Why late-stage collisional orogens are favorable for geothermal exploration

In contrast to convergent plate tectonics during subduction, collisional orogens in their late stages (e.g., Alps, Pyrenees, Himalayas) typically exhibit little to no magmatic activity. Consequently, these settings have historically received limited attention in exploration for geothermal energy. Nevertheless, such orogens do host active, amagmatic geothermal systems owing to the co-occurrence of several key features: (1) ongoing crustal-scale deformation within the brittle upper crust, which generates spatially dispersed, permeable fracture networks; (2) major, steep strike-slip and normal faults with permeable dilation zones that cut across the fracture networks; (3) geothermal gradients of 20–35 °C/km, which provide heat to circulating fluids; (4) pronounced topographic relief, which induces strong hydraulic head gradients between high surface elevations and valley floors.

The interaction of these features produces dynamic geothermal circulation systems: meteoric water infiltrates at high elevations into the dispersed fracture networks and is focused into the major faults, allowing the water to descend to depths up to 10 km and become heated to above 200 °C. Subvertical dilation zones within the major faults link the deep flow paths to the surface or relatively shallow depths, generally at lower elevations, allowing the topographically induced hydraulic gradients to drive the hot water up to discharge sites in the valley floors.

Unfortunately, in orogens where valleys are glacially over-deepened, outflow is commonly hidden under thick sequences of unconsolidated sediments. Therefore, the challenge in exploration in such settings is to locate these blind geothermal systems. To address this challenge, we examined the geothermal favorability of the mountainous Valais region in SW Switzerland, which is one of the most well-known areas of geothermal activity in the European Alps. Through collaboration among structural geologists, hydrogeochemists and seismologists, all available geological and hydrochemical data were compiled in a GIS database to conduct a Play Fairway Analysis. Each data layer was evaluated and rated for its ability to directly or indirectly indicate sites of deep thermal upflow. By weighting and combining these layers, we produced favorability maps displaying areas where active upflow of thermal water is most likely. A notable outcome is that most of the known thermal springs in the study area fall within the identified favorable areas. This success demonstrates the usefulness of the favorability maps in guiding more spatially-targeted exploration in the Valais region. Moreover, our methodology provides a transferable framework for exploration in similar geodynamic settings elsewhere.