Earth and Planetary Science Letters 397 (2014) 159–173

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Earth and Planetary Science Letters

www.elsevier.com/locate/epsl

Steady incision of Grand Canyon at the million year timeframe: A casefor mantle-driven differential uplift

Ryan Crow a,∗, Karl Karlstrom a, Andrew Darling b, Laura Crossey a, Victor Polyak a,Darryl Granger c, Yemane Asmerom a, Brandon Schmandt a

a Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USAb School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USAc Department of Earth and Planetary Science, University of Purdue, West Lafayette, IN 47907, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 September 2013Received in revised form 6 February 2014Accepted 9 April 2014Available online 13 May 2014Editor: T.M. Harrison

Keywords:dynamic typographyGrand Canyon evolutionepeirogenic upliftColorado River incisionU-series geochronology

The Grand Canyon region provides an excellent laboratory to examine the interplay between riverincision, magmatism, and the geomorphic and tectonic processes that shape landscapes. Here we applyU-series, Ar–Ar, and cosmogenic burial dating of river terraces to examine spatial variations in incisionrates along the 445 km length of the Colorado River through Grand Canyon. We also analyze strathterrace sequences that extend to heights of several hundred meters above the river, and integrate thesewith speleothem constrained maximum incision rates in several reaches to examine any temporal incisionvariations at the million-year time frame. This new high-resolution geochronology shows temporallysteady long-term incision in any given reach of Grand Canyon but significant variations along its lengthfrom 160 m/Ma in the east to 101 m/Ma in the west. Spatial and temporal patterns of incision, and thelong timescale of steady incision rule out models where geomorphic controls such as climate oscillations,bedrock strength, sediment load effects, or isostatic response to differential denudation are the first orderdrivers of canyon incision. The incision pattern is best explained by a model of Neogene and ongoingepeirogenic uplift due to an eastward propagating zone of increased upper mantle buoyancy that weinfer from propagation of Neogene basaltic volcanism and a strong lateral gradient in modern uppermantle seismic structure.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Rivers provide sensitive but complex gauges of the interactionsof tectonics and erosion. They incise into underlying bedrock be-cause of the potential energy difference between headwaters andbase level. Incision can be variable through time and along a river’sprofile due to changes in climate, sediment load, bedrock proper-ties, uplift, faulting, and other base level changes (Kirby and Whip-ple, 2001; Pederson and Tressler, 2012; Phillips and Lutz, 2008;Sklar and Dietrich, 2004; Wobus et al., 2010, 2006a, 2006b). Al-though the complex interplay between these drivers of incision isnot completely understood, unique spatial and temporal patternsin river incision can often be used to infer which drivers are dom-inant in a given landscape.

The Colorado Plateau region of western North America (Fig. 1)provides an unparalleled setting for testing models of landscapeevolution in continental interiors. Published models for the inte-gration of the Colorado River and uplift of the Colorado Plateau

* Corresponding author.

http://dx.doi.org/10.1016/j.epsl.2014.04.0200012-821X/© 2014 Elsevier B.V. All rights reserved.

can be tested given sufficient understanding of the temporal andspatial variations in incision as follows: Transient knickzone re-treat, due to headward erosion associated with Colorado River in-tegration (Cook et al., 2009; Pelletier, 2010; Polyak et al., 2008),predicts large incision rate variations in time and space. Variablebedrock strength (Cook et al., 2009; Pederson and Tressler, 2012)predicts that channel slopes should correlate with rock erodabil-ity (Whipple and Tucker, 1999) and that incision rates should besimilar below, within, and above knickpoints. Uplift due to theisostatic response to differential denudation (Lazear et al., 2013;Pederson et al., 2013, 2002b; Roy et al., 2009) predicts higher inci-sion in areas of highest regional denudation (Karlstrom et al., 2013;Pederson et al., 2013). Another class of models postulates tectonicsurface uplift as a result of a variety of mantle-driven processes, in-cluding deep-mantle upwelling (Moucha et al., 2009), edge-drivenupper-mantle convection (van Wijk et al., 2010), and crustal de-lamination (Levander et al., 2011). Uplift models involving large-scale semi-rigid fault blocks (Karlstrom et al., 2007) predict con-stant incision across fault blocks. Mantle-driven epeirogenic up-lift expressed across fault blocks (Karlstrom et al., 2008, 2007),

160 R. Crow et al. / Earth and Planetary Science Letters 397 (2014) 159–173

Fig. 1. (Color online.) Photograph of dated gravels under the landslide at RM 135. AD08-GN135-1 sampled quartzite clasts at the base of the paleo channel shown in thelower left photo.

regional mantle-driven tilting (Liu and Gurnis, 2010; Moucha etal., 2009), and/or tilting across sharp mantle velocity gradients(Karlstrom et al., 2012) should show spatial and temporal associa-tions of incision variation with mantle velocity variations that canpotentially be used to discriminate between the currently diverseinterpretations of mantle dynamics.

