Déformations gravitaires post glaciaires en terrains métamorphiques. Comparaison des indices de déformation du versant sud de la Toura (Saint-Christophe-en-Oisans, France) avec le phénomène de rupture interne du versant sud du Mont Sec (Séchilienne, France)

Resumé

Le versant sud du Mont Sec (Séchilienne, France), composé de micaschistes, est l'objet d'un mouvement gravitaire très étudié depuis 1985, puisqu'il représente un risque majeur pour la vallée de la Romanche, en amont de Grenoble. Les nombreuses mesures disponibles ont pu être modélisées par un mécanisme de déformation particulier, sans surface de rupture prédéfinie par la structure géologique.

L'observation géomorphologique et structurale du versant sud, gneissique, de la Toura (Saint-Christophe-en-Oisans, France) a révélé de si nombreuses similitudes avec le cas de Séchilienne que le mécanisme proposé paraît ici aussi adéquat. Le poli glaciaire observé sur les contrepentes permet de faire remonter le début des mouvements aux interstades glaciaires du Würm.

La poursuite des études sur ce site faiblement actif où les conditions d'observation sont excellentes pourra apporter des éclairages utiles pour la maîtrise de mouvements de versants plus sensibles comme celui de Séchilienne. Plus largement, ce phénomène semble pouvoir concerner de nombreux versants des socles métamorphiques alpins.

Abstract

 Introduction: The Sechilienne slope movement, near Grenoble, causes a majour risk and is intensively monitored since 1985. An original deformation mechanism has been proposed to explain the data.

Some morphologicial similarities have been observed upstream on air photographs of the south slope of la Toura, naer Saint-Christophe-en-Oisans (France). The question is: is the same phenomenon occuring there too?

The site of Saint-Chistophe-en-Oisans (France): The site is located in the Vénéon valley, 13 km upstream from Bourg d'Oisans (Fig. 1), under the Pointe de la Toura peak. There is no permanent stream flowing down the slope. The slope is constituted of gneiss of different formations separeted by North-South faults (Fig. 2). The foot of the slope is covered by alluvial deposits. On the slope itself, there are some recent moraines.

The average slope angle is 36° (Fig. 6), which is around 10° lower than the mean value of the valley. The slope is much steeper near the crest, which could correspond to the summit scarp of a slope movement. The middle part of the slope is marked by huge transversal cracks with up-facing scarps, alternating with vertical cliffs. The screes are stopped by the up-facing scarps and underline the cracks on air photographs (Fig. 4).

The geological structure is dominated by (Fig. 5):

– north-south fractures corresponding to the main faults and to the foliation;

– hectometric cracks dipping of 65° in direction NE (N60). They show up-facing scarps and determine tilted-looking rock domains, as seen on the side thanks to the cut made by the "Torrent du Diable" stream;

– two less persistent discontinuity families, one dipping 65° toward SE (N 245) and the other one vertical, in direction N 80.

The opening of the transversal cracks, their upfacing scarps and their curvature in the central part of the slope suggests a tilting of the slices. This phenomenon could date from the end or from low-level periods of the Würm glacial age (80000 to 10000 yr B.P.) The manual monitoring of five cracks shows that the phenomenon is still slowly active: mean opening 2 mm in two years, maximum 5 mm (Fig. 7 and 8).

Tectonic hypothesis (Fig. 9) may explain the genesis of the atypical transversal cracks and will be tested by geochronological studies (fission traces on apatites) and geodetic measurements.

The "Internal Rupture" deformation mechanism: The movement of the south slope of Mont-Sec (Séchilienne, France), constituted by micaschists causes a major risk near Grenoble (rockslide, damming of the Romanche valley, floods...), and is monitored since 1985. The main observations are: a settlement near the crest with opening of local cracks, no localised disorders on foot of the slope, the opening of transversal continuous cracks (with up-facing scarps), and the low inclination of the displacement vectors, whose direction is normal to the continuous cracks. A 240 m long investigation gallery revealed the rotation of rigid blocks (up to 80 m large) with opening or shearing of the contact planes (Fig. 10).

These data do not fit with classical translationnal mechanisms and indicate a strong structural control. The mean direction of the rock anisotropy is not favourable neither to a generalised toppling mechanism. Data have been modelled with two main continuous joint families issued from the simplified geological model (one is vertical and the other steeply dipping toward the valley). The computation with the distinct elements method (code UDEC) reveals two distinct zones (Fig. 11): the upper zone is settling and horizontally expanding by shearing of the two joints families while the lower zone is retaining it with a "block toppling like" flexion deformation.

The simulation reveals a very deep zone of yielding joints (extending down to the valley level), stress accumulations at the foot of each "cliff" of the final topography and important internal stresses inside the blocks in the flexion zone. Combined with the fluctuation of hydraulic conditions, this subcritical state of stress can, at the geological scale of time, induce secondary fracture propagation. The progressive localisation of this time-defered non reversible damaging phenomenon can lead to global failure of the slope on a non predetermined rupture surface.

Conclusion: The morphological and structural observation of the gneissic south slope of la Toura (Saint-Christophe-en-Oisans, France) has revealed so many similarities with the Séchilienne case that the proposed mechanism appears to fit here too. Pursuing the studies on this lowly active site where the observation conditions are excellent will help understanding and controlling more dangerous sites like Sechilienne. Moreover, this phenomenon seems to concern many metamorphic slopes of the Alps.