a late holocene deep-seated landslide in the northern french pyrenees
TRANSCRIPT
Geomorphology 208 (2014) 1–10
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Geomorphology
j ourna l homepage: www.e lsev ie r .com/ locate /geomorph
A Late Holocene deep-seated landslide in the northern French Pyrenees
T. Lebourg a,⁎, S. Zerathe a, R. Fabre b, J. Giuliano a,c, M. Vidal a
a Univ. Nice Sophia Antipolis, CNRS, IRD, Observatoire de la Côte d’Azur, Géoazur UMR 7329, 250 rue Albert Einstein, Sophia Antipolis 06560 Valbonne, Franceb UMR 5295, I2M-GCE, Avenue des facultés, 33405 Talence, Francec Aix-Marseille Université, CNRS-IRD-Collège de France, UM 34 CEREGE, Technopôle de l’Environnement Arbois-Méditerranée, BP80, 13545 Aix-en-Provence, France
⁎ Corresponding author. Tel.: +33 4 83 61 86 70; fax: +E-mail address: [email protected] (T. Lebourg)
0169-555X/$ – see front matter © 2013 Elsevier B.V. All rhttp://dx.doi.org/10.1016/j.geomorph.2013.11.008
a b s t r a c t
a r t i c l e i n f oArticle history:Received 23 July 2012Received in revised form 1 October 2013Accepted 11 November 2013Available online 23 November 2013
Keywords:Deep-seated landslidePyreneesCosmic ray exposure datingClimate changeCoseismic triggering
A very large deep-seated landslide (DSL) in the northern Pyrenees with over ∼1.4 × 109 m3 was mapped anddated based on sedimentation rates and 10Be terrestrial cosmogenic radionuclide surface exposure (CRE) dating.Our analysis on the landslide reveals the role of inherited structures in the landslide process, and highlighted typ-ical gravitational morpho-structures and a small lake trapped at the toe of the landslide head scarp. The rate oflake sedimentation (0.86 ± 0.57 mm yr−1) also provided us with the approximate age of the landslide:1106 ± 540 yr. The CRE dating result highlights two main slope destabilization phases. Then we discussed thehistory of DSL activity and its controlling factors. Information related to historic markers and the absence of par-ticular climate markers and changes during the Medieval Dark Ages point to a single event in AD 1380 due to amajor seismic event (Lavedan earthquake).
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Over the past decade, many researchers of gravitational mass move-ments have developed and improved the methods of morphologicalcharacterization, observation of dynamics, kinematic modeling, andgeochronological dating, to better understand landslide processes. Inmountain areas, large-scale landslide structures known as “deep-seatedlandslides” (DSLs) or “deep-seated gravitational slope deformations”(DSGSDs) often take place. DSGSDs are gravity-induced processeswhich evolve over a very long time and usually affect entire slopes,displacing rock volumes up to the order of 108 m3 over areas of severalkm2 with thicknesses of several 101 m. The main feature of these pro-cesses is the absence of a continuous surface of rupture and the presenceof a deep zone where displacement takes place mostly through rockmicro-fracturing (Radbruch-Hall, 1978). DSGSDs, defined by Malgot(1977), have been documented almost everywhere in the world butwith different terms such as sackung, gravity faulting, deep-reachinggravitational deformation, deep-seated creep deformation, gravitationalblock-type movement, gravitational spreading and gravitational creep.In spite of the variety of terms used, the most frequently used termsare sackung and lateral spreading. Sackung can be described as asagging of a slope due to visco-plastic deformations at depth which af-fect high and steep slopes made up of rocks with brittle behavior(Zischinsky, 1969; Crosta, 1996). Lateral spreading is the lateral expan-sion of a rock masse along shear or tensile fractures. Two main types of
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ights reserved.
rock spreading, under different geological situations, can be distin-guished: (1) affecting brittle formations overlying ductile units, oftendue to the deformation of the underlying material, and (2) in homoge-neous rock masses (usually brittle) without a well-defined basal shearsurface or a zone of visco-plastic flow (Pasuto and Soldati, 1996). Trig-gering and causal mechanisms of DSGSDs and DSLs (Gutiérrez et al.,2008; Agliardi et al., 2009) include: (1) postglacial debuttressing ofoversteepened slopes and associated changes in groundwater flow(Bovis, 1982; Agliardi et al., 2001; Ballantyne, 2002), (2) topographicstresses (Radbruch-Hall, 1978; Varnes et al., 1989), (3) regional tectonicor locked-in stresses (Miller and Dunne, 1996), (4) earthquake groundshaking or co-seismic slip along faults (e.g. Beck, 1968; Harp andJibson, 1996; McCalpin and Hart, 2003; Gutiérrez-Santolalla et al.,2005; Hippolyte et al., 2006; Guttiérrez et al., 2008), and (5) fluvial ero-sion of the toe of slopes (Crosta and Zanchi, 2000).
Recent studies on gravitational instabilities in the SouthernEuropean Alps (Mercantour–Argentera Massif and Subalpine chains)have allowed us to highlight large deep-seated landslides with specificmorphologies and their relationships with geology (Jomard, 2006; ElBedoui et al., 2009; Zerathe and Lebourg, 2012). These studies have in-dicated that geological structures andweathering processes significant-ly affect the development of the deep-seated landslides. However, thetriggering factors of such large-scale instabilities often remain un-known, especially for those occurred in the Late or Middle Holocene(Zerathe and Lebourg, 2012).
Most of the time large landslides undergo progressive rock degrada-tion, but sometimes affected by extreme events such as earthquakes, icemelt, and heavy precipitation (Crozier et al., 1995). This study aims toidentify such triggering factors of a large landslide in the Pyrenees
2 T. Lebourg et al. / Geomorphology 208 (2014) 1–10
based on cosmic ray exposure (CRE) dating, sediment analysis, and geo-morphological observations.
Only a few DSLs or DSGSDs have been identified in the Pyreneanmountain chains (Gutiérrez-Santolalla et al., 2005; Hürlimann et al.,2006; Gutiérrez et al., 2008), although the Pyrenees are broad steepareas with active tectonics (Barnolas and Chiron, 1996) and erosionthatwould promote gravitational processes. In the northern French Pyr-enees, previous studies identified some shallow rockslides (Fabre et al.,2001, 2002; Lebourg et al., 2003a,b), and a few DSGSDs were also re-ported there (Gutiérrez et al., 2012) as well as in the central SpanishPyrenees (Gutiérrez-Santolalla et al., 2005; Hürlimann et al., 2006;Gutiérrez et al., 2008).
In the present study, we focus on the recognition and characteriza-tion of the largest deep-seated landslide ever identified in the FrenchPyrenees (Gutiérrez et al., 2012) located in the Aspe Valley (AtlanticPyrenees). This Cristallere DSL involved 1.2–1.6 × 109 m3 of rock in acomplex geological setting of various lithologies, affected by faults andfolds, past glacial processes, and surrounding steep slopes (Figs. 1Band 2A). To constrain the age of the Cristallere DSL, we applied CRE dat-ing and studied the sediment deposits of a small lake trapped within alarge gravitational morpho-structure associated with the landslide.We also discuss the triggering factors of the DSL.
