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Precambrian Research xxx (2008) xxx–xxx

Contents lists available at ScienceDirect

Precambrian Research

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dentifying the lithospheric structure of a Precambrian orogen usingagnetotellurics: The Capricorn Orogen, Western Australia

. Selwaya,∗, S. Sheppardb, A. Thorneb, S. Johnsonb, B. Groenewaldb

Continental Evolution Research Group, School of Earth and Environmental Sciences, University of Adelaide, South Australia 5005, AustraliaGeological Survey of Western Australia, 100 Plain St, East Perth, Western Australia 6004, Australia

r t i c l e i n f o

rticle history:eceived 19 September 2007eceived in revised form2 September 2008ccepted 29 September 2008

eywords:agnetotellurics

apricornascoyne

a b s t r a c t

A 300-km long magnetotelluric (MT) survey was carried out across the Capricorn Orogen in Western Aus-tralia. The Capricorn Orogen includes reworked crust of the Archaean Yilgarn and Pilbara Cratons, and theallochthonous latest Archaean to Palaeoproterozoic Glenburgh Terrane in the southern Gascoyne Complexand the Palaeoproterozoic northern Gascoyne Complex. The survey aimed to increase our understandingof the juxtaposition of these crustal elements by obtaining information about their electrical structurewith depth. Phase tensor analysis of the data showed that 2D MT inversion could only be carried out forstations in the centre of the profile, crossing the Gascoyne Complex. The resulting model showed thatthere is no electrical distinction between the Glenburgh Terrane and the northern Gascoyne Complex,suggesting that the Glenburgh Terrane may form basement to the whole of the Gascoyne Complex. 3D

lectromagnetic forward modelling was carried out for the entire survey line, incorporating the basement blocks as wellas the surrounding ocean and sedimentary basins. Station data are most closely reproduced by a model inwhich the margin between the Glenburgh Terrane and the Yilgarn Craton (the Errabiddy Shear Zone) dipssouth, with crust of the Glenburgh Terrane wedged beneath the northern Yilgarn Craton. Forward mod-elling results also suggest that the main boundary between the Gascoyne Complex and the Pilbara Cratonto the north is a steeply dipping structure beneath the Edmund and Collier Basins that may correlate with

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. Introduction

The primary architecture of many Phanerozoic orogens cane either directly observed or confidently inferred (e.g. Busby etl., 2006; Goldfarb, 1997). In contrast, this architecture is com-only obliterated in Precambrian orogens, which can complicate

he identification of the main crustal elements and the con-truction of even first-order sequences of tectonic events. Theapricorn Orogen in central Western Australia records the jux-aposition of the Archaean Yilgarn and Pilbara Cratons (Fig. 1,awood and Tyler, 2004). The orogen includes, in addition to theeformed craton margins, latest Archaean to earliest Neoprotero-oic metamorphic and igneous rocks of the Gascoyne Complex,nd a number of Palaeoproterozoic to Mesoproterozoic sedi-

Please cite this article in press as: Selway, K., et al., Identifying the lithospCapricorn Orogen, Western Australia. Precambrian Res. (2008), doi:10.1016

entary basins, including the Edmund and Collier Basins. Therogen is a rare Australian example of a Proterozoic collisionone where both cratonic margins are preserved and exposed, andhere the intervening geology is well exposed and relatively well

∗ Corresponding author. Tel.: +61 8 8303 5910; fax: +61 8 8303 6222.E-mail address: katherine.selway@adelaide.edu.au (K. Selway).

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301-9268/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2008.09.010

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nderstood. Nevertheless, despite systematic regional mappingrograms, several fundamental deep structural problems remainnresolved.

First, although the boundary between the southern Gascoyneomplex (the Glenburgh Terrane) and the Archaean Yilgarn Craton

s interpreted as a suture (the Errabiddy Shear Zone) (Occhipintit al., 2004; Sheppard et al., 2004), its dip at depth is unknownCawood and Tyler, 2004; Sheppard et al., 2004). Second, there iso evidence at the surface for the location of the suture between theascoyne Complex and the Archaean Pilbara Craton to the north.his suture may be hidden beneath the Edmund and Collier Basins,eaving it impossible to identify with surficial geological or geo-hemical techniques. Third, the northern Gascoyne Complex doesot contain any rocks at the present level of exposure that correlateith the older lithologies that dominate the Glenburgh Terrane in

he southern Gascoyne Complex (Fig. 1). It is not clear whether thelenburgh Terrane is exotic, or forms basement, to the northern

heric structure of a Precambrian orogen using magnetotellurics: The/j.precamres.2008.09.010

ascoyne Complex.Constraints on crustal architecture may be obtained using mag-

etotellurics (MT), an electromagnetic technique that measures thelectrical resistivity of the Earth. MT has been shown to be veryffective in determining lithospheric structure in the Australian

ARTICLE IN PRESSG ModelPRECAM-3020; No. of Pages 12

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nvironment, including delineating exotic crustal regions based onheir contrasting resistivity (Selway et al., 2006), identifying theocation and dip of a relict Proterozoic subduction zone (Selway,006) and imaging regions of fossil fluid flow (Heinson et al., 2006;elway, 2006). Surveys carried out in Australia have shown that MTs able to model major basement structures even beneath very thickounger sedimentary basins (Selway, 2006; Selway et al., 2006).ore broadly, MT has been used extensively throughout the world

o determine the structure at depth of regions such as the Trans-udson Orogen (e.g. Ferguson et al., 2005; Jones et al., 2005), theimalayas (e.g. Spratt et al., 2005; Unsworth et al., 2005) and the

apetus Suture Zone (e.g. Tauber et al., 2003). MT is ideally suited tonvestigating the unresolved structural questions in the Capricornrogen due to the broad scale of the crustal units being investigatednd the presence of sufficient exposure to constrain the geophysicalesults.

