The tectonised zone within the Mont Blanc granite

The Mont Blanc Tunnel intersects a major tectonised zone within the crystalline granite massif, characterized by intense fracturing and exceptionally high groundwater inflows. During tunnel construction, discharge from this zone reached approximately 1,084 L/s, far exceeding the inflows observed along most of the tunnel. Several decades later, the discharge remains substantial, at around 200 L/s.

This exceptional water circulation highlights the role of tectonic structures in creating highly permeable pathways within otherwise low-permeability crystalline rock. The persistence of large inflows over decades also demonstrates the long-term hydraulic connectivity of the fractured system and its capacity to transmit groundwater from high-altitude recharge areas.

Hydrogeological interpretation of the Mont Blanc tectonised zone

The exceptionally high groundwater inflows encountered during construction of the Mont Blanc Tunnel are interpreted as evidence of a highly transmissive fracture network within the granite massif. The tectonised zone acts as a preferential drainage pathway, connecting deep tunnel levels to groundwater recharge areas at high elevations in the Alpine massif.

The persistence of substantial discharge decades after construction suggests that this network remains hydraulically connected to a large recharge area, rather than being supplied solely by local groundwater storage. High-altitude infiltration provides the water, while the connected fractures and tectonic structures facilitate its deep circulation through the crystalline basement.

The associated thermal anomaly provides further evidence for this circulation pattern. The relatively low temperatures observed at depth, compared with those expected from the regional geothermal gradient, can be explained by the downward circulation of cold meteoric water infiltrating at high elevations. The thermal regime therefore reflects the combined effects of Alpine topography, deep groundwater circulation and heat exchange between water and rock.

This case illustrates how tectonic structures can create highly transmissive pathways in crystalline massif, enabling large, persistent groundwater flows and substantial perturbations of the natural thermal field.