Caractérisation de l’évolution géomorphologique de la basse vallée de la Romanche (Isère, France) en relation avec les instabilités gravitaires de ses versants rocheux

Résumé

La basse vallée de la Romanche est localisée au Sud du massif cristallin externe de Belledonne. Les versants de cette vallée glaciaire présentent de nombreux indices de déstabilisations gravitaires, dont un mouvement majeur des Alpes françaises, le mouvement de Séchilienne. Afin d’évaluer les facteurs prédominants dans l’initiation de ces instabilités et d’obtenir une vue spatio-temporelle des processus en action, une étude multidisciplinaire combinant des données géologiques, morphologiques, géophysiques et chronologiques a été menée à plusieurs échelles spatiales. Cette étude a montré l’importance de l’héritage litho-structural et de l’érosion glaciaire sur la localisation des instabilités gravitaires. A ces facteurs internes et préparatoires, s’ajoute le facteur externe hydrologique apparaissant comme déclencheur de ces instabilités.

Abstract

One of the most significant geomorphological consequences of deglaciation in mountainous valleys is the exposure of steepened rock slopes which have been identified as gravitationally unstable areas. Lateral stress release resulting from glacial oversteepening of the valley and ice melting (debuttressing) has been frequently recognized as a major cause triggering rock slope failure in deglaciated areas. A time lag between deglaciation and instabilities’ initiation may however exist and depends on the mechanical characteristics of the rock mass which are controlled by its lithology and structural framework. External factors like tectonic stresses, earthquakes or climatic changes could also trigger them. The present work aims at understanding how geomorphological evolution of a glacial valley is linked to the initiation and localization of gravitational instabilities. To achieve such a goal, I focused on the lower Romanche valley, situated in the Southern part of the Belledonne massif. The Belledonne massif, one of the Paleozoic external crystalline massifs of the French Alps, extends over more than 120 km in a N30 direction. Its basement is constituted by a complex of different metamorphic rocks (gneisses, amphibolites and micaschists). This substratum is covered by non-metamorphic detrital sediments. The Belledonne massif is divided in two major tectonic domains, the external domain to the west and the internal one to the east. These two blocks are separated by a major Late Paleozoic sub-vertical fault so-called Belledonne Middle Fault (BMF). The southern part of the Belledonne massif is carved by the east–west trending lower Romanche River. The slopes of this glacial valley, which went through glaciation and deglaciation phases during Quaternary, show many signs of gravitational destabilization. Of major concern is the large Séchilienne landslide whose more active part corresponds to several million cubic meters. In order to evaluate the main processes controlling the localization and the initiation of the gravitational instabilities in this zone, a multidisciplinary study coupling geological, morphological, geophysical and chronological data was conducted at different spatial scales. The geomorphological and structural study of the massif, performed from surface morphological investigation and from outcrops’ measurements, reveals that all the mass movements are localized in the micaschists which are characterized by lower mechanical properties and by a structural framework differing from the one observed in the amphibolites. Lithological and structural characteristics are thus significant factors in the localization of these instabilities. Moreover, an inherited N80 sinistral strike–slip fault zone so-called Séchilienne Fault Zone (SFZ) was identified on the right bank of the amphibolite gorges just upstream Séchilienne village. The extension to the West of the Séchilienne Fault Zone is confirmed by the Belledonne Middle Fault eastward shift determined by electrical tomography. Seismic and electrical data also revealed a significant glacial overdeepening of the valley in the micaschists which mainly results from the Quaternary glacial erosion across a major lithological contrast magnified by the intersection of these two inherited structures. This overdeepening created a major redistribution of stresses within the rock mass where the instabilities are localized, which shows the role of glacial erosive control. However, this erosive process, associated with the debuttressing of the massif and the litho-structural characteristics, has apparently not been sufficient to trigger gravitational instabilities. Indeed, cosmic ray exposure data acquired from vertical sampling profiles along the Séchilienne and Fau Laurent head scarp provide chronological constraints on the failure time of these major alpine landslides. Exposure ages at 1,100 m a.s.l. in the Mont Sec head scarp area indicate that the glacier retreated at least at 16.6 ± 0.6 ¹⁰Be ka and that the onset of theses landslides began between 5.0 and 7.8 ¹⁰Be ka. Comparing the date of the rock slopes’ failure initiation to the estimated age of total downwastage of the valley yields a minimal pre-failure endurance of 5,400 years. High spatial resolution cosmic ray exposure data collected on vertical profiles also provide an innovative contribution to understand the Séchilienne landslide kinematics during the Holocene. Results show the Séchilienne head scarp exposure to be progressive from failure initiation to present with an increase of the exposure rate between 2.3 and 1 ka up to around 1.1 cm/year. After this acceleration phase, the exposure rates are similar to those obtained by present day monitoring. Moreover, the initiation phase of the Séchilienne and Fau Laurent landslides occurred during the Holocene Climatic Optimum, a warmer and wetter interval. This result suggests that the hydrological external factor seems to have played and still play a significant, triggering or worsening, role on the landslides’ failure in the glacial Romanche Valley.