QUANTITATIVE MODELING OF THE JURASSIC-HOLOCENE SUBSIDENCE HISTORY OF THE VULCAN SUBBASIN, NORTH-WEST SHELF - CONSTRAINTS ON LITHOSPHERE EVOLUTION DURING CONTINENTAL BREAKUP

Flexural isostatic basin modelling techniques allow an insight into th e development of subsidence mechanisms associated with crustal extensi on and the evolution of rift basins. The Vulcan Sub-basin, which is lo cated in the Timer Sea on the northwest Australian passive continental margin, underwent a period of rifting during the Middle to Late Juras sic with upper crustal extension of beta = 1.1. Additional, more regio nal extension by low-displacement domino faulting occurred on the Ashm ore Platform during the Early Jurassic and has a similar stretching fa ctor. Thermal and flexural isostatic models have been developed for th e post-Triassic structural evolution and subsidence histories across t hese provinces. These models show a different post-rift subsidence his tory to that predicted by McKenzie (1978)-type subsidence models and s uggest that an additional regional thermal anomaly was overprinted on the lithosphere temperature field during Late Jurassic rifting. This p roduced an initial uplift mechanism. which allowed erosion of the Ashm ore Platform. followed by increased post-rift thermal subsidence, whic h allowed the development of accommodation space for Cretaceous-Holoce ne post-rift sediments. This thermal anomaly is estimated to have a ma gnitude equivalent to stretching values of beta = 1.5-1.6 across the A shmore 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 low er crust developed during breakup and which increased towards the cont inent-ocean boundary. Therefore, the relationship between upper crusta l faulting and total lithosphere stretching, as predicted by the isost atic response and the development of accommodation space, is not a sim ple one and the discrepancy observed suggests a breakup mechanism alon g the margin which involves partitioning of upper and lower plate defo rmation.