Upper mantle compressional velocity structure beneath the West Mediterranean Basin

P waveforms of regional crustal earthquakes have been modeled to obtain an upper mantle compressional velocity model for the West Mediterranean Basin. Data come from long-period stations of the World-Wide Standardized Seismograph Network and broadband stations located in the Iberian Peninsula. Synthetic waveforms have first been computed for published 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 zone and a 3% discontinuity at 368 km where an increase of the velocity gradient also occurs. These features could be explained either as a deflection of the olivine-to-spinel phase transition, regionally detected at about 395 km, resulting from the lower temperature produced by the subduction of the African plate, or as being due to the presence below 300 km depth of a layer of silicate melt, producing a strong reflection from its bottom, and a more usual depth for the olivine-spinel transition. In both cases the occurrence of relatively low velocities beneath 300 km is interpreted as being caused by the presence of melt associated with the northward dipping subduction of the African plate.