SEISMIC IMAGE OF THE CANTABRIAN MOUNTAINS IN THE WESTERN EXTENSION OFTHE PYRENEES FROM INTEGRATED ESCIN REFLECTION AND REFRACTION DATA

Integrated analysis of normal-incidence and large-aperture seismic ref lection data collected in 1992 and 1993 within the Spanish ESCIN and c omplementary projects provide a first complete NS crustal transect acr oss the Northern Iberian Peninsula and continental margin. Images of t he crustal structure of the Cantabrian Mountains and their transition to the Duero basin and to the Cantabrian margin are obtained from: (a) a 65-km-long vertical reflection profile ESCIN-2 on land; (b) a 200-k m-long reversed refraction profile; and (c) wide-angle recordings of t he marine ESCIN-4 profile. Consistent results between reflectivity pat tern and velocity-depth distribution reveal important lateral variatio ns in the deep structure. The reflective crust imaged in the ESCIN-2 p rofile changes its attitude from sub-horizontal beneath the Duero basi n to noah-dipping beyond the Mountain front. Basement thrusts are obse rved in the upper crust merging into a detachment at 6 s (TWT) and may have triggered the Alpine uplift of the range. The Moho is identified at the bottom of the reflective lower crust and deepens from 12 to 15 s at the northern end of the profile, about 35 km inland. Modelling o f the refraction data laterally extends the seismic image and provides evidence for Variscan crustal features beneath the Duero basin. North wards, the velocity in the lower crust decreases and the Moho, constra ined by the wide-angle data from profile ESCM-2, deepens to about 60 k m ending abruptly at the shoreline. The velocity-depth model is constr ained along the Asturian platform up to the continental slope, where t he crust-mantle boundary is located at 24 km depth. This 'margin Moho' shows a progressive deepening southwards, and extends to the coast wh ere it is found at 30 km depth. The present seismic data support an im portant Alpine reworking and thickening of the crust under the Cantabr ian Mountains. The onshore/offshore transition is marked by an imbrica tion of two crusts of very different thicknesses. This signature offer s a strong parallelism with the one previously observed further east a cross the Pyrenees in the ECORS seismic profile.