Fossil oceanic core complexes recognized in the blueschist metaophiolites of Western Alps and Corsica

International audienceTethyan ophiolites show an apparent poorly organized association of ultramafic andmafic rocks. By contrast tothe complete mantle-crustal sections of Semail-type ophiolite sheets, Tethyan ophiolites are characterized by asmaller amount of mafic rocks (gabbros and basalts), by the absence of any sheeted dyke complex and by thefrequent occurrence of oceanic sediments stratigraphically overlying mantle-derived peridotites and associatedgabbroic intrusions. Therefore, they are considered as typical remnants of oceanic lithosphere formed in slowspreadingenvironment or in ocean–continent transition at distal passive margins. In the very first models offormation of the Tethyan ophiolites, in the years 1980, the geodynamical processes leading to mantle unroofingwere poorly understood due to the paucity of data and concepts available at that time from the present-dayoceans. In particular, at that time, little work had focused on the distribution, origin and significance ofmafic rocks with respect to the dominant surrounding ultramafics. Here, we reconsider the geology of sometypical metaophiolites from the Western Alps and Corsica, and we show how results from the past decadeobtained in the current oceans ask for reassessing the significance of the Tethyan ophiolites in general. Revisitedexamples include a set of representative metaophiolites from the blueschists units of the Western Alps (Queyrasregion) and from Alpine Corsica (Golo Valley). Field relationships between the ophiolitic basement andthe metasedimentary/metavolcanic oceanic cover are described, outlining a typical character of the Tethyanophiolite lithological associations. Jurassic marbles and polymictic ophiolite metabreccias are unconformablyoverlying the mantle-gabbo basement, in a way strictly similar to what is observed in the non-metamorphicAppennine ophiolites or Chenaillet massif. This confirms that very early tectonic juxtaposition of ultramafic andmafic rocks occurred in the oceanic domain before subduction. This juxtaposition resulted from tectonic activitythat is now assigned to the development of detachment faults and to the formation of Oceanic Core Complexes(OCCs) at the axis of slow spreading ridges. This fundamental Plate Tectonics process is responsible for the exhumationand for the axial denudation of mantle rocks and gabbros at diverging plate boundaries. In addition, fieldrelationships between the discontinuous basaltic formations and the ultramafic–mafic basement indicate that thistectonic stage is followed or not by a volcanic stage.We discuss this issue in the light of available field constraints