Crustal structure of the external variscides in northwest spain from deep seismic reflection profiling

Abstract

Recent deep-crustal seismic reflection profiling escicantabrica-1 provides a detailed image of the external parts of the Variscan Belt in northwest Spain. These seismic data together with surface geological information provide constraints on the crustal structure of the foreland thrust and fold belt (Cantabrian Zone) and the transition to the hinterland areas (Narcea Antiform area). The image in the Cantabrian Zone includes strong inclined reflections in the upper 5 s that correspond to mappable Paleozoic rocks at the surface which have been deformed by thrusting and folding during the Carboniferous. Some of the major thrust faults imaged show a ramp-flat geometry at depth and merge into a highly reflective band between 5 and 6 s that corresponds to the Cantabrian Zone detachment. The Precambrian basement is almost transparent underneath the Cantabrian Zone detachment. There are a few subhorizontal discontinuous reflections at 11 s that may correspond to the reflection Moho. At the transition to the hinterland, the seismic profile includes short inclined reflections grouped in several reflective bands in the upper 6 s. These correspond to the Narcea Antiform, which comprises at the surface a stack of thrust sheets containing Precambrian rocks. The Narcea Antiform is placed above a crustal-scale ramp imaged by strong W-dipping reflections between 6 and 9 s. These reflections merge upwards into the Cantabrian Zone detachment at 6 s, and downwards into a reflective lower-crustal zone located between 9 and 12 s. These midcrustal reflectors are interpreted as a major Variscan shear zone that marks the transition between the thin-skinned tectonics of the external areas and the thick-skinned tectonics of the hinterland areas. The lower crust is highly reflective below the Narcea Antiform area and reflectivity fades out to the east beneath the foreland thrust and fold belt. The different seismic responses at middle- and lower-crustal levels in the foreland and hinterland areas suggest that Variscan tectonics strongly affected deep-crustal levels in the hinterland areas, whereas they remained undeformed beneath the foreland.