crustal structure of the ordovician macquarie arc, eastern lachlan orogen, based on...

Australian Journal of Earth Sciences (2002) 49, 323–348

INTRODUCTION

The Eastern Lachlan Orogen contains four belts ofOrdovician volcanic, volcaniclastic and intrusive rocks(Figure 1) that most recently have been interpreted to haveformed in a subduction-related island arc (the MacquarieArc: name modified from Webby 1978) that was dismem-bered by extension in the Middle Silurian to MiddleDevonian (Glen et al. 1998a).

New stratigraphic and structural interpretations of thethree belts in central New South Wales (on the Dubbo,Bathurst and Forbes 1:250 000 sheets) are provided byWarren et al. (1996), Raymond et al. (1998, 2000b), Morgan et al. (1999) and Glen and Fleming (2000). Despite this new work, cross-sections of the volcanic belts are very variable, reflecting gaps in our knowledge due to lack ofoutcrop, as well as an absence of data on geometry and kinematics of faults. This variability also reflects authorbiases and, as a result, the 3-D geometries of the belts andflanking faults are somewhat uncertain.

Knowledge of the 3-D nature of these belts is needed, notjust from a regional tectonic point of view, but also because

of the economic imperatives imposed by exploration for more porphyry, epithermal and skarn copper and gold deposits. As a result, the Australian GeodynamicsCooperative Research Centre, in conjunction with theGeological Survey of New South Wales and GeoscienceAustralia, undertook seismic-reflection profiling across the Junee–Narromine and Molong Volcanic Belts and seismic refraction profiling along the Molong Volcanic Belt in order to examine several aspects of Lachlan Orogentectonics and metallogenesis: (i) the third dimension ofthe volcanic belts, their internal fault architecture in relation to known mineral deposits, and the geometries of the boundaries of the volcanic belts; (ii) evidence for any collision between the Macquarie Arc and the coevalbackarc turbidite basin, inferred by Glen (1998a, 1998b) tobe expressed as the Benambran deformation (sensu lato);

Crustal structure of the Ordovician Macquarie Arc, Eastern Lachlan Orogen, based on seismic-reflection profilingR. A. GLEN,1,2* R. J. KORSCH,1,3 N. G. DIREEN,1,3 L. E. A. JONES,1,3 D. W. JOHNSTONE,1,3

K. C. LAWRIE,3 D. M. FINLAYSON1,3† AND R. D. SHAW2

1Australian Geodynamics Cooperative Research Centre.2Geological Survey of New South Wales, PO Box 536, St Leonards, NSW 1590, Australia.3Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia.

In the Eastern Lachlan Orogen, the mineralised Molong and Junee–Narromine Volcanic Belts are twostructural belts that once formed part of the Ordovician Macquarie Arc, but are now separated byyounger Silurian–Devonian strata as well as by Ordovician quartz-rich turbidites. Interpretation of deepseismic reflection and refraction data across and along these belts provides answers to some of thekey questions in understanding the evolution of the Eastern Lachlan Orogen—the relationshipbetween coeval Ordovician volcanics and quartz-rich turbidites, and the relationship between separate belts of Ordovician volcanics and the intervening strata. In particular, the data provideevidence for major thrust juxtaposition of the arc rocks and Ordovician quartz-rich turbidites, with Wagga Belt rocks thrust eastward over the arc rocks of the Junee–Narromine Volcanic Belt, and the Adaminaby Group thrust north over arc rocks in the southern part of the Molong VolcanicBelt. The seismic data also provide evidence for regional contraction, especially for crustal-scaledeformation in the western part of the Junee–Narromine Volcanic Belt. The data further suggest that this belt and the Ordovician quartz-rich turbidites to the east (Kirribilli Formation) were togetherthrust over ?Cambrian–Ordovician rocks of the Jindalee Group and associated rocks along west-dipping inferred faults that belong to a set that characterises the middle crust of the EasternLachlan Orogen. The Macquarie Arc was subsequently rifted apart in the Silurian–Devonian, withOrdovician volcanics preserved under the younger troughs and shelves (e.g. Hill End Trough). TheMolong Volcanic Belt, in particular, was reworked by major down-to-the-east normal faults that werethrust-reactivated with younger-on-older geometries in the late Early – Middle Devonian and againin the Carboniferous.

KEY WORDS: geophysical models, Lachlan Orogen, Ordovician, seismic reflection surveys, tectonics,volcanic rocks.

*Corresponding author: [email protected]†Present address: 6 Neilson Street, Garran, ACT 2605, Australia.

(iii) evidence for Middle Palaeozoic crustal extension in the Lachlan Orogen that led to rifting of the arc (Glen et al. 1998a) and the formation of the Hill End, Cowra and Jemalong Troughs (the last after Raymond et al. 2000c);and (iv) evidence in the north–south refraction and reflection data through the Molong Volcanic Belt for expres-sion of the Lachlan Transverse Zone of Glen and Walshe(1999).

JUNEE–NARROMINE VOLCANIC BELT

The Junee–Narromine Volcanic Belt is the longest, mostwesterly and most poorly exposed of the Ordovician volcanic belts (Figure 1). It consists of a northern north- to north-northeast-trending ‘Narromine’ part that lieslargely east of the Tullamore Fault, and a southern north-

northwest-trending ‘Junee’ part that lies largely east of theGilmore Fault Zone. A central part between the two con-tains both north-northwest- and north-trending volcanicbodies on both sides of the Tullamore Syncline (Figure 2).Details of stratigraphy in this belt can be found in Sherwin(1996) and Lyons et al. (2000a).

Three seismic-reflection lines were acquired across this belt (Figure 2) using vibroseis sources provided by theAustralian National Seismic Imaging Resource (ANSIR).Acquisition and processing details are described by Jonesand Johnstone (2001). Line 99AGS-L3 (line 3) is 90 km long across the southern part of the belt. Lines 99AGS-L1(line 1) and 99AGS-L2 (line 2) provide a staggered transect98 km long across the central part of the belt, with line 2just south of the Cowal gold–copper deposit.

Data from the two transects are shown firstly in uppercrustal sections [down to 6 s TWT (two-way time) or

324 R. A. Glen et al.

Figure 1 Simplified map of the Lachlan Orogen in New South Wales showing the two main Ordovician units, the intra-oceanicMacquarie Arc, subsequently dismembered and now preserved in four structurally controlled belts, and the coeval turbidite basinfilled by quartz-rich turbidites and Upper Ordovician black shales. The locations of Figures 2 and 10 are also shown.

approximately 18 km] to discuss correlations of seismicdata with outcrop or interpretations inferred from poten-tial field (magnetics and gravity) data. The deeper 20 s TWT data are then discussed in order to draw tectonic interpretations of this belt. In drawing the cross-sectionsit is important to note that all the seismic sections are 2-Dand only give apparent dips of reflections. The thirddimension can only be obtained by linking reflections withsurface structures to generate 3-D surfaces

Upper crustal shallow (6 s TWT) seismic data

ORDOVICIAN QUARTZ-RICH TURBIDITES (WAGGA GROUP)AND SILURIAN GRANITES

The western parts of lines 2 and 3 (Figures 3–5) consist oftriangular-shaped packets of weakly reflective material

with internal east-dipping reflections lying above a strongseries of west-dipping reflections. This packet correlateswith Ordovician quartz-rich turbidites of the Wagga Groupand intrusive Early Silurian granites forming the WaggaBelt. It has not been possible to differentiate granites frommetasedimentary rocks in the seismic data due to lowreflective contrast. Granites are picked up by gravity lows,with densities of 2.65–2.67 t m–3 for Ungarie Granite and2.70 t m–3 for migmatites and metasedimentary rocks. Theseand other gravity interpretations are based on densitiesmeasured on surface rocks given in Direen et al. (2001) and, as yet, there is no control on density variation withdepth.

We interpret the strong west-dipping basal reflection as a floor fault, and the internal narrow east-dippingreflective zones as faults because they truncate the background short reflections. We think these faults are

Crustal structure, Lachlan Orogen 325

Figure 2 Simplified solid geology of part of the Junee–Narromine Volcanic Belt showing the location of the 1999 seismic reflectionlines (based on Glen & Fleming 2000; Raymond et al. 2000a). Selected Ordovician volcanic units are named. See Figure 1 for location.

contractional because they are associated with hangingwallantiforms in background reflections (e.g. above reflectionC in Figures 3a, 4a) and because reflection C correlates with an east-dipping contractional fault on the surface (R. A. Glen unpubl. data).

ORDOVICIAN QUARTZ-RICH TURBIDITES (KIRRIBILLI FORMATION) WEST OF WHEOGA SYNCLINE

Near the eastern end of line 1, there are multiply deformedquartz-rich turbidites of the Ordovician KirribilliFormation (Raymond & Wallace 2000). Seismically the unitis weakly reflective, with a subhorizontal envelope con-taining small-amplitude open folds (Figures 3b, 4b). Thereappears to be little correlation with inferred surface

geology, which is characterised by steep dips on the limbsof inferred tight, upright folds.

IMBRICATE ZONE

Interleaved zones of Ordovician volcanic and Silurian sedi-mentary rocks lie east of the Wagga Belt. Line 3 crosses theOrdovician Gidginbung and Belimebung Volcanics, whichare highly elongated with north-northwest trends parallelto the Gilmore Fault Zone (Figure 2). The seismic fabric ofthe Gidginbung Volcanics (Figure 5) consists predomin-antly of folded subhorizontal to shallow east-dipping back-ground reflections above a narrow zone of east-dippingreflections that separates the volcanics from inferredunderlying granite.


