Movimenti gravitativi diffusi e ripetuti nel versante sinistro della Val Cenischia (Alpi Graie) e loro relazione con il sistema di fratture Cenischia-Nizza

Repeated, extensive gravitational movements in the left slope of the Val Cenischia (Alpi Graie) and their relations to the Cenischia-Nice fracture system

Abstract

Among the current studies on the relations between mass movements and recent tectonic evolution corresponding to both differential areal uplifting of the Alps and movements along disjunctive structures the Val Cenischia, in which the important Cenischia-Nice fracture system is particularly evident has been analysed. Its left flank (prevalently modelled in the «calcescisti con pietre verdi» complex) displays numerous landslide accumulations derived from a succession of detachment niches. They cover an area of about 13 km² and constitute almost the whole slope rising from the valley floor (650-950 m) to 2600 m. Individual accumulations are of considerable size (up to 3.5 km²) and visible thickness (from several tens of metres to a hundred metres). Most of them are composite and hence the outcome of several episodes. Their component bodies are usually «landslide fans» with a varying average slope. They are often separated from the much steeper uphill flank by a change in the slope angle. Undulations can be seen inside these bodies, usually at right angles to their direction of movement. A sector with a gentler slope is generally present on the upper edge. Their original shape has been variously maintained, since they have been more or less deeply channeled by the stream net and remodelled more or less markedly by runoff; the importance of this remodelling varies in function of both the age of a single accumulation and its component portions. Although they are predominantly covered with vegetation, local remobilisation is evident in many places. The deposit consists of a mixture of angular fragments measuring from a few cm3 to about 1000 m³, formed solely of the lithotypes outcropping in the uphill slope, and various amounts of a usually loose, sandy-silty matrix. There are local heaps of rocks larger than some tens of thousands of cubic metres, which appear to be disjointed. The detachment niches are usually situated near the water-shed crest (between 2400 and 3200 m) and have been distinctly remodelled. They are the source of the heavy falls of debris that formed a continuous cover on the slope that has developed above the accumulations. Particular attention has been to reconstruction (where possible) of the chronological relations between the accumulations themselves, between their component bodies and between the accumulations and the recent forms of other kinds. The most ancient bodies are deeply dissected and remodelled, and are related to the terraced relies of the glacial valley floor preserved on the right flank of the valley, tentatively datable to the last glaciation: their age is the same or more probably immediately subsequent and hence belong to the very early Holocene (if not still to the Würm). The more recent bodies have much more clearly kept their original shape and can be related to the present valley floor: they have thus been formed during the Holocene and up the present time. Lastly it has been possible to establish a genetic relation between the gravitational phenomena taken into consideration and, on the une hand, the presence of the Cenischia-Nice system of fractures, and, on the other, the recent uplifting of the sector of the area of the Alps, in which Val Cenischia is included. Both the system taken into consideration, with which the intense state of fracturing of the rocks is connected, and the documented strong recent uplifting, responsible for the materialization of high relief, are among the predisposing causes of the gravitational phenomena, together essentially with the lithology and the attitude; the recent evolution of the system of fractures, furthermore, causing distensive movements within the flank parallel to its elongation, also figures among the determining causes.