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Australian Journal of Earth Sciences (2003) 50, 827–833
Discussion and Reply
Sequence and kinematics of multiple deformation around
Taemas Bridge, Eastern Lachlan Fold Belt, New South Wales
DISCUSSION
G. H. PACKHAM
School of Geosciences, University of Sydney, NSW 2006,
Australia (gpackham@mail.usyd.edu.au).
Hood and Durney (2002) have analysed the sequence of
events in the deformation of Early Devonian rocks in the
Black Range Synclinorium southwest of Yass in the northeastern
Lachlan Fold Belt. The detailed structural analysis
of folds, faults, veins and stylolites has resulted in the
identification of four deformations to which they ascribe a
Carboniferous age based on regional structural relationships.
These relationships and the dating of the deformation
are the subjects of this discussion.
The Black Range Synclinorium contains three conformable
Devonian units. The early Lochkovian to late Pragian
felsic Black Range Volcanics at the base are 2300 m thick,
overlain by 800 m of late Pragian to late Emsian shallowmarine
limestone and shale of the Murrumbidgee Group
and 2900 m of fluvial to lacustrine Hatchery Creek
Conglomerate. The age limits above and below the Black
Range Volcanics are well circumscribed by conodonts. The
lower limit of the Hatchery Creek Conglomerate has been
dated by conodonts in the Murrumbidgee Group. The
Hatchery Creek Conglomerate, the youngest Devonian
sedimentary formation in the area, is a succession of
conglomerate sandstone and shale in upward-fining cycles
1–20 m thick (Owen & Wyborn 1979). A fossil fish fauna has
been found in the formation 2000 m above the base. While
the fauna cannot be dated with the same precision as
conodonts faunas, a late Emsian to middle Eifelian age has
been ascribed to the fish fauna (Young & Turner 2001). This
fish fauna is distinctly older than that from the Late
Devonian Hervey Group (Figure 1), the base of which is
Late Devonian (late Frasnian to Famennian: Pickett 1993;
Young & Turner 2001).
In discussing the regional aspects and dating the
deformation, Hood and Durney (2002 pp. 306, 307) said:
“A recent palaeontological review by Mawson and Talent
(2000) places the Hatchery Creek Conglomerate in the
latest Emsian to middle Eifelian, which theoretically
allows Tabberabberan folding and thrusting after deposition
of the Hatchery Creek Conglomerate and before the
Upper Devonian Hervey Group (Owen & Wyborn 1979).
However, this would have produced significant discordance
between, or possibly within either of, the Middle and
Upper Devonian successions. No such discordances have
been reported from the northern Eastern Lachlan Fold
Belt, nor any internal discordances or fold-related thickness
variations within these groups of the kind that would
be expected if deposition occurred during thrusting and
folding.
The conclusion we draw from these observations is that all
four of our convergent folding events, D1 to D 4, together
with associated or later faulting, post-date the Middle to
Upper Devonian sequences. That is, the deformation
occurred during the Carboniferous Kanimblan event.
This accords with the major Kanimblan deformation
suggested by Powell et al. (1976) for the Hill End Trough,
except that they noted some gentle Middle Devonian
angular discordances in that area. Only mild topographic
uplift, signalled by a change from shallow marine to
terrestrial environment, can be inferred for the Wee
Jasper area at this time.”
These paragraphs state their views on the lack of
medial Devonian deformation in the northeastern Lachlan
Fold Belt. Before making a few more general remarks, I
would like to point out a few relationships from the region
to the north of the Black Range Synclinorium.
Koorawatha Syncline
Referring to the structural relationships in the Koorawatha
Syncline, Hood and Durney (2002 p. 293) say:
“There is, however, evidence of a very gentle angular
relationship regionally at about this level as the Upper
Devonian Hervey Group rests on Illunie Rhyolite (a correlative
of the Mountain Creek Volcanics?) in an extension of
the Taemas Synclinorium approximately 90 km to the
north”
Mapping by Johnston et al. (2001), refining earlier work,
has shown that the Upper Devonian Hervey Group and the
Illunie Rhyolite (now mapped as the Mountain Creek
Volcanics—the lowest of the three formations of the Black
Range Volcanics) are preserved in a north-northwesttrending
zone 1–5 km wide. All the boundaries between
them are faulted. The volcanics, intruded by small granitic
bodies, are confined to this zone. The Hervey Group
extends well beyond the narrow fault zone unconformably
overlying the middle to Late Silurian Young Granodiorite
immediately east and west of the fault zone. Lyons et al.
