| Northern margin of the Western Qinling Fault Zone is an important tectonic boundary fault zone in the northeast of the Tibetan Plateau.Quantitative research on this fault is helpful to deepen the scientific understanding of the fault quaternary activity behavior and reasonably evaluate the seismic risk of the fault.In this paper,the Huangxianggou fault segment in the middle fault of the northern margin of the West Qinling Mountains is selected as the research object.In-depth research and quantitative analysis of the dislocation geomorphology along the fault line and the characteristics of the Late Quaternary tectonic activity are mainly carried out.Human-machine lidar measurement,field survey,fault geomorphology measurement,field sample collection and testing,etc.,the following conclusions were obtained:1.The Huangxianggou section of the northern margin of the Western Qinling fault zone starts from Zhangxian in the east,passes Chechanggou to the west,and ends at the west bank of Taohe River.a total length of about 70 km,towards NWW.It is the main active fault section in the middle of the northern Qinling fault zone.The tendency is different.Since the late Quaternary,it has changed from the thrust to the left-lateral strike-slip feature.The new activity characteristics of this fault are more obvious than other secondary faults.The eastern end of the fault is the pull-apart basin-Zhangxian basin,and the western end is the Lianlu basin formed by the discontinuous step area.2.The tectonic activity of the Huangxianggou fault since the late Pleistocene has formed rich tectonic landforms,which are characterized by fault trenches,the sagponds,fault scarp,alluvial fan lateral stacks,gullies,and left-lateral dislocations in mountains,and A typical fractured valley landform is formed in the main section of Huangxianggou.In this study,the high-resolution terrain data of the Huangxianggou section was obtained by using the LiDAR technology on the UAV,and the displacements of different levels of topography of the main section of Huangxianggou were interpreted and measured in detail.The maximum displacement value was 65.5±0.6 m,the minimum displacement value is 6.2±0.5m.According to the spatial distribution of dislocation values and the cumulative characteristics of the amount of dislocations,the fractures show obvious characteristics of grouping activity.3.Based on the data obtained by UAV LiDAR,the use of Ladicaoz_v2.1 measurement can effectively improve the accuracy and reliability of displacement measurement.Compared with field measurement,software measurement has the following advantages:(1)Can repeatedly measure the dislocation landform;(2)Can better grasp the overall characteristics of the offset landform;(3)Not affected by vegetation coverage,no measurement conditions and other factors;(4)Clearly identify the faulted landform in the fault Projection position on the profile;(5)Reduce the influence of subjective factors of the surveyor;(6)Rebuild the original shape of the faulty landform before the earthquake through the "back-slip" of the amount of dislocation,and visually evaluate the quality of reconstruction.The disadvantage is that it is limited by the accuracy of DEM,and the displacement value needs to be measured as many times as possible to reduce the error range.4.The characteristics of new activities in the Huangxianggou section since the Holocene are obvious.Through the UAV LiDAR measurement and the dating of the gully terrace,the slip rate of the Huangxianggou section was accurately determined.In Huangxiangtai,the second-order dislocation of alluvial and alluvial products is obvious.Through dislocation measurement and age limitation,the upper limit of the horizontal sliding rate of fracture is 3.63±0.07 mm / a.In Yangjia Dazhuang,the diluvial upland is dislocated,and the lower limit of the horizontal sliding rate of the fault is 1.85 ± 0.11 mm /a through dislocation measurement and age limitation.Finally,the average horizontal strike-slip rate since the late Quaternary of the Huangxianggou fault segment was 2.74 ±0.09mm/a.The upper limit of the vertical sliding velocity at Huangxiangtai is 0.64±0.08 mm / a.The lower limit of the vertical sliding velocity at the luanchaigou point was 0.13±0.06mm/a,and the average vertical strike-slip rate was 0.38±0.07 mm /a.The results are roughly the same as the slip rates determined by the predecessors using gully dislocations and faulted pond sediments,which can be complemented with each other.5.According to historical earthquake research,a moderately destructive earth-quake occurred in Lintao South in 1765.The polar earthquake zone is located in the Liulin and Zongshi areas near the Lianlu basin,with a magnitude of about 5?-6 and an epicenter intensity of up to ?.According to field investigations,there is a new geological and geomorphic phenomenon of faults near the polar earthquake area,and the topography has retained the topography marks such as fault scarp and left-handed dislocations of gullies.The left pull-apart zone between the Huangxianggou fault segment is caused by the stress concentration of the fault zone at the intersection and end point caused by the regional stress.6.The western segment of the Huangxianggou fault and the Guomatan fault form the left pull-apart area—the Lianlu basin.When the TaoHe River crosses this area,T1-T6 terraces are developed,and the cumulative horizontal fault of the Tao river is about 3 ± 0.3km.According to the formation time of the modern river valley in the lower Tao He River(1.4-0.97 Ma)and the estimated start time of the Tao River strike-slip(1.09-1.1 Ma),it shows that the early period of the TaoHe River passing through the Haidian Gorge and the downstream section was affected Due to the influence of left-lateral strike-slip activity in the fault zone on the northern margin of the Qinling Mountains,TaoHe began to shift,and the cumulative dislocation of 3± 0.3km represents the largest cumulative dislocation since left-lateral strike-slip occurred in the fault. |
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