Heat flux and seismicity in the Fennoscandian Shield

Abstract

The regional thermal regime in the Fennoscandian Shield is outlined, and the consequent rheological structure is analysed from the Kola Orogen to the Sorgenfrei–Tornquist zone. Moho temperatures and the heat flux from the mantle 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 shield. The thickness of the thermal lithosphere is maximum (200–220 km), where the mantle heat flux is minimum (about 15–20 mW m −2). Three lithospheric cross-sections illustrate the expected lateral variation in viscosity and failure mode throughout the lithosphere as a consequence of the different geothermal conditions and the rheological stratification. In the upper crust, 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 60 km, the upper mantle viscosity is maximum (2.5×10 23 to 25.0×10 23 Pa s) beneath the Archean province and minimum (2.5×10 21 to 10.0×10 21 Pa s) below the 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 provinces. There is a difference both in number of earthquakes and distribution versus depth between areas of different age. The occurrence of larger magnitude seismic events near the base of the seismogenic zone, in the region 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.