TEMPERATURE AND HEAT-FLOW DENSITY IN A THICK CRATONIC LITHOSPHERE - THE SVEKA TRANSECT, CENTRAL FENNOSCANDIAN SHIELD

Lithospheric temperatures and heat flow densities (HFD) were simulated in the 650 km long SVEKA transect in the central Fennoscandian Shield . The area is characterized by anomalously thick crust (up to 55 km) a nd lithosphere (170 200 km). The investigated transect extends from th e Archaean granite-greenstone terrain in the NE to an Early-Middle Pro terozoic domain in the SW composed of metasediments, metavolcanics, gr anitoids and other igneous rocks. The significance of heat transfer by circulating fluids in the crust was investigated with numerical simul ations. Significant deviations from conductive conditions are possible given that hydraulic permeability and hydraulic gradient are sufficie ntly big. Measured values of in situ permeability and the low topograp hic variation in the transect area, however, do not support the existe nce of flow systems which would be thermally relevant in the crustal s cale. Therefore, heat transfer is considered mainly conductive in the crust. In the mantle, radiative heat transfer is assumed to be active in addition to conduction. Thermal conductive simulations which were b ased on the available information on geological, seismic, potential fi eld and HFD investigations of the area suggest the following results: 1. the seismic lithosphere/asthenosphere boundary at the depth of 170- 200 km is at the solidus of volatile-bearing peridotite (about 1100 +/ - 100 degrees C); 2. Surface HFD is to a large extent controlled by he at production in the upper crust, which is responsible for about 30-45 % of the surface HFD signal; 3. lithosphere thickness variations are n ot reflected in HFD variations on the studied transect, mainly because the L/A depth varies only about 20 km along the transect; 4. mantle H FD is low (about 12 +/- 5 mW m(-2)) along the transect and 5. temperat ure at 50 km level (approximately at Moho) increases from the Archaean domain (about 400 degrees C) to the Proterozoic (500 degrees C) end o f the transect. Accuracy of temperature estimation is affected by the applied conductivity and heat production values, and the extreme bound s of the estimated Moho temperatures may be either about 100 K lower o r 200 K higher than the values above. These limits correspond to condu ctivities varied by +/- 1 W m(-1) K-1 and heal production values by +/ - 50% around the normal values. (C) 1998 Elsevier Science Ltd. All rig hts reserved.