong> to recent landform<br />
evolution.<br />
The main rivers are generally orient<strong>ed</strong> from 320°-330°<br />
(NW) to 20°-30° (NE).<br />
The first-order streams also show two main peaks, orient<strong>ed</strong><br />
NE-SW and NW-SE. In some places they swing towards ENE-<br />
WSW and WNW-ESE respectively. Instead, the orientation of<br />
the second-order streams swing more frequently from the two<br />
main peaks (i.e. NW-SE and NE-SW towards NNW-SSE and<br />
NNE-SSW) respectively. The NNW-SSE and NNE-SSW<br />
orientations are well represent<strong>ed</strong> in the third- fourth, fifth- and<br />
sixth-order stream networks.
NEOTECTONIC DEFORMATIONS IN NORTHEST SICILY<br />
Fig. 1 - (A) Schematic structural map of Northern Sicily and locations of the main neotectonic faults. Stereonets show the isodensities of fault poles of the<br />
neotectonic faults which form the shear zones. 1) Etna lavas, 2) Plio-Quaternary deposits, 3) Upper Miocene for<strong>ed</strong>eep deposits, 4) Hyblean-Pelagian, Panormide<br />
and Imerese-Sicanian tectonic units, 5) Sicili<strong>di</strong> tectonic units, 6) Peloritani tectonic units, 7) main neotectonic faults. (B) index map and stereonets of the minor<br />
faults sampl<strong>ed</strong> between the main fault zones. (B) Lateral shift tendency of the main rivers of Northern Sicily elaborat<strong>ed</strong> using Cox’ method (1994). The length<br />
of the vectors is proportional to the magnitude of the process. For each drainage basin the asymmetric factor (AF) is also in<strong>di</strong>cat<strong>ed</strong>. (C) graph in which the<br />
elevations of the MIS 5.5 markers and the main neotectonic faults outcropping in the coastal sectors of Northern Sicily are report<strong>ed</strong>. The comparison between<br />
the non-uniform elevat<strong>ed</strong> MIS 5.5 markers, the kinematic characteristics of the fault zones and the lateral shift tendency of the main rivers have allow<strong>ed</strong> us to<br />
identify several crustal blocks which may <strong>di</strong>vide the Northern Sicily Chain, shown in map view (D).<br />
155
156 NIGRO F. ET ALII<br />
The comparison between the fault network and the fluvial<br />
steps suggests that the recent fault activity in Northern Sicily<br />
strictly controls the drainage pattern and, as a consequence,<br />
landform evolution. The fluvial steps are very well develop<strong>ed</strong><br />
in NE Sicily but gradually become more r<strong>ed</strong>uc<strong>ed</strong> westwards.<br />
This trend is coherent with the <strong>di</strong>stribution of the uplift rate.<br />
The fluvial steps are locat<strong>ed</strong> along incis<strong>ed</strong> tectonic<br />
lineaments mainly orient<strong>ed</strong> NW-SE, N-S/NE-SW. The activity<br />
of the neotectonic faults determines repeat<strong>ed</strong> changes in<br />
inclination of the main river so that it assumes convex-up<br />
geometries.<br />
Regar<strong>di</strong>ng the hypsometric integrals/area ratios, most of the<br />
drainage basins in Northern Sicily are locat<strong>ed</strong> in the range<br />
which in<strong>di</strong>cates equilibrium, as suggest<strong>ed</strong> by the OHMORI<br />
(1993) method.<br />
The hypsometric profiles generally show mass<br />
inequilibrium in <strong>di</strong>fferent sectors of each drainage basin of<br />
Northern Sicily.<br />
The transverse topographic symmetry factor is also us<strong>ed</strong> to<br />
evaluate basin asymmetry to in<strong>di</strong>cate preferr<strong>ed</strong> stream<br />
migration (COX, 1994; Fig. 1B).<br />
DISCUSSION<br />
The above-describ<strong>ed</strong> data suggest that:<br />
• in northern Sicily are present an neotectonic faults systems<br />
superimpos<strong>ed</strong> on the Oligo-Miocene thrust system;<br />
• the neotectonic faults grid is defin<strong>ed</strong> by strike-slip<br />
kinematics;<br />
• the strike-slip deformations is consistent with the seismicity<br />
<strong>di</strong>stribution and with the fault-plane solutions of the main<br />
earthquakes. The orientation of the neotectonic faults is<br />
coherent with the seismogenic pattern defin<strong>ed</strong> by the recent<br />
earthquakes that occurr<strong>ed</strong> offshore Northern Sicily;<br />
• transcurrent faults dominate in the shear zones, which are<br />
locat<strong>ed</strong> at the <strong>ed</strong>ges of the mountain reliefs;<br />
• neotectonic deformations seem to control the morphological<br />
evolution of Northern Sicily quite strongly. The orientation<br />
of the neotectonic faults is coherent with the main<br />
morphostructures, defining a group of mountains;<br />
• The orientation of the neotectonic faults is coherent with the<br />
orientation of the <strong>di</strong>fferent orders of fluvial channels;<br />
• the neotectonic setting is consistent with the morphometric<br />
pattern;<br />
• the fault activity also influences the development of the<br />
drainage network and determines a non-uniform uplift of<br />
the group of mountains;<br />
• the segment<strong>ed</strong> rock masses outcropping in the drainage<br />
basins seem to be relat<strong>ed</strong> to the recent/active faulting that<br />
affects the chain;<br />
• the hypsometric integrals mainly fall within the equilibrium<br />
stage or within the monadnock phase fields of STRAHLER. But<br />
all the hypsometric curves show complex sinuosity. Different<br />
inflection points and slopes can be recognis<strong>ed</strong>, which reflect<br />
varying ground-slope characteristics dominat<strong>ed</strong> by recent<br />
tectonic structures. The elevations/area ratios for each Northern<br />
Sicilian mountain in<strong>di</strong>cate that the mountain belt is undergoing<br />
active tectonics.<br />
The high-angle fault grid defines many of the mountain<br />
<strong>ed</strong>ges in the Palermo, Trapani and Peloritani Mts.<br />
The comparison between the amplitude relief map, the<br />
fault-grid map and the uplift rate <strong>di</strong>stribution map suggests that<br />
the recent tectonic structures have a prominent role in the<br />
evolution of the landforms and mountain elevations, as defin<strong>ed</strong><br />
by the high coherence in the spatial <strong>di</strong>stribution of the three<br />
parameters.<br />
The spatial variability in the relief amplitude has allow<strong>ed</strong> us<br />
to take into consideration tectonic elements of the area, which<br />
fit well into the morphotectonic features.<br />
The fluvial profiles wi<strong>del</strong>y show convex-up segments,<br />
which suggests that the rock mass <strong>di</strong>stribution is strictly relat<strong>ed</strong><br />
to the recent faulting and uplift of the Sicilian belt. In fact, the<br />
convex-up geometry of the longitu<strong>di</strong>nal profiles of the rivers is<br />
very common in the drainage basins of Northern Sicily and is<br />
independent from the pr<strong>ed</strong>ominant outcropping lithology.<br />
Fluvial channels frequently develop in fault zones,<br />
especially in the Nebro<strong>di</strong> and Peloritani Mts. The longitu<strong>di</strong>nal<br />
profiles of the rivers are wi<strong>del</strong>y characteris<strong>ed</strong> by steps, which<br />
are generally align<strong>ed</strong> to form straight channels.<br />
By comparing the non-uniform magnitude and <strong>di</strong>rection of<br />
the lateral shift of the main rivers and the uplift rate<br />
<strong>di</strong>stribution, it is possible to identify the crustal blocks of<br />
Northern Sicily, their relative vertical movement (Fig. 1C), the<br />
<strong>di</strong>rection of tilting and possible large-scale fol<strong>di</strong>ng.<br />
By using the comparison between these two data sets,<br />
Northern Sicily can be <strong>di</strong>vid<strong>ed</strong> into crustal blocks (Fig. 1D),<br />
each of which possesses <strong>di</strong>fferent uplift rates and tilting<br />
<strong>di</strong>rections, as determin<strong>ed</strong> by the shift tendency of the main<br />
rivers.<br />
Some of the neotectonic faults that border the <strong>di</strong>fferent<br />
chain blocks appear to be the physical onland prolongation of<br />
those submerg<strong>ed</strong> structures that in recent years have underlin<strong>ed</strong><br />
an elevat<strong>ed</strong> seismogenic potential. The analyz<strong>ed</strong> neotectonic<br />
fault system can therefore compete in the formation of the<br />
seismogenic volume.<br />
REFERENCES<br />
ANTONIOLI F., KERSHAW S., RENDA P. & RUST D. (2004) - Contrasting<br />
patterns of Late Quaternary tectonic uplift around the coastline of<br />
Sicily. 32 nd Intern. Geol. Congr., Florence (Italy), August 20-28<br />
2004, Field Trip Guide Book - P10, pp. 20.<br />
COX R. T. (1994) - Analysis of drainage-basin symmetry as a rapid<br />
tecnique to identify areas of possible Quaternary tilt-block<br />
tectonics: An example from the Mississippi Embayment. Geol.<br />
Soc. Am. Bull., 106, 571-581.<br />
GIUNTA G., NIGRO F., RENDA P. & GIORGIANNI A. (2000) - The<br />
Sicilian-Maghrebides Tyrrhenian Margin: a neotectonic<br />
evolutionary mo<strong>del</strong>. Boll. Soc. Geol. It., 119, 553-565.<br />
GIUNTA G., LUZIO D., TONDI E., DE LUCA L., GIORGIANNI A., D’ANNA<br />
G., RENDA P., CELLO G., NIGRO F. & VITALE M. (2004) - The<br />
Palermo (Sicily) seismic cluster of September 2002, in the<br />
seismotectonic framework of the Tyrrhenian Sea-Sicily border<br />
area. Ann. of Geoph., 47 (6): 1755-1770.<br />
OHMORI H. (1993) - Changes in the hypsometric curve through<br />
mountain buil<strong>di</strong>ng resulting from concurrent tectonics and<br />
denudation. Geomorphology, 8, 263-277.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 157-159<br />
Exhumation and uplift of the Peloritani Mts (south Italy): constraints<br />
from apatite fission-tracks and (U-Th)/He thermochronometry<br />
VALERIO OLIVETTI (*), MARIA LAURA BALESTRIERI (**), FINLAY M. STUART (°) & CLAUDIO FACCENNA (*)<br />
RIASSUNTO<br />
Esumazione e sollevamento dei monti Peloritani (Sicilia, Italia): vincoli<br />
termocronologici m<strong>ed</strong>iante l’analisi <strong>di</strong> tracce <strong>di</strong> fissione e metodo (U-<br />
Th)/He<br />
Presentiamo nuovi dati termocronologici ottenuti da analisi <strong>di</strong> tracce <strong>di</strong><br />
fissione e (U-Th)/He su cristalli <strong>di</strong> apatite, eseguiti su campioni provenienti da<br />
rocce <strong>del</strong> basamento cristallino dei Monti Peloritani, Sicilia, Italia. Le età<br />
ottenute dall’analisi <strong>del</strong>le fission track variano tra 29.0±5.5 e 5.5±0.9 Ma<br />
mentre le età (U-Th)/He variano tra 20.1 e 6.2 Ma.<br />
Questi dati costituiscono un vincolo cronologico utile all’interpretazione<br />
<strong>del</strong>l’evoluzione tettonica <strong>di</strong> questa porzione <strong>del</strong>l’arco calabro peloritano. Infatti<br />
le età ottenute risultano più giovani <strong>del</strong>la deposizione <strong>del</strong>la formazione <strong>di</strong><br />
Stilo-Capo d’Orlando, che segna la fine <strong>del</strong>la strutturazione principale <strong>del</strong>le<br />
falde tettoniche. I dati termocronologici mettono in luce un evento tettonico<br />
compressivo fuori sequenza datato Miocene m<strong>ed</strong>io. Le età recenti ottenute con<br />
il metodo (U-Th)/He vincolano la fase finale <strong>del</strong>l’esumazione.<br />
Key words: Apatite fission-track, (U-Th)/He<br />
thermochronometry, Peloritani-Calabrian Arc,<br />
denudation, uplift.<br />
INTRODUCTION<br />
The Peloritani Mountains constitute the westward<br />
termination of the Calabria–Peloritani Arc of southern Italy, a<br />
portion of an orogenic w<strong>ed</strong>ge growth in response to tertiary<br />
convergence between the African and European plates.<br />
The Peloritani Mountains form<strong>ed</strong> when the continental<br />
collision occurring during Oligocene – early Miocene times,<br />
caus<strong>ed</strong> the tectonic superposition of the Kabilo-Peloritan<br />
Calabrian belt onto the rock succession of the Sicilian –<br />
Maghrebian belt (CATALANO et alii, 1996; LENTINI et alii,<br />
1996; GRASSO, 2001). Since Late Miocene times this process<br />
was accompani<strong>ed</strong> by back-arc extension of the Tyrrhenian Sea<br />
(e.g. FACCENNA et alii, 1997).<br />
_________________________<br />
(*) Dipartimento Scienze Geologiche, Università “Roma Tre”, Roma, Italy.<br />
E-mail: volivetti@uniroma3.it<br />
(**) CNR, Istituto <strong>di</strong> Geoscienze e <strong>Georisorse</strong>, <strong>Sezione</strong> <strong>di</strong> Firenze, Italy.<br />
(°) Isotope Geosciences Unit, Scottish Universities Environmental Research<br />
Centre, East Kilbride, Glasgow, UK.<br />
Combining (U-Th)/He and fission-track thermochronometry<br />
and s<strong>ed</strong>imentation history we show a complex picture of<br />
several tectonic events since Oligocene time. The obtain<strong>ed</strong> ages<br />
point out a new important Miocene burial event.<br />
GEOLOGICAL SETTING<br />
The Peloritani structural <strong>ed</strong>ifice (e.g. Amo<strong>di</strong>o-Morelli et al.,<br />
1976; Bonar<strong>di</strong> et al., 1976) consists of a set of thrust sheets<br />
compos<strong>ed</strong> of m<strong>ed</strong>ium- to high-grade Paleozoic metamorphic<br />
rocks with remnants of Mesozoic s<strong>ed</strong>imentary cover. The<br />
stacking process occurr<strong>ed</strong> during Alpine south-verging<br />
deformational phases (e.g. CIRRINCIONE & PEZZINO, 1994;<br />
GIUNTA & SOMMA, 1996; VIGNAROLI et alii, 2008). Alpine<br />
metamorphic imprinting was detect<strong>ed</strong> by <strong>di</strong>fferent Authors<br />
(BONARDI et alii, 1976; CIRRINCIONE & PEZZINO, 1994;<br />
ATZORI et alii, 1994; VIGNAROLI et alii, 2008). The age of the<br />
Alpine metamorphic climax is tentatively confin<strong>ed</strong> to the Late<br />
Oligocene (26 Ma) from Rb-Sr dating on mica (ATZORI et alii,<br />
1994) in agreement with previous zircon and apatite fission<br />
track data which points out that substantial exhumation<br />
occurr<strong>ed</strong> between late-Oligocene to early-Miocene (THOMSON,<br />
1994).<br />
The whole Peloritani <strong>ed</strong>ifice is unconformably overlain by<br />
the Stilo-Capo d’Orlando Fm (BONARDI, 1980), a w<strong>ed</strong>ge-top<br />
thick Oligocene(?)–Early Miocene terrigenous clastic<br />
formation that has been partly involv<strong>ed</strong> in the late tectonogenic<br />
phases of the construction of the nappe <strong>ed</strong>ifice (GIUNTA &<br />
SOMMA, 1996; GIUNTA & NIGRO, 1997; LENTINI et alii, 2000,<br />
and references therein).<br />
The Antisilicide nappe, a Mesozoic-Early Tertiary<br />
stratigraphic sequence, took place over the Peloritani <strong>ed</strong>ifice<br />
through a process of north-ward back-thrusting relat<strong>ed</strong> to<br />
collisional stage (BONARDi et alii, 1980). From the Langhian<br />
time the s<strong>ed</strong>imentary history proce<strong>ed</strong><strong>ed</strong> rather regularly in time<br />
with deposition of mainly terrigenous sequences within basins<br />
form<strong>ed</strong> in response to combin<strong>ed</strong> effect of tectonics and sea<br />
level fluctuation.<br />
The age of inception of extensional tectonics is unclear, and<br />
has been propos<strong>ed</strong> to be Serravallian (LENTINI et alii, 1995) or<br />
Late Tortonian (CATALANO et alii, 1996).
158 V. OLIVETTI ET ALII<br />
RESULTS<br />
Fission-track and (U-Th)/He analyses were carri<strong>ed</strong> out on<br />
apatite from crystalline rocks of the Paleozoic units of the<br />
Kabilo-Peloritan Calabrian belt from two north-south orient<strong>ed</strong><br />
traverses and one transect parallel to the Tyrrhenian coast and<br />
one perpen<strong>di</strong>cular to the Ionian coast. Fission-track ages span<br />
29.0±5.5 to 5.5±0.9 Ma while (U-Th)/He ages vary from 20.1<br />
to 6.2 Ma.<br />
Tyrrhenian coast:<br />
AFT ages vary from 5.5±0.9 to 27.3±2.5 Ma, the latter is<br />
the oldest AFT age found in the whole study area. AHe ages<br />
range between 6.6 and 19.4 Ma. The older AHe ages<br />
accompany the older AFT ages.<br />
Central zone:<br />
Samples were collect<strong>ed</strong> along two north-south <strong>di</strong>rect<strong>ed</strong><br />
transects. AFT ages span between 5.8±1.3 and 15.4±3.6. AHe<br />
ages range from 3.3 and 6.7 Ma, the youngest in the Peloritani<br />
region.<br />
Ionian coast:<br />
Along a vertical profile from 20 m to 550 m asl, AFT ages<br />
decrease from sea level going upward and toward the Central<br />
zone, from 18.0±3.8 to 7.7±0.9. The only AHe analysis yield<strong>ed</strong><br />
a mean age of 5.7 Ma.<br />
The most striking feature of both the He and AFT ages from<br />
Peloritani Mountain is the fact that they post-date the<br />
deposition of Stilo-Capo d’Orlando formation, what<br />
unconformably overlies the basement Units.<br />
Along the Ionian vertical profile the ages decrease going<br />
upwards. This trend is contrary to that expect<strong>ed</strong> in a crustal<br />
block during exhumation process. We retain that actual position<br />
is due to tectonic movement after FT cooling ages.<br />
COOLING HISTORY AND TECTONIC IMPLICATIONS<br />
The thermochronologic data of representative samples<br />
obtain<strong>ed</strong> from fission track and (U-Th)/He analysis were<br />
insert<strong>ed</strong> in a time versus temperature/depth plot.<br />
We suggest that the older fission-track and He ages in the<br />
Aspromonte Unit correspond to the time of the early<br />
exhumation of this unit during in-sequence thrusting lea<strong>di</strong>ng to<br />
the formation of the orogenic w<strong>ed</strong>ge (Vignaroli et al., 2008).<br />
Meanwhile the Capo d’Orlando flysch was deposit<strong>ed</strong> on top of<br />
the Aspromonte Unit.<br />
The Tortonian fission-track ages point out a thermal<br />
resetting which should be occurr<strong>ed</strong> between the deposition of<br />
the basal Stilo-Capo d’Orlando Fm and the Langhian deposits.<br />
We propose that the resetting is due to burial produc<strong>ed</strong> by overri<strong>di</strong>ng<br />
thrust sheets emplac<strong>ed</strong> during out-of-sequence thrusting.<br />
Actually <strong>di</strong>fferent generation of thrusts are already recogniz<strong>ed</strong><br />
from the Tyrrhenian coast toward south involving both the<br />
Aspromonte Unit and the inner portion of the Stilo-Capo<br />
D’Orlando Fm (Giunta and Nigro, 1999; Lentini et al., 2000).<br />
We speculate that this out-of-sequence thrusting process was a<br />
dynamic response to progressive foreland-ward thrust<br />
propagation for maintaining the critical taper con<strong>di</strong>tion of the<br />
inner-interm<strong>ed</strong>iate portions of the Peloritani belt.<br />
In this interpretation the young (U-Th)/He ages represent<br />
the timing of the last exhumation probably link<strong>ed</strong> to moderate<br />
post-orogenic extensional collapse. Combining two cooling<br />
ages of <strong>di</strong>fferent closure temperature and stratigraphic<br />
constraint we calculate a value of denudation rate for a<br />
representative sample. The denudation rate shows an increasing<br />
from 0.3 mm/yr to 1-3 mm/yr.<br />
REFERENCES<br />
AMODIO-MORELLI L., BONARDI G., COLONNA V., DIETRICH D.,<br />
GIUNTA G., IPPOLITO F., LIGUORI V., LORENZONI S.,<br />
PAGLIONICO A., PERRONE A., PICCARETTA G., RUSSO M.,<br />
SCANDONE P., ZANETTIN-LORENZONI E., & ZUPPETTA A.,<br />
(1976) - L’arco Calabro-peloritano nell’orogene<br />
appenninico-maghrebide. Mem. Soc. Geol. It., 17, 1-60.<br />
ATZORI P., CIRRINCIONE R., DEL MORO A., & PEZZINO A.,<br />
(1994) - Structural, Metamorphic and geochronologic<br />
features of the Alpine event in the south-eastern sector of<br />
the Peloritani Mountains (Sicily). Perio<strong>di</strong>co <strong>di</strong> Mineralogia,<br />
63, 113-125.<br />
BONARDI G., GIUNTA G., PERRONE V., RUSSO M., & ZUPPETTA<br />
A. (1976) - Schema geologico dei Monti Peloritani. Boll.<br />
Soc. Geol. It., 95, 49-74.<br />
CATALANO R., DI STEFANO P., SULLI A., & VITALE F.P. (1996)<br />
- Paleogeography and structure of the central<br />
M<strong>ed</strong>iterranean: Sicily and its offshore area.<br />
Tectonophysics, 260, 291-323.<br />
CIRRINCIONE R. & PEZZINO A. (1994) - Nuovi dati strutturali<br />
sulle successioni mesozoiche metamorfiche dei M.<br />
Peloritani orientali. Bollettino <strong>del</strong>la Società Geologica<br />
Italiana 113, 195-203.<br />
FACCENNA C., MATTEI M., FUNICIELLO R., JOLIVET L. (1997) -<br />
Styles of back-arc extension in the Central M<strong>ed</strong>iterranean.<br />
Terra Nova 9, 126-130.<br />
GRASSO M. (2001) - The Apenninic-Maghrebian orogen in<br />
southern Italy, Sicily and adjacent areas. In Vai, G.B., and<br />
Martini, I.P, <strong>ed</strong>s., Anatomy of an orogen: the Apennines<br />
and Adjacent M<strong>ed</strong>iterranean Basins: Great Britain, Kluwer<br />
Academic Publishers, p. 255-286.<br />
GIUNTA G., AND NIGRO F. (1999) - Tectono-s<strong>ed</strong>imentary<br />
constraints to the Oligocene-to-Miocene evolution of the<br />
Peloritani thrust belt (NE Sicily). Tectonophysics 315, 287–<br />
99.<br />
GIUNTA G., AND SOMMA R. (1996) - Nuove osservazioni sulla<br />
struttura <strong>del</strong>l’Unità <strong>di</strong> Alì (M.ti Peloritani, Sicilia). Boll.<br />
Soc. Geol. It., 115, 489–500.
LENTINI F., CARBONE S., CATALANO S., & GRASSO M. (1996) -<br />
Elementi per la ricostruzione strutturale <strong>del</strong>la Sicilia<br />
Orientale. Mem. Soc. Geol. It., 51, 179-195.<br />
LENTINI F., CARBONE S., CATALANO S. (2000) - Carta<br />
geologica <strong>del</strong>la provincia <strong>di</strong> Messina: Servizio Geologico,<br />
S.EL.CA., Provincia Regionale <strong>di</strong> Messina, Assessorato<br />
Territorio, scale 1:50,000, 3 sheets.<br />
THOMSON S.N. (1994) - Fission track analysis of the crystalline<br />
basement rocks of the Calabrian Arc, southern Italy:<br />
evidence of Oligo-Miocene late orogenic extension and<br />
erosion. Tectonophysics, v. 238, p. 331-352.<br />
VIGNAROLI G., ROSSETTI F., THEYE T., & FACCENNA C. (2008)<br />
- Styles and regimes of orogenic thickening in the<br />
Peloritani Mountains (Sicily, ITALY): new constraints on the<br />
tectono-metamorphic evolution of the Apennine belt. Geol.<br />
Mag., 1-18.<br />
EXHUMATION AND UPLIFT OF THE PELORITANI MTS<br />
159
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 160-163, 4 ff.<br />
Geologia e geofisica <strong>del</strong> plutone <strong>del</strong> Cerro Trapecio - Tierra <strong>del</strong> Fuego -<br />
Argentina<br />
JAVIER PERONI (*), ALEJANDRO TASSONE (*), MARCO MENICHETTI (**), HORACIO LIPPAI (*) & JUAN FRANCISCO VILAS (*)<br />
ABSTRACT<br />
Geology and geophysics of the Cerro Trapecio pluton – Tierra <strong>del</strong> Fuego -<br />
Argentina<br />
In the eastern part of the Bahìa Ushuaia outcrops dark schists pertains to<br />
the Yaghàn Fm. of the Lower Cretaceous, strongly deform<strong>ed</strong> by Upper<br />
Cretaceous Andean compresional tectonics. They are intrud<strong>ed</strong> by a basaltic<br />
rock with tholeitic-calc-alkaline affinity with a prevalent lithology of the<br />
pyroxenites and monzonites. The aereomagnetic survey and magnetic<br />
measurements of rocks samples, integrat<strong>ed</strong> with the geological informations<br />
permitt<strong>ed</strong> to mo<strong>del</strong> a N-S cross section through the area.<br />
Key words: Andes, structural geology, geophysics, Tierra <strong>del</strong><br />
Fuego.<br />
Il Cerro Trapecio è localizzato circa 16 km ad est <strong>del</strong>la città<br />
<strong>di</strong> Ushuaia, nella Sierra Sorondo nella Cor<strong>di</strong>llera <strong>del</strong>le Ande in<br />
Tierra <strong>del</strong> Fuego. Questa catena si estende per oltre 40 km in<br />
<strong>di</strong>rezione WNW-ESE lungo la costa settentrionale <strong>del</strong> settore<br />
orientale <strong>del</strong> Canal de Beagle.<br />
In questa regione <strong>del</strong> Canal de Beagle, sono presenti <strong>di</strong>versi<br />
corpi intrusivi, costituiti da rocce basaltiche con affinità<br />
toleiitica-calcalcalina intruse nella Formazione Yaghàn <strong>del</strong><br />
Cretaceo Sup.. Queste intrusioni sono connesse al grande<br />
Batolite Patagonico che affiora con continuità lungo il margine<br />
<strong>di</strong> retro-arco <strong>del</strong> continente sud-americano (HERVÈ et alii,<br />
1984).<br />
Nell’area affiora principalmente la Fm. Yaghàn che<br />
rappresenta il riempimento vulcano-clastico <strong>del</strong> bacino<br />
marginale <strong>di</strong> Rocas Verdes, depositatasi al <strong>di</strong> sopra <strong>del</strong>la Fm<br />
Lemaire <strong>del</strong> Giurassico Sup. ( DALZIEL et alii, 1974). La Fm.<br />
Yaghàn è costituita da <strong>di</strong>verse facies s<strong>ed</strong>imentarie con marne<br />
scure, torbi<strong>di</strong>ti fini e grossolana, rare arenarie da massive a<br />
gradate e tufi (OLIVERO & MALUMIAN, 2008). E’ interessata da<br />
un metamorfismo <strong>di</strong> basso grado, in facies a scisti ver<strong>di</strong>, con<br />
una intensa deformazione prodotta dalla fase tettonica an<strong>di</strong>na<br />
<strong>del</strong> Cretaceo Sup., all’interno <strong>del</strong>la quale è possibile<br />
riconoscere almeno due fasi deformative (MENICHETTI et alii,<br />
_________________________<br />
(*) CONICET-INGEODAV. Dpto. de Ciencias Geológicas. Facultad de<br />
Ciencias Exactas y Naturales. Universidad de Buenos Aires - Argentina.<br />
(**) Istituto <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Urbino – Italy<br />
Javier Peroni : e-mail peroni@gl.fcen.uba.ar<br />
2008). La prima (D1) è costituita da pieghe isoclinaliche, a<br />
scala decimetrica, con piano assiale subperpen<strong>di</strong>colare alla<br />
stratificazione; è interessata da un intenso slaty cleavage<br />
immergente a SE e NW. Una seconda fase deformativa (D2) è<br />
rappresentata da un crenulation cleavage e da pieghe<br />
mesoscopiche con lunghezza d’onda decametrica, con<br />
geometrie principalmente chevron, con piano assiale<br />
debolmente immergente a SSW. Tutta l’area è interessata da<br />
faglie inverse e sovrascorrimenti vergenti a nord, con geometrie<br />
a basso angolo <strong>di</strong> incidenza sulla stratificazione. Infine, un<br />
sistema <strong>di</strong> faglie trascorrenti sinistre e faglie normali con piani<br />
subverticali, con immersione prevalente verso sud, sono ben<br />
sviluppate lungo tutto il margine settentrionale e meri<strong>di</strong>onale<br />
<strong>del</strong> Canal de Beagle e tagliano e in parte riattivano, le strutture<br />
compressive (MENICHETTI et alii,, 2004, 2007, 2008) (Fig. 2).<br />
I due sistemi <strong>di</strong> faglie principali sono costituiti da quelli<br />
orientali <strong>del</strong> Canal de Beagle da cui <strong>di</strong>parte la faglia Andorra<br />
verso settentrione. Queste faglie hanno un controllo sulla<br />
morfologia e sull’idrografia <strong>del</strong>l’area, con blocchi ribassati e<br />
Fig. 1 – Ubicazione <strong>del</strong>l’area <strong>di</strong> stu<strong>di</strong>o nella parte più meri<strong>di</strong>onale <strong>del</strong> Sud<br />
America (A) . Localizzazione <strong>del</strong>la carta geologica <strong>di</strong> Fig. 2, <strong>del</strong>le carte aereo<br />
magnetiche e <strong>del</strong> DEM <strong>di</strong> Fig. 4 (B).<br />
basculati nei versanti meri<strong>di</strong>onali <strong>del</strong>la Serra Sorondo. Alcuni<br />
corsi d’acqua e laghi sono allineati secondo la <strong>di</strong>rezione<br />
WNW-ESE e sono localizzati al bordo dei blocchi in<br />
corrispondenza <strong>del</strong>le zone <strong>di</strong> faglie immergenti a SW (Fig. 2).<br />
Nel settore occidentale <strong>del</strong>l’area <strong>di</strong> stu<strong>di</strong>o, si hanno i<br />
maggiori affioramenti <strong>del</strong> corpo intrusivo, noto come plutone <strong>di</strong><br />
Ushuaia, il quale copre una superficie <strong>di</strong> 9 km 2 . Esso è<br />
composto principalmente da una orneblen<strong>di</strong>te <strong>di</strong> tessitura
GEOLOGIA E GEOFISICA DEL PLUTONE DEL CERRO TRAPECIO<br />
Fig. 2 – Carta geologica schematica <strong>del</strong>la parte orientale <strong>del</strong>la Bahìa <strong>di</strong> Ushuaia. 1 – Depositi quaternari; 2 - corpo intrusivo; 3 – Formazione Yaghan (Cretaceo<br />
inf.) ; 4 – Faglie <strong>di</strong>rette; 5 – Faglie trascorrenti; 6 – Giacitura <strong>del</strong>la stratificazione, inclinazione degli strati: a 0-20°; b 20-45°; c 45-75°; d >75° ; 7 – Limite<br />
<strong>del</strong>l’anomalia magnetica ridotta al polo; 8 – Aree ricoperte da ghiacciai; 9 – Laghi. Nel Canal de Beagle è in<strong>di</strong>cata la batimetria in metri (SERVICO DE<br />
HIDROGRAFIA NAVAL ARGENTINA, 1988). A-A’ traccia parziale <strong>del</strong>la sezione <strong>di</strong> Fig. 3 . (Mo<strong>di</strong>ficato da MENICHETTI et alii, 2007).<br />
grossolana con abbondante clino-anfibolo, scarso<br />
clinopirosseno, <strong>di</strong>orite e facies monzo<strong>di</strong>oritica, attraversato da<br />
numerose vene e <strong>di</strong>cchi <strong>di</strong> composizione sienitica (ACEVEDO et<br />
alii, 1989). Include termini da ultrabasici a interm<strong>ed</strong>i con un<br />
contenuto <strong>di</strong> SiO2 variabile dal 40 al 65% (CERREDO et alii,<br />
2000, 2005). Di questo plutone è stato realizzato un mo<strong>del</strong>lo<br />
utilizzando dati aereomagnetici (MENICHETTI et alii, 2007;<br />
PERONI et alii., 2008). Il corpo intrusivo ha una forma<br />
laccolitica con uno spessore massimo <strong>di</strong> 2 km nella sua parte<br />
centrale che tende ad assottigliarsi ai bor<strong>di</strong> fino a circa 500 m.<br />
In pianta il corpo presenta una forma ellittica con l’asse<br />
maggiore N-S, lungo circa 12 km e quello inferiore con<br />
<strong>di</strong>rezione E-W, <strong>di</strong> circa 10 km, per un volume totale stimato <strong>di</strong><br />
140 km 3 (PERONI et alii, 2008) .<br />
Nella zona <strong>del</strong> Cerro Trapecio gli affioramenti sono scarsi e<br />
sono localizzati esclusivamente nel versante meri<strong>di</strong>onale. Il<br />
plutone è rappresentato principalmente da una roccia<br />
monzo<strong>di</strong>oritica, simile a quella <strong>del</strong> plutone Ushuaia, con<br />
presenza <strong>di</strong> filoni <strong>di</strong> andesiti con spessore inferiore al metro,<br />
concordanti con la stratificazione degli scisti <strong>del</strong>la Fm.Yahgan.<br />
Nella mappa aereomagnetica ridotta al polo <strong>del</strong>l’area<br />
(5569-II <strong>del</strong> SEGEMAR 1998), è possibile osservare che il<br />
plutone <strong>di</strong> Ushuaia è separato da quello <strong>del</strong> Cerro Trapecio (Fig<br />
4). Quest’ultimo presenta una anomalia <strong>di</strong> forma ellittica<br />
<strong>di</strong>stribuita su <strong>di</strong> una superficie <strong>di</strong> 13 km 2 , con l’asse maggiore<br />
in <strong>di</strong>rezione N-S per 5,2 km, mentre l’asse minore, in <strong>di</strong>rezione<br />
E-W, è <strong>di</strong> 3.3 km. La suscettività magnetica massima è <strong>di</strong> 24 nT<br />
nella zona centrale fino ad un minimo <strong>di</strong> -50nT . L’anomalia<br />
riproduce abbastanza f<strong>ed</strong>elmente la morfologia <strong>del</strong> Cerro<br />
Trapecio, con un minimo magnetico compreso tra la valle <strong>del</strong><br />
Rio Encajonado ad ovest e quella <strong>del</strong> Rio Escape ad est (Fig.<br />
2).<br />
161<br />
La carta aereomagnetica che riporta il campo totale,<br />
presenta una anomalia localizzata nella zona <strong>del</strong> Cerro<br />
Trapecio con una superficie <strong>di</strong> 8.5 km 2 , sempre <strong>di</strong> forma<br />
ellittica con assi <strong>di</strong> 3,4 e 2,2 km rispettivamente. I valori <strong>del</strong><br />
campo magnetico variano da un massimo nella parte<br />
settentrionale <strong>di</strong> -52 nT fino a -85 nT nella parte meri<strong>di</strong>onale<br />
(Fig. 4). Rispetto alle anomalie ridotte al polo, il campo<br />
magnetico totale è leggermente spostato verso nord. Un<br />
comportamento simile si osserva nelle anomalie magnetiche <strong>del</strong><br />
Fig. 3 – <strong>Sezione</strong> geologica e profilo <strong>del</strong>le anomalie magnetiche ridotte al<br />
polo in nT. Ubicazione riportata in Fig. 2 . 1 – Fm Yahgan; 2 – Fm Lemaire;<br />
3 – Scistosità principale; 4 – Corpi d’acqua; 6 – Faglia <strong>di</strong>retta; 6 – Corpo<br />
intrusivo.
162 J. PERONI ET ALII<br />
Fig. 4 – Carta aereomagnetica ridotta al polo <strong>del</strong>l’area <strong>di</strong> Ushuaia, con in<strong>di</strong>cata la topografia con curve <strong>di</strong> livello con equi<strong>di</strong>stanza <strong>di</strong> 100 m . E’ in<strong>di</strong>cata la<br />
traccia <strong>del</strong>la sezione <strong>di</strong> Fig. 3 (A); carta aereomagnetica <strong>del</strong> campo totale (B); immagine SPOT con vista obliqua <strong>del</strong>l’area (C). AF : Faglia Andorra; EBF :<br />
Faglia orientale <strong>del</strong> Canal de Beagle.<br />
plutone <strong>di</strong> Ushuaia.<br />
I dati geologico-strutturali e geofisici <strong>di</strong>sponibili, sono stati<br />
integrati in una sezione geologica orientata NNE-SSW, che<br />
permette <strong>di</strong> localizzare, attraverso le anomalie magnetiche, la<br />
posizione <strong>del</strong> corpo intrusivo (Fig. 3). Sia il campo magnetico<br />
totale che l’anomalia magnetica ridotta al polo sono attraversati<br />
dal sistema <strong>di</strong> faglie <strong>di</strong>rette. Queste strutture <strong>di</strong>stensive<br />
appartenenti al sistema trascorrente sinistro <strong>del</strong> Canal de<br />
Beagle (MENICHETTI et alii, 2008), immergono a SW, con un<br />
rigetto <strong>di</strong> molte centinaia <strong>di</strong> metri. Il rigetto complessivo è<br />
<strong>di</strong>stribuito su almeno tre blocchi parzialmente ruotati in senso<br />
orario lungo i piani <strong>di</strong> faglia. L’anomalia magnetica ridotta al<br />
polo, passa da un valore minimo regionale <strong>di</strong> -50nT ad un<br />
massimo <strong>di</strong> 24 nT con un gra<strong>di</strong>ente simile a quello topografico.<br />
Il corpo intrusivo è stato mo<strong>del</strong>lato in profon<strong>di</strong>tà fino a circa<br />
1.5 km, posizionato al <strong>di</strong> sopra <strong>del</strong>la Fm. Lemaire e intruso<br />
all’interno <strong>del</strong>la F. Yahgan.<br />
BIBLIOGRAFIA<br />
ACEVEDO R.D., QUARTINO G.P. & COTO C.D. (1989) - La<br />
intrusión ultramáfica de Estancia el Tunel y el significado<br />
de presencia biotita y granate en la Isla Grande de Tierra<br />
<strong>del</strong> Fuego. Acta Geológica Lilloana. 42 (1), 21-36<br />
CERREDO M.E., TASSONE A., COREN F., LODOLO E. & LIPPAI H.<br />
(2000) - Postorogenic, alkaline magmatism in the
Fuegian Andes: The Jeujepen intrusive (Tierra <strong>del</strong> Fuego<br />
Island). IX Congreso Geológico Chileno, Puerto Varas,<br />
Acta (2), 192-196.<br />
CERREDO M.E., REMESAL M.B., TASSONE A. & LIPPAI H.<br />
(2005) - The shoshonitic suite of Hewhoepen pluton,<br />
Tierra <strong>del</strong> Fuego, Argentina. XVI Congreso Geológico<br />
Argentino, Actas I, 539-544, La Plata.<br />
DALZIEL I. W. D., DE WIT M. J.& PALMER, F. K. (1974) -<br />
Fossil marginal basin in the southern Andes. Nature 250,<br />
291–294.<br />
HERVÉ M., SUÁREZ M., PUIG A. (1984) - The Patagonian<br />
Batholith S of Tierra <strong>del</strong> Fuego, Chile: timing and tectonic<br />
implications. Journal Geological Society, London, 141,<br />
909-917.<br />
MENICHETTI M., ACEVEDO R.D., BUJALESKY G.G., CENNI M.,<br />
CERREDO M.E., CORONATO A., HORMACHEA J.L., LIPPAI,<br />
H., LODOLO E., OLIVERO E.B., RABASSA J., TASSONE A.<br />
(2004) – Geological field trip guide of the Tierra <strong>del</strong><br />
Fuego. Geosur meeting Buenos Aires 2004, 39 p.<br />
MENICHETTI M., TASSONE A., PERONI J., GONZALES GUIXOT M.<br />
& CERREDO M.E. (2007) - Assetto strutturale, petrografia e<br />
geofisica <strong>del</strong>la Bahía Ushuaia – Argentina. Rend. Soc.<br />
Geol. It., 4, Nuova Serie, 259-262.<br />
MENICHETTI M., LODOLO, E. & TASSONE A. (2008) - Structural<br />
geology of the Fuegian Andes and Magallanes fold-andthrust<br />
belt – Tierra <strong>del</strong> Fuego Island. Geologica Acta, 6, 1,<br />
19-42.<br />
OLIVERO, E. B., MALUMIÁN, N. (2008) - Mesozoic – Cenozoic<br />
Stratigraphy of the Fuegian Andes, Argentina. Geologica<br />
Acta, 6 (1): 5-18 ISSN 1695-6133.<br />
PERONI J. I., TASSONE A., CERREDO M., LIPPAI H., MENICHETTI<br />
M., LODOLO E., ESTEBAN F. & VILAS J. F. (2008) - 3D<br />
Geophysic mo<strong>del</strong> of Ushuaia Pluton. Tierra <strong>del</strong> Fuego.<br />
Argentina. GeoMod 2008, Bollettino <strong>di</strong> Geofísica teorica<br />
<strong>ed</strong> applicata (2) sup., 263-267.<br />
SERVICO DE HIDROGRAFIA NAVAL ARGENTINA (1988) - Cartas<br />
Náuticas Hoja H-478, Bahia Ushuaia – Paso Chico. Scala<br />
1:15.000.<br />
SEGEMAR - SERVICIO GEOLÓGICO MINERO ARGENTINO –<br />
(1998) - Levantamiento geofísico aéreo magnetometría<br />
área de Tierra <strong>del</strong> Fuego. Proyecto PASMA. Hoja Ushuaia<br />
5569 II, scala 1:250.000.<br />
GEOLOGIA E GEOFISICA DEL PLUTONE DEL CERRO TRAPECIO<br />
163
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 164-167, 1 f.<br />
RIASSUNTO<br />
Pseudotachiliti profonde nei metagabbri <strong>del</strong>la Zona d’Ivrea (Alpi<br />
Meri<strong>di</strong>onali, Italia)<br />
Lo stu<strong>di</strong>o <strong>di</strong> pseudotachiliti (fusi <strong>di</strong> frizione generati da scivolamento<br />
sismico in rocce silicatiche) associate a miloniti in faglie esumate dalla crosta<br />
inferiore può fornire informazioni sulla meccanica dei terremoti a profon<strong>di</strong>tà<br />
inferiori o corrispondenti alla transizione elastico-frizionale/plastico-viscoso<br />
(SIBSON, 1977) e sulla reologia <strong>del</strong>la crosta continentale (v<strong>ed</strong>. HANDY &<br />
BRUN, 2004).<br />
Nei metagabbri <strong>di</strong> Premosello, nella Zona d’Ivrea, compaiono<br />
pseudotachiliti associate ad ultramiloniti molto localizzate. I metagabbri si<br />
sono formati per underplating magmatico alla base <strong>del</strong>la crosta continentale<br />
pre-alpina, poi sono stati sottoposti a metamorfismo in facies da granulitica ad<br />
anfibolitica durante l’estensione litosferica Permiana, ma senza registrare il<br />
successivo metamorfismo alpino <strong>di</strong> più bassa temperatura (HANDY et alii,<br />
1999).<br />
Le pseudotachiliti rinvenute nei metagabbri si sono formate in facies<br />
anfibolitica, come <strong>di</strong>mostrato dalla ricristallizzazione e deformazione plastica<br />
<strong>del</strong>la matrice, dalla crescita <strong>di</strong> neoblasti <strong>di</strong> granato e pirosseno, dalla<br />
ricristallizzazione <strong>di</strong>namica <strong>di</strong> plagioclasio ricco in An e da stime<br />
geotermometriche. Le stesse con<strong>di</strong>zioni metamorfiche sono state attribuite alle<br />
ultramiloniti a plagioclasio-orneblenda-pirosseno associate. L’estrema<br />
localizzazione <strong>di</strong> tali ultramiloniti <strong>ed</strong> il m<strong>ed</strong>esimo spessore <strong>ed</strong> orientazione<br />
<strong>del</strong>le pseudotachiliti suggeriscono che le pseudotachiliti abbiano agito come<br />
precursori fragili per la nucleazione <strong>del</strong>le successive ultramiloniti, durante il<br />
rilassamento post-sismico e creep propri <strong>del</strong>la crosta inferiore.<br />
Dei <strong>di</strong>versi mo<strong>del</strong>li proposti per spiegare la formazione <strong>di</strong> pseudotachiliti<br />
nella crosta interm<strong>ed</strong>io-profonda (instabilità duttile durante creep, HOBBS et<br />
alii, 1986, WHITE, 1996; HANDY & BRUN, 2004; propagazione in profon<strong>di</strong>tà<br />
<strong>di</strong> rotture sismiche dalla crosta superiore fragile, SIBSON, 1980; TSE & RICE,<br />
1986; SCHOLZ, 1988), e shear heating a<strong>di</strong>abatico, KELEMEN & HIRTH, 2007;<br />
BRAECK & PODLADCHIKOV, 2007)), le osservazioni <strong>di</strong> terreno e <strong>del</strong>le<br />
microstrutture sembrano suggerire cicli successivi <strong>ed</strong> alterni <strong>di</strong> pseudotachiliti<br />
<strong>ed</strong> ultramolioniti in con<strong>di</strong>zioni anidre nella crosta interm<strong>ed</strong>ia/inferiore durante<br />
l’esumazione pre-Alpina.<br />
Key words: pseudotachylyte, mylonite, metagabbro, Ivrea<br />
Zone.<br />
INTRODUCTION<br />
Pseudotachylytes are soli<strong>di</strong>fi<strong>ed</strong> melts produc<strong>ed</strong> by<br />
_________________________<br />
Deep-seat<strong>ed</strong> pseudotachylyte in the Ivrea Zone metagabbros<br />
(Southern Alps, Italy).<br />
PITTARELLO LIDIA (*), PENNACCHIONI GIORGIO (* °) & DI TORO GIULIO (* °)<br />
(*) Dipartimento <strong>di</strong> Geoscienze, Università degli Stu<strong>di</strong> <strong>di</strong> Padova<br />
(°) Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia, <strong>Sezione</strong> <strong>di</strong> Roma.<br />
Lavoro eseguito bell’ambito <strong>del</strong> progetto Cariparo<br />
coseismic frictional sli<strong>di</strong>ng in faults within silicate-built rock<br />
(SIBSON, 1975). Studying pseudotachylytes in exhum<strong>ed</strong> faults<br />
might give information on several aspects of earthquake<br />
mechanics (SIBSON, 1975; DI TORO et alii, 2005; PITTARELLO<br />
et alii, 2008) complementary to seismological information.<br />
Though pseudotachylytes are mostly generat<strong>ed</strong> in the upper<br />
brittle crust, they have also been report<strong>ed</strong> from the ductile field<br />
developing in association with mylonites. Pseudotachylyte<br />
overprint<strong>ed</strong> by low grade mylonites can be expect<strong>ed</strong> in the high<br />
<strong>di</strong>fferential stress region close to the plastic-brittle transition in<br />
the crust as result of plastic instabilities (HOBBS et alii, 1986),<br />
but pseudotachylytes have also been describ<strong>ed</strong> at higher<br />
metamorphic grades up to granulite facies con<strong>di</strong>tions (SIBSON,<br />
1980; PASSCHIER, 1982; CLARKE & NORMAN, 1993;<br />
AUSTRHEIM & BOUNDY, 1994; WHITE, 1996; PENNACCHIONI &<br />
CESARE, 1997; LIN et alii, 2005). Different mechanisms have<br />
been propos<strong>ed</strong> to explain production of pseudotachylytes in the<br />
middle and lower crust: plastic instability during creep (HOBBS<br />
et alii, 1986, WHITE, 1996; HANDY & BRUN, 2004), downward<br />
propagation of seismic ruptures from the upper brittle crust<br />
(TSE & RICE, 1986; SCHOLZ, 1988), and shear heating (SIBSON,<br />
1980; KELEMEN & HIRTH, 2007).<br />
Here we describe pseudotachylytes associat<strong>ed</strong> with<br />
localiz<strong>ed</strong> amphibolite facies mylonites within granulitic<br />
metagabbros of the Ivrea Zone. These metagabbros were<br />
form<strong>ed</strong> by underplating at the base of the Pre-Alpine<br />
continental crust and were exhum<strong>ed</strong> during Permian<br />
lithospheric stretching without later strong involvement in<br />
Alpine metamorphism (HANDY et alii, 1999). Therefore, these<br />
pseudotachylytes must have develop<strong>ed</strong> under high-grade<br />
con<strong>di</strong>tions during the Pre-Alpine exhumation path. The study of<br />
this type of deep-seat<strong>ed</strong> pseudotachylyte may <strong>di</strong>scriminate<br />
between the <strong>di</strong>fferent mechanism describ<strong>ed</strong> above which have<br />
potential strong implications on the interpretation of the<br />
rheology of the continental crust as, for example, on the cut-off<br />
depth of propagation of seismic ruptures from the upper brittle<br />
crust (see HANDY & BRUN, 2004).<br />
GEOLOGICAL SETTING<br />
Samples of the stu<strong>di</strong><strong>ed</strong> pseudotachylytes were collect<strong>ed</strong> in<br />
the Premosello metagabbros of the Ivrea Zone (Italian Southern<br />
Alps). The Ivrea Zone consists of pre-Alpine continental lower<br />
crust and lithospheric mantle involv<strong>ed</strong> in Permo-Mesozoic
DEEP-SEATED PSEUDOTACHYLYTE IN THE IVREA ZONE METAGABBROS<br />
extension, thinning and high temperature metamorphism<br />
(GIESE, 1968; BRODIE & RUTTER, 1987; ZINGG et alii, 1990).<br />
The metagabbros were emplac<strong>ed</strong> by magmatic underplating<br />
about 299±5 Ma ago (VAVRA et alii, 1999; HANDY et alii,<br />
1999) at the base of the crust and were pervasively deform<strong>ed</strong><br />
along high-temperature shear zones (280-270 Ma old). These<br />
shear zones develop<strong>ed</strong> either during prograde isobaric heating<br />
from amphibolite to granulite facies, accor<strong>di</strong>ng to some authors<br />
(BRODIE, 1981; BRODIE & RUTTER 1985; HANDY, 1999), or by<br />
isothermal decompression (CHOUDHURI & SILVA, 2000).<br />
Between 230 and 180 Ma localiz<strong>ed</strong> mylonites develop<strong>ed</strong> in<br />
metagabbros during retrograde amphibolite facies con<strong>di</strong>tions<br />
associat<strong>ed</strong> with extensional tectonics (BRODIE & RUTTER, 1987;<br />
HANDY et alii, 1999). Alpine (50-20 Ma) faulting and fol<strong>di</strong>ng<br />
affect<strong>ed</strong> the metagabbros, without a significant metamorphic<br />
overprint (HANDY et alii, 1999).<br />
METHODS<br />
Metagabbros and the crosscutting fault rocks were<br />
investigat<strong>ed</strong> with the optical microscope, scanning electron<br />
165<br />
microscope (SEM), field-emission scanning electron<br />
microscope (FE-SEM), electron probe microanalyzer (EPMA)<br />
and X-ray powder <strong>di</strong>ffraction (XRPD). Image analysis was<br />
perform<strong>ed</strong> with Adobe Photoshop and Image SXM (BARRETT,<br />
see references). Geothermobarometry was conduct<strong>ed</strong> using the<br />
software GTB (SPEAR & KOHN, see references) and<br />
Thermocalc (HOLLAND & POWELL, 1998).<br />
RESULTS<br />
The Premosello metagabbros consist of garnet (37% vol.)<br />
(Alm47, Pyr34, Gro15, And3, Spe2), An85-54 plagioclase (31%),<br />
clino- and orthopyroxene (23%) (En37 Fs29 Wo28 <strong>di</strong>opside and<br />
En63 orthopyroxene), magnetite-ilmenite and apatite.<br />
Metagabbros are locally deform<strong>ed</strong> to flaser gabbros with a<br />
foliation outlin<strong>ed</strong> by ribbons of dynamically recrystalliz<strong>ed</strong><br />
aggregates (grain size of 10-50 µm) of An50 plagioclase, by<br />
elongate domains of cataclastic garnet and pyroxene and thin<br />
layers of ilmenite-magnetite. Garnet and pyroxene commonly<br />
show rims of An85 plagioclase-orthopyroxene-magnetite<br />
kelyphite and symplectite (clinopyroxene/amphibole,<br />
Fig. 1 – Microstructures of the Premosello pseudotachylyte. A) Photo of a thin section under optical microscope showing a pseudotachylyte vein (E-W black<br />
layer) in plane polariz<strong>ed</strong> light. Pseudotachylyte fault vein (E-W black layer) with two injection veins within flaser gabbros. Both the fault and injection veins cut<br />
<strong>di</strong>scordantly the foliation of the host rock. B) Back-scatter<strong>ed</strong> SEM image showing the detail of the intersection between the fault vein and central injection vein<br />
in (A). The pseudotachylyte fine grain of the matrix includes elongate plagioclase clasts (dark gray) defining a foliation in the fault vein, slightly oblique to the<br />
vein boundary, that <strong>di</strong>sappears in the injection vein. In the lower left side of the fault vein a cluster of small i<strong>di</strong>oblastic garnets (grt) is present. C) Back-scatterd<br />
SEM image of the matrix in the pseudotachylyte fault vein consisting of plagioclase (pl; dark gray), a femic mineral phase (mainly px; light gray) and small<br />
bright grains (white) of ilmenite/magnetite D) Back-scatter<strong>ed</strong> SEM image of garnet grains in (B). Garnets have i<strong>di</strong>oblastic shape and show inclusions of<br />
ilmenite/magnetite align<strong>ed</strong> with the matrix foliation. E) Back-scatter<strong>ed</strong> FE-SEM image of poikilitic single i<strong>di</strong>oblastic garnet in the pseudotachylyte. with small<br />
inclusions of pyroxene (px?). F) Back-scatter<strong>ed</strong> SEM image of plagioclase microlites in the injection vein. Microlites are decorat<strong>ed</strong> along the <strong>ed</strong>ges by bright<br />
small grains of oxides.
