Tracking geothermal anomalies along a crustal fault using (U-Th)/He apatite thermochronology and REE analyses, the example of the Têt fault (Pyrenees, France)

Abstract. The Têt fault is a crustal scale major fault in the eastern Pyrenees along which 29 hot springs emerge mainly within the footwall damage zone of the fault. In this study, (U-Th)/He apatite (AHe) thermochronology is used in combination with REE analyses to investigate the imprint of hydrothermal activity nearby two main hot spring clusters and in between in an attempt to better define the geometry and intensity of the recent thermal anomalies along the fault and to compare them with previous results from numerical modelling. This study displays 99 new AHe ages and 63 REE analyses on samples collected in the hanging wall (18 to 43 Ma) and footwall (8 to 26 Ma) of the Têt fault. In the footwall, the results reveal AHe age resetting and apatite REE depletion due to hydrothermal circulation along the Têt fault damage zone, nearby the actual hot springs (Thuès-les-Bains and St-Thomas) but also in areas lacking actual geothermal surface manifestation. These age resetting and element depletions are more pronounced around Thuès-les-Bains hot spring cluster and are spatially restricted to a limited volume of the damage zone. Outside this damage zone, the modelling of thermochronological data in the footwall suggests the succession of two main phases of exhumation, between 30 and 24 Ma and a second one around 10 Ma. In the hanging wall, few evidences of hydrothermal imprint on AHe ages and REE signatures have been found and thermal modelling records a single exhumation phase at 35–30 Ma. Low-temperature thermochronology combined with REE analyses allows to identify the spatial distribution of a recent geothermal perturbation related to hydrothermal flow along a master fault zone in the eastern Pyrenees, opens new perspectives for the exploration of geothermal fields and provides at the regional scale new constraints on the tectonic uplift of the footwall and hanging wall massifs.