Australian Journal of Earth Sciences (2002) 49, 719–736 Geodynamic evolution of the Vulcan Sub-basin, Timor Sea, northwest Australia: a pre-compression New Guinea analogue? G. CHEN, 1 * K. C. HILL, 1† N. HOFFMAN 1 AND G. W. O’BRIEN 2 1 Australian Geodynamics Cooperative Research Centre, Department of Earth Science, La Trobe University, Vic 3083, Australia. 2 Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia. The Vulcan Sub-basin lies immediately inboard of the incipient arc–continent collision in the Timor Sea and comprises part of the Bonaparte Basin system, the northernmost basin on Australia’s North West Shelf. Given the high level of preservation of its extensional fabric, the region can provide important analogues for the likely pre-orogeny architecture of New Guinea, which enables a better under- standing of the onset of, and response to, orogenesis. Structural restoration of regional, depth- converted 2-D seismic lines shows that although the Late Jurassic Swan Graben is significant and contains a thick source-rock section, the principal phase of crustal extension took place in the Triassic to Middle Jurassic. Within the Vulcan Sub-basin, the southern Tilted Fault Block Domain records ~10% Triassic to Middle Jurassic extension, whereas <5% upper crustal extension has been measured in the northern Hourglass Domain. Similarly, while Jurassic extension in the Tilted Fault Block Domain is both deep and focused, the Hourglass Domain is expressed as a broad sag to the northeast, indicating a strong underlying basement influence on compartmentalisation. The Vulcan Sub-basin shows four principal stages of evolution: (i) regional, evenly spaced crustal faulting and subsidence in the Triassic – Middle Jurassic; (ii) focused faulting in the Late Jurassic that created grabens with uplift of the shoulders; (iii) regional subsidence from the Middle Valanginian; and (iv) minor extensional and contractional reactivation in the Mio-Pliocene. The measured brittle extension is much less than that suggested by modelling of lithospheric subsidence, which suggests long wavelength distribution of strain in the ductile lower crust, with upper crustal extension mainly focused along the continent– ocean boundary. Along the North West Shelf and on a smaller scale within the Vulcan Sub-basin per se, the obvious, basement-involved, rectilinear compartments defined by prominent offsetting of both extensional fault systems and abyssal plains have important implications for the development of the New Guinea orogen. Similar scale compartments are recognised in New Guinea and display different structural styles and hydrocarbon prospectivity. The transfer zones separating the compart- ments are the sites of the major copper–gold deposits in New Guinea. Using the Vulcan Sub-basin – Timor area as an analogue, it can be seen that an arc could originally collide with a promontory, such as what is now Timor, and reactivate the lineaments allowing local extension and mineralisation. In addition, interpretation of the structure of the New Guinea Fold Belt may be aided by considering the effects of compression on the geometry of the Vulcan Sub-basin and of the similar Carnarvon Basin and adjacent extended and broken Exmouth Plateau. KEY WORDS: geodynamic evolution, rifts, structural extension, Vulcan Sub-basin. INTRODUCTION The aim of the present study is to construct a model of the structural evolution of the Vulcan Sub-basin, Timor Sea (Figure 1), that assesses the influence of pre-existing crus- tal fabric and helps to address the exploration risk associ- ated with migration and trap leakage. In a wider context, it is hoped that the lessons learnt can be used to form part of a broader Timor – New Guinea study, which aims to better define the crustal architecture of New Guinea prior to Neogene orogenesis. This improved understanding could then be used to forward model orogenesis to attempt to determine the geodynamic processes responsible for copper–gold mineralisation and trapping hydrocarbons in New Guinea. Tectonically the Vulcan Sub-basin is part of the Bonaparte Compartment (O’Brien et al. 1996a), which is the northernmost compartment of Australia’s North West Shelf (Figure 1). The Vulcan Sub-basin lies between the Proterozoic Kimberley Block in northwest Australia and Timor, which was formed by the early stages of arc– continent collision in the Mio-Pliocene. The Neogene tectonism and attendant foreland development in the Bonaparte Basin has been critically important and has *Corresponding author: g.chen@latrobe.edu.au † Present address: 3D-GEO Structural Laboratory, School of Earth Sciences, University of Melbourne, Vic. 3010, Australia.