Thermal Influence of an Alpine Deep Hydrothermal Fault on the Surrounding Rocks

Résumé En

Major fault zones in mountain areas are often associated with cold-water circulations and hydrothermal pathways. Comparedwith the massif as a whole, the deep groundwater flows in these high hydraulic-conductivity zones modify the thermal state of thesurrounding rock. This paper examines the thermal effects of groundwater flow in the area around the steeply dipping La L´ech`eredeep fault zone (LFZ, French Alps) and associated shallow decompressed zone. We used a 3D numerical model drawn up fromgroundwater circulation data to investigate the La L´ech`ere hydrothermal system and the thermal state of the rock in the valley sides.Hydrothermal simulations showed that convective flow into the LFZ cools the valley sides and creates a thermal upwelling under thevalley floor. An unsteady thermal regime that continues for about 10,000 years is also needed to obtain the temperatures currentlyfound under the valley floor in the LFZ. Temperature-depth profiles around the LFZ show disturbances in the thermal gradients inthe valley sides and the valley floor. Convective heat transfer into the LFZ and the decompressed zone, and conductive heat transferin the surrounding rocks produce an unsteady, asymmetric thermal state in the rock on both sides of the LFZ.