# The fingerprint of structural inheritances in the W-Pyrenees revealed by machine learning detection of seismicity

R. Lescoutre, J. -B. Ammirati, Sébastien Chevrot, F. Mouthereau, M. Ducoux, N. Saspiturry, A. Villaseñor, M. Sylvander, E. Masini

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► To cite this version:

R. Lescoutre, J. -B. Ammirati, Sébastien Chevrot, F. Mouthereau, M. Ducoux, et al.. The fingerprint of structural inheritances in the W-Pyrenees revealed by machine learning detection of seismicity. Tectonophysics, 2025, 903, 10.1016/j.tecto.2025.230685. insu-05100387v2

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# The fingerprint of structural inheritances in the W-Pyrenees revealed by machine learning detection of seismicity

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R. Lescoutre<sup>a</sup>, J.-B. Ammirati<sup>b,c,1</sup>, S. Chevrot<sup>c,*</sup>, F. Mouthereau<sup>c</sup>, M. Ducoux<sup>a</sup>, N. Saspiturry<sup>d</sup>, A. Villaseñor<sup>e</sup>, M. Sylvander<sup>f</sup>, E. Masini<sup>a</sup>

## ARTICLE INFO

### Keywords:

Seismicity

Orogeny

Transfer zone

Rift inheritance

Necking zone

Pyrenees

## ABSTRACT

The orogenic evolution of the western Pyrenees and its relationship with ancient rift architecture are well-studied and therefore represent a natural laboratory for investigating the link between structural inheritance and the distribution of present-day seismicity. In this study, we use an automated method to detect and pick P and S waves from local earthquakes to characterize the distribution of seismicity in the region. The P and S picks are used to obtain a catalog of earthquakes and to perform a local earthquake tomography. We analyze 2-D sections and depth slices of the tomographic model and of the seismicity catalog, and confront them to the 3-D geological architecture of the inverted Mauléon rift system. Our results reveal that present-day deformation is mainly extensional, with a main cluster of aligned, steeply north-dipping seismicity localized along the former rift necking domain. In addition, the rift-inherited NNE-SSW Saison and Barlanès transfer zones either offset or disrupt the seismicity cluster and delimit three seismically active segments. The Chaînons Béarnais segment shows localized seismicity, whereas the eastern and western Mauléon segments show a westward extension of earthquake distribution in relation with the widening of both the orogenic wedge and the mantle body preserved at shallow depth. Our results suggest that the rift-inherited necking zone played a predominant role in the localization of the deformation during both orogenic and post-orogenic evolution, and as such represents a critical zone for seismic hazard assessment. Earthquakes diffusely distributed in and around the high-velocity mantle body could be partly related to ongoing serpentinization.

## 1. Introduction

The relationships between rift inheritance and the tectonic evolution of mountain building to post-orogenic collapse has been studied in several orogenic systems such as the Alps (Mohn et al., 2014; Manatschal et al., 2022), the Caledonides (Andersen et al., 2022) or Taiwan (Conand et al., 2020). These studies, supplemented by numerical and analogue experiments of rift inversion, have evidenced the role played by the architecture and rheology of the former rifted margin on the style of deformation (Erdős et al., 2014; Grool et al., 2019; Jourdon et al., 2019, 2020; Jammes and Huismans, 2012) and on the temporal and thermal

evolution of orogens (Célini et al., 2022; Ternois et al., 2019; Waldner et al., 2021; Vacherat et al., 2016; Ducoux et al., 2021a). However there is a lack of understanding on the influence of tectonic inheritance on the present-day deformation pattern as revealed by present-day seismicity (Tarayoun et al., 2019). Indeed there is only a few natural examples where the present-day deformation of an orogenic system can be determined, and for which the pre-orogenic template is well constrained. The northern western Pyrenees (south-west of France) correspond to a mid-Cretaceous hyper-extended rift system which underwent mild reactivation (in comparison to the northern central and eastern Pyrenees) during the Cenozoic convergence and where inherited

<footnote><sup>a</sup> Mantle8 sas, 6 rue de Chamechaude, 38360 Sassenage, France</footnote>

<footnote><sup>b</sup> Departamento de Geología, Universidad de Chile, Santiago, Chile</footnote>

<footnote><sup>c</sup> Géosciences Environnement Toulouse, Observatoire Midi-Pyrénées, Université Paul Sabatier, CNRS, IRD, Toulouse, France</footnote>

<footnote><sup>d</sup> Géosciences Montpellier, Université de Montpellier, CNRS, Montpellier, France</footnote>

<footnote><sup>e</sup> Institute of Marine Sciences, CSIC, Barcelona, Spain</footnote>

<footnote><sup>f</sup> IRAP, UMR 5277, Observatoire Midi-Pyrénées, Université Paul-Sabatier, CNRS, Toulouse, France</footnote>

<footnote>* Corresponding author.
E-mail address: sebastien.chevrot@get.omp.eu (S. Chevrot).</footnote>

<footnote n="1">¹ Deceased.</footnote>
structures controlling the former Mauléon rift basin are well preserved (Souquet et al., 1985; Masini et al., 2014; Teixell et al., 2018; Saspiturry et al., 2020a; Lescoutre et al., 2021). The extensive geological datasets and interpretations acquired in this area during the PYROPE and OROGEN research projects as well as by former studies reveal N110 striking orogenic structures segmented by N20 transfer zones, both largely inherited from the rift evolution as shown by the syn-rift stratigraphy and the location of the main depocenters (Masini et al., 2014; Ducoux et al., 2021b; Saspiturry et al., 2021). While the sedimentary cover was decoupled from the tectonic basement by a thick sequence of

