An integrated geophysical investigation of the midcontinent rift system: Western Lake Superior, Minnesota, and Wisconsin

David Jennings Allen, Purdue University


Integrated geophysical investigations of the Middle Proterozoic (Keweenawan) North American Midcontinent Rift System (MCR) have resulted in a new understanding of the structure, stratigraphy, and evolution of this 1100 Ma aborted continental rift. Synergistic interpretation of seismic reflection, gravity and magnetic anomaly, seismic refraction, rock physical properties, and geologic data has identified a great degree of structural heterogeneity of the MCR in Minnesota, Wisconsin, and western Lake Superior. In the western Lake Superior region, the rift's volcanic rocks are locally relatively thin or absent, the lower portion of the rift's sedimentary sequence was not deposited, and major rift reverse faults terminate above two ridges of pre-Keweenawan basement rocks. Three-dimensional gravity modeling, constrained by seismic reflection profiles, suggests that both ridges are underlain by a belt of granitic rocks within the buried Archean greenstone-granite province beneath the rift basin, testifying to the significant influence of ancestral structures throughout the evolution of the MCR. In southeastern Minnesota, the rift basin is continuous but is deflected to the south-southeast along the Belle Plaine Fault, a normal growth fault that may have developed along an ancestral zone of weakness. The relatively minor modification of this fault by late-stage compressional forces and the minimal uplift of the MCR in southeastern Minnesota suggest that this segment of the rift system may have been nearly parallel to the direction of maximum late-stage compression. Magnetic anomaly modeling indicates that magmatism did not occur uniformly along the length of the MCR. In Minnesota and Wisconsin, the great majority of the Keweenawan igneous rocks are normally polarized and therefore younger than the magnetic reversal at $\sim$1098 Ma, in contrast to western Lake Superior, where the lower half of the volcanic sequence is reversely polarized and was therefore erupted before $\sim$1098 Ma. Gravity modeling suggests that the mass deficiency associated with crustal thickening along the MCR is probably compensated by the positive effect of dense Keweenawan intrusive rocks in the lower crust. The volume of magma trapped in the lower crust may be similar to that which was erupted into the rift basin. ^




Major Professor: William J. Hinze, Purdue University.

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