Figure 3 Uninterpreted versions of migrated shallow (6 s TWT) seismic profiles (a) 99AGS-L2 and (b) 99AGS-L1 across the westernand eastern parts of the Junee–Narromine Volcanic Belt respectively. Vertical scale � horizontal scale assuming an average crustalvelocity of 6000 m s–1.

We interpret this zone as a shallow high-level thrust,with an associated ramp anticline at a depth of approxi-mately 1.5 km (0.5 s TWT) at approximately CDP 4100(Figure 5). The eastern boundary of the GidginbungVolcanics cuts low-dipping reflections and is also inter-preted as an east-dipping fault. Internal reflections withinthe Silurian–Devonian succession (Derriwong Group)

are subhorizontal to gently east-dipping, consistent withthe relatively open folded, gently dipping stratigraphy indicated by field observations.

In line 2, the imbricate zone also includes Ordovicianquartz-rich turbidites of the Girilambone Group and highly deformed Silurian–Devonian conglomerate in theBooberoi Fault Zone, which has west-block-up contractional


Figure 4 Interpreted versions of migrated shallow (6 s TWT) seismic profiles (a) 99AGS-L2 and (b) 99AGS-L1 across the western andeastern parts of the Junee–Narromine Volcanic Belt, respectively. Vertical scale � horizontal scale assuming an average crustal veloc-ity of 6000 m s–1.

kinematics where crossed by line 2 (Scott et al. 2000) (Figure 2). Although there are several possible interpre-tations for line 2 in this area, we suggest that theGirilambone Group is overthrust on its margins byOrdovician volcanics. The outcrop extent of the Girilam-bone Group coincides with a significant gravity low that appears to originate at upper to mid-crustal levels,below the slice of Girilambone Group that appears to be <1 km thick. The source of the gravity low is problematical.Using the modelling outlined in Direen et al. 2001), we areforced to conclude that it is either a layered granite or low-density sedimentary rocks, such as the Silurian–Devonian (� = 2.55 t m–3). In Figure 4a, we have interpreted

the source of the gravity low to consist of an upper package of Silurian–Devonian rocks underlain by a reflective, possible igneous body. In Figure 6, all this low-density rock is shown as Silurian–Devonian sedimentaryrocks.

In summary, therefore, the imbricate zone in line 3 con-sists of a west-vergent thrust slice of Gidginbung Volcanicsand an east-vergent slice of Belimebung Volcanics separ-ated by Silurian–Devonian strata of the Derriwong Group.In line 2 the imbricate zone consists of high-level thrustslices of Ordovician volcanics thrust west over WaggaGroup as well as a thin sliver of Girilambone Group thrustover Silurian–Devonian sedimentary rocks.


Figure 5 Migrated shallow (6 s TWT) seismic profiles of 99AGS-L3 (west and east) across the Junee–Narromine Volcanic Belt in thevicinity of Barmedman showing both uninterpreted (top) and interpreted (bottom) versions. Vertical scale � horizontal scale assum-ing an average crustal velocity of 6000 m s–1.

COWAL IGNEOUS COMPLEX

Although the Cowal Igneous Complex [we prefer the termCowal Igneous Complex to the ‘Lake Cowal VolcanicComplex’ of Raymond et al. (2000b) because the complexcontains significant volumes of intrusive and volcani-clastic material] was crossed by line 2 (Figures 3a, 4a) it has negligible outcrop. Interpretation of its shape fromaeromagnetic data (Glen & Fleming 2000) suggests it ismuch less deformed than the other volcanics crossed to thewest and south. Results from drilling suggest that much ofthe southern part of the complex consists of granodioriteand diorite, with minor primary volcanics (Miles & Brooker1998).

Seismically, the Cowal Igneous Complex is non-reflectivenear the surface, but with some deeper short reflectionsthat outline open folds. The depth extent is uncertain,but it is put at the upper boundary of unit 2c in line 2(Figure 4a). A key feature is the presence of a steep west-dipping reflection that extends from 2 s TWT at approxi-mately CDP 2600 up to the surface where it coincides withan aeromagnetic lineament separating highly deformedfrom less-deformed rocks just east of the Cowal gold–copper deposit (Glen & Fleming 2000; Raymond et al. 2000a).Thus, we infer that this reflection represents a major shearzone and that it may have represented the pathway for goldmineralisation in the structurally controlled Cowal deposit(Miles & Brooker 1998).


UPPER DEVONIAN HERVEY GROUP

The Upper Devonian Hervey Group occurs in the core ofthe Tullamore Syncline and in the core of the Wheoga

Syncline (Figures 3b, 4b), both of which are crossed by line1. Based on interpretation of line 3, we also recognise a newbelt of Upper Devonian rocks east of the Springdale Fault(Figures 5, 7b). All three occupy synclines bounded by faults


Figure 6 Gravity and magnetic forward models of 1999 seismic sections, displayed at a scale of 1:1: (a) 99AGS-L2, (b) 99AGS-L1 and (c)99AGS-L3. Properties (density in t m–3; magnetic susceptibility in SI): (a) granite, (2.67–2.70; 0); �w, (2.72; 0.002–0.005); �v, (2.70–2.78; 0–0.025);CIC, (2.68; 0.005–0.01); Silurian–Devonian, (2.55–2.57; 0); Girilambone Group, (2.62; –0.02): (b) �v, (2.60–2.80; 0–0.05); Late Devonian clas-tics, (2.59; 0) (2.52; –0.025); Silurian–Devonian sediments, (2.59; 0) (2.66; 0); Early Devonian volcanics, (2.80; 0.03–0.05); �k, (2.63; –0.02): (c)granite, (2.65; –0.005); �w, (2.75; 0); �v, (2.75–2.85; –0.14–0.02); Silurian–Devonian, (2.52–2.65; –0.006–0); Dgb, (2.50–2.55; –0.06 to –0.3); Dgt,(2.60; –0.015); Jindalee Group, (2.80; 0.07); Late Devonian clastics, (2.47; 0).

on their western sides (Figure 2). Seismically, the UpperDevonian is very recognisable, being characterised by longcontinuous reflections.

A detailed view of the Upper Devonian rocks in theTullamore Syncline (Figure 7a) shows parallel continuousreflections that outline a very asymmetrical syncline witha gently dipping eastern limb and a steep, structurallycomplex western limb. Reflections in the western limb are truncated by steep east- and west-dipping inferredfaults, which merge (at 0.9 s TWT under CDP 3930) into awest-dipping reflection that separates partly continuous

subhorizontal reflections in the Hervey Group from west-dipping more discontinuous reflections in CowalIgneous Complex. Thus, we interpret these relations as a flower structure developed in a strike-slip zone. The eastern limb of this syncline shows strong discordancebetween subhorizontal reflections in older rocks and dipping reflections in basal Hervey Group. However, thepresence of a minor truncation of basal reflections in the Hervey Group (Figure 7a) and the presence of a smallantiform just below the surface (Figure 4b at CDP 3500),suggests the contact is not just a simple unconformity,


Figure 7 (a) Enlargement of part of seismic line 99AGS-L1 showing the eastern margin of the Cowal Igneous Complex defined by aflower structure at the Marsden Thrust. (b) Enlargement of part of seismic line 99AGS-L3 showing the west-dipping Springdale Fault,with Carboniferous displacement thrusting Silurian–Devonian rocks over Upper Devonian rocks. Vertical scale � horizontal scaleassuming an average crustal velocity of 6000 m s–1. Thick black lines, faults; thin black lines, geological contacts; 2C, Ordovician arcvolcanics; �vj, Jindalee Group and equivalents; SDs, Silurian–Devonian sedimentary rocks.

332 R. A. Glen et al.Ta

ble

1D

escr

ipti

on o

fth

e n

atu

re o

fth

e se

ism

ic r

efle

ctio

ns

in t

he

Ord

ovic

ian

vol

can

ic-a

rc p

acke

t.

Des

crip

tion

Nat

ure

of

refl

ecti

ons

Inte

rpre

tati

ons

and

ques

tion

s

Pa

cket

2A

in

ferr

ed O

rdov

icia

n a

rc p

ack

et (

incl

ud

es v

olc

an

ic,

vo

lca

nic

last

ic a

nd

in

tru

siv

e ro

cks)

1.W

est-

dipp

ing

ban

dsP

rom

inen

t w

est-

dipp

ing,

sub-

plan

ar,n

ear-

para

llel

Wh

at a

re t

he

ban

ds?

Are

th

ey p

rim

ary

zon

es o

fco

her

ent

ban

ds o

fre

flec

tion

s,a

few

ms

wid

e,an

d bu

ilt

up

ofvo

lcan

ics

vsvo

lcan

icla

stic

s,or

com

plex

sil

ls?

Are

th

eysu

bpar

alle

l an

d lo

w-a

ngl

e in

ters

ecti

ng

inte

rnal

ref

lect

ion

s.st

ruct

ura

lly

enh

ance

d or

are

th

ey p

ure

ly s

tru

ctu

ral?

