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349

Foster, D.A., and Gray, D.R., 2007, Strain rate in Paleozoic thrust sheets, the western Lachlan Orogen, Australia: Strain analysis and fabric geochronology, in Sears, J.W., Harms, T.A., and Evenchick, C.A., eds., Whence the Mountains? Inquiries into the Evolution of Orogenic Systems: A Volume in Honor of Raymond A. Price: Geological Society of America Special Paper 433, p. 349–368, doi: 10.1130/2007.2433(17). For permission to copy, contact [email protected]. ©2007 The Geological Society of America. All rights reserved.

The Geological Society of AmericaSpecial Paper 433

2007

Strain rate in Paleozoic thrust sheets, the western Lachlan Orogen, Australia: Strain analysis and fabric geochronology

David A. Foster*Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA

David R. Gray†

School of Earth Sciences, University of Melbourne, Melbourne 3010, Victoria, Australia

ABSTRACT

Average orogenic strain rates may be calculated when it is possible to date mica cleavage or syndeformational veins and estimate fi nite strain. Deformation of accretionary-style thrust sheets in the western Lachlan Orogen occurred by chevron folding and faulting over an eastward propagating décollement. Based on 40Ar/39Ar dates of white micas, which grew below the closure temperature, this deformation started ca. 457 Ma in the west and ended ca. 378 Ma in the east, with apparent “pulses” of deformation ca. 440, 420, and 388 Ma. The 40Ar/39Ar data from thrust sheets in the Bendigo structural zone show that deformation progressed from early buckle folding, which started at 457–455 Ma, through to chevron fold lock-up and thrusting at 441–439 Ma. Based on retrodeformation, the total average strain for this thrust sheet is −0.67, such that the bulk shortening across the thrust sheet is 67%. This amount of strain accumulated over a duration of ~16 m.y. gives a minimum strain rate of 1.3 × 10−15 s−1 and a maximum strain rate of 5.0 × 10−15 s−1, based on fan thickness considerations. The total shortening is between ~310 km and ~800 km, which gives a décollement displacement rate between ~19 mm yr−1 (minimum) and ~50 mm yr−1 (maximum). If deformation occurred in pulses ca. 457–455 and ca. 441–439 Ma, then the calculated strain rate would be on the order of 1 × 10−14 s−1. These strain rates are similar to convergence rates in western Pacifi c backarc basins and shortening rates in accretionary prisms and turbidite-dominated thrust systems as in Taiwan.

Keywords: strain rate, Lachlan Orogen, 40Ar/39Ar thermochronology, strain analysis.

*dafoster@ufl .edu†[email protected]

350 Foster and Gray

INTRODUCTION

In ancient orogens, quantifying strain rates and displace-ment rates has been more challenging than determining the timing of discrete geological events. Deformation within oro-genic belts can be quantifi ed as a strain rate (e⋅ = e/t, where e is the elongation, and t is the time in seconds) or as displacement rate (velocity) on structures or the deformation front. Short-term strain rates in active tectonic environments have been determined by geodic measurements using global positioning system (GPS) instrument arrays (e.g., Puntodewo et al., 1994; Bennett et al., 1999; Chang et al., 2003). In ancient orogens, however, determining rates of deformation has been problem-atical, because precise chronologies of fabric development are limited (Ramsay, 2000). Strain rates in inactive orogens have been estimated assuming that deformation pulses have an average duration of <5 m.y. (Pfi ffner and Ramsay, 1982), and paleodisplacement rates on faults have been estimated by thermochronologic analyses (e.g., Foster and John, 1999; Wells et al., 2000; Brichau et al., 2006), but there are only a small number of direct estimates of internal deformation rates within the lower or middle crust of an orogen (e.g., Ramsay, 2000; Müller et al., 2000).

Accretionary orogens, or orogenic belts formed from the addition of material to continents from plate margin processes at convergent or transpressive plate boundaries, are one of the most important “factories” for producing and maturing conti-nental crust (Şengör and Natal’in, 1996; Windley et al., 2001; Foster and Gray, 2000; Gray et al., 2007). Although the set-ting for accretionary orogens is relatively well understood, the rates of deformation within these orogens and in accretionary thrust systems is poorly defi ned. In this paper, we summa-rize data from the Lachlan Orogen of southeastern Australia, which is a Paleozoic, turbidite-dominated accretionary orogen (e.g., Gray and Foster, 2004). Our analysis of paleodeforma-tion rates in the western Lachlan Orogen combines detailed geochronology of fabrics and faults with restoration of strain in metaturbidites. These data suggest that strain rates for the orogen were on the order of 10−14–10−15 s−1 for regional-scale fold-thrust sheets.

GEOLOGIC SETTING

The eastern part of Australia formed along the Paleozoic margin of Gondwana due to accretion of crust of oceanic affi ni-ties, recycled crustal-derived turbidite, and oceanic volcanic arcs (Foster and Gray, 2000; Cawood, 2005). Eastern Australia is now >1000 km wide and consists of three distinct orogenic belts, the Delamerian, Lachlan/Thomson, and New England orogens, col-lectively referred to as the Tasmanides or the Tasman Orogenic Belt (Fig. 1) (Scheibner, 1978; Gray et al., 2006b). Accretion of these orogenic belts occurred in a stepwise fashion with an east-ward decrease in the age of peak deformations from Cambrian through Triassic times.

New England Orogen

500 km

Sydney

Melbourne

Tasmania

Del

amer

ian

Oroge

n

Narooma Accretionary Complex

Howqua Accretionary Complex

Georgetown inlier

Gawler craton

Thomson

Orogen

Lachlan Orogen

Coen inlier

Fig. 2

Adelaide

EASTERN

CENTRAL

150°E40°S

140°E40°S

140°E15°S

WESTERN

MtIsa inlier

New England OrogenLachlan/Thomson Orogen

Delamerian Orogen

structure form lines

major faults

Permo-Triassic foreland basin

Ordovician volcanic rocksPrecambrian basement

Figure 1. Tectonic province map of eastern Australia showing the oro-genic belts of the Tasmanides. Western, central, and eastern refer to the three subprovinces of the Lachlan Orogen. The box outlined by the dashed line shows the area of the map in Figure 2.

Strain rate in Paleozoic thrust sheets 351

Lachlan Orogen

The Lachlan Orogen is the central orogenic belt in the Tas-manides and consists of three subprovinces (Fig. 1) (e.g., Gray and Foster, 2004). The western and central Lachlan Orogen are dominated by a turbidite succession consisting of quartz-rich sandstones and black shales (Fig. 2) (VandenBerg et al., 2000). These are laterally extensive over 800 km present-width and have a current thickness upward of 10 km. The eastern Lachlan Orogen consists of andesitic volcanics, volcaniclastic rocks, and limestone, as well as quartz-rich turbidites and extensive black shale in the easternmost part (VandenBerg and Stewart 1992; Glen et al., 1998).

The turbidite fan was deposited within a Cambrian to early Ordovician oceanic backarc basin (Fig. 3) that was fl oored mainly by crust of oceanic affi nity but may have also contained small continental ribbons and/or older rifted magmatic arcs (e.g., Foster et al., 2005). In the western province, Cambrian mafi c vol-canic rocks of oceanic affi nities underlie the quartz-rich turbidite succession, whereas in the eastern province, the oldest rocks observed are Ordovician arc volcanic rocks and a late Cambrian–Early Ordovician chert-turbidite-mafi c volcanic sequence. The three provinces of the Lachlan were juxtaposed along regional fault systems both within and bounding the central belt (Fergusson et al., 1986; Morand and Gray, 1991; VandenBerg and Stewart, 1992; Glen, 1992; Gray and Foster, 1998; Foster et al., 1999; Spaggiari et al., 2003, 2004a).

Closing of the Lachlan backarc basin caused Silurian to Devonian deformation of the turbidite fan and underlying mafi c crust by underthrusting on both sides (Gray and Foster, 1997; Foster et al., 1999; Fergusson, 2003; Spaggiari et al., 2004b). This took place behind the major Pacifi c-Gondwana subduction zone, in a supra-subduction zone position (Foster et al., 1999; Collins, 2002). The thickest part of the sediment fan system, in the west-ern Lachlan Orogen, was deformed into dominantly east-vergent structures over a west dipping décollement system associated with west-directed underthrusting and subduction (Fig. 3).

