Application of quartz and calcite microstructures and fluid inclusions on estimation of deformation condition and origin of the Zagros orogen thrust sheets in the Bakhtyari area

Structural analysis of the High Zagros Zone and southwestern rim of the Sanandaj-Sirjan Zone in the Chaharmahal-va-Bakhtyari area resulted in recognition of a thrust system comprises the Ben, Hafshejan, Farsan, Kuhrang and Bazoft thrust sheets from NE to SW. In this study, quartz and calcite microstructures as well as fluid inclusions have been utilized to inspect deformation conditions of the thrust sheets. Microstructural study of quartz grains from the Ben and Hafshejan thrust sheets and their basal thrust zone show evidence of ductile deformation that demonstrate the deformation temperature from 250º to 350ºC for the sheets. Geothermometry study of the sheets using calcite mechanical twins also reveals temperature of 250ºC for the sheets. Fluid inclusions study on quartz veins from these thrust sheets display homogenization temperature up to 220ºC for primary inclusions. This geothermometry survey demonstrates that physical conditions of deformation do not change significantly but increases gradually from the southwestern rim of the Sanandaj-Sirjan Zone toward High Zagros Zone. Based on present data, the area between the Ben and Hafshejan thrust sheets where is considered as the internal part of the Zagros orogen, display characteristics of a transitional zone. Therefore, the Farsan thrust sheet or the Main Zagros Reverse Fault which is known as the margin between the Arabian Plate and Central Iran do not pose this character. The boundary between the Sanandaj-Sirjan Zone as the internal part and the High Zagros Zone as the external part of the Zagros orogene is not abrupt and occurs across a transitional zone comprises a thrust system where basement is also partially involved.