Kinematic nature and origin of regional-scale ductile shear zones in the central Yilgarn Craton, Western Australia

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Tectonophysics 394 (

Kinematic nature and origin of regional-scale ductile shear zones

in the central Yilgarn Craton, Western Australia

She Fa Chena,*, John W. Libbyb, Stephen Wychea, Angela Rigantia

aGeological Survey of Western Australia, Department of Industry and Resources, 100 Plain Street, East Perth,

Western Australia 6004, AustraliabDigital Rock Services, Pty. Ltd., 13 Beverley Terrace, South Guildford, WA 6055, Australia

Received 1 April 2004; accepted 2 August 2004

Available online 22 October 2004

Abstract

Regional-scale, northwest- and northeast-trending ductile shear zones, 1- to 6-km wide and over 100-km long, are the most

conspicuous structures in the central Yilgarn Craton, Western Australia. They are commonly developed along granite–

greenstone contacts, but are centred in strongly foliated granite and granitic gneiss. A wide range of kinematic indicators,

including well-developed S–C fabrics, CV-type shear band cleavage, and abundant asymmetric porphyroclasts, consistently

indicate sinistral movement on northwest-trending shear zones and dextral movement on northeast-trending shear zones. Major

planar fabrics within the ductile shear zones include (1) a northwest- or northeast-trending foliation parallel to the shear zones,

(2) a northerly trending gneissic banding, and (3) a northerly trending foliation. The northwest- and northeast-trending foliations

with associated asymmetric structural elements and a shallowly plunging mineral lineation were developed during a

progressive, inhomogeneous east–west shortening event (D3) characterized by low-temperature, noncoaxial deformation.

Formation of the northerly trending gneissic banding and associated symmetric structural elements is attributed mainly to east–

west coaxial shortening (D2) in a high-temperature state during, or shortly after, granite intrusion. The northerly trending

foliation within the ductile shear zones commonly forms the S-fabric of S–C fabrics that was either developed during D3, same

time as the C-fabric, or was reactivated from S2 foliations. Therefore, major planar fabrics in the regional-scale ductile shear

zones of the central Yilgarn Craton can be ascribed to two tectonic events (D2 and D3), and deformation within the shear zones

includes both coaxial and noncoaxial components.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Yilgarn; Shear zone; Shear sense; Coaxial deformation; Noncoaxial deformation

0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2004.08.001

* Corresponding author. Tel.: +61 8 9222 3207; fax: +61 8

9222 3633.

E-mail address: [email protected] (S.F. Chen).

1. Introduction

Regional-scale ductile shear zones in the Archaean

Yilgarn Craton are very prominent structures, some of

which coincide with terrane boundaries (Fig. 1; Tyler

2004) 139–153

Fig. 1. Simplified geological map of the Yilgarn Craton, showing major shear zones and the study area of this paper. Terrane subdivision after

Tyler and Hocking (2001): EGGGT—Eastern Goldfields Granite–Greenstone Terrane; MGGT—Murchison Granite–Greenstone Terrane; NT—

Narryer Terrane; SCGGT—Southern Cross Granite–Greenstone Terrane; SWT—South West Terrane. MDGB—Marda–Diemals greenstone

belt.

S.F. Chen et al. / Tectonophysics 394 (2004) 139–153140

and Hocking, 2001). In the central Yilgarn Craton

(Fig. 2), regional-scale ductile shear zones are 1- to 6-

km wide and commonly over 100-km-long zones of

high strain and represent the best-exposed shear zones

within the craton (Eisenlohr et al., 1993). They

contain a wide range of well-developed shear sense

indicators, such as S–C fabrics (cf. Lister and Snoke,

1984; Hanmer and Passchier, 1991), CV-type shear

band cleavage (cf. Passchier and Trouw, 1996; CV-band of Berthe et al., 1979a,b) and asymmetric

porphyroclasts (cf. Passchier and Simpson, 1986),

and provide a natural laboratory for kinematic studies

of regional-scale ductile shear zones in the Yilgarn

Craton.

Deformation in rocks is commonly subdivided

into pure shear (coaxial) and simple shear (non-

coaxial; Hobbs et al., 1976; Ramsay, 1980; Hanmer

and Passchier, 1991; Passchier and Trouw, 1996),

but some structures are the result of a combination of

both deformation mechanisms (Choukroune et al.,

1987). Based on detailed structural studies on the

regional-scale ductile shear zones in the central

Yilgarn Craton, we suggest in this contribution that

both coaxial and noncoaxial deformations have been

Fig. 2. First vertical derivative aeromagnetic image of the central

Yilgarn Craton. Locations of Figs. 3 and 7 also shown.

S.F. Chen et al. / Tectonophysics 394 (2004) 139–153 141

involved in the formation of major planar fabrics

within the shear zones during progressive, bulk,

inhomogeneous shortening.

This contribution will describe the kinematic

nature and common structural features of six

regional-scale ductile shear zones in the central

Yilgarn Craton. It will then discuss the timing of

shear zone development and the interrelationship

and origin of major planar fabrics within the shear

zones. The observations and interpretations pre-

sented in this contribution are not only critical to

a more comprehensive understanding of the crustal

evolution in the central Yilgarn Craton, but also

have implications for understanding regional-scale

ductile shear zones in other Archaean granite–

greenstone terrains.

2. Regional geological setting

The Meso- to Neoarchaean Yilgarn Craton is

subdivided into five terranes from west to east (Fig.

