TESTING MODELS FOR THE PRE-RIFT CONFIGURATION OF AUSTRALIA AND ANTARCTICA

Uncertainty surrounds the pre-rift configuration and early rifting history between Australia and Antarctica, and hence the evolution of sedimentary basins along each of these margins. The plate boundary system during the Cretaceous includes the evolving Kerguelen-Broken Ridge Large Igneous Province in the west as well as the conjugate passive and transform margin segments of the Australian and Antarctic continents. Previous reconstruction models have highlighted the difficulty in satisfying all the available observations within a single coherent reconstruction history. We investigate a range of scenarios for the early rifting history of these plates in the light of a variety of geological and geophysical constraints. Potential field data are used to define the boundaries of stretched continental crust on a regional scale. Integrating crustal thickness along tectonic flowlines provides an estimate of the pre-rift location of the continental plate boundary. We then use the pre-rift plate boundary positions to compute ‘full-fit’ poles of rotation for Australia relative to Antarctica. Our workflow integrates kinematic constraints from restoration of extended continental crust with additional constraints from geological structures and large igneous provinces within the same Australian and Antarctic plate system. We test alternative reconstruction scenarios by emphasising different constraints in order to develop a model that best conforms to the available geological observations. Our preferred model implies that the Leeuwin and Vincennes Fracture Zones are conjugate features within Gondwana, but that the direction of initial opening between Australia and Antarctica does not follow the orientation of these features – rather, these features are likely related to the earlier rifting of India away from Australia-Antarctica. Previous full-fit reconstructions, based on qualitative estimates of continental margin overlaps, generally yield a tighter fit than our preferred reconstruction based on palinspastic restoration.