2. Geological setting
The study area is located in the Aspe Valley (Fig. 1), which is an oldglacial valley of the Atlantic Pyrenees.We have been studying this valleyfor 15 years mainly in relation not only to numerous shallow landslidesdeveloped in tills but also to the Cristallere DSL. Since the beginning ofthe 20th century when railway tracks were installed, there have beennumerous signs of active deformation, particularly on railway tracks,tunnels and houses.
The Aspe Valleywas heavily affected by the LastGlaciation, as shownby numerous morphological markers along the entire valley (Debourleand Deloffre, 1977; Lebourg et al., 2004). On about 40% of the outcrops,we have found the deposits of glacial moraines, unsorted and heteroge-neous materials of “till”. These formations are older than 10 kyr BP(Taillefer, 1969), and their deformations occurred in the Holocene.After the glacier retreat, the Aspe River cut the glacial valley throughby about 200 m (Fabre et al., 2002), leaving an alluvial terrace at thebase of the Cristallere slope.
Geologically the Aspe Valley belongs to the Pyrenean Axial Zone(Barnolas and Chiron, 1996) composed of Palaeozoic rocks (Fig. 2A)
France
Paris
Bordeaux
SW Baralet 2052 m a.s.l.
1 km
(A)
(B)
StudiedArea
Fig. 1. Location of the studied area. (A) Location of theAspeValley in France. (B) Three-dimensioboundary of the deep-seated landslide.
that have been folded and faulted during the Variscan and the Alpineorogeny. On the slopewith the Cristallere DSL, the fold axes are orientedfrom 90°N to 140°N. These are recumbent folds that are overturned tothe south (Lebourg et al., 2003a), with their axes oriented east towest, with low westward axial plunge. The Cristallere slope consists ofthree Palaeozoic units (Ternet et al., 2004). From top to bottom:
(i) The Devonian units characterized by limestone, forming a glacialrock bar near the Baralet Peak. These constitute an anticline foldwith a 120°N axis that is bounded by two 130°N sub-verticalfaults;
(ii) The Carboniferous units characterized by alternating black shaleand blue-gray sandstone. This lithology constitutes the majorpart of the Cristallere slope, occupying between 700 and1400 m a.s.l., and;
(iii) The Permian units characterized by alternating sandstone andred shale. They outcrop at the top of the Cristallere slope(1854 m a.s.l.). The upper part of the units consists of limestonewith siliceous breccias and conglomerates. We collected thehydro-thermal quartz used for 10Be dating from this formation.
3. Materials and methods
Our study began with an analysis of high-resolution satellite imagesto identify lineaments and typical topographic anomalies. This wasfollowed by field surveys and geological analysis. During the field inves-tigations, we performed structural measurements of faults, fracturesand bedding planes, together with a precise analysis and mapping ofgravitational morpho-structures, to reveal the role of inherited struc-tures in landsliding (Margielewski, 2006). We paid particular attentionto gravitational morpho-structures to identify landslide boundaries. Toconstrain the timing of landsliding and to understand its kinematics,we used two dating methods: analysis of lake deposits and CRE datingusing 10Be.
3.1. Lake sediment analysis
Lake sediments have recorded climatic events (Appleby, 2000).Lakesmay be formed after a glacial retreat or a landslide, leading to sed-imentation (Wojciech, 2004). Analysis of lake sediments including theestimation of sedimentation rates may indicate when the lake wasformed. Table 1 gives some examples of sedimentation rates obtainedfor European and Canadian mountain lakes. For lakes close to
Aspe
river
NE
Urdos
Cristallere 1874 m a.s.l.
nal viewof the Cristallere slope (fromGoogle Earth).White dashed line corresponds to the
Asperiver
2000
1600
1200
800
m a.s.l.
2000
1600
1200
800
m a.s.l. A A’W E
400 m
Fig. 3AC
Asp
e riv
er
Bar
alat
riv
er
Bel
once
rive
r
Lac ofEstaens
Asp
e ri
ver
0 1 km
Urdos
42.86°
42.89°
42.83°
42.80°
42.86°
42.89°
42.83°
42.80°
-0.55°-0.60°
-0.55°-0.60°
Permian unit
Unconformable alluvium
Carboniferous unit
Devonian unit
Fault
Anticline axis
Syncline axisRiver
Cristalleredeep-seated landslide
Baralet 2052 m
Cristallere 1874 m
Arlet 2207 m
A
Baralet 2052 m
Cristallere 1874 m
Sampledscarp
0
A
-0.575° -0.560°
-0.575° -0.560°
42.865°
42.855°
42.845°
42.865°
42.855°
42.845°
Main scarp
Second scarp Fan
Counter slope scarp Studied lake
Scarp facet Sagging area
Cross sectionFig. 2C
B
500 m
Fig. 2.Morpho-structural settings of the studied area. (A) Geo-structural map around the Cristallere landslide. (B) Morpho-structural map of the Cristallere slope showing the main fea-tures of the gravitational slope deformation. (C) Simplified cross section of the Cristallere deep-seated landslide.
3T. Lebourg et al. / Geomorphology 208 (2014) 1–10
our study area, Appleby (2000) and Catalan et al. (2002) reported sed-imentation rates of 0.83 and 0.67 mm yr−1, respectively. Similarly, inSwitzerland, Appleby (2000) and Schimd et al. (2010) obtained valuesfrom 0.58 to 2.0 mm yr−1. To obtain the approximate age of theCristallere landslide lake sediments, we used a calculation based on sed-iment depth and sedimentation rate estimation considering a biblio-graphical review (Table 1).
3.2. Cosmic ray exposure dating
The main landslide scarp was dated using the CRE dating method.The method quantifies various geomorphological processes such aspaleo-landslide kinematics (Le Roux et al., 2009). It is based on the
accumulation of cosmogenic nuclides produced through nuclear reac-tions between high-energy cosmic radiation and stable isotope compo-nents of rock. As the in-situ production rate of cosmogenic nuclidesdecreases exponentially with depth, the concentration of cosmogenicnuclides in the rock can be linked to the history of near-surface rock ex-posure (see Gosse and Phillips, 2001 for a review).
To determine the initiation age and the kinematics of the Cristallereslope failure, five hydrothermal quartz samples were extracted follow-ing a vertical profile along the head-scarp surface (Figs. 4A and 5).Thesewere used for the 10Bemeasurements (Table 2). Three partial dis-solutions in 48% hydrofluoric acid were used to dissolve ~10% of eachsample, to decontaminate them from potential atmospheric 10Be, andthus to ensure that only the in-situ produced 10Be was considered.
Table 1Example of sedimentation rates recorded for some glacial lakes.
Lake Country Altitude(m a.s.l.)
