Title: Pushing Back the Age of the Grand Canyon
Author: Rebecca M . Flowers, University of Colorado, Boulder
Publication: The Outcrop, September 2009, p. 8, 12-13
When and why the Grand Canyon was carved has puzzled scientists for nearly 150 years (Figure 1). The popular view is that downcutting of the Colorado River and associated incision of the canyon was a consequence of the rise of the Colorado Plateau to its current mean elevation of ~2 km at ca. 6 Ma. However, numerous ideas have been proposed for Grand Canyon evolution, and this has been a hotly debated topic (see Ranney, 2005 for overview).
A new study suggests that the Grand Canyon may have initially formed ~50 million years earlier than generally thought (Flowers et al., 2008 ). Rebecca Flowers, Brian Wernicke and Kenneth Farley at the California Institute of Technology carried out this work; Flowers is now on the faculty at the University of Colorado, Boulder.
This effort used the (U-Th)/He thermochronology method to date apatite crystals in Paleozoic and Proterozoic rocks of the southwestern Colorado Plateau into which the Grand Canyon was carved. Apatite grains contain uranium and thorium that radioactively decay to helium. Helium is completely lost from the apatite crystal by diffusion until the mineral cools below temperatures of ~70° C, at which point helium starts being retained in the crystal. After making assumptions about the geothermal gradient, dating apatite by the (U-Th)/He method can be used to decipher the cooling of rocks as they are unroofed through the upper 1-3 kilometers of crust and thereby constrain regional erosion histories (Wolf et al., 1998). Lateral age variations detected by low temperature thermochronometry can also be used to detect paleotopography because of the manner in which shallow level isotherms mimic the shape of the Earth’s surface (e.g. House et al., 1998). Such information on paleo-relief also imposes a minimum constraint on past surface elevations. “Unroofing” is referred to here as the thickness of rock removed through erosion or tectonism, and “surface uplift” or “uplift” as the increase in surface elevation.
This study used (U-Th)/He thermochronology on apatite crystals in 36 samples from the bottom of the Grand Canyon and across the surface of the southwestern Colorado Plateau (Figure 2) with the goal of resolving the patterns, timing, and relationships between the regional unroofing history, Grand Canyon incision, and plateau surface uplift. The data document a complex history of overall southwest to northeast unroofing from plateau margin to plateau interior, including multiple erosional phases in Late Cretaceous/Early Tertiary (80 to 55 Ma), mid-Tertiary (28 to 16 Ma), and Late Tertiary (<6 Ma) times. A key finding is that thermal history simulations of apatite dates from the bottom of the eastern Grand Canyon and the plateau surface nearby suggest that they were characterized by similar Early to mid-Tertiary thermal histories, despite their ~1500 m of stratigraphic separation. These models imply that a kilometer-scale paleo-Grand Canyon was carved in post-Paleozoic sediments in the eastern Grand Canyon during early Tertiary time. If substantial relief were not present, then a temperature difference of ~35°C between canyon bottom and plateau surface samples would be expected assuming a geothermal gradient of 25°C/km.
The new results suggest that a significant Early Tertiary gorge in the eastern third of the Grand Canyon was subsequently lowered into the current canyon configuration. In other words, if you stand on the rim of the Grand Canyon today, the bottom of the ancestral gorge would have been in sedimentary rocks above your head that were later eroded away. This ancient canyon is unlikely to have had the exact shape, course and dimensions as the modern canyon, but rather may have been modified and exploited during integration of the modern Colorado River to etch out the current 450-km long canyon system. Earlier work on the deeply incised Peach Springs, Milkweed, and Salt River paleocanons along the southwestern margin of the Colorado Plateau was used to support the idea of an extensive paleo-drainage system that included portions of a paleo-Grand Canyon (Potochnik, 2001; Young, 2001). The new data fit the idea that rivers in the plateau margin paleocanyons may have been tributary to an ancestral Colorado River in an Early Tertiary paleo-Grand Canyon.
