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Contribution à l'étude géologique de la Vanoise - Alpes occidentales le massif de la Grande Sassière et la région de Tignes - Val d'Isère - Alpes françaises

Roger Marion 1
1 Département des Sciences de la Terre et de l'Environnement
CISM - Centre Interdisciplinaire Scientifique de la Montagne
Abstract : This study concerns the northen part of the Vanoise massif (French Alps) and more 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 paleozolc basernent, it consists of mesozolc and cenozolc terrains from Trias up to Paleocene 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 mesozolc 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.- The " Chevril series". The small outcrops that they present, show, us incomplete sequences d- The "haut val de Rhêmes seires" in an internal positlon and limited to a little thick of unconformable Marbres Chloriteux (Upper cretaceous - Paleocene) laying directly on the paleozolc basement of the Pointe de la Traversière and the haut Val de Rhèmes . e- The. "Vanoise occidentale serie", in an external position, doesn't outcrop in the studied region. It is characterized by a marin Dogger with "Mytilus" (F. Ellenberger, 1958), and represents the stratigraphic blanket of the "Mont Pourri " and "Archeboc" basement. 2- The " schistes lustrès" napp : Coming from the inner "piémontais " or "Ligura-piémontais " realm it doesn't have, in the studied region, a proper stratigraphie serie : it is a powerful sequence of dlark calcschists more or less marmorean in which metabasits are interbedded. In addition, serpentinites peg the base of the nappe. The precise age of this mesozolcal formation is still unknown. 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 internaI briançonnaise zone. The different structural sets that were born during this first thrusting phase are : - The paleozolc basement of the "Mont Pourri" and "Archeboc" massifs. - The paleozolc slice of the "Brévières". - The blanket slices of the "Val d 'Isère, Clou" and "Chevril" regions. - The paleozolc basement of the "Pointe de la Traversière" and that of the "haut Val de Rhêmes " fit with its cretaceous blanket. This phase, synchron or stitly posterior to the metamorphic foliation (blue schists) of the internaI briançonnaise zone, leads to the elaboration of a complex pilling up of over thrust nappes. 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 sett ing of the nappes on to 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 building 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.
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Submitted on : Thursday, August 22, 2013 - 9:40:01 AM
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Roger Marion. Contribution à l'étude géologique de la Vanoise - Alpes occidentales le massif de la Grande Sassière et la région de Tignes - Val d'Isère - Alpes françaises. Tectonique. Université de Savoie, 1984. Français. ⟨tel-00616588v2⟩

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