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 mountian 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 deltat (1.3 to 1.5 s) and uniform fast polarization directions phi (N100E) at three stations located on the Iberian plate. On the other hand, deltat are smaller (0.5-1.0 s) and the phi scattered at stations located north of the NPF. The small scale variability, the relation with surface geology and the correlation of the deltat 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.