HALF-GRABEN MODEL FOR THE STRUCTURAL EVOLUTION OF THE FITZROY TROUGH, CANNING BASIN, AND IMPLICATIONS FOR RESOURCE EXPLORATION

Abstract

The Fitzroy Trough is a north-west/south-east trending rift along the north-east margin of the Canning Basin. The major crustal extension in the trough occurred in the Middle Devonian to Early Carboniferous. Most idealised cross-sections show down-to-trough normal faults bounding both sides of the trough. In contrast, we show the trough to have a half- graben style, with one side a hinge zone or flexure, and the other side bounded by normal faults. Thus, the basin has marked structural asymmetry. The sense of asymmetry switches several times along strike with the hinged margin on the north-eastern margin in some places and the south-west margin in others. The switching in asymmetry occurs at transfer faults. This structural style is expected in extensional tectonic models where the extension occurs on a detachment surface and is typical of many continental and passive margin rifts. The asymmetry of the Palaeozoic structure has implications for resource exploration because of its influence on facies development in and subsequent structural evolution of the trough. Quite different syn-rift clastic and carbonate facies are expected on faulted and hinged margins of a half-graben. Post-rift subsidence will also be somewhat asymmetric, influencing the carbonate reef geometry in particular. Mesozoic deformation in the basin reactivated many of the Palaeozoic normal and transfer faults, and induced reverse slip up basement surfaces on the hinged margin segments.