Plume-lithosphere Interaction beneath southwestern Africa – Insights from Rayleigh Wave Tomography

Abstract:

A stationary mantle plume heats the overlying lithosphere from below, causing lithospheric erosion and delamination. When the lithosphere moves away from the plume, it can be reestablished by melt residues that have attached to the base of the lithosphere. The Etendeka flood basalt region in northwest Namibia provides an ideal place to examine the plume-lithosphere interaction. The study area used to overly the Tristan da Cuhna mantle plume in the Early Cretaceous at time of the Gondwana breakup. In this study, we determine the upper mantle seismic velocity structure of southern Africa by waveform inversion of Rayleigh waves in order to find the imprints left by the Tristan da Cunha mantle plume and the continental breakup. A high resolution 3D model was achieved by fitting 7248 multi-mode waveforms recorded at more than 150 stations in and around southern Africa. The procedure involves combination of 1-D path average models obtained by modeling each Rayleigh waveform up to the 4th higher mode in a tomographic inversion scheme. The inclusion of a local seismic network (WALPASS array) in northwest Namibia and the use of multi-mode analysis help to achieve a lateral resolution of a few hundred kilometers from ~50 km down to 400 km depth. Thick lithosphere with high velocity is observed beneath the Congo and Kalahari cratons, extending down to a depth of 200 km. The thickest lithosphere (> 200 km) is found beneath the Limpopo Belt. The thick Congo cratonic lithosphere extends farther south toward the Walvis Ridge than in most of the previous models. The Damara Belt that separates the Congo and Kalahari cratons seems to be a superficial phenomenon underlain by a high-velocity body similar to the cratonic body. We propose that the southward extension of the thick Congo cratonic lithosphere towards the Walvis Ridge is contributed by the melt residue from the Tristan da Cuhna mantle plume, which has reconstructed the deep lithospheric root by melt depletion and dehydration. A high-velocity anomaly beneath the northwest coast of Namibia down to a depth of ~80 km coincides with the distributed magma at the surface. At greater depths (250-300 km) beneath the same area, a high-velocity anomaly is pronounced and is interpreted as the plume residues and/or previously destroyed lithospheric materials.