(2000) quoted a SHRIMP zircon U–Pb age with a possible
minor inheritance component of 428.8 ± 1.9 Ma for the
Young Granodiorite and a K–Ar biotite age 417 ± 6 Ma
(presumably cooling). Although the relationship between
the Hervey Group and the Young Granodiorite probably
results from the sum of Silurian to Middle Devonian movements,
the absence of the Black Range Volcanics and later
Early Devonian formations beneath the basal conglomerate
of the Hervey Group where it overlies the grano-

828 Discussion and Reply
diorite is indicative of a significant unconformity between
the Hervey Group and the volcanics rather than a regional
gentle angular relationship.
Southern Cowra Trough
The Moura Formation, a ?Pridoli turbiditic unit, in the
western Cowra Trough has been intruded by the Eugowra
Suite with zircon SHRIMP ages of 393.0 ± 2.5 and
394.2 ± 2.1 Ma (Lyons et al. 2000). The zircon ages are close
to the Tucker et al. (1998) Emsian–Eifelian boundary of
394 Ma and substantially younger than the Silurian–
Devonian boundary placed at 418 Ma. The Hervey Group
overlies the Eugowra Suite granites unconformably
(Figure 1). The Moura Formation has been correlated
lithologically by Lyons et al. (2000) with the Pridoli
Gooligal Group 15 km to the east. The Gooligal Group is
overlain by formations of the Gregra Group, which
establishes that deposition in the Cowra Trough continued
until at least into early Pragian time and the top could be
as young as late Emsian (Meakin & Morgan 1999).
The Silurian formations of the western part of the
trough are overlain unconformably by the bimodal Dulladerry
and Warrumba Volcanics in what has been called the
Dulladerry Rift. The Warrumba Volcanics unconformably
overlie the Young Granodiorite and Silurian Volcanics at
the southern end of the Dulladerry Rift. SHRIMP zircon
ages of 384.4 ± 3.4 and 382.7 ± 2.8 Ma (Lyons et al. 2000)
confirm the late Givetian to earliest Frasnian age for the
Warrumba Volcanics on the Tucker et al. (1998) Devonian
time-scale suggested by the fish and plant remains.
Powell et al. (1980) have documented a discordance of 3–30
between the Dulladerry Volcanics and the Hervey Group
and a much greater discordance of 27–97 with the Moura
Formation (?Pridoli).
The relationships indicate that there were a number of
orogenic events following the early Pragian (or later) end
of deposition in the Cowra Trough. Deformation of the
trough in Early Devonian time preceded the emplacement
of the Eugowra Suite granites around the Emsian–Eifelian
boundary. Upper Givetian to basal Famennian volcanics
were deposited unconformably over the deformed Cowra
Trough succession. The Eugowra granites were exposed
either prior to or after the Dulladerry–Worrumba Volcanics,
and both the volcanics and the granite were overlain
unconformably by the Hervey Group fluvial deposits. Final
deformation was in Carboniferous time. Allowing for the
uncertainties in both the time-scale and zircon ages, the
Eugowra Suite granites could be Eifelian and younger than
the Hatchery Creek Conglomerate.
Currowong and Tullamore Synclines
Clear evidence of mid-Devonian tectonism is recorded
60 km west of the Eugowra Granite where the Tullamore
Syncline, containing the Hervey Group, trends a few
degrees east of north (Figure 1). Unconformable beneath
the Hervey Group on its southeastern flank trending a
little west of north is the Currowong syncline containing
Lower Devonian rocks of the Trundle Group (Lyons et al.
