Heat flux and seismicity in the Fennoscandian Shield

The regional thermal regime in the Fennoscandian Shield is outlined, and th e consequent rheological structure is analysed from the Kola Orogen to the Sorgenfrei-Tornquist zone. Moho temperatures and the heat flux from the man tle are typical of cratonic areas. The deep thermal field shows a cold root in the north-eastern sector. Larger lateral variations of mantle heat flux and Moho temperatures occur in the southern area and at the edges of the s hield. The thickness of the thermal lithosphere is maximum (200-220 km), wh ere the mantle heat flux is minimum (about 15-20 mW m(-2)). Three lithosphe ric cross-sections illustrate the expected lateral variation in viscosity a nd failure mode throughout the lithosphere as a consequence of the differen t geothermal conditions and the rheological stratification. In the upper cr ust, the depth of the brittle-ductile transition varies on average from 30, in the north-east, to 15 km. in the south-west. In the subcrustal mantle. the models predict a mainly ductile behaviour. At a depth of 60km, the uppe r mantle viscosity is maximum (2.5 x 10(23) to 25.0 x 10(23) Pas) beneath t he Archean province and minimum (2.5 x 10(21) to 10.0 x 10(21) Pas) below t he Svecofennian and Sveconorwegian units. The comparison of the rheological calculations with the seismic activity shows a general agreement with the brittle-ductile transition depth expected in the Archaean and Proterozoic p rovinces. There is a difference both in number of earthquakes and distribut ion versus depth between areas of different age. The occurrence of larger m agnitude seismic events near the base of the seismogenic zone, in the regio n of inferred peak of shear resistance, suggests some causal relationship. The Proterozoic areas show a more regular decrease of shocks with depth and , compared to the Archean provinces, a tendency to shallowing. (C) 2001 Els evier Science B.V. All rights reserved.