ABSTRACT

This study concerns the eastern Vanoise units in the western Alps, at the internal part of the Briançonnais zone and at the front of the "Schistes lustrés" complex nappe.

A. FROM THE STRATIGRAPHIC AND PALEOGEOGRAPHIC POINT OF VIEW three sets of units are distinguishad :

1 A group of units born of the European passive continental paléo-margin for which the alpine sedimentary evolution is similar from Trias to Lower Cretacoous; this evolution is characterized by :
- A pre-rift stage (Trias) marked by a subsident platform sedimentation whose deposits (quartrites and carbonates) are preserved from the later erosions only in the Val d' Isère, Grande Motte, les Fours and Prariond units.
- The rifting stage ( Lias -Dogger ) characterized by important erosions on higher parts of the margin (Les Lombards, Calabourdane units and probably a part of the Grand Paradis massif) and by the deposit of liasic sediments in several throughs (Grande Motte, les Fours and Prariond units).
- A post-rift stage (Upper Dogger-Lower Cretacaous) corresponding with the subsidence of the margin leading to the generalization of the pelagic sedimentation during the Upper Jurassic.
After a generalized sedimentary gap during the Lower Cretacaous, the margin becomes different in two paleographic realms at the beginning of the Late Cretacaous.
- The Briançonnais realm (the most external one) stays in a pelagic environment until Early Eocene. In this realm, deposits of alcaline pyroclastites take place towards the Cretaceous-Tertiary limit and synmetamorphics deformations occur during the Middle-Upper Eocene.
- The Piémontais realm (the most internal one) is affected by a calcareous detritic sedimentation during the late Cretaceous. This realm is involdad in synmetamorphic deformations as early as the Senonian (Campanian ?).

2 A group of units born from the Tethysian Paleo-ocean constituted by slices of oceanic substratum and of its sedimentary cover.
- The ophiolitic basement, born during Callovian-Oxfordian times, is made up of serpentinites and more rarely of gabbros.
- The cover locally begins by ophiolitic brecchias coming from the destruction of the substratum. Then, it comprises carbonates with ophiolitic elements (attributable to Upper Jurassic-Berriasian) and thick calcarsous detritic sediments, dated from late Cretaceous (Cenomanian-Early Senonian), in which are scattered coarse detritic materials (turbidites, debris-flows and blocks) of mixed origin (oceanic and continental).
The most part of the oceanic units is involved in synmetamorphic deformations as early as the Senonian.

3 A group of units whose stratigraphic substratum is unknown constituted by terrigenous synorogenic metasediments with flysch facies, without ophiolitic detritic material, dated from the latest Cretaceous ("Schistes lustrés" of the Pointe du Grand Vallon.

B. FROM THE TECTONIC POINT OF VIEW are distingued different deformation stages :

1. An event at the beginning of the Late Cretaceous interpreted as the beginning of the ocean slicing. It has been detected only by its sedimentary consequences (mixed oceanic and continental detritism).

2 An event during the Senonian (Campanian) charaterized by magabrecchias at the internal part of the Briançonnais realm and by the thrusting of oceanic units on the border of the european continent (Piemontais realm) in the conditions of the "eoalpine" HP-LT metamorphism (dated between 90 and 65 M.a.). The latest Cretaceous detritism is probably the sedimentary consequence of this event.

3. A succession of events from Middle-Upper Eocene to present causing the thrusting of the complex nappe of the "Schistes lustrés" (including the Piemontais, oceanic and "Pointe du Grand Vallon" units) on the Briançonnais realm, then the deformation of the nappes edifice in penninic folds in the conditions of the "alpine s.str. or lopontine" HPLT metamorphism (dated towards 40-38 M.a.). Finally, late westhwerds thrustings shear the earlier structures before being affected by large Mio-Pliocene bulgings (Arpont and Grand Paradis domes) and by different generations of faults.

N° 8- 1987 Eric DEVILLE : Etude géologique en Vanoise orientale (Alpes occidentales françaises, Savoie). De la naissance à la structuration d'un secteur de la paleomarge européenne et de l'océan têthysien: aspects stratigraphiques, pétrographiques et tectoniques. 296 p. (200 francs ou 30,48 euros + port). ISBN 2-904431-17-9. Résumé français.