SEISMIC PROPERTIES OF ROCKS EXPOSED IN THE POLAR PROFILE REGION - CONSTRAINTS ON THE INTERPRETATION OF THE REFRACTION DATA

The POLAR profile region comprises the northern part of the Precambria n Baltic Shield. The region is subdivided into a number of tectonic un its, which are made up of Archaean gneiss terrains, locally hosting Ar chaean and/or Palaeoproterozoic supracrustal sequences and belts. We h ave measured the seismic properties of representative rock samples rec overed from surface exposures. The experimental data include the measu rement of elastic wave velocities ( V-P, V-S) and densities at confini ng pressures up to 600 MPa and temperatures up to 600 degrees C (at 60 0 MPa), the determination of the pressure and temperature derivatives of V-P and V-S, and the determination of velocity anisotropy. The petr ophysical data are compared with the chemical and mineral composition of the rocks. Using a regional geotherm, velocity-depth profiles have been calculated for the different lithologies, and the calculated in-s itu velocity data have been used to interpret lithologically the seism ic models for V-P, V-S, and V-P/V-S evaluated from the seismic refract ion data. There is experimental evidence, that the upper crust (<10 km depth) is dominated by quartzo-feldspatic gneisses. The high-velocity body below the surface comprising the Lapland Granulite Belt and part s of the Kittila Greenstone Belt can be correlated with amphibolite an d granulite lithologies. The velocity structure of the seismically hom ogeneous middle crust (10-30 km) is best explained by a uniform quartz -dioritic composition. The boundary between the middle and the lower c rust (at about 30 km) which is marked by a pronounced increase of the P-wave velocity, is due to a change in the overall chemical compositio n from acidic to more mafic and to increasing metamorphic grade, i.e. the transition from amphibolite facies to more mafic granulite facies lithologies.