New Constraints on Deformation History of the Zagros Hinterland: Evidences from Calcite Twin Morphology and Geothermometery in Sargaz Complex, Sanandaj-Sirjan Zone, SE Iran

In order to investigate deformation history preserved in the calcite twins in the hinterland of Zagros, several samples have been collected from calcschists of the Paleozoic Sargaz complex in the Faryab area, SE Iran. These samples are related to F1-recumbent folding and thrust shear zones. The twin width is about 1 and 3.8 µm (thin-twin regime); the twin strain amounts 5 and 10%; twin intensity is about 24.8 and 72.2 twins.mm-1. Based on the upper crust frictional stress equilibrium hypothesis, calculated differential stress values, of about 200 MPa, suggest that twinning occurred at depth about 7-8 ± 1 Km. Together with twin morphology which indicates that twinning developed under low T conditions (between 160 and 230°C), this suggest that twinning occurred at lower temperature and shallower depth than the reported greenschist facies. Calcite C-axis fabrics therefore show a low - temperature, post-tectonic (post F1-folding) pattern. The results of Calcite C-axis fabrics reveal that the compressional stress axes are oriented NE-SW. Based on the age of samples together with timing of twinning relative to F1-folding and thrusting, this regional compressional stress is likely related to oblique subduction of NeoTethys beneath Central Iran in Middle Triassic. All of results support that twinning developed as a late, post-metamorphism deformational event and reveal that the Paleozoic Sargaz complex is jacked-up by underplating process toward the lower-temperatures and shallower depths during the oblique subduction of Neo-Tethys beneath Central Iran.