Elsevier

Geomorphology

Volume 278, 1 February 2017, Pages 329-344
Geomorphology

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

https://doi.org/10.1016/j.geomorph.2016.11.014Get rights and content

Abstract

The 60 × 106 m3 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 (> 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 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 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 (~ 2 mm·year 1) and focused around the head scarp, leading to a general slope subsidence. After 2 ka, the exposure rates increased significantly (~ 8 mm·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 8 ka, associated with a continuous activity of a deep-seated surface failure.

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 deposition of moraines (Kelly et al., 2004, Anderson and Anderson, 2012). Numerous 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, MacGregor et al., 2000, Anderson et al., 2006, Herman and Braun, 2008). In particular, Harbor, 1992, Harbor, 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 (MacGregor 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, tectonic 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 triggering rock-slope instability remains a strongly debated question (Korup, 2008, Zerathe et al., 2014) and the timing of events is a key issue to better understand the most important mechanism(s) driving instabilities in a post-glacial period.

In the last decade, CRE (Cosmic Ray Exposure) dating has been increasingly and extensively used for assessing the timing of large rock-slope failures in the Alps (Ivy-Ochs et al., 1998, Bigot-Cormier et al., 2005, Hippolyte et al., 2006, Hormes et al., 2008, Le Roux et al., 2009, Ivy-Ochs et al., 2009, Delunel et al., 2010a, Ostermann et al., 2012; among others). The same technique was also applied to constrain the timing of the last deglaciation by dating late glacial moraine deposits and glacially polished bedrock surfaces (e.g. Darnault et al., 2012), allowing the chronology of both valley deglaciation and rock-slope instability to be established at specific sites (Bigot-Cormier et al., 2005, Cossart et al., 2008, Hormes et al., 2008, Ivy-Ochs et al., 2009, Prager et al., 2009, Le Roux et al., 2009, Martin et al., 2014). Major findings are that the last glacial retreat occurred simultaneously across the Alps (Darnault et al., 2012) and that some large landslides were not triggered during deglaciation but after a delay of several thousand years after the valley is ice-free (Le Roux et al., 2009, McColl, 2012). A recent synthesis of the failure-age chronicles obtained for the large-scale landslides throughout the Alps (Zerathe et al., 2014) identified two main periods of landslide activity, from 11 to 8 ka (Pre-Boreal and Boreal periods) and from 5 to 3 ka (Subboreal period). The authors related the first activity period (11 to 8 ka) to two alpine-glacier recession phases at 10.9 ± 1.1 ka (end of the Younger Dryas period) and 8.4 ± 0.9 ka (beginning of the Holocene climatic optimum) (Darnault et al., 2012). They proposed that the isostatic rebound following the glacial retreat is the main triggering factor. However, during those two periods, the lower elevation valleys were totally deglaciated, as shown by Le Roux et al. (2009) and Martin et al. (2014), and have then been unaffected by the subsequent glacial retreats, which occurred at higher elevations. Indeed, even for higher elevations, the documented cases (McColl, 2012) indicate that most large post-glacial failures have typically occurred some thousands of years after ice retreat (Hormes et al., 2008, Ivy-Ochs et al., 2009, Prager et al., 2009, Le Roux et al., 2009, Martin et al., 2014). Three main reasons have been generally used to explain this delay (McColl, 2012): the time lag between local slope stress-redistribution and the development of sheeting joints, the lag between regional glacio-isostatic rebound and a potential period of enhanced seismicity, and the effect of climatic factors, such as warmer temperatures and increased rainfall which were more significant in the middle and early Holocene (Le Roux et al., 2009, Zerathe et al., 2014). Recently, Lebrouc et al. (2013) also proposed that the persistence of permafrost could have played a role in delaying instability initiation. Modeling the thermal response of the Séchilienne slope (Romanche valley, Western Alps) during the last 21,000 years, they suggested that the permafrost probably vanished around 10 to 11 ka, at least 3000 to 4000 years after the total ice down-wastage in the valley.

Most of the CRE studies on large landslides were focused on the dating of head scarps, sliding planes or blocks resulting from a rock avalanche process (see Zerathe et al., 2014 and the references herein). To our knowledge, no attempt has been made so far to date the internal scarps of a landslide in order to obtain its kinematics. This paper presents a comprehensive study to constrain the chronology of the large Séchilienne landslide (Western Alps) that affects the right slope of the Romanche valley between 450 m and 1140 m a.s.l. and its relation with the Romanche glacier retreat.

The head scarp and a few polished rock surfaces located in the upper part of the landslide were already locally dated from 23 samples (Le Roux et al., 2009). Application of the CRE method along the 35 m high head scarp yielded an initiation of the rock-slope failure at 6.4 ± 1.4 10Be ka and a continuous rock-slope failure activity with a mean head scarp exposure rate of 0.6 cm·year 1. Glacier retreat at ~ 1100 m a.s.l. was estimated at 16.6 ± 0.6 10Be ka, with total deglaciation of the valley achieved by at least 13.3 ka. This chronological constraint was obtained from the Tinée valley located 130 km South-East from the Romanche Valley (Bigot-Cormier et al., 2005).

