Shear velocity anisotropy distribution beneath southern Africa’s cratons: Lithospheric structure, deformation and discontinuities

Seismic-velocity structure and anisotropy of cratonic lithosphere offer important clues on the enigmatic formation and stabilization of cratons. To investigate the formation and the evolution of old continents, we focused our study on imaging the azimuthal and radial anisotropy beneath southern Africa. We have measured thousands of inter-station, Rayleigh- and Love-wave, phase-velocity curves across southern Africa (Adam and Lebedev 2012) and inverted the very-broadband dispersion data for profiles of the shear speed and azimuthal and radial anisotropy beneath different parts of the Kaapvaal Craton and the Limpopo Belt. Systematic model space mapping is used to evaluate parameter trade-offs and to ensure the robustness of the anisotropy profiles (Adam and Lebedev, 2013). Our results, firstly, reconcile the long-debated previous models based on different interpretations of SKS-splitting measurements in southern Africa (one end-member model placing anisotropy primarily into the lithosphere and attributing it to Archean deformation and the other placing it into the asthenosphere, with recently developed fabric). We show that the depth distribution of anisotropy comprises elements of both models. Secondly, our results reveal the layering of shear speed anomalies and anisotropy within the lithosphere and asthenosphere. We interpret this jointly with receiver-functions from the region (Sodoudi et al. 2013), which suggest the presence of interfaces at around 100, 200 and 300 km depths. Significant high-velocity anomaly characteristic of cold cratonic lithosphere is constrained by surface waves and bottoms at around 200 km depth; the S-to-P conversions at this depth can thus be attributed to the LAB. At around 90-100 km depth, surface-wave data require substantial changes in anisotropy beneath the Kaapvaal Craton. The discontinuity at 90-100 km seen in receiver functions can thus be attributed to the relatively sharp change in anisotropy. Because the anisotropic fabric is a record of flow during the last episode of pervasive deformation experienced by the rock, its differing orientations in the upper and lower parts of the mantle lithosphere have important implications for the mechanism of the formation and stabilization of the craton. J.M.-C. Adam and S. Lebedev. Azimuthal anisotropy beneath southern Africa from very broad-band surface-wave dispersion measurements. Geophys. J. Int., 191:155–174, 2012. doi: 10.1111/j.1365-246X.2012.05583.x. F. Sodoudi, X. Yuan, R. Kind, S. Lebedev, J.M.-C. Adam, F. Tilmann. G3, Seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of Kalahari Craton. 2013. Under revision. J.M.-C. Adam and S. Lebedev. Shear velocity and anisotropy distributions beneath southern Africa's cratons: Lithospheric structure, deformation, LAB and other discontinuities. 2013. In prep.