Thermochronology and REE analyses as new tools to track thermal anomaly and fluid flow along a crustal scale fault (Têt fault, French Pyrenees)

The Têt fault is a crustal scale major fault in the eastern Pyrenees that displays about 30 hot springs along its surface trace with temperatures between 29°C and 73°C. The regional process of fluid circulation at depth has previously been highlighted by thermal numerical modelling supported by hydrochemical analyses and tectonic study. Numerical modelling suggests the presence of a strong subsurface anomaly of temperature along-fault (locally > 90°C/km), governed by topography-driven meteoric fluid upflow through the fault damage zone (advection). On the basis of this modelling, we focused our thermochronological study on 30 samples collected close and between two hot spring clusters in both the hanging wall and the footwall of the Têt fault, where the most important thermal anomaly is recorded by models. We analysed apatite using (U-Th)/He (AHe) dating combined with REE analyses on the same dated grains.

Along the fault, AHe ages are in a range of 26 to 8 Ma in the footwall and 43 and 18 Ma in the hanging wall, and only few apatite grains have been impacted by hydrothermalism near the St-Thomas hot spring cluster. By contrast, particularly young AHe ages below 6 Ma, correlated to REE depletion, are found around the Thuès-les-bains hot spring cluster. These very young ages are therefore interpreted as thermal resetting due to an important hydrothermal activity. A thermal anomaly can be mapped and appears restricted to 1 km around this cluster of hot springs, i.e. more restricted than the size of the anomaly predicted by numerical models. These results reveal that AHe dating and REE analyses can be used to highlight neo- or paleo-hydrothermal anomaly recorded by rocks along faults.

This study brings new elements to discuss the onset of the hydrothermal circulations and consequences on AHe and REE mobilisation, and suggest a strong heterogeneity of the hydrothermal flow pattern into the fault damage zone. Moreover, this study suggests that crustal scale faults adjacent to reliefs can localise narrow high hydrothermal flow and important geothermal gradient.  Besides these results, this study provides new constraints for geothermal exploration around crustal faults, as well as a discussion on the use of thermochronometers into fault damage zones.