Geothermal modelling of the lithosphere in the central Baltic Shield and its southern slope

Abstract

Lithospheric temperature and heat flow density (HFD) were studied in the central Baltic (Fennoscandian) Shield and its subsurface continuation to the south, along a transect trending from eastern Finland to southern Estonia. The transect represents an example of a low HFD (≤ 30 mW m −2) Archaean craton on a thick (150–190 km) lithosphere surrounded by Early and Middle Proterozoic mobile belts on a thinner (110–150 km) lithosphere with slightly elevated HFD (35–55 mW m −2). Numerical 2-D conductive models were constructed in which peridotite solidus temperatures were assigned to those depths which correspond to the seismically determined lithosphere/asthenosphere boundary. This technique was found to reduce the effect of uncertainties in heat production and thermal conductivity values on the simulation results. Upper crustal heat production values for the Finnish terrain were taken from published geochemical analyses of outcropping rocks. For the Estonian terrain new heat production values were measured from core samples representing nineteen deep boreholes. Middle and lower crustal lithologies were estimated with the aid of the deep seismic V P V S data, and corresponding heat production values were adapted from global xenolith averages and from data for granulites cropping out in other Precambrian areas. The results of the modelling suggest that the lithosphere and Moho depth variations are only weakly reflected in the measured surface heat flow data, which are mainly controlled by heat sources in the upper crust. The simulated heat flow densities at 50 km depth (approximately at the Moho) are relatively low and range from 12 mW m −2 at the Archaean northeastern end to 19 mW m −2 on the Proterozoic southwestern end of the transect. Simulated temperatures at 50 km depth increase from northeast to southwest, ranging from 450–550°C in eastern Finland to about 650°C in Estonia. Sensitivity of the simulations to parameter changes was studied by varying the heat production and thermal conductivity values. The extreme values for the Moho temperature estimates thus obtained may be about 50 K lower or 100 K higher than the values above. The corresponding sensitivity of the Moho HFD is about ±6 mW m −2 and of the surface HFD ±5–20 mW m −2, respectively.