UPPER-MANTLE ANISOTROPY IN SW IBERIA FROM LONG-RANGE SEISMIC PROFILESAND TELESEISMIC SHEAR-WAVE DATA

Deep seismic profiles of the ILIHA experiment have provided evidence f or the presence of anisotropy in the deeper levels of the lithosphere beneath the Ossa-Morena zone of the Hercynian domain of SW Iberia, all owing estimation of its main characteristics-a N15 degrees E fast P-wa ve velocity direction and a degree of anisotropy of around 4%. The con sistency of these results is tested against an anisotropy analysis usi ng teleseismic shear waves. Data recorded in a dense network of up to 13 digital three-component stations, deployed in the area for about 2 months, are used to study the spatial coherence of wave fields, in par ticular of shear-wave splitting observations, over an array. Significa nt small-scale lateral variations are observed in the data, suggesting that care should be taken in using single station results as represen tative for geodynamical interpretations. In spite of the short duratio n of the test, a few teleseismic events provide useful data. The analy sis of SKS and S phases shows evidence of the presence of anisotropy. The results from the different events depict a rather consistent aniso tropic structure, with roughly a NE-SW to east-west fast velocity dire ction. The anisotropic patterns derived from DSS crossing profiles and shear-wave splitting over the Ossa-Morena zone are compared. Mechanis ms classically suggested to be responsible for the origin of the aniso tropy, such as past deformations of the lithosphere or relative motion between lithosphere and asthenosphere, are discussed to test their co nsistency with the geophysical and geological data available in the ar ea.