Potential fields and subsurface models of Suvasvesi North impact structure, Finland
Introduction
The long and stable lifetime of Precambrian shields is mirrored in the large number of meteorite impact structures preserved within them. In the Fennoscandian shield, the number of proven structures reaches 30 (Abels et al., 2002) and is result of an intensive search activity during the last few decades (e.g., Henkel and Pesonen, 1992; Pesonen, 1996). Often, geophysical techniques have been used to identify and investigate possible impact structures, e.g., the Suvasvesi doublet in central-east Finland (Fig. 1). The Suvasvesi North (N) structure (centred at 62°41′N and 28°11′E) was originally found by high-altitude (150 m) aeromagnetic mapping, which indicated a roundish and smooth magnetic relief with a diameter of about 3.5 km (Fig. 2). The high-altitude aeromagnetic anomaly was later confirmed by a low-altitude (40 m) airborne survey, which revealed a distinct circular negative anomaly at the centre of the reduced magnetic pattern. A shipborne magnetic survey by the Malmikaivos Oy (Finland) verified the existence of this central peak anomaly.
Microscopic studies of impact melt samples recovered from the drill core (bore specimens) (Pesonen, 1996; Pesonen et al., 1996), revealed planar deformation features (PDFs) in quartz grains and proved the impact origin of Suvasvesi N. The drilling was performed on the flank of the distinct bathymetric minimum (ca. 90 m) and on top of the small (0.8 km in diameter) negative magnetic anomaly that is produced by a reversely magnetised impact melt body.
The Suvasvesi lake is situated at the boundary between two terranes (Fig. 1): the Svecofennian terrane in the south (∼1.88 Ga) and the Archean (∼2.7 Ga) Karelian terrane in the north, both being parts of the Fennoscandian shield. These terranes are separated by the few kilometre wide NW–SE-trending Suvasvesi fault, which is part of the Raahe-Ladoga belt. The Suvasvesi fault appears to be truncated by the Suvasvesi N structure. The target rocks of Suvasvesi N are Svecofennian medium grained granite, migmatites and schists to the south, and Archaean granites north of the fault (Korsman et al., 1997). The most prominent topographic features of the region belong to the Suvasvesi fault zone, forming several NW striking ridges visible also in the magnetic and electromagnetic data (Fig. 2, Fig. 3). The northern part of Lake Suvasvesi is very deep (maximum depth of ∼90 m in Kukkarinselkä open lake) compared to typical lakes (usually <10 m) in that region. It appears as an almost circular structure but neither a topographic nor bathymetric rim, which could be remanent of the crater, has been found.
The present work describes potential field (gravity, magnetics and electromagnetics) data and an improved (see also Werner, 1999; Werner et al., 2000) three dimensional (3D) subsurface model of the Suvasvesi N structure. The aims were to find a model based on combined gravity and magnetics analysis, which is constrained by morphometric cratering rules and petrophysical properties of rocks, and to obtain an age estimate for the impact event using magnetic modelling and paleomagnetic dating techniques.
Section snippets
Methods
To investigate the Suvasvesi N structure, results of airborne magnetic and electromagnetic surveys together with gravimetric profiling were analysed. Sampling of the small, unoriented pieces from the drill core and several outcrops were carried out (for locations see Fig. 1).
The gravity survey is based on two perpendicular profiles (Fig. 4, and for the location Fig. 3), both about 9 km in length, measured on the frozen lake Suvasvesi in 100 m intervals in winter 1996 by the Geological Survey of
Gravity
The Bouguer gravity data show a minimum of ∼5.5 mGal on both profiles (Fig. 4). After subtracting the regional trend and a contribution to the anomaly produced by water, the maximum amplitude of the anomaly is reduced to ∼2.8 mGal. This residual anomaly Δg is considered to result from formations affected by the impact. The maximum gravity anomaly Δg plotted versus diameter has a general trend as shown by Pilkington and Grieve (1992). Both this trend and the diameter of the gravity anomaly
Discussion and conclusion
From the Suvasvesi lakes (the Suvasvesi N and the Suvasvesi) the northern one is a proven impact structure (Pesonen et al., 1996). Recently, new evidences for the impact origin of Suvasvesi South have been provided (Lehtinen et al., 2002). In case the both structures turn out to have an impact origin, there is a high possibility that they also represent a doublet. The importance of the new doublet in terrestrial impact cratering record would be remarkable, especially because of relatively small
Acknowledgements
We thank Mark Pilkington and Jean Pohl for their detailed and constructive reviews and the Geological Survey of Finland for the permission to use its data. S.C. Werner acknowledges the mobility grant of the ESF–IMPACT programme.
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