Mathieu Bellanger

Romain Augier
email: romain.augier@univ-orleans.fr

Nicolas Bellahsen

Laurent Jolivet

Patrick Monié

Thierry Baudin

Olivier Beyssac

Shortening of the European Dauphinois margin (Oisans Massif, Western Alps): New insights from RSCM maximum temperature estimates and 40Ar/39Ar in situ dating

come

Introduction

The involvement of basement units in the external zones of collisional orogens is still poorly understood because the associated deformation is not intense and metamorphic conditions not severe enough to erase the effects of earlier deformation episodes, rendering the syn-collision deformation difficult to recognize. The deformation and kinematic history of these continental units thus often remains partly enigmatic and the rheology of the crust in the early stages of collision poorly known.

The Oisans Massif, which belongs to the External Crystalline Massifs (ECM) of the Western Alps is a key-place to understand the first stages of continental collision. Major basement brittle/ductile shear zones reworking the earlier Variscan fabric have been recognized which have accommodated most of the collisional shortening [START_REF] Wibberley | Are feldspar-to-mica reactions necessarily reaction-softening processes in fault zones?[END_REF][START_REF] Wibberley | Initiation of basement thrust detachments by fault-zone reaction weakening[END_REF][START_REF] Bellahsen | Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology[END_REF]Bellahsen et al., , 2014;;[START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF]Boutoux et al., 2014a), but precise P, T, time data are still lacking to better constrain the sequence of shortening and thickening.

The Oisans Massif also comprises a Mesozoic sedimentary cover, mainly deposited during the Early Jurassic rifting episode, which has not seen the Variscan metamorphic conditions and can thus be used to assess the Alpine history. [START_REF] Bellahsen | Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology[END_REF] and [START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF] have shown that localised west-verging Alpine shear zones developed in the basement during collision while the cover was strongly folded above without a major décollement in between. The geometry and kinematics of these shear zones is thus now established and it remains to estimate the P-T conditions of their development and to date them.

A c c e p t e d M a n u s c r i p t

In this contribution, we first provide a map of the maximum temperature (Tmax) reached by the cover between Grenoble and the Galibier pass (E-W) and between Saint Jean de Maurienne and Embrun (N-S) (Fig. 1) using the RSCM method [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF]. It constrains the thermal history as well as the burial dynamics around the Oisans Massif.

Then, we compare these Tmax with temperatures obtained in both basement and cover, using two chlorite geothermometers [START_REF] Cathelineau | A chlorite solid solution geothermometer the Los Azufres (Mexico) geothermal system[END_REF][START_REF] Cathelineau | Cation Site Occupancy in Chlorites and Illites as a Function of Temperature[END_REF][START_REF] Vidal | A Thermodynamic Model for Fe-Mg Aluminous Chlorite Using Data from Phase Equilibrium Experiments and Natural Pelitic Assemblages in the 100° to 600°c, 1 to 25 kb Range[END_REF][START_REF] Vidal | Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation[END_REF][START_REF] Vidal | P-Tdeformation-Fe3+/Fe2+ mapping at the thin section scale and comparison with XANES mapping: application to a garnet-bearing metapelite from the Sambagawa metamorphic belt (Japan)[END_REF]. In a second step, we provide EPMA analyses of white micas coupled to detailed microstructural and textural analysis of samples collected from various shear zones, in order to document the nature of these white micas, i.e. inherited vs. synkinematic. Finally, in a third step we performed in situ dating of white micas using the 40 Ar/ 39 Ar method. These results constrain the scenario of collisional shortening in the external zones of the Western Alps.

Geological setting

The Oisans Massif within the Western Alps

The Belledonne, Grandes Rousses, and Oisans massifs belong to the external zones and more particularly to the Dauphinois zone of the Western Alps (Fig. 1). External zones, which mainly recorded the Liassic rifting stage and the Oligo-Miocene collision event, were affected by a low-grade metamorphism after the end of Eocene. These massifs are parts of the External Crystalline Massifs (noted ECM, thereafter) that correspond to uplifted Variscan basement covered by Mesozoic to Cenozoic sedimentary series. The ECM were underthrusted below an orogenic prism made of stacked continental (Briançonnais zone) and oceanic (Piemontais zone) nappes transported along the Penninic Frontal Thrust (PFT). The PFT, also called the Crustal Pennine Thrust or the Embrunais Basal Thrust in [START_REF] Dumont | Lateral termination of the north-directed Alpine orogeny and onset of westward escape in the Western Alpine arc: Structural and sedimentary evidence from the external zone[END_REF][START_REF] Dumont | Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc[END_REF] was active with top-to-the-(north-)west motion during the Early Oligocene [START_REF] Ceriani | Multi-stage thrusting at the "Penninic Front" in the Western Alps between Mont Blanc and Pelvoux massifs[END_REF][START_REF] Tricart | La denudation tectonique de la zone ultradauphinoise et l'inversion du front brianconnais au sud-est du Pelvoux (Alpes occidentales); une dynamique miocene a actuelle[END_REF][START_REF] Tricart | The late extension in the inner western Alps: a synthesis along the south-Pelvoux transect[END_REF]. The internal zones, east of the PFT (Fig. 1), corresponds to the subducted continental margin of Europe and the Ligurian Ocean. They are pervasively affected by high-pressure low-temperature (HP/LT) metamorphism, which occurred between 70 and 35 Ma [START_REF] Rosenbaum | The Western Alps from the Jurassic to Oligocene: spatio-temporal constraints and evolutionary reconstructions[END_REF][START_REF] Berger | Subduction-related metamorphism in the Alps: review of isotopic ages based on petrology and their geodynamic consequences[END_REF].

During the Palaeozoic, the Oisans Massif experienced a complex tectonometamorphic history [START_REF] Debon | Evolution of Mg/Fe Ratios in Late Variscan Plutonic Rocks from the External Crystalline Massifs of the Alps (France, Italy, Switzerland)[END_REF][START_REF] Guillot | Nappe stacking and first evidence of Late Variscan extension in the Belledonne Massif (External Crystalline Massifs, French Alps)[END_REF][START_REF] Von Raumer | The Palaeozoic metamorphic evolution of the Alpine external massifs, Schweiz. mineral. petrogr[END_REF][START_REF] Guillot | Suture zones and importance of strike-slip faulting for Variscan geodynamic reconstructions of the External Crystalline Massifs of the western Alps[END_REF]. High-grade Carboniferous metamorphism has strongly overprinted previous tectonometamorphic records. The Oisans Massif mainly consists of late Variscan A c c e p t e d M a n u s c r i p t granites and earlier Devono-Dinantian plutonic and younger volcano-sedimentary sequences, including Visean conglomerates, volcano-sedimentary and coal intercalations [START_REF] Barféty | Geological map and explanatory text of the sheet[END_REF]. The Belledonne and Rochail massifs were then structured by nappe stacking during the Visean and later affected by late Carboniferous to early Permian late-orogenic gravitational collapse [START_REF] Guillot | Nappe stacking and first evidence of Late Variscan extension in the Belledonne Massif (External Crystalline Massifs, French Alps)[END_REF], contemporaneous with partial-melting and plutons emplacement between 330-290 Ma and the formation of extensional Stephano-Westphalian continental basins (i.e. in Grandes Rousses, Belledonne and La Mure massifs) [START_REF] Barféty | Geological map and explanatory text of the sheet[END_REF]Barféty et al., 1984[START_REF] Barféty | Geological map and explanatory text of the sheet n°821[END_REF][START_REF] Guillot | Suture zones and importance of strike-slip faulting for Variscan geodynamic reconstructions of the External Crystalline Massifs of the western Alps[END_REF]. This zone was ultimately eroded during Permian to Triassic times as evidenced by the continental Permo-Triassic deposits (sandstones and conglomerates together with coal intercalations) and middle to late Triassic shallow marine sediments (dolomitic limestone with evaporitic layers). These formations remained attached to the basement during Alpine deformation, and the contact can thus be used as a reference surface to quantify both the extensional strain during the rifting episode and the later compressional deformation during collision [START_REF] Dumont | Multistage shortening in the Dauphine zone (French Alps): the record of Alpine collision and implications for pre-Alpine restoration[END_REF][START_REF] Dumont | Lateral termination of the north-directed Alpine orogeny and onset of westward escape in the Western Alpine arc: Structural and sedimentary evidence from the external zone[END_REF][START_REF] Bellahsen | Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology[END_REF][START_REF] Bellahsen | Collision kinematics in the western external Alps[END_REF][START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF]Boutoux et al., 2014a).

During early Liassic times, the post-Variscan basement was stretched during the rifting episode [START_REF] Dumont | Late Triassic-early Jurassic evolution of the western Alps and of their European foreland; initiation of the Tethyan rifting[END_REF][START_REF] Dumont | Sea-level changes and early rifting of a European Tethyan margin in the Western Alps and SE France. In : Mesozoic and Cenozoic sequence stratigraphy of European basins[END_REF], as evidenced by basaltic flows ("spilites") [START_REF] Laurent | Les épisodes magmatiques filoniens basiques du Massif des Pelvoux-Pelvoux entre Carbonifère et Lias[END_REF] and a series of large-scale fault-bounded tilted blocks, limited by major normal faults that still shape the present-day geometry of the external zones [START_REF] Barféty | La stratigraphie et la structure de la couverture dauphinoise au Sud de Bourg d'Oisans. Leurs relations avec les déformations synsédimentaires jurassiques[END_REF][START_REF] Lemoine | The continental margin of the Mesozoic Tethys in the Western Alps[END_REF][START_REF] Lemoine | Histoire d'une marge continentale passive: les Alpes occidentales au Mésozoïque: introduction, Introduction[END_REF]. During the early Liassic, the Meije block (Fig. 3) was a horst, as evidenced by a condensed sedimentary cover and shallow paleobathymetric conditions [START_REF] Roux | Bathymetric evolution of the Tethyan margin in the western Alps (data from stalked crinoids): a reappraisal of eustatism problems during the Jurassic[END_REF][START_REF] Bouillin | Les filons sédimentaires jurassiques du Pelvoux, enregistreurs des déformations alpines de la bordure orientale du massif cristallin externe de l'Oisans (Alpes occidentales)[END_REF][START_REF] Corna | Les faunes d'ammonites du Jurassique inférieur (Hettangien, Sinémurien et Pliensbachien) au nord du massif des Écrins (Oisans, Alpes occidentales)[END_REF].

After the rifting stage, the Liguro-Piemontais oceanic domain formed between the European and Apulian continental domains from the Bajocian onward, as evidenced by the age of the first sediments deposited above the ocean floor [START_REF] De Wever | Datation de la base des schistes lustrés postophiolithiques par des radiolaires (Oxfordien supérieur? Kimmeridgien moyen) dans les Alpes Cottiennes (Saint Véran, France)[END_REF][START_REF] De Wever | Datations des radiolarites post-ophiolitiques de Corse alpine à l'aide des Radiolaires[END_REF]. Subduction of the Ligurian domain, which started during late Cretaceous times was followed by subduction of the leading edge of the European continental margin under the Apulian microplate as evidenced by the HP to UHP metamorphism in the internal zones (e.g., [START_REF] Chopin | Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences[END_REF][START_REF] Agard | Tectonometamorphic evolution of the Schistes Lustres Complex; implications for the exhumation of HP and UHP rocks in the Western Alps[END_REF]. The more proximal margin was subducted to shallow depths and shortened and the deformation propagated outward.

The "Pyreneo-Provençal phase", Late Cretaceous to Early Eocene in age, responsible for E-W folds and thrusts especially in the southern part of the external zone, is the first post-A c c e p t e d M a n u s c r i p t

Variscan shortening event [START_REF] Lemoine | Rythme et Modalités des Plissements Superposés dans les Chaînes Subalpines Méridionales des Alpes Occidentales Française[END_REF][START_REF] Caby | Les plis transversaux dans les Alpes occidentales: implications pour la genèse de la chaîne alpine[END_REF]. Ante-Senonian E-W fold axes described in the Dévoluy massif, were recently interpreted as due to submarine sliding [START_REF] Michard | Cretaceous folding in the Dévoluy mountains (Subalpine Chains, France): gravity-driven detachment at the European paleomargin versus compressional event[END_REF]. The Nummulitic series are deposited within the foreland flexural basin from Lutetian/Bartonian to Priabonian/Rupelian [START_REF] Sinclair | Tectono-metamorphic evolution of the Briançonnais zone (Modane-Aussois and Southern Vanoise units, Lyon Turin transect, Western Alps)[END_REF][START_REF] Du Fornel | The southern Grès d'Annot outcrops (French Alps): an attempt at regional correlation[END_REF][START_REF] Mulder | High-resolution analysis of submarine lobes deposits: Seismic-scale outcrops of the Lauzanier area (SE Alps, France)[END_REF]. They uncomformably overly reverse faults associated to N-S shortening, south of the Oisans Massif [START_REF] Ford | Kinematics and geometry of early Alpine, basement-involved folds, SW Pelvoux massif, SE France[END_REF]. The timing and the kinematics of other thrusts north of the Oisans Massif, such as the Meije or Combeynot thrusts, are debated, whether they are pre- [START_REF] Barbier | La tectonique de la zone ultradauphinoise au NE du Pelvoux[END_REF][START_REF] Bravard | La structure du revers oriental du Massif du Pelvoux: Observations et interprétations nouvelles[END_REF][START_REF] Ford | Kinematics and geometry of early Alpine, basement-involved folds, SW Pelvoux massif, SE France[END_REF], or syn-to post-Nummulitic [START_REF] Beach | Thrust tectonics and cover-basement relations on the northern margin of the Pelvoux massif, French Alps[END_REF][START_REF] Dumont | Lateral termination of the north-directed Alpine orogeny and onset of westward escape in the Western Alpine arc: Structural and sedimentary evidence from the external zone[END_REF][START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF]. The only evidence of N-S to NW-SE Oligocene shortening within the basement is restricted to a narrow area around the PFT [START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF].

