Title: Thermochronologic constraints on the origin of the Great Unconformity
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. more »« less
McDannell, Kalin T.; Keller, C. Brenhin(
, Geology)
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.
Flowers, Rebecca M.; Macdonald, Francis A.; Siddoway, Christine S.; Havranek, Rachel(
, Proceedings of the National Academy of Sciences)
The Great Unconformity marks a major gap in the continental geological record, separating Precambrian basement from Phanerozoic sedimentary rocks. However, the timing, magnitude, spatial heterogeneity, and causes of the erosional event(s) and/or depositional hiatus that lead to its development are unknown. We present field relationships from the 1.07-Ga Pikes Peak batholith in Colorado that constrain the position of Cryogenian and Cambrian paleosurfaces below the Great Unconformity. Tavakaiv sandstone injectites with an age of ≥676 ± 26 Ma cut Pikes Peak granite. Injection of quartzose sediment in bulbous bodies indicates near-surface conditions during emplacement. Fractured, weathered wall rock around Tavakaiv bodies and intensely altered basement fragments within unweathered injectites imply still earlier regolith development. These observations provide evidence that the granite was exhumed and resided at the surface prior to sand injection, likely before the 717-Ma Sturtian glaciation for the climate appropriate for regolith formation over an extensive region of the paleolandscape. The 510-Ma Sawatch sandstone directly overlies Tavakaiv-injected Pikes granite and drapes over core stones in Pikes regolith, consistent with limited erosion between 717 and 510 Ma. Zircon (U-Th)/He dates for basement below the Great Unconformity are 975 to 46 Ma and are consistent with exhumation by 717 Ma. Our results provide evidence that most erosion below the Great Unconformity in Colorado occurred before the first Neoproterozoic Snowball Earth and therefore cannot be a product of glacial erosion. We propose that multiple Great Unconformities developed diachronously and represent regional tectonic features rather than a synchronous global phenomenon.
Ricketts, Jason W.; Roiz, Jacoup; Karlstrom, Karl E.; Heizler, Matthew T.; Guenthner, William R.; Timmons, J. Michael(
, Geology)
Abstract The Great Unconformity of the Rocky Mountain region (western North America), where Precambrian crystalline basement is nonconformably overlain by Phanerozoic strata, represents the removal of as much as 1.5 b.y. of rock record during 10-km-scale basement exhumation. We evaluate the timing of exhumation of basement rocks at five locations by combining geologic data with multiple thermochronometers. 40Ar/39Ar K-feldspar multi-diffusion domain (MDD) modeling indicates regional multi-stage basement cooling from 275 to 150 °C occurred at 1250–1100 Ma and/or 1000–700 Ma. Zircon (U-Th)/He (ZHe) dates from the Rocky Mountains range from 20 to 864 Ma, and independent forward modeling of ZHe data is also most consistent with multi-stage cooling. ZHe inverse models at five locations, combined with K-feldspar MDD and sample-specific geochronologic and/or thermochronologic constraints, document multiple pulses of basement cooling from 250 °C to surface temperatures with a major regional basement exhumation event 1300–900 Ma, limited cooling in some samples during the 770–570 Ma breakup of Rodinia and/or the 717–635 Ma snowball Earth, and ca. 300 Ma Ancestral Rocky Mountains cooling. These data argue for a tectonic control on basement exhumation leading up to formation of the Precambrian-Cambrian Great Unconformity and document the formation of composite erosional surfaces developed by faulting and differential uplift.
Interactions between the solid Earth and climate system represent a frontier area for geoscientific research that is strongly emphasized in the International Ocean Discovery Program (IODP) Science Plan. The continental margin of India adjoining the Arabian Sea offers a unique opportunity to understand tectonic-climatic interactions and the net impact of these on weathering and erosion of the Himalaya. Scientific drilling in the Arabian Sea is designed to understand the coevolution of mountain building, weathering, erosion, and climate over a range of timescales. The southwest monsoon is one of the most intense climatic phenomena on Earth. Its long-term development has been linked to the growth of high topography in South and Central Asia. Conversely, the tectonic evolution of the Himalaya, especially the exhumation of the Greater Himalaya, has been linked to intensification of the summer monsoon rains, as well as to plate tectonic forces. Weathering of the Himalaya has also been linked to long-term drawdown of atmospheric CO2 during the Cenozoic, culminating in the onset of Northern Hemisphere glaciation. No other part of the world has such intense links between tectonic and climatic processes. Unfortunately, these hypotheses remain largely untested because of limited information on the history of erosion and weathering recorded in the resultant sedimentary prisms. This type of data cannot be found on shore because the proximal foreland basin records are disrupted by major unconformities, and depositional ages are difficult to determine with high precision. We therefore propose to recover longer records of erosion and weathering from the Indus Fan that will allow us to understand links between paleoceanographic processes and the climatic history of the region. The latter was partially addressed by Ocean Drilling Program (ODP) Leg 117 on the Oman margin, and further studies are proposed during IODP Expedition 353 (Indian Monsoon Rainfall) that will core several sites in the Bay of Bengal. Such records can be correlated to structural geological and thermochronology data in the Himalaya and Tibetan Plateau to estimate how sediment fluxes and exhumation rates change through time. The drilling will be accomplished within a regional seismic stratigraphic framework and will for the first time permit an estimation of sediment budgets together with quantitative estimates of weathering fluxes and their variation through time. Specific goals of this expedition include
1. Testing whether the timing of the exhumation of Greater Himalaya correlates with an enhanced erosional flux and stronger chemical weathering after ~23 Ma,
