Propagation of regional seismic phases (Lg and Sn) and Pn velocity structure along the Africa-Iberia plate boundary zone: tectonic implications

We used over 1000 regional waveforms recorded by 60 seismic stations located in northwest Africa and Iberia to map the efficiency ofL gandSnwave propagation beneath the Gulf of Cadiz, Alboran Sea and bounding Betic, Rif and Atlas mountain belts. Crustal attenuation is inferred from the tomographic inversion of L g/Pg amplitude ratios. Upper mantle attenuation is inferred from maps ofSnpropagation efficiency derived by inversion of well‐defined qualitative efficiency assignments based on waveform characteristics. Regions ofL gattenuation correlate well with areas of thinned continental or oceanic crust, significant sedimentary basins, and lateral crustal variations. Comparison of theSnefficiency results with velocities obtained from an anisotropicPntraveltime inversion shows a fairly good correlation between regions of poorSnefficiency and lowPnvelocity. A lowPnvelocity (7.6–7.8 km s−1 ) and significantSnattenuation in the uppermost mantle is imaged beneath the Betics in southern Spain, in sharp contrast to the relatively normalPnvelocity (8.0–8.1 km s−1 ) and efficientSnimaged beneath the Alboran Sea. SlowPnvelocity anomalies are also imaged beneath the Rif and Middle Atlas in Morocco. We do not identify any conclusive evidence of lithospheric‐scale upper mantle attenuation beneath the Rif, although the crust in the Gibraltar region appears highly attenuating, making observations at stations in this region ambiguous. Paths crossing the Gulf of Cadiz, eastern Atlantic and the Moroccan and Iberian mesetas show very efficientSnpropagation and are imaged with highPnvelocities (8.1–8.2 km s−1 ). The spatial distribution of attenuation and velocity anomalies lead us to conclude that some recovery of the mantle lid beneath the Alboran Sea must have occurred since the early Miocene episode of extension and volcanism. We interpret the low‐velocity and attenuating regions beneath the Betics and possibly the Rif as indicating the presence of partial melt in the uppermost mantle which may be underlain by faster less attenuating mantle. In the light of observations from other geophysical and geological studies, the presence of melt at the base of the Betic crust may be an indication that delamination of continental lithosphere has played a role in the Neogene evolution of the Alboran Sea region.