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^40 Ar/^39 Ar 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

1. Karl E. Karlstrom
*
1 ,
2. Ryan S. Crow
1 ,
3. Lisa Peters
2 ,
4. William McIntosh
2
,
5. Jason Raucci
3 ,
6. Laura J. Crossey
4
,
7. Paul Umhoefer
5
and
8. Nelia Dunbar
6 

+  Author Affiliations

1.
^1 Department of Earth and Planetary Sciences, University of New
Mexico, Albuquerque, New Mexico 87131, USA

2.
^2 New Mexico Bureau of Geology and Mineral Technology,
Geochronology Lab, 801 Leroy Place, New Mexico Institute of
Technology, Socorro, New Mexico 87801, USA

3.
^3 Department of Geology, 4099, Northern Arizona University,
Flagstaff, Arizona 86011, USA

4.
^4 Department of Earth and Planetary Sciences, University of New
Mexico, Albuquerque, New Mexico 87131, USA

5.
^5 Department of Geology, 4099, Northern Arizona University,
Flagstaff, Arizona 86011, USA

6.
^6 New Mexico Bureau of Geology and Mineral Technology,
Geochronology Lab, 801 Leroy Place, New Mexico Institute of
Technology, Socorro, New Mexico 87801, USA


Abstract

^40 Ar/^39 Ar dates on basalts of Grand Canyon provide one of the best
records in the world of the interplay among volcanism, differential
canyon incision, and neotectonic faulting. Earlier ^40 K/^40 Ar dates
indicated that Grand Canyon had been carved to essentially its present
depth before 1.2 Ma. But new ^40 Ar/^39 Ar data cut this time frame
approximately in half; new ages are all <723 ka, with age probability
peaks at 606, 534, 348, 192, and 102 ka. Strategic sampling of basalts
provides a semicontinuous record for deciphering late Quaternary
incision and fault-slip rates and indicates that basalts flowed into and
preserved a record of a progressively deepening bedrock canyon.

The Eastern Grand Canyon block (east of Toroweap fault) has bedrock
incision rates of 150–175 m/Ma over approximately the last 500 ka;
western Grand Canyon block (west of Hurricane fault) has bedrock
incision rates of 50–75 m/Ma over approximately the last 720 ka. Fault
displacement rates are 97–106 m/Ma on the Toroweap fault (last 500–600
ka) and 70–100 m/Ma on the Hurricane fault (last 200–300 ka). As the
river crosses each fault, the apparent incision rate is lowest in the
immediate hanging wall, and this rate, plus the displacement rate, is
sub-equal to the incision rate in the footwall. At the reach scale,
variation in apparent incision rates delineates ∼100 m/Ma of cumulative
relative vertical lowering of the western Grand Canyon block relative to
the eastern block and 70–100 m of slip accommodated by formation of a
hanging-wall anticline.

Data from the Lake Mead region indicate that our refined fault-dampened
incision model has operated over the last 6 Ma. Bedrock incision rate
has been 20–30 m/Ma in the lower Colorado River block in the last 5.5
Ma, and displacement on the Wheeler fault has resulted in both lowering
of the Lower Colorado River block and formation of a hanging-wall
anticline of the 6-Ma Hualapai Limestone. In modeling long-term incision
history, extrapolation of Quaternary fault displacement and incision
rates linearly back 6 Ma only accounts for approximately two-thirds of
eastern and approximately one-third of western Grand Canyon incision.
This “incision discrepancy” for carving Grand Canyon is best explained
by higher rates during early (5- to 6-Ma) incision in eastern Grand
Canyon and the existence of Miocene paleocanyons in western Grand Canyon.

Differential incision data provide evidence for relative vertical
displacement across Neogene faults of the Colorado Plateau-Basin and
Range transition, a key data set for evaluating uplift and incision
models. Our data indicate that the Lower Colorado River block has
lowered 25–50 m/Ma (150–300 m) relative to the western Grand Canyon
block and 125–150 m/Ma (750–900 m) relative to the eastern Grand Canyon
block in 6 Ma. The best model explaining the constrained reconstruction
of the 5- to 6-Ma Colorado River paleoprofile, and other geologic data,
is that most of the 750–900 m of relative vertical block motion that
accompanied canyon incision was due to Neogene surface uplift of the
Colorado Plateau.