The magmatic activity mechanism of the fossil spreading center in the Southwest sub-basin, South China Sea

Petrogeochemical data indicate that after the end of seafloor spreading, residual magmatic activity still exists in the deep basin of the South China Sea. By using different viscous structure models beneath the fossil spreading center of the Southwest sub-basin we simulated the amount of melt produced, the length of the melting period, and the thermal evolution process in terms of geothermics and the buoyant decompression melting mechanism. We compared the results of our model with observed heat flow, seismic, and petrogeochemistry data. The results show that depletion buoyancy induced by buoyant decompression melting plays an important role in the melting process, while retention buoyancy, thermal buoyancy, and viscous shear force have only a weak influence on the melting process. From the length of the melting period, we determined that for the three viscous structures models the magmatic activity lasted about 5, 12, and 15 Ma. Under the effect of buoyant depression melting, local high-temperature areas will develop under the basin, which can explain the low-velocity layer detected by seismic exploration in the middle and upper lithosphere of the Southwest sub-basin. We also simulated the possible lithology distribution beneath the fossil spreading center with the physical conditions of different viscous structure, different temperature structure, and different melting fraction, which provided a greater understanding of the rock petrogeochemical data of the deep sea basin in the South China Sea.