Major Structures in the Western Lachlan Orogen

The western Lachlan Orogen is composed of a deformed turbidite sequence cut by a series of major west-dipping, reverse faults that link to an inferred, gently west-dipping, mid-crustal décollement (e.g., Cox et al., 1991; Gray and Willman, 1991a, 1991b; Gray et al., 1991, 2006a, 2006b; Glen 1992, Fergusson and Coney 1992; Gray, 1995; Gray and Foster, 1998). Part of a composite, east-vergent, leading-imbricate fan fold- and thrust-belt, the reverse faults expose Cambrian mafi c volcanics, cherts, and volcaniclastics in their immediate hanging walls (Gray and Willman, 1991b; Gray, 1995; Gray and Foster, 1998; Gray et al., 2006b). Thrust sheets largely consist of chevron-folded sandstone and mudstone layers that refl ect up to 65% short-ening above the mid-crustal décollement (Gray and Willman, 1991a, 1991b). Deep crustal seismic refl ection profi les (Gray

et al., 1991, 2006b; Korsch et al., 2002) and microseismicity studies (Gibson et al., 1981) indicate that the discontinuity is now at ~15–17 km depth (Fig. 2C).

The western Lachlan comprises three structural zones with variations in strike, style of deformation, and timing of defor-mation. These are, from west to east, the Stawell, Bendigo, and Melbourne zones (Fig. 2A). The western part of the Stawell zone is transitional to the Cambrian Delamerian Orogen through a series of faults between the Coongee and Moyston faults (Foster et al., 1999; Miller et al., 2005). Major strike-parallel faults in the structural zones have linear map traces from 25 to 100 km in length and generally dip at 60° to 70° west (Figs. 2A and 2B). Minor faults with throws of <100 m are abundant throughout the western Lachlan Orogen and show both dips to the east and to the west. NW-SE– and NE-SW–trending cross-faults, generally with strike displacements of meters to tens of meters, are also present within the zone (Gray and Foster, 1998).

Stawell ZoneThe Stawell zone consists of Cambrian volcanic rocks

(>4 km thickness) of tholeiite-boninite association and vol-cano genic sediments overlain by unfossiliferous Cambrian-Ordovician , or most likely late Cambrian, quartz-rich turbidites with a distinct 500 Ma detrital zircon population (VandenBerg et al., 2000). West-dipping, high-strain zones separate regions of lower strain, characterized by symmetrical chevron folds and a single main cleavage (Wilson et al., 1992; Gray and Foster, 1998; VandenBerg et al., 2000; Gray et al., 2003).

Bendigo ZoneThe Bendigo zone consists of an Ordovician turbidite

package (~3–4 km original thickness), Upper Cambrian shales and cherts (0.9 km maximum original thickness), and the Mid- to Lower Cambrian volcanic and volcaniclastic rocks (2–2.5 km original thickness) (VandenBerg et al., 2000; Gray et al., 2003). The major faults dip west, juxtapose older rocks over younger rocks, and contain Lowermost Ordovician (Lance-fi eldian) strata in their immediate hanging walls. This suggests that fault propagation and detachment within the Lancefi eldian strata was associated with an easterly transport of the folded and telescoped cover over the Cambrian metavolcanic succession. Fault-bounded slices of Cambrian rocks within the Heathcote fault zone (see Figure 3 in Spaggiari et al., 2004a), the leading fault, suggest duplexing and serial detachment in the Cambrian succession (Gray and Willman, 1991b). Inferred décollements developed at 4–6 km depth within the Lancefi eldian strata (the Campbelltown, Muckleford, and Whitelaw faults are splays off this level) and at 7–10 km depth within the lower part of the Cambrian succession (Heathcote Fault Zone).

Melbourne ZoneThe Melbourne zone consists of chevron-folded Silurian and

Devonian interbedded mudstone and sandstone overlying Ordo-vician black shale and Cambrian andesitic lavas, agglomerates,

AF

Z

CFZ

LFZ

H F

Z

MW

FZ

AVO

CA FZ

CAMPBELL

TOW

N

FAULT

MUCKLE

FORD

FAULT

HEATHCO

TE F

Z

Silurian /Devonianturbidites, mudstones, reworked sandstones

COO

NGEE

FAULT

LANDSBO

ROUG

H

F

AULT

MT

WELL

ING

TON

F

Z

Early Carbon- iferous

EW

Ordovician turbidites

Cambrian

Cambrian

GraniteSedimentary coversequences

Volcanics Cambrian mafic volcanics Fault trace Form lines/ bedding

MFZ

greenschist faciesstrong to intense

cleavage

prehnite -pumpellyite facies/ weak or no cleavage

greenschist facies/ well-developed cleavage

Cambro-Ordovician turbidites

A

B

50 km

Melbourne

146˚E 147˚E145˚E144˚E 148˚E

37˚S 37˚S

WHIT

ELAW

FAULTM

OYSTO

N

FZ

GO

VERNO

R

FAU

LT

LF WF

MF

CFG

FZ

WFZ

KFZ

PF

BALLARAT

EAST

FA

ULTM

EFZG

FZ

455-440 Ma bedding parallel & upright fabrics

430-410 Ma reactivation of faults,minor crenulation cleavage & exhumation

388-378 Mafolding & thrusting/ steep cleavage in east

378-372 Ma reactivation & brittle fracturing

Cambrian andesite

50 km

EF

0

4

8

12

TWT (s)

MOHO

0

4

8

12

TWT

(s)

vvv

vvv

C

(~ 500)

(~ 500)440±2

443±2439±2453±2

455±2

439±2

441±2457±4

440±4

410±3

416±4

388±4372±2

374±2

378±4388±4

374±2

375±2

426±4

375±2

382±2

379±2417±2

426±2

~ 420

Bendigo Zone

Melbourne Zone

Stawell Zone

Tabberabbera

Zone

STA

WEL

L-

AR

ARAT

FZ


boninites, pillowed and massive tholeiitic basalts, and gabbroic-doleritic intrusive rocks (VandenBerg et al., 2000; Gray et al., 2003). The Cambrian metavolcanic rocks occur as fault-bounded inliers within the structurally lowest part of the Mount Welling-ton fault zone and have a complex internally imbricated stratig-raphy suggestive of the upper part of a duplex system (Gray and Foster, 1998; Spaggiari et al., 2003). Fault-bounded slices of Late Ordovician black mudstone and slate occur above the Cambrian inliers and are elongated subparallel to the regional strike with tapered ends (Gray, 1995; Gray and Foster, 1998).

Style of Folds, Faults, and Cleavage

Stawell ZoneQuartz-rich turbidites in the Stawell zone are tightly

chevron-folded and cleaved and cut by major steeply (~60°) west-dipping NW-trending faults or high strain zones (Wilson et al., 1992; Cayley et al., 2002; Gray and Foster, 1998; Phillips et al., 2002; Miller et al., 2005). These include the Stawell-Ararat (incorporating the Mount Ararat, Cathcart, and Coongee faults), Landsborough, Percydale, St Arnaud, and Avoca fault zones, which are regularly spaced at ~15–20 km across the structural zone. The fault zones are characterized by broad (up to 2 km wide) zones of polydeformation with hanging-wall struc-tures, including crenulation folds, crenulation cleavage, boudi-nage, and S-C fabrics indicating west-over-east displacement.

The variable geometry of the upright, north-trending chev-ron folds defi ne large-scale synclinorial and anticlinorial clo-sures (Cayley and MacDonald, 1995). Antiformal culminations are coincident with the hanging walls of the major faults where metamorphic grade is locally biotite zone. This succession has undergone 70%–85% shortening through regional folding and cleavage development, with an additional component due to thrust faulting within the sheet (Wilson et al., 1992; Cayley and MacDonald, 1995).

The consistent low-greenschist metamorphic grade indi-cates a fl at enveloping surface for the tight folds, where the major faults have listric form, rooting into an underlying décolle ment.

The lower (volcanic) parts of the stratigraphy are only brought to the surface on some of the major faults, including the Mount Ararat, Stawell-Ararat, and Avoca fault zones (see Figure 3 in Wilson et al., 1992).