To test these models we present new U-series geochronologyon calcite (travertine) that occurs in terrace gravels both as de-trital cobbles and as inter-cobble cement and cosmogenic burialages on landslide-capped gravel deposits. We apply this dating toterrace sequences in several reaches of the river to extend the in-cision record back to 1.5 Ma. In addition, we extend the incisionhistory back to 2–4 Ma using U–Pb dating of speleothems (Polyaket al., 2008). The new incision rates constrained from these datesare precise analytically, combine maximum and minimum ages tofully bracket the age of gravel deposition, and use multiple strathlevels to determine temporal variations in bedrock incision, at un-precedented resolution, in several reaches that span most of the445-km-long Grand Canyon.

2. Methods

Samples for U/Th dating were obtained from strath terraces atmultiple heights above the river at 4 locations in eastern and cen-tral Grand Canyon where well-exposed strath terrace sequencesare preserved. Samples of clean calcite coatings around mainstemColorado River gravels were sampled along with detrital clasts oftravertine that were deposited with the gravel (see Table 1 for lo-cations, heights, and descriptions). 50–70 mg of calcite powder wasmicro drilled from laminations within the travertine samples andmixed with a 229Th–233U–236U spike after dissolution in HNO3.U and Th were separated using conventional anion exchange chro-matography. U and Th isotopes were measured using a ThermoNeptune multi-collector inductively coupled plasma mass spec-trometer (MC-ICPMS) at the University of New Mexico which wasoptimized for U-series analytical work as described by Asmerom etal. (2006). 234U was measured on a secondary electron multiplier(SEM) with high abundance filter or on the center Faraday cup,while the other isotopes of uranium were measured on Faradaycups. Mass fractionation was monitored using the 238U/235U ratio,while SEM/Faraday gain was set using sample standard bracket-ing and uranium standard NBL-112. A similar procedure was used

for Th isotope measurements with 230Th measured in the SEMand 229Th and 232Th measured in Faraday cups. An in-house 230Thstandard was used to measure the SEM/Faraday gain, and massfractionation was corrected using 238U/235U or 236U/233U. All anal-yses used new half-lives for 234U and 230Th from Cheng et al.(2013).

For samples outside of U/Th range (i.e. > ca. 650 ka), 234Umodel ages were calculated. These model ages are reported asa range from the minimum age possible (i.e. lowest (234U/238U)i(ratios in parentheses are activity ratios; activity is equal to thenumber of atoms times the decay constant of the nuclide of inter-est) that could produce the measured (234U/238U) and (230U/238U)activities) to an upper bound defined by the model age calculatedusing a maximum (234U/238U)i value of 8; this value is slightlyhigher than the upper bound used by Polyak et al. (2008), who cal-culated U–Pb and 234U model ages on cave mammillaries in GrandCanyon. If an analysis plotted off the asymptote of a (234U/238U)vs. (230U/238U) evolution plot they were disregarded.

In the Surprise Valley area where no travertine has been found,we applied cosmogenic burial dating to gravels shielded underlandslide deposits. Cosmogenic burial samples in river sedimentbelow Surprise Valley landslide deposits were buried by 10–30 mof landslide material providing sufficient post-burial shielding. Thesample at river right (river mile (RM) (Stevens, 1983) 135; Fig. 1)consisted of several quartzite clasts crushed and analyzed together.Thus the age is an average of the Al/Be ratios of the 5–8 clasts. Thesample at river left, (RM 136.9; see Supplementary Data for pho-tos) was processed by separating quartz grains from a bag of riversand collected from the ceiling of an overhang which was over-lain by sand and landslide material. Thus the sample represents anaverage nuclide inventory over thousands of grains. The amalgama-tion of several crushed quartzite clasts or sand from the depositswere processed together (Granger and Muziker, 2001) and ana-lyzed for 26Al and 10Be by Accelerator Mass Spectrometry at thePurdue Rare Isotope Measurement Laboratory (PRIME Lab) at theUniversity of Purdue. Minimum burial ages were calculated ignor-ing postburial production and using a 10Be production rate in thesediment source area of 20 at g−1 yr−1 and a 26Al/10Be produc-tion rate ratio of 6.8, calculated for latitude 37◦ , elevation 2 km.Burial ages are insensitive to the choice of production rate. Ra-dioactive meanlives are taken to be 1.02 My for 26Al and 2.005 Myfor 10Be. Uncertainties reflect measurement error only. Postburial

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±2σ error Gravelcharacter

Notes and references

±3 Sparsepebbles insandydeposit

Travertine drape thatextends to river level;incision rate 2 inKarlstrom et al. (2007);updated pool depth(Table S1)

±3 Sparsepebbles intravertineaccumula-tion

Travertine in upperM4? deposit; incisionrate 1 in Karlstrom etal. (2007); updatedpool depth (Table S1)

