New insights on the predisposing factors and geomorphic response to the largest landslide on emerged earth surface. The Seymareh rock slide - debris avalanche (Zagros Mts., Iran)

The Seymareh landslide, detached ~10 ka from the north-eastern flank of the Kabir-kuh fold (Zagros Mts., Iran), is worldwide recognized as the largest massive rock slope failure (44 Gm3) ever recorded on the emerged Earth surface. Understanding the hazard conditions and the risk associated to this out-of-scale event would provide important pin points for risk mitigation strategies in case of extreme landslide scenarios. Controversial theories have been proposed so far by the scientific community to explain the generation of such an exceptional event and different scenarios have been proposed for explaining the induced changes of landscape. This study provides new constraints to the evolution of the Seymareh river valley, before and after the Seymareh landslide occurrence, to correctly identify the predisposing factors, to suggest possible triggers and deduce the geomorphic response to the slope failure. We performed detailed geological and geomorphological surveys and mapping of the Seymareh valley and dated with optically stimulated luminescence (OSL) two suites of fluvial terraces (one older and one younger than the Seymareh landslide) as well as a lacustrine terrace (formed after the temporary landslide damming), as useful geomorphic markers of the valley evolution. River profile metrics showed the evidence of a transient landscape and the plano-altimetric distribution of the geomorphic markers has been correlated to the detectable knickpoints along the Seymareh river longitudinal profile. We thus provide time constraints to the main evolutionary stages of the valley before and after the emplacement of the landslide, to be used as inputs for future stress-strain time-dependent numerical modelling in the perspective of calibrating the rock mass viscosity and verifying the possible earthquake trigger of the Seymareh landslide as an ultimate scenario of ongoing mass rock creep processes.