Cross-propagation of the western Alpine orogen from early to late deformation stages: Evidence from the Internal Zones and implications for restoration

Cross-propagation of the western Alpine orogen from early to late deformation stages: Evidence from the Internal Zones and implications for restoration Thierry Dumont, S. Schwartz, S. Guillot, M. Malusà, M. Jouvent, P. Monié, A. Verly To cite this version: Thierry Dumont, S. Schwartz, S. Guillot, M. Malusà, M. Jouvent, et al.. Cross-propagation of the western Alpine orogen from early to late deformation stages: Evidence from the Internal Zones and implications for restoration. Earth-Science Reviews, 2022, 232, pp.104106. �10.1016/j.earscirev.2022.104106�. �hal-03847932� HAL Id: hal-03847932 https://hal.science/hal-03847932 Submitted on 10 Nov 2022 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Earth-­‐Science  Reviews  232  (2022)  104106                              doi:10.1016/j.earscirev.2022.104106     Cross-­‐propagation   of   the   western   Alpine   orogen   from   early   to   late   deformation   stages:  evidence  from  the  Internal  Zones  and  implications  for  restoration     Dumont  T.*,  Schwartz  S.*,  Guillot  S.*,  Malusà  M.**,  Jouvent  M.***,  Monié  P.****,  Verly  A.*     *  Univ.  Grenoble  Alpes,  Univ.  Savoie  Mont  Blanc,  CNRS,  IRD,  Univ.  Gustave  Eifel,  ISTerre,  38000  Grenoble,   France   **Department  of  Earth  and  Environmental  Sciences,  University  of  Milano-­‐Bicocca,  Italy   ***Institute  of  Petrology  and  Structural  Geology,  Charles  University  in  Prague,  Czech  R.   ****Dynamique  de  la  Lithosphère,  Géosciences  Montpellier,  France     Abstract   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  nappe  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   nappe   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.  39Ar/40Ar  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  opening.       1-­‐Introduction     Despite  being  one  of  the  most  extensively  studied  mountain  ranges  in  the  world,  the  Western  Alps  are  a   very  specific  part  of  the  Alpine  orogen  whose  kinematic  evolution  is  markedly  different  from  the  rest  of   the   chain.   Whereas   the   Alps   trend   approximately   E-­‐W   from   Austria   to   Switzerland,   a   shape   easily   understandable   considering   N-­‐S   Africa-­‐Europe   convergence   during   the   Cenozoic   (Rosenbaum   et   al.,   2002),  the  western  arc  shows  a  180°  shift  across  western  Switzerland,  SE  France  and  N  Italy.  This  shape   was   partly   inherited   from   the   Mesozoic   rifting   stage,   and   mainly   developed   progressively   from   the   late   Eocene  onwards  (Caby,  1996;  Ford  et  al.,  2006;  Vignaroli  et  al.,  2008;  Dumont  et  al.,  2012;  Malusà  et  al.   2015),  as  an  accommodation  of  westward  extrusion,  possibly  combined  with  anticlockwise  rotation  of  the   northern  part  of  the  Adria  plate  (Laubscher,  1988,  1991;  Malusà  et  al.,  2009;  Eva  et  al.  2020).  This  non-­‐ cylindrical  propagation  produced  a  complex  and  polyphase  internal  deformation  of  the  subduction  wedge   preserved  between  the  Adria  plate  and  the  European  foreland,  including  fast  exhumation  and  changes  in   tectonic  transport  direction  through  time  (Platt,  1986),  which  are  both  characteristic  of  the  Western  Alps.     This   complex   3D   and   polyphase   deformation   history   makes   the   initial   architecture   of   the   precursor   continental  margin,  presently  involved  in  the  western  Alpine  arc  difficult  to  restore,  particularly  because     1   the   most   prominent   structures  which   define   the   present-­‐day   trend   of   the   arc   probably   postdate   the   initial   contractional   features   as   they   overprint   the   evidence   of   the   earliest   orogenic   propagation   developed   during  Eocene  times.  Moreover,  the  Adria  plate  first  collided  with  terranes  connected  to  the  eastern  Iberia   plate,  such  as  the  Briançonnais  domain  (Stampfli  et  al.,  2002;  Handy  et  al.,  2010),  so  that  an  early  part  of   the   convergence   history   is   likely   to   have   been   accommodated   by   oblique   contraction   and   reactivation   along  the  original  eastern  extent  of  the  Pyrenean  orogen,  resulting  in  complex  interference  between  pre-­‐ existing  Pyrenean  structures  and  newly  evolving  Alpine  deformation  (Lacombe  &  Jolivet,  2005;  Schreiber   et   al.,   2011;   Balansa   et   al.,   2022).   Finally,   the   Alpine   structures   have   experienced   more   recent   fragmentation,   in   the   southern   part   of   the   arc,   through   the   development   of   the   Ligurian   and   Tyrrenian   breakups   and   by   the   growth   of   the   Apenninic   chain,   driven   by   the   complex   lithospheric   motion   of   various   lithosphere  slabs  (Jolivet  et  al.,  2008;  Zhao  et  al.,  2016;  Salimbeni  et  al.,  2018).     The   surface  geology   of   the  Western   Alps   arc  gives   a   misleadingly   simple   expression   of   this   history.   Radial   transects   have   been   regarded   as   more   or   less   equivalent   and   comparable   with   little   regard   for   their   relative   orientation.   However,   this   approach   does   not   incorporate   consideration   of   the   magnitude   of   oblique  to  lateral  transfer  and  tectonic  transport  oblique  or  parallel  to  the  modern  orogenic  trend,  which   were   potentially   of   major   importance   considering   the   evidence   for   oblique-­‐slip   motions   in   the   southern   part  of  the  western  Alpine  arc  (Butler  et  al.,  1986;  Ricou  &  Siddans,  1986;  Laubscher,  1991;  Malusà  et  al.,   2009).   This   current   work   is   focused   on   deciphering   the   structural   and   tectono-­‐sedimentary   features   related  to  the  early  Alpine  orogenic  stages,  which  were  active  before  the  development  of  the  present-­‐day   arcuate   shape,   and   which   consist   of   multi-­‐scale   evidence   for   different   tectonic   transport   directions   through   time,   and   possible   interference   structures.   Since   the   early   orogenic   propagation   is   also   characterised   by   surficial   interactions   between   relief,   gravity   and   flexural   basin   distribution,   the   potential   link  between  selected  tectono-­‐sedimentary  breccias  and  the  major  tectonic  contacts  is  also  examined.     2-­‐Geological  setting,  overview  of  the  western  Alpine  arc     The  Western  Alps  (fig.  1a)  results  from  the  Cenozoic  continental  collision  between  the  Adria  microplate,  a   northern   portion   of   the   Africa   plate   (Channel   et   al.,   1979),   and   the   European   plate   s.l.,   including   the   Iberia   microplate.   The   orogen   incorporated   the   late   Cretaceous   oceanic   accretionary   wedge   (Deville   et   al.,   1992;   Schwartz,   2000;   Dal   Piaz   et   al.,   2003;   Tricart   &   Schwartz,   2006;   Herviou   et   al.,   2022,   and   refs   therein)   produced   by   the   south-­‐verging   subduction   and   closure   of   the   Ligurian   Tethys,   a   small   slow-­‐spreading   oceanic   domain   which   opened   in   middle   Jurassic   times   (Bernoulli   &   Lemoine,   1980;   Dal   Piaz,   1999;   de   Graciansky  et  al.,  2011).  It  also  contains  parts  of  the  European  continental  margin  of  this  ocean,  variably   affected  by  syn-­‐rift  crustal  thinning  (Lemoine  et  al.,  1986;  Manatschal  et  al.,  2007;  Le  Breton  et  al.,  2021),   and   an   attenuated   crust   and/or   exhumed   mantle   area,   the   Valais   domain,   whose   age   and   extension   are   still  debated  (Bousquet  et  al.,  2002;  Beltrando  et  al.,  2007;  Pfiffner,  2014).  The  remnants  of  units  derived   from   the   Adria   continental   plate   margin   are   relatively   scarce   in   the   Western   Alps   due   to   syn-­‐orogenic   erosion,  although  their  discovery  by  Argand  (1911)  laid  the  foundation  of  modern  Alpine  geology.     The  Western  Alps  are  conveniently  divided  into  external  and  internal  zones,  separated  by  the  «  Penninic   thrust  »  (Schmid  et  al.,  2004),  a  crustal-­‐scale  tectonic  contact  following  the  arc  (fig.  1b)  which  is  E-­‐dipping   in   the   central   part   of   the   arc   (Guellec   et   al.,   1990;   Lardeaux   et   al.,   2006).   To   the   west,   the   contractional   footwall  units  (External  zone)  consist  of  Dauphiné-­‐Helvetic  Variscan  basement  and  Meso-­‐Cenozoic  series   lacking   significant   Alpine   metamorphism.   They   are   locally   overlain   by   exotic   sedimentary   cover   nappes   emplaced  before  the  activation  of  the  Penninic  thrust  (Prealps,  Embrunais-­‐Ubaye  nappes,  Ligurian  flysch   nappes;  Dumont  et  al.,  2012).  The  External  continental  crust  is  deeply  subducted  eastwards  beneath  the   Internal   zones   and   the   Adria   plate   (Zhao   et   al.,   2015;   Nouibat   et   al.,   2022).   To   the   east   of   the   Penninic   thrust,   the   hangingwall   nappe   stack   of   the   Internal   zones   is   highly   heterogeneous   both   regarding   palaeogeographic   provenance   (de   Graciansky   et   al.,   2011)   and   Alpine   metamorphism   (Oberhänsli   et   al.,   2004;   Bousquet   et   al.,   2008;   Agard,   2021),   ranging   from   upper   greenschist   to   eclogite   facies   metamorphism.     Thus,   three   main   units   have   to   be   considered   in   this   area,   that   is,   from   W   to   E,   the   Extenal   zone,   the   Embrunais-­‐Ubaye  nappes,  and  the  Internal  zones  (Fig.  1b):     -­‐   The   external   zone   s.s.   is   composed   of   Variscan   basement   whose   local   exhumation   and   uplift   due   to   several   thick-­‐skinned   shortening   events   provides   some   of   the   major   Alpine   relief,   up   to   >4km.   The   top   basement   is   locally   buried   to   ~10km   depth   in   the   center   of   the   SE   France   Basin   following   several   Mesozoic   rifting   events.   The   Mesozoic   sedimentary   cover   shows   strongly   variable   thickness   and   facies   across  the  Jura  platform,  the  Dauphiné  rifted  margin  (Lemoine  et  al.,  1986),  the  Vocontian  basin  and  the   Provence   platform.   The   Mesozoic   paleogeographic   trends   of   the   External   zone   are   crosscut   by   the   arcuate   Alpine  structures.  The  sedimentary  cover  is  locally  involved  in  thin-­‐skinned  fold  and  thrust  deformation       2      Figure  1:    a-­‐  Location  of  the  Western  Alps  arc  in  the  European  framework.   b-­‐  Geological  map  of  the  southern  part  of  the  Western  Alps  arc,  with  location  of  the  following  figures.  Colors   refer   mainly   to   the   paleogeographic   origin   of   structural   units,   with   overlay   to   distinguish   basement   and   Paleozoic  from  younger  sedimentary  cover.       3   of   different   ages   and   orientations   due   to   Pyrenean   and   Alpine   contractional   propagation   (Gidon,   1997;   Philippe  et  al.,  1998;  Espurt  et  al.,  2012;  Schwartz  et  al.,  2017).  The  occurrence  of  soft  evaporitic  Triassic   layers  played  a  major  control  on  the  location  of  detachments  (Lickorish  et  al.,  2002;  Espurt  et  al.,  2019;   Balansa  et  al.,  2022).  Remnants  of  synorogenic  basins  record  different  stages  of  propagating  lithospheric   flexure  through  foreland  basin  and  forebulge  development,  with  flysch  and  molasse  deposition  (Joseph  &   Lomas,  2004;  Ford  &  Lickorish,  2004;  Kempf  &  Pfiffner,  2004;  Schwartz  et  al.,  2012;  Kalifi  et  al.,  2020).   -­‐   The   Embrunais-­‐Ubaye   nappes   were   transported   in   a   superficial   setting   over   the   flexural   basin   on   the   External  zone  during  the  late  Eocene  to  early  Oligocene  (Kerckhove,  1969;  Gupta  &  Allen,  2000).  They  are   dominantly   composed   of   «  Helminthoid   Flysch  »,   late   Cretaceous   deep   marine   sequences   detached   from   the  Tethys  oceanic  floor  and  transported  over  the  Briançonnais  domain  whose  thin  remnant  thrust  sheets   are  often  observed  at  their  base  (Kerckhove,  1969).  The  Embrunais-­‐Ubaye  nappes  record  only  low-­‐grade   metamorphism   from   sub   greenschist   to   lower   greenschist   facies   (Oberhänsli   et   al.,   2004),   similar   to   the   Ligurian  and  Prealps  nappe  stack.  They  bear  evidence  for  changes  in  transport  direction  (Merle  &  Brun,   1984)  from  an  original  southeastern  origin.  They  are  locally  preserved  over  the  external  foreland  in  the   footwall   of   the   Penninic   thrust,   which   demonstrates   the   polyphase   and   non-­‐coaxial   character   of   Alpine   orogenic   propagation.   Their   emplacement   is   dated   as   late   Eocene-­‐earliest   Oligocene,   with   intial   NW-­‐ directed  deformation  beneath  the  Embrunais  basal  thrust  (Dumont  et  al.,  2011;  fig.  1b,  Fig.  2),  and  they   are   overprinted,   deformed   and   crosscut   in   out-­‐of-­‐sequence   mode   by   the   Penninic   Thrust   propagating   towards  the  SW  from  early  Oligocene  onwards.     -­‐   The   Internal   nappe   stack   includes   parts   of   the   distal   European   margin   (detached   sedimentary   cover   and   basement   of   the   Briançonnais   domain   s.l.),   exhumed   remnants   of   the   Ligurian   Tethys   ocean   (metasediments  and  ophiolites,  the  so-­‐called   «  Schistes   Lustrés  »),  and  scarce  overthrust  pieces  of  Adria   continental  crust  (Dent  Blanche  and  Cervin  units;  Dal  Piaz,  1999;  Schmid  et  al.,  2004).  The  Internal  nappes   can  be  classified  according  to  different  criteria:  their  dominant  lithology,  either  upper  crustal  basement  or   sediments,   their   metamorphic   signature   (Alpine   HP-­‐LT   and/or   Variscan   HT-­‐LP;   Handy   &   Oberhänsli,   2004;   Bousquet   et   al.,   2008;   von   Raumer   et   al.,   2012;   Schwartz   et   al.,   2013),   and   their   paleogeographic   provenance  with  respect  to  the  Tethyan  framework  (Lemoine  et  al.,  1986;  Schmid  et  al.,  2004;  Handy  et   al.,  2010).  In  Fig.  1  and  2  we  define  structural  units  with  respect  to   their   origin,   either   from   the   European   continental  margin  s.l.  (including  parts  of  the  Iberian  plate)  or  from  the  oceanic  domains.  Concerning  the   continental   margin   units,   we   follow   the   definitions   of   Lemoine   et   al.   (1986),   which   distinguishes   the   Subbriançonnais,  Briançonnais,  internal  Briançonnais  and  Prepiedmont  type  units.  Contrary  to  Mohn  et  al.   (2010)  or  Ribes  et  al.  (2019),  we  have  to  maintain  the  distinction  between  the  internal  Briançonnais  and   Prepiedmont  units,  which  have  a  very  different  Mesozoic  record  inherited  from  their  syn-­‐rift  history.  The   ocean-­‐derived  nappes  still  occupy  hangingwall  locations  on  both  sides  of  the  Penninic  thrust  (fig.  2),  and   in  the  highly  metamorphosed  core  of  the  arc.  The  latter  include  the  Monviso  and  Voltri  units,  which  have   been  interpreted  as   remnants  of  an  oceanic  subduction  channel  (Schwartz  et  al.,  2001;  Guillot  et  al.,  2004;   Federico   et   al.,   2007)   but   may   also   result   from   different   intra-­‐oceanic   structural   inheritance   and   decoupling   processes   in   a   complex   plate   interface   (Balestro   et   al.,   2018;   Agard,   2021;   Herviou   et   al.,   2022,   and   refs.   therein).   The   subduction   channel   process   is   also   involved   during   the   continental   subduction   stage   (Ganne   et   al.,   2006;   Bousquet,   2008b,   Federico   et   al.,   2005),   although   alternative   processes   may   explain  the  exhumation  of  HP  units,  such  as  corner-­‐flow  (Polino  et  al.,  1990)  or  transtension  (Malusà  et  al.   2015).  Soon  after  the  Eocene-­‐Oligocene  boundary,  the  initial  suture  was  crosscut  by  the  Penninic  thrust   and   is   thus   strongly   affected   by   backfolding   and   tilting   due   to   vertical   extrusion   of   the   Internal   crystalline   massifs   from   beneath   the   orogenic   wedge   (Schwartz,   2000;   Rolland   et   al.,   2000;   Avigad   et   al.,   2003;   Schwartz  et  al.,  2009).  At  a  smaller  scale,  the  «  pop  –up  »  structural  style  of  the  Briançonnais  zone  s.l.    (fig.   1b,   fig.   2)   is   composed   of   a   stack   of   nappes   involving   upper   Paleozoic   to   Cenozoic   sediments,   probably   cored  by  basement  at  depth.  The  exhumation  of  this  «  pop-­‐up  »  and  of  the  associated  structures  (forward-­‐ directed   Penninic   thrust   and   backward-­‐directed   folds   and   thrusts),   which   was   initiated   during   early   Oligocene   (Jourdan   et   al.,   2013),   post-­‐dates   the   preceding   Eocene   nappe   stacking   phase.   As   emphasised   previously,   this   is   consistent   with   the   observed   overprint   (cross   cutting)   of   the   Embrunais   basal   thrust   by   the  Penninic  thrust  (fig.  1b,  Fig.  2).             Describing   the   structure   of   the   Western   Alps   mainly   on   the   basis   of   the   «  Penninic   thrust  »   is   an   oversimplification,  because  this  tectonic  boundary  crosscuts  the  initial  subduction  wedge  and  developed   relatively   recently   (since   early   Oligocene   onwards;   Simon-­‐Labric   et   al.,   2009;   Maino   et   al.,   2015),   coeval   with   the   formation   of   the   arc   (Dumont   et   al.,   2012).   Its   «  out-­‐of-­‐sequence  »   character   can   be   observed   south   of   Briançon   city,   where   it   cuts   across   an   earlier   nappe   stack   involving   ocean-­‐derived   flysch   sediments,   the   Embrunais-­‐Ubaye   nappes   (fig.   1b),   also   represented   in   the   Ligurian   and   Prealps   nappes.   Moreover,   parts   of   the   oceanic   accretionary   wedge   deformed   together   with   distal   European   continental   margin  units  are  exhumed  in  its  hangingwall  (Schmid  et  al.,  2004).  It  is  important  to  consider  that  the       4        Figure  2:  Radial  cross-­‐sections  along  the  Durance  (A),  Guil  (B)  and  Ubaye  (C)  valleys,  illustrating  the  double-­‐ verging   structure   of   the   Briançonnais   zone,   exhumed   in   between   the   Tethyan   oceanic   nappes   (non-­‐ metamorphic   «  Helminthoid   Flyschs  »   westwards,   and   metamorphic   «  Schistes   Lustrés  »   eastwards).   Such   radially   oriented   sections   emphasize   the   late   Alpine   structures   (red)   such   as   the   Penninic   Thrust   and   conjugates  backthrusts,  which  overprint  the  early  structures  of  the  initial  nappes  stack  (black).     building   of   the   Western   Alps   arc   required   polyphase   deformation   and   displacement   directions   changing   through   time   (Ford   et   al.,   2006)   as   opposed   to   a   continuous   process   driven   by   strain   partitioning   and   maintenance  of  specifically  orientated  convergence  directions  (Fry,  1986).       The  structure  of  the  Internal  zones  of  the  Western  Alps  arc  underlined  by  the  «  Penninic  »  curved  frontal   sole   thrust   and   by   prominent   backthrusting   is   a   possible   consequence   of   Adria   mantle   indentation,   that   is   particularly  evident  in  the  northernmost  Western  Alps  (Schmid  et  al.,  2017;  Malusà  et  al.,  2021;  Nouibat  et   al.,  2022).  Both  follow  the  curvature  of  the  arc.  In  the  study  area,  radial  interpretative  cross-­‐sections  (fig.   2)   illustrate   this   double-­‐vergent   structural   style,   along   with   the   large-­‐scale   characteristics   of   the   Briançonnais   nappe   stack.   Radial   sections   are   commonly   used   to   represent   bulk   Alpine   deformation   across  the  arc  (i.e.  Schmid  et  al.,  2017)  and  have  been  used  as  the  basis  for  restored  models  of  pre-­‐Alpine   paleogeography   or   shortening   estimates   (Fry,   1989;   Seno   et   al.,   2004;   Bellahsen   et   al.,   2014).   These   existing  restorations  are  not  necessarily  compatible  together,  precisely  because  of  their  radial  distribution   over   nearly   180°.   A   key   consideration   which   is   often   overlooked   when   examining   radial   sections   is   the   amount   of   out-­‐of-­‐plane   movement,   oblique   or   perpendicular   to   cross-­‐section   orientations.   