Xisha Trough Research Articles

Large debris dumps in the northern South China Sea

Knowledge of the abundance, source, and fate of marine debris in the deep sea is largely constrained thus far. Here, we report the existence of large deep-sea debris dumps that have not been reported before on the seafloor worldwide. Marine debris remarkably accumulated at ~1700–1800 m in the tributary submarine canyons of the Xisha Trough, northern South China Sea (SCS). Although marine debris in Xisha Trough is patchy, the debris abundance was as high as 36,818 and 51,929 items/km2 at locations SY78 and SY82, respectively, which is one order of magnitude higher than that in other submarine canyons. We propose that most of the debris came from fishery and navigation activities, as indicated by the categories of debris collected from the seafloor dumps. Seasonal surface ocean currents of the SCS and geomorphology of submarine canyons possibly influence the movement of the debris from coasts to the deep seafloor.

A rapid shift in the sediment routing system of Lower-Upper Oligocene strata in the Qiongdongnnan Basin (Xisha Trough), Northwest South China Sea

Abstract The Oligocene is a period of continental breakup in the South China Sea and of earth surface topographic change in the southeastern margin of the Tibetan Plateau. In previous studies, Oligocene sediment from the Qiongdongnan Basin (QDNB) in the NW South China Sea has seldom been recovered, which limited understanding of basin evolution at this time. Here, we apply detrital zircon U-Pb dating to Oligocene sedimentary rocks from the QDNB. The U-Pb age populations in Lower Oligocene highlight the distinctive signatures of several competing sources providing material from nearby structural highs within the basin and Hainan island. In contrast, detrital zircon U-Pb ages of the Upper Oligocene in the central QDNB shows similarities with the Red River and/or coastal rivers in northern Vietnam. These sediments are significantly different from the Lower Oligocene. This abrupt change in the QDNB may imply that its distal sedimentary sources were initially established in the Late Oligocene. Our reconstruction of sediment discharge from proximal to distal sources in the Oligocene improves understanding of the sediment distribution in the QDNB, which will implicate the river drainage evolution and benefit the petroleum exploration of the Northwest South China Sea.

Deep-water bottom current evolution in the northern South China Sea during the last 150 kyr: Evidence from sortable-silt grain size and sedimentary magnetic fabric

Deep water bottom current (DWBC) plays a central role in global climate. Relative to the Atlantic, due to poor preservation of marine carbonate in the Pacific sediments, the evolution of Pacific deepwater circulation is still unclear. The South China Sea (SCS), connection to the Pacific through Luzon Strait, at a depth of about 2400m, offers a unique opportunity to monitor the changes of western Pacific deep water circulation. We present a magnetic and grain size analysis of two sediment cores (PC111 and PC83) located in Xisha Trough, northern SCS in order to reconstruct past changes of DWBC during the last 150kyrs. Variations in the mean size of sortable silt are interpreted to indicate past changes of DWBC, which suggest abrupt, millennial-scale increasing DWBC strength during Heinrich Stadials (HS1, HS2, HS3, HS4, HS5, HS6, HS7, HS8, HS9, HS10, HS11) for past 150kyr. The flow orientation of DWBC revealed by the anisotropy of magnetic susceptibility (AMS) are mostly NW–SE direction in Core PC111 and Core PC83, which are both parallel to the local topography. Combined with the published studies, we suggest the increase bottom current in northern SCS during the Heinrich Stadials (HS1 to HS11), times of weak North Atlantic Deep Water formation, demonstrating the tight coupling between the Atlantic and Pacific Oceans.

