PhD Thierry Dumont

Dr Stephane Schwartz

Stephane Guillot

Marco Malusà

Marine Jouvent

Patrick Monié

Aude Verly

Mancktelow, N M Campani

Müller W & Guerra

western Alpine orogen from early to late deformation stages: evidence from the Internal Zones and implications for restoration

Keywords:

The internal zones of the Western Alps arc are derived from an oceanic and continental subduction wedge developed beneath the Adria plate during its paleogene northward drift. Exhumation of the internal zones proceded from early Oligocene onwards due to westward extrusion of the Adria plate. The prominent fold-and-thrust structures which follow the arc shape, either forward or backward verging, postdate the initial nappes stacking and overprint differently oriented older deformations which are relevant to proper retoration of this arcuate orogen to minimise overlap problems. We document this early stacking phase through outcrop-scale structural analysis at 55 sites between the Maurienne and Ubaye valleys, along with larger-scale examples of early structures. They consistently show an initial N-to NW tectonic transport, whose kinematic indicators are overprinted by either forward (W-to SW-directed) or backward (E-to NE-directed) deformation associated with post-nappe transport along the Penninic thrust. Accordingly, restoring the Briançonnais fold/thrust system must incorporate reconstruction of the nappes stack along the initial top N-NW direction of orogenic propagation, with careful consideration of their paleogeographic origin towards the S-SE. This stack was built during the Eocene Adria-Iberia collision, and overthrust the Subbriançonnais-Valaisan trough to the NW before involving the Dauphiné-Helvetic foreland. It includes different types of Paleozoic units, either Permo-Carboniferous sediments towards its base, or polymetamorphic basement above, which can be explained by inversion of a late Variscan basin and of its southern shoulder, whereas the uppermost Prepiedmont units result from inversion of the Tethyan margin toe. Mixed breccia, locally preserved close to the tectonic contact between the latter units and the overlying "Schistes Lustrés" oceanic nappes, are interpreted as olistostromes fed by both units in a very early collision stage. 39 Ar/ 40 Ar dating suggests that these shallow tectono-sedimentary formations were involved in the subduction wedge during the early Eocene, whereas younger (late Eocene) equivalent olistostromes mark the propagation of the Briançonnais stack over the external (Dauphiné/Helvetic) foreland. The Eocene orogenic wedge was rapidly exhumed during Oligocene westward indentation and radial spreading, in a markedly different tectonic context driven by extrusion around an Adriatic upper mantle indenter, which controlled development of the Western Alps arc in relation with the Ligurian sea

follow the arc shape, either forward or backward verging, postdate the initial nappe 16 stacking and overprint differently oriented older deformations which are relevant to 17 proper retoration of this arcuate orogen to minimise overlap problems. We document 18 this early stacking phase through outcrop-scale structural analysis at 55 sites between 19 the Maurienne and Ubaye valleys, along with larger-scale examples of early structures. 20 They consistently show an initial N-to NW tectonic transport, whose kinematic 21 indicators are overprinted by either forward (W-to SW-directed) or backward (E-to 22 NE-directed) deformation associated with post-nappe transport along the Penninic 23 thrust. Accordingly, restoring the Briançonnais fold/thrust system must incorporate 24 reconstruction of the nappe stack along the initial top N-NW direction of orogenic 25 the south-verging subduction and closure of the Ligurian Tethys, a small slow-spreading 100 oceanic domain which opened in middle Jurassic times (Bernoulli & Lemoine, 1980;Dal 101 Piaz, 1999;[START_REF] De Graciansky | The Western Alps, from rift to passive margin to 1680 orogenic belt, an intergated overview[END_REF]. It also contains parts of the European continental 102 margin of this ocean, variably affected by syn-rift crustal thinning [START_REF] Lemoine | The continental margin of the Mesozoic Tethys in the Western Alps[END_REF]103 Manatschal et al., 2007;Le Breton et al., 2021), and an attenuated crust and/or exhumed 104 mantle area, the Valais domain, whose age and extension are still debated (Bousquet et 105 al., 2002;[START_REF] Beltrando | Was the Valaisan basin floored by oceanic 1507 crust? Evidence of Permian magmatism in the Versoyen unit (Valaisan domain, NW Alps)[END_REF]Pfiffner, 2014). The remnants of units derived from the 106 Adria continental plate margin are relatively scarce in the Western Alps due to syn-107 orogenic erosion, although their discovery by [START_REF] Argand | Les nappes de recouvrement des Alpes Pennines et leurs prolongements structuraux. 1449 Matériaux pour la Carte Géologique de la Suisse[END_REF] laid the foundation of 108 modern Alpine geology. 109

The Western Alps are conveniently divided into external and internal zones, separated 110 by the « Penninic thrust » [START_REF] Schmid | Tectonic map and overall architecture of the 2135 Alpine orogen[END_REF], a crustal-scale tectonic contact following 111 the arc (fig. 1b) which is E-dipping in the central part of the arc [START_REF] Guellec | Neogene evolution of the western Alpine foreland in 1784 the light of Ecors data and balanced cross-section[END_REF]112 Lardeaux et al., 2006). To the west, the contractional footwall units (External zone) 113 consist of Dauphiné-Helvetic Variscan basement and Meso-Cenozoic series lacking 114 significant Alpine metamorphism. They are locally overlain by exotic sedimentary cover 115 nappes emplaced before the activation of the Penninic thrust (Prealps, Ubaye nappes, Ligurian flysch nappes; [START_REF] Dumont | 2525 c-Recontructed part of the profile across the Briançonnais Paleozoic basins before 2526 beeing affected by thrust sequence and overlain by the Prepiedmont and oceanic 2527 nappes[END_REF]. The External continental 117 crust is deeply subducted eastwards beneath the Internal zones and the Adria plate 118 [START_REF] Zhao | First seismic evidence for continental subduction beneath the 2263[END_REF][START_REF] Nouibat | Lithospheric transdimensional ambient-noise tomography of W-Europe: implications for 2049 crustal-scale geometry of the W-Alps[END_REF]. To the east of the Penninic thrust, the 119 hangingwall nappe stack of the Internal zones is highly heterogeneous both regarding 120 palaeogeographic provenance [START_REF] De Graciansky | The Western Alps, from rift to passive margin to 1680 orogenic belt, an intergated overview[END_REF] and Alpine metamorphism 121 [START_REF] Oberhänsli | Metamorphic structure of the Alps 1:1'000000[END_REF][START_REF] Bousquet | Metamorphism of metasediments at the scale of an orogen: a key to the 1565 Tertiary geodynamic evolution of the Alps[END_REF][START_REF] Agard | Subduction of oceanic lithosphere in the Alps: Selective and archetypal from (slow-1442 spreading) oceans[END_REF], ranging from upper 122 greenschist to eclogite facies metamorphism. 123 Thus, three main units have to be considered in this area, that is, from W to E, the 124 Extenal zone, the Embrunais-Ubaye nappes, and the Internal zones (Fig. 1b): 125 -The external zone s.s. is composed of Variscan basement whose local exhumation and 126 uplift due to several thick-skinned shortening events provides some of the major Alpine 127 relief, up to >4km. The top basement is locally buried to ~10km depth in the center of 128 the SE France Basin following several Mesozoic rifting events. The Mesozoic 129 sedimentary cover shows strongly variable thickness and facies across the Jura 130 platform, the Dauphiné rifted margin [START_REF] Lemoine | The continental margin of the Mesozoic Tethys in the Western Alps[END_REF], the Vocontian basin and the 131 Provence platform. The Mesozoic paleogeographic trends of the External zone are 132 crosscut by the arcuate Alpine structures. The sedimentary cover is locally involved in 133 thin-skinned fold and thrust deformation of different ages and orientations due to 134

Pyrenean and Alpine contractional propagation [START_REF] Gidon | Les chaînons subalpins au nord-est de Sisteron et l'histoire tectonique de la nappe de 1775 Digne[END_REF][START_REF] Philippe | Thin-skinned inversion tectonics at oblique basin margins: 2062 example of the western Vercors and Chartreuse Subalpine massifs (SE France)[END_REF]135 Espurt et al., 2012;[START_REF] Schwartz | Foreland exhumation controlled by crustal thickening in the Western Alps[END_REF]. The occurrence of soft evaporitic Triassic 136 layers played a major control on the location of detachments [START_REF] Lickorish | Arcuate thrust systems in sandbox experiments: 1925 A comparison to the external arcs of the Western Alps[END_REF]137 Espurt et al., 2019;[START_REF] Balansa | Structural evolution of the superimposed 1459 Provençal and Subalpine fold-thrust belts (SE France)[END_REF]. Remnants of synorogenic basins record 138 different stages of propagating lithospheric flexure through foreland basin and 139 forebulge development, with flysch and molasse deposition (Joseph & Lomas, 2004;140 Ford & Lickorish, 2004;Kempf & Pfiffner, 2004;[START_REF] Schwartz | Source tracing 2165 of detrital serpentinite in the Oligocene molasse deposits from Western Alps (Barrême basin): 2166 implications for relief formation in the internal zone[END_REF]Kalifi et al., 2020). 141 -The Embrunais-Ubaye nappes were transported in a superficial setting over the 142 flexural basin on the External zone during the late Eocene to early Oligocene 143 [START_REF] Sagnes | [END_REF][START_REF] Gupta | Implications of foreland paleotopography for stratigraphic development in 1795 the Eocene distal Alpine foreland basin[END_REF]. They are dominantly composed of 144 « Helminthoid Flysch », late Cretaceous deep marine sequences detached from the 145 Tethys oceanic floor and transported over the Briançonnais domain whose thin remnant 146 thrust sheets are often observed at their base [START_REF] Sagnes | [END_REF]. The Embrunais-Ubaye 147 nappes record only low-grade metamorphism from sub greenschist to lower greenschist 148 facies [START_REF] Oberhänsli | Metamorphic structure of the Alps 1:1'000000[END_REF], similar to the Ligurian and Prealps nappe stack. They 149 bear evidence for changes in transport direction (Merle & Brun, 1984) from an original 150 southeastern origin. They are locally preserved over the external foreland in the 151 footwall of the Penninic thrust, which demonstrates the polyphase and non-coaxial 152 character of Alpine orogenic propagation. Their emplacement is dated as late Eocene-153 earliest Oligocene, with intial NW-directed deformation beneath the Embrunais basal 154 thrust (Dumont et al., 2011;fig. 1b, Fig. 2), and they are overprinted, deformed and 155 crosscut in out-of-sequence mode by the Penninic Thrust propagating towards the SW 156 from early Oligocene onwards. 157 -The Internal nappe stack includes parts of the distal European margin (detached 158 sedimentary cover and basement of the Briançonnais domain s.l.), exhumed remnants of 159 the Ligurian Tethys ocean (metasediments and ophiolites, the so-called « Schistes 160 Lustrés »), and scarce overthrust pieces of Adria continental crust (Dent Blanche and 161 Cervin units;[START_REF] Dal Piaz | The Austroalpine-Piedmont nappe satck and the puzzle of Alpine Tethys[END_REF][START_REF] Schmid | Tectonic map and overall architecture of the 2135 Alpine orogen[END_REF]. The Internal nappes can be classified 162 according to different criteria: their dominant lithology, either upper crustal basement 163 or sediments, their metamorphic signature (Alpine HP-LT and/or Variscan HT-LP; 164 [START_REF] Oberhänsli | Metamorphic structure of the Alps 1:1'000000[END_REF][START_REF] Bousquet | Metamorphism of metasediments at the scale of an orogen: a key to the 1565 Tertiary geodynamic evolution of the Alps[END_REF][START_REF] Von Raumer | Pre-Mesozoic Alpine basements 2256 -Their place in the European Paleozoic framework[END_REF]Schwartz et 165 al., 2013), and their paleogeographic provenance with respect to the Tethyan 166 framework [START_REF] Lemoine | The continental margin of the Mesozoic Tethys in the Western Alps[END_REF][START_REF] Schmid | Tectonic map and overall architecture of the 2135 Alpine orogen[END_REF][START_REF] Handy | Reconciling plate-tectonic 1802 reconstructuons of Alpine Tethys with the geological-geophysical record of spreading and subduction in 1803 the Alps[END_REF]. In Fig. 1 and2 167 we define structural units with respect to their origin, either from the European 168 continental margin s.l. (including parts of the Iberian plate) or from the oceanic 169 domains. Concerning the continental margin units, we follow the definitions of Lemoine 170 et al. (1986), which distinguishes the Subbriançonnais, Briançonnais, internal 171 Briançonnais and Prepiedmont type units. Contrary to [START_REF] Mohn | Unravelling th interaction between 2037 tectonic and sedimentary processes during lithospheric thinning in the Alpine Tethys margins[END_REF] or Ribes et al. 172 (2019), we have to maintain the distinction between the internal Briançonnais and 173

Prepiedmont units, which have a very different Mesozoic record inherited from their 174 syn-rift history. The ocean-derived nappes still occupy hangingwall locations on both 175 sides of the Penninic thrust (fig. 2), and in the highly metamorphosed core of the arc. 176

The latter include the Monviso and Voltri units, which have been interpreted as 177 remnants of an oceanic subduction channel [START_REF] Schwartz | Numerical model of the effect of serpentinites on the 2153 exhumation of eclogitic rocks: insights from the Monviso ophiolitic massif (Western Alps)[END_REF][START_REF] Guillot | The Montviso ophiolitic massif 1791 (Western Alps), a section through a serpentinite subduction channel[END_REF]178 Federico et al., 2007) but may also result from different intra-oceanic structural 179 inheritance and decoupling processes in a complex plate interface (Balestro et al., 2018;180 Agard, 2021;Herviou et al., 2022, and refs. therein). The subduction channel process is 181 also involved during the continental subduction stage (Ganne et al., 2006;Bousquet, 182 2008b, Federico et al., 2005), although alternative processes may explain the 183 exhumation of HP units, such as corner-flow [START_REF] Polino | Tectonic erosion at the Adria margin and accretionary processes 2085 for the Cretaceous orogeny of the Alps[END_REF] or transtension 184 [START_REF] Malusà | Contrasting styles of (U) HP rock exhumation along the Cenozoic Adria-Europe plate boundary 1952 (Western Alps, Calabria, Corsica)[END_REF]. Soon after the Eocene-Oligocene boundary, the initial suture was 185 crosscut by the Penninic thrust and is thus strongly affected by backfolding and tilting 186 due to vertical extrusion of the Internal crystalline massifs from beneath the orogenic 187 wedge [START_REF] Schwartz | La zone piémontaise des Alpes occidentales: un paléocomplexe de subduction. 2151 Arguments métamorphiques, géochronologiques et structuraux[END_REF][START_REF] Rolland | Extension syn-convergence, poiçonnement 2108 vertical et unités métamorphiques contrastées en bordure ouest du Grand Paradis (Alpes franco-2109 italiennes)[END_REF][START_REF] Avigad | Thrusting and extension in the Southern Dora-Maira ultra-1453 high-pressure massif (Western Alps): view from below the cohesite-bearing unit[END_REF][START_REF] Schwartz | Late tectonic and metamorphic 2171 evolution of the Piedmont accretionary wedge (Queyras Schistes lustrés, Western Alps): Evidences for 2172 tilting during Alpine collision[END_REF]. 188 199 Simon-Labric et al., 2009;[START_REF] Maino | Dating 1933 shallow thrusts with zircon (U-Th)/He thermochronometry -The shear heating connection[END_REF], coeval with the formation of the arc 200 [START_REF] Dumont | 2525 c-Recontructed part of the profile across the Briançonnais Paleozoic basins before 2526 beeing affected by thrust sequence and overlain by the Prepiedmont and oceanic 2527 nappes[END_REF]. Its « out-of-sequence » character can be observed south of 201

