Strong seismic anisotropy in the crust of southern African catons

Abstract:

An enigmatic feature of Precambrian continental lithosphere is its long-term stability, which depends on the degree of coupling between the crust and mantle since cratonisation, despite being affected by mantle flow and plate tectonic forces. Earlier studies of the Kalahari Craton infer deformation of upper mantle by flow with fast direction of seismic anisotropy being parallel to present plate motion, and/or report anisotropy frozen into the lithospheric mantle. We present evidence for very strong seismic anisotropy in the crust of southern Africa, including the cratons. The crustal anisotropy is 30-40% of the total anisotropy as measured by SKS splitting. Our analysis is based on calculation of receiver functions for the data from the SASE experiment which shows strong splitting between the SV and SH components. The direction of the fast axes is uniform within tectonic units and parallel to orogenic strike in the Limpopo and Cape fold belts. It is further parallel to the strike of major dyke swarms which indicates that a large part of the observed anisotropy is controlled by lithosphere fabrics and macroscopic effects. The directions of the fast axes for the crustal anisotropy are parallel to the general directions determined from SKS splitting, although the directions from our analysis of receiver functions is more homogeneous for than for SKS splitting. This analysis indicates parallel fast axes in the crust and in the mantle, which suggests that the crust and lithospheric mantle have been coupled since cratonisation. If so, the apparent match between mantle anisotropy and the present plate motion is coincidental.