Features of a pair of long-wavelength (>250 km) and short-wavelength (<250 km) Bouguer gravity maps of the United States
Published:January 01, 1985
M. F. Kane, R. H. Godson, 1985. "Features of a pair of long-wavelength (>250 km) and short-wavelength (<250 km) Bouguer gravity maps of the United States", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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A complementary pair of wavelength-filtered regional and residual gravity maps of the United States has been compiled from a digital gravity base. Processing of simple model anomalies with the same filters as those used on the maps shows that the short-wavelength (SWL) anomalies, although exaggerated in shape and distorted in amplitude, give a clearer indication of source geometry than the total-field Bouguer anomalies. Side lobes created by the filtering process appear to be the most serious deficiency of the SWL maps, but they can often be distinguished by using simple criteria. The model studies also show that the horizontal-gradient amplitude map is a useful adjunct for interpreting the SWL anomalies. Maximum horizontal-gradient/amplitude relations indicate that most of the anomalies of the SWL map, particularly the prominent ones, have their source in the crust. There is considerable leakage of long-wavelength (LWL) energy from relatively narrow but intense anomalies into the LWL field so that moderate-wavelength anomalies in the LWL map must be interpreted with care. The SWL map defines a series of zones that are based on anomaly characteristics, primarily anomaly trend. These zones correspond in many places to previously identified geologic terranes. It is concluded that rift systems can account for many of the SWL gravity features of the eastern United States; a model of a rift deformed by compression is proposed to explain many of these features. Other gravity-geologic correlations, notably over igneous features, are discernible.
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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.