166 L. PITTARELLO ET ALII<br />
magnetite/ilmenite and rare quartz), respectively, form<strong>ed</strong> at<br />
800-900°C and 0.7-0.8 GPa (ZINGG, 1990; HANDY et alii,<br />
1999).<br />
The metagabbros and the flaser gabbros are cut by sharply<br />
bound<strong>ed</strong> ultramylonites and pseudotachylytes. Ultramylonite (a<br />
few millimeters to centimeters thick) consists of a matrix (grain<br />
size of 1-5 µm) of An54 plagioclase, hornblende and<br />
magnetite/ilmenite inclu<strong>di</strong>ng round<strong>ed</strong> clasts of plagioclase,<br />
pyroxene and garnet. Plagioclase-hornblende pairs from the<br />
matrix yield temperatures of 600-650°C using the calibration of<br />
HOLLAND AND BLUNDY (1994).<br />
Pseudotachylytes (Fig. 1a-b) are rather common and consist<br />
of a granoblastic matrix aggregate (grain size of 1-10 µm) of<br />
plagioclase and pyroxene, ilmenite/magnetite and rare<br />
i<strong>di</strong>oblastic garnet (Alm64, Gro12, Pyr11, And8, Spe5) (Fig. 1b-cd).<br />
The garnet is poikilitic with small (~1-2 µm) inclusions of<br />
matrix minerals. Locally (mainly in injection veins) plagioclase<br />
(An47) shows microlite crystal shapes typical of<br />
pseudotachylyte (Fig. 2f). The pseudotachylyte matrix includes<br />
clasts of the host rock plagioclase (most common), pyroxene<br />
and apatite. Garnet is almost absent between clasts.<br />
Some pseudotachylytes show a crystal-plastic overprint with<br />
development of a foliation, outlin<strong>ed</strong> by elongat<strong>ed</strong> and<br />
recrystalliz<strong>ed</strong> plagioclase clasts. Some typical localiz<strong>ed</strong><br />
ultramylonites in the metagabbros also preserve non-foliat<strong>ed</strong><br />
portion of the matrix, inclu<strong>di</strong>ng angular clasts and showing<br />
cataclastic relationships with the host metagabbros in<br />
embayment in the host rock.<br />
DISCUSSION AND CONCLUSIONS<br />
The fine grain size of the pseudotachylyte matrix <strong>di</strong>d not<br />
allow<strong>ed</strong> a precise estimate of the temperature of formation of<br />
the frictional melts in the Premosello metagabbros. However,<br />
the presence of a crystal –plastic overprint of pseudotachylyte,<br />
the dynamic recrystallization of an An-rich (An47) plagioclase<br />
and the growth of garnet and pyroxene on the pseudotachylyte<br />
matrix suggest amphibolite facies ambient con<strong>di</strong>tions for the<br />
pseudotachylyte development. Pseudotachylytes were already<br />
describ<strong>ed</strong> in the Ivrea gabbros by BRODIE & RUTTER (1987)<br />
and by ZINGG et alii (1990), but they were interpret<strong>ed</strong> as having<br />
form<strong>ed</strong> during the low temperature Alpine history. Our new<br />
observations in<strong>di</strong>cate that a part of these pseudotachylytes must<br />
be referr<strong>ed</strong> to as pre-Alpine consistently with the assumption of<br />
HANDY & BRUN (2004).<br />
The widespread production of frictional melts, at<br />
temperatures under which metagabbros should be capable of<br />
ductile flow and well below the typical plastic-brittle transition<br />
of crustal rocks, is consistent with <strong>di</strong>fferent processes as list<strong>ed</strong><br />
above (HANDY & BRUN, 2004).<br />
The relatively high ambient temperature of generation of<br />
pseudotachylytes has been referr<strong>ed</strong> to relatively dry con<strong>di</strong>tions<br />
in the middle-lower crust (PENNACCHIONI & CESARE, 1997)<br />
affecting the temperature of the brittle-plastic transition. There<br />
is not a straightforward criterion to <strong>di</strong>scriminate between the<br />
above list<strong>ed</strong> mechanics actually responsible for generation of<br />
deep-seat<strong>ed</strong> pseudotachylytes although some microstructural<br />
observations apparently lend support for the concept of a<br />
relatively strong middle-lower crust due to the presence of<br />
water-deficient rocks (MANCKTELOW & PENNACCHIONI, 2004;<br />
FITZ GERALD et alii, 2006). Discriminating between these<br />
processes is of paramount importance in the understan<strong>di</strong>ng the<br />
rheology of the continental crust and will be the topic of<br />
forthcoming work.<br />
The association between very localiz<strong>ed</strong> ultramylonite and<br />
pseudotachylyte of similar thickness and orientation, the<br />
presence of an amphibolite facies overprint of some mylonites<br />
and the relict cataclastic features preserv<strong>ed</strong> in some domains of<br />
ultramylonites suggest that pseudotachylyte may have act<strong>ed</strong> as<br />
structural heterogeneities for the nucleation and localization of<br />
some mylonitic deformation after post-seismic stress relaxation.<br />
The primary role of pre-existing structural and compositional<br />
heterogeneities in the nucleation of ductile shear zones is well<br />
establish<strong>ed</strong> in several metamorphic environments (e.g.<br />
MANCKTELOW & PENNACCHIONI, 2005; PENNACCHIONI &<br />
MANCKTELOW, 2007).<br />
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generat<strong>ed</strong> during seismic faulting and eclogitization of the<br />
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BARRETT, S.: http://www.liv.ac.uk/~sdb/ImageSXM/<br />
BRODIE, K. H. (1981) - Variation in amphibole and<br />
plagioclase composition with deformation. Tectonophysics,<br />
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BRODIE, K.H. & RUTTER, E.H. (1987) - Deep crustal<br />
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BRODIE, K.H. & RUTTER, E.H. (1985) - On the relationship<br />
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ZURBRIGGEN, R. (1999) - Multistage accretion and<br />
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Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 168-171, 5 ff.<br />
La carta geologica <strong>del</strong> massiccio <strong>del</strong>la Berna<strong>di</strong>a<br />
(Prealpi Giulie meri<strong>di</strong>onali, Friuli, Italia NE)<br />
ABSTRACT<br />
The geological map of the Berna<strong>di</strong>a Massif (Southern Julian Prealps<br />
Friuli, NE Italy).<br />
The Southern Julian Prealps (SJP) belong to the sector of the External<br />
Dinarides includ<strong>ed</strong> in the WSW-ENE tren<strong>di</strong>ng, SSE-verging thrust belt of the<br />
eastern Southalpine Chain in evolution from the Middle Miocene to the<br />
present. On the basis of new geological mapping and mesostructural analysis,<br />
the geological framework of the Berna<strong>di</strong>a carbonate massif was point<strong>ed</strong> out. In<br />
the Berna<strong>di</strong>a area only cover sequences outcrop ranging from Early<br />
Creataceous to Early Eocene. Scanty outcrops of glauconitic arenites (Lower<br />
Miocene) are present in the footwall of the major Neogene thrusts. During the<br />
Mesozoic the Berna<strong>di</strong>a massif was part of the Friulian Carbonate Platform<br />
(FCP). Starting from Late Cretaceous to Early Eocene the study area was part<br />
of the WSW–verging Dinaric chain-for<strong>ed</strong>eep system. The Paleogene imbricate<br />
thrust-system shows staircase trajectory and duplex-geometries; thrusts are<br />
mainly NNW-SSE tren<strong>di</strong>ng and show WSW tectonic transport. Inherit<strong>ed</strong><br />
Upper Cretaceous normal faults (NNE-SSW- and NW-SE-tren<strong>di</strong>ng) are often<br />
invert<strong>ed</strong> in lateral and frontal ramps respectively. Starting from Middle<br />
Miocene the S-SSE verging thrust system of the Southalpine Chain start<strong>ed</strong>,<br />
giving rise to a low-angle, approximately WSW-ENE to WNW-ESE tren<strong>di</strong>ng<br />
thrusts. Neoalpine compression deform<strong>ed</strong> the Paleogene structural belt,<br />
producing dome-basin structural pattern. Moreover the Neogene–Quaternary<br />
thrusts often re-utilis<strong>ed</strong> the Dinaric ramps as frontal or lateral ones.<br />
Key words: thrust tectonics, Paleogene External Dinarides,<br />
Neogene-Quaternary eastern Southalpine Chain, Friuli.<br />
INTRODUZIONE<br />
Il massiccio calcareo <strong>del</strong>la Berna<strong>di</strong>a rappresenta una <strong>del</strong>le<br />
ultime propaggini meri<strong>di</strong>onali <strong>del</strong>le Prealpi Giulie essendo<br />
posto a ridosso <strong>del</strong>l’alta pianura friulana orientale. Dal punto <strong>di</strong><br />
vista geologico le Prealpi Giulie meri<strong>di</strong>onali, limitate verso<br />
settentrione dal sovrascorrimento Periadriatico (o linea Barcis -<br />
Staro Selo Auct.), fanno parte <strong>del</strong>la Catena Dinarica esterna che<br />
è stata incorporata nella Catena Sudalpina orientale a partire<br />
dal Miocene m<strong>ed</strong>io (CASTELLARIN et al., 1992; DOGLIONI &<br />
BOSELLINI, 1987).<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> <strong>Georisorse</strong> e Territorio, Università <strong>di</strong> U<strong>di</strong>ne, via<br />
Cotonificio, 114 – 33100 U<strong>di</strong>ne; e-mail: eliana.poli@uniud.it<br />
M. ELIANA POLI *<br />
Nuovi rilevamenti geologico-strutturali <strong>di</strong> dettaglio,<br />
effettuati nell’ambito <strong>del</strong> Progetto CARG-FVG, hanno<br />
consentito <strong>di</strong> ricostruire la storia geologica <strong>del</strong> massiccio<br />
carbonatico a partire dal Cretacico superiore fino ad oggi. Sono<br />
stati definiti i rapporti stratigrafici <strong>del</strong>le unità presenti e<br />
riconosciuti due principali eventi contrazionali. Ogni evento è<br />
stato caratterizzato per geometria e cinematica; i rapporti <strong>di</strong><br />
interferenza e <strong>di</strong> sovrapposizione fra le strutture hanno<br />
consentito <strong>di</strong> ricostruire il timing <strong>del</strong>le <strong>di</strong>verse fasi deformative.<br />
IL CONTESTO PALEOGEOGRAFICO E LA SUCCESSIONE<br />
STRATIGRAFICA<br />
Nel quadro <strong>del</strong>la paleogeografia giurassico-cretacica l’area<br />
prealpina giulia faceva parte <strong>del</strong>la zona <strong>di</strong> transizione fra il<br />
Bacino Sloveno (o Giulio) e la Piattaforma Carbonatica<br />
Friulana (PCF) (Fig. 1). Tale situazione, instauratasi a partire<br />
dal Lias, si mantenne praticamente inalterata sino al Cretacico<br />
superiore.<br />
Fig. 1 – Paleogeografia giurassico-cretacica <strong>del</strong>l’area veneto-friulana. Nel<br />
riquadro l’area in esame. LP: Lineamento Periadriatico; LG: Linea <strong>del</strong>le<br />
Giu<strong>di</strong>carie; VFS: Valsugana–Fella–Sava..<br />
I terreni più antichi presenti nell’area in esame affiorano<br />
lungo le profonde incisioni dei torrenti Torre e Cornappo. Si<br />
tratta <strong>di</strong> calcari bioclastici <strong>di</strong> margine <strong>di</strong> piattaforma <strong>del</strong><br />
Valanginiano (VENTURINI & TUNIS, 1998). La successione<br />
carbonatica continua poi con i calcari <strong>di</strong> piattaforma interna <strong>del</strong><br />
Calcare <strong>del</strong> Cellina (Hauteriviano - Albiano p.p.) e termina con
le facies bioclastiche <strong>di</strong> piattaforma aperta <strong>del</strong> Calcare <strong>del</strong> M.<br />
Cavallo (Albiano sup. – Cenomaniano) (Fig. 2).<br />
A partire dal Campaniano-Maastrichtiano (COUSIN, 1981,<br />
PIRINI RADRIZZANI et al., 1986; SARTORIO et al., 1987;<br />
VENTURINI & TUNIS, 1992), lo slope e il margine <strong>del</strong>la<br />
piattaforma furono interessati da un'intensa tettonica<br />
sins<strong>ed</strong>imentaria. Questi processi, legati alla migrazione verso<br />
WSW <strong>del</strong> sistema catena-avanfossa <strong>del</strong>le Dinari<strong>di</strong> Esterne,<br />
determinarono il sollevamento <strong>del</strong>la PCF che venne a trovarsi<br />
in posizione <strong>di</strong> peripheral buldge, subendo dapprima<br />
un’intensa erosione subaerea e poi un rapido annegamento<br />
anche ad opera <strong>di</strong> faglie normali. Tutto ciò produsse nell’area<br />
prealpina giulia potenti corpi <strong>di</strong> brecce carbonatiche<br />
sottomarine contenenti livelli <strong>di</strong> emipelagiti a globotruncane<br />
(Scaglia rossa friulana) che si depositarono nell’avanfossa e<br />
sui blocchi <strong>di</strong> piattaforma annegati (Fig. 2).<br />
Fig. 2 – I rapporti stratigrafici fra le formazioni affioranti nel massiccio<br />
<strong>del</strong>la Berna<strong>di</strong>a. Legenda: PRP - Arenaria <strong>di</strong> Preplans (Aquitaniano);<br />
SVO – Arenarie e marne <strong>di</strong> Savorgnano (Ypresiano p.p.); GRI –<br />
Flysch <strong>del</strong> Grivò (Selan<strong>di</strong>ano sup. – Ypresiano p.p.); SRF – Scaglia<br />
rossa friulana (Maastrichtiano); CMC – Calcare <strong>del</strong> Monte Cavallo<br />
(Albiano sup. – Cenomaniano); CEL – Calcare <strong>del</strong> Cellina<br />
(Hauteriviano - Albiano p.p.).<br />
A partire dal Paleocene m<strong>ed</strong>io-superiore anche l’area in<br />
esame cominciò ad essere interessata dalla s<strong>ed</strong>imentazione<br />
torbi<strong>di</strong>tica con il Flysch <strong>del</strong> Grivò (qui <strong>del</strong> Selan<strong>di</strong>ano<br />
superiore - Ypresiano p.p.). Tale unità è caratterizzata da<br />
imponenti megastrati formati in gran parte da materiale<br />
proveniente dallo smantellamento <strong>del</strong>la piattaforma carbonatica<br />
<strong>ed</strong> intercalati con depositi torbi<strong>di</strong>tici silicoclastici più <strong>di</strong>stali<br />
(CATANI & TUNIS, 2000). La s<strong>ed</strong>imentazione torbi<strong>di</strong>tica<br />
proseguì nel resto <strong>del</strong>l’Ypresiano con la deposizione <strong>del</strong>le<br />
facies prevalentemente fini appartenenti alle Marne e arenarie<br />
<strong>di</strong> Savorgnano (ZANFERRARI et al., 2008).<br />
L’area prealpina giulia venne raggiunta dal fronte <strong>di</strong>narico<br />
nell’Eocene m<strong>ed</strong>io-superiore, incorporata in esso e sollevata,<br />
subendo quin<strong>di</strong> erosione subaerea fino alla fine <strong>del</strong>l’Oligocene.<br />
Localmente, alcuni isolati lembi <strong>di</strong> “molassa” <strong>di</strong> età<br />
aquitaniana (Arenaria <strong>di</strong> Preplans) poggiano in <strong>di</strong>scordanza o<br />
in paraconcordanza sulle successioni torbi<strong>di</strong>tiche paleogeniche<br />
(Fig. 2).<br />
ASSETTO STRUTTURALE<br />
Il massiccio <strong>del</strong>la Berna<strong>di</strong>a è stato per lungo tempo<br />
interpretato come una anticlinale asimmetrica ad asse WNW–<br />
ESE e vergenza meri<strong>di</strong>onale, definito come "ellissoide" a causa<br />
<strong>del</strong>le evidenti chiusure laterali sia verso SE che verso NW<br />
(FERUGLIO, 1925). L’assetto strutturale è in realtà dato da una<br />
LA CARTA GEOLOGICA DEL MASSICCIO DELLA BERNADIA<br />
169<br />
pila <strong>di</strong> unità tettoniche <strong>di</strong>nariche, successivamente rideformate<br />
nell’evento contrazionale Neoalpino.<br />
Il primo e fondamentale evento deformativo è quello<br />
connesso alla propagazione <strong>del</strong>la Catena Dinarica esterna <strong>di</strong> età<br />
cretacica superiore - paleogenica, che nelle Prealpi friulane<br />
centro-orientali ha dato origine ad un complesso sistema <strong>di</strong><br />
sottili sovrascorrimenti <strong>di</strong> copertura con vergenza occidentale<br />
(POLI, 1994; MERLINI et alii, 2002; PONTON, 2007). Il fronte<br />
<strong>di</strong>narico esterno è oggi rappresentato dal sovrascorrimento<br />
sepolto <strong>di</strong> Palmanova che corre, sigillato dalla successione<br />
miocenica, con <strong>di</strong>rezione NNW-SSE nel settore SW <strong>del</strong>la<br />
pianura friulana centrale. L’evento deformativo <strong>di</strong>narico ha<br />
determinato un raccorciamento crostale in <strong>di</strong>rezione est-ovest<br />
che nell’area prealpina giulia può essere valutato<br />
complessivamente <strong>del</strong>l’or<strong>di</strong>ne <strong>di</strong> alcune decine <strong>di</strong> km.<br />
I thrust e le strutture minori ad essi associate coinvolgono<br />
intensamente tutte le unità stratigrafiche presenti. Essi hanno<br />
una <strong>di</strong>rezione compresa fra NW-SE e NNW-SSE e trasporto<br />
tettonico verso SW-WSW (Figg. 3 e 5). Generalmente<br />
presentano una traiettoria a gra<strong>di</strong>ni, in cui rampe, anche a forte<br />
angolo, e flat si susseguono con <strong>di</strong>verse geometrie anche in<br />
funzione <strong>del</strong>la litologia attraversata.<br />
I thrust sono accompagnati da ampie zone <strong>di</strong> taglio sulle<br />
quali si osservano strutture s/c e duplex. Entro il flysch<br />
arenitico-marnoso la deformazione si è propagata<br />
principalmente lungo i piani <strong>di</strong> strato, con zone <strong>di</strong> taglio a<br />
basso angolo. Nelle sequenze a stratificazione più massiccia<br />
(calcari <strong>di</strong> piattaforma e livelli calciru<strong>di</strong>tico-calcarentici dei<br />
megastrati torbi<strong>di</strong>tici) le geometrie <strong>del</strong>le rampe possono<br />
presentarsi spesso ad alto angolo accompagnate da intenso<br />
clivaggio stilolitico (POLI, 1994).<br />
Frequente è la tettonica <strong>di</strong> inversione: le superfici <strong>di</strong> faglia<br />
estensionale a m<strong>ed</strong>io-alto angolo <strong>del</strong> Cretacico superiore sono<br />
spesso invertite come rampe frontali dalla tettonica<br />
compressiva <strong>di</strong>narica (Fig. 4).<br />
Fig. 3 – Particolare <strong>del</strong>la rampa frontale a vergenza SW <strong>del</strong> sistema <strong>di</strong><br />
thrust <strong>del</strong> M. Lanta rappresentato in Fig. 4 (Ponte <strong>del</strong> Brisicul, T.<br />
Cornappo). Sono evidenti le strutture <strong>di</strong> duplicazione. La struttura è<br />
impostata nei calcari <strong>di</strong> piattaforma interna (Calcare <strong>del</strong> Cellina).<br />
La struttura più interessante e meglio esposta è quella che si<br />
sviluppa lungo la valle <strong>del</strong> T. Cornappo, presso il M. Lanta<br />
(Figg. 3 e 4). Qui una serie <strong>di</strong> superfici <strong>di</strong> movimento con<br />
geometria a gra<strong>di</strong>ni, che coinvolgono sia i carbonati <strong>di</strong><br />
piattaforma che le successioni torbi<strong>di</strong>tiche, forma un foreland<br />
<strong>di</strong>pping duplex plurichilometrico con inversione <strong>del</strong>le strutture
170 M. ELIANA POLI<br />
Fig. 4 – <strong>Sezione</strong> geologica attraverso il massiccio <strong>del</strong>la Berna<strong>di</strong>a. Si noti presso il M. Lanta la zona <strong>di</strong> raccorciamento <strong>di</strong>narica SW-vergente che appila le<br />
successioni cretaciche <strong>di</strong> piattaforma interna (CEL) sulle unità <strong>di</strong> piattaforma esterna (CMC) e sulle successoni torbi<strong>di</strong>tiche paleocenico - eoceniche.<br />
Legenda: SVO – Arenarie e marne <strong>di</strong> Savorgnano (Ypresiano p.p.); GRI – Flysch <strong>del</strong> Grivò (Selan<strong>di</strong>ano sup. – Ypresiano p.p.); CMC – Calcare <strong>del</strong><br />
Monte Cavallo (Albiano sup. – Cenomaniano); CEL – Calcare <strong>del</strong> Cellina (Hauteriviano - Albiano p.p.).<br />
<strong>di</strong> collasso tardo cretaciche e una traslazione stimata <strong>di</strong> oltre<br />
due km. Tali orizzonti <strong>di</strong> duplicazione sembrano avere come<br />
scollamento basale il sovrascorrimento <strong>del</strong>la Berna<strong>di</strong>a,<br />
in<strong>di</strong>viduato nel pozzo AGIP Berna<strong>di</strong>a 1 (MARTINIS, 1966) ad<br />
una profon<strong>di</strong>tà dal piano campagna <strong>di</strong> circa 1.235 m. Tale<br />
superficie <strong>di</strong> accavallamento sovrappone le facies <strong>di</strong> rampa <strong>del</strong><br />
Calcare ad Ellipsactinie (Giurassico sup.) alle successioni<br />
torbi<strong>di</strong>tiche <strong>del</strong> Flysch <strong>del</strong> Grivò. Una serie <strong>di</strong> anticlinalisinclinali<br />
<strong>di</strong> rampa, anche a sviluppo chilometrico, accompagna<br />
la propagazione <strong>del</strong>la deformazione per thrust.<br />
A partire dal Miocene m<strong>ed</strong>io e fino all’Attuale l’area<br />
friulana è stata coinvolta in un secondo evento contrazionale,<br />
quello Neoalpino. All’interno <strong>del</strong>l’evento Neoalpino sono state<br />
riconosciute due fasi deformative principali. La prima è<br />
collocata nel Miocene superiore (forse anche parte <strong>del</strong> Pliocene<br />
inferiore) <strong>ed</strong> è responsabile degli accavallamenti SSE-vergenti<br />
a <strong>di</strong>rezione m<strong>ed</strong>ia N70°E che caratterizzano il settore veneto e<br />
friulano. La seconda (Pliocene - Olocene) ha dato origine alle<br />
strutture compressive più esterne e recenti (spesso ancora<br />
cieche nella pianura) ad andamento tra WSW-ENE e WNW-<br />
ESE. Dal Miocene superiore al Pleistocene σ1 è oscillato<br />
ripetutamente fra NW-SE a NNW-SSE (CAPUTO et alii, 2003).<br />
Nell’area in esame l’associazione strutturale neoalpina è<br />
data da sovrascorrimenti pellicolari a basso angolo, a <strong>di</strong>rezione<br />
tra WSW-ENE e WNW-ESE e senso <strong>di</strong> trasporto verso<br />
meri<strong>di</strong>one (Fig. 5), cui si accompagnano anticlinali <strong>di</strong> rampa e<br />
pieghe per propagazione <strong>di</strong> faglia. La deformazione è <strong>di</strong> solito<br />
accompagnata da un sistema <strong>di</strong> faglie verticali coniugate, con<br />
movimento trascorrente destro sulle faglie a <strong>di</strong>rezione NW-SE<br />
e sinistro su quelle a <strong>di</strong>rezione prevalente N-S.<br />
La compressione neoalpina, quasi ortogonale alla <strong>di</strong>rezione<br />
<strong>di</strong> propagazione paleogenica, ha dato luogo alla deformazione<br />
<strong>del</strong>le unità tettoniche <strong>di</strong>nariche, che presentano sia rotazioni<br />
degli assi <strong>di</strong> piega che geometrie a duomo-bacino a tutte le<br />
Fig. 5 – Proiezioni stereografiche <strong>di</strong> superfici <strong>di</strong> movimento e relativi in<strong>di</strong>catori cinematici misurate nel massiccio <strong>del</strong>la Berna<strong>di</strong>a. A sinistra, quelli legati<br />
all’evento <strong>di</strong>narico (1= 232/10); a destra quelli legati alle fasi deformative neogeniche: 1 = 300/08 (fase deformativa miocenico – ?pliocenica inf.) e<br />
1 = 161/02 (fase pliocenico-quaternaria).<br />
scale. Spesso la propagazione <strong>del</strong>la deformazione neoalpina<br />
avviene riutilizzando come rampe oblique o frontali le superfici<br />
<strong>di</strong> accavallamento <strong>di</strong>narico a basso angolo.<br />
La presenza alla base dei sovrascorrimenti <strong>di</strong> Buia e<br />
Periadriatico (segmento Gemona–Kobarid) <strong>di</strong> isolati lembi <strong>di</strong><br />
Arenaria <strong>di</strong> Preplans (Miocene inferiore), in<strong>di</strong>ca che queste<br />
strutture hanno agito durante l’evento contrazionale neoalpino.
L’analisi <strong>del</strong>le linee sismiche industriali a riflessione <strong>del</strong>la<br />
pianura ha confermato che anche il sovrascorrimento Susans -<br />
Tricesimo, ritenuto da molti AA. la sorgente sismogenica <strong>del</strong><br />
terremoto <strong>del</strong> Friuli <strong>del</strong> 1976, ha agito in epoca neogenicoquaternaria<br />
in quanto <strong>di</strong>sloca <strong>di</strong> oltre 2 km le sequenze basali<br />
<strong>del</strong>la “molassa” sudalpina (BURRATO et al., 2008).<br />
BIBLIOGRAFIA<br />
BURRATO P., POLI M.E., VANNOLI P., ZANFERRARI A., BASILI<br />
R. & GALADINI F. (2008) - Sources of Mw 5+ earthquakes<br />
in northeastern Italy and western Slovenia: An updat<strong>ed</strong><br />
view bas<strong>ed</strong> on geological and seismological evidence.<br />
Tectonophysics, 453, 157-176, doi: 10.1016/j.tecto.<br />
2007.07.009.<br />
CAPUTO R., POLI M.E. & ZANFERRARI A. (2003) - Neogene-<br />
Quaternary Twist Tectonics in the Eastern Southern Alps,<br />
Italy. Mem. Sci. Geol., 54, 155-158.<br />
COUSIN, M. (1981) Les rapports Alpes-Dinarides - Les confins<br />
de l'Italie et de la Yougoslavie. Soc. Géol. du Nord, 1: 1-<br />
521, 2: 1-521, Villeneuve d'Asq.<br />
CATANI & TUNIS (2001) – Caratteristiche s<strong>ed</strong>imentologiche dei<br />
megabanchi cartonatici paleogenici <strong>del</strong> Bacino Giulio<br />
(Valli <strong>del</strong> Natisone, Friuli orientale). St. Trentini Sc. Nat. –<br />
Acta Geologica, 77, 81-102.<br />
CASTELLARIN, A., CANTELLI, L., FESCE, A.M., MERCIER, J.L.,<br />
PICOTTI, V., PINI, G.A., PROSSER, G. & SELLI, L., (1992) -<br />
Alpine compressional tectonics in the Southern Alps.<br />
Relationships with the N-Apennines. Annales Tectonicae, 6,<br />
62-94.<br />
DOGLIONI C.& BOSELLINI A. (1987) - Eoalpine and mesoalpine<br />
tectonics in the Southern Alps. Geol. Rundsch., 76, 735-<br />
754.<br />
FERUGLIO E. (1925) - Le Prealpi tra l’Isonzo e l’Arzino. Boll.<br />
Assoc. Agr. Friulana, ser. 7, 39-40, 1-301, U<strong>di</strong>ne.<br />
MARTINIS B. (1966) - Prove <strong>di</strong> ampi sovrascorrimenti nelle<br />
Prealpi friulane e venete. Mem. Ist. Geol. Miner. Univ.<br />
Padova, XXV, 1-31.<br />
MERLINI S., DOGLIONI C., FANTONI R. & PONTON M. (2002) -<br />
Analisi strutturale lungo un profilo geologico fra la linea<br />
Fella-Sava e l'avampaese adriatico (Friuli Venezia Giulia -<br />
Italia). Mem. Soc. Geol. It., 57, 293-300.<br />
POLI M.E. (1994) - Evidenze <strong>di</strong> tettonica a thrust <strong>di</strong>narica nelle<br />
Prealpi Giulie meri<strong>di</strong>onali (Italia Nord-orientale). Atti<br />
Ticinensi Sci. Terra, ser. spec., 3, (1995), 99-114.<br />
PIRINI RADRIZZANI C., TUNIS G. & VENTURINI S.( 1986) –<br />
Biostratigrafia e paleogeografia <strong>del</strong>l’area sud-occidentale<br />
<strong>del</strong>l’anticlinale M. Mia – M. Mataiur (Prealpi Giulie). Riv.<br />
It. Paleont. Strat., 92, 327-382.<br />
LA CARTA GEOLOGICA DEL MASSICCIO DELLA BERNADIA<br />
171<br />
PONTON M. (2007) - Un’area polideformata nelle Prealpi<br />
Carniche: il Monte Broili e il Cuel dal Melooc. Gortania,<br />
28, 7-18.<br />
SARTORIO D. TUNIS G. & VENTURINI S. (1987) – Nuovi<br />
contributi per l’interpretazione geologica e paleogeografia<br />
<strong>del</strong>le Prealpi Giulie (Friuli orientale): il Pozzo Span . Riv.<br />
It. Paleont. Strat., 93, 181-200.<br />
TUNIS G. & VENTURINI S. (1992) - Evolution of the Southern<br />
Margin of the Julian Basin with Emphasis on the Megab<strong>ed</strong>s<br />
and Turbi<strong>di</strong>tes Sequence of the Southern Julian Prealps<br />
(NE Italy). Geologia Croatica, 45, 127-150.<br />
VENTURINI S. & TUNIS G. (1997) - Il canyon campanianomaastrichtiano<br />
<strong>del</strong>la Val Torre (Prealpi Giulie). Atti<br />
Ticinensi. Sci. Terra, ser. sp. 7, 7-16.<br />
ZANFERRARI, A., AVIGLIANO, R., MONEGATO, G., PAIERO, G.,<br />
POLI, M.E. (2008) - <strong>Note</strong> illustrative <strong>del</strong>la Carta geologica<br />
dItalia alla scala 1:50,000 Foglio 066 U<strong>di</strong>ne; 176 pp.<br />
Graphic Linea, Tavagnacco (UD).
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 172-175, 5 ff.<br />
Geometria, cinematica e attività pliocenico-quaternaria <strong>del</strong> sistema <strong>di</strong><br />
sovrascorrimenti Arba-Ragona (Alpi Meri<strong>di</strong>onali orientali, Italia NE).<br />
ABSTRACT<br />
Geometry, kinematics and Pliocene-Quaternary activity of the Arba-<br />
Ragogna thrust-system (Eastern Southern Alps, NE Italy)<br />
New stratigraphic, geomorphologic and structural data let to <strong>del</strong>ineate the<br />
Pliocene - Quaternary activity of the Arba-Ragogna thrust-system that belongs<br />
to the front of the eastern Southalpine Chain, a S-SE verging thrust-belt in<br />
evolution from middle Miocene to Present. Geometry and kinematic of the<br />
Arba-Ragogna thrust-system were recognis<strong>ed</strong> both using structural survey and<br />
industrial reflection seismic profiles. The system is arrang<strong>ed</strong> as a S-SE verging<br />
and about WSW-ENE striking imbricate fan of m<strong>ed</strong>ium to low-angle thrusts.<br />
The outermost is the Arba-Ragogna blind thrust. Its long–lasting activity is<br />
testifi<strong>ed</strong> by the angular unconformities between the Upper Miocene, Upper<br />
Pliocene and Pleistocene stratigraphic units. A vertical slip rate of about 0.19<br />
mm/a was calculat<strong>ed</strong> for the Arba –Ragogna thrust during the last 21000<br />
years. The Arba-Ragogna thrust is one of the seismogenic sources capable to<br />
generate earthquakes with M6 in the Veneto-Friuli region, but it cannot be<br />
relat<strong>ed</strong> to historical earthquakes. It in<strong>di</strong>cates that it is one of the “silent”<br />
seismogenic source of the eastern Southern Alps.<br />
Key words: thrust tectonics, embricate thrust-system,<br />
seismogenic sources, eastern Southern Alps, Friuli.<br />
INQUADRAMENTO REGIONALE<br />
Il sistema strutturale Arba-Ragogna fa parte <strong>del</strong> fronte<br />
pliocenico-quaternario <strong>del</strong>la catena Sudalpina orientale (ESC),<br />
caratterizzata da sovrascorrimenti a <strong>di</strong>rezione m<strong>ed</strong>ia WSW-<br />
ENE e WNW-ESE e vergenza meri<strong>di</strong>onale che si estendono dai<br />
Monti Lessini fino all’area <strong>di</strong> confine italo-slovena (Fig. 1). La<br />
catena Sudalpina orientale è una tipica catena a<br />
sovrascorrimenti sia <strong>di</strong> basamento che pellicolari con pieghe<br />
per propagazione <strong>di</strong> faglia, fault bend folds, zone a triangolo e<br />
ventagli embricati <strong>di</strong> faglie come strutture prevalenti.<br />
Secondo MASSARI (1990), FANTONI et alii (2002),<br />
CASTELLARIN et alii (1992), CASTELLARIN & CANTELLI (2000),<br />
CASTELLARIN et alii (2006), l’ESC ha acquisito il suo assetto<br />
strutturale fondamentale durante la fase deformativa<br />
serravalliano - tortoniana. Questa fase deformativa, sviluppatasi<br />
con un σ1 orientato circa NNW-SSE, ha dato luogo ad una<br />
_________________________<br />
M. ELIANA POLI (*), ADRIANO ZANFERRARI (*) & GIOVANNI MONEGATO (**)<br />
(*)Dipartimento <strong>di</strong> <strong>Georisorse</strong> e Territorio - Università <strong>di</strong> U<strong>di</strong>ne; e-mail:<br />
eliana.poli@uniud.it<br />
(**)Dipartimento <strong>di</strong> Geoscienze – Università <strong>di</strong> Padova; e-mail:<br />
giovanni.monegato@unipd.it<br />
serie <strong>di</strong> thrust a <strong>di</strong>rezione m<strong>ed</strong>ia WSW - ENE e vergenza SSE,<br />
tra i quali il principale per importanza regionale è quello <strong>del</strong>la<br />
Valsugana, che nella zona <strong>di</strong> Agordo e lungo la valle <strong>del</strong> Piave<br />
fa sovrascorrere le sequenze metamorfiche varisiche al <strong>di</strong> sopra<br />
dei terreni permo-mesozoici. Durante il Serravalliano -<br />
Messiniano il rapido avanzare verso SSE <strong>del</strong> fronte <strong>del</strong>la<br />
catena Sudalpina orientale in forte sollevamento ha determinato<br />
la formazione <strong>di</strong> una avanfossa con depocentro nell’area<br />
prealpina veneta orientale e friulana e <strong>di</strong> un cuneo clastico,<br />
potente oltre 3000 m nelle Prealpi venete e oltre 2.500 m in<br />
quelle carniche (MASSARI et alii, 1986b; FANTONI et alii, 2002;<br />
ZANFERRARI et alii, 2008). La successione serravallianomessiniana<br />
che attualmente affiora nell’area collinare sia veneta<br />
che friulana, è stata intensamente coinvolta nel sistema <strong>di</strong><br />
sovrascorrimenti frontali neogenico-quaternari. Con la<br />
deposizione <strong>del</strong> conglomerato <strong>del</strong> Montello (Tortoniano<br />
superiore-Messiniano inferiore) l’avanfossa viene colmata e<br />
<strong>di</strong>venta, nel Pliocene e soprattutto nel Quaternario, un bacino <strong>di</strong><br />
avampaese moderatamente subsidente.<br />
Fig. 1 – Schema tettonico <strong>del</strong>l’Italia NE e <strong>del</strong>la Slovenia occidentale (da<br />
BURRATO et alii, 2008).<br />
Il successivo intervallo pliocenico-quaternario è<br />
contrad<strong>di</strong>stinto dall’attivazione e dall’evoluzione <strong>del</strong> sistema <strong>di</strong><br />
sovrascorrimenti che caratterizzano il margine meri<strong>di</strong>onale dei<br />
rilievi prealpini (Montello - Friuli belt in CASTELLARIN et alii,<br />
2006) e parte <strong>del</strong> sottosuolo <strong>del</strong>la pianura friulana centroorientale,<br />
formando il fronte più esterno <strong>del</strong>la catena. I thrust<br />
sono ciechi nella pianura la cui superficie presenta comunque<br />
varie evidenze <strong>di</strong> deformazioni e <strong>di</strong>slocazioni anche
ecentissime. In generale evidenze <strong>di</strong> deformazioni recenti sono<br />
date da anticlinali <strong>di</strong> crescita, paleosuperfici quaternarie<br />
sollevate e tiltate e anomalie <strong>di</strong> drenaggio (Gala<strong>di</strong>ni et alii,<br />
2005). La <strong>di</strong>rezione <strong>di</strong> raccorciamento pliocenicoquaternaria,<br />
legata ad un 1 <strong>di</strong>stribuito a ventaglio tra<br />
NNW-SSE e NNE-SSW proc<strong>ed</strong>endo da ovest a est, varia da<br />
NW-SE a NNW-SSE. Questo fatto, legato alla forma <strong>del</strong><br />
cuneo <strong>di</strong> avampaese che si affonda sotto al fronte sudalpino<br />
orientale e alle numerose er<strong>ed</strong>ità strutturali mesoalpine<br />
presenti, trova una conferma nella <strong>di</strong>stribuzione <strong>del</strong>la<br />
sismicità minore (Bressan et alii, 1998).<br />
Lungo il margine meri<strong>di</strong>onale <strong>del</strong>l’ESC il raccorciamento<br />
e l’inspessimento crostale in atto sono testimoniati anche<br />
<strong>del</strong>l’intensa attività sismica che caratterizza il Veneto e il<br />
Friuli centrale (SLEJKO et alii, 1989, POLI et alii, 2002;<br />
GALADINI et alii, 2005; BURRATO et alii, 2008). A questo<br />
proposito il catalogo Working Group CPTI (2004) riporta per<br />
l’area veneto-friulana numerosi terremoti storici e strumentali<br />
con magnitudo compresa fra il 6 e il 7: 1117 (Verona), 1348<br />
(Carnia), 1695 (Asolo), 1873 (Belluno), 1936 (Bosco <strong>del</strong><br />
Cansiglio) e 1976 (Friuli).<br />
IL SISTEMA ARBA – RAGOGNA<br />
Nuovi dati stratigrafici, geomorfologici e strutturali<br />
(Progetti CARG–FVG, PRIN e GNDT) hanno permesso <strong>di</strong><br />
<strong>del</strong>ineare le caratteristiche geometriche e cinematiche e<br />
l’evoluzione pliocenico- quaternaria <strong>del</strong> sistema <strong>di</strong><br />
sovrascorrimenti Arba-Ragogna, che interessa la zona prealpina<br />
carnica fra Maniago e Ragogna e l’antistante pianura.<br />
I dati strutturali sono stati ottenuti sia avvalendosi <strong>del</strong><br />
rilevamento geologico-strutturale <strong>di</strong> terreno che dall’analisi <strong>di</strong><br />
a)<br />
b)<br />
Pli<br />
Aq - La<br />
linee sismiche industriali <strong>del</strong>l’area. Il sistema Arba-Ragogna è<br />
Q<br />
IL SISTEMA DI THRUST ARBA-RAGOGNA (FRIULI)<br />
Se - Me<br />
J -K<br />
Pal - Eoc<br />
Fig. 2 – <strong>Sezione</strong> sismica industriale al fronte <strong>del</strong> sovrascorrimento Arba–<br />
Ragogna (a) e sua interpretazione geologica (b). Il sistema è formato da più<br />
superfici <strong>di</strong> accavallamento SSE-vergenti.<br />
173<br />
composto da un fascio embricato <strong>di</strong> sovrascorrimenti Svergenti<br />
a <strong>di</strong>rezione m<strong>ed</strong>ia WSW-ENE. La geometria dei thrust<br />
più esterni è caratterizzata da forte inclinazione nel tratto superficiale,<br />
localmente fino a subverticale (Fig. 2). Le faglie <strong>di</strong>ventano a m<strong>ed</strong>iobasso<br />
angolo tra 1-3 km <strong>di</strong> profon<strong>di</strong>tà, in quanto tendono a formare flat<br />
nelle litologie meccanicamente più deboli come la marna <strong>di</strong> Tarzo (con<br />
scollamento al top <strong>del</strong> “Gruppo <strong>di</strong> Cavanella”) e il flysch <strong>di</strong> Clauzetto<br />
(con scollamenti al tetto <strong>del</strong>la Piattaforma Carbonatica Friulana).<br />
In pianta le tip line dei sovrascorrimenti descrivono un<br />
andamento arcuato, formando talora locali strutture<br />
transpressive nelle zone <strong>di</strong> interferenza e <strong>di</strong> sovrapposizione tra<br />
le rampe.<br />
Una caratteristica fondamentale <strong>del</strong> sistema <strong>di</strong><br />
accavallamenti è la presenza <strong>di</strong> faglie fuori-sequenza (Fig. 3).<br />
Si tratta in vari casi <strong>di</strong> faglie inverse che si originano nella zona<br />
<strong>di</strong> cerniera <strong>di</strong> anticlinali frontali; in altri casi si tratta <strong>di</strong><br />
accavallamenti a basso angolo (20-30°) che tagliano le strutture<br />
Fig. 3 – Profilo geologico NW-SE attraverso il Monte <strong>di</strong> Ragogna. Si notano:<br />
l’anticlinale <strong>di</strong> rampa frontale <strong>del</strong> sovrascorrimento Arba-Ragogna che è<br />
tagliata da un thrust fuori sequenza nella zona <strong>di</strong> cerniera e il<br />
sovrascorrimento cieco Arba Ragogna il cui movimento ha determinato una<br />
serie <strong>di</strong> <strong>di</strong>scordanze angolari fra i terreni miocenico-quaternari.<br />
Legenda: Mon 1, 2,3, conglomerato <strong>del</strong> Montello (Tortoniano – Messiniano<br />
superiore); SPX: conglomerato <strong>di</strong> San Pietro <strong>di</strong> Ragogna (Pliocene superiore-<br />
Pleistocene inferiore); URP, SF, SPB: depositi pleistocenici; POI, UIN:<br />
depositi olocenici .<br />
plicative e <strong>di</strong>sgiuntive regionali (<strong>di</strong> regola a forte inclinazione<br />
fino a subverticali), con rigetti, per quanto è stato possibile<br />
osservare, <strong>di</strong> or<strong>di</strong>ne decametrico-ettometrico. Nell’area<br />
Sequals-Castelnovo-Ragogna, l’anticlinale <strong>di</strong> rampa frontale<br />
presenta una immersione assiale <strong>di</strong> circa 15-20° verso WSW.<br />
I rapporti <strong>di</strong> sovrapposizione degli in<strong>di</strong>catori cinematici<br />
misurati sulle superfici <strong>di</strong> faglia mostrano la tendenza alla<br />
rotazione <strong>del</strong>la <strong>di</strong>rezione <strong>di</strong> contrazione dal Miocene superiore<br />
(attorno a N135°) al Quaternario (fino a N 180-190°) (Fig. 4).<br />
La faglia basale <strong>del</strong> sistema, il sovrascorrimento Arba –<br />
Ragogna (AR), è cieca nella sua porzione occidentale, mentre<br />
affiora per alcuni km all’estremità orientale, dove risulta essere<br />
tagliato dal sovrascorrimento Susans-Tricesimo (sorgente <strong>del</strong><br />
terremoto <strong>del</strong> Friuli <strong>del</strong> 1976). La faglia AR è SSE-vergente,<br />
con <strong>di</strong>rezione m<strong>ed</strong>ia ENE-WSW, e mostra un rigetto verticale<br />
attorno ai 250 m <strong>del</strong>la base dei depositi quaternari (Fig. 2).<br />
Nella zona <strong>di</strong> Ragogna (Fig. 3), la faglia e l’anticlinale <strong>di</strong><br />
rampa frontale in crescita fra il Messiniano e l’Attuale,<br />
evidenziano rispettivamente una serie <strong>di</strong> rigetti decrescenti e <strong>di</strong><br />
<strong>di</strong>scordanze angolari fra le varie unità stratigrafiche <strong>di</strong> età<br />
compresa fra il Messiniano e il Pleistocene superiore (PAIERO<br />
& MONEGATO, 2003). Dai rapporti stratigrafici si ricava una<br />
velocità m<strong>ed</strong>ia <strong>di</strong> sollevamento <strong>di</strong> circa 0,19 mm/a per<br />
l’intervallo LGM-Attuale (GALADINI et alii, 2005).
174 POLI M. ELIANA ET ALII<br />
Fig. 4 – proiezione stereografica dei vettori <strong>di</strong> movimento misurati sulle<br />
superfici <strong>di</strong> faglia <strong>del</strong> fronte Neoalpino nell’area Maniago-Ragogna.<br />
Lungo la tip line <strong>del</strong>la faglia AR sono evidenti importanti<br />
deformazioni morfotettoniche, come la scarpata <strong>di</strong> circa 4 m <strong>di</strong> altezza<br />
che per oltre 2 km corre nella pianura tra Lestans e Valeriano e le forti<br />
anomalie <strong>del</strong> drenaggio dei torrenti Gerchia, Cosa e <strong>del</strong> Rugo <strong>di</strong><br />
Valeriano.<br />
Recentemente GALADINI et alii (2005) e BURRATO et alii<br />
(2008) hanno stu<strong>di</strong>ato le sorgenti sismogeniche presenti al<br />
fronte <strong>del</strong> Sudalpino orientale. Sulla base <strong>del</strong>le osservazioni<br />
effettuate, gli AA ritengono che il sovrascorrimento AR sia<br />
potenzialmente sismogenico, ma alla corrispondente sorgente<br />
Fig. 5 – Effetti <strong>di</strong> fenomeni <strong>di</strong> liquefazione nei depositi lacustri <strong>del</strong><br />
Pleistocene m<strong>ed</strong>io nel tetto <strong>del</strong> sovrascorrimento Arba -Ragogna (Rugo <strong>di</strong><br />
Valeriano).<br />
sismica parametrizzata (sorgente <strong>di</strong> Sequals) non è stato<br />
possibile associare alcun terremoto storico. La struttura, quin<strong>di</strong>,<br />
nonostante le forti evidenze morfotettoniche, fra cui anche<br />
sismiti (Fig. 5), risulta essere silente perlomeno nell’arco si<br />
tempo indagato e coperto dai cataloghi.<br />
BIBLIOGRAFIA<br />
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ZANFERRARI A., (2002) - New seismotectonic evidence from<br />
the analysis of the 1976-1977 and 1977-1999 seismicity in<br />
Friuli (NE Italy). Boll. Geof. Teor. Appl., 43, 53-78.<br />
SLEJKO D., CARULLI G.B., NICOLICH R., REBEZ A. ZANFERRARI<br />
A., CAVALLIN A., DOGLIONI C., CARRARO F., CASTALDINI<br />
D., ILICETO V., SEMENZA E. & ZANOLLA C. (1989) -<br />
Seismotectonics of the eastern Southern-Alps: a review.<br />
Boll. <strong>di</strong> Geof. Teor. Appl., 31, 109-136.
WORKING GROUP CPTI, 2004. Catalogo Parametrico dei<br />
Terremoti Italiani, v. 2004 (CPTI04). INGV, Bologna.<br />
Available at: http://emi<strong>di</strong>us.mi.ingv.it/CPTI/.<br />
ZANFERRARI A., AVIGLIANO R., CARRARO F., GRANDESSO P.,<br />
MONEGATO G., PAIERO G., POLI M.E., ROGLEDI S., STEFANI<br />
C., TOFFOLON G. - Carta geologica d’Italia alla scala<br />
1:50.000: Foglio 065 “Maniago”. APAT-Servizio<br />
Geologico Nazionale - Regione A. Friuli Venezia Giulia.<br />
Graphic Linea, Tavagnacco (UD). In stampa.<br />
IL SISTEMA DI THRUST ARBA-RAGOGNA (FRIULI)<br />
175
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 176-178, 3ff.<br />
Evidenze <strong>di</strong> tettonica recente <strong>ed</strong> attiva nel settore esterno sepolto<br />
<strong>del</strong>l’Appennino centrale abruzzese.<br />
ABSTRACT<br />
Evidence of recent and active tectonics in the buri<strong>ed</strong> external sector of<br />
the Abruzzi central Appennines.<br />
In order to identify evidence of active tectonics in the buri<strong>ed</strong> external<br />
sector of the Abruzzi central Appennine chain a combin<strong>ed</strong> geologicalstructural<br />
and morphometric study has been carri<strong>ed</strong> out east of the Maiella<br />
thrust front (Orsogna area). In this area, inde<strong>ed</strong>, although historical seismicity<br />
has been document<strong>ed</strong> (e.g., Orsogna 1881, Maw 5.6), the location and nature<br />
of the seismogenic source is still largely debat<strong>ed</strong>. The detail<strong>ed</strong> geologicalstructural<br />
mapping, the <strong>di</strong>stribution of various orders of Quaternary alluvial<br />
terraces and the evaluation of geomorphic in<strong>di</strong>ces, allow<strong>ed</strong> us to assume the<br />
existence of an area characteris<strong>ed</strong> by Quaternary "localis<strong>ed</strong> uplift", superpos<strong>ed</strong><br />
to the “regional doming” affecting the overall Apennine chain.<br />
Bas<strong>ed</strong> on these results, although still preliminary, we interpret the<br />
localis<strong>ed</strong> uplift in the Orsogna area as connect<strong>ed</strong> to the growth of a blind fault,<br />
active at least from the Middle Pleistocene.<br />
Key words: Abruzzi central Appennines, Active tectonics,<br />
external sector of the Chain, morphometric analyses, uplift<br />
and drainage response.<br />
INTRODUZIONE<br />
L’area <strong>di</strong> stu<strong>di</strong>o è ubicata subito ad oriente <strong>del</strong>l’anticlinale<br />
carbonatica <strong>del</strong>la Maiella, in corrispondenza <strong>del</strong> fronte sepolto<br />
<strong>del</strong>la catena centro-appenninica abruzzese (Fig. 1). I terreni<br />
affioranti in tale settore sono costituiti, principalmente, dai<br />
depositi silicoclastici argilloso-sabbiosi e conglomeratici <strong>del</strong>la<br />
Formazione Mutignano (Pliocene superiore - Pleistocene<br />
inferiore). In generale, i termini più recenti <strong>di</strong> questa<br />
Formazione sono tiltati verso l’Adriatico costituendo una<br />
blanda monoclinale immergente verso (N)NE.<br />
L’assetto geologico-strutturale <strong>del</strong> sottosuolo, ottenuto<br />
dall’interpretazione dei dati <strong>di</strong> sismica a riflessione, invece,<br />
evidenzia una serie <strong>di</strong> anticlinali, la più interna <strong>del</strong>le quali è<br />
nota in letteratura come struttura <strong>di</strong> Casoli-Bomba (CASNEDI<br />
1981; PATACCA et alii, 1992). Tali pieghe, orientate<br />
prevalentemente NNW-SSE, si sono sviluppate a seguito <strong>del</strong>la<br />
propagazione verso oriente <strong>di</strong> un accavallamento principale con<br />
_________________________<br />
(*) Università «G. D’Annunzio» <strong>di</strong> Chieti. Dipartimento <strong>di</strong> Scienze <strong>del</strong>la<br />
Terra - Campus Universitario Madonna <strong>del</strong>le Piane - Via dei Vestini 30<br />
- 66013 Chieti Scalo (CH).<br />
Lavoro eseguito con il contributo finanziario <strong>del</strong>l’Università.<strong>di</strong> Chieti (resp.<br />
A. Pizzi).<br />
GIUSEPPE POMPOSO (*) & ALBERTO PIZZI (*)<br />
geometria ramp-flat-ramp dal quale si <strong>di</strong>partono una serie <strong>di</strong><br />
splay minori. In particolare le strutture più esterne coinvolgono<br />
le unità alloctone molisane e le coperture silicoclastiche<br />
neogeniche (CALAMITA et alii, 2002).<br />
Tuttavia la carenza <strong>di</strong> dati sismici risolutivi nei livelli più<br />
pellicolari non ha permesso <strong>di</strong> vincolare il coinvolgimento, o<br />
meno, <strong>del</strong>le coperture tardoquaternarie nella deformazione e<br />
quin<strong>di</strong> <strong>di</strong> <strong>di</strong>agnosticare un’eventuale attività recente <strong>di</strong> tali<br />
elementi.<br />
Inoltre, sulla base <strong>di</strong> stu<strong>di</strong> storico-archivistici tale zona è<br />
stata proposta come l’area epicentrale <strong>del</strong>l’evento sismico <strong>del</strong><br />
1881, cui è stata attribuita una Maw pari a 5.6 e Io pari a 8 con<br />
massimi effetti ad Orsogna (BOSCHI et alii, 1997; MONACHESI<br />
e STUCCHI, 1997 Gruppo <strong>di</strong> Lavoro CPTI, 2004; GALADINI et<br />
alii, 2006).<br />
Tuttavia, non esistono ancora ipotesi chiare sulla<br />
localizzazione e sulla natura geologica <strong>del</strong>la sorgente<br />
sismogenetica responsabile per questo evento. Infatti, l’attività<br />
dei sovrascorrimenti in questo settore è generalmente<br />
considerata terminata al Pleistocene inferiore (CASNEDI 1981;<br />
PATACCA et alii, 1992). Per contro, non si hanno evidenze,<br />
nell’area, degli effetti <strong>del</strong> regime estensionale proprio <strong>del</strong>le aree<br />
<strong>del</strong>la dorsale appenninica (PIZZI et alii, in stampa).<br />
Fig. 1 – Schema geologico-strutturale <strong>del</strong>l’area analizzata (mo<strong>di</strong>ficato da<br />
Calamita et alii, 2002).
EVIDENZE DI TETTONICA RECENTE ED ATTIVA NEL SETTORE DELL’APPENNINO CENTRALE ABRUZZESE<br />
METODOLOGIA<br />
Al fine <strong>di</strong> valutare la possibile attività tardoquaternaria <strong>del</strong>le<br />
strutture osservate, sono stati eseguiti dettagliati stu<strong>di</strong><br />
geologico-strutturali <strong>di</strong> terreno affiancati da analisi <strong>di</strong> tipo<br />
morfometrico sui principali elementi <strong>del</strong> reticolo idrografico<br />
<strong>del</strong>l’area.<br />
Analisi geologico-strutturale<br />
Il rilevamento geologico <strong>di</strong> dettaglio ha permesso <strong>di</strong><br />
in<strong>di</strong>viduare, nell’area localizzata a sud-ovest <strong>del</strong>l’abitato <strong>di</strong><br />
Osogna, un settore nel quale le giaciture misurate nella<br />
Formazione Mutignano hanno un’immersione verso NW (Fig.<br />
2a-b), in netta contrapposizione con il quadro giaciturale<br />
misurato nell’ambito <strong>del</strong> rilevamento CARG <strong>del</strong> Foglio Chieti<br />
(AA.VV., in stampa), che denota, per tali depositi, un regionale<br />
assetto in monoclinale con immersione verso (N)NE (Fig.2c).<br />
Inoltre è stato effettuato uno stu<strong>di</strong>o <strong>di</strong> dettaglio sui depositi<br />
alluvionali terrazzati relativi al Torrente Moro il cui bacino<br />
idrografico attuale risulta totalmente inscritto all’interno dei<br />
depositi argilloso-sabbiosi <strong>del</strong>la Formazione Mutignano. In<br />
particolare, è stato osservato come i depositi alluvionali relativi<br />
ai due or<strong>di</strong>ni <strong>di</strong> terrazzo più antichi – attribuiti rispettivamente<br />
alla parte basale <strong>ed</strong> a quella sommitale <strong>del</strong> Pleistocene m<strong>ed</strong>io –<br />
risultino costituiti quasi esclusivamente da ciottoli <strong>di</strong> natura<br />
carbonatica e quin<strong>di</strong> correlabili con un’area <strong>di</strong> alimentazione<br />
esterna a quella <strong>del</strong> bacino idrografico attuale.<br />
Fig. 2 – Panoramica dei depositi argillosi <strong>del</strong>la Formazione Mutignano a<br />
sud <strong>del</strong>l’abitato <strong>di</strong> Orsogna (a). Particolare <strong>del</strong>le argille immergenti verso<br />
NW (b). Proiezione stereografica <strong>del</strong>le giaciture misurate nell’ambito <strong>del</strong><br />
rilevamento CARG <strong>del</strong> Foglio Chieti per la Formazione Mutignano (c)<br />
(AA. VV. in stampa).<br />
177<br />
Analisi Morfometriche<br />
La <strong>di</strong>fficoltà <strong>di</strong> in<strong>di</strong>viduare evidenze <strong>di</strong> deformazione<br />
tardoquaternaria sulla base dei dati geologici <strong>di</strong> superficie,<br />
imputabile principalmente alla natura argillosa dei depositi<br />
affioranti, ha fatto si che lo stu<strong>di</strong>o geologico-strutturale <strong>di</strong><br />
terreno venisse integrato con una serie <strong>di</strong> analisi morfologiche e<br />
morfometriche effettuate sui reticoli <strong>di</strong> drenaggio.<br />
Attraverso l’interpretazione <strong>di</strong> foto aeree e l’analisi <strong>di</strong> DEM<br />
(risoluzione pixel 20mx20m) m<strong>ed</strong>iante l’utilizzo <strong>di</strong> softwares<br />
d<strong>ed</strong>icati, è stato creato un database contenente i principali<br />
elementi <strong>del</strong> reticolo idrografico.<br />
Tra gli in<strong>di</strong>ci calcolati durante l’analisi morfometrica, lo<br />
Stream Length Gra<strong>di</strong>ent Index (SL) (HACK, 1973; KELLER &<br />
PINTER, 2001) estratto per circa 30 corsi d’acqua, è risultato<br />
particolarmente utile al fine <strong>di</strong> avere un quadro regionale e<br />
dettagliato <strong>del</strong>le anomalie presenti nelle variazioni <strong>del</strong><br />
gra<strong>di</strong>ente topografico <strong>del</strong> reticolo idrografico (Fig.3).<br />
Tutti i dati ricavati sia dal rilevamento geologico-strutturale<br />
Fig. 3 – Schema litologico <strong>del</strong>l’area con sovrapposizione dei valori<br />
<strong>del</strong>l’SL calcolati. L’ellisse in tratteggio evidenzia i valori anomali.<br />
che dalle analisi geomorfiche quantitative sono stati<br />
successivamente integrati in un geo-database con lo scopo <strong>di</strong><br />
poter fruire contemporaneamente <strong>di</strong> tutte le informazioni<br />
<strong>di</strong>sponibili.<br />
DISCUSSIONE<br />
Partendo dai dati <strong>di</strong> terreno è possibile quin<strong>di</strong> ricostruire<br />
l’evoluzione recente <strong>del</strong> settore analizzato considerando la<br />
presenza <strong>di</strong> un paleo-reticolo <strong>di</strong> drenaggio che costituiva il<br />
collettore tra le zone <strong>di</strong> approvvigionamento carbonatiche<br />
(anticlinale <strong>del</strong>la Maiella) <strong>ed</strong> i depositi alluvionali terrazzati <strong>del</strong><br />
Pleistocene m<strong>ed</strong>io attualmente localizzati all’interno <strong>del</strong> bacino<br />
idrografico <strong>del</strong> Torrente Moro. Questo paleo-reticolo <strong>di</strong>
178 G. POMPOSO & A. PIZZI<br />
drenaggio, qui denominato “Paleo-Aventino”, avrebbe avuto un<br />
asse orientato NNE-SSW, coerente con l’andamento regionale<br />
<strong>del</strong>le principali aste fluviali in questo settore <strong>del</strong>la fascia<br />
periadriatica abruzzese. Successivamente, la presenza <strong>di</strong> un<br />
settore in sollevamento, avrebbe costituito un ostacolo per lo<br />
scorrimento <strong>del</strong> “Paleo-Aventino” il quale è stato quin<strong>di</strong><br />
soggetto ad una <strong>di</strong>versione. A tale fenomeno è associabile la<br />
cattura <strong>del</strong> “Paleo-Aventino” da parte <strong>del</strong> Fiume Sangro, tuttora<br />
osservabile.<br />
Inoltre l’ipotesi <strong>di</strong> un’anticlinale in crescita riuscirebbe a<br />
giustificare: i) la presenza <strong>di</strong> patterns ra<strong>di</strong>ali che<br />
<strong>del</strong>imiterebbero il settore frontale e meri<strong>di</strong>onale <strong>del</strong>la piega; ii)<br />
la presenza <strong>di</strong> deflessioni localizzate nel fianco orientale <strong>del</strong>la<br />
struttura, verosimilmente associabili al plunge <strong>del</strong>la piega; iii)<br />
la serie <strong>di</strong> catture osservabili nella porzione meri<strong>di</strong>onale<br />
<strong>del</strong>l’area stu<strong>di</strong>ata; iv) la presenza <strong>di</strong> estesi tratti <strong>del</strong> reticolo ad<br />
andamento E-W associabili a fenomeni <strong>di</strong> tilting <strong>di</strong>fferenziale.<br />
CONCLUSIONI<br />
Nel settore esterno sepolto <strong>del</strong>l’Appennino centrale<br />
abruzzese lo stu<strong>di</strong>o geologico-strutturale dei depositi marini<br />
plio-pleistocenici e la <strong>di</strong>stribuzione dei vari or<strong>di</strong>ni <strong>di</strong> terrazzi<br />
alluvionali quaternari hanno permesso <strong>di</strong> ipotizzare l’esistenza<br />
<strong>di</strong> un “sollevamento localizzato” nell’area <strong>di</strong> Orsogna durante il<br />
tardoquaternario, concomitante al doming regionale <strong>del</strong>l’intera<br />
area appenninica.<br />
I risultati <strong>del</strong>le analisi geomorfiche quantitative ottenuti per<br />
il Torrente Moro e per i corsi d’acqua circostanti mostrano la<br />
presenza <strong>di</strong> anomalie <strong>del</strong> reticolo idrografico. La polarità e la<br />
<strong>di</strong>stribuzione <strong>di</strong> queste anomalie sono coerenti con una risposta<br />
<strong>del</strong> reticolo idrografico ad un settore in sollevamento nell’area<br />
<strong>di</strong> Orsogna.<br />
In base ai risultati ottenuti, sebbene ancora preliminari,<br />
interpretiamo tali fenomeni <strong>di</strong> sollevamento come associabili<br />
all’attività <strong>di</strong> crescita <strong>di</strong> una struttura tettonica cieca, attiva<br />
nell’area almeno dal Pleistocene m<strong>ed</strong>io.<br />
Si sottolinea, inoltre, come l’area <strong>di</strong> massima intensità<br />
<strong>del</strong>l’evento sismico <strong>del</strong> 10 settembre 1881 troverebbe<br />
collocazione all’interno <strong>del</strong> settore evidenziato dal presente<br />
lavoro.<br />
BIBLIOGRAFIA<br />
AA.VV. (in stampa)- <strong>Note</strong> illustrative <strong>del</strong>laCarta Geologica<br />
d’Italia alla scala 1:50.000 - Foglio 361 Chieti. APAT -<br />
Dipartimento Difesa <strong>del</strong> Suolo-Servizio Geologico d’Italia,<br />
S.EL.CA. – Firenze.<br />
ADAMOLI L., BIGOZZI A., CIARAPICA G., CIRILLI S., PASSERI L.,<br />
ROMANO A., DURANTI F. & VENTURI F. (1990) - Upper<br />
Triassic bituminous facies and Hettangian pelagic facies in<br />
the Gran Sasso Range. Boll. Soc. Geol. It., 109 (1), 219-<br />
230.<br />
BOSCHI E., GUIDOBONI E., FERRARI G., VALENSISE G. &<br />
GASPERINI P. (a cura <strong>di</strong>) (1997) - Catalogo dei forti<br />
terremoti in Italia dal 461 a.C. al 1990. Istituto Nazionale<br />
<strong>di</strong> Geofisica e SGA Storia Geofisica Ambiente. Bologna,<br />
SGA, pp. 644. [http://storing.ingv.it/cft/].<br />
CASNEDI R., CRESCENTI U., D'AMATO C., MOSTARDINI F. &<br />
ROSSI U. (1981) - Il Plio-Pleistocene <strong>del</strong> sottosuolo<br />
molisano. Geologica Romana, 20, 1-42.<br />
GRUPPO DI LAVORO CPTI (2004) - Catalogo Parametrico dei<br />
Terremoti Italiani, vers. 2004 (CPT104). INGV Bologna,<br />
http://emi<strong>di</strong>us.mi.ingv.it/CPTI.<br />
GALADINI F., MASTINO F., PIZZI A., SAVARESE F., SCISCIANI V.,<br />
TERTULLIANI A. (2006) - Il terremoto <strong>del</strong> 10 settembre<br />
1881 e la sismicità <strong>del</strong> settore “esterno” <strong>del</strong>la regione<br />
abruzzese. Abstract 25° Convegno Nazionale GNGTS -<br />
Roma 28-30 novembre 2006.<br />
HACK J. (1973) - Stream profile analysis and stream gra<strong>di</strong>ent<br />
index. U. S. Geol. Surv. J. Res. 1, 421-429.<br />
KELLER E. (1986) - Investigation of active tectonics: use of<br />
surficial earth processes. In: Wallace, R. E. (<strong>ed</strong>s), Active<br />
Tectonics stu<strong>di</strong>es in Geophysics. Nat. Acad. Press,<br />
Washington, Dc, 136-147.<br />
KELLER E. & PINTER, N. (2001) - Active Tectonics,<br />
Earthquakes, Uplift and Landscape. Earth Sciences Series,<br />
Prentice – Hall, Englewood Cliffs, NJ, 121-156.<br />
MONACHESI G. & STUCCHI M. (a cura <strong>di</strong>) (1997) - DOM4.1 un<br />
database <strong>di</strong> osservazioni macrosismiche <strong>di</strong> terremoti <strong>di</strong><br />
area italiana al <strong>di</strong> sopra <strong>del</strong>la soglia <strong>del</strong> danno. CNR-<br />
GNDT. [http://emi<strong>di</strong>us.mi.ingv.it/DOM]<br />
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SANTINI U. (1992) - La zona <strong>di</strong> giunzione tra l’arco<br />
appenninico settentrionale e l’arco appenninico<br />
meri<strong>di</strong>onale nell’Abruzzo e nel Molise. In: Tozzi, M.,<br />
Cavinato, G.P., Parotto, M., (Eds.), Stu<strong>di</strong> preliminari<br />
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Vasto. Stu<strong>di</strong> Geologici Camerti, 1991/2, 417-441.<br />
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VINCENZO M., ESESTIME P., GIACCIO B., SPOSATO A.<br />
(2006). Faglie attive nell’area <strong>del</strong> Massiccio <strong>del</strong>la Maiella<br />
(Appennino abruzzese, Italia centrale). Abstract 25°<br />
Convegno Nazionale GNGTS - Roma 28-30 novembre<br />
2006.<br />
PIZZI A., FALCUCCI E., GORI S., GALADINI F., MESSINA P., DI<br />
VINCENZO M., ESESTIME P., GIACCIO B., POMPOSO G.,<br />
SPOSATO A. (in stampa) - Active faulting in the Maiella<br />
Massif (central Apennines, Italy). GeoActa (Bologna).
