Block structure of continental crust derived from gravity and magnetic maps, with Australian examples
Published:January 01, 1985
Peter Wellman, 1985. "Block structure of continental crust derived from gravity and magnetic maps, with Australian examples", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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A continent is considered to consist of a mosaic of crustal blocks each 500 to 1 000 km across, the blocks having different cratonization history. Within each block, gravity anomalies of wavelength 20 to 100 km are generally elongated and subparallel. The relative ages of the gravity trends in the adjacent blocks can be determined because those blocks with trends oblique to the boundary are likely to be older than the boundary, whereas those parallel to the boundary are likely to be younger. These major gravity trends are thought to be caused by large-scale folding initiated at the first major deformation of each block. The boundaries between Precambrian blocks commonly coincide with a gravity gradient between a major high and a major low anomaly, the total amplitude being more than 50 mGal. The dipole anomaly is interpreted as due to a large and abrupt change in mean upper-crustal density, the crust being in regional isostatic equilibrium. Within the crustal blocks, major cross fractures can be mapped using the combined gravity and magnetic data. The cross fractures cause minor gaps or offsets of the primary gravity and primary magnetic trends, and are coincident along part of their length with the axes of magnetic bodies. These cross fractures are commonly subparallel and are generally not at right angles to the major trends. Where basement geology is known, the cross fractures coincide with geologically mapped faults and, rarely, folds.
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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.