Geomorphic and structural assessment of active tectonics in NW Alborz

Alborz Mountains is a region of active deformation within Arabia-Eurasia collision zone. The study fault system in western Alborz comprises abundant evidence of active faulting accompanied by occurrence of historical earthquakes. Active tectonics of Manjil-Rudbar fault zone whose movement caused destructive 1990 Manjil-Rudbar earthquake was concentrated in this article through geomorphic and structural analyses. Satellite images were used to improve the visualization of fault traces in order to constrain their geometry considering structural linkage between different fault segments. Faults are supported by field geologic data and kinematic measurements. There are left-lateral strike-slip and oblique reverse movements observed all over the fault zone corresponding to recent fault activities of Rudbar, Manjil, Kelishom and Jirandeh (Kashachal) faults. A more complete catalogue of earthquake focal mechanisms is presented to consider general seismic framework of the region. Fault plane solutions indicate a radial pattern of thrusting in western Alborz. We analyzed drainage offsets and Quaternary alluvial fans along major structures in order to understand precise role of major faults in ongoing deformation processes. Most river offsets along active faults are small (about 100m), while left-lateral displacement of up to 500m is also observed in streams cut along major faults.