Petrological records of major tectono-magmatic events since Oligocene in the southeastern segment of Xianshuihe fault zone in the eastern margin of Tibetan Plateau

As the boundary between the Bayan Kara and the Sichuan-Yunnan blocks on the eastern Tibetan Plateau, the Xianshuihe left strike-slip fault zone is considered to be the eastern boundary adjusting the large-scale southeastward extrusion of crustal materials beneath the plateau and the rotation of the blocks.The study of the structural deformation characteristics and evolution history of Xianshuihe fault zone is of great scientific significance to deeply understand the continental deformation process in the eastern margin of the Tibetan Plateau.Based on detailed outcrop - microscale structural analysis, the deformation characteristics of the southeast segment of Xianshuihe fault zone have been studied in this paper.Five samples of granitic mylonites and leucogranites were chosen for LA-ICP-MS zircon U-Pb dating, giving ages of 34.4±0.9 Ma, 31.0±1.2 Ma, 15.2±0.3 Ma, 14.9±0.1 Ma and 5.93±0.14 Ma respectively.Two granitic mylonite samples were selected for Ar-Ar dating of muscovite.As a result, the plateau ages of the two samples are 17.21±0.30 Ma and 3.21±0.16 Ma.Based on the previous geochronological data and the results of this paper, three stages of tectonic evolution of Xianshuihe fault zone since Eocene are established.Ⅰ: ca.32 ~ 25 Ma, the deformation of Xianshuihe fault zone concentrated in the deep crust, showing weak magmatism and anatexis; Ⅱ: ca.20 ~ 13 Ma, the peak deformation period of left lateral strike slip of Xianshuihe fault zone, the deformation spread throughout the whole crust, showing a strong left lateral ductile shear deformation and large-scale magmatism.Ⅲ: ca.10 Ma, the deformation is concentrated in the upper crust, which is characterized by block rotation and strong uplift, fault weakening and strong seismicity.The tectonic evolution and seismicity of the Xianshuihe fault zone are constrained by the eastward extrusion of the eastern margin of the Tibetan Plateau since the collision of the Indian Eurasian Plate.