Fracture-controlled water flow in a granitic, high-mountain aquifer: the Panticosa massif (Axial Zone, Pyrenees)

The detailed analysis of the fracture geometry in the Panticosa granitoid (Axial Zone, Pyrenees) highlights the key role of structural analysis studies in characterizing water flow within fractured aquifers. Several fracture sets (striking ENE-WSW, NW-SE, and N-S) were identified through photogrammetry (with nearly 30,000 mapped fractures) and fieldwork. A hierarchical classification of the fracture systems was established based on their continuity along strike. The most prominent set, comprising individual faults that extend over several kilometres and exhibiting normal displacements of 10–50 m, also coincides with the dominant ENE–WSW orientation observed across multiple scales. These large, continuous structures likely result from recent tectonic activity or gravitational collapse. A second-order system includes NE-SW and NW-SE fractures ranging from meters to hundreds of meters in length. The distribution of the third-order set is more scattered and links longer fractures. Additionally, a set of dykes (majorily W-E striking) displays discontinuity contacts with the granite. We interpret that the studied fault sets are responsible for the up flow of thermal water (∼35 °C at the sources) of the deep aquifer from depths of 2 km. The higher continuity and transmissivity of the recent ENE-WSW fault system (showing thicker fault zones) made them more relevant for conditioning water flow.