Michel Seranne
email: michel.seranne@umontpellier.fr

Renaud Couëffé

Michel Séranne

Céline Baral

Justine Villard

Eglantine Husson

The transition from Pyrenean shortening to Gulf of Lion rifting in Languedoc (South France) -A tectonic sedimentation analysis

Keywords: Pyrenees, Gulf of Lion, strike-slip basins, syn-depositional deformation, Priabonian

The pyrenean orogen extended eastward, across the present-day Gulf of Lion margin. The late or post-orogenic dismantling of this orogen segment, contemporaneous with ongoing shortening in the Spanish Pyrenees, is still debated. Understanding the transition between the two geodynamic events requires to document the precise timing of the succession of the tectonic processes involved. We investigate the superposition of rifting structures over pyrenean thrusts and folds in the onshore Languedoc. Compilation and reassessment of the regional chronostratigraphy, in the light of recent biostratigraphic dating and new mapping of Paleogene basins, lead to date the transition to the Priabonian. Tectonic-sedimentation relationship in the Eocene to Oligocene depocentres are analysed in surface exposures as well as in seismic reflection surveys. Bed-to bed mapping allowed us to: i) characterise an intermediate sequence of Priabonian age, bounded at the base and the top by unconformities; ii) evidence syn-depositional deformation within the Priabonian; iii) define the axes of Priabonian deformation. Interpretation of seismic reflection profiles, across the onshore basins covered by syn-and post-rift sequences, reveal the existence of an intermediate sequence displaying similar features, and that is correlated to the Priabonian. Syn-depositional deformation of some Priabonian basins correspond to extensional structure, whereas neighbouring, contemporaneous basins, reveal compressional deformation. The distribution of such apparently conflicting observations across the studied area provides evidence for leftlateral strike-slip deformation between two major regional faults (Cévennes and Nîmes faults). Left-lateral strike-slip along NE-trending faults accommodates E-W extension of the West European Rift (ECRIS) and part of the ongoing N-S shortening in the Central and Western Pyrenees. Priabonian clastic sedimentation and deformation in Languedoc witness the initial stages of the dismantling of the Languedoc-Provence Pyrenees, prior to Oligocene-Aquitanian back-arc rifting.

du rifting sur les structures compressives Pyrénéennes exposées à terre, dans le Languedoc. Tout d'abord, nous compilons et révisons la chronostratigraphie régionale, à la lumière de datations postérieures à la publication des cartes au 1/50.000-du BRGM, et de la cartographie de plusieurs bassins paléogènes. Nous datons cette transition du Priabonien. Nous analysons les relations tectonique-sédimentation des dépocentres Eocène à Oligocène sur le terrain et en sismique réflexion. La cartographie banc-par-banc permet de : i) caractériser une séquence intermédiaire d'âge Priabonien encadrée par deux discordances, ii) mettre en évidence une déformation syn-dépôt dans le remplissage Priabonien, iii) définir les axes de la déformation priabonienne. L'interprétation de profils de sismique réflexion à travers les bassins paléogènes à terre, recouverts par les séries syn-et post-rift, révèle l'existence d'une séquence intermédiaire présentant des caractéristiques similaires et que nous corrélons avec le Priabonien. La déformation syn-dépôt de certains bassins est extensive, alors que dans des bassins voisins contemporains, elle est compressive. La distribution régionale de ces observations apparemment contradictoires est interprétée comme liée à un couloir de déformation en décrochement senestre entre les failles majeures des Cévennes et de Nîmes, d'orientation NE-SW. Ce décrochement accommode l'extension EW des rifts ouest-européens et contribue au raccourcissement N-S des Pyrénées franco-espagnoles, toujours actif au Priabonien. La sédimentation syn-tectonique clastique du Priabonien du Languedoc enregistre les premiers stades du démantèlement de la chaine pyrénéo-provençale, plusieurs millions d'années avant l'initiation de l'extension d'arrière-arc.

Mots-clé:

Pyrénées, Golfe du Lion, bassins sur décrochement, déformation syn-sédimentaire, Priabonien,

Introduction

The Pyrenean orogen results from the inversion of Mesozoic extensional basins, mostly formed during Mid-Cretaceous rifting and mantle denudation [START_REF] Chelalou | New sedimentological, structural and paleo-thermicity data in the Boucheville Basin (eastern North Pyre-nean Zone, France)[END_REF][START_REF] Cochelin | Structural inheritance in the Central Pyrenees: theVariscan to Alpine tectonometamorphic evolution of the Axial Zone[END_REF][START_REF] Lagabrielle | Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites[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] Ternois | Thermochronological Evidence of Early Orogenesis[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] Vergés | Mesozoic evolution and Cainozoic invertion of the Pyrenean Rift[END_REF]. The orogenic shortening was accommodated in the upper crust by reactivation of the inherited Cretaceous extensional faults, as thrust faults [START_REF] Debroas | Le flysch noir albo-cénomanien temoin de la structuration albienne à sénonienne de la Zone nord-pyreneenne en Bigorre (Hautes-Pyrenees, France)[END_REF], while the middle crust and exhumed mantle were subducted beneath the European crust [START_REF] Teixell | Crustal structure and evolution of the Pyrenean-Cantabrian belt: A review and new interpretations from recent concepts and data[END_REF][START_REF] Teixell | The crustal evolution of the west-central Pyrenees revisited: Inferences from a new kinematic scenario[END_REF]. The orogen extended eastwards [START_REF] Ford | Evolution of a salt-rich transtensional rifted margin[END_REF], in Corbières [START_REF] Lamotte | Early record of tectonic magnetic fabric during inversion of a sedimentary basin Short review and examples from the Corbières transfer zone (France)[END_REF][START_REF] Viallard | Un modèle de charriage épiglyptique: la nappe des Corbières orientales (Aude, France)[END_REF], Languedoc [START_REF] Arthaud | Contraintes, déformation et déplacement dans l'avant-pays Nord-pyrénéen du Languedoc méditerranéen[END_REF] and southern Provence [START_REF] Bestani | Structural style and evolution of the Pyrenean-Provence thrust belt[END_REF][START_REF] Lacombe | Structural and kinematic relationships between Corsica and the Pyrenees-Provence domain at the time of the Pyrenean orogeny[END_REF], across the present-day Gulf of Lion (Arthaud and Séguret, 1981;[START_REF] Gorini | Structure of the Gulf of Lions (Northwest Mediterranean Sea): A review[END_REF]. It was later dismantled in Oligocene time to give way to the Gulf of Lion rifted margin (Arthaud et al., 1981;Arthaud and Séguret, 1981;[START_REF] Gorini | Structure of the Gulf of Lions (Northwest Mediterranean Sea): A review[END_REF][START_REF] Guennoc | Histoire géologique du Golfe du Lion et cartographie du rift oligoaquitanien et de la surface messinienne[END_REF][START_REF] Lacombe | Structural and kinematic relationships between Corsica and the Pyrenees-Provence domain at the time of the Pyrenean orogeny[END_REF][START_REF] Mascle | The petroleum systems of the South-East Basin and Gulf of Lions (France)[END_REF][START_REF] Mauffret | Structural style and geodynamic evolution of Camargue and Western Provençal basin, southeastern France[END_REF][START_REF] Séranne | The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview[END_REF][START_REF] Séranne | Structural style and evolution of the Gulf of Lion Oligo-Miocene rifting : Role of the Pyrenean orogeny[END_REF]. In the onshore Corbières area, upper crust extension is characterised by "negative inversion" of the Pyrenean thrusts, associated with deposition of Oligocene-Aquitanian syn-rift sediments in the hanging-wall [START_REF] Gorini | Modèle d'inversion négative : la tectonique extensive post-nappe du fossé de Narbonne-Sigean (Corbières, sud de la France)[END_REF]. The crustal roots of the Pyrenees disappear eastwards [START_REF] Chevrot | The non-cylindrical crustal architecture of the Pyrenees[END_REF] and the orogen brutally gives way to the Gulf of Lion rift and margin, within only tens of kilometres [START_REF] Jolivet | Fast dismantling of a mountain belt by mantle flow: late-orogenic evolution of Pyrenees and Liguro-Provençal rifting[END_REF][START_REF] Mauffret | Structural geometry in the eastern Pyrenees and western Gulf of Lion (Western Mediterranean)[END_REF]. Although documented both onshore and offshore, tectonics and kinematics of the superimposition of Pyrenean structures by later rifting (Fig. 1) remains incompletely understood. Transition of the compression to extension could be driven by collapse of the elevated mountain range [START_REF] Séranne | Structural style and evolution of the Gulf of Lion Oligo-Miocene rifting : Role of the Pyrenean orogeny[END_REF], it could be related to the southern propagation of the European Cenozoic Rift System (ECRIS) [START_REF] Dèzes | Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere[END_REF], or to the onset of back-arc extension due to the NW-dipping subduction roll-back of the old Tethyan lithosphere [START_REF] Jolivet | Fast dismantling of a mountain belt by mantle flow: late-orogenic evolution of Pyrenees and Liguro-Provençal rifting[END_REF][START_REF] Séranne | The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview[END_REF]. Better constraints on tectonics and precise timing of the transition from the Pyrenean orogeny to the Gulf of Lion rifting are needed in order to better understand the origin and driving mechanism of the transition between the two major geodynamics events. Such data are also needed to constrain paleotectonic restorations (e.g. [START_REF] Bestani | Structural style and evolution of the Pyrenean-Provence thrust belt[END_REF][START_REF] Christophoul | Tectono-stratigraphic evolution of the northeastern Pyrenean Foreland[END_REF], and kinematic reconstructions of the Mediterranean (e.g. [START_REF] Romagny | Detailed tectonic reconstructions of the Western Mediterranean region for the last 35 Ma, insights on driving mechanisms[END_REF].

