The crustal structure of the transition zone between the Pyrenees and the Cantabrian Mountains was investigated from a set of seismic refraction / wide angle reflection profiles acquired in 1997. An E-W profile of 540 km shows important variations in crustal thickness and velocity distribution from the Cantabrian Mountains to the western Pyrenees. A continuous Alpine crustal root is observed, with Moho depths of 46-48 km, except for the western part of the Basque Cantabrian Basin, Where the Moho is located at about 40 km, Another outstanding feature is the inferred presence of a discontinuous layer of 6.40-6.90 km s. at midcrustal depths beneath the cantabrian mountains and the Eastern Basque -Cantabrian basin. Some geological and geophysical constraints suggest that it can represent a wedge of the lower crust of the Cantabrian Margin identing the Iberian crust durin the almipe compression. This indentation was conditioned and /or affected by some NE-SW structures and this explains its discontinuous presence along the profile. The anomalously low velocity for this lower crustal material could be a consequence of its intense deformation during the collision and crustal indentation.

Estas investigaciones se han financiado con cargo a los proyectos CICYT AMB95-0922 y AMB98-1012

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