Formation mechanism on geothermal anomalies of Leiqiong area, South China

Leiqiong area, as a land part of Beibuwan Basin is located on the northern continental shelf of South China Sea. According to the geologic structures, Beibuwan Basin is a stretch faulting-depression basin, most of the Cenozoic strata developed on the basement of the Paleozoic and the Yanshanian granites. The geothermal field is controlled mainly by the pattern of basement tectonics and influenced by groundwater activity, but it cannot be ignored that the heat generation of radioactive elements in blind granites contributes as well. The remaining heat of the Pleistocene volcanic activities seems to have disappeared and thus a temperature balance is reached between magmatic rocks and their surrounding rocks. The volcanic eruptions formed both the southern and the northern basaltic terraces, which are the inflow areas of groundwater. By the effect of downward cold water flow, the geothermal gradients are below 3°C/100m in the shallow part of the Cenozoic cover, showing negative thermal anomalies. In the areas dominated by heat conduction, the geothermal field is similar to that in North China Basin, and the gradient of the Cenozoic cover is closely related to the depth of the bedrock roof. Because the uniform heat flow from the deeper crust was redistributed in the shallower Crust, the gradients in the cover on several basement uplifts are in a range of 4-5°C/100m, showing positive thermal anomalies. In some fault zones bounded the uplifts, there are heat sources caused by upward hot water, which are superposed on the heat conduction, and the gradients are up to 5-8°C/100m, showing apparently positive thermal anomalies.