# A long history of ductile and brittle deformation of Eastern Pyrenees fault zones: coupling $^{40}$Ar/$^{39}$Ar geochronology, chlorite geothermometry and structural observation

Gaétan Milesi, Philippe Münch, Delphine Charpentier, Arthur Iemmolo, Aoutmane Bouaoudi, Andreï Lecomte, Virginie Moutarlier, Renaud Gley, Michael Bonno, Patrick Monié

<figure><img src="image_1.png" /></figure>

## ► To cite this version:

Gaétan Milesi, Philippe Münch, Delphine Charpentier, Arthur Iemmolo, Aoutmane Bouaoudi, et al. A long history of ductile and brittle deformation of Eastern Pyrenees fault zones: coupling $^{40}$Ar/$^{39}$Ar geochronology, chlorite geothermometry and structural observation. Journal of Structural Geology, 2025, 199, pp.105465. 10.1016/j.jsg.2025.105465. hal-05175124

HAL Id: hal-05175124

https://hal.science/hal-05175124v1

Submitted on 22 Jul 2025

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# A long history of ductile and brittle deformation of Eastern Pyrenees fault zones: coupling $^{40}$Ar/$^{39}$Ar geochronology, chlorite geothermometry and structural observation

Gaétan Milesi<sup>a,∗</sup> ◆, Philippe Münch<sup>b</sup>, Delphine Charpentier<sup>c</sup>, Arthur Iemmolo<sup>b</sup>, Aoutmane Bouaoudi<sup>c</sup>, Andreï Lecomte<sup>a</sup>, Virginie Moutarlier<sup>d</sup>, Renaud Gley<sup>e</sup>, Michael Bonno<sup>b</sup>, Patrick Monié<sup>b</sup>

## ARTICLE INFO

**Keywords:**

Phyllosilicates

$^{40}$Ar/$^{39}$Ar dating

Chlorite thermometry

Ductile/brittle deformation

Polyphase tectonic

Eastern Pyrenees

## ABSTRACT

In orogenic settings, fault networks accommodate crustal deformation during the evolution of mountain ranges. Fault zones exhibit multiphase ductile and brittle activity, complicating their interpretation. Dating deformation in orogenic basement remains a challenge. In this study, we conducted structural and microstructural observations, coupled with $^{40}$Ar/$^{39}$Ar dating on encapsulated mineral fractions and chlorite thermometry, on major fault zones in the Eastern Pyrenees. To overcome the presence of K-feldspar within fault gouge, a major issue for fault gouge argon dating, we present a method to estimate the contribution of two mixed K-rich phases from $^{40}$Ar/$^{39}$Ar step heating. This method provides limit or maximum age for the deformation recorded by fault gouges in which illite polytypes are mixed with K-feldspar. Our results reveal a ductile-to-brittle transition between 40 and 35 Ma (Priabonian-Bartonian), characterized by 2M1 muscovite and evidence of strike-slip movement along the Py NE-SW fault and NW-SE secondary faults. The Py fault gouge contains muscovite formed at temperatures exceeding 200–250 °C. In the Tét NE-SW fault gouge, the coexistence of 2M1 and 1M illite polytypes suggests late-stage crystallization of 1M illite at 22.1 ± 1.4 Ma, at temperatures between 100 and 150 °C, as determined by chlorite thermometry. These findings confirm significant normal faulting activity on the Tét fault during the Oligo-Miocene, consistent with published low-temperature thermochronology data indicating early exhumation of the Canigou massif relative to the Carança massif, facilitated by normal displacement along the Py fault, and a later exhumation of both massifs in relation to the Tét normal fault activity.

## 1. Introduction

In an orogenic belt, fault zones appear as segments of metric to multi-kilometric scale of crustal weakening that concentrate lithospheric deformation during the different tectonic stages of building and dismantling of the mountain range (e.g. Allègre et al., 1984). Most of them have a multiphase activity in ductile and/or brittle conditions (Hartz et al., 2000), which renders challenging their analysis in terms of

kinematics, deformation and fluid circulation that may lead to various interpretations of their role during the orogenic cycle (Tartaglia et al., 2020). The situation becomes more complex when the studied belt results from a polyorogenic evolution that can end with a more or less complete erasement of the characteristics of the earliest deformations along the fault zones. Among the different tools that can help to decipher the stepwise activity of fault zones through time, geochronology has been widely used to provide quantitative constraints on the timing of

<footnote><sup>a</sup> GeoResources, CNRS, Université de Lorraine, LabCom, CREGU, 54506, Vandœuvre-les-Nancy, France</footnote>

<footnote><sup>b</sup> Géosciences Montpellier, Université de Montpellier, CNRS, Université des Antilles, Montpellier, France</footnote>

<footnote><sup>c</sup> Chrono-Environnement, UMR 6249-CNRS, Université de Franche-Comté, Besançon, France</footnote>

<footnote><sup>d</sup> Institut UTINAM – UMR 6213-CNRS, Université de Franche-Comté, Besançon, France</footnote>

<footnote><sup>e</sup> Université de Lorraine, CNRS, LIEC, Nancy, France</footnote>

<footnote>This article is part of a special issue entitled: Faulting Processes published in Journal of Structural Geology.</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: gaetan.milesi@univ-lorraine.fr (G. Milesi).</footnote>
pressure-temperature-deformation episodes (e.g. Tsukamoto et al., 2020). However, as many studies show, interpretation of isotopic data is not so straightforward, as different processes: thermal event, fluid circulation ... can obscure the geochronological message (e.g., Mulch and Cosca, 2004; Villa et al., 2014; Di Vincenzo et al., 2007; Oriolo et al., 2018; Bosse and Villa, 2019; Beaudoin et al., 2020; Roberts and Holdsworth, 2022; Monié et al., 2024). Combining various thermo-geo-chronological methods with various mineralogical indicators (e.g. Stipp et al., 2002; Lacroix et al., 2012) at a scale of a fault core allows better constraining the significance of the dates obtained and processes associated (e.g. Siebel et al., 2010; Duvall et al., 2011; Mottram et al., 2020; Hueck et al., 2020; Münch et al., 2021).

The Pyrenees is an E-W trending mountain belt resulting from the superposition of the Variscan and Alpine orogenic events and along which several fault zones separate crustal units that emerge mainly in the Axial Zone of the Central and Eastern Pyrenees (Fig. 1). Few geochronological studies have focused on these fault zones and uncertainties persist as to the relative importance of Variscan and Alpine deformations on the current structure of the Pyrenees. Some evidence of localized Cretaceous mylonitization has also been found in the Millas and Querigut massifs from the Axial Zone (Monié et al., 1994; Maurel, 2003). In the first use of ⁴⁰Ar/³⁹Ar dating in the Pyrenees, McCaig and Miller (1986) suggest that the Mérens fault, one of the main south-verging thrust faults extending from the Central to the Eastern Pyrenees, was active in the Eocene. Cochelin et al. (2018) challenged this hypothesis and claimed that Alpine deformations are restricted to the northern and southern borders of the Axial Zone. However, thermochronological data on both sides of the Mérens fault, in the Eastern Pyrenees, provide arguments for an Eocene exhumation of the northern hanging wall of this fault (Milesi et al., 2022). Further West, another thermochronological study in the Central Pyrenees also suggests that the Axial zone underwent two periods of shortening, a first one at 70-40 Ma that affected all the Axial Zone and a second one at 40-20 Ma only localized along a few shear zones (Waldner et al., 2021).

In the eastern Pyrenees, the Tét and Py NE-SW to NNE-SSW trending faults are major crustal faults with a long deformation history during the Pyrenean orogeny (Maurel et al., 2008; Calvet et al., 2014, 2021; Taillefer et al., 2021; Milesi et al., 2019, 2020, 2022), locally accommodating the Gulf of Lion opening. In addition, a second order NW-SE fault

network is very present in the Eastern Pyrenees (Taillefer et al., 2021, Fig. 2a). A detailed thermochronological study showed that the activity on the two main faults, i.e. the Tét and Py faults, and other ones of different orientations and lengths resulted in the complex exhumation of crustal blocks of various sizes delimited by crustal faults (Milesi et al., 2022, 2023). Currently, there is no geochronological data to relate the motion of these main faults to the exhumation/cooling path in their footwall and hanging wall. This is partly due to the fact that these faults have a multiple activity from slightly above the ductile-brittle transition to brittle conditions as witnessed by the presence of some thick fault gouges, which makes their dating difficult. In the present contribution, we report ⁴⁰Ar/³⁹Ar geochronological data coupled with chlorite thermometry data on the two main brittle fault zones from the Eastern Pyrenees Axial Zone and more specifically in the area of Canigou-Carança massifs where it has been proposed that they accommodated the exhumation of the eastern Pyrenees massifs. Our results provide a better understanding of the complex tectonic evolution of the Eastern Pyrenees and information about the significance of geochronological dates obtained on polyphased crustal scale fault zones.

## 2. Geological setting

### 2.1. Review of tectonic evolution of the Eastern Pyrenean Axial Zone

The Axial Zone of the Eastern Pyrenees consists of exhumed Paleozoic basement (Fig. 2a). In this area, the complex fault network records a polyphase tectonic history. Major deformation events have been highlighted in this part of the Pyrenees: i) Variscan deformation structures affected the various basement units highlighted by the presence of South dipping ductile shear zones (Cochelin et al., 2017, 2018), mainly oriented NW-SE to E-W (Gourinard, 1971) like the North Canigou Mylonite Thrust (NCMT) visible along the western segment of the Tét fault (Guitard et al., 1992, Fig. 2c) and Mérens shear zone (Denèle et al., 2009; Mezger et al., 2012). ii) Mesozoic Pyrenean rifting related to 10's km of sinistral displacement along the North Pyrenean fault (Roure et al., 1989). During this deformation event, an activation of preexisting structures in the Axial Zone is likely (Costa and Maluski, 1988; Monié et al., 1994; Maurel, 2003; Asti et al., 2019; Odlum and Stockli, 2020). iii) Eocene collision between the European and Iberic plates resulting in

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Geological map showing the main structural domains of the Pyrenees (modified after Taillefer et al., 2017). The main Neogene normal faults of the Eastern Pyrenees are reported in red. The main massifs of the Eastern Pyrenees: 1: Aston, 2: Hospitalet, 3: Querigut, 4: Millas, 5: Mont-Louis, 6: Carança, 7: Canigou, 8: Albères and 9: North-Catalan ranges. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** a) Structural sketch map of the eastern part of the Axial Zone showing the different regional massifs and basins (modified from Taillefer et al., 2021). Samples collected along the Têt and Py fault zones are indicated by a star. b) Thermal histories based on low-temperature thermochronology since 40 Ma for the Mont-Louis, Canigou and Carança massifs (modified after Milesi et al., 2022), fault and kinematic proposed for the different cooling events are indicated. c) Zoom on the western segment of the Têt fault (modified from Milesi et al., 2019); Stars: dated samples from the Carança valley.</figcaption></figure>

a doubly vergent orogeny (Choukroune, 1989; Roure et al., 1989, Fig. 1). The maximum of shortening occurred in the axial part of the Pyrenees (Vergés et al., 1995; Gibson et al., 2007; Sinclair, 2005; Metcalf et al., 2009; Whitchurch et al., 2011; Fillon and van der Beek, 2012). This period was characterized by the emplacement of south-verging nappes rooted in the northern part of the range (Vergés et al., 1995; Sibuet et al., 2004; Teixell et al., 2016). iv) Onset of the extensional tectonics has been proposed to occur along the NE-SW to E-W structures since Priabonian in the Eastern Pyrenees (Milesi et al., 2022) with successive episodes of normal faulting mainly associated to the Gulf of Lion opening (Fig. 2b and Section 2.3 for further details). The polyphase evolution of the Eastern Pyrenees lead to the occurrence of two main structural directions (Taillefer et al., 2021): a NW-SE fault network related to Mesozoic and Eocene tectonic phases and the NE-SW fault network mainly related to Cenozoic extensional phases.

## 2.2. The NW-SE fault network

The NW-SE fault network is obvious in the Eastern Pyrenees but is absent further west (Fig. 2a). The major faults are described as ductile

inherited regional faults (Guitard et al., 1992, 1998; Autran et al., 2005; Laumonier et al., 2015, 2017) with late-Hercynian dextral offset clearly visible in the Mont-Louis granite (Bouchez and Gleizes, 1995; Autran et al., 2005, Fig. 2a). Maurel et al. (2008) propose that these faults may have been reactivated during the Pyrenean compression, as for the similarly oriented Mérens fault (McCaig and Miller, 1986). Taillefer et al. (2021) proposed normal displacement along Pyrenean NW-SE faults associated to brittle deformation as observed further South in the North-Catalan Ranges (Fig. 1). In the eastern Pyrenees, Mio-Pliocene normal displacement along some NW-SE faults is also supported by thermochronological dataset of Milesi et al. (2022). Timing of this deformation between Burdigalian (19 Ma) to Tortonian (9 Ma) is not well constrained and partly associated to the opening of the Cerdagne basin (Cabrera et al., 1988). Minor reverse and strike-slip movements are recorded in Plio-Quaternary series along these structures (Calvet, 1999; Goula et al., 1999; Delcaillau et al., 2004; Lacan and Ortuño, 2012; Cantarero et al., 2014).
## 2.3. The Tèt and Py crustal faults (NE-SW)

Two main crustal normal faults are recognized in the studied area: the Tèt and the Py faults (Figs. 1 and 2a), both are 60–70° North dipping faults. The major one, the Tèt fault is a 100 km long crustal-scale fault (NE-SW) that separates the Canigou-Carança massif in its footwall from the Mont-Louis massif in its hangingwall, while the Py fault (NNE-SSW) constitutes the tectonic limit between the Carança and the Canigou massifs in the Tèt fault footwall.

