PALEO-GEODYNAMIC EVOLUTION OF THE EAST AFRICAN RIFT SYSTEM BASED ON RECONSTRUCTION MODELING OF CRUSTAL THICKNESS AND PALEOELEVATION FROM 30 MA TO PRESENT

Using the method of Bahadori et al. (2018) we generate a time-dependent kinematic and dynamic model for East African Rift system (EARS) back to 30 Ma. We use paleo-plate motion boundary conditions and GPS point constraints for initial distributions of strain rates for present-day to obtain model estimates of continuous strain rates and velocities through time within northeast Africa. We then use these strain rate estimates to produce quantitative models of crustal thickness and paleoelevation through time. To account for the role of mantle upwelling and lithosphere heating associated with the super plume, we use published dynamic topography estimates to produce updated estimates of paleoelevation and upper mantle temperature and density. Our paleo-elevation model satisfies the observation of a Miocene whale fossil in the Turkana Basin region and shows a paleo-coastline in northeast Africa that is shifted 700 km inland to the west. Based on the paleo-crustal thickness and elevation models, we then solve the force-balance equations for vertically integrated horizontal deviatoric stress, which are used to obtain a variable distribution of effective lithospheric viscosity through time. The dynamic model involves deviatoric stresses associated with gradients in GPE along with stress boundary conditions and shows that GPE gradients yield a dominant component of the deviatoric stress field that produces rifting. Based on our results, additional extensional deviatoric stresses embedded in the boundary conditions are also needed to match plate motions and interior strain distributions. The extensional stresses embedded in the boundary condition solutions are consistent with studies that have investigated the contribution in deviatoric stress associated with divergent and upwelling mantle flow below EARS. However, our model predicts that boundary condition stresses gradually decrease over time and GPE contributions become progressively more important, owing to the elevation gains from mantle heating.