Seismic imaging of a mid-crustal low-velocity layer beneath the northern coast of the South China Sea and its tectonic implications

Abstract

Low-velocity layer (LVL) within the continental crust is usually closely associated with tectonic events, thus it may reflect ongoing processes or offer a chance to backtrack the evolutional history. Mid-crustal LVL has been reported in the northern coast of the South China Sea (SCS) by several geophysical studies, but the 3D distribution of LVL in the whole coastal area is still poorly known. Here, we perform a receiver function study based on 67 broadband seismological stations. We use the common conversion point method to construct structures of the Moho as well as intra-crustal interfaces. We image sharp intra-crustal negative velocity gradient broadly distributed in the eastern Cathaysia block with depth increasing from ~10 km in the NW to 12–16 km in the SE. The corresponding amplitude of the negative interface shows variable values from −0.03 to −0.1, with an evidential trend of strengthening eastwards. We interpret the negative gradient as top of the LVL produced by accumulation of fluids from dehydration of the paleo-Pacific subduction plate. The mid-crustal LVL tends to act as weak domains during continental rift and the lateral variation of the LVL may be one possible reason explaining the along-strike variation in crustal stretching in the northern SCS rifted margin.