COMPARISON OF THE TECTONICS AND GEOPHYSICS OF THE MAJOR STRUCTURAL BELTS BETWEEN THE NORTHERN AND SOUTHERN CONTINENTAL MARGINS OF THE SOUTHCHINA SEA

A comparison of the tectonics and geophysics of the major structural b elts of the northern and the southern continental margins of South Chi na Sea has been made, on the basis of measured geophysical data obtain ed by ourselves over a period of 8 years (1984-1991). This confirmed t hat the northern margin is a divergent one and the southern margin is characterized by clearly convergent features. The main extensional str uctures of the northern margin are, from north to south: (1) The Litto ral 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) T he Northern and Southern Depression zones (i.e., the Pearl River Mouth Basin), this strikes NE-ENE and is a very large Cenozoic depression w hich extends from offshore Shantou westwards to Hainan Island. (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 continent al slope. It is characterized by high magnetic anomalies. (4) Southern Boundary Fault Belt of the transitional crust. This has positive grav ity anomalies on the land side and negative ones on the sea side. (5) The Magnetic Quiet Zone. This is located south of the southern Boundar y Fault Belt and between the continental margin and the Central Basin of the South China Sea. Magnetic anomalies in this belt are of small a mplitude and low gradient. We consider the Magnetic Quiet Zone to be a very important tectonic zone. The major structures of southern contin ental margin southwards are: (1) The Northern Fault Belt of the Nansha Block. This extends along the continental slope north of the Liyue sh oal (Reed Bank) and Zhongye reef, and is a tectonic boundary between o ceanic crust and the Nansha Block continental crust. (2) The Nansha Bl ock Uplift Zone. Due to the development of reefs and shoals, there are many channels and valleys. Our long-distance multichannel seismic pro files indicated that there are thick Paleogene sediments and thin Neog ene sediments all over the central part of the block. (3) The Nansha T rough, a nappe structure formed by the southeastward drifting of Nansh a Block and northwestward overthrusting of Palawan-northwest Borneo. ( 4) Zengmu Shoal Basin, southwest of the Nansha Block; the maximum thic kness of Cenozoic strata is over 9 km in this important petroliferous basin.