Nicolas Saspiturry
email: saspiturry.nicolas@gmail.com

Philippe Razin

Thierry Baudin

Benoit Issautier Olivier Serrano

Eric Lasseur

Cécile Allanic

Isabelle Thinon

Sophie Leleu

Symmetry vs. asymmetry of a hyper-thinned rift: Example of the Mauleon Basin (Western Pyrenees, France)

Keywords:

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Since the 1980's, conceptual rifting models have evolved from the pure-shear model [START_REF] Mckenzie | Some remarks on the development of sedimentary basins[END_REF], to the simple-shear model [START_REF] Wernicke | Low-angle normal faults in the Basin and Range Province: nappe tectonics in an extending orogen[END_REF](Wernicke , 1985;;Wernicke & Burchfiel 1982;[START_REF] Davis | Shear-zone model for the origin of metamorphic core complexes[END_REF][START_REF] Spencer | Role of tectonic denudation in warping and uplift of low-angle normal faults[END_REF][START_REF] Davis | Structural evolution of the Whipple and South mountains shear zones[END_REF]Wernicke & Axen 1988;[START_REF] Brun | Extensional gneiss domes and detachment fault systems; structure and kinematics[END_REF] and, more recently, to numerical polyphasing models [START_REF] Huismans | Symmetric and asymmetric lithospheric extension: Relative effects of frictional-plastic and viscous strain softening[END_REF][START_REF] Canérot | Les Pyrénées: Histoire géologique: Atlantica[END_REF], 2011, 2014;[START_REF] Tirel | Thermomechanical modeling of extensional gneiss domes[END_REF][START_REF] Lavier | A mechanism to thin the continental lithosphere at magma-poor margins[END_REF][START_REF] Brune | Rift migration explains continental margin asymmetry and crustal hyper-extension[END_REF][START_REF] Brune | Abrupt plate accelerations shape rifted continental margins[END_REF][START_REF] Dias | Conjugate rifted margins width and asymmetry: The interplay between lithospheric strength and thermomechanical processes[END_REF]. Much recent work describes the crustal thinning and modalities of mantle exhumation at actual hyper-thinned passive margins such as the Iberia-Newfoundland [START_REF] Péron-Pinvidic | Tectonosedimentary evolution of the deep Iberia-Newfoundland margins: Evidence for a complex breakup history: Tectonics[END_REF][START_REF] Haupert | Upper-plate magma-poor rifted margins: Stratigraphic architecture and structural evolution[END_REF], Angola-Brazil [START_REF] Unternehr | Hyper-extended crust in the South Atlantic: in search of a model[END_REF][START_REF] Péron-Pinvidic | Unravelling the along-strike variability of the Angola-Gabon rifted margin: a mapping approach[END_REF], and Australia-Antarctic [START_REF] Gillard | Tectonomagmatic evolution of the final stages of rifting along the deep conjugate Australian-Antarctic magma-poor rifted margins: Constraints from seismic observations: Australian-Antarctic margins evolution: Tectonics[END_REF] margins. These have been modelled numerically in order to reproduce the geometry of the continental margins and to understand the development of crustal thinning detachment faults. Although the distal part of these extensional systems is well constrained offshore by seismic interpretation, few studies currently describe the proximal onshore domain. One of the most studied examples is the Jurassic Adriatic margin in the Alps [START_REF] Masini | The tectonosedimentary evolution of a supra-detachment rift basin at a deep-water magma-poor rifted margin: the example of the Samedan Basin preserved in the Err nappe in SE Switzerland: Tectono-sedimentary evolution of a supra-detachment rift basin[END_REF][START_REF] Masini | Anatomy and tectonosedimentary evolution of a rift-related detachment system: The example of the Err detachment (central Alps[END_REF][START_REF] Masini | The Alpine Tethys rifted margins: Reconciling old and new ideas to understand the stratigraphic architecture of magmapoor rifted margins[END_REF]. These authors describe the crustal detachment faults and sedimentary evolution of this starved system. Unlike the situation in the Mauléon rift basin, the hyper-thinning crustal models are based on real continental margins bordering oceanic domains. Where the Pyrenees are concerned, development of these conceptual models has led some authors to propose different models to explain the location of the Pyrenean mantle outcrops (Lagabrielle [START_REF] Puigdefàbregas | Tecto-sedimentary cycles and depositional sequences of the Mesozoic and Tertiary from the Pyrenees[END_REF][START_REF] Olivet | La cinématique de la plaque ibérique: Bull[END_REF][START_REF] Rosenbaum | Relative motions of Africa, Iberia and Europe during Alpine orogeny[END_REF][START_REF] Sibuet | Pyrenean orogeny and plate kinematics[END_REF][START_REF] Gong | The rotation of Iberia during the Aptian and the opening of the Bay of Biscay[END_REF]. The deformation of this intracontinental domain is linked to the northward migration of the Iberian plate and to inversion of the previous Cretaceous north-Pyrenean basin [START_REF] Ducasse | Géotraverse dans la partie occidentale des Pyrénées, de l'avant-pays aquitain au bassin de l'Ebre: effet d'une inversion structurale sur l'édification d'une chaîne intracontinentale[END_REF]. The central and eastern parts of the Pyrenees can be divided into three structural zones: the North-Pyrenean Zone (NPZ), the Axial Zone, and the South-Pyrenean Zone (SPZ) (Fig. 1A; [START_REF] Choukroune | Strain patterns in the Pyrenean chain[END_REF]. The NPZ -located between two major faults, the North Pyrenean Fault (NPF) to the south, and the North Pyrenean Frontal Thrust (NPFT) to the north (Fig. 1B; [START_REF] Choukroune | The ECORS Pyrenean deep seismic profile reflection data and the overall structure of an orogenic belt: Tectonics[END_REF][START_REF] Daignières | The Arzacq-western Pyrenees ECORS deep seismic profile[END_REF] -is composed of folded Mesozoic cover and Paleozoic units. The commonly named Metamorphic Internal Zone (MIZ), along the southern part of the NPZ, corresponds to a narrow, east-west, vertically metamorphosed and severely deformed zone [START_REF] Ravier | Le métamorphisme des terrains secondaires des Pyrénées[END_REF]. The MIZ is characterized by high-temperature and low-pressure metamorphism related to Albian-Cenomanian rifting [START_REF] Albarède | Age and Significance of the North Pyrenean Metamorphism[END_REF][START_REF] Montigny | K-Ar Study of cretaceous magmatism and metamorphism in the Pyrenees: Age and length of rotation of the Iberian Peninsula[END_REF][START_REF] Golberg | Données nouvelles et mise au point sur l'âge du métamorphisme pyrénéen[END_REF][START_REF] Golberg | High temperature-low pressure Cretaceous metamorphism related to crustal thinning (Eastern North Pyrenean Zone[END_REF][START_REF] Boulvais | Carbonate alteration associated with talc-chlorite mineralization in the eastern Pyrenees, with emphasis on the St[END_REF][START_REF] Clerc | High-temperature metamorphism during extreme thinning of the continental crust: a reappraisal of the North Pyrenean passive paleomargin[END_REF].

In the western Pyrenees, the NPZ is represented by the Cretaceous Mauléon Basin, thrusted to the north onto the Aquitaine Basin and bordered to the south by the Axial Zone, where Paleozoic rocks are unconformably overlain by Late Cretaceous shallow marine carbonates [START_REF] Souquet | Le Crétacé supérieur Sud-Pyrénéen, en Catalogne[END_REF]. The Mauléon Basin is bounded to the west by Paleozoic blocks commonly known as the "Massifs Basques" [START_REF] Heddebaut | Etudes géologiques dans les massifs paléozoïques basques[END_REF][START_REF] Muller | L'Evolution structurale des Pyrénées (Domaine central et occidental) Le segment hercynien, la chaîne de fond alpine[END_REF]. The MIZ and the NPF disappear in the western part of the Pyrenees [START_REF] Choukroune | Strain patterns in the Pyrenean chain[END_REF][START_REF] Hall | Apparent western termination of the North Pyrenean fault and tectonostratigraphic units of the western north Pyrenees, France and Spain[END_REF][START_REF] Canérot | Nouvelle interprétation structurale de la « faille Nord-Pyrénéenne » en vallée d'Aspe (Pyrénées-Atlantiques)[END_REF]. The compressive phase is responsible for a greater shortening rate in the central and eastern Pyrenees [START_REF] Muñoz | Evolution of a continental collision belt: ECORS-Pyrenees crustal balanced cross-section[END_REF][START_REF] Vergés | Eastern Pyrenees and related foreland basins: pre-, syn-and post-collisional crustal-scale cross-sections[END_REF][START_REF] Beaumont | Factors controlling the Alpine evolution of the central Pyrenees inferred from a comparison of observations and geodynamical models[END_REF][START_REF] Mouthereau | Placing limits to shortening evolution in the Pyrenees: Role of margin architecture and implications for the Iberia/Europe convergence[END_REF] than in the western Pyrenees [START_REF] Teixell | The Anso transect of the southern Pyrenees: basement and cover thrust geometries[END_REF][START_REF] Teixell | Crustal structure and orogenic material budget in the west central Pyrenees: Tectonics[END_REF], inducing better preservation of the Mauléon Basin. The minor deformation of the western NPZ is linked to diachronism at onset of the thrusting of the Pyrenean compressive phase, older to the east (Santonian) and younger to the west (Middle Eocene), due to Iberian plate kinematics [START_REF] Souquet | Le Crétacé supérieur Sud-Pyrénéen, en Catalogne[END_REF][START_REF] Séguret | Crustal scale balanced cross-sections of the Pyrenees[END_REF][START_REF] Olivet | La cinématique de la plaque ibérique: Bull[END_REF]. This diachronism is the result of the configuration of the plates before convergence, with a more stretched domain in the western Pyrenees. The MIZ re-appears in the commonly named "Nappes des Marbres" corresponding to the center of the Albian basco-cantabric basin [START_REF] Rat | Vue sur le Cretacé basco-cantabrique et nord-ibérique: Une marge et son arriere-pays, ses environnements sedimentaires[END_REF].

The Mauléon Basin

The Mauléon Basin corresponds to a Cretaceous subsiding domain, filled by thick Albian to Upper Cretaceous deep basin deposits. The Mendibelza-Igountze southern Unit correspond to the inverted elements of the paleo-southern Mauléon Basin margin [START_REF] Muller | L'Evolution structurale des Pyrénées (Domaine central et occidental) Le segment hercynien, la chaîne de fond alpine[END_REF][START_REF] Canérot | Présence d'une marge méridionale à l'emplacement de la zone des chainons béarnais (Pyrénées basco-béarnaises)[END_REF][START_REF] Puigdefàbregas | Tecto-sedimentary cycles and depositional sequences of the Mesozoic and Tertiary from the Pyrenees[END_REF]. This unit is composed of a Devonian-Carboniferous sedimentary substratum [START_REF] Laverdière | Les formations paléozoïques de la vallée du Laurhibar[END_REF][START_REF] Paris | Étude d'une partie du massif d'Igounce et de ses abords septentrionaux en Barétous et Basse-Soule[END_REF][START_REF] Casteras | Sur les terrains paléozoïques et sur la structure du massif de Mendibelza (Basses-Pyrénées)[END_REF][START_REF] Mirouse | Le Dévonien des Pyrénées occidentales et centrales (France): International Symposium on the Devonian System[END_REF], and a thick Albian synrift Mendibelza Formation [START_REF] Lamare | Les formations détritiques crétacées du massif de Mendibelza[END_REF].