1; Tyler and Hocking, 2001). The Narryer and South

West Terranes are dominated by granite and granitic

gneiss, while the Murchison, Southern Cross, and

Eastern Goldfields Granite–Greenstone Terranes are

composed of north-trending greenstone belts separa-

ted by extensive granite and granitic gneiss (Fig. 1).

The lithostratigraphy and tectonic history of the

Murchison and Southern Cross Terranes are broadly

similar, whereas the Eastern Goldfields Terrane has

younger greenstones and deformation events (Chen

et al., 2003).

In the central Yilgarn Craton (central Southern

Cross Granite–Greenstone Terrane), greenstones are

subdivided into two successions (Chen et al., 2003). A

ca. 3.0 Ga lower succession is characterized by mafic

volcanic rocks and banded iron-formation, with a

local basal quartzite (Wyche et al., 2004). A ca. 2.73

Ga upper succession is composed of felsic-intermedi-

ate volcanic rocks and clastic sedimentary rocks. It

unconformably overlies the lower succession and it is

preserved mainly in the Marda–Diemals greenstone

belt (MDGB in Fig. 1). Granite and granitic gneiss are

dominated by monzogranite and were emplaced

during ca. 2756–2654 Ma (Nelson, 1999, 2001,

2002; Cassidy et al., 2002).

Three principal deformation events have been

recognized in the central Yilgarn Craton (Chen et al.,

2001a, 2003), built on previous work (Griffin, 1990;

Bloem et al., 1994; Dalstra et al., 1999; Greenfield

and Chen, 1999) and based on overprinting relation-

ships, deformation styles and structural orientations.

North–south compression in D1 (Griffin, 1990;

Dalstra et al., 1999) produced shallowly dipping

foliations and thrusts, and tight to isoclinal folds.

East–west shortening in D2 formed macroscopic

upright folds with an axial planar foliation in


greenstones, and a north-trending gneissic banding in

granite, with local evidence for D2 thrusting (Chen et

al., 2003). In a deep crustal seismic traverse that

crossed from the Eastern Goldfields Granite–Green-

stone Terrane into the southeastern part of the

Southern Cross Granite–Greenstone Terrane, Drum-

mond et al. (2000) recognized east-dipping thrusts

and duplexes in the middle crust and both east- and

Fig. 3. Interpretive geological map, based on recently published geolo

interpretation of aeromagnetic images. Locations of Figs. 4a–d, 5, 6a–d, 8

west-dipping thrusts in the upper crust. These

structures were also attributed to D2 deformation

(Drummond et al., 2000). Progressive and inhomo-

geneous east–west shortening in D3 developed

northwest-trending sinistral and northeast-trending

dextral shear zones that are linked by north-trending

high-strain zones, forming large arcuate structures in

the central Yilgarn Craton (Chen et al., 2001a, 2003).

gical maps by the Geological Survey of Western Australia, and

d and 9a–d also shown.


3. Kinematic nature of ductile shear zones in the

central Yilgarn Craton

3.1. Edale shear zone

The northwestern section of the 4- to 6-km-wide

Edale shear zone is situated on the eastern margin of

the Sandstone greenstone belt (Fig. 3). The shear zone

is mainly developed in granite that contains a

pervasive, northwest-trending foliation, with a shal-

lowly (28–208) plunging mineral lineation. The

foliation dips steeply to the southwest adjacent to

the greenstone margin, and towards the northeast in

the shear zone centre. A northerly trending foliation

with a local steeply (608–758) plunging mineral

Fig. 4. (a) A boudinaged pegmatite vein parallel to gneissic banding, with a

asymmetric porphyroclast of quartzofeldspathic aggregate, with attenuated

restraining jog bound by two northwest-trending, right-stepping discrete sh

the bounding shear zones indicates a sinistral shear sense. See Fig. 3 for

lineation is less prominent than the northwest-trending

foliation. S–C fabrics and asymmetric feldspar por-

phyroclasts indicate a sinistral shear sense. Structures

in the North Cook Well greenstone belt, east of the

shear zone (Fig. 3), and in strongly foliated mon-

zogranite farther east include a northerly (3458–0108)trending foliation that is axial planar to macroscopic

folds in the greenstone belt. These structures and

fabrics curve into the Edale shear zone with an

apparent sinistral sense (Eisenlohr et al., 1993).

In the middle section of the Edale shear zone, a

northerly trending gneissic banding is parallel to

numerous pegmatite veins, some of which are strongly

boudinaged (Fig. 4a). Asymmetric feldspar porphyr-

oclasts, quartzofeldspathic aggregates with strongly

perpendicular quartz vein filling in an extensional gash. (b) A large

tails. (c) S–C fabrics in quartz–muscovite schist. (d) A small-scale

ear zones. The curvature of internal foliation and shear zones against

locations.

Fig. 5. A map showing the relationships between high-strain zones

of simple shear and pure shear in the middle part of the Yuinmery

shear zone. See Fig. 3 for location.


attenuated tails (Fig. 4b), S–C fabrics (Fig. 4c) and CV-type shear band cleavage in granitic gneiss and quartz–

muscovite schist, and S-shaped asymmetric folds in

quartzite provide unequivocal evidence of sinistral

shear. A northerly (3408–3608) trending steep foliationin foliated granite is sinistrally displaced by a 3258- to3358-trending foliation that commonly contains a

shallowly plunging mineral lineation. A third set of

discontinuous foliations (CV-type shear band cleavage)

trends 3108–3208. Where the shear zone curves into a

north–south orientation at the southern end, a well-

developed, north-trending foliation wraps around

flattened and symmetric feldspar porphyroclasts, and

deformation is characterized by flattening or pure

shear. Several north–northeast-trending shear zones

that splay off the Edale shear zone (Fig. 3) are

dominated by flattening with a subsidiary sinistral

shear component (Chen et al., 2001a).

