Crustal structure of Liyuexi Trough in the Nansha Block

OBS2019-2 with a total length of 300 km, passing through Liyue Bank, Liyuexi Trough and South China Sea (SCS) Basin in the north, is located near Liyuexi Trough in the Nansha Block, which is a critical region in the transition between the Southwest Subbasin (SWSB) and the East Subbasin (ESB). The final velocity structure model, obtained by analyzing and modeling the OBS2019-2 data, suggests a crust thickness of 4 similar to 7 km in the oceanic crust and 9 similar to 19 km in the continental crust. With a crustal thickness of 6 similar to 9 km, the Continent-Ocean Transition (COT) zone is about 50 km wide within the model range of 120 similar to 170 km. And the model reveals a low-velocity zone that could be the Zhongnan-Liyue Fault, which belongs to the lithospheric fault and is assumed to pass southward through the Liyuexi Trough. Compared to the velocity structure of Dongsha Block, the velocity model of OBS2019-2 shows similar velocity structure features to the Ocean Bottom Seismometer (OBS) velocity model traversing the Zhongsha Block. We infer that the Liyue Block and Zhongsha Block are adjacent to each other and conjugation is more likely. Both were part of the South China Continent before the opening of the South China Sea.