Sedimentation rate(mm yr−1)
Core depth(mm)
Mean annual rainfall(mm yr−1)
Reference
Saanajarvi Finland 679 0.26 56 422 Appleby (2000)Barrancs Lake Spain 2360 1.85 700 ? Larrasoaña et al. (2010)Ovre Neadalsvatn Norway 728 0.55 118 1500 Appleby (2000)Nizne Terianske Slovakia 1941 0.36 60 1775 Appleby (2000)Gossenkôllesee Austria 2417 0.56 108 1300 Appleby (2000)Hagelsee Switzerland 2339 0.58 110 1820 Appleby (2000)Jezero v Ledvicah Spain 1830 0.83 190 2619 Appleby (2000)Redo Spain 2240 0.67 90 1328 Catalan et al. (2002)Engstlen Switzerland 1853 2.0 100 1456 Schimd et al. (2010)Stein Switzerland 1756 1.6 100 1456 Schimd et al. (2010)Tantare Canada 450 0.9–1.3 1500 ? Feyte et al. (2012)Bédard Canada 680 0.5–2.6 1000 ? Feyte et al. (2012)Holland Canada 475 0.5.4 1100 ? Feyte et al. (2012)
? = data not reported.
4 T. Lebourg et al. / Geomorphology 208 (2014) 1–10
After the addition of 100 μl of a 3.025 * 10−3 g 9Be·g−1 Be spiking so-lution (Merchel and Herpers, 1999; Merchel et al., 2008), the purifiedquartz was totally dissolved in an excess of 48% hydrofluoric acid, andthe beryllium was separated by two successive solvent extractions(Bourlès et al., 1989). The target purified beryllium oxide was preparedfor 10Bemeasurements at the ASTER (Accelerator for Earth Sciences, En-vironment and Risk) mass spectrometry accelerator National Facility inFrance, at the CEREGE laboratories (Aix en Provence, France).
The 10Be data were calibrated directly against the National Instituteof Standards and the Technology Standard Reference (material 4325)using an assigned 10Be/9Be ratio of 2.79 ± 0.03 × 10−11 (Nishiizumiet al., 2007) and a 10Be half-life (T1/2) of 1.387 ± 0.012 × 106 years;i.e. a decay constant of 4.997 ± 0.057 × 10−7 yr−1 (Chmeleff et al.,2010; Korschinek et al., 2010). The analytical uncertainties here includ-ed the counting statistics, themachine stability (ca. 0.5%), and the blankcorrection (10Be/9Be blank = 0.256 × 10−14).
A 10Be production rate at sea level and at high latitudes of 4.49 ±0.39 atoms g−1 yr−1 was computed using the Stone scaling scheme(Stone, 2000). The CRE ages were determined from the 10Be concentra-tions using the following equation:
C x;t;εð Þ ¼St � Pn � Snε∧n
þ λ� e− x
∧n � 1−e−t ε∧nþλ� �� �
þ Pμs � St � Sμsε∧μs
þ λ:e−
x∧μs : 1−e
−t ε∧μsþλ
� �" #
þ Pμf � St � Sμfε∧μf
þ λ:e
− x∧μf : 1−e
−t ε∧μf
þλ
� �" #
ð1Þ
where C(x,t,ε) is the 10Be concentration as a function of the depthx (g cm−2), taking into account a rock density of 2.6 g cm−3, the expo-sure time t (yr) and the erosion rate ε (g cm−2 yr−1); St is the topo-graphic shielding (Dunne et al., 1999); Pn is the spallation; Pμs and Pμfare the spallation, slow and fast muon production rates, respectively,at sea level and high latitudes; Sn, Sμs and Sμf are the scaling factors forneutron, slow and fast muons, respectively; and ⋀n, ⋀μs and ⋀μf arethe attenuation lengths for neutrons (160 g cm−2), slow muons(1500 g cm−2) and fast muons (4320 g cm−2), respectively. Themuon schemes followed Braucher et al. (2011) and there scaling factorswere calculated for only for elevation. The sea-level and high-latitudespallogneic production rate (Pn) was scaled for the sampling altitudesand latitudes of the Cristallere head-scarp, using the scaling factors pro-posed by Stone (2000). The quartz erosion rate was assumed to be10 μm yr−1 (Ivy-Ochs et al., 2006), although erosion has little influenceover a time range of few thousand years. Indeed, assuming no erosioninstead of a 10 μm yr−1 erosion rate lowers the calculated CRE ages
by less than 1%. All analytical and chemical data for CRE dating are pre-sented in Table 2.
4. Results
The strategy of our study focused on three aspects: analysis of grav-itational slope deformation, deformation ‘clocks’, and time scale of trig-gering factors. The clocks we used are the comparative markers of themost recent gravitational deformations (landslide failure responsiblefor scarp exhumation) and the dating of scarps by the cosmonucleides10Be method.
4.1. Gravitational slope deformations
In the literature (Zischinsky, 1966; Beck, 1968; Bovis and Evans,1995), DSLs have similar morphological characteristics, withcounterslope, double ridges, escarpments and large extensions thatcan affect an entire slope (Varnes, 1978; El Bedoui et al., 2009). We lo-cated such morphostructures in the study area, including a doubleridge with a lake located 60 m below the Cristallere peak.
The recognition and mapping of the morphological structures of theCristallere landslide slope are based onfive particular geomorphologicalfeatures. Defined from the top to the bottom of the slope:
(i) At the top of the Cristallere slope, below the ridge line, there is asignificant main escarpment oriented from north to south, withan offset ranging from a few meters to more than 120 m belowthe Cristallere peak. At this level, there is a 1.6-km-long breakin the slope with a counter-slope, against which a lake is trapped(Figs. 2B and 3A,B).
(ii) The north-to-south oriented counter-scarp between theCristallere peak and the Courmette peak (1950 m a.s.l.) is associ-ated with fractures oriented from 10°N to 170°N, with a dip of45°E to 50°E. It corresponds to the upper part of the landslidefracture surface that affects the entire slope. The failure plane isdeeply eroded and stripped in the south near the peak of Baralet,where it has been feeding a 50-m-wide alluvial fan. The erosionreveals a series of listric faults that penetrate into the deeperside. These listric faults are associated with the main failure.
(iii) At the Cristallere peak, there is a north-to-south double crest.This double crest corresponds to the collapse of the easternside along the failure plane oriented north to south.
(iv) Immediately below the main escarpment and at the top of thehillside, there is a steep slope with several dihedral structureswith orientations of 50°N to 60°N, 90°N, and 120°N to 130°N,which correspond to fracturesmeasured in the Permian and Car-boniferous geological formations.
Baralet 2052 m EW
100 m
WE
Drilledlake
Fig. 4
Baralet 2052 m
Double ridge
40 m
A
B
C
Cristallere1874 m EW
Drilledlake
Fig. 4
Fig. 5
50 m
Fig. 3. Interpretedfield photos illustrating themain gravitationalmorpho-structures of the Cristallere DSL. (A and B)Views of the Cristallere head-scarp, its associateddouble-ridge and thetrapped lake. Red dashed line in (A) highlights a benchmark level in the Permian layers and the white line in (A) shows a counter-slope scarp. (C) View of the southern part of theCristallere slope showing the main scarps and counter-slope scarp.
5T. Lebourg et al. / Geomorphology 208 (2014) 1–10
(v) Aflat area in the valley corresponds to the toe of the landslide. Thistoe is characterized by a Quaternary alluvial terrace that covers upto 200 m east of the Aspe River, corresponding to the maximumforward shift of the Cristallere landslide (Fig. 2B). This deformationaffects the Last Glacial (Würm) moraines and distorts the entireslope bottom (Barnolas and Chiron, 1996), indicating that the ageof this movement is younger than 10 kyr BP (Fabre et al., 2003).
