For permission to copy, contact [email protected]© 2007 Geological Society of America
1283
ABSTRACT
40Ar/39Ar dates on basalts of Grand Can-yon provide one of the best records in the world of the interplay among volcanism, differential canyon incision, and neotectonic faulting. Earlier 40K/40Ar dates indicated that Grand Canyon had been carved to essentially its present depth before 1.2 Ma. But new 40Ar/39Ar data cut this time frame approxi-mately 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 deci-phering late Quaternary incision and fault-slip rates and indicates that basalts fl owed into and preserved a record of a progres-sively 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 Hur-ricane 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 inci-sion rates delineates ~100 m/Ma of cumula-tive 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 indi-cate that our refi ned fault-dampened inci-sion 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 forma-tion 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 Neo-gene 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 paleoprofi le, and other geologic data, is that most of the 750–900 m of relative verti-cal block motion that accompanied canyon
40Ar/39Ar and fi eld 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
Karl E. Karlstrom*Ryan S. CrowDepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
Lisa PetersWilliam McIntoshNew Mexico Bureau of Geology and Mineral Technology, Geochronology Lab, 801 Leroy Place, New Mexico Institute of Technology, Socorro, New Mexico 87801, USA
Jason RaucciDepartment of Geology, 4099, Northern Arizona University, Flagstaff, Arizona 86011, USA
Laura J. CrosseyDepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
Paul UmhoeferDepartment of Geology, 4099, Northern Arizona University, Flagstaff, Arizona 86011, USA
Nelia DunbarNew Mexico Bureau of Geology and Mineral Technology, Geochronology Lab, 801 Leroy Place, New Mexico Institute of Technology, Socorro, New Mexico 87801, USA
GSA Bulletin; November/December 2007; v. 119; no. 11/12; p. 1283–1312; doi: 10.1130B26154.1; 16 fi gures; 2 tables; Data Repository Item 2007263.
Karlstrom et al.
1284 Geological Society of America Bulletin, November/December 2007
incision was due to Neogene surface uplift of the Colorado Plateau.
Keywords: Grand Canyon, river incision, Ar-Ar dating, Quaternary basalts, tectonic geomor-phology.
INTRODUCTION: BACKGROUND AND GOALS
In spite of over a century of work on the Grand Canyon, there are still fundamental questions about the age of the canyon and the processes that have formed it. There is consensus (e.g., Young and Spamer, 2001) that the present Colo-rado River system through Grand Canyon took its shape only in the last 6 Ma, ca. 65 Ma after Laramide uplift of the Colorado Plateau and 10–20 Ma after the Sevier/Laramide highlands collapsed to form the Basin and Range province in the Miocene. Miocene topographic inversion left the Colorado Plateau higher, reversed some drainages (Potochnik, 2001), and created sig-nifi cant fault scarps at the western edge of the Colorado Plateau, but the Colorado River did not become integrated across the Kaibab Plateau and through western Grand Canyon until after deposition of the Hualapai Limestone (ending 5.97 ± 0.07 Ma; Spencer et al., 2001). Carving of Grand Canyon began after 6 Ma due to inte-gration of a river system that took drainage from the elevated Colorado Plateau, through basins in the Basin and Range province, to a lowered base level in the Gulf of California that began open-ing 6.5–6.3 Ma (McDougall et al., 1999; Oskin and Stock, 2003). Sediments from the Colorado Plateau fi rst reached the Gulf of California 5.36 ± 0.06 Ma (Dorsey et al., 2005), marking a Col-orado River system that had achieved approxi-mately its present course (Fig. 1). By 4.41 ± 0.03 Ma (Faulds et al., 2001), basalts at Sandy Point on Lake Mead (Fig. 1) were emplaced on top of Colorado River gravels in a paleochannel in about the same place as the modern channel.
For the critical period 5–10 Ma, there are few deposits and no accurate paleoelevation data. For this time period, major uncertainties include: (1) the relative importance of headward erosion from the Gulf to drive incision across the Grand Wash cliffs onto the Colorado Plateau (Lucchi-tta, 1972, 1979, 1990; Buising, 1990; Lucchitta et al., 2001) versus lake spill over and integra-tion from the plateau downward (Spencer and Patchett, 1997; Faulds et al., 2001; Meek and Douglas, 2001; Spencer and Pearthree, 2001; House et al., 2005); (2) the role of Neogene sur-face uplift of the Colorado Plateau (Lucchitta et al., 2001; Sahagian et al., 2002), if any (Spencer and Patchett, 1997; Spencer et al., 2001; Patch-ett and Spencer, 2001; Pederson et al., 2002a);
(3) relative vertical displacement and timing of movements of normal faults near the Colo-rado Plateau-Basin and Range boundary and their effect on incision processes (Hamblin et al., 1981; Willis and Biek, 2001; Pederson and Karlstrom, 2001); and (4) the depth and shape of pre–6-Ma paleocanyons that may have been reused, linked, and deepened in the process of carving Grand Canyon (Young, 2001, 2007).
Datable basalts of the western Grand Canyon offer the opportunity to better constrain the inci-sion of Grand Canyon and to understand the neo-tectonic and geomorphic interactions of volca-nism, canyon incision, and normal faulting. The Uinkaret volcanic fi eld (Fig. 1; Uinkaret Plateau block of Beus and Morales, 2003) is a north-south–trending fi eld of cinder cones and basalt fl ows that is situated between the Hurricane and Toroweap faults (Fig. 1). Although some vents existed within Grand Canyon, basalt fl owed into Grand Canyon mainly from the north rim, with some fl ows traveling >120 km down the river corridor (from RM 179–2541; Fig. 1). Flows on the Uinkaret Plateau range in age from 3.4 to 3.7 Ma on Mount Trumbull (40K/40Ar; Best et al., 1980; Billingsley, 2001) to ca. 1 ka (Fenton et al., 2001). But, as reported here, intracanyon basalt fl ows range from ca. 700 to ca. 100 ka.
The fi rst goal of this paper is to present new 40Ar/39Ar dates on basalts from western Grand Canyon (Fig. 2). The new 40Ar/39Ar dates offer a signifi cant advance over both 40K/40Ar dates, which tend to be too old because of undetected excess 40Ar, and cosmogenic surface ages, which tend to be too young due to degradation of surfaces. The 40Ar/39Ar method, coupled with sample character-ization and preparation techniques designed to recognize and remove incorporated clays, allows us to eliminate the elevated ages seen at high- and/or low-temperature steps and arrive at a better estimate of the eruption age (Fig. 3).
The second goal of this paper is to use the new geochronologic data to provide better estimates of Quaternary incision history of Grand Canyon. Earlier workers thought that Grand Canyon had been deepened to essentially its present depth before 1.2 Ma based on 40K/40Ar dates (Hamb-lin, 1970b, 1974, 1994), but new 40Ar/39Ar data and incision studies presented here indicate that basalts fl owed into and preserved a record of a progressively deepening bedrock canyon. This incision is recorded by basalt remnants in the
river corridor that overlie river gravels, which, in turn, rest on top of elevated bedrock straths. This paper presents new, high-quality, incision-rate points, along with a comprehensive summary of published incision-rate data.
The third goal of this paper is to understand the interaction of canyon incision with active normal faulting and refi ne the model of fault-dampened incision fi rst presented by Peder-son and Karlstrom (2001) and Pederson et al. (2002b). Various workers have noted that can-yon incision, basaltic volcanism, and exten-sional faulting have all interacted (Hamblin et al., 1981; Jackson, 1990; Stenner et al., 1999; Fenton et al., 2001; Pederson et al., 2002b); this paper offers a synthesis of these interacting pro-cesses, and their rates, based on new geochro-nology and fi eld studies. The refi ned differential incision model presented in this paper quantifi es the relative roles of vertical-block motion versus hanging-wall fl exure in causing lowered appar-ent incision rates as the river crosses several west-down Neogene normal faults.
The last part of the paper applies the differ-ential incision data to develop a model for the long-term incision history of Grand Canyon. We combine incision rates, fault displacement rates, slip durations on different faults, and differen-tial-incision patterns to extend the differential incision model back to 6 Ma. Differential inci-sion due to faulting was an important process throughout the Neogene tectonic development of the Colorado Plateau-Basin and Range transi-tion and one that has been left out of most mod-els for carving Grand Canyon.
40Ar/ 39Ar Results
We performed a total of 63 incremental, step-heating analyses at the New Mexico Tech Geochronology Research Laboratory on 44 Grand Canyon basalt samples collected mainly during 2000 and 2001 (Figs. 2 and 4; Table 1; Table DR1)2. Twenty-six samples (based on 44 analyses) yielded reliable new dates (2 sigma error <±150 ka) that we interpret to be accu-rate eruption ages (Fig. 2; Table 1). The char-acterization of samples by electron microprobe has been critical for successful dating, both for identifying the most promising samples, and for guiding preparation and treatment of problem samples. Microprobe observations reveal vari-able amounts of matrix glass, alteration of glass or phenocrysts, and/or abundant clay (Fig. 3), unusual for late Quaternary basaltic lavas in arid
2GSA Data Repository Item 2007263, Table DR1 (40Ar/39Ar analytical data) and Table DR2 (displace-ments across major faults and of fault-slip rates), is available at www.geosociety.org/pubs/ft2007.htm. Requests may also be sent to [email protected].
1For locations throughout this paper, we use the conventional nomenclature of river miles (RM) down-stream from Lees Ferry, using Stevens’s (1983) river miles. The river profi les, showing elevations of the river surface, are from the detailed Birdsey survey (1924), with bathymetry added from sonar studies of Wilson (1986), and a schematic representation of geo-logic units modifi ed from Moores (in LaRue, 1925).
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1285
BRIGHT
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Karlstrom et al.
1286 Geological Society of America Bulletin, November/December 2007
1111 2 3
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102 ka
192 ka 348 ka
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Nu
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Age (ka)
Figure 2. Histogram and age-probability chart of 40Ar-39Ar ages in western Grand Canyon. Numbers correspond to samples listed in Table 1.
0 10 20 30 40 50 60 70 80 90 100-1
0
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4 hr ultrasonic treated sample
normally prepared sample
Cumulative 39Ar Released
App
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Figure 3. Backscattered electron images (fi eld of view ~500 microns) of Grand Canyon basalt samples containing: (A) infi ltrated clay and (B) matrix glass. (C) Age spectra for clay-rich sample improved dramatically with extended ultrasonic treatment.
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1287
Pe
N
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ges
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blin
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4)
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ore
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p V
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y fil
l
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row
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ws
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der
Co
ne
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= Q
uat
ern
ary
dis
pla
cem
ent
(m)
73 =
str
atig
rap
hic
sep
arat
ion
(m)
541
± 17
ka
Ar/
Ar d
ate
Exp
lana
tion
Fig
ure
4. M
aps
of fl
ow r
emna
nts
(mod
ifi ed
fro
m H
ambl
in, 1
994
and
Hun
toon
et
al.,
1981
), lo
cati
ons
of n
ew d
ated
sam
ples
, and
age
est
imat
es f
or u
ndat
ed r
emna
nts.
(A
) To-
row
eap
faul
t an
d V
ulca
n’s
Thr
one
area
. (B
) H
urri
cane
fau
lt a
nd W
hitm
ore
Can
yon
area
. See
Fig
ure
1 fo
r m
ap lo
cati
ons.
Karlstrom et al.
1288 Geological Society of America Bulletin, November/December 2007
TABLE 1. 40Ar/39Ar DATES ON BASALTS FROM WESTERN GRAND CANYON
Plotted ages
Basalt flow Sample number River mile Ar/Ar age (ka)
stnemmoC secnerefeR
1 Upper Gray Ledge Mean of two samples 188-189 111 ± 30 n = 2
yduts sihT
2002 ,.la te nosredeP 62 ± 79 R1.881 30-881-00W egdeL yarG reppU
2n = 2
Upper Gray Ledge LP01-189-01 mean of two analyses
189.1L 127 ± 27 n = 2
yduts sihT
yduts sihT 92 ± 311 L1.981 a10-981-10PL LP01-189-01b 189.1L 140 ± 29 This study Longer ultrasonic cleaning
3n = 2
Whitmore Cascade Mean of two samples 187.6R 186 ± 26 n = 2
4002 ,iccuaR
Colonade 1 187.6R 191 ± 30 Raucci, 2004 Colonade 2 187.6R 171 ± 50 Raucci, 2004
Float blocks of large columns collected in Whitmore Canyon
Lower Gray Ledge Mean of two samples 184-188 195 ± 34 n = 2
yduts sihT
2002 ,.la te nosredeP 93 ± 491 L7.781 20-881-00W egdeL yarG rewoL 4
5 Lower Gray Ledge LP01-184-01 184.6L 200 ± 72 This study Longer ultrasonic cleaning, Black Ledge of Hamblin, 1994
2002 ,.la te nosredeP 75 ± 892 R8.491 10-591-00W esabaid evissaM 6 Whitmore Mean of two samples 188-190 319 ± 62
n = 2 2002 ,.la te nosredeP
2002 ,.la te nosredeP 96 ± 813 R6.981 20-091-00W eromtihW 7
yduts sihT 141 ± 323 R2.881 10-881-10PL eromtihW 8
9n = 2
Layered diabase LP01-192-01 mean of two analyses
192.0L 332 ± 39 n = 2
This study
LP01-192-01a 192.0L 309 ± 20 This study LP01-192-01b 192.0L 348 ± 17 This study
Longer ultrasonic cleaning
10n = 3
Mile 177L W00-177-02 mean of three analyses
177.3L 351 ± 25 n = 3
This study
W00-177-02a 177.3L 349 ± 29 Pederson et al., 2002 W00-177-02b 177.3L 385 ± 47 This study W00-177-02c 177.3L 334 ± 36 this study
Pillow basalt blocks intermixed with river sand and gravel
11 Black Ledge Sample from Fenton et al., 2004
4002 ,.la te notneF 08 ± 384 L5.981
12 Toroweap LP01-179-04 179.1R 487 ± 48 This study Toroweap C flow
Upper Prospect Mean of five samples 179.6L 518 ± 22 n = 5
2002 ,.la te nosredeP
13 Upper Prospect K00-179-PR3 179.6L 530 ± 23 Pederson et al., 2002 14 Upper Prospect K00-179-PR4 179.6L 541 ± 53 Pederson et al., 2002 15 Upper Prospect K00-179-PR5 179.6L 486 ± 21 Pederson et al., 2002 16 Upper Prospect K00-179-PR10 179.6L 533 ± 20 Pederson et al., 2002 17 Upper Prospect K00-179-PR6 179.4L 533 ± 82 Pederson et al., 2002
Upper Prospect flows are listed in stratigraphic order, indicating that the age for #15 seems incorrect, despite its analytical precision
18n = 2
Prospect dike LP01-179-12 mean of two analyses
179.4L 521 ± 59 n = 2
yduts sihT
yduts sihT 82 ± 894 L4.971 a21-971-10PL ekid tcepsorP yduts sihT 63 ± 955 L4.971 b21-971-10PL ekid tcepsorP
4002 ,iccuaR 03 ± 045 R2.781 10-4240CW eromtihW redlO 91 Lower Prospect Mean of three samples 179.6L 568 ± 52
n = 3 yduts sihT
20n = 5
Lower Prospect LP01-179-07 mean of five analyses
179.6L 541 ± 22 n = 5
This study Upthrown side of fault
Lower Prospect LP01-179-07a 179.6L 580 ± 79 This study Lower Prospect LP01-179-07b 179.6L 527 ± 39 This study Lower Prospect LP01-179-07c 179.6L 541 ± 28 This study Lower Prospect LP01-179-07d 179.6L 528 ± 31 This study Lower Prospect LP01-179-07e 179.6L 608 ± 66 This study
(continued)
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1289
environments. Samples were selected for analy-sis based on minimal glass, alteration, and clay. Microprobe evaluation included backscattered electron imaging to investigate degree of crys-tallinity and alteration, potassium distribution within the sample, and quantitative geochemical analysis of a range of phases. We also developed acid leaching and ultrasonic treatments that were effective in removing clay, thereby improv-ing precision and, in some cases, reducing the apparent age of samples (Fig. 3C; Table DR1).
