# Origin of Helium and Associated Fluid in Fault-Related Hydrothermal Systems of the Eastern Pyrenees

Gaétan Milesi, Carolina Dantas Cardoso, Raphaël Pik, Manuel Moreira, Delphine Charpentier, Julien Mercadier

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Gaétan Milesi, Carolina Dantas Cardoso, Raphaël Pik, Manuel Moreira, Delphine Charpentier, et al. Origin of Helium and Associated Fluid in Fault-Related Hydrothermal Systems of the Eastern Pyrenees. Terra Nova, 2025, 37 (2), pp.93-102. 10.1111/ter.12753. hal-04746837

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1 Origin of helium and fluid reservoirs in fault geothermal
systems of the Eastern Pyrenees

2

3 Gaétan Milesi¹, Carolina Dantas Cardoso², Raphaël Pik², Manuel Moreira³,

4 Delphine Charpentier⁴, Julien Mercadier¹

5 ¹GeoRessources, CNRS, Université de Lorraine, LabCom CREGU, 54506 Vandœuvre-lès-Nancy, France

6

7 ²Université de Lorraine, CNRS, CRPG, UMR7358, F-5400, Nancy, France

8 ³Université d'Orléans, CNRS, ISTO, UMR 7327, F-45071 Orléans, France

9 ⁴Chrono-Environnement, UMR 6249-CNRS, Université Bourgogne Franche-Comté, Besançon, France

10

11

12 Correspondence

13 Milesi Gaétan, Faculté des Sciences et Technologies, Campus Aiguillettes "Entrée 3B"

14 Rue Jacques Callot, BP 70239, 54506 Vandœuvre-lès-Nancy, France

15 Email: gaetan.milesi@univ-lorraine.fr

16

17 This version is an accepted version of the manuscript without the publisher's

18 typographical corrections. DOI: https://doi.org/10.1111/ter.12753

19

20

21
22 **ABSTRACT**

23 An intense hydrothermal activity is reported in the Eastern part of the Pyrenees,
24 hosted by regional tectonic structures. In this study, we investigated the helium isotopic
25 ratio (³He/⁴He) in different hot springs. The results obtained along the Têt (0.033 -0.099
26 Ra) and the Tech (0.171 - 0.375 Ra) faults is indicative of crustal signature. The comparison
27 with helium datasets from hydrothermal systems in the Alps allows to discuss the origin of
28 higher helium isotopic ratios obtained along the Tech fault. ³He/⁴He ratios in hydrothermal
29 fluids for the different hot spring clusters show an increase towards the East, consistent
30 with the observed thinning of the continental crust in this part of the range. These results
31 suggest compartmentalization of geothermal reservoirs, even at the scale of the same fault
32 system.
33

33

34 **1. INTRODUCTION**

35 Helium isotopes (³He/⁴He) allow to reveal the source of water in various geological
36 contexts (e.g. Craig and Lupton, 1976; Andrews, 1985; Marty et al., 1993; Pinti and Marty,
37 1995,1998; Pik and Marty, 2009; Moreira et al., 2019; Pinti et al., 2022). The two isotopes
38 have different origins, ³He is primordial and located in the Earth's mantle (Craig and
39 Lupton, 1976); whereas ⁴He originates from U and Th radiogenic decay in minerals present
40 in the continental crust (Ballentine and Burnard, 2002; Moreira, 2013). The helium isotopic
41 signature of fluids is therefore a proxy of the original sources in which they were dissolved
42 (e.g. Pinti et al., 2022), more or less modified by additional potential components during
43 their transfer to the surface.
44
In geothermal systems, fluids reservoirs can be regional (Roche et al., 2018) or at the scale of a massif (Taillefer et al., 2018) or a fault (Duwiquet et al., 2019; Guillou-Frottier et al., 2020). In the eastern part of the Pyrenees, an intense hydrothermal activity is localized along regional crustal scale faults (Taillefer et al., 2017). An important thinning of the continental crust in this part of the range (Lacan and Ortuno, 2012) questioned on the origin of hydrothermalism and the size of geothermal reservoirs. In this study, we measured helium isotopes in hot springs along the Têt and Tech faults to characterize the geothermal reservoir(s) and investigate the potential impact of regional crustal thinning on those fluids origin and characteristics.