3.2. Yuinmery shear zone

The northwestern section of the Yuinmery shear

zone is developed along the eastern margin of the

Youanmi greenstone belt (Fig. 3) and is a 2- to 3-km-

wide high-strain zone in granitic rocks. Across the

strike of the shear zone from east to west, a steep

foliation gradually changes from northwest to due

north in strike and from northeast to due west in dip

direction. The plunge of a prominent mineral lineation

on the foliation surface also gradually becomes

steeper from east to west. Locally developed S–C

fabrics, CV-type shear band cleavage, and the geometry

of small-scale restraining jogs (e.g., Fig. 4d) indicate a

sinistral shear sense.

The middle section of the Yuinmery shear zone is

developed entirely within granite and granitic gneiss,

and the shear zone boundaries (particularly the

southwestern boundary) are poorly defined. In some

places, the Yuinmery shear zone is a composite

structure made up of at least two shear zones (1.5-

to 2-km wide), showing simple shear-related deforma-

tional features, that are separated and flanked by zones

of pure shear (Fig. 5). The simple shear zones are

characterized by a well-developed northwest-trending,

steep foliation that contains a prominent, shallowly

plunging mineral lineation. This foliation sinistrally

offsets northerly (3458–0058) trending gneissic band-

ing and foliation. The northerly trending foliation (S-

fabric) rotates progressively to become subparallel

with the northwest-trending foliation (C-fabric)

towards the shear zone centre (Fig. 6a). Well-

developed S–C fabrics (Fig. 6b), CV-type shear band

cleavage (Fig. 6c), and abundant asymmetric porphyr-

oclasts of feldspar and quartzofeldspathic aggregates

(Fig. 6d) consistently indicate sinistral movement in

the simple shear zones. In contrast, the pure shear

zones characterized by flattening only contain mod-

erate to intense northerly trending foliation and

gneissic banding with associated symmetric feldspar

porphyroclasts. They generally lack the northwest-

trending foliation and S–C fabrics, although their

boundaries with simple shear zones are typically

gradational.

The southeastern section of the Yuinmery shear

zone truncates the southern end of the Mount Elvire

greenstone belt (Fig. 3), and is locally well exposed in

granite with a pervasive northwest-trending foliation.

Asymmetric porphyroclasts of feldspar and S-shaped

asymmetric folds of gneissic banding indicate a

sinistral shear sense. In thin section, grain-size

Fig. 6. (a) S-fabric is subparallel to C-fabric that is associated with asymmetric feldspar porphyroclasts in granitic gneiss. (b) S–C fabrics in

foliated granite. (c) CV-type shear band cleavage in granitic gneiss. (d) Asymmetric porphyroclasts of feldspar and quartzofeldspathic aggregates

in foliated granite. See Fig. 3 for locations.


reduction is common in the foliation. Coarser quartz

grains or grain aggregates are commonly fractured,

and asymmetric feldspar porphyroclasts have recrys-

tallized tails.

3.3. Mount Dimer shear zone

The Mount Dimer shear zone (MDSZ in Fig. 7) on

the eastern margin of the Marda–Diemals greenstone

belt is up to 5-km wide. The northwestern section of

the shear zone truncates the northeastern limb of the

regional-scale Bungalbin syncline and juxtaposes

greenstones against foliated granite. Adjacent to the

granite–greenstone contact, a steep foliation dips to

the southwest in basaltic and pelitic schists, and

granite is moderately to strongly foliated. The shear

zone is centred 1.5–2 km into foliated granite, where a

pervasive northwest-trending foliation dips steeply to

the northeast and contains a shallowly plunging

mineral lineation. S–C fabrics (Fig. 8a) and asym-

metric porphyroclasts of feldspar are well developed,

and consistently indicate a sinistral shear sense. A

gradation from moderately to weakly foliated mon-

zogranite marks the northeastern margin of the shear

zone.

The southeastern section of the Mount Dimer shear

zone is poorly exposed but truncates the Hunt Range

greenstone belt on aeromagnetic images with an

apparent sinistral shear sense (Figs. 2 and 7).

Mesoscopic, steeply plunging, S-shaped asymmetric

folds in chert and banded iron-formation also indicate

a sinistral movement on the shear zone. Mineral-

Fig. 7. Interpretive geological map, based on recently published maps by the Geological Survey of Western Australia, and interpretation of

aeromagnetic images. Greenstone belts: HRGB—Hunt Range; MDGB—Marda–Diemals; MMGB—Mount Manning. Shear zones: ESZ—

Evanston; KSZ—Koolyanobbing; MDSZ—Mount Dimer. Fold: BS—Bungalbin syncline. Locality names: MJ—Mount Jackson Homestead;

OR—Olby Rock monzogranite; RRM—Rainy Rocks monzogranite; YYD—Yacke Yackine Dam. Locations of Fig. 8a–c also shown.


exploration drillhole data south of the Hunt Range

greenstone belt reveal a 2- to 3-km-wide high-strain

zone composed of foliated ultramafic, mafic, and

granitic rocks.