A 300-km long MT survey was carried out over the northern Yil-arn Craton, Glenburgh Terrane, northern Gascoyne Complex andhe Bangemall Supergroup, with the aim of developing the geo-ogical understanding of the region in three major ways. The firstf these was to investigate the nature and dip of the Errabiddyhear Zone, which marks the suturing of the Glenburgh Terranend Yilgarn Craton (Occhipinti et al., 2004). The second aim waso investigate the relationship between the Glenburgh Terrane andhe northern Gascoyne Complex, specifically to determine whetherhe basement to the northern Gascoyne Complex appears to corre-

Please cite this article in press as: Selway, K., et al., Identifying the lithospCapricorn Orogen, Western Australia. Precambrian Res. (2008), doi:10.1016

ate with the Glenburgh Terrane or with a different unit, such ashe Pilbara Craton. The third aim was to determine the locationnd nature of the boundary between the northern Gascoyne Com-lex and the Pilbara Craton, which is buried beneath the Bangemallupergroup.

YtStP

black lines show the location of the MT profile. The enlarged inset map shows the

. Geological background

Early interpretations of the Capricorn Orogen were of a geosyn-line that formed in an ensialic setting, with no evidence foundor any plate tectonic processes (Gee, 1979). However, Tyler andhorne (1990) showed that the Pilbara and Yilgarn Cratons have dif-erent geological histories and that the Capricorn Orogeny thereforeeflects the collision of two previously unrelated crustal volumes.yler and Thorne (1990) proposed that a long-lived subduction-elated arc existed in the Gascoyne Complex leading up to obliqueollision during the Capricorn Orogeny, loosely constrained to ca.000–1600 Ma. The suture was thought to be marked by the Min-ie Creek batholith. Most subsequent models for the orogen (Evanst al., 2003; Powell and Horwitz, 1994) have argued for variationsn this theme.

More recently, this interpretation has been challenged throughhe implementation of routine SHRIMP U–Pb geochronology. Theseata have shown that the Capricorn Orogen actually underwentumerous individual tectonothermal events, most importantly thea. 2200 Ma Ophthalmian Orogeny, the 2005–1950 Ma Glenburghrogeny, the 1830–1780 Ma Capricorn Orogeny, the 1680–1620angaroon Orogeny and the 1030–950 Ma Edmundian Orogeny

Cawood and Tyler, 2004; Kinny et al., 2004; Occhipinti etl., 2001; Sheppard et al., 2005, 2007). Furthermore, thesetudies have shown that the southern part of the Gascoyneomplex, the Glenburgh Terrane, is allochthonous to both the

heric structure of a Precambrian orogen using magnetotellurics: The/j.precamres.2008.09.010

ilgarn and Pilbara Cratons and collided with the Yilgarn Cra-on during the Glenburgh Orogeny (Occhipinti et al., 2004;heppard et al., 2004). The Capricorn Orogen is therefore nothe product of a single, long-lived collisional event between theilbara and Yilgarn Cratons, but formed initially due to col-

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isional processes, followed by several events of intracratoniceworking.

.1. Lithotectonic elements

.1.1. Yilgarn CratonThe Archaean Yilgarn Craton comprises several granite–

reenstone terranes, including, at its northwestern edge, thearryer Terrane (Fig. 1), which includes one of the largest frag-ents of early Archaean (>3300 Ma) crust on Earth (Kinny et al.,

990; Nutman et al., 1991). The Narryer Terrane consists of earlyrchaean granitic gneisses, which contain lenses and fragmentsf anorthositic and mafic to ultramafic rocks as well as metased-mentary and metavolcanic rocks (Myers, 1990b; Williams and

yers, 1987). These rocks were intruded by late Archaean gran-te and gabbro sheets (Cawood and Tyler, 2004; Occhipinti et al.,001). The northern margin of the Narryer Terrane was intrudedy biotite monzogranite between 1965 and 1945 Ma, and theneformed, metamorphosed and intruded by granites during the830–1780 Ma Capricorn Orogeny (Kinny et al., 2004; Sheppard etl., 2003).

.1.2. Gascoyne ComplexThe latest Archaean to Proterozoic Gascoyne Complex is sep-

rated from the northwestern Yilgarn Craton by the Errabiddyhear Zone, which is steeply dipping at the surface, has a strikeength of 200 km, and is up to 20 km wide (Occhipinti et al.,001). The shear zone formed during the 2005–1950 Ma Glenburghrogeny, but records episodic reactivation until the Neoprotero-oic (Occhipinti et al., 2004). The Errabiddy Shear Zone containsedium- to high-grade metasedimentary rocks, in addition to

lements of the Narryer Terrane. The shear zone was intrudedy granites at between 1965 and 1945 Ma and again during the830–1780 Ma Capricorn Orogeny.

The Gascoyne Complex may be divided into a southern part (thelenburgh Terrane) and a northern part (northern Gascoyne Com-lex) (Fig. 1). The Glenburgh Terrane comprises a southern domainf 2005–1970 Ma calc-alkaline granites and a northern domain ofranitic gneiss with protolith ages of ca. 2550–2450 Ma and ca.000 Ma, as well as Palaeoproterozoic siliciclastic metasedimen-ary rocks (Kinny et al., 2004; Occhipinti et al., 2001; Sheppardt al., 2004). The Archaean component of the granitic gneiss isounger than any granites dated in the Yilgarn Craton, and it mayorm basement to the Glenburgh Terrane (Sheppard et al., 2004).he 2005–1970 Ma calc-alkaline granites have been interpreted asn Andean-type batholith above a northwest dipping subductionone before collision of the Glenburgh and Narryer Terranes dur-ng the Glenburgh Orogeny (Sheppard et al., 2004). The Glenburgherrane was deformed and metamorphosed during the Glenburghrogeny and then intruded by granites of the Bertibubba Supersuitet 1965–1945 Ma. These granites are the first event common to thelenburgh Terrane, Errabiddy Shear Zone and the Narryer Terrane,roviding a lower age limit for their juxtaposition. Neodymium iso-opic data suggest that the source for the Bertibubba Supersuite washe 2005–1970 Ma calc-alkaline granites along the southern mar-in of the Glenburgh Terrane, implying that crust of the Glenburgherrane may be wedged beneath the Narryer Terrane (Sheppard etl., 2004).

