# Fault Zones Intersections and High-Temperature Geothermal Exploration: 3D Modeling of the Chaudes-Aigues Area (French Massif Central)

Emmy Penhoët, Laurent Guillou-Frottier, Laurent Arbaret, Charles Gumiaux

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Emmy Penhoët, Laurent Guillou-Frottier, Laurent Arbaret, Charles Gumiaux. Fault Zones Intersections and High-Temperature Geothermal Exploration: 3D Modeling of the Chaudes-Aigues Area (French Massif Central). European Geothermal Congress 2025, Oct 2025, Zurich, Switzerland. hal-05049811

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# Fault Zones Intersections and High-Temperature Geothermal Exploration: 3D Modeling of the Chaudes-Aigues Area (French Massif Central)

Emmy Penhoët,<fnref n="1" />,<fnref n="2" /> Laurent Guillou-Frottier,<fnref n="1" />,<fnref n="2" /> Laurent Arbaret,<fnref n="2" /> Charles Gumiaux<fnref n="2" />

Corresponding author: e.penhoet@brgm.fr

**Keywords:** Fault zone, intersection, hot springs, numerical modeling, geothermal energy.

## ABSTRACT

This study investigates the role of fault zone intersections in enhancing geothermal potential, focusing on the Chaudes-Aigues hydrothermal system in the French Massif Central. Located at the intersection of major Variscan fault zones, the site hosts over 30 hot springs, including the Par Spring (82 °C). Despite the low natural permeability of crystalline rocks, intersecting fault zones are thought to significantly increase fluid flow and heat transport. To assess this, we conducted a series of 3D numerical models simulating different fault zone configurations (single fault zone, X-shaped, and Y-shaped geometries), varying intersection angles and permeability values. Results reveal that fault intersections facilitate thermal convection at lower permeabilities than single fault zones. X-shaped and Y-shaped configurations enhance flow compared to isolated fault zones, although they may lead to cooler surface temperatures at the extremities due to dominant cold inflows at higher permeabilities at the intersection. Back to the Chaudes-Aigues case, the Y-shaped model explains the spring emergence location but underestimates surface temperatures and flow rates. Our findings highlight the structural influence of fault intersections on geothermal systems and offer new insights for targeting high-temperature resources in amagmatic, crystalline settings.

## 1. INTRODUCTION

Currently, French geothermal power plants are limited to tectonically active areas such as the Rhine graben (Soultz-sous-Forêts) or magmatic zones (Bouillante, French West Indies). In order to increase the number of new geothermal resources (heat or power), geothermal exploration must move towards other geological contexts, such as basement areas. However, direct exploitation remains complex because of low porosity and permeability values (Achtziger-Zupancic 2017).

Fault zones are known to increase permeability locally, and their effects on fluid circulation and temperature anomalies have been documented (Forster and Smith, 1989; Barton et al. 1995; Faulds et al. 2010; Guillou-Frottier et al., 2020; Duwiquet et al., 2021; Penhoët et

al., 2025). Few studies exist on fluid circulation at the intersection of faults (Person et al., 2012; Patterson et al., 2018), even though these are shown to increase permeability and concentrate fluid flow (Curewitz and Karson, 1997; Craw 2000; Micklethwaite and Cox, 2006; Bense et al., 2013). Contrary to the Person et al. (2012) study where faults are 10-20 m wide, we studied the intersection of “fault zones”, which differ from “faults” by their large width (several hundreds of meters).

To enhance the understanding of the geothermal potential of these unusually permeable targets, our study will concentrate on the Chaudes-Aigues hydrothermal system. Located at the intersection of 2 major Variscan fault zones as illustrated by dashed black lines in Figure 1b, the town is known for hosting 30 hot springs (e.g. the Par Spring, with an emerging fluid temperature of 82°C, Figure 1a) that would be supplied by a deep reservoir whose temperature would reach over 190°C (Fouillac, 1983; Vasseur et al., 1997). Spring temperatures range from 50 to 82°C, with a total flow rate of 500 l/min (8 l/s), constant throughout the year. This configuration provides a demonstrator for understanding the impact of fault zone intersection on fluid circulation and the hydrothermal system. Our study aims to determine whether the structural particularity of the emergence area could explain the temperatures and flow rates of the springs.

