Implications of dynamic topographic measurements along Africa’s passive margins

Earth’s topography is both isostatically and dynamically supported. Sub-crustal density anomalies, caused by convective mantle processes, generate transient vertical motion at Earth's surface, producing the dynamic component of topographic support. Residual depth measurements are a well-established proxy for quantifying dynamic topography on oceanic crust and provide an observation-led approach to probing mantle dynamics. A global database of residual depth anomalies compiled from seismic reflection profiles and wide-angle seismic experiments is augmented with results obtained from interpreting further seismic experiments in the oceans surrounding the African continent. Residual depth anomalies are calculated by isolating and removing isostatic signals arising from sediment loading and crustal heterogeneity. Following these corrections, observed water-loaded depth-to-basement is compared to that predicted by the plate cooling model, with deviation equal to the residual depth anomaly. Coverage surrounding the African continent is improved, particularly in the Gulf of Guinea and Mozambique Channel. Results are consistent with previous observations, showing dynamic support of ± 1 km out to and including spherical harmonic degree l = 40 (i.e. ~ 1000 km). Results are corroborated by independent geologic and geophysical markers of subsidence and uplift. For example, volcanism and slow shear-wave upper mantle velocity anomalies associated with the Cameroon Volcanic Line indicate dynamic support. Improving the spatial sampling of residual depth anomalies provides insight into the influence of convective circulation on Earth’s surface, culminating in a more robust database against which geodynamic models of mantle convection can be benchmarked.