Réf. Sanchez & al. 2010 - A

Référence bibliographique complète

SANCHEZ, G., ROLLAND, Y., CORSINI, M., BRAUCHER, R., BOURLÈS, D., ARNOLD, M., AUMAÎTRE, G. 2010. Relationships between tectonics, slope instability and climate change: Cosmic ray exposure dating of active faults, landslides and glacial surfaces in the SW Alps. Geomorphology, Vol. 117, 1–2, 1–13.

Abstract: In the Argentera massif (French Southern Alps), large active landslides develop along strike of an active corridor of dextral strike-slip faults revealed by shallow ongoing seismicity. Glacially polished bedrock outcrops are offset by right-lateral strike-slip faults. Gravitational structures appear to be spatially connected to these active faults. Dating using the in situ-produced 10Be cosmogenic nuclide performed on glacial, tectonic and gravity surfaces. The late glacial–interglacial Holocene transition is constrained by 10Be ages between 12 and 15 ka obtained on glacially polished surfaces. The main tectonic activity closely post-dates the main deglaciation event and is constrained by 10Be ages of 11 and 7–8 ka obtained on fault scarps. Three successive periods of landsliding are recognized, at 11–12, 7–9 and 2.5–5.5 ka. These Holocene ages were obtained on right-lateral strike-slip fault scarps indicating that recent Alpine tectonics are expressed by transcurrent movements. The discussed close age relationship between deglaciation and a tectonic pulse may suggest that post-glacial rebound and enhanced pore water pressure do influence seismogenic tectonic activity. Gravitational destabilizations at 11–12 and 7–9 ka are coincidental with the main tectonic activity, and suggest tectonic shaking as a landslide trigger. The third gravitational destabilization at 2.5–5.5 ka could be attributed either to slope weakness resulting from multiple low-magnitude earthquake events, as currently revealed by the seismic activity or to climatic causes during the wetter optimum climatic period. These early and middle Holocene ages coincide with a phase of large landslide throughout the Alps scale which suggests that these large gravitational mass movements could be related to combined effects of intense tectonic activity and transitions form cold and dry period to warm and wetter phase.

Mots-clés

Tectonics - Landslide - Climate changes - Cosmic ray exposure dating - South-western Alps

 

Organismes / Contact

• GEOSCIENCE AZUR, UMR 6526, Université de Nice Sophia-Antipolis, 28 Av de Valrose, BP 2135, 06108 Nice, France
• CEREGE, UMR CNRS 6635, Université Aix-Marseille, P.O. Box 80, Europôle méditerranéen de l'Arbois, 13345 Aix-en-Provence Cedex 04, France
• CEA/DSM/LSCE, L'Orme des Merisiers 91198 Gif sur Yvette, France

 

(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

 

 

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

 

 

 

 

 

 

 

(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

 

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

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

 

 

 

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

Reconstitutions

10Be dating

10Be dating of polished glacial surfaces gave approximate ages ranging from 15.36 ± 0.54 ka to 8.13 ± 0.28 ka, which provides constraints on the timing of the last deglaciation (transition between the Last Glacial Maximum (LGM) and the interglacial Holocene).

10Be dating of fault scarps yielded approximate ages between 11.12 ± 1.01 ka and 7.68 ± 0.64 ka for the two dated faults. These ages were corrected using a fault scarp model, which takes into account the evolving geometry of the active fault. These absolute ages confirm the field relationships, as the main fault scarp dated at 11.12 ± 1.01 ka cross-cuts polished glacial surfaces dated at ~ 13 ka. The ages along the striae direction of the neighbouring minor parallel fault scarp are in good agreement and give a mean age of 7.68 ± 0.64 ka for the middle part and the outer edge of the scarp face. The sample Pra.06.08 located in the inner part gave an anomalous 10Be concentration which may be due to its confined position (16.35 ± 0.86 ka).

