Gravity Inversion and Thermal Modeling for the Crust‐Mantle Structure of the Southwest Subbasin in the South China Sea

Abstract

AbstractThe crust‐mantle structure and magmatic activity are one of the key problems in the study on formation and evolution of the southwest subbasin in the South China Sea. According to gravity data from the NH973‐1 profile, we construct a density model of crust‐mantle structure with the gravity inversion method. And then, we make modeling of the temperature structure and thermal evolution in the basin after sea‐floor spreading. Gravity inversion indicates that there is a deeper sunken sedimentation area and a depressed Moho surface region under the southwest subbasin center. Thermal modeling shows that temperature variation is not consistency degradation between crust and mantle, but in a way of crust warming with mantle cooling. After the termination of floor spreading, the crust warming lasted for about 7 Ma, followed by slow cooling, also with the mantle. Then, crust thermal evolution mode was changed into a new way that its temperature declined as same as mantle. In this way, crust and mantle entered the stage of thermal subsiding synchronously. Along profile NH973‐1, the characteristics of crustal warming are different among the south, middle and north parts. At the depth of 7~9 km under the place, about 70 km to the north of the oceanic basin spreading center, the rate of temperature increase sustained above 200°C between 9.6 Ma and 4.6 Ma. This appending temperature would drive up the local temperature value to 1100°C which was of advantage to crust magmatic activity in the end time of seabed spreading, supported by the P‐T diagram. We infer that magmatic partial melting would be likely to happen at this time if there was water‐bearing and decompressing conditions in crust produced by local faults.