Lithospheric thermal structure in the Baltic shield

Abstract

Steady-state heat conduction modelling was carried out to calculate the crustal temperature field and thermal lithosphere thickness in the Baltic shield. The radiogenic sources at the surface were fixed depending on the age of the crust. Below the uppermost 10 kilometres, a relationship between heat generation and P-wave velocity was applied and seismic structure was used to define individual crustal blocks of specific thermal parameters. Small-scale surface heat-flow density anomalies are interpreted as lateral variations of heat generation within the upper part of the crust, whereas the large anomaly in the southern part of the shield is attributed to an anomalously high mantle heat-flow density. The results are shown on maps outlining mantle heat-flow density, Moho temperature and thermal lithosphere thickness. A relationship between subcrustal temperature and Moho depth was found, except for southern Sweden. The lithospheric thickness is found to exceed 200 km in the Bothnian Gulf-northern-central Finland and part of the Kola peninsula, where temperatures at the Moho are less than 500C; the temperature gradient at the top of the mantle has, on the average, a value of 7.5 mK m-' and the mantle heat-flow density varies from 19 to 25mWm-2. Towards the south, the lithospheric thickness decreases until it attains a value lower than 100km under southern Sweden. In this latter area, the Moho temperature and mantle heat-flow density are remarkably high, 700"-900 "C and 30-45 mW m-2, respectively, as well as the temperature gradient, which amounts to 10 mK m-'.