Th

eS

ome

ban

ds d

ie o

ut

to t

he

east

(de

fin

ing

the

wes

tern

seco

nd

idea

is

supp

orte

d by

not

es o

n p

acke

t 2c

bel

ow,b

y th

ebo

un

dary

of

pack

et 2

B),

alth

ough

dee

per

ones

per

sist

ban

ds t

run

cati

ng

wea

ker

type

3 r

efle

ctio

ns

(wh

ich

may

be

fart

her

eas

t be

low

pac

ket

2b a

nd

defi

ne

a la

rge

anti

form

.st

rati

grap

hic

),an

d by

th

e cu

rvat

ure

of

type

3 r

efle

ctio

ns

into

Ban

ds c

ut

refl

ecti

ons

ofle

ss i

nte

nsi

ty (

type

s 2

and

3th

ese

ban

ds,w

hic

h i

s re

min

isce

nt

ofsh

ear

zon

es.T

he

sen

se o

fw

ith

in t

his

pac

ket)

.sh

ear

is t

op-d

own

-to-

the-

wes

t,i.

e.n

orm

al f

ault

ing.

2.B

ack

grou

nd

ban

ds o

fre

flec

tion

sB

road

su

bhor

izon

tal

to g

entl

y ea

st-d

ippi

ng

ban

ds t

hat

Are

th

ese

prim

ary,

hav

e th

ey b

een

rea

ctiv

ated

,or

are

form

bri

dges

bet

wee

n t

he

wes

t-di

ppin

g zo

nes

an

d th

est

ruct

ura

l?se

para

te l

ozen

ge-s

hap

ed p

acke

ts o

fle

sser

ref

lect

ivit

y.T

hes

e ba

nds

are

var

iabl

y pe

rsis

ten

t an

d le

ss p

rom

inen

t th

anth

e w

est-

dipp

ing

ban

ds,b

ut

like

th

em t

hey

con

tain

in

tern

alre

flec

tion

s th

at a

re p

aral

lel

to o

r at

low

an

gles

to

the

exte

rnal

bo

un

dari

es.

3.B

ack

grou

nd

shor

t-le

ngt

h r

efle

ctio

ns

Th

ese

dip

eith

er e

ast,

subh

oriz

onta

lly,

or w

est,

prod

uci

ng

a lo

zen

ge p

atte

rn t

hat

ou

tlin

es z

ones

of

low

ref

lect

ivit

y.E

ast-

dipp

ing

refl

ecti

ons

are

eith

er t

run

cate

d by

th

e w

est-

dipp

ing

ban

ds,o

r be

com

e as

ympt

otic

to

them

,pro

duci

ng

fuzz

y bo

un

dari

es t

o th

ose

ban

ds.O

ther

s cu

rve

into

an

d ou

t of

it i

n p

lace

s.

4.B

reak

s in

sh

ort

refl

ecti

ons

Eas

t-di

ppin

g fa

ult

s cu

ttin

g th

e sh

ort-

len

gth

ref

lect

ion

s.

Pa

cket

2B

in

ferr

ed O

rdov

icia

n a

rc p

ack

et—

up

-dip

ch

an

ge

fro

m 2

A,

ass

oci

ate

d i

n p

art

wit

h a

bru

pt

dip

ch

an

ge

Upp

er p

art

Not

mu

ch s

eism

ic c

har

acte

r.

Low

er p

art

Sh

ort-

len

gth

ref

lect

ion

s ou

tlin

e a

larg

e-sc

ale

anti

form

an

dS

ee 2

A a

bove

.sy

nfo

rm t

o th

e ea

st (

also

ou

tlin

ed b

y ba

nds

in

2A

bel

ow,

Fig

ure

4a).

Th

e ea

st l

imb

ofth

is a

nti

form

pas

sin

g ea

st i

nto

a co

mpl

emen

tary

syn

form

is

cut

by a

n e

ast-

dipp

ing

fau

lt.

Pa

cket

2C

in

ferr

ed O

rdov

icia

n a

rc p

ack

et

Ext

ends

acr

oss

mu

ch o

fli

ne

99A

GS

-L1.

Th

icke

ns

to e

ast

and

deve

lops

in

to a

bro

ad b

and

that

P

acke

t 2C

th

icke

ns

to e

ast.

(Fig

ure

4b),

but

also

rec

ogn

ised

as

a w

est-

dipp

ing

con

tain

lot

s of

tru

nca

tin

g an

d cr

ossi

ng

inte

rnal

ref

lect

ion

s th

at

ban

d on

th

e ea

st s

ide

ofli

ne

99A

GS

-L2,

wh

ich

outl

ine

loze

nge

s of

wea

kly

ref

lect

ive

mat

eria

l li

ke i

n p

acke

t 2A

m

ay e

xten

d to

an

eas

t-di

ppin

g fa

ult

at

2s

to t

he

wes

t.W

her

e n

arro

wes

t (w

est

part

of

lin

e 99

AG

S-L

1),

un

der

CD

P 2

600.

asso

ciat

ed w

ith

tru

nca

tion

s of

un

der-

an

d ov

erly

ing

refl

ecti

ons.

Pa

cket

2D

in

ferr

ed O

rdov

icia

n a

rc p

ack

et

Mig

ht

corr

elat

e w

ith

low

er p

art

ofpa

cket

2B

in

lin

e 99

AG

S-L

2.

Pa

cket

2E

in

ferr

ed O

rdov

icia

n a

rc p

ack

et

but has undergone some later fault movement. This is supported by the ability to extend the discordance far to the west of the Tullamore Syncline.

LOWER DEVONIAN STRATA

Line 1 crosses the Carawandool Volcanics, which occupythe core of the north-trending late Early DevonianCurrowong Syncline that is unconformably overlain by theHervey Group in the west (Figure 2). In its eastern limb,the volcanics pass down section into the subvertical to overturned beds of the Silurian–Devonian DerriwongGroup.

Seismically, the Carawandool Volcanics consist ofstrongly reflective intervals, inferred to reflect primary volcanic units, and less reflective packets, thought to be sediment-rich. Together they extend to a depth of ~4 km (1.4 s TWT) and outline a broad synform that correlateswith the Currowong Syncline (Figures 3b, 4b).

SILURIAN–DEVONIAN STRATA

Mapped Silurian–Devonian strata grouped into the Derri-wong Group occur in all three lines. On line 2, this groupoccurs in the imbricate zone (Figure 4a). It occurs on line1 as a small packet discordantly below the CarawandoolVolcanics in the west limb of the Currowong Syncline(Figures 4b, 7a). The Derriwong Group, with subverticaldips, occurs in the eastern limb of the syncline on the surface, but neither the boundary with the Lower Devonianstrata nor their steep dips are apparent in the seismic data (Figures 3b, 4b). However, we do infer a weak, steeplyeast-dipping reflection, interpreted to be a fault, along the eastern mapped edge of these Silurian rocks. We alsointerpret Silurian–Devonian rocks to be present beneaththe Hervey Group in the Wheoga Syncline at the easternend of line 1 (Figure 4b) and beneath the Springdale Syncline in line 3 (Figure 5), although their depth extentsare uncertain.

In line 3 (Figure 5), the packet extending from theSpringdale Fault west to the Belimebung Volcanics consistsof moderate to weak, relatively continuous, mainly gentlywest-dipping reflections that are very similar to the seismiccharacter of the Upper Silurian – Lower Devonian rocksseen in line 1 (Figures 3b, 4b). This differs from the interpre-tation of Warren et al. (1995) and Bacchin et al. (1999), whoinferred Upper Devonian Hervey Group in this area.

Gravity data for line 3 show a significant negative anom-aly (–200 µm s–2) superimposed on negative Bouguer anom-aly gradient, dropping from a background of +50 µm s–2

to one of –300 µm s–2 over a distance of 24 km (Figure 6).As a result, we extend these Silurian–Devonian rocks (� = 2.55 t m–3) tentatively down to a depth of 18 km, althoughthere could be granites here instead, or as well. Althoughthe reflectivity matches that of Ordovician volcanics, thegravity low rules out this possibility.

INFERRED ORDOVICIAN ARC ROCKS AT DEPTH

Large amounts of reflective material occur in the upper and middle crust. In Table 1 this material is subdivided into different packages based on variations in reflectivity,

especially the presence or absence of strong west-dippingreflective bands. In lines 1 and 2, these packets are sub-divided into sub-packets 2A–E, based on different internalfeatures, particularly the presence of strong west-dippingreflective bands (Figure 4). These bands, characteristic ofpacket 2A, cut across background continuous reflections aswell as shorter length reflections, and are interpreted to becontractional fault zones: local curvature of backgroundreflections suggests minor extensional geometry. Thesefault zones, together with shorter reflections in domain 2B,are folded around a major antiform (Figure 4a), the dimen-sions of which are best seen in the full crustal-scale sectiondescribed below.

The eastern limb of this antiform is cut by a faint,steeply east-dipping reflection, interpreted to be a majorfault, that separates it from west-dipping reflections (packets 2C and 2D) in the east. Internally, packet 2D is characterised by gentle broad folds in the backgroundreflections, suggestive of much lower total strain thaninferred further west. The upper part of packet 2C narrowsmarkedly from east to west (Figure 4b). Although some ofthis thinning occurs below the Hervey Group in theTullamore Syncline, the top of packet 2C is probably adécollement fault itself because the packet is only partiallyfolded around the Currowong Syncline and it does not takeon the subvertical dips that occur in the eastern limb of thesyncline.

Line 3 (Figure 5) also displays the same banded reflec-tive character as seen in lines 1 and 2. Curved bands ofreflections are interpreted as ramp antiforms above west-dipping contractional faults and suggest major internalimbrication.

