Seppo Elo

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 Kittila 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 Vainospaa 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 Sorvaranger Terrains and Varanger Peninsula) there are many separate depth values and clusters exist only at depths between 13 and 15 km.

Geophysical investigations of the Lake Lappajärvi impact structure, western Finland

Abstract A circular negative Bouguer anomaly approximately −10 mgal in amplitude and 17 km in diameter is associated with the Lappajarvi impact structure. The related mass deficit is −440 × 10 11 kg. There are irregularities in the anomaly pattern due to differentially faulted bedrock blocks and lateral density contrasts between bedrock, shocked bedrock, breccia, suevite, impact melt rock and overburden. The minimum amount of impact melt rock was interpreted, from the positive residual Bouguer anomaly between the centre and the rim in the northwestern part of the crater, as being 1.5–2.0 km 3 . There are a few small aeromagnetic anomalies (up to 200 nT at an altitude of 40 m) within the crater, which is mostly characterized by a lack of magnetized material. Strong magnetic anomalies (a few thousand nT) which are abundant in the surroundings disappear in the crater area. The aero-electromagnetic (AEM) data show strong anomalies with inphase /quadrature ratios close to 1, indicating moderately conducting material within the crater. An electromagnetic wide-band (SAMPO EM) profile was measured across the annular depression between the centre and the rim of the crater. Three layers were interpreted. The thickness of the uppermost layer, mostly conductive overburden, is about 100–200 m and the resistivity 60–170 Ωm. In the annular depression, beneath the uppermost layer, there is a 50–250 m thick layer of suevites and breccias with resistivity of about 17–50 Ωm. The resistive lowest layer follows the base of breccia and suevite. Some separate soundings were carried out near the centre of the crater. According to one of these soundings, there is a conductive layer of suevite and breccia, with a resistivity of 30–40 Ωm, beginning at depth of 150 m beneath the exposed impact melt rock. The results of geophysical interpretations, such as the annular depression between the centre and the rim of the crater, the small thickness/diameter ratio of the geophysically anomalous part of the crater, and the calculated amount of impact melt rock, indicate that the Lappajarvi structure belongs to the category of complex craters. Rhomboidal geophysical anomaly patterns (in particular AEM anomalies), parallel to old bedrock structures suggest that the collapse of the transient crater and/or subsequent crater modification were controlled by NW-SE and NE-SW trending pre-existing fracture and fault zones.

Potential fields and subsurface models of Suvasvesi North impact structure, Finland

Abstract The Suvasvesi North (N) impact structure, central-east Finland, has a diameter of about 3.5 km. It is covered by Lake Suvasvesi and lacks a topographic or bathymetric rim. The structure has a pronounced geophysical response, i.e. gravity, magnetics and electromagnetics. It is characterised by a ∼3.5 km wide negative Bouguer gravity anomaly of 5.5 mGal. The impact-related anomaly is about 2.8 mGal whereas regional geological features, water, and post-impact Quaternary sediments produce the difference. The physico-chemical processes due to the impact event have reduced and smoothed the airborne magnetic anomalies at the crater area. A small and intensive circular negative anomaly of ∼160 nT in amplitude and ∼0.8 km in diameter has been drilled and proven to be caused by impact melt. This central magnetic anomaly nearly coincides with the distinct bathymetric depth maximum (ca. 90 m). The Suvasvesi lake and Suvasvesi N structure are characterised also by electromagnetic anomalies. This article presents the potential fields, subsurface models, and an age estimate of the Suvasvesi N impact structure.