±5 Colluvium+ minorCR gravel

Lowest strath ina stepped sequence;incision rate 6 inKarlstrom et al. (2007);updated pool depth(Table S1)

+16 −13 CR gravel Travertine rind onm-scale river clast atstrath; incision rate 4of Karlstrom et al.(2007); updated pooldepth (Table S1)

+9 −8 CR gravel Between M5 and M4,57L; travertine drapecovers gravels 2 mabove strath; incisionrate 5 in Karlstrom etal. (2007); updatedpool depth (Table S1)

+9 −10 CR gravel This study

±4 CR gravel This study

+9 −8 CR gravel This study

+10 −9 CR gravel Heights remeasured2012 with laser rangefinder; incision rate 3of Karlstrom et al.(2007); updated pooldepth (Table S1)

(continued on next page)

Table 1Incision rates for Grand Canyon

Sample number Rivermile

Latitude Longitude Age(ka)

Strathheight(m)

Sampleheight(m)

Meanwaterdepth(m)

Meanpooldepth(m)

Maxpooldepth(m)

Infilling /detrital

Incisionrate(m/Ma)

KWAGUNT AREA1 Weighted mean of

56L-3A-1 & 2,56L-3B-1 & 2, andGC9/01-56L-A2 andB1

56.2 36.26115 −111.8247 153 ± 3 0 5 8.2 12.2 22.4 U/Th ageon infilling

max. 146

2 GC5/01-56L-A2 56.2 36.26115 −111.8247 119 ± 2 0 26 8.2 12.2 22.4 U/Th ageon infilling

max. 188

3 Weighted mean ofK02-056-6A and B

56.8 36.251806 −111.8243 125 ± 2 14 14 8.4 12.4 22.4 U/Th ageon infilling

max. 291

4 Weighted mean ofK02-056-4A & B

56.8 36.2528 −111.8245 351 ± 38 23 24 8.4 12.4 22.4 U/Th ageon infilling

max. 129

5 GC5/01-57L-E 56.8 36.2524 −111.8245 283 ± 14 24 24 8.4 12.4 22.4 U/Th ageon infilling

max. 164

6 RC12-56.7-1A 56.7 36.253841 −111.8246 396 + 29 − 24 33.5 35.25 8.4 12.3 22.4 U/Th ageon detritalclast

min. 141

7 RC12-56.7-1A (2) 56.7 36.253841 −111.8246 364 + 9.8 − 9.2 33.5 35.25 8.4 12.3 22.4 U/Th ageon detritalclast

min. 154

8 Weighted mean ofRC12-56.7-1A and(2)

56.7 36.253841 −111.8246 368 ± 21* 33.5 35.25 8.4 12.3 22.4 U/Th ageon detritalclast

min. 152

9 Weighted mean ofK02-056-2A & B

56.7 36.253841 −111.8246 386 ± 25 33.5 35.5 8.4 12.3 22.4 U/Th ageon infilling

max. 145

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±2σ error Gravelcharacter

Notes and references

+19 −18 CR gravels This study

+3 −4 CR gravels This study

±3 CR gravels Sample is 4 m laterallyfrom RC10-Palisades-1Bat same height; thisstudy

±7 CR gravels Sample of calciteinfilling near top of5–6 m thick gravel;this study

+21 −30 CR gravels ∼20 cm thick infillingbetween gravel clasts;this study

±3 Almostentirelycolluvium

Tread 13.5 m aboveriver, deposit almosttotally colluvial withscarce gravels, heightremeasured 2012;this study

±3 CR gravels This study

+10 −11 CR gravels Infilling right at strathwhich is overlain bygravel in a sandmatrix; this study

±5 CR gravels Infilling right at strathwhich is overlain bygravel in a sandmatrix; this study

+6 −5 CR gravels Infilling right at strathwhich is overlain bygravel in a sandmatrix; this study

+5 −6 CR gravels+colluvium

30 cm above strath,infilling in blackenedsand; this study

Table 1 (continued)

Sample number Rivermile

Latitude Longitude Age(ka)

Strathheight(m)

Sampleheight(m)

Meanwaterdepth(m)

Meanpooldepth(m)

Maxpooldepth(m)

Infilling /detrital

Incisionrate(m/Ma)

