Three-dimensional upper-mantle S-velocity model for the Eurasia-Africa plate boundary region

A new regional S-velocity study resolving the Eurasia-Africa plate boundary region from the Azores to the eastern Mediterranean Sea is presented. The resolution of existing velocity models has been complemented by using new seismic broad-band data recorded by the temporary MIDSEA network and at permanent European seismic stations. Following the partitioned waveform inversion method, we interactively fitted the waveforms of S and Rayleigh wave trains of more than 1100 seismograms. The linear constraints on upper-mantle S velocity provided by the waveform fits have been combined with independent estimates of Moho depth in a linear damped least-squares inversion for S velocity and crustal thickness. The resulting S-velocity structure for the Mediterranean Sea shows strong lateral variations, confirming the complex evolution of this plate boundary region. The upper mantle along the Eurasia-Africa suture zone is characterized by high-velocity material representing subducted oceanic lithosphere. This signature can be followed to depths of 300-500 km, depending on the region and resolution. A high-velocity body, possibly representing a fragment of subducted lithosphere, has been imaged beneath eastern Spain at a depth between 250 and 500 km. Not only convergence has been recorded in the upper mantle, but extension also has its own signature beneath the Mediterranean region. This is particularly clear for the Algero-Provencal and Tyrrhenian basins, where a shallow asthenospheric layer is observed. The lithosphere-asthenosphere system of the western Mediterranean clearly differentiates itself from the structure of the older eastern Atlantic Ocean. Differences in comparison to the structure of a 4 to 20 Ma ocean are also present. These observations support the idea that, rather than a young ocean, the western Mediterranean could be a strongly stretched continent, partly affected by spreading, formed at the back of a slab. The structure characterizing the eastern Mediterranean points to a continuation of the northern African continental lithosphere beneath the sea. Major structural differences in the eastern Atlantic Ocean are imaged between the mid-Atlantic ridge and the older oceanic basins: the north Atlantic lithosphere is characterized by lower velocities beneath the spreading ridge than under the old ocean basins. Despite the strong differences observed in the crustal structure between the mid-Atlantic ridge and the Azores, no significant differences are observed in the S-velocity structure of the upper mantle.