10Be dating of the landslide from the trench and landslide scarp provided approximate ages ranging from 5.65 ± 0.95 to 0.24 ± 0.06 ka. The landslide surface yielded an age of 5.02 ± 0.69 ka. A profile sampled down the main sliding surface, where it is exposed on the upper part of slope, provides ages decreasing from 4.50 ± 0.62 ka to 2.62 ± 0.24 ka. Thus, the observed age decrease pattern is in good agreement with a progressive exposure of the landslide footwall in response to rock mass failure propagation. The dating of the opened cracks yielded 10Be ages of 5.65 ± 0.95 ka, 3.12 ± 0.47 ka, 2.66 ± 0.41 ka, 0.84 ± 0.35 ka and 0.24 ± 0.06 ka, evidencing ongoing gravity motions since at least 5.70 ka.

In addition, the 10Be ages of the “La Clapière” landslide obtained by Bigot-Cormier et al. (2005) have been recalculated using a sea-level 10Be production rate of 4.5 atoms g− 1 year− 1 and re-evaluated according to the recently revised 10Be half-life (Nishiizumi et al., 2007). The recalculated 10Be ages for glacial polished surfaces range from 21.93 ± 5.32 to 13.65 ± 1.49 ka, while those of gravitational scarps range from 2.50 ± 0.68 to 12.75 ka ± 1.39.

Discussion

The main question addressed in this paper is “what are the triggering factors of Holocene landslides in the Alps”? Uplift combined with bedrock incision by rivers or glaciers, slope over-steepening, heavy precipitation, snow melting, and favourably oriented rock mass discontinuities such as joints, faults or schistosity are thought to produce sufficient relief to predispose slopes to catastrophic failures (Korup et al., 2007). However, triggering factors of large landslides are not well understood. The main processes proposed in the literature are the following:

(1) Decompression of slopes or ‘debutressing’ and subsequent stress release following glacier retreat (Augustinus, 1995, Ballantyne, 2002 and Cossart et al., 2008).

(2) Enhanced seismicity due to regional isostatic glacial rebound (Muir-Wood, 1989, Persaud and Pfiffner, 2004 and Hormes et al., 2008)

(3) Increased temperature and wetter conditions during middle Holocene climatic optimum (Ivy-Ochs et al., 2009 and Le Roux et al., 2009).

Phases of more frequent landsliding in the Alps are generally ascribed to climate change. Up to now, the impact of Holocene tectonics on rock slide initiation had rarely been discussed in this region despite any convincing dating of Holocene tectonic activity (Persaud and Pfiffner, 2004, Hippolyte et al., 2006 and Agliardi et al., 2009). In the following, we discuss these hypotheses in the light of new evidence and dating of active tectonic features in the south-western Alps.

1. Holocene tectonics activity and related landslides

From geological observations in the south-western Alps, offsets of LGM moraines and of recent alluvial sediments, fault breccias and speleothems have been identified along active seismic faults by Jomard, 2006 and Larroque et al., 2001. Nevertheless, no precise ages of such tectonic markers have been obtained so far. Further, Jomard (2006) proposed that these features are related to gravity movements.

Thus, for the first time in the south-western Alps, Early Holocene (11 and 8 ka) N140°E dextral kilometre-scale faults with up to 15 m of cumulative offset have been evidenced by both field analysis (glacially polished surface offsets) and 10Be dating.

Tectonic studies undertaken in the Western Alps (Sue and Tricart, 2003 and Champagnac et al., 2006) reported recent N–S normal faults resulting from brittle E–W extension. Here we show that in the south-western Alps these N–S extensional faults branch into a network of NW–SE (N140°E) right-lateral strike-slip faults. There are clear indications from ongoing seismicity that this dextral transtensive fault splay remains active (Jenatton et al., 2007). These data indicate that Alpine extensional movements can be associated with transtensional tectonics at the SW boundary of the rotating Apulian block (Collombet et al., 2002 and Delacou et al., 2008).

Moreover, the 10Be age profile undertaken on a single fault scarp allows an estimation of the earthquake slip history and average magnitude of individual seismic events. An age of 7.68 ± 0.64 ka for the fault activity has been estimated for the two outermost samples. Thus, given that the fault rupture behavior is a complex process, several cases have to be considered. (1) If an individual seismic event has occurred, the observed 1.5 m fault motion, which corresponds to the offset between the two outermost samples, would imply a magnitude M > 7 as predicted by the empirical equation of Wells and Coppersmith (1994). (2) It is also possible that some of the fault displacement may be attributed to multiple seismic events. This would require seismic recurrence of relatively high magnitude (M > 5) earthquakes as to produce such displacements at the surface, in less than 1 ka (as suggested by the individual 1σ error of the age estimates). (3) The fault offset may result from a slow creeping motion over the 1 ka age range. Such a hypothesis is supported by the occurrence of continuous seismic activity during periods of ≥2 year along the fault strike in the Jausiers area (Jenatton et al., 2007). It is difficult to choose between these different scenarios, and the displacement observed at the surface may result from a combination of coseismic slip and slow creep processes.

