Mapping structures of the South China Sea; a synthesis

Rifting of continental margins is generally diachronous along the zones where continents break due to various factors including the boundary conditions which trigger the extensional forces, but also the internal physical boundaries which are inherent to the composition and thus the geological history of the continental margin. Being opened quite recently in the Tertiary in a scissor-shape manner, the South China Sea (SCS) offers an image of the rifting structures which varies along strike the basin margins. The SCS has a long history of extension, which dates back from the Late Cretaceous, and allows us to observe an early stretching on the northern margin onshore and offshore South China, with large low angle faults which detach the Mesozoic sediments either over Triassic to Early Cretaceous granites, or along the short limbs of broad folds affecting Palaeozoic to Early Cretaceous series. These early faults create narrow troughs filled with coarse polygenic conglomerate grading upward to coarse sandstone. Because these low-angle faults reactivate older trends, they vary in geometry according to the direction of the folds or the granite boundaries. A later set of faults, characterized by generally EeW low and high angle normal faults was dominant during the Eocene. Associated half-graben basement deepened as the basins were filling with continental or very shallow marine sediments. This subsequent direction is well expressed both in the north and the SW of the South China Sea and often reactivated earlier detachments. At places, the intersection of these two fault sets resulting in extreme stretching with crustal boudinage and mantle exhumation such as in the Phu Khanh Basin East of the Vietnam fault. A third direction of faults, which rarely reactivates the detachments is NEeSW and well developed near the oceanic crust in the southern and southwestern part of the basin. This direction which intersects the previous ones was active although sea floor spreading was largely developed in the northern part, and ended by the Late Miocene after the onset of the regional Mid Miocene unconformity known as MMU and dated around 15.5 Ma. Latest Miocene is marked by a regional basement drop and localized normal faults on the shelf closer to the coast. The SE margin of the South China Sea does not show the extensional features as well as the Northern margin. Detachments are common in the Dangerous Grounds and Reed Bank area and may occasionally lead to mantle exhumation. The sedimentary environment on the extended crust remained shallow all along the rifting and a large part of the spreading until the Late Miocene, when it suddenly deepened. This period also corresponds to the cessation of the shortening of the NW Borneo wedge in Palawan, Sabah, and Sarawak. We correlate the variation of margin structure and composition of the margin; mainly the occurrence of granitic batholiths and Mesozoic broad folds, with the location of the detachments and major normal faults which condition the style of rifting, the crustal boudinage and therefore the crustal thickness.

Marine and Petroleum Geology, 2014

Rifting of continental margins is generally diachronous along the zones where continents break due to various factors including the boundary conditions which trigger the extensional forces, but also the internal physical boundaries which are inherent to the composition and thus the geological history of the continental margin. Being opened quite recently in the Tertiary in a scissor-shape manner, the South China Sea (SCS) offers an image of the rifting structures which varies along strike the basin margins. The SCS has a long history of extension, which dates back from the Late Cretaceous, and allows us to observe an early stretching on the northern margin onshore and offshore South China, with large low angle faults which detach the Mesozoic sediments either over Triassic to Early Cretaceous granites, or along the short limbs of broad folds affecting Palaeozoic to Early Cretaceous series. These early faults create narrow troughs filled with coarse polygenic conglomerate grading upward to coarse sandstone. Because these low-angle faults reactivate older trends, they vary in geometry according to the direction of the folds or the granite boundaries. A later set of faults, characterized by generally EeW low and high angle normal faults was dominant during the Eocene. Associated half-graben basement deepened as the basins were filling with continental or very shallow marine sediments. This subsequent direction is well expressed both in the north and the SW of the South China Sea and often reactivated earlier detachments. At places, the intersection of these two fault sets resulting in extreme stretching with crustal boudinage and mantle exhumation such as in the Phu Khanh Basin East of the Vietnam fault. A third direction of faults, which rarely reactivates the detachments is NEeSW and well developed near the oceanic crust in the southern and southwestern part of the basin. This direction which intersects the previous ones was active although sea floor spreading was largely developed in the northern part, and ended by the Late Miocene after the onset of the regional Mid Miocene unconformity known as MMU and dated around 15.5 Ma. Latest Miocene is marked by a regional basement drop and localized normal faults on the shelf closer to the coast. The SE margin of the South China Sea does not show the extensional features as well as the Northern margin. Detachments are common in the Dangerous Grounds and Reed Bank area and may occasionally lead to mantle exhumation. The sedimentary environment on the extended crust remained shallow all along the rifting and a large part of the spreading until the Late Miocene, when it suddenly deepened. This period also corresponds to the cessation of the shortening of the NW Borneo wedge in Palawan, Sabah, and Sarawak. We correlate the variation of margin structure and composition of the margin; mainly the occurrence of granitic batholiths and Mesozoic broad folds, with the location of the detachments and major normal faults which condition the style of rifting, the crustal boudinage and therefore the crustal thickness.