The northern Gascoyne Complex consists of medium- to high-rade metamorphic rocks intruded by granitic plutons, including

Please cite this article in press as: Selway, K., et al., Identifying the lithospCapricorn Orogen, Western Australia. Precambrian Res. (2008), doi:10.1016

he 1810–1780 Ma Minnie Creek batholith. No evidence has beenound in the northern Gascoyne Complex for rocks that correlateith the Glenburgh Terrane, leaving the relationship between the

wo unknown (Cawood and Tyler, 2004). The boundary betweenhe northern Gascoyne Complex and Glenburgh Terrane coincides

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PRESSearch xxx (2008) xxx–xxx 3

ith a series of faults including the Chalba Shear Zone (Fig. 1).he northern part of the complex is divided into several structuralnd metamorphic zones, which reflect repeated late Palaeopro-erozoic to early Neoproterozoic reworking (Martin et al., 2006;heppard et al., 2007). At the northern end of the Gascoyne Com-lex, metasedimentary rocks probably grade northwards into lowerrade metasedimentary rocks of the upper Wyloo Group, in par-icular the Ashburton Formation (Myers, 1990a; Williams, 1986),hich was deposited between about 1830 and 1800 Ma.

The tectonic setting of the Minnie Creek batholith, a majorlongate plutonic complex, is enigmatic. Tyler and Thorne (1990)uggested that this batholith stitched the suture between the Pil-ara and Yilgarn cratons, a result of collision during the Capricornrogeny. However, more recent interpretations suggest it mayave formed during the collision between a previously assem-led Yilgarn Craton-Glenburgh Terrane with the Pilbara Cratone.g. Hackney, 2004), or that it may have formed entirely throughntracratonic processes, if the Gascoyne Complex was sutured tohe Pilbara Craton prior to the Glenburgh Orogeny (Sheppard et al.,001).

.1.3. Ashburton BasinThe Ashburton Basin (Thorne, 1990; Thorne and Seymour, 1991)

orresponds to the present day outcrop of the Wyloo Group,12-km thick succession of low-grade, metasedimentary andetavolcanic rocks. Tyler and Thorne (1990) and Thorne and

eymour (1991) interpreted the Ashburton Basin to have formeds a response to the transition from an active tectonic margin to aoreland basin, during the convergence of the Pilbara and Yilgarnratons. The strongly diachronous nature of sedimentation duringhe foreland basin stage was highlighted by Evans et al. (2003) inheir comparison of geochronological data from the upper Wylooroup and the overlying Capricorn Group. Krapez (1999) consid-red the Ashburton Basin fill in terms of two megasequences thatecord the opening and closure of an Atlantic-type ocean. Otherorkers have suggested that the lower part of the Ashburton Basin

uccession, along with the underlying Turee Creek Group of theount Bruce Supergroup, were deposited in the McGrath Trough

Blake and Barley, 1992; Horwitz, 1982; Martin and Thorne, 2004;owell and Horwitz, 1994), a foreland basin related to the devel-pment of the Ophthalmia Fold Belt (Cawood and Tyler, 2004).ocks of the Ashburton Basin were subject to generally low-gradeetamorphism and moderate deformation during the during the

830–1780 Ma Capricorn Orogeny (Martin et al., 2005)

.1.4. Edmund and Collier BasinsThe Edmund and Collier Basins are the youngest tectonic units

ithin the Capricorn Orogen and correspond to the present dayutcrop of the Edmund and Collier Groups that together make uphe Bangemall Supergroup. These rocks unconformably overlie theshburton Basin to the northeast and the Gascoyne Complex to

he southwest. The Edmund Group has a maximum thickness ofbout 4.5 km. It is younger than 1620 Ma granites in the underlyingascoyne Complex and older than ca. 1465 Ma dolerite sills that

ntrude it (Martin and Thorne, 2004). The overlying 1465–1070 Maollier Group has a maximum thickness of about 2.5 km (Martinnd Thorne, 2004).

The Edmund Basin was initiated during intracratonic exten-ional reactivation of structures formed during earlier stages ofhe Capricorn Orogen. These structures, which include the Talga

heric structure of a Precambrian orogen using magnetotellurics: The/j.precamres.2008.09.010

ault and fault system associated with the Wanna Syncline (Fig. 1),ere intermittently active throughout the evolution of the basin,ut do not appear to have had a significant influence on the Collierasin (Martin and Thorne, 2004). Edmund and Collier Basin rocksere metamorphosed at low and very low grades and deformed

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uring the 1030–950 Ma Edmundian Orogeny (Martin et al., 2005;heppard et al., 2007). Cawood and Tyler (2004) interpreted thedmundian Orogeny as a far-field reactivation of the Capricornrogen related to the break up of Rodinia and with events asso-iated with Gondwana assembly (Fitzsimons, 2003). Occhipinti etl. (2004) proposed that it may relate to collision of the Kalahariraton with the western margin of Australia, along the Pinjarrarogen.

