Seismic anisotropy — A key to resolve fabrics of mantle lithosphere of Fennoscandia

Abstract

By merging different regional sets of body-wave data, we compile a relief of the lithosphere–asthenosphere boundary in one of the oldest continental regions on the planet: the Fennoscandian Shield. A maximum lithospheric thickness of 200–220 km is observed in the central part of the Shield and a significant thinning at its margins, particularly in the southwest at the Trans-European Suture Zone. Fabric of the mantle lithosphere is modelled from independent sets of anisotropic parameters, relative P-residual spheres and shear-wave splitting. The mantle lithosphere of Fennoscandia consists of several domains consistent olivine fabrics of dipping foliations, in 3D anisotropic models of the Proterozoic lithosphere, or, predominantly dipping lineations, in models of the Archean lithosphere. The strength of seismic anisotropy agrees with findings from mantle xenoliths. We interpret the anisotropic domains as fragments of mantle lithosphere retaining an old fossil olivine fabric, which was created before these micro-continents assembled. We envisage an early form of plate tectonic processes, such as successive subduction episodes, stacking of oceanic plates, and accretion of magmatic arcs. Such processes could create the generally observed fossil dipping fabrics of the thick Precambrian mantle lithosphere, starting as early as the Archean.