Variations in Shear Wave Splitting Beneath Southern Arabia and the Gulf of Aden

Abstract:

Mantle flow beneath Southern Arabia and the Gulf of Aden remains enigmatic due to a paucity of seismic measurements in the region. Potential processes contributing to mantle flow include northward progression of the African Superplume, radial flow from the Afar plume and vertical flow from small-scale convection along the margins of the Gulf of Aden. These would result in characteristic mantle flow directions, creating mantle anisotropy that can be detected by shear wave splitting. We analyse SKS, SKKS & PKS phases for shear wave splitting at 141 stations deployed throughout Yemen, Oman and Socotra along the margins of the Gulf of Aden. Large numbers of null measurements from a range of back azimuths are found beneath the entire region. These may indicate that vertical anisotropy is present in the upper mantle beneath the region, consistent with models of small-scale convection. The null measurements may also be due to complicated layering of crustal anisotropy interfering destructively and precluding measurement of shear wave splitting. Splitting measurements bordering the Red Sea show North-South orientations that may result from shallow aligned melt along the Red Sea or from variations in lower mantle flow. Fast polarization directions of splitting measurements along the Northern margin of the Gulf of Aden are rift parallel suggesting a shallow source such as rift related faulting might be responsible. These results show that anisotropy beneath the region is not controlled by the northward progression of the African Superplume or radial flow from the Afar plume. Upper mantle flow is likely vertical with splitting occurring either in the crust or the lower mantle.