MAGNETOMETRY OF THE MARINE, ORDOVICIAN LOCKNE IMPACT STRUCTURE, JAMTLAND, SWEDEN

Magnetic modelling of the Middle Ordovician Lockne impact crater revea ls its morphology and the lack of a magnetically sufficient melt body. Suevite and melt are often strongly remanent and stand out in magneti c surveys. However, exceptions occur, e.g. the Lappajarvi impact struc ture. From aeromagnetic data a total intensity and a horizontal gradie nt map were generated. They show a structure with a calm interior. The absence of short wave length anomalies is interpreted to be an indica tion of the lack of a melt sheet with a prominent magnetic signature. Neither can short wave length anomalies be seen at the expected centra l uplift of the structure. However, this kind of anomalies are commonl y associated with larger impact structures. The modelling produced a b owl-shaped structure which is slightly tilted westwards. This is in go od agreement with the field observation of the structure. Based on the absence of magnetic anomalies, together with field observations and d rill-core information we are convinced that no prominent melt body was formed in connection with the Lockne impact, despite the size of the structure. This fact may be explained by the type of target. The targe t material in the Lockne area was seawater and sedimentary strata over lying a crystalline basement. The abundant water in the area possibly contributed to the explosive dispersion of the molten silicates. (C) 1 998 Elsevier Science B.V.