Geomorphology 289 (2017) 44–59

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Geomorphology

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The Benner pass rock avalanche cluster suggests a close relation between long-term slope deformation (DSGSDs and translational rock slides) and catastrophic failure

Marc Ostermann ⁎, Diethard Sanders

Institute of Geology, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria article info abstract

Article history: In mountain ranges deep-seated gravitational slope deformations (DSGSDs) and extremely rapid mass wastings Received 3 April 2015 of rock N105 m3 in volume (catastrophic rock-slope failures, CRF) are present, yet their mutual relation is poorly Received in revised form 19 December 2016 documented. Near the Brenner Pass (1370 m asl) in the eastern Alps, five catastrophic rock-slope failures of me- Accepted 20 December 2016 dium- to high-grade metamorphites are clustered (‘Brenner Pass Cluster’; BPC), and three of them are related to Available online 23 December 2016 DSGSDs. The catastrophic rock-slope failures involved volumes from 12 to 110 Mm3 and show fahrboeschung an- – 14 234 230 Keywords: gles of 10 27°. Numerical dating ( C, U/ Th) suggests that all catastrophic slope failures of the BPC occurred ≤ – Catastrophic rock-slope failure between 13.5 and 6.2 ka. Three of the CRF events may have occurred during the Younger Dryas (12.7 11.7 ka), 2 Rock avalanche whereas two events occurred during the Holocene. Backwater basins dammed up by the CRFs range from 2.5 km Deep-seated gravitational slope deformation (Ridnaun rock avalanche) to 15.5 km2 (Stilfes rock avalanche). Brenner pass Three of the catastrophic rock-slope failures are associated with and developed as a partial failure of a DSGSD. This suggests that progressively slow deformation of slopes ultimately exceeded a stability threshold, resulting in catastrophic rock-slope failures. The initial kinematic mechanisms of failure vary between large-scale toppling, wedge sliding, and planar sliding and are strongly controlled by the structural setting of the slopes. A direct connection of catastrophic mass wasting with specific palaeoclimatic conditions (e.g., phases of en- hanced precipitation) is not indicated; however, this does not exclude specific meteorological situations (e.g., oc- currence of short-term heavy rainfall) that may have expedited slope instability and perhaps even triggered catastrophic events. Attempts to correlate catastrophic rock-slope failures with specific palaeoclimatic regimes are still encumbered by substantial methodical uncertainties and imprecisions as well as the scarcity of dated CRF events. The mapped distribution of CRFs unequivocally indicates that structural predisposition is the most significant long-term con- trol in forming CRF clusters. © 2016 Elsevier B.V. All rights reserved.

1. Introduction (Terzaghi, 1962) are, in turn, mass movements affecting areas up to N10 km2 and show distinctive morphostructures such as ridges, trench- The presence of high-velocity (‘catastrophic’) displacements of large es, downhill- and uphill-facing scarps, and toe bulging (e.g., Agliardi et rock masses and slow, deep-seated gravitational slope deformations al., 2009). The DSGSDs are characterized by a small degree of total dis- (DSGSDs) in mountain ranges is common, yet how they interact is poor- placement relative to the extent of the releasing slope (Massironi et ly understood. In this connection we are using the term “catastrophic al., 2003) and propagate very slowly (0.4–5mmy−1; Varnes et al., rock-slope failures” (CRF) where substantial fragmentation of the rock 1990). mass during runout is involved and where the impact covers an area A quick glance at maps of the Alps clearly indicates that CRFs tend to larger than that of a rockfall (Hermanns and Longva, 2012). This term be ‘clustered’ along major fault zones (Prager et al., 2008; Ostermann also includes rock avalanches, which are gravity-driven, extremely and Sanders, 2012; Zerathe et al., 2014), whereas DSGSDs show a rapid mass movements comprising volumes of ≥105 m3 (cf. Cruden much more scattered distribution (Crosta et al., 2013). As outlined and Varnes, 1996; Hungr et al., 2001; Evans et al., 2006; Hermanns below in more detail, these fault zones were given their unique charac- and Longva, 2012). Deep-seated gravitational slope deformations ter during the Neogene, when northward indentation of the Dolomite continental block led to lateral escape of the eastern-Alpine edifice ⁎ Corresponding author. (e.g., Ratschbacher et al., 1991; Frisch et al., 2000a). In the area under in- E-mail address: [email protected] (M. Ostermann). vestigation, abundant macroseismic clusters along the fault zones

http://dx.doi.org/10.1016/j.geomorph.2016.12.018 0169-555X/© 2016 Elsevier B.V. All rights reserved. M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59 45 indicate that they are still active (Reinecker and Lenhardt, 1999; Reiter as shallow-water carbonate rocks or gneiss, tend to form CRFs (e.g., et al., 2005; Lenhardt et al., 2007; Brückl et al., 2010; Nasir et al., 2013), Abele, 1974). In a few cases, DSGSDs (or portions thereof) suddenly ac- as also indicated by GPS-derived surface displacements (Caporali et al., celerated to become a catastrophic rock-slope failure (Evans and 2013). Furthermore, fault planes and other deformation features identi- Couture, 2002; Crosta and Agliardi, 2003). fied in Quaternary deposits underscore neotectonic activity (Sanders, Herein we characterize a cluster of five CRFs near the Brenner Pass of 2015, 2016). Because their origin was in the Neogene but their activity the eastern Alps. Three of these events — described here for the first continued or rejuvenated, the fault zones represent both ‘inherited’ time in detail — are superposed on and developed from larger underly- structures as well as active features. ing Sackung-type DSGSDs (cf. Zischinsky, 1969) in schistose metamor- The DSGSD formation is favoured by high, glacially oversteepened phic rocks. Apart from the determination of cornerstone parameters valley flanks (Agliardi et al., 2009) combined with relatively incompe- (e.g., rock volume, fahrböschung angle), event ages are constrained by tent rocks rich in structural weaknesses (e.g., schistosity, fractures; two methods (14C, 234U/230Th disequilibrium dating), and the influence Radbruch-Hall, 1978; Crosta, 1996; Massironi et al., 2003). Conversely, of catastrophic rock-slope failures on valley-floor development is competent lithologies with widely spaced structural weaknesses, such assessed. We discuss a potential correlation of CRFs with climatic phases

Fig. 1. (A) Location map of catastrophic rock-slope failures and associated former and still-existing backwater lakes in the Brenner Pass area (Austria/Italy). In addition to the five catastrophic rock-slope failures (Brenner Pass Cluster), important slow-moving, deep-seated gravitational slope deformations (DSGSDs) are indicated and denoted: (a) Gschließegg DSGSD, (b) Trenser Joch DSGSD, (c) Telfer Weissen DSGSD, (d) Wetterspitz DSGSD, and (e) Padauner Berg translational rockslide. (B) Simplified geological map of the expanded research area. The units of the Tauern Window (TW) in the east are separated from the Oetztal-Stuai crystalline basement (OSB) and its sedimentary cover in the west by the Brenner normal fault. In the south, the Periadriatic Lineament (PL) marks the boundary toward the Dolomite indenter, here represented by Brixen Granite (BG) and south Alpine basement with sedimentary cover (SAB). 46 M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59 and active seismogenic faulting in the area and suggest that fault-relat- Subpenninic units derived from European basement, rifted margin sed- ed deformation is the dominant control on catastrophic mass wasting. iments, and from Mesozoic oceanic seaways (e.g., Schmid et al., 2004; Pfiffner, 2009; Handy et al., 2015). East of the Brenner Pass, the 2. Setting Penninic-Subpenninic units are exposed in the Tauern tectonic window (Fig. 1B). The Brenner Pass (1370 m asl) is the lowest and one of the most The central sector of the southern Alps and of the associated PL is a frequented north-south passages across the Alps. The environs of the promontory (or indenter) pushed north into the eastern Alpine edifice. pass are characterized by narrow, steep-flanked valleys between moun- North of the indenter tip, near the Brenner Pass (Fig. 1B), a N-S shorten- tain ranges up to N3200 m in altitude. The trunk valley of the region — ing of 61 km since the Oligocene is deduced (Linzer et al., 2002). The the Wipp Valley — runs from Innsbruck in the north over Brenner Pass present compression by the indenter tip is toward NNW-NW to the Sterzing basin in the south and is fed by several tributaries (e.g., (Jiménez-Munt et al., 2005; Kummerow and Kind, 2006; Heidbach et Obernberg Valley, Pflersch Valley, Ridnaun Valley, Pfitsch Valley; Fig. al., 2008; Bokelmann et al., 2013). Because of persistent indentation 1A). In this area, the tectonic units of the eastern Alpine nappe stack along the western margin of the Tauern Window, surface uplift of nearly are separated along the Periadriatic lineament (PL) from southern Al- 2 mm/a indicates active exhumation (Brückl et al., 2010). The northern pine units (Fig. 1B). In the eastern Alpine edifice, the structurally higher frame of the Tauern Window consists of relatively incompetent quartz- nappes pertaining to the Austroalpine unit are underlain by Penninic- phyllites (Innsbruck quartzphyllite, IQ in Fig. 1B); the southern frame, in

Fig. 2. (A) Stilfes rock avalanche. Hillshade image of Stilfes rock avalanche detachment area, accumulation area, and a part of the backwater area. Since the 1970s numerous drillings have been carried out mainly in the backwater area because of the very heterogeneous internal composition of the silted-up former impoundment. Some of the modern drillings have been sampled for radiocarbon dating. The central part of the rock avalanche accumulations are covered with up to 30-m-thick backwater sediments. Well-developed morpho structures (double-crested ridge, trenches, counterscarps) of a DSGSD are visible around the pink-framed scarp area. (B). Cross section through the Wipp Valley at Stilfes (vertical double scale). The continuous red lines indicate known and the dashed red lines indicate supposed major faults. NB represents the nappe boundary between the Upper Schieferhülle of the Tauern Window and the Oetztal-Stuai crystalline basement (OSB). M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59 47 turn, consists of eastern-Alpine gneissic basement with small, late-oro- (i) Radiocarbon dating (14C) was applied to organic material in CRF- genic Tertiary plutonite bodies (Periadriatic intrusives, PI in Fig. 1B). dammed backwater deposits. Radiocarbon dating was carried The Brenner normal fault (Fig. 1B) comprises an older, top-west, out by Accelerator Mass Spectrometry at the ETH Laboratory of ductile low-angle detachment cross-cut by younger, westward-dipping, Ion Beam Physics in Zurich, Switzerland. Radiocarbon laboratory brittle normal faults (e.g., Behrmann, 1988; Fügenschuh et al., 1997, dates were calibrated to calendar years (quoted BP). Age ranges 2012; Scharf et al., 2013). Along the Brenner normal fault, a roughly were based on the statistical 1σ (corresponding to 68.2% proba- N-S striking valley (Wipp Valley, see above) was excavated that culmi- bility) and 2σ (corresponding to 95.4% probability) standard de- nates at the Brenner Pass (Fig. 1B). Active top-NW extension along the viation. Brenner normal fault is consistent with fault-plane solutions of earth- (ii) 230Th/234U disequilibrium methods were used to date post-depo- quakes and with GPS-derived horizontal displacement vectors (cf. sitional calcite cements precipitated in the interstitial pore space Reiter et al., 2005; Caporali et al., 2013; Nasir et al., 2013). At its northern or along fractures within the Pfitsch rock avalanche mass. Analy- end, the brittle Brenner fault zone is linked to the sinistral Inntal shear ses were carried out at the Institute of Geological Sciences in zone (Linzer et al., 2002; Ortner et al., 2006). West of the Brenner Bern, using multicollector ICP mass spectrometry. For more de- Pass, the hanging wall of the brittle Brenner normal fault consists of tails on this method see Ostermann et al. (2007). eastern-Alpine polymetamorphic basement (Oetztal-Stubai basement; (iii) In addition, for the Ridnaun rock avalanche we attempted to OSB in Fig. 1B) and a belt of micaschists and marbles (Schneeberg com- apply Optically-Stimulated Luminescence dating (OSL) on lacus- plex, SC in Fig. 1B). The basement is overlain by a parautochthonous Tri- trine silts and sandy backwater sediments — however without assic-Jurassic metacarbonate succession (Brenner Mesozoic; BM, with success. Obernberg rock avalanche labelled E in Fig. 1B; Ostermann et al., 2012). The seismic activity along all mentioned fault zones is discussed further below. 4. Catastrophic rock-slope failures of the Brenner Pass region During the Last Glacial Maximum (LGM; ~22–19 ka), except for nun- ataks, the entire area shown in Fig. 1B was buried under coalesced ice 4.1. Stilfes rock avalanche streams. During the LGM, the Brenner Pass divided northward from southward-flowing ice streams (Van Husen, 1987). On the pass, the Approximately 5 km downstream of Sterzing, erosional remnants of LGM ice thickness attained 1000–1100 m. During the penultimate a rock avalanche are exposed over an area of about 2.1 km2 along the peak glaciation (Riss glaciation in the Alps corresponding to marine iso- southern side of the Wipp Valley at Stilfes (Figs. 1 and 2). The CRF tope stage 6), a similar or higher ice thickness than that of the LGM can mass is clearly manifested by an array of hummocks near the eastern be inferred (cf. Van Husen, 1999; Van Husen and Reitner, 2011), which margin of the accumulation area (Fig. 2A). The deposits consist almost underlines the repeated influence of glaciations in the area. The post- exclusively of calcphyllites (Bünderschiefer) derived from the left, north- glacial development of the area was mainly influenced by deglacial iso- ern valley flank (Fig. 2). The assumed head scarp of the rock avalanche is static uplift (Norton and Hampel, 2010) with an average uplift rate of about 1 mm/a (Höggerl, 2001) and slope gravitational processes from oversteepened valley flanks (e.g., Eberhardt et al., 2004). Rapid differen- tial isostatic uplift combined with neotectonic coseismic deformation render the area vulnerable for catastrophic rock-slope failures and DSGSDs (cf. Massironi et al., 2003).

3. Methods

The catastrophic rock-slope failure masses, the scarps, and the back- water successions were investigated during several field campaigns from 2009 to 2014. Field mapping was conducted on a scale of 1:5000 using topographic maps and LiDAR-derived hillshade images. Field maps were implemented into a GIS and combined with a digital eleva- tion model (DEM), orthophotos, topographic maps, and geological maps. For each CRF, the initial rock volume before failure and the volume of CRF deposit were determined. The pre-failure surface of each CRF was reconstructed by prolongation and extrapolation of contour lines over the CRF scarp in 10-m altitude intervals. Because the scarps are morpho- logically distinct and well defined, an uncertainty of estimate of the ini- tial rock volume of b20% was assumed. For volume estimates of CRF deposits, information on depth grounding is crucial; at two sites (Pfitsch, Ridnaun), fluvial incision depth to bedrock provided informa- tion on thickness. Additionally, information from several drilling cam- Fig. 3. (A) Stilfes rock avalanche. Lithological cross section along the Wipp Valley at the paigns was integrated. Areas between data points were interpolated. Stilfes rock avalanche accumulation area and the associated backwater sediments. The Depending on the quality of subsurface information, we assume that numbers correspond to radiocarbon dates (cal YBP) from organic material taken from drill cores (S2-S6, S9, S10) and natural outcrops (STI_03). Some drillings are projected the volume estimates of the CRF deposits contain an uncertainty of up into the cross section. The complex geometry of the impoundment caused sediment to 20%. The failed rock volume has been compared with the deposited traps at different locations within the former lake. This can cause situations where older rock volume. The LiDAR-interpretations, DEM extrapolations, integra- material is situated above younger material in different drillings (S10). (B) Three- tion of drill log data, three-dimensional modelling, and parameter cal- dimensional reconstruction of the Sterzing backwater basin dammed by the Stilfes rock avalanche. Because the depth of the bedrock is not exactly known, the underground has culations have been performed with AutoCAD 2014 and ArcMap 10. been reconstructed through extrapolation of the slopes and drill log information. To deduce the age of mass wasting events, direct and indirect dating Damming of the river not only caused a big impounded lake but also led to an increased methods were used: alluvial fan build up downriver of the dam because of reduced river erosion. 48 M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59 situated at ~1600 m asl, 660 m above the present valley floor, and can be below the present valley floor was positioned at the contact between traced laterally over at least 2.5 km (Fig. 2A). Near the toe of the left val- angular calcphyllite clasts (considered as rock avalanche material) and ley flank, the Penninic Bündnerschiefer group of the Tauern Window a fine-grained lacustrine succession above. A 14C age of the wood of (Upper Schieferhülle in Fig. 2B) is overthrust by paragneisses and am- 11,663 ± 308 cal YBP is interpreted as a proxy for the CRF event age phibolites of the Austroalpine unit (OSB in Fig. 2B). The left valley (Fig. 3A; Table 1). Another drilling (S6) closer to the margin of the flank is cut by steep-dipping normal faults, and the entire slope reveals basin reached fluvio-glacial sediments of the pre-slope failure valley deep-seated gravitational deformation indicated by typical floor 25 m below the actual surface. Organic matter from a well-devel- morphostructures like trenches, scarps, and counterscarps (Massironi oped palaeosoil on top of the fluvio-glacial deposits yielded 13,636 ± et al., 2003). In the following we refer to this mass movement as 143 cal YBP, providing an upper age limit (post quam age) for the cata- ‘Gschließegg DSGSD’ (Fig. 1). The central part of the Gschließegg strophic failure event. DSGSD that formerly encompassed about 8.1 km2 failed catastrophical- Radiocarbon ages indicate that, in the distal part of the lake, the first ly. Because of well-developed surface manifestations and the fact that complete shoaling and peat accumulation had required roughly a 4.3 ka regional schistosity is dipping steeply and subvertically into the slope, of sedimentation and was attained at least at 9.28 ka cal YBP (Fig. 3A). lateral spreading leading to a large-scale toppling failure is assumed to Since that first shoaling, the backwater area continued to aggrade. be the deformation mechanism of the Gschließegg DSGSD (Massironi Peat layers ranging from 5 cm to 3 m in thickness typically are sharply et al., 2003). overlain by relatively thin intervals of lake sediments. A higher-up sec- Along the left toe-of-slope of the valley, rock avalanche deposits are tion toward more proximal positions, the lacustrine sediments covered to an unknown extent by alluvial fans and talus. In addition, the interfinger with alluvial deposits (Fig. 3A). central segment of the rock avalanche mass is onlapped and overlain by backwater sediments (Fig. 3A and B). The morphological overprint of 4.2. Pfitsch rock avalanche the detachment area by slow gravitational slope deformation and the partial cover of the CRF deposit impeded precise reconstructions of vol- The Pfitsch Valley is a NE-SW striking tributary of Wipp Valley to the umes. In view of these limitations, the initial rock volume of the Stilfes east of Sterzing. Between the villages of Afens and Ried, the valley is im- event is reconstructed as ≥80 Mm3, whereas the preserved volume of pressively blocked by a steep step composed of a CRF mass that covers rock avalanche deposit is estimated at ≥90 Mm3. an area of 0.92 km2 (Fig. 4A). The CRF mass is incised by a stream The rock avalanche mass dammed the backwater lake with a maxi- gorge down to 180 m in depth and with steep, near-vertical sidewalls mum extent of 15.5 km2 (Sterzing Basin; Fig. 3B) and with a catchment stabilized by cementation (Sanders et al., 2010). On the northern, 516 km2 in size. The lake basin is filled with an upward-shoaling succes- right flank of the gorge the rock avalanche mass is up to 265 m in thick- sion, starting with lacustrine deposits in the lower part to fluvial-alluvial ness (Fig. 4B; Hering et al., 1992). The southern, left flank of Pfitsch Val- sediments and peat layers in the upper part (Fig. 3A). In the backwater ley shows a well-developed headscarp between 2400 and 2100 m asl at succession that overlies the CRF dam, a piece of wood drilled 30 m (S10) Mount Überseilspitz (2493 m; Fig. 4). The rock avalanche here consists

Table 1 Results of radiocarbon and U/th dating at the Brenner Pass CRF cluster.

Radiocarbon dating results

Slope Sample Laboratory Sampling location Elevation sample AMS 14C age δC13 ± 1σ C/N Calibrated age Calibrated age failure code code material Easting Northing [m asl] [yr BP ±1σ] ‰ [BC/AD, 2σ] [cal. yr BP, 2σ]

Stilfes STI_03 ETH-38822 689909.99 5193044.76 926 Paleosol 4405 ± 45 −22.3 ± 1.1 3327 − 2909 5277 − 4859 Stilfes S2_15.3 ETH-49541 685191.02 5195317.56 924.7 Peat 3995 ± 31 −30.8 ± 1.1 15.9 2577 − 2466 4527 − 4416 Stilfes S4_17.6 ETH-49542 687491.56 5194281.40 922.4 Peat 3846 ± 30 −28.0 ± 1.1 18.3 2457 − 2205 4407 − 4155 Stilfes S4_19.3 ETH-49543 687491.56 5194281.40 920.7 Macrofossils 4784 ± 32 −25.9 ± 1.1 74.7 3645 − 3518 5595 − 5468 Stilfes S4_19.9 ETH-49544 687491.56 5194281.40 920.1 Peat 5493 ± 32 −26.1 ± 1.1 21.0 4445 − 4263 6395 − 6213 Stilfes S5_15.4 ETH-49545 688192.00 5193951.32 924.6 Wood 4437 ± 30 −28.4 ± 1.1 77.2 3330 − 2929 5280 − 4879 Stilfes S5_16.5 ETH-49546 688192.00 5193951.32 923.5 Wood 8299 ± 34 −26.8 ± 1.1 222.4 7483 − 7192 9433 − 9142 Stilfes S5_17.4 ETH-49547 688192.00 5193951.32 922.2 Wood 5340 ± 30 −31.2 ± 1.1 54.9 4314 − 4052 6264 − 6002 Stilfes S6_25 ETH-49548 684777.04 5195187.87 917 Paleosol 11841 ± 58 −24.6 ± 1.1 8.8 11828 − 11543 13778 − 13493 Stilfes S9_17.5 ETH-49549 684369.59 5195137.38 932.5 Wood 3804 ± 29 −23.1 ± 1.1 163.8 2340 − 2141 4290 − 4091 Stilfes S10_20.7 ETH-49550 683884.98 5195099.78 929.3 Wood 5760 ± 33 −20.8 ± 1.1 277.7 4706 − 4530 6656 − 6480 Stilfes S10_29.8 ETH-49551 683884.98 5195099.78 920.2 Wood 10082 ± 56 −31.4 ± 2.1 23.6 10020 − 9405 11970 − 11355 Ridnaun RD_OR_01 ETH-38820 676066.59 5197774.94 1323 Paleosol 6990 ± 40 −25.9 ± 1.1 5984 − 5771 7934 − 7721 Ridnaun RD_OR_02 ETH-38819 676066.59 5197774.94 1320 Paleosol 8865 ± 50 −30.7 ± 1.1 8227 − 7799 10177 − 9749 Brennersee BR_01_A VERA-4983 689988.35 5210947.72 1254 Paleosol 5462 ± 36 −30.1 ± 1.1 4361 − 4246 6311 − 6196

