# Serpentinization and Magmatic Distribution in a Hyperextended Rift Suture: Implication for Natural Hydrogen Exploration (Mauléon Basin, Pyrenees)

Nicolas Saspiturry, Cécile Allanic, Aurélie Peyrefitte

► To cite this version:

Nicolas Saspiturry, Cécile Allanic, Aurélie Peyrefitte. Serpentinization and Magmatic Distribution in a Hyperextended Rift Suture: Implication for Natural Hydrogen Exploration (Mauléon Basin, Pyrenees). Tectonics, 2024, 43 (8), 10.1029/2024tc008385. hal-04673379

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Distributed under a Creative Commons Attribution 4.0 International License
RESEARCH ARTICLE

10.1029/2024TC008385

**Key Points:**

* We address the distribution of serpentinization in an inverted hyperextended rift system based on joint gravimetric and magnetic modeling

* The mantle is gradually serpentinized on an overall thickness of around 15 km reaching a maximum amount of nearly 76%

* The mantle and the basement top are respectively in optimal thermal windows to generate H2 by serpentinization and trap it

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

N. Saspiturry, nicolas.saspiturry@umontpellier.fr

**Citation:**

Saspiturry, N., Allanic, C., & Peyrefitte, A. (2024). Serpentinization and magmatic distribution in a hyperextended rift suture: Implication for natural hydrogen exploration (Mauléon basin, Pyrenees). *Tectonics*, 43, e2024TC008385. https://doi.org/10.1029/2024TC008385

Received 19 APR 2024

Accepted 26 JUL 2024

**Author Contributions:**

**Conceptualization:** N. Saspiturry, C. Allanic

**Data curation:** A. Peyrefitte

**Formal analysis:** N. Saspiturry

**Methodology:** A. Peyrefitte

**Resources:** C. Allanic

**Software:** A. Peyrefitte

**Validation:** C. Allanic, A. Peyrefitte

**Writing – original draft:** N. Saspiturry

**Writing – review & editing:** C. Allanic, A. Peyrefitte

© Wiley Periodicals LLC. The Author(s). This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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# Serpentinization and Magmatic Distribution in a Hyperextended Rift Suture: Implication for Natural Hydrogen Exploration (Mauléon Basin, Pyrenees)

N. Saspiturry¹ ID, C. Allanic², and A. Peyrefitte²

¹Géosciences Montpellier, CNRS, University of Montpellier, Montpellier, France, ²BRGM-French Geological Survey, Orléans, France

**Abstract** The Mauléon basin is a world-class example of hyperextended rift suture. The basin possesses key attributes of an optimal hydrogen target, namely mantle, at shallow depth with tectonic structures rooted into it. Natural H2 seepages have been recognized at the surface in the foothills. Yet distribution and quantification of serpentinization within the mantel piece representing the potential H2 source has not been addressed while this aspect is crucial to consider further exploration. We discuss these aspects using joint gravimetric and magnetic 2D forward modeling along two orthogonal transects. 2D forward modeling shows that serpentinization gradually increases from bottom (20 km depth) to top reaching a maximum amount of nearly 76% (8 km depth). The N-S transect evidence that serpentinization fronts are northward inclined, suggesting a N-S serpentinization gradient responsible for the long wavelength gravity and magnetic anomalies. This orientation matches that of detachment within the former hyperextended domain, which exhumed the mantle during the Cretaceous. The W-E transect shows that serpentinization also increase toward the east reaching its maximum amount against the Barlanès lithospheric structure. The latter also coincides with the main short wavelength magnetic anomaly recognized in the basin. Forward geophysical modeling reveals that this anomaly could be linked to the presence, at shallow depth, of an alkaline magmatic body or a shallower piece of highly serpentinized subcontinental mantle both attesting for the paroxysm of the Cretaceous rifting phase. Finally, we propose a conceptual model of the H2 life cycle in the Mauléon basin and discuss the implications for H2 exploration.

## 1. Introduction

Natural hydrogen (H2) is emerging as a promising possible source of energy for a future low-carbon society. However, our current knowledge of the generation, migration, consumption, and accumulation of natural hydrogen is limited. In the absence of any consensus on optimal exploration strategy or resource assessment, it is not yet possible to consider exploitation of this resource. As a prerequisite, it is essential to estimate the potential of natural hydrogen with a sufficient level of reliability to prefigure future uses. In recent decades, natural hydrogen seepages have been described in mid-oceanic ridges (Marcailou et al., 2011; Welhan & Craig, 1979), in onshore ophiolitic complexes (Abrajano et al., 1988, 1990; Deville & Prinzhofer, 2016; Neal & Stanger, 1983), and in sedimentary basins in Russia (Larin et al., 2015), USA (Zgonnik et al., 2015), Mali (Prinzhofer et al., 2018), Brazil (Moretti et al., 2021; Prinzhofer et al., 2019) and France (Lefeuvre et al., 2021, 2022). Where ultramafic rocks are exposed to water at temperatures lower than 400°C, they inevitably undergo serpentinization reactions that form serpentine ± brucite ± talc ± magnetite, releasing substantial amounts of natural hydrogen (Cannat et al., 2010; Coveney et al., 1987; Klein et al., 2013). Among the numerous processes known to generate natural hydrogen, serpentinization is among the most efficient (Cannat et al., 2010; Coveney et al., 1987; Klein et al., 2009, 2013; McCollom & Bach, 2009). Serpentinized subcontinental mantle is known to be widely present beneath hyperextended rifts and passive margins, making these sites potential targets for natural hydrogen exploration (Boillot et al., 1987; Manatschal, 2004; Manatschal et al., 2006). By estimating the area of exhumed mantle from wide-angle seismic profiles at North Atlantic magma-poor margins, Liu et al. (2023) calculated that the accumulated H2 production could have been as high as ~8.6 × 1,012 metric tons prior to North Atlantic Ocean opening, thus highlighting the significance of H2 generation at magma-poor margins.

The Mauléon basin in the west Pyrenean realm represents an Early Cretaceous hyperextended rift that developed at the Iberia-Eurasia plate boundary (Asti et al., 2022; Jammes et al., 2009; Lagabrielle et al., 2020; Masini et al., 2014; Saspiturry, Razin, et al., 2019; Saspiturry, Allanic, et al., 2020) (Figure 1). Serpentinization occurred in mantle material exhumed at the seafloor, as evidenced in outcrops of tectonic slices in the easternmost part of
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Structural map of the Cantabrian-Pyrenean orogenic system. PB: Parentis basin, BCB: Basque-Cantabrian basin, MB: Mauléon basin, BB: Baronnies basin, CB: Columbrets basin.</figcaption></figure>

the basin (Fabriès et al., 1998; Lagabrielle, Asti, Fourcade, Corre, Poujol, et al., 2019; Lagabrielle, Asti, Fourcade, Corre, Uzel, 2019; Lagabrielle et al., 2020) (Urdach locality in Figure 2a). This domain subsequently underwent tectonic inversion, and serpentinized subcontinental mantle is supposed to remain preserved at shallow depth (~10 km) within the Mauléon basin pop-up (Lehujeur et al., 2021; Saspiturry et al., 2022; Wang et al., 2016) (Figure 2a). Indeed, the strong Labourd positive gravity anomaly (Casas et al., 1997; Grandjean, 1994), is localized upon a shallow high velocity body characterized by P-wave velocities of ~7.5–8 km/s in its core (Chevrot et al., 2018; Lehujeur et al., 2021; Wang et al., 2016). This anomaly is mainly localized in the western part of the Mauléon basin and ends on the Barlanès transfer zone (BTZ) (Figure 2b). However, the main magnetic anomaly recognized into the basin appears shifted from the Labourd gravimetric anomaly questioning their respective sources (Figure 2c).