The landslide analysis shows the external and internal structures ofthe Cristallere landslide:
a. The mass is largely fractured in decametric elements;b. The landslide front is not eroded by the glacial erosion, and there are
a topographic anomaly at the front and a lift against the slide top,with a release of more than 200 m;
12345678910
? ?
1794
m a.s.l.
1790
1786
1794
m a.s.l.
1790
1786
x
EW
Core X Gravitationalfault
Lake
Lake sediment
Permian substratum
2 m
20 m
EWCristallere
1874 m Fig. 5Collapse
Fig. 4B
A
B
Fig. 4. Cristallere Lake. (A) Photo showing the drilled lake and the location of counter-slope scarps (or antithetic gravitational faults). (B) Cross-section of the lake, showing the locations ofthe sample cores taken in the lake.
6 T. Lebourg et al. / Geomorphology 208 (2014) 1–10
c. The interpolation of the lateral boundaries of the landslide, aswell asthe observed deep gravitational morpho-structures, led us to pro-pose a propagation of the gravitational failure until 300 to 400 mdeep (Fig. 2C).
Fig. 5. Hydrothermal quartz sampled for the cosmogenic 10Be dating (hammer size isabout 30 cm).
For the landslide length of about 3600 m, the width of 2200 m andthe thickness of about 350 m, the rock volume involved based on ap-proximation using a half ellipsoid is about 1.4 ± 0.2 × 109 m3.
4.2. Lake sediment benchmark
The Cristallere Lakewas formed after the slope collapse, so the age ofthe sediment will give a lower time limit. The lake is located on the topof the main scarp at an altitude of 1785 m a.s.l. The cumulated muddydeposits in the lake allowed us to determine their age and sedimenta-tion rate. Ten cores with a mean depth of 950 mmwere taken (Fig. 4B).
From some high mountain lakes in temperate zones, sedimentationrates between 0.75 and 1.9 mm yr−1 have been reported (Appleby,2000; Catalan et al., 2002; Morellón et al., 2009, 2011; Larrasoaña etal., 2010; Schimd et al., 2010; Feyte et al., 2012). Using this interval,we calculate an average sedimentation rate of 0.86 ± 0.57 mm yr−1.From this rate and amean sediments thickness of 950 mm, we estimat-ed the earliest formation of the lake at 1106 ± 540 yrs ago.
4.3. Cosmic ray exposure dating
Measuring the in-situ produced 10Be concentrations gave valuesranging from 1.32 × 104 to 2.44 × 104 atoms g−1. The 10Be CRE ages,calculated with the methods described in Section 3, are presented inthe Fig. 6. These ages are given according to the locations of the samplesalong the scarp. As we had only four age samples due to the lack of
Table 2Cosmogenic 10Be analytical data.
Sample Latitude(°)
Longitude(°)
Z(m)
DT
(m)St P0
(atm g−1 yr−1)
10Be/9Be(×10−14)
10Be(×103 at g−1)
Exposure age(yr)
CR10Be-C 42.8536 0.5722 1850 27.9 0.79 18.63 3.401 ± 0.28 16.18 ± 1.16 1083 ± 78CR10Be-D 42.8536 0.5722 1852 25.7 0.79 18.63 3.178 ± 0.22 13.88 ± 0.96 929 ± 64CR10Be-E 42.8536 0.5722 1869 9.4 0.79 18.63 2.594 ± 0.2 13.19 ± 1.01 881 ± 68CR10Be-F 42.8536 0.5722 1875 3 0.95 18.63 5.266 ± 0.05 24.49 ± 1.16 1299 ± 62
Z: altitude; DT: distance from the top of the scarp; St: topographic shielding; P0: production rate.
7T. Lebourg et al. / Geomorphology 208 (2014) 1–10
quartzite outcrops, a classical approach using linear regression does notfit the data well (Fig. 6A). Nevertheless, the results provide interestingconstraints on the kinematics and the timing of the failure. First, theresults highlight the very young ages, ranging between ca. 880and 1300 yrs ago (Table 2). Second, according to the χ2 statistics(Ward andWilson, 1978),we found that the three 10Be CRE ages obtain-ed in the lower part of the scarp represent a single population(χ2
95% = 4.09), with a weighted mean of 960 ± 40 yrs ago (Fig. 6B).These data suggest two destabilization phases: (i) a failure around1300 ± 60 yrs ago whose kinematics remains speculative but mightbe progressive; and (ii) a second phase around 960 ± 40 yrs ago forwhich the10Be CRE age distribution (Fig. 6B) indicates the occurrenceof a single gravitational event with an instantaneous rupture.
5. Discussion
The age of a landslide can be constrained by morphological observa-tions as well as relative and absolute ages. Themainmorphological ele-ments in the study site onwhichwe base our analysis are: (i) preservedmorphology of the landslide (no glacial action); (ii) presence of the lakeat the top of the landslide with undisturbed sediments; and (iii) rapidmovement of a mass. In this study, we analyzed a very large DSL witha vertical offset of around 120 m at the top of the slope, and a horizontaldisplacement of over 200 m along the river.
The results from the CRE dating and the lake sediment analysis sug-gest the following two scenarios:
1. Landslide initiation around 1300 ± 60 yr, followed by a slow andprogressive deformation, and fast deposition around 960 ± 40 yrwith a new triggering factor (seismic or climatic).
30
25
20
15
10
5
00 500 1000 1500 2000
Exposure age (yr)
Dis
tanc
e fr
om th
e to
p of
the
scar
p (m
)
A
-2
Fig. 6. CRE results. (A) Plot of 10Be CRE ages versus distance along the scarp. (B) Gaussian analyssample F. The black curve corresponds to the summed probability of samples C, D and E.
2. A single landslide event around 1300 ± 60 yr, and erosion at thebase of the main scarp.
To examine these two scenarios, wemust analyze paleoclimatic andpaleoseismological data for the northern Pyrenees and analyze as possi-ble landslide triggering factors.
5.1. Seismological factor
Triggering factors of landslides include internal rockweathering andexternal seismic or climatic forcing. Most previous studies deal with theeffects of earthquakes and rainfall for recent shallow landslides (Keefer,1984, 2000, 2002; Stark and Hovius, 2001). However, such studies onlarge and older landslides have been limited.
Landslides induced by earthquakes have been studied for a longtime, using historical documents (e.g., Seed, 1968; Keefer, 2002;Malamud et al., 2004). Most of these studies have described shallowlandslides (less than 106 m3). Very large landslides such as DSLs causedby earthquakes have not been well described (Sing et al., 2008), exceptsome such as Jibson et al. (2004) on landslides in Alaska triggered in2002.
Rodriguez et al. (1999) and Keefer (2002) investigated relationshipsof the characteristics of shallow landslides with earthquakes, the areainfluenced (Keefer, 2002), the distance from the epicenter (Keefer,2002), and the intensity of shaking. However, as noted above, the influ-ence of seismic events on large and deep landslides remains uncertain.
