A structural, geomorphological and InSAR study of the unstable rock slope in Oksfjellet, Kåfjord, Troms

Abstract

The main focus of this study has been to combine field observations of structural and geomorphological elements and interferometric synthetic aperture radar (InSAR) to investigate the unstable rock slope in Oksfjellet. The site is located along a NNE faced slope in a glacially eroded valley in Kåfjord, Troms. The Caledonian bedrock comprises mainly mica schists and display ductile (Caledonian) and brittle (post-Caledonian) structures. Brittle faults and fractures trending NE-SW and NW-SE and a ductile shear zone/thrust seem to have controlled the location of the unstable rock slope. Displacements detected by InSAR correlated well to bedrock structures and morphological elements observed in the field. Satellite InSAR detected displacements, down to a rate of less than 1 mm/year, that is thought to reflect geological processes. It can be concluded that the combination of satellite InSAR and ground-based InSAR provided additional and important information that complemented field observations, and proved valuable in order to assess displacements patterns at different temporal and spatial scales. Toppling and wedge failures along pre-existing post-Caledonian fractures are the main failure mechanisms in Oksfjellet, but sliding along foliation surfaces is also a viable failure mechanism. A deep-seated sliding surface is indicated along the thrust parallel foliation or along the intersection between foliation/thrust and fractures, producing an overall down to the NNE movement of the unstable area. The slope is classified as a complex field with similarities to deep-seated gravitational slope deformations. Fractures and faults delimiting the unstable rock slope at Oksfjellet coincides with NNE-SSW to NE-SW and NW-SE trending regional Mesozoic brittle structures.