Crustal shear wave velocity and radial anisotropy beneath Southern Africa from ambient noise tomography

Abstract

Southern Africa is an amalgamation of a distinct Archean core, several Proterozoic mobile belts, and large igneous provinces, making it an ideal natural laboratory for studying Precambrian crustal evolution and deformation. Here we present a new radially anisotropic shear-velocity model of the crust and uppermost mantle beneath Southern Africa using Rayleigh- and Love-wave ambient noise tomography from multiple seismic networks. In the upper and middle crust, the distribution of the isotropic shear velocity correlates well with known surface geology. However, in the lower crust there is no systematic lateral variation of shear velocity between the Archean and Proterozoic terranes, which is likely due to the intensive modifications of Precambrian crust by several later thermo-tectonic events through processes such as lower crustal flow. The strong positive radial anisotropy (VSH > VSV) in the lower crust, when combined with weak azimuthal anisotropy imaged previously, suggests a complex flow pattern, the directions of which likely varied on local-scale due to inhomogeneous shortening or extension during the multiple post-cratonization thermo-tectonic events.