Earth and Planetary Science Letters 365 (2013) 263–274

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Earth and Planetary Science Letters

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Giant non-catastrophic landslides and the long-term exhumationof the European Alps

Federico Agliardi n, Giovanni B. Crosta, Paolo Frattini, Marco G. Malus�a

Department of Earth and Environmental Sciences, University of Milano-Bicocca, Piazza della Scienza 4, Milano I-20126, Italy

a r t i c l e i n f o

Article history:

Received 7 August 2012

Received in revised form

24 January 2013

Accepted 26 January 2013

Editor: T.M. HarrisonEuropean Alps using the first orogen-scale inventory of DSGSDs (4900 over an area 4105 km2) and a

Available online 8 March 2013

Keywords:

deep-seated gravitational slope

deformation

relief

geomorphometry

exhumation

fission-track dating

European Alps

1X/$ - see front matter & 2013 Elsevier B.V.

x.doi.org/10.1016/j.epsl.2013.01.030

espondence to: Department of Earth and Env

ity of Milano-Bicocca, Piazza della Scienza

I-20126, Italy. Tel.: þ39 0264 482 006; fax: þ

ail address: federico.agliardi@unimib.it (F. Ag

a b s t r a c t

Landslides influence local slope morphology, affect sediment flux from hillslopes to rivers, and mass

wasting in response to tectonics and climate forcing. However, the links between giant, non-

catastrophic landslides known as Deep-Seated Gravitational Slope Deformations (DSGSDs) and the

long-term evolution of orogenic landscapes are almost unknown. We explore these links in the

dataset of published apatite fission-track ages (41000) that provides an estimate of the long-term

exhumation patterns of the orogen. We show that DSGSDs are more widespread than previously

considered, and exhibit an orogen-scale distribution not explained by well-known local lithological and

structural controls. We test the hypothesis that this orogen-scale distribution correlates to the long-

term evolution of the Alps by subdividing the study area into 37 square sub-areas (50�50 km),

classified according to combinations of long-term exhumation and mean annual rainfall. On each sub-

area we perform a morphometric analysis of topography (hypsometry, relief, slope). Excluding local and

regional controls due to rock type and structure, DSGSDs tend to cluster in areas with intermediate

exhumation rates (fission-track age between 10 and 40 Ma), where large-scale topography is less

dissected and incision is localised along major valleys. Here DSGSD abundance correlates positively

with the degree of valley incision and related relief. Instead, DSGSDs lack in areas which underwent

either low exhumation rates, resulting in insufficient relief production, or high exhumation rates

associated to rapid uplift or higher erosional dissection of topography. Negative correlation between

DSGSD abundance and mean annual rainfall suggests that effective hydrological surface processes

contribute, on the long-term timescale, to the development of large-scale topography unfavourable to

DSGSDs, especially in areas of high exhumation rates. Where DSGSDs are abundant, long-lasting slope

deformations effectively adjust post-glacial relief by reducing slope inclination values, and are thus

expected to significantly contribute to the long-term denudation of active orogens.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Competing tectonic and surface processes build and destroytopography in active orogens. Thrusting, crustal thickening andisostatic response result in rock uplift and relief production.Glacial, fluvial, and mass wasting processes erode topography,leading to rock exhumation and sediment redistribution underlithological and climatic controls (England and Molnar, 1990;Burbank and Anderson, 2001). Understanding the relative con-tributions and possible interplays between different processeswhich control the long-term evolution of orogenic landscapes isrequired to validate and improve existing geomorphological

All rights reserved.

ironmental Sciences,

4, Building U4, room 2016,

39 026 4482 073.

liardi).

models of landscape evolution, outline new research directions,and provide useful hints in a geohazard perspective. While thefeedbacks between tectonics and fluvial erosion have been widelystudied (Ahnert, 1970; Whipple, 2004), the links between land-slides and landscape development in active orogenic belts aremuch less clear and often neglected in quantitative geomorpho-logical models (Densmore et al., 1997; Korup et al., 2010).The geomorphic role of landslides has only been accounted forin the framework of fluvial systems and processes, with thresholdhillslopes rapidly adjusting to changes in incision rates, in turnresponding to climate change or tectonic processes (Burbanket al., 1996; Montgomery, 2001). In such framework, landslidesaffect hillslope and channel profiles, as well as sediment transferby soil or bedrock landslides and rock avalanches (Hewitt, 1998;Densmore and Hovius, 2000; Montgomery, 2001; Korup, 2006;Korup et al., 2007; Brardinoni et al., 2009). Instead, very little isknown about the relationships between giant, non-catastrophic

F. Agliardi et al. / Earth and Planetary Science Letters 365 (2013) 263–274264

landslides known as Deep-Seated Gravitational Slope Deforma-tions (DSGSDs, sackungen) and the long-term evolution of topo-graphic relief in active orogens.

In the mid-twentieth century this class of slope instabilitieswas almost completely unknown and sometimes neglected.Starting from the 1970s, DSGSDs were recognised and classified,but still considered as ‘‘outliers’’ of alpine morphogenesis, rareprocesses limited to peculiar lithological or structural settings.Today, the increasing availability of local to regional-scale DSGSDdatasets (Ambrosi and Crosta, 2006; Agliardi et al., 2009;El Bedoui et al., 2011; Pedrazzini, 2012), extensive geologicaldata and advanced investigation tools allow recognising thatDSGSDs are widespread, often active, and long-lasting slopeinstability phenomena (Crosta et al., 2008; Agliardi et al., 2012).Thus, their potential contribution to landscape evolution deservesto be evaluated and eventually accounted for in geomorphologicalmodels. Nevertheless, the dramatic variability of the geologicaland morphological features, geometry, and mechanisms ofDSGSDs has often hampered a sound understanding of the factorscontrolling their distribution, especially when the spatial scale ofentire mountain belts is considered. This points to the need forlarge DSGSD datasets to investigate the local and regional controlson their distribution, geomorphic significance at orogen scale, andrelationships with the long-term geological and topographicevolution of mountain ranges. Such large datasets lack, sincemost landslide inventories presented in the literature werefocused on large, catastrophic landslides occurred in the EuropeanAlps (i.e. rockslides/rock avalanches; Montandon, 1933; Abele,1974; Eisbacher and Clague, 1984; Heim, 1932), but did notaccount for DSGSDs.

