Tectonic implications of metamorphism and gravity field in the Penokean Orogen of northern Michigan

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.