Reed Bank Research Articles

The Law of the Sea and PRC Gray-Zone Operations in the South China Sea

A growing number of incidents—particularly since 2009—highlight the South China Sea (SCS) as the preeminent venue for the People's Republic of China (PRC) maritime gray zone operations. “Gray zone operations” are, in essence, operations that are designed to exploit or create legal (and other) uncertainties for a military or strategic advantage. A prominent example is the way that China has used the so-called nine(ten)-dash line without fully explaining the legal basis for it. There are other examples, as well. China has deliberately cultivated uncertainty about the sovereign status of maritime militia vessels—and thus about whether and how the conduct of these vessels might be directly attributable to the PRC. It has harassed U.S. survey vessels seventy-five nautical miles (nm) south of Hainan Island in an effort to disrupt military survey operations that it claims are impermissible and has sunk a Philippine fishing vessel at Reed Bank in an effort to enforce Chinese claims to exclusive fisheries rights in this area. It has also asserted claims and enforcement rights in zones that clearly belong to other states—including actions against Indonesian fishing vessels in seas in the vicinity of the Natuna Islands, which are part of the Indonesian Exclusive Economic Zone (EEZ) (and Continental Shelf). And it has conducted operations on the basis of inapplicable maritime zone rights, such as by asserting a territorial sea and thus the right to control innocent passage around low-tide elevations with artificial installations built upon them—such as with Mischief Reef. The list goes on.

Early Cretaceous compressive structures in the Nansha block (Dangerous Grounds): Implications for the late Mesozoic tectonic regime on the southern margin of the South China Sea

The Mesozoic and Cenozoic tectonic evolution of South China is important for understanding the transition from subduction to extension related to the Paleo-Pacific plate. Here, the compressive structures of Lower Cretaceous sequences in the Liyue Bank (Reed Bank) and Nansha block (Dangerous Grounds) are presented using drilling data from Mesozoic strata and seismic data. Both south-directed and north-directed thrust nappes developed on the southern margin of the South China Sea. In addition, we observed a series of Early Cretaceous compressive structures in the Chaoshan Depression along the northern margin of the South China Sea. The compressive structures are accompanied by sedimentary environment changes from abyssal to continental deposition between 128.5 Ma and 79.5 Ma, which is perhaps associated with the collision between the South China block and Luconia block (Luconia-Dangerous Grounds-Reed Bank). The late Mesozoic thrust structures in the Chaoshan Depression and Liyue Bank (Reed Bank) and Nansha block (Dangerous Grounds) are related to the subduction of the Paleo-Pacific Ocean.

Cenozoic tectonic events and their implications for constraining the structure and stratigraphic styles from rifting to collision at the southeastern margin of the South China Sea

The southeastern margin of the South China Sea (SCS) connects the Southwest sub-basin of the SCS to the north and the collision zone of Nansha–Liyue blocks and Palawan Island to the south, and it records a complex process of the superimposed regional tectonics. Based on recently collected 2D seismic profiles, constraint by wells and outcrop data, our study establishes a stratigraphic framework across different structural units related to various tectonic settings. We identified six major unconformities from the Cenozoic sediment succession and analyzed their geological significance through a comprehensive interpretation of the regional seismic profiles and related well data. Results show the unconformity T7 represents the cessation of the initial rifting along the whole continental margin, which corresponds to the N-S spreading in the central part of the SCS oceanic basin. Reflector T4 is the breakup unconformity that represents the structural evolution transitioning from rifting to post-rifting in the study area. It also synchronizes with the initial deposition of the Nido carbonate platform. Reflector T3 is the most significant regional structural unconformity that corresponds to the cessation of seafloor spreading in the Southwest sub-basin and collision between the Nansha–Liyue blocks and the Cagayan Arc, choking the proto-SCS subduction zone. These tectonic events, also referred to as Sabah Orogeny, indicate the inception of peripheral foreland basin developed from northwest Palawan to the Reed Bank. Major tectonic events recorded in the study area include southward subduction and closure of the proto-SCS as well as opening and cessation of ridge spreading of the SCS, all of which lead to the Nansha–Liyue–North Palawan blocks successively rifting from the South China continent, subsequently drifting southward and ultimately colliding with the Cagayan volcanic arc. Therefore, we propose that the southeastern margin of the SCS has undergone a multi-stage tectonic evolution from the rifting basin through the post-rifting depression to the collisional foreland basin in the Cenozoic.

