10: Gravity and magnetic anomaly studies of Lake Superior
William J. Hinze, Richard J. Wold, Norbert W. O’Hara, 1982. "10: Gravity and magnetic anomaly studies of Lake Superior", Geology and Tectonics of the Lake Superior Basin, Richard J. Wold, William J. Hinze
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Gravity and magnetic anomalies in Lake Superior are useful in tracing out the subsurface extent and geometry of igneous rocks of the Keweenawan Supergroup that crop out around the lake and on islands within the lake. The data show that the Lake Superior structural basin generally is outlined by the shoreline of the lake. The limbs of the elongated basin are delineated by gravity and magnetic maxima that occur over outcropping and buried mafic volcanic rock. The basin anomalies at the southwestern and southeastern ends of the lake extend into the midcontinent and mid-Michigan geophysical anomalies respectively. This pattern of anomalies is interpreted to be the geophysical expression of a failed rift complex, the Midcontinent Rift System, that extends in an arcuate pattern for more than 2,000 km over mid-America. The anomaly pattern differs along its length, and particularly in Lake Superior reflecting the complex crustal manifestations of the rift. Several faults are indicated in the lake that roughly parallel the outline of the basin or that transect it. However, the anomaly maps provide no evidence of an axial graben. Rather, the inferred axis of the basin is generally a magnetic minimum reflecting the increased thickness of sedimentary rocks that have low magnetic susceptibility. Gravity and magnetic anomalies locally are caused by structural deformation and variation in thickness of volcanic and sedimentary rock related to pre-Keweenawan topography and to Keweenawan fault blocks that strike into the basin from the margins. A major north-northeast trending fault divides the basin into contrasting eastern and western units. By comparison with the western basin, the eastern unit is characterized by subdued gravity and magnetic anomalies that are due to a combination of less diastrophism, decreased volume of volcanic rock, and more extensive sedimentary cover.
Modeling of the gravity data using constraints provided by the geologic, magnetic, and seismic data indicates that volcanic units overlain by relatively nonmagnetic low-density clastic sedimentary rocks fill the Lake Superior Basin. This model assumes the entire crust to be abnormally dense beneath both eastern and western Lake Superior. This increased density is interpreted to be the result of extension along an axial zone associated with pervasive intrusions from the mantle. The modeling also is consistent with a broad thickening of the crust by a few kilometers along the axis of the basin.
Stripping off the gravitational effects of mass deficiency due to the low-density upper Keweenawan clastic sedimentary rocks from a smoothed Bouguer gravity anomaly map results in a residual anomaly map that attenuates and restricts the gravity minima. This map shows the entire lake to be a positive gravity anomaly; however, the minima over the Bayfield Peninsula and Keweenaw Bay are retained, suggesting that mafic lavas are absent or thin in these areas.
These interpretations of the gravity and magnetic anomaly data in the Lake Superior Basin are consistent with the concept that mid-America was subjected to tensional forces in Keweenawan time concurrently with the Grenville orogeny. These forces caused extension of the crust resulting in a range of igneous activity and diastrophism that today is represented by a more dense and perhaps thickened crust overlain by volcanic and sedimentary rock basins that have been disturbed primarily by vertical movements.