2. Determining the amplitude and direction of the environmental change at 8 Ma, and
3. Dating the age of the base of the fan and the underlying basement to constrain the timing of India-Asia collision.
Drilling through the fan base and into the underlying basement in the proposed area will permit additional constraints to be placed on the nature of the crust in the Laxmi Basin (Eastern Arabian Sea), which has a significant bearing on paleogeographic reconstructions along conjugate margins in the Arabian Sea and models of continental breakup on rifted volcanic margins.
Thurston, Olivia G.; Guenthner, William R.; Karlstrom, Karl E.; Ricketts, Jason W.; Heizler, Matthew T.; Timmons, J. Michael(
, Geology)
Abstract Our study used zircon (U-Th)/He (ZHe) thermochronology to resolve cooling events of Precambrian basement below the Great Unconformity surface in the eastern Grand Canyon, United States. We combined new ZHe data with previous thermochronometric results to model the <250 °C thermal history of Precambrian basement over the past >1 Ga. Inverse models of ZHe date-effective uranium (eU) concentration, a relative measure of radiation damage that influences closure temperature, utilize He diffusion and damage annealing and suggest that the main phase of Precambrian cooling to <200 °C was between 1300 and 1250 Ma. This result agrees with mica and potassium feldspar 40Ar/39Ar thermochronology showing rapid post–1400 Ma cooling, and both are consistent with the 1255 Ma depositional age for the Unkar Group. At the young end of the timescale, our data and models are also highly sensitive to late-stage reheating due to burial beneath ∼3–4 km of Phanerozoic strata prior to ca. 60 Ma; models that best match observed date-eU trends show maximum temperatures of 140–160 °C, in agreement with apatite (U-Th)/He and fission-track data. Inverse models also support multi-stage Cenozoic cooling, with post–20 Ma cooling from ∼80 to 20 °C reflecting partial carving of the eastern Grand Canyon, and late rapid cooling indicated by 3–7 Ma ZHe dates over a wide range of high eU. Our ZHe data resolve major basement exhumation below the Great Unconformity during the Mesoproterozoic (1300–1250 Ma), and “young” (20–0 Ma) carving of Grand Canyon, but show little sensitivity to Neoproterozoic and Cambrian basement unroofing components of the composite Great Unconformity.
McDannell, Kalin T., Keller, C. Brenhin, Guenthner, William R., Zeitler, Peter K., and Shuster, David L. Thermochronologic constraints on the origin of the Great Unconformity. Retrieved from https://par.nsf.gov/biblio/10329119. Proceedings of the National Academy of Sciences 119.5 Web. doi:10.1073/pnas.2118682119.
McDannell, Kalin T., Keller, C. Brenhin, Guenthner, William R., Zeitler, Peter K., & Shuster, David L. Thermochronologic constraints on the origin of the Great Unconformity. Proceedings of the National Academy of Sciences, 119 (5). Retrieved from https://par.nsf.gov/biblio/10329119. https://doi.org/10.1073/pnas.2118682119
McDannell, Kalin T., Keller, C. Brenhin, Guenthner, William R., Zeitler, Peter K., and Shuster, David L.
"Thermochronologic constraints on the origin of the Great Unconformity". Proceedings of the National Academy of Sciences 119 (5). Country unknown/Code not available. https://doi.org/10.1073/pnas.2118682119.https://par.nsf.gov/biblio/10329119.
@article{osti_10329119,
place = {Country unknown/Code not available},
title = {Thermochronologic constraints on the origin of the Great Unconformity},
url = {https://par.nsf.gov/biblio/10329119},
DOI = {10.1073/pnas.2118682119},
abstractNote = {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.},
journal = {Proceedings of the National Academy of Sciences},
volume = {119},
number = {5},
author = {McDannell, Kalin T. and Keller, C. Brenhin and Guenthner, William R. and Zeitler, Peter K. and Shuster, David L.},
}
Warning: Leaving National Science Foundation Website
You are now leaving the National Science Foundation website to go to a non-government website.
Website:
NSF takes no responsibility for and exercises no control over the views expressed or the accuracy of
the information contained on this site. Also be aware that NSF's privacy policy does not apply to this site.