.1.5. Pilbara CratonThe Pilbara Craton consists of early- to mid-Archaean

ranite–greenstone terranes, and the unconformably overlyingamersley Basin. Hamersley Basin rocks are intruded by mafic

o ultramafic sills and the entire succession was deformed dur-ng the Ophthalmian and Capricorn Orogenies (Tyler and Thorne,990). The Ophthalmian Orogeny is characterised by north-vergingolds and thrusts (Tyler and Thorne, 1990), and has been datedsing SHRIMP U–Pb monazite geochronology at 2215–2145 MaRasmussen et al., 2005). The driving forces behind the Oph-halmian Orogeny are unknown, although it has been suggestedhat it was related to the collision of the Gascoyne Complex withhe Pilbara Craton (Occhipinti et al., 2004). This is consistent withhe overall northward decrease in the monazite ages (Rasmussent al., 2005).

. Methods

.1. MT data collection

MT is a passive electromagnetic geophysical technique thattilises naturally induced currents within the Earth to image thelectrical structure of the subsurface. Data are gathered by record-ng the naturally occurring magnetic and electric fields at thearth’s surface. Processing of the magnetic data alone yields induc-ion arrows, which are vectors that point towards accumulationsf electrical current in the Earth. Processing of the magnetic dataith the electric data yields apparent resistivity and phase informa-

ion, which can be analysed to indicate the electrical dimensionalityf the subsurface and the geoelectric strike direction and can alsoe inverted to produce a model of the electrical resistivity of theubsurface with depth.

An approximately 300-km long MT survey was carried out inentral-western Western Australia in September and October 2006.he southern portion of the profile was carried out along two roadshat run approximately perpendicular to the strike of the Errabiddyhear Zone and the Chalba Shear Zone, with a 20-km east–west off-et between them (Fig. 1). Only one accessible road extends into therea of interest across the Edmund and Collier Basins. This road runspproximately perpendicular to the strike of the Talga Fault, whichs approximately 50◦ different to the strike of the Errabiddy andhalba Shear Zones where they cross the southern profile. There is65 km, approximately east–west offset between the southern andorthern profiles.

Instrumentation consisted of five-component MT systemseveloped in-house, which record the two horizontal compo-ents of the electric field and the three components of theagnetic field. Magnetic fields were measured using a Barting-

on three-component fluxgate magnetometer and electric fields

Please cite this article in press as: Selway, K., et al., Identifying the lithospCapricorn Orogen, Western Australia. Precambrian Res. (2008), doi:10.1016

ere measured using pairs of copper/copper sulphate porous pots.agnetic and electric recording axes were oriented at a default of

eomagnetic north–south and east–west for data collection. Dataere recorded for 3–4 days and were successfully recorded at a

otal of 38 stations along the profile.

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PRESSearch xxx (2008) xxx–xxx

.2. Data processing

The code Robust Remote Reference Magnetotellurics (RRRMT)Chave et al., 1987) was used to process the MT data. All stationsere remote referenced with the magnetic fields of a simulta-eously recording station to improve the signal to noise ratio.

nduction arrow data were produced at all 38 stations and are ofconsistently good quality along the line in the period range of

pproximately 10s to 4000s. Real induction arrow magnitudes andirections at periods of 100s, 500s, 1000s and 2000s are shown inig. 2.

Processing produced apparent resistivity and phase data at 27f the 38 stations. Apparent resistivity and phase data are shownt five representative stations in Fig. 3. Of the 11 stations that didot produce apparent resistivity and phase data, 5 were as a resultf the electric field recording failing. The remaining 6 stations arell north of station 32 and lie in an area of significant topographicomplexity in the Bangemall Supergroup, characterised by ridgesf sedimentary rocks cut by riverbeds, which can often severelyistort the electrical currents generated deeper within the Earth inprocess called galvanic distortion (e.g. Groom and Bailey, 1991).here is a gradual decrease in data quality from approximatelytation 27 northwards and the 6 stations north of station 32 didot produce any usable quantity of apparent resistivity and phaseata. The absence of apparent resistivity and phase data at these 11tations means that dimensionality analysis could not be carriedut.

.3. Dimensionality analysis

The electrical dimensionality of the Earth is the number of direc-ions in which the Earth’s resistivity changes. For MT data to be

odelled along a profile with a 2D inversion scheme, the data mustespond to a subsurface that is approximately geoelectrically 2D orD. The phase tensor (Bibby et al., 2005; Caldwell et al., 2004) wassed to determine the dimensionality of this dataset. The phaseensor is characterised by three rotation-invariant values, the max-mum (˚max) and minimum (˚min) phase values and the skew angle. The skew angle is a measure of the tensor’s asymmetry and there-

ore of dimensionality. The fourth parameter that defines the phaseensor is the angle ˛ that expresses the tensor’s dependence on theoordinate system. A 2D tensor should have skew values less thanpproximately 5◦. The azimuth of the major axis of the phase tensors related to the maximum direction of current flow in the Earth andhould therefore be period independent in a 2D setting; however,t periods where ˚max and ˚min are similar, the azimuth is ill-efined. The difference between ˚max and ˚min is represented byhe ellipticity, which is a measure of how elliptical the phase tensors. Ellipticities of less than 0.1 suggest one-dimensionality. In a 3Detting, ˇ /= 0 and the azimuth will likely be period independent.