PALISADES AREA10 RC10-LC-1A 65.5 36.1391 −111.8175 50 ± 2 1.8 1.8 7.2 10.9 21.4 U/Th age

on infillingmax. 464

11 RC10-PAL-1B 65.2 36.142192 −111.8131 337 + 6 − 5 55 57 7.2 10.9 21.4 U/Th ageon infilling

max 227

12 RC12-65.2-2A(2) 65.2 36.142192 −111.8131 326 ± 4 55 57 7.2 10.9 21.4 U/Th ageon infilling

max. 234

13 RC12-65.3-3A(2) 65.2 36.14224 −111.8127 402 + 16 − 14 55 60.5 7.2 10.9 21.4 U/Th ageon infilling

max. 190

14 RC10-PAL-2B 65.2 36.14314 −111.8116 623 + 162 − 78 70 83 7.2 10.9 21.4 U/Th ageon infilling

max. 147

TANNER AREA15 K08-68.4-1A 68.4 36.1023 −111.8315 98 ± 1 7 7 6.9 10.5 20.5 U/Th age

on infillingmax. 281

16 K08-68.7-2C 68.7 36.1007 −111.8343 251 ± 3 42 45 7.0 10.6 20.5 U/Th ageon infilling

max. 249

17 RC12-69.2-4A 69.2 36.0999 −111.8344 484 + 37 − 31 52 52 7.0 10.6 20.5 U/Th ageon infilling

max. 150

18 RC12-69.2-4A(2) 69.2 36.0999 −111.8344 463 + 15 − 14 52 52 7.0 10.6 20.5 U/Th ageon infilling

max. 157

19 Weighted mean ofRC12-69.2-4A and(2)

69.2 36.0999 −111.8344 467 ± 16* 52 52 7.0 10.6 20.5 U/Th ageon infilling

max. 155

20 LC08-68.9-3A 68.9 36.0994 −111.8339 420 + 12 − 11 65 65 7.0 10.6 20.5 U/Th ageon infilling

max. 204

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±2σ error Gravelcharacter

Notes and references

CR gravels+colluvium

This study

CR gravels+colluvium

This study

±11 CR gravels+colluvium

This study

±7 CR gravels+colluvium

Vein that cross cuts amixed gravel/colluvialdeposit; this study

±19 CR gravels+colluvium

Drape overlying gravelterrace; strath heightremeasured in 2012;this study

+13 −18 CR gravel Hugh blocks oftravertine in basalriver gravel; strathheight remeasured in2012; this study

+15 −20 CR gravel This study

CR gravel Strath heightremeasured in 2012;this study

±10 1 mlaterallyfrom CRgravel

Micritic layer; thisstudy

±9 CR gravels Sample from a massivetravertineaccumulation withisolated well-roundedclasts; this study

Rind incolluviumdirectly ongood CRgravels

Height above strathestimated; this study

(continued on next page)

Table 1 (continued)

Sample number Rivermile

Latitude Longitude Age(ka)

Strathheight(m)

Sampleheight(m)

Meanwaterdepth(m)

Meanpooldepth(m)

Maxpooldepth(m)

Infilling /detrital

Incisionrate(m/Ma)

ELVES CHASM AREA21 RC12-116.2-3BA 116.2 36.1972 -112.4425 617–1413 33.5 33.5 10.0 13.5 22.1 234U

model ageon detritalclast

min. 39–90

22 RC12-116.2-3BA(2) 116.2 36.1972 −112.4425 564–1423 33.5 33.5 10.0 13.5 22.1 234Umodel ageon detritalclast

min. 39–99

23 RC12-116.2-1A(2) 116.2 36.1972 −112.4425 467 + 49 − 38 33.5 33.5 10.0 13.5 22.1 U/Th ageon infilling

max. 119

24 RC12-116.2-4A 116.2 36.1972 −112.4425 55 ± 0.4 33.5 33.5 10.0 13.5 22.1 U/Th ageon infilling

max. 1012

25 K03-116.2-1A 116.2 36.1972 −112.4425 12 ± 0.05 33.5 41 10.0 13.5 22.1 U/Th ageon infilling

max. 4636

26 K03-116.5-2A 116.5 36.1975 −112.4514 678 + 147 − 79 46 46 9.9 13.5 22.1 U/Th ageon detritalclast

min. 101

27 RC08-EC-1A(3) 116.5 36.1975 −112.4514 651 + 151 − 82 46 46 9.9 13.5 22.1 U/Th ageon infilling

max. 105

28 K03-116.5-1B 116.5 36.1975 −112.4514 727–1019 46 46 9.9 13.5 22.1 234Umodel ageon infilling

max. 67–94

29 K08-116.6-10B 116.6 36.1975 −112.4527 304 ± 7 107 110 9.9 13.5 22.2 U/Th ageon infilling

max. 425

30 RC08-116.6-4A 116.6 36.1978 −112.4562 135 ± 1 135 142 9.9 13.5 22.2 U/Th ageon infilling

max. 1165

31 RC08-116.6-5A 116.6 36.1978 −112.4562 667–1877 142 145 9.9 13.5 22.2 234Umodel ageon infilling

max. 87–246

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±2σ error Gravelcharacter

Notes and references

±19 Mostlycolluvialnear CRgravels

This study

+22 −26 Mostlycolluvial

Rind coating atravertine block;this study

+64 −26 CR gravel This study

+106 −35 CR gravel This study

+46 −23 CR gravel This study

±2 CR gravel Depth to bedrockof 12 m estimatedfrom dam survey;pre-dam max. pooldepth unknownassuming 22 m forconsistency with otherWGC incision rates;hyaloclastite in thebase of the flowindicates flowemplacement wassynrenous with graveldeposition; Crow(2012)

