3D Crustal structure of the Basque-Cantabrian Zone (N Spain) through a nonlinear joint inversion of surface wave phase velocities, teleseismic receiver functions and Rayleigh wave ellipticity

The Basque-Cantabrian Zone (BCZ) is a large, inverted Mesozoic basin (the Basque-Cantabrian basin or BCB) forming part of the Pyrenean-Cantabrian mountain belt, in the north of Iberian Peninsula. The Mesozoic basin developed in one of the most subsident regions between the European plate and the Iberian sub-plate during the stage of crustal hyperextension linked to the rifting of the Central-North Atlantic and the opening of the Bay of Biscay. The high subsidence rate led to the accumulation of more than 15 km of sediments according to some estimates, and the significant crustal extension caused the exhumation of the mantle in the easternmost sector of the BCB. The Alpine orogeny caused the closure and inversion of the BCB and its incorporation to the Pyrenean-Cantabrian orogen. In this work, we studied the crustal structure of the BCZ resulting from this long and complex tectonic evolution using five years of continuous seismic recordings gathered by a local network of broadband stations, most of them deployed in the framework of projects SISCAN and MISTERIOS. A total of 66 locations were used (not all of them simultaneously), with an average spacing of ~30 km between stations. From this dataset, we extracted the multi-mode phase velocities of surface waves and the ellipticity of Rayleigh waves from cross-correlations of the seismic ambient noise. This allowed us to retrieve the shear-wave velocity structure of the crust, especially at shallow to intermediate depths. To better constrain the deeper crustal structure, we also extracted teleseismic P-wave receiver functions for all suitable events. Each dataset was carefully analyzed before performing a nonlinear, joint inversion using the simulated annealing technique. The result is a set of 1D shear-wave velocity models that represent a compromise between all three datasets. These 1D models were then used in a linear interpolation to build a 3D model of the BCZ. The main feature of the 3D model is a thickened crust of up to 50 km beneath the Cantabrian Mountains. A discontinuous, intracrustal level of high-velocities is identified in the northern part of the model, coherently with previous geological and geophysical observations, suggesting that the thick crustal root would be caused by the indentation of the Cantabrian Margin lower crust into the Iberian crust, as has been already proposed. This new 3D model fills a gap in the knowledge of the study area, whose seismic characterization was primarily based on active source studies, which often only provide estimates of the P-wave velocities along 2D profiles.