We   provide   evidence   for   complex,   non-­‐coaxial   deformation   histories   and   geometrically   cross-­‐cutting     structures,   based  on  examination  of  numerous  field  localities.     3-­‐  Stratigraphic  and  structural  setting  of  the  study  area     Variable   stratigraphic   characteristics   are   observed   in   the   Internal   nappes,   whose   outcropping   elements   are   dominantly   composed   of   sedimentary   cover.   They   range   from   ‘pre-­‐rift’,   rift   and   starved   continental   margin  sequences,  to  oceanic  sediments  and  remnants  of  their  slow-­‐spreading  oceanic  floor  (Lemoine  et   al.,  1986;  Lagabrielle,  1994).  The  marginal  stratigraphy  includes  late  Paleozoic  detrital  and  volcanoclastic   formations   which   demonstrate   the   transition   from   late   Variscan   foreland   basins   to   Permo-­‐Triassic   incipient   crustal   thinning,   Triassic   shallow   marine,   carbonate   to   evaporitic   series,   and   highly   condensed   Jurassic  to  Cretaceous  sediments  capped  by  Paleocene  to  late  Eocene  flysch.  The  successive  geodynamic   settings  which  have  controlled  the  facies  and  thickness  of  these  sedimentary  sequences  are  as  follows:   -­‐   Late   Paleozoic:   continental   clastic   and   volcaniclastic   sequences   which   may   reach   2.5   km   in   total   thickness,   were   deposited   in   late   Variscan   foreland   basins   in  a   transtensional   setting,   in   the   framework   of     5   a  major  dextral  transcurrent  shear  zone  south  of  the  Variscan  orogenic  belt  (Guillot  &  Ménot,  2009;  von   Raumer   et   al.,   2012;   Festa   et   al.,   2020).   Strong   lateral   variations   indicate   the   activity   of   depocenters   controlled   by   extensional   faulting   (Cortesogno   et   al.,   1993),   and   widespread   Permian   volcanic   activity,   thermal   evolution   and   underplating   document   the   initiation   of   post-­‐Variscan   crustal   attenuation   in   the   entire   Alpine   area   (Cortesogno   et   al.,   1998;   Rottura   et   al.,   1998;   Marotta   &   Spalla,   2007;   Sinigoi   et   al.,   2010).       -­‐  Triassic:  the  classical  siliciclastic-­‐carbonate-­‐evaporitic  cycle  is  widely  developed  in  the  Internal  units  of   both   European   and   Adria   origin,   with   kilometre-­‐scale   thickness.   Middle   Triassic   shallow   marine   sequences   can   be   traced   over   the   entire   Alpine-­‐Carpathian   area.   In   the   Western   Alps,   little   evidence   for   brittle   extension   has   been   reported   to   date   despite   significant   subsidence.   More   important   Triassic   rift   basins  were  located  further  East  /  Southeast  as  part  of  the  Neotethyan  basin  propagation  (Stampfli  et  al.,   2002).  However,  evaporites  deposited  during  the  late  Triassic  played  a  major  role  during  Alpine  orogeny   as  they  controlled  the  detachment  of  large  sedimentary  cover  units  within  the  collision  wedge.       -­‐  Early  to  Middle  Jurassic:  all  the  marginal  cover  units  of  the  Internal  Western  Alps   show  evidence  of  a  rift   setting  from  early  Jurassic  onwards  (Dumont,  1998).  Contrasting  subsidence  patterns  during  early  Liassic   suggest   tectonic   subsidence   followed   by   uplift   over   the   whole   Briançonnais   domain   during   late   Liassic-­‐ middle   Jurassic,   which   produced   emergence   and   continental   erosion   coeval   with   extensional   block   faulting  (Claudel  &  Dumont,  1999).  This  process,  which  consists  of  a  long-­‐wavelength  uplift  with  a  vertical   amplitude   reaching   about   1   km,   could   be   explained   by   rift   shoulder   uplift   (Stampfli   &   Marthaler,   1990).   Alternatively,   it   could   result   from   different   processes   such   as   thermal   influence   linked   with   the   upper   mantle   boudinage   and   impregnation   during   hyperextension   («  thermal   erosion  »,   Mohn   et   al.,   2012)   or   extensional   ribbon   uplift   (Tavani   et   al.,   2021).   At   a   shorter   wavelength,   the   maximum   amplitude   of   erosion   and   uplift   is   observed   in   the   internal   Briançonnais   units,   initially   located   close   to   the   paleogeographic   boundary   with   the   strongly   subsiding   Prepiedmont   domain.   This   feature   could   correspond  to  flexural  uplift  (Basile  &  Allemand,  2002)  or  to  the  influence  of  lithospheric  necking  (Ribes   et   al.,   2019).   The   resulting   unconformity   is   a   widely   recognised   characteristic   of   the   Briançonnais   marginal   plateau,   which   was   emerged   and   increasingly   uplifted   towards   the   incipient   Tethyan   breakup   (Lemoine   et   al.,   1986).   The   magnitude   of   the   associated   erosional   gap   increases   towards   the   rift,   that   is   from   the   external   to   the   internal   Briançonnais   units.   Syn-­‐rift   continental   erosion   removed   the   whole   Triassic  sequence  in  the  most  internal  Briançonnais  units,  allowing  the  post-­‐rift  sediments  to  rest  on  the   late  Variscan  volcaniclastics  or  directly  on  the  basement.  Conversely,  this  unconformity  is  not  recognised   in   the   more   proximal   rift   basins   of   the   External   zone,   nor   closer   to   the   breakup,   in   the   Prepiedmont   domain   which   was   fed   by   turbidites   sourced   from   the   emerged   Briançonnais   shoulder   (Dumont   et   al.,   1984).  This  strongly  subsiding  domain  may  represent  the  most  hyperextended  part  of  the  distal  margin   (Mohn  et  al.,  2012)  and  play  a  key  role  to  locate  the  early  inversion  processes  (Tavani  et  al.,  2021).     -­‐   late   Middle   Jurassic   to   early   Cretaceous:   consequent   to   the   erosional   events   described   above,   a   widespread   unconformity   is   spectacularly   exposed   in   the   Briançonnais,   characterised   by   deep   marine   post-­‐rift  (syn-­‐spreading)  sediments  overlying  a  variety  of  older  (pre-­‐spreading)  stratigraphic  units.  This   unconformity  has  traditionally  been  described  as  the  "breakup  unconformity"  (Lemoine  et  al.,  1986;  see   discussion   in   Masini   et   al.,   2013).   Following   the   initial   breakup   in   the   late   Middle   to   early   Late   Jurassic   (Li   et   al.,   2013,   and   refs   therein),   a   uniform   post-­‐rift   series   covered   the   whole   margin,   including   the   Briançonnais   marginal   plateau,   preserving   a   transgressive   lag   over   the   erosional   surface.   These   starved   post-­‐rift  pelagic  sediments  record  thermal  subsidence.  However,  some  restricted  extensional  deformation   is  reported  locally  within  the  Briançonnais  area  (Claudel  &  Dumont,  1999),  as  in  Provence  (Dardeau  et  al.,   1988),  and  possibly  due  to  incipient  rifting  between  Europe  and  areas  connected  to  the  Iberian  plate.     -­‐  Late  Cretaceous  to  Eocene:  still  in  a  deep  marine  setting,  several  domains  of  the  internal  zones  record  the   diachronous   onset   of   flysch   sedimentation.   The   oceanic   sediments   show   evidence   of   margin   sourced   turbidites  from  the  Cenomanian  onwards  (Caron  et  al.,  1989;  Durand-­‐Delga  et  al.,  2005;  Catanzariti  et  al.,   2007),   with   increasing   clastic   input   from   the   Campanian   (Helminthoid   Flysch   fm.)   related   to   the   active   Adria   margin   (Di   Giulio,   1992;   Marroni   et   al.,   1992).   Local   sourcing   from   the   European   margin   (Gottero   flysch,   Nilsen   1984;   Marroni   et   al.   2010)   indicate   that   denudation   occurred   before   involvement   in   the   Adria-­‐Europe   collision,   possibly   in   relation   to   the   onset   of   Pyrenean   orogeny.   The   continental   margin   series   are   also   affected   by   tectono-­‐sedimentary   disturbances,   for   example   erosional   unconformities   and   breccias,   especially   in   the   Briançonnais   units   (Gidon   et   al.,   1994).   Also   recorded   in   the   Alpine   foreland   (Michard  et  al.,  2010)  and  in  Provence  (Espurt  et  al.,  2012),  these  features  can  be  interpreted  either  as  a   response  to  active  transcurrent  deformation  (Bertok  et  al.,  2012),  Pyrenean  forebulge  propagation  (Thum   et  al.,  2015),  or  Alpine  forebulge  propagation  (Michard  &  Martinotti,  2002).     Despite   the   metamorphic   overprint,   (litho)stratigraphic   correlations   between   many   units   presently   included  in  the  Internal  Zones  of  the  western  Alpine  arc  remain  possible.  These  units  generally  display  a     6   sedimentary   record   markedly   different   from   the   External   Zone   (Dauphiné-­‐Hevetic,   Vocontian   and   Provence   domains),   both   in   terms   of   stratigraphic   thickness   and   environment,   from   post-­‐Varican   to   Eocene   times   (Lemoine   et   al.,   1986).   This   supports   the   occurrence   of   a   major   lithospheric-­‐scale   displacement  along  the  boundary  between  the  Internal  and  External  Zones,  whose  large  paleogeographic   areas  were  likely  overthrust  from  the  earliest  Oligocene  onwards.     As   previously   stated,   the   dominant   structures   following   the   Western   Alps   arc   (Penninic   thrust,   Briançonnais   zone,   metamorphic   zonation   in   Schistes   Lustrés,   internal   cristalline   massifs   trend)   were   produced   during   the   westward   extrusion   stage   from   the   early   Oligocene   onwards,   and   they   crosscut,   deformed   and   exhumed   the   initial   stack   formed   during   the   north-­‐   to   northwestward   Eocene   orogenic   propagation.   Thus,   despite   this   initial   phase   accommodated   important   tectonic   transports,   its   structural   effects   are   presently   obscured   due   to   further   overprint.   Some   evidence   is   provided   by   interference   structures  at  different  scales,  especially  in  the  Western  Alps  where  the  directions  of  early  and  late  Alpine   orogenic  propagation  are  markedly  different.       4-­‐Polyphase   deformation:   large-­‐scale   overprint   between   differently   oriented   structures  in  the  Internal  Zones  (interference  structures?)     Large-­‐scale   superposed   deformation   due   to   crossed   shortening   episodes   is   suggested   in   the   external   zone   by   the   circular   shape   of   the   Pelvoux-­‐Ecrins   cristalline   massif   (Dumont   et   al.,   2011).   Interference   shortening   structures   are   reported   from   the   internal   Western   Alpine   arc   (e.g.   Jaillard,   1984;   Ganne,   2003;   Bucher   et   al.,   2004).     In   the   Central   Alps,   the   deeply   exhumed   Lepontine   area   shows   complex   curved   shapes   which   are   interpreted   to   result   from   superposed   deformations   with   different   strain   patterns   (Merle,   1987).   Steck   (2008)   and   Steck   et   al.   (2013,   2015,   2019)   document   an   Eocene-­‐earliest-­‐Oligocene   initial   stage   of   N-­‐directed   fold-­‐nappe   thrusting   by   ductile   detachment   of   the   upper   European   crust,   overprinted  by  later  Oligocene  extensional  extrusion  of  the  Lepontine  dome  structure.  No  occurrences  of   interference  structures  are  described  in  the  literature  from  the  study  area  south  of  the  Ambin  massif.  Here   we   provide   some   examples   of   subperpendicular   fold-­‐and-­‐thrust   structures   in   the   east   of   the   Briançonnais   zone,  which  are  scarce  because  the  older  deformation  has  been  largely  overprinted  by  the   younger  phase   which  is  responsible  for  the  arcuate  trend  of  the  modern  chain.         Figure   3:   Schematic  situation  of  the  main  fold-­‐and-­‐thrust  interference  structures  observed  on  the  internal   side  of  the  Briançonnais  zone,  described  in  §  4,  and  their  location  in  the  map  (perspective  viw  towards  the   SW).  Early  phase:  N-­‐  to  NW-­‐directed;  late  phase:  E-­‐  to  NE-­‐directed.     7   4.1-­‐Superposed  fold  structures  in  the  internal  Briançonnais  units  of  Ubaye  valley  (a,  b,  fig.  3)   In   the   southern   part   of   the   Western   Alpine   arc,   the   "Roure"   zone   (Le   Guernic,   1967)   and   the   Acceglio-­‐ Longet   (AL)   zone   (Debelmas   &   Lemoine,   1957;   Lefèvre   &   Michard,   1976)  belong   to   internal   Briançonnais.   They   are   composed   of   late   Paleozoic   and   Mesozoic   series,   characterized   by   very   thin   Triassic-­‐Jurassic   sediments  compared  to  the  external  Briançonnais  units.  They  outcrop  in  two  SSE-­‐NNW  oriented  strips  (a   and   b   fig.   3,   respectively)   tangential   to   the   trend   of   the   arc,   which   are   formed   mainly   by   ENE-­‐WSW   shortening  and  eastward  backfolding.  However,  in  both  areas  it  is  possible  to  detect  earlier  deformation   criteria,  roughly  perpendicular  to  the  youngest  deformation.     At  Col  du  Longet  (a,  fig.  3),  the  northern  termination  of  the  Acceglio-­‐Longet  strip  (Schwartz  et  al.,  2000)   becomes  buried  beneath  the  Rocca  Bianca  ophiolitic  massif  and  the  surrounding  metasediments  (fig.  4).   Outcrop-­‐scale  structures  consist  of  a  pervasive  lineation  and  metre-­‐scale,  top-­‐to-­‐the  N  to  NW  overturned   folds   trending   WSW-­‐ENE,   both   within   the   Permian   to   Mesozoic   clastic   formations   of   the   AL   unit   and   within   the   oceanic   units   above   (fig.   4;   stereogram   site   39,   see   §5;   Verly,   2015).   These   NNW-­‐verging   structures   postdate   the   initial   stacking   of   oceanic   and   continental   units,   and   are   clearly   deformed   by   backfolding.  The  latter  occurred  close  to  the  Eocene-­‐Oligocene  boundary  according  to  U-­‐Th-­‐Pb  dating  on   allanite  in  this  locality  (Verly,  2015),  implying  that  oceanic  and  continental  margin  units  had  been  stacked   earlier   during   the   Eocene.   We   propose   that   this   termination   of   the   Acceglio-­‐Longet   strip   corresponds   to   a   large-­‐scale   hinge   of   a   transverse,   recumbent   or   isoclinal   fold   trending   SW-­‐NE   to   EW   (section   c,   fig.   4),   which  was  further  refolded  by  NE-­‐ward  recumbent  backfolding,  similarly  to  the  conceptual  sketch  of  fig.  3   (a,  upper  left  cartoon).         Figure   4:   Structural   framework   of   the   northern   Acceglio-­‐Longet   «  ultrabriançonnais  »   continental   margin   unit   (purple),   uplifted   from   beneath   the   oceanic   Schistes   Lustrés   nappes   (green)   to   the   east   of   the   Briançonnais  zone  (yellow).  Location  on  fig.  1b.  This  tectonic  window  is  interpreted  as  resulting  from  large-­‐ scale  interference  between  early,  N-­‐NW  directed  fold-­‐thrusting  (D1,  F1)  and  late  E-­‐NE  directed  backfolding   (D2,   F2).   Note   the   location   of   the   Pelvo   d’Elva   and   Longet   megabreccia   slivers   (grey)   along   the   boundary   between  continental  margin  and  oceanic  units.   a-­‐  Perspective  view  towards  the  SW  of  a  simplified  geological  map  drapped  over  DEM.     b-­‐  Vertical  view  of  the  same  map  with  approximative  orientation  of  D1  and  D2  deformations.   c-­‐   Schematic   cross-­‐section   trending   subparallel   to   D1   tectonic   transport,   reconstructed   before   D2   backfolding,   and   able   to   explain   the   northward   interruption   of   the   Acceglio-­‐Longet   outcrops   beneath   the   Rocca  Bianca  massif.       Different   types   of   breccia   are   reported   in   the   Col   du   Longet   area   (Lemoine,   1967;   Gout,   1987   and   refs   therein).  The  significance  of  these  breccia  is  discussed  in  §8,  but  their  interpretation  and  their  assignment   to  either  continental  or  oceanic  units  is  dependant  on  the  structural  setting.  The  two  main  end-­‐members   are   (i)   thin   beds   of   siliciclastic   microbreccias   overlying   the   Permian   siliciclastic   and   volcaniclastic     8   formations   of   the   AL   unit,   with   minor   occurrence   of   dolomitic   clasts   indicating   a   post-­‐Triassic   age.   (ii)   mixed   siliciclastic/carbonate   megabreccia   containing   metric   to   decametric   blocks   of   Triassic   dolostones   together  with  reworked  Permian  clasts  of  various  size.  They  occur  as  lenses  or  slices  above  the  Acceglio-­‐ Longet   unit   or   within   the   base   of   the   Schistes   Lustrés   oceanic   units   (grey,   fig.4).   The   first   type   (i)   is   stratigraphically   linked   with   the   AL   unit,   and   must   be   Jurassic   to   early   Cretaceous   in   age   because   it   is   overlain  by  a  Late  Cretaceous  hard-­‐ground  (Lemoine,  1960a).  The  second  type  (ii)  is  associated  with  late   Jurassic-­‐Cretaceous,   strongly   folded   marbles   and   calcschists   whose   continental   or   oceanic   origin   is   hardly   distinguishable  (Gout,  1987).  However,  the  megabreccia  does  not  appear  to  contain  any  ophiolitic  clasts   and   is   dominated   by   reworking   of   Permian   or   Mesozoic   formations   found   in   the   AL   series.   This   megabreccia  is  comparable  to  the  Pelvo  d'Elva  breccia  (Michard,  1967;  Lefèvre  et  Michard,  1976)  which   occurs  10  km  to  the  SE  in  a  cover  thrust  sheet  pinched  in  between  the  AL  unit  and  the  oceanic  units  (fig.  4)   and  which  is  also  sourced  from  continental  margin  rocks  only  (Michard,  pers.  comm.  2016).  The  Longet   megabreccia  occurs   within  the  NW  equivalent  of   the   Pelvo   d'Elva   thrust   sheet   (c,  fig.  4),  and  its   present   situation  within  the  Schistes  Lustrés  can  be  explained  by  interference  between  N   to  NW  directed  tectonic   transport,   isoclinal   folding,   and   further   backfolding.   The   Longet   and   Pelvo   d'Elva   breccia   occurs   presently   in  the  normal  and  reverse  limbs  of  the  AL  backfold.   9  km  SW  of  Col  du  Longet,  near  Maljasset,  the  Ubaye  river  crosscuts  the  tectonic  boundary  between  the   continental   margin   (Roure)   and   oceanic   (Schistes   Lustrés)   units   (b,   fig.   3).   This   contact   is   presently   dipping   southwestwards   and   the   most   internal   Briançonnais   nappe   (Roure)   is   overriding   the   oceanic   units  due  to  backfolding.  The  asymmetry  of  the  minor  folds  associated  with  the  initial  stacking  indicate  an   apparent   southward   transport   (a,   fig.5,   present   structure),   which   is   inconsistent   with   the   other   regional   data.  However,  taking  into  account  the  overall  reversal  and  once  restored  from  backfolding,  it  becomes  N-­‐ NW  directed  (b,  fig.  5,  restored  structure),  similarly  as  in  Col  du  Longet  (see  §5,  site  39).           Figure  5:  Fold  interference  in  the  Alpet  area,  southern  side  of  the  Ubaye  valley  (location  fig.  1b).   a-­‐  Synthetic  aerial  view  towards  SW  of  the  backfolded  eastern  limit  of  the  Briançonnais  zone,  bounding  the   most   internal   Briançonnais   unit   (Roure   unit;   Le   Guernic,   1967)   above,   from   the   serpentinite-­‐bearing   oceanic   Schistes  Lustrés  nappes  below.  The  contact  in  reverse  polarity  is  affected  by  a  medium  scale  isoclinal  F1  fold   (red)  displaying  an  apparent  top-­‐to-­‐the-­‐south  asymmetry.   b-­‐   Once   restored   from   F2   backfolding   (red   unfolding   axis),   the   F1   asymmetry   gets   N-­‐   to   NW-­‐directed,   consistently  with  our  other  regional  observations.   c-­‐   Stereogram   (Wulff,   lower   hemisphere)   showing   the   unfolding   axis   (D2   fold),   with   present   (white)   and   unfolded  (black)  attitude  of  the  D1  microstructures  (fold  axix  and  intersection  lineations).  Site  n°44,  location   Table  I.       4.2-­‐Backfold  and  backthrust  overprint  structures  in  the  Piemont  units  east  of  Briançon  (c,  d,  e,  fig.  3)   The  Piemont  units  of  Rochebrune  (c,  fig.3)  and  Chaberton  are  thrust  eastwards  over  the  oceanic  blueschist   units,   composed   of   ophiolites   and   Mesozoic   metasediments   (Dumont   et   al.,   1984).   