The main Alpine collisional tectonometamorphic event in the external zones was thus due to its burial under the internal units immediately after the deposition of a Rupelian olistostrome topping the Nummulitic series, the so-called "schistes à blocs" [START_REF] Kerckhove | La « zone du Flysch » dans les nappes de l'Embrunais-Ubaye (Alpes occidentales)[END_REF][START_REF] Gidon | Nouvelles données sur la structure des écailles de Soleil Boeuf (bordure sud du massif du Pelvoux)[END_REF][START_REF] Sinclair | Tectono-metamorphic evolution of the Briançonnais zone (Modane-Aussois and Southern Vanoise units, Lyon Turin transect, Western Alps)[END_REF][START_REF] Mulder | High-resolution analysis of submarine lobes deposits: Seismic-scale outcrops of the Lauzanier area (SE Alps, France)[END_REF]. In the Oisans Massif, E-W shortening was responsible for folding in the cover with axial plane cleavage development in the west (Gratier and Vialon, 1980;[START_REF] Bellahsen | Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology[END_REF][START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF]. The metamorphic peak reached 300 to 350°C for 1-5 kbar [START_REF] Waibel | Sedimentology, petrographic variability and very-low-garde metamorphism of the Champsaur sandstone (Paleogene, Hautes-Alpes, France)[END_REF][START_REF] Jullien | Cookeite and pyrophyllite in the Dauphinois black shales (Isères, rance) : implications for the conditions of metamorphism in the Alpine external zones[END_REF][START_REF] Crouzet | High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit[END_REF][START_REF] Crouzet | Experimental evaluation of thermal recording of successive polarities during uplift of metasediments[END_REF][START_REF] Ceriani | Tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between low-grade metamorphism and tectonic phases[END_REF], which implies a tectonic overburden of around 10km when considering a normal geothermal gradient. The temperature in the Bourg d'Oisans area [START_REF] Crouzet | High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit[END_REF][START_REF] Crouzet | Experimental evaluation of thermal recording of successive polarities during uplift of metasediments[END_REF] reached 330°C after the end of deformation. The metamorphic peak was dated with large uncertainties around 26-24 Ma by K-Ar method in fine clay fraction from the cover [START_REF] Nziengui | Excès d'argon radiogénique dans les quartz des fissures tectoniques : implications pour la datation des séries métamorphiques[END_REF], zircon fission-tracks ages ranging between 27 Ma (alt. 3065m) and 13 Ma (alt. 1310m) [START_REF] Van Der Beek | Inversion of thermochronological age-elevation profiles to extract independent estimates of denudation and relief history -II: Application to the French Western Alps[END_REF]. A deformation stage was dated between 34 and 27 Ma by 40 Ar/ 39 Ar phengite ages from strike-slip shear zones related to E-W shortening in the eastern part of the Oisans Massif (Simon-Labric et al., 2009). [START_REF] Bellahsen | Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology[END_REF] and [START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF] have further proposed that the E-W shortening in the Oisans Massif was mainly accommodated by kilometer-scale top-to-thewest reverse shear zones, which are summarized in the next section.

Inherited or newly formed: the Oisans shear zones revisited

Contrary to the internal zones where deformation is too intense to preserve former structures, the amount of shortening of the Oisans Massif was less than ca. 30-40% during the Alpine collision [START_REF] Dumont | Multistage shortening in the Dauphine zone (French Alps): the record of Alpine collision and implications for pre-Alpine restoration[END_REF][START_REF] Bellahsen | Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology[END_REF][START_REF] Bellahsen | Collision kinematics in the western external Alps[END_REF][START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF] A c c e p t e d M a n u s c r i p t 2014). As a consequence, inherited structures are still preserved, which allows studying relationships between inherited and Alpine newly formed structures. The detailed structural evolution of the Oisans Massif, which is beyond the scope of the present paper, was recently investigated by [START_REF] Bellahsen | Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology[END_REF] and [START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF] in order to describe the distribution of the deformation in the basement.

The Oisans Massif presents unambiguous evidence of intense shortening either at small (outcrop) scale or at large (kilometric) scale. Maps of shortening axes of small-scale structures together with transport directions over the whole Oisans Massif reveal only one major E-W shortening stage (Fig. 2). Strain localization in the basement leads to the formation of a set of newly-formed, top-to-the-west, reverse shear zones that fold the basement-cover interface to the west and form large basement thrusts near the Penninic Frontal Thrust [START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF]. Alpine shear zones can be separated from the Variscan deformation by the lower-temperature, more localised character of the deformation that cross-cuts a former high-temperature, pervasive Variscan fabrics. Moreover, an Alpine age is ensured by the deformation with consistent top-to-the-west kinematics within the Triassic-Liassic tegument [START_REF] Bartoli | Le chevauchement Meije-Muzelle et la répartition des domaines structuraux alpins du massif de l'Oisans (partie Nord du Haut-Dauphiné cristallin)[END_REF][START_REF] Barféty | Geological map and explanatory text of the sheet[END_REF][START_REF] Bellahsen | Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology[END_REF][START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF].

While similar Alpine shear zones of Argentera or Mont Blanc massifs were recently dated from the Oligo-Miocene [START_REF] Rolland | 40Ar/39Ar dating of synkinematic white mica: insights from fluid-rock reaction in low-grade shear zones (Mont Blanc Massif) and constraints on timing of deformation in the NW external Alps[END_REF]Sanchez et al., 2011a;[START_REF] Cenki-Tok | Direct dating of mid-crustal shear zones with synkinematic allanite: new in situ U-Th-Pb geochronological approaches applied to the Mont Blanc massif[END_REF], the only recent constraints on the timing of the Alpine deformation were performed on strike-slip shear zones of the eastern Oisans Massif (Simon-Labric et al., 2009). The most prominent structures accommodating most of the shortening, W-directed reverse shear zones remain therefore undated.

Aims and methods

To put age-constraints on reverse shear zones developed during collision in the Oisans Massif, deformed rocks of four main shear zones were sampled across the W-E profile. These are, from west to east: the Col de Cluy, Plan du Lac, Col du Lac and Combeynot shear zones (Fig. 2), all sharing a common and consistent E-W to SE-NW stretching lineation and an overall top-to-the-west sense of shear. In situ 40 Ar/ 39 Ar laser ablation method [START_REF] Maluski | 40Ar-39Ar laser probe multi-dating inside single biotites of a Variscan orthogeneiss (Pinet, Massif Central, France)[END_REF] was chosen as it allows linking radiochronology to petrology and particularly to textures/structures [START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps[END_REF][START_REF] Müller | Strengthening the link between geochronology, textures and petrology[END_REF][START_REF] Augier | Exhumation, doming and slab retreat in the Betic Cordillera (SE Spain): in situ40Ar/39Ar ages and P-T-d-t paths for the Nevado-Filabride complex[END_REF][START_REF] Gerber | Evolution tectono-métamorphique du Briançonnais interne (Alpes Occidentales, massifs de Vanoise Sud et d'Ambin): comportement du socle et de sa couverture dans un contexte de subduction continentale profonde[END_REF]. This link is particularly important as A c c e p t e d M a n u s c r i p t different parameters such as temperature, deformation or fluid circulations may drastically impact the behaviour of the K/Ar system. It is therefore crucial is to decipher the respective contribution of these parameters to isolate the signal related to deformation.

To ensure a reliable T record, temperatures were constrained at the scale of the Oisans Massif and the neighbouring areas using the Raman Spectrometry of Carbonaceous Material method (RSCM; [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF][START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF]. These data were supplemented by local temperature estimates using the chemical composition of chlorite in the chloritequartz-water equilibrium [START_REF] Vidal | A Thermodynamic Model for Fe-Mg Aluminous Chlorite Using Data from Phase Equilibrium Experiments and Natural Pelitic Assemblages in the 100° to 600°c, 1 to 25 kb Range[END_REF][START_REF] Vidal | Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation[END_REF][START_REF] Vidal | P-Tdeformation-Fe3+/Fe2+ mapping at the thin section scale and comparison with XANES mapping: application to a garnet-bearing metapelite from the Sambagawa metamorphic belt (Japan)[END_REF] either to control RSCM results or to put direct constraints within the shear zones. In parallel, impact of the deformation was evaluated by a combined textural/petrological study of the dated samples and particularly the behaviour of white-micas. Besides, the potential impact of the fluid percolations was evaluated by a careful study of output isotopic ratios. The next sections are dedicated to a brief description of the methods used in this study.

Raman spectrometry of carbonaceous material (RSCM)

The Raman Spectrometry of Carbonaceous Material method [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF] is based on the quantitative study of the degree of graphitization of carbonaceous material (CM), which is a reliable indicator of metamorphic T. Because of the irreversible character of graphitization, CM structure is not sensitive to the retrograde reactions during exhumation and therefore depends on maximum T (noted Tmax, thereafter) reached during metamorphism [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF]. Tmax can be determined in the range 200-650°C by using two empirical geothermometers which link the degree of graphitisation measured from Raman Spectra (RS) to the highest temperature ever experienced by the rock [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF][START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF]. [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF] geothermometer was used when the D4 defect band was present in the RS, for the low-grade rocks. Absolute precisions of ca. 50°C between 650 and 300°C and ca. 25°C between 330 and 200°C are related to uncertainties on petrological data used for the calibration. However, relative uncertainties on Tmax appear however much smaller, probably around 10-15°C and variations of that order of magnitude may be detected [START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF][START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF][START_REF] Gabalda | Thermal structure of a fossil subduction wedge in the Western Alps[END_REF][START_REF] Vitale Brovarone | Stacking and metamorphism of continuous segments of subducted lithosphere in a high-pressure wedge: the example of Alpine Corsica (France)[END_REF].

All Raman spectra were measured and analysed following procedures described in [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF][START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF][START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF] and [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF]. Raman spectra were obtained using a Renishaw InVIA Reflex microspectrometer (IMPMC, Paris, France) with a 514 nm argon laser. RSCM analyses were conducted on thin sections prepared on CM-rich metasediments A c c e p t e d M a n u s c r i p t particularly abundant in the cover series in the studied area, cut in the structural X-Z plane (i.e. orthogonal to foliation and parallel to lineation). To avoid defects on the CM related to thin-section preparation, analyses were all performed below the surface of the section by focusing the laser beam beneath a transparent crystal (i.e. dominantly quartz, calcite, albite and occasionally phyllosilicates). Before each session, the spectrometer was calibrated with a silicon standard. 10-20 spectra were routinely recorded for each sample to smooth out the inner structural heterogeneity of CM within samples.

Microprobe and independent temperature estimates

Mineral analyses were performed with a Cameca SX50 electron microprobe (ISTO-BRGM, Orléans). Analytic conditions were 15 kV accelerating voltage and 10 nA beam current using Fe2O3 (Fe), MnTiO3 (Mn, Ti), diopside (Mg, Si), CaF2 (F), orthoclase (Al, K), anorthite (Ca), albite (Na) as standards. Elements, which are measured in %weight of oxide, are SiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, MnO, CaO, Na 2 O, K 2 O, TiO 2 , F and Cl. Chemical composition of chlorite and the white-micas were particularly studied. Composition of chlorite has been used to retrieve independent T constraints using a thermodynamic approach [START_REF] Vidal | A Thermodynamic Model for Fe-Mg Aluminous Chlorite Using Data from Phase Equilibrium Experiments and Natural Pelitic Assemblages in the 100° to 600°c, 1 to 25 kb Range[END_REF][START_REF] Vidal | Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation[END_REF][START_REF] Vidal | P-Tdeformation-Fe3+/Fe2+ mapping at the thin section scale and comparison with XANES mapping: application to a garnet-bearing metapelite from the Sambagawa metamorphic belt (Japan)[END_REF]. Besides, composition of white-micas was used to distinguish different mineral generations to characterise the dated minerals.

Chlorite, which is abundant in the studied low-grade metamorphic rocks, was used as a geothermometer. Temperatures were estimated by two ways: i) traditional empirical geothermometry based on temperature dependency of the Si, Al and vacancy content of the tetrahedral and octahedral sites within chlorite [START_REF] Cathelineau | A chlorite solid solution geothermometer the Los Azufres (Mexico) geothermal system[END_REF][START_REF] Cathelineau | Cation Site Occupancy in Chlorites and Illites as a Function of Temperature[END_REF], and ii) a thermodynamical approach [START_REF] Vidal | A Thermodynamic Model for Fe-Mg Aluminous Chlorite Using Data from Phase Equilibrium Experiments and Natural Pelitic Assemblages in the 100° to 600°c, 1 to 25 kb Range[END_REF][START_REF] Vidal | Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation[END_REF][START_REF] Vidal | P-Tdeformation-Fe3+/Fe2+ mapping at the thin section scale and comparison with XANES mapping: application to a garnet-bearing metapelite from the Sambagawa metamorphic belt (Japan)[END_REF]) that integrates multiple substitutions (FeMg -1 , Al IV Al VI Si -1 Mg -1 , (Mg, Fe 2+ ) 3 □ -1 Al -2 where □ is the vacancy). In the last case, metamorphic temperatures that prevailed during the formation of chlorite were estimated upon 4 equilibria at fixed water activity and pressure (a w =1 and P=4kbar (Simon-Labric et al., 2009)). Standard deviations are routinely close to ±10°C (from ±4°C to ±32°C).

