Proterozoic–Archean boundary in the mantle lithosphere of eastern Fennoscandia as seen by seismic anisotropy

Abstract

Abstract Lateral variations of seismic anisotropy investigated by body waves allow us to detect the Archean–Proterozoic boundary in the upper mantle beneath the south-eastern Fennoscandia, though isotropic P-velocity perturbations in teleseismic P tomography or shear-velocity variations retrieved by inversion of surface waves by other authors do not noticeably differ in the Proterozoic and Archean mantle beneath the SVEKALAPKO array. The boundary seems to be inclined to the SW, in general, and very complex, forming a broad transition zone. This zone appears in the P residuals, which accumulate the velocity deviations along the ray path, as almost isotropic structure due to superposition of pieces of the mantle lithosphere with differently oriented anisotropy. The shear-wave splitting is consistent for groups of stations within the Archean and Proterozoic domains, and detects anisotropy even in the central transitional domain, which may reflect anisotropy of the thickest lithosphere wedge. In general, variations of the splitting parameters indicate a very complicated structure, which cannot be approximated by a single layer with horizontal symmetry axis or a simple contact of two mantle lithosphere blocks. We propose three potential candidates for a mantle lithosphere model around the Proterozoic–Archean contact.