https://doi.org/10.1130/G49116.1

Manuscript received 5 April 2021 Revised manuscript received 16 June 2021 Manuscript accepted 3 July 2021

© 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license.

Zircon (U-Th)/He thermochronology reveals pre-Great Unconformity paleotopography in the Grand Canyon region, USA B.A. Peak1, R.M. Flowers1, F.A. Macdonald2 and J.M. Cottle2 1Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA 2Earth Science Department, University of California, Santa Barbara, California 93106, USA

ABSTRACT units, which indicates that Precambrian tecto- The Great Unconformity is an iconic geologic feature that coincides with an enigmatic nism is responsible for most of the observed period of Earth’s history that spans the assembly and breakup of the supercontinent Rodinia displacement. In the LGG, the Great Uncon- and the Snowball Earth glaciations. We use zircon (U-Th)/He thermochronology (ZHe) to formity is defined by Tonto Group Tapeats explore the erosion history below the Great Unconformity at its classic Grand Canyon locality ­Sandstone overlying basement, whereas in the in Arizona, United States. ZHe dates are as old as 809 ± 25 Ma with data patterns that differ UGG, ca. 1255 Ma, Unkar Group rests on base- across both long (∼100 km) and short (tens of kilometers) spatial wavelengths. The spatially ment. It is unclear whether the Supergroup origi- variable thermal histories implied by these data are best explained by Proterozoic syn- nally extended over the LGG and was largely depositional normal faulting that induced differences in exhumation and burial across the removed by the sub-Tapeats unconformity or if region. The data, geologic relationships, and thermal history models suggest Neoproterozoic the unconformity in the LGG is a composite sur- rock exhumation and the presence of a basement paleo high at the present-day Lower face with the Tapeats capping older topography. Granite Gorge synchronous with Grand Canyon Supergroup deposition at the present-day Previous studies have suggested that the Chuar Upper Granite Gorge. The paleo high created a topographic barrier that may have limited basin was restricted in mid-Chuar time from the deposition to restricted marine or nonmarine conditions. This paleotopographic evolution proposed Tonian intracontinental seaway (e.g., reflects protracted, multiphase tectonic activity during Rodinia assembly and breakup that Dehler et al., 2017; Rooney et al., 2017). This induced multiple events that formed unconformities over hundreds of millions of years, all restriction could have been caused by paleoto- with claim to the title of a “Great Unconformity.” pography. Throughout the Grand Canyon, the Tapeats is succeeded by Paleozoic strata with an INTRODUCTION Sandstone (spanning ca. 730–520 Ma; Karlstrom Ordovician-Devonian hiatus. These units were The Great Unconformity is exposed along et al., 2020). The Lower Granite Gorge (LGG) buried by Mesozoic foreland deposits that were the length of the Grand Canyon in northwestern does not preserve the Grand Canyon Supergroup, later removed (DeCelles, 2004). Previous apatite Arizona, United States (Fig. 1) and separates which makes it unclear whether the LGG and fission-track and apatite (U-Th)/He data docu- the Cambrian Tonto Group from the underlying UGG share a common Neoproterozoic history. ment Phanerozoic burial temperatures >80 °C Paleoproterozoic basement or Mesoproterozoic- Together, these geologic relationships suggest a for river-level samples and help constrain subse- Neoproterozoic Grand Canyon Supergroup. It multiphase and possibly spatially variable his- quent erosion history (e.g., Dumitru et al., 1994; represents as much as 1.2 b.y. of missing time tory of Great Unconformity development. Here Flowers et al., 2008; Flowers and Farley, 2012; (Timmons and Karlstrom, 2012). Recent studies we present ZHe data to decipher the origin of Lee et al., 2013; Winn et al., 2017). have identified various events potentially asso- this feature in its iconic Grand Canyon exposure. ciated with the Great Unconformity erosion ZHe THERMOCHRONOLOGY surface that include >800 Ma Rodinia amal- GEOLOGIC SETTING Rocks cool as they are exhumed, and this gamation, ca. 800 Ma early Rodinia breakup, The UGG and LGG of the Grand Canyon cooling history—and by proxy, exhumation his- 717–635 Ma Cryogenian Snowball glaciations, expose 1.8–1.4 Ga basement, which remained tory—can be recorded by ZHe thermochronology and ca. 580–500 Ma late Rodinia breakup and at depths consistent with temperatures >400 °C (e.g., Reiners et al., 2002). This method exploits the Pan-African Orogeny (e.g., DeLucia et al., (∼12–15 km) until ca. 1.4 Ga (Williams and the radioactive decay of U and Th to He. At tem- 2018; Keller et al., 2019; Flowers et al., 2020). Karlstrom, 1996; Dumond et al., 2007). In the peratures >220 °C, He will diffuse completely­ Evidence of erosion during all of these periods is UGG, the Proterozoic Grand Canyon Super- out of a zircon crystal; at lower temperatures, preserved in the Grand Canyon Supergroup of the group occurs on top of basement, and the full the He will be retained. The exact temperature-­ Upper Granite Gorge (UGG; Fig. 1C); in uncon- Supergroup and Sixtymile Formation (∼3 km diffusion relationship varies due to radiation formities within the Unkar Group (>800 Ma), thick in total) are only preserved in the east- damage, which accumulates and anneals with tim disconformities between the Cardenas Basalt, ernmost part of the gorge (Fig. 1). The region as a function of temperature (Guenthner et al., Nankoweap Formation and the Chuar Group (ca. is cut by faults that offset the basement and 2013; Ginster et al., 2019). Damage is proxied by 800 Ma), and the unconformity separating the Supergroup (Timmons et al., 2005), but only effective uranium ­concentration (eU) for a zircon Chuar Group and Sixtymile Formation/Tapeats small offsets are apparent in the Phanerozoic suite that underwent the same thermal history,