The Mendibelza-Igountze Unit is thrusted towards the south onto the Larrau-Saint Engrâce Triassic window [START_REF] Galharague | Étude géologique de la zone de relais des massifs d'Igounce et de Mendibelza (Basses-Pyrénées[END_REF] and the Late Cretaceous carbonate cover of the Axial Zone, along the Lakhoura Thrust [START_REF] Lamare | Remarques sur la structure du Pays Basques[END_REF][START_REF] Casteras | Sur la structure de la partie orientale des Pyrénées basques[END_REF][START_REF] Ducasse | Glissement de couverture et panneaux basculés dans la région des Arbailles (Pyrénées occidentales): Un modèle évolutif crétacé de la marge nord-ibérique à l'Est de la transformante de Pamplona[END_REF][START_REF] Teixell | Coupe géologique du massif d'Igountze: implications sur l'évolution structurale de la bordure sud de la zone nord-pyrénéenne occidentale[END_REF].

The Arbailles Unit is separated from the Mendibelza Unit by the steeply dipping Arbailles Thrust [START_REF] Lamare | Sur le passage latéral des faciès détritiques grossiers du Crétacé du massif de Mendibelza aux faciès schisto-gréseux classiques de l'Albien des Pyrénées[END_REF], also called the "Licq Fault" [START_REF] Teixell | The crustal evolution of the west-central Pyrenees revisited: Inferences from a new kinematic scenario[END_REF]. The Arbailles Unit is characterized by a preserved Jurassic-Lower Cretaceous carbonate sequence folded in a N110° trending syncline. This syncline is bounded to the north by a major normal fault separating it from the Saint-Palais Unit. This latter is represented by a thick (~ 6,000 m) Albian to Late Cretaceous flysch sequence deformed in N110° trending folds of various wave lengths. The north-eastern part of this unit is affected by the Saint-Palais, Bellevue and Sainte-Suzanne (NPFT) northward Pyrenean thrusts [START_REF] Daignières | The Arzacq-western Pyrenees ECORS deep seismic profile[END_REF][START_REF] Teixell | Crustal structure and orogenic material budget in the west central Pyrenees: Tectonics[END_REF].

"Grand-Rieu" Ridge [START_REF] Serrano | Le bassin d'Aquitaine: valorisation des données sismiques, cartographie structurale et potentiel pétrolier[END_REF].

The Labourd and Aldudes Units are composed of an Ordovician to Carboniferous sedimentary basement [START_REF] Laverdière | Les formations paléozoïques de la vallée du Laurhibar[END_REF][START_REF] Lamare | La terminaison orientale du massif des Aldudes, aux environs d'Arnéguy[END_REF][START_REF] Heddebaut | Observations tectoniques sur le massif des Aldudes (Basses-Pyrénées)[END_REF][START_REF] Heddebaut | Etudes géologiques dans les massifs paléozoïques basques[END_REF], overlain respectively by Permian-Triassic continental deposits (Lucas, 1985) and an Upper Cretaceous carbonate platform [START_REF] Souquet | Le Crétacé supérieur Sud-Pyrénéen, en Catalogne[END_REF]. The Labourd-Aldudes N20° Units, forming the western margin of the Mauléon rift Basin (Fig. 2), coincide with the commonly named Pamplona Fault identified by the authors [START_REF] Richard | Structure et évolution alpine des massifs paléozoïques du Labourd (Pays Basque français[END_REF][START_REF] Razin | Evolution tecto-sédimentaire alpine des Pyrénées basques à l'ouest de la transformante de Pamplona[END_REF][START_REF] Claude | Etude stratigraphique, sédimentologique et structurale des dépôts mésozoïques au nord du massif du Labourd: rôle de la faille de Pamplona (Pays Basque[END_REF][START_REF] Larrasoaña | Paleomagnetic, structural, and stratigraphic constraints on transverse fault kinematics during basin inversion: The Pamplona Fault[END_REF][START_REF] Pedreira | Three-dimensional gravity and magnetic modeling of crustal indentation and wedging in the western Pyrenees-Cantabrian Mountains[END_REF] The northern part of the Labourd Unit is composed of the Ursuya granulites [START_REF] Viennot | Observations pétrographiques dans le massif cristallin du Labourd (Basses Pyrénées[END_REF][START_REF] Lamare | La série paléozoïque du massif du Baygoura et de la vallée de la Nive; ses relations avec les terrains secondaires environnants[END_REF][START_REF] Boissonnas | Carte géologique de la France au 1/50000; feuille d'Iholdy[END_REF][START_REF] Hart | Provenance evolution during progressive rifting and hyperextension using bedrock and detrital zircon U-Pb geochronology, Mauléon Basin[END_REF].

The eastern part of the Mauléon Basin is materialized by the "Chaînons Béarnais", corresponding to a N110° anticline and syncline system, affecting a Jurassic to Early Cretaceous carbonate cover [START_REF] Canérot | Manifestations de l'halocinèse dans les chaînons béarnais (zone Nord-Pyrénéenne) au Crétacé inférieur[END_REF]. This sedimentary cover is associated with lherzolite outcrops: "Urdach, Tos de la Coustette, Saraillé, Turon de la Técouère" (Fabriès et al., 1991(Fabriès et al., , 1998)). The Urdach mantle body is reworked into Late Albian to Cenomanian synrift breccias, composed of Paleozoic basement and mantle clasts [START_REF] Roux | Recherches stratigraphiques et sédimentologiques sur les flyschs crétacés pyrénéens au sud d'Oloron[END_REF][START_REF] Fortané | Lherzolites and the western "Chaînons Béarnais[END_REF][START_REF] Jammes | Tectono-sedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees: Tectonics[END_REF]Debroas et al., 2010;[START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies: Tectonics[END_REF].

Tectono-sedimentary evolution of the Mauléon Basin

The Permian-Triassic post-Hercynian continental deposits filled extensional basins, such as the Bidarray Basin (Fig. 2), located in the Labourd-Aldudes Units (Bixel andLucas, 1983, 1987). The Late Triassic deposits are characterized by a Lower Triassic Sandstone Unit, a Middle Triassic Carbonate Unit and an Upper Triassic shale, evaporite and ophite complex [START_REF] Curnelle | Evolution structuro-sédimentaire du Trias et de l'Infra-Lias d'Aquitaine[END_REF]Lucas, 1985;[START_REF] Rossi | Datation U-Pb sur zircons des dolérites tholéiitiques pyrénéennes (ophites) à la limite Trias-Jurassique et relations avec les tufs volcaniques dits « infra-liasiques » nord-pyrénéens[END_REF]. During the Jurassic, a carbonate platform developed in a relatively stable tectonic context [START_REF] Delfaud | Evolution des bassins jurassiques dans la zone nordpyrénéenne occidentale: 64ème congrès AFAS Bordeaux[END_REF][START_REF] Lenoble | Les plates-formes carbonatées ouest-pyrénéennes du dogger à l'Albien, stratigraphie séquentielle et évolution géodynamique[END_REF][START_REF] James | La plate-forme carbonatée ouest-pyrénéenne au jurassique moyen et supérieur stratigraphie séquentielle, stades d'évolution, relations avec la subsurface en aquitaine méridionale[END_REF]. The end of the Jurassic is characterized by large scale exposure of the Aquitain-Pyrenean domain. The Neocomian is lacking on this part of Pyrenees and is recorded by the development of the bauxites [START_REF] Combes | Altérites et bauxites, témoins des marges européenne et ibérique des Pyrénées occidentales au Jurassique supérieur-Crétacé inférieur, à l'ouest de la vallée d'Ossau (Pyrénées-Atlantiques, France)[END_REF][START_REF] James | La plate-forme carbonatée ouest-pyrénéenne au jurassique moyen et supérieur stratigraphie séquentielle, stades d'évolution, relations avec la subsurface en aquitaine méridionale[END_REF][START_REF] Canérot | Les Pyrénées: Histoire géologique: Atlantica[END_REF]. During Early Barremian, localized subsidence of the previously emerged domain favored the transgression of carbonate platform deposits that continued until the earliest Albian [START_REF] Delfaud | Evolution des bassins pendant le Crétacé inférieur dans les Pyrénees occidentales et la bordure de l'Aquitaine: 64ème congrès AFAS Bordeaux[END_REF][START_REF] Arnaud-Vanneau | Paléogéographie des calcaires urgoniens du sud de la France[END_REF]. The end of this period is marked locally by extensive halokinetic deformation, associated with the first normal faults and the development of distal spicules marls [START_REF] Canérot | Manifestations de l'halocinèse dans les chaînons béarnais (zone Nord-Pyrénéenne) au Crétacé inférieur[END_REF][START_REF] Canérot | Rifting éocrétacé et halocinèse sur la marge ibérique des Pyrénées Occidentales (France)[END_REF][START_REF] Canérot | Diapirisme crétacé sur la marge ibérique des Pyrénées occidentales; exemple du pic de Lauriolle; comparaisons avec l'Aquitaine, les Pyrénées centrales et orientales[END_REF][START_REF] James | Diapirisme et structuration post-triasique des Pyrénées occidentales et de l'Aquitaine méridionale[END_REF][START_REF] Canérot | Mesozoic diapirism in the Pyrenean orogen: Salt tectonics on a transform plate boundary[END_REF].

The deposition of a thick, deep-marine conglomeratic sequence on the southern margin (Mendibelza-Igountze Unit) of the newly formed Mauléon Basin is considered as marking the onset of the Albian rifting basin, with a strong differentiation between an uplifted southern domain being the source of the conglomerates and a strongly subsiding deep-marine basin to the north [START_REF] Boirie | Etude Sédimentologique des Poudingues de Mendibelza (Pyrénées Atlantiques)[END_REF][START_REF] Boirie | Les poudingues de Mendibelza: dépôts de cônes sousmarins du rift albien des Pyrénées[END_REF]Fixari, 1984;[START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF]Fig. 2).

In the basin axis, more distal turbidites of the Black Flysch group, time-equivalent to the Mendibelza Conglomerates on the southern margin, constitute the first stratigraphic unit of the North-Pyrenean rift. At the same time, along the more gently-dipping northern margin, the albian shallow-marine carbonate deposits grade southwards to more distal marl-dominated sedimentation [START_REF] Biteau | The Aquitaine Basin: Petroleum Geoscience[END_REF].

