Déstabilisation de reliefs, d'érosion en terrain volcanique. Exemples pris dans le Massif Central français. [ Collapse of erosional relief in volcanic terrains. Examples from the Massif Central, France.

Abstract

Volcanic products are generally deposited in topographical lows and are eroded in inverted relief The structures formed have often well defined contacts with their surrounding rocks: e.g. vertical contact (volcanic pipe, cut-and-fill structure), or horizontal contact (lava flows, pyroclastic or mud flow deposition) and thus are potentially unstable and susceptible to landslides and gravitational collapse. The instability can be further emphasized if the structure formed lies on, or next to, a ductile, water saturated substratum (marl, clay, altered pyroclastic deposits), or tectonic (seismic) activity occurs. Evidence of both these analogies are difficult to detect in the resulting deposit. Within the Massif Central, France, many examples are illustrated indicating different scenarios: (1) Slow landslide followed by gravitational collapse (rapid phase and catastrophic deposition), linked to vertical contacts: - vertical wall (contact between two different formations) and clayey bedrock, e.g. Pardines (near Issoire), - volcanic pipe, fractured country rock and ductile underlying substratum, e.g. Dent du Marais (Monts Dore), Châteaugay (Limagne), (2) Simple mass movement with incorporation of volcanic products, favoured by: - clayey substratum, with gentle slope, e.g. Murol (Monts Dore), - clayey substratum, with steep slope, e.g. Montfoulhoux (Limagne), - brittle and altered pyroclastic deposits, e.g. Montagne de la Plate (Monts Dore), - clayey substratum horizontal contact (lava flow - bedrock) triggered by weight of overlying formations and aquiferous horizon, e.g. Gergovie (near Clermont-Ferrand). Debris avalanches associated with landslide events have the same morphological and textural characteristics as volcanic debris avalanches («hummocky» surface) and produce «chaotically mixed» deposits. However, these debris avalanches travel short distances and therefore do not develop matrix dominated faciès, unlike their volcanic debris avalanche counterparts. In the Limagne rift, some volcanic edifices are constructed on a ductile «soap-like-layer» substratum. Evidence of slow moving landslide can be seen, and given their position, close to an urban built-area, observation should be maintained as these landslides could evolve, instantaneously, into debris avalanches.