CITATION: Peak, B.A., et al., 2021, Zircon (U-Th)/He thermochronology reveals pre-Great Unconformity paleotopography in the Grand Canyon region, USA: Geology, v. 49, p. XXX–XXX, https://doi.org/10.1130/G49116.1

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Figure 1. (A) Map of the Grand Canyon region (Arizona, USA) showing the extent of exposed Proterozoic basement and Neoproterozoic Grand Canyon Supergroup with sample locations marked. (B) Inset of Upper Granite Gorge. Major Proterozoic normal faults are highlighted with balls on the downthrown side, after Timmons et al. (2001) and Shoemaker et al. (1978). (C) Simplified stratigraphic column of Grand Canyon Supergroup is modified from Timmons et al. (2005) with dates from Dehler et al. (2017) and Rooney et al. (2017). (D) Schematic cross section with relative elevations along A-A′ in A. Samples are projected to section line.

or by α-dose estimates. With increasing eU, or to this study is eU zonation. (U-Th)/He dates before zonation analysis. See the Supplemental α-dose up to ∼1 × 1018, zircon becomes more He for zoned grains may differ from their unzoned Material for details. retentive, but at higher damage the He retentivity counterparts with the same bulk eU. Variability The LGG ZHe data fall on a single nega- decreases. This can cause positive and negative in zonation patterns between grains can intro- tive date-eU trend spanning 740 ± 27 Ma to date-eU correlations at low and high damage,­ duce dispersion into date-eU relationships, and 69 ± 4 Ma (Fig. 2A). There is no correlation respectively. Thermal histories to explain a these effects are magnified by small grain size between date and grain radius (Fig. S1A). Most given ZHe data set can be explored using radia- (e.g., Hourigan et al., 2005; Farley et al., 2011; zircon zonation profiles for these samples have tion damage accumulation and annealing mod- Ault and Flowers, 2012). rims enriched in parent nuclides relative to cores els for He diffusion, which can include various We acquired ZHe data for four samples each and there is limited intrasample variability in eU damage annealing kinetics (Guenthner, 2021). from the LGG and UGG (Tables S1 and S2 in the zonation patterns (Figs. S2 and S3). Other factors can affect the (U-Th)/He date and Supplemental Material). Seven of these samples ZHe data patterns vary among the UGG include α-ejection, He implantation, inclusions, are Precambrian granitoid basement collected samples (Fig. 2B). Samples CP06–52 and eU zonation, and grain size. With appropriate near river level, and one is the 729 ± 0.9 Ma UG90–2 yield low eU zircon with maximum information, some of these effects can be cor- Walcott Member Tuff near the top of the Chuar dates >700 Ma and lack obvious date-eU cor- rected for or avoided (see the Supplemental Group (Fig. 1D). To better understand the effects relations. In contrast, despite zircon with compa- Material1 for more detail). Especially important of eU zonation on ZHe dates and their inter- rably low eU, the other UGG samples (UG96–1 pretation, we obtained single U, Th, and Sm and EGC1) yield ZHe dates all <400 Ma with concentration profiles for 7–8 zircon grains one exhibiting a negative date-eU trend and the 1Supplemental Material. Analytical methods, data per basement sample using depth-profiling by other a positive trend. As with the LGG sam- tables, thermal history modeling method, and results. laser ablation–inductively coupled plasma–mass ples, there is no apparent relationship between Please visit https://doi.org/10.1130/G​ EOL.S.15078975 to access the supplemental material, and contact spectrometry (LA-ICP-MS) (Fig. S2). Zona- ZHe date and grain radius (Fig. S1B), and [email protected] with any questions. Data are tion data were not acquired for the tuff sample most zircon have rims higher in eU than cores available at https://doi​.org/10.17605/OSF.IO/D8B2Q. because all zircon of sufficient size were dated (Figs. S2 and S3). Sample UG90–2 shows high

