South China Sea Margin Research Articles

Lithospheric structure of the Southwest South China Sea: implications for rifting and extension

ABSTRACT The South China Sea (SCS) is an excellent site for studying the process of conjugate margin rifting, and the origin and evolution of oceanic basins. Compared with the well-defined northern margin of the SCS, the western and southern segments of the SCS margin have not been researched in significant detail. To investigate the regional structure of the southwestern SCS, a gravity model is constructed, along with the lithospheric thermal structure along a wide-angle seismic profile. The profile extends across the conjugate margins of the Southwest Sub-Basin (SWSB) of the SCS and is based on the latest multiple geophysical measurements (including heat flow and thermo-physical parameters). The results show that the average thicknesses of the crust and thermal lithosphere along the profile are about 15 km and 57 km, respectively. The overall amount of extension of continental crust and lithosphere is more than 200 km. Thermal structure of the lithosphere shows that the continental margins are in a warm thermal state. The southwest SCS is characterized by ultra-wide, thinned continental crust and lithosphere, high Moho heat flow, early syn-rift faulted basins, undeformed late syn-rifting, and high seismic velocities in the lower crust. These various pieces of evidence suggest that the break-up of the mantle lithosphere occurred before that of the continental crust favouring a depth-dependent extension of the southwestern SCS margin.

Main controlling factors of hydrocarbon accumulation in Baiyun Sag at northern continental margin of South China Sea

Abstract To examine the distribution pattern and controlling factors of oil and gas accumulation in Baiyun Sag systematically, this study analyzes the key factors controlling hydrocarbon accumulation using a large amount of seismic, drilling and geologic data. It is concluded that the oil and gas accumulation is controlled by the following factors: (1) Massive source rock is the physical basis of the formation of medium and large oil and gas fields. (2) Orderly distribution and accumulation of hydrocarbon is regulated by the differential generation of hydrocarbon and differential hydrocarbon accumulation: spacial differential leads to “inner gas, outer oil” pattern, differential accumulation of hydrocarbon controlled the pattern of “inner gas, outer oil” in the east subsag of Baiyun; the timing differential leads to the “upper gas, lower oil” pattern. (3) Late fault activity, diapirs and inherited structural ridges controlled the favorable migration in the Neogene: the activity of faults and diapirs regulated the trap formation and vertical migration of hydrocarbon; inherited structural ridges controlled the Neogene hydrocarbon accumulation. The result effectively guides the exploration in the Baiyun Sag and surrounding area, and provides experience and evidence for the exploration in deepwater area of Pearl River Mouth Basin and other similar basins.

Characteristics and genetic analysis of submarine landslides in the northern slope of the South China Sea

Although the structural characteristics and triggering mechanisms of submarine landslides have been investigated, little attention has been paid toward the submarine landslides in canyons. In the Baiyun canyon, there are many small submarine landslides that could be an important sediment transport process of the canyons, but few have been studied. This study aims to examine the characteristics and mechanisms of submarine landslides in the Baiyun canyon on the northern continental slope of the South China Sea using multibeam depth data, acoustic sub-bottom profiles, and borehole data. The results suggest that the submarine landslides are mainly distributed in the head and side valley walls of the canyons. The surface morphology of the landslides is broken-stepped, tongued, or round, and the area of the landslides varies between 0.04 and 15.13 km2. Nevertheless, most of the landslides have an area of less than 2 km2. The scarp height of most landslides is between 20 and 40 m. Based on the internal acoustic structures of the landslides, the submarine landslides can be divided into brittle and plastic. The brittle landslides have clear landslide walls or sliding surfaces, whereas the interior of the landslides in the upper slope area of 650–740 m near the source of submarine canyon has the original stratigraphic structure. The interior of the plastic landslides is typically chaotic and has no clear landslide walls or sliding surfaces. The plastic landslides mainly occur in the head and sides of the canyon. We determined the slope stability using the software GeoSlope and concluded that the strength of the soil on the seabed slopes and the slope gradient are the main factors that control the landslides in the study area. Landslides occur when the slope gradient is greater than 9°. In addition, when the horizontal seismic load is not zero, slopes with a gradient of less than 9° are unstable. This study will be helpful in geological risk assessments and understanding of sediment transportation in the northern margin of the Northern South China Sea.

