Cosmic ray exposure dating on the large landslide of Séchilienne (Western Alps): A synthesis to constrain slope evolution Stéphane Schwartz a, ⁎, Swann Zerathe a , Denis Jongmans a , Laurent Baillet a , Julien Carcaillet a , Laurence Audin a , Thierry Dumont a , Didier Bourlès b , Régis Braucher b , Vincent Lebrouc a a Université Grenoble Alpes, ISTerre, CNRS, Q4 IRD, F-38041 Grenoble, France b Université Aix-Marseille, CEREGE, CNRS, IRD, Coll. de France, F-13545 Aix-en-Provence, France abstract article info Article history: Received 27 June 2016 Accepted 16 November 2016 Available online 24 November 2016 The 60 × 10 6 m 3 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 (N 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 ad- ditional 10 Be 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 ob- tained 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 (~2 mm·year -1 ) and focused around the head scarp, leading to a gen- eral slope subsidence. After 2 ka, the exposure rates increased signiﬁcantly (~8 mm·year -1 ) with the develop- ment of pervasive internal deformation of the landslide mass. This new scenario for the Séchilienne slope reﬂects a progressive rock-slope weakening since 8 ka, associated with a continuous activity of a deep-seated sur- face failure. © 2016 Elsevier B.V. All rights reserved. Keywords: Western Alps Séchilienne landslide Cosmic ray exposure dating Slope evolution 1. Introduction The morphology of Alpine valley has been strongly inﬂuenced by glaciations that have probably been the most important erosional mechanism affecting glaciated mountain belts over the Quaternary (Montgomery, 2002 and references therein). The inﬂuence of glacial erosion on the landscape is expressed by typical features such as U- shaped and overdeepened valleys, hanging valleys, stepped proﬁles, polished and striated surfaces and grooves, associated with the deposi- tion of moraines (Kelly et al., 2004; Anderson and Anderson, 2012). Nu- merous erosion models have been developed to explain how ice is able to shape relief, both on longitudinal and transverse valley proﬁles (Harbor, 1992; Augustinus, 1995; McGregor et al., 2000; Anderson et al., 2006, Herman and Braun, 2008). In particular, Harbor (1992 and 1995) simulated the evolution of a transverse valley proﬁle during steady occupation by a glacier, showing that the valley propagates vertically as a U-shape form with lateral steepened rock-slopes. Numerous studies of longitudinal proﬁles of glaciated valleys have shown the presence of steps that usually coincide with coalescence of headwater valleys, tributary junctions or variations in rock resistance (McGregor et al., 2000; Anderson et al., 2006). Glacial erosion and the resulting landscape are strongly controlled by the bedrock hardness and strength, as well as by the fracturing processes (Dunforth et al., 2010, Krabbendam and Glasser, 2011; Salcher et al., 2014). Deglaciation in the Alps left many slopes oversteepened, which have been subsequently affected by large rock-slope instabilities (Erismann and Abele, 2001). Rock-slope failure in deglaciated mountain areas is mostly triggered by the lateral stress release resulting from ice melting (debutressing) (among others, Cruden and Hu, 1993; Blair, 1994; Evans and Clague, 1994; Holm et al., 2004; Cossart et al., 2008). However, the initiation of large-scale landslides in the Alps has also been associated with other factors like earthquakes, subsequent climatic changes, tec- tonic stresses, uplift rate and river and bedrock erosion (e.g., Ballantyne, 2002; Seijmonsbergen et al., 2005; Cossart et al., 2008; Hormes et al., 2008; Le Roux et al., 2009; Sanchez et al., 2009, Zerathe et al., 2014). Identifying the major cause responsible for trigger- ing rock-slope instability remains a strongly debated question (Korup, Geomorphology 278 (2017) 329–344 ⁎ Corresponding author. E-mail address: stephane.schwartz@univ-grenoble-alpes.fr (S. Schwartz). http://dx.doi.org/10.1016/j.geomorph.2016.11.014 0169-555X/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph