UPPER-MANTLE S-VELOCITIES BENEATH AFAR AND WESTERN SAUDI-ARABIA FROM RAYLEIGH-WAVE DISPERSION

Two-station methods are employed to determine Rayleigh wave phase velo cities along three paths crossing the Afro-Arabian dome. Phase velocit ies are modeled using a grid-search technique, and the range of succes sful models indicate that the uppermost mantle beneath Afar and wester n Saudi Arabia is characterized by pronounced negative S wave velocity gradients and S wave velocities that are 0.2-0.8 km/s lower than PREM at 100 km depth. These results are consistent with a thermal plume mo del for Afar, provided that there is a few percent of partial melt pre sent within the mantle. The low velocity anomaly beneath western Saudi Arabia could be caused by migrating plume material from Afar, upwelli ng mantle material associated with Red Sea rifting, or isolated small- scale convection.