3.4. Koolyanobbing shear zone

Libby et al. (1991) and Eisenlohr et al. (1993)

presented detailed descriptions for the whole Koolya-

Fig. 8. (a) S–C fabrics and asymmetric porphyroclasts of feldspar in foliated granite. (b) A small-scale, northwest-trending discrete shear zone

sinistrally displaces a north-trending foliation. (c) Well-exposed, foliated granite with a steeply northeast-dipping foliation, on the north shore of

Lake Deborah East, looking towards northwest. (d) A short, discontinuous CV-type shear band cleavage develops at a small angle to a dominant

foliation (C-fabric) that is parallel to the Youanmi shear zone. Locations of a–c shown in Fig. 7, and d in Fig. 3.


nobbing shear zone (KSZ in Fig. 7), and this study

focuses on its northwestern part. On the western

margin of the Marda–Diemals greenstone belt, granite

and greenstones are both strongly foliated within the

shear zone. Here, an intense foliation dips steeply to

the northeast and contains a moderately plunging

mineral lineation. To the southeast, the shear zone is

developed entirely in granite and granitic gneiss, in

which north- to north–northwest-trending gneissic

banding and foliation are sinistrally displaced by a

series of small-scale (3- to 20-cm wide), northwest-

trending, discrete ductile shear zones (Fig. 8b). Some

of these have been subsequently reactivated as brittle

fractures filled by quartz veins, but the sigmoidal

deflection patterns of the northerly trending foliation

across the quartz veins indicate a sinistral shear sense.

The Koolyanobbing shear zone is best exposed in

foliated granite and granitic gneiss, about 6 km wide,

on the north shore of Lake Deborah East (Fig. 8c).

The western margin of the shear zone is characterized

by a weak north-trending foliation that increases in

intensity eastwards to become a dominant foliation

that wraps into a moderately developed, northwest-

trending foliation. Towards the shear zone centre,

mylonite and ultramylonite form narrow (up to 2-m

wide), evenly spaced, high-strain zones between areas

of protomylonite (Libby et al., 1991). The north-

trending foliation progressively rotates into parallel-

ism with the shear zone, whereas the northwest-

trending foliation has a relatively consistent trend

across the shear zone. In the shear zone centre, the

dominant northwest-trending foliation contains a


prominent shallowly plunging mineral lineation.

Asymmetric porphyroclasts of feldspar with recrystal-

lized tails indicate a sinistral shear sense (Eisenlohr et

al., 1993). Farther east, as simple shear strain

decreases, the north-trending foliation gradually

becomes the dominant fabric, and the northwest-

trending foliation gradually disappears.

3.5. Youanmi shear zone

The northeastern section of the Youanmi shear

zone, adjacent to the Sandstone greenstone belt, is a 5-

to 6-km-wide high-strain zone in foliated granite (Fig.

3). In the western part of the shear zone, a northerly

trending foliation is dextrally displaced by a moder-

ately developed, northeast-trending foliation. In the

Fig. 9. (a) Asymmetric porphyroclasts of feldspar with strongly attenuated

lineation on a northeast-trending foliation surface in granite. (c) Z-shaped

with an axial planar foliation in granitic gneiss. See Fig. 3 for locations.

shear zone centre, a discontinuous CV-type shear band

cleavage (Fig. 8d) dextrally offsets a dominant

foliation that is subvertical or dips steeply to the

northwest. Numerous asymmetric feldspar porphyr-

oclasts (Fig. 9a) indicate dextral shear, while a

consistently shallow-plunging mineral lineation (Fig.

9b) suggests a dominant strike-slip shear component.

The eastern boundary of the shear zone coincides with

the granite–greenstone contact. Strongly foliated

basalt and amphibolite east of the contact contain a

north-trending foliation that dips steeply to the east,

although a northeast-trending foliation is seen locally.

The southwestern section of theYouanmi shear zone

contains several major quartz veins in poorly exposed,

foliated granite and mafic schist. Asymmetric feldspar

porphyroclasts in granite and Z-shaped asymmetric

tails in foliated granite. (b) A prominent shallowly plunging mineral

asymmetric fold in mafic schist. (d) A tightly folded pegmatite vein


folds with an axial planar foliation in mafic schist (Fig.

9c) indicate a dextral shear sense. These folds were

formed in response to strike-slip shearing on already

steeply inclined foliations and typically have steeply

plunging hinges. Where the strike of the shear zone

changes to north–south, a north-trending foliation is

wrapped around flattened and symmetric feldspar

porphyroclasts (Eisenlohr et al., 1993).

3.6. Evanston shear zone

The Evanston shear zone (Fig. 3 and ESZ in Fig. 7)

is a 1- to 3-km-wide high-strain zone. Its northeastern

section truncates several north-trending shear zones,

in which foliation trends 3508–0058 and locally

contains a steeply plunging to down-dip mineral

lineation. The middle section of the shear zone is

marked by numerous northeast-trending quartz veins

in foliated granite and granitic gneiss that preserve S–

C fabrics indicative of dextral shear. The Mount Elvire

and Mount Manning greenstone belts have apparently

been dragged into the Evanston shear zone with a

dextral sense (Figs. 2, 3 and 7).

The southwestern section of the Evanston shear

zone lies on the eastern margin of the Marda–Diemals

greenstone belt and separates it from a large area to

the east dominated by granite (Fig. 7). The Rainy

Rocks monzogranite (RRM in Fig. 7) is an elongate

intrusion parallel to the shear zone. Its southeastern

part is strongly deformed in the shear zone and

contains numerous asymmetric porphyroclasts of

feldspar that indicate a dextral shear sense. East of

the Rainy Rocks monzogranite, amphibolite and

mafic–ultramafic schists within the shear zone contain

a pervasive northeast-trending foliation and a shal-

lowly plunging mineral lineation. En echelon quartz

veins and Z-shaped asymmetric folds also indicate a

dextral shear sense (Greenfield and Chen, 1999).

