Geological map of the Mount Ciantiplagna rock avalanche (Chisone Valley, Italian Western Alps)

Abstract

The 1:10,000 scale geological map of the Mount Ciantiplagna rock avalanche, located in the upper Chisone Valley, encompasses an area of 21.4 km2 affected by the most extensive rock slope failure identified in the Italian Western Alps. The landslide, originated at 2605 m a.s.l., involved massive calcschist with subordinate intercalations of serpentinite and metabasite of the Cerogne-Ciantiplagna Unit (Piedmont Zone, Pennidic Domain). The blocky accumulation covers an area of 3.88 km2 with a corresponding volume of 157 × 106 m3. The rock avalanche traveled a horizontal distance of 4.6 km with a vertical drop of 1460 m and stopped at 1150 m a.s.l. against a transverse moraine situated at the mouth of the Rio del Laux Valley. The landslide deposits created a cross-valley barrier with the development of a large fluviolacustrine basin extending 4 km upstream and with a maximum depth of 120 m. Detailed field mapping and numerous drillholes highlighted that the rock avalanche deposits rest above an earlier rock slide that involved a huge ophiolitic mass of serpentinite and metabasite. In the valley bottom the original surface of the rock avalanche accumulation is not preserved. Several fluvial terraces suspended up to 120 m above the Chisone river have been interpreted as the product of erosion by the stream network at the expense of the blocky deposit. Destabilization of the Mount Ciantiplagna southern slope due to the emplacement of the rockslide can be reasonably invoked as the most likely triggering factor. Extreme rainfall and the seismic activity (I o = VII–VIII) that affects the central sector of the Italian Western Alps can be regarded as further potential triggers. The precise age of the rock avalanche is not known: nevertheless the overlapping of the distal portion of the accumulation over late-glacial moraines, the presence of some late Copper Age archaeological sites close and within the landslide perimeter and the deep dissection of the blocky deposit are in agreement with an early post-glacial age. The geological map presented here provides detailed morphological and stratigraphical information allowing the reinterpretation of the kinematics of the Mount Ciantiplagna landslide.