Tectonophysics 474 (2009) 250–270

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Tectonophysics

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Tectonic vs. gravitational morphostructures in the central Eastern (): Constraints on the recent evolution of the mountain range

Federico Agliardi, Andrea Zanchi ⁎, Giovanni B. Crosta

Dipartimento di Scienze Geologiche e Geotecnologie, Università degli Studi di Milano-Bicocca, Piazza della Scienza, 4, Milano 20126, Italy article info abstract

Article history: Deep-seated gravitational slope deformations (DSGSDs) influence landscape development in tectonically Received 7 March 2008 active mountain ranges. Nevertheless, the relationships among tectonics, DSGSDs, and topography are poorly Received in revised form 22 December 2008 known. In this paper, the distribution of DSGSDs and their relationships with tectonic structures and active Accepted 22 February 2009 processes, surface processes, and topography were investigated at different scales. Over 100 DSGSDs were Available online 28 February 2009 mapped in a 5000 km2 sector of the central Eastern Alps between the Valtellina, Engadine and Venosta valleys. Detailed lineament mapping was carried out by photo-interpretation in a smaller area (about Keywords: 750 km2) including the upper Valtellina and Val Venosta. populations were also analysed in the field Faulting Active tectonics and their mechanisms unravelled, allowing to identify different structural stages, the youngest being Deep-seated gravitational slope deformation consistent with the regional pattern of the ongoing crustal deformation. Finally, four DSGSD examples have Relief been investigated in detail by geological and 2D geomechanical modelling. Numerical modelling DSGSDs affect more than 10% of the study area, and mainly cluster in areas where anisotropic fractured rock Central Eastern Alps mass and high local relief occur. Their onset and development is subjected to a strong passive control by mesoscopic and major tectonic features, including regional boundaries as well as NW–SE, N–S and NE–SW trending recent brittle structures. The kinematic consistency between these structures and the pattern of seismicity suggests that active tectonics may force DSGSDs, although field evidence and numerical models indicate slope debuttressing related to deglaciation as a primary triggering mechanism. © 2009 Elsevier B.V. All rights reserved.

1. Introduction mountain ranges is played by large-scale, deep-seated gravitational slope deformations (DSGSD). The occurrence of dynamic feedbacks between tectonic and DSGSDs are large mass movements involving high-relief slopes surface processes has been emphasized by a number of authors from their toe up to or beyond the ridge. They can involve nearly all (Molnar and England, 1990; Burbank and Anderson, 2001). Surface rock types, and are characterised by poorly defined lateral boundaries, erosion and sediment transport by glacial, fluvial and gravitational large volumes (often N0.5 km3) and thickness, and relatively low processes modify the topography and redistribute large masses over present-day displacement rates (few to tens mm/yr; Varnes et al., significant distances. This may result in changes of the style and rate of 1990; Ambrosi and Crosta, 2006). Further diagnostic features of tectonic deformation (Burbank and Anderson, 2001). On the other DSGSD are the development of persistent linear morpho-structural hand, the rate and type of erosion are dominated by climate, rock type, features (Agliardi et al., 2001). Scarps, trenches, grabens, double or fracturing, and topography. The latter is strongly controlled by vertical multiple ridges are the more frequently described tensional features tectonic uplift related to flexure, isostasy, thrusting and crustal in the upper slopes, whereas counterscarps (i.e. antislope scarps) are thickening processes (Molnar and England, 1990). Thus, tectonics mainly observed in the middle part. Bulging, buckling folds or highly drives surface processes, which in turn could force tectonic processes fractured rock masses are compressional features occurring at the evoking a positive feedback. The relationships between tectonic slope toe. Large catastrophic slope instabilities (rockslides and rock- processes and surface erosion by fluvial, glacial, and catastrophic avalanches) also often occur in DSGSD areas. processes have been studied by a number of investigators In general, DSGSDs appear closely related to specific geological and (Densmore et al., 1998; Burbank and Anderson, 2001; Brocklehurst structural features (bedding, foliations, joints, faults, etc.) and and Whipple, 2007; Korup et al., 2007). On the other hand, an morpho-topographic factors, and frequently occur in tectonically important role in landscape development in tectonically active active areas (McCalpin, 1999; Crosta and Zanchi, 2000; Agliardi et al., 2009). Phenomenological and mechanical descriptions of the pro- cesses leading to DSGSD formation have been proposed by several ⁎ Corresponding author. Tel.: +39 02 6448 2028; fax: +39 02 6448 4273. fi E-mail addresses: [email protected] (F. Agliardi), [email protected] authors, interpreting features observed in the eld as representative (A. Zanchi), [email protected] (G.B. Crosta). of specific mechanisms including: postglacial debuttressing of

0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.02.019 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 251

Fig. 1. Tectonic sketch map of the central Eastern Alps (modified after Martin et al., 1991 and Froitzheim et al., 1997). Rectangles refer to the location of studied areas: 1) Regional study area; 2) Upper Val Venosta; 3) Upper Valtellina. Keys to symbols in the inset: EF: European Foreland; MF: ; JU: Jura; PF: Front; HE: units; AU: Austroalpine units; EW: Engadine Window; TW: Tauern Window; PN+WA: Penninic units and Austroalpine of the Western Alps; SA: ; Ad: Adamello intrusion; MA: Bergell intrusion; M: Monferrato Arc; LT: Ligurian–Piedmont basin; DL: Delphinese units. Keys to symbols in the figure: Ja: Jaggl – Cima Termine Permo-Mesozoic rocks; Ma: Matsch unit; SB: Schneeberg Unit; BL: Brenner Line; JL: Jaufen Line; PS: Pennes Line; PA: Passiria Line; PL: Pejo Line; SF: Schlinig Fault; ZL: Zebrù Line; VSZ: Vinschgau Shear Zone. oversteepened slopes and associated groundwater changes (Agliardi their relationships with tectonic structures, active tectonic processes, et al., 2001; Ballantyne, 2002), topographic stresses in high-relief surface processes, and topography. Aerial photo-interpretation over slopes (Radbruch-Hall, 1978; Varnes et al., 1989), effects of tectonic 5000 km2 between Valtellina, the Engadine valley and Val Venosta stresses (Miller and Dunne, 1996), ground shaking (central Eastern Alps, Fig.1) allowed mapping of more than 100 DSGSDs. (Radbruch-Hall, 1978; McCalpin, 1999), and fluvial erosion of the Detailed morpho-structural lineament mapping was carried out by slope toe (Crosta and Zanchi, 2000). photo-interpretation and field surveys in smaller areas (about 750 km2) A major pitfall in the analysis of the relationships between tectonic including Upper Valtellina and Upper Val Venosta, where mesoscopic and DSGSD-related features is their morphological convergence. fault populations have also been analysed. These data were compared to Widespread morpho-structural evidence of postglacial surface rup- seismological, geodetic and in situ stress data, to discuss the relation- tures in the Alps (Forcella et al., 1982; Crosta and Zanchi, 2000; Sue ships among structural settings, active faulting, and DSGSD. Finally, four and Tricart, 2002; Persaud and Pfiffner, 2004) could be related to DSGSD have been analysed in detail by geomechanical numerical recent or active tectonic processes, to the gravitational deformation of modelling, in order to evaluate the influence of tectonic and surface large rock slopes, or to a combination of them (McCalpin, 1999; processes on the development of these phenomena. Agliardi et al., 2009). Gravitational morpho-structures have been often interpreted as the morphological expression of active faults, 2. Geological setting and active tectonics possibly biasing the overall interpretation of the tectonic evolution of some areas (Forcella and Orombelli, 1984; Bovis and Evans, 1995; 2.1. Geological setting and evolution Hippolyte et al., 2006). Misinterpreting morpho-structural evidence could also influence the assessment of the seismic potential of active The study area is part of the Austroalpine nappe stack, exposed faults (McCalpin, 1999), the reconstruction of the glacial/paraglacial between the Engadine and the Periadriatic Lineament (Fig. 1). The history, as well as the evaluation of postglacial sediment budgets in pre-Alpine and Alpine tectono-metamorphic evolution of these units large alpine catchments. On the other hand, tectonic controls on has been described by several authors (Ratschbacher, 1986; Thöni and DSGSD can be difficult to ascertain, especially in tectonically active Hoinkes, 1987; Ratschbacher et al., 1989; Martin et al., 1991; Thöni, areas with low magnitude seismicity (Mb5; Slejko et al., 1989). 1999; Sölva et al., 2005). This study aims at mapping and characterising tectonic and The Oetztal and the underlying Sesvenna-Campo , which gravitational morpho-structures at different scales (i.e. regional to form most of the area, mainly consist of poly-metamorphic basements single slope scale), in order to investigate the distribution of DSGSDs and including gneiss and schists, with subordinate orthogneiss, amphibolite, 252 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270

Fig. 2. Epicenter distribution for with moment magnitude Mw N2, instrumentally recorded in the 1975–2008 period (source: Earthquake Catalogue of Switzerland; Fah et al., 2003). Base map includes elevation (SRTM topographic dataset, cell size about 90 m) and the major tectonic features. Present-day surface uplift contour lines (modified after Kahle et al., 1997) are also shown. Square encloses area 1 in Fig. 1.