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 179-182, 2 ff.<br />
Analisi in profon<strong>di</strong>tà <strong>di</strong> strutture mesoalpine e neoalpine nelle Alpi<br />
Meri<strong>di</strong>onali orientali<br />
ABSTRACT<br />
Depth analysis of mesoalpine and neoalpine structures<br />
To better analyse the superimposition between structures of mesoalpine<br />
(1-NE-SW) and neoalpine (1 from N-S to NW-SE) phases and to study the<br />
complex system of reactivations, some deep geologic sections have been<br />
realiz<strong>ed</strong>. Five sections are about N-S or NW-SE orient<strong>ed</strong>, they cut the principal<br />
neoalpine structures; two of them are remakes from already publish<strong>ed</strong> sections<br />
(Carulli & Ponton,1992 e Merlini et alii, 2002).<br />
A long section SW-NE cuts orthogonally the others sections and the<br />
mesoalpine structures. So we can reconstruct a more correct reason<strong>ed</strong> scheme<br />
of this part of South-alpine orogenic belt. For the first time the deep interaction<br />
between orthogonally fault planes and their three-<strong>di</strong>mensional wideness has<br />
been analyz<strong>ed</strong>. It is clear, for all the principal structures, the basement<br />
involvement and the common decollement horizons in the Permian and<br />
Carnian evaporitic units. Mesoalpine frontal ramps became lateral or oblique<br />
in the first successive phases and then they are cut and translate.<br />
In this work only one section is present<strong>ed</strong>, the Pontebba-Berna<strong>di</strong>a well-<br />
Tricesimo, mo<strong>di</strong>fi<strong>ed</strong> from Merlini et alii (2002) where are represent<strong>ed</strong> the<br />
principal structures: Val Dogna and M. S. Simeone lines, mesoalpine in<br />
origin, are fold<strong>ed</strong> from the neoalpine anticline ramp, while the Berna<strong>di</strong>a line is<br />
cut from the neoalpine plane and the Cividale line is partly a reactivation of<br />
the mesoalpine plane.<br />
Key words: Friuli, Southern Alps, thrust tectonics, poliphasic<br />
deformations.<br />
Parole chiave: Friuli, Alpi Meri<strong>di</strong>onali, tettonica compressiva,<br />
deformazioni polifasiche.<br />
INTRODUZIONE<br />
MAURIZIO PONTON (*)<br />
Il problema <strong>del</strong>la ricostruzione in profon<strong>di</strong>tà <strong>del</strong>le strutture<br />
tettoniche nel settore orientale <strong>del</strong>le Alpi Meri<strong>di</strong>onali emerge<br />
perio<strong>di</strong>camente e fino ad oggi è stato affrontato ricostruendo<br />
sezioni geologiche più o meno bilanciate orientate<br />
generalmente N-S (CASTELLARIN et alii, 1980; ROEDER, 1992;<br />
ROEDER & LINDSEY 1992; CASTELLARIN et alii, 2006) cioè<br />
analizzando i piani <strong>di</strong> faglia come <strong>di</strong> prevalente origine<br />
neoalpina (1 da N-S a NW-SE) e solo in parte considerando<br />
anche i piani <strong>di</strong> origine mesoalpina (1 NE-SW) (CARULLI &<br />
PONTON, 1992; MERLINI et alii, 2002; POLI et alii, 2002).<br />
Vari sono i lavori che provano la polifasatura <strong>di</strong> questo<br />
settore <strong>di</strong> catena (DOGLIONI & BOSELLINI, 1987; VENTURINI,<br />
1990; DOGLIONI, 1992; CAPUTO, 1996, 1997, 2008; POLI<br />
1995,1996; POLI & ZANFERRARI, 1995; PONTON, 2002;<br />
PONTON, 2007).<br />
L’interferenza fra strutture orientate <strong>di</strong>versamente fra loro,<br />
impostatesi lungo piani i cui angoli <strong>di</strong> inclinazione sono<br />
fortemente influenzati dalle varie strutturazioni prec<strong>ed</strong>enti, sta<br />
alla base <strong>del</strong> complesso <strong>ed</strong>ificio <strong>del</strong>le Alpi Meri<strong>di</strong>onali orientali<br />
e in particolare <strong>del</strong>l’area friulana e giulia.<br />
Tutti gli autori concordano sulla presenza <strong>di</strong> deformazioni<br />
determinate da tre fasi orogeniche principali: ercinica,<br />
mesoalpina e neoalpina. Per quest’ultima si sono poi<br />
in<strong>di</strong>viduati vari sta<strong>di</strong> fra i quali il più <strong>di</strong>scusso, per le poche<br />
evidenze è quello insubrico <strong>del</strong> Cattiano-Bur<strong>di</strong>galiano<br />
(CASTELLARIN et alii, 1992; CAPUTO et alii, 2002; PONTON &<br />
VENTURINI, 2002; ZANFERRARI et alii, 2002; GALADINI et alii,<br />
2005).<br />
METODOLOGIA<br />
Nel presente lavoro e in quelli che seguiranno si è cercato <strong>di</strong><br />
ricostruire in profon<strong>di</strong>tà le relazioni geometriche tra le strutture<br />
attivate durante le fasi mesoalpina e neoalpina prestando<br />
attenzione alle zone <strong>di</strong> interferenza fra i piani <strong>di</strong> faglia.<br />
Assunto che per tutti i piani <strong>di</strong> scorrimento i livelli <strong>di</strong><br />
scollamento principali sono gli stessi e cioè quelli nelle unità<br />
evaporitiche <strong>del</strong> Permiano superiore e <strong>del</strong> Triassico superiore,<br />
per quanto riguarda le rampe, spesso si impostano in<br />
corrispondenza <strong>di</strong> paleofaglie con componente <strong>di</strong>stensiva <strong>del</strong><br />
Triassico sup.- Giurassico inf. e <strong>del</strong> Cretacico sup. e nelle fasi<br />
compressive successive giocano un ruolo <strong>di</strong>verso a seconda<br />
<strong>del</strong>la <strong>di</strong>rezione <strong>di</strong> massima compressione per cui una stessa<br />
porzione <strong>di</strong> piano già esistente passa da rampa frontale a<br />
laterale o obliqua e viceversa.<br />
Al fine <strong>di</strong> analizzare queste relazioni sono state realizzate<br />
(fig. 1) più sezioni geologiche profonde in gran parte orientate<br />
circa N-S, che tagliano ortogonalmente le principali strutture<br />
neoalpine; esse seguono quelle già realizzate da CARULLI &<br />
PONTON (1992) e da MERLINI et alii, 2002 che per l’occasione<br />
sono state riviste e rimo<strong>del</strong>late. Infine una sezione orientata<br />
circa SW-NE taglia tutte queste sezioni intercettando in<br />
ortogonale le principali strutture mesoalpine nei loro tratti più<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, Ambientali e Marine, Via Weiss 2<br />
- 34127 Trieste – ponton@units.it
180 M. PONTON<br />
Fig. 1 – Schema strutturale aggiornato <strong>del</strong>le Alpi Meri<strong>di</strong>onali orientali e ubicazione <strong>del</strong>le sezioni geologiche. Structural updat<strong>ed</strong> scheme of Eastern Southern<br />
Alps and location of the geological sections.<br />
1 M. Palombino-M. Cavallo-Caneva; 2 M. Peralba-Forni <strong>di</strong> Sotto-Carpacco; 3 M. Polinik-M.Dauda-Flaibano (da:Carulli & Ponton, 1992 mo<strong>di</strong>f.); 4 Weidegg-<br />
M.Amariana-M.S. Simeone-San Daniele; 5 Pontebba-Pozzo Berna<strong>di</strong>a-Tricesimo (da: Merlini et alii, 2002 mo<strong>di</strong>f.); 6 Tarvisio-M.Canin-M.Cavallo.<br />
evidenti in affioramento e cioè nella fascia p<strong>ed</strong>emontana.<br />
I dati immessi sono quelli <strong>di</strong> superficie e bibliografici più<br />
aggiornati (oltre ai già citati, anche: PLACER, 1999; CARULLI,<br />
2006; ZANFERRARI et alii, 2006a, 2006b; PONTON, 2008) e<br />
quelli in<strong>di</strong>retti gravimetrici e magnetici (CATI et alii, 1989),<br />
sismici a riflessione e sismologici (MERLINI et alii, 2002; POLI<br />
et alii, 2002; NICOLICH et alii, 2004; GALADINI et alii, 2005).<br />
La ricostruzione in profon<strong>di</strong>tà <strong>del</strong>le geometrie con sezioni<br />
geologiche incrociate è risultata complessa ma abbastanza<br />
sod<strong>di</strong>sfacente nei settori prealpini, mentre per la porzione più<br />
interna <strong>del</strong>la catena le variabili sono notevolmente più<br />
numerose. Entrano maggiormente in gioco le variazioni <strong>di</strong><br />
spessore e <strong>di</strong> facies <strong>del</strong>le formazioni, le strutture estensionali<br />
mesozoiche (CARULLI et alii, 2002) e, nei settori settentrionali,<br />
le unità erciniche che interessano le successioni precarbonifere.<br />
Queste ultime unità, insieme a quelle <strong>del</strong> Permo-<br />
Carbonifero, si sono assunte come un basamento generalizzato<br />
come quello a bassa suscettività magnetica presente al <strong>di</strong> sotto<br />
<strong>del</strong>le evaporiti permiane e <strong>del</strong>la successione mesozoica<br />
<strong>del</strong>l’avampaese.<br />
In sintesi questo lavoro attraverso un’analisi <strong>di</strong> geometrie<br />
non piane e non cilindriche si propone un primo livello <strong>di</strong><br />
interpolazione <strong>del</strong>le strutture <strong>del</strong>le due fasi deformative,<br />
sezionate in più punti e in <strong>di</strong>rezioni <strong>di</strong>verse, in<strong>di</strong>viduando i<br />
piani <strong>di</strong> scollamento e <strong>di</strong> scorrimento, i punti <strong>di</strong> cut-off e quin<strong>di</strong><br />
i più probabili raccorciamenti. Naturalmente se un obbiettivo è<br />
quello <strong>di</strong> rendere bilanciabili le sezioni, per il momento questo<br />
lavoro rappresenta un primo tentativo; l’obbiettivo finale è<br />
quello <strong>di</strong> realizzare una mappa palinspastica <strong>del</strong>l’area.<br />
DISCUSSIONE<br />
Uno dei risultati più imm<strong>ed</strong>iati derivanti dall’incrocio fra<br />
più sezioni è stato quello <strong>del</strong>la revisione <strong>del</strong>le sezioni<br />
prec<strong>ed</strong>entemente pubblicate.<br />
In questa s<strong>ed</strong>e si propone una prima rielaborazione <strong>del</strong>la
Fig.2 – <strong>Sezione</strong> geologica n°5 <strong>di</strong> fig.1 (da MERLINI et alii, 2002 mo<strong>di</strong>f.)<br />
sola sezione <strong>di</strong> MERLINI et alii (2002) nella quale (sezione 5 <strong>di</strong><br />
fig. 1 e fig.2) meglio si <strong>del</strong>ineano le ra<strong>di</strong>ci <strong>del</strong>le strutture<br />
mesoalpine, quali: la linea <strong>del</strong> M Berna<strong>di</strong>a, troncata e traslata in<br />
fase neoalpina, la linea <strong>di</strong> Cividale, il cui piano è in parte<br />
riattivato, e la linea <strong>di</strong> Palmanova, struttura frontale <strong>del</strong> sistema<br />
mesoalpino.<br />
Durante l’elaborazione <strong>del</strong>le sezioni sono emerse<br />
interessanti osservazioni che vengono <strong>di</strong> seguito elencate<br />
brevemente.<br />
Il basamento è praticamente sempre coinvolto in tutte le<br />
strutture analizzate. C’è una strettissima relazione fra il sistema<br />
<strong>del</strong>le cosiddette linee <strong>di</strong> Sauris e la linea (o sistema <strong>di</strong> linee) M.<br />
Dof-M. Auda-M.S. Simeone–Saga; infatti quest’ultima<br />
rappresenterebbe lo scollamento superiore e scorrimento più<br />
avanzato <strong>del</strong> sistema <strong>di</strong> Sauris. Le strutture mesoalpine, i cui<br />
piani <strong>di</strong> scollamento vengono in parte utilizzati nelle fasi<br />
successive, vengono prese in carico all’interno <strong>del</strong>le strutture<br />
neoalpine e traslate verso S o SE assieme a parti dei loro bacini<br />
<strong>di</strong> avanfossa; in questo contesto il sistema <strong>di</strong> Sauris (in origine<br />
mesoalpino e/o insubrico) viene piegato dall’anticlinale da<br />
rampa <strong>del</strong> sistema neoalpino nella zona <strong>del</strong>l’alto Tagliamento.<br />
La rotazione antioraria progressiva <strong>del</strong>le <strong>di</strong>rezioni <strong>di</strong> massima<br />
compressione da NE-SW (fase mesoalpina) fino a NW-SE<br />
degli sta<strong>di</strong> più recenti (fase neoalpina) con oscillazioni finali fra<br />
NW-SE e N-S, si riflette nelle complesse relazioni dei piani in<br />
profon<strong>di</strong>tà.<br />
BIBLIOGRAFIA<br />
CAPUTO R. (1996) - The polyphase tectonics of Eastern<br />
Dolomites, Italy. Mem. Sci. Geol., 48: 93-106.<br />
CAPUTO R. (1997) - The puzzling regmatic system of Eastern<br />
dolomites. Mem. Sci. Geol., 49, 1-10.<br />
ANALISI IN PROFONDITÀ DI STRUTTURE MESOALPINE E NEOALPINE<br />
181<br />
CAPUTO R. (2008) – Tettonica polifasica nei <strong>di</strong>ntorni <strong>di</strong><br />
Cortina d’Ampezzo, Dolomiti orientali. Ren<strong>di</strong>conti online<br />
SGI, 1, note <strong>brevi</strong>, 57-61.<br />
CAPUTO R., POLI M.E. & ZANFERRARI A. (2002) - Neogene-<br />
Quaternary Twist Tectonics In The Eastern Southern Alps,<br />
Italy. Mem. Sci. Geol., 54: 155-158.<br />
CARULLI G.B. (a cura <strong>di</strong>) (2006) – Carta geologica <strong>del</strong> Friuli<br />
Venezia Giulia (scala 1:150.000 con note ill.). Servizio<br />
Geologico <strong>del</strong>la Regione FVG, SELCA.<br />
CARULLI G.B., COZZI A., MASETTI D., PERNARCIC E.,<br />
PODDA F. & PONTON M. (2002) – Middle Triassic-<br />
Early Jurassic extensional tectonics in the Carnian<br />
Prealps (eastern Southern Alps). Mem. Sci. Geol., 54,<br />
151-154.<br />
CARULLI G.B. & PONTON M. (1992) – Interpretazione<br />
strutturale profonda <strong>del</strong> settore centrale carnicofriulano.<br />
Stu<strong>di</strong> Geologici Camerti, vol. spec., 2, CROP<br />
1-1°, 275-284, Camerino.<br />
CASTELLARIN A., CANTELLI L., FESCE A.M., MERCIER J.L.,<br />
PICOTTI V., PINI G.A., PROSSER G. & SELLI L. (1992) -<br />
Alpine compressional tectonics in Southern Alps.<br />
Relationships with the N-Apennines. Annales Tectonicae, 6<br />
(1), 62-94.<br />
CASTELLARIN A., FRASCARI F. & VAI G.B. (1980) – Problemi<br />
<strong>di</strong> interpretazione geologica profonda <strong>del</strong> Sudalpino<br />
orientale. Rend. Soc. Geol. It., 2 (1979), 55-60.<br />
CASTELLARIN A., NICOLICH R., FANTONI R., CANTELLI L.,<br />
SELLA M. & SELLI L. (2006) – Astructure of the litosphere<br />
beneath the Eastern Alps (southern sector of the<br />
TRANSALP transect). Tectonophisics, 414 , 259-282.<br />
CATI A., FICHERA R. & CAPPELLI V. (1989) - Northeastern<br />
Italy. Integrat<strong>ed</strong> processing of geophysical and
182 M. PONTON<br />
geological data. Mem. Soc. Geol. It., 40: 273-288,<br />
Roma.<br />
DOGLIONI C. (1992) – Relationschips between Mesozoic<br />
extensional tectonics, stratigraphy and Alpine<br />
inversion in the Southern Alps. Eclogae Geol. Helv.,<br />
85/1, 105-126.<br />
DOGLIONI C.& BOSELLINI A. (1987) - Eoalpine and mesoalpine<br />
tectonics in the Southern Alps. Geol. Rundsch., 76, 735-<br />
754.<br />
GALADINI F., POLI M.E. & ZANFERRARI A. (2005) -<br />
Seismogenic sources potentially responsible for<br />
earthquakes with M?6 in the eastern Southern Alps<br />
(Thiene-U<strong>di</strong>ne sector, NE Italy). Geoph. J. Int., 161: 739-<br />
762, Oxford.<br />
MERLINI S., DOGLIONI C., FANTONI R. & PONTON M. (2002) -<br />
Analisi strutturale lungo un profilo geologico tra la linea<br />
Fella Sava e l’avampaese adriatico (Friuli Venezia Giulia<br />
- Italia). Mem. Soc. Geol. It., 57: 293-300, Roma.<br />
NICOLICH R., DELLA VEDOVA B., GIUSTINIANI M. & FANTONI<br />
R. (a cura <strong>di</strong>) (2004) – Carta <strong>del</strong> sottosuolo <strong>del</strong>la Pianura<br />
Friulana (con note ill.). Regione FVG e Univ. Trieste, 32<br />
pp.<br />
PLACER L. (1999) - Contribution to the macrotectonic<br />
sub<strong>di</strong>visionof the border region between Southern Alps and<br />
External Dinarides. Geologija 41, 223-255, Ljubljana.<br />
POLI M.E. (1995) - Evidenze <strong>di</strong> tettonica a thrust <strong>di</strong>narica nelle<br />
Prealpi Giulie meri<strong>di</strong>onali (Italia Nord-orientale). Atti<br />
Ticinensi Sci. Terra, ser. spec., 3 (1995): 99-114.<br />
POLI M.E. (1996) - Analisi strutturale <strong>del</strong> Monte <strong>di</strong> M<strong>ed</strong>ea.<br />
(Friuli Orientale - Gorizia). Atti Ticinensi Sci. Terra, ser.<br />
spec., 4<br />
POLI M.E., PERUZZA L., REBEZ A., RENNER G., SLEJKO D.,<br />
ZANFERRARI A. (2002) - New seismotectonic evidence from<br />
the analysis of the 1976-1977 and 1977-1999 seismicity in<br />
Friuli (NE Italy). Boll. Geof. Teor. Appl., 43: 53-78.<br />
POLI M.E., ZANFERRARI A. (1995) - Dinaric thrust tectonics in<br />
the Southern Julian Prealps (Eastern Southern Alps, NE<br />
Italy). In: First Croatian Geological Congress, Opatija 18-<br />
21 october 1995, Proce<strong>ed</strong>ings, 2: 465-468, Zagreb.<br />
PONTON M. (2002) - La tettonica <strong>del</strong> gruppo <strong>del</strong> M. Canin e la<br />
linea Val Resia-Val Coritenza (Alpi Giulie occidentali).<br />
Mem. Soc. Geol. It., 57: 283-292.<br />
PONTON M. (2007) – Un’area polideformata nelle Prealpi<br />
Carniche:il Monte Broili e il Cuel dal Meloc. Gortania , 28<br />
(2006), 7-18 , U<strong>di</strong>ne.<br />
PONTON M. (2008) – <strong>Note</strong> geologiche sulle Prealpi Giulie<br />
nord-occidental. Il fenomeno carsico <strong>del</strong>le Prealpi Giulie<br />
settentrionali-Mem.Isr. It. Spel., s II, XX, 53-71.<br />
PONTON M. & VENTURINI C. (2002) – Il ciclo alpino. In: Vai<br />
G.B. et alii (a cura <strong>di</strong>) “Alpi e Prealpi Carniche e Giulie”.<br />
Guide geologiche Regionali <strong>del</strong>la Soc. Geol. It., BE-MA<br />
<strong>ed</strong>., 76-81, Roma.<br />
ROEDER D. (1992) – Thrusting and w<strong>ed</strong>ge growth, Southern<br />
Alps of Lombar<strong>di</strong>a (Italy). Tectonophysics, 207, 199-243.<br />
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Friuli, Slovenia): Architecture and geodynamics. Nafta,43<br />
(11) , 509-548.<br />
VENTURINI C. (1990) – Geologia <strong>del</strong>le Alpi Carniche centroorientali.<br />
Museo Friulano St. Nat., Pubbl. 36, 220 pp.<br />
ZANFERRARI A., AVIGLIANO R., CALDERONI G., GRANDESSO P.,<br />
MARCHESINI A., MONEGATO G., PAIERO G., POLI M.E.,<br />
RAVAZZI C. & STEFANI C. (in prep.-a) – <strong>Note</strong> illustrative<br />
<strong>del</strong>la Carta Geologica d’Italia alla scala 1:50.000: Foglio<br />
065 “Maniago”. Servizio Geologico d’Italia – Regione<br />
Autonoma Friuli – Venezia Giulia.<br />
ZANFERRARI A., AVIGLIANO R., CALDERONI G., GRANDESSO P.,<br />
MARCHESINI A., MONEGATO G., PAIERO G., POLI M.E.,<br />
RAVAZZI C. & STEFANI C. (in prep.-b) – <strong>Note</strong> illustrative<br />
<strong>del</strong>la Carta Geologica d’Italia alla scala 1:50.000: Foglio<br />
066 “U<strong>di</strong>ne”. Servizio Geologico d’Italia – Regione<br />
Autonoma Friuli – Venezia Giulia.<br />
ZANFERRARI A., POLI M.E. & ROGLEDI S. (2002) - The external<br />
thrust-belt of the Eastern Southern Alps in Friuli (NE Italy).<br />
Mem. Sci. Geol., 54: 159-162.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 183<br />
Seismic Evidence of multiple intrusion episodes within the shallow<br />
subsurface of Campi Flegrei and Ischia Island<br />
The reprocessing of old seismic reflection profiles represents<br />
an useful and cost-effective tool to constrain the geologic<br />
structure of a complex tectonic area such as the inner shelf of<br />
Bay of Naples. BRUNO et al., 2002 reprocess<strong>ed</strong> a seismic<br />
dataset collect<strong>ed</strong> by OGS (Osservatorio Geofisico Sperimentale<br />
– Trieste) during 1973, r<strong>ed</strong>ucing some of the limiting effects of<br />
the acquisition phase to gain more informations on the deep<br />
part of Bay of Naples and Campi Flegrei structure. In this<br />
study, the interpretation concern<strong>ed</strong> 13 seismic lines reprocess<strong>ed</strong><br />
by BRUNO et al., 2002 that spread over an area of about 2000<br />
Km² exten<strong>di</strong>ng from the northern coast of Ischia and Procida<br />
Island up Sorrento Peninsula. The interpretation of seismic<br />
profiles allow<strong>ed</strong> the <strong>del</strong>ineation of a broad geo-volcanological<br />
and structural framework of the Campi Flegrei area. This<br />
region is well-know for its active volcanism but the crustal<br />
structure beneath the Campi Flegrei – Ischia ridge is still<br />
debat<strong>ed</strong>.<br />
Our study confirms previous investigations (FINETTI et al.,<br />
1974; PESCATORE et al., 1984; FLORIO et al., 1999; BRUNO,<br />
2003; AIELLO et al., 2005) on the structure of the Gulf of<br />
Naples basin, but bring new insights into the way magma is<br />
stor<strong>ed</strong> within the crust.<br />
In particular, we found evidence of multiple episodes of sill<br />
emplacement at <strong>di</strong>fferent levels within the crust. Igneous sills<br />
are extraor<strong>di</strong>narily well imag<strong>ed</strong> on these seismic data because<br />
of a significant contrast in acoustic properties (velocity and<br />
density) between the intru<strong>di</strong>ng magma and the s<strong>ed</strong>imentary<br />
host-rock. From the stratigraphic relationships between<br />
s<strong>ed</strong>imentary and intrud<strong>ed</strong> rock, we define two episodes of<br />
magma emplacement occurring during Upper Pliocene and Late<br />
Pleistocene, respectively.<br />
Intrusive processes, particularly well imag<strong>ed</strong> below Ischia<br />
Island, could have ultimately be responsible for the rapid uplift<br />
of Ischia resurgent structure. Eventually, we show that<br />
volcanism in the Neapolitan area is a long process operating<br />
episo<strong>di</strong>cally from the Upper Pliocene onwards.<br />
_________________________<br />
MICHELE PUNZO (*), PIER PAOLO BRUNO (°) & CLAUDIO FACCENNA (*)<br />
(*)Dipartimento <strong>di</strong> Scienze Geologiche, Università degli Stu<strong>di</strong> ROMA<br />
TRE, L.go S. Leonardo Murialdo, 1- 00146 Roma, Italy.<br />
(°)Osservatorio Vesuviano, Istituto Nazionale <strong>di</strong> Geofisica e<br />
Vulcanologia, Via Diocleziano 328 – 80124, Napoli, Italy<br />
REFERENCES<br />
BRUNO P.P., DI FIORE V. & VENTURA G. (2000) - Seismic study<br />
of the “41st Parallel” Fault System offshore the Campanian-<br />
Latial continental margin, Italy; Tectonophysics 324; 37-55.<br />
BRUNO P.P.G., DI FIORE V. & RAPOLLA G. (2002) - Seismic<br />
reflection data processing in active volcanic areas: an<br />
application to Campi Flegrei and Somma Vesuvius offshore<br />
8southern Italy). Annals of Geophysics, 45/6, 753-768.<br />
BRUNO P.P., RAPOLLA A. & DI FIORE V. (2003) - Structural<br />
setting of the Bay of Naples (Italy) seismic reflection data:<br />
implication for campanian volcanism; Tectonophysics 372;<br />
193-213
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 184-187, 3 ff<br />
Influenza <strong>del</strong>l’idratazione <strong>del</strong> cuneo <strong>di</strong> mantello sull’evoluzione <strong>di</strong><br />
un sistema <strong>di</strong> subduzione oceano/continente: una simulazione<br />
numerica<br />
ABSTRACT<br />
Influence of hydration in the mantle w<strong>ed</strong>ge on the evolution of an<br />
ocean/continent system: a numerical simulation<br />
The evolution of an ocean/continent subduction system is simulat<strong>ed</strong> by<br />
using a 2D finite elements thermomechanical mo<strong>del</strong>. The effects of hydration<br />
in mantle w<strong>ed</strong>ge on mantle flow and on crustal recycling is stu<strong>di</strong><strong>ed</strong> for<br />
<strong>di</strong>fferent hydration rates and maximum depth of dehydration of the oceanic<br />
crust for two select<strong>ed</strong> typical subduction velocities (1 cm/a and 5 cm/a). We<br />
found a <strong>di</strong>rect relationship between the amount of recycl<strong>ed</strong> material and the<br />
maximum depth of dehydration. Moreover, the hydration rate and hydration<br />
depth have an important impact on peak pressure and temperature of recycl<strong>ed</strong><br />
continental crust for a subducion rate of 5 cm/a.<br />
Key words: mantle w<strong>ed</strong>ge, mantle hydration, ocean/continent<br />
subduction, numerical mo<strong>del</strong>ing.<br />
Parole chiave: cuneo <strong>di</strong> mantello, idratazione <strong>del</strong> mantello,<br />
subduzione oceano/continente, mo<strong>del</strong>lizzazione numerica.<br />
INTRODUZIONE<br />
Numerosi stu<strong>di</strong> su rocce <strong>di</strong> alta e ultra alta pressione,<br />
connesse allo sviluppo <strong>di</strong> un margine convergente, hanno<br />
evidenziato come porzioni <strong>di</strong> crosta continentale e oceanica<br />
coinvolte nella subduzione siano frequentemente associate a<br />
peridotiti idrate. I loro percorsi P-T e i dati geocronologici<br />
suggeriscono una rapida esumazione indotta da un flusso<br />
forzato presente all’interno <strong>del</strong> cuneo <strong>di</strong> mantello<br />
suprasubduttivo.<br />
Mo<strong>del</strong>li petrologici (SCHMIDT & POLI, 1998) e le<br />
simulazioni numeriche (GERYA & STOCKHERT, 2005) rivelano<br />
come il processo <strong>di</strong> deidratazione che coinvolge la litosfera<br />
oceanica in subduzione e la conseguente idratazione <strong>del</strong> cuneo<br />
<strong>di</strong> mantello abbiano un contributo fondamentale per<br />
l’instaurarsi <strong>di</strong> un flusso forzato in grado <strong>di</strong> trasportare porzioni<br />
<strong>di</strong> crosta presenti all’interno <strong>del</strong> cuneo <strong>di</strong> mantello da elevate<br />
profon<strong>di</strong>tà verso la superficie. Su molti aspetti, comunque, la<br />
<strong>di</strong>scussione è ancora aperta.<br />
Per indagare ulteriormente sul ruolo <strong>del</strong> processo <strong>di</strong><br />
idratazione sul meccanismo <strong>di</strong> ricircolo <strong>ed</strong> esumazione <strong>di</strong><br />
_________________________<br />
MANUEL RODA (*), ANNA MARIA MAROTTA (*) & MARIA IOLE SPALLA (°)<br />
(*)<strong>Sezione</strong> <strong>di</strong> Geofisica, Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra “Ar<strong>di</strong>to<br />
Desio”, Università degli Stu<strong>di</strong> <strong>di</strong> Milano, Italia.<br />
(°)<strong>Sezione</strong> <strong>di</strong> Geologia, Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra “Ar<strong>di</strong>to Desio”,<br />
Università degli Stu<strong>di</strong> <strong>di</strong> Milano, Italia.<br />
materiale crostale, è stato sviluppato un mo<strong>del</strong>lo numerico bi<strong>di</strong>mensionale<br />
per simulare la subduzione forzata <strong>di</strong> litosfera<br />
oceanica al <strong>di</strong> sotto <strong>di</strong> un margine continentale. Il mo<strong>del</strong>lo<br />
prev<strong>ed</strong>e la progressiva deidratazione <strong>del</strong>la crosta oceanica e<br />
l’instaurarsi <strong>di</strong> un cuneo <strong>di</strong> mantello idrato. Lo stu<strong>di</strong>o è stato<br />
condotto variando la velocità <strong>di</strong> convergenza, l’angolo <strong>del</strong><br />
cuneo <strong>di</strong> idratazione e la massima profon<strong>di</strong>tà <strong>di</strong> deidratazione<br />
<strong>del</strong>la crosta oceanica, al fine <strong>di</strong> valutare l’influenza dei succitati<br />
parametri nei confronti <strong>del</strong>la <strong>di</strong>namica all’interno <strong>del</strong> cuneo e<br />
<strong>del</strong>l’evoluzione termo-barometrica <strong>di</strong> porzioni <strong>di</strong> litosfera<br />
continentale riciclata nel canale <strong>di</strong> subduzione.<br />
MODELLO NUMERICO<br />
Lo stato fisico <strong>del</strong> sistema, per ogni intervallo temporale, è<br />
determinato risolvendo le equazioni <strong>di</strong> continuità, <strong>di</strong><br />
conservazione <strong>del</strong> momento e <strong>di</strong> conservazione <strong>del</strong>l’energia,<br />
espresse nella forma:<br />
∇ •( u ) = 0 (1)<br />
−∇p + ∇ •τ + ρ g = 0 (2)<br />
ρCp<br />
<br />
∂T <br />
+ u •∇T<br />
=∇• K∇T<br />
∂t <br />
( )+ ρH (3)<br />
secondo l’approccio descritto in MAROTTA & SPALLA,<br />
2007, dove <br />
u rappresenta la velocità, p la pressione, τ lo<br />
stress deviatorico, ρ la densità, <br />
g l’accelerazione <strong>di</strong> gravità,<br />
Cp la capacità termica a pressione costante, T la temperatura,<br />
K la conducibilità termica e H la produzione <strong>di</strong> calore per<br />
unità <strong>di</strong> massa.<br />
La griglia <strong>di</strong> riferimento <strong>di</strong> 1400 km <strong>di</strong> lunghezza e 700 km<br />
<strong>di</strong> profon<strong>di</strong>tà (fig. 1) è composta da 5761 no<strong>di</strong> e 2808 elementi<br />
tringolari biquadratici <strong>di</strong> <strong>di</strong>mensioni variabili. La<br />
<strong>di</strong>fferenziazione composizionale dei vari materiali<br />
caratterizzanti il sistema è stata ottenuta m<strong>ed</strong>iante la tecnica<br />
<strong>del</strong>le particelle lagrangiane (MAROTTA & SPALLA, 2007),<br />
utilizzando circa 1 milione <strong>di</strong> marcatori <strong>di</strong>stribuiti con densità<br />
<strong>di</strong> 1 marcatori ogni 0.25 km 2 . Le con<strong>di</strong>zioni al contorno sono<br />
in<strong>di</strong>cate in figura 1. Al fine <strong>di</strong> permettere la variazione<br />
temporale <strong>del</strong>la topografia, sopra la litosfera si considera uno<br />
strato <strong>di</strong> aria spesso 8 km. Il mo<strong>del</strong>lo prev<strong>ed</strong>e che la topografia
INFLUENZA DELL’IDRATAZIONE DEL CUNEO DI MANTELLO SULL’EVOLUZIONE DI UN SISTEMA DI<br />
SUBDUZIONE OCEANO/CONTINENTE: UNA SIMULAZIONE NUMERICA<br />
venga istantaneamente erosa, per s<strong>ed</strong>imentare all’interno <strong>del</strong><br />
cuneo <strong>di</strong> accrezione.<br />
Per simulare la progressiva idratazione <strong>del</strong> cuneo <strong>di</strong><br />
mantello suprasubduttivo, conseguente alla deidratazione <strong>del</strong>la<br />
placca oceanica, si è scelto <strong>di</strong> intervenire sul parametro<br />
viscosità dei singoli materiali che lo caratterizzano. La brusca<br />
riduzione <strong>del</strong>le velocità sismiche in questa porzione <strong>di</strong> mantello<br />
fa presupporre l’esistenza <strong>di</strong> una regione a minore viscosità<br />
rispetto a quelle circostanti, con valori variabili tra 10 18 e 10 20<br />
(BILLEN & GURNIS, 2001; ABERS et alii, 2006; RONDENAY et<br />
alii, 2008) L’ambiguità riguardo i meccanismi deformativi<br />
agenti nel cuneo <strong>di</strong> mantello idrato ci suggerisce <strong>di</strong> considerare<br />
una viscosità uniforme e in<strong>di</strong>pendente dalla temperatura per<br />
tutti i materiali presenti nel canale <strong>di</strong> subduzione idrato, con<br />
valore uguale 10 19 (BILLEN & GURNIS, 2001; ARCAY et alii,<br />
2005; GERYA & STOCKHERT, 2005; MEDA et alii, 2007). La<br />
profon<strong>di</strong>tà massima <strong>del</strong> cuneo è determinata, per ogni intervallo<br />
temporale, dall’ascissa e dall’or<strong>di</strong>nata <strong>del</strong> marcatore <strong>di</strong> crosta<br />
oceanica più profondo, fino ad un valore limite fissato (Ylim),<br />
oltre al quale si può considerare un contributo <strong>di</strong> idratazione<br />
trascurabile (SCHMIDT & POLI, 1998; ARCAY et alii, 2005). Il<br />
cuneo <strong>di</strong> mantello idrato è approssimanto geometricamente con<br />
un triangolo acuto, <strong>del</strong>imitato alla base dalla retta<br />
Yi = mXi + q , assimilabile al piano <strong>di</strong> subduzione, dove Yi e<br />
Xi sono le coor<strong>di</strong>nate <strong>del</strong> limite inferiore <strong>di</strong> idratazione<br />
ricalcolate per ogni intervallo temporale e m è il coefficiente<br />
angolare iniziale <strong>del</strong>la subduzione. Il limite <strong>di</strong> tetto <strong>del</strong> cuneo <strong>di</strong><br />
mantello idrato è variabile in funzione <strong>del</strong> grado <strong>di</strong> idratazione<br />
iniziale <strong>del</strong>la crosta oceanica <strong>ed</strong> è descritto dalla funzione<br />
Yi = m ′ Xi + q ′ , dove m ′ rappresenta un multiplo <strong>del</strong>l’angolo<br />
<strong>di</strong> subduzione.<br />
RISULTATI E DISCUSSIONE DELLA SIMULAZIONE<br />
Le prove condotte nella simulazione numerica sono state<br />
eseguite per velocità <strong>di</strong> subduzione <strong>di</strong> 1 e 5 cm/a e per m ′<br />
variabile tra 2.5 e 3 volte m , al fine <strong>di</strong> verificare l’influenza<br />
<strong>del</strong>l’idratazione <strong>del</strong> cuneo mantello sulla <strong>di</strong>namica <strong>del</strong>l’intero<br />
margine attivo e in particolare sullo sviluppo <strong>di</strong> un riciclo <strong>di</strong><br />
crosta continentale da porzioni profonde <strong>del</strong>la subduzione a<br />
Fig. 1 – Con<strong>di</strong>zioni iniziali e al contorno <strong>del</strong> mo<strong>del</strong>lo numerico (in grigio<br />
sono rappresentate le velocità fissate per la subduzione forzata; la linea<br />
tratteggiata orizzontale rappresenta la profon<strong>di</strong>tà <strong>del</strong>l’isotema 1600 K<br />
all’inizio <strong>del</strong>la simulazione).<br />
185<br />
regioni strutturalmente più superficiali. Inoltre sono stati<br />
considerati due limiti inferiori <strong>di</strong> idratazione rispettivamente <strong>di</strong><br />
-150 e -200 km. I risultati dalle simulazioni e dei corrispondenti<br />
percorsi P-T-t <strong>di</strong> un marcatore <strong>di</strong> crosta inferiore<br />
rappresentativi per ogni prova effettuata sono rappresentati<br />
nelle figure 2 e 3. Al fine <strong>di</strong> facilitare la localizzazione dei<br />
<strong>di</strong>fferenti livelli crostali coinvolti nel riciclo subduttivo, colori<br />
<strong>di</strong>fferenti sono stati utilizzati per rappresentare i marcatori <strong>di</strong><br />
crosta superiore e inferiore, sebbene fisicamente essi abbiano le<br />
m<strong>ed</strong>esime proprietà. Una prima osservazione può essere fatta in<br />
riferimento alla profon<strong>di</strong>tà limite a cui è posta la deidrataizone<br />
<strong>del</strong>la crosta oceanica:, per qualsiasi velocità <strong>di</strong> subduzione e<br />
grado <strong>di</strong> idratazione, il numero <strong>di</strong> marcatori <strong>di</strong> crosta<br />
continentale coinvolti nel riciclo è maggiore nei mo<strong>del</strong>li con<br />
Ylim= -200 km rispetto a quelli con Ylim = -150 km; Per una<br />
velocità <strong>di</strong> convergenza <strong>di</strong> 5 cm/a è osservabile un’influenza<br />
<strong>del</strong>la massima profon<strong>di</strong>tà <strong>di</strong> idratazione sulla profon<strong>di</strong>tà <strong>di</strong><br />
picco dei marcatori <strong>di</strong> crosta riciclati: infatti con una Ylim più<br />
elevata le pressioni <strong>di</strong> picco variano tra i 2.8 e i 2.3 GPa in<br />
confronto ai valori <strong>di</strong> 1.7 e 2.0 GPa ottenuti con un limite <strong>di</strong><br />
idratazione più superficiale (fig. 2b-d-f, 3b-d). La pressione <strong>di</strong><br />
picco è influenzata anche dall’ampiezza <strong>del</strong> cuneo <strong>di</strong><br />
idratazione ( ′<br />
m ), a parità <strong>di</strong> Ylim: passando da 2.5 a 3 volte m<br />
la pressione passa da 0.3 a 0.5 GPa. La temperatura massima<br />
raggiunta dai marcatori crostali presenti nel cuneo <strong>di</strong><br />
idratazione è funzione sia <strong>del</strong>la Ylim che <strong>del</strong>l’ampiezza <strong>del</strong><br />
cuneo <strong>di</strong> idratazione. All’aumento <strong>di</strong> entrambi i fattori si ha un<br />
aumento <strong>del</strong>la temperatura (fig. 3d) con maggiore influenza,<br />
però, <strong>del</strong>l’ampiezza <strong>del</strong> cuneo <strong>di</strong> mantello idrato e, quin<strong>di</strong>, <strong>del</strong><br />
grado <strong>di</strong> idratazione imposto. Al variare <strong>di</strong> ′<br />
m da 2.5 a 3 m si<br />
passa da valori <strong>di</strong> picco <strong>di</strong> circa 1100 °C e 980°C per Ylim = -<br />
200 km a valori variabili tra 900°C e 650°C per Ylim = -150<br />
km. Le traiettorie P-T in<strong>di</strong>cano come il riciclo <strong>di</strong> materiale<br />
crostale possa essere attivo fino a livelli crostali superficiali<br />
(
186 RODA M. ET ALII<br />
a b<br />
c<br />
e f<br />
Fig. 2 – Risultati <strong>del</strong>la simulazione numerica: le immagini sul lato sinistro si riferiscono ad una velocità <strong>di</strong> subduzione <strong>di</strong> 1cm/a, quelle sul lato destro ad<br />
una velocità <strong>di</strong> 5 cm/a; in nero e grigio scuro è rappresentata la crosta oceanica, in grigio scuro e grigio è rappresentata la crosta continentale e in grigio<br />
chiaro i s<strong>ed</strong>imenti; la linea retta rappresenta il limite superiore <strong>del</strong> cuneo <strong>di</strong> idratazione per ogni simulazione; le linee tratteggiate sono le isoterme.<br />
d
INFLUENZA DELL’IDRATAZIONE DEL CUNEO DI MANTELLO SULL’EVOLUZIONE DI UN SISTEMA DI<br />
SUBDUZIONE OCEANO/CONTINENTE: UNA SIMULAZIONE NUMERICA<br />
Fig. 3 – a,b) Risultati <strong>del</strong>la simulazione numerica: le immagini sul lato sinistro si riferiscono ad una velocità <strong>di</strong> subduzione <strong>di</strong> 1cm/a, quelle sul lato<br />
destro ad una velocità <strong>di</strong> 5 cm/a (rif fig. 2). c,d) Percorsi PTt riferiti a marcatori <strong>di</strong> crosta continentale inferiore per 1 cm/a (sinistra) e 5 cm/a<br />
(destra); linea piena Ylim = -200 km, linea tratteggiata Ylim = -150 km, nero m’ = 2.5m, grigio m’ = 3m, il quadrato mostra l’ultimo intervallo<br />
temporale.<br />
at active continental margins. Int. J. Earth Sci. (Geol.<br />
BIBLIOGRAFIA<br />
Rundsch). 95, (2).<br />
ABERS A., VAN KEKEN P. E., KNELLER E. A., FERRIS A. &<br />
STACHNIK J. C., (2006) - The thermal structure of<br />
subduction zones constrain<strong>ed</strong> by seismic imaging:<br />
Implications for slab dehydration and w<strong>ed</strong>ge. Earth Planet.<br />
Sci. Lett., 241, 387-397.<br />
ARCAY D., TRIC E. & DOIN M.P., (2005) - Numerical<br />
simulation of subduction zones. Effect of slab dehydration<br />
on the mantle w<strong>ed</strong>ge dynamics. Physics of the Earth and<br />
Planetary Int., 149, 133-153.<br />
BILLEN M. I. & GURNIS M., (2001) - A low viscosity w<strong>ed</strong>ge in<br />
subduction zones. Earth Planet. Sci. Lett., 193, 227-236.<br />
GERYA T. V. & STOCKHERT B., (2005) - Two-<strong>di</strong>mensional<br />
numerical mo<strong>del</strong>ing of tectonic and metamorphic histories<br />
a b<br />
c<br />
187<br />
MAROTTA A. M. & SPALLA M. I., (2007) - Permian-Triassic<br />
high thermal regime in the Alps: Result of late Variscan<br />
collapse or continental rifting? Validation by numerical<br />
mo<strong>del</strong>ing. Tectonics. 26.<br />
MEDA M., MAROTTA A. M. & SPALLA M. I., (2007) – Recycling<br />
of the Sesia-Lanzo Zone (Italian Western Alps) continental<br />
lithosphere during the Alpine subduction.Rend. Soc. Geol.<br />
It., 5, 152-153.<br />
RONDENAY S., ABERS G. A. & VAN KEKEN P. E., (2008) -<br />
Seismic imaging of subduction zone metamorphism.<br />
Geology, 36, (4), 275-278.<br />
SCHMIDT M.W. & POLI S., (1998) - Experimentally bas<strong>ed</strong> water<br />
budgets for dehydrating slabs and consequences for arc<br />
magma generation. Earth Planet. Sci. Lett, 163, 361-379.<br />
d
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 188-189, 1 f.<br />
Early Cretaceous granulite facies migmatites from the Sabzevar Range<br />
ophiolitic mélange (NE Iran) and its bearing on the closure of Neotethys<br />
FEDERICO ROSSETTI (*), MOHSEN NASRABADY (**), GIANLUCA VIGNAROLI (*), THOMAS THEYE (°) & AXEL GERDES (°°)<br />
Migmatiti in facies granulitica <strong>di</strong> età Cretacico inferiore nel mélange<br />
ofiolitifero <strong>del</strong>la catena <strong>di</strong> Sabzevar (NE Iran); implicazioni per le<br />
ricostruzioni <strong>del</strong>la chiusura <strong>del</strong> dominio oceanico <strong>del</strong>la Neotetide.<br />
The tectono-metamorphic signature of the oceanic-deriv<strong>ed</strong><br />
units marking orogenic suture zones provides key elements to<br />
decipher modes and regimes of oceanic subduction and<br />
continental accretion, and to constrain paleotectonic<br />
reconstructions at paleo-convergent margins The remnants of<br />
the Tethyan oceanic realm are the most remarkable of these<br />
suture zones, running from the M<strong>ed</strong>iterranean through East<br />
Europe, Middle East to Asia (Fig. 1a). These ophiolitic rocks<br />
record a polyphase and prolong<strong>ed</strong> history of oceanic<br />
construction (the Paleozoic-Early Mesozoic Paleo-Tethys and<br />
the Mesozoic-Cenozoic Neo-Tethys oceanic realms) and<br />
consumption during a sequence of Late Paleozoic to Cenozoic<br />
subduction/obduction/collision stages localiz<strong>ed</strong> along the<br />
Eurasian active plate margin (e.g., RICOU, 1994; DERCOURT et<br />
al., 2000; STAMPFLI & BOREL, 2002). The Iranian ophiolites<br />
(Fig. 1b) are an integrant part of this evolving scenario, with<br />
the Neotethyan remnants <strong>di</strong>stribut<strong>ed</strong> to mark <strong>di</strong>achronous<br />
closures of various oceanic branches during the Alpine-<br />
Himalayan convergence history (e.g, MCCALL, 1997). Despite<br />
these peculiar characteristics, few modern stu<strong>di</strong>es have<br />
address<strong>ed</strong> the characterization of the tectono-metamorphic<br />
evolution of the Neotethyan Iranian ophiolites. Furthermore,<br />
most of these stu<strong>di</strong>es focus<strong>ed</strong> on the Zagros orogen (e.g,<br />
AGARD et alii, 2006), and the ophiolitic mélanges surroun<strong>di</strong>ng<br />
the Central East Iranian Microcontinent (CEIM, Fig. 1b) are<br />
still lacking of a full petrological description and modern age<br />
data are completely missing. The ophiolitic mélange expos<strong>ed</strong> in<br />
the Sabzevar Range (NE Iran; Fig. 1c) is a remnant of one the<br />
Neo-Tethyan oceanic branches surroun<strong>di</strong>ng the Central East<br />
Iranian Microcontinent that clos<strong>ed</strong> during the Paleocene-<br />
Eocene phase of Arabia-Eurasia convergence (BAROZ ET alii,<br />
1984; MC CALL, 1997; SHOJAAT et alii, 2003). Occurrence of<br />
___________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, Università degli Stu<strong>di</strong> Roma Tre,<br />
Roma, Italy<br />
(**) Department of Geology, Tarbiat Moalem University, Tehran, Islamic<br />
Republic of Iran<br />
(°) Institut für Mineralogie und Kristallchemie, Universität Stuttgart,<br />
Stuttgart, Germany<br />
(°°) Institut für Geowissenschaften, J. W. Goethe Universität, Frankfurt,<br />
Germany<br />
Fig. 1 – (a) Distribution of the remnants of the Tethyan oceanic realm along<br />
the Alpine-Himalayan convergence zone. (b) Simplifi<strong>ed</strong> geological map<br />
showing the main tectonic domains in Iran, with the main ophiolitic belts<br />
in<strong>di</strong>cat<strong>ed</strong> (mo<strong>di</strong>fi<strong>ed</strong> after BAGHERI & STAMPFLI, 2008). (c) Geological map<br />
of the Sabzevar Range (mo<strong>di</strong>fi<strong>ed</strong> and readapt<strong>ed</strong> after LENSCH et alii, 1977),<br />
with location of the granulite-facies metabasites. The location of the sample<br />
(NG353) stu<strong>di</strong><strong>ed</strong> for U/Pb geochronology together with its geographic<br />
coor<strong>di</strong>nates are also in<strong>di</strong>cat<strong>ed</strong>.<br />
km-scale, variably retrogress<strong>ed</strong> mafic HP granulitic slices are<br />
report<strong>ed</strong> in this study. The granulite bo<strong>di</strong>es are dark, m<strong>ed</strong>ium to<br />
fine-grain<strong>ed</strong> rocks showing granoblastic groundmass or weak<br />
foliation. The matrix mineralogy consists of Am + Grt + Cpx +<br />
Pl + Qtz (with Ilm, Rt and Ap as main accessory phases) and<br />
makes up the bulk of rocks (>90%). Texture is characteriz<strong>ed</strong> by<br />
occurrence of submillimetric to millimetric leucocratic patches<br />
and layers interlayer<strong>ed</strong> within the granoblastic mineral matrix.<br />
Quartz mostly occurs as interstitial, xenoblastic grains. Matrix<br />
garnets are poikiloblastic, hosting Cpx, Am, Pl, Qtz, Rt, Ilm,<br />
Ttn as inclusions. In the heavily retrogress<strong>ed</strong> samples, Gt<br />
instead forms porphyroclasts, which usually show strong<br />
evidence for resorption with development of retrogressive Pl +<br />
Amp symplectites. The leucocratic segregations invariably<br />
consist of Qtz + Pl-rich continuous and concordant<br />
segregations of broadly tonalitic/trondhjemitic composition<br />
(Qtz/Pl modal proportions 50-35/50-65). They show a<br />
systematic intragranular connectivity, with Pl and Qtz showing<br />
a coarse granoblastic texture. The Pl + Qtz associations also<br />
form film-like intergrowth surroun<strong>di</strong>ng matrix amphibole and<br />
quartz usually show xenomorphic habit. Titanite and zircon are
EARLY CRETACEOUS GRANULITE FACIES MIGMATITES FROM THE SABZEVAR RANGE<br />
the main accessory phases in the leucocratic segregations. Little<br />
evidence for solid-state deformation is document<strong>ed</strong> in the<br />
leucocratic segregations, mostly attest<strong>ed</strong> by patchy undulose<br />
extinction in some quartz grains. The peak mineral assemblage<br />
(Grt + Cpx + Pl + Am + Qtz) is in<strong>di</strong>cative of the Opx-free,<br />
high-pressure granulite facies (PATTISON, 2003). Textures such<br />
as those describ<strong>ed</strong> above show strong similarities with those<br />
report<strong>ed</strong> by (HARTEL & PATTISON, 1996), who interpret<strong>ed</strong> the<br />
Qtz + Pl segregations as remnants of melt. In particular, the<br />
skeletal nature of the quartz and the Qtz + Pl films argue for an<br />
in situ origin of such melts, and hence product of migmatisation<br />
of a basic protolith. A likely scenario is amphibole<br />
dehydratation melting during prograde granulite facies<br />
metamorphism accor<strong>di</strong>ng the following generalis<strong>ed</strong> reaction<br />
(HARTEL & PATTISON, 1996): Am + Pl = Grt + Cpx + Ttn +<br />
melt (trondhjemitic melt) (1). This textural evidence documents<br />
that the Sabzevar granulites record an episode of amphibole<br />
dehydratation melting and trondhjemite melt segregation during<br />
anatexis of an oceanic slab. The peak P-T estimates are 1.1 ±<br />
0.1 GPa and 810 ± 80 °C (THERMOCALC average P-T<br />
calculations; HOLLAND & POWELL, 1998), which conform to a<br />
paleogeothermal gra<strong>di</strong>ent within the subduction channel of ca.<br />
20-25 °C km -1 . In situ U(-Th)-Pb LA-ICP-MS geochronology<br />
was carri<strong>ed</strong> at Goethe University Frankfurt using the analytical<br />
protocol describ<strong>ed</strong> in GERDES & ZEH (2006; 2008). Analysis of<br />
zircon and titanite grains host<strong>ed</strong> in trondhjemite segregations<br />
point to an Early Cretaceous (Albian, c. 105 Ma) age for the<br />
peak granulite facies metamorphism.<br />
These new data provide evidence for an unknown episode of<br />
high-grade subduction zone metamorphism in the region and<br />
argue for juxtaposition of an older ophiolitic suture along the<br />
Paleocene-Eocene Sabzevar orogen. When fram<strong>ed</strong> within the<br />
existing reconstructions, these new data impose reconsideration<br />
of the paleotectonic configuration of the Eurasian convergent<br />
margin during the Early Cretaceous since they (i) support a<br />
scenario of onset of subduction of a young oceanic lithosphere<br />
creat<strong>ed</strong> in the back-arc domain of the Neotethyan subduction,<br />
and (ii) document the existence of two, north-facing paleosubduction<br />
zones active during the Early Cretaceous closure of<br />
the Neo-Tethyan oceanic realm.<br />
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AGARD P., MONIÉ P., GERBER W., OMRANI J., MOLINARO M.,<br />
LABROUSSE L., VRIELYNCK B., MEYER B., JOLIVET L. &<br />
YAMATO P. (2006) - Transient, syn-obduction exhumation of<br />
Zagros blueschists inferr<strong>ed</strong> from P–T–deformation–time and<br />
kinematic constraints: implications for Neotethyan w<strong>ed</strong>ge<br />
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10.1029/2005JB004103.<br />
AZIZI H., MOINEVAZIRI H., MOHAJEL M. & YAGOBPOOR A.<br />
(2005) - P-T-t path in metamorphic rocks of the Khoy region<br />
(northwest Iran) and their tectonic significance for<br />
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BAGHERI S. & STAMPFLI G.M. (2008)- The Anarak, Jandaq and<br />
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geological data, relationships and tectonic implications.<br />
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OHNENSTETTER M. & ROCCI G. (1984) - Ophiolites and<br />
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(Iran) and possible geotectonic reconstructions. Neues J.<br />
Geol. Paläont. Abh., 168, 358–388.<br />
DERCOURT J., GAETANI M., VRIELYNCK B., BARRIER E., BIJU-<br />
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SANDULESCU M. (2000) - Atlas Peri-Tethys,<br />
Paleogeographical Maps, 24 maps and explanatory notes.<br />
Comm. For the Geol. Map of the World, Paris.<br />
GERDES A. & ZEH A. (2006) - Combin<strong>ed</strong> U-Pb and Hf isotope<br />
LA-(MC-) ICP-MS analyses of detrital zircons: Comparison<br />
with SHRIMP and new constraints for the provenance and<br />
age of an Armorican metas<strong>ed</strong>iment in Central Germany.<br />
Earth Planet. Sci. Lett., 249, 47-62.<br />
GERDES A. & ZEH A. (2006) - Zircon formation versus zircon<br />
alteration – New insights from combin<strong>ed</strong> U-Pb and Lu-Hf insitu<br />
La-ICP-MS analyses of Archean zircons from the<br />
Limpopo Belt. Chem. Geol., doi<br />
10.1016/j.chemgeo.2008.03.005.<br />
GHASEMI A. & TALBOT C. J. (2006) - A new tectonic scenario<br />
for the Sanandaj–Sirjan Zone (Iran). J. Asian Earth Sci., 26,<br />
683-693.<br />
HARTEL T. H.D. & PATTISON D.R.M. (1996) - Genesis of the<br />
Kapuskasing (Ontario) migmatitic mafic granulites by<br />
dehydratation melting of amphibole: the importance of<br />
quartz to reaction progress. J. Metam. Geol., 14, 591-611.<br />
HOLLAND T. J. B. & POWELL R. (1998) - An internally<br />
consistent thermodynamic data set for phases of petrological<br />
interest. J. Metam. Geol, 16, 309-343.<br />
LENCH G., MIHMAND A. & ALAVI-TEHRANI N. (1977) -<br />
Petrography and geology of the ophiolite belt north of<br />
Sabzevar/Khorasan (Iran). Neues Jahrb. Mineralogie. Abh,<br />
131, 156–178.<br />
MC CALL G.J.H. (1997) - The geotectonic history of the<br />
Makran and adjacent areas of southern Iran. J. Asian Earth<br />
Sci., 15, 517-531.<br />
PATTISON D.R.M. (2003) - Petrogenetic significance of<br />
orthopyroxene – free garnet +clinopyroxene + plagioclase ±<br />
quartz-bearing metabasites with respect to amphibolite and<br />
granulite facies. J. Metam. Geol., 21, 21-34.<br />
RICOU L.E. (1994) - Tethys reconstruct<strong>ed</strong>: plates continental<br />
fragments and their boundaries since 260 Ma from Central<br />
America to Southeastern Asia. Geodyn. Acta, 7, 169-218.<br />
SHOJAAT B., HASSANIPAK A.A., MOBASHER K. & GHAZI A.M.<br />
(2003) - Petrology, geochemistry and tectonics of the<br />
Sabzevar ophiolite, North Central Iran. J. Asian Earth Sci.,<br />
21, 1053–1067.<br />
STAMPFLI G. M. & BOREL G.D. (2002) - A plate tectonic mo<strong>del</strong><br />
for the Paleozoic and Mesozoic constrain<strong>ed</strong> by dynamic plate<br />
boundaries and restor<strong>ed</strong> synthetic oceanic isochrones. Earth<br />
Planet. Sci. Lett., 196, 17–33.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 190-193, 4ff.<br />
Preliminary results about mechanical stratigraphy of Oligo-Miocene<br />
carbonate grainstones (Majella Mountain, Abruzzo)<br />
ANDREA RUSTICHELLI (*), EMANUELE TONDI (*) & FABRIZIO AGOSTA (*)<br />
RIASSUNTO<br />
Risultati preliminari sulla stratigrafia meccanica dei grainstones<br />
carbonatici oligo-miocenici <strong>del</strong>la Formazione Bolognano (Montagna <strong>del</strong>la<br />
Majella, Abruzzo).<br />
La capacità <strong>del</strong>le rocce carbonatiche <strong>di</strong> contenere o far migrare i geoflui<strong>di</strong><br />
è fortemente influenzata dallo stato <strong>di</strong> “fratturazione”, o meglio dalla presenza<br />
e dalle caratteristiche <strong>di</strong> elementi strutturali come joint, stiloliti (andate in<br />
taglio o meno), bande <strong>di</strong> compattazione/taglio e zone <strong>di</strong> faglia.<br />
Il ruolo <strong>del</strong>la stratigrafia meccanica è quello <strong>di</strong> proc<strong>ed</strong>ere ad una<br />
<strong>di</strong>fferenziazione degli ammassi rocciosi sulla base <strong>del</strong>le loro caratteristiche <strong>di</strong><br />
fratturazione, con particolare riferimento alla lunghezza e alla spaziatura degli<br />
elementi strutturali. In letteratura, inoltre, tali stu<strong>di</strong> hanno evidenziato una<br />
stretta correlazione tra lo spessore degli strati e la lunghezza e la spaziatura<br />
<strong>del</strong>le fratture (intese quasi sempre come joint). Poche ricerche, tuttavia, sono<br />
in<strong>di</strong>rizzate a comprendere il ruolo che le proprietà petrografiche e petrofisiche<br />
<strong>del</strong>le rocce carbonatiche, acquisite durante la loro storia deposizionale e<br />
<strong>di</strong>agenetica, hanno nel con<strong>di</strong>zionare i meccanismi deformativi responsabili<br />
<strong>del</strong>le <strong>di</strong>fferenti tipologie, e quin<strong>di</strong> <strong>di</strong>stribuzione, degli elementi strutturali.<br />
Con l’obiettivo <strong>di</strong> colmare il suddetto gap conoscitivo, in questo lavoro<br />
vengono presentati i risultati preliminari <strong>di</strong> uno stu<strong>di</strong>o volto alla<br />
caratterizzazione <strong>del</strong>la stratigrafia meccanica in <strong>di</strong>fferenti litofacies<br />
appartenenti alla Formazione Bolognano (Montagna <strong>del</strong>la Majella, Abruzzo).<br />
Questa formazione è costituita da calcari bioclastici e marne <strong>di</strong> età oligomiocenica<br />
riferibili ad un ambiente <strong>di</strong> rampa; nella porzione inferiore sono<br />
state riconosciute più litofacies caratterizzate da <strong>di</strong>fferenti tipologie e<br />
<strong>di</strong>mensioni <strong>di</strong> grani (bioclasti <strong>di</strong> prevalenti Briozoi, Foraminiferi bentonici <strong>ed</strong><br />
Echini<strong>di</strong>), percentuali <strong>di</strong> matrice e cementi, e valori <strong>di</strong> porosità. Quest’ultima<br />
proprietà, inoltre, sembra strettamente <strong>di</strong>pendente dalle prime.<br />
Gli elementi strutturali presenti all’interno <strong>del</strong>le <strong>di</strong>verse litofacies sono<br />
principalmente: (i) superfici <strong>di</strong> <strong>di</strong>ssoluzione per pressione, e (ii) bande <strong>di</strong><br />
compattazione/taglio. La <strong>di</strong>scriminante tra l’una e l’altra sembra essere il<br />
valore 15% <strong>di</strong> porosità. Inoltre, per valori molto bassi <strong>di</strong> porosità (al <strong>di</strong> sotto<br />
<strong>del</strong>l’ 1-2%) la spaziature <strong>del</strong>le superfici <strong>di</strong> <strong>di</strong>ssoluzione per pressione, parallele<br />
alla stratificazione, risulta minore. Le correlazioni tra proprietà petrografiche e<br />
petrofisiche, e strutture tettoniche osservate nel corso <strong>di</strong> questo stu<strong>di</strong>o, per ora<br />
<strong>di</strong> natura prettamente qualitativa, verranno integrate da stu<strong>di</strong> quantitativi<br />
mirati alla definizione <strong>del</strong>le relative proprietà <strong>di</strong>mensionali dei vari elementi<br />
strutturali rilevati (spaziatura e lunghezza).<br />
Key words: compaction and shear band, petrography,<br />
petrophysics, pressure solution seams.<br />
Parole chiave: bande <strong>di</strong> compattazione e <strong>di</strong> taglio, petrografia,<br />
petrofisica, superfici <strong>di</strong> <strong>di</strong>ssoluzione per pressione.<br />
___________________________________________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Camerino.<br />
Lavoro eseguito nell’ambito <strong>del</strong> progetto Faults & Fractures in<br />
Carbonates (F&FC) <strong>del</strong>l’Università <strong>di</strong> Camerino.<br />
INTRODUCTION<br />
The containment and migration capacity of geofluids in<br />
carbonate rocks is strongly influenc<strong>ed</strong> by “fracturing”, which is<br />
the presence and the characteristic of <strong>di</strong>scontinuities mainly<br />
represent<strong>ed</strong> by stratification, joints, stylolites, compaction<br />
/shear bands and fault zones (AYDIN, 2000).<br />
During the last decades, sub-seismic mechanical<br />
stratigraphy was mainly mean as a positive correlation between<br />
b<strong>ed</strong> thickness and fracture spacing and length, at <strong>di</strong>fferent<br />
scales (UNDERWOOD et alii, 2003), in particular regar<strong>di</strong>ng<br />
mode 1 fractures (joints).<br />
Only a few recent papers (i.e. DI NACCIO et alii, 2005)<br />
focus<strong>ed</strong>, using a metho<strong>di</strong>c petrographical and petrophysical<br />
approach, on how compositional, depositional and <strong>di</strong>agenetical<br />
carbonate rock features, relat<strong>ed</strong> to <strong>di</strong>fferent depositional<br />
environments and processes, and subsequently <strong>di</strong>agenetical<br />
pathways, can influence deformative mechanisms to determine<br />
<strong>di</strong>fferent tectonic structures typologies and arrangements. In<br />
particular, these papers dealt mainly development and<br />
arrangement of mode 1 fractures (joints) in platform shallow<br />
water carbonates.<br />
Unlike aforemention<strong>ed</strong> works, this study deals with antimode<br />
I fractures (pressure solution seams) and deformation<br />
bands present within temperate carbonate grainstones<br />
characteriz<strong>ed</strong> by various amounts of porosity.<br />
STUDY AREA<br />
The study area (Fig. 1), nearby Lettomanoppello town, is<br />
the northernmost sector of the Majella Mountain. This<br />
mountain is an east-vergent thrust-relat<strong>ed</strong>, box-shape anticline,<br />
part of the eastern domain of the Apennine Chain, made up by<br />
Meso-Cenozoic carbonate rocks of platform, slope and ramp<br />
settings (GHISETTI & VEZZANI, 1998; VECSEI & SANDERS,<br />
1999).<br />
This current study focuses on the Bolognano Formation<br />
(Upper Oligocene – Miocene), in particular on its lowermost<br />
portion, made up by bioclastic carbonate grainstones (Bryozoan<br />
Limestone; VECSEI & SANDERS, 1999). This lower portion is<br />
characteriz<strong>ed</strong> by the highest primary porosity values, as well as<br />
by the presence of several hydrocarbon shows both in the<br />
fractures and in the host rock (AGOSTA et alii, present volume;<br />
AGOSTA et alii, in press, ALESSANDRONI, 2008).