The structural architecture of the Mauléon pop-up is controlled by the coeval development of N120° thrust systems and N20° syn-collisional transfer zones (Figure 2a; Saspiturry, Allanic, et al., 2020; Saspiturry et al., 2022). Both sets of structures root at depth in the high velocity body interpreted as subcontinental mantle (Lehujeur et al., 2021; Saspiturry et al., 2022; Wang et al., 2016). Thus, the Mauléon basin possesses three key attributes of a natural hydrogen exploration target: (a) serpentinized mantle (b) at shallow depth (c) with tectonic structures deeply rooted into it. Indeed, Lefeuvre et al. (2021, 2022) have documented several H2 hotspots in a soil-gas survey along major faults such as the North Pyrenean frontal thrust (NPFT) system (Figure 2d). They suggested that these structures represent potential pathways for natural hydrogen produced in the Mauléon basin mantle body. They also proposed that hydrogen could be trapped under salt diapirs in the hanging wall of the NPFT. However, our present knowledge of the Mauléon basin H2 source only comes down to the presence of a dense body with a high P-wave velocity, interpreted as sub-continental mantle. In this work we present joint gravimetric and magnetic forward 2D modeling along two orthogonal geological transects through the upper lithospheric structures of the Mauléon basin. This combined geological/geophysical approach allows to: (a) question the nature of the rocks behind the main gravimetric and magnetic anomalies recognized in the basin and
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Geological, geophysical, and geochemical features of the Mauléon basin. Location in Figure 1. Black dashed lines are profiles for joint gravimetric and magnetic 2D forward modeling (Figures 3 and 4). (a) Bedrock and structural map highlighting the pop-up structure of the Mauléon basin with its flanking thrust faults (purple and red lines; teeth on upthrown side). Main north-vergent thrust faults (purple) are, from north to south, the Sainte-Suzanne thrust, North Pyrenean frontal thrust, Bellevue thrust, and Saint-Palais thrust. Main south-vergent thrust faults (red) are, from south to north, the Lakhoura thrust, South Arbailles thrust, and North Arbailles thrust. (b) Complete Bouguer anomaly map showing a strong positive anomaly centered upon the Mauléon basin. Contour interval, 10 mGal. (c) Reduced-to-the-pole anomaly map showing positive anomalies centered upon the Mauléon basin transfer zones. (d) Map of natural H2 seepages measured in the Mauléon basin (Lefeuvre et al., 2021). LA: Labourd gravity anomaly. CB: Chaînons béarnais. Transfer zones: ITZ Iholdy, STZ Saison, BTZ Barlanès. Drill holes: AI-1 Ainhice-1, OR-102 Orthez-102, BE-1 Bellevue-1, HA-101 Hasparren-101, CH-1 Chéraute-1. Salt diapirs: Sa Salies, Iy Iholdy, Ro Roquiague. Geologic features: Ug Ursuya granulites, Ur Urdach mantle body.</figcaption></figure>

(b) address the distribution and amount of serpentinization within the mantle. Finally, these two geophysical transects offer new perspectives on: (a) the presence/absence of mantle in the optimum thermal window for generating H2 by serpentinization and (b) the identification of new potential H2 plays.

## 2. Geological Setting

The Pyrenean Mountain belt results from north-south convergence at the Iberian-Eurasian plate boundary from the Late Santonian to the Early Miocene (Angrand & Mouthereau, 2021; Olivet, 1996; Puigdefàbregas & Souquet, 1986; Rosenbaum et al., 2002). In its western part, the belt is made of four different structural units. To the north, the Aquitaine foreland basin is separated from the North Pyrenean Zone across the NPFT. To the south, the south verging Lakora thrust delimits the North Pyrenean Zone from the Axial Zone which corresponds to the
present-day high range of the Pyrenees. Finally, the South Pyrenean foreland basin is delimited from the Axial Zone by a set of south-verging thrusts. While the Axial Zone and the South Pyrenean foreland basin result from the antiformal nappe-stacking of the Early Cretaceous Iberian proximal rift margin, the North Pyrenean Zone consists in the shortening of the former Early Cretaceous necking zone and hyperextended rift domains (Labaume & Teixell, 2020; Masini et al., 2014; Saspiturry, Allanic, et al., 2020; Teixell et al., 2016; Tugend et al., 2014). In the north-western Pyrenees, the North Pyrenean Zone corresponds to the inverted Mauleon rift basin that is thrusted on top of the Aquitaine foreland basin by a set of north-verging thrust, namely from south to north: the Saint-Palais, Bellevue, and Sainte-Suzanne thrusts (Figure 2a; Daignières et al., 1994; Saspiturry, Razin, et al., 2019). At the front of the Sainte-Suzanne thrust, and buried underneath syn-orogenic sediments, the Arzacq basin corresponds to another Early-Cretaceous rift basin preserved from orogenic deformation (Figure 2a; Issautier et al., 2020). To the south, the inverted Iberian margin is affected by a set of south-verging thrusts, namely from north to south: the North Arbailles, South Arbailles and Lakora thrusts (Figure 2a; Saspiturry, Razin, et al., 2019; Saspiturry, Allanic, et al., 2020). The pre-orogenic rift template as well as the kinematics of rifting are still debated within the Mauleon and Arzacq basins. However, it is well established that these basins undergo continental crust thinning during the Early Cretaceous (Gomez-Romeu et al., 2019; Jammes et al., 2009; Labaume & Teixell, 2020; Lescoutre & Manatschal, 2020; Masini et al., 2014; Saspiturry, Razin, et al., 2019; Saspiturry, Allanic, et al., 2020; Teixell et al., 2016). These two basins are currently separated by a structural high, the Grand Rieu high, made of a thicker crustal block within the preserved early Cretaceous European proximal margin (Masini et al., 2014; Saspiturry, Razin, et al., 2019; Serrano et al., 2006). Although both basins underwent crustal thinning, the maximum rift intensity occurred within the Mauleon basin where local mantle exhumation associated with high temperature/low pressure metamorphism of the sedimentary cover occurred (Albarède & Michard-Vitrac, 1978; Lescoutre et al., 2019; Saspiturry, Lahfid, et al., 2020). Tectonic exhumation of mantle rocks was proposed to be accommodated by a set of N120° detachment faults (Gomez-Romeu et al., 2019; Jammes et al., 2009; Lescoutre et al., 2019; Masini et al., 2014; Saspiturry, Razin, et al., 2019; Saspiturry, Allanic, et al., 2020). However, final denudation of the mantle rocks seems to only occur along the Barlanès transverse structure. Indeed, intense extension was well demonstrated by the occurrence of deep crustal and mantle rocks material as clasts into the Urdach breccias interbedded within the late synrift turbiditic sequence in the vicinity of the BTZ (Debros et al., 2010; Duée et al., 1984; Fortané et al., 1986; Jammes et al., 2009; Lagabrielle, Asti, Fourcade, Corre, Poujol, et al., 2019; Lagabrielle, Asti, Fourcade, Corre, Uzel, 2019; Roux, 1983; Saspiturry, Issautier, Razin, Andrieu, et al., 2021). Such observations show that these rocks were exposed at the seafloor before the onset of orogenesis. Although the basin does not reach the stage of oceanic accretion, it is characterized, in its easternmost part, by the development of postrift alkaline magmatism (Azambre & Rossy, 1976; Azambre et al., 1992; Thiébaut et al., 1979). The relics of this significant Cretaceous rifting stage are still visible today. Indeed, the western part of the Mauléon basin coincides with the large positive Labourd gravity anomaly, which is bounded to the east by the BTZ. Although this anomaly was initially attributed to the presence of lower crustal rocks at shallow depth (Grandjean, 1994; Pedreira et al., 2007; Vacher & Souriau, 2001), it has also been interpreted as a shallow piece of dense lithospheric mantle (Casas et al., 1997; Jammes et al., 2010). This interpretation has gained support from recent works documenting P-wave velocities of more than 7.3 km/s in this deep material (Chevrot et al., 2018; Lehujeur et al., 2021; Wang et al., 2016). The two geophysical modeled transects presented in this work will bring crucial information on the nature of the high velocity body localized under the Mauléon basin.

# 3. Data and Methodology

## 3.1. Gravimetric and Magnetic Data

Our area of interest is covered by gravity surveys archived in the Banque Gravimétrique de la France (Martelet et al., 2009). This data set totals 8,975 measurement points aligned with the IGSN71-Hayford 1930 gravity reference system. Classical gravity data reduction was performed to compute the complete Bouguer anomaly, including free-air and Bouguer corrections. For Bouguer slab reduction and terrain corrections, we chose a reference density of 2.6 g/cm³, considering the orogenic geological context. Free air and Bouguer corrections were made using the IGN digital terrain model topographic data set (250 m resolution). Terrain corrections were computed for radii from 200 m to 10 km around the gravity stations, using the topographic grid resampled every 200 m, and from 10 to 167 km using topographic grids resampled at 1 km intervals (Martelet et al., 2002). The complete Bouguer anomaly map (Figure 2b) is interpretable in terms of density contrasts below the topography.
The complete Bouguer anomaly map ranges from -60 to +25 mGal. Our study area is also covered by the New Magnetic compilation of the Bay of Biscay and surrounding continental shelves (Le Maire et al., 2021), which brings together all airborne magnetic data set available in our area of interest. Authors produced the magnetic anomaly map and the reduced-to-the-pole (RTP) magnetic anomaly map, both computed at 500 m and 3,000 m height. For the joint gravimetric and magnetic forward modeling, we used the RTP magnetic anomaly map computed at 500 m height, which relocates magnetic anomalies above their sources. The RTP magnetic anomaly field ranges from -96 to 111 nT.

