Thermochronologic constraints on the origin of the Great Unconformity

Kalin T. McDannell, C. Brenhin Keller, William R. Guenthner, Peter K. Zeitler, David L. Shuster

EarthArXiv preprint, 2021:


The Great Unconformity represents a gap of hundreds of millions to over a billion years in the geologic record. The cause of this missing time has eluded explanation, yet recently two opposing hypotheses claim either a glacial or plate tectonic origin in the Neoproterozoic. We provide thermochronologic evidence of rock cooling and multiple kilometers of exhumation in the Cryogenian Period in support of a glacial origin for the composite basement nonconformity found across the North American interior. The broad synchronicity of this cooling signal at the continental scale can only be readily explained by glacial denudation.

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 of more than 3–4 km during Cryogenian (717–635 Ma) snowball Earth glaciations, or alternatively, diachronous episodic exhumation throughout the Neoproterozoic (1000–540 Ma) due to plate tectonic reorganization from supercontinent Rodinia assembly and breakup. To test these hypotheses, the temporal pattern 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 modelling framework that record a consistent signal of relatively rapid, high magnitude cooling of ~120–200°C interpreted as erosional exhumation of upper crustal basement during the Cryogenian. These models imply widespread, synchronous cooling consistent with at least ~3–5 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.

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