.3.1. Dimensionality analysis resultsAnalysis of the phase tensor data shows that stations 6–21 dis-

lay dominantly 2D characteristics. Phase tensor strike, skew andllipticity data from two representative stations within this sec-ion are shown in Fig. 4. Between periods of ∼100s and ∼700s,trike directions are reasonably period independent and consistentetween stations with an average value of approximately 60◦. Mostkews for this period range are between ±5◦, suggesting that theubsurface is 2D at these periods. At periods greater than ∼700s,

heric structure of a Precambrian orogen using magnetotellurics: The/j.precamres.2008.09.010

he phase tensor strike becomes quite erratic. Skew values areuite variable, with some being erratic but others sitting between5◦. Ellipticity plots also show some variation but ellipticities fromany stations in this period range drop below a value of 0.1. This

uggests that the data may be 1D, which would explain the sudden

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ig. 2. Induction arrow data at periods 100s, 500s, 1000s and 2000s for all MT stanhanced conductivity with a length that is proportional to the conductivity gradie

nconsistency of strike data, although the scatter and large errors inome of the data prevent confidence in this interpretation for someeriod ranges and stations. From stations 11 to 21, phase tensor

Please cite this article in press as: Selway, K., et al., Identifying the lithospCapricorn Orogen, Western Australia. Precambrian Res. (2008), doi:10.1016

trikes at periods less than 100s continue the trend seen at longereriods, being period independent and averaging between 50◦ and0◦. However, between stations 6 and 10, the phase tensor strikesre more period dependent and trend on average from ∼20◦ at ∼20s

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ig. 3. Apparent resistivity (in � m) and phase (in degrees) against period for five reprepresent TM mode data. Data quality is highest in the centre of the profile, as demonstrtations 06, 10 and 18. Data quality decreases at the southern and northern ends of the prohan 90◦ for stations 02 and 31.

Induction arrows in the displayed Parkinson convention point towards regions ofreasing periods correspond to increasing distance from the MT station.

o ∼50◦ at ∼100s. Although the skews for this period range from sta-ions 6 through 10 are between ±5◦, the period dependence of thetrikes shows that this period range is 3D.

heric structure of a Precambrian orogen using magnetotellurics: The/j.precamres.2008.09.010

The pattern demonstrated in stations 6–21 of 2-dimensionalityith a 60◦ strike direction continues in stations 22–31 but only foreriods less than 200s. At longer periods the strike direction is moreeriod dependent and the skew deviates from 0. Phase tensor data

esentative stations along the profile. Circles represent TE mode data and squaresated by the smooth apparent resistivity and phase curves and small error bars forfile, as demonstrated by the larger error bars, less smooth curves and phases greater

Please cite this article in press as: Selway, K., et al., Identifying the lithospheric structure of a Precambrian orogen using magnetotellurics: TheCapricorn Orogen, Western Australia. Precambrian Res. (2008), doi:10.1016/j.precamres.2008.09.010

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6 K. Selway et al. / Precambrian Research xxx (2008) xxx–xxx

Fig. 4. Phase tensor strike, skew and ellipticity against period for two representative stations along the section of the profile included in the 2D MT inversion. The strike is adetermination of the geoelectric strike and is undefined in 1D environments, period independent in 2D environments and period dependent in 3D environments. The skewis a measure of dimensionality and should be between ±5 in 1D or 2D environments. The ellipticity is a measure of how strongly the electrical currents preferentially flowin one direction and magnitudes less than 0.1 indicate 1-dimensionality.

Fig. 5. Simplest 2D MT model arrived at after testing, which inverted to an rms of 3.47. The data are fit by a three-layer model, with no electrical distinction between theGlenburgh Terrane and the northern Gascoyne Complex.

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F squares TM mode) for four representative stations included in the model. Vertical offsetsb

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ig. 6. Fit of 2D MT model data (solid lines) to station data (diamonds TE mode andetween the station and model data are due to corrections for static shift.

rom stations 1 to 5 and 32–38 demonstrate that these regions areeoelectrically 3D. At all periods, the data from these stations showeriod-dependent strike directions and skew values that deviateignificantly from 0.

.3.2. Dimensionality analysis for geological featuresThere are several areas in which major changes observed in the

ehaviour of the phase tensor or induction arrows correspond tohanges between geological regions as recognised on the surface.

.3.2.1. Errabiddy Shear Zone. At the surface, the boundary betweenhe Yilgarn Craton and the southern Gascoyne Complex (specificallyhe Glenburgh Terrane) is marked by the Errabiddy Shear Zone. The

T profile crosses the Errabiddy Shear Zone near stations 4 and 5.tations 1–4 lie on outcropping Yilgarn Craton and stations 6–21 lien outcropping Gascoyne Complex. At periods less than ∼100s, realnduction arrows show a slight reversal across the Errabiddy Shearone, with arrows to the south of it pointing to the north-west andrrows at stations 6 and 7 to the north of it pointing toward theest or south-west (Fig. 2). The arrow at station 5 points approxi-ately along the strike of the Errabiddy Shear Zone. This behaviour

uggests that at the depths corresponding to the affected periods,he Errabiddy Shear Zone is a more conductive feature than theurrounding subsurface.

The phase tensor data suggest a significant electrical differenceetween the Yilgarn Craton and the Gascoyne Complex. Phase ten-or data from stations on the Yilgarn Craton suggest that the regions electrically 3D while stations on the Gascoyne Complex show

uch more evidence of 2-dimensionality. Stations show increas-ng 2-dimensionality with distance from the Yilgarn Craton and,t stations close to the Yilgarn Craton, it is the shorter periodata that show 3-dimensionality, suggesting that these stations areeing affected by the 3-dimensionality of the Yilgarn Craton as theecorded fields penetrate into it.

.3.2.2. Gascoyne Complex. Stations 6–21 lie within the Gas-oyne Complex. Apart from phase tensor data suggesting

Please cite this article in press as: Selway, K., et al., Identifying the lithospheric structure of a Precambrian orogen using magnetotellurics: TheCapricorn Orogen, Western Australia. Precambrian Res. (2008), doi:10.1016/j.precamres.2008.09.010

-dimensionality at periods less than 100s at stations towardhe extreme south of the Gascoyne Complex, the majority of theascoyne Complex appears electrically 2D, possibly trending toore 1D at depths corresponding to periods greater than 700s

Fig. 4).