## 3.2. 2D Geophysical Modeling

The complete Bouguer anomaly map (Figure 2b) and the RTP magnetic anomaly map (Figure 2c) were used as the observed geophysical data in our joint forward modeling. Only induced magnetization is considered for modeling. Gravimetric and magnetic processing and modeling were done using Oasis Montaj software. Gravimetric and magnetic direct modeling consists of simulating the distributions of density and magnetic susceptibility that best fit the observed geophysical data. The first step consists of positioning the profiles to be modeled and sampling the geophysical data along them. Next, a starting 2D geological model is created. It assigns values of density and magnetic susceptibility to each layer to allow the computation of gravimetric and magnetic response generated by the model. Modeling proceeds by modifying the geometry of the starting model to minimize the differences between the model output and the observed data. The wavelengths, intensity and shape of gravimetric and magnetic anomalies yield information about the depth and size of geological bodies who generate them. Joint modeling of magnetic and gravimetric signals allows a better understanding of the different sources to be modeled and increases the robustness of the results. Because magnetic and gravimetric methods yield nonunique solutions, different structures and bodies can equally well explain the observed signal. Therefore, knowledge of the geological context is essential for testing hypotheses and introducing constraints that reduce the inherent uncertainty of the method. The next section provides some information on the significant amount of data supporting the models presented in this paper (boreholes, seismic reflection profiles, passive seismic acquisitions, crustal balanced cross-sections).

## 3.3. Data Supporting the Models and Hypotheses To Be Tested

The spatial position of the modeled cross-sections was chosen to intersect the main gravity and magnetic anomalies recognized on the Mauléon basin (Figures 2b and 2c). They have been calibrated by numerous data some of which are presented in Figure S1 in Supporting Information S1. The north-south section follows the crustal-scale balanced cross-section published by Saspiturry, Allanic, et al. (2020) (Figure S1A in Supporting Information S1). P-wave velocity ($V_p$) lithospheric model constructed by full-waveform inversion allowed to constrain the lithospheric part of the north-south section (Figure S1A in Supporting Information S1; Wang et al., 2016) while the sedimentary pile as well as the basement top, has been calibrated thanks to a composite seismic reflection profile (lines MT104, MT112, and 83HBS02) (Figure S1B in Supporting Information S1; Saspiturry, Razin, et al., 2019). On this section, the Iberian antiformal nappe-stack (Pyrenean Axial Zone) is cropping in the southern part of the basin (Ducasse & Vélasque, 1988; Saspiturry, Razin, et al., 2019). The quasi-entire sedimentary pile has also been drilled. Indeed, this section is also calibrated by four boreholes, from south to north: Ainhice-1 (3,540.85 m depth); Uhart-Mixe-1 (1,868.8 m depth); Bellevue-1 (6,909 m depth), and Orthez-102 (5,489.10 m depth) (all these data are available in Saspiturry, Razin, et al., 2019). Only the Ainhice-1 borehole reaches the Paleozoic basement of the Mauléon Basin (Figure S1B in Supporting Information S1). The West-East cross section follows exactly the P-wave velocity ($V_p$) lithospheric model constructed by full-waveform inversion published by Saspiturry et al. (2022) allowing to constrain the lithospheric part of the section (Figure S1C in Supporting Information S1). The basement top and the sedimentary cover are partly calibrated in between the Iholdy and Barlanès transverse structures thank to a composite seismic reflexion profile available in Saspiturry et al. (2022) (lines MT116 and 82BAA12). This section is also calibrated by two boreholes, from west to east: Hasparren-101 (6,250 m depth) and Chéraute-1 (6,050 m depth) (all these data are available in Saspiturry, Lahfid, et al., 2020). On both sections, the Moho, the basement top, and the sedimentary cover have been calibrated using the 3D Maupassacq passive seismic acquisition (Lehujeur et al., 2021). The west-east section displays some early cretaceous diapirs reactivated during the Pyrenean compression. They are both visible at the outcrop and seismic scales. To the west of the BTZ, salt tectonic is recognized within the Iholdy and
<table><caption>**Table 1**

Parameter Values Used for the Modeling Hypotheses</caption><thead><tr><th>Models</th><th></th><th>Density (g/cm³)</th><th>Susceptibility (SI)</th><th>MAG</th><th>GRAVI</th></tr></thead><tbody><tr><td rowspan="8">North-South</td><td>1A Uniformly serpentinized mantle</td><td>2,900</td><td>0.0100</td><td>☒</td><td>☑</td></tr><tr><td rowspan="7">1B Serpentinized mantle with a petrophysical gradient of serpentinization increasing from bottom to top</td><td>2,700</td><td>0.06</td><td>☑</td><td>☑</td></tr><tr><td>2,750</td><td>0.05</td><td></td><td></td></tr><tr><td>2,800</td><td>0.04</td><td></td><td></td></tr><tr><td>2,850</td><td>0.03</td><td></td><td></td></tr><tr><td>3,000</td><td>0.0020</td><td></td><td></td></tr><tr><td>3,100</td><td>0.0010</td><td></td><td></td></tr><tr><td>3,200</td><td>0.0005</td><td></td><td></td></tr><tr><td>3,300</td><td>0.0000</td><td></td><td></td><td></td></tr><tr><td rowspan="6">West-East</td><td>2A Uniformly serpentinized mantle</td><td>1A</td><td>1A</td><td>☒</td><td>☑</td></tr><tr><td>2B Gradient of serpentinization increasing from bottom to top</td><td>1B</td><td>1B</td><td>☒</td><td>☑</td></tr><tr><td>3A Uniformly serpentinized mantle—shallow mantle against the Barlanès transfer zone</td><td>1A</td><td>1A</td><td>☒</td><td>☒</td></tr><tr><td>3B Gradient of serpentinization increasing from bottom to top—shallow mantle against the Barlanès transfer zone</td><td>1B</td><td>1B</td><td>☑</td><td>☑</td></tr><tr><td>4A Uniformly serpentinized mantle—shallow alkaline magmatism against BTZ</td><td>Mantle: 1A<br>Magmatism: 2,900</td><td>Mantle: 1A<br>Magmatism: 0.05</td><td>☒</td><td>☒</td></tr><tr><td>4B Gradient of serpentinization increasing from bottom to top—shallow alkaline magmatism against BTZ</td><td>Mantle: 1B<br>Magmatism: 2,900</td><td>Mantle: 1B<br>Magmatism: 0.05</td><td>☑</td><td>☑</td></tr></tbody></table>

Note. Green arrow: very well adjusted; Red arrow: poorly adjusted. BTZ: Barlanès transfer zone. MAG: Modeled magnetic signal. GRAVI: Modeled gravimetric signal.

Roquiague diapirs (Canérot, 1988, 1989; Canérot et al., 2005; Saspiturry et al., 2022) while to the east, it is materialized by the Chaînons béarnais (CB) thrust welds and mini basins (Labaume & Teixell, 2020).

The complete Bouguer anomaly map displays a large positive anomaly, named the Labourd anomaly (Figure 2b). The anomaly is oriented WNW-ESE and is separated into two maxima. One of the working hypotheses tested and discussed in the results section consists of determining whether these maxima are linked to different sources or to the same sources which would be segmented by 2N0° transverse structures. Among the potential candidates to be at the origin of this anomaly, there are the Ursuya granulites (UrG) which outcrop in the westernmost part of the basin (Saspiturry, Cochelin, et al., 2019) and the subcontinental mantle imaged at around 10 km depth by passive seismic data (Lehujeur et al., 2021; Saspiturry et al., 2022; Wang et al., 2016). Some mantle slices corresponding to remnant bodies of the early cretaceous hyperextension are known at the outcrop scale, the Urdach body localized on the Barlanès structure and the Saraillé, Tos de la Coustette and Turon de la Técouère bodies localized on the CB segment (Figure 2b) (Corre et al., 2016; Fabriès et al., 1991, 1998; Labaume & Teixell, 2020; Lagabrielle, Asti, Fourcade, Corre, Poujol, et al., 2019, 2020). The RTP magnetic anomaly field features a WNW-ESE large positive anomaly at the eastern part of the basin (+111 nT) (Figure 2c). The two main magnetic and gravity anomalies therefore have the same orientation (WNW-ESE) but are shifted with respect to each other (Figures 2b and 2c). Another working hypotheses is therefore to test if these anomalies are generated by the same source. Eight geological models were produced to test their impact on the gravity and magnetic signal (Table 1). These models have the same petrophysical characteristics for the sedimentary cover and the continental crust. However, they differ in the distribution and amount of serpentinization: (a) uniformly serpentinized (models 1A, 2A, 3A, and 4A, Table 1) versus (b) gradually serpentinized (models 1B, 2B, 3B, and 4B, Table 1). The west-east models also differ in the presence or absence of heterogeneity at shallow depth against the BTZ: (a) mantle slice (models 3A and 3B; Table 1) versus (b) alkaline magmatic body (models 4A and 4B; Table 1).