Upper Triassic evaporites during both rifting and convergence, field observations and restored balanced cross-sections allow to identify the main structures controlling crustal thinning (Ducoux et al., 2021b). Recent passive seismic data acquired in the framework of the MAUPA-SACQ experiment (Lehujeur et al., 2021; Chevrot et al., 2022; Sylvander et al., 2023) provided new insight into the crustal architecture of the orogen and the present-day seismicity in the region, highlighting in particular the occurrence and the geometry of a mantle body at shallow depth below the Mauléon basin. The good knowledge about the former, partly reactivated rift structures and the present-day orogenic

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** a) Geological map of the western Pyrenees with main tectonic units and structures discussed in the text. The historical Arette earthquake event is located on the map. Insert map shows the location of the study area between France and Spain. b) Rift domain map showing the distribution of the former rift structures and depocenters (modified after Lescoutre and Manatschal, 2020). Faults: St-ST Sainte-Suzanne Thrust; BeT Bellevue Thrust; St-PT Saint-Palais Thrust; NPFT North Pyrenean Frontal Thrust, NPF North Pyrenean Fault; SaT South Arbailles Thrust; Lak Lakora Thrust; Ron Roncesvalles Thrust. Massifs: Arb Arbailles; Mdb Mendi-belza; Igz Igountze. Basin: BCB Basque-Cantabrian Basin. Wells: Oz Orthez; Be Bellevue; Ce Cheraute; Lc Les Cassières 2; Um Uhart-Mixe; Ai Ainhice. The Scientific colour map batlow (Crameri, 2023) is used in this study.</figcaption></figure>
architecture in the western Pyrenees allows us to investigate the relationship between inheritance and the present-day seismicity in the orogen. In this study, we process the continuous seismic data acquired thanks to the MAUPASACQ large-N array to detect and characterize the seismicity of the Mauléon basin. We then analyze the obtained seismicity catalog along with geological observations from previous studies to gain better insights into the active deformations and on their accommodation by existing regional structures and discuss their relations with structural inheritance.

## 2. Geological setting

### 2.1. Main geological structures and link to rift inheritance

The Pyrenees form a typical asymmetric doubly verging collisional orogen in between France and Spain (Fig. 1) resulting from the collision between the Iberian and European rifted margins from the Late Cretaceous to the early Miocene (Roure et al., 1989; Muñoz, 1992; Teixell et al., 2018). The central part of the mountain belt comprises the Axial Zone, made of stacked crustal units of Paleozoic crystalline basement (Fig. 1a). It is flanked by two opposite vergent fold-and-thrust belts, the South and North Pyrenean Zones (SPZ and NPZ, respectively), and two foreland basins, the Ebro and the Aquitaine basins. The structure of the E-W trending NPZ results from the inversion of the early Cretaceous rift system (Golberg and Leyreloup, 1990; Choukroune, 1992; Mouthereau et al., 2014; Teixell et al., 2018; Espurt et al., 2019). The Mauléon basin and the Chaînons Béarnais are located in the western part of the NPZ. These basins are delimited from the Arzacq basin to the north by the south-dipping N120 North Pyrenean Frontal Thrust (NPFT), westward subdivided into the St-Palais, Bellevue and St-Suzanne thrusts, and to the south from the Axial Zone by the north-dipping N120 to N90 Lakora thrust, eastward corresponding to the North Pyrenean Fault, thus shaping a pop-up structure (Tugend et al., 2014; Teixell et al., 2016; Labaume and Teixell, 2020). In detail, the 3-D structural architecture of the study area can be divided into 3 segments: the western Mauléon, eastern Mauléon and the Chaînons Béarnais segments, delimited by the N20 Saison and Barlanès transfer zones (Fig. 1a).

Whereas the Lakora thrust dips steeply toward the north below the allochthonous sedimentary cover in the interpretation of Velasque et al. (1989) and Dumont et al. (2015), field observations show that it is only gently dipping northward in its southern continuation where it juxtaposes slices of Paleozoic basement mainly on top of the Upper Cretaceous carbonates unconformably covering the Axial Zone (Teixell, 1990). South of the Mauléon basin, the north-dipping South Arbailles thrust (Saspiturry et al., 2019) corresponds westward to a merely reactivated Early Cretaceous extensional detachment fault (or South Mauléon Detachment) above which lies a basement tilted block and its overlying rift basin (Ducasse et al., 1986; Masini et al., 2014; Hart et al., 2016; Lescoutre et al., 2021). To the north, the NPFT/St-Palais/ Bellevue/Ste-Suzanne thrusts dip southwards within the sedimentary cover where they detach in the Upper Triassic evaporites. At depth, these thrusts are thought to root within the crystalline/mantelic basement in the core of the pop-up structure (Masini et al., 2014; Teixell et al., 2016; Lehujeur et al., 2021; Saspiturry et al., 2020a). Field observations (Velasque et al., 1989; Johnson and Hall, 1989; Jammes et al., 2009; Teixell, 1998; Masini et al., 2014; Saspiturry et al., 2021), cross-section restorations (Masini et al., 2014; Saspiturry et al., 2020a; Lescoutre and Manatschal, 2020; Ducoux et al., 2021b) as well as 2-D forward kinematic modeling (Gómez-Romeu et al., 2019) show that the Lakora/South Arbailles and NPFT/St-Palais/Ste-Suzanne structures as well as the transfer zones correspond to former Early Cretaceous rift structures. These extensional faults/detachment systems thinned the crust and eventually lead to mantle exhumation and to high temperature/low pressure metamorphism of the overlying sedimentary cover during hyper-extension (Albarède and Michard-Vitrac, 1978; Montigny et al., 1986; Clerc et al., 2015; Lescoutre et al., 2019; Lagabrielle et al.,

2019a, 2019b; Saspiturry et al., 2020b; Ducoux et al., 2021a).

