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.

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大地水准面揭示克拉通岩石圈的密度结构

大地水准面是了解地球深部动力学的关键观测，但被认为在很大程度上对长波长浅层密度结构是透明的，尤其是克拉通岩石圈的那些。在这里，我们证明了观察到的平坦克拉通海洋大地水准面模式，传统上被解释为反映中性浮力克拉通龙骨，对克拉通地幔岩石圈的净浮力和深度相关密度分布提供了关键约束。使用简单的理论计算和定量数值模型，我们表明最近关于岩石圈结构的地震数据需要存在致密的克拉通地幔岩石圈来解释观测到的地形和大地水准面。在实践中，地形揭示了克拉通岩石圈的净浮力，而大地水准面进一步描述了超密度的深度依赖性。我们发现，大克拉通下方的地幔岩石圈具有接近纯热岩石圈的净负浮力，大部分过剩密度分布在岩石圈的下半部分。由于周围造山带的强烈边缘效应，小克拉通的密度分布更难约束，除了它们的地幔岩石圈也具有负浮力。