Download Crustal architecture of the Capricorn Orogen, Western Australia and

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Australian Journal of Earth Sciences (2013)
60, 681–705, http://dx.doi.org/10.1080/08120099.2013.826735
Crustal architecture of the Capricorn Orogen, Western
Australia and associated metallogeny
S. P. JOHNSON1*, A. M. THORNE1, I. M. TYLER1, R. J. KORSCH2, B. L. N. KENNETT3,
H. N. CUTTEN1, J. GOODWIN2, O. BLAY1, R. S. BLEWETT2, A. JOLY4, M. C. DENTITH4,
A. R. A. AITKEN4, J. HOLZSCHUH2, M. SALMON3, A. READING5, G. HEINSON6, G. BOREN6,
J. ROSS6, R. D. COSTELLOE2 AND T. FOMIN2
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1
Geological Survey of Western Australia, Department of Mines and Petroleum, Mineral House, 100 Plain Street,
East Perth, WA 6004, Australia
2
Minerals and Natural Hazards Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
3
Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia
4
Centre for Exploration Targeting, School of Earth and Environment, The University of Western Australia, 35 Stirling
Highway, Crawley, WA 6009, Australia
5
School of Earth Sciences and CODES Centre of Excellence, University of Tasmania, Private Bag 79, Hobart, TAS
7001, Australia
6
Centre for Tectonics, Resources and Exploration, University of Adelaide, Adelaide, SA 5005, Australia
A 581 km vibroseis-source, deep seismic reflection survey was acquired through the Capricorn Orogen
of Western Australia and, for the first time, provides an unprecedented view of the deep crustal architecture of the West Australian Craton. The survey has imaged three principal suture zones, as well as several other lithospheric-scale faults. The suture zones separate four seismically distinct tectonic blocks,
which include the Pilbara Craton, the Bandee Seismic Province (a previously unrecognised tectonic
block), the Glenburgh Terrane of the Gascoyne Province and the Narryer Terrane of the Yilgarn Craton.
In the upper crust, the survey imaged numerous Proterozoic granite batholiths as well as the architecture of the Mesoproterozoic Edmund and Collier basins. These features were formed during the punctuated reworking of the craton by the reactivation of the major crustal structures. The location and
setting of gold, base metal and rare earth element deposits across the orogen are closely linked to the
major lithospheric-scale structures, highlighting their importance to fluid flow within mineral systems by
the transport of fluid and energy direct from the mantle into the upper crust.
KEYWORDS: Ashburton Basin, Capricorn Orogen, Collier Basin, Edmund Basin, Fortescue Basin, Gascoyne Province, Glenburgh Orogeny, Glenburgh Terrane, Hamersley Basin, intracontinental reworking,
mineralisation, Narryer Terrane, Ophthalmian Orogeny, Pilbara Craton, seismic reflection study, suture
zone, West Australian Craton, Yilgarn Craton.
INTRODUCTION
Like most continental regions on Earth, the Australian
continent is particularly well endowed with several
large, high-quality (or giant) orebodies including iron
ore in the Hamersley Ranges of the Pilbara, lead–zinc at
Broken Hill, uranium–copper–gold at Olympic Dam and
lead–zinc–copper–gold at Mount Isa, as well as numerous
high-quality gold and nickel deposits. The large range of
commodities and abundance of deposits has been the
main driver for Australia’s recent economic success and
stability; however, these resources are being depleted at
a rate greater than they are being discovered. The
decline in exploration success is due mostly to the
inaccessibility of the bedrock, up to 80% of which is covered by a thick layer of regolith, or is locally deeply
weathered (Australian Academy of Science 2012). It also
suffers from a lack of detailed geological knowledge in
some underexplored (greenfields) parts of the continent.
A variety of geological evidence has shown that the formation of giant orebodies or deposit clusters in the
upper crust, are directly related to specific tectonic settings (e.g. Groves et al. 2005; Kerrich et al. 2005; Begg
et al. 2010; Hronsky et al. 2012), and that they commonly
occur above deep crustal structures that mark the edges
of discrete continental blocks, such as fossil suture
zones or the extended margins of rifted continents.
*Corresponding author: [email protected]
Ó 2013 Crown Copyright
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Regional-scale geophysical surveys, including aeromagnetic, gravity and a range of passive and active seismic
techniques, provide a relatively rapid method for greatly
increasing our geological understanding of deeply buried or weathered geological terranes, as well as providing critical first-order constraints on crustal
architecture (e.g. Golbey et al. 2006; Aitken & Betts 2009;
Cayley et al. 2011). When these geophysical methods are
combined with some degree of geological knowledge,
regions of preferential mineral endowment can be identified, providing a locus for mineral exploration and
targeting.
The Capricorn Orogen of Western Australia is a
1000 km long, 500 km wide region of variably deformed
rocks located between the Pilbara and Yilgarn Cratons
(Figure 1) and records the Paleoproterozoic assembly of
these cratons to form the West Australian Craton, as
well as over one billion years of subsequent intracratonic reworking (Cawood & Tyler 2004; Sheppard et al.
2010a, b; Johnson et al. 2011a). Apart from the extensive
high-grade iron-ore deposits along the southern Pilbara
margin, the region has few working mines and has had
little history of recent world-class orebody discovery.
The western part of the orogen is particularly well
exposed, and as a result the surface geology, geological
history and plate tectonic setting of the orogen is mostly
well understood (e.g. Sheppard et al. 2010b; Johnson
et al. 2011a). However, owing to extensive crustal reworking, the location of major crustal structures and broad
architecture of the orogen are poorly constrained. To
improve the exploration potential of the region, a better
understanding of the crustal architecture across the orogen is critical, especially identifying the location and
orientation of major crustal structures and craton edges,
as well as any island arc, or exotic accreted crustal
material.
In April and May 2010, 581 km of vibroseis-source,
deep seismic reflection data were acquired along three
traverses (10GA–CP1, 10GA–CP2, and 10GA–CP3)
through the well-exposed western part of the orogen.
These data were interpreted in conjunction with 400 mline-spaced aeromagnetic and 2.5 km-spaced regional
gravity data, as well as detailed gravity and magnetotelluric data that were collected along the length of the survey. The traverses started in the southern part of the
Pilbara Craton, crossed the Gascoyne Province, and
ended in the Narryer Terrane of the Yilgarn Craton
(Figures 2, 3). The project was a collaboration between
the Geological Survey of Western Australia (GSWA),
AuScope (a component of NCRIS, the National Collaborative Research Infrastructure Strategy), and Geoscience
Australia. The seismic data were processed by the Seismic Acquisition and Processing team of the Onshore
Energy and Minerals Division at Geoscience Australia
and interpreted by the authors of this paper during several interpretation sessions. The seismic processing
steps, 2.5D geophysical forward modelling and geological
interpretation of the seismic data are outlined in detail
in Johnson et al. (2011b) and in the Supplementary
Paper. This paper presents the results of that final interpretation in order to construct a rigid architectural and
geological framework for the orogen, and to better
understand the nature and geometry of events associated with both the assembly and punctuated reactivation
of the West Australian Craton. This improved and integrated geological dataset also provides critical
Figure 1 Elements of the Capricorn Orogen and surrounding cratons and basins, showing the location of the Capricorn Orogen seismic transect. Modified from Martin & Thorne (2004). The thick grey lines delimit the northern and southern boundaries of the Capricorn Orogen. Inset shows location of the Capricorn Orogen and Paleoproterozoic crustal elements (KC,
Kimberley Craton; NAC, North Australian Craton; SAC, South Australian Craton; WAC, West Australian Craton) and Archean
cratons (YC, Yilgarn Craton; PC, Pilbara Craton; GC, Gawler Craton). Other abbreviations: MI, Marymia Inlier; YGC, Yarlarweelor Gneiss Complex.
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Architecture of the Capricorn Orogen
Figure 2 Geological map of the western Capricorn Orogen, showing the location of the Capricorn Orogen seismic lines 10GA–
CP1, 10GA–CP2, and 10GA–CP3.
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Figure 2 Continued.
geodynamic information that may be used to identify
regions of the upper crust that may contain a preferential mineral endowment or be more prospective for particular commodities.
GEOLOGICAL SETTING
Early interpretations of the Capricorn Orogen (e.g.
Horwitz & Smith 1978; Gee 1979) favoured an intracratonic setting. These were followed by plate tectonic models in which the Proterozoic orogeny was driven by
collision between the hitherto unrelated Pilbara and Yilgarn Cratons (Muhling 1988; Tyler & Thorne 1990; Blake
& Barley 1992; Powell & Horwitz 1994; Evans et al. 2003).
More recent work, however, has highlighted the complexity of the Capricorn Orogen, which has now been
shown to record at least seven major orogenic events
(Figure 4; e.g. Sheppard et al. 2010b), the oldest two of
which, the 2215–2145 Ma Ophthalmian Orogeny and the
2005–1950 Ma Glenburgh Orogeny, relate to the punctuated assembly of the West Australian Craton (Blake &
Barley 1992; Powell & Horwitz 1994; Occhipinti et al.
2004; Sheppard et al. 2004; Rasmussen et al. 2005; Johnson et al. 2011a), while the younger five, record over one
billion years of intracratonic reworking (Cawood &
Tyler 2004; Sheppard et al. 2010b; Johnson et al. 2011a).
The orogen includes the deformed margins of the Pilbara and Yilgarn Cratons and associated deformed continental margin rocks of the Fortescue, Hamersley, Turee
Creek and lower Wyloo Groups in the Ophthalmia Fold
Belt; medium to high-grade meta-igneous and metasedimentary rocks of the Gascoyne Province that form the
core of the orogen; and numerous, variably deformed
low-grade metasedimentary rocks—including the upper
Wyloo, Bresnahan, Mount Minnie, Padbury, Bryah, Yerrida, Earaheedy, Edmund and Collier Groups—that overlie the West Australian Craton (Figures 1–4; Cawood &
Tyler 2004; Sheppard et al. 2010b). The geological data
are summarised in a time–space plot in Figure 4, and a
detailed geotectonic history of the orogen, in the vicinity
of the seismic traverse, is provided below.
