Kanyuan Xia

Crustal structure across the Xisha Trough, northwestern South China Sea

Located at the northwestern part of the South China Sea (SCS) between the Hainan and Xisha (Paracel) Islands, the Xisha Trough represents a failed rift in conjunction with the opening of the SCS between 32 and 17 Ma. From west towards east within a scale of several hundred kilometers, it presents all major stages of the rifting process, and thus, provides an ideal place to study the rifting process in great details. In the autumn of 1996, a joint team of Sino-German scientists carried out a wide-angle seismic experiment across the Xisha Trough with 10 ocean bottom hydrophones (OBH) along a 237-km NNW-SSE-oriented profile, which was surveyed in 1987 with multi-channel seismic (MCS) method by BGR of Germany and SOA of China. Favorable weather conditions and the powerful 4 x 12-1 air gun array rendered very good quality data with seismic signals observed at the offset of up to 110 km, A detailed velocity-depth model was obtained by using an interactive trial-and-error 2D ray-tracing method. Interpretation of the NICS data published by BGR provides very good geometrical constraints of the complex upper crustal structure, which is characterized by fault blocks, half-horsts and half-grabens filled with syn- and post-rift Cenozoic sediments. The velocity model in turn confirms the major structure outlined by the interpretation of the MCS data, showing a varying sedimentary layer between I and 4 km of thickness and velocities between 1.7 and 4.5 km/s. The P-wave velocity of 5.5 km/s on the top of the crystalline basement is relatively low, suggesting strong weathering. Within the crystalline crust, the velocity increases downward continuously to 6.8 km/s at the bottom of the crust without a clear differentiation in the middle crust, showing clearly its continental nature even beneath the Xisha Trough. The Moho is marked by a sharp first-order interface with a velocity of 8.0-8.1 km/s at the uppermost mantle. The Moho depth is 15 km beneath the center of the trough and increases gradually to more than 25 kin towards north and south, corresponding to an extreme thinning of the pre-rift continental crust from more than 25 kin under the coast line to only 8 kin beneath the Xisha Trough. The similar velocity structure of the continental nature on both sides of the Xisha Trough suggests a homogeneous pre-rift continental setting. The sharp Moho and the lack of high velocity body (HVB) in the lower crust imply no magmatic underplating, which is very different to interpretations across the eastern part of the continental margin in northern South China Sea. The intense faulting in the upper crust, the strong but rather symmetrical crustal thinning centered at the Xisha Trough and the close neighborhood to the open SCS NW subbasin suggest a pure shear rifting, which failed most likely at the phase shortly before the continental breakup

Characteristics of surface heat flow in the South China Sea

A total of 592 heat flow measurements, ranging from 8 to 192 mW/m(2), 78 percent of which are between 50 and 100 mW/m(2), have been collected in the South China Sea (SCS). To overcome shortcomings such as the uneven distribution of the measurements and the occurrence of abnormal heat flow values, the tectonic evolution of different areas and their crustal thickness have been combined in analyzing geothermal characters. The results show that the oceanic basins, where they are floored by oceanic crust, the western part of the southern margin and the western fault system have high heat flow values. Heat flow along the northern margin of the SCS increases from 61 mW/m(2) on the shelf to 73-80 mW/m(2) in the slope area, and in the Xisha-Zhongsha area increases from about 70 mW/m(2) to about 85 mW/m(2) from the NW to the SE. Heat flow in the Nansha (Spratley) Islands is about 60 mW/m(2) or even higher, and decreases from the NW to SE; The average heat flow of the Mekong Basin is about 60 mW/m(2), which is similar to that of the Beibuwan Basin on the northern margin. Heat flow on the eastern margin and on the eastern part of the southern margin is lower, especially in the Luzon Trough, where the average heat flow is lower than 40 mW/m(2). Observed heat flow values in the SW subbasin are generally lower than predicted theoretically from the age of the ocean floor, unlike values in the eastern subbasin. A high heat flow zone in the lower slope area of the northern margin is recognized for the first time

Comparison of the tectonics and geophysics of the major structural belts between the northern and southern continental margins of the South China Sea