230Th/234U disequilibrium dating results

Slope Sample Laboratory Sampling location Elevation ppb U 2SE ppb 2SE 234U/238U 2SE 230Th/234U 2SE Single age failure code code Th Easting Northing [m asl]

Pfitsch PFT 11 PFT_11_1 691,962.03 5,199,506.74 1314 337.082 0.606 34.900 0.288 0.793 0.001 0.098 0.002 11,230 ± 250 Pfitsch – PFT_11_2 –– –325.158 0.603 33.694 0.354 0.788 0.001 0.123 0.007 11,290 ± 200 Pfitsch – PFT_11_4 –– –357.388 0.700 35.900 0.409 0.782 0.001 0.097 0.004 11,100 ± 300 Pfitsch – PFT_11_5 –– –328.978 0.500 33.800 0.401 0.777 0.002 0.101 0.002 11,550 ± 300 Pfitsch PFT 15 PFT 15_1 691,951.41 5,199,528.25 1328 1088.594 1.978 4.005 0.030 1.009 0.001 0.024 0.0002 2502 ± 310 Pfitsch – PFT 15_2 –– –932.042 1.653 4.332 0.033 1.009 0.001 0.025 0.0003 2670 ± 460 Pfitsch – PFT 15_3 –– –929.577 1.659 4.990 0.037 1.010 0.001 0.026 0.0002 2716 ± 200 Pfitsch – PFT 15_4 –– –1106.678 1.980 3.714 0.028 1.009 0.001 0.026 0.0002 2747 ± 400 Pfitsch – PFT 15_5 –– –1135.932 2.008 5.717 0.042 1.009 0.001 0.023 0.0003 2365 ± 200 Pfitsch – PFT 15_6 –– –1099.950 1.973 5.063 0.038 1.009 0.001 0.024 0.0002 2472 ± 150

OxCal 4.2 Bronk Ramsey (2013). Radiocarbon Calibration Curve IntCal 13 (Reimer et al., 2013). M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59 49 of calcphyllites (Bündnerschiefer) and forms both valley flanks. In the 5B). The lake volume is estimated at 0.64 km3. Surveys for the Brenner scarp, schistosity dips 30–35° toward the NW (Fig. 4). Additionally, base tunnel showed that, in the area of the backwater lake, the bedrock the scarp is delimited by a set of WNW-ESE striking faults along its valley floor is located ~300 m below the present valley bottom flanks and a N-S striking fault at its headscarp. The intersection of the (Brandner and Reiter, 2002). The top of bedrock is overlain by a succes- schistosity with the faults forms a dihedron that intersects with the sion of glacial and fluvioglacial deposits 30– 70 m in thickness. The fill- very steep valley flank (~40°; Massironi et al., 2003). The rock mass de- ing of the backwater lake, in turn, is dominated by an upward- fined by this dihedron descended as a catastrophic slope failure. Recon- coarsening/shoaling succession from lacustrine silts into fluvial gravels, struction of the pre-rock avalanche scarp area by interpolation of and documents the progradation of a delta system. The entire NW-fac- contour lines resulted in a rock volume of 30 Mm3. In contrast, the vol- ing flank of the outer Pfitsch valley that also comprises the scarp of the ume of CRF debris before fluvial incision is estimated at 36 Mm3. rock avalanche is a DSGSD (~10 km2, Trenser Joch DSGSD) with well- The CRF completely dammed the Pfitsch River (Fig. 5A), and a back- developed morphostructures such as double ridges, counter-scarps, water lake 11 km2 in area developed (Fig. 5B). A former lake shore pre- and trenches (Massironi et al., 2003; Figs. 1Aand5A). The rock ava- served at 1425 m asl and knickpoints within alluvial fan channels at the lanche originated out of the eastern lateral part of the slope deformation same elevation aid in the reconstruction of the former lake extent (Fig. where the local schistosity changes from steep N-dipping to low-angle N-dipping (Massironi et al., 2003). The orientation of schistosity seems to be the main controlling factor along this slope where sackung occurs at the steep N-dipping parts (Trenser Joch DSGSD) and a cata- strophic large-scale wedge slide, bounded by E-W and N-S to NNE- SSW striking faults, happened at the dip-slope dominated areas (Pfitsch rock avalanche; Massironi et al., 2003). The 234U/230Th age dating of post-CRF calcite cements within the rock avalanche breccia indicates a minimum age (ante quam age) of 11,290 ± 500 YBP for this catastrophic slope collapse (Table 1). The lith- ified rock avalanche mass is dissected by subvertical fractures that might result from neotectonic deformation (Fig. 5C). Calcite cement found on a flanking wall of one of these fractures yielded a mean precip- itation age of 2690 ± 50 YBP; this indicates that at least by that time the CRF mass was sufficiently lithified to react in a brittle fashion and that the river has incised at least 75 m since then. Unfortunately, no indica- tors for sense of displacement were found along the fractures.

4.3. Ridnaun rock avalanche

The left slope of Ridnaun Valley—the major tributary in the west of Sterzing—is characterized by DSGSDs (Telfer Weissen and Wetterspitz DSGSD; Fig. 1A) over an extent of at least 6 km (Zorzi, 2013). The rock avalanche described herein detached from a sector of that unstable slope, and today covers an area of 2.4 km2 between Mareit and Ridnaun (Fig. 6A). The failed slope sector is comprised of Oetztal-Stubai base- ment (OSB) in the upper part and the Schneeberg complex (SC) in the lower part (Figs. 6Band7A). The SC/OSC boundary is a NW-dipping brittle/ductile overthrust with distributed brittle strain recorded by cataclasite zones, slickensides, and pseudotachylites (Sölva et al., 2005). This thrust belt obliquely intersects the NW-SE trending slope and is the main brittle structural element along this valley flank. The en- tire slope shows low angle (15–20°) of NNE-dipping schistosity (Fig. 6B). The CRF deposit consists mainly of micaschists derived from the SC; paragneisses from the OSB are subordinate in abundance. The initial rock mass is estimated to 100–110 Mm3, whereas the rock avalanche deposit ranges from 110 to 120 Mm3 in volume. The valley flank up- slope and on both sides of the CRF headscarp display well-defined scarps, counterscarps, and trenches (Zorzi, 2013; Fig. 7B). Zorzi (2013) subdivided the noncatastrophically failed sections in the Telfer Weissen DSGSD (eastern part) and the Wetterspitz DSGSD (western part). At both areas schistosity generally dips into the slope with a low angle, but in the surroundings of the rock avalanche headscarp small-scale folding causes subvertical SE-dipping schistosity (Zorzi, 2013). Al- though a complex structural setting, it is estimated that significant

Fig. 4. (A) Pfitsch rock avalanche. Hillshade image of Pfitsch rock avalanche area and a part parts the Ridnaun rock avalanche initially developed as a deep-seated of the backwater area. Upslope and sideways of the CRF scarp morpho structures toppling failure (Zorzi, 2013). Zorzi (2013) assigned the Telfer Weissen associated with a DSGSD are well developed. A deep gorge is incised into the widely DSGSD as an analogue for the Ridnaun rock avalanche before cata- lithified rock avalanche accumulations that allowed sampling for U/Th-dating. (B) Cross strophic failure. Polished slickensides and widening of tension cracks section through the Pfitsch rock avalanche (vertical double scale). The drillings have highlight ongoing gravitational instability of this slope. In the steep reached the bedrock only at the site where the Pfitsch River enters the gorge through the CRF material; therefore the exact depth of the bedrock at the cross section is not crown area and along the sidewalls of the scarp area, rockfalls and top- known. pling of blocks are common. 50 M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59

Fig. 5. (A) Pfitsch rock avalanche. Overview photo of the outer Pfitsch Valley downriver of the Pfitsch rock avalanche. The whole slope behind and sideways of the rock avalanche scarp is affected by a DSGSD. (B) Overview photo of the inner Pfitsch Valley upstream of the rock avalanche dam. The recent valley floor is characterized by backwater sediments of the former impoundment interfingering with alluvial fans. The dashed white line represents the maximum extent of the former backwater lake. Here the whole northern slope of the valley is affected by a DSGSD as well. (C) Outcrop photo of the Pfitsch rock avalanche deposits within the gorge. The river is deeply incised into lithified rock avalanche deposits that show some features of brittle deformation (dashed lines). The orange star represents the U/Th-sampling site of sample PFT_15 (2690 ± 50 YBP).

Similar to the Pfitsch Valley, the Ridnaun River was completely preserved rock avalanche accumulation area is 0.22 km2 and blocked by the CRF mass. At its maximum extent, the lake covered an ~0.29 km2 of backwater sediments. The CRF deposit consists of area of at least 2.5 km2. calcphyllites derived from the frame zone of Tauern Window (Upper Erosional remnants of the filling of the former Ridnaun backwater Schieferhülle in Fig. 8B). The schistosity of the calcphyllite series dips lake are well identifiable, and shore terraces can be followed for over 30–45° toward NW, i.e., roughly parallel to the slope of the valley 3.5 km (Fig. 7C). The highest shore terrace is located about 15 m flank (Fig. 8B). above the present valley bottom, which is dominated by fluviatile The scarp area of the CRF is situated along the upper part of the slope gravels. The lake deposits are composed mainly of laminated clays called Padauner Berg (Fig. 1A). The scarp shows a stepped headwall, in- with intercalated laminae to very thin beds of silts and sands (Fig. dicating ongoing deformation in the upper part of the slope (Fig. 8A). 7D). Thin paleosols and layers rich in organic remnants are intercalated The shape of the scarp is controlled by faults: a set of ENE-WSW striking at irregular intervals. The development of paleosols within the terraces faults merges with a NW-SE striking fault set, delimiting a dihedron indicate a fluctuating lake level. Samples taken from two organic-rich (Fig. 8A). In addition, the calcphyllite series here overlies a package, layers vertically separated by 3.1 m yielded radiocarbon ages of ~100 m in thickness, of marbles that are karstified (Fig. 8B). Spring 9963 ± 214 cal YBP and 7828 ± 107 cal YBP, fitting with stratigraphy tufa deposits at the mouth of the Venn Valley may be related to active (Table 1). At the sampling site, relatively constant depositional condi- karstic dissolution in the marbles. Reconstruction of the pre-failure tions in the early phase of lacustrine accumulation is suggested by mo- slope indicates an initial rock volume of 10–12 Mm3. Because of anthro- notonous lamination and the absence of layers rich in organic debris pogenic landscape modification, the volume of the rock avalanche de- (Fig. 7D). Near the distal end of the former rock avalanche lake, along posit is difficult to assess but may be in the order of 12–14 Mm3.A the incision of the Ridnaun river, the contact between the rock ava- palaeosoil found on top of the rock avalanche deposits provided an lanche deposits and the lacustrine succession is exposed. Projecting age of 6120 ± 40 cal YBP. Based on this proxy we assume an event this stratigraphic contact toward the location with the radiocarbon- age of about 6.2– 6.5 ka for the Brennersee rock avalanche that devel- dated layers farther west (Fig. 7D) suggests that the lake bottom there oped from a dip-slope failure (translational rockslide) of large parts of may have been positioned ~3 m below the lower, older sample the Padauner Berg. (9960 ± 214 cal YBP). Under the assumption of a constant rate of sedi- ment accumulation at the site, back-extrapolation of the accumulation 5. Discussion rate from the radiocarbon-dated section to the projected base of the lake succession suggests that the onset of lacustrine sedimentation oc- 5.1. Catastrophic rock-slope failures and climate curred at roughly 12.1 ± 1 ka BP. 5.1.1. Brenner Pass cluster 4.4. Brennersee rock avalanche The catastrophic rock-slope failures of Ridnaun, Pfitsch, and Stilfes are all older than 11.5 ka and likely took place during the Younger At Brenner Pass, Lake Brenner (Brennersee) represents another back- Dryas climatic phase (12.7–11.7 ka; Fig. 9). Within this time span the water lake dammed by a CRF (Fig. 8A). The Brennersee rock avalanche source areas of the mentioned CRFs were free of glacial ice (cf. Van mass has been strongly modified by human activity in the form of Husen, 1999). Reconstructed precipitation patterns show that the con- road construction over centuries, but especially upon construction of trast between the northern fringe of the Alps and the central, inner-Al- the Brenner railway (1864–1867) and a motorway in the 1960s. The pine valleys was stronger during the early Younger Dryas than today M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59 51

Fig. 6. (A) Ridnaun rock avalanche. Hillshade image of Ridnaun rock avalanche area and the associated backwater area. On both sides of the CRF scarp, morpho structures of DSGSDs are visible. (B) Cross section through the Ridnaun rock avalanche (vertical double scale). SC: Schneeberg complex, OSB: Oetztal-Stubai crystalline basement, NB: nappe boundary, ssf: small- scale folds.

