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The Beaver Bay Complex (BBC) is a hypabyssal, multiple-intrusive igneous complex exposed over a 600 km 2 area in northeastern Minnesota. It was emplaced into comagmatic volcanic rocks at 1.1 Ga during the development of the Midcontinent Rift System. Thirteen intrusive units have been identified that represent a minimum of six major intrusive events. With the exception of a body of granophyre, most intrusions were formed from gabbroic to dioritic parental magmas with successive intrusions generally involving less evolved compositions. Compositional variations within intrusive units developed as a result of in situ magmatic differentiation, assimilation of footwall rocks, and composite intrusions of evolved magma from deeper staging chambers. Over the exposed extent of the BBC, intrusion shapes appear to have been controlled by a shallow crustal ridge that trends northwesterly across the BBC. The focus of emplacement appears to have migrated toward the rift axis and toward higher stratigraphic levels with time perhaps reflecting plate drift and thickening of the volcanic pile. In the larger context of the Midcontinent Rift System, geologic, geochronologic, geophysical, and geochemical data consistently indicate that the BBC, particularly the youngest Beaver River diabase dike and sheet network, acted as a magma conduit and structural boundary to the development and infilling of the western end of the Portage Lake Volcanic basin during the main stage of rift volcanism and graben formation.

A 70‐km‐long seismic reflection profile in western Michigan provides new insight into the nature, distribution, and structure of the Keweenawan Supergroup volcanic and overlying sedimentary rocks and the controversial Keweenaw fault along the southern boundary of the Midcontinent Rift System in the Lake Superior basin. Interpretation of the 5‐s reflection data constrained by surface geology, magnetics, and gravity modeling shows that volcanic rocks which cropout north of the Keweenaw fault dip northerly to depths of the order of 17 km. South of the fault, volcanic rocks overlain by ∼2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the South Range. The volcanic pile within the basin thickens rapidly to the north of the Keweenaw fault, suggesting that the volcanics were deposted in an extensional fault‐bounded basin. Clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event. The thickness of the volcanic‐filled basin implies that the upper crust was almost completely broken during the rifting event. The available evidence is interpreted to show the Keweenaw fault as a moderate‐ to high‐angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence. There is no evidence from the seismic profiling for major faulting (except for the Keweenaw fault), intrusions, or folding of the Keweenawan Supergroup in this region.

The Minnesota River Valley subprovince of the Superior Province is an Archean gneiss terrane composed internally of four crustal blocks bounded by three zones of east-northeast-trending linear geophysical anomalies. Two of the block-bounding zones are verified regional-scale shears. The geological nature of the third boundary has not been established. Potential-field geophysical models portray the boundary zones as moderately north-dipping surfaces or thin slabs similar in strike and dip to the Morris fault segment of the Great Lakes tectonic zone at the north margin of the subprovince. The central two blocks of the subprovince (Morton and Montevideo) are predominantly high-grade quartzofeldspathic gneiss, some as old as 3.6 Ga, and late-tectonic granite. The northern and southern blocks (Benson and Jeffers, respectively) are judged to contain less gneiss than the central blocks and a larger diversity of syntectonic and late-tectonic plutons. A belt of moderately metamorphosed mafic and ultramafic rocks having some attributes of a dismembered ophiolite is partly within the boundary zone between the Morton and Montevideo blocks. This and the other block boundaries are interpreted as late Archean structures that were reactivated in the Early Proterozoic. The Minnesota River Valley subprovince is interpreted as a late accretionary addition to the Superior Province. Because it was continental crust, it was not subductible when it impinged on the convergent southern margin of the Superior Craton in late Archean time, and it may have accommodated to convergent-margin stresses by dividing into blocks and shear zones capable of independent movement.