Long-lived brittle reactivation of crossing fault networks enhancing upwellings in hydrothermal systems: The case study of Chaudes-Aigues spring swarm

• Relationships between faults and fluid flow at Chaudes-Aigues, French Massif Central. • Compared to fault zone permeability, topography plays a secondary role for fluid flow. • Faults acting as drains in the past may be present-day barriers. • Once reactivated, inherited structures act as drains and determine spring location. • Geothermal exploration lies on the knowledge of long-lived tectonic history. Identifying the structures that allow hydrothermal fluids to rise from a deep reservoir to surface thermal springs is a major geothermal challenge. Faults play an ambivalent role in this process: they can be effective drains or, on the contrary, impermeable barriers, depending on their geometry and their reactivation over geological time. The locations of thermal springs are directly linked to these structures, their architecture and particularly in fault zones intersections. Exploring this variability is essential to constrain the circulation mechanisms in geothermal systems. The Chaudes-Aigues hydrothermal system, French Massif Central, is a particularly suitable natural laboratory for exploring the relationship between faults and fluid flow. It is characterised by more than 30 thermal springs, some of the hottest in Europe (up to 82°C for the Par Spring), located at the intersection of three Variscan fault zones: N150E, N050E and N00E. These intersections create networks of concentrated fractures, likely to act as preferential conduits for fluid flow. Despite these structural indications, the precise upwelling drains remain poorly identified. This study, aimed at identifying these drains, combines field structural analysis, fault density mapping, lineament analysis and gravity inversion modeling. Fault density mapping reveals a clear contrast across the study area. The southwestern Margeride Granitic Complex (MGC) shows sparse faulting, while its northeastern extent exhibits high fault density in N150E-oriented corridors, particularly near Chaudes-Aigues and La Chaldette. Outcrop-scale fault zone analysis reveals three major types. One of them, made of thick ductile-to-brittle N050E trending shear zones, assumed to be highly permeable, would be the main upwelling drains of the system. Gravity modeling reveals that the MGC would be located at shallow depth beneath the thermal springs. Altogether, results demonstrate that inherited ductile fabrics, reactivated fault systems, and fracture connectivity exert first-order control on granite emplacement and hydrothermal flow. These findings refine the structural model of the Chaudes-Aigues system and highlight the critical role of tectonics in geothermal fluid circulation. This hydrothermal system illustrates how inherited structures may act either as impermeable barriers or efficient drains, once reactivated, a duality that controls the location and persistence of thermal springs.