In this work, we exploit the first orogen-scale inventory ofDSGSDs (Crosta et al., 2008; Agliardi et al., 2012) prepared for theEuropean Alps, one of the most studied orogenic belts worldwide.The inventory includes more than 900 individual DSGSD, mappedin different lithological, structural and morphological settingsacross the whole belt. We analyse the spatial distribution ofDSGSDs and their relationships with lithology and topography,and outline different types of spatial clustering of these phenom-ena. Here we test the hypotheses that (1) distribution of DSGSDsat the orogen scale correlates to the patterns of long-termevolution of the belt; and that (2) DSGSD has a significant impacton topographic relief.

2. Distribution of DSGSD in the European Alps

DSGSD is a class of mass movement involving entire highslopes from their toes to or beyond the ridges, characterisedby relatively low displacement rates (few to tens of mm/a; Varneset al., 1990; Ambrosi and Crosta, 2006; Agliardi et al., 2012) butlarge cumulative displacements testifying non-catastrophic, long-term activity. Typical morpho-structural expressions of DSGSDinclude multiple ridges, trenches, scarps, counterscarps, and half-grabens which dominate in the upper slope sectors, whilebulging, buckling folds, enhanced rock fracturing, and secondary,potentially catastrophic rockslides may occur in the lower slopesectors (Fig. 1). The kinematic interpretation of individualmorpho-structural features and their associations allows recon-structing the geometry and mechanisms of DSGSD, especiallywhen subsurface site investigation data are lacking. DSGSDswith typical morpho-structural expressions usually involve areasexceeding 10 km2, whereas smaller phenomena show featurestransitional to those of typical large landslides (Agliardi et al.,2012).

Several possible triggers of DSGSD have been proposed. Inter-acting topographic and tectonic stresses in both isotropic and

anisotropic rock masses induce shear stress concentrations atslope toes and extensive tensile damage along ridges. Theseeffects scale with topographic relief and may induce large scaleinstability depending on rock mass strength and local structuralcontrols (Varnes et al., 1989; Miller and Dunne, 1996; Kinakin andStead, 2005; Ambrosi and Crosta, 2011). In alpine areas, thedeglaciation of high, oversteepened valley sides carved by Pleis-tocene glaciers caused slope debuttressing, increased stress con-centrations, fracture unloading and damage, and changes in slopehydrology (Augustinus, 1995; Crosta, 1996; Ballantyne, 2002).Fluvial erosion has also been suggested as a DSGSD trigger inlayered sedimentary rocks (Crosta and Zanchi, 2000), whereasground shaking and coseismic displacements during moderate tostrong earthquakes have been suggested to contribute to ongoingDSGSD (Moro et al., 2007).

DSGSD distribution and morpho-structural features on local toregional scale reflect significant structural controls, pointed out atseveral study sites thanks to structural field data and numericalmodelling (Hippolyte et al., 2006; Agliardi et al., 2009). Inheritedtectonic features control (at least passively) DSGSD on threedifferent scales. At the outcrop scale, rock fabric and structureinfluence rock mass strength, deformability and kinematicdegrees of freedom. At slope scale, master fractures constrainthe occurrence, type, and geometry of major morpho-structures,and thus the localisation and kinematics of DSGSD (Agliardi et al.,2001). At regional scale, major features including nappe bound-aries and regional faults act either as loci of preferential linearclustering of DSGSD (e.g. low-angle regional faults and relatedrocks; Agliardi et al., 2009) or as geometrical barriers (e.g.juxtaposition of different lithological domains along subverticalfaults; Ambrosi and Crosta, 2006).

Our DSGSD inventory extends over an area exceeding 105 km2

across Italy, France, Switzerland, and Austria, and encompassesmost of the European Alps (Fig. 2a and supplementary Fig. S1).It is based on interpretation of satellite imagery, cross-validatedby literature data, aerial photos (both stereo and orthophotos,nominal scales ranging between 1:10,000 and 1:30,000), high-resolution LIDAR topography, and local field surveys. DSGSDswere initially mapped in GoogleEarthTM and then incorporatedinto a GIS platform for further processing. To avoid operatorbiases, DSGSDs were mapped as polygons by a single expertwithout using a priori geological or historical information.DSGSDs were recognised and distinguished from either tectonicfeatures or large landslides s.s. using diagnostic criteria including:involvement of entire valley slopes and ridge splitting bylaterally-continuous upper limiting scarps; occurrence of gravita-tional morpho-structures reactivating inherited tectonic features(but mostly less persistent and rectilinear); large displacementslocalised along individual morpho-structures (up to tens ofmetres); dislocation of slope toes and changes in valley crossprofiles, but limited evacuation of unstable masses from the slope(typical feature of landslides s.s.); evidence of controls on localdrainage networks (ponding, lakes, captures) and Quaternarylandforms and deposits, especially related to glacial and perigla-cial environments. Inventory sub-samples were cross-validatedby comparison with existing local inventories, compiled withdifferent techniques (i.e. historical data collection, aerial photo-interpretation, field surveys) on local to regional scales (Mortaraand Sorzana, 1987; APAT, 2008; Ambrosi and Crosta, 2006;Agliardi et al., 2009; Korup and Schlunegger, 2009; El Bedouiet al., 2011; Pedrazzini, 2012).