(Kerschner et al., 2000). In the inner-Alpine zone, where all of these be more obscure because earthquakes caused by post-glacial fault CRFs are located, precipitation was reduced by 20% or more. Conversely, movements from tectonically-driven uplift and/or crustal stresses may modern amounts of annual precipitation prevailed along the northern trigger catastrophic rock-slope failures largely independent of the fringe of the Alps and, probably, also along their southern margin timing of deglaciation (e.g., Hewitt et al., 2008, 2011; Antinao and (Kerschner and Ivy-Ochs, 2007; Kerschner, 2009; Heiri et al., 2014). Sev- Gosse, 2009; Sanchez et al., 2009; Hermanns and Niedermann, 2011; eral publications have indicated a connection between major mass Penna et al., 2011, Ballantyne et al., 2014). movement clusters and wet climatic periods, for example in the NW Ar- In the more distant surroundings of the Brenner, only two CRFs ap- gentine Andes (Trauth et al., 2000) and the Himalaya (Bookhagen et al., pear to have occurred during or around the Younger Dryas. These in- 2005); but at Brenner we have a relative aridification, which is normally clude (i) the 24-Mm3 orthogneiss rock avalanche of Habichen in the considered as disadvantageous or without influence on mass wasting. lower Oetz Valley, which occured N11.5 ka ago (ante quam 14Cage Glacier erosion and debuttressing are important factors in conditioning from lacustrine backwater deposits; unpubl. data of Wahlmüller, in the slopes for failure (e.g., Penna et al., 2011); the failures then occurred Ostermann and Prager, 2014), and (ii) the ~25-Mm3 rock avalanche of with a delay of several thousand years after ice retreat because of pro- Pflach in the Northern Calcareous Alps, which occurred before 12.6 ± gressive rock mass weakening in different structural settings. For the 1 ka (M. Ostermann, unpubl. 234U/230Th ante quam disequilibrium age Scottish Highlands (Ballantyne, 1997) and Iceland (Cossart et al., of diagenetic cement). Relative to data sets from the entire Alps, the 2013) the occurrence of large landslides has been attributed to glacial early events of the BPC might coincide with the onset of potentially en- steepening and seismic activity caused by rapid glacio-isostatic re- hanced CRF activity from 12 to 11 ka (Sanchez et al., 2009; Ostermann bound. In tectonically active orogens like the Alps, this effect seems to and Sanders, 2012). The relationship between landslide activity and 52 M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59

Fig. 7. (A) Ridnaun rock avalanche. Overview photo of the Ridnaun rock avalanche scarp area. The dashed line indicates the nappe boundary (NB) between the Oetztal-Stubai crystalline basement (OSB) and the Schneeberg complex (SC). The head scarp and the CRF deposits in the foreground are marked. (B) The slopes on both sides and above the Ridnaun rock avalanche scarp are characterized by numerous morpho structures associated with DSGSDs, here a row of counterscarps. (C) Overview photo of the Ridnaun backwater area toward the CRF dam. The dashed line indicates a major terrace within the backwater sediments and the continuous line marks the maximum extent of the backwater sediments. The direction of rock avalanche motion is indicated with a pink arrow. (D) Photo of the lacustrine backwater sediments at Ridnaun. Yardstick = 1 m. the Younger Dryas has been examined in regions such as Scotland palaeoclimatic records; and (iii) that ~80% of the post-glacial CRFs of (Ballantyne and Stone, 2013; Ballantyne et al., 2014) and Norway the Alps are of an undetermined age (Ostermann and Sanders, 2012). (e.g., Böhme et al., 2015); however, no enhanced rock-slope failure ac- tivity during this period was observed there. 5.2. Catastrophic rock-slope failures, lithologies, and active deformation The Obernberg rock avalanche took place during a relatively well- defined phase of high CRF activity from 10 to 8 ka (Sanchez et al., 5.2.1. General remarks 2009; Borgatti and Soldati, 2010; Ostermann and Sanders, 2012). Most Fig. 10 shows that CRFs are most common along and near major palaeoclimatic records indicate a Northern Hemisphere climatic opti- faults in terrains of relatively competent lithologies. The CRFs N108 m3 mum during the middle Holocene between 7.5 and 5.9 ka (e.g., in volume prevail on carbonate and metacarbonate rocks, respectively Vollweiler et al., 2006). Paleoclimate records from Spannagel cave, lo- (Northern Calcareous Alps; Obernberg; Ostermann et al., 2012), and cated ~15 km east of the Brenner Pass, suggest above average summer on gneiss along the Oetz Valley (e.g., Köfels, Tumpen, Habichen; Abele, temperatures during this time (Vollweiler et al., 2006). We therefore 1974; Prager et al., 2008). Most metamorphites such as mica schists, conclude that the Brennersee event occurred 6.5– 6.2 ka BP (Fig. 9), phyllites, and foliated gneiss are prone to DSGSD and to comparatively which agrees with the terminal part of the relatively warm climatic small CRFs. In these lithologies, intersections of denselyspaced schistos- phase recorded in Spannagel. ity with joints and additional types of fractures often permit slow, quasi- The backwater sediments of the Stilfes rock avalanche document continuous deformation favouring DSGSD (e.g., Agliardi et al., 2013; several peat layers overlain by lacustrine sediments suggesting a local Crosta et al., 2013). Most CRFs of the Brenner Pass cluster are thus un- base-level rise and/or climatic humidification. Potential causes for usual in that they are detached from preceding DSGSDs in terrains of base-level rise might be multiphase failures of the valley flank, second- phyllites (Table 2). ary mass movements from the rock avalanche deposit, or pulses of ag- The fault zones shown in Fig. 10 are active, with historical earth- gradation on large alluvial fans directly downstream of the failed mass quake records indicating magnitudes M up to ≤5.5, but mostly less (cf. Fig. 2A). The latter hypothesis would fit with climatic humidification (see below). In mountain ranges subject to high-rate active deformation as a favourable factor for the drowning of peat swamps and intermittent and strong earthquakes, such as the central Apennines, surface evidence reestablishment of a shallow lake. for faulting is abundant (e.g., Ascione and Cinque, 1997; Cheloni et al., With respect to any potential correlation between post-glacial cli- 2010; Moro et al., 2012; Ortner et al., 2014). For the fault zones consid- mate and mass wasting, several major questions are identified. With re- ered herein, this suggests that the rate of geomorphic processes exceeds spect to climate, the most pertinent problems include (i) definition of structural deformation in near-surface settings; this fits the observation palaeoclimatic parameters relevant to mass wasting, (ii) quantitative that earthquakes of M N 5–5.5 are required to induce any ‘primary’ faults estimation of these parameters, and (iii) regional differences of to cut to surface (cf. Harp and Wilson, 1995; Persaud and Pfiffner, 2004). palaeoclimatic change (see e.g., discussion in Ostermann et al., 2012). Although the seismic cycle of these fault zones exceeds historical re- In addition, relevant problems regarding mass wasting include (i) that cords and includes quakes of M N 5.5–6 (cf. Schnellmann et al., 2002; mass wastings of highly different types and dynamics are implicit in a Hinsch and Decker, 2003; Gisler et al., 2004; Nasir et al., 2013), such database (ranging from small soilslides a few tens of cubic metres in vol- events appear to be too rare to produce surface deformation preserved ume to larger rockfalls to genuine rockslides); (ii) that the dating preci- over time intervals of over thousands of years and more; however, sion of CRF events in general only allows for proxy comparison to such quakes are triggers of masswasting (cf. Wells and Coppersmith, M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59 53

Stock and Uhrhammer, 2010; Claude et al., 2014). The bulking values for the Brenner Pass CSFs range from 8 to 17% and are more in the range of 7–26% volume increase as estimated by Hungr (1981).Theini- tial rock volume and the accumulated rock volume can only be estimat- ed; but well-defined source areas, DEMs, and sometimes drill cores within the CRF accumulations allow approximations of b20% uncertain- ty in volumetric calculations. Considering all uncertainties, the bulking values for the Brenner Pass CRFs range from 8 up to 40%.

5.2.2. Clusters of catastrophic rock-slope failures Brenner Pass cluster (BPC in Fig. 10): The present seismotectonic stress field between Innsbruck and the Brenner Pass is characterized by NW-SE compression and NE-SW directed extension (Drimmel, 1980), which is consistent with continued or rejuvenated compression by the south-Alpine indenter (Reiter et al., 2005). The southern part of the Brenner fault zone is characterized by high-frequency macro- and microseismicity (Drimmel, 1980); a few events with M 4–4.9 are docu- mented (Reiter et al., 2005). All of the discussed zones of seismicity and CRF activity are linked with each other via the fault zones related to the south-Alpine indenter. The mapped distribution of CRFs in Fig. 10 shows their association with seismogenic zones. Thus, we conclude that CRFs are prepared and, in some cases, may have been directly triggered by earthquakes and/or by effects indirectly related to tectonism (e.g., growth of microcracks, progressive opening of joints and establishment of con- nected joint networks, slope oversteepening, modified groundwater pressure; e.g., Hermanns and Strecker, 1999; Hermanns et al., 2001; Penna et al., 2011). However, this does not exclude any influences by cli- mate or meteorological events. Meteorological events able to trigger CRFs (e.g., extreme rains) are more probable in specific climatic regimes but can occur in many climatic regimes — and will impact only those rock slopes that are near failure at any given time by primary and/or secondary structural deformation. It is this superposition of at least three space/time variables (climate, meteo-events, rock slope status) and associated probabilities that complicates any simple cause-effect approach to predict CRF activity. Inn Valley cluster (IVC in Fig. 10): For the Inntal shear zone (ISZ in Fig. 10), an early Oligocene to recent sinistral displacement of 40– 50 km is estimated (Linzer et al., 2002; Ortner et al., 2006). Whereas Fig. 8. (A) Brennersee rock avalanche. Hillshade image of Brennersee rock avalanche area. at least one of the CRF masses along the Inn Valley resulted from repeat- The valley floor at Brenner Pass has been strongly modified by several construction campaigns during the last centuries. Within the backwater sediments a lake still exists ed mass wasting, another one clearly detached well before the LGM (Brennersee). The area above the headscarp of the Brennersee rock avalanche is (Gruber et al., 2009). Along the Inn Valley, the strongest historical characterized by a row of counterscarps and trenches. (B) Cross section through the quakes are reconstructed at M 5.2–5.5 (Reinecker and Lenhardt, 1999; Brennersee rock avalanche area. Lenhardt et al., 2007). Oetz valley cluster (OVC in Fig. 10): Another cluster of CRFs is pres- 1994; Harp and Wilson, 1995; Schnellmann et al., 2002; Lenhardt, 2007; ent near the northern fringe of the Oetztal-Stubai crystalline basement, McCalpin, 2009, Hermanns and Niedermann, 2011). Furthermore, even aligned along the Oetz Valley. The CRF detachment was controlled by if deformation is slow and accompanied by low-magnitude quakes, its the intersection of foliation in folded metamorphites with exfoliation accumulative presence along fault zones causes secondary deformation, joints and brittle fracture sets (see Prager et al., 2009). Field data and such as opening and propagation of cracks and fractures, and gravity-in- seismic fault-plane solutions suggest that the present deformation of duced sliding (cf. Persaud and Pfiffner, 2004; Ustaszewksi et al., 2008). the Oetztal-Stubai basement is partitioned in W-E extension, dextral In the following, we will discuss CRF clusters from the south near the strike-slip, and local WNW-directed thrusting, depending on location Brenner Pass toward the northern part of the Northern Calcareous (Reiter et al., 2005). The OVC to the Brenner Pass represents an area Alps. The clusters were delimited according to (i) association with with one of the highest frequencies and magnitudes of historical earth- fault zones of known kinematics (dextral, sinistral); (ii) domains with quakes of the eastern Alps (Drimmel, 1980; Reinecker and Lenhardt, a specific style of near-surface deformation (e.g., E-W extension, as for 1999; Nasir et al., 2013). the Brenner Pass cluster); or (iii) in one case, where the active structural Fern Pass–Telfs cluster (FPC in Fig. 10): The northernmost group of regime is not fully clarified at present, with respects to the geographic CRFs is the Fern Pass–Telfs cluster, located along an array of NNE-SSW area (Oetz Valley cluster). striking sinistral faults (LF in Fig. 10) (Loisach–Fern Pass fault zone; When a detached rock mass disintegrates and fragments in the pro- e.g., Linzer et al., 1995, 2002; Prager et al., 2008). For this fault zone, cess of becoming a rock avalanche, an initial volume increase (bulking) Linzer et al. (1995, 2002) deduced a total offset of 10–15 km. The occurs (e.g., Hungr and Evans, 2004). Pierson (1998) assumed a 20% vol- Loisach-Fern Pass fault zone exhibits an exceptionally high frequency ume increase, whereas Hungr and Evans (2004) assumed 25%, based on of earthquakes with M up to 5.3 (Drimmel, 1980; Reinecker and a mean value of porosity measurements of crushed rock by Sherard et al. Lenhardt, 1999; Nasir et al., 2013). The Loisach-Fern Pass fault zone is (1963; 18–35% bulking). In the literature, typical values for bulking linked via a dextral fault system that disburdens the detachment of range between 25 and 30% (e.g., Hungr and Evans, 2004; Pirulli, 2009; rock avalanches (Westreicher et al., 2014; Sanders, 2015) and large 54 M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59

