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

Abstract

Tethyan ophiolites show an apparent poorly organized association of ultramafic and mafic rocks. By contrast to the complete mantle-crustal sections of Semail-type ophiolite sheets, Tethyan ophiolites are characterized by a smaller amount of mafic rocks (gabbros and basalts), by the absence of any sheeted dyke complex and by the frequent occurrence of oceanic sediments stratigraphically overlying mantle-derived peridotites and associated gabbroic intrusions. Therefore, they are considered as typical remnants of oceanic lithosphere formed in slow-spreading environment or in ocean–continent transition at distal passive margins. In the very first models of formation of the Tethyan ophiolites, in the years 1980, the geodynamical processes leading to mantle unroofing were poorly understood due to the paucity of data and concepts available at that time from the present-day oceans. In particular, at that time, little work had focused on the distribution, origin and significance of mafic rocks with respect to the dominant surrounding ultramafics. Here, we reconsider the geology of some typical metaophiolites from the Western Alps and Corsica, and we show how results from the past decade obtained in the current oceans ask for reassessing the significance of the Tethyan ophiolites in general. Revisited examples include a set of representative metaophiolites from the blueschists units of the Western Alps (Queyras region) and from Alpine Corsica (Golo Valley). Field relationships between the ophiolitic basement and the metasedimentary/metavolcanic oceanic cover are described, outlining a typical character of the Tethyan ophiolite lithological associations. Jurassic marbles and polymictic ophiolite metabreccias are unconformably overlying the mantle-gabbo basement, in a way strictly similar to what is observed in the non-metamorphic Appennine ophiolites or Chenaillet massif. This confirms that very early tectonic juxtaposition of ultramafic and mafic rocks occurred in the oceanic domain before subduction. This juxtaposition resulted from tectonic activity that 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 exhumation and for the axial denudation of mantle rocks and gabbros at diverging plate boundaries. In addition, field relationships between the discontinuous basaltic formations and the ultramafic–mafic basement indicate that this tectonic stage is followed or not by a volcanic stage. We discuss this issue in the light of available field constraints.