The Mauléon Basin widened during the Cenomanian to Santonian [START_REF] Canérot | The pull apart-type Tardets-Mauléon Basin, a key to understand the formation of the Pyrenees[END_REF]. Active carbonate turbiditic systems supplied by the Aquitain platform to the north are responsible for the deposition of a thick carbonate flysch sequence at the basin axis [START_REF] Razin | Evolution tecto-sédimentaire alpine des Pyrénées basques à l'ouest de la transformante de Pamplona[END_REF]. At the southern margin, the transgressive "Calcaires des Cañons" carbonate platform onlaps the previously emerged Paleozoic basement, currently exposed in the Axial Zone [START_REF] Casteras | Sur la constitution et sur la structure de la couverture crétacée de la zone primaire axiale pyrénéenne à l'ouest du Pic d'Anie[END_REF][START_REF] Souquet | Le Crétacé supérieur Sud-Pyrénéen, en Catalogne[END_REF][START_REF] Alhamawi | Sédimentologie, Pétrographie Sédimentaire et Diagenèse des Calcaires du Crétacé supérieur de la Marge Ibérique[END_REF]. Chaotic gravity-flow sedimentation characterized the southern tectonically-controlled erosional slope [START_REF] Durand-Wackenheim | ): faciès de résedimentation en milieu profond de matériaux d'une Fixari, G[END_REF]. From Late Santonian to Eocene times, a thick deep-water sequence deriving from eastern syn-tectonic siliciclastic systems accumulated in the Mauléon Basin. Finally, the basin was deformed and inverted by the Pyrenean compression from the Middle Eocene to the Miocene [START_REF] Puigdefàbregas | Tecto-sedimentary cycles and depositional sequences of the Mesozoic and Tertiary from the Pyrenees[END_REF][START_REF] Bosch | Timing of Eocene-Miocene thrust activity in the Western Axial Zone and Chaînons Béarnais (west-central Pyrenees) revealed by multi-method thermochronology[END_REF][START_REF] Labaume | Exhumation sequence of the basement thrust units in the west-central Pyrenees[END_REF].

Present day deep structure of the Mauléon Basin

Gravimetric studies highlight a strong positive gravity anomaly under the Mauléon Basin [START_REF] Grandjean | Mise en évidence des structures crustales dans une portion de chaîne et de leur relation avec les bassins sédimentaires[END_REF][START_REF] Grandjean | Etude des structures crustales dans une portion de chaîne et de leur relation avec les bassins sédimentaires. Application aux Pyrénées occidentales[END_REF][START_REF] Casas | Gravity anomaly map of the Pyrenean region and a comparison of the deep geological structure of the western and eastern Pyrenees[END_REF]. This was previously considered as a dense intracrustal anomaly [START_REF] Grandjean | Etude des structures crustales dans une portion de chaîne et de leur relation avec les bassins sédimentaires. Application aux Pyrénées occidentales[END_REF] but is now interpreted, by some authors, as being induced by the proximity of the lithospheric mantle below a thinned crust [START_REF] Wang | The deep roots of the western Pyrenees revealed by full waveform inversion of teleseismic P waves[END_REF][START_REF] Wang | High resolution imaging of lithospheric structures by full waveform inversion of short period teleseismic P waves: Université Toulouse 3 Paul Sabatier[END_REF]. Recent research efforts focusing on mantle exhumation have linked denudation of the "Chaînons Béarnais" mantle outcrops to Albian hyper-thinning crustal mechanisms along a low angle detachment fault [START_REF] Jammes | Tectono-sedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees: Tectonics[END_REF][START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies: Tectonics[END_REF][START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF][START_REF] Tugend | Formation and deformation of hyperextended rift systems: Insights from rift domain mapping in the Bay of Biscay-Pyrenees: Tectonics[END_REF][START_REF] Corre | Deformation associated with mantle exhumation in a distal, hot passive margin environment: New constraints from the Saraillé Massif (Chaînons Béarnais, North-Pyrenean Zone)[END_REF][START_REF] Teixell | The crustal evolution of the west-central Pyrenees revisited: Inferences from a new kinematic scenario[END_REF]. The inversion of this rift basin is responsible for the Iberian northward underplating [START_REF] Mattauer | Présentation d'un modèle lithosphérique de la chaîne des Pyrénées: Comptes-rendus des séances de l'Académie des sciences[END_REF]Engeser and Schwentke, 1986;[START_REF] Mccaig | Deep geology of the Pyrenees: Nature[END_REF][START_REF] Choukroune | Main results of the ECORS Pyrenees profile[END_REF].

Available geodynamic models of the Mauléon rift basin

The crustal thinning process of the Mauléon rift basin during Albian times has been debated recently [START_REF] Jammes | Tectono-sedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees: Tectonics[END_REF][START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF][START_REF] Tugend | Formation and deformation of hyperextended rift systems: Insights from rift domain mapping in the Bay of Biscay-Pyrenees: Tectonics[END_REF]Tugend et al., , 2015a;;[START_REF] Corre | Deformation associated with mantle exhumation in a distal, hot passive margin environment: New constraints from the Saraillé Massif (Chaînons Béarnais, North-Pyrenean Zone)[END_REF][START_REF] Teixell | The crustal evolution of the west-central Pyrenees revisited: Inferences from a new kinematic scenario[END_REF]Fig. 3). Rifting reconstructions differ from crustal thinning models, salt tectonics and synrift filling geometries. The first models describe this basin as a classical rift, thinned by basinward-dipping, crustal, normal faults, responsible for the development of tilted blocks on the southern margin of the rift [START_REF] Boirie | Les poudingues de Mendibelza: dépôts de cônes sousmarins du rift albien des Pyrénées[END_REF]Fixari, 1984;[START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF][START_REF] Canérot | Tectonique et sédimentation sur la marge nord-ibérique des chaînons béarnais (Pyrénées-béarnaises). Remise en question de la signification des lherzolites du sommet de Saraillé[END_REF][START_REF] Canérot | Manifestations de l'halocinèse dans les chaînons béarnais (zone Nord-Pyrénéenne) au Crétacé inférieur[END_REF][START_REF] Ducasse | Géotraverse dans la partie occidentale des Pyrénées, de l'avant-pays aquitain au bassin de l'Ebre: effet d'une inversion structurale sur l'édification d'une chaîne intracontinentale[END_REF][START_REF] Souquet | Evolución del margen noribérico en los Pyreneos durante el Mesozoico[END_REF]. In recent models of crustal hyper-thinning processes, sub-continental mantle exhumation is interpreted as being related to northward-dipping detachment faults in the distal part of the basin [START_REF] Jammes | Tectono-sedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees: Tectonics[END_REF][START_REF] Tugend | Formation and deformation of hyperextended rift systems: Insights from rift domain mapping in the Bay of Biscay-Pyrenees: Tectonics[END_REF]Tugend et al. , 2015a;;[START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF]Fig 3A). According to these models, the southern margin of the Mauléon Basin corresponds to the "lower plate" and the northern Grand-Rieu Ridge to the "upper plate" (sensu Wernicke, 1985;[START_REF] Lister | Detachment faulting and the evolution of passive continental margins[END_REF]. The Ursuya granulites are also considered as lower to middle crust exhumed during this Albian extensional stage. Two extensional fault systems, the north and south Mauléon detachments, could be responsible for the southward tilting of the Mendibelza and Arbailles Units [START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF]). An alternative model suggests that the Albian thinning of the continental crust was caused by a process of crustal boudinage and led to the creation of a symmetric basin and sliding of the Mesozoic cover from the margins towards the basin axis on each margin [START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies: Tectonics[END_REF][START_REF] Corre | Deformation associated with mantle exhumation in a distal, hot passive margin environment: New constraints from the Saraillé Massif (Chaînons Béarnais, North-Pyrenean Zone)[END_REF][START_REF] Teixell | The crustal evolution of the west-central Pyrenees revisited: Inferences from a new kinematic scenario[END_REF] Fig. 3B).

Our working approach was to reconstruct a N-S cross-section of the present-day Mauléon Basin, using a field approach on the southern margin and seismic data to illustrate the northern one. The Albian reconstitution of this cross-section allows to consider the tectonosedimentary evolution of the basin, focusing on the Albian-Cenomanian rifting phase. The

The southern margin of the Mauléon Basin from field observations

Field work has enabled updating of the 1/ 50 000 BRGM geological maps. The latter image the Iberian margin of the Mauléon Basin: (1) Mendibelza Unit; and (2) Arbailles Unit (Fig. 4A). The southern margin of the Mauléon Basin was deformed during the Eocene alpine compression along several southward-verging thrusts. The most significant of these, known as the Lakhoura Thrust, is responsible for the under-thrusting of the Axial Zone beneath the North-Pyrenean units (Fig. 4). Five structural units bounded by tectonic contacts have been defined from south to north on this inverted margin (Fig. 4B): ( 1

The Axial Zone

The Palaeozoic basement of the Axial Zone is stratigraphically covered by a shallow marine carbonate sequence, Cenomanian to Santonian in age, known as the "Calcaires des Cañons" [START_REF] Souquet | Le Crétacé supérieur Sud-Pyrénéen, en Catalogne[END_REF]Fig. 6A). This carbonate platform succession is overlain by deeper Late Santonian to Maastrichtian marine sediments composed of argillaceous mudstone ("calcshists") containing planktonic foraminifera, grading upward and laterally to siliciclastic deposits. This deep marine depositional setting continues at least until the Early Lutetian in this domain. The Paleozoic rocks are not greatly deformed near the contact with the Late Cretaceous carbonates (Ehujarré and Kakoueta areas).

The Saint-Engrâce -Bedous Unit

This unit has extensive eastward continuity (Aspe Valley), justifying its name Bedous. The Saint-Engrâce-Bedous Unit observed in the Larrau tectonic window is a discontinuous thin band of Triassic rocks delimited by two thrusts and squeezed between the Axial Zone to the south and the Mendibelza -Igountze Unit to the north [START_REF] Ducasse | Géotraverse dans la partie occidentale des Pyrénées, de l'avant-pays aquitain au bassin de l'Ebre: effet d'une inversion structurale sur l'édification d'une chaîne intracontinentale[END_REF][START_REF] Teixell | Coupe géologique du massif d'Igountze: implications sur l'évolution structurale de la bordure sud de la zone nord-pyrénéenne occidentale[END_REF]. The Saint-Engrâce-Bedous Unit is composed of the Middle Triassic Carbonate Unit and the Upper Triassic shale, evaporite and ophite complex. These Triassic rocks are affected by EW-trending folds indicating a southward displacement of the Lakhoura Thrust system.

The Mendibelza-Igountze Unit: contacts between the Paleozoic basement and the synrift deposits

The Mendibelza-Igountze Unit is composed of Albian deep-marine synrift deposits onlapping the Paleozoic sedimentary basement. The Mendibelza synrift conglomeratic gravity flow deposits have already been studied, by the following authors [START_REF] Fournier | Études géologiques sur la partie occidentale de la chaîne des Pyrénées entre la vallée d'Aspe et celle de la Nive[END_REF][START_REF] Fournier | Etudes sur les Pyrénées Basques:(Basses-Pyrénées, Navarre et Guipuzcoa)[END_REF][START_REF] Lamare | La série paléozoïque du massif du Baygoura et de la vallée de la Nive; ses relations avec les terrains secondaires environnants[END_REF][START_REF] Lamare | Les formations détritiques crétacées du massif de Mendibelza[END_REF][START_REF] Lamare | Sur le passage latéral des faciès détritiques grossiers du Crétacé du massif de Mendibelza aux faciès schisto-gréseux classiques de l'Albien des Pyrénées[END_REF][START_REF] Gubler | Sur l'âge des poudingues dits de Mendibelza dans le bassin du Laurhibar, au SE de Mendive (Basse Pyrénées[END_REF][START_REF] Magné | Nouvelles observations relatives à l'âge des poudingues du pic Errozaté (massif de Mendibelza)[END_REF][START_REF] Viers | Observations structurales sur les Pyrénées occidentales[END_REF][START_REF] Paris | Étude d'une partie du massif d'Igounce et de ses abords septentrionaux en Barétous et Basse-Soule[END_REF][START_REF] Poignant | Révision du Crétacé inférieur en Aquitaine occidentale et méridionale[END_REF][START_REF] Galharague | Étude géologique de la zone de relais des massifs d'Igounce et de Mendibelza (Basses-Pyrénées[END_REF][START_REF] Merle | Recherches sur les relations paléogéographiques et structurales entre les massifs basques au sud de Saint-Jean-Pied-de-Port (Pyrénées occidentales[END_REF][START_REF] Boirie | Etude Sédimentologique des Poudingues de Mendibelza (Pyrénées Atlantiques)[END_REF]Fixari 1984;[START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF]. The Palaeozoic substratum of the Mendibelza Formation (Fm.) is composed of Late Devonian to Carboniferous sedimentary rocks (Fig. 5). This substratum is exposed at two highs: the Esterençuby high to the north and the Occabe high to the south. The Albian conglomeratic Mendibelza Fm. is very thick (2,000 m; Fig. 6A) at the northern block, much thinner at the southern one. The pebbles in this conglomerates are made up of Paleozoic sedimentary basement and Early Albian carbonate platform in the Igountze area ("Floridées limestones").

