Abstract

The Penola Trough, which forms part of the Otway Basin system of southern Australia, developed during a protracted phase of extension from the Late Jurassic to late Cretaceous, in response to continental rifting between Australia and Antarctica.

Interpretation of 3D seismic data shows that the synrift sequences within the Penola Trough are dominated by E-W and NW-SE trending structures which form a complex, structurally coherent linked fault system. Displacement is transferred between different components of the fault system by a series of relay ramps and a change in polarity of the rift is marked by a spectacular example of a diffuse accommodation zone. Hard-linkage also occurs across breached relay ramps and via NE-SW trending transfer faults which comprise a third, minor component of the rift system.

The post rift sequence is dominated by NW-SE trending structures, implying a change in extension direction towards the end of the Lower Cretaceous. Comparison with analogue models suggests that many features of the linked fault system can be explained as a consequence of two phase extension. A change in the extension direction also resulted in segmentation of the major listric faults which cut basement and form the boundary of the central part of the trough. This segmentation exerts a major control on the development of the accommodation zones observed in the syn-rift sequence.

Minor N-S trending faults can also be observed, predominantly in the footwalls of the trough-bounding listric faults, but have little effect on the overall rift architecture. These faults parallel the N-S structural grain of the Palaeozoic basement inferred from aeromagnetic maps, and imply that basement fabrics have relatively little effect on the development of the rift system.