The sedimentary record of the Sanshui Basin:Implication to the Late Cretaceous tectonic evolution in the northern margin of South China Sea

Whether the South China continental margin had shifted from active subduction to passive extension in the Late Cretaceous remains controversial.Located in the northernmost of the South China Sea continental margin,Sanshui Basin developed continuous stratigraphy from Lower Cretaceous to Eocene and provides precious outcrops to study the regional tectonic evolution during the Cretaceous.Therefore,we conducted field observations,petrology,clay mineralogy,geochemistry,and detrital zircon chronology analyses of sedimentary rocks from the Upper Cretaceous Sanshui Formation in Sanshui Basin.Results suggest that the Sanshui Basin is characterized as an intermoutane basin with multiple provenances,strong hydrodynamic environment,and proximal accumulation in the Late Cretaceous.An angular unconformity at the boundary between the Lower and Upper Cretaceous was observed in the basin.The sedimentary facies of the northern basin changed from lacustrine sedimentary environment in the Early Cretaceous to alluvial facies in the Late Cretaceous.The zircon U-Pb ages of granitic gravelly sandstone from Sanshui Formation prominently range from 100 Ma to 300 Ma,which is close to the deposition age of Sanshui Formation.The major and trace elements of the Late Cretaceous sedimentary samples show characteristics of active continental margin,and are different from the Paleogene rifting sequences.Hence,we propose that the northern South China Sea margin underwent an intense tectonic uplift at the turn of the Early and Late Cretaceous(around 100 Ma).Afterward,the northern South China Sea margin entered a wide extension stage in the Late Cretaceous(~100 to~80 Ma).This extensional phase is related to the back-arc extension in the active continental margin environment,which is different from the later passive rifting in the Cenozoic.The transition from active subduction to passive extension in the northern South China Sea may occur between the late Late Cretaceous and the Paleogene.

Crustal S-wave velocity structure across the northeastern South China Sea continental margin: implications for lithology and mantle exhumation

The northeastern margin of the South China Sea (SCS), developed from continental rifting and breakup, is usually thought of as a non-volcanic margin. However, post-spreading volcanism is massive and lower crustal high-velocity anomalies are widespread, which complicate the nature of the margin here. To better understand crustal seismic velocities, lithology, and geophysical properties, we present an S-wave velocity (VS) model and a VP/VS model for the northeastern margin by using an existing P-wave velocity (VP) model as the starting model for 2-D kinematic S-wave forward ray tracing. The Mesozoic sedimentary sequence has lower VP/VS ratios than the Cenozoic sequence;in between is a main interface of P-S conversion. Two isolated high-velocity zones (HVZ) are found in the lower crust of the continental slope, showing S-wave velocities of 4.0–4.2 km/s and VP/VS ratios of 1.73–1.78. These values indicate a mafic composition, most likely of amphibolite facies. Also, a VP/VS versus VP plot indicates a magnesium-rich gabbro facies from post-spreading mantle melting at temperatures higher than normal. A third high-velocity zone (VP : 7.0–7.8 km/s;VP/VS: 1.85–1.96), 70-km wide and 4-km thick in the continent-ocean transition zone, is most likely to be a consequence of serpentinization of upwelled upper mantle. Seismic velocity structures and also gravity anomalies indicate that mantle upwelling/ serpentinization could be the most severe in the northeasternmost continent-ocean boundary of the SCS. Empirical relationships between seismic velocity and degree of serpentinization suggest that serpentinite content decreases with depth, from 43% in the lower crust to 37% into the mantle.

Mesozoic Deformation and Its Geological Significance in the Southern Margin of the South China Sea

The pre-Eocene history of the region around the present South China Sea is not well known. New multi-channel seismic profiles provide valuable insights into the probable Mesozoic history of this region. Detailed structural and stratigraphic interpretations of the multi-channel seismic profiles, calibrated with relevant drilling and dredging data, show major Mesozoic structural features. A structural restoration was done to remove the Cenozoic tectonic influence and calculate the Mesozoic tectonic compression ratios. The results indicate that two groups of compressive stress with diametrically opposite orientations, S(S)E– N(N)W and N(N)W–S(S)E, were active during the Mesozoic. The compression ratio values gradually decrease from north to south and from west to east in each stress orientation. The phenomena may be related to the opening of the proto-South China Sea(then located in south of the Nansha block) and the rate at which the Nansha block drifted northward in the late Jurassic to late Cretaceous. The Nansha block drifted northward until it collided and sutured with the southern China margin. The opening of the present South China Sea may be related to this suture zone, which was a tectonic zone of weakness.