4. Common structural features of ductile shear

zones in the central Yilgarn Craton

Most regional-scale ductile shear zones in the

central Yilgarn Craton are developed along granite-

greenstone contacts, but are commonly centred in

strongly foliated granite and granitic gneiss away from

the contacts. The northwest- and northeast-trending

foliations that define the ductile shear zones typically

dip towards greenstones adjacent to the contacts, but

towards granite away from the contacts. This variation

in foliation orientations may reflect the original

disposition of granite–greenstone contacts, and sub-

sequent modification due to impingement of granite

blocks into greenstone belts (Chen et al., 2001a).

Both northwest- and northeast-trending ductile

shear zones contain northerly trending gneissic band-

ing and foliation. Granitic gneiss comprises alternat-

ing leucocratic and mesocratic bands of largely

monzogranitic to granodioritic composition. The

compositional bands have sharp to diffuse contacts

and range from less than 1 mm up to 30 cm in

thickness. They are defined by variations in biotite

content and grain size and by numerous pegmatite

veins and, less commonly, by aplite and schlieren

lenses. The northerly trending gneissic banding and

foliation are typically parallel to fold axial planes.

They are not confined to the northwest- and northeast-

trending ductile shear zones, but are also present in

areas away from the shear zones (Fig. 3).

A wide range of shear sense indicators, such as S–

C fabrics, CV-type shear band cleavage, asymmetric

feldspar porphyroclasts, and small-scale discrete shear

zones, restraining jogs, and S- and Z-shaped asym-

metric folds, consistently indicate sinistral movement

on northwest-trending shear zones and dextral move-

ment on northeast-trending shear zones. Development

of these asymmetric structural elements indicates a

noncoaxial deformation within the shear zones (cf.

Passchier and Trouw, 1996).

S–C fabrics are well developed in most regional-

scale ductile shear zones of the central Yilgarn Craton.

The S-fabric is commonly defined by segregation of

mafic and quartzofeldspathic minerals and by pre-

ferred alignment of biotite and ellipsoidal deformed

quartzofeldspathic grains and grain aggregates. It has

a general northerly trend adjacent to shear zone

boundaries, but progressively rotates into subparallel

with, or is transposed by, the C-fabric in shear zone

centres. The C-fabric is typically defined by grain size

reduction and by deflection of S-fabric into the

orientation of C-fabric. It is relatively straight and

continuous and trends northwest or northeast, parallel

to the shear zone boundaries.

In shear zone centres, a CV-type shear band

cleavage (Passchier and Trouw, 1996) develops at

Table 1

Tectonic history of the central Yilgarn Craton


low angles to the shear zone margins. It has the same

shear sense as the C-fabric, but is typically short and

discontinuous. In granitic gneiss, the CV-type shear

band cleavage fails to continue into more weakly

deformed compositional bands (e.g., pegmatite

dykes). The CV-type shear band cleavage is similar to

the shear band foliation described by White et al.

(1980) and to the extensional crenulation cleavages

described by Platt (1984).

Numerous asymmetric porphyroclasts of feldspar

and quartzofeldspathic aggregates have been found in

the ductile shear zones of the central Yilgarn Craton.

They are dominated by j-type porphyroclasts,

although y-type porphyroclasts have been occasion-

ally observed (see Passchier and Simpson, 1986 for

the definitions of j- and y-type porphyroclasts).

Asymmetric porphyroclasts are commonly aligned

along the northwest- or northeast-trending foliations.

The northwest- and northeast-trending ductile

shear zones in the central Yilgarn Craton do not

intersect each other to form conjugate structures but

are linked by north-trending contractional zones

dominated by coaxial deformation, forming large

arcuate structures (Chen et al., 2001a). At their

terminations, the shear zones either change into a

northerly trend, where they dissipate in a broad zone

of coaxial deformation containing flattened and

symmetric porphyroclasts (Eisenlohr et al., 1993;

Chen et al., 2001a), or split into a network of shear

zones or faults that are dominated by flattening with a

subsidiary strike-slip component (Chen et al., 2001a).

Eruption of mafic volcanic rocks and deposition of banded

iron-formation in lower greenstone succession (ca. 3.0 Ga)

D1 N–S compression: tight to isoclinal folds, layer–parallel

foliation, thrusts

Granite intrusion (ca. 2.75 Ga)

Eruption of Marda andesite–rhyolite (ca. 2.73 Ga)

D2 E–W coaxial shortening (ca. 2.73–2.68 Ga): upright folds,

N-trending foliation, thrusts

Deposition and folding of Diemals Formation clastic rocks

(ca. 2.73 Ga)

Granite intrusion (ca. 2.71–2.68 Ga), formation of N-trending

gneissic banding

D3 progressive, inhomogeneous E–W shortening (ca. 2.68–2.65

Ga): NW- and NE-trending ductile shear zones, arcuate

structures

Granite intrusion (ca. 2.68–2.65 Ga), formation of N-, NW- and

NE-trending foliations in shear zones

5. Timing of ductile shear zones in the central

Yilgarn Craton

It has been commonly recognized that formation of

ductile strike-slip shear zones in the Yilgarn Craton

postdates a regional folding event (e.g., Watkins and

Hickman, 1990; Eisenlohr et al., 1993; Swager, 1997;

Chen et al., 2001a,b; Weinberg et al., 2003). For

example, in the Eastern Goldfields Granite–Greenstone

Terrane (Fig. 1), the north–northwest-trending D3

sinistral shear zones were formed after D2 regional

folding and thrusting (Swager, 1997; Chen et al.,

2001b; Weinberg et al., 2003). In the Murchison

Granite–Greenstone Terrane (Fig. 1), development of

D4 north–northeast-trending dextral shear zones also

succeeded the D3 regional folding event (Watkins and

Hickman, 1990).

