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South Africa Cratons; Thermal model; Gravity anomalies; Compositional model; iron depletion
Abstract:
Tomographic models of seismic velocity perturbations provide valuable insights into the Earth's interior. Yet, because seismic velocities are primarily temperature-dependent, they mostly capture thermal anomalies. Combining them with gravity data enables a joint interpretation that can also account for compositional variations in the upper mantle beneath Precambrian cratons. The South African cratonic region is composed of the Archean Kaapvaal and Zimbabwe craton and, according to xenolith analysis, underlain by a heterogeneous upper mantle both in terms of temperature and composition.
To unravel the temperature and compositional anomalies with corresponding density variations and to link them to the tectonic history of that area, we apply an integrative technique based on a joint interpretation of the seismic tomography and gravity data. We combine a global shear seismic tomography model with an embedded high-resolution regional model and we invert it for temperature, assuming an initial composition, representative of a refertilized upper mantle. 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, and GOCE gravity gradients. The results show significant lithospheric compositional variations consistent with the tectonic history of the area. The most depleted lithosphere is located in the Southeastern Terrane of the Kaapvaal craton, at depth < 100 km, generating a temperature higher than ∼150°C, with respect to that of a refertilized lithosphere.
