Probing seismic anisotropy in North Iberia from shear wave splitting

摘要

The mantle anisotropic features at the northwestern part of the Iberian Peninsula have been investigated by the shear-wave splitting technique from different temporary array deployments during the last 6 years. Successive seismic transects were instrumented from east to west, to sample areas affected by the Alpine compressional tectonics (Western Pyrenees and Cantabrian Mountains), as well as hinterland parts of the Iberian Variscan belt. A remarkable consistency is found in the retrieved anisotropic parameters throughout the study area, with an average fast velocity direction close to E/W. Delay times up to 1.5 s are observed in most transects, but lower values, not exceeding 1 s are measured at the westernmost part. Although the averaged values are compatible among the different stations, a significant variation of the splitting parameters is observed in each station with respect to the backazimuth direction. This azimuthal dependence denotes a complex distribution at depth of the anisotropic features, and cannot be explained by considering single-layer anisotropic models, either with hexagonal or orthorhombic symmetry systems, and/or dipping axes of symmetry. Synthetic models including two distinct anisotropic layers with an orthorhombic symmetry provide a satisfactory fit. In all the retained models the two layers exhibit different thicknesses; the most prominent contribution to the observed anisotropy comes from a lower layer consistently oriented close to E/W, whereas the thinner second layer, located above, 'modulates' the result, and its anisotropic parameters may change along the transect to account for the observed differences in splitting delay times. The dominant E-W layer throughout could not be associated to major compressional events, as in the westernmost part the Variscan terrains exhibit N-S main lineaments. It may rather correspond to an anisotropic imprint around the lithosphere-asthenosphere transition related to the eastward displacement of the Iberian plate due to the Mesozoic extensional processes during the opening of the North Atlantic and Bay of Biscay domains. The weaker anisotropic layer could be associated either with crustal anisotropic materials, or more likely, to an additional anisotropic signature within the lithosphere led by major Variscan and Alpine orogenic processes. © 2006 Elsevier B.V. All rights reserved.