# 3D Structural Modelling and Restoration of a Deformed Alpine Karst Reservoir: Insights into the Groundwater Flows of the Dévoluy Massif (French Alpine Foreland)

Oriane Parizot, Nicolas Espurt, Benoît Viguier, Guillaume Duclaux, Alexandre Zappelli, Abdeltif Lahfid, Nathan Rispal, Ludovic Mocochain, Pierre Henry, Marianna Jagercikova, et al.

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## To cite this version:

Oriane Parizot, Nicolas Espurt, Benoît Viguier, Guillaume Duclaux, Alexandre Zappelli, et al. 3D Structural Modelling and Restoration of a Deformed Alpine Karst Reservoir: Insights into the Groundwater Flows of the Dévoluy Massif (French Alpine Foreland). Basin Research, 2025, 37 (5), pp.e70065. 10.1111/bre.70065. hal-05316112

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RESEARCH ARTICLE

OPEN ACCESS

# 3D Structural Modelling and Restoration of a Deformed Alpine Karst Reservoir: Insights into the Groundwater Flows of the Dévoluy Massif (French Alpine Foreland)

Oriane Parizot<fnref n="1" /> ID | Nicolas Espurt<fnref n="1" /> ID | Benoît Viguier<fnref n="1" /> ID | Guillaume Duclaux<fnref n="1" />,² ID | Alexandre Zappelli³ ID |
Abdeltif Lahfid⁴ ID | Nathan Rispal³,⁵ ID | Ludovic Mocoinch⁶ ID | Pierre Henry³ ID | Marianna Jagercikova⁷ |
Philippe Audra⁵ ID | Bruno Arfib³ ID | Laurent Cadilhac⁸

**Correspondence:** Oriane Parizot (oriane.parizot@geoazur.unice.fr)

**Received:** 8 March 2025 | **Revised:** 4 August 2025 | **Accepted:** 29 September 2025

**Funding:** Agence de l'Eau—Rhône Méditerranée Corse and the Région Sud-Provence-Alpes-Côte d'Azur.

**Keywords:** 3D structural modelling | Dévoluy Massif | hydrostructural approach | karst | Senonian carbonates | syncline

## ABSTRACT

In foreland fold-thrust belts, tectonically deformed aquifers remain underexplored, largely due to the complex architecture of their hydrogeological reservoirs. This limited understanding prevents the development of sustainable management policies to face the current decline in groundwater availability. In the southwestern Subalpine Chains, we characterised the structure of the folded Senonian karst aquifer of the Dévoluy Massif, which is transected by the Median Dévoluy Thrust. Using surface structural data, we performed an original 3D structural model of the massif. Quantitative analyses of this model indicate a total Senonian package volume of 114 km³, of which 25 km³ lie below the elevation of Gillardes Spring, the main karst discharge. A structural restoration of the base of the Senonian indicates Alpine shortening of ~4.4 km in the southern part of the massif and ~1.2 km in the northern part. The deformation of the karst reservoir into two N-trending synclines, separated by the Median Dévoluy Thrust, is identified as a key control on the northward groundwater drainage pattern, culminating at Gillardes Spring. This 3D structural modelling approach also allowed for identification of suitable areas for the exploration of new deeper resources. Finally, this study highlights the potential of a hydrostructural approach and 3D structural modelling for assessing the hydrogeology in tectonically deformed karst reservoirs.

## 1 | Introduction

In faulted and folded foreland systems, rising hydroclimatic pressures (e.g., longer droughts, cryosphere vanishing) and increasing water demands have led to many concerns about the availability and the sustainable use of water resources (Vivioli et al. 2020), including groundwater (Jódar et al. 2021; Kuang et al. 2024; Somers and McKenzie 2020). However, in such areas,

hydrogeological understanding remains limited due to a low density of monitored sources (boreholes or springs), which prevents the development of accurate groundwater flow modelling, and hence appropriate management policies. As is typical with different types of aquifers, including karst, several factors govern the groundwater flow (topography, surface-water exchange, intern hydraulic properties variations, karst development, etc.). But in mountain systems (e.g., Alps) and tectonically deformed

This is an open access article under the terms of the [Creative Commons Attribution License](https://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *Basin Research* published by International Association of Sedimentologists and European Association of Geoscientists and Engineers and John Wiley & Sons Ltd.

