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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. Tectonic controls on Quaternary landscape evolution in the Ventura basin, southern California, USA, quantified using cosmogenic isotopes and topographic analyses

   https://doi.org/10.1130/B36076.1  

   Hughes, A.; Rood, D.H.; DeVecchio, D.E.; Whittaker, A.C.; Bell, R.E.; Wilcken, K.M.; Corbett, L.B.; Bierman, P.R.; Swanson, B.J.; Rockwell, T.K. (January 2022, GSA Bulletin)

   Abstract The quantification of rates for the competing forces of tectonic uplift and erosion has important implications for understanding topographic evolution. Here, we quantify the complex interplay between tectonic uplift, topographic development, and erosion recorded in the hanging walls of several active reverse faults in the Ventura basin, southern California, USA. We use cosmogenic 26Al/10Be isochron burial dating and 10Be surface exposure dating to construct a basin-wide geochronology, which includes burial dating of the Saugus Formation: an important, but poorly dated, regional Quaternary strain marker. Our ages for the top of the exposed Saugus Formation range from 0.36 +0.18/-0.22 Ma to 1.06 +0.23/-0.26 Ma, and our burial ages near the base of shallow marine deposits, which underlie the Saugus Formation, increase eastward from 0.60 +0.05/-0.06 Ma to 3.30 +0.30/-0.41 Ma. Our geochronology is used to calculate rapid long-term reverse fault slip rates of 8.6–12.6 mm yr–1 since ca. 1.0 Ma for the San Cayetano fault and 1.3–3.0 mm yr–1 since ca. 1.0 Ma for the Oak Ridge fault, which are both broadly consistent with contemporary reverse slip rates derived from mechanical models driven by global positioning system (GPS) data. We also calculate terrestrial cosmogenic nuclide (TCN)-derived, catchment-averaged erosion rates that range from 0.05–1.14 mm yr–1 and discuss the applicability of TCN-derived, catchment-averaged erosion rates in rapidly uplifting, landslide-prone landscapes. We compare patterns in erosion rates and tectonic rates to fluvial response times and geomorphic landscape parameters to show that in young, rapidly uplifting mountain belts, catchments may attain a quasi-steady-state on timescales of <105 years even if catchment-averaged erosion rates are still adjusting to tectonic forcing. 

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3. Rates of bedrock canyon incision by megafloods, Channeled Scabland, eastern Washington, USA

   https://doi.org/10.1130/B36975.1  

   Lehnigk, Karin E; Larsen, Isaac J; Lamb, Michael P; David, Scott R (April 2024, Geological Society of America Bulletin)

   Abstract Pleistocene outburst floods from the drainage of glacial Lake Missoula carved bedrock canyons into the Columbia Plateau in eastern Washington, USA, forming the Channeled Scabland. However, rates of bedrock incision by outburst floods are largely unconstrained, which hinders the ability to link flood hydrology with landscape evolution in the Channeled Scabland and other flood-carved landscapes. We used long profiles of hanging tributaries to reconstruct the pre-flood topography of the two largest Channeled Scabland canyons, upper Grand Coulee and Moses Coulee, and a smaller flood-eroded channel, Wilson Creek. The topographic reconstruction indicates floods eroded 67.8 km3, 14.5 km3, and 1.6 km3 of rock from upper Grand Coulee, Moses Coulee, and Wilson Creek, respectively, which corresponds to an average incision depth of 169 m, 56 m, and 10 m in each flood route. We simulated flood discharge over the reconstructed, pre-flood topography and found that high-water evidence was emplaced in each of these channels by flow discharges of 3.1 × 106 m3 s−1, 0.65–0.9 × 106 m3 s−1, and 0.65–0.9 × 106 m3 s−1, respectively. These discharges are a fraction of those predicted under the assumption that post-flood topography was filled to high-water marks for Grand and Moses Coulees. However, both methods yield similar results for Wilson Creek, where there was less erosion. Sediment transport rates based on these discharges imply that the largest canyons could have formed in only about six or fewer floods, based on the time required to transport the eroded rock from each canyon, with associated rates of knickpoint propagation on the order of several km per day. Overall, our results indicate that a small number of outburst floods, with discharges much lower than commonly assumed, can cause extensive erosion and canyon formation in fractured bedrock. 

