( a ) Bathymetric map of the Southern ocean showing major fracture zones and their inferred along-strike basement equivalents in the adjacent Australian and Antarctic continents. ( b ) Flow lines show relative plate motion determined by various researchers for successive stages of rifting and crustal extension (adapted from Williams et al . 2011). The absence of a common plate vector during the earlier stages of rifting reflects differences in the assumed direction of extension prior to 47 Ma, at which time the north–south fracture zones began to develop. The red line shows the location of deep seismic reflection survey p137–01. 

Contexts in source publication

Context 1

... a late Mesozoic-early Cenozoic intra-continental shear zone located above an even older reactivated Palaeozoic basement struc- ture previously identified as the Woorndoo (Foster & Gleadow 1992) or Moyston Fault ( Fig. 1; Hill et al. 1995;Miller et al. 2002) but interpreted here to be the Avoca-Sorell Fault Zone (Figs 1 and 3). Bathymetric images (Fig. 3) show various strands of this frac- ture zone continuing southward all the way to the ocean-continent boundary in Antarctica, where they assume a position directly along strike from two of the most important basement structures in northern Victoria Land, the steeply dipping Lanterman and Leap year faults (Fig. 1). Both of these ...

Context 2

... large- offset, dextral fracture zones that span the entire Southern ocean between SE Australia and Antarctica ( Fig. 1) and marks a sharp break from east-west-trending, normally rifted continental margin between the Great Australian Bight and Terre Adélie-Wilkes Land in Antarctica and a generally NNW-SSE-oriented continental mar- gin farther east (Fig. 3) in which oblique-and strike-slip segments developed between Tasmania and George V Land (Stagg & Reading 2007). It is no less prominent than the Tasman Fracture Zone but thus far no suggestion has ever been made that it too may be located along strike from a major reactivated basement structure in SE Australia. Published geological ...

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... reflection data collected off the coast of southern Australia by Geoscience Australia ( Fig. 3; line p137-01) confirm the presence of a crustal-penetrating structure in this general location and show that it coincides with a prominent step in the Moho as well as a sig- nificant thinning of the middle to lower crust (Fig. 5a). This struc- ture has produced significant offsets in some of the more prominent horizons making up the ...

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... east, seismic line p137-01 ( Fig. 3a) crosses a reactivated basement structure identified here and elsewhere ( Gibson et al. 2011) as the Avoca-Sorell Fault Zone (Fig. 5b) and with which the Coorong Shear Zone might be usefully compared. The Avoca- Sorell Fault Zone shares the same steep dip as the Coorong Shear Zone and is associated with an even more pronounced step in ...

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... structures and the Avoca-Sorell Fault Zone cannot strictly be regarded as a single dis- crete subvertical structure (Fig. 5b). Both it and the Coorong Shear Coorong and Avoca-Sorell shear zones captured in seismic reflection profile p137-01 oriented parallel to the otway coast and orthogonal to structures of interest (for location of profile see Fig. 3). The thinning of the middle to lower crust beneath the deepest part of the sedimentary (otway) basin should be noted. The basin profile and stratigraphic surfaces are constrained by additional unpublished seismic sections oriented at a high angle to ...

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... share a common north-south orientation and both are continu- ous along strike with prominent fracture zones (Tasman and George V fracture zones) that can be traced all the way to Antarctica (Fig. 3). However, whereas the Avoca-Sorell Fault Zone has an obvious correlative in either the Lanterman or Leap year fault in northern Victoria Land (Fig. 1), the Coorong structure is best matched with the Mertz Shear Zone, which up till now has been regarded as a correlative of the Kalinjala Mylonite Zone (Talarico & Kleinschmidt 2003;Di ...

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... Whittaker et al. (2007) andWilliams et al. (2011) recon- structions are both based on the premise that crustal extension barely deviated from its initial NW-SE or NNW-SSE trajectory during the earlier stages of rifting before assuming a north-south azimuth in the Cenozoic (Fig. 3). Seismic and sequence strati- graphic studies along Australia's southern margin (Norvick & Smith 2001;Krassay et al. 2004;Totterdell & Bradshaw 2004;Blevin & Cathro 2008) indicate that this assumption is unwarranted and point to a more complicated record of rifting wherein NW-SE or NNW-SSE extension in the Bight Basin was superseded by ...

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... continental breakup and eastward shift in spreading, the Coorong Shear Zone lay directly opposite the Mertz Shear Zone (Fig. 6a), either because these two structures had remained contiguous throughout the ear- lier stages of rifting or because any offset induced by initial NW-SE- directed extension had been restored during later NNE-SSW rifting (Fig. 3b). Irrespective of which interpretation is correct, fracture zone development in the Southern ocean off the otway Basin is not uniformly distributed but reaches its maximum expression and intensity directly along strike from the Coorong and Avoca-Sorell fault zones (Figs 1 and 3), indicating some form of basement con- trol. This in turn ...

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... 5. Coorong and Avoca–Sorell shear zones captured in seismic reflection profile p137–01 oriented parallel to the otway coast and orthogonal to structures of interest (for location of profile see Fig. 3). The thinning of the middle to lower crust beneath the deepest part of the sedimentary (otway) basin should be noted. The basin profile and stratigraphic surfaces are constrained by additional unpublished seismic sections oriented at a high angle to p137–01.  ...