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G49116.1/5379007/g49116.pdf by guest on 29 September 2021 slightly ­westward (Beus and Morales, 2003; Grand Canyon Supergroup is the most likely A Timmons and Karlstrom, 2012), and Mesozoic explanation for the data set. burial thickened eastward (Robinson Roberts In the Neoproterozoic, grabens and half-­ and Kirschbaum, 1995; DeCelles, 2004; Wer- grabens offsetting the basement and Supergroup nicke, 2011) but both over spatial wavelengths (Timmons et al., 2005) created ­conditions for dis- too large to explain the variability in ZHe data parate mid-late Proterozoic burial and exhuma- patterns. Instead, we suggest that variable Neo- tion histories across major faults. Fault ­systems proterozoic burial and exhumation histories in the UGG (Fig. 1B) were activated multiple across small spatial scales induced by Neo- times during the Proterozoic and culminated in proterozoic faulting during deposition of the normal faults during the ­Neoproterozoic based

B

Figure 3. Inverse time- temperature (t-T) model- Figure 2. ZHe date-eU plots for (A) Lower ing results for the Lower Granite Gorge and (B) Upper Granite Gorge. Granite Gorge (LGG) for Uncertainties are reported as 15% eU and 2s (A) the Neoproterozoic analytical ZHe date uncertainty. Where the exhumation (NeoExh) error bar is not visible, uncertainty is smaller hypothesis, and (B) the than the symbol. A Supergroup (SG) hypoth- esis. Imposed geologic constraints are marked. ­intrasample variation in its eU zonation pattern Path color corresponds (Fig. S2E), which may help explain its substan- to minimum goodness tial ZHe date-eU scatter. of fit between input data and path-predicted dates. Gray bars represent when PROTEROZOIC FAULTING AND the models allow Great DISTRIBUTION OF GRAND CANYON Unconformity erosion SUPERGROUP surface development. ZHe data for the LGG and UGG document (C) Date-eU patterns predicted by the best fit differences in basement thermal history. In the t-T paths yielded by the LGG, the Neoproterozoic dates record a por- inverse models in A and B tion of the Proterozoic time-temperature (t-T) B (black curves) or assum- path, and the consistency in the date-eU pattern ing endmember zonation across samples suggests a shared thermal his- and grain size profiles for each bin (colored curves). tory (Fig. 2A). In the UGG, data patterns differ (D) Representative zona- from those in the LGG (Fig. 2B), which implies tion profiles used in modeling for each input differences in t-T paths across the ∼100 km C ­separating these sample suites. In addition, inter- bin normalized to bin eU and radii (black) or end- sample variability in the UGG data, with spa- member values (colors). tially alternating basement samples with low eU See the Supplemental zircon that yield either Neoproterozoic results or Material (see footnote 1) much younger ZHe dates, points to more abrupt for full model details. differences in t-T paths at the tens of kilometers AFT—apatite fission- track; AHe—apatite (U-Th/ scale. Moreover, Chuar sample EGC1 is strati- He) thermochronology; graphically higher and younger (729 Ma) than UGG—Upper Granite the other samples in the UGG (1.7 Ga; Fig. 1D) Gorge. but yields post-729 Ma ZHe dates, which also suggests differing thermal histories across short spatial wavelengths. Broad uniformity in Phanerozoic sedi- D mentary thickness and resultant burial heat- ing across the region implies that the spatial differences in thermal history recorded by ZHe must date to the Proterozoic. Paleozoic sedimentary rocks across the region thicken

