Structural anatomy and dynamics of evolution of the Qikou Sag, Bohai Bay Basin: Implications for the destruction of North China craton

The Qikou Sag is a complex Cenozoic faulted basin at the center of the Bohai Bay Basin. The Paleogene sediments in the basin show a maximum thickness of 11,000 m in Paleogene representing intense crustal extension. The sag has a typical dish-like structure in the upper part, with the central domain represented by a deep half graben, and surrounded by different types of gentle slopes and sub-sags linked with the basin-margin uplifts. The inner structure of the sag is jointly controlled by the basin-margin extensional faults, intrabasinal extensional tectonics and transfer tectonics. The sag is divided into four different extensional tectonic domains by the basement fault along the coastline, the Haihe–Xingang Fault and the Qidong Fault. These sub-sags are in turn controlled by different detachment systems. The listric Cangdong and Hangu faults mark the major boundary faults controlling the extensional structure of the basin.The structure and tectonics of the Qikou Sag provide a direct record of the late stages of rifting and destruction of the North China Craton. The Mesozoic basement structures of the sag determine the tectonic styles in different sub-sags. The complete inversion of the structures in Mesozoic profoundly influenced the development and evolution of the basin during Paleogene. We identify three main stages in the tectonic evolution of the Qikou: the Oligocene faulting stage, the Miocene–Pliocene thermal subsidence stage and the Pleistocene–Quaternary depression stage. In the Oligocene, fault activities and the basin architecture were mainly controlled by the listric boundary fault system of the basin and the secondary fault system in the hanging wall under the detachment and extension in the upper crust. Since Miocene, the boundary fault of the basin underwent weak extension. The upwelling of the upper mantle and the formation of corresponding shallow depressions contributed to the evolution and tectonic architecture of the Qikou Sag.

► The Qikou Sag is a complex Cenozoic faulted basin at the center of the Bohai Bay Basin.
► The structure and tectonics of the sag are evaluated from geological and seismic data.
► The formation mechanism of the sag provides a record of the late stages of rifting and destruction of the North China Craton.