Australia and Nuna

Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 Australia and Nuna P. G. BETTS1*, R. J. ARMIT1, J. STEWART1,5, A. R. A. AITKEN2, L. AILLERES1, P. DONCHAK3, L. HUTTON3, I. WITHNALL3 & D. GILES4 1 School of Earth, Atmosphere and Environment, Monash University, Clayton Campus, VIC 3800, Australia 2 Centre for Exploration Targeting, The University of Western Australia (M006), Crawley, WA 6009, Australia 3 Geological Survey of Queensland, Level 10, 119 Charlotte Street, Brisbane, QLD 4000, Australia 4 Centre for Mineral Exploration Under Cover, School of Earth and Environmental Sciences, University of Adelaide, SA, Australia 5 Present address: PGN Geoscience, GPO BOX 1033, Melbourne, VIC 3001, Australia *Corresponding author (e-mail: Peter.Betts@monash.edu) Abstract: The Australian continent records c. 1860– 1800 Ma orogenesis associated with rapid accretion of several ribbon micro-continents along the southern and eastern margins of the proto-North Australian Craton during Nuna assembly. The boundaries of these accreted microcontinents are imaged in crustal-scale seismic reflection data, and regional gravity and aeromagnetic datasets. Continental growth (c. 1860– 1850 Ma) along the southern margin of the proto-North Australian Craton is recorded by the accretion of a micro-continent that included the Aileron Terrane (northern Arunta Inlier) and the Gawler Craton. Eastward growth of the North Australian Craton occurred during the accretion of the Numil Terrane and the Abingdon Seismic Province, which forms part of a broader zone of collision between the northwestern margins of Laurentia and the proto-North Australian Craton. The Tickalara Arc initially accreted with the Kimberley Craton at c. 1850 Ma and together these collided with the proto-North Australian Craton at c. 1820 Ma. Collision between the West Australian Craton and the proto-North Australian Craton at c. 1790– 1760 Ma terminated the rapid growth of the Australian continent. Nuna The geological record shows episodes of rapid continental assembly to form supercontinents. There is a degree of confidence in the configuration of supercontinents formed in the last billion years of Earth’s history (Evans 2013); however, determining the supercontinent configurations beyond a billion years is subjective (Cawood & Hawkesworth 2014) because the rock record becomes more difficult to decipher and high-quality palaeomagnetic data is limited (Pisarevsky et al. 2014). Evidence for a Palaeoproterozoic supercontinent Nuna/Columbia (herein referred to as Nuna) (Rogers & Santosh 2002; Zhao et al. 2002, 2004; Pisarevsky et al. 2014) is supported by palaeomagnetic constraints (Evans & Mitchell 2011; Zhang et al. 2012a; Pisarevsky et al. 2014), tectonic criteria (Karlstrom et al. 2001; Betts et al. 2008, 2011; Doe et al. 2012) and geochemical and geochronology data (Condie & Aster 2010; Condie et al. 2011). However, Nuna’s exact configuration remains uncertain. In most Nuna configurations Australia occupies an important component of the supercontinent (Zhao et al. 2002; Zhang et al. 2012a), with most reconstructions placing eastern Australia and western Laurentia next to each other (Fig. 1a –c). Uncertainty remains about the timing of Australia and Laurentia amalgamation during Nuna formation. For example several Nuna reconstructions have Australia and Laurentia together by c. 1740 Ma (Betts et al. 2008; Zhang et al. 2012a), whereas others have proposed that Australia existed as an isolated continent that did not amalgamate to Laurentia until c. 1600 –1500 Ma (Eglington et al. 2013; Pisarevsky et al. 2014). The Palaeoproterozoic assembly of Australia is the most significant period of crustal amalgamation, with approximately two-thirds of the continent forming between c. 1860 and 1800 Ma (Betts et al. 2002). The internal structure of the Australian continent comprises a collage of cratons, microcontinents and arc terranes that had amalgamated by c. 1700 Ma (Li 2000; Betts & Giles 2006; Cawood From: Li, Z. X., Evans, D. A. D. & Murphy, J. B. (eds) Supercontinent Cycles Through Earth History. Geological Society, London, Special Publications, 424, http://doi.org/10.1144/SP424.2 # 2015 The Geological Society of London. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics. Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. Fig. 1. Possible reconstruction of Nuna (Columbia) highlighting the position of Australia. (a) Columbia supercontinent after Zhao et al. (2002). (b) Columbia reconstruction after Yakubchuk (2010). (c) Palaeomagnetically constrained Nuna configuration after (Zhang et al. 2012a). Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA Fig. 1. Continued. & Korsch 2008). Debate about the exact position of major cratons with respect to each other (see Li & Evans 2011; Williams et al. 2012) and the amount of tectonic modification subsequent to assembly (see Giles et al. 2004; Li & Evans 2011; Smits et al. 2014) is ongoing. In this paper we show how Australia records rapid continental amalgamation during the assembly of Nuna Supercontinent and outline the geological events responsible for accretion of the continent. The key geological constraints that need to be considered when reconstructing Nuna are also discussed. North, South and West Australian cratons The geography-based North Australian Craton, South Australian Craton and West Australian Craton nomenclature of Myers et al. (1996) has been used to conceptualize the evolution of the Australian plate during the Proterozoic (Betts & Giles 2006; Cawood & Korsch 2008; Li & Evans 2011). Major geological provinces comprising the North Australian Craton include the Archaean to Palaeoproterozoic Kimberley Craton, Pine Creek Inlier, Aileron Terrane (Arunta Inlier), Davenport Province, Mount Isa Terrane and Etheridge Province (Fig. 2). Recent tectonic reconstructions also suggest that the Curnamona Province and parts of the Gawler Craton, which make up the South Australian Craton, were contiguous with the North Australian Craton (Betts & Giles 2006; Cawood & Korsch 2008; Payne et al. 2009; Howard et al. 2011b; Armit et al. 2012; 2014). Cawood & Korsch (2008) coined the term Diamantina Craton to describe a contiguous North and South Australian Craton. The South Australian Craton became a separate and Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. Fig. 2. Continental-scale composite aeromagnetic image of total magnetic intensity and first vertical derivative data with a superimposed map of major Archaean and Palaeoproterozoic geological provinces. Magnetic data provided by Geoscience Australia. distinct cratonic element during Mesoproterozoic reconfiguration of the Australian continent (Giles et al. 2004; Cawood & Korsch 2008). For the purpose of this paper, the North Australian Craton and South Australian Craton, including its continuation into East Antarctica as part of the Mawson Continent (Payne et al. 2009), are treated as a single entity prior to their separation in the Mesoproterozoic (Giles et al. 2004). We use the term proto-North Australian Craton to describe the largest of the pre-assembly cratonic elements upon which smaller elements accreted. Within the larger Diamantina Craton there are many smaller crustal elements. These are separated by orogenic systems that evolved between c. 1860 and 1800 Ma and include the King Leopold, Halls Creek and Top End orogens in the northwestern part of the proto-North Australian Craton (Tyler & Griffin 1990; Bodorkos et al. 1999, 2000; Carson et al. 2008; Worden et al. 2008) and the Barramundi Orogeny in the eastern proto-North Australian Craton (Bierlein & Betts 2004; Etheridge et al. 1987). Furthermore, the Cornian Orogeny, preserved in the southeastern Gawler Craton (Reid et al. 2008) may also form part of the orogenic system responsible for the amalgamation of the Diamantina Craton. The Western Australian Craton remains the same as initially defined (Myers et al. 1996) and was formed during the c. 1960 Ma Glenburgh Orogeny (Johnson et al. 2013) before it was amalgamated with the rest of the Australian continent. Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA Amalgamation of the Diamantina Craton: the geological and geophysical record Early interpretations of the geological evolution of the Diamantina Craton involved cycles of intraplate tectonic events correlated across the entire Diamantina Craton (Etheridge et al. 1987). Nd isotopic data was used to suggest that granitic suites distributed over a larger area of the Australian continent were predominantly derived from a mafic lower crustal material extracted from the mantle between c. 2400 and 2100 Ma (Wyborn & Etheridge 1988). These data were used to argue that the cratonic elements and geological provinces that comprise Proterozoic Australia were intact by c. 2100 Ma. This, along with the lack of geological features typical of modern collisional settings (e.g. HP metamorphic rocks, remnant oceanic crust and igneous suites with unequivocal arc geochemistry), were used to argue against plate tectonic processes during the Palaeoproterozoic. Consequently, continental sutures were not on the geological agenda and the tectonic evolution of the Australian plate was interpreted as cycles of intraplate orogenesis and extensional-sag basin development driven by small-scale mantle convection (Etheridge et al. 1987). Subsequent mapping programmes along the Halls Creek and King Leopold orogens (Fig. 2) by the Geological Survey of Western Australia (Sheppard et al. 1999, 2001) recognized arc-related igneous suites and interpreted the amalgamation of the Kimberley Craton and the North Australian Craton (Fig. 2) in the context of modern plate tectonics. Since then several other arc terranes have been recognized (Wade et al. 2006; Swain et al. 2008; Korsch et al. 2011b; Kirkland et al. 2013b) and numerous synthesis papers have re-interpreted the evolution of Proterozoic Australia in the context of plate tectonic processes (Myers et al. 1996; Giles et al. 2002, 2004; Betts & Giles 2006; Cawood & Korsch 2008; Payne et al. 2009; Betts et al. 2011). The geological evolution associated with the Palaeoproterozoic assembly of Australian and the geophysical response of major suture zones are summarized below. Arunta Inlier Critical to unravelling the Proterozoic evolution of Australia is the Arunta Inlier in central Australia (Figs 2 & 3). This inlier records a protracted and complex geological evolution that includes major episodes of tectonism that initiated in the Palaeoproterozoic and continued episodically until the Devonian (Maidment et al. 2013). The Arunta Inlier is separated into two provinces: the southern Warumpi Province (Scrimgeour et al. 2005), which amalgamated with Australia at c. 1640 Ma, and the larger Aileron Province in the north (Figs 2 & 3). The eastern part of the Aileron Province preserves the c. 1880–1860 Ma Atnarpa Igneous Complex, which is characterized by calc-alkaline and trondjhemite geochemical associations (Zhao & Cooper 1992), which have been interpreted as the remnants of plate margin magmatism. Evidence for an older crustal