Kinematics, strain pattern, and temperature environment of the Yeba shear zone and multistage structural evolution of the Yeba Group

The Yeba ductile shear zone is located on the southern margin of the middle Gangdese magmatic belt and southeast of Lhasa, China. In this paper, we systematically investigate the structural deformation history in the Yeba Group and provide the kinematics, strain pattern, mean vorticity number and temperature environment of the Yeba ductile shear zone. Three tectonic events are identified in the Yeba Group of the study area. The Yeba shear zone represents the late stage of the northward subduction of the Neo-Tethys Ocean (D 1 stage, 94–85 Ma). Since the collision of the Indian and Eurasian plates, principal compressive stress ( σ 1) in the north–south direction was generated in the Yeba Group and formed composite folds (D 2 stage, ~ 50 Ma). The D 3 -stage structural deformation event of the Yeba Group in the study area is represented by the Woka ductile shear zone (22.38–14.6 Ma). Electron backscattering diffraction and chlorite geothermometry demonstrate that upper greenschist facies (213–295 °C) and lower greenschist facies (400–550 °C) conditions developed in the Yeba shear zone. Finite strain measurements of the Yeba ductile shear zone indicate that almost all deformed rocks exhibit an oblate ellipsoid consistent with near-flattening strain. Kinematic vorticity analysis of the Yeba shear zone yielded values (Wm) of 0.49–0.78, indicating a bulk general shear deformation regime with a combination of 58% pure shear and 42% simple shear. The kinematic vorticity number (58% pure shear), flattening strain pattern, and presence of opposite indicators in the same mylonitic foliation support the occurrence of a transpressive structure.