Analysis Of Deformation Characteristics Of The Brittle-ductile Shear Zone In Zhongba Microterrane Insouthern Tibet

In the western Yarlung Zangbo suture zone,there are widspread outcrops of Zhongba microterrane,in which the Ordovician- Triassic strata were generally deformed and metamorphic. It has been identified that there is a brittle-ductile shear zone striking for NWW in the contact site of Silurian- Permian strata.The brittle-ductile shear zone extends about tens of kilometers, width about 0.2-1kilometers.Formation tends to 190 °-220 °, with tilt angle between 20 °-40 °. The deformation of deformed rocks wans from south to north.Through the investigation on the brittle-ductile shear zone in Zhongba microterrane and microstructural analysis indoor, combining with EBSD analysis and structural analysis methods,we come to know the following aspects initially:(1)The foliation in mylonite within the brittle- ductile shear zone tends to the south.There are stretching lineation, augen porphyroclastic, pressure shadows, asymmetric small folds, book oblique, cleavage refraction and other macro-structure within the brittle- ductile shear zone.Deformed rocks can be seen quartz, calcite wavy extinction,mechanical twinning, ―eye-like‖ structure, kinking under a microscope.Quartz, calcite particles recrystallized obviously, most of which wre dynamic recrystallization.Most of the deformation of the quartz are bulging recrystallisation and subgrain rotation recrystallisation. The static recrystallization of calcite only takes up a very small proportion.(2)In dynamics, we calculate by the fractal method in the brittle- ductile and recieve the result of deformation temperature which range of 330 ℃ ~ 570 ℃,with ancient stress difference between 46.32 ~ 74.78 MPa, and strain rate of between 8.03×10-13s-1 ~1.41×10-11s-1.(3)Through the quartz and calcite EBSD fabric analysis combining with mineral deformation thermometers, we conclude that the typical mineral deformation is divided into three phases: the dextral shear events during high temperature �'sinistral shear activity during low-temperature�' Sinistral shear activity in hypothermiaperiod in Zhongba microterrane brittle- ductile shear zone.The change of the deformation temperature and pressure environment was from high greenschist facies to lower amphibolite facies�'lower greenschist facies metamorphic environment.(4)According to the EBSD analysis and the tectonic setting of the study area,we believe that the formation of the brittle- ductile shear zone was related to the northward subduction of the Tethys Ocean, and experienced the collision between the Indian plate and Eurasian plate and the uplift in Tibetan flateau.which are the three major causes of tectonic activity the shear slip shear zone varying degrees.These three major tectonic events have caused a shear slip of the shear zone to varying degrees.The brittle- ductile shear zone was resulted from the shear layer formed by the strong compression and lateral stretch of the leading edge of the subduction.During the collision of the Indian plate and Eurasian plate,the NE-SW 、near N-S crushing stress was formed by the collision- continental internal succession contractions squeeze.The nature of shear slip shear zone was extruded shear slide at that time.With the uplift of the southern Tibetan Plateau, the brittle- ductile shear zone slided once again.