Réf. Le Roux & al. 2009

Référence bibliographique complète

LE ROUX, O., SCHWARTZ, S., GAMOND, J.F., JONGMANS, D., BOURLÈS, D., BRAUCHER, R., MAHANEY, W., CARCAILLET, J.,  LEANNI, L. 2009. CRE dating on the head scarp of a major landslide (Séchilienne, French Alps), age constraints on Holocene kinematics. Earth and Planetary Science Letters, Vol. 280 (1–4), 236–245. doi:10.1016/j.epsl.2009.01.034 [PDF]

Abstract: Cosmic Ray Exposure (CRE) dating applied to the active Séchilienne landslide (Romanche valley, Belledonne Massif, French Alps) provides information about its Holocene dynamics from initiation to present day activity. Glacier retreat at 1100 m a.s.l. is estimated at 16.6±0.6 10Be ka from glacially polished bedrock samples, with total deglaciation of the valley achieved at least by 13.3 ka. Application of the CRE method along vertical profiles sampled with a high spatial resolution of 3 m on the head scarp yields: (1) an initiation of the rock-slope failure at 6.4±1.4 10Be ka and (2) a continuous rock-slope failure activity with a mean head scarp exposure rate of 0.6 cm/yr. The data suggest an increase of the head scarp exposure rate between 2.3 and 1.0 ka. After this acceleration phase, the exposure rate is similar to that obtained by the present day monitoring data over 20 years in the depletion zone (1.3 cm/yr). Since the failure initiation occurred more than 5400 yr after the total deglaciation of the valley the slope failure does not appear as an immediate consequence of debutressing in the Romanche valley. This result is consistent with studies of other large alpine rockslides in the Alps. Failure initiation of the Séchilienne landslide occurred during the Holocene Climatic Optimum, a hot and wet period. The temperature and precipitation changes of this climatic optimum seem to have a worsening effect at the regional scale to trigger large mass wasting in this glacial alpine valley.

Mots-clés
Cosmic ray exposure method, Séchilienne landslide kinematics, Head scarp dating, Holocene climatic optimum

Organismes / Contact
• Laboratoire de Géophysique Interne et Tectonophysique (CNRS, UMR 5559), Observatoire des Sciences de l'Univers, Université Joseph Fourier, BP 53, F-38041, Grenoble cedex 09, France
• Centre Européen de Recherche et d'Enseignement des Géosciences de l'Environnement (CNRS, UMR 6635), Université Aix-Marseille, BP 80, F-13545 Aix en Provence cedex 04, France
• Geomorphology and Pedology Lab, York University, 4700 Keele St.,N. York, Ontario, Canada M3J 1P3
• Laboratoire de Géodynamique des Chaînes Alpines (CNRS, UMR 5025), Observatoire des Sciences de l'Univers, Université Joseph Fourier, BP 53, F-38041, Grenoble cedex 09, France – Corresponding author (S. Schwartz): stephane.schwartz@ujf-grenoble.fr

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Temperature, precipitation (Romanche Glacier) Landslide Deep landlside

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
France (Isère) Southwestern part of the
Belledonne Massif / lower Romanche valley
Séchilienne landslide   1080 - 1121m (sampling sites) Holocene...

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 


(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions

Discussion:
[The authors] interpret the older age [obtained] (16.6±0.6 10Be ka) as the minimal age of the last retreat of the Romanche Glacier around 1100ma.s.l. in the Séchilienne landslide area. The timing of glacial retreat is consistent with the onset of the formation of the Mont Sec peat bog dated by 14C at 11.5±0.5 ka BP (Muller et al., 2007) in the vicinity [of the sampling site], in a place that was likely previously covered by moraine. [...]

[They] propose to transpose to the Romanche Valley, data available from another large alpine glacial valley, the Tinée Valley. Indeed, according to Vincent et al. (2004), temperature variations recorded over the last century at the scale of the Alpine Arc generate similar ice ablation dynamics for glaciers located several hundreds kilometres apart. The Tinée valley is located 130 km southeast from the Romanche Valley, in a similar geological setting (External Crystalline Massif of Argentera), and displays a similar geometry with a 1000 m height between the bottom of the valley and the glacial shoulder. This valley provides chronological constraints on its total downwastage. The bottom of the valley (1100 m a.s.l.) dates to 13.3±0.1 ka BP (Bigot-Cormier et al., 2005) by 14C age on travertine. At 13.3 ka BP the bottom of the Romanche Valley at 400 m a.s.l. was probably already ice-free. This estimation is consistent with 10Be dates on glacial retreat ranging from 7 to 12 ka (Cossart et al., 2008) obtained in the neighbouring valleys (External Crystalline Massif of Pelvoux) for elevations above 2000 m a.s.l. These chronological constraints suggest that for the Romanche Valley, between the 16.6±0.6 10Be ka exposure age of glacially polished surface at 1100 m a.s.l. and the 13.3±0.1 ka BP downwast age at 400 m a.s.l., 750 m of ice melted in 3300 yr.

