Mesozoic Fault System Analysis And Paleostress Field Evolution In Western Hills Of Beijing,North China

The Western Hills of Beijing is located in the intersection of Yinshan-Yanshan orogenic belt and Taihang Tectonomagmatic Belt.It contains complicated tectonic deformations as a result of multiple tectonic movements.Folds and faults with varied orientations and kinematic features as well as plutons intrusion together with volcanic sedimentations are well preserved in this area.In this paper,we have analyzed the geometry and kinematics features of the faults,and determined their timing sequence of formation based on the superposition of structures.The stress tensor data were acquired by inversion analyses with Win-Tensor program on fault slip vectors collected from outcrop-scale fault surfaces near the main faults.These data are arranged into different groups in order after a comprehensive investigation on the data location in the structural levels,the relations between the fault-striaes,the characteristics of the faults,and regional geological setting as well,Six generations of paleostress fields have been identified in this study.The first generation of paleostress field is inferred to be the Early-Middle Jurassic NE compression(D1)after the intrusion of the dyke in the center of Qingbaikou dome and previous to that of the Jiulongshan Formation(?177 Ma-154 Ma).The second generation is believed to be the Middle Jurassic NW compression(D2)after the sedimentation of the Jiulongshan Formation and previous to that of the Tiaojishan Formation volcanism(154 Ma-147 Ma).The third generation is inferred to be the NE compression(D3)after the sedimentation of the Tiaojishan Formation volcanism(147-138Ma).The fourth generation is believed to be the Latest Jurassic and Earliest Cretaceous NW compression(D4)after the sedimentation of the Tiaojishan Formation volcanism(138-128Ma).The fifth generation is believed to be the SE extension(D5)after the Zhangjiakou Formation volcanism(128-118Ma).The sixth generation of paleostress field is inferred to be later than 118 Ma and characterized by a widespread multidirectional extension(D6).The north-south principle compression direction of the paleostress field and its timing implies that the collision of the Mongolian arc terranes along the Solon suture against the North China plate and the Siberian plate against the Mongolian-North China plate collision across the Mongolo-Okhotsk Sea could be responsible for the tectonic deformation in this region.The NE extending tectonic deformation under the NW compression in the Middle Jurassic and Latest Jurassic might be interpreted as the far-field effects of the westward subduction of the Paleo-Pacific plate.The extension in Early Cretaceous combined with extensive magmatism could be treated as the collapse of the orogenic belt after the cession of plate collisions,and perhaps accompaned with partial melting in the lower crust of North China craton under the impaction of a suspected mantle plume.The widespread multidirectional extension deformations indicate a peak period of the destruction of North China craton.