One Billion Years of Stability in the North American Midcontinent Following Two-Stage Grenvillian Structural Inversion

Abstract

The North American craton interior preserves a >1 Ga history of near surface processes that inform ongoing debates regarding timing and drivers of continental-scale deformation and erosion associated with far-field orogenesis. We tested various models of structural inversion on a major segment of the Midcontinent Rift along the Douglas Fault (DF) in northern Wisconsin, which accommodated ≳10 km of total vertical displacement. U-Pb detrital zircon and vein calcite Δ₄₇/U-Pb thermochronometry from the hanging wall constrain the majority of uplift (≳8.5 km) and deformation to 1052–1036 Ma during the Ottawan phase of the Grenvillian orogeny. Combined U-Pb zircon dates, Δ₄₇/U-Pb calcite thermochronometry, and field data that document syn- to early post-depositional deformation in the footwall constrain a second stage of uplift (1–1.5 km) ca. 995–980 Ma during the Rigolet phase of the Grenvillian orogeny. A minor phase of Appalachian far-field orogenesis is associated with minimal thrust reactivation. Our combined analyses identified the 995–980 Ma Bayfield Group as a Grenvillian foreland basin with an original thickness 0.5–2 km greater than currently preserved. By quantifying flexural loading and other subsidence mechanisms along the Douglas Fault, we identify dynamic subsidence as a mechanism that could be consistent with the development of late-Grenvillian transcontinental fluvial systems. Minimal post-Grenvillian erosion (0.5–2 km) in this part of the craton interior has preserved the Bayfield Group and equivalent successions, limiting the magnitude of regional erosion that can be attributed to Neoproterozoic glaciation.