STRATIGRAPHY OF OLIGO-MIOCENE CARBONATE GRAINSTONES (MAJELLA MOUNTAIN)<br />
METHODOLOGY<br />
The work has been carri<strong>ed</strong> out using an integrat<strong>ed</strong><br />
stratigraphic – structural approach, both at outcrop and at<br />
microscope scales.<br />
The field work involv<strong>ed</strong> the geological mapping, at a scale<br />
1:10,000, the detail<strong>ed</strong> stratigraphic analysis of key sections of<br />
the lower portion of the Bolognano Formation (Bryozoan<br />
Limestone), and overly qualitative structural analysis. The<br />
laboratory work follow<strong>ed</strong> a careful collection of representative<br />
hand-samples. We perform<strong>ed</strong> petrographical and petrophysical<br />
characerization of the hand-samples, which was achiev<strong>ed</strong><br />
thanks optical microscope, catho<strong>del</strong>uminescence, HCl test, and<br />
image analyses.<br />
The resulting stratigraphic, petrographical and petrophysical<br />
data have been integrat<strong>ed</strong> with the structural data in order to<br />
define the mechanical stratigraphy of the lower portion of the<br />
Bolognano Formation.<br />
Fig. 1 – a) Geological map of the Majella Mountain and location of the study<br />
area (mo<strong>di</strong>fi<strong>ed</strong> by GHISETTI & VEZZANI, 1998). b) Stratigraphic succession of<br />
the Majella Mountain.<br />
-Carta geologica <strong>del</strong>la Montagna <strong>del</strong>la Majella <strong>ed</strong> ubicazione <strong>del</strong>l’area <strong>di</strong><br />
stu<strong>di</strong>o (mo<strong>di</strong>ficata da GHISETTI & VEZZANI, 1998). b) Successione stratigrafica<br />
<strong>del</strong>la Montagna <strong>del</strong>la Majella.<br />
PRELIMINARY RESULTS<br />
The present study has broadly allow<strong>ed</strong> us to sub<strong>di</strong>vide the<br />
lowermost portion (Bryozoan Limestone) of the Bolognano<br />
Formation in two lithofacies (A and B), relat<strong>ed</strong> to <strong>di</strong>fferent<br />
paleogeographic settings and characteriz<strong>ed</strong> by <strong>di</strong>fferences in<br />
b<strong>ed</strong> arrangement and thickness, in petrographical features (Fig.<br />
2) and consequently, in petrophysical properties.<br />
The more spread lithofacies (A), thick about 100 m, is made<br />
up by shallow water, m<strong>ed</strong>ium-to-very coarse, bioclastic<br />
carbonate grainstones, poor-cement<strong>ed</strong>, characteriz<strong>ed</strong> by wellconnect<strong>ed</strong><br />
high primary porosity (average value 15 %). Despite<br />
rather constant b<strong>ed</strong> geometries (lens-shape clinob<strong>ed</strong>s) and<br />
thickness range (5-80 cm), the nature of bioclasts changes from<br />
the bottom to the top of the stratigraphic succession (Fig. 2).<br />
The other lithofacies (B) forms few m-thick <strong>di</strong>scontinuous<br />
191<br />
bo<strong>di</strong>es interb<strong>ed</strong>d<strong>ed</strong> with the lithofacies A. Lithofacies B is<br />
made up by deeper water, more or less marly, fine-to-m<strong>ed</strong>ium<br />
bioclastic grainstones and packstones, well-cement<strong>ed</strong>,<br />
characteriz<strong>ed</strong> by poor-connect<strong>ed</strong> low porosity (1-2 %),<br />
arrang<strong>ed</strong> in 5-50 cm-thick planar b<strong>ed</strong>s (Fig. 2).<br />
On the whole, the study rocks are interest<strong>ed</strong> by a fewkilometre<br />
or less long, and m-to-10’s m-thick vertical throw,<br />
high-angle normal to strike-slip faults, arrang<strong>ed</strong> in a main set<br />
orient<strong>ed</strong> NO-SE and in a secondary set orient<strong>ed</strong> NE-SO.<br />
Pressure solution seams, joints and compaction and shear bands<br />
are present also. Once the petrographical and petrophysical<br />
properties of the aforemention<strong>ed</strong> lithofacies are been defin<strong>ed</strong>,<br />
these are been compar<strong>ed</strong> with the fracturing associat<strong>ed</strong> to them,<br />
using a qualitative approach.<br />
Is noticeable how average spacing and length of b<strong>ed</strong>perpen<strong>di</strong>cular<br />
and oblique pressure solution seams in the<br />
lithofacies A are greater than in the lithofacies B, due mainly to<br />
minor b<strong>ed</strong> thicknesses of the latter. Instead, regar<strong>di</strong>ng the<br />
lithofacies B, b<strong>ed</strong>-parallel pressure solution seam development<br />
increases as grain size of rock decreases (Fig. 3). Would be<br />
important to understand if grain size really influences the<br />
pressure solution process, or, instead, an higher clay and/or<br />
cement content.<br />
As concerns the <strong>di</strong>fferent structural elements presents in the<br />
two lithofacies, compaction and shear bands are present only in<br />
the lithofacies A. Regar<strong>di</strong>ng to compaction, it often occur close<br />
to and parallel to the b<strong>ed</strong> surfaces (Fig. 4). It is demonstrat<strong>ed</strong><br />
that the development of this type of geological structures is<br />
relat<strong>ed</strong> to grain juxtaposition, which determinates a higher<br />
packing of the rock, with consequent porosity losing under a<br />
compressive stress action, until to reach a limit configuration<br />
where grains can’t be further mov<strong>ed</strong> and the pressure solution<br />
mechanism is favourite (TONDI et alii 2006; TONDI, 2007). The<br />
presence of compaction/shear bands or pressure solution seams<br />
appear to be controll<strong>ed</strong> by primary porosity values; if porosity<br />
values are >15% compaction and shear band are enhanc<strong>ed</strong>,<br />
otherwise pressure solution process seem to be facilitat<strong>ed</strong>. If<br />
porosity is the key factor in order to determine the development<br />
of the aforemention<strong>ed</strong> structures, our data demonstrat<strong>ed</strong> that<br />
porosity values depends by intragranular porosity of<br />
components (relat<strong>ed</strong> to the ratio between porous bioclastic<br />
grains: Bryozoans and Larger Bentic Foraminifera such as<br />
Lepidocyclina and, less important, Amphistegina and<br />
Operculina, and no-porous grains, such us Echinoids plates;<br />
Fig. 2), intergranular porosity (depen<strong>di</strong>ng by grain size, shape<br />
and sorting) and cementation. Further, quantitative analyses<br />
concerning petrophysics and fracturing parameters may better<br />
clear up the aforemention<strong>ed</strong> issues.<br />
In conclusion, the preliminary results of this work show<br />
how <strong>di</strong>fferences in compositional, depositional and <strong>di</strong>agenetical<br />
features (typology, abundance, size, shape and sorting of<br />
grains, cement and clay minerals contents, porosity and pore<br />
structure) relat<strong>ed</strong> to <strong>di</strong>fferent environmental setting,<br />
s<strong>ed</strong>imentary processes and, probably, to evolutive biota<br />
changes, can influence the development of <strong>di</strong>fferent fracture<br />
elements in more or less porous, ramp carbonate grainstones.<br />
Further work, extend<strong>ed</strong> to other carbonate grainstones of<br />
<strong>di</strong>fferent places, ages and paleogeographical setting, would be<br />
necessary to give a world-wide vali<strong>di</strong>ty to the aforemention<strong>ed</strong><br />
insights.
192 A. RUSTICHELLI ET ALII<br />
Fig. 3 – Anastomosing, b<strong>ed</strong>-parallel pressure solution seams, more abundant in<br />
fine-grain<strong>ed</strong> carbonates b<strong>ed</strong>s (minor competence) of lithofacies B. Pressure<br />
solution seams are absent in fine-to-m<strong>ed</strong>ium-grain<strong>ed</strong> b<strong>ed</strong> of the same<br />
lithofacies, visible in the upper part of the photo (more competent, dark color).<br />
- Stiloliti anastomizzate, parallele alla stratificazione, maggiormente <strong>di</strong>ffuse<br />
negli strati carbonatici a grana fine (meno competenti) <strong>del</strong>la litofacies B. Si<br />
noti lo strato più competente (colore più scuro), a grana m<strong>ed</strong>io-fine, privo <strong>di</strong><br />
stiloliti.<br />
Compaction band<br />
Fig. 2 – Sequence stratigraphic scheme of Bryozoan Limestone (Bolognano Formation) outcropping in the study area.<br />
Schema stratigrafico-sequenziale dei Calcari a Briozoi (Formazione Bolognano) affioranti nell’area <strong>di</strong> stu<strong>di</strong>o.<br />
Shear band<br />
Fig. 4 – Compaction and shear bands (white) in the carbonate grainstones,<br />
making up the lithofacies A (dark, due tar-inva<strong>di</strong>ng). Compaction bands are<br />
b<strong>ed</strong>-parallel. The open voids represent tail joints localiz<strong>ed</strong> at the extensional<br />
quadrants and within releasing jogs of the shear bands.<br />
- Bande <strong>di</strong> compattazione e <strong>di</strong> taglio (bianche) nei grainstones carbonatici<br />
costituenti la litofacies A (scuri, poiché impregnati da bitume). Le bande <strong>di</strong><br />
compattazione sono parallele alla stratificazione. Le cavità aperte entro le<br />
bande <strong>di</strong> taglio rappresentano dei tail join, ubicati nei quadranti estensionali<br />
e nei releasing jogs <strong>di</strong> queste strutture.
STRATIGRAPHY OF OLIGO-MIOCENE CARBONATE GRAINSTONES (MAJELLA MOUNTAIN)<br />
REFERENCES<br />
AGOSTA F., ALESSANDRONI M., & TONDI E. (2009) - Oblique<br />
normal faulting along the northern <strong>ed</strong>ge of the Majella<br />
anticline, central Italy: inferences on hydrocarbon<br />
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in press.<br />
AGOSTA F., ALESSANDRONI M., ANTONELLINI M, & TONDI E.<br />
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of Structural Geology, 29, 614-628.<br />
UNDERWOOD C. A., COOKE M. L., SIMO L. & MULDOON M. A.<br />
(2003) – Stratigraphic controls on vertical fracture<br />
patterns in Silurian dolomites, Northeastern Wisconsin.<br />
AAPG Bullettin, 87 (1), 121-142.<br />
VECSEI A. & SANDERS D. (1999) – Facies analysis and<br />
sequence stratigraphy of a Miocene warm-temperate<br />
carbonate ramp, Montagna <strong>del</strong>la Maiella, Italy.<br />
S<strong>ed</strong>imentary Geology, 23, 103-127.<br />
193
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 194-195<br />
Inferring the Fracturing Intensity Patterns in Tectonic Structures<br />
by the Computation of the Time Stress Integral (TSI)<br />
As is well known, stress can produce failure and hence<br />
fracturing in rocks either in traction, when the least stress<br />
component become equal to the rock cohesion or along shear<br />
planes (faults).<br />
In the latter case, the failure con<strong>di</strong>tion is reach<strong>ed</strong> independently<br />
from the mean stress intensity (within the elastic behaviour of<br />
rocks!) and as the stress tensor reaches the proper asymmetry,<br />
accor<strong>di</strong>ng to several failure criteria (e.g. Navier-Coulomb,<br />
Griffith).<br />
This results from the fact that on the one hand this asymmetry<br />
generate a simple shear stress on a given plane within the rock,<br />
and on the other hand is responsible for a pressure on that plane<br />
that increases its strength to break and slip.<br />
The effective capacity of a given stress to induce failure in<br />
shear con<strong>di</strong>tions is evaluat<strong>ed</strong> by the Deformation Function DF<br />
parameter, that represents the <strong>di</strong>fference, measur<strong>ed</strong> in Pa,<br />
between the shear acting on a plane and its strength.<br />
Maximum values of DF correspond to ideal failure plane as<br />
pr<strong>ed</strong>ict<strong>ed</strong> by the aforemention<strong>ed</strong> criteria.<br />
The stress acting on a rock in tectonic environments is call<strong>ed</strong><br />
the static stress and results from the (tensorial) ad<strong>di</strong>tion of a<br />
series of components that includes overburden, regional and<br />
local stresses.<br />
The effective stress will be the result of this ad<strong>di</strong>tion subtract<strong>ed</strong><br />
by the pore fluid pressure that releases the static stress by the<br />
pressure exert<strong>ed</strong> on the rock by the pressure, thus r<strong>ed</strong>ucing the<br />
rock strength.<br />
In this simple introduction we will neglect the pore elasticity<br />
component induc<strong>ed</strong> by the elasticity of the rock/pore system.<br />
As it is well known, rocks present an odd spatial <strong>di</strong>stribution of<br />
fractures, despite the rather homogeneous <strong>di</strong>stribution of<br />
regional and overburden stress values.<br />
This variability depends from the local rheological variations<br />
(even negligible, as this asymmetry is observ<strong>ed</strong> also in perfectly<br />
homogeneous rocks) and local ad<strong>di</strong>tional stress components.<br />
In tectonic environments, local stresses are induc<strong>ed</strong> by the<br />
relative movements and deformations of rock undergoing<br />
translations during the changes in geometry of the geological<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, Università degli Stu<strong>di</strong> Roma Tre<br />
Largo S. Leonardo Murialdo 1, 00146 ROMA<br />
Lavoro eseguito nell’ambito <strong>del</strong>l'attività <strong>del</strong> Laboratorio <strong>di</strong> Geo<strong>di</strong>namica<br />
Quantitativa e Telerilevamento<br />
F. SALVINI*<br />
structures. These local components derive therefore form the<br />
kinematics induc<strong>ed</strong> by the geological processes and are referr<strong>ed</strong><br />
as kinematic stresses.<br />
Kinematic stresses are group<strong>ed</strong> into two categories:<br />
(i) stress induc<strong>ed</strong> by elastic accumulation nearby slip planes<br />
with friction;<br />
(ii) stress induc<strong>ed</strong> by internal elastic deformation of the rock<br />
during its movement.<br />
The first component develops in flexural slip processes in<br />
fol<strong>di</strong>ng as well as in fault damage zones.<br />
In the latter case it is the main responsible of the fracturing in<br />
fault damage zones (and in the earlier fault cores) with the<br />
development of well known fracture cleavages (e.g. Ri<strong>ed</strong>el<br />
planes, Har<strong>di</strong>ng fractures).<br />
As an example, the second kinematic stress category is present<br />
on rocks during their movement along active axial planes and is<br />
produc<strong>ed</strong> by the induc<strong>ed</strong> torsion.<br />
Please note that in the cit<strong>ed</strong> example the first category is also<br />
present, resulting from the flexural slip processes.<br />
The final fracturing state of a rock relates to its stress history<br />
through time during all the tectonic events that it suffer<strong>ed</strong>.<br />
As a consequence, the estimate of the fracturing intensity of a<br />
rock unit cannot be deriv<strong>ed</strong> simply by the analysis of its final<br />
geometry compar<strong>ed</strong> to the initial one, as is done with the<br />
curvature analysis in fol<strong>di</strong>ng.<br />
A better estimate will derive from the computation of the<br />
accumulation of all the stresses (torsion and shear deriv<strong>ed</strong>)<br />
during all its geological history.<br />
To mo<strong>del</strong> this accumulation, we consider that fracturing<br />
develops in a very long time with very low velocities and can<br />
be <strong>di</strong>vid<strong>ed</strong> into a long series of re-equilibrat<strong>ed</strong> steps that fully<br />
compensate static stress accumulation in between (i.e. a fold<strong>ed</strong><br />
layer will not unbend if you extract it from he rock!). Therefore<br />
in this mo<strong>del</strong>, we accumulate through time only the brittle<br />
effects of the stresses that develop<strong>ed</strong> at each step.<br />
In other words, a fractur<strong>ed</strong> rocks will possible accumulate other<br />
fracturing episodes during its geological evolution. If the new<br />
fractures are not dynamically compatible with the pre-existing<br />
ones, then a new fracture set will develop. On the other hand, if<br />
the existing fracturing is compatible with the new failure<br />
con<strong>di</strong>tions, it will increase the fracturing intensity by infilling<br />
processes.<br />
With these considerations, we can state that the final fracturing<br />
of a rock relates to its history of suffer<strong>ed</strong> failure-capable
stresses, that is the history integrat<strong>ed</strong> through time of its DF, the<br />
parameter that describes the capacity of stresses to induce<br />
fractures.<br />
The resulting value is the Time-Stress Integral (TSI), that is<br />
defin<strong>ed</strong> as the integral through time of the DF and has the<br />
<strong>di</strong>mensions of Pa*year:<br />
TSI = DF dt<br />
This integral, between the instant t1 and t2, can be numerically<br />
approximat<strong>ed</strong> by assuming that few changes occur between<br />
contiguous steps through time.<br />
Therefore we can convert it into:<br />
TSI = Σ DF ∆t<br />
By the forward mo<strong>del</strong>ling of the evolution of the structures it is<br />
possible to compute the DF at each step and numerically<br />
integrate it to compute the TSI.<br />
Stu<strong>di</strong>es of outcrops in tectonic environments, together with data<br />
from deep drillings compar<strong>ed</strong> to forward mo<strong>del</strong>s of their<br />
evolution reveal<strong>ed</strong> that a relationship exists between the<br />
fracture intensity and its spatial <strong>di</strong>stribution in tectonic<br />
environments and the comput<strong>ed</strong> TSI values in forward<br />
numerical mo<strong>del</strong>ling.<br />
Fracture intensity can be measur<strong>ed</strong> with the H/S parameter<br />
(among many methods), being H the fracture extension normal<br />
to the (null) axis of the fracture/structure (e.g. in case of fold<strong>ed</strong><br />
structures, along the normal to the line of intersection of the<br />
fracture with the surface of the fractur<strong>ed</strong> layer). S is the<br />
geometrical <strong>di</strong>stance between adjacent, homologous fractures.<br />
These stu<strong>di</strong>es reveal<strong>ed</strong> that<br />
H/S = f(TSI)<br />
The TSI in fault damage zones is numerically comput<strong>ed</strong> by<br />
<strong>di</strong>scretising the fault surface into cells and computing the DF<br />
on each of them for both sides, since DF is a function also of<br />
the rock rheology. The TSI is then comput<strong>ed</strong> by integrating the<br />
DF along the path of <strong>di</strong>splacement of each cell on the fault<br />
surface. The computation of the TSI is obtain<strong>ed</strong> by the FRAP<br />
software, that was successfully us<strong>ed</strong> to pr<strong>ed</strong>ict fault<br />
compartmentalisation in oil reservoirs.<br />
The TSI in fault-relat<strong>ed</strong> structures is comput<strong>ed</strong> by forward<br />
mo<strong>del</strong>ling through time of a series of 2D sections using a<br />
Layer<strong>ed</strong>-HCA numerical tool, the FORC software, that is<br />
capable of kinematically replicating the evolution of geological<br />
structures and computing the local dynamical stresses inclu<strong>di</strong>ng<br />
torsion and simple-shear ones.<br />
INFERRING THE FRACTURING INTENSITY PATTERNS<br />
195
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 196-198, 3 ff.<br />
The 100-150 Ma Apparent Polar Wander Path for Adria/Africa<br />
RIASSUNTO<br />
La curva <strong>di</strong> deriva dei poli <strong>di</strong> Adria/Africa fra 100 e 150 Ma fa<br />
È stata calcolata una curva <strong>di</strong> deriva dei poli per Adria/Africa per<br />
l’intervallo 100-150 Ma, utilizzando campioni raccolti in 6 sezioni <strong>del</strong><br />
Kimmeridgiano-Albiano <strong>del</strong>l’Appennino settentrionale.<br />
Il problema <strong>del</strong>le rotazioni tettoniche introdotte durante l’orogenesi<br />
appenninica è stato risolto calcolando le rotazioni relative fra le sezioni<br />
confrontando i valori <strong>di</strong> declinazione provenienti da intervalli temporali<br />
comuni. La curva calcolata mostra due bruschi cambi <strong>di</strong> <strong>di</strong>rezione, uno a circa<br />
140 Ma e l’altro a circa 105 Ma, probabilmente dovuti ad episo<strong>di</strong> <strong>di</strong> True Polar<br />
Wander.<br />
Inoltre, la curva calcolata da dati <strong>del</strong>l’Appennino è in buon accordo con<br />
quella calcolata per Africa nello stesso intervallo <strong>di</strong> tempo (BESSE &<br />
COURTILLOT (2002), rafforzando l’ipotesi che Adria si sia mossa<br />
coerentemente con Africa, almeno durante l’intervallo <strong>di</strong> tempo stu<strong>di</strong>ato.<br />
Key words: Adria, Apennines, APWP, magnetostratigraphy,<br />
paleomagnetism.<br />
INTRODUCTION<br />
The accurate determination of Apparent Polar Wandering<br />
Paths (APWPs) is a prerequisite to constrain paleogeographic<br />
reconstructions. Nevertheless, despite the large effort<br />
perform<strong>ed</strong> by the paleomagnetic community over the last five<br />
decades, one of the major problems that plague the<br />
determination of the APWP is the partial or total lack of data<br />
for certain periods.<br />
The Umbro-Marchean pelagic succession is well known for<br />
its exceptional magnetostratigraphic results and could give the<br />
possibility to reconstruct well dat<strong>ed</strong> APWP segments.<br />
However, the use of data from orogenic belts is hamper<strong>ed</strong> by<br />
the presence of local tectonic rotations.<br />
We test<strong>ed</strong> a method to solve the problem of tectonic<br />
rotations and reconstruct<strong>ed</strong> a 150 to 100 Ma high-resolution<br />
APWP for Adria by studying 4 Kimmeridgian-Lower Aptian<br />
(Bosso, Arcevia, Contessa and Gorgo a Cerbara, SATOLLI et<br />
_________________________<br />
SARA SATOLLI (*,°), FERNANDO CALAMITA (°) & JEAN BESSE (*)<br />
(*) Laboratoire de Géomagnétism et Paléomagnétism, Institut de Physique<br />
du Globe de Paris, France.<br />
(°) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università “G. d’Annunzio” <strong>di</strong><br />
Chieti-Pescara, Italy.<br />
alii, 2007) and 3 Aptian-Albian (Gorgo a Cerbara, La<br />
Roccaccia and Poggio le Guaine, SATOLLI et alii, 2008)<br />
magnetostratigraphic sections in the Northern Apennines (Fig.<br />
1).<br />
GEOLOGICAL SETTING AND PALEOMAGNETIC<br />
ANALYSIS<br />
The analyz<strong>ed</strong> sections are locat<strong>ed</strong> in the Northern Apennines<br />
fold-and-thrust belt. The Triassic-Cenozoic succession<br />
deposit<strong>ed</strong> above the Adria margin was deform<strong>ed</strong> during the<br />
Tertiary orogenesis.<br />
We sampl<strong>ed</strong> the Calcari ad Aptici, the Maiolica and the<br />
Marne a Fucoi<strong>di</strong> Formations by collecting 2.5-cm-<strong>di</strong>ameter<br />
cores with portable drilling equipment. Samples were orient<strong>ed</strong><br />
in-situ with a magnetic compass and analys<strong>ed</strong> in the<br />
paleomagnetic laboratories of the Istituto Nazionale <strong>di</strong><br />
Geofisica and Vulcanologia of Rome and of the Institut de<br />
Physique du Globe of Paris with a 2G-DC SQUID<br />
magnetometer.<br />
The magnetic properties of the analyz<strong>ed</strong> rocks (natural<br />
remanent magnetization, susceptibility and thermal<br />
demagnetization of a three-component isothermal remanent<br />
magnetization) and the range in which the characteristic<br />
remanent magnetization components were isolat<strong>ed</strong> strongly<br />
depend upon lithologies.<br />
Thermally demagnetiz<strong>ed</strong> samples generally <strong>di</strong>splay two or<br />
three components (Fig. 2): a low blocking temperature<br />
component of normal polarity close to the geocentric axial<br />
<strong>di</strong>pole field; an interm<strong>ed</strong>iate temperature component with a<br />
revers<strong>ed</strong> polarity (also detect<strong>ed</strong> by TARDUNO et alii, 1992); the<br />
characteristic component of magnetization (both normal and<br />
reverse polarity).<br />
THE APWP RECONSTRUCTION<br />
In order to solve the problem of tectonic rotations link<strong>ed</strong> to<br />
Alpine and Apennine orogenesis, we comput<strong>ed</strong> relative<br />
rotations between sections by considering paleomagnetic<br />
<strong>di</strong>rections from common time overlaps, and realign<strong>ed</strong> them into<br />
a common declination reference frame (Bosso section).<br />
The sections are characteriz<strong>ed</strong> by similar rotations (between<br />
ca. 24° and ca. 34° CCW) with respect to the value expect<strong>ed</strong><br />
for Africa from BESSE & COURTILLOT (2002), taken as a proxy<br />
for stable Adria, with exception of Gorgo a Cerbara which is
THE 100-150 MA APPARENT POLAR WANDER PATH FOR ADRIA/AFRICA<br />
Fig. 1 – Digital elevation mo<strong>del</strong> and schematic geological map of the study area, with location of the stu<strong>di</strong><strong>ed</strong> sections. White arrows show the rotation of the<br />
sections with respect to the African reference frame (BESSE & COURTILLOT, 2002).<br />
the more strongly rotat<strong>ed</strong> (ca. 51° CCW). Particularly, La<br />
Roccaccia, Poggio le Guaine and Bosso sections show no<br />
relative rotations, thus belong to the same structural unit, as<br />
also support<strong>ed</strong> by field structural evidence and geological<br />
maps. Conversely, the <strong>di</strong>fference in inclination remains always<br />
small between the sections, and includ<strong>ed</strong> in their respective<br />
error bars.<br />
We reconstruct<strong>ed</strong> a 100–150 Ma APWP segment in Bosso<br />
coor<strong>di</strong>nates (Fig. 3) by computing an average pole every 5<br />
Myr in the Kimmeridgian-lower Aptian sections, and a<br />
pole for each sampl<strong>ed</strong> Member in the lower Aptian-Albian<br />
Marne a Fucoi<strong>di</strong> Formation.<br />
Moreover, we comput<strong>ed</strong> a 25° counterclockwise rotation<br />
between the 100-150 Ma segment in Bosso coor<strong>di</strong>nates and the<br />
equivalent age 10 Myr sli<strong>di</strong>ng window APWP from BESSE &<br />
COURTILLOT (2002), by minimizing the sum of angular <strong>di</strong>stance<br />
between poles. The APWP comput<strong>ed</strong> from Apennines data and<br />
the equivalent age segment from BESSE & COURTILLOT (2002)<br />
are in very good agreement, showing the same temporal<br />
evolution and being most often statistically in<strong>di</strong>stinct after the<br />
rotation correction.<br />
The new segment records an APWP loops close to the<br />
Jurassic/Cretaceous boundary (around 140 Ma) that document a<br />
fast and abrupt change in plate motion, without any significant<br />
standstill. The global change in plate motion could be either<br />
link<strong>ed</strong> to large scale collisions (closure of the Mongol-Okhotsk<br />
Fig. 2 – Example of vector <strong>di</strong>agram of typical demagnetization data, in<br />
situ coor<strong>di</strong>nates, showing the three recogniz<strong>ed</strong> magnetization components.<br />
Open and solid symbols represent projection onto the vertical and<br />
horizontal planes, respectively. Demagnetization steps values are<br />
express<strong>ed</strong> in °C.<br />
197
198 S. SATOLLI ET ALII<br />
Fig. 3 – The 150–100 Ma APWP segment reconstruct<strong>ed</strong> from Northern Apennines data. a) The 150–100 Ma segment (black and green symbols) before and<br />
after a 25° CW rotation, compar<strong>ed</strong> with the APWP from BESSE & COURTILLOT (2002); c) the 150–100 Ma APWP segment (open black symbols and black<br />
arrow) compar<strong>ed</strong> with 150–90 Ma segment from BESSE & COURTILLOT (2002) (open grey symbols and dark grey arrow).<br />
Ocean), True Polar Wander (TPW) events, or possibly to the<br />
South Atlantic opening.<br />
A smaller event is detect<strong>ed</strong> at 100-110 Ma, with apex at ca.<br />
105 Ma, and could evidence a fast southward motion of Adria<br />
(maybe due to TPW), undetect<strong>ed</strong> from classical African<br />
APWPs.<br />
CONCLUSIONS<br />
We reconstruct<strong>ed</strong> a 150 to 100 Ma high-resolution APWP,<br />
using 6 magnetostratigraphic sections from the Northern<br />
Apennines (Fig. 3). The segment document two loops, at 140<br />
and 105 Ma, most probably due to TPW events.<br />
The good agreement found between the 100-150 Ma segment<br />
comput<strong>ed</strong> from data collect<strong>ed</strong> in the Apennines and the<br />
equivalent time segment from BESSE & COURTILLOT (2002)<br />
strengthens the fact that Adria was part of Africa at the<br />
lithospheric level at least during the analyz<strong>ed</strong> period. Whereas,<br />
rotations between sections were induc<strong>ed</strong> later on, most<br />
probably during the Apennines orogenesis.<br />
Finally, the high quality of the obtain<strong>ed</strong> segment<br />
demonstrates that is possible to reconstruct APWP using data<br />
from orogenic belts, when it is possible to solve tectonic<br />
rotations link<strong>ed</strong> to the orogenesis.<br />
REFERENCES<br />
BESSE, J. & COURTILLOT, V. (2002) - Apparent and true polar<br />
wander and the geometry of the geomagnetic field over the<br />
last 200 Myr. J. Geophys. Res., 107 (B11), 2300.<br />
SATOLLI, S., BESSE, J., SPERANZA, F. & CALAMITA, F. (2007) -<br />
New 125-150 Ma high-resolution Apparent Polar Wander<br />
Path for Adria from magnetostratigraphic sections in<br />
Umbria-Marche (Northern Apennines, Italy): Timing and<br />
duration of the global Jurassic-Cretaceous hairpin turn.<br />
Earth Planet. Sci. Lett. 257, 329-342.<br />
SATOLLI, S., BESSE, J. & CALAMITA, F. (2008) -<br />
Paleomagnetism of Aptian-Albian sections from the<br />
Northern Apennines (Italy): Implications for the 150-100<br />
Ma apparent polar wander of Adria and Africa. Earth<br />
Planet. Sc. Lett., 276, 115-128.<br />
TARDUNO, J.A., LOWRIE, W., SLITER, V., BRALOWER, T.J. &<br />
HELLER, F. (1992) - Revers<strong>ed</strong> polarity characteristic<br />
magnetizations in the Albian Contessa sections, Umbrian<br />
Apennines, Italy: implications for the existence of a Mid-<br />
Cretaceous mix<strong>ed</strong> polarity interval. J. Geophys. Res. 97,<br />
241–271.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 199-202, 3 ff.<br />
Risultati preliminari <strong>del</strong>l’analisi sismica <strong>del</strong> Golfo <strong>di</strong> Squillace<br />
(Calabria ionica): ricostruzione 3D<br />
ABSTRACT<br />
Preliminary results of seismic analysis of Squillace Gulf (Ionian<br />
Calabria): 3D reconstruction<br />
In this paper we report the first results of a three-<strong>di</strong>mensional seismic<br />
analysis of geometries and architectures in the subsurface of Squillace Gulf<br />
locat<strong>ed</strong> in the Ionian margin of Catanzaro Trough (Southern Calabria).<br />
The goal of this research is to find main details on the temporal and spatial<br />
<strong>di</strong>stribution of trascurrent faulting in the central portion of the CA that are still<br />
lacking.<br />
The interpretation of seismics profiles give us the possibility to remark<br />
<strong>di</strong>fferent evolution phase in this sector of Calabrian Arc (CA).<br />
In particular we recognize, over Messinian succession, two <strong>di</strong>fferent<br />
tectono-stratigraphics units represent<strong>ed</strong> by Pliocene and Pleistocene s<strong>ed</strong>iments.<br />
The chronological value to the most evidence seismic anomalies was given<br />
by the calibration of seismic profiles with deep well cores.<br />
At first sight, the tectonic activity of eastern side of CA seems to be<br />
dominant during the period between Upper Miocene and Pliocene despite the<br />
last 1My ca. during witch the vertical movement can be consider<strong>ed</strong> as a result<br />
of the interaction between tectonic uplift and Quaternary sea-level changes.<br />
Key words: seismic reflection profiles, sequence stratigraphy,<br />
3D images.<br />
INTRODUZIONE<br />
Il Tirreno, il più giovane bacino <strong>del</strong> M<strong>ed</strong>iterraneo<br />
occidentale, costituisce insieme alla catena appenninica una tra<br />
le aree più stu<strong>di</strong>ate <strong>del</strong> M<strong>ed</strong>iterraneo.<br />
Esso rappresenta un bacino <strong>di</strong> retro-arco formatosi dal<br />
Miocene fino all’attuale. Durante quest’intervallo temporale il<br />
rifting tirrenico e la compressione nella regione appenninica<br />
hanno coesistito, e tuttora coesistono, con una progressiva<br />
migrazione verso E (MALINVERNO & RYAN, 1986).<br />
La bibliografia esistente mostra come esistano <strong>di</strong>verse linee<br />
<strong>di</strong> pensiero circa la sua genesi <strong>ed</strong> evoluzione (REHAULT et alii,<br />
1984; DEWEY et alii, 1989; KNOTT & TURCO, 1991; VAN DIJK<br />
et alii, 2000; TURCO et alii, 2006).<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Camerino, Via<br />
Gentile III da Varano – 62062 Camerino (MC), Italy.<br />
valeriasbrescia@unicam.it; Tel. 0737-402600<br />
(°) CNR – I.A.M.C., Napoli, Italy<br />
SBRESCIA VALERIA (*), TURCO EUGENIO (*) & MILIA ALFONSA (°)<br />
In questo contesto si inserisce l’Arco Calabro che<br />
costituisce l’elemento <strong>di</strong> congiunzione tra il sistema orogenico<br />
appenninico a N e la catena siciliano-maghrebide a S,<br />
occupando quin<strong>di</strong> una posizione centrale nel quadro evolutivo<br />
tirrenico.<br />
In particolar modo va sottolineata l’importanza dei bacini<br />
peri-tirrenici (i.e il Bacino <strong>di</strong> Paola, Golfo <strong>di</strong> S.Eufemia, Golfo<br />
<strong>di</strong> Policastro) <strong>ed</strong> i bacini posti sul margine ionico <strong>del</strong>la Calabria<br />
(Golfo <strong>di</strong> Taranto, Bacino <strong>di</strong> Crotone e Golfo <strong>di</strong> Squillace) che<br />
registrano la successione stratigrafica e le strutture associate<br />
agli eventi tettonici responsabili <strong>del</strong>l’apertura <strong>del</strong> Tirreno.<br />
Essendo l’evoluzione strutturale <strong>di</strong> questi bacini legata alla<br />
migrazione verso SE <strong>del</strong>l’Arco Calabro iniziata a seguito<br />
<strong>del</strong>l’apertura <strong>del</strong> bacino Tirrenico, <strong>di</strong>viene chiaro il motivo per<br />
il quale la Stretta <strong>di</strong> Catanzaro e la sua continuazione nel Golfo<br />
<strong>di</strong> Squillace assumono una posizione chiave per la definizione<br />
dei processi che controllano l’evoluzione <strong>di</strong> questa porzione <strong>di</strong><br />
bacino dal Tortoniano all’attuale.<br />
INQUADRAMENTO GEOLOGICO<br />
L’arco Calabro, parte emergente <strong>del</strong> prisma <strong>di</strong> accrezione<br />
associato alla subduzione <strong>del</strong>lo slab litosferico ionico,<br />
rappresenta un frammento <strong>del</strong> margine europeo che si<br />
sovrappone all’Appennino e alla catena Maghrebide durante la<br />
collisione tra Europa e Apulia nel tardo Miocene inferiore<br />
Fig. 1 – Mappa strutturale semplificata <strong>del</strong>l’Arco Calabro (Italia meri<strong>di</strong>onale)<br />
e localizzazione <strong>del</strong> Golfo <strong>di</strong> Squillace.
200 V. SBRESCIA ET ALII<br />
(fig.1).<br />
Con il termine “Arco Calabro” si intende l’area compresa<br />
tra due gran<strong>di</strong> lineamenti trascorrenti rappresentati dalla linea<br />
<strong>del</strong> Pollino a NE, con cinematica sinistra, e dalla linea <strong>di</strong><br />
Taormina a SW, con cinematica destra (AMODIO MORELLI et<br />
alii, 1976; COLELLA, 1988; VAN DIJK & OKKES, 1991; WELTJE,<br />
1992).<br />
Sebbene la migrazione verso SE <strong>del</strong>l’Arco Calabro è<br />
continua dal Tortoniano fino all’attuale, negli ultimi 700ka la<br />
sua velocità <strong>di</strong> spostamento <strong>di</strong>minuisce poiché <strong>di</strong>venta<br />
pr<strong>ed</strong>ominante il movimento verticale (uplift = 1.67mm/a,<br />
WESTAWAY, 1993; ANTONIOLI et alii, 2006).<br />
Per l’Arco Calabro sono stati suggeriti <strong>di</strong>versi mo<strong>del</strong>li<br />
cinematici che possono essere così riassunti:<br />
- Translation mo<strong>del</strong>s (MOUSSAT, 1983; MEULEKAMP et<br />
alii, 1986; DEWEY et alii, 1989)<br />
- Sphenochasm mo<strong>del</strong>s (SCANDONE, 1979; DEWEY et<br />
alii, 1989)<br />
- Ben<strong>di</strong>ng mo<strong>del</strong>s (GHISETTI et alii, 1982; LUONGO et<br />
alii, 1988)<br />
- Ra<strong>di</strong>al Drift mo<strong>del</strong>s (DUBOIS, 1976; FINETTI & DEL<br />
BEN, 1986)<br />
E’ generalmente riconosciuto che, basandosi<br />
sull’evoluzione tettono-s<strong>ed</strong>imentaria neogenica, l’Arco Calabro<br />
può essere sud<strong>di</strong>viso in una serie <strong>di</strong> segmenti limitati da set <strong>di</strong><br />
faglie strike-slip orientate NW-SE (MEULENKAMP et alii, 1986;<br />
VAN DIJK & OKKES, 1991; DEL BEN et alii, 2008).<br />
Il Catanzaro Trough rappresenta uno <strong>di</strong> questi segmenti<br />
bordato, a N e S, da due fasce trascorrenti sinistre<br />
(MEULENKAMP et alii, 1986; VAN DIJK, 1994; ZECCHIN et alii,<br />
2006).<br />
La scelta <strong>di</strong> analizzare le perforazioni esistenti e la fitta<br />
maglia <strong>di</strong> profili sismici che coprono sia il versante ionico<br />
(Golfo <strong>di</strong> Squillace) che quello tirrenico (Golfo <strong>di</strong> S. Eufemia)<br />
<strong>del</strong>la Stretta <strong>di</strong> Catanzaro è dettata, oltre che dalla poca<br />
chiarezza degli affioramenti per il verificare la supposta<br />
tettonica trascorrente sinistra in questa porzione <strong>di</strong> catena,<br />
anche dalla capacità <strong>del</strong>le aree depresse <strong>di</strong> registrare l’intera<br />
s<strong>ed</strong>imentazione (dall’apertura <strong>del</strong> bacino all’attuale) e le<br />
strutture associate alla tettonica neogenica.<br />
DATI E METODOLOGIA<br />
Il presente lavoro si basa sull’interpretazione <strong>di</strong> circa<br />
500km <strong>di</strong> profili sismici multicanale a riflessione acquisiti<br />
durante la campagna oceanografica condotta dall’AGIP S.p.A.<br />
durante la metà degli anni ’70 <strong>ed</strong> i profili CROP esistenti in<br />
questa porzione <strong>del</strong> Mar Ionio (fig. 2).<br />
I profili acquisiti nella zona F coinvolgono il versante<br />
ionico <strong>del</strong>l’Italia Meri<strong>di</strong>onale e, più in particolare, l’offshore<br />
<strong>del</strong>l’area compresa tra la penisola <strong>del</strong> Gargano e lo Stretto <strong>di</strong><br />
Messina.<br />
Oltre ai dati sismici, lungo il versante ionico sono presenti<br />
anche una serie <strong>di</strong> perforazioni per l’esplorazione <strong>di</strong> idrocarburi<br />
Fig. 2 – Localizzazione <strong>del</strong> grid sismico (linee multicanale CROP e F) e dei pozzi per l’esplorazione petrolifera in corrispondenza <strong>del</strong> Golfo <strong>di</strong> Squillace.
RISULTATI PRELIMINARI DELL’ANALISI SISMICA DEL GOLFO DI SQUILLACE<br />
prodotti dall’AGIP <strong>di</strong>stribuiti in maniera <strong>di</strong>somogenea:<br />
maggiormente presenti al largo <strong>di</strong> Crotone, meno densi nel<br />
Golfo <strong>di</strong> Squillace e quasi <strong>del</strong> tutto assenti a largo <strong>di</strong> Locri.<br />
Lo scopo <strong>del</strong>l’interpretazione dei suddetti profili è quello <strong>di</strong><br />
in<strong>di</strong>viduare e descrivere i principali aspetti geologicostratigrafici,<br />
strutturali e tettonici dei bacini posti sul versante<br />
tirrenico e su quello ionico <strong>del</strong>la Stretta <strong>di</strong> Catanzaro per poi<br />
poterli inserire all’interno <strong>di</strong> una ricostruzione temporale<br />
<strong>del</strong>l’apertura tirrenica cui la migrazione <strong>del</strong>l’Arco Calabro è<br />
strettamente legata.<br />
La successione stratigrafica registrata nel bacino<br />
estensionale posto in corrispondenza <strong>del</strong> Golfo <strong>di</strong> Squillace<br />
comprende circa 3200m <strong>di</strong> depositi Plio-Pleistocenici che<br />
poggiano su depositi Messiniani con spessore variabile da<br />
poche decine a circa 1200m.<br />
Le perforazioni esistenti (fig. 3) in quest’area mostrano, dal<br />
basso verso l’alto, le seguenti litologie:<br />
- Basamento metamorfico; - Depositi Miocenici <strong>del</strong><br />
Serravalliano-Tortoniano, Formazione <strong>del</strong>le Argille marnose<br />
<strong>del</strong> Ponda (poche decine <strong>di</strong> metri) e Formazione arenaceoconglomeratica<br />
<strong>di</strong> San Nicola <strong>del</strong>l’Alto (da 30m a 600m <strong>di</strong><br />
spessore), e <strong>del</strong> Messiniano, Gessoso-Solfifera (da 60m a<br />
1300m) e Formazione <strong>del</strong>le Cavarne (da poche decine a<br />
~300m); - Depositi Pliocenici, Formazione <strong>del</strong>la Molassa <strong>di</strong><br />
Zinga (~60m) e Formazione <strong>del</strong>le Argille <strong>di</strong> Crotone (da 300m<br />
a ~600m); - Depositi Pleistocenici, Formazione <strong>di</strong> San Mauro<br />
(poche decine <strong>di</strong> metri).<br />
Fig. 3 – Stratigrafie semplificate <strong>di</strong> alcuni pozzi per l’esplorazione petrolifera<br />
ubicati nel Golfo <strong>di</strong> Squillace.<br />
201<br />
I terreni presenti possono essere associati a 3 <strong>di</strong>versi cicli<br />
s<strong>ed</strong>imentari: il bacino si sviluppa durante il Serravalliano-<br />
Pliocene inf. seguendo i maggiori sistemi <strong>di</strong> faglie; dopo la sua<br />
formazione comincia la seconda fase, quella <strong>di</strong> riempimento,<br />
che avviene secondo <strong>di</strong>versi impulsi tettonici <strong>ed</strong> infine si passa<br />
alla fase <strong>di</strong> chiusura <strong>del</strong> bacino stesso.<br />
L’interpretazione dei profili sismici ha permesso la<br />
<strong>di</strong>stinzione <strong>di</strong> cinque unità tettono-stratigrafiche: Pliocene,<br />
Pleistocene, Miocene (sud<strong>di</strong>viso in Messiniano e Serravalliano-<br />
Tortoniano) e Basamento Metamorfico.<br />
Tale <strong>di</strong>fferenziazione è stata possibile grazie, oltre alle<br />
informazioni fornite dalle stratigrafie <strong>del</strong>le perforazioni<br />
presenti, anche dal riconoscimento <strong>del</strong>le unconformities a bassa<br />
frequenza.<br />
Inoltre, all’interno <strong>di</strong> queste unità si riconoscono ulteriori<br />
sequenze deposizionali separate da unconformities ad alta<br />
frequenza.<br />
Le faglie, per lo più normali, interessano le unità<br />
mioceniche (Tortoniano-Serravalliano, Messiniano) e<br />
plioceniche.<br />
I depositi pleistocenici, invece, non sembrano essere<br />
interessati da attività tettonica se non in minima parte; ciò<br />
testimoniato dal fatto che gli orizzonti <strong>di</strong> queste unità poggiano<br />
in onlap su quelli pliocenici seguendo le normali fasi <strong>di</strong><br />
riempimento <strong>di</strong> un bacino.<br />
Successivamente alla fase <strong>di</strong> apertura <strong>del</strong> bacino <strong>di</strong><br />
Squillace i s<strong>ed</strong>imenti che lo hanno riempito sono stati piegati<br />
dalla tettonica trascorrente sinistra agente lungo questa<br />
porzione <strong>di</strong> arco.<br />
CONCLUSIONE<br />
L’analisi dei dati a <strong>di</strong>sposizione ha permesso una<br />
ricostruzione temporale <strong>del</strong>l’evoluzione <strong>del</strong> bacino <strong>di</strong><br />
Squillace, la creazione <strong>di</strong> una mappa strutturale e, grazie<br />
all’utilizzo <strong>di</strong> software specifici, la realizzazione <strong>di</strong> immagini<br />
tri<strong>di</strong>mensionali<br />
Il record stratigrafico <strong>del</strong> bacino mostra la presenza <strong>di</strong> una<br />
successione stratigrafica più o meno continua dal Serravalliano<br />
fino all’attuale che poggia su <strong>di</strong> un basamento metamorfico.<br />
La fase tettonica più intensa registrata ha avuto luogo<br />
durante il Pliocene inferiore. Qui la fase deformativa è<br />
associata all’attività <strong>di</strong> faglie normali orientate W-E che<br />
bordano a N e S il bacino così creatosi nell’offshore <strong>di</strong><br />
Catanzaro. Segue una fase <strong>di</strong> deposizione <strong>di</strong> s<strong>ed</strong>imenti argillosiltosi<br />
e quin<strong>di</strong>, una volta terminata l’intensa fase tettonica<br />
<strong>di</strong>stensiva, il bacino viene progressivamente riempito da<br />
s<strong>ed</strong>imenti sabbioso-arenacei con presenza <strong>di</strong> ciottoli e talora <strong>di</strong><br />
argilla siltosa.<br />
Alla tettonica <strong>di</strong>stensiva si aggiunge, dal Pleistocene m<strong>ed</strong>iosuperiore,<br />
un forte uplift che in buona parte è legato alla<br />
tettonica regionale.<br />
Quanto mostrato fornisce quin<strong>di</strong> un quadro temporale<br />
<strong>del</strong>l’evoluzione <strong>di</strong> una porzione <strong>del</strong>l’Arco Calabro<br />
rappresentata dalla stretta <strong>di</strong> Catanzaro, elemento trasversale<br />
alla catena, <strong>ed</strong> in particolare <strong>del</strong>la sua prosecuzione a mare<br />
ovvero <strong>del</strong> Golfo <strong>di</strong> Squillace.
