Expedition 367 Preliminary Report: South China Sea Rifted Margin

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.

2021

Recommended Citation Jian, Z., Larsen, H., Zarikian, C., Jian, Z., Christian Larsen, H., Alvarez Zarikian, C., Bowden, S., Cukur, D., Dadd, K., Ding, W., Ferré, E., Ferreira, F., Gewecke, A., Huang, E., Jiang, S., Jin, H., Kurzawski, R., Li, Y., Li, B., Lin, J., Liu, C., Liu, C., Mohn, G., Ningthoujam, L., Osono, N., Peate, D., Persaud, P., Qiu, N., Satolli, S., Schindlbeck, J., Straub, S., Tian, L., & Van Der Zwan, F. (2018). International ocean discovery program expedition 368 preliminary report: South China Sea Rifted Margin Testing hypotheses for lithosphere thinning during continental breakup: Drilling at the South China Sea rifted margin. Integrated Ocean Drilling Program: Preliminary Reports (368), 1-54. https://doi.org/10.14379/iodp.pr.368.2018

Journal of Geophysical Research: Solid Earth, 2015

Coring/logging data and physical property measurements from International Ocean Discovery Program Expedition 349 are integrated with, and correlated to, reflection seismic data to map seismic sequence boundaries and facies of the central basin and neighboring regions of the South China Sea. First-order sequence boundaries are interpreted, which are Oligocene/Miocene, middle Miocene/late Miocene, Miocene/Pliocene, and Pliocene/Pleistocene boundaries. A characteristic early Pleistocene strong reflector is also identified, which marks the top of extensive carbonate-rich deposition in the southern East and Southwest Subbasins. The fossil spreading ridge and the boundary between the East and Southwest Subbasins acted as major sedimentary barriers, across which seismic facies changes sharply and cannot be easily correlated. The sharp seismic facies change along the Miocene-Pliocene boundary indicates that a dramatic regional tectonostratigraphic event occurred at about 5 Ma, coeval with the onsets of uplift of Taiwan and accelerated subsidence and transgression in the northern margin. The depocenter or the area of the highest sedimentation rate switched from the northern East Subbasin during the Miocene to the Southwest Subbasin and the area close to the fossil ridge in the southern East Subbasin in the Pleistocene. The most active faulting and vertical uplifting now occur in the southern East Subbasin, caused most likely by the active and fastest subduction/obduction in the southern segment of the Manila Trench and the collision between the Northeast Palawan and the Luzon arc. Timing of magmatic intrusions and seamounts constrained by seismic stratigraphy in the central basin varies and does not show temporal pulsing in their activities.

Nature Communications, 2020

During extension, the continental lithosphere thins and breaks up, forming either wide or narrow rifts depending on the thermo-mechanical state of the extending lithosphere. Wide continental rifts, which can reach 1,000 km across, have been extensively studied in the North American Cordillera and in the Aegean domain. Yet, the evolutionary process from wide continental rift to continental breakup remains enigmatic due to the lack of seismically resolvable data on the distal passive margin and an absence of onshore natural exposures. Here, we show that Eocene extension across the northern margin of the South China Sea records the transition between a wide continental rift and highly extended (<15 km) continental margin. On the basis of high-resolution seismic data, we document the presence of dome structures, a corrugated and grooved detachment fault, and subdetachment deformation involving crustal-scale nappe folds and magmatic intrusions, which are coeval with supradetachment ba...

Petroleum Geoscience, 2010

The Cenozoic evolution of SE Asia records a diverse array of tectonic processes with rifting, subduction, terrane collision and large-scale continental strike-slip faulting occurring in spatially and temporally complex relations. Oligocene seafloor spreading and rift propagation in the South China Sea are critical tectonic events that overprint an earlier phase of regional extension. Two end-member models proposed to explain the opening of the South China Sea differ in the relative importance of extrusion versus subduction as the driving mechanism. This paper treats the South China Sea region as a large multi-phase continental rift basin. Synthesizing recently published studies and using filtered Bouguer gravity data, we make a series of observations and possible interpretations to advance the notion that a hybrid tectonic models need to be proposed and tested. We present an example from the Phu Khanh Basin where flexural backstripping supports our interpretation that an 'out-of-sequence' rifting event was of sufficient magnitude to completely attenuate the continental crust in the ultra deep water part of the basin. The complex rift history of the region leads us to believe that future frontier hydrocarbon exploration will carry large uncertainties from basin to basin. Red triangles show location of age dates (reviewed by . AC, possible attenuated crust; G10 marks prominent north-trending structurally-controlled carbonate build.