In this study, we sampled rock outcrops within the landslide, at an elevation between 840 and 1140 m a.s.l. with a four-fold objectives: (1) to confirm the timing of the triggering of the failure over the head scarp length, including the main lateral scarp, (2) to assess the vertical glacial retreat rate in the Romanche valley from dating of polished and striated rock surfaces at lower elevations, (3) to date internal scarps of different sizes and types (valley facing and counter scarps) in order to assess their origin (glacial erosion or gravitational movement) and (4) to get some insight in the landslide kinematics. A total of 63 samples were taken in the upper part of the landslide. In contrast with the major near-vertical head scarp studied by Le Roux et al. (2009), internal scarps were frequently affected by rockfalls, rejuvenating the outcrops and locally providing young ages. The glacially polished outcrops in the landslide often exhibit a wide range of 10Be concentrations, with unexpected low values that suggest that some outcrops have been temporary covered by deposits and subsequently exposed after their erosion. Exposure of rocks to cosmic rays on the Séchilienne slope is influenced by multiple processes including the glacial retreat, the erosion of till deposits, the local fall of blocks and gravitational movements. The analysis of this large dataset allows to assess the influence of the different phenomena and to constrain the kinematics of the Séchilienne slope since the Last Glacial Maximum (LGM) around 21 ka (Clarke et al., 2009).

Section snippets

Geological setting and dynamics of the landslide

The Séchilienne landslide is located in the southern part of the Belledonne massif (Western Alps) along the East-West trending Romanche valley at 20 km South-East of Grenoble City (Fig. 1). This massif, which extends over more 120 km in a N30 direction with a maximum elevation of 3000 m a.s.l., is one of the Paleozoic External Crystalline Massifs of the French Western Alps. It is a part of the Variscan orogen that has been overprinted by Alpine shortening and uplift (Guillot et al., 2009). The

Main morphological features

The Séchilienne slope dominates the right bank of the Romanche valley. Three distinct morphological zones can be distinguished from North to South (Fig. 2, Fig. 3a): the near-horizontal preserved glacial plateau, the depleted part of the plateau affected by the landslide and the steeper glacial slope mainly destabilized by recent rockfalls. The preserved glacial plateau (Mont Sec Plateau in Fig. 2, Fig. 3) with an elevation of 1140 m a.s.l. shows North-South elongated depressions carved in

In-situ 10Be dating

Cosmic Ray Exposure (CRE) is a geochronological dating method that is based on the accumulation of Terrestrial Cosmogenic Nuclides (TCNs, such as 36Cl, 10Be, or 26Al) in superficial rocks. TCNs are produced within mineral lattice through nuclear reactions between the nucleus of the elements that form the minerals and the incident secondary cosmic ray particles derived from the high-energy galactic cosmic radiation (see a review in Gosse and Phillips, 2001). Because the production rate of these

Glacially polished surface exposure ages

The dating results obtained along the glacially polished rock surfaces show a large variability in apparent exposure ages (Table 2). For the glacial Mont Sec Plateau, the obtained exposure ages range from 1.9 ± 0.2 ka (P18) to 17.5 ± 1.1 ka (P17), while they vary from 2.4 ± 0.3 ka (P14–2) to 15.4 ± 1 ka (P15–3) on the glacial slope. The analysis of the two datasets leads to the probability density plots shown in Fig. 6, Fig. 7c. On the Mont Sec Plateau, despite the data scattering, two main peaks of higher

Deglaciation scenario of the Séchilienne slope

In order to discuss the deglaciation history of the study area, we compare the mean annual air temperature curve (Tmaa) over the last 21,000 years (Lebrouc et al., 2013) with the summed probability curves for 10Be ages measured on polished surfaces (Fig. 9). Two cold thermal periods are distinguished (labeled A and YD) from the temperature curve fluctuations (Fig. 9a). The A period extends from 21 ka (LGM) to 14.7 ka with Tmaa around − 8 °C. It corresponds to the end of the LGM while the YD period

Conclusion

In this study we bring 63 new CRE ages to decipher the kinematics of the large Séchilienne landslide and its relationship with the deglaciation scenario of the Romanche valley. The previously acquired CRE data (23 samples) were recalculated and combined with the new dataset. Exposure ages acquired on glacially polished surfaces allow to date two main glacier retreat events, (1) the early ice melting event on the Mont Sec Plateau at 17.5 ka above 1140 m a.s.l., and (2) the late down-wastage

Acknowledgements

This study was supported by the project “ANR-09-RISK-008” (SLAMS) funded by Agence Nationale de la Recherche (France). We thank R. Delunel and an anonymous reviewer for helpful comments that significantly improved the manuscript and N. Vögeli for improving English writing.

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