Briançon city, where it cuts across an earlier nappe stack involving ocean-derived flysch 202 sediments, the Embrunais-Ubaye nappes (fig. 1b), also represented in the Ligurian and 203 Prealps nappes. Moreover, parts of the oceanic accretionary wedge deformed together 204 with distal European continental margin units are exhumed in its hangingwall (Schmid 205 et al., 2004). It is important to consider that the building of the Western Alps arc 206 required polyphase deformation and displacement directions changing through time 207 [START_REF] Ford | [END_REF] as opposed to a continuous process driven by strain partitioning and 208 maintenance of specifically orientated convergence directions (Fry, 1986). 209

The structure of the Internal zones of the Western Alps arc underlined by the 210 « Penninic » curved frontal sole thrust and by prominent backthrusting is a possible 211 consequence of Adria mantle indentation, that is particularly evident in the 212 northernmost Western Alps [START_REF] Schmid | Ivrea mantle wedge, arc of the 2139 Western Alps, and kinematic evolution of the Alps-Apennines orogenic system[END_REF][START_REF] Malusà | The deep 1955 structure of the Alps base on the CIFALPS seismic experiment: A synthesis[END_REF]Nouibat et al., 213 2022). Both follow the curvature of the arc. In the study area, radial interpretative cross-214 sections (fig. 2) illustrate this double-vergent structural style, along with the large-scale 215 characteristics of the Briançonnais nappe stack. Radial sections are commonly used to 216 represent bulk Alpine deformation across the arc (i.e. [START_REF] Schmid | Ivrea mantle wedge, arc of the 2139 Western Alps, and kinematic evolution of the Alps-Apennines orogenic system[END_REF] and have been 217 used as the basis for restored models of pre-Alpine paleogeography or shortening 218 estimates [START_REF] Fry | Southwestward thrusting and tectonics of the western Alps[END_REF][START_REF] Seno | A kinematic evolutionary model for the Penninic sector of 2175 the central Ligurian Alps[END_REF][START_REF] Bellahsen | Collision kinematics in the western external Alps[END_REF]. These existing 219 restorations are not necessarily compatible together, precisely because of their radial 220 distribution over nearly 180°. A key consideration which is often overlooked when 221 examining radial sections is the amount of out-of-plane movement, oblique or 222 perpendicular to cross-section orientations. We provide evidence for complex, non-223 'pre-rift', rift and starved continental margin sequences, to oceanic sediments and 230 remnants of their slow-spreading oceanic floor [START_REF] Lemoine | The continental margin of the Mesozoic Tethys in the Western Alps[END_REF]Lagabrielle, 1994). 231

The Variscan orogenic belt [START_REF] Guillot | paleogeography[END_REF][START_REF] Von Raumer | Pre-Mesozoic Alpine basements 2256 -Their place in the European Paleozoic framework[END_REF]Festa et al., 241 2020). Strong lateral variations indicate the activity of depocenters controlled by 242 extensional faulting [START_REF] Cortesogno | Elements of the Paleozoic history of the 1638 Ligurian Alps[END_REF], and widespread Permian volcanic activity, 243 thermal evolution and underplating document the initiation of post-Variscan crustal 244 attenuation in the entire Alpine area (Cortesogno et al., 1998;[START_REF] Rottura | Origin and 2115 significance of the Permian high-K calc-alkaline magmatism in the central-eastern Southern Alps[END_REF]245 Marotta & Spalla, 2007;Sinigoi et al., 2010). 246 -Triassic: the classical siliciclastic-carbonate-evaporitic cycle is widely developed in the 247 Internal units of both European and Adria origin, with kilometre-scale thickness. Middle 248 Triassic shallow marine sequences can be traced over the entire Alpine-Carpathian area. 249

In the Western Alps, little evidence for brittle extension has been reported to date 250 despite significant subsidence. More important Triassic rift basins were located further 251 East / Southeast as part of the Neotethyan basin propagation [START_REF] Stampfli | Western Alps geological constraints on western 2188 Tethyan reconstructions[END_REF]. 252 However, evaporites deposited during the late Triassic played a major role during 253 Alpine orogeny as they controlled the detachment of large sedimentary cover units 254 within the collision wedge. 255 -Early to Middle Jurassic: all the marginal cover units of the Internal Western Alps show 256 evidence of a rift setting from early Jurassic onwards (Dumont, 1998). Contrasting 257 subsidence patterns during early Liassic suggest tectonic subsidence followed by uplift 258 over the whole Briançonnais domain during late Liassic-middle Jurassic, which 259 produced emergence and continental erosion coeval with extensional block faulting 260 (Claudel & Dumont, 1999). This process, which consists of a long-wavelength uplift with 261 a vertical amplitude reaching about 1 km, could be explained by rift shoulder uplift 262 [START_REF] Stampfli | Divergent and convergent margins in the North-Western Alps, 2191 confrontation to actualistic models[END_REF]. Alternatively, it could result from different processes such 263 as thermal influence linked with the upper mantle boudinage and impregnation during 264 hyperextension (« thermal erosion », [START_REF] Mohn | Necking of continental crust in 2034 magma-poor rifted margins: Evidence from the fossil Alpine Tethys margins[END_REF] or extensional ribbon uplift 265 [START_REF] Tavani | Rift inheritance controls the switch from thin-to thick-skinned thrusting and basal 2214 décollement re-localization at the subduction-to-collision transition[END_REF]. At a shorter wavelength, the maximum amplitude of erosion and 266 uplift is observed in the internal Briançonnais units, initially located close to the 267 paleogeographic boundary with the strongly subsiding Prepiedmont domain. This 268 feature could correspond to flexural uplift [START_REF] Basile | Erosion and flexural uplift along transform faults[END_REF] or to the influence 269 of lithospheric necking [START_REF] Ribes | Long-lived mega fault-scarps and related breccias at distal rifted margins: insight from present-day and 2097 fossil analogues[END_REF]. The resulting unconformity is a widely 270 recognised characteristic of the Briançonnais marginal plateau, which was emerged and 271 increasingly uplifted towards the incipient Tethyan breakup [START_REF] Lemoine | The continental margin of the Mesozoic Tethys in the Western Alps[END_REF]. The 272 magnitude of the associated erosional gap increases towards the rift, that is from the 273 external to the internal Briançonnais units. Syn-rift continental erosion removed the 274 whole Triassic sequence in the most internal Briançonnais units, allowing the post-rift 275 sediments to rest on the late Variscan volcaniclastics or directly on the basement. 276

Conversely, this unconformity is not recognised in the more proximal rift basins of the 277 External zone, nor closer to the breakup, in the Prepiedmont domain which was fed by 278 turbidites sourced from the emerged Briançonnais shoulder [START_REF] Lemoine | Découverte de Foraminifères planctoniques du Crétacé supérieur dans les 1917 Schistes lustrés du Queyras (Alpes Occidentales). Conséquences paléogéographiques et tectoniques[END_REF]. This 279 strongly subsiding domain may represent the most hyperextended part of the distal 280 margin [START_REF] Mohn | Necking of continental crust in 2034 magma-poor rifted margins: Evidence from the fossil Alpine Tethys margins[END_REF] and play a key role to locate the early inversion processes 281 [START_REF] Tavani | Rift inheritance controls the switch from thin-to thick-skinned thrusting and basal 2214 décollement re-localization at the subduction-to-collision transition[END_REF]. 282 -late Middle Jurassic to early Cretaceous: consequent to the erosional events described 283 above, a widespread unconformity is spectacularly exposed in the Briançonnais, 284 characterised by deep marine post-rift (syn-spreading) sediments overlying a variety of 285 older (pre-spreading) stratigraphic units. This unconformity has traditionally been 286 described as the "breakup unconformity" [START_REF] Lemoine | The continental margin of the Mesozoic Tethys in the Western Alps[END_REF]; see discussion in Masini 287 et al., 2013). Following the initial breakup in the late Middle to early Late Jurassic (Li et 288 al., 2013, and refs therein), a uniform post-rift series covered the whole margin, 289 including the Briançonnais marginal plateau, preserving a transgressive lag over the 290 erosional surface. These starved post-rift pelagic sediments record thermal subsidence. 291 However, some restricted extensional deformation is reported locally within the 292 Briançonnais area (Claudel & Dumont, 1999), as in Provence [START_REF] Dardeau | Jeu de blocs et 1657 tectonique distensive au Callovien et à l'Oxfordien dans le bassin du Sud-Est de la France[END_REF], and 293 possibly due to incipient rifting between Europe and areas connected to the Iberian 294 plate. 295 -Late Cretaceous to Eocene: still in a deep marine setting, several domains of the 296 internal zones record the diachronous onset of flysch sedimentation. The oceanic 297 sediments show evidence of margin sourced turbidites from the Cenomanian onwards 298 (Caron et al., 1989;Durand-Delga et al., 2005;Catanzariti et al., 2007), with increasing 299 clastic input from the Campanian (Helminthoid Flysch fm.) related to the active Adria 300 margin [START_REF] Di Giulio | The evolution of the Western Ligurian Flysch units and the role of mud diapirism in 1693 ancient accretionary prisms (Maritime Alps, Northwestern Italy)[END_REF][START_REF] Marroni | [END_REF]. Local sourcing from the European margin 301 (Gottero flysch, [START_REF] Nilsen | Submarine-fan facies associations of the Upper Cretaceous and Paleocene 2046 Gottero Sandstone, Ligurian Apnennines, Italy[END_REF][START_REF] Marroni | Anatomy of the Ligure-Piemontese subduction system: 1979 evidence from Late Cretaceous-Middle Eocene convergent margin deposits in the Northern Apennines, 1980 Italy[END_REF] indicate that denudation occurred 302 before involvement in the Adria-Europe collision, possibly in relation to the onset of 303 Pyrenean orogeny. The continental margin series are also affected by tectono-304 sedimentary disturbances, for example erosional unconformities and breccias, 305 especially in the Briançonnais units [START_REF] Gidon | Carte géol[END_REF]. Also recorded in the Alpine 306 foreland [START_REF] Michard | Cretaceous folding in the Dévoluy mountains 2026 (Subalpine Chains, France): gravity-driven detachment at the European paleomargin versus 2027 compressional event[END_REF] and in Provence (Espurt et al., 2012), these features can 307 be interpreted either as a response to active transcurrent deformation (Bertok et al., 308 2012), Pyrenean forebulge propagation [START_REF] Thum | The Piolit, Pelat and Baiardo Upper Cretaceous flysch 2223 formations (Western Alps): geodynamic implications at the time of the Pyrenean tectonic phases[END_REF], or Alpine forebulge 309 propagation [START_REF] Michard | The Eocene unconformity of the Briançonnais domain in French-Italian 2029 Alps, revisited (Marguareis massif, Cuneo); a hint for a Late Cretaceous-Middle Eocene frontal bulge 2030 setting[END_REF]. 310 Despite the metamorphic overprint, (litho)stratigraphic correlations between many 311 units presently included in the Internal Zones of the western Alpine arc remain possible. 312

These units generally display a sedimentary record markedly different from the External 313 Zone (Dauphiné-Hevetic, Vocontian and Provence domains), both in terms of 314 stratigraphic thickness and environment, from post-Varican to Eocene times (Lemoine 315 et al., 1986). This supports the occurrence of a major lithospheric-scale displacement 316 along the boundary between the Internal and External Zones, whose large 317 paleogeographic areas were likely overthrust from the earliest Oligocene onwards. 318

As previously stated, the dominant structures following the Western Alps arc (Penninic 319 thrust, Briançonnais zone, metamorphic zonation in Schistes Lustrés, internal cristalline 320 massifs trend) were produced during the westward extrusion stage from the early 321 Oligocene onwards, and they crosscut, deformed and exhumed the initial stack formed 322 during the north-to northwestward Eocene orogenic propagation. Thus, despite this 323 initial phase accommodated important tectonic transports, its structural effects are 324 presently obscured due to further overprint. Some evidence is provided by interference 325 structures at different scales, especially in the Western Alps where the directions of 326 early and late Alpine orogenic propagation are markedly different. 327 328 4-Polyphase deformation: large-scale overprint between differently oriented 329 structures in the Internal Zones (interference structures?) 330 331 Large-scale superposed deformation due to crossed shortening episodes is suggested in 332 the external zone by the circular shape of the Pelvoux-Ecrins cristalline massif (Dumont 333 et al., 2011). Interference shortening structures are reported from the internal Western 334

Alpine arc (e.g. Jaillard, 1984;[START_REF] Ganne | Les dômes de socle HP-BT dans le domaine Pennique des Alpes nord-occidentales (massifs 1765 d'Ambin et de Vanoise Sud): modalités de leur exhumation[END_REF][START_REF] Bucher | Tectonic 1574 evolution of the Briançonnais units alonf a transect (ECORS-CROP) through the Italian-French Western 1575 Alps[END_REF]. In the Central Alps, the 335 deeply exhumed Lepontine area shows complex curved shapes which are interpreted to 336 result from superposed deformations with different strain patterns [START_REF] Merle | Histoire de la déformation dans les Alpes Lépontines occidentales[END_REF]. Steck 337 (2008) and [START_REF] Steck | Tectonics of the Lepontine 2196 Alps: ductile thrusting and folding in the deepest tectonic levels of Central Alps[END_REF][START_REF] Steck | Tectonics of the Monte Rosa and surrounding nappes (Switzerland 2201 and Italy): Tertiary phases of subduction, thrusting and folding in the Penninic Alps[END_REF][START_REF] Steck | The Maggia nappe: an extruding sheath fold basement nappe in 2199 the Lepontine gneiss dome of the Central Alps[END_REF]

3) 348

In the southern part of the Western Alpine arc, the "Roure" zone [START_REF] Le Guernic | La zone du Roure: contribution à l'étude du Briançonnais interne et du Piémontais en 1885 Haute Ubaye[END_REF] and 349 the Acceglio-Longet (AL) zone [START_REF] Debelmas | Calcschistes piémontais et terrains à faciès briançonnais dans les hautes 1662 vallées de la Maira et de la Varaita[END_REF][START_REF] Lefèvre | [END_REF]) 350 belong to internal Briançonnais. They are composed of late Paleozoic and Mesozoic 351 series, characterized by very thin Triassic-Jurassic sediments compared to the external 352 Briançonnais units. They outcrop in two SSE-NNW oriented strips (a and b fig. 3, 353 respectively) tangential to the trend of the arc, which are formed mainly by ENE-WSW 354 shortening and eastward backfolding. However, in both areas it is possible to detect 355 earlier deformation criteria, roughly perpendicular to the youngest deformation. 356 At Col du Longet (a, fig. 3), the northern termination of the Acceglio-Longet strip 357 [START_REF] Schwartz | The Acceglio zone (Cottian Alps): a new example of 2168 eclogitized continental crust in the Western Alps[END_REF] becomes buried beneath the Rocca Bianca ophiolitic massif and 358 the surrounding metasediments (fig. 4). Outcrop-scale structures consist of a pervasive 359 lineation and metre-scale, top-to-the N to NW overturned folds trending WSW-ENE, 360 both within the Permian to Mesozoic clastic formations of the AL unit and within the 361 oceanic units above (fig. 4; stereogram site 39, see §5; [START_REF] Verly | Timing of the Alpine collision of the European margin units: new constraints from tectonic 2249 analysis and P-T-t paths of the Acceglio-Longet zone[END_REF]. These NNW-verging 362 structures postdate the initial stacking of oceanic and continental units, and are clearly 363 deformed by backfolding. The latter occurred close to the Eocene-Oligocene boundary 364 according to U-Th-Pb dating on allanite in this locality [START_REF] Verly | Timing of the Alpine collision of the European margin units: new constraints from tectonic 2249 analysis and P-T-t paths of the Acceglio-Longet zone[END_REF], implying that 365 oceanic and continental margin units had been stacked earlier during the Eocene. We 366 propose that this termination of the Acceglio-Longet strip corresponds to a large-scale 367 hinge of a transverse, recumbent or isoclinal fold trending SW-NE to EW (section c, fig. 368 4), which was further refolded by NE-ward recumbent backfolding, similarly to the 369 conceptual sketch of fig. 3

