Recent deformations of the deep continental root beneath southern Africa

Abstract

SEISMIC anisotropy, manifested by the splitting of shear waves into two orthogonal polarizations, provides evidence for deformation in the Earth's upper mantle. Where such deformation has been inferred beneath Precambrian cratons, it has been interpreted either as related to recent plate motion1,2, or inherited from the Precambrian3,4. Here we present data from a portable seismograph array in the Kaapvaal craton of South Africa, which indicate that the inferred direction of mantle flow underneath the array is close to the direction of absolute plate motion for southern Africa since the end of the Jurassic period. Such an alignment has also been reported for the North American craton2, suggesting that the flow in both regions is related mainly to shearing of the sublithospheric mantle by the plate above. The old continental roots that are likely to exist in this depth range5,6 must therefore be deformed by the plate motion, but the deformations are not strong enough to be seen in the presently available seismic tomography data.