Etude Géologique du Massif du Ruitor ( Alpes Franco-italiennes) : évolution structurale d'un socle briançonnais.

Abstract : This structural study is based on a mapping of the Ruitor Massif which constitutes the basement of the "zone houillère briançonnaise" (Savoie Alps). Petrographic study shows that this basernent is cornposed of pelitie rocks tending towards graywacke in composition, intruded by tholeitie and silicic (anatectic) magmas. This basernent underwent a Hercynian or even earlier amphibolite-facies metarnorphism (with an associated foliation), then a greenschist to amphibolite-facies metamorphism (with an associated crenulation c1eavage). Il is similar to the Gd-St-Bernard, South Vanoise and Ambin basement massifs. However the Alpine retrograde metamorphism was less pervasive in the Ruitor. During the Late Carboniferous this basement limited the eastern border of the Houiller basin. Four Alpine deformation phases have been observed. 1) A first foliation FI, linked to a glaucophanitic greenschist facies (1ineation FI : NW), occurred after the HP peak. 2) Isoclinal folds P2 have NNE axes and WNW f (lineation L2 = WNW). P2 is synchronous with the greenschist facies of the Mesoalpine metamorphism. 3) A recumbent regional fold P3, with a crenulation c1eavage S3, post-dates the last phase of albite cristallization. Therefore P3 would be late-mesoalpine. Extensional structures late-P3 indicate a movement towards the WNW and could be linked to a final overthrusting of the Houiller-basement (Ruitor) on to the outer Houiller. 4) Minor folds P4, have ESE-WNW axes and vertical axial planes. In unfolding P3 and then P2, we discover that the "Schistes lustrés d'Avise" wedge was pre-mesoalpine (pre-D2). It formed wedge opening towards the East, with an N-S axis. The Schistes lustrés wedge resulted therefore from an early backthrust which probably happened under HP ; its relation with Dl remains unknown. The other "Schistes lustrés" wedges of the Alpes Graies probably have the same origin. Note that in tbis evolutional hypothesis, no late backthrusting occurred. If in spite of the appearances P3 constituted the inverted limb of a late backfold, the "Schistes lustrés" wedge in the basement would only mean an early overthrusting. The structural evolution happened under decreasing pressure and could reflect the history (Eocene) of the Penninic unit uplift in the alpine accretionar prism, as defined by Platt (1986).
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Applied geology. Université Scientifique et Médicale de Grenoble, 1987. French


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Thierry Baudin. Etude Géologique du Massif du Ruitor ( Alpes Franco-italiennes) : évolution structurale d'un socle briançonnais.. Applied geology. Université Scientifique et Médicale de Grenoble, 1987. French. <tel-00539864>

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