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(U-Th)/He Thermochronology Reveals Pre-Great Unconformity Paleotopography in the Grand Canyon Region, USA B.A

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(U-Th)/He Thermochronology Reveals Pre-Great Unconformity Paleotopography in the Grand Canyon Region, USA B.A 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 Geological Society of America | GEOLOGY | Volume XX | Number XX | www.gsapubs.org 1 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G49116.1/5379007/g49116.pdf by guest on 29 September 2021 A C B B D 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.
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