Unravelling the evolution of an Alpine to post-glacially active fault in the Swiss Alps Michaela Ustaszewski a, * , Marco Herwegh a , Alastair F. McClymont b , O. Adrian Pfiffner a , Robyn Pickering a , Frank Preusser a a Institute of Geological Sciences, University of Bern, Baltzerstrasse 1-3, 3012 Bern, Switzerland b Institute of Geophysics, Swiss Federal Institute of Technology, Ho ¨ nggerberg, 8093 Zu ¨ rich, Switzerland Received 12 February 2007; received in revised form 16 August 2007; accepted 5 September 2007 Available online 29 September 2007 Abstract The Gemmi fault is a prominent NWeSE striking lineament that crosses the Gemmi Pass in the central Swiss Alps. A multidisciplinary investigation of this structure that included geological mapping, joint profiling, cathodoluminescence and scanning electron microscopy, stable isotope measurements, luminescence- and U-TH-dating, 3D ground penetrating radar (GPR) surveying and trenching reveals a history of fault movements from the Miocene to the Holocene. The main fault zone comprises a 0.5e3 m thick calcite cataclasite formed during several cycles of veining and brittle deformation. Displaced Cretaceous rock layers show an apparent dextral slip of w10 m along the fault. A detailed study of a small sediment-filled depression that crosses the fault provides evidence for a post-glacial reactivation of the fault. A trench excavated across the fault exposed a Late-Glacial-age loess layer and late Holocene colluvial-like slope-wash deposits that showed evidence for fault displacement of a few centimeters, indicating a recent strike-slip reactivation of the fault. Focal mechanisms of recent instru- mentally recorded earthquakes are consistent with our findings that show that the fault at the Gemmi Pass, together with other parallel faults in this area, may be reactivated in today’s stress field. Taking together all the observations of its ancient and recent activity, the Gemmi fault can be viewed as a window through geological space and time. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Swiss Alps; Neotectonics; Active faulting; Fault dating; Brittle reactivation; Fluid flow in faults 1. Introduction In the central Alps, the collision of the Adriatic and the European continental margins initiated crustal thickening dur- ing the Cenozoic (Schmid et al., 1997; Escher et al., 1997; Pfiffner et al., 2002). GPS measurements show that the conver- gence of the two plates is ongoing at a rate of <2 mm/year (Calais et al., 2002), which is likely to have caused recent rock uplift and seismicity (Sue and Tricart, 2003; Persaud and Pfiffner, 2004; Ustaszewski and Pfiffner, in press). The Alps have been studied for over 150 years and their evolution has been recorded in great detail (e.g. Tru ¨mpy, 1998, 2003; Pfiffner et al., 1997). In contrast, little is known about their recent history. This is surprising, because active and potentially seismogenic faults represent a significant haz- ard to life and constructions. Knowledge of their neotectonic history (<15 ka) is therefore a key issue and a pre-requisite for any assessment of seismic hazard. Few previous researchers have touched on the topic of re- cent tectonic activity in the Alps. High-precision levelling measurements have been conducted since 1917 (Kahle et al., 1997; Funk and Gubler, 1980; Gubler et al., 1981; Schlatter and Marti, 2002), and the Swiss Seismologic Survey publishes an annual report of the seismic activity in Switzerland (e.g. Baer et al., 2005; Deichmann et al., 2006). Today’s stress re- gime, inferred from focal mechanisms of instrumentally recorded earthquakes shows a contrast between the Penninic units south of the Rho ˆne river, where NeS extension * Corresponding author. Tel.: þ41 31 631 4832; fax: þ41 31 631 4843. E-mail address: michaela.ustaszewski@geo.unibe.ch (M. Ustaszewski). 0191-8141/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2007.09.006 Journal of Structural Geology 29 (2007) 1943e1959 www.elsevier.com/locate/jsg