The Cenozoic evolution of SE Asia records a diverse array of tectonic processes with rifting, subduction, terrane collision and large-scale continental strike-slip faulting occurring in spatially and temporally complex relations. Oligocene seafloor spreading and rift propagation in the South China Sea are critical tectonic events that overprint an earlier phase of regional extension. Two end-member models proposed to explain the opening of the South China Sea differ in the relative importance of extrusion versus subduction as the driving mechanism. This paper treats the South China Sea region as a large multi-phase continental rift basin. Synthesizing recently published studies and using filtered Bouguer gravity data, we make a series of observations and possible interpretations to advance the notion that a hybrid tectonic models need to be proposed and tested. We present an example from the Phu Khanh Basin where flexural backstripping supports our interpretation that an 'out-of-sequence' rifting event was of sufficient magnitude to completely attenuate the continental crust in the ultra deep water part of the basin. The complex rift history of the region leads us to believe that future frontier hydrocarbon exploration will carry large uncertainties from basin to basin. Red triangles show location of age dates (reviewed by . AC, possible attenuated crust; G10 marks prominent north-trending structurally-controlled carbonate build.

The South China Sea has increasingly become a place for the study of continental break-up and especially an example of rifting where the lithosphere is known to be warmer than usual and weak compared to many classic, Atlantic-type continental margin settings (Cullen et al., 2010). The South China Sea has several advantages over larger ocean basins for the study of rifting and break-up in that it is much easier to match conjugate rifted margins against one another because extension and seafloor spreading ended during the Miocene when the SW-directed propagation of the spreading axis ceased (Barckhausen and Roeser, 2004; Briais et al., 1993). Matching conjugate margins makes reconstruction of strain accommodation much easier and more accurate and allows us to test models of pure versus simple shear in modern and ancient rifted margin settings (Lister et al., 1986; Stampfli et al., 1991). These are central to understanding problems, such as why subsidence in rifted margins is often much more than can be predicted from simple rift models and the degree of extension seen in seismic profiles (Ceramicola et al., 2005; Hall and White, 1994).

Marine and Petroleum Geology, 2013

Seismic reflection data imaging conjugate crustal sections at the South China Sea margins result in a conceptual model for rift-evolution at conjugate magma-poor margins in time and space. The wide Early Cenozoic South China Sea rift preserves the initial rift architecture at the distal margins. Most distinct are regular undulations in the crustemantle boundary. Individual rift basins are bounded to crustal blocks by listric normal faults on either side. Moho uplifts are distinct beneath major rift basins, while the Moho is downbended beneath crustal blocks, with a wavelength of undulations in the crust emantle boundary that approximately equals the thickness of the continental crust. Most of the basinbounding faults sole out within the middle crust. At the distal margins, detachment faults are located at a mid-crustal level where a weak zone decouples crust and mantle lithosphere during rifting. The lower crust in contrast is interpreted as being strong. Only in the region within about 50 km from the Continent eOcean Transition (COT) we suggest that normal faults reach the mantle, enabling potentially a coupling between the crust and the mantle. Here, at the proximal margins detachment fault dip either seaward or landward. This may indicate the presence of exhumed mantle bordering the continental margins. Post-rift shallow-water platform carbonates indicate a delay in subsidence during rifting in the South China Sea. We propose that this is an inherent process in highly extended continental margins and a common origin may be the influx of warm asthenospheric material into initially cool sub-lithospheric mantle. On a crustal-scale largely symmetric process predominate in the initial rifting stage. At the future COT either of the rift basin-bounding faults subsequently penetrates the entire crust, resulting in asymmetry at this location. However, asymmetric deformation which is controlled by large scale detachment faulting is confined to narrow areas and does not result in a margin-wide simple-shear model. Rather considerable along-margin variations are suggested resulting in alternating "upper and lower plate" margins.

Seismic reflection data imaging conjugate crustal sections at the South China Sea margins result in a conceptual model for rift-evolution at conjugate magma-poor margins in time and space.