THE POLAR PROFILE REVISITED - COMBINED P-WAVE AND S-WAVE INTERPRETATION

The POLAR Profile Project studied the crustal structure in a 440 km lo ng swath extending across the northern Baltic Shield in a southwest-no rtheasterly direction. The obtained seismic data comprise P-waves and very clear S-waves. The results of forward modelling using the raytrac ing technique together with synthetic seismograms for the S-waves nece ssitated 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 recognize d 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 sligh tly lower than that in the surrounding Archaean gneisses which also pr evail in the middle crust down to a depth of about 30 km. The lower cr ust (30-45 km) is marked by a pronounced velocity contrast of the P-wa ves 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, unu sually high velocities (8.6-8.7 km/s) are found in the upper mantle at depths of 46 to 50 km.