(a, upper left cartoon). 370

Different types of breccia are reported in the Col du Longet area [START_REF] Lemoine | Brèches sédimentaires marines à la frontière entre les domaines briançonnais et 1902 piémontais dans les ALpes occidentales[END_REF]Gout, 371 1987 and refs therein). The significance of these breccia is discussed in §8, but their 372 interpretation and their assignment to either continental or oceanic units is dependant 373 on the structural setting. The two main end-members are (i) thin beds of siliciclastic 374 microbreccias overlying the Permian siliciclastic and volcaniclastic formations of the AL 375 unit, with minor occurrence of dolomitic clasts indicating a post-Triassic age. (ii) mixed 376 siliciclastic/carbonate megabreccia containing metric to decametric blocks of Triassic 377 dolostones together with reworked Permian clasts of various size. They occur as lenses 378 or slices above the Acceglio-Longet unit or within the base of the Schistes Lustrés 379 oceanic units (grey, fig. 4). The first type (i) is stratigraphically linked with the AL unit, 380 and must be Jurassic to early Cretaceous in age because it is overlain by a Late 381 Cretaceous hard-ground (Lemoine, 1960a). The second type (ii) is associated with late 382 Jurassic-Cretaceous, strongly folded marbles and calcschists whose continental or 383 oceanic origin is hardly distinguishable [START_REF] Gout | Tectonique océanique et tectonique alpine dans les schistes lustrés de Haute Ubaye[END_REF]. However, the megabreccia does 384 not appear to contain any ophiolitic clasts and is dominated by reworking of Permian or 385 Mesozoic formations found in the AL series. This megabreccia is comparable to the Pelvo 386 d'Elva breccia [START_REF] Michard | Etudes géologiques dans les zones internes des Alpes cottiennes[END_REF][START_REF] Lefèvre | [END_REF] which occurs 10 km to the SE 387 in a cover thrust sheet pinched in between the AL unit and the oceanic units (fig. 4) and 388 which is also sourced from continental margin rocks only (Michard, pers. comm. 2016). 389

The Longet megabreccia occurs within the NW equivalent of the Pelvo d'Elva thrust 390 sheet (c, fig. 4), and its present situation within the Schistes Lustrés can be explained by 391 interference between N to NW directed tectonic transport, isoclinal folding, and further 392 backfolding. The Longet and Pelvo d'Elva breccia occurs presently in the normal and 393 reverse limbs of the AL backfold. 394 9 km SW of Col du Longet, near Maljasset, the Ubaye river crosscuts the tectonic 395 boundary between the continental margin (Roure) and oceanic (Schistes Lustrés) units 396 (b, fig. 3). This contact is presently dipping southwestwards and the most internal 397 Briançonnais nappe (Roure) is overriding the oceanic units due to backfolding. The 398 asymmetry of the minor folds associated with the initial stacking indicate an apparent 399 southward transport (a, fig.5, present structure), which is inconsistent with the other 400 regional data. However, taking into account the overall reversal and once restored from 401 backfolding, it becomes N-NW directed (b, fig. 5, restored structure), similarly as in Col 402 du Longet (see §5, site 39). 403 404 4.2-Backfold and backthrust overprint structures in the Piemont units east of Briançon (c, 405 d, e, fig. 3) 406 The Piemont units of Rochebrune (c, fig.3) and Chaberton are thrust eastwards over the 407 oceanic blueschist units, composed of ophiolites and Mesozoic metasediments (Dumont 408 et al., 1984). This backward thrusting crosscuts an initial stack characterised by oceanic 409 units resting in thrust contact with underlying Prepiedmont units, a situation visible 410 beneath the Chenaillet ophiolitic massif (Gimont tectonic window; [START_REF] Barféty | Notice explicative, carte 1490 géologique de France (1/50000è)[END_REF]. 411 Some ophiolite bearing slices found along the backthrust beneath the Rochebrune and 412

Chaberton units represent remnants of the sheared backfold. Further east, the Piemont 413 units outcrop in two subcircular tectonic windows surrounded by the oceanic 414 metasediments with rare ophiolitic rocks, the Gran Roc and the Mte Banchetta windows 415 (d,e,fig. 3,respectively). These continental margin units suffered deeper burial than 416 those of the Rochebrune and Chaberton units (Caron, 1971), and we interpret their high 417 elevation (~3000m and ~2800m, respectively) as dome structures resulting from large-418 scale interference folding similar to the conceptual sketch of fig. 3 (upper sketch, 419 centre). In both cases, these structures are markedly asymmetric, their eastern limb 420 being steeper or overturned. 421

The Gran-Roc window (d, fig. 3) is composed of, from bottom to top, Carnian gypsum 422 [START_REF] Megard-Galli | Découverte de Carnien fossilifère associé à du gypse en pays 2004 piémontais (massif du Gran Roc, Alpes cottiennes septentrionales, Italie)[END_REF] which is the detachment layer, a thick pile of Norian 423 dolostones (800m) with dolomitic breccia at their base (Caron, 1971), and Rhaetian to 424 Liassic limestones and shales typical of the Prepiedmont series [START_REF] Dumont | Le chaînon de Rochebrune au sud-est de Briançon: évolution paléogéographique et 1695 structurale d'un secteur de la zone piémontaise des Alpes occidentales[END_REF]. The 425 western side of the massif provides a natural N-S cross section showing an asymmetric 426 anticline cored by the Carnian evaporites. This structure may have been initiated as a 427 north-verging ramp anticline (fig. 6). It is furthermore possible that the location of this 428 fold was initially localised by a pre-existing diapir, because of the apparent truncation of 429 dolostones by the underlying evaporites. 430

Further east, the Mte Banchetta tectonic window (e, fig. 3; fig. 7) is also cored by a thick 431 pile of Triassic dolostones, locally covered by alternating carbonate and shale which 432 have Rhaetian characteristics (site 13, §5, and eastern slopes of Mte Banchetta). A 433 calcschist formation similar to the "Lias prépiémontais" [START_REF] Lemoine | Découverte de Foraminifères planctoniques du Crétacé supérieur dans les 1917 Schistes lustrés du Queyras (Alpes Occidentales). Conséquences paléogéographiques et tectoniques[END_REF] is exposed to 434 the SW of Mte Banchetta (b, fig. 7;Jouvent, 2017). The occurrence of these three 435 diagnostic lithologies indicates that the Mte Banchetta tectonic window is underlain by a 436 Prepiedmont Mesozoic series similarly to the nearby Gran Roc window. 437 A 'mixed' megabreccia occurs between these continental margin formations and the 438 oceanic units, first described by Caron (1971), whose significance will be discussed in § 439 8.3. The 3D map-scale geometry of the Mte Banchetta window is consistent with an 440 interference structure (fig. 3,e). Similarly to the Gran Roc window, the eastern slopes 441 show obvious backward fold-and-thrusting of oceanic and continental unit imbricates 442 (F2, fig. 7), a feature comparable to large-scale backfolding and backthrusting at the 443 eastern edge of the Rochebrune and Chaberton-Grande Hoche Prepiedmont units 444 [START_REF] Lemoine | Découverte de Foraminifères planctoniques du Crétacé supérieur dans les 1917 Schistes lustrés du Queyras (Alpes Occidentales). Conséquences paléogéographiques et tectoniques[END_REF][START_REF] Tricart | Faulted backfold versus reactivated backthrust: the role of inherited structures 2239 during late extension in the frontal Piémont nappes east of Pelvoux (Western Alps)[END_REF]. To the N-NW of Mte Banchetta, the 445 Prepiedmont unit is buried beneath the oceanic metasediments with no evidence of any 446 normal fault. We explain this structure by the occurrence of a N-to NW-overturned 447 anticline involving both the continental and oceanic units and interfering with backfolds 448 to produce the circular-shape window. Small scale folds consistent with this 449 interpretation are observed (sites 13 and 14, § 5). 450

4.4-Tectonic cover of the Ambin massif (g, fig. 3) 474

The Ambin internal crystalline massif is a 20 km-wide dome surrounded by the 475 "Schistes lustrés" oceanic units (fig. 1, fig. 9). It has a Variscan metamorphic crystalline 476 core showing evidence of north-vergent HP early Alpine deformation overprinted by 477 retromorphic E-directed shear (Clarea nappe, Ganne et al., 2004 and refs. therein). This 478 core is covered by a nappe stack including, from bottom to top, late Paleozoic 479 volcaniclastics ("Ambin group", or Ambin nappe), Mesozoic continental derived units 480 (Briançonnais, Piemont), and oceanic units (Schistes Lustrés). The Mesozoic cover 481 nappes pinch-out northwards over the Ambin nappe on top of the dome, as shown in a 482 N-S cross section (fig. 9a,9c; Jouvent, 2017). The culmination coincides with a shortened 483 fault block with locally preserved Triassic strata, truncated by the cover nappes. The 484 lower Triassic quartzites in the footwall of the nappes display evidence of north-485 directed shear (stratification/cleavage relationships, fig. 9d; site 3, § 5). The top of the 486 Ambin nappe displays a regional angular unconformity, with northward-younging 487 middle Triassic carbonate strata beneath it, towards a more complete Briançonnais 488 series (Bellecombe area, Jaillard, 1989). In contrast, towards the south, an extremely 489 reduced Mesozoic cover overlies the lowest Triassic to late Paleozoic clastics (Polino et 490 al., 2002), as usually observed in the Internal Briançonnais domains. This regional 491 unconformity together with the fault block (fig. 9a,9c) suggest that the location of the 492 Ambin dome is partly inherited from the Mesozoic passive margin history, and that the 493 A synthetic outcrop-scale analysis in the southwestern part of the western Alpine arc 501 [START_REF] Tricart | Tectoniques superposées dans les Alpes occidentales, au sud du Pelvoux: évolution 2230 structurale d'une chaîne de collision[END_REF] demonstrated the occurrence of three deformation episodes in the 502 Briançonnais zone. The second and the third phases (D2, D3) correspond respectively to 503 outward thrusting of the nappe stack associated with the activation of the Penninic 504 thrust, and to inward (backward) fold-and-thrusting. Both are kinematically linked with 505 the formation and the propagation of the Western Alps arc, initiated approximately at 506 the Eocene-Oligocene boundary (Dumont et al., 2011). D2 and D3 sensu Tricart (1980) 507 are overlapping in time because they occurred on both sides of the "Briançonnais fan" 508 whose axis propagated outwards. D1 corresponds to the initial underthrusting of the 509 Briançonnais and Prepiedmont domains beneath the Ligurian-Piemont oceanic 510 accretionary wedge. It was previously regarded as late Cretaceous (Caron, 1977;Tricart, 511 1980). A late Cretaceous age was assigned to the Eoalpine orogeny (85-60 My; Hunziker 512 et al., 1992, and refs therein;[START_REF] Dal Piaz | Review of radiometric dating in the Western Italian Alps[END_REF][START_REF] Polino | Tectonic erosion at the Adria margin and accretionary processes 2085 for the Cretaceous orogeny of the Alps[END_REF]

based on K-513

Ar dating, which is now recognised to have involved substantial uncertainty and 514 scattering due to excess argon analytical problems. HP radiometric ages were still 515 controversial in the 1980's [START_REF] Lemoine | Découverte de Foraminifères planctoniques du Crétacé supérieur dans les 1917 Schistes lustrés du Queyras (Alpes Occidentales). Conséquences paléogéographiques et tectoniques[END_REF]) but were progressively refined to 516 Cenozoic (65-38 Ma; [START_REF] Liewig | Geoghemical and K-Ar isotopic behaviour of Alpine sheet silicates 1927 during polyphased deformation[END_REF][START_REF] Takeshita | White mica K-Ar ages of blueschist-facies rocks from the 2211 Piemonte 'calc-schists' of the western Italian Alps[END_REF]. From a structural point 517 of view, the discrepancies in the orientation of early small-scale structures with respect 518 to younger deformation phases was reported by several authors [START_REF] Vialon | Etude géologique du massif cristallin de Dora Maira, Alpes cottiennes internes[END_REF]Caron, 519 1973Caron, 519 , 1974;;Caby, 1973;[START_REF] Malavieille | Etude tectonique et microtectonique de la déformation ductile dans de grands 1941 chevauchements crustaux: exemple des Alpes franco-italiennes et de la Corse[END_REF][START_REF] Mahwin | Relations entre structures longitudinales et transverses au 1938 coeur de l'arc alpin occidental: exemple du Val Germanasca (Massif cristallin Dora-Maira)[END_REF][START_REF] Platt | Ramsay[END_REF]. This well-520 known feature has been interpreted in different ways, namely as Variscan inheritance 521 [START_REF] Vialon | Etude géologique du massif cristallin de Dora Maira, Alpes cottiennes internes[END_REF], resulting from shear coeval with block rotation (Caron, 1974), as a 522 rotation of early structures [START_REF] Boudon | L'arc alpin occidental: réorientation de structures primitivement E-W par glissement et étirement dans un 1541 système de compression global N-S? Eclogae[END_REF], or as change in deformation regime 523 [START_REF] Malavieille | Etude tectonique et microtectonique de la déformation ductile dans de grands 1941 chevauchements crustaux: exemple des Alpes franco-italiennes et de la Corse[END_REF]. Conversely, Caby (1973[START_REF] Breton | This 1113 evolution involved major changes in collision kinematics through time, which have been 1114 identified in many localities of the Internal Zones, in the Prealpine and Embrunais 1115 nappes, and in the Helvetic-Dauphinois domain[END_REF] interpreted the transverse trends of 524 the early Alpine deformation phase to result from northward translation of the orogenic 525 wedge, having preceded the development of the arc curvature. A northward translation 526 of the early Alpine wedge was also promoted by [START_REF] Maury | Le décrochement subbriançonnais: une nouvelle interprétation de la limite 2000 interne-externe des Alpes franco-italiennes[END_REF] and Ricou & 527 Siddans (1986), and later documented by Schmid & Kissling (2000) and Ceriani et al. 528 (2001). The latter publication reported initial N to NW-directed tectonic transport in the 529 Internal nappe stack using their microstructural signature in the vicinity of the Penninic 530 thrust. 531 I). Most of the 55 sites are located in the Briançonnais zone, and some are within 537 the oceanic units on both sides of it. The deformation criteria measured are outcrop-538 scale axes, intersection lineations and stretching lineations. At each site, we identified 539 the pro-and/or retro-verging deformations whose trends are broadly following the 540 shape of the arc, and we restored (unfolded) the preceding tectonic features which 541 generally consist of folds and stratification/schistosity intersection lineations. 542

Compared to the chronology of [START_REF] Tricart | Tectoniques superposées dans les Alpes occidentales, au sud du Pelvoux: évolution 2230 structurale d'une chaîne de collision[END_REF], the partly overlapping outward and 543 inward deformation episodes referred to as D2 and D3, respectively, are merged in D2 in 544 this study, which aims to focus on the early part of deformation history. D2 outward or 545 inward structures are used to restore the previous kinematic indicators, assigned to D1 546 deformation. 547