The Tèt fault is parallel to the high-temperature Variscan foliation of the NCMT, particularly well expressed along the Western segment of the fault (Fig. 2c). During the early Eocene, thrusting of the Aston–Mont-Louis unit onto the Canigou massif is proposed according to balanced cross sections (Ternois et al., 2019). Based on regional thermochronological studies (Maurel et al., 2008; Milesi et al., 2022), the Tèt fault has been proposed to accommodate this Eocene contractional tectonic event. Thermal modeling based on the low-T thermochronological data also points to the occurrence of multiple cooling events associated to normal displacement along the Tèt fault (Maurel et al., 2008; Milesi et al., 2022) probably linked to the opening of Gulf of Lion (Séranne et al., 1995) and related crustal thinning (Chevrot et al., 2018; Milesi et al., 2024). The recovery of thermal histories in the different massifs based on low-T thermochronology (Fig. 2b) indicates that a major normal displacement along the Tèt fault occurred between upper Oligocene and lower Miocene (26–19 Ma), followed by a less prominent tectonic activity during the mid to late-Miocene times (15–9 Ma) along the western segment of the Tèt fault (Milesi et al., 2023). Minor extensional tectonic events (not recorded by (U-Th)/He thermochronology) along the Tèt fault are proposed during the Plio-Quaternary (Carozza and Baize, 2004; Lacan and Ortuño, 2012; Petit and Mouthereau, 2012; Clauzon et al., 2015).

Timing of activity of the Py fault is less documented (Calvet et al., 2021). Temperature-time modeling (Fig. 2b) shows an earlier onset of cooling related to the Py fault activity in the late Eocene and thus suggests an earlier normal faulting along the Py fault compared to the Tèt fault (Milesi et al., 2022, Fig. 2a). This earlier exhumation and coeval extensional tectonic phase do not appear to be related to the rifting phase leading to the opening of the Liguro-Provençal domain but rather to the West European Rifting (Ziegler, 1992; Romagny et al., 2020; Angrand and Mouthereau, 2021; Jolivet et al., 2021; Mouthereau et al., 2021; Séranne et al., 2021).

# 3. Methodology

## 3.1. Petrographic observations and mineralogical characterization

### 3.1.1. Scanning electron microscope

Petrographic observations were performed at the FEMTO-ST Institute (Université de Bourgogne Franche-Comté). Microstructures and mineral assemblages were analysed with a JEOL JSM5600 (SEM) equipped with Secondary Electron (SEI), backscatter electron (BSE) and Energy Dispersive X-ray Spectroscopy (EDX) detectors.

### 3.1.2. XRD diffraction

The bulk mineralogy was characterized at the Université de Bourgogne Franche-Comté by using a Bruker D8 Advance diffractometer system, Cu-Kα radiation equipped with a fast LynxEye linear detector. Samples were hand ground to <50 µm. X-ray diffractograms were acquired between 3 and 70°2θ at a step size of 0.02, a time/step of 1 s at 40 kV and 40 mA on disoriented powder. The <2 µm fractions were obtained by sedimentation and oriented specimen preparations were analysed in air dried conditions and after ethylene-glycol saturation between 5 and 30°2θ at a step size of 0.01 and time/step 0.5 s. No expandable or interstratified minerals were found. The <2 µm fractions were then submitted to centrifugation in order to extract the finer

fractions, this protocol, unlike the continuous flow ultracentrifuge (e.g. Gerardin et al., 2024), limits the amount of extracted material, especially the finer fractions (<0.2 µm). X-ray diffractograms were acquired between 20 and 30°2θ at a step size of 0.035, or 0.009 (when sufficient material was available), a time/step of 3 s at 40 kV and 40 mA on disoriented powder using a Bruker D8 Discover, Cu-Kα radiation equipped with a fast LynxEye linear detector at LIEC, Nancy laboratory. The raw XRD data are available in Appendix 1.

### 3.1.3. Mica polytype characterization

For mica polytypes determination, the X-ray diffractograms were used between 20 and 32°2θ on powder. Polytype quantifications were performed by using the Bruker-AXS's TOPAS© software and the Profex-BGMN Rietveld refinement software (Doebelin and Kleeberg, 2015).

## 3.2. Chlorite chemical composition

### 3.2.1. Microprobe analyses

Quantitative chemical analyses of chlorite were performed at SCMEM (GeoRessources laboratory - Université de Lorraine) with a CAMECA SXFive TACTIS EPMA equipped with five WDS spectrometers using an accelerating voltage of 15 kV, a probe current of 10 nA and a beam diameter of 1 µm. The peak and background counting times were 10 and 5s, respectively. The standards were natural minerals and synthetic oxides as follows: orthoclase (Si, K), wallastonite (Ca) albite (Al, Na), hematite (Fe), forsterite (Mg), MnTiO3 (Mn, Ti), Chromite (Cr). The analyses were performed at 15 keV accelerating voltage, 10 nA specimen current.

### 3.2.2. Iron oxidation state estimate by mössbauer spectroscopy

Fe(II) and Fe(III) proportion in the bulk rock were estimated at the LCPME Laboratory (Université de Lorraine). Mössbauer spectroscopy was performed at 298 K with a constant acceleration. Mössbauer spectrometer is equipped with 512 multichannel analyzer (Halder Electronic Gmbh) and a 50 mCi source of ⁵⁷Co in Rh matrix. Data were obtained from appropriate amounts (10 mg of Fe per cm²) of solid samples to get optimal experimental conditions. The 50 mCi source of ⁵⁷Co in Rh matrix was maintained at room temperature (RT) and mounted at end of a Mössbauer velocity transducer. The spectrometer was calibrated with a 25 µm foil of α-Fe at RT. Analysis of the Mössbauer spectra consisted of least-square fitting of data with a combination of two-peak quadrupole components (doublets) based of the Lorentzian model fitting method (Lagarec and Rancourt, 1997) was used to fit spectra. The center shifts were reported with respect to that of 25 µm-thick α-Fe foil at RT.

### 3.2.3. Iron oxidation state estimate by µ-XANES spectroscopy

To characterise the distribution and determine, in situ, the oxidation state of iron in chlorite crystals, X-ray fluorescence (XRF) and Fe K-edge XANES data were, respectively, collected at the BM23 beamline of the European Synchrotron Radiation Facility (ESRF; Grenoble, France) using the micro-focused experimental setup (Mathon et al., 2015). The µ-XANES spectra were collected and treated following the procedure described in Muñoz et al. (2006) for Fe-chlorite study and in Muñoz et al., 2013.

## 3.3. Chlorite thermometry

In the current study, chlorite crystallization temperatures have been estimated using the microprobe analyses coupled with the µ-XANES and Mössbauer spectroscopy Fe speciation analyses. This coupling allows us to consider the real amount of Fe³⁺, to better constrain the formation temperatures of chlorites for which redox can vary under hydrothermal conditions (Charpentier et al., 2024). Indeed, at low-temperature conditions, chlorite incorporates large amount of Fe³⁺ (up to 50 % of the total Fe, Vidal, 2005; Lanari et al., 2014a).
Since the analyses of newly formed chlorite should be expressed using the Al-free chlorite endmember with clinochlore, chamosite, Mg-sudoite and amesite endmembers (see results below), temperatures could not be estimated using classical thermodynamic models of Vidal (2001, 2005), Vidal et al. (2006) or Lanari et al. (2014b). We therefore used the semi-empirical thermometer developed Bourdelle and Cathelineau (2015). However, this thermometer is not effective for chlorite closed to the sudoite endmember, that is the case for some analysed chlorite (see results below).

### 3.4. $^{40}$Ar/$^{39}$Ar dating

After sample preparation, the different mineral fractions were encapsulated in high-vacuum evacuated quartz vials, except for large biotites from sample 18CAR07 that were wrapped in Al foils and irradiated for 20 h in the core of the Triga Mark II nuclear reactor of Pavia (Italy) with several aliquots of the Taylor Creek sanidine standard (28.61 ± 0.03 Ma, Renne et al., 2011) as flux monitor. After loading in a stainless-steel chamber equipped with a viewpoint, the samples were baked at about 120 °C for 24 h to remove atmospheric argon from the sample surface. $^{40}$Ar/$^{39}$Ar analyses were conducted in the Noble Gas laboratory of Géosciences at the University of Montpellier (France) using an analytical system composed of three assemblies maintained under ultra-high vacuum: 1) a gas extraction system made of two sample chambers and different laser devices: a) a 30W CO$_2$ laser for step-heating analysis in the first chamber is focused to a 1 mm diameter spot; b) an ultra-violet laser used to drill the quartz tubes containing the fine-grained samples in the second chamber and a 20W infra-red fiber laser for their step-wise degassing; 2) a stainless steel gas purification line including two getting, a cold trap, and an automated pipette to monitor mass discrimination and detector bias on air aliquots; 3) a multicollection mass spectrometer (Thermo Scientific Argus VI MS) with a nominal mass resolution of ~200 and a sensitivity for argon measurements of 3.55 × 10$^{-17}$ mol/fA at 200 µA trap current. The mass spectrometer houses a Nier-type source and a fixed array of 5 F detectors and one CDD detector used to measure $^{36}$Ar (Mark et al., 2009). For the dating of encapsulated samples, a hole is drilled in quartz vials in vacuo prior to the step-heating experiments to release the amount of $^{39}$Ar lost by recoil during the irradiation. Then the samples are progressively degassed with the fiber laser beam using a heating duration of 60 s at each step. The same duration is used for the unencapsulated samples degassed with the CO$_2$ laser beam. Custom-made software is used to control the laser intensity, the timing of extraction/purification and the data acquisition. ArArCalc software© v2.5.2 (Koppers, 2002) was used for data reduction and plotting, with the decay constants of Renne et al. (2011). All the data were corrected for system blanks made every three steps, radioactive decay, isotopic interferences and mass discrimination. Atmospheric $^{40}$Ar is estimated using a value of the initial $^{40}$Ar/$^{36}$Ar ratio of 298.6 (Lee et al., 2006). Argon isotopic interferences on K and Ca were determined from irradiation of KF and CaF$_2$ pure salts from which the following correction factors were determined: ($^{40}$Ar/$^{39}$Ar)$_K$ = 0.00945 ± 0.00075, ($^{38}$Ar/$^{39}$Ar)$_K$ = 0.0127 ± 0.00030, ($^{39}$Ar/$^{37}$Ar)$_Ca$ = 0.0006135 ± 0.000061 and ($^{36}$Ar/$^{37}$Ar)$_Ca$ = 0.000259 ± 0.000025.

For the Variscan gneiss, a micro-population of about twenty biotite grains was analysed following a classical procedure of stepwise heating. For the other samples, $^{40}$Ar/$^{39}$Ar dating was carried out on different grain size fractions (<0.1 µm to < 50 µm) using the encapsulation technique for an evaluation of fast neutron induced $^{39}$Ar recoil on the fine-grained phyllosilicates (e.g. Dong et al., 1995; Abd Elmola et al., 2018; Monié et al., 2024). For these samples a retention age (RA) discarding the $^{39}$Ar recoil fraction and a total gas age (TGA, equivalent to a K-Ar age) including this recoil fraction were calculated. Detailed characteristics of these fractions are given in the mineralogical section 4.2 and full $^{40}$Ar/$^{39}$Ar result tables are available in Appendix 2.

## 4. Results

### 4.1. Structural analysis

Along the Tét fault, within the Carança valley, we sampled the E-W Variscan mylonitic gneisses from the NCMT (18CAR07) as it may represent the main Variscan host rock and early deformation events with E-W foliation dipping to the North (Fig. 3a). We also sampled a thin, subvertical NW-SE mylonitic zone (18CAR06, Fig. 3b) oblique to the gneisses foliation and exhibiting some reactivation in the brittle regime, i.e. brittle tectoglyphs and cataclastic rocks. The ductile deformation is marked by stretching lineation indicative of a strike slip movement. The brittle deformation is characterized by slickenlines showing normal and dextral movement (Fig. 3b). Finally, we sampled a fault 1-m-thick, homogeneous and clay-rich gouge from the core zone of the main WSW-ENE trending Tét normal fault (T4a, Fig. 3c and d). The border of the sampled fault gouge shows subvertical slickensides without clear indication on the nature of displacement.

Along the Py fault, we have selected a fault exhibiting a large multicore zone within gneissic protolith with well-exposed fault clay-rich gouge levels. We sampled two fault gouges from the complex core zone (Fig. 3e), one along a fault plane in the middle of fault core (PyG) and the second one in a highly deformed leucocratic aplite (PyP). Striated fault planes inside the fault core indicate lateral and vertical displacement without sense criteria.

### 4.2. Petrography and mineralogy

The petrography and microstructure of studied samples have been observed by scanning electron microscopy (SEM) shown in Fig. 4. XRD results obtained on the bulk and <2 µm powders associated with observations from backscattered electron images, allow a qualitative characterization of the mineralogy (Table 1). Observations were also performed under SEM also to determine the different types of phyllosilicates, chlorite and/or mica, regarding their chemistry (EDX analysis) and their morphology, size and microstructural position in the studied samples (Fig. 5).

