Seismic image of the Cantabrian Mountains in the western extension of the Pyrenees from integrated ESCIN reflection and refraction data

Abstract

Integrated analysis of normal-incidence and large-aperture seismic reflection data collected in 1992 and 1993 within the Spanish ESCIN and complementary projects provide a first complete N-S crustal transect across the Northern Iberian Peninsula and continental margin. Images of the 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-km-long reversed refraction profile; and (c) wide-angle recordings of the marine ESCIN-4 profile. Consistent results between reflectivity pattern and velocity-depth distribution reveal important lateral variations in the deep structure. The reflective crust imaged in the ESCIN-2 profile changes its attitude from sub-horizontal beneath the Duero basin to north-dipping beyond the Mountain front. Basement thrusts are observed 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 of the refraction data laterally extends the seismic image and provides evidence for Variscan crustal features beneath the Duero basin. Northwards, the velocity in the lower crust decreases and the Moho, constrained by the wide-angle data from profile ESCIN-2, deepens to about 60 km ending abruptly at the shoreline. The velocity-depth model is constrained along the Asturian platform up to the continental slope, where the crust-mantle boundary is located at 24 km depth. This 'margin Moho' shows a progressive deepening southwards, and extends to the coast where it is found at 30 km depth. The present seismic data support an important Alpine reworking and thickening of the crust under the Cantabrian Mountains. The onshore/offshore transition is marked by an imbrication of two crusts of very different thicknesses. This signature offers a strong parallelism with the one previously observed further east across the Pyrenees in the ECORS seismic profile.