The geophysical characteristics and evolution of northern and southern margins of the South China Sea

Data of about 40,000 km gravity and magnetic measurements, 10,000 km multichannel seismic lines, 20 sonobuoy refraction seismic stations, and 5 heatflow stations have been obtained by our institute in the South China Sea (SCS) during the passed ten years. Based on these and other data, the present paper discusses the tectono-geophysical characteristics and evolution of SCS and put forward some new ideas. On the northern margin of SCS, five parallel structure zones have been recognized and named as, from the north to the south, the Littoral Fault Zone, Northern Depression Zone, Central Uplift Zone, Southern Depression Zone, and Slope Fault Zone. The basement of the margin is the extension of that inland Southeast China and consists of the Hercynian fold belt in the east and the Caledonian fold belt in the west. Both belts were extensively disturbed by Yanshanian magmatic emplacements. Based on gravity data, the crustal thickness is 25-26 km for the Northern Depression Zone, greater than 29 km for the Central Uplift zone, and 20-22 km for the Southern Depression Zone. A gradient belt of Moho surface is observed along the Slope Fault Zone. A belt of high magnetic anomalies is found extending from the Dongsha Uplift NE-ward to the Penghu Islands, and a magnetic quiet zone extending along the lower slope. The former is interpreted as the manifestation of a basalt belt of 1-3 km in depth. These were intraplate eruptions and indicate the Cenozoic extension. The Slope Fault Zone is represented by a northward dipping lithospheric fault that penetrates the basement down to the Moho. We prefer the interpretation of the fault as a segment of the Mesozoic East Asia subduction zone. The I-type granites (76-130 Ma) and associated volcanic rocks in the basement of the continental shelf might be the associated magmatic arc, while the thickened crust in the Central Uplift Zone might be the accretional wedge. The magnetic quiet zone might indicate the remains of the Paleo-Pacific oceanic crust. In the southern margin of SCS three seismic sequences have been identified. In particular, a thick Paleogene sequence consists of neritic clastics with clear stratification and widespread distribution. This sequence was strongly deformed into asymmetric folds and eroded in the western Nansha Block. Based on the correlation with limited and distant wells, this compressional event, called as the Nansha Movement, is tentatively dated as Late Eocene, which was contemporaneous with the first phase of the Zhu-Qong movement in the northern margin of SCS, but under totally different stress regime. Unconformably overlying Neogene sequence is thin and mainly as fillings of small and discrete half grabens in the Dangerous Grounds, but thick and continuous on the Liyue Bank and the NW Palawan shelf (1.5 to over 2 km), and even thicker in the Zengmu Basin (the Great Sarawak Basin) and to the west. This indicates that the Dangerous Grounds have stayed mainly in the uplift state since the Nansha Movement, different from the shelf area to the east, south, and southwest. In light of the above-mentioned findings, we suggest that this block collided with Paleo-Borneo and uplifted in Late Eocene time, long before the start of the opening of SCS. Before the collision, most areas of the Nansha block was the shallow sea bordering the Paleo South China continent, where marginal rifting had been continuing since the Late Cretaceous. Not until the Late Oligocene the extension extended southward to the Nansha Block and became strong enough to open the SCS. Later in the Early Miocene, Borneo rotated counterclockwisely, probably about a pole near the center of Borneo rather than to the west. The narrow Nansha Trough Basin was formed by the elastic downwarp under the load of the NW-ward overthrusting Sabah nappe, while the Zengmu Basin was formed by the extension west of the pole of rotation