The continental extension discrepancy and anomalous subsidence pattern in the western Qiongdongnan Basin, South China Sea

Passive margins are first-order tectonic features on earth, formed by polyphase extension and breakup of continental lithosphere. It is accepted that mildly extended basins in the proximal margin form largely by uniform continental thinning, but the continental extension in deepwater is much debated. Here we choose a line from the western Qiongdongnan Basin in the magma-poor South China Sea margin to further explore the style of continental extension and the corresponding subsidence patterns, where high resolution seismic and heat flow data are available. Structural analysis reveals that the basin architecture is extremely asymmetric and mainly controlled by the SE directed normal faults, soling into a low-angle detachment fault. However, fault timings nicely show that the faults form through polyphase rifting and from inside outwards, and do not follow a simple sequential faulting like the West Iberia margin, where the upper and lower crustal deformation are strongly coupled. Perhaps the fault order here reflects a much weaker and decoupled lower crust. Then we divide the basin into five zones and calculate the average fault derived extension factors. Moreover, the whole crustal extension factors are also estimated after assessing the initial crustal thickness on basis of the backstripped subsidence. And the lithospheric extension factors are numerically modeled using a coupled lithosphere and basin scale model constrained by the heat flow and sedimentation data. Results show that along the profile, the overall lithospheric extension is much less than that of the whole crust, and only at the south end of the profile does lithospheric extension exceeds the whole crust, showing lithospheric depth-dependent thinning. The whole crustal extension is roughly equal to the faults derived extension in the northern part, displaying no extension discrepancy. In the middle part, the whole crustal extension is less than the faults derived extension, denoting inverse extension discrepancy. While the whole crustal extension is in far excess of the faults derived extension in the southern part, showing obvious positive extension discrepancy. We infer that the inverse and positive discrepancy of crustal extension was likely to be caused by landward directed lower crustal flow, driven by the horizontal pressure arising from the combination of the differential extension in the faulted upper crust, in the mantle lithosphere, and the differential sedimentation during the syn-rift period.

Rapid transition from continental breakup to igneous oceanic crust in the South China Sea

Continental breakup represents the successful process of rifting and thinning of the continental lithosphere, leading to plate rupture and initiation of oceanic crust formation. Magmatism during breakup seems to follow a path of either excessive, transient magmatism (magma-rich margins) or of igneous starvation (magma-poor margins). The latter type is characterized by extreme continental lithospheric extension and mantle exhumation prior to igneous oceanic crust formation. Discovery of magma-poor margins has raised fundamental questions about the onset of ocean-floor type magmatism, and has guided interpretation of seismic data across many rifted margins, including the highly extended northern South China Sea margin. Here we report International Ocean Discovery Program drilling data from the northern South China Sea margin, testing the magma-poor margin model outside the North Atlantic. Contrary to expectations, results show initiation of Mid-Ocean Ridge basalt type magmatism during breakup, with a narrow and rapid transition into igneous oceanic crust. Coring and seismic data suggest that fast lithospheric extension without mantle exhumation generated a margin structure between the two endmembers. Asthenospheric upwelling yielding Mid-Ocean Ridge basalt-type magmatism from normal-temperature mantle during final breakup is interpreted to reflect rapid rifting within thin pre-rift lithosphere.

Possible Spatial Distribution of the Mesozoic Volcanic Arc in the Present‐Day South China Sea Continental Margin and Its Tectonic Implications

AbstractThe distribution of the Late Mesozoic volcanic arc in the South China Sea (SCS) continental margin has long been a controversial topic due to its significance in understanding the transition mechanism of a margin from subduction to extension. Here a comprehensive analysis was conducted in the margin using reprocessed magnetic data, newly collected drilling/dredging samples, depositional environment, and deformation style inferred from multichannel seismic profiles to jointly constrain the possible distribution of the Mesozoic volcanic arc. From the map of reduced‐to‐the‐pole magnetic anomaly, several high‐positive magnetic anomaly belts can be discriminated, which cross the central Pearl River Mouth Basin, extend southwestward to the Zhongsha (Macclesfield Bank) and Xisha Islands (Paracel Islands), and distribute discontinuously around the southwest subbasin. Geophysical analysis shows that these belts have similar amplitudes and magnetic depths to known volcanic arcs, such as the Luzon, Cagayan, and Sulu arcs. Furthermore, the high‐amplitude positive magnetic anomaly belts coincide with the distribution of Late Mesozoic arc‐like granites, intermediate rocks, and agglomerates, suggesting that the belts possibly originated from the existence of Late Mesozoic volcanic arc. Accretion and compression environment located in front of the inferred arc provides independent supports to our interpretation. Results indicate that the southwest part of the arc is distributed on both sides of the southwest SCS subbasin, whereas the northeast part remains nearly in its original location, further suggesting that the breakup locations for the SCS margin might be the volcanic front/forearc in the northeast and the arc in the southwest during the opening of the SCS basin.

