GPS constraints on broad scale extension in the Ethiopian Highlands and Main Ethiopian Rift

Measurements from GPS sites spanning the Ethiopian Highlands, Main Ethiopian Rift, and Somali Platform in Ethiopia and Eritrea show that present-day finite strain rates throughout NE Africa can be approximated at the continent scale by opening on the MER. Most sites in the Ethiopian Highlands are consistent with the motion of the Nubian plate at the level of 1mm/yr with 95% confidence. However, sites at least as far as 60km west of the rift show higher velocities relative to the stable Nubian frame of 1-2mm/yr, requiring a combination of localized and distributed deformation to accommodate the African extensional domain. Off-rift velocities are consistent with ongoing strain related to either high gravitational potential energy or intrusive magmatism away from midrift magmatic segments either on the western rift margin or within the Ethiopian Highlands, especially when combined with likely rheological differences between the Ethiopian Rift and Highlands. Velocities from the Somali Platform are less well determined with uncertainties and residuals from a Somali frame definition at the level of 2-3mm/yr but without spatially correlated residuals.