±2 No gravel Age from Polyak et al.(2008); heightmodified to accountfor Lake Meadaggradation

+16 −12 No gravel Age from Polyak et al.(2008); heightmodified to accountfor Lake Meadaggradation

Table 1 (continued)

Sample number Rivermile

Latitude Longitude Age(ka)

Strathheight(m)

Sampleheight(m)

Meanwaterdepth(m)

Meanpooldepth(m)

Maxpooldepth(m)

Infilling /detrital

Incisionrate(m/Ma)

32 RC08-116.8-11A 116.8 36.1983 −112.4570 382 + 14 − 13 180 180 10.0 13.5 22.2 U/Th ageon infilling

max. 529

33 RC08-116.8-8A 116.8 36.1983 −112.4574 223 + 6 − 5 195 195 10.0 13.5 22.2 U/Th ageon infilling

max. 975

SURPRISE VALLEY34 K10-136.1-SV 136.6 36.3891 −112.5162 980 ± 420 61 61 10.3 13.9 22.7 cosmo-

genicburial age

max. 85

35 AD08-GN135-1 135 36.3810 −112.4881 880 ± 440 70 70 10.7 14.3 23.5 cosmo-genicburial age

max. 106

36 Weighted meanof AD08-GN135-1and K10-136.1-SV

135–136.6

932 ± 304 65.5 65.5 10.5 14.1 23.1 cosmo-genicburial age

max. 95

WESTERN GRAND CANYON37 RC07-246-1C 246 35.8240 −113.6464 574 ± 15 30 30 22 Ar/Ar age

on basalt95

38 Cave B 266 3870 ± 100 363 U/Pb ageon cavemammil-lary

max. 94

39 Dry Canyon 265 2170 ± 340 187 U/Pb ageon cavemammil-lary

max. 86

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±2σ error Gravelcharacter

Notes and references

+112 −73 No gravel Polyak et al. (2008);max. pool depth notused in incision ratecalculation

+30 −24 No gravel Polyak et al. (2008);max. pool depth notused in incision ratecalculation

+37 −26 No gravel Polyak et al. (2008);max. pool depth notused in incision ratecalculation

+24 −21 No gravel Polyak et al. (2008);max. pool depth notused in incision ratecalculation

+68 −44 No gravel Polyak et al. (2008);max. pool depth notused in incision ratecalculation

+172 −90 No gravel Polyak et al. (2008);max. pool depth notused in incision ratecalculation

Table 1 (continued)

Sample number Rivermile

Latitude Longitude Age(ka)

Strathheight(m)

Sampleheight(m)

Meanwaterdepth(m)

Meanpooldepth(m)

Maxpooldepth(m)

Infilling /detrital

Incisionrate(m/Ma)

EASTERN GRAND CANYON CAVES40 Gavain Abyss 93 2190 ± 470 900 10.0 13.3 23.9 U/Pb age

on cavemammil-lary

max. 411

41 Tsean Bida 80 3430 ± 430 726 9.9 14.1 22.5 U/Pb ageon cavemammil-lary

max. 212

42 Butte Fault 57 2680 ± 490 445 8.3 12.1 22.1 U/Pb ageon cavemammil-lary

max. 166

43 Bedrock Cave 32 830 ± 50 310 10.4 14.4 24.6 U/Pb ageon cavemammil-lary

max. 374

44 Shinumo Creek 94 3720 ± 800 920 9.9 13.4 23.8 U/Pb ageon cavemammil-lary

max. 247

45 Mother Cave 90 1600 ± 500 605 10.4 13.9 23.9 234Umodel ageon cavemammil-lary

max. 378

Note. Bold rows indicate data used in regressions.* Weighted mean calculated using minimum errors and individual errors were multiplied by the squareroot of the MSWD if the MSWD was greater than 1.

166 R. Crow et al. / Earth and Planetary Science Letters 397 (2014) 159–173

Fig. 2. Tomographic map of the study area. Background image shows mantle-velocity structure at 80 km depth (Schmandt and Humphreys, 2010). Also shown are >0.6 Maincision rates (Darling et al., 2012; Karlstrom et al., 2008, 2007; Pederson et al., 2006, 2002a; Polyak et al., 2008; Willis and Biek, 2001), color coded by method, and NAVDAT(www.navdat.org) data showing the migration of volcanism. See U.S. Geological Survey et al. (2011) for quaternary faults.

production by muons would make the true burial age older thanreported.

