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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 (