Variable inversion of polyphase rift basins impacts the Triassic sequence architecture of the NW Shelf, Australia

Inversion structures are commonly observed in continental rift basins and can exert important controls on basin evolution, sequence stratigraphic architecture and associated petroleum systems. Deep seismic interpretation shows that three principal modes of rifting developed during multiple phases of extension and continental breakup on Australia‘s North West Shelf. Early phase rifting (Late Proterozoic & Early Paleozoic) is characterised by low-angle detachment faults (soling out at the Moho), crustal necking and hyper-extension. These thin-skinned fault systems were reactivated during subsequent rift events in the Late Paleozoic and Mesozoic. However, major narrow rift basins, bound by high-angle normal faults that cut into the upper mantle, also formed and progressively localised extensional deformation. In some cases the narrow rifts matured into seafloor spreading centres during breakup, typically outboard of the thin-skinned systems. An intermediate period of wide rift mode extension, wherein extensional deformation was distributed over broad areas, may have occurred prior to the development of discrete narrow rifts in areas such as the Northern Carnarvon and Browse basins. The rifting events were punctuated by periods of thermal sag, often in conjunction with pulses of compressional deformation. For example, inversion during the Mid to Late Triassic has been widely recognised on the NW Shelf. However, there is a marked difference in the manner in which the different types of rift basin responded to these compressional events. The low-angle detachment fault systems were relatively well oriented for reverse movement during reactivation, leading to the development of large inversion anticlines towards the inboard basin margin. 2D restorations (using Move) show that uplift and erosion associated with these compressional events resulted in forced regression of clastic deltas, driven by a reduction in accommodation and increased sediment supply, linked to a series of 2nd and 3rd order sequence boundaries. In contrast, the high-angle fault systems associated with the narrow rift basins are poorly oriented for reactivation during compression and tend to be characterised by wrench/flower structures, generally with relatively limited uplift and erosion. In addition, the sub-basins themselves tend to be associated with well-developed synclinal geometries. This is interpreted to be the result of lithospheric downwarping due to a combination of loading at the base of the lithosphere (due to thermal decay) and in-plane compressional stress. The increased accommodation in these rapidly subsiding synclinal basins often coincides with a marked change (deepening) in depositional setting and development of aggrading shelf margins.