A southward-trending axis of Bouguer anomaly maximums as high as +53 mgal flanked by subparallel axes of anomaly minimums as low as -75 and -89 mgal has been demonstrated in southeastern Minnesota by ten westward-trending traverses consisting of a total of 2572 gravity observations taken at half-mile intervals. The maximum coincides with a belt of Middle Keweenawan basic igneous rocks as thick as about 29,000 feet near Minne apolis and prob ably thicker southward; the minimums are considered to reflect adjacent belts of less dense Upper Keweenawan sedimentary rocks as thick as about 11,000 feet. Two narrow zones of high gravity gradients that parallel the anomaly maximum northeast of Minneapolis are interpreted as defining steep faults which bound a major crustal feature, the St. Croix horst; this block has been elevated thousands of feet, mainly in late Precambrian time. An abrupt bend in the anomaly maximum southwest of Minneapolis may result from sinistral strike-slip movement o.f about 30 miles along a fault of Precambrian age that strikes N35°W.Minor rejuvenation of these Precambrian tectonic elements can account for many structural features developed in the Paleozoic strata. Near the southern Minnesota border the anomaly curve can be interpreted as resulting from a root of basic igneous rocks about ten miles thick below Upper Keweenawan strata as much as two miles thick. The Middle Keweenawan igneous province extends southward at least to Le Sueur County, Minnesota, and probably into Iowa.
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