Variability in sinking fluxes and composition of particle-bound phosphorus in the Xisha area of the northern South China Sea

Export fluxes of phosphorus (P) by sinking particles are important in studying ocean biogeochemical dynamics, whereas their composition and temporal variability are still inadequately understood in the global oceans, including the northern South China Sea (NSCS). A time-series study of particle fluxes was conducted at a mooring station adjacent to the Xisha Trough in the NSCS from September 2012 to September 2014, with sinking particles collected every two weeks by two sediment traps deployed at 500m and 1500m depths. Five operationally defined particulate P classes of sinking particles including loosely-bound P, Fe-bound P, CaCO3-bound P, detrital apatite P, and refractory organic P were quantified by a sequential extraction method (SEDEX). Our results revealed substantial variability in sinking particulate P composition at the Xisha over two years of samplings. Particulate inorganic P was largely contributed from Fe-bound P in the upper trap, but detrital P in the lower trap. Particulate organic P, including exchangeable organic P, CaCO3-bound organic P, and refractory organic P, contributed up to 50–55% of total sinking particulate P. Increase of CaCO3-bound P in the upper trap during 2014 could be related to a strong El Niño event with enhanced CaCO3 deposition. We also found sediment resuspension responsible for the unusual high particles fluxes at the lower trap based on analyses of a two-component mixing model. There was on average a total mass flux of 78±50mgm−2 d−1 at the upper trap during the study period. A significant correlation between integrated primary productivity in the region and particle fluxes at 500m of the station suggested the important role of biological production in controlling the concentration, composition, and export fluxes of sinking particulate P in the NSCS.

Hyper-extended rift systems in the Xisha Trough, northwestern South China Sea: Implications for extreme crustal thinning ahead of a propagating ocean

From the apex of the NW Sub-basin of the South China Sea, we present new long-cable deep-penetration seismic profiles and deepwater drillings together with corrected seismic interpretations from previous studies. Structural mapping demonstrates that the Xisha Trough is characterized by a hyper-extended rift system that shows evidence of extremely thinned crust and rotated fault blocks. A series of NE, NEE and W-E striking listric normal faults along the Xisha Trough, which merge at depth into Moho reflection, record strongly extensional deformation. Based on the absence of an acoustic basement beneath a wedge-shaped rifted sequence, we document three diapiric magma structures exiting the Moho and penetrating into the extremely thinned continental crust, which is covered by a Cenozoic sequence. For the Xisha Trough we can demonstrate, based on seismic data and calculated fault slip rates, the faults bounding the Xisha Trough were initially formed by distributed rifting, and were reactivated and focused on faulting along the axis of the Xisha Trough. However, the peak faulting migrated along the margin strike from east to west and terminated at the apex of the Xisha Trough. One of the key processes, controlling the evolution of the Xisha Trough, is depth-dependent extension, which may arise from differential lithospheric stretching by regional dynamic lower crustal/mantle flow.

Crustal structure and extension mode in the northwestern margin of the South China Sea

AbstractCombining multi‐channel seismic reflection and gravity modeling, this study has investigated the crustal structure of the northwestern South China Sea margin. These data constrain a hyper‐extended crustal area bounded by basin‐bounding faults corresponding to an aborted rift below the Xisha Trough with a subparallel fossil ridge in the adjacent Northwest Sub‐basin. The thinnest crust is located in the Xisha Trough, where it is remnant lower crust with a thickness of less than 3 km. Gravity modeling also revealed a hyper‐extended crust across the Xisha Trough. The postrift magmatism is well developed and more active in the Xisha Trough and farther southeast than on the northwestern continental margin of the South China Sea; and the magmatic intrusion/extrusion was relatively active during the rifting of Xisha Trough and the Northwest Sub‐basin. A narrow continent‐ocean transition zone with a width of ∼65 km bounded seaward by a volcanic buried seamount is characterized by crustal thinning, rift depression, low gravity anomaly and the termination of the break‐up unconformity seismic reflection. The aborted rift near the continental margin means that there may be no obvious detachment fault like that in the Iberia‐Newfoundland type margin. The symmetric rift, extreme hyper‐extended continental crust and hotter mantle materials indicate that continental crust underwent stretching phase (pure‐shear deformation), thinning phase and breakup followed by onset of seafloor spreading and the mantle‐lithosphere may break up before crustal‐necking in the northwestern South China Sea margin.

