ERET - Écroulements Rocheux dans les parois à permafrost de haute montagne : quelles Évolutions dans le Temps ?

Rocky slope collapses and sub-glacial erosion: what temporal changes in the Alps?

• Auteur / Autrice : Léa Courtial-manent
• Direction : Jean-louis Mugnier, Ludovic Ravanel
• Type : Projet de thèse
• Discipline(s) : Géographie Physique et Environnementale
• Date : Inscription en doctorat le 01/10/2021
• Etablissement(s) : Chambéry
• Ecole(s) doctorale(s) : École doctorale Sciences de la terre, de l’environnement et des planètes (Grenoble ; 199.-....)
• Partenaire(s) de recherche : Laboratoire : Institut des Sciences de la Terre

Mots clés

Mots clés libres

Aléas

Haute-montagne

Réchauffement climatique

Hazards

High alpine moutains

Global warming

Résumé

Le nombre des écroulements rocheux associés au réchauffement climatique va-t-il continuer à augmenter en haute montagne, avec son cortège de nouveaux risques et une diminution de l'habitabilité des hauts massifs alpins ? Ou assiste-t-on à une sorte de « purge temporaire » avant un retour à des valeurs indépendantes de ce réchauffement mais plutôt contrôlées par des paramètres géologiques ? Pour répondre à ces questions, nous coupleront trois approches : 1) une quantification à long terme de l'érosion basée sur la concentration en isotopes cosmogéniques des sédiments issus des parois et amalgamés lors du transport glaciaire ; 2) une quantification à court terme basée sur la réitération de mesures topographiques à haute résolution ; 3) une modélisation des conditions thermiques des parois durant le dernier millénaire. Ceci permettra de mieux quantifier spatialement les risques de déstabilisation de versants rocheux et leurs évolutions temporelles.

The alpine high mountain is confronted with deep changes connected to the global warming today driving an important glacial retreat. To understand the current evolution of the relief in this context, it is crucial to determine the respective role of the rock-falls from walls above glaciers, of periglacial and of sub-glacial erosions. For that purpose, we shall analyze the characteristics of sediments resulting from these processes of erosion and transported by the glaciers and the systems of sub-glacial drainage. Sediments in sub-glacial position correspond to a mixture resulting from the erosion of the sub-glacial substratum and from clastes resulting from zones dominating the glacier stemming from hillslope processes (rocky collapses, avalanche). The used method will be based on the measure of the concentration in cosmogenic isotopes in sediments. The isotopic signal results then from a very different mixture of two poles: the first one corresponding to the sub-glacial substratum which is protected from the cosmic radiation by the thickness of the glacier, the second results from hillsides and it concentration in cosmogenic isotope is dependent on their exposure time, thus on the average rate of erosion of hillsides. The glacier works at the same time as an agent of transport of sediments and a crusher so reducing the size of the sediments and mixing them perfectly; so by analyzing the sandy fraction, easy to sample, it is possible from some kilograms of representative rocks to reconstitute the evolution of a whole hillside. We shall study the glaciers and their walls in various contexts (massifs of the Mont Blanc, the Mount Rose, the Vanoise and the Ecrins) for which would be measured the concentration in 10Be isotope of sands under and above glaciers. This will allow to consider for these glaciers: a) the proportion of sediments stemming from the sub-glacial erosion; b) the average rate of erosion of hillsides; c) the rate of sub-glacial erosion by taking into account respective surfaces of zones sources in the equation of mixture. The results of the isotopic approach will be compared with direct measures of the sedimentary flow monitored on torrents stemming from these glaciers, allowing to consolidate by a direct measure the estimations obtained indirectly. The comparison of the average rates of erosion of the various units of hillsides and the regional rate of uplift will also allow to specify the evolution, in the millennial scale, the height and the relief in the considered zone. Finally, The comparison between the estimations of the rates of erosion, the morphology of the rock-slope, the models of flow of the glaciers and the thermal evolution of the permafrost will allow to better understand the processes of sub-glacial erosion as well as hillside erosion.