In the central Yilgarn Craton, at least two phases of

regional folding related to east–west shortening (D2)

have been recognized in greenstones (Table 1). An

early phase of folding and thrusting deformed the

lower greenstone succession dominated by mafic

volcanic rocks and banded iron-formation, and a later

phase resulted in deposition and folding of clastic

sedimentary rocks (Diemals Formation) that uncon-

formably overlie the lower greenstone succession. The

maximum age of the later D2 folding phase is

constrained by the maximum depositional age of

2729F9 Ma indicated by SHRIMP (sensitive high-

resolution ion microprobe) U–Pb zircon dating on

detrital zircons in sandstone from the Diemals

Formation (Nelson, 2001; Chen et al., 2003). Con-

tinued east–west shortening during D2 in the central

Yilgarn Craton resulted in the formation of a northerly

trending gneissic banding (S2) in granitic gneiss

(Table 1) that has SHRIMP U–Pb zircon ages ranging

from ca. 2714 to ca. 2699 Ma (Group 1 in Table 2;

Nelson, 2001, 2002; Cassidy et al., 2002).

Regional-scale, northwest- and northeast-trending

ductile shear zones in the central Yilgarn Craton were

developed during D3 progressive and inhomogeneous

east–west shortening (Table 1; Chen et al., 2001a,

2003). Strongly foliated granite within the shear zones

Table 2

SHRIMP U–Pb zircon ages

Group 1 Granitic gneiss

Willow Bore: 2714F10 Ma (Nelson, 2002)

Yacke Yackine Dam: 2711F4 Ma (Nelson, 2001)

Yuinmery Homestead: 2701F8 Ma (Nelson, 2002)

Elvire Rock: 2699F6 Ma (Nelson, 2002)

Group 2 Foliated granite in shear zones

Mountain Well: 2683F4 Ma (Nelson, 2001)

Bell Chamber Well: 2667F8 Ma (Nelson, 2002)

Evanston Mine: 2654F6 Ma (Nelson, 2001)

Group 3 Granite intruding shear zones

Koolyanobbing: 2656F3 Ma (Qiu et al., 1999)


contains a northwest- or northeast-trending S3 folia-

tion and has SHRIMP U–Pb zircon ages ranging from

ca. 2683 to ca. 2654 Ma (Group 2 in Table 2; Nelson,

2001, 2002). The Koolyanobbing shear zone is

intruded by an undeformed granite body that has a

SHIMP U–Pb zircon age of 2656F3 Ma (Qiu et al.,

1999). This age is within error of the 2654F6 Ma date

obtained for the foliated monzogranite in the Evanston

shear zone (Nelson, 2001), and probably represents

the minimum age of D3 ductile strike-slip shearing.

6. Origin of ductile shear zones in the central

Yilgarn Craton

The most important planar fabrics in the regional-

scale, northwest- and northeast-trending ductile shear

zones of the central Yilgarn Craton include (1) a

northerly trending gneissic banding, (2) a northerly

trending foliation, and (3) a northwest- or northeast-

trending foliation. A clear understanding of the

kinematic nature, timing, and interrelationship of

these planar fabrics is critical to understanding the

origin and evolution of ductile shear zones in the

central Yilgarn Craton.

The northerly trending gneissic banding in granitic

gneiss is typically parallel to pegmatite veins and

dykes, some of which are strongly boudinaged (Fig.

4a), while others are deformed into symmetric and

tight folds (Fig. 9d). Although granoblastic texture is

common in granitic gneiss due to recrystallization that

outlasted deformation, it can be seen that symmetric

porphyroclasts of feldspar and quartzofeldspathic

aggregates are elongate parallel to the gneissosity.

Augen gneiss (Passchier et al., 1990) with flattened K-

feldspar porphyroclasts in a fine-grained matrix is

observed locally. A northerly trend of the gneissic

banding and its close relationship with symmetric

structural elements suggest that the banding was

probably developed mainly during east–west coaxial

shortening (cf. Myers, 1978; Passchier et al., 1990)

and that it cannot be attributed solely to primary

compositional inhomogeneity. Eisenlohr et al. (1993)

suggested that the gneissic banding in granitic gneiss

was formed by deformation at high temperatures. The

granite was probably still hot and plastic or was

deeply buried when east–west coaxial shortening took

place in D2 to form the gneissic banding.

In the central Yilgarn Craton, granitic gneiss is

either distributed along, or isolated from, the north-

west- and northeast-trending ductile shear zones (Fig.

3). The gneissic banding typically maintains a north-

erly trend in areas unaffected by strike-slip shearing,

but diverges into a north–northwest or north–northeast

trend within and adjacent to the ductile shear zones,

locally parallel to the shear zones (e.g., in shear zone

centres). The change of gneissic banding orientations

within the ductile shear zones is due to strike-slip

shearing in D3 that resulted in (1) rotation of preexisting

gneissic banding, and (2) development of new gneissic

banding in the orientation of local strain field within

syn-D3 granite.

A northerly trending foliation is widespread in the

central Yilgarn Craton and appears to have developed

during D2 and D3. In greenstone belts, it forms an axial

planar foliation to F2 folds whose axial traces are

locally deflected by D3 ductile shear zones. In granitic

gneiss, the northerly trending foliation is typically

parallel to the gneissic banding. A northerly trending

foliation is also locally developed in large areas of less-

deformed granite (Fig. 3), for example, the Olby Rock

monzogranite (OR in Fig. 7) that has a SHRIMP U–Pb

zircon age of 2697F8 Ma (Nelson, 1999).

