Quantitative modelling of the Jurassic-Holocene subsidence history of the Vulcan Sub-basin, North West Shelf: constraints on lithosphere evolution during continental breakup

Flexural isostatic basin modelling techniques allow an insight into the development of subsidence mechanisms associated with crustal extension and the evolution of rift basins. The Vulcan Sub-basin, which is located in the Timer Sea on the northwest Australian passive continental margin, underwent a period of rifting during the Middle to Late Jurassic with upper crustal extension of beta = 1.1. Additional, more regional extension by low-displacement domino faulting occurred on the Ashmore Platform during the Early Jurassic and has a similar stretching factor. Thermal and flexural isostatic models have been developed for the post-Triassic structural evolution and subsidence histories across these provinces. These models show a different post-rift subsidence history to that predicted by McKenzie (1978)-type subsidence models and suggest that an additional regional thermal anomaly was overprinted on the lithosphere temperature field during Late Jurassic rifting. This produced an initial uplift mechanism. which allowed erosion of the Ashmore Platform. followed by increased post-rift thermal subsidence, which allowed the development of accommodation space for Cretaceous-Holocene post-rift sediments. This thermal anomaly is estimated to have a magnitude equivalent to stretching values of beta = 1.5-1.6 across the Ashmore Platform, decreasing to the southwest to beta = 1.2 beneath the Londonderry High. The development of this anomaly is coincident with estimates of the timing of breakup of the Australian plate and implies that a regional ductile stretching in the lithospheric mantle and lower crust developed during breakup and which increased towards the continent-ocean boundary. Therefore, the relationship between upper crustal faulting and total lithosphere stretching, as predicted by the isostatic response and the development of accommodation space, is not a simple one and the discrepancy observed suggests a breakup mechanism along the margin which involves partitioning of upper and lower plate deformation.