53

## 2. GEOLOGICAL SETTING

### 2.1 Geological context of the Eastern Pyrenees

In the Pyrenees, the fault network is inherited from multiple tectonic events with an original Hercynian ductile deformation (e.g. Cochelin et al., 2017), overlaid by Late Cretaceous to the Middle Miocene collision associated to the displacement of the European and Iberic plates (Muñoz, 1992). During this period, the Pyrenean Axial Zone has undergone a stack of crustal nappes essentially during the main Eocene-early Oligocene orogenic build up (Jolivet et al., 2007; Labaume et al., 2016; Vacherat et al., 2016). The Eastern part of the Axial Zone is affected by Late Priabonian to the Quaternary regional extensional tectonics, associated with the (re-)activation of major structures as normal faults in response to the Gulf of Lion opening (e.g. Jolivet et al., 2021; Milesi et al., 2022, 2023). This regional-scale extension has led to an important thinning of the crust from 45 km to 25 km from West to East at the scale of the Eastern Pyrenees (Lacan and Ortuno,
67 2012; Chevrot et al., 2018; Diaz et al., 2018). A link between crustal thinning and post-
68 Oligocene mantle flow due to the Apennine slab retreat is proposed (Jolivet et al., 2020;
69 2021; Romagny et al., 2020), which could be expressed by the syn-rift Plio-quaternary
70 volcanism observed along NW-SE faults of El Emporda or Transverse ranges (Fig. 1).

71 In the Eastern Pyrenees, the Têt and Tech faults are the two main NE-SW crustal
72 faults (Fig. 1), displaying offsets in the Moho (Calvet et al., 2021). These faults are also
73 marked by pyrenean polyphased tectonics (Milesi, 2020, Ternois et al., 2019; Calvet et al.,
74 2021). Late normal faulting event is associated to puri-kilometric displacement
75 accommodated on these faults (Maurel, 2003; Milesi et al., 2022, 2023). Taillefer et al.
76 (2021) emphasizes an important brittle fault network in the Eastern Pyrenees particularly
77 well expressed along the western segment of the Têt fault that play an important role on
78 the present geothermal system location.

## 2.2 Hydrothermal circulation model

80 The Têt and the Tech fault localize active hydrothermal circulations that have been
81 widely studied. The hot spring clusters are essentially localized in the footwall fault
82 damage zone (Taillefer et al., 2017; Milesi et al., 2019). Figure 2 shows a schematic view
83 of hydrothermal circulations based on an initial meteoric origin for both faults (Krimissa,
84 1995; Taillefer et al. 2018). The water chemistry was investigated in terms of relative
85 concentrations of major cations and anions. All the spring fluids are highly concentrated in
86 sodium, chloride and sulphate and have pH values higher than 9, that characterize
87 hydrothermal fluids. We can note a slight difference between Tech and Têt hot springs
88 regarding Cl⁻ +NO₃⁻ and HCO₃⁻+CO₃²⁻ ratio due to a protolith difference (Appendix 1). For
89 the Têt fault hydrothermal system, altitude of infiltration is above 2000 m for
90 Thuès/Canaveilles (TH) and St-Thomas-les-Bains/Prats-Balaguer (ST) hot spring clusters,
91 between 1800 and 2000 m for Llo and Vernet-les-Bains (VB) hot spring clusters (Taillefer
92 et al., 2018). For Amélie-les-Bains (AM) hot spring cluster, along the Tech fault, the
93 altitude of infiltration is lower, between 1100 and 1200 m (Krimissa, 1995). For all those
94 hydrothermal systems, fluids reached, at depth, variable temperatures between 80°C and
95 135°C, according to the hot spring cluster. Water residence times based on ¹⁴C are similar
96 for the Têt and Tech faults between 5 and 13 ka (Krimissa, 1995). Taillefer et al. (2018)
97 proposed a numerical model of hydrothermal circulations along the Têt fault, and the main
98 results show that (i) the topography appears to be the main parameter to control maximum
99 fluid temperature at depth, and that (ii) hydrothermal circulations along the Têt fault
100 hydrothermal system is divided into three parts. The hydrothermal fluids at TH and ST hot
101 spring clusters come from the Carança range and geothermal activity is maximum. The
102 negative thermal anomaly along the fault plane between ST and Llo hot spring clusters is
103 interpreted as a fluid recharge area. Different origin for Llo and VB hydrothermal fluids is
104 suspected from Puigmal and the Canigou range, respectively.

