The Téné Fault (SW Helvetics, Switzerland): Architecture and permeability evolution of a carbonate strike-slip fault system with hints of low to moderate energy paleoseismogenic activity

The SW Helvetic Alps represent the most seismically active region of Switzerland. For this area, the precise location of earthquake hypocenters and subsurface 3D fault structures, extending to a scale of several hundred meters, have been mapped. However, little is known about the seismic faulting processes and detailed fault geometries, as information at a higher spatial resolution is required. Here, we study the fault and permeability architecture of the 900 m long NE-SW trending carbonate-hosted Téné Fault (TF), an exceptionally well exposed strike-slip fault, by means of remote sensing and field mapping in combination with structural analysis. TF shows a protracted structural evolution with early strain localization occurring at the ductile to brittle transition, which is subsequently overprinted by cycles of brittle deformation combined with dissolution/precipitation events during exhumation. In addition, a pronounced along-strike variability in structures and strain localization can be observed. While both ends of TF show fault strand patterns of >100 m width, strain in its center is extremely localized (width < 0.4 m) consisting of multiple events of micro-brecciation, gouge injection, cataclasis, and/ or veining. Prominent are a multitude of absolutely planar and knife-sharp rupture planes of 1.5-27.5 m length, which cross-cut all older fault core structures at slight angles (5-10°) to the fault plane, in an en echelon type manner. Together with the occurrence of injection veins of granular carbonatic material this hints towards a seismic origin of these fault strands. A comparison of the rupture lengths of the fault planes with those of recent earthquakes supports our inference. The formation, effect on dynamic permeability and role as potential proxy of the “supposed seismic fault” strands for recent low to moderate energy seismic activity is discussed.