Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.276p&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 276
Mathematics
Logic
Scientific paper
A compilation of circular topographic, morphological, or geophysical structures in Fennoscandia and adjacent areas reveals 62 craterform structures of which 15 (class A or a) appear to be of extraterrestrial origin due to meteorite impact (Henkel and Pesonen, 1992) The majority of the structures are probable (class B, 9) and possible (class C, 34) impact craters for which there is not yet sufficient proof for impact origin. Four of the proven impact craters (Lappajarvi, ~77 Ma old; Dellen, ~90 Ma old; Mien, ~120 Ma old; and Janisjarvi, ~700 Ma old) contain large volumes of impact melt and many other features of intense shock metamorphism. The age of the recognized impact craters varies from prehistoric (3500 B.C, Kaali) to Late Precambrian (~1210 Ma, Bjorko). The histogram of the ages (although the number of proven impact craters is still very small) shows two possible peaks: one group consisting of impact craters less than 150 Ma old and the second one with ages between 350 and 600 Ma. There is so far a deficiency of impact craters in Fennoscandia with ages between 200 and 350 Ma. The majority of the proven impact craters have diameter (rim to rim) between 5 and 20 km; the largest meteorite impact crater in Fennoscandia, the Siljan (age ~360 Ma), has a diameter of 55 km. The impact cratering rate for Fennoscandia, in the region where craters occur, is 2.4x10^-14 km^-2a^-1 and includes twelve proven impact craters with diameters from 3 to 55 km. This amounts to ~2 events per every 100 Ma during the last 700 Ma. There is increasing evidence that some (3, class E) of the large circular geological, morphological, or geophysical features (the Uppland, the Nunjes, and the Marras structures) represent deeply eroded scars of Early Proterozoic impact craters, but impact- generated rocks or fall-out ejecta layers have not yet been identified with these structures. No craterform structure of Archean age has so far been discovered in Fennoscandia although statistically remnants of Archean cratering events should be found in the Fennoscandian Shield. New ways of searching for these craters are proposed and discussed. In addition to changes in the petrophysical properties of rocks like density, magnetization, and electrical conductivity, redistribution of large volumes of rocks are associated with large impacts. Such changes in structures and rock properties may be identified by integrated interpretations of regional high resolution geophysical data. The Siljan impact case shows, however, that the impact overprinting can be very slight in comparison to geophysical anomalies caused by pre-impact lithological and structural variations. We review the record of Fennoscandian impact craters giving examples of geophysical signatures of impact craters of different sizes and ages. Henkel H. and Pesonen L.J. In Terrestrial impact craters and craterform structures with a special focus on Fennoscandia (eds. L.J. Pesonen and H. Henkel), Tectonophysics, Special Issue (in print).
Henkel Hartmut
Pesonen Lauri J.
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