Craton Formation by Underplating and Development of the MLD: Evidence from Bayesian Surface Wave Inversion

Craton formation and evolution remains enigmatic because observations from long and short period seismic waves and geochemical data are inconsistent. For example, both internal layering and radial anisotropy are poorly constrained. By inverting cratonic Rayleigh and Love surface wave dispersion curves for shear-wave velocity and radial anisotropy using a flexible Bayesian scheme, we show that these inconsistencies can be reconciled. Our methodology does not require any vertical smoothing and only includes anisotropic layers where necessary to fit the data. Results show all cratons possess a positively radially anisotropic upper lithospheric layer that is best explained by Archean underplating. An isotropic layer lies beneath, likely indicative of two-stage craton formation. We find a variable amplitude low velocity zone (LVZ) may exist within the upper anisotropic layer of up to 9 of 12 cratons studied. This LVZ is well correlated to observed Mid-Lithospheric Discontinuities (MLDs). Our results suggest the MLD is best explained by post formation modification within cratons.