<footnote n="1">¹Géoazur, Univ. Côte d'Azur, CNRS, Observatoire de la Côte d'Azur, IRD, Valbonne, France | ²School of Earth Sciences, The University of Western Australia, Perth, Western Australia, Australia | ³Aix Marseille Univ., CNRS, IRD, INRAE, CEREGE, Aix-en-Provence, France | ⁴Bureau des Recherches Géologiques et Minières, Orléans, France | ⁵Univ. Côte d'Azur, Polytech'Lab-UPR 7498, Sophia-Antipolis, France | ⁶1797 Rte des Puys-F05200, Puy-Sanières, France | ⁷La Tour, Saint-Maurice-en-Valgodemard, France | ⁸Agence de l'Eau Rhône Méditerranée et Corse, Lyon, France</footnote>
## Summary

* The 3D structural model highlights the hydrostructural architecture of the Senonian karst reservoir

* Shortening in the reservoir ranges from 1.2 to 4.4 km and is archived by two synclines separated by the Médian Dévoluy Thrust

* The restoration illustrates the pre-contractional geometry of the Senonian basin and inheritance

* The modelled volume of Senonian reservoir is 114 km³, of which 25 km³ lies under the Gillardes spring

* Close relationship between underground water circulation and reservoir structural architecture

aquifers, the complex architecture of the faulted and folded reservoirs and related hydrostructures strongly influences groundwater flow (Ball et al. 2014; De La Torre et al. 2020; Klaba et al. 2024; Labat et al. 2021). Fracture networks and fault zones associated with geological structures are particularly important as they locally modify the hydraulic properties and act as barriers and/or drains (Bense et al. 2013; Boura et al. 2024; Figueroa et al. 2021). Although few (Goldscheider 2005; Goldscheider and Neukum 2010), some studies have highlighted the control of fold geometries on groundwater flow directions for tectonically deformed aquifers, such as synclines and anticlines in convergent and divergent flows, respectively. Consequently, a better understanding of the present and past 3D geometry of the geological structures in fold-thrust belts will significantly improve assessments of groundwater systems (Abirifard et al. 2022; Ball et al. 2014; Fernández-Ibáñez et al. 2019; Legarreta-González et al. 2024; Morante-Carballo et al. 2022).

In this study, a 3D geological approach was carried out on a major hydrogeological reservoir in the southwestern French Cenozoic Subalpine Chains, the Senonian karst formation of the Dévoluy Massif (Figure 1). Both geological and hydrogeological aspects of the Dévoluy Massif have been seldom studied over the last decades (Arnaud 1981; Dubois 1962; Dubois and Fontes 1962; Gidon et al. 1970; Gidon and Pairis 1976; Lory 1900; Meckel 1997; Meckel et al. 1996; Mercier 1958; Michard et al. 2010). Although the Gillardes Spring, which drains the Senonian karst reservoir (Bonhomme 1972; Luparini 1975), is one of the most important springs in France with an interannual average discharge of ~5 m³/s, karst and speleological studies were only recently carried out (Audra 1996; Audra et al. 2009; Jagercikova et al. 2022, 2021; Lemot et al. 2023; Lismonde et al. 2008; Zappelli et al. 2018).

Therefore, the aim of this study is to produce a balanced and restored 3D structural model of the deformed Senonian carbonate formation of the Dévoluy Massif to better constrain the current reservoir's 3D structural characteristics (e.g., geometry, paleogeometry and volume) and to quantify the distribution of the Alpine shortening in 3D space. Based on this original modelling and a hydrostructural approach, we aim to decipher the internal architecture of the tectonically deformed Dévoluy karst aquifer and identify hydrogeological boundaries and dominant groundwater flows directions at the massif scale.

## 2 | Geological Setting

The Dévoluy Massif is an isolated topographic relief spanning ~200 km² in the southwestern Subalpine Chains of France, bounded by the Champsaur Valley to the east, the Bochaine Massif to the southwest and the Trièves area to the northwest (Figure 1). It forms a perched syncline characterised by spectacular vertical and lateral cliff sections of deformed Senonian limestone reservoirs with altitudes ranging from ~900 m to ~2800 m a.s.l. The massif includes mountain peaks such as the Pic de Bures (2709 m a.s.l.) to the south, the Pic Pierroux (2377 m a.s.l.) to the east, the Grande Tête de l'Obiou (2789 m a.s.l.) to the northwest and the Grand Ferrand (2758 m a.s.l.) to the west (Figure 1). At the surface, the massif is mainly drained northward by Souloise and Ribière rivers which outflow to the Drac River (Figure 1).

