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

Abstract

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 (D₁ stage, 94–85 Ma). Since the collision of the Indian and Eurasian plates, principal compressive stress (σ1) in the north–south direction was generated in the Yeba Group and formed composite folds (D₂ stage, ~ 50 Ma). The D₃-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.

Appendix

Rigid grain net (RGN) is a graphical vorticity calculation method based on the shape and direction orientation of porphyroclasts representing rigid rotated objects in a flowing matrix. According to its shape factor (B*) and the angle between the long axis and the foliation (θ), each porphyroclast is drawn on the RGN. The shape factor (B*) used here is defined by Bretherton (1962) and applied by Passchier (1987), as presented by Jessup et al. (2007):

$$B* \, = \, \left( {{{{\text{Mx}}^{{2}} {-}{\text{ Mn}}^{{2}} } \mathord{\left/ {\vphantom {{{\text{Mx}}^{{2}} {-}{\text{ Mn}}^{{2}} } {\left( {{\text{Mx}}^{{2}} + {\text{ Mn}}^{{2}} } \right)}}} \right. \kern-\nulldelimiterspace} {\left( {{\text{Mx}}^{{2}} + {\text{ Mn}}^{{2}} } \right)}}} \right),$$

where Mx and Mn are the lengths of the long and short axes of the porphyroclasts, respectively. For the RGN, positive and negative semihyperbolas are plotted at 0.025 increments of Wm (Jessup et al. 2007). The maximum B*, as the critical threshold (Rc) between a freely rotated grain and a grain in a stable sinking position, is defined by the transition from semihyperbolas to vertical lines. Based on the position where the critical shape factor (B*) is equal to the aspect ratio (R; R = Mx/Mn) of the rotated porphyroclasts at the critical threshold (Rc), the average kinematic vorticity (Wm) is directly estimated from the RGN. Therefore, the kinematic vorticity of each porphyroclast is equal to its critical threshold.