2000). The difference in strike between the two synclines is
~10. The Currowong Syncline contains the Carrawandool
Volcanics dated at 403.2 ± 2.1 Ma (Emsian) that are
intruded by two granite bodies: the Dalrida Granite, dated
by SHRIMP at 395.4 ± 1.7 Ma and the Bundaburrah
Granodiorite, which is unconformably overlain by the
Hervey Group.
Broader considerations
Figure 1 Location of the Hatchery Creek Conglomerate, the
distribution of Upper Devonian “Lambie Facies” formations in
the eastern part of the Lachlan Fold Belt and the rocks unconformably
underlying them. C, Cambrian; O, Ordovician; S,
Silurian; Smg, mid-Silurian granite; Dl, Lower Devonian; Dlg,
Lower Devonian granite; Dm, Middle Devonian; Dmv, Middle
Devonian volcanics.
On a broader scale in the eastern Lachlan Fold Belt, contrary
to what is implied by Hood and Durney (2002) in the
first paragraph quoted above, there is no location where
deposition has been continuous from the Early Devonian
to Late Devonian and the Early Middle Devonian to Late
Devonian.
Only two early Middle Devonian sedimentary formations
are known in the region east of the Gilmore Fault.
One is the Hatchery Creek Conglomerate, discussed above,
and the other is on the northwestern corner of the Mt
Frome Syncline (Figure 1) where the Mt Frome Limestone
extends into the first conodont zone of the Middle Devonian
and is overlain conformably by the Boogledie Formation,

Multiple deformation around Taemas Bridge 829
a succession of feldspatholithic sandstone, shale and
conglomerate (Meakin & Morgan 1999). This is overlain
unconformably by the basal beds of the Upper Devonian Mt
Frome Group. Elsewhere, east of the Gilmore Fault, the last
recorded Early Devonian marine sediments are variously
late Lochkovian to Emsian.
Possible late Middle to early Late Devonian formations
occur in two areas: (i) the Dulladerry Rift post-dating the
Cowra Trough discussed above; and (ii) the Comerong
Volcanics and the Boyd Volcanic Complex, which are suites
that underlie the coastal strip of the Upper Devonian
Merrimbula Group. The latter volcanics overlie Ordovician
quartz-rich facies rocks and locally part of the Early
Devonian Bega Batholith. There are no older Devonian
sedimentary formations underlying these volcanics. In
both cases, the volcanics are bimodal and are supposedly
deposited in an extensional tectonic environment (Scheibner
& Basden 1998).
The Upper Devonian formations in the Lachlan Fold Belt
east of the Gilmore Fault post-dating the volcanics (‘Lambie
Facies’) are universally unconformable on older rocks
(Figure 1). They are predominantly quartz-rich fluvial
and alluvial facies deposits with a marine transgression
early in the depositional history. On present biostratigraphic
data, the base is late Frasnian to early Famennian.
If it is diachronous, it could be a little older in the east. The
Hatchery Creek Conglomerate, rather than being part of a
diachronous ‘molasse’ overlap sequence, is better regarded
as a short-term incursion of the fluvial facies of the
Mulga Downs Group onto the western flank of the eastern
Lachlan Fold Belt.
An assessment of Middle Devonian deformation around
the Hill End Trough was made in a series of studies by
Powell and coworkers (Powell et al. 1976; Powell & Edgecombe
1978) that focused on the angular discordances
between Upper and Lower Devonian rocks in thinner and
less-disturbed successions on the palaeogeographical
highs east and west of the trough. Even there they found
that half of the 130 angular discordances located were
between 10 and 20. The significant basal Upper Devonian
unconformities on both highs that cut across the Lower
Devonian and Silurian strata to expose Ordovician formations
(Figure 1) were not taken into account.
The question of the timing of deformation of the Hill
End Trough was an issue I revisited (Packham 1999),
reasserting my long-held view that it occurred in Middle
Devonian time. The trough contains a conformable thick
succession of Ludlow to upper Emsian turbidite facies
rocks, the last deep-water sediments in the northeast
Lachlan Fold Belt. As Upper Devonian formations are
confined to the flanking highs, indirect arguments have
been used to deduce the time of its deformation.
