Deformation Pattern Based On Geometry And Kinematics Of Active Tectonics In The Southwestern Ordos Block

The southwestern Ordos is located between the Tibetan Plateau,Ordos,Gobi-Alashan blocks,and the Qinling orogenic belt,where several groups of active faults with different trending were developed.The major block-boundary fault,(i.e.,the Haiyuan-Liupanshan-Qinling fault system)has undergone a significant transition in kinematics along the strike.From northwest to southeast,the Haiyuan fault is characterized by left-lateral strike-slip,the Liupanshan fault is thrust dominated,and the Qinling fault is of left-lateral strike-slip with remarkable normal component.Meanwhile,the SW Ordos is located at the northernmost margin of the northeastward growth of the Tibetan Plateau.Therefore,it is an ideal place to constrain the deformation patterns of the northeastern Tibetan Plateau,SW Ordos and their adjacent regions.Whether current eastward extrusion of the Tibetan Plateau mainly occurred along the major block-boundary fault system?How the strain are accommodated,transformed,or absorbed? Whether the northeastward extension of the Tibetan Plateau has affected the active tectonics of the southwestern Ordos? If so,how is the deformation pattern and its spatial extent? Thus,it is very important to study the geometry,kinematics,and deformation pattern of the SW Ordos and its adjacent regions,based on the geological and geodetic method.However,it remains unclear of the fault motion,fault slip rate and its tectonic implications of Longxian-Baoji fault zone,which located within the Tibet-Ordos transition zone.In this study,we focused on the geometry and kinematics of the Qishan-Mazhao fault,which is the longest and most active fault within the Longxian-Baoji fault zone,to study the geometry,kinematics,and deformation pattern of the SW Ordos and its adjacent regions.We have analyzed the slip-rate distribution along the major block-boudanry fault(i.e.,the Haiyuan-Liupanshan-Qinling fault system)and discussed the deformation transition model of the study area,which improved our understanding of the deformation of the Tibet-Ordos transition zone and the dynamics between the Tibetan Plateau,Ordos,and Qinling orogenic belt.The main findings are shown as following:The 180-km-long northwest-striking Qishan-Mazhao fault is a left-lateral strike-slip fault with normal component,which is located at the southwest margin of the Ordos block.TheQishan-Mazhao fault can be divided into 4 subsegments,from northwest to southeast,such as the Dazhuangke-Yanjiawan,Sangyuan-Wuyanquan,Dagouwan-Mimatai,and Mimatai-Mazhao subsegments.Hereinto,part of the northernmost subsegment(i.e.,Dazhuangke-Dengjiacao subsegment)is the most active fault strand.Meanwhile,the paleoseismic research suggests that the most recent earthquake has occurred after 1830±30 a BP,which possibly indicates that this earthquake event may be corresponding to the historical earthquake of 600 AD occurred in this area.The late Quaternary left-lateral slip rate of the Qishan-Mazhao fault is approximately 0.5-1.2mm/yr,which is consistent with that observed from the GPS measurement,indicating that this fault strand is very active during the late Quaternary.Spatially,the slip rate of the Qishan-Mazhao fault decreases from the northwest to southeast,according to the offset distribution measured from the displaced geomorphic features.The fault slip rate shows eastward decrease along the Haiyuan-Liupanshan-Qinling fault system from ~4-6 mm/yr to 0 mm/yr.Most left-lateral slip(~3 mm/yr)of the Haiyuan fault may have been absorbed by the crustal shortening across the Liupanshan mountain,the Qishan-Mazhao fault absorbed the remained portion of ~0.5-1.2 mm/yr,hence the counterclockwise rotation of the Ordos block should also accumulated part of the strain.Meanwhile,the approximately 1 mm/yr slip rate difference between the South China,Ordos,and North China Plain blocks has probably been absorbed by the NWW-SEE extension of the Shanxi graben,namely,the relative eastward movement of the North China Plain block in respect to the Ordos block.In sum,the deformation pattern in the SW Ordos and its adjacent regions is characterized by crustal shortening,relative movement,and block rotation.The left-lateral motion of the Qishan-Mazhao fault could be driven by the northeastward motion of the Tibetan Plateau.Meanwhile,we suggest that the northeasternmost forefront of the northeastward growth of the Tibetan Plateau has reached the boundary zone bounded by the Qishan-Mazhao,Ayouqi-Yabrai,and Sanguankou-Niushoushan faults.The spatial consistence of the surficial and upper mantle forefronts of the northeastward growth of the Tibetan Plateau suggests that the surfical deformation in SW Ordos and its adjacent regions might be resulted from the deformation in depth,such as the mantle.The lacking of severely deformation in upper-middle crust may suggest there is a spatial and temporal delay between the upper-middle crust and upper mantle in depth in SW Ordos and its adjacent regions.Our GPS block modeling in the SW Ordos and its adjacent regions presents the sense of motion and slip rates of the active faults,showing that the decadal slip rate measured from GPS matches very well with the long-term late Quaternary slip rate obtained in geological investigation.Meanwhile,the GPS block model also shows the rotation and vertical-axis rotation rates of the micro-blocks,suggesting that deformation in the SW Ordos is not only concentrate on the major block-bounding strike-slip faults,but also occurred within the intracontinental blocks,which is characterized by the fault slip,rotation of micro-blocks,and internal deformation.The slip rate distribution along the major block-boundary faults and the GPS block modeling results in the study area do not support the “Eastward extrusion model”(i.e.,“Rigid block model”).In other words,our results do not support the hypothesis that the Tibetan Plateau and South China block extrude eastward along the Altyn Tagh-Haiyuan-Liupanshan-Qinling fault system,and neither along the eastern Kunlun-Taibaishan-Qinling fault system.In contrast,the deformation in the northeastern Tibetan Plateau is consistent with the “continuum model”.