Interpretation of Precambrian geology in Minnesota using low-altitude, high-resolution aeromagnetic data
Published:January 01, 1985
Val W. Chandler, 1985. "Interpretation of Precambrian geology in Minnesota using low-altitude, high-resolution aeromagnetic data", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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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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The Utility of Regional Gravity and Magnetic Anomaly Maps
The first composite magnetic-anomaly map of the conterminous United States and adjacent offshore areas has been published at a color-contour interval of 200 gammas and at the scale and projection of other national geologic and geophysical maps for easy comparison. This map, despite the inconsistent characteristics of the surveys from which it was compiled, is useful in providing a regional framework for the interpretation of magnetic studies of limited areas, in selecting areas for more detailed magnetic investigations, and in studying the distribution and character of regional geologic features.
The map has a wide variation of magnetic-anomaly patterns, trends, and types, thus reflecting the diversity of the geologic terranes of the United States. In general, the anomaly pattern east of the Cordillera in the craton and in the Appalachian Mountains consists of more and greater intensity anomalies. The muted nature of the anomalies of much of the Cordillera is a result of several factors but appears to be primarily related to a decreased crustal magnetization caused by an abnormally shallow Curie isotherm. The anomalies of the Appalachian Mountains and the Cordilleran system primarily reflect the major structural patterns of the orogens, but important exceptions occur, such as those associated with rocks underlying thrust sheets in the Appalachian Mountains and westerly-striking anomaly trends in the Cordillera, which are correlated with igneous intrusives, faults, and mineral deposits.
The buried southern and eastern edges of the Pre-cambrian craton are indicated by changes in the magnetic anomalies and their dominant trends. Within the central United States, numerous regional magnetic-anomaly provinces are observed that reflect the long, complex history of the Precambrian basement rocks of the craton. These provinces are transected by conspicuous, intense, long, generally linear anomalies that originate from mafic extrusive or shallow intrusive igneous bodies within failed rifts, such as the Midcontinent rift system, the Southern Oklahoma aulacogen, and the Reelfoot rift buried beneath the Mississippi embayment. These are only a few of the many interesting regional geologic features that are observed on the composite magnetic-anomaly map of the United States.