The axial trough fill has been metamorphosed up to
biotite grade. In samples from the Hill End goldfield, Lu
et al. (1996b) obtained
40
Ar/ 39 Ar dates on biotite of
359.8 ± 0.6 and 358.2 ± 0.4 Ma, close to the Devonian–
Carboniferous boundary, for the cooling of regional metamorphic
biotite through the closure temperature for argon.
Lu et al. (1996a, b) had found from palaeothermometry and
palaeobarometric studies together with dating of the mineralisation
phases that pressure had remained high and
temperatures elevated well into Carboniferous time. I
argued that these data indicated Middle Devonian deformation,
metamorphism and limited exhumation followed by
consequential slow cooling during and after the deposition
of the flanking Upper Devonian formations prior to deformation
later in the Early Carboniferous.
The structural relationships in the Koorawatha Syncline,
the Southern Cowra Trough the Currowong and
Tullamore Synclines provide clear evidence of deformation
in the middle part of the Devonian. This is supported
by the unconformable relationship of Upper Devonian
formations to older units on the Molong and Capertee
High, a deduced Middle Devonian deformation of the Hill
End Trough and the regional unconformity at the base of
the Upper Devonian “Lambie Facies” throughout the
eastern Lachlan Fold belt. This clear evidence for regional
Tabberabberan events means that the four deformations
identified by Hood and Durney (2002) in the Black Range
Synclinorium cannot be definitively dated as Carboniferous
in age. Some, and perhaps all of them, could have
occurred in Middle Devonian time.
REPLY
D. W. DURNEY 1 and D. I. A. HOOD 2 *
1
Department of Earth & Planetary Science, Macquarie
University, NSW 2109, Australia.
2
4 Cheriton Avenue, Castle Hill, NSW 2154, Australia.
Packham raises some fundamental points about interpretation
of the sub-Upper Devonian unconformity in the
Lachlan Fold Belt. He questions the Carboniferous age of
the four fold- and cleavage-forming episodes in Lower to
lower Middle Devonian units of the Black Range Synclinorium
(Hood & Durney 2002) that we had deduced from
structural correlation with deformed Upper Devonian
rocks and suggests that ‘Some, and perhaps all of them’
could be Middle Devonian. He evidently regards this
method as ‘indirect’ or not ‘definitiv[e]’ and points,
instead, to the widespread unconformable relations of
Upper Devonian units east of the Gilmore Fault (Figure 1)
and to a thermochronological interpretation (Packham
1999) of analytical data (Lu et al. 1996b, c) from the Hill
End area.
This general subject was discussed by Powell and coworkers
and Packham (1999), but we find those assessments
to be incomplete. (For example, they discussed mainly a
single fold episode; we four. They assumed that the mild
angular unconformable relations around the Hill End
Trough were due to fold deformations; we do not.) Packham’s
comments raise questions about what causes unconformities,
a point that we feel must be addressed if further
progress is to be made. We therefore explain what we
consider to be the primary tools for assessing whether
unconformities are due to folding—geometric and
structural criteria—and enquire how they apply to the
ca Middle Devonian discordances that he mentions.
Likewise, we examine the evidence and reasoning for the
*Corresponding author: david_hood@royalsun.com.au

830 Discussion and Reply
ca Middle Devonian metamorphism that he (Packham 1999)
assumed for the Hill End Goldfield.
Geological ages here allow for uncertainties in radiometric
time-scales referred to in Packham (1999).
Folding significance of unconformities
Packham describes the process that produces unconformities
as ‘deformation’ or ‘orogenic events’. However, these
terms are not sufficient to indicate the kind of movement
(which may be uplift, doming, sag, tilting, faulting or
folding (possibly due to no horizontal strain), extension,
contraction, wrench motion, intrusion, volcanic collapse,
etc.). Alternatively, they may perhaps assume a particular
kind of movement (say, folding).