component of the Aileron Province is derived from Nd-isotopic data, which have both juvenile and evolved components, and basement rocks that comprise highly attenuated Archaean crust with more juvenile arc or backarc components (Zhao & McCulloch 1995). An Archaean basement is partially supported by the presence of Archaean detrital zircon in the Lander Package (Claoué-Long et al. 2008) and inherited zircon populations between c. 2600 and 1900 Ma from younger igneous suites (Zhao & Bennett 1995; Wade et al. 2008). The oldest sedimentary rocks are the c. 1840–1820 Ma Lander Package in the west, and the slightly younger Ongeva Package in the east (1810–1800 Ma; Claoué-Long et al. 2008). The Lander Package comprises interbedded pelite and psammite interpreted to reflect turbidite sedimentation with interlayered basalt and dolerite units that represent lavas or sills (Claoué-Long et al. 2008). The Ongeva Package comprises marine clastic sediments and felsic and mafic volcaniclastic rocks. Both these packages are intruded by bimodal magmas associated with the c. 1800 Ma Stafford Igneous Event (Collins & Shaw 1995; Claoué-Long et al. 2008), constraining the minimum deposition age. Tanami Province The Tanami Province is preserved within the protoNorth Australian Craton (Fig. 2) and comprises Neoarchaean paragneiss basement rocks of the the Billabong Complex (c. 2514 Ma) and metasedimentary rocks of the Browns Range Metamorphics (Crispe et al. 2007). Palaeoproterozoic pelagic and turbidite successions of the Stubbins Formation (c. 1864 Ma) and Dead Bullock Formation (c. 1864– 1844 Ma) are interpreted to record back-arc basin development above a north-dipping subduction zone (Bagas et al. 2008). Intercalated basaltic volcanic rocks and dolerite intrusions have tholeiitic to calc-alkaline affinities (Bagas et al. 2008). Turbidites of the Killi Killi Formation are correlated with the Lander Package in the Arunta Inlier (Crispe et al. 2007; Goleby et al. 2009). An basin inversion in the Tanami Province (Joly et al. 2010) is characterized by the development of a layer-parallel foliation and associated north– south-trending isoclinal folds (Bagas et al. 2010). Regional metamorphism associated with this event is variable between greenschist facies in the NW Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. Fig. 3. Continental-scale composite gravity (colour) and first vertical derivative of the total magnetic intensity (grey) image showing the location of major suture zones associated with the amalgamation of the Nuna Supercontinent in Australia. Locations of seismic reflection profiles mentioned in the text are shown. Gravity and magnetic data provided by Geoscience Australia. and amphibolite facies in the SE (Crispe et al. 2007; Bagas et al. 2008). Unconformably overlying this deformed basin succession are fluvial and lacustrine sedimentary successions of the c. 1825–1810 Ma Ware Group (Crispe et al. 2007). Inversion of the Ware Group occurred between c. 1810 and 1800 Ma (Bagas et al. 2010). This event is characterized by open folding associated with NW–SE transitional to east– west-directed crustal shortening (Crispe et al. 2007), followed by an episode of crustal extension, which was followed by renewed south- to SE-directed and east –west-directed shortening at c. 1800 Ma (Bagas et al. 2010). This deformation probably reflects the distal expression of the collision between the Kimberley Craton and the proto-North Australian Craton. Western Willowra Suture Collision between the western Aileron Province (Arunta Inlier) and the Tanami Province (protoNorth Australian Craton) is constrained by the deposition of the c. 1864–1844 Ma Stubbins Formation and the c. 1840 Ma Lander Package and Killi Killi Formation, which stitch the two terranes (Goleby et al. 2009). The Willowra Suture Zone (Figs 3 & 4) defines the crustal boundary between the proto-North Australian Craton and the Aileron Province. This suture was first documented in seismic reflection data (Goleby et al. 2009; seismic line 05GA-T1; Fig. 3). The suture zone is defined by a wide zone (c. 10 km) of southdipping seismic reflections that bound dominantly Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA Fig. 4. (a) Bouguer Gravity map across the Willowra Suture between the proto-North Australian Craton and the Aileron Terrane. (b) Reduced to the Pole aeromagnetic map across the Willowra Suture between the proto-North Australian Craton and the Aileron Terrane. (c) Forward model of the Willowra Suture constrained by regional seismic reflection data. The model shows the suture is south-dipping and separated thick crust in the Aileron Terrane from thinner crust in the Tanami Province (proto-North Australian Craton). In the seismic profile, the Dead Bullock Formation (2865 + 195 kg m23 mean +1 SD) dominates the Tanami Group, although the Killi Killi group (2588 + 107 kg m23) is preserved in a synformal feature to the NW of the Coomarie Dome granite (2311 + 115 kg m23). The Frankenia Dome granite has a density of 2424 + 178 kg m23. Other units are either absent from the seismic section, or are preserved only as thin veneers, and are not modelled. Basement beneath the Tanami Group is unsampled, but is assumed to have a density of 2800 kg m23. Structuring within the Tanami Group, constrained by both gravity and magnetic modelling, is shown to be consistent with the large-scale structuring of the basement. In the Tanami Province, this structuring is characterized by numerous SE-dipping crustal boundaries interpreted as thrust faults, with minimal evidence of folding, although one synformal feature is recognized immediately NW of the Coomarie Granitoid. In the Aileron Terrane, structuring is divergent, with several NW- and SE-dipping features interpreted as imbricate thrust zones and duplex structures. Within the Tanami Group, the suture is identified by a magnetic and gravity high, with strongly asymmetrical anomalies indicative of a SE dip. Aeromagnetic and gravity data provided by Geoscience Australia. Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. north-dipping reflectors in the Aileron Province from dominantly south-dipping reflectors in the Tanami Province (Goleby et al. 2009). Where the suture intersects the Moho there is significant thickening of the crust from c. 35 –42 km beneath the Tanami Province to c. 60 km beneath the Aileron Province (Goleby et al. 2009; Fig. 4). The Moho is offset with a normal sense of movement. A 2D joint forward modelling of gravity and magnetic data (Fig. 4) along transect line 05GAT1 (Goleby et al. 2009) used the depth-migrated seismic profile as the a-priori structure of the region. For the upper crust, initial densities were set according to petrophysical constraints from surface/ drillcore samples. The basement of the Tanami Province consists of a series of SE-dipping, faultbounded blocks that dissect the Tanami Group and granitoids in the upper crust, and the underlying basement. The structuring of the Aileron Province is somewhat different, with divergent thrusts indicating an uplifted orogenic wedge. The modelling suggests that the basement to the Aileron Province must have comprised anomalously high-density material (c. 2900 kg m23). Within this province there is a further requirement for a large, highdensity NW-dipping mid-crustal body, with a density of 3100 kg m23, beneath the Willowra Gravity Ridge. The boundaries of this feature are coincident with prominent seismic reflectors and a regional gravity high. Eastern Willowra Suture The regional seismic reflection transect (Line 09GAGA1) also images the Willowra suture (Fig. 5) between the eastern Aileron Province and the protoNorth Australian Craton. The suture is coincident with the Atuckera Fault Zone, which separates crustal blocks with distinctive seismic reflectance character (Korsch et al. 2011a). The Atuckera Fault Zone dips moderately to the south, similar to the geometry as the Western Willowra Suture imaged along transect 05GA-T1 (Goleby et al. 2009). The Atuckera Fault is innocuous in regional gravity data, but broadly defines a regional boundary in aeromagnetic data where crust in the Davenport Province (Fig. 2) in the north is more magnetic than Aileron Province to the south. Depth to the Moho beneath the Davenport Province is 45 km whereas it is 60 km beneath the Aileron Province (Korsch et al. 2011a), similar to the crustal architecture to the west. Mount Isa Terrane Basement rocks of the Mount Isa Terrane comprise supracrustal successions preserved in the central Kalkadoon–Leichhardt Belt and Ardmore–May Downs Domain (Fig. 6a, b). These rocks include migmatitic to gneissic rocks (e.g. c. 1857 Ma Kurbayia Migmatite: Bierlein et al. 2008) in the Kalkadoon–Leichhardt Belt, and greenschist facies schist and phyllite units of the pre-1874 Ma Yaringa Metamorphics, and St Ronans Metamorphics in the Ardmore–May Downs Domain (Bierlein & Betts 2004; Foster & Rubenach 2006; Bierlein et al. 2008). Basement units were deformed and metamorphosed during the c. 1870–1850 Ma Barramundi Orogeny (Page 1983; Etheridge et al. 1987; Page & Williams 1988; Betts et al. 2006). Deformation was characterized by the development of Fig. 5. Conceptual section of (a) the Atnarpa arc formation in the Aileron Terrane and (b) the collision of the Aileron Terrane and proto-North Australian Craton resulting in the formation of the Willowra Suture. Left side of section is to the north. Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA Fig. 6. Geophysical data, and the seismically constrained gravity model of the Isa– Numil Suture Zone, showing (a) Bouguer gravity and (b) Reduced to the pole magnetic data. The location of the seismic data and the Bouguer gravity transect is shown in (a). The thickest black line in (a) and (b) indicates the interpreted suture trace; thick black lines indicate seismically constrained structures; and thin, dotted lines indicate unconstrained structures. (c) Seismically constrained gravity model of the suture zone. Model geometry, including the modelled density distribution is shown in the bottom panel. Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. an intense bedding-parallel gneissic foliation (Etheridge et al. 1987) and north–south-trending upright folds with axial-planar foliation formed during a regional east– west shortening (Blake 1987; Blake & Stewart 1992). Following the Barramundi Orogeny, tonalitic, granodioritic and granitic rocks of the c. 1855– 1850 Ma Kalkadoon and Ewen Batholiths and the Leichhardt Volcanics were emplaced in a north– south-trending belt in the central part of the terrane. The nature and significance of the Kalkadoon– Ewen Batholiths has been a topic of some controversy. Wyborn & Page (1983) suggested that the relatively low initial 87Sr/86Sr ratios and the absence of inherited zircons in granites indicated that the age of the source was not greatly older than their emplacement age, and were derived from continental crust that differentiated from the mantle between 2100 and 1900 Ma. However, McDonald et al. (1997) interpreted c. 2500–2420 Ma SHRIMP U– Pb ages from zircon cores of the Kurbayia Migmatite to represent an igneous crystallization event, hence suggesting that magma intruded country rock with a minimum age of 2750 Ma. McDonald et al. (1997) also noted that the supracrustal basement and the Kalkadoon Batholith have strong arc-like geochemical characteristics, similar to Archaean tonalite –trondhjemite–granodiorite magma suites, and interpreted the Kalkadoon Batholith to be the product of lower crustal melting with arc affinities. However, it is possible, given the lack of inherited zircons, their relatively juvenile character and the arc-like geochemistry and inherent linear geometry of the batholiths, that they may in fact represent the remnants of an ancient continental arc or back-arc batholiths. Gidyea Suture Geophysical interpretation, seismic reflection data and gravity forward modelling suggest that the eastern margin of the Mount Isa Terrane (geophysical extent of the Mount Isa Inlier) is bounded by a major suture zone (Gidyea Suture Zone; Korsch et al. 2012). The Gidyea Suture Zone is buried beneath the Palaeoproterozoic (c. 1660–1600 Ma) supracrustal successions (Jackson et al. 2000; Foster & Austin 2008), and records the collision between the Numil Terrane (Korsch et al. 2012) to the east and the basement rocks of the Mount Isa Terrane. Two-dimensional forward modelling of Bouguer gravity data, parallel to the crustal-scale seismic reflection line (line 07GA-IG1; Korsch et al. 2012), was undertaken to better characterize the crustal structure and geophysical signature of the Gidyea Suture Zone (Korsch et al. 2012). The structuring defined by the depth-migrated seismic profile was used to determine a-priori crustal structure, including the thickness of the sedimentary basins and some granitoids, as well as the large-scale basement structure. The west-dipping Gidyea Suture Zone has a prominent linear gravity response located to the east of the present-day Cloncurry Fault (Korsch et al. 2012; Fig. 6c). This linear anomaly has a strike-length of several hundred kilometres (Fig. 6a, b). The suture zone is modelled with a density of 2800 kg m23 with a 458 dip to the west. The suture extends to the Moho (Fig. 6c), is approximately 5 km thick and is interpreted to represent mafic and ultramafic oceanic lithosphere that has been entrained between the Mount Isa Terrane and the Numil Terrane. In the NE Mount Isa Terrane, the Bouguer gravity anomaly has a steep gradient (2.5 mGal/km) because the suture is present at a depth of c. 1.8 km. The amplitude of the anomaly decreases to the south, suggesting the suture is deeper beneath younger Palaeoproterozoic to Phanerozoic basins. Western Mount Isa The western margin of the Mount Isa Terrane is imaged in regional aeromagnetic and gravity datasets (Fig. 6a, b), which show a prominent truncation of NW-trending structural grain of the proto-North Australian Craton against the north–south structural grain of the Mount Isa Terrane. The structural grain of the Ardmore–May Downs Domain aligns with the regional structural and geophysical grains of the Davenport Province and Tennant Creek Block (Figs 1 & 2) in the interior of the proto-North Australian Craton. The north –south structural grain of the Mount Isa Terrane to the east of the Mount Isa Fault largely reflects post-Barrumundi tectonic overprint (O’Dea et al. 1997a; Betts et al. 2006), although this is likely to have been superimposed on the north– south-trending structural grain of the Barramundi Orogeny (O’Dea et al. 1997b) and the Kalkadoon–Leichhardt Belt (McDonald et al. 1997; Wyborn & Page 1983; Bierlein et al. 2008, 2011). This boundary between the Ardmore–May Downs Domain and the Leichhardt River Domain (Fig. 6a, b) occurs along a prominent splay of the regional Mount Isa Fault and is interpreted as a major crustal discontinuity, active during the Barramundi Orogeny, possibly in a continental back-arc setting behind the Kalkadoon Batholith. The apparent arcuate trace of the fault system probably reflects modification during later orogenic events. Sm–Nd isotope model ages of igneous and sedimentary rocks from the Ardmore–May Downs Domain are indistinguishable from the pre-1800 Ma magmatic rocks of the Kalkadoon Batholith, which are characterized by depleted mantle model Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA ages (TDM) ranging from 2380 to 2820 Ma (Bierlein & Betts 2004; Bierlein et al. 2011). These rocks have stronger within-plate affinities (Bierlein et al. 2011) and are quite distinct from the rocks to the east of the Kalkadoon Batholith. This data was used to reject the notion that the Mount Isa Fault Zone represented a suture (Bierlein & Betts 2004). Eastern Gawler Craton The Archaean nucleus of the Gawler Craton forms a 150 –200 km wide, .1000 km belt folded around an apparent orocline (Fig. 7a). The oldest known rocks are c. 3150 Ma granites (Fraser et al. 2010), which form a narrow belt along the eastern edge of the craton. The majority of the Archaean core of the Gawler Craton comprises supracrustal sedimentary and volcanic rocks with interpreted ages between 2560 and 2520 Ma, detrital source ages of c. 2720–2535 Ma and TDM model ages between 3200 and 2800 Ma (Swain et al. 2005). Calc-alkaline volcanic and granitic rocks in the region are the remnants of a c. 2558 and 2500 Ma magmatic arc. These arc-related rocks are coincident with isotopically juvenile komatiitic volcanic rocks of the c. 2520 and 2510 Ma Harris Greenstone Belt and possible correlatives of the Hall Bay Volcanics (Swain et al. 2005) (Fig. 7a). These volcanic successions are interpreted to represent back-arc magmatism associated with west- to south-dipping subduction (Swain et al. 2005). Flanking the Archaean basement terranes is the c. 1850 Ma Donington Suite. (Fig. 7). This suite forms a prominent 600 km north-trending belt of granitoids along the eastern Gawler Craton (Hand et al. 2007). These rocks correlate with the Kalkadoon Batholith in the Mount Isa Terrane, and in the reconstruction of Giles et al. (2004) form a continuous magmatic belt. The Donington Suite has an elevated incompatible element signature, 1Nd(1850Ma) values between –2 and –4, and 1Hf(1850Ma) values between 24 and +5.3, suggesting derivation from fractionated mafic crust, with contamination from Archaean crust (Hand et al. 2007; Reid et al. 2008). Reid et al. (2008) suggested that the Donington Suite formed in a continental back-arc region based on the absence of subduction-related geochemical characteristics. An alternative interpretation is that the wide range of radiogenic isotope values reflects the mixing of Archaean and juvenile magma sources. In either situation, magmatism occurred proximal to a plate margin, consistent with the interpretation of the Kalkadoon Batholith. Immediately following the emplacement of the Donington Suite, the eastern Gawler Craton underwent an episode of compressional orogenesis during the c. 1850– 1845 Ma Cornian Orogeny (Reid et al. 2008). This event is characterized by north-directed transport and the development of non-coaxial folds. The Cornian Orogeny is also characterized by a clockwise P–T path with peak metamorphic granulite facies conditions of 7308C and 6 kbar (Reid et al. 2008) followed by decompression associated with south-block-down extension. Given the post-granite emplacement timing of the Cornian Orogeny (c. 1850–1845 Ma), it may correlate with the collision between the Numil Terrane and the Mount Isa Terrane following the emplacement of the Kalkadoon Batholith. Kalinjala Shear Zone: a suture? The north- to NNW-trending Kalinjala Shear Zone extends for several hundred kilometres from the southern to the central Gawler Craton. It is interpreted to define the suture between the Archaean nucleus of the Gawler Craton (Sleaford Complex) and the continental ribbon (comprising the Donington Suite and c. 1960–1920 Ma Corny Point Paragneiss; Zang 2002; Fig. 7). This suture is stitched by sedimentary successions deposited between c. 1790 and 1740 Ma. These successions are extensively preserved in the Nawa terrane (Fig. 7; Payne et al. 2006), and the upper Hutchison Group and Wallaroo Group in the eastern Gawler Craton (Szpunar et al. 2011). The Kalinjala Shear Zone was subsequently overprinted by structures formed during the c. 1740–1690 Ma Kimban Orogeny (Vassallo & Wilson 2002; Stewart 2010). Regional seismic data (line 08GA-G1: Figs 7 & 8a) suggests the Kalinjala Shear Zone is coincident with a significant disruption of the Moho (Fig. 8b & c). Two sutures are interpreted with in the broader shear zone (Fig. 8c). The Kalinjala Shear Zone defines several major fault splays, which resemble a crustal-scale positive flower structure bounded by the suture zone (Fig. 8c). The Donington Suite is only preserved to the east of the Kalinjala Shear Zone and has no temporal equivalent west of the shear zone, suggesting that the Kalinjala Shear Zone is a c. 1850–1840 Ma suture that separates the Cornian Orogen from the Archaean nucleus of the Gawler Craton. A second west-dipping suture, separating the Cornian Orogen from a basement terrane to the east, is characterized by a major step in the Moho with reverse offset (Fig. 8c). Kimberley Craton – Pine Creek Inlier The Kimberley Craton and the Pine Creek Inlier (Fig. 9) record a period of complex arc accretion that culminated with collision between the Kimberley Craton and proto-North Australian Craton during the c. 1850–1820 Ma Halls Creek Orogeny (Bodorkos et al. 1999; Sheppard et al. 1999, 2001; Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. Fig. 7. Simplified tectonic element map of the Gawler Craton showing the location of the major terranes and terrane boundaries. Location of seismic line interpreted in Figure 8 is also shown. The map shows the oroclinal geometry of ribbon terranes defining the Gawler Craton with superimposed Mesoproterozoic magmatism in the central part of the craton. Tyler et al. 1999) and the King Leopold Orogeny (Fig. 9). This orogen comprises three distinct tectono-metamorphics zones (Fig. 9a, b). The western and central zones of the Halls Creek Orogen form part of the Kimberley Craton, whereas the eastern zone is part of the proto-North Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA Fig. 8. (a) Line diagram seismic interpretation of the eastern Gawler Craton superimposed on seismic data collected along traverse 08GA-G1 showing major structural elements and the geological boundaries. (b) Line diagram seismic interpretation superimposed on the MT data. (c) Geological interpretation along traverse 08GA-G1showing major sutures between the Archaean nucleus of the craton, the Donington Suite and the Numil Terrane. Location of the seismic traverse is shown in Figure 7. Australian Craton. The Halls Creek Orogen and the King Leopold Orogen vary in strike by c. 908 and form a regional apparent orocline around the Kimberley Craton (Fig. 9). The oldest rock recorded in the Western Zone are turbidites of the Marboo Formation, which were deposited between c. 1870 and 1865 Ma (Bodorkos et al. 2002), which underwent low- to medium-grade metamorphism at c. 1860 