Thermochronologic constraints on the origin of the Great Unconformity

Abstract

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.

Existing thermochronologic data do not constrain Snowball glacial erosion below the Great Unconformities

“…In reality, no data examined by McDannell et al. ( 2 ) limit substantial erosion to the 717- to 635-Ma span of Snowball glaciation. We show that thermal histories corresponding to pre-Snowball (pre-717 Ma) and/or post-Snowball (post-635 Ma) exhumation of Precambrian basement, with no rock cooling/erosion during Snowball, also reasonably reproduce the data ( Fig.…”

“…The Great Unconformity is an iconic geologic feature, commonly defined by a substantial time gap between Cambrian and Precambrian units. Recent studies propose sub-Great Unconformity erosion was due to multiple tectonic and geodynamic drivers over a protracted period ( 1 ) or to synchronous glacial erosion from Snowball Earth ( 2 , 3 ). This debate also highlights conflicting views on the value of incorporating geologic knowledge into thermal history reconstructions.…”

“…McDannell et al. ( 2 ) forego geologic evidence and use inversion modeling of “the thermochronologic data alone” from four locations to argue for “rock cooling and multiple kilometers of exhumation in the Cryogenian Period in support of a glacial origin for erosion” and state that glacial erosion is “the only plausible mechanism that satisfies the required timing, magnitude, and broad spatial pattern of continental erosion.” However, their model outcomes are artifacts of their converging algorithm, thermal history priors, and output representation scheme. They attribute end-Tonian to Cambrian (850 Ma to 541 Ma) rock cooling to glacial erosion, although Snowball Earth was limited to ∼25% of this interval (717 Ma to 635 Ma); they show no data or metric to assess how well their preferred time–temperature ( t–T ) paths replicate the observations or to illustrate that they outperform alternative t–T paths; and many of their favored histories are not at surface temperatures in Cambrian time, thus violating the very geologic feature that the paths supposedly reproduce.…”

“… The thermochronologic data ( Left ) for the four locations modeled by McDannell et al. ( 2 ) do not require rock cooling/erosion during Snowball Earth ( Right ). No new data are published by ref.…”

Supplemental Material: Cryogenian glacial erosion of the central Canadian Shield: The “late” Great Unconformity on thin ice

“…The 100 ¶ C isotherm is also preferred because the cooling path midpoint is less influenced by varying the number or position of t-T points in the accepted thermal histories (i.e., inflection point biases at higher temperatures or rate changes), or from nuances that may be introduced by thermochronologic data sensitivity/resolution. Note: the Athabasca QTQt inversion with applied geologic constraints published in McDannell et al (10) shows 120-130 ¶ C of total resolved Neoproterozoic cooling. The FDHM = 644 +24 ≠19 Ma at the half-maximum 65 ¶ C isotherm within the 800-500 Ma window, whereas the symmetric mean and modal values from GMT are 648 ± 21 Ma (1 ‡) and 646 ± 20 Ma (scale), respectively.…”