Bendigo ZoneRegional anticlinoria and synclinoria in the Bendigo zone,

as inferred from graptolite biostratigraphic zones, have wave-lengths of 10–15 km and amplitudes on the order of 1–2 km (Cox et al., 1991; Gray and Willman, 1991a, 1991b; Gray et al., 2006a). Folding consists of extremely regular parallel trains of gently plunging, upright chevron folds with their axial surface traces spaced at ~8 per kilometer. They show several orders of folding, with lower order fold-wavelengths ranging from 100 to 300 m and fold-amplitudes from 50 to 100 m (Gray and Willman, 1991a, 1991b; Fowler and Winsor, 1996). Strike-lengths of the parallel chevron folds are on the order of tens of kilometers. The folds have typical chevron form with long, straight limbs and narrow hinge zones. Hinge zones are rounded in sandstone-dominant facies to a more angular form in mudstone-dominant sections (Fowler and Winsor, 1996). Limb-thrusts, bedded veins, and quartz saddle reefs are rela-tively common (Cox et al., 1991; Gray et al., 1991; Jessell et al., 1994; Fowler, 1996; Fowler and Winsor, 1996, 1997).

Cleavage varies from divergent-fanning slaty cleavage or primary crenulation cleavage (White and Johnston, 1981) in pelites to a convergent-fanning spaced cleavage in sandstones (Yang and Gray, 1994). It consists of thinly spaced zones of subparallel white mica and chlorite alternating with inter-cleavage zones of less-ordered quartz and white mica aggre-gates ( Glasson and Keays, 1978; Stephens et. al., 1979; Gray and Willman, 1991a, 1991b; Yang and Gray, 1994). Abundant evidence of dissolution is shown by truncated quartz grains along cleavage, stripy differentiated layering, pressure shadows , and mica beards (cf. Glasson and Keays, 1978; Stephens et al., 1979; Waldron and Sandiford, 1988; Yang and Gray, 1994). Cleavage in pelitic layers at the structurally lowest levels of individual thrust sheets, or in the immediate hanging walls to major faults, is more intense with closer spacing of mica seams and low (<20°) bedding-cleavage angles such that slates have a distinct phyllitic sheen (Gray and Willman, 1991a, 1991b).

Major intra-zone faults of the Bendigo zone include the Campbelltown, Leichardt, Muckleford, Sebastian, and White-law faults. The faults have N-S trends, spacing of ~20 km, steep (>60°) westerly dip, and throws on the order of 1–2 km (Gray and Willman, 1991a, 1991b; Gray and Foster, 1998). These faults generally place lowermost Ordovician (Lancefi eldian) over either mid-Early Ordovician (Chewtonian-Castlemainian) or latest Early Ordovician (Darriwilian-Yapeenian) rocks. Chev-ron folds in the turbidite succession of their hanging walls show increased fold tightness (<30°) and markedly higher strains (X/Z > 9:1), as well as a change in axial surface dip direction within 3 km of the fault trace (Table 1) (see Figure 6 in Gray and Willman, 1991b).

Figure 2. (A) Structural trend map and (B) structural profi le, with degree of cleavage development and grade of metamorphism of the western the Lachlan Orogen incorporating the Stawell, Bendigo, and Mel-bourne zones (after Foster et al., 1999). Faults are shown as bold lines: MFZ—Moyston fault; CFZ—Congee fault; LFZ—Landsborough fault zone; PF—Percydale fault; AFZ—Avoca fault zone; BEF—Ballarat East fault; CF—Campbelltown fault; LF—Leichardt fault; MF—Muckleford fault; WF—Whitelaw fault; HFZ—Heathcote fault zone; MEFZ—Mount Easton fault zone; MWFZ—Mount Wellington fault zone; GFZ—Governor fault zone; WFZ—Wonnangatta fault zone. Circles show positions for 40Ar/39Ar samples with ages in million years and one-sigma errors: bold black type refers to primary mica cleav-age or syntectonic vein mica age; white type on black shading refers to fault reactivation, or post-tectonic vein mica age. (C) Crustal cross section for the western Lachlan orogen based on surface exposures and geophysical data (from Gray et al., 2006b). TWT—two-way time.

AFZ

HFZ

MW

FZ

KFZ

GiFZ

SAFZ

AFZ

HFZ

SAFZ

A 4

50

–44

0 M

a

C 4

30

–42

0 M

a

BZ

SZM

ZTZ

OZ

~70

0–80

0 km

Doo

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502

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Ma

Lico

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vv

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vv

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crat

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380

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Figu

re 3

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and

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ate

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FZ—

Avo

ca f

ault

zone

; BZ

—B

endi

go z

one;

GFZ

—G

over

nor

faul

t zo

ne;

HFZ

—H

eath

cote

fau

lt zo

ne;

KFZ

—K

iaw

a fa

ult

zone

; M

WFZ

—M

ount

Wel

lingt

on f

ault

zone

; M

Z—

Mel

bour

ne z

one;

OZ

—O

meo

zo

ne; S

AFZ

—St

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l-A

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t fau

lt zo

ne; S

Z—

Staw

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TZ

—Ta

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Melbourne ZoneThe Melbourne structural zone shows a transition from open,

upright folds with weak or no cleavage to inclined and overturned, tight to isoclinal folds with strong axial surface cleavage, into a 10–15-km-wide zone of intense deformation as part of the Mount Wellington fault zone (Gray, 1995; Gray and Foster, 1998). Major anticlinoria and synclinoria are spaced at ~20 km intervals and have wavelengths of ~40 km and amplitudes of ~4 km. Regionally, the folds tend to be upright, open to tight chevron folds cut by steeply dipping reverse faults and have 70°–100° fold interlimb angles (Table 1) showing either a weak reticulate fi ssility or no cleavage fi ssility at all (Gray, 1995; Gray and Mortimer, 1996). The amount of internal strain recorded by regional folding, based on arc length determinations, and penetrative fabric development in the higher strained areas (see below), approximates ~32% shortening (Gray, 1995). Where cleavage is more strongly developed, the folds are tight, have more regular interlimb angles ranging between 60° and 70°, and refl ect a shortening of ~53%. Where cleavage is intense and folds are cut by signifi cant numbers of faults, the fold inter-limb angles range between 18°–42°, and shortening approaches 64%. Within the Mount Wellington fault zone, narrow elongate fault-bounded slivers of Cambrian metavolcanic rocks and Ordo-vician black slate, up to 20 km in strike-length and 4 km outcrop width, occur associated with strongly foliated phyllonite (Gray, 1995; Gray and Foster, 1998).

P-T Conditions of Metamorphism and Deformation

The eastern boundary of the complex transition zone between the Delamerian and Lachlan orogenic belts (Korsch et al., 2002; Miller et al., 2005) is gradational into a wide zone

of greenschist facies turbidites metamorphosed under medium-pressure conditions (Offl er et al., 1998; Phillips et al., 2002). Exposed rocks in the eastern Stawell zone and the northwestern part of the Bendigo zone were metamorphosed to biotite zone conditions, with a biotite-muscovite-quartz-albite assemblage in pelitic rocks (Morand et al., 1995). Greenschist facies meta-basalts along the Avoca fault zone are composed of actinolite-chlorite-epidote-albite-sphene or chlorite-sericite-albite ± car-bonate assemblages. In most of the Bendigo zone, the meta-morphic grade is chlorite zone with some subgreenschist facies (anchizone) rocks in the east (Offl er et al., 1998).

Metamorphic pressure for the low-grade rocks (apart from contact zones of Devonian granitic intrusions) in the Stawell and Bendigo zones, based on lattice parameter (b

0) measure-

ments of phengitic micas, was ~0.4 GPa, assuming a tem-perature (T ) of ~360 °C, indicating an intermediate pressure (P) series (Offl er et al., 1998). The Fe/Mg compositions of coexist ing actinolite and chlorite from metabasalt in the Avoca fault zone indicate moderately high-P metamorphism (Offl er et al., 1998). Most of the Heathcote fault zone meta basalt is prehnite-pumpellyite facies, with assemblages of albite-chlorite-prehnite-pumpellyite-titanite in the tholeiitic rocks (Spaggiari et al., 2002; Crawford et al., 2003). Metamorphic grade increases up to lower greenschist facies toward the base of the Heathcote fault zone (Spaggiari et al., 2002; Crawford et al., 2003). Blueschist facies metavolcanic rocks occur as blocks in serpentinite-matrix mélange in the central segment, where deeper structural levels are exposed (Spaggiari et al., 2002). They consist of winchite (Na-Ca blue amphibole) + albite + stilpnomelane + chlorite + Mg-Cr spinel, with acces-sories titanite + quartz ± apatite ± talc, which give estimated

TABLE 1. THRUST WEDGE COMPONENTS, WESTERN LACHLAN OROGEN W E

TLEBNRETSAETLEBNRETSEWenoZenruobleMenoZogidneBenoZllewatS

DtnemgeSCtnemgeSBtnemgeSAtnemgeSGooseneck synclinorium

Mount Wellington Fault Zone

Age of sediments Cambrian Ordovician Silurian, Devonian Silurian, Devonian Late Ordovician, Silurian

Deformation style chevron folds, cleavage, reverse

faults

chevron folds, cleavage, reverse

faults

open chevron folds

close to tight chevron folds,

cleavage

transposition layering, isoclinal

folds Fold interlimb angle

Thrust sheet Basal fault zone

30°–60° >30°

30°–60° >30°

60°–100° 50°–70° >30°

Background X/Z strain

(4:1) 4:1 zero ~1.4:1 ~10:1

Maximum X/Z strain undetermined ~34:1 — 1.4:1 ~15:1Percent shortening (~66%) ~66% ~34 % ~40% ~69%Deformed width ~62 km ~96 km ~85 km ~40 km ~16 km

enoztluaflasaBLithology metavolcanic rocks Lower Ordovician

turbidites — — Ordovician-Silurian

turbidites X/Z strain >15:1 34:1 15:1 Metamorphism epizone epizone anchizone epizone epizone

356 Foster and Gray

pressures and temperatures of 0.6–0.7 GPa and <450 °C and formed ca. 455–440 Ma (Spaggiari et al., 2002).

