Composite faults in the Swiss Alps formed by the interplay of tectonics, gravitation and postglacial rebound: an integrated field and modelling study

Abstract

Along the flanks of several valleys in the Swiss Alps, well-preserved fault scarps occur between 1900 and 2400 m altitude, which reveal uplift of the valley-side block relative to the mountain-side block. The height of these uphill-facing scarps varies between 0.5 m and more than 10 m along strike of the fault traces, which usually trend parallel to the valley axes. The formation of the scarps is generally attributed either to tectonic movements or gravitational slope instabilities. Here we combine field data and numerical experiments to show that the scarps may be of composite origin, i.e. that tectonic and gravitational processes as well as postglacial differential uplift may have contributed to their formation. Tectonic displacement may occur as the fault scarps run parallel to older tectonic faults. The tectonic component seems, however, to be minor as the studied valleys lack seismic activity. A large gravitational component, which is feasible owing to the steep dip of the schistosity and lithologic boundaries in the studied valleys, is indicated by the uneven morphology of the scarps, which is typical of slope movements. Postglacial differential uplift of the valley floor with respect to the summits provides a third feasible mechanism for scarp formation, as the scarps are postglacial in age and occur on the flanks of valleys that were filled with ice during the last glacial maximum. Finite-element experiments show that postglacial unloading and rebound can initiate slip on steeply dipping pre-existing weak zones and explain part of the observed scarp height. From our field and modelling results we conclude that the formation of uphill-facing scarps is primarily promoted by a steeply dipping schistosity striking parallel to the valley axes and, in addition, by mechanically weaker rocks in the valley with respect to the summits. Our findings imply that the identification of surface expressions related to active faults can be hindered by similar morphologic structures of non-tectonic origin.