Thermal plume models and melt generation in East Africa: A dynamic modeling approach

The hypothesis that thermal plumes contribute to the Cenozoic magmatism in East Africa is now widely accepted. The controversy lies on how many plumes exist and where they may be located. In this study we show numerical experiments of mantle convection models for a number of thermal plume models and discuss the implications for the melt generation in East Africa. We investigate how the plume(s), the Tanzania craton, and the African lithospheric structure may interplay to result in the magmatism distribution in East Africa since the Eocene. Our results demonstrate that the variable thickness of the lithosphere modulates melt generation. A single-plume model cannot reproduce the observations consistently. Double-plume models with plumes located at Afar and Kenya regions are viable with reasonable physical properties. The distribution of the plume material, however, is sensitive to the angle at which the Tanzania craton and regions of thick lithosphere approach the plume, as the African plate moves. Models that have present-day location of the second plume (Kenya plume) under the Eastern rift or the interior of the Tanzania craton can best match the basalt distribution. Our model results suggest that the basaltic eruptions associated with the Afar plume tap a relatively deep source of the plume body in general, whereas melting occurs at shallower depths for the Kenya plume except for the Eocene episode. The magmatism is derived from a more depleted mantle source in the low-Ti basalt province of northwestern Ethiopia. Our experiments indicate the thermal influence of the Afar plume but predict an absence of plume-derived melts, suggesting the melt generation within lithosphere triggered by thermal influence of Afar plume in this region. Our model results suggest that plume plays an active role on the initiation of the rifting process in East Africa.