Rock type control on DSGSD distribution was evaluated bycontrasting the inventory map with a regional lithological map,obtained by combining 1:500,000 geological maps available forSwitzerland and Italy (Swisstopo, 2007; Bigi et al., 1983; availableover 82,500 km2, 725 DSGSDs covered). The lithological map was

Fig. 1. Examples of typical morpho-structural features associated to DSGSD, from different lithological and structural settings in the European Alps: (a) Colle dell’Assietta

(Susa–Chisone valley divide, WGS84 lat 45.0601, lon 6.9531; structural domain: Piedmont zone of the axial Western Alps; lithology: calcschist); (b) Mt. de la Saxe (Aosta

valley, WGS84 lat 45.8181, lon 6.9861; structural domain: ultra-Helvetic; lithology: slate); (c) Mt. Watles (Venosta valley, WGS84 lat 46.7171, lon 10.4961; structural

domain: Austroalpine lid; lithology: paragneiss); (d) Punta Vallaccia (Fassa valley, WGS84 lat 46.3891, lon 11.7021; structural domain: Southern Alps; lithology: limestone,

sandstone and marl); and (e and f) Piz Dora (Mustair valley, WGS84 lat 46.6061, lon 10.3211; structural domain: Austroalpine lid; lithology: (meta) conglomerate,

sandstone and siltstone).

F. Agliardi et al. / Earth and Planetary Science Letters 365 (2013) 263–274 265

reclassified by grouping rock types, according to their expectedgeomechanical behaviour (i.e. average rock mass properties andanisotropy), into granitoid/metabasite (GM), volcanic (VO),orthogneiss (OG), metapelite/metapsammite (MP), bedded sand-stones and marls including flysch (FL), and carbonate rocks (CA).Quaternary deposits (e.g. alluvial valley fill, thick glacial cover)were considered separately. DSGSD density (i.e. the ratio ofinvolved area and total area) was then computed for each rocktype (inset in Fig. 2a).

The inventory includes 904 DSGSDs affecting a total area of5472 km2 (about 5% of total, Fig. 2a). Individual phenomena rangein area between 0.2 and 108 km2 (average value: 6 km2). SmallerDSGSDs usually occur in minor tributary valleys, but sometimesrepresent more deformed sectors nested inside larger phenom-ena. DSGSDs showing typical morpho-structural features oftenexceed 10 km2 in area. Despite the uncertainties related to

mapping scale, our inventory shows that DSGSD mainly occursin moderately strong anisotropic rock masses, including phyllites,micaschists and paragneisses (areal density: 12.7%; Fig. 2a),orthogneisses (density: 6.5%), and flysch-type units includingsandstones and marls (density: 3.9%). Volcanic rocks are alsoprone to DSGSD (density about 5%), but their exposure area isnegligible. Conversely, carbonate and granitoid rocks are verypoorly affected.

Our large dataset reflects a preferred, regional-scale linearclustering of DSGSDs along major regional tectonic features (i.e.large shear zones or nappe boundaries), with some outstandingexamples occurring in the Tarentaise (France), Rhone and Rhein(Switzerland), Susa, Valtellina and Venosta (Italy) valleys(Figs. 2 and 3). In some sectors of the European Alps, thesecontrols dominate, and most DSGSDs occur along major regionalstructures, although they are uncommon in neighbouring areas

Fig. 2. DSGSD inventory and orogen-scale rainfall and exhumation patterns: (a) inventory of mapped DSGSDs (dark-red polygons), major structural features (red lines) and

distribution of mean annual rainfall (after Frei and Schar, 1998) over the 90 m SRTM DEM. Inset: rock type controls on DSGSD distribution expressed by areal density (GM:

granitoid/metabasite; VO: volcanics; OG: orthogneiss; MP: metapelite/metapsammite; FL: bedded sandstones and marls, including flysch; CA: carbonate rocks). Swath

profiles of Fig. 4 are also shown; and (b) apatite FT dataset (dots, classified by age), and arbitrary 50�50 km square sub-areas classified according to exhumation and

rainfall patterns (see text for explanation). Inset: simplified tectonic sketch map of the European Alps.

F. Agliardi et al. / Earth and Planetary Science Letters 365 (2013) 263–274266

(Fig. 3). However, the inspection of our dataset also raises apreviously unrecognised element of surprise: a higher-order,orogen-scale distribution of DSGSDs seems superimposed to thebetter known local and regional structurally-controlled

distributions. In fact, we observed that DSGSDs are missing orvery sparse over large areas where local lithological, structuraland geomorphological favourable conditions would indeed exist,e.g. Ossola valley or the Tauern range (Fig. 2). We test the

Fig. 3. Areas with dominant control of regional tectonic features on the linear

clustering of DSGSDs (sub-areas ‘‘HRC*’’, locations in Fig. 2): (a) Tarentaise valley

(France); (b) Upper Rhein valley (Switzerland); and (c) Gotthard and Upper Rhone

valley (Switzerland).

F. Agliardi et al. / Earth and Planetary Science Letters 365 (2013) 263–274 267

hypothesis that such orogen-scale distribution correlates to thelong-term evolution of the European Alps, by analysing theoccurrence of DSGSD in sectors characterised by different long-term exhumation patterns.