Crustal plumbing system of post-rift magmatism in the northern margin of South China Sea: New insights from integrated seismology

The origin and evolution of magmatism in rifting margins are a fundamental geological subject, and remain the focus of intense studies. Unlike the classical magma-poor and magma-rich rifting margins, the northern margin of the South China Sea (SCS) exhibits a strong post-rift magmatism, but the evolution and mechanism of this behavior are still poorly understood. In this work we investigate the detailed structural features of the post-rift magmatism in the crust of the SCS northern margin. Our analysis is based on wide-angle and multichannel reflection seismic data, combined with previous seismological results. Our results show a wide distribution of shallow igneous sills and intrusions. These features are spatially and tectonically linked to the lower crustal high-velocity bodies (HVBs) and submarine volcanism, indicating a causative connection between the three features. Considering the existence of an obvious low-velocity branch of the Hainan plume in the northern SCS and uplift of the lithosphere-asthenosphere boundary, we propose that the HVBs reflect consolidated mafic intrusions which formed the lower crustal reservoirs feeding the overlying igneous sills and intrusions. In the SCS northern margin where abundant extensional and detachment faults can act as magmatic channels, such a dynamic process might cause post-rift volcanism. Our results suggest that the crustal magmatic system of post-rift volcanism has a multilevel upward migration mode, and the HVBs in the lower crust could be the product of post-rift magmatism, further indicating that the SCS northern margin had a magma-poor property at the rifting phase but has undergone a strong magmatic rejuvenation by the subsequent mantle plume.

Hafnium isotopic constraints on the origin of late Miocene to Pliocene seamount basalts from the South China Sea and its tectonic implications

Post-spreading intraplate volcanism has widely affected the South China Sea (SCS) region including Indochina, the northern margin of the SCS, and the SCS basin itself. In the SCS basin, several off- and on-fossil spreading center seamounts formed between 3.8 and 7.9 Ma. Based on previously published geochemical and Sr-Nd-Pb isotopic data, the intraplate volcanism is widely related to the Hainan mantle plume, whose existence has been evidenced by recent geophysical studies. To test this petrogenetic model, new Hf isotope data have been obtained from volcanic rocks from a suite of compositionally representative seamounts in the SCS basin. Compared to published Nd isotope ratios (0.512675–0.512965, 5 εNd units), 176Hf/177Hf ratios span a much larger range (0.282876–0.283097, 8 εHf units), even within individual seamounts (e.g., Zhangzhong seamount). These features, combined with previous studies, clearly confirm mantle heterogeneity beneath the SCS region. Similar to the trends in Sr-Nd-Pb isotope space, Nd-Hf isotope ratio show a relatively narrow, elongate mixing trend between a depleted Indian MORB-type mantle endmember and an enriched EMII-type mantle end member. We propose that this narrow trend is inconsistent with the origin of the enriched end member from a heterogeneous sub-continental lithospheric mantle (SCLM) and instead suggests a plume-related origin. As a conceptual model for the post-spreading tectonic scenario of the Hainan plume affecting the SCS region, we propose that a plume ascends to the bottom of the lithosphere beneath Hainan and its northern Leizhou peninsula at the northern margin of SCS, from where it migrates along sloping rheologic boundary layers to lithospheric faults under an extensional setting towards the central SCS where the magmas erupt at the young spreading centers.

