Kinematics and dynamics of the East African Rift from GPS geodesy and thin-sheet modeling

Although the process of continental rifting is of primary importance for seismic and volcanic hazards, ocean formation, and vital natural resources, the underlying physics remains debated. Some dynamic models suggest that large-scale mantle convection controls rifting, however there is evidence that localized asthenospheric processes such as small-scale convection and/or added buoyancy from melt intrusions also play an important role. Strain distribution across and along active rifts, measurable from GPS observations, is a key data set needed to elucidate the dynamics of rifting. The East African Rift (EAR) is the planet’s archetype divergent plate boundary offering a favorable opportunity to quantify strain distribution across an active continental rift system. Here, we present a new velocity field for the EAR that includes GPS observations from continuous sites and updated (2010) campaign GPS measurements. We combine the new velocity field with seismic moment tensors to develop a refined kinematic model, from which we derive a strain-rate field. We then model the large-scale dynamics of Africa and surroundings using a thin-sheet approach to estimate vertically averaged stresses. We show that the majority of vertically averaged stresses (~15-25 MPa) acting to rupture the EAR result from gradients in gravitational potential energy (GPE). Additional stresses from large-scale mantle convection (i.e. the African Superplume) prove small (~5 MPa) compared to stresses derived from gradients in GPE. Therefore we argue the major forces driving continental rifting are derived from gradients in GPE.