Fossil mantle-sediments interface recognized in the Western Alps metaophiolites : a key to unravel the accretion mechanism of the Jurassic Tethys ocean

ABSTRACT In the southern Aosta Valley (Italian Northwestern Alps), meta-ophiolites are mainly composed of serpentinized mantle-derived peridotites intruded by gabbros and rodingitic dykes, well exposed in the Mount Avic area, and of smaller amounts of mafic rocks and metatrondhjemite. This rock assemblage recalls the "slow-spreading" lithosphere created at modern mid-ocean ridges. Meta-ophiolites show a dominant early Alpine subduction-related metamorphic imprint under eclogite/blueschist facies conditions, variously retogressed under greenschists facies conditions. In the high Champorcher Valley (SW of Mount Avic) serpentinites are directly covered by a serpentinite mélange followed by flysch-like calcschists with detrital ophiolitic interbeds. Despite the pervasive Alpine tectonic deformation and metamorphic recrystallization through subduction-related stretching and boudinage and collision-related folding, the mélange internal fabric still retains records of a block-in-matrix structure, well consistent with mass-transport processes related to an active oceanic tectonic setting in which mantle rocks were progressively and continuously exhumed by faulting. The products of mass-transport processes and faulting are unconformably sealed by flysch-type calcschists embedding cm-sized clasts of actinolite/tremolite-schists interpreted as detrital ophiolitic material. The serpentinite mélange is interpreted as syn-extensional sedimentary rocks produced at the mantle-sediments interface on the Jurassic Tethys ocean floor and subsequently overprinted by subduction zone tectonics.