Bulletin of the Geological Society of Malaysia

The NW Sabah continental margin can be subdivided into 6 tectono-stratigraphic provinces on the basis of differences in structural styles and sedimentation histories: (1) Rajang Group Fold-Thrust Belt, (2) Inboard Belt, (3) Baram Delta, (4) Outboard Belt, (5) NW Sabah Trough and (6) NW Sabah Platform. The Tertiary sedimentary sequence was deposited during two main phases of basin development: (1) A pre-early Middle Miocene phase of generally deep-marine clastic sedimentation. (2) A post-early Middle Miocene phase of clastic she1ti'slope deposition, which prograded northwestward over the underlying sediment wedge and is separated from the latter by a major regional unconformity. Evidence for Palaeogene subduction of South China Sea oceanic crust beneath NW Sabah is incomplete. The subduction hypothesis (Haile, 1973; Hamilton, 1979) is based on the following main elements: (1) The Rajang Group Fold-Thrust Belt of turbidites and associated ophiolites, interpreted to represent an accretionary prism. (2) The sub-linear NW Sabah Trough, interpreted to represent the sea-bed expression of a subduction trench. A major missing element in this hypothesis is a volcanic arc of the correct age. The NW Sabah Trough can be seen on seismic data to represent the downfaulted SE margin of the NW Sabah Platform. An additional important element in the morphology of the Trough is the Baram Delta toe thrust zone, bounding the Trough to the SE, which was emplaced post Middle Miocene. Furthermore, the Trough terminates abruptly against the Luconia Block. These factors suggest that the present NW Sabah Trough is a relatively young feature, and that if an older, Palaeogene, trench is present, it would occur landward of, and with a different orientation to the NW Sabah Trough. The Palaeogene trench would no longer have a surface expression. Hamilton's (1979) geodynamic interpretation is supported with the proposed modification of a Palaeogene subduction system broadly parallel to the structural strike of the Rajang Group Fold-Thrust Belt. The Inboard Belt wrench tectonics trend parallel to the postulated trench (cf. the Semangko fault of the Sumatra subduction system) as a result of oblique subduction. Counterclockwise rotation of Borneo from Eocene to Middle Miocene resulted in increasingly oblique subduction. The Baram Delta sediment prism masks the Palaeogene trench. The following three-stage model for the tectonic evolution of the NW Sabah continental margin is proposed: (1) Late Eocene to early Middle Miocene oblique subduction of South China Sea oceanic crust beneath NW Sabah with deposition and subsequent imbrication of deep-marine sediments into an accretionary prism. (2) Diachronous collision of the South China Sea attenuated continental crust (NW Sabah Platform) with Sabah, and associated cessation of ocean-floor spreading in the early Middle Miocene, led to regional uplift and erosion of the accretionary prism, resulting in a major regional unconfornlity (the Deep Regional Unconformity). This was followed by northwestward progradation of clastic sediments over the Inboard Belt from Middle Miocene to early Late Miocene. (3) Resumption of convergent forces between Borneo and the NW Sabah Platform in middle Late Miocene was accompanied by major tectonic activity. The Inboard Belt was subjected to strong compressional deformation associated with major N-S wrench zones, resulting in the formation of a localised to semi-regional unconformity (the Shallow Regional Unconformity). Two new depocentres were formed seaward of the Inboard Belt: the Baram Delta and the Outboard Belt. A thick prograding clastic wedge built out towards NW in both depocentres, whilst the Inboard Belt was continuously eroded. A Late Pliocene/Pleistocene phase of locally developed deformation affected the Inboard Belt, Outboard Belt and Baram Delta.