202 V. SBRESCIA ET ALII<br />
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Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 203-206, 3ff.<br />
Active intraplate deformation within Adria: examples from the<br />
Adriatic region<br />
RIASSUNTO<br />
Deformazione attiva all’interno <strong>del</strong>la placca Adria: esempi nell’area<br />
adriatica.<br />
L’avampaese adriatico costituisce una <strong>del</strong>le poche aree preservate dalla<br />
subduzione all’interno <strong>del</strong>l’intero M<strong>ed</strong>iterraneo e tale dominio viene<br />
generalmente considerato asismico e indeformato. Tuttavia, i dati <strong>di</strong> sismica a<br />
riflessione e la recente sismicità registrata nell’area permettono <strong>di</strong> <strong>del</strong>ineare<br />
una serie <strong>di</strong> zone <strong>di</strong>screte in deformazione (Dorsale M<strong>ed</strong>io-Adriatica, sistema<br />
<strong>di</strong> faglie <strong>del</strong>le Tremiti e Gondola-Mattinata).<br />
In questo stu<strong>di</strong>o, basato sull’integrazione <strong>di</strong> dati geologici, strutturali e<br />
geofisici, è stata caratterizzata la deformazione recente e il regime tettonico<br />
all’interno <strong>del</strong>la placca Adria, oltre alle relazioni con le aree attive circostanti<br />
la regione Adriatica.<br />
Parole chiave: placca Adria; sismicità; tettonica<br />
intraplacca; tettonica attiva; Adriatico.<br />
ABSTRACT<br />
Recent seismicity record<strong>ed</strong> in the Adriatic region suggests that active<br />
deformation occurs within the Adria plate. Earthquakes are common in the<br />
adjoining chains and in particular along the Northern Apennines, in the<br />
Southern Alps and along the Dinaric-Albanian-Hellenic thrust fronts; in<br />
ad<strong>di</strong>tion, m<strong>ed</strong>ium-to-low grade seismicity also affects the foreland and is<br />
cluster<strong>ed</strong> along two main transects through the centre of the Adriatic region.<br />
The two zones of active deformation correspond to the Mid-Adriatic Ridge and<br />
the Gargano region, and approximately trend NW-SE and E-W, respectively.<br />
Several interpretations have been propos<strong>ed</strong> to explain the deformation<br />
observ<strong>ed</strong> within the Adriatic foreland, inclu<strong>di</strong>ng a variety of contrasting<br />
kinematic and geodynamic mo<strong>del</strong>s.<br />
We attempt<strong>ed</strong> to unravel the complicat<strong>ed</strong> structural evolution of the<br />
Central Adriatic region by interpreting industrial and ministerial (shallow and<br />
deep) seismic profiles and the recent seismicity record<strong>ed</strong> in the Adriatic. The<br />
subsurface study allow<strong>ed</strong> us to better understand the structural style, timing<br />
and rates of deformation affecting the Mid-Adriatic Ridge and the Gargano<br />
region, and to unravel the relationships of active deformation with respect to<br />
the adjacent chains. The results of our study in<strong>di</strong>cate that the structural setting<br />
and type of seismicity of the Central Adriatic can be explain<strong>ed</strong> in terms of<br />
active intraplate deformation with the reactivation of inherit<strong>ed</strong> <strong>di</strong>scontinuities.<br />
_________________________<br />
VITTORIO SCISCIANI (*) & FERNANDO CALAMITA (*)<br />
(*)Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università “G. D'Annunzio” Chieti-<br />
Pescara, Campus Universitario Madonna <strong>del</strong>le Piane, Via dei Vestini n° 30,<br />
66013 Chieti Scalo (CH) – Italy. E-mail: scisciani@unich.it<br />
Lavoro eseguito con il contributo finanziario <strong>del</strong>l’Università “G.<br />
D'Annunzio” Chieti-Pescara.<br />
Key words: Adria plate; seismicity, intraplate tectonics; active<br />
tectonics; Adriatic.<br />
TECTONIC FRAMEWORK<br />
The Adria microplate is a continental block that, at present,<br />
extends NW-SE, mainly occupi<strong>ed</strong> by the Adriatic basin but also<br />
inclu<strong>di</strong>ng the Po Plain, Istria, the Gargano Promontory and the<br />
Apulian Peninsula (Fig.1). The microplate is surround<strong>ed</strong> by<br />
several strongly deform<strong>ed</strong> belts originating from the collision<br />
with the Eurasian plate (e.g., the Dinaric-Albanian-Hellenic<br />
chain during the Cretaceous-Paleocene; the Southern Alpine<br />
chain from Oligocene to the present), from post-collisional<br />
rotation of the Sar<strong>di</strong>nia Block (Northern Apennine belt) and<br />
from subduction-relat<strong>ed</strong> migration towards the SE (Southern<br />
Apennine belt) of the Calabrian arc (BOCCALETTI et al., 1971;<br />
Fig. 1 – Tectonic map of the Adriatic region and surroun<strong>di</strong>ng chains.<br />
_________________________
204 V. SCISCIANI & F. CALAMITA<br />
MALINVERNO & RYAN, 1986; PATACCA & SCANDONE, 1989;<br />
CARMIGNANI & KLIGFIELD, 1990; FINETTI, 2005).<br />
The Adria microplate, inclu<strong>di</strong>ng the Adriatic region, has<br />
generally been consider<strong>ed</strong> a single, rigid and nearly aseismic<br />
block (ANDERSON & JACKSON, 1987; ROYDEN et al., 1987),<br />
with seismicity mostly confin<strong>ed</strong> to its margins.<br />
In the last three decades, increasing acquisition of seismic<br />
reflection profiles and deep penetration well-logs for<br />
hydrocarbon exploration has strongly improv<strong>ed</strong> the<br />
understan<strong>di</strong>ng of subsurface geology of the Adriatic basin. In<br />
ad<strong>di</strong>tion, more and better seismological stations (MONTONE et<br />
al., 2004; CASTELLO et al., 2006; PONDRELLI et al., 2006 and<br />
references therein) have record<strong>ed</strong> moderate-to-strong seismicity<br />
concentrat<strong>ed</strong> in the Central Adriatic and in the Gargano<br />
Promontory. Bas<strong>ed</strong> on the new available data, several papers<br />
have describ<strong>ed</strong> the geological-structural setting and the seismotectonics<br />
of the Adriatic region, and two main zones of<br />
deformation have been document<strong>ed</strong> in the Central Adriatic and<br />
the Gargano region (Fig. 2).<br />
In the Central Adriatic, compressive deformation is<br />
concentrat<strong>ed</strong> along a NW-SE tren<strong>di</strong>ng ridge that transects the<br />
foreland (Mid-Adriatic Ridge - FINETTI, 1982; DE ALTERIIS,<br />
1995; ARGNANI & FRUGONI, 1997; ARGNANI, 1998; BERTOTTI<br />
et al., 2001). The Mid-Adriatic Ridge (Fig. 1) is either regard<strong>ed</strong><br />
as a zone of interaction between the frontal zones of the<br />
respectively NE-verging Apennine and the SW-verging Dinaric<br />
fold-and-thrust belts (BALLY et al., 1986; CASERO et al., 1990;<br />
FINETTI & DEL BEN, 2005; SCROCCA, 2006) or as a foreland<br />
deformation zone (ARGNANI & FRUGONI, 1997; ARGNANI,<br />
1998). Moreover, some authors view this area as the peripheral<br />
bulge of the two neighbouring opposite-verging chains (KRUSE<br />
& ROYDEN, 1994; ARGNANI & FRUGONI, 1997) in which the<br />
structural setting has been complicat<strong>ed</strong> by salt <strong>di</strong>apirism of<br />
Triassic evaporites (DE ALTERIIS, 1995).<br />
In the Gargano Promontory, several fault systems orient<strong>ed</strong><br />
E-W (e.g., the Mattinata Fault system), NW-SE (e.g., the<br />
Apricena and the Cerignola-Foggia Fault systems – SELLA et<br />
al., 1988; PATACCA & SCANDONE, 2004) and NE-SW (i.e. the<br />
Tremiti Fault and the Sannicandro Garganico-Apricena Fault<br />
system – Argnani et al., 1993; Salvi et al., 1999) have been<br />
identifi<strong>ed</strong> both onshore and in the adjacent offshore. Some<br />
authors have emphasis<strong>ed</strong> the role of the E-W tren<strong>di</strong>ng<br />
<strong>di</strong>scontinuities, suggesting a large amount of <strong>di</strong>splacement on<br />
these strike-slip structures (e.g., SCROCCA, 2006 cum bibl.),<br />
while others have interpret<strong>ed</strong> some of these structures as southvergent<br />
reverse faults (ORTOLANI & PAGLIUCA, 1987), salt<br />
swells (DE DOMINICIS & MAZZOLDI, 1987 or in terms of<br />
inversion tectonics induc<strong>ed</strong> by the propagation of stress<br />
Fig. 2 – Seismicity and structural sketch map of the Central Adriatic, inclu<strong>di</strong>ng the Gargano region (structural data mo<strong>di</strong>fi<strong>ed</strong> from DE ALTERIIS (1995)<br />
and PATACCA & SCANDONE (2004) for the Apulian region, and from IVANCIC et al. (2006) for the Croatia off-shore). The epicenters are from<br />
NEIC-USGS Catalogue (period 1977-May 2007) with M >= 2.5 and earthquakes focal mechanisms from PONDRELLI et al., (2006); QRCMT CATALOGUE<br />
and HARVARD-CMT CATALOGUE.<br />
transmitt<strong>ed</strong> from adjacent chains (ARGNANI et al., 1993; 1994).<br />
Moreover, both senses of strike-slip motion have been<br />
identifi<strong>ed</strong> from field stu<strong>di</strong>es along the faults and the timing of<br />
activity report<strong>ed</strong> in literature is usually inconsistent (for a<br />
general review, see PATACCA & SCANDONE, 2004).<br />
Furthermore, a complex polyphase history with the
Fig. 3 – Tectonic scheme of the Adriatic basin and surroun<strong>di</strong>ng chains.<br />
The NW-<strong>di</strong>rect<strong>ed</strong> convergence of Nubia with respect to Eurasia transmits<br />
NW-SE orient<strong>ed</strong> stress in the Adria plate. The latter reactivates zones of<br />
weakness within the Central Adriatic (Gargano region and the Mid-<br />
Adriatic Ridge) and parts, mainly ESE-WNW orient<strong>ed</strong>, of the chains<br />
(Maghrebides, Tyrrhenian sector off-shore of Sicily, Calabrian arc,<br />
Northern Apennines, Southern Alps, Southern Dinarides). The arrow is the<br />
velocity of Nubia with respect to stable Eurasia (6 mm/yr) pr<strong>ed</strong>ict<strong>ed</strong> from<br />
the GPS bas<strong>ed</strong> REVELIT97- 2000 mo<strong>del</strong> (SELLA et al., 2002). The circl<strong>ed</strong><br />
arrows in<strong>di</strong>cate GPS velocities with respect to Eurasia from permanent<br />
stations and 95% confidence ellipse (from SERPELLONI et al., 2005).<br />
superposition of <strong>di</strong>fferent strike-slip kinematics and<br />
reactivation of pre-existing Mesozoic extensional fault systems<br />
has been propos<strong>ed</strong> (CHILOVI et al., 2000; MORELLI, 2002).<br />
MAIN RESULTS<br />
In this study, we analys<strong>ed</strong> the active deformation in the<br />
Adriatic region and in the chains surroun<strong>di</strong>ng the Adria plate.<br />
Our results are bas<strong>ed</strong> on the interpretation of recently acquir<strong>ed</strong><br />
deep seismic reflection profiles, on instrumental seismicity and<br />
on a critical review of previous geological and geophysical data<br />
collect<strong>ed</strong> in the area, and allow<strong>ed</strong> us to better constrain the<br />
structural style and modes of present-day deformation. The<br />
main results of this research are summariz<strong>ed</strong> as follows:<br />
(1) In Central Italy, the thrust emplacement of the Northern<br />
Apennine belt occurr<strong>ed</strong> mostly during the Pliocene, when most<br />
of the shortening by compressive structures was achiev<strong>ed</strong>;<br />
during the Quaternary, thrust activity strongly decreas<strong>ed</strong>, and at<br />
ACTIVE INTRAPLATE DEFORMATION WITHIN ADRIA<br />
205<br />
present, the area shows only limit<strong>ed</strong> seismicity, especially with<br />
respect to the rest of the Northern Apennine arc (from Ancona<br />
to the Po Plain), where <strong>di</strong>ffuse earthquakes are promot<strong>ed</strong> by<br />
active crustal thrusts (Figs. 2 and 3).<br />
(2) In the Adriatic foreland, active deformation is<br />
concentrat<strong>ed</strong> along two main linear trends, locat<strong>ed</strong> in the<br />
Central Adriatic region (Mid-Adriatic Ridge) and close to the<br />
Gargano Promontory. The former is a zone of crustal<br />
compression and at present represents the linkage between the<br />
two opposite-verging and active Northern Apennine and South<br />
Dinarides-Albanides belts (Figs. 1, 2, and 3), while the latter is<br />
a region affect<strong>ed</strong> by strike-slip faults and by transpression<br />
generat<strong>ed</strong> by an active stress field characteriz<strong>ed</strong> by a 1 axis<br />
orient<strong>ed</strong> roughly NW–SE and a 3 axis tren<strong>di</strong>ng NE–SW<br />
inferr<strong>ed</strong> from structural data, in-situ stress measurements and<br />
focal mechanism solutions of recent earthquakes (Figs. 3).<br />
(3) In the Mid-Adriatic Ridge and the Apulian foreland, the<br />
positive reactivation of inherit<strong>ed</strong> zones of weakness (i.e.<br />
previously affect<strong>ed</strong> by Mesozoic extensional tectonics) is the<br />
dominant mechanism controlling deformation within the<br />
Adriatic foreland.<br />
(4) The interpretation of seismic reflection profiles and the<br />
crustal seismicity record<strong>ed</strong> in the Central Adriatic region<br />
(inclu<strong>di</strong>ng hypocentral depths > 20 km) suggest that<br />
compressive deformation is not restrict<strong>ed</strong> to the shallow<br />
s<strong>ed</strong>imentary cover but affects the entire crust.<br />
Accor<strong>di</strong>ng to the above results, we consider the Central<br />
Adriatic a zone of weakness within the rigid Adria plate that<br />
experienc<strong>ed</strong> intraplate contractional deformation multiple<br />
times, produc<strong>ed</strong> by the “coupl<strong>ed</strong>” behaviour of the Adriatic<br />
foreland with the adjoining orogens and induc<strong>ed</strong> by the presentday<br />
NW <strong>di</strong>rect<strong>ed</strong> convergence of Nubia with respect to the<br />
stable Eurasian plate (Fig. 3).<br />
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Multiple mechanisms of fault-zone weakening along a continental<br />
low-angle normal fault: The Zuccale fault, Elba Island<br />
STEVEN A.F. SMITH (*,**), C. COLLETTINI (***), R.E.HOLDSWORTH (*), C.G. MACPHERSON (*) & C. VITI (****)<br />
RIASSUNTO<br />
Meccanismi <strong>di</strong> indebolimento multipli su <strong>di</strong> una faglia <strong>di</strong>retta a basso<br />
angolo: la faglia <strong>del</strong>lo Zuccale, Isola d’Elba<br />
Le faglie <strong>di</strong>rette a basso angolo sono probabilmente <strong>del</strong>le faglie deboli, ma<br />
vi sono pochi stu<strong>di</strong> che documentano la reologia <strong>del</strong>le rocce <strong>di</strong> faglia su queste<br />
strutture. In questo contributo presentiamo un lavoro <strong>di</strong> analisi microstrutturale<br />
e geochimica sulla faglia <strong>di</strong>retta a basso angolo <strong>del</strong>lo Zuccale ben esposta<br />
all’isola d’Elba. Tale faglia è stata attiva come struttura a basso angolo durante<br />
alcune intrusioni ignee nel blocco <strong>di</strong> letto <strong>ed</strong> è caratterizzata da una nucleo con<br />
una stratigrafia tettonica ben <strong>di</strong>stinta.<br />
Stu<strong>di</strong> microstrutturali mostrano che durante l’attività <strong>del</strong>la faglia sono stati<br />
attivi <strong>di</strong>versi meccanismi deformativi, in particolare: 1) cataclasi; 2) creep per<br />
<strong>di</strong>ssoluzione e riprecipitazione; 3) plasticità intracristallina, e; 4) particulate<br />
flow. Questi meccanismi deformativi implicano <strong>di</strong>versi processi <strong>di</strong><br />
indebolimento trai cui: i) scivolamento per attrito accomodato da fillosilicati<br />
deboli come talco e clorite; ii) processi plastici influenzati dalla granulometria<br />
all’interno <strong>del</strong>le calco-miloniti; iii) particulate flow accomodato da minerali<br />
argillosi estremamente fini e iv) incremento <strong>di</strong> pressione <strong>di</strong> flui<strong>di</strong> su <strong>brevi</strong><br />
intervalli <strong>di</strong> tempo e su porzioni <strong>di</strong> faglia limitate.<br />
Durante l’attività <strong>del</strong>la faglia questi meccanismi deformativi non si<br />
escludono a vicenda ma la loro importanza è funzione <strong>del</strong>la struttura <strong>del</strong>la zona<br />
<strong>di</strong> faglia, <strong>del</strong>la litologia coinvolta nella deformazione, e <strong>del</strong>la velocità <strong>di</strong><br />
deformazione locale. Analisi degli isotopi stabili su vene e cementi<br />
suggeriscono che durante l’intrusione dei plutoni ignei si sono sviluppate<br />
pressioni dei flui<strong>di</strong> sopralitostatiche.<br />
Key words: Fault mechanics, Fault-zone weakening, Lowangle<br />
normal faults, Microstructures, Stable isotopes.<br />
Despite extensive research in to the mechanical significance<br />
and geometric evolution of low-angle normal (detachment)<br />
faults, few stu<strong>di</strong>es have focus<strong>ed</strong> on the importance of the fault<br />
rock material which is generat<strong>ed</strong> during prolong<strong>ed</strong> slip and<br />
exhumation (COWAN et alii, 2003; COLLETTINI &<br />
_________________________<br />
(*) Department of Earth Sciences, University of Durham, Durham,<br />
England, DH1 3LE, UK<br />
(**) Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia (INGV), 605 Via <strong>di</strong><br />
Vigna Murata, 00143, Rome<br />
(***) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università degli Stu<strong>di</strong> <strong>di</strong><br />
Perugia, 06120, Perugia, Italy.<br />
(****) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università degli Stu<strong>di</strong> <strong>di</strong><br />
Siena, Siena, Italy.<br />
Financial assistance was provid<strong>ed</strong> by European Research Council Starting<br />
Grant “USEMS” 2008-2013, to G. Di Toro.<br />
HOLDSWORTH, 2004; HAYMAN, 2006). The Zuccale fault on<br />
the Island of Elba is closely associat<strong>ed</strong> with syn-tectonic<br />
igneous intrusions (SAUPE et alii, 1982; KELLER & COWARD,<br />
1996; DINI et alii, 2002), and possesses a complex fault rock<br />
`stratigraphy’ that records the interaction between multiple<br />
deformation mechanisms and fluids deriv<strong>ed</strong> from <strong>di</strong>stinct<br />
crustal reservoirs.<br />
Optical- and scanning-electron microscopy, combin<strong>ed</strong> with<br />
XRD and stable isotope analyses, reveal systematic changes in<br />
fault rock chemistry and texture: 1) Cataclasis and <strong>di</strong>ssolutionprecipitation<br />
creep were the dominant deformation mechanisms<br />
during the early stages of fault activity. Cataclasis facilitat<strong>ed</strong><br />
the influx of chemically active fluids, lea<strong>di</strong>ng to widespread syn<br />
Fig 1 - Mappa geologica semplificata <strong>del</strong>l’Elba e sezione geologica che mostra<br />
la geometria <strong>del</strong>la faglia <strong>del</strong>lo Zuccale. I-V in<strong>di</strong>cano i cinque complessi <strong>di</strong><br />
TREVISAN et alii (1967; mo<strong>di</strong>ficata da BORTOLOTTI et alii, 2001). La stella<br />
in<strong>di</strong>ca l’affioramento <strong>di</strong> Punta <strong>di</strong> Zuccale.<br />
Simplifi<strong>ed</strong> geological map and cross-section of Elba, highlighting the location<br />
and geometry of the Zuccale fault. Labels I-V refer to the tectonic ‘complexes’<br />
of TREVISAN et alii (1967; mo<strong>di</strong>fi<strong>ed</strong> by BORTOLOTTI et alii, 2001). The star<br />
marks the location of the type locality, Punta <strong>di</strong> Zuccale.
208 S.A.F. SMITH ET ALII<br />
Fig 2 - La faglia <strong>di</strong> Zuccale a Punta <strong>di</strong> Zuccale. Il nucleo <strong>del</strong>la zona <strong>di</strong> faglia è<br />
evidenziato dalle linee tratteggiate bianche, è spesso 3-8 m <strong>ed</strong> è caratterizzato<br />
da una tipica stratigrafia tettonica costituita da rocce foliate. Il blocco <strong>di</strong> tetto e<br />
<strong>di</strong> letto sono caratterizzati da deformazione fragile. Il <strong>di</strong>agramma in alto a<br />
destra riassume il fabric principale <strong>del</strong>la zona <strong>di</strong> faglia caratterizzato da<br />
foliazione P e zone <strong>di</strong> taglio Y e R.<br />
The Zuccale fault at the type locality, Punta <strong>di</strong> Zuccale. The ‘core’ of the<br />
Zuccale fault is outlin<strong>ed</strong> by white dash<strong>ed</strong> lines, and is 3-8 meters thick. The<br />
fault core contains a <strong>di</strong>stinct fault rock ‘stratigraphy’, sandwich<strong>ed</strong> between<br />
footwall and haningwall blocks characteriz<strong>ed</strong> by intense brittle deformation.<br />
The inset <strong>di</strong>agram summarizes the main Ri<strong>ed</strong>el fabric components (P<br />
foliation, Y and R shears) observ<strong>ed</strong> within the fault core.<br />
tectonic growth of weak phyllosilicate minerals, inclu<strong>di</strong>ng talc<br />
and chlorite. Crystal-plasticity was important within calcite-rich<br />
fault rocks. Calcite grains (~10µm in <strong>di</strong>ameter) possess a strong<br />
C-axis preferr<strong>ed</strong> orientation, suggesting that they experienc<strong>ed</strong><br />
dynamic recrystallisation by <strong>di</strong>slocation creep. The calcitemylonites<br />
are crosscut by vein material that was progressively<br />
shear<strong>ed</strong> and recrystallis<strong>ed</strong>, in<strong>di</strong>cating cyclic brittle-plastic<br />
deformation. During the later stages of fault activity, particulate<br />
flow became an important deformation mechanism. Rolling and<br />
sli<strong>di</strong>ng of grains past one another was accommodat<strong>ed</strong> along<br />
clay-lin<strong>ed</strong> grain boundaries; 2) During progressive exhumation,<br />
dolomite was supers<strong>ed</strong><strong>ed</strong> by calcite as the dominant syntectonic<br />
fault cement. Sub-horizontal dolomite veins within the<br />
fault core record transient supra-lithostatic fluid pressures<br />
follow<strong>ed</strong> by mineral sealing and fault strengthening. The 13 CV-<br />
PDB signature of such vein dolomite is strongly cluster<strong>ed</strong> around<br />
a mean value of -5.7‰, 18 OV-SMOW varies between 10‰ and<br />
14‰, and 87 Sr/ 86 Sr is between 0.70958 and 0.71001. The fluid<br />
in equilibrium with the dolomite veins was probably deriv<strong>ed</strong><br />
from a meteoric source that interact<strong>ed</strong> with local plutonic<br />
intrusions carrying mantle-CO2. Later calcite veins have 13 CV-<br />
PDB values of -6.5‰ to -10.5‰, 18 OV-SMOW between 25‰ and<br />
27‰, and 87 Sr/ 86 Sr cluster<strong>ed</strong> around a mean value of 0.70908.<br />
In this case, the low-temperature fluid in equilibrium with the<br />
calcite veins appears to have been deriv<strong>ed</strong> from seawater.<br />
Potential fault zone weakening mechanisms that have been<br />
identifi<strong>ed</strong> along the Zuccale fault include: (i) <strong>di</strong>ssolutionprecipitation<br />
creep and frictional slip within phyllosilicate-rich<br />
fault rocks, (ii) a switch to grain-size sensitive deformation<br />
mechanisms within calcite-mylonites, (iii) particulate flow<br />
accommodat<strong>ed</strong> by fine-grain<strong>ed</strong> clay phases, and (iv) transiently<br />
high fluid pressures that probably occurr<strong>ed</strong> over short<br />
timescales and across localiz<strong>ed</strong> fault patches. These weakening<br />
mechanisms were not mutually exclusive, but their relative<br />
importance must have vari<strong>ed</strong> as a function of fault zone<br />
structure and composition, local strain rates, and the availability<br />
of fluid before, during, and following igneous intrusion. Their<br />
combin<strong>ed</strong> effects may have result<strong>ed</strong> in significant long-term<br />
weakening of the Zuccale fault, facilitating its exhumation<br />
towards the surface.<br />
Fig 3 - Isotopi stabili per le vene nella sona <strong>di</strong> faglia: i) vene sub orizzontali in<br />
dolomite formatesi durante le fasi iniziali <strong>del</strong>la faglia; ii) vene in calcite che<br />
attraversano il nucleo e che sono relazionate alle fasi finali <strong>del</strong>l’attività <strong>del</strong>la<br />
faglia. I flui<strong>di</strong> in equilibrio con le vene in dolomite sono relazionati ad una<br />
sorgente meteorica <strong>ed</strong> idrotermale mentre quelli in equilibrio con le vene <strong>di</strong><br />
calcite derivano da acqua marina infiltrata nella zona <strong>di</strong> faglia attraverso le<br />
fratture presenti nel blocco <strong>di</strong> tetto e letto.<br />
Stable isotope compositions of the early, sub-horizontal dolomite veins within<br />
the fault core, and the later calcite veins that cross-cut the fault core. The<br />
fluid in equilibrium with the dolomite veins appears to have been deriv<strong>ed</strong><br />
from a meteoric-hydrothermal source, whilst the fluid in equilibrium with the<br />
calcite veins was deriv<strong>ed</strong> from seawater that infiltrat<strong>ed</strong> downwards through<br />
the fault<strong>ed</strong> hangingwall and footwall.
REFERENCES<br />
BORTOLOTTI, V., FAZZUOLI, M., PANDELI, E., PRINCIPI, G.,<br />
BABBINI, A. & CORTI, S. (2001) - Geology of Central and<br />
Eastern Elba Island, Italy. Ofioliti, 26 (2a), 97-150.<br />
COLLETTINI, C. & HOLDSWORTH, R. E. (2004) - Fault zone<br />
weakening and character of slip along low-angle normal<br />
faults: insights from the Zuccale fault, Elba, Italy. Journal<br />
of the Geological Society, 161, 1039-1051.<br />
COWAN, D. S., CLADOUHOS, T. T. & MORGAN, J. K. (2003) -<br />
Structural geology and kinematic history of rocks form<strong>ed</strong><br />
along low-angle normal faults, Death Valley, California.<br />
Geological Society of America Bulletin, 115(10), 1230-<br />
1248.<br />
DINI, A., INNOCENTI, F., ROCCHI, S., TONARINI, S. &<br />
WESTERMAN, D. S. (2002) - The magmatic evolution of the<br />
late Miocene laccolith-pluton-dyke granitic complex of<br />
Elba Island, Italy. Geological Magazine, 139(3), 257-279.<br />
HAYMAN, N. W. (2006) - Shallow crustal fault rocks from the<br />
Black Mountain detachments, Death Valley, CA. Journal of<br />
Structural Geology, 28(10), 1767-1784.<br />
KELLER, J. V. A. & COWARD, M. P. (1996) - The structure and<br />
evolution of the Northern Tyrrhenian Sea. Geological<br />
Magazine, 133(1), 1-16.<br />
SAUPE, F., MARIGNAC, C., MOINE, B., SONET, J. &<br />
ZIMMERMANN, J. L. (1982) - K/Ar and Rb/Sr Dating of<br />
Rocks from the Eastern Part of Elba Island (Province of<br />
Livorno, Italy). Bulletin De Mineralogie, 105(3), 236-245.<br />
TREVISAN, L., MARINELLI, G., BARBERI, F., GIGLIA, G.,<br />
INNOCENTI, F., RAGGI, G., SQUARCI, P., TAFFI, L. & RICCI,<br />
C. A. (1967) - Carta Geologica <strong>del</strong>l’Isola d’Elba. In:<br />
Consiglio Nazionale <strong>del</strong>le Ricerche, Gruppo <strong>di</strong> Ricerca per<br />
la Geologia <strong>del</strong>l’Appennino centro-settentrionale e <strong>del</strong>la<br />
Toscana, Pisa, Consiglio Nazionale <strong>del</strong>le Ricerche, Gruppo<br />
<strong>di</strong> Ricerca per la Geologia <strong>del</strong>l’Appennino centrosettentrionale<br />
e <strong>del</strong>la Toscana.<br />
THE ZUCCALE FAULT, ELBA ISLAND<br />
209
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 210-213, 4 ff.<br />
RIASSUNTO<br />
Evoluzione P-T nella parte occidentale <strong>del</strong> complesso <strong>del</strong> Thor-O<strong>di</strong>n,<br />
Monti Monashee, Cor<strong>di</strong>gliera Canadese.<br />
Nuovi dati microstrutturali e minerochimici sono presentati con lo scopo<br />
<strong>di</strong> contribuire alla ricostruzione <strong>del</strong>la storia tettono-metamorfica nel complesso<br />
metamorfico <strong>del</strong> Thor-O<strong>di</strong>n, appartenente ad un insieme <strong>di</strong> complessi che<br />
espongono le rocce più profonde <strong>del</strong>la Cor<strong>di</strong>gliera Canadese. Questi dati<br />
integrano le conoscenze litostratigrafiche e strutturali incluse in una carta<br />
geologica in scala 1:35000, in via <strong>di</strong> raffinamento. Stime sull’evoluzione P-T,<br />
per un transetto E-W <strong>di</strong> circa 5 km nella parte occidentale <strong>del</strong> complesso <strong>del</strong><br />
Thor-O<strong>di</strong>n, in<strong>di</strong>cano che l’esumazione <strong>di</strong> questo complesso si è sviluppata in<br />
con<strong>di</strong>zioni geotermiche che ecc<strong>ed</strong>ono quelle <strong>del</strong>la geoterma rilassata e che<br />
suggerirebbero un contesto <strong>di</strong> tettonica estensionale.<br />
Key words: Cana<strong>di</strong>an Cor<strong>di</strong>llera, HT metamorphism, P-T<br />
evolution, Thor-O<strong>di</strong>n dome.<br />
INTRODUCTION<br />
We present new micro-structural and mineral chemical data<br />
for high temperature metamorphic rocks from the Thor-O<strong>di</strong>n<br />
dome (Fig. 1) to reconstruct its tectono-metamorphic evolution.<br />
We analys<strong>ed</strong> in detail metapelites and amphibolite bou<strong>di</strong>ns<br />
from a 5 km long E-W transect from the core of the dome to the<br />
Greenbush Lake normal shear-band zone, which rims the<br />
western margin of the dome (Fig. 2).<br />
GEOLOGICAL SETTING<br />
From east to west the Cana<strong>di</strong>an Cor<strong>di</strong>llera consists of five<br />
lithostratigraphic complexes, which are parallel to the strike of<br />
the chain: Foreland, Omineca, Intermontane, Costal and Insular<br />
belts (Fig. 1). The Omineca belt, between the Foreland and<br />
Intermontane belts, consists of Precambrian intrusive and<br />
metamorphic rocks belonging to the North American craton.<br />
_______________________<br />
P-T evolution in the western Thor-O<strong>di</strong>n dome, Monashee<br />
Mountains, Cana<strong>di</strong>an Cor<strong>di</strong>llera.<br />
M. IOLE SPALLA (*), DAVIDE ZANONI (°), PAUL F. WILLIAMS (°) & GUIDO GOSSO (*)<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra “A. Desio”, Università <strong>di</strong> Milano<br />
and CNR-IDPA, Via Mangiagalli, 34, 20133 Milano, Italy.<br />
iole.spalla@unimi.it<br />
(°) Department of Geology, University of New Brunswick, 2 Bailey Dr.<br />
Fr<strong>ed</strong>ericton, NB, Canada E3B 5A3. dzanoni@unb.ca<br />
Lavoro eseguito con il contributo finanziario <strong>del</strong> MIUR-COFIN 2005 e <strong>del</strong><br />
NSERC Discovery Grant.<br />
This belt has been exhum<strong>ed</strong> as a consequence of the collisional<br />
accretion of the allocthonous terranes (Intermontane, Costal<br />
and Insular belts) on the North American margin (MONGER et<br />
alii, 1982; GABRIELSE et alii, 1991).<br />
Fig. 1 – Geologic sketch map of the Thor-O<strong>di</strong>n dome; the insect shows the<br />
lithostratigraphic complexes of the Cana<strong>di</strong>an Cor<strong>di</strong>llera. Rectangle in<strong>di</strong>cate<br />
the position of Fig. 2. Mo<strong>di</strong>fi<strong>ed</strong> after KRUSE and WILLIAMS (2007).<br />
The deepest metamorphic core complexes of the whole<br />
Cana<strong>di</strong>an Cor<strong>di</strong>llera are expos<strong>ed</strong> in the Omineca belt and<br />
include the Monashee complex, in South East British<br />
Columbia. The southernmost part of the Monashee complex is
P-T EVOLUTIONS IN THE MONASHEE MOUNTAINS<br />
Fig. 2 – Geologic sketch map of the study area; sample positions are shown. Close up from the compilation map of KRUSE et alii (2004).<br />
the Thor-O<strong>di</strong>n dome, which consists of a Proterozoic basement<br />
sequence (migmatitic orthogneisses and paragneisses, with<br />
amphibolites) and of a Proterozic to Palaeozoic metas<strong>ed</strong>imentary<br />
sequence (quartzites, schists, marble, calcsilicates,<br />
gneisses, and amphibolites). Both sequences are characteris<strong>ed</strong><br />
by a penetrative transposition foliation (ST). The margins of<br />
the Thor-O<strong>di</strong>n complex are bound<strong>ed</strong> by a system of normal<br />
shear zone known as Thor-O<strong>di</strong>n detachment, which is believ<strong>ed</strong><br />
to extend down to the Moho (KRUSE & WILLIAMS, 2007).<br />
MESO-STRUCTURES<br />
The transposition foliation of the Thor-O<strong>di</strong>n dome typically<br />
<strong>di</strong>splays two generation of intrafolial (mature) folds and is<br />
overprint<strong>ed</strong> by more open asymmetric (immature) folds. Most<br />
of the folds in<strong>di</strong>cate top-to-the-NE flow (JOHNSTON et alii,<br />
2000; WILLIAMS & JANG, 2005) and are a product of a<br />
progressive deformation involving repeat<strong>ed</strong> cycles of<br />
perturbation by fol<strong>di</strong>ng of ST, follow<strong>ed</strong> by tightening of the<br />
folds to transpose fold<strong>ed</strong> surfaces back into ST. The foliation<br />
contains a gently southwest-plunging Ky-bearing lineation.<br />
In the western part of the Thor-O<strong>di</strong>n dome there is a<br />
regional westerly <strong>di</strong>pping normal shear zone (Fig. 2),<br />
(Greenbush Lake shear-band zone; JOHNSTON et alii, 2000),<br />
which is part of the Thor-O<strong>di</strong>n detachment and which<br />
overprints the transposition foliation. The main foliation in this<br />
211<br />
shear zone, overprinting the immature folds, is the reactivat<strong>ed</strong><br />
transposition foliation (RST), which is rotat<strong>ed</strong> into a steep<br />
orientation and overprints the immature folds. The shear zone is<br />
associat<strong>ed</strong> with extensional tectonics, during which a westerly<br />
plunging Sil-bearing lineation, develop<strong>ed</strong> as a result of to a topto-the-W<br />
shear on RST (JOHNSTON et alii, 2000; KRUSE &<br />
WILLIAMS, 2007). This lineation overprints the Ky-bearing<br />
lineation. A prominent micro-fracturing develop<strong>ed</strong><br />
perpen<strong>di</strong>cular to the lineation in the shear zone.<br />
The asymmetric immature folds are overprint<strong>ed</strong> by melt<br />
fill<strong>ed</strong> faults, which locally are fold<strong>ed</strong> and intersect<strong>ed</strong> by top-tothe-W<br />
shear surfaces or zones.<br />
Extensional structures of the shear zone are overprint<strong>ed</strong> by<br />
brittle structures, which are mainly relat<strong>ed</strong> to the N-S tren<strong>di</strong>ng<br />
Victor Creek Fault (Fig. 2) (KRUSE & WILLIAMS, 2005). Large<br />
open folds with a northerly tren<strong>di</strong>ng vertical axial plane,<br />
overprint the ST and are believ<strong>ed</strong> to pre-date the extension.<br />
MICRO-STRUCTURES<br />
Samples were collect<strong>ed</strong> on the northern slope of Blanket<br />
Mt. (core of the dome), between Blanket Mt. and the Victor<br />
Creek Fault and in the Greenbush Lake shear-band zone (flank<br />
of the dome) (Fig. 2).<br />
Grt Amphibolite bou<strong>di</strong>ns show a dominant foliation mark<strong>ed</strong><br />
mainly by Bt and by SPO of brown AmpII (Fig. 3a). AmpI
212 M. I. SPALLA ET ALII<br />
Fig. 3 – a) SPO foliation mark<strong>ed</strong> by AmpII and Pl in amphibolites; plane polaris<strong>ed</strong> light. b) Grt with internal foliation at a high angle to the external foliation;<br />
cross<strong>ed</strong> polars. c) Ky porphyroclast wrapp<strong>ed</strong> by RST in metapelites; cross<strong>ed</strong> polars. d) Wm in the necks of bou<strong>di</strong>nag<strong>ed</strong> Sil; cross<strong>ed</strong> polars.<br />
shows a <strong>di</strong>fferent orientation with respect to the pervasive<br />
foliation. Parallel to the foliation there are Qtz lenses, with<br />
minor PlI and layers rich in Cpx. PlI also occurs as interstitial<br />
crystals between AmpII or as granoblastic aggregates of grains<br />
showing deformation twinning. Coarse-grain<strong>ed</strong> Grt is in contact<br />
with PlI, Cpx and AmpII, and locally has an internal foliation at<br />
a high angle to the matrix foliation (Fig. 3b). Ttn mainly shows<br />
a SPO parallel to the foliation. Locally coarse-grain<strong>ed</strong> Oamp,<br />
interpret<strong>ed</strong> as coeval with Grt, is partially replac<strong>ed</strong> by green<br />
AmpIII and Chl. Green AmpIII grows along the rims and the<br />
cleavages of AmpII; AmpII can be greener towards the Grt and<br />
Cpx rims. AmpIII and PlII form<strong>ed</strong> symplectites at the Grt rim.<br />
AmpIII also overgrows Cpx. Chl overgrows the symplectites at<br />
the Grt rim and, with sagenitic Rt, replaces Bt.<br />
In Grt-free amphibolite bou<strong>di</strong>ns SPO of Amp (I and II), PlI<br />
and rare BtI define the foliation. Pl-rich layers, with rare<br />
skeletal Cpx, also mark the foliation. Ttn crystals are scatter<strong>ed</strong><br />
and enclos<strong>ed</strong> in AmpI and II and are roughly parallel to the<br />
foliation. Amp is greener towards the rim (AmpIII and IV).<br />
Saw-shap<strong>ed</strong> <strong>ed</strong>ges between Amp and Pl are interpret<strong>ed</strong> as due<br />
to a re-crystallisation postdating the foliation (AmpIII and PlII).<br />
AmpIV fills fractures intersecting the foliation at a high angle.<br />
Metapelites between the Victor Creek Fault and Blanket<br />
Mt. preserve Ky porphyroclasts, which show SPO parallel and<br />
inclusion trails oblique with respect to the RST (Fig. 3c). In<br />
these rocks the RST is defin<strong>ed</strong> by SPO and LPO of BtII and Sil<br />
and feldspar-rich layers. Coarse-grain<strong>ed</strong> Ky and Grt are<br />
wrapp<strong>ed</strong> by RST and contain inclusions of Qtz, BtI and Rt,<br />
rimm<strong>ed</strong> by Ilm; Ky also encloses WmI. BtI is transversal to the<br />
RST, which wraps it. Grt (I = core and II = rim) pr<strong>ed</strong>ates or is<br />
coeval with RST, accor<strong>di</strong>ng to inclusion trails, which are at a<br />
high angle and progressively asymptotic to this foliation. GrtII<br />
forms also fine-grain<strong>ed</strong> crystals in the RST films. Between the<br />
coarse-grain<strong>ed</strong> GrtII and BtII reaction rims with BtIII, Qtz, PlII<br />
and rare Kfs occur; in these reaction rims GrtIII forms<br />
symplectites, which are elongat<strong>ed</strong> in the foliation. Fibrous Sil<br />
replaces Ky. The feldspar-rich layers consist of Qtz, PlI and<br />
Kfs, which forms coarse-grain<strong>ed</strong> crystals showing a rough SPO<br />
parallel to the RST. PlI shows growth twining and locally Kfs<br />
forms interstitial crystals between PlI and round<strong>ed</strong> Qtz crystals<br />
or fills Ky fractures. These structures may in<strong>di</strong>cate partial<br />
melting. Fractures in Grt are fill<strong>ed</strong> mainly by Chl and minor<br />
BtIV. WmII ± Chl ± BtIII fill necks of micro-bou<strong>di</strong>nag<strong>ed</strong> Sil<br />
(Fig. 3d); Bt is partially replac<strong>ed</strong> by Chl, sagenitic Rt and Kfs.
In Ky-free metapelites BtI, Sil, PlI, Kfs and Qtz have a SPO<br />
defining the RST. PlI shows growth twinning and rarer<br />
deformation twinning; mirmekites develop<strong>ed</strong> at the Kfs rims.<br />
Grt forms coarse-grain<strong>ed</strong> crystals, which are wrapp<strong>ed</strong> by the<br />
RST, partially replac<strong>ed</strong> by BtII, PlII and Qtz, and shows corerim<br />
zoning (GrtI, II and III); where Grt is almost totally<br />
replac<strong>ed</strong> green BtII and PlII occurs as coarse-grain<strong>ed</strong> crystals.<br />
Rare Wm grew at the Sil rim.<br />
MINERAL CHEMISTRY AND P-T ESTIMATES<br />
In Grt amphibolites XNa in M4 and XAl in T1 decrease from<br />
AmpII (pargasite and Mg-hornblende) to AmpIII (Fehornblende),<br />
XCa in Grt increase towards the rim and XAn<br />
shows similar value in PlI and II; this is compatible with a<br />
slight decrease in pressure and temperature.<br />
In the Grt-free amphibolites AmpI to AmpII (K-pargasite)<br />
show a similar content in Ti and Al tot compatible with<br />
temperature of about 700°C for about 0.7 GPa; AmpIII, in the<br />
symplectites (<strong>ed</strong>enite and Mg-hornblende), shows lower Al tot<br />
content, compatible with a slight decompression at about 0.5 -<br />
0.7 GPa for temperatures between 700 – 760°C. AmpIV (Mghornblende<br />
and actinolite) shows the lowest content in Ti and<br />
Al VI , in<strong>di</strong>cating temperature and pressure lower than 660°C and<br />
0.4 GPa.<br />
In metapelites XFe increases from GrtI to GrtIII, while XAn<br />
in Pl is constant and the Ti content decreases from BtI to BtIV,<br />
suggesting a decompression during cooling. In particular in Ky-<br />
Sil bearing metapelites the variation of Ti content from BtI to<br />
BtIV in<strong>di</strong>cates a variation in temperature from 700 - 750°C to<br />
600 - 700°C to lower than 500°C (Fig. 4).<br />
Fig. 4 – Ti content (a.p.f.u.) in Bt vs temperature estimat<strong>ed</strong> accor<strong>di</strong>ng to<br />
HENRY et alii, 2005. Bt enclos<strong>ed</strong> in Ky prophyroblasts has Ti 0.45 a.p.f.u.<br />
in<strong>di</strong>cating temperatures between 700 and 750°C. Bt lying in the RST has 02<br />
< Ti < 0.5 a.p.f.u. in<strong>di</strong>cating that it recrystallis<strong>ed</strong> at a lower temperature (600<br />
-700°C). Late Bt filling Grt microfractures grew at T 500°C.<br />
CONCLUSION<br />
In the polydeform<strong>ed</strong> metamorphic rocks outcropping from<br />
Blanket Mt. to the Greenbush Lake shear-band zone, P-T<br />
estimates have been inferr<strong>ed</strong> by using mineral assemblages<br />
coeval with superpos<strong>ed</strong> fabrics in rocks of <strong>di</strong>fferent bulk<br />
P-T EVOLUTIONS IN THE MONASHEE MOUNTAINS<br />
213<br />
compositions. Results in<strong>di</strong>cate that P-T con<strong>di</strong>tions chang<strong>ed</strong><br />
under a high thermal regime, which exce<strong>ed</strong>s the range limit<strong>ed</strong><br />
by the stable continental geotherm and a maximally relax<strong>ed</strong><br />
geotherm for reasonable heat supply after crustal thickening.<br />
Such a high geothermal gra<strong>di</strong>ent is compatible with an<br />
extensional tectonic setting.<br />
REFERENCES<br />
GABRIELSE H., MONGER J.W.H., WHEELER J.O., & YORATH<br />
C.J. (1991) - Part A. Morphogeological belts, tectonic<br />
assemblages and terranes. In: H. Gabrielse and C.J. Yorath.<br />
(Eds.) - Geology of the Cor<strong>di</strong>lleran Orogen in Canada. 4,<br />
15–28.<br />
HENRY D.J., GUIDOTTI C.V. & THOMSON J.A. (2005) - The Tisaturation<br />
surface for low-to-m<strong>ed</strong>ium pressure metapelitic<br />
biotites: Implications for geothermometry and Tisubstitution<br />
mechanisms Am. Mineral. 90, 316-328.<br />
JOHNSTON D.H., WILLIAMS P.F., BROWN R.L., CROWLEY J.L. &<br />
CARR S.D. (2000). - Northwestward extrusion and<br />
extensional exhumation of crystalline rocks of the<br />
Monashee complex, southeastern Cana<strong>di</strong>an Cor<strong>di</strong>llera. J.<br />
Struct. Geol., 22, 603-625.<br />
KRUSE S., MCNEILL P.D., & WILLIAMS P.F., (2004) -<br />
Geological map of the Thor–O<strong>di</strong>n dome, southern<br />
Monashee complex, BC. http://www.unb.ca/fr<strong>ed</strong>ericton<br />
/science/geology/monashee.<br />
KRUSE S. & WILLIAMS P.F. (2005) - Brittle faulting in the<br />
Thor–O<strong>di</strong>n culmination, Monashee complex, southern<br />
Cana<strong>di</strong>an Cor<strong>di</strong>llera: constraints on geometry and<br />
kinematics. Can. J. Earth Sci., 42, 2141–2160.<br />
KRUSE S. & WILLIAMS P.F. (2007) - The Monashee reflection:<br />
Re-examination of a Lithoprobe crustal-scale seismic<br />
reflection in the southern Cana<strong>di</strong>an Cor<strong>di</strong>llera. Geosphere,<br />
3, doi: 10.1130/GES00049.1.<br />
MONGER J.W.H., PRICE R.A. & TEMPELMAN-KLUIT D.J. (1982)<br />
- Tectonic accretion and the origin of the two major<br />
metamorphic and plutonic welts in the Cana<strong>di</strong>an<br />
Cor<strong>di</strong>llera. Geology, 10, 70-75.<br />
WILLIAMS P.F. & JANG D. (2005) - An investigation of lower<br />
crustal deformation: Evidence for channel flow and its<br />
implications for tectonics and structural stu<strong>di</strong>es. J. Struct.<br />
Geol., 27, 1486–1504.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 214-217, 6 ff.<br />
Origine <strong>del</strong>l’attuale campo <strong>di</strong> sforzo nella zona <strong>del</strong>l’Arco Calabro<br />
ABSTRACT<br />
Origin of the present-day stress field in the surroun<strong>di</strong>ngs of the Calabrian<br />
arc<br />
Present day stress fields in the Tyrrhenian area is the results of a complex<br />
interplay of various dynamic processes acting at various scales, either local<br />
and regional, such as Africa-Eurasia convergence and Calabrian subduction. In<br />
order to investigate the role play<strong>ed</strong> by each dynamic process in driving the<br />
tectonic and geodynamic setting of the area, we use a finite element approach<br />
appli<strong>ed</strong> on both a thermal mo<strong>del</strong> and a tectonic mo<strong>del</strong>. Pr<strong>ed</strong>ict<strong>ed</strong> stress and<br />
strain in the Central M<strong>ed</strong>iterranean area are compar<strong>ed</strong> to complementary data<br />
presently available in the area, such as geological, geophysical and geodetic<br />
data. The results of our mo<strong>del</strong>ing support the hypothesis that Africa-Eurasia<br />
convergence and Calabrian subduction are the controlling mechanism of the<br />
present-day stress field in the southernmost part of the Tyrrhenian.<br />
Key words: mo<strong>del</strong>ling, tectonic setting, Tyrrhenian, rheology<br />
INTRODUZIONE<br />
L’attuale campo <strong>di</strong> sforzo nell’area <strong>del</strong> Tirreno è il risultato<br />
<strong>del</strong>l’azione combinata <strong>di</strong> <strong>di</strong>fferenti processi <strong>di</strong>namici che<br />
agiscono sia alla scala locale che a quella regionale, come la<br />
convergenza Africa-Eurasia e la subduzione Calabra.<br />
Per stimare i campi <strong>di</strong> deformazione e sforzo nella zona<br />
<strong>del</strong>l’Arco Calabro è stato utilizzato un mo<strong>del</strong>lo agli elementi<br />
finiti basato sull’approccio thin sheet (MAROTTA et alii, 2004).<br />
Un confronto tra le pr<strong>ed</strong>izioni <strong>del</strong> mo<strong>del</strong>lo con dati<br />
complementari attualmente <strong>di</strong>sponibili nell’area, come dati<br />
geologici, geofisici, GPS e <strong>di</strong> sforzi tettonici, consente <strong>di</strong><br />
vincolare meglio il mo<strong>del</strong>lo geofisico e <strong>di</strong> investigare il ruolo<br />
dei <strong>di</strong>versi meccanismi tettonici attivi. Nel mo<strong>del</strong>lo sono<br />
considerate le maggiori complessità strutturali <strong>del</strong>l’area <strong>del</strong><br />
Tirreno, mentre le proprietà reologiche <strong>del</strong>le rocce sono il<br />
risultato <strong>di</strong> un’analisi termica.<br />
_________________________<br />
RAFFAELE SPLENDORE (*), ANNA MARIA MAROTTA (*) & RICCARDO BARZAGHI (**)<br />
(*) Università degli Stu<strong>di</strong> <strong>di</strong> Milano, Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra<br />
“Ar<strong>di</strong>to Desio”, <strong>Sezione</strong> <strong>di</strong> Geofisica<br />
(**) DIAR, Politecnico <strong>di</strong> Milano<br />
Lavoro eseguito nell’ambito <strong>del</strong> progetto SISMA (Seismic Information<br />
System for Monitoring and Alert), finanziato da ASI.<br />
MODELLO<br />
Mo<strong>del</strong>lo tettonico<br />
Per calcolare il campo <strong>di</strong> deformazione nell’area <strong>del</strong> Tirreno<br />
è stato utilizzato l’approccio thin sheet in coor<strong>di</strong>nate sferiche<br />
(MAROTTA et alii., 2004), applicato su una griglia<br />
bi<strong>di</strong>mensionale, costituita da elementi lineari <strong>di</strong> forma<br />
triangolare (Fig. 1) e che si sviluppa sull’area <strong>del</strong><br />
Fig. 1 – Griglia bi<strong>di</strong>mensionale ad elementi triangolari lineari utilizzata per il<br />
mo<strong>del</strong>lo tettonico. Ogni elemento ha 3 no<strong>di</strong>, posti sui vertici <strong>del</strong> triangolo.<br />
M<strong>ed</strong>iterraneo.<br />
Il mo<strong>del</strong>lo risolve l’equazione <strong>del</strong> momento integrata sullo<br />
spessore litosferico, espressa nella forma:<br />
∂ ∂<br />
2µ<br />
∂θ ∂θ u <br />
θ + ur +<br />
<br />
1 ∂ 1 ∂<br />
µ<br />
sinθ ∂Φ sinθ ∂Φ uθ + ∂<br />
∂θ u <br />
<br />
<br />
Φ − uΦ cotθ<br />
<br />
<br />
<br />
+ 2µ ∂<br />
∂θ u 1 ∂<br />
θ −<br />
sinθ ∂Φ uΦ − uθ cotθ<br />
<br />
<br />
<br />
cotθ =<br />
<br />
<br />
gρcR 2L 1− ρ <br />
c<br />
<br />
<br />
∂ 2<br />
S<br />
∂θ<br />
(1)<br />
e<br />
∂ 1 ∂<br />
µ<br />
∂θ sinθ ∂Φ u ∂<br />
θ +<br />
∂θ uΦ − uΦ cotθ<br />
<br />
<br />
<br />
<br />
<br />
<br />
+ 1 ∂ 1 ∂<br />
2µ<br />
sinθ ∂Φ sinθ ∂Φ u ∂<br />
θ +<br />
∂θ uΦ − uΦ cotθ<br />
<br />
<br />
<br />
cotθ =<br />
<br />
<br />
gρcR <br />
<br />
<br />
ρ m<br />
2L 1− ρc ρm <br />
<br />
<br />
1 ∂ 2<br />
S<br />
sinθ ∂Φ<br />
(2)<br />
Con uθ, uφ <strong>ed</strong> ur componenti <strong>del</strong>la velocità lungo la<br />
colatitu<strong>di</strong>ne, la longitu<strong>di</strong>ne e ra<strong>di</strong>ale rispettivamente, θ
ORIGINE DELL’ATTUALE CAMPO DI SFORZO NELLA ZONA DELL’ARCO CALABRO<br />
colatitu<strong>di</strong>ne, φ longitu<strong>di</strong>ne, µ viscosità m<strong>ed</strong>ia <strong>del</strong>la litosfera, S<br />
spessore crostale, L spessore litosferico, ρc e ρm densità <strong>di</strong><br />
crosta e mantello rispettivamente, g accelerazione <strong>di</strong> gravità <strong>ed</strong><br />
R raggio terrestre.<br />
Le con<strong>di</strong>zioni al contorno lungo il bordo meri<strong>di</strong>onale <strong>del</strong><br />
mo<strong>del</strong>lo (Fig. 2) sono rappresentate dalle velocità <strong>di</strong><br />
convergenza Africa-Eurasia calcolate seguendo la proc<strong>ed</strong>ura <strong>di</strong><br />
NOCQUET et alii (2001), per la stima <strong>del</strong> Polo Euleriano, e<br />
assumendo le stazioni PENC, BOR1, BRUS, VILL, ZIMM,<br />
POTS, HERS, TOUL, MADR, YEBE per definire l’Europa<br />
stabile (DEVOTI et alii, 2002), e GOUG, SUTH, MAS1 per<br />
Fig. 2 – Velocità fissate al bordo meri<strong>di</strong>onale <strong>del</strong>la griglia come con<strong>di</strong>zioni al<br />
contorno <strong>del</strong> mo<strong>del</strong>lo tettonico, rappresentanti la convergenza africa-Eurasia e<br />
la subduzione Egea.<br />
definire l’Africa stabile (MCCLUSKY et alii, 2003). Lungo gli<br />
altri bor<strong>di</strong> <strong>del</strong> mo<strong>del</strong>lo sono state utilizzate le con<strong>di</strong>zioni al<br />
contorno descritte in MAROTTA et alii (2004).<br />
La viscosità effettiva <strong>di</strong> ogni elemento <strong>del</strong>la griglia nella<br />
zona <strong>di</strong> stu<strong>di</strong>o è stata ricavata attraverso uno stu<strong>di</strong>o termico e<br />
reologico tri<strong>di</strong>mensionale.<br />
Mo<strong>del</strong>lo termico<br />
La viscosità effettiva è stata calcolata sulla base <strong>del</strong> campo<br />
termico <strong>del</strong>la litosfera e i conseguenti profili verticali <strong>di</strong><br />
resistenza ottenuti assumendo una composizione granitica per<br />
la crosta e dunitica per il mantello. Il campo termico è stato<br />
calcolato utilizzando un mo<strong>del</strong>lo termico agli elementi finiti<br />
che risolve l’equazione per la conduzione <strong>di</strong> calore:<br />
∇ ⋅ ( k∇T)+<br />
ρH = 0 (3)<br />
su una griglia tri<strong>di</strong>mensionale, ricavata proiettando lungo la<br />
profon<strong>di</strong>tà la griglia bi<strong>di</strong>mensionale utilizzata per il mo<strong>del</strong>lo<br />
tettonico. k è la conduttività termica, T la temperatura, ρ la<br />
densità e H la velocità <strong>di</strong> produzione <strong>di</strong> calore ra<strong>di</strong>ogenico per<br />
unità <strong>di</strong> massa. La superficie superiore <strong>del</strong> mo<strong>del</strong>lo termico e la<br />
base <strong>del</strong>la crosta sono state definite interpolando sui no<strong>di</strong> <strong>del</strong>la<br />
griglia la topografia e la profon<strong>di</strong>tà <strong>del</strong>la Moho ottenute da<br />
BASSIN et alii (2000) (Fig. 3). Il mo<strong>del</strong>lo termico raggiunge la<br />
profon<strong>di</strong>tà massima <strong>di</strong> 200 km. La base <strong>del</strong>la litosfera è definita<br />
come profon<strong>di</strong>tà <strong>del</strong>l’isoterma 1600°K.<br />
L’equazione (3) è risolta assumendo come con<strong>di</strong>zioni al<br />
contorno una temperatura fissata in superficie (Ts=300°K) e un<br />
215<br />
flusso <strong>di</strong> calore residuo qr fissato alla base <strong>del</strong> mo<strong>del</strong>lo e<br />
ricavato a partire dal flusso <strong>di</strong> calore superficiale (ARTEMIEVA,<br />
Fig. 3 – Spessori crostali interpolati sugli elementi <strong>del</strong>la griglia <strong>di</strong> riferimento,<br />
utilizzati sia nel mo<strong>del</strong>lo tettonico che in quello termico.<br />
Fig. 4 – Flusso <strong>di</strong> calore in superficie interpolato sugli elementi <strong>del</strong>la griglia<br />
utilizzata nel mo<strong>del</strong>lo termico.<br />
2006; POLLACK et alii.,1993, Fig. 4), secondo la proc<strong>ed</strong>ura<br />
descritta da POLLACK & CHAPMAN (1977).<br />
Per ricavare i profili <strong>di</strong> resistenza abbiamo assunto che le<br />
rocce si comportino come materiale fragile o duttile a seconda<br />
<strong>del</strong> loro stato termico e variando la velocità <strong>di</strong> deformazione da<br />
10 -19 a 10 -16 s -1 . Per il comportamento fragile assumiamo la<br />
legge <strong>di</strong> Byerlee nella forma (LYNCH & MORGAN, 1987)<br />
σ B = β ⋅ r (4)<br />
con r profon<strong>di</strong>tà e β=16 MPa/km o 40 MPa/km per<br />
compressione <strong>ed</strong> estensione rispettivamente. Per il<br />
comportamento duttile assumiamo la legge <strong>di</strong> potenza nella<br />
forma (WEERTMAN & WEERTMAN, 1975)<br />
σ D = ε.<br />
ε .<br />
1<br />
n<br />
<br />
<br />
o<br />
<br />
dove ε .<br />
<br />
⋅ exp<br />
<br />
<br />
E a<br />
<br />
nRT <br />
(5)<br />
è la velocità <strong>di</strong> deformazione, R la costante dei gas, T<br />
la temperatura assoluta e ε .<br />
o, n e Ea costanti <strong>di</strong>pendenti dal tipo<br />
<strong>di</strong> rocce (Tab 1). Il profilo verticale <strong>di</strong> resistenza è determinato
216 R. SPLENDORE ET ALII<br />
come<br />
( ) min{<br />
σ ( y)<br />
σ ( y)<br />
}<br />
y y B D<br />
σ = ,<br />
(6)<br />
mentre la viscosità effettiva è calcolata come<br />
µ eff = 1<br />
L<br />
(7)<br />
ε . ⋅ σ ydy 0<br />
Fig. 5 – Viscosità effettiva calcolata a partire dai profili <strong>di</strong> resistenza e<br />
interpolata sugli elementi <strong>del</strong>la griglia utilizzata nel mo<strong>del</strong>lo termico.<br />
RISULTATI E DISCUSSIONI<br />
La Fig. 5 mostra la <strong>di</strong>stribuzione <strong>del</strong>la viscosità effettiva<br />
Fig.6 – Profon<strong>di</strong>tà <strong>del</strong>la base termica <strong>del</strong>la litosfera (isoterma 1600°K) riferita<br />
ad ogni elemento <strong>del</strong>la griglia, calcolata a partire dai profili termici stimati dal<br />
mo<strong>del</strong>lo termico.<br />
nella regione <strong>del</strong> M<strong>ed</strong>iterraneo Centrale. Da un confronto con<br />
le figure 3 e 4 si <strong>di</strong>stinguono tre regioni principali <strong>di</strong> elevata<br />
resistenza, una a sud <strong>del</strong>l’Arco Calabro, dove la viscosità<br />
effettiva raggiunge valori <strong>di</strong> 10 28 Pa·s, e le altre due in<br />
corrispondenza dei due bacini oceanici <strong>del</strong> Tirreno, dove la<br />
viscosità effettiva è <strong>del</strong>l’or<strong>di</strong>ne <strong>di</strong> 10 27 Pa·s. Il valore elevato a<br />
sud <strong>del</strong>l’Arco Calabro è correlabile con il bassissimo flusso <strong>di</strong><br />
calore regionale (circa 20 mW/m 2 ) associato ad una crosta<br />
sottile (fra 10 e 15 km) e ad una litosfera molto spessa (fino a<br />
170 km, Fig. 6). Nonostante un flusso <strong>di</strong> calore notevolmente<br />
più elevato nel Tirreno settentrionale (oltre 100 mW/m 2 ), il<br />
Fig. 7 – Autovettori <strong>del</strong>la Strani Rate calcolati per i tre elemnti<br />
triangolari che hanno ai vertici le stazioni permanenti LAMP-<br />
CAGL-NOT1, NOT1-AQUI-CAGL e MATE-AQUI-NOT1. In<br />
grigio le velocità <strong>di</strong> deformazione ottenute a partire dalle soluzioni<br />
<strong>del</strong>le serie temporali giornaliere, in nero quelle ottenute dalla<br />
mo<strong>del</strong>lazione. Il mo<strong>del</strong>lo riproduce ottimamente il regime e la<br />
<strong>di</strong>rezione <strong>del</strong>la deformazione osservata.<br />
contributo <strong>del</strong> mantello alla resistenza <strong>del</strong>la litosfera rimane<br />
comunque importante, garantendo localmente un valore elevato<br />
<strong>del</strong>la viscosità effettiva rispetto all’area circostante.<br />
In Fig. 7 sono rappresentati gli autovettori <strong>del</strong>la velocità <strong>di</strong><br />
deformazione pr<strong>ed</strong>etti dall’integrazione <strong>del</strong>le equazioni (1) e<br />
(2) con le con<strong>di</strong>zioni al contorno specificate nel paragrafo<br />
prec<strong>ed</strong>ente e utilizzando la viscosità effettiva in Fig. 5 e gli<br />
spessori crostali in Fig. 4. Queste previsioni sono confrontate<br />
con i corrispondenti autovettori ottenuti a partire dalle soluzioni<br />
<strong>del</strong>le serie temporali giornaliere ricavate dalle stazioni<br />
permanenti NOT1, LAMP, MATE, AQUI, GAGL <strong>ed</strong> elaborate<br />
con il software Bernese.<br />
Il mo<strong>del</strong>lo tettonico riproduce con buona approssimazione<br />
la deformazione osservata a scala regionale, sia nel regime che<br />
nella <strong>di</strong>rezione, supportando l’ipotesi che la convergenza<br />
Africa-Europa e la subduzione Calabra siano i principali<br />
meccanismi responsabili <strong>del</strong>l’attuale campo <strong>di</strong> sforzo e<br />
deformazione nella parte più meri<strong>di</strong>onale <strong>del</strong> Tirreno, dove la<br />
porzione settentrionale <strong>del</strong>l’indenter Africano contribuisce alla<br />
compressione SE-NW <strong>ed</strong> all’estensione SW-NE, spingendo<br />
lateralmente la porzione orientale <strong>del</strong>la penisola Italiana.<br />
Relativamente al modulo degli autovettori, mentre la<br />
componente compressiva degli autovettori mo<strong>del</strong>lizzati risulta<br />
confrontabile con l’osservazione, per lo meno nel Tirreno<br />
meri<strong>di</strong>onale, l’estensione viene ancora fortemente sottostimata.