Since most of the collapsed orogen is presently buried beneath the thick Neogene sequence of the Gulf of Lion margin, the onshore Languedoc provides the only geological outcrops that are relevant to address this question. There, the rare available stratigraphic data suggested that the Pyrenean orogeny lasted until "early Oligocene", as reported on the Saint-Martin-de-Londres 1/50.000-scale geological map [START_REF] Philip | Carte Géologique de la France au 1/50 000[END_REF], while rifting took place in the late Rupelian according to the Montpellier map [START_REF] Andrieux | [END_REF]. Consequences of such a short delay implies: i) that Pyrenean compression was polyphase, including a last compressive phase, following a Late Eocene period of tectonic quiescence, and ii) rifting interrupted folding and thrusting, with no transition. [START_REF] Séranne | Structural style and evolution of the Gulf of Lion Oligo-Miocene rifting : Role of the Pyrenean orogeny[END_REF]. Na: Narbonne, Pe: Perpignan, Mo: Montpellier, Ma: Marseille. Directions of Pyrenean shortening from [START_REF] Arthaud | Contraintes, déformation et déplacement dans l'avant-pays Nord-pyrénéen du Languedoc méditerranéen[END_REF], [START_REF] Gaviglio | Fracturation et histoire tectonique du bassin de Gardanne (Bouches-du-Rhone)[END_REF][START_REF] Lacombe | Determining paleostress orientations from faults and calcite twins: a case study near the Sainte-Victoire Range (southern France)[END_REF][START_REF] Lamotte | Early record of tectonic magnetic fabric during inversion of a sedimentary basin Short review and examples from the Corbières transfer zone (France)[END_REF] and Oligocene extension from : [START_REF] Arthaud | Cadre tectonique de quelques bassins sédimentaires[END_REF][START_REF] Benedicto | Modèles tectono-sédimentaires de bassins en extension et style structural de la marge passive du Golfe du Lion (Partie Nord), sud-est de la France[END_REF][START_REF] Hippolyte | Tectonic-stratigraphic record of paleostress time changes in the Oligocene basins of the Provence[END_REF].

In this paper, we examine five Paleogene basins located in Languedoc, which developed at the front of the Eocene external thrusts of the Pyrenean belt and onshore of the Gulf of Lion passive margin. The main objective of this contribution is to document, thanks to the revision of available biostratigraphic data and to detailed sedimentological, tectonic and seismic analyses, the key period of the transition between the last stages of Pyrenean orogenic (mountain building) events and the earliest stages of rifting associated with the opening of the Gulf of Lion. We aim at constraining the tectonic setting associated with the deposition of Priabonian continental units by analysing the tectonic-sedimentation relationships in 5 distinctive basins (Fig. 2). Analyses are based on field observations (Guzargues, Les-Matelles, Saint-Martin-de-Londres basins) and subsurface data (Hérault and Saint-Chaptes Gardon basins). This study also relies on: i) new original investigation in the Guzargues basin, ii)

Regional geological setting

2-1 Location of the studied area within the Pyrenean orogen

Languedoc, in the south of France, is located between the Variscan basement of the Massif Central and the shores of the Mediterranean Sea; it links the Pyrenees (in the SW) to the Alps (to the NE), across the Rhône Valley (Fig. 1). Together with the Corbières, Languedoc forms a segment of the north foreland of the Pyreneo-Provençal fold and thrust belt (Arthaud and Séguret, 1981;[START_REF] Bestani | Structural style and evolution of the Pyrenean-Provence thrust belt[END_REF].

The Paleozoic (Variscan) basement is exposed in the Massif Central to the NW, in the Maure and Esterel massifs to the east. To the south, the basement has been reactivated during the Late Cretaceous to Eocene Pyrenean orogeny. It is exposed in the Pyrenees and buried beneath the thick Neogene sequences of the Gulf of Lion margin (Arthaud and Séguret, 1981;[START_REF] Gorini | Structure of the Gulf of Lions (Northwest Mediterranean Sea): A review[END_REF][START_REF] Guennoc | Histoire géologique du Golfe du Lion et cartographie du rift oligoaquitanien et de la surface messinienne[END_REF] (Fig. 1). Onshore, the Variscan basement is unconformably covered by Mesozoic sequence, including at the base variable amounts of Triassic detritals and evaporites [START_REF] Debrand-Passard | Synthèse géologique du Sud-Est de la France -Stratigraphie et paléogéographie[END_REF], which have been used as decollement level during later tectonic phases. The Jurassic through to Neocomian marine marls and carbonates were deposited during the Tethyan rifting and subsequent thermal subsidence, controlled by NE-trending normal faults such as the Cévennes, Nîmes and Durance faults. Thickness of the Jurassic increases southeastwards from the Cévennes fault (less than 1,5km), and maximum subsidence is recorded in the hanging-wall of the Nîmes Fault, where seismic profiles show up to 10km of Trias to Neocomian sequences [START_REF] Le Pichon | Geodynamics of the France Southeast Basin[END_REF][START_REF] Séguret | Structure profonde de la région du Gard Rhodanien -Apport du retraitement et de la réinterprétation de données sismiques régionales[END_REF]. This Mesozoic sedimentary cover has been folded and thrusted in the Pyrenean foreland, detached over Triassic decollement. Thick-skinned thrusting presumably occurred close to the present day shoreline (Arthaud and Séguret, 1981;[START_REF] Lacombe | Structural and kinematic relationships between Corsica and the Pyrenees-Provence domain at the time of the Pyrenean orogeny[END_REF][START_REF] Séranne | Structural style and evolution of the Gulf of Lion Oligo-Miocene rifting : Role of the Pyrenean orogeny[END_REF], The style of the thrust and fold belt is mostly controlled by the thickness of the detached Mesozoic sedimentary cover [START_REF] Arthaud | Contraintes, déformation et déplacement dans l'avant-pays Nord-pyrénéen du Languedoc méditerranéen[END_REF]: north-verging, EW-oriented thrusts dominate areas of thin cover (≤ 2,5 to 3 km) whereas thicker cover developed series of E-W folds. Combined thin-and thick-skinned shortening exceed 20km in the Languedoc [START_REF] Arthaud | Contraintes, déformation et déplacement dans l'avant-pays Nord-pyrénéen du Languedoc méditerranéen[END_REF] and 40km in the Provence segment of the orogen [START_REF] Bestani | Reconstruction of the Provence Chain evolution[END_REF][START_REF] Bestani | Structural style and evolution of the Pyrenean-Provence thrust belt[END_REF]. The Pyrenees-Provence fold and thrust belt is segmented by NE-to NNEtrending faults which separate areas of distinct structural styles and amounts of shortening.

Left-lateral strike-slip along the Cévennes Fault accommodated shortening of its hanging-wall while the NW footwall remained mostly undeformed during the Pyrenean orogeny [START_REF] Arthaud | Contraintes, déformation et déplacement dans l'avant-pays Nord-pyrénéen du Languedoc méditerranéen[END_REF]. In Provence, the Durance Fault separates thin-and thick-skinned Pyrenean deformation [START_REF] Bestani | Géométrie et cinématique de l'avant-pays provençal : Modélisation par coupes équilibrées dans une zone à tectonique polyphasée[END_REF][START_REF] Bestani | Reconstruction of the Provence Chain evolution[END_REF].

2-2. Location of the studied area within the Gulf of Lion rift and passive margin

The studied area is affected by NE-trending normal faults bounding syn-rift Oligocene basins (Fig. 2). At least, some of these faults -if not all -are reactivated previous Pyrenean left-lateral strike-slips faults (i.e. the Cévennes fault, Matelles Fault [START_REF] Mattauer | La faille normale oligocène des Matelles (environs de Montpellier). Une difficile confrontation entre un modèle à la mode et les observations de terrain[END_REF]. Seismic reflection profiling has shown that, during Oligocene rifting, such faults detached in the Triassic decollement [START_REF] Benedicto | Modèles tectono-sédimentaires de bassins en extension et style structural de la marge passive du Golfe du Lion (Partie Nord), sud-est de la France[END_REF][START_REF] Benedicto | Interaction between faulting, drainage and sedimentation in extensional hanging-wall syncline basins: example of the Oligocene Matelles Basin, Gulf of Lion margin[END_REF][START_REF] Husson | Interaction géodynamique/karstification et modélisation géologique 3D des massifs carbonatés : Implication sur la distribution prévisionnelle de la karstification. Exemple des paléokarsts crétacés à néogènes du Languedoc montpelliérain[END_REF][START_REF] Sanchis | Structural style and tectonic evolution of a polyphase extensional basin of the Gulf of Lion passive margin: the Tertiary Alès Basin[END_REF]. The onshore syn-rift basins NW of the Nîmes Fault were formed by thin-skinned extensional tectonics [START_REF] Séranne | The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview[END_REF]. The basement-cover decollement ramps down in the Nîmes basement fault [START_REF] Benedicto | Low-angle crustal ramp and basin geometry in the Gulf of Lion passive margin: the Oligocene-Aquitanian Vistrenque graben[END_REF] and into the basement ramp of the Montpellier thrust to the south and southeast of the study area.

Revised chronology of events

The chronology of the transition between Pyrenean orogeny and the Gulf of Lion rifting is still discussed. The stratigraphy of the 1/50.000-scale geological maps of the study area is confusing: all maps are consistent in identifying the Bartonian (e6) as a Pyrenean syn-tectonic sequence, but they greatly diverge in dating the end of the orogeny and the onset of the rifting.

The comparison of stratigraphy of the adjacent maps of the study area reveals contradicting ages for identical and spatially continuous formations (Fig. 3 a).

Another unconsistency arises from the outstanding question: what was the geodynamic context during the time interval between the Pyrenean syn-orogenic deposits and the syn-rift formations? Adjacent maps carry contradicting interpretations: the « g1 » is a continuity of the syn-orogenic Bartonian on the Saint-Martin-de-Londres map [START_REF] Philip | Carte Géologique de la France au 1/50 000[END_REF], whereas the Montpellier map correlates the « g1 » with the early stages of the Gulf of Lion rifting [START_REF] Andrieux | [END_REF].

The mammalian reference levels developed for the Paleogene continental record of Europe [START_REF] Sigé | Un outil de la stratigraphie du Tertiaire continental : L'échelle de niveaux-repères de Mammifères[END_REF], allowed to improve the stratigraphy and to revise the tectonic agenda of the area, especially around the transition from orogeny to rifting. Figure 3b uses chronology of [START_REF] Vandenberghe | The Geologic Time Scale[END_REF] and mammal reference levels of [START_REF] Aguilar | Actes du Congrès BiochroM'97 -Montpellier[END_REF].

