Structural and microstructural analyses of the involved-basement along the Zagros Thrust System, western Iran

The Zagros Hinterland Fold-and-Thrust Belt is located in the central portion of the Zagros Thrust System and consists of the exhumed basement windows associated with NW-striking and NE-dipping flexural duplex structures that contain in-sequence thrusting and related folding. Mylonitic nappes of the basement were exhumed along deep-seated sole thrusts of the Zagros Thrust System. Lattice Preferred Orientations (LPOs) c-axis of quartz show asymmetric Type-I crossed girdles that demonstrate a non-coaxial deformation under plane strain conditions. Based on the opening angles of the quartz c-axis fabric skeletons, deformation temperatures vary from 425° ± 50℃ to 540° ± 50℃ which indicate amphibolite facies conditions. The estimated mean kinematic vorticity evaluated from quartz c-axis of the quartzo-feldspathic mylonites (Wm = 0.55 ± 0.06) indicates the degree of non-coaxiality during mylonite exhumation. This indicates that the M2 mylonitic nappe was formed by a combination of 62% pure shear and 38% simple shear during oblique convergence. The estimated angle θ between the maximum instantaneous strain axis (ISA1) and the transpressional zone boundary is 17° and the angle of oblique convergence is 57° in the M2 nappe of the involved-basement. Estimated kinematic vorticity number by PHD and PAR techniques in the M1 mylonitic nappe is 0.68 ± 0.03. Therefore 52% pure-shear component is demonstrated of pure-shear dominated transpressinal zone. The operation of pure-shear dominated transpressional system in the mid-to lower crustal levels is a significant process for developing deep rooted ductile shear zones that involved-basement and exhumation of the high-grade facies to the surface of Earth. The estimated values of dip-parallel elongation and thinning perpendicular to the flow plane are range from 41 to 59.5% and 29 to 37.5% respectively.