In summary, packet 2 links into, or is a depth extent of,Ordovician arc volcanics (discussed above) and is thusinferred to consist of Ordovician arc rocks (igneous rocksand volcaniclastics, including plutonic roots of the arc)with their strong reflectivity attributed to the presence ofcoherent volcanics and igneous intrusions (Figures 4, 5;packets 2A–E in Table 1). On lines 2 and 3, the western partsof these inferred arc rocks underlie Ordovician turbiditesand intrusive granites of the Wagga Belt, being separatedfrom it by a major inferred west-dipping fault (Figures 4a,5). On line 1, the eastern margin of these Ordovician arcrocks stop at depth at a steep fault at CDP 2650 (Figure 4b).In line 3, the eastern edge of the arc rocks at depth is inter-preted to be a steep fault under CDP 4350 (Figure 5).

DEVONIAN GRANITES

The Barmedman and Thurungly Granites are not wellimaged in the seismic data (Line 3, Figure 5), and are diffi-cult to interpret. Strong, subhorizontal reflections limit themaximum thickness of the Barmedman Granite to approxi-mately 2.7 km (~0.9 s TWT: Figure 5), although it may beonly approximately 1.5 km (0.5 s TWT) thick. The LateDevonian Thurungly Granite appears to be more equantthan the Barmedman Granite (Figure 5). The low densitiesimplied for these bodies (2.45–2.55 t m–3) may indicate pervasive alteration. The Thurungly Granite consists oftwo magnetically distinct phases, both with a density of~2.55 t m–3 and a floor modelled at approximately 1.8 km(Figure 6).



Figure 8 (a) Uninterpreted version of migrated deep (20 s TWT) seismic profiles 99AGS-L1 and 99AGS-L2 as a staggered profile acrossthe Junee–Narromine Volcanic Belt. Note that there is a gap between the two lines, with line 99AGS-L2 being located approximately 7 km south of line 99AGS-L1 (see Figure 2 for location). (b) Uninterpreted version of migrated deep (20 s TWT) seismic profile 99AGS-L3 across the Junee–Narromine Volcanic Belt in the vicinity of Barmedman. For each section, vertical scale � horizontal scaleassuming an average crustal velocity of 6000 m s–1.


Figure 9 Interpreted versions of migrated deep (20 s TWT) seismic profiles (a) 99AGS-L1 and 99AGS-L2, and (b) 99AGS-L3 across theJunee–Narromine Volcanic Belt. Vertical scale � horizontal scale assuming an average crustal velocity of 6000 m s–1. Note that 2A–2Eare facies within the Ordovician arc.

INFERRED ?CAMBRIAN–ORDOVICIAN VOLCANICS

A moderately reflective packet characterised by shortreflections and strong west-dipping reflective bands occursin the subsurface at the eastern ends of lines 1 and 3(Figures 3b, 5). This packet has the same banded type ofreflectivity as the overlying Ordovician arc rocks (packet2), and is much more reflective than the Ordovician quartz-rich turbidites. Therefore, we think that it consists largelyof igneous material. Gravity and magnetic modelling(Figure 6) require a dense (2.80 � t m–3), magnetically sus-ceptible package (7000 � 10–5 SI), consistent with packets ofmafic igneous rocks. We suggest that this packet correlateswith volcanic rocks of the Jindalee Group (Lyons et al.2000b) that crop out east of the seismic lines (Figure 2).

Crustal-scale seismic sections across theJunee–Narromine Volcanic Belt

The interpretations from shallow 6 s TWT data can now beput into a crustal-scale perspective by examining thedeeper level seismic data across the Junee–NarromineVolcanic Belt. Figures 8 and 9 consist of data to a depth ofapproximately 50 km (17 s TWT) and illustrate a number ofkey points.

(1) The seismic profiles are dominated by a blind,major west-dipping thrust system that puts the quartz-richturbidites of the Wagga Belt over the Junee–NarromineVolcanic Belt. That is, the Lower to Middle Ordovician turbidites hosting Silurian granites have been thrust over the Lower to Upper Ordovician volcanics.

(2) This west-dipping fault forms the floor fault to east-dipping imbricate faults in the Wagga Belt.

(3) On line 3 (Figure 9b), the western boundary ofthe Gidginbung Volcanics, corresponding to the mappedGilmore Fault Zone, is a small east-dipping thrust that solesoff the Ordovician arc rocks at depth and is a back-thrustoff the floor fault described above.

(4) Ordovician arc rocks, inferred to be part of theMacquarie Arc, extend at depth across line 2, most of line1 and half of line 3.

(5) We infer that a major internal imbrication eventhas affected the western part of the arc package that liesstructurally below rocks of the Wagga Belt. On line 2, theseearly thrusts are folded around large crustal-scaleantiforms that persist to depths of ~8 s (Figure 9a) andwhich lie in the hangingwalls of west-dipping faults (withinferred contractional displacements) that project upwardsto the east of the seismic section.

(6) The crustal packet from 1 s (TWT) on the easternside of line 1 down to 11 s in line 2 contains west-dipping,moderately strong reflections that we interpret as faultsthat thickened the pile by thrust stacking (Figure 9a).Because this packet projects up dip towards the JindaleeGroup, a belt of Ordovician chert (Percival 1999) andassociated volcanic rocks (Lyons et al. 2000b) (Figure 2), weinfer that it consists of ?Cambrian–Ordovician volcanicand associated intrusive rocks that lie below the arc rocks.

(7) Upper parts of the sections contain MiddlePalaeozoic rocks that may have formed in extensional half-grabens that were later partially inverted in the ?late Early Devonian in high-level imbricate zones.

(8) Upper Devonian sedimentary units were deformedin the Carboniferous by thrusting and folding, with somestrike-slip faulting in accommodation zones in the MarsdenThrust (Figure 2).

MOLONG VOLCANIC BELT

The Molong Volcanic Belt is the best studied part of theMacquarie Arc (Pogson & Watkins 1998; Meakin & Morgan1999), with very good outcrop except for the part northeastof Wellington. Major copper and gold resources occur at Cadia–Ridgeway, Cargo, Browns Creek and Copper Hill (Figure 10). Seismic-reflection profiles (105 km) wereacquired along three lines cutting the Molong Volcanic Beltin 1997, with the energy sources supplied by 10 kg explosivecharges placed in ~20 m-deep drillholes (see Figures 1, 10for line locations). Acquisition parameters are listed inJohnstone et al. (1998) and processing procedures aredescribed in Jones and Johnstone (2001).

Line 97AGS-EL1 (line 1) is a 31 km long, south–north linethat passes from Ordovician turbidites of the AdaminabyGroup northwards into the Molong Volcanic Belt. Line97AGS-EL3 (line 3: 47 km), and 97AGS-EL2 (line 2: 27 km)together provide a 74 km-long staggered transect across theMolong Volcanic Belt from the Cowra Trough in the westto the Mumbil Shelf and Hill End Trough in the east.The original interpretations of the 1997 seismic reflectionlines (Glen et al. 1998b, 1998c) are improved here due to recent reprocessing and by new insights provided by the Junee–Narromine data. These reflection seismic lineswere augmented by the acquisition in 1997 of a 350 kmsouth–north refraction line passing from Ordovician turbidites and intrusive granitoids in the south into andalong the Molong Volcanic Belt (Finlayson et al. 2002).

Data from the west–east transect are shown firstly inupper crustal sections (down to 3 s TWT or approximately9 km: Figures 11, 12) to discuss correlations of seismic datawith outcrop data. The deeper 20 s data are then discussedfor the east–west and then south–north lines in order todraw tectonic interpretations of this belt.

Upper crustal shallow (6 s TWT) seismic data forlines 97AGS-EL2 and 97AGS-EL3

COWRA TROUGH AND INTRUSIVE DEVONIAN GRANITE

The western part of line 3 is occupied by deformed fill ofthe Cowra Trough intruded by Early Devonian granites(Figure 12a). The Gumble Granite at the western edge ofline 3 is non-reflective, but we infer it to have a sub-horizontal base. Although the seismic data suggest itextends only to a depth of 1.2–1.8 km (400–600 ms TWT),modelling of a negative Bouguer anomaly of 50–100 µm s–2

associated with the granite (� = 2.57 t m–3) (Figure 13) suggests a floor at ~3.5 km.

Seismic data show that an antiform, outlined by twodifferent sets of reflections, lies east of the Gumble Granite.Short, weak reflections correspond to the Lower DevonianGregra Group in the eastern limb. Below these lies a mixture of continuous and short reflections that corres-pond to the Cudal Group. A shallow, west-dipping zone


of thin reflections cuts the antiform at a depth of 2 km (600 ms TWT) and projects towards the surface at CDP 1580near the contact between the Gregra Group and the Lambiefacies (Catombal Group). The zone is interpreted to be anew gently west-dipping thrust, with the mapped CudalFault as a splay from it.

UPPER DEVONIAN STRATA

Mapping indicates the presence of tight folds and thrustfaults cutting the Upper Devonian Catombal Group in thefootwall of the Molong Fault (shown as the northern partof the Columbine Mountain Fault on the Molong 1:100 000scale geological map: Krynen et al. 1997). However, we cannot recognise this unit in the reflection data. Gravity

modelling (Figure 13) supports structural interpretationsthat the Catombal Group extends to a depth of ~2 km.

