1) The northwest Australian margin was formed as a consequence of four extension events and one inversion event: (i) a broadly-distributed late Permian event, (ii) a predominantly localized late Triassic to early Jurassic event responsible for the formation of the Exmouth, Barrow, and Dampier sub-basins and the deposition of the lower Dingo claystone, (iii) a predominantly localized Callovian fault reactivation within the Exmouth, Barrow, and Dampier sub-basins that led to the deposition of the upper Dingo claystone, and (iv) a major Tithonian-Valanginian event that created accommodation space for the Barrow group but much more importantly, generated large post-Valanginian regional subsidence. This pattern of subsidence indicates significant lower crustal and lithospheric mantle extension below a major intracrustal, eastward dipping detachment.
2) Crucial in the development of the upper and lower Dingo claystones was the segmented nature of the border faults that delineated the Exmouth, Barrow, and Dampier sub-basins and how the extensional deformation was accommodated by these border fault systems. For example, in the Dampier sub-basin, the Callovian extension was accommodated along the Madeleine Trend and the Lewis Trough and the Lambert Shelf represented the collapsed hangingwall block, whereas in the central Barrow sub-basin, the extension was accommodated along the Flinders Fault System with the Peedamullah Shelf being the footwall block. This along-strike variation in the deformational style of the Peedamullah and Lambert Shelves from the footwall in the Barrow sub-basin to the hangingwall in the Dampier Barrow sub-basin plays an important role in governing where and when fluvial networks gained access to the evolving rift. Consequently, the deformational style affects the quality of source and reservoir rocks along and across the sub-basin as well as hydrocarbon migration pathways.
3) The Tithonian-Valanginian extensional event generated large post- Valanginian regional subsidence across the Carnarvon basin with only minor accompanying brittle deformation and erosional truncation. The regional distribution and amplitude of the post-Valanginian subsidence is not consistent with the minor amounts of Tithonian-Valanginian brittle upper crustal extension observed along the margin. Facies and microfossil analyses suggest that prior to the extensional event large portions of the platform were emergent or at very shallow water depths. To match the distribution and magnitude of the post-Valanginian "thermal-type" subsidence requires significant lower crustal and mantle extension across the Northern Carnarvon basin. If the extension was limited to the mantle, there would be no net subsidence after the rift-induced heat dissipated. This is because in extensional settings, basin subsidence is only the consequence of crustal thinning. Such a distribution of extension implies the existence of a eastward dipping, intracrustal detachment having a ramp-flat-ramp geometry that effectively thinned the lower crust and lithospheric mantle. The detachment breached the surface close to the position of the continent-ocean boundary, west of the Exmouth Plateau. The flat component of the detachment occurred at mid-crustal depths (~15 km) across the Plateau and ramped beneath the Australian continent near the Pilbara block. Lower crustal ductile extension provides a viable mechanism to generate large "thermal-type" subsidence with little attendant brittle deformation observed along many conjugate margins. The fault reactivation associated with the Tithonian extensional event increases to the south and is responsible for the formation of the Jurabi Horst. During the first three phases of extension along the northwest Australian margin, the "present-day" Jurabi Horst was a structural low (i.e., Permian, late Triassic, and Callovian) and during the fourth phase of extension (Tithonian) was uplifted and became a structural high covered by thick Jurassic syn-rift deposits.
4) Reorganization of the Indo-Australian plates during the Turonian (Gearle horizon) and the consequent lithospheric in-plane force variation caused minor reactivation of fault systems within the Exmouth, Barrow, and Dampier sub-basins (e.g., uplift of Barrow Island). The amplitude of these structures is consistent with inversion structures observed within other inverted rift basins. In addition to the Turonian inversion, a marked change in depositional style occurred along the margin from gravity-dominated to current-controlled deposition at this time.
5) A first-order implication of this Tithonian-Valanginian detachment is that significant lower-plate extension will be associated with a large injection of rift-related heat. We suggest that this last phase of rifting dominates the maturation history of the Exmouth, Barrow, and Dampier sub-basins as well as the Exmouth Plateau, Rankin Trend, Alpha Arch, and Kangaroo Trough. The importance of this rifting event in terms of the tectonic, stratigraphic and maturation history of the northwest Australian margin has not been previously recognized. The thermal implications of our derived tectonic model, coupled with the thermal blanketing effect of the Dingo claystones and Muderong shale, explains many of the first-order observations of the distribution and maturity of hydrocarbons in the Barrow sub-basin and Rankin platform.
