THE MOHO DEPTH DISTRIBUTION IN FENNOSCANDIA AND THE REGIONAL TECTONICEVOLUTION FROM ARCHEAN TO PERMIAN TIMES

From a variety of seismic refraction and reflection profiling results, a new detailed Moho depth map for the Fennoscandian part of the Balti c Shield has been compiled. A complementary crystalline crustal thickn ess map was compiled for Denmark, the Skagerrak Sea, offshore western Norway and adjacent areas where sedimentary strata reach maximum thick nesses of 10 km and even more. These maps are discussed in relation to tectonic evolution and crustal ages of Fennoscandia, seismic velociti es, Bouguer gravity and seismicity. Areas deviating from the proposed age-thickness relationships are the Kola Peninsula (Archean) and the O slo Rift (Permian). The average crustal thickness in the former area i s around 44 km, while the latter area is associated with a Moho elevat ion of 3-5 km. Maximum crustal thicknesses in excess of 50 km have bee n 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 stro ngly negative. Intracrustal P-velocity variations do not seem to refle ct geological age or specific tectonic features. Although the seismici ty is moderate in Fennoscandia, most of the earthquake activity and, i n particular, the largest earthquakes are confined to areas where the crustal thinning is most pronounced.