In situ 40 Ar/ 39 Ar dating

The 40 Ar/ 39 Ar in situ laser ablation method was first described by [START_REF] Schaeffer | Laser Ar-39-Ar-40 study of Apollo 17 basalts[END_REF] and successively upgraded by [START_REF] York | 40Ar/39Ar dating of terrestrial minerals with a continuous laser[END_REF] and [START_REF] Maluski | 40Ar-39Ar laser probe multi-dating inside single biotites of a Variscan orthogeneiss (Pinet, Massif Central, France)[END_REF].

Procedure was detailed in [START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps[END_REF], [START_REF] Augier | Exhumation, doming and slab retreat in the Betic Cordillera (SE Spain): in situ40Ar/39Ar ages and P-T-d-t paths for the Nevado-Filabride complex[END_REF] or [START_REF] Turrillot | Late orogenic exhumation of the Variscan high-grade units (South Armorican Domain, western France), combined structural and 40Ar/39Ar constraints[END_REF].

A c c e p t e d M a n u s c r i p t

We recall here the main stages for 40 Ar/ 39 Ar in situ sample preparation and analytical procedure.

Measurements were all performed at Géosciences Montpellier (France) with an analytical system that consists of: (a) an IR-CO 2 laser of 100 kHz used at 5-15% during 60-80 sec; (b) a beam shutter for selection of exposure times; (c) a lenses system for beam focusing; (d) a camera connected to the monitor for beam focusing and selection of ablated zones; (e) a steel chamber with a copper plate and the four samples on. This chamber is depressurized to 10 -8 to 10 -9 bar; (f) an inlet line for purification of gases including two Zr-Al getters; (g) a Argus VI (Thermo-Fisher) multi-collector mass spectrometer from; (h) a software which controls the timing of extraction/purification and the data acquisition. Rock sections of 1 mm thick, which had been used to make the petrographic thin-sections, were double polished to c. 1 µm. Whole section and detailed-area photographs of both the rock section and corresponding thin section were taken for an accurate selection of suitable areas for laser experiments. All samples were ultrasonically rinsed in ethanol and distilled water, wrapped in pure aluminium foils and then irradiated in the Triga Mark II nuclear reactor (Pavia, Italia) with several aliquots of the Fisch Canyon sanidine standard (28.03 ± 0.08Ma, [START_REF] Jourdan | Age calibration of the Fish Canyon sanidine 40Ar/39Ar dating standard using primary K-Ar standards[END_REF] as flux monitors. After irradiation, both the monitors and the sections were placed on a Cu-holder inside the sample chamber and heated for 48 h at 150-200 °C.

For each experience, argon was released from a 100 x 300 µm mean surface corresponding very often to aggregates of white micas. Incision of the sample did not exceed 10-20 µm depending on the three-dimensional orientation of the white mica crystals. For each experiment, ages have been obtained after correction with blanks, mass discrimination, radioactive decay of 37 Ar and 36 Ar and irradiation-induced mass interferences. They are reported with 1 uncertainty and were evaluated assuming an atmospheric composition for the initially trapped argon [i.e. ( 40 Ar/ 36 Ar) i = 295.5]. The proportion of 38 Ar and 37 Ar isotopes that result from interference with chlorine and calcium during irradiation reveals the contribution of Cl and Ca-rich minerals or inclusions to the overall argon release and so to the calculated age. A blank analysis was done every two or three analyses to evaluate the argon background within the whole analytical system. Isotopic data are computed with the ArArCalc freeware [START_REF] Koppers | ArArCALC-software for 40Ar/39Ar age calculations[END_REF] based on the algorithms described in [START_REF] York | Least squares fitting of a straight line with correlated errors[END_REF][START_REF] Faure | Principles of Isotope Geology[END_REF][START_REF] Mcdougall | Geochronology and Thermochronology by the 40Ar/39Ar Method[END_REF][START_REF] Renne | Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating[END_REF][START_REF] Min | A test for systematic errors in 40Ar/39Ar geochronology through comparison with U/Pb analysis of a 1.1-Ga rhyolite[END_REF].

Along with a strong dependency with temperature and deformation, the behaviour of the K/Ar system in white micas can be partially altered by fluid circulations. As fluid A c c e p t e d M a n u s c r i p t circulations are hardly detected a priori, output isotopic ratios ( 36 Ar/ 39 Ar, 37 Ar/ 39 Ar and 38 Ar/ 39 Ar ratios) were carefully checked.

Temperature record within the sedimentary cover

A large-scale RSCM Tmax cover in the sedimentary series

First order thermal features

Our dataset consists of 70 samples collected across the external zones, from Vif to the Penninic Frontal Thrust (E-W transect) and from Saint Jean de Maurienne to Chorges (N-S transect) in various lithologies and all structural units of the studied area except Variscan basement rocks (Table 1, Fig. 4). A more systematic sampling was performed along the E-W transect where the thermal structure is so far poorly documented. Results are all presented in maps of various scales and projected along structure-orthogonal crosssections. For the sake of comparison, results of previous studies including illite-crystallinity maps [START_REF] Aprahamian | La cristallinité de l'illite et les minéraux argileux en bordure des massifs cristallins externes de Belledonne et du Pelvoux (Variations et relations possibles avec des événements tectoniques et métamorphiques alpins)[END_REF][START_REF] Barlier | L'évolution des Terres noires subalpine méridionales d'après l'analyse minéralogique des argiles et la réflectométrie des particules carbonées[END_REF] or RSCM data in bounding areas (e.g. [START_REF] Gabalda | Thermal structure of a fossil subduction wedge in the Western Alps[END_REF] are also shown (Fig. 4).

At first glance, an unexpected major result of this study is the relative homogeneity of the Tmax value at the scale of study area, with an average value of ca. 330°C (329±8 for 27 Tmax values along an E-W cross-section), from the Lower Jurassic series close to Vif, all the way to the Galibier pass, at the eastern edge of the study area (Fig. 4,5). Conversely, lower Tmax ca. 200-220°C is recorded to the south either in the Mesozoic series or Eocene flysch (north of Saint Michel de Chaillol; Fig. 4). Moreover, Tmax results recorded further south along an E-W section between Chorges and l'Argentière-la-Bessée (Fig. 4) highlight a drastic southward lateral change in the thermal structure. There, Tmax values present an overall upward temperature increase from 253±10°C to more than 330±5°C in the vicinity of the Penninic Frontal Thrust. Besides, additional important temperature variations were recorded either to the west of the study area (Belledonne and La Mure massifs), to the east in the vicinity of the Penninic Frontal Thrust (Fig. 4,5). These variations are described here in more details.

Thermal structure of the western bound of the Belledonne and La Mure massifs

To the west of the Belledonne and La Mure massifs, Tmax values decrease upward the stratigraphic succession from 334±6°C-329±8 in the Liassic, 325±6-286±5°C in A c c e p t e d M a n u s c r i p t calcareous Dogger and 227±23-<200°C in upper Jurassic black-shales (Fig. 4,5) highlighting a ca. 130°C temperature variation. In details, Tmax values gradually decrease sequence along a ca. 30°C/km gradient but also highlight a localised temperature decrease near the top of the calcareous Dogger (296±5-286±5°C) and the base of the upper Jurassic (227±23-204±14°C). This ca. 80°C temperature gap can be followed 40 km strike, which is consistent with a normal sense west-dipping tectonic contact located within the upper Jurassic black-shales (Oxfordian). However, due to poor local outcrop conditions, this contact has not been observed in the field.

In the same area, the base of thick Jurassic sequences of the Vercors yielded Tmax value close to 335°C (334±6°C) while the thin Jurassic cover of the Belledonne and La Mure massifs yielded only ca. 280°C (279±9°C). In this case, Tmax distribution evidenced an east-verging shear zone/thrust consistent with the description of a large-scale backthrust in the field by [START_REF] Butler | The influence of pre-existing basin structure on thrust system evolution in the Western Alps[END_REF]. The Mesozoic sequence of the Vercors therefore appears i) transported eastward above the La Mure thrust which roots in Triassic or Liassic units, and ii) is partly dismembered by an extensional structure (Fig. 4,5).

Thermal structure in the vicinity of the Penninic Frontal Thrust

To the east of the study area, close to the Lautaret and Galibier passes, Tmax values decrease gradually from ca. 330°C in the easternmost Mesozoic cover of the Oisans Massif to ca. 260°C in the Helmintoids flysch (Tête Noire area) despite the presence of numerous tectonic contacts including the PFT. Tmax results therefore align well along an apparent 20-30°C.km -1 gradient suggesting either that the Tmax imprint was post-tectonic or that extensional movements are not restricted to the PFT and rather distributed over numerous small-scale, normal-sense structures. In any case, in their current position, Briançonnais units did not experience Tmax higher than ca. 270°C which fall well below the ca. 330°C temperatures characterising the internal Dauphinois. Conversely, a ca. 40°C localized temperature drop is recorded further south, across the PFT (around L'Argentière-la-Bessée, Fig. 4) where the Nummulitic series yielded ca. 330±5°C while Briançonnais units only experienced 291±6°C. There, this result is consistent with both field and low-temperature thermochronology evidences of the reactivation of the PFT as an extensional structure [START_REF] Tricart | La denudation tectonique de la zone ultradauphinoise et l'inversion du front brianconnais au sud-est du Pelvoux (Alpes occidentales); une dynamique miocene a actuelle[END_REF][START_REF] Tricart | Evidence of synextension tilting and doming during final exhumation from analysis of multistage faults (Queyras Schistes lustrés, Western Alps)[END_REF][START_REF] Tricart | The late extension in the inner western Alps: a synthesis along the south-Pelvoux transect[END_REF][START_REF] Tricart | Diachronous late-stage exhumation across the western Alpine arc: constraints from apatite fission-track thermochronology between the Pelvoux and Dora-Maira Massifs[END_REF][START_REF] Ceriani | From n-s collision to WNW-directed post-collisional thrusting and folding: Structural study of the Frontal Penninic Units in Savoie (Western Alps, France)[END_REF].

Independent temperature record using chlorite geothermometry

A c c e p t e d M a n u s c r i p t Four samples were selected from the sample collection of the sedimentary cover for the RSCM approach (see location on Table 2 and on Fig. 2). The main aim of this approach is to estimate the temperature recorded by the cover series using independent geothermometers based on the chemical composition of chlorites [START_REF] Cathelineau | A chlorite solid solution geothermometer the Los Azufres (Mexico) geothermal system[END_REF][START_REF] Cathelineau | Cation Site Occupancy in Chlorites and Illites as a Function of Temperature[END_REF][START_REF] Vidal | A Thermodynamic Model for Fe-Mg Aluminous Chlorite Using Data from Phase Equilibrium Experiments and Natural Pelitic Assemblages in the 100° to 600°c, 1 to 25 kb Range[END_REF][START_REF] Vidal | Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation[END_REF][START_REF] Vidal | P-Tdeformation-Fe3+/Fe2+ mapping at the thin section scale and comparison with XANES mapping: application to a garnet-bearing metapelite from the Sambagawa metamorphic belt (Japan)[END_REF]. Results are all presented on figure 6.

Chlorites present different habits and occur in various textures. They are either dispersed in the matrix (samples A-2-2, A-4-232, see location Fig. 2), crystallized within veins (samples A-1-345( 2)) or along the foliation and top-to-the-W shear bands (sample A-1-321).

Irrespectively to their structural position and to the potential impact of the protolith bulk chemical composition, the two geothermometers yielded quite consistent results for all samples within both the analytical and calibration errors and the within-sample internal dispersion. However, the thermodynamical approach generally yield more dispersed temperature results between ca. 210 and 350°C than empirical geothermometry that show an unimodal distribution centrered on the narrow ca. 270-330°C range (Fig. 6). Temperature ranging from 150 to 350°C was retrieved from sample A.2.2, 150-340°C from sample A-4-232, 230-380°C for sample A-1-321 and 240-390°C for sample A-1-345 (Fig. 6).

Interestingly, RSCM Tmax fall systematically within the high temperature value indicating i) the cross-validation of the methods and that ii) chlorites, at variance with the RSCM, do record other discrete metamorphic increments. However, textural observations did not allow distinguishing the relative timing of successive chlorite growth events during the metamorphic evolution (prograde or retrograde (re-)equilibrations).

Phyllosilicate habits, chemical composition and chlorite temperature (TChl) record for dated samples

The crystallisation-deformation relationships of chlorite and white-micas that were first recognised in the field were then strengthened by detailed textural observations in thinsection. Mineral chemical compositions were used to constrain i) the chemistry of dated minerals and ii) the physical conditions by chlorite geothermometry. Together with textural observations, these results are of prime importance to gather accurate a priori constraints and discussion elements for in situ laser-probe investigations.

A c c e p t e d M a n u s c r i p t

First order common features: guidelines for the 40 Ar/ 39 Ar in situ approach

At the scale of the outcrop, rocks present routinely large white-mica crystals.

Conversely, aggregates of very fine-grained white-micas occur only in deformed zones. At the scale of thin-sections, deformed samples present two contrasted habits of white-micas.

Large, up to 2mm variously oriented white-mica flakes appear closely associated with a former high-temperature foliation and high-temperature parageneses (Fig. 7). These crystals coexist with aggregates of fine-grained white-micas growing either at the expense of former large crystals or in cracks of various scales within feldspars. Besides, this population presents a close association with deformation structures and particularly with top-to-thewest shear bands within proto-mylonites, mylonites or phyllonites. This strong correlation has been observed at the scale of strain gradients along which large, well-developed whitemica clasts tend to disappear whereas fine-grained blasts form the frame of the rock (Fig. 7).

