Active Piedmont Zone Deformation, a manifestation of activity on the ‘Master Ray Fault’; insight into the seismic hazard analysis of the Tehran metropolitan area

Abstract. The piedmont zone in the frontal active regions of the orogenic belts exhibits various deformation patterns, which helps unravel the seismic sources for cities that flourish in such tectonic settings. A detailed analysis of the active folding, faulting, and related morphological features of the Quaternary alluvial units disclose prominent thrust faults in the Tehran piedmont zone. These faults are kinematically related and play a vital role in a better understanding of the seismic hazard of the Tehran metropolitan area. The five south-dipping thrust faults, the related hanging-wall folding and subsidiary faulting accommodate a considerable amount of north-south shortening during Quaternary. The shortening is observed in the alluviums and the underlying Eocene volcanic bedrock. Interestingly, in western Tehran, the Chitgar area discloses a type locality for active fault-bend folding, backthrusting, oblique-slip normal faulting and fault inversion in the piedmont zone. Our optically stimulated luminescence dating on the Late Pleistocene alluviums in the Chitgar area constrains the slip rate of the primary and secondary faults. According to our analyses, we introduce the ‘Master Ray Fault’ as a crucial seismogenic fault of the Tehran region, manifested as south-dipping thrust faults in the piedmont zone. We estimate the minimum slip rate on the Master Ray Fault to be ca. 0.50 mm a^-1. Our study offers a crucial methodological framework for improving the existing understanding on Quaternary thrust fault kinematics and associated morphological features, aiding in the unveiling of potential seismic sources in metropolitan areas located in piedmont zones adjacent to active orogenic belts.