In this area, the base of this conglomeratic sequence is composed of discontinuous calciturbidites reworking the Early Albian carbonate platform and the Paleozoic basement. In the Mendibelza area, the base of this formation is characterized by discontinuous base-of-slope sedimentary breccias reworking the Lower Triassic Sandstone Unit (Figs. 7A andB). These discontinuous Igountze calci-turbidites and Mendibelza breccias materialized the base of the Mendibelza Formation, that is overlied by three southward back-stepping mega-sequences defined by [START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF] First rift models described the Mendibelza-Igountze Unit as turbiditic proximal cones installed on the southward tilted blocks affecting the Iberian margin of the Mauléon Albian Basin [START_REF] Boirie | Les poudingues de Mendibelza: dépôts de cônes sousmarins du rift albien des Pyrénées[END_REF]Fixari, 1984;[START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF][START_REF] Canérot | Tectonique et sédimentation sur la marge nord-ibérique des chaînons béarnais (Pyrénées-béarnaises). Remise en question de la signification des lherzolites du sommet de Saraillé[END_REF][START_REF] Canérot | Manifestations de l'halocinèse dans les chaînons béarnais (zone Nord-Pyrénéenne) au Crétacé inférieur[END_REF][START_REF] Ducasse | Géotraverse dans la partie occidentale des Pyrénées, de l'avant-pays aquitain au bassin de l'Ebre: effet d'une inversion structurale sur l'édification d'une chaîne intracontinentale[END_REF][START_REF] Souquet | Evolución del margen noribérico en los Pyreneos durante el Mesozoico[END_REF]. However, in the light of recent understanding of crustal hyper-thinning processes, [START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF] have interpreted the southward tilting of the Mendibelza-Arbailles Unit as resulting from the development of a northward detachment fault. To evaluate these different hypotheses, a detailed field analysis was conducted to determine the nature of the substratum / cover contacts and the synrift deposit geometry. All the contacts observed in the field between the substratum and the synrift deposits are primary contacts that were not reactivated during the Pyrenean orogeny.

Two kinds of contacts have been distinguished: (1) depositional surfaces; and (2) syn-sedimentary faults. All these surfaces have been corrected for post-depositional tilting in order to reconstruct their geometries at the time of the syn-rift deposition (Fig. 8).

Albian depositional surfaces

The depositional surfaces (yellow and blue lines in Fig. 5) are in all cases characterized by an angular unconformity with the Albian synrift strata. This unconformity is between 15-40° (Fig. 8). Our facies analysis of the Mendibelza Fm. confirms that this unit consists of conglomeratic gravity-flow deposits [START_REF] Boirie | Etude Sédimentologique des Poudingues de Mendibelza (Pyrénées Atlantiques)[END_REF][START_REF] Boirie | Les poudingues de Mendibelza: dépôts de cônes sousmarins du rift albien des Pyrénées[END_REF]Fixari, 1984;[START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF]. The composition, the very coarse granulometry and the roundness of the clasts suggest that this base-of-slope gravity system was fed by non-preserved shelf-edge fandeltas, reworking the Paleozoic meta-sedimentary units in an uplifted southern area located in the present day Axial Zone. In this context, these deep marine conglomerates are considered to have been deposited in a close to horizontal position. In addition, the Mendibelza Fm. is currently stacked with an average regional dip of N120°-30 to 40°SW, without any wedgeshaped syn-tectonic growth strata. These characteristics make it possible to reconstruct the inclination of the depositional surfaces of the synrift albian deposits on the Palaeozoic substratum (Fig. 8). The surfaces are generally tilted from 15° to 40° towards the NE to NNE, with an average value of 30°, once the Mendibelza Fm. strata have been reseated in the original horizontal position. This shows that the Albian substratum of the proximal part of the Mauléon Basin was inclined towards the basin axis at the time of the synrift Mendibelza Fm. deposition. Locally, the Esterençuby high is characterized by an albian surface inclined at 30° to 50° towards the east.

Albian synsedimentary vertical normal faults

Two fault systems have been identified: (1) the N110°-45°NNE North Occabe Fault (NOF, in orange in Figs. 5 and8); and (2) the N20° vertical normal Esterençuby faults (in purple in Fig. 5). These faults have an angular relationship of between 75° and 90° with the strata of Mendibelza conglomerates. Reconstruction of these faults indicates an original orientation and dip of N120°-75 to 85° NE for the NOF and an unchanged N20° vertical for the Esterençuby faults. The Early-Middle Albian megasequences 1 and 2 of the Black Flysch [START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF] were deposited during the activity of the NOF which is sealed by the upper part of megasequence 2 (Fig. 5). The stratal geometries and geological mapping show that the activity of the N20° Esterençuby fault system was contemporary with the NOF, i.e. during the Early-Middle Albian.

Megasequence 3 onlapped the Occabe high during the Late Albian. The occurrence of chaotic gravity flow deposits from the Late Albian to Santonian, on the Occabe domain implies the presence of a tectonically controlled scarp in between the emerged to shallow marine Axial Zone and the deep marine Mendibelza domain. However this fault scarp is now overprinted by the Lakhoura Thrust. Therefore, these faults are diachronous and increasingly younger towards the southern margin of the Mauléon Basin, since the North Occabe Fault is sealed by Middle Albian deposits. These observations are in agreement with the southward backstepping of the Albian deposits [START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF].

The Palaeozoic rocks are affected locally by low temperature weak deformations (Fig. 7C).

But several of these cataclasites are sealed by Permian sediments indicating a Late-Hercynian age for this deformation (Fig. 5). These observations do not therefore confirm the occurrence of cataclasites punctuating a detachment fault between the Hercynian basement and the Albian synrift deposits, as proposed by [START_REF] Jammes | Tectono-sedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees: Tectonics[END_REF], [START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF].

The Mendibelza Unit: preliminary interpretations Most previous publications describe the Mendibelza Unit as an Early Albian southward tilted block, characterized by a stratigraphic or tectonic contact between the substratum and the Albian Mendibelza conglomerates [START_REF] Ducasse | Glissement de couverture et panneaux basculés dans la région des Arbailles (Pyrénées occidentales): Un modèle évolutif crétacé de la marge nord-ibérique à l'Est de la transformante de Pamplona[END_REF][START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF][START_REF] Ducasse | Géotraverse dans la partie occidentale des Pyrénées, de l'avant-pays aquitain au bassin de l'Ebre: effet d'une inversion structurale sur l'édification d'une chaîne intracontinentale[END_REF][START_REF] Souquet | Evolución del margen noribérico en los Pyreneos durante el Mesozoico[END_REF][START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF]. However, the present study, based on field analysis and structural reconstruction, does not support these interpretations. Quite the opposite in fact: it shows an absence of wedge-shaped syn-tectonic growth strata in the Albian synrift sequence and depositional surface on the hercynian substratum inclined from 15 to 40° towards the NE. The hercynian substratum of the Mendibelza Unit dips towards the Mauléon Basin axis. The tilt of this unit towards the NE is marked by the development of diachronous N120° normal faults that are increasingly younger towards the south. The present day tilting of the Albian sequence, about 45° towards the south, postdates the Mendibelza Formation.

The Arbailles Unit: tectonic and stratigraphic framework

The contact between the Arbailles and the Mendibelza Units corresponds currently to a vertical fault. Due to the overturned and northward-dipping position of the Albian sequence of the Mendibelza Unit to the south, this fault is interpreted as a verticalized southward-directed thrust fault called the Arbailles Thrust (Ducasse and Vélasque, 1988, Fig. 4). This thrust is locally retro-thrusting towards the north, indicating a later northward displacement of the Mendibelza Unit responsible for the verticalization of the previous Arbailles Thrust [START_REF] Dumont | Microseismicity of the Béarn range: Reactivation of inversion and collision structures at the northern edge of the Iberian plate: SEISMICITY AND STRUCTURES IN W. PYRENEES[END_REF]. The Arbailles Unit is bounded northward by a set of northward-dipping to subvertical normal faults, known as the North Arbailles Fault (NAF, Fig. 9). These N120° faults result in a juxtaposition of Barremian -Aptian shallow-marine carbonate deposits to the south, and deeper marine marl-dominated Albian deposits to the north. This set of faults disappears eastward. In fact, the Arbailles Unit is overlapped eastward by the Albian-Cenomanian Tardets Black Flysch group deposits [START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF]. Westward, the Arbailles Unit is bounded by the Saint-Jean-Pied-de-Port Upper Triassic Unit.

The Arbailles Unit consists of a carboniferous basement, a Late Triassic sequence and a Mesozoic cover (Figs. 4 and9). In this sector, the Late Triassic shales and evaporites overlie the Lower Triassic sandstone unit, without any Middle Triassic carbonate Unit. The Upper Triassic Unit is widely exposed on the south-west and consists of ophites and numerous cargneules indicating a wide zone of deformation. There is a classical Jurassic sequence above the Upper Triassic Unit [START_REF] Lenoble | Les plates-formes carbonatées ouest-pyrénéennes du dogger à l'Albien, stratigraphie séquentielle et évolution géodynamique[END_REF]. This Jurassic sequence is limited at the top by an erosional surface marked by greater and greater truncation towards the south-west, explaining the lack of most of the Late Jurassic in the southern part of the Arbailles Unit (Fig. 6A). This unconformity is linked to the Late Jurassic -Early Cretaceous emersion phase, associated regionally with the development of bauxites [START_REF] Combes | Altérites et bauxites, témoins des marges européenne et ibérique des Pyrénées occidentales au Jurassique supérieur-Crétacé inférieur, à l'ouest de la vallée d'Ossau (Pyrénées-Atlantiques, France)[END_REF][START_REF] James | La plate-forme carbonatée ouest-pyrénéenne au jurassique moyen et supérieur stratigraphie séquentielle, stades d'évolution, relations avec la subsurface en aquitaine méridionale[END_REF][START_REF] Canérot | Le cadre stratigraphique et géodynamique des altérites et des bauxites sur la marge ibérique des Pyrénées occidentales (France)[END_REF]. The geometry of this surface reflects the tilt of the Jurassic deposits towards the north during this period. This emersion extends as far as the base of the Barremian, as evidenced by the lack of Neocomian deposits in the Arbailles Unit (Fig. 6A).