Fig. 9. Compilation of the age data from the Brenner Pass CRF cluster in relation to GRIP (Greenland Ice Core Project) and NGRIP (North Greenland Ice Core Project) δ18O curves. The timing of the Gschnitz stadial is indicated according to Ivy-Ochs et al. (2006). rockfalls (cf. Preh and Sausgruber, 2015) connected to the compression up to ~10 ka might nevertheless be related to unloading and slope and overthrusting along the NW fringe of the Oetztal-Stubai crystalline steepening (see also Agliardi et al., 2013). For catastrophic mass wast- basement (Fig. 10)(Inntalthrust;cf.Linzer et al., 1995, 2002; Reinecker ings, Prager et al. (2008) inferred similar long reaction times required and Lenhardt, 1999; Frisch et al., 2000b; Reiter et al., 2005). For the Telfs for ‘structural expression’ of stress from unloading and valley-flank fault, based on stratigraphic marker levels in a cover-thrust nappe, a steepening through the growth of (micro) cracks into a network of dextral displacement of 10–15 km is indicated by Linzer et al. (2002) joints that ultimately provide a detachment scar. With such potentially (TF in Fig. 10). Apart from seismicity, deformation and fracturation of long reaction times involved before relaxation either as a CRF or as in- Quaternary (pre-LGM) talus breccias preserved along the trace of the ception of a DSGSD, no clear cause-effect relationship between a specific Telfs fault system further underscores that faulting is still active climatic regime and large-volume mass wastings is apparent. Indeed, to (Sanders, 2015). their expressed surprise, Agliardi et al. (2013, p. 272) found that the dis- tribution of mean annual rainfall correlates negatively with DSGSD 5.3. Deep-seated gravitational slope deformations abundance; to them, this suggests that “effective surface hydrological processes contribute to a large-scale topography hampering DSGSD”. The DSGSDs are widespread in the Brenner Pass region, where they Their interpretation indicates that the controls of time scales and reac- affect entire high-relief valley flanks involving huge rock volumes (Fig. tion times to the impact of climatic change are often overlooked. Sim- 10). Although Zorzi (2013) investigated the DSGSDs in Ridnaun Valley plistically speaking, single-dimension cause-effect scenarios and Massironi et al. (2003) studied a DSGSD in Pfitsch Valley, no sys- correlating large-volume mass wastings to climate changes — common- tematic investigation on these slope failures exists for the area under ly in the context of temperature and/or precipitation regimes — remain consideration. hard to prove. From the data set for the entire Alps presented in Crosta et al. (2013), The role of seismicity in the development and rate of movement for the DSGSDs obviously are not clearly associated with active faults. the DSGSDs in the Alps is unclear to date (Crosta et al., 2013). Long-term Among diverse factors, steep and high (toporelief N1000 m) flanks of insitu observations and/or analysis of high-resolution remote sensing valleys in densely foliated metamorphites are the most prone to data were required to unequivocally prove the effects of seismic shak- DSGSD (Agliardi et al., 2013; Crosta et al., 2013). This is also evident in ing. Given the overall seismic activity, some amount of seismic shaking Fig. 10, wherein valley flanks of phyllites show numerous DSGSDs. A can be inferred for nearly every location on timescales of hundreds to comparison of rates of long-term (106–107 years) uplift with distribu- perhaps N10 ka. In the central Apeninnes, DSGSDs set to move or accel- tion of DSGSDs revealed that these are most common in areas of inter- erate in velocity were documented by Moro et al. (2007) for the 1997 mediate uplift rate and where fluvial incision is confined to major Colfiorito seismic sequence (Mw = 5.7 and 6.0) by satellite Interferom- valleys (Agliardi et al., 2013). With respect to glacial unloading on etry and GPS data. A similar, highly probable correlation of DSGSD ve- DSGSD development, Crosta et al. (2013) reviewed that time delays of locity and seismicity was established during the 1915 Avezzano quake M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59 55

Fig. 10. Clusters of catastrophic rock-slope failures (FPC = Fern Pass–Telfs cluster; OVC = Oetz Valley cluster; IVC = Inn Valley cluster; BPC = Brenner Pass cluster) and DSGSD distribution at the western part of the eastern Alps and the northern part of the southern Alps (DSGSD inventory from Agliardi et al., 2013,modified). The white arrows represent the gridded directions and values of surface velocities modelled by GPS data and an elastic dislocation model according to Caporali et al. (2013).

(Mw =6.7;Moro et al., 2009). The Alps-wide compilations of DSGSDs part of the former slope, laterally delimited by a subvertical WSW- (Agliardi et al., 2013; Crosta et al., 2013) and the subsample shown in ENE striking fault set that sits at both sides. Fig. 10 clearly show that DSGSDs are much less related to seismogenic The Pfitsch rock avalanche detached from the eastern lateral part of fault zones than CRFs. In the Brenner Pass area, different stages of slowly the Trenser Joch DSGSD, spanning about 10 km2. The schistosity within deforming rock masses to CRFs detaches thereof are present, and render the area affected by gravitational processes changes from steep N-dip- the area a showcase for a progression from DSGSDs into catastrophic ping at the western parts of the DSGSD where sackung occurs rock-slope failures. (Massironi et al., 2003) to low-angle N-dipping at the Pfitsch rock ava- At the Brenner Pass cluster, four CRFs are closely related to DSGSDs lanche scarp area. The dip-slope failure is bounded by E-W and N-S to (Fig. 1; Table 3). At the central region of Stilfes, the Gschließegg NNE-SSW striking faults and failed as a wedge slide (Massironi et al., DSGSD, spanning about 8.1 km2, failed catastrophically resulting in the 2003). Stilfes rock avalanche. The Gschließegg DSGSD shows significant dis- The Ridnaun rock avalanche originated from the central part of a placements along the crestline and both sides of the Stilfes rock ava- DSGSD and affected large parts of the left slope of the Ridnaun Valley. lanche headscarp area (Fig. 2A). Regional schistosity is dipping steeply Zorzi (2013) subdivided the noncatastrophically failed sections in to subvertically into the slope not favouring a slope failure. The whole the Telfer Weissen DSGSD (eastern part) and the Wetterspitz ridge is dissected by E-W and WSW-ENE striking fault sets and a steep DSGSD (western part). In this area, the schistosity generally dips nappe boundary. Supersaturated waters with respect to calcite along into the slope with a low angle, but around the CRF headscarp the lower part of the slope indicate a deep-seated water flow from the small-scale folding causes subvertical SE dipping (Zorzi, 2013). The crest that is characterized by double ridges, scarps, open trenches, and Ridnaun rock avalanche is the result of deep-seated toppling origi- flat-bottomed depressions (Massironi et al., 2003). Lateral spreading is nating from its upper part. The same type of failure, although not cat- assumed to be the deformation mechanism of the Gschließegg DSGSD astrophic is size, can be observed at the neighbouring Telfer Weissen (Massironi et al., 2003) leading to a toppling failure of the uppermost DSGSD (Zorzi, 2013). 56 M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59

The Brennersee rock avalanche developed out of a dip-slope failure (translational rockslide) from large parts of the Padauner Berg. The ge- 15,500 15,500 13,800 – – – 9200 6500 ometry of the failed rock mass was bounded by NW-SE and WSW-ENE – – striking steep fault sets. Age range CRF event 6. Conclusions

• In the area of the Brenner Pass (eastern Alps), five catastrophic rock-

volume increase I to II [%] [yrs BP] slope failures (CRFs) involving medium to high-grade metamorphites are clustered together with deep-seated gravitational slope deforma- tions (DSGSDs). The CRFs involved volumes from 12 to 110 Mm3 and

] Bulking factor show fahrböschung angles of 10 to 27°. The volume/fahrböschung re- 3 lations plot within the range of most catastrophic mass wastings. Nu- merical dating (14C, 36Cl, 234U/230Th disequilibrium) suggests that, all together, the CRFs took place between ≤13.5 and 6.2 ka BP. CRF volume [Mm 120 [II] [I initial failure volumen] [II volume of deposites] 90 [II] 53 [II] 13 [II] • Each catastrophic rock-slope failure dammed up a backwater basin with a lake that has existed over thousands of years, or that still exists. The basins range in area from 2.5 km2 (Ridnaun rock avalanche) to 11 km2 (Pfitsch rock avalanche) to 15.5 km2 (Stilfes rock avalanche; Sterzing basin) and, together, contain a sediment record that may ) span much of the Younger Dryas to the Holocene time. Pulsed sedi- Hermanns et al. Type of CRF dam ( 2011 ment aggradation includes phases of lacustrine sedimentation, progradation of deltas and alluvial fans, braided stream systems, and phases of paludification. Within the Sterzing basin, three phases of peat aggradation were 14C–dated to 6.3–6.1, ~5.1, and 4.5–4.2 ka BP. The longevity of the dams before a partial breach occurred can be es- Fahrböschung [°] 13 IIa/ii/2 80 [I] 11 11,300 timated to at least 4.3 ka at Stilfes and 4–5 ka at Ridnaun, both based on the radiocarbon dates from the impoundments. For the Pfitsch dam, no age data from the backwater is available; but the time for silting up the enormous impoundment there might be at least in the same order as at Stilfes. The U/Th-dated calcite precipitations at fi fi Schistosity in relation to the slope Into the slope-subvertical Dip-slope 17Into the slope 17 IIa/iii/2 IIa/ii/2 30 [I] 110 [I] 17 8 10,800 11,700 Out of the slope 10 IIIa/ii/4 45 [I] 15 7600 Dip-slope 27P IIa/i/2tsch indicate 12 that [I] the river has incised 8 in the 6200 lithi ed landslide dam at least 75 m in depth within the last 2.7 ka. avalanche avalanche avalanche avalanche avalanche • In the Brenner Pass area, different types of DSGSDs are developed. The structural composition of the slopes can vary at short distances and strongly controls the characteristics of the instability phenomena. Large-scale toppling from lateral spreading of at least parts of the fi Metamorphic overprint Process Medium-grade Rock Medium-grade Rock High-grade Rock Medium-grade Rock slope occurred at two sites (Stilfes, Ridnaun). A wedge failure (P tsch) occurred alongside a slope affected by sackung processes (Trenser Joch DSGSD) and large parts of the Padauner Berg failed as a transla- tional rockslide (Brennersee).