Characteristics and distribution patterns of reef complexes on the carbonate platform margin in deep water areas:the western South China Sea

As a potential oil and gas reservoir, reef complexes have been a research focus from petroleum geologists for a long time. There are favorable conditions for the development of reef complexes in the South China Sea; however, their internal structures, evolution and distribution are still poorly understood. Based on 2D and 3D seismic data, the internal structures and evolution patterns of the reef complexes on the carbonate platform margin in the deep water areas over the western South China Sea were studied in detail. The result shows that two types of reef complexes, i.e., fault controlling platform margin reef complexes and ramp reef complexes have been developed in the study area. The reef complexes have independent or continuous mound or lenticular seismic reflections, with three internal structures （i.e., aggrading, prograding and retrograding structures）. There are different growth rates during the evolution of the reef complexes, resulting in the formation of catch-up reefs, keep-up reefs and quick step reefs. The study also reveals that different platform margin reef complexes have different internal structures and distributions, because of the different platform types. These results may be applied to the exploration and prediction of carbonate platform margin reef complexes in other areas that are similar to the study area.

Peat formation and accumulation mechanism in northern marginal basin of South China Sea

In the present study,the coal-rock organic facies of Oligocene Yacheng Formation of the marginal basin in the South China Sea were classified and divided.In addition,through the correlations of the large-scale coal-bearing basins between the epicontinental sea and the South China Sea,it was concluded that the coal forming activities in the South China Sea presented particularity and complexity.Furthermore,the coal forming mechanisms also presented distinctiveness.The marginal basins in the South China Sea consist of several large and complex rift or depression basins,which are distributed at different tectonic positions in the South China Sea.Therefore,the marginal basins in the South China Sea are not simple traditional units with onshore continental slopes extending toward the deep sea.The marginal basins are known to consist of multi-level structures and distinctive types of basins which differ from the continental regions to the sea.During the Oligocene,the existing luxuriant plants and beneficial conditions assisted in the development of peat.Therefore,the Oligocene was the significant period for the formation and aggregation of the peat.However,the peat did not form in unified sedimentary dynamic fields,but instead displayed multi-level geographical units,multiple provenance areas,instability,and nonevent characteristics.As a result,the marginal basins in the South China Sea are characterized by non-uniform peat aggregation stages.In another words,the majority of the peat had entered the marine system in a dispersive manner and acted as part of the marine deposits,rather than during one or several suitable coal-forming stages.These peat deposits then became the main material source for hydrocarbon generation in all of the marginal basins of the South China Sea.The study will be of much significance for the hydrocarbon exploration in the marginal basins of the South China Sea.

Sectional distribution of salinity and its indication of Kuroshio'sintrusion in the southern Taiwan Strait and northeastern South China Sea late summer, 1994

Using the CTD data investigated at about 330 stations in the southern Taiwan Strait, the northeastern South China Sea and their adjacent seas in August and September of 1994, this paper anayses the sectional distributions of salinity in 10 selected sections and discusses the possibility of the Kuroshio's intrusion into the northeastern South China Sea and the southern Taiwan Strait. The results show that: as the main stream of Kuroshio passes by the northern Luzon Strait during the survey period, the 'Kuroshio-influenced water' with some hydrological features of the Kuroshio may extend through the Luzon Strait to the sea areas near both the Dongsha Islands and the Taiwan Shoal of the southern Taiwan Strait.