Interestingly, EPMA analyses reveal the existence of two texturally controlled groups of white-mica of contrasted compositions reminding the results of [START_REF] Lanari | Diachronous evolution of the alpine continental subduction wedge: Evidence from P-T estimates in the Briançonnais Zone houillère (France -Western Alps)[END_REF] in the Briançonnais domain, further east. The first group, which plots close to the muscovite endmember composition, is mainly represented by the large white-micas clasts population carried by an earlier foliation (Fig. 7). Besides, the Na-content of this population is systematically higher than 0.05 a.p.f.u. (atoms per formula unit) while the Si content is lower than 3.1 a.p.f.u. (Fig. 7). The second group, mostly corresponding to fine-grained aggregates of white-micas, has a phengite chemistry, i.e. between muscovite and celadonite end-member due to Tschermak substitutions. This group presents variable silica-content scattered between 3.2 and 3.6 a.p.f.u. and Na-content systematically lower than 0.05 a.p.f.u.

(Fig. 7,8). However, the small size of crystals and large internal variations of compositions suggest a partial mixing between two end-members.

EPMA analysis of the chemical compositions confirm the presence of two unrelated generations of white-micas recognised on the basis of textural observations and characterized by contrasted chemical compositions. Being conspicuously involved with the greenschist facies partial reequilibration of former high-temperature parageneses and closely associated with a top-to-the-W asymmetry, phengites therefore appear as a synkinematic mineral particularly suitable to put age-constraints on the deformation. Conversely, the habits of pre-kinematic clasts and the muscovite composition of most of the large whitemica population are likely inherited crystals pertaining to Variscan parageneses.

A c c e p t e d M a n u s c r i p t

Besides, temperature, which is the most important factor controlling Argon diffusion, must be estimated to make a difference between cooling and deformation (crystallisation) ages. Similarly to temperature estimates performed in the cover series, temperature recorded by basement rocks was estimated using two different chlorite geothermometers. Results are all presented in figure 8.

Textural and chemical characteristics of dated samples

Following this structural and chemical guideline, white-micas present in the dated samples were carefully studied. Besides, chemical composition of chlorite was used to constrain the temperature experienced by the rocks. Results are all presented from west to east on figure 8.

Col de Cluy shear zone

Samples from Col de Cluy are Variscan micaschists and gneisses with greenschist to amphibolite facies associations with quartz, white-micas, feldspar, biotite and/or chlorite.

Rocks show generally only an incipient recrystallization. Former biotite and K-feldspar are still preserved and locally abundant. White-micas, which are present in all samples, mostly occur as ca. 1-2mm weakly deformed flakes and locally as clasts involved in shear bands.

This population coexists with fine-grained white-micas that occur as rare isolated aggregates. Grain-size is generally less than 10 x 50 µm and white-mica crystals are often closely associated with chlorite. EPMA analyses of the white-micas mostly plot within a rather narrow composition range close to the muscovite end-member. Analyses of finegrained white-micas plot along continuous pattern from muscovite (Si(3.05); Na(0.09;0.12)) to low-silica content phengite (Si(3.15;3.25); Na(0;0.02)) (Fig. 8). Chlorite occurs as numerous small-scale aggregates. Chlorite shows variable composition with 70 to 85% of clinochlore and XMg values close to 0.45 for variable silica content. Geothermometric results yield quite contrasting results. While the thermodynamic approach yielded dispersed temperature between 150 to 370°C, empirical geothermometry showed quite consistent ca. 230 to 320°C temperature results (Fig. 8). Being devoid of conspicuous Alpine reequilibrations, these samples appear, at a first glance poorly suitable to allow dating the deformation and were therefore not used for the 40 Ar/ 39 Ar in situ approach.

A c c e p t e d M a n u s c r i p t

Plan du Lac shear zone

Thanks to good outcrop conditions allowing the recognition of the strain distribution, the Plan du Lac shear zone was particularly studied. Four samples were selected over the strain gradient. Sample A-4-74b is a proto-mylonite sample showing a rather weak Alpine deformation by comparison to samples A-4-68 and 68b that are phyllonites. Sample A-4-66b was selected from an intermediate position within the strain gradient. All samples show a clear top-to-the-west asymmetry except sample A-4-66b picked along a localized top-to-the-E shear zone.

All samples derive from a quartz, feldspar, biotite and white mica Variscan gneiss.

Chlorite pervasively replaces biotite either as static pseudomorphs in low strain domains or as synkinematic crystallisations in mylonites. In low strain domain, white mica occur as large, up to ca. 1-2mm clasts with clear disequilibrium textures while fine-grained whitemica are rare and clearly secondary. Conversely, mylonite and phyllonite are almost devoid of large clasts and exclusively made of fine-grained white-micas. EPMA composition of white-micas reveals two distinct poles with very few intermediate values (Fig. 8). The first pole that corresponds to muscovite (Si(3.15); Na(0.06;0.08)) and the second one to a silicarich phengite (Si(3.2;3.4); Na(0;0.02)) (Fig. 8). Interestingly, mineral composition shows a conspicuous dependency to deformation. Mylonite and phyllonite samples yield mostly phengitic white-micas. Conversely, less-deformed proto-mylonite sample yield texturallycontrolled chemical compositions with muscovite compositions for the large clasts and phengite composition for the fine-grained blasts.

Chlorite is locally abundant. It occurs in the matrix, growing at the expense of biotite, crystallized in syn-deformation veins or associated with fine-grained white-micas along shear planes. Most analyses yield consistent results with 70 to 90% of clinochlore content, an XMg close to 0.45 for a variable silica content (Fig. 8). Regardless of strain intensity, structural position and habits, most results cluster between 260 to 290°C (Fig. 8).

Col du Lac shear zone

Sample A-1-355 was collected from the core of a top-to-the-W shear band characterized by a thin layer of phengite-quartz phyllonites. This sample derives from a Variscan high-grade gneiss still preserved a few decimetres apart on the same outcrop. The rock is made of quartz, feldspar and biotite partly turned into chlorite. Interestingly, white A c c e p t e d M a n u s c r i p t mica is absent from the mineralogical composition of the protolith. Small-scale but abundant white-mica aggregates from the phyllonites are therefore newly-crystallized as the result of strain and/or fluid-induced destabilization of the K-feldspar (e.g. Gueydan et al., 2003).

Accordingly, EPMA analyses of the white-micas all yielded phengitic compositions with a very low dispersion (Si(3.2;3.4); Na(0;0.02)).

Chlorite occurs as small-scale aggregates and rare dispersed larger crystals that may derive from former biotites. Chlorite shows almost constant composition close to the clinochlore end-member with XMg values around 0.6. Most geothermometric results cluster between 250 and 290°C for both geothermometers (Fig. 8).

Combeynot shear zone

Sample A-1-345 was selected in the most deformed outcropping ductily-deformed basement rocks of the Combeynot shear zone. Mylonites are scarce and proto-mynolites, derived from orthogneiss, which belongs to a volcano-sedimentary complex [START_REF] Barbier | Geological map and explanatory text of the sheet n°798[END_REF], are often directly overthrusted onto the sedimentary cover. White micas occur generally as large clasts surrounded by very fine-grained blasts. Aggregates of fine-grained white-micas also occur in tension gashes in K-feldspar. Composition of white-micas shows a rather continuous pattern from muscovite (Si(3.05;3.15); Na(0.06;0.08)) to phengite (Si(3.25;3.5); Na(0;0.02)) (Fig. 8). Despite a clear partially reequilibration within Alpine physico-chemical conditions, Alpine white-micas occur as very fine grained aggregates.

Chlorite is rare, small and often closely associated with white-micas. Thus, most chlorites show mixed composition with white-micas (i.e. K-rich chlorite) and were thus discarded.

Data are thus only presented in table 2. Synkinematic fine-grained aggregates of phengite appear particularly adapted to put age-constraints on the activity of the shear zones. Accordingly, samples from Col de Cluy shear zone which may record only a weak Alpine re-equilibration have not been selected from the in situ 39 Ar/ 40 Ar approach. For all other samples, success of the study relies on our ability to effectively analyze newly formed crystals whose typical size is often small (150µm max.).

Temperatures recorded in the shear zones yielded very consistent results close to 250-300°C which falls below RSCM Tmax recorded in the cover series. Chlorite therefore probably recorded retrograde metamorphic increments during exhumation and cooling of the Oisans Massif. It is noteworthy that temperatures recorded at peak-T conditions also fall below the lower bound of the closure temperature of white-micas (e.g. [START_REF] Dodson | Closure temperature in cooling geochronological and petrological systems[END_REF][START_REF] Freeman | Deformation migration in an orogen-scale shear zone array: an example from the Basal Briançonnais Thrust, internal Franco-Italian Alps[END_REF][START_REF] Villa | Isotopic closure[END_REF].

6 40 Ar/ 39 Ar age constraints Our dataset consists of 199 in situ 40 Ar/ 39 Ar ages performed on 7 samples collected over three main Alpine shear zones, namely (from west to east), the Plan du Lac, Col du Lac, and the Combeynot shear zones (Table 3, see location on Fig. 2). To ensure the coverage of the different strain facies recognized in the field, a more systematic sampling was performed over the Plan du Lac shear zone. Results of the in situ 40 Ar/ 39 Ar approach are all shown and discussed in Fig. 9 as apparent ages for which an initial 40 Ar/ 39 Ar ratio of 295.5 is assumed for the non-radiogenic argon component. These age results are described in the light of the textural/structural observations, chemical composition results and the output isotopic ratios ( 36 Ar/ 39 Ar, 37 Ar/ 39 Ar and 38 Ar/ 39 Ar) informative of potential disturbances of the K/Ar system. For small-scale phengite aggregates located in the vicinity of chlorite, calcium or fluid inclusions-bearing minerals, the release of the 38 Ar and 37 Ar isotopes interfering with chlorine and calcium revealed that argon was extracted from both phengites and K-poor minerals [START_REF] Brereton | Corrections for interfering isotopes in the 40 Ar/ 39 Ar dating method[END_REF][START_REF] Mcdougall | Geochronology and Thermochronology by the 40Ar/39Ar Method[END_REF]. The quoted errors represent one sigma deviation and do not include uncertainty on the monitor. Errors are routinely inferior to ca. 1 Ma while larger error (ca. 2 to 15 Ma) are generally due to both a weak argon signal and a high atmospheric contribution (Table 3).

At a first glance, an important conclusion is that the successive phengite generations recognized on the basis of textures (e.g. Fig. 7), compositions (Figs 8) effectively yield successive ages. Fine-grained aggregates of synkinematic phengites yield Alpine ages sensu lato. Conversely, white-micas characterized by muscovite chemical compositions and/or presenting clear pre-kinematic (i.e. inherited) habits yielded pre-alpine ages or even sometimes Variscan ages. Interestingly, a clear dependency to the intensity of the deformation may even be detected either at the scale of the sample or at the scale of the strain gradient within a shear zone (e.g. Fig. 10). Therefore, most ages are tentatively interpreted either as, at least partially, re-opened Variscan isotopic systems for "old" ages or as genuine crystallization ages for the "young" ca. 35-25 Ma ages. More locally, results however show important age variations that may also record fluid percolations.

Plan du Lac shear zone

A c c e p t e d M a n u s c r i p t

In order to study the potential impact of the finite Alpine deformation, four samples were selected over a single strain gradient toward the core of the Plan du Lac shear zone.

The relative structural position of samples is given on figure 10.

A-4-74b (Proto-mylonite)

In this sample, discrete ca. 1mm-thick shear bands partially overprint a hightemperature inherited texture composed by large, brittlely fractured feldspar clasts and more ductilely stretched quartz and muscovite crystals (Fig. 10). Conversely, this sample contains Alpine phyllosilicates including phengite and chlorite either carried along a weak vertical foliation, or crystallized along top-to-the-W shear bands. A total of 40 apparent in situ analyses were performed to cover all situations. Apparent in situ ages are scattered between 207 ± 5.2 Ma and 24.3 ± 1.8 Ma (Fig. 10). 22 analyses corresponding to the oldest ages are correlated with high 38 Ar/ 39 Ar ratio and atmospheric content (Table 3) but ages fall within the age range of uncontaminated phengites. Interestingly, the core of the shear bands provides ca. 25 Ma consistent ages (24.3 ± 1.8 Ma and 26.7 ± 0.6 Ma).

A-4-66b (Top-to-the-E Mylonite)

This sample is characterized by a schistosed high-grade gneiss traversed by a 3 mmthick top-to-the-E shear band (Fig. 10). While Variscan fabrics are still well developed in most of the rock volume, Alpine synkinematic recrystallizations are almost complete within shear bands. There, former muscovites and feldspars are absent and a ca. 2mm thick phengitized fringe is developed (Fig. 10). 34 in situ analyses were performed. No relationship between apparent ages and the 37 Ar/ 39 Ar and 38 Ar/ 39 Ar variations were detected, except for the 3 three apparent ages older than ca. 100Ma, which have high 38 Ar/ 39 Ar values (Table 3). Analyses show a first order textural control of the age. Large muscovites, at least partially preserved within the matrix yielded 301.4 ± 6.6 and 62.3 ± 1.4 Ma. Conversely, small-scale phengites within the shear band yielded 30 alpine ages ranging between 40.5 ± 1.1 Ma and 29.5 ± 0.6 Ma plus an older age of 47.6 ± 1.2 Ma (Fig. 10). Interestingly, while ages depict a clear first order younging toward and within the shear band, older ages occur within the core of the shear band over a ca. 1-2mm-thick fringe (Fig. 10). In this case, a late gradient of 40 Ar excess seems to overprint an initial age gradient related to deformation.

A-4-68 (Phyllonite)

A c c e p t e d M a n u s c r i p t Sample A-4-68 is a phyllonite sample. 15 in situ analyses were performed and yielded ages ranging between 93.5 ± 2.0 and 22.4 ± 0.9 Ma (Table 3). Most of apparent ages cluster between 32 and 22 Ma (Fig. 10). There is no relationship of apparent ages with 37 Ar/ 39 Ar unlike with 38 Ar/ 39 Ar, suggesting some interaction with chlorine-rich fluids that may occur as trapped inclusions (Fig. 10).

A-4-68b (Phyllonite)

Sample A-4-68b was cut in two distinct rock slabs (section 8 and 5) selected only a few centimeters distant; section 5 being a little less deformed than section 8. Analyses performed on section 5 yielded 21 apparent ages, ranging between 114.2 ± 4.3 and 25.9 ± 0.5 Ma (Fig. 10). Analyses present a strong relationship between apparent ages and 37 Ar/ 39 Ar, 38 Ar/ 39 Ar and atmospheric content. In the absence of Ca-rich and Cl-rich phases in the chip, contamination may be rather related with fluid inclusions trapped in the analysed phengites (Table 3). Despite contamination by Ca-rich and Cl-rich fluids, analyses yielded ages of about 25-27 Ma. Interestingly, analyses performed on section 8 yielded 44 apparent ages, which do not show any evidence of a direct relationship with the 37 Ar/ 39 Ar, 38 Ar/ 39 Ar and atmospheric content. These ages are scattered between 37.2 ± 1.0 Ma and 23.2 ± 0.6 Ma (with two old ages of 46.4 and 47.2 Ma) and define mainly two age groups at 31-33 Ma ( 7ages) and 25-27 Ma (18 ages) (Fig. 10).

Col du Lac shear zone (Sample A-1-355, phyllonites)

Sample A-1-355 was collected from the core of a top-to-the-W shear band characterized by a ca. 1 meter-thick layer of phyllonites. This sample derives from a Variscan high-grade gneiss still preserved a few decimetres apart on the same outcrop and turned into a phengite-quartz rock in the most deformed zone. It contains few chlorites and no remnant of inherited muscovites present in the undeformed protolith. The 30 apparent ages are scattered between 110 ± 7 Ma and 15 ± 10 Ma and 25 of them range between 42.68 ± 2.22 and 23.88 ± 1.6 Ma (Table 3). No correlation between the apparent age and 38 Ar/ 39 Ar and 37 Ar/ 39 Ar ratios was observed. However, some analyses, particularly the "old", up to 50 Ma ages present high 38 Ar/ 39 Ar values and/or an important atmospheric contribution (Table 3). Interestingly, these old ages were found in micas from C'-type shear bands and therefore argue for a 40 Ar excess. These results are quite similar to those obtained in the most deformed parts of sample A-4-66b. Despite this potential contamination, analyses with low A c c e p t e d M a n u s c r i p t 38 Ar/ 39 Ar ratio and low atmospheric content yielded 15 internally consistent apparent ages scattered between 32.9 ± 0.9 and 24.8 ± 0.6 Ma (Fig. 10).

Combeynot shear zone (Sample A-1-345, proto-mylonite)

Sample A-1-345 was selected in the most deformed outcropping ductilely-deformed basement rocks of the Combeynot shear zone. Mylonites are scarce and protomylonites, derived at the expense of orthogneiss (former leucogranite) are often directly overthrusted onto the sedimentary cover. It is the less deformed sample of the data set. None of the 15 in situ analyses provide Alpine age (Table 3, Fig. 10). Instead, apparent ages are distributed between 389 ± 27 Ma and 156 ± 3 Ma with no clear textural control. Part of the apparent ages is correlated with the 38 Ar/ 39 Ar ratio, which suggests a possible contribution of Cl-rich phases or fluid inclusions. High 37 Ar ⁄ 39 Ar ratios (Table 3) are probably due to the degassing of Variscan epidote or titanite, but again no systematic correlation between the variation of these ratios and the age dispersion was observed.

Discussion

Reliability, significance of the RSCM Tmax mapping

RSCM Tmax estimates has proven an efficient mean to put extensive temperature constraints on the thermal structure of coherent units or on the importance of tectonic contacts [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF][START_REF] Augier | Exhumation, doming and slab retreat in the Betic Cordillera (SE Spain): in situ40Ar/39Ar ages and P-T-d-t paths for the Nevado-Filabride complex[END_REF][START_REF] Negro | Thermal structure of the Alboran Domain in the Rif (northern Morocco) and the Western A c c e p t e d M a n u s c r i p t Betics (southern Spain). Constraints from Raman spectroscopy of carbonaceous material[END_REF][START_REF] Gabalda | Thermal structure of a fossil subduction wedge in the Western Alps[END_REF][START_REF] Angiboust | Couplages profonds et comportement de la lithosphère océanique dans les zones de subduction: approches pétrologiques et thermomécaniques[END_REF]. In the Oisans Massif, RSCM Tmax fall in the same temperature range as more punctual results issued from other methods available in literature. The most important result is the uniformity of Tmax values at the scale of study area, with an average value of ca. 330°C with a relative error that may not exceed 10-15°C [START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF].

Thermopaleomagnetic studies indicate that cooling started from 335°C in the north of the Emparis formations and in the Bourg d'Oisans basin, after the main tectonic phase at ca. 25 Ma [START_REF] Ménard | Utilisation de reaimantations postmetamorphiques pour une etude de l'evolution tectonique et thermique tardive dans les Alpes occidentales (France)[END_REF][START_REF] Crouzet | High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit[END_REF][START_REF] Crouzet | Experimental evaluation of thermal recording of successive polarities during uplift of metasediments[END_REF]) (Fig. 11). The stability field of paragenesis observed in the Jurassic cover (quartz-cookeite-margarite-pyrophyllite) that most likely recorded peak-metamorphism conditions in the Emparis basin is restricted from 270 to 340°C and from 1 to 5kbar [START_REF] Jullien | Cookeite and pyrophyllite in the Dauphinois black shales (Isères, rance) : implications for the conditions of metamorphism in the Alpine external zones[END_REF]) (Fig. 11). Besides, the overall A c c e p t e d M a n u s c r i p t thermal structure of the Oisans Massif as recorded by the distribution of RSCM Tmax is consistent with illite-crystallinity maps [START_REF] Aprahamian | La cristallinité de l'illite et les minéraux argileux en bordure des massifs cristallins externes de Belledonne et du Pelvoux (Variations et relations possibles avec des événements tectoniques et métamorphiques alpins)[END_REF]1988;[START_REF] Barlier | L'évolution des Terres noires subalpine méridionales d'après l'analyse minéralogique des argiles et la réflectométrie des particules carbonées[END_REF] (Fig. 4).

Related to the irreversible character of graphitization, and the insensitivity of the CM structure to retrograde reactions (e.g. [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF][START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF][START_REF] Aoya | Extending the applicability of the Raman carbonaceous-material geothermometer using data from contact metamorphic rocks[END_REF], RSCM record the highest temperature ever undergone by the rock sample. In principle, RSCM Tmax results issued from a continuous section may therefore be diachronous and represent the superimposition of distinct thermal events.

In the Bourg d'Oisans and Emparis basins, metamorphic peak-T conditions of ca. 335°C where reached at ca. 25 Ma (i.e. [START_REF] Crouzet | High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit[END_REF][START_REF] Crouzet | Experimental evaluation of thermal recording of successive polarities during uplift of metasediments[END_REF]. As the restored sedimentary cover of this area is generally thinner than ca. 2 km [START_REF] Barféty | Geological map and explanatory text of the sheet n°797[END_REF][START_REF] Barbier | Geological map and explanatory text of the sheet n°798[END_REF][START_REF] Crouzet | Experimental evaluation of thermal recording of successive polarities during uplift of metasediments[END_REF][START_REF] Bellahsen | Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology[END_REF], these temperatures are clearly ascribed to an Alpine tectonometamorphic event and particularly to the tectonic burial of the Dauphinois domain under internal units [START_REF] Crouzet | Experimental evaluation of thermal recording of successive polarities during uplift of metasediments[END_REF][START_REF] Dumont | Multistage shortening in the Dauphine zone (French Alps): the record of Alpine collision and implications for pre-Alpine restoration[END_REF]. Besides, the progressive temperature decrease toward the top of the overthrusting units close to the Galibier pass is consistent with a late to post-tectonic temperature imprint, as suggested by thermopaleomagnetic results [START_REF] Crouzet | High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit[END_REF](Crouzet et al., , 2000)). Interestingly, the upward RSCM Tmax evolution indicates a ca. 20-30°C.km -1 gradient (Fig. 4C), which is consistent with the gradient around 20 to 25°C.km -1 proposed by [START_REF] Crouzet | High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit[END_REF] but lower than the ca. 40-50°C.km -1 gradient deduced from fluid inclusion data (Gratier et al., 1973;[START_REF] Nziengui | Excès d'argon radiogénique dans les quartz des fissures tectoniques : implications pour la datation des séries métamorphiques[END_REF] (Fig. 12). However, high geothermal gradients deduced from fluid inclusions may result from an underestimated pressure due to reequilibriation of fluid inclusion densities during an isothermal decompression as proposed by [START_REF] Boullier | Fluid inclusions: tectonic indicators[END_REF] or [START_REF] Crouzet | Experimental evaluation of thermal recording of successive polarities during uplift of metasediments[END_REF].

Conversely, in the sedimentary cover west of the Belledonne Massif, Tmax values show large temperature variations from 335°C to less than 200°C (Fig. 4,5). Tilted with the bedding, this ca. 130°C Tmax upward sequence decrease may witness a post-metamorphic peak westward tilting of the sedimentary succession (Fig. 4,5). Moreover, Tmax recorded at the base of the succession is consistent with the ca. 8 km thickness of the Mesozoic series assuming a ca. 40°C.km -1 geothermal gradient [START_REF] Philippe | Thin-skinned inversion tectonics at oblique basin margins: example of the western Vercors and Chartreuse Subalpine massifs (SE France)[END_REF]. Besides, this result is consistent with the 60 to 80 mW.m -2 basal heat flow estimated in this area [START_REF] Deville | Contrasting thermal evolution of thrust systems: An analytical and modeling approach in the front of the western Alps[END_REF]) and a bulk thermal conductivity close to 1.5 W.m -1 .K -1 [START_REF] Beardsmore | Crustal heat flow: a guide to measurement and modelling[END_REF].

Minor and local fluid circulations along the basement-cover interface cannot be ruled out but an overall thermal imprint related to large-scale fluid circulations is unlikely as available A c c e p t e d M a n u s c r i p t studies show that the fluid flow operated only at small scale in the studied area [START_REF] Henry | Evolution of synmetamorphic veins and their wallrocks through a Western Alps transect: no evidence for large-scale fluid flow. Stable isotope, major-and trace-element systematics[END_REF][START_REF] Kirschner | Fluid migration through thrust faults in the Helvetic nappes (Western Swiss Alps)[END_REF][START_REF] Boutoux | Fluid systems above basement shear zones during inversion of pre-orogenic sedimentary basins (External Crystalline Massifs, Western Alps)[END_REF].

Significance of the 40 Ar/ 39 Ar in situ ages

Geochronological data and particularly 40 Ar/ 39 Ar data are traditionally interpreted as recording closure of the isotopic system at a specific temperature depending on the isotopic diffusion properties of the mineral (e.g. [START_REF] Dodson | Closure temperature in cooling geochronological and petrological systems[END_REF][START_REF] Freeman | Deformation migration in an orogen-scale shear zone array: an example from the Basal Briançonnais Thrust, internal Franco-Italian Alps[END_REF][START_REF] Villa | Isotopic closure[END_REF][START_REF] Harrison | Diffusion of 40Ar in muscovite[END_REF]. Accordingly, many studies therefore assume that the closure temperature of the K/Ar system for muscovite argon retention is around 350-450°C depending on cooling-rates, grain-size and other controlling factors such as the geometry of the diffusion domain (e.g. [START_REF] Wijbrans | 40Ar/39Ar dating of white micas from an Alpine high-pressure metamorphic belt on Naxos (Greece): the resetting of the argon isotopic system[END_REF][START_REF] Hames | An empirical evaluation of the argon diffusion geometry in muscovite[END_REF][START_REF] Reddy | A 40Ar/39Ar laser probe study of micas from the Sesia Zone, Italian Alps: implications for metamorphic and deformation histories[END_REF][START_REF] Villa | Isotopic closure[END_REF]. Using recent results of modeling, the closure temperature for muscovite would fall in the range of 400-420°C for a 10-30°C.Ma -1 cooling-rate and a diffusion radius of 100µm [START_REF] Harrison | Diffusion of 40Ar in muscovite[END_REF]. However, closure temperature for phengite is still discussed as it may largely vary as a function of both intrinsic (e.g. grainsize) and extrinsic (cooling-rates, temperature, pressure and deformation or fluid circulations) parameters. Recent studies suggest that phengite is more retentive for argon than muscovite and that phengite ages are interpretable as crystallisation ages to temperature as high as 500-550°C (e.g. [START_REF] Dunlap | Neocrystallization or cooling? 40Ar/39Ar ages of white micas from low-grade mylonites[END_REF][START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps[END_REF][START_REF] Müller | Strengthening the link between geochronology, textures and petrology[END_REF][START_REF] Mulch | Recrystallization or cooling ages: in situ UV-laser 40Ar/39Ar geochronology of muscovite in mylonitic rocks[END_REF][START_REF] Augier | Exhumation, doming and slab retreat in the Betic Cordillera (SE Spain): in situ40Ar/39Ar ages and P-T-d-t paths for the Nevado-Filabride complex[END_REF][START_REF] Mulch | Time scales of deformation and exhumation in extensional detachment systems determined by high-spatial resolution in situ UV-laser 40Ar/39Ar dating[END_REF][START_REF] Gerber | Evolution tectono-métamorphique du Briançonnais interne (Alpes Occidentales, massifs de Vanoise Sud et d'Ambin): comportement du socle et de sa couverture dans un contexte de subduction continentale profonde[END_REF][START_REF] Rolland | 40Ar/39Ar dating of synkinematic white mica: insights from fluid-rock reaction in low-grade shear zones (Mont Blanc Massif) and constraints on timing of deformation in the NW external Alps[END_REF][START_REF] Rolland | Constraining deformation stages in brittleductile shear zones from combined field mapping and Ar-40/Ar-39 dating: The structural evolution of the Grimsel Pass area (Aar Massif, Swiss Alps)[END_REF]Sanchez et al., 2011a). Temperature experienced during the Alpine orogeny by dated samples did not exceeded 330°C ± 30°C since they initially cooled at the end of the Variscan Orogeny. Ages of the fine-grained synkinematic phengites therefore likely represent crystallization ages. Conversely, large, inherited clasts of muscovite may reflect Variscan cooling ages in the absence of additional disturbing factors. Stratigraphic constraints indicate that the flexural basin of the external zones was diachronously filled up from Lutetian to Bartonian (45 to 33Ma) [START_REF] Du Fornel | The southern Grès d'Annot outcrops (French Alps): an attempt at regional correlation[END_REF][START_REF] Mulder | High-resolution analysis of submarine lobes deposits: Seismic-scale outcrops of the Lauzanier area (SE Alps, France)[END_REF][START_REF] Salles | Migration of a synclinal depocentre from turbidite growth strata: the Annot syncline, SE France[END_REF]. Burial of this basin below internal units occurred during Rupelian (34 to 28 Ma; [START_REF] Fry | Southwestward thrusting and tectonics of the western Alps[END_REF][START_REF] Sinclair | Tectono-metamorphic evolution of the Briançonnais zone (Modane-Aussois and Southern Vanoise units, Lyon Turin transect, Western Alps)[END_REF][START_REF] Ford | Foreland basin evolution around the western Alpine Arc[END_REF][START_REF] Mulder | High-resolution analysis of submarine lobes deposits: Seismic-scale outcrops of the Lauzanier area (SE Alps, France)[END_REF]. In Champsaur and Saint-Antonin basins, amphiboles reworked from Oligocene volcanism coeval with deposition yielded ca. 34-31 Ma Priabonian ages by 40 Ar/ 39 Ar methods [START_REF] Féraud | Nouvelles données géochronologiques sur le volcanisme paléogène des Alpes occidentales: existence d'un événement magmatique bref généralisé[END_REF][START_REF] Ivaldi | Contexte lithostructural, âges 40 K/ 40 Ar et géochimie du volcanisme calcoalcalin tertiaire de Cap-d'Ail dans le tunnel ferroviaire de Monaco[END_REF], giving a maximum age for the burial of the basins under the A c c e p t e d M a n u s c r i p t internal nappes. In addition, part of the Oisans Massif formed a 500 m high relief in the Oligocene [START_REF] Gupta | Implications of foreland paleotopography for stratigraphic development in the Eocene distal Alpine foreland basin[END_REF] and dated samples, restored in their inferred initial position, were probably less than 2km deep, between ca. 40 and 34 Ma. It therefore appears unlikely that phengites were formed under these P-T conditions and the ca. 34-33 Ma age clearly fixes the upper bound of the possible ages for the deformation of the Oisans Massif. 97 in situ experiments yielded ages younger than 34 Ma.

Interestingly, young ages are clearly correlated with the intensity of fine-grained whitemicas blastesis particularly important in high-strain zones. It is therefore proposed that ages younger than 34 Ma reflect synkinematic crystallisation ages and thus deformation ages, in a broad sense. In addition, apparent ages show a bimodal distribution with a first age group settled at ca. 33-30 Ma and a second group at ca. 28-25 Ma ages (Fig. 11). These results are consistent with results of a conventional 40 Ar/ 39 Ar approach performed on wrench shear zones of the same area (Simon-Labric et al., 2009). However, our interpretation differs.

Based on age spectra scattered between 32 and 24 Ma, Simon-Labric et al. ( 2009) proposed the superimposition of two distinctive shearing events (one at ca. 32 and one ca. 24 Ma). In this study, the use of an in situ mean over strain gradients allow linking deformation ages and the amount of finite strain experienced by the rocks. Accordingly, proto-mylonite and mylonite samples yielded older, ca. 33-30Ma ages while highly strained samples (phyllonites) yielded younger ca. 28-25 Ma ages, suggesting that the deformation progressively localized along the phyllonite bands thus leaving large rock volumes devoid of further recrystallizations. In any case, intense E-W shortening started with or just after the burial of external zones under the internal nappes around ca. 34-33 Ma and was still active at ca. 25 Ma. Synchronous deformation ages of either top-to-the-W reverse shear zones or wrench shear zones thus confirm the small-scale mutual cross-cutting relationships observed in the field [START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF].

102 in situ experiments yielded ages mostly clustering between 100 and 40 Ma with ages as old as 380 Ma. Older than ca. 34 Ma and presenting sometimes pre-Alpine or even genuine Variscan ages, theses apparent ages clearly do not record alpine collisional deformation.

Relying on temperature estimates throughout the studied area, the temperature concept predicts that all white-micas sharing common pre-Alpine textures and inherited muscovite chemical composition should yield the same age corresponding to the time elapsed since initial cooling at the end of the Variscan Orogeny. Yet this is not the case for most analyses.

On the contrary, the correlation of in situ 40 Ar/ 39 Ar ages and finite strain intensity at any scale reveals that ages are texturally-controlled and therefore independent of closure At first glance, as stated above, an overall correlation with the intensity of the finite strain can be drawn between samples. Weakly strained samples (from the Combeynot or the Plan du Lac shear zones) yielded rather old, sometime only pre-Alpine ages while phyllonites yielded much younger ages that tend to crystallization ages of newly formed crystals. Such control by the deformation is also perceptible at the scale of some dated rock samples with an overall age decrease toward the core of a shear band from ca. 300 Ma to less than 30 Ma in less than 1 centimetre (Fig. 11). It appears therefore tempting to ascribe to deformation the ability to facilitate argon diffusion and thus to partially reset (i.e. re-open) inherited muscovite isotopic system. Intensity of deformation, whether it occurs by grain-size reduction during mylonitization or by presence and distribution of high-diffusivity pathways lattice-modifying microstructures would therefore be the main controlling factor explaining the age scatter between an inherited cooling ages and the age of a tectonometamorphic event (i.e. [START_REF] Scaillet | MgFe and ((Mg,Fe)Si-Al2) compositional control on argon behaviour in high-pressure white micas: A 40Ar39Ar continuous laser-probe study from the Dora-Maira nappe of the internal western Alps, Italy[END_REF][START_REF] West | Dating mylonitic deformation by the 40Ar-39Ar method: An example from the Norumbega Fault Zone[END_REF][START_REF] Hames | On the loss of 40Ar * from muscovite during polymetamorphism[END_REF][START_REF] Kramar | Heterogeneous 40Ar* distributions in naturally deformed muscovite: in situ UV-laser ablation evidence for microstructurally controlled intragrain diffusion[END_REF]Kramar et al. , 2003;;[START_REF] Mulch | In-situ UV-laser 40Ar/39Ar geochronology of a micaceous mylonite : an example of defect-enhanced argon loss[END_REF]. As suggested by the linear relationships observed between some apparent ages and corresponding 38 Ar/ 39 Ar ratios arguing for interactions with Cl-rich fluids, minor excess argon cannot be simply ruled out. Besides, a limited yet conspicuous input of extraneous argon within and in the vicinity of shear bands is clearly detectable for some samples (Fig. 10). In this case, results of absolute chronology are markedly inconsistent with the relative chronology established independently on the basis of microtextures. For example, mylonite sample A-4-66b presents a series of ca. 50-40 Ma old ages occurring within the core of the shear band while other analyses show a clear age decrease toward the shear band. Excess argon was moreover reported within fluid inclusions from Oisans basement quartz veins [START_REF] Nziengui | Excès d'argon radiogénique dans les quartz des fissures tectoniques : implications pour la datation des séries métamorphiques[END_REF]. In this study seven K/Ar experiments on whitemicas from the sedimentary cover yielding initially 48 to 30 Ma ages were corrected to 27- (Fig. 11 and12) in order to discuss a new tectonic scenario (see also in [START_REF] Bellahsen | Collision kinematics in the western external Alps[END_REF]) (Fig. 13).

Among the new results, this study constrains the thermal structure of a large volume of the external basement. Results, despite local variations, show a constant Tmax around 330°C. This constant temperature along several tens of kilometres suggests that the basement and the cover preserved in half-grabens have been first quickly underthrust with rather limited internal shortening. Deformation ages from this study and from Simon-Labric et al. (2009) are consistent with a burial of the external zones around between 34 and 25 Ma (Fig. 12, 13). The main shortening seems to have occurred at or around the metamorphic peak (Fig. 12) as suggested in [START_REF] Boutoux | Fluid systems above basement shear zones during inversion of pre-orogenic sedimentary basins (External Crystalline Massifs, Western Alps)[END_REF]. The distribution of isotherms obtained with thermopaleomagnetic method, which cross-cut the folds, indicates that the main shortening had ended before cooling started in the Bourg d'Oisans basin [START_REF] Crouzet | High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit[END_REF]. The sampled profile may therefore correspond to an east-dipping isotherm (around 330°C) during underthrusting of the Oisans Massif, which would then suggest that the whole area behaved as a more or less rigid block below the internal units during part of its burial history, before it was ultimately shortened. Similarly, the geothermal gradient observed close to the Lautaret and Galibier passes (around 20-30°C.km -1 ) seems to seal the PFT activity dated between 32 and 27 Ma [START_REF] Freeman | Deformation migration in an orogen-scale shear zone array: an example from the Basal Briançonnais Thrust, internal Franco-Italian Alps[END_REF]. Thus, deformation mainly occured just prior or during metamorphic-peak conditions and seems, in any case, to have ceased before the onset of the main cooling event. During this time period, and around the metamorphic peak conditions, the external zone crust underwent homogeneous shortening until approximately 25 Ma. Shortening was progressively localized into phyllonite bands. At 25 Ma, the crust was progressively decoupled from the subducting lithosphere and started to exhume. The PFT was then inactivated. These results argue against a progressive westward thrust slice development that lead to the exhumation of units after that they have reached A c c e p t e d M a n u s c r i p t diachronously temperatures close to 330°C. However, this would imply that the basement top has been heated up uniformly from 50-100°C to 300-350°C uniformly over ca. 70km, which in turn suggests a rather slow burial.

At the scale of individual shear zones, high-strain samples (phyllonite) are younger (28-25Ma) than proto-mylonite and mylonite . This suggests that deformation progressively localized along phyllonite bands. Moreover, ages of deformation reveal that the deformation is contemporaneous all along the profile from the PFT to the Plan du Lac shear zone, i.e. over more than ca. 30km. This synchronicity of deformation shows that shortening in the ECM was distributed in space and time and confirms that the crust was relatively weak during shortening as proposed in [START_REF] Bellahsen | Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology[END_REF]. This is in agreement with the formation of newly formed shear zones rather than normal faults reactivation.

Later, shortening reached the Vercors fold and thrust belt during Burdigalian to Langhian times (Fig. 13). This shortening appears linked to crustal ramps below the Belledonne Massif, which suggests that the deformation localized in frontal parts after the end of the main shortening phase of the ECM at around 20 Ma [START_REF] Ménard | Relations entres structures profondes et structures superficielles dans le Sud-Est de la France[END_REF][START_REF] Butler | The influence of pre-existing basin structure on thrust system evolution in the Western Alps[END_REF][START_REF] Philippe | Thin-skinned inversion tectonics at oblique basin margins: example of the western Vercors and Chartreuse Subalpine massifs (SE France)[END_REF]. Between 25 and 20 Ma, the Oisans Massif has undergone fast cooling at a rate of 33°C.Ma -1 [START_REF] Crouzet | Experimental evaluation of thermal recording of successive polarities during uplift of metasediments[END_REF] and was exhumed at a rate of 0.4 km.Ma -1 (van der [START_REF] Van Der Beek | Inversion of thermochronological age-elevation profiles to extract independent estimates of denudation and relief history -II: Application to the French Western Alps[END_REF]. Final cooling, whose age is estimated from fission tracks ages [START_REF] Vernon | Increase in late Neogene denudation of the European Alps confirmed by analysis of a fission-track thermochronology database[END_REF], occurred after 15 Ma, which corresponds to the age of thrusting within the Vercors Massif. During this more external fold and thrust belt shortening, only slight deformation occurred in the Oisans Massif [START_REF] Dumont | Multistage shortening in the Dauphine zone (French Alps): the record of Alpine collision and implications for pre-Alpine restoration[END_REF]. The new argon data reported here thus complete the scenario of burial and first shortening at ca. 33 Ma, progressive localisation of shear zones until 25 Ma, and final exhumation above a crustal ramp until after 15-20 Ma.