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G49116.1/5379007/g49116.pdf by guest on 29 September 2021 on observations such as offsets in basement and Supergroup-equivalent units to the north A of the Grand Canyon, reverse offsets within the Unkar Group units, and reconstruction of pre- Laramide extensional offsets (Shoemaker et al., 1978; Timmons et al., 2001, 2005; Beus and Morales, 2003). The Chuar Syncline and bound- Figure 4. Schematic pale- ing Butte Fault in the eastern UGG (Fig. 1B) B otopographic evolution of were active during the Tonian as documented by the Grand Canyon region stratigraphic thinning and were reactivated in the (not to scale and with late Neoproterozoic to Early Cambrian as indi- present-day, east-west coordinates). Time slices cated by incision below the Cambrian Sixtymile correspond to periods of Formation (Elston and McKee, 1982; Timmons faulting and unconfor- et al., 2001; Karlstrom et al., 2020). mity formation shown in Figure 1C: (A) Pre-Unkar In the LGG, the absence of Grand Canyon C Supergroup suggests that Proterozoic depo- Group, (B) Top Unkar Group, (C) Pre-Chuar sition may have been restricted to the UGG Group, and (D) Pre-Tonto east of the Sinyala Fault System. To test this Group. Wavy black line in hypothesis, we carried out inverse thermal D indicates approximate history simulations of the LGG data using the base level of pre-Tonto Group Great Unconfor- HeFTy software package (Ketcham, 2005) and mity erosion. the ZRDAAM model (Guenthner et al., 2013) D for two endmember t-T histories (Fig. 3): (1) the Supergroup hypothesis (SG), applying the Supergroup burial and exhumation history as preserved in the eastern UGG (Elston and McKee, 1982; Timmons et al., 2005; Dehler et al., 2017; Rooney et al., 2017; Karlstrom et al., 2018), and (2) the Neoproterozoic exhumation hypothesis (NeoExh), in which Material text and Tables S8–S12). The out- Our study outcomes are consistent with the LGG was exhumed synchronously with comes illustrate that differences in the Pro- multiphase faulting and erosion in the Grand Supergroup deposition in the UGG. Exhuma- terozoic thermal history are required to explain Canyon region contributing to Great Unconfor- tion begins at 823 ± 26 Ma and represents the the UGG data if the same Phanerozoic thermal mity development over a protracted Proterozoic likely onset of normal faulting that accommo- history is assumed (Figs. S4 and S5). This is interval. Figure 4 shows our schematic recon- dated the Chuar Group, as dated by K-Ar in consistent with Neoproterozoic fault-induced struction of the deposition, erosion, faulting, and the UGG (Elston and McKee, 1982), and is variability in Supergroup burial, as also implied paleotopographic history, with each time slice consistent with ca. 782 Ma detrital zircon in the by the ZHe data patterns and preserved geologic corresponding to a known faulting period. The base of the Chuar Group (Dehler et al., 2017). constraints. Unkar Group was deposited in a fault-bounded Phanerozoic constraints are the same in both basin at ca. 1255 Ma (Fig. 4A), and syn-dep- models. LGG samples were modeled together PROTEROZOIC PALEOTOPOGRAPHY ositional tectonic activity continued through (Table S3), and representative eU zonation AND ORIGIN OF THE GREAT ca. 1100 Ma (Fig. 4B). After Unkar deposi- profiles for each sample were used (Fig. 3D; UNCONFORMITY tion, normal faulting and erosion occurred at Table S4). The HeFTy implementation of the We interpret the different thermal histories ca. 830–800 Ma with the onset of Chuar Group widely used ZRDAAM model with fission- of the LGG and UGG and within the UGG as deposition at ca. 780 Ma (Elston and McKee, track annealing kinetics enables inclusion of caused by late Meso-Neoproterozoic fault- 1982; Dehler et al., 2017), while normal-fault zonation profile inputs, so modeling was done ing that produced paleotopography and syn- exhumation of the present-day LGG began using this approach to honor this complexity. tectonic deposition and erosion. The “Upper” simultaneously (Fig. 4C). This