The Pyrenean range is characterized bymoderate seismic activity, al-though there have been strong earthquakes (Fig. 7; Lambert and Levret-Albaret, 1996), including some with epicentral intensities greater thanVIII near the Aspe Valley during the last 1500 years (Lambert and
Exposure age (yr)
Pro
baili
ty (
10)
1.4
1.2
1
0.8
0.6
0.4
0.2
00 500 1000 1500
B
is of CRE ages. The solid blue lines show samples C, D and E, and the dashed blue line shows
M > 54 < M < 53 < M < 42 < M < 3 M < 2
580
I = IXI = VIII
Historical seismicity:
Instrumental seismicity:
Cristallerelandslide
without calculatedintensity
1543
580
1518
1302
1
2
3 4
-2° -1° 0° 1° 2° 3°44°
43°
42°
44°
43°
42°
-2° -1° 0° 1° 2° 3°
Fig. 7. Instrumental seismicity, historical seismicity and uncalculated seismicity of the Pyrenees range (modified fromHonoré et al., 2011) and other coseismic DSGSDs (sackungs and lat-eral spreading) in the Pyrenees range. 1: Zarroca et al. (2012); 2: Gutiérrez et al. (2012); 3: Gutiérrez-Santolalla et al. (2005); and 4: Gutiérrez et al. (2008).
8 T. Lebourg et al. / Geomorphology 208 (2014) 1–10
Levret-Albaret, 1996), and some other potentially relevant ones de-scribed by BRGM (2004). The most important examples are shown inTable 3.
The difficulty of working with such past events is the lack of infor-mation about events such as large earthquakes (Honoré et al., 2011).These are rarely identified clearly beyond the last millennium in non-populated mountain areas. Indeed, written and verbal memories rarelydate back to 1000 yrs ago except some exceptional events such as theLavedan earthquake 1380 yrs ago. A document related to this earth-quake (BRGM, 2004; www.sisfrance.net) states that: “The city ofBordeaux was severely shaken by the earthquake, leading to critical condi-tions of the city including its walls. Most peoplewere afraid of death and be-lieved that the city was opened in two. The same applied to many othercities. In the Pyrenees Mountains huge stones fell and many persons andcattle died….”
We have very little quantitative information on this event, and the“intensity” and the “evidence” in some historical records lead us tonote the positive correlation between this event and the landslide. It
Table 3Major earthquakes with epicentral intensities N VIII, occurred near the Aspe Valley duringthe last 1500 yrs.After BRGM (2004).
Date yr cal. AD Location in France Distance from Aspe Valley(km)
Intensity
580 Lavedan 45 N 81302 Bigorre 50 ± 5 N 81518 Bigorre 50 ± 5 N 81543 Mauleon 48 ± 5 N 81660 Bigorre 50 ± 5 8.51750 Bigorre 50 ± 5 7.51854 Lavedan 45 7.51967 Arette (Bearn) 29 8
would be presumptuous to directly link this earthquake to the trigger-ing factor of the Cristallere landslide. However, this cannot be totally ex-cluded because this area, ca. 50 km from Aspe Valley, has been the siteof many large earthquakes (Table 3).
5.2. Climatic factor
It is also important to examine other possible triggering factors oflandslides such as climatic events. Indeed, many studies such asBorgatti and Soldati (2002) and Soldati et al. (2004) have found positivecorrelations between changes inweather patterns that can cause distur-bances of the hydraulic system and major slip failures (Bigot-Cormieret al., 2005; El Bedoui et al., 2009).
The impact of climate change since the Last Glacial Maximum(18 kyr) on hydrological processes can be significant, and the literatureoffers many interpretations in relation to both temperature and rainfall.Throughout the world, most landslides are caused by rainfall. However,unlike shallow landslides, DSLs need more than heavy rainfall over ashort period such as specific climate change. If a landslide depth ismore than 300 m, particular conditions are needed (Van Beck, 2002).Hydrological processes that trigger landslides significantly vary withtime and affect weathering processes (Lebourg et al., 2011). Potentiallandslide activitymust be assessed in terms of its spatial extent, tempo-ral frequency, and weathering conditions (Van Beck, 2002). For timescales of less than 100 years, environmental conditions controllinglandslide activity are associated with the power law of landslide sizeand frequency (Dai and Lee, 2001; Guzzetti et al., 2002). On longertime scales, the activity of larger landslides may be more influencedby climate change (Borgatti and Soldati, 2002; Soldati et al., 2004).
Previous studies in mountain regions using cosmogenic dating haveshown the impact of Holocene climate change on landslides (Bigot-Cormier et al., 2005; El Bedoui et al., 2009). The largest slope move-ments in the Alps (e.g. Clapière, Séchilienne, and Marbrière (Zerathe
Table 4Climate change periods of the Late Holocene epochs (modified afterDesprat et al., 2003; Morellón et al., 2009, 2011). Bold periods correspondto the potential timing of the Cristallere landslide initiation.
Late Holocene epoch Time period(yr)
Recent Warming 100Little Ice Age 100 to 550/600Medieval Warm Epoch 550/600 to 1000Dark Ages 1000 to 1500Romain Warm Epoch 1500 to 2500
9T. Lebourg et al. / Geomorphology 208 (2014) 1–10
and Lebourg, 2012) have been linked with the melting and retreat ofglaciers that initiated DSLs due to decompression relaxation. However,even for slopes that were not covered by glaciers, increased waterflow in the rock has also related to landslide initiation (e.g., in the Mar-itime Alps; Zerathe and Lebourg, 2012).
Our literature review on climatic changes in the Pyrenees providesinformation related to temperature evolution, but that directly relatedto the Late Holocene rainfall has been limited. The most propitioustimes related to the Cristallere landslide, however, appear to be theMe-dieval Warm Epoch and the Dark Ages (Table 4). The Medieval WarmEpoch was first defined by Lamb (1965) based on historic informationand paleoclimatic data from western Europe. He found evidence forwarm, dry summers and mild winters around 1100 to 1200 yr BP.This epoch or Medieval Climate Anomaly (Moreno et al., 2012) wasalso associated with widespread hydrological anomalies from 1150 to700 yr BP (Stine, 1994; Moreno et al., 2012). For this epoch, Bradleyand Jones (1993) indicatedprolongeddrought in some areas and excep-tional rains in other areas, suggesting widespread changes in the watercirculation regimes. Recent studies on global warming (Mann et al.,1998, 1999; Goosse et al., 2006) related these changes in precipitationand hydrologic conditions to the temperature rise. Furthermore, severalstudies have suggested that such global warming will be associatedwith abrupt changes in precipitation or temperature rather than slow-phase transitions (Alley et al., 1997; Anderson et al., 1998; Campbellet al., 1998). Bradley and Jones (1993) and Mann et al. (1998) indicatethat reconstructing such hydrological changes is more difficult thanreconstructing paleo-temperatures, because the former is more geo-graphically restricted. Consequently, precipitation and the associatedhydrological variability must be considered on a regional scale. This isproblematic for us because information on regional climate changes islimited.
6. Conclusions
The Cristallere DSLwas a landslide of about 1.4 × 109 m3, and one ofthe largest in the French Pyrenees. This landslide is characterized byspecific gravitational morphostructures including counter slopes anddouble ridges partly under the lake. This lake is located near the top ofCristallere Mountain, and was created after the landslide event.
