Interpretations of the Gravity Anomaly Map of Finland

An introduction to the Bouguer anomaly map of Finland is presented. Anomaly sources from surface to upper mantle are discussed both qualitatively and quantitatively. Related density variations are reviewed. The article is based on gravity data from the Finnish Geodetic Institute supplemented by petrophysical and gravity data from the Geological Survey of Finland. The Finnish Geodetic Institute has established a national network with an average station interval of 5 km. In the regional gravity measurements of the Geological Survey of Finland average station intervals have varied from 0.4 to 1.0 km. A number of details have been ascertained by gravity profiles with station spacings from 5 to 40 m. The overall range of Bouguer anomaly in Finland is 120 mGal. The distribution of rock densities within the exposed crust is not unimodal. The main component is the granodioritic one, 2685 2720 kg/m, consisting of mainly plutonic rocks as in Central Finland Granitoid area, or metamorphosed rocks in schist and migmatite belts. The granitic component, 2600 2660 kg/m, is common. The high density mode from 2740 to 2920 kg/m comprises high-grade metamorphic, mafic intrusive and volcanic rocks and has the largest standard deviation of the components. Distinct Bouguer anomaly maxima in Finland are caused by greenstones and other mafic volcanic rocks, ultramafic and mafic intrusions, diabase dykes and sills, carbonatite intrusions, granulite belts and upthrusted blocks from middle crust in general, and schist and migmatite belts when surrounded by a gneissgranite basement or granites. Distinct Bouguer anomaly minima are caused by granite intrusions, rapakivi granites in particular and potassium rich granitoids in general, thick layers of quartzites in metamorphosed schist belts, granitegneiss basement domes or blocks in metamorphosed volcanosedimentary surroundings, preserved basins of Mesoproterozoic and younger sedimentary rocks, meteorite craters, strongly weathered bedrock and occasionally by thick overburden. Mass anomalies due to crustal thickness variations are more or less compensated for by mass distributions within the crust. The gravity minimum associated with the land uplift has its source in the lithosphere upper mantle transition zone.