Lithospheric structure of the South China Sea and adjacent regions: Results from potential field modelling

This work aims to investigate the crustal and lithospheric mantle thickness of the South China Sea (SCS) and adjacent regions. The crust-mantle interface, average crustal density, and lithospheric mantle base are calculated from free-air gravity anomaly and topographic data using an iterative inversion method. We construct a three-dimensional lithospheric model with different hierarchical layers. The satellite-derived gravity is used to invert the average crustal density and Moho (crust-mantle interface) undulations. The average crustal density and LAB (lithosphere–asthenosphere boundary) depths are further adjusted by topographic data under the assumption of local isostasy. The average difference in Moho depths between this study and the seismic measurement results is <1.5 km. The results show that in oceanic regions, the Moho depths are 7.5–30 km and the LAB depths are 65–120 km. The lithospheric thickness of the SCS basin and the adjacent regions increases from the sea basin to the continental margin with a large gradient in the ocean-continent transition zones. The Moho depths of conjugate plots during the opening of SCS, Zhongsha Islands and Reed Bank, reveal the asymmetric spreading pattern of SCS seafloor spreading. The lithospheric thinning pattern indicate two different spreading directions during seafloor spreading, which changed from N-S to NW-SE after the southward transition of the spreading axis. The lithosphere of the SCS basin and adjacent regions indicate that the SCS basin is a young basin with a stable interior lithosphere.

S-wave velocity structure and tectonic implications of the northwestern sub-basin and Macclesfield of the South China Sea

Based on the optimum P-wave model, the S-wave velocity structure of a wide angle seismic profile (OBS2006-1), across the northwestern sub-basin (NWSB) and the Macclesfield, is simulated by a 2-D ray-tracing method. The results indicate the S-wave velocities in the upper and lower crust of the NWSB are 3.2–3.6 km/s and 3.6–4.0 km/s, with Vp/Vs ratios of 1.82–1.88 and 1.74–1.82, respectively, which reflect typical oceanic crust characteristics. The S-wave velocity in the upper crust of the NWSB is a little higher in the NNW segment than that in the SSE segment, while the lateral variation of Vp/Vs ratio is in the opposite. We suggest that the NWSB might have experienced asymmetrical magma flows during sea floor spreading, which may have blurred the magnetic anomaly lineation. The comparison of S-wave velocities along the northern margin of the SCS shows that the west section is different from the east section, and the northwestern margin has a non-volcanic crust structure. The S-wave structures and P-wave velocity models along the northern margin, Macclesfield and Reed Bank show that the Macclesfield might have a conjugate relationship with the Reed Bank.

Sequential patterns in Cenozoic marginal basins of the Northwest Pacific

AbstractBy using magnetic anomaly identifications, onshore and offshore exploratory wells, ODP drillings, isotopic dating and orthochronology results, this paper summarizes the tectonic event sequence (including extension, back‐arc spreading, subsidence, convergence/compressive deformation and thrusting) of the marginal basins of the NW Pacific and extracted sequence patterns of the marginal basins according to the tectonic event sequence from an array of oil company regional seismic lines and other geological data. Three important evolutionary stages of the Northwestern Pacific (NW Pacific) marginal sea basins are distinguished: from the Palaeocene to Eocene, from Oligocene to Middle Miocene and from the later Middle Miocene to the present day. Each stage corresponds to a rifting sequence (Palaeocene to Eocene), a drifting sequence (Oligocene to Middle Miocene) and a subsidence/convergence sequence (later Middle Miocene to the present day). The rifting sequence and drifting sequence developed sequentially in the South China Sea Basin, from Palaeocene to Eocene and from Oligocene to Middle Miocene, while in the Japan and Okhotsk Sea basins, an early stage rifting sequence was not developed in the Palaeocene to Eocene interval. However, the drifting and rifting sequence developed synchronously from the Oligocene to Middle Miocene. From the Later Miocene time, all the marginal basins of the Northwest Pacific basically stopped drifting and began to enter the subsidence (post‐drifting) stage. An arc‐continent collision from the south caused the formation of a convergence sequence in the southern margin of each marginal basin. The NW Pacific Plate collision with the Kuril–Kamchatka Arc led to the deformation of the subsidence sequence in the southern margin of the Kuril Sea Basin. The NW Pacific Plate convergence with the Japan Arc led to the formation of a convergence sequence in the southern margin of the Japan Sea Basin. The Borneo–Palawan Arc convergence with the Dangerous Grounds–Reed Bank Block in the South China Sea Basin from 15 Ma led to the formation of the convergence sequence in the southern margin of the South China Sea Basin. The degree of thrusting deformation of the convergent sequence increased gradually from north to south in the marginal basins of the Northwest Pacific. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea

The shear wave velocity structure of a wide angle seismic profile (OBS973-2) across Reed Bank in the southern continental margin of the South China Sea (SCS) is simulated by 2-D ray-tracing method, based on its previous P-wave model. This profile is 369-km-long and consists of fifteen three-component ocean bottom seismometers (OBS). The main results are as follows.(1) The model consists of seven layers and the shear wave velocity increases from 0.7km/s at the top of sediment layer to 4.0km/s in the lower crust. (2) The Moho depth decreases from 20-22km at the Reed Bank to 9-11km at the deep oceanic basin with the shear wave velocity of 4.2km/s below the Moho. (3) The Vp/Vs ratio decreases with depth through the sedimentary layers, attributed to increased compaction and consolidation of the rocks. (4) In the continental upper crust (at model distance 90-170km), S-wave velocity (2.5–3.2km/s) is relatively low and Vp/Vs ratio (1.75–1.82) is relatively high compared with the other parts of the crust, corresponding to the lower P-wave velocity in the previous P-wave model and normal faults revealed by MCS data, indicating that a strong regional extensional movement had occurred during the formation process of the SCS at the Reed Bank area. (5) The S-wave structures indicate that Reed Bank crust has different rock compositions from that in the east section of the northern margin, denying the presence of conjugate relationship of Reed Bank with Dongsha islands. According to P-wave models and other data, we inferred that Reed Bank and Macclesfield were separated from the same continental crust during the rifting and break-up process.

Oligocene–Miocene carbonates in the Reed Bank area, South China Sea, and their tectono-sedimentary evolution

We have analyzed two recently acquired multi-channel seismic profiles across the Reed Bank area, South China Sea. A detailed seismic interpretation coupled with drilling data has proved the occurrence of a wide carbonate platform, developed between the Late Oligocene and the Early Miocene (32–20 Ma). The top of these carbonates is an important regional unconformity, corresponding to a strong and continuous seismic reflector. An age of about 20 Ma is inferred for this reflector, acting as a regional unconformity marking the cessation of seafloor spreading with an erosional/non-depositional hiatus, spanning 3 or 4 m.y. The time interval during which the carbonate platforms formed is concurrent with the opening of the South China Sea. The reconstruction of the tectonic subsidence in this area has shown a decrease in rate of subsidence during the drifting stage (32–17 Ma), indicating that the whole Reed Bank area was in a tectonically stable and shallow water environment for more than 10 m.y., favoring the development of shallow water carbonates. With a sharply increased subsidence rate after the end of spreading (17 Ma) and a continuously increased sea level, the carbonate platforms were drowned and died, except some structural highs with reef buildups, where the carbonate sedimentation continued to Middle Miocene. We suggest that delayed tectonic subsidence in Reed Bank area might have been related to a secondary mantle convection under the rift lasting more than about 10 m.y.

Different expressions of rifting on the South China Sea margins

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 E–W 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 NE–SW 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.

Mapping crustal thickness using marine gravity data: Methods and uncertainties

Crustal thickness is a critical parameter for understanding the processes of continental rifting and breakup and the evolution of petroleum systems within passive margins. However, direct measurements of crustal thickness are sparse and expensive, highlighting the need for methodologies using gravity anomaly data, jointly with other geophysical data, to estimate crustal thickness. We evaluated alternative gravity inversion methodologies to map crustal thickness variations at rifted continental margins and adjacent oceanic basins, and we tested our methodology in the South China Sea (SCS). Different strategies were investigated to estimate and remove the gravity effect of density variations of sediments and the temperature and pressure variations of the lithospheric mantle from the observed free air gravity anomaly data. Sediment density was calculated using a relationship between sediment thickness, porosity, and density. We found that this method is essential for crustal thickness inversion in the presence of a thick sedimentary cover by comparing the Moho depths obtained from gravity inversion and seismic interpretation in the Yinggehai Basin where sediments are up to 13 km thick; the inversion accuracy depended on the parameters of the exponential equation between porosity and the buried depth. We modeled the lithospheric mantle temperature field based on oceanic crustal age, continental crustal stretching factors, and other boundary conditions. We tested three different methods to calculate the thermal expansion coefficient, which is either held constant or is a linear/polynomial function of temperature, for applying a thermal correction and found that the inversion results were relatively insensitive to alternative methods. We compared inversion results with two recent deep seismic profiles that image the rifted continental edge at the northern margin of the SCS and the continental Liyue Bank (Reed Bank) at the southern margin, and we found that the inversion accuracy was improved considerably by removing sediment, thermal, and pressure gravity effects.

Enriched and depleted characters of the Amnay Ophiolite upper crustal section and the regionally heterogeneous nature of the South China Sea mantle

The volcanic section of the Middle Oligocene Amnay Ophiolite in Mindoro, Philippines has previously been shown to be of normalmid-oceanic ridge basalt (NMORB) composition. Here we report for the first time an enriched mantle component that is additionally recorded in this crustal section. New whole rock major and trace element data are presented for nine mafic volcanic rocks from a section of the ophiolite that has not been previously examined. These moderately evolved tholeiitic basalts were found to have resulted from the bulk mixing of ∼10% ocean island basalt components with depleted mantle. Drawing together various geochemical characteristics reported for different rock suites taken as representatives of the South China Sea crust, including the enriched MORB (EMORB) and NMORB of the East Taiwan Ophiolite, the NMORB from previous studies of the Amnay Ophiolite and the younger ocean floor eruptives of the Scarborough Seamount–Reed Bank region, a veined mantle model is proposed for the South China Sea mantle. The NMORB magmatic products are suggested to have been derived from the more depleted portions of the mantle whereas the ocean island basalt (OIB) and EMORB-type materials from the mixing of depleted and veined/enriched mantle regions.

