Age distribution of fossil landslides in the Tyrol (Austria) and its surrounding areas

Abstract

Abstract. Some of the largest mass movements in the Alps cluster spatially in the Tyrol (Austria). Fault-related valley deepening and coalescence of brittle discontinuities structurally controlled the progressive failure and the kinematics of several slopes. To evaluate the spatial and temporal landslide distribution, a first comprehensive compilation of dated mass movements in the Eastern Alps has been made. At present, more than 480 different landslides in the Tyrol and its surrounding areas, including some 120 fossil events, are recorded in a GIS-linked geodatabase. These compiled data show a rather continuous temporal distribution of landslide activities, with (i) some peaks of activity in the early Holocene at about 10 500–9400 cal BP and (ii) in the Tyrol a significant increase of deep-seated rockslides in the Subboreal at about 4200–3000 cal BP. The majority of Holocene mass movements were not directly triggered by deglaciation processes, but clearly took a preparation of some 1000 years, after ice withdrawal, until slopes collapsed. In view of this, several processes that may promote rock strength degradation are discussed. After the Late-Glacial, slope stabilities were affected by stress redistribution and by subcritical crack growth. Fracture propagating processes may have been favoured by glacial loading and unloading, by earthquakes and by pore pressure fluctuations. Repeated dynamic loading, even if at subcritical energy levels, initiates brittle fracture propagation and thus substantially promotes slope instabilities. Compiled age dating shows that several landslides in the Tyrol coincide temporally with the progradation of some larger debris flows in the nearby main valleys and, partially, with glacier advances in the Austrian Central Alps, indicating climatic phases of increased water supply. This gives evidence of elevated pore pressures within the intensely fractured rock masses. As a result, deep-seated gravitational slope deformations are induced by complex and polyphase interactions of lithological and structural parameters, morphological changes, subcritical fracture propagation, variable seismic activity and climatically controlled groundwater flows.

Paraglacial rock-slope stability

“…This finding was both a surprise and a concern to researchers in the European Alps, who had previously assumed that the ancient large landslides were a relic of deglaciation and not a phenomenon that had occurred under climatic conditions similar to present (Abele, 1997). While some major landslides initiated prior to deglaciation (Ambrosi and Crosta, 2006), and some were synchronous with deglaciation (Agliardi et al, 2001;Smith, 2001), the majority of documented cases indicates that large post-glacial failures have typically occurred some thousands of years after ice retreat ( This is a view that is consistent amongst several researchers who link climate changes with increased landslide activity, including debris-and earth-flows Matthews et al, 1997;Dapples et al, 2002;Bertolini et al, 2004;Soldati et al, 2004;Prager et al, 2007;Prager et al, 2008). Abele (1997) however, suggested that it is unlikely that temporal patterns of large, deep-seated rock slope failures would match climatic patterns.…”

The Chironico landslide (Valle Leventina, southern Swiss Alps): age and evolution

“…Some of these preparatory conditions include gradual rock weathering, frost action, changes in pore water pressure and stress redistribution during or after glacial unloading (e.g. Gunzburger et al 2005;Prager et al 2008;Korup et al 2010). Important triggering factors are heavy rainfall or earthquakes, which have as a consequence a direct collapse of the slope, as shown by more recent events in Alpine realm (Fritsche and Fäh 2009).…”

The Gradenbach Observatory—monitoring deep-seated gravitational slope deformation by geodetic, hydrological, and seismological methods

“…Brückl and Parotidis (2005) introduced the concept of subcritical crack growth as the process which controls the velocity of quasicontinuous deformation of the main body of a mass movement, but also the sliding velocity at the basal surface of rupture. Prager et al (2008Prager et al ( , 2011) also considered subcritical crack growth as a process controlling the development of DGSD. According to Brückl and Parotidis (2005), one fundamental idea is that asperities at the sliding surface must be overcome by subcritical crack growth.…”

“…The other deformation network points are at most 150 m away from the strain rosette. Network surveys were carried out in June of 2007, 2008, 2009and in December 2011. Five sets of slant distances, directions, and zenith distances were observed at three network points during each survey.…”