Rheological consequences of the lithospheric thermal structure in the Fennoscandian Shield

The surface heat-flow density of the Fennoscandian Shield, after removing the disturbances due to palaeoclimatic changes, shows a remarkable contrast from the Archaean terrains to the Late Proterozoic provinces which derive from tectonic reactivation and reheating of older materials. This involves lateral variations in the rheological behaviour of the lithosphere. On the basis of seismic structural data and assumptions about the petrological composition and flow parameters of steady-state dislocation creep, strength profiles and lateral viscosity variations have been deduced for several sites. In the northern and northeastern parts of the shield, where the Moho temperature and mantle heat-flow density are typically cratonic, the rheological thickness of the lithosphere, given by the depth at which the strength is reduced to 1 MPa, ranges from 120 to 140 km. In the southwestern shield, were enhanced Moho temperatures and mantle heat-flow density occur, the rheological thickness is reduced to 60-80 km. The depth of the brittle-ductile transition in the upper crust, above which most earthquakes occur, varies on average from 30 km in the northeast to 18 km in the southwest. The limiting temperature of the brittle uppermost layer on average is 365 ± 70°C. The essentially aseismic behaviour of the mantle agrees with the subcrustal predominating ductile deformations predicted by the models. The average lithospheric strength falls within the range 70-200 MPa, typical of the older stable areas. The viscosity of the upper mantle, at a reference depth level of 60 km, ranges from 1021 to 1024 Pa s and increases with the geological age, being maximum beneath the Archaean nucleus.