To allow for York-style regression (York et al., 2004), strathages with unequal errors (positive and negative errors were dif-ferent) were adjusted such that the smaller error was inflated tothe magnitude of the larger. For 234U model ages, the median agewas used such that the error reflects the difference between themedian value and the maximum and minimum values computed.Both detrital and infilling ages were weighted the same. Strathheights measured with a laser range finder were conservatively as-sumed to have a ±2 m error; heights estimated from topo mapswere assumed to have a ±10 m error. The height of the SpencerCanyon sample (see below) above pre-Lake Mead river level wasestimated based on dam survey reports (LaRue, 1925) and thatheight is assumed to have an error of ±6 m. When only one datedstrath is present, incision rates were calculated using the mean ofthe 10 deepest pools along the modern river in a 15 mile (24 km)reach assuming that the deepest pools are scoured to near bedrock(Table S1; Karlstrom et al., 2007).

3. Results

U/Th dating in strath-terrace sequences, at four locations inGrand Canyon (Fig. 2), yielded 21 ages that ranged from 11 ka toca. 650 ka (see the Supplementary Data for analytical results (Ta-ble S2) and photos of each site). 234U model ages were calculatedfor 4 samples that were outside of the upper limit for U/Th dat-ing and these ranged from ca. 500 to 2 Ma, including errors. Twocosmogenic burial ages from separate locations with similar strathheights give less precise ages of ca. 1 Ma (see the Supplemen-tary Data Table S3 for analytical results). Averaged strath-to-strath

bedrock incision rates were calculated by regression of a line (Yorket al., 2004) or lines (in the case of non-steady incision) through aplot of strath height versus age (e.g. Fig. 3), yielding not only theincision rate (slope of the line) but also an estimate of the moderndepth to bedrock, the y-intercept. We improve on earlier efforts bycombining ages on travertine infillings around river cobbles withages of detrital travertine clasts. These data provide minimum andmaximum ages, respectively, on gravel deposition so we regressonly the youngest detrital travertine clast ages and oldest infill-ing ages from a given strath level. The geologic validity of theregression can then be tested by comparing the y-intercept to anindependent measure of the modern depth to bedrock below theriver’s surface. In the case of Grand Canyon, bathymetry and drillhole data indicated that bedrock is typically about 20–30 m belowriver level in the thalweg (Karlstrom et al., 2007). We also per-formed sensitivity tests to determine the effect of point selectionand the method of handling asymmetric errors on the regressions,which are minimal (see Supplementary Data Table S4). The nextsections discuss the new Grand Canyon incision rate data fromwest to east.

3.1. Western Grand Canyon

Incision rates in western Grand Canyon are well constraineddue to basalt flows which entered Grand Canyon near the Toroweapand Hurricane faults, flowed over 135 km down the river channeland capped and preserved now-perched river gravels (Crow, 2012;Crow et al., 2008; Karlstrom et al., 2007; Pederson et al., 2002a).West-down faulting has dampened apparent incision rates in thedownthrown blocks, with the strongest dampening within 10 kmof Quaternary faults due to the combination of block lowering

R. Crow et al. / Earth and Planetary Science Letters 397 (2014) 159–173 167

Fig. 3. Strath-to-strath incision plot for western Grand Canyon. The slope of theregressed line (using the red points) gives the western Grand Canyon incision rateof 101 m/Ma. U/Pb ages from Polyak et al. (2008).

and development of hangingwall flexures (Karlstrom et al., 2007;Pederson et al., 2002a). Thus, to evaluate regional trends, we focuson background incision rates that can be determined away fromfaults. Near Spencer Canyon (RM 246), ∼ 25 km west of the Hur-ricane fault (Fig. 2), new 40Ar/39Ar dating of a basalt flow (Crow,2012) that caps mainstem river gravel yields an incision rate of95 ± 2 m/Ma over ca. 574 ka. Speleothem-constrained rates inthis area are 86–94 m/Ma (Cave B and Dry Canyon (Polyak et al.,2008)) once heights of caves above the pre-dam river are correctedfor post-Lake Mead aggradation (Cave B is at an elevation of 654 mand the 1923 river level was 291 m above sea level; Dry Canyonis at an elevation of 484 m and the 1923 river level was 298 mabove sea level (LaRue, 1925)). Combining the new data at Spencer

Canyon and the speleothem data for western Grand Canyon (Polyaket al., 2008) gives a background strath-to-strath bedrock incisionrate of 101 ± 4.6 m/Ma with a y-intercept of −28 ± 7.6 m (Fig. 3),in agreement with independent estimates of the depth to bedrockbelow the Colorado River in Grand Canyon. The mean square ofweighted deviation (MSWD) on the regression is 0.05, indicatingthat the errors are overestimated and/or that by chance the strathheights and ages have less scatter about the regression than wouldbe anticipated. Because of relatively large analytical uncertaintyon the Dry Canyon U/Pd date (Polyak et al., 2008), it is possiblethat incision rates changed around 2 Ma, however consistent in-cision rates calculated over ca. 0.5 and 4 Ma using more precisegeochronology suggest steady long-tern incision.

