Lithospheric Deformation, Structural Styles and Depocentre Distribution Across a Slowly Evolving Passive Margin—the Otway Basin, Southeast Australia

Daniel B. Palmowski¹, Nick Hoffman², and Kevin C. Hill^2
1 3D-Geo, Geelong-West, Australia
² 3D-Geo, Melbourne, Australia

Late Jurassic to Paleocene rifting and continental break up between Antarctica and Australia is well-recorded on deep regional seismic data across the Otway Basin non-volcanic, oblique passive margin. A clear record of progressive extension of the lower crust is preserved in a series of marginward-propagating tectonically-controlled depocentres. Underlying laminated lower crust thins southwards over ~150 km as the reflection MOHO rises to subcrop a thin veneer of sediments. Serpentinized mantle material is almost exposed at the seafloor in Outer Margin Highs, with true oceanic crust a further 120 km outboard. A succession of structural styles records the thinning and extension of the lower crust with closely-spaced faulting linking into ductile lower crust, followed by widely spaced brittle faulting, spanning the whole crust, as cooling dominated in the lower crust and lithospheric mantle at high extension.

The Otway margin is proposed as a type example of progressive rifting in a slow-spreading non-volcanic setting. The margin preserves a record of successive failure of a layered continental lithosphere, with rifting of the upper crust exposing lower crust which in turn fails and exposes lithosheric mantle in a passively-emplaced unroofed mantle core complex, with the final establishment of a new spreading centre. The evolution of structural and deposition style, and their links to lower-crustal and mantle rheology evolution is well documented and suggests that the Otway Basin can be used as an analogue to infer lower crustal processes in other margins where the lower crust is concealed by volcanics or poor seismic penetration.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005