The Mauléon basin is also characterized by a prominent positive Bouguer gravity anomaly (Fig. 2) (Grandjean, 1994; Casas et al., 1997). This anomaly is commonly attributed to a shallow depth high-density body (≥ 2900 Kg/m³), which has been interpreted either as high density lower crust or as a variably serpentinized mantle body, and either as an allochthonous mantle slice (e.g. Teixell et al., 2016) or as a continuous mantle rise (e.g. Wang et al., 2016), in both cases formerly located in the hyper-extended domain of the Cretaceous rifted margin. The latter interpretation (continuous mantle rise) was proposed on the basis of the PYROPE data (Chevrot et al., 2014, 2015; Wang et al., 2016) and the more recent MAUPASACQ passive seismic acquisition (Polychronopoulou et al., 2018), both supporting a continuous mantle rise beneath a thin continental crust (Lehujeur et al., 2021). Details of the density anomaly have been recently modeled using a petro-physical approach (Pajang et al., 2025). It confirms that the Mauléon basin is floored by a serpentinized mantle that results from hydration wt. 5 % H₂O of a lherzolitic mantle. This high density anomaly vanishes toward the east across the Barlanès transfer zone (Fig. 2).

During early convergence, tectonic inversion started with the northward subduction of the hyper-extended crust and exhumed mantle domains underneath the European continental crust while the decoupled sedimentary cover was tightened and uplifted over the converging margins (Lescoutre and Manatschal, 2020). Surface observations and NNE-SSW cross-sections suggest that the alpine deformation in the NPZ decreases westwards across the Barlanès and Saison transfer zones. This segmentation is thought to be related to the structural reorganization in the vicinity of the Cantabrian-Pyrenean accommodation zone across the Basque Massifs (Lescoutre and Manatschal, 2020; Lescoutre et al., 2021). Indeed, as proposed by Lescoutre and Manatschal (2020), structural inversion of the overlapping rift Basque-Cantabrian and Mauléon basins lead to the formation of early orogenic shortcutting structures such as represented by the E-W trending Lakora and Roncesvalles faults within the Basque Massifs (Fig. 1a). Such thick-skinned, early orogenic structures are thus responsible for the formation of a proto-orogenic wedge within which exhumed mantle rocks can be preserved and escape subduction.

<figure><img src="image_5.png" /><figcaption>Fig. 2. Map showing Bouguer anomalies (from Wehr et al., 2018) and station locations of the MAUPASACQ experiment. PBB Permanent broadband stations; BB Broadband stations; SP Short-period stations; GN 3C 10 Hz geophone nodes.</figcaption></figure>
All together, these stratigraphic, structural and lithological varia-
tions across the western Pyrenees allow a reconstruction of the rift do-
mains within the present-day orogenic system (Tugend et al., 2015;
Lescoutre and Manatschal, 2020; Ducoux et al., 2021a). The transition
between these domains corresponds to major sedimentary and structural
variations in relation to crustal thinning (e.g., Tugend et al., 2015;
Lescoutre and Manatschal, 2020; Miró et al., 2021; Manatschal et al.,
2021). As such, the transition from the proximal to the necking domains
corresponds to a progressive deepening of the basin associated with the
thinning of the crust to < 20 km. The transition from the necking to the
hyper-extended domain corresponds to the zone of maximum accom-
modation space where generally deep sedimentary facies lie over a low-
angle detachment fault and a basement composed of hyper-thinned crust
or subcontinental mantle. In the western Pyrenees (Fig. 1b), such do-
mains roughly correspond to the Axial Zone (proximal domain), the
Arbailles, Mendibelza and Igountze massifs (necking domain) and the
Mauléon basin or the Chaînons Béarnais to the north of these massifs
(hyper-extended domain). Based on this approach, the Lakora thrust
(Fig. 1) and its hanging wall faults such as the South Arbailles thrust,
delimit the former proximal domain to the south from the necking
domain to the north and corresponds to reactivated rift structures which
controlled major crustal thinning during Cretaceous hyper-extension.

## 2.2. Historical seismicity

The largest recorded crustal instrumental event in the Pyrenees is the 1967 Arette earthquake (Mᵢ 5.3–5.7, Rigo et al., 2015, Fig. 1a). It occurred in a narrow E-W corridor in the NPZ of the western Pyrenees where most of seismic activity in the Pyrenees is concentrated (Souriau and Pauchet, 1998; Sylvander et al., 2021). Past analyses of present-day strain patterns derived from focal mechanisms indicate a dominant orogen-normal extension (Chevrot et al., 2011; Mazzotti et al., 2021; Rigo et al., 2015), concordant with, although poorly-resolved, slow N-S extension of 0.1–0.4 mm.yr⁻¹ defined by GNNS data (Mazzotti et al., 2020). The extensional strain pattern is commonly thought to result from a local reactivation of a large-scale thrust sheet (namely the Lakora-Eaux-Chaudes fault system), which remains structurally poorly defined (Velasque et al., 1989; Dumont et al., 2015). Another scenario involves the activity of a series of ESE-WNW en-échelon normal fault segments, aligned with the so-called North Pyrenean Fault in the central-eastern Pyrenees, that accommodate the differential uplift between the Axial Zone and the NPZ at lower elevation (Fillon et al., 2021). Temporal constraints in the Axial Zone and adjacent foreland regions (Aquitaine and Ebro basins) for the onset of recent exhumation suggest that extension coincides with a large-scale dynamic change that occurred around 10 Myr ago in response to the cessation of slab retreat in the West Mediterranean (Rat et al., 2022). The distribution of seismic activity therefore appears to be controlled by inherited structural and density contrasts. Yet, the lack of precise knowledge of the 3-D crustal structures, and of the relation between the seismicity distribution and geological structures, has precluded a better understanding of the role of inheritance in the distribution of earthquakes in the Pyrenees.

