Seismic Evidence for Craton Formation by Underplating and Development of the MLD.

Inconsistencies between observations from long and short period seismic waves and geochemical data mean craton formation and evolution remains enigmatic. Specifically, internal layering and radial anisotropy are poorly constrained. Here, we show that these inconsistencies can be reconciled by inverting cratonic Rayleigh and Love surface wave dispersion curves for shear‐wave velocity and radial anisotropy using a flexible Bayesian scheme. This approach requires no explicit vertical smoothing and only adds anisotropy to layers where required by the data. We show that all cratonic lithospheres are comprised of a positively radially anisotropic upper layer, best explained by Archean underplating, and an isotropic layer beneath, indicative of two‐stage formation. Within the positively radially anisotropic upper layer, we find a variable amplitude low velocity zone within 9 of 12 cratons studied, that is well correlated with observed Mid‐Lithospheric Discontinuities (MLDs). The MLD is best explained by metasomatism after craton formation. Plain Language Summary: The ancient cores of the continents, or cratons, are a treasure‐trove of >2.5 billion years of Earth's history. However, scientists disagree on the processes that led to their formation, or whether they have evolved significantly through time. This is because the geological and geophysical methods used to investigate cratons often yield conflicting results. By capitalizing on an up‐to‐date global long‐wavelength seismic data set, we image the cores of 12 cratons using an advanced statistical method, Bayesian inference. The flexible method requires few choices to be made a priori, is driven by the quality of the data itself and measures uncertainties on results. By detecting velocity differences between horizontally and vertically vibrating seismic waves, we show that all cratons likely comprise an upper layer formed during the hot early Earth, by a process that strongly aligns the constituent minerals of the rocks in the horizontal plane. Below this ∼125 km thick upper layer, a lower layer (∼150 km thick) shows no clear alignment of minerals and so was likely formed by a different process, at a later time. Variable slow wavespeed zones exist within the upper layer that match previous results from short‐wavelength seismic data. Key Points: Bayesian surface wave inversion reconciles existing differences between seismic images of craton structure from long and short period dataCratons show a shallow low velocity zone (LVZ) within a layer of positive radial anisotropy and a high velocity isotropic layer beneathCratons are formed in two stages shown by anisotropic structure and are later modified producing a LVZ and mid lithosphere discontinuities [ABSTRACT FROM AUTHOR]