Segnalazione di un gruppo di «paleofrane» nel tratto della Valle d’Ayas compreso tra gli abitati di Lignod ed Extrepiera (Valle d’Aosta)

Report of an assembly of « paleo-landslides » in the section of the Valle d’Ayas between the villages of Lignod and Extrepiera (Valle d’Aosta)

Abstract

The accumulation of ancient landslides extending over an area of more than 3 km2 and with visible thickness of about 100 m, preserved on both sides of the Valle d’Ayas, is reported. Upstream, an extensive lake basin (now filled) caused by subsequent damming up of the valley is clearly visible. Three bodies can be discerned and attributed to three distinct detachment niches. Each body can also be divided into sections corresponding to different landslides episodes. The accumulation is composed of angular elements measuring up to 1000 m³ immersed in a matrix formed of small rock fragments. The lithology is monotonous (prasinites, serpentinites, and subordinate « schistes lustrées ») and varies in function of the differences in origin. The original shape of the accumulation has been prevalently well preserved and the contribution of subsequent events can be distinguished: at one point only, corresponding to T. Evançon, there is an incision of more than 80 m. Avalanche fans are also superposed on the edges. The relations between the accumulation and the surrounding glacial deposits show that the mass movements responsible for its formation are of Holocenic age. The fact that these ancient landslides lie some 3 km N of the Colle di Joux, which is the morphological expression of the « Aosta-Colle della Ranzola » Line at the point where it relates to the Valle d’Ayas, suggests that they may be associated with recent activity of an auxiliary of this structure.