### 3.4. Rock Density and Magnetic Susceptibility Calibration

#### 3.4.1. Jurassic to Cenozoic Sedimentary Pile

To calibrate the gravimetric forward modeling of the sedimentary cover, we used a compilation of rock density values available in the vicinity of the Iberia-Eurasia plate boundary; the: North Pyrenean Zone (Torné
et al., 1989), Basque-Cantabrian basin (García-Senz et al., 2020; Pedreira et al., 2007), Mauléon basin (García-Senz et al., 2020; Grandjean, 1994; Martin et al., 2017; Vacher & Souriau, 2001; Wehr et al., 2018), Ebro basin (Pedreira et al., 2007; Pueyo et al., 2015; Torné et al., 1989), Aquitaine basin (Torné et al., 1989; Vacher & Souriau, 2001), and South Pyrenean Zone (Torné et al., 1989) (see compilation available in Table S1 in Supporting Information S1). Thus, the following rock densities were assigned according to previous published work: 2.60 g/cm³ for Jurassic to Aptian carbonate rocks, 2.40 g/cm³ for the lower part of the Cretaceous to Cenozoic sedimentary pile and 2.20 g/cm³ for the upper part. Since, the sedimentary series do not contain magnetic material, a magnetic susceptibility of 0 SI was assigned to them.

### 3.4.2. Upper Triassic

The upper Triassic is composed of a wide range of rocks, that is, shale, evaporite and alkaline magmatic rock regionally named “ophites” (Curnelle, 1983; Lucas, 1985; Rossi et al., 2003). This unit is also widely involved in halokinetic deformation, associated with the Cretaceous rifting as well as the Pyrenean compression (Canérot, 1988, 1989; Canérot & Lenoble, 1993; Canérot et al., 2005; James & Canérot, 1999; Labaume & Teixell, 2020). The upper Triassic series are currently mainly composed of shale and alkaline magmatic rocks in the core of diapirs as the evaporites were dissolved during diapirs growth (Canérot et al., 2005; Labaume & Teixell, 2020). Thus, to take these heterogeneities into account as best as possible, two units were defined for the upper Triassic: a first unit dominated by evaporites and a second one mainly composed of alkaline magmatic rocks and shales. According to the regional data listed in Table S1 in Supporting Information S1 (García-Senz et al., 2020; Pedreira et al., 2007; Pueyo et al., 2015), a rock density of 2.20 g/cm³ and a magnetic susceptibility of 0 SI has been attributed to the first upper Triassic salt-dominated unit. The second unit has been modeled with a rock density of 2.50 g/cm³ and a magnetic susceptibility of 0.007 SI since it's dominated by ophites, and shales.

### 3.4.3. Continental Crust

To calibrate the gravimetric forward modeling of the continental crust, we used the wide regional data set listed in Table S1 in Supporting Information S1 (García-Senz et al., 2020; Grandjean, 1994; Martin et al., 2017; Neres et al., 2018; Pedreira et al., 2007; Pueyo et al., 2015; Torné et al., 1989; Vacher & Souriau, 2001; Wehr et al., 2018). The values used in our models also consider the fact that recent regional studies revealed that the Iberian crust is less dense than the European one (Wehr et al., 2018). Thus, for gravimetric modeling of continental crust, we used density values of 2.8 g/cm³ (lower crust) to 2.7 g/cm³ (upper crust) for Iberia and 2.9 g/cm³ (lower crust) to 2.8 g/cm³ (upper crust) for Europe. On the north-south profile, the uppermost part of the Axial Zone, mainly composed of metasediments (Saspiturry, Razin, et al., 2019), has been modeled with a rock density of 2.4 g/cm³. On the west-east profile, the uppermost part of the UrG, widely altered by weathering (Saspiturry, Cochelin, et al., 2019), has also been modeled with a rock density of 2.4 g/cm³. For magnetic modeling of continental crust, we used magnetic susceptibility values of 0.008 SI for the Iberian lower crust; 0.004 SI for the Iberian upper crust; 0.01 SI for the European lower crust and 0.005 to 0.001 SI for the European upper crust based on Pedreira et al. (2007) in the Basco-cantabric basin (upper crust: 0.001 SI; middle crust: 0.0035 SI; lower crust: 0.007 SI) and Neres et al. (2018) in the SW Iberian margin (upper crust: 0.005 SI; lower crust: 0.01 SI).

### 3.4.4. Alkaline Magmatism

The hypothesis of an alkaline magmatic intrusion in the vicinity of the BTZ (models 4A and 4B; Table 1) has been tested with a rock density of 2.9 g/cm³ and a magnetic susceptibility of 0.05 SI. These values are consistent with rock density of 2.7–3 g/cm³ and magnetic susceptibility of 0.04–0.06 SI reported for similar magmatic intrusion in the Gulf of Lion (Canva et al., 2019), the Bilbao synclinorium (Pedreira et al., 2007), the Lusitanian Basin (Neres et al., 2012, 2014), the Sines massif (Ribeiro et al., 2013) or the Portimão and the Guadalquivir Banks in the ocean-continent transition of the SW Iberia margin (Neres et al., 2018).

### 3.4.5. Sub-Continental Mantle

Serpentinization process induces the linear decrease of mantle rock density (Bina & Henry, 1990; Christensen, 1978; Krammer, 1990; Nazarova, 1994; Oufi et al., 2002; Toft et al., 1990). Mantle density thus vary from 3.3 g/cm³ for fresh peridotites up to 2.55 g/cm³ for totally serpentinized ones as evidenced by numerous studies on samples from Oman, Greece and Alps outcropping mantle body as well as mid Atlantic ridges (Bonnemains
et al., 2016); Indian ridge (Chen et al., 2021); and Tibet ophiolites (Li et al., 2017). Mantle density rock values listed in Table S1 in Supporting Information S1 and used for regional 2D forward modeling show a wide range of density varying from 3.3 to 2.90 g/cm³ (García-Senz et al., 2020; Grandjean, 1994; Martin et al., 2017; Pedreira et al., 2007; Pueyo et al., 2015; Torné et al., 1989; Vacher & Souriau, 2001; Wehr et al., 2018). The magnetic susceptibility of serpentinized peridotites increases with modal amounts of magnetite (Bina & Henry, 1990; Nazarova, 1994). Based on a large data set of variably serpentinized peridotites, Maffione et al. (2014) reported a wide range of magnetic susceptibility values, ranging from 0.0007 up to 0.0758 SI, with an average value of 0.014 SI. They show that values higher than 0.02 SI were observed only for samples with a degree of serpentinization higher than 55%-60%. Other studies shows that magnetic susceptibility of highly serpentinized mantle can exceptionally reach 0.12 to 0.15 SI (Bonnemains et al., 2016; Chen et al., 2021; Krammer, 1990). According to all the above-mentioned literature, we modelized mantle serpentinization hypotheses with a wide range of density (3.3-2.7 g/cm³) and magnetic susceptibility (0-0.06 SI). We discuss the signification of these values in terms of serpentinization degree in the discussion section.

# 4. Joint Gravimetric and Magnetic Modeling Results

## 4.1. North-South Section

On the north-south section, both the gravimetric and magnetic signals display a high amplitude and long wavelength positive anomaly of respectively +24.17 mGal and +10.02 nT (Figures 3 and 4). Because of the wavelength and intensity of these anomalies, modeling shows that the source body must be deeper than 8 km. Two models were produced along the N-S cross-section. The first model 1A is characterized by a homogeneously serpentinized mantle, modeled with a density of 2.9 g/cm³ and a magnetic susceptibility of 0.01 SI (Figure 3). Although the model 1A correctly matches the gravimetric signal, it fails to reproduce the magnetic one (Table 1). The second model 1B (Figure 4), reveals that the mantle needs to be gradually serpentinized with different serpentinization fronts characterized by decreasing density (from 3.3 to 2.7 g/cm³) and increasing magnetic susceptibility (from 0 SI to 0.06 SI) to reproduce both the gravimetric and magnetic signals (Table 1). One major outcome arising from model 1B is that both the high amplitude and long wavelength magnetic and gravimetric positive anomalies can be explained by the presence of the mantle at a depth varying from 8 km to the south (near the Axial Zone) to 10 km to the north (near to the preserved European margin). Even if these two anomalies are aligned with the mantle at depth, the magnetic anomaly is slightly shifted from 4 km toward the south with respect to the gravimetric anomaly. Model 1B also shows that the only way to generate this offset is to tilt the serpentinization fronts from around 10° to 25° toward the north (Figure 4). The magnetic signal displays a second positive anomaly of 4.95 nT localized to the south of the section. Considering the short wavelength of this anomaly, the modeling result shows that its source needs to be relatively shallow. The latter seems to coincide with the presence of the exposed basement of the Axial Zone (Figures 3 and 4). Finally, model 1B reveals that the former Early Cretaceous European rifted margin has been slightly inverted and mostly preserve its pre-collisional configuration while the Iberian margin suffered significant continental crustal thickening due to collisional nappe-stacking.

