Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

approach the long term (Quaternary) behavior of this
active structure.
RESULTS AND DISCUSSION
Onshore Carboneras Fault
La Serrata is a linear range bounded by faults, which front
shows geomorphological evidence of Quaternary tectonic
activity: deflected drainage, beheaded alluvial fans,
alluvial fans without a source area and faulted and folded
Quaternary deposits. Slip rate estimates were obtained by
dating the sediments involved in deformation. At El
Puntal site, a remarkable outcrop shows a pierced
segment of well stratified Early Pleistocene sediments
uplifted a minimum of 50 metres with respect to the
same sediments outcroping in nearby channel bottoms
(Fig. 1). Assuming a minimum age for the Early
Pleistocene alluvial unit of 870 ka BP (upper limit of the
Early Pleistocene) and a maximum age of 2358 ka BP
according to ERS results for related travertine, (Wenzens
1992), we estimate a minimum dip‐slip rate between 0.02
and 0.06 mm/a. Further 10 Be results will better constrain
the age of the Early Pleistocene alluvial fan.
Fig. 1: Panoramic view of the pierced and uplifted Early
Pleistocene alluvial fan sediments at La Serrata segment.
Offset channels (Fig. 2) observed all along La Serrata have
accumulated about 100 metres of displacement along
strike. We estimate that the channels are younger than
the interfluvial Late Pleistocene alluvial units but older
than the calcrete crust formed on Mid Pleistocene alluvial
fans (between 210.2 – 130.8 ka BP according to U/Th
results) where channels are incised, which suggests a
minimum strike‐slip rate of 0.54 mm/a. Thus, from
geomorphological observations, strike‐slip displacement
seems to be around 10 times larger than dip‐slip rate.
Fig. 2: Example of offset drainage along La Serrata range,
at Los Trances site.
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology)
92
At El Hacho site trenches perpendicular to the fault were
open across an evidenced fault scarp to detect
paleoearthquake evidence (Moreno et al., 2007). The
trenches exposed two fault zones, one of them reaching
up to the surface faulting all Pleistocene units. Chaotic
and wedged deposits were interpreted as colluvial
wedges related to sharp fault movements proving the
seismic behaviour of the fault. A minimum of 4 events
were identified in the trenches, two at the base of the
colluvial wedges: E h1 and E h2 constrained by TL between
Mid Pleistocene age and 49.9 ka BP; and two younger
events between faulted and unaltered units: Eh3
constrained by TL between 41.5 ka BP and 26.6 ka BP, and
E h4 constrained between 30.8 and present time in this
tench.
Also, at El Hacho site, a 3D trenching survey with
trenches parallel to the fault showed a faulted and buried
paleochannel (Moreno et al., 2008). Radiocarbon analyses
date this paleochannel as 772‐937 AD. Two events are
infered from this faulted paleochannel, the first one
would have displaced the drainage course and the second
one would have faulted the channel infill. The first event
is interpreted to be event E h4 deduced in trenches across
to the fault, adjusting the its upper age in 1061 BP. The
second event afecting the paleochannel (Eh5) has an age
constrained between the age of the infilling material
(1313 ka BP) and present time. The horizontal maximum
offset of the paleochannel is 3 m, and although part of
this offset could have not a tectonic origin, a minimum
strike‐slip rate of 0.1 mm/a for the last 30 ka can be
estimated taking into account the age of the incised
alluvial fan.
Trenches digged in fluvial sediments (El Hacho gully, Cerro
Blanco and Pecho de los Cristos sites) show fluvial
deposits overlaying the fault zone. Near El Hacho site, the
fluvial sediment was dated by 14 C (Moreno et al., 2007)
suggesting seismic silence since 688‐889 AD.
Thus, the faulted paleochannel and unfaulted fluvial
sediments seem to precisely constrain the age of the last
earthquake (Eh5) in this segment of the fault between
1313 BP and 1061 BP, and suggest an elapsed time since
the last earthquake of 1061 to 1178 yrs. A fault relay to a
parallel fault trace could also explain the similar ages
between faulted and indisturbed sediments.
At Los Trances site, an artificial wall shows older faulted
Quaternary sediments depicting at least 2 events: E t1 and
E t2 delimiting faulted and non faulted Mid Pleistocene
sediments and constrained between Mid Pleistocene and
180,1 ka BP (Fig. 3 below) according to U/Th results. A
trench was open parallel to the wall to observe the upper
faulted sediments and a different fault zone structure was
shown. Here two younger events were defined and ages
were constrained by TL: E t3 (83 – 69.1 ka BP) and E t4 (38 –
Present) (Fig. 3 above). From these four events; three of
them correlate well with events observed at El Hacho.