Context 10

... and include a tectonically reworked Neoproterozoic–early Cambrian passive margin sequence overlying continental crust of Mesoproterozoic or older age (Berry et al . 1997, 2008; Meffre et al . 2000). Slices of mafic and ultramafic rock overlying this deformed passive margin sequence (not shown in Fig. 1), and bearing a striking resemblance to the island arc–forearc assemblage preserved in the Grampians– Stavely terrane farther west, have been interpreted as parts of a tectonically dismembered ophiolite complex (Crawford et al . 2003). However, neither this dismembered ophiolite nor the underlying passive margin sequence has any obvious physical connection to the rest of the Delamerian Fold Belt. Several faults and a strip of Lachlan Fold Belt intervene (Fig. 1). This has led several researchers to conclude that the rocks of western Tasmania represent an allochthonous crustal block that has been tectonically transported into its present position from elsewhere either through collision and accretion from the east (Cayley 2011) or through orogen-parallel strike-slip faulting from the south during the closing stages of the Delamerian orogeny (Gibson et al . 2011). Irrespective of which interpretation is correct, the rocks of western Tasmania are clearly bounded in the west by the Sorell Fault, a reactivated early Palaeozoic basement structure whose along-strike onshore counterpart is the west-dipping Avoca Fault (Fig. 1). The northern limits of the Tasmanian crustal block are unknown although deep seismic reflection data for the southern part of the Lachlan Fold Belt in central Victoria (Cayley et al . 2011) support the idea that rocks of Tasmanian affinity are not restricted to the region south of Bass Strait but continue northward in the subsur- face for some significant distance beneath parts of southern Victoria (Fig. 1). The western limits of this buried Tasmanian crust (Selwyn Block) are defined by the west-dipping Heathcote Fault, which farther south terminates against the Avoca Fault, thereby juxtaposing rocks of the Delamerian orogen directly against those of the younger Lachlan Fold Belt (Fig. 1). The Avoca Fault and its along- strike offshore equivalent (Sorell Fault) might therefore be expected to constitute a boundary of some considerable tectonic significance across which there was a commensurate and equally abrupt change in crustal rheology and mechanical behaviour at the time of continental breakup between Australia and Antarctica. No less importantly, from the perspective of this paper, is the observation that this crustal boundary merges southward into the Tasman Fracture Zone (Figs 1 and 3). The Tasman Fracture Zone is part transform boundary and evolved from a late Mesozoic–early Cenozoic intra-continental shear zone located above an even older reactivated Palaeozoic basement structure previously identified as the Woorndoo (Foster & Gleadow 1992) or Moyston Fault (Fig. 1; Hill et al . 1995; Miller et al . 2002) but interpreted here to be the Avoca–Sorell Fault Zone (Figs 1 and 3). Bathymetric images (Fig. 3) show various strands of this fracture zone continuing southward all the way to the ocean–continent boundary in Antarctica, where they assume a position directly along strike from two of the most important basement structures in northern Victoria Land, the steeply dipping Lanterman and Leap year faults (Fig. 1). Both of these structures originated in the early Palaeozoic but have since been modified by several phases of later deformation, including an episode of strike-slip faulting in the Cenozoic that affected much of northern Victoria Land and has the same sense of dextral shear (Figs 1 and 3) as the Tasman Fracture Zone offshore (Salvini et al . 1997; Capponi et al . 1999; Rossetti et al . 2002; Kleinschmidt & Läufer 2006; Storti et al . 2007). The Lanterman Fault preserves an even earlier phase of late Cambrian– early ordovician strike-slip faulting and now separates island arc rocks (Bowers terrane) from older continental crust to the west (Wilson terrane) (Weaver et al . 1984; Capponi et al . 1999; Gibson et al . 2011). It is has been widely interpreted as a former subduction zone or palaeosuture (Weaver et al . 1984; Gibson & Wright 1985; Rocchi et al . 1998; Tessensohn & Henjes-Kunst 2005), which, together with the Leap year Fault farther east, is one of the principal structures along which the terranes of northern Victoria Land were assembled and accreted onto the east Gondwana margin (Weaver et al . 1984; Gibson & Wright 1985; Capponi et al . 1999; Tessensohn & Henjes-Kunst 2005). The George V Fracture Zone is the most westerly of the large- offset, dextral fracture zones that span the entire Southern ocean between SE Australia and Antarctica (Fig. 1) and marks a sharp break from east–west-trending, normally rifted continental margin between the Great Australian Bight and Terre Adélie–Wilkes Land in Antarctica and a generally NNW–SSE-oriented continental margin farther east (Fig. 3) in which oblique- and strike-slip segments developed between Tasmania and George V Land (Stagg & Reading 2007). It is no less prominent than the Tasman Fracture Zone but thus far no suggestion has ever been made that it too may be located along strike from a major reactivated basement structure in SE Australia. Published geological maps for this part of Australia (e.g. Cowley 2006) show no such basement structure, although the same cannot be said of Antarctica, where a 5 km wide mylonite zone (Mertz Shear Zone) separating late Archaean–Palaeoproterozoic cratonic basement from early Palaeozoic rocks of the Ross orogen occurs directly along strike from this same fracture zone (Fig. 1; inset). However, its inferred correlative in South Australia is the late Palaeoproterozoic Kalinjala Mylonite Zone (Talarico & Kleinschmidt 2003; Di Vincenzo et al . 2007; Goodge & Fanning 2010) and lies much too far west to be collinear with the George V Fracture Zone. Published flow lines for relative plate motion (Fig. 3) between Australia and Antarctica (Powell et al . 1988; Tikku & Cande 1999; Williams et al . 2011) would also appear to be incom- patible with this correlation. The alternative is to argue for a different palaeogeographical reconstruction in which the Mertz Shear Zone is matched against a different structure in southern Australia. Possibilities include the Gawler Shear Zone and hitherto previously unmapped Coorong Shear Zone in the southern part of the Delamerian Fold Belt (Fig. 4). The Coorong structure occurs along strike from the George V Fracture Zone (Figs 1 and 3) and extends northwards as far as the Curnamona Craton (Fig. 1), where it overlaps a parallel, and presumably temporally related, structural discontinuity (Anabama– Redan Shear Zone) for which a Rodinia breakup age has been proposed (Preiss 2000). It would appear to be the more important structure but owing to little or no outcrop along its length, is best mapped and characterized through geophysical data (Fig. 4) and the interpretation of deep seismic reflection data collected across the structure offshore (Fig. 5). The Gawler Shear Zone is even less well exposed and lies wholly within the southern Gawler Craton (Figs 2 and 4), where it defines the western edge of a basement tilt block upon which shallow water shelf sediments (Ardrossan Shelf) were subsequently deposited (Belperio et al . 1998). Aeromagnetic data for the Coorong Shear Zone and surrounding region, including part of the adjacent continental shelf, are pre- sented as an image in Figure 4. This image is a vertical derivative of the total magnetic intensity aimed at capturing abrupt changes in the potential field gradient that might reveal the presence of major geological structures or discontinuities. The Coorong Shear Zone is particularly conspicuous in this image and juxtaposes two domains with opposing structural trends (Fig. 4). To the west of this shear zone are inverted and tightly folded platform sequences of the Adelaide Supergroup (Nackara Arc) in which structural fabrics generally strike north–south to NNW–SSE, whereas to the east and south are even more intensely deformed rocks of the Kanmantoo Group and its correlatives in which structures trend NNE–SSW (Fig. 4). Cutting across this NNE–SSW fabric, and intruded into the Kanmantoo Group and its correlatives, are variably magnetized post-kinematic granites and minor gabbro for which a late Cambrian–early ordovician age has been determined (Foden et al . 2002 a ). Magmatic rocks of comparable age and magnetic character have also intruded along the Coorong structure itself (Fig. 4), indicating that this structure is no younger in age than late Cambrian– early ordovician and served as a conduit for post-orogenic magmatic intrusion. Geochemical and isotopic data further indicate that the granites were sourced from the Kanmantoo Group, which evidently extends to considerable depth beneath this part of the Delamerian Fold Belt (Foden et al . 2002 a , b ). In contrast, granites of post-Delamerian age are not widely developed west of the Coorong Shear Zone and none have been mapped in the Nackara Arc (Fig. 4). A few granite outcrops occur between the Coorong and Gawler shear zones farther south but they are hosted by the Kanmantoo Group (Belperio et al . 1998) and far removed from the linear belt of post-tectonic granites and subsidi- ary gabbro intruded along the Coorong structure (Fig. 4). Indeed, except for this one area, the Coorong Shear Zone would appear to serve not only as the western limit of late Cambrian–early ordovician bimodal magmatism in the Delamerian Fold belt but also as an important crustal boundary within the original sedimentary basin across which there was an abrupt change from platform (Adelaide Supergroup, Normanville Group) to deeper water sedimentary facies (Kanmantoo Group). Early Cambrian mafic magmatism is similarly much more common east of the Coorong Shear Zone (Belperio et al . ...

