Structural Pattern,Evolution And Dynamic Mechanism Of The Mesozoic Strata In The Huanghua Depression And Jiyang Depression

Mesozoic strata of Bohai bay basin are an important and new oil-gas exploration objective to search for Mesozoic buried hill, but their Mesozoic structural pattern and evolution are not clear yet. The Huanghua and Jiyang depressions are important and new oil-gas exploration areas, and are also mark a change of Mesozoic–Cenozoic structural patterns in the Bohai Bay basin. Moreover, these two areas were sited at the center of a Mesozoic plateau in north-eastern China craton during the Mesozoic. Their structural evolution is a key problem to understand lithospheric thinning in eastern China and the formation and subsequent collapse of this plateau. Therefore, the study area has important implication for future research. A large number of seismic profiles were interpreted to analyze the distribution of relic Mesozoic strata. Combining them with balanced sections, we discuss their Mesozoic structural evolution,dynamic mechanism and original basin characteristics. Finally we got the following conclusion:1.During the Indosinian period,Huanghua and Jiyang depressions developed gentle EW-NEE folds and thrust belts under NS compression. Triassic sediments are found the syncline and were seriously influenced by a sinistral strike-slip. The original basin is a compressed flexural basin. Later, during early Yanshannian period, the NWW-SEE compression developed NE thrust belts and folds, middle to lower Jurassic sediments are found in a compressed flexural basin highly influenced by dextral slipping.2. The Lankao-Liaocheng-Shenyang fault is a set of NE inverse-slip faults system and controls the structure of Bohai bay basin during the middle Yanshannian, as deduced from the contrast of relic strata from both the sides of the fault. Meanwhile, Cangdong fault was dextral slipped, and the sinistral slip NEE Tan-Lu fault strengthened gradually. All of these resulted in the structure of Huanghua depression and Jiyang depression took on different aspect. During the Late Jurassic, the upper Jurassic sediments accumulated in a compressional pull-apart basin and in a flexural syncline, respectively at southern range and middle-northern range of Huanghua depression; the two basins were all seriously influenced by compression and dextral slipping. In the same time, Jiyang depression developed escape tectonic, the NW inverse faults changed into normal faults, the upper Jurassic accumulated in escape-apart basin. In the Bohai bay basin, early Cretaceous is the key period of change in tectonism; almost all inverse faults changed into normal faults. During the upper crustal uplift, Huanghua depression was located at the center of this Mesozoic plateau; lower Cretaceous only accumulated in a dextral slip faulted rift basin at the middle of Huanghua depression. The escape tectonic continued in the latter Yanshannian period in the Jiyang depression.3. We deduced that the Mesozoic plateau in north-eastern China craton changed from NWW to SEE tectonism. The Huanghua depression was located at the center of this Mesozoic plateau in the early Cretaceous. Furthermore, based on the structural contrast of Huanghua and Jiyang depression, and from previous results, we put forward the hypothesis that maximum lithospheric thinning and delamination in eastern China happened before early Cretaceous. So, the Mesozoic plateau probably collapsed in early Cretaceous. Due to Yanshannian fifth episode of folding and thrusting, there were no upper Cretaceous sediments during late Yanshannian period.