The 40Ar/39Ar ages reported here are weighted-mean plateau ages for the fl at central portions of age spectra (Fig. 5). Isochrons for these fl at portions generally have atmospheric intercepts and have isochron ages statistically indistin-guishable from plateau values. Many of the age spectra have elevated ages at high and/or low temperatures (Fig. 5), attributed to extraneous 40Ar, either as inherited 40Ar in infi ltrated clay or within incompletely degassed xenocrysts, or as excess 40Ar in phenocrysts (Fig. 3). All of the 40Ar/39Ar ages are less than 723 ka, and all studied fl ows have normal paleomagnetic polarity (Hamblin, 1994), consistent with their eruption within the Gauss normal polarity chron (780 ka to present). Thus, complex mechanisms
of post-eruptive reheating previously proposed to explain the normal polarity of fl ows with 40K/40Ar ages >780 ka are no longer required (cf. Hamblin, 1994). Note that 40Ar/39Ar ages reported here are slightly older (0.6%) than those reported in Pederson et al. (2002b) and Fenton et al. (2004) because ages have been recalculated using the calibration of Renne et al. (1998; Fish Canyon Tuff sanidine age = 28.02 Ma).
Quaternary (<1 Ma) basalts are diffi cult to date in general, and Grand Canyon basalts have been more diffi cult than many in the Southwest, in part due to their interaction with river water and clays. Although these dates are dramati-cally better than older 40K/40Ar dates, there are relatively large uncertainties in age measure-ments, both in terms of precision and accuracy. Two sigma analytical precision is typically ±5%–30% (Table 1), but the quoted precision associated with individual analyses may not adequately refl ect the accuracy of the measure-ments. For example, the two analyses of sample 9 yielded ages of 309 ± 20 and 348 ± 17, but the ages do not overlap within the calculated 2 sigma error. There are also a few instances where the geologic context shows that the accuracy of ages is not refl ected in the reported precision. For
example, samples 13–17 were sampled in strati-graphic order in a fl ow stack on Upper Prospect fl ows in Prospect Canyon (Fig. 5. Four of these ages are in close agreement and indicate that this sequence of fl ows was likely emplaced rap-idly (close to 533 ka), but the 486 ± 21-ka age on sample 15, in spite of its high precision, is incompatible with its stratigraphic position and is outside the 2 sigma precision of its neighbors. For samples like these, with MSWD (Mean Square Weighted Deviate) values >1, we follow the method of Dalrymple and Hamblin (1998) of recalculating errors to better refl ect scatter of the dates beyond analytical error.
Another way to evaluate accuracy is to com-pare multiple analyses from the same sample. In most cases (samples 2, 10, 18, 20, 27, but not 9), the multiple analyses overlap within the reported 2 sigma precision. Also, in most cases, when two or more samples were taken from the same fl ow (sample 3 and samples 13/14) or fl ows were believed to be correlative (samples 16 and 17), ages also overlap within 2 sigma precision.
The results are shown in Figure 2. Nineteen of the well-dated samples range in age from 480 to 723 ka, with age-probability peaks at 534, 606, and 723 ka (Fig. 2). These samples come
TABLE 1. 40Ar/39Ar DATES ON BASALTS FROM WESTERN GRAND CANYON (continued)
Plotted ages
Basalt flow Sample number River mile Ar/Ar age (ka)
stnemmoC secnerefeR
21 Lower Prospect LP01-179-08 179.2L 602 ± 37 This study D—Dam of Hamblin, 1994
22 Lower Prospect LP01-179-06 179.6L 632 ± 45 This study Downthrown side of fault
Black Ledge Mean of nine analyses (eight samples)
207-208 572 ± 31 n = 9
Lucchitta et al., 2000
23 Black Ledge GC-29-93 207.5L 525 ± 26 Lucchitta et al., 2000
24n = 2
Black Ledge Mean of next two samples
207.5L 605 ± 12 n = 2
Lucchitta et al., 2000
Black Ledge GC-26-93 207.5L 604 ± 16 Lucchitta et al., 2000 Black Ledge GC-26b-93 207.5L 607 ± 18 Lucchitta et al., 2000
25n = 2
Black Ledge Mean of next two samples
207.7L 522 ± 57 n = 2
Lucchitta et al., 2000
Black Ledge GC-34-93 207.7L 559 ± 18 Lucchitta et al., 2000 Black Ledge GC-35-93 207.7L 500 ± 14 Lucchitta et al., 2000
26n = 2
Black Ledge Mean of next two samples
208-209 605 ± 17 n = 2
Lucchitta et al., 2000
Black Ledge GC-24-93 208.2R 609 ± 12 Lucchitta et al., 2000 Black Ledge GC-22-93 208.6R 585 ± 28 Lucchitta et al., 2000
27n = 2
Black Ledge LP01-208-01 mean of two analyses
208.3R 528 ± 39 n = 2
This study
Black Ledge LP01-208-01a 208.3R 525 ± 49 Pederson et al., 2002 (location modified)
Black Ledge LP01-208-01b 208.3R 534 ± 64 This study Longer ultrasonic cleaning
28n = 2
176.9-high remnant Mean of two samples 176.9L 613 ± 38 n = 2
This study
K01-177-01 176.9L 643 ± 54 This study K01-177-05 176.9L 601 ± 35 This study
29 Spencer Canyon K01-246-01 246.0R 723 ± 31 This study Black Ledge of Hamblin, 1994
30 Sandy Point basalt JF-97-76 ~290 4410 ± 30 Faulds et al., 2001 Sandy Point basalt
Karlstrom et al.
1290 Geological Society of America Bulletin, November/December 2007
Figure 5. 40Ar/39Ar spectra for dated basalt samples. Reported ages are weighted-mean plateau ages for the fl at central portions of age spectra. (Continued on following two pages.)
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1291
Figure 5 (continued).
Karlstrom et al.
1292 Geological Society of America Bulletin, November/December 2007
mainly from fl ows in the vicinity of the Prospect Canyon/Toroweap fault (Fig. 4A). The oldest 40Ar/39Ar age is 723 ± 31 ka (2 sigma error) for a single sample of a >17-m-thick fl ow remnant at RM 246 at the mouth of Spencer Canyon in western Grand Canyon (Figs. 1 and 6). Based on our correlation of fl ow remnants, we inter-pret this fl ow to have traveled ~110 km down the river from a series of edifi ces within Grand Canyon and along Toroweap fault (Fig. 4A; Crow et al., 2007).
Basalts ranging in age from 525 to 650 ka include Lower Prospect fl ows of Prospect Canyon (LP, Fig. 4A), high remnants of basalt upstream of Toroweap fault (HR, Fig. 4A), and fl ows near RM 208 named Black Ledge (Fig. 1; Hamblin, 1994; Lucchitta et al., 2000). Units ranging in age from 480 to 540 ka include
Upper Prospect fl ows (UP, Fig. 4A), Prospect Cone dike, Toroweap A fl ow, and Black Ledge remnants near RM 189.5 (Fig. 4B) and at Gran-ite Park near RM 208. Numerous dated Black Ledge fl ows at Granite Park come from four separate outcrop remnants (both sides of the river) that are likely correlative fl ows. We were unable to match the high precision reported by Lucchitta et al. (2000). Multiple-age fl ows may indeed be present (Lucchitta et al., 2000), but existing ages are not decipherable in terms of just two ages of fl ows.
There is abundant fi eld evidence for multiple fl ows in the 480- to 723-ka age range, accu-mulating to thicknesses of >500 m in proximal areas (Prospect Canyon) and also present as superposed fl ows in distal areas (Granite Park). The farthest-traveled fl ow in Grand Canyon
(RM 254) is undated, but is assumed to be in this age range. A single 540 ± 30 ka dated basalt ~3 km northwest of the Colorado River at RM188 (Fig. 4B) erupted from a cinder cone near the Hurricane fault (Raucci, 2004) and demonstrates that there was also volcanism elsewhere in the Uinkaret Volcanic fi eld in this interval. It is tempting to designate the 534- and 606-ka peaks (Fig. 2) as Lower and Upper Pros-pect fl ows, respectively, and correlate them with two different age fl ows at Granite Park, but this remains unproven. Thus, pending additional dat-ing of geologically well constrained samples, we view the multiple peaks at 534, 606, and 723 ka in the age-probability plot (Fig. 2) to be part of a broad time span of 480- to 723-ka volcanism, but not necessarily an accurate representation of discrete fl ow events.
Figure 5 (continued).
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1293
A younger set of fl ows has a range in age from 298 to 325 ka and an age probability peak of 348 ka (Fig. 2). These fl ows include the Whitmore fl ow (W, Fig. 4B), Layered Dia-base (RM 192, Fig. 4B), Massive Diabase (RM 195, Fig. 1), and the 177-mile basalt that fl owed upstream to its present location (Fig. 4A; Crow et al., 2007). The largest volume of 300- to 350- ka basalt was erupted along the Hurricane fault in the area of Whitmore Canyon.
The youngest fl ows we have dated are 100- to 200- ka fl ows near Whitmore Canyon. Age probability peaks are at 192 and 102 ka (Fig. 2). The “Gray Ledge” fl ow of Hamblin (1994) has a lower (ca. 200-ka) and upper (ca. 100-ka) com-ponent. Upper Gray Ledge fl ows at river mile 188.1 and 189.1 are 97 ± 26 and 127 ± 21 ka, respectively. A lower Gray Ledge (mile 187.7) and one fl ow previously referred to as “Black Ledge” (mile 184.7, now identifi ed as lower Gray Ledge) give ages of 194 ± 39 and 200 ± 72 ka, respectively (Table 1).
Methods of Calculating Bedrock Incision Rates
The new basalt ages allow us to calculate incision rates in numerous places in western Grand Canyon. The Colorado River system in Grand Canyon preserves a series of inset Qua-ternary alluvial terraces at various heights that record climatically controlled aggradation and incision episodes superimposed on a history of overall exhumation and deepening of the bedrock canyon (Pederson et al., 2002b, 2006; Anders et al., 2005). Methods for calculating incision rates are refi ned from those of Ped-erson et al. (2002b) and need elaboration to help evaluate the variable quality of incision data points. Basalt fl ows locally rest on top of river gravels that overlie bedrock straths within the river corridor, near the modern river chan-nel (Figs. 4 and 7). The straths represent times of erosion of the bedrock channel by the river (Bull, 1991) and hence times of canyon deep-ening. Dates obtained from materials directly above the straths, such as basalt fl ows or trav-ertine, thus provide a close approximation of the time of formation of the strath (Pederson et al., 2002b; Pazzaglia et al., 1998). Height of the strath above the 10,000 cubic feet per second (283 m3/s) reference river level was measured with a Jacob staff and/or estimated from LIDAR (Light Detection and Ranging) measurements of height of the base of fl ows. Straths are inter-preted as a past position of part of the bedrock river channel before emplacement of the basalt or other dated material.
Our fi rst method of estimating bedrock inci-sion is to compare the height of the strath relative
to the inferred position of the bedrock surface beneath the modern river (method A in Fig. 6). Our preferred estimate of depth to bedrock under the river is the “maximum pool depth,” defi ned as the mean of the ten deepest pools for a 15-mile-long reach centered on the dated remnant.
These values are calculated using bathymetry data that were generated using sonar (Wilson, 1986). This method uses slightly deeper bed-rock depths than Pederson et al. (2002b), and provides systematic estimates of bedrock inci-sion and deepening of Grand Canyon.
bedrock incision rate =
45 m/ 723 ka = 62 m/Ma
base of basalt
granite
~15 m to bedrock below river
Present level of river
as backed up by Lake Mead
~30 m
Figure 6. Photo of Lava Cliff Rapids (RM 246) showing base of 723-ka basalt remnant and its ~30-m pre-dam height above the river. Maximum depth to bedrock of 15 m was deter-mined by drilling at Bridge Canyon Dam site ~8 river miles (13 km) upstream, suggesting a bedrock incision rate of 62 m/Ma. Northern Arizona University, Cline Library, Special Collections and Archives, Julius F. Stone collection.
Karlstrom et al.
1294 Geological Society of America Bulletin, November/December 2007
There are inherent geologic uncertainties in our bedrock incision calculations that also apply to other bedrock incision studies (e.g., Merritts et al., 1994; Burbank et al., 1996; Pazzaglia and Brandon, 2001). The fi rst uncer-tainty is that the dated material provides only a minimum age of the strath, with unknown hia-tus between beveling of the strath, deposition of river gravels, and deposition of the dated material. In this regard, our quoted rates may be maximum bedrock incision rates.
A second and probably larger uncertainty is that the mean depth to bedrock beneath the present river remains poorly known. Bathy-metric data (Figs. 7 and 8) suggest that the
river channel, like the river banks at lowest fl ows and like most side-stream tributaries, is highly pot-holed and is fl oored by a mixture of bedrock and alluvial fi ll. The resulting modern bedrock “strath” is highly nonplanar both lat-erally (Fig. 7, river cross section from Hanks and Webb, 2006) and longitudinally (Fig. 8). For incision calculations, we defi ne “pools” as areas where water depths are greater than the mean water depth. Mean pool depth in Grand Canyon is 9–17 m; maximum pool depth (mean of the ten deepest pools in a 15-mile-long reach) is 19–24 m, and maximum depth to bedrock based on drilling and seismic studies is ~28 m (Fig. 8).
