Shear-wave splitting in Ethiopia: Precambrian mantle anisotropy locally modified by Neogene rifting

Twenty-six broadband seismic stations in an areal array spanning 500 x 500 km across Ethiopia were used for shear-wave splitting studies. Our results show small-to-moderate delay times (0.5 - 1.7s) with fast-polarization azimuths sub-parallel to the orientation of the East African Rift (NNE-SSW) and also to the Proterozoic tectonic fabric across the entire studied area. Our results imply Ethiopian upper-mantle anisotropy is controlled largely by the Proterozoic accretion of the Mozambique belt, with possible minor effects within the rift due to aligned cracks or melt pockets parallel to the rift axis. Our observations are not consistent with anisotropy created by asthenospheric flow parallel either to the Cenozoic extension direction (NW-SE) or to the modern absolute plate motion direction (NNW-SSE), or to asthenospheric radial flow from the "Afar'' plume.