Magnetometry of the marine, Ordovician Lockne impact structure, Jamtland, Sweden

Magnetic modelling of the Middle Ordovician Lockne impact crater reveals its morphology and the lack of a magnetically sufficient melt body. Suevite and melt are often strongly remanent and stand out in magnetic surveys. However, exceptions occur, e.g. the Lappajarvi impact structure. From aeromagnetic data a total intensity and a horizontal gradient map were generated. They show a structure with a calm interior. The absence of short wave length anomalies is interpreted to be an indication of the lack of a melt sheet with a prominent magnetic signature. Neither can short wave length anomalies be seen at the expected central uplift of the structure. However, this kind of anomalies are commonly associated with larger impact structures. The modelling produced a bowl-shaped structure which is slightly tilted westwards. This is in good agreement with the field observation of the structure. Based on the absence of magnetic anomalies, together with field observations and drill-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 target material in the Lockne area was seawater and sedimentary strata overlying a crystalline basement. The abundant water in the area possibly contributed to the explosive dispersion of the molten silicates. (C) 1998 Elsevier Science B.V.