A third uncertainty is that some basalt rem-nants may have fl owed onto alluvial terraces rest-ing on elevated bedrock benches rather than into the paleothalwag. In this regard, our rates may tend to be maximum bedrock incision rates.
We portray the geologic uncertainty in depth to bedrock in our incision vectors (Fig. 9) by showing mean pool depth (which gives the min-imum bedrock depth/minimum incision rate), maximum pool depth (preferred bedrock depth/preferred incision rate), and maximum bedrock depth (maximum incision rate). The 2-sigma analytical precision of the age analysis is used in the same way as previous workers (Fenton et al., 2001; Pederson et al., 2002b). The combined
487±48 ka
351±25 ka n=3
74 m
60 m
49 m
37 m32 m
C BA
326 ka
385 ka125 ka
283 ka153 ka
M2?M1 M1
161 ka
68-64 ka
55 ka
94 m
133 m
>183 m
52 m44 m
38 m
24 m
8 m
ColoradoRiver
10,000 cfs
Max pool depth at RM 57 (see below)
M5?
Toroweap C flow
-50
0
50
100
150
200
250
-50
50
100
150
200
250
Met
ers
abov
e ri
ver (
refe
ren
ce s
tag
e =
10,
000
cfs)
D
Tread Heights
0
69 ka71 ka
Strath Heights
175 m
117 m
28 m
E
River cross section from drill dataat RM 32.8 (Hanks and Webb, 2006, Fig. 3)
Dep
th b
elo
w
wat
er s
urf
ace
(m)
Basalts of Western Grand CanyonRM 177-179
Terraces of Eastern Grand CanyonRM 55-70
177 mile
M7
M6
M5
M4
M3
0
10
20
30
0
10
20
30
63 61 59 57 55 53 51River mile downstream from Lees Ferry
Maximum Pool Depth
Maximum Bedrock Depth
Mean Pool DepthMean Water Depth
A
Uncertainty in inferred depthto bedrock used in Fig. 9
B
74 m
M4
Explanation
Bedrock
River Gravelsshowing terrace number
Basalt
U-series dates
TCN dates
40Ar/39Ar dates
M1-7 Mainstem fillterracesMeasured heightabove river level
OSL dates
Drill hole
Figure 7. (A) Schematic summary of Grand Canyon terraces, strath and tread heights, and geochronol-ogy. The right side shows U-series dates on terrace fi lls from eastern Grand Canyon (from Anders et al., 2005; Pederson et al., 2006); left side shows Ar-Ar basalt dates from western Grand Canyon (this paper). Methods for calcu-lating incision amounts based on dated samples are shown at lower left: A—from bedrock strath to inferred depth of bedrock below river level based on pool depths (Pederson et al., 2002b and this paper); B—from bedrock straths to 10,000 ft3/s (283 m3/s) river level; C—from height of dated sample to river; D—from top of aggra-dational terrace or basalt fl ow to present river level (Lucchitta et al., 2000); E—from strath to strath height differences in a given reach plus an understanding of duration of fi ll events (Pederson et al., 2006 and this paper). An example of lat-eral variation of depth to bedrock beneath the river is shown based on drill data at RM 32.9 (Hanks and Webb, 2006). (B) Example of bathymetry data (Wilson, 1986), showing mean water depth, mean pool depth, maximum pool depth, and maximum bedrock depth used in Figure 9 to infer uncertainty in depth to bedrock.
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1295
estimates of both geologic and analytical uncer-tainties (Fig. 9) show that the large and system-atic incision-rate variations between eastern and western Grand Canyon that lead to the main conclusions of this paper are robust.
A second method for estimating bedrock incision rates is to compare the ages of straths of different heights in the same reach (Peder-son et al., 2006). This method provides an esti-mate of bedrock incision that is independent of depth to bedrock and also allows data from fi ll terraces to be considered. This analysis empha-sizes that the times when the Colorado River was incising bedrock may have been relatively short, less than half of its Quaternary history, and that the rest of the time it is aggrading its bed and, hence, not incising the canyon. Using this method, Pederson et al. (2006) reported an average incision rate of ~142 m/Ma in eastern Grand Canyon for the last 385 ka. Addition of new data points from this study refi nes this esti-mate to apparent rates of 172 m/Ma in eastern Grand Canyon and 55 m/Ma in western Grand Canyon (Fig. 10). By projecting the regressed lines through our best incision points to below
river level (Fig. 10), this method suggests that average depth to bedrock in the eastern Grand Canyon is 28 m (in agreement with the deep-est measurements found so far from drilling; Fig. 7), and in the western canyon is 9 m (shal-lower than the 15-m depths determined from drilling at Bridge Canyon dam site). Additional high-quality incision points, once obtained, will help refi ne these numbers such that this method shows great promise for estimating both long-term average apparent incision rates and average depth to bedrock in different reaches.
Differential Bedrock Incision Rates
Figure 11 and Table 2 summarize bedrock incision-rate data points from Grand Can-yon, most of which are newly reported in this paper. The eastern Grand Canyon rates are uni-form from RM 57 (with points 2, 3, 4, and 5 of Table 2 giving a mean incision rate of 150 m/Ma), to RM 177–179 (with points 7 and 8 of Table 2 giving a mean of 155 m/Ma). These rates are somewhat less than the regressed line through the same points (172 m/Ma) because
of depth to bedrock assumptions. The points at RM 177 and 179, just east, and in the imme-diate footwall, of the Toroweap fault, are based on dated basalt remnants that overly river grav-els, with evidence for basalt-water interactions in the form of pillows and sand-fi lled fractures in the basal basalt. Importantly for this study, the combined fi ll terrace dates and basalt data (Fig. 10) indicate that there was a uniform aver-age bedrock incision rate for the entire reach of eastern Grand Canyon (RM 56–179) for the time interval 153–487 ka, in spite of aggrada-tion and incision episodes (Fig. 10; Pederson et al., 2006). Basalt data in several other places (e.g., RM 177–179, RM 188, RM 204–208, and RM 246) also suggest a uniform bedrock inci-sion rate within a given reach of Grand Canyon back to 723 ka, as discussed below.
Well-constrained, measured incision rates (Fig. 9; Table 2) change abruptly across the Toroweap fault, to values of 50–70 m/Ma. This is interpreted to indicate that lowering of the western block by faulting results in dampen-ing of the eastern Grand Canyon incision rate to produce a lowered “apparent incision
Lee’
s Fe
rry
Gle
n C
anyo
n D
am
Mar
ble
Can
yon
Dam
Sit
e
Bri
dg
e C
anyo
n D
am S
ite
RM 5
2
50
45
40
35
30
25
20
15
10
5
0
250 200 150 100 50 0
Mean pool depth
Maximum pool depth
Maximum bedrock depth
Measured maximum depth to bedrock from drill data(cited in Hanks and Webb, 2006)
Measured maximum depth to bedrock from seismicdata (cited in Hanks and Webb, 2006)
Mean water depth
River Water Surface
Maximum Pool Depth
Maximum Bedrock Depth
Mean Pool Depth
Mean Water Depth
River Miles (downstream from Lees Ferry)
Wat
er D
epth
(m
)
Uncertainty in inferreddepth to bedrock used in Fig. 9
Figure 8. Bathymetry, mean water depth, mean pool depth, maximum pool depth, and measured maximum depth to bedrock in the Colorado River of Grand Canyon. Bathymetry from Wilson (1986) from Lees Ferry to RM 235 based on sonar studies (no data below RM 235 where river bottom has been silted in by Lake Mead). Measured maximum depth to bedrock (from Hanks and Webb, 2006) is based on drilling done to evaluate dam sites and seismic studies of river banks. Maximum pool depth is used in this paper as the best proxy for mean depth to bedrock.
Karlstrom et al.
1296 Geological Society of America Bulletin, November/December 2007
rate.” Figure 11 shows a systematic variation in apparent incision rates within the Uinkaret block: rates increase progressively westward from the fault and nearly regain the eastern Grand Canyon rate ~6 km west of the Toroweap fault. Immediately west of the Hurricane fault, apparent incision rates diminish again abruptly to 60–70 m/Ma, and remain relatively uniform; they do not again approach the eastern Grand Canyon rates for the entire western Grand Can-yon block (to RM 246).
An important new data point is the 723 ± 28-ka remnant exposed opposite the mouth of Spencer Canyon (Fig. 6, RM 246) that fl owed ~100 km along the Colorado River bed. The basalt is perched on Precambrian granite directly above the river channel. Although no gravel is exposed at the strath, the remnant is directly opposite a major side canyon (Spencer Canyon) at the head of what used to be Lava
Cliff rapids (now silted in by Lake Mead) and is interpreted to have been emplaced in the paleo-thalwag. The top of this fl ow is at an elevation of 381 m (based on LIDAR), and the base of the fl ow, at 350 m, is just exposed above the pres-ent lake-controlled river level. This reach of the river has been fl ooded by Lake Mead, but historic photographs (Fig. 6) as well as pre-dam contour maps and surveyor’s descriptions of the Spencer Canyon Power Site (LaRue, 1925, plate LIX) show that the base of the fl ow was ~30 m above the river level at the head of Lava Cliff rapids (Fig. 6). Dam-site surveyors guessed that bedrock at the dam site would be less than 12 m below river level based on the presence of bed-rock outcrops in the rapid (LaRue, 1925, p. 94), in good agreement with the 15-m depth of bed-rock near Bridge Canyon dam site (RM 238). Hence, bedrock incision has averaged 58–62 m/Ma since this fl ow was emplaced (Table 2). In
this case, because of the nearby drill data, the maximum bedrock depth is probably most accurate; nevertheless, using the maximum pool depth method for consistency (Figs. 9 and 11), the incision value increases to 75 m/Ma.
The dashed incision vectors in Figure 11 are less well constrained than the solid ones, but are also considered important data points. The Lava Falls (RM 182.8) and Buried Canyon fl ows (RM 182.5; Hamblin, 1994) represent basalt fl ows that completely fi lled the paleothalwag plug-ging the river and shifting the river to its present more southerly location. The basal Buried Can-yon basalt fl ow (fl ow “A” of Hamblin, 1994), the covered base of which is 66 m above river level gives a maximum incision rate of 155 m/Ma, if we assume an age of 550 ka, consistent with a correlation to the 475- to 625-ka Prospect and Black Ledge fl ows, as suggested by LIDAR correlations (Crow et al., 2007). Similarly, at
0
20
40
60
80
100
120
140
160
180
200
56L
57L
57L
57L
177.
3L
178.
9R
179.
3R
181.
6R
182.
5R
182.
8R
188.
1R
188.
85L
194.
2R
195.
2R
195.
3L
203.
4R
207.
3L
208.
35R
223.
1R
246R
290L
425R
2345789101112161821222324252627282930
Incision Point (from Table 2)
Inci
sio
n R
ate
(m/M
a)
27 m
/Ma
20 m
/Ma
75 m
/Ma
max
64 m
/Ma
max
150
m/M
a m
ax
151
m/M
a
131
m/M
a
163
m/M
a
145
m/M
a
162
m/M
a
66 m
/Ma
68 m
/Ma
73 m
/Ma
80 m
/Ma
136
m/M
a
111
m/M
a
58 m
/Ma
53 m
/Ma
64 m
/Ma
76 m
/Ma
76 m
/Ma
73 m
/Ma
746361
83max bedrock depth
mean pool depth
Wel
l-co
nst
rain
ed in
cisi
on
rate
Esti
mat
ed in
cisi
on
rate
Wel
l-co
nst
rain
edm
axim
um
inci
sio
n ra
te
max pool depth reported age
+2σ
-2σ68
m/M
a
Geologic uncertainty in incision rates based on proxies for depth to bedrock:
Age uncertainty in incision rates based on analytical error in agedetermination:
preferredrate
max
152
148
183
85
156
145
166
127
143
121
149
105
168
158
184
131
156
136
171
123
180
148
181
146
7360
8449
7363
8254
8464
8960
9370
9667
158
120
153
122
127
9612
898
6751
7544
7954
9437
6146
6938
7456
8251
7875
8861
8271
9161
8464
8051
7872
8058
2727
Figure 9. Incision data points from Table 2, arranged by river mile, showing both analytical and geologic uncertainties for incision rates. Gray vector shows preferred incision vectors based on reported age and maximum pool depth. Upper right boxes show incision rate uncer-tainty based on geochronological uncertainty (± 2σ). Upper left boxes show incision-rate uncertainty based on use of mean pool depth and maximum bedrock depth (from Fig. 8).
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1297
the mouth of Whitmore Wash (RM 188.1), a rate of 136 m/Ma would result, assuming the fl ow mapped as “Massive Diabase” by Hamblin (1994; see Table 2) correlates instead with the 475- to 625-ka Prospect and Black Ledge fl ows as suggested by LIDAR heights.
Fault Displacement: Magnitudes and Rates
Toroweap FaultThe Toroweap fault is part of a several hun-
dred-km-long, N-S–striking normal fault system (Hamblin, 1970a), which is part of the distributed system of normal faults that forms the microseis-mically active neotectonic edge of the Colorado Plateau (Fig. 1; Brumbaugh, 1987). South of Grand Canyon, the Toroweap fault links with the Aubrey fault; north of Grand Canyon, it extends ~250 km as the active Sevier/Toroweap fault zone (Fig. 1; Pearthree et al, 1983; Pearthree,
1998; U.S. Geological Survey and Arizona Geo-logical Survey, 2006). Total west-down strati-graphic separation of Paleozoic units is variable along strike. For the location where it crosses Grand Canyon, stratigraphic separation has been variably reported (177–370 m; Table DR2), but we use the value of 193 m of McKee and Schenk (1942), based on offset of key horizons in the Cambrian part of the section.
Total post-basalt (post–600-ka) slip on the Toroweap fault where it crosses Grand Canyon has been reported as 44 m (McKee and Schenk, 1942), 46 m (Hamblin, 1970a), and 60 m (this paper). Late Quaternary separation is about one-half (Billingsley, 2001) to one-third (this study) of total stratigraphic separation (Table DR2). This has been interpreted to mean that the fault is one of the youngest and most active normal faults in western Grand Canyon (Jackson, 1990), likely less than 2–3 million years old.