#### 4.2.1. Ductile to ductile/brittle deformation: 18CAR07 gneisses and 18CAR06 mylonite

The Variscan mylonitic gneisses from NCMT (18CAR07) present a foliation (Fig. 4a and b) with an important amount of quartz, muscovite and biotite, and traces of K-feldspar and rutile (Table 1). The mineralogy of the sample 18CAR07 is representative of the protolith in the vicinity of the Tét fault zone in the Carança massif.

Fault rocks from the small NW-SE mylonitic zone (sample 18CAR06) and associated cataclastic rocks show a restricted mineralogical assemblage made of quartz, muscovite and rutile compared to variscan gneisses (Table 1). The lack of K-feldspar and biotite is noticeable, and the lack of biotite gives their withish colour to outcropping fault rocks (Fig. 3b). Within sample 18CAR06, quartz, blocky muscovite and rutiles are arranged parallel to the planar deformation of the rocks (Fig. 4c). They are considered as syn-kynematic phases. However, evidence of cataclastic deformation can also be observed in this sample such as a matrix of disoriented fine-grained phyllosilicate platelets, i.e. muscovite (Fig. 4d). This confirms that this fault zone was first active in a ductile regime and was later reactivated at the ductile/brittle transition.

#### 4.2.2. Brittle deformation: Tét and Py fault gouge samples

Mineral assemblage of the main Tét fault zone (T4a) reveals a mineralogical composition of mainly quartz, and chlorite, and in less extent muscovite/illite and rutile (Table 1). Similarly to the NW-SE small fault rocks (18CAR06), biotite and K-feldspar are lacking within the Tét fault gouge. Fig. 4e and f shows that T4a gouge consists of clasts made of quartz, large patches of chlorite + muscovite/illite and rutile surrounded by a matrix of fine phyllosilicate platelets. Some clasts
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Photo of sampled fault zones: a) South verging Variscan mylonitic gneisses from the NCMT (stereograph shows the S1 foliation; 18CAR07) representing the protolith of the Tét fault. b) Sample 18CAR06, a WNW-ESE fine-grained mylonitic zone (white); note the presence of intercalated cataclastic zones (brown) that reworks mylonites. This brittle deformation is associated to strike slip and normal motion of the fault. c) Fault zone of the Tét fault at the entrance of the Carança valley; DZ: damaged zone and d) Sample T4a, a clay rich fault gouge along ENE-WSW fault plan and e) Py fault zone showing a complex core zone composed of cataclasites and gouges close to the village of Mantet (PyG and PyP samples); striated fault planes measured in the core zone show vertical and lateral displacement. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

(caption on next page)
**Fig. 4.** Backscattered electron images (BSE). Qz refers to quartz, Bt = biotite, Kfs = K-feldspar, Ms = muscovite/illite, and Rt = rutile (a,b) Global view of 18CAR07 mylonite (NCMT) showing assemblage of quartz, biotite, muscovite and K-feldspar in foliation (S1). (c) Global view of 18CAR06 fine grained fault zone showing a main assemblage of quartz, large muscovite (Ms1) with rutile locally and foliation (S1). (d) Matrix of disoriented fine-grained muscovite/illite platelets (Ms2) in 18CAR06 sample. (e, f) Tét fault gouge (T4a) showing clasts of quartz, surrounded by a patch of phyllosilicates, and fractures filled with mica and chlorite. Rutile and zircon are also visible. (g) Py fault gouge 1 (PyG) showing large clasts of quartz, K-feldspar and muscovite/illite associated with patches of phyllosilicates and chlorite. Rutile is also present. (h) Py fault gouge 2 (PyP) showing large clasts of quartz, K-feldspar, albite and muscovite. A fine assemblage of phyllosilicates, probably mica, filled fractures. Rutile is also present.

<table><caption>**Table 1**
Qualitative mineralogy based on petrography, SEM and XRD results.</caption><thead><tr><th></th><th>18CAR07<br>Gneisses</th><th>18CAR06<br>Mylonite</th><th>T4a Tét<br>fault<br>gouge</th><th>PyG Py<br>fault<br>gouge</th><th>PyP Py<br>fault<br>gouge</th></tr></thead><tbody><tr><td>Quartz (Qz)</td><td>XX</td><td>XX</td><td>XX</td><td>XX</td><td>XX</td></tr><tr><td>K-feldspar<br>(Kfs)</td><td>X</td><td></td><td></td><td>X</td><td>XX</td></tr><tr><td>Albite (Ab)</td><td></td><td></td><td></td><td>X</td><td>XX</td></tr><tr><td>Muscovite/<br>Illite (Ms)</td><td>XX</td><td>XX</td><td>X</td><td>XX</td><td>XX</td></tr><tr><td>Chlorite (Chl)</td><td></td><td></td><td>XX</td><td>X</td><td></td></tr><tr><td>Biotite (Bt)</td><td>XX</td><td></td><td></td><td></td><td></td></tr><tr><td>Rutile (Rt)</td><td>(X)</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Apatite (Ap)</td><td>(X)</td><td></td><td></td><td>(X)</td><td>(X)</td></tr></tbody></table>

(Fig. 5a) are crosscut by fractures filled with the same matrix of phyllosilicate platelets surrounding the clasts (Fig. 5b), these particles that can be very thin, few micrometres to less than a micrometre (Fig. 5c). The matrix is composed of a mixture of fine grained, elongated and oriented chlorite and illite particles (Fig. 5b).

Two samples from the Py fault zone were studied: samples PyG and PyP. PyG fault gouge revealed a mineralogical composition of mainly mica, quartz, K-feldspar and with smaller amounts of albite, chlorite and rutile and rare apatite (Fig. 4g and h, Table 1). PyP fault gouge revealed a very similar mineralogical composition to the PyG sample excepting that chlorite is absent (Table 1). In both cases, patches of thin phyllosilicate particles surround clasts made of muscovite/illite, quartz, K-feldspar, albite and rutile and infill the fractures within the clasts (Fig. 5d–g). It is noticeable that large chlorite is lacking within the clasts contrary to Tét fault gouge. The size of thin phyllosilicate particles is from few micrometres to less than a micrometre and these particles are mainly mica with few chlorite particles within PyG and solely mica within PyP.

### 4.2.3. Illite/muscovite polytypes

The different fractions of 18CAR06 mylonite show only the presence of 2M1 muscovite (Fig. 6a). For the Tét fault gouge (T4a) Fig. 6b shows the reflection peaks at 3.88 Å, 3.73 Å, 3.46 Å, 3.30 Å, 3.20 Å and 2.99 Å corresponding to the 2M1 muscovite polytype. The occurrence of 1M illite polytype is indicated by the reflection peaks at 4.12 Å, 3.60 Å and 3.07 Å in fraction <2 µm. The proportion of 2M1 and 1M polytypes were calculated by using Rietveld refinement method on the 20–30 °2θ interval (Table 2). We can note the presence of chlorite associated to the muscovite in all fractions, that complicates the illite/muscovite polytypes quantification. XRD results confirm the association of chlorite and muscovite particles with each other shown in the photographs (Figs. 4 and 5).

For PyG and PyP fault gouges, we observe a decrease in quartz and feldspar, in association with phyllosilicates (mainly 2M1 muscovite and minor chlorite in PyG) enrichment, towards the finer fractions. 1M illite is identified in the fractions <2 µm of PyG sample (Fig. 6c and d) limited to 10 % in the smallest fraction <0.1 µm regarding the poor quality of the reflection peaks the uncertainty on the polytypes quantification, we consider that only the 2M1 polytype is represented within all the grain-size fraction.

## 4.3. Chlorite mineralogy

### 4.3.1. Iron oxidation states in chlorites

Mössbauer analyses permit an estimate of the iron speciation in the bulk rock sample, whereas µ-Xanes spectroscopy permits an estimate of the oxidation state of iron in chlorite crystals. 4 paramagnetic components have been distinguished in both Py and Tét fault gouge (Fig. 7) by Mössbauer analyses. Regarding their center shift (CS) and distance (D), components 1 and 2 are attributed to $Fe^{2+}$ in the octahedral position of chlorite. Components 3 and 4 are attributed to $Fe^{3+}$ in the octahedral position of chlorite and muscovite respectively. Thanks to the component areas, proportion of muscovite and chlorite in the phyllosilicate fraction were calculated (Fig. 8). As observed by SEM and XRD, chlorite proportion is dominant in Tét fault gouge (up to 90 % of phyllosilicate phases) whereas it is smaller in the PyG fault gouge (less than 40 % of phyllosilicate phases). The average $XFe^{3+}$ ($XFe^{3+} = Fe^{3+}/(Fe^{2+}+Fe^{3+})$) in chlorite of the Tét fault gouge is of 0.25. Chlorite from the PyG fault gouge displays a very high $XFe^{3+}$ value (about 0.55).

Complementary analyses have been performed on 6 chlorites of the Tét fault gouge (T4a) by µ-XANES spectroscopy. Results confirmed that all iron cations of the measured chlorite are located in the octahedral sites. The proportions $XFe^{3+}$ in chlorite vary from 0.3 to 0.4 (Table 2), slightly higher than the average value obtained by Mössbauer spectroscopy. That can be explained by the fact that only chlorite from fractures have been analysed by µ-XANES spectroscopy whereas Mössbauer spectroscopy give an average the $XFe^{3+}$ of the chlorites identified within the T4a fault gouge, i.e. within fractures and in patches. $XFe^{3+}$ in chlorite from the fractures is 0.35 and it must be lower in chlorite around the patches.

### 4.3.2. Chlorite structural formulae

Microprobe analyses were performed on chlorite from both Py fault gouge (PyG) and Tét fault gouge (T4a). Chlorites from the Py gouge are very small and closely associated with mica, and therefore difficult to analyse. The two types of chlorites from Tét gouge were analysed. Mössbauer and µ-Xanes spectroscopy, and microprobe results were coupled to calculate the structural formulae of chlorite, considering an $XFe^{3+}$ of 0.55 for Py fault gouge chlorite, and 0.35 and 0.25 for the populations of chlorite from the fractures and the patches in the Tét fault gouge, respectively (see data in Appendix 3).

For Tét fault gouge, the chlorite composition in each analysed population is very homogeneous, enabling the calculation of average structural formulae on a 14 oxygens basis. [Si$_{2.82}$ Al$_{1.17}$ O$_{10}$] (Al$_{1.35}$ Fe$_{0.43}$ Fe$_{1.29}$ Mg$_{2.80}$ Mn$_{0.02}$ □$_{0.12}$) (OH)$_{8}$ and [Si$_{3.06}$ Al$_{0.94}$ O$_{10}$] (Al$_{1.22}$ Fe$_{0.76}$ Fe$_{1.424}$ Mg$_{2.36}$ Mn$_{0.05}$ □$_{0.19}$) (OH)$_{8}$ are average structural formulae for chlorite from the Tét fault gouge in patches and in the fractures respectively. [Si$_{2.94}$ Al$_{1.05}$ O$_{10}$] (Al$_{1.47}$ Fe$_{1.40}$ Fe$_{1.14}$ Mg$_{1.66}$ Mn$_{0.04}$ □$_{0.29}$) (OH)$_{8}$ is the average structural formulae for the few chlorites we managed to analyse in the Py fault gouge. The chemical compositions provide evidence differences between the chlorite from the two fault gouges.

On IVAl vs Fe$_{total}$ + Mg + Mn diagram (Fig. 8a), chlorite from both Py and Tét fault gouge lies between the clinochlore-daphnite and suдоite endmembers; however, chlorite from the Tét fault gouge has composition closer to clinochlore-daphnite endmember than chlorite from the Py fault gouge. Differences in chemical composition between the two populations from the Tét fault gouge can also been observed. Indeed, chlorite in the patches and fractures from the Tét fault gouge can be
<figure><img src="image_14.png" /><figcaption>**Fig. 5.** Backscattered electron images (BSE). Qz refers to quartz, Chl = chlorite, Ms = muscovite/illite, Rt = rutile, Ab = albite, Kfs = K-feldspar. (a) Têt fault gouge showing clasts made of quartz and rutile surrounded by chlorite. (b) Têt fault gouge fracture filled with newly formed chlorite and mica. (c) Têt fault gouge brittle fracture in quartz filled by few micrometres to less than a micrometre chlorite and mica (d) PyG fault gouge. Large mica and quartz inherited from the protolith surrounded by small detrital grains of quartz and thin platelets of mica from few micrometres to less than a micrometre. (e) PyG fault gouge. Fracture filled with mica, and in less extent chlorite, platelets of various size from few micrometres to less than a micrometre. (f) PyP fault gouge. Large, elongated mica associated with small stocky disoriented mica. (g) PyP fault gouge. Fractures filled with mica platelets of various size.</figcaption></figure>

distinguished regarding their tetrahedral content (Fig. 8a): chlorite from the patches have higher ⅣAl than those from the fractures, 1.2 and 0.9 apfu, respectively. On Fe vs Mg diagram (Fig. 8b) the distinction between all analysed chlorite is very clear, with higher Fe content (about 2.5) for chlorite from the PyG fault gouge than for those from the Têt fault (about 2 in chlorite from the fractures and about 1.5 in chlorite from the patches). This iron content variation is anti-correlated to Mg,

following the amosite-chamosite replacement.

