11: Long-wavelength gravity and magnetic anomalies of the Lake Superior region
V. W. Chandler, P. L. Bowman, W. J. Hinze, N. W. O’Hara, 1982. "11: Long-wavelength gravity and magnetic anomalies of the Lake Superior region", Geology and Tectonics of the Lake Superior Basin, Richard J. Wold, William J. Hinze
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Long-wavelength gravity and magnetic anomalies isolated by upward continuation to 50-km and 150-km levels have been useful in investigating regional-scale and deep crustal structure of the Lake Superior region. The 50-km-level data over the Keweenawan rift zone are characterized by an arcuate belt of gravity maxima that corresponds to a somewhat varied magnetic expression. Modeling results of the 50-km data along the rift axis are consistent with a thick, high-density crust and an upper crustal magnetization that may be regionally dominated by Keweenawan remanence. Analysis using Poisson’s theorem of the 50-km-level data along the St. Croix horst and over the Michigan Basin yield magnetization-to-density contrast ratios consistent with basaltic or gabbroic rocks surrounded by granitic rocks. The general long-wavelength anomaly configuration in northern Lake Superior and environs favors the Sibley Basin-Nipigon Plate, rather than the Kapuskasing Subprovince, as a possible failed arm of an rrr triple junction.
Away from the rift zone numerous crustal features are associated with long-wavelength anomalies. Belts of gneissic rocks with high metamorphic grades, including the gneissic rocks south of the Great Lakes tectonic zone, the English River Subprovince, and the Kapuskasing Subprovince, are associated with long-wavelength gravity maxima. A major crustal block over the eastern Superior Province, which has a western boundary near or along the Kapuskasing Subprovince, is characterized by broad gravity and magnetic minima. A long-wavelength magnetic maximum extending southwesterly from northern Lake Huron may reflect a belt of l, 500-m.y.-old anorogenic plutons. The Moose River and Michigan Basins of Phanerozoic age are associated with long-wavelength gravity and magnetic maxima. Strike-pass filtering of the 50-km-level data is useful for investigating the distribution of anomaly trends in the area and for determining the extent of geologic features along a particular strike.