Rocks in the west of the Melbourne zone are unmetamor-phosed (diagenetic zone) (Offl er et al., 1998), and those in the east are greenschist grade. There is a transition from uncleaved or weakly cleaved in the west and north (Gray and Mortimer 1996) to strongly cleaved in the east in the Mount Welling-ton fault zone, where slate and phyllite are the dominant rocks (Vanden Berg et al., 1995; Gray, 1995). Cambrian mafi c to inter-mediate vol canic rocks in the Mount Wellington fault zone show pumpellyite-actinolite and lower greenschist facies assemblages (VandenBerg et al., 1995). Chlorite-actinolite assemblages defi n-ing foliated margins and shear zones within fault slices of meta-volcanic rocks indicate temperatures of 350–400 °C and pressures of 0.2–0.4 GPa during deformation and internal fault duplexing, suggesting that the décollement initiated at a depth of 7–12 km.

Chronology of Deformation

The geochronology of metamorphism and deformation in the western Lachlan has been defi ned mainly by 40Ar/39Ar dating of white mica growth in the low- to intermediate-grade metamor-phic rocks (Foster et al., 1996a, 1996b, 1998, 1999; Bierlein et al., 1999, 2001; Gray et al., 2003; Miller et al., 2005). The data come from several types of structures and lithologies within the thrust wedges, including mica concentrates and whole-rock analyses of mica-rich cleavage bands, mica separates, single grains from syndeformational quartz veins, and mica separates and single grains from fault fi bers. Within the Bendigo and Stawell struc-tural zones, data from the different structures are very consistent. The data from the Melbourne zone are more variable in quality due to the relatively lower grade of metamorphism and deforma-tion outside of the frontal fault zone.

At low greenschist and prehnite-pumpellyite facies P-T conditions, metamorphic neocrystalline white mica grows at or below the argon closure temperature of ~330–400 °C. The ages obtained from white mica grown in these conditions, there-fore, refl ect the timing of deformation-induced crystallization or recrystallization (e.g., Dunlap et al., 1991, 1997; Kirschner et al., 1996; Foster et al., 1999; Mulch and Cosca, 2004). In the metaturbidites of the western Lachlan, there is a distinct mica-preferred orientation related to fabric development during thrust-sheet emplacement and to cleavage formation during folding (White and Johnston, 1981; Cox et al., 1991; Yang and Gray, 1994; Tan et al., 1995). The timing of deformation can be deter-mined by isotopic dating of the metamorphic mica when sam-ples contain only neocrystalline white mica. This requirement is met in some, but not, all of the low-grade rocks. Rocks with very high strains have the best chance of complete recrystallization and therefore most applications of this dating method for the western Lachlan orogen have concentrated on the fault zones. At higher metamorphic grades, and for muscovite growing in syn-tectonic quartz veins, individual grains of metamorphic white mica grow to larger sizes and are easily extracted and measured

as pure phases, thereby removing potential mixed-ages caused by detrital mica. Complications arise when analyzing very fi ne grained, <10 micron-sized mica, which include recoil redistri-bution of 39Ar during sample irradiation. Metamorphic mica grains for most of the greenschist facies rocks in the Bendigo and Stawell structural zone are generally large enough (>10–50 microns) that recoil has not been a problem. This is not the case with samples from parts of the Melbourne structural zone, where the fi ner-grain size resulted in many samples yielding discordant age spectra as a result of 39Ar recoil.

The results summarized in this section are based upon data presented Foster et al. (1996a, 1996b, 1998, 1999), Bucher (1998), Bierlein et al. (1999, 2001), Spaggiari et al. (2003), and Miller et al. (2005). A summary of the results is given in Fig-ure 4 and locations of selected samples and their ages are shown the map and cross section in Figure 2.

Stawell ZoneOrdovician 40Ar/39Ar plateau ages of ca. 453–439 Ma are

recorded by cleavage mica from the Stawell-Ararat fault zone and Landsborough fault, and by sericite from syntectonic quartz veins in the Stawell gold mine (Foster et al., 1999; Bierlein et al., 2001; Miller et al., 2005). These late Ordovician to early Silurian white mica dates record the peak of metamorphism and tectonic short-ening. Minimum ages for low-temperature release steps from the white micas in the Stawell zone range from ca. 426 to ca. 405 Ma and are related to fault reactivation, exhumation during Silurian to Early Devonian times, and heating by ca. 400 Ma post-tectonic plutons (Foster et al., 1996a; Arne et al., 1998; Bucher, 1998).

Bendigo ZoneThe oldest 40Ar/39Ar plateau ages (ca. 457–455 Ma) for

samples in the Bendigo zone are given by sericite from early quartz veins within thrust faults at Ballarat East (Foster et al., 1998; Bierlein et al., 1999) and cleavage mica from early fabrics in the Heathcote fault zone (Fig. 5) (Foster et al., 1999). Sericite samples from early laminated quartz veins in the Bendigo gold mine give ages ca. 445–442 Ma (Fig. 6) (Bierlein et al., 2001). Most samples from the Bendigo zone that show well-developed subvertical cleavage give 40Ar/39Ar plateau ages of ca. 440 Ma, including (1) phyllite and slate samples from the Avoca fault zone (ca. 439 Ma) (Fig. 7); (2) white mica from bedding-parallel quartz-mica fi bers within the hanging wall of the Whitelaw thrust sheet (ca. 444 Ma) (Fig. 8); and (3) sericite from syntectonic quartz veins in the Central Deborah Mine and Wattle Gully Mine

Figure 4. Time-space diagram for the western Lachlan Orogen (modi-fi ed from Gray and Foster, 2004). References for geochronology data (inset box): 1Foster et al. (1998, 1999), Spaggiari et al. (2003), Miller et al. (2005); 2Foster et al. (1996a, 1998, 1999), Bierlein et al. (1999, 2001); 3Arne et al. (1998), Foster et al. (1996b), Bucher (1998), Spag-giari et al. (2003). bt—biotite; hb—hornblende; LFZ—Landsborough fault zone; MWFZ—Mount Wellington fault zone.

e

STAWELL ZONE BENDIGO ZONE MELBOURNE ZONE

Western Lachlan Orogen

Ben

dig

o

early bedding parallel fabrics & veins

fault reactivation

Peak metamorphism & faulting

igneous crystallizationor cooling age

U/Pb & Ar/Ar3

Ar/Ar2

vein formatiom

Ar/Ar1

cleavage orfault mica

sed

imen

tati

on

sed

imen

tati

on

deformation &metamorphism

sedimentation

Staw

ell z

on

ese

dim

enta

tio

n

sed

imen

tati

on

Ben

dig

o z

on

e

zon

ew

est

Mel

bo

urn

e

zon

eea

st M

elb

ou

rne

MAIN DEFORMATION

EARLY

DEFORMATION

in MWFZ

Au

358 Foster and Gray

(ca. 440–439 Ma) (Foster et al., 1998, 1999). Cleavage mica from other fault zones in the Bendigo zone, including the Muckleford fault, also give plateau dates of ca. 440 Ma (Bucher, 1998). Mini-mum, low-temperature, ages for four of the age spectra from cleav-age mica, the plateau age of one slate sample, and the minimum ages from many of the vein sericites from the Bendigo zone are between 417 and 426 Ma, suggesting a fault reactivation and exhu-mation history similar to the Stawell zone. Finally, a Devonian date of 375 ± 2 Ma (revised from the ca. 382 Ma date in Foster et al., 1999) is given by sericite alteration surrounding a late syn-tectonic dike intruding en-echelon fractures in the Heathcote fault zone. This is consistent with post-early Devonian thrusting along

the eastern margin of the Heathcote fault zone, which is required to explain deformed strata of this age in the footwall where it over-rides the Melbourne structural zone (Fig. 4) (VandenBerg, 1999).

