Geostructural and geomorphic constraints for landscape evolution modeling and stress-strain numerical analysis of the giant Seymareh landslide (Zagros Mts., Iran)

The Seymareh rock slide-debris avalanche is the largest known subaerial non-volcanic landslide on Earth (44 Gm3), occurred ~10 ka in the Zagros Mountain Range along the NE side of the Kabir Kuh fold (Iran). Because of the giant dimensions and the exceptional nature of the event the landslide was studied by several authors also to identify the triggering mechanisms. The present study is part of that scientific debate and its purpose is to properly describe the geostructural system elements to be triggers for this kind of gravitational instability starting from the reconstruction of the evolutionary and the geotechnical model of the Seymareh river valley before and after the exceptional event. This study is finalized to implement and improve numerical Landscape Evolution Models, an important numerical modelling technique for understanding the coupled tectono-geomorphic evolution of mountain belts that simulate the evolution of the Earth surface in response to different driving forces, such as tectonics, climate and human activity. LEMs encompass empirical data and conceptual models into a set of mathematical equations that can be used to reconstruct or predict terrestrial landscape evolution and corresponding sediment fluxes, as better inputs for numerical, time-dependent stress-strain numerical modeling in slope stability. The study was carried out performing detailed geotechnical and geomorphological surveys and mapping of the Seymareh valley and dating 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. The analysis has leaded to the morphological and evolutionary modeling of five sectors of the valley, by whose study it was possible to synthesize the geostructural triggering elements of such kind of slope instabilities. Finally, the geotechnical model has laid the foundation for future numerical modeling works which explore in more detail the evolution of rock mass creep process of rock slope covered in this work. Grounding on the evolutionary and geotechnical reconstruction of the valley, the following geostructural predisposing elements have been identified: i) stratigraphic setting, ii) structural setting, iii) relief energy, iv) kinematic releases, v) creeping time. 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.