Consequently, what is the relationship between these tectonic events and landslide development? In the Alps, this relationship has rarely been proposed, except around the Insubric Line (Ambrosi and Crosta, 2006) and along the Belledonne fault (Hippolyte et al., 2006). In addition, it is generally considered that faults have a passive role in the development of large gravitational deformations (Di Luzio et al., 2004, Bois et al., 2008 and Agliardi et al., 2009). The hypothesis of seismic triggering in landslide development has yet not been supported by any clear temporal correlations of fault displacement and gravity features.

For the first time in the Alps we show spatial and temporal relationships between faults and landslides. Indeed, in the study area gravity structures nucleate along main active faults. The main sliding surface starts on a fault scarp suggesting a strong spatial link between the active fault and the Le Pra landslide. Cosmogenic dating shows three periods of correlated tectonic and gravitational activities:

(1) A first age group at 11–12 ka corresponds to both faults and gravitational scarps being exposed to cosmic rays. The errors associated to the 10Be dating recalculated from Bigot-Cormier et al. (2005) do not allow discrimination of the ages of faulting from those of gravitational destabilization, both being similar within error (< 1 ka).

 

(2) A second age group, corresponding to fault scarp exposure 7–8 ka ago, is compared to landslides structures dated at 7–9 ka. The fault ages are similar (within error) to the gravitational initiation age obtained in the La Clapière area while those of the le Pra area precede it by 2.0 ± 0.5 ka.

 

(3) Finally, a third 10Be age group mainly comprises surfaces exposed due to landsliding between 5.5 and 2.5 ka. This last age group includes (i) data obtained in the Le Pra area showing the propagation of a landslide surface between 5 ka and 2.6 ka, and (ii) ages of 2.5–3.7 ka obtained on gravitational scarps in La Clapière landslide.

These three age groups show:

(i) a coincidence of tectonic and gravity displacements (age groups 1 and 2). This coincidence suggests that some tectonic events did trigger landslides. As large surface fault displacements along active faults suggest, the strong seismic shaking originating from large rupture fault induces progressive movements along active or locally pre-existing faults that progressively break up the rock slopes and lead to their gravitational destabilization.

 

(ii) the propagation of gravity motions without any evidence of strike-slip displacements (age group 3 and current situation of most landslides in the valley). These large mass movements may develop in response to slope weakening subsequent to multiple low-magnitude earthquake events. Such ongoing low-magnitude shaking, which is evidenced by current seismicity (Jenatton et al., 2007), induces an increase in rock slope fracturing, specifically along the active fault and leading to gradual, long-term reduction in rock strength. Thus, this progressive rock slope softening is thought to lead to long-lasting gravitational slope deformations over periods of thousands of years.

At the scale of the Alps, these early Holocene fault ages (≈ 8 ka) coincide with a high landslide frequency (8–9 ka) in the whole Alpine arc which is commonly interpreted as the result of climate change, coinciding with a phase of global warming (Soldati et al., 2004, Hippolyte et al., 2006, Deplazes et al., 2007, Hippolyte et al., 2009, Ivy-Ochs et al., 2009 and Le Roux et al., 2009). In the light of active tectonics related to landslide triggering, we suggest that the development of large landslide events could be related to a phase of intense tectonic activity.

2. Impact of climate change on slope morphology

The previous section shows that active faulting in the Alps promotes slope instability and landslide generation. However, the impact of climate change in a current low seismicity area as the Alps cannot be excluded.