The Pilbara Craton and Archean to Proterozoic
sedimentation in the northern Capricorn Orogen
During the late Archean and Paleoproterozoic, prior to
the collision of the Pilbara Craton with the Glenburgh
Terrane of the Gascoyne Province, the southern margin
of the craton evolved from an intracontinental rift to a
passive margin, before being converted to an active margin, and subsequently into a series of foreland basins
(Blake & Barley 1992; Krapez 1999; Martin et al. 2000;
Thorne & Trendall 2001; Trendall et al. 2004; Martin &
Morris 2010). This history is marked by the deposition of
the Fortescue, Hamersley, Turee Creek, and lower Wyloo
Groups. Following the assembly of the West Australian
Craton, Paleoproterozoic intracontinental reactivation
resulted in the deposition of the upper Wyloo and Capricorn Groups (Figures 4, 5) in a possible foreland basin
setting. Based on the known stratigraphy, stacking of all
the Archean and Paleoproterozoic supracrustal units
present in the northern Capricorn Orogen gives a maximum cumulative thickness of 26 km (Figure 5).
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Architecture of the Capricorn Orogen
Figure 3 Regional geophysical data for the western Capricorn Orogen with 2.5 km bouguer anomaly gravity data (colour)
draped over 300–500 m line-spaced TMI aeromagnetic data (black and white). The location of the structural-metamorphic zone
boundaries of the Gascoyne Province, major fault and mineral deposits is also shown.
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Figure 4 Time–space plot for the Capricorn Orogen. Abbreviations: AB, Ashburton Basin; BBZ, Boora Boora Zone; ESZ, Errabiddy Shear Zone; HFTB, Hamersley, Fortescue and Turee Creek basins; LZ, Limejuice Zone; ManZ, Mangaroon Zone; MooZ,
Mooloo Zone; MutZ, Mutherbukin Zone; NT, Narryer Terrane; PC, Pilbara Craton; PCS, Petter Calc-silicate; PWd, Paradise
Well diatexite PZ, Paradise Zone; QP, Quartpot Pelite; YC, Yilgarn Craton; YGC, Yarlarweelor Gneiss Complex.
PILBARA CRATON BASEMENT
FORTESCUE GROUP: RIFTING OF THE PILBARA CRATON
Within the northern Capricorn Orogen, granite–greenstone basement rocks of the Pilbara Craton are exposed
in the Wyloo, Rocklea and Milli Milli Domes, and in the
Sylvania Inlier (Figures 1, 2). Greenstones are dominated by low-grade metamorphosed mafic volcanic and
siliciclastic sedimentary rocks. The minimum age of the
granite–greenstones is fixed by the ca 2775 Ma age of the
overlying basal Fortescue Group (Trendall et al. 2004).
Their maximum age is unknown, although comparison
with similar granite–greenstone assemblages in the
northern Pilbara Craton (Van Kranendonk et al. 2007;
Hickman & Van Kranendonk 2008) suggests they probably formed sometime between ca 3800 and ca 2830 Ma.
Granite–greenstone rocks are unconformably overlain
by mixed volcano-sedimentary rocks of the Fortescue
Group, formed during protracted rifting of the Pilbara
Craton between ca 2775 and ca 2630 Ma (Blake 1993, 2001;
Thorne & Trendall 2001; Blake et al. 2004; Trendall et al.
2004). In the northwestern and northeastern Pilbara,
Fortescue Group sedimentation and volcanism was controlled by a northeast-striking extensional fault system
(Blake 2001), whereas in the southern Pilbara, Thorne &
Trendall (2001) argued that the fault system was principally oriented east-southeast.
In the South Pilbara Sub-basin (Thorne & Trendall
2001), the Fortescue Group is up to 6.5 km thick
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Architecture of the Capricorn Orogen
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Figure 5 Generalised stratigraphy and deformation history of the northern Capricorn Orogen. The tectonic interpretation is
modified after Martin & Morris (2010).
(Figure 5), and is subdivided, into seven formations,
which are grouped into four major tectono-stratigraphic
units. From the base upwards, Unit 1 consists of the Bellary Formation and Mount Roe Basalt; Unit 2 is the
Hardey Formation; Unit 3 consists of the Boongal, Pyradie and Bunjinah formations; and Unit 4 is the Jeerinah
Formation. Thick mafic to ultramafic sills intrude much
of the succession above the Mount Roe Basalt. The origin
of the four major tectono-stratigraphic units of the Fortescue Group are interpreted to result from deposition
in an extensional tectonic setting (Thorne & Trendall
2001).
HAMERSLEY GROUP: PASSIVE- TO ACTIVE-MARGIN
SEDIMENTATION AND IGNEOUS ACTIVITY
The 2.5–3.0 km thick Hamersley Group conformably
overlies the Fortescue Group (Figure 5), and was deposited between ca 2630 and ca 2450 Ma, when the southern
Pilbara evolved from a rift setting to a passive continental margin and finally to an active margin (Morris &
Horwitz 1983; Blake & Barley 1992; Krapez 1999; Thorne
& Trendall 2001; Trendall et al. 2004; Martin & Morris
2010). Rocks of the Hamersley Group reflect this largely
deeper-marine distal setting, and are dominated by
banded iron-formation, shale, chert and fine-grained
carbonate.
Seven major lithostratigraphic units are recognised
within the Hamersley Group. In ascending order, these
are the Marra Mamba Iron Formation, Wittenoom Formation, Mount Sylvia Formation, Mount McRae Shale,
Brockman Iron Formation, Weeli Wolli Formation, and
Boolgeeda Iron Formation (Trendall & Blockley 1970;
Simonson et al. 1993; Trendall et al. 2004).
TUREE CREEK GROUP AND LOWER WYLOO GROUP:
FORELAND BASIN SEDIMENTATION AND VOLCANISM DURING
THE OPHTHALMIAN OROGENY
Deposition of the Turee Creek and lower Wyloo Groups,
took place during the early foreland-basin stage of the
2215–2145 Ma Ophthalmian Orogeny, when the Pilbara
Craton collided with the Glenburgh Terrane to the south
(Blake & Barley 1992; Martin et al. 2000; Occhipinti et al.
2004; Sheppard et al. 2004, 2010b; Rasmussen et al. 2005;
Johnson et al. 2010, 2011a; Martin & Morris 2010). An
alternative view by Krapez & McNaughton (1999), considered the post-Turee Creek Group history in terms of two
mega-sequences that record the opening and closure of
an Atlantic-type ocean.
Along the southern Pilbara margin, the Turee Creek
and lower Wyloo groups have maximum thicknesses of
about 4 and 3 km, respectively, although the Turee Creek
Group was not crossed during the survey. The upper
part of the lower Wyloo Group is dominated by the
2 km thick, ca 2210 Ma Cheela Springs Basalt, and
both the Turee Creek and lower Wyloo Groups are
intruded by similar-aged dolerite sills and dykes (Martin
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€ ller et al. 2005; Martin & Morris 2010). The
et al. 1998; Mu
two groups are separated by a significant angular unconformity (Figure 5), and, together with another occurring
within the upper Turee Creek Group, are interpreted to
reflect the northward propagation of the Ophthalmia
fold-and-thrust belt into the foreland basin (Martin &
Morris 2010).
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UPPER WYLOO AND CAPRICORN GROUPS: FORELAND-BASIN
SEDIMENTATION AND VOLCANISM DURING THE CAPRICORN
OROGENY
The upper Wyloo Group (Figure 5) has an estimated
thickness of about 7.5 km, and, in ascending order, consists of the Mount McGrath Formation, Duck Creek
Dolomite, and Ashburton Formation (Thorne &
Seymour 1991). A 120 m-thick, mixed mafic and felsic
volcanic unit, the June Hill Volcanics, overlies the Duck
Creek Dolomite north of the Wyloo Dome, although this
unit was not crossed during the survey. The Ashburton
Formation is unconformably overlain by siliciclastic,
carbonate and felsic volcanic rocks of the Capricorn
Group, deposited following the early deformation stage
of the 1820–1770 Ma Capricorn Orogeny.
The age of the upper Wyloo Group is still poorly constrained. Evans et al. (2003) obtained an age of ca
1800 Ma for the June Hill Volcanics, whereas a tuffaceous
unit in the overlying Ashburton Formation has been
dated at ca 1829 Ma (Sircombe 2003). The apparent contradiction in these ages, coupled with the ca 1804 Ma age
of felsic volcanic rocks in the Capricorn Group (Hall
et al. 2001), has led Evans et al. (2003) to suggest that
deposition of the upper part of the upper Wyloo Group
and of the Capricorn Group were strongly diachronous
owing to oblique basin closure during the Capricorn
Orogeny.
The tectonic setting of the upper Wyloo Group is considered to be a retro-foreland basin associated with the
onset of the 1820–1770 Ma Capricorn Orogeny (Tyler &
Thorne 1990; Thorne & Seymour 1991; Evans et al. 2003),
although Blake & Barley (1992) and Martin & Morris
(2010) suggest that the setting may have been extensional
in its early stages.
The Narryer Terrane of the Yilgarn Craton
The Yilgarn Craton is an extensive region of Archean
continental crust dominated by granite–greenstone terranes. On the basis of recent geological mapping and a
re-evaluation of geological data at all scales (Cassidy
et al. 2006), this craton has been divided into several distinct tectonic entities—the Narryer, Youanmi and South
West Terranes, and the Eastern Goldfields Superterrane.
The Narryer and South West Terranes are dominated by
granite and granitic gneiss with only minor supracrustal greenstone inliers, whereas the Youanmi Terrane
and the Eastern Goldfields Superterrane contain substantial greenstone belts separated by granite and granitic gneiss. The Narryer Terrane contains the oldest
rocks in Australia—the ca 3730 Ma Manfred Complex—
as well as the oldest detrital zircons on Earth, at ca 4400
Ma (Froude et al. 1983; Compston & Pidgeon 1986; Wilde
et al. 2001). Part of the Narryer Terrane was subject to
extensive metamorphism and reworking during the
1820–1770 Ma Capricorn Orogeny and these strongly
deformed parts have been termed the Yarlarweelor
Gneiss Complex (Figures 1, 2; Sheppard et al. 2003). The
Murchison Domain consists of granite–greenstones and
contains rocks as old as ca 3000 Ma. All of the northern
Yilgarn Craton terranes were intruded by granitic rocks
between ca 2800 and ca 2600 Ma. All of the northern Yilgarn Craton (super)terranes were intruded by high-Ca
TTG granitic rocks between ca 2690 and ca 2660 Ma, and
low-Ca granites between ca 2660 and ca 2620 Ma (Cassidy
et al. 2006; Champion & Cassidy 2007).
