Control of lithospheric to crustal fault architecture on Cenozoic Rift geothermal systems (Upper Rhine, Bresse and Limagne, France)

Hydrothermal groundwater systems are key to renewable energy due to their high thermal efficiency. Effective geothermal resources require not only a heat source but also permeable geological formations and appropriate flow patterns. Fault zones, play a critical role in this process, potentially acting as either drains or barriers. This study investigates the origins of heat flow in geothermal reservoirs by comparing the Bresse, Limagne, and Upper Rhine Grabens within the European Cenozoic Rift System (ECRIS), using a multidisciplinary approach to examine thermal properties, subsurface structure, and the role of fault zones in surface heat flow anomalies. Surface heat flow data from 943 points, geological maps, seismicity databases, and stress orientation data characterize crustal structures and fault networks, while seismic tomography, gravimetric maps, Moho and LAB depths, and SKS splitting measurements provide insights into mantle density variations, structural boundaries, and flow directions—together informing the model of heat distribution and high-temperature fluid migration. This synthesis highlights different scale of heat flow anomalies above the continental average of 67 mW/m²: (i) a long-wavelength anomaly, (ii) three high-wavelength anomalies, and (iii) seven areas with very high-wavelength anomalies exceeding 175 mW/m². High surface heat flow anomalies are linked to fault zones, such as the Sillon Houiller fault. In contrast, the long wavelength anomaly at the scale of the French Massif Central (FMC) is probably not linked to a lithospheric fault. Characterizing the lithosphere-asthenosphere architecture reveals that the long-wavelength thermal anomaly aligns with a thin lithosphere-crust corridor and a positive seismic velocity anomaly, indicating an increase in the mantle heat flux beneath the thin lithosphere of the FMC and the Rhine Graben. In Upper Rhine and Bresse Grabens areas, high crustal fault density network connecting with major lithospheric discontinuities often corresponds to high surface heat flow. The active and crustal faults (Sillon Houiller, La Marche, and Vittel) facilitate the upward migration of high temperature fluid. Finally, this study highlights the importance of integrating multidisciplinary methods to understand surface heat flow and its implications for geothermal energy.