Evolution of Geological Survey of Canada magnetic-anomaly maps: A Canadian perspective
Published:January 01, 1985
P. J. Hood, P. H. McGrath, D. J. Teskey, 1985. "Evolution of Geological Survey of Canada magnetic-anomaly maps: A Canadian perspective", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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Aeromagnetic surveying by Canadian government agencies commenced in 1947, and by 1967 a sufficient area of the country had been covered so that it was feasible to produce a Magnetic Anomaly Map (MAM) of Canada. The first 1:5 000 000-scale national MAM was published by the Geological Survey of Canada in 1967, subsequent versions were published in 1972 and 1977, and a fourth edition has been compiled. The colored Magnetic Anomaly Map of Canada demonstrated that features of large areal extent, such as geologic provinces, were readily delineated and proved the usefulness of producing such regional compilations. Consequently, in carrying out aeromagnetic surveys overseas as contributions to Canadian aid programs, the preparation of regional magnetic-anomaly maps has usually been made part of the contract at the interpretation stage. The first such MAM published under contract was a colored 1:1 000 000 MAM of Guyana, which was the first of its kind in South America. Subsequently a colored 1:1 000 000 MAM of Baluchistan, Pakistan, was published, and a MAM for the Liptako-Gourma area of West Africa is being prepared. It is abundantly clear that such magnetic-anomaly maps should be made a standard end product of regional aeromagnetic sur-veys because of their several uses. They serve not only as index maps to the aeromagnetic-survey coverage available on a national basis (as well as to gaps in the coverage) but also stimulate comparison of the regional magnetic features with those appearing on similar-scale geologic and geophysical (e.g., gravity) maps and with Land sat imagery. They also make excellent office wall displays for hand-waving geo-physicists.
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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.