Geodynamics of the South China Sea

The beginning of seafloor spreading in the South China Sea (SCS) is now established from IODP drilling Leg 349 at 33Ma. Chron 12 (32Ma) is the oldest chron identified in the SCS. The nature of the crust of the northeastern part of the SCS located north of chron C12, where chrons 15 to 17 were previously identified, is not oceanic but thinned continental crust intruded by volcanic elongated features emplaced 17–22Ma ago. Based on magnetic anomaly identifications, the end of the SCS spreading could be either 15.5, 20.5Ma (Briais et al., 1993; Barckhausen et al., 2014) or something else. However, as post-spreading magmatic activity (~13–3.5Ma) largely masks the spreading fabric in particular near the axis of the east sub-basin, published locations of the axial magnetic anomaly (extinct spreading axis) and spreading rates are not reliable. A contoured map of the extremely dense set of magnetic data shows that a few magnetic lineations belonging to the magnetic seafloor spreading fabric are still preserved and parallel to the N055° bathymetric seafloor spreading trends identified on swath-bathymetric maps in the central part of the SCS, suggesting that the extinct ridge axis is N055° trending with potential N145° transform faults. Based on published swath-bathymetric data, oceanic domains with different seafloor spreading lineaments have been delimited (N055°, N075° and N085°) and provide important constraints used to propose a kinematic sketch of the SCS opening. As a consequence, the Zhongnan faults zone, located between the east and southeast sub-basins, acted as a major fracture zone system during the SCS opening, with horizontal offsets varying from ~50km to ~140km.The flow-line pattern defined from the seafloor spreading lineaments and the few identified FZs have been used to highlight conjugate segments of continental margins. During the first phases of opening of the SCS, from the fit of continents to chron C10 (30Ma), the N175° extension observed in the Xisha trough and eventually south of the Macclesfield Bank extends to the Qui Nhon ridge, located along the eastern margin of Vietnam, in the southward prolongation of the Red River fault system. Normal faults curve toward the south with a horsetail geometry interpreted as evidence for a few tens of kilometers of dextral motion along the Qui Nhon ridge. Since chron 10 and until the end of SCS opening, the plate boundary located between the southern South China Sea (SSCS) and EU plates jumped westward several times from the location of the Ulugan fault near Palawan to the western limit of the southwest basin, explaining the progressive formation of the SCS from east to west and giving the characteristic V-shape of the SCS. The opening of the whole SCS is linked and occurred simultaneously with the northward subduction of the proto-SCS whose suture is located south of Palawan and extends westwards in north Borneo.

Deep-water sedimentary systems and their relationship with bottom currents at the intersection of Xisha Trough and Northwest Sub-Basin, South China Sea

Based upon 2D reflection seismic data and numerical modelling, this study confirms the presence of a complex deep-water sedimentary system on the present-day seafloor at the intersection of the Xisha Trough and the Northwest Sub-Basin (South China Sea) and investigates their relationship with bottom currents. The deep-water sedimentary system consists of submarine canyons, slides and slumps, wave-like successions, mounded drifts and two groups of marginal depressions (those with erosional features and those appearing as morphological sediment sinks). Three-dimensional process-based modelling is applied to investigate sediment dynamics induced by a combined effect of tidal currents and a quasi-steady geostrophic current (South China Sea Deep Water Circulation). Simulation results show that the South China Sea Deep Water Circulation at the southeastern flank of the seamount plateau could reach velocities of 15cm/s during flood tides, enabling erosion and transport processes. In contrast, the rest of the plateau area is favoured for deposition, since current velocities in this region are persistently lower than 10cm/s. The current velocities at the feet of the obstacles (where the morphological depressions are located) are strengthened and are several cm/s higher than that in adjacent flat areas (e.g. where the mounded drifts are located). The flow is constricted and accelerated after being deflected by the obstacles, resulting in contrasting higher sedimentation rates within the mounded sediments and lower rates at the morphological depressions. A comparison between the seismic stratigraphy and the simulated fluid dynamics enables a decoding of the pathway, identifying the current regime as well as unravelling the relationship between depositional processes and the deep-sea water circulation. This study provides new insights and exposes new challenges in understanding the dynamics of deep-sea sedimentation processes in South China Sea.