and marble, separated by poorly metamorphic Permo-Mesozoic with lateral continental extrusion along strike-slip and normal faults sedimentary rocks. Both units show well-preserved amphibolite-facies occurred since Late Oligocene (i.e. Periadriatic, DAV, Brenner Linea- associations overprinted by greenschist-facies assemblages. The main ment) marking the recent evolution of the belt. E–W to ESE–WNW difference between the two units is the occurrence of Variscan eclogitic dextral motion was associated with back-folding and related back- relics in the Oeztal nappe. A high-pressure Eo-Alpine metamorphic thrusting along the North Giudicarie and Merano–Mules NE–SW event was also recognized north of Merano in the Texel Massif, which is trending branch of the Periadriatic Lineament, now interpreted as an tectonically interposed between the two main units. The Campo nappe, original restraining bend of this fault, leading to the formation of consisting of phyllites in the region around , lacks these peculiar ductile to brittle shear zones between 30 and 20 Ma ago (Martin et al., assemblages and shows more preserved Variscan associations. The 1991; Prosser, 2000; Mancktelow et al., 2001; Müller et al., 2001; Viola sedimentary cover of the Sesvenna-Campo nappe (i.e. Ortler and S-charl et al., 2001, 2003). Exhumation of the Penninic units now exposed in units) mainly consists of Upper Palaeozoic terrigenous metasediments, the Tauern window occurred at the same time along the N–S trending passing to thick Triassic carbonate successions exposed north of the Brenner normal fault (Selverstone, 1988). Zebru Line. During the Middle , back-thrusting in the Southern Alps Major regional structures occurring in the area are the Schlinig Line was coeval with the northward propagation of the North Giudicarie (Schmid and Haas, 1989; Froitzheim et al., 1997) and the Zebrù Line fault along the Passiria valley (north of Merano), as a NNE–SSW (Conti et al., 1994). The former gently dips to the E–SE along the Upper sinistral strike-slip fault. The Engadine Line, a sinistral oblique-slip Val Venosta, stacking the Oeztal nappe above the Sesvenna-Campo fault bounding the Engadine window to the east, also became active at nappe. Schmid and Haas (1989) interpreted this structure as an intra- this time (Schmid and Froitzheim, 1993) causing the exhumation of basement shear zone (Fig.1), associated with the Early Cretaceous west- the Penninic units in the Engadine window. Both lines still show directed tectonic transport of the Oetztal Nappe. The same structure was seismic activity (Caporali et al., 2005). reactivated during the Late Cretaceous as a shallow normal fault post- dating nappe stacking (Froitzheim et al., 1997). The Zebrù Line occurs 2.2. Recent and active tectonics north of Bormio and steeply deeps to the North, separating the Campo nappe from the overlying Ortler sedimentary cover. Several minor units Recent and active tectonics was recognised in the central Eastern Alps (e.g. the Grosina and Chavalatsch nappes) are also included within the by several authors, based on seismological data (Slejko et al., 1989; Austroalpine stack. Thick mylonitic and cataclastic layers occur along the Deichmann and Baer, 1990; Braunmiller et al., 2002; Caporali et al., main tectonic contacts. 2005), GPS permanent network and precise levelling data (Kahle et al., Following nappe emplacement, exhumation of HP units, and at 1997;D'Agostino et al., 2005; Tesauro et al., 2006), as well as by least two stages of post-nappe folding, N–S shortening synchronous photogeological analyses (Forcella et al., 1982), mesoscopic structural F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 253

Fig. 3. (a) Moment tensors of recent earthquakes, with numbers indicating hypocentral depth in kilometres. Squares in (a) are the studied areas as in Fig. 1. (b) principal stress directions obtained by the analysis of fault mechanism solutions using the P–T dihedra method: horizontal hatches indicate high compatibility for extension, vertical hatches high compatibility for compression. Black filled symbols indicate the preferred axes of consistency regions. Convergent arrows indicate the direction of compression, divergent arrows the direction of extension (modified after Agliardi et al., 2009). analyses of fault populations and in situ stress determinations (Zanchi based on full-waveform inversion of the main shocks, indicate normal et al., 1995; Persaud and Pfiffner, 2004; Reinecker et al., 2005; Agliardi faulting mechanisms with nodal planes striking NNW–SSE (Braun- et al., 2009). miller et al., 2002). Another notable event was recorded on 17 July The region is characterised by upper crustal microseismicity, 2001 near Merano (M=4.8; Caporali et al., 2005). For this event, focal increasingly more frequent toward the internal sector of the belt and planes suggest strike-slip motion along a NE–SW left-lateral fault particularly clustered in the Engadine, Valtellina and Venosta areas parallel to the North Giudicarie Line. Recent tectonic activity along (Slejko et al., 1989; Fah et al., 2003; ECOS, ETHZ-Red Puma, Harvard NNE–SSW sinistral and NW to WNW–ESE dextral strike-slip faults had CMT sources; Fig. 2). In spite of the large uncertainty affecting the already been suggested by Forcella et al. (1982) based on photo- localisation of small (Mb2) and shallow events, epicentres cluster in geological investigations. the Graubunden between and the Engadine, in the Upper We tested the entire set of available fault planes solutions (Fig. 3a) Valtellina and Mustair valley, and along the western flank of the Upper through the P and T dihedra method (Angelier, 1984), to evaluate

Val Venosta (Fig. 2). Earthquakes within the internal Alps are seismic data in terms of stress regime. The compatibility areas for σ1 restricted to the upper 15 km of the crust, contrary to the northern and σ3 and their centroids (Fig. 3b), obtained by using 12 couples of foreland where earthquakes occur throughout the crust down to the nodal planes, suggest a horizontal N–S trending σ1 axis and a Moho at 30 km (Deichmann and Baer, 1990). In the study area, the horizontal E–W trending σ3 axis. These results agree with crustal Earthquake Catalogue of Switzerland (Fah et al., 2003) reports about deformation measurement by GPS permanent networks (D'Agostino

200 earthquakes with moment magnitude (Mw)N2 in the 1975–2008 et al., 2005; Tesauro et al., 2006). These show a rotation of the period, with maximum values reaching 4.9. Two earthquakes with compressional geodetic strain axes to N–S in the Eastern Alps, possibly

Mw N4 occurred on 29–31 December 1999 north of Bormio (maximum as a result of the counter-clockwise rotation of the Adria microplate Mw =4.9; Braunmiller et al., 2002). Moment tensor determinations, (D'Agostino et al., 2005), causing a compression consistent with the 254 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 255

Fig. 5. Analysis of DSGSDs mapped at the regional scale. a) DSGSD frequency – area distribution; b) DSGSD areal distribution versus lithology. The areal frequency of each lithology, the fraction of total DSGSD area involving each lithology, and the percentage of outcrop area involved in DSGSD (i.e. DSGSD density) are shown; c) DSGSD frequency versus minimum, average and maximum values of local relief, extracted from the map in Fig. 4; d) DSGSD frequency versus uplift rate (data after Kahle et al., 1997). present stress regime in the central Eastern Alps (Müller et al., 1992; 30 DSGSDs occurring near two major structural features, i.e. the Mariucci and Müller, 2003; Reinecker et al., 2005). Schlinig and the Gallo Faults. Clustering of DSGSDs suggests that their Available geodetic levelling data (Kahle et al., 1997) suggest that the development is constrained by some combinations of local conditions. area between Chur and the Upper Valtellina and Venosta valley is Thus, we analysed the distribution of DSGSD versus lithology, local presently undergoing a surface uplift up to 1.2–1.4 mm/yr (Fig. 2). This relief, active seismicity, and the present-day rate of uplift. rate is consistent with the high values of topographic relief (N1500– The distribution of DSGSDs with respect to lithology was 2000 m) of the area, which are typical of landscapes developed under investigated by intersecting the DSGSD map with a reclassified tectonic forcing (Kühni and Pfiffner, 2001; Montgomery and Brandon, version of the 1:500,000 Geological Map of Switzerland (Swisstopo, 2002). Persaud and Pfiffner (2004) related the uplift rate in the area to 2007; Fig. 4a). Rock types were grouped according to their typical ongoing tectonic processes, although other researchers proposed that it geomechanical behaviour (i.e. average rock mass properties and could be related to isostatic reaction to crustal overthickening and anisotropy; Bieniawski, 1989; Hoek and Brown, 1997) into: granitoid, erosion (Schlunegger and Hinderer, 2001), or Last Glacial Maximum metabasite, orthogneiss, metapelite (mainly phyllite), paragneiss, (LGM) glacier removal (Gudmundsson, 1994). marble, carbonate and terrigenous (mainly sandstone and marl) sedimentary rock. deposits, mapped only when occurring 3. Spatial distribution of DSGSD in large accumulations (e.g. alluvial valley fill, thick glacial cover), were considered separately (Fig. 5b). Notwithstanding the uncertain- DSGSDs were mapped at the regional scale to compare their ties due to map scale, about 68% of the mapped DSGSD occur in distribution to the local topographic, geological and tectonic settings. metapelite or paragneiss, with a DSGSD density up to 15% (Fig. 5b). Mapping was based on the analysis of aerial and satellite images at 13% of DSGSD area involves orthogneiss (density over 9%). Sandstone different scales. This allowed to recognise and map 105 DSGSDs and marl are also significantly affected by DSGSD (density up to 13%), (Fig. 4), each one extending between 0.3 km2 and 34 km2 (Fig. 5a), although their exposure is here limited. Carbonates and granitoids are whereas the total area involved in DSGSD is about 460 km2 (9.2% of poorly affected by DSGSD, whereas metabasite and marble are the study area). Three main DSGSD clusters were recognised north of involved only when interlayered in metapelite and paragneiss. Tirano (Valtellina), around Bormio (Upper Valtellina) and between The distribution of local relief in DSGSD areas was analysed using the Mustair valley and the Upper Val Venosta. The first cluster the SRTM digital elevation model (cell size ∼90 m). Local relief includes the Padrio-Varadega DSGSD, i.e. the largest observed in the (Fig. 4b) was calculated as the difference between the minimum and Alps, covering an area of about 34 km2 (Ambrosi and Crosta, 2006). maximum elevation within circular windows with a radius of 5 km for The Bormio cluster includes about 25 DSGSDs aligned WNW–ESE each grid cell for the studied area. The size of the counting circle south of the Zebrù Line. The Mustair-Venosta cluster consists of about accounts for the wavelength of major valley spacing and allows

