4D EVOLUTION OF SALT DIAPIR REACTIVATION IN AN OBLIQUE EXTENSIONAL SYSTEM, NW AUSTRALIA: INSIGHTS FROM SCALED PHYSICAL MODELS

Vulcan Sub-basin in NW Australia reveals a unique and interesting geological phenomenon of one salt diapir reactivated in an oblique extensional system. This structure formed in a “subducting rifted margin” setting where the north-facing Timor Trough subduction zone generated upper-crust flexural extension on the Australian Plate. 3D seismic interpretation provides good constraints on the present-day geometry of this structure. However, the kinematics and deformation mechanisms of how the system evolved through time, and how the salt diapir interacted with the pre-existing structures and influenced the normal faulting during oblique extension, are still poorly understood.

In this study, scaled sandbox models were used to investigate the 4D evolution of this natural example. Model sectioning and 3D structural reconstruction allowed detailed analysis of the 3D fault systems. Modeling results show that extension obliquity controls the overall pattern of normal fault arrays. En echelon normal faults developed during the extension, formed step-like local depocenters, indicating strong influence of the underlying oblique pre-existing structures. The diapir, due to its weak mechanical property, profoundly modified local deformation patterns and created a narrow deformation zone near it by absorbing extension, indicating the “stress/strain concentration” effect of the diapir. Two areas near the diapir with less deformation represent the stress/strain shadows. Models with different initial diapir configurations (i.e. isolated diapir vs. diapir with base layer) revealed contrasting diapir behaviors during extension. The roof of isolated diapirs repeatedly collapsed as the diapirs widened during extension. In the cases of diapir with base layer, however, the roof of the diapirs subsided less than the surrounding graben areas, resulting in higher elevations over the top of the diapirs. This shows that these diapirs can be supported by salt withdrawals from surrounding subsiding areas during extension. These modeling results not only simulate the evolution of the natural example but also provide valuable insights on mechanical controls of pre-existing fabrics and salt structures.