The Suvasvesi South Structure, Central Finland: Further Evidences of Impact

Introduction: The Suvasvesi South structure is located in Central East Finland (62°41’N, 28°11’E) and correlates with the Haapaselka open lake, the southern of the two circular Suvasvesi lakes. The target rock consists mainly of Proterozoic mica schists and migmatites in its easternmost part and granitoids in the western part. Suvasvesi S was first perceived in satellite images [1], and might form a crater doublet with the Suvasvesi N impact structure [2]. We have previously presented evidences, such as presence of fractured target rocks and shatter-cone boulders on the eastern shore of Haapaselka, which suggest that the Suvasvesi S is also an impact structure [2]. During the summer 2002 we carried out a new field survey in the area. The new bathymetric and geophysical data, coupled with thin section studies of the discovered melt boulders confirm that the Suvasvesi S is a new impact structure in Finland. Target and Impact Rocks: The target rock of the Suvasvesi South crater consists mainly of Proterozoic mica schists and migmatites in its easternmost part and granitoids in its western part.The morphology is strongly effected by Quaternary ice movements: the outcrops are smoothed, rounded and show at the surface distinct glacial striations. New Field Discoveries: All the investigated outcrops show mainly closed fractures. The two dominant fracture families are subvertical, N-S and E-W oriented. In particular, Takunluoto islet, located near the centre of the Suvasvesi South structure, consists mainly of heavily fractured granitoids. Shatter cone-like features were observed in outcrops at three different places. Their orientation is clearly different from the tectonic deformation trends. However, decimeter long striated structures may be the result of glacial erosion, even though the linear direction of both structures points toward the centre of the crater. Still, it is unsure if those are the result of glacial erosion or impact. At Takunluoto islet, possible in situ shatter cones of 50 cm length were observed. Also in this case, however, the cones are weakly developed and may be slickenslide structures originated by the heavy fracturing. Several boulders proving impact evidence were collected in the easternmost part of the Suvasvesi S structure. These shatter cones were found in mica schist and granitoid boulders along the NW coast of Haapasaari island and at Lusikkaniemi bay. Further cutting in the laboratory revealed that such boulders might consist of breccia containing granitoid clasts of few cm in size. In such a case the cones developed on the granitoid clasts. Finding of such boulders is restricted to particular areas: the Lusikkaniemi and the Haapasaari region, about one km away from each other. We collected several impact melt boulders along in gravel pits along the roadsides at Mannamaki hill, about 5 km southeast of the structure (Fig. 1). The rock consists of a fine-grained black matrix with fluidal texture, alternated with cm-sized clasts of target rock. Both the impact lithologies showed planar deformation features (PDFs) in quartz grains and a breccia structure. PDFs can be distinguished as two different sets and are mainly decorated, meaning that syngenetic impact fluid inclusions were present. Breccia structure is distinguishable because of the fine-grained matrix surrounding the bigger quartz or felspar grains. Instead, the granite sample collected in situ did not show any impact evidence. It should be noticed that the thin sections were prepared also for Takunluoto granitic sample, which represents the strongly fractured outcrop on the islet near to the centre of impact. Clasts are mainly melted, vesiculated or crushed.