OCEAN BOTTOM SEISMOMETER AND ONSHOREOFFSHORE SEISMIC EXPERIMENT IN NORTHEASTERN SOUTH CHINA SEA

Three-component Ocean Bottom Seismometers, portable land stations and marine air gun seismic sources were used to carry out an onshore-offshore deep seismic profile in northeastern South China Sea. This profile, orientated in NNW-SSE, was as long as 500 km and perpendicular to the strike of regional tectonics. The offshore data were processed in Taiwan Ocean University using a number of available software and the onshore data were analyzed in South China Sea Institute of Oceanology by new-written programs and public software. Preliminary results show that the seismic data are in good quality and contain rich information of deep structure. Seismic phases, e.g. Pg, PmP and Pn, are identified in the offset range 5～220 kin, which will provide an important dataset for the deep crustal structure and oil-gas basin evolution studies of this region.

Geothermal investigation of the thickness of gas hydrate stability zone in the north continental margin of the South China Sea

The exploration of unconventional and/or new energy resources has become the focus of energy research worldwide,given the shortage of fossil fuels.As a potential energy resource,gas hydrate exists only in the environment of high pressure and low temperature,mainly distributing in the sediments of the seafloor in the continental margins and the permafrost zones in land.The accurate determination of the thickness of gas hydrate stability zone is essential yet challenging in the assessment of the exploitation potential.The majority of previous studies obtain this thickness by detecting the bottom simulating reflectors（BSRs） layer on the seismic profiles.The phase equilibrium between gas hydrate stable state with its temperature and pressure provides an opportunity to derive the thickness with the geothermal method.Based on the latest geothermal dataset,we calculated the thickness of the gas hydrate stability zone（GHSZ） in the north continental margin of the South China Sea.Our results indicate that the thicknesses of gas hydrate stability zone vary greatly in different areas of the northern margin of the South China Sea.The thickness mainly concentrates on 200–300 m and distributes in the southwestern and eastern areas with belt-like shape.We further confirmed a certain relationship between the GHSZ thickness and factors such as heat flow and water depth.The thickness of gas hydrate stability zone is found to be large where the heat flow is relatively low.The GHSZ thickness increases with the increase of the water depth,but it tends to stay steady when the water depth deeper than 3 000 m.The findings would improve the assessment of gas hydrate resource potential in the South China Sea.

Petroleum geology controlled by extensive detachment thinning of continental margin crust: A case study of Baiyun sag in the deep-water area of northern South China Sea

The relationships between crustal stretching and thinning,basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic,gravity,magnetic,ocean bottom seismic(OBS),deep-water exploration wells and integrated ocean drilling program(IODP).During the early syn-rifting period,deep-water area was a half-graben controlled by high angle faults influenced by the brittle extension of upper crust.In the mid syn-rifting period,this area was a broad-deep fault depression controlled by detachment faults undergone brittle-ductile deformation and differentiated extension in the crust.In the late syn-rifting period,this area experienced fault-sag transition due to saucer-shaped rheology change dominated by crustal ductile deformation.A broad-deep fault depression controlled by the large detachment faults penetrating through the crust is an important feature of deep-water basin.The study suggests that the broad-deep Baiyun sag provides great accommodation space for the development of massive deltaic-lacustrine deposition system and hydrocarbon source rocks.The differentiated lithospheric thinning also resulted in the different thermal subsidence during post-rifting period,and then controlled the development of continental shelf break and deep-water reservoir sedimentary environment.The high heat flow background caused by the strong thinning of lithosphere and the rise of mantle source resulted in particularities in the reservoir diagenesis,hydrocarbon generation process and accumulation of deep-water area in northern South China Sea.

Characteristics and origin of continental marginal fault depressions under the background of preexisting subduction continental margin,northern South China Sea,China

Based on the new seismic and drilling data and the recent related research results,this paper systematically analyzes the diversity and complexity of evolution process of crustal lithosphere structure and basin structure in the Pearl River Mouth Basin on the northern margin of the South China Sea.Three types of detachment faults of different structural levels exist:crust-mantle detachment,inter-crust detachment and upper crust detachment.It is considered that different types of extensional detachment control different subbasin structures.Many fault depressions controlled by upper crust detachment faults have been found in the Zhu I Depression located in the proximal zone.These detachment faults are usually reformed by magma emplacement or controlled by preexisting faults.Baiyun-Liwan Sag located in the hyperextension area shows different characteristics of internal structure.The Baiyun main sag with relative weak magmatism transformation is a wide-deep fault depression,which is controlled by crust-mantle detachment system.Extensive magmatism occurred in the eastern and southwest fault steps of the Baiyun Sag after Middle Eocene,and the crust ductile extensional deformation resulted in wide-shallow fault depression controlled by the upper crust detachment fault.Based on the classical lithosphere extensional breaking and basin tectonic evolution in the Atlantic margin,it is believed that the magmatic activities and pre-existing structures in the Mesozoic subduction continental margin background are important controlling factors for the diversified continental margin faulted structures in the northern South China Sea.

