Deformation conditions and quantifying strain in the Chai-Kour shear zone in the Zagros orogenic belt of Iran

Strain analyses and kinematic studies in orogenic belts are essential for understanding their deformation mechanisms. The Chai-Kour brittle-ductile shear zone is located in the central Sanandaj-Sirjan metamorphic belt, southeast of the Zagros orogen. The present study analyzes deformed mylonitic rocks to clarify the deformation kinematics, temperature, and strain geometry of the Chai-Kour shear zone. Previously developed kinematic shear indicators such as S-C shear bands, deformed porphyroclasts, asymmetric boudins, and folds provide an overall top-to-the-southeast sense of shear in the Chai-Kour shear zone. Quartz micro-thermometry confirms that the Chai-Kour rocks were initially affected by a high-temperature (550–650 °C) deformation that was followed by a postsuperimposed low-temperature (380–420 °C) deformation. Rf/ϕ strain measurements reveal a plane-strain deformation geometry. The estimated Wm by the Rigid Grain Net technique ranges from 0.54 to 0.73, suggesting simultaneous contributions of simple (38–54%) and pure (46–62%) shear components. Spatial variation of deformation indicates that the simple shear component increases towards the northeast and southwest boundaries of the shear zone. The present study suggests that this shear zone experienced a 55–75% shortening perpendicular to its boundaries. These deformation characteristics in the Chai-Kour shear zone have been influenced primarily by a transpressive tectonic regime with lateral extrusion, resulting from the continuing oblique convergence of the Afro-Arabian plate with the continental crust of central Iran. This tectonic evolution resembles that experienced in other collisional zones, such as the Alpine and Himalayan orogens.