Stochastic alluvial fan and terrace formation triggered by a high-magnitude Holocene landslide in the Klados Gorge, Crete
Authors: Bruni Elena T. , Ott Richard F. , Picotti Vincenzo , Haghipour Negar , Wegmann Karl W. , Gallen Sean F. 
Published: July 2021

Abstract. Alluvial fan and terrace formation is traditionally interpreted as a
fluvial system response to Quaternary climate oscillations under the
backdrop of slow and steady tectonic activity. However, several recent
studies challenge this conventional wisdom, showing that such landforms can
evolve rapidly as a geomorphic system responds to catastrophic and
stochastic events, like large-magnitude mass wasting. Here, we contribute to
this topic through a detailed field, geochronological, and numerical
modelling investigation of thick (>50 m) alluvial sequences in
the Klados catchment in southwestern Crete, Greece. The Klados River
catchment lies in a Mediterranean climate, is largely floored by carbonate
bedrock, and is characterised by well-preserved alluvial terraces and inset
fans at the river mouth that exceed the volumes of alluvial deposits in
neighbouring catchments of similar size. Previous studies interpreted the
genesis and evolution of these deposits to result from a combination of
Pleistocene sea-level variation and the region's long-term tectonic
activity. We show that the >20 m thick lower fan unit,
previously thought to be late Pleistocene in age, unconformably buries a
paleoshoreline uplifted in the first centuries CE, placing the depositional
age of this unit firmly in the late Holocene. The depositional timing is
supported by seven new radiocarbon dates that indicate middle to late Holocene
ages for the entire fan and terrace sequence. Furthermore, we report new
evidence of a previously unidentified valley-filling landslide deposit that
is locally 100 m above the modern stream elevation, and based on
cross-cutting relationships, it predates the alluvial sequence. Observations
indicate the highly erodible landslide deposit as the source of the alluvial
fill sediment. We identify the likely landslide detachment area as a large
rockfall scar at the steepened head of the catchment. A landslide volume of
9.08×107 m3 is estimated based on volume reconstructions of the
mapped landslide deposit and the inferred scar location. We utilise
landslide runout modelling to validate the hypothesis that a high-magnitude
rockfall would pulverise and send material downstream, filling the valley up
to ∼100 m. This partial liquefaction is required for the
rockfall to form a landslide body of the extent observed in the valley and
is consistent with the sedimentological characteristics of the landslide
deposit. Based on the new age control and the identification of the
landslide deposit, we hypothesise that the rapid post-landslide aggradation
and incision cycles of the alluvial deposits are not linked to long-term
tectonic uplift or climate variations but rather stochastic events such as
mobilisation of sediment in large earthquakes, storm events, or ephemeral
blockage in the valley's narrow reaches. The Klados case study represents a
model environment for how stochastically driven events can mimic
climate-induced sedimentary archives and lead to deposition of thick alluvial
sequences within hundreds to thousands of years, and it illustrates the
ultrasensitivity of mountainous catchments to external perturbations after
catastrophic events.