Combined teleseismic imaging of the structure of southern African cratons using P-receiver functions and P- and S- finite-frequency tomography

We present a seismic model of the South-African cratonic region obtained by combining receiver functions and teleseismic P and S traveltime tomography. We determined receiver functions (RFs) for 82 stations by iterative deconvolution. Based on HK analysis (Zhou and Kananmori, JGR,2001), we measure a relatively standard crustal thickness (~38 km) with a flat and sharp Moho discontinuity within the Kaapvaal and Zimbabwe cratons. The lowest Vp/Vs values ~1.69, are found near the locations of diamondiferous kimberlite pipes in the heart of the Kaapvaal craton. We also observe strong crustal anisotropy in the lower crust. Our best-fit RF model has an average fast polarization oriented at 30° to 40° and a crustal fabric that produces, in some cases, around 50% of the total anisotropy inferred from SKS splitting (Paul G. Silver et al., GRL,2001). We also performed finite-frequency tomography in 3 frequency bands (1, 0.5 and 0.25 Hz for P and 0.1, 0.05 and 0.02 Hz for S) to obtain 3-D P- and S-wave perturbation models for the upper mantle. Crustal corrections are based on the RF models. Tests showed that our dataset is able to resolve structure of 3°x3° up to 400 km depth. The high-velocity cratonic roots extend to 300-350 km depth. Lower velocities are detected below the Bushveld complex and the mobile belts. The model also suggests a stratified structure, since we found a low velocity zone (LVZ) at about 170 km depth in the cratonic areas. SdP RFs and surface-wave tomography are in progress and should improve imaging of the LVZ and resolving the unusual structures within the cratonic lithosphere.