105

106 3. SAMPLING AND ANALYTICAL METHOD

107 3.1 Sampling

108 For this study, hydrothermal fluids of Têt and Tech crustal NE-SW faults with
109 similar tectonic history have been investigated. 11 hot springs have been sampled (Table
110 1), and those located in the damage zone of the major fault have been favored. 4 hot spring
111 clusters along the Têt fault have been sampled from the West to the East: Llo (1 spring),
112 ST (2), TH (3) and VB (1). Further East, AM (4) hot spring cluster along the Tech fault
113 have been investigated.

109

110

111

112

113
## 3.2 Helium measurement

The analyses of ⁴He/²⁰Ne and ³He/⁴He ratios of gases sampled in copper tubes were performed, respectively, on a MKS Microvision 2 quadrupole mass spectrometer (QMS) and a split flight tube noble gas mass spectrometer (Helix SFT - Thermo Fisher Scientific) at the Centre de Recherches Pétrographiques et Géochimiques (CRPG) noble gas analytical facility (Mabry et al., 2013; Cardoso et al., 2023). The standard deviation of the ³He/⁴He ratios ranged from 2.0 to 3.7 %, based on the replicate analysis of standard He (atmospheric). Results are corrected for blanks, which contributed <0.1 % to the total He abundances (Table 1).

## 4. RESULTS

⁴He/²⁰Ne ratios against ³He/⁴He ratios (Craig et al., 1988) obtained on the Eastern Pyrenees hot springs are plotted in Figure 3. For these samples, the ⁴He/²⁰Ne ratios range from 1.63 to 85.78, higher than the expected ASW value at 10°C (0.276; Smith and Kennedy, 1983). ⁴He/²⁰Ne ratios of hot springs of the Têt fault are homogeneous (28.1 to 61.2) for each local clusters except VB (5.8) hot spring; in contrast, this ratio is variable (1.63 to 85.78) for the AM cluster along the Tech fault. The helium isotopic ratios measured in the studied hydrothermal water range from 0.033 to 0.375 Ra, where Ra is the atmospheric ³He/⁴He (1.39 x 10⁻⁶, Graham, 2002). Two distinct groups of samples can be distinguished: (i) Têt fault samples ranging between 0.033 and 0.099 Ra, with typical crustal signatures (Marty et al., 1992, Oxburgh et al., 1986, Cardoso et al., 2023), and (ii) Tech fault samples ranging between 0.171 and 0.375 Ra, which display values higher than typical continental crust. In the ⁴He/²⁰Ne against ³He/⁴He graph, all the Têt hot spring
clusters plot coherently along mixing lines between the ASW component (Ne-rich) and deep components (He-rich) characterized by various R/Ra isotopic compositions. Along the Têt fault, a clear increase of the helium isotopic ratios of the deep component implicated for the various clusters is observable towards the East (Fig. 4), suggesting the gradual participation of an additional ³He-rich source in their genesis. For the Tech fault cluster, the implicated deep component displays the highest ³He/⁴He of the survey but heterogeneous content at the scale of the cluster and is therefore coherent with the eastward increase of the ³He-rich source participation. However, on Figure 4, the Tech fault data do not follow the Têt fault clusters trend, suggesting a significant decoupling between the two fault systems.

## 5. DISCUSSION

### 5.1 Origin of helium isotopic ratio in the Eastern Pyrenees

The difference in helium isotopic ratio between the Têt fault (0.025-0.060 Rc/Ra) and Tech fault (>0.17 Rc/Ra) hydrothermal systems can be explained by the differential addition of an ³He-rich component to the local crustal component which is generally characterized by radiogenic ⁴He signatures (~0.01-0.03 Ra). Such an ³He-rich component can originate from two different types of processes: (i) fluids/magmas transferred from the mantle (primordial ³He), or (ii) specific crustal production in presence of high amounts of Li (nucleogenic production of ³He on ⁶Li). Given the very specific conditions and important Li concentrations in the crust necessary to produce such anomalous R/Ra signatures (600 ppm for 0.2 Ra, Cardoso et al., 2023), the addition of 2-2.5 % of mantle component in the generation of the Tech fault hydrothermal fluids is the most probable
hypothesis. Equivalent R/Ra isotopic compositions have already been encountered in the southern Paris Basin (Marty et al., 1993, Pinti and Marty, 1995, Cardoso et al., 2023).

