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Analogue Modelling of Gravity-Driven Deformation in the Western Otway Basin, South Australia

Abstract

The Otway Basin is one of a series of Late Jurassic – Early Cretaceous extensional basins that formed the South Australian passive margin when Antarctica and Australia rifted apart. The basin trends generally NW-SE and is commonly divided into eastern and western zones based on changes in fault orientations. Previous seismic studies carried out in the Western Otway basin proposed that the basin was formed by two major rift phases according to the change of dominant fault orientations from E-W to NW-SE up section. New seismic studies using 3D seismic data indicates that the evolution of the basin may have been more complex. A new evolutionary model with three major phases of extension is proposed for the Western Otway basin. The first N-S directed Early Cretaceous rift event formed by E-W striking extensional faults. This was followed by a subsidence phase leading to the burial of the early rift faults. Then second extensional phase also attributed to continue N-S rifting was accommodated by E-W faults that affect the landward part of the basin. The last, Upper Cretaceous extensional event was post-rift gravity driven detached extension above a ductile detachment that formed a series of linked, NW-SE striking listric, down-to the basin fault systems. The faults of this phase are confined to the upper Cretaceous – Neogene strata. Older basin models proposed that this fault was related to a major change in the direction of rifting. This third NW-SE fault system displays evidence of early linkages with the underlying second generation of faults and appears to be largely disconnected from the older, buried first rift phase faults. The new interpretation proposes that the heavily faulted upper sequences on the passive margin were the result of a post-rift, late-stage, thin-skinned gravity-driven system formed above an over-pressured shale detachment in the Western Otway basin. The system seems to have slided towards the southwest down a slope formed as the passive margin tilted to the southwest. This new model was tested using scaled sandbox models that simulated the gravity sliding of progradation sand delta above silicone detachments.