A weakly coupled data assimilation system of a coupled physical–biological model for the northeastern South China Sea

南海东北部沿岸海域在每年夏季都存在显著的物理过程(如上升流,珠江冲淡水)和活跃的生物过程,这些基本的物理场和生物场要素对沿岸海域的研究起关键作用。本文在一适应于南海东北部的物理生物耦合模式的基础上建立了一个弱约束的EnOI耦合同化系统。开展的SST同化试验结果表明,物理过程存在合理的响应,且近海上升流区的海洋生态变量对物理变量的同化也有响应。

Middle and Late Tertiary Palaeogeography and Palaeoenvironment on the Northern Continental Margin of the South China Sea

Based essentially on research results of calcareous nannofossils, combined with some other microfossil da-ta and several secondary depositional breaks, this paper discusses the criteria of division and comparison of themiddle and late Tertiary marine sediments, palaeogeographical and palaeoenvironmental evolution andpalaeoclimates on the northern continental margin of the South China Sea, comprising the Tainan basin, PearlRiver Mouth basin. Southeast Hainan basin and Beibu Gulf basin. Study shows that the upper Oligocene toPliocene strata in the whole area consist essentially of marine sediments except in the Beibu Gulf basin. Theyinclude littoral. neritic and deltaic sediments as well as carbonate rock-bioherm limestone. The sea advancedfrom southeast to northwest. During the transgression there appeared three culminations coinciding to thestages of deposition of nannofossil zones NN4-5, NN11 and 13-15.

Differences in the Tectonic Evolution of Basins in the CentralSouthern South China Sea and their Hydrocarbon Accumulation Conditions

To reveal the causes of differences in the hydrocarbon accumulation in continental marginal basins in the centralsouthern South China Sea,we used gravity-magnetic,seismic,drilling,and outcrop data to investigate the tectonic histories of the basins and explore how these tectonic events controlled the hydrocarbon accumulation conditions in these basins.During the subduction of the Cenozoic proto-South China Sea and the expansion of the new South China Sea,the continental margin basins in the central-southern South China Sea could be classified as one of three types of epicontinental basins:southern extensional-foreland basins,western extensional-strike slip basins,and central extensional-drift basins.Because these basins have different tectonic and sedimentary histories,they also differ in their accumulated hydrocarbon resources.During the Cenozoic,the basin groups in the southern South China Sea generally progressed through three stages:faulting and subsidence from the late Eocene to the early Miocene,inversion and uplift in the middle Miocene,and subsidence since the late Miocene.Hydrocarbon source rocks with marine-continental transitional facies dominated byⅡ-Ⅲkerogen largely developed in extremely thick Miocene sedimentary series with the filling characteristics being mainly deep-water deposits in the early stage and shallow water deposits in the late stage.With well-developed sandstone and carbonate reservoirs,this stratum has a strong hydrocarbon generation potential.During the Cenozoic,the basin groups in the western South China Sea also progressed through the three developmental stages discussed previously.Hydrocarbon source rocks with lacustrine facies,marine-continental transitional facies,and terrigenous marine facies dominated byⅡ2-Ⅲkerogen largely developed in the relatively thick stratum with the filling characteristics being mainly lacustrine deposits in the early stage and marine deposits in the late stage.As a reservoir comprised of self-generated and self-stored sandstone,this unit also has a high hydrocarbon generation potential.Throughout those same three developmental stages,the basin groups in the central South China Sea generated hydrocarbon source rocks with terrigenous marine facies dominated byⅢkerogen that have developed in a stratum with medium thicknesses with the filling characteristics being mainly sandstone in the early stage and carbonate in the late stage.This reservoir,which is dominated by lower-generation and upper-storage carbonate rocks,also has a high hydrocarbon generation potential.