Context 11

... the east (Cayley 2011) or through orogen-parallel strike-slip faulting from the south during the closing stages of the Delamerian orogeny (Gibson et al . 2011). Irrespective of which interpretation is correct, the rocks of western Tasmania are clearly bounded in the west by the Sorell Fault, a reactivated early Palaeozoic basement structure whose along-strike onshore counterpart is the west-dipping Avoca Fault (Fig. 1). The northern limits of the Tasmanian crustal block are unknown although deep seismic reflection data for the southern part of the Lachlan Fold Belt in central Victoria (Cayley et al . 2011) support the idea that rocks of Tasmanian affinity are not restricted to the region south of Bass Strait but continue northward in the subsur- face for some significant distance beneath parts of southern Victoria (Fig. 1). The western limits of this buried Tasmanian crust (Selwyn Block) are defined by the west-dipping Heathcote Fault, which farther south terminates against the Avoca Fault, thereby juxtaposing rocks of the Delamerian orogen directly against those of the younger Lachlan Fold Belt (Fig. 1). The Avoca Fault and its along- strike offshore equivalent (Sorell Fault) might therefore be expected to constitute a boundary of some considerable tectonic significance across which there was a commensurate and equally abrupt change in crustal rheology and mechanical behaviour at the time of continental breakup between Australia and Antarctica. No less importantly, from the perspective of this paper, is the observation that this crustal boundary merges southward into the Tasman Fracture Zone (Figs 1 and 3). The Tasman Fracture Zone is part transform boundary and evolved from a late Mesozoic–early Cenozoic intra-continental shear zone located above an even older reactivated Palaeozoic basement structure previously identified as the Woorndoo (Foster & Gleadow 1992) or Moyston Fault (Fig. 1; Hill et al . 1995; Miller et al . 2002) but interpreted here to be the Avoca–Sorell Fault Zone (Figs 1 and 3). Bathymetric images (Fig. 3) show various strands of this fracture zone continuing southward all the way to the ocean–continent boundary in Antarctica, where they assume a position directly along strike from two of the most important basement structures in northern Victoria Land, the steeply dipping Lanterman and Leap year faults (Fig. 1). Both of these structures originated in the early Palaeozoic but have since been modified by several phases of later deformation, including an episode of strike-slip faulting in the Cenozoic that affected much of northern Victoria Land and has the same sense of dextral shear (Figs 1 and 3) as the Tasman Fracture Zone offshore (Salvini et al . 1997; Capponi et al . 1999; Rossetti et al . 2002; Kleinschmidt & Läufer 2006; Storti et al . 2007). The Lanterman Fault preserves an even earlier phase of late Cambrian– early ordovician strike-slip faulting and now separates island arc rocks (Bowers terrane) from older continental crust to the west (Wilson terrane) (Weaver et al . 1984; Capponi et al . 1999; Gibson et al . 2011). It is has been widely interpreted as a former subduction zone or palaeosuture (Weaver et al . 1984; Gibson & Wright 1985; Rocchi et al . 1998; Tessensohn & Henjes-Kunst 2005), which, together with the Leap year Fault farther east, is one of the principal structures along which the terranes of northern Victoria Land were assembled and accreted onto the east Gondwana margin (Weaver et al . 1984; Gibson & Wright 1985; Capponi et al . 1999; Tessensohn & Henjes-Kunst 2005). The George V Fracture Zone is the most westerly of the large- offset, dextral fracture zones that span the entire Southern ocean between SE Australia and Antarctica (Fig. 1) and marks a sharp break from east–west-trending, normally rifted continental margin between the Great Australian Bight and Terre Adélie–Wilkes Land in Antarctica and a generally NNW–SSE-oriented continental margin farther east (Fig. 3) in which oblique- and strike-slip segments developed between Tasmania and George V Land (Stagg & Reading 2007). It is no less prominent than the Tasman Fracture Zone but thus far no suggestion has ever been made that it too may be located along strike from a major reactivated basement structure in SE Australia. Published geological maps for this part of Australia (e.g. Cowley 2006) show no such basement structure, although the same cannot be said of Antarctica, where a 5 km wide mylonite zone (Mertz Shear Zone) separating late Archaean–Palaeoproterozoic cratonic basement from early Palaeozoic rocks of the Ross orogen occurs directly along strike from this same fracture zone (Fig. 1; inset). However, its inferred correlative in South Australia is the late Palaeoproterozoic Kalinjala Mylonite Zone (Talarico & Kleinschmidt 2003; Di Vincenzo et al . 2007; Goodge & Fanning 2010) and lies much too far west to be collinear with the George V Fracture Zone. Published flow lines for relative plate motion (Fig. 3) between Australia and Antarctica (Powell et al . 1988; Tikku & Cande 1999; Williams et al . 2011) would also appear to be incom- patible with this correlation. The alternative is to argue for a different palaeogeographical reconstruction in which the Mertz Shear Zone is matched against a different structure in southern Australia. Possibilities include the Gawler Shear Zone and hitherto previously unmapped Coorong Shear Zone in the southern part of the Delamerian Fold Belt (Fig. 4). The Coorong structure occurs along strike from the George V Fracture Zone (Figs 1 and 3) and extends northwards as far as the Curnamona Craton (Fig. 1), where it overlaps a parallel, and presumably temporally related, structural discontinuity (Anabama– Redan Shear Zone) for which a Rodinia breakup age has been proposed (Preiss 2000). It would appear to be the more important structure but owing to little or no outcrop along its length, is best mapped and characterized through geophysical data (Fig. 4) and the interpretation of deep seismic reflection data collected across the structure offshore (Fig. 5). The Gawler Shear Zone is even less well exposed and lies wholly within the southern Gawler Craton (Figs 2 and 4), where it defines the western edge of a basement tilt block upon which shallow water shelf sediments (Ardrossan Shelf) were subsequently deposited (Belperio et al . 1998). Aeromagnetic data for the Coorong Shear Zone and surrounding region, including part of the adjacent continental shelf, are pre- sented as an image in Figure 4. This image is a vertical derivative of the total magnetic intensity aimed at capturing abrupt changes in the potential field gradient that might reveal the presence of major geological structures or discontinuities. The Coorong Shear Zone is particularly conspicuous in this image and juxtaposes two domains with opposing structural trends (Fig. 4). To the west of this shear zone are inverted and tightly folded platform sequences of the Adelaide Supergroup (Nackara Arc) in which structural fabrics generally strike north–south to NNW–SSE, whereas to the east and south are even more intensely deformed rocks of the Kanmantoo Group and its correlatives in which structures trend NNE–SSW (Fig. 4). Cutting across this NNE–SSW fabric, and intruded into the Kanmantoo Group and its correlatives, are variably magnetized post-kinematic granites and minor gabbro for which a late Cambrian–early ordovician age has been determined (Foden et al . 2002 a ). Magmatic rocks of comparable age and magnetic character have also intruded along the Coorong structure itself (Fig. 4), indicating that this structure is no younger in age than late Cambrian– early ordovician and served as a conduit for post-orogenic magmatic intrusion. Geochemical and isotopic data further indicate that the granites were sourced from the Kanmantoo Group, which evidently extends to considerable depth beneath this part of the Delamerian Fold Belt (Foden et al . 2002 a , b ). In contrast, granites of post-Delamerian age are not widely developed west of the Coorong Shear Zone and none have been mapped in the Nackara Arc (Fig. 4). A few granite outcrops occur between the Coorong and Gawler shear zones farther south but they are hosted by the Kanmantoo Group (Belperio et al . 1998) and far removed from the linear belt of post-tectonic granites and subsidi- ary gabbro intruded along the Coorong structure (Fig. 4). Indeed, except for this one area, the Coorong Shear Zone would appear to serve not only as the western limit of late Cambrian–early ordovician bimodal magmatism in the Delamerian Fold belt but also as an important crustal boundary within the original sedimentary basin across which there was an abrupt change from platform (Adelaide Supergroup, Normanville Group) to deeper water sedimentary facies (Kanmantoo Group). Early Cambrian mafic magmatism is similarly much more common east of the Coorong Shear Zone (Belperio et al . 1998), further supporting the idea that the Coorong Shear Zone is a much older inherited structure that first became active during deposition of the Kanmantoo Group. Seismic reflection data collected off the coast of southern Australia by Geoscience Australia (Fig. 3; line p137-01) confirm the presence of a crustal-penetrating structure in this general location and show that it coincides with a prominent step in the Moho as well as a significant thinning of the middle to lower crust (Fig. 5a). This structure has produced significant offsets in some of the more prominent horizons making up the Early Cretaceous synrift section of the otway Basin, indicating some degree of reactivation at the time of basin formation (Fig. 5a). However, its more obvious attribute and defining feature is its steep to subvertical attitude (Fig. 5a). This attitude, combined with the considerable strike-length observed in the aeromagnetic image (Fig. 4), indicates that the Coorong Shear Zone is unlikely to be related to the ...

