Be-10 ages reveal > 12 ka of gravitational movement in a major sackung of the Western Alps (France)

The Rognier sackung, which extends NE-SW for more than 9 km, is a major gravitational morpho-structure located in the Western Alps. We used the Be-10 cosmic ray exposure dating method on 23 samples to unravel its history and better understand its genetic mechanism. We found that the average slip rate on sackung faults varies from 0.3 to 3.2 mm yr(-1). Conjugate sackung faults are contemporaneous, in agreement with a model in which the whole mountain ridge is involved in the gravitational deformation (600 m thick). Multiple samples on single faults suggest that the development of this sackung started before 12 ka and that it is still active. We cannot exclude the possibility that the dissolution of the underlying Triassic evaporites participated in the sagging of Mt. Rognier. However, the age of this sackung and the presence of the Lauziere sackung on the other side of the Arcs valley support glacial erosion and debuttressing following the retreat of the valley glacier as the main causes of these large gravitational structures. The long and permanent activity of the Rognier sackung contrasts with that of the neighbouring Arcs sackung, which we previously dated using the same method and showed that it is a result of flexural toppling. Along with the proximity of the Rognier sackung to the Belledonne border active fault, its long history suggests that earthquake shaking contributed to maintain its activity. Additionally, two ages confirm that in the western Alps, rock glaciers formed mainly during the Younger Dryas. This study allowed the determination of the best sampling strategy for future CRE dating of these types of gravitational structures