Imaging Geothermal Anomalies Using Low-temperature (U-Th)/He Thermochronometry: A Case Study from the Active Têt Fault Hydrothermal System (Eastern Pyrenees, France)

Abstract

In the eastern Pyrenees (France), the Têt Neogene normal fault localizes 29 hot springs (from 29°C to 73°C) mainly distributed along the footwall damage zone. The presence along a weakly active fault of a strong subsurface temperature anomaly is also supported by geochemical analyses of hot spring waters and by numerical models of fluid circulations (Taillefer et al., 2018). The activity of the hydrothermal loop is essentially under the control of the adjacent topography with a fluid upflow channelized along the ca. 400 m thick damage zone of the Têt fault footwall (Taillefer et al., 2017). Apatite (U-Th)/He (AHe) ages were obtained on samples from this footwall, within and outside the damage zone affected by hydrothermal circulations. The results reveal a large variability of ages as a function of sample location with respect to this damage zone and hot spring occurrence. Outside the influence of hot springs, the AHe age range (10–25 Ma) is consistent with the regional miocene exhumation history of the eastern Pyrenees. In the damage zone near the hot spring cluster, samples in the first 400 m from the fault contact show AHe ages that do not fit with this regional exhumation history. In the outer damage zone, they are significantly younger ( less than 6 Ma) than AHe ages (10-14 Ma) of samples in the same structural position with respect to the fault but unaffected by hydrothermalism and coeval fracturation. In the inner damage zone, where intense fracturation and hydrothermal alteration prevail, AHe ages obtained in the vicinity of hot springs are strongly dispersed and essentially too old (up to 40 Ma) regarding the sample altitude and the regional thermal history. These data show that AHe ages can be used to constrain the dimension of a thermal anomaly related to hot fluid upflow along a fault. In the studied area, this thermal anomaly is more restricted (