Fig. 4. Spatial distribution of DSGSD phenomena mapped at the regional scale in the outlined area with respect to lithology and local relief. The lithological map (a) was obtained by reclassification of the 1:500,000 Geological Map of Switzerland (Swisstopo, 2007). The local relief map (b) was obtained by the SRTM digital elevation model (cell size ∼90 m). See text for explanation. Squares refer to area 1 in Fig. 1. Labels A, B, C and D refer to case studies: A) Mt. Watles; B) Ruinon; C) Bosco del Conte; D) Cima di Mandriole. 256 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270

Fig. 6. Morpho-structural features related to DSGSD in the studied sector of the central Eastern Alps. a) double-crested ridge at Cima 10 (Upper Val Venosta), showing an apparent downthrow of about 100 m; b) complex morpho-structural association related both to tectonic lineaments and DSGSD morpho-structures in the Viola valley (Upper Valtellina); c) small ponds dammed by counterscarps in the Mt. Watles DSGSD area (Upper Val Venosta); d) Ruinon DSGSD area, involving the entire right flank of the Valfurva (Upper Valtellina). In the lower part of the slope, the active Ruinon landslide area is evident; e) view from the SW of the Bosco del Conte DSGSD (Viola valley, Upper Valtellina); f) aerial view of the Cima di Mandriole DSGSD, impending on the Pian Palù reservoir and dam (Peio, Noce valley). comparison to other studies (Kühni and Pfiffner, 2001; Montgomery 30 DSGSDs. We used black and white (1:33,000 scale, 1954; 1:20,000 and Brandon, 2002). scale, 1983) and colour aerial photos (1:20,000 scale, 1980–82), and In the study area, DSGSDs are associated with average local relief colour orthophotos (2000). Lineaments have been classified as either frequently exceeding 1500–2000 m (Fig. 5c). The minimum local relief tectonic or gravitational according to a variety of morphological detected in DSGSD areas also exceeds 1000 m (Fig. 5c). criteria (McCalpin, 1999; Persaud and Pfiffner, 2004) including: The distribution of DSGSDs was also compared to the pattern of the persistence, plan shape, continuity across major ridges or valleys, present-day rate of rock uplift (Kahle et al., 1997). Although the study relationships with major tectonic structures (e.g. nappe boundaries, area is located at the boundary of their dataset, available data suggest thrusts, faults), association to other tectonic structures or gravitational that the entire area is undergoing a high uplift rate (N1.2 mm/yr), with a features, kinematics inferred by different indicators (e.g. relations with maximum north of Bormio (about 1.4 mm/yr), where recent earth- glacial, periglacial and alluvial landforms and deposits), and amount of quakes also cluster (Fig. 2). In the study area, the absolute frequency of displacement (when possible). DSGSD shows a positive correlation to uplift rate (Fig. 5h). Tectonic lineaments (i.e. master fractures, faults) are generally persistent and rectilinear, with low associated topographic relief. These 4. Detailed mapping of morpho-structural features features strongly condition the drainage pattern and the localization of minor rock slope instabilities (e.g. rock falls, block slides). Recent tectonic 4.1. Mapping and classification featurescrossmajortopographicfeaturesasridgesorwatersheddivides. Postglacial faults appear as solid outcrop zones, crossing glacial 1A detailed photogeological survey was carried out over about landforms or deposits, and can evenly occur across slopes. 750 km2, including the Upper Valtellina (600 km2) and Val Venosta Gravitational morpho-structures are related to DSGSD phenomena (150 km2). We mapped both tectonic lineaments and morpho- (Figs. 4, 6–8), and include double-crested ridges, scarps, counterscarps, structural evidence of gravitational processes related to more than gravitational half-grabens with associated block tilting, and open trenches F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 257

Fig. 7. Comparison between tectonic lineaments and DSGSD morpho-structures in the Upper Val Venosta (area 2 in Fig. 1). Rose diagrams refer to cumulative number and length of fractures in 5° azimuth classes. The maximum relative percentage is indicated below each rose. The total number of analysed segments is reported for each diagram. Square encloses the area in Fig. 11a(“A” in Fig. 4) (modified after Agliardi et al., 2009).

(Agliardi et al., 2001). These structures often show a trend similar to that of valleys; see Fig. 7). These fractures cross nappe boundaries (e.g. tectonic features, but are commonly less continuous, arcuate, and occur as Schlinig fault) and bound the NW margin of the Permo-Mesozoic swarms of multiple associated scarps. These can have individual offsets up massif of Cima di Termine (Vallunga Line). Nevertheless, the main to several tens of meters, and are usually confined in specificslopesectors morphological features (e.g. the Upper Val Venosta) follow N–S and showing evidence of dislocation with respect to neighbouring slopes. NNW–SSE, trends, which are less evident from the analysis of fracture lineaments (Fig. 7). N–S and NNW–SSE fractures occur along both 4.2. Tectonic vs. gravitational morpho-structures valley flanks, along the double ridge connecting Cima 10 and Cima 11, where a possibly active fault has been detected, east of Curon Venosta 4.2.1. Upper Val Venosta: the Resia Pass area and S. Valentino alla Muta up to Plan dei Morti (Fig. 7). Here N–S The photogeological survey along the Upper Val Venosta covers the trending normal faults were recognised. NW–SE fractures are also area from Glorenza to the Resia Pass (Fig. 7). Over 1500 lineaments common in the area, especially around Malles Venosta and in the were mapped, including fractures as well as scarps, counterscarps and upper Schlinig valley. E–W fractures locally occur, i.e. west of Lago trenches related to DSGSD. della Muta and south of Cima Pian del Lago. The main fracture system trends NE–SW, i.e. the same trend as the Postglacial gravitational reactivations of NE–SW, N–S, NNW–SSE, NE–SW nodal planes of seismic events (Fig. 3). Several valleys also and E–W fractures are widespread, in particular on the western flank show this trend (e.g. Mustair, Zerzer, Roja, Planol, and Plawenn of the Upper Val Venosta (Fig. 7). Double and multiple ridges, scarps 258 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270

Fig. 8. Comparison between (a) tectonic lineaments and (b) DSGSD morpho-structures along the upper part of Valtellina south of Bormio (area 3 in Fig. 1). Same symbols as in Fig. 7. F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 259 and counterscarps related to DSGSDs are particularly frequent along deep among mountain peaks up to 3000 m a.s.l., consisting of Late the Cima 10–11 ridge, where they trend N–S, and along the Mt. Watles Palaeozoic Sondalo Gabbro, Bormio Phyllites, and Grosina Gneiss ridge, where NE–SW trending morpho-structures occur. The Cima 11 units. We mapped over 4000 lineaments and prepared two separate double-crested ridge suggests a downthrow of about 100 m (Fig. 6a). maps for structural features and DSGSD-related gravitational morpho- NW–SE trending DSGSD morpho-structures occur in the Schlinig structures (Fig. 8a and b). valley along the Mt. Watles and Mt. Rodes ridge. Less frequent E–Wto Trends of mapped lineaments (Fig. 8a) are close to those recognised ENE–WSW trending DSGSD morpho-structures occur above Laudes in nearby areas. Frequent and persistent NW–SE fractures occur south of and along the northern slope of the Zerzer valley (Fig. 7). DSGSDs are Bormio, whereas NE–SW fractures generally occur more discontinu- not as significant along the eastern flank of the Upper Val Venosta, ously. N–S trending fracture lineaments are evident along the Upper where the Permo-Triassic sedimentary cover crops out, and where Valtellina as along the Upper Val Venosta. E–W trending faults south of lineaments are generally less persistent. Mt. Zandila could be related to the Insubric Line (Figs. 1 and 4), the most important alpine structure occurring a few kilometres to the south. The 4.2.2. Upper Valtellina relationships between NW–SE trending systems and the N–S and NNW– The investigated sector of the Upper Valtellina (Fig. 8) is bounded SSE ones may suggest that they are synchronous and related to dextral to the north by the Italian–Swiss border, to the east by the Gavia Pass transtension. NW–SE fractures correspond to dextral strike-slip fault, area, to the south by the upper Val Grosina and to the west by the whereas N–S and NNW–SSE faults are pure normal and normal/oblique Foscagno Pass area. Near Bormio, the valley is cut more than 2000 m faults forming extensional stepovers and duplex structures. These are

Fig. 9. (a) Mesoscopic structural analysis of fault populations related to the oldest stage recognized in the study area. Great circles are faults, slickenside lineations are dots with arrows indicating the sense of motion. Convergent arrows correspond to the horizontal direction of the σ1 axis; divergent arrows to the horizontal direction of the σ3 axis. Principal stress eigenvectors are represented as stars with five (σ1), four (σ2) and three branches (σ3). Outline stars represent the solution obtained with the R4DT inversion function (Angelier, 1984), filled stars the solution for INVD (Angelier, 1990). (b) Mesoscopic structural analysis of fault populations related to the youngest stage recognized in the study area. Same symbols as in panel (a). 260 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270