The deep mantle upwelling beneath the northwestern South China Sea:Insights from the time-varying residual subsidence in the Qiongdongnan Basin

Deep hot mantle upwelling is widely revealed around the Qiongdongnan Basin on the northwestern South China Sea margin. However, when and how it influenced the hyper-extended basin is unclear.To resolve these issues, a detailed analysis of the Cenozoic time-varying residual subsidence derived by subtracting the predicted subsidence from the backstripped subsidence was performed along a new seismic reflection line in the western Qiongdongnan Basin. For the first time, a method is proposed to calculate the time-varying strain rates constrained by the faults growth rates, on basis of which, the predicted basement subsidence is obtained with a basin-and lithosphere-scale coupled finite extension model, and the backstripped subsidence is accurately recovered with a modified technique of backstripping to eliminate the effects of later episodes of rifting on earlier sediment thickness. Results show no residual subsidence in 45–28.4 Ma. But after 28.4 Ma, negative residual subsidence occurred, reached and remained ca. -1000 m during 23–11.6 Ma, and reduced dramatically after 11.6 Ma. In the syn-rift period(45–23 Ma), the residual subsidence is ca. -1000 m, however in the post-rift period(23–0 Ma),it is positive of ca. 300 to 1300 m increasing southeastwards. These results suggest that the syn-rift subsidence deficit commenced at 28.4 Ma, while the post-rift excess subsidence occurred after 11.6 Ma.Combined with previous studies, it is inferred that the opposite residual subsidence in the syn-and post-rift periods with similar large wavelengths(>10^(2) km) and km-scale amplitudes are the results of transient dynamic topography induced by deep mantle upwelling beneath the central QDNB, which started to influence the basin at ca. 28.4 Ma, continued into the Middle Miocene, and decayed at ca.11.6 Ma. The initial mantle upwelling with significant dynamic uplift had precipitated considerable continental extension and faulting in the Late Oligocene(28.4–23 Ma). After ca. 11.6 Ma, strong mantle upwelling probably occurred beneath the Leizhou–Hainan area to form vast basaltic lava flow.

Western fault zone of South China Sea and its physical simulation evidences

The western fault zone of the South China Sea is a strike-slip fault system and consists of four typical strike-slip faults. It is the western border of the South China Sea. The formation of the system is due to the extrusion of Indo - China Peninsula caused by the collision of India with Tibet and the spreading of the South China Sea in Cenozoic. There are five episodes of tectonic movement along this fault zone, which plays an important role in the Cenozoic evolution of the South China Sea. By the physical modeling experiments, it can be seen the strike-slip fault undergoes the sinistral and dextral movement due to the relative movement velocity change between the South China Sea block and the Indo - China block. The fault zone controls the evolution of the pull basins locating in the west of the South China Sea.

A Comparative Study of the Coal-forming Characteristics of Marginal Sea Basins and Epicontinental Sea Basins

The coal-forming characteristics,as well as the similarities and differences between epicontinental sea basins and continental marginal sea basins developed during different time periods,were analyzed in this study by adopting comparative analysis thoughts and methods.The results obtained in this study revealed that epicontinental basins and marginal sea basins are both characterized by the main development of thin coal seams or extremely thin coal seams.In addition,changes in sea levels were determined to be the main controlling factors for coal formation,and there were similarities in the continent-sea interactions and coal-forming sedimentary systems of the different basins.However,there were also significant differences observed in the sea level change events,basin basement structural characteristics,coal seam stability levels,accumulation and aggregation characteristics,and the migration patterns of coal-forming materials.For example,the marginal sea basins in the South China Sea were found to be characterized by strong tectonic activities,diversity and complexity.The basin structures showed complex patterns of depressions,uplifts and concave or sag uplifts,which tended to lead to greater complexity in the paleogeographic patterns of the coal formations.This had subsequently resulted in complex coal-forming processes and paleogeographic characteristics,in which the coal-forming zones displayed bead-like distributions,and the enrichment areas and centers were scattered.The practical significance of studying the similarities and differences of the coal-forming characteristics between epicontinental basins and marginal sea basins is that the results can potentially be used to guide the predictions of coal-measure coal seam distributions in South China Sea,as well as provide valuable guidance for future explorations of natural gas reservoirs related to coal measures in the South China Sea area.

