Morphoevolution of the Seymareh River valley in the tectonically active Zagros Mts. (Iran): Predisposing factors and Geomorphic response to the largest landslide on the Earth surface

The Seymareh Landslide, detached 10-9 ka from the 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. Controversial theories have been developed by the scientific community to explain the exceptional nature of the event and different scenarios have been proposed. This work provides constraints to the Seymareh River valley morphoevolution, before and after the giant landslide occurrence, to correctly identify the predisposing factors and the geomorphic response to the slope failure. This kind of research is framed in an already tested multi-modeling approach that includes also contributions from engineering-geological modelling and stress-strain numerical modelling to analyze the time-dependent evolution of valley slopes related to mass rock creep processes. Field activities have been aimed at detailed geological and geomorphological mapping, as well as at sampling of the terraced alluvial and lacustrine deposits of the Seymareh River valley, for OSL dating as geomorphic markers. A suitably constructed, high-resolution DEM allowed detailed geomorphological and morphometric analyses. River longitudinal profiles have been analyzed to find evidence of the transient landscape in response to active tectonics and to the emplacement of the Seymareh landslide. Several orders of alluvial terraces (both older and younger than the Seymareh landslide) as well as a lacustrine terrace (linked to the temporary landslide damming of the Seymareh Valley) have been recognized and projected along the present longitudinal profile of the Seymareh river. The plano-altimetric distribution and the OSL ages of such geomorphic markers, correlated to the detectable knickpoints along the river longitudinal profiles, allowed to constrain the main morpho-evolutionary stages of the valley. Four sectors of the valley have been defined, where different predisposing, preparatory and triggering conditions for massive rock slope failure have been recognized based on the related landforms.