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The compilation of regional Bouguer gravity-anomaly and magnetic-anomaly maps, the gridding of those data, and the development of numerous filtered maps, together with detailed petrographic analysis of basement-rock drillhole samples, have provided significant insight into the Precambrian basement. The geophysical data have yielded important clues to the tectonic framework and the regional distribution of basement-rock types. In parts of the east-central Midcontinent the basement drillhole data have been extremely useful in the evaluation of geophysical anomalies and in the interpretation of the Precambrian geology. However, in other parts of the region, correlation of the drillhole data with the geophysical anomalies has been poor. This poor correlation has led to a consideration of factors that can produce ambiguities in correlating geophysical and geologic data.

This investigation has shown the value of an integrated geophysical and geologic approach to studying the tectonic framework of basement rocks in the east-central Midcontinent. As a result of this study, four principal basement zones are recognized: an anorogenic granite-rhyolite terrane, several basement rift zones underlain primarily by mafic volcanic rock, the southern continuation of the Grenville province, and the New Madrid rift complex.

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