Recently, Cardoso et al. (2023) discussed the origin of those specific signatures in the light of new precise Xenon isotopic data and confirmed the hypothesis of a mantle origin for this ³He-rich component. They pointed out the role of the local major faults system to explain the northward decrease of mantle-derived helium, from the Massif Central where magmas have been recently advected into the crust (with signatures up to 6.4 Ra), toward the southern Paris Basin where cryptic magmatism must be present below (or at the base of) the crust in areas with no apparent volcanic manifestations at the surface.

The transfer of mantle-derived helium in the hydrothermal system of the Tech fault, and on a minor proportion of the Têt fault system, could be related to the presence of magmas recently advected in the crust. However, the most recent evidence of local magmatism is represented by Tortonian to Plio-quaternary volcanism (along NW-SE grabens in the South of the Tech fault, (Cebrià et al., 2000; Maillard et al., 2020), which cannot acknowledge still active volatiles degassing. Thus, the presence of such an ³He-rich component, and the difference of isotopic signature between the Têt and Tech fault hydrothermal systems, should be tentatively related to the local geo-tectonic context, which is characterized by (i) an extensional fault system, and (ii) differential associated thinning of the continental crust.

Previous helium isotopic data obtained on hydrothermal fluids from the Alps (Marty et al., 1992) have already been associated with differential crustal thickness (Fig. 5). The main results indicated a first order correlation between isotopic helium ratio and crustal thickness. These results are consistent with the crustal thickness observed in this
region and major mantle dynamic (Granet et al., 1995; Goes et al., 1999). For continental crust > 30 km, the isotopic helium ratio is below 0.02 Rc/Ra, except for 3 sites (Fig. 5b): In the Alps, Pyrawarth (1.62 Rc/Ra) spring is located in the Vienna Basin (Austria) where the water flow and temperature variation has been correlated with active seismicity along the Vienna Basin Transfer fault (Barön et al. 2019; Hardege et al., 2019) and Villach in the Eastern Alps for which the 3 hot springs show isotopic ratios between 0.58 and 0.60 Rc/Ra, for a continental crust of ~45 km. On this site, the spring of Urquelle has the highest net heat flux of the Alps (Luijendijk et al., 2020), suggesting extremely pronounced hydrothermal activity. This site localized also active faults (Atanackov et al., 2021) with the largest historical regional earthquakes (M>6, Galadini et al., 2005; Hammerl, 2017). In the Alps, helium isotopic signature seems influenced by seismically active faults as proposed by Toutain and Baubron (1999); this isotopic record in fault hydrothermal systems appear possible even for continental crust >30 km. Heterogeneous isotopic values with some values above 0.2 Rc/Ra obtained along the Tech fault can be linked to the moderate seismic activity (Souriau and Pauchet, 1998; Sylvander et al., 2022; Ruiz et al., 2023).

**5.2 Implications for geothermal reservoirs**

Figure 4 shows that the Tech fault helium data do not follow the Têt fault clusters trend (~2 times higher). These results confirm the lack of regional geothermal reservoir and the presence of fault-hosted orogenic geothermal systems and fluid reservoirs associated at the scale of a massif and a fault (e.g. Diamond et al., 2018; Taillefer et al., 2018; Wanner et al., 2019). Water residence times estimated from helium data not impacted by mixing with surface water ($^4\text{He}/^{20}\text{Ne}$ ratio) are between 30 and 70 ka for the Têt fault
206 hot springs and between 25 and 100 ka for the Tech fault (Fig. 3), longer than water
207 residence times predicted by ¹⁴C method (Krimissa, 1995).

208 Results obtained for the Têt fault show an increase of helium isotopic ratios toward
209 the East for the different hot spring clusters that seems to coincide with the regional crustal
210 thinning (Fig. 6). However, result of VB must be interpreted carefully due to the lower
211 ⁴He/²⁰Ne ratio compared to the other samples along the Têt fault. The helium isotopic ratios
212 observed on the same fault segment for Llo, ST and Th hot spring clusters cannot be
213 explained by differences of water residence times (not consistent with ³He content). The
214 contrast tends to indicate different geothermal reservoirs at the scale of this fault segment.
215 These different geothermal reservoirs can be due to the presence of NW-SE secondary fault
216 network along the western segment of the fault (Taillefer et al., 2021) that localize
217 hydrothermal loop at the scale of the massif. Results obtained are in favor of local fault
218 geothermal systems (Milesi et al., 2020) and a compartmentalization of hydrothermal
219 circulation (Duwiquet et al., 2019; Guillou-Frottier et al., 2024) and geothermal reservoir.
220

