文摘
The Otway Basin is one of a number of basins formed along Australia's southern continental margin during the Late Jurassic-Early Cretaceous rifting of Australia from Antarctica. It lies on a transitional area of extended lithosphere between the margin to the west, where continental separation took place close to the Australian craton, and the region to the east where separation failed to develop through Bass Strait, but instead switched to the south of Tasmania. Seven AGSO regional deep seismic profiles recorded during 1992 across onshore parts of the basin provide images of synrift basin bounding faults that dip predominantly towards the continent, basin sequences in half-graben rift segments at two-way times greater than 4 s, and mid-crustal detachments with ramp and flat geometry at mid-crustal levels. When combined with industry seismic data, trend variations on faults indicate that extensional strain direction varied along the rift system, probably locally controlled by pre-existing Palaeozoic geology but consistent with general north-south lithospheric extension. Basement highs between some of the rift segments are interpreted as accommodation (or transfer) zones. Multiple basin-bounding faults in some places mirror features within basement and may indicate a component of local strike-slip during early rifting. The syn-rift segments form part of the first-stage Early Cretaceous failed rift system in the Otway-Bass strait region. Crustal thickness is interpreted to be 31 km (10.3-10.5 s TWT) near the northern basin margin, thinning to about 25 km (9 s TWT, 40 % reduction in thickness of Palaeozoic crust) seaward of the Tartwaup fault zone. This fault zone, and the Timboon fault system farther east, are interpreted to be headwall faults (landward limit) of a Late Cretaceous second-stage rifting system along a lower plate margin which ultimately separated Australia from Antarctica.