A low-angle normal fault and basement structures within the Enping Sag, Pearl River Mouth Basin: Insights into late Mesozoic to early Cenozoic tectonic evolution of the South China Sea area

The basement structure of the Cenozoic Enping Sag, within the Pearl River Mouth Basin on the northern margin of South China Sea, is revealed by borehole-constrained high-quality 3D seismic reflection data. Such data suggest that the Enping Sag is bounded in the north by a low-angle normal fault. We interpret this low-angle normal fault to have developed as the result of the reactivation of a pre-existing thrust fault part of a pre-Cenozoic thrust system. This is demonstrated by the selective reactivation of the pre-existing thrust and by diffuse contractional deformation recognized from the accurate analysis of basement reflections. Another significant result of this study is the finding of some residual rift basins within the basement of the Enping Sag. Both the thrust system and the residual basins are interpreted to have developed after the emplacement of continental margin arc-related granitoids (J3–K1) that define the basement within the study area. Furthermore, seismic sections show that the pre-existing residual rift basins are offset by the main thrust fault and they are both truncated by the Tg unconformity. These structural relationships, interpreted in the frame of previous studies, help us to reconstruct a six-event structural evolution model for the Enping Sag from the late Mesozoic to the early Cenozoic. In particular, we interpret the residual rift basins to have formed as the result of back-arc extension due to the slab roll-back of the Paleo-Pacific Plate subduction in the early K2. The thrust system has recorded a compressional event in the late K2 that followed the back-arc extension in the SCS area. The mechanism of this compressional event is still to be clarified, and might be related to continuous subduction of the Paleo-Pacific Plate or to the continent-continent collision between a micro-continental block and the South China margin.

The structure and evolution of deepwater basins in the distal margin of the northern South China Sea and their implications for the formation of the continental margin

Abstract Although it is generally accepted that the South China Sea margin can be classified as a magma-poor rifted margin, little is known about the structural style of the distal margin and how the distal deep basins evolved. Observations and data from the Northern South China Sea margin suggest that the deepwater basins (distal margin domain) are commonly underlain by a highly extended continental crust, and detachment fault systems have been interpreted in places. We have interpreted a large seismic data set covering deepwater basins in the Northern South China Sea. The interpreted Moho on reflection seismic profiles indicates that the crustal thickness varies along sections perpendicular to the coast, which is the first-order criterion to divide the whole margin into a proximal domain, necking zone and distal domain. Furthermore, the underlying extremely stretched crust (

Crustal structure and rifting of the northern South China Sea margin: Evidence from shoreline‐crossing seismic investigations

Seismic reflection and refraction data acquired from both onshore and offshore in the South China area and on the continental shelf of the northern South China Sea passive margin provide new constraints on the P‐wave velocity structure beneath the continental shelf and contribute to constructing a continental rift evolution model. Based on the crustal thicknesses and P‐wave velocities derived from the velocity structure of the seismic profile OBS2010‐2 across the continental shelf, two distinct domains are identified: (a) the thinned/extended continental crust and (b) the transitional crust. The thinned continental crust has a thickness of 25–27 km across the Hainan Rise, decreasing to 21–23 km further south in the rift and faulted depression zone. The transitional crust is characterized by a thickness of 20–21 km. High‐velocity values (>7.0 km/s) in the lower crust of the transitional domain may indicate possible magmatic underplating during rifting. Our results show that the Cathaysia Block extends from onshore South China to offshore into the South China Sea and is bounded by the transitional crust of the continental shelf. The initial rifting of the continental margin occurred in the rift and faulted depression zone and might be explained by the upwelling of the lithosphere based on a depth‐dependent extension mechanism during the late Cretaceous or Paleocene rifting.

Tectonic implications of Mesozoic magmatism to initiation of Cenozoic basin development within the passive South China Sea margin

The South China Sea (SCS) is one of the classical example of a non-volcanic passive margin situated within three tectonic plates of the Eurasian, Indo-Australian and Philippine Sea plate. The development of SCS resulted from interaction of various types of plate boundaries, and complex tectonic assemblage of micro blocks and accretionary prisms. Numerous models were proposed for the formation of SCS, yet none can fully satisfy different aspects of tectonic forces. Temporal and geographical reconstruction of Cretaceous and Cenozoic magmatism with the isochrones of major basins was conducted. Our reconstruction indicated the SE margin of Asia had gone through two crustal thinning events. The sites for rifting development are controlled by localized thermal weakening of magmatism. NW–SE extension setting during Late Cretaceous revealed by magmatism distribution and sedimentary basins allow us to allocate the retreated subduction of Pacific plate to the cause of first crustal thinning event. A magmatic gap between 75 and 65 Ma prior to the initiation of first basin rifting suggested a significant modification of geodynamic setting occurred. The Tainan basin, Pearl River Mouth basin, and Liyue basins started to develop since 65 Ma where the youngest Late Cretaceous magmatism concentrated. Sporadic bimodal volcanism between 65 and 40 Ma indicates further continental extension prior to the opening of SCS. The E–W extension of Malay basin and West Natuna began since late Eocene followed by N–S rifting of SCS as Neotethys subducted. The SCS ridge developed between Pearl River Mouth basin and Liyue basin where 40 Ma volcanic activities concentrated. The interaction of two continental stretching events by Pacific followed by Neotethys subduction with localized magmatic thermal weakening is the cause for the non-volcanic nature of SCS.

