UPPER-MANTLE COMPRESSIONAL VELOCITY STRUCTURE BENEATH THE WEST MEDITERRANEAN BASIN

P waveforms of regional crustal earthquakes have been modeled to obtai n an upper mantle compressional velocity model for the West Mediterran ean Basin. Data come from long-period stations of the World-Wide Stand ardized Seismograph Network and broadband stations located in the Iber ian Peninsula. Synthetic waveforms have first been computed for publis hed velocity models corresponding to different tectonic provinces and obtained with analogous techniques. A model that strongly improves the fits to the data is then derived. The proposed model is characterized by a 100-km-thick lid overlaying a not very pronounced low-velocity z one and a 3% discontinuity at 368 km where an increase of the velocity gradient also occurs. These features could be explained either as a d eflection of the olivine-to-spinel phase transition, regionally detect ed at about 395 km, resulting from the lower temperature produced by t he subduction of the African plate, or as being due to the presence be low 300 km depth of a layer of silicate melt, producing a strong refle ction from its bottom, and a more usual depth for the olivine-spinel t ransition. In both cases the occurrence of relatively low velocities b eneath 300 km is interpreted as being caused by the presence of melt a ssociated with the northward dipping subduction of the African plate.