Some effects of regional metamorphism and geologic structure on magnetic anomalies over the Carolina Slate belt near Roxboro, North Carolina
Published:January 01, 1985
E. S. Robinson, P. V. Poland, L. Glover, III, J. A. Speer, 1985. "Some effects of regional metamorphism and geologic structure on magnetic anomalies over the Carolina Slate belt near Roxboro, North Carolina", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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Aeromagnetic anomalies over the Carolina Slate belt near Roxboro, North Carolina, are influenced by geologic structure and regional metamorphism. Anomaly amplitudes diminish with the transition from greenschist- to amphibolite-facies rocks along a north west-trending metamorphic gradient. The northeast-trending Virgilina synclinorium is the principal structure in the area. A greenschist-facies metasedimentary rock unit is the principal magnetic-anomaly source in the northeast part of the synclinorium. Magnetization of this unit appears to diminish in a southwesterly direction along the structure, apparently because of the alteration of magnetite to hematite by oxidation.
Field-intensity profiles crossing linear anomalies were compared with profiles computed for two-dimensional models to determine the subsurface configuration of the Virgilina synclinorium. These comparisons indicate that the greenschist-grade metasedimentary rocks extend to depths of at least 2.5 km along the axis of the synclinorium. This implies that the amphibolite-greenschist isograd surface, which is exposed approximately 8 km northwest of the synclinorium axis, cannot dip toward the southeast at less than 20 degrees.
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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.