### 4.4. Ar-Ar dating

#### 4.4.1. Ductile to ductile/brittle deformation: 18CAR07 gneisses and 18CAR06 fault zone

About 20 grains of biotite were extracted from the NCMT mylonitic
<figure><img src="image_15.png" /><figcaption>**Fig. 6.** X-ray diffraction patterns for randomly oriented preparations for different size fractions for samples 18CAR06 fault (a), T4a (b) from Têt fault zone and for samples PyG (c) and PyP (d) from the Py fault zone. Chl = chlorite, 1M = 1M illite, 2M1 = 2M1 muscovite, Ab = Albite, Kfs = K-feldspar, Rt=Rutile and Qz = Quartz. The raw XRD data is available in Appendix 1.</figcaption></figure>

**Table 2**

Analyses obtained by µ-XANES spectroscopy on chlorite particles of Têt fault gouge fractures.

<table><thead><tr><th></th><th>Analyse</th><th>Centroid energy (eV)</th><th>Integrated pre-edge intensity</th><th>XFe3+</th></tr></thead><tbody><tr><td>Chlorite 1</td><td>1</td><td>7113.73</td><td>0.1164</td><td>0.44</td></tr><tr><td rowspan="2">Chlorite 2</td><td>1</td><td>7113.61</td><td>0.1096</td><td>0.35</td></tr><tr><td>2</td><td>7113.53</td><td>0.0803<br><i>Mean value</i><br><i>St Dev</i></td><td>0.30<br>0.33<br>0.04</td></tr><tr><td>Chlorite 3</td><td>1</td><td>7113.58</td><td>0.0979</td><td>0.33</td></tr><tr><td rowspan="2">Chlorite 4</td><td>1</td><td>7113.55</td><td>0.1056</td><td>0.31</td></tr><tr><td>2</td><td>7113.56</td><td>0.089<br><i>Mean value</i><br><i>St Dev</i></td><td>0.32<br>0.32<br>0.01</td></tr><tr><td>Chlorite 5</td><td>1</td><td>7113.63</td><td>0.0905</td><td>0.37</td></tr><tr><td rowspan="3">Chlorite 6</td><td>1</td><td>7113.62</td><td>0.0828</td><td>0.36</td></tr><tr><td>2</td><td>7113.66</td><td>0.075</td><td>0.39</td></tr><tr><td>3</td><td>7113.55</td><td>0.0906<br><i>Mean value</i><br><i>St Dev</i></td><td>0.31<br>0.35<br>0.04</td></tr></tbody></table>

gneisses (sample 18CAR07) in the grain-size fraction 80–100 µm. Their progressive degassing provides a discordant age spectrum that increases steadily from ~30 Ma to 305.5 ± 0.7 Ma without leading to a plateau age (Appendix 4). For more than 80 % of the <sup>39</sup>Ar outgassing, the apparent ages are older than 285 Ma and the total gas age of this biotite sample is 288.7 ± 0.7 Ma. The <sup>37</sup>Ar<sub>Ca</sub> content of this sample is negligible while <sup>38</sup>Ar<sub>Cl</sub> is degassed in constant proportion with <sup>39</sup>Ar<sub>K</sub> corresponding to a Cl/K ratio of ~0.011 (deduced from the relation Cl/K = (<sup>38</sup>Ar<sub>Cl</sub>/<sup>39</sup>Ar<sub>K</sub>) x 0.559).

The <sup>40</sup>Ar/<sup>39</sup>Ar age spectra obtained on four encapsulated grain-size fractions of the ductile/brittle 18CAR06 quartz-muscovite rich fault zone are shown in Fig. 9. These age spectra are all corrected for the amount of <sup>39</sup>Ar lost by recoil during irradiation, which corresponds to the first step on the graphics with a zero-age due to the absence of a radiogenic <sup>40</sup>Ar component in the quartz capsule. It is observed that the amount of <sup>39</sup>Ar loss decreases with increasing grain size, from 15.55 % for the <0.2 µm fraction to 2.75 % for the larger 2–50 µm fraction as previously reported (cf. Dong et al., 1995; van der Pluijm et al., 2001). The four age spectra have comparable profiles with apparent ages increasing steadily during sample degassing. Only the coarsest fraction (2–50 µm) shows a slight decrease in apparent ages at the end of degassing. For these four fractions, a decrease of the Total Gas Age (TGA) with the grain size is observed, between 40.9 ± 0.1 Ma (fraction 2–50 µm) and 33.6 ± 0.2 Ma (fraction <0.2 µm, Table 3). Note that the maximum apparent ages are close for these four fractions, between 45 and 48 Ma. No chlorine contamination is revealed by these analyses
<figure><img src="image_16.png" /><figcaption>Fig. 7. Paramagnetic components obtained on bulk rock samples of Py and Têt fault gouge by Mössbauer spectroscopy with derived proportion of muscovite and chlorite in the phyllosilicate fraction, and XFe³⁺ and XFe²⁺ in chlorite and muscovite (nd: undetermined value).</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 8. Structural formulas values of newly formed chlorite from the Py fault gouge (PyG) in red and from the Têt fault gouge (T4a) in green (dark green: clasts; light green: matrix-fractures). The graphs show (a) [IV]Al vs. Fe + Mn + Mg and (b) Fe vs Mg. Note Fe is regarded as total iron, i.e. Fe²⁺ and Fe³⁺. apfu: atoms per formula unit, [IV]Al: aluminium in tetrahedral sheets. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

while a gentle increase in Ca/K values (deduced from the relation Ca/K=(³⁷ArCa/³⁹ArK) x 1.887) is concomitant with grain-size reduction.

### 4.4.2. Brittle fault zones Ar-Ar dating: Têt and Py fault gouge samples

For the Têt fault gouge (T4a), six grain-size fractions were separated, and their age spectra are reported on Fig. 10a. We can note that besides the coarsest fraction (2–50 µm), these age spectra have more discordant profiles than those exhibited by sample 18CAR06. They are characterized by a regular increase of apparent ages until 75–90 % of the total amount of ³⁹Ar released, followed an age decrease at the end of degassing. The maximum ages on the spectra are higher for the three finest fractions <0.3 µm (~60–65 Ma) than for the coarser ones (~55–60 Ma). The relative amount of ³⁹Ar lost by recoil effect increases

with decreasing size fraction and reaches up to 30 % in the finest particles <0.1 µm. TGA's increase with grain-size from 30.7 ± 0.8 Ma to 44.7 ± 0.4 Ma. Note that increase of ³⁹Ar lost by recoil effect and decrease of TGA with decreasing grain size is regular except for fractions 0.3–0.5 µm and 0.2–0.3 µm. Cl/K ratios remain close to zero for all dated fractions, while Ca/K values increase slightly toward the finest grain size fractions.

Six grain-size fractions were dated from the PyG fault gouge sample and the spectra are presented in Fig. 10b. The three coarse fractions (2–50, 0.5–2 and 0.2–0.5 µm) provide similar age profiles steadily increasing until about 35–40 % of the ³⁹Ar released, followed by a flat segment at ~60 Ma and an increase of apparent ages at the end of degassing, up to 130.5 Ma for the coarsest fraction. These three fractions
<figure><img src="image_18.png" /><figcaption>**Fig. 9.** Age spectra of four grain-size fractions from 18CAR06 NW-SE fault dated using encapsulation technique.</figcaption></figure>

provide decreasing TGA's (55.3–48.3 Ma) and increasing of recoiled ³⁹Ar (5.16–13.97 %) for decreasing size fractions. The three finest fractions (<0.2 µm) have similar age spectra corresponding to rather similar TGA's (46.9–45.8 Ma) and amount of recoiled ³⁹Ar (22.4–24.8 %). These spectra differ from those of the coarser fractions notably by the older values at ~70 Ma recorded from 50 to 90 % of the ³⁹Ar released. Cl/K ratios are close to zero for all dated fractions, while Ca/K values are larger in the three finest grain size fractions.

Four grain-size fractions were isolated from the PyP fault gouge sample (Fig. 10c). The four fractions provide similar age profiles increasing until the end of degassing ~80–100 Ma. These fractions provide TGA's decreasing from 60.1 ± 0.5 Ma and 54.5 ± 0.4 Ma and increasing recoil with decreasing size fraction, except for the finest fraction (<0.2 µm) providing a slightly older age of 56.9 Ma. We can observe that Cl/K ratios are close to zero for all dated fractions, while Ca/K ratio is slightly variable.

<caption>**Table 3**</caption>

Synthesis of Ar-Ar results with muscovite polytypes quantification for different grain size fractions.

<table><thead><tr><th rowspan="2">Sample</th><th>Grain size</th><th>Retention age</th><th>Total gas age</th><th>Maximum age</th><th>39Ar recoil</th><th>2M1</th><th>1M</th></tr><tr><th>(µm)</th><th>(Ma)</th><th>(Ma)</th><th>(Ma)</th><th>(%)</th><th>(wt%)</th><th>(wt%)</th></tr></thead><tbody><tr><td rowspan="4">18CAR06</td><td>2-50</td><td>41.8 ± 0.3</td><td>40.9 ± 0.3</td><td>45.8 ± 0.1</td><td>2.75</td><td>100</td><td>0</td></tr><tr><td>0.5-2</td><td>39.9 ± 0.3</td><td>37.7 ± 0.3</td><td>45.1 ± 0.1</td><td>6.69</td><td>100</td><td>0</td></tr><tr><td>0.2-0.5</td><td>39.1 ± 0.4</td><td>36.2 ± 0.4</td><td>45.6 ± 1.4</td><td>8.76</td><td>100</td><td>0</td></tr><tr><td><0.2</td><td>39.2 ± 0.3</td><td>33.6 ± 0.2</td><td>47.6 ± 1.7</td><td>15.55</td><td>100</td><td>0</td></tr><tr><td rowspan="6">T4a</td><td>2-50</td><td>48.7 ± 0.4</td><td>44.5 ± 0.4</td><td>54.9 ± 0.2</td><td>8.81</td><td>70</td><td>30</td></tr><tr><td>0.5-2</td><td>49.0 ± 0.4</td><td>44.7 ± 0.4</td><td>59.5 ± 0.5</td><td>8.98</td><td>68</td><td>32</td></tr><tr><td>0.3-0.5</td><td>47.7 ± 0.4</td><td>35.5 ± 0.3</td><td>60.1 ± 0.3</td><td>21.56</td><td>40</td><td>60</td></tr><tr><td>0.2-0.3</td><td>46.2 ± 0.4</td><td>37.9 ± 0.3</td><td>65.5 ± 0.2</td><td>18.06</td><td>42</td><td>58</td></tr><tr><td>0.1-0.2</td><td>43.1 ± 0.4</td><td>32.0 ± 0.3</td><td>60.0 ± 0.6</td><td>25.99</td><td>28</td><td>72</td></tr><tr><td><0.1</td><td>43.9 ± 1.1</td><td>30.7 ± 0.8</td><td>62.8 ± 2.2</td><td>30.40</td><td>29</td><td>71</td></tr><tr><td rowspan="6">PyG</td><td>2-50</td><td>58.2 ± 0.4</td><td>55.3 ± 0.4</td><td>130.5 ± 3.0</td><td>5.16</td><td>100</td><td>0</td></tr><tr><td>0.5-2</td><td>55.5 ± 0.4</td><td>49.2 ± 0.4</td><td>87.3 ± 1.6</td><td>11.46</td><td>95</td><td>5</td></tr><tr><td>0.2-0.5</td><td>56.0 ± 0.4</td><td>48.3 ± 0.4</td><td>73.9 ± 1.0</td><td>13.97</td><td>97</td><td>3</td></tr><tr><td><0.2</td><td>60.0 ± 0.5</td><td>46.1 ± 0.4</td><td>86.7 ± 2.7</td><td>23.50</td><td>92</td><td>8</td></tr><tr><td>0.1-0.2</td><td>60.3 ± 0.5</td><td>46.9 ± 0.4</td><td>85.3 ± 1.5</td><td>22.42</td><td>-</td><td>-</td></tr><tr><td><0.1</td><td>60.6 ± 0.5</td><td>45.8 ± 0.4</td><td>86.7 ± 2.3</td><td>24.80</td><td>90</td><td>10</td></tr><tr><td rowspan="4">PyP</td><td>2-50</td><td>61.4 ± 0.5</td><td>60.1 ± 0.5</td><td>99.4 ± 0.3</td><td>2.16</td><td>100</td><td>0</td></tr><tr><td>0.5-2</td><td>62.2 ± 0.5</td><td>57.8 ± 0.4</td><td>170.7 ± 0.6</td><td>7.06</td><td>100</td><td>0</td></tr><tr><td>0.2-0.5</td><td>62.1 ± 0.5</td><td>54.5 ± 0.4</td><td>118.1 ± 0.6</td><td>12.32</td><td>100</td><td>0</td></tr><tr><td><0.2</td><td>66.6 ± 0.5</td><td>56.9 ± 0.4</td><td>112.7 ± 1.9</td><td>14.75</td><td>100</td><td>0</td></tr></tbody></table>

# 5. Discussion

## 5.1. Interpretation of 1M illite/2M1 muscovite dates

Two main strategies are used to interpret grain-size fraction dependent illite/muscovite polytypes dates from fault rocks: 1) interpreting the finest and coarsest grain-size fractions as limit ages, the finest fraction date corresponding to the best estimate of the maximum/oldest age of illite/muscovite authigenesis during a deformational event and the coarsest fraction date to the estimate of the minimum/youngest age of the detrital/inherited input from host rocks; and 2) extrapolating end-member ages or “Illite Age Analysis” (IAA, Pevear, 1999; van der Pluijm et al., 2001) that allows estimating the age of authigenic illite by plotting illite dates as a function of the proportion of the inherited/authigenic, or 2M1/1M polytype, illite in the different size fractions and fitting a regression line to the data in order to extrapolate calculated end-member ages of inherited (2M1 muscovite) and authigenic (1M illite). Both strategies have their advantages and limitations and should be used complementarily, and we refer to Hueck et al. (2022) for a complete description and analysis of the two strategies.