Chronological markers such as CRE dating and lake sediments haveprovided two possible scenarios for the occurrence of the Cristallerelandslide: (i) landslide initiation at 1300 ± 60 yrs ago, with slow andprogressive deformation, then around 960 ± 40 yrs ago faster move-ment triggered by an earthquake or a climatic factor; and (ii) a singlelandslide event around 1300 ± 60 yr, and subsequent erosion at thebase of the landslide.
Information related to historicmarkers and the absence of particularclimatemarkers suggest a higher probability of the second scenario. Thesecond scenario can also be supported by a major seismic event around1380 yr (the Lavedan earthquake). By contrast, the absence of a largewatershed above the landslide and the lack of major climatic changebetween 1000 and 1500 yrs ago indicate a limited impact of climatechange.
Acknowledgments
We would like to thank Régis Braucher and Didier Bourlès at theCEREGE for their help with the CRE ages calculation. We also thankFrançoise Courboulex, Anne Deschamp, Laetitia Honoré, MatthieuSylvander, and Alexis Rigaud for fruitful discussion and information onseismological data and historical earthquakes.
References
Agliardi, F., Crosta, G., Zanchi, A., 2001. Structural constraints on deep seated slope defor-mation kinematics. Eng. Geol. 59, 83–102.
Agliardi, F., Crosta, G.B., Zanchi, A., Ravazzi, C., 2009. Onset and timing of deep-seatedgravitational slope deformations in the eastern Alps, Italy. Geomorphology 103,113–129.
Alley, R.B., Mayewski, P.A., Sowers, T., Stuiver, M., Taylor, K.C., Clark, P.U., 1997. Holoceneclimatic instability: A prominent, widespread event 8200 yr ago. Geology 25,483–486.
Anderson, D.E., Binney, H.A., Smith, M., 1998. Evidence for abrupt climatic change innorthern Scotland between 3900 and 3500 calendar years BP. The Holocene 8,97–103.
Appleby, G., 2000. Radiometric dating of sediment record in European mountain lakes.Paleolimnology and ecosystem dynamics at remote European Alpine lakes. Limnol.59, 1–14.
Ballantyne, C., 2002. Paraglacial geomorphology. Quat. Sci. Rev. 21, 1935–2017.Barnolas, A., Chiron, J.C., 1996. Synthèse géologique et géophysique des Pyrénées.
Géophysique. Cycle Hercynien. Introduction, 1. BRGM, Orléans (729 pp.).Beck, A.C., 1968. Gravity faulting as a mechanism of topographic adjustment. N. Z. J. Geol.
Geophys. 11, 191–199.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.
Borgatti, L., Soldati, M., 2002. In: Ryba´r, J., Stemberk, J., Wagner, P. (Eds.), The Influence ofHolocene Climatic Changes on Landslide Occurrence in Europe. Landslides. A.A.Balkema Publishers, Lisse, pp. 111–116.
Bourlès, D.L., Raisbeck, G.M., Yiou, Y., 1989. 10Be and 9Be in marine sediments and theirpotential for dating. Geochim. Cosmochim. Acta 53, 443–452.
Bovis, M.J., 1982. Uphill-facing (antislope) scarps in the Coast Mountains, southwestBritish Columbia. Geol. Soc. Am. Bull. 93, 804–812.
Bovis, M.J., Evans, S.G., 1995. Rock slope movements along the Mount Currie “fault scarp”,southern Coast Mountains, British Columbia. Can. J. Earth Sci. 32, 2015–2020.
Bradley, R.S., Jones, P.D., 1993. “Little Ice Age” summer temperature variations: theirnature and relevance to recent global warming trends. The Holocene 3,367–376.
Braucher, R., Merchel, S., Borgomano, J., Bourlès, D., 2011. Production of cosmogenic ra-dionuclides at great depths: a multi element approach. Earth Planet. Sci. Lett. 309,1–9.
BRGM, 2004. Histoire et caracteristiques des seismes ressentis en France metropolitaineet sur ses abords, SisFrance catalog. Available at http://www.sisfrance.net/ (lastaccess 2012 June).
Campbell, I.D., Campbell, C., Apps, M.J., Rutter, N.W., Bush, A.B., 1998. Late Holocene1500 yr climatic periodicities and their implications. Geology 26, 471–473.
Catalan, J., Pla, S., Rieradevall, M., Felip, M., Ventura, M., Buchaca, T., Camarero, L., Brancelj,A., Appleby, P.G., Lami, A., Grytnes, J.A., Agustí-Panareda, A., Thompson, R., 2002. LakeRedó ecosystem response to an increasing warming in the Pyrenees during the twen-tieth century. J. Paleolimnol. 28, 129–145.
Chmeleff, J., von Blanckenburg, F., Kossert, K., Jakob, J., 2010. Determination of the 10Behalf-life by multicollector ICP–MS and liquid scintillation counting. Nucl. Inst.Methods Phys. Res. B 268, 192–199.
Crosta, G., 1996. Landslide, spreading, deep seated gravitational deformation: analysis, ex-amples, problems and proposals. Geogr. Fis. Din. Quat. 19, 297–313.
Crosta, G.B., Zanchi, A., 2000. Deep-seated slope deformations. Huge, extraordinary, enig-matic phenomena. In: Bromhead, E., Dixon, N., Ibsen, M. (Eds.), Landslides inResearch, Theory and Practice. Thomas Telford, Proc. 8th Int. Symposium Landslides,Cardiff, June 2000. Thomas Telford, pp. 351–358.
Crozier, M.J., Deimel, M.S., Simon, J.S., 1995. Investigation of earthquake triggering fordeep-seated landslides, Taranaki, New Zealand. Quat. Int. 25, 65–73.
Dai, F.C., Lee, C.F., 2001. Frequency–volume relation and prediction of rainfall-inducedlandslides. Eng. Geol. 59, 253–266.
Debourle, A., Deloffre, R., 1977. Pyrénées occidentales. Annales de Géographie 86 (478),739–741.
Desprat, S., Sánchez Goñi, M.F., Loutre, M.F., 2003. Revealing climatic variability of the lastthree millennia in northwestern Iberia using pollen influx data. Earth and PlanetaryScience Letters 213, 63–78.
Dunne, J., Elmore, D., Muzikar, P., 1999. Scaling factors for the rates of production of cos-mogenic nuclides for geometric shielding and attenuation at depth on sloped sur-faces. Geomorphology 27, 3–11.
El Bedoui, S., Guglielmi, Y., Lebourg, T., Pérez, J.L., 2009. Deep-seated failure propagation ina fractured rock slope over 10,000 years: the La Clapière slope, the south-easternFrench Alps. Geomorphology 105, 232–238.
Fabre, R., Desreumaux, C., Lebourg, T., 2001. Les glissements rocheux du versant sud duLayens (Vallée d'Aspe, Pyrénées occidentales). Bull. Soc. Geol. Fr. 171, 407–418.
10 T. Lebourg et al. / Geomorphology 208 (2014) 1–10
Fabre, R., Lebourg, T., Clément, B., 2002. Typologie et modèles géométriques deglissements de terrain: exemples dans les Pyrénées Atlantiques et Centrales. Rev.Fr. Géotech. 99, 35–48.
