Interpretation of gravity and magnetic data, central and eastern Brazil
Published:January 01, 1985
Nicolau L.E. Haralyi, Yociteru Hasui, 1985. "Interpretation of gravity and magnetic data, central and eastern Brazil", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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The Bouguer gravity-anomaly and total-intensity magnetic-anomaly maps covering large areas of central and eastern Brazil define many major structures of the Precambrian basement. Archean granite-gneiss terranes with intervening greenstone belts and high-grade metamorphic zones occur in a series of crustal blocks separated by major discontinuities. This structural pattern essentially was not modified by the Pro-terozoic polycyclic thermo-tectonic processes and Upper Jurassic-Tertiary reactivation of the area. Rather, the Archean block structure greatly influenced the deformation involved in these later events. The magnetic-anomaly data are especially useful in mapping regional fault systems with lengths measured in hundreds of kilometers and mapping of mafic-ultramafic and alkaline intrusions. The distribution of many major mineral deposits is closely related to the major regional crustal structure interpreted from the geophysical data, and thus the regional geophysical data can be used to delimit regional mineralized domains.
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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.