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The origin of the phenomenon known as the Great Unconformity has been a fundamental yet unresolved problem in the geosciences for over a century. Recent hypotheses advocate either global continental exhumation averaging 3 to 5 km during Cryogenian (717 to 635 Ma) snowball Earth glaciations or, alternatively, diachronous episodic exhumation throughout the Neoproterozoic (1,000 to 540 Ma) due to plate tectonic reorganization from supercontinent assembly and breakup. To test these hypotheses, the temporal patterns of Neoproterozoic thermal histories were evaluated for four North American locations using previously published medium- to low-temperature thermochronology and geologic information. We present inverse time–temperature simulations within a Bayesian modeling framework that record a consistent signal of relatively rapid, high-magnitude cooling of ∼120 to 200 ° C interpreted as erosional exhumation of upper crustal basement during the Cryogenian. These models imply widespread, synchronous cooling consistent with at least ∼3 to 5 km of unroofing during snowball Earth glaciations, but also demonstrate that plate tectonic drivers, with the potential to cause both exhumation and burial, may have significantly influenced the thermal history in regions that were undergoing deformation concomitant with glaciation. In the cratonic interior, however, glaciation remains the only plausible mechanism that satisfies the required timing, magnitude, and broad spatial pattern of continental erosion revealed by our thermochronological inversions. To obtain a full picture of the extent and synchroneity of such erosional exhumation, studies on stable cratonic crust below the Great Unconformity must be repeated on all continents. 

Abstract. Diffusion properties of cosmogenic 3He in quartz at Earth surface temperatures offer the potential to directly reconstruct the evolution of pastin situ temperatures from formerly glaciated areas, which is important information for improving our understanding of glacier–climateinteractions. In this study, we apply cosmogenic 3He paleothermometry to rock surfaces gradually exposed from the Last Glacial Maximum(LGM) to the Holocene period along two deglaciation profiles in the European Alps (Mont Blanc and Aar massifs). Laboratory experiments conducted onone representative sample per site indicate significant differences in 3He diffusion kinetics between the two sites, with quasi-linearArrhenius behavior observed in quartz from the Mont Blanc site and complex Arrhenius behavior observed in quartz from the Aar site, which weinterpret to indicate the presence of multiple diffusion domains (MDD). Assuming the same diffusion kinetics apply to all quartz samples along eachprofile, forward model simulations indicate that the cosmogenic 3He abundance in all the investigated samples should be at equilibrium withpresent-day temperature conditions. However, measured cosmogenic 3He concentrations in samples exposed since before the Holocene indicate anapparent 3He thermal signal significantly colder than today. This observed 3He thermal signal cannot be explained with a realisticpost-LGM mean annual temperature evolution in the European Alps at the study sites. One hypothesis is that the diffusion kinetics and MDD modelapplied may not provide sufficiently accurate, quantitative paleo-temperature estimates in these samples; thus, while a pre-Holocene 3Hethermal signal is indeed preserved in the quartz, the helium diffusivity would be lower at Alpine surface temperatures than our diffusion modelspredict. Alternatively, if the modeled helium diffusion kinetics is accurate, the observed 3He abundances may reflect a complexgeomorphic and/or paleoclimatic evolution, with much more recent ground temperature changes associated with the degradation of alpine permafrost. 

The spatial and temporal distribution of Alpine snow is a sensitive gauge of environmental change. While understanding past snow dynamics is essential for reconstructing past climate and forecasting future trends, reliable snowpack data prior to the instrumental record are scarce. We present a novel pairing of cosmogenic paleothermometry and luminescence photochronometry which constrain the temperature and insolation history of bedrock outcrops at the Gotthard Pass, Switzerland, over the last ∼15,000 years. By coupling these results with cosmogenic¹⁴C‐¹⁰Be chronology and modern in situ rock thermometry, we infer a ∼70‐day reduction of snowpack at the topographic mid‐slope. Our data indicate stable environmental conditions throughout the Holocene, followed by a 6.6 ± 2.9°C increase of ground surface temperature, coeval with an order‐of‐magnitude or more increase in ground surface insolation. Bracketing the onset of these changes between 1504 and 1807 CE, our findings tie the snowpack decline with the onset of human industrialization.