What we tried to convey by the expression ‘significant
discordance’ (Hood & Durney 2002 p. 306) was not a ‘lack of
medial Devonian deformation’ in the broad sense nor
‘continuous’ deposition (Packham) but an angular relation
that is significant in terms of ‘folding and thrusting’. This
is the point that we were addressing and which is the crux
of the matter now under discussion: not whether there are
one or more unconformities, even ones that may cut
through large sections of stratigraphy, but whether they
were produced by folding. (For present purposes we
include also folds produced by wrench-kinematic deformation:
i.e. folding that involves horizontal shortening.) This
type of deformation has some specific geometric and kinematic
characteristics.
For example, the folds that we studied around Taemas
Bridge (Hood & Durney 2002), and which are Packham
candidates for causing a Middle Devonian unconformity,
show limb dips that are moderate to steep (>30), that are
widely distributed at those angles, that alternate on
regular wavelengths and have one or more preferred
strikes. These are characteristics of folding by horizontal
shortening—i.e. by buckling (Price & Cosgrove 1990)—as
we had verified from kinematic observations (Hood &
Durney 2002). Ductile strain and associated cleavage
development is another. Primarily, the dip pattern of the
folds should be reflected by a corresponding discordance
pattern at the unconformity, and this pattern and its
associated small-scale deformation structures should be
missing or weaker (i.e. not fully correlatable) in the
younger strata.
Do the Middle Devonian unconformities show fold
movement?
(1) Angle of discordance (sub-Upper Devonian
unconformity) On magnitude of angularity alone, a
typical limb dip of around 45 or more, as found for many
of the folds east of the Coolac–Narromine Fault (or even
30, allowing for some tightening of early folds by later
deformation), if eroded and overlain by Upper Devonian
sedimentary rocks, would be well beyond the majority of
known discordances at the base of the Upper Devonian or
Lambian unconformity. The quoted discordances are: 73%

Multiple deformation around Taemas Bridge 831
appear to be restricted to the Currowong Syncline (Lyons
et al. 2000; Glen et al. 2002; Packham) and Jesse Syncline
(Glen & Watkins 1999) and grade into sub-concordance
north and south, respectively, along strike. Consistent, very
gentle to gentle, tilts over many kilometres (plus or minus
some faulting: Glen & Watkins 1999) seem much more
prevalent. The Warrumba Volcanics and Dulladerry
Volcanics are clear examples representing the early Late
Devonian phase of the unconformity movement in the
Cowra Trough area. These upper Middle to lowermost
Upper Devonian (Packham) volcanics thicken continuously
from 0 m to up to ~2500 m over about 20 km northwards
beneath the Hervey Group (after mapping by
Conolly 1965; Raymond & Pogson 1998; Raymond et al.
2001). The data indicate tilt components of ~6 and ~8 for
the respective volcanics to the north. Significantly, the
discordance components across strike of the dominant
sub-meridional folds are similar or less and are likewise
consistently orientated, implying no folding of the
volcanics prior to deposition of the Upper Devonian.
(5) Sub-upper Middle Devonian unconformity Nonconformable
relationships of the Dulladerry Volcanics and
coeval Comerong Volcanics/Boyd Volcanic Complex with
underlying Emsian–Eifelian and Early Devonian granites
(Packham) represent a further and older erosion phase,
some time within the early Eifelian to early Givetian
interval. Angularity cannot be assessed from these
relations and so must be sought elsewhere. The sites
involving Upper Silurian to lower Middle Devonian strata
mentioned in point (1) would incorporate any movement of
this age. These, together with known conformable
relations of lower Middle Devonian Hatchery Creek
Conglomerate (discussed in Hood & Durney 2002) and
Boogledie Formation (Packham) with underlying Lower
Devonian limestones, suggest generally low to no angularity
for this phase.
(6) Moura Formation The single case of consistently
higher discordance angle noted by Packham—the gentle to
moderate 26 and 42 local-means of Powell et al. (1980)
between the ?Siluro-Devonian Moura Formation and
Upper Devonian in the Parkes Syncline—may not
represent ca Middle Devonian movement as the tilting
may have occurred prior to or during intrusion of the
late Early Devonian Eugowra Granite.
