Coupled Crust-Mantle Evolution for > 2 Gy in Southern Africa from Exceptionally Strong Crustal Anisotropy

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. Earlier studies infer deformation of the lower lithosphere by mantle flow with fast direction of seismic anisotropy being parallel to present plate motion, and/or report anisotropy frozen into the lithospheric mantle. We demonstrate coupled crust-mantle evolution in southern African cratons for more than 2 billion years based on unexpectedly strong crustal azimuthal anisotropy (Thybo et al., 2019). The direction of the fast axis 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. Parallel fast axes in the crust and in the mantle indicate coupled crust-mantle evolution. These conclusions have implications for the rheology of the lower lithosphere and the effects of mantle flow on lithosphere deformation.