## 4.2. West-East Section

Building upon the results of the north-south section, the serpentinization fronts have been incorporated into the three west-east models presented in the paper (models 2B, 3B, and 4B; respectively Figures 5-7). For the sake of clarity, each of the three west-east models presented here were produced without the serpentinization fronts and are available in Figure S2 (model 2A), Figure S3 (model 3A) and Figure S4 (model 4A) in Supporting Information S1. However, none of these models allow the gravimetric and magnetic signals to be correctly adjusted (Table 1). Considering the lobate shape of the gravimetric anomaly that is characterized by two maxima (+20.57 mGal to the west and +24.17 mGal to the central part of the section), the corresponding dense body must consist of two separate parts or two different bodies to coincide with the structure suggested by the gravimetric data. Considering the short wavelength of the western +20.57 mGal anomaly, the modeling result shows that its source needs to be relatively shallow. The latter clearly coincides with the outcropping weathered UrG (Figures 5-7). As for the north-south section, modeling results highlight that the +24.17 mGal high amplitude and long wavelength anomaly is clearly related to the presence of lithospheric mantle at a depth of 8-10 km. As in model 1B, the models 2B, 3B, and 4B reveal that the serpentinization gradually increases from bottom to top as shown by the upward decrease in density and increase in magnetic susceptibility (Figures 5-7). The models 2B,
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Geophysical forward modeling of the Mauléon basin along a SSW-NNE cross-section. Model 1A testing the hypothesis of uniformly serpentinized mantle. UP: Upper Crust; LC: Lower Crust; SD: Salt dominated Triassic. Modeling parameters: D: rock density, MS: magnetic susceptibility. Thrust faults: LAT: Lakhoura, SAT: South Arbailles, NAT: North Arbailles, SPT: Saint-Palais, BET: Bellevue, SST: Sainte-Suzanne. Location in Figure 2.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Geophysical forward modeling of the Mauléon basin along a SSW-NNE cross-section. Model 1B testing the hypothesis of a serpentinized mantle with a petrophysical gradient of serpentinization increasing from bottom to top. Modeling parameters: D rock density, MS magnetic susceptibility. S1-S7: Serpentinized fronts; UP: Upper Crust; LC: Lower Crust; SD: Salt dominated Triassic. Thrust faults: LAT Lakhoura, SAT: South Arbailles, NAT: North Arbailles, SPT: Saint-Palais, BET: Bellevue, SST: Sainte-Suzanne. Location in Figure 2.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Geophysical forward modeling of the Mauléon basin along a WNW-ESE cross-section. Model 2B testing the hypothesis of a serpentinized mantle with a petrophysical gradient of serpentinization increasing from bottom to top. S1-S7: Serpentinized fronts; UP: Upper Crust; LC: Lower Crust; OD: Ophite dominated Triassic; SD: Salt dominated Triassic. Modeling parameters: D: rock density, MS: magnetic susceptibility. UrG: Ursuya granulites; IyD: Iholdy diapir; RoD: Roquiague diapir; BTZ: Barlanès transfer zone; CB: Chaînons Béarnais. Location in Figure 2.</figcaption></figure>

3B, and 4B are consistent with the passive seismic acquisitions data (Lehujeur et al., 2021) showing that the sub-vertical BTZ delimits a western domain where mantle rocks are present at shallow depth from an eastern domain characterized by a thick continental crust. Although the lithological change is abrupt and the Barlanès structure is
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Geophysical forward modeling of the Mauléon basin along a WNW-ESE cross-section. Model 3B testing the hypothesis of a serpentinized mantle with a petrophysical gradient of serpentinization increasing from bottom to top and the presence of shallow mantle against the Barlanès transfer zone. S1–S7: Serpentinized fronts; UP: Upper Crust; LC: Lower Crust; OD: Ophite dominated Triassic; SD: Salt dominated Triassic. Modeling parameters: D: rock density, MS: magnetic susceptibility, UrG: Ursuya granulites; IyD: Iholdy diapir; RoD: Roquiague diapir; BTZ: Barlanès transfer zone; CB: Chaînons Béarnais. Location in Figure 2.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Geophysical forward modeling of the Mauléon basin along a WNW-ESE cross-section. Model 4B testing the hypothesis of a serpentinized mantle with a petrophysical gradient of serpentinization increasing from bottom to top and the presence of shallow alkaline magmatism against the Barlanès transfer zone. S1–S7: Serpentinized fronts; UP: Upper Crust; LC Lower Crust; OD: Ophite dominated Triassic; SD: Salt dominated Triassic. Modeling parameters: D rock density, MS magnetic susceptibility. UrG: Ursuya granulites; IyD: Iholdy diapir; RoD: Roquiague diapir; BTZ: Barlanès transfer zone; CB: Chaînons Béarnais. Location in Figure 2.</figcaption></figure>
sub-vertical, the gravimetric signal gradually decreases crossing the BTZ. Thus, this significant change in upper lithospheric architecture from both side of the BTZ (mantle to the west vs. continental crust to the east) does not allow to explain the progressive decrease in gravimetric signal toward the east by itself. Geophysical modeling shows that to fit the progressive decrease in density toward the east, the serpentinization fronts need to be slightly inclined in this same direction (Figures 5–7). This geometry of the serpentinization fronts induces a gradual density transition crossing the BTZ from decreasing density in the uppermost serpentinized mantle (western Mauléon segment) to low density within the “Chainons Béarnais” continental crust (eastern Mauléon segment). Even if the model 2B reproduces very well the gravimetric signal (Figure 5), it fails to reproduce the magnetic one (Table 1). One major outcome arising from model 2B is that the mantle piece, localized at a depth of 8–10 km, do not allow to reproduce the main magnetic anomaly upon the BTZ (Figure 5). Indeed, this high amplitude anomaly reaching +103.79 nT appears to be shifted toward the east from the main +24.17 mGal gravimetric anomaly. This observation implies that their source is different. Other outcome arising from the failing model 2B is that the short wavelength of this high amplitude magnetic anomaly requires a shallow source located in between 4 and 10 km depth to properly fit the magnetic signal. Considering the strong intensity of the magnetic signal, modeling results also reveal that the magnetic body must either be very thick and/or have an extremely high magnetic susceptibility. Moreover, the absence of any gravimetric anomaly correlated with this magnetic anomaly attests that the magnetized body must have a density close to its surroundings (continental crust properties: density of 2.8 g/cm³). Considering the geological context, two type of lithology could generate such anomaly into the basin, model 3B tested the presence of highly serpentinized mantle (density of 2.7 g/cm³ and magnetic susceptibility of 0.06 SI; Figure 6) while model 4B tested the presence of alkaline magmatism (density of 2.9 g/cm³ and magnetic susceptibility of 0.05 SI; Figure 7). Models 3B and 4B both successfully reproduce the gravimetric and magnetic signals and are both geologically consistent (Table 1). We discuss their implications in terms of basin evolution and hydrogen exploration potential in the discussion section.