10Be ages obtained on polished glacial surfaces in the studied area allow us to estimate the end of the last deglaciation at 13–12 ka. This result is in accordance with climate records at global scale (ice cores — Alley, 2004; global sea-level rise — Waelbroeck et al., 2002) and also at the regional Alpine scale (pollen study — Ortu et al., 2008). This glacial–interglacial Holocene transition (11.5 ka) leading to the total ice cap retreat was altogether a short and complex period (3–4 ka; Waelbroeck et al., 2002) implying that the change to warmer conditions occurred rapidly. However, in the Mediterranean area, it is still not certain if these coincided exactly with warmer conditions. Several studies based on pollen (Ortu et al., 2008) and speleothem data (Zanchetta et al., 2007) have shown the general tendency to a warmer and wetter period at around 8 ka (Climatic Optimum). Paleolake-level data document wetter conditions only from 5 ka (Magny et al., 2007 and Ortu et al., 2008) while caution must be taken as seasonal variations may have influenced these paleolake-level data.

So, did deglaciation and wetter climatic conditions have any effects on tectonics and landslide activities?

This study shows that fault (11–8 ka) and landslide (≈ 10–8 ka) activities closely postdate the retreat of the Argentera massif ice cap (15–12 ka). This spatiotemporal coincidence between large fault displacements and large gravitational mass movements following abrupt climate change leads to the assumption that the melting of Alpine ice cap and residual permafrost should have directly conditioned fault and rock mass stability. It has been shown by some authors that uplift due to post-glacial isostatic rebound could have an influence on both seismogenic activity and gravity motions by rapidly changing the state of lithosphic stress (Muir-Wood, 1989, Stewart et al., 2000, Ballantyne, 2002, Hetzel and Hampel, 2005, Hormes et al., 2008 and Ustaszewski et al., 2008). These effects are shown to reduce lithostatic load (minimal stress axe: σ3) and to allow rupture of faults and facilitate rock mass failure. Increased pore groundwater pressure during glacial melting, permafrost degradation or heavy rainfall events have also been evoked to induce enhanced seismicity (Costain et al., 1987, Davies et al., 2001, Saar and Manga, 2003 and Christiansen et al., 2007) and landsliding (Caine, 1982 and Gruber and Haeberli, 2007). The rock slope in the studied Tinée valley is heavily fractured, structurally and lithologically contrasted (shistosity, shear zone). The water in fissure and tectonically stressed faults exert a fluid overpressure and enable the fault and rock slope to fail more readily. However, for the south-western Alps, the body of these assumptions needs to be further tested by additional CRE dating of Holocene faults because in the studied example only two fault ages have been obtained.

Another landslide triggering factor which has been suggested is increased precipitation (Dortch et al., 2009) during optimum climatic conditions in the Alps (Ivy-Ochs et al., 2009 and Le Roux et al., 2009). The important phase of generalized destabilization in the valley at around 4–5 ka could be correlated with an increase in cleft-water pressure during a phase of intense precipitation. The short-lived hydrologic oscillations during the middle Holocene tending to influence alteration could favour the development of slope instability. However, given that in the Argentera massif it appears that a phase of tectonic destabilization (11–8 ka) occurred prior to landslide development and that the paleohydrologic reconstructions are still a matter of debate, it seems unlikely that wet climatic phases are the only triggering factor for landslides.

3. Summary

The valleys in which the Le Pra and the La Clapière landsides occurred have a maximum height of 2 km, which results from tectonic, glacial, periglacial and fluvial processes. Local lithology and structure as fracture and foliation can play a major role in preparing rock slopes for landslides (Korup et al., 2007). These features induce rock anisotropy that probably leads to slope weakening (Follacci et al., 1988). Also, these large landslides are generally located along the active faults, and do not affect the whole massif. This suggests that lithology and structure of the rock slope are not the main triggering factors of landslides but may be important in predisposing the slope to failure and enabling it to destabilize more easily by seismic shaking, ice melting or heavy precipitation.

Therefore, our preferred evolutionary model would be (1) the occurrence of a major phase of tectonic activity at 11–8 ka leading to widespread slope fracturing; and (2) in the following evolution, the occurrence of the wet ‘climatic optimum’ conditions resulted in the alteration of the fractured bedrock. This weathering process led to a decrease in rock mass resistance and promoted landslide development.