### 2.1 | Lithology

The deformed stratigraphic succession of the Dévoluy Massif consists of three main packages. The basal pre-Senonian package is Jurassic-Turonian in age. Its uppermost unit consists of a marl-rich impermeable layer, the Late Aptian-Cenomanian marlyturbiditic ‘blue marls’ formation, which ranges from 200 to 300 m thick (Bréhéret 1995; de Graciansky and Lemoine 1988; Gidon et al. 1980; Michard et al. 2010) (Figure 2). The intermediate Senonian package is Coniacian-Maastrichtian in age. This is a thick layer of deep-water calci-turbidites, also known as ‘calcaires en Lauzes’ formation, with a variable thickness of almost 1000 m to the northwest (Dubois and Fontes 1962; Gidon et al. 1980; Mercier 1958) (Figure 2). In detail, the Senonian package includes at its base the ‘Lauzes marneuses’ (Campanian) corresponding to deposits of dark clay-limestone. Above these lie the ‘Lauzes siliceuses’, followed by beige limestones at the top, associated with oyster and silex beds from the Late Maastrichtian (Gidon et al. 1971). Furthermore, this Senonian micritic limestone is poorly porous and permeable. It is therefore the caves, karst network, fractures network and stratification joints within the Senonian package that allow water to be transported from the surface to the depths (Bonhomme 1972; Luparini 1975). The Cenozoic package begins with Eocene continental conglomerates followed by nummulitic limestones, Globigerina marls, and finally the Oligocene arenites known as ‘Grès de Saint-Disdier’ (Gidon et al. 1980, 1971; Huet 2024; Meckel et al. 1996). Quaternary deposits, including glacial, fluvial and slope formations (e.g., large screeps), unconformably overlie the entire massif (Gidon et al. 1980).

### 2.2 | Structural Framework

The overall geometry of the massif corresponds to a perched synclinorium armed of Senonian limestones subdivided in two synclines: the NNW-trending Saint-Étienne syncline to the east and the NNE-trending Saint-Disdier syncline to the west (Figure 1). Both synclines are bounded by the NNE-trending, W- to SW-verging Median Dévoluy Thrust (Meckel et al. 1996), which links the Aspres-lès-Corps strike-slip fault to the north and the Digne Nappe to the south (Gidon et al. 1970; Lickorish and Ford 1998) (Figure 1). The massif is also cut by 000° N–050° N fault systems
<figure><img src="image_3.png" /><figcaption>**FIGURE 1** | Geological map of the Dévoluy Massif, modified after 1:50,000 BRGM geological maps of Saint-Bonnet and Gap sheets (Gidon et al. 1980, 1971). The Dévoluy Unconformity (DU) is indicated by blue line and the Eocene Unconformity (EU), by green line. CM, Central Massif; BGCK, The Banards-Grand-Combe klippe system.</figcaption></figure>
discussed in Meckel et al. (1996) (Figure 1). In the southeastern part of the massif, the SW-verging Banards and Grand-Combe klippen (Figure 1) (Gidon et al. 1980; Meckel et al. 1996) are made of overturned E-dipping pre-Senonian, Senonian and Cenozoic strata, which overlie E-dipping Cenozoic strata of the eastern limb of the Saint-Étienne syncline. These klippen are cut by WNW-dipping gravitational normal faults (Meckel et al. 1996).

A key structural feature of the Dévoluy Massif is the Dévoluy Unconformity (DU; Figure 2), located at the base of the Senonian package. This surface overlies a highly deformed pre-Senonian package, inherited from a Turonian tectonic event (Mercier 1958; Michard et al. 2010 and references therein). The Dévoluy Unconformity is a sub-marine unconformity

(Michard et al. 2010) that cuts E-W trending fold-and-thrust systems in the pre-Senonian package. These structures are related to an earlier N-S compressive event (Baudrimont and Dubois 1977), which predates the uppermost Santonian Provençal compression (Philip and Allemann 1982). This older tectonic event, associated with both deformation and major erosion, is interpreted as resulting of S-directed sub-marine thin-skinned gravitational sliding of the northern slope of the Vocontian Basin over the Bathonian-Oxfordian black shales (Michard et al. 2010). Following this collapse and the deep-sea sedimentation of the Senonian calci-turbidites, the massif was subsequently moderately deformed, uplifted and unconformably overlain by a mixed continental to marine Cenozoic package (Figure 2; Gidon et al. (1980, 1971)). This contact corresponds to the Eocene Unconformity (EU; Figure 2), which

<figure><img src="image_4.png" /><figcaption>**FIGURE 2** | Synthetic lithostratigraphic column of the Dévoluy Massif modified from Michard et al. (2010) and Gidon et al. (1980). The Senonian karst formation (Coniacian-Maastrichtian) lies unconformably above the deformed pre-Senonian package (Jurassic-Turonian). The Senonian package is unconformably covered by Cenozoic package (Eocene, Oligocene and Quaternary). The base of the Senonian package, the Dévoluy Unconformity (DU) is indicated by blue line and the top, the Eocene Unconformity (EU), by green line.</figcaption></figure>
separates the Cenozoic package from the underlying Senonian package with a low angular unconformity (10°–20° in central Dévoluy and <10° in the southern part of the massif) (Meckel et al. 1996). A third unconformity is observed within the Cenozoic succession, between the Eocene continental conglomerates and the overlying nummulitic limestones. This contact is associated with transtensional deformation, which can be observed through the presence of syn-sedimentary normal and strike-slip faults (Gidon and Pairis 1976). Afterwards, the Dévoluy Massif and neighbouring areas were buried under sedimentary cover during the Oligocene and the beginning of the Miocene (Boschetti et al. 2025; Lemot et al. 2023; Meckel 1997). During the Neogene, the area experienced a renewed E-W compressive regime, associated with Alpine shortening. This shortening led to the development of the Saint-Étienne and Saint-Disdier synclines, separated by the Median Dévoluy Thrust, and dextral slip along the Aspres-lès-Corps fault (Meckel 1997; Meckel et al. 1996). The Banards and Grand-Combe klippen were also emplaced during this period, and were later tilted due to break-forward folding of the Saint-Étienne syncline.