This   backward   thrusting  crosscuts  an  initial  stack  characterised  by  oceanic  units  resting  in  thrust  contact  with  underlying   Prepiedmont  units,  a  situation  visible  beneath  the  Chenaillet  ophiolitic  massif  (Gimont  tectonic  window;   Barféty   et   al.,   1995).   Some   ophiolite   bearing   slices   found   along   the   backthrust   beneath   the   Rochebrune     9   and   Chaberton   units   represent   remnants   of   the   sheared   backfold.   Further   east,   the   Piemont   units   outcrop   in  two  subcircular  tectonic  windows  surrounded  by  the  oceanic  metasediments  with  rare  ophiolitic  rocks,   the   Gran   Roc   and   the   Mte   Banchetta   windows   (d,   e,   fig.   3,   respectively).   These   continental   margin   units   suffered  deeper  burial  than  those  of  the  Rochebrune  and  Chaberton  units  (Caron,  1971),  and  we  interpret   their   high   elevation   (~3000m   and   ~2800m,   respectively)   as   dome   structures   resulting   from   large-­‐scale   interference  folding  similar  to  the  conceptual  sketch  of  fig.  3  (upper  sketch,  centre).  In  both  cases,  these   structures  are  markedly  asymmetric,  their  eastern  limb  being  steeper  or  overturned.     The   Gran-­‐Roc   window   (d,   fig.   3)   is   composed   of,   from   bottom   to   top,   Carnian   gypsum   (Megard-­‐Galli   &   Caron,   1972)   which   is   the   detachment   layer,   a   thick   pile   of   Norian   dolostones   (800m)   with   dolomitic   breccia   at   their   base   (Caron,   1971),   and   Rhaetian   to   Liassic   limestones   and   shales   typical   of   the   Prepiedmont  series  (Dumont,  1983).  The  western  side  of  the  massif  provides  a  natural  N-­‐S  cross  section   showing   an   asymmetric   anticline   cored   by   the   Carnian   evaporites.   This   structure   may   have   been   initiated   as   a   north-­‐verging   ramp   anticline   (fig.   6).   It   is   furthermore   possible   that   the   location   of   this   fold   was   initially   localised   by   a   pre-­‐existing   diapir,   because   of   the   apparent   truncation   of   dolostones   by   the   underlying  evaporites.         Figure  6:   Western   slopes   of   the   Gran-­‐Roc   massif,   a   tectonic   window   cored   by   a   Prepiedmont   upper   Triassic-­‐ lowermost   Jurassic   series   and   surrounded   by   oceanic   Schistes   Lustrés   nappes   (location   fig.   1b).   It   shows   asymmetric  anticlinal  bending  of  the  upper  dolomitic  layers,  likely  associated  with  a  NE-­‐directed  ramp  over   the  Carnian  evaporites  detachment  layer.  We  interpret  the  interruption  of    the  Prepiedmont  unit  towards  the   left   (NE)   as   a   result   of   hangingwall   cutoff   by   a   D1   NE-­‐directed   thrust   (cartoon).   The   eastern   side   of   the   massif,  behind  the  crest  (visible  on  fig.  7),  is  affected  by  D2  backfolding,  producing  the  rounded  shape  of  this   tectonic  window.     Further  east,  the  Mte  Banchetta  tectonic  window    (e,  fig.  3;  fig.  7)  is  also  cored  by  a  thick  pile  of  Triassic   dolostones,  locally  covered  by  alternating  carbonate  and  shale  which  have  Rhaetian  characteristics  (site   13,   §5,   and   eastern   slopes   of   Mte   Banchetta).   A   calcschist   formation   similar   to   the   "Lias   prépiémontais"   (Dumont,  1984)  is  exposed  to  the  SW  of  Mte  Banchetta  (b,  fig.  7;  Jouvent,  2017).  The  occurrence  of  these   three   diagnostic   lithologies   indicates   that   the   Mte   Banchetta   tectonic   window   is   underlain   by   a   Prepiedmont  Mesozoic  series  similarly  to  the  nearby  Gran  Roc  window.   A   ‘mixed’   megabreccia   occurs   between   these   continental   margin   formations   and   the   oceanic   units,   first   described  by  Caron  (1971),  whose  significance  will  be  discussed  in  §  8.3.  The  3D  map-­‐scale  geometry  of   the  Mte  Banchetta  window  is  consistent  with  an  interference  structure  (fig.  3,  e).  Similarly  to  the  Gran  Roc   window,   the   eastern   slopes   show   obvious   backward   fold-­‐and-­‐thrusting   of   oceanic   and   continental   unit   imbricates   (F2,   fig.   7),   a   feature   comparable   to   large-­‐scale   backfolding   and   backthrusting   at   the   eastern   edge  of  the  Rochebrune  and  Chaberton-­‐Grande  Hoche  Prepiedmont  units  (Dumont  et  al.,  1984;  Tricart  &   Sue,   2006).   To   the   N-­‐NW   of   Mte   Banchetta,   the   Prepiedmont   unit   is   buried   beneath   the   oceanic   metasediments  with  no  evidence  of  any  normal  fault.  We  explain  this  structure  by  the  occurrence  of  a  N-­‐   to   NW-­‐overturned   anticline   involving   both   the   continental   and   oceanic   units   and   interfering   with   backfolds  to  produce  the  circular-­‐shape  window.  Small  scale  folds  consistent  with  this  interpretation  are   observed  (sites  13  and  14,  §  5).           10        Figure   7:   Mte  Banchetta  interference  structure,  near  Sestriere  ski  resort  (location  fig.  1b):  the  continental   margin   series   (Prepiedmont   nappe)   is   overlain   by   a   mixed   megabreccia   described   in   fig.   14,   and   outcrop   in   a   dome-­‐shape  tectonic  window  surrounded  by  the  oceanic  Schistes  Lustrés  nappes.   Upper  part:  Simplfied  geological  map  and  satellite  view  drapped  over  DEM,  perspective  view  towards  SW.     Lower  part:  schematic  cross-­‐section  trending  subparallel  to  D1  tectonic  transport,  reconstructed  before  D2   backfolding,   illustrating   our   interpretation   of   the   northward   interruption   of   Prepiedmont   outcrops   as   a   result  of  top-­‐to-­‐the  north  hangingwall  cutoff  by  a  D1  N-­‐NW  directed  blind  thrust,  similarly  as  the  Gran-­‐Roc   structure  (fig.  6).       4.3  Hanging-­‐wall  cutoff  in  the  Piemont  nappes  (f,  fig.  3)   To   the   East   of   Briançon   city,   the   Montgenèvre   pass   is   located   on   a   Piemont   nappe  which  is  thrust  over  the   Briançonnais   stack   (a,   fig.   8).   This   Piemont   nappe   is   detached   beneath   the   Norian   dolostones.   The   complete   thickness   of   the   Norian   formation   (800m)   is   present   in   the   Janus   massif   to   the   S   (b,   fig.   8),   but   it   decreases  progressively  northward  to  ~300m  (Chalvet)  as  a  result  of  basal  truncation  by  a  thrust  ramp,   and   the   hangingwall   cut-­‐off   reaches   the   Rhaetian   and   the   Jurassic   fm.   further   to   the   north   (fig.   8).   The   orientation   of   this   cut-­‐off   outcropping   on   both   sides   of   the   Chalvet   ridge   is   ENE-­‐WSW.   The   deformation   of   Liassic  strata  in  the  hangingwall  (Col  de  la  Lauze,  e,  fig.  8)  consists  of  ~N80°,  top-­‐to-­‐the  N  trending  folds,     11   whereas   stretching   lineations   in   the   footwall   (Clot   Enjaime,   d   fig.   8)   are   trending   ~N170°.   Thus,   the   large-­‐ scale  structure  and  the  outcrop-­‐scale  deformation  consistently  indicate  a  top  N  to  NW  tectonic  transport   of  the  Piemont  units  over  Internal  Briançonnais  units  of  Clot  Enjaime-­‐Alpet-­‐Rio  Secco,  which  are  in  turn   resting   structurally   on   the   classical   Briançonnais   footwall.   This   relatively   early   thrust   stacking   is   overprinted   by   both   backfolding/backthrusting,   duplicating   the   Piemont   units   in   this   area   (Chalvet   and   Chaberton  units,  separated  by  the  NS  trending  Rio  Secco  syncline),  and  by  recent  extensional  faulting  (b,   fig.  8).     The  Chaberton  unit  itself  terminates  northwards,  possibly  due  to  a  similar  hangingwall  cut-­‐off  truncation   of  the  Norian  dolostones  near  the  Acles  pass  (site  7,  §  5).  Here  the  overlying  Rhaetian-­‐Liassic  strata  are   affected  by  ~EW  trending  deformation  similar  to  the  area  of  Col  de  La  Lauze.         Figure  8:   Fold-­‐thrust  interference  structures  in  the  Chaberton  and  Chalvet  areas,  north  of  Montgenèvre  pass   (location  fig.  1b).  This  area  shows  Prepiedmont  series  displaying  complexe  tectonic  relationships  with  both   the   Briançonnais   continental   margin   units   and   the   Shistes   Lustrés   oceanic   nappes,   due   to   interaction     12   between   two   subperpendicular   fold-­‐thrust   deformations   D1   and   D2.   The   southern   (left)   part   of   the   block-­‐ diagram   shows   the   Chenaillet   ophiolitic   massif   downthrown   along   a   major   late-­‐orogenic   normal   fault   (Tricart  &  Sue,  2006).   a-­‐   Simplified   geological   map   and   satellite   view   drapped   over   the   DEM,   perspective   view   towards   W.   The   Chaberton-­‐Grande   Hoche   massif   is   backthrust   over   the   oceanic   Schistes   Lustrés   units   (green)   and   affected   by   D2   backfolding.   By   contrast,   the   Chalvet   massif   preserves   more   or   less   the   initial   D1   stack   since   the   Prepiedmont  unit  overlains  the  Internal  Briançonnais  unit  of  Rio  Secco  (§  4.3)  through  a  D1  thrust.   b-­‐   Panoramic   view   of   the   Chalvet   area   towards   the   W-­‐SW,   from   the   top   of   Chaberton,   showing   the   ramp   geometry  of  this  thrust,  with  basal  truncation  and  hangingwall  cutoff  of  the  Prepiedmont  Norian  dolostones   from  S  to  N  (right).     c-­‐  Schematic  cross-­‐section  trending  subparallel  to  D1  tectonic  transport,  more  or  less  corresponding  to  the   present  outcrops  with  the  exception  of  late  orogenic  normal  faulting  (white).     d  and  e-­‐  Kinematic  evidence  for  N-­‐directed  transport  along  the  D1  thrust  at  the  bottom  of  the  Prepiedmont   Chalvet  unit  (folds,  cleavage  and  stretching  lineation;  Wulff  stereograms,  lower  hemisphere).  Location  in  figs.   a  to  c  above  (respectively  sites  12  and  11,  fig.  10,  table  I).     4.4-­‐Tectonic  cover  of  the  Ambin  massif  (g,  fig.  3)   The   Ambin   internal   crystalline   massif   is   a   20   km-­‐wide   dome   surrounded   by   the   "Schistes   lustrés"   oceanic   units  (fig.  1,  fig.  9).  It  has  a  Variscan  metamorphic  crystalline  core  showing  evidence  of  north-­‐vergent  HP   early  Alpine  deformation  overprinted  by  retromorphic  E-­‐directed  shear  (Clarea  nappe,  Ganne  et  al.,  2004   and  refs.  therein).           Figure   9:   Nappes  structure  and  tectonic  marks  on  top  of  the  Ambin  dome-­‐shape  massif,  east  of  Bardonecchia   city  (location  fig.  1b),  cored  by  Variscan  basement  and  late  Variscan  clastics  (Clarea  Group  and  Ambin  Group,   respectively;  Ganne  et  al.,  2004).     a-­‐   Schematic   section   across   Ambin   and   the   Maurienne   valley,   illustrating   on   top   of   the   dome   the   tectonically   reduced   character   of   both   the   Mezozoic   sedimentary   cover   and   the   tectonic   cover   by   the   Briançonnais-­‐ Prepiedmont  nappes  beneath  the  oceanic  Schistes  Lustrés  nappes  (green).       b-­‐   Block   diagram   combining   the   southern   part   of   section   a   with   relief,   perspective   view   towards   W,   with   location  of  critical  observations  to  the  W  of  section  a  (rectangle,  section  c  below).   c-­‐   Detailed   section   on   top   of   the   Ambin   dome,   showing   scarse   remnants   of   the   Triassic   dolomitic   cover   of   the   Ambin   Group,   preserved   beneath   a   northward   pinching   stack   of   Briançonnais   and   Prepiedmont   nappes     13   overlain   by   the   Schistes   Lustrés.   This   preservation   probably   occurred,   in   spite   of   intense   northward   shear,   thanks  to  a  pre-­‐orogenic  normal  fault.     d-­‐   Kinematic   evidence   for   N-­‐directed   tectonic   transport   in   the   Triassic   cover   beneath   the   nappes   (D1   intersection  lineation,  present  attitude  and  restored  from  wesward  folding;  site  3,  fig.  10,  table  I).       This  core  is  covered  by  a  nappe  stack  including,  from  bottom  to  top,  late  Paleozoic  volcaniclastics  ("Ambin   group",  or  Ambin  nappe),  Mesozoic  continental  derived  units  (Briançonnais,  Piemont),  and  oceanic  units   (Schistes  Lustrés).  The  Mesozoic  cover  nappes  pinch-­‐out  northwards  over  the  Ambin  nappe  on  top  of  the   dome,   as   shown   in   a   N-­‐S   cross   section   (fig.   9a,   9c;   Jouvent,   2017).   The   culmination   coincides   with   a   shortened   fault   block   with   locally   preserved   Triassic   strata,   truncated   by   the   cover   nappes.   The   lower   Triassic   quartzites   in   the   footwall   of   the   nappes   display   evidence   of   north-­‐directed   shear   (stratification/cleavage  relationships,  fig.  9d;  site  3,  §  5).  The  top  of  the  Ambin  nappe  displays  a  regional   angular   unconformity,   with   northward-­‐younging   middle   Triassic   carbonate   strata   beneath   it,   towards   a   more   complete   Briançonnais   series   (Bellecombe   area,   Jaillard,   1989).   In   contrast,   towards   the   south,   an   extremely   reduced   Mesozoic   cover   overlies   the   lowest   Triassic   to   late   Paleozoic   clastics   (Polino   et   al.,   2002),   as   usually   observed   in   the   Internal   Briançonnais   domains.     This   regional   unconformity   together   with  the  fault  block  (fig.  9a,  9c)  suggest  that  the  location  of  the  Ambin  dome  is  partly  inherited  from  the   Mesozoic   passive   margin   history,   and   that   the   northward   propagation   of   the   early   Alpine   nappes   reactivate  a  transverse  (~EW)  uplifted  rift  structure.       5-­‐Outcrop-­‐scale  structural  record  of  multistage  kinematics  in  the  Penninic  nappes   of  the  Western  Alps      5.1-­‐  Literature  review   A   synthetic   outcrop-­‐scale   analysis   in   the   southwestern   part   of   the   western   Alpine   arc   (Tricart,   1980)   demonstrated  the  occurrence  of  three  deformation  episodes  in  the  Briançonnais  zone.  The  second  and  the   third  phases  (D2,  D3)  correspond  respectively  to  outward  thrusting  of  the  nappe  stack  associated  with  the   activation   of   the   Penninic   thrust,   and   to   inward   (backward)   fold-­‐and-­‐thrusting.   Both   are   kinematically   linked   with   the   formation   and   the   propagation   of   the   Western   Alps   arc,   initiated   approximately   at   the   Eocene-­‐Oligocene   boundary   (Dumont   et   al.,   2011).   D2   and   D3   sensu   Tricart   (1980)   are   overlapping   in   time  because  they  occurred  on  both  sides  of  the  "Briançonnais  fan"  whose  axis  propagated  outwards.  D1   corresponds   to   the   initial   underthrusting   of   the   Briançonnais   and   Prepiedmont   domains   beneath   the   Ligurian-­‐Piemont   oceanic   accretionary   wedge.   It   was   previously   regarded   as   late   Cretaceous   (Caron,   1977;  Tricart,  1980).  A  late  Cretaceous  age  was  assigned  to  the  Eoalpine  orogeny  (85-­‐60  My;  Hunziker  et   al.,  1992,  and  refs   therein;   Dal  Piaz  &  Lombardo,  1985;   Polino  et  al.,  1990)   based   on  K-­‐Ar   dating,  which   is   now   recognised   to   have   involved   substantial   uncertainty   and   scattering   due   to   excess   argon   analytical   problems.   HP   radiometric   ages   were   still   controversial   in   the   1980's   (Lemoine   et   al.   1984)   but   were   progressively  refined  to   Cenozoic  (65-­‐38  Ma;  Liewig  et  al.,  1981;  Takeshita  et  al.,  1994).  From  a  structural   point  of  view,  the  discrepancies  in  the  orientation  of  early  small-­‐scale  structures  with  respect  to  younger   deformation   phases   was   reported   by   several   authors   (Vialon,   1966;   Caron,   1973,   1974;   Caby,   1973;   Malavieille,   1982;   Mahwin   et   al.,   1983;   Platt,   1989).   This   well-­‐known   feature   has   been   interpreted   in   different   ways,   namely   as   Variscan   inheritance   (Vialon,   1966),   resulting   from   shear   coeval   with   block   rotation  (Caron,  1974),  as  a  rotation  of  early  structures  (Boudon  et  al.,  1976),  or  as  change  in  deformation   regime  (Malavieille,  1982).  Conversely,  Caby  (1973,  1975)  interpreted  the  transverse  trends  of  the  early   Alpine   deformation   phase   to   result   from   northward   translation   of   the   orogenic   wedge,   having   preceded   the   development   of   the   arc   curvature.   A   northward   translation   of   the   early   Alpine   wedge   was   also   promoted   by   Maury   &   Ricou   (1983)   and   Ricou   &   Siddans   (1986),   and   later   documented   by   Schmid   &   Kissling   (2000)   and   Ceriani   et   al.   (2001).   The   latter   publication   reported   initial   N   to   NW-­‐directed   tectonic   transport  in  the  Internal  nappe  stack  using  their  microstructural  signature  in  the  vicinity  of  the  Penninic   thrust.       5.2-­‐  Field  data  and  structural  interpretation   Here   we   present   a   comprehensive   field   survey   of   small-­‐scale   superposed   fold   structures   which   affect   various  stratigraphic  layers  of  the  late  Paleozoic-­‐Mesozoic  series,  over  >60km  along  the  southern  part  of   the   Internal   arc   of   the   Western   Alps   (fig.   10;   fig.   11;   table   I).   Most   of   the   55   sites   are   located   in   the   Briançonnais   zone,   and   some   are   within   the   oceanic   units   on   both   sides   of   it.   The   deformation   criteria   measured   are   outcrop-­‐scale   axes,   intersection   lineations   and   stretching   lineations.   At   each   site,   we   identified  the  pro-­‐  and/or  retro-­‐verging  deformations  whose  trends  are  broadly  following  the  shape  of  the   arc,   and   we   restored   (unfolded)   the   preceding   tectonic   features   which   generally   consist   of   folds   and     14   stratification/schistosity   intersection   lineations.   Compared   to   the   chronology   of   Tricart   (1980),   the   partly   overlapping  outward  and  inward  deformation  episodes  referred  to  as  D2  and  D3,  respectively,  are  merged   in  D2  in  this  study,  which  aims  to  focus  on  the  early  part  of  deformation  history.  D2  outward  or  inward   structures  are  used  to  restore  the  previous  kinematic  indicators,  assigned  to  D1  deformation.               15        Figure   11:   Microstructural   data   of   the   55   sites   (location   table   I)   investigated   in   the   Briançonnais/Prepiedmont  zones  and  nearby  in  the  Embrunais-­‐Ubaye  nappes  and  the  Schistes  Lustrés.  Red   and   green   dots   represent   the   forward   and   backward   late   (D2)   fold   trends,   respectively.   Black   dots   are   the   early   (D1)   trends,   and   grey   dots   represent   the   latter   restored   from   D2   deformation   (where   needed).   The   median  trends  at  each  site  are  reported  on  fig.  10.         16   As   an   example,   a   description   of   site   48,   located   in   the   External   Briançonnais,   is   provided   in   fig.   12   (localisation  fig.  10  &  table  I).  Here,  the  initial  contact  between  the  overthrust  oceanic  sediments,  namely   the   Helminthoid   Flysch   nappe   composed   of   early   late   Cretaceous   purple   shales,   and   an   underlying   continental   margin   Briançonnais   unit   (Sautron   nappe,   Gidon   et   al.,   1994)   is   observed.   This   contact   is   affected   by   top-­‐to-­‐the   SW   shear   deformation,   folding   and   thrusting,   in   the   footwall   of   the   Brec   de   Chambeyron   Briançonnais   unit   (a,   fig.   12).   SW-­‐verging   deformation   (D2,   fig.   12)   is   disharmonic,   characterised   by   large   wavelength   folds   in   the   Triassic   dolomitic   limestones   of   the   upper   unit,   and   by   tighter   folding   which   displays   shorter   wavelength   in   the   Cretaceous-­‐Paleogene   calcschists   (b,   fig.   12).   These  ‘D2’  folds  at  different  scales  trend  parallel  to  the  Briançonnais  zone  in  this  part  of  the  arc  (fig.  10).   They   overprint   an   older   lineation,   trending   approximately   perpendicular   to   D2   fold   trends,   which   is   systematically   observed   in   the   calcschists   (c,   fig.   12)   beneath   the   oceanic   nappe.   This   lineation   consists   of   intersection  between  stratification  and  a  pre-­‐D2  schistosity  named  D1,  because  it  is  affected  by  D2  folds   (c,   fig.   12,   and   stereogram).   