Fabre, R., Texier, J.P., Clément, B., Lebourg, T., 2003. Méthode de localisation des morainesde convergence dans une ancienne vallée glaciaire (Pyrénées, France): conséquencessur les instabilités des moraines et reconstruction des glaciers au Würm. Can.Geotech. J. 40, 419–434.
Feyte, S., Gobeil, C., Tessier, A., Cossa, D., 2012. Mercury dynamics in lake sediments.Geochim. Cosmochim. Acta 82, 92–112.
Goosse, H., Arzel, O., Luterbacher, J., Mann, M.E., Renssen, H., Riedwyl, N., Timmermann,A., Xoplaki, E., Wanner, H., 2006. The origin of the European “MedievalWarm Period”.Clim. Past 2, 99–113.
Gosse, J.C., Phillips, F.M., 2001. Terrestrial in situ cosmogenic nuclides: theory and applica-tion. Quat. Sci. Rev. 20, 1475–1560.
Gutiérrez, F., Ortuño, M., Lucha, P., Guerrero, J., Acosta, E., Coratza, P., Piacentini, D., Soldati,M., 2008. Late Quaternary episodic displacement on a sackung scarp in the centralSpanish Pyrenees. Secondary paleoseismic evidence ? Geodin. Acta 21, 187–202.
Gutiérrez, F., Linares, R., Roqué, C., Zarroca, M., Rosell, J., Galve, J.P., Carbonel, D., 2012.Investigating gravitational grabens related to lateral spreading and evaporite dissolu-tion subsidence bymeans of detailedmapping, trenching, and electrical resistivity to-mography (Spanish Pyrenees). Lithosphere 4, 331–353.
Gutiérrez-Santolalla, F., Acosta, E., Ríos, S., Guerrero, J., Lucha, P., 2005. Geomorphologyand geochronology of sackung features (uphill-facing scarps) in the Central SpanishPyrenees. Geomorphology 69, 298–314.
Guzzetti, F., Malamud, B.D., Turcotte, D.L., Reichenbach, P., 2002. Power-law correlationsof landslide areas in central Italy. Earth Planet. Sci. Lett. 195, 18–169.
Harp, E.L., Jibson, R.W., 1996. Landslides triggered by the 1994 Northridge, California,earthquake. Bull. Geol. Soc. Am. 86, 319–332.
Hippolyte, J.-C., Brocard, G., Tardy, M., Nicoud, G., Bourlès, D., Braucher, R., Ménard, G.,Souffaché, B., 2006. The recent fault scarps of the western Alps (France): tectonic sur-face ruptures or gravitational sackung scarps? A combined mapping, geomorphic,levelling, and 10Be dating approach. Tectonophysics 418, 255–276.
Honoré, L., Courboulex, F., Souriau, A., 2011. Groundmotion simulations of a major histor-ical earthquake (1660) in the French Pyrenees using recent moderate size earth-quakes. Geophys. J. Int. 187, 1001–1018.
Hürlimann, M., Ledesma, A., Corominas, J., Prat, P.C., 2006. The deep-seated slope defor-mation at Encampadana, Andorra: representation ofmorphologic features by numer-ical modelling. Eng. Geol. 83, 343–357.
Ivy-Ochs, S., Kerschner, H., Reuther, A., Maisch, M., Sailer, R., Schaefer, J., Kubik, P.W.,Synal, H.A., Schlüchter, C., 2006. The timing of glacier advances in the northernEuropean Alps based on surface exposure dating with cosmogenic 10Be, 26Al, 36Cl,and 21Ne. In: Siame, L.L., Bourlès, D.L., Brown, E.T. (Eds.), In Situ Cosmogenic Nuclidesand their Applications in Earth Sciences. Geol. Soc. America, 415, pp. 43–60.
Jibson, R.W., Harp, E.L., Schulz, W., Keefer, D., 2004. Landslides triggered by the 2002 De-nali Fault, Alaska, Earthquake and the inferred nature of the strong shaking. Earth-quake Spectra 20, 669–691.
Jomard, H., 2006. Analyse multi-échelles des deformations gravitaires du Massif del'Argentera Mercantour. (PhD Thesis) Université Nice Sophia Antipolis (246 pp.).
Keefer, D.K., 1984. Landslides caused by earthquakes. Bull. Geol. Soc. Am. 95, 406–421.Keefer, D.K., 2000. Statistical analysis of an earthquake-induced landslide distribution —
the 1989 Loma Prieta, California event. Eng. Geol. 58, 231–249.Keefer, D.K., 2002. Investigating landslides caused by earthquakes — a historical review.
Surv. Geophys. 23, 510–573.Korschinek, G., Bergmaier, A., Faestermann, T., Gerstmann, U.C., Knie, K., Rugel, G.,
Wallner, A., Dillmann, I., Dollinger, G., von Gostomski Ch, Lierse, Kossert, K., Maitia,M., Poutivtsev, M., Remmert, A., 2010. A new value for the half-life of 10Be byheavy-ion elastic recoil detection and liquid scintillation counting. Nucl. Inst.Methods Phys. Res. B 268, 187–191.
Lamb, H.H., 1965. The early medieval warm epoch and its sequel. Palaeogeogr.Palaeoclimatol. Palaeoecol. 1, 13–37.
Lambert, J., Levret-Albaret, A., 1996. Mille ans de séismes en France: (catalogued'épicentres, paramètres et références), Ouest ed. Presse Academic, Nantes (75 pp.).
Larrasoaña, J.C., Ortuño, M., Birks, H.H., Valero-Garcés, B., Parés, J.M., Copons, R., Camarero,L., Bordonau, J., 2010. Palaeoenvironmental and palaeoseismic implications of a 3700-year sedimentary record from proglacial Lake Barrancs (MaladetaMassif, Central Pyr-enees, Spain). Palaeogeogr. Palaeoclimatol. Palaeoecol. 294, 83–93.
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 Planet.Sci. Lett. 280, 236–245.
Lebourg, T., Fabre, R., Clément, B., Frappa, M., 2003a. Different landslides on mountainsides in relation with the geological and geomorphological inheritance. Bull. Eng.Geol. Environ. 62, 221–229.
Lebourg, T., Riss, J., Fabre, R., Clément, B., 2003b. Morphological characteristics of till for-mations in relation with mechanical parameters. Math. Geol. 35, 835–852.
Lebourg, T., Riss, J., Pirard, E., 2004. Influence of morphological characteristics of heteroge-neous moraines formations on the mechanical behaviour. Eng. Geol. 73, 37–50.
Lebourg, T., Hernandez, M., Jomard, H., El Bedoui, S., Bois, T., Zerathe, S., Tric, E., Vidal, M.,2011. Temporal evolution of weathered cataclastic material in gravitational faults ofthe La Clapiere deep seated landslide bymechanical approach. Landslides 8, 241–252.
Malamud, B.D., Turcotte, D.L., Guzzetti, F., Reichenbach, P., 2004. Landslides, earthquakeand erosion. Earth Planet. Sci. Lett. 229, 5–45.
Malgot, J., 1977. Deep-seated gravitational slope deformations in neovolcanic mountainranges of Slovakia. Bull. Eng. Geol. Environ. 16, 106–109.
Mann, M.E., Bradley, R.S., Hughes, M.K., 1998. Global-scale temperature patterns and cli-mate forcing over the past six centuries. Nature 392, 779–787.
