From incipient slope instability through slope deformation to catastrophic failure — Different stages of failure development on the Ivasnasen and Vollan rock slopes (western Norway)

Abstract

The long-term evolution of rock slope failures involves different stages, from incipience of slope instability to catastrophic failure, through a more or less long-lasting slope deformation phase that also involves creeping or sliding. Topography, lithology, and structural inheritance are the main intrinsic factors that influence this evolution. Here, we investigate the role of these intrinsic factors on the rock slope failure development of the Ivasnasen and Vollan rock slopes (Sunndal Valley, western Norway) using a multitechnique approach that includes geomorphologic and structural field mapping, kinematic analysis, terrestrial cosmogenic nuclide exposure dating, topographic reconstruction, and deformation quantification. Ivasnasen is a rock slope failure complex with several past rock slope failures and a present unstable rock slope, located on a cataclinal NW-facing slope and developed in augen gneiss. Vollan on the opposite valley side is a deep-seated gravitational slope deformation (DSGSD) affecting the whole mountainside, developed in quartzite in the upper part and micaschist in the lower part. These different lithologies belong to different nappe complexes that were emplaced and folded into a series of syn- and anticlines during the Caledonian orogeny. These folds lead to different lithologies being exposed in different structural orientations on the opposite valley flanks, which in turn leads to different types and evolution of rock slope failures. At Ivasnasen the 45°–55° NW-dipping ductile foliation allowed for a fairly simple planar sliding mechanism for the 1.2millionm3 post-glacial rock slope failure. Failure occurred ca. 3.3ka ago after a short period of prefailure deformation. For the present 2.2millionm3 unstable rock slope at Ivasnasen, a steepening of the foliation at the toe impedes such a mechanism and up to 10m of displacement has not lead to a catastrophic failure yet. The Vollan DSGSD is characterized by a steep major back scarp with its up to 130-m-wide graben, which opened along the subvertical foliation in the quartzite, and a conspicuous array of counter-scarps in the micaschist. The morphologic features are explained by a flexural toppling mechanism in the micaschist, which likely initiated in the lower slope section and propagated retrogressively until reaching the massive quartzite at the back scarp. Flexural toppling cannot solely explain the total along-slope displacement of up to 200m, i.e., an elongation of 28%. Creep along a basal shear zone, which may have developed in the hinge zone of the flexural toppling, combined with shallow valley-dipping joints, likely accounts for the large elongation. Implications of this study for the hazard assessment of unstable rock slopes in Norway include the relatively rapid development of instabilities in cataclinal slopes and the important insights on long-term displacement rates gained from cosmogenic nuclide dating for a better understanding of the evolution of unstable rock slopes.