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... Zone and none have been mapped in the Nackara Arc (Fig. 4). A few granite outcrops occur between the Coorong and Gawler shear zones farther south but they are hosted by the Kanmantoo Group (Belperio et al . 1998) and far removed from the linear belt of post-tectonic granites and subsidi- ary gabbro intruded along the Coorong structure (Fig. 4). Indeed, except for this one area, the Coorong Shear Zone would appear to serve not only as the western limit of late Cambrian–early ordovician bimodal magmatism in the Delamerian Fold belt but also as an important crustal boundary within the original sedimentary basin across which there was an abrupt change from platform (Adelaide Supergroup, Normanville Group) to deeper water sedimentary facies (Kanmantoo Group). Early Cambrian mafic magmatism is similarly much more common east of the Coorong Shear Zone (Belperio et al . 1998), further supporting the idea that the Coorong Shear Zone is a much older inherited structure that first became active during deposition of the Kanmantoo Group. Seismic reflection data collected off the coast of southern Australia by Geoscience Australia (Fig. 3; line p137-01) confirm the presence of a crustal-penetrating structure in this general location and show that it coincides with a prominent step in the Moho as well as a significant thinning of the middle to lower crust (Fig. 5a). This structure has produced significant offsets in some of the more prominent horizons making up the Early Cretaceous synrift section of the otway Basin, indicating some degree of reactivation at the time of basin formation (Fig. 5a). However, its more obvious attribute and defining feature is its steep to subvertical attitude (Fig. 5a). This attitude, combined with the considerable strike-length observed in the aeromagnetic image (Fig. 4), indicates that the Coorong Shear Zone is unlikely to be related to the west-vergent thrust faults identified by others (Flöttmann & Cockshell 1996) in earlier offshore seismic surveys conducted by the South Australian Department of Mines and Energy. These faults sole out at much shallower depths in the crust (Flöttmann & Cockshell 1996). The Coorong Shear Zone is an entirely separate structure, the vertical and lateral dimen- sions of which are more consistent with a strike-slip origin. In keeping with this interpretation, no single planar surface can be identified in the seismic data (Fig. 5a) and strain has instead been distributed over several parallel and subvertical structures as is expected of a major strike-slip shear zone. A strike-slip origin has already been proposed for the equally steep Anabama–Redan Fault in the northern part of the Delamerian Fold Belt (Preiss 2000). Previous structural studies of the Delamerian Fold Belt have emphasized that crustal shortening during orogenesis was largely taken up on basement-involved footwall shortcut thrusts and inverted normal faults located along or close to the original western margin of the deep-water Kanmantoo basin (Flöttmann et al . 1994; Flöttmann & James 1997). An east-dipping and downward-flatten- ing basement ramp was thought to exist in this area along which there had been significant strain partitioning during and subsequent to sedimentary basin formation (Flöttmann et al . 1994; Belperio et al . 1998). However, if any such ramp or basement structure does indeed lie at depth beneath the Kanmantoo Group, it is not immediately obvious from the seismic section interpreted here (Fig. 5a), either because this structure does not extend sufficiently far eastward or because it steepens before reaching the Coorong Shear Zone. In either case, this basement ramp and the Coorong Shear Zone are not one and the same structure even though both would appear to date from the time of Rodinia breakup and both are rooted in basement beneath some of the highest grade and most intensely deformed metasedimentary rocks in the Delamerian Fold Belt. Farther east, seismic line p137-01 (Fig. 3a) crosses a reactivated basement structure identified here and elsewhere (Gibson et al . 2011) as the Avoca–Sorell Fault Zone (Fig. 5b) and with which the Coorong Shear Zone might be usefully compared. The Avoca– Sorell Fault Zone shares the same steep dip as the Coorong Shear Zone and is associated with an even more pronounced step in the Moho, along with a corresponding increase in crustal thickness landward (Fig. 5b). This is accompanied at higher structural levels by a decrease in the thickness of the overlying Mesozoic sedimentary basin sequences (Fig. 5b). As with the Coorong Shear Zone, strain has been distributed across several different structures and the Avoca–Sorell Fault Zone cannot strictly be regarded as a single dis- crete subvertical structure (Fig. 5b). Both it and the Coorong Shear Zone share a common north–south orientation and both are continu- ous along strike with prominent fracture zones (Tasman and George V fracture zones) that can be traced all the way to Antarctica (Fig. 3). However, whereas the Avoca–Sorell Fault Zone has an obvious correlative in either the Lanterman or Leap year fault in northern Victoria Land (Fig. 1), the Coorong structure is best matched with the Mertz Shear Zone, which up till now has been regarded as a correlative of the Kalinjala Mylonite Zone (Talarico & Kleinschmidt 2003; Di Vincenzo et al . 2007; Goodge & Fanning 2010). Correlation of Coorong and Mertz shear zones. other than a few isolated outcrops of mylonite, the north–south-striking Mertz Shear Zone (Figs 1 and 3) is mainly known from well-documented differences in the age and geological history of rocks exposed on either side of the structure (Talarico & Kleinschmidt 2003; Ménot et al . 2007; Di Vincenzo et al . 2007; Goodge & Fanning 2010). Neoarchaean–Mesoproterozoic rocks exposed west of the shear zone (Fig. 1) are geologically indistinguishable from formerly contiguous parts of the Gawler Craton (Fitzsimons 2003; Goodge & Fanning 2010; Boger 2011) although neither they nor any of the mylonitic rocks preserve any record of Delamerian–Ross deformation and metamorphism (Di Vincenzo et al . 2007; Ménot et al . 2007). Reported 40 Ar/ 39 Ar cooling ages for these mylonitic rocks are 1500 Ma or older (Di Vincenzo et al . 2007). By way of comparison, rocks immediately east of the Mertz Shear Zone are host to a significant volume of Cambro-ordovician granite (Fanning et al . 2002; Di Vincenzo et al . 2007; Goodge & Fanning 2010) and form part of the Delamerian–Ross orogen (Fig. 1). An important crustal boundary of early Palaeozoic age evidently occurs in this region and would appear to be the Antarctic equivalent of the Coorong Shear Zone in that both structures mark or closely approximate the western limits of Delamerian–Ross orogenesis in their respective continental margins (Fig. 4). In contrast, the Kalinjala Mylonite Zone lies well to the west of the currently accepted limits of Delamerian-age magmatism and deformation in SE Australia based on geochronological as well as geological grounds (Swain et al . 2005). Recently published c . 3150 Ma ages for gneissic granite in the Spencer Domain (Fraser et al . 2010) further indicate that basement of known Archaean age extends east of the Kalinjala Mylonite Zone and is of even greater antiquity than Neoarchaean crust exposed farther west in the neighbouring Cleve Domain (Fig. 2b). unlike the Mertz Shear Zone, the Kalinjala Mylonite Zone is wholly entrained within older cratonic basement and for this reason might be better compared with a structure in Antarctica that is similarly bounded on either side by Archaean crust. The region west of the Mertz Shear Zone is host to several such structures (Fig. 1) and comprises Neoarchaean crust overlain by a late Palaeoproterozoic metasedimentary cover sequence whose record of 1700 Ma metamorphism and deformation followed by granite magmatism at 1590 Ma closely matches that of similar sequences in formerly adjacent parts of the Gawler Craton (Peucat et al . 2002, 1999; Ménot et al . 2005; Goodge & Fanning 2010). Equally significantly, this cover sequence and its inferred metasedimentary counterparts in the Gawler Craton yield near-identical Sm–Nd data and detrital zircon ages consistent with derivation of their protoliths from a source region with a common c . 3.2–3.1 Ga age (oliver & Fanning 2002; Ménot et al . 2005). Either the Spencer Domain, along with its 3150 Ma granitic gneisses, was proximal to the metasedimentary sequences developed in both regions or rocks of Mesoarchaean age are more widely developed in Antarctica than existing geochronological data would suggest. In either case, geological reconstructions of the Australian and Antarctic margins (Talarico & Kleinschmidt 2003; Di Vincenzo et al . 2007; Goodge & Fanning 2010) based on matching the Kalinjala and Mertz shear zones are no longer likely to be tenable and produce an interpretation of continental rifting increasingly at odds with observed extensional basin geometries and continental fits based on other criteria such as ocean-floor fabrics and magnetic anomalies (e.g. Powell et al . 1988; Tikku & Cande 1999). Several researchers have commented on the difficulties of reconcil- ing reconstructions of the Australian and Antarctic continental margins based on ocean-floor fabrics and plate-tectonic considera- tions as opposed to geological grounds (Powell et al . 1988; Hill et al . 1995; Royer & Rollet 1997; Whittaker et al . 2007; Williams et al . 2011). Part of the problem stems from uncertainties in matching geological structures across ocean–continent transition zones for the two conjugate margins that are not only exceptionally wide in some places (Fig. 1) but also preserve little or no magnetic record of the direction of extension. Mismatches are particularly evident in some of the earlier geologically based reconstructions (e.g. Flottman et al . 1993) in which major basement thrust faults of Delamerian–Ross age in northern ...

