GEOTHERMAL MODELING OF THE LITHOSPHERE IN THE CENTRAL BALTIC SHIELD AND ITS SOUTHERN SLOPE

Lithospheric temperature and heat flow density (HFD) were studied in t he central Baltic (Fennoscandian) Shield and its subsurface continuati on to the south, along a transect trending from eastern Finland to sou thern Estonia, The transect represents an example of a low HFD (less t han or equal to 30 mW m(-2)) Archaean craton on a thick (150-190 km) l ithosphere surrounded by Early and Middle Proterozoic mobile belts on a thinner (110-150 km) lithosphere with slightly elevated HFD (35-55 m W m(-2)). Numerical 2-D conductive models were constructed in which pe ridotite solidus temperatures were assigned to those depths which corr espond to the seismically determined lithosphere/asthenosphere boundar y. This technique was found to reduce the effect of uncertainties in h eat production and thermal conductivity values on the simulation resul ts. Upper crustal heat production values for the Finnish terrain were taken from published geochemical analyses of outcropping rocks. For th e 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 d ata, and corresponding hear 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 litho sphere and Moho depth variations are only weakly reflected in the meas ured surface heat flow data, which are mainly controlled by heat sourc es in the upper crust. The simulated heat flow densities at 50 km dept h (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 Proterozo ic southwestern end of the transect, Simulated temperatures at 50 km d epth increase from northeast to southwest, ranging from 450-550 degree s C in eastern Finland to about 650 degrees C in Estonia, Sensitivity of the simulations to parameter changes was studied by varying the hea t production and thermal conductivity values, The extreme values for t he 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.