A transect across Australia's southern margin in the Otway Basin region: crustal architecture and the nature of rifting from wide-angle seismic profiling

Abstract

The Otway Basin in southeastern Australia formed on a triangular-shaped area of extended continental lithosphere during two extensional episodes in Cretaceous-to-Miocene times which ultimately led to the separation of Australia and Antarctica. The velocity structure and crustal architecture of the Otway continental margin has been interpreted from offshore-onshore wide-angle seismic profiling data along a transect extending from near the northern Otway Basin margin with Palaeozoic outcrop to the deep ocean basin under the Southern Ocean. Along this transect, the Otway Continental Margin (OCM) Transect, the onshore half-graben 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 continental 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 sequence overlying 7 km of crustal basement rocks above the Moho at 15 km depth (water depth 4220 m). Conspicuous strong Moho reflections are evident under the continental slope at about 10.2 s TWT. Basement is interpreted 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 overlying deeper basement with a velocity of 6.15–6.35 km/s. Over the same distance 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 profile (water depth 5200 m). The continent-ocean boundary (COB) is interpreted to be at a prominent topographic 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/underplate material in the lower crust nor any syn-rift or early post-rift subaerial volcanics, indicating that the Otway continental margin can be considered a non-volcanic margin, similar in many respects to some parts of the Atlantic Ocean margins, e.g. the Nova Scotia-Newfoundland margin off Canada and the Galicia Bank off the Iberian Peninsula.