[1] Regional seismic tomography provides valuable information on the structure of shields, thereby gaining insight to the formation and stabilization of old continents. Fennoscandia (known as the Baltic Shield for its exposed part) is a composite shield for which the last recorded tectonic event is the intrusion of the Rapakivi granitoids around 1.6 Ga. A seismic experiment carried out as part of the European project Svecofennian-Karelia-Lapland-Kola (SVEKALAPKO) was designed to study the upper mantle of the Finnish part of the Baltic Shield, especially the boundary between Archean and Proterozoic domains. We invert the fundamental mode Rayleigh waves to obtain a three-dimensional shear wave velocity model using a ray-based method accounting for the curvature of wave fronts. The experiment geometry allows an evaluation of lateral variations in velocities down to 150 km depth. The obtained model exhibits variations of up to ±3% in S wave velocities. As the thermal variations beneath Finland are very small, these lateral variations must be caused by different rock compositions. The lithospheres beneath the Archean and Proterozoic domains are not noticeably different in the S wave velocity maps. A classification of the velocity profiles with depth yields four main families and five intermediate regions that can be correlated with surface features. The comparison of these profiles with composition-based shear wave velocities implies both lateral and vertical variations of the mineralogy.

S U M M A R YThe 430 km long DSS Baltic profile in SE Finland runs through the Rapakivi intrusion, the Svecokarelian geosynclinal complex, including the Ladoga-Bothnian Bay zone, and the Archean basement complex. A brief description of field operations in 1982 is presented. Record sections of P-and S-waves were used for the interpretations. A starting model was constructed by inverse methods and improved by dynamic ray tracing modelling. Three blocks are indentified on the cross-section from SW to NE. These are the southern, associated with the Rapakivi Massif, depth to Moho (M) about 40 km; the central, Ladoga-Bothnian Bay zone, with M of approximately 55-60 km, and the northern, Archean crystalline basement, with M about 40-45 km. The average crustal velocity is 6.6-6.7 kms-'. The crustal structure along the Baltic profile is compared with the surrounding DSS profiles, Finnish Sveka and Soviet Kern-Tulos and Ladoga profiles. The eastern border of the Ladoga-Bothnian Bay zone was located and specific features of the crust and uppermost mantle along the profile area are discussed.

The Svecofennian‐Karelian‐Lapland‐Kola Transect (SVEKALAPKO) project is one of the five multidisciplinary key projects of Europrobe, a scientific program of the European Science Foundation (ESF) that studies the tectonic evolution of European continental lithosphere [Gee and Zeyen, 1996]. The SVEKALAPKO project [Hjelt and Daly, 1996] has adopted a multidisciplinary approach that uses geological, penological, and geophysical methods to unravel the evolution of the crust and lower lithosphere in three major crustal segments of the Fennoscandian Shield: the Proterozoic Svecofennian and Lapland‐Kola orogens and the intervening Archaean Karelia craton. Improved knowledge of the structure and evolution of the Fennoscandian Shield should lead to a better understanding of plate‐tectonic processes in the early history of the Earth.

No abstract