Lunar and Planetary Science XXXIII (2002) 1188.pdf

THE SOUTH STRUCTURE, CENTRAL FINLAND: NEW EVIDENCES FOR IMPACT. M. Lehtinen1, L.J. Pesonen2, H. Stehlik3 and M. Kuulusa2, 1Finnish Museum of Natural History, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki, Finland, e-mail: [email protected]; 2Department of Geophysics, Univer- sity of Helsinki, P.O. Box 64, FIN-00014 Helsinki, Finland, e-mail: [email protected], [email protected]; 3Hagedornweg 2 / 12, A-1220 Wien, Austria, e-mail: [email protected]

Introduction: The lake Suvasvesi North and the Lusikkaniemi revealed very weakly developed striated lake Suvasvesi South structures in central Finland form features that could represent “miniature” shatter cones a pair of circular lakes, which appear distinct in geo- (see also [6]). However, these features can also be morphologic, satellite imagery and airborne geophysi- weakly developed slickensides or glacial striations. cal maps. Both lakes contain open lake areas (Kuk- karinselkä in the north and Haapaselkä in the south) with lack of islands in their central parts. Geologically the lakes occur in the Lake Ladoga–Bothnian Bay tec- tonic belt, which borders the Proterozoic (in the south) and the Archean (in the north) terranes in the Fenno- scandian Shield. Previous geological, geophysical and shock metamorphic data have proven that the Suvas- vesi N is an with a probable age, e.g., [1], [2] and [3]. This is consistent with geo- physical and topographic evidences, which show that the Suvasvesi N structure clearly truncates the old Su- vasvesi fracture zone. However, the origin of its south- ern companion has puzzled the researchers for years [1] and [4]. It has often been listed as a possible impact structure, e.g., [1] and [5] but so far there has not been sufficient proofs for the impact origin. Here we present new evidences, which strongly support the Figure 1. Examples of shatter cones in a boulder from Lu- impact origin also for the lake Suvasvesi South struc- sikkaniemi from the east shore of the Haapaselkä open lake ture. If this interpretation is correct the two Suvasvesi of the Suvasvesi South structure. The horizontal edge of the lakes represent a new doublet in the terrestrial impact specimen is 15 cm. Photo M. Lehtinen. cratering record. New impact evidences: The new evidences for the impact origin for Suvasvesi South are outputs of the Intense fracturing. Slightly off from the center of the three recent fields trips to the Suvasvesi lakes, e.g., [4]. Haapaselkä towards west and towards south there are These field trips were carried out in the summer and islands and islets with outcrops and boulders, which we fall of 2001, respectively. visited in our second field trip in September 2001. No Shatter cones. Well-developed shatter cones (Fig. 1) shock features and no shatter cones were observed in were found in the southeastern shoreline of the ca. 3 these outcrops or in boulders. However, the outcrops of km wide Haapaselkä open lake, which forms the cen- one of these islands (islet Takunluoto) were unusually tral part of the lake Suvasvesi South structure (Fig. 2). strongly fractured which could be a signature of im- They were particularly numerous in the Lusikkaniemi pact. Petrographic studies of thin sections of one mica area in the southeastern shore behind the islands of gneiss sample from Takunluoto revealed numerous Suolasaaret and Tattarisaari. It is noteworthy that shat- kink bands in biotite and muscovite. However, no ter cones were not found systematically along the PDF´s were found. shores but rather in restricted areas. Petrologically the Geophysics. The Suvasvesi South, like the Suvasvesi shatter cones occur in medium-grained mica gneiss and North, is exceptional by its geophysical appearance. gneissose granodiorite, which are common Proterozoic Particularly striking is the low altitude airborne mag- target rocks in the area. The lengths of the cones vary netic map (Fig. 2), which shows that the Haapaselkä from a few centimeters to half of a meter. The shatter open lake of the Suvasvesi South is magnetically ex- cones were always very fragile and disrupted easily by ceptionally flat. It does not have a central anomaly as hand. All the ca. twenty shatter cones so far discovered does the Suvasvesi North structure in the center of the were in boulders. A preliminary investigation of the Kukkarinselkä, which anomaly turns out to be caused exposed mica gneiss and granodiorite outcrops south of by impact melt layer [2]. The magnetic weak relief

Lunar and Planetary Science XXXIII (2002) 1188.pdf

SUVASVESI SOUTH STUCTURE: M. Lehtinen et al.

associated with both Suvasvesi lakes can be due to Acknowledgements: We thank Fabio Donadini, Jüri non-magnetic and relatively thick layer of water, gla- Plado and Stephanie Werner for help and discussions cial overburden and post-impact sediments [6]. during the field work.

References: [1] Pesonen L.J. (1996) Earth, Moon, Planets, 72, 377-393. [2] Pesonen L.J et al. (1996) LPS XXVII, 1021-1022. [3] Werner S. et al. (2001a) Geo Abstracts, XXVI EGS General Assembly, Nice, France (CD-ROM). [4] Werner S. et al. (2001b) Meteoritics & Planet. Sci. Suppl. 36, A223. [5] Abels A. et al. (2002) In: Plado J. and Pesonen L.J. (eds.), Lecture Notes in Earth Sciences (in print). [6] Pesonen L.J. et al. (1999) GSA Spec. Pap. 339, 131-147. [7] Bottke M. and Me- losh J. (1996) Icarus, 124, 372–291.

Figure 2. Left: Locations of the Suvasvesi N and S structures in central Finland. Right: Airborne total magnetic field map of the structures.

Alternatively, it can be due to subdued susceptibility and remanence in the rocks caused by impact proc- esses. There are several examples of impact structures (e.g., [6]), which show anomalous weak magnetic char- acteristics due to impact produced alterations of the rocks in impact craters. Paleomagnetic and petrophysi- cal investigations of the fractured target rocks from the shoreline and islands of the Suvasvesi South are now underway to see whether they provide any clues for the origin of the weak magnetic signature associated with this structure. Discussion: The appearance of the Suvasvesi lakes in satellite images resembles those of the Clearwater lakes in Canada (Clearwater West and Clearwater East) although the latter ones are distinctly larger than the Suvasvesi lakes. The Clearwater lakes form a doublet impact structure of ca. 290 Ma old. If it turns out that both of the Suvasvesi lakes have an impact origin, the possibility that they also represent a doublet is en- hanced. To be so, we have to date the southern struc- ture to show that it is contemporaneous with the age of the northern counterpart, which is probably of Permian age. The scientific importance of the new doublet in terrestrial impact cratering record is remarkable since both structures have relatively small diameter (less than 5 km) even though the erosion is taken into account [7].