The Barremian to earliest Albian carbonate platform deposits onlap the Jurassic carbonate platform towards the south-west (Fig. 6A). This onlapping is highlighted by the southward thinning of the Barremian deposits and the interbedding of transgressive conglomerates including pebbles reworked from the pre-existing Kimmeridgian carbonates (Fig. 7D). The Aptian carbonates (Urgonian facies) overlying the Barremian to the north backstep towards the south and onlap the eroded Jurassic sequence directly in this direction. The thickness of the Aptian deposits increases northward without significant facies change, indicating a northward increase in subsidence (towards basin axis, Figs. 4, 6A and9).

From Late Aptian to earliest Albian, algal carbonates ("Mélobésiées" limestone) onlap the eroded Jurassic sequence in the southern part of the Arbailles Unit (Fig. 7E). Intraformational breccias (Fig. 7F) attest to the presence at that time of an unstable depositional setting and probably a slight inclination of the depositional profile. This is confirmed by the northward change in facies from shallow marine carbonates to deeper marine spicule marls with intercalations of some carbonate debrites towards the north (Fig. 7G andH). This Late Aptian-Early Albian sequence is affected by two kinds of progressive unconformity: (1) a generalized and progressive northward thickening of this sequence at the scale of the Arbailles Unit, linked to a northward increase in the subsidence rate; and (2) a growth strata unconformity of shorter wavelength that could result from diapiric movements along the southern edge of the Arbailles Unit [START_REF] Canérot | Manifestations de l'halocinèse dans les chaînons béarnais (zone Nord-Pyrénéenne) au Crétacé inférieur[END_REF][START_REF] Canérot | Rifting éocrétacé et halocinèse sur la marge ibérique des Pyrénées Occidentales (France)[END_REF][START_REF] Canérot | Les Pyrénées: Histoire géologique: Atlantica[END_REF][START_REF] Canérot | Diapirisme crétacé sur la marge ibérique des Pyrénées occidentales; exemple du pic de Lauriolle; comparaisons avec l'Aquitaine, les Pyrénées centrales et orientales[END_REF].

The Saint-Palais Unit

The Saint-Palais Unit is located between the Arbailles Unit and the Saint-Palais Thrust. The The seismic line interpreted is a merger of lines MT104, MT112 and 83HBS02 (acquired by SNEAP in 1981 and reprocessed by BRGM in 2014; Fig. 10). The seismic interpretation was carried out using the Geographix® Discovery Suite that allows for each CDP of the seismic profile to export its X, Y, time, amplitude and depth values. Seismic/well tie is obtained by using the time / depth curves acquired in the boreholes and derived from the recording of checkshots. The time / depth conversion was performed using a mean velocity field calculated on the basis of all the time / depth data of the calibration wells, integrating their deviations (Fig. 11). The top basement has been calibrated taking into account the ECORS seismic reflection profile [START_REF] Daignières | The Arzacq-western Pyrenees ECORS deep seismic profile[END_REF]. The composite line presented in this paper is calibrated with six wells (Fig. 6), from south to north: Ainhice-1 (3,540. Suzanne thrusts involved the European basement. Using the field cross-section (Fig. 4B) and well-velocity models calibrating the interpreted composite seismic reflection profile (Fig. 10), we propose a depth migrated section of the entire Mauléon Basin (Fig. 11).

The Saint-Palais Unit

The Palaeozoic substratum below the Saint-Palais Unit is tilted towards the north, being at a depth of around 2,900 m in the Ainhice-1 well (Fig. 6) and of 7,000 m in the central part of the basin. The base of this well is characterized by Paleozoic basement, overlain by the thick Upper Triassic salt Unit materializing a major decoupling level between the basement and the sedimentary cover. This Triassic salt unit is grading upward to two Mesozoic cover units characterized by a normal polarity and separated by a tectonic contact. The Lower Unit, U1, consists of a complete Jurassic sequence, up to the Kimmeridgian, covered by Late Aptian Urgonian limestones. Unit U1 is characterized by a hiatus of the Neocomian to the Early Aptian. The Upper Unit, U2, is composed of the same Jurassic but the Kimmeridgian marls are thinner than in U1, grading upward to Earliest Albian carbonate platform deposits (Fig. 6A). These cover units are overlain by Middle to Late Albian spicule marls and siliciclastic turbidites. The Albian basinal deposits thicken towards the north and reach their maximum thickness in the Saint-Palais Anticline, clearly identified in the field. The Saint-Palais domain corresponds to the depocenter of the Albian basin, with an estimated thickness of up to 4,000-5,000 m [START_REF] Roux | Recherches stratigraphiques et sédimentologiques sur les flyschs crétacés pyrénéens au sud d'Oloron[END_REF]Fixari, 1984;[START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF]. The upper part of the Saint-Palais Unit is formed by the Upper Cretaceous carbonate-dominated flysch, which reaches a thickness of more than 1,500 m in the Uhart Mixe well.

The Bellevue Unit

The Bellevue Unit is located between the Saint-Palais and the Bellevue Thrusts. The Bellevue Thrust is responsible for the offset of the Jurassic-Cretaceous cover and the displacement of the Bellevue Unit onto the Sainte Suzanne Unit. The Paleozoic basement is located at a depth of around 8,000 m under the Bellevue Unit. This unit is composed of a complex structure whose most obvious feature is the Bellevue Anticline affecting the Mesozoic cover intersected in the Bellevue well. The northward Bellevue Thrust is marked by a 1,800 m thick tectonized zone composed of Late Triassic salt and Barremian carbonate slices. These Barremian units could be tectonically incorporated to the Triassic salt forming the overturned flank of the overlying anticline evidenced in the Bellevue well (Fig. 6). In fact, this tectonized interval is overlain by an overturned Jurassic carbonate unit composed of Dogger to Kimmeridgian deposits. Above, the Bellevue Unit is made up of a complete Jurassic to Late

The Sainte-Suzanne Unit

The Sainte-Suzanne Thrust corresponds to the major north verging thrust responsible for a displacement of more than 17 km of the Mauléon Basin deposits onto the south Aquitain Grand Rieu Unit (Fig. 10). It can be considered as the North-Pyrenean Frontal Thrust in this part of the Pyrenees. The Sainte Suzanne Unit is made up of a very thick Jurassic to Campanian sequence, slightly inclined and thickening towards the south. It has been crosscut by the Orthez-102 well where the Sainte Suzanne Thrust has been identified at a depth of 4,320 m (Fig. 6). The Jurassic is made up of a complete sequence of marine carbonate platform deposits from Hettangian to Kimmeridgian, truncated under the Early Cretaceous deposits, as evidenced by the north-directed erosional truncations. The 1 000m thick Early Cretaceous sequence is characterized by a lack of Neocomian. The Barremian is characterized by shallow marine carbonate deposits with annelids showing that this unit is more proximal than the Bellevue Unit. The Aptian is sub-isopachous and is composed of shallow marine carbonate platform deposits (Urgonian facies). The Albian deposits are also made up of carbonate platform deposits thickening towards the south. These facies are therefore more proximal than the Albian deposits of the Bellevue Unit. A clear truncation of the Albian reflections is seen towards the north, below the Late Cretaceous calcareous flysch. This latter calci-turbidite sequence is thinning and onlapping to the north on top of the eroded Albian sequence.

The Grand-Rieu/Arzacq Unit

The Grand-Rieu -Arzacq Unit is the footwall of the Sainte Suzanne Thrust and corresponds to the southern part of the Aquitain Basin. From the seismic data, the Paleozoic basement below the Sainte Suzanne Thrust can be estimated to be at a depth of 8,500 m. It is assumed to

Top Jurassic to Cenomanian tectono sedimentary and geodynamic evolution of the Mauléon Basin Latest Jurassic to Neocomian uplift phase

The Jurassic is characterized by an extensive and relatively continuous carbonate platform spreading between the southern Mauléon Basin margin (Arbailles Unit) and the Arzacq Basin (Fig. 12A). At its top, the platform is limited by a major unconformity resulting from an emersive and erosional phase dated from the latest Jurassic to Barremian (Fig. 12B). This event is evidenced by the development of bauxites and the lack of Neocomian deposits both in the Pyrenees and the Arzacq Basin [START_REF] Combes | Altérites et bauxites, témoins des marges européenne et ibérique des Pyrénées occidentales au Jurassique supérieur-Crétacé inférieur, à l'ouest de la vallée d'Ossau (Pyrénées-Atlantiques, France)[END_REF][START_REF] James | La plate-forme carbonatée ouest-pyrénéenne au jurassique moyen et supérieur stratigraphie séquentielle, stades d'évolution, relations avec la subsurface en aquitaine méridionale[END_REF][START_REF] Canérot | Le cadre stratigraphique et géodynamique des altérites et des bauxites sur la marge ibérique des Pyrénées occidentales (France)[END_REF].

Before the Early Cretaceous transgression, the Jurassic platform is characterized by a synform morphology extending from the southern margin of the Mauléon Basin to the Arzacq Basin. It is revealed respectively by the south-directed and north-directed erosional truncations affecting the Jurassic in the Arbailles and the Arzacq Units (Fig. 11). This deformation stage implies the uplift of these domains, corresponding partly to the Early Cretaceous Mauléon Basin edges; while in the meantime, subsidence stops in the synform axis. Mechanism responsible for the uplift of the entire Pyrenean-Aquitaine domain might be related to an asthenosphere upwelling preceding the Early Cretaceous rifting.

Barremian to earliest Albian rifting stage 1: symmetric basin

After the Late Jurassic -Early Cretaceous exposure/weathering stage, onset of transgression leads to the development of an extensive restricted carbonate platform during the Barremian.

From Barremian to Aptian time, the balance between carbonate production and creation of available space favors the aggradation of 1 600m thick shallow marine carbonate facies, witnessing a highly subsiding stage controlling a relative-flat depositional profile (Figs. 6 and 12C). This homogeneous subsidence stage evolves during Late Aptian to the earliest Albian. The basin's edges were characterized by the back-stepping of the shallow carbonate platform, while the Mauléon and Arzacq Basins center are composed of deeper spicule marls. In this domain, creation of available space exceeded the carbonate production, resulting in the relocating of the carbonate factory on the basin's edges. Paleogeography reorganization was thus controlled by the differential subsidence regime responsible for a steeper depositional profile.

As a consequence, during the Barremian -earliest Albian time the Mauléon Basin was a symmetric basin, characterized by shallow marine carbonate platforms on each basin edges.

The absence of brittle deformation within the upper-crust excludes the application of the pureshear model developed by [START_REF] Mckenzie | Some remarks on the development of sedimentary basins[END_REF] to explain the subsidence regime of this basin. A symmetric synrift sag basin with no brittle deformation in the upper crust has been proposed from numerical modeling [START_REF] Karner | Syn-rift regional subsidence across the West African continental margin: the role of lower plate ductile extension[END_REF]. In this model, the subsidence of the basin results from lower crustal ductile thinning favored by decoupling between the upper and lower crusts. We propose that the initiation of the Mauléon sag basin results from thinning of the lower crust, without any major deformation of the upper crust. The isostatic response to the lower crustal thinning induces regional sagging and onlapping of the syn-thinning deposits on each side of the basin. The rifting stage 1 can be interpreted as a "ductile crustal pure-shear thinning phase".