As an example, a description of site 48, located in the External Briançonnais, is provided 548 in fig. 12 (localisation fig. 10 &table I). Here, the initial contact between the overthrust 549 oceanic sediments, namely the Helminthoid Flysch nappe composed of early late 550 Cretaceous purple shales, and an underlying continental margin Briançonnais unit 551 (Sautron nappe, [START_REF] Gidon | Carte géol[END_REF] is observed. This contact is affected by top-to-the SW 552 shear deformation, folding and thrusting, in the footwall of the Brec de Chambeyron 553 Briançonnais unit (a, fig. 12). SW-verging deformation (D2, fig. 12) is disharmonic, 554 characterised by large wavelength folds in the Triassic dolomitic limestones of the upper 555 unit, and by tighter folding which displays shorter wavelength in the Cretaceous-556 Paleogene calcschists (b, fig. 12). These 'D2' folds at different scales trend parallel to the 557 Briançonnais zone in this part of the arc (fig. 10). They overprint an older lineation, 558 trending approximately perpendicular to D2 fold trends, which is systematically 559 observed in the calcschists (c, fig. 12) beneath the oceanic nappe. This lineation consists 560 of intersection between stratification and a pre-D2 schistosity named D1, because it is 561 affected by D2 folds (c, fig. 12, and stereogram). Schistosity S1 is subparallel to 562 stratification, and isoclinal D1 microfolds trending parallel to D1 lineation are also 563 observed. In order to provide a map synthesis of similar observations across the study 564 area (fig. 10; fig. 11), we projected horizontally at each site the mean orientation of D2 565 fold trends, which may be forward or backward oriented (red or green, respectively), 566 and the mean orientation of D1 lineations or fold axis, once unfolded from D2 567 deformations. 568

The map shows a striking consistency of restored D1 trends across the southern part of 569 the Alpine arc. Top-to-the north shear sense is observed in many sites, either indicated 570 by fold asymmetry or by angular relationships between stratification and cleavage. The 571 occurrence of fold trends transverse to the western Alpine chain has long been 572 recognised [START_REF] Vialon | Etude géologique du massif cristallin de Dora Maira, Alpes cottiennes internes[END_REF]Bertrand, 1968 and refs therein;Caby, 1973Caby, , 1975;;Tricart & 573 Schwartz, 2006), but has been differently interpreted: either as an evidence of N-S 574 shortening and N-directed transport (Caby, 1973;[START_REF] Tricart | A north-south section across the Queyras Schistes lustrés (Piedmont zone, 2237 Western Alps): syn-collision refolding of a subduction wedge[END_REF], consistent 575 with N-S HP transport identified through high-pressure stretching lineations in the 576 internal crystalline massifs [START_REF] Choukroune | [END_REF][START_REF] Ganne | Geometry and kinematics of early Alpine nappes in a 1768 Briançonnais basement (Ambin Massif, Western Alps)[END_REF]Le Bayon & 577 Ballèvre, 2006;Strzerzyski et al., 2011;[START_REF] Scheiber | Upper crustal deformation in continent-continent collision: 2132 A case stydy from the Bernard nappe complex (Valais, Switzerland)[END_REF], or as more or less parallel 578 to tectonic transport, thus oriented radially towards the exterior of the arc (Caron, 1974;579 Malavieille & Etchecopar, 1981;[START_REF] Malavieille | Signification tectonique des linéations d'allongement dans 1946 les Alpes occidentales[END_REF][START_REF] Philippot | Opposite vergence of nappes and crustal extension in the French-Italian Western Alps[END_REF]. We argue that 580 these transverse trends can be regarded as approximately perpendicular to the 'D1' 581 transport directions of nappes during early stacking stages, like 'D2' trends are for the 582 late stages. 583

The D1 structures are regionally consistent, and are locally observed immediately 584 beneath the base of the oceanic nappes (i.e. fig. 10, sites 2, 3, 7, 13, 20, 48). Thus this 585 widespread deformation is most likely associated with the early Alpine collision and 586 with the emplacement of the oceanic accretionary wedge over the distal continental 587 margin units. Like in Provence, the Briançonnais domain must have been affected by 588 older compressional deformation linked with the Pyrenean orogeny, but there is no 589 evidence of deep burial older than early Eocene and these surficial structures were 590 probably largely overprinted by the D1 deformation. 591

We document the superposition of two deformation stages which can be identified by 592 crossing small-scale structures over the whole area, regardless of the metamorphic 593 grade, from blueschist facies in eastern domains to sub-greenschist facies in the 594 Embrunais-Ubaye areas. Moreover, we observe similar superposed deformations at 595 different scales, from kilometric to outcrop. Many examples of large-scale interference 596 folds have been reported in the Internal Westen Alps (Platt et al., 1989;Steck, 1998;597 Ganne, 2003;Le Bayon, 2005). These observations preclude models involving radial 598 outwards orientations of tectonic transport constant through time, which in addition 599 would make restorations impossible in the core of the arc. We propose that the 600 widespread "transverse" D1 deformation trends result from an early N to NW directed 601 transport and stacking stage, whose propagation appears presently "longitudinal" to the 602 southern part of the Western Alpine arc. The restored D1 trends (fig. 10) show a slight 603 discrepancy in orientations from N (Maurienne vally) to S (Ubaye valley), which 604 suggests that these trends were distorted during the formation of the arc. Considering 605 that the formation of the arc is associated with the activation of the post-nappe Penninic 606 thrust (Dumont et al., 2011), and with the pro-and retro-D2 structures, such a distortion 607 is expected from older (D1) structures (Caby, 1973). 608

Following our interpretation derived from small-scale observations, the most important 609 nappe displacements are expected to have occurred during the early D1 stacking stage, 610 whereas D2 thrusts mostly consist of high-angle, post-nappe structures having 611 accommodated vertical extrusion on both sides of the Briançonnais zone (fig. 2). If so, 612 the internal zones in the Western Alpine arc should display significant along-strike 613 changes as relicts of early stacking process, which is exemplified in the following. 614 615 6-Along-strike variations in the structure and internal composition of nappes: 616 617 Our structural arguments presented in the previous sections show that the D1 early 618 nappe structures are crosscut by D2 folds an thrusts, which follow the shape of the arc, 619 both at an outcrop scale and at a km scale. Since the formation of the arc is a recent 620 feature, similar oblique crosscuting relationships should be observed at a map scale as 621 well. Such obliquity, which may also result from paleogeographic inheritance, can be 622 documented by variations along the strike of the major D2 structures, that is along the 623 different zones following the arc from S to N. To illustrate this, we provide examples 624 from surface geology which demonstrate that the structure of the Western Alpine arc is 625 not concentric in map view. Along-strike variations can be detected within: 626 627 6.1-The stratigraphy and structure of the "zone houillère" in the central part of the arc 628

The "Zone houillère" around the city of Briançon and further north is composed of 629 detrital sedimentary sequences reaching a total thickness of about 2,5 km (Fabre, 1961;630 Feys 1963). It shows two superposed main units separated by the Drayères thrust 631 oriented SW-NE (Fabre et al., 1982;Caby, 1996;[START_REF] Barféty | Carte 1492 géol[END_REF]Lanari et al., 2012). 632

The lower unit is found to the N-NE of the study area, from southern Vanoise massif to 633 northern Briançonnais, and includes upper Westphalian (C and D members) to lower 634

Stephanian clastic sequences (Tarentaise fm., Fabre, 1961;[START_REF] Schade | Nouvelles récoltes de plantes dans la zone houillère (Alpes 2122 françaises) au col de la Ponsonnière (Valloire) et au Mont Vallon (Meribel)[END_REF] overlain 635 by a thick volcano-sedimentary lower-middle Permian series (Ponsonnière area). The 636 upper unit, dominantly outcropping to the S-SE, near Briançon and further south, 637 contains fining upwards Namurian (south of Briançon) to lower Westphalian (A 638 member) clastics and coal measures, unconformably overlain by middle Stephanian 639 and/or upper Permian coarse clastics [START_REF] Barféty | Notice explicative, carte 1490 géologique de France (1/50000è)[END_REF]. These two units were 640 possibly stacked during the latest Variscan orogenic events because both are 641 unconformably overlain by the western Briançonnais Mesozoic cover in the Cerces-642 Grand Area area [START_REF] Barféty | Carte 1492 géol[END_REF]. They have been subsequently affected by Alpine 643 folds and dominantly east-verging thrusts (Fabre et al., 1982) so that the upper unit is 644 split into several slices. However, it is quite clear that this upper unit, found in the 645 southern area, is less complete than the lower one further north, due to an erosional gap 646 of upper Westphalian-lower Stephanian layers unconformably overlain by middle 647

Stephanian clastics. This southward increasing truncation of the infill of the late 648 Carboniferous basin, also illustrated by [START_REF] Mercier | Révision du Carbonifère Briançonnais: stratigraphie et évolution du 2007 bassin[END_REF] and Desmons 649 and Mercier (1993), shows that its southern margin has been deformed and uplifted in 650 relation to late Variscan events. Consistently, a higher late Carboniferous subsidence 651 rate affected the northern part of the Zone Houillère (Manzotti et al., 2014). The 652 observed fluvial drainage patterns in late Carboniferous formations are dominantly 653 northward directed [START_REF] Mercier | Révision du Carbonifère Briançonnais: stratigraphie et évolution du 2007 bassin[END_REF][START_REF] Barféty | Notice explicative, carte 1490 géologique de France (1/50000è)[END_REF] and braided 654 systems developed in the vicinity of source areas located to the south (Manzotti et al., 655 2014). Thus, in spite of the dominantly N-S trend and the narrow width of the presently 656 outcropping Zone Houillère, there are significant latitudinal changes in its stratigraphy 657 and structure, suggesting that the initial trend of the late Variscan basin was markedly 658 oblique with respect to later Alpine shortening. 659 660

6.2-The "Permo-Carboniferous axial zone" in the southern part of the arc 661

The southernmost constraints on the Carboniferous series are found in a tectonic 662 window across the Ubaye valley, within the lowermost nappe of the external 663 Briançonnais stack [START_REF] Gidon | Carte géol[END_REF], structurally equivalent to the « zone houillère » 664 unit near Briançon. Further SE, the Paleozoic formations are only represented by thick 665 Permian volcanics and volcaniclastics, outcropping in both the southeastern extension 666 of this stack, named « Permo-Carboniferous Axial zone » (PCAZ) by [START_REF] Lefèvre | Les nappes briançonnaises internes et ultrabriançonnaises dans les Alpes cottiennes 1878 méridionales[END_REF], and 667 in the more internal « Roure-Acceglio » zone (RAZ). The PCAZ and the RAZ are separated 668 by a major tectonic contact post-dating the initial nappe stacking and having possibly 669 accommodated left-lateral displacement of the Roure-Acceglio units, which are 670 truncated and pinch-out SE-wards along it (Preit fault zone, [START_REF] Lefèvre | Les nappes briançonnaises internes et ultrabriançonnaises entre Maira et Grana (Alpes 1880 Cottiennes méridionales)[END_REF]. A significant 671 part of these southeastern Permian-dominated units, especially in the PCAZ, are devoid 672 of their Mesozoic sedimentary cover for tectonic reasons. This detached cover may 673 correspond in part to the Mesozoic nappe stack developed further N in the region of 674 Briançon above the « Zone Houillère ». Another part, mostly observed in the RAZ, bears 675 evidence of Mesozoic erosional truncation having removed, at least in part, the Triassic 676 carbonate and siliciclastic series [START_REF] Debelmas | Calcschistes piémontais et terrains à faciès briançonnais dans les hautes 1662 vallées de la Maira et de la Varaita[END_REF][START_REF] Michard | Contribution à l'étude géologique de la zone d'Acceglio-Longet dans la haute Varaita 2016 (Alpes cottiennes, Italie)[END_REF] (Lemoine, 1960a). 682 683

6.3-The most internal continental margin units 684

The units named "Prepiedmont", [START_REF] Lemoine | Le Jurassique et le Crétacé prépiémontais à l'Est de Briançon 1910 (Alpes occidentales) et l'évolution de la marge européenne de la Téthys: données nouvelles et 1911 conséquences[END_REF]) are located at the eastern border 685 of the Briançonnais zone (fig. 1). Around the latitude of Briançon, they are detached 686 along the Carnian evaporites and thus only composed of upper Triassic and more recent 687 sediments (fig. 6). The basal thrust ramps up into the Jurassic section in one 688 northwestern location (Chalvet, fig. 8). These units do not outcrop further north than the 689 Maurienne valley, with the exception of the Grande Motte unit, whose paleogeographic 690 origin is debated (Deville, 1986, and refs therein). To the southeast, in contrast, these 691 units are complemented by older strata, due to a lower position of the detachment in the 692 stratigraphic section. In Valgrana (fig. 1;Michard, 1961aMichard, , 1961b[START_REF] Michard | Etudes géologiques dans les zones internes des Alpes cottiennes[END_REF]Megard-Galli & 693 Baud, 1977), the Triassic sequence of the Prepiedmont series include paleontologically 694 dated middle Triassic strata overlain by late Triassic and early Jurassic formations dated 695 by Franchi (1898), with a total thickness of 1 km to 1.5 km (unit III of [START_REF] Michard | Etudes géologiques dans les zones internes des Alpes cottiennes[END_REF]. 696

The thick middle Triassic succession found in Valmaira can also be regarded as a 697 Prepiedmont unit (unit I of [START_REF] Michard | Etudes géologiques dans les zones internes des Alpes cottiennes[END_REF]. Further southeast, in the Ligurian Alps, the 698 Prepiedmont units include late Paleozoic formations and even polymetamorphic 699 basement [START_REF] Vanossi | Guide Geologiche Regionali, Alpi Ligure, itinerario n°8[END_REF][START_REF] Seno | A kinematic evolutionary model for the Penninic sector of 2175 the central Ligurian Alps[END_REF][START_REF] Decarlis | Tectono-thermal evolution of a distal rifted margin: constraints form the Calizzano 1669 Massif (Prepiedmont-Briançonnais domain, Ligurian Alps)[END_REF]. This shows that the 700 detachment of the Prepiedmont units climbs up section from southern regions (Liguria, 701

Valgrana) towards the north-northwest along the central part of the arc (Cottian Alps). 702 703

6.4-The Valaisan domain 704

The occurrence of an oceanic domain sutured between the Briançonnais zone and the 705 external zone, together with its age of opening and closure, remains a topic of strong 706 debate [START_REF] Stampfli | Western Alps geological constraints on western 2188 Tethyan reconstructions[END_REF][START_REF] Bousquet | The tectono-metamorphic history of the 1561 Valaisan domain from the Western to the Central Alps: new constraints on the evolution of the Alps[END_REF][START_REF] Beltrando | Was the Valaisan basin floored by oceanic 1507 crust? Evidence of Permian magmatism in the Versoyen unit (Valaisan domain, NW Alps)[END_REF]Masson et al., 707 2008;[START_REF] Loprieno | The 1929 Valais units in Savoy (France): a key area for understanding the palaeogeography and the tectonic 1930 evolution of the Western Alps[END_REF][START_REF] Beltrando | The Valaisan controversy revisited: 1502 Multi-stage folding of a Mesozoic hyper-extended margin in the Petit St. Bernard pass area[END_REF][START_REF] De Broucker | Early Cretaceous 1666 accretionary complex of the Valaisan ocean[END_REF]. One reason 708 underpinning this famous Alpine controversy is the lack of evidence for any continental 709 breakup south of the Maurienne valley and west of the Briançonnais zone, which is a 710 major difference between the northern and southern parts of the arc. A possible solution 711 is to consider a southwestward transition from an oceanic domain, represented in 712 eastern Switzerland and possibly floored by exhumed subcontinental mantle 713 [START_REF] Malusà | What is the tectono-metamorphic evolution of 1962 continental breakup: The example of the Tasna ocean-continent transition[END_REF][START_REF] Ribes | Tectono-sedimentary evolution of a fossil ocean-continent transition: Tasna nappe, 2100 central Alps (SE Switzerland)[END_REF]LeBreton et al., 2021) to a more or less 714 attenuated continental crust domain pinching out towards the Vocontian basin, SE 715

