Neotectonics in the Swiss Alps—A late Alpine to postglacially active fault at the Gemmi Pass

Abstract

The area of the central and western Swiss Alps reveals not only the highest uplift rates of Switzerland (1.5mma−1 near Brig, Schlatter & Marti 2002), but also shows a strong concentration of earthquakes (e.g. Deichmann et al. 2004). This raised the question, whether the region hosts any linear topographic expressions that can be attributed to motion along potentially seismogenic faults. The area was therefore chosen for the investigation of postglacially active lineaments. Firstly, aerial photographs from the entire area were searched for linear features, which could be of gravitational or tectonic origin. Subsequently, selected lineaments were visited in the field to study their origin. We found scarce but positive evidence for neotectonic fault movements. One particular lineament that exhibited the most promising exposures was investigated in greater detail. This lineament is a prominent NW–SE striking fault located at the Gemmi Pass, runs perpendicularly to the regional fold axes and cuts through the Helvetic nappe stack. The position and orientation of the fault discounts gravitational reactivation. A close examination of the fault rocks reveals a long term evolution of this fault starting already at a late stage of Alpine nappe emplacement and related deformation.