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

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.