The new dates on the Upper Prospect fl ows in Prospect Canyon (Fig. 4A) and on the Toroweap C fl ow (Fig. 12C) help refi ne estimates of dis-placement rate (Jackson, 1990; Fenton et al., 2001). As shown in Figures 12A and 12B, the contact between the highest Prospect Canyon basalt fl ow and the base of the Quaternary side-stream fi ll terraces at the rim of Prospect Canyon basalt is offset a total of 52 m by the fault (three strands). This measurement is comparable to the measurement of 46 m by Huntoon (1977) that was presumably taken on the main strand. Using the 52-m offset (Fig. 12), combined with the 518 ± 22-ka mean age of the Upper Prospect fl ows, yields a displacement rate of 100 m/Ma. Like-wise, a marker red sandstone (Figs. 12A and 12B) just above the lower Prospect fl ow (mean age of 568 ± 52 ka) is offset 60 m, yielding a slip rate of 106 m/Ma. Figure 12 shows that the new Ar-Ar ages for basalts generally agree with
-20
0
20
40
60
80
100
0 100 200 300 400 500 600 700 800
Age (ka)
Hei
gh
t (m
)
Eastern Grand Canyon BlockIncision Rate 172 m/Ma Modern Depth to Bedrock = 28 mR2 = .948
Western Grand Canyon BlockIncision Rate 55 m/MaModern Depth to Bedrock = 9 m R2 = .996
16
28
8
25
21
26
20
22
24
27
28
18
33
5
14
15
17 6
21
19
4
7
13
E Grand Canyon incision rates used in regression
Estimated E Grand Canyon incision rates
W Grand Canyon incision rates used in regression
Estimated W Grand Canyon incision rates
Sample location, showing thickness of gravels
Strath height and age with 2 sigmauncertainty
Maximum rate (see Table 2)
Fill terrace heights (Pederson et al., 2006)
Incision rate data point keyed to Table 2
Ages from fill terraces (Pederson et al., 2006)
3
Figure 10. Alternate method of calculating incision rates (modifi ed from Pederson et al., 2006) uses strath to strath heights and ages (data from Table 2) to give average incision rates of 173 m/Ma in the eastern Grand Canyon and 55 m/Ma in western Grand Canyon. Dated fi ll terraces and heights (light gray) show climatically infl uenced aggradational and incision intervals (modifi ed from Pederson et al., 2006).
Karlstrom et al.
1298 Geological Society of America Bulletin, November/December 2007
2 1.5 1 0.5 0 km
7 6 5 4 3 2 1 0
2 1.5 1 0.5 0 km
7 6 5 4 3 2 1 010
00 ft
.10
00ft.
East
4 km
0
2 k
m
horiz
onta
l sca
le
4x v
ertic
al e
xag.
Tru
e Sc
ale
Cro
ss S
ecti
on
Wes
t
Cal
cula
ted
inci
sio
n ra
te
100
m/M
a
Fau
lt s
lipra
te10
0 m
/Ma
Pk Mr
PcPt Ph Pe Dtb
Cb
aC
t
CmPs
Pk Mr
PcPt Ph Pe Dtb
Cb
aC
t
CmPs
PC
7) RM 177: 145 m/Ma (351 ka)8) RM 178.9: 162 m/Ma (487 ka)Slip rate: ~100 m/Ma (630 ka)
Toroweap fault (RM 179)9) RM 179.3:: 66 m/Ma (487 ka)
10) RM 181.6: 68 m/Ma (607 ka)12) RM 182.8: 80 m/Ma (475-625 ka)13) RM 183: Max 155 m/Ma (475-625 ka)
14) RM 184.55: Max 165 m/Ma (200 ka)
Lava fault (RM 190)
2-5) RM 56-57: 150 m/Ma (153-385 ka) (average of incision points 2-5)
Eastern Grand Canyon
Slip rate: 70 m/Ma (185 ka)Hurricane fault (RM 191)
16) RM 188.1: 136 m/Ma (475-625 ka)18) RM 188.85: 111 m/Ma (475-625 ka)
22) RM 195.2: Max 64 m/Ma (475-625 ka)
27) RM 223.1: 73 m/Ma (475-625 ka)
21) RM 194.2: 58 m/Ma (475-625 ka)
25) RM 207.3: 76 m/Ma (605 ka)
24) RM 203.4: 64 m/Ma (475-625 ka)
26) RM 208.35: 73 m/Ma (528 ka)
28) RM 246: Max 75 m/Ma (723 ka)
Mohawk Fault (RM 171)
Parashant Canyon (RM 198)
Frog and Granite Park faults
Western Grand Canyon
Uin
kare
t b
lock
Wes
tern
Gra
nd
Can
yon
blo
ckEa
ster
n G
ran
d C
anyo
n b
lock
PC
A‘
A
02 134km
02 134km
Un
cert
ain
in
cisi
on
rate
(inco
mp
lete
d
ata)
10
0 m
/Ma
100
50 0
Incision rate(m/Ma) sc
ale
Fig
ure
11. I
ncis
ion-
rate
dat
a an
d fa
ult-
dam
pene
d di
ffer
enti
al r
iver
inci
sion
mod
el f
or G
rand
Can
yon.
Pro
ject
ed r
iver
gra
dien
t an
d ca
nyon
rim
sho
wn
on E
-W c
ross
sec
tion
of
wes
tern
Gra
nd C
anyo
n (l
ocat
ion
show
n in
Fig
. 1).
Eas
tern
Gra
nd C
anyo
n bl
ock
show
s a
fair
ly u
nifo
rm in
cisi
on r
ate
of 1
45–1
62 m
/Ma
from
RM
57
to R
M 1
79. U
inka
ret
bloc
k sh
ows
dim
inis
hed
appa
rent
inci
sion
(58–
66 m
/Ma)
at e
ast e
nd, i
ncre
asin
g to
nea
r th
e ea
ster
n ca
nyon
rat
e at
wes
t end
, int
erpr
eted
to in
dica
te th
at s
lip o
n To
row
eap
faul
t is
bei
ng a
ccom
mod
ated
by
acti
ve f
orm
atio
n of
a h
angi
ng-w
all
anti
clin
e. W
este
rn G
rand
Can
yon
bloc
k al
so s
how
s lo
wes
t ap
pare
nt i
ncis
ion
rate
s (<
58 m
/Ma)
in
imm
edia
te
hang
ing
wal
l of H
urri
cane
faul
t, b
ut r
ates
incr
ease
to o
nly
76 m
/Ma,
indi
cati
ng th
at lo
wer
ed r
ates
are
the
resu
lt o
f bot
h fo
rmat
ion
of a
han
ging
-wal
l ant
iclin
e an
d lo
wer
ing
of
wes
tern
blo
ck b
y 75
–100
m/M
a re
lati
ve t
o ea
ster
n G
rand
Can
yon
bloc
k. S
ee T
able
2 f
or d
escr
ipti
on o
f ea
ch d
ata
poin
t an
d F
igur
e 9
for
grap
hica
l por
tray
al o
f un
cert
aint
ies
for
vect
ors.
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1299
TA
BLE
2. Q
UA
TE
RN
AR
Y IN
CIS
ION
RA
TE
S (
IR)
IN G
RA
ND
CA
NY
ON
* A
ND
UN
CE
RT
AIN
TIE
S
IP
num
ber
RM
N
ame
D
Age
(k
a)
± 2σ
Str
ath
heig
ht
(m)
Sam
ple
heig
ht(m
)
Mea
nPD
(m)
Max
PD
(m)
Max
BD
(m)
IR –
2σ
max
m
/Ma
IR p
m
/Ma
IR +
2σ
min
m/M
a
IR m
ean
PD
m/M
a
IR m
ax
BD
m/M
a
Not
es
1 56
L T
rave
rtin
e
119
±2
<0
26
13
23
28
197
193
190
109
235
Max
imum
inci
sion
rat
e, tr
aver
tine
in
uppe
r M
4? d
epos
it; P
eder
son
et a
l.,
2006
; spa
rse
grav
els,
mai
nly
hills
lope
co
lluvi
um
2 56
L
Tra
vert
ine
15
3 ±
3 <0
5
13
23
28
152
150
148
85
183
Max
imu
m in
cisi
on
rat
e, t
rave
rtin
e d
rap
e o
n b
edro
ck t
hat
ext
end
s to
ri
ver
leve
l; P
eder
son
et
al.,
2006
; sp
arse
peb
ble
s in
san
dy
dep
osi
t b
elo
w t
rave
rtin
e n
ear
rive
r
3 57
L
Tra
vert
ine
38
5 ±
14
36
38
13
22
28
156
151
145
127
166
Ped
erso
n e
t al
., 20
06, a
ge
is m
ean
of
two
an
alys
es, n
ew h
eig
hts
(20
06),
tr
aver
tin
e d
rap
e co
vers
gra
vels
2
m a
bo
ve s
trat
h
4 57
L
Tra
vert
ine
34
3 ±
28
23
24
13
22
28
143
131
121
105
149
Ped
erso
n e
t al
., 20
06;
age
is m
ean
o
f tw
o a
nal
yses
, new
hei
gh
ts
(200
6), t
rave
rtin
e ri
nd
on
m-s
cale
ri
ver
clas
t at
str
ath
5
57L
T
rave
rtin
e
283
±14
24
24
13
22
28
16
8 16
3 15
8 13
1 18
4 P
eder
son
et
al.,
2006
, tra
vert
ine
cem
ente
d g
rave
ls in
set
into
M5,
12
m lo
wer
str
ath
th
an #
3 6
57L
Tra
vert
ine
12
5 ±
2 14
14
13
22
28
29
5 28
8 28
1 21
6 33
6 M
axim
um in
cisi
on r
ate;
Ped
erso
n et
al
., 20
06, n
ew h
eigh
ts (
this
pap
er),
lo
wes
t str
ath
in s
tepp
ed s
eque
nce;
m
ostly
col
luvi
um, m
inor
am
ount
of
grav
el a
bove
str
ath
7 17
7.3L
17
7 re
mn
ant*
–2
35
1 ±
25n
= 3
32
36
11
19
28
15
6 14
5 13
6 12
3 17
1 M
od
ifie
d f
rom
Ped
erso
n e
t al
., 20
02,
pill
ow
s o
f b
asal
t in
riv
er s
and
an
d
gra
vel r
esti
ng
on
str
ath
8
178.
9R
To
row
eap
C
–0
487
± 48
n =
1
60
~74
11
19
28
180
162
148
146
181
To
row
eap
fau
lt—
up
thro
wn
sid
e,
gra
vel a
bo
ve s
trat
h a
nd
bel
ow
bas
alt
is m
ost
ly n
on
-vo
lcan
ic
clas
ts
9 17
9.3R
T
oro
wea
p C
1
487
± 48
n =
1
13
~14.
5 11
19
28
73
66
60
49
84
T
oro
wea
p f
ault
—d
ow
nth
row
n s
ide,
1.
5-m
-th
ick
gra
vel a
t d
ista
nce
500
m
wes
t o
f fa
ult
, th
icke
r g
rave
l at
fau
lt (
~25
m)
10
181.
6R
Po
nd
ero
sa
4 60
7 ±
48n
= 2
22
46
11
19
28
73
68
63
54
82
60
7 ±
48 P
on
der
osa
rem
nan
t o
f D
alry
mp
le a
nd
Ham
blin
(19
98);
fl
ow
un
der
lain
by
gra
vel,
cin
der
s,
collu
viu
m
11
182.
5R
Lava
Fal
ls
5 N
o da
te
21
28.6
12
19
28
84
73
64
60
89
A
ssum
ed a
ge o
f 475
–625
ka
base
d on
LI
DA
R h
eigh
ts o
f top
and
bot
tom
of
flow
(c
ontin
ued)
Karlstrom et al.
1300 Geological Society of America Bulletin, November/December 2007
TA
BLE
2. Q
UA
TE
RN
AR
Y IN
CIS
ION
RA
TE
S (
IR)
IN G
RA
ND
CA
NY
ON
* A
ND
UN
CE
RT
AIN
TIE
S(c
ontin
ued)
IP
num
ber
RM
N
ame
D
Age
(k
a)
± 2σ
Str
ath
heig
ht
(m)
Sam
ple
heig
ht(m
)
Mea
nPD
(m)
Max
PD
(m)
Max
BD
(m)
IR –
2σ
max
m
/Ma
IR p
m
/Ma
IR +
2σ
min
m/M
a
IR m
ean
PD
m/M
a
IR m
ax
BD
m/M
a
Not
es
12
182.
8R
Lava
Fal
ls
5 N
o da
te
25
25
12
19
28
93
80
70
67
96
Fra
ctur
ed b
asal
t mix
ed w
ith r
iver
san
d an
d si
lt fil
ls a
cha
nnel
-like
feat
ure,
flo
w a
lso
tops
mai
n st
em r
iver
gr
avel
s, a
ssum
ed a
ge o
f 475
–625
ka
base
d on
LID
AR
hei
ghts
of t
op a
nd
botto
m o
f flo
w
13
183R
B
urie
d C
anyo
n 6
No
date
66
66
12
19
28
17
9 15
5 13
6 14
2 17
1 M
axim
um in
cisi
on r
ate,
bas
e of
flow
co
vere
d by
talu
s; a
ssum
ed a
ge o
f 47
5–62
5 ka
bas
ed o
n LI
DA
R h
eigh
ts
of to
p an
d bo
ttom
of f
low
14
18
4.55
L G
ray
Ledg
e*
8 20
0 ±
72
n =
1
<15
21
12
19
28
25
8 16
5 12
3 13
0 21
0 M
axim
um in
cisi
on r
ate,
Bla
ck L
edge
re
mna
nt o
f Ham
blin
(19
94),
but
too
youn
g, L
IDA
R fl
ow to
p he
ight
s si
mila
r to
200
ka
laye
red
diab
ase
on th
e op
posi
te s
ide
of th
e riv
er
15
187.
9L
Gra
y Le
dge
12
194
± 3
9n
= 1
0
0 11
18
28
11
6 93
77
57
14
4 Lo
wer
flow
(at
riv
er le
vel)
is 1
94 ±
39,
m
ay b
e sl
umpe
d, b
ase
of u
pper
flow
se
para
ted
from
low
er fl
ow b
y ba
salti
c riv
er g
rave
ls, s
trat
h he
ight
11
m a
t w
est e
nd o
f out
crop
, but
str
ath
is
belo
w r
iver
leve
l at e
ast e
nd; n
ot
used
in in
cisi
on c
alcu
latio
ns b
ecau
se
of u
ncer
tain
ty in
geo
logi
c co
ntex
t 16
18
8.1R
B
lack
Led
ge *
12
N
o da
te
56
64
11
19
28
158
136
120
122
153
3 m
of m
ain
stem
riv
er g
rave
ls, 3
m o
f co
lluvi
um, 2
.5 m
of t
ephr
a un
der
flow
; as
sum
ed a
ge o
f 475
–625
ka
base
d on
LID
AR
hei
ghts
of t
op a
nd b
otto
m
of fl
ow
17
188.
1R
Gra
y Le
dge
12
97 ±
26
n =
1
<13
15
11
19
28
45
1 33
0 26
0 24
7 42
3 M
axim
um in
cisi
on r
ate,
12.
