Cenozoic Faults Systems and Its Geodynamics of the Continental Margin Basins in the Northern of South China Sea

Zhang Yuanze; Qi Jiafu; Wu Jingfu

Abstract：We employ recently updated seismic reflection data to investigate spatial-temporal difference of Cenozoic faults systems and its dynamic mechanism of the Pearl River Mouth Basin and Qiongdongnan Basin in the Northern of South China Sea.It shows geometry, kinematics of extensional faults systems in the Paleogene rift zone of the Pearl River Mouth basin and Qiongdongnan basin change significantly along strike, which is composed of NE-and EW-trending normal basement faults.The con-cealed regional and local NW-trending faults with associated deformation zone constitute the transfer zones between the extensional faults systems.The "non-detachment" extensional fault system consists of NE-and NEE-trending normal faults to the west of NW-trending Yunkai and Songtao-Songnan transfer zones, and the detachment fault system is composed of NE-and EW-trending normal/strile-slip faults to the east.Paleogene rifting experienced three distinguished stages.In the early period (Wenchang Formation), simple (half) grabens in western basins was controlled by planar normal faults with relatively small extension, and composite (half) grabens in eastern basins are dominated by listric normal faults with large extension, where normal faults converge and terminate at detachment fault within mid-crust.In the middle period (Enping/Yacheng Formation), faults in the west are characterized by inherited activity, whereas NWW-and EW-trending faults in the east of Pearl River Mouth Basin form deep detachment system by cutting antecedent mid-crust detachment fault, with inversion along the Yunkai transfer zone.In the late stage (Zhuhai/Lingshui Formation), the activities of faults decline in the west, whereas NWW-and EW-trending faults form deep detachment system in the east of the Qiongdongnan Basin, with inversion and strike-slip structures along the Songtao-Songnan transfer zone.The influencing factors of differential fault systems and tectonic evolution include pre-existing basement fabrics, rheology of crust and lithosphere, and extension amount.The detachment systems are associated with the NWW-trending throughgoing crustal faults within basement weak belts and present local crustal thinning zone, the geodynamic mechanisms for differential extension on both sides of the transfer zones result from the strike-slip movement along the NWW-trending pre-existing faults induced by the opening of the South China Sea from east to west, and changes of the stress field and lithospheric thermal structure account for the depth variation of the detachment systems.