CRUSTAL STRUCTURE OF THE FENNOSCANDIAN SHIELD - A TRAVELTIME INTERPRETATION OF THE LONG-RANGE FENNOLORA SEISMIC REFRACTION PROFILE

The 1979 Fennoscandian Long-Range Project (FENNOLORA) was aimed at the determination of the detailed structure in the earth's mantle down to a depth of about 400 km. Observation distances reached almost 2000 km within Scandinavia between shotpoints off the North Cape and the southern coast of Sweden. To achieve an unbiased regarding the upper mantle structure, a careful crustal survey was carried out along the entire profile at the same time. Beneath the Fennoscandian Shield, i.e. the central section of the profile, the crust is characterized by quite a smooth increase in P-wave velocity down to the Moho which lies at a depth of about 50 km in the southern half of the shield and at about 45 km further north. The mean crustal velocity is 6.6-6.7 km/s. At the base of the crust the velocity increases gradually from about 7 km/s to 8.0-8.4 km/s in a 5-10 km thick crust-mantle transition zone. Both in the south and in the north, the relatively homogeneous crust of the Baltic Shield borders on areas with a more differentiated velocity structure. First-order discontinuities at shallower depth characterize the crust-mantle boundary near the southeastern tip of Sweden (38 km) and under the Caledonides in the north (43 km).