1 Introduction

Compiled field and literature data show that landslides are geologically pre-disposed by the lithological, structural and morphological setting. Concerning the Alps, radiometric age dating data suggest that several active moving landslides represent "old" slope deformations that have been (re-) activated for some 1000 yrs. At several locations, densely foliated and fractured metamorphic rocks (phyllites, schists, gneisses) are widely spread and affected by slowly moving deep-seated rock slides.

According to Cruden and Varnes (1996) rock slides are characterized by a downslope movement of a rock mass on surfaces of rupture i.e. relatively thin zones of intensive shear strain. The landslides presented herein may be classified as "rock compound slides" (according to Varne's classification updated by Hungr et al. (2014)). A rock compound slide is characterized by sliding of a mass of rock on a rupture surface consisting of several planes, or an uneven surface, so that motion is kinematically feasible only if accompanied by significant internal distortion of the moving mass. Distinct extensional features i.e. horst- and graben structures at the head as well secondary shear surfaces are characteristic features of this type of landslide.

Furthermore, this type of landslides shows fully persistent basal shear zones, and in some cases also internal shear zones formed through progressive failure processes. It is commonly agreed that shear zones of rock slides originate from to the growth, propagation and coalescence of pre-existing and new fractures as well as rock cataclasis. Based on a variety of field surveys and monitoring data, several slopes show actual gravitational deformation that is determined by sliding processes resulting from shear deformation along discrete shear zones.

Generally, there is little knowledge about the temporal and spatial evolution of shear zones. In addition, there is also a lack in understanding how shear zones form by progressive failure processes. As a consequence, the assessment if a fully persistent basal shear zone has formed is still difficult when direct subsurface investigations such as core drillings or investigation adits are not available.

This contribution aims to highlight open questions for future research rather than to provide final outcomes and conclusions. Beside others, one aim is to initiate a fruitful discussion focusing on the formation and geometry of shear zones of deep-seated rock slides. Field observations focusing on rock mass failure processes of slides and conceptual considerations focusing on the formation of shear zones are presented. Emphasis is given on surface investigations based on mapping and monitoring methods and their ability to develop reliable 3D rock slide models for slope analyses and hazard assessments.

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