Seismic evidence of Alpine crustal thickening and wedging from the western Pyrenees to the Cantabrian Mountains (north Iberia)

The Alpine collision between the Iberian and European plates resulted in a complex crustal structure beneath the northern Iberian Peninsula, as revealed from a new set of seismic refraction/wide-angle reflection profiles. The study area is characterized by two major E-W ranges, the Cantabrian Mountains and the Pyrenees, which are relayed to the south by the Iberian Chain. Important variations in crustal thickness and velocity distribution are found in a 560-km-long E-W transect. In contrast to the typical 30- to 32-km-thick European Variscan crust of the western end of the profile, a continuous Alpine crustal root is evidenced from the Cantabrian Mountains to the central Pyrenees, with a Moho depth of 46–48 km, locally rising to 40 km depth in between, beneath the Basque-Cantabrian Basin. Another outstanding feature is the inferred presence of high velocities of 6.40–6.75 km/s at midcrustal depths, which can be associated with portions of a lower crustal wedge from the northern (European) domain indenting the southern (Iberian) crust during the Alpine stage of compression. This indentation produces the delamination of the Iberian crust, with northward underthrusting of its lower half and the consequent crustal thickening. The indenting wedge has a discontinuous presence along the longitudinal section, as it was controlled and/or affected by N-S to NE-SW structures. Further evidence of the northward subduction of the Iberian plate is provided by another profile toward the Aquitaine Basin, while a N-S profile across the Iberian Chain reveals an Alpine midcrustal thickening beneath this belt with Moho depths of ~42 km.