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The Identification and Forming Mechanism of Paleogene Oblique Extension System in Middle Bohai Bay Basin


Huanghua, Linqing and Dongpu Depressions, which are localized in the middle of Bohai Bay basin, are characterized by Cenozoic NNE- trending rifting. They are successively distributed from north to south. These three depressions are separated by several EW to NWW- trending transfer zones which were possibly controlled by pre-existing structures. So far there are several studies on a single depression. However no works studied the depressions as a whole. In our study, based on the seismic and drilling data, the quantitative analysis was used to determined the Paleogene activity of fault system and the sedimentary formation thickness of Kongnan Area, South Huanghua Depression, then we recover paleostructure framework and structural evolution processes of this area. The result shows that the activity of the faults in this area migrated from middle to north and south sides in Palaeocene-Oligocene Epoch. The center of subsidence showed left-step oblique distribution which indicated that the whole area represented the attribute of oblique stretch. The comparative analysis of the Paleogene faults system in Linqing Depression and Dongpu Depression indicate that the two depressions have the same characteristic of oblique rifting. Meanwhile, the distribution of residual sedimentary strata in the three depressions shows the fact that in the Palaeocene the total subsidence center localized in South HuangHua and Linqing Depressions. However in the Oligocene the center moved to the Northern HuangHua Depression and the Dongpu Depression respectively. The evidence indicated that the tectonic system and evolution of the three depressions in the Paleogene have unitive characteristic, it could be contributed to one oblique extension system. Gravity, magnetotelluric and P-wave velocity data confirm that a deep fracture fault zone-Lanliao Fault zone developed under the oblique extension system, the dextral strike-slip movement of the fault in the Paleogene influenced the evolution of the system. The active of Lanliao Fault zone is the response of the episodic process of subduction of the Pacific plate beneath the Eurasia plate in Paleogene.