The last phase of the Midcontinent Rift system was characterized by a transformation of original graben bounding normal faults into reverse faults. The Douglas Fault of northern Wisconsin and Keweenaw Fault of Upper Michigan are now low- to high-angle reverse faults, but they were originally major graben-bounding growth faults (Cannon and others, 1989). Reverse displacement along the Keweenaw Fault resulted in steepening of volcanic and sedimentary units. Faults, fractures, and broad open folds within rift-filling strata developed in response to tectonic adjustment of the subsiding volcaniclastic section and to the compressional event (Butler and Burbank, 1929;White, 1968). Reset Rb-Sr biotite ages within older Precambrian basement rocks near the Michigan-Wisconsin border suggest that the high-angle reverse faulting occurred about 1060+/20 Ma (Cannon and others, 1993b), although it may have started as early as 1080 Ma and was likely completed by about 1040 Ma (Cannon, 1994). The timing and probable cause of this compressional event is continental collision within the Grenville Province (Hoffman, 1989; Cannon and Hinze, 1992; Cannon, 1994).