Crust and upper mantle structure of South China Sea and its surrounding regions from Rayleigh wave tomography

The South China Sea (SCS) region is geologically and tectonically complex with interactions from several major plates (Eurasian, Philippine Sea, Pacific, and Indian-Australian). The SCS itself consists of the SCS Basin, which is oceanic in nature, and the northern and southern continental margins around the SCS. Due to its complex geological structure and its importance to mineral resources, this region has become a key interest in geological investigations. However, the origin of SCS Basin is still under debate. In this study, we investigate the crust and upper mantlebeneaththe SCS and its surrounding areas from Rayleigh wave tomography. From earthquake surface waves as well as Green functions from ambient noise correlations,we obtain Rayleigh wave group velocity maps for periods 10 to 70 seconds, which is then inverted for the 3D S-wave velocity structure of the study region. The combination of the two data sets greatly increases the number of surface wave paths, improves the path coverage, and thus greatly increases the resolution of seismic tomography. The tomographic results reveal major features, which correlate with surface geology and tectonics in the region (including major plates, basins, and island arcs). The structure of the lithosphere and asthenosphere exhibits strong lateral variation, confirming that the SCS region is composed of a number of small blocks sutured together. The Red River Fault (RRF) is probably a lithosphere scale feature as a transform fault in the extrusion tectonics of the region between Indochina block and South China block. The S-wave velocity structure of the SCS Basin varies greatly with depth, showing a thick sedimentary layer, thin crust, and shallow asthenosphere; it is a remnant sea after the expansion ended. The lithosphere is thin in most areas of the SCS. Further implications about the tectonic of the region will be discussed.