THE ROLE OF THE PRE-RIFT STRUCTURE IN THE ARCHITECTURE OF THE DAMPIER BASIN AREA, NORTH WEST SHELF, AUSTRALIA

Abstract

The Dampier Sub-basin shows many faults oblique to the basin axis. Previous explanations for this range from syn-rift transfer systems through to deep seated wrenching.

Multiple rift episodes, with differing stress directions, occur in the area's history, each utilising the pre-existing fault patterns. As basement is difficult to interpret beneath thick sedimentary cover, the initial architecture is interpreted from the tectonic setting.

The sub-basin lies adjacent to the Archean Pilbara Craton, a stable crustal block surrounded by ancient mobile belts. The East Africa rift system has also formed in a Craton margin setting. In East Africa earthquake data and detailed seismic interpretation show the rift utilises faults within the mobile belt systems.

In the Dampier area, the three different extension vectors combined with the pre-rift fabric and the East Africa analogue, are used to build an alternate model for the basin genesis. Permo-Carboniferous extension sets up a rift system partitioned by the Precambrian fabric. Jurassic extension reactivates these faults but with oblique slip and dip slip movement caused by the new extension direction. This oblique slip causes complex branching arrays of new faults within the cover section. A third extension vector in the Cretaceous subsequently modifies the fabric. The Dampier Sub-basin is seen as a complex failed rift utilising a Precambrian tectonic fabric. The structural inheritance of the pre-rift fabric by each rift episode has affected the geometry of hydrocarbon-bearing structures of the sub-basin.