Structural evolution of Ashkhaneh fault zone in northwestern Kopeh-Dagh (NE Iran)

The East-West to Northwest-Southeast trending Ashkhaneh fault zone is located in northeast of Iran between the Alborz and Kopeh-dagh tectonic zones. Geometric and kinematic characteristics of Ashkhaneh fault zone was revealed by measuring of fault kinematic indicators such as: S-C structures, asymmetric folds, slip lines and fault steps along eight cross-sections across this fault zone and perpendicular to main structures. Stereographic analysis of the measured structural elements and constructing cross-sections show a dominant reverse mechanism with left-lateral strike-slip component for the Ashkhaneh fault zone. Donghozdagh Anticline in the hanging wall of Ashkhaneh fault zone is a fault-propagation fold. To the east and in the hanging wall of the fault zone, several thrust faults such as Kuh-e-Docheng fault, North Beruj fault and South Beruj fault have the same geometrical characteristics of the Ashkhaneh fault zone. These thrusts in hanging wall of the Ashkhaneh fault zone with dip to north (same as Ashkhaneh fault) are evidences for of the occurrence of greater convergence in southeast than the northwest part of the Ashkhaneh fault zone. Eight structural cross-sections perpendicular to the Ashkhaneh fault zone show evolution of thrusts and fault-related folds, as well as migration of rock units toward south-southwest in this area. The arrangement of these thrust fault systems emphasis that structural growth took place from north to south-southwest in this part of the Kopeh-dagh.