Mapping a geothermal anomaly using apatite (U‐Th)/He thermochronology in the Têt fault damage zone, eastern Pyrenees, France

Abstract

(U‐Th)/He ages on apatite obtained in the vicinity of the Têt fault hydrothermal system show a large variability. In the inner damage zone adjacent to the fault core, where fluid flows are concentrated, AHe ages display a large scatter (3–41 Ma) and apatite ageing. Samples from the outer damage zone show young ages with less dispersion (0.9–21.1 Ma) and apatite rejuvenation. Outside the damage zone, ages are consistent with the regional exhumation history between 20 and 12 Ma. The important age dispersion found in the damage zone is interpreted as the result of 4He mobility during fluid infiltration. Our results show that thermochronological data close to a fault should be interpreted with caution, but may offer a new tool for geothermal exploration.

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

“…In this study, we propose an extended analysis of (U-Th)/He on apatite (AHe) thermochronometer (a LT thermochronometer sensitive for a range of temperature between 30 and 90 °C, Gautheron et al, 2009;Shuster and Farley, 2009), in association to Rare Earth Elements (REE) analysis of dated apatites, to track the effects of surface and hidden thermal systems along the Têt normal fault (Pyrenees). We showed in a preliminary work (Milesi et al, 2019) that the analysis of REE mobility can help to interpret AHe ages scattering and thus evidence hydrothermal imprints on measured ages. In this previous work, we focused on an area located in the vicinity of an active hot spring cluster along the dormant Têt fault in the eastern Pyrenees.…”

“…Low temperature thermochronometers revealed that the Canigou massif was mainly exhumed, and rapidly cooled, between ~30-20 Ma in relation with the Têt fault normal activity (Fig. 2) (Maurel et al, 2008;Milesi et al, 2019). The Cenozoic exhumation of the Carança and Canigou Dashed lines correspond to 95% confidence interval.…”

“…1B). Previous studies, at the scale of the eastern Pyrenees, using various thermochronometers such as muscovite and biotite 40 Ar/ 39 Ar, zircon fission track (ZFt), (U-Th)/He on zircon (ZHe), apatite fission track (AFT) and (U-Th)/He on apatite (AHe) provide an important age database to constrain the regional thermal history of eastern Pyrenees (Maurel et al, 2008;Gunnell et al, 2009;Milesi et al, 2019).…”

“…The DZ is a rock volume adjacent to the CZ characterised by an important amount of fractures (Caine et al, 1996;Sibson, 2000;Kim et al, 2004;Agosta et al, 2007;Faulkner et al, 2010). Close to Thuès-les-Bains, on the basis of field measurements of fractures and small faults in the Carança valley, the half-thickness of the DZ is estimated at 400 m with an inner highly fractured DZ of 75 m (Milesi et al, 2019). The half-thickness of the Têt fault DZ is estimated between 200 m and 700 m using lineament analysis of satellite SPOT images with 5 m resolution (Taillefer et al 2017).…”

“…In places where heat flow data are not available (no bore hole), geothermal exploration mainly relies on surface manifestations of hot hydrothermal fluids (hot springs, tuffa deposits, recent hydrothermal mineralisation, …). It has however recently been proposed that Low Temperature (LT) thermochronology can be used as a low-cost tool to support or extend the exploration of such geothermal systems (Gorynski et al, 2014;Milesi et al, 2019). Actually, alongside these thoughts, the past decades revealed an increasing amount of studies about the influence of hydrothermal circulations on LT thermochronometers, rendering difficult their interpretation in terms of exhumation history along faults (e.g.…”