Structural and kinematic analyses of the basement window within the hinterland fold-and-thrust belt of the Zagros orogen, Iran

Abstract

-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 folds. Mylonitic nappes of the basement were exhumed along deep-seated sole thrusts of the Zagros Thrust System. Lattice preferred orientation (LPO) c-axes of quartz show asymmetric type-1 crossed girdles that demonstrate a non-coaxial deformation under plane strain conditions. Based on the opening angles of quartz c-axis fabric skeletons, deformation temperatures vary from 425 ± 50°C to 540 ± 50°C, indicating amphibolite facies conditions. The estimated mean kinematic vorticity evaluated from quartz c-axis of the quartzo-feldspathic mylonites (W m = 0.55 ± 0.06) indicates the degree of noncoaxiality during mylonite exhumation. The estimated angle θ between the maximum instantaneous strain axis (ISA 1 ) and the transpressional zone boundary is 17°, and the angle of oblique convergence is 57°in the M 2 nappe of the basement involved. This indicates that the mylonitic nappe was formed by a combination of 62 % pure shear and 38 % simple shear during oblique convergence.