• In general, catastrophic rock-slope failures and DSGSDs tend to occur Predominant lithology pyrite-bearing calcphyllites pyrite-bearing calcphyllites Phyllithic garnet-mica schists Calcitic marbles Medium-grade Rock pyrite-bearing calcphyllites separate from each other. Near the Brenner Pass, however, four of the five CRFs are associated with and developed from different types of underlying DSGSDs. These CRFs were slowly formed by progressive slope deformation that gradually approached an instability threshold. Upon being triggered (e.g., by seismicity or heavy rains), the threshold was surpassed and the rock mass descended by catastrophic failure. While multiple case studies around the world have shown that rock slope failures occur without any external trigger mechanism, our con- clusion that three Brenner Pass CRF events occurred coetaneously during the Younger Dryas suggests a shared trigger. Early Jurassic to Paleogenecomplex & Schneeberg complex Pre-variscan and Variscan Büdnerschiefer Wedge failure 36 [II] Austroalpine unit Early Jurassic to Paleogene Büdnerschiefer Lower Triassic to Lower Jurassic Büdnerschiefer Early Jurassic to Paleogene Büdnerschiefer

• The Brenner Pass CRF cluster formed in a seismogenic W-E extension- al stress field accompanied with normal faulting, exhumation, and surface uplift. This stress field is sustained by northward compression of the southern Alpine continental indenter a few kilometres to the tsch Penninic unit Quartzose,

fi south. From Brenner Pass at the orogenic crestline toward the fringe Catastrophic rock-slope failure (CRF) Tectonostratigraphic unit Ridnaun Ötztal-Stubai-crystalline basement Obernberg Central-Alpine Mesozoic; Upper Stilfes Penninic unit Quartzose, P Brennersee Penninic unit Quartzose,

Table 2 Compilation of some key parameters of the CRFs from the Brenner Pass cluster. of the eastern Alps ~60 km farther north, tectonic indentation led to M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59 57

compartmentalization of the eastern-Alpine edifice along major active strike-slip faults; over this entire area, CRFs are clustered in close vi- cinity to and along these fault zones (~80% CRFs within a 0–5kmdis- Zorzi (2013) Massironi et al. (2003) Massironi et al. (2003) tance to fault zones).

• The clear-cut association of seismogenic faults with catastrophic rock- Supersaturated waters Supersaturated waters slope failures suggests a cause-effect relation. Mass wasting was pre- pared and maybe triggered by coseismic deformation and/or in re- sponse to cumulative secondary deformation (jointing, micro-/ cracking, toppling) related to faulting. Because (i) some 80% of the nal CRF fi propagation Remarks References Rock avalanche avalanche Rock avalanche avalanche CRFs of the Alps are still undated and because of (ii) imprecise age- bracketing of most ‘dated’ CRF events and (iii) a lack of palaeoseismic records of sufficient resolution in time and precision in space, a de- monstrable correlation between CRF activity and palaeoseismicity or phases of increased rate of deformation is impossible at present. Kinematics of initial CRFs Upper part: toppling failure; Lower part rotational rocksliding Planar sliding Rock toppling failure; Lower part rotational rocksliding Wedge sliding Rock Acknowledgements

This research project was funded by the Autonomous Province of

⁎ Bolzano/Bozen – South Tyrol (17/40.3). We gratefully acknowledge Luca Zorzi (University of Padua, Italy), Ulrich Burger (Brenner Base Tunnel Project), Icilio Starni (Geoconsulting International, Italy), Susan Telfer Weissen: deep-seated toppling Wetterspitz: deep-seated sliding Translational rocksliding Failed part: wedge failure, non-failed part: sackung Lateral spreading Upper part: Ivy-Ochs (ETH Zurich, Switzerland), Igor Villa (University of Berne, Switzerland), and Helena Rodnight (formerly at University of Innsbruck, Austria). Special thanks to Kathleen Wendt (University of Innsbruck, Austria) for English proofreading. ⁎ LiDAR data and a digital elevation model were provided by the De- partment of Regional Planning of the Autonomous Province Bozen- at-bottomed DSGSD morphostructural features DSGSD mechanism Double ridge, scarps, counterscarps, trenches Scarps, open trenches Double ridge, scarps, counterscarps, trenches Double ridge, scarps, open trenches, fl depressions South Tyrol (http://www.provinz.bz.it/) for the Italian part of the study area and by the Government of Tyrol (www.tirol.gv.at/) for the Austrian Sto

\ fi

\ part. Drill logs were kindly provided by the Of ce of Geology and Mate- rial Control, Bozen, Italy. Major Faultsets (and Jointsets) N-S to NNE-SSW striking faults faults & WSW-ENE striking faults faults & N NNE-SSW striking faults E-W striking faults & WSW-ENE striking faults References

Abele, G., 1974. Bergstürze in den Alpen — ihre Verbreitung. Morphologie und Folgeerscheinungen vol. 25. Wissenschaftliche Alpenvereinshefte 230 p. Agliardi, F., Crosta, G.B., Zanchi, A., Ravazzi, C., 2009. Onset and timing of deep-seated gravitational slope deformations in the eastern Alps, Italy. Geomorphology 103, 113–129. Agliardi, F., Crosta, G.B., Frattini, P., Malusà, M.G., 2013. Giant non-catastrophic landslides Schistosity in relation to the slope Generally into the slope; at CRF headscarp subvertical SE dipping Dip-slope NW-SE striking Dip-slope E-W striking Into the slope-subvertical and the long-term exhumation of the European Alps. Earth Planet. Sci. Lett. 365: 263–274. http://dx.doi.org/10.1016/j.epsl.2013.01.030. Antinao, J., Gosse, J., 2009. Large rockslides in the southern Central Andes of Chile (32– 34.5°S): tectonic control and significance for quaternary landscape evolution. Geo-

45° morphology 104:117–133. http://dx.doi.org/10.1016/j.Geomorph.2008.08.008. – Ascione, A., Cinque, A., 1997. Le scarpate su faglia dell’ Apennino meridionale: Genesi, età Regional schistosity N-dipping; partly small-scale folded NW-dipping steep N-dipping steep N-dipping esignificato tettonico, Il Quaternario. Ital. J. Quat. Sci. 10, 285–292. Ballantyne, C.K., 1997. Holocene rock slope failures in the Scottish Highlands. Paläoklimaforschung 19, 197–205. Ballantyne, C.K., Stone, J.O., 2013. Timing and periodicity of paraglacial rock-slope failures in the Scottish Highlands. Geomorphology 186:150–161. http://dx.doi.org/10.1016/j. CRF headscarp orientation ESE-WNW Low angle NNE-SSW Low angle to N-S 30 ESE-WNW Subvertical or geomorph.2012.12.030. Ballantyne, C.K., Sandeman, G.F., Stone, J.O., Wilson, P., 2014. Rock-slope failure following ⁎ Late Pleistocene deglaciation on tectonically stable mountainous terrain. Quat. Sci. – CRF source within the DSGSD part Central part lateral part part Rev. 86:144 157. http://dx.doi.org/10.1016/j.quascirev.2013.12.021. Behrmann, J.H., 1988. Crustal-scale extension in a convergent orogen: the Sterzing- ]

2 Steinach mylonite zone in the eastern alps. Geodyn. Acta 2, 63–73. ⁎ Böhme, M., Oppikofer, T., Longva, O., Jaboyedoff, M., Hermanns, R.L., Derron, M.H., 2015. [km 1.4 DSGSD area 7.2 Central Analyses of past and present rock slope instabilities in a fjord valley: implications for hazard estimations. Geomorphology 248:464–474. http://dx.doi.org/10.1016/j. geomorph.2015.06.045. Bokelmann, G., Qorbani, E., Bianchi, I., 2013. Seismic anisotropy and large-scale deforma- tion of the Eastern Alps. Earth Planet. Sci. Lett. 383:1–6. http://dx.doi.org/10.1016/j. ⁎ epsl.2013.09.019. Bookhagen, B., Thiede, R.C., Strecker, M.R., 2005. Late Quaternary intensified monsoon phases control landscape evolution in the northwest Himalaya. Geology 33, 149–152. Deep-seated gravitational slope deformation translational rockslide Ridnaun Valley (Telfer Weissen & Wetterspitz DSGSD) Borgatti, L., Soldati, M., 2010. Landslides as a geomorphological proxy for climate change: a record from the Dolomites (northern Italy). Geomorphology 120, 56–64. Brandner, R., Reiter, F., 2002. Die geologische Prognose zur Unterquerung des Pfitschtales/ Val di Vizze (Südtirol, Italien) – eine Herausforderung für Strukturgeologen, Stratigraphen und Quartärgeologen. Abstracts PANGEO Austria 2002, Salzburg, tsch Trenser Joch DSGSD 10 Eastern Not a DSGSD. fi pp. 24–25. Catastrophic rock-slope failure Brennersee Padauner Berg CRF DSGSD P Ridnaun Left slope of the Stilfes Gschließegg DSGSD 8.1 Central ⁎

Table 3 Compilation of the major CRF and DSGSD characteristics at the Brenner Pass cluster. Bronk Ramsey, C., 2013. OxCal 4.2. Web Interface Build. 58 M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59

