The crustal structure and evolution of the Bunguran Trough, offshore Sarawak, Malaysia

Abstract

The Bunguran Trough is a deep Tertiary basin depocentre in the West Luconia Delta Province, Sarawak Basin, offshore Malaysia. At the centre of the trough, the top of the acoustic basement is ∼15 km deep, buried beneath a thick pile of Late Eocene to Quaternary coastal-deltaic sediments of the proto-Rajang-Lupar River delta system. Sediment thickness, bathymetry and gravity data were utilized to re-construct the crustal structure of the West Luconia Delta and the Bunguran Trough. The results indicate an average crustal thickness of 5.3 km beneath the Bunguran Trough, which is extremely thin compared to the extended continental crust (∼15–22 km) beneath the adjacent West, Central and North Luconia provinces. Extreme crustal thinning is confined to the Bunguran Trough, which appears to have steeply faulted boundaries, but the nature of the faults is obscured by the thick sedimentary pile.Structural evidences from reflection seismic data suggest widespread extensional faulting throughout Sarawak Basin although some major tectonic lineaments may have had significant strike-slip movements due to later reactivation and basin inversion. Both extensional and strike-slip faulting may have contributed to the formation and subsidence of the Bunguran Trough. West Balingian Line is a major fault zone that seems to have had a significant strike-slip displacement history and probably had an important role during basin initiation. Seismic data suggests that this fault zone extends from the Balingian and Tatau provinces into West Luconia as a major strike-slip fault with a releasing bend along which the Bunguran Trough may have developed. Crustal extension in a pull-apart basin may have been initiated by sinistral motion along the fault during Late Eocene-Oligocene. Alternatively, the widespread normal faulting indicates that the Bunguran Trough may have developed as a rift basin which experienced at least two phases of extension before a major inversion event which is marked by the Middle Miocene Unconformity (MMU). The main phase of subsidence in the Bunguran Trough had occurred during the active extensional phase before the MMU.