## 2. GEOLOGICAL SETTING

The Chaudes-Aigues thermal springs are located in the basement made of metamorphic units (Vanderhaeghe et al., 2020) close to the northern edge of the Margeride Granite emplaced 323 Ma ago by NW-SE faults (playing the role of feeder zone, Talbot et al., 2005). On a regional scale, the area is characterized by a very high fracturing density (Penhoët et al., 2025). The emergence area is located at the intersection of 3, including 2 main fault zones forming a complex Y-shaped structure. The first fault zone, N140, characterized by the presence of another thermal spring (La Chaldette, 35°C), is more than 2.5 km thick and 10 km long. The second, oriented N50, is relatively thinner (1.5 km) but is highly pronounced in the topography, as evidenced by the orientation of the valley. The river, the Remontalou, is deviated from its initial orientation (N150) by these faults. These Variscan directions have been reactivated many times since the Paleozoic to the

<footnote n="1">¹BRGM, Georesources Division, Orléans, France</footnote>

<footnote n="2">²ISTO, UMR 7327, University of Orléans, CNRS, BRGM, Orléans, France</footnote>
present day (Penhoët et al., 2025). The third trend, which is less significant than the previous ones, corresponds to several faults and fractures oriented N0. These trends are even more visible due to the numerous

microgranite and quartz veins present in the study area (red segments in Figure 1). These fault zones have been reactivated several times over geological time (De Goer de Herve et al., 1990; Faure 1995; Penhoët et al., 2025).

<figure><img src="image_2.png" /><figcaption>Figure 1: (a) The Par Spring (82°C), one of the hottest in Europe. (b) Location of the study area simplified geological map. The geometry used for models is highlighted by the black dotted lines.</figcaption></figure>

## 3. METHODS

Chaudes-Aigues corresponds to a sector with a possible high geothermal potential. As demonstrated in previous studies, the geometry of the fault zone, more specifically its thickness over height ratio, directly influences the setting up of thermal convection (Malkovsky and Magri 2016, Duwiquet et al., 2024; Penhoët et al., 2025).

This site will be used for parametric studies. Using Comsol Multiphysics, Darcy law was coupled with heat transfer equation and mass conservation as in Guillou-Frottier et al. (2013).

### 3.1 Meshing and geometry

The first series of models corresponds to a single fault, 15 km long, 1 km wide and 5 km thick, zone with no intersection (Figure 2a). Dimensions are the same for all configurations. Next, X-shaped models (Figure 2b) were made to represent an intersection. The angle between the two fault zones forming the X is variable (Figure 2b). Finally, the specific case of Chaudes-Aigues is illustrated by a Y-shaped model with a 90° angle between the two fault zones (Figure 2c).

For parametric study, a simplified parallelepidic geometry is chosen for the fault zone and the basement. The size of mesh elements is based on permeability value. In areas of low permeability (host rocks), the mesh element size is coarse but extremely fine in the fault zone (high permeability) (Figure 2).

### 3.2 Boundary conditions

For all models, a mixed thermal condition is imposed at the surface of the fault zone (single, X or Y) to reproduce potential thermal variations corresponding to hot springs (Magri et al., 2015; Taillefer et al., 2018, 2024). It is given by:

$$Q = h (T_0 - T) \quad (1)$$

where Q is the surface heat flow, h a heat transfer coefficient and T0 the fixed temperature at the surface, away from the fault zones. The value h (heat transfer coefficient), controlling this equation, is supposed to reproduce (i) a surface heat flux of 100 mW/m² under conductive conditions and (ii) a temperature that could reach ~80 °C at the spring emergence. Outside the fault zones, a surface temperature of T0 of 10°C is applied. The basal heat flux is imposed at 85 mW/m² and bulk heat production is imposed at 3 µW/m³ to reach the measured surface heat flow of 100 mW/m² in the French Massif Central (Lucazeau et al., 1984). The surface pressure is equivalent to the atmospheric pressure (10⁵ Pa), and its value increases gradually with depth. A no flow condition was imposed on the lateral sides and bottom of the model. Lateral boundaries are thermally insulating.
<figure><img src="image_3.png" /><figcaption>**Figure 2** : Geometries and meshes used for the numerical models: (a) simple fault zone, (b) variable-angle X intersection, (c) Y intersection the case of Chaudes-Aigues.</figcaption></figure>

**3.3 Physical properties**

Fluid dynamic viscosity is a function of temperature following the equation of Smith and Chapman (1983) or Rabinowicz et al. (1998). The density of water is also a function of pressure and temperature (Guillou-Frottier et al., 2020). The main unknown in our models remains the permeability. The Margeride Granitic Complex is considered (without fracturing) as impermeable (10⁻¹⁸ m²). In the natural environment, however, the permeability of fractured rock can reach values of the order of 10⁻¹³ m² (Brace 1984; Evans et al. 1997; Sonney and Vuataz 2009; Ranjram et al. 2015; Walter 2016; Achtziger-Zupancic et al. 2016 & 2017; Neuzil 2019), so the value assigned to the fault zone will vary. With depth, the increase in pressure tends to seal the fractures, and a decrease in permeability with depth is therefore applied (Garibaldi et al., 2010).