# 3. Data and methods

## 3.1. *The MAUPASACQ experiment*

The MAUPASACQ experiment is a large-N array deployed in the Mauléon basin from March to October 2017 (Polychronopoulou et al., 2018; Lehujeur and Chevrot, 2020; Sylvander et al., 2023). It consists of a total of 442 three-components sensors: 54 broad-band stations, including 6 stations from the French and Spanish permanent networks, 197 short period seismometers, and 191 three-component 10-Hz geophone nodes (Fig. 2). The denser part of the network forms a 50 × 30 km rectangular grid. The geophone nodes were installed along five WNW-ESE profiles with a spacing of 1 km and three NNE-SSW profiles

with a spacing of 1.5 km. The short-period and broad-band sensors are
uniformly distributed with a spacing of 3 km and 7 km, respectively. The
network is surrounded by two rings of 10 short-period (inner-ring) and
11 broad-band stations (outer-ring).

## 3.2. Characterization of the local seismicity

3.2.1. Automated earthquake detection

Due to the size of the MAUPASACQ dataset, classic semi-
automatic approaches based on manual phase identification (Allen,
1982; Baer and Kradolfer, 1987) are simply not applicable. We therefore
analyzed the continuous, 3-component waveforms recorded by the
MAUPASACQ array using PhaseNet (Zhu and Beroza, 2019), a deep
convolutional neural network (DCNN) trained to automatically identify
and pick P and S phases from local earthquakes (epicentral distances
comprised between 0 and 150 km). For each sensor, PhaseNet outputs
the corresponding probability density functions of P and S picks. Here
we keep the phase picks with a probability higher than 30 %. This rather
low threshold ensures that we get as many picks as possible from the
automatic picking performed with PhaseNet (Ammirati et al., 2022).
The model identified more than 57 Million phases (~30 Million P phases
and ~27 Million S phases). This represents ~130,000 detections per
sensor for the duration of the experiment (~6 months). Most of these
picks are false positives, but this does not pose a problem because they
will be systematically filtered out during the next processing step (as-
sociation). In that stage, only picks that are consistent with true seismic
events will be retained, ensuring the reliability of the final dataset.

Although many applications showed that PhaseNet performs well in
identifying seismic phases even from low signal-to-noise ratio signals
(Münchmeyer et al., 2022; García et al., 2022), it remains a single-
station approach. Hence it is necessary to evaluate the consistency of
the detections across the array before they can be associated to a com-
mon seismic event. To do so, we use the source scanning algorithm REAL
of Zhang et al. (2019). Briefly, this method compares theoretical travel-
times calculated for arbitrary source positions with observed travel-
times corresponding to real events (in our case, the P and S phases
identified by PhaseNet). Using the regional 1-D velocity model (Fig. S1;
Kissling et al., 1994) described in Souriau and Pauchet (1998), we first
computed P- and S-waves theoretical travel-times, every 0.02 degrees
(~2 km) for distances ranging from 0 to 0.2 degrees and every 1 km, for
depths between 0 and 30 km. Taking advantage of the large number of
sensors of the MAUPASACQ deployment, we only consider events with a
minimum of 20 P phases and 10 S phases. Less stringent criteria would
allow more detections but at the price of more false and/or replicated
events (several detections of the same event). This step resulted in the
detection of ~2700 events.

3.2.2. Validation of the seismicity catalog

As mentioned above, because the MAUPASACQ array has a very
large number of sensors, we could not perform manual picks on the
whole dataset. However, the performance of the automatic phase pick-
ing and phase association processes can be assessed by plotting a Wadati
diagram (Fig. S2). On this diagram, incorrect phase associations are
easily identified as they will not follow the linear relationship between P
and P—S travel-times. Moreover, we were also able to compare our
seismicity catalog with another catalog obtained from the same dataset
(Fig. S3), but with a different automatic picker, based on Kurtosis
analysis (Polychronopoulou et al., 2018). Both catalogs were relocated
in the same 1-D velocity model (Fig. S3a) in order to also compare the
event locations. Both catalogs are very similar in terms of hypocenter
locations, but the comparison reveals that the new catalog contains 75 %
more events (Fig. S3b). Note that a significant number of these events,
absent from the initial catalog, corresponds to quarry blasts. Based on
geographic and temporal criteria (proximity to an identified quarry site,
shallowness of the focus and occurrence during working hours), at least
400 of the events present in our catalog can be identified as artificial
events. They have been removed from the seismicity maps and cross-sections, so as not to pollute seismotectonic interpretations. The picks of common events in the two catalogs are very similar although significant differences can appear at some stations (Fig. S4). Previous studies showed that, in general, PhaseNet performs well in identifying phases even for very small earthquakes (Ammirati et al., 2022; García et al., 2022).

### 3.3. Local earthquake tomography

We considered only those earthquakes and quarry blasts whose preliminary location (according to the 1-D velocity model) had been obtained with a minimum of 8 P, and 4 S picks and an azimuthal gap smaller than 200°, resulting in a selection of 1472 events with 183,923 P phases and 152,312 S phases recorded by 438 seismic stations (Fig. S5). The box containing all the earthquakes and stations is 150 × 140 km in the N-S and E-W directions respectively, extending from 4 km above sea level to 50 km depth. The relatively small size of the model allows us to use a flat-Earth approximation. Travel time calculations are performed in a Cartesian grid and thus require the conversion of geographical coordinates into Cartesian coordinates. We use the finite-difference method of Podvin and Lecomte (1991) with 0.2 km cubic cells to compute P and S travel times. The model is parameterized with constant velocity cubic cells of 1.2 km. The cell size was then decreased by a factor of 3 using the offset-and-average interpolation approach (e.g., Fig. 10 in Evans and Zucca, 1988). The final model is thus parameterized with 0.4 km cubic cells. The total number of cells in the final interpolated model is 375 × 351 × 138 cells in the X, Y, and Z directions, respectively. Checkerboard reconstructions (Figs. S6, S7 and S8) are obtained using the same methodology.

