Structural architecture and tectonic evolution of the Cenozoic Zhanhua Sag along the Tan–Lu Fault Zone in the eastern North China: Reconciliation of tectonic models on the origin of the Bohai Bay Basin

Abstract

The Zhanhua Sag, a major depocenter within the larger Bohai Bay Basin (BBB) along the Tan–Lu Fault Zone (TLFZ) in the eastern North China, developed through two successive phases with different stress regimes during the Paleogene. The kinematics and the slip history of the NW–SE– and N–S–striking normal fault systems strongly controlled the internal structure and stratigraphy of the Paleocene–Lower Eocene strata in the Zhanhua Sag during Phase 1. Isopach maps of the Paleocene–Lower Eocene stratigraphy show ~4000–m–thick clastic rock sequences in these structurally controlled depocenters. The average dip–slip rates of the NW–SE– and N–S–striking faults were 78.96 and 50.82 m/m.y. (respectively) in the Paleocene–Early Eocene, but decreased to 45.87 and 35.81 m/m.y. in the Middle–Late Oligocene. The NE–SW–striking fault systems and their slip history controlled the development of the Middle Eocene–Oligocene depocenters and their strata during Phase 2. The average dip–slip rate of the NE–SW–striking faults was 10.47 m/m.y. in the Paleocene–Early Eocene but increased to 93.49 m/m.y. in the Middle–Late Oligocene. The NE–SW–oriented faults cut across and displace the NW–SE–striking faults, indicating their relatively younger ages. These fault patterns and kinematics recorded in the Zhanhua Sag reflect an abrupt change in extension directions from ENE–WSW to NW–SE before and after the Middle Eocene that was related to a reversal in the slip direction from sinistral to dextral along the TLFZ. This slip reversal was, in turn, a result of a major change in the subduction direction of the Pacific Plate beneath eastern Asia and in the mode of strain partitioning across the TLFZ in the continental upper plate.