Schistosity   S1   is   subparallel   to   stratification,   and   isoclinal   D1   microfolds   trending  parallel  to  D1  lineation  are  also  observed.           Figure  12:   Field   example   of   D1/D2   microstructural   interference   in   the   external   Briançonnais   zone,   south   of   the  Ubaye  valley  (Stroppia  pass  near  Fouillouse;  site  48,  location  fig.  10  and  table  I).     a-­‐   Overall   outcrop   view   towards   NW,   showing   the   early   (D1)   initial   stack   with   the   oceanic   Helminthoid   Flysch  nappe  overlying  the  Briançonnais  Chambeyron  nappe  of  continental  origin.  This  early  thrust  (black)  is   crosscut  by  top-­‐to-­‐the  SW  D2  thrusts  (red).  The  stereogram  shows  that  D1  folds/intersection  lineations  and   S1  schistosity  are  scattered  and  deformed  by  D2  folding,  both  beeing  perpendicular  (projection).  D1  and  D2   folds  are  associated  with  NW-­‐directed  and  SW-­‐directed  tectonic  transport,  respectively.   b-­‐  Detail  of  D2  folds  associated  with  mesoscale  folding  of  D1  thrust.     c-­‐  Close  view  of  D1  intersection  lineation  and  microfolds  involved  by  perpendicular  D2  folds,  corresponding  to   the  stereogram  above.     17      In   order   to   provide   a   map   synthesis   of   similar   observations   across   the   study   area   (fig.   10;   fig.   11),   we   projected   horizontally   at   each   site   the   mean   orientation   of   D2   fold   trends,   which   may   be   forward   or   backward   oriented   (red   or   green,   respectively),   and   the   mean   orientation   of   D1   lineations   or   fold   axis,   once  unfolded  from  D2  deformations.     The   map   shows   a   striking   consistency   of   restored   D1   trends   across   the   southern   part   of   the   Alpine   arc.   Top-­‐to-­‐the  north  shear  sense  is  observed  in  many   sites,  either  indicated  by  fold  asymmetry  or  by  angular   relationships  between  stratification  and  cleavage.  The  occurrence  of  fold  trends  transverse  to  the  western   Alpine  chain  has  long  been  recognised  (Vialon,  1966;  Bertrand,  1968  and  refs  therein;  Caby,  1973,  1975;   Tricart   &   Schwartz,   2006),   but   has   been   differently   interpreted:   either   as   an   evidence   of   N-­‐S   shortening   and   N-­‐directed   transport   (Caby,   1973;   Tricart   &   Schwartz,   2006),   consistent   with   N-­‐S   HP   transport   identified   through   high-­‐pressure   stretching   lineations   in   the   internal   crystalline   massifs   (Choukroune   et   al.,  1986;  Ganne  et  al.,  2004;  Le  Bayon  &  Ballèvre,  2006;  Strzerzyski  et  al.,  2011;  Scheiber  et  al.,  2013),  or   as  more  or  less  parallel  to  tectonic  transport,  thus  oriented  radially  towards  the  exterior  of  the  arc  (Caron,   1974;   Malavieille   &   Etchecopar,   1981;   Malavieille   et   al.,   1984;   Philippot,   1990).   We   argue   that   these   transverse   trends   can   be   regarded   as   approximately   perpendicular   to   the   ‘D1’   transport   directions   of   nappes  during  early  stacking  stages,  like  ‘D2’  trends  are  for  the  late  stages.     The   D1   structures   are   regionally   consistent,   and   are   locally   observed   immediately   beneath   the   base   of   the   oceanic   nappes   (i.e.   fig.   10,   sites   2,   3,   7,   13,   20,   48).   Thus   this   widespread   deformation   is   most   likely   associated   with   the   early   Alpine   collision   and   with   the   emplacement   of   the   oceanic   accretionary   wedge   over   the   distal   continental   margin   units.   Like   in   Provence,   the   Briançonnais   domain   must   have   been   affected  by  older  compressional  deformation  linked  with  the  Pyrenean  orogeny,  but  there  is  no  evidence   of  deep  burial  older  than  early  Eocene  and  these  surficial  structures  were  probably  largely  overprinted  by   the  D1  deformation.       We  document  the  superposition  of  two  deformation  stages  which  can  be  identified  by  crossing  small-­‐scale   structures   over   the   whole   area,   regardless   of   the   metamorphic   grade,   from   blueschist   facies   in   eastern   domains   to   sub-­‐greenschist   facies   in   the   Embrunais-­‐Ubaye   areas.   Moreover,   we   observe   similar   superposed   deformations   at   different   scales,   from   kilometric   to   outcrop.   Many   examples   of   large-­‐scale   interference  folds  have  been  reported  in  the  Internal  Westen  Alps  (Platt  et  al.,  1989;  Steck,  1998;  Ganne,   2003;   Le   Bayon,   2005).   These   observations   preclude   models   involving   radial   outwards   orientations   of   tectonic   transport   constant   through   time,   which   in   addition   would   make   restorations   impossible   in   the   core  of  the  arc.  We  propose  that  the  widespread  "transverse"  D1  deformation  trends  result  from  an  early   N  to  NW  directed  transport  and  stacking  stage,  whose  propagation  appears  presently  "longitudinal"  to  the   southern   part   of   the   Western   Alpine   arc.   The   restored   D1   trends   (fig.   10)   show   a   slight   discrepancy   in   orientations   from   N   (Maurienne   vally)   to   S   (Ubaye   valley),   which   suggests   that   these   trends   were   distorted  during  the  formation  of  the  arc.  Considering  that  the  formation  of  the  arc  is  associated  with  the   activation   of   the   post-­‐nappe   Penninic   thrust   (Dumont   et   al.,   2011),   and   with   the   pro-­‐   and   retro-­‐D2   structures,  such  a  distortion  is  expected  from  older  (D1)  structures  (Caby,  1973).     Following   our   interpretation   derived   from   small-­‐scale   observations,   the   most   important   nappe   displacements   are   expected   to   have   occurred   during   the   early   D1   stacking   stage,   whereas   D2   thrusts   mostly  consist  of  high-­‐angle,  post-­‐nappe  structures  having  accommodated  vertical  extrusion  on  both  sides   of   the   Briançonnais   zone   (fig.   2).   If   so,   the   internal   zones   in   the   Western   Alpine   arc   should   display   significant  along-­‐strike  changes  as  relicts  of  early  stacking  process,  which  is  exemplified  in  the  following.       6-­‐Along-­‐strike  variations  in  the  structure  and  internal  composition  of  nappes:     Our   structural   arguments   presented   in   the   previous   sections   show   that   the  D1   early   nappe   structures   are   crosscut  by  D2  folds  an  thrusts,  which  follow  the  shape  of  the  arc,  both  at  an  outcrop  scale  and  at  a  km   scale.  Since  the  formation  of  the  arc  is  a  recent  feature,  similar  oblique  crosscuting  relationships  should  be   observed  at  a  map  scale  as  well.  Such  obliquity,  which  may  also  result  from  paleogeographic  inheritance,   can   be   documented   by   variations   along   the   strike   of   the   major   D2   structures,   that   is   along   the   different   zones  following  the  arc  from  S  to  N.  To  illustrate  this,  we  provide  examples      from  surface  geology  which   demonstrate   that   the   structure   of   the   Western   Alpine   arc   is   not   concentric   in   map   view.   Along-­‐strike   variations  can  be  detected  within:       6.1-­‐  The  stratigraphy  and  structure  of  the  "zone  houillère"  in  the  central  part  of  the  arc     The  "Zone  houillère"  around  the  city  of  Briançon  and  further  north  is  composed  of  detrital  sedimentary   sequences  reaching  a  total  thickness  of  about  2,5  km  (Fabre,  1961;  Feys  1963).  It  shows  two  superposed   main  units  separated  by  the  Drayères  thrust  oriented  SW-­‐NE  (Fabre  et  al.,  1982;  Caby,  1996;  Barféty  et  al.,     18   2006;  Lanari  et  al.,  2012).  The  lower  unit  is  found  to  the  N-­‐NE  of  the  study  area,  from  southern  Vanoise   massif  to  northern  Briançonnais,  and  includes  upper  Westphalian  (C  and  D  members)  to  lower  Stephanian   clastic   sequences   (Tarentaise   fm.,   Fabre,   1961;   Schade   et   al.,   1985)   overlain   by   a   thick   volcano-­‐ sedimentary  lower-­‐middle  Permian  series  (Ponsonnière  area).  The  upper  unit,  dominantly  outcropping  to   the  S-­‐SE,  near  Briançon  and  further  south,  contains  fining  upwards  Namurian  (south  of  Briançon)  to  lower   Westphalian   (A   member)   clastics   and   coal   measures,   unconformably   overlain   by   middle   Stephanian   and/or  upper  Permian  coarse  clastics  (Barféty  et  al.,  1995).  These  two  units  were   possibly   stacked  during   the  latest  Variscan  orogenic  events  because  both  are  unconformably  overlain  by  the  western  Briançonnais   Mesozoic  cover  in  the  Cerces-­‐Grand  Area  area  (Barféty  et  al.,  2006).  They  have  been  subsequently  affected   by   Alpine   folds   and   dominantly   east-­‐verging   thrusts   (Fabre   et   al.,   1982)   so   that   the   upper   unit   is   split   into   several  slices.  However,  it  is  quite  clear  that  this  upper  unit,  found  in  the  southern  area,  is  less  complete   than  the  lower  one  further  north,  due  to  an  erosional  gap  of  upper  Westphalian-­‐lower  Stephanian  layers   unconformably  overlain  by  middle  Stephanian  clastics.  This  southward  increasing  truncation  of  the  infill   of   the   late   Carboniferous   basin,   also   illustrated   by   Mercier   and   Beaudouin   (1987)   and   Desmons   and   Mercier   (1993),   shows   that   its   southern   margin   has   been   deformed   and   uplifted   in   relation   to   late   Variscan   events.   Consistently,   a   higher   late   Carboniferous   subsidence   rate   affected   the   northern   part   of   the   Zone   Houillère   (Manzotti   et   al.,   2014).   The   observed   fluvial   drainage   patterns   in   late   Carboniferous   formations   are   dominantly   northward   directed   (Mercier   &   Beaudouin,   1987;   Barféty   et   al.,   1995)   and   braided   systems   developed   in   the   vicinity   of   source   areas   located   to   the   south   (Manzotti   et   al.,   2014).   Thus,   in   spite   of   the   dominantly   N-­‐S   trend   and   the   narrow   width   of   the   presently   outcropping   Zone   Houillère,   there   are   significant   latitudinal   changes   in   its   stratigraphy   and   structure,   suggesting   that   the   initial  trend  of  the  late  Variscan  basin  was  markedly  oblique  with  respect  to  later  Alpine  shortening.         6.2-­‐  The  "Permo-­‐Carboniferous  axial  zone"  in  the  southern  part  of  the  arc     The   southernmost   constraints   on   the   Carboniferous   series   are   found   in   a   tectonic   window   across   the   Ubaye   valley,   within   the   lowermost   nappe   of   the   external   Briançonnais   stack   (Gidon   et   al.,   1994),   structurally  equivalent  to  the  «  zone  houillère  »  unit  near  Briançon.  Further  SE,  the  Paleozoic  formations   are  only  represented  by  thick  Permian  volcanics  and  volcaniclastics,  outcropping  in  both  the  southeastern   extension  of  this  stack,  named  «  Permo-­‐Carboniferous  Axial  zone  »  (PCAZ)  by  Lefèvre  (1982),  and  in  the   more   internal   «  Roure-­‐Acceglio  »   zone   (RAZ).   The   PCAZ   and   the   RAZ   are   separated   by   a   major   tectonic   contact  post-­‐dating  the  initial  nappe  stacking  and  having  possibly  accommodated  left-­‐lateral  displacement   of  the  Roure-­‐Acceglio  units,  which  are  truncated  and  pinch-­‐out  SE-­‐wards  along  it  (Preit  fault  zone,  Lefèvre   1984).   A   significant   part   of   these   southeastern   Permian-­‐dominated   units,   especially   in   the   PCAZ,   are   devoid  of  their  Mesozoic  sedimentary  cover  for  tectonic  reasons.  This  detached  cover  may  correspond  in   part   to   the   Mesozoic   nappe   stack   developed   further   N   in   the   region   of   Briançon   above   the   «  Zone   Houillère  ».   Another   part,   mostly   observed   in   the   RAZ,   bears   evidence   of   Mesozoic   erosional   truncation   having   removed,   at   least   in   part,   the   Triassic   carbonate   and   siliciclastic   series   (Debelmas   &   Lemoine,   1957;   Michard,   1959).   The   gap   is   increasingly   important   from   the   lower   unit   (core   of   the   Acceglio   anticline)  to  the  upper  one  (Pelvo  d’Elva  nappe,  Lefevre  &  Michard  1976),  and  from  SE  to  NW  within  the   Pelvo   d'Elva   nappe   (Lefevre,   1982):   the   Jurassic   to   Cretaceous   strata   rest   over   lower   Triassic   quartzites   near  Acceglio  (Lefevre,  1962)  and  over  Permian  volcanoclastics  near  Col  du  Longet  (Lemoine,  1960a).       6.3-­‐  The  most  internal  continental  margin  units       The   units   named   "Prepiedmont",   (Lemoine   et   al.,   1978)   are   located   at   the   eastern   border   of   the   Briançonnais   zone   (fig.   1).   Around   the   latitude   of   Briançon,   they   are   detached   along   the   Carnian   evaporites   and   thus   only   composed   of   upper   Triassic   and   more   recent   sediments   (fig.   6).   The   basal   thrust   ramps   up   into   the   Jurassic   section   in   one   northwestern   location   (Chalvet,   fig.   8).   These   units   do   not   outcrop   further   north   than   the   Maurienne   valley,   with   the   exception   of   the   Grande   Motte   unit,   whose   paleogeographic   origin   is   debated   (Deville,   1986,   and   refs   therein).   To   the   southeast,   in   contrast,   these   units   are   complemented   by   older   strata,   due   to   a   lower   position   of   the   detachment   in   the   stratigraphic   section.   In   Valgrana   (fig.   1;   Michard,   1961a,   1961b,   1967;   Megard-­‐Galli   &   Baud,   1977),   the   Triassic   sequence  of  the  Prepiedmont  series  include  paleontologically  dated  middle  Triassic  strata  overlain  by  late   Triassic  and  early  Jurassic  formations  dated  by  Franchi  (1898),  with  a  total  thickness  of  1  km  to  1.5  km   (unit  III  of  Michard,  1967).  The  thick  middle  Triassic  succession  found  in  Valmaira  can  also  be  regarded  as   a   Prepiedmont   unit   (unit   I   of   Michard,   1967).   Further   southeast,   in   the   Ligurian   Alps,   the   Prepiedmont   units  include  late  Paleozoic  formations  and  even  polymetamorphic  basement  (Vanossi,  1991;  Seno  et  al.,   2004;  Decarlis  et  al.,  2017).  This  shows  that  the  detachment  of  the  Prepiedmont  units  climbs  up  section   from  southern  regions  (Liguria,  Valgrana)  towards  the  north-­‐northwest  along  the  central  part  of  the  arc   (Cottian  Alps).       19      6.4-­‐  The  Valaisan  domain     The   occurrence   of   an   oceanic   domain   sutured   between   the   Briançonnais   zone   and   the   external   zone,   together   with   its   age   of   opening   and   closure,   remains   a   topic   of   strong   debate   (Stampfli   et   al.,   2002;   Bousquet   et   al.,   2002;   Beltrando   et   al.,   2007;   Masson   et   al.,   2008;   Loprieno   et   al.,   2011;   Beltrando   et   al.,   2012;   De   Broucker   et   al.,   2021).   One   reason   underpinning   this   famous   Alpine   controversy   is   the   lack   of   evidence   for   any   continental   breakup   south   of   the   Maurienne   valley   and   west   of   the   Briançonnais   zone,   which  is  a  major  difference  between  the  northern  and  southern  parts  of  the  arc.  A  possible  solution  is  to   consider   a   southwestward   transition   from   an   oceanic   domain,   represented   in   eastern   Switzerland   and   possibly  floored  by  exhumed  subcontinental  mantle  (Manatschal  et  al.,  2006;  Ribes  et  al.,  2020;  LeBreton   et   al.,   2021)   to   a   more   or   less   attenuated   continental   crust   domain   pinching   out   towards   the   Vocontian   basin,  SE  France  (Dumont  et  al.,  2012).  The  disappearance  of  the  Valaisan  zone  towards  the  southern  part   of  the  western  Alps  arc  would  then  occur  for  paleogeographic  reasons.  To  the  N(W)  of  the  Briançonnais   domain,   the   occurrence   of   a   transitional   paleogeographic   domain   towards   this   thinned   crust   area   is   testified   by   pinched   units   bearing   specific   "intermediate"   Mesozoic   stratigraphic   signatures,   frequently   assigned  to  "subbriançonnais",  all  along  the  arc  (Galster  et  al.,  2010;  Ceriani  et  al.,  2001;  Maury  &  Ricou,   1983;   Barbier,   1963;   Barale   et   al.,   2017).   This   transition   would   have   been   incorporated   within   the   western   Alpine   arc,   creating   complex   lateral   relationships   between   the   Helvetic-­‐Dauphinois,   Valaisan,   Subbriançonnais  and  Provençal  units,  as  an  expression  of  the  lateral  paleogeographic  transitions  around   the  scissors-­‐shape  Valaisan  incipient  breakup.       These   lateral   variations   within   the   internal   nappe   stack   of   the   Western   Alps   arc   demonstrate   that   its   structure   is   not   concentric,   and   that   radial   profiles   are   not   equivalent   laterally   and   should   not   be   considered   as   the   optimal   visualisation   of   a   complex   kinematic   evolution   during   the   Cenozoic.   These   features  can  be  due  to  the  obliquity  of  the  orogenic  wedge  with  respect  to  the  paleogeographic  trends,  but   can   also   be   explained   by   oblique   to   subperpendicular   crosscutting   of   the   early   orogenic   nappe   stack   by   recent  (Oligo-­‐Miocene)  collision  driven  by  westward  extrusion  (Dumont  et  al.,  2012).  The  superposition   order  of  structural  units  is  a  key  argument  for  restoration,  but  in  radial  profiles  this  order  is  frequently   disturbed   and   modified   by   recent   outward   "out-­‐of-­‐sequence"   thrusting,   i.e.   in   the   Guil   valley   (Claudel   &   Dumont,  1999).  The  knowledge  of  the  structural  geometry  of  the  initial  nappe  stack  emplaced  during  the   early  stages  of  thrusting  is  thus  a  key  criterion  for  a  reliable  restoration.       7-­‐  Sequence  and  superposition  of  nappes  in  the  early  orogenic  stages     Since   the   building   of   the   Internal   Zones   is   polyphase,   our   aim   in   this   section   is   to   enlight   some   characteristic   large-­‐scale   structural   features   associated   with   the   early   stacking   phase   (D1),   with   due   consideration   of   their   subsequent   modification   by   younger   deformation   pulses   (D2).   This   assists   reconstruction  of  the  continental  subduction  wedge  large-­‐scale  geometry  during  the  early  orogenic  stages,   as  discussed  in  §  9.2.       7a-­‐  Upper  part  of  the  continental  subduction  wedge,  overlain  by  oceanic  accretionary  units:   A   key   feature   for   the   understanding   the   structure   of   the   internal   Alpine   nappe   stack     is   the   distinction   between   ophiolite   bearing   series   and   Triassic-­‐soled   series,   now   referred   to   as   oceanic   and   continental   margin   units,   respectively.     This   was   recognized   several   decades   ago   (Lemoine   and   Michard,   1963;   Lemoine,   1964;   Michard   and   Schumacher,   1973;   Bourbon   et   al.,   1979),   leading   to   the   identification   of   imbrications   of   oceanic   and   continental   units,   especially   in   the   southern   part   of   the   arc   (Michard,   1967;   Henry  et  al.,  1993;  Balestro  et  al.,  2020).  Serpentinite-­‐bearing  layers  occur,  for  example,  between  the  main   Dora-­‐Maira   basement   complex   containing   UHP   relicts   and   the   blueschist   Permian-­‐Triassic   Dronero-­‐ Sampeyre  unit  (Henry  et  al.,  1993).  Some  others  are  found  between  the  middle  Triassic  series  of  Val  Maira   and  the  Triassic-­‐Jurassic  series  of  Val  Grana  (units  I  and  III  of  Michard,  1967,  respectively),  both  derived   from   the   Prepiedmont   domain.   The   imbricated   structures   can   result   from   different   processes:   (1)   rift   inheritance,   with   inversion   of   a   complicated   ocean-­‐continent   transition   domain   featuring   continental   allochthons   separated   by   exhumed   mantle   patches   (Beltrando   et   al.,   2010b;   Festa   et   al.,   2020),   or   (2)   subduction-­‐collision  processes,  with  polyphase  deformation  of  an  initially  simple  ocean/continent  thrust   contact  at  the  bottom  of  the  accretionary  wedge.     