Study On The Lateral Motion Of Northeastern Tibetan Plateau

The onset of lateral motion of the Tibetan Plateau has been fixed on late Cenozoic(20~12 Ma)by almost recent studies.And the large-scale lateral motion tectonics has been found in SE Tibetan Plateau,which can be also supported by the Global Positioning System(GPS)horizontal velocity field.By contrast,the lateral motion of NE Tibetan Plateau is still uncertain.Firstly,the lateral motion is restricted within NE Tibetan Plateau,or the scale of lateral motion is so large that can associate with the Cenozoic extension tectonics of North China block that is still controversial.Secondly,controversies focus on route of lateral motion.Some studies argued that the lateral motion along the Qinling with the E-W trending,but some results show that the lateral motion is possible NEE extruding into the southern Ordos block.The challenge lies in the lack of geological constraints of the spatiotemporal crustal deformation across the NE Tibetan Plateau,including southern Ordos block,Qinling and Weihe graben.Nowadays,the Global Positioning System(GPS)becomes the important method to study the temporal crustal deformation.Although it represents the current state of the crustal deformation,it is a useful supplement to the tectonic study by traditional geological ways.Especially,the long-term and stable GPS velocity fields have played important roles in Geodynamic studies.Based on the previous research,which has revealed the sketchy lateral motion of NE Tibetan Plateau in three-dimensional,the most recent achieved high-dense and long-term 3-D crustal deformation velocity fields(GPS horizontal and leveling vertical measurements)provide us a good occasion to further study the present lateral motion of NE Tibetan Plateau and its relative geodynamics.This study will follow those ideas and the contents are as follows,1.The evolution of lateral extrusive tectonic in NE Tibetan Plateau: I summarized the images of Lateral extrusive evolution in NE Tibetan Plateau and its neighboring region,based on the most recent studies on low-temperature thermochronometry and magnetostratigraphy.This work is important to further study on the present lateral motion and its geodynamics.2.Study on the spatial distribution of present lateral motion across the NE Tibetan Plateau and its nearby area by long-term(1999~2014)GPS horizontal velocity field:The study works mainly include the GPS reference conversion and strain rate field calculation.3.Using the long-term(1970~2014)leveling vertical velocity field to test and further study the present kinematic model of lateral motion revealed by the GPS data of our study.4.Study on the fault slip rates on main active faults of NE Tibetan Plateau and its vicinity: firstly,the spherical linear elastic block theory(Blocks)with refined block geometry of study area that constraining with long-term(1999~2014)GPS horizontal velocity field is employed to inverse the present fault slip rates.And then,considering the Weihe graben is mainly controlled by the normal faults,so the half-space elastic dislocation model constraining with long-term(1970~2014)leveling vertical velocity field is employed to inverse the fault slip rates on most important fault within the Weihe graben.5.Using the long-term(1999~2014)GPS horizontal velocity field to explore the mechanism of Ziwuling small earthquake cluster with the trending of NNW,located in the southern margin of the Ordos block.6.Based on the spatiotemporal evolution of the lateral motion and the present crustal deformation kinematic model of NE Tibetan Plateau,some questions relative to the lateral motion have been further studied,including the active features of southern Ordos block,the south and north boundaries of present lateral motion,the relationships between the spatiotemporal evolutions of the lateral motion and regional faults activities,and the involved depth beneath the earth surface of lateral motion.Conclusions are as follows based on the above studies.1.The detailed present crustal deformation among the different terrans in the study area are revealed by the GPS velocity field respected to the North China block(Euler vector: 57.06979oN,136.10122oE,0.11670o/Ma).It shows that the NE Tibetan Plateau is NE moving towards the SE Ordos block,and when it meets the Ordos block,the crustal deformation shows differentiations.Except the northward flowing to the Yinchuan graben,the lateral extrude across southern Ordos and the Weihe graben with NEE-NE direction.And the frontier of the lateral motion is reach to mid-south part of Shanxi graben.The southern boundary of later motion is a right lateral diffused shearing zone,relative