Geomorphological study of the rock-slope failure at Adjet, Storfjord, Troms

Abstract

Norway is known for its steep mountains and many fjords. This landscape provides the potential for large rock-slope failures (RSFs) which, in turn, can cause other catastrophic hazards, e.g. tsunamis. In Troms County, northern Norway, many RSFs are found east and west of the Lyngen peninsula, within an area of discontinuous permafrost. The RSF at Adjet is located southeast of the Lyngen peninsula. It is not a hazard for people as such, but it is of interest for better understanding RSFs in general, due to its complexity. The aim of the present study is to gain an understanding of the rock-slope failure at Adjet, its geomorphological features, and the origin and movement of these features. The features were identified and analyzed using different methods such as field mapping, evaluation of aerial and orthophotographs, as well as InSAR data. The investigated RSF area is situated on a west facing mountainside, 500 – 1340 m a.s.l., with a ca. 5 km long ridge delimiting the RSF area to the east. The downslope length of the RSF is ca. 2 km. The failure site is of great size and complexity and contains several different geomorphological and structural features. The predominant lithology is mica schist with various amounts of garnet and/ or quartz. The mountain ridge character above the RSF changes from north to south, from a peak-like ridge and cliffs to a relatively gradual sloping ridge. The study area can therefore be divided into two provinces: province A in the north and province B in the south. These provinces are separated by an ENE-WSW trending fault. Geomorphological features within the RSF area are rock glaciers, debris fields, talus fan deposits, and slide blocks. They originate through different failure processes, such as rockfall, toppling and rock slide. In province A, mainly rock glaciers and debris fields are prominent. In province B, slide blocks and talus fan deposits are dominant features. The source of the rock-slope failure is the mountain wall. Folds, faults and fractures are identified, and include abundant steeply (NE-SW, NW-SE) and moderately (NW-SE) dipping fracture sets. These form conjugate fracture sets. Folds are mainly found in the far north of the study area. It is postulated that disintegrated rocks slide on moderately dipping fault/ fractures and that their extent is determined by steeply dipping fracture sets. In the north, a sliding surface is also provided by fold limbs. Rockfall, toppling and rock slide were identified as failure processes. Movement is indicated by geomorphological features such as transverse ridges and furrows, lobes, and steep barren slopes at rock glaciers. Rockfall at the front of rock glaciers was observed during field work. InSAR data was additionally used to document movement and support movement hypotheses. The study site displays a broad range of velocity values. ranging from no movement to velocities of ca. 15 cm per day. Most prominent are the rock glaciers, which also display the fastest velocities. Several rock-slope failure processes are supposed to have occurred at Adjet. They are alternating with periods of no failure... This is indicated by overlapping of geomorphological features of the same or other category. Thus an evolution of the rock-slope failure at Adjet, of 6 relative ages is suggested at the end.