France [START_REF] Dumont | 2525 c-Recontructed part of the profile across the Briançonnais Paleozoic basins before 2526 beeing affected by thrust sequence and overlain by the Prepiedmont and oceanic 2527 nappes[END_REF]. The disappearance of the Valaisan zone towards the 716 southern part of the western Alps arc would then occur for paleogeographic reasons. To 717 the N(W) of the Briançonnais domain, the occurrence of a transitional paleogeographic 718 domain towards this thinned crust area is testified by pinched units bearing specific 719 "intermediate" Mesozoic stratigraphic signatures, frequently assigned to 720 "subbriançonnais", all along the arc [START_REF] Galster | The Soja and Lyzzone-Terri nappes: discovery of a Briançonnais 1762 element below the front of the Adula nappe (NE Ticino, Central Alps)[END_REF][START_REF] Ceriani | The total 1126 estimate of radial shortening remains significantly higher in the northern half of the arc, 1127 including the foreland and the Helvetic nappes[END_REF]Maury & 721 Ricou, 1983;[START_REF] Barbier | La zone subbriançonnaise dans le région du col du Galibier[END_REF][START_REF] Barale | Stratigraphy, sedimentology and 1481 syndepositional tectonics of the Jurassic-Cretaceous succession at the transition between Provençal and 1482 Dauphinois domains (Maritime Alps, NW Italy)[END_REF]. This transition would have been 722 incorporated within the western Alpine arc, creating complex lateral relationships 723 between the Helvetic-Dauphinois, Valaisan, Subbriançonnais and Provençal units, as an 724 expression of the lateral paleogeographic transitions around the scissors-shape Valaisan 725 incipient breakup. 726

These lateral variations within the internal nappe stack of the Western Alps arc 727 demonstrate that its structure is not concentric, and that radial profiles are not 728 equivalent laterally and should not be considered as the optimal visualisation of a 729 complex kinematic evolution during the Cenozoic. These features can be due to the 730 obliquity of the orogenic wedge with respect to the paleogeographic trends, but can also 731 be explained by oblique to subperpendicular crosscutting of the early orogenic nappe 732 stack by recent (Oligo-Miocene) collision driven by westward extrusion (Dumont et al., 733 2012). The superposition order of structural units is a key argument for restoration, but 734 in radial profiles this order is frequently disturbed and modified by recent outward 735 "out-of-sequence" thrusting, i.e. in the Guil valley (Claudel & Dumont, 1999). The 736 knowledge of the structural geometry of the initial nappe stack emplaced during the 737 early stages of thrusting is thus a key criterion for a reliable restoration. 738 739 7-Sequence and superposition of nappes in the early orogenic stages 740 741 Since the building of the Internal Zones is polyphase, our aim in this section is to enlight 742 some characteristic large-scale structural features associated with the early stacking 743 phase (D1), with due consideration of their subsequent modification by younger 744 deformation pulses (D2). This assists reconstruction of the continental subduction 745 wedge large-scale geometry during the early orogenic stages, as discussed in § 9.2. 746 747 7a-Upper part of the continental subduction wedge, overlain by oceanic accretionary 748 units: 749

A key feature for the understanding the structure of the internal Alpine nappe stack is 750 the distinction between ophiolite bearing series and Triassic-soled series, now referred 751 to as oceanic and continental margin units, respectively. This was recognized several 752 decades ago [START_REF] Lemoine | Sur le problème stratigraphique et tectonique des Schistes Lustrés dans 1920 les Alpes Cottiennes[END_REF]Lemoine, 1964;Michard and Schumacher, 753 1973;[START_REF] Bourbon | Stratigraphie des Schistes lustrés piémontais 1546 dans les Alpes cottiennes (Alpes occidentales franco-italiennes): nouvelle interprétation et conséquences 1547 géodynamiques[END_REF], leading to the identification of imbrications of oceanic and 754 continental units, especially in the southern part of the arc [START_REF] Michard | Etudes géologiques dans les zones internes des Alpes cottiennes[END_REF]Henry et al., 755 1993;[START_REF] Balestro | Geology of the southern 1473 Dora-Maira Massif: insights from a sector with mixed ophiolitic and continental rocks (Valmala tectonic 1474 unit, Western Alps)[END_REF]. Serpentinite-bearing layers occur, for example, between the 756 main Dora-Maira basement complex containing UHP relicts and the blueschist Permian-757 Triassic Dronero-Sampeyre unit [START_REF] Henry | Geometry and structural evolution of ultra-high pressure and 1807 high-pressure rocks from the Dora-Maira massif, Western Alps, Italy[END_REF]. Some others are found between 758 the middle Triassic series of Val Maira and the Triassic-Jurassic series of Val Grana 759 (units I and III of Michard, 1967, respectively), both derived from the Prepiedmont 760 domain. The imbricated structures can result from different processes: [START_REF] Ring | Kinematics of the Alpenrhein-Bodensee graben system in the Central Alps: 2105 Oligocene/Miocene transtension due to the formation of the Western Alps arc[END_REF] rift 761 inheritance, with inversion of a complicated ocean-continent transition domain 762 featuring continental allochthons separated by exhumed mantle patches (Beltrando et 763 al., 2010b;[START_REF] Balestro | Geology of the southern 1473 Dora-Maira Massif: insights from a sector with mixed ophiolitic and continental rocks (Valmala tectonic 1474 unit, Western Alps)[END_REF], or (2) subduction-collision processes, with polyphase 764 deformation of an initially simple ocean/continent thrust contact at the bottom of the 765 accretionary wedge. 766

While not excluding the first hypothesis, we currently prefer the second option 767 considering our observations and structural data from the literature. Our structural and 768 microstructural data ( § 5; fig. 2) between Briançonnais and Ubaye demonstrate that the 769 internal nappe stack has been crosscut and disturbed by radially oriented forward and 770 backward "out-of-sequence" thrusting. Such multistage deformation is also documented 771 in the southern part of the arc (Val Maira and Valgrana, west of Cuneo city) by Michard 772 (1967) and [START_REF] Schumacher | Contribution à l'étude structurale du Haut Val Grana[END_REF], which emphasize multistage deformation and north 773 directed backthrusting. We argue that oceanic/continental units imbrication is most 774 likely due to a tectonic disturbance of the initial superposition of nappes: In the early 775 stages of stacking, the oceanic nappes, either non-metamorphic (Helminthoid flyschs), 776 or metamorphic sediments (Schistes Lustrés) and oceanic basement (Chenaillet, 777 Monviso), were thrust over the continental margin units (Prepiedmont and 778 Briançonnais). Such a superposition is locally preserved in different locations of the 779 internal zones of the Western Alps: the Chenaillet ophiolites and metasediments 780 overlying the Gondran prepiedmont unit [START_REF] Lemoine | Données nouvelles sur la série du Gondran près Briançon (Alpes Cottiennes); 1904 Réflexions sur les problèmes stratigraphique et paléogéographique de la zone piémontaise[END_REF][START_REF] Barféty | Notice explicative, carte 1490 géologique de France (1/50000è)[END_REF], the 781 Monviso metaophiolites overlying the Dora-Maira massif, the Mte Banchetta tectonic 782 window ( §4.2, Fig. 7). 783

Besides imbrications, other anomalous structures consist of continental margin units 784 resting over oceanic units. Such a reverse superposition is shown for example by the 785 Penninic thrust bringing the Briançonnais nappes above the Embrunais-Ubaye 786 Helminthoid flysch nappes near Guillestre (Dumont et al., 2011), or by the Rochebrune 787 Prepiedmont nappe backthrust over the oceanic metasediments of the Lac des Cordes 788 unit [START_REF] Lemoine | Découverte de Foraminifères planctoniques du Crétacé supérieur dans les 1917 Schistes lustrés du Queyras (Alpes Occidentales). Conséquences paléogéographiques et tectoniques[END_REF][START_REF] Tricart | Evolution d'une portion de marge continentale: blocs basculés 2234 et charriages alpins dans la nappe prépiémontaise de Rochebrune (Alpes occidentales)[END_REF]. In both cases, this reversal is due to a 789 tectonic disturbance of the initial nappe stack by the D2 deformation event ( § 4 & 5) and 790 it must be restored to reveal the top of the continental subduction configuration beneath 791 the accretionary wedge in the early orogenic stages. 792 793 7b-Internal structure of the Briançonnais nappe stack: 794

The Carboniferous "Zone Houillère" and associated Mesozoic cover units were initially 795 located towards the base of the Briançonnais pile of nappes sensu stricto, both in 796

Vanoise and in the region of Briançon (Caby, 1996;[START_REF] Barféty | Notice explicative, carte 1490 géologique de France (1/50000è)[END_REF][START_REF] Barféty | Carte 1492 géol[END_REF], as it is the 797 case in the Swiss Alps (Escher et al., 1993). Generally detached from their 798 polymetamorphic basement, these units may have been translated later to a higher 799 structural position by either forward or backward deformation, as for example the Mont 800

Thabor unit around Névache [START_REF] Barféty | Carte 1492 géol[END_REF]Lanari et al., 2012). In the region of 801 Briançon, the "Zone Houillère" is overlain by a refolded stack of Mesozoic cover nappes 802 detached along Triassic evaporite layers, which possibly represent the initial cover of 803 the "Internal Briançonnais" units of the Permo-Carboniferous Axial Zone located further 804 SE (Lefevre, 1982). 805

On top of the Briançonnais thrust sheets scarce evidence of an uppermost 806 polymetamorphic basement nappe are found (fig. 1), which triggered famous debates 807 about identification of thrust-sheets in the Western Alps [START_REF] Termier | Les nappes de recouvrement du Briançonnnais[END_REF]Termier & 808 Kilian, 1920). These uppermost units locally bear an "ultrabriançonnais" highly 809 condensed type of Mezozoic series (Rio Secco; Lemoine, 1961Lemoine, , 1964[START_REF] Lemoine | Brèches sédimentaires marines à la frontière entre les domaines briançonnais et 1902 piémontais dans les ALpes occidentales[END_REF]. To the 810 north, possible equivalents of these units are the Sapey-Ruitor orthogneiss and 811 micaschists in Vanoise, yielding Cambrian-Ordovician ages [START_REF] Guillot | Zircon U-Pb geochronology of 1786 Ordovician magmatism in the polycyclic Ruitor Massif (Internal W Alps)[END_REF] and 812 which are affected by both Variscan and Alpine metamorphism. They are also located in 813 the uppermost position of the Briançonnais stack. To the southeast, pre-Namurian 814 polymetamorphic basement also occurs in the Internal Briançonnais units of Liguria 815 [START_REF] Cortesogno | Genesi ed evoluzione dello 1641 zoccolo pre-Mesozoico du alcuni settori des Brianzonese Ligure interno[END_REF]. These upper basement nappes which override the Permo-816

Carboniferous basins are disconnected from the internal crystalline basement massifs. 817

As discussed later, we propose that the thrust sequence involving, from bottom to top, 818 the Carboniferous Zone Houillère and Permo-Carboniferous units, the Meso-Cenozoic 819 cover nappes, and the polymetamorphic basement thrust sheets, may result from 820 tectonic inversion of late Variscan inherited paleogeography. 821 7c-Olistostromes on top of the early orogenic wedge: 823 Near Briançon, there is evidence for early and shallow emplacement of a basement 824 nappe in an uppermost position. The so-called "quatrième écaille" [START_REF] Termier | Les nappes de recouvrement du Briançonnnais[END_REF] is 825 composed of micaschists similar to Rio Secco, but bearing a condensed and specific 826 Mesozoic cover: thin middle Triassic carbonates of more distal environments than the 827 underlying typical Briançonnais nappes [START_REF] Barféty | Notice explicative, carte 1490 géologique de France (1/50000è)[END_REF] unconformably overlain by 828 the Prorel breccia, which is probably late Cretaceous [START_REF] Barféty | Notice explicative, carte 1490 géologique de France (1/50000è)[END_REF]. The basal 829 thrust of this nappe is marked by an olistostrome of Bartonian age [START_REF] Barféty | La quatrième écaille près de Briançon (Alpes 1496 françaises): un olistostrome précurseur de l'orogenèse pennique éocène[END_REF]) 830 deposited on top of the Briançonnais stack, and containing various exotic blocks, 831 including oceanic sediments (Helminthoid Flysch). These features underline the former 832 connection of a thrust system inside the early orogenic wedge with the surficial part of 833 the wedge and with the floor of the Paleogene flexural basin in front of the orogen. The 834 N-NW-wards propagation of the orogen and of the preceding flexural basin throughout 835 Paleogene times is well documented in the Western Alps (Sinclair, 1997;Kempf & 836 Pfiffner, 2004;[START_REF] Ford | [END_REF][START_REF] Schmid | Ivrea mantle wedge, arc of the 2139 Western Alps, and kinematic evolution of the Alps-Apennines orogenic system[END_REF]. Slope breccia and olistostromes 837 which occur at the transition between the range and the basin are further incorporated 838 as a diachronous marker layer in the footwall of the nappes [START_REF] Sagnes | [END_REF]Ford & 839 Lickorish, 2004). The occurrence of detrital elements of mixed continental (basement, 840 Mesozoic cover) and oceanic provenance (ophiolites, oceanic sediments including 841 Helminthoid Flysch) may allow tracing of the propagation of the obducted accretionary 842 wedge over the Briançonnais foreland in the early stages of continental subduction. 843 However, there is a need to distinguish these thrust-related breccias within the various 844 types of breccia observed in the sedimentary record of the Briançonnais and 845 neighbouring areas. 846 847

8-Breccia and olistostromes 849 850

The scale and significance of synsedimentary breccias interbedded in the Briançonnais 851 Meso-Cenozoic series and in the adjoining Subbriançonnais, Valais and Prepiedmont 852 domains has been a subject of debate for many decades [START_REF] Lemoine | Brèches sédimentaires marines à la frontière entre les domaines briançonnais et 1902 piémontais dans les ALpes occidentales[END_REF]Chaulieu, 1992 853 and refs therein;[START_REF] Ribes | Long-lived mega fault-scarps and related breccias at distal rifted margins: insight from present-day and 2097 fossil analogues[END_REF]. Various types of syntectonic breccia are observed in 854 relation to different geodynamic settings, whose sedimentary signatures are difficult to 855 distinguish. Scarp or slope breccia can occur in either divergent or convergent setting 856 with similar sedimentological characteristics (Chaulieu, 1992). Apart from their facies, 857 additional criteria to consider are (1) the occurrence of exotic material, which can 858 reflect syn-orogenic exhumation of source areas or tectonic juxtaposition of oceanic and 859 continental thrust-sheets, ( 2) their Alpine structural setting, shear fabric and location 860 with respect to the major Alpine thrusts or plate boundaries [START_REF] Polino | Tectonic erosion at the Adria margin and accretionary processes 2085 for the Cretaceous orogeny of the Alps[END_REF]. 861