Albian rifting stage 2: asymmetric basin

At Albian time, the southern margin of the Mauléon Basin was affected by differential vertical movements: (1) uplift of the southern part of the basin; and (2) tectonic subsidence towards the north (Fig. 12E). They are responsible for the onset of the Mendibelza deep gravity flow deposits passing distally to the Black Flysch group towards the Saint-Palais depocenter (Fig. 12E). At this time, the Iberian margin substratum was characterized by a northward dipping surface with an average slope angle of 15-30° (Fig. 8). This surface was affected by syn-sedimentary albian N120° southward-propagating high-angle normal faults, controlling the destabilization of the fan-delta system into the deep basin (Fig. 12E). These faults were sealed by younger synrift back-stepping deposits of the Mendibelza Fm. towards the south.

The conglomerates directly overlain the Paleozoic substratum of the Mendibelza Unit, indicating the entire Mesozoic lacks (Fig. 5), while it was continuous throughout the Pyrenean-Aquitaine domain. Thus, the Mesozoic cover of this unit need to be subtracted prior to the sedimentation of the Albian synrift Mendibelza sequence. It leads to the development of an emerged Axial Zone during the rifting stage 2. The sliding level consists of the thick Upper Triassic shale, evaporite and ophite complex systematically brecciated and / or deformed under the prerift cover of the Iberian margin. This mass-sliding is recorded within the Ainhice-1 well Mesozoic cover unit overlap (Units U1 and U2, Fig. 6). The stratigraphic differences between these two Mesozoic cover units, suggest an origin of the upper one from a southern marginal part of the basin where the Jurassic-Cretaceous hiatus is greatest. Thus, the Early Albian Axial Zone denudation is linked to the northward mass sliding of the Mesozoic cover towards the Mauléon Basin axis in response to the Iberian margin northward tilting (Fig. 12D).

At the same period, the European margin of the Mauléon Basin recorded less vertical movements and tectonic activity than the southern margin. Unlike the Iberian conjugate margin, this northern domain is not subject to gravitational sliding of the Mesozoic cover. Indeed, no regional uplift and associated coarse-grained alluvial-derived gravity-flow deposits are evidenced in the Arzacq, Sainte-Suzanne and Bellevue Units. The Albian period is mainly represented by a shallow marine carbonate platform recognized in the Arzacq and Sainte-Suzanne Units (Figs. 6 and13). The slightly southward-dipping depositional profile is responsible for a facies transition between shallow-marine algae limestones and deeper spicule marls in the Sainte Suzanne Unit. In Saint Palais and Bellevue Units, the Jurassic-Aptian sedimentary cover is significantly normally offset, which allows the accumulation of thick deep Albian facies to the south of the Saint-Palais Thrust, suggesting that this latter corresponds to a southward dipping Albian normal fault (Figs. 11 and 13). This fault is part of the northern Mauléon Albian Basin's margin. The thick interval with very deformed Triassic material intersected at the base of the Bellevue well is interpreted as an Albian diapiric ridge developing on the northern edge of the Albian Mauléon Basin (Fig. 6).

Comparison between the southern and northern margins shows that the Mauléon Basin acquired asymmetry during the Early Albian time: (1) deep basin turbiditic deposits on the Iberian margin and (2) carbonate platform on the European one. According to most of the models that consider a simple-shear deformation process to explain the asymmetry of a rift (Wernicke, 1985;[START_REF] Lister | Detachment faulting and the evolution of passive continental margins[END_REF], the Mauléon Basin's asymmetry is interpreted as resulting from the activation of the southward dipping Saint-Palais Detachment during the Early Albian (Fig. 13). Following [START_REF] Lister | Detachment faulting and the evolution of passive continental margins[END_REF]

Late Cretaceous rifting stage 3: apparent symmetric basin

During the Late Cretaceous period, a carbonate platform developed on both edges of the Mauléon Basin, and the deep basin became wider (Figs. 6 and13). Indeed, the width of the Mauléon deep basin, measured from platform to platform, was around 100 km wide. The Grand-Rieu Ridge represents a major palaeogeographic boundary between the carbonate platform of the European margin and the center of the Mauléon Basin [START_REF] Razin | Evolution tecto-sédimentaire alpine des Pyrénées basques à l'ouest de la transformante de Pamplona[END_REF][START_REF] Serrano | Le bassin d'Aquitaine: valorisation des données sismiques, cartographie structurale et potentiel pétrolier[END_REF]. The transition between the carbonate platform and the basinal carbonate turbidites is governed by a southward dipping normal fault between the Sainte Suzanne Unit and the Arzacq Basin. The calcareous flysch deposited in the Mauléon Basin was derived mainly by gravity flows coming from the northern Aquitain carbonate platform. These calcareous turbidites onlap the previous Albian deposits towards the Sainte-Suzanne Unit to the north and the Saint-Palais to Mendibelza Units to the south (Fig. 13). The development of the Grand-Rieu Anticline attests of salt movement at Early Cenomanian time.

On the southern margin the commonly named Late Cretaceous "Calcaires des Cañons" carbonate platform deposits lie unconformably over the previously exposed Palaeozoic basement of the Axial Zone (Fig. 13). Towards the north, in the Mendibelza Unit, base-ofslope chaotic breccias attest of a normal fault-controlled platform-basin transition between the southern carbonate platform developing on the Axial Zone and the Mauléon Basin (Fig. 13).

Several arguments indicate that a second and opposite detachment occurred at Early Cenomanian time: the northward verging Lakhoura Detachment (Fig. 13). This synsedimentary fault, currently inverted as the Lakhoura Thrust (Fig. 5), is interpreted as being responsible for the southward tilting of the Mendibelza/Arbailles Units. The Early Cenomanian age of this tilting is attested by: (1) the absence of wedge-shaped syn-tectonic growth strata in the Albian synrift sequence of the Mendibelza Unit; (2) the 98 Ma cooling age obtained on the [START_REF] Hart | Thermal evolution of a hyperextended rift basin, Mauléon Basin, western Pyrenees: Thermal evolution of hyperextended rift: Tectonics[END_REF], responsible for the southward tilt of the Lower Triassic Sandstone Unit in the Saint-Jean-Pied-de-Port area [START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF]; and (3) the onlap of the Late Cretaceous deposits on the tilted and eroded Albian sequence in the north of the Arbailles Unit [START_REF] Casteras | Carte géologique de la France au 1/ 50 000[END_REF][START_REF] Ducasse | Géotraverse dans la partie occidentale des Pyrénées, de l'avant-pays aquitain au bassin de l'Ebre: effet d'une inversion structurale sur l'édification d'une chaîne intracontinentale[END_REF]Fig. 9). This observation implies that the northern edge of the Arbailles Unit becomes a structural high in the basin during Early Cenomanian. This structural high is fossilized at Santonian time as highlighted by the age of the first deposits passing over it.

Discussion and Conclusion

The northwestern Pyrenees present a strong positive gravity anomaly under the Mauléon rift Basin [START_REF] Grandjean | Etude des structures crustales dans une portion de chaîne et de leur relation avec les bassins sédimentaires. Application aux Pyrénées occidentales[END_REF][START_REF] Casas | Gravity anomaly map of the Pyrenean region and a comparison of the deep geological structure of the western and eastern Pyrenees[END_REF] resulting from the presence at low depth (~10 km) of lithospheric mantle [START_REF] Wang | The deep roots of the western Pyrenees revealed by full waveform inversion of teleseismic P waves[END_REF]. This high mantle body position implies the existence of a very thin continental crust arguing for a hyper-thinning Cretaceous rifting stage. In this work, we assume that the Saint-Palais and Lakhoura structures affect the lithospheric mantle. They are responsible for the hyper-thinning of the Mauléon Basin continental crust. However, this rift basin have not underwent oceanic spreading as classical basins along the Atlantic Ocean [START_REF] Manatschal | New models for evolution of magma-poor rifted margins based on a review of data and concepts from West Iberia and the Alps[END_REF][START_REF] Péron-Pinvidic | Tectonosedimentary evolution of the deep Iberia-Newfoundland margins: Evidence for a complex breakup history: Tectonics[END_REF][START_REF] Haupert | Upper-plate magma-poor rifted margins: Stratigraphic architecture and structural evolution[END_REF], or in the Alps fossil margins [START_REF] Manatschal | What is the tectono-metamorphic evolution of continental break-up: The example of the Tasna Ocean-Continent Transition[END_REF][START_REF] Masini | The tectonosedimentary evolution of a supra-detachment rift basin at a deep-water magma-poor rifted margin: the example of the Samedan Basin preserved in the Err nappe in SE Switzerland: Tectono-sedimentary evolution of a supra-detachment rift basin[END_REF], Mohn et al., 2014[START_REF] Decarlis | The tectono-stratigraphic evolution of distal, hyper-extended magma-poor conjugate rifted margins: Examples from the Alpine Tethys and Newfoundland-Iberia[END_REF]. Another significant difference with these classical hyperextended magma poor margins is the thickness of the syn-thinning deposits in the hyperextended domain (~ 8 km thick in Mauléon Basin vs. less than 1 km thick on the Atlantic and Alps margins). The thick synrift deposits record a complex polyphasic thinning history from Barremian to Early Cenomanian time.

During stage 1, the Mauléon Basin is symmetric as evidenced by the development of a carbonate platform on each basin edges. The carbonate production counterbalances the high accommodation, resulting in a near flat sedimentary profile. We interpret the creation of accommodation as a result of ductile thinning of the lower crust, without affecting the uppercrust (Fig. 14A). This thinning stage shares similarity with the crustal scale boudinage process, previously proposed at Albian time [START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies: Tectonics[END_REF][START_REF] Corre | Deformation associated with mantle exhumation in a distal, hot passive margin environment: New constraints from the Saraillé Massif (Chaînons Béarnais, North-Pyrenean Zone)[END_REF][START_REF] Teixell | The crustal evolution of the west-central Pyrenees revisited: Inferences from a new kinematic scenario[END_REF]; Fig. 3B). However, only the lower crust is affected by ductile thinning, favored by the presence of a decoupling level in between the lower and upper crusts. The resulting synrift sag basin morphology is quite similar to the Mesozoic Columbrets sag Basin, which does not 29 present any evidence of deep basin gravitary sedimentary infill (Etheve, 2016 andEtheve et al, 2018).

At Albian time (stage 2), the rift fabric totally changed. The basin was affected by "simpleshear thinning" due to the development of the southward dipping Saint-Palais Detachment (Fig. 14B). This crustal thinning phase affected the entire lithosphere, forming an asymmetric rift basin, as previously proposed in the simple-shear models [START_REF] Jammes | Tectono-sedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees: Tectonics[END_REF]Tugend et al, 2014Tugend et al, , 2015a;;[START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF], Fig. 3A). However, unlike the previous models, the Albian rifting stage initiates with simple shear thinning, without distributed extension in the upper crust (pure shear thinning sensus [START_REF] Mckenzie | Some remarks on the development of sedimentary basins[END_REF]. [START_REF] Lavier | A mechanism to thin the continental lithosphere at magma-poor margins[END_REF] have evidenced using numerical modeling that the continental crust was already thin (~20km) when simple shear thinning starts. In the case of the Mauléon Basin we propose that the Early Cretaceous stage favors the ductile thinning of the lower crust before beginning the Albian-Cenomanian simple shear thinning stage.