The new study also bears on the controversial rise of the Colorado Plateau. Evidence for significant Early Tertiary topographic relief would require substantial surface uplift of this portion of the Colorado Plateau by Early Tertiary time, pointing toward buoyancy sources associated with the proposed development of the Laramide flat slab as the initial cause of plateau elevation gain. These results are incompatible with uplift of the entire Colorado Plateau from sea level in late Tertiary time.
Other recent studies using new analytical tools are yielding complementary insights into the longstanding controversy over Grand Canyon evolution. Bedrock incision rates constrained by 40Ar/39Ar dating of young canyon basalts indicate that Quaternary downcutting rates are inadequate to carve the entire canyon in 6 Ma, and exploitation of an ancestral eastern canyon could explain the remaining discrepancy (Karlstrom et al., 2007). Development of a significant gorge in the western Grand Canyon by 17 Ma was suggested based on U-b dates for cave deposits inferred to resolve the lowering of the water table during canyon incision (Polyak et al., 2008). This proposed phase of western canyon incision, although controversial, (Pederson et al., 2008; Pearthree et al., 2008), would fall near the end o the second regional unroofing episode suggested by the (U-Th)/He results of Flowers et al. (2008). The intense debate over the history of the Grand Canyon may be partly attributable to independent evolution of different portions of the canyon system that were subsequently merged into the modern configuration during integration of the present-day Colorado River. These new efforts that push back the age of the Grand Canyon and constrain the history of different canyon segments may ultimately resolve the evolution of one of the most dramatic and well-recognized features on Earth.
Flowers, R.M., Wernicke, B.P., and Farley, K.A., 2008, Unroofing, incision and uplift history of the southwestern Colorado Plateau from apatite (U-Th)/He thermochronometry: Geological Society of America Bulletin, v. 120, p. 571-587.
House, M.A., Wernicke, B.P., and Farley, K.A., 1998, Dating topography of the Sierra Nevada, California, using apatite (U-Th)/He ages: Nature, v. 396, p. 66-69.
Karlstrom, K.E., Crow, R.S., Peters, L., McIntosh, W., Raucci, J., Crossey, L.J., Umhoefer, P., and Dunbar, N., 2007, 40Ar/39Ar and field studies of Quaternary basalts in Grand Canyon and model for carving Grand Canyon: quantifying the interaction of river incision and normal faulting across the western edge of the Colorado Plateau: Geological Society of America Bulletin, v. 119, p. 1283-1312.
Pearthree, P.A., Spencer, J.E., Fauld, J.E., and House, P.K., 2008, “Age and evolution of the Grand Canyon revealed by U-Pb dating of water table-type speleothems”: Science, v. 321, p. 1634c.
Pederson, J., Young, R., Lucchitta, I., Beard, L.S., and Billingsley, G., 2008, Comment on “Age and evolution of the Grand Canyon revealed by U-Pb dating of water table-type speleothems “: Science, v. 321, p. 1634b.
Polyak, V., Hill, C., and Asmerom, Y., 2008, Age ad evolution of the Grand Canyon revealed by U-Pb dating of water tabletype speleothems: Science, v. 319, p. 1377-1380.
Potochnik, A. R., 2001, Paleogeomorphic evolution of the Salt River Region: Implications for Cretaceous-Laramide inheritance for ancestral Colorado River drainage, in Young, R. A. a. S., E.E., ed., Colorado River Origin and Evolution, Grand Canyon Association, p. 17-24.
Ranney, W., 2005, Carving Grand Canyon: Evidence, theories, and mystery: Grand Canyon, Arizona, Grand Canyon Association, 160 p.
Wolf, R.A., Farley, K.A., and Kass, D.M., 1998, Modeling of the temperature sensitivity of the apatite (U-Th)/He thermochronometer: Chemical Geology, V. 148, p. 105-114.
Young, R.A., 2001, The Laramide-Paleogene history of the western Grand Canyon region: Setting the stage, in Young, R. A. a. S., E.E., ed., Colorado River Origin and Evolution, Grand Canyon Association, p. 7-16.