Cretaceous–Eocene provenance connections between the Palawan Continental Terrane and the northern South China Sea margin

The plate kinematic history of the South China Sea opening is key to reconstructing how the Mesozoic configuration of Panthalassa and Tethyan subduction systems evolved into today's complex Southeast Asian tectonic collage. The South China Sea is currently flanked by the Palawan Continental Terrane in the south and South China in the north and the two blocks have long been assumed to be conjugate margins. However, the paleogeographic history of the Palawan Continental Terrane remains an issue of uncertainty and controversy, especially regarding the questions of where and when it was separated from South China. Here we employ detrital zircon U–Pb geochronology and heavy mineral analysis on Cretaceous and Eocene strata from the northern South China Sea and Palawan to constrain the Late Mesozoic–Early Cenozoic provenance and paleogeographic evolution of the region testing possible connection between the Palawan Continental Terrane and the northern South China Sea margin. In addition to a revision of the regional stratigraphic framework using the youngest zircon U–Pb ages, these analyses show that while the Upper Cretaceous strata from the Palawan Continental Terrane are characterized by a dominance of zircon with crystallization ages clustering around the Cretaceous, the Eocene strata feature a large range of zircon ages and a new mineral group of rutile, anatase, and monazite. On the one hand, this change of sediment compositions seems to exclude the possibility of a latest Cretaceous drift of the Palawan Continental Terrane in response to the Proto-South China Sea opening as previously inferred. On the other hand, the zircon age signatures of the Cretaceous–Eocene strata from the Palawan Continental Terrane are largely comparable to those of contemporary samples from the northeastern South China Sea region, suggesting a possible conjugate relationship between the Palawan Continental Terrane and the eastern Pearl River Mouth Basin. Thus, the Palawan Continental Terrane is interpreted to have been attached to the South China margin from the Cretaceous until the Oligocene oceanization of the South China Sea. In our preferred paleogeographic scenario, the sediment provenance in the northeastern South China Sea region changed from dominantly nearby Cretaceous continental arcs of the South China margin to more distal southeastern South China in the Eocene.

Magmatism in the evolution of the South China Sea: Geophysical characterization

Timing, distribution, and intensity of magmatism are critical to understanding deep lithospheric processes in the opening of the South China Sea (SCS). Based primarily on multichannel seismic reflection data, and aided with gravity and magnetic anomalies, we identify various igneous features, such as sills, stocks, volcanic edifices, and volcanoes, which are often characterized by high positive seismic amplitudes, and/or by chaotic or blank reflections beneath them because of their seismic shielding effect. We estimate the timing of igneous activities and analyze their areal distribution in the SCS region based on seismic stratigraphy. We confirm that most igneous emplacements in the SCS margins occurred after the cessation of seafloor spreading, and are sparse and volumetrically small during the rifting and seafloor spreading phases, supporting magma-poor margins prior to the breakup of the continental lithosphere. By contrast, post-spreading magmatic activities are widespread in the continental slope areas and in the central SCS basin, and are most likely triggered by extension in relation to cooling and subsidence of the oceanic and attenuated continental lithosphere, which could accumulate a total thermal contractional displacement up to 24km.

