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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. 

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. 

Knowledge of the kinetic behavior of He in apatite and other U- and Th-bearing minerals comes largely from detailed step-heating experiments, yet such experiments are time consuming and are rarely performed during routine thermochronological studies using the U-Th/He method. We propose a new analytical method for measuring both the bulk 4He abundance and the kinetics of He release in apatite. Using this method He is extracted from samples by continuous heating using a ramped temperature schedule under static vacuum conditions, and the evolved He is measured periodically as it accumulates in the extraction system. Continuous ramped heating (CRH) experiments can be conducted using instrumentation available in most noble-gas ther- mochronology labs but require particular attention to temperature control, measurement linearity and dynamic range, and suppression of active gases co-evolved with He. CRH experiments require little more time than conventional single-step heating measurements but yield a detailed record of He release not provided by con- ventional methods. Kinetic parameters for He diffusion in Durango apatite derived from continuous heating data agree well with those obtained from published step-heating studies. The continuous record of He release ob- tained from CRH experiments also provides important information about the siting of He and the presence of multiple He components in apatite, some of which may be responsible for anomalous U-Th/He ages and high age dispersion. As such the CRH method shows promise as a useful sample screening tool for apatite U-Th/He thermochronology.