While   not   excluding   the   first   hypothesis,   we   currently   prefer   the   second   option   considering   our   observations  and  structural  data  from  the  literature.  Our  structural  and  microstructural  data  (§  5;  fig.  2)   between   Briançonnais   and   Ubaye   demonstrate   that   the   internal   nappe   stack   has   been   crosscut   and   disturbed   by   radially   oriented   forward   and   backward   "out-­‐of-­‐sequence"   thrusting.   Such   multistage   deformation   is   also   documented   in   the   southern   part   of   the   arc   (Val   Maira   and   Valgrana,   west   of   Cuneo     20   city)   by   Michard   (1967)   and   Schumacher   (1972),   which   emphasize   multistage   deformation   and   north   directed   backthrusting.   We   argue   that   oceanic/continental   units   imbrication   is   most   likely   due   to   a   tectonic   disturbance   of   the   initial   superposition   of   nappes:   In   the   early   stages   of   stacking,   the   oceanic   nappes,  either  non-­‐metamorphic  (Helminthoid  flyschs),  or  metamorphic  sediments  (Schistes  Lustrés)  and   oceanic   basement   (Chenaillet,   Monviso),   were   thrust   over   the   continental   margin   units   (Prepiedmont   and   Briançonnais).  Such  a  superposition  is  locally  preserved  in  different  locations  of  the  internal  zones  of  the   Western   Alps:   the   Chenaillet   ophiolites   and   metasediments   overlying   the   Gondran   prepiedmont   unit   (Lemoine,   1971;   Barféty   et   al.,   1995),   the   Monviso   metaophiolites   overlying   the   Dora-­‐Maira   massif,   the   Mte  Banchetta  tectonic  window  (§4.2,  Fig.  7).     Besides   imbrications,   other   anomalous   structures   consist   of   continental   margin   units   resting   over   oceanic   units.  Such  a  reverse  superposition  is  shown  for  example  by  the  Penninic  thrust  bringing  the  Briançonnais   nappes  above  the  Embrunais-­‐Ubaye  Helminthoid  flysch  nappes  near  Guillestre  (Dumont  et  al.,  2011),  or   by   the   Rochebrune   Prepiedmont   nappe   backthrust   over   the   oceanic   metasediments   of   the   Lac   des   Cordes   unit   (Dumont   et   al.,   1984;   Tricart   et   al.,   1985).   In   both   cases,   this   reversal   is   due   to   a   tectonic   disturbance   of  the  initial  nappe  stack  by  the  D2  deformation  event  (§  4  &  5)  and  it  must  be  restored  to  reveal  the  top  of   the  continental  subduction  configuration  beneath  the  accretionary  wedge  in  the  early  orogenic  stages.       7b-­‐  Internal  structure  of  the  Briançonnais  nappe  stack:   The   Carboniferous   "Zone   Houillère"   and   associated   Mesozoic   cover   units   were   initially   located   towards   the   base   of   the   Briançonnais   pile   of   nappes   sensu   stricto,   both   in   Vanoise   and   in   the   region   of   Briançon   (Caby,  1996;  Barféty  et  al.,  1995,  2006),  as  it  is  the  case  in  the  Swiss  Alps  (Escher  et  al.,  1993).  Generally   detached  from  their  polymetamorphic  basement,  these  units  may  have  been  translated  later  to  a  higher   structural   position   by   either   forward   or   backward   deformation,   as   for   example   the   Mont   Thabor   unit   around  Névache  (Barféty  et  al.,  2006;  Lanari  et  al.,  2012).  In  the  region  of  Briançon,  the  "Zone  Houillère"  is   overlain   by   a   refolded   stack   of   Mesozoic   cover   nappes   detached   along   Triassic   evaporite   layers,   which   possibly  represent  the  initial  cover  of  the  "Internal  Briançonnais"  units  of  the  Permo-­‐Carboniferous  Axial   Zone  located  further  SE  (Lefevre,  1982).     On   top   of   the   Briançonnais   thrust   sheets   scarce   evidence   of   an   uppermost   polymetamorphic   basement   nappe   are   found   (fig.   1),   which   triggered   famous   debates   about   identification   of   thrust-­‐sheets   in   the   Western   Alps   (Termier,   1899;   Termier   &   Kilian,   1920).   These   uppermost   units   locally   bear   an   "ultrabriançonnais"   highly   condensed   type   of   Mezozoic   series   (Rio   Secco;   Lemoine,   1961,   1964,   1967).   To   the  north,  possible  equivalents  of  these  units  are  the  Sapey-­‐Ruitor  orthogneiss  and  micaschists  in  Vanoise,   yielding   Cambrian-­‐Ordovician   ages   (Guillot   et   al.,   2002)   and   which   are   affected   by   both   Variscan   and   Alpine  metamorphism.  They  are  also  located  in  the  uppermost  position  of  the  Briançonnais  stack.  To  the   southeast,   pre-­‐Namurian   polymetamorphic   basement   also   occurs   in   the   Internal   Briançonnais   units   of   Liguria   (Cortesogno   et   al.,   1981).   These   upper   basement   nappes   which   override   the   Permo-­‐Carboniferous   basins   are   disconnected   from   the   internal   crystalline   basement   massifs.   As   discussed   later,   we   propose   that   the   thrust   sequence   involving,   from   bottom   to   top,   the   Carboniferous   Zone   Houillère   and   Permo-­‐ Carboniferous  units,  the  Meso-­‐Cenozoic  cover  nappes,  and  the  polymetamorphic  basement  thrust  sheets,   may  result  from  tectonic  inversion  of  late  Variscan  inherited  paleogeography.       7c-­‐  Olistostromes  on  top  of  the  early  orogenic  wedge:   Near  Briançon,  there  is  evidence  for  early  and  shallow  emplacement  of  a  basement  nappe  in  an  uppermost   position.   The   so-­‐called   "quatrième   écaille"   (Termier,   1899)   is   composed   of   micaschists   similar   to   Rio   Secco,   but   bearing   a   condensed   and   specific   Mesozoic   cover:   thin   middle   Triassic   carbonates   of   more   distal  environments  than  the  underlying  typical  Briançonnais  nappes  (Barféty  et  al.,  1995)  unconformably   overlain   by   the   Prorel   breccia,   which   is   probably   late   Cretaceous   (Barféty   et   al.,   1995).   The   basal   thrust   of   this   nappe   is   marked   by   an   olistostrome   of   Bartonian   age   (Barféty   et   al.,   1992)   deposited   on   top   of   the   Briançonnais   stack,   and   containing   various   exotic   blocks,   including   oceanic   sediments   (Helminthoid   Flysch).   These   features   underline   the   former   connection   of   a   thrust   system   inside   the   early   orogenic   wedge  with  the  surficial  part  of  the  wedge  and  with  the  floor  of  the  Paleogene  flexural  basin  in  front  of  the   orogen.   The   N-­‐NW-­‐wards   propagation   of   the   orogen   and   of   the   preceding   flexural   basin   throughout   Paleogene  times  is  well  documented  in  the  Western  Alps  (Sinclair,  1997;  Kempf  &  Pfiffner,  2004;  Ford  et   al.  2006;  Schmid  et  al.,  2017).  Slope  breccia  and  olistostromes  which  occur  at  the  transition  between  the   range  and  the  basin  are  further  incorporated  as  a  diachronous  marker  layer  in  the  footwall  of  the  nappes   (Kerckhove,   1969;   Ford   &   Lickorish,   2004).   The   occurrence   of   detrital   elements   of   mixed   continental   (basement,   Mesozoic   cover)   and   oceanic   provenance   (ophiolites,   oceanic   sediments   including   Helminthoid   Flysch)   may   allow   tracing   of   the   propagation   of   the   obducted   accretionary   wedge   over   the   Briançonnais   foreland   in   the   early   stages   of   continental   subduction.   However,   there   is   a   need   to     21   distinguish  these  thrust-­‐related  breccias  within  the  various  types  of  breccia  observed  in  the  sedimentary   record  of  the  Briançonnais  and  neighbouring  areas.     8-­‐Breccia  and  olistostromes       The   scale   and   significance   of   synsedimentary   breccias   interbedded   in   the   Briançonnais   Meso-­‐Cenozoic   series  and  in  the  adjoining  Subbriançonnais,  Valais  and  Prepiedmont  domains  has  been  a  subject  of  debate   for   many   decades   (Lemoine,   1967;   Chaulieu,   1992   and   refs   therein;   Ribes   et   al.,   2019).   Various   types   of   syntectonic   breccia   are   observed   in   relation   to   different   geodynamic   settings,   whose   sedimentary   signatures  are  difficult  to  distinguish.  Scarp  or  slope  breccia  can  occur  in  either  divergent  or  convergent   setting  with  similar  sedimentological  characteristics  (Chaulieu,  1992).  Apart  from  their  facies,    additional   criteria  to  consider  are  (1)  the  occurrence  of  exotic  material,  which  can  reflect  syn-­‐orogenic  exhumation   of   source   areas   or   tectonic   juxtaposition   of   oceanic   and   continental   thrust-­‐sheets,   (2)   their   Alpine   structural  setting,  shear  fabric  and  location  with  respect  to  the  major  Alpine  thrusts  or  plate  boundaries   (Polino   et   al.,   1990).   Thrust-­‐related   mélanges   and   associated   mass-­‐transport   and   tectonic   processes   are   described  in  Festa  et  al.  (2010),  which  helps  to  distinguish  breccia  related  to  extensional  tectonics  (type  1)   from  those  related  to  subduction  or  collision,  and  to  formalise  the  transition  from  precursor  olistostromes   to  tectonic  mélanges  associated  with  nappes  boundaries.       8.1-­‐Breccia:  a  Review     This   section   classifies   the   numerous   examples   of   syndepositional   breccia   described   in   the   Western   Alps   literature   which   respect   to   the   geodynamic   context   of   their   occurrence.   The   following   stages   are   considered:   a)   Tethyan   rifting   (early   to   early-­‐middle   Jurassic),   b)   Tethyan   passive   margin   overlapping   with  the  Atlantic-­‐Bay  of  Biscay  rifting  (middle  Jurassic  to  early  Cretaceous),  c)  Initiation  of  convergence   related   to   the   Pyrenean   orogeny   (late   Cretaceous   to   early   Eocene),   and   d)   Collision   related   to   Alpine   orogeny  (early  to  late  Eocene).   a)   During   the   early   Jurassic   Tethyan   rift   stage,   the   deposition   of   syn-­‐rift   sequences   occurred   in   an   extensional   setting   before   the   oceanic   opening   of   the   Ligurian   Tethys   (early   middle   Jurassic).   The   preservation  of  such  breccia  is  scarce  in  the  Briançonnais,  due  to  uplift  and  emergence  at  that  time  (Faure   &   Mégard-­‐Galli,   1988;   Claudel   &   Dumont,   1999),   but   lateral   equivalents   are   known   in   the   Prepiedmont   units  of  Liguria  (Mte  Galero  breccia,  member  A;  Dallagiovanna  &  Lualdi,  1984;  Decarlis  &  Lualdi,  2011),  in   the  Cottian  Alps  (Narbona  breccia;  Michard,  1967;  Michard  &  Schumacher,  1973;  Gidon  et  al.,  1978)  and  in   the   Prealps   (lower   member   of   the   Breccia   Nappe:   Hendry,   1972;   Steffen   et   al.,   1993;   lower   member   of   the   Evolène  series,  Mont  Fort  nappe:  Pantet  et  al.,  2020).  Local  supply,  mass  flow,  debris  flow  and  turbidites,   together   with   local   angular   unconformities   and   submarine   erosion   (Dumont   et   al.,   1984;   Deville,   1986)   are  regarded  as  evidence  of  extensional  rift  context.   b)   After   the  initial   opening   of   the   Ligurian   Tethys,   passive   margin   sedimentation   on   the   European   margin   was  affected  by  a  second  extensional  pulse  associated  with  the  Atlantic-­‐Bay  of  Biscay  rifting  and  Iberian   plate   divergence   (late   Middle   to   Late   Jurassic;   Vergés   &   Garcia-­‐Senz,   2001).   Locally   preserved   in   the   early   post-­‐rift  Briançonnais  sediments  (Tissot,  1954;  Bourbon,  1980;  Jaillard,  1987;  Jaillard,  1999;  Claudel  et  al.,   1997),  including  the  Ligurian  Briançonnais  (Bertok  et  al.,  2011),  and  are  widespread  in  nearby  domains,   both   continent-­‐ward   (Subbriançonnais)   and   oceanward   (Prepiedmont).   They   consist   of   the   Telegraphe   breccia  in  the  Subbriançonnais  units  (Barbier,  1963;  Barféty  et  al.,  1977;  Barféty  et  al.,  2006)  and  coeval   lateral  equivalents  (Nielard  breccia,  Barféty  et  al.,  1977;  Neyzets  and  Piolit  breccia;  Latreille,  1954;  Chenet,   1978).  Time-­‐equivalent  turbiditic  breccia  in  Prepiedmont  units  are  found  in  Liguria  (Mte  Galero  breccia,   members   B   and   C,   Decarlis   &   Lualdi,   2011),   in   the   Briançon   region   (Lemoine   et   al.,   1986)   and   in   the   Prealps   (Breccia   Nappe,   upper   member,   Steffen   et   al.,   1993).   All   correspond   to   rifted   margin   pelagic   environments,   indicative   of   increased   transport   and   deeper   erosion   of   the   source   areas   than   during   the   previous  syn-­‐rift  stage.     c)   The   initiation   of   Africa-­‐Europe   convergence   is   recorded   in   the   European   margin   sediments   through   the   propagation   of   the   Pyrenean   foreland   north   of   the   Iberian   plate,   but   before   the   complete   closure   of   the   Tethys   and   the   Adria   collision   (late   Cretaceous   to   Paleocene).   The   pelagic   early   Late   Cretaceous   sedimentary   record   in  the   Briançonnais   contains   breccia   occurrences   which,   together   with   local   erosional   angular   unconformities   (Bourbon   et   al.,   1976),   possibly   reflect   incipient   compressional   reactivation   of   marginal   structures   (Chaulieu,   1992;   de   Graciansky   et   al.,   2011):   the   Cerces   breccia   (Tissot,   1954;   Barféty   et  al.,  2006),  the  Mélézin  breccia  (Barféty  et  al.,  1995),  the  Madeleine  breccia  (Gidon  et  al.,  1994),  and  even   olistoliths  (Bourbon,  1980)  are  reported  within  the  Cenomanian  to  early  Senonian  formations  of  several   external   Briançonnais   nappes.   Significant   tectonic   activity   is   also   indicated   by   the   occurrence   of   coarse   breccia   interbedded   in   the   upper   Cretaceous   calcschists   of   the   internal   Briançonnais   units   of   Vanoise     22   (Fours   unit,   Deville,   1986;   Tsanteleina   and   Chevril   breccia,   Jaillard,   1999).   Further   south,   the   most   internal   Briançonnais   nappes   contain   coarse   breccia   with   olistoliths   interbedded   in   calcschistous   formations  probably  late  Cretaceous  in  age  (Gidon  et  al.,  1994):  the  Longet  breccia  (Lemoine,  1967)  and   similar   breccias   in   the   uppermost   Pelvo   d'Elva   nappe   further   south   (Lefèvre   &   Michard,   1976).   Lateral   equivalents   are   found   in   the   Subbriançonnais   nappes:   Bachelard   and   Pelat   flyschs:   Blanc   et   al.,   1987,   Thum   et   al.,   2015;   l'Argentière   breccia:   Chenet   (1978).   All   these   clastic   formations   are   usually   interpreted   as   derived   from   degraded   submarine   fault   scarps,   but   their   compositions   frequently   suggest   a   deeper   erosion   of   the   source   areas   and   increased   transport   than   the   Jurassic   breccias.   Angular   erosional   unconformities  affecting  the  post-­‐rift  sediments  (i.e.  Guil  lower  nappe  or  Galibier  area)  with  locally  high-­‐ angle   relationships   (Tissot,   1954)   may   result   from   compressional   reactivation   of   rift   structures   (i.e.   Béraudes   fault,   Lemoine   et   al.,   2000).   Alternatively,   Bertok   et   al.   (2012)   describe   early   Late   Cretaceous   kilometric-­‐scale   normal   fault   scarps   in  the   Ligurian   Briançonnais   which   are   interpreted   to   have   formed   in   a   transcurrent   regime.   This   late   Cretaceous   deformation   must   be   related   to   the   motion   of   the   Iberia   microplate   relative   to   Europe   (Le   Breton   et   al.,   2021)   and   to   the   propagation   of   the   Pyrenean   foreland   within  its  eastern  continuation  (Corsica,  Sardinia,  Briançonnais).  Coeval  evidence  of  inversion  is  known  in   Provence,   the   Maritime   Alps   (Schreiber   et   al.,   2011),   further   north   (Dévoluy   pre-­‐Santonian   folding;   Michard  et  al.,  2010)  and  in  northern  Spain  (Soto  et  al.,  2011).     d)   Detrital   sedimentation   finally   occurred   after   the   complete   closure   of   the   Ligurian   Tethys,   and   recorded   the   propagation   of   the   Alpine   orogenic   wedge   resulting   from   the   Adria-­‐Europe   collision   (Eocene).   Sedimentation  ceased  in  early  Oligocene  due  to  involvement  of  the  whole  internal  Alpine  zones  within  the   collision   wedge.   This   propagation   developed   an   underfilled   flexural   basin   over   both   the   Briançonnais   domain  and  the  more  proximal  parts  of  the  margin  (Ceriani  et  al.,  2001;  Sinclair,  1997).  Its  sedimentary   infill   is   diachronous,   spanning   from   Lutetian   to   Bartonian   in   the   Briançonnais   (Barféty   et   al.,   1995;   Michard   &   Martinotti,   2002)   and   from   Lutetian   to   Priabonian   in   autochthonous   Corsica   and   in   the   External   Alpine   zone   (Durand-­‐Delga,   1984;   Joseph   &   Lomas,   2004,   and   refs.   therein).   The   flexural   basin   was   propagating   towards   the   NW   throughout   the   Eocene   (Kempf   &   Pfiffner,   2004;   Ford   et   al.,   2006;   Dumont  et  al.,  2012).  It  is  generally  overlain  by  a  widespread  olistostrome  layer  marking  the  base  of  the   obducted  oceanic  accretionary  wedge.  This  olistostrome  formation,  named  "Schistes  à  blocs"  (Kerckhove,   1969)  is  also  diachronous  and  locally  dated  as  Late  Eocene-­‐Early  Oligocene  beneath  the  non-­‐metamorphic     thrust-­‐sheets  which  overly  the  external  zone  of  the  Western  Alps  (Dumont  et  al.,  2012,  and  refs.  therein).   In  the  external  Briançonnais  nappes,  few  occurrences  of  such  deposits  are  observed,  always  located  on  top   of  the  tectonic  pile.  The  most  striking  example  is  the  "Quatrième  écaille"  near  Briançon,  which  has  been   subject  to  debate  since  ~1900  (Lemoine,  1961),  and  which  is  now  interpreted  as  a  kilometre-­‐scale  block   which   slid   above   the   Briançonnais   basin   infill   in   Early   Bartonian   times   (Barféty   et   al.,   1992).   Its   exotic   character  is  shown  by  a  specific,  polymetamorphic  basement  only  found  in  the  most  internal  zones,  that  is   the  uppermost  unit  of  the  Briançonnais  thrust  stack  (a,  fig.  13;  white  stars),  and  by  open-­‐marine  Middle   Triassic   facies   different   from   the   underlying   nappes.   It   is   comparable   to   distal   ramp   carbonate   facies   described   in   Liguria   (Decarlis   &   Lualdi,   2009).   Nearby   and   beneath   this   block   are   found   conglomerates   and   breccia   fed   by   it,   thus   containing   mostly   basement   (Eychauda   breccia,   Barféty   et   al.,   1992,   1995).   Similar  breccias  are  found  in  several  places  at  the  same  structural  and  stratigraphic  level  (a,  fig.  13,  red   stars;   b,   c,   fig.   13).   Finally,   "mixed"   breccia   consists   of   a   rare   occurrence   of   mafic   or   ultramafic   pebbles   within  coeval  olistostrome  layers  (d,  fig.  13,  blue  stars  22  to  24),  together  with  more  frequent  pebbles  of   Cretaceous   oceanic   sediments   (Helminthoid   Flysch).   They   were   fed   by   both   continental   and   oceanic   fragments  from  the  orogenic  front.  We  argue  that  these  "mixed"  detrital  formations,  always  located  on  top   of  the  Briançonnais  nappe  pile  and/or  beneath  the  exotic  flysch  nappes,  can  be  useful  in  identification  of   the  earliest  stage  of  propagation  of  the  accretionary  wedge.     Figure  13  (next  p.):   Location  of  uppermost  basement  slices  and  of  synsedimentary  breccia  and  olistostromes   on  top  of  the  Briançonnais/Prepiedmont  nappes  stack,  emplaced  during  the  surficial  propagation  of  the  early   orogenic  wedge  (from  literature  and  personal  observations).     a-­‐   Location   map   (legend   of   numbers   below).   White   stars:   polymetamorphic   basement   slices   witnessing   the   widespread  occurrence  of  an  uppermost  "internal"  Briançonnais  nappe  likely  issued  from  an  area  devoid  of   late-­‐Variscan   sediments   (see   §   7).   Black   stars:   Examples   of   upper   Cretaceous   (dated   or   attributed)   detrital   formations  deposited  Adria-­‐Europe  collision,  assumed  to  result  from  Pyrenean  orogenic  propagation  (§  8.1,   c).   