Detecting blind geothermal systems in orogenic belts: Structural controls on paleo-hydrothermal fluid pathways in the Rhône Valley, SW Switzerland

Meteoric water infiltrates deeply into high-relief mountain ranges, heats up along its circulation path due to the background geothermal gradient and eventually discharges at lower elevation as thermal springs. Enabling such topographically-driven circulation depends on parameters such as permeability, hydraulic head, and thermal buoyancy of the rising water. Brittle deformation zones, especially active fault zones, often provide high-permeability fluid pathways due to repeated slip and refracturing. A systematic 4D analysis of such fault systems can therefore aid in identifying prospective sites in orogenic belts for more detailed geothermal exploration by 3D seismics and drilling.

Our ongoing GeoTex project investigates the geothermal potential of the Rhône Valley, SW Switzerland – a geothermally active Alpine zone with thermal springs, regional faults, and enhanced seismicity. Using structural field observations, seismological data, and remote sensing, we characterise fault geometries, kinematics and fault rocks in the vicinity of known thermal springs. Observable paleo-fluid pathways marked by hydrothermal veins and rock alteration are treated as analogues for recent thermal water circulation and are linked to major Alpine structures in the underlying basement units, such as large-scale strike-slip faults or the axial planes of uplifting basement domes. We identify three geodynamic domains with distinct fault characteristics: (1) A domain on the NW flank of the valley floor 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 valley floor with dilatant zones along strike-slip fault segments; and (3) a zone on the southern flank of the valley floor subjected to recent NE–SW extension expressed by dominantly normal to transtensional faulting focal mechanisms.

For each domain, we developed conceptual structural models helping to identify present-day fluid pathways. Integrating hydrochemical data (indicative of deep-geothermal fluid circulation) into our models, allows us to refine our understanding of such fluid pathways and to predict potential locations of blind active geothermal systems throughout the Rhône Valley and other Alpine settings.