to the Pingwu-Qiangchuan-North Hanzhong basin fault,Chengxian-Taibai fault and Lixian-Luojiabao fault.Meanwhile,along the 38oN located in the interior of Ordos block,which also show the left lateral movement,is possible as the latest northern boundary of lateral motion.Our results also show that the Qinling with E-W trending is inactive.So it is not the route of present lateral motion.And the horizontal extension of the mid-east Weihe graben is not controlled by the Qinling,but relative to the counterclockwise rotation of southern Ordos block.2.The characters of present-day vertical crustal deformation coincide to the kinematic model of lateral motion.Firstly,NE Tibetan Plateau is not moving along the E-W Qinling,but moves towards the SE Ordos block.Because of the significant horizontal shortening(almost 6 mm/yr)can match the fast uplift there(4~6 mm/yr).By contrast,the significant difference of vertical deformation,located at the joint of West Qinling and Qinling,also not supports the eastwards flow along the Qinling.Secondly,the differential vertical crustal deformation of Weihe graben are consistent with the lateral motion kinematic model,which is flow cross the south Ordos block and Weihe graben with the direction from NEE in their west to NE in their east.The west part of Weihe graben is uplift with 2~3 mm/yr relative to the mid-east part of Weihe graben,which coincide to the horizontal crustal shortening there.And the subsidence of mid-east part of Weihe graben is consistent with the crustal extension with the direction of NNW.Thirdly,the south and north boundaries of present lateral motion inferred by the GPS data are also well defined the present uplift variation.3.The inversion shows that Pingwu-Qiangchuan-North Hanzhong basin,Chengxian-Taibai and Lixian-Luojiabao faults are all right-lateral strike-slip faults,and the right-lateral slip rate on Lixian-Luojiabao fault is up to 3.0 mm/yr.Meanwhile,this diffused right lateral shearing zone can connect to the right lateral slip on the mid-north Weihe graben.Besides,the result also shows that the faults within the Qinling is inactive and no left lateral slip fault.The left-lateral slip rate on Haiyuan fault is about 5.2 mm/yr,and Liupanshan-Qishan-Mazhao fault zone also has obvious left lateral movement.So it infers that it is still an important boundary for present lateral motion.However,to the north of Haiyuan fault,the Xiangshan-Tianjingshan fault is also a left lateral slip fault with 3~4 mm/yr,which indicate that the north boundary of present lateral motion may has migrated northward to 38oN.4.Based on the GPS velocity field and the GPS profiles across the Ziwuling small earthquake cluster,the NNW small earthquake cluster is caused by left-lateral shear slip,which is prone to strain accumulation.When the strain releases along the weak zone of structure,small earthquakes diffuse within its upper crust.So the different movement character of the inner of Ordos block and Weihe graben(which was demonstrated through GPS measurements)and the small earthquake cluster belt reflect the activated southern margin of the Ordos block,which was generated through readjustment of the present lateral motion kinematic model.5.The discussion on the lateral motion show that the active of southern Ordos is not only performs on the different crustal deformation within the Ordos block,but also performs on the flow in the depth of asthenosphere,the reduction of lithosphere,and the thrust faulting and seismicity within the lower crust.The uplift time of Baiyushan located in the southern side of 38oN is about 1.2 Ma B.P.,which is consistent to the onset time of left-lateral slip on Xiangshan-Tianjingshan fault that lies along 38oN,can be seen as an indirect evidence for the latest northern boundary of lateral motion along 38oN.Lateral motion of NE Tibetan Plateau experienced a processing of evolution from its south to north.The onset of lateral motion firstly touched the Weihe graben is about 20~15 Ma by the left-lateral strike-slip of Chengxian-Taibai fault.In the late Miocene(8~5 Ma)the West Qinling-Qinling fault with left-lateral slip became the northern boundary of lateral motion.And then,in mid-late Pleistocene(1.8 Ma),the northern boundary of lateral motion further migrated to the Haiyuan-Liupanshan-Qishan-Mazhao fault.Present-day the latest northern boundary of lateral motion is along the 38oN within the interior of Ordos block.Crust-mantle coupling vertical coherence and asthenospheric flow model is able to well meet the present lateral motion kinematic model.And the overall active uplifting of Ordos block is attributed to asthenospheric heat erosion beneath the Ordos block.