geometry may Model details are provided in the Supplemen- and “Lower” basins were likely separated by have isolated deposition of the Grand Canyon tal Material and Tables S3–S7. The NeoExh a paleo high bounded on either side by fault Supergroup from areas farther west and thus model yielded t-T paths with better fits to the systems as is suggested by west-dipping nor- from the proposed Tonian intracontinental data than the SG model (Fig. 3A). This remains mal faults between the LGG and UGG (Fig. 1A) seaway (Dehler et al., 2017). Tonian sedimen- true when endmember combinations of grain and supported by inverse t-T modeling. Varia- tary rocks were deposited syn-tectonically in size and observed zonation profile are used tion in thermal history among UGG samples the deepening Chuar Syncline with shallower (Fig. 3C). These outcomes imply that of the can be explained by relationships to paleoto- burial elsewhere in the UGG. The final pre- two hypotheses tested, the NeoExh model is pographical features inferred from preserved Tonto Group tectonic and erosion event occurred most consistent with the LGG ZHe data, com- geology (Fig. 1): UG96–1 was in a paleo low at ca. 520–510 Ma (Fig. 4D). This model pro- patible with the preserved Supergroup extent. in the hanging wall of the Crystal Fault (Tim- poses that the Great Unconformity in the Grand In the UGG, the spatial heterogeneity in data mons et al., 2001), where it underwent greater Canyon developed via multiple erosional events patterns suggests variability in the timing and/ Neoproterozoic burial and associated He loss driven by tectonism differing on the scale of tens or magnitude of Proterozoic burial and exhu- and now yields younger ZHe dates than sample of kilometers, which indicates that small scale mation across normal faults. To test this, we CP06–52, which was located on a Neoprotero- topography played an important role in erosion performed t-T forward and inverse models of zoic paleo high on the footwall of a normal fault and deposition during the protracted breakup of several UGG samples (see the Supplemental (Timmons et al., 2001). the Rodinian supercontinent.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G49116.1/5379007/g49116.pdf by guest on 29 September 2021 ACKNOWLEDGEMENTS Grand Canyon: Science, v. 338, p. 1616–1619, na: Geosphere, v. 9, p. 216–228, https://doi​ This work was supported by U.S. National Science https://doi​.org/10.1126/science.1229390. .org/10.1130/GES00842.1. Foundation grants EAR-1822119 and EAR-1916698 Flowers, R.M., Wernicke, B.P., and Farley, K.A., Reiners, P.W., Farley, K.A., and Hickes, H.J., 2002, He to R. Flowers and F. Macdonald and a University 2008, Unroofing, incision, and uplift history of diffusion and (U-Th)/He thermochronometry of of Colorado–Boulder Chancellor’s Fellowship to the southwestern Colorado Plateau from apatite zircon: Initial results from Fish Canyon Tuff and B. Peak. We thank Karl Karlstrom for organizing (U-Th)/He thermochronometry: Geological So- Gold Butte: Tectonophysics, v. 349, p. 297–308, Grand Canyon trips resulting in sample archives. We ciety of America Bulletin, v. 120, p. 571–587, https://doi.org/10.1016/S0040-1951(02)00058-6​ . thank Jim Metcalf for help with ZHe data acquisi- https://doi​.org/10.1130/B26231.1. Robinson Roberts, L.N., and Kirschbaum, M.A., tion, Emmy Smith for locating archived separates, and Flowers, R.M., Macdonald, F.A., Siddoway, C.S., and 1995, Paleogeography of the late Cretaceous of Mark Pecha at Arizona LaserChron (Tucson, Arizona, Havranek, R., 2020, Diachronous development the western interior of middle North America– USA) for providing reconnaissance data. We thank of Great Unconformities before Neoproterozoic Coal distribution and sediment accumulation: David Foster and two anonymous reviewers for feed- Snowball Earth: Proceedings of the National U.S. Geological Survey Professional Paper 1561, back that improved this manuscript. Academy of Sciences of the United States of https://doi​.org/10.3133/pp1561. 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