Genetic Dynamics of China Offshore Cenozoic Basins

More than 10 Cenozoic oil-rich sedimentary basins have been developed in the offshore area of China. The differences and similarities in their evolution process and dynamic background need to be systematically summarized with unified research ideas and methods. Based on abundant geological and geophysical data in petroleum exploration and production in the offshore basins, this paper analyzes the tectono-stratigraphic framework of the Cenozoic basins located in the Bohai Sea, the East China Sea and the South China Sea. First, we further clarify the obliquely pull-apart evolution stage of the Bohai Bay Basin by demonstrating the regional strike-slip fault system dominated the basin development. After presenting the importance of regional compression on the evolution of the East China Sea Basin, we sub-divide an evolution stage of the retroarc foreland basin. As for the northern South China Sea, we discover large detachment faults and their controlling detachment basins in the deepwater areas; and thus propose that the large-scale detachment faulting is the main mechanism for the crustal thinning, mantle exhumation and the formation of deepwater marginal basins. Our study reveals offshore basins in China had experienced an early-stage extension covering the whole region; then they entered into a differential tectonic evolution stage since Late Eocene. To be specific, the Bohai Sea experiences an evolution process of early rifting through an oblique strike-slip deformation, a thermal subsidence and finally a rapid subsidence. The East China Sea evolved from a rift basin to a retroarc foreland basin, then became a regional subsidence basin. While the South China Sea had a continuing extension evolution process of an early-rift basin, detachment basin, seafloor spreading with continental lithospheric breakup, and the present-day passive continental margin. The kinematics analysis of the regional plate tectonics suggests the global plate reorganization in Late Eocene resulted in the differential evolution of the offshore basins, before which the East China had mainly been under a "double-plate" dynamic system constructed by the interaction between the Eurasian plate and the Pacific plate. The subduction and retreat of the Pacific plate led to the back-arc extension on the continental margin and the formation of a massive Basin-Range type basin system in the eastern margin. After the reorganization, mainland China has been under the "multi-plate system" constructed by the Indian plate, the Eurasian plate, the Pacific plate and the Philippine sea plate. The India-Eurasia collision, the direction change of the Pacific plate subduction, and subduction and collision of the paleo South China Sea, the subduction of the wedge-shaped Philippine Sea plate and its westward migration and subduction with Pacific plate led to more complex plate motion processes and a multi-stage kinematical reorganization. These plate tectonic events caused the diversity of the offshore basin genesis and the differences of their tectonic evolution processes.