Crustal structures of Eastern Fennoscandia

Abstract

The area of the Eastern Fennoscandian Shield is quite well covered by deep seismic sounding profiles, most of which were shot in the last ten years. In general the velocities of the seismic waves in the crust increase continuously with depth, but intense reflections also indicate the presence of distinct velocity boundaries. The velocities of the refracted waves in the various profiles resemble each other but the depths to the boundaries vary considerably. Low-velocity zones were found in the uppermost crust along most of the profiles. The Eastern Karelia profiles also exhibited large low-velocity zones in the lower part of the crust. High-velocity bodies found in the uppermost crust in Eastern Fennoscandia are thought to be associated with mafic intrusions and mafic metavolcanic rocks. The depth to the Moho shows considerable lateral variation, and deep depressions in this boundary are thought to be connected with large fracture zones. High P n-wave velocities of about 8.5 km/s are in many cases associated with these depressions.