Fault-hosted hot springs of the Rhône Valley in the context of varying regional-scale neotectonics

Orogenic geothermal systems develop when meteoric water infiltrates the subsurface at high elevations, heats up along a deep circulation path due to the background geothermal gradient and eventually emerges at the surface in low topographic sites as localized hot springs. Such systems depend on permeable fault geometries; however, in orogenic settings fluid-discharge zones may additionally be controlled by the configuration of topography, nappe geometry, fault patterns and unconsolidated deposits that can conceal the bedrock structure. Hence, it is crucial to study local hot springs in the context of fault structures related to regional tectonics in order to predict the locations of blind geothermal systems. The Rhône Valley is a favourable site for such a study, as it shows the highest seismic activity in Switzerland and hosts several clusters of hot springs aligned along the regional Rhône-Simplon fault system.

Here, we combine data sets on geodynamics such as geodesy of recent crustal movements, regional recent stress fields, relocated hypocenters and focal mechanisms as well as structural field data to interpret the hot spring occurrences in the context of regional geodynamics. Our data suggest the presence of three adjacent structural domains: (1) A domain on the NW flank of the Rhône fault characterized by a NW–SE oriented maximum principal stress, high seismicity, and a pervasive network of strike- slip dominated faults; (2) a zone encompassing the Rhône Valley floor with transtensive, dilatant zones along strike-slip fault segments; and (3) a zone on the southern flank of the valley floor subjected to a recent NE–SW extension expressed by dominantly normal to transtensional faulting focal mechanisms. This southern domain constitutes the SW-extruding hanging wall block of the Simplon low-angle normal fault. The block is bounded by two crustal scale strike-slip faults, the dextral Rhône strike-slip fault in the NW and the sinistral Ospizio Sottile line in the SE.

In summary, our study highlights the importance of the large-scale tectonic setting for understanding and exploring fault controlled and hence, strongly localized geothermal resources in orogenic settings.