The gravity field and tectonics of the Nansha Islands (Dangerous Grounds)

Abstract

Gravity surveys of the Nansha Islands in the southern part of the South China Sea (the Dangerous Grounds) have been undertaken by the China Sea Institute of Oceanology, Academy Sinica. More than 19,000 line kilometres of gravity profiles were measured between 1984 and 1994 in the region 106-118°E, and 4-15°N. The accuracy of the gravity data is approximately + 2.5 mgal. The area surveyed can be split into three zones according to the observed gravity variations. These three zones are: 1) the Zengmu Basin (Sarawak Basin) Province where the gravity variation is mostly controlled by variations in sediment thickness, 2) the Reed Bank Gravity High Province which includes the highest measured gravity values in the South China Sea and where gravity variations are influenced mostly by changes in deep crustal structure, and 3) the Nansha Trough (Palawan Trough) Gravity Low Province with the lowest gravity values in the Nansha Islands. Crustal thickness has been modelled from the gravity data along several profiles. The results indicate a crustal thickness of approximately 25 km for the Reed Bank, a thickness of 20-25 km in the reef areas, a thickness of approximately 20 km in the trough areas, and a thin and denser crust of 17-20 km thickness beneath the Zengmu Basin.