Structural characteristics and mechanism of the Hengshui accommodation zone in the southern Jizhong subbasin, Bohai Bay basin, China

Based on seismic data, we document the differences in the geological structure and evolution on the two sides of the Hengshui Accommodation Zone (HAZ), identify the timing of faulting on faults, and analyze the mechanism of the HAZ. The northern part of the HAZ is characterized by a half-graben controlled by double detachment faults with opposite dips, and the southern part of the HAZ is characterized by a compound half-graben controlled by a single detachment fault with multiple faults. The structural evolution of the southern Jizhong subbasin can be divided into the following five stages: the basement evolution stage; Rift Episode I (Ek-Es4); Rift Episode II (Es3–Es2); Rift Episode III (Es1-Ed); and the Postrift stage (Nm-Ng). Three segments were identified along the HAZ, namely, the Xinle Anticline (XLA), the Ningjin Segment (NJS) and the Hengshui Transfer Fault (HSTF). The XLA is a transverse anticline produced by the fault section of the Taihang Mountain Piedmont Fault (TMPF). The NJS is a transfer fault formed by the differential extension of the Ningjin Fault. As a major part of the HAZ, there are obvious differences in the faults on both sides of the HSTF, and NE-trending Cenozoic faults were mainly on its north side, while the NNE-trending faults formed in the basement stage were distributed on its south side. During the basement evolution stage, NNE-trending basement faults and the HSTF developed by WNW extension. During Rift Episode Ⅰ (Ek-Es4), under the NW extension, NE-trending fault segments formed, and pre-existing faults were reactivated. During Rift Episode Ⅱ (Es3–Es2), pre-existing faults were subjected to oblique extension, and the NE-trending fault segments on both sides of the HSTF grew into the main fault and intersected with the HSTF. From Rift Episode III (Es1-Ed) to the present, some pre-existing faults have been reactivated, and many new secondary faults in the basin have finally produced the present structural pattern. These results suggest that the continuous transfer of the regional stress field during the Mesozoic and Cenozoic was the driving force for the formation and evolution of the HAZ.