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Abstract

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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