## 2.3 | Groundwater Resources

In the Dévoluy Massif, two rivers drain surface runoff to the north: the Souloise and the Ribière in the eastern and western part, respectively (Figure 1). The Senonian karst reservoir constitutes the main hydrogeological resource of the massif (Bonhomme 1972; Luparini 1975) and is generally bounded at depth by the Dévoluy Unconformity, which acts as a no-flux boundary condition (Figure 2) (Luparini 1975). It is widely accepted that most of the groundwater drains northwards, emerging at a single major point: the Gillardes Spring (Bonhomme 1972; Luparini 1975) (Figure 1). This has been confirmed by dye tracing tests carried out by the dye injection in the caves (Zappelli et al. 2018). These tests also revealed a secondary, southward drainage, beneath the Pic de Bures down to the Sigouste Spring (Figure 1). The Gillardes Spring is one of the most significant karst springs in France, with an interannual average discharge of ~5 m³/s, ranging from 0.2 and 38 m³/s. Despite its importance, studies on the hydrogeological functioning of the Senonian karst reservoir are long-standing (Bonhomme 1972; Luparini 1975). More recent speleological investigations have focused on the significant development of the karst within the Senonian formation (Audra 1996; Audra et al. 2009; Jagercikova et al. 2022, 2021; Lemot et al. 2023; Lismonde et al. 2008; Zappelli et al. 2018): the caves and shafts are predominantly vertical, controlled by the dip, and some are several hundred meters deep, up to 960 m for the Chourum (shaft) des Aiguilles. Ancient, inactive drains are known at various altitudes, the highest of which have been dated to the Middle Miocene. Most of the caves are located on high slopes where Senonian limestone outcrops, mainly on the western flank and its outer walls, as well as on the south-eastern flank (Aurouze and Bure plateaus). All shaft are developed within the vadose zone. The phreatic zone at the bottom of the western syncline is only reached in the bottom of the Puits des Bans and the Tune des Renards. The Puits des Bans, located on the eastern flank, 5 km upstream of the Gillardes Spring, operates as an overflow chimney, discharging during flood periods after a backflooding of more than 220 m. In addition, several

springs within the Dévoluy Massif discharge from shallow, irregular aquifers hosted in the Cenozoic and Quaternary layers, which serve as important sources of drinking water for both humans and livestock. The disconnection of shallow aquifers and Senonian karst by the Eocene Globigerina marls is still unclear due to the irregular thickness of marls, which can be partially lacking or thick up to 100 m (Gidon et al. 1971; Figure 2). This study aims to provide new findings in the understanding of the hydrogeological conceptual model of the Dévoluy Massif.

## 3 | Methodology

The three-dimensional (3D) reconstruction is a powerful tool for visualising and analysing the geometry of complex tectonic structures (Faure et al. 2024; Fernández et al. 2004; Ramos et al. 2020). Unlike in usual basin exploration, the Dévoluy Massif lacks deep boreholes and seismic reflection data. However, its exceptional outcrop conditions provide extensive surface exposure, making it an ideal field for a 3D structural modelling. In this study, we performed a 3D structural model of the Senonian Dévoluy formation using the MOVE structural modelling and analysis software, based on the flexural-slip restoration algorithm (Ramsay and Huber 1987). This 3D structural model was calibrated using surface structural data derived from previously published 1:50,000 BRGM geological maps (Saint-Bonnet and Gap sheets (Gidon et al. 1980, 1971)), published geological data (Meckel et al. 1996; Richard et al. 2010), Geol-Alp website (www.geol-alp.com), and new data acquired during a 2-week field survey conducted in this area in 2024, including geological mapping, dip and fault-slip measurements, and drone panoramic views (Figures 1 and 3a).

The structural model includes a 3D geological mapping of the Senonian package envelope onto a 25 m resolution digital elevation model (topography), that is, the Dévoluy Unconformity horizon (DU) at its base and the Eocene Unconformity horizon (EU) at its top (Figure 3a). 3D reconstruction of DU and EU horizons, and fault planes was performed using this 3D surface contour mapping from surficial data along 29 serial cross-sections (23 longitudinal cross-sections and 6 latitudinal cross-sections). These were constructed using bed and fault contours, and dip data (Figure 3) which allowed us interpreting the geometry of structures at depth and eroded ‘in the air’ above the topographic surface according to the concepts of thrust tectonics (Boyer and Elliott 1982; Dahlstrom 1969; Suppe 1985) (Figure 3b). The orientations of the longitudinal cross sections are ~ESE-WNW in the Saint-Étienne syncline and ~WNW-ESE in the Saint-Disdier syncline. These two distinct trends are orthogonal to the axis of the synclines. The latitudinal cross-sections are subparallel to the trends of the synclines. The surface contours on the digital elevation model and cross-sections are interpolated with a triangular mesh to create 3D surfaces of DU and EU horizons, Median Dévoluy Thrust, basal thrust of the Banards-Grand-Combe klippe system, as well as second-order faults. The DU and EU surfaces have been smoothed at the intersections with second-order fault planes and the current volume of the Senonian package was modelled using a ~50 m cell size.