Episodic rifting of continental marginal basins and tectonic events since 10 Ma in the South China Sea

Integrated study on the tectonic, basin filling and thermal evolution of the Tertiary basins on the northern margin of the South China Sea has indicated that the rift continental margin on the northern South China Sea is not a passive margin and that the western margin is characteristic of the transformextension. Episodic rifting and the thermal events since 10 Ma in the area have been documented by the integrated analysis of dynamic process. It has been clarified that the tectono-thermal events have exerted a significant influence on basin features and hydrocarbon accumulation, and particularly, the westward collision of the Lusong Island Arc, the generation of densely spaced faults in the Pearl River Mouth Basin, the rapid subsidence, high heat flow and large-scale over pressure and thermal-fluid breakthrough in the Yinggehai and the Qiongdongnan basins have proved to be the important factors determining the formation of oil and gas in these basins.

The deep thermal characteristic of continental margin of the northern South China Sea

Heat flow plays an important role in the study ot thermal structure and thermal evolutionof continental margin of the northern South China Sea. The analysis of heat flow value shows that margin heat flow in the northern South China Sea is relatively high setting, but the percentage of crustal heat flow is lower than 35% in terrestrial heat flow. The terrestrial heat flow exhibited a current of rise from the Northern Continental Margin to the Southern Central Basin. However, the proportion of crustal heat flow in terrestrial heat flow slowly dropped down in the same direction. It is suggested that the main factor causing high heat flow setting is the moving up of hot material from asthenosphere.

Stretching characteristics and its dynamic significance of the northern continental margin of South China Sea

To investigate the thinning of the whole crust, and the contribution of the upper versus lower crust to the stretching since Cenozoic, we calculated the stretching factor of the northern margin of South China Sea with data such as whole crustal thickness, depth of Cenozoic sedimentary basement and the horizontal displacement of faults. An isometric line drawing on whole crustal stretching factor is then obtained. Along the seismic Line 1530 in Baiyun sag, we also calculated the stretching factors of the upper and lower crust. The results suggest that the whole crustal thickness decreases seaward while the whole crustal stretching factor increases from shelf to slope. The Moho upwells highest beneath where the crust is thinned most. The value of the whole crustal stretching factor ranges from 1.5 to 6. Two areas were thinned intensely: the center of Yinggehai Basin, and the Baiyun sag in the Pearl River Mouth Basin. The calculation of the upper and lower crustal stretching factors from DSP1530 in Baiyun sag shows that the original crust of Baiyun sag should be thinned before deformation. Its pre-Cenozoic evolution as well as tectonic position during Cenozoic might be responsible for that.

Patterns and Dynamics of Rifting on Passive Continental Margin from Shelf to Slope of the Northern South China Sea:Evidence from 3D Analogue Modeling

Affected by thermal perturbation due to mantle uprising, the rheological structure of the lithosphere could be modified, which could lead to different rifting patterns from shelf to slope in a passive continental margin. From the observed deformation style on the northern South China Sea and analogue modeling experiments, we find that the rift zone located on the shelf is characterized by half grabens or simple grabens controlled mainly by long faults with large vertical offset, supposed to be formed with normal lithosphere extension. On the slope, where the lithosphere is very hot due to mantle upweUing and heating, composite grabens composed of symmetric grabens developed. The boundary and inner faults are all short with small vertical offset. Between the zones with very hot and normal lithosphere, composite half grabens composed of half grabens or asymmetric grabens formed, whose boundary faults are long with large vertical offset, while the inner faults are relatively short. Along with the thickness decrease of the brittle upper crust due to high temperature, the deformation becomes more sensitive to the shape of a pre-existing weakness zone and shows orientation variation along strike. When there was a bend in the pre-existing weakness zone, and the basal plate was pulled by a clockwise rotating stress, the strongest deformation always occurs along the middle segment and at the transition area from the middle to the eastern segments, which contributes to a hotter lithosphere in the middle segment, where the Baiyun （白云） sag formed.