Shear‐Wave Velocity Structure of Southern Africa's Lithosphere: Variations in the Thickness and Composition of Cratons and Their Effect on Topography

Abstract

Seismic‐wave velocities offer essential constraints on the temperature, thickness, and composition of the lithosphere of cratons. We invert broadband, Rayleigh‐wave phase and Love‐wave phase velocities measured across the Kaapvaal Craton and Limpopo Belt for depth distributions of shear‐wave velocity and radial anisotropy, from the upper‐crust down to deep upper mantle. Our probabilistic, Bayesian inversion addresses model nonuniqueness by means of direct parameter‐space sampling. An increase in Vs between the Moho and 100–150 km depths occurs across the region and can be explained by the gradual emergence of garnet below 80 km, due to the spinel peridotite‐garnet peridotite transformation and due to the exsolution of garnet from mantle orthopyroxene. Lateral variations in this Vs gradient can provide new information on lateral compositional variations. Cold cratonic lithosphere is manifest in very high shear velocities, up to 4.8 km/s. The depth extent of the shear‐velocity anomaly and the inferred lithospheric thickness increase from ∼200 km beneath the central and southwestern Kaapvaal to ∼300 km beneath the Limpopo Belt. Curiously, surface elevation decreases monotonically with the increasing lithospheric thickness. The relationship between the lithospheric thickness and topography depends on the lithospheric composition and, with the crustal structure taken into account, our results imply that the bottom part of the Limpopo lithosphere (200–300 km) is weakly‐to‐moderately depleted (Mg# 89.7–90.8). Our results also show that the central‐southwestern Kaapvaal lithosphere is thinner than it was (according to kimberlites) 100–200 m.y. ago. It may have been thinned by the same mantle plume that, initially, triggered the kimberlite eruptions.

Crustal and Upper Mantle Shear Wave Velocity Structure of Botswana: The 3 April 2017 Central Botswana Earthquake Linked to the East African Rift System

“…The depth-dependent structure of our model is in general agreement with previous studies (Adams & Nyblade, 2011;Li & Burke, 2006;Ravenna et al, 2018) with mantle shear wave velocities that increase to 4.7-4.8 km/s at ∼150 km before decreasing again. The gradual velocity increase to 150 km may be explained by an increase in garnet content as argued by Ravenna et al (2018). Furthermore, our results suggest that the deep root of the Zimbabwe Craton (at ∼200 km depth) has a higher shear wave velocity than the Kaapvaal Craton (Figure 4d).…”

African cratonic lithosphere carved by mantle plumes

“…Cratonic lithosphere is also present beneath the Paleoproterozoic Kheis-Okwa-Magondi Belt west-northwest of Kaapvaal 20 , consistent with it having Archean basement 20 , and beneath central and northern parts of the adjacent Rehoboth Province 54 , the location of the previously proposed, unexposed Maltahohe Craton 20,55 . At shallow mantle-lithosphere depths, the Limpopo Belt stands out with relatively low between the Moho and 100 km 40 . Cratonic lithosphere is notably absent beneath the southern and eastern Kaapvaal and north-eastern Zimbabwe Cratons.…”

“…3), as expected from the relationship between the lithospheric thickness and temperature given by realistic geotherms 38,39 . In the thick cratonic lithosphere, the increase of temperature with depth is relatively slow and the LAB can be expected to be marked by only a subtle change in the slope of the depth dependence of temperature and seismic velocity 40 . For this reason, direct estimates of the LAB depth from seismic tomography models are ambiguous, unless thermodynamic modelling including seismic data or models is performed 41 .…”

Dynamics of the lithosphere and upper mantle in light of seismic anisotropy

“…On those scales, it is also easier to explore uncertainties, and probabilistic 1-D profiles obtained with Monte Carlo inversion schemes can be used, for example, to explore the trade-off between the radial and azimuthal anisotropy layer imaging [e.g. Beghein and Trampert, 2004;Agius and Lebedev, 2014;Bodin et al, 2016;Ravenna et al, 2018].…”