Fig. 9 (continued). evident especially between the Foscagno Pass and Mt. Trela (NW sector analysed and compared (rose diagrams in Fig. 8). The trends of of Fig. 8a). tectonic and gravitational morpho-structures are very close. In both In the Bormio area, major gravitational morpho-structures related cases, the more important sets trend NW–SE (N125°–130°) and NE– to DSGSD (Fig. 8a and b) occur along the Valfurva, Valdidentro and Val SW (N35°–45°), whereas the N–S and E–W sets are less significant. Viola (Fig. 6e), where they mainly follow NE–SW fracture trends. The orientations of gravitational features are more scattered and DSGSDs occur in metamorphic rocks (i.e. phyllite, paragneiss) and are conditioned by topography (i.e. valley trend, drainage pattern). generally characterised by large and complex trenches and aligned downhill facing scarps. Along the high-relief slopes of Upper 5. Mesoscopic analysis of fault populations in the central Alps Valtellina, the occurrence of N–S fractures is frequently associated with DSGSD. NW–SE trending structures also occur, such as a graben- A detailed field study of several hundred of recent faults was like system of trenches and uphill-facing counterscarps that cross the undertaken at 43 survey points in the study area (Fig. 9). Most of the ridges east of Corno di San Colombano (3050 m a.s.l.). Deformed field work was carried out in Val Venosta, whereas in Upper Valtellina rock glaciers and slope deposits suggest recent motions very detailed observations were performed in an underground tunnel, along these structures (e.g. at Ruinon DSGSD in Valfurva, Figs. 6d where in situ stress was measured by hydrofracturing (Canetta et al., and 8b; Agliardi et al., 2001). In other cases, rock glaciers lack their 1992). Paleostress analysis of striated fault surfaces was performed to source areas (e.g. Migiondo at Sondalo, south of Morignone, Fig. 8b; determine the principal stress directions through iterative (R4DT) and Guglielmin and Orombelli, 2003; Crosta and Frattini, 2004). direct inversion (INVD) methods (Angelier, 1984, 1990). Numerical The frequency and length of the mapped linear tectonic features solutions were accepted for values of the average angles between (9692) and DSGSD morpho-structures (6511) were statistically computed maximum shear stress on the fault plane and observed F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 261

Fig. 10. Fault populations analyzed in the Premadio 2 power plant tunnel near Bormio and related stress-tensor determinations (a to f). Results of hydrofracturing at Premadio (Canetta et al., 1992) and comparison with stress-tensor solutions obtained for the most recent stage are shown in plots g and h. Symbols for principal stress eigenvectors: same as in

Fig. 9. Magnitude of principal stresses from hydrofracturing tests: σ1 =5.15 MPa, σ2 =2.27 MPa, and σ3 =1.94 MPa.

striations lower than 10°–12°. Larger deviations were only considered σ1 axis (i.e. when strike-slip faults occur), or with a vertical σ1 axis in the case of separation of complex fault assemblages. The geometry (i.e. when N–S trending normal faults dominate). and relative chronology of slickensides, as well as the geometrical Both mesoscopic observations and mapped lineaments suggest consistency among sets of conjugate faults allowed us to recognise that the Upper Val Venosta developed along a N–S sinistral strike-slip two main brittle tectonic stages. fault, subsequently reactivated as a normal fault. This may be related to the Glorenza fault, which deeply dissects the mylonitic rocks 5.1. NW–SE compression stage related to Mesozoic thrusting and normal faulting along the Schlinig Line. NW–SE and NE–SW conjugated strike-slip faults related to a N–S The oldest fault assemblage found in Val Venosta consists of compression have also been found at several sites indicating the conjugate sets of E–W trending dextral and NNE–SSW to N–S trending importance of recent tectonic deformation. Mesoscopic data suggest sinistral strike-slip faults. Along the Merano–Mules segment of the that N–S trending normal-oblique faults are the most recent ones, Periadriatic Line, in the Passiria valley, these faults are often associated post-dating motions along the conjugated sets of NNW–SSE dextral with NW–dipping NE–SW reverse dip-slip faults and previous mylonitic and NNE–SSW sinistral strike-slip faults. This tectonic regime, shear zones displaying a similar geometry and kinematics (Viola et al., consistent with the present-day seismicity (Fig. 3) and with the left-

2001). Stress-tensor determinations indicate a horizontal NW–SE σ1 lateral motion along major tectonic structures (i.e. North Giudicarie axis associated with a NE–SW σ3 horizontal axis. NW–SE normal dip- Fault, Passiria Fault and Engadine Line, trending between NE–SW and slip faults have been found in association with strike-slip faults. They can NNE–SSW), has probably been active at least since the Late Miocene be related to local permutation of the maximum and intermediate stress (Müller et al., 2001). axes (Fig. 9a). This stage can be related to dextral motion and associated Fault analyses were also carried out into an exploratory tunnel at SE-vergent back-thrusting which occurred along a restraining bend of the Premadio power station, near Bormio (Canetta et al., 1992), at a the Insubric Line (North Giudicarie Line and Meran-Mules Line) during depth of about 200 m within the Bormio Phyllites. The tunnel crosses a the Miocene (Prosser, 2000; Viola et al., 2001). N–S trending normal fault, representing the northern branch of an important fault system occurring along the axis of Upper Valtellina 5.2. N–S compression stage and related E–W extension (Berra et al., 2002). Here a complex polyphase assemblage has been recognised through the analysis of cross-cutting relationships (Fig. Conjugate sets of NNW–SSE trending dextral and NNE–SSW 10a). Two main stages have been distinguished, closely matching the trending sinistral strike-slip faults were recognised at most of the observations made in the nearby Venosta area. The oldest one consists studied sites, associated with N–S trending normal dip-slip faults of a NW–SE compression causing the activation of conjugate sets of E– (Fig. 9b). NNE–SSW sinistral strike-slip faults were also found in W dextral strike-slip faults and NNE–SSW to N–S striking sinistral association with NNE–SSW normal and normal-sinistral faults (e.g.: faults. For this stage, stress-tensor determinations indicate a hor-

Tubre valley). Stress-tensor determinations relative to this stage izontal NW–SE σ1 stress axis associated with a NE–SW σ3 horizontal suggest an E–W horizontal σ3 axis, associated either with a horizontal axis (Fig. 10b). Measured NW–SE normal dip-slip faults can be 262 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 263 geometrically related to the same stage, with numerical solutions 6.2. Ruinon DSGSD (Upper Valtellina) suggesting a local permutation in the σ1 and σ2 axes (Fig. 10c). E–W trending normal faults related to N–S extension possibly reactivating The Ruinon DSGSD (“B” in Figs. 4, 11b) occurs in the Upper dextral strike-slip faults have also been found (Fig. 10d). The second Valtellina, east of Bormio along the middle Valfurva, where both valley stage shows a compression with a N–S trending σ1 axis and a flanks are affected by large DSGSDs. The area consists of phyllites of horizontal E–W σ3 axis, indicated by the activation of conjugate sets of the Campo nappe, including isoclinally folded marble and prasinite NNW–SSE dextral and NNE–SSW sinistral strike-slip faults (Fig. 10e). layers (Agliardi et al., 2001). The DSGSD developed over an area of These fault sets are crossed by N–S trending conjugate normal faults, about 5 km2 along a pre-existing deep-seated surface dipping to again associated with a permutation of the principal stress directions WNW, and reactivated recent WNW–ESE and N–S fractures. Displaced (Fig. 10f). marble layers and observed offset along the main DSGSD headscarp Fault analyses have been integrated by in situ stress determinations suggest a downthrow of about 150 m (Fig. 6d). According to Agliardi provided by 16 hydrofracturing tests carried out at Premadio (Canetta et al. (2001), the geometry and displacement pattern of the DSGSD are et al., 1992). Data from only nine of the 16 tests have been used constrained by WNW trending master fractures, continuously cross- because of the complicated fracture patterns resulting from rock ing most of the northern flank of the Valfurva. Along these fractures anisotropy and intense fracturing. Principal stress directions obtained other gravitational deformations occur (e.g. Pietra Rossa; Fig. 8b). by multiple regression are typical of an extensional regime activating These fractures, consistent with recent dextral strike-slip faults, have normal faults. Stress directions obtained through in situ tests (i.e. been related to neotectonic activity by Forcella and Orombelli (1984). nearly vertical σ1 axis, horizontal σ2 and σ3 axes, the last one trending On the contrary, their recent reactivation is suggested by normal dip- ENE–WSW; Fig. 10g and h) are closely consistent with those obtained slip gravitational shear failures crossing the Holocene rock glacier of by the analysis of faults. the Cavallaro valley (active between 10 and 6 ka, Fig. 11b). The evolution of WNW trending gravitational scarps and half-grabens led 6. DSGSD modelling: selected case studies to the progressive failure of the lower slope sector during the Holocene (Agliardi et al., 2001). This is indicated by large landslide The mechanisms of DSGSD development were studied in detail accumulations and by the occurrence of an active, 30⁎106 m3 complex and investigated through 2D stress-strain numerical modelling at four rockslide at the DSGSD toe (between 1400 m and 2150 m a.s.l.), sites (Fig. 4). These were selected according to their morpho- presently causing major concern (Crosta and Agliardi, 2003). structural evidence, interaction with man-made structures, and Measurements by the Permanent Scatterers technique (Colesanti availability of geomechanical or monitoring data. et al., 2004) suggest that the entire DSGSD and the confining Corna Rossa area are lowering at a vertical rate of 7–24 mm/yr. 6.1. Mt. Watles DSGSD (Upper Val Venosta) 6.3. Val Viola–Bosco del Conte DSGSD (Upper Valtellina) Mt. Watles is located on the western flank of the Upper Val Venosta (“A” in Figs. 4, 11a), and is delimited by the Adige river to the east, the The Bosco del Conte DSGSD (“C” in Fig. 4)occursonthe Schlinig valley to the southwest, and Zerzer valley to the northwest. southeastern flank of the Val Viola (Figs. 8 and 12), a ENE trending Mt. Watles slope has a local relief of about 1500 m with average slopes valley linking the Italian–Swiss border to the Upper Valtellina. The of 20°–30°. The DSGSD (Agliardi et al., 2009) affects an area of about most important structural feature in the area is the Mt. Verva–Corno 10 km2 within paragneiss with thick orthogneiss and amphibolite delle Pecore–Dosso Peneglia thrust, stacking the Grosina Unit above layers of the Oetztal nappe (Figs. 1 and 11). This is juxtaposed to the the Campo Unit (Corradini et al., 1973). The thrust dips to the SSE with sedimentary cover of the Sesvenna-Campo nappe along the Schlinig an average dip of 35°–45°, and is associated with a break in the fault (Froitzheim et al., 1997), which crosses the western flank of Mt. hillslope gradient. Both valley flanks show widespread gravitational Watles. morpho-structures including scarps, counterscarps and trenches, Mt. Watles is crossed by scarps and counterscarps with single mainly reactivating pre-existing NE–SW fractures. N–S, NW–SE and offsets up to some tens of metres (Fig. 6c). A link between recent WNW–ESE fracture sets are also important. Both valley flanks are also brittle structures (Fig. 7) and gravitational morpho-structures is characterised by large landslide accumulations and DSGSDs. evident (Agliardi et al., 2009). These consist of huge NE trending The Bosco del Conte DSGSD extends on about 3 km2 between ridge-top trenches, scarps and counterscarps, sometimes filled with 2515 m a.s.l. and 1550 m a.s.l. (Fig. 6e). The NW slope sector is cut by lacustrine and peat deposits (Fig. 11a). In the upper part of the slope, sharp NE–SW scarps and counterscarps up to 1 km long and up to asymmetrical half graben-like systems of scarps and counterscarps 50 m high. The N and E slopes down to the confluence of the Cardone re-activate NE–SW recent fractures containing cataclastic bands and and Viola valleys are less steep, without large morpho-structures but fault gouge (Agliardi et al., 2009). In the middle and lower slope with widespread evidence of past slope failures. Along the N slope sectors, rockslide scars and accumulations suggest a partial collapse. sector, a large landslide probably up to 100 m thick, interrupts the According to Agliardi et al. (2009), differences in rock rheology DSGSD morpho-structures to the east. A smaller complex landslide, 20 below and above the Schlinig Fault could have localised deforma- to 30 m thick, occurs on the western side of the slope. Counterscarps tions in the metamorphics of the hanging wall, belonging to the are usually filled by the abundant blocky debris masking the bedrock. Oetztal nappe. In the upper part of the dome-shaped slope, some scarps formed along Radiocarbon dating of peat deposits formed along major morpho- dark phyllitic and cataclastic zones. The upslope development of the structures (Agliardi et al., 2009) demonstrates that slope deformation DSGSD seems to be constrained by the exposure zone of the started during the Lateglacial and continued during the Holocene in orthogneiss–phyllite thrust. Some of the mapped morpho-structures some slope sectors. Field evidence includes glacial and periglacial cross-cut glacial deposits at 1950 m a.s.l. along the eastern limit of the landforms and deposits cut or obliterated by gravitational morpho- DSGSD. Deposits related to nivation are also cut by some morpho- structures in the Mt. Watles area and surroundings (e.g. Cima 10, Cima structures in the Mt. Verva area, whereas large along the 11, Cima Termine). slopes postdate most of the DSGSD movements.

