Pull-Apart Basins Within the North China Craton: Structural Pattern and Evolution of Huanghua Depression in Bohai Bay Basin

There are many pull apart basins such as Shanxi graben,Jiao-Lai basin and some sags in Bohai Bay basin within North China craton.The Huanghua depression is such a pull apart basin located at center of Bohai Bay basin.It is NNE-trending depression separating NNE-trending Jizhong depression from NWW-trending Jiyang depression.It developed as a pull apart basin in a large-scale strike-slipping fault system.The western margin of depression is Cangdong fault system,which is Lan-Liao-Yanshan-Yangerzhuang fault system.The Qikou is biggest sag with sedimentary thickness of more than 11 km and under intensive extensional region of Bohai Bay basin area.The Huanghua depression has sharp western and eastern boundary faults controlling pull apart basin.The Qikou is major component of Huanghua depression,its inner structural pattern is controlled by E-W trending structures rather than previously known Cangdong fault.The single sag in Huanghua depression is characterized by graben in lower part and half graben in upper part.The heaves in Huanghua depression developed in Shayi sedimentary stage,separating the Great Qikou into several sub-sags such as present-day Qikou sag,Banqiao sag,Qibei sag and Qinan sag.The Huanghua depression has four major faults including NNE-trending strike-slipping boundary faults,NEE-trending regional extensional faults,E-W trending secondary extensional faults and N-S trending transfer zones such as coastal transfer zone and Kongdian heave.The different extensional domains are controlled by single detachment fault system in basement.The architecture of Qikou is a result of late destruction of North China craton under regional NNW-oriented extension.The basin underwent four stages of evolutions including Eocene propagating rifting called Kongdian event,Early Oligocene pull-apart graben called Jiyang event,Late Oligocene half-graben called Dongying event and Miocence-Pleistocene thermal depression under dish-like bukling.