Studies of gravity anomalies in Georgia and adjacent areas of the southeastern United States
Published:January 01, 1985
Leland Timothy Long, Anton M. Dainty, 1985. "Studies of gravity anomalies in Georgia and adjacent areas of the southeastern United States", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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Summaries of simple Bouguer gravity surveys in Georgia and their interpretations by the School of Geophysical Sciences, Georgia Institute of Technology, are presented. These surveys, primarily at a spacing of about 1 km, cover parts of the folded Appalachians, the Inner Piedmont, the Charlotte and Carolina Slate belts, and the Coastal Plain. In the folded Appalachians, positive anomalies of up to 5 mGal occur over outcrops of the carbonate members of the sedimentary sequence, indicating that these members are denser than the rest of the sequence. Few Bouguer anomalies are found over the fault contact between the folded Appalachians and the Blue Ridge, and these seem to correlate with topography. There is a general lack of anomalies owing to near-surface sources in the Inner Piedmont, although a large regional anomaly there is caused by lower crustal sources. In contrast, large Bouguer anomalies of 20-mGal amplitude are associated with surface exposures of mafic rocks (positive anomalies) and granites (negative anomalies) in the Charlotte and Carolina Slate belts adjacent to the Inner Piedmont. The fault contact between the Inner Piedmont to the northwest and the Charlotte or Carolina Slate belt to the southeast is the site of a steep gradient in the Bouguer gravity, owing to denser, more mafic rocks in the upper 5 km of the crust on the southeast side. In the Coastal Plain, many Bouguer gravity anomalies and total-magnetic-field anomalies are caused by Triassic rift structures. Another feature of the Bouguer gravity field in the Coastal Plain is a correlation of gravity and topography, suggesting that basement structures control topography in some regions.
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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.