Bayesian Joint Inversion of Sp Receiver Functions and Rayleigh Wave Phase Velocities for the Structure of the Cratonic Upper Mantle

The internal structure of the cratonic lithosphere provides insight into its formation and evolution. A wide range of studies have found evidence for shear wave velocity decreases in the shallow upper mantle beneath cratons, often termed mid-lithospheric discontinuities (MLDs). With S-to-P (Sp) receiver functions we have shown that MLDs occur intermittently in cratons globally and that they are more prevalent in mantle lithosphere that is younger than Paleoproterozoic. To better resolve the velocity reductions associated with MLDs, we use a Bayesian approach to jointly invert from Sp receiver functions, which are sensitive to velocity gradients, and Rayleigh wave phase velocities, which provide information on absolute velocities but with lower vertical resolution. In initial inversions we employ Rayleigh wave phase velocities from GDM52 (Ekstrom, 2011), at periods of 25-250s, and Sp receiver function stacks at periods of 2-100s at 35 stations in cratonic regions. In these inversions, the use of adaptive splines allows for smooth and parsimonious velocity profiles, and the parameter space is searched using a Monte Carlo Markov Chain approach. Preliminary results yield a wide range of velocity structures. In some cratonic regions, for example in the interior of the Canadian Shield, velocities remain high throughout the cratonic mantle lithosphere. In contrast, on the edge of the eastern block of the North China craton we see evidence of a thinned and altered lithospheric mantle where a velocity drop of 6.8% indicates the lithosphere-asthenosphere boundary at 90 km depth. We also observe negative velocity gradients in the upper mantle of regions with high-shear velocity upper mantles, such as the North American and Kalahari cratons. In these regions, at stations where clear negative MLD phases are observed in the Sp receiver function stacks, the joint inversions yield low velocity layers with shear velocity drops of at least 3% to 4% in the shallow upper mantle. The largest of these velocity reductions exceed the maximum values inferred to be possible from ancient metasomatism during Archean craton formation (Saha et al., 2021). Thus, contributions from more recent metasomatism or anisotropy are required to explain the full global range of MLD-related shear velocity reductions.