OTWAY CONTINENTAL-MARGIN TRANSECT - CRUSTAL ARCHITECTURE FROM WIDE-ANGLE SEISMIC PROFILING ACROSS AUSTRALIA SOUTHERN MARGIN

The Otway Basin in southeastern Australia formed on a triangular-shape d area of extended continental lithosphere during two extensional epis odes in Cretaceous to Miocene times. The extent of the offshore contin ental margin is highlighted by Seasat/Geosat satellite altimeter data, The crustal architecture and structural features across this southeas t Australian margin have been interpreted from offshore-onshore wide-a ngle seismic profiling data along the Otway Continental Margin Transec t extending from the onshore Lake Condah High, through the town of Por tland, to the deep Southern Ocean. Along the Otway Continental Margin Transect, the onshore half-graben geometry of Early Cretaceous deposit ion gives way offshore to a 5 km-thick slope basin (P-wave velocity 2. 2-4.6 km/s) to at least 60 km from the shoreline. At 120 km from the n earest shore in a water depth of 4220 m, sonobuoy data indicate a 4-5 km sedimentary sequence overlying a 7 km thick basement above the Moho at 15 km depth. Major fault zones affect the thickness of basin seque nces in the onshore area (Tartwaup Fault Zone and its southeast contin uation) and at the seaward edge of the Mussel Platform (Mussel Fault). Upper crustal basement is interpreted to be attenuated and thinned Pa laeozoic rocks of the Delamerian and Lachlan Orogens (intruded with Ju rassic volcanics) that thin from 16 km onshore to about 3.5 km at 120 km from the nearest shore. Basement rocks comprise a 3 km section with velocity 5.5-5.7 km/s overlying a deeper basement unit with velocity 6.15-6.35 km/s. The Moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore, and then to about 12 km in the deep ocean at the limits of the transect (water depth 5200 m). The continent-ocean boundary is interpreted to be at a prominent topograp hic inflection point 170 km from shore at the bottom of the continenta l slope in 4800 m of water. P-wave velocities in the lower crust are 6 .4-6.8 km/s, overlying a thin transition zone to an upper mantle veloc ity of 8.05 km/s beneath the Moho. Outstandingly clear Moho reflection s seen in deep-marine profiling data at about 10.3 s two-way time unde r the slope basin and continent-ocean boundary place further strong co ntrols on crustal thickness. There is no evidence of massive high velo city ( >7 km/s) intrusives/underplate material in the lower crust nor any synrift or early post-rift subaerial volcanics, indicating that th e Otway continental margin can be considered a non-volcanic margin, si milar in many respects to some parts of the Atlantic Ocean margins e.g , the Nova Scotia - Newfoundland margin off Canada and the Galicia Ban k off the Iberian Peninsula. Using this analogue, the prominent gravit y feature trending northwest-southeast at the continent-ocean boundary may indicate the presence of highly serpentinised mantle material ben eath a thin crust, but this has yet to be tested by detailed work.