Abstract

The upper mantle beneath old, stable continents reveals puzzling seismic discontinuities within the cold and strong thermal boundary layer. This is unexpected, as tomographic models depict fast wavespeeds and low energy dissipation, offering no basis for sharp velocity gradients within the boundary layer. Resolving this paradox requires explaining the observations with mechanisms such as metasomatic alteration by volatile-bearing rocks or grain-scale solid-state processes. I present new constraints derived from scattered-wave imaging that significantly improve global coverage and depth resolution. Advanced denoising of body-waves reveals a sharp velocity reduction beneath two-thirds of all continents at _{90 ± 20 km depth, best explained by sub-solidus grain-boundary weakening. In some regions, velocity drops are observed between 120 and 250 km, while in others, velocity increases occur at} 150 ± 50 km. Across all cases, upper mantle discontinuities rarely extend beyond the continental heterosphere. Near continental margins and subduction zones, these features may indicate partial melting, while rare double discontinuities could reflect melt layers or tectonic remnants from continental assembly, paleo-subduction, or underplating. These findings provide crucial seismic clues for resolving the paradox of lithosphere discontinuities, offering new insights into the nature and dynamics of the thermal boundary layer beneath continents