Twenty aeromagnetic profiles over a 100-mile-wide strip along the arc of a great circle passing through Denver, Colorado, and Washington, D. C, reveal large anomalies of major crustal significance. Contoured data disclose several areas of distinct magnetic patterns reflecting basement lithology and structure.
The mafic rocks of the Blue Ridge and Piedmont and the Keweenawan mafic belt in Iowa and Nebraska give rise to strong linear trends. Areas with a more random pattern of closely spaced magnetic anomalies appear in central Ohio, eastern Iowa, and central and western Nebraska. Except in the Blue Ridge and Piedmont areas, crystalline basement rocks are covered by a thick blanket of virtually nonmagnetic sedimentary rocks, and lithology must be inferred from correlations of the magnetic data with scattered drill-hole data and regional gravity data. The area of highly magnetic rocks in central Ohio has a sharp western boundary that coincides with the western limit of metamorphism associated with the probable extension into Ohio of the Grenville province of Canada. This area and a similar one in eastern Iowa are linked by an arcuate, nearly continuous belt of positive gravity anomalies that extends north into Wisconsin and northern Michigan and then swings southeast across central Michigan. This horseshoe-shaped feature is associated with lithologically diverse but highly magnetic basement rocks. A group of linear magnetic anomalies in western Iowa and eastern Nebraska correlates with the well-known midcontinent gravity high. In Nebraska the magnetic data provide a basis for grouping the extremely complex drill-hole data into three over all lithologic terranes.
An analysis of the long-wavelength (>40 miles) variations of the profiles shows that they form a number of large coherent anomalies, many of which show little relation to the major tectonic trends and lithologic patterns of the basement surface. The very broad and less numerous anomalies in the east, which have more or less north-south trends, are significantly different from the more numerous anomalies in the western part of the strip, which tend to trend east-west. One linear anomaly extends for nearly 500 miles across Nebraska and Iowa and may mark a zone of rifting. Heat-flow data show that rocks at the Curie point, which determines the depth below which rock magnetization cannot occur, may be deep enough, at least in shield and other stable parts of the continent, to include a part of the upper mantle. The concentration of the large magnetic features in Iowa and Nebraska may indicate that the thickness of magnetized rock is greater in this area and that perhaps some of these features originate in the upper mantle.