The Study Of The Medog Shear Zone Of The Eastern Himalayan Syntaxis

In this thesis, I present tectonic deformation, metamorphic petrology, rock fabric and isotope chronology for the Medog shear zone, with the aim of constraining the geometry, kinematics and initiation time of the shear zone, as well as the P-T paths of high-grade metamorphic rock in the study area. Then I will compare the Medog shear zone with the Milin-Dongjiu shear zone, which is the western boundary of the eastern Himalyan Syntaxis. Finally the tectonic evolution of the eastern Himalayan Syntaxis will be discussed. The main results are as follows:1. Through the detailed field observations, we find out that the Medog shear zone is complex and shows variations from place to place, depending mainly on its orientation. From south to north, the Medog shear zone can be divided into three segments:Aniqiao-Xirang, Pangxin-Damu and Gandeng-Jialasa. Our research demonstrates that they are a normal shear zone with oblique stretching lineation, a dextral ductile strike-slip shear zone and a right lateral thrust shear zone, respectively.2. Based on the new LA-ICP-MS zircon U-Pb dating for syn-kinematic leucocratic intrusions and40Ar/3Ar ages of muscovite mostly from two micas schists and quartz-mica schists,the initiation time of the Medog shear zone can be well constrained. The geochronology analysis reveals that the Medog shear zone has undergone multistage evolution since India-Asia collision, and its right lateral shearing initiatied no later than～28.6Ma.3. Large amount of high grade metamorphic rocks, e.g. mica schists bearing Ky-Grt and amphibolites, are widely exposed along the Medog shear zone. Thermobaromertric computation shows that these rocks have undergone metamorphism characterized by clockwise P-T paths and nearly isothermal decompression (ITD) with the peak high-presure conditions.4. According to comparisons (geometrics, kinematics and the initiation time) of the two boundaries of the syntaxis, i.e. the Medog shear zone and Dongjiu-Milin shear zone, it suggests that the two shear zones have similar geometry and kinematics: normal slip dominates in the south section, strike slip for the central parts where sinistral in western and dextral in eastern segment, respevtively, and finally thrust slip controls the north section. It indicates that the Namche Barwa metamorphic terrane is controlled by the two shear zones, which moved northward and subducted deeply beneath the Lhasa terrane. On the other hand, the two shear zones started at the same time, and are coeval as the uplifting of Namche Barwa metamorphic terrane.5. Combining with the published data, a three-stage model has been established to explain the formation and evolution of the eastern Himalayan Syntaxis:the subduction-collision stage (65-40Ma), exhumation stage (37-23Ma) and rapid uplift stage (22Ma-present).