**4. RESULTS**

A series of numerical experiments has been performed with different angles between fault zones (30 to 90°), geometries (simple, X-shaped and Y-shaped) and different fault zone permeabilities. Temperature field, positive thermal anomalies (defined as the excess temperature when compared to the conductive regime), velocity field, and convective patterns, are recorded. In order to calculate the flow rates at the surface, the surface of the models is divided into small zones, which

are only used for flow rate calculations. The flow rate is calculated at the final time step as an integral of velocity field over the small surfaces of the fault zone. The results for the simple fault zone are shown in Figure 3, the X-shaped intersection models in Figure 4 and the Y-shaped intersection models in Figures 5 and 7. Details of the various calculations are given in Figure 6.

**4.1 Single fault zone**

In our configuration, above a permeability of 3 10⁻¹⁴ m², convection starts up and temperature anomalies appear. They first appear at the center of the fault zone, then as permeability increases, they become unstable and move from the center to the edges of the fault zone (Figure 3). High temperatures anomalies between 44 and 50°C appear at shallow levels (200-210 m). The temperatures of potential thermal springs are high, reaching 57°C at a flow rate of 10 l/s (52°C and 91/s in Figure 3b). At depth, e.g. 1500 m, positive temperature anomalies remain visible, with values around 30°C.

<figure><img src="image_4.png" /><figcaption>**Figure 3** : (a) Case of a simple fault zone, for high permeability values (3 10⁻¹⁴ m²) the anomaly moves towards the end of the fault zone. (b) Cross-section of the model at the surface, production of 3 potential thermal springs.</figcaption></figure>

**4.2 X-shaped configurations**

In order to reproduce several natural cases, three different angles of intersection were tested: 30, 60 and 90°. In terms of locating positive temperature anomalies, the 3 types of models produced the same results. When the permeability value is low, lower than 3 10⁻¹⁴ m², hot upwellings are concentrated at the
intersection of fault zones (Figure 4). The ends then correspond to infiltration zones dominated by cold flows. However, as the permeability value increases, positive anomalies are found at the 4 ends of the fault zones. The surface common to the two fault zones, the intersection, becomes a major infiltration zone.

In addition, the results in terms of positive temperature anomaly values, thermal springs and flow rates differ. The 30° angle produces relatively low surface temperatures, reaching a maximum of 44.2°C for 3 × 10⁻¹⁴ m² (average of 20.4°C). The associated outflow rates corresponding to these temperatures are high, reaching up to 40 l/s. Maximum temperature anomalies at depth are confined to rather shallow depths, 400 m on average. The associated values reach a maximum of 46.2°C. When the angle is at its maximum, 90°, the springs produced are slightly different. Surface temperatures are higher, up to 53°C, but with lower flow rates (30 l/s maximum). The maximum positive temperature anomalies are deeper and higher than for the previous configuration, reaching up to 55.7°C at average depths of 545 m. The 60° model produced intermediate results. Surface temperatures are lower at an angle of 90° but higher than the angle of 30°. The flow rates show the same behavior as the deep anomalies (temperature and depth).

<figure><img src="image_5.png" /><figcaption>**Figure 4 :** (a) X-shaped configuration, 60°, for low permeability values (9 10⁻¹⁵ m²) the thermal anomaly is confined at the intersection. (b) Cross-section of the model at the surface, production of one potential thermal spring.</figcaption></figure>

## 4.3 Y-shaped configuration

The Y-shaped configuration shows similar behavior as the X-shaped models, with an angle of 90°.

For low permeability values (8 10⁻¹⁵ m²), the positive temperature anomalies are deep and of low amplitude at the intersection of the two fault zones. As permeability increases, they intensify and become more surficial, forming at the 3 extremities of the model (Figure 5). They reach a maximum value of 48°C at a depth of 650 m (10⁻¹⁴ m²). Beyond this value, cold downward flow becomes the predominant feature (Figure 5) and the positive anomaly values decrease. At the surface, the highest temperatures reached are up to 55°C and an associated flow rate of 25 l/s (see green curve in Figure 6).

