Lindley Hanson's List: NP:Grand Canyon

Geology; November 2008; v. 36; no. 11; p. 835-838; DOI: 10.1130/G25032A.1 2008 Geological Society of America

Model for tectonically driven incision of the younger than 6 Ma Grand Canyon
Karl E. Karlstrom1,*, Ryan Crow1, L.J. Crossey1, D. Coblentz2 and J. W. Van Wijk2

1 1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, 87131, USA
2 2Geodynamics Group, Los Alamos National Laboratory, Los Alamos, New Mexico, 87545, USA

Correspondence: *E-mail: kek1@unm.edu.

Accurate models for the incision of the Grand Canyon must include characterization of tectonic influences on incision dynamics such as active faulting and mantle to surface fluid interconnections. These young tectonic features support other geologic data that indicate that the Grand Canyon has been carved in the past 6 Ma. New U-Pb dates on speleothems are reinterpreted here in terms of improved geologic constraints and understanding of the modern aquifer. The combined data suggest that Grand Canyon incision rates have been relatively steady since 3–4 Ma. Differences in rates in the eastern (175–250 m/Ma) and western (50–80 m/Ma) Grand Canyon are explained by Neogene fault block uplift across the Toroweap-Hurricane system. Mantle tomography shows an abrupt step in mantle velocities near the Colorado Plateau edge, and geodynamic modeling suggests that upwelling asthenosphere is driving uplift of the Colorado Plateau margin relative to the Basin and Range. Our model for dynamic surface uplift in the past 6 Ma contrasts with the notion of passive incision of the Grand Canyon due solely to river integration and geomorphic response to base-level fall.

Geology; August 2002; v. 30; no. 8; p. 739-742; DOI: 10.1130/0091-7613(2002)030<0739:DIOTGC>2.0.CO;2
© 2002 Geological Society of America

Differential incision of the Grand Canyon related to Quaternary faulting—Constraints from U-series and Ar/Ar dating
Joel Pederson1, Karl Karlstrom2, Warren Sharp3 and William McIntosh4

1 Department of Geology, Utah State University, Logan, Utah 84322, USA
2 Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
3 Berkeley Geochronology Center, Berkeley, California 94709, USA
4 New Mexico Institute of Mining and Geology, Socorro, New Mexico 87801, USA

Incision of the Colorado River in the Grand Canyon, widely thought to have happened between ca. 6 and 1.2 Ma, has continued at variable rates along the canyon over the past ~500 k.y., based on measurements of bedrock incision combined with U-series and 40Ar/39Ar ages. River incision rates downstream of the Toroweap fault in the western Grand Canyon are about half the ~140 m/m.y. incision rate calculated for a distance of at least 200 km upstream of the fault. We hypothesize that this differential incision is due to west-down slip on the Toroweap fault of 94 ± 6 m/m.y. based on measured offset of the newly dated Upper Prospect basalt flow, which is the major middle-late Quaternary slip evident along the river. Regional incision has been driven mostly by base-level fall related to drainage reversal off the Colorado Plateau ca. 6 Ma. Because local normal faulting is lower in rate than this regional incision and is likely an expression of Basin and Range extension and subsidence rather than uplift, this is a case where active faulting diminishes, but does not drive, incision. Quaternary incision rates are insufficient to have carved the Grand Canyon in 6 m.y., suggesting either that rates have decreased through time as the original base-level signal has attenuated, or that some component of the canyon relief we see today existed prior to Colorado River integration.

Key Words: faulting • geochrono

Geology; June 2003; v. 31; no. 6; p. e16-e17; DOI: 10.1130/0091-7613(2003)312.0.CO;2
© 2003 Geological Society of America

Differential incision of the Grand Canyon related to Quaternary faulting—Constraints from U-series and Ar/Ar dating: Comment and Reply
Thomas C. Hanks1 and J. Luke Blair1

1 U.S. Geological Survey, MS 977, Menlo Park, California 94025, USA

The first 20% of the full text of this article appears below.

Pederson et al. (2002) present bedrock incision rates of the Colorado River at three sites in the Grand Canyon. From west to east, these sites and rates (averaged over the past 300 to 500 k.y.) are Granite Park, 72–92 m/m.y.; Toroweap, 133 ± 16 m/m.y.; and Eastern Canyon, 135 ± 17 to 144 ± 18 m/m.y. Granite Park is located ~10 fault-normal km west of the Hurricane fault, which in turn is located ~15 km west of the Toroweap fault. Toroweap is located just east of the Toroweap fault, where it crosses the Colorado River, while Eastern Canyon is ~110 km east of the Toroweap fault, at the mouth of the Little Colorado River (Pederson et al., 2002, their Figs. 1 and 2). The Hurricane and Toroweap faults are down-to-the-west normal faults, with mid to late Quaternary slip rates of 70–170 and 70–180 m/m.y., respectively (Fenton et al., 2001), which Pederson et al. (2002) propose as the cause of the east-west difference in bedrock incision rates across these faults. We agree that these faults can affect the nearby Granite Park and Toroweap incision

How old is the Grand Canyon? Writing in the journal Science, a group of scientists describes using features found in caves in the canyon's walls to try to put a date on the natural wonder. The researchers studied structures known as mammillaries or “cave clouds” — carbonate deposits that form at or near the water table level, and dated the features using uranium-lead isotope dating techniques.

The new research suggests that the formation of the Grand Canyon, when the first cuts began to form in the rock, began at least 17 million years ago, making the canyon significantly older than previously thought. The canyon channel is thought to have been completely cut through by 5 or 6 million years ago.

The new study is controversial, with several geologists speaking out vocally about the study's methods and its findings. In this segment, Ira talks with one of the scientists behind the new dating study about the team's work and findings.