Late Jurassic subsidence and passive margin evolution in the Vulcan Sub-basin, north-west Australia: constraints from basin modelling

The Vulcan Sub-basin, located in the Timer Sea, north-west Australia, devel oped during the Late Jurassic extension which ultimately led to Gondwanan p late breakup and the development of the present-day passive continental mar gin. This paper describes the evolution of upper crustal extension and the development of Late Jurassic depocentres in this subbasin, via the use of f orward modelling techniques. The results suggest that a lateral variation i n structural style exists. The south of the basin is characterized by relat ively large, discrete normal faults which have generated deep sub-basins, w hereas more distributed, small-scale faulting further north reflects a coll apse of the early basin margin, with the development of a broader, 'sagged' basin geometry. By combining forward and reverse modelling techniques, the degree of associated lithosphere stretching can be quantified. Upper crust al faulting, which represents up to 10% extension, is not balanced by exten sion in the deeper, ductile lithosphere; the magnitude of this deeper exten sion is evidenced by the amount of post-Valanginian thermal subsidence. Rev erse modelling shows that the lithosphere stretching factor has a magnitude of up to beta = 1.55 in the southern Vulcan Sub-basin, decreasing to beta = 1.2 in the northern Vulcan Sub-basin. It is proposed that during plate br eakup, deformation in the Vulcan Sub-basin consisted of depth-dependent lit hosphere extension. This additional component of lower crustal and lithosph ere stretching is considered to reflect long-wavelength partitioning of str ain associated with continental breakup, which may have extended 300-500 km landward of the continent-ocean boundary.