Thrust-related mélanges and associated mass-transport and tectonic processes are 862 described in Festa et al. (2010), which helps to distinguish breccia related to extensional 863 tectonics (type 1) from those related to subduction or collision, and to formalise the 864 transition from precursor olistostromes to tectonic mélanges associated with nappes 865 boundaries. 866 occurred in an extensional setting before the oceanic opening of the Ligurian Tethys 877 (early middle Jurassic). The preservation of such breccia is scarce in the Briançonnais, 878 due to uplift and emergence at that time (Faure & Mégard-Galli, 1988;Claudel & 879 Dumont, 1999), but lateral equivalents are known in the Prepiedmont units of Liguria 880 (Mte Galero breccia, member A; [START_REF] Dallagiovanna | Le brecce di Monte Galero: nuovi dati e interpretazioni[END_REF][START_REF] Decarlis | Synrift sedimentation on the northern Tethys margin: an example from the 1674 Ligurian Alps (Upper Triassic to Lower Cretaceous, Prepiedmont domain, Italy)[END_REF], 881 in the Cottian Alps (Narbona breccia; [START_REF] Michard | Etudes géologiques dans les zones internes des Alpes cottiennes[END_REF][START_REF] Michard | Position des brèches et des ophiolites dans les séries piémontaises 2032 des Vals Grana et Marmora (Alpes cottiennes méridionales, Italie)[END_REF]882 Gidon et al., 1978) and in the Prealps (lower member of the Breccia Nappe: Hendry, 883 1972;[START_REF] Steffen | La Brèche du Chablais à son extrémité 2204 occidentale (Hte. Savoie, France): Sédimentologie, éléments stratigraphiques et interprétation 2205 paléogéographique[END_REF]; lower member of the Evolène series, Mont Fort nappe: Pantet 884 et al., 2020). Local supply, mass flow, debris flow and turbidites, together with local 885 angular unconformities and submarine erosion [START_REF] Lemoine | Découverte de Foraminifères planctoniques du Crétacé supérieur dans les 1917 Schistes lustrés du Queyras (Alpes Occidentales). Conséquences paléogéographiques et tectoniques[END_REF][START_REF] Deville | Données nouvelles sur le cadre stratigraphique et structural de l'unité de la Grande 1687 Motte (Massif de la Vanoise, Alpes de Savoie)[END_REF] are 886 regarded as evidence of extensional rift context. 887 b) After the initial opening of the Ligurian Tethys, passive margin sedimentation on the 888 European margin was affected by a second extensional pulse associated with the 889 Atlantic-Bay of Biscay rifting and Iberian plate divergence (late Middle to Late Jurassic; 890 [START_REF] Vergés | Mesozoic evolution and Cainozoic inversion of the Pyrenean Rift[END_REF]. Locally preserved in the early post-rift Briançonnais 891 sediments [START_REF] Tissot | Etude géologique des Massifs du Grand Galibier et des Cerces (zone briançonnaise[END_REF][START_REF] Bourbon | Evolution d'un secteur de la paléomarge téthysienne en milieu pélagique: la zone 1543 briançonnaise près de Briançon, entre le début du Malm etl'Eocène inférieur[END_REF]Jaillard, 1987;Jaillard, 1999;Claudel et al., 892 1997), including the Ligurian Briançonnais [START_REF] Berger | Subduction-related metamophism in the Alps: review fo isotopic ages[END_REF], and are widespread in 893 nearby domains, both continent-ward (Subbriançonnais) and oceanward 894 (Prepiedmont). They consist of the Telegraphe breccia in the Subbriançonnais units 895 [START_REF] Barbier | La zone subbriançonnaise dans le région du col du Galibier[END_REF][START_REF] Barféty | Carte géologique 1488 1/50000è, feuille 744 St Jean de Maurienne[END_REF][START_REF] Barféty | Carte 1492 géol[END_REF] and coeval lateral equivalents 896 (Nielard breccia, [START_REF] Barféty | Carte géologique 1488 1/50000è, feuille 744 St Jean de Maurienne[END_REF]Neyzets and Piolit breccia;Latreille, 1954;Chenet, 897 1978). Time-equivalent turbiditic breccia in Prepiedmont units are found in Liguria (Mte 898 Galero breccia, members B and C, [START_REF] Decarlis | Synrift sedimentation on the northern Tethys margin: an example from the 1674 Ligurian Alps (Upper Triassic to Lower Cretaceous, Prepiedmont domain, Italy)[END_REF], in the Briançon region 899 [START_REF] Lemoine | The continental margin of the Mesozoic Tethys in the Western Alps[END_REF] and in the Prealps (Breccia Nappe, upper member, Steffen et al., 900 1993). All correspond to rifted margin pelagic environments, indicative of increased 901 transport and deeper erosion of the source areas than during the previous syn-rift stage. 902 c) The initiation of Africa-Europe convergence is recorded in the European margin 903 sediments through the propagation of the Pyrenean foreland north of the Iberian plate, 904 but before the complete closure of the Tethys and the Adria collision (late Cretaceous to 905 Paleocene). The pelagic early Late Cretaceous sedimentary record in the Briançonnais 906 contains breccia occurrences which, together with local erosional angular 907 unconformities [START_REF] Bourbon | Importance et signification des érosions 1549 mésozoïques à la marge externe de la zone briançonnaise (Alpes du Sud)[END_REF], possibly reflect incipient compressional 908 reactivation of marginal structures (Chaulieu, 1992;[START_REF] De Graciansky | The Western Alps, from rift to passive margin to 1680 orogenic belt, an intergated overview[END_REF]: the 909 Cerces breccia [START_REF] Tissot | Etude géologique des Massifs du Grand Galibier et des Cerces (zone briançonnaise[END_REF][START_REF] Barféty | Carte 1492 géol[END_REF], the Mélézin breccia (Barféty et al., 910 1995), the Madeleine breccia [START_REF] Gidon | Carte géol[END_REF], and even olistoliths (Bourbon, 1980) 911 are reported within the Cenomanian to early Senonian formations of several external 912 Briançonnais nappes. Significant tectonic activity is also indicated by the occurrence of 913 coarse breccia interbedded in the upper Cretaceous calcschists of the internal 914 Briançonnais units of Vanoise (Fours unit, [START_REF] Deville | Données nouvelles sur le cadre stratigraphique et structural de l'unité de la Grande 1687 Motte (Massif de la Vanoise, Alpes de Savoie)[END_REF]Tsanteleina and Chevril 915 breccia, Jaillard, 1999). Further south, the most internal Briançonnais nappes contain 916 coarse breccia with olistoliths interbedded in calcschistous formations probably late 917 Cretaceous in age [START_REF] Gidon | Carte géol[END_REF]: the Longet breccia [START_REF] Lemoine | Brèches sédimentaires marines à la frontière entre les domaines briançonnais et 1902 piémontais dans les ALpes occidentales[END_REF] and similar 918 breccias in the uppermost Pelvo d'Elva nappe further south [START_REF] Lefèvre | [END_REF]. 919 Lateral equivalents are found in the Subbriançonnais nappes: Bachelard and Pelat 920 flyschs: [START_REF] Blanc | La formation du Flysch du Bachelard (Neocrétacé-1535 Paléocène) dans l'Unité du Pelat (zone subbriançonnaise des nappes de l'Ubaye, Alpes occidentales 1536 françaises)[END_REF], Thum et al., 2015;l'Argentière breccia: Chenet (1978). All 921 these clastic formations are usually interpreted as derived from degraded submarine 922 fault scarps, but their compositions frequently suggest a deeper erosion of the source 923 areas and increased transport than the Jurassic breccias. Angular erosional 924 unconformities affecting the post-rift sediments (i.e. Guil lower nappe or Galibier area) 925 with locally high-angle relationships [START_REF] Tissot | Etude géologique des Massifs du Grand Galibier et des Cerces (zone briançonnaise[END_REF] may result from compressional 926 reactivation of rift structures (i.e. Béraudes fault, [START_REF] Tricart | De l'océan à la chaîne de montagnes, tectonique des 2232 plaques dans les Alpes[END_REF]. Alternatively, 927 [START_REF] Bertok | Kilometre-scale palaeoescarpments as 1524 evidence for Cretaceous synsedimentary tectonics in the External Briançonnais domain (Ligurian Alps, 1525 Italy)[END_REF] describe early Late Cretaceous kilometric-scale normal fault scarps 928 in the Ligurian Briançonnais which are interpreted to have formed in a transcurrent 929 regime. This late Cretaceous deformation must be related to the motion of the Iberia 930 microplate relative to Europe (Le Breton et al., 2021) and to the propagation of the 931 Pyrenean foreland within its eastern continuation (Corsica, Sardinia, Briançonnais). 932

Coeval evidence of inversion is known in Provence, the Maritime Alps (Schreiber et al., 933 2011), further north (Dévoluy pre-Santonian folding; [START_REF] Michard | Cretaceous folding in the Dévoluy mountains 2026 (Subalpine Chains, France): gravity-driven detachment at the European paleomargin versus 2027 compressional event[END_REF] and in 934 northern Spain [START_REF] Soto | Widespread Cretaceous inversion event in northern Spain: 2185 evidence from subsurface and palaeomagnetic data[END_REF]. 935 d) Detrital sedimentation finally occurred after the complete closure of the Ligurian 936 Tethys, and recorded the propagation of the Alpine orogenic wedge resulting from the 937 Adria-Europe collision (Eocene). Sedimentation ceased in early Oligocene due to 938 involvement of the whole internal Alpine zones within the collision wedge. This 939 propagation developed an underfilled flexural basin over both the Briançonnais domain 940 and the more proximal parts of the margin [START_REF] Ceriani | The total 1126 estimate of radial shortening remains significantly higher in the northern half of the arc, 1127 including the foreland and the Helvetic nappes[END_REF]Sinclair, 1997). Its 941 sedimentary infill is diachronous, spanning from Lutetian to Bartonian in the 942 Briançonnais [START_REF] Barféty | Notice explicative, carte 1490 géologique de France (1/50000è)[END_REF][START_REF] Michard | The Eocene unconformity of the Briançonnais domain in French-Italian 2029 Alps, revisited (Marguareis massif, Cuneo); a hint for a Late Cretaceous-Middle Eocene frontal bulge 2030 setting[END_REF] and from Lutetian to 943 Priabonian in autochthonous Corsica and in the External Alpine zone (Durand-Delga, 944 1984;Joseph & Lomas, 2004, and refs. therein). The flexural basin was propagating 945 towards the NW throughout the Eocene (Kempf & Pfiffner, 2004;[START_REF] Ford | [END_REF]946 Dumont et al., 2012). It is generally overlain by a widespread olistostrome layer marking 947 the base of the obducted oceanic accretionary wedge. This olistostrome formation, 948 named "Schistes à blocs" [START_REF] Sagnes | [END_REF] is also diachronous and locally dated as 949 Late Eocene-Early Oligocene beneath the non-metamorphic thrust-sheets which overly 950 the external zone of the Western Alps (Dumont et al., 2012, and refs. therein). In the 951 external Briançonnais nappes, few occurrences of such deposits are observed, always 952 located on top of the tectonic pile. The most striking example is the "Quatrième écaille" 953 near Briançon, which has been subject to debate since ~1900 (Lemoine, 1961), and 954 which is now interpreted as a kilometre-scale block which slid above the Briançonnais 955 basin infill in Early Bartonian times [START_REF] Barféty | La quatrième écaille près de Briançon (Alpes 1496 françaises): un olistostrome précurseur de l'orogenèse pennique éocène[END_REF]. Its exotic character is shown 956 by a specific, polymetamorphic basement only found in the most internal zones, that is 957 the uppermost unit of the Briançonnais thrust stack (a, fig. 13; white stars), and by open-958 marine Middle Triassic facies different from the underlying nappes. It is comparable to 959 distal ramp carbonate facies described in Liguria [START_REF] Decarlis | A sequence stratigraphic approach to a Middle Triassic shelf-slope complex 1672 of the Ligurian Alps (Ligurian Briançonnais, Monte Carmo-Rialto unit, Italy)[END_REF]. Nearby and 960 beneath this block are found conglomerates and breccia fed by it, thus containing mostly 961 basement (Eychauda breccia, [START_REF] Barféty | La quatrième écaille près de Briançon (Alpes 1496 françaises): un olistostrome précurseur de l'orogenèse pennique éocène[END_REF][START_REF] Barféty | Notice explicative, carte 1490 géologique de France (1/50000è)[END_REF]. Similar breccias are found in 962 several places at the same structural and stratigraphic level (a, fig. 13, red stars; b, c, fig. 963 13). Finally, "mixed" breccia consists of a rare occurrence of mafic or ultramafic pebbles 964 within coeval olistostrome layers (d, fig. 13, blue stars 22 to 24), together with more 965 frequent pebbles of Cretaceous oceanic sediments (Helminthoid Flysch). They were fed 966 by both continental and oceanic fragments from the orogenic front. We argue that these 967 "mixed" detrital formations, always located on top of the Briançonnais nappe pile 968 and/or beneath the exotic flysch nappes, can be useful in identification of the earliest 969 stage of propagation of the accretionary wedge. 970 971 8.2-The significance of mixed breccia 972 Few occurrences of "mixed" detrital formations, namely containing clastic elements 973 derived from both continental margin and oceanic domains, have so far been reported in 974 the Western Alps, apart from their intra-oceanic precursors emplaced during the late 975 Cretaceous closure of the Tethyan domain [START_REF] Deville | From oceanic closure to continental 1690 collision: A synthesis of the « Schistes lustrés » metamorphic complex of the Western Alps[END_REF]Balestro et al., 2015). The 976 Late Eocene olistostrome covered by the Helminthoid Flysch nappes of oceanic origin 977 contains frequent Late Cretaceous oceanic blocks (Autapie type Helminthoid flysch, 978 [START_REF] Sagnes | [END_REF]. Exceptional blocks of oceanic basement (basalts) are observed in two 979 localities (a, fig. 13, sites 23 and 24). Although not dated, the Paneyron mixed detrital 980 formation ("Ophiolite de Sérenne"; Kerckhove, 1969) is probably a lateral equivalent of 981 this layer. Other occurrences are reported from more internal parts of the arc, but 982 remain a subject of debate in the absence of reliable age data. These are, from south to 983 north [START_REF] Ring | Kinematics of the Alpenrhein-Bodensee graben system in the Central Alps: 2105 Oligocene/Miocene transtension due to the formation of the Western Alps arc[END_REF] the Valliera and Tibert megabreccia in Valgrana [START_REF] Michard | Etudes géologiques dans les zones internes des Alpes cottiennes[END_REF], which 984 contain siliciclastic facies, dolomitic blocks and serpentinite elements associated with 985 kilometre-scale ultramafic lenses. Initially interpreted as possibly interlayered within 986 the Mesozoic stratigraphy, these megabreccias occur between the underlying 987 Prepiedmont Triassic-Liassic units and the tectonically superposed oceanic "Schistes 988

Lustrés" nappes [START_REF] Michard | Position des brèches et des ophiolites dans les séries piémontaises 2032 des Vals Grana et Marmora (Alpes cottiennes méridionales, Italie)[END_REF], [START_REF] Polino | [END_REF] the Cula breccia [START_REF] Gout | Tectonique océanique et tectonique alpine dans les schistes lustrés de Haute Ubaye[END_REF] which 989 must be distinguished from the Col du Longet breccia [START_REF] Lemoine | Brèches sédimentaires marines à la frontière entre les domaines briançonnais et 1902 piémontais dans les ALpes occidentales[END_REF]Gidon et al., 990 1994). The latter, fed by nearby internal Briançonnais areas, is linked with the upper 991 Pelvo-d'Elva nappe ( § 4.1; fig. 4) and does not contain any oceanic clasts (Michard, pers. 992 com. 2019, and personal observations), whereas the Cula breccia which contains basalt 993 and serpentine pebbles is located at the base of the oceanic "Schistes Lustrés" pile of 994 nappes (Cula unit; [START_REF] Gout | Tectonique océanique et tectonique alpine dans les schistes lustrés de Haute Ubaye[END_REF] (3) the Prafauchier series [START_REF] Dumont | Le chaînon de Rochebrune au sud-est de Briançon: évolution paléogéographique et 1695 structurale d'un secteur de la zone piémontaise des Alpes occidentales[END_REF], containing 995 continental margin derived clasts (dolostones, micaschists) and serpentinite grains, with 996 locally serpentinite blocks, and interpreted either as part of the Prepiedmont series, or 997 belonging to an overlying unit, (4) the Rocher Renard breccia linked with the Lago Nero 998 unit at the base of the Chenaillet ophiolitic massif (Polino & Lemoine, 1984;Burroni et 999 al., 2003;Principi et al., 2004), (5) the Mte Banchetta breccia (Caron, 1971), containing 1000 blocks and pebbles of serpentinite, oceanic sediments (Jurassic marbles and ophicalcite) 1001 and continental margin sediments (Triassic dolostones and sandstones), in a shaley 1002 matrix. This latter example crops out on top of a folded Prepiedmont unit ( § 4.2; fig. 7), 1003 and is described below. 1004