ORIGINE DELL’ATTUALE CAMPO DI SFORZO NELLA ZONA DELL’ARCO CALABRO<br />
REFERENCES<br />
ARTEMIEVA M. I. (2006) – Global 1°x1° thermal mo<strong>del</strong> Tc1 for<br />
the continental lithosphere: implications for lithosphere<br />
secular evolution. Tectonophysics, 416, 245-277.<br />
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of resolution for surface wave tomography in North<br />
America. Eos. Trans. AGU, 81 (48), Abstract S12A-03.<br />
CRUST 2.0 - http://mahi.ucsd.<strong>ed</strong>u/Gabi/rem.html<br />
Devoti R., Ferraro C., Gueguen E., Lanotte R., Luceri V.,<br />
Nar<strong>di</strong> A., Pacione R., Rutigliano P., Sciarretta C., Vespe F.<br />
(2002) – Geodetic control on recent tectonic movements in<br />
the central M<strong>ed</strong>iterranean area. Tectonophysics, 346, 151-<br />
167.<br />
LYNCH H. D. & MORGAN P. (1987) – The tensile strenght of the<br />
lithosphere and the localisation of extension. In Continental<br />
extensional tectonics (M. P. COWARD et alii), Spec. Publ.<br />
Geol. Soc. London, 28, 53-65.<br />
POLLACK H. N. & CHAPMAN D. S. (1977) – On the regional<br />
variation of heat flow, geotherms, and lithospheric<br />
thickness. Tectonophysics, 38, 279-296.<br />
POLLACK H. N., HURTER S. & JOHNSON J. R. (1993) – Heat<br />
Flow from the Earth's interior: analysis of the global data<br />
set. Reviews of Geophysics., 31(3), 267-280.<br />
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(2004) – Combin<strong>ed</strong> effects of tectonics and glacial isostatic<br />
adjustment on intraplate deformation in central and northern<br />
Europe: Applications to geodetic baseline analyses. Journal<br />
of Geopjysical Research, 109, B01413,<br />
doi:10.1029/2002JB002337<br />
MCCLUSKY S. et alii (2000) – Global Positioning System<br />
constraints on plate kinematics and dynamics in the eastern<br />
M<strong>ed</strong>iterranean and Caucasus. J. Geophys. Res., 105, 5695<br />
– 5719.<br />
MCCLUSKY S., REILINGER R., MAHMOUD S., BEN SARI D.,<br />
TEALEB A. (2003) – GPS constrains on Africa (Nubia) and<br />
Arabia plate motions. Geophys. J. Int., 155, 126 – 138.<br />
NOCQUET J. M., CALAIS E., ALTAMINI A., SILLARD P.,<br />
BOUCHER C. (2001) – Intraplate deformation in western<br />
Europe d<strong>ed</strong>uc<strong>ed</strong> from an analysis of the International<br />
Terrestrial Reference Frame 1997 (ITRF97) velocity field.<br />
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WEERTMAN J. & WEERTMAN J. R. (1975) – High temperature<br />
creep of rock, and mantle viscosity. Ann. Rev. Earth Planet.<br />
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217
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 218<br />
ABSTRACT<br />
Una proc<strong>ed</strong>ura per la determinazione <strong>del</strong>la <strong>di</strong>stribuzione granulometrica<br />
<strong>di</strong> rocce cataclastiche sviluppate in carbonati utilizzando il principio <strong>del</strong>la<br />
<strong>di</strong>ffrazione laser.<br />
Particle size <strong>di</strong>stribution analysis provides a first order<br />
contribution to rock characterization and to the description of<br />
many geological processes such as s<strong>ed</strong>imentation, rock<br />
fragmentation and soil formation. Fast particle size data<br />
acquisition over a wide size range is provid<strong>ed</strong> by <strong>di</strong>ffraction<br />
granulometers, which typically allow using a wide variety of<br />
analytical methods, nam<strong>ed</strong> standard operating proc<strong>ed</strong>ures<br />
(SOP). These ensure the appropriate flexibility for analysing<br />
very <strong>di</strong>fferent granular materials, but deserve accurate<br />
investigations and systematic testing for the possible influence<br />
that <strong>di</strong>fferent analytical methods may exert on results obtain<strong>ed</strong><br />
by the same or <strong>di</strong>fferent instruments. For this purpose, we<br />
perform<strong>ed</strong> specific tests on poorly coherent carbonate platform<br />
fault core rocks by using two <strong>di</strong>fferent instruments with<br />
<strong>di</strong>fferent sample <strong>di</strong>spersion and pumping systems. In particular,<br />
most of our analyses were carri<strong>ed</strong> out by using a Mastersizer<br />
2000 laser <strong>di</strong>ffraction granulometer manufactur<strong>ed</strong> by Malvern<br />
Instruments Ltd., because of the ensur<strong>ed</strong> possibility to test<br />
several wet and dry analytical proc<strong>ed</strong>ures inclu<strong>di</strong>ng <strong>di</strong>fferent<br />
pump spe<strong>ed</strong>s, measure precision tests with and without sample<br />
ultrasonication, <strong>di</strong>fferent <strong>di</strong>spersant liquids, and sampling<br />
precision tests. Results of our work in<strong>di</strong>cate a significant<br />
sensitivity of particle size data to the adopt<strong>ed</strong> analytical<br />
proc<strong>ed</strong>ure and, consequently, their indeterminateness when<br />
standard operating proc<strong>ed</strong>ures are not systematically test<strong>ed</strong> and<br />
critically <strong>di</strong>scuss<strong>ed</strong>. The lack of information on the analytical<br />
proc<strong>ed</strong>ure can contribute to mislea<strong>di</strong>ng data interpretation and<br />
data comparison and, eventually, to wrong inferences on the<br />
progression of cataclasis in fault zones. We propose a workflow<br />
of analytical tests preliminary to the set up of the most<br />
appropriate SOP for systematic laser <strong>di</strong>ffraction granulometry<br />
of cataclastic breccias in carbonate rocks. Application of this<br />
_________________________<br />
A workflow for laser <strong>di</strong>ffraction granulometry of carbonate<br />
cataclastic rocks<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, Università degli Stu<strong>di</strong> Roma Tre,<br />
Largo S. L. Murialdo 1, I-00146 Roma, Italy<br />
FABRIZIO STORTI (*) & FABRIZIO BALSAMO (*)<br />
proc<strong>ed</strong>ure is typically flexible due to the great variability of<br />
analys<strong>ed</strong> materials and the common ne<strong>ed</strong> of several iterations<br />
before reaching the most statistically robust results.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 219-220, 2 ff.<br />
Defomation pattern in a massive pond<strong>ed</strong> lava flow at ODP-IODP<br />
Site 1256 (Pacific Ocean): a core and log approach<br />
PAOLA TARTAROTTI (*), EMANUELE FONTANA (*) & LAURA CRISPINI (°)<br />
RIASSUNTO<br />
Assetto strutturale <strong>di</strong> una colata lavica nel Sito ODP-IODP 1256 (Oceano<br />
Pacifico): integrazione <strong>di</strong> dati <strong>di</strong> carota e <strong>di</strong> log in foro<br />
Una sezione completa “in situ” <strong>di</strong> crosta oceanica è stata recentemente<br />
perforata per la prima volta in una crosta <strong>di</strong> 15 Ma creata in un segmento <strong>del</strong>la<br />
dorsale Est Pacifica, caratterizzata da un’ elevata velocità <strong>di</strong> espansione (>200<br />
mm/anno). La sezione crostale comprende colate basaltiche <strong>di</strong> vario spessore,<br />
scarse lave a cuscini, un complesso <strong>di</strong> <strong>di</strong>cchi basici e la parte superficiale <strong>del</strong><br />
Layer 3 gabbrico. Le colate superficiali includono una colata massiva potente<br />
ca. 70 m, interpretata come un “lava pond” messo in posto al <strong>di</strong> fuori <strong>del</strong>l’asse<br />
<strong>di</strong> espansione <strong>del</strong>la dorsale. L’assetto strutturale <strong>del</strong> lava pond d<strong>ed</strong>otto<br />
dall’integrazione <strong>di</strong> dati <strong>di</strong> carotaggio e <strong>di</strong> log geofisici in foro conferma la<br />
peculiarità <strong>di</strong> questo corpo lavico, che risulta estraneo ai meccanismi noti <strong>di</strong><br />
accrezione crostale all’asse <strong>di</strong> dorsale oceanica.<br />
Key words: fracturing, Integrat<strong>ed</strong> Ocean Drilling Program,<br />
ocean crust, physical properties, superfast ridge, submarine<br />
lava flow.<br />
A complete intact “in situ” section of upper oceanic crust, from<br />
extrusive lavas through the <strong>di</strong>kes and into the underlying<br />
gabbros has been recently drill<strong>ed</strong> for the first time in a 15 Ma<br />
old crust that form<strong>ed</strong> at the East Pacific Rise with a full<br />
sprea<strong>di</strong>ng rate of >200 mm/yr. The study area is ODP-IODP<br />
Site 1256 (6°44.2N, 91°56.1W, Pacific Ocean) where two<br />
holes, Hole 1256C and 1256D, have been drill<strong>ed</strong> into the<br />
basaltic basement during ODP Leg 206, IODP Exp<strong>ed</strong>itions 309<br />
and 312 (WILSON et al., 2003; TEAGLE et al., 2006). Hole<br />
1256D has been deepen<strong>ed</strong> to a depth of ca. 1500 meters below<br />
seafloor (mbsf). The upper section of the igneous basement<br />
consists of thin (3m). The massive flows<br />
include a pond<strong>ed</strong> lava flow, locat<strong>ed</strong> near the top of both Hole<br />
1256C and 1256D, where it has a thickness of 32m and 74m,<br />
respectively. Minor intervals of pillow lavas (20m) and<br />
hyaloclastite (a few meters) were recover<strong>ed</strong> in Hole 1256D,<br />
which are follow<strong>ed</strong> downhole by a 470m-thick section of sheet<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Milano<br />
(°) DIPTERIS, Dipartimento per lo Stu<strong>di</strong>o <strong>del</strong> Territorio e <strong>del</strong>le sue Risorse,<br />
Università <strong>di</strong> Genova<br />
Lavoro eseguito nell’ambito <strong>del</strong> progetto FIRST 2007 con il contributo<br />
finanziario <strong>del</strong>l’Università <strong>di</strong> Milano<br />
and massive flows with subor<strong>di</strong>nate breccia, a transition zone<br />
from massive flows to <strong>di</strong>kes, a ca. 350m-thick sheet<strong>ed</strong> <strong>di</strong>ke<br />
zone, and gabbros (Fig. 1).<br />
The lava pond is the object of this research; it has been<br />
interpret<strong>ed</strong> as a thick lava flow <strong>del</strong>iver<strong>ed</strong> most probably offaxis<br />
and accumulat<strong>ed</strong> in a topographic depression (WILSON et<br />
al., 2003; CRISPINI et al., 2006; TARTAROTTI et al., 2006).<br />
Fine-scale structural analysis was carri<strong>ed</strong> out in orient<strong>ed</strong> core<br />
pieces from the two holes. Structural data were process<strong>ed</strong> by: i)<br />
identifying post-magmatic structures, namely joints, veins,<br />
shear veins, microfaults, late magmatic veins, and measuring<br />
their true <strong>di</strong>p angle; ii) counting the number of structures/dm<br />
(all planar structures record<strong>ed</strong> in orient<strong>ed</strong> and not orient<strong>ed</strong><br />
cores). During ODP Leg 206, the true <strong>di</strong>p angle of 613 planar<br />
structures in Hole 1256C and of 1702 planar structures in Hole<br />
1256D were measur<strong>ed</strong> (SHIPBOARD SCIENTIFIC PARTY, 2003).<br />
Structural results were integrat<strong>ed</strong> with data obtain<strong>ed</strong> by wireline<br />
logging and onboard physical property measurements. The<br />
main results are illustrat<strong>ed</strong> in the following.<br />
Fig. 1 – Simplifi<strong>ed</strong> basement stratigraphy at ODP-IODP Site 1256.<br />
The Lava Pond occurring at the top part of the volcanic section<br />
is much less fractur<strong>ed</strong> than the deeper parts. This observation is<br />
relat<strong>ed</strong> essentially to the spacing of structures that is larger than<br />
in the underlying thinner lava flows. It is suggest<strong>ed</strong> that<br />
dominantly conductive cooling in this upper portion of the crust<br />
result<strong>ed</strong> in less pervasive cracking than that obtain<strong>ed</strong> by<br />
dominantly convective cooling. Brittle, post-magmatic<br />
structures are mainly sub-horizontal in the Lava Pond and<br />
become steeper downhole. This result reflects the <strong>di</strong>stance of
220 P. TARTAROTTI ET ALII<br />
Fig. 2 – Downhole <strong>di</strong>stribution of Qualitative Fracture Intensity* in Holes 1256C and 1256D compar<strong>ed</strong> with downhole logging measurements perform<strong>ed</strong> during<br />
ODP Leg 206 and IODP Exp<strong>ed</strong>itions 309 and 312 (Ocean Drilling Program, Integrat<strong>ed</strong> Ocean Drilling Program and Lamont Doherty Earth Observatory,<br />
available on http://iodp.ldeo.columbia.<strong>ed</strong>u/DATA/) in Hole 1256D.<br />
the lava pond from the stress field produc<strong>ed</strong> by the intrusion of<br />
<strong>di</strong>kes at depth, inducing mostly subvertical eruptive fractures<br />
and fracture network, and/or from the rifting extentional<br />
tectonics. The lower extent of fracturing in the lava pond basalt<br />
with respect to the deeper crust is reflect<strong>ed</strong> in <strong>di</strong>fferent physical<br />
properties values, as obtain<strong>ed</strong> by shipboard as well as<br />
downhole logging measurements. Electrical resistivity,<br />
compressional velocity, and bulk density are higher, whereas<br />
natural ra<strong>di</strong>oactivity and neutral porosity are lower than in the<br />
deeper parts of Hole 1256D (Fig. 2). Within the lava pond,<br />
variations in fracture density mostly correspond to anomalies of<br />
rock physical properties, as the effect of high concentrations of<br />
secondary minerals. The occurrence of a thick massive lava<br />
flow at the top of the volcanic section apparently affects the<br />
deformation pattern of the entire drill<strong>ed</strong> crust and,<br />
consequently, its permeability regime which in turn controls the<br />
geometry and <strong>di</strong>stribution of hydrothermal circulation and<br />
basalt alteration. Our results fit with the interpretation given by<br />
previous works that the lava lpnd is a lava flow emplac<strong>ed</strong> in an<br />
off-axis topographic depression.<br />
* Qualitative estimation of the degree of fracturing bas<strong>ed</strong> only<br />
on a visual inspection of cores.<br />
REFERENCES<br />
CRISPINI L., TARTAROTTI P. & UMINO S. (2006) –<br />
Microstructural features of a subaqueous lava from<br />
basaltic crust off the East Pacific Rise (ODP Site 1256,<br />
Cocos Plate). Ofioliti, Special Issue “Modern and fossil<br />
oceanic lithosphere”, 31(2), 117-127.<br />
SHIPBOARD SCIENTIFIC PARTY (2003), Site 1256. In: D.S.<br />
Wilson, D.A.H. Teagle, G.D. Acton, et al., Proc. ODP, Init.<br />
Repts., 206: College Station, TX (Ocean Drilling Program),<br />
1–396, doi:10.2973/odp.proc.ir.206.103.2003<br />
TARTAROTTI P., CRISPINI L. & THE IODP EXPEDITIONS 309-312<br />
SHIPBOARD SCIENTIFIC PARTY (2006) – ODP-IODP Site<br />
1256 (East Pacific Rise): an in-situ section of upper<br />
oceanic crust form<strong>ed</strong> at a superfast sprea<strong>di</strong>ng rate. Ofioliti,<br />
Special Issue “Modern and fossil oceanic lithosphere”,<br />
31(2), 107-116.<br />
WILSON D.S., TEAGLE D.A.H., ACTON G.D. et alii (2003) – Proc.<br />
ODP, Init. Repts., 206 [CD-ROM]. Available from: Ocean<br />
Drilling Program, Texas A & M University, College Station<br />
TX 77845-9547, USA.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 221-224, 8ff.<br />
Processi <strong>di</strong> fagliazione nei grainstones carbonatici porosi: implicazioni per la<br />
caratterizzazione dei serbatoi naturali <strong>di</strong> geoflui<strong>di</strong><br />
EMANUELE TONDI (*), FABRIZIO AGOSTA (*) & ANTONINO CILONA (*)<br />
ABSTRACT<br />
Faulting in porous carbonate grainstones: implications for the<br />
characterization of natural reservoirs of geofluids<br />
Recently, a new faulting mechanism was document<strong>ed</strong> in porous carbonate<br />
grainstones, in which faults form by strain localization into narrow tabular<br />
bands characteriz<strong>ed</strong> by volumetric and shear strain (so call<strong>ed</strong> compactive shear<br />
bands, TONDI et alii, 2006; TONDI, 2007). In the field, these structures are<br />
easily recognizable because they are lightly color<strong>ed</strong> with respect to the parent<br />
rock and/or show a positive relief due to their increas<strong>ed</strong> resistance to<br />
weathering. Both these characteristics are link<strong>ed</strong> to the compaction processes<br />
acting within the bands.<br />
Starting from single compactive shear bands, which resolve a few mm of<br />
<strong>di</strong>splacement, these structures evolve in zone of compactive shear bands and<br />
then, eventually, in well develop<strong>ed</strong> faults with slip surfaces and fault rocks<br />
(breccia and gouge).<br />
In this work we present new data concerning their growth mechanisms, in<br />
terms of <strong>di</strong>splacement, length and thickness. We also document how the<br />
transition from one deformation process to another, which is likely to be<br />
controll<strong>ed</strong> by the changes in the material properties, is record<strong>ed</strong> by <strong>di</strong>fferent<br />
ratios and <strong>di</strong>stributions of the fault <strong>di</strong>mensional attributes.<br />
Field analysis document<strong>ed</strong> <strong>di</strong>fferent possibility of interaction and linkage<br />
between these structures: (i) a simple <strong>di</strong>vergence of compactive shear bands,<br />
(ii) extensional and contractional jogs, and (iii) eye structures, already<br />
describ<strong>ed</strong> for deformation bands in sandstones. All these types of interaction<br />
and linkage could happen at any deformation stage, single bands, zone of<br />
bands or well develop<strong>ed</strong> faults.<br />
Concerning their <strong>di</strong>mensional parameters, we observ<strong>ed</strong> that length,<br />
<strong>di</strong>splacement and thickness of single bands given values (50 cm of length and<br />
6 mm of both <strong>di</strong>splacement and thickness) and <strong>di</strong>d not show any scale<br />
relationship among them. Conversely, <strong>di</strong>fferent mechanism occur within zones<br />
of shear bands. The <strong>di</strong>splacement value along a zone of shear bands is<br />
maximum near the center of the structure, as it is commonly observ<strong>ed</strong> in faults<br />
with sharp <strong>di</strong>scontinuities (slip surfaces). This <strong>di</strong>fferent mechanical behavior<br />
is due to, the possibility to have <strong>di</strong>fferent <strong>di</strong>splacement envelopes within the<br />
shear band zones thanks to the number of single bands they include. For this<br />
reason, the increas<strong>ed</strong> <strong>di</strong>splacement value in these zones is accompani<strong>ed</strong> to an<br />
increas<strong>ed</strong> number of bands (and of the thickness). Similarly, D-L graphs of<br />
well develop<strong>ed</strong> faults show the same relation, with the <strong>di</strong>fference that in faults<br />
with up to 20-40 cm of <strong>di</strong>splacement, a slip surface (with breccia and gouge) is<br />
present and so the number of shear bands remain constant even for large<br />
<strong>di</strong>splacements.<br />
Concerning the relations among the <strong>di</strong>fferent <strong>di</strong>mensional parameters,<br />
thickness and <strong>di</strong>splacement depict two populations, well develop<strong>ed</strong> faults (with<br />
slip surfaces) resolve much <strong>di</strong>splacement with respect the zone of shear bands.<br />
In conclusion, single shear band can evolve to zone of shear bands and<br />
then to well develop<strong>ed</strong> fault or can interact to another single bands forming<br />
larger structures. Interaction and linkage can occur at any deformation stage.<br />
___________________________________________________________<br />
(*) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Camerino.<br />
Lavoro eseguito nell’ambito <strong>del</strong> progetto Faults & Fractures in Carbonates<br />
(F&FC) <strong>del</strong>l’Università <strong>di</strong> Camerino.<br />
Typical values for their <strong>di</strong>mensional parameters are:<br />
- Single band is 50 cm long, 0.5 cm thick and resolve 0.5 cm of<br />
<strong>di</strong>splacement: the D/T factor is 1.0;<br />
- Zone of shear bands is maximum 10 m long, 20 cm thick and resolve 30<br />
cm of <strong>di</strong>splacement: the D/T factor is 1.5;<br />
- The <strong>di</strong>splacement and the thickness (relat<strong>ed</strong> to the number of bands) are<br />
maximum in the central part of the zone of bands;<br />
- Well develop<strong>ed</strong> fault (with slip surfaces and cataclasis) is more efficient:<br />
the D/T factor is 3.5.<br />
Key words: compaction, compactive shear bands,<br />
porosity, pressure solution seams.<br />
Parole chiave: compactive shear bands, compattazione,<br />
<strong>di</strong>ssoluzione per pressione, porosità.<br />
INTRODUZIONE<br />
Le rocce carbonatiche, <strong>di</strong>fferentemente da altri tipi <strong>di</strong> rocce<br />
serbatoio (i.e. rocce silicoclastiche, plutoniche, etc.), sono<br />
rappresentate da una notevole varietà <strong>di</strong> litotipi, sulla base <strong>del</strong>la<br />
natura e <strong>del</strong>l’organizzazione/forma degli elementi costituenti<br />
(grani, pori, cementi, minerali, etc.), e caratterizzate da <strong>di</strong>versi<br />
valori <strong>di</strong> porosità e permeabilità, anche in relazione alla loro<br />
evoluzione <strong>di</strong>agenetica. Inoltre, la capacità <strong>del</strong>le rocce<br />
carbonatiche nel contenimento e nella migrazione dei geoflui<strong>di</strong><br />
è fortemente influenzata dallo stato <strong>di</strong> “fratturazione”. Allo<br />
scopo <strong>di</strong> meglio comprendere e quantificare quest’ultimo<br />
aspetto, negli ultimi anni numerose ricerche sono state<br />
in<strong>di</strong>rizzate alla comprensione dei processi <strong>di</strong> enucleazione e<br />
sviluppo <strong>di</strong> faglie e fratture nei <strong>di</strong>versi litotipi carbonatici, alla<br />
quantificazione <strong>del</strong>le loro proprietà spaziali e <strong>di</strong>mensionali e<br />
alla caratterizzazione fisico-chimica <strong>del</strong>la roccia deformata<br />
(ANTONELLINI et alii, 2007 e referenze citate).<br />
Se nelle rocce carbonatiche poco porose le <strong>di</strong>scontinuità<br />
vanno a costituire dei networks che, a secondo <strong>del</strong>la loro natura,<br />
connettività e grado <strong>di</strong> evoluzione, rappresentano dei siti<br />
preferenziali per la localizzazione e la migrazione dei flui<strong>di</strong><br />
(AGOSTA & AYDIN, 2006 e referenze citate), in quelle porose,<br />
tali <strong>di</strong>scontinuità rappresentano generalmente <strong>del</strong>le barriere che<br />
inibiscono la migrazione dei flui<strong>di</strong> (TONDI et alii, 2006, TONDI,<br />
2007). Tale <strong>di</strong>fferenza è strettamente correlata ai processi<br />
deformativi che agiscono in quest’ultima tipologia <strong>di</strong> carbonati,<br />
caratterizzati da un’elevata porosità (superiore al 15%);<br />
con<strong>di</strong>zione necessaria per lo sviluppo <strong>di</strong> particolari strutture<br />
tettoniche solo recentemente descritte: le shear bands. Queste<br />
strutture <strong>di</strong> taglio si formano grazie ad un particolare processo
222 E. TONDI ET ALII<br />
deformativo che contempla la rotazione e compattazione dei<br />
grani costituenti il protolite. Sta<strong>di</strong> più evoluti <strong>del</strong>la deformazione<br />
prev<strong>ed</strong>ono l’instaurarsi all’interno <strong>di</strong> tali strutture <strong>di</strong> processi <strong>di</strong><br />
<strong>di</strong>ssoluzione per pressione che portano alla formazione <strong>di</strong> una<br />
vera e propria superficie <strong>di</strong> <strong>di</strong>scontinuità, la quale favorisce poi<br />
il movimento e la relativa formazione <strong>del</strong>la roccia <strong>di</strong> faglia.<br />
In questo lavoro, oltre ad una breve revisione dei processi <strong>di</strong><br />
enucleazione e sviluppo <strong>del</strong>le faglie nei grainstones carbonatici<br />
porosi, vengono presentati nuovi dati riferiti alle proprietà<br />
spaziali e <strong>di</strong>mensionali <strong>del</strong>le shear bands e una <strong>di</strong>scussione sul<br />
loro significato in relazione ai <strong>di</strong>versi meccanismi deformativi<br />
agenti durante la crescita e lo sviluppo <strong>di</strong> queste strutture <strong>di</strong><br />
taglio.<br />
La definizione <strong>di</strong> appropriate leggi matematiche, in grado <strong>di</strong><br />
descrivere le caratteristiche spaziali e <strong>di</strong>mensionali <strong>del</strong>le<br />
strutture tettoniche, risulta <strong>di</strong> fondamentale importanza per la<br />
costruzione <strong>di</strong> mo<strong>del</strong>li strutturali quali quelli DFN (Discrete<br />
Fractures Network). Spesso utilizzati dall’industria petrolifera<br />
per la mo<strong>del</strong>lizzazione dei flui<strong>di</strong> all’interno dei serbatoi naturali<br />
fratturati; i mo<strong>del</strong>li DFN, infatti, utilizzano software in grado <strong>di</strong><br />
produrre la <strong>di</strong>stribuzione <strong>del</strong>le fratture m<strong>ed</strong>iante meto<strong>di</strong><br />
statistici/frattali.<br />
ENUCLEAZIONE E CRESCITA DELLE FAGLIE<br />
Recentemente, TONDI et alii (2006) e TONDI (2007) hanno<br />
descritto una nuova tipologia <strong>di</strong> strutture nei grainstones<br />
carbonatici porosi, le quali sono caratterizzate da deformazione<br />
volumetrica e <strong>di</strong> taglio, e quin<strong>di</strong> chiamate compactive shear<br />
bands. Queste strutture che si presentano sottili e tabulari sono<br />
facilmente riconoscibili in campagna in quanto, <strong>di</strong> colore più<br />
chiaro rispetto alla roccia circostante, sono generalmente<br />
caratterizzate da un rilievo positivo dovuto alla loro maggiore<br />
resistenza all’erosione (Fig. 1). Amb<strong>ed</strong>ue queste caratteristiche<br />
sono legate a processi <strong>di</strong> compattazione e cementazione che<br />
avvengono al loro interno. In fig. 2 viene mostrato il mo<strong>del</strong>lo <strong>di</strong><br />
enucleazione e sviluppo <strong>di</strong> queste strutture <strong>di</strong> taglio e la<br />
variazione, in percentuale, <strong>del</strong>la porosità, dei grani e <strong>del</strong>la<br />
matrice nelle zone costituenti le compactive shear bands<br />
rispetto all’host rock. Tale mo<strong>del</strong>lo evolutivo prev<strong>ed</strong>e una<br />
prima fase <strong>di</strong> compattazione che risolve una piccola entità <strong>di</strong><br />
movimento <strong>di</strong> taglio, <strong>ed</strong> un successivo sviluppo <strong>di</strong> processi <strong>di</strong><br />
<strong>di</strong>ssoluzione per pressione all’interno <strong>del</strong>la zona compattata<br />
(Zona 2), che portano alla formazione <strong>di</strong> una zona <strong>di</strong> debolezza<br />
costituita da materiale residuale (Zona 1) favorendo un sempre<br />
maggiore movimento <strong>di</strong> taglio. Il materiale <strong>di</strong>sciolto nella Zona<br />
2 precipita all’interno dei pori <strong>del</strong>l’host rock, a<strong>di</strong>acente alla<br />
banda <strong>di</strong> deformazione, portando alla formazione <strong>del</strong>la Zona 3<br />
e quin<strong>di</strong> ad un ispessimento <strong>del</strong>la banda stessa.<br />
I rilievi <strong>di</strong> campagna, inoltre, hanno permesso <strong>di</strong> osservare<br />
in questa tipologia <strong>di</strong> rocce carbonatiche sia singole compactive<br />
shear bands, che risolvono pochi mm <strong>di</strong> rigetto e mostrano uno<br />
spessore anch’esso millimetrico, sia zone <strong>di</strong> compative shear<br />
bands, costituite da più bande sub-parallele l’una all’altra, che<br />
faglie ben sviluppate, in cui sono riconoscibili nette superfici <strong>di</strong><br />
taglio associate a roccia <strong>di</strong> faglia (generalmente breccia e<br />
gauge). Queste tre tipologie <strong>di</strong> strutture sono state interpretate<br />
come sta<strong>di</strong> successivi <strong>del</strong>la deformazione che, unitamente a<br />
<strong>di</strong>versi processi <strong>di</strong> interazione e collegamento tra le strutture<br />
(Fig. 3), determinano la crescita e lo sviluppo <strong>del</strong>le faglie nei<br />
grainstones carbonatici porosi.<br />
Fig. 1 – Compactive shear bands affioranti a Favignana (Isole Ega<strong>di</strong>, Sicilia).<br />
Compactive shear bands cropping out in Favignana (Ega<strong>di</strong> Islands, Sicily).<br />
a)<br />
b)<br />
Host rock<br />
Zona 3<br />
Zona 2<br />
Zona 1<br />
Grani (%)<br />
Zona 3<br />
Zona 2<br />
Porosità (%)<br />
Zona 1<br />
Matrice (%)<br />
Fig. 2 – a) Mo<strong>del</strong>lo <strong>di</strong> evoluzione <strong>del</strong>le compactive shear bands; b) variazione<br />
percentuale <strong>di</strong> porosità, grani e matrice nelle <strong>di</strong>verse zone costituenti le<br />
compactive shear bands, rispetto all’host rock.<br />
a) Evolutionary mo<strong>del</strong> of compactive shear bands; b) porosity, grain and<br />
matrix percentage variation in the compactive shear bands, with respect the<br />
host rock.
Fig. 3 – Diverse tipologie <strong>di</strong> interazione e collegamento tra le compactive<br />
shear bands: 1) semplici <strong>di</strong>vergenze; 2) e 3) jogs estensionali e contrazionali;<br />
4) struttura a “occhio”. In figura è rappresentata una compactive shear band<br />
trascorrente destra: le strutture coniugate sono costituite da compactive shear<br />
bands trascorrenti sinistre mentre le <strong>brevi</strong> strutture localizzate alle terminazioni<br />
<strong>del</strong>le prec<strong>ed</strong>enti e/o localizzate nei jogs rappresentano bande <strong>di</strong> compattazione<br />
e/o stiloliti.<br />
Different possibility of interaction and linkage between compactive shear<br />
bands: 1) a simple <strong>di</strong>vergence; 2) and 3) extensional and contractional jogs;<br />
4) eye structure.<br />
PROCESSI DI FAGLIAZIONE NEI GRAINSTONES CARBONATICI POROSI<br />
PROPRIETA’ DIMENSIONALI DELLE FAGLIE<br />
Un analisi sistematica eseguita in campagna, su affioramenti<br />
ben esposti, ha permesso <strong>di</strong> misurare i <strong>di</strong>versi elementi<br />
<strong>di</strong>mensionali quali la lunghezza, il rigetto e lo spessore <strong>del</strong>le<br />
singole compactive shear bands, <strong>del</strong>le zone <strong>di</strong> compactive<br />
shear bands e <strong>del</strong>le faglie ben sviluppate.<br />
Le singole compactive shear bands sono caratterizzate da<br />
ben determinati valori <strong>di</strong>mensionali e, come è possibile<br />
osservare dal grafico <strong>di</strong> fig. 4, questi non mostrano chiare<br />
relazioni <strong>di</strong> scala. Di lunghezza <strong>del</strong>l’or<strong>di</strong>ne dei 50 cm, il rigetto<br />
e lo spessore hanno valori pr<strong>ed</strong>ominanti compresi tra 0, 4 cm e<br />
0,6 cm.<br />
Le zone <strong>di</strong> compactive shear bands presentano, al contrario,<br />
lunghezze variabili, e su alcune <strong>di</strong> esse è stato inoltre possibile<br />
misurare la variazione <strong>del</strong> rigetto lungo la struttura stessa.<br />
Come è possibile notare in fig. 5, il rigetto ha il valore massimo<br />
nella zona centrale, come accade per le faglie con superfici <strong>di</strong><br />
taglio ben evidenti (Fig. 6), con la <strong>di</strong>fferenza che per le zone <strong>di</strong><br />
compactive shear bands la possibilità <strong>di</strong> aumentare il rigetto è<br />
strettamente legata al numero <strong>di</strong> bande che vanno a costituire la<br />
zona stessa. Di conseguenza, oltre ad un aumento <strong>del</strong> rigetto,<br />
nella zona centrale si osserva anche un maggior numero <strong>di</strong><br />
bande e quin<strong>di</strong> un maggiore spessore. Nelle strutture più<br />
evolute, l’aumento <strong>del</strong> rigetto è associato ad un numero<br />
maggiore <strong>di</strong> bande e quin<strong>di</strong> ad un aumento considerevole <strong>di</strong><br />
spessore solo nelle zone prossime alle terminazioni <strong>del</strong>la<br />
struttura. Nella parte centrale, dove le superfici <strong>di</strong> taglio<br />
associate a roccia <strong>di</strong> faglia sono ben sviluppate, questo non è<br />
più evidente e l’aumento <strong>del</strong> rigetto viene pertanto risolto<br />
esclusivamente dallo scorrimento lungo le superfici <strong>di</strong> taglio<br />
stesse. La maggiore efficienza nel risolvere il rigetto <strong>di</strong> queste<br />
ultime rispetto alla zone <strong>di</strong> compactive shear bands è mostrato<br />
dal grafico <strong>di</strong> fig. 7. In funzione dei <strong>di</strong>versi meccanismi <strong>di</strong><br />
deformazione agenti, i rapporti <strong>di</strong> scala tra rigetto e spessore<br />
risultano piuttosto <strong>di</strong>versi tra le due popolazioni <strong>di</strong> strutture.<br />
Fig. 4 – Grafico spessore (S) - rigetto (R) <strong>di</strong> singole compactive shear bands.<br />
Thickness (S) - <strong>di</strong>splacement (R) graph of single compactive shear bands.<br />
a)<br />
b)<br />
223<br />
Fig. 5 – a) Zona <strong>di</strong> compactive shear bands. Per la legenda v<strong>ed</strong>i <strong>di</strong>dascalia <strong>di</strong><br />
Fig. 3. b) Grafico lunghezza (L) rigetto (R) riferito alla zona <strong>di</strong> compactive<br />
shear bands mostrata in a).<br />
a) Zone of compactive shear bands. b) Length (L) <strong>di</strong>splacement (R) graph of<br />
the zone of compactive shear bands shown in a).<br />
Fig. 6 – Grafico lunghezza (L) rigetto (R) <strong>di</strong> una faglia con superfici <strong>di</strong> taglio<br />
ben sviluppate.<br />
Length (L) <strong>di</strong>splacement (R) graph of a well develop<strong>ed</strong> faults.
224 E. TONDI ET ALII<br />
Zone <strong>di</strong> bande<br />
Jogs e steps<br />
Fig. 7 – Grafico rigetto (R) spessore (S) riferito sia a zone <strong>di</strong> compactive shear<br />
bands che a faglie con superfici <strong>di</strong> taglio ben sviluppate. I dati cerchiati si<br />
riferiscono a zone <strong>di</strong> jogs e steps in cui i valori <strong>di</strong> spessore risultano<br />
sovrastimati.<br />
Displacement (R) thickness (S) graph of zone of compactive shear bands and<br />
well develop<strong>ed</strong> faults. Circl<strong>ed</strong> data refere<strong>ed</strong> to jogs and steps zones where<br />
thickness values are not consistent.<br />
CONCLUSIONI<br />
Faglie<br />
Nei grainstones carbonatici porosi, la struttura <strong>di</strong> taglio più<br />
semplice è rappresentata dalla singola compactive shear band<br />
che evolve a zone <strong>di</strong> compative shear bands e successivamente<br />
a faglia evoluta con la formazione <strong>di</strong> una <strong>di</strong>scontinuità <strong>di</strong> taglio<br />
ben evidente associata a roccia <strong>di</strong> faglia. Ad ogni fase <strong>di</strong><br />
crescita, le <strong>di</strong>verse strutture possono interagire tra loro con<br />
fenomeni <strong>di</strong> collegamento in corrispondenza <strong>di</strong> jogs<br />
estensionali e/o contrazionali (Fig. 8).<br />
Per quanto riguarda i valori <strong>di</strong> lunghezza, rigetto e spessore,<br />
le singole compactive shear bands sono caratterizzate da ben<br />
determinati valori <strong>di</strong>mensionali: lunghezza 50 cm, rigetto e<br />
spessore 0,5 cm, con un rapporto tra rigetto e spessore pari a 1.<br />
Le zone <strong>di</strong> compactive shear bands hanno lunghezza massima<br />
<strong>di</strong> 10 m a cui corrisponde un rigetto <strong>di</strong> 30 cm e uno spessore <strong>di</strong><br />
20 cm. Il rapporto rigetto spessore è pari a 1,5. Sia il rigetto che<br />
lo spessore sono massimi nella zona centrale <strong>del</strong>la struttura. Le<br />
faglie ben sviluppate con evidenti superfici <strong>di</strong> taglio e associata<br />
roccia <strong>di</strong> faglia risultano più efficienti <strong>del</strong>le prec<strong>ed</strong>enti strutture,<br />
con un rapporto tra rigetto e spessore pari a 3,5.<br />
In conclusione, una dettagliata analisi <strong>di</strong> campagna ci ha<br />
permesso <strong>di</strong> definire appropriate leggi matematiche in grado <strong>di</strong><br />
descrivere le caratteristiche spaziali e <strong>di</strong>mensionali <strong>del</strong>le faglie<br />
nei grainstones carbonatici porosi, anche in relazione ai <strong>di</strong>versi<br />
meccanismi deformativi agenti durante la loro crescita e<br />
sviluppo.<br />
Fig. 8 – Mo<strong>del</strong>lo <strong>di</strong> crescita <strong>del</strong>le faglie nei grainstones carbonatici porosi. La<br />
struttura <strong>di</strong> taglio più semplice è rappresentata dalla singola compactive shear<br />
band che evolve a zone <strong>di</strong> compative shear bands e successivamente a faglia<br />
evoluta, con la formazione <strong>di</strong> una <strong>di</strong>scontinuità <strong>di</strong> taglio ben evidente associata<br />
a roccia <strong>di</strong> faglia. Ad ogni fase <strong>di</strong> crescita, le <strong>di</strong>verse strutture possono<br />
interagire tra loro con fenomeni <strong>di</strong> collegamento in corrispondenza <strong>di</strong> jogs<br />
estensionali e/o contrazionali.<br />
Growth mo<strong>del</strong> of faults in porous carbonate grainstones. The fundamental<br />
shear structure is represent<strong>ed</strong> by the compactive shear bands that evolves in<br />
zone of compactive shear bands and subsequently the formation of a well<br />
defin<strong>ed</strong> slip surface in well develop<strong>ed</strong> faults. At any growth stage they can<br />
interact and link to each other.<br />
BIBLIOGRAFIA<br />
AGOSTA F. & AYDIN A. (2006) - Architecture and deformation<br />
mechanism of a basin-boun<strong>di</strong>ng normal fault in Mesozoic<br />
platform carbonates, Central Italy. Journal of Structural<br />
Geology, 28, 1445-1467.<br />
ANTONELLINI M., TONDI E., AGOSTA F., AYDIN A. & CELLO G.<br />
(2008) - Failure modes in deep-water carbonates and their<br />
impact for fault development: Majella Mountain, Central<br />
Apennines, Italy. Marine and Petroleum Geology,<br />
DOI:10.1016/j.marpetgeo.2007. 10.008.<br />
TONDI E., ANTONELLINI M., AYDIN A., MARCHEGIANI L. &<br />
CELLO G. (2006) – The role of deformation bands and<br />
pressure solution seams in fault development in carbonate<br />
grainstones of the Majella Mountain, Italy. Journal of<br />
Structural Geology, 28, 376-391.<br />
TONDI E. (2007) – Nucleation, development and petrophysical<br />
properties of faults in carbonate grainstones: Evidence<br />
from the San Vito Lo Capo peninsula (Sicily, Italy). Journal<br />
of Structural Geology, 29, 614-628.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 225-226, 2 ff.<br />
RIASSUNTO<br />
Proprietà fisiche <strong>del</strong>le Evaporiti Triassiche da pozzi profon<strong>di</strong> e da<br />
affioramento<br />
Nell’Appennino Umbro Marchigiano i terremoti <strong>di</strong> maggiore intensità si<br />
enucleano all’interno <strong>del</strong>le Evaporiti Triassiche , ET. Conoscere le proprietà<br />
fisiche <strong>di</strong> queste rocce è <strong>di</strong> fondamentale importanza per poter interpretare<br />
correttamente i dati geofisici.<br />
In questo lavoro vengono mostrati i risultati ricavati dalle analisi <strong>di</strong><br />
campioni <strong>di</strong> ET provenienti sia <strong>di</strong> pozzi profon<strong>di</strong> che da affioramenti toscani.<br />
Le ET si presentano come una sequenza s<strong>ed</strong>imentaria <strong>di</strong> circa 1.5 - 2 km<br />
composta da alternanze <strong>di</strong> solfati e carbonati che hanno subito intensi processi<br />
<strong>di</strong>sgenetici e tettonici. Data la loro complessa storia, attualmente è possibile<br />
osservare ET in affioramento come alternanze <strong>di</strong> gesso e dolomia con rara<br />
anidrite mentre in profon<strong>di</strong>tà sono caratterizzate da alternanze <strong>di</strong> anidrite e<br />
dolomia. Sono state condotte misure <strong>di</strong> densità, porosità, velocità ra<strong>di</strong>ale e<br />
assiale fino a 100 MPa <strong>di</strong> pressione <strong>di</strong> confinamento su campioni<br />
rappresentativi <strong>di</strong> questa formazione. Sono state riscontrate densità maggiori<br />
nei campioni <strong>di</strong> pozzo (~2.79 g/cm 3 per le dolomie e ~2.95 g/cm 3 per le<br />
anidriti) rispetto a quelle <strong>di</strong> affioramento (~2.6 g/cm 3 per le dolomie, ~2.8<br />
g/cm 3 per le anidriti e ~2.3 g/cm 3 per il gesso), basse porosità,
226 F. TRIPPETTA ET ALII<br />
that from tomography or seismic reflection.<br />
Fig. 1 – Location map of the samples. Dots represent Roccastrada Quarry where all Outcrop samples (OA, for Outcrop Anhydrite; OG for Outcrop Gypsum;<br />
OD for Outcrop Dolostone) come from and Daniel1 and Donald1 wells where Borehole samples (BA for Boreholes Anhydrites and BD for Borehole Dolostone)<br />
come from. <strong>Note</strong> that at the surface TE are compos<strong>ed</strong> by gypsum, dolostones and rare anhydrites whilst at depth are compos<strong>ed</strong> by anhydrite and dolostones .<br />
Localizzazione <strong>del</strong>le zone <strong>di</strong> provenienza dei campioni. I pallini rappresentano la cava <strong>di</strong> Roccastrada dalla quale provengono tutti i campioni <strong>di</strong> affioramento<br />
(OA Anidrite <strong>di</strong> affioramento, OG Gesso da affioramento e OD dolomia da affioramento), e i due pozzi Daniel 1 e Donald 1 dai quali provengono i campioni <strong>di</strong><br />
pozzo (BA, Anidrite da pozzo e BD Dolomia da pozzo). È da notare come le ET in affioramento presentino gesso, dolomia e rara anidrite mentre in profon<strong>di</strong>tà<br />
sono caratterizzate da anidrite e dolomia.<br />
Fig. 2 –P-Wave velocity measur<strong>ed</strong> under hydrostatic stress con<strong>di</strong>tions for <strong>di</strong>fferent values of confining pressure Pc. A) Vp for sulphates samples: dots for<br />
borehole and rhombus (gypsum) and square (anhydrite) for outcrop sample, B) Vp for dolostone samples: dots for borehole and square for outcrop samples.<br />
Velocità <strong>del</strong>le onde P misurate a varie pressioni <strong>di</strong> confinamento. A) Vp nei solfati: i pallini rappresentano i campioni <strong>di</strong> pozzo mentre i rombi (gesso) e i<br />
quadrati (anidrite) rappresentano i campioni <strong>di</strong> affioramento. B) Vp nei campioni <strong>di</strong> dolomia: i pallini rappresentano i campioni <strong>di</strong> pozzo mentre quadrati<br />
rappresentano quelli <strong>di</strong> affioramento.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 227-229, 2 ff.<br />
Seismogenic sources in northeastern Italy and western Slovenia: an<br />
overview from the Database of In<strong>di</strong>vidual Seismogenic Sources<br />
(DISS 3.0.4)<br />
PAOLA VANNOLI (*), SALVATORE BARBA (*), ROBERTO BASILI (*), PIERFRANCESCO BURRATO (*), UMBERTO<br />
FRACASSI (*), VANJA KASTELIC (*), MARA MONICA TIBERTI (*) & GIANLUCA VALENSISE (*)<br />
RIASSUNTO<br />
Sguardo d’insieme sulle sorgenti sismogenetiche <strong>del</strong> nordest italiano e<br />
<strong>del</strong>la Slovenia occidentale dal Database <strong>del</strong>le Sorgenti Sismogenetiche<br />
In<strong>di</strong>viduali (DISS 3.0.4)<br />
Il Database <strong>del</strong>le Sorgenti Sismogenetiche In<strong>di</strong>viduali (DISS) contiene<br />
sorgenti capaci <strong>di</strong> generare terremoti con Mw > 5.5 e fornisce una visione<br />
sinottica <strong>del</strong>la sismogenesi in Italia e nelle aree limitrofe. In questo lavoro<br />
presentiamo un dettaglio sulle sorgenti <strong>del</strong>l’Italia <strong>del</strong> nord-est e <strong>del</strong>la Slovenia<br />
occidentale. I terremoti <strong>di</strong>struttivi nell’area veneto-friulana sono generati da<br />
faglie inverse lungo strutture N-NO-immergenti <strong>del</strong>la Catena Subalpina.<br />
Nell’area slovena sono invece generati da faglie trascorrenti destre che<br />
seguono i vecchi lineamenti <strong>di</strong>narici.<br />
Key words: NE Italy, Seismogenic Sources, Slovenia.<br />
We present an overview of the seismogenic sources of<br />
northeastern Italy and western Slovenia, includ<strong>ed</strong> in the latest<br />
version of the Database of In<strong>di</strong>vidual Seismogenic Sources<br />
(DISS, v. 3.0.4; DISS WORKING GROUP, 2007).<br />
DISS is a large repository of geologic, tectonic and active<br />
fault data on Italy and surroun<strong>di</strong>ng areas (Fig. 1) resulting from<br />
its authors’ first-hand experience and from a large amount of<br />
literature data (BASILI et alii, 2008).<br />
Fig. 1 – Screenshot of the web-page of the DISS, ttp://<strong>di</strong>ss.rm.ingv.it/<strong>di</strong>ss.<br />
_________________________<br />
(*) Istituto Nazionale <strong>di</strong> Geofisica e Vulcanologia,<br />
Via <strong>di</strong> Vigna Murata, 605 – 00143 Roma, Italy.<br />
DISS’ main object is the Seismogenic Source. DISS<br />
Seismogenic Sources are active faults capable of generating<br />
Mw > 5.5 earthquakes. We <strong>di</strong>stinguish two main categories of<br />
Seismogenic Sources (BASILI et alii, 2008):<br />
- “In<strong>di</strong>vidual Seismogenic Sources” are obtain<strong>ed</strong> from<br />
geological and geophysical data and are characteriz<strong>ed</strong> by a full<br />
set of geometric (strike, <strong>di</strong>p, length, width and depth),<br />
kinematic (rake) and seismological parameters (average<br />
<strong>di</strong>splacement, magnitude, slip rate, recurrence interval).<br />
In<strong>di</strong>vidual Seismogenic Sources are assum<strong>ed</strong> to exhibit<br />
“characteristic” behavior with respect to rupture length/width<br />
and expect<strong>ed</strong> magnitude. They are test<strong>ed</strong> against worldwide<br />
databases for internal consistence in terms of length, width,<br />
average <strong>di</strong>splacement and magnitude, and can be<br />
complement<strong>ed</strong> with information on fault scarps when present.<br />
This category of sources favors accuracy of the information<br />
suppli<strong>ed</strong> over completeness. As such, they can be us<strong>ed</strong> for<br />
deterministic assessment of seismic hazard, for calculating the<br />
probability of occurrences of strong earthquakes for the sources<br />
themselves (AKINCI et alii, 2008), for calculating earthquake<br />
and tsunami scenarios (LORITO et alii, 2008; TIBERTI et alii,<br />
2008), and for tectonic and geodynamic investigations (e.g.<br />
BURRATO & VALENSISE, 2008).<br />
- “Composite Seismogenic Sources” (also term<strong>ed</strong><br />
“Seismogenic Areas”) are obtain<strong>ed</strong> from geological and<br />
geophysical data and characteriz<strong>ed</strong> by geometric (strike, <strong>di</strong>p,<br />
width, depth) and kinematic (rake) parameters, but their length<br />
is more loosely defin<strong>ed</strong> and spans an unspecifi<strong>ed</strong> number of<br />
In<strong>di</strong>vidual Sources. They are not assum<strong>ed</strong> to be capable of a<br />
specific earthquake but their potential can be deriv<strong>ed</strong> from<br />
existing earthquake catalogues. A Composite Source is<br />
essentially identifi<strong>ed</strong> on the basis of regional surface and<br />
subsurface geological data. This category of sources favors<br />
completeness of the record of potential earthquake sources over<br />
accuracy of source description. In conjunction with seismicity<br />
and modern strain data, Composite Sources can thus be us<strong>ed</strong><br />
for regional probabilistic seismic hazard assessment and for<br />
investigating large-scale geodynamic processes (e.g. BARBA et<br />
alii, 2008; MELETTI et alii, 2008).