Red   stars:   Eocene   synsedimentary   breccia   and   olistostromes   containing   pebbles   of   Variscan   metamorphic   basement,  feeded  by  this  upper  basement  nappe  (examples  b  and  c);  the  host  flysch  sediment  is  locally  dated   from   Bartonian   (Barféty   et   al.,   1992).   Blue   stars:   "mixed"   synsedimentary   breccia   and   olistostromes,   containing   both   continental   margin   and   oceanic   basement   clasts;   at   22   to   24,   located   in   external   Briançonnais,  the  breccia  belong  to  the  "Schistes  à  blocs"  formation,  an  olistostrome  located  at  the  bottom  of       23        (Figure  13,  cont.)   the  oceanic  Helminthoid  Flysch  nappes  (example  d);  sites  20-­‐21,  east  of  the  Briançonnais   zone,  could  be  an  equivalent  of  such  olistostrome  later  involved  in  the  orogenic  wedge.   b-­‐  Polygenic  breccia  containing  pebbles  of  Variscan  metamorphic  basement,  dominantly  gneiss,  interbedded   in  the  Eocene  "Flysch  Noir"  formation  of  the  Chatelet  Briançonnais  nappe,  east  of  Vallon  Laugier,  below  Pic   des  Houerts  (Gidon  et  al.,  1994).   c-­‐  The  Eychauda  breccia,  same  kind  of  breccia  feeded  by  micachists  of  the  Prorel  unit  («  Quatrième  écaille  »,  §   7   and   §   8.1),   a   Bartonian   olistostrome   sitting   on   top   of   the   external   Briançonnais   nappes   stack   (Serre   Chevalier,  near  Briançon).     d-­‐  Mixed  breccia  with  basalt  pebbles  in  the  Eocene  «  Schistes  à  blocs  »  fm.,  SE  of  Fouillouse  village,  left  side  of   Ubaye  valley.  This  olistostrome  formation  is  overlain  by  the  Helminthoid  Flysch  nappe  of  oceanic  origin.   e-­‐   Col   du   Longet   megabreccia,   with   spectacular   decametric   dolostone   blocks   included   in   the   Cretaceous   calcschist  formation  (§8.1,  c).     24   Figure  13:  Location  of  outcrops:     1-­‐  Col  d’Etaches  (Barféty  et  al.,  2006)   2-­‐  Passo  dei  Fourneaux  (Polino  et  al.  2002)   3-­‐  Pian  dei  Morti  fort,  Rho  valley  (Polino  et  al.  2002)   4-­‐  Chalets  des  Acles  (Barféty  et  al.,  2006)   5-­‐  Top  of  Chaberton  peak  (Barféty  et  al.,  1995)   6-­‐  Rio  Secco  (Termier  1903;  Lemoine,  1960c  ou  1961b)   7-­‐  Montgenèvre  (Barféty  et  al.,  1995;  site12,  fig.  10  &  11;  fig.  8d)   8-­‐  Le  Rosier,  La  Vachette  (Barféty  et  al.,  2006)   9-­‐  Cervières  (Barféty  et  al.,  1995)   10-­‐  Brunissard,  S.  Izoard  pass  (Lemoine,  1961a)   11-­‐  Lac  de  Souliers  –  Col  du  Tronchet  (Lemoine,  1961a;  Debelmas  &  Lemoine,  1966)   12-­‐  W.  Arvieux  (Lemoine,  1961a;  Debelmas  &  Lemoine,  1966)   13-­‐  E.  Villargaudin  (Lemoine  1961a;  Debelmas  &  Lemoine,  1966)   14-­‐  E.  Ceillac  (Lemoine,  1961a;  Debelmas  &  Lemoine,  1966)   15-­‐  Colle  delle  Sagneres,  N.  Acceglio  (Lefevre  &  Michard,  1976)   16-­‐  Eychauda  conglomerates  (Barféty  et  al.,  1992;  Barféty  et  al.,  1995)   17-­‐  Prorel  micaschists  (Termier,  1899;  Lemoine,  1960b,  c,  1964;  Barféty  et  al.,  1992)   18-­‐  Furfande  microconglomerates  (site  n°32,  Figs.  10  &  11)   19-­‐  Col  des  Houerts  conglomerates  (Gidon  et  al.,  1994)   20-­‐  Monte  Banchetta,  Sestriere  (Caron,  1971;  fig.  14)   21-­‐  Rocher  Renard,  S.  Chenaillet  (Burroni  et  al.,  2003)   22-­‐  Col  de  Sérenne,  E.  Col  de  Vars  (Kerckhove,  1969:  Gidon  et  al.  1994)   23-­‐  Fouillouse    (Gidon  et  al.,  1994)   24-­‐  Les  Sagnes,  S.  Larche  (Kerckhove,  1969;  Gidon  et  al.,  1978)   25-­‐  La  Madeleine  breccia,  Val  d'Escreins  (Gidon  et  al.,  1994)   26-­‐  Col  du  Longet  breccia,  high  Ubaye  valley  (Lemoine,  1967;  Gout,  1987)   27-­‐  Pelvo  d'Elva  breccia  (Michard,  1967;  Lefèvre  &  Michard,  1976)   28-­‐  Bachelard  and  Pelat  flyschs  (Blanc  et  al.,  1987;  Thum  et  al.,  2015)       8.2-­‐The  significance  of  mixed  breccia   Few   occurrences   of   "mixed"   detrital   formations,   namely   containing   clastic   elements   derived   from   both   continental  margin  and  oceanic  domains,  have  so  far  been  reported  in  the  Western  Alps,  apart  from  their   intra-­‐oceanic  precursors  emplaced  during  the  late  Cretaceous  closure  of  the  Tethyan  domain  (Deville  et   al.,  1992;  Balestro  et  al.,  2015).  The  Late  Eocene  olistostrome  covered  by  the  Helminthoid  Flysch  nappes   of   oceanic   origin   contains   frequent   Late   Cretaceous   oceanic   blocks   (Autapie   type   Helminthoid   flysch,   Kerckhove  1969).  Exceptional  blocks  of  oceanic  basement  (basalts)  are  observed  in  two  localities  (a,  fig.   13,  sites  23  and  24).  Although  not  dated,  the  Paneyron  mixed  detrital  formation  ("Ophiolite  de  Sérenne";   Kerckhove,  1969)  is  probably  a  lateral  equivalent  of  this  layer.  Other  occurrences  are  reported  from  more   internal  parts  of  the  arc,  but  remain  a  subject  of  debate  in  the  absence  of  reliable  age  data.  These  are,  from   south   to   north   (1)   the   Valliera   and   Tibert   megabreccia   in   Valgrana   (Michard,   1967),   which   contain   siliciclastic  facies,  dolomitic  blocks  and  serpentinite  elements  associated  with  kilometre-­‐scale  ultramafic   lenses.   Initially   interpreted   as   possibly   interlayered   within   the   Mesozoic   stratigraphy,   these   megabreccias   occur   between   the   underlying   Prepiedmont   Triassic-­‐Liassic   units   and   the   tectonically   superposed   oceanic   "Schistes  Lustrés"  nappes  (Michard  &  Schumacher,  1973),  (2)  the  Cula  breccia  (Gout,  1987)  which  must   be   distinguished   from   the   Col   du   Longet   breccia   (Lemoine,   1967;   Gidon   et   al.,   1994).   The   latter,   fed   by   nearby  internal  Briançonnais  areas,  is  linked  with  the  upper  Pelvo-­‐d'Elva  nappe  (§  4.1;  fig.  4)  and  does  not   contain   any   oceanic   clasts   (Michard,   pers.   com.   2019,   and   personal   observations),   whereas   the   Cula   breccia   which   contains   basalt   and   serpentine   pebbles   is   located   at   the   base   of   the   oceanic   "Schistes   Lustrés"   pile   of   nappes   (Cula   unit;   Gout,   1987)   (3)   the   Prafauchier   series   (Dumont,   1983),   containing   continental   margin   derived   clasts   (dolostones,   micaschists)   and   serpentinite   grains,   with   locally   serpentinite   blocks,   and   interpreted   either   as   part   of   the   Prepiedmont   series,   or   belonging   to   an   overlying   unit,  (4)  the  Rocher  Renard  breccia  linked  with  the  Lago  Nero  unit  at  the  base  of  the  Chenaillet  ophiolitic   massif  (Polino  &  Lemoine,  1984;  Burroni  et  al.,  2003;  Principi  et  al.,  2004),  (5)  the  Mte  Banchetta  breccia   (Caron,   1971),   containing   blocks   and   pebbles   of   serpentinite,   oceanic   sediments   (Jurassic   marbles   and   ophicalcite)   and   continental   margin   sediments   (Triassic   dolostones   and   sandstones),   in   a   shaley   matrix.   This  latter  example  crops  out  on  top  of  a  folded  Prepiedmont  unit  (§  4.2;  fig.  7),  and  is  described  below.   In  addition,  two  occurrences  of  ophiolite  bearing  tectono-­‐sedimentary  breccia  in  the  western  Alpine  arc   may   have   a   comparable   significance:   the   Montaldo   calcschists   in   Liguria   (Dallagiovanna   et   al.,   1991;     25   Decarlis  et  al.,  2013)  which  contain  olistoliths  of  serpentinites  and  prasinites,  and  the  Gets  flysch  (Caron  &   Weidmann,   1987;   Caron   et   al.,   1989;   Bill   et   al.,   1997)   resting   at   the   top   of   the   Prealps   tectonic   pile   and   containing  ophiolitic  and  granitic  blocks.  Remarkably,  all  these  examples  of  mixed  continental  and  oceanic   breccia   occur   immediately   above   the   highest   continental   margin   units,   generally   the   Prepiedmont   nappes   which   represent   the   most   distal   part   of   the   European   continental   margin,   and   are   overlain   by   the   lowermost  metasedimentary  units  of  the  oceanic  "Schistes  Lustrés"  derived  from  the  accretionary  prism   (Schwartz,   2000).   These   "mixed"   clastic   formations   are   thus   a   useful   marker   of   the   initial   geometry   of   the   subduction  trench,  later  deformed  by  further  collision  phases.  We  argue  that  their  formation  is  related  to   the  translation  of  the  subduction  trench  over  the  distal  European  margin,  and  that  their  significance  may   be  different  from  other  mixed  breccias  deposited  on  the  oceanic  floor  and  related  to  either  subcontinental   mantle  exhumation  near  the  ocean-­‐continent  transition  (Meresse  et  al.,  2012)  or  early  subduction  of  the   Tethyan  floor  (Marroni  et  al.,  2007).       8.3-­‐The  Monte  Banchetta  mixed  breccia,  an  olistostrome  at  the  bottom  of  the  oceanic  accretionary  wedge?   A   key   example   of   mixed   breccia   is   found   in   Mte   Banchetta,   near   Sestriere   (fig.   3,   site   e).   The   structural   setting,   described   in   §   4.2,   is   interpreted   to   be   a   cross-­‐fold   anticline   producing   a   dome.   An   uplifted   continental  margin  unit,  probably  the  same  as  the  Prepiedmont  Gran  Roc  unit  further  SW,  outcrops  in  a   tectonic   window   (fig.   7;   Jouvent,   2017).   The   underlying   series   was   described   by   Caron   (1971)   and   regarded   as   analogous   to   the   Prepiedmont   series   of   Valgrana   (Michard   &   Schumacher,   1973).   A   recent   map   (Corno   et   al.,   2019),   does   not   follow   this   attribution,   simply   calling   it   "continental   succession",   but   three   diagnostic   formations   of   the   Prepiedmont   series   can   be   identified,   namely   the   Norian   dolostones,   the   Rhaetian   dolostones,   limestones   and   schists,   and   the   "Lias   prépiémontais"   calcschists   ("calcschistes   siliceux   ankéritiques"   of   Caron,   1971).   This   series   which   crops   out   in   the   Mte   Banchetta   area   is   not   overturned,   except   on   the   eastern   slopes   due   to   backfolding   (fig.  7).   On   the   west   side   of   Mte   Banchetta   (a,   fig.  14,  site  c),  the  upper  part  of  the  calcschists  is  overlain  by  the  breccia,  through  a  dark  grey  schist  layer   containing   scarce   pebbles   (c,   fig.   14)   and   bearing   a   specific   stilpnomelane   mineralization   (Caron,   1970).   The  same  schist  is  also  found  above  as  a  matrix  of  the  breccia  (Caron,  1971;  b,  d,  fig.  14).  Laterally  (Clot   della   Mutta;   fig.   14,   south   of   site   c),   the   Prepiedmont   calcschists   are   overlain   by   an   hectometric-­‐sized   serpentinite   sliver   in   apparent   tectonic   contact,   but   this   contact   seems   to   pinch   out   beneath   the   schist   layer   at   site   c.   This   feature   can   be   interpreted   in   two   ways:   either   this   sedimentary   layer   is   inserted   along   the  contact,  thus  the  serpentinite  sliver  is  a  larger  block  belonging  to  the  breccia,  or  it  seals  the  tectonic   contact   between   serpentinite   and   calcschists.   In   both   cases,   the   breccia   must   be   regarded   as   an   olistostrome  deposited  over  the  Prepiedmont  series.     With   the   exception   of   the   basal   schist   layer   which   contains   rounded   pebbles,   the   breccia   has   a   very   proximal  character  based  on  the  size  and  shape  of  the  blocks.  This  suggests  that  it  was  fed  by  nearby  and   active  submarine  scarps  involving  both  continental  margin  (Prepiedmont)  and  oceanic  floor  units.  Corno   et  al.  (2019)  propose  an  interpretation  of  this  polymictic  breccia  in  an  hyperthinned  marginal  setting,  in   response   to   the   Jurassic   rifting.   Alternatively,   considering   that   the   breccia   is   overlying   the   Prepiedmont   marginal  series  and  that  it  is  located  in  the  footwall  of  mixed  imbricates  (a,  fig.  14),  we  propose  that  the   Mte   Banchetta   breccia   was   deposited   on   the   European   margin   toe   in   a   subduction   trench   setting,   marking   the  earliest  stage  of  obduction  of  the  accretionary  wedge.     Similar  olistostromes  are  known  beneath  the  Helminthoid  flysch  nappes,  the  non-­‐metamorphic  equivalent   to   the   oceanic   Schistes   Lustrés   ("Schistes   à   blocs"   fm.;   Kerckhove,   1969).   They   locally   contain   ophiolitic   detritus  (§  8.1;  d,  fig.  13),  and  we  propose  that  they  represent  non-­‐metamorphic  lateral  equivalents  to  the   Mte  Banchetta  mixed  breccia.     Some  Ar  dating  was  completed  from  phengites  sampled  in  the  Prepiedmont  Rhaetian  formation  beneath   the   breccia   (f,   fig.   14;   Jouvent,   2017).   Variscan   inheritance   can   be   ruled   out   because   there   is   no   significant   reworking   of   Variscan   material   in   the   late   Triassic   sediments   of   the   Prepiedmont   series   (Dumont   et   al.,   1984).   Hence   the   52.31±1.32   Ma   age   obtained   (g,   fig.   14)   indicates   that   the   deposition   of   the   Mte   Banchetta   mixed   breccia   occurred   during   early   Eocene   or   before.   Although   this   HP   age   needs   to   be   supported   by   further   dating   studies,   the   underthrusting   of   the   Mte   Banchetta   Prepiedmont   unit   which   produced   this   metamorphic   event   seems   consistent   with   the   early   exhumation   of   the   oceanic   Schistes   Lustrés   (Agard   et   al.,   2002;   Herviou   et   al.,   2022),   and   with   the   slightly   younger   involvement   of   the   Briançonnais  units  in  continental  subduction  (Bucher  et  al.,  2004;  Bousquet  &  Berger,  2008;  Strzerzyski  et   al.,  2011).  This  is  also  consistent  with  the  age  of  Adria-­‐Europe  collision  in  the  Western  Alps  according  to   geodynamic  models  (de  Graciansky  et  al.,  2011;  Manzotti  et  al.,  2014;  Pfiffner,  2014;  van  Hinsbergen  et  al.,   2020;  Le  Breton  et  al.,  2021).       26        Figure   14:   The   mixed   megabreccia   of   Mte   Banchetta,   near   Sestriere,   Italy.   The   structure   of   this   area   is   illustrated   in   fig.   7,   location   fig.   1b.   The   outcrops   overlain   a   Prepiedmont   type   series   with   typical   upper   Triassic  to  lower  Jurassic  formations,  and  are  overthrust  by  the  serpentinite-­‐bearing  oceanic  Schistes  Lustrés   nappes,  with  some  mixed  slivers  in  between.  The  breccia  was  feeded  by  both  continental  and  oceanic  series.   a-­‐   Panoramic   view   towards   E,   from   the   top   of   Chaberton   peak,   and   location   of   further   observation   points.   The   central   part   shows   an   oceanic   sliver   tectonically   overlying   the   Prepiedmont   continental   margin   series.   This   stack   grades   laterally   to   the   megabreccia,   which   interrupts   the   tectonic   contact   at   point   c.   These   features  are  overthrust  towards  N-­‐NW  by  continental  and  oceanic  thrust-­‐sheets  (Pta  Rognosa).     b-­‐  Close  view  of  the  block-­‐supported  breccia  with  black  shaley  matrix.       27   c-­‐  Close  view  of  black  shaley  sedimentary  layers  resting  over  the  lower  Liassic  Prepiedmont  fm.  next  to  the   northern   termination   of   the   oceanic   sliver,   and   containing   siliceous   and   carbonate   rounded   pebbles.   This   layer,  described  by  Caron  (1970),  corresponds  to  the  host  sediment  of  the  megabreccia  (b).   d-­‐  Outcrop  view  towards  N  of  the  megabreccia  resting  through  shaley  matrix  over  the  oceanic  sliver,  which   could  be  regarded  as  an  olistolith  in  the  megabreccia.  The  blocks  are  sourced  from  the  upper  Triassic-­‐lower   Jurassic  layers  of  the  Prepiedmont  series.     e-­‐   Example   of   mixed   oceanic   and   continental   metric   blocks   (serpentinites/ophicalcite   and   dolostones)   juxtaposed  in  the  upper  part  of  the  breccia.     f-­‐  N-­‐NW  directed  drag  fold  affecting  the  upper  Triassic  schists  and  dolostones  in  the  footwall  of  an  oceanic   sliver  (site  13,  fig.  11,  table  I),  consistent  with  the  N-­‐NW  directed  thrusting  of  the  Pta  Rognosa  imbricates  (a).     g-­‐   Preliminary   geochonological   data   (39Ar/40Ar   on   phengites   in   upper   Triassic   schistous   dolomitic   layer,   sample  location  f)  suggesting  that  the  Prepiedmont  series  which  floored  the  Mte  Banchetta  megabreccia  was   involved   in   the   collision   wedge   during   early   Eocene.   This   would   provide   a   minimum   age   for   the   deposition   of   the  breccia.  39Ar/40Ar  dating  results  in  Table  II.     9-­‐Discussion  and  geodynamic  implications       Our  data  allow  identification  of  two  main  stages  during  the  building  and  evolution  of  the   internal  zones  of   the   Western   Alps   arc,   corresponding   to   nappe   stacking   and   to   westward   extrusion,   respectively.   The   orientation   of   shortening   and   tectonic   transport   changed   significantly   through   time,   as   shown   by   subperpendicular   fold   and   lineation   trends   in   the   study   area.   The   late   stage   (D2,   §4,   §5)   corresponds   to   the   activation   of   forward   and   backward   thrust   systems   which   accommodate   the   exhumation   of   the   Briançonnais  stack  (fig.  2).  It  is  responsible  for  the  formation  of  the  arc  driven  by  westward  migration  and   indentation  of  Adria  upper  mantle  (Malusà  et  al.,  2016;  Schmid  et  al.,  2017;  Nouibat  et  al.,  2022),  and  the   most   prominent   thrusts   and   fold   trends   result   from   this   late   stage,   such   as   the   Penninic   thrust   near   Briançon.   Deciphering   the   early   stage   (D1)   is   much   more   difficult   because   the   associated   structures   are   overprinted,   deformed   and   crosscut   by   the   second   one   at   all   scales.   However,   proper   integration   of   this   early   deformation   phase   is   critically   important   because   it   recorded   the   absorption   of   N-­‐S   convergence   required   by   plate   tectonics   during   Alpine   orogenesis   (Schmid   &   Kissling,   2000;   Handy   et   al.,   2015;   van   Hinsbergen   et   al.,   2020).   The   large   amount   (several   hundred   km.)   of   early   N-­‐S   shortening   is   well   documented  in  the  Central  and  Eastern  Alps  (Pleuger  et  al.,  2007;  Scharf  et  al.,  2013;  Scheiber  et  al.,  2013;   Steck   et   al.,   2015;   Handy   et   al.,   2015   and   refs.   herein),   but   cannot   be   kinematically   linked   with   the   westward   extrusion   and   radial   spreading   dynamics   of   the   Western   Alps   arc,   whose   formation   mainly   postdates  the  initial  northward  drift  of  Internal  Alpine  nappes.         9.1-­‐Formation  and  kinematic  evolution  of  the  western  Alpine  arc   The  Western  Alpine  arc  is  a  very  specific  feature  of  the  Alpine  chain,  because  its  orogenic  dynamics  cannot   be   directly   linked   with   the   Africa-­‐Europe   motion   path,   but   requires   the   involvement   of   an   intermediate   indenter   between   the   Adria   plate   and   the   subducting   European   plate  (Platt   et   al.,   1989;   Rosenbaum   et   al.,   2005;   Le   Breton   et   al.,   2021;   Nouibat   et   al.,   2022)   that   produced   a   non-­‐cylindric   3D   structure.   This   complex   finite   geometry     makes   restorations   more   complicated   because   indentation   and   extrusion   produced  a  specific  lithospheric  structure  and  metamorphic  record  during  collision  (Schmid  et  al.,  2004;   Bousquet   et   al.,   2008;   Beltrando   et   al.,   2010a;   Handy   et   al.,   2010;   Schmid   et   al.,   2017;   Salimbeni   et   al.,   2018).   The   arc   was   completed   during   the   more   recent   stages   of   Alpine   orogenic   evolution   (Caby,   1996;   Schmid  &  Kissling,  2000;  Ford  et  al.,  2006;  Maffione  et  al.,  2008;  Handy  et  al.,  2010;  Dumont  et  al.,  2011;   Ring  &  Gerdes,  2016;  Le  Breton  et  al.,  2021),  accompanied  by  synorogenic  anticlockwise  rotations  which   increase  in  magnitude  towards  the  south  (Thomas  et  al.,  1999).  