220

221 CONCLUSIONS

222 Helium isotopic data obtained on fault hydrothermal system of the Têt and Tech
223 faults in the Eastern Pyrenees shows an increase of ³He contribution toward the East
224 consistent with the observed thinning of the continental crust in this part of the range. When
225 comparing the helium dataset from hydrothermal systems in the Alps to the results obtained
226 along the Tech fault, we question whether the helium isotopic ratios above typical
227 continental crust values could mark the seismic activity of the fault. These results show
228 compartmentalization of geothermal reservoirs, even at the scale of kilometric fault

229 segments.

230

231 **ACKNOWLEDGMENTS**

232 This research received funding from the program TelluS of the Institut National des

233 Sciences de l'Univers, CNRS and OSU THETA of the Bourgogne Franche-Comté

234 University. G.M and J.M acknowledge the support of the French Agence Nationale de la

235 Recherche (ANR), under grant ANR-21-CHIN-0006 (project Geomin3D). We thank S.

236 Ternois for Tech fault water sampling and helpful discussions, anonymous reviewers and

237 editors for their constructive comments.
<figure><img src="image_2.png" /></figure>

Figure 1: Structural map of Eastern Pyrenees modified after Taillefer et al. (2021). The name of the hot spring clusters studied are indicated: Llo, St-Thomas-les-Bains/Prats-Balaguer (ST), Thuès-les-Bains/Canaveilles (TH), Vernet-les-Bains (VB), Amélie-les-Bains (AM). Moho depth isocontours are modified from Goula et al. (1999), Mauffret et al. (2001) and Nercessian et al. (2001).
<figure><img src="image_3.png" /></figure>

Figure 2: Conceptual model of hydrothermal circulation along the Têt fault for the LLO, ST, TH, VB hot spring clusters and the Tech fault for AM hot spring cluster, modified after Krimissa (1995). The altitude of infiltration and maximum temperature at depth are issued from Taillefer et al. (2018) for the Têt fault and Krimissa (1995) for the Tech fault. The maximal depth of fluid flow paths is not to scale, and we considered a constant geothermal gradient.
Table 1. Helium isotopic data for Eastern Pyrenees hot springs along the Têt and Tech fault. R is the ³He/⁴He ratio and Ra is the atmospheric ³He/⁴He ratio (1.39.10⁻⁶, e.g. Graham, 2002).