3.2. Central Grand Canyon

In the Surprise Valley area of central Grand Canyon a series oflandslides dammed the Colorado River (Huntoon, 1975). We haveobtained new minimum cosmogenic burial ages from strath ter-races directly below two sections of these landslides, dating themass failures for the first time and constraining incision ratessince that time. At RM 135 (north side of the river) cosmogenicburial dating of a 2-m-thick gravel deposit at the base of a paleo-channel filled by ∼30 m of landslide debris yielded an age of880 ± 440 ka (2σ ). At RM 136.6 (south side of the river) a 5-m-thick gravel dated at 980 ± 420 ka (2σ ) was buried by 10–25 mof landslide debris, known as Ponchos Runup (Warme, 2010;Watkins et al., 2007), that initiated on the north side of the canyonand was thrust up the south side. The heights of the straths abovethe river are 70 and 61 m, respectively. Because the dates andheights suggest that the deposits are approximately coeval, we usea mean strath height of 65.5 which yields a maximum incision rateof 95+46

−23 m/Ma, based on a weighted mean age of 932 ± 304 ka.Elves Chasm, at RM 117, is the site of one of the largest traver-

tine accumulations in Grand Canyon; a 250-m-high travertine cliffis present on the south side of the river (Fig. 4). Bedrock straths

Fig. 4. (Color online.) Annotated photograph look upstream at Elves Chasm travertine accumulation. Sample locations and available geochronology shown. The 34 m terracewas sampled ∼0.3 RM upstream.

168 R. Crow et al. / Earth and Planetary Science Letters 397 (2014) 159–173

Fig. 5. Strath-to-strath incision plot for central Grand Canyon. The slope of theregressed line (using the red points) gives the relatively imprecise central GrandCanyon incision rate of 97 m/Ma. All unlabeled points are from Elves Chasm. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

below travertine-cemented gravels are present at heights from 34to 201 m above river level. The heights and ages of 15 newlydated samples are shown in Fig. 5. Dating of both coatings anddetrital travertine blocks within the 46-m terrace yielded U/Thages that are within error of each other and thus constrain theage of the deposit to ca. 650 ka, which yields an incision rateof about 100 m/Ma. At the 34-m strath a U/Th age on a traver-tine that fills space between cobbles gives a minimum age forthe gravel of 467+49

−38 ka, which yields a maximum incision rate ofabout 120 m/Ma. Thus, we infer that incision rates were constantover the last 600 ka. 234U model ages from higher samples, out-side of the upper limit for U/Th dating, have large errors (20–54%),but these data points also suggest similar or higher incision rate of87–246 m/Ma back to about 1.5 Ma.

Combining the new data at Surprise Canyons and Elves Chasmgives a background bedrock incision rate of 97 ± 42 m/Ma with ay-intercept of −13 ± 11 m (Fig. 5) for central Grand Canyon. TheMSWD of 0.9 indicates that a linear regression is appropriate asobserved deviations from the regression are explainable by the an-alytical error.

3.3. Eastern Grand Canyon

In eastern Grand Canyon, at Tanner (RM 69), Palisades (RM 65),and Kwagunt (RM 56), strath terraces sequences have been datedusing U/Th dating of travertine that occurs as both cements be-tween gravel clasts and detrital clasts. The Kwagunt locality hasbeen the cornerstone for eastern Grand Canyon incision ratesyielding rates of 131 to 172 m/Ma depending on the dated strathsused and the incision rate calculation method (Karlstrom et al.,2008, 2007; Pederson et al., 2006, 2002a). New results indicatethat the 34-m Kwagunt terrace has statistically indistinguishableages on travertine infillings and detrital clasts that tightly constrainthe age of gravel deposition to ca. 390 ka, giving an incision rateof ∼150 m/Ma.

As a still-better approach for estimating incision in easternGrand Canyon, we combine these Kwagunt data with data fromsimilar terrace sequences at the two other nearby localities toget the best incision estimate for this section of Grand Canyon.Our strath-to-strath analysis give a bedrock incision rate of 160 ±11 m/Ma (Fig. 6) with a y-intercept of −25 ± 4 m (see the Supple-mentary Data for separate plots from each area), consistent with

Fig. 6. Strath-to-strath incision plot for eastern Grand Canyon. The slope of the re-gressed line (using the red points) gives the eastern Grand Canyon incision rate of160 m/Ma. Kwagunt infilling data from Pederson et al. (2006, 2002a). (For interpre-tation of the references to color in this figure legend, the reader is referred to theweb version of this article.)

independent estimates of the depth to bedrock below the river. TheMSWD of 2.7 indicates that there is slightly more scatter about theregression than would be anticipated at the 95% confidence level,likely due to the combination of maximum and minimum ages.