The Glenburgh Terrane and assembly of the
West Australian Craton
The Glenburgh Terrane of the Gascoyne Province represents a continental fragment, exotic to both the Pilbara
and Yilgarn Cratons. The oldest component of the
Glenburgh Terrane, the 2555–2430 Ma Halfway Gneiss
(Figures 2, 4), consists of heterogeneous granitic
gneisses that contain abundant older inherited zircons,
some of which are as old as ca 3447 Ma (Johnson et al.
2011c). Although no older crust (>2555 Ma) is exposed,
the Lu–Hf compositions and crustal model ages of both
magmatic and inherited zircons indicate a long crustal
history, ranging back to ca 3700 Ma. These isotopic data
also demonstrate that large parts of the unexposed terrane formed via juvenile crustal growth processes
between ca 2730 and ca 2600 Ma. Formation of the 2555–
2430 Ma gneisses occurred mainly by the in situ reworking of these older crustal components (Johnson et al.
2011c). A comparison of the U–Pb zircon ages and zirconHf isotopic compositions of the Halfway Gneiss with
those of the bounding Pilbara and Yilgarn cratons, indicates that the Halfway Gneiss (and thus the Glenburgh
Terrane) is exotic to, and evolved independently from,
these cratons (Johnson et al. 2011c). The Glenburgh Terrane is interpreted to have collided with, and accreted
to, the Pilbara Craton during the 2215–2145 Ma Ophthalmian Orogeny (Blake & Barley 1992; Powell & Horwitz
1994; Occhipinti et al. 2004; Johnson et al. 2010, 2011a, c).
The Moogie Metamorphics (Figures 2, 4) are dominated by psammitic schists, the protoliths to which were
deposited across the Glenburgh Terrane in a time frame
coincident with the 2215–2145 Ma Ophthalmian Orogeny.
These rocks contain detrital zircon sourced from the
southern Pilbara region, and are interpreted to have
been deposited in a foreland basin that formed in
response to uplift of the southern margin of the Pilbara
Craton during the collision of the Glenburgh Terrane
with the Pilbara Craton (Occhipinti et al. 2004; Johnson
et al. 2010, 2011a; Martin & Morris 2010). Deposition of
the Moogie Metamorphics occurred in a similar time
frame to the Beasley River Quartzite of the lower Wyloo
Group, which was also deposited in a foreland basin setting during the Ophthalmian Orogeny (Martin & Morris
2010).
Sometime after the collision between the Glenburgh
Terrane and the Pilbara Craton, continental-margin arcmagmatic activity was initiated along the southern margin of this newly amalgamated block (Sheppard et al.
2004; Johnson et al. 2010, 2011a). The 2005–1975 Ma
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Architecture of the Capricorn Orogen
Dalgaringa Supersuite is exposed in the southern part of
the province (Figures 2, 4), and consists of granitic
gneisses, which have major-, trace-, and rare earth element concentrations consistent with formation in a
supra-subduction zone setting (Sheppard et al. 2004).
Their whole-rock Sm–Nd, and magmatic zircon Lu–Hf,
isotopic signatures indicate the incorporation of Neoarchean granitic gneisses with isotopic compositions
similar to those of the Halfway Gneiss (Sheppard et al.
2004; Johnson et al. 2011a), suggesting that magmatism
occurred in a continental-margin arc, the Dalgaringa
Arc, which formed along the southern margin of the
Glenburgh Terrane. This magmatic event records the
progressive closure and northward subduction of oceanic crust under the combined Pilbara Craton–Glenburgh Terrane.
Terminal ocean closure, the collision between the
amalgamated Pilbara Craton–Glenburgh Terrane and
the Yilgarn Craton, and the formation of the West
Australian Craton, all took place during the 1965–1950
Ma collisional phase of the Glenburgh Orogeny (Occhipinti et al. 2004; Johnson et al. 2010, 2011a). The collision
resulted in the imbrication of the northern margin of
the Yilgarn Craton with slices of the Glenburgh Terrane
along the Errabiddy Shear Zone (Figure 2), and the highgrade tectonometamorphism of metasedimentary and
meta-igneous rocks along the southern margin of the
Glenburgh Terrane. Substantial uplift and exhumation
of the Narryer Terrane also occurred during this event
(Muhling et al. 2008). The collisional event was accompanied by the intrusion of granitic stocks and dykes of the
1965–1945 Ma Bertibubba Supersuite, which are the first
common magmatic element of the northern margin of
the Yilgarn Craton, the Yarlarweelor Gneiss Complex,
the Errabiddy Shear Zone, and the Paradise Zone of the
Glenburgh Terrane (Figures 2, 4).
Intracratonic reworking, magmatism and
sedimentation
Subsequent to the assembly of the West Australian Craton during the Glenburgh Orogeny, the history of the
Capricorn Orogen is dominated by more than one billion
years of episodic intracontinental reworking and reactivation, including sedimentation, magmatism, metamorphism and deformation (Figure 4).
THE 1820–1770 MA CAPRICORN OROGENY
Although the Capricorn Orogeny had been widely interpreted to be the result of oblique collision between the
Yilgarn and Pilbara Cratons (Myers 1990; Tyler &
Thorne 1990; Krapez 1999; Evans et al. 2003), most of
those models were based largely on interpretations of
poorly dated metasedimentary successions in the northern part of the orogen, or else did not take into account
the ages, spatial distribution and composition of granitic
magmatism in the Gascoyne Province. The recognition
of pre-1950 Ma tectonometamorphic events associated
with the assembly of the West Australian Craton also
negates a Capricorn Orogeny-aged collision.
Structures and metamorphic mineral assemblages
related to the Capricorn Orogeny, and granites of the
689
accompanying 1820–1775 Ma Moorarie Supersuite, are
recognised across the province and in adjacent tectonic
units such as the Ashburton Basin to the north, the Yarlarweelor Gneiss Complex and Errabiddy Shear Zone to
the south, and the Padbury, Bryah and Yerrida basins to
the east (Thorne & Seymour 1991; Occhipinti et al. 1998;
Krapez & McNaughton 1999; Occhipinti & Myers 1999;
Sheppard & Swager 1999; Pirajno et al. 2000; Sheppard
et al. 2003, 2010b; Martin et al. 2005; Muhling et al. 2012).
The orogeny is characterised by extensive compressional and possibly strike-slip deformation at low- to
medium-metamorphic grades, and was accompanied by
the intrusion of voluminous, felsic magmatic stocks and
plutons of the Moorarie Supersuite, including the Minnie Creek and Landor batholiths (Figures 2, 4). Intense
structural and magmatic reworking of the northern margin of the Narryer Terrane formed the Yarlarweelor
Gneiss Complex (Figures 1, 2; Sheppard et al. 2003). Sedimentation is recorded across the orogen with the deposition of the upper Wyloo and Capricorn groups in the
Ashburton Basin, and the Leake Spring Metamorphics
in the Gascoyne Province (Figure 4).
THE 1680–1620 MA MANGAROON OROGENY
The Mangaroon Orogeny encompasses complex deformation, metamorphism, sedimentation and granite magmatism. Extensional-related faults and fold structures
and metamorphic assemblages related to the orogeny
appear to be restricted entirely to the Mangaroon Zone
(Figures 2–4) in the northern part of the Gascoyne Province, although granite magmatism (the Durlacher Supersuite) and sedimentation (the Pooranoo Metamorphics)
took place across the entire province (Figures 2, 4).
In the southern part of the Gascoyne Province, the
Pooranoo Metamorphics consist of a lower fluvial succession of sandstones, which are overlain by shallow-marine
sandstones. Farther north, in the Mangaroon Zone, deepwater turbiditic sandstones dominate the succession.
Deformation and medium-grade metamorphism is
restricted to the Mangaroon Zone of the Gascoyne Province, which records high-temperature–low-pressure metamorphism, presumably under extensional crustal regimes
(Sheppard et al. 2005). Metamorphism was accompanied
by the intrusion of voluminous granitic stocks and plutons of the 1680–1620 Ma Durlacher Supersuite. Following
peak metamorphism in the Mangaroon Zone, magmatism
stepped across into the other parts of the Gascoyne Province, especially the Mutherbukin Zone (Figures 3, 4),
where large volumes of megacrystic K-feldspar-phyric
monzogranite (the Davey Well batholith; Figures 2, 4)
were emplaced between ca 1670 and ca 1650 Ma.
Although the driver of orogenesis is currently
unknown, the high-temperature–low-pressure metamorphic conditions, and short duration of metamorphism,
magmatism and sedimentation imply extensiondominated orogeny (Sheppard et al. 2005).
MESOPROTEROZOIC SEDIMENTATION IN THE EDMUND
AND COLLIER BASINS
Following the Mangaroon Orogeny, mostly fine-grained
siliciclastic and carbonate sediments were deposited in
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the Edmund and Colliers basins (Figures 2, 4). The
Edmund and Collier groups consist of 4–10 km of siliciclastic and carbonate metasedimentary rocks that were
deposited under fluviatile to deep marine conditions.
The groups have been divided into six depositional packages, each being separated by an unconformity or basal
marine-flooding surface (Martin & Thorne 2004).
The Edmund Basin extends from the Pingandy Shelf
in the north, and continues south across the Talga Fault
as an extensional basin over 180 km in width (NE–SW)
and 400 km in length (NE–SE) (Figures 1, 2). The sedimentary rocks unconformably overly granites of the
1680–1620 Ma Durlacher Supersuite of the Gascoyne
Province (Sheppard et al. 2010b), and in the uppermost
part of the basin, locally includes volcaniclastic rocks
dated at 1463 8 Ma (Figure 4; Wingate et al. 2010). These
constraints indicate deposition sometime between ca
1620 and ca 1465 Ma. Deposition within the Edmund
Basin was controlled mainly by extensional movements
on the Talga Fault (Martin & Thorne 2004), with the
basin fill dramatically thickening to the southwest from
the Pingandy Shelf. U–Pb SHRIMP dating of detrital zircons from the Edmund Group, and associated paleocurrent directions indicate that detritus was sourced
predominantly from the northern part of the orogen,
including the Fortescue and Hamersley groups of the
southern Pilbara region (Martin et al. 2008). Subsequent
basin inversion took place during the 1385–1200 Ma
Mutherbukin Tectonic Event and the 1030–955 Ma
Edmundian Orogeny, with minor fault reactivation taking place during the ca 570 Ma Mulka Tectonic Event.
