Geoid Reveals the Density Structure of Cratonic Lithosphere

Geoid is a key observable for understanding the dynamics of the deep Earth, but has been considered largely transparent to long-wavelength shallow density structures, especially those of the cratonic lithosphere. Here, we demonstrate that the observed flat craton-ocean geoid pattern, traditionally interpreted as reflecting neutrally buoyant cratonic keels, provides critical constraints on both the net buoyancy and the depth-dependent density distribution of cratonic mantle lithosphere. Using both simple theoretical calculations and quantitative numerical models, we show that the recent seismic data on lithospheric structure require the existence of a dense cratonic mantle lithosphere to explain the observed topography and geoid. In practice, topography reveals the net buoyancy of the cratonic lithosphere, while geoid further delineates the depth-dependence of excess density. We find that the mantle lithosphere below large cratons bears net negative buoyancy close to that of a pure-thermal lithosphere, with most of the excess density distributed within the lower half of the lithosphere. Density profiles of small cratons, due to strong edge effects from surrounding orogenic belts, are harder to constrain, except that their mantle lithosphere is also negatively buoyant.