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Salt Diapir Reactivation and Normal Faulting in an Oblique Extensional System: 3-D Seismic Insights From Vulcan Sub-Basin, Bonaparte Basin, NW Australia


3D seismic and well data from Vulcan Sub-basin provide a unique opportunity to characterize the structural pattern developed by salt diapir reactivation and normal faulting in an oblique extensional system. Resulting from the extensive Jurassic rifting event across the Australian NW Shelf, the Vulcan Sub-basin formed a series of NE–SW-trending horsts and grabens, including the Swan Graben—a Jurassic depocenter seated in the NW part of Vulcan Sub-basin. Two sets of northeast-striking thick-skinned normal faults underlying the Swan Graben offset the Permo-Triassic strata and delineate the graben boundaries. With the deposition of Cretaceous–Cenozoic post-rift succession, the Swan salt diapir, which was originally sourced from pre-Permian evaporites, emplaced upward to the near-Tertiary stratigraphic level at the center of Swan graben. The graben system witnessed widespread fault reactivation and flexural extension in Neogene times when continental collision and subduction occurred between Australia-Indian plate and Southeast Asian microplates. A series of NEE–SWW striking normal fault arrays in the shallower section of Swan graben area reveal a soft-linked, left-stepping en echelon pattern. Normal faults strike 30–45° oblique to underlying border faults, and their length and distribution are wider than the graben width, but still controlled by pre-existing basement fabrics, clearly indicating an oblique extension mechanism. The presence of a salt diapir and its reactivation profoundly influenced the local structural styles in this oblique extensional system due to its exceptionally weak mechanical strength: 1) The strikes of normal faults are deflected near the salt diapir and converge at the top of salt, overall showing a “bowknot-shape” deformation pattern; 2) Two three-way dipping structures formed on the north and south side of salt diapir with little deformation, indicating the stress and strain concentration effect of the salt diapir. The orientation of the “bowknot-shape” structure, coupled with extension obliquity analysis of normal faults, indicates a nearly E–W flexural wave in this area, which is consistent with the Neogene–present subduction zone to the north. This study not only improves our understanding of mechanical role of salt, pre-existing structural fabrics and extension obliquity as well as their interactions in upper-crustal deformation, but also shed lights on the Neogene structural reactivation pattern of western Bonaparte Basin.