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... strike from a major reactivated basement structure in SE Australia. Published geological maps for this part of Australia (e.g. Cowley 2006) show no such basement structure, although the same cannot be said of Antarctica, where a 5 km wide mylonite zone (Mertz Shear Zone) separating late Archaean–Palaeoproterozoic cratonic basement from early Palaeozoic rocks of the Ross orogen occurs directly along strike from this same fracture zone (Fig. 1; inset). However, its inferred correlative in South Australia is the late Palaeoproterozoic Kalinjala Mylonite Zone (Talarico & Kleinschmidt 2003; Di Vincenzo et al . 2007; Goodge & Fanning 2010) and lies much too far west to be collinear with the George V Fracture Zone. Published flow lines for relative plate motion (Fig. 3) between Australia and Antarctica (Powell et al . 1988; Tikku & Cande 1999; Williams et al . 2011) would also appear to be incom- patible with this correlation. The alternative is to argue for a different palaeogeographical reconstruction in which the Mertz Shear Zone is matched against a different structure in southern Australia. Possibilities include the Gawler Shear Zone and hitherto previously unmapped Coorong Shear Zone in the southern part of the Delamerian Fold Belt (Fig. 4). The Coorong structure occurs along strike from the George V Fracture Zone (Figs 1 and 3) and extends northwards as far as the Curnamona Craton (Fig. 1), where it overlaps a parallel, and presumably temporally related, structural discontinuity (Anabama– Redan Shear Zone) for which a Rodinia breakup age has been proposed (Preiss 2000). It would appear to be the more important structure but owing to little or no outcrop along its length, is best mapped and characterized through geophysical data (Fig. 4) and the interpretation of deep seismic reflection data collected across the structure offshore (Fig. 5). The Gawler Shear Zone is even less well exposed and lies wholly within the southern Gawler Craton (Figs 2 and 4), where it defines the western edge of a basement tilt block upon which shallow water shelf sediments (Ardrossan Shelf) were subsequently deposited (Belperio et al . 1998). Aeromagnetic data for the Coorong Shear Zone and surrounding region, including part of the adjacent continental shelf, are pre- sented as an image in Figure 4. This image is a vertical derivative of the total magnetic intensity aimed at capturing abrupt changes in the potential field gradient that might reveal the presence of major geological structures or discontinuities. The Coorong Shear Zone is particularly conspicuous in this image and juxtaposes two domains with opposing structural trends (Fig. 4). To the west of this shear zone are inverted and tightly folded platform sequences of the Adelaide Supergroup (Nackara Arc) in which structural fabrics generally strike north–south to NNW–SSE, whereas to the east and south are even more intensely deformed rocks of the Kanmantoo Group and its correlatives in which structures trend NNE–SSW (Fig. 4). Cutting across this NNE–SSW fabric, and intruded into the Kanmantoo Group and its correlatives, are variably magnetized post-kinematic granites and minor gabbro for which a late Cambrian–early ordovician age has been determined (Foden et al . 2002 a ). Magmatic rocks of comparable age and magnetic character have also intruded along the Coorong structure itself (Fig. 4), indicating that this structure is no younger in age than late Cambrian– early ordovician and served as a conduit for post-orogenic magmatic intrusion. Geochemical and isotopic data further indicate that the granites were sourced from the Kanmantoo Group, which evidently extends to considerable depth beneath this part of the Delamerian Fold Belt (Foden et al . 2002 a , b ). In contrast, granites of post-Delamerian age are not widely developed west of the Coorong Shear Zone and none have been mapped in the Nackara Arc (Fig. 4). A few granite outcrops occur between the Coorong and Gawler shear zones farther south but they are hosted by the Kanmantoo Group (Belperio et al . 1998) and far removed from the linear belt of post-tectonic granites and subsidi- ary gabbro intruded along the Coorong structure (Fig. 4). Indeed, except for this one area, the Coorong Shear Zone would appear to serve not only as the western limit of late Cambrian–early ordovician bimodal magmatism in the Delamerian Fold belt but also as an important crustal boundary within the original sedimentary basin across which there was an abrupt change from platform (Adelaide Supergroup, Normanville Group) to deeper water sedimentary facies (Kanmantoo Group). Early Cambrian mafic magmatism is similarly much more common east of the Coorong Shear Zone (Belperio et al . 1998), further supporting the idea that the Coorong Shear Zone is a much older inherited structure that first became active during deposition of the Kanmantoo Group. Seismic reflection data collected off the coast of southern Australia by Geoscience Australia (Fig. 3; line p137-01) confirm the presence of a crustal-penetrating structure in this general location and show that it coincides with a prominent step in the Moho as well as a significant thinning of the middle to lower crust (Fig. 5a). This structure has produced significant offsets in some of the more prominent horizons making up the Early Cretaceous synrift section of the otway Basin, indicating some degree of reactivation at the time of basin formation (Fig. 5a). However, its more obvious attribute and defining feature is its steep to subvertical attitude (Fig. 5a). This attitude, combined with the considerable strike-length observed in the aeromagnetic image (Fig. 4), indicates that the Coorong Shear Zone is unlikely to be related to the west-vergent thrust faults identified by others (Flöttmann & Cockshell 1996) in earlier offshore seismic surveys conducted by the South Australian Department of Mines and Energy. These faults sole out at much shallower depths in the crust (Flöttmann & Cockshell 1996). The Coorong Shear Zone is an entirely separate structure, the vertical and lateral dimen- sions of which are more consistent with a strike-slip origin. In keeping with this interpretation, no single planar surface can be identified in the seismic data (Fig. 5a) and strain has instead been distributed over several parallel and subvertical structures as is expected of a major strike-slip shear zone. A strike-slip origin has already been proposed for the equally steep Anabama–Redan Fault in the northern part of the Delamerian Fold Belt (Preiss 2000). Previous structural studies of the Delamerian Fold Belt have emphasized that crustal shortening during orogenesis was largely taken up on basement-involved footwall shortcut thrusts and inverted normal faults located along or close to the original western margin of the deep-water Kanmantoo basin (Flöttmann et al . 1994; Flöttmann & James 1997). An east-dipping and downward-flatten- ing basement ramp was thought to exist in this area along which there had been significant strain partitioning during and subsequent to sedimentary basin formation (Flöttmann et al . 1994; Belperio et al . 1998). However, if any such ramp or basement structure does indeed lie at depth beneath the Kanmantoo Group, it is not immediately obvious from the seismic section interpreted here (Fig. 5a), either because this structure does not extend sufficiently far eastward or because it steepens before reaching the Coorong Shear Zone. In either case, this basement ramp and the Coorong Shear Zone are not one and the same structure even though both would appear to date from the time of Rodinia breakup and both are rooted in basement beneath some of the highest grade and most intensely deformed metasedimentary rocks in the Delamerian Fold Belt. Farther east, seismic line p137-01 (Fig. 3a) crosses a reactivated basement structure identified here and elsewhere (Gibson et al . 2011) as the Avoca–Sorell Fault Zone (Fig. 5b) and with which the Coorong Shear Zone might be usefully compared. The Avoca– Sorell Fault Zone shares the same steep dip as the Coorong Shear Zone and is associated with an even more pronounced step in the Moho, along with a corresponding increase in crustal thickness landward (Fig. 5b). This is accompanied at higher structural levels by a decrease in the thickness of the overlying Mesozoic sedimentary basin sequences (Fig. 5b). As with the Coorong Shear Zone, strain has been distributed across several different structures and the Avoca–Sorell Fault Zone cannot strictly be regarded as a single dis- crete subvertical structure (Fig. 5b). Both it and the Coorong Shear Zone share a common north–south orientation and both are continu- ous along strike with prominent fracture zones (Tasman and George V fracture zones) that can be traced all the way to Antarctica (Fig. 3). However, whereas the Avoca–Sorell Fault Zone has an obvious correlative in either the Lanterman or Leap year fault in northern Victoria Land (Fig. 1), the Coorong structure is best matched with the Mertz Shear Zone, which up till now has been regarded as a correlative of the Kalinjala Mylonite Zone (Talarico & Kleinschmidt 2003; Di Vincenzo et al . 2007; Goodge & Fanning 2010). Correlation of Coorong and Mertz shear zones. other than a few isolated outcrops of mylonite, the north–south-striking Mertz Shear Zone (Figs 1 and 3) is mainly known from well-documented differences in the age and geological history of rocks exposed on either side of the structure (Talarico & Kleinschmidt 2003; Ménot et al . 2007; Di Vincenzo et al . 2007; Goodge & Fanning 2010). Neoarchaean–Mesoproterozoic rocks exposed west of the shear zone (Fig. 1) are geologically indistinguishable from formerly contiguous parts of the Gawler Craton (Fitzsimons 2003; Goodge & Fanning 2010; Boger 2011) although neither they nor any of the mylonitic rocks preserve any record of Delamerian–Ross deformation and metamorphism (Di Vincenzo et al . 2007; ...

