琼东南盆地深水区长昌凹陷地壳结构特征

长昌凹陷位于琼东南盆地深水区,向东通过西沙海槽与南海西北次海盆相通,其近东西向的展布形态明显异于深水区其他凹陷的NE—NEE向形态,为了弄清其地壳结构,从而更好地分析凹陷的结构和演化机制,这里根据深反射地震资料、VSP资料和最新重力资料对长昌凹陷的地壳结构进行了综合地球物理模拟。结果显示:长昌凹陷北侧地壳厚度为22~24 km,南侧地壳厚度约20~22 km,从两侧向长昌凹陷中央地壳厚度逐渐减薄,最薄处只有2.8 km;莫霍面深度与沉积基底呈镜像关系,沉积基底最深的地方莫霍面深度最浅,最浅深度距海平面13.8 km;凹陷中央东部存在一层厚约4 km的下地壳高速层,该层在地震剖面和层速度剖面上均可识别。

CRUSTAL STRUCTURE OF CHANGCHANG SAG IN THE DEEPWATER AREA OF QIONGDONGNAN BASIN

The Changchang Sag is located in the deepwater area of the Qiongdongnan Basin. It is linked with the northwestern sub-basin of the South China Sea through the Xisha Trough to the east side. The Changchang Sag trends in a nearly east-west direction,while the other basins in the deepwater area of the Qiongdongnan Basin are in NE—NEE direction. In order to better understand the crustal structure of Chang-chang Sag, deep reflection seismic profiles, VSP data and the latest marine gravity data were used for integrated geophysical simulation. The simulation results show that the crust of the north wing of the Changchang Sag is about 22~24 km in thickness, but 20~22 km of the south wing,decreasing greatly to the center of the Changchang Sag. The minimum thickness of the crystalline crust is 2.8 km. Mirror image relationship occurs between the Moho depth and the depositional basement, and the shallowest Moho is 13.8 km below the sea-level, corresponding to the deepest depositional basement. There is a high-velocity lower crust body in the east of the center of the Changchang Sag,which can be recognized on the deep reflection seismic profiles and layer velocity profiles. The crustal thickness decreases from east to west.