### 5.1.1. NW-SE fault zone

The 18CAR06 sample exhibits two different populations of muscovite that evidence the transition from ductile to ductile/brittle deformation, allowing us to interpret the dates obtained on the different granulometric fractions in two ways.

First, the coarsest fraction (2–50 µm), composed of 100 % 2M1 polytype and corresponding to syn-kinematic largest muscovite aligned parallel to the mylonitic foliation, yields a TGA of 40.9 ± 0.3 Ma. It must be pointed out that the mica (biotite) from the host rock or protolith (sample 18CAR07) of the gouge yields an age of 305.5 ± 0.7 Ma (Appendix 4), that is much older than the retained age for ductile deformation along the NW-SE fault. Albarede et al. (1978) also dated muscovite and biotite from the Carança massif at around 214 Ma and 60–70 Ma, respectively. These authors interpreted these dates as partially reset dates of Variscan gneisses during Pyrenean orogeny, but it is noticeable that inherited muscovite yield much older age than that of the studied mylonite. The shape of the spectrum for the mylonitic muscovite (18CAR06) does not exhibit old individual apparent ages for the high temperature degassing step. Thus, the age of 40.9 ± 0.3 Ma of syn-kinematic 2M1 muscovite is interpreted as the one of pyrenean
<figure><img src="image_19.png" /><figcaption>**Fig. 10.** Age spectra for different grain size fractions dated using encapsulation technique from the Têt fault gouge sample T4a (a) and the Py fault zone: PyG (b) and PyP (c) fault gouges.</figcaption></figure>

ductile strike-slip deformation along this fault.

The finer fractions, also composed of 100 % 2M1 muscovite polytype but corresponding to small, disoriented muscovite within fracture zones through the mylonitic foliation, yields grain-size fraction dependent muscovite dates with TGA decreasing from 37.7 ± 0.3 Ma to 33.6 ± 0.2 Ma (Table 3). As all the fractions are composed of 100 % 2M1 muscovite polytype we cannot apply the IAA and thus we consider that the limit/ maximum age of 33.6 ± 0.2 Ma as the best estimate for the authigenesis of 2M1 polytype during late brittle/ductile reactivation of this second- ary fault.

5.1.2. Py fault zone

All the studied granulometric fractions from the two Py fault gouges have a mineralogy largely dominated by 2M1 illite/muscovite polytypes and following the rough precision of polytype quantification we consider that all fractions contain only 2M1 polytypes. The IAA cannot be applied such as in the 18CAR06 sample. However, the mineralogy of the two Py fault gouges revealed the occurrence of significative amount of K-feldspar even in the finest fraction and peculiarly in sample PyP (Table 2). This is easily detectable on the degassing spectra exhibiting increasing apparent ages, up to 100–150 Ma, for highest temperatures heating steps (Fig. 10b and c). To propose the best estimate for muscovite ages within these fractions, we developed a method based on the Gaussian deconvolution of the curve representing the ³⁹Ar released as a function of the laser power during step heating experiment. We used laser power because temperature cannot be directly measured. To test and validate this method we prepared, and analysed as natural gouge samples, mixtures of <2 µm powders with various proportions of pure illite and K-feldspar. The illite sample corresponds to pure diagenetic illite (Lanson et al., 1996) and the feldspar comes from a variscan granite. We calculated modeled ages for illite and feldspar that are concordant with the measured ages on pure feldspar and illite samples, except for one mixture, this allowing us to apply on gouge samples. The full description of our test on artificial mixture is given in Appendix 5. From each experiment, we extract the curve of normalized released ³⁹Ar against step heating, or laser power, increments. Then we perform a Gaussian peak deconvolution using Fityk software (Wojdyr, 2010). We obtain two different degassing curves, one corresponding to the ³⁹Ar released by a phase degassing only at low laser power, or low-temperature, and another to the ³⁹Ar released by a second phase degassing at low and high laser power, or temperature, up to the fusion. This first curve most probably corresponding to the argon degassing of a low retentivity phase (2M1 muscovite) and the second one to a phase with both low and high retentivities (feldspar). To calculate a modeled TGA for 2M1 muscovite and K-feldspar in mixtures, we first assign recoil ³⁹Ar following the results obtained on pure natural illite and feldspar samples during the test, then we assign all measured isotopes for mix- tures to modeled illite and K-feldspar considering the peaks deconvo- lution. Doing this we approximate that calcium (³⁷Ar) and chlorine (³⁸Ar) are equally distributed between the two phases, which is not necessarily the case. However, the low measured signals on these two isotopes, due to the low calcium and chlorine content of analysed illite and muscovite, support this approximation. Doing this we also approximate the quantity of ³⁹Ar recoiled in the finest fractions because we were not able to prepare K-feldspar test samples for each gran- ulometry. However, the proportion of K-feldspar greatly diminishes in these fractions in all the samples and we consider the calculated TGA for 2M1 muscovite only as a maximum age (Table 4).

The limit/maximum age, obtained for fine fractions, that can be proposed for PyG and PyP samples are 39.8 and 41.6 Ma, respectively. The modeled TGAs of feldspar fluctuate between 51.6 and 73.3 Ma and between 61 and 68.6 Ma for PyG and PyP, respectively. These ages are compatible with the total fusion ages obtained on K-feldspar from the Canigou Massif at 60.4 ± 1.7 Ma (Maurel et al., 2008) and 63 Ma (Albarede et al., 1978). Limit/maximum ages for 2M1 polytype are closed for both dated gouges suggesting that the limit/maximum age of
<caption>**Table 4**</caption>

Summary of modeled ages calculated for pure illite/muscovite and feldspar in each grain-size fraction of PyG and PyP samples. Samples with * fit with three Gaussian instead of two.

<table><thead><tr><th rowspan="2">Sample</th><th rowspan="2">Grain size (µm)</th><th colspan="2">Gouge</th><th colspan="2">Feldspar</th><th colspan="2">2M1 polytype</th></tr><tr><th>TGA (Ma)</th><th>Error (2σ)</th><th>TGA (Ma)</th><th>Error (2σ)</th><th>TGA (Ma)</th><th>Error (2σ)</th></tr></thead><tbody><tr><td rowspan="4">PyP</td><td>2-50*</td><td>60.1</td><td>0.5</td><td>68.6</td><td>0.5</td><td>44.3</td><td>0.4</td></tr><tr><td>2-0.5</td><td>57.8</td><td>0.4</td><td>65.8</td><td>0.5</td><td>43.9</td><td>0.3</td></tr><tr><td>0.5-0.2</td><td>54.5</td><td>0.4</td><td>60.9</td><td>0.5</td><td>41.6</td><td>0.3</td></tr><tr><td>&lt;0.2</td><td>56.9</td><td>0.4</td><td>68.2</td><td>0.5</td><td>43.7</td><td>0.3</td></tr><tr><td rowspan="5">PyG</td><td>2-50</td><td>55.3</td><td>0.4</td><td>62.9</td><td>0.5</td><td>49.0</td><td>0.4</td></tr><tr><td>2-0.5</td><td>49.2</td><td>0.4</td><td>63.4</td><td>0.6</td><td>45.8</td><td>0.4</td></tr><tr><td>0.5-0.2*</td><td>48.3</td><td>0.4</td><td>51.6</td><td>0.5</td><td>47.0</td><td>0.4</td></tr><tr><td>&lt;0.2</td><td>46.1</td><td>0.4</td><td>73.3</td><td>0.9</td><td>43.1</td><td>0.3</td></tr><tr><td>0.1-0.2</td><td>46.9</td><td>0.4</td><td>69.0</td><td>3.6</td><td>45.8</td><td>0.4</td></tr><tr><td></td><td>&lt;0.1</td><td>45.8</td><td>0.4</td><td>52.2</td><td>0.5</td><td>39.8</td><td>0.4</td></tr></tbody></table>

the deformation on the Py fault is ~40 Ma. Even if this estimation must be considered cautiously it is noticeable that this age is significantly older than the one for 2M1 illite polytype from the secondary fault associated to the Têt fault zone (33.6 Ma).

### 5.1.3. Têt fault zone

The T4a gouge from the main Têt fault zone is the only one exhibiting a mineralogical composition with both 2M1 and 1M polytypes allowing the calculation of an illite date with the IAA method. Fig. 11 presents the regression fitting the data from T4a sample. This regression points to an age of 22.1 ± 1.4 Ma for the authigenic endmember and to 55 Ma for the inherited endmember. It is noticeable that this regression has an R² of 0.97 suggesting that the IAA is a robust approach in this case. The age of inherited endmember highlights that this inherited endmember cannot be considered as a homogeneous reservoir. This age may correspond to the one of partially reset Variscan muscovite or to the one of mixture of more or less reset Variscan muscovite with muscovite developed during early ductile pyrenean deformation; therefore, we consider it meaningless. We consider the age calculated for the 1M authigenesis with the

IAA method as the best estimate during the main brittle activity of the Têt fault zone.

### 5.2. Temperature of deformation and regional cooling history

The temperature conditions of chlorite formation in Têt fault gouge (T4a) can be estimated using structural formulae calculated from iron oxidation state measured with Mössbauer and µ-XANES (Section 4.2.1) and chemical analysis (Section 4.2.2). They have been estimated using thermometers working with the Sudoite – Amesite – Clinochlore/Daphnite – Al-free Chlorite endmembers (Bourdelle and Cathelineau, 2015). Regarding the Têt fault gouge (T4a), the two distinguished populations yield different temperatures (Fig. 12). Chlorites localized in clasts (Fig. 5a) display temperature of formation between 150 and 250 °C, whereas chlorites localized in the matrix and fractures (Fig. 5b) display temperature of formation between 100 and 150 °C. SEM observation reveals that coarser muscovite are observed within clasts whereas smaller elongated and oriented muscovite/illite are observed within the matrix surrounding the clasts (Fig. 5a and b), likely associated to mainly 2M1 muscovite and 1M illite polytyptes, respectively. Chlorite temperatures obtained with respect to the structural location fit with the range of temperature proposed for the formation of 2M1 and 1M polytypes (Hueck et al., 2022; Monié et al., 2024). Thus, 1M illite authigenesis within the gouge of the Têt fault zone is clearly associated with the brittle deformation dated at 22.1 ± 1.4 Ma and formed at temperatures of 100–150 °C estimated from associated chlorite (Fig. 11). That is consistent with the main cooling event record by low-temperature thermochronology in the Carança and Canigou massifs interpreted to be related to the main normal motion episode on the Têt fault (Milesi et al., 2022, 2023). Therefore, the timing and temperature of brittle deformation along the Têt normal fault zone suggest temperature at equilibrium between fault zone and host rock (Fig. 13).

In the Py fault zone, no chlorite formation temperature has been possible to determine from PyG fault gouge due to the chemical composition close to the sudoite endmember (Fig. 8), out of the range of the Bourdelle and Cathelineau (2015) thermometer validity. Only 2M1 polytypes is represented in the two gouges of the Py fault zone

<figure><img src="image_20.png" /><figcaption>**Fig. 11.** Illite age analysis plot or plot of % 2M1 (detrital illite) vs. ⁴⁰Ar/³⁹Ar illite Total Gas Ages (TGA) of the different clay-size fractions in samples T4a from the Têt fault zone (blue shape 1σ error, dot line 2σ error). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Fig. 12.** Structural formulas values of newly formed chlorite from the Têt fault gouge (T4a) in green (dark green: patches; light green: fractures) in the Si vs. R²⁺. The isothermic lines determined by Bourdelle and Cathelineau (2015) are surimposed. Temperatures could not be estimated by the model for Py fault gouge (PyG) chlorites. Note that R²⁺ refers to divalent cations (Fe²⁺, Mn²⁺ and Mg²⁺), and R³⁺ refers to trivalent cations (Al³⁺, Fe³⁺). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

suggesting temperature of formation above 250-200 °C. Ar-Ar dating show an estimation for the limit/maximum age of the deformation on the Py fault at around 40 Ma (Section 5.1.2). This episode of deformation appears to be earlier and at slightly higher temperature than that on the Têt fault along the Carança massif. This is consistent with low-temperature thermochronology results suggesting that Canigou massif was exhumed earlier than the Carança massif in relation with normal motion on the Py fault, the Canigou massif and the Carança massif being in the hanging wall and the footwall of the Py fault, respectively.

The lack of 1M polytype in the gouges from the Py fault could suggest that this fault was not, or only incipiently, reactivated during later stages of deformation. Did it play a role during the late stage of exhumation of Canigou massif as suggested by observed vertical slickensides within its fault core (Fig. 3e)? This lack of mineralogical evidence for late brittle deformation can be explained by: (i) focus of the brittle deformation associated to the late exhumation of the Canigou massif on a non-visible core zone along the Py fault is unlikely, but still possible, because of the occurrence of slickensides in the studied core zone; (ii) major redox contrast between the Têt and Py faults (Fig. 7) with very high XFe3+ ~0.55 for chlorites in the Py fault compared to the Têt fault (0.2), this difference can impact the generation of 1M illite/muscovite; (iii) concentration of the deformation on the Têt fault zone which may have accommodated alone the exhumation of both Canigou and Carança massifs while they cooled below 150 °C, the Py fault being inactive at such low temperature as suggested by the occurrence almost exclusively of higher temperature synkinematic 2M1 polytypes. We favor this last hypothesis and traces of 1M polytype and some vertical slickensides suggest a tectonic activity coming to an end.