ORDOVICIAN VOLCANICS

Surface geology of the Molong Volcanic Belt consists ofMiddle and Upper Ordovician volcanics, volcaniclasticsand intrusives separated by Upper Ordovician limestonesand unconformably overlain by Upper Silurian strata. Thevolcanics contain internal east- and west-dipping faults andare bounded on the west by the east-dipping Molong Faultin the vicinity of the seismic traverse (see sections onKrynen et al. 1997; Meakin et al. 1997).

Seismically, the Ordovician volcanics are character-ised by short individual reflections with a prevailing


Figure 10 Simplified geology of part of the Molong Volcanic Belt showing the location of the 1997 seismic reflection lines (based onRaymond et al. 1998). See Figure 1 for location.

subhorizontal to eastward dip, and some open antiformalhinges. There is only minor, discontinuous banding of reflections (Figures 11, 12), thereby precluding ready subdivision into packets of contrasting reflectivity as ispossible for the Junee–Narromine Volcanic Belt. Thesereflections are cut by several east-dipping thin zones ofreflections. The most obvious zone projects to the surfaceat CDP 1380, where it correlates with the Molong Fault thatbounds the Molong Volcanic Belt on the west. The seismicsection shows that this fault can be traced to a depth ofmore than 9 km (3 s TWT). Another weak zone of dippingreflections further east (CDP 1000) projects toward theNurea Fault, which was interpreted as a west-dipping faultfolded succession by Krynen et al. (1997). Although we cannot exclude a west-dipping fault, our preferred solutionis that the fault dips east and links into a moderately east-dipping fault imaged below 3 km (1 s TWT) that truncates reflections in the hangingwall at low angles(Figure 12a).

Between the Molong and Nurea Faults, short back-ground reflections show moderate easterly dips (with hangingwall antiforms above inferred faults) belowapproximately 1.5 s TWT (Figure 12a). Closer to the surface,they outline a broad antiform that correlates moderatelywith surface mapping (Krynen et al. 1997).

Farther east, mapping by Krynen et al. (1997) showsthree additional faults: an unnamed east-dipping thrust on the western edge of the Fairbridge Volcanics, and the Barrakee and Ammerdown Faults to the east, both ofwhich were inferred to be west dipping. The seismic dataprovide some evidence for the unnamed fault. However, wecannot correlate the Barrakee and Ammerdown Faultswith any west-dipping reflections in the seismic sections.Instead, they may correlate with non-persistent east-dipping reflective zones (Figure 12a).

Moderate east- and west-dipping short reflections pre-dominate in the seismic section east of the Nurea Fault,although they are more gently dipping than surface dips(Krynen et al. 1997). Folded reflections east of CDP 560 lieeast of an unnamed east-dipping fault and above a gentlywest-dipping fault.

INFERRED ORDOVICIAN ARC ROCKS

Silurian–Devonian strata of the Cowra Trough are under-lain by a strongly reflective packet that extends from ~1 sTWT down to at least 3 s TWT (Figure 12a) and which con-tains subhorizontal to gently, east-dipping, short reflectionsand reflection bands. The strong, partly banded nature ofthe reflections is similar to the seismic expression of the


Figure 11 Uninterpreted versions of unmigrated shallow (3 s TWT) seismic profiles (a) 97AGS-EL3 across the Cowra Trough and the Molong Volcanic Belt, and (b) profile 97AGS-EL2 across the Mumbil Shelf and Hill End Trough. Vertical scale � horizontal scaleassuming an average crustal velocity of 6000 m s–1.

exposed Ordovician volcanics, and thus we infer that theyrepresent Ordovician arc rocks at depth. Cutting these area series of west-dipping zones of reflections interpreted tobe faults and some of these can be tracked upwards intothe Cowra Trough fill.

Similar reflective crust occurs in line 2 underSilurian–Devonian strata of the Mumbil Shelf and Hill EndTrough (Figures 11b, 12b). This reflective crust, also inter-preted as Ordovician arc rocks, is dominated by bands ofreflections that vary in dip from gently east through to gently west, outlining broad antiforms and synforms.These bands lie parallel to, or cut across, short backgroundreflections (e.g. under CDP 980 at 2 s TWT).

These reflective bands are cut by thin east-dipping zonesof reflections that are associated with clean truncation,curvature or background reflections or development of

broad footwall synforms and hangingwall antiforms (e.g.under CDP 810 at ~1 s TWT). These criteria all suggest thatthese thin zones are east-dipping faults (Figure 12b) andform the basis for recognition in the seismic section ofdepth extensions of the Suma Park Fault System, as wellas the Wilga Glen and Godolphin Faults.

SILURIAN–DEVONIAN ROCKS OF THE MUMBIL SHELF ANDHILL END TROUGH

The upper part of the crust in line 2 is less reflective thanthat occupied by the underlying inferred Ordovician arcrocks and consists of spaced individual reflections withvery few, non-persistent bands (Figure 12b). We correlatethis weakly reflective crust with mapped Upper Silurianand Lower Devonian strata of the Mumbil Shelf and the


Figure 12 Interpreted versions of unmigrated shallow (3 s TWT) seismic profiles (a) 97AGS-EL3 across the Cowra Trough and the MolongVolcanic Belt, and (b) profile 97AGS-EL2 across the Mumbil Shelf and Hill End Trough. Vertical scale � horizontal scale assuming anaverage crustal velocity of 6000 m s–1.

Hill End Trough (Meakin et al. 1997). It is harder to iden-tify faults in these less-reflective Silurian–Devonian rocks,but we have been able to identify narrow reflections thatcorrelate with the east-dipping Sawmill Hill Fault and thewest-dipping Belerada Fault as defined from mapping inMeakin et al. (1997) (Figure 12b).

Background reflections east of the Sawmill Hill Faulthave low dips and outline open folds that correlate with thesynclinorium in the Cunningham Formation (section inMeakin et al. 1997). However, at the eastern end of the linethe seismic data do not clearly image the west-dippingMerrions Formation on the western limb of the Hill EndAnticline. West of the Sawmill Hill Fault, reflections outline west-vergent folds in an east-dipping envelopingsurface that correlate approximately with mapped struc-tures shown in Meakin et al. (1997).

Crustal scale west–east seismic sections acrossthe Molong Volcanic Belt

The deeper crustal structure of lines 3 and 2 (Figure 14)shows a three-layered crust. The lower layer (5) extends from


Figure 13 Gravity and magnetic forward models of the 1997 seismic sections, displayed at scale of 1:1. (a) 97AGS-EL1 (north–south),(b) 97AGS-EL3 (east–west), (c) 97AGS-EL2 (east–west). Properties (density in t m–3; magnetic susceptibility in SI): (a) �v, (2.62–2.73;–0.02–0.025); �v (monzonites), (2.66–2.74; –0.002–0.002); �a, (2.72; –0.017–0.002); Sw, (2.58; –0.015 to –0.012): (b) Dg, (2.57; 0); CT, (2.64–2.65;0.001–0.007); Dc, (2.52; 0.002); �v, (2.60–2.65; 0.005–0.03); Silurian–Devonian, (2.57–2.58; 0–0.005): (c) �v, (2.65–2.66; –0.04–0.06); �v (ultramafic),(2.65; 0.02); MS, (2.45–2.50; –0.005–0.01); HET, (2.55; 0); Cgb, (2.55; –0.025).

Figure 14 Unmigrated deep (20 s TWT) seismic profiles 97AGS-EL3 and 97AGS-EL2 as a staggered profile across the CowraTrough, Molong Volcanic Belt, Mumbil Shelf and Hill EndTrough showing both uninterpreted (top) and interpreted (bottom) versions. Note that there is a gap between the two lines,with line 97AGS-EL2 being located approximately 12 km to thesouth of line 97AGS-EL3 (see Figure 10 for location). Note also that the numbers used for the crustal layers correspond with those used on the refraction model (Figure 16). Vertical scale � horizontal scale assuming an average crustal velocity of 6000 m s–1.

the Moho at approximately 45 km (~15 s TWT), up to a weaklyreflective west-dipping surface that occurs from ~39 km (13 s TWT) in the west to ~21 km (~7 s TWT) in the east.Above this lies a thick, west-dipping layer (4) that consistslargely of internal west-dipping narrow bands of reflectionscutting variably dipping, short background reflections,and is inferred to be igneous in character. In line 3, this isoverlain by a thick upper crustal layer (2a + 3, largelyinferred Ordovician volcanics) that contains west-dippingfaults west of the Molong Fault and east-dipping faults tothe east (Figure 14). In line 2, this upper crustal section consists of Ordovician arc rocks overlain by weakly reflec-tive east-dipping Silurian–Devonian sedimentary rocks.Significantly, background reflections in the lower parts of this layer outline large-wavelength antiforms that lie in the hangingwalls of faults that separate layer 4 from layers 3 + 2.

South–north line (97AGS-EL1) across the southernpart of the Molong Volcanic Belt

Line 1 is a south to north line (Figure 10) passing from the Garland Granodiorite (northern part of the WyangalaBatholith) through quartz-rich turbidites of the Lower–Middle Ordovician Adaminaby Group into the Coombingand Weemalla Formations and Forest Reefs Volcanics of theMolong Volcanic Belt. The line was designed to assess thegeometry of the Forest Reefs Volcanics, interpreted as a volcanic caldera collapse structure by Wyborn (1992), rela-tions at the southern part of the volcanic belt, especiallybetween the belt and the turbidite basin (interpreted as amajor thrust by Glen & Wyborn 1997), and the geometry ofthe Lachlan Transverse Zone.

