![]() ![]() ![]() Fold cores show evidence of internal shear. The folds show oversteepened fold forelimbs, back-rotated backlimbs, and forward-vergent (NW to NNW) “blind” thrust ramps that terminate within the growth strata. We estimate ~8 km minimum shortening since the late Miocene ca. We interpret that the seafloor structures were formed by imbricate thrusts above multiple detachments. Synkinematic sedimentation interactions with growing folds and near-surface strains were analyzed from seismic attribute maps. Nine seismic horizons within growth strata were mapped and correlated to petroleum industry seismostratigraphy. We show fold growth history, deformation styles, along-strike structural variabilities, and synkinematic sedimentation during triangle zone–style fold growth. TIMESLICE SEISMIC CONSULTANTS FULLHere, we performed tectonostratigraphic analyses on a petroleum industry three-dimensional (3-D) seismic volume that imaged the full growth stratal record. Previous studies have suggested the seafloor ridges formed either above shale diapirs produced by mass movement of overpressured shales (i.e., mobile shale) or above an imbricate fold-and-thrust array. The NW Borneo deep-water fold-and-thrust belt, offshore Sabah, southern South China Sea, contains a structurally complex region of three to four seafloor ridges outboard of the shelf-slope break. The main phase of subsidence in the Bunguran Trough had occurred during the active extensional phase before the MMU. 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). Crustal extension in a pull-apart basin may have been initiated by sinistral motion along the fault during Late Eocene-Oligocene. 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. 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. Both extensional and strike-slip faulting may have contributed to the formation and subsidence of the Bunguran Trough. 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. 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. 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. Sediment thickness, bathymetry and gravity data were utilized to re-construct the crustal structure of the West Luconia Delta and the Bunguran Trough. 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. The Bunguran Trough is a deep Tertiary basin depocentre in the West Luconia Delta Province, Sarawak Basin, offshore Malaysia. ![]()
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