Fig. 11. Geomorphological and morpho-structural maps of the Mt. Watles (a) and Ruinon (b) DSGSD areas (location: “A” and “B” in Fig. 4). A simplified geological sketch of the Mt. Watles area is portrayed in the inset: units 1 (orthogneiss), 2 (amphibolite) and 3 (paragneiss) belong to the Oetztal nappe; units 4 (Triassic carbonates), 5 (Permo-Triassic sandstones) and 6 (paragneiss) belong to the Sesvenna – Campo nappe (modified after Agliardi et al., 2001, and Agliardi et al., 2009). 264 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270

Fig. 12. Morphostructural analysis of the Val Viola Bormina area and Bosco del Conte DSGSD (location: “C” in Fig. 4). a) Fracture lineaments from photogeological analysis. b) Geomorphological map with the main structures, morpho-structures and landslides. F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 265

Fig. 13. Geomorphological and morpho-structural map of the Cima di Mandriole DSGSD at Peio. (location: “D” in Fig. 4). Contour interval: 50 m.

6.4. Cima di Mandriole DSGSD (Peio) gravitational movements (Fig.13). These follow a slightly curved trend and terminate in alluvial deposits, which were found below fractured The Cima di Mandriole DSGSD (“D” in Figs. 4, 13) occurs in the rock at the slope toe (Desio, 1961). In the upper slope, steep head upper Noce valley (Trentino–Alto Adige), few kilometres south-east of scarps with downthrow exceeding 100 m occur (Fig. 6e), reactivating the Ruinon area. It extends over about 6 km2, involving the Campo a NE trending fault. Above 2300 m a.s.l., the slope is crossed by swarms nappe metamorphics north of the Peio Line (Fig. 1), a NE trending of rectilinear counterscarps up to 200 m long, cross-cutting glacial Cretaceous normal fault separating the Campo from the Tonale nappe erosional surfaces and rock glacier deposits. Below 2300 m a.s.l., (Martin et al., 1991). Rocks include isoclinally folded paragneiss scarps and counterscarps form gravitational half-grabens. These associated with mylonitic orthogneiss, crossed by ENE–WSW and represent the head of a series of large concentric landslides with NW–SE master joints and normal to oblique dip-slip faults. morphological evidence increasing downslope. The statistical analysis The DSGSD involves the entire slope down to the 53 m high Pian of tectonic and gravitational lineaments revealed that most tectonic Palù dam (Fig. 13). Heavily fractured rock masses on the left bank of fractures trend NE–SW, whereas E–W and N–S ones are few but the dam site were observed during its construction (Desio, 1961). persistent. NE–SW fractures clearly constrain the geometry of the Thus, the dam was built using large concrete blocks with friction DSGSD, showing a trend very close to the slope direction. joints, able to stand large deformations. Cataclastic shear zones with Geological and site investigation data (Crosta et al., 2000) suggest loose coarse gouge were encountered during borehole drilling and that the DSGSD moved over a basal shear zone over 200 m deep, tunnel excavations, indicating discrete failure surfaces related to causing the formation of an active wedge in the upper slope. In the 266 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270

Table 1 Summary of average values of the geomechanical properties adopted in the numerical modelling of DSGSD case studies.

Rock mass Ubiquitous joints DSGSD Mass density Bulk modulus Shear modulus Tensile strength Cohesion Friction angle Tensile strength Cohesion Friction angle kN/m3 GPa GPa MPa MPa ° MPa MPa ° Mt. Watles (A) 27 3 2 0.1 1.5 32 0.05 0.1 40 Ruinon (B) 27 10 5 0.1 0.5 32 0.08 0.1 30 Bosco del Conte (C) 27 7 4.5 0.1 0.3 32 0.05 0.1 28 Cima di Mandriole (D) 27 10 5 0.1 0.2 32 0.1 0.1 33

lower part of the slope, morpho-structures were activated as large thicknesses generally develop later from the slope toe toward the landslides with multiple shear surfaces up to 100 m deep, recognised crest. The development of tensile fracture zones along the slopes is in boreholes as low core recovery and cataclastic layers. Cross-cutting controlled by the slope profile and the orientation of the ubiquitous relations between morpho-structures and glacial/periglacial deposits joints. Model results also suggest that natural rock slopes subjected to suggest a Lateglacial activation and further Holocene activity in the DSGSD remain in a steady-state condition of dynamic equilibrium upper slope. At the slope toe, displacement monitoring revealed with accelerations for changes in controlling factors (e.g. groundwater ongoing activity with average rates of 5–6 mm/yr. table position). This corresponds to creeping conditions along an already formed failure surface. 6.5. Numerical modelling According to numerical models, master joints and faults are major constraints on DSGSD onset and geometry especially in gentle slopes, 2D stress-strain numerical modelling of the analysed DSGSDs was thus confirming the strong passive structural control suggested by carried out using the explicit finite difference code FLAC (Itasca, field investigations. The orientation of planes of weakness (i.e. 2005), to test different onset mechanisms and constrain the role of foliation or closely spaced joints) also significantly controls the inferred controlling factors. Finite difference grids were generated, development of DSGSD. In fact, no large deformations developed in fitted to reconstructed pre-DSGSD slope profiles and constrained by models when ubiquitous joints were not included. Where ubiquitous- displacement boundary conditions. Based on test model runs and joint orientation is favourable to slope instability, failure requires little previous experiences in modelling large rock slope failures (Agliardi et internal deformation (e.g. Watles). Although brittle deformation is al., 2001; Ambrosi and Crosta, 2006), we assumed a Mohr–Coulomb associated to single morpho-structures, at depth the rock mass elasto-plastic yield criterion and used an ubiquitous-joint model experiences plastic deformation along subcircular or composite (Itasca, 2005) to simulate the anisotropy due to pervasive foliation or shear zones. These are often associated with antithetic shear zones closely spaced joints. Major structural features (e.g. nappe boundaries, emerging at different elevations, bounding active wedges or half- regional faults, master joints) were also included, either as regions graben structures (e.g. Cima di Mandriole, Ruinon, Bosco del Conte). with different geomechanical properties (e.g. Mt. Watles), or as This geometry was also observed by other authors through different discrete interfaces crossing the finite difference grids (e.g. Cima di modelling approaches (e.g. analytical solutions: Savage and Smith, Mandriole). Geomechanical properties of rock masses and ubiquitous 1986; physical models: Chemenda et al., 2005). joints (Table 1) were obtained through standard rock mass character- isation based on field and laboratory data (Bieniawski, 1989; Hoek and 7. Discussion Brown, 1997). All the studied DSGSDs occur in valleys heavily glaciated during the Our multidisciplinary analyses considered the relationships among LGM. Thus, stress conditions corresponding to the glacial and tectonic features, active tectonic processes, surface processes, topo- paraglacial evolution of the studied slopes were simulated by graphy, and the development of DSGSD. sequential modelling including ice loading at LGM glacier stage, We showed that the occurrence of major structural features (i.e. subsequent deglaciation and related processes (e.g. changes in fractures, master joints) is essential for explaining the distribution and groundwater conditions). Assumed LGM ice surface elevations general features of DSGSD as well as for modelling their onset. This is (Fig. 14) were based on literature (Florineth and Schlüchter, 2000) confirmed by numerical models, unable to simulate the observed and on the distribution of mapped glacial landforms and deposits, DSGSD deformation patterns if these structural and geological with total ice thickness ranging between 1200 m and 2000 m for the elements are neglected (e.g. Watles, Cima di Mandriole DSGSD). In studied cases. In situ stress was also applied as initial condition when the central Eastern Alps DSGSD develop in anisotropic, moderately available (e.g. seismotectonic and hydrofracturing data). Moreover, strong fractured rock masses, mainly schistose metamorphic rocks cut simulations neglecting the effects of glacial loading and unloading by recent NW–SE, N–S, and NE–SW recent fracture systems. DSGSD were carried out for comparison. Model parameters were calibrated occurs preferentially along high relief (N1000 m), structurally- by comparing simulation results (i.e. cumulative displacements, type controlled valley sides overdeepened by glacial erosion. Main trends of failure, distribution of plasticity indicators) to geomorphological of DSGSD morpho-structures always closely fit the orientation of and morpho-structural constraints. Surface and deep displacement recent brittle structures, thus confirming the significant structural monitoring data both along the slope and the dam were also control on their development pointed out by several authors considered for the Cima di Mandriole DSGSD. (Radbruch-Hall, 1978; Agliardi et al., 2001; Hippolyte et al., 2006). Numerical models simulating deglaciation provided a good Tectonic structures passively control the kinematic degrees of freedom account for the major morpho-structural features recognised and in rock masses and influence rock mass strength, thus constraining mapped at surface, thus confirming their gravitational origin (Fig. 14). DSGSD localisation, geometry, and morpho-structural pattern Model results suggest that tensile fracturing parallel to topography, (Agliardi et al., 2001). Morpho-structures trending close to slope shearing through the slope and slippage along joints are strongly direction are generally more developed, suggesting a primary control controlled by glacial unloading/rebound, whereas these features were of topographic stress on the activation of existing tectonic features. not simulated by “ice free” models. In our simulations, plastic This is evident in upper Valtellina, where DSGSD features mainly trend deformation close to the highest ridge initiate soon after the N–S and NW–SE, as well as in the upper Val Venosta, where NE–SW beginning of deglaciation, whereas deep shear zones of differing and N–S trends dominate. F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 267

