Ecology and archaeology

Abstract

A two-dimensional numerical solution of the heat conduction equation was used to calculate the crustal temperature field along two profiles of the Northern Segment of the European Geotraverse: the FENNOLORA profile (Baltic Shield) and profile 2 of the EUGENO-S Project (Danish Transition Zone). The solution of the 2-D steady-state equation of heat conduction was solved with the integrated finite-difference method.The crustal structure was discretized by a rectangular grid. Seismic refraction results were used to calculate the corresponding distribution of heat generation. We applied empirical equations linking seismic velocity with heat generation to convert the characteristic seismic velocity into heat production.In the upper crust, the temperature field shows no significant lateral variation. The lower crust, on the other hand, is characterized by lateral and vertical variations of the temperature field mainly at tectonic transition zones (i.e. Precambrian to Phanerozoic crustal domains). In the Baltic Shield, temperatures tend to decrease towards the north with increasing age of the shield. As expected, relatively low Moho temperatures were obtained beneath the Baltic Shield (300–500°C), whereas a maximum of 900 to 1000°C at the Moho was found to correspond to high surface heat flow in southern Sweden. In the geologically complex Danish Transition Zone, the calculated Moho temperatures reached values between 550° C in the Shield region and 650°C under the Danish Basin. The variation of mantle heat flow reflects the pattern of surface heat flow. In the Fennoscandian part of the shield, the Moho heat flow is 5–40 mW/m2, in the Danish part 25–30 mW/m2, and below the Danish Basin up to 45 mW/m2.With the present data set the model shows a remarkable coincidence between high Q0, the temperature field, and areas of major tectonic change.Sensitivity analysis shows that the surface heat flow and the heat generation in the upper crust are the most critical thermal parameters. Due to their relatively limited range of variation the heat production in the lower crust, the thermal conductivity and corrections applied to the seismic velocities play only a secondary role on the two-dimensional temperature field. The differences between 1-D and 2-D temperature models are most pronounced in the lower crust, where significant lateral temperature variations are seen.