This  evolution  involved  major  changes  in   collision  kinematics  through  time,  which  have  been  identified  in  many  localities  of  the  Internal  Zones,  in   the  Prealpine  and  Embrunais  nappes,  and  in  the  Helvetic-­‐Dauphinois  domain  (Caby,  1975;  Merle  &  Brun,   1984;  Choukroune  et  al.  1986;  Baird  &  Dewey,  1986;  Platt  et  al.,  1989;  Ramsay,  1989;  Le  Bayon  &  Ballèvre,   2006;  Dumont  et  al.,  2011;  Scheiber  et  al.,  2013).  Nevertheless,  the  structure  of  the  arc  is  often  examined   on  the  basis  of  radial  cross  sections  (i.e.  Schmid  et  al.,  2017),  which  assume  that  the  major  displacements   occurred   perpendicular   to   the   present   trend   of   the   chain.   This   "radial   spreading"   model,   still   broadly   accepted  despite  being  questioned  by  Goguel  (1963),  may  be  more  or  less  valid  in  the  northern  part  of  the   arc,  where  the  change  in  orientation  of  tectonic  transport  through  time  remains  moderate  (Escher  et  al.,   1997;  Steck,  2008;  Steck  et  al.,  2015).  An  anticlockwise  change  in  translation  path  is  more  significant  in   the  central  part  of  the  arc  and  increases  southwards  (Merle  &  Brun,  1984;  Ceriani  et  al.,  2001;  Handy  et  al.,   2010).  The  total  estimate  of  radial  shortening  remains  significantly  higher  in  the  northern  half  of  the  arc,   including   the   foreland   and   the   Helvetic   nappes   (Epard,   1990;   Sinclair,   1997;   Burkhard   &   Sommaruga,     28   1998;  Schmid  &  Kissling,  2000;  Bellahsen  et  al.,  2014;  Pfiffner,  2016)  than  in  the  southern  part  (Ford  et  al.,   1999;   Ford   &   Lickorish,   2004).   In   the   former   part   and   towards   the   Central   Alps,   the   total   shortening   represents  the  cumulative  effects  of  the  early  nappe  stacking  and  of  the  later  extrusion  with  backfolding,   both   oriented   S-­‐N   to   SE-­‐NW   (Escher   et   al.,   1997;   Schmid   et   al.,   1997;   Steck,   2008;   Steck   et   al.,   2015).   Conversely,  there  is  an  increasing  angular  discrepancy  towards  the  southern  part  of  the  arc  between  the   early   (N   to   NW)   and   late   (SW   to   S)   thrusting   stages.   Consequently,   an   increasing   part   of   "along-­‐strike"   tectonic   transport   due   to   the   early   phase,   which   is   difficult   to   detected   on   radial   sections,   should   be   expected   towards   the   southern   part   of   the   arc.   Our   field   observations   presented   above   support   this   interpretation,   and   provide   evidence   for   "orogen-­‐parallel"   (N-­‐   to   NW-­‐directed)   early   nappe   stacking   within   the   Briançonnais   s.l.   zone.   Moreover,   this   orogenic   stage   was   possibly   responsible   for   greater   lateral  displacements  than  the  later  extrusion  stage.     9.2Restoration,  southern  origin  of  the  Internal  nappes   Such   a   perspective   may   have   implications   for   paleogeographic   restorations   and   for   the   interplay   between   pre-­‐Alpine  (Variscan  and  Tethyan)  and  Alpine  structures.  In  the  southern  part  of  the  arc,  it  is  difficult  to   decipher   the   provenance   and   thrusting   sequence   of   the   internal   nappes   simply   on   the   basis   of   their   geometrical   expression   and   structural   relationships   along   radial   cross-­‐sections.   The   arc   needs   to   be   retrodeformed   in   several   steps,   taking   into   account   successive   retrotranslation   paths   (Laubscher,   1988,   1991;   Schmid   &   Kissling,   2000).   Any   unfolding   process   must   consider   an   intermediate   step   of   restoration,   which   reconstructs   the   superposition   of   structural   units   during   the   earliest   orogenic   stages,   and   which   must  be  oriented  appropriately  in  consideration  of  the  early  kinematic  history,  that  is  ~SE-­‐NW  (Michard   et  al.,  2004;  Tricart  &  Schwartz,  2006).  Such  an  attempt  is  illustrated  in  fig.  15  (a).  This  schematic  profile   deals   with   the   late   Eocene   situation,   during   the   S-­‐   SE-­‐directed   continental   subduction   of   the   distal   European   margin.   This   situation   represents   the   early   stage   of   underthrusting   of   the   Briançonnais   and   Prepiedmont  domains  beneath  the  oceanic  accretionary  wedge  and  the  Adria  plate,  consistently  with  the   palinspastic  reconstruction  of  Dumont  et  al.  (2012,  fig.  15b).  Inversion  and  continental  subduction  at  this   stage   must   have   been   influenced   by   rift   inheritance,   that   is   by   the   vertical   and   lateral   distribution   of   lithospheric  thinning  in  the  distal  margin  as  emphasised  by  recent  research  (Mohn  et  al.,  2012;  Masini  et   al.,   2013;   Le   Breton   et   al.,   2021;   Tavani   et   al.,   2021).   Crustal   underthrusting   likely   benefited   from   the   thinned   (Briançonnais   continental   ribbon)   to   hyperthinned   (Prepiedmont   margin   toe)   character   of   the   distal   margin.   Thin-­‐skinned   processes   in   the   upper   part   of   the   nappe   stack   were   enhanced   by   the   occurrence   of   various   potential   detachment   layers   in   the   Late   Carboniferous   to   early   Cenozoic   sedimentary  sequence.  Like  in  the  Provençal  domain  (Decarlis  et  al.,  2014;  Espurt  et  al.,  2019;  Balansa  et   al.,  2022),  the  widespread  Briançonnais  and  Prepiedmont  Triassic  evaporites  allowed  large  fragments  of   the  Meso-­‐Cenozoic  cover  to  detach  and  remain  in  a  shallow  burial  environment.  Interestingly,  this  process   could  not  apply  to  the  internal  Briançonnais  units,  derived  from  the  marginal  uplift  area  (§  3),  where  all   the   potential   detachment   layers   had   been   eroded   during   rift-­‐related   uplift.   This   may   explain   in   part   the   higher   metamorphic   grade   experienced   by   these   units.     This   interpretative   profile   is   also   intended   to   highlight   the   incipient   underthrusting   of   the   European   crust   s.s.     (proximal   margin)   with   the   pinched   Valais-­‐Vocontian   attenuated   crust   area   between   the   European   crust   and   the   Briançonnais   subduction   wedge.     The   nappe   stack   includes,   from   bottom   to   top,   (1)   Late   Paleozoic   sedimentary   and   volcanoclastic   Briançonnais   units   (upper   Carboniferous   "Zone   Houillère"   and   "Permo-­‐Carboniferous   axial   zone");   (2)   Briançonnais   Mesozoic   siliciclastic   and   carbonate   sedimentary   sequences,   either   covering   the   former   units,  or  detached  along  Triassic  evaporitic  layers  and  duplicated.  These  units  contain  some  remnants  of   Eocene   foreland   basins   which   provide   time   constraints   for   thrust   initiation.   They   are   derived   from   a   marginal   area   moderately   affected   by   syn-­‐rift   erosion,   thus   away   from   the   main   marginal   uplift,   (3)   Permian   to   Mesozoic,   mostly   siliceous   nappes,   so-­‐called   "ultrabriançonnais"   or   "Acceglio"   type   units.   These  units  are  derived  from  the  most  active  rift  shoulder  area,  devoid  of  Mesozoic  platform  carbonates   due   to   deep   syn-­‐rift   erosion,   (4)   scarce   occurrence   of   Briançonnais   polymetamorphic   basement   thrust   sheets  with  local  evidence  of  overlying  unconformable  Triassic  sediments,  featuring  inheritance  from  an   elevated   late   Variscan   area   devoid   of   any   post-­‐Variscan   basins,   (5)   Prepiedmont   nappes   transported   from   an   area   close   to   the   continent-­‐ocean   boundary,   beyond   the   rift   shoulder   uplift,   and   providing   evidence   for   northward  thrust  system  propagation.  The  occurrence  of  rift  shoulder  and  of  megabreccia  associated  with   the  uppermost  Briançonnais  units  and  with  some  Prepiedmont  series  (§  8)  suggests  that  the  hinge  zone   between  both  could  correspond  to  a  major  necking  zone  within  the  pre-­‐existing  continental  margin  (Ribes   et  al.,  2019).             29        Figure  15:   a-­‐  Theoretical  profile  of  the  early  Alpine  wedge  during  the  Adria-­‐Iberia  collision  stage,  trending   parallel   to   the   orogenic   propagation   at   this   stage,   that   is   NNW-­‐SSE   (no   vertical   scale).   This   evolution   is   controlled  by  uncoupling  processes,  through  the  ability  of  sedimentary  cover  to  be  detached  and  to  feed  the   shallow   part   of   the   wedge   beneath   the   accretionnary   wedge,   whereas   basement   units   are   diachronously   underplated.  The  Briançonnais  "Zone  Houillère"  and  "Permo-­‐Carboniferous  axial  zone"  Paleozoic  basins  are   represented   at   an   incipient   stage   of   inversion,   and   should   be   overlain   by   thin   polymetamorphic   basement   thrust  sheets  not  visible  at  this  scale.   b-­‐  Paleogeographic  sketch  of  Adria-­‐Iberia  collision  stage  with  location  of  profile  a  (after  Dumont  et  al.,  2012,   adapted).     c-­‐  Recontructed  part  of  the  profile  across  the  Briançonnais  Paleozoic  basins  before  beeing  affected  by  thrust   sequence   and   overlain   by   the   Prepiedmont   and   oceanic   nappes.   This   involves   the   occurrence   of   a   southern   uplifted  border  with  exhumed  basement,  a  potential  origin  for  the  polymetamorphic  thrust-­‐sheets  presently   observed   on   top   of   the   Briançonnais   nappes   stack   (§   7).   Similar   reconstruction   is   provided   from   Ligurian   Briançonnais  units  (§  3).     d-­‐   Paleogeographic   sketch   before   Alpine   collision   (earliest   Eocene),   with   the   postulated   situation   of   the   Paleozoic  basins  within  the  Briançonnais  domain  belonging  to  the  Iberian  plate  (after  Dumont  et  al.,  2012,   adapted).       30   The   original   configuration   of   these   different   units   cannot   be   deduced   from   their   present   metamorphic   signature,   which   results   from   the   activation   of   detachments   through   time   and   on   the   depth   of   their   involvement   within   the   collision   wedge   (i.e.   Michard   et   al.,   2004),   and   also   because   the   initial   metamorphic  record  can  be  overprinted  during  younger  stages  of  collision/extrusion  (Lanari  et  al.,  2014;   Schwartz  et  al.,  2020).     Following   our   interpretation,   the   paleogeographic   origin   of   the   internal   nappes   should   not   be   located   within  the  core  of  the  Western  Alps  arc,  which  is  problematic  to  restore  due  to  overlap,  but  more  probably   in  the  southeast  of  their  present  location.  Consequently,  unfolding  the  profile  of  fig.  15  (a)  would  restore   the   Briançonnais   nappes   far   to   the   south   with   respect   to   the   Dauphiné-­‐Helvetic   domain,   especially   considering  the  gap  represented  by  the  closure  of  the  Valais-­‐Vocontian  basin  presently  squeezed  along  the   Penninic   thrust.   Many   arguments   support   an   original   southern   location   near   to   the   Provence-­‐Corsica   domains   (Maury   &   Ricou,   1983;   Stampfli   et   al.,   2002;   Handy   et   al.,   2010;   Thum   et   al.,   2015).   The   thick   Permo-­‐Carboniferous  volcanic  and  clastic  sequences  characterise  the  southern  Variscan  foreland  and  are   different   from   the   Dauphiné-­‐Helvetic   basement,   which   was   closer   to   the   Variscan   axial   chain   (Guillot   &   Ménot,  2009;  Ballèvre  et  al.,  2018)  This  paleogeographic  situation  persisted  into  the  Permian  (Bourquin  et   al.,  2011).  The  late  Variscan  magmatic  and  volcanic  events  within  the  Briançonnais  units  mark  the  onset  of   post-­‐Variscan   lithospheric   thinning   and   associated   thermal   effects   (Dal   Piaz,   1993),   and   they   are   different   and   more   recent   than   those   in   the   Dauphiné/Helvetic   zone   (Bertrand   et   al.,   2005;   Manzotti   et   al.,   2014;   Ballèvre   et   al.,   2020).   Conversely,   Carboniferous   clastics   and   coal   measures   together   with   Permian   calc-­‐ alcaline   volcanism   show   similarities   to   Provence   and   Corsica   (Basso,   1987;   Toutin-­‐Morin   &   Bonijoly   D.,   1992).   The   Triassic   series   of   the   Maritime   Alps   is   very   similar   to   the   Briançonnais   sequence   (Maury   &   Ricou,   1983;   D'Atri   et   al.,   2016)   and   evaporitic   potential   detachment   layers   which   controlled   the   Briançonnais  cover  deformation  are  widespread  in  Provence  (Bestani  et  al.,  2015).  The  northern  Provence   platform  edge,  running  eastwards  to  crosscut  the  Argentera  massif  cover  (Barale  et  al.,  2017),  is  crosscut   and  transported  northwards  beneath  the  Embrunais-­‐Ubaye  nappes  (Séolane-­‐Cap  unit)  and  further  north   in   the   Prealps   nappes   (Maury   &   Ricou,   1983;   D'Atri   et   al.,   2016).   The   Subbriançonnais   Late   Cretaceous-­‐ Paleogene   flysch   formations   can   be   linked   paleogeographically   both   with   the   Briançonnais   domain   and   with  the  Provence  Pyrenean  foreland  (Kerckhove,  1965;  Blanc  et  al.,  1987;  Thum  et  al.,  2015).       9.3-­‐Pre-­‐Alpine  paleogeography  and  western  termination  of  the  Alps   There   is   thus   strong   evidence   to   consider   the   Briançonnais   as   the   eastern   termination   of   the   Iberia-­‐ Sardinia-­‐Corsica  microplate,  and  the  Prepiedmont  and  Subbbriançonnais  domains  as  transitions  towards   the   Ligurian   Tethys   ocean   and   the   Valais   basin,   respectively.   However,   the   paleogeographic   pattern   of   the   margin   should   not   be   interpreted   as   linear.   The   occurrence   of   lateral   transitions   from   the   Briançonnais   marginal   plateau   to   the   Provence   platform   westwards   (e.g.   Decarlis   et   al.,   2017),   or   towards   its   eastern   termination  within  the  Tethyan  oceanic  floor  (Handy  et  al.,  2010)  is  likely  to  have  complicated  the  thrust   sequence  even  in  the  early  stages  of  inversion.  As  an  example,  some  specific  units  displaying  late  Jurassic   platform   carbonates,   which   are   observed   locally   beneath   the   oceanic   Helminthoid  flysch   nappes   (Dumont   et  al.,  2012),  must  not  be  interpreted  as  being  derived  from  the  most  distal  part  of  the  margin  but  from  its   lateral   transition   to   the   Provence   domain.   These   lateral   variations   may   be   either   progressive,   resulting   from   oblique   opening   and/or   a   scissors-­‐shape   margin,   or   sharp,   due   to   the   occurrence   of   continental   transform   zones   (Lemoine   et   al.,   1989).   Moreover,   the   lateral   termination   of   the   western   Alpine   orogen   coincides   with   a   flip   in   subduction   polarity   of   the   converging   Tethyan   lithosphere   (Lacombe   &   Jolivet,   2005;  Vignaroli  et  al.,  2008;  Argnani,  2009),  possibly  reactivating  an  oceanic  transform  zone  (Dumont  et   al.,   2011,   and   refs   therein).   This   feature,   which   may   have   localised   the   western   termination   of   the   Alps   near  to  the  end  of  the  south-­‐dipping  slab,  may  also  explain  why  the  Helminthoid  Flysch  oceanic  sediments,   which   were   deposited   near   the   European   paleomargin   and   thrust   northwards   or   NW-­‐wards   (Merle   &   Brun,  1984;  Marroni  et  al.,  1992;  Mueller  et  al.,  2019;  Mueller  et  al.,  2020),  could  escape  metamorphism   because   they   had   been   deposited   to   the   west   of   this   transform   boundary,   that   is   in   an   upper-­‐plate   position.   Conversely,   their   time   equivalent   Schistes   Lustrés,   deposited   over   the   southward   subducting   domain  in  lower  plate  position,  were  affected  by  blueschist  metamorphism  within  the  accretionary  wedge   (Agard  et  al.,  2002).     9.4-­‐Early  stages  of  continental  subduction  and  inversion  of  the  distal  margin  structures   Following  the  initiation  of  subduction  at  the  Adria  margin  and  the  intra-­‐oceanic  stacking  stages  (Pleuger   et  al.,  2007),  the  Adria-­‐Europe  continental  collision  was  achieved  in  two  stages  due  to  the  occurrence  of   the  Briançonnais  continental  ribbon  belonging  to  the  Iberian  microplate  (Le  Breton  et  al.,  2021,  and  refs   therein).  The  first  one,  from  the  Lutetian  (possibly  late  Paleocene  in  the  easternmost  Briançonnais  areas;   Bucher   &   Bousquet,   2007)   to   Priabonian,   is   not   recorded   in   the   European   foreland   s.s.   (Dauphinois-­‐   31   Helvetic   domains;   Boutoux   et   al.,   2016),   apart   from   lithospheric   flexure   propagation   (Ford   et   al.,   2006)   and   from   peripheral   consequences   of   the   Pyrenean   foreland   propagation   from   Provence   (Dumont   et   al.,   2011).   This   "Adria-­‐Iberia"   collision   only   affected   the   Briançonnais   marginal   plateau   together   with   its   transition   towards   Tethys,   the   Prepiedmont   margin   toe  (b,   d,   fig.   15;   a,   b,   fig.   16).   S-­‐verging  subduction  of   the  margin  and  northward  motion  of  Adria  driven  by  Africa-­‐Europe  convergence  (Rosenbaum  et  al.,  2002)   was  probably  facilitated  by  the  thinned  character  of  the  Briançonnais  crust  (Mohn  et  al.,  2010;  Le  Breton   et  al.,  2021)  but  the  rift  uplift  geometry  near  to  the  necking  zone,  facing  the  orogenic  wedge  propagation,   played   a   major   role.   Progressive   detachment   within   the   sedimentary   cover   and   the   upper   crust   fed   the   early  collision  prism  (Scheiber  et  al.,  2013;  Tavani  et  al.,  2021),  whose  kinematics  were  controlled  by  the   northward  motion  of  Adria  as  demonstrated  in  the  Central  Alps  (Escher  et  al.,  1993;  Schmid  et  al.,  1997).   This  tectonic  stack,  whose  top-­‐to-­‐bottom  stacking  order  was  likely  representative  of  the  paleogeographic   distribution,  pinches  out  westwards  along  a  sinistral  transfer  zone  presently  incorporated  and  distorted   in   the   southern   part   of   the   arc   (Ricou   &   Siddans,   1986;   Schmid   &   Kissling,   2000;   Schmid   et   al.,   2017).   The   incipient   stages   of   inversion   in   the   Briançonnais   domain   were   probably   analogous   to   the   present   structure   of   Provence,   where   Triassic   evaporites   played   a   major   role  as   a   detachment   layer  (Bestani  et   al.,   2015),  but  the  topographic  surface  of  the  Briançonnais  orogen  also  shows  evidence  of  olistostromes  and   large-­‐scale  gravity  induced  deposits  in  a  deep  flexural  basin  setting.  The  thick  late  Variscan  Briançonnais   basins,  which  possibly  trended  oblique  to  the  propagation  considering  the  late  Variscan  paleogeography   (Guillot   &   Ménot,   2009;   Pfiffner,   2014),   were   also   detached   (Zone   Houillère)   and   overthrust   by   their   southern   margin   (most   internal   Briançonnais   units),   allowing   the   development   of   a   top-­‐to-­‐the   N-­‐NW   thrust   sequence   (c,   fig.   15).   Below   the   detachments,   most   of   the   Variscan   and   older   metamorphic   basement   was   diachronously   underthrust   beneath   the   accretion-­‐collision   wedge   (Bucher   et   al.,   2004;   Malusà  et  al.,  2005;  Berger  &  Bousquet,  2008;  Strzerzynski  et  al.,  2011;   Lanari  et  al.,  2012;  Scheiber  et  al.,   2013;  Pfiffner,  2014),  experiencing  HP  metamorphism  spanning  from  ~50  Ma  to  ~35  Ma,  and  marked  by   N-­‐   to   NW-­‐directed   tectonic   transport   criteria   (Dumont   et   al.,   2012).   The   coeval   northward   drift   of   the   Adria/Briançonnais  continental  subduction  wedge  (b,  fig.  15;  b,  fig.  16)  caused  the  closure  of  the  Valais-­‐ East   Vocontian   basins,   translated   the   Adria   lithosphere   to   a   higher   latitude   and   shallower   depth,   and   eventually  resulting  in  juxtaposition  with  the  Dauphiné/Helvetic  lithosphere.       