Tectonic Deformation In Late Cenozoic Of Liupanshan-Baoji Fault Zone In The NE Margin Of Tibet Plateau

Liupanshan-Baoji fault zone is located at the boundary of NE margin of Tibet Plateau and the SW margin of Ordos Block.It connects the active Haiyuan fault to the north,and dispersed buried in the Weihe graben to the south.The west of the fault zone is Longxi terrain,while the east of it is the stable Ordos Block.The Liupanshan-Baoji fault zone is the forefront edge of the northeastward expansion of Tibet Plateau,which suffered intense deformation since the late Cenozoic.So Liupanshan-Baoji fault zone is the key area to study the uplift mechanism and expansion kinematics of the NE margin of Tibet plateau,as well as the deformation since the late Cenozoic around the SW margin of the Ordos Block.In this study,we quantify analyzed the geometry and kinematics of five faults of Liupanshan-Baoji fault zone,and obtained the following conclusions:The north and south segment of the eastern Liupanshan fault mainly conducts sinistral strike-slip motion with thrust component,the central part of the fault conducts thrust motion,and the whole fault is active since the Late Pleistocene to the Holocene.The horizontal displacement of the gullies along north segment to Haijiazhuang village is between 10-40 m,so the average horizontal slip rate is 1-3 mm/a from the Late Pleistocene to Holocene,and the average vertical slip rate since the Late Pleistocene is 0.9 mm/a according to previous studies.The horizontal and vertical displacement is 7-20 m and 2-6 m since 10000 a respectively along the south segment,the slip rate of this segment is 0.7-2 mm/a in horizontal and 0.2-0.6 mm/a in vertical since the Holocene.The trench along the south segment of the fault discloses a fast motion event between 1180 a BP to 3300 a BP,which in accordance with the collapse found along this segment,combining with the historical earthquake data,the south segment of the fault is the seismogenic structure of the A.D.600 Qin-Long earthquake.The eastern Liupanshan fault off set one limb of an anticline with the distance of 4.5 km,which represents the total displacement since the fault began to sinistral slip.Combining with the average horizontal slip rate of the fault,the onset time of the left-lateral slip motion began at ~2.7 Ma.Longxian-Qishan-Mazhao fault is the most active one of the Longxian-Baoji fault zone since the Late Quaternary,which mainly conducts sinistral strike-slip with normal faulting component.Many displaced the Late Pleistocene first paleosol S1 were found along the fault,all these fault plane demonstrates normal faulting,and the displacement is 1-3 m in most places.The formation age of S1 is 100000 a,and the vertical slip rate since the Late Pleistocene is 0.01-0.03 mm/a.Series of left-lateral displaced gullies were found along the fault with the distance between 70 to 130 m,we analyzed the geomorphic characteristic of the gullies and consider the formation age of gullies should be around 100000 a B.P.,the left-lateral slip rate is 0.7-1.3 mm/a since the Late Pleistocene.The rock bridge between two fault segments was deformed intensively and formed 5 folds,the crustal shortening of the folds is the total displacement of the sinistral strike-slip fault,which is 1 km in total.Combining the horizontal slip rate,the onset time of the sinistral slip motion is about 1 Ma.Substantial data indicate that Guguan-Guozhen fault active in the Middle Pleistocene,and stopped moving since the Late Pleistocene.Combining with stria,gully,normal and thrusting fault plane,the fault conducted sinistral strike-slip with normal fault motion in the early Quaternary,the early Quaternary fault plane inherit the normal fault plane formed in the Late Cretaceous.The time of the faulting activity is unclear of the Taoyuan-Guichuansi fault and Qianyang-Biaojiao fault since the late Cenozoic,based on the stria and the feature of the gullies,Taoyuan-Guichuansi demonstrates dextral strike-slip motion;according to well drilling data,Qianyang-Biaojiao fault mainly conducted normal faulting since Pliocene.The three main faults in Liupanshan-Baoji fault zone all have sinistral strike-slip motion feature,according to the left-lateral onset time of each fault,the sinistral strike-slip motion of Haiyuan-Liupanshan-Baoji fault zone from NW to SE,which indicate that the sinistral strike-slip motion spreading of the faults in the NE Tibet Plateau is from NW to SE.The cause of this fault motion pattern is that,after the remote effective of the consistent uplifting of the Tibet Plateau reached study area,the sub-block west to the fault zone was pushed to move to NE,E,SE and affected the faulting pattern.