Calcite twinning constraints on paleostress patterns and tectonic evolution of the Zagros hinterland: the Sargaz complex, Sanandaj–Sirjan zone, SE Iran

Abstract

Based on analyses of calcite twins, we constrain the tectonic history of the Paleozoic Sargaz complex within the SE part of the Sanandaj–Sirjan zone (hinterland domain of the Zagros orogen), SE Iran. The mean width of measured calcite twins was 1.97 μm, corresponding to the width of type II twins; variations in twin width with twin density indicate that calcite twinning in the study area occurred at temperatures of between 170 and 200°C. These results support the interpretation that the twins developed at a shallower depth and lower temperature than those of greenschist facies metamorphism recorded in this complex, and that twinning is therefore mainly a late, post-metamorphic deformation process. The c-axis fabrics of the studied samples are monoclinic, consisting of an intense point maximum located slightly anticlockwise of the normal to the shear plane; this asymmetry indicates non-coaxial deformation and a dextral component of shear in the thrust zones. The geometric relationship between stress axes and bedding reveals that the reconstructed stress tensors mainly post-date F1-folding. Compressional stress axes are oriented NE–SW. This compressional stress was probably related to the (oblique) subduction of the Neotethys beneath Central Iran by Middle Triassic–Jurassic times, during the Cimmerian orogeny.