Transfer Strctures And Different Intra-plate Deformation In The Sag, Bohai Bay Basin

The intraplate Huanghua Depression is located at the center of the eastern China Bohai Bay Basin, far away the continental margins, which is one of the oil and gas-bearing basin of the Bohai Bay Basin. The Qikou sag is located in north-central area of the Huanghua Depression, it is the juntion of the NE-striking structural system and the NNE-EW-striking structural system in the Huanghua Depression, where are many different types of transfer zones developed. So, the Qikou sag is a key position for studying different intrapate deformation of the Bohai Bay Basin.This paper systematically summarized the definition, type and research trend of transfer zone in domestic and overseas. Combined with seismic profiles, gravity and magnetic data, this paper mainly focuses on the stratigraphical characteristics, fracture characteristics, transfer structures and different intra-plate deformation. Some new conclusions obtained are as follows:1. The"Great Qikou Sag"is the major tectonic unit of the Huanghua Depression, its inner structural pattern is controlled by E-W-trending structures. The single sag in the Huanghua Depression is characterized by graben in the lower part and the half graben in the upper part. The heaves in the Huanghua Depression developed at the Shayi sedimetary stage, separating"the Great Qikou Sag"into several sub-sags such as the present-day Qikou Sub-sag, the Banqiao Sub-sag, the Qibei Sub-sag and the Qinan Sub-sag.2. The margin of the Huanghua Depression is the Cangdong Fault System and the LanLiao-Yangerzhuang Fault System. But the margin of the Qikou Sag is the Hangu Fault in the north, the Haihe-Xingang Fault, and the important southern edge Qizhong Fault. These EW-striking faults directly resulted in the development of the NS-striking transfer zones in the Qikou Sag. 3.Transfer zones in the Great Qikou Sag can be divided into three orderes: three N-S-trending major transfer zones, from east to west are named as the East Kongdian Transfer Zone, the Coastal Transfer Zone and the Yangerzhuang Transfer Zone, which are correspond to three basement-involved strike-slip fault zones, respectively, to convert the different deformation on either side of depression; the secondary transfer zone is near E-W trending, to play an important role of adjusting the differences between south, middle and north areas of the Huanghua Depression; the third level transfer zone is generally perpendicular to its master fault, transmitting the extension amount between the faults.4. There are several causes of transfer zones in the Great Qikou Sag: the influence of boundary master faults, control of basement-invovled structures and differentiation of Cenozoic extension. The existence and development of transfer zones in the Qikou Sag cause different deformation in the different periods and different faulted-blocks in the Huanghua Depression, resulting in differences in structural styles of the Qikou Sag.5. The Huanghua Depression is influenced by the role of the roll-back subduction of the Pacific Plate and the indentation of the Indian Plate toward the Eurasian Plate. The depression was under a dextral extensional stress field in Cenozoic, as a result, it showed a rifting during the early evolutionary period, and pull-apart faulting during the later evolutionary period. Its principal regional stress underwent clockwise roation in Cenozoic, from the NWW-SEE-oriented extension during the Kongdian- Shasi period to the NW-SE-oriented extension during the Shasan-Dongying period, and near N-S-oriented extension during the Neogene. However, such an evolutionary process can be used to successfully explain the development and evolution of the pull-apart Huanghua Depression.