5 m
to fi
rst
basa
lt ric
h gr
avel
(no
str
ath)
18
188.
85L
Bla
ck L
edge
12
N
o da
te
42
75-6
8 11
19
28
12
8 11
1 98
96
12
7 F
ield
rel
atio
nshi
ps c
ompl
ex—
mul
tiple
flo
ws
and
stra
ths;
mas
sive
flow
co
rrel
ated
to 4
83 ±
80
ka A
r/A
r da
te
of F
ento
n et
al.,
200
4 19
18
9.1L
G
ray
Ledg
e 12
12
7 ±
27
n =
2
<7
10
11
19
28
260
205
169
142
276
Max
imum
inci
sion
rat
e, 8
m to
low
est
grav
el @
700
0 cf
s (9
0% b
asal
t cl
asts
—no
str
ath)
20
19
2L
Laye
red
Dia
base
13
33
2 ±
39
n =
2
<18
18
11
19
28
12
6 11
1 10
0 87
13
9 M
axim
um in
cisi
on r
ate,
bas
alt r
ests
di
rect
ly o
n be
droc
k 21
19
4.2R
B
lack
Led
ge
16
No
date
13
22
11
19
28
67
58
51
44
75
A
ssum
ed a
ge o
f 475
–625
ka
base
d on
LI
DA
R h
eigh
ts o
f top
and
bot
tom
of
flow
22
19
5.2R
M
assi
ve D
iab
ase
18
298
± 57
n =
1
<7
7 11
19
28
79
64
54
37
94
M
axim
um
inci
sio
n r
ate,
LID
AR
h
eig
ht
to b
ase
of
flo
w, f
low
res
ts
dir
ectl
y o
n b
edro
ck
(con
tinue
d)
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1301
TA
BLE
2. Q
UA
TE
RN
AR
Y IN
CIS
ION
RA
TE
S (
IR)
IN G
RA
ND
CA
NY
ON
* A
ND
UN
CE
RT
AIN
TIE
S(c
ontin
ued)
IP
num
ber
RM
N
ame
D
Age
(k
a)
± 2σ
Str
ath
heig
ht
(m)
Sam
ple
heig
ht(m
)
Mea
nPD
(m)
Max
PD
(m)
Max
BD
(m)
IR –
2σ
max
m
/Ma
IR p
m
/Ma
IR +
2σ
min
m/M
a
IR m
ean
PD
m/M
a
IR m
ax
BD
m/M
a
Not
es
23
195.
3L
Bla
ck L
edge
18
N
o da
te
10
11.7
5 11
19
28
61
53
46
38
69
A
ssum
ed a
ge o
f 475
–625
ka
base
d on
LI
DA
R h
eigh
ts o
f top
and
bot
tom
of
flow
24
20
3.4R
B
lack
Led
ge
24
No
date
17
21
11
18
28
74
64
56
51
82
A
ssum
ed a
ge o
f 475
–625
ka
base
d on
LI
DA
R h
eigh
ts o
f top
and
bot
tom
of
flow
, riv
er g
rave
ls 1
0% b
asal
t in
clud
ing
exot
ic c
last
s 25
20
7.3L
B
lack
Led
ge
21
605
± 12
n =
2
25
28.5
12
21
28
78
76
75
61
88
A
r/A
r d
ate
fro
m L
ucc
hit
ta e
t al
., 20
00, h
eig
ht
to s
trat
h m
easu
red
in
2002
, flo
w u
nd
erla
in b
y 3.
5 m
of
mai
n s
tem
riv
er g
rave
ls in
clu
din
g
exo
tic
clas
ts
26
208.
35R
B
lack
Led
ge
20
528
± 39
n =
2
20
34.5
12
20
28
82
76
71
61
91
H
eig
hts
to
str
ath
an
d b
ase
of
bas
alt
rem
easu
red
in 2
006,
pre
vio
us
mea
sure
men
ts v
ary
fro
m 2
4–22
m
27
223.
1R
Bla
ck L
edge
12
N
o da
te
16
16
12
24
28
84
73
64
51
80
0.25
m o
f mai
nste
m r
iver
gra
vel u
nder
flo
w, a
ssum
ed a
ge o
f 475
–625
ka
base
d on
LID
AR
hei
ghts
of t
op a
nd
botto
m o
f flo
w
28
246R
B
lack
Led
ge
39
723
± 31
n =
1
<30
<30
12
24
28
78
75
72
58
80
Hei
gh
t fr
om
Sp
ence
r C
anyo
n D
am
Su
rvey
—ap
pro
xim
atel
y 10
0' t
o
stra
th f
rom
riv
er le
vel a
nd
dep
th t
o
bed
rock
is 1
2 m
29
29
0L
San
dy P
oint
4410
± 3
010
5 10
7 15
28
27
27
27
San
dy P
oint
bas
alt;
age
from
Fau
lds
et
al.,
2001
; app
roxi
mat
e he
ight
of
basa
lt ab
ove
river
from
Luc
chita
(1
972)
, bas
ed o
n Lo
ngw
ell,
assu
med
po
ol d
epth
is 1
5 m
30
42
5R
Pan
da g
rave
ls
55
00
43
65
28
20
Str
ath
at b
ase
of P
anda
gra
vels
, P
anda
Gul
ch (
Hou
se e
t al.,
200
5,
p. 3
67);
>65
-m d
epth
to b
edro
ck
7 km
ups
trea
m a
t Dav
is D
am
Ave
rage
Eas
tern
Gra
nd C
anyo
n R
ates
(b
old
rat
es)
159
150
143
119
172
Ave
rage
Wes
tern
Gra
nd C
anyo
n R
ates
(b
old
rat
es)
8073
67
54
87
Not
e: IP
—in
cisi
on p
oint
; RM
—riv
er m
ile b
elow
Lee
s F
erry
(R
—riv
er r
ight
; L—
river
left)
; D—
dist
ance
wes
t of T
orow
eap
faul
t; st
rath
and
sam
ple
heig
ht r
elat
ive
to 1
0,00
0 cf
s le
vel;
PD
—po
ol d
epth
; B
D—
bedr
ock
dept
h; IR
—in
cisi
on r
ate;
IR p
—pr
efer
red
inci
sion
rat
e.
*B
old—
best
con
stra
ined
inci
sion
rat
es; i
talic
s—m
axim
um in
cisi
on r
ates
.
Karlstrom et al.
1302 Geological Society of America Bulletin, November/December 2007
Q
T4T4
T4
T4
52 m
off
set
of T
4 st
rath
colu
mn
arjo
inte
d fl
ow
60 m
off
set
of
red
san
dst
on
e m
arke
r
red,
fin
e-g
rain
ed s
eds
6 10
rub
ble
(10)
rub
ble
(15)
rub
ble
(15)
rub
ble
rub
ble
(21)
rub
ble
(18) se
ds
red,
fin
e-g
rain
edse
ds
(13)
mas
sive
(10)
mas
sive
(10)
mas
sive
(13)
mas
sive
40 m
17) 5
33 ±
82
ka
15) 4
86 ±
21
ka 14) 5
41 ±
53
ka
13) 5
30 ±
23
ka
16) 5
33 ±
20
ka
602
± 3
7 ka
541
± 2
2 ka
n=
5
Esti
mat
es o
f fau
lt s
lip ra
te o
n T
oro
wea
p fa
ult
in P
rosp
ect
Can
yon
2
1
Qu
ater
nar
y te
rrac
e fil
l
Red
sed
imen
tary
mar
ker u
nit
Bas
alt
flow
s
mea
n o
f all
3 lo
wer
Pro
spec
t ag
es
(incl
ud
ing
a re
mn
ant
ori
gin
ally
map
ped
as
“D-D
am”, H
amb
lin, 1
994)
is 5
68 ±
52
ka
This
ag
e is
no
t in
ag
reem
ent
wit
h it
s st
rati
gra
ph
ic p
osi
tio
n
Mea
n a
ge
is
533
± 1
9 ka
Mea
n a
ge
is
532
± 2
1 ka
mea
n o
f all
5 u
pp
er
Pro
spec
tag
es is
518
± 2
2 ka
E
WU
pp
er P
rosp
ect
Flo
w
40m
12m
Fau
lt S
carp
Pro
spec
tC
ind
er C
on
e
Co
lora
do
Riv
er
thic
knes
s in
met
ers
(10)
WE W
E
35°
dip
70°
dip
Stra
th o
ffse
t47
m
Toro
wea
p C
Flo
w
Toro
wea
p A
Flo
w
Toro
wea
p A
Flo
w
rive
r gra
vel
Dik
e
Cam
bri
an B
rig
ht
An
gel
S
hal
e
35°
dip
70°
dip
Stra
th o
ffse
t47
m
Toro
wea
p C
Flo
w
Toro
wea
p A
Flo
w
Toro
wea
p A
Flo
w
rive
r gra
vel
Dik
e
Cam
bri
an B
rig
ht
An
gel
S
hal
e
A
B C
Fig
ure
12. (
A) T
orow
eap
faul
t cr
oss
sect
ion
base
d on
mea
sure
d se
ctio
n in
P
rosp
ect
Can
yon.
Ins
et p
hoto
s sh
ow (
B)
mou
th o
f P
rosp
ect
Can
yon
sout
h of
riv
er (
look
ing
sout
h) w
here
top
of
Upp
er P
rosp
ect
fl ow
(m
ean
age
of
518
± 22
ka)
is
offs
et 5
2 m
giv
ing
a di
spla
cem
ent
rate
of
100
m/M
a an
d th
e L
ower
Pro
spec
t fl o
w (
mea
n ag
e of
568
± 5
2 ka
) is
off
set
60 m
giv
ing
a di
spla
cem
ent
rate
of
106
m/M
a. (
C) V
iew
look
ing
nort
h at
Tor
owea
p fa
ult
on n
orth
sid
e of
riv
er s
how
s lis
tric
geo
met
ry a
nd s
play
s th
at o
ffse
t the
bas
al
Toro
wea
p A
fl o
w a
nd u
nder
lyin
g ri
ver
grav
els
as w
ell
as t
he 4
87 ±
48
ka
Toro
wea
p C
bas
alt;
str
ath
on t
op o
f th
e C
ambr
ian
Bri
ght
Ang
el S
hale
(C
ba)
is o
ffse
t 47
m g
ivin
g a
slip
rat
e of
97
m/M
a.
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1303
their stratigraphic position, with the exception of #15, the 486 ± 21 fl ow, as discussed above. If this sample is ignored, using the mean age of the Upper Prospect fl ows as 533 ka, and a dis-placement of 52 m, yields a displacement rate of 98 m/Ma. For Toroweap C fl ow on the north side of the river near Lava Falls (Fig. 12C), this fl ow overlies Toroweap A fl ow and the underlying gravels and gives a minimum age for the strath. The strath is offset 47 m, about the same amount as Toroweap C fl ow (Hamblin, 1994; Fig. 27), giving a slip rate (over 487 ± 48 ka) of 97 m/Ma. These rates of 97–106 m/Ma are similar to the 111 ± 9-m/Ma rates reported by Fenton et al. (2001). Thus, we interpret the displacement rate in the last 600 ka to have been ~100 m/Ma, and to have been fairly uniform through this time interval (Hamblin, 1970a; Fenton et al., 2001), rather than accelerating (Jackson, 1990).
The Toroweap fault dips 60–65° west based on its map trace as it crosses the canyon (Hunt-oon et al., 1981). Direct measurements in Pros-pect Canyon and along the river show an over-all dip of ~65°, with low angle splays of ~35° (Fig. 12C). Thus, a displacement rate of 100 m/Ma slip translates to a throw (vertical compo-nent of dip slip) of ~90 m/Ma of the western block. However, our new results indicate that this Quaternary displacement is mainly taken up by formation of a hanging-wall fl exure in the Uinkaret half graben, as documented by vari-able apparent incision rates (Fig. 11) and slight upriver dip of the 500- to 600-ka fl ow surfaces determined from LIDAR analysis (Crow et al., 2007). The development of eastward dips of up to 10–18° on the Paleozoic strata (Wenrich et al, 1997) may be explainable by progressive devel-opment of a hanging-wall rollover anticline over the last several million years of normal faulting with steady slip rates, but there may have also been a preexisting Laramide fl exure along the Toroweap fault in this locality (Hamblin, 1994).
Hurricane FaultThe Hurricane fault has a history of Laramide
west-up (reverse) motion (Naeser et al., 1989; Kelley et al., 2001; Huntoon, 2003), including reactivated reverse fault segments (Figs. 4B and 13B). It has a complicated geometry with numer-ous segments along its >250-km-long strike length (Stenner et al., 1999) as well as anasto-mosing strands within Grand Canyon region and complex variation of displacement along and across them (Huntoon et al., 1981; Wenrich et al., 1997). In general, the Hurricane fault has more offset and is older to the north and less off-set and is younger to the south. In Grand Canyon, net west-down stratigraphic separation of Paleo-zoic units is 400–500 m in the Whitmore seg-ment immediately north of the Colorado River,
250–400 m in the area where it crosses the Colo-rado River (near RM 191; Figure 4B; Wenrich et al., 1997, 1981), and 730 m in the Three Springs area ~20 km to the south (Huntoon et al., 1981).
The amount of Neogene slip has been diffi -cult to quantify. Some workers have proposed minimal Quaternary displacement; for exam-ple, Huntoon et al. (1981) and Hamblin (1994) mapped the trace of the main Hurricane fault as covered by unfaulted remnants of the Gray Ledge (100–200 ka) and Whitmore fl ows (300–350 ka; Fig. 4B), suggesting that the Hurricane fault has no post–350-ka displacement. However, Bill-ingsley (2001) reported offset of 610 m on both the 3.6 ± 0.18-ka Bundyville basalt, and for the directly underlying Mesozoic strata, yielding a displacement rate of 169 m/Ma north of Grand Canyon. The amount of Laramide reverse offset remains unconstrained, but if one assumed there were no Laramide west-up ancestry, this would suggest that most of the displacement has taken place in the last 3.6 Ma. Amoroso et al. (2004) found that slip rates of 150–250 m/Ma were relatively constant since 1 Ma in the Shivwits section of the Hurricane fault just north of the Grand Canyon. Recent seismicity attests to con-tinued activity.