In addition, two occurrences of ophiolite bearing tectono-sedimentary breccia in the 1005 western Alpine arc may have a comparable significance: the Montaldo calcschists in 1006 Liguria [START_REF] Dallagiovanna | Itinerario n°7, Da Cuneo a Mondovi, Via Lurisia-1646 Frabosa-Serra[END_REF]Decarlis et al., 2013) which contain olistoliths of 1007 serpentinites and prasinites, and the Gets flysch (Caron & Weidmann, 1987;Caron et al., 1008Caron et al., 1989;;Bill et al., 1997) resting at the top of the Prealps tectonic pile and containing 1009 ophiolitic and granitic blocks. Remarkably, all these examples of mixed continental and 1010 oceanic breccia occur immediately above the highest continental margin units, generally 1011 the Prepiedmont nappes which represent the most distal part of the European 1012 continental margin, and are overlain by the lowermost metasedimentary units of the 1013 oceanic "Schistes Lustrés" derived from the accretionary prism [START_REF] Schwartz | La zone piémontaise des Alpes occidentales: un paléocomplexe de subduction. 2151 Arguments métamorphiques, géochronologiques et structuraux[END_REF]. These 1014 "mixed" clastic formations are thus a useful marker of the initial geometry of the 1015 subduction trench, later deformed by further collision phases. We argue that their 1016 formation is related to the translation of the subduction trench over the distal European 1017 margin, and that their significance may be different from other mixed breccias deposited 1018 on the oceanic floor and related to either subcontinental mantle exhumation near the 1019 ocean-continent transition [START_REF] Meresse | A fossil ocean-continent transition of the 2009 Mesozoic Tethys preserved in the Schistes Lustrés nappe of northern Corsica[END_REF] or early subduction of the Tethyan 1020 floor (Marroni et al., 2007). 1021

8.3-The Monte Banchetta mixed breccia, an olistostrome at the bottom of the oceanic 1023 accretionary wedge? 1024

A key example of mixed breccia is found in Mte Banchetta, near Sestriere (fig. 3, site e). 1025

The structural setting, described in § 4.2, is interpreted to be a cross-fold anticline 1026 producing a dome. An uplifted continental margin unit, probably the same as the 1027 Prepiedmont Gran Roc unit further SW, outcrops in a tectonic window (fig. 7; Jouvent, 1028 2017). The underlying series was described by Caron (1971) and regarded as analogous 1029 to the Prepiedmont series of Valgrana [START_REF] Michard | Position des brèches et des ophiolites dans les séries piémontaises 2032 des Vals Grana et Marmora (Alpes cottiennes méridionales, Italie)[END_REF]. A recent map 1030 (Corno et al., 2019), does not follow this attribution, simply calling it "continental 1031 succession", but three diagnostic formations of the Prepiedmont series can be identified, 1032 namely the Norian dolostones, the Rhaetian dolostones, limestones and schists, and the 1033 "Lias prépiémontais" calcschists ("calcschistes siliceux ankéritiques" of Caron, 1971). 1034

This series which crops out in the Mte Banchetta area is not overturned, except on the 1035 eastern slopes due to backfolding (fig. 7). On the west side of Mte Banchetta (a, fig. 14, 1036 site c), the upper part of the calcschists is overlain by the breccia, through a dark grey 1037 schist layer containing scarce pebbles (c, fig. 14) and bearing a specific stilpnomelane 1038 mineralization (Caron, 1970). The same schist is also found above as a matrix of the 1039 breccia (Caron, 1971;b, d, fig. 14). Laterally (Clot della Mutta; fig. 14, south of site c), the 1040 Prepiedmont calcschists are overlain by an hectometric-sized serpentinite sliver in 1041 apparent tectonic contact, but this contact seems to pinch out beneath the schist layer at 1042 site c. This feature can be interpreted in two ways: either this sedimentary layer is 1043 inserted along the contact, thus the serpentinite sliver is a larger block belonging to the 1044 breccia, or it seals the tectonic contact between serpentinite and calcschists. In both 1045 cases, the breccia must be regarded as an olistostrome deposited over the Prepiedmont 1046

series. 1047

With the exception of the basal schist layer which contains rounded pebbles, the breccia 1048 has a very proximal character based on the size and shape of the blocks. This suggests 1049 that it was fed by nearby and active submarine scarps involving both continental margin 1050 (Prepiedmont) and oceanic floor units. Corno et al. (2019) propose an interpretation of 1051 this polymictic breccia in an hyperthinned marginal setting, in response to the Jurassic 1052 rifting. Alternatively, considering that the breccia is overlying the Prepiedmont marginal 1053 series and that it is located in the footwall of mixed imbricates (a, fig. 14), we propose 1054 that the Mte Banchetta breccia was deposited on the European margin toe in a 1055 subduction trench setting, marking the earliest stage of obduction of the accretionary 1056 wedge. 1057 Similar olistostromes are known beneath the Helminthoid flysch nappes, the non-1058 metamorphic equivalent to the oceanic Schistes Lustrés ("Schistes à blocs" fm.; 1059 [START_REF] Sagnes | [END_REF]. They locally contain ophiolitic detritus ( § 8.1; d, fig. 13), and we 1060 propose that they represent non-metamorphic lateral equivalents to the Mte Banchetta 1061 mixed breccia. 1062 Some Ar dating was completed from phengites sampled in the Prepiedmont Rhaetian 1063 formation beneath the breccia (f, fig. 14;Jouvent, 2017). Variscan inheritance can be 1064 ruled out because there is no significant reworking of Variscan material in the late 1065 Triassic sediments of the Prepiedmont series [START_REF] Lemoine | Découverte de Foraminifères planctoniques du Crétacé supérieur dans les 1917 Schistes lustrés du Queyras (Alpes Occidentales). Conséquences paléogéographiques et tectoniques[END_REF]. Hence the 1066 52.31±1.32 Ma age obtained (g, fig. 14) indicates that the deposition of the Mte 1067 Banchetta mixed breccia occurred during early Eocene or before. Although this HP age 1068 needs to be supported by further dating studies, the underthrusting of the Mte 1069 Banchetta Prepiedmont unit which produced this metamorphic event seems consistent 1070 with the early exhumation of the oceanic Schistes Lustrés [START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser 1446 probe 40Ar ⁄ 39Ar constraints and implications for the Western Alps[END_REF]Herviou et 1071Herviou et al., 2022)), and with the slightly younger involvement of the Briançonnais units in 1072 continental subduction [START_REF] Bucher | Tectonic 1574 evolution of the Briançonnais units alonf a transect (ECORS-CROP) through the Italian-French Western 1575 Alps[END_REF]Bousquet & Berger, 2008;Strzerzyski et al., 1073Strzerzyski et al., 2011)). This is also consistent with the age of Adria-Europe collision in the Western Alps 1074 according to geodynamic models [START_REF] De Graciansky | The Western Alps, from rift to passive margin to 1680 orogenic belt, an intergated overview[END_REF]Manzotti et al., 2014;1075Pfiffner, 2014;;[START_REF] Van Hinsbergen | Orogenic architecture of the Mediterranean region and kinematic reconstruction of its 2243 tectonic evolution since the Triassic[END_REF]Le Breton et al., 2021). 1076 Briançonnais stack (fig. 2). It is responsible for the formation of the arc driven by 1085 westward migration and indentation of Adria upper mantle [START_REF] Malusà | Tracking Adria indentation beneath the Alps by 1948 detrital zircon U-Pb geochronology: Implications for the Oligocene-Miocene dynamics of the Adriatic 1949 microplate[END_REF]Schmid 1086[START_REF] Schmid | Ivrea mantle wedge, arc of the 2139 Western Alps, and kinematic evolution of the Alps-Apennines orogenic system[END_REF][START_REF] Nouibat | Lithospheric transdimensional ambient-noise tomography of W-Europe: implications for 2049 crustal-scale geometry of the W-Alps[END_REF], and the most prominent thrusts and fold trends result 1087 from this late stage, such as the Penninic thrust near Briançon. Deciphering the early 1088 stage (D1) is much more difficult because the associated structures are overprinted, 1089 deformed and crosscut by the second one at all scales. However, proper integration of 1090 this early deformation phase is critically important because it recorded the absorption 1091 of N-S convergence required by plate tectonics during Alpine orogenesis (Schmid & 1092Kissling, 2000;[START_REF] Handy | Reconstructing the Alps-Carpathians-Dinarides as a key to 1797 understanding switches in subduction polarity, slab gaps and sirface motion[END_REF][START_REF] Van Hinsbergen | Orogenic architecture of the Mediterranean region and kinematic reconstruction of its 2243 tectonic evolution since the Triassic[END_REF]. The large amount 1093 (several hundred km.) of early N-S shortening is well documented in the Central and 1094

Eastern Alps [START_REF] Pleuger | Structural evolution of the contact 2078 between two Penninic nappes (Zermatt-Saas zone ans Combin zone, Western Alps) and implications for 2079 the exhumation mechanism and palaeogeography[END_REF][START_REF] Scharf | Peak-temperature patterns of polyphase 2128 metamorphism resulting from accretion, subduction and collision (eastern Tauern Window, European 2129 Alps) -a study with Raman microspectroscopy on carbonaceous material (RSCM)[END_REF][START_REF] Scheiber | Upper crustal deformation in continent-continent collision: 2132 A case stydy from the Bernard nappe complex (Valais, Switzerland)[END_REF]Steck et al., 1095[START_REF] Steck | Tectonics of the Monte Rosa and surrounding nappes (Switzerland 2201 and Italy): Tertiary phases of subduction, thrusting and folding in the Penninic Alps[END_REF]Handy et al., 2015 and refs. herein), but cannot be kinematically linked with the 1096 potential detachment layers which controlled the Briançonnais cover deformation are 1221 widespread in Provence [START_REF] Bestani | Structural style 1533 and evolution of the Pyrenean-Provence thrust belt[END_REF]. The northern Provence platform edge, 1222 running eastwards to crosscut the Argentera massif cover [START_REF] Barale | Stratigraphy, sedimentology and 1481 syndepositional tectonics of the Jurassic-Cretaceous succession at the transition between Provençal and 1482 Dauphinois domains (Maritime Alps, NW Italy)[END_REF], is 1223 crosscut and transported northwards beneath the Embrunais-Ubaye nappes (Séolane-1224 Cap unit) and further north in the Prealps nappes [START_REF] Maury | Le décrochement subbriançonnais: une nouvelle interprétation de la limite 2000 interne-externe des Alpes franco-italiennes[END_REF]D'Atri et al., 1225[START_REF] D'atri | Geological setting of the southern termination 1660 of Western Alps[END_REF]. The Subbriançonnais Late Cretaceous-Paleogene flysch formations can be linked 1226 paleogeographically both with the Briançonnais domain and with the Provence 1227 Pyrenean foreland (Kerckhove, 1965;[START_REF] Blanc | La formation du Flysch du Bachelard (Neocrétacé-1535 Paléocène) dans l'Unité du Pelat (zone subbriançonnaise des nappes de l'Ubaye, Alpes occidentales 1536 françaises)[END_REF][START_REF] Thum | The Piolit, Pelat and Baiardo Upper Cretaceous flysch 2223 formations (Western Alps): geodynamic implications at the time of the Pyrenean tectonic phases[END_REF]. 1228 1229

9.3-Pre-Alpine paleogeography and western termination of the Alps 1230

There is thus strong evidence to consider the Briançonnais as the eastern termination of 1231 the Iberia-Sardinia-Corsica microplate, and the Prepiedmont and Subbbriançonnais 1232 domains as transitions towards the Ligurian Tethys ocean and the Valais basin, 1233 respectively. However, the paleogeographic pattern of the margin should not be 1234 interpreted as linear. The occurrence of lateral transitions from the Briançonnais 1235 marginal plateau to the Provence platform westwards (e.g. [START_REF] Decarlis | Tectono-thermal evolution of a distal rifted margin: constraints form the Calizzano 1669 Massif (Prepiedmont-Briançonnais domain, Ligurian Alps)[END_REF], or 1236 towards its eastern termination within the Tethyan oceanic floor [START_REF] Handy | Reconciling plate-tectonic 1802 reconstructuons of Alpine Tethys with the geological-geophysical record of spreading and subduction in 1803 the Alps[END_REF] is 1237 likely to have complicated the thrust sequence even in the early stages of inversion. As 1238 an example, some specific units displaying late Jurassic platform carbonates, which are 1239 observed locally beneath the oceanic Helminthoid flysch nappes [START_REF] Dumont | 2525 c-Recontructed part of the profile across the Briançonnais Paleozoic basins before 2526 beeing affected by thrust sequence and overlain by the Prepiedmont and oceanic 2527 nappes[END_REF], 1240 must not be interpreted as being derived from the most distal part of the margin but 1241 from its lateral transition to the Provence domain. These lateral variations may be either 1242 progressive, resulting from oblique opening and/or a scissors-shape margin, or sharp, 1243 due to the occurrence of continental transform zones [START_REF] Lemoine | Extension synrift et failles transformantes jurassiques dans les Alpes Occidentales[END_REF]. Moreover, 1244 the lateral termination of the western Alpine orogen coincides with a flip in subduction 1245 such a feature should still be observed in the present lithospheric structure, which has 1346 recently been investigated with increasing resolution [START_REF] Hetényi | The AlpArray seismic network: A large-scale 1810 European experiment to image the Alpine orogen[END_REF]Malusà et al., 1347[START_REF] Malusà | The deep 1955 structure of the Alps base on the CIFALPS seismic experiment: A synthesis[END_REF]. The shape of the Ivrea Body indenter, representing Adria lithospheric mantle 1348 [START_REF] Zhao | Evidence for a serpentinized plate interface favouring continental 2269 subduction[END_REF] is now relatively well constrained [START_REF] Schmid | Ivrea mantle wedge, arc of the 2139 Western Alps, and kinematic evolution of the Alps-Apennines orogenic system[END_REF]refs. 1349 therein) and its regular trend is sharply interrupted southwards to the west of Cuneo 1350 city. This sharp discontinuity at depth contrasts with the curved form of the Penninic 1351 units at surface in the southern part of the arc, which suggests that the latter are 1352 decoupled from the indenter. We propose an interpretation of the sharp southern 1353 termination of the Ivrea body as an anticlockwise rotated relict of the Western Adria 1354 Transform Zone (d, fig. 16). This interpretation is consistent with the occurrence of 1355 anticlockwise rotation of the southern Tertiary Piedmont Basin since the Oligocene 1356 [START_REF] Maffione | A synchronous Alpine 1936 and Corsica-Sardinia rotation[END_REF]. The shallow location of the Ivrea Body, especially to the south 1357 (Lardeaux et al., 2006) could be either inherited from the early stage of Tethyan rifting 1358 or from the main exhumation stage of (U)HP continental units in the Eocene (Malusà et 1359[START_REF] Malusà | The deep 1955 structure of the Alps base on the CIFALPS seismic experiment: A synthesis[END_REF], bringing the Adria upper mantle and subduction wedge into position before 1360 westward indentation. Finally, considering the western boundary of the Alps as 1361 inherited from a transform boundary between two opposed-dip oceanic subduction 1362 zones allows for an early initiation of the asthenospheric counterflow though the tear 1363 zone, subsequently enhanced by Apenninic rollback. Such an asthenospheric 1364 counterflow is documented by mantle anisotropy beneath the Alps-Apennines junction 1365 [START_REF] Salimbeni | Active and fossil mantle flows in the 2119 western Alpine region unravelled by seismic anisotropy analysis and high-resolution P wave tomography[END_REF]. 1366 1367

