CRUSTAL STRUCTURE OF XISHA BLOCK AND ITS TECTONIC ATTRIBUTES

Abstract

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 this line is established employing the ray-tracing and forward travel-time modeling method. The 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 upper mantle is a layer with an velocity of 8.0 km·s^–1. The average crustal thickness of Xisha block is about 23 km. The depths of the upper and lower crusts are 9 km and 14 km. Moho interface lies at the depth of 23∼27 km. Xisha block is a thinned continental crust as compared with the South China continental margin; it has a size about 9.2×10⁵ km³ calculated from two mutually perpendicular seismic profiles crossing the block. Xisha block and Nansha block are distributed on the two sides of the fossil spreading ridge of Northwest Sub-basin. They are conjugate to each other since their velocity structures are very similar.