The Subsidence History Analysis And The Anomalous Subsidence Separation During The Postrift Stage In The Central And Northern Region Of The Bohai Bay Basin

Based on the previous study of the history of basin subsidence and structural evolution process, this paper takes the drilling data and seismic profiles in Bohai bay basin as the research object, uses the theoretical methods and technological means of basin analysis, basin simulation and so on, to obtain the information of the subsidence and the subsidence rate of different periods. We break the previous research on unification of the whole process about the subsidence history of basin, and divide it into the synrift stage and the postrift stage to study alone. Combine with the characteristics of the distribution of the crustal strain rate during the synrift stage, we accomplish the subsidence episodes to be divided again. Most of all, we study the characteristics of the anomalous subsidence during the postrift stage, quantitatively get the characteristics of the anomalous subsidence and the subsidence rate during this stage, and to explore the dynamic relationship between the process of anomalous subsidence and the subduction process of the western Pacific plate. Main ideas and conclusions are as following:(1) The research results show that the curves of the crustal strain rate during the synrift stage in the central and northern region of the Bohai bay basin have obvious three big fluctuations, may indicate three tectonic subsidence events:The first episode(60-42Ma), corresponding to Ek-Es4, the average tectonic subsidence rate is3.5-6.9m/Ma, the second episode(42-36Ma), corresponding to Es3-Es2, the average tectonic subsidence rate is5.5-50m/Ma, the third episode(36-24.6Ma), corresponding to Esl-Ed, the average tectonic subsidence rate is11.54-54.7m/Ma. The stretch curves present a structure style of "a steep dip and a gentle dip", just as a ladder, which show every episode of the subsidence process consists a rapid stretched subsidence process and a slowly thermal decaying subsidence process. The results of the strain rate simulation show that the large-scale tectonic subsidence event also corresponds to the peak value of the strain rate, the maximum can reach400a-1(1/s), which is consistent with the strong stretch in the horizontal direction at that time. The subsidence rate at early stage of the postrift significantly slower than the synrift stage, which shows the typical characteristics of a slowly thermal decaying subsidence. About5.3Ma in the land and12Ma in the sea area, corresponding the sedimentary period of the Minghuazhen formation, the anomalously accelerated subsidence has taken place, this phenomenon has manifestation in the whole study area.(2) There are obvious differences between the observation of tectonic subsidence and the simulated subsidence during the postrift stage in the study region. The anomalous subsidence value is about100-200m in the northern region, the anomalous value is about500-700m in the central Bohai sea region, and is about100-300m in the central land region. The anomalous subsidence value in the study area during the postrift stage has a characteristic of increase from the northern region and the central land to the central Bohai sea region the sea. The anomalous subsidence process during the postrift stage is overlay by two accelerate subsidence processes.(3)The postrift anomalous subsidence is mainly caused by the "slab" subduction of the Pacific plate, which induce the flow of the deep mantle material and lead to a down-drag force. The anomalous subsidence in essence is a kind of dynamic subsidence, which is caused by dynamic factors.