In summary, the Bendigo zone 40Ar/39Ar data indicate that early (generally bedding-parallel) quartz veins and early cleav-age, protected from reactivation in fold hinges, give ages of 457–455 Ma. Several samples from axial planar cleavage and syntectonic quartz veins give ages ca. 445 Ma. The most com-mon age given by the highly transposed fabrics in the major fault zones, and by white mice separated from late-tectonic quartz veins that intrude brittle fractures in fold hinge zone, is 440–439 Ma (Fig. 9). We interpret this to indicate that folding of the turbidites began ca. 457–455 Ma and ended ca. 440–439 Ma.

Based on the style of deformation in the less deformed areas of the Melbourne zone, and the lack of evidence for fault-bend folding, deformation of the Bendigo zone began by sinusoidal buckle folding associated with bedding-parallel veins. Deforma-tion then progressed to chevron folding and through to isoclinal folding and fabric transposition in the high strain zones, along with brittle thrusting in the hinge zones of chevron folds higher in the thrust sheets (Fig. 9). This progressive deformation occurred over a total of ~15–17 m.y., with most of the deformation in the thrust-sheet hanging walls occurring between 445 and 439 Ma, based on the 40Ar/39Ar geochronology.

Melbourne ZoneThe lack of signifi cant cleavage over most of the Melbourne

structural zone precluded the growth of signifi cant metamorphic mica and limits the ability to date deformation by the 40Ar/39Ar method. Stronger fabrics in the frontal thrusts within the Mount Wellington fault zone resulted in better data, but not of the quality obtained from the Stawell and Bendigo zones. Slate with strong transposition foliation from the basal parts of the Mount Welling-ton fault zone gives ages of ca. 410 Ma (Foster et al., 1998). The mica cleavage in this sample contains a small amount of detrital mica has some recoil problems and therefore, ca. 410 Ma is taken as a maximum age for the metamorphic mica growth. White mica from a folded metavolcanic rock in the basal zone gave an age of 416–411 Ma (Foster et al., 1999), and cleavage mica from phyl-lite just above the contact with the metavolcanic rocks gave a plateau age of 419 ± 1 Ma (Spaggiari et al., 2003), which prob-ably records early deformation in the Mount Wellington fault zone. It is unclear how widespread the 419–411 Ma deformation was in the Melbourne zone, because an associated unconformity is only local in extent (VandenBerg, 1999). Foster et al. (1999) interpreted the 419–411 Ma deformation to indicate that the basal décollement, exposed in the Mount Wellington fault zone, was active before signifi cant folding of the hanging wall.

Structurally higher samples of cleavage from the Mount Wellington fault zone give ages ca. 388 Ma (Foster et al., 1999); an age of 388 ± 2 Ma is also given by white mica in veins in a gold deposit in the eastern Melbourne zone (Foster et al., 1998). 40Ar/39Ar dates of hornblende from mafi c dikes (Woods Point dike swarm) that intrude fractures aligned with the fold hinges give

D

Fraction 39Ar released0.0 0.2 0.4 0.6 0.8 1.0

200

220

240

260

280

300

320

340

360

380

400

420

440

460

480

500

457 ± 2 Ma

Age(Ma)

Heathcote cleavage

C

A B

Figure 5. (A) Outcrop photo showing isoclinal folds in a quartz vein and strongly transposed fabric from the hanging wall of the Heathcote fault zone (Australian 20-cent coin for scale). (B) Photomicrograph (width is 800 microns) from the Heathcote fault zone showing curved pressure shadow on pyrite in slate. (C) 40Ar/39Ar age spectrum from a sample of cleavage mica from the Heathcote fault zone (Foster et al., 1999). Inset (D) photomicrograph (width is 10 mm) of the location of the cleavage micas analyzed to produce the age spectrum.

0.0 0.2 0.4 0.6 0.8 1.0300

320

340

360

380

400

420

440

460

480

500

520

Age

(Ma)

CD11-21 sericiteTFA = 445 ± 2 Ma

442 ± 2 Ma

0.0 0.2 0.4 0.6 0.8 1.0300

320

340

360

380

400

420

440

460

480

500

520


452 ±2 Ma

0.0 0.2 0.4 0.6 0.8 1.0

Fraction 39Ar released

300

320

340

360

380

400

420

440

460

480

500

520


437 ± 2 Ma

Age

(Ma)

Age

(Ma)

Sh

ee

psh

ea

d A

nti

cli

ne

De

bo

rah

An

tic

lin

e

CD 11-21

CD 9-18CD 9-15

faults

quartz reef

lamprophyre dykes

shale (Chewtonian & Bendigonian)

siltstone (Chewtonian)

sandstone (Bendigonian)

Bedding parallel vein with visible gold

Massive vein with visible gold

Post-thrusting breccia vein with no gold

440 ± 2 Ma

Figure 6. Age spectra diagrams for white mica separated from laminated and massive, fault-fi lled veins from the Bendigo gold mine (Central Deborah mine). Cross section shows the location of the samples from the hinge of the Deborah anticline (from Bierlein et al., 2001), and the photo shows multiple generations of laminated and massive synorogenic quartz veins from the mine (hammer handle for scale [4 cm wide]). Early bedding-parallel veins in this deposit give ages of ca. 445–442 Ma, massive veins in limb thrusts give ages ca. 440 Ma, and post-thrusting breccia veins give ages ca. 437 Ma. The older high-temperature release steps in sample CD9-18 are due to incorporation of some older mica during brecciation.

360 Foster and Gray

ages of 378–376 Ma (Bierlein et al., 2001), and white mica from mineralization associated with dikes of this swarm give ages of 378–374 Ma (Foster et al., 1996b, 1998, 1999). These data indi-cate that deformation had ended by ca. 378 Ma, which is consistent with the stratigraphy of the Melbourne zone (Fig. 4) (VandenBerg, 1999). The results from the Melbourne zone, therefore, suggest that deformation of the hanging wall of the thrust sheet took place between ca. 388 and 378 Ma, or over ~10 m.y. The error on this duration, however, could be as large as ± 5 m.y.

Strain States in the Western Lachlan Orogen

Strain markers within the western Lachlan Orogen include deformed pillows and amygdales in metavolcanics of the Stawell zone (Wilson et al., 1992), syntectonic quartz fi bers in

pressure shadows on pyrite and graptolites on bedding planes in the Bendigo zone (Gray and Willman, 1991b; Gray, 1997), and syntectonic quartz fi bers in pressure shadows on pyrite and deformed cooling columns in metavolcanics of the Melbourne zone (Gray, 1995) (Fig. 10). Strain analysis techniques are after Durney and Ramsay (1973) and Ramsay and Huber (1983). For description of the methods see Wilson et al. (1992), Gray and Willman (1991b), and Gray (1995) for the Stawell, Bendigo, and Melbourne zones, respectively.

Stawell Zone StrainTotal XZ strain estimates are restricted to the Cambrian

metavolcanic units (see Wilson et al., 1992, for data and loca-tion information). Deformed pillow shapes and quartz-calcite augens, formerly plagioclase phenocrysts, within porphyritic meta volcanics in the strongly deformed parts of steep, east-dipping fault slivers (“Waterloo” structures) at the Stawell gold mine give X:Y:Z = 4.9:1:0.31 with X/Z = 16:1 and a Flinn k value

Avoca Fault Zone crenulation cleavage

0.0 0.2 0.4 0.6 0.8 1.0200

220

240

260

280

300

320

340

360

380

400

420

440

460

480

500

439 ± 2 Ma

Fraction 39Ar released

Figure 7. Field photo of strong crenulation cleavage from the Avoca fault zone. Diagram showing 40Ar/39Ar age spectrum from cleavage mica (data from Foster et al., 1998).

atm

0

0.001

0.002

0.003

0.004

0 0.01 0.02 0.03

443.6 ± 1.2 Ma

MSWD = 1.34

DG84-17B sericite

single grain fusion analyses

36A

r/ 40

Ar

39Ar/ 40Ar

Figure 8. 40Ar/39Ar inverse isochron diagram for analyses of single grains of white mica extracted from a bedding plane fault (photo) from a quarry in Castlemaine, Victoria (Foster et al., 1999). MSWD—mean square of weighted deviations.


of ~0.25 (fl attening fi eld). Volume loss for the western Lachlan slates was <10% (Gray, 1997b) so that the strain remains in the fl attening fi eld close to plain strain, even assuming the maximum volume change. A background strain in the mine exposures of X/Z = ~1.5:1 is recorded within the less-deformed cores of the individual fault slices or shear lozenges (Wilson et al., 1992).