Fig. 14. Sketches comparing the results of field morpho-structural study and numerical modelling for the case-study areas. Left: interpretative cross sections with the main tectonic and morpho-structural features. Right: model results in terms of computed maximum shear strain increment, outlining strain localisation and shear band development in slopes subjected to postglacial unloading. Local LGM ice surface elevations after Florineth and Schlüchter (2000).

The consistency between fault kinematics inferred by mesoscopic may contribute to explain the widespread distribution of DSGSD in the analysis and fault plane solutions of earthquakes suggests that N–S Resia Pass area (Mustair-Venosta DSGSD cluster). We confirm a trending normal dip-slip faults and conjugate NE–SW sinistral and widespread gravitational reactivation of possibly active brittle NW–SE dextral strike-slip faults could be active in the central Eastern structures over a larger area. This could suggest possible active Alps. Forcella and Orombelli (1984) suggested that WNW–ESE scarps controls by ongoing tectonic processes on DSGSD onset, i.e. by active and trenches, related to DSGSD in Valfurva (Bormio DSGSD cluster; fault slip, fault zone weakening, in situ stress or seismic shaking. The Figs. 8 and 11), could be surface expressions of neotectonic faults. On high values of present-day rock uplift (N1 mm/yr) and local relief the other hand, Agliardi et al. (2009) suggested that the occurrence of (often exceeding 1500–2000 m) associated with DSGSD also suggest a fault systems kinematically related to the active stress field in the possible tectonic forcing of DSGSD development. According to central Eastern Alps (Müller et al., 1992; Mariucci and Müller, 2003) Montgomery and Brandon (2002), these relief values are typically 268 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 found in tectonically active areas, where they are associated with suggested that the period of paraglacial slope adjustment (Ballantyne, landscape evolution by large landsliding processes. Nevertheless, the 2002) could span up to thousand years. Finally, the occurrence of distinction between tectonically active structures and fractures continuing slope movements over long time periods could also reactivated by DSGSD is not obvious. In most cases gravity acts at suggest a sort of ongoing positive feedback between slope instability, faster long-term average strain rates than neotectonic faulting, uplift, and valley deepening. In fact, high uplift rates enhance valley masking the occurrence of active fractures moving at low displace- deepening, which in turn causes stress concentration at the slope toe, ment rates (0.1 mm/yr of N–S shortening in the Alps; Westaway, resulting in an increased slope activity and further fracturing. 1992). Radbruch-Hall (1978) introduced a range for the measured rates of large-scale mass rock creep (from 2 cm/yr to 20 cm/d). 8. Conclusions Varnes et al. (1990) and Bovis (1982) report displacement rates ranging respectively between 1.4 and 5.0 mm/yr, and between 0.14 DSGSDs are a common feature in the postglacial morphological and 0.75 mm/yr. Colesanti et al. (2004) and Ambrosi and Crosta evolution of the central Eastern Alps, involving more than 10% of the (2006) report vertical displacement rates ranging between 7 and present surface and thus huge rock volumes. Their extent suggests a 30 mm/yr for DSGSD areas in the Central Italian Alps. These rates are complex interplay with relief. Thus, significant controls on erosion much higher than for any observed active fault in the area. This seems processes at different timescale can be ascribed to DSGSD, which can to confirm that many scarps and counterscarps in alpine environ- remain active for long time periods at low displacement rates. ments must be considered the result of gravitational phenomena Regional clustering of DSGSD suggests that, in general, they are rather than of neotectonics and seismic activity. more frequent when some combination of local factors occurs, In the central Eastern Alps, displacement rates of DSGSD are also including strongly anisotropic rock masses, local relief N1000 m, and much higher than present-day rock uplift rates measured by precise rates of rock uplift N1 mm/yr. This also suggests possible links levelling networks (i.e. up to 1.4 mm/yr in the study area; Kahle et between enhanced relief production forced by tectonics (Montgomery al., 1997). DSGSD rates also greatly exceed erosion rates for the Alps and Brandon, 2002) and the onset of DSGSD. estimated by Hinderer (2001), with average values since the Late Morpho-structures related to DSGSD in the area strictly follow the Glacial in the range 0.25–1.06 mm/yr, e.g. summing up to about 13 m trends of recent NW–SE and N–S fractures in Valtellina and by N–Sto for the basin. Continuing changes in elevation along large slope NE–SW ones in the Upper Val Venosta. This confirms the strong sectors related to DSGSD can result in a significant focused lowering passive structural control on DSGSD already outlined by some authors. of topography over a time scale of 103–104 years. At the aforemen- The kinematic consistency between these structures and the pattern tioned displacement rates, and assuming steady-state DSGSD, this of seismicity also suggests that possibly active brittle structures are could locally sum up to 100–300 m since the Lateglacial. Since subjected to widespread gravitational reactivation. This possibly DSGSDs are widespread (e.g. about 10% of the study area), they could suggests active tectonic forcing of DSGSD, also supported by the significantly influence erosion rates and sediment yield at catchment recorded values of uplift and local relief. scale in areas with high relief (also a proxy of erosion; Ahnert, 1970), Finally, numerical modelling suggests that deglaciation following especially when glacial and fluvioglacial erosion/deposition pro- LGM collapse and related paraglacial processes are the major triggers cesses are exhausted. DSGSD control on erosion may include: of DSGSD. The onset of these phenomena needs some pre-existing exhuming bedrock by reactivation of tectonic faults (Hippolyte et structural features (e.g. pervasive foliation, “Alpine” low-angle al., 2006), damming or modifying drainage networks both on slopes mylonitic to cataclastic fault zones, recent steep to vertical fractures) and valley floors, supplying sediments to stream erosion at different and geological conditions (e.g. occurrence of paraderivatives), timescale (e.g. from catastrophic slope failure to slow toe bulging). especially when related to active fault systems. It has been shown that large DSGSDs occur in areas that underwent glaciation and deglaciation, as well as in unglaciated areas (Crosta and Acknowledgements Zanchi, 2000). Nevertheless, most DSGSDs occur in glacial valleys and that processes related to deglaciation (e.g. slope unloading, ground- The authors are indebted to C. Ambrosi, D. Bassanelli, and M. Villa water level fluctuations and water pressure increase because of for assistance in the collection and analysis of data, A. Azzoni and A. permafrost conditions, ice melting and fracture unloading) are Frassoni of ISMES, and ENEL Hydro S.p.A. and Provincia Autonoma di required for large displacements to occur in numerical models. This Trento for providing field testing and monitoring data. The paper agree with previous findings (Brückl, 2001; Agliardi et al., 2001; benefited from the careful reviews of M. Jaboyedoff and O. Korup. The Ballantyne, 2002). According to the available data, maximum ice research has been partially funded by a FIRB (“Multi-disciplinary surface elevation during LGM in the studied areas was ranging approach for large landslides hazard assessment”) and a MIUR project between 2500 and 3000 m a.s.l. (Florineth and Schlüchter, 2000), thus (“Catastrophic slope failures: characterisation, monitoring and mod- leading to a significant debuttressing of slopes during deglaciation. elling for hazard assessment”). Postglacial DSGSD triggering or evolution is also suggested by morpho-structures cross-cutting glacial and periglacial deposits, and confirmed by radiocarbon dating of peat deposits sealing Mt. Watles References DSGSD features (Agliardi et al., 2009). Agliardi, F., Crosta, G., Zanchi, A., 2001. Structural constraints on deep-seated slope Meigs and Sauber (2000) presented the case of a very large deformation kinematics. Engineering Geology 59, 83–102. instability with features similar to those of DSGSD. This developed in a Agliardi, F., Crosta, G., Zanchi, A., Ravazzi, C., 2009. Onset and timing of deep-seated 13 years period following rapid glacial retreat, with net down-dip gravitational slope deformations in the Eastern Alps, Italy. Geomorphology 103, – displacement at the base of the landslide of about 300–400 m. This 113 129. Ahnert, F., 1970. Functional relationships between denudation, relief, and uplift in large example suggests that the displacement rate can be higher just after mid-latitude basins. American Journal of Science 268, 243–263. deglaciation. Radiocarbon dating (Agliardi et al., 2009) also demon- Ambrosi, C., Crosta, G.B., 2006. Large sackung along major tectonic features in the – strates that sudden activation and largest movements occurred just Central Italian Alps. Engineering Geology 83, 183 200. – Angelier, J., 1984. Tectonic analysis of fault slip data sets. Journal of Geophysical after LGM collapse (18 16 ka; Lambeck et al., 2002; Ivy-Ochs et al., Research 89, 5835–5848. 2004; Kelly, 2006). Nevertheless, available monitoring data (Colesanti Angelier, J., 1990. Inversion of field data in fault tectonics to obtain the regional stress– et al., 2004) suggest that after an initial accelerating stage, slopes III. A new rapid direct inversion method by analytical means. Geophysical Journal International 103, 363–376. movements keep on at slow rates. This is in agreement with the Ballantyne, C.K., 2002. Paraglacial geomorphology. Quaternary Science Reviews 21, results of Cruden and Hu (1993) and Prager et al. (2008), which 1935–2017. F. Agliardi et al. / Tectonophysics 474 (2009) 250–270 269

