Neotectonic faulting, uplift and seismicity in the central and western Swiss Alps

Abstract

Our study aims to characterize the post-glacial neotectonic activity by finding surface expressions of recently active tectonic faults. The central and western Swiss Alps were chosen as the study area because surface uplift rates are very high, indicating ongoing uplift of the external basement massifs. Moreover, the Valais area coincides with enhanced seismic activity. Active faults were searched by mapping lineaments on aerial photographs and subsequent field studies. Three main types of faults could be distinguished: gravitational faults (i.e. faults related to mass movements); tectonic faults; and composite faults (i.e. tectonic faults with a component of gravitational and post-glacial rebound-related reactivation). A large number of tectonic faults were found (over 1700), but only two unequivocally post-glacially active tectonic faults could be distinguished. Indications for their post-glacial (re-)activation are displaced Quaternary landforms or sediments. Large gravitational faults, as well as composite faults often correlate with deep-seated gravitational slope deformations (DSGSD). The latter occur mainly along valley slopes, particularly where a pervasive foliation strikes parallel to the valley. Fault orientations show correlations either with the regional main foliation (e.g. Aar and Gotthard massif), the orientation of valleys (e.g. Bedretto and Urseren valley), or pre-existing tectonic structures (e.g. faults parallel to joints that are perpendicular to the strike of major structures in the Helvetic nappes). Comparisons of fault orientations with orientations of nodal planes of earthquake focal mechanisms of the last 20 years show a poor indicative correlation. The central and western Swiss Alps host a large number of faults prone for reactivation in today's stress field. However, for most of these faults, no indications of their last phase of activity exist. The low number of unambiguously active tectonic faults suggests that the current strain is either predominantly aseismic or, alternatively, cumulated seismic moment is too low for producing surface rupture.

The probabilistic seismic hazard assessment of Germany—version 2016, considering the range of epistemic uncertainties and aleatory variability

“…This continent-continent collision is still ongoing as active uplift of external Alpine basement massifs and is connected with remarkable seismicity. It coincides to a large extent with increased uplift gradients (Ustaszewski and Pfiffner 2008).…”

Cosmogenic nuclide‐derived rates of diffusive and episodic erosion in the glacially sculpted upper Rhone Valley, Swiss Alps

“…In all other side valleys, Pleistocene glaciers were either too small to scour the main valleys or non-erosive, leading to cirque basins with post-glacial, fl uvially incised lower reaches (Figure 3b). Upon the retreat of the Rhone glacier, previously active faults or shear zones were reactivated (Ustaszewski and Pfiffner, 2008). As reported by Ustaszewski and Pfiffner (2008) and Persaud and Pfiffner (2004), such post-glacial surface ruptures are common features throughout the Swiss Alps.…”

“…Upon the retreat of the Rhone glacier, previously active faults or shear zones were reactivated (Ustaszewski and Pfiffner, 2008). As reported by Ustaszewski and Pfiffner (2008) and Persaud and Pfiffner (2004), such post-glacial surface ruptures are common features throughout the Swiss Alps. In most cases, these post-glacial scarps have valley-side-up geometries [e.g.…”

“…Individual faults can be followed for up to 4 km. Faults scarps in the non-glaciated basins in the upper Rhone Valley are continuous from the ridge tops across the valley bottoms and have throws up to 10 m (Ustaszewski and Pfiffner, 2008). In the case of glacially scoured basins, the composite fault scarps are only present on the ridge tops and valley sides, disappearing in the valley bottoms.…”

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

“…1; e.g. Eckardt et al 1983;Ustaszewski 2007;Ustaszewski & Pfiffner 2008). In the Upper Rhone valley, the Urseren valley, the Upper Rhine valley, faults occur over a distance of ~90 km between the towns Brig and Trun, whereas in the Bedretto valley they are present over a distance of ~10 km.…”