Deformation Characteristics And Evolution Periods Of Cenozoic Active Faults In The South Of Rongma Country, Tibet

Since 23Ma,strike-slip faults and normal faults have dominated the plateau.The active structures are different from the north to the south.The south of Tibet is a pure extensional normal fault,while the Qiangtang area is a strike-slip fault and a normal fault.Rongma Country is located in the southern margin of Qiangtang Basin and the northern margin of Bangong Lake-Nujiang suture zone.It is surrounded by Riganpei Co-Yibug-Caka strike-slip fault,Gyaring Co fault and Muga-Purou rift.It is a complex deformation field composed of strike-slip fault and normal fault.Geometric,kinematic and chronological data of faults in the study area are scarce.Previous work neglected the periods of fault activity since Neogene.By means of satellite remote sensing,field geological survey and isotope dating techniques,the active faults in the study area since neotectonics have been systematically studied.Based on Landsat 8 and GF-2 satellite data,three groups of active faults,including strike-slip faults in NEE-SWW and NWW-SEE directions and normal faults in nearly north-south direction,have been interpreted in the study area since Neogene.The results of remote sensing interpretation were validated in the field structural investigation,and detailed geometric and kinematic observations were carried out on the three groups of faults.It is determined that the main strike-slip faults in the left of NEE-SWW and the right strike-slip faults in NWW-SEE are mainly strike-slip faults,and secondary faults are mostly developed.Near N-S trending faults are mainly normal faults,some of which have oblique sliding characteristics,and most of them are grabens.NEE-SWW and NWW-SEE strike-slip faults were active earlier than nearly N-S normal faults,both of which were active in Neogene.The paleotectonic stress field inverted by fault scratches indicates that large near-E W strike-slip faults regulate the shortening of the N-S direction and the extension of the E-W direction,and that the N-S trending grabens represent the extension of the E-W direction.Strike-slip faults cut coarse-grained granite.The minimum age of zircon U-Pb is about 69 Ma.Strike-slip faults cut Cenozoic strata,which limits the starting time of strike-slip faults to Cenozoic.Quanhua dating results show that strike-slip faults have been active since the late Quaternary,with a vertical velocity of about 0.34 mm/yr.Nearly north-south faults are accompanied by hot springs.Regional fault correlation shows that the initiation time of strike-slip faults is 18-7 Ma,and that of normal faults is 6-4 Ma.Fault slip rate inverted by InSAR shows that there is a subsidence rate of 3.5mm/yr-6.5mm/yr in the radar line of sight of the Near-East faults in the study area,which is in good agreement with the results of GPS geodetic observation.Near-N-S normal faults may be caused by deep mantle convection and subsidence,while the eastward extrusion of mantle results in the development of nearly E-W conjugate strike-slip faults.In the past 100 years,there have been many earthquakes with M_S?5 in the adjacent area of the study area.In the field survey,the North-South trending ground fissures and seismic bulges have also been found.