Preparation and overview of the gravity anomaly map of the United States
Published:January 01, 1985
Norbert W. O'Hara, Paul L. Lyons, 1985. "Preparation and overview of the gravity anomaly map of the United States", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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The Gravity Anomaly Map of the United States was published in October 1982 by the Society of Exploration Geophysicists. The regional map was compiled and edited in collaboration with the U.S. Geological Survey, the U.S. Defense Mapping Agency, and the National Oceanic and Atmospheric Administration.
Publication of the conterminous United States Bouguer and offshore free-air gravity data is at a scale of 1:2 500 000. Map scale, base, and projection are identical with the existing geologic, tectonic, basement-rock, and magnetic maps of the United States. The contour interval is 5 mGal, with gravity amplitudes depicted in color intervals of 25 mGal. Anomalies were calculated using the IGSN71 standard and the GRS67 ellipsoid with Bouguer values based on a rock density of 2.67 g/cm3. Nearly 2 million digital gravity-data points were initially examined, then sorted to produce an equally gridded station spacing of 4 km. The screened data were next terrain-corrected, where appropriate, and machine-contoured. Detailed editing, assimilation of nondigital data where necessary, hand contouring, and final cartographic work completed the process.
The most obvious characteristic of the new national gravity map is the spectacular contrast between the generally high anomalous amplitudes observed on the eastern part of the map When compared with the predominance of low anomalous amplitudes illustrated on the western part. Typical regional geologic structural features recognizable from the gravity anomalies in the eastern half of the United States include the Midcontinent rift system, present and possibly past continental margins, Precambrian-basement trends, orogenic belts, buried basins, and more obvious mountainous terrain. Major structural features reflected by the anomalies in the western United States include the Southern Rockies, Colorado Plateau, Idaho batholith, Basin and Range pattern, Columbia Plateau volcanic-rock region, and indications of ancient plate collisions, subduction, and crustal uplift associated with the Pacific Cordillera.
Comparisons of specific geologic structures, represented on the geologic and tectonic maps, with corresponding gravity and magnetic anomalies provide essential information regarding the distribution and configuration of basement crystalline rocks, structural and lithologic provinces, zones of crustal weakness, and the distribution of mafic rocks and sedimentary basins.
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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.