Seasonal flux variability of planktonic foraminifera during 2009–2011 in a sediment trap from Xisha Trough, South China Sea

The flux and seasonality of planktonic foraminifera were studied through a time-series sediment trap deployed in the Xisha Trough, South China Sea from June 2009 to August 2011. The general flux pattern for planktonic foraminifera showed high values during the winter half-year/season (from late September/October to February) and low flux during the rest of the year. This flux pattern contrasts with the bimodal pattern in the central and southern South China Sea, which shows two high flux peaks corresponding to the prevailing summer and winter monsoon periods, respectively. Apparent interannual flux differences, which may be related to the climate change from an El Niño year to a La Niña year, were observed. Monsoonal driven seasonality is most prominent in Pulleniatina obliquiloculata, Neogloboquadrina dutertrei and Globigerina bulloides, with over 80% of their species-specific fluxes (93% for G. bulloides) occurring from late September/October to February. Globigerinoides sacculifer and Globigerinoides ruber show similar seasonality, generally following the trend of the total flux of planktonic foraminifer. G. bulloides may have the potential to be developed as a winter proxy-species. Globigerinoides conglobatus mostly appears in June to August and can be developed as a summer proxy-species.

Full-fit reconstruction of the South China Sea conjugate margins

Although the South China Sea margins have been well-studied, there are no full-fit reconstructions that document the complex rifting history in a regional geodynamic context. We restored conjugate margins to their original Late Cretaceous unstretched geometries, and generated the first full-fit reconstructions of the South China Sea conjugate margins. The impact of magmatic underplating on restoration and full-fit reconstruction was estimated quantitatively. The full-fit reconstruction model includes restoration uncertainties and results in minimum gaps and overlaps between the restored conjugate margin geometries. Based on our restoration, the full-fit reconstruction and the continental basement thickness distribution, we estimate that there was more extension of continental basement on the western part of the conjugate margins compared with the eastern margin prior to the initiation of seafloor spreading. Therefore, the mid ocean ridge initially formed in the east and propagated westward to open the South China Sea. Seafloor spreading propagated along the Xisha Trough during NW Sub-basin formation, but continental breakup occurred along the southern side of the Zhongsha–Xisha Islands where the lithosphere was probably weaker than along the Xisha Trough, consistent with the geometry and evolution of the NW Sub-basin and the southward jump of the mid oceanic ridge.

Mass transport deposits and processes in the north slope of the Xisha Trough, northern South China Sea

Triple mass-transport deposits (MTDs) with areas of 625, 494 and 902 km2, respectively, have been identified on the north slope of the Xisha Trough, northern South China Sea margin. Based on high-resolution seismic reflection data and multi-beam bathymetric data, the Quaternary MTDs are characterized by typical geometric shapes and internal structures. Results of slope analysis showed that they are developed in a steep slope ranging from 5° to 35°. The head wall scarps of the MTDs arrived to 50 km in length (from headwall to termination). Their inner structures include well developed basal shear surface, growth faults, stepping lateral scarps, erosion grooves, and frontal thrust deformation. From seismic images, the central deepwater channel system of the Xisha Trough has been filled by interbedded channel-levee deposits and thick MTDs. Therefore, we inferred that the MTDs in the deepwater channel system could be dominated by far-travelled slope failure deposits even though there are local collapses of the trough walls. And then, we drew the two-dimensional process model and threedimensional structure model diagram of the MTDs. Combined with the regional geological setting and previous studies, we discussed the trigger mechanisms of the triple MTDs.

