Inhomogeneous strain partitioning in the Cenozoic Bohai Bay Basin controlled by pre-existing crustal fabrics and oblique subduction: Insights from the Jizhong and Huanghua subbasins

The tectonic features and geodynamics of the Bohai Bay Basin (BBB) have been extensively studied over several decades. It is generally accepted that the Cenozoic BBB was subjected to a superimposed extensional and strike-slip stress field. However, the specific basin-forming mechanism remains ambiguous. Basin-scale studies with high spatial and temporal resolution reveal how strain migrates and localizes during multiphase rifting. Understanding these strain-related processes is crucial for deciphering the formation mechanisms and controlling factors of rift basins. This study investigates the rift-related evolution and strain partitioning in the Jizhong and Huanghua subbasins, the western half of the BBB. Using high-resolution 3D and 2D seismic datasets and well data, we present detailed geological sections, residual thickness maps, and fault systems, showing two distinct patterns of strain partitioning in the Cenozoic. In the Jizhong Subbasin, strain migrated inwards from the NE-trending boundary faults and finally became localized along the NE-trending rift axis; in the Huanghua Subbasin, strain migrated northeastwards from the southern part and then was largely localized along the near E–W-trending faults in the northeastern part. By integrating our results with previous studies of other subbasins, we identify two separate extensional domains within the Cenozoic BBB. The suborthogonal extensional domain in the Jizhong Subbasin is dominated by relatively stable NW–SE extension. In contrast, the extended areas to the east (i.e., the Huanghua, Bozhong, and Liaohe-Liaodongwan subbasins) comprise the oblique extensional domain, characterized by asymmetric transtensional pull-apart deformation and subjected to a clockwise-rotated extensional stress field from NW–SE to NNW–SSE. We suggest that the spatially inhomogeneous strain partitioning in the BBB has developed since the middle Eocene, and was controlled by the interaction of pre-existing crustal fabrics (e.g., the Tan-Lu Fault Zone) and the oblique subduction of the Pacific Plate. The boundary between the two domains may represent a transition zone where the margin-parallel residual velocity component of the oblique convergence decreased considerably landwards. Our study highlights the Cenozoic strain evolution and the associated geodynamics in the BBB, emphasizing the strain complexity within the back-arc rift basin under the oblique subduction background.