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 locate d in northwest Africa and Iberia to map the efficiency of Lg and Sn wave pr opagation beneath the Gulf of Cadiz, Alboran Sea and bounding Betic, Rif an d Atlas mountain belts. Crustal attenuation is inferred from the tomographi c inversion of Lg/Pg amplitude ratios. Upper mantle attenuation is inferred from maps of Sn propagation efficiency derived by inversion of well-define d qualitative efficiency assignments based on waveform characteristics. Reg ions of Lg attenuation correlate well with areas of thinned continental or oceanic crust, significant sedimentary basins, and lateral crustal variatio ns. Comparison of the Sn efficiency results with velocities obtained from a n anisotropic Pn traveltime inversion shows a fairly good correlation betwe en regions of poor Sn efficiency and low Pn velocity. A low Pn velocity (7. 6-7.8 km s(-1)) and significant Sn attenuation in the uppermost mantle is i maged beneath the Betics in southern Spain, in sharp contrast to the relati vely normal Pn velocity (8.0-8.1 km s(-1)) and efficient Sn imaged beneath the Alboran Sea. Slow Pn velocity 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 observatio ns at stations in this region ambiguous. Paths crossing the Gulf of Cadiz, eastern Atlantic and the Moroccan and Iberian mesetas show very efficient S n propagation and are imaged with high Pn velocities (8.1-8.2 km s(-1)). The spatial distribution of attenuation and velocity anomalies lead us to c onclude that some recovery of the mantle lid beneath the Alboran Sea must h ave 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 uppermos t mantle which may be underlain by faster less attenuating mantle. In the l ight of observations from other geophysical and geological studies, the pre sence of melt at the base of the Betic crust may be an indication that dela mination of continental lithosphere has played a role in the Neogene evolut ion of the Alboran Sea region.