南海区域构造沉降特征

TECTONIC SUBSIDENCE OF THE SOUTH CHINA SEA

以南海区域作为一个整体拉张单元,运用McKenzie盆地模拟方法,分析南海区域的构造沉降史。南海区域的构造沉降以海盆为中心,南北陆缘差异沉降十分明显。北部陆缘在向海盆倾斜沉降的背景上出现波动性,而南部陆缘具有北陡南缓的不对称性。纵向上,南海区域新生代沉降历史具有幕式和非均匀性。南海北部陆缘东西两侧岩石圈区域拉伸状态具有明显不同的特征。

Based on the tectonic subsidence computation of four regional profiles through the South China Sea using the McKenzie basin modeling, the subsidence history of the South China Sea, as a whole extensional unit, was discussed in the present paper.The largest extent of the subsidence in the South China Sea occurred at the center of the sea basin, and rapidly decreased sidewards. The total basal subsidence for the southwest subbasin and east subbasin is 4～6 and 45～5 km respectively. However, the peak extent of the southwest subbasin and the subsidence differences among various places of the subbasin are larger and more complex than those of the east subbasin. In the west of the southwest subbasin, the total basal subsidence is 45～5 km and the difference is smaller than in the east of the subbasin where the subsidence is 4～6 km. It was probably caused by the early time and low extent of the seafloor spreading in the southwest subbasin. The subsidence of the north margin of the southwest subbasin fluctuates with the sea basin getting deeper towards the center, whereas that of the south margin is in asymmetry with the north side steep and the south side gentle. The subsidence fluctuation of the north margin was obviously controlled by extension. Vertically, the Cenozoic subsidence of the South China Sea has a periodic and inhomogenous history. The subsidence mainly shows the extension controlled by the NEtrending fractures in Paleocene, and was characteristic of the seafloor spreading of the southwest subbasin, and the subsidence range was rapidly extending in middle Eocene, and a new subsidence history began in Oligocene. PlioceneQuaternary was the regional subsidence stage. The lithospheric extension states of the east and west sides of the north margin are very different. From the southeast Hainan basin to Xisha Trough basin and from the outer slope to the north of the east subbasin, the original crust thickness was 38～40 km.