The shear-wave velocity structure of the lithosphere-asthenosfhere system in the Iberian area and surroundings

The S-wave velocity model, to the depth of 300 km for the Iberian area and surroundings, is obtained with the non-linear inversion of Rayleigh wave tomographic data in the period range from 5 to 150 s. The three-dimensional model of the region is assembled from 74 juxtaposed one-dimensional cellular structures, sized 1° 9 1°, by means of a local smoothing optimisation. The S-wave velocity model shows the almost constant velocity (*4.40 km/s) in the upper mantle beneath the Iberian Massif, the well- developed low-velocity zones in the Valencia Trough and under northwest Africa, and specific features of the upper mantle layering in the zone of the Iberian range. The thickness of the crust varies between 25 and 35 km with exceptions in the zones of the Valencia Trough (11–18 km) and Pyrenees (35–45 km). Our Vs models are well correlated with the geological structure along Transmed I and evidence several features of the lith- osphere–asthenosphere system in the Betic–Rif domain: quite distinct Iberian, Alboran and Meseta crustal domains; relatively thin, well-developed lithosphere under the Rif system; and clearly defined boundary between the upper and lower asthenosphere. The cellular velocity model of the Betics, where ultrapotassic lamproitic magmatism is observed, has features similar to those of other three areas in the western Mediterranean where ultra- potassic lamproitic magmatism of different age is observed. The crust and upper mantle models in these four areas can be correlated with the age of the magmatism and the local heat flow, and outline how, starting from a naturally quite undifferentiated mantle where magma genesis is currently in progress, the differentiation in lithosphere and asthenosphere can take place in time.