Amygdaloidal strain markers within the Mount Ararat metavolcanic rocks show a marked increase in strain toward the Ararat fault (Table 3 in Wilson et al., 1992), with X/Z strains increasing from 3:1, to 11:1, to 25:1 in the high strain zone of the fault. Flinn k values have k > 1 (constriction fi eld), indicating signifi cant stretching where the principal stretch X-axis is sub-parallel to the stretching lineation (Wilson et al., 1992).

Bendigo Zone StrainTotal XZ strain in slates (R), determined largely from syntec-

tonic quartz fi bers in pressure shadows on pyrite and graptolites on bedding planes (see Gray and Willman, 1991a, 1991b; Gray, 1997a, for methods and terminology), varies regionally across the thrust belt with XZ strain ranging from 2> R ≤ 40 (Gray and Willman, 1991b). The background X/Z strain values are relatively homogenous and range between 2 and 5 in slates away from fault zones. The highest strains (XZ > 40:1) are associated with the hang-ing walls of the major faults (Table 1) (Gray and Willman, 1991b). Strain magnitude also varies with lithology, because XZ strains in sandstones determined by the Fry method range between 1.3 and 1.7 and are signifi cantly lower than those in mudstones (Gray and Willman, 1991a; Yang and Gray, 1994). Psammites show XZ strain magnitude variations dependent on position on folds; XZ strain estimates from fold hinges are generally higher (≈1.6–1.7) than limb estimates (≈1.3) from the same fold.

Melbourne Zone StrainIn the penetratively deformed, chevron-folded, cleaved tur-

bidites of the Melbourne zone, total X/Z strain states range from ~6:1–15:1 (Gray, 1995). Strain magnitude increases from the upright fold zone (X/Z strains <6:1) into the inclined fold zone (X/Z strains between 10:1 and 15:1) and matches the increase in cleavage intensity into the Mount Wellington Fault Zone. The magnitude of total strain varies within the Cambrian mafi c vol-canic inliers, with less deformed parts giving X/Z strains <3.5:1, and the more strongly deformed, foliated parts show X/Z = 13:1. Quartz-fi ber pressure shadows on pyrite in fault slices of Upper Ordovician black slate give XZ strains from 9:1–19:1.

Retrodeformation of the Western Lachlan Orogen

Shortening within western Lachlan Orogen thrusts sheets is partitioned between chevron folding and cleavage development in both mudstones and sandstones, as well as faulting (Cox et al., 1991; Gray and Willman, 1991a, 1991b; Gray et al., 2006a). Palin-spastic restoration (unstraining) of the regional chevron-folded and faulted profi le shown in Figure 10 includes a fault restoration followed by strain removal and then unfolding of the remaining buckle component (see Figure 10 in Gray and Willman, 1991b).

The fault restoration is problematical because of the uni-formity of sedimentary facies and the lack of biostratigraphic markers over segments of the exposed thrust system. Where there is stratigraphic control on fault displacements, particularly in pro-fi les between the intrazone faults, the calculated shortening com-ponents due to faulting are <5% (see Table 1 in Gray and Willman, 1991a). Displacement on the major zone-bounding faults that bring the underlying Cambrian metavolcanics to the present level

oceanic crust

gentle folding

457– 455 Ma

oceanic crust

decollementslip on

445–444 Ma 441–439 Mafolds tighten to chevron forms,cleavage develops

slip on increasing

pervasive cleavage development, chevronfolds tighten and become isoclinal in high strain zones, thrusting in hinges

bedding parallel quartz veins

decollement

folding ofquartz veins

oceanic

crust

quartz veins inthrusts

Figure 9. Timing and sequence of structural shortening within a typical Bendigo zone thrust sheet.

E

Cam

bria

nan

des

ite50

km

W

EF

0 4 8 12TWT (s)

MO

HO

0 4 8 12TWT (s)

v vv v vv

9.6

(XZ

)

12.6

(X

Z)

10.2

(X

Z)10

.2 (

XZ

)

15.2

(X

Z)

2.1

(X

Z)

3.5

(XZ

)1.

4

3.2

(X

Z) 33:

1(X

Z)

2.5

(XZ

)

34:

1(X

Z)

6.9

(XZ

)

4.0

(XZ

) 9.2

(XZ

)

5.2

(XZ

) 3

.8(X

Z)

14.

8 (

XZ

)

4.9

(XZ

)

3.2

(XZ

)

8.9

(XZ

)

~32

% fo

ld s

ho

rten

ing

~60

% fo

ld s

ho

rten

ing

~60

% fo

ld

sho

rten

ing

~53

% fo

ld

sho

rten

ing

~64

% fo

ld

sho

rten

ing

stra

in d

ata

from

Gra

y (1

995)

stra

in d

ata

from

Gra

y an

d W

illm

an (1

991a

,b)

stra

in d

ata

from

Wils

on a

nd o

ther

s (1

992)

crat

on

ized

by

500

Ma

bac

ksto

p

15.

0(X

Z)

10.0

(XZ

)25

.0(X

Z)

k=0.

3

k<1

defo

rmed

am

ygda

les,

pillo

ws

Cam

bria

n m

etav

olca

nics

pre

ssur

e sh

adow

s on

pyr

ite

in tu

rbid

ites

defo

rmed

coo

ling

colu

mns

in C

ambr

ian

met

avol

cani

cs

redu

ctio

n sp

ots

in c

over

mud

ston

es

zero

or v

ery

low

pen

etra

tive

str

ain

pres

sure

sha

dow

s on

pyr

ite

in tu

rbid

ites

,de

form

ed g

rapt

olit

es

50

A B C

Hea

thco

teFa

ult Z

one

Whi

tela

wFa

ult

Mt A

rara

tFa

ult Z

one

40 10203060 XZ strain

zero

or v

ery

low

pen

etra

tive

str

ain

Mt W

ellin

gton

Faul

t Zon

eFi

ddle

rs

Gre

enFa

ult

Bark

lyFa

ult

Mt A

rara

tFa

ult

Avoc

aFa

ult Z

onem

axim

um re

cord

ed s

trai

n va

lue

(in fa

ult z

ones

)

back

grou

nd X

/Z s

trai

n

hyb

rid

z

on

e

MO

HO

Muc

klef

ord

Faul

t

LFC

FM

FC

FW

FFF

MA

FFG

FB

F

MFZ

maj

or f

ault

zo

ne

SAFZ

AFZ

HFZ

MW

FZ

8.7

(XZ

)

no m

arke

rs in

turb

idit

es

3.7

(XZ

)

2.5

(XZ

)

Figu

re 1

0. (

A)

Wes

t to

eas

t pr

ofi le

s th

roug

h th

e w

este

rn L

achl

an O

roge

n sh

owin

g st

rain

dat

a fo

r th

e st

ruct

ural

zon

es (

see

text

for

ref

eren

ces)

. (B

) Po

sitio

ns o

f m

ajor

fau

lts (

Gra

y an

d Fo

ster

, 19

98).

(C)

Cru

stal

sec

tion

(Gra

y et

al.,

200

6b).

MFZ

—M

oyst

on f

ault;

MA

F—M

ount

Ara

rat

faul

t; SA

FZ—

Staw

ell-

Ara

rat

faul

t zo

ne;

CF—

Cam

pbel

ltow

n fa

ult;

LF—

Lei

char

dt f

ault;

AFZ

—A

rara

t fa

ult

zone

; M

F—M

uckl

efor

d fa

ult;

WF—

Whi

tela

w f

ault;

FF—

Fost

ervi

lle f

ault;

HFZ

—H

eath

cote

fau

lt zo

ne;

FGF—

Fidd

lers

Gre

en f

ault;

MW

FZ—

Mou

nt W

ellin

gton

fau

lt zo

ne;

BF—

Bar

kly

faul

t; T

WT

—tw

o-w

ay ti

me.


of exposure requires a minimum displacement (throw) of 17 km (i.e., the projection of the Cambrian metavolcanics above the inferred décollement depth for the Bendigo Zone), which equates to a heave of ~10 km for each of the Stawell-Ararat, Avoca, Heath-cote, and Mount Wellington Fault Zones. This gives a minimum fault shortening across the profi le of 11% (or e = −0.11, where e is the minimum elongation) by restoration of the Cambrian and thereby removing displacement on the major fault zones, given L

1 = 350 km and L

0 = (350 + 40) km (where L is length).

