Tracking geothermal anomalies along a crustal fault using (U − Th)∕He apatite thermochronology and rare-earth element (REE) analyses: the example of the Têt fault (Pyrenees, France)

Gaétan Milesi,Audrey Taillefer,Olivier Bruguier,Patrick Monié,Mathieu Bellanger,Philippe Münch,Roger Soliva,Céline Martin,Michaël Bonno

doi:10.5194/se-11-1747-2020

Gaétan Milesi, Audrey Taillefer + Show 7 more

Open Access

https://doi.org/10.5194/se-11-1747-2020

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Abstract

Abstract. The Têt fault is a major crustal-scale fault in the eastern Pyrenees (France) 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 rare-earth element (REE) analyses in order to investigate the imprint of hydrothermal activity around two main hot spring clusters (Thuès-les-Bains and St Thomas) and between them. The main goal is 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 single apatite grains from 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, near the two hot spring clusters, and also in areas lacking present-day geothermal surface manifestation. These age resettings and element depletions are more pronounced around the Thuès-les-Bains hot spring cluster and are spatially restricted to a limited volume of the damage zone. Outside this damage zone, new modelling of thermochronological data specifies the thermal evolution of the massifs. The footwall model suggests the succession of two main phases of cooling: between 30 and 24 Ma and a second one around 10 Ma. In the hanging wall, little evidence of hydrothermal imprint on AHe ages and REE signatures has been found, and thermal modelling records a single cooling phase at 35–30 Ma. Low-temperature thermochronology combined with REE analyses allows us to identify the spatial extent of a recent geothermal perturbation related to hydrothermal flow along a master fault zone in the eastern Pyrenees, opens new perspectives to constrain the geometry and intensity of geothermal fields, and provides new regional constraints on the cooling history of the footwall and hanging-wall massifs.

Highlights

Global geothermal energy production is expected to grow rapidly over the years due to its low environmental impact and increasing technological developments (Van der Zwaan and Della Longa, 2019)
AHe ages and rare-earth element (REE) patterns from apatite grains of the footwall of the Têt normal fault are summarized in Table 1 and Figs. 7 and 8
All analysed apatite grains from samples either outside or inside the damage zone (DZ) show a typical wing-shaped REE pattern with a marked europium anomaly (Fig. 8) characteristic of apatites from S-type granites (Sha and Chappell, 1999; Belousova et al, 2002) that form the protolith of the Canigou augen gneiss (Cocherie et al, 2005; Castiñeiras et al, 2011)

Summary

Introduction

Global geothermal energy production is expected to grow rapidly over the years due to its low environmental impact and increasing technological developments (Van der Zwaan and Della Longa, 2019). Heat advection requires the presence of water such as that commonly found in continental hydrothermal systems (Deming, 1994). This type of system can show temperature and water discharge rates large enough for electricity production in “geothermal doublets”, such as those developed in the Basin and Range province (Blackwell et al, 2000; Faulds et al, 2010) or in western Anatolia, Turkey (Roche et al, 2018), with temperatures above 200 ◦C (Bertani, 2012).

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