Interpretation of gravity anomalies along the POLAR Profile in the northern Baltic Shield

Abstract

The POLAR Profile in the northern Baltic Shield was analyzed by two- and three-dimensional quantitative gravity modelling, interpretation of Fourier amplitude spectra, qualitative interpretation using Bouguer anomalies and second vertical derivative and horizontal gradient maps, and reference to petrophysical data.Densities of outcropping crustal units show several distinct modes ranging from 2600 to 2990 kg/m3. In relation to the mean background density of 2690 kg/m3 used in the gravity modelling, the granitic crustal units (2600 kg/m3) produce well-defined gravity lows of up to 20 mGals, and the granulites (2750 kg/m3) and greenstones (2940 kg/m3) cause the most pronounced gravity highs of 30 and 45 mGal. The estimated thickness of the Kittilä Greenstone Belt is about 6 km. In the cross section along the POLAR Profile the base of the Lapland Granulite Belt dips to the northeast and reaches a maximum thickness of about 16 km. The thickness of the Vainospää granite body is approximately 6 km.Evidence of buried anomaly sources also exists. Our interpretation predicts units of increased density in the basement under the Varanger Peninsula sedimentary rocks. The general pattern of a gravity high near the Barents Sea coast coupled with a gravity low seawards implies a thick layer of sedimentary rocks under the sea which may be compensated by crustal thinning. According to the seismic refraction interpretation there is a depression in the Moho under the southwestern edge of the Lapland Granulite Belt explaining, together with other features of the seismic model, the regional background on which the gravity anomaly of the granulite belt is superposed.According to the Fourier analysis of gravity anomalies there are two dominant depth clusters of density discontinuities in the southwestern (Karelian Province) and middle parts (Lapland Granulite Belt) of the POLAR profile. The shallower discontinuity varies between 6 and 10 km, and the deeper one rises from about 20 km in the southwest to about 10 km in the middle area. In the northwestern part (Inari and Sørvaranger Terrains and Varanger Peninsula) there are many separate depth values and clusters exist only at depths between 13 and 15 km.