Abstract

Ophiolites occur at several places in the Lower Penninic of the W and Central Alps. They are generally ascribed to oceanic crust of a so-called “Valais ocean” of Cretaceous age which plays a fundamental role in many models of Alpine paleogeography and geodynamics. The type locality and only observational base for the definition of a “Valais ocean” in the W Alps is the Versoyen ophiolitic complex, on the French-Italian boundary W of the Petit St-Bernard col. The idea of a “Valais ocean” is based on two propositions that are since 40 years the basis for most reconstructions of the Lower Penninic: (1) The Versoyen forms the (overturned) stratigraphic base of the Cretaceous-Tertiary Valais-Tarentaise series; and (2) it has a Cretaceous age. We present new field and isotopic data that severely challenge both propositions.

(1) The base of the Versoyen ophiolite is a thrust. It overlies a wildflysch with blocks of Versoyen rocks, named the Méchandeur Formation. This “supra-Tarentaise” wildflysch has been confused with an (overturned) stratigraphic transition from the Versoyen to the Valais-Tarentaise series. Thus the contact Versoyen/Tarentaise is not stratigraphic but tectonic, and the Versoyen ophiolite has no link with the Valais basin. This thrust corresponds to an inverse metamorphic discontinuity and to an abrupt change in tectonic style.

(2) The contact of the Versoyen complex with the overlying Triassic-Jurassic Petit St-Bernard (PSB) series is stratigraphic (and not tectonic as admitted by all authors since 50 years). Several types of sedimentary structures polarize it and show that the PSB series is younger than the Versoyen. Consequently the Versoyen ophiolitic complex is Paleozoic and forms the basement of the PSB Mesozoic sediments. They both belong to a single tectonic unit, named the Versoyen-Petit St-Bernard nappe.

(3) Ion microprobe U-Pb isotopic data on zircons from the main gabbroic intrusion in the Versoyen complex give a crystallization age of 337.0 ± 4.1 Ma (Visean, Early Carboniferous). These zircons show typical oscillatory zoning and no overgrowth or corrosion, and are interpreted to date the Versoyen magmatism.

These U-Pb data are in excellent agreement with our field observations and confirm the Paleozoic age of the Versoyen ophiolite. The existence of a “Valais ocean” of Cretaceous age in the W Alps becomes very improbable. The eclogite facies metamorphism of the Versoyen-Petit St-Bernard nappe results from an Alpine intra-continental subduction, guided by a Paleozoic oceanic suture. This is an example of the long term influence of inherited deep-seated structures on a much younger orogeny. This might well be a major cause of the inherent complexity of the Alps.

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