The Study Of The Deformation Characteristic Of The Helanshan Tectonic Belts

The Helanshan Tectonic Belts(HTB),a south-north directed intracontinental tectonic belt located in the northern segment of the “North-South Tectonic Belt”,and was bounded by Alxa Massif to the west and the Ordos Basin to the east,and stretched to the Yinshan orogenic belt in the north and to the Qilian orogenic belt in the south.The HTB,which has a long-term tectonic evolution history,undergone complex intracontinental tectonic evolution since the Paleozoic era.Abundant structural information provided by the deformation characteristics of the HTB not only records the tectonic evolution history of the Precambrian plate boundaries,but also records detailed information on the intracontinental tectonic evolution processes of the blocks surrounding the HTB since the Paleozoic era,especially during the Mesozoic and Cenozoic.Accordingly the HTB is considered as the natural laboratory for the study of the intracontinental tectonic evolutionary processes of the China continent during the Mesozoic and Cenozoic.Based on the detailed geometric,kinematic and dynamic analysis of the HTB,together with the study of the deformation properties of the Bayanhot Basin and Ordos Basin Thrust Belt on both sides of the HTB,we have drawed the following conclusions: In the east-west direction,it was a “Horst and Graben” structure among the HTB,the Bayanhaote Basin in the west and the Yinchuan Graben in the east.The initial of this structural pattern was in Cretaceous.While in the north-south direction,the HTB can be subdivided into the southern,middle and northern parts.The southern one was characterized by an EW-directed striking and northward thrusting,while the northern one was characterized by a NS-directed striking and eastward thrusting.But the middle one is mainly characterized by a multi-stage of superimposed folding characteristics.In addition,the different parts of the HTB show distinct structural deformation in aspcect of geometric,kinematic and dynamic characteristics.The southern part of the HTB experienced four stages of deformation processes.The first stage of structural deformation(D1)was manifested by the NNE-directed shortening during the Silurian and the end of the Devonian,which leading to the formation of WNWstriking closed compressive structures within the Cambrian and the Devonian strata.The second stage of the structural deformation(D2)was characterized by the nearly NS directed shortening during the Triassic,which led to the formation of the EW-striking closed shortening structures in the Carboniferous and below strata.The third stage of the structural deformation was characterized by end Jurassic WNW-directed shortening,which led to the formation of the NNE-striking shortening structures.The fourth stage of the structural deformation was characterized by the NNE-directed shortening during the Cenozoic and led to the formation of the strike-slipping within the earlier fractures.As to the Middle part of the HTB,it was also characterized by four stages of deformation processes.The first one was WNW-striking shortening structures within the Middle Ordovician strata,which was formed by the NNE-directed shortening during the Silurian and Devonian.The second one was characterized by the NNE-directed shortening during the end of the Triassic and Early Jurassic,leading to the formation of a series of WNW-striking open folds and northward thrusting compression structures in the Triassic and below strata.The third one can be divided into two parts furtherly,the earlier one was characterized by a series of NNE-striking shortening structures within the Jurassic and the below strata,which was formed by the WNW-directed shortening in the end of the Jurassic.And the latter one was characterized by the WNW-directed extension and the formation of a series of NNW-striking normal faults controlling the sedimentation of the Lower Cretaceous.The fourth one of the deformation was characterized by NNE directed shortening during the Cenozoic,leading to the formation of a series of WNW-striking shortening structures within the Cenozoic and below strata.The northern part of the HTB,was in a parallel unconformity contacts between Ordovician and Carboniferous,indicating the absence of intensive shortening deformation in the north HTB during the Silurian and the Devonian,therefore,the first stage of structural deformation(D1)in the northern HTB was characterized by “no shortening deformation”.The second stage of the deformation(D2)is characterized by near NNE-directed shortening in the Late Triassic,leading to the formation of a series of gentle folds extending in the EW direction and southward thrusting in the Triassic and below strata.The third-stage of the deformation(D3)shows a nearly WNW-direction shortening at the end of the Late Jurassic,which led to the formation of a series of NNE-striking compressional structure in the Jurassic and below strata.And the fourth-phase of the deformation(D4)shows nearly northward thrusting in the west of the Huanghe fault and counterclockwise rotation in the east of the Huanghe fault.This characteristics of the distribution of the stress field indicates that an outwardly divergent stress field resulted from the Tibetan Plateau was distributed to the south of the HTB during the Cenozoic.According to the comparison of structural deformations within different tectonic units in the HTB,it is concluded that the first and second stage of structural deformations were progressed from south to north,and the third stage of tectonic deformation is formed in an east-west shortening.Whereas the fourth phase deformation reflects the northeastward expansion of the Tibetan Plateau during Cenozoic.