Restoration of the DU horizon was carried out based on bed length and thickness conservation and the built-in flexural-slip
<figure><img src="image_5.png" /><figcaption>**FIGURE 3** | (a) Geological map of the Senonian package used for 3D structural modelling. The DU horizon is indicated by blue line and the EU horizon by green line. The trace of the 29 geological cross-sections used to produce the 3D structural model are indicated. (b) Examples of geological cross-sections across the massif. The first two, with an approximate north-south orientation, show the pseudo-tabular geometry of the Senonian package (CS01NS) or the dipping towards the north (CS02NS). The three longitudinal cross-sections highlight the geometry of the two synclines and the contribution of the Median Dévoluy Thrust to the current geometry of the massif. The caption is similar to Figure 3a. All the geological cross-sections can be seen in the **Supporting Information**.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>FIGURE 3 | (Continued)</figcaption></figure>

algorithm. The top of the Senonian package (EU horizon) can be considered as a flat template surface (subaerial erosional surface) to restore the model, since no major upper Cretaceous sea-level variations are recorded in the area (Lory 1900). Restoration was performed along a 255° N transport direction, consistent with W- to SW-trending Alpine shortening estimated by Meckel et al. (1996). This 3D unfolding of the Senonian package envelope allows for an independent quantification of both the slip along the Median Dévoluy Thrust and the magnitude of folding affecting the Senonian strata.

# 4 Results and Discussion

## 4.1 Field Data

The data collected in the field enabled to (1) constrain the structural geometry of the massif (2) delineate the limits of the Senonian package envelope (DU and EU horizons), and (3) refine the kinematics and position of key faults.

In the first case, the Saint-Disdier syncline is asymmetrical, with Senonian strata dipping 20 to 30° E on the western flank over a length of 5 km. In contrast, the eastern flank, exposing Cenomanian strata at the surface, is significantly narrower (~1 km wide), with a sub-horizontal dip visible on the cross-sections (Figure 3b). The Saint-Étienne syncline is restricted to the north (2 km wide), clamped between the Gicon relief to the west and the Faraud mountain to the east, and exhibits relatively steep dips (~50°) on both flanks. Further south, the western flank tends to flatten (~20° E) and widens considerably (10 km separates the Median Dévoluy Thrust from the Banards-Grand-Combe klippe system), while the eastern flank straightens and becomes overturned under the Banards klippe (Figure 3b).

With regards to the extent of the perched syncline, drone panoramic views of the outer perimeter of the massif were used to determine its geometrical characteristics. It these images, the DU horizon is clearly visible, juxtaposing deformed pre-Senonian strata with the flat-lying Senonian formation in cliff sections (Figure 4a,b). To the northeast and southeast of the
<figure><img src="image_7.png" /><figcaption>**FIGURE 4** | Illustrations showing interpreted drone panoramic views and the data integrated into the 3D model. See Figures 1 and 2 for the location. The DU horizon is indicated by blue line and the EU horizon by green line. (a) View from Champsaur, looking west towards the Faraud Mountain. (b) View from Trièves, looking east towards Tête de la Cavale and Grand Ferrand. (c) View from south of the Rabou Pass, looking north towards the Banards-Grand-Combe Klippe system and the Faraud Mountain.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**FIGURE 5** | Interpreted drone panoramic views illustrating the structural styles observed in the center part of the Dévoluy Massif. See Figures 1 and 2 for the location. The green line corresponds to the EU horizon. (a) View from the south, looking north, along the axis of the Saint-Étienne syncline, at the Étroits site. (b) View from south of the Saint-Disdier town, looking towards the Gicon relief and the Median Dévoluy Thrust. Fold axes are presented on the equal-area (Schmidt) stereographic lower-hemisphere projection. *n*: data number, ØM: Median Dévoluy Thrust, BGCK: The Banards-Grand-Combe klippe system, Sn: Senonian, pSn: Pre-Senonian, Eo: Eocene, Ol: Oligocene.</figcaption></figure>

massif, the EU horizon can also be observed, allowing for a precise definition of the top Senonian package on the edges of the model (Figures 3 and 4a,c). The EU horizon is also visible within the massif, both along syncline flank and in deeply incised river valleys (Figures 3 and 5a).