The linkage between longitudinal sediment routing systems and basin types in the northern South China Sea in perspective of source-to-sink

Using bathymetric and seismic data, this study describes the morpho-sedimentary features in Qiongdongnan basin and southwest Taiwan collision basin, northern South China Sea and reveals the linkages between sediment routing system and basin types. The modern Central Canyon in the Qiongdongnan basin is located along the rift margin, and subparallel to the shelf-break southeast of Hainan Island. The modern Central Canyon develops along the basin axis (i.e., Xisha Trough) and longitudinally transports sediments eastward which are mainly supplied by northern continental slope. The Penghu Canyon in the southwest Taiwan collision basin is located along the collision boundary parallel to the strike of the adjacent uplifted Taiwan orogen. The Penghu Canyon develops along the tilting basin axis transporting sediments longitudinally southward to the deep-sea basin and Manila Trench. The Penghu Canyon is supplied with sediments from both flank Kaoping and South China Sea slopes where tributary canyons and channels transport sediments down-slope and feed the axial canyon.The certain basin types may be occupied by particular styles of sediment routing system. By comparing the morpho-sedimentary features and basin characteristics associated with the modern Central Canyon to that of the Valencia Channel in NW Mediterranean Sea, the longitudinal sediment routing system in rift basin type can be determined. In contrast, the longitudinal sediment routing systems in collision setting can be represented by the comparable examples of Penghu Canyon in southwest Taiwan collision basin and Markham Canyon in western Solomon Sea. The rift type sediment routing system is characterized by an axial canyon with a single sediment supply from land drainage margin. In contrast, sediment routing system in collision type basins consists of an axial canyon and dual sediment supplies from flank adjacent slopes. The axial canyons in collision basins are more active than that of the rift basin due to abundant sediment supply and active tectonics. The results demonstrate the role of longitudinal sediment transport in delivering terrestrial sediments for a long distance from land drainages (source) to far-field deep-sea basin (sink). This study has implication for better understanding of the linkages between basin mechanics, sediment routing systems and sedimentary processes.

Structure and kinematic analysis of the deepwater area of the Qiongdongnan Basin through a seismic interpretation and analogue modeling experiments

Located at the northwest continental slope of the South China Sea, the Qiongdongnan Basin bears valley-shaped bathymetry deepening toward east. It is separated from the Yinggehai Basin through NW-trending Indo-China-Red River shear zone, and connected with NW subsea basin through the Xisha Trough. Along with the rapid progress of the deepwater exploration, large amounts of high resolution geophysical and geological data were accumulated. Scientific researches about deepwater basins kept revealing brand new tectonic and sedimentary discoveries. In order to summarize the structural features and main controlling factors of the deepwater Qiongdongnan Basin, a series of researches on basin architecture, fault activities, tectonic deformation and evolution were carried out. In reference to analogue modeling experiments, a tectonic situation and a basin formation mechanism were discussed. The researches indicate that: the northern boundary of the Qiongdongnan Basin is strongly controlled by No. 2 fault. The overlapping control of two stress fields from the east and the west made the central depression zone extremely thinned. Combined with the changed stress field, the segmentation of a preexisting weakness zone made the sags in the east experiencing different rifting histories from the west ones. The NE-trending west segment of the Qiongdongnan Basin experienced strong rifting during Eocene, while the roughly EW-trending sags in the east segment show strong rifting during late Eocene and early Oligocene. Local structures such as NW-trending basal fault and inherited uplifts controlled the lateral segmentation. So first order factors such as regional stress field and preexisting weakness zone controlled the basin zonation, while the second order factors determined the segmentation from east to west.

