Crustal Structure of the Baltic Shield Along the Pechenga–Kostomuksha–Lovisa Geotraverse

Abstract

Results of geologic and geophysical modeling are presented, based on detailed seismic studies along two profiles—Pechenga-Kostomuksha and Lieksa-Lovisa. Density, geothermal, magnetic, and geoelectric models were obtained from the interpretations of various geophysical fields and correlated with the reference seismic sections. All the models were combined in order to compile a geologic-geophysical crustal section. The crustal thickness along the Pechenga-Kostomuksha-Lovisa geotraverse varies from 38 to 65 km. Two anomalous structures have been observed that are referred to as the Belomorian-Karelian and Ladoga-Bothnian zones. These zones are characterized by enhanced values of magnetic fields, presence of seismic foci and wave attenuation, and variation of the depth and magnitude of modern crustal movements. These zones are distinguished by the discontinuity M reconstruction, an increase in transitional layer thickness (to 25 km) at the base of the crust, and an increase in depth down to the discontinuity M (50 to 65 km). On average, the crust is thinner (40 km) in the ancient part of the shield than in the younger Svecofennian province (45 km). The velocity differences also are important: for example, the crust of the ancient shield is characterized by lower velocities and the transitional high-velocity layer is absent or thinner. The Karelian granite-greenstone area (a fragment of the Archean craton) has the most simple and balanced deep structure. Within the Karelian area, the layers are nearly horizontal and their thickness is rather constant. The northeastern part of a fragment of the Murmansk block has similar crustal characteristics within the Kola area, where it has undergone Early Proterozoic deformation. Geological and geophysical data for the Pechenga-Varzuga zone suggests that there was intracontinental rifting and a subsequent construction regime during the Svecofennian orogeny that involved a considerable part of the shield. The deep-crustal structure is more complicated to the south. An increase in volume of material with the properties of granulites and basic rocks is observed in the upper crust. The rocks form an inclined alternation of high-density and high-velocity plates and lenses. The packet of tectonic clustering of supracrustal rocks is most conspicuous in the Lapland-Kolvitsa granulite belt. The packet thickness does not exceed 13 km.