The Mont Fort nappe, its Mesozoic cover and relations to the Upper Penninics (Western Swiss Alps)

Résumé 0

This study focuses on the Pennine Alps, in the central part of the Alpine Arc. As this massif offers a wide section through the Alpine nappe stack, and as its geological structure and tectonometamorphic evolution have been extensively studied, it constitutes an ideal setting for the study of continental collision processes. Although the major tectonic units of the Pennine Alps were defined more than a century ago, and despite the countless studies conducted since then, fundamental questions such as the position of some major tectonic boundaries or first order geometries of some nappes, are still debated. In particular, the questions regarding the position of the boundary between the Middle Penninic Mont Fort nappe and the Upper Penninic Tsaté nappe as well as the tectonic attribution and paleogeographic origin of the Frilihorn and Cimes Blanches units have been debated for decades. These questions constitute the main topics of this study. This work is mainly based on an important structural and stratigraphic field study over an area extending between the Bagnes and the Matter valleys. It provides new and detailed data concerning the nature of the studied contacts and allows to revise several tectonic interpretations. Both the basal contact of the Lower Triassic to Lower Cretaceous Evolène Series and the basal contact of the p.p. Upper Cretaceous non-ophiolitic Schistes Lustrés of the Série Rousse are reinterpreted as stratigraphic. These two series would therefore constitute together the autochthonous sedimentary cover of the Mont Fort nappe. Several sections of the basal contacts of these series are discordant. They are interpreted as corresponding to remnants of high-amplitude Jurassic normal paleofaults. The contact between the Série Rousse and the overlying ophiolite-bearing Schistes Lustrés is marked by the presence of the thin Triassic-Jurassic Frilihorn tectonized slices. This contact, located within the Schistes Lustrés complex of the Combin zone, is interpreted as representing the major tectonic boundary separating the Middle and Upper Penninic units. A redefinition of the Tsaté nappe is proposed, excluding the Série Rousse, and including only the ophiolite-bearing Schistes Lustrés and associated meta(ultra-)basites. In the Täschalpen area, east of Zermatt, the Permian-Jurassic Faisceau Vermiculaire series (incl. Cimes Blanches and Frilihorn Series) is in contact with the basement forming the Alphubel anticline. This contact is interpreted as stratigraphic, and the ensemble formed by the basement of the Alphubel anticline, the Faisceau Vermiculaire and the Série Rousse is interpreted as constituting the prolongation of the Mont Fort nappe, east and south of the Dent Blanche klippe, in the lower limb of the Mischabel backfold. The proposed tectonic scheme emphasizes the significance of the Mont Fort nappe. This nappe derived from the Prepiemont distal margin constitutes a link between the Briançonnais units and the Piemont ophiolites which appears to be essential for the understanding of the structure of the Penninic nappe stack.