Within the northwest- and northeast-trending duc-

tile shear zones, the northerly trending foliation

commonly forms the S-fabric that was either devel-

oped during D3, the same time as the C-fabric (cf.

Berthe et al., 1979a,b; Platt, 1984; Hanmer and

Passchier, 1991), or was reactivated from the S2foliation. Some northerly trending foliation surfaces

within the shear zones (e.g., the Edale and Yuinmery


shear zones) contain a steeply plunging to down-dip

L2 mineral lineation that was probably related to D2

thrusting, predating the C-fabric development. Steeply

plunging L2 mineral lineation has also been observed

locally on north-trending S2 foliation within strongly

foliated granite that is unaffected by D3, to the west of

the Mount Elvire greenstone belt (Fig. 3).

The northwest- and northeast-trending foliations

within the ductile shear zones commonly contain a

shallowly plunging mineral lineation. They were

typically associated with various asymmetric struc-

tural elements, including S–C fabrics, CV-type shear

band cleavage and asymmetric porphyroclasts, and

were developed by noncoaxial deformation at rela-

tively low temperatures (D3; Eisenlohr et al., 1993;

Chen et al., 2001a). The C-fabric has a relatively

consistent northwest or northeast trend, parallel to the

shear zone boundaries, whereas the S-fabric typically

shows a progressive rotation in orientation across the

strike of shear zones. In lower-strain zones of strike-

slip shearing, the S-fabric wraps into a weak C-fabric

at higher angles. In high-strain zones, the C-fabric

becomes dominant and closely spaced, and the angle

between S- and C-fabrics decreases.

The transition between D2 and D3 reflects a

variation in crustal temperature gradient, and repre-

sents a change from crustal thickening to strike-slip

shearing. D2 folding and thrusting were accompanied

by intrusion of voluminous granite (Table 1). As

granite bodies gradually cooled and became more

rigid, they impinged into greenstone belts during D3,

with movement accommodated predominantly by

sinistral shear along the northwest-trending margins

and dextral shear along the northeast-trending margins

of granite blocks (Table 1; Chen et al., 2001a).

Deformation in D3 was partitioned into coaxial and

noncoaxial components. In the north-trending high-

strain zones that linked the northwest- and northeast-

trending ductile shear zones, deformation was pre-

dominantly coaxial, while deformation within the

shear zones was noncoaxial. This deformation style

is closely related to rock heterogeneity and compe-

tency contrasts. The centres of solid granite blocks

were relatively resistant to deformation compared

with the adjacent greenstone belts. During progres-

sive, bulk, inhomogeneous shortening (Bell, 1981),

strain was partitioned along the granite margins and

the granite centres were less strained. This resulted in

impingement of rigid granite blocks into less com-

petent greenstone belts along the northwest- and

northeast-trending ductile shear zones.

7. Conclusions

The regional-scale ductile shear zones in the central

Yilgarn Craton contain three major planar fabrics: (1) a

northwest- or northeast-trending foliation, with asso-

ciated asymmetric structural elements and a shallowly

plunging mineral lineation, was developed in D3

characterized by noncoaxial deformation; (2) a north-

erly trending gneissic banding with associated sym-

metric structural elements was developed mainly in D2

coaxial shortening; and (3) a northerly trending

foliation was either developed in D3 or reactivated

from the S2 foliation.

In the central Yilgarn Craton, D2 is dominated by

coaxial shortening resulting in crustal thickening and

D3 is characterized by noncoaxial deformation to

accommodate impingement of competent granite

blocks into less competent greenstone belts. However,

deformation in D3 was apparently partitioned into a

coaxial component in the north-trending high-strain

zones and to a noncoaxial component in the northwest-

and northeast-trending ductile shear zones.

Acknowledgements

We thank Martin Van Kranendonk, Ian Tyler, Terry

Farrell, Ben Goscombe, Bruce Groenewald, and Paul

Morris for their valuable comments on various versions

of this manuscript, and Dellys Sutton, Michael Prause,

Suzanne Dowsett, and Murray Jones for drafting the

figures. Ian Lowrie is thanked for field assistance to

SFC. Excellent comments and suggestions from

dTectonophysicsT reviewers, John Myers and David

Gray, significantly improved themanuscript. Published

with permission from the Director of the Geological

Survey of Western Australia.

References

Bell, T.H., 1981. Foliation development—the contribution, geom-

etry and significance of progressive, bulk, inhomogeneous

shortening. Tectonophysics 75, 273–296.


Berthe, D., Choukroune, P., Jegouzo, P., 1979a. Orthogneiss,

mylonite and non-coaxial deformation of granites: the example

of the South Armorican Shear Zone. Journal of Structural

Geology 1, 31–42.

Berthe, D., Choukroune, P., Gapais, D., 1979b. Orientations

preferentielles du quartz et orthogneissification progressive en

regime cisaillant: l’exemple du cisaillement sudarmoricain.

Bulletin de Mineralogie 102, 265–272.

Bloem, E.J.M., Dalstra, H.J., Groves, D.I., Ridley, J.R., 1994.

Metamorphic and structural setting of Archaean amphibolite-

hosted gold deposits near Southern Cross, Southern Cross

Province, Yilgarn Block, Western Australia. Ore Geology

Reviews 9, 183–208.

