McDannell et al. (1) raise questions about our zircon (U–Th)/He (ZHe) data (2). Several of the same authors argued Cryogenian glacial origin of the Great Unconformity (GUn) in two articles and a letter in PNAS (3–5). Here, we respond to McDannell et al. (1), and explain why thermal overprinting does not obscure the tectonic origin of the GUn.
Previous thermochronologic studies were limited to Laurentia (e.g., refs. 3, 6, and 7), and not global in scope as McDannell et al. (1) claimed. The GUn within continental interiors is not only marked by more than one billion years of missing history but also by an average basement exhumation of ∼25 km (2). Therefore, understanding the origin of the GUn must account for data from higher-temperature systems, rather than rely solely on low-temperature thermochronology.
Basement samples of Zhan et al. (2) were collected from outcrops across a vast region. Thermal history modeling results suggest later thermal overprinting varies from one region to another (2). The reproducibility of ∼1.3-Gy-old single grain dates suggests that samples Qingshuihe and Luotuoshan (2) were not heated through the ZHe partial retention zone (140–200 °C) since the middle Mesoproterozoic. Therefore, not all zircon grains have been fully reset by Phanerozoic reheating. Precambrian ZHe dates recorded in North China include middle Mesoproterozoic to Tonian and Ediacaran (2). If glacial erosion removed at least ∼3 to 5 km of material during “snowball Earth” events (3, 4), why is there a complete absence of Cryogenian ages?
McDannell et al. (1) used different input t–T paths to calculate predicted ZHe dates as function of effective uranium concentration using currently used helium diffusion kinetics. However, none of the input t–T paths is geologically valid. Input paths to near-surface at 1,600 Ma are not valid for three samples from cratonic interior, because sampled basement is overlain by early Cambrian strata (2), rather than by late Paleoproterozoic. Similarly, paths close to the surface at ∼520 Ma are not valid for sample Luotuoshan near the cratonic margin, because there is no stratigraphic evidence for Cambrian overlap. Therefore, it is not surprising to see a relatively large mismatch between observed and model-predicted ages.
Deep-time thermochronology based solely on the ZHe method has proven challenging owing to more complexity in the ZHe system than assumed in currently used kinetics (8, 9), ongoing debate over whether and how geologic data should be used in modeling (5, 10, 11), and inevitable thermal overprinting (1). McDannell et al. (1, 3) criticized the use of constraint boxes. However, the aim of the modeling is to derive meaningful interpretations, rather than to force the model to a desired solution.
No one doubts the role of ice sheets in eroding their substrate. Glacial deposits are full of eroded material. A huge amount of erosion clearly takes place in some circumstances. Glacially eroded topography is in one case nearly 1 km (late Cryogenian in Mineral Fork, Utah). Even so, glacial erosion was still eclipsed in magnitude by tectonically induced exhumation in driving the GUn.
Acknowledgments
Author contributions
L.D. designed research; L.D. and R.-R.Z. performed research; L.D., N.C.-B., M.Z., R.-R.Z., Z.Y., B.W., V.O., and X.Z. analyzed data; and L.D. wrote the paper.
Competing interests
The authors declare no competing interest.
Data, Materials, and Software Availability
Study data are included in the article.
References
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Data Availability Statement
Study data are included in the article.