Additional uncertainties from volume change and out-of-section motion are minor. Gray (1997b) showed that the maximum amount of volume change in the section is <10%. Out-of-section, strike-slip movement along the faults is very minor in the Lachlan Orogen and particularly in the western Lachlan Orogen, as demonstrated by Gray and Foster (1998).

Methods for Removing StrainIn strained rock sections, simple bed-length balancing tech-

niques as part of section balancing (e.g., Dahlstrom, 1969) only provide a minimum estimate of the total rock shortening. The excess area method (Chamberlain, 1910; Gwinn, 1970) and the total area method (Dennison and Woodward, 1963; Hossack, 1979; Mitra and Namson, 1989) overcome this inadequacy (see Merle, 1986), but a more reliable method utilizes the input of strain data and involves the unstraining, segment by segment, of the structural profi les (e.g., Hossack, 1978; Cobbold, 1979; Woodward et al., 1986; Mitra and Namson, 1989; Gray and Willman, 1991b).

In this paper, we have applied the strain reversal technique to a 350-km-long regional profi le across the western Lachlan Orogen (Fig. 11). The profi le was split into four segments, A, B, C, and D, for unstraining (see Fig. 11, component line lengths), to accommodate regional differences in strain, par-ticularly the marked gradient in XZ strain across the Melbourne Zone (Fig. 10). Segment A, or the western belt consisting of the Stawell and Bendigo Zones (see Fig. 10), has an assumed regional background X/Z strain of 4:1 and a deformed length of 200 km (maximum) or 150 km (minimum) (Table 1). The Melbourne Zone (see Fig. 10), or the eastern belt, was split into Segments B, C, and D, with the adopted values of zero X/Z strain (i.e., no penetrative strain), 1.4:1 strain, and 10:1 strain, respectively, over a combined total length of 150 km (Table 1).

Retrodeformation of the individual profi le segments was undertaken in two steps (see Fig. 11). These were (1) removal of the penetrative strain components by applying the X-Y stretch tool (strain transformations) in the vector-based Adobe Illustra-tor drafting package to the line segments of the profi le; and (2) removal of the buckle shortening components to the unstrained profi le segment by the bed-length method involving comparison between the undeformed (layer arc length measurement deter-mined by map measurer tool) and the deformed bed lengths.

Calculated Initial LengthsBased on the above, the predeformation former basin

width is ~740 km, given e = (L1–L

0)/L

0, and e = −0.58, and

L1 = 310 km (see Fig. 11). Adding in the major fault displace-

ments would give a former width of at least 780 km, and there-fore on the order of 800 km.

Crustal scale area balancing of the western Lachlan Orogen, assuming strain compatibility between the upper and lower crust, however, requires a two-times fault duplication of the former tur-bidite fan thickness in the Western Belt (Stawell and Bendigo zones), which is ~6 km (see Gray et al., 2006a). This means that the Western Belt width should be doubled by fault restoration, and therefore the combined original width of the Stawell and Bendigo zones could approximate ~900–950 km.

Treating the western and eastern belts separately (Fig. 11) gives the following:

1. Western Belt (66% shortening) Present profi le width is ~160 km and with e = −0.66 gives a ~470 km width of the former Stawell and Bendigo zones depositional region. This represents a minimum basin width, as fan thickness and area balance con-siderations suggest that the width may be as much as ~950 km (Gray et al., 2006a).

2. Eastern Belt (38% shortening) Present profi le width is ~150 km and with e = −0.38 gives a ~242 km width of the former Melbourne zone depositional region.

Deformation Rate for the Western Lachlan Orogen

By utilizing the retrodeformation and geochronological data from the western Lachlan Orogen, we are able to quantify the timing and rate of deformation across the orogenic belt. Retrodeformation for the western part, the Stawell and Bendigo zones, gave a minimum shortening of 310 km and a maximum of 790 km (see above). Deformation of this zone took an aver-age of 16 m.y. (455–439 Ma). This gives an average displace-ment rate for the basal décollement of ~19 mm yr−1 (minimum) and ~50 mm yr−1 (maximum). We have not assigned errors to these values due to the uncertainty in internal thrust shortening (see discussion in Gray et al., 2006a). The error in the duration of deformation amounts to ±1 mm yr−1. The Melbourne zone shortened ~92 km over ~10 m.y., giving a displacement rate of ~9 mm yr−1. The error in the duration of shortening is up to ±5 m.y., which corresponds with displacement rates ranging from 7 to 23 mm yr−1.

The geochronology and strain data can also be used to calculate internal strain rates for the western Lachlan thrust wedges. With strain rate calculations, it is possible to calculate a conventional strain rate or a natural strain rate (Pfi ffner and Ramsay, 1982). A conventional strain rate is elongation/time (e/t) where e is the elongation (e = (L

1–L

0)/L

0) and t is time (in

seconds). Using this approach, strain estimates for the western belt (Stawell and Bendigo zones) and eastern belt (Melbourne zone) are as follows:

1. Western belt, where shortening of 66% occurred over 16 m.y. (0.51 × 1015 s), gives a strain rate equal to 1.3 × 10−15 s−1.

E

Cam

bria

nan

des

ite50

km

W 2) re

mov

al o

f pen

etra

tive

str

ain

1) A

ssu

me

aver

age

stra

in p

er s

hee

t (X

Z=

4:1)

3) re

mov

al o

f ~30

% b

uck

le s

ho

rten

ing

STA

WEL

L-B

END

IGO

ZO

NES

(No

str

ain

mar

kers

in S

taw

ell Z

on

e b

ut

trea

ted

as

sin

gle

str

uct

ura

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tity

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ne:

as

hav

e si

mila

r str

uct

ura

l sty

les,

sim

liar s

ho

rten

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s et

c.)

L 0= 1

66

L 1= 8

0

equ

ates

to e

(min

)= –

52%

sim

ple

lin

e st

retc

h

L 1=

160

un

its

Mel

bo

urn

e Z

on

e

eq

ua

tes

to e

(min

)= –

0.38

Pre

sen

t wid

th is

~15

0 km

,

wh

ich

eq

ua

tes

to a

n o

cea

n b

asi

n a

t lea

st ~

242

km in

wid

th

L 0= 3

76 u

nit

s (7

20

km

)W

LO C

rust

al s

cale

sh

ort

enin

g=

58%

CO

MP

ON

ENT

LIN

E LE

NG

THS

WES

TERN

LA

CH

LAN

ORO

GEN

retr

o-de

form

ed le

ngth

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form

ed le

ngth

Seg

men

t ASe

gm

ent B

Seg

men

t DSe

gm

ent C

RET

R0-

DEF

OR

MA

TIO

N

Seg

men

t A

MEL

BO

URN

E ZO

NE

com

ple

ted

by

line

len

gth

mea

sure

men

tal

on

g fo

ld a

rceq

uat

es to

e(m

in)=

-34%

1) re

mov

al o

f bu

ckle

co

mp

on

ent

no

cle

avag

eze

ro p

enet

rati

ve s

trai

n

Seg

men

t B

1) M

od

al s

trai

n a

cro

ss G

oo

sen

eck

syn

clin

ori

um

(XZ

=1.

4:1)

X/Z

=1.

4:1

2) re

mov

al o

f pen

etra

tive

str

ain

L 0= 2

6

L 1=21

equ

ates

to e

(min

)= –

16%

3) re

mov

al o

f bu

ckle

rem

ain

ing

co

mp

on

ent

com

ple

ted

by

line

len

gth

mea

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men

tal

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g fo

ld a

rc equ

ates

to e

(min

)= –

40%

Seg

men

t C

L 0= 2

5

L 1= 8

.5

equ

ates

to e

(min

)= –

66%

2) re

mov

al o

f pen

etra

tive

str

ain

1) M

od

al s

trai

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ss M

WFZ

(XZ

=10

:1)

X/Z

=10

:1

3) re

mov

al o

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ckle

rem

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mp

on

ent

com

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ld a

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uat

es to

e(m

in)=

–69

%

Seg

men

t D

Seg

men

t A’

Seg

men

t B’

Seg

men

t D’

Seg

men

t C’

Seg

men

t A’

Seg

men

t B’

Seg

men

t C’

Seg

men

t D’

50 km

then

the

form

er o

cea

n b

asi

n w

as

at l

east

~74

0 km

in w

idth

Pre

sen

t pro

file

wid

th is

~31

0 km

an

d g

iven

e(m

in)=

–0.