Line 1 (Figure 15) shows a three-layered crust similar tothat described above. The lowest layer (5), from the inferredMoho at ~45 km (~15 s TWT) up to approximately 27 km (9 s TWT), is weakly reflective. The overlying stronglyreflective layer (4), extending up to ~9 km (3 s TWT), con-tains north-dipping to subhorizontal bands of reflections.The weakly reflective upper layer (2a + 3) contains shortreflections and some stubby bands of reflections that varyin dip from subhorizontal and gentle to openly folded, witha large antiform present in the north of the traverse,approximately under station number 650. We cannot recog-nise the boundaries to the intrusive monzonites in theForest Reefs Volcanics. In the northern part of the line, weuse cut-offs, zones of weak reflections and the presence of open antiforms to infer the presence of several south-dipping faults (Figure 15).

The seismic-reflection data in the southern end of theline are insufficient to resolve the contact between theAdaminaby Group and the Coombing Formation, inter-preted as a fault by Glen and Wyborn (1997). One asym-metrical magnetic anomaly (?a black shale band) mayindicate a steep northerly ?overturned dip (Figure 13).

Several key features emerge from the seismic lines cutting the Molong Volcanic Belt.

(1) Ordovician volcanic rocks at the surface are muchmore reflective than the flanking Silurian–Devonian units.By analogy, we interpret the reflective packets below theCowra Trough and Lambie Shelf in the west and below the

Mumbil Shelf and Hill End Trough in the east as beingOrdovician arc rocks.

(2) The presence of variably strong, discrete east- orwest-dipping reflections that cut across subhorizontal todipping reflections. Some reflections project towardmapped or inferred faults. The most obvious correspondswith the east-dipping Molong Fault, on which Ordovicianvolcanics were thrust over Upper Devonian rocks. Theseinferred faults dip east in the east, but are west-dippingwest of the Molong Fault and under the Cowra Trough.Thus, it appears that a major change in tectonic transportdirection occurs under the western part of the MolongVolcanic Belt (see below).

(3) Geometrical relations such as hangingwall anti-forms and footwall synforms suggest contractional kine-matics on east-dipping faults. However, using the top ofthe Ordovician volcanics as a datum, the faults show stratigraphic evidence of normal, down-to-the-east separation and thus east-block-down extension. We recon-cile these opposing ideas by suggesting that the faultsunderwent original down-to-the-east extension during formation of the Mumbil Shelf and Hill End Trough,followed by reactivation as east-over-west thrusts (somewith younger-on-older, out-of-sequence relations) duringregional inversion.

(4) The seismic data suggest that the Godolphin Faultpreserves the greatest amount of net stratigraphic exten-sion, with Ordovician volcanics in the hangingwall of thefault lying at depths of ~5 km (1.8 s TWT). If the fault actedas the western bounding fault of the Hill End Trough,rather than the Sawmill Hill Fault as shown in section byMeakin et al. (1997), then the Mumbil Group may pass atdepth into Silurian fill of the Hill End Trough.

(5) The nature of the strongly reflective mid-crustallayer is intriguing. We tentatively interpret the bands ofstrong reflections as thrusts, indicative of an imbricatedmiddle crust. This interpretation is strengthened by thepresence of broad antiforms just above the upper boundingfault at the base of layer 2a + 3.

SEISMIC REFRACTION PROFILING

Details of the 350 km, north–south, 1997 refraction line,passing from Ordovician turbidites and intrusive grani-toids in the south into, and along, the Molong Volcanic Beltin the north, are discussed by Finlayson et al. (2002). Keyfeatures of this line (Figure 16) are the longitudinal changesin thickness of velocity layers that are centred broadlyunder the Lachlan Transverse Zone. These include:

(1) Thickening of the near-surface layer north ofCadia.

(2) Southwards termination of Ordovician arc vol-canics (layer 2a, Figure 16) and metamorphosed arc vol-canics (layer 3) near Wyangala, where they are juxtaposedalong a major south–dipping boundary against layer 2b that corresponds with structurally thickened Ordovicianturbidites (Adaminaby Group) and intrusive granites(?migmatites at depth). The presence of an underthrustlayer of Ordovician volcanics is supported by the changein composition of granites in the northern part ofthe Wyangala Batholith from strongly deformed and elongate S-type granites in the south to a cluster of equant


and largely I-type granites in the north (Chappell et al.1991).

(3) Thickening of layer 4 between Cadia and Wyangala.Layer 4 corresponds to the mid-crustal reflective layer inreflection lines 1–3. The nature of this layer is not clear.Possibilities include metamorphosed Ordovician and/orCambrian ocean crust, partially subducted terranes (Glenet al. 1998a) and/or Palaeozoic, Late Cretaceous or MiddleMiocene mafic magmatic underplates. Because the extrathickness of the bottom of layer 4 under the LachlanTransverse Zone has a low velocity, and thus low density,

it is unlikely to be solely mantle-generated underplatedmaterial.

(4) A southerly dip on the Moho.Figure 16 shows the good correlation between the reflec-

tion data of line 97AGS-EL1 and the refraction line.Reflective crustal layer 4, with north-dipping reflections(Figure 15), correlates with velocity layer 4 (?Cambrian–Ordovician ocean crust ± accreted terranes) in the refrac-tion data (Figure 16). Similarly, reflective layer 2 + 3, withinternal south-dipping reflections, correlates with refrac-tion layers 2 and 3.


Figure 15 Unmigrated deep (20 s TWT) seismic profile 97AGS-EL1 across the Molong Volcanic Belt and the northernmost part of thequartz-rich turbidites (Adaminaby Group) showing both uninterpreted (left) and interpreted (right) versions. Note that the numbersused for the crustal layers correspond with those used on the refraction model (Figure 16). Vertical scale � horizontal scale assumingan average crustal velocity of 6000 m s–1.

DISCUSSION

Contrasting architecture of the volcanic beltsComparison between the deep (20 s TWT) east–west stag-gered sections of lines 99AGS-L1 and 99AGS-L2 across theJunee–Narromine Volcanic Belt and lines 97AGS-EL2 and97AGS-EL3 across the Molong Volcanic Belt indicates significant differences in the crustal architecture of the arc rocks at depth (Figure 17). Whereas both belts show evidence for contraction and extension, the upper part ofthe Molong Volcanic Belt is dominated by extension, witheast-dipping extensional faults that throw Ordovician volcanics down to the east. This is most clearly seen for the east-dipping faults in line 97AGS-EL2, but can be extra-polated westwards to the Nurea and Molong Faults on line97AGS-EL3, both of which contain Upper Silurian strata in their hangingwalls. These east-dipping faults show evidence of later reverse reactivation as contractionalfaults. The presence of Ordovician arc rocks below the fillof the Cowra Trough, in the western part of line 97AGS-EL3 also implies extension west of the Molong Fault, butthe controlling extensional structures have not yet beenidentified.

In contrast, the Junee–Narromine Volcanic Belt isdominated by major crustal contraction, with two large,crustal-scale antiforms lying in the hangingwalls of majorwest-dipping faults. The trailing (western limb) of the eastern antiform is generally of low strain, with open foldsand few big faults. In contrast, the trailing western limb of the western antiform (in the western underthrust part of the belt) is strongly deformed by early and late contractional faults. Simple line-length balancing of these

structures suggests ~30% overall shortening across lines99AGS-L1 and 99AGS-L2, with ~45% of shortening in theunderthrust, western part of the belt. Extensional struc-tures in this belt (reactivation of west-dipping faults in line99AGS-L2 and the half-grabens, especially in line 99AGS-L3)post-date these contractional structures.

Temporally, we broadly correlate the extensional struc-tures cutting the Junee–Narromine Volcanic Belt withthose in the Molong Volcanic Belt because both show evi-dence of later contractional reactivation in the late Earlyand Middle Devonian as well as in the Carboniferous. Ifwe are correct, the major contractional structures in theJunee–Narromine Volcanic Belt are older than those visible at shallow levels in the Molong Volcanic Belt.However, equivalent contractional structures exist deeperin the Molong Volcanic Belt, such as large antiforms in the hangingwalls of east-dipping faults at the base of theOrdovician arc rocks, and the west-dipping fault thatreaches a depth of ~39 km (13 s TWT) under the westernedge of line 97AGS-EL2 (Figure 14). We are emphasising thedifferences in the extensional histories of both belts withgreater extension in the Molong Volcanic Belt, reflected ata high level by the opening of the Hill End Trough, the pres-ence of the Jemalong Trough above the Junee–NarromineVolcanic Belt notwithstanding.

Later contractional events also seem to have been less important in the Junee–Narromine Volcanic Belt. Inlines 99AGS-L1 and 99AGS-L2, they were restricted to‘thin-skinned’ deformation above 2 km, with the deeperarchitecture of the belt left relatively intact, possibly as aresult of a buttressing effect due to the deep-rooted core ofthe arc.


Figure 16 Interpretation of the seismic refraction profile, modified after Finlayson et al. (2002). 1–5 represent the seismic refractionlayers defined by Finlayson et al.; also shown are the density ranges for layers 2a, 3 and 4. The shaded panel represents the area covered by seismic reflection line 97AGS-EL1, and the dipping lines in the panel within layers 2a to 4 represent dips of reflectionstaken from Figure 15.


Figure 17 Partially offset east–west cross sections across the Eastern Lachlan Orogen constrained by the deep crustal structure acrossthe Junee–Narromine and Molong volcanic belts, as interpreted in the seismic sections (see Figure 1 for location). Inferences on thegeology between the seismic profiles and to the east of the 1997 profiles are based on the synthesis maps of Glen and Zhang (2001).Abbreviations (e.g. SDs) are as used on the earlier figures.