Pilbara Craton, moderate resistivity Gascoyne Complex and high resistivity YilgarnCraton. The ocean and marine and continental sedimentary basins were forwardmodelled as low resistivity features. The solid black lines show the location of theMT profile. The boundary between the Gascoyne Complex and the Pilbara Cratonis in a position that correlates with the Talga Fault and the boundary between theGascoyne Complex and the Yilgarn Craton dips south.

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. 2D MT inversion

Full 2D MT inversion can only be carried out where dimensional-ty analysis shows that the data are 2D. Therefore, data from stations–21 were included in the inversion, with individual data pointsither included or excluded based on their phase tensor analysis.n addition, shorter period data from stations 23 and 24, which werelso analysed to be 2D, were included. The remaining stations wereot included in the inversion due to 3-dimensionality.

MT data were collected with axes oriented north–south andast–west. However, for 2D inversion the axes must be mathemat-cally rotated to be parallel and perpendicular to the geoelectrictrike. The average geoelectric strike direction for the 2D sections 60◦ (Fig. 4). Strike direction became erratic in the region con-idered to be 1D but since strike is by its nature undefined in1D region, these data can be rotated in the same manner as

he 2D data. There is an inherent 90◦ ambiguity in the geoelec-ric strike direction (Caldwell et al., 2004), so the induction arrowata were used to constrain the strike direction. Although there

s some variation in the direction of induction arrows at periodshorter than approximately 200s, at longer periods the arrowsoint consistently north/north-west (Fig. 2), supporting a strike

Please cite this article in press as: Selway, K., et al., Identifying the lithospCapricorn Orogen, Western Australia. Precambrian Res. (2008), doi:10.1016

f 060◦. Station data were therefore rotated 30◦ anticlockwise.he 2D MT data were modelled along a profile oriented perpen-icular to geoelectric strike using the inversion code non-linearonjugate gradients (NLCG) (Rodi and Mackie, 2001). The algo-ithm contains a regularisation parameter tau (�) that acts as a

wmw

ig. 8. Synthetic real induction arrow data produced by the final 3D forward model (Fig.00s, 500s, 1000s and 2000s.

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rade-off between smoothness and model fit which was set to. To account for static shift effects, static shift was included asn inversion parameter and additionally the apparent resistivityrror floors were set to 10%, while phase error floors were seto 1.45◦.

Models were run from a starting half-space of 100 � m withesulting features tested for robustness, depth extent, lateral extentnd resistivity. Results of the 2D MT modelling suggest that a 3-ayer model is the simplest and smoothest fit to the data (Fig. 5).his model has an rms error of 3.47 and data fits to four represen-ative stations are shown in Fig. 6. A more conductive mid-lowerrust sits between a moderately resistive upper crust and a moreesistive lower crust and upper mantle. No distinction betweenhe geoelectrical structure of the crust beneath outcropping north-rn Gascoyne Complex and Glenburgh Terrane can be interpretedrom this model, nor has any kind of break or discontinuity been

odelled between them. Therefore no electrical distinction can beade between the crust of the northern Gascoyne Complex and thelenburgh Terrane.

. 3D forward GDS modelling

heric structure of a Precambrian orogen using magnetotellurics: The/j.precamres.2008.09.010

Since so many of the data do not fit the 2D criteria, 3D for-ard models were run to test for electrical structure. 3D forwardodelling involves setting up a 3D resistivity grid and determininghat data that resistivity structure would produce in comparison

7) in grey, compared with the station real induction arrow data in black at periods

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ith the measured station data. Station induction arrow data are ofood quality along the whole profile and are unaffected by galvanicistortion and are therefore used for comparisons with the 3D for-ard modelled data. Since MT data from the majority of stationso not contain 1D sections, the phase tensor cannot identify andemove galvanic distortion (Bibby et al., 2005), so MT data were notncluded in the forward model comparisons. However, the phaseensor data themselves, being unaffected by galvanic distortion,ere compared. 3D forward modelling was carried out using theTD3FWD code (Mackie and Madden, 1993; Mackie et al., 1993).rids were set up that measured 1600 km north–south, 1000 kmast–west and 70 km deep, with an underlying 1D layered structurextending for 1500 km.

A 3D forward model was set up with features derived from aombination of the geological features seen at the surface withhe resistivity features modelled in the 2D MT model, extrapo-ated with depth. The coastline, ocean and sedimentary basins bothn the ocean and on the continent were input into the grid, withhe depths of the sedimentary basins taken from the SEEBASETM

ataset. Seawater and the seawater-saturated marine sedimentaryasins were modelled with a resistivity of 1.2 � m, the iron-richamersley Basin was modelled with a resistivity of 8 � m and the

emaining sedimentary basins were modelled with a resistivity of

Please cite this article in press as: Selway, K., et al., Identifying the lithospCapricorn Orogen, Western Australia. Precambrian Res. (2008), doi:10.1016

0 � m (Fig. 7). Induction arrows reverse direction around the Erra-iddy Shear Zone and increase in magnitude toward the northernnd of the profile (Fig. 2), suggesting that the boundaries betweenhe Gascoyne Complex and the Yilgarn and Pilbara Cratons are

mMbl

ig. 9. Synthetic imaginary induction arrow data produced by the final 3D forward modelt periods 100s, 500s, 1000s and 2000s.