3. Patterns of orogen scale exhumation

Orogenic belts are regions of positive rock uplift triggered byeither tectonics or isostatic response to surface processes(Burbank and Anderson, 2001; Malus�a and Vezzoli, 2006). Sedi-mentary rocks formed below sea level, metamorphic rocks fromthe deep crust and deeply intruded magmatic rocks are nowexposed at remarkable elevations above sea level. When rockuplift is completely or partly counterbalanced by exogenicprocesses, overburden removal produces erosional exhumationthat can be constrained by different geochronological and ther-mochronological methods (England and Molnar, 1990; Reinersand Brandon, 2006; Malus�a et al., 2011a).

In the European Alps, exhumation has been thoroughly inves-tigated starting from the earliest stages of tectonic exhumation inthe Paleogene (e.g. Agard et al., 2002; Malus�a et al., 2011b) to thelatest stages of erosional unroofing, which are well constrained bylow-temperature thermochronological analyses and cosmogenicnuclide studies performed both on bedrock and detritus (e.g.,Hurford et al., 1991; Hunziker et al., 1992; Malus�a et al., 2005,2006, 2011a; Wittmann et al., 2007; Vernon et al., 2008; Nortonet al., 2011; Resentini and Malus�a, 2012; Reverman et al., 2012;Valla et al., 2012). Among all the available datasets, only bedrock

fission-track (FT) data on apatite are evenly distributed andcover the whole orogen (Malus�a et al., 2005; Vernon et al.,2008; Rosenberg and Berger, 2009). They provide an orogen-scale image of the exhumation pattern across the uppermost3–4 km of the crust (Wagner and Van den Haute, 1992; Reinersand Ehlers, 2005), to be possibly detailed with other thermo-chronological methods and with detrital data to fill the gapbetween long-term and short-term timescales (Malus�a et al.,2009; Resentini and Malus�a, 2012). This allows us to explorethe relationships between DSGSD distribution and the orogen-scale pattern of long-term erosion. In our analysis, we haveconsidered all the available apatite FT ages (41000) publishedin geochronological works since the 1960s, as shown in Fig. 2b(the complete reference set is listed as supplementary materialS2). We inspected FT age variations across the orogen along threeswath profiles perpendicular to the local orogen trend (Figs. 2 and4), representative of the structural architecture of the western,central and eastern Alpine sectors. Swaths are 25 km wide with alongitudinal sampling step of 200 m, and have been selectedamong a set of 30 profiles covering the entire orogen.

As outlined by Malus�a et al. (2005), FT age distributions showsharp breaks across the boundaries between major tectonicdomains (Fig. 4). Oldest ages are found in the Southern Alps,where both rock uplift and exhumation rates are low and thesedimentary cover is still largely preserved (e.g., Giger, 1990;Zattin et al., 2003; 2006; Emmerich et al., 2005; D’Adda et al.,2011; Reverman et al., 2012). Youngest ages are found in theExternal Massifs, where huge rock uplift in the Neogene led toremoval of the Helvetic cover and to fast exhumation of theEuropean basement, now exposed at elevations exceeding 4000 ma.s.l. (e.g., Michalski and Soom, 1990; Bigot-Cormier et al., 2006;Glotzbach et al., 2009; Reinecker et al., 2009). Units of theLepontine Dome and the Tauern window, representing the deepbasement of the Central and Eastern axial Alps (Fig. 2b), also showyoung ages (e.g., Grundmann and Morteani, 1985; Hurford, 1986;Staufenberg, 1987; Most-Angelmaier, 2003; Timar-Geng et al.,2004; Keller et al., 2005; Rahn, 2005). Instead, intermediate agescharacterise the Western Axial Alps and the Austroalpine lid (e.g.,Hurford et al., 1991; Fugenschuh et al., 1999; Viola et al., 2001;Steenken et al., 2002; Malus�a et al., 2005; Tricart et al., 2007;Fig. 2b). Erosional exhumation is related to the rainfall patternacross the orogen, which is in turn constrained by orography. Thepresent-day rainfall pattern (Frei and Schar, 1998; Fig. 2a) showsmaximum values both along the external front of the mountainrange, penetrating to the South to include the Lepontine area, andin the Eastern Southern Alps. The spatial pattern of precipitationin the Alps is a result of the interaction of both macroscale (e.g.Atlantic) and mesoscale (orographic) circulation effects (Frei andSchar, 1998). Thus, despite the strong variations of absolutevalues of mean annual rainfall related to Pleistocene glaciations,regional spatial patterns of precipitation can be assumed to haveremained relatively stable since the onset of Alpine morphogenicgrowth, thus during most of the Neogene (Gansser, 1983;Garzanti and Malus�a, 2008; Campani et al., 2012).

4. Exhumation patterns and DSGSD distribution

In order to investigate the relationships among long-termorogen exhumation, topography, and DSGSD distribution,we subdivided the area covered by the inventory into 37 arbitrarysquare sub-areas (50�50 km; Fig. 2b) and classified them accord-ing to four combinations of long-term exhumation and meanannual rainfall (Fig. 2). We choose to perform our analyses inequally-sized, arbitrarily located square sub-areas in order to beable to average out local controls on DSGSD (e.g. lithology, slope-