Within the NW-SE satellite fault of the Têt, the lack of biotite compared to the NCMT (18CAR07) and the presence of rutile (Fig. 4c and Table 1) that may have been formed by the breakdown of biotite is indicative of temperature between 250 and 350 °C (Cabral et al., 2015; Schirra and Laurent, 2021). Such temperatures are consistent with the dynamic crystallization of large (tens of microns) muscovite (2M1

polytype). During ductile/brittle reactivation of this fault, at around 33.6 Ma, rutile is absent and small (<2 µm) 2M1 muscovite may have formed by both comminution of mylonitic muscovite and authigenesis during late reactivation of NW-SE fault at 200–250 °C. However, this deformation temperature during late activity along the NW-SE fault seems higher than regional temperature based on low-temperature thermochronology for the Carança massif (Fig. 13) suggesting a peak of temperature in the fault zone at the ductile/brittle transition (e.g. Janots et al., 2019) potentially driven by fluid circulation in fault zone (e.g. Guillou-Frottier et al., 2024). Later activity on the set of NW-SE faults, suggested by low-temperature thermochronology, has not been recorded within the studied fault zone.

### 5.3. Eastern Pyrenees tectonic implications

At the scale of the eastern Pyrenees fault system, our results show a transition from ductile to brittle deformation between 40.9 and 22.1 Ma, marked by the transition between 2M1 and 1M muscovite/illite polytype within fault gouges and the transition between strike-slip and extensional deformation.

Ar-Ar dating allows to record early stages of deformation (40.9 Ma) under ductile condition (>300 °C) on NW-SE faults for which previous low-temperature thermochronology data do not provide constraints (Maurel et al., 2008; Milesi et al., 2022). These early deformation stages are associated mainly with strike-slip, and with some vertical displacements, under ductile conditions evolving towards ductile/brittle ones while cooling of the massifs was going on.

As already proposed with low-temperature thermochronology (Milesi et al., 2022), the activation of the Py fault in ductile/brittle condition likely between ~40 and 32 Ma might be associated to the beginning of the exhumation of Canigou massif in relation with extensional deformation. Ductile/brittle deformation, associated with vertical and horizontal slickensides, along the NW-SE structures is proposed with a maximum/limit age at 33.6 ± 0.2 Ma that is synchronous with the Canigou exhumation. During the Priabonian, the deformation within eastern Pyrenees may be related to the West European Rifting as is now largely accepted (Ziegler, 1992; Romagny et al., 2020; Angrand and Mouthereau, 2021; Jolivet et al., 2021; Mouthereau et al., 2021; Séranne et al., 2021). However, the occurrence of strike slip movement in ductile condition along the NW-SE fault and even in ductile/brittle condition along the Py and NW-SE faults may also be interpreted in terms of late Pyrenean compression.

1M illite ages obtained on Têt fault and chlorite temperature associated fit with the main cooling event record by low-temperature thermochronology in this part of the range. Thus, the major normal activity of the Têt fault during the early Miocene can be considered responsible of the main Canigou-Carança massif exhumation. Late Miocene cooling phase record along the western segment of the Têt fault as well as Pliocene quaternary strike-slip movements (Carozza and Baize, 2004; Lacan and Ortuño, 2012; Petit and Mouthereau, 2012; Clauzon et al., 2015) have not been evidenced in the explored fault cores, maybe in relation with the lack of crystallization of datable phases. This can be due to the range of temperature reached since late Miocene, below 60 °C according to the regional thermal history (Milesi et al., 2022). Without any thermal input through hydrothermal circulations, such low temperature precludes the crystallization of illite within fault rocks.

## 6. Conclusion

Coupling $^{40}$Ar/$^{39}$Ar dating with chlorite thermometry on the fault zones from Eastern Pyrenees greatly improves the timing and temperature of deformation. Our results reveal major ductile/brittle transition of the deformation between 40 and 33.6 Ma (Priabonian-Bartonian) recorded by the crystallization of 2M1 muscovite both NW-SE and NE-SW (Py) fault zones. Brittle deformation is dated during Oligo-Miocene (22.1 ± 1.4 Ma) and recorded by the authigenesis of 1M illite
<figure><img src="image_22.png" /><figcaption>**Fig. 13.** Thermal histories based on low-temperature thermochronology since 45 Ma for the Mont-Louis, Canigou and Carança massifs (modified after Milesi et al., 2022). Age of deformation based on Ar-Ar dating and range of temperature proposed for the different fault zones are reported with kinematic associated strike slip (SS) and normal displacement (N), respectively.</figcaption></figure>

only on the Têt fault zone.

Py fault zone is composed of 2M1 muscovites formed at temperature above 250-200 °C, due to the presence of k-feldspar in fault zone the age of deformation ~40 Ma can only be regarded as a limit or maximum age. The tectonic event record on the Py fault can explain early exhumation of the Canigou massif observed with low-temperature thermochronology and limit the initiation of faulting along the Py fault. The lack of 1M illite can be explain by no major tectonic activity on the Py fault post 40-32 Ma.

In the Têt fault zone, 1M illite marked the brittle activation at temperature between 100 and 150 °C measured by chlorite thermometry at 22.1 ± 1.4 Ma. These results are consistent with major normal activity of the Têt fault during the Oligo-Miocene associated to the exhumation of Canigou-Carança massif. Results obtained fit with thermal histories proposed for the Carança massif suggesting that brittle deformation is not associated to a major increase of temperature in the Têt fault zone. Dates obtained on NW-SE secondary faults allow to time the activity of these fault at ~33.6 ± 0.2 Ma that suggest tectonic activity also in the crustal blocks between the main crustal scale faults.

Our results highlight the importance to combine different type of geological information to interpret $^{40}$Ar/$^{39}$Ar dates such as structural and micro-structural data, detailed mineralogy to provide thermal constrains and low-temperature thermochronology to link fault activity with massif exhumation.

CRediT authorship contribution statement

**Gaëtan Milesi:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
**Philippe Münch:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
**Delphine Charpentier:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
**Arthur Iemmolo:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Methodology, Investigation, Formal analysis.
**Aoutmane Bouaoudi:** Visualization, Resources, Methodology, Investigation.
**Andreï Lecomte:** Resources, Methodology, Investigation.
**Virginie Moutarlier:** Resources, Methodology, Investigation.
**Renaud Gley:** Resources, Methodology, Investigation.
**Michael Bonno:** Resources, Methodology, Investigation.
**Patrick Monié:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, 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. The author is a Guest Editor for this journal and was not involved in the editorial review or the decision to publish this article.

Acknowledgements

This research received funding from the program Tellus of the Institut National des Sciences de l'Univers, CNRS and OSU THETA of the Bourgogne Franche-Comté University. G.M. acknowledges the support of the French Agence Nationale de la Recherche (ANR), under grant ANR-21-CHIN-0006 (projectGeomin3D). This work was partly done with resources from the Pôle de compétences en physico-chimie de l'environnement, ANATELO, LIEC laboratory, UMR 7360 CNRS – Université de Lorraine. Micro-XANES measurements have been supported European Synchrotron Radiation Facility program ES548. The authors are very grateful to P. Patrier and D. Beaufort for providing the artificial illite and feldspar mixtures. We particularly thank O. Mathon and M. Munoz for their crucial help during data acquisition on the BM23 beamline of the ESRF. We thank the editor E.C. Ferré, S. Haines and an anonymous reviewer for their valuable and constructive comments, which helped improve the quality of this manuscript.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsg.2025.105465.

Data availability

All data included in this article are available in article or in Appendix.

References

Abd Elmola, A., Buatier, M., Monié, P., Labaume, P., Trap, P., Charpentier, D., 2018. 40Ar/39Ar muscovite dating of thrust activity: a case study from the Axial Zone of the Pyrenees. Tectonophysics 745, 412–429. https://doi.org/10.1016/j.tecto.2018.09.004.

Albarede, F., Feraud, G., Kaneoka, I., Allegre, C.J., 1978. 39 Ar-40 Ar dating: the importance of K-feldspar on multi-mineral data of polyorogenic areas. J. Geol. 86, 581–598. https://doi.org/10.1086/649726.

Allégre, C.J., Courtillot, V., Tapponnier, P., Hirn, A., Mattauer, M., Coulon, C., Jaeger, J., Achache, J., Schärer, U., Marcoux, J., Burg, J.P., Girardeau, J., Armijo, R., Gariépy, C., Göpel, C., Tindong, L., Xuchang, X., Chenfa, C., Guangqin, L., Baoyu, L., Jiwen, T., Naiwen, W., Guoming, C., Tonglin, H., Xibin, W., Wanming, D., Huaibin, S., Yougong, C., Ji, Z., Hongrong, Q., Peisheng, B., Songchan, W., Bixiang, W., Yaoxiu, Z., Xu, R., 1984. Structure and evolution of the Himalaya–Tibet orogenic belt. Nature 307, 17–22. https://doi.org/10.1038/307017a0.

Angrand, P., Mouthereau, F., 2021. Evolution of the Alpine orogenic belts in the Western Mediterranean region as resolved by the kinematics of the Europe-Africa diffuse plate boundary. In: Lacombe, O., Tavani, S., Teixell, A., Pedreira, D., Calassou, S. (Eds.), BSGF Earth Sci. Bulletin 192, 42. https://doi.org/10.1051/bsgf/2021031.

Asti, R., Lagabrielle, Y., Fourcade, S., Corre, B., Monié, P., 2019. How do continents deform during mantle exhumation? Insights from the northern iberia inverted Paleopassive Margin, western Pyrenees (France). Tectonics 38, 1666–1693. https://doi.org/10.1029/2018TC005428.

Autran, A., Calvet, M., Delmas, M., 2005. Carte géologique de la France à 1/50,000, feuille Mont-Louis. BRGM.

Beaudoin, A., Scaillet, S., Mora, N., Jolivet, L., Augier, R., 2020. In situ and step-
heating40 Ar/39 Ar dating of white mica in low-temperature shear zones (Tenda
massif, Alpine Corsica, France). Tectonics 39, e2020TC006246. https://doi.org/
10.1029/2020TC006246.

Bosse, V., Villa, I.M., 2019. Petrochronology and hygrochronology of tectono-
metamorphic events. Gondwana Res. 71, 76–90. https://doi.org/10.1016/j.
gr.2018.12.014.

Bouchez, J.L., Gleizes, G., 1995. Two-stage deformation of the Mont-Louis-Andorra granite pluton (Variscan Pyrenees) inferred from magnetic susceptibility anisotropy. J. Geol. Soc. 152, 669–679. https://doi.org/10.1144/gsjgs.152.4.0669.

Bourdelle, F., Cathelineau, M., 2015. Low-temperature chlorite geothermometry: a graphical representation based on a T-R2+–Si diagram. Eur. J. Mineral 27, 617–626. https://doi.org/10.1127/ejm/2015/0027-2467.

Cabral, A.R., Rios, F.J., De Oliveira, L.A.R., De Abreu, F.R., Lehmann, B., Zack, T., Laufek, F., 2015. Fluid-inclusion microthermometry and the Zr-in-rutile thermometer for hydrothermal rutile. Int. J. Earth Sci. 104, 513–519. https://doi.org/10.1007/s00531-014-1120-8.

Cabrera, L., Roca, E., Santanach, P., 1988. Basin formation at the end of a strike-slip fault: the Cerdanya Basin (eastern Pyrenees). J. Geol. Soc. 145, 261–268. https://doi.org/10.1144/gsjgs.145.2.0261.

Calvet, M., 1999. Régimes des contraintes et volumes de relief dans l'est des Pyrénées/
Stress regimes and volumes of reliefs in the Eastern Pyrenees. Géomorphol. Relief,
Process. Environ. 5, 253–278. https://doi.org/10.3406/morfo.1999.991.

Calvet, M., Gunnell, Y., Delmas, M., 2014. The Tèt river valley: a condensed record of long-term landscape evolution in the Pyrenees. In: Fort, M., André, M.-F. (Eds.), Landscapes and Landforms of France, World Geomorphological Landscapes. Springer, Netherlands, Dordrecht, pp. 127–138. https://doi.org/10.1007/978-94-007-7022-5-13.

Calvet, M., Gunnell, Y., Laumonier, B., 2021. Denudation history and palaeogeography of the Pyrenees and their peripheral basins: an 84-million-year geomorphological perspective. Earth Sci. Rev. 215, 103436. https://doi.org/10.1016/j.earscirev.2020.103436.

Cantararero, I., Lanari, P., Vidal, O., Alías, G., Travé, A., Baqués, V., 2014. Long-term fluid circulation in extensional faults in the central Catalan Coastal Ranges: P–T constraints from neoformed chlorite and K-white mica. Int. J. Tect Sci. 103, 165–188. https://doi.org/10.1007/s00531-013-0963-8.

Carozza, J.-M., Baize, S., 2004. L'escarpement de faible de la Tèt est-il le résultat de la tectonique active Plio-Pléistocène ou d'une exhumation Pléistocène. C. R. Geosci. 336, 217–226. https://doi.org/10.1016/j.crte.2003.10.026.

Charpentier, D., Milesi, G., Labaume, P., Abd Elmola, A., Buatier, M., Lanari, P.,
Munoz, M., 2024. Evolution of fluid redox in a fault zone of the Pic de Port Vieux
thrust in the Pyrenees Axial Zone (Spain). Solid Earth 15, 1065–1086. https://doi.
org/10.5194/se-15-1065-2024.

