Kinematics and Dynamics of the Main Ethiopian Rift

Abstract:

Although the East African Rift System (EARS) is often cited as a type example for “narrow” rifting (where strain is localized along the rift axis), the true extent of rift-related deformation remains largely unknown due to sparse geophysical observations outside of the main rift valley. Our study, which takes this large scale approach, investigates the distribution of deformation in the Main Ethiopian Rift (MER) and surrounding regions, including the Ethiopian Highlands to the west of the rift valley, Somali Platform to the east, and Afar Triple Junction. We first construct kinematic, self-consistent strain rate and velocity fields on a 1° by 1° grid using continuous spline interpolations of strain rate observations (earthquake and fault data, plate rotations, and GPS velocities). Next, we calculate the deviatoric stress field associated with gravitational potential energy (GPE) by integrating density as a function of depth using published crustal density structures (CRUST1.0) and newly obtained receiver functions. We then directly solve for the deviatoric stress field associated with the lateral density variations by assuming a minimum energy stress field (e.g. Flesch et al. [2001]). Finally, we look for symmetries and asymmetries in both the strain rate and GPE deviatoric stress fields to assess the source of observed, off-rift deformation. We compare our results to published global and regional models that include the East African Rift and Iceland. Results suggest that the MER is not an end-member, “narrow” type rift, and that heterogeneities in lithospheric strength likely play an important role in governing the kinematics of rifting in Ethiopia.