F. Agliardi et al. / Earth and Planetary Science Letters 365 (2013) 263–274268

scale structure). As suggested by other authors (Brocklehurst andWhipple, 2004; Liu-Zeng et al., 2008; Sternai et al., 2011; Larsenand Montgomery, 2012), arbitrary sub-areas do not capture finelocal variations of the parameters of interest (i.e. lithology,morphometry, climate), but span different conditions dependingon their size and are suitable to explore regional to mesoscalevariations. The location of square sub-areas was selected to fit theextent of major structural domains characterised by fairly con-sistent exhumation history. For the sake of simplicity, referenceexhumation rates for classification were averaged from the timerocks have crossed the partial annealing zone of apatite (Wagnerand Van den Haute, 1992). In order to classify these sub-areas, weconsidered high, intermediate and low exhumation rates on longterm as indicated by ages o10, between 10 and 40, and 440 Ma,respectively (Fig. 2b). The identification of exhumation patternson orogen scale did not require a detailed reconstruction ofthermochronology-based exhumation histories, prone to intro-duce further uncertainty due to assumptions hardly constrainedover the entire dataset. Sub-areas with high exhumation rates anda dominant tectonic control on exhumation (HRT), characterisedby apatite FT ages younger than 10 Ma and by mean annualrainfall (Frei and Schar, 1998) lower than 1200 mm, include theTauern Window, the External Massifs of the southwestern Alpsand the overlying cover sequences. Fast-exhumed sub-areasincluding a strong climatic (rainfall) control (HRC) are charac-terised by apatite FT ages younger than 10 Ma and by meanannual rainfall exceeding 1200 mm, and include the LepontineDome, the External Massifs of the northwestern Alps and relatedsedimentary cover. Sub-areas yielding FT ages between 10 and40 Ma (e.g. in the Western axial Alps and in the Austroalpine lid)were classified as IR, being characterised by intermediate rates ofexhumation. Sub-areas yielding apatite FT ages exceeding 40 Ma,chiefly located in the Southern Alps, were classified as LR, as theyare characterised by low rates of exhumation. The areal extent ofindividual sub-areas (2500 km2) is small enough to ensure fairlyhomogeneous exhumation patterns, but large enough to averageout local lithological, structural and morphological controls.

Interestingly, the orogen-scale distribution of DSGSDs in theEuropean Alps correlates well with the long-term exhumationpattern (timescale: 106–107 a) of different orogen sectors(Figs. 4 and 5). DSGSDs cluster in areas of intermediate exhumationrates (IR, e.g. Western Axial Alps, Austroalpine lid; Figs. 2, 4 and 5).They are abundant in the Western Alps, with density up to 15% inthe Susa valley (Fig. 2b), but also in the Austroalpine lid of theEastern Central Alps (Ambrosi and Crosta, 2006; Agliardi et al.,2009). Instead, DSGSDs are almost lacking (densityo5%) in areascharacterised by very fast exhumation, either related to tectonicforcing (HRT, e.g. External Massifs of the Western Alps, TauernWindow) or associated to significant rainfall controls (HRC, e.g.Lepontine), as well as in slowly exhumed areas (LR; e.g. SouthernAlps). This occurs despite the fact that local lithological and topo-graphic conditions favourable to DSGSD indeed exist. For example,DSGSD density less than 5% is observed in the Tauern range(Penninic units of Eastern Alps; Fig. 2b). In this area, valley slopeswith significant local relief have been carved by glaciers in soundgneiss rocks unfavourable to DSGSD onset, but also in Palaeozoicand Mesozoic metasedimentary rocks and foliated gneiss generallyprone to DSGSD (Fig. 2a). In contrast, surrounding areas enclosedin Austroalpine nappes are densely affected by DSGSD (swathprofile 3 in Fig. 4). A similar situation can be observed in theLepontine area (Penninic units of the Central Alps; Fig. 2b), whereDSGSDs lack in outcropping areas of very hard orthogneiss rocks, butalso where foliated metasedimentary rocks occur (e.g. calcschists).

An exception to the general situation presented above isrepresented by sub-areas (HRCn in Figs. 2b and 3) where DSGSDareal density is almost exclusively related to mega-scale

structural controls resulting in a typical regional linear clusteringof individual phenomena. For example, DSGSD clustering isdirectly controlled by both the Frontal Penninic Fault and theBrianconnais Fault in the lower Tarentaise valley (Figs. 2b and3a); by the Frontal Penninic Fault in the upper Rhein valley(Figs. 2b and 3b); and by the Tavetsch mylonite zone betweenthe Aar and Gotthard massifs (Figs. 2b, and 3c and swath profile2 in Fig. 4). In order to avoid biases related to mega-structuralcontrols, these areas have been excluded from further analyses.

5. DSGSD distribution and large-scale topography

For each square sub-area (Fig. 2b) we performed a geomorpho-metric analysis of bulk topography, based on the NASA Shuttle RadarTopography Mission (SRTM) 3 arc-second DEM, with nominal cellsize of 90 m (Farr et al., 2007). We used the void-filled version of theDEM, projected to UTM-WGS84 (Fig. 2), and prepared standardderivative maps (e.g. slope map) and local relief maps. Local relief isregarded to reflect the interactions between tectonic and erosionalprocesses (Burbank and Anderson, 2001), and was calculated as thedifference between the maximum and minimum elevation withincircular moving windows with 5 km radius. This kernel size issuitable to account for major valley spacing in accordance withseveral previous studies (Montgomery and Brandon, 2002) and,despite the scale dependence of resulting local relief values, providesconsistent results over the entire study area. For each square sub-areawe computed the following geomorphometric descriptors: (1) mini-mum elevation Hmin, representing the local base level of the sub-area;(2) Hmax–Hmin, i.e. local relief; (3) Hmean–Hmin, here used as adescriptor of the degree of valley incision to the local base level (forgiven Hmin and local relief, high values indicate dominant elevatedareas with local, deeply incised valleys; e.g. Pike and Wilson, 1971);(4) the hypsometric curve and the hypsometric integral (i.e. HI inFig. 5a) using the polynomial fit method (Perez-Pena et al., 2009);(5) basic statistics of elevation and slope gradient (minimum, mean,maximum values and dispersion); and (6) mean annual rainfall (Freiand Schar, 1998; Figs. 2a and 5d). The areal density of DSGSD (i.e. theratio between the area involved in DSGSD and the total area) wascalculated for each square sub-area to provide an objective measureof DSGSD abundance (Fig. 4). Mean rainfall and DSGSD density acrossthe orogen were also tracked along the swath profiles in Fig. 4.