Chevrot, S., Sylvander, M., Diaz, J., Martin, R., Mouthereau, F., Manatschal, G., Masini, E., Calassou, S., Grimaud, F., Pauchet, H., Ruiz, M., 2018. The non-cylindrical crustal architecture of the Pyrenees. Sci. Rep. 8. https://doi.org/10.1038/s41598-018-27889-x.

Choukroune, P., 1989. The Ecors Pyrenean deep seismic profile reflection data and the overall structure of an orogenic belt. Tectonics 8, 23–39. https://doi.org/10.1029/TC008i001p00023.

Clauzon, G., Le Strat, P., Duvail, C., Do Couto, D., Suc, J.-P., Molliex, S., Bache, F., Besson, D., Lindsay, E.H., Opdyke, N.D., Rubino, J.-L., Popescu, S.-M., Haq, B.U., Gorini, C., 2015. The Roussillon Basin (S. France): a case-study to distinguish local and regional events between 6 and 3 Ma. Mar. Petrol. Geol. 66, 18–40. https://doi.org/10.1016/j.marpetgeo.2015.03.012.

Cochelin, B., Chardon, D., Denèle, Y., Gumiaux, C., Le Bayon, B., 2017. Vertical strain partitioning in hot Variscan crust: syn-convergence escape of the Pyrenees in the Iberian-Armorican syntax. Bull. Soc. Geol. Fr. 188, 39. https://doi.org/10.1051/bsgf/2017206.

Cochelin, B., Lemirre, B., Denèle, Y., De Saint Blanquat, M., Lahfid, A., Duchène, S., 2018. Structural inheritance in the central Pyrenees: the variscan to Alpine tectonometamorphic evolution of the axial zone. J. Geol. Soc. 175, 336–351. https://doi.org/10.1144/jgs2017-066.

Costa, S., Maluski, H., 1988. Use of the 40Ar-39Ar stepwise heating method for dating mylonite zones: an example from the St. Barthélémy massif (Northern Pyrenees, France). Chem. Geol. Isot. Geosci. 72 (2), 127–144. https://doi.org/10.1016/0168-9622(88)90061-9.

Delcaillau, B., Carozza, J.-M., Font, M., 2004. The northern segment of the Tèt Fault (Pyrenees-Orientales) : Neogene evolution and geomorphic implications. Bull. Soc. Geol. Fr. 175, 257–272. https://doi.org/10.2113/175.3.257.

Denèle, Y., Olivier, P., Gleizes, G., Barbey, P., 2009. Decoupling between the middle and upper crust during transpression-related lateral flow: variscan evolution of the Aston gneiss dome (Pyrenees, France). Tectonophysics 477, 244–261. https://doi.org/10.1016/j.tecto.2009.04.033.

Di Vincenzo, G., Carosi, R., Palmeri, R., Tiepolo, M., 2007. A comparative U–Th–Pb (zircon–monazite) and 40Ar–39 Ar (muscovite–biotite) study of shear zones in northern Victoria Land (Antarctica): implications for geochronology and localized reworking of the Ross Orogen. J. Metamorph. Geol. 25, 605–630. https://doi.org/10.1111/j.1525-1314.2007.00717.x.

Doebelin, N., Kleeberg, R., 2015. Profex : a graphical user interface for the Rietveld refinement program BGMN. J. Appl. Crystallogr. 48, 1573–1580. https://doi.org/10.1107/S1600576715014685.

Dong, H., Hall, C.M., Peacor, D.R., Halliday, A.N., 1995. Mechanisms of argon retention in clays revealed by Laser40 Ar-39 Ar dating. Sci. Technol. Humanit. 267, 355–359. https://doi.org/10.1126/science.267.5196.355.

Duvall, A.R., Clark, M.K., Van Der Pluijm, B.A., Li, C., 2011. Direct dating of Eocene reverse faulting in northeastern Tibet using Ar-dating of fault clays and low-temperature thermochronometry. Earth Planet Sci. Lett. 304, 520–526. https://doi.org/10.1016/j.epsl.2011.02.028.

Fillon, C., van der Beek, P., 2012. Post-orogenic evolution of the southern Pyrenees: constraints from inverse thermo-kinematic modelling of low-temperature thermochronology data. Basin Res. 24, 418–436. https://doi.org/10.1111/j.1365-2117.2011.00533.x.

Gerardin, M., Milesi, G., Mercadier, J., Cathelineau, M., Bartier, D., 2024. Development of an integrated analytical platform for clay mineral separation, characterization and
K-Ar dating. Geosci. Instrum. Methods Data Sys. 13, 309-323. https://doi.org/10.5194/gi-13-309-2024.

Gibson, M., Sinclair, H.D., Lynn, G.J., Stuart, F.M., 2007. Late- to post-organic exhumation of the Central Pyrenees revealed through combined thermochronological data and modelling. Resin Basin. 19, 323-334. https://doi.org/10.1111/j.1365-2117.2007.00333.x.

Goulà, X., Olivera, C., Fleta, J., Grellet, B., Lindo, R., Rivera, L.A., Cisternas, A., Carbon, D., 1999. Present and recent stress regime in the eastern part of the Pyrenees. Tectonophysics 308, 487-502. https://doi.org/10.1016/S0040-1951(99)00120-1.

Gourinard, Y., 1971. Détermination cartographique et géophysique des failles bordières du fossé néogène de Cerdagne (Pyrenees orientales franco-espagnoles). 96ème Congrès National des Sociétés Savantes, Toulouse 1971. Sciences 2, 245-263.

Guillou-Frottier, L., Milesi, G., Roche, V., Duquiet, H., Taillefer, A., 2024. Heat flow, thermal anomalies, tectonic regimes and high-temperature geothermal systems in fault zones. Comptes Rendus. Géoscience 356, 1-33. https://doi.org/10.5802/crgeos.213.

Guitard, G., Geyssant, J., Laumonier, B., Autran, A., Fonteilles, M., Dalmayrac, B., et al., 1992. Carte géologique de la France (1/50,000), feuille prades, p. 1095.

Guitard, G., Laumonier, B., Autran, A., Bandet, Y., Berger, G., 1998. Notice Explicative, Carte Géologique de France (1/50,000), Feuille Prades (1995). Orléans: BRGM.

Hartz, E.H., Andresen, A., Martin, M.W., Hodges, K.V., 2000. U-Pb and 40Ar/39 Ar constraints on the Fjord region detachment zone: a long-lived extensional fault in the central East Greenland caledonians. J. Geol. Soc. 157, 795-809. https://doi.org/10.1144/jgs.157.4.795.

Hueck, M., Wemmer, K., Basei, M.A.S., Philipp, R.P., Oriolo, S., Heidelbach, F., Oyhantçabal, P., Siegesmund, S., 2020. Dating recurrent shear zone activity and the transition from ductile to brittle deformation: white mica geochronology applied to the Neoproterozoic Dom Feliciano Belt in South Brazil. J. Struct. Geol. 141, 104199. https://doi.org/10.1016/j.jsg.2020.104199.

Hueck, M., Wemmer, K., Ksienzyk, A.K., Kuehn, R., Vogel, N., 2022. Potential, premises, and pitfalls of interpreting illite iron dates - a case study from the German Variscides. Earth Sci. Rev. 232, 104133. https://doi.org/10.1016/j.earscirev.2022.104133.

Janots, E., Grand'Homme, A., Bernet, M., Guillaume, D., Gnos, E., Boiron, M.-C., Rossi, M., Seydoux-Guillaume, A.-M., De Ascença Guedes, R., 2019. Geochronological and thermometric evidence of unusually hot fluids in an Alpine fissure of Lauzière granite (Belledonne, Western Alps). Solid Earth 10, 211-223. https://doi.org/10.5194/se-10-211-2019.

Jolivet, L., Menant, A., Roche, V., Le Pourhiet, L., Maillard, A., Augier, R., Do Couto, D., Gorini, C., Thinon, I., Canva, A., 2021. Transfer zones in Mediterranean back-arc regions and tear faults. In: Lacombe, O., Tavani, S., Teixell, A., Pedreira, D., Calassou, S. (Eds.), BSGF Earth Sci. Bulletin 192, 11. https://doi.org/10.1051/bsgf/2021006.

Koppers, A.A., 2002. ArArCALC-software for 40Ar/39Ar age calculations. Comput. Geosci. 28 (5), 605-619. https://doi.org/10.1016/S0098-3004(01)00095-4.

Lacan, P., Ortuño, M., 2012. Active tectonics of the Pyrenees: a review. J. Iber. Geol. 38. https://doi.org/10.5209/rev_JIGE.2012.v38.n1.39203.

Lacroix, B., Charpentier, D., Buatier, M., Vennemann, T., Labaume, P., Adatte, T., Travé, A., Dubois, M., 2012. Formation of chlorite during thrust fault reactivation. Record of fluid origin and P-T conditions in the Monte Perdido thrust fault (southern Pyrenees). Contrib. Mineral. Petrol. 163, 1083-1102. https://doi.org/10.1007/s00410-011-0718-0.

Lagarec, K., Rancourt, D.G., 1997. Extended Voigt-based analytic linearmode method for determining N-dimensional correlated hyperfine parameter distributions in Mössbauer Spectroscopy. Nucl. Instrum. Methods Phys. Res., Sect. B.

Lanari, P., Vidal, O., De Andrade, V., Dubacq, B., Lewin, E., Grosch, E.G., Schwartz, S., 2014a. XMapTools: a MATLAB®-based program for electron microprobe X-ray image processing and geothermobarometry. Comput. Geosci. 62, 227-240. https://doi.org/10.1016/j.cageo.2013.08.010.

Lanari, P., Wagner, T., Vidal, O., 2014b. A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO-FeO-Al2O3-SiO2-H2O: applications to P-T sections and geothermometry. Contrib. Mineral. Petrol. 167, 968. https://doi.org/10.1007/s00401-014-0968-8.

Lanson, B., Beaufort, D., Berger, G., Baradat, J., Lacharpagne, J.C., 1996. Illitization of diagenetic kaolinite-to-dickite conversion series; late-stage diagenesis of the Lower Permian Rotliegend Sandstone reservoir, offshore of The Netherlands. J. Sediment. Res. 663, 501-518. https://doi.org/10.1306/D4268392-2B26-11D7-8648000102C1865D.

Laumonier, B., Calvet, M., Delmas, M., Barbey, P., Lenoble, J., Autran, A., 2017. Notice explicative Carte géologique de la France à 1/50,000, feuille Mont-Louis (1094). Orléans: BRGM, p. 139.

Laumonier, B., Calvet, M., Le Bayon, B., Barbey, P., Lenoble, J.-L., 2015. Notice explicative, Carte géologique de la France à 1/500 000, feuille Prats-de-Mollo-la-Preste (1099). In: Orléans: BRGM, p. 189.

Lee, J.Y., Marti, K., Severinghaus, J.P., Kawamura, K., Yoo, H.S., Lee, J.B., Kim, J.S., 2006. A redetermination of the isotopic abundances of atmospheric Ar. Geochem. Cosmochim. Acta 70 (17), 4507-4512. https://doi.org/10.1016/j.gca.2006.06.1563.

Mathon, O., Beteva, A., Borrel, J., Bugnazet, D., Gatla, S., Hino, R., Kantor, I., Mairs, T., Munoz, M., Pasternak, S., Perrin, F., Pascarelli, S., 2015. The time-resolved and extreme conditions XAS (TEXAS) facility at the European Synchrotron Radiation Facility: the general-purpose EXAFS bending-magnet beamline BM23. J. Synchrotron Radiat. 22, 1548-1554. https://doi.org/10.1107/S1600577515017786.

Maurel, O., 2003. L'exhumation de la Zone Axiale des Pyrénées orientales: Une approche thermo-chronologique multi-méthodes du rôle des failles, vol. 264.

Maurel, O., Monié, P., Pik, R., Arnaud, N., Brunel, M., Jolivet, M., 2008. The Meso-Cenozoic thermo-pectonic evolution of the eastern Pyrenees: an 40Ar/39Ar fission track (U-Th)/He thermochronological study of the Canigou and Mont-Louis massifs. Int. J. Earth Sci. 97, 565-584. https://doi.org/10.1007/s00531-007-0179-x.

McCaig, A.M., Miller, J.A., 1986. 40Ar-39Ar age of mylonites along the Merens Fault, central Pyrenees. Tectonophysics 129, 149-172. https://doi.org/10.1016/0040-1951(86)90250-7.

Metcalf, J.R., Fitzgerald, P.G., Baldwin, S.L., Muñoz, J.-A., 2009. Thermochronology of a convergent orogen: constraints on the timing of thrust faulting and subsequent exhumation of the maladeta pluton in the central pyrenean axial zone. Earth Planet Sci. Lett. 287, 488-503. https://doi.org/10.1016/j.epsl.2009.08.036.

Milesi, G., Cardoso, C.D., Pik, R., Moreira, M., Charpentier, D., Mercadier, J., 2024. Origin of helium and associated fluid in fault-related hydrothermal systems of the eastern Pyrenees. Terra Nova ter, 12753. https://doi.org/10.1111/ter.12753.

Milesi, G., Monié, P., Münch, P., Soliva, R., Taillefer, A., Bruguier, O., Bellanger, M., Bonno, M., Martin, C., 2020. Tracking geothermal anomalies along a crustal fault using (U - Th)/He apatite thermochronology and rare-earth element (REE) analyses: the example of the Tét fault (Pyrenees, France). Solid Earth 11, 1747-1771. https://doi.org/10.5194/se-11-1747-2020.

