Abstract
Regional seismic tomography provides valuable information on the structure
of shields, thereby gaining insight to the formation and stabilization
of old continents. Fennoscandia (known as the Baltic Shield for its
exposed part) is a composite shield for which the last recorded tectonic
event is the intrusion of the Rapakivi granitoids around 1.6 Ga.
A seismic experiment carried out as part of the European project
Svecofennian-Karelia-Lapland-Kola (SVEKALAPKO) was designed to study
the upper mantle of the Finnish part of the Baltic Shield, especially
the boundary between Archean and Proterozoic domains. We invert the
fundamental mode Rayleigh waves to obtain a three-dimensional shear
wave velocity model using a ray-based method accounting for the curvature
of wave fronts. The experiment geometry allows an evaluation of lateral
variations in velocities down to 150 km depth. The obtained model
exhibits variations of up to +-3\\ in S wave velocities. As the thermal
variations beneath Finland are very small, these lateral variations
must be caused by different rock compositions. The lithospheres beneath
the Archean and Proterozoic domains are not noticeably different
in the S wave velocity maps. A classification of the velocity profiles
with depth yields four main families and five intermediate regions
that can be correlated with surface features. The comparison of these
profiles with composition-based shear wave velocities implies both
lateral and vertical variations of the mineralogy.
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