Abstract. The Seymareh landslide, detached ∼10 ka from
the northeastern flank of the Kabir-kuh fold (Zagros Mts., Iran), is
recognized worldwide as the largest rock slope failure (44 Gm3) ever
recorded on the exposed Earth surface. Detailed studies have been performed
that have described the landslide mechanism and different scenarios have
been proposed for explaining the induced landscape changes. The purpose of
this study is to provide still missing time constraints on the evolution of
the Seymareh River valley, before and after the emplacement of the Seymareh
landslide, to highlight the role of geomorphic processes both as
predisposing factors and as response to the landslide debris emplacement. We used optically stimulated luminescence (OSL) to date lacustrine and
fluvial terrace sediments, whose plano-altimetric distribution has been
correlated to the detectable knickpoints along the Seymareh River
longitudinal profile, allowing the reconstruction of the evolutionary model
of the fluvial valley. We infer that the knickpoint migration along the main
river and the erosion wave propagation upstream through the whole drainage
network caused the stress release and the ultimate failure of the rock mass
involved in the landslide. We estimated that the stress release activated a
mass rock creep (MRC) process with gravity-driven deformation processes
occurring over an elapsed time-to-failure value on the order of 102 kyr. We
estimated also that the Seymareh damming lake persisted for ∼3500 years before starting to empty ∼6.6 ka due to lake
overflow. A sedimentation rate of 10 mm yr−1 was estimated for the
lacustrine deposits, which increased up to 17 mm yr−1 during the early
stage of lake emptying due to the increased sediment yield from the lake
tributaries. We calculated an erosion rate of 1.8 cm yr−1 since the
initiation of dam breaching by the Seymareh River, which propagated through the
drainage system up to the landslide source area. The evolutionary model of the Seymareh River valley can provide the
necessary constraints for future stress–strain numerical modeling of the
landslide slope to reproduce the MRC and demonstrate the possible role of
seismic triggering in prematurely terminating the creep-controlled
time-to-failure pathway for such an extremely large case study.
MODELLING OF STRASS-STAIN STATE IN EPICENTRAL ZONE OF STRONG EARTHQUAKE