International Petroleum Technology Conference, 2014

The Bunguran Trough (BT) covering Sarawak Deepwater Block 2F shows a number of largely parallel trends of folded Neogene anticlines, with reverse faults and thrusts in the cores, and blind thrusting and folding in the upper section of the individual mapped anticlines. The Bunguran Fold Belt (BFB), comprising the deepwater deposition setting of the Rajang Delta (synonym: West Luconia Delta), has been historically compared by explorationists/geologists with the Sabah Fold Belt, and accordingly a genetic model related to gravity sliding had been advocated. However, the gravity hypothesis may not necessary hold alone based on the following observations and considerations:

Journal of Structural Geology, 2003

The Baram Delta province evolved during the Middle Miocene to present day from a foreland basin to a shelf margin. Episodic folding events affected the region, causing uplift of the hinterland, delta progradation, and inversion of gravity-related faults. Existing models place the N-Sand NE-SW-trending folds in a strike-slip transpressional setting. New geological mapping and re-interpretation of existing data suggest the region is better understood as the development of a west-verging thrust belt in a Middle Miocene foreland basin (filled by sandstones and shales of the Belait and Setap Formations), with key major folds (Jerudong and Belait anticlines, Belait syncline) forming during the Middle Miocene as fault bend and fault propagation folds. The deformation style is complicated by the predominantly shaley Middle Miocene-Pliocene Setap Formation becoming thicker and more overpressured from south to north. Onshore where the Setap Formation is thin or absent, the Belait formation is attached to the underlying Lower Miocene and older sequences. Offshore and in a narrow onshore strip the overpressured Setap Formation causes deformation in the Beltait Formation to be detached from the underlying Cretaceous-Tertiary accretionary prism 'basement'. The detached style exhibits considerable structural complexity, including lift-off folds, growth faults, shale diapirs and pipes. Onshore thrust and inversion features are dominantly N-S-trending and began activity in the Middle Miocene; deformation is probably associated with an E-W maximum horizontal stress direction. In the Late Miocene (around 7.5 Ma; Watters et al., 1999) NE-SW striking inversion folds developed, located mostly over early counter-regional faults and associated reactive diapirs. Folds verge towards the NW when underlain by counter regional faults, and towards the S or SE when underlain by regional faults. Folds and diapirs along N-S trends were also reactivated. Continuation of this deformation into the Pliocene is largely confined to the offshore area. Onshore the N-S structures (no detachment) were not reactivated during the Pliocene.

Marine and Petroleum Geology, 2013

The attenuated continental crust of the Dangerous Grounds is located in the southeastern part of the South China Sea. It was affected by unconformities as identified by several authors (Cullen et al., 2010; Hinz and Schlüter, 1985; Hutchison, 2010; Hutchison and Vijayan, 2010). In the northeastern Dangerous Grounds, a prominent reflector in seismic data is associated with the top of a widespread Oligocene to Early Miocene (18e20 Ma) carbonate platform. This reflector and the underlying carbonates can be used to constrain the timing of the unconformities and the rifting history of the Dangerous Grounds. By carefully interpreting seismic reflection lines we trace the platform carbonates based upon their appearance in the seismic image. This platform is continuous in the PalawaneBorneo trough and gets patchy toward the Dangerous Grounds. In the Dangerous Grounds the image of this key reflector changes and here it merely forms the top of a clastic layer. Carbonates remain abundant but mainly as isolated reefs that grew on top of tilted fault blocks. In the southwestern Dangerous Grounds the prominent unconformity sealing the tectonic activity is known as the Middle Miocene Unconformity. This in fact is an Early Miocene unconformity, which represents a sequence boundary in the Borneo ePalawan trough and in various parts of the Dangerous Grounds, while in other parts of the Dangerous Grounds, it represents a major angular unconformity. The unconformity characteristics supplemented with tentative ages indicate that Luconia and the southern Dangerous Grounds were sub-aerial during the Early Miocene, while the Reed Bank, the northern Dangerous Grounds and parts of the central Dangerous Grounds were mostly submerged except for some islands concentrated on the western edge of the BorneoePalawan trough. This trough is interpreted as a foreland basin where the flexural forebulge provided shallow marine conditions that promoted reef growth. As the carbonate deposition migrated from the BorneoePalawan trough toward the Dangerous Grounds we suggest that the flexural forebulge provided shallow water conditions for further reef growth on the eastern Dangerous Grounds.

