Crustal structure and fracture zone in the Central Basin of the South China Sea from wide angle seismic experiments using OBS

We present two E-W trending wide-angle seismic profiles (OBS2013-ZN, OBS2014-ZN), which cross the boundary (Zhongnan fault zone) between the east sub-basin and the southwest sub-basin of the South China Sea (SCS). We processed the data and used 2D ray-tracing to determine the oceanic crust thickness, velocity structures and Moho depth variations related to the fault zone. The simulated velocity models show that the oceanic basin of the SCS has a typical oceanic crust covered by a 1-2 km thick sediment layer with a velocity of 2.0-3.5 km/s. The crust has a thickness of 5-8 km, of which the oceanic layer 2 is 1.8-3 km thick, with velocity increasing downward from 4.3 km/s to 6.4 km/s, and the oceanic layer 3 is 3-5 km thick, with velocity increasing downward from 6.4 km/s-7.0 km/s. The Moho depth in the oceanic basin is approximately 6-7 km below seabed. The Moho discontinuity has a prominent upheaval zone with a low velocity of 7.6 km/s, whose location corresponds to the low velocity zone in oceanic layer 2. Our results suggest the presence of a NW-SE-trending fracture zone (40-60 km wide) rather than a major “Zhongnan fault” oriented N-S by connecting the upheaval portions of the Moho in the two profiles. The NW-SE orientation Zhongnan transform fault zone in our study area is consistent with the direction of opening of the South China Sea in the last stage of its evolution. This large transform fault zone connected and offset the spreading centers of the east and southwest sub-basin.