PRESSearch xxx (2008) xxx–xxx 9

ow resistivity features, so they were modelled with resistivitiesf 30 � m. Archaean cratons throughout the world are commonlyore resistive than younger terranes (e.g. Jones et al., 2005), so

he Pilbara and Yilgarn Cratons were forward modelled with a highesistivity of 4200 � m. The Gascoyne Complex was forward mod-lled with a more moderate resistivity of 500 � m, which increaseso a resistivity of 2000 � m below 30 km depth as expected fromhe 2D MT model. No distinction was made in the model betweenhe Glenburgh Terrane and the northern Gascoyne Complex.

Several tests were carried out to determine which geologicaleatures best reproduce the station data. The first group of testsas for the dip of the Errabiddy Shear Zone, which is the boundaryetween the Glenburgh Terrane and the Yilgarn Craton. Forwardodels were run to test whether this boundary has a steep dip, aoderate north dip or a moderate south dip. The station data wereost closely reproduced when the Errabiddy Shear Zone was given

n approximately 45◦ southerly dip, which has also been suggestedy several geological models (e.g. Sheppard et al., 2004; Tyler andhorne, 1990).

The second group of tests was for the location and nature of theoundary between the Gascoyne Complex and the Pilbara Craton.he precise location of this boundary is not known as it is obscuredy the overlying Bangemall Supergroup, but it may be related to

heric structure of a Precambrian orogen using magnetotellurics: The/j.precamres.2008.09.010

ajor structures such as the Wanna Syncline or Talga Fault (Fig. 1,artin and Thorne, 2004). 3D forward models were run with the

oundary between the Pilbara Craton and the Gascoyne Complexocated at both the Wanna Syncline and the Talga Fault. The resul-

(Fig. 7) in grey, compared with the station imaginary induction arrow data in black

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ant synthetic arrows at the northern end of the profile correlateith the station data more closely when the boundary is placed at

he Talga Fault rather than the Wanna Syncline. Tests were then runith the boundary at the Talga Fault dipping both north and south.

ynthetic data from the southerly dipping-boundary test did noteproduce the station data as closely as the synthetic data from bothhe northerly dipping test and the vertically dipping test, whichere almost indistinguishable from each other.

.1. Final model

The features that best fit the station data are (1) highly resistiveilgarn and Pilbara Cratons, (2) a moderately resistive Gascoyneomplex with no distinction between the Glenburgh Terrane andhe northern Gascoyne Complex, (3) a moderate south-dippingrrabiddy Shear Zone and (4) a vertically- or northerly-dippingoundary between the Gascoyne Complex and the Pibara Cratonoinciding with the Talga Fault. The 3D forward model incorpo-ating all these features is shown in depth slices in Fig. 7. The realynthetic induction arrows produced by this forward model ateriods 100s, 500s, 100s and 2000s are shown in Fig. 8, overlain onhe station data. The synthetic data are generally a close match tohe station data but local 3D features which cannot be included inhe 3D forward model mean that the reproduction can not be exact.he most notable deviations are at stations 10–17 at 100s. As wells potential unmodelled local features, these arrows have smallagnitudes and their directions are therefore poorly determined.

he imaginary induction arrows at periods 100s, 500s, 100s and000s are shown in Fig. 9. The synthetic imaginary arrows showore variation with period than the station imaginary arrows,

enerally pointing in a more southerly direction at 100s and in aore northerly direction at 1000s and 2000s. This misfit is again

ikely to be due to 3D features not included in the forward modelhich affect the real and imaginary arrows differently. However,

btaining a good fit to the real arrows gives confidence that theominant structures have been modelled. The overall rms error ofhe synthetic GDS data to the station GDS data is 2.4, showing thatn average the induction arrow data have been closely reproduced.

A comparison between the phase tensors produced by the sta-ion data and the synthetic data at a period of 128s is shown inig. 10. Phase tensors are a more accurate comparison than appar-nt resistivity and phase data since the phase tensors are unaffectedy galvanic distortion. The orientation of the phase tensors has beenell reproduced across the survey line, but the synthetic data pro-uce phase tensors with a lower ellipticity than the station phaseensors, showing that there is a greater degree of preferential cur-ent flow in the station data than the synthetic data. At a period of28s for the resistivities input into the forward model, these phaseensors are responding to features approximately 50 km away,hich includes the Gascoyne Complex, the Bangemall Supergroup

Please cite this article in press as: Selway, K., et al., Identifying the lithospCapricorn Orogen, Western Australia. Precambrian Res. (2008), doi:10.1016

nd the Hamersley and Carnarvon Basins. The greater ellipticitybserved in the station data could be because the basins are moreonductive than has been modelled, or currents could be flowingreferentially along faults or fabrics within the basement that haveot been modelled.

6

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Fig. 11. Interpreted geological cross-section across the southern Pil

ig. 10. Phase tensor data produced by the final 3D forward model (Fig. 7) in grey,ompared with the station phase tensor data in black at period 128s.

Two other important model tests were run: in the first, theppermost 40 km of the model was the same as the initial model,ut below this the entire model space was set to a resistivity of200 � m to test for an electrically homogenous mantle. The result-

ng induction arrows are very similar to those of the model shownn Fig. 7, but at periods greater than 500s they point approximately0◦ more anticlockwise. Since this is a worse fit to the station data,t suggests that the mantle is not electrically homogenous, a resulthat is supported by seismic tomography data (Drummond, 1981;eading et al., 2007) and also shows that the arrows are respondinguch more strongly to crustal structure than mantle structure. In

he second test, the Yilgarn Craton was cut off along a boundaryunning along an approximate longitude of 115◦ since this appearso be the extent of the craton from the surface geology. Mesh spaceo the west of this boundary was set to correspond to the mod-rate resistivity of the Gascoyne Complex. The induction arrowsroduced by this forward model were indistinguishable from thosef the initial model, showing that this region is too far away fromhe profile location for this change in resistivity to affect the data.

heric structure of a Precambrian orogen using magnetotellurics: The/j.precamres.2008.09.010

. Discussion and conclusions

MT data were collected along an approximately 300-km longrofile across the western Capricorn Orogen in Western Aus-

bara Craton, Gascoyne Complex and northern Yilgarn Craton.