Conclusion 1368

Figures captions 2272

  fig.2) is composed of a stack of nappes involving upper Paleozoic to Cenozoic sediments, 190

3 -

 3 coaxial deformation histories and geometrically cross-cutting structures, based on 224 examination of numerous field localities. 225 226 Stratigraphic and structural setting of the study area 227 Variable stratigraphic characteristics are observed in the Internal nappes, whose 228 outcropping elements are dominantly composed of sedimentary cover. They range from 229

  marginal stratigraphy includes late Paleozoic detrital and volcanoclastic formations 232 which demonstrate the transition from late Variscan foreland basins to Permo-Triassic 233 incipient crustal thinning, Triassic shallow marine, carbonate to evaporitic series, and 234 highly condensed Jurassic to Cretaceous sediments capped by Paleocene to late Eocene 235 flysch. The successive geodynamic settings which have controlled the facies and 236 thickness of these sedimentary sequences are as follows: 237 -Late Paleozoic: continental clastic and volcaniclastic sequences which may reach 2.5 238 km in total thickness, were deposited in late Variscan foreland basins in a transtensional 239 setting, in the framework of a major dextral transcurrent shear zone south of the 240

  document an Eocene-earliest-Oligocene 338 initial stage of N-directed fold-nappe thrusting by ductile detachment of the upper 339 European crust, overprinted by later Oligocene extensional extrusion of the Lepontine 340 dome structure. No occurrences of interference structures are described in the literature 341 from the study area south of the Ambin massif. Here we provide some examples of 342 subperpendicular fold-and-thrust structures in the east of the Briançonnais zone, which 343 are scarce because the older deformation has been largely overprinted by the younger 344 phase which is responsible for the arcuate trend of the modern chain. 345 346 4.1-Superposed fold structures in the internal Briançonnais units of Ubaye valley (a, b, fig. 347

4514. 3

 3 Hanging-wall cutoff in the Piemont nappes(f, fig. 3) 452 To the East of Briançon city, the Montgenèvre pass is located on a Piemont nappe which 453 is thrust over the Briançonnais stack (a, fig.8). This Piemont nappe is detached beneath 454 the Norian dolostones. The complete thickness of the Norian formation (800m) is 455 present in the Janus massif to the S (b, fig.8), but it decreases progressively northward 456 to ~300m (Chalvet) as a result of basal truncation by a thrust ramp, and the hangingwall 457 cut-off reaches the Rhaetian and the Jurassic fm. further to the north (fig.8). The 458 orientation of this cut-off outcropping on both sides of the Chalvet ridge is ENE-WSW. 459The deformation of Liassic strata in the hangingwall (Col de la Lauze, e, fig.8) consists of 460 ~N80°, top-to-the N trending folds, whereas stretching lineations in the footwall (Clot 461 Enjaime, d fig. 8) are trending ~N170°. Thus, the large-scale structure and the outcrop-462 scale deformation consistently indicate a top N to NW tectonic transport of the Piemont 463 units over Internal Briançonnais units of Clot Enjaime-Alpet-Rio Secco, which are in turn 464 resting structurally on the classical Briançonnais footwall. This relatively early thrust 465 stacking is overprinted by both backfolding/backthrusting, duplicating the Piemont 466 units in this area (Chalvet and Chaberton units, separated by the NS trending Rio Secco 467 syncline), and by recent extensional faulting (b, fig. 8). 468 The Chaberton unit itself terminates northwards, possibly due to a similar hangingwall 469 cut-off truncation of the Norian dolostones near the Acles pass (site 7, § 5). Here the 470 overlying Rhaetian-Liassic strata are affected by ~EW trending deformation similar to 471 the area of Col de La Lauze. 472

  northward propagation of the early Alpine nappes reactivate a transverse (~EW) 494 uplifted rift structure. 495 496 5-Outcrop-scale structural record of multistage kinematics in the Penninic nappes 497 of the Western Alps 498 5.1-Literature review 500

  532 5.2-Field data and structural interpretation 533 Here we present a comprehensive field survey of small-scale superposed fold structures 534 which affect various stratigraphic layers of the late Paleozoic-Mesozoic series, over 535 >60km along the southern part of the Internal arc of the Western Alps (fig. 10; fig. 11; 536 table

867 8 . 1 -

 81 Breccia: a Review 868 This section classifies the numerous examples of syndepositional breccia described in 869 the Western Alps literature which respect to the geodynamic context of their 870 occurrence. The following stages are considered: a) Tethyan rifting (early to early-871 middle Jurassic), b) Tethyan passive margin overlapping with the Atlantic-Bay of Biscay 872 rifting (middle Jurassic to early Cretaceous), c) Initiation of convergence related to the 873 Pyrenean orogeny (late Cretaceous to early Eocene), and d) Collision related to Alpine 874 orogeny (early to late Eocene). 875 a) During the early Jurassic Tethyan rift stage, the deposition of syn-rift sequences 876

1077 9 -

 9 Discussion and geodynamic implications 1078 Our data allow identification of two main stages during the building and evolution of the 1079 internal zones of the Western Alps arc, corresponding to nappe stacking and to 1080 westward extrusion, respectively. The orientation of shortening and tectonic transport 1081 changed significantly through time, as shown by subperpendicular fold and lineation 1082 trends in the study area. The late stage (D2, §4, §5) corresponds to the activation of 1083 forward and backward thrust systems which accommodate the exhumation of the 1084

Figure

  Figure 1: 2274

Figure 2 :Figure 3 :

 23 Figure 1: 2274

Figure

  Figure 1

Figure 14

 14 Figure 14

Figure 15

 15 Figure 15

  Figure 16

Table

  Click here to access/download;Table;

doi:10.1080/00206814.2014.943307

Balestro, G., Fioraso, G. & Lombardo, B., 2011. Geological map of the upper Pellice Valley (Italian Western

Alps). Journal of maps, p.634-654, doi:10.4113/jom.2011.2013

Balestro, G., Fioraso, G. & Lombardo, B., 2013. Geological map of the Montviso massif (Western Alps).

Journal of maps, doi:10.1080/17445647.2013.842507

based on petrology and their geodynamic consequences.Geol. Soc. London Spec.Publ., 298,

Bernoulli, D. & Lemoine, M., 1980. Birth and early evolution of the Tethys: the overall situation. In:

Géologie des chaînes alpines issues de la Tethys.Bur. Rech. Geol. Min., Mem. 115,

Southern Alps and Sardinia (Italy): a synthesis.Lithos, 45,

Grenoble, 250p.

[START_REF] Lemoine | Découverte de Foraminifères planctoniques du Crétacé supérieur dans les 1917 Schistes lustrés du Queyras (Alpes Occidentales). Conséquences paléogéographiques et tectoniques[END_REF] Le Rhétien et le Lias inférieur prépiémontais dans le chaînon de Rochebrune (Alpes

occidentales): enregistrement sédimentaire du passage des carbonates de plate-forme triasiques au

Jurassique hémipélagique lors du début du rifting téthysien. Géologie Alpine, Grenoble, 60, p.13-25.

Dumont, T.,[START_REF] Lemoine | Découverte de Foraminifères planctoniques du Crétacé supérieur dans les 1917 Schistes lustrés du Queyras (Alpes Occidentales). Conséquences paléogéographiques et tectoniques[END_REF]. Tectonique synsédimentaire triasico-jurassique et rifting

téthysien dans l'unité prépiémontaise de Rochebrune au sud-est de Briançon.Bull. Soc. géol. France,[START_REF] Montgenèvre | [END_REF] 26,

p. 921-933.

Dumont, T., 1998. Sea-level changes and early rifting of a European Tethyan margin in the Western Alps

and Southeastern France. In: de Graciansky, P.C., Hardenbol, J., Jacquin, T. & Vail, P. (Eds.), Mesozoic and

Cenozoic sequence stratigraphy of the European basins. Soc.Econ. Petr. Min. Special publ., 60,

Dumont, T., Simon-Labric, T., Authemayou, C. & Heymes, T., 2011. 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. Tectonics, 30, doi:10.1029/2010TC002836

Dumont, T., Schwartz, S., Guillot, S., Simon-Labric, T.,Tricart, P. & Jourdan, S., 2012. Structural and

sedimentary records of the Oligocene revolution in the Western Alpine arc. Jour. Geodyn.,

doi:10.1016/j.jog.2011.11.006

Durand-Delga, M., 1984. Principaux traits de la Crose Alpine et corrélations avec les Alpes Ligures. Mem.

Soc. Geol. Italiana,[START_REF] Blanc | [END_REF]

Durand-Delga, M., Fondecave-Wallez, M.J. & Rossi, P., 2005. L'unité ophiolitique de Pinetto (Corse):

signification du détritisme continental dans sa couverture de flysch albo-cénomanien. C. R. Géosciences,

337, p. 1084-1095.

Epard, J.L., 1990. La nappe de Morcles au sud-ouest du Mont Blanc. Mémoires de Géologie (Lausanne), 8,

p.

Escher, A., Hunziker, J.C., Marthaler, M., Sartori, M. &Steck, A., 1997. Geologic framework and structural

evolution of the western Swiss-Italian Alps. In: Pfiffner, O.A., Lehner, P., Heitzmann, P.,Mueller, S. & Steck,

A. (Eds.), Deep structure of the Swiss Alps -Results of NRP 20. Birkhäuser Verlag,Basel,

Escher, A.,Masson, H. & Steck, A., 1993. Nappe geometry in the Western Swiss Alps. Jour Struct Geology,

15,3-5, p. 501-509.

Espurt, N., Hippolyte, J.-C., Saillard, M., and Bellier, O., 2012. Geometry and kinematic evolution of a long-

living foreland structure inferred from field data and cross section balancing, the Sainte-Victoire System,

Provence, France, Tectonics, 31, TC4021.

Espurt, N., Wattelier, F., Philip, J., Hippolyte, J.C., Bellier, O. & Bestani, L., 2019. Mesozoic halokinesis and

basement inheritance in the eastern Provence fold-thrust belt. Tectonophysics, 766, p. 60-80.

Eva, E.,Malusà, M. G., & Solarino, S., 2020. Seismotectonics at the transition between opposite-dipping

slabs (western Alpine region). Tectonics, 39(9), e2020TC006086.

Fabre, J., 1961. Contribution à l'étude de la Zone Houllière en Maurienne en Tarentaise. Mémoires du

Bureau de Recherches Géologiques etMinières 2:315

Fabre, R,Gidon, M, Tricart, P, 1982. La structure du Paléozoïque de la Zone brianconnaise axiale au Nord

de Névache. Géologie Alpine 58:31-52

Faccenna, C., Becker, T.W., Auer, L., Billi, A., Boschi, L.,Brun, J.P., Capitanio, F.A., Funiciello, F., Horvàth, F.,

Jolivet, L., Piromallo, C., Royden, L., Rossetti, F. & Serpelloni, E., 2014. Mantle dynamics in the

Mediterranean. Review ofGeophysics, 52, p. 283-332, doi:10.1002/2013RG000444.

Faure, J.L. & Mégard-Galli, J., 1988. L'émersion jurassique en Briançonnais: sédimentation continentale et

fracturation distensive.Bull. Soc. géol. France, 4, 4,

Federico, L., Capponi, G., Crispini, L., Scambelluri, M. & Villa, I., 2005. 39 Ar/ 40 Ar dating of high-pressure

rocks from the Ligurian Alps: Evidence for a continuous subduction-exhumation cycle. Earth and

Planetary Science Letters, 240, p. 668-680.

Federico, L., Crispini, L., Scambelluri, M. & Capponi, G., 2007. Ophiolite mélange zone records exhumation

in a fossil subduction channel.Geology, 35,[START_REF] Secco | [END_REF]

Festa, A., Pini, G.A., Dilek, Yildirim & Codegone, G., 2010. Mélanges and mélange-forming processes: a

historical overview and new concepts. International Geology Review, 52;10-12, p. 1040-1105.

Festa, A.,Balestro, G ., Borghi, A., De Caroli, S. & Succo, A., 2020. The role of inheritance in continental

break-up and exhumation of Alpine Tethyan mantle (Canavese Zone, Western Alps). Geoscience Frontiers,

11, p. 167-188.

Feys, R, 1963. Etude géologique du carbonifère Brianconnais. Mémoires du Bureau de Recherches

Géologiques etMinières 6:387

Ford, M., Duchêne, S., Gasquet, D. & Vanderhaeghe, O., 2006. Two-phase orogenic convergence in the

external and internal SW Alps. Jour. Geol.Soc. London, 163,

Ford, M., Lickorish, W.H. & Kusznir, N.J., 1999. Tertiary foreland sedimentation in the Southern Subalpine

Jaillard, E., 1984. Etude géologique des unités briançonnaises de Vanoise occidentale au sud-ouest de

Pralognan (Savoie): Evolution sédimentaire et structurale. PhD Thesis, Université de Grenoble, 212 p.

Jaillard, E., 1987. Nature, signification et répartition du détritisme dans la zone briançonnaise de Vanoise.

Géologie Alpine, Mémoire H.S. n°13, p. 143-155.

Jaillard, E., 1989. La transition Briançonnais externe-Briançonnais interne en Savoie; l'Aiguille des Aimes,

le Roc du Bourget et le massif d'Ambin. Géologie Alpine,Grenoble, 65,

Jaillard, E., 1999. The Late Cretaceous-Eocene sedimentation in the Internal Briançonnais units of Vanoise

(French Alps): witness of early Alpine movements. EclogaeGeol. Helv., 92,[START_REF] Polino | [END_REF]

Jolivet, L., Augier, R., Faccenna, C., Negro, F., Rimmele, G., Agard, P., Robin, C., Rossetti, F. & Crespo-Blanc,

Paradis, Alpes occidentales). PhD Thesis, University of Rennes, 385 p.

LeBayon, B. & Ballèvre, M., 2006. Deformation history of a subducted continental crust (Gran Paradiso,

Western Alps): continuing crustal shortening during exhumation. Jour. Str. Geology,28, p. 793-815.

Le Breton, E., Brune, S., Ustaszewski, K., Zahirovic, S., Seton, M. & Müller, D., 2021. Kinematics and extent of

Marroni, M., Monechi, S., Perilli, N., Principi, G. & Treves, B., 1992. Late Cretaceous flysch deposits of the

Northerh Apennines, Italy: age of inception of orogenesis-controlled sedimentation. Cretaceous Res.,13, p.

487-504.

Marroni, M. & Pandolfi, L., 2007. The architecture of an incipient ocanic basin: a tentative reconstruction of

the Jurassic Liguria-Piemonte basin along the Northern Apennines-Alpine Corsica transect. Int. J. Earth

Sciences (Geol. Rundsch.), 96, p. 1059-1078.

Ricou, L.E., 1984. Les Alpes occidentales: chaîne de décrochement.Bull. Soc. géol. France, 26, 5,

Ricou, L.E. & Siddans, W.B., 1986. Collision tectonics in the Western Alps. In "Collision tectonics", Coward

M.P. & Ries A.C. (Eds), Geol.Soc. spec. publ. 19,

Termier, P. & Kilian, W., 1920. La signification tectonique des lambeaux de micaschistes qui affleurent près

de Briançon et à la surface des terrains à faciès briançonnais. C. R. Acad. Sc.Paris, 171,

Thomas, J.C., Claudel, M., Collombet, M.,Tricart, P., Chauvin, A. & Dumont, T., 1999. First paleomagnetic

 Balestro, G., Festa, A., Borghi, A., Castelli, D., Gattiglio, M. & Tartarotti, P., 2018