Fenton et al. (2001) estimated an average displacement rate of 81 ± 6 m/Ma over approx-imately the last 200 ka, based on He cosmo-genic surface dates and offsets of Whitmore Cascade (177 ± 9-ka) and Bar Ten (88 ± 6-ka) fl ows and young alluvial fans (29–74 ka). We measured fault displacement of 14 m (Fig. 4B) of a thick, columnar-jointed fl ow in Whitmore Canyon (Fig. 13A); 40Ar/39Ar dating of fallen basalt columns that are probably from this fl ow give 186 ± 26 ka (Table 1; Raucci, 2004), pro-viding a displacement rate of 75 m/Ma in the last 186 ka. We have also identifi ed new splays of the Hurricane fault system with Quaternary displacement along and east of the river near RM 190 (Fig. 4B). The fault west of the river displaces Whitmore-age remnants by 6 m; the fault east of the river has west-down offset of an alluvial deposit (Qfd4 of Fenton et al., 2004) that overlies the 319-ka Whitmore fl ow, with displacement varying from 10 to 15 m along the fault. These new displacements (Fig. 4B), if added to the 14 m along the strand in Whit-more Wash (Fig. 13B), give a cumulative slip of 30–35 m in the last 200–320 ka, and a mini-mum slip rate of 94–109 m/Ma. Further stud-ies of the partitioning of displacement between strands will be needed to refi ne these estimates, but we use the range 75–100 m/Ma as our cur-rent best estimate of late Quaternary slip rate on the Hurricane fault.
Figure 14 summarizes the combined incision- and slip-rate data. For the Hurricane fault, like the
Toroweap fault, Paleozoic rocks defi ne a hang-ing-wall anticline that formed at least in part due to Quaternary slip on listric faults. But, unlike the Toroweap block, this is not as strongly indi-cated by apparent incision-rate data. Additional dating is needed to decipher the extent of hang-ing-wall fl exure in this area. Apparent incision rates west of the Hurricane fault do not return to rates of eastern Grand Canyon and instead are fairly constant at 60–75 m/Ma (Fig. 14). Based on differential incision rates, this suggests that the western Grand Canyon block has subsided vertically ~100 m/Ma relative to the eastern Grand Canyon block, mainly due to move-ment along the Hurricane fault system, which may have been active longer (3–4 Ma) than the Toroweap fault (2–3 Ma) as Neogene extension has migrated eastward into the Colorado Plateau (Jackson, 1990).
Western FaultsThere are also a number of small faults
between the Hurricane and Grand Wash faults (Table DR2). For example, Resor (2007) iden-tifi ed 275 m of normal slip and accompany-ing fl exure across the Frogy fault system (RM 196.4). Huntoon et al. (1981, 1982) mapped approximately 45 faults that cross the river between RM 225 and 275. A majority of these faults have west-up displacement (550 m net separation) that probably took place during Laramide contraction; some have west-down displacement (185 m net west-down separation) and are likely Miocene. The net displacement from these faults is ~365 m of west-up separa-tion. The amount of Quaternary slip on these faults is unconstrained, but any contribution these faults make to lowered apparent incision rates in western Grand Canyon is less than the resolution of our existing data (Fig. 14).
The physiographic boundary between the Colorado Plateau and Basin and Range prov-inces is the Grand Wash cliffs, which marks the abrupt western end of Grand Canyon (Fig. 1). This is a retreating escarpment of Paleozoic rocks formed initially by movement on the Grand Wash fault zone pre-10 Ma (Beard, 1996; Brady et al., 2000; Faulds et al., 2001). Grand Wash fault zone separates the fl at-lying strata of the Colorado Plateau from the east-dipping 30–50° Paleozoic strata of the Wheeler Ridge block (Brady et al., 2000). There is ~3.5 km of stratigraphic separation across this zone, and strata of the Wheeler Ridge block are folded into a hanging-wall fl exure expressed in both the Paleozoic rocks and Neogene rocks. The traces of the Grand Wash fault strands are covered by unfaulted Muddy Creek Formation, indicating that movement ceased before ca. 10 Ma (Beard, 1996; Brady et al., 2000; Faulds et al., 2001).
Karlstrom et al.
1304 Geological Society of America Bulletin, November/December 2007
The Wheeler fault is a 60°-west-dipping normal fault (Longwell, 1936) that is exposed ~5 km west of the Grand Wash fault zone. It has ~2.5 km of normal stratigraphic separation of Paleozoic rocks (Brady et al., 2000). The Wheeler fault splits into several faults to the south, and these show ~300 m (Brady et al., 2000) to 450 m (Howard and Bohannon, 2001) of normal separa-tion on the top of the Hualapai limestone. Paleo-zoic rocks, the Hualapai limestone, and the 4.7-Ma Grand Wash basalts above the Wheeler fault all have east-dips defi ning a hanging-wall fl ex-ure (Howard and Bohannon, 2001). Paleozoic rocks dip 30–40° whereas Hualapai limestone dips <5°. Both slip amount and hanging-wall dip suggest that most slip took place before 6 Ma (Howard and Bohannon, 2001), although there is also signifi cant Neogene slip that is important for regional models (below).
The Iceberg Canyon fault, an additional 5 km west (mapped before the fi lling of Lake Mead; Longwell, 1936), is a 10°-35°–west-dipping, listric, normal fault. It has ~1.2 km of normal separation (Brady et al., 2000). Based on the lower elevation of the 4.4-Ma Sandy Point basalt (105 m above pre-dam river grade) rela-tive to the 4.7-Ma Grand Wash basalts (Howard and Bohannon, 2001), the base of which is up to 260 m above pre-dam river grade, we infer that some post–4.4-Ma slip took place on faults between the Wheeler and Iceberg Canyon fault systems. However, pending further mapping, we lump all post–6-Ma displacements to be part of the combined Wheeler/Iceberg Canyon fault systems.
Refi ned Model for Differential Incision of Grand Canyon Due to Fault Dampening
The new data on incision- and fault-slip rates confi rms and signifi cantly refi nes the model presented by Pederson and Karlstrom (2001) and Pederson et al. (2002b) that west-down displacement on Neogene normal faults in the western Grand Canyon dampens the east-ern Grand Canyon incision rate. The original model (Pederson and Karlstrom, 2001; Peder-son et al., 2002b) was:
footwall incision rate = (apparent hanging-wall incision rate) + (fault-slip rate). (1)
This works well in the immediate vicinity of the Toroweap fault, where the incision rate in the footwall (two closest rates upstream of the fault in Fig. 9) averages 154 m/Ma over the last 500 ka and is subequal to the sum of the aver-age incision rate in the immediate hanging wall of 67 m/Ma (two closest), plus the fault-slip rate of ~100 m/Ma. Across the Hurricane fault,
Whi
tmor
eCa
scad
e
colonnade186±26, n=2 14m
4m
Cba
Cm
Cm
Mr
MrMr
Dtb DtbPs
Ps
Q
W E
E W
B
A
Figure 13. Photos of offset basalts along Hurricane fault in Whitmore Canyon. (A) Looking north, 186 ± 26 colonnade fl ow is offset ~14 m (including fl exure) giving slip rate of 75 m/Ma. (B) Looking south, offset of ~4 m of Quaternary surface (51 ± 9; Fenton et al., 2001) and high-est Whitmore fl ow gives displacement rate of 80 m/Ma; syncline in footwall of one strand of Hurricane fault suggests Laramide contractional ancestry to this strand (shown as reverse fault in Fig. 4B) before extensional reactivation to produce the observed normal separation.
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1305
the relationship also works well: the footwall rate of 131 m/Ma (average of two closest) over approximately the last 550 ka is subequal to the downstream incision rate of 61 m/Ma (two clos-est) plus fault-slip rate of 75–100 m/Ma.
A refi nement of the model addresses what parts of fault slip are accommodated by hang-ing-wall fl exure versus relative vertical-block subsidence, and helps explain the effects of mul-tiple faults dampening a far-fi eld incision rate, and with faults operating over different time spans (Fig. 15A). By our original hypothesis, the combined slip on the faults of 175–200 m/Ma would suggest that apparent incision rates west of both faults would be zero or negative, resulting in the river that should be aggrad-ing. This is not supported by observations for positive bedrock incision of 50–75 m/Ma at all known locations and over all time scales as western blocks have been moving down relative to eastern blocks. This apparent discrepancy can be explained by a revised model:
footwall-incision rate = (apparent hanging-wall incision rate) + [(vertical-block lowering rate) + (slip rate accommodated by hanging-wall flexure)]. (2)
As shown in Figure 14, hanging-wall fl exure in the Uinkaret block accounts for much of the incision dampening, with overall block lower-ing of only ~10–20 m/Ma for the estimated slip of 100 m/Ma. West of the Hurricane fault, the importance of hanging-wall fl exure is less (~10 m/Ma), with vertical block lowering of 60–90 m/Ma accounting for most of the postu-lated slip rate (75–100 m/Ma). For the Wheeler fault, of the ~400 m of offset of the 6-Ma Huala-pai Limestone (Howard and Bohannon, 2001), about half is taken up by hanging-wall fl exure (Fig. 15A). These data suggest that different amounts of the total fault slip are partitioned into hanging-wall fl exure due to differing geom-etries and histories of the fault systems.
Long-Term Incision Models for Grand Canyon
Over longer time spans, fault displacement data and apparent incision rates from the Basin and Range province suggest that the fault damp-ening model and coherent block behavior of fault blocks has operated for the last 6 Ma. Given the pre-dam strath height of 105 m (Lucchitta, 1972), average rate of incision at Sandy Point
(RM 295) over the last 4.41 ± 0.03 Ma (Faulds et al., 2001) is 27 m/Ma (Fig. 15; Table 2). Although less well constrained, bedrock inci-sion rate in the last 6 Ma in the Mojave Val-ley of the Lower Colorado River is ~20 m/Ma (Fig. 15). As reported by House et al. (2005), the fi rst Colorado River gravels, the Panda gravels, fi ll paleovalleys cut into Miocene pre-Colorado River alluvial fan deposits at ~43 m above the present river in an area just a few km south of Davis Dam. Depth to bedrock at the dam is >65 m based on data from dam construction (Bahmoier, 1950). These data suggest appar-ent incision rates for the Lower Colorado River block of ~20 m/Ma, shown in Figure 15A, and suggest that this Basin and Range block has moved down ~180 m (~30 m/Ma) relative to the western Grand Canyon block since 6 Ma.
The data from the Lower Colorado River block (4.4–5.5 Ma) cover a different time frame than our new data from the Grand Canyon (approximately the last 720 ka). It is unlikely that incision was constant from 6 Ma to present due to expected changes in climatic, geomor-phic, and tectonic conditions, but there are few data that link the incision histories between the early history and our late Quaternary data. One
Eastern G.C. rate150-175 m/Ma
EasternGrand Canyon
Block
Lower Colorado River Block
UinkaretBlock
Western Grand Canyon Block
Quaternary block lowering rate of23 m/Ma
Quaternary block lowering rate of100 m/Ma
Toroweap fault
Hurricane fault
Wheeler fault
0
20
40
60
80
100
120
140
160
180175
150
75
50
30
20
65
40
115
-50510152025303540458590
Distance west from the Toroweap fault (m)
Ap
paren
t Incisio
n R
ate (m/M
a)
140
Well-constrained Uinkaret block incision rates
Well-constrained eastern Grand Canyon incision rates
Estimated Uinkaret block incision rates
Well-constrained western Grand Canyon block incision rates
Estimated western Grand Canyon block incision rates
Well-constrained lower Colorado River block incision rates
29 Incision rate data point keyed to Table 2
28
29
24
25 26
22
2321
16
18
8
7
12
1110
27
9
Figure 14. Plot of incision rate versus distance west of the Toroweap fault. Variation in apparent incision rates indicates variable subsidence west of the Toroweap fault due to formation of a hanging-wall anticline above a listric fault.
Karlstrom et al.
1306 Geological Society of America Bulletin, November/December 2007
550
m
330
m
Bous
e Fm
(5.5
Ma)
Moj
ave
Valle
yC
otto
nwoo
dVa
lley
Hua
lapa
i Ls
(6.0
Ma)
700
m
880
m
Lees Ferry
Little Colorado River
Kaibab upwarp
Toroweap fault
Hualapai Plateau
Grand Wash fault
Hurricane fault
Eminence fault
Palisades fault
Iceburg Canyon faultWheeler fault
Gold Butte
S. Virgin Detachment
Black MountainHoover Dam
Topock Divide Mojave Canyon
Needles
Davis Dam - Pyramid Hills
Hoover Dam Potholes
Boulder Canyon Lip
Parker
Glen Canyon Dam
Red
Butt
e ba
salt
(8
.7-9
.4 M
a)
Long
Pt b
asal
t(7
.0 M
a)Sh
ivw
itts
bas
alt
(7.1
- 8.
2 M
a)
Snap
Pt b
asal
t(9
.1 M
a)
Mr
Pk
Pc
Ph
Ps
Cm
Cba
Ct
Mr
Cm
Cba
Ct
Xu
Xu
Yu
450
m
Cu
Mu
East
ern
Gra
nd
Can
yon
Blo
ck
Wes
tern
Gra
nd
C
anyo
n B
lock
Low
er C
olo
rad
o R
iver
Blo
ck
Elevation7,0
00
6,0
00
5,0
00
4,0
00
3,0
00
2,0
00
1,0
00
0 1,0
00
ftkm 2 1.5 1 .5 0 -.5
7,0
00
6,0
00
5,0
00
4,0
00
3,0
00
2,0
00
1,0
00
0 1,0
00
ftkm 2 1.5 1 .5 0 -.5
Elevation
150 m/Ma
150 m/Ma x 6 Ma
175 m/Ma
175 m/Ma x 6 Ma
57 m/Ma x 6 Ma
20 m/Ma x 5.5 Ma
27 m/Ma x 5.5 Ma
Mod
elE
aste
rn
GC
Wes
tern
G
CLo
wer
C
R
Tor
owea
p/H
urric
ane
faul
ts
Whe
eler
/Ic
ebur
g fa
ult
115
0 / 9
0057
/ 34
227
/ 16
293
/ 55
830
/ 18
0
217
5 / 1
050
57 /
342
27 /
162
118
/ 708
30 /
180
Inci
sio
n R
ate
(m/M
a) /
Am
ou
nt
(m)
Fau
lt L
ow
erin
g R
ate
(m/M
a) /
Am
ou
nt
(m)
Bida
hoch
i Fm
(6.
5 M
a)19
50 m
Mio
cene
pal
eoca
nyon
?Yo
ung
(200
7)
Sand
y Pt
bas
alt (
4.4
Ma)
Grand Wash basalts (4.7 Ma)
Tod
ay’s
pro
file
A
550
m
330
m
Bous
e Fm
(5.5
Ma)
Moj
ave
Valle
yC
otto
nwoo
dVa
lley
Hua
lapa
i Ls
(6.0
Ma)
700
m88
0 m
Bida
hoch
i Fm
(6.