Geomorphometric analysis carried out on the 37 arbitrary sub-areas shows that areas with different long-term exhumationpatterns exhibit some distinct characters of large-scale bulktopography (Fig. 5). Sub-areas with intermediate exhumation rate(IR) have the highest values of the hypsometric integral (40.4–0.5; Fig. 5a), here considered as suggesting a relatively lowerosional dissection of topography (Strahler, 1952), the loweststandard deviation of slope gradient (Fig. 5b) and high values ofHmean–Hmin, i.e. degree of valley incision to the local base levels(Fig. 5c). Such incision is localised along major alpine glacialtroughs and exceeds 1000 m. In these areas, the areal density ofDSGSD shows a positive correlation with valley incision (Fig. 5c),confirming that local relief generated by fluvial and glacialerosion is a primary control for the onset of DSGSDs. Lessexpected, in IR areas DSGSD density shows a negative trend withrespect to mean annual rainfall (Fig. 5d), with density valuesdropping below 5% where mean annual rainfall exceeds1200 mm/a. Although the adopted rainfall climatology is basedon modern datasets (Frei and Schar, 1998), its spatial pattern canbe considered representative of longer-term conditions as dis-cussed in Section 3. Observed trends may suggest that morpho-logical conditions less favourable to DSGSD occur in areas wheresurface hydrological processes are more effective, leading to

WesternAxial Alps

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Fig. 4. Selected swath profiles across the Western (1), Central (2) and Eastern

(3) Alps (location in Fig. 2); DSGSD abundance (area, in red) is compared to

elevation (black lines), apatite fission-track ages (dark blue squares), and mean

daily rainfall (thick light blue lines). Major structural units are also indicated (FPF:

Frontal Penninic Fault; IF: Insubric Fault). (For interpretation of the references to

colour in this figure legend, the reader is referred to the web version of this

article.)

F. Agliardi et al. / Earth and Planetary Science Letters 365 (2013) 263–274 269

higher landscape dissection (Fig. 5a), rougher topography(Fig. 5b), and lower degree of major valley incision (Fig. 5c).

Sub-areas characterised by high exhumation rates related totectonic forcing (HRT, e.g. Tauern range) show low DSGSD arealdensity (o5%) despite being characterised by morpho-climatic set-tings similar to IR sub-areas. Sub-areas which underwent highexhumation rates with significant rainfall controls (HRC, e.g. Lepon-tine), also almost lacking DSGSD, are characterised by high valleyincision (Fig. 5c), but more dissected and rougher topography (Fig. 5aand b) with respect to IR. Finally, sub-areas exhumed at lowest rates(LR, e.g. central Southern Alps), lacking DSGSD irrespective of thewide range of mean annual rainfall (Fig. 5d), show the lowest valuesof the hypsometric integral (Fig. 5a), the roughest topography(Fig. 5b) and the minimum degree of valley incision (Fig. 5c).

6. DSGSD impact on slope angle distributions

Where abundant and well developed, DSGSDs may beexpected to leave a signature on topography. We tested thishypothesis by comparing the relative frequency distributions ofslope gradient inside and outside mapped DSGSD areas (Korup,2006). We derived slope angle distributions by considering all thevalues computed from the DEM inside DSGSD areas and within1 km wide buffers around them (Fig. 6). This strategy allowed tosample slope angle values in areas unaffected by DSGSD, butcharacterised by consistent lithological, structural and morpho-climatic conditions. In order to minimise bias on slope angledistributions, we excluded areas with slope angle less than 10degrees, representing valley floors or colluvial toe slopes. Slopeangle distributions were derived for the entire DSGSD dataset(and surroundings) and for sub-samples characterised by specificrock types (metapelites, orthogneisses, flysch s.l., i.e. rock typesmost affected by DSGSD; number of samples in Fig. 2a). Weverified the normality of each distribution, derived the relatedstatistics, and verified that distributions of slope angles inside andoutside DSGSD are significantly different by standard two-samplet-test (Fig. 6).

Notwithstanding the bias affecting absolute values of slopegradient derived from low resolution DEMs (i.e. lower resolutionresults in lower slope gradients; Chang and Tsai, 1991), resultsshow a statistically significant reduction in slope gradient withinDSGSD areas, either for the whole DSGSD population or forsubsets characterised by different rock types (Fig. 6). Modal slopeangle for the whole DSGSD dataset is about 241, compared toabout 261 for neighbouring areas. For DSGSD occurring in orthog-neiss, slope reduction is evident but statistically not significant.This is interpreted as a consequence of the higher rock massstrength of these rocks, likely to limit the occurrence of largeDSGSD displacements.

