A recently compiled 80 000-line-kilometer aeromagnetic survey of the State of Kansas, flown under uniform specifications, provides an excellent database for spectral-enhancement techniques. A suite of spectrally filtered maps of Kansas is proving useful in the regional study of the Precambrian basement. The pole-correction map (i.e., the reduced-to-the-pole map minus the original map) enhances an east-west-trending boundary between Precambrian terranes of different age. The second-vertical-derivative map reveals extensive basement faults trending southwest through central Kansas. The combination of high-frequency-pass and trend-pass filters reveals the bounding faults of the Central North American rift system (CNARS). The upward and downward continuation filters are also useful in delineating basement terranes. These maps reveal parallelism between the Humboldt fault, which bounds the eastern side of the Nemaha uplift, and the CNARS. This suggests that the Humboldt fault probably developed as one of the easternmost faults of the CNARS in late Precambrian time and was reactivated in late Paleozoic time. Recent seismicity indicates that some of the CNARS faults are active today.
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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.