Palaeoclimate and structure: the most important factors controlling subsurface temperatures in crystalline rocks. A case history from Outokumpu, eastern Finland

Abstract

SUMMARY The present work contributes to the study of heat-transfer mechanisms in crystalline bedrock. We present evidence from a thoroughly investigated case history in Outokumpu, eastern Finland, in the Fennoscandian (or Baltic) Shield, which shows that the subsurface temperature field is controlled by the thermal conductivity structure and downward diffusion of palaeoclimatic ground-temperature variations. The subsurface temperature profiles from three continuously cored boreholes (790-1 100 m deep) were used for a detailed 2-D modelling of structural, hydrogeological and palaeoclimatic effects on the subsurface temperature field. The boreholes are situated in a subdued topography and they intersect Early Precambrian folded lithologies, such as mica gneisses, black schists, skarn rocks, quartzites and serpentinite-talc rocks. Finitedifference techniques were used in the numerical solution of the heat- and masstransport equations. The 2-D models of thermal conductivity and hydraulic permeability that were compiled were based on an extensive set of data relating to geological structure, in situ hydraulic permeability, groundwater composition and thermal conductivity of drill-core samples down to about 1 km depth. It was found that the thermal effect of topographically driven groundwater convection is very small, with Peclet numbers typically of the order of 10-4-10-5. The effect of anisotropy of thermal conductivity was found to be one order of magnitude smaller than the effects of heatflow refraction in the inclined rock layers which produce horizontal components of heat-flow density ranging from - 15 to 5 mW m-' (basal heat flow 35 mW m-'). Since the advective heat transfer could be neglected, we tried to find a palaeoclimatic groundtemperature history that would explain the measured data. First, an inversion algorithm based on heat conduction in laterally homogeneous media was used for the reconstruction of palaeoclimatic ground-temperature history, but it yielded spurious results because of the strong 2-D conductivity effects. The only means to reconstruct the past ground-temperature changes was forward modelling using a conductive transient 2-D model with time-dependent surface-temperature variations. The surface temperatures at different times (between 100000 yr BP and the present) and the basal heat-flow density were varied in order to reach a reasonable fit between the modelled and measured borehole temperatures. To reach this goal, the ground-temperature history must include the main climatic events during the last 100000 yr, such as the last glacial epoch and the Little Ice Age (from about 1300 AD to 1700 AD), followed by warmer temperatures in more recent times. Our results suggests that subsurface temperatures in conditions similar to those of the Outokumpu case can yield a wealth of palaeoclimatic information when an appropriate approach for modelling heat transfer in the bedrock is chosen.