The ophiolites of the Alps, of Corsica, and of the Apennines, which originate from the basement of the Ligurian segment of the Mesozoic Tethys ocean, are not consistent with the classical mid-ocean ridge spreading models; neither a continuous and thick basaltic layer nor a true sheeted-dike complex ever existed. The first oceanic floor that appeared between the divergent European and Apulian passive margins was made up of mantle-derived serpentinite and associated minor gabbro bodies; some of the ocean-floor gabbros were already foliated and metamorphosed to amphibolite facies. To explain these features, we suggest use of the model of Wernicke, which postulates a major, oblique, normal detachment fault that cuts across the lithosphere. The model explains some asymmetrical features on both sides of the preoceanic continental rift. An increase of the offset of the detachment fault may lead to the tectonic denudation of the upper mantle and thus give birth to an ultramafic ocean floor.

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