Tectonic Deformation Style、Formation Age And Signihcance Of Western Hills Of Beijing In Mesozoic

The tectonic position of Western Hills of Beijing is located in the intersection ofYinshan-Yanshan orogen(EW) and Taihang tectonic belt(NNE),where is rich instructural deformation. In recent years，the attention on Mesozoic folds structure inthis area is not enough.During the Mesozoic，the area of Western Hills of Beijing mainly developedthree sets of fold structures of different direction. They are folds oriented to NW、folds oriented to NE and folds oriented to NNE. Through the field observation anddata analysis, combined with the age of volcanic rocks, we set out the forming ageand order of the three sets of fold structures. The earliest is the folds oriented to NW,at174~160Ma;then the folds oriented to NE formed at160~135Ma;The last is foldsoriented to NNE, it’s formation age is later than the folds oriented to NE and before135Ma.In addition，we think that the forming of the Qingshui brachyanticline andQingbaikou brachyanticline is the result of tectonic superimposing. The folds orientedto NE acrossed on the folds oriented to NW, forming the brachyanticline. It’s notformed in one tectonic activity.The three sets of fold structures of different direction reflects differentpaleotectonic stress field. The folds oriented to NW reflect a NE-SW compressivefunction. During the Jurassic period, the polymerization of north plate and south platewears off，the subduction to NW of Pacific plate strengthens continuesly. This may bethe caution of that function. The folds oriented to NE reflect a NW-SE compressivefunction. During this period, the NS plate convergence had been very weak, thesubduction from Pacific plate to Eurasian plate plays a key role.In addition，the Yanhecheng fault performs as a normal fault in its final activity,formed in an extensional environment, later than135Ma.