Rift initiation on Australia’s southern margin: insights from the Bremer Sub-basin

Summary The Bremer Sub-basin occupies a unique position on Australia’s south-west margin. It is influenced by two rift events associated with the breakup of Gondwana: rifting along Australia’s western margin in the Early Cretaceous as well as the southern margin rifting in the Late Cretaceous. The basin is underlain by Proterozoic granites, gneisses and sedimentary rocks of the Albany- Fraser Orogen and the structural fabric of the basin is strongly influenced by the basement architecture. Weaknesses along shear zones localised deformation in the early rift phases, resulting in complex structures including ramp-flat faults and associated extensional folds. The western part of the basin contains reactivated NWtrending shear zones in the basement, which are visible onshore in magnetic data and are shown to extend offshore using satellite gravity data. Strike-slip reactivation of these shear zones resulted in isolated deep depocentres and basement highs. In the central and eastern part of the basin, salt diapirs and associated salt withdrawal and expulsion structures were active during rifting and breakup. The salt is likely Proterozoic in age and is tentatively correlated with salt in the Polda Trough, 1500 km east of the Bremer Sub-basin, which has implications for our understanding of the pre-rift architecture of Australia’s southern margin.