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

The 60 x 10(6) m(3) Sechilienne 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 (>35 m 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 Be-10 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 13 ka. Exposure ages obtained on gravitational scarps reveal that the landslide initiation occurred 8 to 6 ka ago. From the initiation until 2 ka the gravitational kinematics was slow (similar to 2 mm" year(-1)) and focused around the head scarp, leading to a general slope subsidence. After 2 ka, the exposure rates increased significantly (similar to 8 min. year(-1)) with the development of pervasive internal deformation of the landslide mass. This new scenario for the 58chilienne slope reflects a progressive rock-slope weakening since 8 ka, associated with a continuous activity of a deep-seated surface failure