Relationship between Macquarie Arc and coevalturbidite basin

The boundary between the Ordovician turbidite basin and the coeval Macquarie Arc is crossed along the westernmargin of the Junee–Narromine Volcanic Belt by lines99AGS-L2 and 99AGS-L3, along the eastern margin of theJunee–Narromine Volcanic Belt at depth in line 99AGS-L1,and across the southern margin of the Molong VolcanicBelt in line 97AGS-EL1. Although the contact in line 97AGS-EL1 is not imaged directly in the seismic reflection data,interpretation of the refraction data suggests that it dipsto the south. The contact along the eastern side of theJunee–Narromine Volcanic Belt does not crop out along the line of section, being obscured by younger basin fill.We infer it to be an east-dipping fault, post-Silurian in age, with little constraint on its earlier geometry. Furthernorth the contact between the turbidites and theOrdovician volcanics has been interpreted as an east-dipping thrust by Glen (in Raymond et al. 2000a).

Seismic data from the Junee–Narromine Volcanic Beltsuggest that the Ordovician turbidites of the Wagga Belthave been thrust eastwards over the arc rocks, along a blind west-dipping fault that forms the floor fault to theimbricated rocks of the Wagga Group. The internal imbri-cation appears to correlate with mapped and inferrednorth-northwest-trending faults on the surface. The floorfault corresponds conceptually with the Gilmore FaultZone, which has been interpreted from outcrop furthersouth as a west-dipping sinistral transpressional myloniteand fault zone (Stuart-Smith 1991). Significantly, the seismic data in line 99AGS-L3 indicate that the westernfaulted margin of the Gidginbung Volcanics, mapped on the surface as part of the Gilmore Fault Zone, dips east not west and is thus not equivalent to the Gilmore Fault of previous correlations. The surface fault is only a high-level east-dipping splay off the major fault at depth.

Lachlan Transverse Zone

Is the Lachlan Transverse Zone real in the subsurface? Twolines of evidence suggest that it is. The first is the presenceof north- and south-dipping reflections that lie oblique tothe east and west-dipping reflections north of the LachlanTransverse Zone in lines 97AGS-EL2 and 97AGS-EL3. Wesuggest that these north- and south-dipping reflections correlate with the east–west to west-northwest-trendingstructures inferred from within the Lachlan TransverseZone by Glen and Walshe (1999).

The second line of evidence is provided by the refrac-tion data that show major changes in crustal velocityoblique to the north–south line, centred approximatelyunder the proposed location of the Lachlan TransverseZone (see Finlayson et al. 2002). Especially marked is thesouth–dipping velocity boundary between layer 2b to thesouth and layers 2a and 3 to the north (Figure 16), whichwe suggest represents the major boundary between theMolong Volcanic Belt and the turbidite basin and whichextends as a ~35 km-long mapped and inferred fault east ofthe seismic line (Glen & Wyborn 1997). According to Glenand Walshe (1999), this fault marks the southern limit of theLachlan Transverse Zone.

Crustal structure

It is now possible to draw an offset east–west section acrosspart of the eastern Lachlan Orogen using the seismic sections (again noting that the dips are only apparent dips unless constrained by surface or near-surface data)and filling in the gaps with information from regional mapping.

The structure of the middle crust is dominated by east-vergent, west-dipping faults (Figure 17). Elements of thesefaults cut the upper crust and rise towards the surface inkey areas: (i) as the fault that carries the turbidite basinfill and S-type granitoids of the Wagga Belt over arc rocksalong the western side of the Junee–Narromine VolcanicBelt; (ii) as the Wiagdon Thrust – Mudgee Fault systemalong the eastern edge of the Hill End Trough; (iii) with lesscertainty, the fault that juxtaposes arc rocks of theJunee–Narromine Volcanic Belt and Ordovician turbiditesof the Kirribilli Formation against the Jindalee Group tothe east; and (iv) as imbricate faults on the eastern side ofthe Cowra Trough seen in the western part of line 97AGS-EL3 and in cross-section on the Dubbo 1:250 000 geologicalsheet (Morgan et al. 1999). In this context, the east-dippingfaults cutting the central and eastern parts of the MolongVolcanic Belt and the Mumbil Shelf to the east are anti-thetic faults developed off this west-dipping imbricatearray (Glen 1999 figure 95) and demonstrate extensionalong the eastern side of the Molong Volcanic Belt.

CONCLUSIONS

Interpretation of seismic data across the Molong VolcanicBelt and Junee–Narromine Volcanic Belt sheds much lighton the assembly of the eastern part of the Lachlan Orogen.In particular, it provides evidence for:

(1) Collision between the Macquarie Arc and itsbackarc basin, with Ordovician quartz-rich turbiditesthrust over the Ordovician volcanics, which now lie atdepth beneath the eastern part of the Wagga Belt, account-ing for a prominent gravity high.

(2) Crustal thickening in the turbidite basin, indicatedby north-northwest-striking east-dipping thrusts. Shorten-ing along the western side of the Junee–NarromineVolcanic Belt was accommodated by imbrication and for-mation of large antiforms of Ordovician arc rocks abovethrust ramps.

(3) Possible thrusting of arc volcanics and Ordovicianturbidites over slices of ?Cambrian–Ordovician oceaniccrust incorporated into the thrust pile.

(4) Crustal thickening of ?Cambrian–Ordovicianoceanic crust, as indicated by west-dipping reflections thatdominate the lower crust, interpreted as expressions ofthrusting.

(5) Earlier structural interpretations that Ordovicianvolcanics underlie the Hill End and Cowra Troughs.

(6) Late Silurian – Early Devonian crustal extensionproducing shelves, troughs and inferred half-grabens.Apparent growth faults, with subsequent reactivation, dipeast on line 99AGS-L3, and both east and west on line99AGS-L2, where they bound sub-basins of the JemalongTrough. Equivalent faults have east dips in the Mumbil


Shelf and western part of the Hill End Trough, but may dipwest along the eastern edge of the Cowra Trough.

(7) Late Early to Middle Devonian contractionexpressed by a marked angular unconformity below UpperDevonian rocks and by the reverse reactivation of theextensional faults with younger-on-older geometries.

(8) Carboniferous contraction, best expressed by defor-mation of Upper Devonian continental sedimentary rocksof the Lambie facies. These rocks generally occupy syn-clines in the footwalls of major thrusts (e.g. line 99AGS-L1).Based on line 99AGS-L1, we infer that jogs in the trends ofthese thrusts represent accommodation zones marked bythe development of flower structures at depth.

(9) In the Junee–Narromine Volcanic Belt, the Silurianto Carboniferous extensional and contractional processesare high level and ‘thin-skinned’. They rework, but do notobliterate, earlier crustal-scale structures that extendthrough the crust. In contrast, in the Molong Volcanic Belt there is considerable reworking of older crust andincorporation of older units into more ‘thin-skinned’extensional and contractional tectonics.

ACKNOWLEDGEMENTS

Acquisition costs for lines 99AGS-L1 and 99AGS-L2 werefunded equally by the New South Wales Department ofMineral Resources and the Australian GeodynamicsCooperative Research Centre (AGCRC); all other lineswere funded by the AGCRC, which also undertook the processing of the data. Energy from the refraction surveywas recorded by 90 Seismic Group recorders on loan fromthe US Geological Survey. We thank Chris Mawer, JohnWalshe and Barry Drummond for input into selection oftargets and lines, Bruce Goleby for many discussionsabout processing and interpretation of the data and MikeHall, Paul Lennox, Patrick Lyons and Bruce Goleby for constructive reviews. We especially thank Joe Mifsud,Geoscience Australia, for cartographic excellence in draft-ing the figures. R. A. Glen and R. D. Shaw publish with per-mission of Director-General, New South Wales Departmentof Mineral Resources. R. J. Korsch, N. G. Direen, L. E. A.Jones, D. W. Johnstone, K. C. Lawrie and D. M. Finlaysonpublish with permission of the Chief Executive Officer,Geoscience Australia.

REFERENCES

BACCHIN M., DUGGAN M., GLEN R., GUNN P., LAWRIE K., LYONS P.,MACKEY T., RAPHAEL N., RAYMOND O., ROBSON D. & SHERWIN L.1999. Geophysical Interpretation Map of Cootamundra, NSW,Scale 1:250 000. Australian Geological Survey Organisation,Canberra.

CHAPPELL B. W., ENGLISH P. M., KING P. L., WHITE A. J. R. & WYBORN D.1991. Granites and Related Rocks of the Lachlan Fold Belt (1:1 250 000 Scale Map). Bureau of Mineral Resources, Canberra.

DIREEN N. G., LYONS P., KORSCH R. J. & GLEN R. A. 2001. Integrated geophysical appraisal of crustal architecture in the easternLachlan Orogen. Exploration Geophysics 32, 252–262.

FINLAYSON D. M., KORSCH R. J., GLEN R. A., LEVEN J. H. & JOHNSTONE

D. W. 2002. Seismic imaging and crustal architecture across theLachlan Transverse Zone, a cross-cutting feature of easternAustralia. Australian Journal of Earth Sciences 49, 311–321.

GLEN R. A. 1998a. Tectonic development of the Lachlan Orogen:a framework for mineral exploration. In: Lewis P. C. ed.Lachlan Fold Belt ‘98, pp. 1–6. Australian Institute of GeoscientistsBulletin 23.