Ma (Tyler et al. 1999). Apparently postcollision high-K plutons were emplaced contemporaneous with metamorphism (Griffin et al. 2000). The Central Zone comprises a package of turbidites (Tickalara Metamorphics) containing zircon populations as young as c. 1865 Ma (Bodorkos et al. 2000; Sheppard et al. 2001). Coincident with turbidite deposition was the emplacement of the Paperbark Granite Suite at c. 1865 Ma and regional high-temperature, low-pressure metamorphism associated with the c. 1865–1850 Ma Hooper Orogeny within the King Leopold Orogen and its contiguous parts of the western and central zones (Tyler et al. 1995). The Kimberley Craton was located outboard of the proto-North Australian Craton at this time (Tyler et al. 1999). Intruding these rocks are c. 1860 and 1850 Ma mafic and ultramafic intrusions (Page et al. 1995). The c. 1850 Ma Dougalls Suite consists of tonalites and trondjemite interpreted as island arc-related magmas (Tickalara Arc) that formed above a SE-dipping subduction zone or on attenuated continental crust along the southeastern margin of the Kimberley Craton (Sheppard et al. 1999). This episode of magmatism continued until c. 1835 Ma. The Halls Creek Orogeny records the accretion of Tickalara Arc with c. 1915–1845 Ma passive margin sedimentary succession of the proto-North Australian Craton (Eastern Zone) (Bodorkos et al. 2002), followed by collision between the Kimberley Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. Fig. 9. (a) Location of the interpreted suture zone between the Kimberley Craton and the North Australian Craton superimposed on composite vertical derivative magnetic anomalies (textured) and reduced to the pole aeromagnetic data (colours) upward continued 6 km to show the mid crustal magnetic features: (b) Simplified geological map of the Hall Creek Orogen. Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA Craton and the proto-North Australian Craton at c. 1820 Ma (Tyler et al. 1995). Peak high-temperature metamorphic conditions of 700 –8008C were attained during this event along with poly-deformation characterized by early recumbent folds overprinted by upright folds (Bodorkos et al. 1999). Collision between the Kimberley Craton and the proto-North Australian Craton is also recorded in the adjacent Pine Creek Inlier (Fig. 2). The Pine Creek Inlier is separated into three domains, the Litchfield Province in the west, the Central Domain and Nimbuwah Domain in the east (Worden et al. 2008). The Litchfield Province is inferred to be part of the Kimberley Craton (Carson et al. 2008). The Central Domain comprises Neoarchaean basement of the Stanley Metamorphics, which are dated at between c. 2545 and 2520 Ma. Worden et al. (2008) suggested that the Central Domain correlated with the Nimbuwah Domain. Several stacked Palaeoproterozoic volcanosedimentary packages deposited between 2050 and 1860 Ma (Worden et al. 2008) in the Pine Creek Inlier. These rocks were deformed during the c. 1861 and 1847 Ma Nimbuwah Event (Worden et al. 2008), which correlated with the Hooper Orogeny. The Nimbuwah Event is characterized by upright folding that increases in intensity in the Nimbuwah Domain to the east (Worden et al. 2008). Peak high-temperature, low-pressure upper amphibolite or granulite facies metamorphism occurred in the Litchfield Province and Nimbuwah Domain, whereas greenschist facies metamorphic conditions occurred in the Central Domain (Carson et al. 2008; Worden et al. 2008). Peak metamorphism is dated at c. 1855 Ma in the Litchfield Domain (Carson et al. 2008). Hollis et al. (2014) interpreted the c. 1870– 1860 Ma Cahill Formation and Nourlangie Schist to represent a sedimentary package derived from a Neoarchean juvenile felsic basement source, which has been correlated with the Gawler Craton. These rocks are interpreted to represent deposition at a plate margin before the accretion of the Litchfield and Central domains with the Nimbuwah Domain during the Nimbuwah event at c. 1865– 1855 Ma (Hollis et al. 2014) before the docking of the Kimberley Craton. Carson et al. (2008) reported upper amphibolite facies assemblages within the Welltree Metamorphics, which is dated at 1813 + 3 Ma and may correlate with the Halls Creek Orogeny, and records the final amalgamation of the Kimberley Craton/Pine Creek Inlier with the proto-North Australian Craton. Halls Creek Orogen suture Unlike many of the sutures zones within central, southern and eastern Proterozoic Australia, there are no seismic constraints on the geometry of the Halls Creek Orogen. The Halls Creek Orogen probably preserves two sutures. The first bounds accreted western and central zones with the Eastern zone. This boundary coincides with the Halls Creek Fault Zone, a large north–south-trending fault zone that bounds the arc-related rocks of the Central Zone from the Eastern Zone (Fig. 9b). Structural analysis of this fault and associated splays suggests early ductile dextral movement associated with the formation of the Kimberley Basin, followed by a sinistral overprint (White & Muir 1989). These movements probably represent post-collisional reactivation of the Halls Creek Fault Zone. The Halls Creek Fault Zone is coincident with a shallow gradient in regional Bouguer gravity data (Fig. 10), which is interpreted to result from the juxtaposition of relatively dense rocks (2790 kg/m3) of the Central Zone with relatively low-density rocks (2720 kg/m3) of the proto-North Australian Craton and the overlying Proterozoic basins (Fig. 10). Forward modelling suggests an east-dipping suture between the Central Zone (Kimberley Craton) and the Eastern Zone (proto-North Australian Craton). The dip varies from moderate (c. 608) to a depth of 7 km before progressively becoming shallower (45 –308) at depth. The boundary between the Western Zone and the Kimberley Craton is modelled as a steep structure with an architecture that resembles a positive flower structure in the upper crust. The significance of this structure is poorly defined. Collision between the West Australian Craton and North Australian Craton Rudall complex The final micro-continent to accrete to the protoNorth Australian Craton during assembly of Australia was the West Australia Craton. The geological record for the collision of the West Australian Craton and the proto-North Australian Craton is preserved in the Rudall Complex on the northern margin of the West Australia Craton (Kirkland et al. 2013a). The Rudall Complex (Fig. 11) comprises the Talbot, Connaughton and Tabletop terranes (Bagas 2004). The two southern-most terranes, the Tabletop and Connaughton terranes, comprise meta-volcanic and meta-sedimentary successions, variably intruded by mafic igneous rocks. The Connaughton Terrane is more intensely deformed and metamorphosed than the Tabletop Terrane. Intrusive rocks of the Connaughton Terrane contain the Kalkan Suite granite plutons intruded during the c. 1800– 1765 Ma Yapungku Orogeny (Smithies & Bagas 1997), whereas the Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. Fig. 10. Bouguer Gravity and regional aeromagnetic forward model of the Halls Creek Orogen showing the location of major crustal sutures. Model geometry, including the modelled density distribution, is shown in the bottom panel. Location of the profile is shown in Figure 9a. Tabletop Terrane contains bimodal and felsic magma suites emplaced between c. 1590 and 1560, 1476, and 1310 Ma (Bagas 2004; Kirkland et al. 2013a). Meta-sedimentary successions from these terranes have c. 2300 Ma maximum depositional ages (Kirkland et al. 2013a). The Talbot Terrane in the northern and western parts of the Rudall Complex comprises poly-deformed metasedimentary rocks and felsic intrusives (Kirkland et al. 2013a). Ortho-gneiss containing Mesoarchaean and earliest Palaeoproterozoic inherited zircon populations suggests that it is underlain by older basement. The maximum depositional age of meta-sedimentary successions is c. 1791 Ma (Kirkland et al. 2013a). The Talbot Terrane was intruded by c. 1800 and 1765 Ma Kalkan Suite granites and smaller plutons at c. 1450 Ma (Kirkland et al. 2013a). Hf isotope data from inherited zircon within the Kalkan Suite and similarities in detrital zircon populations and the timing of sedimentation suggest that the Rudall Complex is part of a broader basinal system that developed on the West Australian Craton (Kirkland et al. 2013a). Hf isotope data also suggest a basement similar to the Pilbara Craton, supporting passive seismic data indicating attenuated Archaean crust to the north of the Pilbara Craton (Reading et al. 2012). Juvenile Hf isotope signatures from the younger granites suites suggest that a crust-forming event occurred at c. 1950 Ma (Kirkland et al. 2013a). Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA and Australia and Laurentia (Zhang et al. 2012a). These correlations are explored from a geological perspective below. Australia– India correlation Fig. 11. Simplified geological map of the Rudall Complex showing the distribution of internal terranes. Deformation during the subsequent Yapungku Orogeny is characterized by isoclinal folding, and the sequential east-verging thrust stacks (Smithies & Bagas 1997). In the Connaughton Terrane, crustal thickening of approximately 35 –40 km was accompanied by medium-pressure metamorphism (12 kPa, 8008C) with a strongly decompressive clockwise P–T path (Clarke 1991). The timing of pre- and post-tectonic granite emplacement constrains the crustal thickening between c. 1790 and 1765 Ma (Smithies & Bagas 1997; Bagas 2004). No seismic reflection data image this orogen; however, given that the Rudall Complex has some of the highest-pressure metamorphic conditions in Proterozoic Australia, a suture is likely to the north or east of the Rudall Complex. Australian connections in Nuna Recent palaeomagnetic constrained reconstructions of Nuna suggest geographic relationships between Australia and India, Australia and Antarctica, There is little geological evidence for a plate margin to the north of Australia during the Palaeoproterozoic and thus the North Australian Craton may have been part of a larger land mass in the interior of Nuna. This is supported by Fe-speciation in c. 1730 Ma sedimentary rocks, which suggests basin conditions similar to the modern-day Black Sea (Planavsky et al. 2011). Some configurations of Nuna place India beside East Antarctica (Zhao et al. 2002; Yakubchuk 2010; Fig. 1a, b). However, the Nuna configurations of Meert et al. (2011) and Zhang et al. (2012a) place southern India (southern Granulite terrane; Dhawar Craton), Bastar Craton and Eastern Ghats Granulite Province (Dasgupta et al. 2013) adjacent to the North Australian Craton (Fig. 1c). Meert et al.’s (2011) configuration is relevant for c. 1850–1840 Ma and, if correct, it is possible that c. 1850 Ma accretionary orogenic belts of the Eastern Ghats Granulite Province (Dasgupta et al. 2013) correlate with the Halls Creek Orogen (Sheppard et al. 1999). Accretion of c. 1850 Ma ophiolite complexes in the Eastern Ghats (Vijaya Kumar et al. 2011) and the Tickalara Arc in Hall Creek Orogen