Brückl, E., Behm, M., Decker, K., Grad, M., Guterch, A., Keller, G.R., Thybo, H., 2010. Crustal Hermanns, R.L., Niedermann, S., Garcia, A.V., Gomez, J.S., Strecker, M.R., 2001. structure and active tectonics in the Eastern Alps. Tectonics 29, TC2011. http://dx.doi. Neotectonics and catastrophic failure of mountain fronts in the southern intra-Ande- org/10.1029/2009TC002491. an Puna Plateau, Argentina. Geology 29, 619–622. Caporali, A., Neubauer, F., Ostini, L., Stangl, G., Zuliani, D., 2013. Meodling surface GPS ve- Hermanns, R.L., Hewitt, K., Strom, A., Evans, S.G., Dunning, S., Scarascia-Mugnozza, G., locities in the Southern and Eastern Alps by finite dislocations at crustal depths. 2011. The Classification of rockslide dams. In: Evans, S.G., Hermanns, R., Strom, A., Tectonophysics 590:136–150. http://dx.doi.org/10.1016/j.tecto.2013.01.016. Scarascia Mugnozza, G. (Eds.), Natural and artificial rockslide damsLecture Notes in Cheloni, D., D'Agostino, N., D'Anastasio, E., Avallone, A., Mantenuto, S., Giuliani, R., Earth Sciences vol. 133. Springer, Heidelberg, pp. 581–593. Mattone, M., Calcaterra, S., Gambino, P., Dominici, D., Radicioni, F., Fastellini, G., Hewitt, K., Clague, J.J., Orwin, J.F., 2008. Legacies of catastrophic rock slope failures in 2010. Coseismic and initial post-seismic slip of the 2009 Mw 6.3 L'Aquila earthquake, mountain landscapes. Earth Sci. Rev. 87, 1–38. Italy, from GPS measurements. Geophys. J. Int. 181:1539–1546. http://dx.doi.org/10. Hewitt, K., Gosse, J., Clague, J.J., 2011. Rock avalanches and the pace of late Quaternary de- 1111/j.1365-246X.2010.04584.x. velopment of river valleys in the Karakoram Himalaya. GSA Bull. 123, 1836–1850. Claude, A., Ivy-Ochs, S., Kober, F., Antognini, M., Salcher, B., Kubik, P.W., 2014. The Hinsch, R., Decker, K., 2003. Do seismic slip deficits indicate an underestimated earth- Chironico landslide (Valle Leventina, southern Swiss Alps): age and evolution. quake potential along the Vienna Basin Transfer Fault System? Terra Nova 15: Swiss J. Geosci. 107, 273–291. 343–349. http://dx.doi.org/10.1046/j.1365-3121.2003.00504.x. Cossart, E., Mercier, D., Decaulne, A., Feuillet, T., Jónsson, H.P., Sæmundsson, Þ., 2013. Im- Höggerl, N., 2001. Bestimmung von rezenten Höhenänderungen durch wiederholte pacts of post-glacial rebound on landslide spatial distribution at a regional scale in geodätische Messungen. In: Hammerl, C., Lenhardt, W.A., Steinacker, R., northern Iceland (Skagafjörður). Earth Surf. Process. Landf. 39:336–350. http://dx. Steinhauser, P. (Eds.), Die Zentralanstalt für Meteorologie und Geodynamik 1851- doi.org/10.1002/esp.3450. 2001. Leykam, Graz, pp. 630–644. Crosta, G.B., 1996. Landslide, spreading, deep seated gravitational deformation: analysis, Hungr, O., 1981. Dynamics of Rock Avalanches and Other Types of Mass Movements. Ph. examples, problems and proposals. Geogr. Fis Din. Quat. 19, 297–313. D. Thesis. University of Alberta, Edmonton 500 pp. Crosta, G.B., Agliardi, F., 2003. Failure forecast for large rock slides by surface displacement Hungr, O., Evans, S.G., 2004. Entrainment of debris in rock avalanches: an analysis of a measurements. Can. Geotech. J. 40, 176–191. long run-out mechanism. Geol. Soc. Am. Bull. 116, 1240–1252. Crosta, G.B., Frattini, P., Agliardi, F., 2013. Deep seated gravitational slope deformations in Hungr, O., Evans, S.G., Bovis, M., Hutchinson, J., 2001. Review of the classification of land- the European Alps. Tectonophysics 605:13–33. http://dx.doi.org/10.1016/j.tecto. slides of the flow type. Environ. Eng. Geosci. 8, 221–238. 2013.04.028. Ivy-Ochs, S., Kerschner, H., Kubik, P.W., Schlüchter, C., 2006. Glacier response in the Euro- Cruden, D.M., Varnes, D.J., 1996. Landslide types and processes. In: Turner, A., Schuster, R. pean Alps to Heinrich event 1 cooling: the Gschnitz stadial. J. Quat. Sci. 21:115–130. (Eds.), Landslides: Investigation and Mitigation (special report) vol. 247. National Re- http://dx.doi.org/10.1002/jqs.955. search Council, Transportation and Research Board Special Report, Washington, DC, Jiménez-Munt, I., Garcia-Castellanos, D., Negredo, A.M., Platt, J.P., 2005. Gravitational and USA, pp. 36–75. tectonic forces controlling postcollisional deformation and the present-day stress Drimmel, J., 1980. Rezente Seismizität und Seismotektonik des Ostalpenraumes. In: field of the Alps: constraints from numerical modeling. Tectonics 24, TC5009. Oberhauser, R. (Ed.), Der geologische Aufbau Österreichs, pp. 507–527. http://dx.doi.org/10.1029/2004TC001754. Eberhardt, E., Stead, D., Coggan, J.S., 2004. Numerical analysis of initiation and progressive Kerschner, H., 2009. Gletscher und Klima im Alpinen Späatglazial und frühen Holozän. In: failure in natural rock slopes – the 1991 Randa rockslide. Int. J. Rock Mech. Min. Sci. Schmidt, R., Matulla, C., Psenner, R. (Eds.), Klimawandel in Österreich. Die letzten 41, 69–87. 20.000 Jahre und ein Blick voraus. Innsbruck University Press, Innsbruck, pp. 5–26. Evans, S.G., Couture, R., 2002. The 1965 Hope Slide, British Columbia; catastrophic failure Kerschner, H., Ivy-Ochs, S., 2007. Palaeoclimate from glaciers: examples from the Eastern of a sagging rock slope. Geological Society of America, Abstracts with Programs vol. Alps during the Alpine Lateglacial and early Holocene. Glob. Planet. Chang. 60, 58–71. 34, pp. 16–26. Kerschner, H., Kaser, G., Sailer, R., 2000. Alpine Younger Dryas glaciers as paleo-precipita- Evans, S.G., Scarascia-Mugnozza, G., Strom, A., Hermanns, R., Ischuk, A., Vinnichenko, S. tion gauges. Ann. Glaciol. 31, 80–84. (Eds.), 2006. Landslides From Massive Rock Slope Failure and Associated Phenomena. Kummerow, J., Kind, R., 2006. Shear wave splitting in the Eastern Alps observed at the Springer, pp. 3–52. TRANSALP network. Tectonophysics 414:117–125. http://dx.doi.org/10.1016/j.tecto. Frisch, W., Dunkl, I., Kuhlemann, J., 2000a. Post-collisional orogen-parallel large-scale ex- 2005.10.023. tension in the Eastern Alps. Tectonophysics 327, 239–265. Lenhardt, W.A., 2007. Earthquake-triggered landslides in Austria – Dobratsch revisited. Frisch, W., Székely, B., Kuhlemann, J., Dunkl, I., 2000b. Geomorphological evolution of the Jahrb. Geol. Bundesanst. 147, 193–199. Eastern Alps in response to Miocene tectonics. Z. Geomorphol., Neue Folge 44, 103- Lenhardt, W.A., Freudenthaler, C., Lippitsch, R., Fiegweil, E., 2007. Focal-depth distribu- 138. tions in the Austrian Eastern Alps based on macroseismic data. Austrian J. Earth Sci. Fügenschuh, B., Seward, D., Mancktelow, N.S., 1997. Exhumation in a convergent orogen: 100, 66–79. the western Tauern window. Terra Nova 9, 213–217. Linzer, H.-G., Ratschbacher, L., Frisch, W., 1995. Transpressional collision structures in the Fügenschuh, B., Mancktelow, N.S., Schmid, S.M., 2012. Comment on Rosenberg and upper crust: the fold-thrust belt of the Northern Calcareous Alps. Tectonophysics 242, Garcia: estimating displacement along the Brenner Fault and orogen-parallel exten- 41–61. sion in the Eastern Alps. Int. J. Earth Sci. 101:1451–1455. http://dx.doi.org/10.1007/ Linzer, H.-G., Decker, K., Peresson, H., Dell'Mour, R., Frisch, W., 2002. Balancing lateral oro- s00531-011-0725-4. genic float of the Eastern Alps. Tectonophysics 354, 211–237. Gisler, M., Fäh, D., Kästli, P., 2004. Historical seismicity in Central Switzerland. Eclogae Massironi, M., Bistacchi, A., Dal Piaz, G., Monopoli, B., Schiavo, A., 2003. Structural control Geol. Helv. 97:221–236. http://dx.doi.org/10.1007/s00015-004-1128-3. on mass-movement evolution: a case study from the Vizze Valley, Italian Eastern Gruber, A., Strauhal, T., Prager, C., Reitner, J., Brandner, R., Zangerl, C., 2009. Die Alps. Eclogae Geol. Helv. 96, 85–98. ‘Butterbichl-Gleitmasse’–eine große fossile Massenbewegung am Südrand der McCalpin, J.P., 2009. Paleoseismology in extensional tectonic environments. In: McCalpin, Nördlichen Kalkalpen (Tirol, Österreich). Swiss Bull. Angewandte Geologie 14, J.P. (Ed.), Paleoseismology. Academic Press, Burlington, pp. 171–269. 103–134. Moro, M., Saroli, M., Salvi, S., Stramondo, S., Doumaz, F., 2007. The relationship between Handy, M.R., Ustaszewski, K., Kissling, E., 2015. Reconstructing the Alps-Carpathians– seismic deformation and deep-seated gravitational movements during the 1997 Um- Dinarides as a key to understanding switches in subduction polarity, slab gaps and bria-Marche (Central Italy) earthquakes. Geomorphology 89:297–307. http://dx.doi. surface motion. Int. J. Earth Sci. 104:1–26. http://dx.doi.org/10.1007/s00531-014- org/10.1016/j.geomorph.2006.12.013. 1060-3. Moro, M., Saroli, M., Tolomei, C., Salvi, S., 2009. Insights on the kinematics of deep-seated Harp, E.L., Wilson, R.C., 1995. Shaking intensity thresholds for rock falls and slides: evi- gravitational slope deformations along the 1915 Avezzano earthquake fault (Central dence from 1987 Whittier Narrows and Superstition Hills earthquake strong-motion Italy), from time-series DInSAR. Geomorphology 112:261–276. http://dx.doi.org/10. records. Bull. Seismol. Soc. Am. 85, 1739–1757. 1016/j.geomorph.2009.06.011. Heidbach, O., Tingay, M., Barth, A., Reinecker, J., Kurfe, D., Müller, B., 2008. The World Moro, M., Saroli, M., Gori, S., Falcucci, E., Galadini, F., Messina, P., 2012. The interaction be- Stress Map Database Release. http://dx.doi.org/10.1594/GFZ.WSM Rel2008 online tween active normal faulting and large scale gravitational mass movements revealed at. www.world-stressmap.org. by paleoseismological techniques: a case study from Central Italy. Geomorphology Heiri, O., Koinig, K., Spötl, C., Barrett, S., Brauer, A., Drescher-Schneider, R., Gaar, D., Ivy- 151–152:164–174. http://dx.doi.org/10.1016/j.geomorph.2012.01.026. Ochs, S., Kerschner, H., Luetscher, M., Moran, A., Nicolussi, K., Preusser, F., Schmidt, Nasir, A., Lenhardt, W., Hintersberger, E., Decker, K., 2013. Assessing the completeness R., Schoeneich, P., Schwörer, C., Sprafke, T., Terhorst, B., Tinner, W., 2014. and instrumental earthquake data in Austria and the surrounding areas. Austrian Palaeoclimate records 60-8 ka in the Austrian and Swiss Alps and their forelands. J. Earth Sci. 106, 90–102. Quat. Sci. Rev. http://dx.doi.org/10.1016/j.Quascirev.2014.05.021. Norton, K.P., Hampel, A., 2010. Postglacial rebound promotes glacial re-advances – acase Hering, G., Langheinrich, G., Wilczewski, N., 1992. Der prähistorische Bergsturz von study from the European Alps. Terra Nova 22:297–302. http://dx.doi.org/10.1111/j. Pfannes/Südtirol/Italien. Eiszeitalter u. Gegenwart vol. 42, pp. 115–120. 1365-3121.2010.00946.x. Hermanns, R.L., Longva, O., 2012. Rapid rock-slope failures. In: Clague, J.J., Stead, D. (Eds.), Ortner, H., Reiter, F., Brandner, R., 2006. Kinematics of the Inntal shear zone–sub-Tauern Landslides: Types, Mechanisms and Modelling. Cambridge University Press, Cam- ramp fault system and the interpretation of the TRANSALP seismic section, Eastern bridge, pp. 59–70. Alps, Austria. Tectonophysics 414:241–258. http://dx.doi.org/10.1016/j.tecto.2005. Hermanns, R.L., Niedermann, S., 2011. Late Pleistocene - early Holocene paleoseismicity 10.017. deduced from lake sediment deformation and coeval landsliding in the Calchaquíes Ortner, H., Pomella, H., Sanders, D., 2014. Formation of cataclasites in shallow-subsurface valleys, NW Argentina. Geological Criteria for Evaluating Seismicity Revisited: Forty settings – meteoric diagenetic processes control fault rock formation at seismogenic Years of Paleoseismic Investigations and the Natural Record of Past Earthquakes. Spe- faults in the Abruzzi Apennines, Italy. Geophysical Research Abstracts 16, EGU2014- cial Paper Geol. Soc. Am. vol. 479:pp. 181–194. http://dx.doi.org/10.1130/2011. 6971-1. 2479(08). Ostermann, M., Sanders, D., 2012. Post-glacial rockslides in a 200 × 130-km area of the Hermanns, R.L., Strecker, M.R., 1999. Structural and lithological controls on large quater- Alps: characteristics, ages, and uncertainties. In: Eberhardt, E., Froese, C., Turner, K., nary rock avalanches (sturzstroms) in arid Northwest Argentina. Geol. Soc. Am. Bull. Leroueil, S. (Eds.), Landslides and Engineered Slopes. Protecting Society through Im- 111, 934–948. proved Understanding. CRC Press, Boca Raton, pp. 659–663. M. Ostermann, D. Sanders / Geomorphology 289 (2017) 44–59 59

