Early Carboniferous age of the Versoyen ophiolites and consequences: non-existence of a “Valais ocean” (Lower Penninic, western Alps)

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.

The Valais units in Savoy (France): a key area for understanding the palaeogeography and the tectonic evolution of the Western Alps

“…5. Hence, the arguments for a stratigraphic contact to the Aroley Formation are beyond any doubt also for all previous authors, except Masson et al (2008). The Complexe Antéflysch Formation predates the BarremianAptian (Aroley Formation) and post-dates conglomerates of presumably post-Early Jurassic age described below (Brèches du Collet des Rousses Formation of Fig.…”

“…Recently, Masson et al (2008) presented stratigraphic and tectonic arguments from our area that apparently denies the existence of a Valais ocean. According to these authors, the ''Versoyen ophiolites'' simply represent a piece of the Variscan orogen; the ophiolites would represent a Carboniferous-age complex that is in Alpine-tectonic contact with the Valais trilogy.…”

“…The latter must be younger since they post-date both the post-Early Jurassic Brèches du Collet des Rousses Formation and the Permian Punta Rossa Complex. Note, however, that Schärer et al (2000) dated other leuco-gabbros from the Aiguille du Clapet area with 309 ± 6 Ma, while Masson et al (2008) obtained an even older age of 337 ± 4.1 Ma. We will comment on these Carboniferous ages that, according to the above-mentioned authors, prove a Variscan age of the ''Versoyen ophiolites'', including the Complexe Antéflysch Formation, in the ''Discussion'' section.…”

“…5) and continental lithosphere (External Valais units) alike. The relative age of the three formations is well-established and undisputed, except for the differing view expressed by Masson et al (2008), since the oldest Aroley Formation directly lies on the older Mesozoic cover of the External Valais units above an angular unconformity (Fig. 3).…”

“…Frisch 1979;Stampfli 1993;Stampfli et al 1998) or non-existence (e.g. Schärer et al 2000;Manatschal 2004;Beltrando et al 2007;Masson et al 2008) of a Cretaceous age Valais ocean. Finally, we will briefly discuss the significance of these new findings for the palaeotectonic and orogenic evolution of the Western Alps.…”