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1. Cryogenian glacial erosion of the central Canadian Shield: The “late” Great Unconformity on thin ice

   https://doi.org/10.1130/G50315.1  

   McDannell, Kalin T.; Keller, C. Brenhin (September 2022, Geology)

   Abstract The Great Unconformity has been recognized for more than a century, but only recently have its origins become a subject of debate. Hypotheses suggest global Snowball Earth glaciations and tectonic processes associated with the supercontinent Rodinia as drivers of widespread kilometer-scale erosion in the late Neoproterozoic. We present new integrated zircon and apatite (U-Th)/He and fission-track thermochronology from Precambrian basement samples of the central Canadian Shield in northern Manitoba to test these ideas. Bayesian inverse modeling indicates that 150–200 °C of cooling (>3 km of exhumation) occurred simultaneously with Cryogenian glaciations at ca. 690–650 Ma within interior North America. This estimate for the timing of unroofing is more precise than previous appraisals and does not align with any known tectonic or magmatic events (i.e., large igneous province eruptions) potentially associated with the supercontinent cycle that occurred during the late Proterozoic along the Laurentian margins. Based on these results and interpretations, the timing and magnitude of exhumation is best explained by glacial erosion, and further establishes the importance of multiple thermochronometers for resolving detailed deeptime thermal histories. 

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2. Grooving in the midcontinent: A tectonic origin for the mysterious striations of L’Anse Bay, Michigan, USA

   https://doi.org/10.1130/GES02603.1  

   Alemu, Tadesse B; Hodgin, Eben B; Swanson-Hysell, Nicholas L (July 2023, Geosphere)

   A striated surface is present at an erosional unconformity between foliated Paleoproterozoic Michigamme Formation and fluvial conglomerate and sandstone of the Neoproterozoic Jacobsville Formation exposed at L’Anse Bay (Michigan, USA). These striations have been interpreted to be the result of ice flow in either the Proterozoic, the Pleistocene, or the modern. Recently, the glacial origin interpretation for this striated surface has been used to argue that it may be related to ca. 717–635 Ma Cryogenian snowball Earth glaciation. This interpretation would make the surface a rare example of a Neoproterozoic glacial pavement, with major chronostratigraphic implications that in turn impose constraints on the timing of intracratonic erosion related to the formation of the Great Unconformity. In this contribution, we present new observations showing that the surface is a tectonic slickenside caused by largely unconformity-parallel slip along the erosional unconformity. We document structural repetition of the Michigamme-Jacobsville contact with associated small-scale folding. The unconformity-parallel slip transitions into thrust faults that ramp up into the overlying Jacobsville Formation. We interpret that the surface records contractional deformation rather than ancient glaciation, recent ice movement, or recent mass wasting. The faulting likely occurred during the Rigolet phase of the Grenvillian orogeny, which also folded the Jacobsville Formation in the footwall of the Keweenaw fault. 

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3. Tectonic controls on basement exhumation in the southern Rocky Mountains (United States): The power of combined zircon (U-Th)/He and K-feldspar 40Ar/39Ar thermochronology

   https://doi.org/10.1130/G49141.1  

   Ricketts, Jason W.; Roiz, Jacoup; Karlstrom, Karl E.; Heizler, Matthew T.; Guenthner, William R.; Timmons, J. Michael (June 2021, Geology)

   Abstract The Great Unconformity of the Rocky Mountain region (western North America), where Precambrian crystalline basement is nonconformably overlain by Phanerozoic strata, represents the removal of as much as 1.5 b.y. of rock record during 10-km-scale basement exhumation. We evaluate the timing of exhumation of basement rocks at five locations by combining geologic data with multiple thermochronometers. 40Ar/39Ar K-feldspar multi-diffusion domain (MDD) modeling indicates regional multi-stage basement cooling from 275 to 150 °C occurred at 1250–1100 Ma and/or 1000–700 Ma. Zircon (U-Th)/He (ZHe) dates from the Rocky Mountains range from 20 to 864 Ma, and independent forward modeling of ZHe data is also most consistent with multi-stage cooling. ZHe inverse models at five locations, combined with K-feldspar MDD and sample-specific geochronologic and/or thermochronologic constraints, document multiple pulses of basement cooling from 250 °C to surface temperatures with a major regional basement exhumation event 1300–900 Ma, limited cooling in some samples during the 770–570 Ma breakup of Rodinia and/or the 717–635 Ma snowball Earth, and ca. 300 Ma Ancestral Rocky Mountains cooling. These data argue for a tectonic control on basement exhumation leading up to formation of the Precambrian-Cambrian Great Unconformity and document the formation of composite erosional surfaces developed by faulting and differential uplift. 

