Crust

The long-term stability of Precambrian continental lithosphere depends on the rheology of the lithospheric mantle as well as the coupling between crust and mantle lithosphere, which may be inferred by seismic anisotropy. Anisotropy has never been detected in cratonic crust. Anisotropy in southern Africa, detected by the seismological SKS-splitting method, usually is attributed to the mantle due to asthenospheric flow or frozen-in features of the lithosphere. However, SKS-splitting cannot distinguish between anisotropy in the crust and the mantle. We observe strong seismic anisotropy in the crust of southern African cratons by Receiver Function analysis. Fast axes are uniform within tectonic units and parallel to SKS axes, orogenic strike in the Limpopo and Cape fold belts, and the strike of major dyke swarms. Parallel fast axes in the crust and mantle indicate coupled crust-mantle evolution for more than 2 billion years with implications for strong rheology of the lithosphere.

We model the structure of the lithosphere, its thickness and seismic anisotropy, of the French Massif Central (MC) and the Bohemian Massif (BM), from spatial variations of P-wave delay times and the shear-wave splitting observed at networks of mobile and permanent seismological stations. Joint analysis of anisotropic parameters of P and S waves provide a good resolution for identification of lithosphere blocks characterized by consistent orientations of olivine fabrics within the mantle lithosphere. We suggest that deep sutures, detected as boundaries between domains with different orientation of anisotropic structures, provided paths for mantle fluids participating in the Late Variscan metallogeny in both massifs. The sutures were partly rejuvenated in the Tertiary, predestined a space for rifts and provided channels for the magmas feeding the Cenozoic volcanism.