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Etude géologique de la suture tethysienne dans les Alpes franco-italiennes Nord-Occidentales de la Doire Ripaire (Italie) à la région de Bourg Saint-Maurice .(France)

Abstract : Materials of ophiolitic suture zones can be used to restore the key steps of the evolution of fossil oceanic areas and of their margins. This study deals with the different components of a part of the Tethyan suture in the north-western French-Italian Alps. It concerns more precisely the lithostratigraphic and structural analysis of the so-called Schistes lustrés complex. The main studied area is the French-ltalian border close to the Mont-Cenis pass (the Susa area in the Piemont Region, Italy and the Lanslebourg area in Savoy, France). The northern part of the peridotitic Lanzo massif (north ofTorino, Italy) within its limits with the pre-alpine basement massif of Sesia, the klippe of the " Schistes lustrés du Mont Jovet" (Moûtiers area, France), and finally the Valaisanne zone within the Bourg Saint Maurice area (France) were also examined. Essentially qualitative, this work takes into account the known petrologic, metamorphic and radiochronologic data. The Schistes lustres complex is resolved into three different litho structural sets of units. The two lower ones, born of the European palaeo-margin for the lowest and of the palaeo-oceanic realm or Tethys for the other, are made of calcschists and marbles labelled by syn-orogenic detrital deposits of composite origin, oceanic and continental. The mass and the height of the reworked materials, ophiolitic or not, notably decrease from the lowest oceanic units to the highest ones. Although the determinations of the rare fossil traces are always in debate, the ages found and the radiochronological intervals of the alpine metamorphic assemblages of the Schistes lustrés lead to a single conclusion: the primary sedimentary rocks belong to the Upper Cretaceous. They represent Upper Cretaceous flyschs. The upper one, not identified within our field study, is made of very late Cretaceous gritty flysch without reworked ophiolitic materials [Deville, 1986]. AlI these Upper Cretaceous deposits are explained as the sedimentary consequences of two major stages of the closure of the Tethyan ocean during the late Cretaceous. The Schistes lustres of the Lanzo area and the peridotitic Lanza massif on one hand and the Schistes lustres of the Mont Jovet klippe on the other hand, can be combined with the previous pattern. The first ones indeed are connected to the Schistes lustres of the structurally oceanic lowest units and the second to the schistes lustres of the highest ones. Also a lithospheric sketch is proposed in which the Lanzo massif is disconnected with the south-alpine mantle, as it was previously interpreted. The valaisanne zone in France in the Bourg Saint Maurice area finally is reexamined. The valaisan Flysch stratigraphically lies upon the " Roches vertes du Versoyen ", a primary volcanogenic complex. The new discovery of microfaunas, which complete the previous ones, leads to give to this Flysch a post Barremo-Aptian age. Moreover, the " Roches vertes du Versoyen "are interpreted as essentially a volcanogenic complex, born of a thinned shield realm becoming partially oceanic along the western limit of the Briançonnaise zone. If these metabasites were really emplaced during cretaceous times, if their 60-70 Ma metamorphism is evidenced, and if the valaisan Flysch has experienced the same metamorphic conditions at the same time, then this Flysch is finally an Upper Cretaceous Flysch. In conclusion, all of the data of the studied transect allowed us to describe the different steps of the evolution of the Tethyan suture within the Alps of Savoy.
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Submitted on : Monday, August 13, 2012 - 10:43:44 AM
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Serge Fudral. Etude géologique de la suture tethysienne dans les Alpes franco-italiennes Nord-Occidentales de la Doire Ripaire (Italie) à la région de Bourg Saint-Maurice .(France). Tectonique. Université de Savoie, 1996. Français. ⟨tel-00723712⟩

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