The Saint-Palais detachment is responsible for the northward Iberian margin roll-over. This interpretation strongly contrasts with previous models arguing for an Albian southward tilt of the Iberian margin along a northward dipping normal fault [START_REF] Boirie | Etude Sédimentologique des Poudingues de Mendibelza (Pyrénées Atlantiques)[END_REF]Fixari., 1984;[START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF][START_REF] Ducasse | Géotraverse dans la partie occidentale des Pyrénées, de l'avant-pays aquitain au bassin de l'Ebre: effet d'une inversion structurale sur l'édification d'une chaîne intracontinentale[END_REF] or crustal scale detachments [START_REF] Jammes | Tectono-sedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees: Tectonics[END_REF][START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF]. In this scheme, we propose that rift asymmetry controls the asymmetric sedimentary profile. Actually, the Iberian margin (roll-over) is composed of a proximal turbiditic s.l. system whereas the European one (southward gently-dipping) is characterized by a carbonate platform. Similarly, to the Columbrets Basin (Etheve., 2016 andEtheve et al., 2018), the onset of simple shear thinning is synchronous with the basinward prerift cover-sliding along the Late Triassic salt décollement. Such Early Albian gravity tectonics have already been reported all along the Iberian rift margin [START_REF] Bouquet | ) : Stratigraphie-Sédimentologie-Structure-Implications Géodynamiques: Pau[END_REF][START_REF] Ducasse | Glissement de couverture et panneaux basculés dans la région des Arbailles (Pyrénées occidentales): Un modèle évolutif crétacé de la marge nord-ibérique à l'Est de la transformante de Pamplona[END_REF][START_REF] Ducasse | Géotraverse dans la partie occidentale des Pyrénées, de l'avant-pays aquitain au bassin de l'Ebre: effet d'une inversion structurale sur l'édification d'une chaîne intracontinentale[END_REF][START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies: Tectonics[END_REF][START_REF] Corre | Deformation associated with mantle exhumation in a distal, hot passive margin environment: New constraints from the Saraillé Massif (Chaînons Béarnais, North-Pyrenean Zone)[END_REF][START_REF] Teixell | The crustal evolution of the west-central Pyrenees revisited: Inferences from a new kinematic scenario[END_REF]; Fig. 3B). However, one of the major discrepancies with the previous models is the absence of gravitational cover sliding along the European rift margin (Fig. 3B). The sliding of the Mesozoic cover led to local diapirism along the Iberian margin, as shown by the lateral facies variations from shallow carbonate platform to spicule marls around the diapiric structures [START_REF] Canérot | Manifestations de l'halocinèse dans les chaînons béarnais (zone Nord-Pyrénéenne) au Crétacé inférieur[END_REF][START_REF] Canérot | Rifting éocrétacé et halocinèse sur la marge ibérique des Pyrénées Occidentales (France)[END_REF][START_REF] Canérot | Diapirisme crétacé sur la marge ibérique des Pyrénées occidentales; exemple du pic de Lauriolle; comparaisons avec l'Aquitaine, les Pyrénées centrales et orientales[END_REF][START_REF] James | Diapirisme et structuration post-triasique des Pyrénées occidentales et de l'Aquitaine méridionale[END_REF][START_REF] Canérot | Mesozoic diapirism in the Pyrenean orogen: Salt tectonics on a transform plate boundary[END_REF].

The paleogeography of the Early Cretaceous Mauléon Basin, derived from the sedimentary record is not compatible with a major strike-slip drift of the Iberian plate as proposed in kinematic reconstructions [START_REF] Le Pichon | Une hypothèse d'évolution tectonique du golfe de Gascogne[END_REF][START_REF] Roest | Kinematics of the plate boundaries between Eurasia, Iberia ,and Africa in the North Atlantic from the Late Cretaceous to the present[END_REF][START_REF] Olivet | La cinématique de la plaque ibérique: Bull[END_REF][START_REF] Rosenbaum | Relative motions of Africa, Iberia and Europe during Alpine orogeny[END_REF][START_REF] Sibuet | Pyrenean orogeny and plate kinematics[END_REF][START_REF] Gong | The rotation of Iberia during the Aptian and the opening of the Bay of Biscay[END_REF]. We assume that the Iberia sinistral strike-slip motion did occur on a more southward structure [START_REF] Malod | Comparaison de l'évolution des marges continentales au nord et au sud de la péninsule ibérique[END_REF][START_REF] Canérot | The Iberian Plate: myth or reality?[END_REF] developed earlier during Late Jurassic (Tugend et al., 2015b) and / or may have been evenly distributed within the Cretaceous rift system. At Early Cenomanian time (final stage 3), the southward tilting of the Mendibelza-Arbailles Units, -formerly considered as Early Albian [START_REF] Boirie | Etude Sédimentologique des Poudingues de Mendibelza (Pyrénées Atlantiques)[END_REF][START_REF] Boirie | Les poudingues de Mendibelza: dépôts de cônes sousmarins du rift albien des Pyrénées[END_REF]Fixari, 1984;[START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF][START_REF] Ducasse | Géotraverse dans la partie occidentale des Pyrénées, de l'avant-pays aquitain au bassin de l'Ebre: effet d'une inversion structurale sur l'édification d'une chaîne intracontinentale[END_REF][START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF] Albian and Cenomanian time can be interpreted as "flip-flop detachment tectonic" [START_REF] Sauter | Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years[END_REF][START_REF] Geoffroy | Flip-flop detachment tectonics at nascent passive margins in SE Afar[END_REF][START_REF] Gillard | Tectonomagmatic evolution of the final stages of rifting along the deep conjugate Australian-Antarctic magma-poor rifted margins: Constraints from seismic observations: Australian-Antarctic margins evolution: Tectonics[END_REF], highlighting the diachronous shift from upper-plate to lower-plate morphology along a same margin section. This double successive asymmetry is responsible for the apparent final symmetry of this hyper-thinned rift. This work reconciliates the previous rift models, i.e. symmetric [START_REF] Souquet | Evolución del margen noribérico en los Pyreneos durante el Mesozoico[END_REF][START_REF] Ducasse | Géotraverse dans la partie occidentale des Pyrénées, de l'avant-pays aquitain au bassin de l'Ebre: effet d'une inversion structurale sur l'édification d'une chaîne intracontinentale[END_REF], asymmetric [START_REF] Jammes | Tectono-sedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees: Tectonics[END_REF][START_REF] Masini | The tectonosedimentary evolution of a hyper-extended rift basin: the example of the Arzacq-Mauléon rift system[END_REF]Tugend et al., 2015a, Fig. 3A) or mixed [START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies: Tectonics[END_REF][START_REF] Corre | Deformation associated with mantle exhumation in a distal, hot passive margin environment: New constraints from the Saraillé Massif (Chaînons Béarnais, North-Pyrenean Zone)[END_REF]Teixell et al., 2016, Fig. 3B). The polyphase thinning history is responsible for the hyper-thinning of the continental crust and the development of a relatively narrow hyper-thinned rift domain (Fig. 14C).

In three dimensions, the Mauléon Basin appears relatively complex and not cylindrical. On our synthetic 2D transect (Fig. 14C), the mantle is systematically overlain by a hyper-thinned crust whereas eastward (Urdach), it has been denuded and partly reworked into synrift deposits [START_REF] Roux | Recherches stratigraphiques et sédimentologiques sur les flyschs crétacés pyrénéens au sud d'Oloron[END_REF][START_REF] Duée | Les lherzolites associées aux Chaînons Béarnais (Pyrénées Occidentales): Mise à l'affleurement anté-dogger et resédimentation albo-cénomanienne: Comptes-rendus des séances de l'Académie des sciences[END_REF][START_REF] Fortané | Lherzolites and the western "Chaînons Béarnais[END_REF]Debroas et al., 2010;[START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies: Tectonics[END_REF][START_REF] Teixell | The crustal evolution of the west-central Pyrenees revisited: Inferences from a new kinematic scenario[END_REF]. We consider the Saint-Palais Detachment as responsible for the hyper-thinning of the continental crust, while the Lakhoura one might be responsible for partial mantle denudation. This is consistent with the Urdach mantle/crust contact that display top north-east shear direction [START_REF] Corre | La bordure nord de la plaque ibérique à l'Albo-Cénomanien[END_REF]. Thus, it appears that the basin morphology changes along strike, due to N0°-20° transverse structures such as the Pamplona [START_REF] Richard | Structure et évolution alpine des massifs paléozoïques du Labourd (Pays Basque français[END_REF][START_REF] Razin | Evolution tecto-sédimentaire alpine des Pyrénées basques à l'ouest de la transformante de Pamplona[END_REF][START_REF] Claude | Etude stratigraphique, sédimentologique et structurale des dépôts mésozoïques au nord du massif du Labourd: rôle de la faille de Pamplona (Pays Basque[END_REF][START_REF] Larrasoaña | Paleomagnetic, structural, and stratigraphic constraints on transverse fault kinematics during basin inversion: The Pamplona Fault[END_REF][START_REF] Pedreira | Three-dimensional gravity and magnetic modeling of crustal indentation and wedging in the western Pyrenees-Cantabrian Mountains[END_REF][START_REF] Díaz | Mapping the indentation between the Iberian and Eurasian plates beneath the Western Pyrenees/Eastern Cantabrian Mountains from receiver function analysis[END_REF], the Saint-Jean-Pied-de-Port, Saison, Barlanès and Ossau structures [START_REF] Canérot | Les Pyrénées: Histoire géologique: Atlantica[END_REF][START_REF] Canérot | The pull apart-type Tardets-Mauléon Basin, a key to understand the formation of the Pyrenees[END_REF]Debroas et al., 2010). This N0°-20° structuration seems to clearly control the 3D geometry of the Saint-Palais hyper-extended rift domain, and will be the purposed of future investigations.

Figures and Captions:

Figure. 5

Geological map of the Mendibelza Unit, modified from the 1/50 000 BRGM geological maps [START_REF] Casteras | Carte géologique de la France au 1/50 000[END_REF][START_REF] Casteras | Carte géologique de la France au 1/ 50 000[END_REF][START_REF] Pochat | Carte Géologique de France au 1/50 000[END_REF]. The Mendibelza Formation is composed of three main synrift mega-sequences, younger and younger towards the south, defined by [START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF]. The stereographic representations show the Albian orientation of the contact between the Mendibelza conglomerates strata and the sedimentary basement. The Mendibelza Unit is separated from the Arbailles Unit by the steep Arbailles fault and is overthrust towards the south by the Lakhoura Thrust. The North Occabe Fault (NOF) is fossilized by the Albian Megasequences 1 and 2, indicating an Early to Middle Albian age for this fault.