Field observations highlighted the kinematics of some faults and thrusts, which were integrated into the 3D model. This is particularly true for the Étroits fault, a dextral reverse fault plane located west of Saint-Étienne town (Figures 1, 3b, and 5a). The Median Dévoluy Thrust and the Banards-Grand-Combe
klippe system have also been studied in detail. The Median Dévoluy Thrust is associated with a ramp anticline, which is more or less developed between north and south. Northward, this thrust uplifts the Gicon anticline, composed of Senonian formations (Figure 3b) and associated with a polyphase tectonic history (Gidon and Pairis 1976; Meckel et al. 1996). South of this feature, beige Eocene strata overthrust red Oligocene strata, which dip 10°–20° eastward (Figures 1 and 5b). In the northern part of the massif, the Median Dévoluy Thrust is characterised by a less developed ramp anticline, reflecting limited shortening. In contrast, farther south, the hanging wall ramp exhibits folded pre-Senonian and Senonian strata emplaced over Oligocene conglomerates of the Saint-Disdier syncline, also known as the Montmaur syncline on this southern part of the massif (Gidon et al. 1971). Here, the ramp anticline is

more advanced and associated with greater apparent shortening. At the outcrop scale, the Median Dévoluy Thrust is defined by (1) small footwall structures in the Oligocene strata and (2) a shear zone that is more indured than the host-rock. Within this zone, shallowly dipping fold axes trending between 90°N and 180°N indicate an oblique orientation of the maximum principal stress $\sigma_1$ (~NE–SW) relative to the N-S trending structure (Figure 5b). These folds show dextral reverse, post-Oligocene activity of the Median Dévoluy Thrust in agreement with the W-directed compression documented by Meckel et al. (1996). Regarding the Banards-Grand-Combe klippe system, it consists of overturned, E-dipping pre-Senonian, Senonian and Cenozoic strata, overlying E-dipping Cenozoic strata (Figures 3b and 4c). However, contrary to the existing geological map (Gidon et al. 1971), field data suggest that the fault

<figure><img src="image_9.png" /><figcaption>**FIGURE 6** | 3D model illustrations. (a) 3D view from the south, showing EU and DU horizons and the plane of the Median Dévoluy Thrust before erosion and in their current configuration. (b) 3D view from the southwest, showing EU and DU horizons and the plane of the Median Dévoluy Thrust before erosion and in their current configuration. (c) 2D and 3D views of the current model (after erosion). The digital elevation model within the massif has been removed to better see the structures at depth. BGCK, The Banards-Grand-Combe klippe system; ØM, Median Dévoluy Thrust; GS, Gillardes Spring.</figcaption></figure>
plane responsible for this structure is located within Eocene marls, rather than between Eocene nummulitic limestones and marls (Figure 4c). This field observation was also used to refine the fault geometry in the structural model.

## 4.2 | The 3D Model of the Dévoluy Massif

Geological cross-sections and surface data allowed the construction of the 3D model of the Senonian formation, as illustrated in Figure 6. In detail, in the current 3D configuration, the DU horizon is present throughout the model, and its outer limits correspond to the model bounds. The EU horizon, on the other hand, can only be identified in the center of the massif, particularly along the axis of the two synclines. Depth extrapolation of the NE-SW oriented fault system that cuts the Senonian package within the Saint-Étienne syncline shows variable mean dips of these planes, ranging from ~45° to 90°. In addition, the morphology of the Median Dévoluy Thrust varies between the north, where it dips strongly (40° E-60° E), and the south, where it appears to flatten at depth (Figure 7).

Regarding the morphology of the DU horizon, its elevation ranges from 186 m a.s.l. (e.g., south of St-Disdier town, Figure 1) to 2336 m a.s.l. (below 'Rocher Rond', Figures 1 and 8a). In general, the highest elevations are on the eastern, western, and southern edges (Bure Plateau) of the massif rather than in its center due to the synclinorium geometry. In addition, according to the 3D model, the southern part of the Saint-Étienne syncline is tilted to the north in contrast to the Saint-Disdier syncline that appears relatively flat along its length (Figure 8b). In both cases,

<figure><img src="image_10.png" /><figcaption>**FIGURE 7** | The depth geometry of the Median Dévoluy Thrust (ØM). BGCK, The Banards-Grand-Combe klippe system; GS, Gillardes Spring.</figcaption></figure>

the lowest elevations of the DU horizon coincide with the axis of the two synclines (Figure 8a).

The current volume of the Senonian package, derived from the DU basal reservoir surface, the EU top formation surface, and the present-day topography, is 114 km³ (Figures 3b and 8a,c,d). This volume is divided into 58 km³ in the Saint-Étienne syncline and 56 km³ in the Saint-Disdier syncline. The thickest preserved portion of the Senonian package is located beneath the Bure Plateau and on the western flank of the Saint-Disdier syncline, where it reaches a thickness of more than 1000 m (Figure 8d). In contrast, it is relatively thin (< 300 m) close to the model boundaries and near the Median Dévoluy Thrust, where it outcrops at the surface.

