The thermal and anisotropic structure of the top 300 km of the mantle

Abstract

From our analyses of fundamental and higher-mode surface waves from ∼1.2×107 seismograms, we constructed tomographic models of Vsv and Vsh. At depths shallower than 300 km, models of the temperature of cooling oceanic lithosphere, pressure and temperature estimates from garnet peridotite nodules, and the estimated average potential temperature of the convecting upper mantle from the thickness of the oceanic crust are combined with laboratory measurements of shear modulus and attenuation to parameterize the dependence of Vsv on temperature and pressure. The resulting parameters agree with those from independent laboratory measurements, which extend over a range of nine orders of magnitude of the dimensionless period. The fitting parameters were then used to produce global estimates of the temperature at depths shallower than 300 km and of the lithospheric thickness. The resulting map is similar to those of previous studies, though the use of such a large number of seismograms has considerably improved the resolution. Extensive regions of the continents are underlain by lithosphere whose thickness is less than that of old oceanic lithosphere. In contrast, the lithospheric thickness beneath most cratons is between 180 and 220 km. Where rifted continental margins formed when Pangea fragmented and cut across the cratonic lithosphere, the present continental margins coincide with the margins of the cratons.