ABSTRACT

This study concerns the northen part of the Vanoise massif (French Alps) and " re precisely the "Grande Sassière" and "Mont Pourri" massifs within the "Sainte-Foy-Tarentaise - Val d'Isère" region.

A. Two paleogeographic realms have been distinguished :

1°/- An internal briançonnais realm :
Laying on a paleosoïc basement, it consists of mesozoïc and cenozoïc terrains from Trias up to Palecocene and may go up to Eocène (?)
This realm is subsident with calm sedimentation during the Trias. It has a complete change from the Lias onwards induced by a distensive event ; which has for consequence the deposit of reduced sedimentary sequences rich in breccial levels and stratigraphic gaps.
In spite of the great diversity of the stratigraphic columns studied, five sets of briançonnaise mesozoïc series can be individualized :

• a- The "Val d'Isère series" generally show a thick Trias and locally imposing breccias (Tsanteleina and Santel breccias) ;
• b- The "Clou series". They generally show, under the posttriassic unconformabilities, a very reduced Trias (by erosion ?) ;
• c- ThQ "Chevril series". The small outcrops that they present, show us incomplete sequences ;
• d- The "haut val de Rhême series" in an internal position and limited to a little thick of unconformable Marbres Chloriteux (Upper cretaceous - Paleocene) laying directly on the paleozoïc basement of the Pointe de la Traversière and the haut Val de Rhemes.
• Q- ThQ "Vanoise occidentale serie", in an external position, doesn't outcrop in the studied region. It is characterizedby a marin Dogger with "Mytilus" (F. Ellenberger, 1958), and represents the stratigraphic blanket of the "Mont Pourri" and "Archeboc" basement.

B. From the structural and metamorphic point of view.
The terrains have been affected by a polyphased compressive tectonic and by important metamorphic recrystallizations. The chronology of these post eocene events remains unfortunately relative.
- We have recongnized :

1°/- An early polyphased thrusting phase inducing the tectonic positioning of the "schistes lustrés" nappe on the internal briançonnaise zone.
The different structural sets that were born during this first thrusting phase are :

• - The paleozoïc basement of the "Mont Pourri" and "Archeboc" massifs.
• - The paleozoïc slice of the "Brévières".
• - The blanket slices of the "Val d'Isère, Clou" and "Chevril" regions.
• - The paleozoïc basement of the "Pointe de la Traversière" and that of the "haut Val de Rhemes" fit with its cretaceous blanket.

2°/- The deformation of the early thrusts :
Assymetric to subisoclinal recumbent E-W or W-NW - E-SE folds are superimposed on the first structures. Locally it is possible to observe other folds NE-SW.
This phase is contemporaneous of metamorphic recrystallizations within the green schists facies. To explain the complex envelopments of the structural units, we propose a model based on the existence of kilometric hoodling structures of which directions have been mentioned above. They deform the nappe setting ("Briançonnaise" and "Schistes Lustrés" nappes) into sheath megafolds (anticlinal and synclinal) of nappes.
Contrarily to the early thrusting phase (obduction of "piémontais" material) we should have here a phase of intense tightening of the margin (hyper-collision).

3°/- A late thrusting phase transports the setting of the nappes onto the external briançonnaise zone ("zone houillère") and finally individualizes the great morphostructural sets of the studied region, including among others the composite klippe of the "Grande Sassière".
The first dexter play of the NW-SE transverse faults of the Isère Valley could be contemporaneous to the advancing of these late thrusts.

4°/- A buldging phase gives birth to large radius ondulations and has for principal consequence the north-westwards tilt of the late thrust plans in the south east of the studied region giving them, therefore, an image of back thrusting.
This phase, probably Miocene (?) precedes or accompanies the transverse or normal play of accidents like the "Chardonnet - Val de Rhêmes " fault and the recent faults of the "Isère" valley.

N° 2- 1984 Roger MARION: Contribution à l'étude géologique de la Vanoise, Alpes Occidentales. Le massif de la Grande Sassière et de la région de Tignes-Val d'lsére.196 p. (150 francs ou 22,86 euros, + port). ISBN 2-904431-07-1 ; Résumé français.

Cet ouvrage est diponible auprès de Andre.Paillet@univ-savoie.fr