Sedimentary rocks within the Collier Basin were
deposited across both basement of the Gascoyne Province and locally deformed sedimentary rocks of the
Edmund Basin. In the western part of the Capricorn
Orogen, in the region of the seismic traverse, the basin
is relatively restricted; although it widens significantly
to the east, to a maximum of 200 km (Figures 1, 2). The
depositional age of the Collier Group is poorly constrained. Faulting associated with the 1385–1200 Ma
Mutherbukin Tectonic Event (Johnson et al. 2011d) does
not appear to affect rocks of the Collier Group, thus providing an upper age constraint for deposition. The lower
age constraint is provided by voluminous dolerite sills of
the ca 1070 Ma Kulkatharra Dolerite (Figure 4)—which
form part of the Warakurna Large Igneous Province
(Wingate et al. 2002)—and intrude rocks of both the
Edmund and Collier basins, thus deposition took place
sometime between ca 1200 and ca 1070 Ma. Deposition
also appears to have been less influenced by significant
fault movements, which characterised the deposition
within the Edmund Basin (Martin & Thorne 2004). U–Pb
SHRIMP dating of detrital zircons from the Collier
Group, and associated paleocurrent directions imply
that detritus was sourced predominantly by the erosion
of the underlying Edmund Basin (Martin et al. 2008).
Subsequent basin inversion took place during the 1030–
955 Ma Edmundian Orogeny.
THE 1385–1200 MA MUTHERBUKIN TECTONIC EVENT
The Mutherbukin Tectonic Event is a poorly defined tectonothermal event, known primarily from the
Mutherbukin Zone in the central part of the Gascoyne
Province (Johnson et al. 2011d), although hydrothermal
alteration and transpressional strike-slip faulting also
affected rocks of the Edmund Basin. This event does not
appear to have been associated with any significant periods of magmatism or sedimentation.
In the Gascoyne Province, the event is characterised
by amphibolite facies metamorphism and deformation,
which is restricted to a narrow 50 km wide corridor,
bounded by the Ti Tree and Chalba shear zones
(Figures 2–4). Dating of metamorphic monazite, mainly
from garnet–staurolite schists, from widely spaced localities provides a range of ages between ca 1280 and ca 1200
Ma, interpreted as the age of deformation and metamorphism (Johnson et al. 2011d). In the overlying Edmund
Group, evidence for Mutherbukin-age deformation is
more cryptic, owing to its very low metamorphic grade
and restriction to narrow shear zones and faults, which
have subsequently been reactivated. Nevertheless,
Mutherbukin-aged hydrothermal monazite and xenotime within these sedimentary rocks indicates that they
were subject to low-grade metamorphism and hydrothermal alteration at this time (Rasmussen et al. 2010;
Johnson et al. 2011d).
Although the driver of tectonism is not precisely
known, it is noted that the timing and duration of deformation and hydrothermal fluid flow were synchronous
with Stages I (1345–1260 Ma) and II (1225–1140 Ma) of the
Albany-Fraser Orogeny (Spaggiari et al. 2011), as well as
the Mount West (1345–1293 Ma) and Musgrave Orogeny
(1220–1150 Ma) in the west Musgrave Province (Howard
et al. 2011). The synchronicity of tectonism across the
West Australian Craton, implies a period of continentwide activity, possibly associated with a major period of
continent-scale plate reorganisation, i.e.during progressive break-up of the Nuna Supercontinent (e.g. Evans &
Mitchell 2011; Johnson 2013).
THE 1030–955 MA EDMUNDIAN OROGENY
The latest Mesoproterozoic to earliest Neoproterozoic
Edmundian Orogeny is best known for widespread folding and low-grade metamorphism in the Edmund and
Collier basins (Martin & Thorne 2004), although the
orogeny was also responsible for reworking a southeast-striking corridor between the Chalba and Ti Tree
shear zones in the Gascoyne Province (Figures 2–4;
Sheppard et al. 2007). Within this corridor, deformation
and metamorphism were accompanied, and post-dated,
by the intrusion of leucocratic granite stocks and
sheets, and rare earth element-bearing pegmatites, of
the 1030–925 Ma Thirty Three Supersuite.
In the Edmund and Collier basins, the Edmundian
Orogeny was responsible for low- to very-low-grade
metamorphism, thrust faulting and transpressionalrelated upright and open folding (Martin & Thorne 2004;
Martin et al. 2005). The fold and fault structures strike
west–east to northwest–southeast, and are concordant
with both the general basin architecture and the
regional-scale structures in the underlying Gascoyne
Province basement (Figure 2).
The interval between ca 1050 and ca 1000 Ma is commonly thought to mark the assembly of the Rodinia
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Architecture of the Capricorn Orogen
691
Figure 6 Migrated seismic section for line 10GA–CP1 across the northern Capricorn Orogen, showing both uninterpreted and
interpreted versions. Display shows vertical scale equal to the horizontal scale, assuming a crustal velocity of 6000 m/s. Common Depth Point (CDP) locations are shown in Figures 2 and 3 (100 CDP ¼ 2 km).
supercontinent (e.g. Li et al. 2008 and references
therein), of which the Australian continent may have
been an integral part. Collision between the eastern
margin of Australia and a partly assembled Rodinia is
estimated at ca 1000 Ma (Li et al. 2008), the timing of
which coincides with the growth of peak metamorphic
phases during the Edmundian Orogeny. In all reconstructions of Rodinia (e.g. Pisarevsky et al. 2003; Li et al.
2008), however, the western margin of the West Australian Craton is shown to face an open ocean. If so, then
the deformation and metamorphism associated with the
Edmundian Orogeny may have been a response to plate
reorganisation and collisions elsewhere in Rodinia,
since no other impinging crustal block was inferred to
be present to the west.
THE CA 570 MA MULKA TECTONIC EVENT
The Mulka Tectonic Event is responsible for a series of
anastomosing shear zones or faults that cut rocks of the
Gascoyne Province and Edmund and Collier groups
across the southwestern part of the Capricorn Orogen
(Figure 4). This tectonic event is characterised by fault
reactivation, rather than reworking. Mulka-aged faults
are generally concentrated within discrete corridors
such as the Chalba and Ti Tree shear zones (Figures 2–4)
and show small dextral offsets in the order of 10–100 m,
although cumulative offsets across the Chalba Shear
Zone are up to 35 km (Sheppard et al. 2010b). The Mulka
Tectonic Event is coeval with the Petermann, Paterson
and King Leopold Orogenies, and reflects an episode of
intracontinental reactivation during the assembly of the
Gondwana Supercontinent (Johnson 2013).
GEOLOGICAL INTERPRETATION OF THE SEISMIC
DATA
The seismic reflection profiling provided very clear
images, identifying many structures that extend right
through the crust (Figures 6–9). The depth to the base of
the crust varies significantly, changing from a shallow
but variable Mohorovi
ci
c (‘Moho’) character visible
beneath the Pilbara Craton (Figures 6, 9), to a deep,
indistinct crust–mantle boundary beneath the southern
part of the Capricorn Orogen (Figures 7–9), and passing
into a shallower Moho once the northern edge of the Yilgarn Craton is reached at the southern end of line
10GA–CP3 (Figures 8, 9). The upper crust can be subdivided into several provinces, basins and zones, based
principally on surface geological mapping and potentialfield data (Johnson et al. 2011b). By comparison,
the lower crust appears to consist of at least three discrete seismic provinces. Following Korsch et al. (2010),
we use the term ‘seismic province’ to refer to a discrete
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Figure 7 Migrated seismic section for line 10GA–CP2 across the Gascoyne Province, showing both uninterpreted and interpreted versions. Display is to 60 km depth, and shows vertical scale equal to the horizontal scale, assuming a crustal velocity
of 6000 m/s. Common Depth Point (CDP) locations are shown in Figures 2 and 3 (100 CDP ¼ 2 km).
volume of middle to lower crust, which cannot be traced
to the surface, and whose crustal reflectivity is different
to that of laterally or vertically adjoining provinces.
The Pilbara Craton (seismic line 10GA–CP1)
The Moho is weakly defined and has a gently undulating
character, at a depth of 11.5–12.3 s two-way travel time
(TWT) (about 34–37 km using an average crustal velocity
of 6000 ms1) (Figures 6, 9). The Baring Downs Fault is a
moderate to steeply north-dipping fault that extends
through the crust to the Moho. The seismic character of
the mid and lower crust on either side of this fault is
markedly different. To the north of the Baring Downs
Fault, beneath the Archean to Paleoproterozoic supracrustal rocks, the crust shows a broad twofold subdivision into a generally weakly reflective upper crust,
corresponding to the exposed granite–greenstones of the
Pilbara Craton, and a moderately reflective lower crust
referred to as the Carlathunda Seismic Province, as its
composition is not known, since it has not been tracked
to the surface. The boundary between these crustal divisions is undulating and offset by the major faults, but
generally occurs at a depth of 4–7 s TWT (12–21 km). To
the south of the Baring Downs Fault, the mid- to lowercrustal levels can be subdivided into a generally strongly
reflective middle crust, and a weakly reflective lower
crust, the boundary undulating at depths of 8.3–9.5 s
TWT (25–27.5 km). The crust to the south of the Baring
Downs Fault is interpreted as a separate seismic entity,
the Bandee Seismic Province, which also is not exposed
at the surface.
In addition to the Baring Downs Fault, other major
crustal structures—the Nanjilgardy, Soda, Moona, Beasley and Blair faults—are imaged in the seismic data (Figures 6, 9). Both the Soda and Nanjilgardy faults are
interpreted to extend through the crust to the Moho. The
Nanjilgardy Fault is a single, steep, north- to northeastdipping structure in middle- to lower-crustal levels, but
splays upwards into a complex, transpressive flower
structure defined by steep to flat-lying, northeast- or
southwest-dipping, minor faults (Figure 6, 9). Both the
Moona and Soda faults are steep to subvertical close to
the surface, but flatten out to form irregular, northeasterly dipping listric structures in the middle to lower
crust. The Blair and Beasley faults are moderately to
steeply southward-dipping in the upper parts of the profile, although become flatter at depth. Although all the
major faults across the orogen are subparallel (Figure 2),
the Baring Downs Fault marks a major axis across
which the dip direction of the major faults change, with
those to the north, all dipping moderate to steeply northeastward, and those to the south, dipping moderately
south-westward (Figure 6).