<figure><img src="image_6.png" /><figcaption>**Figure 5 :** (a) Y-shaped configuration, for high permeability values (3 10⁻¹⁴ m²) 3 thermal anomalies are created at the edges of the fault zones. (b) Cross-section of the model at the surface, production of 3 potential hot springs.</figcaption></figure>

# 5. DISCUSSION

## 5.1 Springs characteristics

All the configurations tested generate positive temperature anomalies at the surface, and therefore the production of potential thermal springs. However, the characteristics of these springs are not identical for an intersection of fault zones or a single fault zone.

In the case of a simple fault zone, the flow rates from the springs remain low despite the increase in the permeability of the fault zone. The maximum value reached is 10 l/s, which remains low (purple, Figure
6a). However, this low flow rate also results in the highest spring temperatures, reaching 57°C.

<figure><img src="image_7.png" /><figcaption>Figure 6 : (a) Outflow rate corresponding to upwelling zones, (b) maximum surface temperature reached and (c) temperature anomaly at 1000 m depth as a function of fault zone permeability.</figcaption></figure>

The X-shaped configurations clearly show the impact of the intersection angle on fluid flow. The smaller the angle, the greater the outflow rates from potential

springs (Figure 6a). Flow rate from a 30° intersection is higher than that from a 90° intersection, although the difference remains small for a permeability of 3 10⁻¹⁴ m², 40 l/s compared with 25 l/s respectively. The differences in outflow between a single fault zone and an intersection are primarily due to geometry. In fact, the geometry of the intersection doubles the volume of a potential infiltration zone, which considerably increases outflow rates. On the other hand, this advantage can quickly become disadvantageous when permeability increases with the preponderance of cold flows. When it comes to surface temperature, the opposite is observed. For any value of permeability, the greater the angle, the higher the positive temperature anomalies (Figure 5b). The 90° configuration gives the highest spring temperature of 53°C.

To summarize, simple fault zones are thought to produce hotter thermal springs but with low flow rates, unlike intersection zones, which are colder but have higher flow rates.

## 5.2 Set up of thermal convection

In parallel with these observations, it is possible to see that the intersection of fault zones facilitates the onset of convection. The simple fault zone produces temperature anomalies only for very high permeability values (K > 3 10⁻¹⁴ m²). In the case of Y or X intersections, convection starts at low permeabilities (as low as 8 10⁻¹⁵ m²). This phenomenon can be seen directly in Figure 6c, where deep anomalies are first produced by fault intersections.

## 5.3 Implications for Chaudes-Aigues area and limits

The Chaudes-Aigues emergence area has been reproduced with the Y-shaped configuration. The results are intermediate, with lower temperature anomalies than a simple fault and lower flow rates than an intersection. For low permeabilities (8 10⁻¹⁵ – 10⁻¹⁴ m²), it is possible to reproduce a spring located at the intersection of the two fault zones (Figure 7a). If the location is correct, the associated flow rates remain too low (0.001 - 3 l/s, Figure 7b) as well as the surface temperature (15 - 43 °C, Figure 7b).

These results show that geometry has an influence on flow rates and emergence temperatures, but that it is not the only parameter. Particularly in the case of Chaudes-Aigues, located in a mountainous area, topography must be taken into account. In addition, the reservoir supposed to be at 190°C at a depth of 5 km (Vasseur et al., 1997) has not been reproduced here.

# 6. CONCLUSIONS

This study highlights the key role of fault intersections in controlling geothermal fluid flow and surface thermal anomalies. Intersections allow convection to occur at lower permeabilities than isolated faults, leading to higher discharge rates despite lower surface temperatures.
<figure><img src="image_8.png" /><figcaption>**Figure 7:** (a) Configuration reproducing the location of Chaudes-Aigues springs (K = $10^{-14}$ m²). (b) Cross-section of the model at the surface, production of an emergence area.</figcaption></figure>

The geometry of the intersection influences both flow intensity and heat distribution. At Chaudes-Aigues, the Y-shaped structure explains the spatial pattern of springs but not the full extent of flow and temperature, suggesting deeper thermal inputs or topographic effects. These results underline the importance of incorporating regional structural complexity into models and geothermal exploration.

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Acknowledgements (optional)

This study has been supported by the GERESFAULT project (ANR-19-CE05-0043) and corresponds to one part of E.P. PhD thesis.