Quartz OSL dating of displaced landforms constrains slip-rates on the Abr and Khij faults (NE Iran) and their role in Arabia–Eurasia collision zone tectonics

Long-term fault slip rates are fundamental to active tectonic studies, as they provide direct constraints on earthquake recurrence, seismic hazard assessment, and risk mitigation. However, slip-rate estimates in northeastern Iran remain limited due to sparse independent chronological constraints on displaced geomorphic markers. Moreover, few studies have tested whether quartz optically stimulated luminescence (OSL) dating yields slip-rate estimates consistent with terrestrial cosmogenic nuclide (TCN) and geodetic data in arid fluvial settings. The Abr and Khij faults, major strands of the Shahrud fault system in northeastern Iran, accommodate significant left-lateral strike-slip motion within the Arabia–Eurasia collision zone. In this study, we measured cumulative offsets of 331 ± 7 m (Abr) and 193 ± 4 m (Khij) from displaced alluvial-fan and erosional-surface deposits. Quartz OSL dating of four sediment samples—supported by dose recovery tests, decay curve analyses, and equivalent dose distribution modeling—yielded depositional ages of 111.0 ± 8.6 ka (Abr) and 118.4 ± 6.9 ka (Khij), providing maximum estimates for surface formation. Using these data, we calculated slip rates of 3.00 ± 0.24 mm/yr and 1.63 ± 0.11 mm/yr for the Abr and Khij faults, respectively. These rates fall within or slightly below previous estimates from TCN dating (Abr: 3–4 mm/yr; Khij: 1–3 mm/yr), indicating overall methodological agreement while highlighting the importance of refining displacement and age constraints. The complementary use of OSL and TCN dating offers a robust chronological framework that reconciles depositional and exposure histories, thereby refining slip-rate estimates for active faults in northeastern Iran. Together, the Abr and Khij faults accommodate ~4.4 mm/yr of left-lateral slip, consistent with regional geodetic strain estimates from InSAR and GPS. This result supports the active tectonic role of the eastern Alborz Mountains within the Arabia–Eurasia collision zone. The study further demonstrates the reliability of quartz OSL dating in arid, fluvial depositional contexts, where weak luminescence signals, early saturation, partial bleaching, and sediment mixing are common challenges. The measured slip rates underscore the key role of the Abr and Khij faults in accommodating Arabia–Eurasia plate convergence and provide new constraints on strain partitioning across northeastern Iran. Our findings also have important implications for seismic hazard assessment and for refining fault activity rates in other transpressional orogenic settings.