Geoid Reveals the Density Structure of Cratonic Lithosphere

Abstract

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.

Cenozoic India-Asia collision driven by mantle dragging the cratonic root

“…The crust has an average density of ~ 2.8 , with the lower crust being weaker than the upper crust to mitigate the effect of deformation from the imposed plate motion to deeper depths. The continental mantle lithosphere consists of a buoyant upper layer, a neutrally buoyant middle layer, and a dense lower layer relative to the ambient mantle that follows recent inferences [53][54][55] . More model information is available in recent studies using similar methods [24][25][26][27] .…”

Continental Delamination Initiates Subduction and Supercontinent Breakup

“…Analog 32 and numerical 33 experiments suggest that a buoyant overriding continental lithosphere with a small effective viscosity and a thin thickness could allow passive margin SI. However, these dynamic properties contradict the recent realization that most passive margin SI events occurred along the craton edges 6 , where the cratonic lithospheric mantle (CLM) is thick, dense 13,14 , and mechanically strong 34,35 . Therefore, a different mechanism is required for passive margin SI prior to supercontinent separation.…”

“…As a result, the average thermal density anomaly in the upper and lower CLM is 57 kg/m 3 and 22 kg/m 3 , respectively. To evaluate the dynamic effect of different CLM density pro les, we vary the compositional density of the upper and lower layers, while maintaining the mean CLM's excess density to be close to 1% of the ambient mantle, as recently constrained 13,14 . The passive margin material is assumed to have a smaller critical plastic strain and yielding stress than normal oceanic lithospheric mantle (Methods) due to the inherent weakness from previous rifting and/or deformation.…”

“…For simulations with different CLM density pro les, we vary the compositional density of the upper CLM. Accordingly, the compositional density of the lower CLM is calculated to make the net buoyancy of the CLM close to 1% denser than the ambient asthenospheric material, as constrained by recent studies 13,14 . The compositional density values tested are shown in Fig.…”

“…While most previous studies assumed the continental lithospheric mantle to be chemically uniform 8-10, 26, 27, 31-33 , recent studies have revealed that the CLM likely consists of a buoyant upper layer and a dense lower layer [11][12][13][14] . The overall net buoyancy of the CLM is notably larger than previously thought (up to 1% denser than the ambient asthenosphere), as required by matching the global topography, geoid, and gravity 13,14 . The two CLM layers are frequently separated by a mid-lithospheric discontinuity (MLD) 12,36 , which is widely observed beneath cratons [36][37][38] .…”