STRUCTURE AND DEVELOPMENT OF THE OTWAY BASIN

Abstract

The Otway Basin developed as a series of extensional basins along the southern margin of Australia during the Late Jurassic and the Early Cretaceous, prior to the break up of Eastern Gondwanaland. The Basin developed initially as a rift which subsequently, Middle Cretaceous time onwards, drifted and separated.

The structural framework of the Basin is very complex due to the superposition of various successive tectonic processes that resulted in a series of predominantly north-west-south-east striking, basin dipping faults. Strike directions of the faults progressively change to a more north-south direction off Tasmania. In addition, in the area east of Cape Otway, the predominant strike trend changes to a north-east- south-west direction.

Previous models for the development of this complex Basin have all invoked an east-west right lateral wrench system across south-eastern Australia. However, results from recent seismic mapping of the basin have indicated that some features present exhibit components of east-west left lateral movement. Many younger features, particularly in the east, also exhibit recent reverse fault movements.

The author interprets that a right lateral east-west couple was active during the early part of the basin development. However, many of the features and trends observed can be better accounted for by the presence of a north-south striking wrench system which has induced an apparent east-west left lateral movement. Such a wrench system is present in the Southern Ocean, where a series of north-south striking transform faults have developed as a result of relative movement at the Southern Ocean spreading centre. Lateral movements across these transform faults have played a major role in the development of some of the observed features within the Basin.