Greenfield, John1; Gilmore, Phil1; Musgrave, Robert1
1Geological Survey of New South Wales, Department of Regional NSW, Maitland, Australia.
Initial development of the Tasmanides in southeastern Australia involved Early Neoproterozoic rifting/break-up of the Rodinian supercontinent, expressed as tholeiitic dyke swarms and continental rifting in the Adelaide Rift Complex of eastern South Australia. This left highly-extended, transitional crust between the Gawler Craton and Curnamona Province, which became the depocentre for extensive platform carbonate and shallow marine clastic sedimentation. By ~700 Ma, a passive margin developed east of the Curnamona Province which saw the initiation of the palaeo-Pacific Ocean. A final NE–SW phase of rifting in the Late Neoproterozoic was associated with shallow marine platform sedimentation and alkaline magmatism (Mount Arrowsmith Volcanics), further attenuating the eastern Curnamona Province crust and presenting an angular continental salient towards the nascent ocean to the east.
This crustal configuration profoundly influenced the palaeogeography and tectonism that followed during the Delamerian Cycle, as passive margin clastic deposition in the early Cambrian gave way to west-facing subduction in the mid Cambrian. Elements of the resulting Cambrian volcanic arcs (Mount Wright and Loch Lily–Kars) are now immediately adjacent to the Curnamona Province margin. However, regional geological mapping in the Koonenberry Belt has shown that the Mount Wright Arc developed in a rift zone within the Curnamona Province that probably initiated during the last phase of Rodinia break-up in the Late Neoproterozoic. In contrast, the strike-equivalent Loch Lily–Kars Arc was developed in an intra-oceanic setting. Mapping, drillhole and geophysical data shows that this arc segment, along with forearc volcanic rocks of the Ponto Group, were oroclinally folded clockwise almost 90°, and thrust against the southeastern Curnamona Province margin during the Late Cambrian Delamerian Orogeny. If the original arc segments were part of a linear belt, it would have extended southeast from the oroclinal hinge at the Grasmere Knee Zone. Recently acquired and modelled AusLAMP magnetotelluric data show a strong lower crustal conductive anomaly aligned along this trend.
The Delamerian Orogeny caused strong ductile deformation of rocks deposited in the Delamerian Cycle. Areas that were highly attenuated during break-up suffered tight upright folding and oroclinal bending, which may also have been affected by clockwise rotation of the Curnamona Province. Proterozoic Olarian Cycle metamorphic rocks along the margins of the Curnamona Province and the eastern edge of the Gawler Craton also suffered upright open to tight folding during the Delamerian, with σ1 perpendicular to the outboard margin. In the Broken Hill Domain, early southwest-directed fold-thrusting switched to northwest-directed fold-thrust and strike-slip deformation at the end of the Delamerian Orogeny in the Early Ordovician.
Clearly the switch to a convergent setting, with arc accretion and terminal deformation in the Delamerian Orogen, caused extensive shortening of a highly attenuated margin that had been in extension for c. 300 Ma. The Curnamona Province, although acting as a salient in the early Delamerian Orogeny, was itself deformed and could not insulate the distal-foreland Flinders Ranges from deformation late in this event. This had cumulative effects on the ensuing cycles of subduction roll-back, extension and contraction that defined the Tasmanides throughout the Palaeozoic.
John leads the Geoscience Acquisition & Synthesis Unit in the Geological Survey of NSW, which collects and interprets geoscientific data from geological mapping, geophysical surveys, and specialist studies in mineral deposits, palaeontology, and petrography.