Conclusions

For the first time in the south-western Alps, dating using in situ-produced 10Be has been successfully performed to estimate ages of recent fault slip. Holocene ages of 11 and 7–8 ka have been obtained on these active NW–SE (N140°E) dextral transtensive faults resulting from the rotation of the Apulian block. Coeval with a phase of intense landslide activity identified at 11–12 and 8 ka, this tectonic activity is also in concordance with large landslide in the Alps.

Moreover, here we have shown a case of spatial and temporal relationships of landslides with active tectonic faults. To summarize, the main contribution of this paper is that:

(1) significant horizontal tectonic displacements (≈ 1.5 m) could have occurred during the early Holocene period (7–8 ka) along an active dextral plurikilometre-scale fault system in the south-western Alps; and

(2) potential high magnitude seismic events or, at least, multiple low-magnitude earthquakes could trigger landslides as large gravity mass movements occurred in the same age range as recent fault activity.

Within the active tectonic framework, climate changes also have an influence on landslide development. However, the understanding of the climate-fault stability relationship in the Alps requires further studies.

Observations

 

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

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

 

Active tectonics and climate change are thought to be key parameters in the development of large slope movements in mountain belts (Keefer, 1984 and Ballantyne, 2002). It has been frequently recognized in high altitude (north Alps) and high latitude (Scandinavia) regions that climate change leads to landslide initiation and development (Soldati et al., 2004, Ivy-Ochs et al., 2009 and Le Roux et al., 2009) and influence tectonic activity (Muir-Wood, 1989 and Stewart et al., 2000). Post-glacial isostatic rebound (Cederbom et al., 2004 and Champagnac et al., 2007) as well as slope decompression due to ice cap retreat (Bovis, 1982, Augustinus, 1995 and Ballantyne, 2002) and increased pore groundwater pressure due to meltwater drainage or heavy rainfall (Caine, 1982, Costain et al., 1987 and Gruber and Haeberli, 2007) are the main climatic factors thought to influence fault and slope stability, especially during periods of climate change.

Earthquake shaking has also been invoked to contribute to large rock slope failure (Keefer, 1984). In zones of intense seismic activity, the relationship between active tectonics and rock slide development is clearly evidenced (San Andreas fault — McCalpin and Hart, 2003; Denali fault — M = 7.8 — Jibson et al., 2006). The strong ground acceleration and high frequency energy resulting from M > 7 earthquakes are shown to trigger large landslide and rockfall events. However, such relationships are more questionable in current low seismicity areas such as the Alps (Persaud and Pfiffner, 2004, Hippolyte et al., 2006 and Agliardi et al., 2009), where it is difficult to ascertain whether tectonic structures play an active or a passive role in slope gravitational deformation.

This present paper deals with the relationships between seismicity, landslide triggering and climate change. Using published data and new 10Be cosmic ray exposure dating (CRE) of fault, landslide and polished glacial surface pertinent features, the authors summarize tectonic activity, large landslide and climate history in the south-western Alps. We focus on the southern flank of the ArgenteraMercantour massif where active faults (Jenatton et al., 2007) and large landslides features (Bigot-Cormier et al., 2005) are spatially closely related. Thus, this area is appropriate to study such feedback relationships. However, no clear temporal correlations have been found between these three processes, mainly owing to quoted uncertainties in the available dating methods (Bigot-Cormier et al., 2005). The ongoing development of the cosmic ray exposure dating method using 10Be now allows the acquisition of ages with precision of a few percent, generally falling below the time recurrence intervals of seismic events and that of major climatic trends. The authors compare these landslides to other large landslides of known age in the Alps to explore possible temporal correlation and reinforce the discussion of possible causes and triggering mechanisms.

CRE dating of morphological and tectonic surfaces was performed using in situ-produced 10Be (Brown et al., 1991 and Siame et al., 2000). Three morphological surfaces have been sampled: (1) Polished glacial surfaces collected to date the last glacier retreat in the Tinée valley ; (2) Two vertical fault scarps with a maximum horizontal displacement of 15 and 5 m, respectively, sampled to date the last post-glacial tectonic offsets ; (3) The landslide surface and trenches formed by gravity motions, sampled in order to date the main landsliding and compare them with ages obtained on fault scarps. [See details in the study]

 

(4) - Remarques générales

 

 

(5) - Syntèses et préconisations

 

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