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... with a common c . 3.2–3.1 Ga age (oliver & Fanning 2002; Ménot et al . 2005). Either the Spencer Domain, along with its 3150 Ma granitic gneisses, was proximal to the metasedimentary sequences developed in both regions or rocks of Mesoarchaean age are more widely developed in Antarctica than existing geochronological data would suggest. In either case, geological reconstructions of the Australian and Antarctic margins (Talarico & Kleinschmidt 2003; Di Vincenzo et al . 2007; Goodge & Fanning 2010) based on matching the Kalinjala and Mertz shear zones are no longer likely to be tenable and produce an interpretation of continental rifting increasingly at odds with observed extensional basin geometries and continental fits based on other criteria such as ocean-floor fabrics and magnetic anomalies (e.g. Powell et al . 1988; Tikku & Cande 1999). Several researchers have commented on the difficulties of reconcil- ing reconstructions of the Australian and Antarctic continental margins based on ocean-floor fabrics and plate-tectonic considera- tions as opposed to geological grounds (Powell et al . 1988; Hill et al . 1995; Royer & Rollet 1997; Whittaker et al . 2007; Williams et al . 2011). Part of the problem stems from uncertainties in matching geological structures across ocean–continent transition zones for the two conjugate margins that are not only exceptionally wide in some places (Fig. 1) but also preserve little or no magnetic record of the direction of extension. Mismatches are particularly evident in some of the earlier geologically based reconstructions (e.g. Flottman et al . 1993) in which major basement thrust faults of Delamerian–Ross age in northern Victoria Land were aligned with similarly verging basement-cored structures developed along the western margin of the Delamerian Fold Belt in SE Australia. Such reconstructions placed northern Victoria Land much too far west of Tasmania to achieve a correspondingly good fit between island arc–forearc assemblages of near-identical age in the Bowers and Grampians–Stavely terranes. Later reconstructions (Royer & Rollet 1997; Hill & Exon 2004) avoided this problem by placing northern Victoria Land, along with formerly contiguous parts of the South Tasman Rise, much closer to Tasmania. This had the effect of aligning the Bowers and Grampians–Stavely terranes (Finn et al . 1999) whilst maintaining some connection between west-vergent, craton-directed structures in the Wilson terrane and relevant parts of SE Australia (Gibson & Nihill 1992; Flottmann et al . 1993, 1998). A recurrent problem with these more geologically constrained reconstructions was the amount of overlap between the different crustal elements farther east, including Tasmania and the South Tasman Rise (Tikku & Cande 1999, 2000). More recently, other plate-tectonic reconstructions have been proposed in which the Australian plate is located farther east with respect to Antarctica (Whittaker et al . 2007; Williams et al . 2011). In these reconstructions, the Mertz Shear Zone is restored to a position between the Kalinjala Mylonite Zone and Coorong Shear Zone at 84 Ma (Fig. 6). yet on geological grounds, an alignment of this shear zone with the Coorong structure might be expected. The Kalinjala Mylonite Zone lies some 300–350 km west of the Coorong structure (Figs 1–3), indicating that Antarctica, along with the Mertz Shear Zone, has been rotated too far west in these reconstructions. A westward shift of near-identical magnitude is also evident in reconstructions for the Lanterman Fault Zone which, at 84 Ma, should have restored to a position along strike from the Avoca (Sorell) Fault (Fig. 6). This discrepancy in the relative lateral motion of Australia versus Antarctica has been raised before (Tikku & Direen 2008) and put down to differences in the choice of rotational (Euler) pole compared with some previous reconstructions (Tikku & Cande 1999, 2000). More specifically, Tikku & Direen (2008) argued that the c . 400 km westward shift in Antarctica could be eliminated from the Whittaker et al . (2007) reconstruction by adopting the previously proposed Tikku & Cande (2000) Euler pole along with the original fracture zone pick upon which this pole was calculated (compare Fig. 6c and d). Tikku & Cande (2000; see also Tikku & Cande 1999) matched the Leeuwin (Perth) fracture zone (Australia) with the Vincennes fracture zone (Antarctica) whereas Whittaker et al . (2007) matched the Leeuwin and Perth South fracture zones. However, even where pref- erence is given to the former, problems persist as shown by the reconstruction of Williams et al . (2011) in which Antarctica is rotated even farther west with respect to Australia. The Whittaker et al . (2007) and Williams et al . (2011) reconstructions are both based on the premise that crustal extension barely deviated from its initial NW–SE or NNW–SSE trajectory during the earlier stages of rifting before assuming a north–south azimuth in the Cenozoic (Fig. 3). Seismic and sequence stratigraphic studies along Australia’s southern margin (Norvick & Smith 2001; Krassay et al . 2004; Totterdell & Bradshaw 2004; Blevin & Cathro 2008) indicate that this assumption is unwarranted and point to a more complicated record of rifting wherein NW–SE or NNW–SSE extension in the Bight Basin was superseded by NNE–SSW extension in the otway Basin during the latest Tithonian ( c . 145 Ma). Basin architecture and observed patterns of normal faulting along Australia’s southern margin are more in accord with this more complicated history of rifting and serve to emphasize the importance of basement structure and the role structural inheritance played in determining the location and geometry of continental breakup. As with most other rifted continental margins, Australia’s southern margin is segmented and subject to marked changes in orientation along strike (Figs 1 and 3). These changes are manifest in both the shelf-break and more distal continent–ocean boundary, and find maximum expression in the prominent re-entrant developed off western Tasmania, where the continental margin undergoes an abrupt change of strike from NW–SE to north–south across the Tasman Fracture Zone (Figs 1 and 3). This fracture zone is continu- ous along strike with the Avoca–Sorell Fault Zone, which was not only optimally oriented for reactivation as a transform boundary during north–south rifting (Hill et al . 1995; Gibson et al . 2011) but also represents the boundary (Fig. 1) between basement blocks of contrasting rheology as reflected in their very different origins and crustal histories. More specifically, whereas rocks to the west of this boundary belong to the Lachlan Fold Belt and comprise mainly rheologically weaker, Cambrian–ordovician turbidites floored by oceanic crust, western Tasmania on the other side of the Avoca– Sorell Fault Zone is predominantly made up of mechanically stronger Precambrian cratonic basement overlain by reworked Neoproterozoic–early Palaeozoic cover rocks (Fig. 1). An analogous transform margin and re-entrant in the Gulf of Guinea off west Africa have similarly been interpreted in terms of pre-existing basement structure and reduced mechanical strength inherited from rocks tectonically reworked between two more rigid crustal blocks during an earlier deformational event (Benkhelil et al . 1995; Mascle et al . 1997). In contrast, no re-entrant or transform boundary developed in the vicinity of the Coorong Shear Zone despite its north–south orientation and many other similarities to the Avoca–Sorell Fault Zone. Some other limiting factor would seem to be involved. one possi- bility is that the Coorong Shear Zone is stitched along nearly its entire length by post-kinematic magmatic intrusions (Fig. 4) whose emplacement inhibited future reactivation because any inherited structural anisotropy or pre-existing crustal weakness was removed or at least significantly reduced. However, if this were the case, it is difficult to see how this structure could have influenced the location and development of the George V Fracture Zone along strike to the south (Figs 1 and 3), let alone have any bearing on later rifting and margin geometry. yet this structure was clearly an important boundary during rifting and ensuing continental breakup because late Jurassic–Early Cretaceous normal faults and half-graben in the Bight and otway basins on either side of the structure show very different orientations and formed under different extensional regimes. Extensional structures in the Bight Basin mainly formed in response to NW–SE- or NNW–SSE-directed crustal extension and typically strike west–east or NE–SW (Norvick & Smith 2001; Bradshaw et al . 2003; Totterdell & Bradshaw 2004; Blevin & Cathro 2008), parallel to basement structures in the adjacent Gawler Craton (e.g. Tallacootra and Karari shear zones), which were presumably favourably oriented for reactivation through tensile failure or left-lateral shear (Fig. 7a and b). In contrast, normal faults and half-graben in the western otway Basin dominantly strike NW–SE and formed in response to NNE–SSW-directed extension (Krassay et al . 2004; Blevin & Cathro 2008). Basement control on the orientation of these structures is not immediately obvious, although it has long been speculated that rocks of the Gawler Craton occur at depth beneath this region, albeit in highly attenuated form following breakup of the Rodinia supercontinent (Preiss 2000; Teasdale et al . 2003). It may therefore be the case that basement beneath the western otway Basin shares a similar history of fault reactivation to other parts of the eastern Gawler Craton, including repeat movements on NW–SE-trending structures that occurred both before and during rifting along Australia’s southern margin. In either event, it is difficult to escape the conclusion that extensional strain has been compartmentalized or partitioned across the Coorong Shear Zone so as to produce ...