5 M
a)19
50 m
800
0
5010
015
020
025
030
035
040
045
050
0
0
100
200
300
400
500
600
700
800
550
900
mile
s
km
Dis
tanc
e D
owns
trea
m fr
om L
ees
Fer
ry
Mr
Pk
Pc
Ph
Ps
Cm
Cba
Ct
Mr
Cm
Cba
Ct
Xu
Xu
Yu45
0 m
Cu
Mu
East
ern
Gra
nd
Can
yon
Blo
ck
Wes
tern
Gra
nd
C
anyo
n B
lock
Low
er C
olo
rad
o R
iver
Blo
ck
Restored elevation of the Lower Colorado River Block
5,0
00
4,0
00
3,0
00
2,0
00
1,0
00
0 1,0
00
ftkm
1.5 1 .5 0 -.5
Mo
del
1
Mo
del
2
Red
Butt
e ba
salt
(8
.7-9
.4 M
a)
Long
Pt b
asal
t(7
.0 M
a)
150 m/Ma
150 m/Ma x 6 Ma
175 m/Ma
175 m/Ma x 6 Ma
Mio
cene
pal
eoca
nyon
?Yo
ung
(200
7)
Shiv
wit
ts b
asal
t(7
.1 -
8.2
Ma)
Snap
Pt b
asal
t(9
.1 M
a)
57 m/Ma x 6 Ma
20 m/Ma x 5.5 Ma
27 m/Ma x 5.5 Ma
Sand
y Pt
bas
alt (
4.4
Ma)
Grand Wash basalts (4.7 Ma)
Rest
ore
d m
od
els
for i
nci
sio
n h
isto
ryB
Elevation Relative to the Colorado Plateau
7,0
00
6,0
00
5,0
00
4,0
00
3,0
00
2,0
00
1,0
00
0ftkm 2 1.5 1 .5 0
Fig
ure
15.
(A)
Col
orad
o R
iver
lon
gitu
dina
l pr
ofi le
wit
h pr
ojec
ted
bedr
ock
unit
s in
Gra
nd C
anyo
n; p
ink—
Pre
cam
bria
n ba
sem
ent,
blu
e—P
aleo
zoic
str
ata,
gr
een—
Mes
ozoi
c st
rata
, yel
low
—C
enoz
oic
depo
sits
; sch
emat
ic r
epre
sent
atio
n of
geo
logi
c un
its
mod
ifi ed
from
Moo
res
(in
LaR
ue, 1
925)
; not
e ~1
0 ti
mes
ver
tica
l ex
agge
rati
on. T
oday
’s r
iver
pro
fi le
show
n w
ith
thre
e bl
ocks
defi
ned
by
inte
rnal
ly c
onsi
sten
t ap
pare
nt i
ncis
ion
rate
s—ea
ster
n G
rand
Can
yon,
wes
tern
Gra
nd
Can
yon,
Low
er C
olor
ado
Riv
er b
lock
. Red
line
s re
pres
ent a
mou
nt o
f inc
isio
n in
6 M
a m
odel
ed b
y ex
tend
ing
Qua
tern
ary
rate
s ba
ck in
tim
e. (B
) F
ault
-dam
pene
d in
cisi
on m
odel
res
tore
s fa
ult
slip
s an
d m
odel
s hy
poth
etic
al r
iver
pal
eopr
ofi le
at
6 M
a us
ing
equa
tion
for
Mod
el 2
(se
e te
xt).
Mod
els
1 an
d 2
diff
er in
ter
ms
of
amou
nt o
f fau
lt lo
wer
ing
on th
e co
mbi
ned
Hur
rica
ne/T
orow
eap
faul
ts s
yste
m n
eede
d to
mat
ch 1
50–1
75 m
/Ma
inci
sion
rat
es in
eas
tern
Gra
nd C
anyo
n. F
or th
ese
mod
els,
the
Low
er C
olor
ado
Riv
er b
lock
has
bee
n lo
wer
ed 7
38–8
88 m
(re
spec
tive
ly)
on n
orm
al f
ault
s si
nce
6 M
a re
lati
ve t
o ea
ster
n G
rand
Can
yon
bloc
k.
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1307
approach, supported by our data, that bedrock incision rates in Grand Canyon have been steady over approximately the last 720 ka, is to extrap-olate Quaternary incision rates back in time to provide at least a fi rst-order comparison of inci-sion histories at short and long time scales.
Figure 15A is drawn with scaled incision vec-tors drawn at six times the vertical scale of the river profi le such that vectors show the amount of bedrock incision that would have occurred over the last 6 Ma at Quaternary rates, com-pared to the depth of Grand Canyon. The upper (dashed) line in Figure 15A shows 6 Ma of bed-rock incision in eastern Grand Canyon at 175 m/Ma (from the regressed line of Fig. 10); the lower (solid) line shows 6 Ma of bedrock inci-sion at 150 m/Ma (using maximum pool depth from Fig. 9). Thus, Quaternary incision rates, extrapolated back 6 Ma, can explain approxi-mately two-thirds of the present depth of eastern Grand Canyon (Fig. 15A). For western Grand Canyon block, fault-dampened Quaternary inci-sion rates, extrapolated back 6 Ma (solid line), would explain only approximately one-third of the depth of western Grand Canyon.
This 200- to 400-m “incision discrepancy” in eastern Grand Canyon and 700- to 900-m dis-crepancy in western Grand Canyon might be used to argue that our reported Quaternary rates are underestimates. Hanks et al. (2001; Hanks and Blair, 2003) reported Quaternary rates of ~500 m/Ma in Glen Canyon in the last 500 ka, and Marchetti and Cerling (2001) reported rates of 380–480 m/Ma in approximately the last 200 ka in the Fremont River tributary of the Col-orado River. But, if incision has taken place at these rates in Grand Canyon, this would require present bedrock depths to be an additional 80 m deeper (to get to 300 m/Ma over 500 ka) than our mean pool depth, which is not supported by any existing drilling measurements of maximum depth to bedrock (Fig. 8). One possibility is that the published Glen Canyon and Fremont River rates are overestimates because the cosmogenic surface ages used are minimum ages, perhaps beyond the useful 100- to 200-ka window for surface ages (Wolkowinsky and Granger, 2004). This interpretation is supported by the rates of 140 m/Ma reported on the San Juan River based on cosmogenic burial dating (Wolkowinsky and Granger, 2004). Hence, the difference between the needed long-term average eastern Grand Canyon incision rate of 275 m/Ma and our data for Quaternary rates of 150–175 m/Ma seems unlikely to be explained in terms of an under-estimate of late Quaternary rates (c.f. Hanks et al., 2001).
Instead, the “incision discrepancy” is best explained by a combination of non-steady, decelerating incision rates and the existence of
previously carved canyons that were used dur-ing integration of the Colorado River. Exploring the fi rst option, steady rates of 275 m/Ma would be needed to carve eastern Grand Canyon (up to 1650 m deep) in 6 Ma. Pre-Grand Canyon, 7- to 10-Ma basalts near the rim of Grand Canyon also provide an approximate incision/denuda-tion datum. South of Grand Canyon, the 8.8- to 9.7- Ma Red Butte basalt rests on Chinle For-mation (without river gravels) at an elevation of 2160 m, and the 7.0-Ma Long Point basalt over-lies Eocene gravels at an elevation of ~1900 m. North of Grand Canyon, the 7.1- to 8.2- Ma Shivwitts basalts rest on Kaibab and Moenkopi formations at elevations of ~1800 m and the 9.1-Ma Snap Point basalt rests on Kaibab Formation at an elevation of 1950 m (Billingsley, 2001) . Assuming the basalts fl owed into relative low spots in the landscape at 7–9 Ma, this basalt datum also suggests a long-term incision/denu-dation rate of ~250 m/Ma over this time interval (Fig. 15A). To explain the difference between the inferred long-term rates and the observed Quaternary rates, we envision that rapid base level fall for Colorado Plateau drainages took place over a geologically short time interval after initial integration of the system ca. 6 Ma. This led to early incision rates >250-300 m/Ma, with a subsequent decline in incision rates as channel slopes decreased, leading to Quaternary incision values of 150–175 m/Ma.
Another contributing factor to explain the incision discrepancy, especially in western Grand Canyon (Fig. 15A), is that initial integra-tion of the Colorado River from the Colorado Plateau to the Basin and Range may have taken advantage of previously carved canyons. Young (2001, 2007 and references therein) has docu-mented paleocanyons (Peach Springs, Milk-weed-Hindu), up to 1000 m deep, that existed in the Eocene to middle Miocene and contained north-fl owing rivers that drained the Mogollon highlands. These canyons were part of an exten-sive Paleogene drainage system possibly involv-ing the ancestral Salt River (Potochnik, 2001) and other deep Miocene canyons that were reused during drainage reversal and integration of the present drainage system. Similarly, sev-eral workers have postulated that western Grand Canyon, in general, and the Esplanade surface, in particular, may have been partly carved before 6 Ma (Scarborough, 2001), perhaps by early drainages that fl owed west from the Kaibab uplift (Young, 2001, 2007). In particular, Young (2007) proposed a Miocene canyon >600 m deep that was present in western Grand Can-yon, having developed in part due to structural relief from normal faulting on the Grand Wash fault system 16.5–10 Ma (Fig. 15A). The top of the 6-Ma Hualapai limestone, at an elevation of
880 m, is inferred to be near its original depo-sitional elevation relative to the Colorado Pla-teau block (Howard and Bohannon, 2001) and may have been the lake level fed by Miocene drainages (Young, 2007). The elevation of the base of the well-documented canyons near Peach Springs is 1100–1200 m (Young, 2001). Together these data (dotted line in Fig. 15A) suggest that perhaps half of the 700- to 900-m “incision discrepancy” in western Grand Can-yon may be explained by the existence of such paleocanyons (Fig. 15B), although the history of which paleocanyons were used and how they were linked remains unconstrained.
Restored Paleoprofi les
Figure 15B restores the profi le in Figure 15A by removing fault slip according to model param-eters in the table (upper left of Fig. 15) to arrive at a modeled 6-Ma river profi le. These models keep three parameters fi xed: (1) the range of eastern Grand Canyon incision rates is fi xed at 150–175 m/Ma to conform to Figures 9 and 10; (2) fault lowering on the Wheeler/Iceberg Can-yon fault is fi xed at 180 m over the last 6 Ma (30 m/Ma) based on observations of offset and fl exure of the Hualapai Limestone (Howard and Bohannon, 2001); (3) fault lowering on the com-bined Toroweap and Hurricane faults are consid-ered together for simplicity (and to conform to Fig. 14). Two possible fault-dampened incision models are shown. In the two models, apparent incision rates of 57 and 27 m/Ma for the west-ern Grand Canyon and Lower Colorado River blocks, respectively, can restore to match the 150- and 175-m/Ma eastern Grand Canyon inci-sion rates via 93 and 118 m/Ma of fault lowering active over 6 Ma on the combined Hurricane/Toroweap fault system. These models demon-strate that the fault-dampened incision model is capable of explaining observed data to fi rst order, especially if the “incision discrepancy” through-out Grand Canyon can be explained by higher 5- to 6-Ma incision rates in combination with the existence of western paleocanyons. These mod-els suggest a cumulative vertical displacement of 750–900 m between the Lower Colorado block and the eastern Grand Canyon block in the last 6 Ma; this displacement is needed to explain the Quaternary incision rate data.
More refi ned models will require better data on temporal and spatial partitioning of slip among different fault strands, as well as refi ned apparent incision rates and their varia-tion through time. In Models 1 and 2 (Fig. 15), slip on the combined Toroweap and Hurricane faults was modeled to last for the total 6 Ma of canyon incision. Shorter durations for fault slip, as perhaps suggested by geologic data (if one
Karlstrom et al.
1308 Geological Society of America Bulletin, November/December 2007
assumes no Laramide reverse slip, see above), require larger fault displacements on multiple strands of the Hurricane than shown in Fig-ure 15. Because of the downstream cumulative effects of fault slip, models that restrict slip to the Toroweap to 2 Ma and slip on the Hurricane to 3.5 Ma, to accomplish the observed inci-sion dampening, also require larger slip on the Wheeler/Iceberg Canyon fault systems. In spite of remaining uncertainties regarding the tempo-ral and spatial variations of both fault slip and apparent incision rates, the differential incision model provides a powerful new constraint that needs to be addressed when framing the con-troversy about the long-term evolution of the Colorado River system and uplift models for the Colorado Plateau.
Evaluating Models for River Integration
Figure 16A extends the modeled 5- to 6- Ma paleoriver profi le to sea level at the Gulf of California, which has formed the base level for the river since 5.36 Ma. The premise of this analysis is that the river profi le of major rivers like the Colorado, although they evolve through time, may be used as an approximate datum for estimating uplift and denudation rates. At largest scale, and million-year time frame, rivers evolve toward a concave-up pro-fi le that refl ects a balance between channel slope, discharge, and sediment load (e.g., Bull, 1979, 1991). Even in young, large rivers in tectonically active landscapes (Burbank et al., 1996), this basic form establishes itself early, albeit with knickpoints and steep gradients that refl ect disequilibrium. Large rivers have ample stream power to erode and essentially erase small fault scarps and spill-over points in short time spans (Pederson et al., 2003).
Figure 16A shows a river profi le that may have resulted from initial integration by progres-sive lake spill over (stepped red line), a model that is supported by emerging geochronology. This profi le depicts spill over from Lake Bida-hochi at ca. 6.5 Ma (Scarborough 2001; Meek and Douglas, 2001), integration of drainage through a Miocene paleocanyon that may have existed west of the Kaibab uplift (Young, 2007), arrival of water to Lake Hualapai at ca. 6 Ma (House et al., 2005; Spencer and Pearthree, 2001), arrival of water at Lake Mojave at 5.5 Ma (House et al., 2005), followed closely by over-topping of Topock gorge to fi ll Lake Havasu and Lake Blythe at ca. 5.5 Ma (House et al., 2005; Spencer et al., 2007), with Colorado Pla-teau sediments reaching the Gulf of California at 5.36 Ma (Dorsey et al., 2005). As discussed above, this hypothetical 5- to 6-Ma paleopro-fi le is 200–300 m higher than the modeled
paleoprofi le reconstructed using steady Qua-ternary incision and slip rates, a difference that may be explainable by higher 5- to 6-Ma inci-sion rates that would likely have resulted from rapid adjustment of the stream profi le to the new knickpoints (spill-over points). The integration process probably involved both spill over (Scar-borough 2001; Spencer and Pearthree, 2001) and headward erosion (Lucchitta, 1990). Head-ward erosion, aided by groundwater sapping and resulting stream piracy (Pederson, 2001), was likely also an important mechanism to con-nect and integrate paleocanyons during initial integration of the Colorado River system across the Grand Wash cliffs and Kaibab uplift.
