Fine crustal structures of Zhangjiakou-Bohai tectonic zone in Tianjin area revealed by a deep seismic reflection profile

A NE-trending deep seismic reflection profile with 86 km long across the Zhangjiakou-Bohai tectonic zone in Tianjin area was completed in 2018. This profile reveals the fine crustal structure image and the tectonic features of the faults in this tectonic zone. It provides new seismic evidence for the research on the deep seismogenic environment and tectonic patterns of the Zhangjiakou-Bohai tectonic zone. These results are of great significance for the discussion on the deep dynamic and evolutionary process of the North China rift basin since the Late Mesozoic. According to this study, the crust of the Zhangjiakou-Bohai tectonic zone in Tianjin area is divided into the upper part and the lower part based on the reflection T-G of the crystalline basement. The reflection wave groups in the upper part of the crust are abundant and present obvious layers. The clear ups and downs of the interfaces clearly depict the Cenozoic sedimentary layers of the Jizhong depression, the bottom boundary of the "dustpan shaped" sedimentary depression, and also the geometry of the Chaobaihe, Jiyunhe and Fengtai-Yejituo faults. On the contrary, there has no distinct seismic phase in the area between the crystalline basement (T-G) and the Moho surface, which has great differences from the results of other parts of North China. The crustal thickness ranges from 30. 0 to 34. 5 km, and the crust in the middle segment is thicker than both the north and south segments. The Moho surface presents prominent inhomogeneity and discontinuity in the transverse direction, and there are two obvious reflection events, R-Lambda and R-C, at the area where the Moho surface is offset. The R-Lambda may be the lateral residue of the upwelling of the asthenosphere. The laminated reflection seismic phase of R-C presents characteristics of the crust-mantle transition zone. This profile reveals 2 deep faults offset the Moho surface (F-D1 and F-D2) and 9 upper crust faults. The deep faults should be caused by the uplift of the upper mantle resulted from the asthenosphere. The upper crust faults are related to the vertical movement and lateral tension of the upper crust. The deep faults (F-D1 and F-D2) provide a channel for the energy exchange and upwelling of deep thermal materials in this area. The corresponding shallow crustal faults (F-3 and F-9) provide possible conditions for energy adjustment. Therefore, the adjacent areas of these faults should be paid attention to for the occurrence of strong earthquakes in the future.