New geologic and geochronologic data clearly establish that the two Archean basement-rock terranes recognized previously in Minnesota also occur in northern Wisconsin and Michigan, on the east side of the midcontinent gravity high. Greenstone-granite complexes (about 2,700 m.y. old) that are remarkably similar to those in the greenstone terrane of northern Minnesota are present on the south side of the Gogebic Range in northern Wisconsin and adjacent Michigan and in the northern complex of the Marquette district. Migmatitic gneisses and amphibolite, which are similar to the high-grade rocks in the gneiss terrane in southern Minnesota, are sporadically exposed south of the greenstone terrane and appear to compose the basement in most of northern and central Wisconsin and northern Michigan. The gneisses in Wisconsin and Michigan have been dated at two widely separated localities; they have radiometric ages of more than 3,000 m.y. and, for one rock type, an age of approximately 3,500 m.y. The boundary between the greenstone and gneiss terranes is covered by younger supracrustal rocks, but is interpreted to trend approximately eastward across northern Wisconsin to the vicinity of Marquette, Michigan, and beyond. Subsequent to its welding, probably 2,600 to 2,700 m.y. ago, abundant mafic dikes of Precambrian X and Y ages were emplaced in a wide zone parallel to the boundary, which indicates that it was predominantly a zone of extensional tectonics throughout much of post-Archean–pre-Phanerozoic time.