A structural, geomorphological and InSAR study of the unstable rock slopes at Mellomfjellet, Nordreisa

Abstract

The focus of this study has been to combine structural data with morphological observations and displacement data to characterise the unstable rock slopes at Mellomfjellet. The site contains two instabilities (Mellomfjellet 1 and 2) and is located along a west-facing slope in Reisadalen, a glacially eroded valley, in Nordreisa, Troms. The bedrock consists mainly of Caledonian amphibolite and hornblende schist, displaying Caledonian ductile and Post-Caledonian brittle structures. NNW-SSE and NNE-SSW-striking backscarps delimit the unstable area and align with the strike of the Post-Caledonian regional brittle structures. The backscarps seem to control the extent of the unstable rock slopes, as displacement from InSAR shows a downslope movement inside the areas delimited by the backscarps. Structural analysis based on traditional field data revealed two structural domains (Mellomfjellet 1 and 2) each with three joint sets: Mellomfjellet 1: J1 (029/72±16), J2 (288/84±21) and J3 (075/67±12); and Mellomfjellet 2: J1 (022/81±14), J2 (108/88±15) and J3 (075/47±17). J1 and J2 were most dominant joint sets, striking parallel to subparallel with the backscarps. J3 was found only found adjacent to two E-W striking lineaments, cross-cutting the two instabilities. The undulating foliation (155/13±17 and 183/11±15, Mellomfjellet 1 and 2 respectively) is shallowly dipping downslope, and gently folded with a fold axis (237->13) also plunging downslope. Structural analysis based on a photogrammetric point cloud identified both J1, J2 and the foliation, and showed the same orientations as the field data when taking the variance into account. Kinematic analysis and field observations suggest planar sliding along a failure surface stepping between sub vertical joints (J1 and J2) and the foliation. Mellomfjellet 1 shows morphological features similar with a DSGSD, complex fields and a biplanar compound slide. Mellomfjellet 2 show features similar with a slide topple, and a biplanar compound slide. A failure scenario for each of the instabilities was determined, based on the delimiting backscarps and morphological features, where both scenarios lie in the medium hazard class. Mellomfjellet 1 has an estimated volume of 1.12x106 m3 and Mellomfjellet 2 has an estimated volume of 3.05x106 m3. The run-out analysis showed that Mellomfjellet 1 will not reach any infrastructure or buildings, while Mellomfjellet 2 is likely to reach a construction site where a new power line is being established. Both of the instabilities are classified as low risk sites.