| Jiali fault zone is a large strike slip fault located in the southeast of Qinghai Tibet Plateau and also a fault which locates in the easternmost end of Karakoram Jiali fault zone.It is an important regulating fault for block movement after the collision between Indian plate and Eurasian plate.With a total length of over 500 km,the fault extends from the interior of the flat plateau to the SE direction,passes through the front edge of the Namjagbarwa tectonic syntax,and then turns to the SSE trend,extending out of Tibet,spanning three main types of geomorphic units.The systematic and in-depth study on the activity of Jiali fault zone in the late Quaternary is helpful to understand the deformation model of the present Qinghai Tibet Plateau,has certain guiding significance for understanding the history of the uplift of the Qinghai Tibet Plateau,and is also the key tectonic fault to test the rigid block model and continuous deformation model.In this paper,through Google Earth image interpretation,DEM analysis of Aster 30 m resolution and DEM analysis and processing of high-precision UAV aerial survey,combined with detailed field survey,the geometric distribution of fault is determined,and more detailed segmentation is carried out on the basis of previous studies.Based on the field survey,UAV high-precision DEM survey and profile drawing,the strike slip and normal fault values of Jiali fault were obtained,and the profile cleaning,OSL and 14 C sample dating were carried out.The late Quaternary activity of Jiali fault was studied,and the following conclusions were obtained:(1)The Jiali fault zone extends out of Tibet from the interior of the plateau with NW-NWW strike,bypassing the east syntax.From remote sensing images,it can be clearly seen that the Jiali fault passes through three geomorphic units,namely,the interior of the flat plateau,the transition zone of U-type Valley zone,and the V-type Canyon zone.Based on the analysis of the differences of geomorphology and activity in different segments of the fault zone,the basic segmentation principles are determined,that is,the Naqu-Lindi segment,the Xiama-Jiali segment and the Jiali-Chayu segment;According to the previous geological work and geodetic survey results,Naqu-Lindi section and Xiama-Jiali section are characterized by tension and torsion.Jiali-Tongmai section is in the northern margin of the eastern tectonic boundary,which is characterized by compression and torsion under the influence of continuous northward movement.Tongmai-Chayu section is speculated to be characterized by dextral strike slip,and the geomorphic characteristics also reflect the property transformation.(2)The Naqu-Lindi section of the NW Jiali fault strikes NW-SE,dips NE,with a dip angle > 60 °,The total length of the fault is nearly 55 km,and the linear characteristics are obvious on the satellite images.There are obvious dislocations in the gullies along the line.A series of reverse scarps and beaded swamps can be seen along the line,indicating that the fault has obvious normal fault characteristics besides obvious dextral strike slip movement.In the field investigation,the clear and identifiable surface fracture zone is from Kejia village to Kemaniya,with a length of over 40 km;At the scarp of Kejia village,the swamp width and slope gradient were measured.By assuming the possible value of fault dip angle,the slip amount of fault dip was calculated to be between 31.7 m and 41.7 m,and the OSL sample age was 24.4 ± 0.5ka,and the dip slip rate of 1.30-1.71mm/a is obtained;Through the restoration of the terraces,the dislocations of 110 m second terrace and 290 m third terrace are obtained.According to the OSL dating results collected by predecessors on the third terrace,the strike slip rate of Naqu fault is about 6.1mm/a.(3)The length of the surface fracture zone is 40 km from Kejia village to Kemaniya.There are many turf tears along the line.According to the crescent tear structure,the amount of single horizontal dislocation is 0.6m,and the amount of vertical dislocation is less than 0.5m.In addition,a 500 m long tensile fracture is developed in the west of Sangdi basin at the east end of the fracture,without obvious horizontal and vertical dislocations.The maximum width of the fracture is 10 cm,According to the recovery period of grassland destruction in the plateau is about 300 years,it is inferred that the earthquake occurred about 200 years ago.According to the relationship between rupture length and magnitude,the magnitude is estimated to be about Ms 6.5;(4)Two small triangular extensional basins are developed at both ends of Naqu fault.The fault trace between the basins is the clearest and the surface fracture zone is mainly in this section.After extending out of the basin,the activity is obviously weakened.The profile survey of the basin shows that the terrain of Kemaniya basin is higher in the northwest and lower in the southeast,which is located in the northwest of the western end of the fault;The terrain of Sangdi basin on the southeast side of the eastern end of the fault decreases slowly from southeast to northwest,while steep cliffs are formed on the west side of the basin.It is speculated that the basin controlling normal fault is developed.Therefore,the kinematic model of Naqu fault is proposed.The Qiangtang block on the north side of the fault moves eastward rapidly,and the fault mainly shows dextral movement.The difference of activity rate is caused by the large amount of strike slip absorbed by the extension of the basins on both sides.(5)After field investigation,the Xiama Jiali fault is subdivided into NiangyaPaduo segment,Quelong segment and Xiama segment from east to west.The fault strike of Niangya-Paduo member is 279-291 °,with a length of 26 km,extending from Niangya base in the southwest of Jiali county to the direction of NWW to Paduo village,with obvious linear characteristics.According to the interpretation of satellite images,its western extension seems to be connected with the Dangxiong-Jiali shear zone;Fault strike 302° in quelong section,Its length is about 12 km.Because of the strong activity and abnormal fault tendency,it is supposed to be a regulating fault in the dextral strike slip step area;Fault strike of Xiama section is 275-280 ° and the length of the fault is about 30 km.UAV aerial survey and profile cleaning work have been carried out on the north and south sides of the Xiama basin.Comprehensive analysis shows that the fault on the south edge of the Xiama basin is the main fault,and the fault on the north side should be the passive branch fault.The scarps inclined to NNE are developed on the three faults,so it is inferred that the faults are dextral strike slip and normal faults.The amount of bedrock dislocation on Niangya-Paduo fault reaches 15 m.Because the elevation of bedrock scarp is consistent with that of regional third-order terrace,the age of normal fault activity is taken as the maximum age of formation of third-order terrace,and the OSL dating result of third-order terrace is 38 ± 6 Ka,and a dip slip rate of about 0.4 mm / A is determined.The gully dislocation on the fourth terrace,9km west of Jiali County,reveals the strike slip characteristics of the fault.The maximum dislocation of the gully is 96 m,and the previous dating result of the fourth terrace in this area is 47.5m ± 4ka,and the strike slip rate is about 2 mm / a.(6)According to the above research results,it shows that the Naqu-Jiali section is uplifted in the South and descended in the north.Combined with the geophysical and GPS research results,a geodynamic model of this section is established: while the Indian block continues to move northeastward,the uplift of Lhasa block in the south of the fault is significantly higher than that of Qiangtang block in the north,resulting in the rapid eastward movement of the north side of the fault.There is also a significant northward dip slip movement.Geological work shows that the strike slip faults in the vast area south of Bangong-Nujiang suture are normal faults;The results of three-dimensional GPS survey on the Qinghai Tibet Plateau reveal the uplift and subsidence characteristics of different regions on the plateau,and confirm the present normal fault characteristics of Jiali fault;The extensional characteristics of Jiali fault may come from the compression and uplift of deep crust and the uneven subsidence of upper crust caused by the existence of weak layer in the middle.Geophysical data show that the Indian crust near 92 ° E subducted and compressed the lower crust of the plateau at a high angle,which may cause the uplift of the lower crust of Lhasa terrane;The existence of horizontal reflector in the crust may represent the existence of weak layer in the middle crust. |
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