MESO-CENOZOIC MULTICYCLIC TECTONIC EVOLUTION AND ITS BEARING ON THE FORMATION OF BURIED HILL SYSTEMS IN THE WESTERN BOHAI SEA

The Meso-Cenozoic tectonic evolution of the western Bohai Sea is characterized by frequent change in tectonic regimes and distinct cyclicity, which caused the formation of diversified and complex buried hills. On the basis of fine interpretation of seismic data, this paper is devoted to the reconstruction of the multicyclic tectonic evolutionary history of the study area during the period of Meso-Cenozoic using the method of structural analysis. Structural deformation, tectonic stages and definition of paleostress field are also studied in details. Starting from remained strata and fracture systems in buried hills, the geological structure of the buried hills is carefully dissected, and the structural style as well as genetic types of the buried hills identified and divided. It is found that there are six tectonic cycles in the region during Mesozoic and Cenozoic, i.e. Indosinian, early Yanshanian, Middle Yanshanian, Late Yanshanian, Early Himalayan and Late Himalayan epochs. Accordingly, the buried hills have experiencecl four stages of evolution, which includes the formation of material basis in the pre-Indosinian epoch, the formation of embryonic form of buried hills by thrusting and napping in the Early Yanshanian epoch, inceptive tectonic framework by tensional rifting in the Middle Yanshanian to Early Himalayan epoch, and the accelerated faulting and subsiding in the Late Himalayan epoch. The structural characteristics of buried hills in the study area are obviously zonal due to the space-time differentiation of tectonic deformation and stacking patterns in each cycle. From west to east, the buried hills can be divided into three types, i.e. inner residual thrust type, reversal block-fault tilting type and complex strike slip fault-block type.