Bacterial diversity in sediments of core MD05-2902 from the Xisha Trough, the South China Sea

A sediment core MD05-2902 was collected from the deep-sea basin of the Xisha Trough. The vertical distribution and diversity of bacteria in the core was investigated through ten sub-sampling with an interval of 1 m using bacterial 16S rRNA gene as a phylogenetic bio-marker. Eighteen phylogenetic groups were identified from 16S rRNA gene clone libraries. The dominant bacterial groups were JS1, Planctomycetes and Chloroflexi, which accounted for 30.6%, 16.6%, and 15.6% of bacterial clones in the libraries, respectively. In order to reveal the relationship between biotic and abiotic data, a nonmetric multidimensional scaling analysis was performed. The result revealed that the δ15N, δ13C, total organic carbon and total organic nitrogen possibly influenced the bacterial community structure. This study expanded our knowledge of the biogeochemical cycling in the Xisha Trough sediment.

Modern transport and deposition of settling particles in the northern South China Sea: Sediment trap evidence adjacent to Xisha Trough

Studies on modern sediment transport and deposition, especially studies analyzing settling particles collected with sediment traps, have rarely been carried out in the northern South China Sea. Using sediment trap time series data from Site XS1 (17°24.5′N, 110°55.0′E, water depth 1690m) adjacent to the Xisha Trough, variations in sediment source through time have been reconstructed. These observations include total particle flux (TPF) and current data, grain size distributions, and clay mineral compositions obtained from two sediment traps deployed in 500m and 1500m water depth, respectively. Time series records at Site XS1 changed seasonally for both sampled layers. TPF in the lower layer (426mg/m2/d) was several times that of the upper layer (113mg/m2/d) and is affected by lateral transport. However, mean grain size (Mz) of the upper layer is greater that of the lower layer (29 vs10μm) due to contributions from biogenic materials. There are no clear seasonal changes in clay mineral assemblage in either the upper or lower layers. The annual percentages of four main clay minerals were 82–83% illite, 7–9% kaolinite, 6–8% chlorite and 1–3% smectite. Taiwan was the dominant sediment source (42–74%), while sediment contributions from the Red River and Annamite Chain account for 23–53% and 0–15%, respectively. Sediment supply from Taiwan could be explained by deep water current flow, while coastal currents may aid sediment transport from the Red River and small mountainous rivers of central Vietnam.

Stress field modeling of northwestern South China Sea since 5.3 Ma and its tectonic significance

Tectonically, the northwestern South China Sea (SCS) is located at the junction between three micro-plates, i.e., the Indochina, South China and Zhongsha-Xisha micro-plates, and involves three basins, i.e., the Yinggehai Basin, the Qiongdongnan Basin and Xisha Trough in the east, and the Zhongjiannan Basin in the south. Since the Pliocene (5.3 Ma), the Yinggehai Basin has experienced repeated accelerating subsidence, high thermal fluid, and widely developing mud-rich overpressure chambers, abundant mud diapers and crust-mantle mixed CO2. While a large central canyon was developed in the Qiongdongnan Basin, new rift occurred in the Xisha Trough. These characteristics demonstrate a single tectonic unit for the northwestern SCS, for which we have undertaken stress field modeling to understand its plate deformations and sedimentary responses. Our results demonstrate that an extension tectonic event occurred after 5.3 Ma in the Yinggehai-Qiongdongnan-Xisha trough area, which is characterized by thinner crust ( 3 500 m). A new rift system subsequently was developed in this area; this event was mainly driven by the combined effects of different movement velocity and direction of the three micro-plates, and the far-field effect of the continental collision between the Indian Plate and the Tibetan Plateau, and subduction of the Pacific Plate underneath the Eurasian Plate.

Petroleum Exploration Potential of Xisha Trough Basin in SCS

Nowadays oil-gas exploration make a great contribution to the world oil-gas reserve increase. A series of deepwater passive continental margin basins are found in Northern Continental Slope of South China Sea. These basins consisted of thick Mesozoic and Cenozoic sedimentary strata with the characteristics of the major world deepwater oil-gas basins. As one of Cenozoic sedimentary basins in deepwater area of Northern Slope of South China Sea, Xisha Trough Basin developed 1500-8000m thick sedimentary strata, which are north-south zoning characteristics of thicker in the center and thinner both in the north and south sides of basin. In its evolutionary history there are two stages: One is Paleocene-Oligocene Rift with Continental River-Lake Facies sedimentary and the other is Miocene-Quaternary Depression with shallow sea-hemiplegic sedimentary. It has good petroleum geological conditions that source rocks consist of lacustrine mudstones, paralic mudstone, and marine mudstone, Tertiary high porosity and permeability deepwater fan reservoirs are the main reservoir, and structural traps and lithologic traps developed. In a word, it has good oil-gas exploration potential.

