Fabrics of Mantle Lithosphere of Fennoscandia Inferred from Seismic Anisotropy

Abstract:

Though crust in Archean cratons often displays a relatively simple architecture in comparison with younger orogens, where Moho topography and deep crustal structure are more complex, differences in structure of the mantle lithosphere are less prominent. The mantle lithosphere of Fennoscandia is built by domains with their own fossil inclined fabrics, which are similar to those we retrieved in younger continental provinces from investigations of seismic anisotropy. Passive seismic experiments SVEKALAPKO and LAPNET provided data for structural studies of the upper mantle beneath Fennoscandia. We evaluate the large-scale anisotropy in the upper mantle from (1) splitting of SKS waves (Vecsey et al., 2008), (2) directional terms of relative P-wave travel time residuals, (3) teleseismic tomography and (4) jointly interpreted body-wave anisotropic parameters (Plomerova et al., 2011).

The domains of mantle lithosphere are sharply bounded both in the Proterozoic and Archean provinces and can be modelled in 3D by peridotite aggregates with dipping lineation a or foliation (a,c). The domains represent lithosphere fragments retaining fossil olivine preferred orientation created before the micro-plates assembled. Wedge-like penetration of the Archean domain into the Proterozoic province in the south-central Finland, supported by alternating ages of mantle xenoliths, seems to continue towards the north, where a westward shift of a boundary between regions with positive and negative velocity perturbations in teleseismic P-wave tomography can indicate an inclination of the Baltic-Bothnia Megashear Zone. We search for a mechanism which could create the observed inclined fabrics within continental assemblages. Such mechanism should differ from simple cooling processes which would lead to a horizontal stratification of the lithosphere without creating domains exhibiting different fabrics.