New Evidence for Impact from the Suvasvesi South Structure, Central East Finland

Abstract

The circular Suvasvesi South structure (diameter about 3.8 km) is located in Central East Finland (62°35.8′N, 28°13′E) and correlates with the Haapaselkä open lake area, the southern of the two Suvasvesi lakes. Suvasvesi S was first noticed in satellite images and might form a crater doublet with the proven Suvasvesi N impact structure. We have previously presented evidence, such as presence of fractured target rocks and shatter-cone boulders on the eastern shore of Haapaselkä, which suggest that the Suvasvesi South is also an impact structure. During the summer 2002 we carried out a field survey in the area of the Suvasvesi lakes, which led to additional discoveries of shatter cones in boulders. We also discovered impact melt boulders in gravel pits along the roadsides, about 5 km southeast of the structure. Microscopic studies of the thin sections of impact melt and of the shatter cone boulders reveal the presence of well developed and decorated PDFs in quartz grains, maskelynite, fluidal textures between impact melt mineral clasts and kink bands in micas. Consequently, the melt rock is considered to be of impact origin. It is unlikely that the boulders with shatter cones and impact melt blocks were derived from the northern structure, because material transported from it by ice drift would not have passed this area. The outcrops on the islets of the Suvasvesi South area are heavily fractured with subvertical NNW-SSE and ENE-WSW trending; however the fracturing may be related to the Svecofennian tectonic deformations occurring in this area. Also, the outcrops show shatter cone features with a maximum of 50 cm in size. We interpret the shatter cone features to be of impact origin because of their shape and because the orientation of their apices differ from the other deformation systems. However, thin section analysis from outcrop specimens has not shown impact evidence so far. The new findings suggest the presence of an eroded impact melt layer in the southern structure. Bathymetric and airborne magnetic data point to a distinct structure of smaller dimension than the northern one. Preliminary paleomagnetic measurements on the granitic host rocks of Suvasvesi South reveal two components, of which one (steep downwards) is probably either a hard viscous remanence of present age or a Svecofennian age feature. The other one is poorly defined, but has a southwest direction similar to that isolated for the northern structure and could be related to impact.