Geodynamic evolution of the Vulcan Sub‐basin, Timor Sea, northwest Australia: A pre‐compression New Guinea analogue?

Abstract

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 understanding 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 compartments 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.

Triassic petroleum system as an alternative exploration concept in offshore western Timor Indonesia

“…Most of the study area is a marginal part of the northern Ashmore Platform and is characterised by downwarping due to lithospheric flexure that has correlation to the development of the Timor Trough (Chen et al 2012). Inside the study area, the deepest part of the Timor Trough can reach up to 2.4 km deep.…”

“…This region experienced a broad basinal sag during the Early Palaeozoic followed by multiple deformations and subsidences in the late Palaeozoic as a prelude to the breakup of Gondwana (Chen et al 2012). The proto-Vulcan sub-basin and Malita graben were intensively developed at this time (Cadman and Temple 2003).…”

“…A subsequent period of significant development was the Late Permian to Middle Triassic thermal subsidence (Chen et al 2012), followed by the Late Triassic compressional event (Cadman and Temple 2003). This event, known as the Fitzroy Movement, was probably related to the breakup events along the Gondwanan margin or the docking of continental blocks along the adjacent Irian Jaya (Papua) subduction margin (Longley et al 2002).…”

“…The Middle to Late Jurassic north-west-south-east extension created major north-east-south-west-trending depocenters, such as Vulcan sub-basin, Malita graben and high areas of the Ashmore Platform and the Londonderry High (Chen et al 2012). This extension corresponds to a continental breakup phase (Cadman and Temple 2003) and syn-rift sediment infilling.…”

“…Late Triassic calcareous shales are the most favourable source rocks because these contain a total organic content (TOC) up to 15.9%, while their hydrogen index (HI) is in the range Fig. 2 Regional stratigraphy, structural style and main tectonic events of the study area (Keep et al 2002;Longley et al 2002;Cadman and Temple 2003;Chen et al 2012 with modification) calibrated with seismic marker and well log analysis of 441-654 mg/g. These shales are likely to be deposited in a restricted marine setting (Livsey et al 1992).…”