The Moho depth distribution in Fennoscandia and the regional tectonic evolution from Archean to Permian times

Abstract

From a variety of seismic refraction and reflection profiling results, a new detailed Moho depth map for the Fennoscandian part of the Baltic Shield has been compiled. A complementary crystalline crustal thickness map was compiled for Denmark, the Skagerrak Sea, offshore western Norway and adjacent areas where sedimentary strata reach maximum thicknesses of 10 km and even more. These maps are discussed in relation to tectonic evolution and crustal ages of Fennoscandia, seismic velocities, Bouguer gravity and seismicity. Areas deviating from the proposed age-thickness relationships are the Kola Peninsula (Archean) and the Oslo Rift (Permian). The average crustal thickness in the former area is around 44 km, while the latter area is associated with a Moho elevation of 3–5 km. Maximum crustal thicknesses in excess of 50 km have been found in southwestern and Central Finland and correlate with a crust of Proterozoic age. The Caledonides of western Norway do not exhibit, as elsewhere, a crustal root. Sub-Moho P n and S n seismic velocities are relatively small in that area and Bouguer gravity values are strongly negative. Intracrustal P-velocity variations do not seem to reflect geological age or specific tectonic features. Although the seismicity is moderate in Fennoscandia, most of the earthquake activity and, in particular, the largest earthquakes are confined to areas where the crustal thinning is most pronounced.