Berra, F., Sciesa, E., Mazzoccola, D., Guglielmin, M., 2002. Carta Geologica d'Italia, Section di roccia e di colate detritiche connesse a deformazioni gravitative profonde di D1d4 Passo dello Stelvio, scale 1:10,000. Progetto CARG – Regione, Lombardia, versante. Ist. Lomb. (Rend. Sc.), B, vol. 135, pp. 87–100 (in Italian). Milano (in Italian). Hinderer, M., 2001. Late Quaternary denudation of the Alps, valley and lake fillings and Bieniawski, Z.T., 1989. Engineering Rock Mass Classification. J. Wiley and Sons, New modern river loads. Geodinamica Acta 14, 231–263. York. 251 pp. Hippolyte, J.C., Brocard, G., Tardy, M., Nicoud, G., Bourlès, D., Braucher, R., Ménard, G., Bovis, M.J., 1982. Uphill-facing (antislope) scarps in the Coast Mountains, southwest Souffaché, B., 2006. The recent fault scarps of the Western Alps (France): Tectonic British Columbia. Geological Society of America Bulletin 93, 804–812. surface ruptures or gravitational sackung scarps? A combined mapping, geo- Bovis, M.J., Evans, S.G., 1995. Rock slope movements along the Mount Currie “fault morphic, levelling, and 10Be dating approach. Tectonophysics 418, 255–276. scarp”, southern Coast Mountains, British Columbia. Canadian Journal of Earth Hoek, E., Brown, E.T.,1997. Practical estimates or rock mass strength. International Journal Sciences 32, 2015–2020. of Rock Mechanics and Mining Sciences & Geomechanics Abstracts 34 (8),1165–1186. Braunmiller, J., Kradolfer, U., Baer, M., Giardini, D., 2002. Regional moment tensor Itasca Consulting Group Inc., 2005. FLAC version 5.0, User Manuals. determination in the European–Mediterranean area – initial results. Tectonophysics Ivy-Ochs, S., Schäfer, J., Kubik, P.W., Synal, H.-A., Schlüchter, C., 2004. Timing of 356, 5–22. deglaciation on the northern Alpine foreland (Switzerland). Eclogae Geologicae Brocklehurst, S.H., Whipple, K.X., 2007. Response of glacial landscapes to spatial Helvetiae 97, 47–55. variations in rock uplift rate. Journal of Geophysical Research 112, F02035. Kahle, H.G., Geiger, A., Burki, B., Gubler, E., Marti, U., Wirth, B., Rothacher, M., Gurtner, W., Brückl, E., 2001. Cause–effect models of large landslides. Natural Hazards 23, 291–314. Beutler, G., Bauersima, I., Pfiffner, O.A., 1997. Recent crustal movements, geoid and Burbank, D.W., Anderson, R.S., 2001. Tectonic Geomorphology. Blackwell Scientific, density distribution: contribution from integrated satellite and terrestrial measure- Oxford. 270 pp. ments. In: Pfiffner, O., Lehner, P., Heitzmann, P.,Müller, S., Steck, A. (Eds.), Results of the Canetta, M., Frassoni, A., Maugliani, V., Tanzini, M., 1992. Appraisal of the rock mass NRP20 Deep structures of the Swiss Alps. Birkhauser Verlag, Basel, pp. 251–259. interested by underground excavations for the extension of a hydroelectric power Kelly, M.A., 2006. Chronology of deglaciation based on 10Be dates of erosional features in plant. In: Broch, E., Lysne, D.K. (Eds.), Proceedings of the 2nd International the Grimsel Pass region, central Swiss Alps. Boreas 35, 634–643. Conference on Hydropower, Lillehammer, Norway, 16–18 June 1992, pp. 81–88. Korup, O., Clague, J.J., Hermanns, R.L., Hewitt, K., Strom, A.L., Weidinger, J.T., 2007. Giant Caporali, A., Braitenberg, C., Massironi, M., 2005. Geodetic and hydrological aspects of landslides, topography, and erosion. Earth and Planetary Science Letters 261, the Merano earthquake of 17 July 2001. Journal of Geodynamics 39, 317–336. 578–589. Chemenda, A.I., Bouissou, S., Bachmann, D., 2005. 3-D physical modelling of deep seated Kühni, A., Pfiffner, O.A., 2001. The relief of the Swiss Alps and adjacent areas and its landslides: new technique and first results. Journal of Geophysical Research 110, F04004. relation to lithology and structure: topographic analysis from 250-m DEM. Colesanti, C., Crosta, G.B., Ferretti, A., Ambrosi, C., 2004. Monitoring and assessing the Geomorphology 41, 285–307. state of activity of slope instabilities by the Permanent Scatterers Technique. In: Lambeck, K., Yokoyama, Y., Purcell, T., 2002. Into and out of the Last Glacial Maximum: Evans, S.G., Scarascia-Mugnozza, G., Strom, A., Hermanns, R. (Eds.), Landslides from sea-level change during oxygen isotope stages 3 and 2. Quaternary Science Reviews Massive Rock Slope Failure. NATO Science Series, IV. Earth and Environmental 21, 343–360. Sciences, vol. 49. Springer, Dordrecht, pp. 175–194. Mancktelow, N., Stöckli, D.F., Grollimund, B., Müller, W., Fügenschuh, B., Viola, G., Conti, P., Manatschal, G., Pfister, H.,1994. Synrift sedimentation, Jurassic and Alpine tectonics Seward, D., Villa, I.M., 2001. The DAV and Periadriatic fault system in the Eastern in the central Ortler nappe (Eastern Alps, Italy). Eclogae Geologicae Helvetiae 87, 63–90. Alps south of the Tauern window. International Journal of Earth Sciences Corradini, M., Notarpietro, A., Potenza, R., 1973. L'assetto geologico degli gneiss di Valle (Geologische Rundschau) 90, 593–622. Grosina nell'alta Valtellina (Sondrio-Italia). Atti Soc. Ital. Sci. Nat. Museo Civ. St. Nat. Mariucci, M.T., Müller, B., 2003. The tectonic regime in Italy inferred from borehole Milano, vol. 114, pp. 135–151 (in Italian). breakout data. Tectonophysics 361, 21–35. Crosta, G.B., Agliardi, F., 2003. Failure forecast for large rock slides by surface Martin, S., Prosser, G., Santini, L., 1991. Alpine deformation along the Periadriatic displacement measurements. Canadian Geotechnical Journal 40 (1), 176–191. lineament in the Italian Eastern Alps. Annales Tectonicae 5, 118–140. Crosta, G.B., Frattini, P., 2004. Controls on modern alluvial fan processes in the central McCalpin, J.P., 1999. Criteria for determining the seismic significance of sackungen and Alps, northern Italy. Earth Surface Processes and Landforms 29 (3), 267–293. other scarp-like landforms in mountainous regions. Techniques for Identifying Crosta, G., Zanchi, A., 2000. Deep seated slope deformations. Huge, extraordinary, Faults and Determining their Origins. U.S. Nuclear Regulatory Commission, enigmatic phenomena. In: Bromhead, E., Dixon, N., Ibsen, M. (Eds.), Landslides in Washington, pp. 2.55–2.59. Research, Theory and Practice, Proceedings of the 8th International Symposium on Meigs, A., Sauber, J., 2000. Southern Alaska as an example of the long term Landslides, Cardiff, June 2000. Thomas Telford, London, pp. 351–358. consequences of mountain building under the influence of glaciers. Quaternary Crosta, G., Zanchi, A., Agliardi, F., Frattini, P., Ambrosi, C., 2000. Convenzione di studio Science Reviews 19, 1543–1562. della frana di “Pian Palù” in sponda sinistra dell'Alta Val di Peio – Torrente Noce. Miller, D.J., Dunne, T., 1996. Topographic perturbations of regional stresses and Università degli Studi di Milano-Bicocca – Provincia Autonoma di Trento, consequent bedrock fracturing. Journal of Geophysical Research 101, 25523–25536. unpublished technical report, 150 pp. (in Italian). Molnar, P., England, P., 1990. Late Cenozoic uplift of mountain ranges and global climate Cruden, D.M., Hu, X.Q., 1993. Exhaustion and steady-state models for predicting change: chicken or egg. Nature 346, 29–34. landslide hazards in the Canadian Rocky Mountains. Geomorphology 8, 279–285. Montgomery, D.R., Brandon, M.T., 2002. Topographic controls on erosion rates in D'Agostino, N., Cheloni, D., Mantenuto, S., Selvaggi, G., Michelini, A., Zuliani, D., 2005. tectonically active mountain ranges. Earth and Planetary Science Letters 201, 481–489. Strain accumulation in the southern Alps (NE Italy) and deformation at the Müller, B., Zoback, M.L., Fuchs, K., Mastin, L., Gregersen, S., Pavoni, N., Stephansson, O., northeastern boundary of Adria observed by CGPS measurements. Geophysical Ljunggren, C., 1992. Regional patterns of tectonic stress in Europe. Journal of Research Letters 32, L19306. Geophysical Research 97, 11783–11804. Deichmann, N., Baer, M., 1990. Earthquake focal depth below the Alps and the northern Müller, W., Prosser, G., Mancktelow, N., Villa, I.M., Kelley, P.S., Viola, G., Oberli, F., 2001. Alpine foreland of Switzerland. In: Freeman, R., Giese, P., Müller, St. (Eds.), The Geochronological constraints on the evolution of the Periadriatic Fault System European Geotraverse: integrative studies. European Science Foundation, Stras- (Alps). International Journal of Earth Sciences (Geologische Rundschau) 90 (3), bourg, France, pp. 277–288. 623–653. Densmore, A.L., Ellis, M.A., Anderson, R.S.,1998. Landsliding and the evolution of normal fault- Persaud, M., Pfiffner, O.A., 2004. Active deformation in the eastern Swiss Alps: post- bounded mountain ranges. Journal of Geophysical Research 103 (B7), 15, 203–15, 219. glacial faults, seismicity and surface uplift. Tectonophysics 385, 59–84. Desio, A.,1961. Geological features of the reservoirs and of the dam foundation rocks. In: Prager, C., Zangerl, C., Patzelt, G., Brandner, R., 2008. Age distribution of fossil landslides Association of Italian electric producer and utility companies (Ed.), Dams for in the Tyrol () and its surrounding areas. Natural Hazards and Earth System hydroelectric power in Italy, 33 p. Sciences 8, 377–407. Fah, D., Giardini, D., Bay, F., Bernardi, F., Braunmiller, J., Deichmann, N., Furrer, M., Prosser, G., 2000. The development of the North Giudicarie fault zone (Insubric Line, Gantner, L., Gisler, M., Isenegger, D., Jimenez, M.J., Kastli, P., Koglin, R., Masciadri, V., Northern Italy). Journal of Geodynamics 30, 229–250. Rutz, M., Scheidegger, C., Schibler, R., Schorlemmer, D., Schwarz-Zanetti, G., Radbruch-Hall, D., 1978. Gravitational creep of rock masses on slopes. In: Voight, B. Steimen, S., Sellami, S., Wiemer, S., Wossner, J., 2003. Earthquake catalog of (Ed.), Rockslides and Avalanches – Natural Phenomena. Developments in Switzerland (ECOS) and the related macroseismic database. Eclogae Geologicae Geotechnical Engineering, vol. 14. Elsevier, Amsterdam, pp. 608–657. Helvetiae 96/2, 219–236. Ratschbacher, L., 1986. Kinematics of Austro-Alpine cover nappes: changing translation Florineth, D., Schlüchter, C., 2000. Alpine evidence for atmospheric circulation patterns path due to transpression. Tectonophysics 125, 335–356. during the Last Glacial Maximum. Quaternary Research 54, 295–308. Ratschbacher, L., Frisch, W., Neubauer, F., Schmid, S., Neugebauer, J., 1989. Extension in Forcella, F., Orombelli, G., 1984. Holocene slope deformations in Valfurva, Central Alps, orogenic belts: the eastern Alps. Geology 17, 404–407. Italy. Geografia Fisica e Dinamica Quaternaria 7, 41–48. Reinecker, J., Heidbach, O., Tingay, M., Sperner, B., Müller, B., 2005. The release 2005 of Forcella, F., Gallazzi, D., Montrasio, A., Notarpietro, A., 1982. Note illustrative relative the World Stress Map (available online at www.world-stress-map.org). all'evoluzione neotettonica dei Fogli 6 – Passo dello Spluga, 7 – Pizzo Bernina, 8 – Savage, W.Z., Smith, W.K., 1986. A model for the plastic flow of landslides. USG Bormio, 17 – , 18 – Sondrio, 19 – Tirano. Contributi alla realizzazione della Professional Paper 1385. 32 pp. Carta Neotettonica d'Italia. Progetto Finalizzato Geodinamica, vol. 513, pp. 239–288 Schlunegger, F., Hinderer, M., 2001. Crustal uplift in the Alps: why the drainage pattern (in Italian). matters. Terra Nova 13, 425–432. Froitzheim, N., Conti, P., Van Daalen, M., 1997. Late Cretaceous, synorogenic, low-angle Schmid, S.M., Froitzheim, N., 1993. Oblique slip and block rotation along the Engadine normal faulting along the Schlinig fault (Switzerland, Italy, Austria) and its Line. Eclogae Geologicae Helvetiae 86, 569–593. significance for the tectonics of the Eastern Alps. Tectonophysics 280, 267–293. Schmid, S.M., Haas, R., 1989. Transition from near surface thrusting to intrabasement Gudmundsson, G.H., 1994. An order-of-magnitude estimate of the current uplift-rates in decollement, Schlinig thrust, Eastern Alps. Tectonics 8, 697–718. Switzerland caused by the Wurm Alpine deglaciation. Eclogae Geologicae Helvetiae Selverstone, J., 1988. Evidence for E–W crustal extension in the Eastern Alps: 87/2, 545–557. implications for the unroofing history of the Tauern Window. Tectonics 7, 87–105. Guglielmin, M., Orombelli, G., 2003. Il cono di deiezione terrazzato allo sbocco della Slejko, D., Carulli, G.B., Nicolich, R., Rebez, A., Zanferrari, A., Cavallin, A., Doglioni, C., Valle di Migiondo presso Sondalo (Valtellina). Un esempio di accumulo di valanghe Carraro, F., Castaldini, D., Iliceto, V., Semenza, E., Zanolla, C., 1989. Seismotectonics of 270 F. Agliardi et al. / Tectonophysics 474 (2009) 250–270

