Abstract: With the statistic results about a total number of 462 heat flow values and geothermal gradient values, the present geothermal field in the northern margin of the South China Sea is characterized with “general higher heat flow values and higher geothermal gradient values”. The heat flow values increase from continental shelf to ocean basin against the trend of the crust thickness and Moho depth. Thermal Curie Point Isotherm (the 575 ℃ isotherm) was obtained according to the static geothermal temperature field calculated from the heat flow and geothermal gradient survey data. Magnetic Curie Point Isotherm was obtained by inversion of the magnetic anomalies. The results show that the two Curie Point Isotherms derived respectively from the prior two methods are inconsistent with significant differences in the middle and lower slope in the northern margin of the South China Sea. The Magnetic Curie Point Isotherm is above the Thermal Curie Point Isotherm between fault F2 and fault F4 in the integrated profiles of OBS93 and ESP-E, which means that some magma under-plating events caused the thermal disequilibrium in this region. Based on the study of the crust structure, crust expansion coefficient, Moho depth, faults and magma activities, the author points out that the variation trend of the heat flow values in the northern margin of the South China Sea is controlled by the magnitude of the lithosphere thinning and Moho uplifting. The magma activities during the Neo-tectonic period might be a factor for the local heat flow anomaly.

Key words: northern margin of the South China Sea, Magnetic Curie Point, Thermal Curie Point, crust expansion coefficient, Moho depth, magma under-plating