Abstract A comparison of the tectonics and geophysics of the major structural belts of the northern and the southern continental margins of South China Sea has been made, on the basis of measured geophysical data obtained by ourselves over a period of 8 years (1984–1991). This confirmed that the northern margin is a divergent one and the southern margin is characterized by clearly convergent features. The main extensional structures of the northern margin are, from north to south: 1. (1) The Littoral Fault Belt, a tectonic boundary between the continental crust and a transitional zone, along the coast of the provinces of Guangdong and Fujian in South China. It is characterised by earthquake activities, high magnetic anomalies and a rapid change in crustal thickness. 2. (2) The Northern and Southern Depression zones (i.e., the Pearl River Mouth Basin), this strikes NE-ENE and is a very large Cenozoic depression which extends from offshore Shantou westwards to Hainan Island. 3. (3) The Central Uplift Zone. This includes the Dongsha Uplift, Shenhu Uplift and may be linked with the Penghu uplift and Taiwan shoals to the east, forming a large NE-striking uplift zone along the northern continental slope. It is characterized by high magnetic anomalies. 4. (4) Southern Boundary Fault Belt of the transitional crust. This has positive gravity anomalies on the land side and negative ones on the sea side. 5. (5) The Magnetic Quiet Zone. This is located south of the southern Boundary Fault Belt and between the continental margin and the Central Basin of the South China Sea. Magnetic anomalies in this belt are of small amplitude and low gradient. We consider the Magnetic Quiet Zone to be a very important tectonic zone. The major structures of southern continental margin southwards are: 1. (1) The Northern Fault Belt of the Nansha Block. This extends along the continental slope north of the Liyue shoal (Reed Bank) and Zhongye reef, and is a tectonic boundary between oceanic crust and the Nansha Block continental crust. 2. (2) The Nansha Block Uplift Zone. Due to the development of reefs and shoals, there are many channels and valleys. Our long-distance multichannel seismic profiles indicated that there are thick Paleogene sediments and thin Neogene sediments all over the central part of the block. 3. (3) The Nansha Trough, a nappe structure formed by the southeastward drifting of Nansha Block and northwestward overthrusting of Palawan-northwest Borneo. 4. (4) Zengmu Shoal Basin, southwest of the Nansha Block; the maximum thickness of Cenozoic strata is over 9 km in this important petroliferous basin.

Recent progress of deep seismic experiments and studies of crustal structure in northern South China Sea

The South China Sea (SCS) is one of the largest marginal seas in the western Pacific. Its northern part has the features of a passive continental margin. The studies of deep crustal structure in this area are very important for understanding the tectonic nature, evolution history, basin formation of the northern margin, and the origin of the SCS. In the past decades, the deep seismic experiments of crustal studies in the northern SCS have gone through three stages, namely the sonobucy, two-ship Expanding Spread Profile (ESP), and Ocean Bottom Hydrophone/Seismometer (OBH/OBS). Along the continental slope, the sonobuoy experiments provided useful information about the velocity structure of the upper crust, while the ESP data recorded for the first time the seismic signals from deep crustal structure and Moho interface. And the OBH/OBS profiles revealed the crustal structure in much greater detail. This paper first gives a brief historical review of these deep seismic experiments and studies, then a summary of the latest progress and important research results. The remaining problems and suggestions for further research work are presented as conclusive remarks.

Numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence: Discussions on the evolution of the Tainan Basin

Existing evidence shows that an Oligocene erosion event occurred on the northern continental margin of the South China Sea, and the Tainan Basin area might be at the center of this event, followed by a rapid tectonic subsidence in the late Oligocene and early Miocene period. The rapid tectonic subsidence is mainly thermal-controlled, and the effect of the Yichu Fault on the Tainan Basin is limited to the basins eastern part. We develop a 2-D thermal-mechanical kinematic numerical model to explore the relationship between thermal uplift and subsequent rapid subsidence in the Tainan Basin. Our modeling indicates that the Oligocene uplift, erosion, and subsequent rapid subsidence could be caused by a thermal event, and the differential subsidence of the basement caused by thermal contraction can initiate the development of small faults. However, it also suggests that other mechanisms might be needed to jointly account for the observed erosions. Citation: Shi, X., H. Xu, X. Qiu, K. Xia, X. Yang, and Y. Li (2008), Numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence: Discussions on the evolution of the Tainan Basin, Tectonics, 27, TC6003, doi: 10.1029/2007TC002163.