constrains collision between the proto-North Australian Craton and India, and Australia after c. 1850 Ma. If India and Australia were connected in the configuration of Zhang et al. (2012a), the Dharwar and Kimberley cratons may be segments of the same land mass (see Fig. 1c). Tentative correlations between the Bangemall Basin (West Australia Craton) and the Cuddapah Basin of eastern India (Piper 2010) suggest that intracratonic basin of the Bastar and Dharwar Cratons may correlate with the Palaeoproterozoic basins systems developed throughout the North Australian Craton. Radhakrishna et al. (2013) suggested a correlation between India and the West Australian Craton, also consistent with these basin correlations. A connection between India and the North Australian Craton is supported by detrital zircon populations from the Ongole Domain in the Eastern Ghats Granulite Province and the North Australian Craton (Henderson et al. 2014), suggesting that they may have been contiguous before docking with the Dharwar Craton at c. 1800 Ma. Dharma Rao et al. (2011) reported c. 1333 Ma ophiolites in Eastern Ghats Granulite Province and suggested a long-lived convergent margin between c. 1800 and 1330 Ma. In this scenario, the Mesoproterozoic basins along the Eastern Ghats Granulite Province may have formed in a foreland setting. An implication of this interpretation is that Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. Eastern Ghats must have faced a long-lived ocean for more than 500 myr, which is used by Pisarevsky et al. (2013, 2014) to support a connection between the Dharwar Craton and Baltica at c. 1460 Ma, a configuration also preferred by Henderson et al. (2014). If correlations between the North Australian Craton and India are correct then India must have been translated along the Nuna margin after 1600 Ma to lie adjacent to the Rayner Terrane of Antarctica during Rodinia times (Boger 2011). Mesoproterozoic ophiolites of the Eastern Ghats Granulite Province may be associated with such a translation and have little relevance with the older c. 1800 Ma ophiolites. A c. 1460 Ma palaeopole from the Lakhna dykes on the Indian continent (Pisarevsky et al. 2013) precludes a link between India and North Australia at this time, if one assumes that North Australia remained directly linked to low-latitude NW Laurentia (Pisarevsky et al. 2014). Australia – Antarctica correlations The connection between Australia and Antarctica has persisted since the Archaean (Boger 2011; Payne et al. 2009). Geological correlations exist between Archaean basement rocks of the Sleaford Complex (Daly et al. 1998) and similar rocks of Terra Adelie Land in Antarctica (Fanning et al. 1995) on the opposing conjugate margins of the Southern Ocean. Betts et al. (2008) correlated the Kimban and Strangways orogenies with the c. 1730–1720 Ma Nimrod Orogeny (Transantarctic Mountains; Goodge et al. 2001) and interpreted these orogens to have formed in an accretionary orogeny at c. 1740 Ma. Circa 1600 Ma granites from Terra Adelie Land (Peucat et al. 2002) are likely to be correlatives with the c. 1600–1580 Ma Hiltaba Granite Suite in the Gawler Craton (Daly et al. 1998). Together the Gawler Craton and East Antarctica are interpreted to form a major crustal element termed the ‘Mawson Continent’ (Fanning et al. 1995; Cawood & Korsch 2008; Payne et al. 2009; Boger 2011). In recent reconstructions the Mawson Continent is interpreted as a microcontinental ribbon that connected orogenic systems of Australia and Laurentia (Betts et al. 2008, 2011). Mesoproterozoic connections between Australia and Antarctica include correlations between the Nornamlup Belt rocks of the Albany Fraser Belt and rocks of the Windmill Island (Zhang et al. 2012b). These rocks preserve evidence for two episodes of orogenesis that correlate to the evolution of the Albany Fraser Orogen (Clark et al. 2000; Duebendorfer 2002; Bodorkos & Clark 2004). Detrital and inherited zircon populations of c. 1400–1450, c. 1600–1500 and c. 1700–1800 Ma from the Windmill Island correlate with major events in Proterozoic Australia. Australia– Laurentia correlation Palaeomagnetic data (Betts et al. 2008; Payne et al. 2009) suggest that Australia was positioned adjacent to Laurentia in a SWEAT-like position (Fig. 1c). This configuration has been interpreted for c. 1740 Ma (Betts et al. 2008) and c. 1590 Ma (Payne et al. 2009). This configuration also reconciles most geological criteria (Karlstrom et al. 2001; Betts et al. 2008; Doe et al. 2012), including correlation with major episodes of accretion at the convergent margin and evolution of interior extensional basins (Thorkelson et al. 2001a; Rainbird et al. 2003; Rainbird & Davis 2007; Betts et al. 2008). Payne et al. (2009) suggested that the Sask Craton (central Laurentia) may have once been contiguous with the Gawler –Terra Adelie crust. This interpretation was based on correlations of evolved Nd-model ages (c. 3400–2800 Ma; Swain et al. 2005), comparison of c. 2720–2600 and c. 3000– 2800 Ma detrital zircon populations from Archaean lithologies of the Gawler Craton with the age of magmatic suites and detrital zircon populations in the Sask Craton (Payne et al. 2009), and similarities in the timing of Neoarchaean to earliest Palaeoproterozoic orogenic events in the Sask Craton and Trans-Hudson Orogen (Chiarenzelli et al. 1998; Rayner et al. 2005) and the Gawler Craton (c. 2450 Ma: Sleafordian Orogeny; Swain et al. 2005). We consider any connection between the Gawler Craton and Sask Terrane at c. 1800 Ma unlikely during the amalgamation of Nuna based on palaeogeographic constraints (Zhang et al. 2012a). The Wopmay Orogen represents a series of arc terranes and micro-continents accreted between c. 1800 Ma and c. 1840 along the Coronation margin of the Slave Craton (Hoffman 1980). The Wopmay Orogen records a transition from passive margin evolution to arc accretion events (e.g. Hottah Terrane; Hildebrand et al. 2010a), which facilitated a switch from east-dipping to westdipping subduction (Calderian Orgeny; Hildebrand et al. 2010a). Arc accretion in the Great Bear Magmatic Zone (Cook et al. 1999; Hildebrand et al. 2010b) and Fort Simpson Terrane (Villeneuve et al. 1991; Pilkington & Saltus 2009; Crawford et al. 2010) during east-dipping subduction (Cook et al. 2005) occurred between c. 1880 and 1840 Ma (Cook 2011). The accretion of these terranes and deformation along the Archaean margin of the Slave Craton is intimately linked to dextral transcurrent movements on the continent-scale Great Slave Shear Zone (Hoffman 1987). Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA In SWEAT-like palaeogeographic reconstructions, the Wopmay Orogen is positioned proximal to the northeastern margin of Proterozoic Australia. The Wopmay Orogeny is temporally similar to the Barramundi Orogeny recorded in the Mount Isa Terrane (c. 1870–1850 Ma). Geochemical trends of volcanic rocks within the Barramundi and Wopmay orogenies display remarkable uniformity (Hildebrand & Bowring 1984; Etheridge et al. 1987). We interpret the Barramundi and Wopmay orogenies to record the amalgamation of several arc and micro-continent terranes, including the Hottah Terrane (Hildebrand et al. 2010a), Fort Simpson Arc (Crawford et al. 2010), Etheridge Province, Numil Terrane and Abingdon Seismic Province (Korsch et al. 2012) between the larger continental masses of the Slave Craton and the proto-North Australian Craton during Nuna assembly. An extensive basin system developed across Australia between c. 1850 and 1600 Ma (Giles et al. 2002; Betts et al. 2003). These basins cover older Archaean cratonic blocks and are incorporated into younger orogenic belts (e.g. c. 1740–1700 Ma Kimban Orogeny and c. 1600–1500 Ma Isan Orogeny; Betts & Giles 2006). Prominent basins include Kimberley Basin, Birrundudu Basin, McArthur Basin and the extensive superimposed superbasins preserved in the Mount Isa Terrane and Etheridge Province (Georgetown Inlier; Jackson et al. 2000). This basin system is correlated with contemporary basins developed in the Curnamona Province (Giles et al. 2002, 2004; Conor & Preiss 2008; Gibson et al. 2008), the Gawler Craton (Payne et al. 2008; Szpunar et al. 2011), and parts of the West Australian Craton (Allen et al. 2015). Early basin development following assembly of the North Australian Craton is preserved in the Tanami Block, Arunta Inlier, Tennant Creek and Davenport Province (Fig. 2). Sedimentation was dominated by turbidite successions (Tanami Region; c. 1845 Ma; Bagas et al. 2008), fluvial to shallow marine clastic successions (Tennant Creek– Davenport Province; c. 1820–1800 Ma; ClaouéLong et al. 2008), and marine sedimentary and felsic and mafic volcaniclastic successions (Arunta Inlier; Claoué-Long et al. 2008). Extensive rift-sag basins formed between c. 1800 and 1740 Ma are preserved over large tracts of the Australian continent (Allen et al. 2015). In the Mount Isa Terrane and the McArthur Basin (Fig. 1; Jackson et al. 2000; Neumann et al. 2006), fluvial, marginal lacustrine and marine successions and extensive basaltic and bimodal volcanism were deposited during multiple extension events between c. 1780 and 1750 Ma (Eriksson & Simpson 1993; O’Dea et al. 1997a; Rawlings 1999; Neumann et al. 2006; Potma & Betts 2006). These are overlain by post-rift carbonate successions. The basin systems of the North Australian Craton are likely to correlate with c. 1780–1730 Ma sedimentary basins preserved in the Nawa Terrane (Payne et al. 2006), Eyre and Yorke peninsulas (Szpunar et al. 2011; Gawler Craton), the Earaheedy Basin (northern Yilgarn Craton; Pirajno et al. 2009), the c. 1780 Ma Cadney and Reynolds Range packages, and the c. 1760 and 1740 Ma Ledan Package in the Arunta Province (Allen et al. 2015). Basins across the North Australian Craton between c. 1740 and 1715 Ma formed in hypersaline and euxinic basin conditions (Shen et al. 2002), suggesting that the basins did not face an open ocean. Subsequent basin inversion (Betts 1999) is related to accretionary tectonics along the southern margin of the continent (Giles et al. 2002; Betts & Giles 2006). Basins developed in the interval between c. 1710 and 1670 Ma are preserved in the Mount Isa Terrane, McArthur Basin, Curnamona Province, Etheridge Province and the central Gawler Craton (Fig. 1). Sedimentation occurred during crustal extension (Daly et al. 1998; Betts et al. 1999; Conor & Preiss 2008; Gibson et al. 2008; Howard et al. 2011a) with an overall sedimentation pattern suggesting greater basin subsidence in the eastern provinces. Continued basin development in Australia (Isa Superbasin cycle: c. 1668–1595 Ma) is preserved from the Curnamona Province through to the Birrindudu Basin and possibly the Kimberley Basin in NW Australia (Allen et al. 2015). Early basin development was preceded by extension and magmatism (Gibson et al. 2008) and deposition hiatus (Conor & Preiss 2008). Rapid subsidence and deposition within shallow-water environments occurred in the eastern parts of the continent (Conor & Preiss 2008; Southgate et al. 2000), whereas fluvial environments prevailed