Hyperextension and polyphase rifting: Impact on inversion tectonics and stratigraphic architecture of the North West Shelf, Australia

Abstract

Restoration of interpreted 2D deep seismic lines across Australia's North West Shelf indicates that rifting style changed significantly during multiple phases of extension and continental breakup. Early phase rifting during the Neo-Proterozoic and Early Paleozoic, characterised by low-angle detachment faults, crustal-scale necking and hyperextension of the crust (Metamorphic Core Complex Mode), was localised over pre-existing Proterozoic orogenic belts formed adjacent to major cratonic blocks. Subsequent extensional events in the Late Paleozoic and Mesozoic reactivated these low-angle rift fault systems. However, a series of narrow rift basins (Narrow Rift Mode), bound by high-angle normal faults that cut into the upper mantle, also began forming due to changes in the rheological architecture of the lithosphere following previous extension, crustal thinning and post-rift cooling. Wide rift mode extension, wherein extensional deformation was distributed over broad areas, may have occurred prior to, or in conjunction with, the development of more localised narrow rifts (dual mode rifting) in areas such as the Northern Carnarvon and Browse basins. Extensional deformation progressively localised into the narrow rift basins, which in some cases, matured into seafloor spreading centres during breakup, typically outboard of the major low-angle detachment fault systems. The resulting general structural architecture evolution is consistent with several other hyperextended margins from around the world.The rift events were punctuated by periods of thermal sag, often in conjunction with pulses of compressional deformation associated with plate-scale tectonic events. Structural restoration of 2D regional seismic lines shows a marked difference in the way the different polyphase rift basin elements responded to shortening. Low-angle detachment faults were relatively well oriented for reactivation under compression, leading to the development of inversion anticlines and associated convergent unconformities towards the inboard basin margin. In contrast, high-angle normal fault systems associated with the pre-existing narrow rift basins were poorly oriented for reactivation during compression and tend to be characterised by minor wrench/flower structures, generally with relatively limited uplift and erosion on the basin margins. Regional in-plane compressional stresses also worked in combination with vertical loading due to thermal decay to downwarp the lithosphere, forming broad rapidly subsiding synclinal basins above axes of the pre-existing rift basins. The evolving structural architecture developed during the compressional events had significant impacts on the associated sequence stratigraphic architecture.