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... and Kanmantoo Groups (Preiss 2000; Foden et al . 2006). Similarly reworked passive margin sequences make up much of the contemporaneous and formerly contiguous Ross orogen in Antarctica (Fig. 1) and, in common with their Australian counterparts, preserve a comparable record of late Cambrian– earliest ordovician high-temperature–low-pressure metamorphism accompanied by widespread granite magmatism (Stump et al . 1986; Borg & DePaolo 1991; Flottmann et al . 1993; Gibson et al . 2011). Platform sequences of the Adelaide Supergroup are best preserved in the Nackara Arc (Fig. 2a) whereas the Kanmantoo Group and its correlatives make up the Kanmantoo–Glenelg zone and extend eastwards into western Victoria, where they are tectonically juxtaposed against the Grampians–Stavely terrane (Fig. 1). The Grampians–Stavely terrane incorporates remnants of an island arc– forearc assemblage (VandenBerg et al . 2000; Kemp 2003) and is bounded on the east by the Moyston Fault, long identified (VandenBerg et al . 2000) as the boundary between the Delamerian and Lachlan fold belts (Fig. 1). Rocks of the Lachlan Fold Belt were not deformed until mid-Palaeozoic time and comprise mainly deep-water, quartz-dominated turbidite sequences of Cambro- ordovician age floored by older c . 500 Ma oceanic crust (VandenBerg et al . 2000; Squire et al . 2006). These turbidite sequences, together with the Grampians–Stavely terrane, extend offshore for some considerable distance and underlie much of the continental shelf off the west coast of Tasmania (Fig. 1). Weakly deformed quartz-rich turbidite sequences of comparable to slightly younger age occur widely in eastern Tasmania and northern Victoria Land east of the Leap year Fault, and most probably represent a continuation of the Lachlan Fold Belt into these two regions (Fig. 1). In contrast, the rocks of western Tasmania show greater affinity with the Delamerian orogen (Fig. 1) and include a tectonically reworked Neoproterozoic–early Cambrian passive margin sequence overlying continental crust of Mesoproterozoic or older age (Berry et al . 1997, 2008; Meffre et al . 2000). Slices of mafic and ultramafic rock overlying this deformed passive margin sequence (not shown in Fig. 1), and bearing a striking resemblance to the island arc–forearc assemblage preserved in the Grampians– Stavely terrane farther west, have been interpreted as parts of a tectonically dismembered ophiolite complex (Crawford et al . 2003). However, neither this dismembered ophiolite nor the underlying passive margin sequence has any obvious physical connection to the rest of the Delamerian Fold Belt. Several faults and a strip of Lachlan Fold Belt intervene (Fig. 1). This has led several researchers to conclude that the rocks of western Tasmania represent an allochthonous crustal block that has been tectonically transported into its present position from elsewhere either through collision and accretion from the east (Cayley 2011) or through orogen-parallel strike-slip faulting from the south during the closing stages of the Delamerian orogeny (Gibson et al . 2011). Irrespective of which interpretation is correct, the rocks of western Tasmania are clearly bounded in the west by the Sorell Fault, a reactivated early Palaeozoic basement structure whose along-strike onshore counterpart is the west-dipping Avoca Fault (Fig. 1). The northern limits of the Tasmanian crustal block are unknown although deep seismic reflection data for the southern part of the Lachlan Fold Belt in central Victoria (Cayley et al . 2011) support the idea that rocks of Tasmanian affinity are not restricted to the region south of Bass Strait but continue northward in the subsur- face for some significant distance beneath parts of southern Victoria (Fig. 1). The western limits of this buried Tasmanian crust (Selwyn Block) are defined by the west-dipping Heathcote Fault, which farther south terminates against the Avoca Fault, thereby juxtaposing rocks of the Delamerian orogen directly against those of the younger Lachlan Fold Belt (Fig. 1). The Avoca Fault and its along- strike offshore equivalent (Sorell Fault) might therefore be expected to constitute a boundary of some considerable tectonic significance across which there was a commensurate and equally abrupt change in crustal rheology and mechanical behaviour at the time of continental breakup between Australia and Antarctica. No less importantly, from the perspective of this paper, is the observation that this crustal boundary merges southward into the Tasman Fracture Zone (Figs 1 and 3). The Tasman Fracture Zone is part transform boundary and evolved from a late Mesozoic–early Cenozoic intra-continental shear zone located above an even older reactivated Palaeozoic basement structure previously identified as the Woorndoo (Foster & Gleadow 1992) or Moyston Fault (Fig. 1; Hill et al . 1995; Miller et al . 