 

Abstract The provocative hypothesis that the Shinumo Sandstone in the depths of Grand Canyon was the source for clasts of orthoquartzite in conglomerate of the Sespe Formation of coastal California, if verified, would indicate that a major river system flowed southwest from the Colorado Plateau to the Pacific Ocean prior to opening of the Gulf of California, and would imply that Grand Canyon had been carved to within a few hundred meters of its modern depth at the time of this drainage connection. The proposed Eocene Shinumo-Sespe connection, however, is not supported by detrital zircon nor paleomagnetic-inclination data and is refuted by thermochronology that shows that the Shinumo Sandstone of eastern Grand Canyon was >60 °C (∼1.8 km deep) and hence not incised at this time. A proposed 20 Ma (Miocene) Shinumo-Sespe drainage connection based on clasts in the Sespe Formation is also refuted. We point out numerous caveats and non-unique interpretations of paleomagnetic data from clasts. Further, our detrital zircon analysis requires diverse sources for Sespe clasts, with better statistical matches for the four “most-Shinumo-like” Sespe clasts with quartzites of the Big Bear Group and Ontario Ridge metasedimentary succession of the Transverse Ranges, Horse Thief Springs Formation from Death Valley, and Troy Quartzite of central Arizona. Diverse thermochronologic and geologic data also refute a Miocene river pathway through western Grand Canyon and Grand Wash trough. Thus, Sespe clasts do not require a drainage connection from Grand Canyon or the Colorado Plateau and provide no constraints for the history of carving of Grand Canyon. Instead, abundant evidence refutes the “old” (70–17 Ma) Grand Canyon models and supports a <6 Ma Grand Canyon. 

Fe‐ and Mn‐oxides are common secondary minerals in faults, fractures, and veins and potentially record information about the timing of fluid movement through their host rocks. These phases are difficult to date by most radioisotopic techniques, but relatively high concentrations of U and Th make the (U‐Th)/He system a promising approach. We present new petrographic, geochronologic and thermochronologic analyses of secondary oxides and associated minerals from fault zones and fractures in southeastern Arizona. We use these phases in attempt to constrain the timing of fluid flow and their relationship to magmatic, tectonic, or other regional processes. In the shallowly exhumed Galiuro Mountains, Fe‐oxide (U‐Th)/He dates correspond to host‐rock crystallization and magmatic intrusions from ca. 1.6 to 1.1 Ga. Step‐heating⁴He/³He experiments and polydomain diffusion modeling of³He release spectra on these samples are consistent with a crystallite size control on He diffusivity, and little fractional loss of radiogenic He since formation in coarse‐grained hematite, but large losses from fine‐grained Mn‐oxide. In contrast to Proterozoic dates, Fe‐ and Mn‐oxides from the Catalina‐Rincon and Pinaleño metamorphic core complexes are exclusively Cenozoic, with dates clustering at ca. 24, 15, and 9 Ma, which represent distinct cooling or fluid‐flow episodes during punctuated periods of normal faulting. Finally, a subset of Fe‐oxides yield dates of ca. 5 Ma to 6 ka and display either pseudomorphic cubic forms consistent with oxidative retrogression of original pyrite or magnetite, or fine‐grained botryoidal morphologies that we interpret to represent approximate ages of recrystallization or pseudomorphic replacement at shallow depths.

 

Using offshore detrital apatite (U‐Th)/He thermochronometry and 3D thermo‐kinematic modeling of the catchment topography, we constrain the timing of major topographic change at Bourgeois Fjord, Antarctic Peninsula (AP). While many mid‐latitude glacial landscapes developed primarily in response to global cooling over the last ~2.6 Ma, we find that kilometer‐scale landscape evolution at Bourgeois Fjord began ~30–12 Ma ago and <2 km of valley incision has occurred since ~16 Ma. This early onset of major topographic change occurred following the initiation of alpine glaciation at this location and prior to the development of a regional polythermal ice sheet inferred from sedimentary evidence offshore of the AP. We hypothesize that topographic change relates to (i) feedbacks between an evolving topography and glacial erosion processes, (ii) effects of glacial‐interglacial variability, and (iii) the prevalence of subglacial meltwater. The timing and inferred spatial patterns of long‐term exhumation at Bourgeois Fjord are consistent with a hypothesis that glacial erosion processes were suppressed at the AP during global Plio‐Pleistocene cooling, rather than enhanced. Our study examines the long‐term consequences of glacial processes on catchment‐wide erosion as the local climate cooled. Our findings support the hypothesis that landscapes at different latitudes had different responses to global cooling. Our results also suggest that erosion is enhanced along the plateau flanks of Bourgeois Fjord today, which may be due to periglacial processes or mantling via subglacial till. If regional warming persists and meltwater becomes more pronounced, we predict that enhanced erosion along the plateau flank will accelerate topographic change.