Geology, geomorphology and geochronology of the coseismic? Emad Deh rock avalanche associated with a growing anticline and a rising salt diapir, Zagros Mountains, Iran

Abstract

This work documents for the first time the prehistoric, but morphologically pristine, Emad Deh rock avalanche of the Zagros Mountains. The ca. 420 Mm3 rock avalanche was initiated as a rockslide on a dip slope affecting a weak unit of marls with interbedded limestones overlain by a thick and competent limestone formation. The slope is located on the northern limb of the growing Gavbast Anticline and at the edge of the inflating Gavbast Dome, related to a buried salt diapir of Hormuz salt. The unconfined rock avalanche deposits, covering 32 km2, were accumulated on coalescing alluvial fans and a floodplain environment. The depositional lobe shows sectors with distinctive morphological features attributable to the variable flow-depositional behavior of the debris stream, controlled by the nature of the substrate: (1) proximal continuous breccia flanked by levees; (2) intermediate depression; and (3) flat-topped polygonal hills and conical hills. A morphometric and spatial distribution analysis has been performed with the 550 conical hills (hummocks) mapped in the distal sector. The rock avalanche, with 914 m of maximal height drop (H) and 9280 m of runout (L), displays an extraordinarily high mobility (H/L index of 0.09), that can be attributed to the combined effect of dynamic rock fragmentation and basal lubrication by the soft substrate in the floodplain. Relief rejuvenation and slope over-steepening related to the growth of the anticline and the inflation of the Gavbast Dome are considered the main long-term preparatory factors that shifted the slope to a state of marginal stability. The slope failure was likely triggered by a large M ≥ 6.5–7 earthquake at ca. 5.4 ka, as indicated by OSL ages obtained from folded alluvium situated just beneath the rock avalanche deposit. The source of the earthquake was probably a blind reverse fault situated beneath the asymmetric Gavbast Anticline, as support the structural and geomorphic features of the fold. The identification and dating of large coseismic landslides in the Zagros Mountains, where large earthquakes are rarely accompanied by primary surface ruptures, could help to improve both landslide and seismic hazard assessments.