Palaeomagnetism of the Lappajärvi impact structure, western Finland

Abstract

Palaeomagnetic and rock magnetic results are presented for rocks from the Lappajärvi meteorite impact structure, western Finland. Oriented samples were collected from the melt rocks in the central island of the crater and of the Palaeoproterozoic (Svecofennian) target rocks at the crater rim. The natural remanent magnitization (NRM) of the melt rocks is generally weak and metastable during demagnetization. The characteristic NRM component, carried by pyrrhotite, yields a palaeomagnetic pole (latitude 55.4°N, longitude 152.6°E, dp = 6.2°, dm = 8.7/ dg, 6 sites) suggesting an age of ~ 195 Ma for the impact. The results from a deep core drilled from the impact melt rock display similar palaeomagnetic directions but with slightly shallower inclinations. The palaeomagnetic age differs from the 40Ar- 3939Ar age of 77 Ma for the impact. Assuming that the 40Ar- 39Ar age is correct, the discrepancy between palaeomagnetic and radiometric ages is best explained by post-impact (and post-cooling) tilting of the melt layer. The tilting (15°) may be due to isostatic rebounds along joints in the melt. Impact remanence has also been isolated in a few target rocks at the rim. This poorly defined overprint could be a mild shock remanent magnetization superimposed on the 1.9 Ga old Svecofennian direction. This interpretation is supported by the observation that the NRM intensity is enhanced in specimens having the impact overprint. The palaeomagnetic results are consistent with earlier interpretations based on geophysical, morphological, mineralogical and geochemical data that the Lappajärvi structure is a complex meteorite impact site less than 200 Ma old.