to the west (e.g. McArthur Basin; Southgate et al. 2000; Conor & Preiss 2008; c. 1665 and 1650 Ma). An overlying carbonate blanket developed on a regional platform (c. 1645 Ma) with shallow- to moderately deep-water, anoxic basins conditions (Shen et al. 2002; Planavsky et al. 2011). Basins deepen to the east and are dominated by outer-shelf and deep-water environments (Conor & Preiss 2008), consistent with passive margin interpretations (Betts et al. 2003; Lambeck et al. 2012). A basin inversion (Southgate et al. 2000) and associated tectonic uplift are coincident with the accretion of the Warumpi Province (Scrimgeour et al. 2005) at c. 1645 –1640 Ma followed by an influx of deep-marine turbidites and shelf siliciclastic and carbonate sediments (Southgate et al. 2000). Equivalent basins in Laurentia (c. 1850– 1700 Ma) include the Thelon, Baker Lake, Athabasca and Wernecke basins (Thorkelson et al. 2001b; Rainbird et al. 2003; Rainbird & Young 2009). These basins are preserved in the western Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. Churchill Province and the Yukon Territories of northern Canada. The Thelon Basin, Baker Lake Basin and Athabasca Basin were deposited during c. 1845 and c. 1720 Ma crustal extension and contain predominantly fluvial clastic successions (Rainbird 2004). Basin development after c. 1720 Ma is characterized by massive sand sheets with transient marine incursions (Rainbird & Young 2009) interpreted to represent the onset of post-rift sedimentation. The Wernecke Basin (Thorkelson et al. 2001a, 2001b) in NW Canada contains a thick (c. 13 km) pile of c. 1800 and 1700 Ma mudstone and carbonate sediments. The c. 1735 and 1665 Ma Amundsen Basin correlates with the Wernecke Basin (MacLean & Cook 2004) but is dominated by fluvial and shallow marine sedimentary rocks overlain by carbonates, shales and bimodal volcanic rocks (Bowring & Ross 1985; Ross 1986; Thorkelson et al. 2001a; MacLean & Cook 2004). Unlike Australia, there is little record of c. 1670–1600 Ma basins in northern Laurentia, which has been interpreted to support interpretations that Australia and Laurentia may have separated by this time (Betts et al. 2003; Lambeck et al. 2012). Australia and southern Laurentia record a protracted evolution of crustal accretion during the Palaeoproterozoic (Karlstrom et al. 2001; Betts et al. 2008, 2011; Cawood & Korsch 2008; Payne et al. 2009). This accretionary orogen is characterized by protracted episodes of crustal extension, interrupted by transient episodes of crustal and island arc accretion and associated orogenic events (Betts et al. 2011). Correlation of orogenic events in Australia and Laurentia are presented in Karlstrom et al. (2001) and Betts et al. (2008, 2011). Regional orogenic correlations include: (1) the c. 1800–1770 Ma Yapunku–Yamba events in Australia (Collins & Shaw 1995; Smithies & Bagas 1997; Bagas 2004) and the c. 1780 Ma Medicine Bow event in Laurentia (Duebendorfer & Houston 1986, 1987; Chamberlain et al. 1993); (2) the c. 1735– 1710 Ma Strangways and Kimban orogenies in Australia and the c. 1740–1720 Ma deformation events (Yavapai Orogeny) in the Mojave and Yavapai provinces (Laurentia) (Karlstrom & Bowring 1988; Duebendorfer et al. 2001; Whitmeyer & Karlstrom 2007); and (3) c. 1640 Ma Leibig Orogeny (Scrimgeour et al. 2005) and its correlatives in the western Gawler Craton (Oldean event of Hand et al. 2007) with the Mazatzal Orogeny (c. 1650–1600 Ma) in the Mazatzal Province, and the Labradorian Orogeny in Labrador, Canada (Gower et al. 1992; Whitmeyer & Karlstrom 2007). The later stages of the Maztazal Orogeny overlaps with arc-related magmatism of the St Peter Suite (Swain et al. 2008), the Wartakan Orogeny in the Gawler Craton (Fig. 2; Hand et al. 2007; Stewart & Betts 2010) and the Olarian Orogeny in the Curnamona Province (Fig. 2; Willis et al. 1983; Forbes & Betts 2004; Forbes et al. 2008). Accretion along southern Laurentia mainly involved accretion of juvenile arc terranes, although some isotopically evolved crust was also accreted (Hill & Bickford 2001; Karlstrom et al. 2001; Duebendorfer et al. 2006; Bickford et al. 2008). Accretion along the southern margin of Australia appears to have involved terranes that were derived from the proto-North Australian Craton (Betts et al. 2011). Between c. 1800 and 1500 Ma southern Australia and southern Laurentia formed a continuous accretionary orogenic system that faced an external ocean. Discussion Amalgamation of the North Australian Craton We discuss the amalgamation using the proto-North Australian Craton as the reference frame and the nucleus of accretion, although we acknowledge that the proto-North Australia Craton probably represents a small micro-continental fragment of Nuna. Assembly of Australia was relatively rapid with two-thirds of the continent amalgamating in a 70 – 80 myr period. Assembly involved Archaean cratons, ribbon micro-continents and possible arc terranes. Our envisaged reconstruction of Australian terranes is represented in Figure 12. Collision of the Aileron Province and the proto-North Australian Craton pre-dates c. 1840 Ma (Fig. 13a). The size of the accreted microcontinental ribbon remains uncertain. Isotopic evidence suggests a link between the Aileron Province and northern Gawler Craton (Payne et al. 2006) during the Proterozoic. This interpretation is supported by the lack of a clear crustal discontinuity in regional seismic data (Korsch & Kositcin 2010). We propose that the combined Aileron Province and the Nawa terrane of the Gawler Craton represents a single micro-continental ribbon herein referred as the ‘Aileron Terrane’. We interpret the Willowra Suture to represent the remnants of a south-dipping subduction zone with the proto-North Australian Craton positioned in the down-going plate and the Aileron Terrane located in the overriding plate (Fig. 5). This interpretation is consistent with the presence of arc magmatism in the Aileron Terrane (Atnarpa Igneous Complex: Zhao & Cooper 1992) and the absence of arc magmatism in the Tanami Province. The Kalkadoon Batholith and the Donington Suite granites form a .600 km linear belt (Fig. 12) developed in an arc or back-arc tectonic setting along the eastern margin of the proto-North Australian Craton and the Gawler Craton. This magmatic belt is interpreted to have formed in response to Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA Fig. 12. Reconstruction of Australia fragments merged during Nuna amalgamation. The reconstruction joins major suture zones and retro-deforms the Gawler Orocline into linear ribbons. The long-wavelength (mid crust) magnetic response of individual terranes is used to refine the reconstruction. The reconstruction demonstrates how the assembly of Australia involved accretion of a series of ribbon micro-continental terranes along the southern and eastern margins of the continent. (a) The final interpretation. (b) Interpretation with superimposed magnetic data. Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. Fig. 13. Interpreted tectonic map of the northwestern margin of Australia: (a) c. 1870–1850 Ma before the collision of the Aileron Terrane onto the southern margin of the proto-North Australian Craton. Formation of an arc terrane between c. 1910 and 1880 Ma above a south-dipping subduction zone. (b) c. 1850–1840 Ma. Accretion of the Aileron Terrane to the proto-North Australian Craton. Deposition of the Killi Killi Formation and Lander Package, which stitches these terranes together. Formation of the Tickalara Arc above a SE-dipping subduction zone. Extension in the Tanami Province is caused by trench retreat (roll-back) along the convergent margin to the NW. (c) Initial collision of the Kimberley Craton with the proto-North Australian Craton and the onset of the Halls Creek Orogeny. Deposition of the Ware Group in the overriding plate and the onset of escape tectonics of the proto-North Australian Craton around the tip of the Kimberley Craton. (d) Formation of the Hall Creek–King Leopold Orocline as subduction retreats around the rear of the Kimberley Craton resulting in highly attenuated crust to the south of the Kimberley Craton (King Leopold Orogen) and to the west of the Aileron Terrane. This interpretation is highly speculative but explains the regional structural architecture and the relationships between basins developed on the proto-North Australian Craton and orogenesis on the plate margin. the west-dipping subduction of oceanic lithosphere that separated the Mount Isa and Numil terranes. This interpretation requires the Gawler Craton and the proto-North Australian Craton to be amalgamated before c. 1845 Ma, which is consistent with the Gawler Craton accreting with the Aileron Terrane to the proto-North Australian Craton. Accretion of the Numil Terrane along the eastern margin of the Mount Isa Terrane must post-date the emplacement of the Kalkadoon Batholith at c. 1850 Ma. A minimum age for the timing of accretion is c. 1800 Ma, which is constrained by the age of extensional basins that overprint the west-dipping Gidyea Suture. The eastern margin of the Numil Terrane is bounded by a fossil west-dipping subduction zone, termed the Rowe fossil subduction zone, imaged in regional seismic reflection data (Korsch et al. 2012). This subduction zone separates the Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA Numil Terrane from the buried Abingdon Seismic Province (Korsch et al. 2012). The suture associated with this fossil subduction zone projects to the upper crust beneath the Etheridge Province (Korsch et al. 2012; Fig. 3) and therefore has a minimum age of c. 1700 Ma (the oldest ages of the basins succession of the Etheridge Province), although accretion of these two terranes could be significantly older. We speculate that the Abingdon Seismic Province represents a piece of Laurentia stranded on the Australian continent during c. 1660 Ma break-up. The easternmost suture zone imaged in the Gawler Seismic line (08GA-G1; Fig. 7a) has a comparable geometry and spatial association with c. 1850 Ma granites as the Gidyea Suture, suggesting that they may correlate. An implication of this interpretation is that the Cornian Orogen is equivalent to the Mount Isa Terrane. The correlation of the Gidyea Suture Zone and the suture along the eastern edge of the Cornian Orogen requires Proterozoic basement terranes to the east of the Cornian Orogen, such as the Curnamona Province, which we interpret to be equivalent to the Numil Terrane. A west-dipping suture is interpreted from geophysical data (Williams et al. 2010) and may represent a segment of this suture in the Curnamona Province. Betts et al. (2008) proposed the accretion of the Gawler Craton and the along-strike equivalent terranes (Mawson Continent) onto the edge of the proto-North Australian Craton during the Kimban Orogeny. This interpretation is not supported by the basin record, which suggests that the Gawler Craton was connected to the North Australian Craton at c. 1780 Ma (Allen et al. 2015). To explain this, we suggest that the Archaean component of the Gawler Craton was rifted