Magnetic terranes in the central United States determined from the interpretation of digital data
Published:January 01, 1985
Thomas G. Hildenbrand, 1985. "Magnetic terranes in the central United States determined from the interpretation of digital data", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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A color magnetic-anomaly map compiled from digital data provides a synoptic view of major magnetic anomalies and corresponding geologic features of the central United States. The availability of the data in digital form allowed application of a variety of analytical techniques to enhance the anomalies and provide new interpretive information. Derivative and directional filters were applied to the data to help identify lithologic and structural boundaries. A magnetization-density ratio map was used to delineate major geophysical provinces. In addition, the data were draped over the Precambrian surface to remove the effects of basement relief.
The amplitudes and wavelengths of magnetic anomalies exhibited on the compiled maps vary considerably within the Midcontinent. Magnetic anomalies correlate well with major Precambrian and Paleozoic tectonic features and aid in delineating the features' lateral extent and associated structures. Coherent patterns of anomalies observed on the maps prove useful in delineating basement domains. Of particular interest are several linear magnetic features that may reflect continental-scale geologic features. Two northwest-trending lineaments that show remarkable linearity— the south-central and Great Lakes magnetic lineaments—coincide with several structural and lithologic boundaries in the Midcontinent and apparently express major crustal discontinuities.
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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.