Rapid post-rift tectonic subsidence events in the Pearl River Mouth Basin, northern South China Sea margin

Data from 26 drill wells and 27 regional seismic profiles were integrated to investigate the timing, phase and origin of the post-rift subsidence in the middle to eastern Pearl River Mouth Basin (PRMB) of the northern South China Sea using the traditional 1-D backstripping technique. Different from previous research of backstripped tectonic subsidence in the basin, we calculated the tectonic subsidence using the newly built local porosity-depth relationships for decompaction and updated sedimentological and paleontological data for paleobathymetry reconstruction. Well-data based subsidence curves reveal a roughly decaying pattern in both the magnitude and rate of the post-rift subsidence in the PRMB, which is in accordance with the general decreasing trend of the thermal subsidence typical of a passive margin. Two events of rapid post-rift tectonic subsidence were identified, which occurred in the Early to early Middle Miocene and the Pliocene. The timing of the first rapid post-rift subsidence event varies and is earlier in the southern rather than northern part of the basin. Additionally, the amplitude of contemporaneous tectonic subsidence is greater in the southern part of the basin. The second rapid tectonic subsidence event occurred simultaneously in both the southern and northern parts of the basin, with the amplitude of subsidence being much greater in the southern part. We associate the first rapid subsidence event with the southward jump of the South China Sea spreading ridge, which occurred between the Oligocene and Early Miocene, while the second event with the arc-continent collision at Taiwan since the latest Late Miocene. The southern PRMB in the deep-water slope area shows a much higher magnitude of tectonic subsidence in both events than its northern counterpart in the shelf area, which could be associated with its much thinner lithosphere. The latter could cause upswelling of denser lower crust and upper mantle material, resulting in more rapid tectonic subsidence in the southern PRMB.

Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

U-Pb isotope geochronology and geochemistry of granites from Hainan Island (northern South China Sea margin): Constraints on late Paleozoic-Mesozoic tectonic evolution

Hainan Island is a key component of the South China Sea region and provides insights on regional geological evolution since the Paleozoic. Ten new LA-ICPMS zircon U-Pb ages from granites of Hainan Island include Late Permian (254±3Ma; 252±3Ma), Middle-Late Triassic (243±2Ma; 242±3Ma; 240±2Ma; 228±2Ma) and late Early to early Late Cretaceous (105±1Ma; 101±1Ma; 96±2Ma; 95±3Ma) ages. All samples in the present study, including late Permian, Middle-Late Triassic, and late Early to early Late Cretaceous granitic rocks show geochemical characteristics similar to those of calc-alkaline to high-K calc-alkaline I-type granites. Major and trace element geochemical variations show that during petrogenesis, these granites experienced fractional crystallization of minerals (e.g., Ti oxides and apatite). Compared to the late Permian and Middle-Late Triassic granitic rocks, middle to late Cretaceous granites generally have lower initial 87Sr/86Sr ratios of 0.70594 to 0.70886, lower TDM2 ages of 1314 to 1382Ma, and higher εNd(t) of −4.94 to −5.96, implying that the magmatic source for Cretaceous granites received more significant contribution from juvenile material relative to that for Permo-Triassic granites. These new data, combined with data from literature for Hainan Island and the South China Sea (SCS) region underpin a conceptual model for late Paleozoic to Mesozoic tectonic evolution for Hainan Island and the general SCS region as follows, (a) Late Permian (272–252Ma), the initiation and development of continental arc related to the subduction of Palaeo-Tethys ocean; (b) Triassic (249–228Ma), continued arc magmatism, the gradual cessation of Palaeo-Tethys subduction and subsequent development of an extensional setting; (c) Early Jurassic to early Cretaceous (190–130Ma), the development of an Andean-type continental arc, and regional tectonic regime switch to the westward subduction of the Palaeo-Pacific plate; (d) Middle to late Cretaceous (128–70Ma), the continuation of the Andean-type arc, the development of an extensional setting due to slab rollback, and the cessation (~70Ma) of Palaeo-Pacific plate subduction.