Along the west-east transect five shorts wavelength and low amplitude magnetic signals are observed. As we move westwards, away from the magnetic mantle, the magnetic signal becomes negative. Despite being negative, a peak of -3.36 nT is observable in the most eastern part of the transect. This peak is correlated with the presence at the outcrop of the UrG, which also generate the short wavelength gravity anomaly located to the west of the transect (+20.57 mGal) (Figures 6 and 7). Moving toward the east, three pics are visible on the magnetic signal between the UrG and the BTZ: from west to east +3.92, +13.26, and +21.62 nT (Figures 6 and 7). Another peak is localized to the east of the Barlanès structure: +13.26 nT (Figures 6 and 7). Due to the very short wavelength of these magnetic anomalies, modeling shows that the source must be shallower than 4 km, so within the sedimentary cover. Models 3B and 4B reveal that these anomalies are linked to the very well-known early Cretaceous diapirs developed above preserved syn-rift normal faults (Figures 6 and 7). These diapirs have been both recognized at the outcrop scale and in seismic reflexion profiles as the Iholdy, Roquiague and CB diapiric systems (Canérot, 1988, 1989; Canérot et al., 2005; Labaume & Teixell, 2020; Saspiturry et al., 2022). As for the north-south profile (model 1B; Figure 4), the models 3B (Figure 6), and 4b (Figure 7) reveal that the mantle is responsible for a long wavelength and low amplitude magnetic anomaly of around +10.02 nT. This anomaly is less visible in the west-east profile due to the low wavelength and high amplitudes magnetic anomalies which slightly mask this signal.

# 5. Discussion

## 5.1. Distribution of Serpentinization and Alkaline Magmatism

Joint gravimetric and magnetic forward modeling reveals the occurrence of serpentinized mantle being at the origin of the long-wavelength gravimetric (+24.17 mGal) and magnetic (+10.02 nT) anomalies recognized in the basin. The north-south model 1B (Figure 4) highlights that the serpentinization is more pronounced toward the north as shown by: (a) the presence of the uppermost front characterized by a density of 2.7 g/cm³ and a magnetic susceptibility of 0.06 SI only at the foot of the inverted European margin and (b) the fact that the serpentinization fronts are inclined toward the north of around 10°–25°. This orientation matches that of detachment systems recognized within the former Mauléon basin hyperextended domain, which exhumed the mantle rock and exposed it to serpentinization during the Early Cretaceous rifting (Jammes et al., 2009; Masini et al., 2014;
Saspiturry, Razin, et al., 2019; Saspiturry, Allanic, et al., 2020). The geometry of these northward dipping de-
tachments is consistent with the northward increase in serpentinization inferred from our north-south geophysical
model 1B (Figure 4).

The west-east model 2B highlights that serpentinization gradually increases toward the east reaching its maximum value in the vicinity of the BTZ (Figure 5). This observation is supported by the facts that the: (a) uppermost serpentinization front is only localized in the easternmost part of the mantle body and (b) serpentinization fronts are all inclined toward the east toward the BTZ. The eastward increase of serpentinization revealed by 2D geophysical forward modeling is consistent with paleogeographic reconstructions (Saspiturry, Issautier, Razin, Andrieu, et al., 2021) and restored cross-sections (Lescoutre and Manatschal., 2020; Saspiturry, Allanic, et al., 2020; Teixell et al., 2016) demonstrating that denudation of mantle rock was only achieved in the easternmost part of the Mauléon basin. Indeed, clasts of subcontinental mantle rocks have been reworked into the uppermost Albian to lower Cenomanian Urdach breccias only in the vicinity of the Barlanès structure (Debroas et al., 2010; Duée et al., 1984; Fortané et al., 1986; Roux, 1983). This inferred increase in serpentinization amount both toward the north and the east may be correlated with the Late Jurassic to Cenomanian change in kinematics of the Iberia and Eurasia plates from NE-SW orthogonal rifting to more transtensional WNW-ESE rift opening (Asti et al., 2022) generating respectively the northward and eastward serpentinization gradient recognized into the Mauléon basin. Thus, these observations suggest that at least part of the serpentinization is related to the Early Cretaceous hyperextension since the two recognized serpentinization gradients fit the palinspastic reconstructions of the syn-rift basin (Canérot, 2017; Canérot & Mediavilla, 2023; Saspiturry, Issautier, Razin, Andrieu, et al., 2021).

Models 3B (Figure 6) and 4B (Figure 7) show that two geological hypotheses allow to reproduce the short wavelength and high amplitude Barlanès magnetic anomaly (+103.79 nT): highly serpentinized mantle versus alkaline magmatism both localized at shallow depth and being shifted from the main serpentinized mantle body. The hypothesis of a mantle piece pinched along the BTZ is consistent with the lithospheric nature of this structure. Indeed, it localized mantle exhumation during the paroxysm of the Cretaceous rifting stage (Debroas et al., 2010; Saspiturry, Issautier, Razin, Andrieu, et al., 2021) and then segmented the orogenic prism during the Pyrenean compression (Lehujeur et al., 2021; Saspiturry et al., 2022). Based on syn-rift paleogeography, some authors have already proposed that the Barlanès structure could materialized a tilted block of mantle during the final stage of Early Cenomanian crustal break-up (Debroas et al., 2010; Saspiturry, Issautier, Razin, Andrieu, et al., 2021). The hypothesis defended in model 4B is also consistent since the North Pyrenean Zone underwent widespread Mesozoic magmatic activity, attested by Cretaceous alkaline magmatism (Montigny et al., 1986; Rossy et al., 1992). The Cretaceous igneous rocks consist of alkaline basalt and trachyte, as well as intrusive teschenyte and syenite (Azambre & Rossy, 1976; Azambre et al., 1992; Thiébaut et al., 1979). The Cretaceous volcanism was predominantly submarine, characterized by pillow basalt and pyroclastic rocks. The position of these rocks within the Cretaceous sedimentary deposits ensures reliable biostratigraphic ages for this magmatic event, being late Albian (synrift) to Santonian (postrift) in the western Pyrenees (Casteras et al., 1970; Castañares et al., 1997; Lamolda et al., 1983; López-Horgue et al., 1999; Rat, 1959; Rat et al., 1983; Schoeffller et al., 1964). This magmatism is very well-known at the outcrop scale in the eastern part of the Mauléon basin as sills localized within the Cretaceous turbidites in the footwall of the Barlanès structure (Casteras et al., 1970). If the Barlanès magnetic anomaly is effectively related to a Cretaceous magmatic body, the latter would be the largest ever identified within the north-Pyrenean rift system. Such alkaline magmatic bodies have been modeled in the ocean-continent transition of passive margin such as the SW Iberian margin (Neres et al., 2018) and the Gulf of Lion margin (Canva et al., 2019) in a similar palinspatic position, that is, in between the domains of hyper-extended continental crust and exhumed sub-continental mantle. The alkaline magmatic body of model 4B is characterized by a dimension of around 8 × 8 km and generates a magnetic anomaly of +103.79 nT (Figure 7). It shares close similarities with the Portimão bank in terms of size (dimension of 10 × 5 km) and amplitude of inferred magnetic signal (+160 nT) (Neres et al., 2018). So, model 3B implies the presence of a tilted block of mantle localized along the Barlanès structure at the end of the rifting stage (Figure 8 top) while model 4B suggests that the Barlanès structure localized the premises of the lithospheric break-up aborted before oceanic spreading (Figure 8 bottom). However, even if this supposed magmatic body has a strong chance of being Cretaceous in age considering the paleogeography of the Albian-Cenomanian rifting, we cannot exclude that it could be late Variscan in age as the Pyrenees are also affected by a major phase of alkaline magmatism during Permian time (Bixel & Lucas, 1983; Gretter et al., 2015; Lago et al., 2004; Lloret et al., 2018; Lucas, 1985; Rodriguez-Mendez et al., 2014).
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Schematic restoration of the WNW-ESE cross-section through the Mauléon basin hyperextended domain during the upper Cretaceous postrift stage. On top: current magnetic and gravimetric signal; On the middle: schematic restoration according to geophysical model 3B; On the bottom: schematic restoration according to geophysical model 4B.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Graph showing the rate of serpentinization as a function of rock density. Data reported in this graph are from Chibati et al. (2022) (n = 22; blue data's), Li et al. (2017) (n = 34; yellow data's), Chen et al. (2021) (n = 20; gray data's) and Bonnemains et al. (2016) (n = 61; orange data's). The density of the serpentinization fronts S1–S7 from models 1B, 3B, and 4b have been reported on the graph to evaluate their amount of serpentinization.</figcaption></figure>

