Étude géomophologique des flux glaciaires dans les Alpes nord-occidentales au Pléistocène récent. Du maximum de la dernière glaciation aux premières étapes de la déglaciation

Abstract : This work takes place in the spatio-temporal context of the last glacial cycle (Würm), the paleogeographic setting being the intra-würm area according to Bourdier (1961). The reconstitution of ice flows in north-western Alps during the Würm ice-surface maximum is the main scope if this study. Thanks to the acquisition of new geomorphologic observations, a consistent palaeogeographic cartography of the glacial system during the Würm Glacial Maximum (WGM) has been achieved, for upstream watersheds (feeding zones) and downstream zones (piedmont lobes), Second, the study has been applied to determine the origin of the different ice flows that have fed the piedmont lobes in the north-western Alps (Swiss lobe of the Rhône glacier, Lyon lobe and Moirans lobe). To conduct this work, the use of several complementary methods has been necessary: (i) a glacio-morphologic approach allowing to define the altitude of the glacial equilibrium line; (ii) petrographic analyses of erratic boulders to identify from which watersheds they originate; (iii) and analysis of the heavy minerals contained in the fine fraction of ablation tills, to complete and confirm the petrographic analyses. The first stages of the deglaciation associated with the individualization of the flows coming from the four main systems of the Rhône, the Arve, the Isère and the Romanche valleys have been reconstructed. They are followed by the first stages of the lower Late glacial of the Arve and Rhône valleys. Lastly, a chronological approach has been conducted, in particular to date the Würm glacial maximum. It is based on the analysis of the available data and tends to expose the main trends. The obtained results provide a significant contribution to the paleogeographic reconstructions and to the study of ice flow organization during the Würm maximum. These new data/elements allow a novel interpretation of the glacial network organization in north-western Alps and questions/ challenges/disputes, in particular, the traditional vision of the Rhône glacier reaching the “internal moraines complex “. Indeed, the analyses prove that the whole ices of Lyon piedmont lobe were coming from the accumulation zones of the internal part of the French northern Alps (geographic meaning): south of the Mont-Blanc massif, Beaufortin, Tarentaise and a part of the Maurienne. In addition, they confirm the strength of the glacial systems that have filled in the big transverse valleys of the subalpine massifs (transverse valleys of Annecy-Faverges and Chambéry) and emphasise the essential role of the transverse glaciers coming from the central Alps in the feeding of the Lyon piedmont lobe. As for the feeding of Moirans lobe, the petrographic analyses and the counting of heavy minerals demonstrate a major contribution of the Romanche glacier. These results confirm the hypotheses outlined in the XIXth century by some authors (Lory, 1860; Falsan et Chantre, 1879). The study of the deglaciation stages highlights the relative significance of the different systems. In particular, it shows the predominnace and the late influence of the Isère glacier and the significance of which being often undervalued by authors. Lastly, a chronologic approach of the WGM has been undertaken. Although it does not provide decisive new information, it suggests a diachronism between the extension of the systems of the northern Alps (late maximum) and of the western Alps (early maximum).
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Sylvain Coutterand. Étude géomophologique des flux glaciaires dans les Alpes nord-occidentales au Pléistocène récent. Du maximum de la dernière glaciation aux premières étapes de la déglaciation. Géographie. Université de Savoie, 2010. Français. ⟨tel-00517790v3⟩

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