Oceanic lithosphere at the edge of a cenozoic active continental margin (northwestern slope of Galicia Bank, Spain)

Abstract

A basaltic seafloor was surveyed and sampled by the French submersible Nautile on the northwestern slope of the Galicia Bank (Spain); the morphology and petrology of the volcanic rocks indicate an oceanic crust, although rather enriched in incompatible elements. The radiometric age of the basalts (about 100 Ma) suggests that they were emplaced a short time after the onset of seafloor spreading in that part of the North Atlantic. This oceanic seafloor remained attached to the overriding Iberian plate when the north and northwest passive Galicia margin was turned into an active margin in the Paleocene-Eocene. Mesozoic basalts now form the inner wall of the fossil northern Galicia trench. Mesozoic structures of the margin, including the northern continuation of the peridotite ridge, drilled to the west of Galicia, were slightly deformed by Cenozoic tectonism, although some inversion and shortening did occur locally. We consider this remnant basaltic seafloor to have been joined to the Galicia basement since the Mesozoic. Consequently, it cannot be interpreted as a wedge of oceanic lithosphere accreted to the Iberian plate as a result of Cenozoic subduction. Indeed, the initial oceanic lithosphere and the ocean-continent boundary are preserved at the edge of the former active continental margin. This setting can help to understand the processes of ophiolite emplacement in the case of further collision.