Geomorphology and geochronology of the coseismic? Emad Deh rock avalanche of the Zagros MountainsGeomorphology and geochronology of the coseismic? Emad Deh rock avalanche of the Zagros Mountains

Abstract

<p>This work documents for the first time the prehistoric, but morphologically pristine, Emad Deh rock avalanche, located in the Fars Arc of the Zagros Mountains. The Zagros orogen is one of the most seismically active regions in the world. However, large earthquakes are very rarely accompanied by surface ruptures, restricting the possibility of integrating paleoseismic data in seismic hazard analyses. This limitation could be partially overcome by identifying and dating secondary paleoseismic evidence such as coseismic landslides. The source of the Emad Deh rock avalanche is located on a dip slope at the northern limb of the Gavbast anticlinal ridge. The slope, with a local relief of around 800 m, is essentially a hogback with a thick caprock of the competent Asmari Limestone, dipping around 35 degrees and underlain by marls with interbedded limestones of the Pabdeh Formation. The collapse of a bedrock slab around 2x2 km and its disintegration, resulted in the highly mobile Emad Deh rock avalanche, which accumulated on the unconfined piedmont an unstratified, unsorted and highly angular bouldery gravel deposit with an estimated minimum volume of around 220 million m<sup>3</sup>. The rock avalanche has a maximal height drop (H) of 914 m, a runout measured from the headscarp crown of 9280 m, and a H/L mobility index of 0.09. The cohesionless flow of fragmented rock formed a spatulate-shaped lobe around 32 km<sup>2</sup> in area with well-defined lateral ridges in the proximal sector, expanding over an arc of 40 degrees and reaching a width of 6.1 km. The distal sector of the lobe displays polygonal hummocks that grade into progressively smaller, and locally radially aligned, conical hummocks. In the terminal sector, the rock avalanche was emplaced on soft floodplain alluvium, which displays folding with vergence consistent with the impacting flow direction. Two OSL ages in correct stratigraphic order obtained from the deformed alluvium indicate an age of ca. 5.5 ka for the Emad Deh rock avalanche, attributable to a major paleoearthquake. Subsequently, four additional planar failures occurred in the rock avalanche scar, mainly affecting marls and limestones of the Paddeh Fmormation, and resulting in more cohesive flows. The deposits of these later landslides, with volumes ranging between 10 and 45 million m<sup>3</sup>, are confined by the lateral ridges of the rock avalanche and its scar. Cartographic relationships allow inferring their relative chronology.</p>