EXTENSION OF THE ZHONGNAN-SILING FAULT ZONE IN SOUTH CHINA SEA AND ITS BEARING ON SEAFLOOR SPREADING

In order to detect the extension pattern of the Zhongnan-Siling fault zone in the oceanic basin, to define the southern margin of South China Sea, and to explore its relationship with the evolutionary history of seafloor spreading of the South China Sea, comprehensive geological and geophysical studies are made by the authors based on the data of gravity and magnetic anomalies, seismic profiles, depth of Moho, seismic P-wave velocity, drilling and dredging samples. The Zhongnan-Siling fault zone, which cuts down to the Moho-boundary, extends in N-S trend between the Eastern Sub-basin and Northwest Sub-basin along the southern margin of the South China Sea, and can be traced to the southern continental margin of the South China Sea. According to the variation in the trend of the magnetic anomaly belt in the oceanic basin of the South China Sea and the mutual relationship of the magnetic anomaly belt with the strike-slip or transform fault and the seafloor spreading direction, combined with previous research results on the "scissors-like" collision, seafloor spreading evolution and the tectonic stress of the South China Sea, the authors proposed in this paper that in the period of 32Ma to 25Ma, the South China Sea basin and the Nansha block had experienced a clockwise rotation along with the spreading of the Eastern Sub-basin in NNW trend, which caused the change in the direction of the Zhongnan-Siling fault from NNW in the early stage to N-S later on. After 23.5Ma, the clockwise rotation was stopped, while the Eastern Sub-basin continued to spread in NNW trend, the southwestern Sub-basin began spreading in a NW-SE trend with SW-ward propagation. The three crust-cutting deep faults, the Zhongnan-Siling fault, together with the other two regional crust-cutting deep faults, the Red River-East Vietnam fault and the Manila trench fault, formed a regional "slip line field", which constrained the seafloor spreading of the South China Sea and the southward drifting of the Nansha block.