S-wave Velocity Structure and Vp/Vs Ratios Beneath Liyuexi Trough, Nansha Block, the South China Sea: Implications for Crustal Lithology and Tectonic Nature of the Southern Continental Margin

By processing the vertical component data of OBS2019-2, a wide angle seismic line situated adjacent to the Liyuexi Trough in the Nansha Block of the South China Sea (SCS), we obtained a crustal model of P-wave velocity structure beneath the line and acquired information about the deep crustal structure in the Liyuexi Trough region. However, the lithology remains to be fully constrained. This paper presents the results of processing the horizontal component data of OBS2019-2 to obtain the S-wave velocity structure model and Vp/Vs ratios. The results show that the Vp/Vs ratios of sedimentary layer range from 1.90 to 4.60, while the corresponding S-wave velocities range from 0.4 to 1.8 km/s. As for the crust, the Vp/Vs ratios fall between 1.70 and 1.89, while the Swave velocities range between 2.9 and 4.2 km/s. The lithological compositions of oceanic crust from top to bottom may consist of basalt, diabase, and gabbro. In contrast, the lithologies of continental crust transition from felsic to mafic downwards, including andesite, granodiorite, granite, greenschist, and anorthosite. The rock compositions of the Continent-Ocean Transition (COT) region are intermediate between the continental and oceanic crusts, exhibiting transitional features. The rock composition of the high-velocity layer (HVL) in the lower crust may consist of amphibolite, and the presence of magma intrusions and underplating in this region could be associated with the Zhongnan-Liyue Fault. Compared to the Dongsha Block, the lithologies of the Liyue Block and Zhongsha Block are more similar, indicating a higher possibility of conjugation between the Liyue Block and the Zhongsha Block.