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


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Early Carboniferous age of the Versoyen ophiolites and consequences: non-existence of a "Valais ocean'' (Lower Penninic, western Alps)
Bulletin de la Société Géologique de France
Masson H., Bussy F., Eichenberger M., Giroud N., Meilhac C., Presniakov S.
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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 Mechandeur
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 lone
term influence of inherited deep-seated structures on a Much younger
orogeny. This might well be a major cause of of the inherent complexity
of the Alps.
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01/10/2012 20:07
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