The Study Of Deformation And Metamorphism About The Qingchuan-Pingwu Fault In The Northern Segment Of Longmenshan

The Qingchuan-Pingwu Fault lies at the northern segment of Longmen Shan thrust belt and the south of The Bikou terrane in the SW Qinling-Dabie orogenic belt. In the regional tectonic position, it is located in the junction of Qinling-Dabie orogenic belt, Songpan-Garze orogenic belt and Yangtze Plate. On the basis of previous researches, we investigated the deformation and metamorphism of the study area through regional geological survey, micro-structural analysis, EBSD fabric analysis and mineral chemical analysis and to constrain metamorphic pressure and temperature conditions using conventional geothermobarometers.Three deformational episodes D1, D2and D3of in the Qingchuan-Pingwu Fault are identified on the basis of detailed structural analysis. D1is represented by thrust with sinistral strike-slip during Indosinian. D2has characteristics of extensional deformation with detachment deformation in Yanshanian. There are some micro-structural characteristics in ductile detachment fault, such as bookshelf structure, core-mantle structure, the rotational porphyroclast system, pressure shadow, mineral fish, S-C fabric, dynamic recrystallization and static recrystallization and so on. D3is thrust with dextral strike-slip during Himalaya period. Three deformational episodes are corresponded with three tectonic foliations S1, S2, S3which are indentified on macroscale and microscale. The foliation S1is marked with the tight fold and the spaced crenulation cleavage. It is replaced almost by the foliation S2. The foliation S2is well developed with the penetrative cleavage. The foliation S3is the spaced axial cleavage that dislocates S2foliation. EBSD quartz fabric analysis indicates that the tectonite of the Qingchuan-Pingwu Fault generally experienced a high temperature to low temperature deformation process from D1-D3stage. D2is the main phase deformation, the second is D3and the last is D1.The author found the muscovite of these samples was generally phengite by mineral chemical analysis. The average Si of muscovites in Bikou tectonite is3.37, and the average Si of Maoxian is3.19. So these muscovites are almost phengites. Using the related geothermobarometers, we get the metamorphic temperature and pressure of the tectonite which lies the north of the Qingchuan-Pingwu Fault. The temperature is230℃～260℃, the pressure is0.53-0.83GPa. It indicates that this area has experienced high-pressure metamorphism in D1stage. Use the same method, we get the metamorphic temperature and pressure of the tectonite which lies the south of the Qingchuan-Pingwu Fault, they are300℃～460℃and0.38-0.42GPa. It probably belong to D2stage or D1-D2stageThe Qingchuan-Pingwu Fault is active at229±22ka through ESR dating of fault gouge in D3stage. It shows that The Qingchuan-Pingwu Fault has a strong movement in Neotectonic period.