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

Abstract

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.

Thick-skinned, south-verging backthrusting in the Felch and Calumet troughs area of the Penokean Orogen, northern Michigan

“…In this report we point out that although the Bush Lake fault is a major north-verging structure, consistent with the overall sense of northward tectonic transport of the Penokean orogen in northern Michigan and adjacent Wisconsin (Sims and others, 1985;Klasner, Ojakangas, and others, 1988;Klasner, Sims, and others, 1988;Attoh and Klasner, 1989;Klasner and Cannon, 1989;and Klasner and others, 1991), the structures in the Felch and Calumet troughs area are south-verging (Klasner and Sims, 1989). The south-verging structures, which deform Archean basement as well as rocks of the Early Proterozoic Marquette Range Supergroup, are interpreted as out-of-sequence backthrusts and backfolds that developed to accommodate abrupt changes in crustal thickness along the continental margin caused by thrusting.…”

“…Klasner (1972) and Bayley (1959) showed that the metamorphism mostly postdates DI deformation, and Klasner (1978) showed that metamorphism peaked during D2 deformation. Through combined metamorphic, structural, and gravity modeling studies, Attoh and Klasner (1989) suggested that the high-grade metamorphism was caused by tectonic stacking during thrusting in the Penokean orogen.…”

Tectonic imbrication and foredeep development in the Penokean orogen, east-central Minnesota: An interpretation based on regional geophysics and the results of test-drilling. The Penokean orogeny in Minnesota and upper Michigan; a comparison of structural geology

“…In the last decade, however, a plate collisional environment has become a principal feature of tectonic models proposed for the Penokean orogeny, both for the segment in Minnesota (Hoist, 1984a(Hoist, , 1984b(Hoist, , 1984cMorey and Southwick, 1984;Southwick and others, 1988;Southwick and Morey, this volume, ch. C) and the segment in Wisconsin and Upper Michigan Cambray, 1977Cambray, , 1978Larue, 1983;LaBerge and others, 1984;Schulz, 1983;Schulz and others, 1984;Sims and others, 1987;Klasner and Attoh, 1986;Attoh and Klasner, 1989).…”

“…The boundary between these two assemblages is the Niagara fault zone. A number of tectonic models identifying plate collision (some, multiple collisions) in this area cite the Niagara fault zone as the collision boundary or suture zone Cambray, 1977Cambray, , 1978Larue, 1983;LaBerge and others, 1984;Sims and others, 1985;Klasner and Attoh, 1986;Klasner, Ojakangas, and others, 1988;Klasner, Sims, and others, 1988;Attoh and Klasner, 1989).…”

Petrography and geochemistry of early Proterozoic granitoid rocks in Wisconsin magmatic terranes of Penokean Orogen, northern Wisconsin

“…Early Proterozoic deformation involved north-directed thrusting and folding of the supracrustal rocks and, at least locaJly, the Archean basement rocks (Gregg, I993;Klasner and Sims, 1993) on the continental margin. Crustal thickening resulted in metamorphism and development of local basement gneiss domes (Sims and Peterman, 1983;Klasner and others, I988;Holm and others, 1988;Attoh and Klasner, 1989).…”