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 Timor Sea, north-west Australia, developed during the Late Jurassic extension which ultimately led to Gondwanan plate breakup and the development of the present-day passive continental margin. This paper describes the evolution of upper crustal extension and the development of Late Jurassic depocentres in this subbasin, via the use of forward modelling techniques. The results suggest that a lateral variation in structural style exists. The south of the basin is characterized by relatively large, discrete normal faults which have generated deep sub-basins, whereas more distributed, small-scale faulting further north reflects a collapse 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 crustal faulting, which represents up to 10% extension, is not balanced by extension in the deeper, ductile lithosphere; the magnitude of this deeper extension is evidenced by the amount of post-Valanginian thermal subsidence. Reverse modelling shows that the lithosphere stretching
factor has a magnitude of up to β=1.55 in the southern Vulcan Sub-basin, decreasing to β=1.2 in the northern Vulcan Sub-basin. It is proposed that during plate breakup, deformation in the Vulcan Sub-basin consisted of depth-dependent lithosphere extension. This additional component of lower crustal and lithosphere stretching is considered to reflect long-wavelength partitioning of strain associated with continental breakup, which may have extended 300–500 km landward of the continent–ocean boundary.