OCEANIC LITHOSPHERE AT THE EDGE OF A CENOZOIC ACTIVE CONTINENTAL-MARGIN (NORTHWESTERN SLOPE OF GALICIA BANK, SPAIN)

A basaltic seafloor was surveyed and sampled by the French submersible Nautile on the northwestern slope of the Galicia Bank (Spain); the mo rphology and petrology of the volcanic rocks indicate an oceanic crust , although rather enriched in incompatible elements. The radiometric a ge of the basalts (about 100 Ma) suggests that they were emplaced a sh ort time after the onset of seafloor spreading in that part of the Nor th 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 basalt s now form the inner wall of the fossil northern Galicia trench. Mesoz oic structures of the margin, including the northern continuation of t he peridotite ridge, drilled to the west of Galicia, were slightly def ormed by Cenozoic tectonism, although some inversion and shortening di d occur locally. We consider this remnant basaltic seafloor to have be en joined to the Galicia basement since the Mesozoic. Consequently, it cannot be interpreted as a wedge of oceanic lithosphere accreted to t he Iberian plate as a result of Cenozoic subduction. Indeed, the initi al 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.