Evaluating Alternative Uplift Models
The post–6-Ma evolution of the Colorado River profi le can be considered in terms of two end-member uplift models (Fig. 16B). In both, we assume that by 5–6 Ma the river was developing a regionally concave-up profi le from the east side of the Kaibab uplift to the Gulf of California. The models coincide upstream of the Davis Dam-Mojave Valley area, our last fi rm incision point (Fig. 16A). Upstream of this point, the fault-dampening model seems to explain the differential incision data over the last 6 Ma, albeit needing the resolutions for the “incision discrepancy” discussed above. The two models differ in how the 5- to 6-Ma Colo-rado River profi le may have been graded to sea level in late Miocene time south of the Mojave basin. The Lower Colorado River region is structurally complex near Yuma because of the San Andreas fault system (including the Algo-dones fault, Fig. 16A), but most workers have noted an absence of Quaternary normal faulting in much of the Lower Colorado River corridor (House et al., 2005). Although early Colorado River gravels (units A and B of Metzger et al., 1973) are present both at the surface and in the subsurface between Mojave Valley and Yuma, we know of no defi nitive strath at the base of the fi rst Colorado River gravels such as exists in the Mojave Valley region. The following discussion highlights the importance of continued neo-tectonic and geomorphic studies of the Lower Colorado River region to help evaluate whether faulting across the Plateau-Basin and Range boundary caused the Colorado Plateau to go up (Fig. 16B, Model 1), or the 5- to 6-Ma, sea-level datum to go down (Fig. 16B, Model 2) relative to today’s mean sea level.
As shown in Figure 16B, Model 1 lets the river profi le evolve by keeping the left side (sea level) relatively fi xed and allowing uplift of the Colorado Plateau (e.g., Powell, 1875; Dut-ton, 1882; Lucchitta, 1972, 1979; Sahagian et
al., 2002). It assumes that much of the Lower Colorado River profi le has remained close to sea level (Metzger, 1968; Lucchitta et al., 2001), with minor vertical movements on faults related to the San Andreas system. Note that global sea level was 10–20 m higher during the early Plio-cene warm period (5–3 Ma; Ravelo et al., 2004), such that global changes in sea level are not a major consideration for this time period.
Model 1 is supported by the observations that bedrock straths are observed above the pres-ent river level in many places. The presence of Colorado River gravels (by themselves) at elevations up to 250 m above the present river (House et al., 2005) is likely due to the history of aggradation from 5.5 to 3.3 Ma followed by a series of aggradation and incision events (Metzger et al., 1973, House et al., 2005). How-ever, the observation that bedrock straths for these various events are commonly above the modern river level and at progressively lower elevations between Lake Mead and Yuma may suggest modest but still positive bedrock inci-sion for the entire length of the profi le (House et al., 2005), as shown in Model 1. For example, early Colorado River gravels near Blythe (Unit B, correlated by House et al. (2005) with the >4-Ma gravels of Bullhead City) rest on bedrock at elevations up to 150 m above current river level. Model 1 suggests a stepped, but regionally con-cave-up, profi le for the newly integrated 5- to 6-Ma Colorado River and is compatible with a relative lack of late Miocene to Quaternary faulting and subsidence between Davis Dam and Parker (House et al., 2005). As a driver for Model 1, epeirogenic uplift of the Colorado Plateau is consistent with geodynamic models that suggest that a component of extension and plateau uplift in the southwestern USA is taking place via ongoing mantle-driven surface uplift (Karlstrom et al., 2005).
Model 2 keeps the right side of Figure 16B fi xed, consistent with models for no Neogene surface uplift of the Colorado Plateau (Spencer, 1996; Spencer and Patchett, 1997; Pederson et al., 2002a), and lowers the western end of the 6-Ma river profi le and the 5- to 6-Ma, sea-level datum relative to the fi xed Colorado Plateau ele-vation. This model requires that the paleo–6-Ma sea-level position is now ~800 m below present sea level near Yuma and that post–6-Ma his-tory of the lower Colorado River corridor was strongly aggradational. In this model, Bouse Formation that was encountered at 150 m depths in drill holes (Howard and Bohannon, 2001) near Yuma would have to be non-marine saline lake deposits, well above paleo-sea level (Spen-cer and Patchett, 1997; Patchett and Spencer, 2001). The Bouse/Imperial Formations south of Yuma interfi nger with Colorado River gravels
Grand Canyon incision and neotectonics
Geological Society of America Bulletin, November/December 2007 1309
Lees Ferry
Little Colorado River
Kaibab upwarp
Toroweap fault
Hualapai Plateau
Grand Wash fault
Hurricane fault
Eminence fault
Palisades fault
Iceburg Canyon faultWheeler fault
Gold Butte
S. Virgin Detachment
Black MountainHoover Dam
Topock Divide Mojave Canyon
Needles
Davis Dam - Pyramid Hills
Hoover Dam Potholes
Boulder Canyon Lip
Parker
CibolaBlythe
Laguna
YumaAlgodones fault
Gulf of California
Glen Canyon Dam
Today’s elevation relative to Colorado Plateau
7,0
00
6,0
00
5,0
00
4,0
00
3,0
00
2,0
00
1,0
00
0ftkm
2 1.5 1 .5 0
Bous
e Fm
(5.5
Ma)
Dep
th to
ear
ly C
olor
ado
Rive
r dep
osit
s
Hua
lapa
i Ls
(6.0
Ma)
Bida
hoch
i Fm
(6
.5 M
a)
800
Chocolate Mountains
0
5010
015
020
025
030
035
040
045
050
0
0
100
200
300
400
500
600
700
800
550
900
mile
s
km
Dis
tanc
e D
owns
trea
m fr
om L
ees
Fer
ry
Mr
Pk
Pc
Ph
Ps
Cm
Cba
Ct
Mr
Ph
Pc
Ps
Cm
Cba
Ct
Xu
Xu
Yu
Cu
Mu
East
ern
Gra
nd
Can
yon
Blo
ck
Wes
tern
Gra
nd
C
anyo
n B
lock
Low
er C
olo
rad
o R
iver
Blo
ck
Today’s elevation relative to Gulf of California
5,0
00
4,0
00
3,0
00
2,0
00
1,0
00
0 1,0
00
2,0
00
3,0
00
ftkm 1.5 1 .5 0 -.5 -1
5.5
Ma
6.0
Ma
5.36
Ma
6.5
Ma
Bid
aho
chi L
ake
Hu
alap
ai L
ake
Mo
jave
bas
in
Bly
the
bas
in
Lake
Hav
asu
bas
inm
ioce
ne
pal
eoca
nyo
ns
rest
ore
d 6
.0 M
a p
rofil
e
Mo
del
1
Mod
el 2
600
650
700
750
800
1000
1100
1200
1300Re
sto
red
6.0
Ma
Pro
file
Pres
ent
Riv
er P
rofil
e
Gra
nd
Can
yon
Tod
ay’s
Pro
file
Alt
ern
ate
Up
lift
/ Su
bsi
den
ce M
od
els
to R
esto
re 6
.0 M
a p
aleo
pro
file
Pres
ent
Pro
file
Day
5.5
Rive
r Pro
file
Lake
Sp
ill o
ver l
ine
Mo
del
1: U
plif
t o
f Co
lora
do
Plat
eau
rela
tive
to fi
xed
sea
le
vel a
t G
ulf
of C
alifo
rnia
S.L.
Pres
ent
Pro
file
Day
5.5
Rive
r Pro
file
Lake
Sp
ill o
ver l
ine
Mo
del
2: S
ub
sid
ence
of L
ow
er
Co
lora
do
Riv
er re
lati
ve to
fixe
d
Co
lora
do
Pla
teau
S.L.
Sub
sid
ence
Up
lift
A B Fig
ure
16. S
umm
ary
of r
egio
nal
faul
t di
spla
cem
ent
mod
el a
nd l
ong-
term
evo
luti
on f
or t
he C
olor
ado
Riv
er. P
rogr
essi
ve l
ake
spill
ove
r (s
tepp
ed r
ed l
ine)
fro
m
Bid
ahoc
hi L
ake
(6.5
Ma)
, to
Hua
lapa
i Lak
e (6
Ma)
to
Moj
ave
and
Bly
the
basi
ns (
5.5
Ma)
may
hav
e fa
cilit
ated
dra
inag
e in
tegr
atio
n ac
ross
the
Kai
bab
uplif
t an
d G
rand
Was
h cl
iffs
and
est
ablis
hmen
t of
the
Col
orad
o R
iver
pro
fi le
alon
g it
s pr
esen
t co
urse
by
5.36
Ma.
(A
) R
esto
rati
on o
f N
eoge
ne f
ault
dis
plac
emen
t us
ing
Mod
el 1
of
Fig
ure
15, a
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Karlstrom et al.
1310 Geological Society of America Bulletin, November/December 2007
(Buising, 1990) and sit on marine rocks at up to 1000 m below sea level (Olmstead et al., 1973), but these are on the other side of the San Andreas fault, the vertical motion across which remains poorly constrained. Model 2 may be supported by possible early Colorado River gravels (units A and B of Metzger) and the pre-Colorado River Bouse Formation encountered at various depths in drill holes (Olmstead et al., 1973; Metzger et al., 1973; Spencer et al., 2007), as shown in Fig-ure 16A, although we know of no defi nitive 5- to 6-Ma Colorado River straths that have been positively identifi ed.
Model 2 would require a change, somewhere south of Mojave Valley, from a river profi le to the north that has had modest net incision since 6 Ma (20–30 m/Ma), to a 5- to 6-Ma river profi le datum to the south that has subsided markedly below sea level in the last 6 Ma. The reported variable depths of the pre-Colorado units sug-gest vertical components of fault displacement on the Algodones of several hundred meters (Olmstead et al., 1973), but even restoring this offset (Fig. 16) does not create a reasonable con-cave-up, 5- to 6-Ma, paleoriver profi le for Model 2. Thus, if Model 2 is correct, there would have to be other, presently unrecognized, Neogene faults along the profi le that would allow recon-struction of a reasonable 5- to 6-Ma profi le, the gradient for which, in this downstream part of the profi le, must have been lower than upstream gradients.
Aspects of each model remain viable, and components of each may have operated. For example, there may have been several hundred meters of actual uplift of eastern blocks accom-panied by similar magnitude subsidence in the Gulf region. Nevertheless, the combined geo-logic data (profi le analysis, bedrock straths above the modern river level, and reported absence of Quaternary faulting in most of the Lower Colo-rado River corridor) seem best explained by Model 1, where most of the 750–900 m of rela-tive displacement resulted from surface uplift of the Colorado Plateau.
CONCLUSIONS
New 40Ar/39Ar dates on basalts in western Grand Canyon provide one of the best records of canyon incision in the world. Different appar-ent incision rates in different reaches of Grand Canyon, when combined with new fault-slip rates, lead to our new model for fault-dampened incision and provide fi rst-order constraints on how active faulting interacts with the incision of a major river/canyon system. Dated basalt fl ows and travertine deposits associated with old river gravels indicate average incision rates of 150–175 m/Ma for the 290-km-long eastern
Grand Canyon block (Lees Ferry to Toroweap fault) over the last 350 ka. The Uinkaret block (8-km-wide block from Toroweap to Hurricane faults) shows variable bedrock incision rates: 66 m/Ma in the immediate hanging wall of the Toroweap fault increasing westward to 136 m/Ma in the immediate footwall of the Hurricane fault. This suggests that fault-dampened inci-sion in this block is being accomplished mainly by formation of a hanging-wall anticline above a listric Toroweap fault. The western Grand Canyon block (140-km-wide block from Hurri-cane to Grand Wash faults) shows bedrock inci-sion rates of 50–75 m/Ma over the last 723 ka. This is less than half of the eastern canyon rate and indicates lowering of western Grand Can-yon block by ~100 m/Ma relative to the eastern Grand Canyon block over the last 723 ka. New dates on offset basalts indicate ~100 m/Ma slip rate on the Toroweap fault (last 600 ka) and 70- to 100-m/Ma slip on the Hurricane fault (last 186 ka). This requires modifi cation of previous models for fault-dampened incision. In our new model, slip rate plus downstream incision rate are subequal to upstream rates only immediately across faults. At longer spatial scales, approxi-mately half of the cumulative slip on these two faults (170–200 m/Ma) is expressed as relative vertical displacement between the Colorado Pla-teau and Basin and Range blocks, the rest being accommodated by fl exure of the hanging walls. Mechanistically, this is due to a listric character of Neogene normal faulting combined with par-titioning of slip between fault strands.
Dated basalts and new data on neotectonic fault block geometry provide insight on lon-ger term incision history of Grand Canyon and processes at the boundary between the Colo-rado Plateau and Basin and Range provinces. Throughout Grand Canyon, Quaternary bed-rock incision rates appear to have been nearly constant in a given reach for the last 720 ka, but these are minimum rates for long-term canyon incision, which requires ~275 m/Ma to carve eastern Grand Canyon in 6 Ma. Long-term aver-age apparent incision rates in the upper Lake Mead region of ~27 m/Ma in the last 4.4 Ma and ~20 m/Ma in the last 5.5 Ma near Davis Dam also suggest coherent block behavior of the Colorado River corridor block and net inci-sion for the entire profi le north of the Mojave Valley. Using steady incision rates and prelimi-nary models, the Colorado River corridor block has lowered ~27 m/Ma relative to the western Grand Canyon block, which, in turn, has been lowering at ~100 m/Ma relative to the Colorado Plateau averaged over 6 Ma. The combined fault displacement caused 750–900 m of relative ver-tical displacement between the Basin and Range and Colorado Plateau provinces in the last 6 Ma.
Of the two models, surface uplift of the Colo-rado Plateau by 750–900 m better reconstructs a reasonable 6-Ma paleoprofi le and better explains straths that are above sea level between the Mojave Valley and Yuma. Such Quaternary epeirogenic uplift may have been driven by buoyant low-velocity mantle upwelling beneath the tectonically active western United States.
ACKNOWLEDGMENTS
Research support for this study came from National Science Foundation grant EAR- 9706541 (to Karl-strom and Pederson). Logistical support came from EAR-0612310 (to Karlstrom) for the University of New Mexico whitewater equipment. We thank Joel Pederson, Mike Timmons, Shari Kelly, Stacy Wagner, Matt Heizler, Scott Miner, Phil Reasor, and many oth-ers who have interacted with the broader University of New Mexico Grand Canyon collaboration for their help with the fi eldwork and for informal discussions. We thank Grand Canyon National Park for continued support in the form of river and sampling permits and access to the river corridor. The New Mexico Tech Geochronology Research Laboratory makes data available to the public and the scientifi c community online at www.ees.nmt.edu/Geol/labs/Argon_Lab/NMGRL_homepage.html. Formal reviews by Marty Grove and Greg Stock, and Associate Editor John Wakabayashi are appreciated. Informal reviews by Dick Young, Kyle House, Jim O’Connor, Tom Hanks, and Jon Spencer also helped improve the manuscript.
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