Episodes, 2018

The South China Sea (SCS) is presented here as a case example to demonstrate the evolution of basins developed at convergent boundaries. The structural map published in 2017 by CGMW at the 1:3 million scale allows to visualize the location of the rifting faults from a normal to hyper-extended crust, the shape and structure of the oceanic crust and their late involvement in a convergent margin. It highlights the reactivation of the Mesozoic tectono-stratigraphic setting such as broad folds and granitic plutons during the rifting, and the effect of the resulting architecture on the NW Borneo accretionary wedge.

Proceedings, Deepwater and Frontier Exploration in Asia & Australasia Symposium

The history and development of the peri-Borneo basins are controlled by major regional tectonic events. Tectonism has also had an important effect on geomorphology and hydrology of the Neogene delta systems around the periphery of the island. These systems formed as a result of the ability to accommodate massive amounts of sediment that became available due to Neogene uplift. In gross geometry, the deltas are broadly similar, characterised by an inner onshore deformed zone, the modern delta and shallow shelf, a zone dominated by growth faults which may extend down the delta slope and an deepwater outer zone of folding and associated thrust faults. From the Oligocene the geological evolution has been the result of two opposing forces; the opening of the South China Sea which commenced in the Oligocene and westerly-directed compression as micro-continental material initially sourced from the Australian Plate ("Australoid material") moved westwards since the Miocene. Overall sinistral wrenching has produced zones of deformation extending through Borneo that are the loci of the Neogene delta systems. In western Sulawesi, Neogene compression produced the West Sulawesi Fold Belt. * TGS-NOPEC Geophysical Company ** Brunei Petroleum Co. Sdn. Bhd.

Geological Society of Malaysia Bulletin, 2017

This review combines a wide range of onshore and offshore data from Oligocene to Pliocene sediments of northern Borneo to address the following topics: (i) the stratigraphic conditions before and after the Top Crocker Unconformity [TCU], (ii) Early Miocene palaeogeography, (iii) events during later Early to mid Middle Miocene times, including the Deep Regional Unconformity [DRU], and (iv) the waning of the Sabah Orogeny in Early Pliocene times. Emphasis is placed on dating the key events, in order to consistently identify the stratigraphic and tectonic changes observed in the different data sets. The data shows a period of uplift and deformation in the north, perhaps focussed in southern Palawan, during the Oligocene then its sudden cessation at the TCU, with strong contrast in both facies and deformation style of Late Oligocene outcrops on the Kudat Peninsula compared to the Early Miocene sediments drilled offshore. This termination of tectonic deformation coincides with published estimates for the onset of ridge-jump and sea-floor spreading in the South China Sea northwest of Sabah. Following this unconformity the deposition of sediments during the Early Miocene appears to have consisted of a fluvio-deltaic high in the south (from central Borneo) and a broad deep marine basin in the north, from west of the Kudat Peninsula to at least the western part of the Sulu Sea. For the subsequent DRU, its timing and character appears to coincide with large scale sedimentary changes in eastern Sabah and this suggests a pause in regional compression during the early part of the long lasting Sabah Orogeny. The stratigraphic description of these events has important implications in plate tectonic reconstructions for northern Borneo and the South China Sea. The role of subduction on the eastern side of Sabah may be a key component of revised tectonic models and further evaluation of the area from the Dent Peninsula south to the Indonesian border is recommended.