The age of the lacustrine limestone was defined as Lutetian or slightly older. In the Pic-Saint-Loup foreland, fauna allowed to place this formation between MP12 and MP13 reference levels of the mammal reference levels [START_REF] Crochet | Découverte dans le bassin de Saint-Martin-de-Londres (Hérault, Sud de la France) d'un gisement à vertébrés continentaux d'âge Eocène moyen[END_REF]. In different localities of Les-Matelles basin, several lignite beds provided a diversified fauna, which correlates with the Lutetian (MP13) [START_REF] Crochet | Découverte de vertébrés continentaux de l'Eocène moyen et supérieur dans le bassin des Matelles (Hérault, Sud de la France) et redécouverte du[END_REF]. A distinctive marly facies including 1 to 20cm diametre oncolithes, occurs at the top. It has yielded fauna ascribed to the MP14 reference level [START_REF] Crochet | Découverte de vertébrés continentaux de l'Eocène moyen et supérieur dans le bassin des Matelles (Hérault, Sud de la France) et redécouverte du[END_REF] of latest Lutetian age. This facies passes southwards and upwards to the Bartonian syn-orogenic breccia, exposed along the Montpellier thrust [START_REF] Andrieux | [END_REF]). In the Sommières and Saint-Chaptes basins the reference sites of Robiac provided a late Bartonian age for the latest Pyrenean syn-tectonic formations [START_REF] Remy | Les Périssodactyles (Mammalia) du gisement Bartonien supérieur de Robiac (Éocène moyen du Gard[END_REF]. ? ?

Guzargues (Lihoreau & Tabuce, pers.comm.) Les Matelles [START_REF] Crochet | Géologie et paléontologie de la partie septentrionale du fossé oligocène des Matelles (Hérault, sud de la France[END_REF])

Paleotherium

Gervais (Remy pers. comm) Poteau 276 [START_REF] Crochet | Découverte de vertébrés continentaux de l'Eocène moyen et supérieur dans le bassin des Matelles (Hérault, Sud de la France) et redécouverte du[END_REF] Camplong [START_REF] Crochet | Découverte de vertébrés continentaux de l'Eocène moyen et supérieur dans le bassin des Matelles (Hérault, Sud de la France) et redécouverte du[END_REF] Nozières (Rémy & Lesage,2005) Robiac (Rémy & al 2015) Pont de Ners (Rémy, 1999[START_REF] Rémy | Un nouveau gisement de vertébrés d'âge Priabonien et son contexte géologique (Tranchée de Nozières, Gard)[END_REF]) Faveirol (Rémy, 1999) St Dézéry (Rémy, 1994) Aumelas (Rémy & al 2016) St Martin [START_REF] Crochet | Découverte dans le bassin de Saint-Martin-de-Londres (Hérault, Sud de la France) d'un gisement à vertébrés continentaux d'âge Eocène moyen[END_REF] lignite 'le Pont' [START_REF] Crochet | Découverte de vertébrés continentaux de l'Eocène moyen et supérieur dans le bassin des Matelles (Hérault, Sud de la France) et redécouverte du[END_REF] lignite 'exploité' (Crochet pers.com Biostratigraphy scale according to mammal reference levels (M.Z.). Black stars and black hexagons correspond to mammal and charophyte sites, respectively. Simplified lithology of the formations is indicated. Colour code as in the following maps and sections of the Paleogene basins analysed in this study. White areas corresponds to non-depositional and erosional hiatuses; the Priabonian interval from St Chaptes-Gardon-Alès basins is documented in [START_REF] Lettéron | Reconstruction of a saline, lacustrine carbonate system (Priabonian, St-Chaptes Basin, SE France): depositional models, paleogeographic and paleoclimatic implications[END_REF].

They pass upward to continental marls and limestones dated to the Bartonian-Priabonian transition (MP17) [START_REF] Remy | Les Périssodactyles (Mammalia) du gisement Bartonien supérieur de Robiac (Éocène moyen du Gard[END_REF][START_REF] Rémy | Un nouveau gisement de vertébrés d'âge Priabonien et son contexte géologique (Tranchée de Nozières, Gard)[END_REF]. In the Saint-Martin-de-Londres basin, similar lithologies correspond to time equivalent, as suggested by charophytes (Feist in [START_REF] Philip | Carte Géologique de la France au 1/50 000[END_REF].

The overlying formation mapped as « g1 » in Les-Matelles basin, displays polygenic conglomerates and sandstones within yellow marls and silts, including several lignite beds.

They are interpreted as alluvial plain deposits [START_REF] Benedicto | Interaction between faulting, drainage and sedimentation in extensional hanging-wall syncline basins: example of the Oligocene Matelles Basin, Gulf of Lion margin[END_REF][START_REF] Egerton | Inherited drainage pathways sediment distribution and provenance in rift basinsPhD[END_REF]. The age of this formation was subjected to discussion due to the controversial location of the discovery by Gervais, in the late XIX th Century, of a Lophiodon skull (see review in [START_REF] Hartenberger | Les gisements de vertébrés de la région montpellieraine -1[END_REF]. Reappraisal of the specimen, exhumed from Lyon university collections by J. A. Remy (personal communication), relates the specimen to MP 18 (middle Priabonian). Another Lophiodon specimen has recently been discovered in the correlative « g1 » formation in the Guzargues basin, and has been related to the MP19 reference level (R. Tabuce and F. Lihoreau, personal communication). The age of this conglomerate formation is therefore attributed to the Priabonian. As first suggested by [START_REF] Andrieux | [END_REF], it is correlated with the detrital continental formation « Grès de Célas » of upper Priabonian, in the Sommières and Saint-Chaptes -Gardon basins, northeast of the study area, which spans the MP18 to MP19 [START_REF] Rémy | Nouveaux gisements de Mammifères et Reptiles dans les Grès de Celas[END_REF][START_REF] Remy | Mammifères fossiles des grès de Célas (Eocène supérieur du Gard): découvertes récentes[END_REF][START_REF] Rémy | Un nouveau gisement de vertébrés d'âge Priabonien et son contexte géologique (Tranchée de Nozières, Gard)[END_REF].

At Les-Matelles and Saint-Vincent-de-Barbeyrargues localities, yellow to orange marls interfingering with syn-rift breccia, contain charophytes [START_REF] Grambast | Aperçu des charophytes tertiaires du Languedoc et leur signification stratigraphique[END_REF], teeth of small mammals [START_REF] Thaler | Premiers résultats d'une recherche systématique des dents de Rongeurs, par lavage de marnes de l'Oligocène[END_REF] and gastropods [START_REF] Rey | Sur l'importance paléontologique des faunes malacologiques continentales de très petites dimensions comme celles de la région de Montpellier[END_REF] that yielded mid-to late-«Stampian» age, which corresponds to the Late Rupelian. This age was later confirmed by the discovery of small mammals in Les-Matelles half-graben, that relate to MP25 mammal scale [START_REF] Crochet | Géologie et paléontologie de la partie septentrionale du fossé oligocène des Matelles (Hérault, sud de la France[END_REF]. In addition, breccia pipes and dykes of intracontinental alcaline basalt, that intrude Lutetian and Priabonian formations, were dated 24 to 28 Ma [START_REF] Gastaud | Les systèmes filoniens des Causses et du Bas Languedoc (Sud de la France) : géochronologie, relations avec les paléocontraintes[END_REF], and have been related to the Late Rupelian rifting event [START_REF] Dautria | 160 Ma of sporadic basaltic activity on the Languedoc volcanc line (southern France) : A peculiar case of lithosphere-asthenosphere interplay[END_REF][START_REF] Séranne | The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview[END_REF]. In the northern basins, this unconformable formation has not yet provided mammal remains, but Charophytes from the alluvial environments of Saint-André-de-Cruzières in the north of Alès Basin yielded an Oligocene age [START_REF] Feist-Castel | Sur les Charophytes fossiles du bassin Tertiaire d[END_REF].

Reappraisal of ancient specimens and new discoveries thus indicate that the sequence made of marls, sandstones beds and polygenic channelized conglomerates, which is exposed in separated basins across the study area, belongs to the same interval, dated to the Priabonian.

In particular, the fluvial conglomerate and yellow sandy marls (reported on the published maps as "g1", Fig. 3a), correlate with, and are the proximal equivalent of the lacustrine delta sandstones "Grés de Célas" of the Alès Basin [START_REF] Lettéron | Reconstruction of a saline, lacustrine carbonate system (Priabonian, St-Chaptes Basin, SE France): depositional models, paleogeographic and paleoclimatic implications[END_REF]. This formation is intermediate between the syn-orogenic Pyrenean (Bartonian) and the syn-rift (Late Rupelian) deposits. Furthermore, it is bounded by two marked angular unconformities, the upper one corresponding to a several-million-years hiatus.

In the following, we examine key field evidence in the north-Montpellier area (Guzargues, Les-Matelles and St-Martin-de-Londres basins, respectively) where the tectonic sedimentation relationships allow to characterise this intermediate formation. It can be interpreted as the record of the transition between the Pyrenean orogeny to the Gulf of Lion rifting.

Tectonic and sedimentation in the Guzargues Basin

4-1. Lithostratigraphy of Eocene deposits in the Guzargues area

The Guzargues basin (Fig. 2) has not been the topic of any specific study and remains poorly documented. The Paleogene sedimentary succession contains 4 main lithological units representative of the regional geodynamic history since the early stages of the Pyrenean compression to the main rifting phase (Fig. 4). When possible, all of them were precisely mapped to analyse the geometrical relationships of Priabonian deposits with under-and overlying units. (2) Massive whitish Lutetian limestones, up to 100m thick in the area, form an easily recognised, continuous unit displaying folds produced by late stages of the Pyrenean compression.

(3) Lutetian limestones are conformably capped by a singular unit, up to 30m thick, made of lacustrine to shallow marine, marly facies containing oncoliths and calcarenitic layers, Bartonian in age.

(4) Topmost deposits of the Paleogene succession in the Guzargues area correspond to Priabonian fluvial deposits showing alternations of conglomerates, sandstones and marls, up to 300 m thick. Pebbles are supplied both from distant Pyrenean sources and from local Mesozoic carbonates. These deposits rest unconformably above the Pyrenean folds consisting of Lutetian to Bartonian units.

One kilometre to the south, the northern edge of the neighbouring Assas basin shows a sedimentary unit of strong geodynamic significance, which is missing in the Guzargues basin.

Close to the Assas village, Oligocene breccias and marls, usually interpreted as syn-rift deposits, unconformably overlie Priabonian continental facies.

Thus, the Priabonian deposits of the Guzargues basin, bounded below and above by unequivocal syn-tectonic units, represent a key unit to study the transition between two main regional geodynamic events.

4-2. Sedimentological features of Priabonian deposits in the Guzargues basin

As observed in the 10 sections studied in the Guzargues basin, Priabonian deposits are basically made of eight lithofacies units distinguished by their lithology and sedimentary structures. Each lithofacies is described and interpreted in terms of depositional processes (Table 1), and most are illustrated on Figure 5.