228 P. VANNOLI ET ALII<br />
Seismogenic sources are complement<strong>ed</strong> by:<br />
- Comments on critical issues, and summaries of publish<strong>ed</strong><br />
papers;<br />
- Sets of original figures, pictures, maps and sections taken<br />
from the literature or drawn by the compiler of the source;<br />
- A list of pertinent references.<br />
Fig. 2 – Overview of the seismogenic sources of northeastern Italy and western<br />
Slovenia includ<strong>ed</strong> in DISS 3.0.4. The In<strong>di</strong>vidual Seismogenic Sources are<br />
represent<strong>ed</strong> with yellow rectangles, Composite Seismogenic Sources with r<strong>ed</strong><br />
ribbons.<br />
DISS supplies a synoptic view of seismogenesis in<br />
northeastern Italy and western Slovenia (Fig. 2; BURRATO et<br />
alii, 2008). In the Veneto-Friuli area destructive earthquakes<br />
are generat<strong>ed</strong> by thrust faulting along N-NW<strong>di</strong>pping structures<br />
of the Eastern Southalpine Chain (ESC). Thrusting along the<br />
mountain front responds to about 2 mm/y of regional<br />
convergence, and it is associat<strong>ed</strong> with growing anticlines, tilt<strong>ed</strong><br />
and uplift<strong>ed</strong> Quaternary palaeolandsurfaces and forc<strong>ed</strong><br />
drainage anomalies. In western Slovenia, dextral strike-slip<br />
faulting along the NW-SE tren<strong>di</strong>ng structures of the Idrija fault<br />
system dominates the seismic release. Activity and style of<br />
faulting are defin<strong>ed</strong> by recent earthquakes (e.g. the Ms 5.7,<br />
1998 Bovec-Krn Mt. and the Mw 5.2, 2004 Kobarid<br />
earthquakes), whereas the relat<strong>ed</strong> recent morphotectonic<br />
imprint is still a debat<strong>ed</strong> matter.<br />
DISS supplies a segmentation mo<strong>del</strong> for the outermost ESC<br />
thrust front, and the association of the four major shocks of the<br />
1976 Friuli earthquake sequence with in<strong>di</strong>vidual segments of<br />
major thrust fronts. In western Slovenia DISS contains several<br />
Composite Sources that follow the main strike-slip faults (i.e.<br />
Rasa, Idrija, and Ravne faults), and two In<strong>di</strong>vidual Sources<br />
associat<strong>ed</strong> with the 1511 and 1998 earthquakes.<br />
This paper cannot substitute a complete in-depth visit of the<br />
DISS web site (http://<strong>di</strong>ss.rm.ingv.it/<strong>di</strong>ss).<br />
REFERENCES<br />
AKINCI A., PERKINS D., LOMBARDI A. M. & BASILI R. (2008) -<br />
Uncertainties in probability of occurrence of strong<br />
earthquakes for fault sources in the Apennines, Italy.<br />
Journal of Seismology. doi: 10.1007/s10950-008-9142-y.<br />
BARBA S., CARAFA M.M.C. & BOSCHI E. (2008) - Experimental<br />
evidence for mantle drag in the M<strong>ed</strong>iterranean. Geophys.<br />
Res. Lett., 35, L06302, doi:10.1029/2008GL033281.<br />
BASILI R., VALENSISE G., VANNOLI P., BURRATO P., FRACASSI<br />
U., MARIANO S., TIBERTI M.M. & BOSCHI E. (2008) - The<br />
Database of In<strong>di</strong>vidual Seismogenic Sources (DISS),<br />
version 3: summarizing 20 years of research on Italy's<br />
earthquake geology. Tectonophysics, 453, 20–43,<br />
doi:10.1016/j.tecto.2007.04.014.<br />
BURRATO P., POLI M.E., VANNOLI P., ZANFERRARI A., BASILI<br />
R. & GALADINI F. (2008) - Sources of Mw 5+ earthquakes<br />
in northeastern Italy and western Slovenia: An updat<strong>ed</strong><br />
view bas<strong>ed</strong> on geological and seismological evidence.<br />
Tectonophysics, 453, 157-176, doi: 10.1016/j.tecto.<br />
2007.07.009.<br />
BURRATO P. & VALENSISE G. (2008) - Rise and fall of a<br />
hypothesiz<strong>ed</strong> seismic gap: source complexity in the 16<br />
December 1857, Southern Italy earthquake (Mw 7.0). Bull.<br />
Seism. Soc. Am., 98 (1), 139-148, doi:<br />
10.1785/0120070094.<br />
DISS WORKING GROUP (2007) - Database of In<strong>di</strong>vidual<br />
Seismogenic Sources (version 3.0.4): A compilation of<br />
potential sources for earthquakes larger than M 5.5 in Italy<br />
and surroun<strong>di</strong>ng areas. Available at:<br />
http://<strong>di</strong>ss.rm.ingv.it/<strong>di</strong>ss.<br />
LORITO S., TIBERTI M.M., BASILI R., PIATANESI A. &<br />
VALENSISE G. (2008) - Earthquake-generat<strong>ed</strong> tsunamis<br />
in the M<strong>ed</strong>iterranean Sea: scenarios of potential threats to<br />
Southern Italy. Journal of Geophysical Research, 113,<br />
B01301, doi:10.1029/2007JB004943.<br />
MELETTI C., GALADINI F., VALENSISE G., STUCCHI M., BASILI<br />
R., BARBA S., VANNUCCI G. & BOSCHI E. (2008) - A seismic<br />
source zone mo<strong>del</strong> for the seismic hazard assessment of the<br />
Italian territory, Tectonophysics, 450 (1-4), 85-108,<br />
doi:10.1016/j.tecto.2008.01.003.<br />
TIBERTI M.M., LORITO S., BASILI R., KASTELIC V., PIATANESI<br />
A. & VALENSISE G. (2008) - Scenarios of earthquakegenerat<strong>ed</strong><br />
tsunamis in the Adriatic Sea. P. Cummins, L.<br />
Kong and K. Satake (Eds): Topical Issue on Tsunamis. Pure<br />
and Appli<strong>ed</strong> Geophysics, doi: 10.1007/s00024-008-0417-6.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 229-231, 1 f.<br />
P-T con<strong>di</strong>tions of mylonitic shearing in the Granite Harbour<br />
Intrusive complex of the Wilson Terrane, Deep Freeze Range,<br />
northern Victoria Land, Antarctica<br />
GIANLUCA VIGNAROLI (*), FEDERICO ROSSETTI (*), THOMAS THEYE (**) & FABRIZIO BALSAMO (*)<br />
RIASSUNTO<br />
Con<strong>di</strong>zioni P-T <strong>del</strong>la deformazione milonitica nel complesso “Granite<br />
Harbour Intrusive” <strong>del</strong> Wilson Terrane, Deep Freeze Range, Terra<br />
Vittoria settentrionale, Antartide.<br />
Il “Granite Harbour Intrusive” (GHI) rappresenta un complesso <strong>di</strong> rocce<br />
intrusive <strong>di</strong> età Cambro-Ordoviciana associato alle sequenze metamorfiche (<strong>di</strong><br />
m<strong>ed</strong>io e alto grado) che caratterizzano il sistema orogenico <strong>di</strong> Ross nel Deep<br />
Freeze Range (Wilson Terrane). Il complesso GHI risulta essere interessato da<br />
una serie <strong>di</strong> zone <strong>di</strong> taglio milonitiche ad andamento NW-SE. Uno stu<strong>di</strong>o<br />
tessiturale-petrologico <strong>di</strong> dettaglio ha permesso (i) <strong>di</strong> in<strong>di</strong>viduare le paragenesi<br />
mineralogiche correlate alle zone <strong>di</strong> taglio, e (ii) <strong>di</strong> stimare le con<strong>di</strong>zioni <strong>di</strong><br />
pressione e temperatura <strong>di</strong> formazione <strong>del</strong> fabric milonitico nella facies<br />
anfibolitica. Questi risultati, integrati con prec<strong>ed</strong>enti lavori <strong>di</strong> geocronologia,<br />
suggeriscono <strong>di</strong> attribuire lo sviluppo <strong>del</strong>le zone milonitiche alle fasi finali <strong>del</strong><br />
ciclo orogenico <strong>di</strong> Ross descritte altrove per il Wilson Terrane.<br />
Key words: Granite Harbour Intrusive, metamorphic<br />
petrology, pseudosection, textural analysis, Wilson Terrane.<br />
An integrat<strong>ed</strong> structural and petrological study has been<br />
address<strong>ed</strong> to examine a set of ductile shear zones cutting<br />
through the Cambrian-Ordovician Granite Harbour Intrusive<br />
(GHI) rocks of the Wilson Terrane expos<strong>ed</strong> in the Deep Freeze<br />
Range (northern Victoria Land, Antarctica).<br />
The Wilson Terrane constitutes (together with the Bowers<br />
and the Robertson Bay terranes; GANOVEX TEAM, 1987 and<br />
references therein), part of the Neoproterozoic to Early<br />
Paleozoic in age Ross-Delamerian Orogeny in northern<br />
Victoria Land (Fig. 1a,b) (e.g. KLEINSCHMIDT & TESSENSOHN,<br />
1987; FLÖTTMAN, 1993). There is increasing consensus in<br />
considering the terrane mo<strong>del</strong> of north Victoria Land as a fossil<br />
arc-trench system link<strong>ed</strong> to a westward-<strong>di</strong>rect<strong>ed</strong> subduction<br />
system at the paleo-Pacific active margin of Gondwana (e.g.<br />
RICCI et alii, 1997; FEDERICO et alii, 2006).<br />
In the Deep Freeze Range, the Ross–Delamerian orogenic<br />
cycle is typifi<strong>ed</strong> by regional amphibolite grade metamorphism<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze Geologiche, Università degli Stu<strong>di</strong> “Roma<br />
Tre”, Lg. S.L. Murialdo, 1 - 00146 Roma<br />
(**) Institut für Mineralogie der Universität, Azenbergstr. 18 - 70174,<br />
Stuttgart, Germany<br />
This work was support<strong>ed</strong> by the Italian Antarctic Research Program (PNRA<br />
project coor<strong>di</strong>nat<strong>ed</strong> by S. Rocchi).<br />
and the emplacement of the GHI complex (e.g. CARMIGNANI et<br />
alii, 1988; TALARICO et alii, 2004). In the metamorphic<br />
sequences, variably retrogress<strong>ed</strong> granulitic facies rocks and<br />
associat<strong>ed</strong> migmatites have been describ<strong>ed</strong> (e.g. CASTELLI et<br />
alii, 1991; 1994; PALMERI et alii, 1991; 1994; TALARICO &<br />
CASTELLI, 1995; PALMERI, 1997; TALARICO et alii, 2004). The<br />
GHI complex dominantly consists of a huge exposure of<br />
foliat<strong>ed</strong> and non-foliat<strong>ed</strong> intrusions, sandwich<strong>ed</strong> in-between the<br />
high- and the m<strong>ed</strong>ium-grade metamorphic rocks. Emplacement<br />
of the GHI is consider<strong>ed</strong> to be syn-to-post-tectonic with respect<br />
to development of the Ross ag<strong>ed</strong> structures (CARMIGNANI et<br />
alii, 1988; PALMERI et alii, 1991; MUSUMECI & PERTUSATI,<br />
2000). The age of emplacement event spans from ca.<br />
530 to 480 Ma (ARMIENTI et alii, 1990; TONARINI & ROCCHI,<br />
1994; DI VINCENZO & ROCCHI, 1999; BOMPAROLA et alii,<br />
2006). The lower age boundary is commonly consider<strong>ed</strong> as<br />
in<strong>di</strong>cative of the cessation of tectonic assembly in the Deep<br />
Freeze Range, although reactivation of some shear zones at<br />
410-440 Ma has been recently document<strong>ed</strong> (DI VINCENZO et<br />
alii, 2007).<br />
During the 2005-2006 field campaign, a set of ductile shear<br />
zones cutting the GHI complex of the Deep Freeze Range area<br />
has been recognis<strong>ed</strong>. They are arrang<strong>ed</strong> to form subparallel,<br />
NW-SE tren<strong>di</strong>ng belts of high shear strain. The gradual change<br />
from host granitic rocks to shear zone is invariably associat<strong>ed</strong><br />
to intensely foliat<strong>ed</strong> (solid-state) rocks and mark<strong>ed</strong> by intense<br />
grain size r<strong>ed</strong>uction, with the progressive transition from<br />
protomylonites to ultramylonites. Solid-state shear fabric<br />
systematically overprints the magmatic foliation and is<br />
commonly associat<strong>ed</strong> with NE-SW tren<strong>di</strong>ng stretching<br />
lineations provid<strong>ed</strong> by the alignment of quartz, biotite and<br />
plagioclase + sillimanite + phengite composite associations.<br />
The kinematics in<strong>di</strong>cators invariably in<strong>di</strong>cate top-to-the-NE<br />
sense of shear (Fig. 1c).<br />
At the micro-scale, two <strong>di</strong>stinctive textural domains can be<br />
recognis<strong>ed</strong>: (i) an early, pre-kinematic assemblage, which we<br />
refer to a magmatic fabric, and (ii) a transpositive mylonitic<br />
fabric, syn-kinematic with respect to the main deformational<br />
fabric. Porphyroclasts of K-feldspar, Ca-rich plagioclase and<br />
garnet constitute the previous magmatic assemblage. Magmatic<br />
garnet has <strong>di</strong>mensions up to one centimetre, shows a general<br />
round<strong>ed</strong> shape and is alman<strong>di</strong>ne rich with nearly flat chemical<br />
profiles (core-to-rim: Alm83-82Grs3-2Prp10-12Sps4-4). K-feldspar<br />
and Ca-rich plagioclase appear with irregular shapes and
230 G. VIGNAROLI ET ALII<br />
wrapp<strong>ed</strong> by the external mylonitic foliation. Myrmekite<br />
structures commonly occur at the boundary of feldspars. All<br />
porphyroclasts are wrapp<strong>ed</strong> by the mylonitic fabric<br />
characteris<strong>ed</strong> by biotite, phengite, albite and sillimanite within<br />
a matrix of recrystallis<strong>ed</strong> ribbon quartz aggregates. Euh<strong>ed</strong>ral<br />
garnet crystals up to 1 millimetre in <strong>di</strong>ameter form<br />
porphyroblasts within the shear zone assemblage. This garnet<br />
type is also alman<strong>di</strong>ne-rich, showing a bell-shap<strong>ed</strong> Mn profile<br />
moving from core to rim (from 10-12 mol % to 12-19 mol %),<br />
in conjunction with a slight depletion in the pyrope (from 5-7<br />
mol % to 3-5 mol %) and alman<strong>di</strong>ne (from 80-82 to 75-82 mol<br />
Fig. 1 – (a) The Ross Orogen in Antarctica; (b) sketch map showing the<br />
<strong>di</strong>fferent terranes in northern Victoria Land; (c) structural sketch map of the<br />
Deep Freeze Range area (after Musumeci & Pertusati, 2000, mo<strong>di</strong>fi<strong>ed</strong> and<br />
r<strong>ed</strong>rawn) illustrating localisation of the stu<strong>di</strong><strong>ed</strong> ductile shear zones in GHI and<br />
the associat<strong>ed</strong> sense of shear (black arrows; hanging wall movement).<br />
%) components. Accessory minerals consist of zircon,<br />
monazite, allanite, xenotime and rutile. Monazite and xenotime<br />
are typically align<strong>ed</strong> along the mylonitic foliation.<br />
Metamorphic con<strong>di</strong>tions of shearing have been estimat<strong>ed</strong> by<br />
comparing inverse (thermobarometric estimates) and forward<br />
(P-T pseudosections) mo<strong>del</strong>ing techniques. Thermobarometric<br />
estimates integrate conventional thermo-barometry and results<br />
from thermodynamic softwares: THERMOCALC 3.26 of<br />
HOLLAND and POWELL (1998), in the form of the July 2006<br />
update, and TWEEQ 2.34 of BERMAN (1991), in the updat<strong>ed</strong><br />
version of the BERMAN & ARANOVICH (1996) thermodynamic<br />
database. Phase relationships were mo<strong>del</strong><strong>ed</strong> in the<br />
MnNCKFMASHT system by using the PERPLE_X program<br />
(CONNOLLY, 1990). The integration of results allow depicting a<br />
retrogressive metamorphic evolution during a nearly isobaric<br />
cooling from anhydrous peak con<strong>di</strong>tions (P>8.4 kbar and<br />
T>720° C) towards the activation of the fluid-assist<strong>ed</strong> ductile<br />
shearing at amphibolite facies con<strong>di</strong>tions (ca. 4 kbar and 620°<br />
C).<br />
We attribute the document<strong>ed</strong> syn-amphibolitic shearing<br />
deformation in the GHI complex to the waning stages of the<br />
Ross orogenic cycle in the region. These structures correlate<br />
with the ones describ<strong>ed</strong> along the Pacific coast of north<br />
Victoria Land by LÄUFER & ROSSETTI (2003) and argue for<br />
processes active during the final tectonic assembly of the<br />
metamorphic and igneous rocks of the Wilson Terrane.<br />
REFERENCES<br />
ARMIENTI P., GHEZZO C., INNOCENTI F., MANETTI P., ROCCHI<br />
S., TONARINI S. (1990) - Isotope geochemistry of granitoids<br />
suites from Granite Harbor Intrusives of the Wilson<br />
Terrane, northern Victoria Land, Antartica. Eur. J.<br />
Mineral., 2, 103-123.<br />
BERMAN R.G. (1991) - Thermobarometry using<br />
multiequilibrium calculations: a new technique with<br />
petrologic applications. Can. Mineral., 29, 833-855.<br />
BERMAN R.G. & ARANOVICH L.Y. (1996) - Optimiz<strong>ed</strong><br />
standard state and solution properties of minerals I. Mo<strong>del</strong><br />
calibration for olivine, orthopyroxene, cor<strong>di</strong>erite, garnet,<br />
and ilmenite in the system FeO-MgO-CaO-Al2O3-TiO2-<br />
SiO2. Contr. Mineral. Petrol., 126, 1-24.<br />
BOMPAROLA R.M., BELOUSOVA E., GHEZZO C., GRIFFIN W.L.,<br />
O’REILLY Y.O. (2006) - Resetting of the U–Pb zircon<br />
system in Cambro-Ordovician intrusives of the Deep Freeze<br />
Range, northern Victoria Land, Antarctica. J. Petrol., 48,<br />
327-364.<br />
CARMIGNANI, L., GHEZZO C., GOSSO G., LOMBARDO B.,<br />
MECCHERI M., MONTRASIO A., PERTUSATI P.C., SALVINI F.<br />
(1988) - Geology of the Wilson Terrane in the area between<br />
David and Mariner Glaciers, Victoria Land (Antartica).<br />
Mem. Soc. Geol. It., 33, 77-97.<br />
CASTELLI D., LOMBARDO B., OGGIANO G., ROSSETTI P.,<br />
TALARICO F. (1991) - Granulite facies rocks of the Wilson<br />
Terrane (northern Victoria Land): Campbell glacier. Mem.<br />
Soc. Geol. It., 46, 197-204.<br />
CASTELLI D., OGGIANO G., SCAMBELLURI M. AND TALARICO F.<br />
(1994) — Peak metamorphic con<strong>di</strong>tions and retrograde P-<br />
T paths in the Wilson Terrane and the Dessent Unit<br />
(northern Victoria Land, Antarctica): new constraints to<br />
tectonic mo<strong>del</strong> for the Wilson Terrane and the Wilson<br />
Terrane - Bowers Terrane boundary. Terra Antartica, 1, 51-<br />
53.<br />
CONNOLLY J.A.D. (1990) - Multivariable phase <strong>di</strong>agrams: an<br />
algorithm bas<strong>ed</strong> on generaliz<strong>ed</strong> thermodynamics. Am. J.<br />
Sci., 290, 666-718.<br />
DI VINCENZO G., CAROSI R., PALMERI R., TIEPOLO M. (2007) -<br />
A comparative U–Th–Pb (zircon–monazite) and 40Ar–39Ar
(muscovite–biotite) study of shear zones in northern<br />
Victoria Land (Antarctica): implications for geochronology<br />
and localiz<strong>ed</strong> reworking of the Ross Orogen. J. Metamorph.<br />
Geol., 25, 605-630.<br />
DI VINCENZO G. & ROCCHI S. (1999) - Origin and interaction<br />
of mafic and felsic magmas in an evolving late orogenic<br />
setting: the Early Paleozoic Terra Nova Intrusive Complex,<br />
Antarctica. Contr. Mineral. Petrol., 137, 15-35.<br />
FEDERICO L., CAPPONI G., CRISPINI L. (2006) - The Ross<br />
orogeny of the transantarctic mountains: a northern<br />
Victoria Land perspective. Int. J. Earth Sci., 95, 759-770.<br />
FLÖTTMANN T., GIBSON G.M., KLEINSCHMIDT G. (1993) -<br />
Structural continuity of the Ross and Delamerian orogens<br />
of Antarctica and Australia along the margin of the paleo-<br />
Pacific. Geology, 21, 319-322.<br />
GANOVEX TEAM (1987) - Geological Map of northern Victoria<br />
Land, Antarctica, 1/500.000. Explanatory notes, Geol.<br />
Jahrb., B 66, 7-79.<br />
HOLLAND T.J.B. & POWELL R. (1998) - An internally consistent<br />
thermodynamic data set for phases of petrological interest.<br />
J. Metamorph. Geol., 16, 309-343.<br />
KLEINSCHMIDT G. & TESSENSOHN F. (1987) - Early Paleozoic<br />
westward <strong>di</strong>rect<strong>ed</strong> subduction at the Pacific margin of<br />
Antarctica. In: McKenzie G.D. (Ed.) - Gondwana Six:<br />
structure, tectonics and geophysics. Amer. Geophys. Union,<br />
Geophys. Monogr. Series, 40, 89-105.<br />
LÄUFER A.L. & Rossetti F. (2003) - Late-Ross ductile<br />
deformation features in the Wilson Terrane of northern<br />
Victoria Land (Antarctica) and their implications for the<br />
western front of the Ross Orogen. Terra Antarctica, 10,<br />
179-196.<br />
MUSUMECI G. & PERTUSATI P. (2000) - Structure of the Deep<br />
Freeze Range-Eisenhower Range of the Wilson Terrane<br />
(North Victoria Land, Antarctica): emplacement of<br />
magmatic intrusions in the early Palaeozoic deform<strong>ed</strong><br />
margin of the East Antarctic Craton. Antarctic Sci., 12, 89-<br />
104 .<br />
PALMERI R. (1997) - P-T paths and migmatite formation: an<br />
example from Deep Freeze Range, northern Victoria Land,<br />
Antarctica. Lithos, 42, 47-66.<br />
PALMERI R., PERTUSATI P.C., RICCI C.A., TALARICO F. (1994) -<br />
Late Proterozoic (?)-Early Paleozoic evolution of the active<br />
pacific margin of Gondwana: evidence from the southern<br />
Wilson Terrane (northern Victoria Land, Antarctica). Terra<br />
Antarctica, 1, 5-9.<br />
PALMERI R., TALARICO F., MECCHERI M., OGGIANO G.,<br />
PERTUSATI P.C., RASTELLI N., RICCI C.A. (1991) -<br />
Progressive deformation and Low Pressure/High<br />
Temperature metamorphism in the Deep Freeze Range,<br />
Wilson Terrane, northern Victoria Land, Antarctica. Mem.<br />
Soc. Geol. It, 46, 179-195.<br />
SHEAR ZONES IN THE GHI, WILSON TERRANE<br />
231<br />
RICCI C.A., TALARICO F., PALMERI R. (1997) - Tectonothermal<br />
evolution of the Antarctic Paleo-Pacific margin of<br />
Gondwana: a northern Victoria Land perspective. In: Ricci<br />
C.A. (Ed.) - The Antarctic region: geological evolution and<br />
Processes. Terra Antartica Publication, Siena, 213-218.<br />
TALARICO F.M., PALMERI R., RICCI C.A. (2004) - Regional<br />
metamorphism and P-T evolution of the Ross Orogen in<br />
northern Victoria Land (Antartica): a review. Period.<br />
Mineral., 73, 185-196.<br />
TALARICO F. & CASTELLI D. (1995) - Relict granulites in the<br />
Ross orogen of northern Victoria Land (Antarctica), I.<br />
Field occurrence, petrography and metamorphic evolution.<br />
Precambr. Res., 75, 141-156.<br />
TONARINI S. & ROCCHI S. (1994) - Geochronology of Cambro-<br />
Ordovician intrusives in northern Victoria Land; a review.<br />
Terra Antartica, 1, 46-50.<br />
F. R. and F. B. wish to thank all the colleagues that shar<strong>ed</strong> the field<br />
activity during the 2005-2006 exp<strong>ed</strong>ition, for their friendly support<br />
and useful advice in the field. Advice and support by F. Storti is also<br />
acknowl<strong>ed</strong>g<strong>ed</strong>. F. B. and G. V. acknowl<strong>ed</strong>ge a PNRA fellowship. We<br />
gratefully acknowl<strong>ed</strong>ge all the personnel of the Mario Zucchelli base<br />
and the exp<strong>ed</strong>ition leader A. Della Rovere for the logistical support<br />
during the field activity.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 232-234, 4 ff.<br />
Ruolo <strong>del</strong>lo strain hardening nell’evoluzione <strong>del</strong>le zone <strong>di</strong> taglio: da<br />
deformazione omogenea ad eterogenea<br />
ABSTRACT<br />
Role of strain hardening in shear zone evolution: homogeneous to<br />
heterogeneous deformation<br />
In this paper a mathematical mo<strong>del</strong> is present<strong>ed</strong> describing the evolution<br />
of a homogeneous shear zone to a heterogeneous one as a result of strain<br />
hardening affecting its margins. The mo<strong>del</strong> is characteriz<strong>ed</strong> by (2N-1) layers,<br />
each of them being homogeneously deform<strong>ed</strong> in a <strong>di</strong>fferent manner due to<br />
strain hardening describ<strong>ed</strong> by an exponential law. Different parameters – such<br />
as elongation along the axes of the reference frame or shear strain hardening<br />
rate – are vari<strong>ed</strong>, producing <strong>di</strong>fferent finite effective shear strain profiles across<br />
the shear zone. This mo<strong>del</strong> can effectively describe an exhuming asymmetric<br />
shear zone in the footwall to an extensional detachment.<br />
Key words: shear strain, shear zones, tectonic exhumation.<br />
INTRODUZIONE<br />
Le zone <strong>di</strong> taglio duttili sono strutture molto comuni<br />
all’interno <strong>del</strong>la litosfera e sono luogo <strong>di</strong> accumulo e<br />
concentrazione <strong>del</strong>la deformazione. Le prime schematizzazioni<br />
matematiche <strong>di</strong> queste strutture hanno fornito mo<strong>del</strong>li che<br />
descrivono zone <strong>di</strong> taglio caratterizzate da deformazioni<br />
omogenee come taglio semplice, con o senza variazioni <strong>di</strong><br />
volume (RAMSAY & GRAHAM, 1970), o dall’azione simultanea<br />
<strong>di</strong> taglio semplice, taglio puro e variazione <strong>di</strong> volume (FOSSEN<br />
& TIKOFF, 1993). Tuttavia, la maggior parte <strong>del</strong>le zone <strong>di</strong> taglio<br />
duttili naturali è caratterizzata da deformazione eterogenea che<br />
può essere definita dall’andamento sigmoidale <strong>del</strong>la foliazione<br />
o dalla variazione lungo la zona <strong>di</strong> taglio <strong>di</strong> alcuni parametri<br />
deformativi come lo shear strain o lo strain finito (VITALE et<br />
alii, 2007; VITALE & MAZZOLI, 2009).<br />
Alcuni Autori hanno messo in relazione l’eterogeneità <strong>del</strong>la<br />
deformazione con la <strong>di</strong>versa evoluzione temporale <strong>del</strong>la stessa<br />
all’interno <strong>del</strong>la zona <strong>di</strong> taglio. Tale variazione temporale si<br />
esprime attraverso il cambiamento <strong>del</strong>lo spessore durante lo<br />
sviluppo <strong>del</strong>la zona <strong>di</strong> taglio e la variazione, ad esempio, <strong>del</strong>lo<br />
shear strain (MEANS, 1995; VITALE & MAZZOLI, 2008). In base<br />
_________________________<br />
(*)Università <strong>di</strong> Napoli “F<strong>ed</strong>erico II”, Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra,<br />
Largo San Marcellino 10, 80138, Napoli<br />
STEFANO VITALE (*) & STEFANO MAZZOLI (*)<br />
a questa caratteristica, HULL (1988) e MEANS (1995) hanno<br />
sud<strong>di</strong>viso queste strutture in tre gruppi (Fig. 1): le zone <strong>di</strong><br />
taglio <strong>di</strong> tipo I e II sono caratterizzate, rispettivamente, da<br />
spessori che aumentano o <strong>di</strong>minuiscono nel tempo e profili<br />
<strong>del</strong>lo shear strain appiattiti (tipo I) e appuntiti (tipo II), mentre<br />
le zone <strong>di</strong> taglio <strong>di</strong> tipo III mantengono spessore costante<br />
durante il loro sviluppo e mostrano profili <strong>del</strong>lo shear strain<br />
Fig. 1 – classificazione <strong>del</strong>le zone <strong>di</strong> taglio in base alla variazione <strong>del</strong>lo<br />
spessore nel tempo.<br />
uniformi. MEANS (1995) ipotizza, inoltre, che a guidare<br />
l’evoluzione dei tipi I e II siano, rispettivamente, i processi <strong>di</strong><br />
strain hardening e softening.<br />
VITALE & MAZZOLI (2008), riprendendo quest’ultima<br />
ipotesi, hanno mo<strong>del</strong>lizzato matematicamente l’evoluzione<br />
<strong>del</strong>le zone <strong>di</strong> taglio <strong>di</strong> tipo I e II. Il mo<strong>del</strong>lo <strong>del</strong> tipo I è<br />
caratterizzato da un singolo livello deformato omogeneamente<br />
sottoposto a strain hardening che nel corso <strong>del</strong> tempo include<br />
nuovi livelli lungo i margini, aumentando lo spessore <strong>del</strong>la<br />
zona <strong>di</strong> taglio, mentre il mo<strong>del</strong>lo <strong>del</strong> tipo II è descritto da una<br />
zona <strong>di</strong> taglio omogenea che, sottoposta a strain softening<br />
nella parte centrale, esclude i margini preesistenti, <strong>di</strong>minuendo<br />
lo spessore <strong>del</strong>la zona <strong>di</strong> taglio. In questa mo<strong>del</strong>lizzazione,<br />
<strong>di</strong>versamente da MEANS (1995), la presenza <strong>di</strong> una componente<br />
<strong>di</strong> taglio puro o <strong>di</strong> variazione <strong>di</strong> volume può far <strong>di</strong>minuire o<br />
aumentare lo spessore in<strong>di</strong>pendentemente dall’inclusione <strong>di</strong><br />
nuova roccia (tipo I) o esclusione dei vecchi margini (tipo II).<br />
Scopo <strong>di</strong> questo lavoro è applicare la stessa metodologia <strong>di</strong><br />
mo<strong>del</strong>lizzazione alle zone <strong>di</strong> taglio <strong>di</strong> tipo III, ovvero<br />
analizzare l’evoluzione <strong>di</strong> una zona <strong>di</strong> taglio da omogenea ad<br />
eterogenea attraverso l’hardening dei suoi margini e stu<strong>di</strong>are<br />
come i parametri deformativi principali variano attraverso la<br />
zona <strong>di</strong> taglio.
Fig. 2 –schema che illustra la mo<strong>del</strong>lizzazione <strong>di</strong> una zona <strong>di</strong> taglio<br />
eterogenea <strong>di</strong> tipo III controllata da shear strain hardening.<br />
RUOLO DELLO STRAIN HARDENING NELL’EVOLUZIONE DELLE ZONE DI TAGLIO<br />
MODELLIZZAZIONE<br />
Consideriamo una zona <strong>di</strong> taglio caratterizzata da<br />
deformazione omogenea che nel tempo <strong>di</strong>minuisce la sua<br />
intensità lungo i margini (strain hardening). Al tempo n = 1<br />
(Fig. 2) la zona <strong>di</strong> taglio omogenea è formata da (2N-1) livelli,<br />
ognuno deformato in maniera omogenea; negli sta<strong>di</strong> successivi<br />
(n = 2, 3, … , N) i livelli posti ai margini sono interessati da<br />
strain hardening, mentre quelli posti nella parte centrale<br />
continuano a deformarsi con la stessa intensità. Il livello jesimo<br />
sarà dunque caratterizzato da una deformazione finale<br />
omogenea espressa dalla matrice A*(j,n) che sarà il risultato <strong>del</strong><br />
prodotto <strong>del</strong>le matrici <strong>del</strong>la deformazione incrementale (Fig. 2):<br />
con ;<br />
dove k1, k2 e k3 sono le elongazioni lungo gli assi x, y e z <strong>del</strong><br />
sistema <strong>di</strong> riferimento solidale alla zona <strong>di</strong> taglio (asse x<br />
parallelo alla <strong>di</strong>rezione <strong>di</strong> taglio e z ortogonale ai piani <strong>di</strong><br />
taglio), γ è lo shear strain, Γhard è lo shear strain effettivo, ∆ è<br />
la variazione <strong>di</strong> volume e j è un in<strong>di</strong>ce che in<strong>di</strong>ca il livello<br />
deformato e varia tra 1 e n. La con<strong>di</strong>zione per avere shear<br />
strain hardening è che lo shear strain <strong>di</strong>minuisca nel tempo<br />
ovvero γ(i+1) < γ(i). Le matrici che esprimono una<br />
deformazione incrementale descrivono un general strain<br />
risultante dall’azione simultanea <strong>di</strong> taglio semplice, taglio puro<br />
e variazione <strong>di</strong> volume (FOSSEN & TIKOFF, 1993).<br />
Al fine <strong>di</strong> simulare lo shear strain hardening, è stata usata<br />
una funzione esponenziale negativa:<br />
con α
234 S. VITALE & S. MAZZOLI<br />
deformazione eterogenea può essere mo<strong>del</strong>lizzata assumendo<br />
che l’eterogeneità <strong>del</strong>la deformazione <strong>ed</strong> eventuali variazioni<br />
<strong>del</strong>lo spessore siano da attribuire alla presenza <strong>di</strong> shear strain<br />
hardening lungo i margini, con o senza taglio puro e variazione<br />
<strong>di</strong> volume simultanei. L’approccio matematico proposto in<br />
questo lavoro permette, inoltre, <strong>di</strong> ottenere profili <strong>del</strong>lo shear<br />
strain effettivo finito e <strong>di</strong> altri parametri come lo strain finito o<br />
il <strong>di</strong>splacement che possono essere confrontati con i dati <strong>di</strong><br />
zone <strong>di</strong> taglio eterogenee naturali.<br />
Il mo<strong>del</strong>lo proposto <strong>di</strong> evoluzione <strong>di</strong> una zona <strong>di</strong> taglio<br />
Fig. 4 – esempio <strong>di</strong> zona <strong>di</strong> taglio omogenea che evolve ad eterogenea a<br />
letto <strong>di</strong> un detachment estensionale.<br />
omogenea in una eterogenea attraverso l’hardening dei margini<br />
potrebbe efficacemente descrivere, ad esempio, l’evoluzione <strong>di</strong><br />
una zona <strong>di</strong> taglio posta a letto <strong>di</strong> un detachment estensionale<br />
(Fig. 4a) che, durante l’esumazione tettonica <strong>del</strong> blocco <strong>di</strong> letto,<br />
mette a contatto il margine superiore <strong>del</strong>la zona <strong>di</strong> taglio con<br />
rocce sempre più fr<strong>ed</strong>de innescando l’hardening <strong>del</strong>le rocce<br />
sottostanti la faglia (Fig. 4b) e la creazione <strong>di</strong> una zona <strong>di</strong> taglio<br />
eterogenea asimmetrica (Fig. 4c).<br />
REFERENCES<br />
FOSSEN H. & TIKOFF B. (1993) - The deformation matrix for<br />
simultaneous simple shearing, pure shearing and volume<br />
change, and its application to transpression-transtension<br />
tectonics. Journal of Structural Geology 15, 413-422.<br />
HULL J. (1988) - Thickness–<strong>di</strong>splacement relationships for<br />
deformation zones. Journal of Structural Geology 10, 431–<br />
435.<br />
MEANS W.D. (1995) - Shear zones and rock history.<br />
Tectonophysics 247, 157-160.<br />
RAMSAY J.G. & GRAHAM R.H. (1970) - Strain variation in<br />
shear belts. Cana<strong>di</strong>an Journal of Earth Sciences 7, 786-813.<br />
VITALE S., WHITE J.C., IANNACE A. & MAZZOLI S. (2007) -<br />
Ductile strain partitioning in micritic limestones, Calabria,<br />
Italy: the roles and mechanisms of intracrystalline and<br />
intercrystalline deformation. Cana<strong>di</strong>an Journal of Earth<br />
Sciences 44, 1587–1602.<br />
VITALE S. & MAZZOLI S. (2008) - Heterogeneous shear zone<br />
evolution: the role of shear strain hardening/softening.<br />
Journal of Structural Geology, 30, 1363-1395,<br />
doi:10.1016/j.jsg.2008.07.006<br />
VITALE S. & MAZZOLI S. (2009) - Finite strain analysis of a<br />
natural ductile shear zone in limestones: insights into 3-D<br />
coaxial vs. non-coaxial deformation partitioning. Journal of<br />
Structural Geology, 31, doi: 10.1016/j.jsg.2008.10.011
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 2 (2008), 235-237, 4 ff.<br />
Analisi strutturale <strong>del</strong>l’Unità Parasicilide affiorante nell’area <strong>di</strong><br />
Castelnuovo Cilento (Campania)<br />
STEFANO VITALE (*), FABIO LAIENA (*), ANGELO NOVIELLO (*), SABATINO CIARCIA (*), STEFANO MAZZOLI (*)<br />
& MARIO TORRE (*)<br />
ABSTRACT<br />
Structural analysis of the Parasicilide Unit, Castelnuovo Cilento area,<br />
Campania region, Italy<br />
This paper is focus<strong>ed</strong> on the structural analysis of the Parasicilide Unit<br />
basin succession cropping out in the Cilento area (southern Apennines), where<br />
it is expos<strong>ed</strong> in the footwall to the Nord-Calabrese Unit. The succession is<br />
characteriz<strong>ed</strong> by superpos<strong>ed</strong> deformation relat<strong>ed</strong> to three fol<strong>di</strong>ng events. The<br />
first produc<strong>ed</strong> tight to isoclinal folds associat<strong>ed</strong> with dominant NW-SE<br />
shortening resulting in regional, SE verging recumbent fol<strong>di</strong>ng accompani<strong>ed</strong> by<br />
the development of parasitic folds. The second deformation stage,<br />
characteriz<strong>ed</strong> by roughly E-W shortening, produc<strong>ed</strong> open to tight mesoscopic<br />
folds. The third deformation episode was characteriz<strong>ed</strong> by vertical shortening,<br />
which l<strong>ed</strong> to the development of open folds with sub-horizontal axial planes.<br />
The first and second fol<strong>di</strong>ng events are best relat<strong>ed</strong> to the overthrusting of the<br />
Nord-Calabrese Unit onto the Parasicilide Unit and to a later, <strong>di</strong>fferently<br />
orient<strong>ed</strong> shortening event, respectively. On the other hand, the third fol<strong>di</strong>ng<br />
stage, being associat<strong>ed</strong> with vertical shortening, may be interpret<strong>ed</strong> as a result<br />
of stretching and thinning of a previously overthicken<strong>ed</strong> accretionary w<strong>ed</strong>ge.<br />
Key words: southern Apennines, accretionary w<strong>ed</strong>ges, w<strong>ed</strong>getop<br />
basins.<br />
INTRODUZIONE<br />
In Cilento (Campania) affiorano estese successioni bacinali<br />
<strong>di</strong> origine oceanica o <strong>di</strong> transizione tra l’oceano (Neotetide) e il<br />
continente (margine Apulo) riferibili all’Unità Nord-Calabrese<br />
<strong>ed</strong> all’Unità Parasicilide (BONARDI ET AL., 1988; CIARCIA ET<br />
AL., 2009). Queste successioni fanno parte <strong>del</strong> cuneo<br />
d’accrezione appenninico formatosi a partire dal Miocene<br />
inferiore e accavallatosi successivamente sui domini <strong>di</strong><br />
piattaforma carbonatica (CIARCIA ET AL., 2009). Tali unità sono<br />
ricoperte in <strong>di</strong>scordanza dai depositi <strong>di</strong> w<strong>ed</strong>ge-top <strong>del</strong> Gruppo<br />
<strong>del</strong> Cilento (AMORE ET AL., 1988).<br />
L’area <strong>di</strong> stu<strong>di</strong>o (Fig. 1) è ubicata tra i comuni <strong>di</strong><br />
Castelnuovo Cilento e Salento dove l’Unità Parasicilide affiora<br />
in finestra tettonica al <strong>di</strong> sotto <strong>del</strong>l’Unità Nord-Calabrese<br />
(APAT, 2005).<br />
_________________________<br />
(*) Dipartimento Scienze <strong>del</strong>la Terra, Università <strong>di</strong> Napoli F<strong>ed</strong>erico II,<br />
Largo San Marcellino 10, 80138 Napoli<br />
La successione <strong>del</strong>l’Unità Parasicilide è formata, dal basso<br />
verso l’alto, da quattro unità stratigrafiche: (i) argilliti, argille e<br />
marne <strong>del</strong>la Formazione <strong>di</strong> Postiglione; (ii) calcari marnosi,<br />
marne e calcareniti <strong>del</strong>la Formazione <strong>di</strong> Monte<br />
Sant’Arcangelo; (iii) marne e calcari biancastri <strong>del</strong>la<br />
Fig. 1 – Carta geologica <strong>del</strong>l’area stu<strong>di</strong>ata.<br />
Formazione <strong>di</strong> Contursi; (iv) depositi silicoclastici <strong>del</strong>le<br />
Arenarie <strong>di</strong> Albanella. Tali unità stratigrafiche sono riferibili ad<br />
analoghe successioni riconosciute nel Foglio 503-Vallo <strong>del</strong>la<br />
Lucania e raggruppate nell’unità tettonica <strong>di</strong> Castelnuovo<br />
Cilento (APAT, 2005); in particolare, la prima corrisponde in<br />
parte alle ‘argilliti <strong>di</strong> Genesio’; la seconda e la terza alle ‘marne<br />
e calcareniti <strong>del</strong> T. Trenico’; mentre la quarta corrisponde alle<br />
‘arenarie <strong>di</strong> Pianelli’. La successione <strong>del</strong>l’Unità Parasicilide è<br />
attribuita al Cretacico Superiore-Bur<strong>di</strong>galiano (CIARCIA ET AL.,<br />
2009).<br />
In questo lavoro vengono presentati i primi risultati<br />
<strong>del</strong>l’analisi strutturale <strong>del</strong>l’Unità Parasicilide affiorante in<br />
Cilento.
236 S. VITALE ET ALII<br />
ANALISI STRUTTURALE<br />
La successione esaminata è caratterizzata dalla<br />
sovrapposizione <strong>di</strong> tre fasi plicative. La prima fase è<br />
contrad<strong>di</strong>stinta da pieghe (F1) da strette ad isoclinali, con<br />
forme da chevron a sinusoidali (Fig. 2a,b). È presente un<br />
clivaggio <strong>di</strong> piano assiale poco sviluppato nelle peliti e spaziato<br />
e convergente nelle litologie più competenti. Secondo la<br />
classificazione <strong>di</strong> RAMSAY (1967), tali pieghe rientrano nella<br />
classe 1B o 1C per gli strati più competenti e nella classe 3 per<br />
gli strati argillitici meno competenti.<br />
Le pieghe isoclinali <strong>di</strong> prima fase (i cui dati <strong>di</strong> orientazione<br />
sono riportati in Fig. 3a-b) sono ripiegate da strutture in<br />
Fig. 2 – (a) interferenza tra il secondo <strong>ed</strong> il primo piegamento. (b) pieghe<br />
<strong>di</strong> terza fase.<br />
prevalenza da aperte a strette e generalmente angolari (Fig. 2a).<br />
Codeste pieghe <strong>di</strong> seconda fase (F2) si mostrano a forma <strong>di</strong><br />
kink con piani assiali spesso coniugati (Fig. 2a).<br />
Gli assi <strong>di</strong> piega <strong>di</strong> seconda fase (A2) mostrano una<br />
<strong>di</strong>rezione prevalente N-S (Fig. 3c) <strong>ed</strong> i poli dei relativi piani<br />
assiali (PA2; Fig. 3d) in<strong>di</strong>cano piani immergenti con varia<br />
inclinazione a W e ad E. Il pattern d’interferenza tra i due<br />
piegamenti (Fig. 2a) è compreso tra le classi 2 e 3 <strong>del</strong>la<br />
classificazione <strong>di</strong> RAMSAY (1967).<br />
Entrambe le pieghe F1 <strong>ed</strong> F2 sono ripiegate da un terzo<br />
evento plicativo (F3), caratterizzato da pieghe generalmente<br />
aperte <strong>ed</strong> osservabili esclusivamente lungo i fianchi corti subverticali<br />
<strong>del</strong>le mesopieghe F1 (Fig. 3b). Gli assi <strong>di</strong> piega <strong>di</strong><br />
terza fase (A3; Fig. 3e) sono generalmente poco inclinati, così<br />
come i relativi piani assiali (PA3; Fig. 3f).<br />
Fig. 3– Proiezioni stereografiche <strong>del</strong>le giaciture <strong>del</strong>le strutture analizzate.<br />
DISCUSSIONE E CONCLUSIONI<br />
Il lavoro presentato rappresenta il primo stu<strong>di</strong>o strutturale<br />
dei terreni appartenenti all’Unità Parasicilide affioranti in<br />
Cilento. L’analisi strutturale ha permesso il riconoscimento <strong>di</strong><br />
tre fasi plicative sovrapposte. Le prime due fasi, per le loro<br />
caratteristiche geometriche e strutturali, possono essere<br />
associate al sovrascorrimento <strong>del</strong>l’Unità Nord-Calabrese<br />
sull’Unità Parasicilide e ad un successivo minore evento <strong>di</strong><br />
raccorciamento non coassiale rispetto primo. La terza fase<br />
plicativa in<strong>di</strong>ca, invece, un raccorciamento verticale<br />
probabilmente associato all’estensione orizzontale in risposta al<br />
prec<strong>ed</strong>ente ispessimento <strong>del</strong> cuneo d’accrezione.<br />
Il rilevamento strutturale ha evidenziato come l’Unità<br />
Parasicilide affiori in finestra tettonica a letto <strong>del</strong>l’Unità Nord-<br />
Calabrese. Il contatto tettonico a basso angolo taglia le strutture<br />
plicative sia a letto sia a tetto. La sezione geologica (Fig. 4)<br />
evidenzia come la successione analizzata sia caratterizzata da<br />
una deformazione alla mesoscala descritta da un treno <strong>di</strong><br />
antiformi e sinformi (F1) con asimmetria a s. Questa<br />
caratteristica, unita alla generale polarità stratigrafica inversa <strong>di</strong><br />
tali terreni, in<strong>di</strong>ca che la successione affiorante forma il fianco<br />
rovesciato <strong>di</strong> una piega a scala regionale vergente a SE<br />
formatasi durante la prima fase plicativa. Tale struttura è stata<br />
successivamente raccorciata sia lungo la <strong>di</strong>rezione E-W, sia<br />
verticalmente. Infine, la piega a scala regionale è stata <strong>di</strong>slocata<br />
da un contatto a basso angolo che, nell’area <strong>di</strong> stu<strong>di</strong>o, ha eliso<br />
il fianco a polarità normale.
Fig. 4 – <strong>Sezione</strong> geologica, la traccia e la legenda sono in<strong>di</strong>cate nella Fig. 1<br />
REFERENCES<br />
AMORE O., BONARDI G., CIAMPO G., DE CAPOA P., PERRONE<br />
V. & SGROSSO I. (1988) - Relazioni tra "flysch interni" e<br />
domini appenninici: reinterpretazione <strong>del</strong>le formazioni <strong>di</strong><br />
Pollica, San Mauro e Albidona e il problema<br />
<strong>del</strong>l'evoluzione inframiocenica <strong>del</strong>le zone esterne<br />
appenniniche. Mem. Soc. Geol. It., 41, 285-299.<br />
APAT (2005) - Carta Geologica d’Italia alla scala 1:50.000,<br />
Foglio 503 ‘Vallo <strong>del</strong>la Lucania’. SELCA, Firenze.<br />
BONARDI G., AMORE F.O., CIAMPO G., DE CAPOA P.,<br />
MICONNET P. & PERRONE V. (Eds) (1988) - Il Complesso<br />
Liguride Auct.: stato <strong>del</strong>le conoscenze e problemi aperti<br />
sulla sua evoluzione pre-appenninica <strong>ed</strong> i suoi rapporti con<br />
l'Arco Calabro. Mem. Soc. Geol. Ital., 41, 17-35.<br />
CIARCIA S., VITALE S., DI STASO A., IANNACE A., MAZZOLI S.,<br />
TORRE M. (2009) - Stratigraphy and tectonics of an Internal<br />
Unit of the southern Apennines: implications for the<br />
geodynamic evolution of the peri-Tyrrhenian mountain belt.<br />
Terra Nova, in stampa.<br />
RAMSAY J.G. (1967) - Fol<strong>di</strong>ng. and fracturing of rock.<br />
McGraw-Hill, New York.<br />
ANALISI STRUTTURALE DELL’UNITÀ PARASICILIDE<br />
237
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 238-240, 3 ff.<br />
Structural constraints to the Euganean Geothermal Field (NE Italy)<br />
RIASSUNTO<br />
Vincoli strutturali <strong>del</strong> Campo Geotermico Euganeo (Italia <strong>di</strong> NE)<br />
Il campo geotermico Euganeo presso Padova (dal quale nel 2007 sono stati<br />
estratti circa 17 Mm 3 <strong>di</strong> flui<strong>di</strong> ad un temperatura compresa tra 60
STRUCTURAL CONSTRAINTS TO THE EUGANEAN GEOTHERMAL FIELD<br />
Fig. 1 - Structural sketch of the central-eastern Southern Alps showing the location of the Euganean Geothermal Field (EGF) and the S. Vito <strong>di</strong><br />
Leguzzano quarry, where the structural data of Fig. 2 were acquir<strong>ed</strong>.<br />
Fig. 2 – Sketch of the S. Vito <strong>di</strong> Leguzzano quarry, near Schio. Not to scale.<br />
The outcrop shows a prominent drag fold of the footwall of the Schio-<br />
Vicenza fault develop<strong>ed</strong> during a Tertiary extensional deformation. The<br />
stereoplots are the equal angle projection of strike-slip faults and the fault<br />
plane solution. The principal axes of infinitesimal strain from fault slip<br />
analysis are labell<strong>ed</strong> “1” for extension and “3” for shortening (software:<br />
FaultKin v. 5.4 by R.W.ALLMENDINGER, R.A. MARRETT & T. CLADOUHOS,<br />
Cornell Univ.). The shortening axis corresponds to the kinematic sigma 1<br />
develop<strong>ed</strong> within a steep shear zone tren<strong>di</strong>ng NW-SE (Schio-Vicenza fault).<br />
sinistral movements (Fig. 2). These fin<strong>di</strong>ngs point to a<br />
poliphase deformation of the SVFS characteriz<strong>ed</strong> by extension<br />
follow<strong>ed</strong> by strike-slip regime, i.e. the same sequence of the<br />
geologic evolution illustrat<strong>ed</strong> above.<br />
PROPOSED STRUCTURAL MODEL OF THE EGF<br />
Hydrothermal outflow occurs most commonly at the<br />
terminations of in<strong>di</strong>vidual faults and where multiple faults<br />
interact (CUREWITZ & KARSON, 1997).<br />
239<br />
Fig. 3 – SW of Padova the Schio-Vicenza fault system shows a relay zone<br />
between two segments. The structure is coincident with the Euganean<br />
hydrothermal outflow, which can be explain<strong>ed</strong> as the vertical fluid flow<br />
along a conduit produc<strong>ed</strong> by local tensional stress development.
240 D. ZAMPIERI ET ALII<br />
Strike-slip fault systems are affect<strong>ed</strong> by local complexities such<br />
as bends and stopovers. Releasing bends and <strong>di</strong>lational<br />
stepovers are typically complex sites of fracturing, veining and<br />
fluid flow. The link between hydrothermal outflow and strikeslip<br />
tectonics has been recently demonstrat<strong>ed</strong> for example in the<br />
western Alps (BAIETTO et alii, 2008).<br />
The revision of the SVFS architecture by using unpublish<strong>ed</strong><br />
seismic lines has permitt<strong>ed</strong> to unravel the complex structure of<br />
the fault system, which is not a single fault as is usually<br />
depict<strong>ed</strong> in structural and geological maps. The fault zone is<br />
compos<strong>ed</strong> of some synthetic normal fault segments buri<strong>ed</strong><br />
beneath the alluvial cover. A left stepover structure (relay zone)<br />
between two <strong>di</strong>stinct fault segments of the SVFS has been<br />
recogniz<strong>ed</strong> just in coincidence with the thermal area (Fig. 3).<br />
Given the Neogene to Quaternary sinistral strike-slip<br />
kinematics superimpos<strong>ed</strong> on the fault system, this structure<br />
have accommodat<strong>ed</strong> along-strike local extension and may be<br />
responsible for rock fracturing and permeability development,<br />
enhancing migration to surface of thermal waters.<br />
REFERENCES<br />
BAIETTO A., CADOPPI P., MARTINOTTI G., PERELLO P.,<br />
PERROCHET P. & VUATAZ F.D., 2008. Assessment of<br />
thermal circulation in strike-slip fault systems: the Terme<br />
<strong>di</strong> val<strong>di</strong>eri case (Italian western Alps). In: WIBBERLEY et<br />
al. (<strong>ed</strong>s) The internal Structure of Fault Zones:<br />
Implications for mechanical and Fluid-Flow properties.<br />
Geol. Soc. Sp. Publ. 299, 317-339.<br />
CASTELLARIN A., VAI G. B. & CANTELLI L., 2006. The Alpine<br />
evolution of the Southern Alps around the Giu<strong>di</strong>carie<br />
faults: A Late Cretaceous to Early Eocene transfer zone.<br />
Tectonophysics, 414, 203-223.<br />
CUREWITZ D. & KARSON J.A., 1997. Structural setting of<br />
hydrothermal outflow: Fracture permeability maintain<strong>ed</strong><br />
by fault propagation and interaction. J. Volcan. and<br />
Geotherm. Res., 79, 149-168.<br />
DOGLIONI C. & BOSELLINI A., 1987. Eoalpine and mesoalpine<br />
tectonics in the Southern Alps. Geol. Rundsch., 76, 735-<br />
754.<br />
FABBRI P. & TREVISANI S., 2005. Spatial <strong>di</strong>stribution of<br />
Temperature in the geothermal Euganean field (NE,Italy):<br />
a simulat<strong>ed</strong> annealing approach. Geothermics, 34, 617-<br />
631.<br />
FANTONI R., CATELLANI D., MERLINI S., ROGLEDI S. &<br />
VENTURINI S. (2002). La registrazione degli eventi<br />
deformativi cenozoici nell’avampaese Veneto-Friulano.<br />
Mem. Soc. Geol. It., 57, 301-313.<br />
PICCOLI G., BELLATI R., BINOTTI C., DI LALLO E., SEDEA R.,<br />
DAL PRÀ A., CATALDI R., GATTO G.O., GHEZZI G.,<br />
MARCHETTI M., BULGARELLI G., SCHIESARO G., PANICHI<br />
C., TONGIORGI E., BALDI P., FERRARA G.C., MASSARI F.,<br />
MEDIZZA F., ILICETO V., NORINELLI A., DE VECCHI GP.,<br />
GREGNANIN A., PICCIRILLO E.M. & SBETTEGA G., 1976. Il<br />
sistema idrotermale euganeo-berico e la geologia dei<br />
Colli Euganei. Mem. Ist. Geol. Miner. Univ. Padova, 30,<br />
266 pp.<br />
SEMENZA E., 1974. La fase giu<strong>di</strong>cariense, nel quadro <strong>di</strong> una<br />
nuova ipotesi sull’orogenesi alpina nell’area italo<strong>di</strong>narica.<br />
Mem. Soc. Geol. It. 13, 187-226.<br />
ZANFERRARI A., BOLLETTINARI G., CAROBENE L., CARTON A.,<br />
CARULLI G.B., CASTALDINI D., CAVALLIN A., PANIZZA M.,<br />
PELLEGRINI G.B., PIANETTI F. & SAURO U., 1982.<br />
Evoluzione neotettonica <strong>del</strong>l'Italia nord-orientale. Mem.<br />
Sci. Geol. 35, 355-376.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 241-244, 5 ff.<br />
Pseudotachylytes along the Orobic Thrust, Southern Alps, Bergamo:<br />
a proxy for the Eo-Alpine deformation ?<br />
STEFANO ZANCHETTA, PAOLO D’ADDA , VALENTINA BARBERINI, NICOLETTA FUSI & ANDREA ZANCHI<br />
RIASSUNTO<br />
Le pseudotachiliti <strong>del</strong> Thrust Orobico (BG): possibili in<strong>di</strong>catori <strong>di</strong> una<br />
deformazione Eo-Alpina nelle Alpi Meri<strong>di</strong>onali ?<br />
Il Thrust Orobico rappresenta la più importante struttura Alpina che<br />
interessa il basamento <strong>del</strong>le Alpi Meri<strong>di</strong>onali tra Bergamo e Sondrio. Lungo<br />
questo sovrascorrimento il basamento cristallino <strong>di</strong> età Varisica è sovrascorso<br />
verso sud, al <strong>di</strong> sopra <strong>del</strong>le coperture s<strong>ed</strong>imentarie <strong>di</strong> età Permo-Triassica. La<br />
determinazione <strong>del</strong>l’età <strong>del</strong> movimento dei vari segmenti che costituiscono il<br />
thrust è <strong>di</strong> fondamentale importanza ai fini <strong>del</strong>la ricostruzione <strong>del</strong>l’evoluzione<br />
strutturale <strong>del</strong>le Alpi Meri<strong>di</strong>onali durante le prime fasi <strong>del</strong>l’orogenesi Alpina.<br />
Nell’area <strong>del</strong> Passo <strong>di</strong> San Marco, la zona <strong>di</strong> faglia correlata al Thrust<br />
Orobico mostra uno spessore variabile da 150 a 200 m e risulta caratterizzata<br />
dalla presenza <strong>di</strong> numerose vene <strong>di</strong> pseudotachiliti formatesi, per quanto<br />
emerso dalle analisi strutturali <strong>di</strong> dettaglio eseguite lungo il thrust, durante le<br />
ultime fasi evolutive <strong>del</strong>la struttura, in un regime deformativo <strong>di</strong> tipo fragile. In<br />
particolare, le pseudotachiliti mostrano caratteristiche giaciturali comparabili<br />
con quelle <strong>del</strong>le faglie inverse, <strong>di</strong>rette ENE-OSO, che rappresentano gli<br />
elementi strutturali dominanti all’interno <strong>del</strong>la zona <strong>di</strong> faglia.<br />
Date le relazioni strutturali fra le pseudotachiliti e le altre strutture<br />
connesse alla zona <strong>di</strong> faglia <strong>del</strong> Thrust Orobico, la caratterizzazione<br />
petrografico-mineralogica <strong>del</strong>le vene <strong>di</strong> pseudotachiliti e la loro datazione<br />
ra<strong>di</strong>ometrica offrono la possibilità <strong>di</strong> investigare l’evoluzione <strong>di</strong> questa<br />
importante struttura paleosismica e <strong>di</strong> porre dei vincoli temporali all’attività<br />
<strong>del</strong>la stessa..Le datazioni ad oggi <strong>di</strong>sponibili <strong>del</strong>le pseudotachiliti <strong>del</strong> Thrust<br />
Orobico e <strong>del</strong>la Linea <strong>del</strong> Porcile (MEIER, 2003) in<strong>di</strong>cano un’età tardo-<br />
Cretacica. L’affidabilità <strong>di</strong> tali datazioni risulta però limitata dalla mancanza<br />
<strong>di</strong> una dettagliata analisi strutturale e dall’assenza <strong>di</strong> una caratterizzazione<br />
petrografico-mineralogica approfon<strong>di</strong>ta, in<strong>di</strong>spensabile ai fini <strong>del</strong>la sicura<br />
interpretazione dei dati ra<strong>di</strong>ometrici. Il presente lavoro, tutt’ora in corso, si<br />
propone <strong>di</strong> sopperire a tale lacuna, fornendo una stima più precisa e più solida<br />
<strong>del</strong>l’evoluzione temporale <strong>del</strong> Thrust Orobico e <strong>del</strong>la Line <strong>del</strong> Porcile.<br />
Key words: Eo-Alpine deformation, Orobic Thrust, Porcile<br />
Thrust, pseudotachylites, Southern Alps.<br />
INTRODUCTION<br />
The Alpine structural evolution of the Southern Alps still<br />
poses several unsolv<strong>ed</strong> problems especially on its oldest history<br />
and its possible relationships with the Eo-Alpine tectonic<br />
events affecting the Austroalpine domain. DOGLIONI &<br />
BOSELLINI (1987) and BERSEZIO et alii (1993) emphasiz<strong>ed</strong> the<br />
occurrence of a pre-Tertiary orogeny bas<strong>ed</strong> on the development<br />
of a deep Late Cretaceous E-W tren<strong>di</strong>ng for<strong>ed</strong>eep basin along<br />
_________________________<br />
Dipartimento <strong>di</strong> Scienze Geologiche e Geotecnologie, Università degli<br />
Stu<strong>di</strong> <strong>di</strong> Milano Bicocca, Piazza <strong>del</strong>la Scienza 4 – 20126 Milano.<br />
correspon<strong>di</strong>ng author S. ZANCHETTA: stefano.zanchetta@unimib.it<br />
the southern margin of the belt which were infill<strong>ed</strong>, in the<br />
Lombar<strong>di</strong>an basin by turbi<strong>di</strong>tic units rich in olistoliths. ZANCHI<br />
et alii (1990) also obtain<strong>ed</strong> K/Ar ra<strong>di</strong>ometric ages around the<br />
Cretaceous-Palaeogene boundary for <strong>di</strong>ke swarms crossing<br />
previous thrust surfaces form<strong>ed</strong> in the Presolana area, Orobic<br />
Alps, Bergamo, within the Lower to Middle Triassic units.<br />
Although later works (FANTONI et alii, 1999) obtain<strong>ed</strong> slightly<br />
Fig. 1 – Structural map of the Orobic Alps, Lombardy, Italy. 1: Upper<br />
Cretaceous turbi<strong>di</strong>tes; 2: Jurassic to Lower Cretaceous basinal units; 3:<br />
Upper Triassic units; 4: Lower to Middle Triassic successions; 5: Orobic<br />
Anticlines inclu<strong>di</strong>ng crystalline basement, Permian and lowermost Triassic<br />
units; 6: MG to LG Variscan basement of the Orobic Alps; 7: Pre-Alpine HT<br />
units of the Orobic basement; 8: Metamorphic units of the Central Alps north<br />
of the Insubric Line; 9: Masino-Bregaglia (Brg, T); 10: Adamello-Presanella.<br />
O-A: Orobic Anticline; TC-A: Trabuchello-Cabianca Anticline; CE-A:<br />
C<strong>ed</strong>egolo Anticline. PSM: Passo San Marco, also shown with a star.<br />
younger ages for the same <strong>di</strong>kes, the matter of Late Cretaceous<br />
thrust stacking remains still open.<br />
The opportunity to date the oldest thrust structures of the<br />
Southern Alps occurs in the western Orobic region, where the<br />
occurrence of pseudotachylyte veins has been describ<strong>ed</strong> by<br />
previous authors (CARMINATI & SILETTO, 2005) along the<br />
Orobic and Porcile thrusts. The Orobic Thrusts strikes E-W and<br />
is the main Alpine structure of the Orobic Southern Alps.<br />
Along this thrust the Variscan crystalline basement is stack<strong>ed</strong><br />
southwards on the Permo-Triassic cover units. The Porcile<br />
Thrust trends ENE-WSW, is cut by the Orobic Thrust and<br />
stacks m<strong>ed</strong>ium to low grade metapelitic rocks on the Morbegno<br />
Gneiss and on the Gneiss Chiari units; thin slices of the Permo-<br />
Triassic s<strong>ed</strong>imentary cover often occur along this structure,
242 S. ZANCHETTA ET ALII<br />
Fig. 2 – a) Geological scheme of the Passo San Marco area. b) pre-Alpine D2 structure in the crystalline basement: fold axes (A2) and S2 foliations (poles to<br />
planes, lower emisphere). c) Axial planes of D3 folds of Alpine age in the crystalline basement. d) Fold axes of D3 folds; the high <strong>di</strong>spersion is possibly relat<strong>ed</strong><br />
to block rotation along the Orobic Thrust. e) Stress tensor obtain<strong>ed</strong> with reverse faults measur<strong>ed</strong> along the Orobic Thrust.<br />
which may be an invert<strong>ed</strong> Permian normal fault.<br />
Accor<strong>di</strong>ng to LAUBSCHER (1985), the Orobic Thrust sheets<br />
corresponds to the Northern Grigna Unit in the s<strong>ed</strong>imentary<br />
cover, which represents the highest and consequently oldest<br />
structural unit of the s<strong>ed</strong>imentary cover, roughly correspon<strong>di</strong>ng<br />
to the thrust sheets forming the Presolana thrust stack. As<br />
preliminary Ar/Ar ages on the pseudotachylytes of the Orobic<br />
thrust have given a very Late Cretaceous age (MEIER, 2003),<br />
these results can fit with the data obtain<strong>ed</strong> from the <strong>di</strong>ke<br />
swarms of the Presolana area, both suggesting a strong<br />
evidence for the Late Cretaceous compressive events.<br />
In this paper we present the results of petrological<br />
investigations concerning the pseudotachylytes form<strong>ed</strong> along<br />
the two main thrusts. Petrological investigations have been<br />
perform<strong>ed</strong> in close connection with a detail<strong>ed</strong> structural<br />
analysis of faults and folds develop<strong>ed</strong> in the San Marco Pass<br />
area. These stu<strong>di</strong>es are aim<strong>ed</strong> to obtain a better understan<strong>di</strong>ng<br />
of the genesis and textures of these complex rocks, in order to<br />
optimize the results of new ra<strong>di</strong>ometric age determinations<br />
which will be perform<strong>ed</strong> in the next future.<br />
FAULT ANALYSIS ALONG THE OROBIC THRUST<br />
About 130 fault planes have been measur<strong>ed</strong> along the<br />
Orobic Thrust in the Ca’ San Marco area. The best<br />
observations refer to the road taking to the Pass, where fresh<br />
and continuous outcrops of retrogress<strong>ed</strong> mica schists show<br />
Fig. 3 – Rose <strong>di</strong>agrams relative to reverse faults measur<strong>ed</strong> along the Orobic<br />
Thrust in the Passo San Marco area.
