New thermochronometric constraints on the Tertiary landscape evolution of the central and eastern Grand Canyon, Arizona

Content Provider	Semantic Scholar
Author	Lee, Jeanette Park Ehlers, Todd A.
Copyright Year	2013
Abstract	Thermal histories are modeled from new apatite (U-Th)/He and apatite fi ssion-track data in order to quantitatively constrain the landscape evolution of the Grand Canyon region. Fifty new samples and their associated thermochronometric ages are presented here. Samples span from Lee’s Ferry in the east to Quartermaster Canyon in the west and include four age-elevation transects into Grand Canyon and borehole samples from the Coconino Plateau. Twenty-seven samples are inversely modeled to provide continuous thermal histories. This represents the most extensive and complete dataset on patterns of long-term exhumation in the Grand Canyon region, and it enables us to constrain the timing and magnitude of erosion and also discriminate between canyon incision and broader planation. The new data suggest that the early Cenozoic landscape in eastern Grand Canyon was low in relief and does not indicate the presence of an early Cenozoic precursor to the modern Grand Canyon. However, there is evidence for the incision of a smaller-scale canyon across the Kaibab Uplift at 28–20 Ma. This middle-Cenozoic denudation event was accompanied by the removal of a majority of remaining Mesozoic strata west of the Kaibab Uplift. In contrast, just upstream in the area of Lee’s Ferry, ~2 km of Mesozoic strata remained over the middle Cenozoic and were removed after 10 Ma. INTRODUCTION Near the end of Cretaceous time, the area of the eastern Grand Canyon was buried under 2–3 km of sediment shed from the Sevier and Mogollon highlands to the west and south (Elston and Young, 1991; Burchfi el et al., 1992; Dumitru et al., 1994; Flowers et al., 2008). Today, most of the Mesozoic and Cenozoic sediment has been removed and the Colorado River has incised the 1.6-km-deep Grand Canyon into the Paleozoic and Precambrian rocks of the southwestern Colorado Plateau. The earliest studies by J.W. Powell and C.E. Dutton noted the curious course of the Colorado River as it bisects topographic highs and largely ignores structural relief, yet appears to have integrated along its present course since 6 Ma (Hunt, 1956, 1969; McKee et al., 1967). Because of erosion, the lack of a Cenozoic rock record has hampered the descriptions of fl uvial systems and landscape morphology that are critical to identifying the scenarios and processes that carved the Grand Canyon; for this reason the canyon has remained an enigmatic and controversial landscape feature since the fi rst scientifi c description more than 100 years ago. Given that numerical constraints on landscape evolution are essential for resolving the controversial origin of the Grand Canyon, the goal of this study is to present new low-temperature thermochronometric constraints on landscape development utilizing both apatite fi ssion track (AFT) and (U-Th)/He (AHe) data. This new synthesis helps provide substantial resolution to recent debates about the age of the Grand Canyon (Pederson, 2008; Polyak et al., 2008; Karlstrom et al., 2008; Flowers et al., 2008; Wernicke, 2011; Luchitta et al., 2011). BACKGROUND The modern Colorado River fl ows westward across the southwestern margin of the Colorado Plateau. However, paleocurrent direction indicators and provenance studies in the early to middle Cenozoic gravels that fi ll the older paleocanyons indicate a regional northeast-fl owing drainage system (Potochnik, 2001; Young, 2001), as shown in Figure 1. Rim gravels from the surrounding plateau margin contain Precambrian basement clasts shed from the eroding source terrain of the Cretaceous Mogollon highlands to the west and south (Bilodeau, 1986; Elston and Young, 1991; Burchfi el et al., 1992). These rim gravels are the few remaining sedimentary relicts of a denudation event that is well documented from earlier thermochronometer studies (Dumitru et al., 1994; Kelley et al., 2001; Flowers et al., 2008) and supported by our results here. Flowers et al. (2008) presented extensive thermochronometric data from the region that indicate late Mesozoic to middle Cenozoic denudation and outline the broad southwest to northeast erosional patterns of the southwestern Colorado Plateau. However, the lack of high-density data sets that target areas of high topographic relief results in limited insight when deciphering the distinct exhumation histories across the length of the Grand Canyon area. Most workers agree that the Colorado River system as it is today did not exit the Colorado Plateau until slightly before 5.5 Ma (Lucchitta , 2003; Lucchitta and Richard, 2001; Pederson, 2008, Karlstrom et al., 2012). The western Grand Canyon took its shape after emplacement of the Late Miocene Shivwits basalt (Lucchitta and Richard, 2001) and after deposition of the 12–6 Ma Muddy Creek Formation and Hualapai Limestone (Castor and Faulds, 2001; Faulds For permission to copy, contact editing@geosociety.org © 2013 Geological Society of America Geosphere; April 2013; v. 9; no. 2; p. 216–228; doi:10.1130/GES00842.1; 9 fi gures; 2 tables; 3 supplemental fi les. Received 4 July 2012 ♦ Revision received 18 December 2012 ♦ Accepted 20 December 2012 ♦ Published online 5 February 2013 *Emails: Lee: jplee@usgs.gov; Stockli: stockli@ jsg.utexas.edu; Kelley: sakelley@nmbg.nmt.edu; Pederson: joel.pederson@usu.edu; Karlstrom: kek1@ unm.edu; Ehlers: todd.ehlers@uni-tuebingen.de CRevolution 2: Origin and Evolution of the Colorado River System II themed issue Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/2/216/3342695/216.pdf by guest on 08 December 2018 Thermochronometric constraints on the Tertiary landscape evolution of Grand Canyon Geosphere, April 2013 217 et al., 2001; Spencer et al., 2001). Following introduction of the Colorado River to the Grand Wash trough, eventual integration quickly followed multiple spillover events in the Basin and Range to a lowered base level in the Gulf of California that had been opening since the latest Miocene (McDougall et al., 1999; Oskin and Stock, 2003). Sediments from the Colorado Plateau fi rst reached the Gulf of California ca. 5.4 Ma (Dorsey et al., 2005), indicating that the Colorado River system as we defi ne it today had fi nally achieved its modern course by that time. However, a few studies have suggested the Colorado River (or a precursor river) may have exited the Colorado Plateau at the present location much earlier (Hill and Ranney, 2008; Polyak et al., 2008); Wernicke (2011) hypothesized that the Grand Canyon is actually Late Cretaceous in age and was carved into the nowabsent Mesozoic strata by one of the northeastfl owing drainages that are well documented on the southwest Colorado Plateau boundary. The data used to provide evidence for the Late Cretaceous paleocanyon of the California River hinges on the thermal modeling of a few AHe data samples. Luchitta et al. (2011) presented evidence for Middle Miocene drainage to the east of the Grand Canyon. This proposed drainage system fl owed to the southwest, possibly crossing the Kaibab Uplift through a paleocanyon. This is consistent with other work (Flowers et al., 2008; Pederson, 2008) and new evidence (in the following) for broad middle Cenozoic planation of the region and a more modest and somewhat younger paleocanyon dissecting the Kaibab Uplift.
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