255

<table><thead><tr><th></th><th>Hot spring cluster</th><th>Hot spring name</th><th>Lat<br>N</th><th>Lon<br>£</th><th>Cond.<br>µg/cm</th><th>T<br>°C</th><th>pH</th><th>⁴He<br>mol</th><th>⁴He<br>mol/g</th><th>⁴He²⁸Ne</th><th>error</th><th>⁴He/⁶He (R/Ra)</th><th>error</th><th>Re/Ra</th></tr></thead><tbody><tr><td rowspan="7">Têt fault</td><td>LLO</td><td>Llo</td><td>42.45113</td><td>2.06108</td><td>342</td><td>29.1</td><td>9.4</td><td>3.39E-09</td><td>5.47E-10</td><td>28.1</td><td>0.93</td><td>0.033</td><td>0.001</td><td>0.025</td></tr><tr><td>ST</td><td>Baraquette</td><td>42.50079</td><td>2.16746</td><td>303</td><td>46.8</td><td>9.3</td><td>3.90E-09</td><td>4.88E-10</td><td>60.7</td><td>2.15</td><td>0.040</td><td>0.001</td><td>0.037</td></tr><tr><td>ST</td><td>Aigues</td><td>42.50425</td><td>2.18247</td><td>317</td><td>60.0</td><td>9.2</td><td></td><td></td><td>61.2</td><td>1.79</td><td>0.039</td><td>0.001</td><td>0.037</td></tr><tr><td>TH</td><td>Cascade bas</td><td>42.52726</td><td>2.24521</td><td>323</td><td>72.5</td><td>9.0</td><td>1.13E-09</td><td>1.45E-10</td><td>46.4</td><td>1.49</td><td>0.047</td><td>0.002</td><td>0.043</td></tr><tr><td>TH</td><td>Lukas</td><td>42.52678</td><td>2.24156</td><td>291</td><td>43.2</td><td>9.4</td><td>1.78E-09</td><td>2.22E-10</td><td>38.5</td><td>1.20</td><td>0.053</td><td>0.002</td><td>0.043</td></tr><tr><td>TH</td><td>Rivière</td><td>42.53142</td><td>2.25015</td><td>324</td><td>60.1</td><td>9.1</td><td>3.45E-09</td><td>4.37E-10</td><td>34.9</td><td>1.04</td><td>0.047</td><td>0.002</td><td>0.043</td></tr><tr><td>VB</td><td>Du parc</td><td>42.54451</td><td>2.38911</td><td>299</td><td>54.8</td><td>9.1</td><td>6.94E-10</td><td>1.12E-10</td><td>5.8</td><td>0.16</td><td>0.099</td><td>0.003</td><td>0.060</td></tr><tr><td rowspan="4">Tech fault</td><td>AM</td><td>Escaladou</td><td>42.46868</td><td>2.66789</td><td>444</td><td>62.5</td><td>8.7</td><td>9.91E-10</td><td>4.50E-10</td><td>11.4</td><td>0.33</td><td>0.216</td><td>0.004</td><td>0.205</td></tr><tr><td>AM</td><td>Amelie</td><td>42.46999</td><td>2.66735</td><td>442</td><td>55.5</td><td>8.9</td><td>1.02E-09</td><td>5.36E-10</td><td>1.6</td><td>0.05</td><td>0.375</td><td>0.008</td><td>0.264</td></tr><tr><td>AM</td><td>En Comes</td><td>42.46868</td><td>2.66789</td><td>444</td><td>52.6</td><td>8.9</td><td>2.49E-09</td><td>1.13E-09</td><td>22.8</td><td>0.75</td><td>0.189</td><td>0.004</td><td>0.174</td></tr><tr><td>AM</td><td>Petit Monjollet</td><td>42.46868</td><td>2.66789</td><td>441</td><td>57.6</td><td>8.8</td><td>1.15E-08</td><td>5.22E-09</td><td>85.8</td><td>2.71</td><td>0.171</td><td>0.003</td><td>0.170</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Figure 3: Mixing diagram showing the ³He/⁴He ratio reported against ⁴He/²⁰Ne with the results obtained on the hot springs along the Têt and Tech faults (ASW: Air Saturated Water). Hot springs along Tech fault show are almost 2 times higher ³He/⁴He ratio compared to samples of the Têt fault. Water residency time model is calculated from Zuber et al. (1997), based on a middle crust composition (Rudnick and Gao, 2003) representative of the Eastern part of the Pyrenees lithologies (Hoÿm de Marien et al., 2019). A mean porosity of 0.005 % consistent with basement lithologies and ⁴He flux of 3.4E-11 cm³/cm².yr and R/Ra of the flux of 7 were considered in this model.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4: Graph of atmospheric Rc/Ra vs. longitude for the eastern Pyrenees hot springs. Vernet les Bains (VB) point is indicated with an arrow due to the lowest value of ⁴He/²⁰Ne obtained compared to the other hot springs of the Têt fault (see Figure 3). This difference can lead to an uncertainty on the Rc/Ra obtained, which is the same for Amelie and Escaladou hot springs from Amélie-les-Bains (AM) cluster.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5: Synthesis of helium data on hydrothermal water from the Eastern Pyrenees (this study), and Alps (Marty et al., 1992). The Alps data are available in Appendix 2. A) Mixing diagram showing the $^3\text{He}/^4\text{He}$ ratio reported against $^4\text{He}/^{20}\text{Ne}$. B) Rc/Ra for the different crustal thickness. Crustal thickness for the Alps hot springs are issued of Grab et al. (2009). Points with Rc/Ra above 0.2 for normal or thickened continental crust (> 30 km) are from Pyrawarth and Villach hot springs in Austria and Tech fault (Eastern Pyrenees).</figcaption></figure>
<figure><img src="image_7.png" /></figure>

Figure 6: Simplified structural map of Eastern Pyrenees showing helium isotopic ratio (Rc/Ra) obtained and Moho depth isocontours. The map shows the increase of helium isotopic ratio consistent with the crustal thinning towards the West.
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