from the margin of Australia during subduction roll back between c. 1840 and 1750 Ma. Seismic data (line 08GA-G1) indicate significant crustal thinning within the broader Kalinjala Shear Zone (Fig. 8c). This possibly represents a zone of intense crustal extension, micro-continent break-off and mafic underplating that was superimposed on the existing Cornian suture. We suggest that this micro-continental fragment (Gawler Craton) was re-accreted onto the southern margin of Australia during the Kimban Orogeny, resulting in relatively high-pressure metamorphism preserved along the Kalinjala Shear Zone (Dutch et al. 2008). The Kalinjala Shear Zone therefore may represent a reactivated suture zone inherited from the Cornian Orogeny. Along the northwestern margin of Australia, subduction is indicated by the formation of the Tickalara Arc between the Kimberley Craton and protoNorth Australian Craton at c. 1850 Ma (Fig. 13b). The polarity of the subduction zone is uncertain. Our preference is that the arc formed in an intraoceanic setting above a SE-dipping subduction zone (Fig. 13b). Alternatively the arc may have formed on attenuated crustal rocks along the southeastern margin of the Kimberley Craton, above a NW-dipping subduction zone. The c. 1850 Ma Dougalls Suite magmas, emplaced in the central zone, resemble crustal-derived adakitic, low- to medium-K calc-alkaline magmas from modern convergent margins (Sheppard et al. 2001). These magmas require over-thickened crust (Sheppard et al. 2001), suggesting that arc accretion and crustal stacking had initiated by 1850 Ma. A correlation between the Nimbuwah Domain and the Gawler Craton is supported by detrital zircon populations for the Corny Point Paragneiss of the eastern Gawler Craton, which have similar Lu –Hf isotopic signatures to the Nimbuwah Domain (Howard et al. 2009). If this correlation is correct, the Nimbuwah Domain and the Gawler Craton must have been separated between 2000 and 1860 Ma, before collision of the Aileron and proto-North Australia Craton. The Nimbuwah Event has been correlated with the Hooper Orogeny in the Halls Creek Orogen (Worden et al. 2008). This correlation requires connectivity between the entire Pine Creek Inlier and the Kimberley Craton, with both provinces located outboard of the Australian continent during the Hooper Orogeny/Nimbuwah event(?). Alternatively, the absence of ductile deformation during the c. 1855 Ma Nimbuwah event is markedly different from the crustal shortening associated with the Hooper Orogeny (Tyler et al. 1995) and any correlation may be coincidence. Interpretation that c. 1840–1820 Ma basin systems in the Tanami Province (Fig. 2) represent a continental back-arc environment (Bagas et al. 2008) may be valid, but related to subduction along the northwestern margin of the proto-North Australian Craton, rather than the southern margin as initially proposed (Bagas et al. 2008). Amalgamation of the Kimberley Craton and proto-North Australian Craton during the c. 1820 Ma Halls Creek Orogeny (Tyler et al. 1995) defines the completed assembly of the Diamantina Craton (combined North and South Australian cratons). This event may be part of a larger continental collisional orogen in which Indian and Australian continents amalgamated and the orogenic belts of NW Australia extend into the Eastern Ghats in India. We propose a Halls Creek –King Leopold orocline (sensu-stricto), which formed at the southern tip of the Kimberley Craton (and its continuation into India). Collision between the Kimberley Craton and the proto-North Australian Craton resulted in a domain characterized by intense crustal shortening in the Halls Creek Orogen. However, dolerite dyke emplacement at c. 1820–1800 Ma in the King Leopold Orogen (Tyler et al. 1999) is Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. consistent with crustal extension. In the overriding plate a second domain (Fig. 13c), developed distal from the collision zone, is characterized by crustal extension associated with trench retreated and subduction roll-back beyond the southern tip of the Kimberley Craton (Fig. 13c). Wrapping of the convergent margin around the southern Kimberley Craton would have created a highly arcuate convergent margin as subduction attempted to re-establish behind the Kimberley Craton (Fig. 13d). Large areas of the overriding plate may have been translated into the space created by the retreating trench. This region is preserved to the west of the Arunta Inlier and the north of the Mesoproterozoic Paterson Orogen and is now buried beneath the Palaeozoic Canning Basin. However, the long-wavelength geophysical anomalies suggest that the structural grain of the basement in the overriding plate is parallel with the trend of the orocline. This result is comparable to micro-continent dynamic models of Moresi et al. (2014), which show how congested margins re-establish into stable plate margins. The last major collision event during the amalgamation of the Australia portion of Nuna was the accretion of the West Australian Craton and the North Australian Craton. Palaeomagnetic data suggest that amalgamation pre-dates 1700 Ma (Li 2000), with the most likely geological event that records this accretion represented by the Yapungku Orogeny, which is characterized by medium-pressure metamorphic conditions (Clarke 1991) within the Rudall Complex (Fig. 11). The Rudall Complex has been interpreted as a collision zone (Betts & Giles 2006), but this notion is dispelled by geochronology and isotopic data that indicate the Rudall Complex has affinities with the eastern Pilbara Craton and the Capricorn Orogen, both of which reside within the West Australian Craton (Kirkland et al. 2013a). No seismic reflection data image this orogen; however, given that the Rudall Complex has some of the highest-pressure metamorphic conditions in Proterozoic Australia, a suture is likely to occur to the north and east of the Rudall Complex. The Yapungku Orogen temporally overlaps with the Yambah Event in the Arunta Inlier (Betts & Giles 2006). The Yambah Event has been interpreted in the context of crustal shortening in several reconstructions (e.g. Betts & Giles 2006), although the high-temperature granulite facies metamorphism associated with this event may reflect regional lithospheric extension (Cutts et al. 2013), and heralds the onset of extensive continental back-arc basin development throughout the Diamantina Craton (Giles et al. 2002). Coincident crustal shortening and extension along the same convergent margin occurs during accretionary orogenesis when subduction attempts to retreat along the convergent margin where accretion has not occurred. This results in intense crustal extension in the overriding plate before a stable convergent margin is re-established (Moresi et al. 2014). Ribbon tectonics v. collisional events A complex issue related to unravelling the amalgamation and evolution of the Australian continent is the uncertainty of the architecture and position of terranes and cratons at the time of amalgamation. There is a propensity to use the present-day geometry of the these terranes and cratons to constrain palaeogeographic reconstructions (Myers et al. 1996; Wade et al. 2006). However, many of the terranes and cratons accreted during the Nuna assembly have been subjected to intense overprinting and modification. For example, the Arunta Inlier has a geometry that trends east –west, which is the assumed architecture during Nuna accretion, despite being significantly modified during Palaeoproterozoic (Scrimgeour et al. 2005), Mesoproterozoic (Morrissey et al. 2011) and Palaeozoic (Maidment et al. 2013). The South Australian Craton has long been considered its own palaeogeographic entity (Myers et al. 1996), but almost certainly came into existence during Mesoproterozoic reconfiguration of the continent (Giles et al. 2004; Cawood & Korsch 2008). The interior structure of the Gawler Craton is characterized by several thin and linear microcontinental fragments, juvenile back-arc basins and arc terranes that are folded to form the regional Gawler orocline (Fig. 7). When unfolded these terranes become relatively simple linear geological belts or ribbons of different age and tectonic derivation. Figure 12 shows our preferred reconstruction of Australia at c. 1800 Ma following the assembly of the continent. The reconstruction is refined by correlating long-wavelength magnetic characteristics of individual terranes. The reconstruction links the major suture zones. The Donington Suite and the Kalkadoon Batholith form a continuous magmatic belt that connects the eastern Gawler Craton with the Mount Isa Terrane. The Peake and Denison Inlier (NE Gawler Craton) is translated such that it honours correlations with the eastern Mount Isa Terrane (Wilson 1987). The Gawler orocline is retro-deformed into a series of linear ribbon terranes. The Nawa Terrane (Fig. 2) and the Aileron Province (Fig. 2) are aligned to form a single linear terrane (Aileron Terrane in Fig. 12), which reconciles their similar geological evolutions (Baines et al. 2011) and isotopic characteristics (Payne et al. 2006; Howard et al. 2011b; Hollis et al. 2014). In this reconstruction the Numil Terrane (Fig. 2) and Curnamona Province (Fig. 2) are correlated. A relatively simple geological pattern of accretion emerges from the Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA reconstruction. Proterozoic assembly of the continent occurred as series of ribbons that initially accreted onto the southern margin of the ProtoNorth Australian Craton. The Aileron Terrane and Gawler Craton are micro-continental ribbons that have comparable isotopic characteristics with the North Australian Craton (Howard et al. 2009; Hollis et al. 2014), suggesting they may have been initially derived from the proto-North Australian Craton. Assembly was then accommodated via accretion of a series of relatively long and narrow micro-continental ribbons, including the Numil Terrane and Abingdon Seismic Province (Fig. 2) onto the eastern margin of Australia (Fig. 14). Supercontinent amalgamations record major continent–continent collision events (Squire et al. 2006; Bradley 2011). From an Australian Nuna perspective the largest of these events are likely to be the collision between Laurentia and Australia following the accretion of the Numil Terrane at c. 1840–1800 Ma and collision between Australia and India (including the Kimberley Craton) at c. 1820 Ma. Both of these collision events are sparsely preserved in the geological record. In eastern Australia the basement terranes likely to record this collision are recognized in seismic reflection data and are now buried beneath Palaeo- and Mesoproterozoic sedimentary basins. The Halls Creek Orogen in NW Australia is likely to represent a small portion of a larger orogenic belt formed during collision of India and Australia. The dimension of the West Australian Craton is comparable to present-day large continental micro-continents such as the Campbell Plateau in the Pacific Ocean. The collision between the West and North Australian Craton is therefore considered to be hybrid between ribbon accretion and true continent– continent collision. Australia faces an external ocean Using a modified-SWEAT configuration (Betts et al. 2008), Betts et al. (2011) argued that the southern margin of Australia and Laurentia faced an external ocean (Murphy & Nance 2003, 2005), unlike many Nuna reconstructions (Zhang et al. 2012a). In this geodynamic setting, subduction rollback (Collins 2002; Cawood et al. 2009) would Fig. 14. Interpreted amalgamation sequence of Australia during Nuna formation. (a) c. 1880 Ma interpreted distribution of individual tectonic elements before amalgamation. (b) c. 1860 Ma accretion of the Mount Isa Terrane onto the eastern margin of the Proto-North Australian Craton. (c) c. 1850–1840 Ma accretion of the Aileron Terrane (Arunta Inlier) and the Gawler Craton (Aileron Terrane) onto the combined proto-North Australian Craton and the western and eastern Mount Isa terranes. (d) 1825– 1815 Ma collision between the Kimberley Craton and the proto-NAC during the Halls Creek Orogeny in the northwestern part of the continent and the accretion of the Numil Terrane onto the margin of the eastern Mount Isa Terrane. (e) c. 1800–1790 Ma collision between the Diamantina Craton and the West Australian Craton in the south and the amalgamation the Abingdon Seismic Province (western Laurentia) and the Numil Terrane along the eastern margin of the North Australian Craton. All reconstructions were undertaken using G-plates (Williams et al. 2012). Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. have been the dominant mode of subduction and this is evident by the protracted extensional basin evolution in Diamantina Craton (Giles et al. 2002) and the development of back-arc basins along the southern Laurentian margin (Doe et al. 2012). This extensional accretionary orogen was punctuated by major, although transient, orogenic events that can be correlated in East Antarctica and Laurentia, which together formed the margin of Nuna. Betts et al. (2011) recognized differences in the isotopic character of the accreted crustal elements on the accretionary margin of Australia and Laurentia. Australia’s accretionary orogen was dominated by reworking of existing continental ribbons or micro-continents that had originated from the Australian parts of Nuna, whereas Laurentia’s accretionary margin was characterized by accretion of juvenile crust (Karlstrom & Bowring 1988) and minor continental lithosphere (Hill & Bickford 2001; Bickford et al. 2008). The southern margin of the Australian continent persisted as an extensional accretionary orogen, not dissimilar to the modern SW Pacific, for more than 350 myr between 1800 and 1450 Ma. For the most part, convergence was accommodated along a principal north-dipping subduction zone, although episodic plate margin instability and accretion may have triggered transient subduction polarity switches that accommodated back-arc inversion (Selway et al. 2009). Interior ocean between Australia and Laurentia? Several authors have recently suggested that Australia (+Antarctica) was an isolated continent for much of the Palaeoproterozoic, with amalgamation with Nuna during the earliest Mesoproterozoic (Eglington et al. 2013; Pisarevsky et al. 2014). Palaeoproterozoic arc-related magmatism (Bonnetia arc terrane) identified in the Wernecke Mountains, northwestern Laurentia (Furlanetto et al. 2013) and the Etheridge Province (Georgetowm Inlier), eastern Australia (Champion 1991; Murgulov et al. 2007; Betts et al. 2009) suggests that an ocean existed between Laurentia and Australia between c. 1710 Ma (Bonnetia arc; Furlanetto et al. 2013) and c. 1550 Ma (Forest Home arc; Champion 1991). A passive margin to this ocean has been interpreted along the eastern margin at Australia at c. 1650 Ma (Lambeck et al. 2012), with increased magmatic activity indicated by juvenile Nd-isotopic responses in volcanic tuffs and sedimentary successions of the Isa Superbasins and equivalent rocks (Lambeck et al. 2012). Interpretations of Australia as a sole wanderer during the Palaeoproterozoic are not supported by the anoxic environments in the McArthur Basin (Shen et al. 2002; Planavsky et al. 2011), which suggests that this basin was not connected to an open ocean north of Australia at c. 1730 or c. 1640 Ma, and must have been part of a larger continental mass. The alternative model is that Australia had amalgamated with Laurentia and was part of Nuna by c. 1740 Ma (Zhang et al. 2012a). This model is supported by correlations of the Barramundi and Wopmay orogenies at c. 1850 Ma (O’Dea et al. 1997b). Interior basins of Laurentia and Australia are correlated between c. 1800 and 1700 Ma (Betts et al. 2008) and several orogenic events are correlated along the convergent margin of southern Laurentia and Australia (Betts et al. 2008; 2011). Nd isotopic data has been used to correlate the Transantarctic Mountains and Miller Ranges (Antarctica) with the Yavapai/Mazatzal provinces (Borg & DePaolo 1994) and Mojave Province (Bennett & DePaolo 1987), respectively. These correlations are consistent with palaeomagnetic data, which allows Australia to be positioned proximal to Laurentia in a SWEAT-like position at c. 1740 Ma (Betts et al. 2008). In order to reconcile all of the geological and palaeomagnetic constraints, we suggest that a connection between Australia and Laurentia may not have persisted for the entire interval following their initial amalgamation during Nuna assembly between c. 1850 and 1740 Ma. Following assembly an interior ocean developed between Australia and Laurentia between c. 1740 and 1710 Ma. Consumption of the ocean had initiated by c. 1710 Ma along the Laurentian margin, whereas the Australian margin remained a passive margin until at least c. 1640 Ma (Lambeck et al. 2012; Fig. 15a). A switch in the polarity of subduction occurred between c. 1640 and 1555 Ma with convergence along the eastern margin of Australia (Fig. 15b). Palaeomagnetic data suggest that Australia and Laurentia were in close proximity at c. 1595 Ma (Payne et al. 2009; Hamilton & Buchan 2010), indicating that the ocean was relatively small. Final closure of this ocean and the re-amalgamation of Australia and Laurentia must have post-dated arc magmatism in the Etheridge Province and is therefore likely to be recorded by the waning stages of the Isan Orogeny (Betts et al. 2006; Giles et al. 2006; Blenkinsop et al. 2008; Fig. 15c). Detrital zircon populations from the lower Fifteenmile Group in Yukon, Canada (Medig et al. 2014) imply derivation from c. 1500 Ma granite suites of the Mount Isa Terrane, suggesting that Laurentia and Australia were connected at this time. We interpret the ocean separating Australia and Laurentia between c. 1710 Ma and c. 1550 Ma as a relatively small ocean that formed during introversion (Murphy & Nance 2003) of Nuna. The configuration of Laurentia and Australia may have been modified during this introversion, although Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 AUSTRALIA AND NUNA Fig. 15. Palaeogeographic reconstructions of Australia and surrounding continents modified after Zhang et al. (2012a). (a) Development of an internal ocean separating Australia and Antarctica from Laurentia at c. 1660 Ma. Passive margin development along eastern Australia and western Laurentia. (b) Onset of the closure of the internal basin at c. 1600 Ma, with subduction beneath the Laurentian and Australian continents. Speculated break-up of India and Australia based on high temperature metamorphism in the Eastern Ghats (Henderson et al. 2014). (c) Continental collision between Australia and Laurentia after c. 1555 Ma. Break-up of Siberia and India from Laurentia and Australia respectively. Position of Siberia modified after Pisarevsky et al. (2014). palaeomagnetic data suggest a modified-SWEAT configuration at the beginning and end of the Wilson cycle (Betts et al. 2008; Payne et al. 2009). If the connection between the Kimberley Craton and the Dhawar Craton in the Nuna configuration of Zhang et al. (2012a) is correct, it requires the break-up of India and North Australia before the Early Mesoproterozoic, when the Eastern Ghats Granulite Province records a protracted history of subduction and accretion (Dharma Rao et al. 2011; Pisarevsky et al. 2013; Henderson et al. 2014). The timing of the break-up is poorly constrained, but ultra-high-temperature metamorphism in the Eastern Ghats Granulite Terrane at c. 1760 and 1600 Ma (Bose et al. 2011) may indicate the lower crustal expression of extension, leading to the separation of Australia and India, with the implication of a post-1600 Ma break-up (Fig. 15c). Alternatives to this model, where India is placed next to Baltica (Pisarevsky et al. 2014), assumes Downloaded from http://sp.lyellcollection.org/ at Monash University on June 4, 2015 P. G. BETTS ET AL. that the Eastern Ghats Granulite Province was continuously adjacent to a convergent margin based on preserved ophiolites dated at c. 1850 and 1330 Ma. However, it is possible that these ophiolites are not related to the same ocean. The 1850 Ma ophiolite is remnant from India–North Australia Craton collision during Nuna assembly, whereas the 1330 Ma ophiolite formed after Nuna break-up. Conclusions The Australian record of Nuna Formation is characterized by a combination of continent– continent collision events and the accretion of micro-continental ribbons. The majority of the continent had accreted during a relatively short interval between c. 1860 and 1800 Ma. Accretion occurred: (1) along the southern margin of the proto-North Australian Craton (Aileron Terrane and Western Australian Craton); (2) along the eastern margin of the protoNorth Australian Craton (Numil Terrane, and Abingdon Seismic Province) terminating with collision with Laurentia; and (3) along the northwestern margin of proto-North Australian Craton (Kimberley Craton). Australia faced a large external ocean to the south (relative to present coordinates), which evolved into a long-lived convergent accretionary orogen dominated by extensional tectonics between c. 1800 and 1450 Ma. This accretionary orogen is not considered in most reconstructions of Nuna, which have Australia occupying the interior part of the Nuna supercontinent. This extensional accretionary orogen was largely controlled by a principal north-dipping subduction zone that consumed the external ocean and played a major role in the reworking and modification of the plate margin, as well as the addition of new juvenile arc and back-arc terrane. Break-up between eastern Australia and western Laurentia is indicated by the intensification of passive margin volcanism followed by oceanic basin formation at c. 1660 Ma. This ocean was subsequently closed by subduction initially along the Laurentian margin. West-dipping subduction along eastern Australia initiated between c. 1640 and 1555 Ma led to the termination of the basin between c. 1550 and 1500 Ma. The break-up history of Nuna is poorly constrained to the north and west of the North Australian Craton. 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