Research On The Active Characteristics Of The Daluo Fault Since The Late Quaternary In Southwestern Region Of Yunnan Province

The Daluo Fault in Southwest Yunnan is located in the southeastern of the Tibet Plateau,which records the latest active tectonic information of lateral extrusion of the Tibet Plateau.Based on satellite image analysis,on-site tracking investigation,trench excavation and,dating sample collection and testing,as well as large scale micro-geomorphology surveying of fault dislocation,the study extract the geometric structure of the Daluo Fault,and analyze the characteristics of its activity since the Late Quaternary,furthermore the structural transformation relationship between the Daluo Fault and the Longling-Lancang Fault zone a.According to the study contents,the conclusions are as following:(1)The Daluo Fault in southwestern Yunnan is a Holocene sinistral strike-slip fault with the length of 150 km,and its trend direction is N70°E and have a steep dip angle,the general dipping direction is SE when NW partial.It originates from the northern margin of Keng Tung Basin in Myanmar in the southwest and enters Yunnan Province through Mengla in the Northeast direction,then extends to Jinghong Basin through Daluo,Mengban,Menghun,Gelanghe.The Daluo Fault is distributed obliquely or clustered by a number of branch faults.(2)The Daluo Fault has been relatively active since the late Quaternary,and new activities have formed abundant tectonic landforms.The manifestations are: fault trench,fault saddle,fault ridge,bedrock cliff,fault ridge,fault depression pit,linear ridge(thin ridge extending linearly along fault strike)and left-lateral gully.The fault dislocation of ridges,water systems and platform vary from several meters to more than ten kilometers.According to 1:200,000 regional geological map,the faults leftlateral disrupted the late Hercynian granite and the giant quartz vein,with horizontal dislocations of(11.9±0.3)km and(10.6±0.8)km,respectively.Meanwhile,the faults also disrupted the Nanleihe River and Nanlanhe River,the second tributaries of the upper Mekong River,with horizontal dislocations of(11.3±0.3)km and(10.8±0.6)km,they represent the largest accumulated dislocation since the sinistral strike-slip of the fault.There are abundant left-lateral dislocation fault landforms along the fault.The profiles of exploration trenches also reveal the latest Holocene and modern sediments,indicating that the fault has been active since the late Quaternary.Jinghong and Keng Tung pull-apart basins are located at both ends of the fault,along which the development of Daluo,Mengpan,Mengman,Granghe and other Quaternary basins are controlled.(3)The Daluo fault has been sliding since the Holocene.Through the study of several fault profiles,we can conclude that the last active time of the Daluo fault is(482—850)a B.P.Based on the study of typical deformed terrace and landforms,the slip rate of Daluo fault is determined.In the north of Papencun,there are obvious sinistral dislocations on alluvial terraces.Through the measurement of dislocations and dating samples,the upper limit of horizontal slip rate of faults is(2.5±0.1)mm/a.In Manmapang,the ridge is dislocated by faults.The lower limit of slip rate is(0.8±0.1)mm/a by measuring ridge dislocations and dating samples.If the slip rate of faults remains constant,according to the average maximum dislocation along the faulted geological body about(11.2±0.5)km,it is estimated that the time of structural transformation of strike-slip activity should be(4.4—14.9)Ma B.P.Combining with previous research results,the transition time of Daluo fault from compression to sinistral strike-slip should be(4.4—5.5)Ma,related in the late Miocene Neogene.(4)Three paleoearthquakes have been revealed in this study.Their ages are(482—850)a B.P,(3640±30)—(3680±30)a B.P and(7025—7250)a B.P.It appears a certain periodicity.The recurrence period of earthquakes is about 3000 a.(5)Since the Late Cretaceous,a large-scale clockwise rotation has taken place in the southeastern margin of the Tibet Plateau.Since about 16 Ma B.P,with the continuous northward intrusion of the Indian block to the eastern Himalayan syntaxis,the Sichuan-Yunnan block has escaped to the Southeast fastest,and the fault system in southwestern of Yunnan Province has undergone structural transformation as a whole.The Redriver fault began to dextral strike-slip movement,the NE-trending fault system began to sinistral strike-slip movement,these small block rotated clockwise of bookshelf and accumulated strain energy.In the late Early Pleistocene,the Longling-Lancang Fault Zone began to form.The left-lateral strike-slip of the Daluo fault is the result of the action of the right-lateral strike-slip of the Redriver Fault.The southern boundary of the lateral extrusion of the Tibet Plateau should be the dextral strike-slip system confined by the Longling-Lancang Fault and Redriver Fault,rather than the traditional Redriver Fault.