Cosmic ray exposure dating on the large landslide of Séchilienne (Western Alps): A synthesis to constrain slope evolution

The 60×106m3 Séchilienne landslide (Belledonne Massif, Western Alps) is located on the right bank of the East-West trending Romanche valley which is shaped by glacial and alluvial processes during the Quaternary. Its head scarp (>35m high) was dated by Le Roux et al. (2009) using the cosmic ray exposure (CRE) method. Even though these previous results revealed that the initiation of the instability occurred several thousand years after ice down-wastage in the valley, the internal landslide evolution is not constrained. In this paper, we provide 63 additional 10Be samples collected from the internal scarps and the main scarp, as well as on glacially polished rock surfaces. The aim is to constrain the global landslide kinematics (internal and head scarps) and its relationship with glacier retreat. Results from glacially polished surfaces point out that complex shielding processes (relict moraines, soil deposits and seasonal snow cover) might have affected rock dating. Despite scattering of the resulting ages, the dataset shows that the glacial retreat was achieved between 17.5 and 13ka. Exposure ages obtained on gravitational scarps reveal that the landslide initiation occurred 8 to 6ka ago. From the initiation until 2ka the gravitational kinematics was slow (~2mm·year−1) and focused around the head scarp, leading to a general slope subsidence. After 2ka, the exposure rates increased significantly (~8mm·year−1) with the development of pervasive internal deformation of the landslide mass. This new scenario for the Séchilienne slope reflects a progressive rock-slope weakening since 8ka, associated with a continuous activity of a deep-seated surface failure.