P- and S-wave velocity models are inverted simultaneously, following the methodology described in Benz et al. (1996) and modified by Tryggvason et al. (2002) to consider S-wave travel times. The method consists in linearizing the non-linear tomographic problem in order to jointly invert the velocity structure and earthquake locations (Spencer and Gubbins, 1980). The initial model used for the iterative inversion is the same 1-D model used for phase association and preliminary earthquake location (Fig. S1). Velocity changes from this reference model are solved using the iterative LSQR (least-square) method described in Paige and Saunders (1982). A smoothing constraint is applied to control the model roughness (Benz et al., 1996). After testing values ranging from 200 (overdamped) to 10 (underdamped), we found that a smoothing parameter of 20 produces the best variance reduction without degrading the appearance of the models. After 10 iterations the residual mean square (RMS) of the P- and S-wave residuals did not decrease significantly. The initial RMS of the P-wave arrival times was 0.135 s, decreasing to 0.091 s after 10 iterations. The initial RMS for S-waves was 0.153 s and 0.095 s after 10 iterations.

### 3.4. Earthquake location

Although the tomographic inversion implies iterative earthquake relocation, this operation is only applied to the sub-selection of events used for tomography. We used NonLinLoc, a non-linear approach based on the probabilistic reformulation of the inverse event location problem (Lomax and Snieder, 1994; Tarantola and Valette, 1982) to re-calculate the 3-D locations for the entire seismicity catalog. The resulting hypocenters are represented by a probability density function, thus giving more comprehensive location uncertainties. We relocated the ~2700 events obtained using the DCNN PhaseNet within our 3-D tomographic P- and S-wave models. To ensure the quality of our seismicity catalog and remove poorly located earthquakes (such as unlikely false events generated from erroneous picks or events outside the network) we retained only those with an RMS value below 0.5 s and horizontal and vertical location uncertainties under 10 km. As a result, the final dataset consists of 2654 well-constrained events. These remaining events are

characterized by low uncertainties. Overall, location uncertainties (estimated from the 68 % confidence ellipsoid error) are on average 1.9 km (within one standard deviation $\sigma$ of 1.1 km) and 2.3 km ($\sigma$ = 1.3 km) in the horizontal and vertical directions, respectively. The mean RMS is 0.21 s ($\sigma$ = 0.06s). Each event was located using an average of 155 ($\sigma$ = 81) phases. This catalog contains events that occurred between April 1st and October 4th, 2017. We used S-wave maximum amplitudes to estimate the local magnitude (Ml). We followed the scheme described by Duverger et al. (2021), resulting in magnitude values coherent with those provided by the French Atomic Energy Commission (CEA). Local magnitudes range from -1.99 < Ml < 2.98. We note that Ml values generally increase with increasing distance from the MAUPASACQ array and that hypocenters are better constrained where the station coverage is denser (Fig. 3). The resulted earthquake location is displayed in Fig. 3.

## 4. Wave velocity variations and crustal seismicity

Thanks to the dense distribution of stations of the MAUPASACQ deployment (Fig. 2), our tomographic inversion produced images of the Mauléon Basin with an unprecedented level of details (Fig. 4). In the first 3 km (Fig. 4a) Vp are ~5 km/s beneath the Arzacq basin and ~6 km/s beneath the Basque Massifs. The Mauléon basin is characterized by wave velocities ranging from 5 to 5.5 km/s. We note that shallow velocity variations are in good agreement with well-identified regional structures such as the NPFT or the St-Palais thrust (Fig. 4a).

At ~6 km depth (Fig. 4b), seismic velocity shows sharp variations across N110 and N20 lineaments which correspond to the main rift and orogenic structure orientations in the inverted rift system. The basement is also signed by higher velocities ($Vp$ ~ 6.5 km/s) beneath the Basque Massifs. P-wave velocities in the Arzacq basin show in general lower values ($5 < Vp < 5.5$ km/s) than in the Mauléon basin ($6 < Vp < 6.5$ km/s).

At even deeper levels (~16 km), decreasing ray density at the periphery of our study area (e.g., Fig. S8) limits the resolution of velocity variations beneath the Mauléon basin (Fig. 4c). Abnormally high velocities are observed beneath the Mauléon basin with value greater than $Vp > 8$ km/s.

## 5. Comparison between velocity model, seismicity and surface geology

The distribution of seismicity in map view shows at least two different patterns (Fig. 5a). From the western Mauléon basin segment to the Chaînons Béarnais segment, a roughly continuous and localized seismicity cluster is observed along the southern margin of the basin, trending roughly parallel to the trace of the Lakora thrust at the surface (former limit between the proximal and the necking domains). Note that this E-W band of localized seismicity cluster was already shown to propagate eastward to longitude 0.3° (west of the Saint-Gaudens gravity anomaly) (Souriau et al., 2014; Chevrot et al., 2022). A second pattern can be observed, characterized by a scattered seismicity within a broader area encompassing the entire former rifted area to the north, but only in the western and eastern Mauléon basin segments (i.e., west of the Barlanès transfer zone, Fig. 5a). In addition to these two main patterns, subsidiary events related to either depleted oil fields (Arzacq basin at 43.45° latitude and -0.6° longitude) or local structures/events (e.g., north-west of the Basque Massifs) are observed. Note also that some of the earthquakes located in the South Pyrenean Zone (previously identified in the regional catalog from Chevrot et al., 2022) could be attributed to either the reactivation of structures from the South Pyrenean Frontal Thrust System or to the reactivation of former rift structures in the eastern termination of the Basque-Cantabrian rift system (Lescoutre et al., 2021).

