Mapping basement magnetization zones from aeromagnetic data in the San Juan basin, New Mexico
Published:January 01, 1985
Lindrith Cordell, V.J.S. Grauch, 1985. "Mapping basement magnetization zones from aeromagnetic data in the San Juan basin, New Mexico", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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Two new techniques are employed in analysis of aeromagnetic data from the San Juan basin, New Mexico, to enhance the expression of buried basement structure and lithology: (1) the data are analytically continued downward onto the irregular basement surface in order to reduce the effect of variable depth to basement, and (2) magnetization boundaries are delineated by a linear filter based on the gradient of pseudogravity. Reduction to the basement involves three procedures that, together, provide a practical method for continuation of potential fields between general surfaces. Data are continued from the nonlevel flight-elevation surface onto a level surface (drape-to-level continuation) by means of a system of successive approximations based on expansion of a Taylor series. Data are continued from the new level surface downward to another level surface at the mean elevation of the basement (level-to-level continuation) by analytical downward continuation based on the fast Fourier transform, incorporating a high-cut filter. Data are continued from this level onto the nonlevel basement surface (level-to-drape continuation) by direct evaluation of a truncated Taylor series expansion, in the vertical dimension, of the field represented on the level surface.
Magnetization boundaries are determined by evaluating the magnitude of the horizontal gradient of the pseudogravity transform of the magnetic data. Lines drawn along ridges of the horizontal gradient mark inferred basement-magnetization boundaries.
After application of these techniques, the aeromagnetic data from the San Juan basin, New Mexico, revealed features of the Precambrian basement not evident, or only vaguely so, in the original data. Together with scanty drill data, the aeromagnetic data indicate a 70-km-wide belt of predominantly metasu-pracrustal rocks trending east-northeast across the center of the basin. Predominantly granitic terranes border this belt on the northwest and southeast. Numerous magnetization boundaries tens of kilometers in length are identified. Structural grain is strongly east-northeast; an area of prominent northerly trends occurs in the north-central part of the basin and locally elsewhere. Cenozoic intrusive rocks are unquestionably aligned with basement structural grain. Laramide and Neogene structures in general are not so aligned. There is no evidence of large strike-slip displacement of basement structures within the area surveyed.
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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.