Upper mantle structure of southern Africa from Rayleigh wave tomography

Abstract

Rayleigh wave phase velocities in southern Africa have been obtained at 18 periods from 20 to 167 s using a two-plane wave method. These phase velocities are utilized in subsequent inversions to solve for shear wave structure in the crust and upper mantle of southern Africa. A fast lithosphere lid is imaged beneath most parts of southern Africa. The lid thickness is estimated on the basis of the variation of shear wave velocity with depth and ranges from ∼80 km beneath the Namaqua-Natal belt to 180 ± 20 km beneath the Kaapvaal craton. Relative low velocities are observed under the Cape Fold Belt and beneath the tectonic border regions in the shallow upper mantle. The Bushveld Complex is relatively slower than its surroundings above 100 km, probably because of high iron content that resulted from an intracratonic intrusion at 2.05 Ga. A relative low-velocity zone (LVZ) is observed at the depths of 160-260 km across southern Africa with an average velocity of ∼4.5 km/s. This layer, although not absolutely slow, is ∼4% slower than the fast lithosphere above it. The velocity reduction in this layer varies from 4.8% at the edges of the Kaapvaal craton to 1.5% at its center. Such a large velocity reduction and a strong lateral variation in the LVZ cannot be totally attributed to compositional and petrologic effects. Our preferred interpretation is that the LVZ is largely caused by high temperature. It is probably associated with sublithospheric mantle convection and should make contributions to supporting the high elevation of southern Africa. Copyright 2006 by the American Geophysical Union.