Exhumation of the ECM was also accompanied by extensional reactivation of the PFT since the late Oligocene [START_REF] Tricart | Diachronous late-stage exhumation across the western Alpine arc: constraints from apatite fission-track thermochronology between the Pelvoux and Dora-Maira Massifs[END_REF] (Fig. 13). Thus, during the late Oligocene and early Miocene, the ECM was a crustal unit bordered by a normal fault to the east and a reverse fault to the west, transported with low internal deformation, similarly to the models of syncollisional rock exhumation described by [START_REF] Chemenda | A mechanism for syn-collisional rock exhumation and associated normal faulting: Results from physical modelling[END_REF] (Fig. 14). Moreover, numerous veins witnessing for fluids circulation have been documented between 17 to 5Ma in the Aar Massif [START_REF] Mullis | Fluid regimes during late stages of a continental collision: Physical, chemical, and stable isotope measurements of fluid inclusions in fissure quartz from a geotraverse through the Central Alps, Switzerland[END_REF][START_REF] Mullis | P-T-t path of quartz formation in extensional veins of the Central Alps[END_REF][START_REF] Challandes | P-T-t modelling, fluid circulation, and 39Ar-40Ar and Rb-Sr mica ages in the Aar Massif shear zones (Swiss Alps)[END_REF], in the Mont Blanc Massif [START_REF] Leutwein | Age des cavités à cristaux du granite du Mont Blanc[END_REF][START_REF] Marshall | Fluid inclusion, stable isotope and Ar-Ar evidence for the age and origin of gold-bearing quartz veins at Mont Chemin, Switzerland[END_REF] or in the Belledonne Massif A c c e p t e d M a n u s c r i p t [START_REF] Gasquet | Miocene to Messinian deformation and hydrothermal activity in a pre-Alpine basement massif of the French western Alps: new U-Th-Pb and argon ages from the Lauziere massif[END_REF], indicating that this event was coeval with a fluid-rich process which affected all the orogen.

Conclusion

Following the description of Alpine shear zones responsible for shortening of the Oisans External Massif of the Western Alps by [START_REF] Bellahsen | Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology[END_REF] and [START_REF] Bellanger | Basement shear zones development and shortening kinematics in the Ecrins Massif[END_REF], the alpine thermal structure of the massif is established here thanks to the RSCM method and the shear zones were dated. The main outcome is that the interface between the cover and the basement has recorded a maximum temperature of ~330°C, constant across the whole massif and that the shortening was accommodated by top-to-the west shear zones that progressively localized from 34 to 25 Ma, before the ECM were finally exhumed above a crustal ramp after 15-20 Ma, which in turn controlled shortening in the deformed cover further west (Vercors Massif). 2012) and [START_REF] Plunder | How continuous and precise is the record of P-T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustrés, W. Alps)[END_REF]. Detailed results are given in table 1. [START_REF] Deville | Contrasting thermal evolution of thrust systems: An analytical and modeling approach in the front of the western Alps[END_REF] transformed into temperatures using the equation proposed in [START_REF] Allen | Basin analysis: principles and applications[END_REF]. The distribution of the Tmax in the sedimentary pile is aligned along a ca. 30-50°C.km -1 gradient. Besides, temperature gaps across narrow zones highlight localized tectonic contacts. (b) Distribution of the RSCM Tmax results close to the Lautaret and Galibier passes. The structural data come from [START_REF] Depardon | Les déformations de la couverture mésozoïque au Nord de l'Oisans[END_REF], Beach (1981a[START_REF] Beach | Strain analysis in a cover thrust zone, external French Alps[END_REF], [START_REF] Ceriani | Multi-stage thrusting at the "Penninic Front" in the Western Alps between Mont Blanc and Pelvoux massifs[END_REF][START_REF] Bellanger | Raccourcissement alpin du massif des Ecrins: cinématique, calendrier tectonique et conditions pression-température[END_REF]. Note that the distribution of the Tmax within the tectonic pile is consistent with a ca. 20-30°C.km -1 post-tectonic field gradient. Detailed results are given in table 1. both thermodynamic and empirical approaches [START_REF] Cathelineau | A chlorite solid solution geothermometer the Los Azufres (Mexico) geothermal system[END_REF][START_REF] Vidal | Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation[END_REF], 2006) are compared to RSCM data. Chlorite probably records a part of the cooling (retrograde) evolution. Detailed results are given in table 2. Muscovite/Celadonite/Pyrophyllite ternary plot together with results of chlorite geothermometry using both thermodynamic and empirical approaches [START_REF] Cathelineau | A chlorite solid solution geothermometer the Los Azufres (Mexico) geothermal system[END_REF][START_REF] Vidal | A Thermodynamic Model for Fe-Mg Aluminous Chlorite Using Data from Phase Equilibrium Experiments and Natural Pelitic Assemblages in the 100° to 600°c, 1 to 25 kb Range[END_REF][START_REF] Wibberley | Initiation of basement thrust detachments by fault-zone reaction weakening[END_REF], 2006). Detailed results are given in table 2. Pressure-Temperature (P-T) constraints for the Oisans Massif. The P-T data come from (Gratier et al., 1973;[START_REF] Bernard | Microthermometrie des inclusions fluides de cristaux syn-cinematiques[END_REF] and references therein (Thermobarometry from fluids inclusions and paragenesis from cover and basement veins), [START_REF] Jullien | Cookeite and pyrophyllite in the Dauphinois black shales (Isères, rance) : implications for the conditions of metamorphism in the Alpine external zones[END_REF] (conventional metamorphic petrology from the Emparis basin Mesozoic cover, [START_REF] Crouzet | High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit[END_REF] (thermopaleomagnetism from the Bourg d'Oisans basin), [START_REF] Ceriani | Tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between low-grade metamorphism and tectonic phases[END_REF] (Zircon fission track and Kübler index and b-cell dimension of white mica fom the Nummulitic cover of the Cheval noir unit), this study (RSCM from Mesozoic and Nummulitic cover). Temperature-deformation-time (T-d-t) constraints for the Oisans Massif. The T-d-t data come from (1) [START_REF] Nziengui | Excès d'argon radiogénique dans les quartz des fissures tectoniques : implications pour la datation des séries métamorphiques[END_REF]; (2) [START_REF] Jullien | Cookeite and pyrophyllite in the Dauphinois black shales (Isères, rance) : implications for the conditions of metamorphism in the Alpine external zones[END_REF](3) this study; (4) [START_REF] Crouzet | High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit[END_REF]; (5) [START_REF] Ménard | Utilisation de reaimantations postmetamorphiques pour une etude de l'evolution tectonique et thermique tardive dans les Alpes occidentales (France)[END_REF]; (6) [START_REF] Crouzet | Experimental evaluation of thermal recording of successive polarities during uplift of metasediments[END_REF]; (7) (van der Beek et al., 2010); (8) [START_REF] Sabil | La datation par traces de fission: aspects méthodologiques et applications thermochronologique en contexte Alpin et de Marge Continentale[END_REF][START_REF] Van Der Beek | Inversion of thermochronological age-elevation profiles to extract independent estimates of denudation and relief history -II: Application to the French Western Alps[END_REF][START_REF] Beucher | Exhumation and relief development in the Pelvoux and Dora-Maira massifs (western Alps) assessed by spectral analysis and inversion of thermochronological age transects[END_REF].

Figure 12: Large-scale compilation of existing time-constraints for the External zone and for the Western Alps close to the 45th parallel. ( 1) [START_REF] Riche | Tectonique synsédimentaire sur la bordure orientale du bassin tertiaire de Valreas[END_REF] (2) [START_REF] Clauzon | Genèse et évolution du piémont néogène subalpin du Bas-Dauphiné. Livret guide de l'excursion préforum du 3ème forum national de géomorphologie, Aix en Provence, Unité de recherche associé[END_REF] (3) [START_REF] Philippe | Thin-skinned inversion tectonics at oblique basin margins: example of the western Vercors and Chartreuse Subalpine massifs (SE France)[END_REF] (4) [START_REF] Du Fornel | The southern Grès d'Annot outcrops (French Alps): an attempt at regional correlation[END_REF] (5) [START_REF] Mulder | High-resolution analysis of submarine lobes deposits: Seismic-scale outcrops of the Lauzanier area (SE Alps, France)[END_REF]) (6) [START_REF] Féraud | Nouvelles données géochronologiques sur le volcanisme paléogène des Alpes occidentales: existence d'un événement magmatique bref généralisé[END_REF] [START_REF] A C C E P T E D M A | n u s c r i p t[END_REF] [START_REF] Marignac | Alpine fault sealing at the contact between a crystalline basement and its sedimentary cover : La Gardette (French Alps)[END_REF] [START_REF] Nziengui | Excès d'argon radiogénique dans les quartz des fissures tectoniques : implications pour la datation des séries métamorphiques[END_REF]) (12) [START_REF] Crouzet | High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit[END_REF][START_REF] Crouzet | Experimental evaluation of thermal recording of successive polarities during uplift of metasediments[END_REF]) (13) (van der Beek et al., 2010) (14) [START_REF] Sabil | La datation par traces de fission: aspects méthodologiques et applications thermochronologique en contexte Alpin et de Marge Continentale[END_REF][START_REF] Van Der Beek | Inversion of thermochronological age-elevation profiles to extract independent estimates of denudation and relief history -II: Application to the French Western Alps[END_REF]Beucher et al., 2012) (15-16) [START_REF] Fügenschuh | Late stages of deformation and exhumation of an orogen constrained by fission-track data: A case study in the Western Alps[END_REF] (17) [START_REF] Ceriani | From n-s collision to WNW-directed post-collisional thrusting and folding: Structural study of the Frontal Penninic Units in Savoie (Western Alps, France)[END_REF] [START_REF] Rolland | Constraining deformation stages in brittleductile shear zones from combined field mapping and Ar-40/Ar-39 dating: The structural evolution of the Grimsel Pass area (Aar Massif, Swiss Alps)[END_REF] [START_REF] Ceriani | Tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between low-grade metamorphism and tectonic phases[END_REF][START_REF] Fügenschuh | Late stages of deformation and exhumation of an orogen constrained by fission-track data: A case study in the Western Alps[END_REF] (19) [START_REF] Fügenschuh | Late stages of deformation and exhumation of an orogen constrained by fission-track data: A case study in the Western Alps[END_REF] (20) [START_REF] Féraud | Nouvelles données géochronologiques sur le volcanisme paléogène des Alpes occidentales: existence d'un événement magmatique bref généralisé[END_REF] (21) [START_REF] Corsini | Alpine and late-hercynian geochronological constraints in the Argentera Massif (Western Alps)[END_REF] (22) (Sanchez et al., 2011a) (23) [START_REF] Bigot-Cormier | Dénudations différentielles du massif cristallin externe alpin de l'Argentera (Sud-Est de la France) révélées par thermochronologie traces de fission (apatites, zircons)[END_REF] (24) [START_REF] Bigot-Cormier | Dénudations différentielles du massif cristallin externe alpin de l'Argentera (Sud-Est de la France) révélées par thermochronologie traces de fission (apatites, zircons)[END_REF][START_REF] Bogdanoff | Apatite fission track analysis in the Argentera massif: evidence of contrasting denudation rates in the External Crystalline Massifs of the Western Alps[END_REF]Sanchez et al., 2011b) (25) [START_REF] Pfiffner | A Continent Revealed: The European Geotraverse[END_REF]) (26) (Ruffini et al., 1997) (27-29) (Leutwein et al., 1970) (30) [START_REF] Marshall | Fluid inclusion, stable isotope and Ar-Ar evidence for the age and origin of gold-bearing quartz veins at Mont Chemin, Switzerland[END_REF]) (31) (Crespo-Blanc et al., 1995) (32) (Kirschner et al., 1996) (33-34) (Rolland et al., 2008) (35) [START_REF] Krummenacher | Déterminations d'âge isotropique faites sur quelques roches des Alpes par la méthode Potassium-Argon[END_REF]) (36) (Leutwein et al., 1970) (37-40) [START_REF] Leloup | Alpine thermal and structural evolution of the highest external crystalline massif: The Mont Blanc Tectonics[END_REF] (41) (Baggio et al., 1967) (42-43) [START_REF] Seward | Ar-40/Ar-39 dating of Penninic Front tectonic displacement (W Alps) during the Lower Oligocene (31-34 Ma)[END_REF][START_REF] Leloup | Alpine thermal and structural evolution of the highest external crystalline massif: The Mont Blanc Tectonics[END_REF] (44-45) (Fügenschuh and Schmid, 2003) (46) (Freeman et al., 1998) (47) (Beucher, 2009) (48-49) [START_REF] Fügenschuh | Late stages of deformation and exhumation of an orogen constrained by fission-track data: A case study in the Western Alps[END_REF][START_REF] Tricart | Diachronous late-stage exhumation across the western Alpine arc: constraints from apatite fission-track thermochronology between the Pelvoux and Dora-Maira Massifs[END_REF] (50) [START_REF] Ellenberger | Etude géologique du pays de Vanoise (Savoie), Mémoire servant à la carte géologique de la France[END_REF] (51) [START_REF] Ganne | Les dômes de socle HP-BT dans le domaine Pennique des Alpes Nord-Occidentales: modalité de leur exhumation[END_REF] (52) [START_REF] Gerber | Evolution tectono-métamorphique du Briançonnais interne (Alpes Occidentales, massifs de Vanoise Sud et d'Ambin): comportement du socle et de sa couverture dans un contexte de subduction continentale profonde[END_REF] (53) [START_REF] Malusà | Miocene to Present differential exhumation in the Western Alps: Insights from fission track thermochronology[END_REF] (54-56) [START_REF] Ganne | Les dômes de socle HP-BT dans le domaine Pennique des Alpes Nord-Occidentales: modalité de leur exhumation[END_REF] (57-61) [START_REF] Gerber | Evolution tectono-métamorphique du Briançonnais interne (Alpes Occidentales, massifs de Vanoise Sud et d'Ambin): comportement du socle et de sa couverture dans un contexte de subduction continentale profonde[END_REF] (Freeman et al., 1997) (64) [START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps[END_REF] (65) [START_REF] Duchêne | The Lu-Hf dating of garnets and the ages of the Alpine high-pressure metamorphism[END_REF] (66) [START_REF] Rubatto | Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones[END_REF] (67) [START_REF] Monié | 39Ar-40Ar evidence for mid-Eocene high-pressure metamorphism in the Monviso ophiolitic massif[END_REF] (68-69) [START_REF] Freeman | Deformation migration in an orogen-scale shear zone array: an example from the Basal Briançonnais Thrust, internal Franco-Italian Alps[END_REF] (70-71) [START_REF] Schwartz | Diachronous exhumation of HP-LT metamorphic rocks from south-western Alps: evidence from fission-track analysis[END_REF][START_REF] Tricart | Diachronous late-stage exhumation across the western Alpine arc: constraints from apatite fission-track thermochronology between the Pelvoux and Dora-Maira Massifs[END_REF][START_REF] Beucher | Évolution Néogène de l'Arc Alpin sud-occidental. Approches sismotectonique et thermochronologique[END_REF] (72) [START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps[END_REF] (73) (Duchêne et al., 1997) (74) [START_REF] Gebauer | 35 Ma old ultrahigh-pressure metamorphism and evidence for very rapid exhumation in the Dora Maira Massif[END_REF] (75) [START_REF] Rubatto | Exhumation as fast as subduction?[END_REF] (76 et 78) [START_REF] Tilton | Pb-Sr-Nd isotopic behavior of deeply subducted crustal rocks from the Dora Maira Massif, Western Alps, Italy-II: what is the age of the ultrahigh-pressure metamorphism?[END_REF] (77) [START_REF] Scaillet | MgFe and ((Mg,Fe)Si-Al2) compositional control on argon behaviour in high-pressure white micas: A 40Ar39Ar continuous laser-probe study from the Dora-Maira nappe of the internal western Alps, Italy[END_REF]) (79) [START_REF] Gebauer | 35 Ma old ultrahigh-pressure metamorphism and evidence for very rapid exhumation in the Dora Maira Massif[END_REF] (80) [START_REF] Beucher | Exhumation and relief development in the Pelvoux and Dora-Maira massifs (western Alps) assessed by spectral analysis and inversion of thermochronological age transects[END_REF] (81 et 83) [START_REF] Chopin | 40Ar-39Ar dating of high pressure metamorphic micas from the Gran Paradiso area (Western Alps): Evidence against the blocking temperature concept[END_REF] (82) [START_REF] Ganne | Les dômes de socle HP-BT dans le domaine Pennique des Alpes Nord-Occidentales: modalité de leur exhumation[END_REF] (84) (Meffan-Main et al., 2004) (85-86) [START_REF] Hurford | A revised thermal history for the Gran Paradiso massif[END_REF][START_REF] Hurford | Constraints on the late thermotectonic evolution of the western Alps: EVidence for episodic rapid uplift[END_REF].

