Geodynamic Characteristics in the Southwest Margin of South China Sea

The strike-slip fault system in the southwestern margin of South China Sea (SCS) lies on the transition zone between the continental shelf and slope of SCS, which is an important ocean-continent boundary. By using submarine heat flow data, a seismic shear wave tomography model, and gravity potential field data, this paper investigates the distribution of submarine heat flow in the southwestern margin of SCS, the thermal-rheological structure of the crust and mantle, the temperature-viscosity characteristics of the upper mantle Vs low-velocity layer, the tangential stress field of the rheological boundary layer at the lithosphere base, and the convective velocity structure of the mantle asthenosphere. Our new results show that the deep geothermal activity in the southwestern margin of SCS is intense, and the high heat flow area of the mantle with Q(m)/Q(s) >70% is distributed along an NNE-trending strip. Moreover, both the east and west sides of the strike-slip fault zone correspond to two low-value areas with a viscosity coefficient of 10(21)-10(22) Pa.s at Moho depth, and beneath the Nansha Block are strong and cold blocks with a viscosity coefficient of 10(24)-10(25) Pa.s. The northward and eastward shear stress components tau(N) and tau(E) of the rheological boundary layer at the base of the lithosphere mantle decrease with depth. At 65-km depth, both tau(N) and tau(E) are greater than 5.5 x 10(8) N/m(2). At 100-km depth, both tau(N) and tau(E) are less than 1 x 10(8) N/m(2). The calculation results based on the seismic shear wave model of the upper mantle and the gravimetric geoid model indicate that the depth of 120-250 km is the low-velocity layer, and the average temperature of the mantle at 180-km depth can be up to 1,300 degrees C. Moreover, the average effective viscosity coefficient is close to 10(18) Pa.s, which satisfies the temperature and viscosity conditions for partial melting or convective migration of mantle material. The mantle convection calculation results show that the average flow rate is 8.5 cm/a at 200-km depth and 2.2 cm/a at 400-km depth.