Evidencing a large body of ice in a rock glacier, Vanoise Massif, Northern French Alps
Résumé
The Sachette rock glacier is an active rock glacier located between 2660 and 2480m a.s.l. in the Vanoise Massif, Northern French Alps (45 degrees 29 N, 6 degrees 52 E). In order to characterize its status as permafrost feature, shallow ground temperatures were monitored and the surface velocity measured by photogrammetry. The rock glacier exhibits near-surface thermal regimes suggesting permafrost occurrence and also displays significant surface horizontal displacements (0.6-1.3 +/- 0.6myr-1). In order to investigate its internal structure, a ground-penetrating radar (GPR) survey was performed. Four constant-offset GPR profiles were performed and analyzed to reconstruct the stratigraphy and model the radar wave velocity in two dimensions. Integration of the morphology, the velocity models and the stratigraphy revealed, in the upper half of the rock glacier, the good correspondence between widespread high radar wave velocities (>0.15-0.16mns-1) and strongly concave reflector structures. High radar wave velocity (0.165-0.170mns-1) is confirmed with the analysis of two punctual common mid-point measurements in areas of exposed shallow pure ice. These evidences point towards the existence of a large buried body of ice in the upper part of the rock glacier. The rock glacier was interpreted to result from the former advance and decay of a glacier onto pre-existing deposits, and from subsequent creep of the whole assemblage. Our study of the Sachette rock glacier thus highlights the rock glacier as a transitional landform involving the incorporation and preservation of glacier ice in permafrost environments with subsequent evolution arising from periglacial processes.