Geologic significance of regional gravity and magnetic anomalies in the east-central Midcontinent
Published:January 01, 1985
E. G. Lidiak, W. J. Hinze, G. R. Keller, J. E. Reed, L. W. Braile, R. W. Johnson, 1985. "Geologic significance of regional gravity and magnetic anomalies in the east-central Midcontinent", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
Download citation file:
The compilation of regional Bouguer gravity-anomaly and magnetic-anomaly maps, the gridding of those data, and the development of numerous filtered maps, together with detailed petrographic analysis of basement-rock drillhole samples, have provided significant insight into the Precambrian basement. The geophysical data have yielded important clues to the tectonic framework and the regional distribution of basement-rock types. In parts of the east-central Midcontinent the basement drillhole data have been extremely useful in the evaluation of geophysical anomalies and in the interpretation of the Precambrian geology. However, in other parts of the region, correlation of the drillhole data with the geophysical anomalies has been poor. This poor correlation has led to a consideration of factors that can produce ambiguities in correlating geophysical and geologic data.
This investigation has shown the value of an integrated geophysical and geologic approach to studying the tectonic framework of basement rocks in the east-central Midcontinent. As a result of this study, four principal basement zones are recognized: an anorogenic granite-rhyolite terrane, several basement rift zones underlain primarily by mafic volcanic rock, the southern continuation of the Grenville province, and the New Madrid rift complex.
Figures & Tables
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.