OBS Survey and Crustal Structure of the SW Sub‐Basin and Nansha Block, South China Sea

Abstract

AbstractOBS973‐1 is a wide angle reflection and refraction deep seismic line using Ocean Bottom Seismometers (OBS), collected for the first time in the southern part of South China Sea. The line crosses the southern margin of the Southwest Sub‐basin and the middle part of Nansha Block. By seismic phase analysing and travel time fitting, a 2D P‐wave velocity model along the line is constructed. The modeling results show that the surface sedimentary layer has a velocity of 2.5~4.5 km/s and a thickness of 1000~3000 m, with locally rough basement interface. The crystalline basement has a velocity of 4.5~5.5 km/s on the top, increasing to 6.8~6.9 km/s near the bottom of the crust. In the middle crust there is a small velocity discontinuity (0.1~0.2 km/s), while the Moho on the bottom crust has a large velocity contrast (1.2 km/s). In the top mantle the velocity is 8.0~8.1 km/s. The Moho depth and the crustal thickness have much difference between the northern and southern segments along the line. The northern segment is in the deep sea basin, where the Moho is 11 km deep and the crystalline crust is only 5~6 km thick, which are typical characteristics of oceanic crust. In contrast, the southern segment is on the continental block, where the Moho has a maximum depth of 24 km and the crust has a corresponding thickness of 20 km, which are representative characteristics of thinned continental crust. From the deep sea basin to the continental block, the Moho depth and the crustal thickness increase rapidly. In the continental area, the upper crust and lower crust have similar thickness and variation. No High Velocity Layer (HVL) is seen in the lower crust. These may imply that pure‐shear extension and uniform thinning are dominating inside the crust, and magma underplating is not developed in the lower crust. Comparing with the structure models of OBS973‐2 and OBS973‐3, it is inferred that the middle and eastern parts of Nansha Block have similar tectonic characters, and the two sides of Southwest Sub‐basin are a pair of non‐volcanic asymmetric conjugate continental margins.