At the northeastern end of 10GA–CP1 (Figure 6),
granite–greenstones of the Pilbara Craton imaged in the
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Architecture of the Capricorn Orogen
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Figure 8 Migrated seismic section for line 10GA–CP3, showing both uninterpreted and interpreted versions. Display is to
60 km depth, and shows vertical scale equal to the horizontal scale, assuming a crustal velocity of 6000 m/s. Common Depth
Point (CDP) locations are shown in Figures 2 and 3 (100 CDP ¼ 2 km).
Rocklea Dome are generally weakly reflective, although
a number of prominent southwest-dipping to flat-lying
reflections are recorded at depth. These reflections are
interpreted as discontinuous greenstone sheets within
the granitic crust.
The Fortescue Group is most prominent in the northeastern part of the section (Figures 6, 9) where it dips
gently to the southwest, and is imaged as a series of
weak to strong, layered reflections. Based on their seismic reflectivity, three units can be recognised within the
Fortescue Group: a lower layer of weak reflections 0.25 s
TWT (0.8 km) thick that possibly corresponds to sedimentary rocks of the Hardey Formation; a middle layer
of strong reflections 0.7 s TWT (2.2 km) thick that
equates to the mostly basaltic rocks of the Boongal, Pyradie and lower Bunjinah formations; and an upper layer
of weak reflections 0.5 s TWT (1.4 km) thick, which
includes the upper Bunjinah Formation and Jeerinah
Formation. Although the seismic reflections lose some
of their definition beneath the Turner Syncline, they
suggest a southwesterly thickening of the Fortescue
Group from about 1.5 s TWT (4.5 km) to about 2 s TWT
(6 km) approaching the Moona Fault. The seismic data
also suggest that a section of the Hamersley Group about
1 s TWT (3 km) thick is preserved in the Turner
Syncline.
A thick, almost complete, succession of the Fortescue
Group crops out on the southwestern limb of the Rocklea
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Figure 9 Cross-section of the transect across the Capricorn Orogen, combining seismic lines 10GA–CP1, 10GA–CP2, and 10GA–
CP3, and showing key faults, terranes, zones, basins and seismic provinces.
Dome, and is cut by the Karra Well and Soda faults (Figures 6, 9). Farther west, between the Soda and Nanjilgardy faults, a succession of weakly to strongly reflective
lower and middle Fortescue Group rocks 1.7 s TWT
(5 km) thick is interpreted to overlie the weakly reflective granite–greenstone rocks.
The stratigraphic succession between the Nanjilgardy Fault and southwestern end of 10GA–CP1 (Figures 6, 9) has been difficult to interpret from the seismic
reflection data alone. The surface geology consists of
polyphase-deformed fold- and fault-repeated succession
of rocks of the upper Wyloo Group, consisting of the
Mount McGrath Formation, Duck Creek Dolomite, and
Ashburton Formation, although this structural complexity is not imaged in the subsurface, probably because of a
lack of continuous reflective markers within the stratigraphy. In the surface geology, the presence of both Fortescue and Hamersley group rocks to the south of the
Nanjilgardy Fault along strike of the seismic traverse,
implies that both successions should be present beneath
the Ashburton Basin. The 2.5D forward modelling of
gravity and aeromagnetic data (Johnson et al. 2011b)
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suggest that banded iron formation of the Hamersley
Group must be present to the south of the Baring Downs
Fault and so these units, although somewhat thinned,
have been extend southward onto the northern part of
line 10GA–CP2 (Figures 7, 9). Although these layers do
not have the same reflective signature as the exposed
Hamersley and Fortescue groups observed at the northern end of 10GA–CP1 (Figure 6), this may be due to an
increased structural complexity and, in particular, the
presence of numerous low-angle faults, beneath the Ashburton Basin. On the northern part of 10GA–CP2 (Figures 7, 9), these layers are dissected by a series of northverging thrust faults which terminate within the lower
Wyloo Group.
The Gascoyne Province and Yilgarn Craton
(seismic lines 10GA–CP2 and 10GA–CP3)
The location and character of the Moho varies considerably across 10GA–CP2 and 10GA–CP3 (Figures 7–9).
Under the Gascoyne Province, in 10GA–CP2 (Figure 7),
the Moho is deep and undulating, varying between 12
and 15.3 s TWT (36–46 km), and continues this trend into
the northern part of 10GA–CP3 (Figure 8), where the
Moho is at 15 s TWT (45 km) depth. In the central part
of 10GA–CP3, the Moho is interpreted to be at about 16 s
TWT (48 km) depth, but at the southernmost end of the
seismic line, the Moho is difficult to interpret. Line
10GA–CP3 ties to another, more recently acquired, line
across the southern Carnarvon Basin, 11GA–SC1
(Korsch et al. 2013), and ends 40 km to the west of the
start of the Youanmi seismic line, 10GA–YU1 (Zibra et al.
2013). Based on these three seismic lines, the Moho at the
southern end of 10GA–CP3 is interpreted to have been
faulted and duplicated by crustal-scale thrusting (Figure 8) marking the subsurface boundary between the Yilgarn Craton and the Glenburgh Terrane. Therefore, the
Glenburgh Terrane may be present below the Narryer
695
Terrane on seismic line 10GA–YU1 at around 15–16 s
TWT (45–48 km).
CRUSTAL TERRANES, SEISMIC PROVINCES AND SUTURE ZONES
On the basis of seismic character and surface geology,
two terranes—the Glenburgh and Narryer Terranes—
and three seismic crustal provinces—the Yarraquin,
MacAdam and Bandee Seismic Provinces—have been
identified within lines 10GA–CP2 and 10GA–CP3 (Figures 7–9). Most of these are separated by major crustal
structures, which coincide with mapped surface faults
or shear zones (Figures 2, 3).
Within line 10GA–CP2, the Glenburgh Terrane and
MacAdam Seismic Province are separated from the Bandee Seismic Province by the moderately south-dipping
Lyons River Fault (Figures 7, 9), which in the middle and
upper crust splays into the Ti Tree Shear Zone. Since
rocks of the Bandee Seismic Province are not exposed at
the surface, it is not possible to determine if this province
is similar to the Glenburgh Terrane in terms of its age,
lithological makeup and composition. However, the contact between the two, along the Lyons River Fault, is
marked by a zone of strong seismic reflections that parallel the fault (areas A and B in Figure 10); there is a significant step in the Moho where the Lyons River Fault
intersects the upper mantle (area C in Figure 10); and
although the seismic characters of the middle crust of
both the Glenburgh Terrane and Bandee Seismic Province are relatively similar, the Glenburgh Terrane is characterised by a non-reflective lower crust—the MacAdam
Seismic Province—that is not evident in the Bandee Seismic Province immediately north of the Lyons River Fault
(Figures 7, 10). These observations suggest that the Lyons
River Fault represents a major suture zone that separates
two, distinct pieces of continental crust.
Based on the interpretation of deep seismic reflection
data in 11GA–SC1 (Korsch et al. 2013), which crosses the
Figure 10 Interpreted migrated seismic section of part of line 10GA–CP2, showing the location of the suture zone between the
Glenburgh Terrane and Bandee Seismic Province at the Lyons River Fault. Display shows vertical scale equal to the horizontal scale, assuming a crustal velocity of 6000 m/s.
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southern part of 10GA–CP3, and 10GA–YU1 (Zibra et al.
2013) to the east, the Yilgarn Craton at the southern end
of 10GA–CP3 has been subdivided into the Narryer Terrane and the Yarraquin Seismic Province, the two separated by the Yalgar Fault. The Yarraquin Seismic
Province (Korsch et al. 2013) forms a highly reflective
lower crustal package that underlies the entire Youanmi
Terrane (Zibra et al. 2013). The position of the Yalgar
Fault in 10GA–CP3 has been tied from 11GA–SC1
(Korsch et al. 2013) where the fault is imaged as an eastverging, west-dipping thrust. The flat geometry of the
Yalgar Fault in this section is due to the thrust fault
being imaged normal to its movement direction, with
the Narryer Terrane moving out of the plane of section.
Movement on this fault and the juxtaposition of the
Narryer Terrane with the Yarraquin Seismic Province
is interpreted to predate the final collision of the Yilgarn
Craton and the Glenburgh Terrane since the Yalgar
Fault is cut by Glenburgh Orogeny-aged (ca 1950 Ma)
structures (Korsch et al. 2013).
The Narryer Terrane and Yarraquin Seismic Province are separated from the Glenburgh Terrane by a
series of anastomosing, north-dipping faults known as
the Errabiddy Shear Zone, and a single, moderately
south-dipping fault named the Cardilya Fault, which is
interpreted to be the main suture (Figures 8, 9). These
two fault systems also segment the Glenburgh Terrane
into two crustal elements:
The Dalgaringa Supersuite, including the Nardoo Granite, which at the surface, represents the exhumed midcrustal portions of a continental-margin arc known as
the Dalgaringa Arc (Sheppard et al. 2004; Johnson et al.
2010, 2011a), and which occurs structurally above (the
hangingwall) both the Errabiddy Shear Zone and the Cardilya Fault.
The remainder of the Glenburgh Terrane, including the
basement gneisses into which the continental-margin arc
magmas were intruded, occur in the footwall of the Cardilya Fault, lying structurally below the Narryer Terrane.
The Cardilya Fault, which separates the Yilgarn Craton (the Narryer Terrane–Yarraquin Seismic Province)
from the Glenburgh Terrane and MacAdam Seismic
Province, transects the entire crustal profile and can be
imaged down to 15.5 s TWT (46.5 km). It is interpreted
to offset and duplicate the Moho, which under the
Narryer Terrane, is much shallower at 12.5 s TWT
(37.5 km) depth (Figures 8, 9). These results are comparable to those obtained by passive-seismic methods
(Reading et al. 2012) and gravity modelling (Hackney
2004). At the northern end of line 10GA–CP3, the footwall
of the Cardilya Fault is marked by a thick package, up to
3 s TWT (9 km) thick, of strong seismic reflections
(area A in Figure 8) that are parallel to the fault. Similar
packages are also observed adjacent to the boundary
with the MacAdam Seismic Province (area B in Figure 8), and these areas are interpreted to represent
strongly deformed parts of the Glenburgh Terrane.
Lithological variations within the Errabiddy Shear
Zone, such as imbricate slices of Narryer Terrane, or
intrusions of the Bertibubba Supersuite, cannot be differentiated seismically.