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ralia. 2D MT modelling was confined to stations on the Gascoyneomplex (Fig. 4). In this region the Gascoyne Complex has previ-usly been subdivided into a southern terrane dominated by Laterchaean and Early Palaeoproterozoic gneisses and metasediments

the Glenburgh Terrane) that do not appear to outcrop to the northf the Chalba Shear Zone (the northern Gascoyne Complex), whichs dominated by Mid-Palaeoproterozoic gneisses.

2D MT modelling results show that the simplest model to fit theata is a three-layer model (Fig. 5) and that there is no evidenceor an electrical boundary or a major electrical difference betweenhe Glenburgh Terrane and the northern Gascoyne Complex. Thisbservation leads to three main possible geological interpretations.irst, the Glenburgh Terrane could form basement to the north-rn Gascoyne Complex, with no major tectonic boundary betweenhe two regions. This interpretation suggests that the resistivityayers would be at a greater depth beneath the northern Gas-oyne Complex than the Glenburgh Terrane, but testing of theodel showed that the data cannot distinguish whether such a

epth offset exists (Fig. 5). Second, the two pieces of crust couldave evolved separately, but coincidentally have the same electricalharacteristics and therefore appear to be continuous. This scenarios unlikely since major geological boundaries are generally associ-ted with changes in resistivity (e.g. Jones et al., 2005; Selway etl., 2006). However, as with all geophysical techniques, MT simplyeasures a physical property and it is always possible that geolog-

cally distinct regions share the same geophysical characteristics.he third possibility is that the observed electrical characteristicsould be due to various episodes of crustal reworking such as mag-atism, metamorphism or fluid flow after amalgamation of thelenburgh Terrane and the northern Gascoyne Complex, whichould obliterate their previous, and possibly contrasting, electri-al characteristics. Although this cannot be ruled out, there doesot appear to be any correlation between zones of reworking andhe electrical character of the crust. For instance, the southern partf the Glenburgh Terrane was not reworked after the Glenburghrogeny, yet it has the same electrical character as crust northf the Chalba Shear Zone that was strongly reworked during the030–950 Ma Edmundian Orogeny. The most likely interpretationf the MT model is that the Glenburgh Terrane and the northernascoyne Complex are a contiguous piece of crust, with the olderlenburgh Terrane forming basement to the northern Gascoyneomplex and the younger lithologies that outcrop in the north-rn Gascoyne Complex structurally overlying this older basementFig. 11).

Hypotheses regarding the lithospheric structure of the regionere tested through 3D forward modelling. A model grid was setp in which seawater and sedimentary basins were modelled with

ow resistivities, the Pilbara and Yilgarn Cratons with high resistiv-ties, the Gascoyne Complex with an intermediate resistivity thatncreases with depth, and the boundaries between the Gascoyneomplex and the Archaean Cratons with low resistivities. The Erra-iddy Shear Zone is the suture between the Yilgarn Craton and thelenburgh Terrane, and 3D forward modelling was utilised to inves-

igate its dip at depth. Results suggest that the Errabiddy Shear Zoneas a moderate southerly dip (Fig. 8), implying that the Glenburgherrane extends beneath the Narryer Terrane of the Yilgarn Cra-on (Fig. 11). This result supports the conclusions of Sheppard etl. (2004), who suggested that granites of the 1965–1945 Ma Bert-bubba Supersuite, which intruded into the northern edge of thearryer Terrane after the Glenburgh Orogeny, were largely derived

Please cite this article in press as: Selway, K., et al., Identifying the lithospCapricorn Orogen, Western Australia. Precambrian Res. (2008), doi:10.1016

rom melting of the 2005–1970 Ma calc-alkaline granites within thelenburgh Terrane.

While the Errabiddy Shear Zone is well constrained as theuture between the Glenburgh Terrane and the Yilgarn Craton (e.g.cchipinti et al., 2004), the location of the suture between the

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ascoyne Complex and the Pilbara Craton is uncertain. There iso suggestion in any of the electrical data that the Pilbara Craton

orms basement to any portion of the Gascoyne Complex, indi-ating that this suture does not lie within the currently definedascoyne Complex. The 2D MT modelling has shown no evidence

or a major suture at the Chalba Shear Zone or the Minnie Creekatholith (Fig. 5). This indicates that the suture may lie beneathhe Edmund and Collier Basins and is possibly related to one ofhe primary structural controls on the basin, the Talga Fault or the

anna Syncline (Fig. 1). This possibility was tested with 3D for-ard modelling and results showed that the station data are best

eproduced by an electrical boundary located at the Talga Fault withither a vertical or a northerly dip (Figs. 8 and 11). If this electricaloundary does represent the suture between the Gascoyne Com-lex and the Pilbara Craton, as seems likely, its location beneath theangemall Supergroup makes further analysis of its age and natureifficult, although the 2215–2145 Ma Ophthalmian Orogeny coulde consistent with collisional orogenesis (Occhipinti et al., 2004).

cknowledgements

The authors gratefully acknowledge the pastoralists and Abo-iginal communities from the survey region for allowing us accesso the land. The phase tensor code used in this project was writteny S. Thiel. We appreciate helpful comments on the manuscript by. Spaggiari. S. Sheppard, A. Thorne, S. Johnson and B. Groenewaldublish with the permission of the Director, Geological Survey ofestern Australia. Reviews of the manuscript by Max Meju and an

nonymous reviewer have greatly improved its quality.

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