Geoscientific Colloquium

Although mantle convection at Earth-like vigor is a chaotic process, it is possible to constrain its flow history back in time for periods comparable to a mantle overturn (? 100million years). Such retrodictions, enabled by computationally demanding adjoint methods, are an exciting new tool to improve our understanding of deep Earth processes, and to link uncertain geodynamic modeling parameters explicitly to geologic observables. Here we present the first mantle flow retrodictions for geodynamically plausible, compressible, high resolution Earthmodels going back in time to the Mid Paleogene. Our retrodictions involve the dynamic effects from a low viscosity zone in the upper mantle, assimilate a past plate motion model for the tangential surface velocity field, and probe the influence from uncertain modeling parameters by using two different estimates for the present-day heterogeneity state of the mantle as imaged by two recent seismic tomographic studies, and two different values for deep mantle viscosity. Focusing on the African hemisphere, we find that our retrodictions produce a spatially and temporally highly variable asthenosphere flow with faster-than-plate velocities, and a history of dynamic topography characterized by local doming events, in agreement with considerations on plate driving forces, and regional scale uplifts reported in the geologic literature. Our results suggest that improved constraints on non-isostatic vertical motion of the Earth surface — provided, for instance, by basin analysis, seismic stratigraphy, landform studies, or the sedimentation record— will play a crucial role in furthering our understanding mantle dynamics.