Ridge-like upwelling in the uppermost lower mantle beneath eastern Africa from finite-frequency seismic tomography

Global seismic tomographic imaging has shown a large and coherent low-velocity anomaly in the lower part of the mantle beneath Africa. It has been suggested that this anomalous feature may be responsible for the unique geological history of Africa. But the link between the African superplume in the deep mantle and surface tectonics remains unclear. In this study we carry out tomographic inversions for the seismic velocity structure beneath southern Africa, utilizing "banana-doughnut" traveltime sensitivity kernels of body waves recorded by temporary and permanent broadband seismic stations in the region. Preliminary results show a ridge-like low-velocity anomaly in the upper part of the lower mantle (900 - 1200 km depth) beneath eastern Africa. It is 300-400 km wide beneath the Kaapvaal craton and becomes broader beneath Tanzania. At shallower depth and in the mantle transition zone, the low-velocity anomaly becomes localized and cylindrical beneath Tanzania. This is consistent with an anomalously thin mantle transition in the area. A weak, localized low-velocity anomaly is also found in the transition zone beneath the western end of the Limpopo Mobile Belt (between the Zimbabwe and Kaapvaal cratons). Our observations may have important implications for the deep earth engine. Models of mantle convection driven by a combination of basal and internal heating predict a transition in the shape of a buoyant upwelling from tabular to cylindrical near the basal boundary layer. The ridge-like low-velocity anomaly in the upper part of the lower mantle beneath southern Africa may thus represent upwelling from a boundary layer in the mid-mantle, possibly above the top of the African superplume. The fact that the ridge-like low-velocity anomaly is present beneath southern African cratons suggests that volcanism in eastern Africa is not the result of passive upwelling driven by plate separation but mainly the consequence of deep, buoyant mantle convection.