## 5.2. Quantification of Serpentinization Degree

Geological models 1B and 2B show that the mantle is gradually serpentinized with seven different serpentinization fronts characterized by decreasing density (from 3.3 to 2.7 g/cm³) and increasing magnetic susceptibility (from 0 SI to 0.06 SI) from 25 km depth to 10 km depth. It is very well established that increasing serpentinization degree is linearly correlated with decreasing mantle rock density from 3.35 g/cm³ for fresh peridotites up to 2.45 g/cm³ for totally serpentinized (Bina & Henry, 1990; Christensen, 1978; Krammer, 1990; Miller & Christensen, 1997; Nazarova, 1994; Oufi et al., 2002; Toft et al., 1990). It is therefore possible to use the modeled densities as an effective proxy for determining the average percentage of serpentinization within the modeled mantle fronts. The graph in Figure 9 shows mantle density as a function of serpentinization percentage and includes four recent studies with data from the Pyrenees (22 samples; Chibati et al., 2022), the Tibet (34 samples; Li et al., 2017), the Indian ridge (20 samples; Chen et al., 2021), the Oman, the Alps and the Atlantic Ocean (61 samples; Bonnemains et al., 2016). The linear regression line derived from these data (density = (-0.0079 × Serpentinization amount) + 3.3016) is similar to those proposed by Miller and Christensen (1999) and Oufi et al. (2002). By plotting in Figure 9, the density values used in the modeled serpentinization fronts, we estimate an average percentage value for the serpentinization rate for each front. Thus the mantle unit modeled under the Mauléon basin is gradually serpentinized on an overall thickness of around 15 km from 13% at the bottom up to 76% on top as follow: (Front 1) density of 3.2 g/cm³ = ~13%; (Front 2) density of 3.1 g/cm³ = ~26%; (Front 3) density of 3 g/cm³ = ~38%; (Front 4) density of 2.85 g/cm³ = ~57%; (Front 5) density of 2.8 g/cm³ = ~63%; (Front 6) density of 2.75 g/cm³ = ~70%; (Front 7) density of 2.7 g/cm³ = ~76% (Figure 9). Thus, the preserved Mauléon mantle unit share close similarities with the Gorringe Bank, in the distal domain of the SW Iberian margin, itself gradually serpentinized from 10% to 70% on a thickness of around 12 km (Sallarès et al., 2013). Despite the very high amplitude of its gravity anomaly, one of the largest on Earth, the Gorringe bank shows no significant magnetic anomaly, except for some short wavelength peaks likely due to local magmatic intrusions. The magnetic susceptibility of serpentinized peridotites increases with modal amounts of magnetite but without a clear linear relationship as for the density (Bina & Henry, 1990; Nazarova, 1994). Based on a large data set of variably serpentinized peridotites, Maffione et al. (2014) show that magnetic susceptibility values higher than 0.02 SI were observed only for samples with a
degree of serpentinization higher than 55%-60%. Our geophysical models 1B, 3B, and 4B appear consistent with these results since magnetic susceptibilities of more than 0.03 SI are only reported for fronts 4 to 7, characterized by a serpentinization degree higher than 57% according to their density.

Our results agree with recent studies of serpentinization quantification degree performed on the north-Pyrenean outcropping peridotites. Indeed, based on 22 samples along the whole north-Pyrenean zone, Chibati et al. (2022) have highlighted that mantle rocks are characterized by rock density of 3.29 g/cm³ for fresh peridotites up to 2.5 g/cm³ for totally serpentinized ones. Loiseau et al. (2024) have shown that the Turon de la Técouère mantle outcrop, in the eastern part of the Mauléon basin, is heterogeneously serpentinized from 3% up to 62%. For this same range of serpentinization degree, they measured magnetic susceptibility from 10-6 SI to 10-2 SI in terms of order of magnitude and rock density of 2.81-3.28 g/cm³. Published works on the north-Pyrenean mantle bodies all agree on the fact that the mantle is serpentinized in a very heterogeneous way at the outcrop scale, ranging from an almost fresh mantle to a completely serpentinized one (Chibati et al., 2022; de Saint-Blanquat et al., 2016; Loiseau et al., 2024; Tichadou et al., 2021). Thus, given the variability of serpentinization amount at the outcrop scale, the percentage of serpentinization deduced from our study for each front must be considered as an average value. The significant amount of serpentinization reached in the uppermost part of the mantle (76% in the serpentinization front 7) could have been favored by preferential fluid-circulation within the main exhuming tectonic contact during early Cretaceous rifting. Indeed, the preserved north-Pyrenean zone detachments are known to favor the development of intense serpentinization on top of the mantle during continental crust hyperextension, as described in the Urdach and Saraillé massifs in the eastern Mauléon basin (Asti et al., 2019; Lagabrielle, Asti, Fourcade, Corre, Poujol, et al., 2019; Lagabrielle, Asti, Fourcade, Corre, Uzel, 2019), the Moncaup (Lagabrielle et al., 2010), and the Bestiac peridotite(s) in the central Pyrenees (de Saint Blanquat et al., 2016).

$V_p$ seismic velocities of peridotites with a serpentinization degree higher than 75% can reach 4.5-5.5 km/s (Canales et al., 2000; Carlson & Miller, 2003; Christensen, 1978; Horen et al., 1996; Miller & Christensen, 1997; Sallarès et al., 2013). So, it is challenging to differentiate crustal and upper mantle rocks using only P-wave seismic velocities because serpentinized upper mantle can show a wide-range of velocities (~4.5-8.5 km/s), depending on the degree of serpentinization (Boddupalli et al., 2022; Chian et al., 1995; Christensen, 2004; Keen et al., 2018; Sallarès et al., 2013). Carlson and Miller. (2003) demonstrated that $V_p$ velocities are linearly correlated with the serpentinization amount. In models 3B and 4B, the mantle is serpentinized at 13% (Front 1) at 20 km depth and 63% (Front 5) at 10 km depth in the western Mauléon mantle piece. Consequently, the corresponding expected velocities should decrease from 7.8 km/s at 20 km depth to 6.0 km/s at 10 km depth. On the same way, in the eastern part of the west-east section, the mantle is serpentinized at 38% (Front 3) at 20 km depth and reaches 76% (Front 7) in between 12 and 5 km depth depending on the models with corresponding expected $V_p$ seismic values of respectively ~6.8 km/s at the bottom and ~5.4 km/s on top of the mantle. Passive seismic acquisitions present consistent $V_p$ values with the expected ones deduced from the serpentinization amount evaluation (Figure S1C in Supporting Information S1; Lehujeur et al., 2021; Saspiturry et al., 2022). Indeed, they show decreasing $V_p$ seismic velocities from ~8 km/s at 20 km depth up to ~6.5 km/s at 10 km depth in the western part of the mantle body while $V_p$ velocities decrease from ~7 km/s at 20 km depth up to ~5.5 km/s at 5 km depth in its eastern part.

### 5.3. Implication for H2 Exploration

Joint forward geophysical modeling reveals that the high amplitude and long wavelength gravimetric and magnetic anomalies are due to the presence of gradually serpentinized mantle over 15 km thick. To consider future H2 exploration, one major question must be addressed: is the mantle currently in optimal thermal conditions to generate H2? Indeed, recent studies reveal that H2 production derived from mantle serpentinization mainly depends on temperature conditions. Serpentinization reactions are fast at temperatures higher than 315°C, but H2 concentrations may be limited by the attainment of stable thermodynamic equilibrium between olivine and aqueous fluid (McCollom & Bach, 2009). Conversely, below 150°C, H2 generation is severely limited by slow reaction kinetics and partitioning of Fe(II) into brucite (McCollom & Bach, 2009). Consequently, rates of H2 generation increase steeply from 200 to 300°C, but drop off at lower and higher temperatures (McCollom et al., 2016). To investigate if the mantle is within the optimum thermal conditions, we positioned the current isotherms on both modeled sections (Figure 10) considering the 25.0 ± 2.7°C/km geothermal gradient estimated by Saspiturry, Lahfid, et al. (2020). Thus, following the linear regression derived from these data (T°C = (0.0252 × Depth) + 7.1291; Saspiturry, Lahfid, et al., 2020; Figure S1 in
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Conceptual model of the Mauléon basin H2 life cycle. (a) SSW-NNE cross-section from geophysical model 1B; (b) WNW-ESE cross-section from geophysical model 3B and (c) WNW-ESE cross-section from geophysical model 4B showing H2 production window (H2PW), H2 reservoir window (H2RW) and potential hydrogen plays. (S): serpentinization amount. Modeling parameters: D rock density, MS magnetic susceptibility. Thrust faults: LAT Lakhoura, SAT South Arbailles, NAT North Arbailles, SPT Saint-Palais, BET Bellevue, SST Sainte-Suzanne. Transfer zones: ITZ Iholdy, STZ Saison, BTZ Barlanès. BTZ: Barlanès transfer zone; IyD: Iholdy diapir; RoD: Roquiague diapir.</figcaption></figure>
Supporting Information S1): the 100, 200, and 300°C isotherms are expected to be encountered at ~3,700, ~7,700, and ~11,600 m depth. The 200 and 300°C isotherms representing the optimum H2 production window (H2PW) are thus restricted to depths between 7.7 and 11.6 km (Figure 10). According to models 1B (Figures 10a), 3B (Figures 10b), and 4B (Figure 10c), a mantle thickness of 2.5–4 km is currently included in the H2PW. Estimated H2PW serpentinization amount (Section 5.2) is between 38% (Front 4) and 76% (Front 7) on the north-south section while is ranging between 63% and 76% on the west-east section. Even if these estimations could appear relatively high, there is still a significant amount of H2 to be produced since the mantle is not fully serpentinized. Water is required to perform the serpentinization reactions. Thus, it remains to be seen whether the water supply is effective to H2PW depths. This issue has recently been discussed. Indeed, based on electrical resistivity data published by Campanyà et al. (2012), Lefeuvre et al. (2021) suggested that the very low electrical resistivity at a depth of 13 km could be related to the presence of fluids infiltrating along the Lakora thrust fault until the contact between the Axial Zone and the mantle. This interpretation implies that serpentinization could currently be active as suggested as well by the seismicity. Lehujeur et al. (2021) have shown a distributed seismicity within the mantel piece. The latter could be linked to active serpentinization. Indeed, the increase in volume that occurs during serpentinization can build up stress and trigger cracking that would promote seismicity within the mantle (Malvoisin et al., 2017). Thus, the seepages recorded at the surface today have a great chance of being the consequence of very recent production (Lefeuvre et al., 2022).

