Teleseismic Shear Wave Splitting Research Articles

Anisotropy beneath the Pyrenees Range from teleseismic shear wave splitting: Results from a test experiment

We have investigated the seismic anisotropy from SKS and SKKS wave splitting along a N‐S profile perpendicular to the Pyrenean mountain belt. The line crossed the North Pyrenean Fault (NPF), which probably represents the plate boundary between the European and the Iberian plates. We observed large delay times δt (1.3 to 1.5 s) and uniform fast polarization directions ϕ (N100E) at three stations located on the Iberian plate. On the other hand, δt are smaller (0.5–1.0 s) and the ϕ scattered at stations located north of the NPF. The small scale variability, the relation with surface geology and the correlation of the δt with lithospheric thickness suggest that the anisotropy could be predominantly of lithospheric origin. However, an asthenospheric contribution cannot be ruled out. In the North Pyrenean Zone, an important thermal event before the Pyrenean compression has likely erased the former mantle fabric. The observed anisotropy is then likely related to the Pyrenean orogeny. On the Iberian plate, our splitting parameters are similar to those obtained at two stations located on Hercynian structures in Central Spain. This suggests that two contributions are likely superposed to that of the Pyrenean built up: one related to the Hercynian orogeny and the other one related to the rotation of the Iberian plate.

Teleseismic shear-wave splitting and deformations in Central Europe

We analyse the results of measurements of shear-wave splitting due to azimuthal anisotropy in the upper mantle of Central Europe, based on the SKS technique. The observations at seismograph stations in the eastern part of Germany are combined with the published estimates of the parameters of azimuthal anisotropy at neighbouring locations. These data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the upper mantle. Directions of polarization of the fast wave counted clockwise from north change from 50–70° in the western part of the region to 100–120° in the eastern part. These directions are close to the trend of the Hercynian fold belt and might suggest that anisotropy is frozen in the subcrustal lithosphere. However, the observed traveltime delays between the split waves are generally too large to be easily reconciled with the thermal and seismic estimates of thickness of the subcrustal lithosphere underneath Central Europe.