Lithofacies units are often associated together, defining three main facies associations (FA1, FA2, and FA3), the distribution of which varies depending on the spatial location in the Guzargues basin and on the vertical position into the Priabonian stratigraphic succession.

FA1 is represented by the alternations of (i) channel-shaped, poorly to weakly sorted conglomerates (Gu, Gt; Table 1), (ii) matrix-rich, stratified conglomerates (Gm; Table 1), and

(iii) medium-to coarse-grained, cross-bedded sandstones (Sp; Table 1). A general trend in the vertical evolution of FA1 facies units is recognised through most of the studied sections. In the lower part of sections, predominant Gu/Gt conglomeratic channels, up to 3m thick and 3 to 5m Gu conglomerates can be interpreted as channel lag [START_REF] Miall | A review of the braided river depositional environment[END_REF](Miall, , 1978;;[START_REF] Nemec | Quaternary alluvial fans in the southwestern Crete: Sedimentation processes and geomorphic evolution[END_REF].The Gm and Gt facies units present strong similarities with gravel bars in braided streams [START_REF] Mack | Climatic and tectonic controls on alluvial fan and axial fluvial sedimentation in the Plio-Pleistocene Palomas half graben, southern Rio Grande Rift[END_REF][START_REF] Miall | A review of the braided river depositional environment[END_REF][START_REF] Nemec | Quaternary alluvial fans in the southwestern Crete: Sedimentation processes and geomorphic evolution[END_REF] . Sp cross-bedded sandstones are typical of megaripples deposited by a unidirectional upper to lower flow regime [START_REF] Simons | Sedimentary structures generated by flow in alluvial channels[END_REF][START_REF] Southard | Experimental determination of bed-form stability[END_REF] produced by subaerial to subaqueous, stream and/or sheet flows [START_REF] Hampton | Sheetflow fluvial processes in rapidly subsiding basin, Altiplano plateau[END_REF][START_REF] Mack | Climatic and tectonic controls on alluvial fan and axial fluvial sedimentation in the Plio-Pleistocene Palomas half graben, southern Rio Grande Rift[END_REF]. Inferred FA1 depositional environments are thought to belong to an alluvial fan setting, first supplied by a local catchment area (lower part of sections), and then by a regional catchment area (upper part of sections). Note the occurrence of several pedogenetic carbonate nodules above the irregular top surface of the carbonated sandy horizon.

Table 1: Summary of the main facies features observed in the Priabonian deposits of the Guzargues basin, and interpretations in terms of depositional processes. Gm, Gt, Sp, Sr, Fm and P, are abbreviations from the facies code provided by (Miall, 1978). Gu and Sst are abbreviations defined by [START_REF] Barrier | Control of alluvial sedimentation at foreland-basin active margins: a case study from the Northeastern Ebro basin[END_REF].

FA2 corresponds to the alternation between (i) moderately sorted, medium-to coarse-grained sandstones with sub-rounded clast lenses (Sst; Table 1), and (ii) medium-to coarse-grained cross-bedded sandstones (Sp; Table 1). If local (Mesozoic but also Lutetian) calcareous elements still represent an important proportion of clasts (granules, gravels, pebbles), siliceous component (quartz and lydite gravels) is more abundant than in FA1. FA2 depositional processes are dominated by tractional transport under a unidirectional upper flow regime, with episodic sheet flows. Inferred depositional environments correspond to the proximal part of a subaqueous alluvial fan to fan delta, with sediment supplied from both local and regional catchment areas.

FA3 is mainly represented by yellowish, greenish or reddish silty clays, with plastic texture at some places, in which are interbedded thin layers of well-sorted siltstones and fine-grained sandstones (Fm; Table 1). Carbonate pedogenic nodules locally form scarce, up to 1m thick, distinctive layers (P; Table 1). Interbedded massive fine-grained sandstones (Sr; Table 1) usually show vertical root traces and more rarely asymmetrical ripple cross-laminations several centimetres thick at the top.

FA3 deposition must be the result of vertical settling from detrital suspensions in standing water [START_REF] Horton | Sedimentology of a lacustrine fan-delta system, Miocene Horse Camp Formation[END_REF][START_REF] Miall | A review of the braided river depositional environment[END_REF]. Fine-grained sandstones showing asymmetrical ripples and interbedded silty and sandy layers suggest episodic flow regime, related to sheet flood events. The occurrence of carbonate nodules records occasional pedogenic processes and temporary aerial exposure. Assuming these depositional processes, FA3 should have been deposited in the distal part of a subaqueous to subaerial alluvial fan, or in a floodplain laterally to major braided distributaries. The 10 studied sections show vertical variations in the FA succession (basically FA1-FA2 or FA2-FA3 alternations) forming elementary sequences between several metres and tens of metres thick (Fig. 6). As indicated by the various inferred depositional environments for FA, these elementary sequences, made up of alternatively proximal and distal deposits, are arranged into overall trends measuring several tens of metres thick, recording progradational and retrogradational evolutions.

At the scale of the Guzargues basin, the large FA alternations are describing a whole progradational, then retrogrational, and finally progradational evolution, which is easily recognised on the field. These trends were used to correlate Priabonian deposits at basin scale and, combined with aerial photograph interpretation, to produce a detailed geological map that allows building cross-sections.

4-3. Tectonic-sedimentation relationships in the Guzargues basin

The detailed geological map (Fig. 7) and cross-sections (Fig. 8) of Guzargues basin show sequences of Priabonian conglomerates interfingering with marls, that are involved in gentle folding of complex geometry. To the south, the basin is separated from the contemporaneous Assas Basin by a NW-SE anticline, cored by Valanginian (Fig. 7 and Fig. 8 -Section 1). Northsouth sections of Guzargues basin (Fig. 8 -Sections 2, 3) display an asymmetrical syncline structure: the southern limb is steeply dipping north, whereas the northern limb extends over 3 to 4 km in a gently dipping or subhorizontal position. In addition, the basal part of the Priabonian is affected by ENE-trending anticlines, while the youngest conglomeratic Priabonian seals the anticline (Fig. 8 -Section 3).

The Lirou anticline -The Bartonian and the base of the Priabonian sequence (isolated sandstone and conglomerate beds in marls) are wedging towards an anticline with an EWoriented, W-plunging axis. Conglomerate beds isolated in the silt and marls are observed onlapping both flanks of the anticline (Fig. 6), and they display diverging bedding away from the anticline (Fig. 7, Fig. 9a). The upper conglomeratic interval, corresponding to the final progradational trend recorded by Priabonian deposits, progrades northwards across the whole basin, over a silt and shale interval and seals the Lirou anticline (Fig. 6). Folding affects the Lutetian lacustrine limestones, and the top of the Lutetian is eroded in the crest of the anticline. paleocurrent in the paleo-valley. Very thin (several tens of metres) in the transverse valley, the Priabonian formations thickens rapidly northward within 1 kilometre distance, from less than 300 m to 450 m, and it exceeds 600 m in the syncline axis. In addition, fluvial conglomerate belts pass laterally and distally (northwards) to silts and marls of alluvial plain facies. These observations clearly show that: i) the syncline was formed during deposition of the Priabonian ; ii) the basin was fed by a north-flowing paleoriver, incised into the growing Lutetian limestone E-W anticline. Serre de Moujes -The northern part of the basin corresponds to a syncline trending N020°, that involves Lutetian limestones and Priabonian conglomerates (Fig. 7). The conglomerates are more developed on the steep western flank, where they unconformably rest over the steeply dipping Lutetian limestones, and they grade eastwards to distal silts and marls (Fig. 8 -Section 4). Along the eastern flank of the syncline, Priabonian formations are onlapping onto the Lutetian limestone; in addition, the Bartonian oncolith formation is transgressed towards the north by the Priabonian. Onlapping surfaces point to a pre-existing synform of Lutetian limestones prior to Priabonian deposition. The conglomerate belt is separated from that of the main part of the basin, suggesting the presence of a smaller, northern, sub-basin with a distinct feeder that supplies clastic sediments across the western boundary (Fig. 8 -Section 2).

Unfortunately, no clear evidence of paleocurrent were found. North-south correlations across the entire Guzargues basins shows that the northern sub-basin represents the youngest depocentre of the whole basin (Fig. 8 -Section 1). To the NW, Priabonian is directly in contact with the Mesozoic carbonates through a sub-vertical faulted boundary. In the Serre de Moujes, outcrop in the damage zone of the bordering fault reveals sets of associated fractures and dissolution surfaces, consistent with a left-lateral strike-slip motion (Fig. 9b). Unfortunately, no slickensides could be observed to constrain the respective vertical and horizontal components of slip. Saint-Bauzille-de-Montmel borehole (located 1.5 km north, along strike) constrains a curved geometry for this fault and provides evidence for a 700m vertical offset (Fig. 8 of such offset may be inherited from earlier (Pyrenean) tectonic phases. These observations argue for i) deposition of Priabonian syn-tectonic sediments in a hanging-wall syncline, controlled by the activity of the N020°-trending bounding fault, and ii) accommodation of a significant amount a left-lateral strike-slip, by this inherited fault.

4-4. Tectonics -paleoenvironment relationships in the Guzargues basin

5-1. Les-Matelles basin

The overall geometry of Les-Matelles basin (LMB) is that of an Oligocene syn-rift hanging-wall syncline, controlled by a NNE-trending normal fault (Fig. 2), known as the Matelles-Corconne Fault [START_REF] Benedicto | Interaction between faulting, drainage and sedimentation in extensional hanging-wall syncline basins: example of the Oligocene Matelles Basin, Gulf of Lion margin[END_REF]. The Priabonian sequence mostly consists of marls and siltstones with rare conglomerate beds. Paleocurrent measurements in the channelised fluvial conglomerates indicate northwest to northeast flowing channels [START_REF] Egerton | Inherited drainage pathways sediment distribution and provenance in rift basinsPhD[END_REF]. Detailed mapping [START_REF] Benedicto | Modèles tectono-sédimentaires de bassins en extension et style structural de la marge passive du Golfe du Lion (Partie Nord), sud-est de la France[END_REF] revealed that the underlying Priabonian sequence also presents growth structures in an asymmetric syncline, that displays a NW limb steeper (≥ 40°) and thinner than the SE limb (Fig. 12). Sequential tectonic restoration of transverse sections indicates an extensional offset across Les-Matelles-Corconne Fault of some 375 m, during Priabonian [START_REF] Benedicto | Interaction between faulting, drainage and sedimentation in extensional hanging-wall syncline basins: example of the Oligocene Matelles Basin, Gulf of Lion margin[END_REF].