In the Chaînons Béarnais segment the main E-W oriented seismicity cluster shows, as a general trend, a deepening toward the west. Across
<figure><img src="image_6.png" /><figcaption>Fig. 3. Earthquake location and magnitude projected over the rift domain map. St-ST Sainte-Suzanne Thrust; BeT Bellevue Thrust; St-PT Saint-Palais Thrust; NPFT North Pyrenean Frontal Thrust; SaT South Arbailles Thrust; Lak Lakora Thrust.</figcaption></figure>

the Barlanès and the Saison transfer zones, the main seismicity cluster (along the southern margin of the NPZ) appears abruptly shifted southward and locally discontinuous, respectively. These local variations of seismicity in the transfer zones are well observed at ~6 km depth (Fig. 4b). In addition, earthquake distribution at around 6 km depth (Fig. 4b) displays two WNW-ESE and W-E lineaments in between the Saison and Barlanès transfer zones which appear to merge across the Barlanès transfer zone. Figs. 4c and 5b also show that seismicity is more widely distributed west of the Barlanès transfer zone below the Mauléon basin where crustal velocities are >~7 km/s and where the high velocity body is observed. Approaching the Basque Massifs, the seismicity cluster changes orientation from ~N100 to E-W inside the Basque Massifs where the cluster appears more discontinuous (Fig. 5a).

We have extracted 5 sections from the velocity model (Fig. 6), three NNE-SSW profiles (parallel to the shortening direction) and two WNW-ESE profiles (parallel to the orogen axis). Of these 5 sections, 3 are closely parallel to the profiles displayed and interpreted in Lehujeur et al. (2021), in which a more comprehensive comparison of velocity models with regional geology is presented. Profile 1 (Fig. 6a) corresponds to profile C1-C3 in Lehujeur et al. (2021), profile 2 (Fig. 6b) corresponds to section A1-A2 in Lehujeur et al. (2021), and profile 5 (Fig. 6e) corresponds to section B1-B2 in Lehujeur et al. (2021).

The other two sections correspond to a SSW-NNE profile in the Chaînons Béarnais segment (Fig. 6c) and to a WNW-ESE profile located southward of profile 5 (Fig. 6d). On the SSW-NNE cross-sections (Figs. 6a, b and c), seismicity is mainly localized along a steeply north-dipping plane, with a few earthquakes located at a depth < 10 km to the north (within the basin) or south of the profile. In the western (Fig. 6a) and eastern Mauléon (Fig. 6b) segments, seismicity is localized between the high velocity body to the north and the lower velocity crust to the south and below the trace of the Lakora/South Arbailles thrust system at

the surface (Fig. 5a). Diffuse seismicity can be observed in the high-velocity body attributed to mantle rock by Lehujeur et al. (2021) (Vp >~7 km/s in Figs. 4c and 6a). In the Chaînons Béarnais segment (Fig. 6c), seismicity is localized along the southern boundary of the basin and north of the trace of the Lakora thrust at the surface. On the WNW-ESE cross-sections (Figs. 6d and e), the main seismicity cluster appears to follow a W-dipping plane to a depth of 20 km and rises below the trace of the North Pyrenean Fault (i.e., the Lakora thrust equivalent in the Chaînons Béarnais segment) at the surface. While the WNW-ESE profiles are slightly oblique to the orientation of the seismicity cluster on the map and to the southern orogenic structures, the W-dipping cluster appears to stop westward across the Barlanès transfer zone on profile 5 (Fig. 6e). On the same profile 5 and to the west of the Barlanès transfer zone, seismicity appears to be localized above the high-velocity body, but also along its apparent western termination at ~18 km depth. On southern profile 4 (Fig. 6d), the W-dipping seismicity cluster appears slightly disrupted across the Saison transfer zone.

# 6. Discussion

## 6.1. Mode and origin of the deformation

In summary, two main types of seismicity have been recognized in this study: (i) localized E-W trending seismicity cluster to the south of the NPZ (and locally south of the high velocity anomaly) and parallel to the trace of the Lakora (North Pyrenean Fault to the east)/Roncesvalles thrusts, and (ii) scattered seismicity within and around the high-velocity anomaly. Focal mechanisms highlight a predominant normal component along the main structure south of the NPZ (Mauléon basin and Chaînons Béarnais) as well as some dextral strike-slip component (Fig. 7). The results are consistent with INSAR observations (Berthelon
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Horizontal slices at 3 km (a), 6 km (b), and 16 km (c) depth across the tomographic model. Projected seismicity (< 2 km) and surface structural geology are displayed. d) Seismicity and its magnitude projected over the rift domain map. St-ST Sainte-Suzanne Thrust; BeT Bellevue Thrust; St-PT Saint-Palais Thrust; NPFT North Pyrenean Frontal Thrust; SaT South Arbailles Thrust; Lak Lakora Thrust.</figcaption></figure>

et al., 2019) which suggest a downward displacement of the NPZ (former early Cretaceous basins) relative to the Axial Zone (former proximal margin). The reason for such displacement is yet unclear and many factors could be at play such as far-field plate motion, regional post-orogenic stress field, or local effects (fluids, density/rheological contrast). In the latter, the density contrast between the dense mantle body and the buoyant crustal rocks of the Axial Zone has been proposed to account for such relative displacement (Souriau et al., 2014; Fillon et al., 2021; Rat et al., 2022). In particular, petro-physical models from a recent study (Pajang et al., 2025) show that seismic velocity below the Mauléon basin can be reproduced by a dry mantle (density of 3.3–3.4) overlain by a serpentinized mantle (density 2.9) to around 40% between 20 and 10 km depth, and that this is in sharp contrast to the adjacent lighter column of rocks in the mountain range (densities ranging from 2.5 to 2.9). These models further predict that, in order to fit the current elevation in the Mauléon region, the excess subsurface load must be supported by a plate with an effective elastic thickness of 10 to 20 km. However, these models alone fail to explain why the E-W anomaly seismicity cluster extends far beyond the limits of the mantle body

(Fig. 5), implying that other processes must be at play in the region.

