GEOTHERMAL REGIME AND TECTONO-THERMAL EVOLUTION OF THE SEDIMENTARY BASINS IN THE NORTHERN CONTINENTAL MARGINS OF THE SOUTH CHINA SEA

Continental margin is a unique geological unit that records the lithospheric deformation process from extension, rifting to drifting, and develops a large number of sedimentary basins with affluent hydrocarbon potential. The South China Sea is one of typical marginal seas in the western Pacific area and attracts more attention during the past decades due to its special tectonic settings. Knowledge of thermal regime is vital for understanding the rupturing process of continental lithosphere, and hydrocarbon resource assessment as well. Here we integrated all types of heat flow data and basin modeling to investigate the thermal regime of the northern continental margin of the South China Sea and the associated tectono-thermal evolution of the Cenozoic basins there. Our results show that the heat flow gradually increases from the shelf area with a mean of 69.8±9.3 mW/m² to the slope areas with the average of 86.7±25.5 mW/m². This seaward trend in heat flow indicates the differential crustal extension beneath this area. However, the heat flow of different basins demonstrates obvious variation, for example, the heat flow in the Yinggehai basin is high as 84 mW/m², while that of the Beibuwan basin is only 61 mW/m², suggesting the influence of local tectonics on thermal regime. Three rapid subsidence patterns can be clearly identified in the back-stripping analysis of the basins; the first rapid subsidence coincides well with the Late Cretaceous-Paleocene extension of the continental margins in SE Asia, along with the increase of heat flow. The second rapid subsidence could match the Eocene-Oligocene seafloor spreading in the South China Sea, also accompanying the increase of base heat flow in these basins. The third rapid subsidence since Pliocene is speculated as the result of the strike-slip activities of the Red River Fault Zone which is one important accommodation boundary in SE Asian tectonics. We concluded that the multistage extension and associated heat flow increase determine the present-day thermal regime of this area, and rapid sedimentation and correspondingly huge sediments would accelerate the maturity of source rocks concerned to form oil and gas, under the aid of high thermal condition.