The thickness of the lithosphere from the dispersion of surface waves

Summary. The inversion of regional Rayleigh and Love wave dispersion data in the period range 30–200 or 250 s leads to an inference that high S ‐wave velocities in the mantle are to be found to depths of the order of 300 km or greater under the ancient Precambrian shields of the world. Under the other parts of the world, a low S ‐wave velocity channel is found at depths up to 100 km below the surface. The depth to the onset of the low‐velocity channel is consistent with the onset of partial melting of a mantle of peridotitic composition. Under the shields there is no evidence for partial melting to depths of the order of 300km. No statement can be made from these data about the presence or absence of a low‐velocity channel in subduction zones. If the lithosphere is defined to be that region of the mantle that moves with the same velocity of flow as the surface above it, then the Pacific and Nazca plates have uniform thicknesses of only 100 km. For all the other major plates of the world, two models can be proposed that cannot be resolved with the present data. In the first model, all the other major plates, including their oceanic parts, have uniform thicknesses of at least 300 km and may have thicknesses as great as 700 km. In the second model, all the other major plates have deep roots under the ancient Precambrian shields, and an eddy of flow around these roots begins at a depth of about 100 km.