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International Journal of Earth Sciences 99 (2010) 427-441
Interaction between tectonic and erosion processes on the morphogenesis of an Alpine valley: geological and geophysical investigations in the lower Romanche valley (Belledonne massif, western Alps)
Olivier Le Roux 1, Stéphane Schwartz 1, Jean François Gamond 1, Denis Jongmans 1, Pierre Tricart 2, Michel Sébrier 3

In the Belledonne massif, the steep Paleozoic Belledonne Middle Fault (BMF) separates micaschists, displaying numerous landslides, from amphibolites. The massif is incised by the lower Romanche river valley. When crossing the BMF, the valley widens into a lozenge-shaped basin recently interpreted as an active pull-apart type structure associated with a major N110 striking Quaternary fault. Multidisciplinary investigations were carried out in the basin to check if this model has implications on the seismic and landslide hazard assessment. This study demonstrated the existence of a N80 sinistral strike slip Séchilienne Fault Zone (SFZ). This fault zone is suspected to offset the BMF by 375 m across the basin. Geophysical experiments revealed that the bedrock depth increases strongly in the basin, up to 350 m. Our study invalidates the active pull-apart origin of the basin and suggests it results from Quaternary glacial and fluvial erosion processes, magnified by the intersection of two inherited structures, the BMF and the SFZ.
1 :  Laboratoire de géophysique interne et tectonophysique (LGIT)
CNRS : UMR5559 – Institut de recherche pour le développement [IRD] – LCPC – OSUG – INSU – Université de Savoie – Université Joseph Fourier - Grenoble I
2 :  Laboratoire de géodynamique des chaines alpines (LGCA)
CNRS : UMR5025 – OSUG – INSU – Université de Savoie – Université Joseph Fourier - Grenoble I
3 :  Laboratoire de tectonique (LT)
CNRS : UMR7072 – INSU – Université Pierre et Marie Curie (UPMC) - Paris VI – Université de Cergy Pontoise
Planète et Univers/Sciences de la Terre/Géomorphologie
Western Alps – Alpine valley – Alluvial lozenge-shaped basin – Geophysical prospecting – Quaternary erosion processes