EVIDENCE FOR AZIMUTHAL ANISOTROPY IN SOUTHWEST IBERIA FROM DEEP SEISMIC-SOUNDING DATA

Data from the Iberian Lithospheric Heterogeneity and Anisotropy (ILIHA) deep seismic sounding experiment have been used to study the seismic structure of the lower lithosphere beneath the Ossa-Morena zone (southwest Iberia). The interpretation of the mantle phases observed in the record sections evidences a layered lower lithosphere, with alternating high- and low-velocity zones. Two to three main reflectors are observed in the uppermost mantle, between 30 and 90 km depth. A simple three-layered crust, with no significant lateral inhomogeneities over the area explored, means that the mantle phases are unlikely to be affected by variations in crustal structure. One reversed line and two intersecting unreversed lines define an area beneath southwest Iberia where the same lithospheric reflectors' segments are sampled along different azimuths. The independent analysis of the different lines locates the reflectors at the same depths, but significant variations appear between the velocities of the corresponding layers derived from each profile. This fact is interpreted as evidence for azimuthal anisotropy in the lower lithosphere. The anisotropy orientation deduced from our data is not consistent with results derived from other techniques, such as SKS splitting. This fact, already reported for other continental areas, must be interpreted as evidence of the existence of changes with depth in the anisotropic properties of the mantle. The different hypotheses suggested to explain the origin of the continental mantle anisotropy are discussed and related to the result of our study.