Abstract:

There are many pull apart basins such as Shanxi graben, Jiao-Lai basin and some sags in the Bohai Bay basin within the North China craton. The Huanghua depression is such a pull apart basin located at the center of the Bohai Bay basin. It is NNE-trending depression separating the NNE-trending Jizhong depression from the NWW-trending Jiyang depression. It developed as a pull apart basin in a large-scale strike-slipping fault system. The western margin of the depression is the Cangdong fault system, which is the Lan-Liao-Yanshan-Yangerzhuang fault system. The “Great Qikou sag” is the biggest sag with sedimentary thickness of more than 11 km and under intensive extensional region of the Bohai Bay basin area. The Huanghua depression has sharp western and eastern boundary faults controlling pull apart basin. The “Great Qikou sag” is the major component of the Huanghua depression, its inner structural pattern is controlled by E-W trending structures rather than the previously known Cangdong fault. 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 in the Shayi sedimentary stage, separating “the Great Qikou sag” into several sub-sags such as the 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 the coastal transfer zone and the Kongdian heave. The different extensional domains are controlled by the single detachment fault system in the basement. The architecture of the “Great Qikou sag” is a result of the late destruction of the 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.

Key words: Bohai Bay basin, pull apart basin, Huanghua depression, Oligocene, Qikou sag, North China craton, tectonics