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.

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Spatial occurrence, structural architecture and formation of brittle fault zones and joints are investigated by outcrop observations, scanline mapping, and light-and scanning electron microscopy in an anisotropic crystalline rock mass (e.g. granites, para-gneisses and schists) of the central Gotthard massif in the Swiss Alps. The analysis presented illustrates that several pre-fault anisotropic features (i.e. dykes, ductile shear zones, foliation and presumably a pre-existing meso-scale fracture set) control the nucleation and propagation of brittle faults. Three sets of brittle fault zones striking NE-SW, NNE-SSW and WNW-ESE can be distinguished. They formed through cataclasis at temperatures below 300 °C, and were activated predominately in a strike-slip regime. Up to five joint sets were mapped and characterized according to orientation, frequency, spacing and formation. Finally a regional fan structure was established in the Gotthard Pass area, encompassing the main foliation, steeply dipping joints and brittle fault zones, each of which shows the same orientation and location of the symmetry plane (NE-SW orientated). ZUSAMMENFASSUNG Der vorliegende Artikel präsentiert Untersuchungen über den Aufbau, die räumliche Verteilung und die Entstehung von spröden Störungszonen und Kluftstrukturen in anistropen Gesteinen des zentralen Gotthardmassivs der Schweizer Alpen. Dabei wurde auch der Einfluss unterschiedlichster bereits existierender Strukturen, wie z.B. Intrusionsgänge, duktile Scherzonen, Schieferungsflächen oder Klüfte auf die Entstehung und Ausbildung von jungalpinen spröden Störungszonen im Detail untersucht. Die Ergebnisse basieren auf Aufschlussbeobachtungen, "Scanline-Aufnahmen" und Licht-bzw Elektronenmikroskopischen Untersuchungen. Drei Störungssysteme, ein NE-SW streichendes Hauptsystem, ein NNE-SSW und ein WNW-ESE streichendes System, welche vorwiegend als Blattverschiebungen fungieren, wurden kartiert. Entsprechend den lithologischen Einheiten konnten bis zu 5 verschiedene Kluftfamilien unterschieden und im Hinblick auf deren geometrische Parameter und Entstehung analysiert werden. Die im zentralen Gotthardmassiv bekannte Fächerstruktur der Hauptfoliation wird auch durch ein Kluftsystem und spröde Störungszonen mit gemeinsamer NE-SW streichender Symmetrieebene abgebildet. to existing hydroelectric dams. In the Gotthard Pass area, the spatial relationship between large water inflow rates into the Gotthard A2 highway tunnel (built in the 1970's), flowing from two distinct brittle fault zones intersecting the tunnel, and the maximum surface subsidence measured along the Gotthard pass road, above and adjacent to the tunnel, can be directly correlated (Zangerl et al. 2003). A hypothesis was thus developed that these brittle fault zones may act as highly permeable conduits, which can undergo large normal-and shear displacements through rock mass consolidation processes as the stress