GEOPHYSICAL MAPPING OF THE GREAT LAKES TECTONIC ZONE IN THE CENTRAL UPPER PENINSULA, MICHIGAN

The Great Lakes Tectonic Zone (GLTZ) forms the boundary between the Wawa-Abitibi subprovince (north side) and Minnesota River Valley subprovince (south side) within the Archean Superior Province. The GLTZ is concealed for all of its 1100 km length, except near Marquette in the central Upper Peninsula of Michigan. There, it is exposed as a NW-striking, 2.3 km wide mylonite zone along a strike length of about 11 km, with a mylonitic foliation that dips steeply to the SW. The location of the GLTZ has been unknown to the east, where it is concealed by Paleozoic sedimentary rocks. Here, we use legacy aeromagnetic data in combination with modern and recently collected aeromagnetic data and ground gravity data to develop new interpretations of the geophysical expression of the GLTZ, to define its eastward extent under cover, and to identify other nearby covered tectonic elements. Numerous discontinuous, NW-striking aeromagnetic gradients observed over the mylonite zone are interpreted to be produced by structurally juxtaposed rocks with varying magnetizations, relations observed locally in outcrops. Mapping of similar gradients across the region shows that they are widely distributed, but irregularly concentrated in greatest numbers within 3 km of the center of the GLTZ. Gravity data show a steep regional gradient along the GLTZ trend, likely produced by the juxtaposition of a dense greenstone belt on the north against lower-density gneisses and granites on the south. Using the distribution of aeromagnetic gradients, broader aeromagnetic patterns, and the regional gravity gradient, the GLTZ is interpreted to extend about 55 km under cover to the east, where it changes strike to E-W and NE. The GLTZ may have influenced the formation of structural features with similar strikes nearby, including an undated (Paleoproterozoic?) dike swarm, and a possible concealed, elongated trough of variably magnetic and dense Paleoproterozoic strata. Geophysical patterns along the GLTZ are terminated on the east by broad aeromagnetic and gravity highs produced by volcanic rocks of the buried eastern arm of the 1.1 Ga Midcontinent Rift. The rift’s intersection with the GLTZ is the location of a change in the rift margin’s strike, from crudely N-S north of the GLTZ to NW south of the GLTZ.