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4. Evidence for the Neoproterozoic Rifting of Rodinia in the Rocky Mountain Front Range

   https://doi.org/10.1029/2023TC008216  

   Murray, Kendra_E; Niemi, Nathan_A; Clark, Marin_K (January 2025, Tectonics)

   Abstract Recent advances in low‐temperature thermochronology enable the recovery of deep‐time thermal histories from Precambrian crystalline rocks shaped by multiple tectonic events, offering unprecedented opportunities to test tectonic hypotheses and links to significant biologic and climatic episodes. In particular, the late Neoproterozoic breakup of supercontinent Rodinia profoundly shaped the western margin of Laurentia, leaving a geologic record along the Cordilleran hingeline that temporally associates continental rifting with biological change at the Ediacaran‐Cambrian transition and may explain the unusual eastern extent of the Laramide orogeny. However, sedimentary evidence east of the Cordilleran hingeline is lacking, leaving postulated links untested. Here we interpret Neoproterozoic to recent tectonic histories from the Colorado Front Range using thermal history modeling of zircon (U‐Th)/He (ZHe) ages (50–607 Ma), which vary with grain U‐Th composition. These models are constrained by geologic records that place basement rocks near Earth's surface at ca. 700, 500, and 300 Ma, and they resolve late Neoproterozoic heating to 240–285°C followed by cooling. Sensitivity tests confirm this heating signal depends on fitting Mesoproterozoic⁴⁰Ar/³⁹Ar ages and a ZHe data set that includes high‐U‐Th grains with reproducible 61 ± 7.5 Ma ages that correspond to Colorado Mineral Belt magmatism and Laramide exhumation. We interpret the Neoproterozoic heating as direct evidence that intracontinental rifting in the Front Range region drove kilometer‐scale burial coeval with global glaciation and the fragmentation of Rodinia. The magnitude and duration of reheating are well constrained, but resolving subsequent cooling during Neoproterozoic‐Paleozoic time strongly depends on surface constraints from the geologic record. 

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5. Calibrating the Russøya excursion in Svalbard, Norway, and implications for Neoproterozoic chronology

   https://doi.org/10.1130/G49593.1  

   Millikin, Alexie E.G.; Strauss, Justin V.; Halverson, Galen P.; Bergmann, Kristin D.; Tosca, Nicholas J.; Rooney, Alan D. (February 2022, Geology)

   Abstract The Tonian–Ediacaran Hecla Hoek succession of Svalbard, Norway, represents one of the most complete and well-preserved Neoproterozoic sedimentary successions worldwide. With diverse fossil assemblages, an extensive carbonate δ13C record, and sedimentary evidence for two distinct Cryogenian glaciations, this succession will continue to yield insights into the Neoproterozoic Earth system; however, at present there are no direct radiometric age constraints for these strata. We present two new Re-Os ages and initial Os isotope data that constrain the timing of Neoproterozoic glaciation in Svalbard, providing further support for two globally synchronous Cryogenian glaciations and insight into pre- and post-snowball global weathering conditions. An age from the Russøya Member (Elbobreen Formation) facilitates correlation of the negative carbon isotope excursion recorded therein with the pre-glacial “Islay” excursion of the Callison Lake Formation of northwestern Canada and the Didikama and Matheos Formations of Ethiopia. We propose that this globally synchronous ca. 735 Ma carbon isotope excursion be referred to as the Russøya excursion with northeastern Svalbard as the type locality. This new age provides an opportunity to construct a time-calibrated geological framework in Svalbard to assess connections between biogeochemical cycling, evolutionary innovations within the eukaryotes, and the most extreme climatic changes in Earth history. 

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