Lithospheric Boundaries and Upper Mantle Structure Beneath Southern Africa Imaged by P and S Wave Velocity Models

Abstract

We report new P and S wave velocity models of the upper mantle beneath southern Africa using data recorded on seismic stations spanning the entire subcontinent. Beneath most of the Damara Belt, including the Okavango Rift, our models show lower than average velocities (−0.8% Vp; −1.2% Vs) with an abrupt increase in velocities along the terrane's southern margin. We attribute the lower than average velocities to thinner lithosphere (~130 km thick) compared to thicker lithosphere (~200 km thick) immediately to the south under the Kalahari Craton. Beneath the Etendeka Flood Basalt Province, higher than average velocities (0.25% Vp; 0.75% Vs) indicate thicker and/or compositionally distinct lithosphere compared to other parts of the Damara Belt. In the Rehoboth Province, higher than average velocities (0.3% Vp; 0.5% Vs) suggest the presence of a microcraton, as do higher than average velocities (1.0% Vp; 1.5% Vs) under the Southern Irumide Belt. Lower than average velocities (−0.4% Vp; −0.7% Vs) beneath the Bushveld Complex and parts of the Mgondi and Okwa terranes are consistent with previous studies, which attributed them to compositionally modified lithosphere resulting from Precambrian magmatic events. There is little evidence for thermally modified upper mantle beneath any of these terranes which could provide a source of uplift for the Southern African Plateau. In contrast, beneath parts of the Irumide Belt in southern and central Zambia and the Mozambique Belt in central Mozambique, deep-seated low velocity anomalies (−0.7% Vp; −0.8% Vs) can be attributed to upper mantle extensions of the African superplume structure. In the interpretation of our models, we not only reexamine the Precambrian tectonic framework of southern Africa, but also investigate if there is evidence for thermally perturbed upper mantle beneath southern Africa, particularly beneath the uplifted regions of the Southern African Plateau. There has been much discussion (e.g.,

Shear‐Wave Velocity Structure of the Southern African Upper Mantle: Implications for Craton Structure and Plateau Uplift

“…In other words, our models show that the northern boundary of the greater Kalahari Craton lies along the southern margin of the Damara Belt, and that the western boundary lies along the eastern margin of the Gariep and Namaqua-Natal belts. This finding is consistent with body wave tomography models showing a similar transition in P wave and S wave velocity structure across these terrane boundaries (Ortiz et al, 2019;White-Gaynor et al, 2020), and also with MT studies (Miensopust et al, 2011;Muller et al, 2009).…”

Rifting of the Kalahari Craton Through Botswana? New Seismic Evidence

“…This low velocity anomaly in the uppermost mantle may be interpreted as the mantle source of fluids. The low velocity anomaly beneath the ORZ was also found in several other (lower resolution) tomographic P and S wave velocity models (Ortiz et al, 2019;White-Gaynor et al, 2020), and may be related to decompression melting caused by lithospheric thinning in response to tensional stresses (Yu et al, 2017). The normal-faulting focal mechanisms are consistent with the tensional stress regime that is required for such a scenario.…”

Probing the Southern African Lithosphere With Magnetotellurics: 2. Linking Electrical Conductivity, Composition, and Tectonomagmatic Evolution

“…Similarly, Proterozoic terranes assembled near the Archean cratons do not show consistent characteristics and architectures that would provide a simple relationship between lithospheric thickness, age, and composition from the electrical conductivity distribution. The south‐central Rehoboth Terrane, for instance, demonstrates lower resistivities ( K 2 ‐ K 3 , Figure 3b) and higher velocity bodies (White‐Gaynor et al., 2020). In contrast, the Gibeon area, which includes one of the clusters of Cretaceous kimberlitic volcanism, is imaged as a highly resistive region (Figure 3f), with relatively lower velocities (White‐Gaynor et al., 2020).…”

“…The south-central Rehoboth Terrane, for instance, demonstrates lower resistivities (K 2 -K 3 , Figure 3b) and higher velocity bodies (White-Gaynor et al, 2020). In contrast, the Gibeon area, which includes one of the clusters of Cretaceous kimberlitic volcanism, is imaged as a highly resistive region (Figure 3f), with relatively lower velocities (White-Gaynor et al, 2020). Similarly, the northern end of the Rehoboth Terrane and adjacent Ghanzi-Chobe belt possibly shows the greatest depths at which 1,000 Ωm resistivity is observed (Figure 12b).…”