Cosmogenic @u1@u0Be dating of a sackung and its faulted rock glaciers, in the Alps of Savoy (France)

• Published in: Geomorphology (Amsterdam, Netherlands) Netherlands), 2009-07, Vol.108 (3-4), p.312-320
• Main Authors: Hippolyte, J C, Bourles, D, Braucher, R, Carcaillet, J, Leanni, L, Arnold, M, Aumaitre, G
• Format: Article
• Language: English
• ISSN: 0169-555X
• DOI: 10.1016/j.geomorph.2009.02.024

Sackungs are the largest gravitational deformation observed in mountains. They are characterized by the long term slowness of the movements, but their mechanism is still not well understood. Nowadays cosmic ray exposure (CRE) dating methods allow dating the morphologic structures involved in sackung and can contribute at the understanding of their origin. In the Alps, the 5.3 km long Arcs sackung initiated during the activity of rock glaciers. Three samples from these faulted rock glaciers provide their first CRE ages and show that at 2000 m elevation these block accumulations moved during the Younger Dryas and stopped in early Holocene. Six @u1@u0Be ages of faults scarps show that the Arcs sackung lasted only a few thousand years and stopped at about 8,462 +/- 432 @u1@u0Be yrs. They also reveal that deformation migrated upslope in agreement with a mechanism of flexural toppling of vertical layers. This unique and long gravitational event, characterized by migration of the deformation, does not support earthquake shaking as triggering mechanism for individual faults. It shows that, in the upper Isere valley, slope deformation was delayed of several thousand years after glacial debuttressing, and is not anymore active despite its fresh morphology.