9.5-­‐Late  stages  of  continental  collision   The  second  part  of  Alpine  orogenic  evolution,  from  the  Eocene-­‐Oligocene  boundary  onwards  (Dumont  et   al.,   2012),   is   dominated   by   westward   extrusion   (WNW   to   WSW)   of   the   previously   elevated   Adria   upper   mantle   towards   the   European   foreland   (c,   d,   fig.   16),   causing   both   steep   subduction   of   the   European   lithosphere  (Zhao  et  al.,  2015;  Malusà  et  al.,  2021)  and  indentation  of  the  previous  continental  subduction   wedge   by   the   Ivrea   body   (Schmid   et   al.,   2017),   shaping   the   arc   of   the   Western   Alps   and   exhuming   the   HP-­‐ LT  evidence  of  the  early  stage.  This  kinematic  stage  was  accommodated  by  strike-­‐slip  motion  and  orogen-­‐ parallel  extension  (Mancktelow,  1992;  Steck,  2008;  Campani  et  al.,  2010;  Ring  &  Gerdes,  2016)  with  local   thermal   overprint   (Bousquet,   2008a;   Wiederkehr   et   al.,   2010)   in   the   Central   and   Eastern   Alps.   In   the   Western   arc,   radial   spreading   and   forward   propagation   involving   newly   formed   crustal-­‐scale   thrusts   facilitated   the   exhumation   of   the   external   foreland   until   recently   (Schwartz   et   al.,   2017).   This   "modern"   Alpine   orogenic   stage   accommodates   the   Apenninic   dynamics   and   the   Corsica-­‐Sardinia   breakoff   (Gidon,   1974;  Laubscher,  1991;  Maffione  et  al.,  2008),  suggesting  that  coupled  driving  forces  should  be  sought  in   the   Mediterranean   and   Alpine   slab   dynamics   (Jolivet   et   al.,   2008;   Vignaroli   et   al.,   2008;   Faccenna   et   al.,   2014;   Salimbeni   et   al.,   2018;   Eva   et   al.   2020)   rather   than   in   Africa-­‐Europe   convergence,   and   should   incorporate   consideration   of   gravitational   spreading.   The   "Adria-­‐Europe"   collision   stage,   which   is   responsible  for  the  most  obvious  structures  of  the  Internal  Zones,  in  particular  the  arc  and  the  "Penninic   thrust",   disturbed   the   initial   stacking   order   and   overprinted   the   metamorphic   record   of   the   internal   units.   The   dome   shape   of   the   main   internal   cristalline   massifs   (Gran   Paradiso,   Vanoise,   Ambin,   Dora-­‐Maira)   could   partly   result   from   crossed   shortening   episodes   between   early   and   late   stages   of   continental   collision,   as   proposed   in   the   external   zone   (Dumont   et   al.,   2011).   Despite   this   overprint,   we   argue   that   the   early  stage  structures  remain  preserved  at  all  scales.       9.6-­‐Insights  for  large-­‐scale  3D  structure   The  main  geophysical  transects   imaging  the  finite  lithospheric  structure  in  the  Western  Alps  are  oriented   perpendicular  to  the  trend  of  the  arc  (NRP  20:  Pfiffner  et  al.,  1997;  ECORS:  Guellec  et  al.,  1990;  CIFALPS:   Malusà   et   al.,   2021).   Following   our   interpretation,   the   geometrical   expression   of   convergence   must   be   different  depending  on  their  location  and  orientation.  In  the  northern  part  of  the  arc  (NFP  20),  the  NW-­‐SE   to  NNW-­‐SSE  orientation  is  able  to  take  into  account  most  of  the  early  stage  nappe  stacking  together  with   later  backfolding.  However,  such  a  profile  is  not  adequate  to  observe  the  late  stage  of  dextral  shear       32        Figure  16:   4D  sketch  from  the  initiation  of  the  Adria-­‐Iberia  collision  (a)  to  the  westward  extrusion  atage  (d),   illustrating   the   behaviour   of   the   Tethyan   slabs   and   the   key   role   of   the   subduction   flip   across   the   western   Adria  transform  zone.     a-­‐   The   northward   drift   of   Adria   (yellow)   and   the   southward   subduction   of   the   E-­‐Tethyan   slab   (blue)   are   bounded  westwards  by  a  transform  zone,  and  the  accretionnary  wedge  is  reaching  the  easternmost  margin   of  the  Iberian  plate.  The  northwestern  part  of  Adria  lithospheric  mantle  will  become  the  uplifted  Ivrea  body   in  the  late  stages  of  extrusion.   b-­‐   The   easternmost   part   of   the   Iberian   plate   (Briançonnais   and   Prepiedmont   domains,   respectively)   are   undethrust   beneath   the   northward   moving   Adria   plate,   leading   to   detachment,   inversion   and   nappes   stacking  processes  in  the  subducted  upper  crust,  marked  by  deformation  D1.  The  Alpine  orogen  is  preceeded   by   the   development   of   a   flexural   basin   also   propagating   northwards   and   pinching   westwards   along   the   transform   boundary.   This   stage   is   also   responsible   for   the   progressive   closure   of   the   Valais   trough.   By   that   time,  convergence  transmitted  through  Iberia  plate  also  activates  or  re-­‐activates  the  Pyrenean  orogen  and   foreland.         c-­‐  The  N-­‐  to  NW-­‐directed  propagation  ceases  the  end  of  Eocene,  after  complete  closure  of  the  Valais  trough,   and   a   sharp   kinematic   change   occurs   with   both   initiation   of   extrusion   of   the   northern   part   of   Adria,   and   development   of   slab   rollback   beneath   the   eastern   part   of   Iberia   (green),   marked   by   the   western   European   Cenozoic  rifts.     d-­‐   The   westward   propagation   of   the   Western   Alps,   accomodated   in   the   Central   Alps   by   orogen-­‐parallel   extension   (Simplon   fault)   and   dextral   strike-­‐slip   faulting   (Insubric   fault),   leads   to   the   formation   of   the   arc   with   radial   spreading   deformation.   The   Internal   Zones,   containing   the   accretionary   and   continental   subduction   wedges   built   during   the   previous   stages,   are   exhumed   and   pinched   between   forward   and   backward  structures  (D2,  red  and  green,  respectively),  among  which  the  Penninic  Thrust.  This  stage  is  driven   by  indentation  at  depth  by  the  Ivrea  body,  a  shallow  isolated  piece  of  Adria  lithospheric  mantle  in  the  core  of   the   arc,   well   identified   by   geophysics   (Malusà   et   al.,   2021).     The   abrupt   southwestern   boundary   of   this   lithospheric  indenter  could  derive  from  the  western  Adria  transform  zone  having  accomodated  its  northward   drift  during  the  previous  stages.     33      coupled   with   extrusion.   On   the   other   hand,   a   NE-­‐SW   oriented   profile   in   the   southern   part   of   the   arc   (CIFALPS  1)  shows  a  nice  expression  of  the  late  westward  extrusion  phase  but  fails  to  see  the  earlier  N-­‐  to   NW-­‐directed   nappe   stacking.   Consequently,   any   attempt   of   restoration   using   such   radially   oriented   lithospheric   profiles   must   follow   a   sequential   approach,   first   based   on   the   southern   profiles   to   retrodeform  extrusion,  then  considering  northern  profiles  to  restore  the  main  part  of  stacking  due  to  N-­‐ NW-­‐ward  convergence.   A   key   feature   of   our   model   is   the   location   of   the   western   termination   of   the   Alps   along   a   lithospheric   sinistral  strike-­‐slip  boundary  named  the  "Western  Adria  Transform  Zone"  (Dumont  et  al.,  2012;  fig.  16a),   which   was   subsequently   involved   and   distorted   in   the   arc   (Malusà   et   al.   2015;   Schmid   et   al.,   2017;   fig.   16d).   Despite   the   overprint,   some   relicts   of   such   a   feature   should   still   be   observed   in   the   present   lithospheric   structure,   which   has   recently   been   investigated   with   increasing   resolution   (Hetényi   et   al.,   2018;  Malusà  et  al.,  2021).  The  shape  of  the  Ivrea  Body  indenter,  representing  Adria  lithospheric  mantle   (Zhao  et  al.,  2020)  is  now  relatively  well  constrained  (Schmid  et  al.,  2017  and  refs.  therein)  and  its  regular   trend   is   sharply   interrupted   southwards   to   the   west   of   Cuneo   city.   This   sharp   discontinuity   at   depth   contrasts   with   the   curved   form   of   the   Penninic   units   at   surface   in   the   southern   part   of   the   arc,   which   suggests   that   the   latter   are   decoupled   from   the   indenter.   We   propose   an   interpretation   of   the   sharp   southern   termination   of   the   Ivrea   body   as   an   anticlockwise   rotated   relict   of   the   Western   Adria   Transform   Zone   (d,   fig.   16).   This   interpretation   is   consistent   with   the   occurrence   of   anticlockwise   rotation   of   the   southern  Tertiary  Piedmont  Basin  since  the  Oligocene  (Maffione  et  al.,  2008).  The  shallow  location  of  the   Ivrea  Body,  especially  to  the  south  (Lardeaux  et  al.,  2006)  could  be  either  inherited  from  the  early  stage  of   Tethyan   rifting   or   from   the   main   exhumation   stage   of   (U)HP   continental   units   in   the   Eocene   (Malusà   et   al.   2021),  bringing  the  Adria  upper  mantle  and  subduction  wedge  into  position  before  westward  indentation.   Finally,   considering   the   western   boundary   of   the   Alps   as   inherited   from   a   transform   boundary   between   two   opposed-­‐dip   oceanic   subduction   zones   allows   for   an   early   initiation   of   the   asthenospheric   counterflow  though  the  tear  zone,  subsequently  enhanced  by  Apenninic  rollback.  Such  an  asthenospheric   counterflow  is  documented  by  mantle  anisotropy  beneath  the  Alps-­‐Apennines  junction  (Salimbeni  et  al.,   2018).         Conclusion     The   occurrence   of   an   early   phase   of   along-­‐strike   tectonic   transport   criteria   in   the   southern   part   of   the   Internal  Western  Alps  arc  is  indicative  of  an  early  stage  of  N-­‐  to  NW-­‐directed  nappe  stacking  associated   with   the   involvement   of   the   easternmost   domains   of   the   Iberia   plate   (Briançonnais,   Prepiedmont)   in   continental   subduction   beneath   the   Adria   plate   since   early   Eocene.   We   propose   that   this   early   stage   had   a   major   impact   on   both   metamorphic   imprint   and   translation   of   nappes,   through   the   control   of   delamination  processes  within  the  upper  crustal  section,  and  that  it  is  chiefly  responsible  for  "inversion"   of   not   only   Mesozoic   marginal   rift   structures,   but   also   of   late   Variscan   foreland   structures   (Scheiber   et   al.,   2013;  Ballèvre  et  al.,  2018).  The  associated  structures  were  later  (early  Oligocene  onwards)  overprinted   and  distorted  during  the  formation  and  bending  of  the  arc,  and  were  crosscut  by  the  Penninic  Thrust,  an   expression   of   the   westward   extrusion   and   exhumation   of   the   previously   formed   continental   subduction   wedge.   Westward   extrusion   was   accommodated   in   the   Central   Alps   by   dextral   displacement   along   the   Insubric   line   as   part   of   the   Periadriatic   fault   zone   (Laubscher,   1991;   Schärer   et   al.,   1996),   by   ductile   shear   and  extension   along  the  Simplon  fault  zone  (Mancktelow,  1992;  Escher  et  al.,  1997;  Steck,  2008;  Campani   et  al.,  2010),  and  by  exhumation  of  the  Lepontine  dome  (Wiederkehr  et  al.,  2008;  Steck  et  al.,  2013,  2019).   The   Western   Alps   result   from   a   succession   of   orogenic   phases:   firstly,   during   the   Eocene,   S   to   SE   subduction   of   the   easternmost   part   of   the   Iberia   plate,   namely   the   Prepiedmont   and   Briançonnais   domains,   beneath   the   Adria  plate  and  the  oceanic  accretionary  wedge;   secondly,   from   the   early   Oligocene   onwards,  WNW  to  WSW  extrusion  of  the  previous  orogenic  wedge  over  the  subducted  European  plate.     The  first  stage  was  accommodated  by  a  major  sinistral  transcurrent  boundary  between  Corsica-­‐Provence   and   the   Briançonnais,   which   was   possibly   inherited   from   a   tear   boundary   between   two   opposed-­‐dip   subduction  areas  within   the   residual  Tethyan  oceanic  domain  (a,  fig.  16).   The   development   of   this   early   orogenic   wedge   was   controlled   by   the   northward   drift   of   Adria   (b,   fig.   16),   beneath   which   the   Prepiedmont   and   Briançonnais   "distal"   continental   margin   units   were   diachronously   involved,   likely   activating  crustal  uncoupling  processes  similar  to  those  described  in  Provence  (Bestani  et  al.,  2015)  and  in   the  Pyrenean  foreland  (Lacombe  &  Mouthereau,  1999),  together  with  detachment  of  the  upper  Paleozoic   and   Mesozoic   sedimentary   cover,   partly   controlled   by   evaporites   (Michard   et   al.,   2004).   The   European   domains   s.s.   (Dauphiné-­‐Helvetic)   were   only   lightly   affected   by   this   deformation,   mainly   through   reactivation  of  the  Pyrenean-­‐Provence  structures  at  the  northern  margin  of  the  Iberian  plate  s.l.    However,     34   this  N-­‐NW  propagation  of  the  early  Alpine  orogen,  with  a  minimum  translation  of  approximately  200km   (Schmid   &   Kissling,   2000;   Ford   et   al.,   2006),   accommodated   the   closure   of   the   eastern   part   of   the   Vocontian-­‐Valais   basin.   This   propagation   was   fringed   to   the   north   and   NW   by   the   development   of   a   flexural   basin,   over   which   the   surficial   record   of   this   early   orogenic   wedge   is   locally   preserved   (Swiss   and   French   Prealps,   Embrunais-­‐Ubaye   nappes,   Ligurian   flysch   nappes).   It   consists   of   various   tectono-­‐ sedimentary  breccias  and  olistostromes,  sometimes  reworking  mixed  oceanic-­‐continental  material  in  the   vicinity  of  the  sole  thrust  of  the  lowermost  oceanic  nappes.     At  the  initiation  of  the  second  stage,  close  to  the  Eocene-­‐Oligocene  boundary,  some  parts  of  lithospheric   mantle   of   the   Adria   upper   plate   had   been   brought   to   a   shallow   depth   in   front   of   the   Dauphiné-­‐Helvetic   crust,  due  to  underplating  of  the  Briançonnais  crustal  elements  and  in  response  to  its  steep  transcurrent   western  boundary  (c,  fig.  16).  Thus,  the  western  Adria  lithospheric  mantle  was  suitably  located  to  indent   the   European   crust   in   the   southern   Western   Alps.   This   stage   was   driven   by   westward   extrusion   of   the   northern   Adria   plate,   accommodated   by   the   dextral   activation   of   part   of   the   Periadriatic   fault   zone   (Insubric   line)   and   extension   in   the   Simplon-­‐Lepontine   areas.   The   western   Alpine   extrusion   occurred   coeval   with   rifting   and   breakup   of   the   Ligurian,   then   Thyrrenian   oceanic   domains   and   with   the   propagation   of   the   Apennine   orogen   in   a   slab   rollback   framework   (Jolivet   et   al.,   2008),   suggesting   the   occurrence   of   an   asthenospheric   counterflow   responsible   for   coupling   these   opposite   dynamics   (Salimbeni  et  al.,  2018).    The  expression  of  extrusion  in  surface  geology  consists  of  exhumation,  forward   and   backward   thrust-­‐folding   and   distortion   of   the   initial   stack   along   the   arc,   activation   of   the   Penninic   Thrust  and  radial  outward  propagation  of  thin-­‐  and  thick-­‐skinned  deformation  in  the  external  foreland  (d,   fig.   16).   This   largely   overprinted   the   initial   structures   in   the   Internal   zones,   although   the   amount   of   horizontal  displacement  was  possibly  a  lower  order  of  magnitude  than  during  the  first  stage.       Aknowledgements     Adrian   Pfiffner   and   an   anonymous   reviewer,   as   well   as   the   Editor   Carlo   Doglioni,   are   gratefully   aknowledged  for  thoughtful  and  constructive  reviews,  which  significantly  improved  the  manuscript.  The   authors   are   grateful   to   the   Cifalps   project   team   for   stimulating   collaboration   focused   on   the   present   lithospheric  structure  of  the  Western  Alps  ,  and  to  Steve  Matthews  for  many  field  discussions  and  debates   in  the  Briançonnais  and  adjoining  areas  over  the  past  2  decades.     References     Agard,  P.,  2021.  Subduction  of  oceanic  lithosphere  in  the  Alps:  Selective  and  archetypal  from  (slow-­‐spreading)  oceans.  Earth-­‐Science   Reviews,  214,  doi.org/10.1016/j.earscirev.2021.103517   Agard,   P.   &   Lemoine,   M.,   2003.   Visage   des   Alpes:   Structure   et   évolution   géodynamique.   Commission   de   la   Carte   Géologique   du   Monde,  50p.,  ISBN  2-­‐9517181-­‐1-­‐X   Agard,   P.,   Monié,   P.,   Jolivet,   L.   &   Goffé,   B.,   2002.   Exhumation   of   the   Schistes 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Site%N°% Locality% Latitude% Longitude% 1" 2" 3" 4" 5" 6" 7" 8" 9" 10" 11" 12" 13" 14" 15" 16" 17" 18" 19" 20" 21" 22" 23" 24" 25" 26" 27" 28" 29" 30" 31" 32" 33" 34" 35" 36" 37" 38" 39a" 39b" 40" 41" 42" 43" 44" 45" 46" 47" 48" 49" 50" 51" 52" 53" 54" 55" 45°06'50"" 45°06'15"" 45°07'35"" 45°04'30"" 45°03'34"" 45°02'43"" 45°01'12"" 44°59'10"" 44°59'30"" 44°59'36"" 44°58'00"" 44°56'15"" 44°56'59"" 44°56'38"" 44°55'39"" 44°54'16"" 44°54'44"" 44°54'22"" 44°51'09"" 44°53'32"" 44°52'26"" 44°50'15"" 44°48'47"" 44°47'48"" 44°47'03"" 44°47'03"" 44°47'30"" 44°47'41"" 44°46'22"" 44°45'21"" 44°43'02"" 44°43'57"" 44°43'12"" 44°43'13"" 44°43'16"" 44°40'38"" 44°40'44"" 44°39'22"" 44°38'04"" 44°38'07"" 44°36'30"" 44°35'53"" 44°35'07"" 44°34'36"" 44°35'09"" 44°34'25"" 44°33'56"" 44°32'08"" 44°30'18"" 44°33'39"" 44°34'03"" 44°33'00"" 44°26'11"" 44°20'08"" 44°17'36"" 44°15'13"" 6°36'30"" 6°38'42"" 6°48'20"" 6°50'21"" 6°30'10"" 6°38'35"" 6°41'45"" 6°38'22"" 6°32'50"" 6°41'25"" 6°42'47"" 6°42'18"" 6°55'20"" 6°48'32"" 6°40'06"" 6°38'54"" 6°32'44"" 6°32'56"" 6°32'47"" 6°43'16"" 6°43'31"" 6°41'00"" 6°41'21"" 6°40'50"" 6°40'58"" 6°39'00"" 6°36'57"" 6°45'17"" 6°43'24"" 6°38'40"" 6°40'08"" 6°42'42"" 6°43'18"" 6°44'48"" 6°45'57"" 6°40'33"" 6°47'57"" 6°40'30"" 6°55'53"" 6°56'52"" 6°53'11"" 6°50'53"" 6°51'27"" 6°51'39"" 6°52'02"" 6°48'50"" 6°48'29"" 6°50'43"" 6°51'30"" 6°38'31"" 6°36'54"" 6°36'40"" 6°26'17"" 6°41'47"" 6°37'01"" 6°43'27"" Col"de"la"Vallée"Etroite" Valle"di"Rho,"NW"Bardonecchia" Valle"di"Rochemolles,"NE"Bardonecchia" Mte"Seguret,"N"Oulx" Lac"des"Beraudes,"Cerces"massif" l'Aiguille"Rouge,"NE"Névache" Col"des"Acles,"E"Névache" Forts"de"l'OliveOLenlon,"S"Névache" Tête"Noire,"NE"Monêtier" Pointe"de"Pécé,"ESE"Névache" Rio"Secco,"Col"de"la"Lauze,"N"Montgenèvre" Clot"Enjaime,"NW"Montgenèvre" Mte"Banchetta,"SE"Sestriere" Champlas"Seguin,"W"Sestriere" W."slopes"of"Clarée,"N"La"Vachette" Fort"des"Salettes,"Briançon" N."Serre"Chevalier,"WNW"Briançon" S."Serre"Chevalier,"WNW"Briançon" Tenailles"de"Montbrison,"N"l'Argentière" Fort"du"Gondran,"Chenaillet,"SE"Briançon" Cervières,"SE"Briançon" W."Crête"des"Granges,"S"Briançon" Col"des"AyesOBeaudouis,"S"Briançon" Chalets"de"Clapeyto,"NW"Arvieux" Pic"de"Balart,"NW"Arvieux" Crête"de"la"Moulière,"E"l'Argentière" Pic"du"Bonhomme,"E"l'Argentière" Col"du"Tronchet,"NE"Arvieux" Le"Coin"d'Arvieux,"N"Château"Queyras" Pic"du"Grand"Vallon,"SE"l'Argentière" Col"du"Lauzet,"NE"Guillestre" Col"de"Furfande,"SW"Château"Queyras" Col"de"la"Lauze,"SW"Château"Queyras" Les"Escoyères,"SW"ChâteauOQueyras" Road"to"Montbardon,"SW"Château"Queyras" Gros,"NE"Guillestre" Col"du"Fromage,"Ceillac,"E"Guillestre" Combe"Chauve,"E"Guillestre" W"Col"du"Longet,"NE"St"Paul"s/Ubaye" E"Col"du"Longet,"NE"St"Paul"s/Ubaye" S."Péouvou,"NE"St"Paul"s/Ubaye" Combe"Brémond,"NE"St"Paul"s/Ubaye" lower"Vallon"de"Mary,"NE"St"Paul"s/Ubaye" upper"Vallon"de"Mary,"Roure,"NE"St"Paul"s/Ubaye" Bergerie"de"l'Alpet,"NE"St"Paul"s/Ubaye" La"Barge,"NE"St"Paul"s/Ubaye" S"Tête"du"Sanglier,"NE"St"Paul"s/Ubaye" N"Brec"de"Chambeyron,"E"St"Paul"s/Ubaye" Col"de"Stroppia,"E"St"Paul"s/Ubaye" Pic"de"Crevoux,"W"Vars" Pic"de"Chabrieres,"W"Vars" Crevoux,"E"Embrun" Le"LauzetOUbaye,"W"Barcelonnette" SuperOSauze,"S"Barcelonnette" Petit"Cheval"de"Bois,"Col"d'Allos,"S."Barcelonnette" Col"de"la"Cayolle,"S"Barcelonnette" ! ! Table  I:    Location  of  the  microstructural  data  sites  listed  in  figs.  10  and  11.           45            Table   II:   Results   of   step-­‐heating   on   phengite   sample   BLA-­‐2016-­‐10   (location   site   13   fig.   10,   and   fig.   14f).   40Ar/39Ar  analyses  were  conducted  in  the  Noble  Gas  laboratory  of  Géosciences  at  the  University  of  Montpellier   2,   France,   using   a   multicollector   mass   spectrometer   (Thermo   Scientific   Argus   VI   MS)   with   a   nominal   mass   resolution   of   200   and   a   sensitivity   for   argon   measurements   of   3.55 × 10−17 moles/fA   at   200 μA   trap   current.     46