In the southernmost part of the basin, Priabonian unconformably overlies the syn-orogenic breccia of Paleocene age, related to the activity of the Montpellier thrust [START_REF] Andrieux | [END_REF]).

In the central segment of the basin, Priabonian is conformable over the distal syn-orogenic marls and oncoliths of Bartonian age. Finally, in the northern extension of this basin, Priabonian is unconformable over Neocomian marly limestones, which are deformed by EW-trending compressional structures. Such folds and thrusts are correlated to the Pic-Saint-Loup thrust, active during Bartonian [START_REF] Philip | Carte Géologique de la France au 1/50 000[END_REF]. Considered at the scale of the basin, the Priabonian is thus unconformable over pre-Pyrenean deformed series, as well as syn-orogenic Pyrenean sedimentation. Finally, the syn-rift Late Rupelian breccia are unconformably overlying the Priabonian [START_REF] Crochet | Géologie et paléontologie de la partie septentrionale du fossé oligocène des Matelles (Hérault, sud de la France[END_REF]. syn-extension deposition of Priabonian units. Modified after [START_REF] Benedicto | Interaction between faulting, drainage and sedimentation in extensional hanging-wall syncline basins: example of the Oligocene Matelles Basin, Gulf of Lion margin[END_REF].

5-2. Saint-Martin-de-Londres basin

Saint-Martin-de-Londres basin (SMLB) extends in an EW asymmetric syncline, immediately north of the Pic-Saint-Loup thrust, a structure of the Pyrenean fold and thrust belt (Fig. 2).

Above an unconformity over the Neocomian, the basin sequence starts with continental marls and thin beds of lacustrine limestones of latest Cretaceous to Paleocene age [START_REF] Crochet | Géologie et paléontologie de la partie septentrionale du fossé oligocène des Matelles (Hérault, sud de la France[END_REF][START_REF] Freytet | Les dépôts continentaux et marins du Crétacé supérieur et des couches de passage à l'Eocène en Languedoc[END_REF][START_REF] Philip | Carte Géologique de la France au 1/50 000[END_REF], which are the distal correlative of the syntectonic « Vitrollian » breccia, deposited along the Montpellier thrust.

Lutetian lacustrine limestones, transgressive over Late Jurassic to Paleocene formations, form an 80 m to 100 m-thick slab that designs a wide, low amplitude, EW-oriented syncline (Fig. 13). The overlying breccia, dated to the Bartonian [START_REF] Philip | Carte Géologique de la France au 1/50 000[END_REF], consists of local, late Jurassic, Neocomian and Lutetian limestones, derived from erosion of the active Pic-Saint-Loup thrust front. They pass northwards to distal continental marls and silts, which present syn-tectonic growth structures. The southern boundary of the basin has been intensely folded and faulted in relation with the Pyrenean Pic-Saint-Loup thrust, during Bartonian time (Fig. 14).

The Priabonian sequence unconformably overlies the Bartonian breccia; to the east, it seals an intra-Bartonian thrust propagation fold (Fig. 13), which requires erosion of the Pyreneanrelated structure prior to Priabonian deposition. Along the southern margin of the SMLB, the Priabonian sequence designs an asymmetric syncline with sub-vertical to overturned southern limb (Fig. 15a,b.). The sequence starts with a thin level of finely laminated limestone that contains bioclasts and charophytes characterising a lacustrine / palustrine environment. Laterally, the limestone passes to thicker and more massive beds of lacustrine mudstone. The thin basal limestone is correlated to the north of the basin, with 10 m to 15 m thick marls and carbonates deposited in an arid, evaporitic environment. Above the limestones, along the central southern margin only, are found grey marls including several thin beds of glauconitic sandstones with bioclasts and rare foraminifera [START_REF] Egerton | Inherited drainage pathways sediment distribution and provenance in rift basinsPhD[END_REF]. This shallow marine sandstone passes upwards to marls and siltsones including thin bioturbated sandstone beds, deposited in a floodplain, similar to those described in LMB. In the upright, southern limb of the syncline, these sandstone beds are affected by small scale reverse faults (Fig. 15c). They also display soft penetrative deformation of initially cylindrical root traces (Fig. 15d), which indicate an early, pre-lithification, N-S shortening prior to folding. Finally, up to 50 m stacked, channelized conglomerates, are deposited in severalmetres-thick levels, interbedded with siltstones and marls. The high ratio of channelized conglomerates vs floodplain marls and siltstones is more in line with Guzargues than with Les-Matelles basins. Conglomerates are clast-supported and rather well-sorted. Clasts are polygenic, including significant number of quartz veins and black cherts, derived from the Paleozoic deformed basement. Distinctive clasts of glauconitic sandstones and calcarenites containing Albian orbitolines cannot be related to any local source, as the Mesozoic sequence is interrupted above the Valanginian due to the regional erosion of the Durancian Isthmus [START_REF] Husson | Interaction géodynamique/karstification et modélisation géologique 3D des massifs carbonatés : Implication sur la distribution prévisionnelle de la karstification. Exemple des paléokarsts crétacés à néogènes du Languedoc montpelliérain[END_REF]. Such clasts, also been identified in Les-Matelles and Montferrier Priabonian outcrops are imbricated under the influences of north to northwestwards paleocurrents [START_REF] Egerton | Inherited drainage pathways sediment distribution and provenance in rift basinsPhD[END_REF], and thus correspond to a southerly derived material [START_REF] Freytet | Les dépôts continentaux et marins du Crétacé supérieur et des couches de passage à l'Eocène en Languedoc[END_REF].

The Priabonian sequence of SMLB unconformable over Pyrenean structures, and containing clasts supplied by a southerly derived sedimentation was deposited in a N-S compressional setting, related to reactivation of the Pyrenean Pic-Saint-Loup Thrust.

Seismic sequence analysis of the Hérault Basin

The Hérault Basin (HB) is a NNE-SSW asymmetric half-graben developed along the Cévennes Fault, formed in relation with the rifting of the Gulf of Lion (Arthaud et al., 1981;[START_REF] Maerten | Extensional tectonics of the Oligo-Miocene Hérault Basin (S France), Gulf of Lion margin[END_REF][START_REF] Séranne | The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview[END_REF]. It covers the western part of the Pyrenean Montpellier thrust (Fig. 2) and is filled by up to 1500m-thick succession of rift and post-rift sequences. The synrift and earlier Pyrenean structures are therefore mostly covered, which lead us to investigate the Pyrenean to syn-rift transition through subsurface data.

The basin sequence consists of (i) Rupelian to Aquitanian, continental and lacustrine formations, and (ii) Burdigalian to Langhian, marine to fluvial post-rift deposits, forming syn-rift and post-rift sequences separated by a major unconformity. The pre-rift substratum consists of Permian to Jurassic sequence, as well as mid-Cretaceous bauxite deposits, which have been sampled by several boreholes [START_REF] Berger | Carte Géologique de la France au 1/50 000[END_REF][START_REF] Lajoinie | Les formations bauxitiques de la Provence et du Languedoc[END_REF]. In addition, syn-tectonic Pyrenean Paleocene to Eocene formations (poorly exposed along the eastern margin of the basin) and Mesozoic carbonates deformed during Pyrenean compression can be extrapolated beneath the basin, and several Pyrenean thrusts repeating parts of the Jurassic sequence are documented [START_REF] Alabouvette | Carte Géologique de la France au 1/50 000[END_REF][START_REF] Berger | Carte Géologique de la France au 1/50 000[END_REF]. Two geophysical surveys acquired in [START_REF] Gastaud | Les systèmes filoniens des Causses et du Bas Languedoc (Sud de la France) : géochronologie, relations avec les paléocontraintes[END_REF]and 1984, then reprocessed in 2005[START_REF] Lagabrielle | Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites[END_REF][START_REF] Serrano | Le bassin Oligo-Miocène de l'Hérault: un exemple de rétrocharriage des structures pyrénéennes[END_REF], provide a regular seismic coverage (Fig. 2) of sufficient quality to examine the seismic units underlying the syn-rift sequence (Fig. 16). Some of these seismic sections were already interpreted by [START_REF] Maerten | Extensional tectonics of the Oligo-Miocene Hérault Basin (S France), Gulf of Lion margin[END_REF] who focused their study on the geometry of syn-and post-rift units. Reinterpretation of the seismic profiles allows us to identify, in several parts of the basin, a distinctive seismic unit below the syn-rift deposits (Fig. 16). It is bounded by angular unconformities both at the top and at the base and displays significant thickness variations and the Cevennes-fault [START_REF] Maerten | Extensional tectonics of the Oligo-Miocene Hérault Basin (S France), Gulf of Lion margin[END_REF]. Our data do not support such a transfer zone active during the Priabonian.

Seismic sequence analysis in the Gardon -Saint-Chaptes basin

The Priabonian Gardon -Saint-Chaptes basin (GSCB) sits at the junction of an almost continuous NE-trending strip of Priabonian outcrops from the foreland of the Montpellier thrust (Fig. 2), to the large syn-rift Alès basin, parallel to the NE-trending Cévennes Fault. Syn-rift Oligocene sediments are observed, unconformable over the Priabonian, in the Alès Basins [START_REF] Sanchis | Structural style and tectonic evolution of a polyphase extensional basin of the Gulf of Lion passive margin: the Tertiary Alès Basin[END_REF] and in a distinct outcrop in the southern part of the GSCB [START_REF] Lettéron | Reconstruction of a saline, lacustrine carbonate system (Priabonian, St-Chaptes Basin, SE France): depositional models, paleogeographic and paleoclimatic implications[END_REF]. Transgression of the post-rift Burdigalian tidal sequences, restricted to the E-W paleo-ria of Uzès basin [START_REF] Reynaud | Topographic controls on production and deposition of tidal cool-water carbonates[END_REF], did not reached the Alès basin. The Gardon segment, between Alès and Saint-Chaptes, consists of a large asymmetrical syncline, with a distinctive NW-trend, contrasting with the general NE-trend of the contemporaneous basins (Fig. 2).

