Search

Inversion of satellite-derived free-air gravity-anomaly data has been used to map crustal thickness and continental lithosphere thinning in the South China Sea. Using this, we determine the ocean–continent transition zone structure, the distal extent of continental crust, and the distribution of oceanic lithosphere and continental fragments in the South China Sea. We construct a set of regional crustal cross-sections, with Moho depth from gravity inversion, spanning the South China Sea from offshore China and Vietnam to offshore Malaysia, Brunei and the Philippines to examine variations in ocean–continent transition structure and ocean-basin width. Our analysis shows a highly asymmetrical conjugate margin structure. The Palawan margin shows a narrow transition from continental to oceanic crust. In contrast, the conjugate northern margin of the South China Sea shows a wide region of thinned continental crust and an isolated block of continental crust (the Macclesfield Bank) separated from the Chinese margin by a failed oceanic rift. The Dangerous Grounds are predicted to be underlain by fragmented blocks of thinned continental crust. We use maps of crustal thickness and continental lithosphere thinning from gravity inversion together with free-air gravity- and magnetic-anomaly data to identify structural trends and to show that rifting and the early seafloor-spreading axis had an ENE–WSW trend while the later seafloor-spreading axis had a NE–SW trend.

Tectonic models for Borneo’s Cenozoic evolution differ in several aspects, particularly in the extent to which they include paleomagnetic data suggestive of strong counterclockwise rotation between 30 and 10 Ma. Key areas are undersampled. We present the results of a paleomagnetic study of Eocene to Early Miocene sandstones from northwest Sabah, principally from the Crocker Formation. We obtained reliable site means from 11 locations along a 250 km northeast-southwest transect using thermal demagnetization to isolate characteristic remanent magnetization (ChRM) directions. The Crocker Formation sandstones are pervasively remagnetized; pyrrhotite dominates the ChRM signal. Locations can be grouped into different domains on the basis of the relative sense of rotation about a vertical axis. Mean ChRM directions for seven locations between Kota Kinabalu and Keningau (declination, dec 12°–19°; inclination, inc –22°–23°) indicate minor clockwise rotation and modest tilting, whereas two locations near Tenom (dec 321°–345°, inc –6°–24°) record counterclockwise rotation and modest tilting. Although we cannot precisely date the age of remagnetization, the results of fold tests from 4 locations, interpreted within the regional structural framework, strongly indicate that remagnetization occurred between 35 and 15 Ma, the waning stages of the Sarawak orogeny to an early phase of the Sabah orogeny. Our results pose serious difficulties for current tectonic models in which Borneo rotates 50° counterclockwise as a rigid block between 30 and 10 Ma. With respect to prior paleomagnetic studies, we suspect that an early episode of strong regional counterclockwise rotation (before 35 Ma) was overprinted not only by differential clockwise rotation of crustal blocks during opening of the South China Sea (32–23 Ma), but also locally by a younger (after 10 Ma) counterclockwise rotation.

The West Baram Line separates NW Borneo’s two petroleum systems. Oligocene sandstone and Lower Miocene carbonate reservoirs of the gas-prone Luconia system lie SW of that line. Northeast of the West Baram Line, the oil-rich Baram-Balabac Basin produces from Middle Miocene to Early Pliocene sandstones deposited in a foreland basin on the western side of the mountainous interior of Borneo. On the present-day shelf exploration efforts spanning nearly four decades have focused on the Champion and Baram deltas and associated extensional growth fault structures. Many of these structures have experienced youthful inversion owing to ongoing tectonic shortening. Recent discoveries prove this petroleum system extends into deep water beyond the modern shelf edge where an active fold-thrust belt has formed above autochthonous rifted continental crust of the Dangerous Grounds province in the South China Sea. New regional mapping (c. 100 000 km2) integrating seismic, borehole and gravity data shows that the Baram-Balabac Basin is segmented into four structural domains whose boundary zones trend NW-SE similar to the strike of the West Baram Line. Domain boundaries appear to control the position of the basin’s palaeoshelf edges, turbidite depositional systems, major unconformities and the position of the basin’s largest fields. These observations suggest that the domain boundaries are the expression of deep structures, probably within the underlying rifted continental crust. Two of the domain boundaries can be projected onshore to align with fault systems separating contrasting geological elements indicating they represent features of tectonic interest. The onshore geology of NW Borneo represents the early history of the Baram-Balabac Basin. The onshore geology is reviewed and new outcrop, biostratigraphic and palaeomagnetic data from Sabah are discussed in light of current models for the region’s tectonic evolution. A hybrid model is proposed in which the Baram-Balabac Basin post-dates the Sarawak Orogeny. The Sarawak Orogeny, which appears to be more regionally extensive than previously believed, is attributed to Eocene to Early Oligocene collision of the Dangerous Grounds and Reed Bank with Sabah and Palawan. In the proposed model Oligo-Miocene subduction of oceanic crust under NW Borneo is minimal. The Sabah Orogeny and younger inversion events are related to underthrusting of the Dangerous Grounds driven by both the opening of the South China Sea and by NW-directed subduction beneath SE Sabah in the Semporna-Dent Peninsula. In this context the structural complexity of the Baram-Balabac foreland basin reflects the adjustment of deep crustal blocks to far-field tectonic stress.

Tectonic subsidence analysis of borestratigraphic package is immature, consisholes north of the Bewani-Torricelli-Prince tent with observed low present-day geother Alexander Mountain (BTPA) range of Northern ma1 gradients. Present day oi l seeps in Papua New Guinea reveals rapid post-Eocene Northern New Guinea are anomalous when tectonic subsidence punctuated by two discompared to these computations. We suggest t inc t episodes of tec tonic upl i f t . These two hypotheses: Either the oil was generatpulses resulted from near-field changes in ed as a result of local (igneous) thermal strain produced by segmented evolution of events, or as our seismic data suggests, the the sinistral Northern New Guinea wrench oil migrated along conduits associated with fault system. Periods of slow tectonic subm a j o r t h r u s t f a u l t s y s t e m s f r o m d e e p e r sidence are marked by carbonate-dominated buried source rocks to the south. sediment deposition whereas episodes of greatest tectonic subsidence are predomina t e ly s i l i c i c l a s t i c . Our outcrop-to-borehole reconnaissance demonstrates that these sequences are primarily fine-grained, protected shelf-to-slope facies with uncommon biolithites, grainstones, and coarse-grained s i l i c i c l a s t i c s .

Synthesis of available geologic and geophysical data for Papua New Guinea yields a series of kinematically constrained tectonic reconstructions. These reconstructions document the post-Jurassic evolution of the northern margin of the Australian plate from a rifted, passive continental margin to one composed of accreted, tectonostratigraphic terranes undergoing sinistral oblique transpression. The allochthonous terranes represent marginal basins and fringing island arcs which began docking to the Australian plate in the Miocene. At present, assemblage forms a diffuse suture zone between the Australian and Pacific plates, structurally responding in a complex fashion to the regional oblique compression.