The Western Otway Basin ? a tectonic framework from new seismic, gravity and aeromagnetic data

Abstract

Interpretation of new seismic, gravity and aeromagnetic data sets from the western Otway Basin has contributed signif­icantly towards a better understanding of the regional tectonic framework of the basin's evolution. Geosat data from the Southern Ocean and gravity data onshore have defined what is here termed the Otway-Sorell microplate, a triangular region of extended continental crust and lithosphere bounded by the onshore northern limits of the Otway Basin, the eastern limits of the Sorell Basin, and a prominent Geosat lineament (?fault zone) extending from the offshore Crayfish Platform to southern Tasmania. The Geosat data also redefine the Spencer and Tasman Fracture Systems within the Southern Ocean lithosphere, and the existence of the previously named Gambier, St. Vincent and George V fracture zones is now doubtful. Fragmentation of Australia's continental crust during Jurassic lithospheric extension led to the development of a series of rift segments and intervening crustal blocks in the western Otway Basin. Long wavelength aeromagnetic anomalies outline what is interpreted as the extent of Palaeozoic crustal fragments and a possible lithospheric triple junction, including the newly-named Coorrong Trough. This trough is along strike from the Scopes Range ? Padthaway Gravity High extending south from the western margin of the Murray Basin, suggesting a significant onshore-offshore structural correlation near the traditional Precambrian-Phanerozoic boundary, known as the Tasman Line. The boundaries of Palaeozoic crustal fragments can be correlated with significant changes in the character of deep seismic profiling data and a thinning of the crust south of the Tartwaup Fault Zone. The Tartwaup fault zone is interpreted as the cratonic boundary for a lower-plate margin during the episode of lithospheric extension which eventually resulted in separation of Australia from Antarctica. Seismic mapping of the Otway Group sequences defines the early rift segments and the regional scale of later deposition. The earliest rifting developed in a series of half graben with most of the major bounding faults dipping towards the craton. These rift segments did not develop fully. However, rifting southwest of the Tartwaup fault zone did progress to the stage of continental separation. There are significant northwest and northeast structural trends which can also be identified in magnetic and gravity images. Fault reactivation occurred in Aptian, Late Albian, Late Maastrichtian, Eocene and Miocene times in response to intra-plate stress being transmitted from plate boundaries.