Investigating Layered Cratonic Lithosphere in the Fennoscandian Shield

Abstract

Cold, thick, and chemically depleted lithosphere with high seismic velocity and low attenuation characterizes continental cratons. However, discrepancies between xenolith‐based thermal constraints and those inferred from surface‐wave seismic models, together with the widespread observation of mid‐lithosphere discontinuities, challenge the traditional view of a simple cratonic lithospheric structure. Improving constraints on lithospheric layering is therefore essential for understanding the thermal and compositional structure of the cratonic upper mantle. In this study, we develop a three‐dimensional model of vertically polarized shear‐wave velocity (Vsv) in Fennoscandia through the joint inversion of Rayleigh wave phase velocities and S‐to‐P receiver functions, resolving both absolute velocities and strong vertical gradients. Compared to models derived from phase velocities alone, our results show that the highest velocities (>4.7 km/s) are located deeper, predominantly at ∼140–210 km depth. We compare the seismic model of the Fennoscandia craton interior with Vsv predictions based on depleted peridotite compositions along xenolith‐derived geotherms. After accounting for anelasticity and radial anisotropy, we find that the high velocities observed deeper than ∼150 km can be explained with a moderate diamond fraction (∼0.5 vol.%) combined with a colder geotherm than estimates based on standard xenolith thermometry, while remaining consistent with xenolith constraints. In the shallow lithosphere, slower‐than‐predicted shear‐wave velocities are best explained by the combined effects of radial anisotropy and the presence of carbonate and/or hydrous minerals introduced by metasomatism. The layered seismic structure revealed in this study provides new insights into the thermal and compositional architecture of cratonic lithosphere and its formation and modification.