Figure. 9

Geological map of the Arbailles Unit and the southern part of the Saint-Palais Unit, using updated BRGM 1/50 000 maps [START_REF] Casteras | Carte géologique de la France au 1/ 50 000[END_REF][START_REF] Boissonnas | Carte géologique de la France au 1/50000; feuille d'Iholdy[END_REF][START_REF] Pochat | Carte géologique de France au 1/50 000[END_REF][START_REF] Mckenzie | Some remarks on the development of sedimentary basins[END_REF]

  first part of this paper focuses on a description of the southern margin of the Mauléon rift basin based on outcrop data. The second part focuses on the deep geometry of the Mauléon Basin, using the interpretation of a N-S reprocessed composite seismic reflection profile. This line shows the deep geometry of the conjugate rift margins. The third part of the paper presents a new N-S composite crustal section of the entire Mauléon Basin, and a reconstitution at Cenomanian time. We propose a new model of crustal thinning highlighting the individualization of the Iberian and European conjugate rift margins.

  ) The Axial Zone, formed by the Paleozoic basement and its Late Cretaceous cover (over-thrust by the northern units along the Lakhoura Thrust); (2) The Saint-Engrâce/Bedous Unit, composed of a complex assemblage of Triassic rocks thrusted onto the Axial Zone to the south; (3) The Mendibelza-Igountze Unit, including Albian deep-marine synrift deposits onlapping the Paleozoic sedimentary basement (Fig. 5); (4) The Arbailles Unit, represented by carboniferous rocks overlain by a continental to shallow marine Triassic to earliest Albian sequence; and (5) the Saint-Palais Unit formed, essentially, of a thick Albian to Late Cretaceous turbiditic sequence overlying a deformed Jurassic to Early Cretaceous Carbonate unit.

  Mendibelza conglomerates are overlain by Cenomanian to Santonian polymictic chaotic breccias reworking the "Calcaires des Cañons" carbonate platform of the Axial Zone and angular blocks of the Paleozoic sedimentary basement. These deposits attest a base-of-slope palaeogeographic boundary between the Axial Zone and the Mendibelza-Igountze Unit during the Late Cretaceous. Along the N-S geological cross-section (Fig. 4), the Mendibelza-Igountze Unit is overthrust southward by the Lakhoura Thrust, onto the Axial Zone and the Saint-Engrâce Unit. The Mendibelza-Igountze Unit is also thrusted towards the west, onto the Aldudes Unit. The Cretaceous to Paleocene synrift and post-rift strata are affected by the Lakhoura Thrust, as shown by their vertical to overturned position along the front of the Lakhoura Thrust on the southern limit of the Mendibelza Unit, indicating that this thrust is post Cretaceous.

  Saint-Palais Unit formed the Albian -Late Cretaceous depocenter of the Mauléon Basin. At outcrop scale, this unit is folded into N120° synclines / anticlines of pluri-kilometric wavelength, composed of Early Albian spicule marls and Middle Cenomanian -Turonian carbonate distal turbidites. The northern part of this unit is affected by the N110-120° overturned Saint-Palais anticline. Field work confirms that the distal Late Cretaceous carbonate turbidites are sourced from the north by the European carbonate platform, as shown by the N150-200° directed paleocurents measured in the field. These Late Cretaceous carbonate deposits overlaid the Albian sequence unconformably. This is shown by the southward distal onlap of the Late Cretaceous deposits on the Albian synrift sequence, on the southern edge of the Saint-Palais Unit.4. Seismic interpretation of the Mauléon Basin and the southern part of theAquitain BasinUnlike the southern margin that has been observed directly in the field (Mendibelza and Arbailles Units), the architecture of the central part and the northern conjugate margin of the This section focuses on the deep geometry of the Mauléon Basin from the interpretation of a N-S composite seismic reflection profile, reprocessed by the BRGM in 2017 (MT104 and MT112 acquired by ESSO-Rep in 1969 on the Mauléon-Tardets exploration permit, cf 4.2).

  85 m depth); Uhart-Mixe-1 (1,868.8 m depth); Bellevue-1 (6,909 m depth); Orthez-102 (5,489.10 m depth); Amou-1 (5,543 m depth); and Bastennes-Gaujacq-1.bis (4,442.10 m depth). Only the Ainhice-1 well reaches the Paleozoic basement of the Mauléon Basin. The studied SSW-NNE seismic line through the Mauléon Basin cuts across four distinct structural units, separated from one another by thrust faults (Fig. 10). From south to north, the units are as follows: (1) Saint-Palais; (2) Bellevue; (3) Sainte-Suzanne; and (4) Grand-Rieu/Arzacq (Figs. 6 and 10). The Saint-Palais Unit corresponds to the central and deepest part of Mauléon Basin, while the Bellevue and Sainte Suzanne Units belong to its northern margin. The Bellevue and Sainte

  Cretaceous sequence in a normal position. The Early cretaceous sequence is characterized by a lack of Neocomian. The Barremian to Aptian carbonate platform unit lying unconformably on the Portlandian is more than 1,500 m thick. The Barremian deposits are composed of limestones with annelids and 500 m thick of more distal Sainte-Suzanne marls. The Aptian is mainly composed of Urgonian limestones. The Albian thins towards the North and is represented by spicule marls including intercalations of carbonate mudstone indicating the proximity of the northern Aquitanian carbonate platform. The Late Cretaceous sequence is made up of Cenomanian to Coniacian carbonate turbidites.

  be overlain by a more or less continuous Triassic Unit. The inferred Liassic Unit present in the Aquitain Basin to the north appears to pinch-out southwards against a diapiric structure associated with a normal fault. This Mesozoic tectonic structure marks the boundary between the Grand-Rieu Ridge to the South and the commonly named Arzacq Basin corresponding to a highly subsiding domain during Albian times. The overlying Middle to Late Jurassic carbonates are more continuous and isopachous. The top of the Jurassic sequence corresponds to a major exposure surface marked by north-directed erosional truncations under the Barremian transgressive carbonate deposits, as indicated in the Amou well. This unconformity is time-equivalent to the one describe in the Arbailles Unit on the southern margin of the Mauléon Basin. During the latest Jurassic and the Neocomian period, the northern margin of the Mauléon Basin was tilted towards the south, unlike the southern margin that was tilted towards the north. The Barremian to Aptian sequence of the Grand-Rieu Unit is relatively isopachous and consists mainly of shallow marine carbonate platform deposits. The Albian deposits form a thick unit of fairly deep marine spicule marls indicating a strong subsidence rate of the Arzacq Basin, as indicated in the Amou and Bastennes-Gaujacq (Fig.6). A large wave-length anticline affects the Jurassic -Albian sequence at the transition between theGrand-RieuRidge and the Arzacq Basin (Grand-Rieu Anticline). A major truncation of the folded strata below the Cenomanian carbonate platform deposits attests to the Late-Albian to Early Cenomanian age of this deformation. In the Grand-Rieu -Arzacq Unit the Late Cretaceous is represented by sub-isopachous carbonate platform deposits. During the Tertiary, this domain was characterized by a very high subsidence rate, increasing towards the South, as attested by the very thick Tertiary sequence at the front of the Sainte Suzanne Thrust. Local southward dipping clinoforms can be observed south of the Bastennes-Gaujacq well, probably related to the growth of this diapiric structure. The very great thickness of the Tertiary sequence attests to the strong flexural subsidence of the Grand-Rieu -Arzacq Unit but also to the large displacement and uplift of the Mauléon Basin along the Sainte-Suzanne Thrust, since no Tertiary deposits have been preserved in the Mauléon Basin (even if they were deposited over this entire flexure domain). Most of the Tertiary flexural subsidence occurs before the Late Lutetian, as shown by the thickening of the Danian to Lower Ypresian deposits towards the south.

  the Albian Iberian margin corresponds to the upper plate and the European one to the lower plate. The Saint-Palais Detachment might be responsible for the northward Iberian margin tilting, this former being part of a major crustal scale roll-over. The roll-over structure is accommodated by N120° oriented and steeply dipping normal faults that propagate towards the south (i.e. NAF, NOF). They are antithetic to the Saint-Palais structure. During Albian time, the width of the Mauléon deep basin, measured from platform to platform, was around 50 km.

  -is induced by the change of detachment vergence and the development of the Lakhoura northward dipping Detachment. The Arbailles Unit thus becomes a structural high bordering the southern hyperthinned Mauléon Basin domain. Consequently, we assume that the Arbailles Unit becomes a tilted block at Early Cenomanian time. Therefore, the tectono-sedimentary evolution of the Mauléon rift Basin during the stages 2 and 3 is defined by the development of two antithetic and diachronous detachments, i.e. the Albian southward dipping Saint-Palais detachment during Stage 2 and the Early Cenomanian northward dipping Lakhoura one during Stage 3. The change in detachment vergence between

Figure. 1 (

 1 Figure. 1

  . The Arbailles Unit is separated from the Saint-Palais Unit by N120° normal faults known as the North Arbailles Fault (NAF). These maps highlight the southward erosion of the Jurassic cover and the southward back stepping of the Barremian to Aptian carbonate platform, onlapping the previous eroded Jurassic deposits. In the North of the Arbailles Unit, the Late Cretaceous deposits onlapped the Albian rift sequence towards the south from Cenomanian to Santonian time. 67 Interpretation of a N-S petroleum reflection composite seismic line, reprocessed by the BRGM in 2017 (MT104, MT112 and 83HBS02). This composite seismic line shows the deep geometry of the Mauléon Basin and its European margin. This seismic line through the Mauléon Basin cuts across four distinct structural units, separated from one another by thrusting overlaps. From south to north, the units are as follows: (1) Saint-Palais, (2) Bellevue, (3) Sainte Suzanne, and (4) Grand-Rieu/Arzacq. The Saint-Palais Unit corresponds to the Albian rift depocenter. The Bellevue and Sainte Suzanne Units materialized the northern conjugate margin of the Albian Cretaceous Mauléon rift basin, and the Barremian-Aptian depocenter. emersive phase was responsible for erosion of the previous carbonate platform, as shown by the southward erosional truncations affecting the Jurassic under the Early Cretaceous deposits. (C) The Early Cretaceous is characterized by a lack of Neocomian and the development of a flat carbonate platform during Barremian-Early Aptian time. During Late Aptian time, the previous carbonate platform distalized towards the north into more distal spicule marls, showing a slight tilt of the depositional profile. (D) At Early Albian time, the southern Mauléon margin was affected by the gravitational sliding of the Mesozoic cover towards the north leading to local diapirism. (E) The Albian time was characterized by differential vertical movements responsible for the uplift of the southern margin and tectonic subsidence towards the north. This stage is materialized by the sedimentation of the Mendibelza Fm. proximal turbiditic s.l. siliciclastic system, onlapping towards the south a Paleozoic basement inclined towards the north. The northward tilt of the Iberian margin is affected by southward propagating N120° normal faults. (F) At Cenomanian time, the Mendibelza-Arbailles Unit was affected by a tilt towards the south.

apparent symmetric rift basin. St-P: Saint-Palais, Lak: Lakhoura.

Acknowledgements

This study was conducted within the framework of the integrated geological Orogen research project (TOTAL, BRGM, and INSU). We are very grateful to the reviewers and to the editor, who contributed to the improvement of this manuscript. We would like to thank the Orogen project managers Sylvain Calassou (Total), Emmanuel Masini (Total), Olivier Vidal (CNRS) and French geological survey -BRGM. We also thank Joseph Canérot for the numerous scientific discussions on the tectono-sedimentary evolution of the Mendibelza and Arbailles Units. The authors thank Geoffrey Bird for the English review.

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