SUBSIDENE AND TECTONIC EVOLUTION OF THE BEIKANG BASIN, THE SOUTH CHINA SEA

The Beikang basin lies in the middle of the Nansha sea area and its geotectonic location belongs to the southwest boundary of the Nansha massif.The north part of the basin is separated from Nanweixi and Nanweidong basins by faults,3 000 m isopach and island reefs,the Tingjia fault bounds the southwest of the basin,and the northwest edge fault of the Nansha Trough acts as the east boundary of the basin.The basin covers an area of 62×104 km2.Tectonic subsidence is determined and the relation between the tectonic evolution and the regional tectonic movements in the Beikang basin studied on the basis of the geologic tectonic features and by using the backstripping technique and 1D Airy isostasy correction The results show that the Beikang basin is an extensional basin that has experienced three phases of subsidence and three stages of tectonic evolution. Rapid subsidence in middle Eocene made up 28%～34% of the total subsidence with a subsidence rate of 234～325 m/Ma and the maximum extensional coefficient is 172. And it was associated with the collision of the Eurasia and Pacific plates. Tectonic subsidence during late Eoceneearly Oligocene was slower and associated with the strikeslip of the eastern VietnamWan'an fault. Rapid subsidence in PlioceneQuaternary made up 40% of the total subsidence and was controlled by the regional subsidence of the South China Sea influenced by subduction of the Pacific plate beneath the Eurasia plate.