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4. Geodynamic and Climatic Forcing on Late‐Cenozoic Exhumation of the Southern Patagonian Andes (Fitz Roy and Torres del Paine massifs)

   https://doi.org/10.1029/2023TC007914  

   Muller, Veleda_A_P; Sue, Christian; Valla, Pierre_G; Sternai, Pietro; Simon‐Labric, Thibaud; Gautheron, Cécile; Cuffey, Kurt_M; Grujic, Djordje; Bernet, Matthias; Martinod, Joseph; et al (July 2024, Tectonics)

   Abstract High‐relief glacial valleys shape the modern topography of the Southern Patagonian Andes, but their formation remains poorly understood. Two Miocene plutonic complexes in the Andean retroarc, the Fitz Roy (49°S) and Torres del Paine (51°S) massifs, were emplaced between 16.9–16.4 Ma and 12.6–12.4 Ma, respectively. Subduction of oceanic ridge segments initiated ca. 16 Ma at 54°S, leading to northward opening of a slab window with associated mantle upwelling. The onset of major glaciations caused drastic topographic changes since ca. 7 Ma. To constrain the respective contributions of tectonic‐mantle dynamics and fluvio‐glacial erosion to rock exhumation and landscape evolution, we perform inverse thermal modeling of a new data set of zircon and apatite (U‐Th)/He from the two massifs, complemented by apatite⁴He/³He data for Torres del Paine. Our results show rapid rock exhumation recorded only in the Fitz Roy massif between 10 and 8 Ma, which we ascribe to local mantle upwelling forcing surface uplift and intensified erosion around 49°S. Both massifs record a pulse of rock exhumation between 7 and 4 Ma, which we interpret as enhanced erosion during the beginning of Patagonian glaciations. After a period of erosional and tectonic quiescence in the Pliocene, increased rock exhumation since 3–2 Ma is interpreted as the result of alpine glacial valley carving promoted by reinforced glacial‐interglacial cycles. This study highlights that glacial erosion was the main driver to rock exhumation in the Patagonian retroarc since 7 Ma, but that mantle upwelling might be a driving force to rock exhumation as well. 

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5. The timing of Missoula floods: Implications for the age of Grand Coulee (eastern Washington, USA)

   https://doi.org/10.1130/G52505.1  

   Lehnigk, Karin E; Larsen, Isaac J; Quirk, Brendon J; David, Scott R (September 2024, Geology)

   Abstract The Channeled Scabland of eastern Washington (USA) was formed by outburst floods from glacial Lake Missoula. Despite chronological advances, the timing of erosion in the main flood channels is unresolved. In particular, it is still uncertain whether upper Grand Coulee, the largest canyon in the Channeled Scabland, was incised during or prior to the last glaciation. We report 10Be exposure ages from erratics in upper Grand Coulee, glacial Lake Columbia, and surrounding flood routes. Flood-transported boulders on the high-elevation east rim of Grand Coulee date to ca. 17–15 ka. Ages from boulders on the floor of Grand Coulee indicate later flooding at ca. 14 ka, which post-dated canyon incision and occurred after inundation of the Telford-Crab Creek scabland at ca. 15–14.5 ka. Prior hydraulic modeling and dating suggest the entrance to Grand Coulee was blocked by rock and that canyon incision was incomplete at ca. 17 ka; hence, we interpret the 17–15 ka exposure ages on the east rim to coincide with flow over a retreating cataract during canyon incision. Our results indicate incision of Grand Coulee was completed between 17 ka and 14 ka. The short duration of canyon incision suggests that glacial Lake Missoula generated some of the most erosive outburst floods in Earth's history. 

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