Seismic constraints on dynamics of the mantle of the Kaapvaal craton

Abstract

Shear-wave splitting in the upper mantle of the Kaapvaal craton in southern Africa is measured by applying the SKS technique to the records of a portable array of digital broad-band seismographs. The data are interpreted in terms of lattice preferred orientation in olivine. Fast wave polarization directions at all stations of the array are close to the absolute plate motion (APM) direction of southern Africa since the end of the Jurassic. This alignment is similar to that previously reported for the North American craton and suggests that the large-scale component of mantle anisotropy on both sides of the Atlantic can be related to resistive drag at the base of the plates. A likely depth range of the corresponding deformation is 150–400 km. Analysis of the mantle converted phases suggests that the high-velocity mantle root of the Kaapvaal craton, which translates coherently with the plate, resides in the same depth range: the bottom of the root is found at a depth close to 380 km. We conclude that the root is deformed by the recent plate motion, but the deformations are not strong enough to be seen in the available tomographic models.