Seismic properties of rocks exposed in the POLAR profile region—constraints on the interpretation of the refraction data

Abstract

The POLAR profile region comprises the northern part of the Precambrian Baltic Shield. The region is subdivided into a number of tectonic units, which are made up of Archaean gneiss terrains, locally hosting Archaean and/or Palaeoproterozoic supracrustal sequences and belts. We have measured the seismic properties of representative rock samples recovered from surface exposures. The experimental data include the measurement of elastic wave velocities (VP, VS) and densities at confining pressures up to 600 MPa and temperatures up to 600°C (at 600 MPa), the determination of the pressure and temperature derivatives of VP and VS, and the determination of velocity anisotropy. The petrophysical 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-situ velocity data have been used to interpret lithologically the seismic models for VP, VS, and evaluated from the seismic refraction data. There is experimental evidence, that the upper crust (< 10 km depth) is dominated by quartzofeldspatic gneisses. The high-velocity body below the surface comprising the Lapland Granulite Belt and parts of the Kittila Greenstone Belt can be correlated with amphibolite and granulite lithologies. The velocity structure of the seismically homogeneous middle crust (10-30 km) is best explained by a uniform quartz-dioritic composition. The boundary between the middle and the lower crust (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 composition from acidic to more mafic and to increasing metamorphic grade, i.e. the transition from amphibolite facies to more mafic granulite facies lithologies.