The occurrence of earthquakes within and around the high-velocity body raises questions on the origin of the scattered seismicity in the former hyper-extended domain. This could be due to the reactivation of rift/orogenic structures in the vicinity of the dense mantle body and, or in addition to, the ongoing serpentinization of the mantle body as this process is known to generate a significant rock volume increase (Klein and Le Roux, 2020). This later interpretation is supported by identified hydrogen seeps at the surface along the NPFT which could be related to ongoing serpentinization (Lefeuvre et al., 2021, 2022).

## 6.2. 3-D inheritance and present-day deformation

Our results underline the control of inherited rift architecture on the distribution of the present-day deformation in the western Pyrenees (Fig. 8) and on the segmentation of active faults. Indeed, to the west, the southward shift of the seismicity across the Barlanès transfer zone appears to be linked to the reorientation of the structure along a transfer fault or relay ramp that transfers deformation to the Lakora fault
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Seismicity distribution in the western Pyrenees projected on the geological map (a) and on the Bouguer anomaly map (b). Profile traces from Fig. 6 are displayed on a).</figcaption></figure>

(Fig. 5a). Further to the southwest, seismicity tends to follow the orientation of the newly formed Roncesvalles fault which transfers a part of the deformation between the NPZ and the Basque-Cantabrian basin (Lescoutre and Manatschal, 2020; Lescoutre et al., 2021). The transition between the Chaînons Béarnais segment and the east and west Mauléon segments is also characterized by the presence of the high-velocity mantle body and the widespread distribution of seismicity within the NPZ (Figs. 5b and 8). This suggests that present-day deformation is localized in and around the mantle body, and that the zone of deformation is progressively widening westward, as suggested by the apparent divergent lineaments of seismicity (WNW-ESE and E-W, Fig. 4b) across the Barlanès transfer zone. These observations are in line with surface geology, which shows a widening of the NPZ toward the west, and with tectonic models, which propose a widening pop-up structure toward the Basque Massifs across rift-inherited transfer zones (Masini et al., 2014; Lescoutre and Manatschal, 2020; Saspiturry et al., 2022). In that aspect, the general westward deepening of the seismicity cluster to the east of the Barlanès transfer zone could reflect the

deepening of the crustal wedge (in particular its S point, see e.g., Lescoutre et al., 2021) in relation with its widening westward. Between the eastern and western Mauléon segments, the E-W seismicity cluster south of the basin also shows a disturbance through the Saison transfer zone. In summary, our results show that seismicity distribution can vary along transfer zones, suggesting that transfer zones should be regarded as critical domains for seismic hazard prediction in orogenic systems.

In the southwestern part of the Mauléon basin, it is interesting to note that the seismicity cluster changes orientation from ~N100 to ~E-W at longitude -1.2° (Fig. 5a) and propagates westwards across the Basque Massifs and along the northern margin of the inverted Basque-Cantabrian Basin (Lescoutre et al., 2021). This longitude (-1.2°) corresponds, further to the south, to the apparent intersection between the rift-inherited ~N110 Lakora thrust and the newly-formed ~N80-striking Roncesvalles thrust. Note also that, west of this longitude, the South Arbailles fault shows only minor contractional reactivation in comparison to its eastern part. These observations support that orogenic to present-day deformation is mainly controlled by rift-inherited structures
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** SSW-NNE (a, b, c) and WNW-ESE (d, e) tomographic sections across the western Pyrenees (see location on Fig. 5). Profile 1: western Mauléon segment; profile 2: eastern Mauléon segment; profile 3: Chaînons Béarnais segment. Surface geology, borehole and nearby seismicity (< 5 km) are projected on each profile. Vertical lines with arrows pointing outward or inward show the location of main anticlinal and synclinal structures in the basin, respectively.</figcaption></figure>

in the NPZ. It locally varies and localizes along newly-formed structures at rift segment boundaries where it transfers deformation to the Basque-Cantabrian Basin, as proposed by Lescoutre and Manatschal (2020).

If we consider only the main E-W seismicity cluster, it is interesting to note that seismicity at depth follows the southern limit of the former necking zone. On the surface, this domain corresponds to the Lakora structure (Fig. 3) which is, together with the Arbailles fault system, reactivated rift-related detachments responsible for continental crust hyper-extension and mantle exhumation during the Early Cretaceous (Jammes et al., 2009; Masini et al., 2014; Saspiturry et al., 2019; Labaume and Teixell, 2020; Lescoutre and Manatschal, 2020). However, the nature of the vertical contact between the mantle body and the crust where the earthquakes occur is debated. Two interpretations are proposed below (Fig. 9), which differ in the importance given at depth to the reactivation of the faults bounding the necking zone.

In the first hypothesis (Fig. 9, hypothesis 1), the deepest earthquakes are caused by strain localization at the Iberian Moho that was originally south-dipping in the necking zone (Cretaceous), then later rotated and

verticalized during collision (Paleogene) and finally reactivated as a normal fault recently (Neogene). Thus, in this hypothesis, earthquakes occur on a newly formed normal fault related to the post-orogenic stress field. In agreement with some previous tectonic interpretations (Wang et al., 2016; Saspiturry et al., 2020a), the shallowest part of the distal Iberian margin, including e.g., the Lakora and South Arbailles thrusts, forms shallow-dipping detachments that were only slightly inverted above the mantle indenter. Thick-skinned thrusts that root in the middle-lower crust of the Iberian proximal margin are responsible for the main phase of orogenic growth, which conforms with some predictions of numerical models of rifted margin inversion (Jourdon et al., 2019; Mouthereau et al., 2021). In summary, the shallow seismicity reflects a newly formed steeply-dipping normal fault cutting across the former Iberian margin necking zone. This fault root at depth along the verticalized Iberian Moho, which is responsible for the deepest seismicity.

