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Abstract: One of the largest mass movements in the Alps, the fossil Fernpass rockslide in the Northern Calcareous Alps (Tyrol, Austria), was dated absolutely for the first time. Three independent radiometric dating methods were applied to geologically individual sample sites and enabled a cross-checking of the results. Close to the scarp-front, rockslide-dammed torrent deposits indicate a 14C minimum age of at least 3380–3080 cal. yrs BP. The chronostratigraphic base of this backwater sequence has not been dated yet, but is assumed to date somewhat older into the middle Holocene. However, this coincides with two cosmogenic radionuclide 36Cl ages of sliding planes at the steep and rugged scarp. Exposure data thus obtained are 3400±900 and 4800±1200 yrs respectively with an arithmetic mean of 4100±1300 yrs. Additional age data were acquired from post-depositional calcareous cements, which locally lithified the rockslide debris. These meteoric cements contain remarkably high Uranium total contents and yielded a 230Th/234U minimum age of about 4150±100 yrs for the accumulation of the rockslide deposits. All dating data agree well and indicate that the Fernpass rockslide is of Subboreal age. Thus the intensively structured present morphology, characterised by the well-known cone-shaped Toma-hills and several lakes in the depressions, is indicative of primary rockslide morphology and clearly not shaped by late-glacial ice as assumed formerly. [Copyright &y& Elsevier]

This study presents a multi-disciplinary approach for combining geological-geomorphological field mapping with different surface deformation monitoring techniques (i.e. extensometer, tachymetry, photogrammetry, airborne laser scanning (ALS), terrestrial laser scanning (TLS), uncrewed aerial vehicle (UAV)) to evaluate the impact of structural features on the formation process and deformation behaviour of the deep-seated Marzellkamm rock compound slide. The investigated rock slide is located in a high-alpine, glacier retreat environment in Northern Tyrol (Ötztal Alps, Austria) and formed in a well-foliated, fractured metamorphic rock mass. The total volume of the rock slide of approximately 13 Mm3 was estimated based on geographic information system (GIS) analysis by comparing the surface topography and reconstructed geometry of the basal shear zone. In the upper part of the rock slide, annual mean velocities of up to 0.3 m/year were measured by a combined tachymetric-global navigation satellite system (GNSS) procedure. Based on multi-temporal terrain model analyses, the highest sliding velocities were obtained at the central slope foot area, reaching more than 1 m/year. Geological-geomorphological field mapping showed that the rock slide can be subdivided into two large rock slide systems with different geometry, formation age, and current activity. Furthermore, structural and geomorphological features (i.e. main and minor scarps, graben-structures, downhill- and uphill-facing scarps, tensile fractures, trenches), as well as surface deformation data, indicate the formation of six rock slide slabs. These slabs move downwards at different velocities and are separated by discrete shear zones, where slope displacement primarily accumulates. Large offsets along these shear zones indicate that internal slab deformation has only limited influence on the overall rock slide behaviour. At the Marzellkamm rock slide, pre-existing tectonic fault zones and planes were reactivated and used as weakness zones, playing a crucial role in the overall rock slide geometry and internal separation of the rock mass into slabs. Field survey and monitoring data suggest that a fully persistent, curved, non-circular basal shear zone has developed. Based on comprehensive analyses of mapping and deformation monitoring data, a geological-geometrical and kinematical model of the Marzellkamm rock slide was developed, providing a basis for site-specific hazard assessment and numerical modelling. • Two rock slide systems of six rock slide slabs of different deformation behaviour • Significant influence of pre-existing geological structures on rock slide formation • Deformation is controlled by the magnitude of shearing on separating shear zones. • Field survey indicates a fully persistent, curved, non-circular basal shear zone. • Two rock fall/avalanche events occurred at the lower part of the most active slabs. [ABSTRACT FROM AUTHOR]