## 4.3 | 3D Model Restoration

The 3D and 2D restorations of the DU horizon show an average thickness of the Senonian package of ~500–600 m (Figure 9a). The latter appears to become thicker at the present location of the Grand Ferrand and the Bure Plateau, reaching more than 1000 m in these areas. In contrast, at the present-day Gicon position, as well as to the southwest of the Median Dévoluy Thrust and at the Banards-Grand-Combe klippe system, the Senonian package is much thinner, in the order of a hundred meters (Figure 9a). These variations in thickness suggest the development of small NNW-SSE sedimentary depocenters during the Senonian times consistent with the vertical motion indicated by Lory (1900) and the N120° fold system discussed by Gidon et al. (1970). We consider a tectonic origin characterized by a relay system related to a dextral strike-slip activity of the Median Dévoluy Thrust (Figure 9a). This tectonic activity is considered to be coeval with the onset of the Provençal orogen (Grool et al. 2018; Handy et al. 2010; Roest and Srivastava 1991) and an ante-Eocene folding phase highlighted by Flandrin (1966) based on observations of angular unconformities in the Diois and Baronnies.

The Alpine shortening responsible for the current geometry of the Senonian package was calculated from the restored 3D model, taking into account the model boundaries before and after deformation (Figure 9b). The datum of the formations on either side of the Median Dévoluy Thrust corresponds to an intermediate step, allowing quantification of the amount of shortening accommodated by the fold-thrust system. The amount of shortening was calculated at 10 different locations along a central N-S axis (Figure 9b). Restoration results show that the total shortening is significantly greater to the south (4.4 km) than to the north (1.2 km). This total shortening is subdivided into the amount of slip along the Median Dévoluy Thrust and the amount of internal shortening accommodated by the Saint-Étienne and Saint-Disdier synclines, as well as by the Median Dévoluy-Gicon anticline. The results show a zero horizontal offset on either side of the Median Dévoluy Thrust to the north and almost 4.2 km to the south. In contrast, internal folding accounts for 1.2 km of shortening to the north and only ~200 m in the south (Figure 9b).

These variations in shortening are consistent with the structural geometry of the Median Dévoluy Thrust, which controls the style of deformation accommodation: in the north, the
sub-vertical thrust acts as a buttress, promoting internal folding of the sedimentary pile; in the south, where the thrust flattens at depth, the sedimentary sequence is able to move westward, localising shortening primarily along the thrust. These results

are consistent with the estimates of Meckel et al. (1996), who proposed a minimum horizontal displacement of 2–3 km along the Median Dévoluy Thrust in the southern part of the massif, decreasing towards the north.

<figure><img src="image_11.png" /><figcaption>**FIGURE 8** | Characteristics of the Senonian package envelop illustrated on 2D maps. (a) Elevation of the DU horizon (m a.s.l.) and delimitation of the surface under the Gillardes Spring (at 880 m a.s.l.). (b) 2D map showing the strike of the DU horizon within the Dévoluy Massif. (c) Elevation of the current Senonian package top (m a.s.l.). (d) Current Senonian package thickness (m). BGCK: The Banards-Grand-Combe klippe system, ØM: Median Dévoluy Thrust.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**FIGURE 8** | (Continued)</figcaption></figure>

5 | Hydrogeological Implications

Former studies (Bonhomme 1972; Lismonde et al. 2008; Luparini 1975) provided insights into the karst hydrology of the Dévoluy Massif. The outcropping karst features promote rapid infiltration of rainwater and snowmelt, particularly on the edges of the massif (Luparini 1975), allowing for the recharge of groundwater at depth. Hydrodynamic investigations, complemented by dye tracing experiments (Zappelli et al. 2018), revealed well-developed karst drains that facilitate rapid groundwater transfer down to the Gillardes Spring.

This study provides an original view of the geological controls
on groundwater flows into this tectonically deformed aquifer.
Indeed, the deformation of the Senonian karst reservoir into
two N-trending synclines (Saint-Étienne and Saint-Disdier
synclines), separated by the Median Dévoluy Thrust, is iden-
tified as a key control on northward-directed groundwater
flow towards the Gillardes Spring (Figure 10). Our findings
are supported by observations from Goldscheider (2005) and
Goldscheider and Neukum (2010), who demonstrated that

