Thermo-compositional model of the South African cratonic mantle obtained from seismic and gravity data

Xenolith data reveal lateral and vertical compositional variations of the upper mantle of the Precambrian cratons, indicating a different degree of refertilization with respect to the most depleted mantle in iron components, characterizing the oldest Archean cratons. The South African cratonic region is composed of the Kaapvaal and Zimbabwe craton, both of Archean age, having deep and fast lithospheric roots, which are likely depleted in heavy constituents. In contrast, there exist regions, such as the Limpopo belt, a terrane that was trapped between the Kaapvaal and Zimbabwe cratons during their collision (2.6–2.7 Ga), and Bushveld Complex, an area characterized by intraplate magmatism occurred 2.05 Ga, whose negative velocity anomalies in the upper mantle, indicate a more fertile composition due to metasomatism. To unravel the origin of these anomalies and link them to the tectonic history of the area, we apply an integrative technique based on a joint interpretation of the seismic tomography and gravity data, which can discern temperature and compositional variations. To this aim, we combine the global surface seismic tomography model [1] with the embedded regional model [2], derived from teleseismic tomographic inversion of the S-body wave dataset recorded by the Southern African Seismic Experiment. The combined seismic model is inverted for temperature, assuming an initial composition, representative of a refertilized upper mantle [3], using a mineral physics approach [4]. The composition and temperature of the upper mantle are iteratively changed, increasing progressively the amount of iron depletion, to fit the residual density, obtained from the joint inversion of the residual gravity and residual topography. The great advantage of using both the gravity field and residual topography lies in their different dependence on the distribution of density heterogeneities (depth and size). In a second type of inversion we included the GOCE gravity gradient [5]. The obtained results show that the most depleted lithosphere is confined at depth lower than 100 km, generating a temperature higher than ~200, with respect to that of a refertilized lithosphere. The Southeastern Terrane of the Kaapval craton are characterized by thicker and more depleted cratonic roots than the Zimbawe craton. The presence of a depleted mantle below the cratonic crust may indicate that the crust and mantle have been connected since the craton formation. These results, related to the different structures and properties of the upper mantle, improve our understanding of the evolution of the South African cratonic lithosphere.

References

[1] Schaeffer and Lebedev, 2013. https://doi.org/10.1093/gji/ggt095

[2] Youssof et al., 2015. http://dx.doi.org/10.1016/j.epsl.2015.01.034

[3] Griffin et al., 2004. doi:10.1016/j.chemgeo.2004.04.007

[4] Conolly, 2005. doi:10.1016/j.epsl.2005.04.033

[5] Kaban et al., 2022. doi.org/10.1007/s00024-021-02925-6