GLEN R. A. 1998b. The Eastern Belt of the Lachlan Orogen. In:Finlayson D. M. & Jones L. E. A. eds. Mineral Systems and Crust – Upper Mantle of Southeast Australia, pp. 80–82. AustralianGeological Survey Organisation Record 1998/2.

GLEN R. A. 1999. Structure. In: Meakin N. S. & Morgan E. J. eds. Dubbo1:250 000. Geological Sheet SI/55-4, Explanatory Notes (2nd edition),pp. 366–392. Geological Survey of New South Wales, Sydney.

GLEN R. A. & FLEMING G. D. 2000. Interpreted Basement Geology, centralpart of the Junee–Narromine Volcanic Belt, NSW (S/I, 55-3, 55-7,55-11) 1:500 000 scale. Geological Survey of New South Wales,Sydney.

GLEN R. A., KORSCH R. J., FINLAYSON D. M., JOHNSTONE D. W. & WALSHE

J. L. 1998c. New data on the Ordovician intraoceanic MacquarieArc: preliminary interpretation of seismic refraction and reflection profiling along and across the Molong Volcanic Belt,Lachlan Orogen, New South Wales. In: Lewis P. C. ed. Lachlan Fold Belt ‘98, pp. 93–96. Australian Institute of GeoscientistsBulletin 23.

GLEN R. A., KORSCH R. J. & JOHNSTONE D. W. 1998b. Crustal structure ofthe eastern Lachlan Orogen based on a preliminary interpretationof 4 sec TWT seismic reflection data. In: Finlayson D. M. & Jones L. E. A. eds. Mineral Systems and Crust – Upper Mantle ofSoutheast Australia, pp. 83–84. Australian Geological SurveyOrganisation Record 1998/2.

GLEN R. A. & WALSHE J. L. 1999. Cross-structures in the LachlanOrogen: the Lachlan Transverse Zone example. AustralianJournal of Earth Sciences 46, 641–658.

GLEN R. A., WALSHE J. L., BARRON L. M. & WATKINS J. J. 1998a.Ordovician convergent-margin volcanism and tectonism in theLachlan sector of east Gondwana. Geology 26, 751–754.

GLEN R. A. & WYBORN D. 1997. Inferred thrust imbrication, deformationgradients and the Lachlan Transverse Zone in the eastern belt ofthe Lachlan Orogen, New South wales. Australian Journal ofEarth Sciences 44, 49–68.

GLEN R. A. & ZHANG W. 2001. Syntheses maps Central Eastern New SouthWales. 1—Geology. (S/I55-3, 55-4, 55-7, 55-8, 55-11). Scale 1:500 000.Geological Survey of New South Wales, Sydney.

JOHNSTONE D. W., OWEN A. J. & NICOLL M. G. 1998. AGCRC EasternLachlan seismic survey 1997: operational report. AustralianGeological Survey Organisation Record 1998/30.

JONES L. E. A. & JOHNSTONE D. W. 2001. Acquisition and processing ofthe 1997 Eastern Lachlan (L146) and 1999 Lachlan (L151) seismicreflection surveys. In: Korsch R. J. & Lyons P. eds. IntegratedGeophysical and Geophysical Studies of the Northeastern LachlanOrogen, New South Wales, pp. 26–35. Australian Geological SurveyOrganisation Record 2001/09.

KRYNEN J. P., MORGAN E. J., RAYMOND O. L., SCOTT M. M. & WARREN

A. Y. E. 1997. Molong First Edition (1:100 000 geological map 8631).NSW Department of Mineral Resources, Sydney and AustralianGeological Survey Organisation, Canberra.

LYONS P., DUGGAN M. B. & WALLACE D. A. 2000b. Jindalee Group.In: Lyons P., Raymond O. L. & Duggan M. B. eds. Forbes 1:250 000Geological Sheet, SI55-7 (2nd edition). Explanatory Notes,pp. 7–9. Australian Geological Survey Organisation Record2000/20.

LYONS P., RAYMOND O. L. & DUGGAN M. B. 2000a. Forbes 1:250 000Geological Sheet Si55-7 (2nd edition) Explanatory Notes.Australian Geological Survey Organisation Record 2000/20.

MEAKIN N. S. & MORGAN E. J. (Compilers). 1999. Dubbo 1: 250 000 Geological Sheet SI/55-4, Explanatory Notes (2nd edition).Geological Survey of New South Wales, Sydney.

MEAKIN S., SPACKMAN J. M., SCOTT M. M., WATKINS J. J., WARREN A. Y. E.,MOFFIT R. S. & KRYNEN J. P. 1997. Orange (1:100 000 Geological Map 8731) (1st edition). Geological Survey of New South Wales,Sydney and Australian Geological Survey Organisation,Canberra.

MILES I. N. & BROOKER M. R. 1998. Endeavour 42 deposit, Lake Cowal,New South Wales: a structurally controlled gold deposit.Australian Journal of Earth Sciences 45, 837–847.

MORGAN E. J., CAMERON R. G., COLQUHOUN G. P., MEAKIN N. S., RAYMOND

O. L., SCOTT M. M., WATKINS J. J., BARRON L. M., HENDERSON

G. A. M., KRYNEN J. P., POGSON D. J., WARREN A. Y. E., WYBORN D.,YOO E. K., GLEN R. A. & JAGODZINSKI E. 1999. Dubbo 1:250 000Geological Sheet SI/55-4 (2nd edition). Geological Survey of NewSouth Wales, Sydney and Australian Geological SurveyOrganisation, Canberra.


PERCIVAL I. G. 1999. The age of the Jindalee Group: evidence from conodonts preserved in cherts. Geological Survey of New SouthWales Palaeontological Report 99/02 (GS1999/513) (unpubl.).

POGSON D. J. & WATKINS J. J. 1998. Bathurst 1:250 000 Geological SheetSI/55-8: Explanatory Notes. Geological Survey of New SouthWales, Sydney.

RAYMOND O. L., DUGGAN M. B., GLEN R. A., LEYS M., LYONS P., SCOTT

M. M., SHERWIN L. & WALLACE D. A. 2000a. Forbes (SI/55-7)Simplified Basement Geology 1:250 000 scale map (1st edition).Australian. Geological Survey Organisation, Canberra andGeological Survey of New South Wales, Sydney.

RAYMOND O. L., DUGGAN M. B., LYONS P., SCOTT M. M., SHERWIN L. A.,WALLACE D. A., KRYNEN J. P., YOUNG G. C., WYBORN D., GLEN R. A.,PERCIVAL I. P. & LEYS M. 2000b. Forbes (1:250 000 Geological Map SI/55–7) (2nd edition). Australian Geological SurveyOrganisation, Canberra and Geological Survey of New SouthWales, Sydney.

RAYMOND O. L., POGSON D. J., WYBORN D., HENDERSON G. A. M., KRYNEN

J., MEAKIN S., MORGAN E. J., SCOTT M. M., STUART-SMITH P., WALLACE

D. A., WARREN A. Y. E. & WATKINS J. J. 1998. Bathurst 1:250 000Geological Sheet SI/55-8. Geological Survey of New South Wales,Sydney and Australian Geological Survey Organisation,Canberra.

RAYMOND O., SHERWIN L., LYONS P. & SCOTT M. M. 2000c. The JemalongTrough: an extension of Silurian-Devonian rifting in the easternLachlan Fold Belt. Geological Society of Australia Abstracts 59,409.

RAYMOND O. L. & WALLACE D. A. 2000. Kirribilli Formation. In: LyonsP., Raymond O. L. & Duggan M. B. eds. Forbes 1:250 000. Geological

Sheet, SI/55-7, Explanatory Notes (2nd edition), pp. 30–33.Australian Geological Survey Organisation Record 2000/20.

SCOTT M. M., SHERWIN L., RAYMOND O. L. & LYONS P. 2000. MannaConglomerate. In: Lyons P., Raymond O. L. & Duggan M. B. eds.Forbes 1:250 000. Geological Sheet, SI/55-7, Explanatory Notes (2ndedition), pp. 62–64. Australian Geological Survey OrganisationRecord 2000/20.

SHERWIN L. 1996. Narromine 1:250 000 Geological Sheet SI/55-3,Explanatory Notes. Geological Survey of New South Wales,Sydney.

STUART-SMITH P. G. 1991. The Gilmore Fault Zone—the deformationalhistory of a possible terrane boundary within the Lachlan FoldBelt New South Wales. BMR Journal of Australian Geology &Geophysics 12, 35–49.

WARREN A. Y. E., GILLIGAN L. B. & RAPHAEL N. M. 1995. Geology of theCootamundra 1:250 000 Map Sheet. Geological Survey of NewSouth Wales, Sydney.

WARREN A. Y. E., GILLIGAN L. B. & RAPHAEL N. M. 1996. Cootamundra1:250 000 Geological Sheet SI/55-11 (2nd edition). Geological Surveyof New South Wales, Sydney.

WEBBY B. D. 1978. History of the Ordovician continental platform shelfmargin of Australia. Journal of the Geological Society of Australia25, 41–63.

WYBORN D. 1992. The tectonic significance of Ordovician magmatismin the eastern Lachlan Fold belt. Tectonophysics 214, 287–303.

Received 13 August 2001; accepted 2 December 2001


crustal structure of the ordovician macquarie arc, eastern lachlan orogen, based on...

Documents