What are the relationships between folds in Upper
Devonian and Lower to Middle Devonian strata?
Figure 2 Early Carboniferous multiple folding of the Hervey
Group, Gilgandra–Cowra–Yass Zone. Interpreted fold groups are
identified by line symbols; faults and regolith are omitted for
clarity. B, Bumberry Syncline; H, Hervey Syncline; K, Koorawatha
Syncline; M, Mandagery Syncline; P, Parkes Syncline.
Geology after Conolly (1965) and references in Durney (2000);
interpretation by Durney.
Where the preserved Upper Devonian does not rest directly
on the older Devonian units, folds and cleavages should be
structurally correlatable between the two successions if
they had both been affected by the same folding (as
opposed to tilting) at a later time. To minimise possible
complications from (i) different degrees of folding due to
different underlying crustal competencies and (ii) spatial
variation of dominant fold trend, we compare folds and
cleavage in large areas from the same structural zone—the
Gilgandra–Cowra–Yass Zone of Scheibner and Basden
(1998)—as mentioned briefly in Hood and Durney (2002).
These are (i) the predominantly Lower Devonian rocks of
the Black Range Synclinorium (Hood & Durney 2002) and
(ii) the Upper Devonian Hervey Group to the north
(Figure 2: south of the Cowra Trough in Figure 1). Here, we
summarise the correlation for seven independent structural
attributes, with between one and four orientation
subsets each.
(1) Fold trend As shown in Figure 2, the Hervey Group
contains an identical range of fold trends and very similar
orientationally groups [northeast–north-northeast, northnortheast–north
(possibly separate north-northeast and
north trends in the Koorawatha Syncline) and northnorthwest–west-northwest]
to those in the Black Range
Synclinorium (northeast, north-northeast–north, northnorthwest–northwest
and west-northwest–west: Hood &
Durney 2002 figure 13).
(2) Fold sequence Overall fold sequence in the Hervey
Group has not yet been fully established owing to a predominance
of dome–basin interference patterns in these rocks
(Durney 2000). However, a reliable time relation (in Scott
2000 p. 60) was obtained from steep tilting of northeast-foldrelated
vein arrays on the east limb of the dominant north–
north-northeast-trending Mandagery Syncline (Figure 2)

832 Discussion and Reply
at its northern end. As this is the same orientation and
sequence as F 1 (northeast) and F 2 (north–north-northeast)
folds in the Black Range Synclinorium (Hood & Durney
2002), it implies that the earliest known folding in the Black
Range Synclinorium affected also the Hervey Group in this
area.
(3) Fold tightness Interlimb-angle ranges from gentle
to tight in both the Hervey Group (Powell et al. 1980;
Raymond & Pogson 1998; Durney 2000 table 2.1) and the
Black Range Synclinorium (Hood & Durney 2002 table 1).
The dominant folds (north-northeast to north in the
Hervey Group and north-northeast to north-northwest in
the Black Range Synclinorium) are mainly open to close in
both areas (Durney 2000; Hood & Durney 2002). Gentle limb
dips in parts of the Parkes and Bumberry Synclines
(Figure 2) may be attributable to competent granite basement
near those sites (Powell et al. 1980).
(4) Fold continuity The major north-northwest (F 3)
syncline on the eastern side of the Black Range Synclinorium
(the Taemas Synclinorium) continues north-northwestwards,
via a strip of downfaulted Mountain Creek
Volcanics, into similarly folded Hervey Group rocks of the
Koorawatha Syncline (Brunker & Offenberg 1970; Johnston
et al. 2001). These structures are part of the one major
downwarp: Scheibner’s (1976) Cowra–Yass Synclinoral
Zone.
(5) Kinematics Kinematics of the inferred earliest
(northeast–north-northeast or northeast) folds are thruststyle
in both areas. For the inferred second (north-northeast–north)
generation, they are thrust-style in the
Hervey Group and wrench-style in the Black Range
Synclinorium (Durney 2000 table 2.1; Hood & Durney 2002;
figure 13).
