A TRANSECT ACROSS AUSTRALIA SOUTHERN MARGIN IN THE OTWAY BASIN REGION- CRUSTAL ARCHITECTURE AND THE NATURE OF LIFTING FROM WIDE-ANGLE SEISMIC PROFILING

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 which ultimately led to the separa tion of Australia and Antarctica. The velocity structure and crustal a rchitecture of the Otway continental margin has been interpreted from offshore-onshore wide-angle seismic profiling data along a transect ex tending from near the northern Otway Basin margin with Palaeozoic outc rop to the deep ocean basin under the Southern Ocean. Along this trans ect, the Otway Continental Margin (OCM) Transect, the onshore half-gra ben geometry of Early Cretaceous deposition gives way to a 5-km-thick basin sequence (P-wave velocity 2.2-4.6 km/s) extending down the conti nental slope offshore to at least 60 km from the shoreline. At 120 km from the nearest shore, sonobuoy data indicate a 4-5 km sedimentary se quence overlying 7 km of crustal basement rocks above the Moho at 15 k m depth (water depth 4220 m). Conspicuous strong Moho reflections are evident under the continental slope at about 10.2 s TWT. Basement is i nterpreted to be attenuated/faulted Palaeozoic rocks of the Delamerian and Lachlan Orogens (intruded with Jurassic volcanics) that thin from 16 km onshore to about 3.5 km at 120 km from the nearest shore. These rocks comprise a 3 km section that has a velocity of 5.5-5.7 km/s ove rlying deeper basement with a velocity of 6.15-6.35 km/s. Over the sam e distance the Moho shallows from a depth of 30 km onshore to 15 km de pth at 120 km from the nearest shore, and then to about 12 km in the d eep ocean at the limits of the profile (water depth 5200 m). The conti nent-ocean boundary (COB) is interpreted to be at a prominent topograp hic inflection point at the bottom of the continental slope in 4800 m of water. P-wave velocities in the lower crust are 6.4-6.8 km/s above a transition to the Moho, with an upper mantle velocity of 8.05 km/s. There is no evidence of massive high-velocity (>7 km/s) intrusives/und erplate material in the lower crust nor any syn-rift or early post-rif t subaerial volcanics, indicating that the Otway continental margin ca n be considered a non-volcanic margin, similar in many respects to som e parts of the Atlantic Ocean margins, e.,g. the Nova Scotia-Newfoundl and margin off Canada and the Galicia Bank off the Iberian Peninsula. (C) 1998 Elsevier Science B.V. All rights reserved.