Lithospheric anisotropy beneath the Pyrenees from shear wave splitting

We investigate upper mantle anisotropy beneath the Pyrenean range along three N-S profiles across the mountain belt. The results of a first profile that operated in 1993 in the central Dart of the belt have been presented elsewhere. We present the results of two other profiles that ran in 1995-1996 and 1996-1997 in the eastern and western part of the belt, respectively and propose an interpretation of the whole results. Teleseismic shear waves (SKS, SKKS, and PKS) are used to determine splitting parameters: the fast polarization direction phi and the delay time delta t. Teleseismic shear wave splitting in the eastern Pyrenees displays homogeneous phi values trending N100 degrees E and delta t values in the range 1.1 to 1.5 s. A station located in the southern Massif Central, 100 km north of the range, is characterized by different splitting parameters (phi = N70 degrees E, delta t = 0.7 s). In the western part of the belt, anisotropy parameters are similar across the whole belt (phi = N1 10 degrees E and delta t = 1.3 to 1.5 s). Most of the measured delay times, including those obtained in the central parr of the range, are above the global average of the SKS splitting (around 1 s). At the belt scale, phi is generally poorly correlated with recent estimations of the absolute plate motion, which predicts a fast direction ranging between N50 degrees E and N80 degrees E. Instead, the orientation of phi (N100 degrees E) is parallel to the trend of the Pyrenean belt but also to Hercynian preexisting structures. This parallelism supports an anisotropy primarily related to frozen or active lithospheric structures. We show that a signature related to the Pyrenean orogeny is likely for the stations located in the internal domains of the belt. By contrast, the anisotropy measured at the stations located on the external parts of the belt could reflect a pre-Pyrenean (Hercynian) deformation. We suggest that a late Hercynian strike-slip deformation is responsible for this frozen upper mantle anisotropy and that the Pyrenean tectonic fabric developped parallel to this preexisting fabric. Finally, no particularly strong splitting is related to the North Pyrenean Fault, commonly believed to represent the plate boundary between Iberia and Eurasia.