THE TALGA AND GODFREY FAULTS
Although the Talga and Godfrey faults do not appear to
separate crust of differing seismic character, they are
important structures because they transect the entire
crustal profile, joining together before intersecting
the Moho (Figures 7, 9, 11). Both faults are listric in the
upper 10 s TWT ( 30 km) of crust offsetting rocks as
young as the 1620–1465 Ma Edmund Group, indicating
they are Paleoproterozoic to Mesoproterozoic extensional structures which accommodated the deposition of
sediments into the Edmund Basin. However, from 3 to
7 s TWT (9–21 km), the Talga Fault is parallel to an
imbricate set of faults (area A in Figure 11) that are
defined by series of parallel reflectors (area B in Figure 11) indicating that it is a zone of intense deformation. The faults offset units of both the Fortescue and
Hamersley groups (area C in Figure 11) and terminate
within the lower Wyloo Group, suggesting the faulting
may be related to thrusting during the Ophthalmian
Orogeny. The northward-directed thrust sense of movement is parallel to the transport direction of exposed
northward-verging thrusts in the Ophthalmia Fold and
Thrust Belt (Tyler 1991). Therefore, the Talga Fault
appears to be a major crustal shear zone that forms part
of a north-verging fold and thrust system that was subsequently reactivated as an extensional fault, accommodating the deposition of the Edmund Group.
BATHOLITHS AND GRANITE INTRUSIONS
Within the Glenburgh Terrane and upper part of the
Bandee Seismic Province, the upper 5 s TWT (15 km)
of the crust is defined by numerous, irregular and seismically non-reflective bodies (Figures 7, 9, 10), which are
indicated by the mapped surface geology (Figure 2) to be
granite plutons of the 1820–1775 Ma Moorarie Supersuite
and 1680–1620 Ma Durlacher Supersuite.
In the Mutherbukin Zone (Figures 2, 3, 9, 10), a single,
large intrusion—the Davey Well batholith—belonging to
the 1680–1620 Ma Durlacher Supersuite is imaged
between the Chalba and Ti Tree shear zones (CDPs
11075–13325). The intrusion has a concave basal contact
with the underlying Glenburgh Terrane (Figure 10) and
ranges in thickness from 0.6 s TWT (1.8 km) in the centre of the Mutherbukin Zone, to 3.8 s TWT (11.5 km) at
its northerly contact where it is truncated by the Ti Tree
Shear Zone. The pluton thins rapidly to about 2 s TWT
(6 km) on the southern side of the Chalba Shear Zone,
where it has been downthrown across this fault to the
south. Weak seismic reflections within and beneath the
intrusion are parallel to the concave basal contact, suggesting that the shape and orientation of the pluton is
due to folding during the Mutherbukin Tectonic Event.
Prior to folding, the pluton would have been a flat-lying
or gently dipping, tabular body greater than, or equal to,
11.5 km at its thickest.
In the Limejuice Zone, the Minnie Creek batholith
(Figures 2, 3, 7, 9, 10) is imaged north of the Ti Tree Shear
Zone at CDP 11075, and is interpreted to continue northward, underlying rocks of the Edmund Basin in the
Cobra Syncline to CDP 8975 (Figure 10). North of the
Edmund Fault at CDP 9450, the batholith is cut by
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Figure 11 Interpreted migrated seismic section of part of line 10GA–CP2, showing the attitude and orientation of the Talga and
Godfrey faults. Display shows vertical scale equal to the horizontal scale, assuming a crustal velocity of 6000 m/s.
numerous listric normal faults, which have produced a
series of rotated (southwest-side down) half grabens. The
Minnie Creek batholith has not been imaged north of
these half grabens (between CDP 8975 and the Lyons
River Fault at CDP 8725), where the area is dominated by
rocks of high seismic reflectivity (Figures 7, 10). The
base of the Minnie Creek batholith is mostly interpreted
as a fault contact, but between CDP 9900 and 10400, the
batholith has a relatively sharp, flat, possibly intrusive
contact with the underlying Glenburgh Terrane. In this
section, the batholith has a thickness of 2.25 s TWT
(6.75 km), but its maximum thickness, immediately
north of the Ti Tree Shear Zone, is 4.5 s TWT (13.5 km);
nevertheless, as the basal contact in this region is tectonic in origin, it is possible that the Minnie Creek batholith has been thickened during folding or faulting.
At several localities within the batholith (e.g. CDP
10300, 10500, and 11000), regions of highly planar seismically reflective material dip at moderate angle toward
the batholith centre (Figure 10). At the surface, these
bodies have been shown to be kilometre-scale rafts of
low-metamorphic grade pelitic and semipelitic schist
belonging to the 1840–1810 Ma Leake Spring Metamorphics (Figure 2). These packages, some of which are up
to 8 km in length, most likely represent vestiges of a formerly coherent sedimentary succession, into which the
granites were intruded. The preservation of these tabular metasedimentary packages suggest that the batholith
may have formed by a series of sheet-like plutons, an
interpretation supported in part by field evidence, which
indicates that many of the granites in the Moorarie
Supersuite, in the Limejuice Zone, have sheet-like geometries (Sheppard et al. 2010a, b).
Weakly reflective crust is imaged in the Mangaroon
Zone (Figures 7, 11, 12), which is bounded by the Lyons
River Fault in the south (CDP 8720), and the Godfrey
Fault to the north (CDP 6575). The area north of CDP
8300 is covered by sedimentary rocks of the Edmund
Basin, and there are no surface exposures of basement
rocks in this region (Figure 2). The crust in the Mangaroon Zone is interpreted as mostly granitic rocks of the
1680–1620 Ma Durlacher Supersuite, and metasedimentary rocks of the 1760–1680 Ma Pooranoo Metamorphics.
In the centre of the zone (CDP 8300–8550), a package of
moderately north-dipping, seismically reflective material (Figure 12) is interpreted to be a 6 km long raft of
metasedimentary material of the Pooranoo Metamorphics. The reflective package also parallels numerous
other weak seismic reflections that occur throughout
the zone, and which are interpreted to reflect relict sedimentary or lithological layering within the Pooranoo
Metamorphics. The presence of parallel seismic reflections down to 2.0 s TWT indicates that the Pooranoo
basin was at least 6 km thick in the Mangaroon Zone.
Farther north, in the area underlying the Edmund
Basin (south of the Godfrey Fault; Figure 11), it is difficult to determine if the non-reflective packages are granites of the Moorarie or Durlacher Supersuites, and for
simplicity they are tentatively shown as a continuation
of the interlayered granitic and metasedimentary material
of
Durlacher
Supersuite
and
Pooranoo
Metamorphics.
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ARCHITECTURE OF THE EDMUND AND COLLIER BASINS
Although the Collier Basin succession in the Wanna Syncline is relatively thin, both Edmund and Collier basins
are well imaged in line GA10–CP2 (Figures 7, 11). The
maximum thickness of the Edmund Basin is 2.25 s TWT
(6.75 km) thick on the southern side of the Godfrey
Fault, and in the core of the Wanna Syncline, the Collier
Basin is <0.25 s TWT (750 m) thick. A relatively large
package, up to 1 s TWT (3 km) thick, of highly seismically reflective material occurs between the base of Package 2 and the top of Package 4 (Figure 11); these
reflections are interpreted to be abundant dolerite sills
of the ca 1465 Ma Narimbunna Dolerite and ca 1070 Ma
Kulkatharra Dolerite, which intrude the upper parts of
the Edmund Basin.
Three principal southwest-dipping structures, the
Talga, Godfrey and Lyons River faults define three half
grabens (Figures 7, 9, 11), into which sediments of the
Edmund Basin were deposited (Martin & Thorne 2004),
and across which significant variations in sediment
thickness are evident. On the Pingandy Shelf, to the
north of the Talga Fault (Figure 11), the maximum thickness of Packages 1 and 2 are 0.5 s TWT (1.5 km),
whereas to the south they increase to 1.25 s TWT
(3.75 km). Across the Godfrey Fault, Packages 1 and 2
increase from 2.0 s TWT (6 km) to >2.75 s TWT
(8.25 km) thick. Packages 1 and 2 are consistently
thicker in the hangingwall of the basin-bounding extensional faults, suggesting that extensional downthrow on
these major faults was toward the southwest. Packages 1
and 2 are not present south of CDP 8300, presumably having been incised and eroded away prior to the deposition
of Packages 3 and 4 (Martin et al. 2008). In this region,
Package 3 is at least 1 s TWT (3 km) thick.
Between the Godfrey Fault (CDP 6575) and the
Edmund Fault (CDP 9465), the Edmund Basin (Package
3) is intensely folded, with numerous imbricate thrust
faults truncating the limbs of the tight, upright folds
(Figure 11).
DISCUSSION
Crustal architecture, assembly and reactivation
of the West Australian Craton
The new deep seismic reflection imaging, extending
from the southern Pilbara Craton to the northern Yilgarn Craton, provides, for the first time, a holistic view
of the crustal architecture of the Capricorn Orogen
(Figure 9). These data are important because they not
only provide critical information on the timing, assembly and reworking history of the West Australian Craton,
but also provide a rare insight into the anatomy of continental collision zones in general.
THE PILBARA CRATON–BANDEE SEISMIC PROVINCE SUTURE
The Pilbara Craton granite–greenstones and underlying
Carlathunda Seismic Province are separated from the
Bandee Seismic Province by the steeply north-dipping
Baring Downs Fault. The timing of amalgamation is
uncertain, but must pre-date the deposition of the 2775–
2630 Ma Fortescue Group, since these rocks were deposited across the suture. The pre-ca 2775 Ma age for the collisional episode suggests that it may be contemporary
with other suturing events during the 3070–3060 Ma Prinsep Orogeny, which amalgamated the East Pilbara Terrane and the numerous terranes of the West Pilbara
Superterrane (Van Kranendonk et al. 2007; Hickman &
Van Kranendonk 2008; Hickman et al. 2010).
Deposition of over 20 km of sediments into the Fortescue, Hamersley, Turee Creek, and Ashburton basins has
covered the original suture, but the seismic reflection
data indicate that reactivation of this structure has
extended the fault to the surface, where it is mapped as
the present-day Baring Downs Fault. The seismic data
also show evidence of post-1800 Ma (the minimum age of
the upper Wyloo Group within the Ashburton Basin)
extensional displacements on this structure with significant downthrow on the northeastern side (Figures 6, 9).