(6) Cleavage trend The dominant penetrative cleavage
in mudrocks and penetrative and stripy cleavages in sandstones
locally trend about north-northeast or northwest in
both areas (Scott 2000 figures 2.13, 4.9; D. W. Durney unpubl.
data).
(7) Fissility anisotropy type Moderately to strongly
cleaved mudrocks of both successions show the same,
rather unusual, type of three-dimensional fissility anisotropy
(Durney & Kisch 1994): steeply plunging, beddingtransverse
blades, in places verging on transverse pencils
(Scott 2000 figures 2.9–2.12, 4.6). Both regions have therefore
experienced similar, special, deformation histories,
explained as accumulated non-coaxial shortening associated
with successive folding on widely divergent trends
(Durney 2000; Durney & Hood 2002).
Is there Middle Devonian metamorphism in the
Hill End Trough?
Packham refers to his 1999 interpretation of Lu et al.’s
(1996b) radiometric dating in the Hill End Trough as an
example of ‘Middle Devonian’ (375–364 Ma) folding and
metamorphism. [Those authors considered their data to
indicate ‘Carboniferous’ (363–359 Ma) folding and metamorphism.]
The Hill End Goldfield, where Lu et al.’s (1996b) samples
come from, shows evidence of both hydrothermal activity
(bedding-parallel quartz–gold vein mineralisation) and
regional metamorphism (biotite-bearing rocks with penetrative
cleavage). The bedding-parallel veins, together with
bedding, are folded and boudinaged and are internally
deformed and recrystallised to varying degrees (Seccombe
& Hicks 1989; Windh 1995). The veins thus began to
form ‘before, or during the earliest stage of, the regional
folding’ (Windh 1995 p. 1768). (They also show later
development by slip and dilation during the folding.) Lu
et al. (1996b) gave 380–370 Ma (about late Middle to early
Late Devonian, depending on the time-scale used) as the
age of this stage, based on 40 Ar/ 39 Ar dating of ‘early veinmuscovite’.
(1) Peak temperature The 420C peak temperature
that Seccombe et al. (1993 p. 424) and Packham (1999 p. 26)
equated to their respective peak metamorphisms was
obtained from alteration-carbonate porphyroblasts (Seccombe
& Hicks 1989) associated with the mineralised
bedding-parallel veins (Seccombe & Hicks 1989; Windh
1995). Therefore, the porphyroblasts definitively represent
a hydrothermal event, not a metamorphic one. As they are
‘pretectonic or early syntectonic’ with respect to cleavage
(Seccombe & Hicks 1989 p. 261; Windh 1995 figure 6D), they
must pre-date the regional metamorphism, which should
post-date (Packham 1999) or perhaps syn-date the main
deformation. Thus we find no basis for assigning the early
420C event to metamorphism.
(2) Peak pressure The 2.9 kb pressure estimate (290
MPa or ~11 km burial: Packham 1999) that Seccombe and
Hicks (1989) and Lu and Seccombe (1993) linked to the 420C
temperature was based on ‘metamorphic white mica’ from
the host rocks (Lu & Seccombe 1993 p. 311) rather than on
muscovite from the veins. It therefore relates to the metamorphism.
As the 420C hydrothermal event was not
coincident with the metamorphism, burial during the
former is undefined. It could thus have been much less than
11 km, consistent with its hydrothermal nature and timing
before any deposition of Upper Devonian sediment and
later fold-related thickening of the sediments.
Conclusions
We find the terms ‘deformation’ and ‘orogeny’ insufficient
to convey the nature of the Tabberabberan movement in
the northeastern Lachlan Fold Belt. The two oldest Middle
Devonian stratigraphical formations appear to be conformable
on their respective late Emsian and earliest
Eifelian limestone substrates. The Middle to early Late
Devonian movements that were responsible for the
Lambian unconformity in the region show predominantly
very gentle (

Multiple deformation around Taemas Bridge 833
Acknowledgements
Pat Conaghan and Phil Seccombe are thanked for comments
that led to improvements.
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