Two tectonic systems in the Cenozoic Bohai Bay basin and their genetic interpretation

Abstract：The Paleogene Bohai Bay basin may be divided into three rift zones and one rift area, which correspond to the updoming zone of the upper mantle. The tectonic deformation in the basin may be classified into two relatively independent and interconnected Cenozoic tectonic systems: the extensional tectonic system and strike-slip tectonic system. The extensional tectonic system is a linked fault system formed by extensional faults of different scales and transform faults perpendicular or oblique to the extensional faults. In the basin they are dispersed and penetrate into the upper crust and control the distribution and evolution of the Paleogene rifting. Three NE-NNE right-lateral strike-slip faults are superposed on the extensional system. These strike-slip faults and their associated structural elements form the Cenozoic strike-slip tectonic system. The extensional system is a “horizontal layered” thin-skinned tectonics at the crustal scale. The normal faults converge toward great depths or terminate at the detachment fault surfaces within the mid-crust. The strike-slip system is a “vertical belt-like“ thick-skinned tectonics at the crust scale. The strike-slip faults at the shallow level of the crust are merged into the deep fault zones in several ways. The two tectonic systems are concrete manifestations of the active rifting and passive rifting mechanisms in the basin during the Cenozoic.