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Ostermann and Sanders 2017.Pdf Geomorphology 289 (2017) 44–59 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph 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.
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