THE STRUCTURAL FEATURES OF XISHA TROUGH BASIN

The Xisha Trough Basin,located on the continental slope of the northern South China Sea,is a Cenozoic deepwater basin.The sediment fill in the basin is about 1500 ~ 8000m in thickness,and becomes thinner from the center towards north and south.The sediments show a zonal distribution pattern in north-south direction.In seismic profiles,there is double-layer structure,of which the upper is formed with subsidence and the lower with rifting.The structures formed in the rifting stage were mainly of Domino-style of half-graben,a series of half-graben controlled by major faults on one side(F1,F2,F3,F4,F5).The basin has experienced two stages of evolution: the first stage underwent rifting through Paleocene-Oligocene and filled with lacustrine deposits,the second stage underwent subsidence through Miocene-Quaternary and filled shallow or bathyal sediments.

PLANKTONIC FORAMINIFERA IN THE XISHA TROUGH OF THE SOUTH CHINA SEA DURING THE LAST 120 KA AND THEIR PALEOCLIMATIC IMPLICATIONS

Planktonic foraminifera from the core 83PC in the Xisha Trough of the northern South China Sea(SCS)have been quantitatively analyzed.The microfaunal analytical results,combined with oxygen and carbon isotopes,carbonate content and AMS14C dating data,are used in this paper to reveal paleoceanographic characteristics of the study area during the last ~120ka.The distribution of planktonic foraminifera shows that the abundance decreased in glacial periods,warm water species were sensitive to temperature changes,and the percentages of cool water species have decreased gradually since MIS5.Sea surface paleotemperature(SST)was estimated via transfer function FP-12E.The calculation results suggest that the winter SST ranged from 21.3 to 26.4℃,the summer SST from 28.2 to 29.5℃,and the SST seasonality from 2.7 to 6.9℃.By comparison with the results from the southern SCS,the SST differences were evident from MIS2 to MIS5,which probably resulted from the growth of the Northern Hemisphere ice sheet and the strong East Asian Monsoon,the higher SST differences in glacial periods indicated enhanced winter monsoon.Several obvious short-term SST descending events,including the Younger Dryas and Heinrich events,have been recognized since the last glaciation,suggesting the sedimentary records in response to the global climatic changes in the SCS.

Preliminary Results of Earthquake Observations and Onshore‐Offshore Seismic Experiments on Islands of the Xisha Block, South China Sea

AbstractTemporary earthquake stations were installed on islands of the Xisha block, South China Sea and onshore‐offshore deep seismic experiments were carried out to study the crustal structure beneath this region. The seismic data were analyzed and processed. A 1‐D shear wave velocity model beneath the Chenhang island (CHH) was constructed by applying the receiver function method to the teleseismic data. A 2‐D P‐wave velocity crustal structure was modeled by ray‐tracing and travel‐time simulation. The result shows that a 2 km‐thick low‐velocity sediment which is attributed to Cenozoic deposition exists on the top of CHH. In the lower crust, there exists a prominent low‐velocity layer (LVL) with average shear wave velocity of 3.5 km/s; Moho interface lies at the depth of 24~26 km right beneath CHH. The crustal thickness gradually decreases outward along the onshore‐offshore seismic line. The crustal structure has the typical characteristics of rift valley on the northwestern side of CHH, which belongs to the extension of the Xisha trough. The presentation of the anisotropy of upper mantle minerals and the ductile rheological structure arising from hot mantle activities may result in the generation of the LVL in lower crust.