Studies On The Late Quaternary Activity Of The Yanggao-Tianzhen Fault,Northern Shanxi Graben

The Jin-Ji-Meng Basin and Range Province(JJMBRP)is located at the northern Shanxi Graben(SG),northeastern corner of the Ordos Block.It has the tectonic conditions for strong earthquakes because it is the conjunction of multiple active tectonic blocks and many large Holocene active normal faults are distributed in this region.However,no M>7 event has been documented in historical times.Thus,it is of great significance to assess the seismic risks in this region.In addition,the JJMBRP is also the locus where two main tectonic belts in North China,the NE-trending extensional SG and the NW-trending,left-laterally strike-slip Zhangjiakou-Bohai Fault Zone(ZBFZ),intersect.The studies on the geometrical and kinematic characteristics of the faults in the intersection area are key for understanding the tectonic transformation between these two tectonic belts.The Yanggao-Tianzhen Fault(YTF)is an important NEE-trending active normal fault in the transition zone of the end of the SG and the ZBFZ,defining the northern boundary of the Yanggao-Tianzhen Basin(YTB).In views of its relatively complex fault geometry,characterizing the fault geometry and kinematics of the YTF accurately will thus have important instructive significance for understanding regional geodynamics.However,the late Quaternary activity features of the YTF are still poorly understood.Based on the traditional mature theories of active tectonics,this contribution utilizes remote sensing,field investigation to study the late Quaternary activity characteristics and paleoseismic history of the fault,and gets the followings:(1)The YTF extends for more than 75 km from Dashagou,Fengzhen,Inner Mongolia in the west to Yiqingpo,Tianzhen,Shanxi Province in the east.The overall strike of the fault is N78°E,but the strike gradually changes from ENE to NE,then to WNW from the west to the east,with dips ranging from 30° to 80°,most are between 40° and 70°.In most cases,the YTF lies in the contact zone between the bedrock mountain and the sediments in the basin,but the fault grows into the basin where the fault geometry is irregular.The YTF can be divided into the following four segments according to the variations in fault geometry like changes in strikes and stepover,named the Dashagou-Shoukoubu segment,Xiaolongwangmiao-Baiyangkou segment,Baiyangkou-Hujiawa segment,and the Yuanjialiang-Yiqingpo segment from the west to the east.The 58km-long Xiaolongwangmiao-Yiqingpo section constitutes the northern margin of YTB,which is traditionally known as the Yanggao-Tianzhen Basin North Margin Fault.(2)The YTF is dominated by normal faulting,but its sense of motion changes along its strike.The NEE and NE-striking segments are mainly normal dip-slip faults,while the NWW-striking segment act as a normal fault with a minor sinistral strike-slip component.The sense of motion of the YTF determined by geologic and geomorphic evidence is consistent with the relationship between the regional NNW-SSE extension regime and the fault geometry.(3)By measuring and dating the displaced geologic markers and geomorphic surfaces utilizing UAV photography and OSL dating at five sites with well-developed displaced landforms and simple fault geometry along the YTF,the fault slip rates were estimated to be 0.1-0.3 mm/a over the late Pleistocene.To compare the slip rate determined by the geological method with the extension rate constrained by geodetic measurement,the vertical slip rates were converted into horizontal slip rates using the dips of the fault planes measured in the field.Assuming that the fault dips measured at the surface are representative of the depth-averaged fault dip,the corresponding extensional rate of 0.1-0.2 mm/a roughly accounts for 10-20 % of the total crustal deformation rate of the northern Shanxi Graben.(4)The cumulative displacement and slip rate pattern of the YTF indicates that the fault activity is strongest approximately at the fault center and taper towards the fault tips.Combining with the footwall topography of the YTF,it is believed that the central part of the YTF may be the locus of initial deformation during the opening of the northern SG and the fault likely propagated from its fault center to its two sides since its formation.Evidence such as the spatial variation in fault activity,the footwall topography and the sense of motion of the YTF support more the previously proposed "Double Direction Shear" model for the formation of the JJMBRP,that is,the formation of the JJMBRP was jointly controlled by both the right-lateral shear from the Shanxi Graben and the left-lateral shear from the Zhangjiakou-Bohai Fault Zone.(5)The faulted geomorphic units such as the late Pleistocene alluvial fans and the Holocene sedimentary strata indicate that the YTF is a late Pleistocene to Holocene active fault.The fresh small fault scarps developed in the northwest of the Yanjiabu Village and the east of the Xijiagou Village suggest that the fault has been active in the late Holocene.According to the empirical formula between maximum displacement and magnitude for normal fault,the magnitudes corresponding to the two surface ruptures are estimated to be Mw6.8 and Mw6.75,respectively.In views of these two places are far away from the fault center,the actual magnitude may be slightly larger than the estimated value.Besides,the two surface ruptures are relatively fresh,so they should be related to the same surface-rupturing event which ruptured the whole Yanggao-Tianzhen Basin North Margin Fault.Using the empirical formula between the magnitude and the rupture length,the magnitude of this event is estimated to be Mw7.1,which indicates that the YTF had generated an M7 earthquake in the late Holocene and the YTF is capable of producing M ?7 earthquakes.(6)Four trench profiles were obtained after cleaning the natural fault outcrops in the Xiaolongwangmiao-Baiyangkou segment and the Baiyangkou-Hujiawa segment.Two paleo-seismic events have been interpreted in both the trenches at Zhuanlou Village and Xuesandun Village,and the OSL samples used to determine the time of these events are still being tested.Five events have been recorded in Yanjiabu Village Trench,among which the latest one produced the surface rupture mentioned above.Two surface rupturing events were interpreted in Pingshan Village trench,of which the latest one occurred between B.P.4500a-2280 a.(7)The Huashan Basin Fault is located to the east of YTF,which is connected with the latter near Xunyaokou Village.The overall trend of the two faults tends to be consistent.The former is a late Pleistocene active fault,which has been active since B.P.44 ka but was inactive in Holocene.