北康盆地沉降作用与构造运动

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

在地质构造特征研究的基础上,采用地层回剥法和局部均衡模式,研究北康盆地的构造沉降作用,并探讨该盆地构造演化与区域构造运动的关系。北康盆地是一个拉张性盆地,经历了3次快速沉降作用。中始新世沉降作用占总构造沉降量的28%～34%,沉降速率为234～325m/Ma,拉伸系数最大达1 72,它是43Ma时印度板块和欧亚板块碰撞的结果。上始新世—早渐新世构造沉降作用速率较慢,幅度也较小,为西卫运动影响下的越东—万安走滑断裂发生走滑拉张活动所致。上新世—第四纪的沉降速率极快,幅度很大,可达整个构造沉降的40%以上,可能是太平洋板块与欧亚板块相互作用所引起的整个南海区域沉降在盆地中的反映。

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.