Transverse segmentation of the Baram-Balabac Basin, NW Borneo: refining the model of Borneo's tectonic evolution

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.

Mesozoic and Cainozoic rocks dredged from the South China Sea (Reed Bank area) and Sulu Sea and their significance for plate-tectonic reconstructions

Outcrops of pre-Quaternary rocks were sampled by dredging and coring during two cruises of RV ‘Sonne’ in the South China Sea and Sulu Sea (1982, 1983). Late Triassic deltaic sandstones wih plant remains are the oldest sediments recovered. Late Jurassic/Early Cretaceous regional metamorphism is indicated by gneiss and garnet-mica schists exposed in the northwestern flanks of Reed Bank. These rocks, and occurrences of gabbro, diorite, dacite, rhyolite and siliceous shale prove that the Dangerous Grounds and Reed Bank are underlain by a continental fragment. During the period of seafloor spreading in the South China See a Late Oligocene to Early Miocene carbonate platform developed on this southwards-drifting continental fragment, which subsided in the Middle Miocene. Middle Miocene andesite and Early to Middle Miocene reefal carbonates were recovered from the Cagayan Ridge in the Sulu Sea. The new data are in accord with the plate-tectonic history developed by Taylor and Hayes (1980, 1983) and Holloway (1981).

Geology of Southern Palawan-Balabac Area: Results of Marine Geophysical Studies in Northwest Sulu Sea and in South China Sea: ABSTRACT

According to our interpretation, two major terranes are superimposed in the Balabac-south Palawan area. An autochthonous Palawan continental terrane extends from the Dangerous Grounds and Reed Bank to beneath the islands of Palawan and Balabac, and parts of the Sulu Sea. An allochthonous imbricated terrane covers an area of approximately 170 x 350 km between the southwestern prolongation of the Cagayan Ridge in the Sulu Sea and the Palawan Trough in the South China Sea, including southern and central Palawan and the Balabac Island. This originally accreted terrane formed outside the Balabac-southern Palawan area, but has been thrust onto the Palawan continental terrane since the early Miocene. If the allochthonous terrane is underlain by the continuation of the Palawan continental terrane, as the authors believe, new possibilities for hydrocarbon exploration will develop.

Profiler‐sonobuoy measurements in the South China Sea Basin

Analyses of single‐channel seismic reflection profiles, and wide‐angle reflection and refraction data from sonobuoys, indicate that the deep part of the South China Sea basin is typically oceanic in structure, except that layer 2 is about 1 km thicker than usual and layer 3 is only about half the usual thickness. The top of layer 2 has a rough upper surface and becomes deeper in the northern part of the basin, where it is overlain by sediments of velocity 1.7–3.8 km/s up to 3 km thick. A 6‐km‐thick sequence of sediments fills a basement depression just north of the outer subsurface (peripheral) ridge of the Sunda shelf that may be structurally part of the Palawan trough. Thick sediments were measured at the outer parts of the Sunda shelf, Palawan shelf, Northeast Borneo shelf, and in the Taiwan straits (Pescadores channel) south of the Penghu‐Peikang basement high. The Manila trench is divided into two sections, a sediment‐filled trench between southernmost Taiwan and Stewart bank off central northern Luzon and a topographic trench between Stewart bank southward into the Mindoro straits. Burial of part of the trench by sediments that thicken to the north and the thick sediment cover of the northern South China Sea basin indicate provenance from the north. Each of the margins of the South China Sea basin is interpreted to have a different structural style. The northern margin of the basin is predominantly a zone of tension (graben and horst block structures), the southern margin is predominantly a zone of compression (fold structures), the western margin may be a zone of shear, and the eastern margin is a subduction zone (Palawan trough and Manila trench). The geological and geophysical characteristics of the basin suggest a middle Tertiary to Recent history of development that involved spreading of the sea floor. It seems probable that the southern margin of the basin, called the Reed bank crustal block, was formerly attached to mainland Asia along the present eastern edge of Macclesfield bank. Sometime during the Paleogene the Reed bank crustal block broke away from the continent, and new sea floor was created between the southeastward migrating block and the Asian mainland. Old ocean crust in front of the advancing block was subducted along the northwestern sides of Borneo and Palawan. Subduction ceased in the early Miocene with the collision of the crustal block and the Borneo‐Palawan isthmian ridge.