58

giv

es to

tal s

ho

rten

ing

e(m

in)=

– 6

6%

L 0= 2

37 u

nit

s

L 0= 3

5 L 0=

27

Cam

bria

nm

etab

asal

t

L 1=

44 L 0

= 6

7

Sta

wel

l an

d B

end

igo

Zo

nes

eq

ua

tes

to e

(min

)= –

0.66

Pre

sen

t wid

th is

~16

0 km

,

wh

ich

eq

ua

tes

to a

n o

cea

n b

asi

n a

t lea

st ~

470

km in

wid

th

Figu

re 1

1. D

efor

med

and

ret

rode

form

ed c

rust

al s

ectio

ns f

or t

he w

este

rn L

achl

an O

roge

n. S

egm

ent A

inc

lude

s th

e se

ctio

n in

the

Sta

wel

l an

d B

endi

go z

ones

. The

Mel

bour

ne z

one

is d

ivid

ed in

to s

egm

ents

B, C

, and

D b

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se o

f sig

nifi c

ant v

aria

tions

in s

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—M

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rain

; L—

leng

th.


2. Eastern belt, where shortening of 43% occurred over 10 m.y. (0.32 × 10−15 s), gives a strain rate equal to 1.3 × 10−15 s−1.

Another approach to derive strain rate involves displacement rate/distance (Twiss and Moores, 1992). Using this approach, the western belt, where the minimum displacement rate is 19 km m.y.−1/310 km gives a minimum strain rate of 2.0 × 10−15 s−1, and the maximum displacement rate is consid-ered to be 50 km m.y.−1/310 km gives a maximum strain rate of 5.0 × 10−15 s−1. The eastern belt, where the average displacement rate is 9 km m.y.−1/92 km, gives a strain rate of 3.1 × 10−15 s−1.

With either calculation method, the data indicate that fold-ing and thrusting of turbidite in this oceanic backarc basin set-ting occurred at average strain rates between 1 × 10−15 s−1 and 5 × 10−15 s−1. The concentrations of 40Ar/39Ar vein and cleavage mica ages ca. 445–439 Ma in the Bendigo zone may indicate that most deformation in the thrust sheets occurred over a shorter interval of more rapid deformation. In fact, most of the intense cleavage development occurred ca. 441–440 Ma, and the limb thrust veins, which give ages of ca. 440–439 Ma, suggest that most deformation ended by 439 Ma. Given the errors in the analy-ses and ranges of ages, a pulse of deformation ~2 m.y. may have resulted in much of the 66% shortening. If this were the case, the strain rate would have been on the order of 1 × 10−14 s−1.

DISCUSSION AND CONCLUSIONS

Modern deformation rates within orogenic belts range from ~5–15 mm yr−1 (Hyndman et al., 2005) based on GPS measure-ments and longer term geological studies (e.g., Hindle et al., 2002; McQuarrie and Wernicke, 2005). These rates are a fraction of plate tectonic rates, which typically range from 10 to 200 mm yr−1, although individual faults and shear zones may take on a larger fraction of plate tectonic rates for short intervals of time (e.g., Carter et al., 2004). Corresponding strain rates for tectonic and orogenic processes, therefore, typically range from 10−12 to 10−15 s−1 (Pfi ffner and Ramsay, 1982; van der Pluijm and Marshak, 2004).

Internal rates of deformation in the ductile middle and lower crust of orogens are more diffi cult to evaluate (Ramsay, 2000). Strain rate calculations for deformation within ancient orogenic belts are limited by the existing geochronology of the fabric-forming events, as well as knowledge of the incremental strains (Pfi ffner and Ramsay, 1982; Ramsay, 2000; Müller et al., 2000). These diffi culties have been addressed by a small num-ber of studies that succeeded in combining structural and micro-structural analysis with radioisotopic analyses on mineral phases grown during deformation. The challenge is fi nding areas where deformation and metamorphism occurred at temperatures below the closure temperature for the isotopic system in the phases of interest. This is a rapidly growing fi eld of research with signifi -cant potential (Müller, 2003).

Our results from the western Lachlan Orogen give minimum average strain rates of 1 to 5 ×10−15 s−1, which are at the low end

of the range of deformation rates estimated for orogenic belts. The concentrations of 40Ar/39Ar vein and cleavage mica ages ca. 457–455 and 440–439 Ma in both the Stawell and Bendigo zones suggest that deformation occurred dominantly in two more rapid pulses of shortening. Most of the intense cleavage develop-ment occurred ca. 441–440 Ma, and the limb thrust veins, which give ages of ca. 440–439 Ma, suggest that deformation ended by 439 Ma. A pulse of deformation of ~2 m.y. duration may have resulted in a considerable fraction of the 66% shortening. If this were the case, the strain rate would have been ~1 × 10−14 s−1.

The 40Ar/39Ar data from the thrust sheets in the Bendigo zone suggest that thrust sheet deformation occurred over a total of ~16 m.y. and progressed from early buckle folding at 457–455 Ma through to chevron fold lock-up and thrusting at 440–439 Ma. Deformation and fabric development in the thrust sheets were progressive during this time interval, and folding and ductile strain probably developed diachronously (cf. Pinan et al., 2004). It is also possible that the thrust sheet strain accumulated in a pulse-like manner due to a caterpillar-style movement on the basal décollement, linked to stop-start movements as the deform-ing sedimentary wedge achieved the necessary wedge taper requirements for slippage on the basal fault, but this requires fur-ther research.

Comparing the deformation rates with other estimates from Phanerozoic convergent orogens shows distinct similarities. For example, Kligfi eld et al. (1981, 1986) integrated measured fi nite strain and the total time of deformation, based on K/Ar and 40Ar/39Ar geochronology, to calculate average strain rates of 10−14 to 10−15 s−1 for the Northern Apennines. In the Apennine case, simple shear components are a major part of the incremental strain accumulation, whereas in the western Lachlan Orogen, the largest volume of the thrust sheets (up to 94% per volume) has undergone pure shear accumulation of incremental strains (see Figure 12 in Gray and Willman, 1991b).

Müller et al. (2000) used microscale Rb-Sr dating of fi brous strain fringes on pyrites in the Pyrenees, which gave strain rates up to 1.1 × 10−15 to 7.7 × 10−15 s−1. Three separate studies of Rb-Sr and Nd-Sm dating of large metamorphic garnets have suggested shear strain rates ~2–3 × 10−14 s−1. van der Pluijm et al. (2006) used 40Ar/39Ar dating of clay in fault gouge from the Canadian Rockies to suggest pulses of deformation with rates of ~10−14 s−1.

The western Lachlan shortening rate estimates are similar to rates estimated for the Robertson Bay terrane of North Vic-toria Land, where 40Ar/39Ar ages of cleavage mica give conver-gence rates of ~4–10 mm yr−1 when fold shortening is considered (Dallmeyer and Wright, 1992). It is also similar to fold propa-gation and cleavage formation in fl ysch across the Rheinesche Schiefergebirge, where K-Ar ages of sericite indicate a rate ~5 mm yr−1 (Ahrendt et al., 1983). Moreover, the calculated dis-placement rates are within the range of modern plate tectonic velocities in similar backarc settings, like the western Pacifi c backarc basins and accretionary prisms (e.g., McCaffrey, 1996), as well as the rates of shortening in the turbidite-dominated thrust belt of Taiwan (Yu et al., 1997).

366 Foster and Gray

ACKNOWLEDGMENTS

Reviews by D. De Paor and C. Fergusson helped improve the clarity of the paper. Fieldwork for this research was supported by Australian Research Council Grant E8315675 and Monash Uni-versity Special Research Funds (awarded to DRG). The 40Ar/39Ar geochronology was supported by the Australian Geodynamics Cooperative Research Centre (1995-97) and the National Science Foundation grant EAR0073638 to DAF. Support for the write-up for DRG was from an Australian Professorial Fellowship as part of Australian Research Council Discovery Grant DP0210178.

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