Ar/ Ar and ﬁ eld studies of Quaternary basalts in Grand Canyon …geomorphology.sese.asu.edu/Papers/Karlstrom_GSAB_07.pdf · 2019-08-27 · 40Ar/39Ar and ﬁ eld studies of Quaternary

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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

*[email protected]

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

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Karlstrom et al.


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)



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


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


207.7L 522 ± 57 n = 2


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


208-209 605 ± 17 n = 2


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.


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.)



Figure 5 (continued).

Karlstrom et al.


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).



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.


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

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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

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30

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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.



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

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40

35

30

25

20

15

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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.


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

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120

140

160

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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

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ax

151

m/M

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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).



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

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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

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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.


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.



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.


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)



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

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Karlstrom et al.


Q

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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.


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.



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

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150

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50

30

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-50510152025303540458590

Distance west from the Toroweap fault (m)

Ap

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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.


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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.



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.


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



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

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r dep

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s

Hua

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(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

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Cm

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Ct

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Xu

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East

ern

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nd

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Blo

ck

Wes

tern

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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

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ake

Hu

alap

ai L

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Mo

jave

bas

in

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the

bas

in

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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

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e

Gra

nd

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Tod

ay’s

Pro

file

Alt

ern

ate

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lift

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den

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to R

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re 6

.0 M

a p

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pro

file

Pres

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Pro

file

Day

5.5

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file

Lake

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ill o

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del

1: U

plif

t o

f Co

lora

do

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eau

rela

tive

to fi

xed

sea

le

vel a

t G

ulf

of C

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file

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file

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ill o

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del

2: S

ub

sid

ence

of L

ow

er

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lora

do

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er re

lati

ve to

fixe

d

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lora

do

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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

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ado

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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

nd a

ssum

ing

stea

dy Q

uate

rnar

y in

cisi

on r

ates

, res

ults

in a

rea

sona

ble

6-M

a pa

leop

rofi l

e (a

s in

Fig

. 15)

, but

wit

h a

seve

ral h

undr

ed m

eter

di

scre

panc

y w

ith

the

infe

rred

5.5

-Ma

lake

spi

ll-ov

er p

rofi l

e (s

ee t

ext)

. Das

hed

lines

at

the

left

are

dep

ths

to p

ossi

ble

earl

y C

olor

ado

Riv

er d

epos

its

as in

terp

rete

d by

Olm

stea

d et

al.

(197

3) a

nd M

etzg

er e

t al

. (19

73).

(B

) B

elow

Moj

ave

basi

n, a

lter

nate

upl

ift/

subs

iden

ce m

odel

s im

ply—

Mod

el 1

: ~7

50–9

00 m

upl

ift

of C

olor

ado

Pla

teau

rel

ativ

e to

sea

leve

l, ve

rsus

Mod

el 2

: ~7

50–9

00 m

of

subs

iden

ce o

f th

e la

te M

ioce

ne s

ea-l

evel

dat

um a

t th

e G

ulf

of C

alif

orni

a.

Karlstrom et al.


(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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Ar/ Ar and ﬁ eld studies of Quaternary basalts in Grand Canyon …geomorphology.sese.asu.edu/Papers/Karlstrom_GSAB_07.pdf · 2019-08-27 · 40Ar/39Ar and ﬁ eld studies of Quaternary