ON THE STRUCTURE OF THE CRUST AND UPPER MANTLE BENEATH THE AFRO-ARABIAN REGION FROM SURFACE-WAVE DISPERSION

The deep structure beneath the Afro-Arabian region was investigated using Rayleigh and Love waves in the period range 20-250 s. Models of crustal and upper mantle structure were inferred by simultaneous inversion of both phase and group velocities. Our choice of model parametrization allowed for a transverse isotropic medium and a variable depth of the Moho. The crustal thicknesses were found to vary from 43 to 45 km on the African plate. The Moho is deeper in the Arabian plate (45-47 km), but in the vicinity of the Red Sea the crust becomes much thinner (30 km). The upper mantle under Arabia is more similar to that of an African mobile zone than to that of a shield. A low velocity zone (LVZ) with an S-wave velocity of 4.14-4.2 km/s was found in the depth range 120-140 km. However, this LVZ is less pronounced than that found in the vicinity of the Red Sea (S-wave velocity 4.08 km/s). In agreement with previous studies, the anisotropy (V(SH)-V(SV))/V(SV)) has been verified for the Afro-Arabian region. We interpret the anisotropies using the hypothesis of lattice-preferred orientation of the olivine crystals in the upper mantle. The inference is that a vertical flow exists up to near the surface in the vicinity of the Red Sea rift (V(SV) is 8-9% faster than V(SH). Under the African and Arabian sides the flow is essentially vertical below depths of 200 +/- 40 km and becomes essentially horizontal above with a V(SH) from 6% to 14% faster than V(SV).