Crustal and upper mantle structure beneath Afar

Abstract

The interaction of the Arabian, Nubian, and Somalian plates in the Afar triple junction is characterized by diverging plate dynamics along the Red Sea, Gulf of Aden, and East African rifts. Previous investigations using shear-wave splitting measurements and geodetic analyses have found that the average anisotropy aligns with the observed surface motion. However, the vertical layering of anisotropy in this region is yet to be accurately determined. In this study, we use earthquake seismic data to map Rayleigh-wave azimuthal anisotropy in the crust and lithospheric mantle beneath the East African Rift System. Our findings indicate the presence of layered anisotropy around the East African Rift System.  At shorter periods, which sample the crust, we identified rift-parallel anisotropy near the rift, but in the central portion of the rift, rift-normal anisotropy was observed. At longer periods, which sample the lithospheric mantle, the anisotropic pattern exhibited significant differences. These observations suggest that the crust and lithospheric mantle are mechanically distinct below the East African Rift System. Likewise, these results imply the presence of complex dynamics within the crust and lithosphere in the region of the Afar triple junction.