Table captions

Table 1: Detailed results from the RSCM approach. Given are the GPS position (latitude (Lat.) and longitude (Long.) in decimal degrees (WGS84) and the elevation in meter), the tectonic unit, the stratigraphic age, the lithology, the number of Raman spectra (Sp.), the R2 or the RA1 ratio, the RSCM temperature (mean and standard deviation). R2 ratio, in regular font [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF] or the RA1 ratio, in italic font [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF]) was preferred depending on the shape of the spectra and the presence of the D4 peak [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF].

Table 2: Detailed results from the chlorite geothermometry approach. Given are the GPS position (latitude (Lat.) and longitude (Long.) in decimal degrees (WGS84) and results of thermodynamic and empirical approaches [START_REF] Cathelineau | A chlorite solid solution geothermometer the Los Azufres (Mexico) geothermal system[END_REF][START_REF] Vidal | Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation[END_REF], 2006). Results are presented separately for the Mesozoic cover and the basement (Alpine shear zones). 5.6 5.7 5.8 5.9 6 6.1 6.2 6.3 6.6 6.7 6.8 6.9 7 7.1 Longitude (°) Elevation (m) Longitude (°) 5.7 5.8 5.9 6 6.1 6.2 6.3 6.6 6.7 6.8 6.9 2012) [START_REF] Aprahamian | La cristallinité de l'illite et les minéraux argileux en bordure des massifs cristallins externes de Belledonne et du Pelvoux (Variations et relations possibles avec des événements tectoniques et métamorphiques alpins)[END_REF]; [START_REF] Barlier | L'évolution des Terres noires subalpine méridionales d'après l'analyse minéralogique des argiles et la réflectométrie des particules carbonées[END_REF] [START_REF] Butler | The influence of pre-existing basin structure on thrust system evolution in the Western Alps[END_REF] [START_REF] Nziengui | Excès d'argon radiogénique dans les quartz des fissures tectoniques : implications pour la datation des séries métamorphiques[END_REF] RSCM (This study) > on deformation: Thermopalaeomagnetism [START_REF] Crouzet | Experimental evaluation of thermal recording of successive polarities during uplift of metasediments[END_REF] TChl-basement (This study) Apatite Fission Tracks [START_REF] Sabil | La datation par traces de fission: aspects méthodologiques et applications thermochronologique en contexte Alpin et de Marge Continentale[END_REF][START_REF] Van Der Beek | Inversion of thermochronological age-elevation profiles to extract independent estimates of denudation and relief history -II: Application to the French Western Alps[END_REF][START_REF] Beucher | Exhumation and relief development in the Pelvoux and Dora-Maira massifs (western Alps) assessed by spectral analysis and inversion of thermochronological age transects[END_REF] Mesozoic cover

B e l l e d o n n e C h a r t r e u s e

Nummulitic cover

Veins in Basement

Veins in mesozoic cover (Gratier et al., 1973;[START_REF] Bernard | Microthermometrie des inclusions fluides de cristaux syn-cinematiques[END_REF] Age (Ma) Priabonian Flysch deposit [START_REF] Sinclair | Tectono-metamorphic evolution of the Briançonnais zone (Modane-Aussois and Southern Vanoise units, Lyon Turin transect, Western Alps)[END_REF] Metamorphic petrology (Jullien and Go é, 1993) Temperature (°C) Depth (km) and Pressure (kbar) [START_REF] Crouzet | High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit[END_REF][START_REF] Ceriani | Tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between low-grade metamorphism and tectonic phases[END_REF]Le Fort, 1973Saliot, 1973 P-T estimates

Cooling

  A c c e p t e d M a n u s c r i p t

  deviation to the closure temperature concept could be simply explained in terms of presence of 40 Ar excess either due to (1) partial deformation-induced resetting of Variscan muscovites (inherited argon), or (2) genuine incorporation of extraneous argon related to fluid circulations. Besides, a straightforward yet possible source of geologically meaningless ages could be simply explained by mixtures between inherited muscovite and synkinematic phengite argon reservoirs during lasering.

23

  Ma by removing excess40 Ar content from fluid inclusions. If excess argon exists, it should have a homogeneous composition at the scale of the whole study area, which seems particularly unlikely. Excess argon is therefore present but does not explain the first order features of the results.

  This study brings primarily T-t-d (Temperature-time-deformation) results that further constrain the tectonometamorphic evolution of the external zones of the Western Alps. In this section, these results are all integrated into a compilation of published P-T-t-d results

Figure 1 :

 1 Figure captions

Figure 2 :

 2 Figure 2: Simplified structural map of the studied area modified from Bellanger et al.(2014). Indicated is the location of the studied samples either form the Mesozoic cover for RSCM and chlorite geothermometry or from the basement shear zones for chlorite geothermometry and for the 40 Ar/ 39 Ar in situ approach. See tables for precise locations of samples.

Figure 3 :

 3 Figure 3: Deformation features of the Oisans Massif at decreasing scales. (a) kilometer-scale view as illustrated by a geological cross-section from the Penninic Frontal Thrust (PFT) to La Mure Massif. Note the distribution of the Alpine shear zones and the overall eastward increasing of the finite strain intensity; (b) landscape-scale view as illustrated by a picture of a part of the Plan du Lac shear zone in the Vénéon Valley. Note the anastomosed relationships of the shear planes, the folded shape of the basement-cover interface and the pinched part of Mesozoic cover. Field-sketch diagrams represent macroscopic features of

Figure 4 :

 4 Figure 4: First-order results from the Raman Spectrometry of Carbonaceous Material approach. (a) RSCM Map showing mapping of the T max results and inferred isotherms. The isotherm shape was drawn from interpolated map and from the distribution of the Illite crystallinity; (b) Illite crystallinity map modified from Aprahamian (1974) and Barlier et al. (1974). (c) Projection of the RSCM Tmax over a classical E-W cross-section from the west of the Belledonne Massif to the Penninic Frontal Thrust. (d and e) RSCM Tmax results vs. longitude and elevation along the cross-section (c). Data represented with smaller symbols to the east of the Penninic Frontal Thrust come from Beyssac et al. (2002); Gabalda et al. (2009); Lanari et al. (2012) and[START_REF] Plunder | How continuous and precise is the record of P-T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustrés, W. Alps)[END_REF]. Detailed results are given in table1.

Figure 5 :

 5 Figure 5: Local-scale features of results from the Raman Spectrometry of Carbonaceous Material approach. (a) Distribution of the Tmax to the west of the Belledonne Massif from RSCM (circle) and vitrinite reflectance results (star; data from[START_REF] Deville | Contrasting thermal evolution of thrust systems: An analytical and modeling approach in the front of the western Alps[END_REF]

Figure 6 :

 6 Figure 6: Chlorite composition and results from chlorite geothermometry the Mesozoic cover. (a) XMg vs. Silica content diagram. (b) Clinochlore/Amesite/Sudoite ternary plot. Note that chlorite composition is first controlled by the bulk composition of the protolith. (c) Internal consistency of the geothermometric results. Chlorite geothermometry results using

Figure 7 :

 7 Figure 7: Example of microstructural and composition of sample A.4.64 used as a guideline for dated samples. (a) Contrasted habits of white-micas as seen on several close-up views from the same thin-section. (b) White-mica chemical composition in ternary plots and XMg vs. Silica content diagram. Fine-grained, often syn-kinematic white-micas present in C-type and S-type foliation have phengite compositions. Well-developed white-mica clasts have a muscovite composition.

Figure 8 :

 8 Figure 8: White-micas composition and chlorite geothermometry from dated samples (basement shear zones). For each of the dated shear zone, presented is the composition of white-micas as a function to their textural position in Si vs. Na content diagram and

  (a) corresponds to results for the Col de Cluy shear zone while (b), (c) and (d) correspond to Plan du Lac, Col du Lac and Combeynot shear zones, respectively. Due to a very poor dataset, results for the Combeynot shear zone are not shown.

Figure 9 :

 9 Figure 9: Time-chart showing all results of the in situ 40 Ar/ 39 Ar approach. Detailed results are given in table 3. Stratigraphic age for the Nummulitic flysch deposit is from Mulder et al. (2010); zircon and apatite fission-tracks results from Van der Beek et al. (2010).

Figure 10 :

 10 Figure 10: Pictures showing the distribution of in situ laser ablation of the 40 Ar/ 39 Ar approach. Note the overall dependency of the K/Ar system to the finite strain intensity.Detailed results are given in table 3.

Figure 11 :

 11 Figure 11: Pressure-Temperature-deformation-time evolution of the Oisans Massif. (a)

  (8)[START_REF] Gasquet | Miocene to Messinian deformation and hydrothermal activity in a pre-Alpine basement massif of the French western Alps: new U-Th-Pb and argon ages from the Lauziere massif[END_REF] (9) (Simon-Labric et al., 2009) (a: Combeynot sinistral strike-slip shear zone; b: Pelvoux dextral strike-slip shear zone; c: Ailefroide dextral strike-slip shear zone); (10) this study; (11)

(

  Strzerzynski et al., 2012) (63)

Figure 13 :

 13 Figure 13: Lithospheric reconstructions of the Alps along a cross-section close to the 45th parallel. See text for explanations.

  Figure 1

Figure 3 A

 3 Figure3

  Figure 5

  Figure 10

  dimension data of white micas

  Figure11

Figure 12 ~34

 12 Figure12

Table 3 :

 3 Detailed results from the 40 Ar/ 39 Ar approach. Given are the GPS position (latitude

	(Lat.) and longitude (Long.) in decimal degrees (WGS84) in decimal degrees (WGS84) and
	different output isotopic ratio.

Table 3

 3

	Page 50 of 66

Acknowledgments

This research was funded by the European Research Council (ERC) under the seventh Framework Programme of the European Union (ERC Advanced Grant, grant agreement No 290864, RHEOLITH) and has received funding from the BRGM (contract L10 U 044).

We would like to thank M. Sellos and M. Vacelet for constructing discussions and for their help in the field; J.-G. Badin, S. Janiec for thin-section preparation. Y. Rolland and T.

Dumont are thanked for constructive comments that greatly improved the quality of a previous manuscript. A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p t Gratier, J. P., and P. Vialon (1980), Deformation pattern in a heterogeneous materialfolded and cleaved sedimentary cover immediately overlying a crystalline basement (Oisans, French Alps), Tectonophysics, 65 [START_REF] Gabalda | Thermal structure of a fossil subduction wedge in the Western Alps[END_REF], 151-180.

Gratier, J.-P., B. Lejeune, and V. Jean-Louis (1973), Etude des déformations de la couverture et des bordures sédimentaires des massifs cristallins externes de Belledonne, des Grandes Rousses et du Pelvoux (depuis des Aravis jusqu'à le région de Remollon), Université de Grenoble.

Gueydan, F., Leroy, Y. M., Jolivet, L., & Agard, P. (2003)