Cassidy, K.F., Champion, D.C., McNaughton, N.J., Fletcher, I.R.,

Whitaker, A.J., Bastrakova, I.V., Budd, A.R., 2002. Character-

isation and metallogenic significance of archaean granitoids of

the Yilgarn Craton, Western Australia. Report no. 222. Minerals

and Energy Research Institute of Western Australia (MERIWA).

514 pp.

Chen, S.F., Libby, J.W., Greenfield, J.E., Wyche, S., Riganti, A.,

2001a. Geometry and kinematics of large arcuate structures

formed by impingement of rigid granitoids into greenstone belts

during progressive shortening. Geology 29, 283–286.

Chen, S.F., Witt, W.K., Liu, S., 2001b. Transpression and

restraining jogs in the northeastern Yilgarn Craton, Western

Australia. Precambrian Research 106, 309–328.

Chen, S.F., Riganti, A., Wyche, S., Greenfield, J.E., Nelson, D.R.,

2003. Lithostratigraphy and tectonic evolution of contrasting

greenstone successions in the central Yilgarn Craton, Western


Choukroune, P., Gapals, D., Merle, O., 1987. Shear criteria and

structural symmetry. Journal of Structural Geology 9, 525–530.

Dalstra, H.J., Ridley, J.R., Bloem, E.J.M., Groves, D.I., 1999.

Metamorphic evolution of the central Southern Cross Province,

Yilgarn Craton, Western Australia. Australian Journal of Earth

Sciences 46, 765–784.

Drummond, B.J., Goleby, B.R., Swager, C.P., 2000. Crustal

signature of Late Archaean tectonic episodes in the Yilgarn

Craton, Western Australia: evidence from deep seismic sound-

ing. Tectonophysics 329, 193–221.

Eisenlohr, B.N., Groves, D.I., Libby, J., Vearncombe, J.R., 1993.

The nature of large scale shear zones and their relevance to gold

mineralisation, Yilgarn Block. Report no. 122. Minerals and

Energy Research Institute of Western Australia. 161 pp.

Greenfield, J.E., Chen, S.F., 1999. Structural evolution of the

Marda–Diemals area, Southern Cross Province. Geological

Survey of Western Australia Annual Review 1998–99, 68–73.

Griffin, T.J., 1990. Southern Cross Province. Geology and Mineral

Resources of Western Australia, Memoir 3. Geological Survey

of Western Australia, pp. 60–77.

Hanmer, S., Passchier, C., 1991. Shear-sense indicators: a review.

Geological Survey of Canada Paper 90-17 (72 pp.).

Hobbs, B.E., Means, W.D., Williams, P.F., 1976. An Outline of

Structural Geology. Wiley and Sons. 571 pp.

Libby, J., Groves, D.I., Vearncombe, J.R., 1991. The nature and

tectonic significance of the crustal-scale Koolyanobbing shear

zone, Yilgarn Craton, Western Australia. Australian Journal of

Earth Sciences 38, 229–245.

Lister, G.S., Snoke, A.W., 1984. S–C mylonites. Journal of

Structural Geology 6, 617–638.

Myers, J.S., 1978. Formation of banded gneisses by deformation of

igneous rocks. Precambrian Research 6, 43–64.

Nelson, D.R., 1999. Compilation of geochronology data, 1998.

Geological Survey of Western Australia Record 1999/2

(222 pp.).


Geological Survey of Western Australia Record 2001/2

(205 pp.).


Geological Survey ofWestern Australia Record 2002/2 (282 pp.).

Passchier, C.W., Simpson, C., 1986. Porphyroclast systems as

kinematic indicators. Journal of Structural Geology 8, 831–843.

Passchier, C.W., Trouw, R.A.J., 1996. Microtectonics. Springer.

289 pp.

Passchier, C.W., Myers, J.S., Krfner, A., 1990. Field Geology of

High-Grade Gneiss Terrains. Springer-Verlag. 150 pp.

Platt, J.P., 1984. Secondary cleavages in ductile shear zones. Journal

of Structural Geology 6, 439–442.

Qiu, Y.M., McNaughton, N.J., Groves, D.I., Dalstra, H.J., 1999.

Ages of internal granitoids in the southern cross region, Yilgarn

Craton, Western Australia, and their crustal evolution and

tectonic implications. Australian Journal of Earth Sciences 46,

971–981.

Ramsay, J.G., 1980. Shear zone geometry: a review. Journal of


Swager, C.P., 1997. Tectono-stratigraphy of late Archaean green-

stone terranes in the southern Eastern Goldfields, Western


Tyler, I.M., Hocking, R.M., 2001. Tectonic Units of Western

Australia (Scale 1:2,500,000). Geological Survey of Western

Australia.

Watkins, K.P., Hickman, A.H., 1990. Geological evolution and

mineralisation of the Murchison Province Western Australia.

Geological Survey of Western Australia Bulletin 137 (267 pp.).

Weinberg, R.F., Louis, M., van der Borgh, P., 2003. Timing of

deformation in the Norseman–Wiluna Belt, Yilgarn Craton,

Western Australia. Precambrian Research 120, 219–239.

White, S.H., Burrows, S.E., Carreras, J., Shaw, N.D., Humphreys,

F.J., 1980. On mylonites in ductile shear zones. Journal of


Wyche, S., Nelson, D.R., Riganti, A., 2004. 4350–3130 Ma detrital

zircons in the Southern Cross Granite–Greenstone Terrane,

Western Australia: implications for the early evolution of the

Yilgarn Craton. Australian Journal of Earth Sciences 51, 31–45.

Documents

Kinematic nature and origin of regional-scale ductile shear zones in the central Yilgarn Craton, Western Australia