In the second hypothesis (Fig. 9, hypothesis 2), seismicity is localized on verticalized orogenic structures, which have themselves partially
<figure><img src="image_10.png" /><figcaption>Fig. 7. a) Selection of focal mechanisms in the western Pyrenees. b) Triangle diagram (Frohlich, 1992) for the earthquakes localized along the northern margin of the Axial Zone (between latitude 43.2 and 43.0) highlighting the main normal component of the deformation. Profiles 1 (c) and 3 (d) across the western Mauléon segment and the Chaînons Béarnais segment, respectively, with projected focal mechanisms.</figcaption></figure>

reused rift-related necking structures. In detail, the present-day deformation is interpreted as reactivation of the verticalized ramp (D2 in Dumont et al., 2015) of the Lakora thrust. Early in the orogeny, this thrust (among others) was proposed by Lescoutre and Manatschal (2020) to be part of a fault system allowing, in 3-D, the transfer of deformation from the NPZ to the Basque-Cantabrian basin to the southwest across the Basque Massifs. Consistent with kinematic forward modeling (Gómez-Romeu et al., 2019), this early thick-skinned orogenic structure was guided by former detachment faults of the necking zone and propagated downward into the exhumed lower crust and mantle. This early thick-skinned deformation (prior to Eocene crustal nappes stacking) enabled the anomalous preservation of mantle rocks at shallow crustal depth in the western Pyrenees orogenic wedge. This scenario

establishes a 3-D genetic link between rift-related necking structures, orogenic architecture and present-day deformation. This interpretation is supported by the spatial distribution of the localized seismicity cluster, which extends both to the west in the Basque Massifs and to the east in the Chaînons Béarnais), and which follows the geometry of the inverted overlapping rift systems and cross-cutting structures described in Lescoutre and Manatschal (2020).

Independently of the disputed structural interpretations mentioned above, this study emphasizes the key role played by the rift-inherited necking zone in defining the distribution of deformation during both the orogenic and post-orogenic evolution of alpine-type orogens. Further attempts to explore and test the link between former necking zones and present-day seismicity in other alpine-type orogenic systems
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** 3-D bloc diagram of the western Pyrenees showing the relationship between the former rift domains and structures, the present-day seismicity and the rift-inherited segmentation. Simplified geological interpretations of profiles 1, 2 and 3 (Fig. 6) are displayed as well as earthquakes projection and isovelocity lines from the tomographic model. The figure highlights the role of the former rift necking zone in localizing present-day deformation as well as the westward spreading of the seismicity distribution in relation with the increasing width of the orogenic wedge and the occurrence of the mantle body at shallow crustal depth (high velocity - high density body). For hypothesis 1 or 2 regarding the age and origin of the structure at depth along which seismicity localizes, please see discussion and Fig. 9.</figcaption></figure>

should be conducted. If supported by additional studies, this would represent a critical information for seismic hazard assessment in post-orogenic systems.

## 7. Conclusion

Results from automated earthquake detection and location obtained with the large-N MAUPASACQ array allows us to explore the link between the present-day seismicity in the western Pyrenees and inheritance. Seismicity, from which focal mechanisms reveals mostly the normal to strike-slip components of the deformation, shows strong distribution variation from east to west across major, rift-inherited transfer zones, and from south to north along rift-inherited orogenic structures. In map view, the main seismicity cluster follows the segmented southern margin of the rift basins between the Axial Zone and the NPZ. In 2-D sections, this cluster is localized at shallow crustal depth along the boundary of the former proximal and necking domains. At depth in the Mauléon segment, this seismicity cluster localizes between the high

velocity mantle body and the crustal rocks of the Axial Zone (or equivalent crustal nappe stack just west of the Axial Zone). While the density contrast between the dense mantle body and the surrounding buoyant crustal rocks has been previously suggested to represent a potential trigger of the present-day deformation in the western Pyrenees, others mechanisms could be involved to account for the E-W extent of the seismic cluster beyond the limits of the gravity anomaly. In the NPZ, seismicity becomes more widely distributed westward across transfer zones, mimicking the shape of the positive Bouguer anomaly and the widening of the orogenic wedge. In details, these earthquakes are mainly distributed around and in the high-velocity anomaly interpreted as partly serpentinized mantle rocks. As a consequence, we propose that the strongly segmented, rift-inherited orogenic architecture and the mantle body are the main factors controlling the distribution of the seismicity in the western Pyrenees. In particular, we highlight the role of the former rift necking zone in localizing deformation during both orogenic and post-orogenic evolution.
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Comparison between hypothesis 1 and hypothesis 2 for the role of inheritance on the location of the localized seismicity cluster in the western Pyrenees. See text for details.</figcaption></figure>

## Credit authorship contribution statement

**R. Lescoutre:** Writing – original draft, Visualization, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **J.-B. Ammirati:** Writing – original draft, Visualization, Software, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation. **S. Chevrot:** Writing – original draft, Visualization, Investigation, Formal analysis, Data curation, Conceptualization. **F. Mouthereau:** Writing – original draft, Validation, Investigation, Conceptualization. **M. Ducoux:** Writing – original draft, Visualization, Validation, Investigation, Conceptualization. **N. Saspiturry:** Writing – original draft, Investigation, Conceptualization. **A. Villaseñor:** Writing – original draft, Methodology, Formal analysis, Data curation. **M. Sylvander:** Writing – original draft, Visualization, Resources, Investigation, Formal analysis, Data curation. **E. Masini:** Writing – original draft, Validation, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

J.-B.A. acknowledges funding from the Agencia Nacional de Investigación y Desarrollo (ANID) of Chile (FONDECYT grant N°3200633). This publication is dedicated to Jean-Baptiste Ammirati, who suddenly passed away during the development of this work. We thank D. Pedreira, A. Teixell and an anonymous reviewer for the constructive reviews on the manuscript. The authors declare no competing interests.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.tecto.2025.230685.

## Data availability

Seismicity data used in this study are available upon request.

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