reservoir geometry (convergent structures like synclines versus
divergent ones like anticlines) plays a critical role in controlling
groundwater flow patterns in other tectonically deformed
Alpine karst systems. In the Dévoluy Massif, the geometry
of the Median Dévoluy Thrust allows characterising the E-W
connection of the karst reservoir, between the Saint-Étienne
and the Saint-Disdier synclines. This connection is disrupted
in the southern part due to the contact with the impermeable
Eocene Globigerina marls (Figure 10.3) but remains continu-
ous farther north (Figure 10.1,2,5,6). This hydrogeological link
has been confirmed by dye tracing experiments (Figure 10)
that demonstrated flow paths from the Aurouze Plateau down
to Gillardes Spring (Zappelli et al. 2018). In the southern part
of the massif, the geometry of the DU (Figure 8a,b) explains
the observed southward groundwater discharge (Figure 10).
Beneath the Bure Plateau, the anticline geometry of the DU,
combined with topographically driven flow, is responsible for
the N-S groundwater flow separation and contributes to the
discharge at the Sigouste Spring (Figure 10.5). Additionally,
on the southern edge of the Saint-Disdier syncline, the flat ge-
ometry of the DU may contribute to southward groundwater
<figure><img src="image_13.png" /><figcaption>FIGURE 9 | Legend on next page.</figcaption></figure>
**FIGURE 9** | Restoration of the DU horizon above the Senonian package top. (a) Illustration showing the presumed thickness of the Senonian package after restoration in three (3D plane) and two (cross-sections) dimensions. The caption of the cross-sections is similar to Figure 3a. (b) Quantification of the amount of shortening within the Dévoluy Massif from the model boundaries before, after, and during the deformation, distinguishing the amount of folding from the amount of slip related to the Median Dévoluy Thrust (ØM). All the geological cross-sections can be seen in the Supporting Information.

<figure><img src="image_14.png" /></figure>

**FIGURE 10** | 3D illustration of the Dévoluy Massif highlighting the groundwater-flow direction inferred from the architecture of the reservoir and cross-validated by dye tracing. Saint-Étienne S.: Saint-Étienne syncline, Saint-Disdier S.: Saint Disdier syncline, ØM: Median Dévoluy Thrust.
flows, potentially disconnected from the Bure reservoir by the
Median Dévoluy Thrust (Figure 3b CS11EW).

Based on the 3D structural modelling, we estimate that ~22% (25km³) of the Senonian reservoir lies below the elevation of the Gillardes Spring (<880 m a.s.l.; Figure 8a). This volume is distributed as follows: 6.1 km³ in the Saint-Étienne syncline and 18.8 km³ in the Saint-Disdier syncline (Figure 8a). The presence and extent of an aquifer at depth, however, are still debated (Bonhomme 1972; Luparini 1975). The present volume of saturated reservoir rock may be greater if we consider the position of the water table located above the Gillardes Spring elevation within the Senonian reservoir. However, further interpretation of groundwater availability is currently constrained by the limited understanding of the water table position, its hydraulic gradient, and the distribution of karst cavities below the present water table. Finally, this study highlights the importance of integrating hydrostructural analysis and 3D structural modeling for the development of a robust hydrogeological conceptual model of tectonically deformed karst reservoirs.

6 | Synthesis and Conclusion

The integration of surface structural data into modelling software highlights the 3D geometry of a structure through the construction of characteristic 3D horizons and fault planes. In this study, focused on the Dévoluy Massif, we aimed to better understand the present and past configuration of its karst reservoir, revealing a total volume of approximately 114 km³ of Senonian rocks. The Senonian package initially formed within small sedimentary basins, likely related to the early deformation phase of the Provençal compression. During the Alpine compression, two key deformation modes were identified: (1) dextral reverse motion along the Median Dévoluy Thrust, and (2) differential shortening (<1 km in the north due to folding, and up to ~4 km in the south due to slip along the thrust). These results highlight the major role of the Median Dévoluy Thrust in the accommodation of deformation. The 3D reconstruction also highlights the N-dipping morphology of the Senonian package, which strongly influences groundwater flow dynamics. Regarding hydrogeological findings, this study highlights the importance of considering a hydrostructural approach and particularly the reservoir morphology characterisation, for assessing groundwater flow directions and aquifer limits in tectonic-deformed karst reservoirs. Groundwater drainage patterns of the Dévoluy Massif, confirmed through dye tracing experiments, can be inferred from the architecture of the Senonian reservoir and Median Dévoluy Thrust. Structural modelling allowed us to identify areas for exploration of new deep resources, while uncertainties remain about karst development in the deep saturated zone. Finally, this 3D hydrostructural model, based on surface data, offers a valuable but interpretative framework. It will need to be refined and validated using future subsurface data, particularly deep boreholes, to fully constrain the hydrogeological potential of the Dévoluy Massif.

Acknowledgements

This research has been supported by the Agence de l'Eau—Rhône
Méditerranée Corse and the Région Sud-Provence-Alpes-Côte d'Azur

as part of a one-year post-doctoral project. The 3D structural model was
performed using the MOVE Software Suite donated by PE Limited. The
authors would like to thank the two reviewers for their constructive
comments that helped improve the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are openly available in ResearchGate at https://www.researchgate.net.

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**Supporting Information**

Additional supporting information can be found online in the Supporting Information section. Data S1: bre70065-sup-0001-DataS1.pdf.