The exact age or setting for reactivation is not known,
nor is it clear whether any fault reactivation accompanied deposition of the upper Wyloo Group in the Ashburton Basin.
Although the Nanjilgardy Fault, which lies to the
north of the Baring Downs Fault (Figures 6, 9), does not
represent a suture zone, the fault transects the entire
crustal profile, rooting in the Moho. At the surface, the
fault forms the major lithological boundary between
the 1840–1800 Ma upper Wyloo Group in the Ashburton
Basin and older >ca 2200 Ma metasedimentary and metavolcanic rocks of the lower Wyloo, Turee Creek, Hamersley and Fortescue groups. The age of initiation, or
significance of the Nanjilgardy Fault, is not known, but
the present-day architecture as a positive flower structure must have been imparted during or after the 1820–
1770 Ma Capricorn Orogeny as rocks of the upper Wyloo
Group have been displaced.
THE BANDEE SEISMIC PROVINCE–GLENBURGH TERRANE
SUTURE
The Glenburgh Terrane–MacAdam Seismic Province
and Pilbara Craton–Bandee Seismic Province are
sutured along the moderately south-dipping Lyons
River Fault (Figures 2, 7, 9). From a variety of surface
geological information, the collision is interpreted to
have occurred during the 2215–2145 Ma Ophthalmian
Orogeny (Occhipinti et al. 2004; Johnson et al. 2010,
2011a, c; Martin & Morris 2010). However, intense
crustal reworking of the entire northern Gascoyne
Province and the intrusion of voluminous plutonic granitic rocks during episodic reworking (including the
1820–1770 Ma Capricorn and 1680–1620 Ma Mangaroon
orogenies) has obscured any evidence of this collision
(Sheppard et al. 2005, 2010a, b). The suture zone is now
represented at the surface by faults that have propagated through the younger magmatic and metamorphic
rocks. To the north of the Lyons River Fault, metasedimentary and metavolcanic rocks of the Fortescue and
Hamersley groups are interpreted to underlie much of
the Ashburton Basin, occurring as far south as the
Talga Fault in the Gascoyne Province (Figures 6, 7, 9).
Both groups appear to be faulted and dissected by a
series of north-directed thrusts that terminate within,
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Figure 12 Interpreted migrated seismic section of part of line 10GA–CP2, showing the architecture of the Mangaroon Zone,
including the raft of metasedimentary material that is parallel to other weak seismic reflections (shown as dashed form lines)
within the granitic body. Display shows vertical scale equal to the horizontal scale, assuming a crustal velocity of 6000 m/s.
Abbreviations: D, Durlacher Supersuite; E, Edmund Group.
and are progressively truncated by, the lower parts of
the 2450–2210 Ma lower Wyloo Group, suggesting that
thrusting was related to collision during the Ophthalmian Orogeny. Uplift and erosion of the Fortescue and
Hamersley groups along this axis during thrusting,
may have provided much of the detritus for the foreland basin deposits of the lower Wyloo Group (Martin
& Morris 2010) and the Moogie Metamorphics in the
Gascoyne Province (Johnson et al. 2011a).
During intracontinental reactivation, the Lyons
River and Geegin faults (Figure 12) were reactivated
as extensional structures allowing the deposition of up
to 6 km of turbiditic sedimentary rocks (the 1740–1680
Ma the Pooranoo Metamorphics) into the Mangaroon
Zone, immediately prior to the 1680–1620 Ma Mangaroon Orogeny. Outside the Mangaroon Zone, the Pooranoo Metamorphics consist of less than 700 m of
medium- to coarse-grained fluvial to shallow-marine
sedimentary rocks.
Following the Mangaroon Orogeny, the Lyons River,
Talga and Godfrey faults, appear to have controlled
much of the depositional history of the 1620–1465 Ma
Edmund Basin (Johnson et al. 2011a). These three Ophthalmian-aged structures were reactivated in an extensional setting to form a series of half grabens (Figures 7,
9, 11) into which sediments of the Edmund Group were
deposited. Package thickness variations across the
major faults, especially from the Pingandy Shelf southward across the Talga Fault (Figure 11), imply intermittent and variable amounts of extensional movements on
all three faults. The progressive truncation of older packages by units higher in the succession indicates syndepositional uplift and erosion during periods of short-
lived and punctuated reverse movements. Regional-scale
post-depositional basin inversion is evident as a series of
thrusts and hangingwall anticlines on fault splays
between the Lyons River and Godfrey faults (Figure 11).
This corridor of intense deformation has been mapped
in the surface geology (Figures 2, 3), and does not appear
to have significantly affected rocks of the Collier Basin,
suggesting either that the deformation is related to the
1385–1200 Ma Mutherbukin Tectonic Event, or that the
Godfrey Fault acted as a backstop to the northwardpropagating thrust system during the 1030–955 Ma
Edmundian Orogeny.
THE GLENBURGH TERRANE–YILGARN CRATON SUTURE
The final assembly of the West Australian Craton
occurred when the northern margin of the Yilgarn Craton was sutured to the composite Pilbara–Bandee–Glenburgh craton at about ca 1965 Ma, during the Glenburgh
Orogeny, along the Cardilya Fault. Owing to the intrusion of voluminous post-collisional granitic rocks of the
1820–1775 Ma Moorarie Supersuite across the Cardilya
Fault during the Capricorn Orogeny (Sheppard et al.
2010a), the significance of this structure as the main
suture zone was not recognised until the interpretation
of the seismic data. Prior to the seismic investigation,
the moderately well-exposed north-dipping Errabiddy
Shear Zone, which contains an imbricate assemblage of
lithologies from both the Glenburgh and Narryer Terranes, was thought to be the main suture zone (e.g.
Sheppard et al. 2010b).
Although there is little evidence of post-collisional
reactivation on the Cardilya Fault and Errabiddy Shear
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Zone in the seismic data, the Cardilya Fault truncates
and displaces plutonic magmatic rocks, specifically the
Landor batholith, of the 1820–1775 Ma Moorarie Supersuite, indicating reactivation during, or after, the 1820–
1770 Ma Capricorn Orogeny. Uplift and reworking across
the Errabiddy Shear Zone are also recorded by several
40
Ar/39Ar mica dates between ca 960 and ca 820 Ma
(Occhipinti 2007), indicating a long-lived post-collisional
history for this fault system.
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IMPLICATIONS FOR CONTINENTAL COLLISION AND
SUBSEQUENT CRUSTAL REWORKING
The West Australian Craton was built progressively
over one billion years from the punctuated collision of
continental blocks (the Bandee Seismic Province, the
Glenburgh Terrane and the Yilgarn Craton) to the
southern Pilbara Craton margin. Owing to significant
post-collisional tectonomagmatic reworking and vertical extension of the major fault structures into younger rock packages, the principal crustal sutures only
have a weak expression at the surface, and ophiolites,
high-pressure rocks or melanges, which are usually
considered key components of continental suture
zones, are either not preserved or not exposed.
However, the suture zones, as well as other major
crustal-scale structures related to the assembly of the
West Australian Craton, are well preserved and
imaged to great depth in the seismic reflection data
(Figures 6–9). The preservation of ancient reworked
sutures and crustal-scale structures within the mid- to
lower-crust appears to be a common feature of many
continental suture zones, where examples have been
documented in seismic reflection data from the TransHudson Orogen (e.g. White et al. 2005), Slave Province
(e.g. Clowes et al. 2005; Cook & Erdmer 2005) and
numerous Australian orogenic belts (e.g. Korsch et al.
2010, 2012; Cayley et al. 2011; Wyche et al. 2013).
The preservation of ancient suture zones in the deep
crust is therefore critical for understanding the crustal
architecture and evolution of the collision zone, but does
it reliably preserve pre-collisional geometric relationships such as the subduction polarity? Furthermore, in
complex collision zones, what stage during the collisional/orogenic cycle does the present-day architecture
reflect? In the Capricorn survey, both the 2215–2145 Ma
Ophthalmian and 2005–1950 Ma Glenburgh Orogeny
suture zones (the Lyons River and Cardilya faults,
respectively) dip moderately southward (Figure 9). The
simplest interpretation implies south-directed subduction of oceanic crust during both events. However, the
wealth of whole-rock geochemical, isotopic and in situ
zircon Lu–Hf isotopic data (Sheppard et al. 2004; Johnson
et al. 2011a) demonstrates that during the 2005–1950 Ma
Glenburgh Orogeny, the subduction of oceanic crust was
north-directed with the formation of the Dalgaringa Arc
within the southern margin of the Glenburgh Terrane,
opposite to the present-day dip of the suture zone. The
current crustal architecture implies significant syn- to
post-collisional modification of the crustal profile either
by a style of ‘crocodile tectonics’ (e.g. Meissner 1989;
Sadowial et al. 1991; Xu et al. 2002) or by the interplay of
complex syn- to post-collisional events such as crustal
obduction, backthrusting and crustal delamination.
This implies that the crustal architecture was preserved
relatively late in the orogenic cycle and that precollisional features may not be inherently preserved in
the orogenic architecture. Additional information, such
as whole-rock geochemical and isotopic data (e.g.
Sheppard et al. 2004), or where the subduction-related
magmatic rocks are not accessible, in situ Lu–Hf isotopic
composition of appropriately aged inherited zircon
grains, are thus required to constrain pre-collisional
relationships. However, such information is not available to constrain the subduction polarity leading up to
the pre-ca 2775 Ma collision between the Pilbara Craton
and Bandee Seismic Province or the 2215–2145 Ma Ophthalmian-aged collision between the Bandee Seismic
Province and the Glenburgh Terrane.
Following the final assembly of the West Australian
Craton, the orogen was subject to over one billion years
of punctuated intracontinental reactivation and reworking (e.g. Sheppard et al. 2010b), including present-day
seismic movements (Revets et al. 2009; Keep et al. 2012).
The three principal sutures associated with the assembly of the West Australian Craton, as well as most of the
other crustal structures, show evidence for multiple
reactivation (Figure 9), in extensional, compressional
and oblique-slip settings. Reactivation of these major
crustal structures has led to their vertical extension into
younger sedimentary or plutonic rocks (a process common to ancient collision zones; e.g. O’Driscoll 1986;
Hronsky et al. 2012), and so the suture zones, and other
associated crustal-scale faults, are commonly only
weakly expressed at the surface by much younger faults
or fault systems (Figures 2, 3, 9). The accommodation of
regional-scale strain (extensional, compressional and
oblique-slip) during the numerous reworking events
appears to have been accommodated mostly by the reactivation of the pre-existing faults and shear zones, rather
than by the generation of new crustal-scale structures.
