Quartz c-axis fabric development associated with shear deformation along an extensional detachment shear zone: Chapedony Metamorphic Core Complex, Central-East Iranian Microcontinent

摘要

Lattice preferred orientations (LPOs) of quartz were used to establish differences in deformation geometry, finite strain and temperature across an extensional detachment shear zone within the Chapedony Metamorphic Core Complex in the Central-East Iranian Microcontinent along the northern flank of Gondwana. Quartz c-axis data show a continuous evolution across the core complex from asymmetric Type Ⅰ crossed girdles at the southwest margin, to broken, asymmetric Type Ⅰ crossed girdle and single girdle with a large concentration of axes plotted in the center of the stereoplot at the central parts of the core complex and small circle girdle pattern at the northeast margin. These variations in quartz c-axis patterns imply change in strain geometry during deformation from plane strain to general flattening and pure flattening. Integrating analyses of quartz c-axis opening angles, quartz c-axis patterns and recrys-talization regimes of quartz and feldspar suggests deformation temperatures range between less than 400 ℃ and 650 ℃, which yield greenschist to amphibolite facies conditions. Mean kinematic vorticity number (Wm) measured in the mylonite samples ranges between 0.67 and 0.71, which indicates that exhumation of the metamorphic rocks of the CMCC was facilitated by a significant component of pure shear strain within a general shear regime.