Tectonic characteristics, formation and evolution of the Yidong region in the Yihezhuang Uplift, Bohai Bay Basin

Multi-stage tectonic movements increase the complexity of structural geometry within a fault trough, therefore, it is critical to understand the deformation mechanism of fault trough. We show here that the interpretation of the Eastern Yihezhuang Uplift (Jiyang Depression, East China) helped to understand the deformation mechanism of the Yidong fault trough. We investigated the structural characteristics and evolution of the Yidong region using high-resolution 3D seismic and drilling data. The results suggested that the NW- and NE-striking faults cut through the region to form a "two hill-one trough-two step" structural pattern, with the NW-striking Yidong fault trough sandwiched between the Yigu 991 fault and the Yigu 56 fault. In the section perpendicular to the fault trough, the residual thickness of the Paleozoic sediments in the fault trough remains constant, while the Mesozoic is thinning from north to south, and so is the Kongdian Formation from east to west. In the section parallel to the fault trough, multiple imbricated reverse faults formed on the front of the Yigu 991 fault with series of associated SN-trending stepwise normal faults forming at the back margin. This "compress at the front and pull at the back" deformation is the characteristics of gravity sliding tectonics. The analysis of balanced cross sections showed that the region has experienced five stages of evolution. In the rifting stage, the Yigu 991, Yigu 56 and Yidong faults dissected the basement rock into an isolated block, allowing the stratum to glide westward under the action of the tilted block to form the Yidong fault trough. Thus, we propose that the gravity sliding determined the formation of the Yidong fault trough, by which, the rifting at the early Himalayan laid a foundation for the gravity sliding, the wide and gentle Yigu 991 fault plane provided essential condition, and formation tilting derived from the uplift of footwall of the Yidong fault triggered the gravity sliding. © 2019, Editorial Office of Earth Science Frontiers. All right reserved.