西沙地块地壳结构及其构造属性

西沙地块作为在南海形成演化过程中形成的微陆块,记录了南海演化历史的重要信息,其地壳结构、物质组成及构造属性是探讨南海形成演化的关键.基于采集到的OBS2013-3测线海底地震仪数据,用射线追踪和正演走时拟合方法,获得了西沙地块的二维纵波速度模型.模型显示沉积层速度为2.2~3.2 km·s-1,厚度为0.8~3.0 km,局部基底面起伏较大,上地壳顶部速度为5.0~5.5 km·s-1,下地壳底部速度为6.9 km·s-1,上地幔顶部速度为8.0 km·s-1.西沙地块的地壳厚度平均为23 km,上地壳厚度约为9 km,下地壳厚度约为14 km,莫霍面埋深为23~27 km.从穿过西沙地块的纵、横两条大剖面推算,块体大小约为9.2×105 km3,与华南陆缘相比,表现为整体减薄的陆壳特征.西沙地块与南沙地块垂直于西南次海盆扩张脊分布,根据二者地壳结构的特征对比,二者互为共轭关系.

Crustal structure of Xisha block and its tectonic attributes

Xisha block, as a micro-continental massif formed during the process of the formation of the South China Sea (SCS), has recorded important information about the evolutionary history of the SCS. The crustal structure, petrological properties and tectonic attributes of Xisha block are critical to the theory of formation and evolution of the SCS. Based on Ocean Bottom Seismometers (OBS) data acquired from Line OBS2013-3, the P-wave velocity model of Xisha block along the line is established employing the ray-tracing and forward travel-time modeling method. This velocity structure shows that the sedimentary layer is various with a velocity of 2.2~3.2 km·s^-1 and a thickness of 0.8~3.0 km. The basement interface is locally rugged and rough. The velocity values of the upper and lower crusts are 5.0~6.4 km·s^-1 and 6.5~6.9 km·s^-1,and the velocity at the top of upper mantle is 8.0 km·s^-1. The average crustal thickness of Xisha block is about 23 km. The thickness of the upper and lower crust is 9 km and 14 km, respectively. Moho interface lies at the depth of 23~27 km. Xisha block is a type of thinned continental crust comparing with the South China continental margin and has a size of about 9.2×10⁵ km³ calculated from the two perpendicular seismic profiles. Xisha block and Nansha block are distributed on the two sides of the spreading ridge of NW sub-basin. They are conjugate with each other since their velocity structures are very similar.