2002) but interpreted here to be the Avoca–Sorell Fault Zone (Figs 1 and 3). Bathymetric images (Fig. 3) show various strands of this fracture zone continuing southward all the way to the ocean–continent boundary in Antarctica, where they assume a position directly along strike from two of the most important basement structures in northern Victoria Land, the steeply dipping Lanterman and Leap year faults (Fig. 1). Both of these structures originated in the early Palaeozoic but have since been modified by several phases of later deformation, including an episode of strike-slip faulting in the Cenozoic that affected much of northern Victoria Land and has the same sense of dextral shear (Figs 1 and 3) as the Tasman Fracture Zone offshore (Salvini et al . 1997; Capponi et al . 1999; Rossetti et al . 2002; Kleinschmidt & Läufer 2006; Storti et al . 2007). The Lanterman Fault preserves an even earlier phase of late Cambrian– early ordovician strike-slip faulting and now separates island arc rocks (Bowers terrane) from older continental crust to the west (Wilson terrane) (Weaver et al . 1984; Capponi et al . 1999; Gibson et al . 2011). It is has been widely interpreted as a former subduction zone or palaeosuture (Weaver et al . 1984; Gibson & Wright 1985; Rocchi et al . 1998; Tessensohn & Henjes-Kunst 2005), which, together with the Leap year Fault farther east, is one of the principal structures along which the terranes of northern Victoria Land were assembled and accreted onto the east Gondwana margin (Weaver et al . 1984; Gibson & Wright 1985; Capponi et al . 1999; Tessensohn & Henjes-Kunst 2005). The George V Fracture Zone is the most westerly of the large- offset, dextral fracture zones that span the entire Southern ocean between SE Australia and Antarctica (Fig. 1) and marks a sharp break from east–west-trending, normally rifted continental margin between the Great Australian Bight and Terre Adélie–Wilkes Land in Antarctica and a generally NNW–SSE-oriented continental margin farther east (Fig. 3) in which oblique- and strike-slip segments developed between Tasmania and George V Land (Stagg & Reading 2007). It is no less prominent than the Tasman Fracture Zone but thus far no suggestion has ever been made that it too may be located along strike from a major reactivated basement structure in SE Australia. Published geological maps for this part of Australia (e.g. Cowley 2006) show no such basement structure, although the same cannot be said of Antarctica, where a 5 km wide mylonite zone (Mertz Shear Zone) separating late Archaean–Palaeoproterozoic cratonic basement from early Palaeozoic rocks of the Ross orogen occurs directly along strike from this same fracture zone (Fig. 1; inset). However, its inferred correlative in South Australia is the late Palaeoproterozoic Kalinjala Mylonite Zone (Talarico & Kleinschmidt 2003; Di Vincenzo et al . 2007; Goodge & Fanning 2010) and lies much too far west to be collinear with the George V Fracture Zone. Published flow lines for relative plate motion (Fig. 3) between Australia and Antarctica (Powell et al . 1988; Tikku & Cande 1999; Williams et al . 2011) would also appear to be incom- patible with this correlation. The alternative is to argue for a different palaeogeographical reconstruction in which the Mertz Shear Zone is matched against a different structure in southern Australia. Possibilities include the Gawler Shear Zone and hitherto previously unmapped Coorong Shear Zone in the southern part of the Delamerian Fold Belt (Fig. 4). The Coorong structure occurs along strike from the George V Fracture Zone (Figs 1 and 3) and extends northwards as far as the Curnamona Craton (Fig. 1), where it overlaps a parallel, and presumably temporally related, structural discontinuity (Anabama– Redan Shear Zone) for which a Rodinia breakup age has been proposed (Preiss 2000). It would appear to be the more important structure but owing to little or no outcrop along its length, is best mapped and characterized through geophysical data (Fig. 4) and the interpretation of deep seismic reflection data collected across the structure offshore (Fig. 5). The Gawler Shear Zone is even less well exposed and lies wholly within the southern Gawler Craton (Figs 2 and 4), where it defines the western edge of a basement tilt block upon which shallow water shelf sediments (Ardrossan Shelf) were subsequently deposited (Belperio et al . 1998). Aeromagnetic data for the Coorong Shear Zone and surrounding region, including part of the adjacent continental shelf, are pre- sented as an image in Figure 4. This image is a vertical derivative of the total magnetic intensity aimed at capturing abrupt changes in the potential field gradient that might reveal the presence of major geological structures or discontinuities. The Coorong Shear Zone is particularly conspicuous in this image and juxtaposes two domains with opposing structural trends (Fig. 4). To the west of this shear zone are inverted and tightly folded platform sequences of the Adelaide Supergroup (Nackara Arc) in which structural ...