7. Discussion

Large, non-catastrophic slope instabilities known as Deep-Seated Gravitational Deformations are far more widespread thanpreviously recognised. They preferentially occur in moderatelystrong, anisotropic rock masses (mainly foliated metamorphics),able to sustain high-relief hillslopes without failing catastrophi-cally. In the European Alps, DSGSDs occur in areas with local reliefexceeding 1000 m (average: about 1800 m), mostly alongstructurally-controlled high valley sides oversteepened by glacialerosion. Local relief values computed in the European Alps aretypical of tectonically active mountain belts, and possibly asso-ciated to significant erosion rates related to large landslides(Montgomery and Brandon, 2002; Korup et al., 2007). Duringthe Last Glacial Maximum (LGM), most areas affected by DSGSDwere covered by glaciers with surface elevation exceeding1700 m and up to 3000 m (Van Husen, 1987; Bini et al., 2009),and in some cases experienced repeated re-advances after LGMcollapse (18–16 ka; Ivy-Ochs et al., 2004). This is in agreementwith the idea of predominant triggering of alpine DSGSD byprocesses related to valley deglaciation (e.g. debuttressing ofhigh-relief slopes, tensile damage and fracture unloading, ground-water changes related to ice melting and permafrost formation),suggested in the literature on the basis of crosscutting relation-ships with glacial features, absolute geochronology, and numer-ical modelling (Augustinus, 1995; Bruckl, 2001; Hippolyte et al.,2006; Cossart et al., 2008; Ambrosi and Crosta, 2011).

Our large inventory dataset allowed to investigate the controlson DSGSD distribution beyond the lithological and local(i.e. slope-scale) structural controls which have been widely studied

9 10 11 12 13 140.02.55.07.5

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2.55.07.5

10.012.515.017.5

Hmean - Hmin (m)

R2= 0.368

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10.012.515.017.5

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0.02.55.07.5

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

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LRIRHRTHRC

DS

GS

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ty (%

)D

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sity

(%)

DS

GS

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ty (%

)D

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sity

(%)

Hypsometric integral, HI (-)

R2= 0.321

Fig. 5. DSGSD distribution and bulk topography; DSGSD density computed for the 37 square sub-areas (Fig. 2b) is plotted against (a) hypsometric integral (HI);

(b) standard deviation of slope angles in the local sub-area, representing topographic roughness; (c) Hmean–Hmin, representing valley incision to the local sub-area base

level; and (d) mean annual rainfall (mm/a).

F. Agliardi et al. / Earth and Planetary Science Letters 365 (2013) 263–274270

in the past. Using arbitrary square sub-areas allowed to average outlocal controls and recognise an orogen-scale clustering of DSGSDs,suggesting possible relationships between these phenomena andthe large-scale, long-term evolution of the orogen. We tested thishypothesis by relating long-term exhumation patterns of differentorogen sectors, large-scale bulk characteristics of topography, andthe spatial distribution of DSGSDs. These slope instability phenom-ena clearly tend to be more frequent in areas characterised byintermediate exhumation rates (IR; apatite FT ages between 10 and40 Ma), showing a lower degree of erosional dissection andsmoother topography with respect to other sub-areas (Fig. 5). In IRareas, topographic incision is particularly high along major alpineglacial troughs, along which high relief and resulting topographicstress concentrations favour DSGSD clustering in moderately strongrocks. In IR areas, higher mean annual rainfall values correspond tomore dissected and rougher topography, associated to a decrease inDSGSD abundance. This suggests that, in the long-term, moreeffective surface hydrological processes contribute to the develop-ment of large-scale topographic features unfavourable to DSGSD,which is in turn promoted where glacial overprint on topographyand the associated effects are more preserved. Thus, a long-termexhumation control on the orogen-scale distribution of DSGSD is notin contrast, but complements the mechanistic concept ofparaglacial–postglacial DSGSD triggering by deglaciation, with orwithout lithological and stress distribution constraints (Augustinus,1995; Harbor, 1995; Ambrosi and Crosta, 2011).

Excluding favourable lithological and slope-scale to regionalscale structural controls, DSGSDs lack in areas which underwentexhumation at either low rates (LR; apatite FT ages 440 Ma) orhigh rates (HRC and HRT; apatite FT ages o10 Ma), irrespective ofthe extent of glaciation. In LR areas, geological evidence (low rockuplift, substantial preservation of thick Mesozoic sedimentarycover) and geomorphometric analysis (Figs. 4 and 5) suggest thatthe lack of DSGSDs is due to insufficient relief production topromote large-scale instability of rock masses (Fig. 7) unlesssharp stratigraphic controls (Crosta and Zanchi, 2000) or weakrocks (e.g. shales) promoting large slope instabilities occur. InHRC, low DSGSD density may be partly related to a contributionof effective surface hydrological processes to large-scale topogra-phy, in the same way discussed for IR areas. Nevertheless, DSGSDsalso lack in some HRC and HRT areas where valley incision and

associated relief are comparable to those observed in IR areas(Fig. 5c). Different hypotheses can be submitted to further testingto explain this evidence, including (a) mega-scale lithologicaleffects: deep crustal levels are exposed in the Penninic andHelvetic units of HRC and HRT areas, increasing the probabilitythat slopes are carved in hard rocks unfavourable to DSGSD,although large area of outcropping DSGSD-prone rock-typesindeed occur (see Section 4); and (b) rapid rock uplift in HRTareas: according to geological evidence, the External Massifs andthe Tauern range underwent fast tectonic unroofing and reliefproduction, favourable to high long-term erosion rates by cata-strophic landsliding (Montgomery and Brandon, 2002; Korupet al., 2007).