thick ultracataclastic zones inject<strong>ed</strong> with pseudotachylyte veins.<br />
Rose <strong>di</strong>agrams (Fig.3) relative to reverse faults (68) show a<br />
mark<strong>ed</strong> E-W trend with a northward <strong>di</strong>p showing a wide range<br />
of <strong>di</strong>p values (20°-60°) due to the occurrence of secondary R<br />
shears. Fault striations occurring along most of the measur<strong>ed</strong><br />
surfaces (53) are mainly <strong>di</strong>p-slip. Fault planes bearing<br />
pesudotachylytes and injection veins show very similar<br />
geometrical features with a m<strong>ed</strong>ium-angle <strong>di</strong>p (50°-60°)<br />
suggesting that the genesis of pseudotachylytes is relat<strong>ed</strong> to the<br />
main brittle stage of thrust faulting. Stress tensor<br />
determinations obtain<strong>ed</strong> with striat<strong>ed</strong> fault planes in<strong>di</strong>cates a<br />
N10° <strong>di</strong>rection of the main stress axis (Angelier, 1990).<br />
Successive both left- and right-lateral strike-slip faults<br />
reactivate some of the main thrust faults, suggesting a complex<br />
evolution through the entire Alpine history of the Orobic<br />
Thrust.<br />
TEXTURE AND PETROGRAPHY OF<br />
PSEUDOTACHYLYTES<br />
It is now wi<strong>del</strong>y accept<strong>ed</strong> that the occurence of<br />
pseudotachylytes within a fault zone suggests coseismic<br />
friction-induc<strong>ed</strong> melting along fault planes (SIBSON, 1975;<br />
SPRAY, 1995; WENK et alii, 2000). In this context<br />
pseudotachylytes represent unique opportunities to study fault<br />
processes along exhum<strong>ed</strong> palaeoseismic structures and also<br />
offer the possibility to pose absolute time-constraints on the<br />
fault activity by means of ra<strong>di</strong>ometric dating of the meltderiv<strong>ed</strong><br />
cryptocristalline or glassy matrix.<br />
The pseudotachylyte veins occurring along the Orobic and<br />
the Porcile Thrusts (Fig. 4) <strong>di</strong>scontinuosly decorate fault planes<br />
and typically <strong>di</strong>splay a thickness from a few millimeters to 20-<br />
25 mm, with rare reservoir (injection) veins reaching 100-150<br />
mm along the Porcile Thrust, east of Laghi <strong>del</strong> Porcile. They<br />
are found inside <strong>di</strong>fferent lithologies, two-mica gneiss, mica<br />
schists, K-feldspar gneiss, always showing cataclastic textures<br />
develop<strong>ed</strong> before pseudotachylyte formation. Pseudotachylyte<br />
veins <strong>di</strong>splay only minor reactivations along a successive set of<br />
<strong>di</strong>screte reverse fault planes subparallel to the main reverse<br />
planes but with a <strong>di</strong>p angle that never exce<strong>ed</strong>s 20°-25°. Locally<br />
<strong>di</strong>latational epidote-chlorite veins, 1-5 mm thick, crosscut all<br />
the previous thrust-relat<strong>ed</strong> structures.<br />
Pseudotachylytes occurring along melt generation surfaces<br />
(mgs) are typically zon<strong>ed</strong> with a thin black wall along contacts<br />
with the wall rock, and a lighter dark gray to brass-like<br />
colour<strong>ed</strong> part in the vein center. Clasts deriv<strong>ed</strong> from the wall<br />
rock are made of quartz and minor plagioclase and titanite. The<br />
clast/matrix ratio decreases towards the vein center contrary to<br />
the increase of the clast grain size. Reservoir pseudotachylyte<br />
veins, deriv<strong>ed</strong> from the melt injections from the mgs towards<br />
the wall rock, usually occurring along existing fractures or<br />
veins, are typically unzon<strong>ed</strong> and <strong>di</strong>splay a clast/matrix ratio<br />
close to zero, probably due to the “bottle-neck” effect<br />
(O’HARA, 2001).<br />
Quartz and plagioclase clasts are round<strong>ed</strong> or sub-round<strong>ed</strong>,<br />
with an aspect ratio between 1 and 1.5. Elongat<strong>ed</strong> clasts are<br />
PSEUDOTACHYLYTES ALONG THE OROBIC THRUST<br />
243<br />
usually align<strong>ed</strong> parallel to the vein margins as an effect of melt<br />
flow. Isoclinal and convolute folds in<strong>di</strong>viduat<strong>ed</strong> by alternating<br />
light and dark colour<strong>ed</strong> matrix layers are common features<br />
observ<strong>ed</strong> in most of the analys<strong>ed</strong> samples (fig. 4).<br />
The textures and mineralogy of both wall rock and<br />
pseudotachylyte veins has been determin<strong>ed</strong> by SEM and EMPA<br />
analyses together with qualitative XRPD perform<strong>ed</strong> on<br />
microsampl<strong>ed</strong> pseudotachylyte matrix and MicroCT on micro<br />
cores to unravel the 3D relations between veins and wall rocks.<br />
The pseudotachylyte veins matrix consists of a<br />
cryptocrystalline polymineralic aggregate made of quartz,<br />
biotite, plagioclase, titanite and magnetite as ubiquitous phases,<br />
whereas K-feldspar, ilmenite, apatite and sulfides only occur in<br />
samples deriv<strong>ed</strong> from K-feldspar gneisses and epidote-bearing<br />
gneisses (fig. 5). In samples where both ilmenite and titanite<br />
crystalliz<strong>ed</strong> from the pristine melt, ilmenite is found only within<br />
the black wall rimming the pseudotachylyte/wall rock contact,<br />
whereas titanite is present only in the central part of the vein.<br />
Fig. 4 – a) Pseudotachylyte injection vein in a K-feldspar gneiss wall rock<br />
(Gneiss <strong>di</strong> Pizzo Meriggio, Porcile Thrust. b) 3d image slices of a<br />
pseudotachylyte vein in a two-mica gneiss obtain<strong>ed</strong> with MicroCT (X-ray<br />
microcomputeriz<strong>ed</strong> tomography). Bright spots are epidote and titanite<br />
crystals.
244 S. ZANCHETTA ET ALII<br />
The veins matrix <strong>di</strong>splays a chemical composition characteristic<br />
of interm<strong>ed</strong>iate silicate melts, with SiO2 wt% compris<strong>ed</strong><br />
between 52 and 64 and alkali (Na2O + K2O) ranging between 5<br />
and 11 wt% . More <strong>di</strong>fferentiat<strong>ed</strong> composition, with SiO2<br />
reaching 70-75 wt% were found only in some samples deriv<strong>ed</strong><br />
from epidote-free gneisses. An inverse relation between the<br />
total alkali content and Si was observ<strong>ed</strong> in all samples.<br />
Biotite represents the most abundant phase crystalliz<strong>ed</strong> from<br />
the melt. It is present both as 1 to 15 m laths and spherulithic<br />
Fig. 5 – BSE image of a pseudotachylite vein. Partially melt<strong>ed</strong> lithic clast<br />
are present together with quartz clast, often with a thin K-feldspar rim.<br />
Biotite and titanite (ttn) are crystalliz<strong>ed</strong> from the melt. <strong>Note</strong> the weak shape<br />
preferr<strong>ed</strong> orientation of bt laths around wall rock survivor clasts (sample<br />
SM6Q, Passo San Marco road.<br />
aggregates reaching 50 m in <strong>di</strong>ameter. Biotite crystallytes are<br />
typically more abundant close to the pseudotachylyte/wall rock<br />
contact and decrease towards the vein center. The XMg and the<br />
Ti-content of biotite crystallytes are always higher than the<br />
ones of wall rock biotite, suggesting higher equilibrium<br />
temperatures for pseudotachylyte biotites. The coupl<strong>ed</strong> Si<br />
increase and biotite crystallites abundance decrease towards the<br />
vein center possibly suggest the occurence of melt<br />
<strong>di</strong>fferentiation during the crystallization process.<br />
CONCLUSIONS<br />
The occurrence of pseudotachylyte veins within fault rocks<br />
correlat<strong>ed</strong> to the Orobic Thrust is of fundamental importance as<br />
pseudotachylytes offer the opportunity to better constrain the<br />
dominant fault processes and the time of activity of some of the<br />
main Alpine structures in the Southern Alps. Previous<br />
ra<strong>di</strong>ometric dating of pseudotachylyte veins from the Orobic<br />
and Porcile Thrusts (MEIER, 2003) yield a Late Cretaceous age<br />
for fault activity along these structures, suggesting the<br />
occurrence of pre-Tertiary (Eo-Alpine) deformation in this<br />
sector of the Southern Alps. The detail<strong>ed</strong> structural anlayses of<br />
pseudotachylytes relationships with other fault structures and<br />
petrographic-mineralogical characterization of them, constitute<br />
the starting point to obtain in the next future reliable constraints<br />
on the age of the Orobic and Porcile Thrust fault activity.<br />
The unquestionable dating of an Eo-Alpine orogenic phase<br />
record<strong>ed</strong> within the Southern Alps and its characterization both<br />
in structural and metamorphic terms could carry important<br />
consequences also on the evolution mo<strong>del</strong>s of the entire Alpine<br />
belt, mainly on the relations between the Southern Alps and the<br />
Asustroalpine domain, where the presence of a Eo-Alpine<br />
deformation and metamorphic phase it is now wi<strong>del</strong>y accept<strong>ed</strong>.<br />
REFERENCES<br />
ANGELIER, J. 1990. Inversion of field data in fault tectonics to<br />
obtain the regional stress – III. A new rapid <strong>di</strong>rect<br />
inversion method by analytical means. Geophys. Journ. Int.<br />
103(1), 363–376.<br />
BERSEZIO R., FORNACIARI M., GELATI R., NAPOLITANO A. &<br />
VALDISTURLO A. (1993) – The significance of the Upper<br />
Cretaceous to Miocene clastic w<strong>ed</strong>ges in the deformation<br />
history of the Lombar<strong>di</strong>an Southern Alps. Geol. Alpine, 69,<br />
3-20, Grenoble.<br />
CARMINATI E. & SILETTO G.B. (2005) – The Central Souther<br />
Alps (N Italy) paleoseismic zone: a comparison between<br />
field observations and pr<strong>ed</strong>ictions of fault mechanics.<br />
Tectonophysics, 401, 179-197.<br />
DOGLIONI C. & BOSELLINI A. (1987) – Eoalpine and<br />
Mesoalpine tectonic in the Southern Alps. Geol. Rundsch.,<br />
76, 735-754.<br />
FANTONI R., BERSEZIO R., FORCELLA F., GORLA L., MOSCONI<br />
A. & PICOTTI V. (1999) – New dating of the Tertiary<br />
magmatic products of the Central Southern Alps, bearings<br />
on the interpretations of the Alpine tectonic history. Mem.<br />
Scie. Geol., 51/1, 47- 61.<br />
LAUBSCHER H.B. (1985) – Large scale, thin-skinn<strong>ed</strong> thrusting<br />
in the Southern Alps: kinematic mo<strong>del</strong>s. Geol. Soc. Amer.<br />
Bull., 76, 710-718.<br />
MEIER A. (2003) – The Periadriatic Fault System in Valtellina<br />
(N-Italy) and the evolution of the Southwestern Segment of<br />
the Eastern Alps. PhD Thesis, Diss. Eth. No. 15008, Zurich.<br />
O’HARA K.D. (2001) - A pseudotachylytes geothermometer. J.<br />
Struct. Geol., 23, 1345-1357.<br />
SIBSON R.H. (1975) – Generation of pseudotachylyte by<br />
Ancient Seismic Faulting. Geophys. Jour. Royal. Astr. Soc.,<br />
43, 775-794.<br />
SPRAY J.G. (1995) – Pseudotachylyte controversy: fact or<br />
friction ? Geology, 23, 1119-1122.<br />
ZANCHI A., CHIESA S. & GILLOT P.Y. (1990) – Tectonic<br />
evolution of the Southern Alps in the Orobic chain:<br />
structural and geochronological in<strong>di</strong>cations for pre-<br />
Tertiary compressive tectonic. Mem. Soc. Geol. It., 44, 77-<br />
82.<br />
WENK H.R., JOHNSON L.R. & RATSCHBACHER L. (2000) –<br />
Pseudotachylytes in the Eastern Peninsular Ranges of<br />
California. Tectonophysics, 321, 253-277.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 245<br />
Utilizzo <strong>di</strong> dati geologico-strutturali <strong>di</strong> terreno<br />
per la mo<strong>del</strong>lazione 3D: esempi <strong>di</strong> strutture Alpine<br />
ANDREA ZANCHI (*), STEFANO ZANCHETTA (*) , PAOLO D’ADDA (*), FEDERICO AGLIARDI (*), MASSIMO CONTI<br />
(*), FRANCESCO GALBIATI(*), FABRIZIO BERRA (**), FRANCESCA SALVI (°)<br />
RIASSUNTO<br />
La possibilità <strong>di</strong> integrare informazioni cartografiche a<br />
carattere geologico–strutturale attraverso l’utilizzo dei GIS e <strong>di</strong><br />
poterle utilizzare <strong>di</strong>rettamente in appositi software per la<br />
mo<strong>del</strong>lazione 3D <strong>di</strong> corpi geologici complessi rappresenta oggi<br />
una realtà a <strong>di</strong>sposizione <strong>di</strong> molti enti accademici. L’attivazione<br />
<strong>di</strong> consorzi internazionali, per lo sviluppo <strong>di</strong> queste<br />
applicazioni (gOcad) e la messa a <strong>di</strong>sposizione per utilizzo<br />
<strong>di</strong>dattico e <strong>di</strong> ricerca (Move, Midland Valley), ha favorito la<br />
<strong>di</strong>ffusione e migliorato le capacità <strong>di</strong> utilizzo <strong>di</strong> questi<br />
complessi strumenti <strong>di</strong> elaborazione.<br />
La messa a punto <strong>di</strong> una serie <strong>di</strong> proc<strong>ed</strong>ure, da noi testate in<br />
<strong>di</strong>fferenti situazioni geologico-strutturali nel corso degli ultimi<br />
anni, ha reso possibile la ricostruzione 3d <strong>di</strong> elementi geologici<br />
<strong>di</strong> ogni tipo a partire da informazioni prevalentemente <strong>di</strong> tipo<br />
cartografico e strutturale (ZANCHI et alii, 2009a, 2009b). Le<br />
ricostruzioni così ottenute possono poi essere successivamente<br />
implementate attraverso dati provenienti da indagini <strong>di</strong> altro<br />
genere (pozzi, sondaggi, sezioni sismiche, ecc.).<br />
Le ricostruzioni presentate sono basate essenzialmente<br />
sull’utilizzo <strong>di</strong> <strong>di</strong>fferenti software: ArcGis per l’archiviazione<br />
dei dati geologici <strong>di</strong> terreno, gOcad e Move per la<br />
mo<strong>del</strong>lazione 2d, 3d <strong>ed</strong> eventualmente 4d (retrodeformazione).<br />
Nell’ambito <strong>del</strong>la presentazione verranno illustrati alcuni<br />
esempi significativi tratti da ricostruzioni effettuate nell’ambito<br />
<strong>di</strong> strutture presenti nella copertura <strong>del</strong> Sudalpino Orobico e nel<br />
basamento Austroalpino <strong>del</strong>la zona <strong>di</strong> Merano (BZ).<br />
Le applicazioni relative al Sudalpino si riferiscono<br />
principalmente a due esempi:<br />
- strutture a pieghe e faglie sviluppate nell’ambito dei<br />
sistemi <strong>di</strong> sovrascorrimenti che interessano le unità <strong>del</strong><br />
Triassico Inferiore e M<strong>ed</strong>io a sud <strong>del</strong>la Linea Valtorta-Val<br />
Canale;<br />
- deformazione gravitativa <strong>di</strong> versante sviluppata nelle unità<br />
<strong>del</strong> Triassico Superiore nei pressi <strong>del</strong> bordo meri<strong>di</strong>onale <strong>del</strong>la<br />
_________________________<br />
(*) Dipartimento <strong>di</strong> Scienze e Tecnologie Geologiche, Piazza <strong>del</strong>la Scienza<br />
4 Milano Università degli stu<strong>di</strong> <strong>di</strong> Milano-Bicocca<br />
(**) Dipartimento <strong>di</strong> Scienze <strong>del</strong>la Terra, Piazzale Gorini 34 Milano<br />
Università degli stu<strong>di</strong> <strong>di</strong> Milano<br />
(°) Istituto ENI, Milano.<br />
Andrea.zanchi@unimib.it 02/64482028<br />
catena (DGPV <strong>di</strong> Fuipiano Imagna). Le ricostruzioni sono state<br />
effettuate utilizzando in prevalenza dati cartografici <strong>di</strong><br />
superficie e numerose sezioni geologiche seriate appositamente<br />
costruite. Nel caso <strong>del</strong>la deformazione gravitativa è stato<br />
possibile retrodeformare attraverso 2Dmove alcune <strong>del</strong>le<br />
sezioni realizzate, allo scopo <strong>di</strong> verificare la congruenza<br />
geometrica <strong>del</strong> mo<strong>del</strong>lo realizzato. Questo tipo <strong>di</strong> approccio si<br />
è rivelato estremamente utile per lo sviluppo <strong>di</strong> mo<strong>del</strong>li<br />
numerici relativi all’evoluzione <strong>del</strong> fenomeno gravitativo.<br />
Per quanto riguarda la ricostruzione effettuata nell’ambito<br />
<strong>del</strong> basamento Austroalpino <strong>del</strong>la zona <strong>di</strong> Merano, sono state<br />
ricostruite alcune strutture plicative polifasiche <strong>di</strong> età Alpina<br />
che interessano il Complesso <strong>del</strong>le Cime <strong>di</strong> Tessa. In questo<br />
caso la ricostruzione è basata su una cartografia geologicostrutturale<br />
<strong>di</strong> dettaglio condotta a scala 1:5.000. Grazie<br />
all’ottima esposizione <strong>del</strong>le strutture è stato possibile ricostruire<br />
queste complesse pieghe attraverso proc<strong>ed</strong>ure simili a quelle<br />
adottate nei prec<strong>ed</strong>enti casi, ma realizzate attraverso l’utilizzo<br />
<strong>di</strong> gOcad.<br />
Oltre ai risultati ottenuti nell’ambito <strong>di</strong> temi <strong>di</strong> ricerca,<br />
verranno illustrati anche alcuni esempi <strong>di</strong> semplici applicazioni<br />
sviluppate, a carattere sperimentale, nell’ambito <strong>di</strong> alcuni<br />
insegnamenti <strong>di</strong> 2° livello.<br />
BIBLIOGRAFIA<br />
ZANCHI A., DE DONATIS M., GIBBS A. & MALLET J.-L. (2009a)<br />
- Imaging Geology in 3D. Computers and Geosciences,<br />
doi:10.1016/j.cageo.2007.09.009.<br />
ZANCHI A., SALVI F., ZANCHETTA S., STERLACCHINI S. & GUERRA<br />
G. (2009b) - 3D reconstruction of complex geological bo<strong>di</strong>es:<br />
examples from the Alps. Computers and Geosciences,<br />
doi:10.1016/j.cageo.2007.09.003.
Ren<strong>di</strong>conti online Soc. Geol. It., Vol. 5 (2009), 246-248, 4 ff.<br />
FReDNet: un sistema per il monitoraggio <strong>del</strong>le deformazioni crostali<br />
e per il posizionamento <strong>di</strong> precisione in tempo reale in Friuli-V. G.<br />
ABSTRACT<br />
FReDNet: un sistema per il monitoraggio <strong>del</strong>le deformazioni crostali e per<br />
il posizionamento <strong>di</strong> precisione in tempo reale in Friuli-V. G.<br />
The Friuli Regional Deformation network (FReDNet) of continuously<br />
operating Global Positioning System (GPS) receivers monitors crustal<br />
deformation in the Friuli region. The principal goals of the FReDNet program<br />
are to determine the <strong>di</strong>stribution of deformation in this region, to estimate<br />
interseismic strain accumulation on its active faults to better assess seismic<br />
hazards, to monitor hazardous faults for emergency response management, and<br />
to provide infrastructure for geodetic data management and processing. The<br />
network is also design<strong>ed</strong> to serve as a real-time surveying infrastructure (GPS-<br />
RTK).<br />
Key words: crustal deformation, FReDNet, geodetic data<br />
management, GPS, GPS-RTK<br />
INTRODUZIONE<br />
Le misurazioni geodetiche permettono <strong>di</strong> ottenere stime<br />
<strong>di</strong>rette <strong>ed</strong> accurate sulla velocità <strong>di</strong> deformazione in un'area<br />
geografica. Assieme ad altre osservazioni <strong>di</strong> natura geologica e<br />
sismologica, le misurazioni geodetiche sono <strong>di</strong> rilevanza<br />
centrale per stu<strong>di</strong> a lungo termine <strong>di</strong> pericolosità sismica. Esse,<br />
infatti, offrono i migliori vincoli per valutare quantitativamente<br />
la velocità con cui si accumulano le deformazioni. La<br />
tecnologia GPS (Global Positioning System) offre l'accuratezza<br />
necessaria per monitorare in maniera continua le deformazioni<br />
in zone tettonicamente attive. Inoltre, m<strong>ed</strong>iante l'utilizzo <strong>di</strong><br />
misurazioni <strong>di</strong>fferenziali <strong>di</strong> GPS, è possibile rilevare,<br />
localmente, spostamenti, con elaborazioni <strong>di</strong> post-processing,<br />
<strong>del</strong>l'or<strong>di</strong>ne <strong>del</strong> millimetro.<br />
FReDNet (Friuli Regional Deformation Network) è il nome<br />
dato alla rete <strong>di</strong> ricevitori permanenti GPS <strong>del</strong> Friuli-Venezia<br />
Giulia realizzata dal <strong>di</strong>partimento Centro <strong>di</strong> Ricerche<br />
Sismologiche-CRS <strong>di</strong> U<strong>di</strong>ne <strong>del</strong>l’Istituto Nazionale <strong>di</strong><br />
_________________________<br />
DAVID ZULIANI (*), PAOLO FABRIS (*), FRANCESCO PALMIERI (*) & ENRICO PRIOLO (*)<br />
(*) Istituto Nazionale <strong>di</strong> Oceanografia e <strong>di</strong> Geofisica Sperimentale – OGS,<br />
Borgo Grotta Gigante 42/c, 34010 Sgonico (TS)<br />
Oceanografia e <strong>di</strong> Geofisica Sperimentale-OGS <strong>di</strong> Trieste<br />
(BATTAGLIA et alii., 2003, BATTAGLIA et alii., 2004, ZULIANI et<br />
alii. 2003). Gli obiettivi primari <strong>del</strong>la rete FReDNet sono:<br />
• monitoraggio <strong>del</strong>le deformazioni crostali;<br />
• <strong>di</strong>stribuzione <strong>di</strong> un servizio <strong>di</strong> posizionamento <strong>di</strong><br />
precisione in tempo reale (GPS-RTK).<br />
I ricevitori <strong>del</strong>la rete FReDNet sono attivi dall'estate 2002 e<br />
raccolgono il dato GPS "in continuo". Fino ad oggi sono state<br />
installate 12 stazioni permanenti sul territorio <strong>del</strong> Friuli-<br />
Venezia Giulia e in parte su quello <strong>del</strong> Veneto.<br />
Sono presentate alcune serie temporali e le relative velocità<br />
<strong>di</strong> deformazione finora elaborate e si descrive il servizio <strong>di</strong><br />
posizionamento in tempo reale.<br />
LA RETE FREDNET<br />
I ricevitori <strong>del</strong>la rete FReDNet sono attivi dall'estate 2002 e<br />
raccolgono il dato GPS "in continuo" con campionamento a 1s.<br />
Fino ad oggi sono state installate 12 stazioni permanenti<br />
(Fig.1).<br />
Fig. 1 – Schema <strong>del</strong>la rete FReDNet.<br />
La <strong>di</strong>sposizione <strong>del</strong>le stazioni sul territorio è stata stu<strong>di</strong>ata<br />
in modo tale da permettere il monitoraggio <strong>del</strong>le deformazioni<br />
in atto lungo i tre principali sistemi <strong>di</strong> faglie presenti in Friuli<br />
(GALADINI et alii, 2002):
FReDNet: UN SISTEMA PER IL MONITORAGGIO DELLE DEFORMAZIONI CROSTALI<br />
• faglie <strong>di</strong>nariche, orientate in <strong>di</strong>rezione NW-SE, secondo<br />
l'asse definito da Caneva-Monte Prat-Monte Acomizza;<br />
• faglie periadriatiche, orientate in <strong>di</strong>rezione E-W,<br />
secondo l'asse definito da Zouf Plan-Monte Prat;<br />
• faglie orientate in <strong>di</strong>rezione NE-SO, secondo l’asse<br />
definito da Alpe Faloria-Monte Prat-M<strong>ed</strong>ea-Trieste.<br />
Ogni stazione <strong>del</strong>la rete fissa è costituita da:<br />
• un pilastrino in cemento solidamente ancorato a roccia<br />
stabile per garantire l’inamovibilità <strong>del</strong>la struttura. Il<br />
pilastrino è dotato <strong>di</strong> un sistema <strong>di</strong> accoppiamento su cui<br />
installare l’antenna GPS;<br />
• un ricevitore GPS <strong>di</strong> alta qualità a doppia frequenza (L2,<br />
L1);<br />
• un’antenna GPS <strong>di</strong> tipo Choke Ring dotata <strong>di</strong> borchia <strong>di</strong><br />
riferimento;<br />
• una cupola <strong>di</strong> protezione per l’antenna Choke Ring;<br />
• cavi <strong>di</strong> connessione fra antenna e ricevitore <strong>ed</strong> eventuali<br />
interfacce aggiuntive;<br />
• un sistema <strong>di</strong> alimentazione affidabile (attraverso la rete<br />
elettrica nazionale o sistemi alternativi quali quello<br />
fotovoltaico od eolico) dotato anche <strong>di</strong> un sistema<br />
sostitutivo <strong>di</strong> emergenza (batterie tampone) che permetta<br />
un funzionamento in continuo <strong>del</strong> ricevitore;<br />
• eventuali <strong>di</strong>spositivi per la sicurezza e la protezione<br />
<strong>del</strong>le strumentazioni (scaricatori elettrici, etc);<br />
• un sistema per la trasmissione, in tempo reale, dei dati<br />
satellitari raccolti dal ricevitore GPS. La trasmissione è<br />
effettuata verso un centro <strong>di</strong> raccolta dati <strong>ed</strong> è attuabile<br />
in <strong>di</strong>versi mo<strong>di</strong>:<br />
o sistema satellitare;<br />
o modem gsm/gprs;<br />
o sistemi WiFi d<strong>ed</strong>icati (apparati <strong>di</strong> trasmissione<br />
Wireless in tecnologia Spread Spectrum per lunga<br />
<strong>di</strong>stanza);<br />
o apparati ra<strong>di</strong>o con frequenze fornite su concessione.<br />
Il centro <strong>di</strong> raccolta dati, gestione <strong>del</strong>la rete e <strong>di</strong><br />
elaborazione è ubicato presso il CRS <strong>di</strong> U<strong>di</strong>ne. Il sistema si<br />
avvale <strong>di</strong> un software per la configurazione da remoto <strong>del</strong>le<br />
stazioni permanenti e per il calcolo e la <strong>di</strong>stribuzione in tempo<br />
reale <strong>di</strong> correzioni <strong>di</strong>fferenziali.<br />
La stazione ZOUF è inserita nella rete geodetica GPS<br />
europea EPN (EUREF Permanent Network) e i suoi dati sono<br />
anche <strong>di</strong>stribuiti da ASI (Agenzia Spaziale Italiana).<br />
I dati <strong>del</strong>la rete FReDNet sono <strong>di</strong>sponibili sul sito<br />
http://www.crs.inogs.it/fr<strong>ed</strong>net.<br />
L’obiettivo scientifico che il sistema si propone <strong>di</strong><br />
conseguire è la caratterizzazione dei processi tettonici regionali<br />
e <strong>di</strong> alcune aree più critiche dal punto <strong>di</strong> vista sismico.<br />
L’attività prev<strong>ed</strong>e il monitoraggio <strong>del</strong>le deformazioni crostali<br />
<strong>del</strong>l’area regionale, grazie alle misurazioni <strong>del</strong>la rete <strong>di</strong> stazioni<br />
GPS permanenti <strong>ed</strong> in alcune campagne episo<strong>di</strong>che <strong>di</strong><br />
misurazione svolte all’uopo. In particolare, le misurazioni GPS<br />
permetteranno <strong>di</strong>:<br />
247<br />
• stimare entità, <strong>di</strong>rezione e velocità <strong>di</strong> deformazione <strong>del</strong>le<br />
principali strutture crostali presenti;<br />
• stimare lo sforzo intersismico che si accumula nelle<br />
strutture sismogeniche;<br />
• definire il potenziale sismico e la pericolosità <strong>del</strong>le<br />
faglie identificate come attive.<br />
La rete GPS permanente FReDNet rappresenta<br />
l’infrastruttura <strong>di</strong> base necessaria per la misurazione geodetica<br />
<strong>del</strong> territorio regionale e <strong>del</strong>le aree imm<strong>ed</strong>iatamente a<strong>di</strong>acenti.<br />
Oltre alle misurazioni in continuo, sono tuttavia necessarie<br />
<strong>del</strong>le misurazioni episo<strong>di</strong>che <strong>di</strong> dettaglio concentrate in alcune<br />
aree <strong>di</strong> maggiore interesse.<br />
SERIE TEMPORALI E VELOCITÀ DI<br />
DEFORMAZIONE FINORA ELABORATE<br />
I mo<strong>del</strong>li cinematici e <strong>di</strong>namici, vincolati con i dati<br />
geodetici raccolti dalla rete, aiuteranno a definire con maggiore<br />
precisione il mo<strong>del</strong>lo geo<strong>di</strong>namico <strong>del</strong>l'area friulana e <strong>del</strong>le<br />
zone orientali <strong>del</strong>la microplacca adriatica. Grazie a questo più<br />
accurato mo<strong>del</strong>lo sarà possibile descrivere meglio sia le zone<br />
sismogeniche, che i processi tettonici che controllano la<br />
deformazione attiva <strong>del</strong>la regione.<br />
Finora sono state calcolate, m<strong>ed</strong>iante il software<br />
GAMIT/GLOBK, le serie temporali giornaliere e le velocità <strong>di</strong><br />
deformazione in atto. In Fig. 2 è riportata la serie temporale <strong>del</strong><br />
sito ZOUF.<br />
Fig. 2 – Serie temporale <strong>del</strong> sito ZOUF, le coor<strong>di</strong>nate sono calcolate nel<br />
sistema <strong>di</strong> riferimento ITRF.
248 D. ZULIANI ET ALII<br />
In Fig. 3 e 4 sono rappresentate le velocità <strong>di</strong> deformazione<br />
finora calcolate.<br />
I sistemi <strong>di</strong> riferimento utilizzati per la determinazione <strong>del</strong>le<br />
Fig.3 – Velocità <strong>di</strong> deformazione dei siti FReDNet. Il sistema <strong>di</strong><br />
riferimento è quello globale ITRF.<br />
Fig.4 – Velocità <strong>di</strong> deformazione dei siti FReDNet. Il sistema <strong>di</strong><br />
riferimento è quello europeo EURA.<br />
velocità sono:<br />
• ITRF - International Terrestrial Reference Frame (Fig.<br />
3)<br />
• European Reference Frame of McClusky (Fig. 4)<br />
Questi sono alcuni dei risultati <strong>di</strong>sponibili anche sul sito web<br />
sopra menzionato.<br />
IL SERVIZIO DI POSIZIONAMENTO IN TEMPO<br />
REALE (GPS-RTK)<br />
La rete FReDNet rappresenta il supporto per l’istituzione <strong>di</strong><br />
un servizio <strong>di</strong> posizionamento in tempo reale che in fase<br />
realizzativa nell’ambito <strong>di</strong> un progetto <strong>del</strong>l’OGS, co-finanziato<br />
dalla Regione Autonoma Friuli Venezia Giulia m<strong>ed</strong>iante<br />
contributi assegnati per la realizzazione <strong>di</strong> progetti <strong>di</strong> ricerca<br />
scientifica e applicata e <strong>di</strong> iniziative <strong>di</strong> trasferimento e <strong>di</strong><br />
<strong>di</strong>ffusione dei risultati <strong>del</strong>la ricerca (ai sensi <strong>del</strong>la L.R. 11/2003<br />
e <strong>del</strong> D.P. Reg. n. 0324/Pres. Del 8/10/2004). A fine progetto<br />
(tre anni), sarà pertanto <strong>di</strong>sponibile, per la Regione FVG, un<br />
servizio innovativo in grado <strong>di</strong> fornire all’utenza pubblica,<br />
privata e scientifica, gli strumenti essenziali per una<br />
navigazione georeferenziata <strong>di</strong> alta precisione in tempo reale. Il<br />
servizio sarà fruibile in ogni istante e in qualsiasi luogo <strong>del</strong>la<br />
Regione con una precisione, per l’utenza finale, <strong>di</strong> qualche<br />
centimetro, <strong>ed</strong> avrà una notevole ricaduta su una varietà <strong>di</strong><br />
potenziali applicazioni commerciali (aviazione, operazioni<br />
marittime, trasporti terrestri, turismo, ecc.), produttive (gestione<br />
<strong>del</strong> territorio, agricoltura e ambiente) e <strong>di</strong> miglioramento dei<br />
servizi (trasporti, emergenze e protezione civile).<br />
BIBLIOGRAFIA<br />
BATTAGLIA M., ZULIANI D., PASCUTTI D., MICHELINI A.,<br />
MARSON I., MURRAY M.H., & BÜRGMANN R. (2003) -<br />
Network Assesses Earthquake Potential in Italy's Southern<br />
Alps. Eos, 84 (28), 262-264.<br />
BATTAGLIA M., ZULIANI D., MURRAY M. H., BURGMANN R.,<br />
AND PRIOLO E. (2004) - Using GPS to assess the earthquake<br />
potential in Friuli (NE Italy). UNAVCO Annual Meeting,<br />
Boulder (CO), February 26-27, 2004. Poster session.<br />
GALADINI F., POLI M.E. E ZANFERRARI A. (2002) - Sorgenti<br />
sismogenetiche responsabili <strong>di</strong> terremoti <strong>di</strong>struttivi<br />
nell’Italia nord-orientale. Atti 21° Convegno GNGTS,<br />
Roma. CD-ROM.<br />
ZULIANI D., PASCUTTI D., BATTAGLIA M., MURRAY M.H.,<br />
MICHELINI A., BÜRGMANN R., MARSON I. (2003) -<br />
FReDNet: Una rete <strong>di</strong> ricevitori GPS per la valutazione <strong>del</strong><br />
potenziale sismico nell’area sud orientale <strong>del</strong>le Alpi<br />
italiane. 22° Convegno GNGTS, Roma, 18-20 Novembre<br />
2003.
In<strong>di</strong>ce<br />
Presentazione.................................................................................................................................................................. Pag. 3<br />
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sheet, central Italy................................................................................................................................................... » 9<br />
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approach..................................................................................................................................................... » 17<br />
Ar g n A n i A. & ro g l e d i s. - The tectonic evolution of the Pisa-Viareggio basin: implications for the Neogene basins<br />
of Tuscany................................................................................................................................................................ » 21<br />
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of extensional fault zones develop<strong>ed</strong> in poorly lithifi<strong>ed</strong>, low-porosity sandstones of the Barreiras Formation,<br />
NE Brazil............................................................................................................................................. » 23<br />
BA r C h i M. r., Co l l e t t i n i C., de PA o l A n., FA u l k n e r d.,lu PAt t e l l i A., Mi r A B e l l A F. & tr i P P e t tA F. - Lithological<br />
and Mechanical Control on the Base of the Crustal Seismogenic Zone: a Case-Study from the Northern Apennines<br />
of Italy............................................................................................................................................................. » 24<br />
BA r n A B A C., MA r e l l o l., vu A n A., PA l M i e r i F., ro M A n e l l i M., Pr i o l o e. & Br A i t e M B e r g C. - Subsurface structure<br />
of the Tagliamento valley (NE Italy) using Microtremors and Gravity Anomaly................................................... » 27<br />
BA r r e C A g. & Mo n A C o C. - Neogene rotations in the Sicilian-Maghrebian Chain: new structural data from the<br />
Madonie Mountains................................................................................................................................................. » 28<br />
Ben s i s., FA n u C C i F. & Po d d A F. - Strutture a macro e mesoscala <strong>del</strong>le Dinari<strong>di</strong> triestine (carta GEO-CGT<br />
<strong>del</strong> FVG)................................................................................................................................................................ » 35<br />
Bo n i n i l., <strong>di</strong> Bu C C i d. se n o s., to s C A n i g., vA l e n s i s e g. - Compatibilità <strong>del</strong>l’assetto strutturale superficiale con la<br />
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(Monte Amiata)....................................................................................................................................................... » 40<br />
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trough:the high Agri Valley, Southern Apennines, Italy.......................................................................................... » 47<br />
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in the Montello-Cansiglio area from geologic and geodetic data (Eastern Southalpine Chain, NE Italy).............. » 48<br />
CA l A M i tA F., es e s t i M e P., PA C e P., PA lt r i n i e r i W.,sC i s C i A n i v. & tAvA r n e l l i e. - The Pliocene-Quaternary salient<br />
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250<br />
in d i C e<br />
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physical properties and new perspectives................................................................................................................ » 65<br />
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Victoria Land (Antarctica): a clue for the Paleo-Pacific margin of Gondwana....................................................... » 66<br />
d’Ad d A P., zA n C h i A., Be r r A F., MA l u s à M., Be r g o M i M. & tu n e s i A. - Structural setting of the Gan<strong>di</strong>no –Sovere<br />
thrust system and new dating of Tertiary magmatic bo<strong>di</strong>es (Orobic Alps, Italy)..................................................... » 68<br />
<strong>di</strong> nA C C i o d., Bo n C i o P., Br o z z e t t i F., & PA z z A g l i A F.j. - Morphotectonics of the Lunigiana-Garfagnana Plio-<br />
Quaternary grabens (Northern Apennines).............................................................................................................. » 70<br />
<strong>di</strong> to r o g., de l gA u d i o P., hA n r., hi r o s e t., ni e l s e n s., sh i M A M o t o t., CAvA l l o A. - Frictional properties of<br />
mantle rocks during earthquakes............................................................................................................................. » 73<br />
<strong>di</strong> PA s q u A l e M. & oC C h i P i n t i r. - Altopiano Ragusano (Sicilia Sud-Orientale): note <strong>di</strong> geologia strutturale................ » 76<br />
es t e B A n F., tA s s o n e A., Me n i C h e t t i M., Ce r r e d o M. e., rA PA l i n i A., li P PA i h. & vi l A s j.F. - Suscettività magnetica<br />
e assetto strutturale nelle Ande inTierra <strong>del</strong> Fuego.................................................................................................. » 80<br />
FA B B r i n i l., Br o g i A. & li o t tA d. - Faglie transtensive e mineralizzazioni a solfuri nell’area meri<strong>di</strong>onale <strong>del</strong> Monte<br />
Amiata: la struttura <strong>del</strong> Monte Civitella (Toscana meri<strong>di</strong>onale).............................................................................. » 84<br />
FA C C e n n A C., Br u n j.P. & ro s s e t t i F. - Exhumation of high-pressure rocks driven by slab rollback........................... » 85<br />
FAC C e n n A C., de FiliPPis l., so l i g o M., Bi l l i A., Fu n i C i e l l o r., ro s s e t t i C. & tu C C i M e i P. - Cicli deposizionali<br />
<strong>del</strong> travertino Lapis Tiburtinus durante il tardo Pleistocene (Tivoli, Italia centrale): influenze<br />
climatiche e tettoniche........................................................................................................................................... » 86<br />
FA n t o n i r. & sC o t t i P. - Time of hydrocarbon generation vs trasforming age in Mesozoic oil play in Po Plain......... » 89<br />
FA n u C C i F., vA r g A s Co r d e r o i., lo r e t o M. F. & ti n i v e l l A u. - Mo<strong>del</strong>li <strong>di</strong> accrezione <strong>del</strong> margine cilenocentro-meri<strong>di</strong>onale.................................................................................................................................................<br />
» 93<br />
Fe d e r i C o l., CrisPini l. & CA P P o n i g. - Paleozoic strike-slip tectonics in northern Victoria Land, (Antarctica) and<br />
the Borchgrevink Orogeny: is there a link?............................................................................................................. » 95<br />
Fer r A n t i l., MA z z e l l A M. e., Mo n A C o C., Mo r e l l i d.& sA n t o r o e. - Active transpression within the frontal<br />
zone of the Southern Apennines in northern Calabria by integration of geomorphologic, structural and<br />
marine geophysical data......................................................................................................................................... » 97<br />
Fe r r A r i n i F., Bo n C i o P., PA P P o n e g., Ce s A r A n o M. & Au C e l l i P. - Nuovi dati su geometria, cinematica e segmentazione<br />
<strong>del</strong> sistema <strong>di</strong> faglie attive lungo il margine nord-orientale <strong>del</strong> Matese (Molise).......................................... » 100<br />
gu e r r i e r o v., MA z z o l i s. & vi tA l e s. - Analisi statistica multi-scala <strong>di</strong> dati <strong>di</strong> scan line condotte su analoghi <strong>di</strong><br />
reservoir <strong>di</strong> idrocarburi............................................................................................................................................. » 104<br />
kA s t e l i C v., Bu r r At o P. & vr A B e C M. - Influence of inherit<strong>ed</strong> geometry and fault history on the seismogenic activity<br />
and potential of strike-slip fault system in NW Slovenia: the case study of Ravne Fault................................ » 108<br />
lA u r i tA s., CAvA l C A n t e F., Be lv i s o C. & Prosser g. - The Liguride complex of the Pollino area (Southern Apennines):<br />
tectonic setting and preliminary mineralogical data....................................................................................... » 111<br />
livio F., si l e o g., Be r l u s C o n i A., Mi C h e t t i A.M., Mu e l l e r k., CA r C A n o C. & ro g l e d i s. - Quaternary evolution<br />
of “blind” fault-relat<strong>ed</strong> folds in the Central Po Plain (Northern Italy).................................................................... » 115<br />
MA F F i o n e M., sP e r A n z A F. & FA C C e n n A C. - Ben<strong>di</strong>ng and growth of the Central Andeanplateau: paleomagnetic and<br />
structural constraints from the Eastern Cor<strong>di</strong>llera(22-24°S, NW Argentina).......................................................... » 121<br />
MA g g i M., ro s s e t t i F., te C C e F. & vi g n A r o l i g. - Fluid assist<strong>ed</strong> shearing at the depth of the Brittle-Ductile Transition:<br />
an integrat<strong>ed</strong> structural, petrological, fluid inclusions study of the Erbalunga shear zone, Schistes Lustrés<br />
Nappe, Alpine Corsica (France)............................................................................................................................... » 125<br />
MA n n i n o i., Ci A n FA r r A P., sA lv i n i F. - Mo<strong>del</strong>ling the fractur<strong>ed</strong> carbonatic rocks of Lepini Mountains to infer deep<br />
hydrogeological pathways........................................................................................................................................ » 126
in d i C e<br />
MA r r o n i M., PA n d o l F i l., gi u n tA g. & MA l A s o M A A. - Tectono-metamorphic history of the Duarte Terrane (Jarabacoa<br />
Area, Hispaniola Island): insights on the tectonic evolution of the northern rim of the Caribbean Oceanic Plateau...... Pag. 127<br />
MA s s i r o n i M., Me n e g o n l. & Bi s tA C C h i A. - On fault misorientation in exhum<strong>ed</strong> metamorphic complexes: sliptendency<br />
analysis of faults in the Alpine orogenic w<strong>ed</strong>ge....................................................................................... » 131<br />
Me n e g o n l. & Pe n n A C C h i o n i g. - Local shear zone pattern and bulk deformation in the Gran Para<strong>di</strong>so metagranite<br />
(NW Italian Alps).................................................................................................................................................... » 133<br />
Min e l l i l., FA C C e n n A C. & CA s e r o P. - Structure and evolution of the Calabrian accretionary w<strong>ed</strong>ge (Southern<br />
Italy).............................................................................................................................................................. » 135<br />
Mi n z o n i n. - Nuovi dati sul ciclo varisico in Calabria................................................................................................... » 137<br />
Mi t t e M P e r g h e r s., dA l l A i l., <strong>di</strong> to r o g. & Pe n n A C C h i o n i g. - Involvement of pore fluids in frictional melting from<br />
stable isotopes study of pseudotachylytes............................................................................................................... » 139<br />
Mo l l i g., Co rt e C C i g., vA s e l l i l. & ot t r i A g. - Caratteri strutturali e interazione fluido-roccia in una faglia normale<br />
ad alto angolo nei marmi <strong>di</strong> Carrara............................................................................................................... » 142<br />
ni g r o F., sA lvA g g i o g., FAvA r A r. & re n d A P. - From compression to extension during the Sicily chain buil<strong>di</strong>ng.... » 144<br />
ni g r o F., sA lvA g g i o g., FAvA r A r. & re n d A P. - Evoluzione tettonica mesozoico-terziaria <strong>del</strong>la Sicilia centrosettentrionale............................................................................................................................................................<br />
» 148<br />
ni g r o F., FAvA r A r., re n d A P., sA lvA g g i o g., Ar i s C o g. & Pe r r i C o n e M. - Neotectonic uplift and tilting of crustal<br />
blocks in Northern Sicily......................................................................................................................................... » 153<br />
ol i v e t t i v., BA l e s t r i e r i M. l., st u A rt F. M. & FA C C e n n A C. - Exhumation and uplift of the Peloritani Mts (south<br />
Italy): constraints from apatite fission-tracks and (U-Th)/He thermochronometry................................................. » 157<br />
Pe r o n i j., tA s s o n e A., Me n i C h e t t i M., li P PA i h. &vi l A s j.F. - Geologia e geofisica <strong>del</strong> plutone <strong>del</strong> Cerro Trapecio<br />
- Tierra <strong>del</strong> Fuego - Argentina.................................................................................................................................. » 160<br />
Pi t tA r e l l o. l., Pe n n A C C h i o n i g. & <strong>di</strong> to r o g. - Deep-seat<strong>ed</strong> pseudotachylyte in the Ivrea Zone metagabbros (Southern<br />
Alps, Italy)...................................................................................................................................................... » 164<br />
Po l i M.e. - Carta geologica dei Monti la Berna<strong>di</strong>a (Prealpi Giulie meri<strong>di</strong>onali, Friuli)............................................... » 168<br />
Po l i M.e., zA n F e r r A r i A. & Mo n e g At o g. - Geometria, cinematica e attività pliocenico-quaternaria <strong>del</strong> sistema <strong>di</strong><br />
sovrascorrimenti Arba-Ragona (Alpi Meri<strong>di</strong>onali orientali, Italia NE)................................................................... » 172<br />
PoM P o s o g. & Pi z z i A. - Evidenze <strong>di</strong> tettonica recente <strong>ed</strong> attiva nel settore esterno sepolto <strong>del</strong>l’Appennino centrale<br />
abruzzese......................................................................................................................................................... » 176<br />
Po n t o n M. - Analisi in profon<strong>di</strong>tà <strong>di</strong> strutture mesoalpine e neoalpine nelle Alpi Meri<strong>di</strong>onali orientali...................... » 179<br />
Pu n z o M., Br u n o P.P. & FA C C e n n A C. - Seismic Evidence of multiple intrusion episodes within the shallow subsurface<br />
of Campi Flegrei and Ischia Island.................................................................................................................. » 183<br />
ro d A M., MA r o t tA A.M. & sPA l l A M.i. - Influenza <strong>del</strong>l’idratazione <strong>del</strong> cuneo <strong>di</strong> mantello sull’evoluzione <strong>di</strong> un<br />
sistema <strong>di</strong> subduzione oceano/continente: una simulazione numerica.................................................................... » 184<br />
ro s s e t t i F., nA s r A B A d y M., vi g n A r o l i g., th e y e t. & ge r d e s A. - Early Cretaceous granulite facies migmatites<br />
from the Sabzevar Range ophiolitic mélange (NE Iran) and its bearing on the closure of Neotethys.................... » 188<br />
ru s t i C h e l l i A.,to n d i e. & Ag o s tA F. - Preliminary results about mechanical stratigraphy of Oligo-Miocene carbonate<br />
grainstones (Majella Mountain,Abruzzo)........................................................................................................ » 190<br />
sA lv i n i F. - Inferring the Fracturing Intensity Patterns in Tectonic Structures by the Computation of the Time Stress<br />
Integral (TSI)........................................................................................................................................................... » 194<br />
sAto l l i s., CA l A M i tA F. & Besse j. - The 100-150 Ma Apparent Polar Wander Path for Adria/Africa.......................... » 196<br />
sB r e s C i A v., tu r C o e. & MiliA A. - Risultati preliminari <strong>del</strong>l’analisi sismica <strong>del</strong> Golfo <strong>di</strong> Squillace (Calabria ionica):<br />
ricostruzione 3D............................................................................................................................................... » 199<br />
sC i s C i A n i v. & CA l A M i tA F. - Active intraplate deformation within Adria: examples from the Adriatic region............ » 203<br />
sM i t h s.A.F., Co l l e t t i n i C., ho l d s W o rt h r.e., MA C P h e r s o n C.g. & vi t i C. - Multiple mechanisms of fault-zone<br />
weakening along a continental low-angle normal fault: The Zuccale fault, Elba Island........................................ » 207<br />
251
252<br />
in d i C e<br />
sPA l l A M. i., zA n o n i d., Wi l l i A M s P.F. & gosso g. - P-T evolution in the western Thor-O<strong>di</strong>n dome, Monashee<br />
Mountains, Cana<strong>di</strong>an Cor<strong>di</strong>llera.............................................................................................................................. Pag. 210<br />
sPl e n d o r e r., MA r o t tA A. M. & BA r z A g h i r. - Origine <strong>del</strong>l’attuale campo <strong>di</strong> sforzo nella zona <strong>del</strong>l’Arco<br />
Calabro.................................................................................................................................................................... » 214<br />
st o rt i F. & BA l s A M o F. - A workflow for laser <strong>di</strong>ffraction granulometry of carbonate cataclastic rocks...................... » 218<br />
tA rtA r o t t i P., Fo n tA n A e. & CrisPini l. - Defomation pattern in a massive pond<strong>ed</strong> lava flow at ODP-IODP Site<br />
1256 (Pacific Ocean): a core and log approach........................................................................................................ » 219<br />
to n d i e., Ag o s tA F. & Ci l o n A A. - Processi <strong>di</strong> fagliazione nei grainstones carbonatici porosi: implicazioni per la<br />
caratterizzazione dei serbatoi naturali <strong>di</strong> geoflui<strong>di</strong>.................................................................................................. » 221<br />
tr i P P e t tA F. Co l l e t t i n i C., vi n C i g u e r r A s. & Me r e d i t h P.g. - Physical properties of triassic evaporites from boreholes<br />
and outcrops.................................................................................................................................................... » 225<br />
vA n n o l i P., BA r B A s., BA s i l i r., Bu r r Ato P., Fr A C A s s i u., kA s t e l i C v., ti B e rt i M.M. &vA l e n s i s e g. - Seismogenic<br />
sources in northeastern Italy and western Slovenia: an overview from the Database of In<strong>di</strong>vidual Seismogenic<br />
Sources (DISS 3.0.4)................................................................................................................................................ » 227<br />
vi g n A r o l i g., ro s s e t t i F., th e y e t. & BA l s A M o F. - P-T con<strong>di</strong>tions of mylonitic shearing in the Granite Harbour<br />
Intrusive complex ofthe Wilson Terrane, Deep Freeze Range, northern Victoria Land, Antarctica........................ » 229<br />
vi tA l e s., lA i e n A F., no v i e l l o A., Ci A r C i A s., MA z z o l i s.& to r r e M. - Analisi strutturale <strong>del</strong>l’Unità Parasicilide<br />
affiorante nell’area <strong>di</strong> Castelnuovo Cilento (Campania).......................................................................................... » 232<br />
vi tA l e s.& MA z z o l i s. - Ruolo <strong>del</strong>lo strain hardening nell’evoluzione <strong>del</strong>le zone <strong>di</strong> taglio: da deformazione omogenea<br />
ad eterogenea................................................................................................................................................. » 235<br />
zA M P i e r i d., FA B B r i P. & Po l A M. - Structural constraints to the Euganean Geothermal Field (NE Italy).................... » 238<br />
zA n C h e t tA s., d’Ad d A P., BA r B e r i n i v., Fu s i n. & zA n C h i A. - Pseudotachylytes along the Orobic Thrust, Southern<br />
Alps, Bergamo: a proxy for the Eo-Alpine deformation?........................................................................................ » 241<br />
zA n C h i A., zA n C h e t tA s., d’Ad d A P., Ag l i A r d i F., Co n t i M.,gA l B i At i F., Be r r A F., sA lv i F. - Utilizzo <strong>di</strong> dati geologico-strutturali<br />
<strong>di</strong> terreno per la mo<strong>del</strong>lazione 3D: esempi <strong>di</strong> strutture Alpine........................................................ » 245<br />
zu l i A n i d., FA B r i s P., PA l M i e r i F. & Pr i o l o e. - FReDNet: un sistema per il monitoraggio <strong>del</strong>le deformazioni crostali<br />
e per il posizionamento <strong>di</strong> precisione in tempo reale in Friuli-V. G................................................................. » 246