Deep thermal structure of Southeast Asia constrained by S-velocity data

Southeast Asia, located in the southeastern part of the Eurasian Plate, is surrounded by tectonically active margins, exhibiting intense seismicity and volcanism, contains complex geological units with a perplexing evolution history. Because tectonic evolution is closely related to the deep thermal structure, an accurate estimation of the lithosphere thermal structure and thickness is important in extracting information on tectonics and geodynamics. However, there are significant uncertainties in the calculated deep thermal state constrained only by the observed surface heat flow. In this study, in order to obtain a better-constrained deep thermal state, we first calculate the deep thermal structure of Southeast Asia by employing an empirical relation between S-velocity and temperature, and then we estimate the base of the thermal lithosphere from the calculated temperature-depth profiles. The results show that, in general, the temperature is higher than the dry mantle solidus below the top of the seismic low-velocity zone, possibly indicating the presence of partial melt in the asthenosphere, particularly beneath oceanic basins such as the South China Sea. The temperature at a depth of 80 km in rifted and oceanic basins such as the Thailand Rift Basin, Thailand Bay, Andaman Sea, and South China Sea is about 200 °C higher than in plateaus and subduction zones such as the Khorat Plateau, Sumatra Island, and Philippine Trench regions. We suggest that the relatively cold and thick lithosphere block of the Khorat Plateau has not experienced significant internal deformation and might be extruded and rotated as a rigid block in response to the Indo-Eurasia collision. Our results show that the surface heat flow in the South China Sea is mainly dominated by the deep thermal state. There is a thermal anomaly in the Leiqiong area and in the areas adjacent to the northern margin of the South China Sea, indicating the presence of a high-temperature and thin lithosphere in the area of the well-known and controversial Hainan plume. The thermal lithosphere-asthenosphere boundary uplift area along the Xisha and southeastern Vietnam margin, in the western margin of South China Sea, which corresponds to the volcanic belt around this area, might indicate upwelling of hot mantle materials. The temperature values at 100 and 120 km depths through most regions of Southeast Asia are about 1400–1500 and 1550–1600 °C, respectively, which are nearly uniform with a small temperature difference. Our results also show that the lithosphere becomes thinner from the continent blocks toward the oceanic basins, with the smaller thickness values of 65–70 km in the South China Sea. The estimated base of the lithosphere corresponds approximately to the 1400 °C isotherm and shows good correlation with the tectonic setting.

Neogene residual subsidence and its response to a sinking slab in the deep mantle of eastern China

Mantle convection could have a significant effect on basin evolution; however, research quantifying this relationship is controversial. To understand the formation mechanism and evolution of the Cenozoic rift basins in eastern China, we applied the back-stripping technique and strain rate inversion modeling to 119 wells from Sangjiang Basin, Bohai Bay Basin, North Yellow Sea Basin, South Yellow Sea Basin, East China Sea Shelf Basin, and four basins within the northern South China Sea margin. The modeled results can be used to reconstruct the tectonic subsidence history and further assess the potential subsidence mechanisms of eastern China. Residual subsidence is defined as the difference between the theoretical and observed tectonic subsidences. Our results show that the residual subsidence since 20Ma in eastern China generally increases from ∼100 to 300m in terrestrial areas to ∼1.2–1.8km onthe continental shelf. Our observed residual subsidence is generally consistent with the present-day dynamic topography that is predicted from mantle flow models, and it is associated with a stagnant slab within the mantle that has been observed by seismic tomography. The migration pathway of the residual subsidence since 20Ma is consistent with the movement direction of the Pacific and Philippine plates. Therefore, we suggest that the residual subsidence might be a dynamic subsidence induced by a negative buoyancy of the sinking slab in the deep mantle beneath eastern China.

Deep seismic structure of the northeastern South China Sea: Origin of a high‐velocity layer in the lower crust

AbstractWe present a 2‐D seismic tomographic image of the crustal structure along the OBS2012 profile, which delineates the Moho morphology and magmatic features of the northeastern South China Sea margin. The image was created by forward modeling (RayInvr) and traveltime tomographic inversion (Tomo2D). Overall, the continental crust thins seaward from ~27 km to ~21 km within the continental shelf across the Zhu I Depression and Dongsha Rise, with slight local thickening beneath the Dongsha Rise accompanying the increase in the Moho depth. The Dongsha Rise is also characterized by ~4–7 km thick high‐velocity layer (HVL) (~7.0–7.6 km/s) in the lower crust and exhibits a relatively high velocity (~5.5–6.4 km/s) in the upper crust with a velocity gradient lower than those of the Zhu I Depression and Tainan Basin. Across the continental slope and continent‐ocean transition (COT), which contain the Tainan Basin, the crust sharply thins from 20 km to 10 km seaward and a ~2–3 km thick HVL is imaged in the lower crust. We observed that volcanoes are located only within the COT, but none exist in the continental shelf; the Dongsha Rise exhibits a high magnetic anomaly zone and different geochemical characteristics from the COT. Based on those observations, we conclude that the HVL underlying the COT is probably extension related resulting from the decompression melting in the Cenozoic, whereas the HVL beneath the Dongsha Rise is probably arc related and associated with the subduction of the paleo‐Pacific plate. These findings are inconsistent with those of some previous studies.