Converted waves reveal a thick and layered tectosphere beneath the Kalahari super-craton

Thick and high-velocity roots are generally observed beneath the Archean cratons. Inside these high-velocity keels, velocity contrasts are detected neither by surface nor by body waves tomographies. We present here evidences based on the S-to-P and P-to-S converted waves for the existence of an irregularly stratified and thick keel beneath the Kalahari super-craton. To satisfy surface wave data and S-to-P conversions, the velocity model should have beneath the Moho a ∼ 160 km thick anisotropic structure with vertical slow axis and decreasing anisotropic parameters with depth. Such a structure may stem from the preferred orientation of olivine [100] axis in the horizontal plane under shearing deformation. This structure, together with the ∼ 100 km thick layer underlying it, forms a ∼ 300 km thick continental root beneath the cratons. Inside this root, the P and S velocities should be higher on average respectively by an amount of 6% and 4% than the AK135 velocity model. Beneath ∼ 300-350 km depth, a low velocity zone is clearly detected that may be either the remainder of large magma reservoirs related to cratonic flood basalts or a melted silicate layer created by the transformation, just above the 410-km deep discontinuity, of wadsleyite assembly to olivine assembly.