Ostermann, M., Prager, C., 2014. Major Holocene Rock Slope Failures in the Upper Inn- Sanders, D., Ostermann, M., Brandner, R., Prager, C., 2010. Meteoric lithification of cata- and Ötz valley region (Tyrol, Austria). In: Kerschner, H., Krainer, K., Spötl, C. (Eds.), strophic rockslide deposits: diagenesis and significance. Sediment. Geol. 223, From the foreland to the Central Alps – Field trips to selected sites of Quaternary re- 2150–2161. search in the Tyrolean and Bavarian Alps. Geozon, Berlin, pp. 116–127. Scharf, A., Handy, M.R., Favaro, S., Schmid, S.M., Bertrand, A., 2013. Modes of orogen-par- Ostermann, M., Sanders, D., Prager, C., Kramers, J., 2007. Aragonite and calcite cementa- allel stretching and extensional exhumation in response to microplate indentation tion in ‘boulder-controlled’ meteoric environments on the Fern pass rockslide (Aus- and roll-back subduction (Tauern Window, Eastern Alps). Int. J. Earth Sci. 102: tria): implications for radiometric age-dating of catastrophic mass movements. 1627–1654. http://dx.doi.org/10.1007/s00531-013-0894-4. Facies 53, 189–208. Schmid, S.M., Fügenschuh, B., Kissling, E., Schuster, R., 2004. Tectonic map and overall ar- Ostermann, M., Sanders, D., Ivy-Ochs, S., Alfimov, V., Rockenschaub, M., Römer, A., 2012. chitecture of the Alpine orogen. Eclogae Geol. Helv. 97, 93–117. Early Holocene (8.6 ka) rock avalanche deposits, Obernberg (Eastern Alps): aspects Schnellmann, M., Anselmetti, F.S., Giardini, D., Mac Kenzie, J., Ward, S.N., 2002. Prehistoric of landform interpretation and kinematics of rapid mass movement. Geomorphology earthquake history revealed by lacustrine slump deposits. Geology 30, 1131–1134. 171–172, 83–93. Sherard, J.L., Woodward, R.J., Gizienski, S.F., Clevenger, W.A., 1963. Earth and Earth-rock Penna, L.M., Hermanns, R.L., Niedermann, S., Folguera, A., 2011. Multiple slope failures as- Dams. Wiley, New York 722 pp. sociated with neotectonic activity in the Southern Central Andes (37°–37°30′S), Pat- Sölva, H., Grasemann, B., Thöni, M., Thiede, R., Habler, G., 2005. The Schneeberg normal agonia, Argentina. Geol. Soc. Am. Bull. 123:1880–1895. http://dx.doi.org/10.1130/ fault zone: normal faulting associated with cretaceous SE-directed extrusion in the B30399.1. Eastern Alps (Italy/Austria). Tectonophysics 401, 143–166. Persaud, M., Pfiffner, A., 2004. Active deformation in the eastern Swiss Alps: post-glacial Stock, G.M., Uhrhammer, R., 2010. Catastrophic rock avalanche 3600 years BP from El faults, seismicity and surface uplift. Tectonophysics 385, 59–84. Capitan, Yosemite Valley, California. Earth Surf. Process. Landf. 35, 941–951. Pfiffner, A., 2009. Geologie Der Alpen. Haupt Verlag, Bern 359 pp. Terzaghi, K., 1962. Stability of steep slopes on hard weathered rock. Géotechnique 12, Pierson, T.C., 1998. An empirical method for estimating travel times for wet volcanic mass 251–270. flows. Bull. Volcanol. 60, 98–109. Trauth, M.H., Alonso, R.A., Haselton, K.R., Hermanns, R.L., Strecker, M.R., 2000. Climate Pirulli, M., 2009. The Thurwieser rock avalanche (Italian Alps): description and dynamic change and mass movements in the NW argentine Andes. Earth Planet. Sci. Lett. analysis. Eng. Geol. 109, 80–92. 179:243–256. http://dx.doi.org/10.1016/S0012-821X(00)00127-8. Prager, C., Zangerl, C., Patzelt, G., Brandner, R., 2008. Age distribution of fossil landslides in Ustaszewski, M., Hampel, A., Pfiffner, A., 2008. Composite faults in the Swiss alps formed the Tyrol (Austria) and its surrounding areas. Nat. Hazards Earth Syst. Sci. 8, 377–407. by the interplay of tectonics, gravitation and postglacial rebound: an integrated field Prager, C., Ivy-Ochs, S., Ostermann, M., Synal, H.-A., Patzelt, G., 2009. Geology and radio- and modelling study. Swiss J. Geosci. 101:223–235. http://dx.doi.org/10.1007/ metric 14C-, 36Cl- and Th−/U-dating of the Fernpass rockslide (Tyrol, Austria). Geo- s00015-008-1249-1. morphology 103, 93–103. van Husen, D., 1987. 24 pp. Die Ostalpen in Den Eiszeiten. Populärwissenschaftliche Preh, A., Sausgruber, J.T., 2015. The extraordinary rock-snow avalanche of Alpl, Tyrol, Aus- Veröffentlichungen Der Geologischen Bundesanstalt. Geologische Bundesanstalt, tria. Is it possible to predict the runout by means of single-phase Voellmy- or Cou- Vienna. lomb-type models? In: Lollino, G., Giordan, D., Crosta, G.B., Corominas, J., Azzam, R., van Husen, D., 1999. Geological processes during the quaternary. Mitt. Österr. Geol. Ges. Wasowski, J., Sciarra, N. (Eds.), Engineering Geology for Society and 92, 135–156. TerritoryLandslide Processes vol. 2. Springer, Cham, Switzerland, pp. 1907–1911 van Husen, D., Reitner, J., 2011. An outline of the Quaternary stratigraphy of Austria. E&G Radbruch-Hall, D., 1978. Gravitational creep of rock masses on slopes. In: Voight, B. (Ed.), Quat. Sci. J. 60:366–387. http://dx.doi.org/10.3285/eg.60.2-3.09. Rockslides and Avalanches — Natural PhenomenaDevelopments in Geotechnical En- Varnes, D.J., Radbruch-Hall, D., Varnes, K.L., Smith, W.K., Savage, W.Z., 1990. Measuremt of gineering vol. 14. Elsevier, Amsterdam, pp. 608–657. ridge-spreading movements (sackungen) at Bald Eagle Mountain, Lake Country, Col- Ratschbacher, L., Frisch, W., Linzer, H.-G., Merle, O., 1991. Lateral extrusion in the eastern orado, 1975-1989. USGS Open-File Report, pp. 90–543. Alps, Part 2: Structural analysis. Tectonics 10, 257–271. Vollweiler, N., Scholz, D., Mühlinghaus, C., Mangini, A., Spötl, C., 2006. A precisely dated Reimer, P.J., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Grootes, P.M., climate record for the last 9 kyr from three high alpine stalagmites, Spannagel Guilderson, T.P., Haflidason, H., Hajdas, I., Hatté, C., Heaton, T.J., Hoffmann, D.L., Hogg, Cave, Austria. Geophys. Res. Lett. 33, L20703. A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., Manning, S.W., Niu, M., Reimer, R.W., Wells, D.L., Coppersmith, K.J., 1994. New empirical relationships among magnitude, rup- Richards, D.A., Scott, E.M., Southon, J.R., Staff, R.A., Turney, ChSM., van der Plicht, J., ture length, rupture width, rupture area, and surface displacement. Bull. Seismol. Soc. 2013. IntCal13 and Marine13 radiocarbon age calibration curves 0-50,000 years cal Am. 84, 974–1002 Cambridge. BP. Radiocarbon 55, 1869–1887. Westreicher, F., Kerschner, H., Nicolussi, K., Ivy-Ochs, S., Prager, C., 2014. Ein Bergsturz am Reinecker, J., Lenhardt, W.A., 1999. Present-day stress field and deformation in eastern Mieminger Plateau oder wie aus einer "postglazialen Moräne" ein holozäner Austria. Int. J. Earth Sci. 88, 532–550. Bergsturz wurde. In: Koinig, K.A., Starnberger, R., Spötl, C. (Eds.), Deuqua 2014, Reiter, F., Lenhardt, W.A., Brandner, R., 2005. Indications for activity of the Brenner Nor- Hauptversammlung der Deutschen Quartärvereinigung, Innsbruck 2014 37. Inns- mal Fault zone (Tyrol, Austria) from seismological and GPS data. Austrian J. Earth bruck University Press, Conference Series, Universität Innsbruck, pp. 149–150. Sci. 97, 16–23. Zerathe, S., Lebourg, T., Braucher, R., Bourlès, D., 2014. Mid-Holocene cluster of large-scale Sanchez, G., Rolland, Y., Corsini, M., Braucher, R., Bourlès, D., Arnold, M., Aumaître, G., landslides revealed in the Southwestern Alps by 36Cl dating. Insight on an Alpine- 2009. Relationships between tectonics, slope instability and climate change: cosmic scale landslide activity. Quat. Sci. Rev. 90, 106–127. ray exposure dating of active faults, landslides and glacial surfaces in the SW Alps. Zischinsky, U., 1969. Über sackungen. Rock Mech. 1, 30–52. Geomorphology 117, 1–13. Zorzi, L., 2013. From Deep Seated Gravitational Movements to Rock Avalanches: The Role Sanders, D., 2015. Post-glacial rock avalanche causing epigenetic gorge incision of Failure Mechanism in Sudden Rock Collapse. Unpublished PhD-thesis. University (Strassberg gorge, Eastern Alps). Geophysical Research Abstracts vol. 17. EGU2015, of Padua 220 pp. p. 2283. Sanders, D., 2016. Neotectonic deformation in quaternary deposits, Northern Calcareous Alps (abstr.). GeoTirol 2016–Annual Meeting DGGV, Abstract, p. vol. 294.