Surface wave tomography: Imaging of the lithosphere–asthenosphere boundary beneath central and southern Africa?

Abstract

The lithosphere–asthenosphere boundary (LAB) remains a controversial subject in Earth sciences, and beneath cratonic regions appears to be a particularly difficult boundary to consistently image. Seismic methods give different indicators on the velocity structure of the upper mantle: tomographic models provide estimates of the velocity variations at a variety of lateral scales, but have limited vertical resolution; receiver function techniques provide good indication of the depth to seismic discontinuities, but less information on the absolute velocities. This study assesses whether the different methods give consistent estimates for the depth of the LAB in southern Africa. Using a surface wave dataset with nearly 12,000 paths in the African region, new tomographic models of central and southern Africa are calculated. To show the non-unique nature of tomography, results are presented for two different parameterisations. The models indicate varying velocity structure beneath the cratonic regions of central and southern Africa, which yield estimates of the LAB depth from around 150 km depth in Tanzania, to approximately 200 km depth beneath the Kalahari Craton, down to depths of 225–250 km beneath parts of the Congo. At a broad-scale these depth estimates are compatible with geothermometry from kimberlite xenoliths. In regions such as Tanzania, the kimberlite magmatism is observed to occur along strong horizontal gradients in upper mantle seismic velocity structure — potentially edge features in lithospheric structure. In contrast, a detailed comparison beneath the Kalahari Craton indicates that in this region, and given the present resolution of the tomography, there is no clear link between the kimberlites and velocity gradients. However, in general the kimberlites do not sample the regions of fastest seismic velocities. The relationship between LAB depth estimates from the tomographic modelling and those estimates from receiver functions is not clear. Results from receiver function techniques beneath southern Africa tend to place discontinuities at either shallower (100–150 km), or deeper (300–350 km), depths than the thermally defined LAB estimates. As such, particular care should be taken in automatically associating a discontinuity from fast to slow seismic velocities as the same location as a thermally defined lithosphere–asthenosphere boundary.