Rock-slope failures in Norway - temporal development and climatic conditioning

Abstract

In previously glaciated mountain regions rock-slope failure processes contribute significantly to landscape development and may pose a direct or indirect threat to the population and infrastructure in inhabited areas. In Norway, most of the population lives along fjords and valleys which are highly exposed to rock-slope failures and potential secondary effects, such as displacement waves and catastrophical flooding due to the breaching of landslide dams. To minimise potential consequences, it is important to understand the preparatory factors destabilising rock slopes before they fail catastrophically. Rock-slope destabilisation in paraglacial landscapes is driven by several internal and external factors, adding to the structural pre-conditions, which are a significant component in crystalline rocks. In this thesis, it is demonstrated that the temporal distribution of catastrophic rock-slope failures (CRSF) in Norway is strongly linked to climatic factors, such as ground temperatures and permafrost, as well as to debuttressing effects after deglaciation. For the age determination of rock-slope failure events, terrestrial cosmogenic nuclide (TCN) dating techniques were applied, which was complemented with Quaternary geological mapping. As a first step, uncertainty of inheritance was evaluated on a recent rock avalanche in Patagonia (Chile), determining the amount of inherited 10Be concentrations in each sample. The analysis showed that CRSF boulders are likely to by affected by inheritance, leading to a general age overestimation. The effect, however, is dependent on the real exposure age and the exposure and burial history of the pre-failure surface. In northern and western Norway, the failure timing of several pre-historic CRSFs at Rombakstøtta in Nordland, at Mannen in Møre og Romsdal and at Ramnanosi in Sogn og Fjordane was determined. During the last decades, 10Be dating has become increasingly popular for dating such rock-slope failure deposits. In this thesis, the approach was taken one step further, targeting near vertical sliding surfaces of actively deforming rock-slope instabilities for surface exposure dating. This allows for an estimation of the timing of initial failure and the subsequent rates of progressive deformation. The deformation history of six rock-slope instabilities was analysed, including Skjeringahaugane, Oppstadhornet and Mannen in western Norway, and Revdalsfjellet 1, Revdalsfjellet 2 and Gamanjunni 3 in northern Norway. To set this into the context of climatic variations, the results of three temperature and precipitation reconstructions are analysed using one of them as forcing for a long-term reconstruction of the permafrost distribution in rock-walls. The temporal distribution of CRSF events in Norway generally peaks shortly after deglaciation. At five sites, rock-slope failure activity was observed to follow the deglaciation closely, including the initial failure timing of at least two rock-slope instabilities. This early destabilisation is most likely related to debuttressing effects, when the glacial ice as counterweight was removed from the oversteepened rock slopes. At the Mannen site, a ‘stability crisis’ was identified, where the same slope failed 3-6 times within a few hundred years. Climatic conditions related to permafrost degradation and increased precipitation may have added to the sudden slope destabilisation 4.9±0.6 ka ago. The initial failure timing of four rock-slope instabilities fall into the period of the Holocene thermal maximum (HTM). At Mannen (62◦N, 1295 m asl.) and Revdalsfjellet 2 (69◦N, 650 m asl.), the modelled ground temperatures were close to or above 0◦C, when deformation started early in the HTM. Late permafrost degradation at Gamanjunni 3 (69◦N, 1200 m asl.), which is located close to Revdalsfjellet, explains a time lag of over two millennia before initial failure. The results of this thesis strengthen the hypothesis that climatic variability and related permafrost fluctuations have an effect on rock-slope stability in Norway. The temporal distribution generally reflects the results of other independent studies, with a peak shortly after deglaciation, high rock-slope failure activity during the HTM and a third period of activity between 5 and 2 ka ago.