Mann, M.E., Bradley, R.S., Hughes, M.K., 1999. Northern hemisphere temperatures duringthe past millennium: inferences, uncertainties, and limitations. Geophys. Res. Lett. 26,759–762.
Margielewski,W., 2006. Structural control and types ofmovements of rockmass in anisotropicrocks: case studies in the Polish Flysch Carpathians. Geomorphology 77, 47–68.
McCalpin, J.P., Hart, E.W., 2003. Ridge-top spreading features and relationship to earth-quakes, San Gabriel Mountains region, Southern California—Parts A and B. In: Hart,E.W. (Ed.), Ridge-Top Spreading in California; Contributions Toward Understandinga Signifi cant Seismic Hazard: Sacramento, California, California Geological Survey,CD 2003–05, 2 CD-ROMs.
Merchel, S., Herpers, U., 1999. An update on radiochemical separation techniques for thedetermination of long-lived radionuclides via accelerator mass spectrometry.Radiochim. Acta 84, 215–219.
Merchel, S., Arnold, M., Aumaitre, G., Benedetti, L., Bourlès, D.L., Braucher, R., Alfimov, V.,Freeman, S.P.H.T., Steier, P., Wallner, A., 2008. Towards more precise 10Be and 36Cldata from measurements at the 10–14 level: influence of sample preparation. Nucl.Inst. Methods Phys. Res. B 266, 4921–4926.
Miller, D.J., Dunne, T., 1996. Topographic perturbations of regional stresses and conse-quent bedrock fracturing. J. Geophys. Res. 101, 1–25.
Morellón, M., Valero-Garcés, B., Vegas-Vilarrúbia, T., González-Sampériz, P., Romero, Ó.,Delgado-Huertas, A., Mata, P., Moreno, A., Rico, M., Corella, J.P., 2009. Late glacialand Holocene paleohydrology in thewesternMediterranean region: the Lake Estanyarecord (NE Spain). Quat. Sci. Rev. 28, 2582–2599.
Morellón, M., Valero-Garcés, B.L., González-Sampériz, P., Vegas-Vilarrúbia, T., Rubio, E.,Rieradevall, M., Delgado-Huertas, A., Mata, P., Romero, O., Engstrom, D.R., LópezVicente, E., Navas, A., Soto, J., 2011. Climate changes and human activities recordedin the sediments of Lake Estanya (NE Spain) during the Medieval Warm Period andLittle Ice age. J. Paleolimnol. 46, 423–452.
Moreno, A., Pérez, A., Frigola, J., Nieto-Moreno, V., Rodrigo-Gámiz, M., Martrat, B.,González-Sampériz, P., Morellón, M., Martín-Puertas, C., Corella, J.P., Belmonte, Á.,Sancho, C., Cacho, I., Herrera, G., Canals, M., Grimalt, J.O., Jiménez-Espejo, F.,Martinez-Ruiz, F., Vegas-Vilarrúbia, T., Valero-Garcés, B.L., 2012. The Medieval Cli-mate Anomaly in the Iberian Peninsula reconstructed from marine and lake records.Quat. Sci. Rev. 43, 16–32.
Nishiizumi, K., Imamura, M., Caffee, M.W., Southon, J.R., Finkel, R.C., McAninch, J., 2007.Absolute calibration of 10Be AMS standards. Nucl. Inst. Methods Phys. Res. B 258,403–413.
Pasuto, A., Soldati, M., 1996. Rock spreading. In: Dikau, R., Brunsden, D., Schrott, L., Visen,M.-L. (Eds.), Landslide Recognition. Identification, Movement and Causes. Wiley,Chichester, pp. 122–136.
Radbruch-Hall, D., 1978. Gravitational creep of rock masses on slopes. In: Voight, B. (Ed.),Rockslides and Avalanches— Natural Phenomena. Developments in Geotechnical En-gineering, 14, pp. 608–657.
Rodrıguez, C.E., Bommer, J.J., Chandler, R.J., 1999. Earthquake-induced landslides: 1980–1997. Soil Dynamics and Earthquake Eng. 18, 325–346.
Schmid, M., Busbridge, M., Wüest, A., 2010. Double-diffusive convection in Lake Kivu.Limnol. Oceanogr. 55, 225–238.
Seed, H.B., 1968. Landslides during earthquakes due to liquefaction. J. Soil Mech. Found.Div. 94, 1055–1122.
Sing, R.G., Botha, G.A., Richards, N.P., McCarthy, T.S., 2008. Holocene landslides inKwaZulu-Natal, South Africa. S. Afr. J. Geol. 11, 39–52.
Soldati, M., Corsini, A., Pasuto, A., 2004. Landslides and climate change in the Italian Dolo-mites since the Late glacial. Catena 55, 141–161.
Stark, C.P., Hovius, N., 2001. The characterization of landslide size distributions. Geophys.Res. Lett. 28, 1091–1094.
Stine, S., 1994. Extreme and persistent drought in California and Patagonia during medi-aeval time. Nature 369, 546–549.
Stone, J.O., 2000. Air pressure and cosmogenic isotope production. J. Geophys. Res. 105,23753–23759.
Taillefer, F., 1969. Les glaciations des Pyrénées. Bull. AFEQ., INQUA, Paris 19–32.Ternet, Y., Majesté-Lenjoulas, C., Guerrot, C., Rossi, P., 2004. BRGM-Laruns Somport
Geological Map, Volume 1069.Van Beck, L.P.H., 2002. Assessment of the Influence of Changes in Land Use and
Climate on Landslide Activity in a Mediterranean Environment. (PhD Thesis)ProefschriftUniversiteit Utrecht (360 pp.).
Varnes, D.J., 1978. Slope movements types and processes. In: Schuster, R.L., Krizek, R.J.(Eds.), Landslides Analysis and Control. Transportation Research Board Special Re-port, 176. National Research Council, Washington D.C., pp. 11–33.
Varnes, D.J., Radbruch-Hall, D.H., Savage, W.Z., 1989. Topographic and structural condi-tions in areas of gravitational spreading of ridges in the western United States. U.S.Geol. Surv. Prof. Pap. 1496, 1–28.
Ward, G.K., Wilson, S.R., 1978. Procedures for comparing and combining radiocarbon agedeterminations: a critique. Archaeometry 1, 19–31.
Wojciech, T., 2004. Estimating recent sedimentation rates using 210pb on the example ofmorphologically complex lake (Upper Lake Raduñskie, Poland). Geochronometria 23,21–26.
Zarroca, M., Linares, R., Bach, J., Roqué, C., Moreno, V., Font, L.L., Baixeras, C., 2012.Integrated geophysics and soil gas profiles as a tool to characterize activefaults: the Amer fault example (Pyrenees, NE Spain). Environ. Earth Sci. 67,889–910.
Zerathe, S., Lebourg, T., 2012. Evolution stages of large deep-seated landslides at the frontof a subalpine meridional chain (Maritime-Alps, France). Geomorphology 138,390–403.
Zischinsky, U., 1966. On the deformation of high slopes. Proc. lst Conf. Int. Soc. RockMech.,Lisbon, Vol. 2, pp. 179–185.
Zischinsky, U., 1969. Uber sackungen. Rock Mech. 1, 30–52.