Furthermore, in the Gascoyne Province, the orientation
of most tectonometamorphic fabrics and syn-tectonic
magmatic intrusions such as the Minnie Creek, Landor
and Davey Well batholiths (Figure 2) are also coaxial to
the main crustal-scale structures. The depositional
architecture of the 1620–1465 Ma Edmund and 1200–1070
Ma Collier basins, and subsequent style of basin inversion were controlled by extensional and transpressional
movements on these structures and, more importantly,
their orientation (Johnson et al. 2011a). Therefore, the
crustal architecture, imparted during the assembly of
the West Australian Craton ca 2200 to ca 1950 million
years ago, has strongly influenced the partitioning of
deformation, metamorphism and magmatism during
the numerous intracratonic reworking events.
Mineral prospectivity
The Capricorn seismic survey has identified a series of
major crustal structures, many of which cut through the
crust to the mantle and show evidence for multiple reactivation (Figure 9). Numerous mineral deposit types
have been recognised throughout the orogen (Figure 3)
and include the world-class hematite iron-ore deposits of
the Hamersley Basin; volcanic-hosted massive sulfide
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Figure 13 Summary cross-section through the Capricorn Orogen, showing the spatial relationship between known mineral
deposits and the location of suture zones and other major lithospheric-scale faults.
(VHMS) copper–gold deposits in the Bryah Basin on the
Yilgarn Craton margin; orogenic lode-gold mineralisation, such as that at Peak Hill in the southern Capricorn
Orogen margin, Glenburgh and the Star of Mangaroon
in the Gascoyne Province, and Paulsens and Mount
Olympus along the northern Capricorn Orogen margin;
various intrusion- and shear zone-related base metal,
tungsten, rare earth element, uranium and rare-metal
deposits in the Gascoyne Province; and lead–copper–
zinc sediment-hosted mineralisation at Abra within the
Edmund Basin (Figure 3). However, the tectonic setting
of these deposits or their relation to the crustal architecture has yet to be adequately assessed.
Central to the orogen’s prospectivity is the growing
understanding that the formation of ore deposits is an
expression of much larger earth-system processes,
which operate on a variety of scales to focus mass and
energy flux. Many factors including the geodynamic setting, architecture, metal source, fluid-flow drivers and
pathways, and depositional mechanisms (Wyborn et al.
1994; Knox-Robinson & Wyborn 1997), influence the type,
style and location of mineralisation. One of the aims of
the seismic survey was to identify large-scale crustal
structures, including those that cut through the crust to
the mantle, which may form pathways for fluid flow to
mineral systems. A recent mineral prospectivity study
of the Gascoyne Province, using the mineral systems
approach, has also highlighted the significance of these
major crustal structures to sites of potential mineralisation (Aitken et al. 2013). Although factors such as rock
type, rock composition and metal source are important
for mineralisation, the faults and, in particular, their
constant reactivation during subsequent and punctuated
orogenesis are key to focussing the fluids and energy
flux of the mineral system. Especially important are the
structures that transect the entire crustal profile (the
Nanjilgardy, Baring Downs, Talga, Lyons River, and Cardilya faults; Figures 2, 3, 9, 13), as they provide a lithospheric-scale deep plumbing system to allow the
transport of fluid and energy direct from the mantle into
the upper crust.
In most cases, known sites of mineralisation lie along,
close to or within fault splays related to the major crustal
structures (Figures 3, 13). The Soda and Karra faults in
the northern Capricorn Orogen are associated with the
premium martite–microplaty hematite ores. The
Nanjilgardy Fault hosts significant gold deposits in both
the Fortescue Group of the Wyloo Dome at Paulsens and
Proterozoic rocks of the Ashburton Basin such as at
Mount Olympus. The Baring Downs Fault hosts several
gold and base metal deposits in the Ashburton Basin
such as Star of the West, Glen Florrie and Soldiers
Secret. In the Gascoyne Province, the composite Lyons
River–Minnie Creek–Minga Bar Fault system hosts rare
earth elements at Gifford Creek, and gold at the Star of
Mangaroon. An along-strike extension of the Lyons
River Fault, the Quartzite Well Fault, hosts the polymetallic Abra base metal deposit. The Ti Tree and Chalba
shear zones host numerous tungsten, molybdenum and
copper–gold base metal deposits, and the Cardilya–Deadman Fault hosts gold at Glenburgh. Apart from the Mt
Olympus gold deposit, which has been dated at ca
1740 Ma (Şener et al. 2005), the age of the various mineral
deposits across the orogen are not precisely known, and
so it is difficult to determine whether they represent primary deposits related directly to tectonic setting (i.e.craton edge or island arc), or whether they are younger,
secondary hydrothermal deposits related to fluid flow
during intracontinental reactivation.
In the case of the Mt Olympus deposit, which lies
along the Nanjilgardy Fault, the gold is interpreted to be
of hydrothermal origin (Şener, et al. 2005). The seismic
data show the Nanjilgardy Fault to have been (re)activated during the 1820–1770 Ma Capricorn Orogeny consistent with the age dating and demonstrate that
hydrothermal fluid flow within this major-crustal structure is an important mechanism for gold mineralisation.
The polymetallic Pb–Cu–Zn deposit at Abra in the
lower part of the Edmund Group (which lies on an extension of the Lyons River Fault) shows a complex history
of monazite and xenotime growth between ca 1385 and ca
1235 Ma (Rasmussen et al. 2010). The deposit has been
interpreted to be SEDEX in origin (Vogt & Stumpf 1987;
Collins & McDonald 1994), but the polyphase growth of
phosphate minerals implies significant secondary
upgrading associated with hydrothermal fluid flow
(Pirajno et al. 2009; Rasmussen et al. 2010) during fault
reactivation.
The Lyons River Fault also hosts significant rare
earth elements within the Gifford Creek Carbonatite
Suite (formerly the Gifford Creek Complex; Pearson
1996; Pearson & Taylor 1996; Pearson et al. 1996)
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(Figures 3, 13). The suite consists of a series of ferroan
carbonatite sills that are distributed within a 30 km long
northwest-trending belt that parallels the Lyons River
Fault (Pearson 1996; Pearson & Taylor 1996; Pearson
et al. 1996). The sills were emplaced into metasedimentary and granitic rocks of the Pooranoo Metamorphics
and Durlacher Supersuite, respectively and so are younger than ca 1620 Ma. During subsequent tectonometamorphism, hydrothermal alteration of the carbonatites
led to extensive upgrading of rare earth elements into a
series of mineralised ironstone veins and dykes.
Although the ages of the carbonatite sills and subsequent rare earth element-rich ironstones are not precisely known (<ca 1620 Ma), the emplacement of the
alkalic rocks implies the interaction of mantle-derived
melts with thinned, metasomatised, subcontinental lithospheric mantle (Foley 2008). Emplacement of the sills in
the upper crust was controlled by ?extensional movements on the Lyons River Fault, and subsequent rare
earth element-upgrading took place during hydrothermal activity.
These three examples highlight the importance and
interplay between hydrothermal activity, the reactivation of lithospheric-scale faults, and gold, base metal and
rare earth element mineralisation. Although the ages or
structural settings of other deposits in the orogen are
not precisely known, it is noted that the main lithospheric-scale structures (at the surface) host most of the
known deposits. This association highlights the importance of major crustal-scale structures as conduits of
fluid flow to mineral systems by the transport of fluid
and energy direct from the mantle into the upper crust.
However, these structures also coincide with former craton edges and possible island arcs (areas of preferentially endowed crust) and so, with regard to the source of
metals, there may be a deeper, underlying ‘tectonic’ control (e.g. Groves et al. 2005; Kerrich et al. 2005; Begg et al.
2010; Hronsky et al. 2012) on the location of particular
commodities within the orogen. The clustering of gold
and base metal deposits around the three main suture
zones (Figure 13) may reflect this underlying tectonic
control, but some deposits, such as the rare earth element deposits at Gifford Creek, show a direct mantlederived component for the metal source (the carbonatite
sills). In reality, mineralisation is likely to reflect both
end-member components and various combinations of
the two. However, since most of the former island arcs
and craton edges are now deeply buried or reworked, the
most important factor for mineral prospectivity is the
presence and location of repeatedly reactivated lithospheric-scale faults (Figure 13).
CONCLUSIONS
The Capricorn Orogen deep seismic reflection survey
has imaged the crust and upper mantle structure, providing, for the first time, a holistic view of the crustal
architecture of the Western Australian Craton. The survey demonstrates that the craton was built from four
main crustal components: the Pilbara and Yilgarn Cratons, the Glenburgh Terrane and the Bandee Seismic
Province, by the progressive and punctuated collision of
continental blocks to the southern Pilbara Craton margin. This collisional architecture has fundamentally controlled the style and orientation of over one billion years
of intracontinental reactivation, including deformation,
metamorphism, magmatism and sedimentation.
The three main crustal suture zones, as well as other
major crustal structures, are well preserved in the mid
and lower crust, but are only weakly expressed at the
surface by vertical extensions of these structures
through younger (meta)sedimentary and (meta)igneous
rock packages during intracontinental reactivation. The
present-day architecture is the culmination of numerous
collision-related processes that may have been preserved
relatively late in the orogenic cycle. Therefore, the
imaged architecture may not be used to reconstruct the
pre-collisional tectonic settings such as the subduction
polarity.
The majority of known mineral deposits in the Capricorn Orogen are spatially associated with the surface
expression of the major crustal suture zones and other
lithospheric-scale faults. These faults show evidence of
having been multiply reactivated in an intracontinental
setting, and highlight the importance of major crustalscale structures and fluid flow to mineral systems by the
transport of fluid and energy direct from the mantle into
the upper crust.
ACKNOWLEDGEMENTS
This work forms part of a collaborative project between
GSWA, AuScope and Geoscience Australia. GSWA funding was sourced from the Exploration Incentive Scheme
(EIS) under the Royalty for Regions program. Many
thanks go to the two reviewers, Dr Peter Betts and
Dr David Martin, for greatly improving the manuscript.
Thanks also go to Michael Prause for drafting of figures.
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Received 18 December 2012; accepted 16 July 2013