The Structural Geometry And Kinemics Of Extensional Basin- An Example From Langfang-Gu’an Depression, Bohai Gulf Basin

We analysed the the structural geometry and kinemics of extensional basin, which is directed by tectonic geology, basin analysis and fault-bend fault-bending fold theory, and based on the 3D seismic interpretation, setting Langfang-Gu’an Depression for example, and finally, we the originesis mechanism of Langfang-Gu’an Depression.Langfang-Gu’an Depression whose boundary fault is Daxing Fault is located in the northwest of Jizhong Depression in Bohaiwan Gulf Basin. Daxing Fault plane is divided into 10 zones by 4 transversal axial planes and 2 vertical axial planes, and each zone has its own configuration and feature. We also have discussed the mechanism of Daxing Fault’s movement, and consider that Daxing Fault’s movement is Domino normal pattern at first, and then the fault rotates to form the fault which is flatten from top to bottom. The stratum of the hanging wall rotates with fault at first, and then moves in incline shear pattern to form rollover anticline when the fault has to be flatten from top to bottom.After the map-view restoration of the fault-blocks of Paleogene in Langgu Sag, we have found that, the fault-blocks in the south rotate by clockwise while the fault-blocks in the north rotate by anticlockwise. We think that the different rotation between Middle-south Zone and Middle-east Zone results from the unbalanced stress which decreases from middle to the tips.We describe an analytical method for interpreting the geometry and structural history of asymmetric half grabens in rift basins with extensional fault-bend fold theory. The fault active times can be identified by the feature of growth axial surfaces and active axial surface. The growth axial surface can be displaced by nonconformity. We can calculate the denudation thickness by prolonging the former growth axial surface.According to the extensional fault-bending fold, we have establishment the basic models of extensional structural wages, and we think that the exist of wages may cause the deformation of upper formation. The local deformation in Gu’an is result from extensional structural wages and the collapse anticline in Langgu Depression has relationship with extensional structural wages.At last, we think Langfang-Gu’an Depression is a combined depression which is formed by Domino normal pattern, rotation pattern and incline shear pattern and has twice episodic riftings.