Giant, non-catastrophic landslides reduce slope angle values,typical of neighbouring areas, which are already well belowvalues typically suggested for fluvial threshold hillslopes(Burbank et al., 1996; Montgomery, 2001) due to the severeoverprint of Quaternary glaciations on alpine topography. Thispoints to a long-term dynamic equilibrium condition attained byDSGSDs in the context of orogenic landscapes severely signed byglacial and paraglacial processes, very differently from the rapidslope adjustment by fluvial undercutting. This is supported bygeomorphological and geochronological evidence, suggesting thatDSGSDs are long-lasting phenomena, in some cases able tosurvive multiple glacial pulses. Increasingly available displace-ment data derived by satellite SAR interferometry also show thatseveral DSGSDs undergo present-day activity. For example, ver-tical displacement rates up to 10–30 mm/a along ridges havebeen measured in several DSGSD areas in the Upper Valtellina(central Eastern Alps; Colesanti et al., 2006; Ambrosi and Crosta,2006; Agliardi et al., 2012). These values are much higher than thepresent-day rock-uplift rates (Kahle et al., 1997). At the afore-mentioned displacement rates, and assuming DSGSD activity atconstant displacement rates, such focused lowering of topographylocally sum up to over 200–300 m since the Lateglacial, largelyexceeding average erosion rates estimated for this sector of theAlps (Hinderer, 2001). Where DSGSDs are widespread, theycould thus significantly contribute to denudation and masstransfer at the catchment scale over a time scale of 103–104 a,by bedrock exhumation by gravitational reactivation of inheritedtectonic structures (Hippolyte et al., 2006), modification of river

Axial AlpsHelvetic Southern Alps

N S0

20Km

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landslidesGiant non-catastrophiclandslides (DSGSD)

Insufficient relief production

HRT HRC IR LR

Fig. 7. Cartoon summarising the relationships among exhumation patterns, large-scale topography, and occurrence of DSGSDs (see text for discussion). Arrows are

proportional to long-term exhumation rates.

10 20 30 40 50 60 700.00

0.01

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0.06t-teststatisticst = -5.42p > |t| = 6.72⋅10-8

slope gradient (°)

0.00

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0.06t-test statisticst = -0.97p > |t| = 0.33

slope gradient (°)

R2=0.989R2=0.995

R2=0.974 R2=0.987

10 20 30 40 50 60 70

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0.00

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slope gradient (°)

t-test statisticst = -5.35p > |t| = 9.76⋅10-8

R2=0.998 R2=0.998

all data

1 km buffer outside DSGSD

10 20 30 40 50 60 700.00

0.01

0.02

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0.06 t-test statisticst = -4.36p > |t| = 1.38⋅10-5

slope gradient (°)

DSGSD areas

metapelite

orthogneiss flyschs.l.

rela

tive

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ency

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Fig. 6. Relative frequency distributions of slope angle; these have been derived considering all the values computed from the DEM inside mapped DSGSDs areas (red

triangles) and in neighbouring areas within 1-km buffers (blue circles). Fitted Gaussian distribution functions are statistically significant for the whole dataset and for

DSGSD subsets occurring in metapelite and flysch s.l. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this

article.)

F. Agliardi et al. / Earth and Planetary Science Letters 365 (2013) 263–274 271

networks on slopes and valley floors, and sediment supply torivers by processes occurring on different timescales (e.g. fromcatastrophic collapse of DSGSD sectors to slow toe bulging andundercutting).

8. Conclusions

Working in the European Alps, one of the most studied activeorogens worldwide, we achieved three major, innovative findings:

F. Agliardi et al. / Earth and Planetary Science Letters 365 (2013) 263–274272

(1)

giant, non-catastrophic slope instabilities are not rare phe-nomena. We delivered a unique DSGSD inventory on theorogen scale, mapping over 900 individual phenomena in theAlps, over an area exceeding 105 km2 in Italy, France, Switzer-land, and Austria. This is the first-ever orogen-scale inventoryof DSGSDs, and provides unique opportunities to interpret thedistribution of these phenomena at the scale of an entireactive orogenic belt, as well as to outline their impacts onAlpine morphogenesis;

(2)

notwithstanding the local and regional controls by rock typeand structure, the orogen-scale distribution of DSGSDs nicelycorrelates with the long-term evolution (time scale106–107 a) of different orogen sectors. We show that differentcombinations of long-term exhumation rates and rainfallpatterns are associated to different characteristics of large-scale bulk topography in the post-glacial landscape. DSGSDoccurrence and clustering is favored in areas of intermediateexhumation rates, where landscape is poorly dissected andincision is localised along major alpine glacial troughs.A surprising negative correlation between DSGSD abundanceand mean annual rainfall possibly suggests that effectivesurface hydrological processes contribute to a large-scaletopography hampering DSGSD, which is instead an effectiveplayer of relief adjustment in areas where the glacial over-print dominates. This hypothesis is here proposed for furthertesting;

(3)

DSGSD is an effective player of post-glacial relief adjustment:the occurrence of DSGSDs significantly reduces slope gradientto values consistently below not only those typical of fluvialthreshold hillslopes, but also below those resulting from thestrong glacial imprint affecting alpine valleys, and preservedoutside DSGSD areas. Since DSGSDs are long-lasting phenom-ena, they are expected to significantly contribute to denuda-tion where they are widespread, and should be properlyconsidered in geomorphological evolutionary models of land-scape evolution.

Acknowledgements

This work was partially supported by the Italian Ministry ofEducation, University and Research (MIUR) through FIRB andPRIN research Programmes. We thank C. Ambrosi, G. Fioraso,F. Gianotti, R. Polino, and A. Zanchi for fruitful discussions,F. Brardinoni for his careful revision of an earlier draft, andE. Valbuzzi for support during inventory mapping and checking.DSGSD inventory data are available upon request.

Appendix A. Supporting information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.epsl.2013.01.030.

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