Structural mapping of the Åknes Rockslide, Møre and Romsdal County, Western Norway

NGU Rapport 2008.042

Åknes is known as the most hazardous rockslide area in Norway at present, and is among the most investigated rockslides in the world, representing an exceptional natural laboratory. This study focuses on structural geology on five zones to interpret and understand the geometry of the rockslide area. The interpretations are further used to build a geological model of the site. This is a large rockslide with an estimated volume of 35-40 million m3 (Derron et al., 2005), defined by a back scarp, a basal shear zone at about 50 meters depth and an interpreted toe zone where the sliding plane daylights the surface. This study resulted in a division of the central zone of the rockslide into four sub-domains, experiencing extension in the upper part and compression in the lower part. Structural mapping of the area indicates that the foliation of the gneiss plays an important role in the development of this rockslide. The upper boundary zone of the rockslide is seen as a back scarp that is controlled by, and parallel to, the pre-existing, steep foliation planes. Where the foliation is not favourably orientated in regard to the extensional trend, the back scarp follows a pre-existing fracture set or forms a relay structure. The foliation in the lower part, dipping 30° to 35° to S-SSE, seems to control the development of the basal sliding surface with its subordinate low angle trusts surfaces, which daylights at different levels. The sliding surfaces are sub-parallel to the topographic slope and are located along mica-rich layers in the foliation