Observations
 
Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

[see (3) - Effets du changement climatique sur l'aléa]

(3) - Effets du changement climatique sur l'aléa
Reconstitutions

Introduction:
[...] Several studies recently focused on the chronology of valley deglaciation and rock-slope instability 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). Surface exposure ages tend to indicate that the largest landslides in the Alps did not occur during deglaciation but in mid-Holocene time when climate became warmer and wetter. The Fernpass (Austria, 4100 yr), Flims (Switzerland, 8900 yr), Kandertal (Switzerland, 9600 yr), Köfels (Austria, 9800 yr), La Clapière (France, 10,300 yr), Valtellina (Italy, 7430 yr) landslides occurred at least a few thousand years after deglaciation. The Séchilienne landslide, which affects the south-facing crystalline slope of the glaciated Romanche Valley, is one of the largest in the French Alps. Based on CRE dating, this study first aims to determine the age of release of bedrock and emplacement and to compare it to timing of deglaciation. The paper is also focused on placing temporal constraints on the Séchilienne motion (kinematics) by regularly sampling the head scarp surface along vertical profiles. Exposure ages suggest a change in the kinematics of the landslide during the late Holocene.

[see Results and interpretation: Glacially polished bedrock dating (p.6) ; and Head scarp dating (p.7)...]

Discussion:
[...] At the scale of the Alpine Arc, the recent CRE studies of major landslides show that the initiation of instability does not immediately follow deglaciation but occurs several thousand years after the valley is totally ice-free. The estimated pre-failure endurance is thus more than 3000 yr for la Clapière landslide (French Western Alps, Bigot-Cormier et al., 2005), more than 2500 yr for the Flims rockslide (Swiss Central Alps, Ivy-Ochs et al. 2009), more than 4000 yr for the Val Viola rockslide (Italian Central Alps, Hormes et al., 2008), and more than 2000 yr for the Fernpass rockslide (Austrian Eastern Alps, Prager et al., 2009). Considering data available for major alpine landslides, Ivy-Ochs et al. (2009) suggest slope failure did not occur during deglaciation but during mid-Holocene time when climate became markedly warmer and wetter. In the case of the Séchilienne landslide, the CRE data show that the initiation stage occurred during the Holocene Climatic Optimum, an event ranging from 9.0 to 5.0 ka BP. In the case of the La Clapière landslide, the initiation stage also occurred during the Climatic Optimum. In the western Alps, this period is characterized by an increase of the mean temperature of 1 to 2° (Davis et al., 2003), an increase of the density of forest cover (de Beaulieu, 1977) and an increase in lake level due to heavy annual precipitation (Magny, 2004, 2007). The combination of these different climatic characteristics indicates that the Climatic Optimum was a warmer and wetter period. The exposure rates obtained from the Séchilienne head scarp show that the activity of the landslide is continuous from its triggering until present day, with an acceleration phase between 2.3 and 1.0 ka, which coincides with the subatlantic chronozone. This warmer and wetter phase allows displacement rates to reach 0.75 to 1.80 cm/yr. [The authors] suggest that slide acceleration is due to the decrease of mechanical properties within the rock mass or along slip surfaces, resulting from progressive slope motion. These high displacement rates are similar to those recorded during the last twenty years by instrumental survey (0.7 to 1.6 cm/yr; Vengeon et al., 1999). Monitoring data suggest a correlation between the increase of displacement rate of the whole slope with heavy rainfall thus clearly pointing out the major role of precipitation in slope instability. Therefore, an external hydrological parameter seems to have aworsening effect, adding to the decrease of the mechanical properties of the rock mass to trigger and maintain the Séchilienne landslide dynamics. Nevertheless the mechanical properties of a massif can also be weakened by seismic events. In fact the Séchilienne landslide is located in a zone (Romanche Valley, Belledonne Massif) affected by recurring seismicity. The potential maximal magnitude estimated from the instrumental seismicity is lower than ML=3.5 (Thouvenot et al., 2003). It cannot be excluded that seismic activity played a role in the Séchilienne landslide movement but this link is not demonstrated at the present time.

Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Time of initiation and triggering

One of the most significant geomorphological consequences of deglaciation in mountainous valleys is the exposure of steepened rock slopes which have been identified as gravitationally unstable areas. Lateral stress release resulting from ice melting (debutressing) has been frequently recognized as a major cause triggering rock-slope failure in deglaciated mountain areas. However, other factors like tectonic stresses, uplift rate, river and bedrock erosion, earthquakes and subsequent climatic changes have also been evoked, as contributing to large rock-slope instabilities. [see references in the introduction of the paper]

Sampling strategy and methodology:
Cosmic Ray Exposure dating is based on the accumulation of rare nuclides produced through nuclear reactions induced by high-energy cosmic radiation when rock is exposed at the earth's surface. [The authors] used in situ produced 10Be resulting from spallation reactions on Si and O in quartz. [...] Three sites were sampled on the glacial shoulder of the Mont Sec Plateau between 1080 and 1121m a.s.l., two in the stable area and one in the landslide. The resulting CRE ages range from 7.5 to 16.6 10Be ka. [...] The samples [were divided] into two series. In the first series, samples collected from glacially polished bedrock surfaces allowed determination of the approximate timing of the last deglaciation on top of the Séchilienne landslide. In the second series, samples taken from 3 vertical profiles down the head scarp allow to determine the timing of the head scarp exposure and denudation. [...] A model of the the exposure history [were] considered to estimate the deglaciation (Tg) and the destabilization (Td) ages. Exposure ages were computed [with the model] assuming no erosion since initiation of the landslide and with negligible chemical weathering. [...] [see details in the study, p. 4]


