![]() ![]() This method cannot provide depth control, and it is mostly assumed that the main anisotropy resides in the lithospheric mantle or the asthenosphere 10. It is most often detected by the so-called SKS-splitting method that identifies the accumulated anisotropy between the Earth’s core and the surface by measuring the travel-time difference between the horizontally (SH) and vertically (SV) polarised waves. Surface wave inversion taking anisotropy into account indicates that the upper crust is highly anisotropic in the Limpopo Belt, and that both the mantle lithosphere and the asthenosphere are anisotropic with different directions of the fast axes 9.Īnisotropy is direction-dependent seismic velocity. Seismic surface wave tomography indicates that the lithosphere thickness is around 175 km when possible anisotropy is not taken into account 8. Seismic body-wave tomography, by assuming an isotropic mantle, indicates lithosphere thicknesses of up to 250 km by application of ray-theory methods 6 and up to 350 km by application of finite-frequency methods 7, respectively. The depth extent of the southern African cratons has been studied by several methods that indicate substantially different estimates for the thickness of the lithosphere. Surface geologic mapping and chronology, together with results from isotope studies on xenoliths, show that the cratonic crust of southern Africa formed and stabilised in the Archaean 4 and that it was reworked by a series of Proterozoic and Phanerozoic tectonomagmatic events 5. The early-to-late Archaean cratons in southern Africa constitute a natural laboratory for studies of the formation of cratons and their retention for billions of years. ![]() The stability of the lithosphere system of crust and mantle depends primarily on the rheology of the lithospheric mantle 2, 3 as the part of the lithosphere directly affected by mantle flow, and also on the degree of coupling between the crust and mantle since cratonisation. ![]() An enigmatic feature of Precambrian continental lithosphere is its long-term stability 1 after formation and amalgamation more than 2 billion years ago. ![]()
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