Pyrenean structures in this area consist of a succession of E-W folds affecting the thick Triassic to Neocomian sequence [START_REF] Arthaud | Contraintes, déformation et déplacement dans l'avant-pays Nord-pyrénéen du Languedoc méditerranéen[END_REF], dated with syn-tectonic Paleocene to Bartonien terrigenous sediments in the syncline axes. The later are buried to the west beneath Priabonian and Oligocene sequences, which provides a first-order evidence for a major post-Pyrenean discontinuity (Fig. 2). Detailed mapping of the northeastern border of the GSCB also defines a marked angular unconformity of the Priabonian over the synpyrenean breccia sequences [START_REF] Sanchis | Méthodologie d'imagerie 3D des bassins, des exemples dans le bassin du Sud Est[END_REF]. [START_REF] Sanchis | Méthodologie d'imagerie 3D des bassins, des exemples dans le bassin du Sud Est[END_REF]. Main markers are identified; Priabonian interval in purple overlay; intra-Priabonian marker corresponds to the base of "Grès de Celas" Formation. Location on Fig. 2 and 21.

Chronostratigraphy is rather well constrained due to numerous mammalian fossils (Fig. 3) reviewed in [START_REF] Lettéron | Reconstruction of a saline, lacustrine carbonate system (Priabonian, St-Chaptes Basin, SE France): depositional models, paleogeographic and paleoclimatic implications[END_REF]. Priabonian deposits form a wide asymmetric syncline, including a thick NE and reduced SW limbs, respectively. Seismic reflection survey of the Alès basin included a southern profile (85LGC11), which reveals the structure and mode of formation of this syncline [START_REF] Sanchis | Méthodologie d'imagerie 3D des bassins, des exemples dans le bassin du Sud Est[END_REF] (Fig. 19). In section, the GSCB displays Priabonian growth syncline migrating WNW-ward, which characterises dip-slip movement of a hangingwall flat made of Mesozoic sequence detached above an eastward, low-dipping, decollement, above the Paleozoic basement. Such low-dipping decollement, the "Alès Fault", has been identified in the neighbouring Alès basin and beneath the Mesozoic sequence, folded during the Pyrenean [START_REF] Benedicto | Modèles tectono-sédimentaires de bassins en extension et style structural de la marge passive du Golfe du Lion (Partie Nord), sud-est de la France[END_REF][START_REF] Sanchis | Méthodologie d'imagerie 3D des bassins, des exemples dans le bassin du Sud Est[END_REF][START_REF] Sanchis | Structural style and tectonic evolution of a polyphase extensional basin of the Gulf of Lion passive margin: the Tertiary Alès Basin[END_REF]. This decollement at the basement-cover interface is distinct from the Cévennes basement fault, although both the Cévennes and Alès faults emerge along the same trace. This seismic profile (Fig. 19) also provides evidence for 4km E-SE migration of the depocentre, therefore suggesting similar displacement of the hanging-wall across the Alès Fault, in this direction, during Priabonian. [START_REF] Baral | Caractérisation géologique du réservoir karstique profond du bassin de Saint-Chaptes[END_REF]. Location on Fig. 2 and 21.

The Saint-Chaptes part of the GSCB has been investigated by seismic reflection profiling, aiming at hydrogeological exploration of the covered Barremian Urgonian karst [START_REF] Josnin | Influence de la tectonique quaternaire sur la structure de deux aquifères karstiques du Languedoc (France)[END_REF].

The sections tied to 4 wells sampling the Oligocene and Priabonian interval (Fig. 2) [START_REF] Baral | Caractérisation géologique du réservoir karstique profond du bassin de Saint-Chaptes[END_REF][START_REF] Baral | Geological characterization and formation of a covered karstic reservoir in the Urgonian limestones of the Saint Chaptes Basin (Gard, South of France): towards predicting the distribution and state of karstification[END_REF] and correlated with surface exposures providing sedimentology and stratigraphy controls [START_REF] Lettéron | Reconstruction of a saline, lacustrine carbonate system (Priabonian, St-Chaptes Basin, SE France): depositional models, paleogeographic and paleoclimatic implications[END_REF], allowed to identify, date and map the major seismic markers across the basin (Fig. 20). In borehole, the Priabonian sequence consists of lacustrine marls and carbonates, that pass upward to sandstones. It rests on a thin interval of undated detrital continental sediment, which can be tentatively correlated with the syn-tectonic Bartonian observed in outcrops north of the borehole. Priabonian displays growth structures generated by extensional faults which ramp down into the Barremian (Urgonian facies) massive limestones. Such faults belong to the N-NE trending fault-network that truncates the Pyrenean folds, and is sealed by the Rupelian syn-rift sequence (Fig. 2). The dip-slip component of such faults observed in seismic, is not associated with any significant horizontal displacement, since the pyrenean fold axes [START_REF] Berger | [END_REF] are not laterally offset (Fig. 2). Time-depth conversion was achieved using velocities stack provided with the seismic profiles and controlled with the borehole data [START_REF] Baral | Caractérisation géologique du réservoir karstique profond du bassin de Saint-Chaptes[END_REF], in order to make an isopach maps of the Priabonian depocentre Fig. 21a). It provides evidence for Priabonian tectonics across N-NEtrending active faults, observed north of the basin, and southward, parallel to the Sommières basin (Fig. 2). The Gardon segment of the depocentre, although less well constrained by subsurface data, designs the NNW-trending asymmetrical syncline imaged in seismic (Fig. 19). The overlying Oligocene (syn-rift) depocentre (Fig. 21b) is restricted to the Saint-Chaptes part of the GSCB and it is not affected by the earlier faults, nor controlled by any active boundary faults. It designs a wide depocentre, whose subsidence axis strikes N050, oblique with respect with the Priabonian depocentre. This suggests a Rupelian syn-rift basin formed as a hanging-wall syncline [START_REF] Benedicto | Interaction between faulting, drainage and sedimentation in extensional hanging-wall syncline basins: example of the Oligocene Matelles Basin, Gulf of Lion margin[END_REF] above a detachment, which we interpret to be the Paleozoic basement-Mesozoic cover interface, i.e. the Alès Fault in this area. The structural relationships in the GSCB and the contrasted orientation of the Priabonian and

Rupelian depocentres indicate two distinct events with different extension directions.

Regional synthesis and discussion

Field observations show that the GB, LMB and SMLB display similar Paleogene successions and stratigraphic settings. In all exposed basins, the syn-tectonic pyrenean continental formations consist of breccia along the active thrusts that pass distally, i. -LMB: Syn-tectonic growth-structures are consistent with a hanging-wall syncline, resulting from extensional faulting along the NNE-striking bounding normal fault.

-SMLB: Structures are related to syn-depositional compressional folding across the Etrending Pic-Saint-Loup thrust.

-GB: The tectonic sketch derived from the detailed geological map Fig. 7 displays syndepositional folding, which characterises both NE-SW compression and EW extension; in addition, there is evidence of left-lateral strike-slip along NNE-trending faults (Fig. 11b).

-HB: Depocentre distribution with respect with controlling faults suggests left-lateral motion along the NNE-trending Cévennes Fault, which accommodates normal faulting across N-trending splays (Fig. 22).

-GSCB: The Priabonian sequence was deposited as a hanging-wall syncline controlled Restoration of the LMB points to 375m Priabonian extension [START_REF] Benedicto | Interaction between faulting, drainage and sedimentation in extensional hanging-wall syncline basins: example of the Oligocene Matelles Basin, Gulf of Lion margin[END_REF] across a releasing bend of the Matelles-Corconne Fault. This could represent the order of magnitude of displacement across the other identified NE-trending faults that bound the Guzargues and Sommières basins (Fig. 2). We thus suggest that the Priabonian cumulated left-lateral motion distributed across the 40 km wide NE-trending wrenching zone remained moderate, with a minimal value of 4 to 5 kilometres. Even though some wrenching may have occurred without any syn-tectonic deposits recorded, the cumulated Priabonian movement unlikely exceeded 10km. There is no data to document the offset across the major Nîmes fault. Late Eocene was also documented in southern Provence, defined by both strike-slip or reverse faults [START_REF] Lacombe | Determining paleostress orientations from faults and calcite twins: a case study near the Sainte-Victoire Range (southern France)[END_REF]. N-S shortening along the regional NE-trending faults accommodated transpression of the oblique Africa-Eurasia convergence in the Languedoc-Provence-Corsica domain [START_REF] Lacombe | Structural and kinematic relationships between Corsica and the Pyrenees-Provence domain at the time of the Pyrenean orogeny[END_REF] Fig. 23 1999), which activated the inherited NE-trending strike-slip faults as extensional faults. This Rupelian change in stress orientation reflects a change at plate boundaries, which we relate to the onset of subduction roll-back and back-arc rifting above the Apennine subduction [START_REF] Jolivet | Mediterranean extension and the Africa-Eurasia collision[END_REF][START_REF] Jolivet | Continental breakup and the dynamics of rifting in back-arc basins: The Gulf of Lion margin[END_REF][START_REF] Réhault | The western Mediterranean basin geological evolution[END_REF][START_REF] Romagny | Detailed tectonic reconstructions of the Western Mediterranean region for the last 35 Ma, insights on driving mechanisms[END_REF][START_REF] Séranne | The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview[END_REF]. Southeast-trending transfer faults zones are a distinctive feature of the rifting of the Gulf of Lion [START_REF] Canva | The Catalan magnetic anomaly: Its significance for the crustal structure of the Gulf of Lion passive margin and relationship to the Catalan transfer zone[END_REF][START_REF] Gorini | Contribution to the structural and sedimentary history of the Gulf of Lions (western Mediterranean), from the ECORS profiles, industrial seismic profiles and well data[END_REF][START_REF] Guennoc | Histoire géologique du Golfe du Lion et cartographie du rift oligoaquitanien et de la surface messinienne[END_REF][START_REF] Maillard | Transfer zones and associated volcanic province in the eastern Valencia Basin: Evidence for a hot rifted margin?[END_REF][START_REF] Mauffret | Structural geometry in the eastern Pyrenees and western Gulf of Lion (Western Mediterranean)[END_REF]. However, there is no evidence of active SE-oriented structure in the onshore study area, until the Oligocene rifting episode [START_REF] Maerten | Extensional tectonics of the Oligo-Miocene Hérault Basin (S France), Gulf of Lion margin[END_REF][START_REF] Sanchis | Structural style and tectonic evolution of a polyphase extensional basin of the Gulf of Lion passive margin: the Tertiary Alès Basin[END_REF]. The Priabonian strike-slip deformation documented in the present study therefore constitutes a distinct tectonic phase, corresponding to different kinematics and driving forces than the rifting of the Gulf of Lion [START_REF] Séranne | The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview[END_REF]. We suggest that the development of SE trending transfer zones in the back-arc basin, as from the Rupelian, dates the initiation of slab tearing in the underlying subduction zone [START_REF] Romagny | Detailed tectonic reconstructions of the Western Mediterranean region for the last 35 Ma, insights on driving mechanisms[END_REF]. Paleogeographic reconstruction at 35 Ma from [START_REF] Romagny | Detailed tectonic reconstructions of the Western Mediterranean region for the last 35 Ma, insights on driving mechanisms[END_REF]; Position of the North Alpine foreland from [START_REF] Dumont | Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc[END_REF][START_REF] Ford | Deep-Water Sedimentation in the Alpine Basin of SE France: New perspectives on the Grbs d'Annot and related systems[END_REF] and of the ECRIS from [START_REF] Dèzes | Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere[END_REF].

