Saeede Keshavarz

Microstructural, finite strain and vorticity analyses of quartz-rich mylonites were used in order to investigate kinematics of rock flow and deformation temperature in the Sirjan thrust sheet exposed in a structural window within the Sanandaj–Sirjan High Pressure – Low Temperature (HP–LT) metamorphic belt that forms part of the hinterland of the Zagros orogenic belt of Iran. A dominant top-to-the-SW sense of shear in the study area is indicated by several shear sense indicators such as asymmetric boudins, rotated porphyroclasts, mica fish and S/C fabrics. Quantitative analyses reveal approximately plane strain deformation conditions with R_xz values ranging from 2.5 to 4.3 and increasing towards the Sirjan thrust. Opening angles of quartz c-axis fabrics and recrystallization regimes suggest deformation temperatures vary from 430 to 625 ± 50°C in the hanging wall rocks. Oblique grain shape and quartz c-axis fabrics were used to estimate the degree of non-coaxiality during deformation. The obtained vorticity profile indicates a down-section increase in kinematic vorticity number (W_m) from 0.6 to 0.89. This range of vorticity numbers confirms contributions of both simple (41–68 %) and pure shear (32–59 %) deformation components. The structural characteristics of the study area ultimately were controlled by oblique motion of the Afro-Arabian plate relative to the Iranian plate.

The Sirjan mylonite nappe is part of the HP-LT Sanandaj-Sirjan Metamorphic Belt (Zagros Orogenic Belt, Iran). This nappe was emplaced in a transpressional regime during oblique convergence between the Afro-Arabian Continent and Central Iranian Microcontinent. In order to investigate the type of flow, fabrics from deformed quartz and K-feldspar porphyroclasts were analyzed. Quantitative finite strain (R XZ = 1.9-3.8) and mean kinematic vorticity number (W m = 0.60-0.85) analyses revealed that both the strain ratio and the simple shear component of deformation increase towards SE along a 2 km long NW-SE-oriented transect. Analysis of quartz and K-feldspar textures suggests shearing at c. 400-650 °C. We also conclude that deformation in the Sirjan mylonite nappe began with nearly pure shear flow but became a more simple-shear flow in character as strain accelerated.