Heterogeneous ductile deformation and quartz c-axis fabric development within the HP-LT Sanandaj-Sirjan Metamorphic Belt, Iran

Quartz c-axis fabrics, finite strain analysis, and kinematic vorticity number (W K) were used to investigate the heterogeneous nature of ductile deformation along an extruding crustal shear zone — along the HP-LT Sanandaj-Sirjan Metamorphic Belt and Zagros Thrust System in the Zagros Mountains of Iran. Quartz c-axis fabric transitions were studied in the well-exposed quartz mylonites within this belt. Asymmetric quartz c-axis fabrics confirm localized top-to-the-southeast sense of shear. Quantitative data were used to establish differences in deformation history, finite strain and deformation temperature during ductile deformation. Quantitative finite strain (R XZ =2–4.7) and mean kinematic vorticity number (W K =0.76±0.14) analyses demonstrate that both the strain ratio and the simple shear component of deformation increase towards the basal thrust of the Zagros Thrust System (i.e. from northeast to southwest). On moving towards the basal thrust of the Zagros Thrust System, the quartz c-axis fabrics become progressively and gradually change from orthorhombic to monoclinic. The sequential variation in quartz c-axis fabrics is explained in terms of increasing finite strain and vorticity which is compatible with progressive increase of ductile shear strain and dynamic recrystallization during transpressional deformation.