When considering H2 exploitation, plays need to be properly identified. Numerous works shed light on the fact that temperature has a significant impact on the reactivity of H2 at depth. The optimal H2 reservoir window (H2RW) is between 100 and 200°C as H2 is inert within this temperature range due to the kinetic inhibition of both biological and abiotic reactions. Indeed, at temperatures lower than 100°C, H2 is consumed by microbial activity while above 200°C, it can react with sulfate and carbonate (Lefeuvre et al., 2022; McCollom & Shock, 1997; Schwartz & Friedrich, 2006; Truche et al., 2009, 2010, 2020). Considering the current thermal gradient, the 100 and 200°C isotherms representing the H2RW are restricted to depths between 3.7 and 7.7 km (Figure 10). Some plays have already been proposed in a north-south section parallel to model 1B at around 20 km westward. These plays are localized at a depth of 2.8–4 km within the anticlines made up of folded Mesozoic to Triassic formations related to the NPFT system (Lefeuvre et al., 2022). Our WNW-ESE models 3B (Figure 6) and 4b (Figure 7) offer an original point of view along strike of the basin featuring new potential H2 plays. Both models reveals that the Barlanès structure is lithospheric and deeply rooted within the sub-continental mantle. The main difference between these two models lies in the nature of the lithology of the body against the uppermost part of the structure. Given the size and importance of the BTZ, there is a strong chance that it can act as an effective pathway to carry H2 upwards toward the basement/cover interface at ~4 km depth and ~110°C. Such a temperature is ideal for long term H2 trapping. The top of the Barlanès structure thus materialized a potential optimal play for trapping H2. However, this structure is localized in the vicinity of the largest recorded crustal instrumental event in the Pyrenees: the 1967 Arette earthquake (MI 5.3–5.7, Souriau & Pauchet, 1998; Rigo et al., 2015; Sylvander et al., 2021). Thus, seismic hazard needs to be properly addressed for future exploration in the vicinity of this structure. Models 3B and 4B also reveal the presence of potential preserved normal faults affecting the basement. On Model 3B, the faulted basement top is localized at depth of 4 km (~95°C) to 4.5 km (~107°C) and is included in the uppermost part of the H2PW (Figure 10b). On Model 4B, the basement top is slightly deeper as it is between 4.5 km (~107°C) and 5.5 (~132°C) km depth (Figure 10c). These normal faults appear to be sealed by the upper Triassic sequence. Since no H2 seepages have yet been measured above these inherited N20° normal faults (Figure 2b) probably reflects that the Triassic salt-sequence represents a potential efficient caprock (Figures 10b and 10c). All these targets represent optimal plays because they (a) are connected to the 200–300°C H2PW, (b) are localized within the 100–200°C H2RW, and (c) offer potential permeable pathways through the fault damage zone beneath an effective Triassic caprock. However, many questions remain unanswered: (a) what is the extent of these structures? (b) do they have a high permeability? (c) what is the overall volume of mantle present in the H2PW and the proportion of mantle to be serpentinized and (d) how much hydrogen would still be produced if the mantle were entirely serpentinized? To assess this resource further, some of these questions could be addressed by creating a 3D geometrical model of the mantle present in the H2PW.

Following the Pyrenean belt toward the east, two other positive Bouguer anomalies (Lourdes and Saint-Gaudens) are localized within the North Pyrenean Zone and aligned parallel to the trend of the chain (Angrand
et al., 2018; Casas et al., 1997; Chevrot et al., 2018). Such as the Labourd gravity anomaly described in our paper, these anomalies are related to the presence at shallow depth of dense material (Casas et al., 1997; Grandjean, 1994; Muñoz, 1992; Roure et al., 1989). A growing scientific consensus proposes that the Saint-Gaudens anomaly, upon the Baronnies-Comminges cretaceous rift basin (Figure 1), is due to the presence of a shallow body of allochthonous mantle (~8–16 km depth) pushed northward onto the Aquitaine Basin on the NPFT system (Espurt et al., 2019). Thus, this area shares close similarities with the Mauléon Basin as a potential optimal fertile H2 system. It fulfills the following three conditions: potential (a) source zone for H2 generated by serpentinization at shallow depth, (b) mantle rocks within the H2PW considering a geothermal gradient of 25°C/km and (c) effective migration pathways rooted in the mantle source such as the NPFT as well as Triassic caprock. The Lourdes and Saint-Gaudens anomalies therefore represent key targets for replicating the joint gravimetric and magnetic modeling method applied in the Mauléon basin to gain a more precise understanding of the potential H2 source. Other adjacent sedimentary basins could be explored for their natural H2 potential. Indeed, the Late Jurassic to Early Cretaceous transtensional deformation at the Iberia-Eurasia diffuse plate boundary led to the development of several sedimentary basins (e.g., Asti et al., 2022) that have been mostly preserved from the Pyrenean compression, including the Parentis (Bois et al., 1997; Ferrer et al., 2008; Jammes et al., 2010), Basque-Cantabrian (García-Senz et al., 2020; Pedrera et al., 2017, 2020), Bay of Biscay (Thinon et al., 2003) and Columbrets (Etheve et al., 2018; Roma et al., 2018) syn-rift basins (Figure 1). They all share similar characteristics with the Mauléon basin: (a) the presence of a significant gravimetric anomaly aligned with the rift axis, (b) efficient caprock of Late Triassic to earliest Jurassic evaporites, (c) potential H2 source zones of serpentinized mantle at shallow depth, and (d) tectonic structures rooted in the mantle offering potential H2 pathways.

## 6. Conclusion

In this work we show that the mantle beneath the Mauléon basin is gradually serpentinized over a thickness of almost 15 km. Several kilometers of mantle are present in the H2PW. Although the H2PW mantle is serpentinized between 38% and 76%, there is still a significant amount of H2 to be produced if the entire mantle were to be serpentinized. The faulted basement/cover interface located within the H2RW represents a major target for trapping H2. Many questions remain to be answered, but there is no doubt that the basin has great potential for natural H2 exploration. This work demonstrates that joint gravimetric and magnetic forward modeling represents a proven efficient tool to study the overall architecture and amount of serpentinization of preserved mantle upon reactivated rift system. A similar approach should be used to explore the natural H2 potential of others sedimentary basins in the Pyrenean and analogous regions.

## Data Availability Statement

The authors declare that the data supporting this study are available within the following data repository: Saspiturry et al. (2024), https://doi.org/10.17632/kmh5mr4dvp.1. The dataset includes the original gmsys files from Oasis Montaj software of 8 joint gravimetric and magnetic forward 2D modeling along two orthogonal geological transects through the upper lithospheric structures of the Mauléon basin. The dataset also contains the complete Bouguer anomaly, RTP anomaly and elevation grids used for the modeling.

## Acknowledgments

This work is part of Saspiturry's postdoctoral research conducted under the supervision of Cécile Allanic as part of the OROGEN project, cofunded by Total S.A., BRGM, and CNRS. We thank OROGEN project managers Sylvain Calassou (Total), Emmanuel Masini (Total), Olivier Vidal (Centre National de Recherche Scientifique) and Isabelle Thinon (BRGM). We thank the French Geological Survey (BRGM) for supporting this work. We greatly thank the two reviewers and especially Marta Neres who allowed us to significantly improve this work thanks to her constructive comments. Finally, we would like to thank the editors Dr. Laurent Jolivet and Dr. Jacqueline Halpin.

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