Use of 87 Sr/ 86 Sr ratio as indicator of flowpaths on a large-scale instability : the Séchilienne landslide (Isère, France)

International audienceThe Séchilienne landslide is located on the right bank of the Romanche River, South East of Grenoble (Isère, France). The active zone of the gravitational instability involves several millions of cubic meters. The geology consists of fractured hard rocks (micaschists) with double permeability and strong spatial heterogeneities, covered by carbonates deposits at the top of the slope. Water plays two major roles. A first as a rheologic agent by worsening the instability, and the second one being to trigger water-rock interactions, changing the nature of the material and making it less cohesive. These two factors interact through time and modify porosity and permeability and therefore the flow of water within the rock. Major elements chemistry and Sr isotopes where measured in different outflows draining both the stable and instable part of the Séchilienne instability in order to determine the water origin and flowpaths along the slope. Sr isotopic variations provide natural fingerprints of rock-water interactions and have been largely used in studies of weathering and hydrogeology. Differences in isotopes ratios reflect only mixing, thus providing information on sources and pathways of water. Sr water composition will depend on the solubity of each mineral phase constituting the rock that react with water along its flowpath and on the residence time of those waters. Coupled with major element concentrations and ratios, Sr istopes allow us to constrain the water-rock interaction processes and water circulation through the instability provided the different mineral phases that interact with water have contrasted isotope signatures. In Sechilienne, variable 87Sr/86Sr ratios where measured in the sampled waters, reflecting different water flowpaths. These results indicate a favorable situation where the slope is mainly composed of fractured micaschists, covered in uncomformity by carbonates (Lias) at the top of the slope, both formations having very different isotope compositions. By using Sr isotopic ratios coupled with major elements chemistry, it was possible to represent the main mixing trends between the main water masses contributing to the stable and instable parts of the instability