Priabonian left-lateral wrenching in Languedoc links the active Pyrenean orogen to the E-W rifting of the European Cenozoic Rift System (ECRIS) [START_REF] Dèzes | Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere[END_REF] and the North Alpine Foreland Basin [START_REF] Ford | Deep-Water Sedimentation in the Alpine Basin of SE France: New perspectives on the Grbs d'Annot and related systems[END_REF]. Such left-lateral wrenching during Priabonian most probably extended southward in the Corbières (Fig. 1); however, lack of Priabonian sedimentary record in this area does not allow to confirm this. Eocene northward motion of the Adriatic plate was accommodated by left-lateral oblique convergence along the SW Alps until continental collision in Oligocene [START_REF] Dumont | Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc[END_REF]Ford et al., 2006) (Fig. 24).

Convergence between the Iberia and Europe respectively lasted until early Miocene in the Pyrenees e.g. [START_REF] Mouthereau | Placing limits to shortening evolution in the Pyrenees: Role of margin architecture and implications for the Iberia/Europe convergence[END_REF][START_REF] Teixell | Crustal structure and evolution of the Pyrenean-Cantabrian belt: A review and new interpretations from recent concepts and data[END_REF] and circa 85km shortening is computed for the Late Eocene to Early Oligocene interval [START_REF] Teixell | The crustal evolution of the west-central Pyrenees revisited: Inferences from a new kinematic scenario[END_REF]. Pyrenean shortening in Corbières, Languedoc and Provence terminated during the Bartonian [START_REF] Alabouvette | Montpellier -Carte Géologique de la France à 1/250000eme[END_REF][START_REF] Bestani | Structural style and evolution of the Pyrenean-Provence thrust belt[END_REF][START_REF] Séguret | Le duplex à plis de propagation de rampes de Cazedarnes (arc de Saint-Chinian, avant-pays nord-pyrénéen, France)[END_REF]. Northward flowing clastic sedimentation pattern in Languedoc, Provence and the SE Alps shows that this segment of the orogen, located in the present-day Gulf of Lion, was being dismantled at that time [START_REF] Egerton | Inherited drainage pathways sediment distribution and provenance in rift basinsPhD[END_REF][START_REF] Guieu | Arguments for the pre-rift uplift and rift propagation in the Ligurian-Provençal basin (Northwestern Mediterranean) in the light of Pyrenean Provençal orogeny[END_REF][START_REF] Joseph | Deep-water sedimentation in the Alpine Foreland Basin of SE France: New perspectives on the Grès d'Annot and related systems -An introduction[END_REF].

The left-lateral movement along the Nîmes and Cévennes fault system extended northward to the Jura region where sinistral strike-slip is well documented during Eocene [START_REF] Homberg | Structural inheritance and cenozoic stress fields in the Jura fold-and-thrust belt[END_REF]. It is also consistent with the transfer zone between Rhine and Bresse grabens [START_REF] Bergerat | Extensional tectonics and subsidence of the Bresse basin: an interpretation from ECORS data[END_REF][START_REF] Lacombe | Eocene-Oligocene tectonics and kinematics of the Rhine-Saone continental transform zone[END_REF][START_REF] Madritsch | Multiple fault reactivations within the intracontinental Rhine-Bresse Transfer Zone[END_REF], which both record a distinctive Priabonian pulse of sedimentation predating a mid-Oligocene extensional tectonics [START_REF] Sissingh | Comparative Tertiary stratigraphy of the Rhine Graben, Bresse Graben and Molasse Basin: correlation of Alpine foreland events[END_REF]. Crustal extension across these rift basin amounts to 7 km [START_REF] Ziegler | Evolution of the lithosphere in the area of the Rhine Rift System[END_REF], a value in agreement with the offset measured across the strike-slip faults.

During the Priabonian, the Languedoc-Provence segment of the Pyrenean orogen was thus surrounded by two active convergence zones, while it was undergoing collapse. This different behaviour may be due to the interaction with the west-European rifting [START_REF] Dèzes | Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere[END_REF], which propagated southward beneath the Languedoc-Provence-Pyrenees.

Conclusion

The transition from the Pyrenean orogeny to the Gulf of Lion rifting in onshore Languedoc

Fig. 1 :

 1 Fig. 1: Structural map southern France showing the relation between the Pyrenees and Gulf of Lionmargin; modified from[START_REF] Séranne | Structural style and evolution of the Gulf of Lion Oligo-Miocene rifting : Role of the Pyrenean orogeny[END_REF]. Na: Narbonne, Pe: Perpignan, Mo: Montpellier, Ma:

  Fig.1

  Fig.18

Fig. 3

 3 Fig.3

Fig. 3 a:

 3 Fig. 3 a: Correlation chart of the Eocene to Oligocene interval, and inferred ages, according to the BRGM 1/50.000-scale geological maps of the study area. The inset in the lower part gives the position of the BRGM 1/50.000-scale geological maps. b: Revised chronostratigraphy of the basins north of Montpellier (covered by the geological maps of Montpellier, St Martin-de-Londres and part of Sommières and Anduze) and of the St Chaptes-Gardon basins (maps of Alès and part of Anduze and Uzès). This is based on new mapping of the Paleogene basins (this study) and on the compilation of biostratigraphy data, including recent discoveries of mammal fossils (references in the figure and in the text).

( 1 )

 1 Fig. 4: Generalised lithostratigraphy of the Late Mesozoic and Paleogene in the Guzargues Basin and immediate surrounding.

  Fig.4

  wide, contain heterometric, sub-rounded to well-rounded clasts, mainly composed of local Mesozoic (Upper Jurassic to Lower Cretaceous) carbonates. In some places, they also show abundant oncoliths (as fragments or complete specimens) probably originated from the underlying Bartonian formation. In the upper part of sections, conglomeratic bodies in Gm facies unit are thinner (< 1m) and more laterally continuous (usually more than 10 metres in width) than in the lower part. In addition to local Mesozoic carbonates, quartz and lydite, originated from Cévennes and/or Montagne Noire Paleozoic peripheral units, and ginger (Aptian?) sandstones represent a significant proportion of clasts in channel fillings.

Fig. 5 :

 5 Fig. 5: Photographs of outcrops illustrating some typical facies observed in the Priabonian deposits of the Guzargues basin. A: Facies Gu with carbonate clast alignment underlining tenuous cross-beddings. Note scours at the base of the channel (arrows). B: Facies Gt showing progressive transition between (base) unsorted, highly heterometric massive, stratified conglomerates, and (top) weakly sorted, crossbedded conglomerates. C: Alternations between (Sst) cross-stratified sandstones with lenses of scattered subrounded gravels and pebbles, and (Gm) matrix-rich, graded, stratified conglomerates with weakly sorted pebbles mainly made of Mesozoic carbonates. D: Reddish to yellowish laminated silty claystones (facies Fm) with intercalation of a decimetric layer of massive fine-grained sandstones (facies Sr). E: Medium-to coarse-grained cross-bedded sandstones typical of facies Sp. F: 50cm-thick carbonated horizon showing well-preserved, vertical root casts, probably derived from vegetalization on top of massive fine-grained sandstones (Sr).

Fig. 6 :

 6 Fig. 6: Detailed stratigraphic correlation of Priabonian deposits along a 2km-long N-S transect at the southeastern border of the Guzargues basin, showing differential subsidence and onlap geometries induced by the syndepositional growth of the Lirou anticline. See Fig. 7 for location of logs. Horizontal correlation lines set at the maximum of progradation recorded by several meters-thick, stacked conglomeratic channels that closely rests on Lutetian limestones at the apex of the Lirou anticline.

  Fig.6

.

  Fig. 7: Detailed geological and structural map of the Guzargues basin (north of Highway D68, whileAssas sub-basin is located south of the highway). Location of the sedimentological logs 1 to 9 reported on Fig.6are indicated by fine black rectangles. Position of the structural cross-sections extremities (Fig.8 and 9) are given by black arrows.

Fig. 10 :

 10 Fig. 10: Detailed structural section in the south of Guzargues basin showing growth syncline and internal angular unconformities. See location of Section 5 on Fig. 7.

  Fig. 10

Fig. 11 :

 11 Fig. 11: a: Sketch map of the paleoenvironments in the Guzargues basin, in relation with the Priabonian tectonics. b: Conceptual tridimensional representation (no scale intended) of the interaction between Priabonian kinematics and sedimentation in the Guzargues basin. Braided fluvial conglomerates accumulate in the most subsiding parts, i.e. the actively deforming synclines that affect the Mesozoic-Lutetian cover, detached over the basement. See text for explanations.

Figure

  Figure 11a provides a synthetic view of the paleoenvironment distribution across the Guzargues Basin during Priabonian. Fluvial sediments were delivered through an incised canyon across a growing anticline, which separates Assas (to the south) and Guzargues basins. The belt of northward-flowing fluvial channels preferably occupied the most subsiding parts of the basin: i) in the south, parallel to the bordering growing anticline and, ii) in the overall N-trending axis of the syn-depositional syncline. The latter was controlled by a set of NNEtrending faults which display map-and outcrop-scale evidences of normal faulting and leftlateral strike-slip. The secondary fluvial conglomerates depocenter of Serre de Moujes, in the northern part of the basin, witnesses an additional fluvial input to the basin, across a relay or

Fig. 12 :

 12 Fig. 12: Structural section across Les-Matelles basin (from surface data and seismic profile) showing

  Fig.12

  Fig. 13: Detailed geological map of Saint-Martin-de-Londres basin. Position of sections Fig. 14 indicated as white line. The black dot and cross points to the churches of the villages.

