# Low-temperature (U-Th)/He thermochronometry applied to an active hydrothermal system: The example of the Têt fault (Eastern Pyrénées, France)

Gaétan Milesi, Roger Soliva, Patrick Monie, Mathieu Bellanger, Philippe Munch, Audrey Taillefer, Michaël Bonno

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## ► To cite this version:

Gaétan Milesi, Roger Soliva, Patrick Monie, Mathieu Bellanger, Philippe Munch, et al.. Low-temperature (U-Th)/He thermochronometry applied to an active hydrothermal system: The example of the Têt fault (Eastern Pyrénées, France). EGU General Assembly 2018, Apr 2018, Vienne, Austria. Copernicus, 20, pp.7354, 2018. hal-05240858

HAL Id: hal-05240858

https://hal.science/hal-05240858v1

Submitted on 26 Sep 2025

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# Low-temperature (U-Th)/He thermochronometry applied to an active hydrothermal system.
The example of the Têt fault (Eastern Pyrénées, France)

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Milesi Gaétan₁ (gaetan.milesi@umontpellier.fr), Monié Patrick₁, Soliva Roger₁, Münch Philippe₁, Bellanger Mathieu₂, Taillefer Audrey₁, Bruguier Olivier₁ and Bonno Michael₁

¹Géosciences Montpellier, UMR5243, Montpellier, France

²TLS Geothermics, Toulouse, France

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## 1/ Geological setting and new field data

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▲ Structural map with sample location (modified from Taillefer et al., 2017). Samples TET1, TET2 and TET6 are located in the damage zone.

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▲ Synthetic cross section with projected location of samples associated and numerical model of the hydrothermal anomaly (Taillefer et al., 2017).

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▲ Scanline of mineralized fractures in the Carança valley. We observe two types of fractures: red bars represent the number of fractures cemented by quartz and blue bars the number of fractures filled by calcite (see photography). Red rose diagram shows strike orientations for quartz cemented fractures, green rose diagram shows orientations for all mineralized fractures.

In the Eastern part of the Pyrénées, the Neogene Têt normal fault displays 29 hot springs in 4 clusters mainly located in its footwall damage zone. Field data and numerical modeling of the hydrothermal system of the Têt fault suggest that a poorly active normal fault adjacent to landform can localize thermal anomaly, more than three times the geothermal

gradient. This geothermal anomaly apparently extends up to 1500 meters away from the Têt fault in its footwall (Taillefer et al., 2017). We focused our study on the warmer cluster near Thuès-entre-Valls, showing surface water temperature of about 75°C. We present the results of a joint analysis of structural data and AHe ages and discuss the influence of the thermal anomaly within the footwall of the Têt fault.

## 2/ (U-Th)/He ages on apatite

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▲ AHe ages as a function of distance from the Têt fault. Samples adjacent to the fault contact are the most affected by hydrothermalism and age scattering.

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▲ AHe ages as a function of eU values and Th/U ratios. Samples TET1, TET2 and TET6 are from the damage zone.

Samples far from the fault zone, unaffected by incision, provide ages that are consistent with the regional denudation and thermal history of the Carança massif. In the damage zone influenced by hot spring thermal anomaly, the young ages of sample TET2 strongly affected by incision suggest that apatites were reset under a high geothermal gradient of about 100°C/km.

Very close to the fault core, the age scattering between 36.6 ± 1.7 Ma and 2.7 ± 0.3 Ma is interpreted to result from U leaching during hydrothermalism coevally with He remobilization.

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AHe ages are reported as a function of altitude in the Carança massif (this study) and in the Canigou massif (Maurel et al., 2008). This diagram shows a decrease of denudation rate between 20 to 12 Ma.

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▲ Evolution of erosion amount since the middle Miocene, in which AHe mean ages are reported. The red horizontal lines are values of erosion calculated for different geothermal gradients assuming closure temperature of 45°C (Ehlers et al., 2001).

The denudation and incision domains are delimited according to the literature using thermochronological data, cross section restoration and cosmonucleides data.

## 3/ Conclusion

▲ Hot spring temperature up of 70°C
▲ Hot spring temperature up of 60°C
▲ Hot spring temperature up of 50°C
▲ 3D view of study area that shows field data, sampling location, and distribution of isotherms at depth from Taillefer et al. (2017).

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AHe ages obtained in the fault contact TET1 and TET6 suggest that hydrothermal activity induced a large scattering of ages related to U (and Y) leaching combined with possible He remobilization. This indicates that AHe ages in the vicinity of (paleo-) hydrothermal hot springs associated with crustal fault that are used as hot fluid drains have to be interpreted with caution.

In the distal damage zone (TET2), very low AHe ages in the range of 1 to 6 Ma are inconsistent with the local erosion rate, implying the existence of a strong recent thermal anomaly (±100°C/km) into the fault damage zone (<225 m of the fault). On the other hand, samples outside the damage zone provide AHe ages that are fitting with a previous study combining fission tracks and AHe ages (Maurel et al., 2008). Our study suggests that thermochronometry can be used as a new tool for geothermal exploration to estimate recent thermal anomaly. This preliminary study will be extended to other clusters of hot springs along the Têt fault, using (U-Th)/He on apatite, zircon and fluorite.

## References

Ehlers, T.A., Armstrong, P.A., and Chapman, D.S., 2001, Normal fault thermal regimes and the interpretation of low-temperature thermochronometers: Physics of the Earth and Planetary Interiors, v. 126, p. 179-194.

Maurel, O., Monié, P., Pik, R., Arnaud, N., Brunel, M., and Jolivet, M., 2008, The Meso-Cenozoic thermo-pectonic evolution of the Eastern Pyrenees: an 40Ar/39Ar fission track and (U-Th)/He thermochronological study of the Canigou and Mont-Louis massifs: International Journal of Earth Sciences, v. 97, p. 565-584, doi: 10.1007/s00531-007-0179-x.

Taillefer, A., Soliva, R., Guillou-Frottier, L., Le Goff, E., Martin, G., and Seranne, M., 2017, Fault-Related Controls on Upward Hydrothermal Flow: An Integrated Geological Study of the Têt Fault System, Eastern Pyrénées (France): Geofluids, v. 2017, p. 1-19, doi: 10.1155/2017/8190109.