Thermal anomalies and geological structures in the Provence basin: Implications for hydrothermal circulations at depth
Résumé
Deep temperature estimates previously made in France show three main positive thermal anomalies, one of them centred on the Provence basin (south-east France) between Marseille and Montpellier. This study presents newly corrected temperature data and improved temperature maps in order to (i) validate or to invalidate the thermal anomalies previously identified and (ii) relate deep temperatures with major geological structures of the area. Although thermal gradient varies from place to place, it averages 31.3°C/km in the Provence basin (from 30.6 to 32.5 °C/km in average for France according to the chosen database), but some places show gradients reaching 36°C/km. To characterize thermal anomalous areas, a three-dimensional model of the temperatures was built between the surface and 6 km depth, allowing us to elaborate thermal maps and cross-sections. The identified thermal anomalies are different from those obtained by former works. New other “hot” anomalous areas (Montpellier, Lodève and Drôme areas) and cold anomalous areas (Aix-en-Provence and Cévennes areas) have been highlighted. At depth, thermal cross-sections show 50 km-scale anomalies, which are parallel with the major faults (Cévennes, Nimes, Salon-Cavaillon and Moyenne-Durance faults) whereas more elongated (roughly 100 km) anomalies are associated with perpendicular cross-sections. On these cross-sections each major fault is associated with a thermal anomaly. In addition, a cold area may overlie a warm one, and vice versa. Among different possible explanations, these thermal signatures could correspond to convective fluid circulation within the faults. Simple numerical models of hydrothermal convection within fault zones appear to reproduce similar amplitudes and vertical variations of thermal anomalies as those observed in the Provence basin