Convergence depths of tectonic regions from an ensemble of global tomographic models

Abstract

AbstractWe project 3‐D variations in shear wave (vS) velocity from 21 whole‐mantle tomographic models onto global tectonic regionalizations. The vS profiles of oceanic and continental regions show strong upper mantle variations, as expected from plate tectonics, and most are consistent with lower mantle heterogeneity uncorrelated with surface tectonics. We construct probabilistic lower bounds for a set of convergence depths , where the vS profiles for regions X and Y become statistically indistinguishable. The lower bounds account for intraregional variances and are corrected for vertical‐smearing bias. Ensemble probability curves yield > 146 km and > 178 km at 90% confidence, where A, B, and C comprise oceanic crust of ages < 25 Ma, 25–100 Ma, and >100 Ma. These intra‐oceanic convergence depths are consistent with half‐space cooling models, showing that the mantle more than 100 km below the G discontinuity participates in conductive cooling, and they imply that the upper part of the oceanic asthenosphere is advected by the plate flow. At 95% confidence, > 350 km, where region PS comprises stable continents (platforms P and shields S) and BC comprises mature ocean basins (>25 Ma). Stable‐continent heterogeneities show high radial correlation, indicating that marks the base of a coherent tectosphere. Our results call into question the interpretation of the lithosphere‐asthenosphere boundary as the kinematic base of the tectonic plates. They are more consistent with a thicker tectosphere decoupled from mantle flow by a low‐viscosity layer immediately above the 410 km discontinuity.