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Abstract

High-resolution aeromagnetic data recently acquired over northeastern and east-central Minnesota are useful in interpreting the Archean and Proterozoic bedrock beneath a widespread mantle of Pleistocene glacial materials. The new aeromagnetic data, gathered at a 400-m line spacing and a 150-m ground clearance, allow a much improved structural interpretation of many Archean and Proterozoic features, including the Archean migmatitic rocks of the Vermilion Granitic Complex, the Vermilion fault zone, the Keweenawan (Middle Proterozoic) lavas on the St. Croix horst, and the Keweenawan Duluth Complex. The data have delineated two poorly exposed dike swarms, one of Early Proterozoic age striking northwest in north-central Minnesota, and another of Keweenawan affinity striking northeast in east-central Minnesota. Several prominent anomaly lineaments occur over the region and may indicate previously unknown faults. Extremely weak but spatially coherent maxima define curvilinear and dendritic patterns over the subdued anomaly signature of the Lower Proterozoic Animikie basin and reflect near-surface sources, perhaps within the overlying Cretaceous or Pleistocene (glacial) materials.

Detailed interpretation using frequency-domain filtering was conducted on data grids from northern (Hibbing area), east-central (Carlton County area), and northeastern (Gabbro Lake area) Minnesota, and a variety of magnetic interpretational problems were addressed. High-pass filtering in the Hibbing area revealed internal structures within a large Archean batholith and also defined several Lower Proterozoic dikes. Strike-sensitive filtering over the Lower Proterozoic Biwabik Iron Formation in the Hibbing area was useful in separating stratigraphy-related anomaly components along strike from components associated with crosscutting structures containing natural ore zones. Calculation of the second vertical derivative over the Carlton County aeromagnetic grid was useful in tracing and determining the magnetic polarity of Keweenawan dikes. The second-derivative data also enhanced anomalies that may be related to large-scale folds in Lower Proterozoic rocks, and possibly in some cases to sources in the overlying younger materials. Reduction to the pole of the Gabbro Lake aeromagnetic data was effective in eliminating the effect of a strong remanent component, which is characteristic of many Keweenawan igneous rocks, and significantly improved the correspondence of anomaly patterns to known geology. The results observed over the Gabbro Lake area demonstrate the potential of this method for investigating the Keweenawan terranes of the Lake Superior region.

Although much work remains to be done, the preliminary results of this study demonstrate the great utility and geologic resolution of the data and should provide some general guidelines for future studies using filtering-enhancement techniques.

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