Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Tectonics
The cores of continents or cratons host the planet’s oldest rocks and a rich record of plate tectonic events that can be poorly preserved on active continental margins. A portion of the North American craton was dissected by a large rift system, the Midcontinent Rift, that was active in the late Mesoproterozoic, or ~1.1 billion years ago. Cessation of extension and the development of rift basins and subsequent inversion – or the reversal of basin subsidence due to compressional tectonic forces – are linked to mountain building at the edge of North America. The long-lived Grenvillian orogeny involved continent-continent collision at a scale that rivals the Alpine-Himalayan mountain belt. Connecting these seemingly disparate but significant tectonic events in Earth’s history is challenging because the timing of inversion and the magnitude of uplift of Midcontinent rift basins are poorly constrained.
Hodgin et al. [2024] address these outstanding questions with a holistic approach: they present and integrate detailed field-based observations, sedimentology, structural geology, radiogenic and stable isotope geochemistry, and outcomes from numerical models. They target the Douglas fault and adjacent sedimentary rocks in northern Wisconsin that are inferred to accommodate structural inversion of the Midcontinent Rift and deposition in the foreland of the Grenvillian orogeny, respectively.
At the core of their study are emergent geochronologic approaches to reconstruct this deep-time tectonic history. The authors use a tandem geochronology approach that combines high- throughput but low-precision laser ablation inductively coupled plasma mass spectrometry to identify the youngest detrital zircons in their target samples. They then use chemical abrasion isotope dilution thermal ionization mass spectrometry to precisely date these specific grains thereby fingerprinting maximum depositional ages of key sedimentary rocks. The authors pair U-Pb geochronology and clumped isotope thermochronology to constrain the timing and depth of formation of mineral veins along the Douglas fault that signal fault activity.
Integrated data track the timing and magnitude of two episodes of uplift during structural inversion of the rift system linked to specific phases of the Grenvillian orogeny. A far-field collision event approximately 1.05 billion years ago resulted in greater than 8.5 kilometers of uplift associated with thrusting on the Douglas fault, followed by a second phase of minor uplift approximately 0.98 billion years ago during ongoing Grenvillian orogenesis. These events produced a large sedimentary basin in the foreland of the mountains produced by the continent-continent collision.
The authors suggest that following structural inversion of the rift system and creation of a vast foreland basin, this part of the Midcontinent experienced nearly a billion years of continental stability. Encapsulated within these billion years is a significant gap in geologic time or the rock record known as the Great Unconformity. The origins of this iconic geologic feature found throughout North America are widely debated. The authors argue their observations preclude significant erosion of the Midcontinent during Neoproterozoic global glaciations or subsequent activity on the Douglas fault system. Interpretations of continental stability assume that the preserved geologic record fully captures the history of continental erosion, which can be further interrogated with low-temperature thermochronology.
Citation: Hodgin, E. B., Swanson-Hysell, N. L., Kylander-Clark, A. R. C., Turner, A. C., Stolper, D. A., Ibarra, D. E., et al. (2024). One billion years of stability in the North American Midcontinent following two-stage Grenvillian structural inversion. Tectonics, 43, e2024TC008415. https://doi.org/10.1029/2024TC008415
—Alexis Ault, Associate Editor, Tectonics
