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REPORT

Age and Carving of Grand Canyon:

Toward a Resolution of 150 Years of Debate

14–21 September 2019 | Grand Canyon, Arizona, USA

CONVENERS aquifer system shows mixing of meteoric, karst, and CO -rich
2
Karl E. Karlstrom, University of New Mexico, Albuquerque, “lower world” waters, and complex water pathways (LC). Grand
New Mexico 87131, USA; kek1@unm.edu Canyon National Park, in its 100th anniversary and beyond, is
Laura J. Crossey, University of New Mexico, Albuquerque, working to establish a better baseline for understanding water
New Mexico 87131, USA quantity and quality as well as complex groundwater flow paths
Eugene Humphreys, University of Oregon, Eugene, Oregon in order to continue to provide water for the park’s more than
97403, USA six-million annual visitors (JC).
David Shuster, University California Berkeley, Berkeley, Regional uplift of the Rocky Mountain–Colorado Plateau
California 94720, USA region took place in three episodes (Laramide, mid-Tertiary, and
Kelin Whipple, Arizona State University, Tempe, Arizona 85287, USA past 10 Ma), each related to a phase of carving of paleocanyons,
Ryan Crow, U.S. Geological Survey, 2255 N. Gemini Road, but the relative uplift amounts have yet to be well quantified;
Flagstaff, Arizona 86001, USA ~thirds for each is one estimate (KK, GH). These lithospheric
modifications started with flat slab subduction of the buoyant con-
PARTICIPANTS jugate of the Shatsky Rise (GH). Middle Tertiary removal of the
Andres Aslan (AA); Thorsten Becker (TB); Whitney Behr slab initiated the ignimbrite flare-up volcanism with uplift and
(WB); Jeanne Calhoun (JC); Kristen Cook (KC); Laurie Crossey heating consequences that still need to be deconvolved (PR).
(LC); Ryan Crow (RC); Andy Darling (AD); Rebecca Dorsey Young and ongoing uplift of the Rockies relative to the Colorado
(RD); Madison Douglas (MD); Claudio Faccenna (CF); Anke Plateau (EK, AA) and of western Colorado Plateau relative to sea
Friedrich (AF); Arjun Heimsath (AH); Gene Humphreys (GH); level (KK, RC) may be driven by mantle convection at global
Karl Karlstrom (KK); Eric Kirby (EK); Alan Levander (AL); scales (DR) but is probably dominated by changes in lithospheric
M. Qasim Mahmood (MQM); Juliet McKenna (JM); Peter density structure (GH), for example, by lithosphere delamination
Reiners (PR); Marisa Repasch (MR); David Rowley (DR); Leah and asthenospheric return flow (AL) that is driving inboard migra-
Sabbeth (LS); Taylor Schildgen (TS); David Shuster (DS); Peter tion of basaltic volcanism (RC). Mantle xenoliths were seen near
van der Beek (PvdB); Brian Wernicke (BW); Kelin Whipple (KW).* Lava Falls that show deformation features and give pressure and
temperature information about mantle tectonism (WB).
This GSA Thompson Field Forum involved 28 researchers and Geodynamics of uplift involved both isostatic and dynamic
young scientists who discussed the evidence for the age, geomor- forces. We used the term “dynamic topography” for the compo-
phic evolution, and incision history of Grand Canyon. The field nents of topography not explained by crustal isostasy (DR). The
forum was centered around an eight-day, 280-mile field confer- western U.S. upper mantle contains very large gradients in seismic
ence by raft through the Grand Canyon. Each of the participants velocity that likely reflect marked buoyancy variations that affect
gave a field seminar presentation. topography. Estimates of dynamic topography for the region in
Water in Grand Canyon’s dissected aquifer system includes recent papers range from several kilometers (TB) to near zero.
the Colorado River plus indigenous groundwaters that emerge as Empirical estimates of differential uplift over the past 5–10 Ma of
springs. John Wesley Powell, in an 1893 address to irrigation ~1 km are based on differential incision studies of rivers, which is
advocates in Los Angeles, foresaw: “… a heritage of conflict and observed where rivers cross sharp mantle velocity gradients (KK,
litigation over water rights for there is not enough water to supply RC, AA, EK).
the land.” As we face a hotter and drier future, our task is to figure Age of the Colorado River: The oldest known deposits of a
out how to make do with less water and more people while pre- major river draining the western Rockies are the 11 Ma gravels
serving our quality of life, lands, groundwater resources, and below the Grand Mesa basalt (AA). Downward integration of the
groundwater-dependent ecosystems (JM). Grand Canyon’s incised system is suggested by onset of rapid cooling near Rifle, Colorado,

GSA Today, v. 30, no. 2, https://doi.org/10.1130/GSAT-TFF-GC.1.
*Participant initials are referenced throughout this report based on contributions to discussions during the forum.

8 GSA Today | February 2020