Milesi, G., Monié, P., Soliva, R., Münch, P., Valla, P.G., Richaud, S., Bonno, M., Martin, C., Bellanger, M., 2022. Deciphering the cenozoic exhumation history of the eastern Pyrenees along a crustal-scale normal fault using low-temperature thermochronology. Tectonics 41. https://doi.org/10.1029/2021TC007172.

Milesi, G., Soliva, R., Monié, P., Münch, P., Bellanger, M., Bruguier, O., Bonno, M., Taillefer, A., Mayolle, S., 2019. Mapping a geothermal anomaly using apatite (U-Th)/He thermochronology in the Tét fault damage zone, eastern Pyrenees, France. Terra Nova 31, 569-576. https://doi.org/10.1111/ter.12429.

Milesi, G., Valla, P.G., Münch, P., Huyghe, D., 2023. Tectono-geomorphological evolution of the eastern Pyrenees: insights from thermo-kinematic modeling. Tectonophysics 866, 230057. https://doi.org/10.1016/j.tecto.2023.230057.

Monié, P., Münch, P., Milesi, G., Bonno, M., Iemmolo, A., 2024. 40 Ar/39 Ar geochronology of crustal deformation. Comptes Rendus. Géoscience 356, 1-29. https://doi.org/10.5802/crgeos.209.

Monié, P., Soliva, J., Brunel, M., Maluski, H., 1994. Les cisaillements mylonitiques du granite de Millas (Pyrenees, France); age Cretac 40 Ar/39 Ar et tectonique. Bulletin de la Société Géologique de France 165 (6), 559-571.

Mottram, C.M., Kellett, D.A., Barresi, T., Zwingmann, H., Friend, M., Todd, A., Percival, J.B., 2020. Syncing fault rock clocks: Direct comparison of U-Pb carbonate and K-Ar illite fault dating methods. Geology 48, 1179-1183. https://doi.org/10.1130/G47778.1.

Mouthereau, F., Angrand, P., Jourdon, A., Ternois, S., Fillon, C., Calassou, S., Chevrot, S., Ford, M., Jolivet, L., Manatschal, G., Masini, E., Thinon, I., Vidal, O., Baudin, T., 2021. Cenozoic mountain building and topographic evolution in Western Europe: impact of billions of years of lithosphere evolution and plate kinematics. In: Lacombe, O., Tavani, S., Teixell, A., Pedreira, D., Calassou, S. (Eds.), BSGF Earth Sci. Bulletin 192, 56. https://doi.org/10.1051/bsgf/2021040.

Mark, D.F., Barford, D., Stuart, F.M., Imlach, J., 2009. The ARGUS multicounter noble gas mass spectrometer: performance for 40Ar/39Ar geochronology. Geochem. Geophys. Geosystems 10 (10). https://doi.org/10.1029/2009GC002643.

Mezger, J.E., Schnapperelle, S., Rölke, C., 2012. Evolution of the Central Pyrenean Mérens Fault Controlled by Near Collision of Two Gneiss Domes.

Mulch, A., Cosca, M.A., 2004. Recrystallization or cooling ages: in situ UV-laser40 Ar/39 Ar geochronology of muscovite in mylonitic rocks. J. Geol. Soc. 161, 573-582. https://doi.org/10.1144/0016-764903-110.

Münch, P., Caillaud, J., Monié, P., Grauby, O., Corsini, M., Ricci, J., Romagny, A., Philippon, M., Lanson, B., Azdimousa, A., Ben Moussa, A., Arnaud, N., 2021. Direct dating of brittle extensional deformation contemporaneous of Neogene exhumation of the internal zones of the Rif Chain. Tectonophysics 807, 228800. https://doi.org/10.1016/j.tecto.2021.228800.

Muñoz, M., De Andrade, V., Vidal, O., Lewin, E., Pascarelli, S., Susini, J., 2006. Redox and speciation micromapping using dispersive X-ray absorption spectroscopy: application to iron in chlorite mineral of a metamorphic rock thin section. Geochem. Geophys. Geosystems 7. https://doi.org/10.1029/2006GC001381, 2006GC001381.

Munoz, M., Vidal, O., Marcaillou, C., Pascarelli, S., Mathon, O., Farges, F., 2013. Iron oxidation state in phyllosilicate single crystals using Fe-K pre-edge and XANES spectroscopy: effects of the linear polarization of the synchrotron X-ray beam. Am. Mineral. 98, 1187-1197. https://doi.org/10.2138/am.2013.4289.

Odium, M.L., Stockli, D.F., 2020. Geochronologic constraints on deformation and metasomatism along an exhumed mylonitic shear zone using apatite U-Pb, geochemistry, and microtextural analysis. Earth Planet Sci. Lett. 538, 116177. https://doi.org/10.1016/j.epsl.2020.116177.

Oriolo, S., Wemmer, K., Oyhantçabal, P., Fossen, H., Schulz, B., Siegesmund, S., 2018. Geochronology of shear zones – a review. Earth Sci. Rev. 185, 665-683. https://doi.org/10.1016/j.earscirev.2018.07.007.

Petit, C., Mouthereau, F., 2012. Steep topographic slope preservation by anisotropic diffusion: an example from the Neogene Tét fault scarp, eastern Pyrenees. Geomorphology 171-172, 173-179. https://doi.org/10.1016/j.geomorph.2012.05.016.

Peverar, D.R., 1999. Illite and hydrocarbon exploration. Proc. Natl. Acad. Sci. 96, 3440-3446. https://doi.org/10.1073/pnas.96.7.3440.

Roberts, N.M.W., Holdsworth, R.E., 2022. Timescales of faulting through calcite geochronology: a review. J. Struct. Geol. 158, 104578. https://doi.org/10.1016/j.jsg.2022.104578.
Renne, P.R., Balco, G., Ludwig, K.R., Mundil, R., Min, K., 2011. Response to the comment by WH Schwarz et al. on "Joint determination of 40K decay constants and 40Ar*/ 40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology" by PR Renne et al. (2010). Geochem. Cosmochim. Acta 75 (17), 5097-5100. https://doi.org/10.1016/j.gca.2011.06.021.

Romagny, A., Jolivet, L., Menant, A., Bessière, E., Maillard, A., Canva, A., Gorini, C., Augier, R., 2020. Detailed tectonic reconstructions of the Western Mediterranean region for the last 35 Ma, insights on driving mechanisms. In: Lacombe, O., Tavani, S., Teixell, A., Pedreira, D., Calassou, S. (Eds.), BSGF - Earth Sciences Bulletin, vol. 191, p. 37. https://doi.org/10.1051/bsgf/2020040.

Roure, F., Choukroune, P., Berastegui, X., Munoz, J.A., Villien, A., Matheron, P., Baret, M., Seguret, M., Camara, P., Deramond, J., 1989. Econs deep seismic data and balanced cross sections: geometric constraints on the evolution of the Pyrenees. Tectonics 8, 41-50. https://doi.org/10.1029/TC08i001p00041.

Schirra, M., Laurent, O., 2021. Petrochronology of hydrothermal rutile in mineralized porphyry Cu systems. Chem. Geol. 581, 120407. https://doi.org/10.1016/j.chemgeo.2021.120407.

Séranne, M., Benedicto, A., Labaum, P., Truffert, C., Pascal, G., 1995. Structural style and evolution of the Gulf of lion oligo-miocene rifting: role of the pyrenean orography. Mar. Petrol. Geol. 12, 809-820.

Séranne, M., Couéffé, R., Husson, E., Baral, C., Villard, J., 2021. The transition from Pyrenean shortening to Gulf of Lion rifting in Languedoc (South France) - a tectonic-sedimentation analysis. In: Lacombe, O., Tavani, S., Teixell, A., Pedreira, D., Calassou, S. (Eds.), BSGF - Earth Sciences Bulletin, vol. 192, p. 27. https://doi.org/10.1051/bsgf/2021017.

Sibuet, J.-C., Srivastava, S.P., Spakman, W., 2004. Pyrenean orography and plate kinematics: pyrenean orography and plate kinematics. J. Geophys. Res. Solid Earth 109. https://doi.org/10.1029/2003JB002514.

Siebel, W., Hann, H.P., Danišk, M., Shang, C.K., Berthold, C., Rohrmüller, J., Wemmer, K., Evans, N.J., 2010. Age constraints on faulting and fault reactivation: a multi-chronological approach. Int. J. Earth Sci. 99, 1187-1197. https://doi.org/10.1007/s00531-009-0474-9.

Sinclair, H.D., 2005. Asymmetric growth of the Pyrenees revealed through measurement and modeling of orogenic fluxes. Am. J. Sci. 305, 369-406. https://doi.org/10.2475/ajs.305.5.369.

Stipp, M., Stünitz, H., Heilbronner, R., Schmid, S.M., 2002. The eastern Tonale fault zone: a 'natural laboratory' for crystal plastic deformation of quartz over a temperature range from 250 to 700°C. J. Struct. Geol. 24, 1861-1884. https://doi.org/10.1016/S0191-8141(02)00035-4.

Taillefer, A., Milesi, G., Soliva, R., Monnier, L., Delorme, P., Guillou-Frottier, L., Le Goff, E., 2021. Polyphased brittle deformation around a crustal fault: a multi-scale approach based on remote sensing and field data on the mountains surrounding the Tét hydrothermal system (Eastern Pyrénées, France). Tectonophysics 804, 228710. https://doi.org/10.1016/j.tecto.2020.228710.

Taillefer, A., Soliva, R., Guillou-Frottier, L., Le Goff, E., Martin, G., Seranne, M., 2017. Fault-related controls on upward hydrothermal flow: an integrated geological study of the Tét Fault system, eastern Pyrénées (France). Geofluids 1-19. https://doi.org/10.1155/2017/8190109, 2017.

Tartaglia, G., Viola, G., van der Lelij, R., Scheiber, T., Ceccato, A., Schönenberger, J., 2020. "Brittle structural facies" analysis: a diagnostic method to unravel and date multiple slip events of long-lived faults. Earth Planet Sci. Lett. 545, 116420. https://doi.org/10.1016/j.epsl.2020.116420.

Teixell, A., Labaume, P., Lagabrielle, Y., 2016. The crustal evolution of the west-central Pyrenees revisited: inferences from a new kinematic scenario. C. R. Geosci. 348, 257-267. https://doi.org/10.1016/j.crte.2015.10.010.

Ternois, S., Odlum, M., Ford, M., Pik, R., Stockli, D., Tibari, B., Vacherat, A., Bernard, V., 2019. Thermochronological evidence of early Orogenesis, eastern Pyrenees, France. Tectonics 38, 1308-1336. https://doi.org/10.1029/2018TC005254.

Tsukamoto, S., Tagami, T., Zwingmann, H., 2020. Direct Dating of Fault Movement. Understanding Faults. Elsevier, pp. 257-282. https://doi.org/10.1016/B978-0-12-815985-9.00007-2.

van der Pluijm, B.A., Hall, C.M., Vrolijk, P.J., Pevear, D.R., Covey, M.C., 2001. The dating of shallow faults in the Earth's crust. Nature 412, 172-175. https://doi.org/10.1038/35084053.

Vergés, J., Millán, H., Roca, E., Muñoz, J.A., Marzo, M., Cirés, J., Den Bezemer, T., Zoetemeijer, R., Cloetingh, S., 1995. Eastern Pyrenees and related foreland basins: pre-, syn- and post-collisional crustal-scale cross-sections. Mar. Petrol. Geol. 12, 903-915.

Vidal, O., 2005. Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: application to natural examples and possible role of oxidation. Am. Mineral. 90, 347-358. https://doi.org/10.2138/am.2005.1554.

Vidal, O., 2001. A thermodynamic model for FE-MG aluminous chlorite using data from phase equilibrium experiments and natural pelitic assemblages in the 100 to 600 C, 1 to 25 kb range. Am. J. Sci. 301, 557-592. https://doi.org/10.2475/ajs.301.6.557.

Vidal, O., De Andrade, V., Lewin, E., Munoz, M., Parra, T., Pascarelli, S., 2006. P-T -deformation-Fe3+/Fe2+ mapping at the thin section scale and comparison with XANES mapping: application to a garnet-bearing metapelite from the Sambagawa metamorphic belt (Japan). J. Metamorph. Geol. 24, 669-683. https://doi.org/10.1111/j.1525-1314.2006.00661.x.

Villa, I.M., Bucher, S., Bousquet, R., Kleinhanns, I.C., Schmid, S.M., 2014. Dating polygenetic metamorphic assemblages along a transect across the western Alps. J. Petrol. 55, 803-830. https://doi.org/10.1093/petrology/egu007.

Waldner, M., Bellahsen, N., Mouthereau, F., Bernet, M., Pik, R., Rosenberg, C.L., Balvay, M., 2021. Central Pyrenees mountain building: constraints from new LT thermochronological data from the axial zone. Tectonics 40. https://doi.org/10.1029/2020TC006614.

Wojdyr, M., 2010. Fityk: a general-purpose peak fitting program. J. Appl. Crystallogr. 43, 1126-1128. https://doi.org/10.1107/S0021889810030499.

Whitchurch, A.L., Carter, A., Sinclair, H.D., Duller, R.A., Whittaker, A.C., Allen, P.A., 2011. Sediment routing system evolution within a diachronously uplifting orogen: insights from detrital zircon thermochronological analyses from the South-Central Pyrenees. Am. J. Sci. 311, 442-482. https://doi.org/10.2475/05.2011.03.

Ziegler, P.A., 1992. European Cenozoic rift system. Tectonophysics 208 (1-3), 91-111. https://doi.org/10.1016/0040-1951(92)90338-7.