the eastern-southern Alps: a review. Bollettino di Geofisica Teorica ed Applicata 31 Varnes, D.J., Radbruch-Hall, D., Savage, W.Z., 1989. Topographic and structural (122), 109–136. conditions in areas of gravitational spreading of ridges in the Western United Sölva, H., Grasemann, B., Thöni, M., Thiede, R., Habler, G., 2005. The Schneeberg Normal States. U.S. Geological Survey Professional Paper 1496. 28 pp. Fault Zone: normal faulting associated with Cretaceous SE-directed extrusion in the Varnes, D.J., Radbruch-Hall, D., Varnes, K.L., Smith, W.K., Savage, W.Z., 1990. Eastern Alps (Italy/Austria). Tectonophysics 401 (3–4), 143–166. Measurement of ridge-spreading movements (sackungen) at Bald Eagle Mountain, Sue, C., Tricart, P., 2002. Widespread post–nappe normal faulting in the internal Lake County, Colorado, 1975–1989. U.S. Geological Survey Open-File Report 90-543. Western Alps: a new constraint on arc dynamics. Journal of the Geological Society 13 pp. (London) 159, 61–70. Viola, G., Mancktelow, N.S., Seward, D., 2001. Late Oligocene–Neogene evolution of Swisstopo, 2007. GeoKarten (Geologie, Tektonik, Hydrogeologie) 1:500000 – Vektor- Europe–Adria collision: new structural and geochronological evidence from the daten. Swiss Federal Office of Topography, CD-ROM. Giudicarie fault system (Italian Eastern Alps). Tectonics 20, 999–1020. Tesauro, M., Hollenstein, C., Egli, R., Geiger, A., Kahle, H.G., 2006. Analysis of central Viola, G., Mancktelow, N.S., Seward, D., Meier, A., Martin, S., 2003. The Pejo fault system; western Europe deformation using GPS and seismic data. Journal of Geodynamics an example of multiple tectonic activity in the Italian Eastern Alps. Geological 42, 194–209. Society of America Bulletin 115 (5), 515–532. Thöni, M., 1999. A review of geochronological data from the Eastern Alps. Schweizerische Westaway, R., 1992. Seismic moment summation for historical earthquakes in Italy: Mineralogische und Petrographische Mitteilungen 79, 169–201. tectonic implications. Journal of Geophysical Research 97 (B11), 15437–15464. Thöni, M., Hoinkes, G., 1987. The Austroalpine unit west of the Hohe Tauern: The Otztal– Zanchi, A., Gregnanin, A., Corona-Chavez, P., Frassoni, A., Guido, S., 1995. Recent brittle Stubai complex as an example for the Eoalpine metamorphic evolution. The deformation in the Central-Western Alps, Italy. IGCP378, Circumalpine Quaternary southern Otztal basement: geochronological and petrological consequences of Correlations. Southern Alps Quaternary Geology, Lugano, 2–6 October 1995. Eoalpine metamorphic overprinting. In: Flugel, H.W., Faupl, P. (Eds.), Geodynamics of the Eastern Alps, Verlag 1.418, pp. 200–213.