Abstract
Abstract The POLAR Profile Project studied the crustal structure in a 440 km long swath extending across the northern Baltic Shield in a southwest-northeasterly direction. The obtained seismic data comprise P-waves and very clear S-waves. The results of forward modelling using the raytracing technique together with synthetic seismograms for the S-waves necessitated slight modification of a previously published velocity model obtained from P-waves only. The profile crosses the Lapland Granulite Belt with granulite facies rocks at the surface. The Lapland Granulite Belt and a part of the Karasjok-Kittila Greenstone Belt are recognized as high-velocity bodies in the upper crust, both for P- and S-waves. The V P V S -ratio (and Poisson-ratio, respectively) seems to be slightly lower than that in the surrounding Archaean gneisses which also prevail in the middle crust down to a depth of about 30 km. The lower crust (30–45 km) is marked by a pronounced velocity contrast of the P-waves in respect to the middle crust, but only a small velocity contrast for the S-waves is seen. In the southwestern part of the profile, unusually high velocities (8.6–8.7 km/s) are found in the upper mantle at depths of 46 to 50 km.
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