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Group picture at the Little Colorado River. Photo by Laurie Crossey. Front row, from left: Marisa Repasch; M. Qasim Mahmood; Taylor Schildgen; Andy
Darling; Arjun Heimsath; Karl Karlstrom; Laurie Crossey; Peter Reiners; Juliet McKenna. Standing, from left: Thorsten Becker; Kristen Cook; Kelin Whipple;
Jeanne Calhoun; Whitney Behr; Eric Kirby; Andres Aslan; David Rowley; Gene Humphreys; Alan Levander; Peter van der Beek; Madison Douglas; David
Shuster; Ryan Crow; Leah Sabbeth; Anke Friedrich; Brian Wernicke; Becky Dorsey; and Claudio Faccenna.

(MWX well) at 6–8 Ma, before the Colorado River was integrated Canyon is a young (post–5 Ma) canyon segment based on thermo-
through Grand Canyon (EK). The Green River was integrated with chronology. Eastern Grand Canyon may have been partially
the Colorado between 8 and 2 Ma, but the lack of terraces older than carved 25–15 Ma by a paleo–Little Colorado River. Muav Gorge
2–3 Ma and steady incision documented by detrital sanidine dating “looks young” like Marble Canyon but has little incision rate data.
data suggest a young 2–3 Ma Green River integration (AA). New A 65–50 Ma north-flowing Hualapai paleoriver (Music Mountain
detrital sanidine dating combined with magnetostratigraphy show Formation) and Hindu paleovalley have been long recognized;
that the oldest Colorado River sediment was first delivered to the these may have followed the Hurricane fault segment. Western-
Gulf of California between 4.8 and 4.63 Ma (RC). Recent studies most Grand Canyon has recent thermochronologic data that are
provide new evidence for a multistage history of punctuated sedi- most consistent with it being carved below the Esplanade surface
ment discharge and complex marine-river interactions during inte- in the past 5 Ma (DS, KK). A Wheeler Ridge ca. 20 Ma paleocan-
gration of the Colorado River to the ocean (RD). yon and a ca. 20 Ma paleoriver that supplied clasts from Grand
Thermochronology allows us to reconstruct past, now-eroded, Canyon’s Shinumo Sandstone to the Sespe Formation of California
landscapes. Lees Ferry and Marble Canyon rocks were >60 °C were presented and debated (BW, LS).
until after 5 Ma, indicating that this area was beneath ~2 km of Bedrock incision rates in the northern Colorado River basin
Jurassic and Cretaceous strata (Vermillion cliffs) and hence was have been 100–160 m/Ma over the past 10 Ma and somewhat
not carved until the past 5 Ma (KK). All thermochronology mod- faster (200–300 m/Ma) over the past 0.3 to 1 Ma. Short-term
els for the eastern Grand Canyon segment show rim- and river- (100 ka) incision rates are variable, reflecting complexities of flu-
level samples at 50–80 °C until 25–15 Ma, indicating this segment vial processes at glacial-interglacial scales. Incision rates in Grand
of Grand Canyon was also not carved in its present location and Canyon show semi-steady incision at 160 m/Ma over the past
depth. Rim- and river-level samples that are now separated verti- 1.2 Ma in the east; 100–110 m/Ma over 1.2 Ma in central Grand
cally by 1.5 km show different rim (~55 °C) and river (~85 °C) Canyon; and 90–100 m/Ma over 3–4 Ma in the west (RC). Steady
temperatures until their temperatures converged 25–15 Ma, indi- incision in a given reach at the million-year timescale suggests
cating that an East Kaibab paleocanyon was carved across the steady forcings, the absence of major knickpoint passage, and a
Kaibab uplift at this time (KK). Best time-temperature histories tectonic uplift driver (RC, KK, AA, EK). Differences reach-to-
need to account for the long radiation damage history and that reach have been interpreted by some researchers to reflect active
lattice damage by alpha particle decay has different annealing differential uplift (RC, KK, AA, EK), although geomorphic damp-
characteristics than fissioning of radioactive nuclei (DS). ening from landslides, such as the three-million-year history of
Age of Grand Canyon: Endmember “Young Canyon” models landsliding near Surprise Valley (KK), may have dampened bed-
(all post–6 Ma) and “Old Canyon” models (70–50 Ma) were not rock incision in central Grand Canyon. Seventeen lava damming
strongly supported on the trip. A “paleocanyon solution” is that events from ca. 800 to 100 ka are recorded in western Grand
integration of the Colorado River at 5–6 Ma deepened older Canyon; these dams quickly failed by overtopping, then the
paleocanyon segments as it carved Grand Canyon (KK). Marble system returned to semi-steady bedrock incision rates (RC).
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