The influence of lithospheric thickness variations on continental evolution

The shear wave velocity Vs as a function of depth z can be obtained from surface wave tomography, using the phase velocities of fundamental and higher mode Rayleigh waves. Since Vs is principally controlled by temperature, rather than by composition, it can be used to map the lithospheric thickness. Extensive regions of thick lithosphere underlie some, but not all, cratons. Conversely, thick lithosphere underlies some platforms and belts of active deformation. Because of this lack of correspondence, and because their age cannot be determined from seismology, we refer to regions of thick lithosphere as ‘cores’ rather than ‘cratons’. The shape of such cores has controlled the geometry of continental deformation and the distribution of diamond-bearing kimberlites. The strength of the cores resides in the dry crust, which is insulated from the hot convecting mantle by the thick buoyant lithosphere. The most surprising feature is the presence of thick lithosphere beneath Tibet and Iran, whose velocity structure closely resembles that of the cores beneath cratons, though they have a thicker hotter crust. Tibet and Iran appear to be places where cratons are now being formed.