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Combined analysis of drill-hole, gravity, and magnetic data indicates that the buried Precambrian basement rocks of the Dakotas can be divided into several lithotectonic terranes. Eastern North Dakota and northeastern South Dakota are underlain by Archean gneiss. Except for the Black Hills region of South Dakota, where Archean rocks are also exposed, the western third of both Dakotas is underlain mainly by Early Proterozoic gneiss and metasedimentary rocks. Part of this region is underlain by Archean crust with an Early Proterozoic tectonic overprint. A broad transition zone of strongly overprinted Archean crust occurs between the Proterozoic rocks to the west and the Archean rocks to the east. South central South Dakota is underlain by an Early Proterozoic batholith. Early Proterozoic felsic volcanic rocks occur in southeast South Dakota. The bootheel portion of South Dakota contains a diverse assemblage of basement rocks that are partly Archean in age.Churchill Province rocks of the Trans-Hudson foldbelt project into the western Dakotas. The Thompson nickel belt and the Pickwitonei gneiss belt correlate with the western and eastern halves, respectively, of the transition between Archean and Proterozoic crust, and the Archean Glennie – Hanson Lake microcontinent of the Churchill Province likely extends into western North Dakota. Archean rocks of Minnesota extend into the eastern Dakotas, and the Wyoming craton extends to the Black Hills region. The Cheyenne foldbelt projects into southwest South Dakota. The Penokean foldbelt of Michigan and Wisconsin does not extend into the Dakotas, but it most likely extends into northwest Iowa.Tectonic evolution of the Early Proterozoic terrane in the Dakotas was most likely similar to plate tectonic models for the evolution of the Trans-Hudson foldbelt in the Churchill Province. As in the Churchill Province, the western Dakotas are underlain by Early Proterozoic rocks, but it is not known whether these rocks formed as a result of rifting and subsequent closure of a once extensive Archean crust or as a result of collision of once widely separated blocks of Archean crust.

Published geologic data, together with new metamorphic pressure and temperature data and gravity data, provide strong evidence for northward directed thrusting of Lower Proterozoic rocks in northern Michigan and Wisconsin. Metamorphic studies indicate pressures of 7 kbar at Watersmeet node and 4 kbar at the Peavy node. The Republic metamorphic node, located 30 km to the north of Peavy, attained pressures of 2–3 kbar. All three metamorphic highs reached temperatures in the range of 600°–650°C and are correlated with areas of negative gravity anomalies relative to surrounding regions. In the Peavy high the center of the gravity anomaly lies on the sillimanite isograd, and we suggest that the source of this negative anomaly may be remobilized underlying Archean gneiss. The highest grade rocks of the Republic node lie well within a residual gravity anomaly, which is interpreted to have a very shallow source, and the Watersmeet high lies above gneiss domes which show negative gravity anomalies. A north‐south geologic cross section constructed from a two‐dimensional gravity model, which extends northward from the Dunbar dome in the magmatic arc terrane of Wisconsin, across a terrane boundary interpreted as a suture and marked by the Niagara fault, and across the Peavy metamorphic node, suggests that the Dunbar dome was transported northward onto the continental foreland during the Penokean orogenic event. These north verging fold‐thrust sheets or nappes produced prominent tectonic thickening at the Peavy and Watersmeet, as indicated by the relatively high metamorphic pressures there. The thrust sheets evidently thinned to the north, as suggested by the relatively low pressures at Republic. We suggest that the Archean continental edge, defined by the steep gradient in the regional northeast trending gravity high, lies beneath the magmatic rocks of Wisconsin and thus extends south of the Niagara fault suture.

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