(4) - Remarques générales
 

(5) - Syntèses et préconisations

Conclusions:
CRE data acquired from vertical sampling profiles along the Séchilienne head scarp provide chronological constraints on the failure time of this major alpine landslide. Exposure ages at 1100 m a.s.l. in the head scarp area indicate that the glacier retreated at 16.6±0.6 10Be ka and that the head scarp of the landslide was triggered at 6.4±1.4 10Be ka. Comparing the date of the rock-slope failure initiation to the estimated age of total downwastage of the valley yields a minimal pre-failure endurance of 5400 yr. Therefore slope destabilization does not appear as an immediate consequence of debutressing in the Romanche Valley. This result is consistent with those obtained for other large alpine landslides (Flims, Val Viola, Fernpass, La Clapière). The initiation phase of the Séchilienne landslide occurred during the Holocene Climatic Optimum, a warmer and wetter interval. This result suggests that temperature and precipitation changes at that period had a significant – triggering or worsening – effect on the Séchilienne failure in the glacial Romanche Valley. High spatial resolution CRE data (3 m spacing) collected on vertical profiles provides an innovative contribution to understanding the Séchilienne landslide kinematics during the Holocene. Results show the head scarp exposure to be progressive – and then the sliding process to be continuous – from failure initiation to present with a mean rate of about 0.6cm/yr. Exposure age distribution, however, suggest an increase of the exposure rate between 2.3 and 1 ka up to 1.08 cm/yr, mean value. After this acceleration phase, the exposure rates are similar to those obtained by present day monitoring (1.3 cm/yr).

Références citées :

Bigot-Cormier, F., Braucher, R., Bourlès, D., Guglielmi, Y., Dubar, M., Stéphan, J.F., 2005. Chronological constraints on processes leading to large active landslides. Earth Planet. Sci. Lett. 235, 141–150.

Cossart, E., Braucher, R., Fort, M., Bourlès, D.L., Carcaillet, J., 2008. Slope instability in relation to glacial debutressing in alpine areas (upper durance catchment, southeastern France): Evidence from field data and 10Be cosmic ray exposure ages. Geomorpholgy 95, 3–26. [Fiche Biblio]

Davis, B.A.S., Brewer, S., Stevenson, A.C., Guiot, J., 2003. The temperature of Europe during the Holocene reconstructed from pollen data. Quat. Sci. Rev. 22, 1701–1716.

de Beaulieu, J.L., 1977. Contribution pollenanalytique à l'histoire tardiglaciaire et holocène de la végétation des Alpes méridionales françaises. PhD thesis, Université Aix-Marseille III, Marseille, 391 pp.

Hormes, A., Ivy-Ochs, S., Kubik, P.W., Ferreli, L., Michetti, A.M., 2008. 10Be exposure ages of rock avalanche and a late glacial moraine in Alta Valtellina, Italian Alps. Quat. Int. 190, 136–145.

Ivy-Ochs, S., Poschinger, A.V., Synal, H.A., Maisch, M., 2009. Surface exposure dating of the Flims landslide, Graubünden, Switzerland. Geomorphology 103, 104–112.

Magny, M., 2004. Holocene climate variability as reflected by mid-European lake-level fluctuations and its probable impact on prehistoric human settlements. Quat. Int. 113 (1), 65–79.

Magny, M., 2007. Climate oscillations and hydrological variations in Europe over the last 15,000 years. Lettre pigb-pmrc-France 20, 72–76.

Muller, S.D., Nakagawa, T., de Beaulieu, J.L., Court-Picon, M., Carcaillet, C., Miramont, C., Roiron, P., Boutterin, C., Ali, A.A., Bruneton, H., 2007. Postglacial migration of silver fir (Abies alba Mill.) in the southwestern Alps. J. Biogeography 34, 876–899.

Prager, C., Ivy-Ochs, S., Ostermann, M., Synal, H.A., Patzelt, G., 2009. Geology and radiometric 14C-, 36Cl- and Th-/U-dating of the Fernpass rockslide (Tyrol, Austria). Geomorphology 103, 93–103.

Thouvenot, F., Frechet, J., Jenatton, L., Gamond, J.F., 2003. The Belledonne Border Fault: identification of an active seismic strike-slip fault in the western Alps. Geophys. J. Int. 155, 174–192.

Vengeon, J.M., Giraud, A., Antoine, P., Rochet, L., 1999. Analysis of the deformation and toppling of rock slopes in crystallophyllian terrain. Can. Geotech. J. 36, 1123–1136.

Vincent, C., Kappenberger, G., Valla, F., Bauder, A., Funk, M., Le Meur, E., 2004. Ice ablation as evidence of climate change in the Alps over the 20th century. J. Geophys. Res. 109, D10104. doi:10.1029/2003JD003857