Regional plate tectonics and the evolution of Australia's passive continental margins

The divergent, passive continental margins around Australia have evolved through the progressive dissection of eastern Gondwanaland in five separate seafloor-spreading episodes. The earliest episode occurred 155 m.y. ago off northwestern Australia. The latest episode started 55 m.y. ago south of Australia and continues on the Southeast Indian spreading ridge to the present. The geological and structural evolution of each passive margin was a protracted process. The onset of seafloor spreading (breakup) was preceded by more or less extensive sedimentary basin subsidence. Non-volcanic rift-grabens, with fairly clear boundary faults, began evolving 40-50 m.y. before breakup. Such rifting was often preceded by a broader intra-cratonic style of basin subsidence straddling the incipient continent-ocean boundary between 50 and 100 m.y. before breakup. Rift and infrarift sedimentation rates declined exponentially towards the breakup unconformity. Postbreakup subsidence was again rapid, but exponentially decreased with time. Immediate postbreakup sedimentation was usually interrupted by submarine erosion in the shallow, but rapidly subsiding, ocean basin. Prograding marine shelf conditions prevailed from about 20-40 m.y. after breakup along all margins. The dominant driving mechanism of postbreakup subsidence seems clearly to be lithospheric thermal contraction caused by the removal, by seafloor spreading, of a thermal anomaly from beneath the continental margin. The driving mechanism of the rift-phase subsidence is far less clear. Simple surface observations do not unambiguously discriminate between sudden pull-apart rifting, finite rate stretching, and deep crustal metamorphic contraction. All processes may occur to at least some extent and dominate at different times. However, the apparent absence of both extensive rift volcanics, and transform / transcurrent faulting at rift offsets, plus the non-conformity of rift, breakup, and basement structural patterns, argue against stress-driven stretching as a crucial mechanism. Specific model predictions of crustal structure, heat flow and cumulative sedimentation pattern favour a dominantly metamorphic subsidence mechanism, although this fails to account fully for some thin continental crust near continent-ocean boundaries.