The new series of 1:1 000 000-scale magnetic anomaly maps of the Geological Survey of Canada: Compilation techniques and interpretation
Published:January 01, 1985
S. D. Dods, D. J. Teskey, P. J. Hood, 1985. "The new series of 1:1 000 000-scale magnetic anomaly maps of the Geological Survey of Canada: Compilation techniques and interpretation", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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In 1977 the Geological Survey of Canada initiated a project to produce colored magnetic-anomaly maps at a scale of 1:1 000 000 for those areas of Canada covered by published aeromagnetic maps at a scale of 1:50 000 or 1 inch: 1 mile. An additional objective of this project was to create a magnetic data bank for Canada which could be used at scales from 1:250 000.
One of the more interesting uses of these data has been the generation of magnetic shaded-relief maps in which the magnetic-field values are assigned a vertical scale to represent topography and illuminated from a light source (usually considered to be the sun). This procedure has the effect of enhancing features such as dikes and contacts that are not seen on the original magnetic-anomaly maps because of the coarseness of the color scale. The data can also be used with standard continuation or derivative techniques.
It is intended that the 1:1 000 000-scale magnetic-anomaly maps will be utilized essentially as basic building blocks in compiling future editions of the 1:5 000 000-scale Magnetic Anomaly Map of Canada, which will in turn be used for the Magnetic Anomaly Map of North America being compiled as part of the Geological Society of America's Decade of North American Geology program.
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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.