Radial anisotropy and S-wave velocity depict the internal to external zone transition within the Variscan orogen (NW Iberia)

Abstract

Abstract. The cross-correlation of ambient noise records registered by seismic networks has proven to be a valuable tool to obtain new insights into the crustal structure at different scales. Based on 2 to 14 s period Rayleigh and Love dispersion data extracted from the seismic ambient noise recorded by 20 three-component broadband stations belonging to two different temporary experiments, we present the first (i) upper-crustal (1–12 km) high-resolution shear wave velocity and (ii) radial anisotropy variation models of the continental crust in NW Iberia. The area of study represents one of the best-exposed cross sections along the Variscan orogen of western Europe, showing the transition between the external eastern zones towards the internal areas in the west. Both the 2-D maps and an E–W transect reveal a close correspondence with the main geological domains of the Variscan orogen. The foreland fold-and-thrust belt of the orogen, the Cantabrian Zone, is revealed by a zone of relatively low shear wave velocities (2.3–3.0 km s−1), while the internal zones generally display higher homogeneous velocities (> 3.1 km s−1). The boundary between the two zones is clearly delineated in the models, depicting the arcuate shape of the orogenic belt. The velocity patterns also reveal variations of the elastic properties of the upper crust that can be linked to major Variscan structures, such as the basal detachment of the Cantabrian Zone, the stack of nappes involving pre-Variscan basement, or sedimentary features such as the presence of thick syn-orogenic siliciclastic wedges. Overall, the radial anisotropy magnitude varies between −5 % and 15 % and increases with depth. The depth pattern suggests that the alignment of cracks is the main source of anisotropy at < 8 km depths, although the intrinsic anisotropy seems to be significant in the West Asturian–Leonese Zone, the low-grade slate belt adjacent to the Cantabrian Zone. At depths > 8 km, widespread high and positive radial anisotropies are observed, which we attribute to the presence of subhorizontal alignments of grains and minerals in relation to the pre- or syn-orogenic deformation associated with the Variscan orogenesis.