  Fig.14

  Fig. 15: a and b: general view of the eastern end of St-Martin-de-Londres Priabonian basin, an asymmetrical growth syncline, unconformable over the synorogenic Bartonian breccia, which were generated along the pyrenean Pic-St-Loup thrust. c: overturned Priabonian sandstone along the southern limb of the asymmetric syncline. d; detail of the previous outcrop displaying small scale reverse faults that provide evidence for N-S compressive deformation. E: deformed root traces, evidence for pre-lithification N-S compressive deformation of the Priabonian sandstone bed, prior to its folding. Outcrop location on Fig. 13.

Fig. 16 :

 16 Fig. 16: Seismic profiles representative of the Hérault basin (H84D, H84F, H84X). The purple sequence interpreted as Priabonian (see discussion in text) is intermediate between the Pyrenean synorogenic units and the Oligocene syn-rift sequence. See location on Fig. 2 and 18.

Fig. 17 :

 17 Fig. 17: Line drawings of parts of selected seismic profiles in the Hérault basin showing the lower and upper erosional boundaries of the intermediate sequence. See location on Fig.18.

  complex internal geometries. Geometrical relationships with underlying and overlying seismic sequences define its relative stratigraphy: (i) It is truncated by typical syn-rift sequences and is affected by the extensional syn-rift faults; (ii) It unconformably rests on pre-rift units affected by Pyrenean reverse faults (Fig.17

  Fig.19

Fig. 19 :

 19 Fig.19: Interpreted line drawing of a seismic reflection profile across the Gardon Basin showing the syn-depositional migration of the hanging-wall syncline. Modified after[START_REF] Sanchis | Méthodologie d'imagerie 3D des bassins, des exemples dans le bassin du Sud Est[END_REF]. Main markers are

Fig. 20 :

 20 Fig. 20: Seismic reflection profile and corresponding line drawing in the Gardon -Saint-Chaptes basinshowing evidence for syn-depositional (Priabonian) extensional faulting sealed by the Rupelian sequence. Maisonnette borehole allows to interpret and to date the sequence. Modified after(Baral,

Fig. 21 :

 21 Fig. 21: Isopach maps of Priabonian (a) and Rupelian (b) sequences in the Gardon -Saint-Chaptes basin. Note the different orientation of the Rupelian depocentre axis with respect to the previous Priabonian subsidence axis. Location in Fig. 2.

  Fig. 22: Synthetic regional map of Priabonian structures and kinematics in Languedoc. For location of specific features and localities, please refer to Fig. 2, which shares the same frame. The study area corresponds to a zone of sinistral shearing between the Cévennes and Nîmes faults, with a minimal value of 5 km (grey overlay symbolises the simple shear).

  by a SE-trending extensional low-angle decollement of the Mesozoic cover over the Paleozoic basement.Such apparently contradicting kinematics can be understood in an overall left-lateral deformation along the regional NNE-trending faults, such as the Cevennes, Les-Matelles-Corconne and the Nîmes faults (Fig.22). When submitted to a stress regime characterised by a horizontal, N-S, maximum principal stress s1 and a horizontal E-W minimum principal stress s3, inherited faults oriented NNE are reactivated as left-lateral strike-slip faults, with restraining bends[START_REF] Christie-Blick | Deformation and basin formation along strike-slip[END_REF] along more northerly oriented segments, as in LMB, HB, GSCB. East-west oriented fault splays such as the Pic-Saint-Loup correspond to restraining bends. Syn-tectonic deposition in the syncline north of Uzès (Fig.2, Fig.22) also provide additional evidence for N-S shortening across EW oriented structures, during Priabonian.The amount of left-lateral displacement during Priabonian is poorly constrained, as it reactivates, and is superimposed, onto Pyrenean left-lateral ramps, resulting in a combined finite strain, which hampers the distinction of the two contributions to the strike-slip offset. In addition, our study shows that i) the strain was distributed across the whole area between the Cévennes and Nîmes faults and ii) possible discrepancies between the amount of deformation recorded in the Mesozoic cover and the basement may occur. However, the Priabonian tectonic-sedimentation relationships recorded in some of the studied basins provide minimum values for the offset. The extensional growth structure in the Hérault basin (Fig.16 c) suggests a minimum extension of some 3 km, corresponding to the minimum value of Priabonian leftlateral offset across this segment of the Cévennes fault. The migration of the Priabonian hanging-wall syncline in the GSCB (Fig.19) indicates NE-ward displacement of 4 km.

Fig. 23 :

 23 Fig. 23: Evolution of stress regime from mid-Eocene Pyrenean shortening, through Priabonian strikeslip regime, to Rupelian rifting.

Fig. 24 :

 24 Fig. 24: Simplified map Pyrenees / European Cenozoic Rift System /Alps during Priabonian.

  Fig.24

  occurred during Priabonian. Prior to Priabonian time, every structural and tectonicsedimentation evidence argue for pyrenean compressional deformation. In contrast, after Priabonian, all structures indicate NW-SE extension related to the Gulf of Lion rifting. This makes the Priabonian interval a key intermediate period of kinematic change. Reassessment of the chronostratigraphic evolution of the area allowed us to constrain the calendar of events. This period is recorded by deposition of a distinctive sedimentary sequence, which is documented both by surface analyses and seismic profiling of Priabonian basins. Such intermediate sequence is bounded at the base by an angular unconformity over the Pyrenean structures, and at the top by the Oligocene syn-rift unconformity, respectively. Continental sedimentation ranges from lacustrine/ palustrine limestone and evaporitic marls at the base, to marls and silts of alluvial plain, including variable proportion of sandstone sheets and channelised fluvial conglomerates, toward the top. The Priabonian series progrades north-or northeastward and is derived from a southern source, presently located offshore, which indicates the existence of a remaining pyrenean topography in the present-day Gulf of Lion. Mapping and seismic analyses of the tectonic-sedimentation relationships in the Priabonian basins give evidence for syn-depositional deformation against bounding faults and in growth synclines. Contrasted modes of deformation are observed, within short distance, in the Priabonian basins: i) controlled by extensional movement of NNE to NNW-trending faults or segments of faults or ii) controlled by E-W-oriented thrusts and folds. Regional mapping of these structures shows a distribution of the compressional and extensional structures which are consistent with a left-lateral shearing of the area comprised between the major, NEtrending, Cévennes and Nîmes faults. Such sinistral strike-slip of moderate offset (± 5 km) accommodates the E-W extension of the West European rifts basins that are located to the north of the study area. To the south, the strike-slip faults are connected to the ongoing shortening in the Spanish Pyrenees. Priabonian deformation and sedimentation records the initial stages of the dismantling of the Languedoc-Provence segment of Pyrenees, located between two active orogens: the Spanish Pyrenees and the Western Alps. The late orogenic dismantling of the eastern Pyrenees (Jolivet et al., 2020) therefore started up to 5 My earlier than the Rupelian onset of Apennine subduction retreat and associated back-arc NW-SE extension.

St Chaptes-Gardon-Alès Basins North Montpellier Basins

	(Guzargues, Les Matelles, St Martin de Londres Basins)
	MP25	MP24	MP23	MP22	MP21	MP20	MP19	MP18	MP17	MP16	MP15	MP14	MP13	MP12
	Chat.	Rupelian			Priabonian		Bartonian			Lutetian
		Oligocene								Eocene
		30					35			40			45

Revised chronostratigraphy b M.Z.

	Mudflat/saline lake	(clay, dolomite & gypsum)	Lacustrine/palustrine	(limestone and marls)	Lacustrine	(mudstone, silts, shales)
	Fluvio-deltaic	(sandstones sheets)	Fluvial/alluvial plain	(channelized conglomerates)	Alluvial fan	(breccia & conglomerate)
	Alluvial plain	(silty marls, sandstones)	Alluvial plain	(pedogenic marls)	Alluvial plain	(algal mats, oncoliths)

50000 geological maps chronostratigraphy a

			St Sauveur Cruzières (Feist-Castel 1971)	(Charophytes)
																				St Martin L. (Charophytes) Feist in (Philip & al 1978)					.)
		Alès				g1-3						e7b «Calc.	bitumineux»		e7bG «Grès	de Célas»	e7a «marnes	feuilletées»		e6	e5
		Uzès			g2	g1c «calc.	marneux»	g1b «Grès	de Célas»	g1a		e7b	«Calc. blanc»			e7a «Marnes	bleues»			e6			Uzès	939	North
		Sommières Anduze	g3	g2-3		g2-3	g1b «Calc. g1b «Calc.de	à chailles» Martignargues»		g1a «Grès g1a «Grès	de Célas» de Célas»	e7a2	e7 «Calcaire	de Monteils»	«Ludien»		e7a1	«calc. à lignite»		e6 e6	e5	e3-5	«calc. lacustre»	Alès	912	Anduze	938	Sommières	964
	1/	Montpellier St Martin L.		g2-3 g2-3	«Brêche des «Brêche des	Matelles» Matelles»			g1 g1	«Oligocène inf» «Gonglomérats	Oligocène inf.»									e6b «marnes «marnes à	gréseuses» oncolithes»	e6	e5-6	e3-5	«calc. lacustre» e3-5 «Calc.	à Planorbes»	1/50000 Geol. maps setting	St Martin de Londres	963	Montpellier	990

m SECTION 1 SECTION 3 SECTION 4 SECTION 2

Acknowledgements:

This study was carried out during the OROGEN project, funded by TOTAL, BRGM and CNRS-INSU. It benefited from the stimulating discussions amongst the OROGEN community.

We acknowledge the BRGM for providing the reprocessed lines of the H83 and H84 seismic surveys, presented in this study. We are indebted to SMAGE (Syndicat Mixte pour

We are extremely grateful to J.Y. Crochet, J.A. Remy, F. Lihoreau, and R. Tabuce (ISEM-University of Montpellier) for discussing palaeontology and mammal biostratigraphy, and for guiding us to the precise locations of ancient and more recent paleontological discoveries.

The paper benefited from fruitful exchanges with Flavia Girard and Renaud Caby (Géosciences Montpellier), François Fournier (CEREGE), Alexandre Letteron (IFP-CEREGE), Hubert Camus (Cenote) and Pascal Fénart (Hydrofis).

The manuscript has been improved (and extended!) thanks to the insightful comments of François Guillocheau, Sylvain Calassou, and of an anonymous reviewer. The precious suggestions and corrections of the Guest Editor, Olivier Lacombe, are greatly appreciated.