U/Pb dated cave mammillaries currently provide the best con-straints on pre-600 ka incision in eastern Grand Canyon (Polyak etal., 2008). Cave mammillaries are interpreted to form as the watertable passes through a cave. Hence the elevation of the mam-millary calcite can be related to canyon depth at the time themammillary formed if one assumes a flat water table and thatwater table decline tracks canyon deepening. These assumptionsmay be reasonable for caves within a few km of the mainstem;whereas incision rates from caves at large distances from the riverare considered maximum rates as the water table is known toslope towards the river (Karlstrom et al., 2008; Pearthree et al.,2008; Pederson et al., 2008). We regress only the mammillary-constrained incision rates that yield the lowest rates (assuminga water table sloping towards the river) and combine them withthe highest dated straths in eastern Grand Canyon to calculate astrath-to-strath incision rate of 236 ± 31 m/Ma for eastern GrandCanyon for the period from 4 Ma to 600 ka (see SupplementaryData Fig. S1). Taken together the available data for eastern GrandCanyon permit either semi-steady incision over 4 Ma, or post-0.6-Ma rates (160 m/Ma) somewhat slower than 0.6–4 Ma rates(236 m/Ma).

4. Discussion

4.1. Implications for knickzone transience

The new data on incision rates through time in a given reachare important in placing constraints on when pulses of incisionand resulting knickzones could have passed through Grand Canyon.Our new data suggest that major transient knickzones, definedbroadly as zones separating reaches with discrete channel slopes,could not have passed through western Grand Canyon after 4 Maor through eastern Grand Canyon after 600 ka. The temporally

R. Crow et al. / Earth and Planetary Science Letters 397 (2014) 159–173 169

Fig. 7. Diagram showing the lithologies present immediately above river level ateach incision rate location. Thicknesses from Beus and Billingsley (1989). The redline shows the height of various strath levels based on modern depth to bedrockestimates and the new incision rates. (For interpretation of the references to colorin this figure legend, the reader is referred to the web version of this article.)

steady incision rates show conclusively that knickzone migrationcannot explain the spatial variation in incision through the canyon.Incision rates in the Lees Ferry area, where the prominent mod-ern knickzone of the Colorado River system is located, suggestan increase in incision rates there from 60 m/Ma to 400 m/Main the last 200 ka which is interpreted to represent transient in-cision through the area (Cook et al., 2009; Darling et al., 2012;Karlstrom et al., 2012). If so, an ancestral Lees Ferry knickzonecould have only migrated through eastern Grand Canyon (RM 56)prior to about 600 ka, implying a maximum knickzone migrationrate of 150 km/Ma.

4.2. Processes modulating bedrock incision

River incision through bedrock is driven by potential energy dif-ferences between headwaters and base level and the river’s abilityto apply work on its bed. The former is controlled in large part byrock uplift driven by tectonics and isostatic rebound. The latter iscontrolled by geomorphic and hydrologic factors, including bedrockstrength, sediment load, and stream power. Few geomorphic stud-ies are able to look at million-year timescales for continental-scalerivers and still fewer are able to examine changes in rates ofbedrock incision through time. The following section discusses theimplication of steady incision and, as importantly, steady differ-ential incision, for understanding the interplay between tectonic,hydraulic, and geomorphic influences in carving Grand Canyon.

Bedrock strength (erodability) has been used to explain varia-tions in bedrock incision and landscape morphology (Cook et al.,

2009; Pederson and Tressler, 2012; Sklar and Dietrich, 2004). Thedegree to which incision is modulated in Grand Canyon by bedrockstrength can be tested with our dataset by comparing newly de-termined incision rates to the nature of the rock in different strati-graphic layers being incised through time (Fig. 7). Over the last600 ka, eastern Grand Canyon was incising through relatively weakBright Angel Shale and Dox Sandstone; central Grand Canyon wasincising through the cliff-forming and hence intermediate strengthTapeats sandstone, then harder basement granodiorites; and west-ern Grand Canyon was incising (more slowly) through Muav Lime-stone, Bright Angel Shale and basement schists and granites. At allthree sites, which are distributed throughout the 475 km length ofGrand Canyon, average bedrock incision rates were steady throughtime even though the river was incising through rocks of vari-able strength. This suggests that bedrock erodability was not thedominant control on long-term incision rate. One might argue thatslower bedrock incision in western Grand Canyon (101 m/Ma) rel-ative to eastern Grand Canyon (160 m/Ma) over the last 600 kamight reflect harder rocks in the west as both basement and nu-merous post-600 ka lava dams are present there and that theriver is adjusting its slope in response to the exposure of theseharder rocks. However, the data for steady incision rates over thelast 4 Ma in western Grand Canyon indicate that the rates havebeen semi-steady through Bright Angel Shale, Muav Limestone andbasement rocks, which have markedly different bedrock strengths(Mackley, 2005), with no observed change at 600 ka that marksthe onset of a period of repeated basaltic lava damming. A majorconclusion of lava dam studies (Crow, 2012) is that lava dams didnot significantly affect the average long-term bedrock incision ratesin western Grand Canyon over the last half million years. Tem-porally steady incision at all the sites in Grand Canyon, despitemarked differences in bedrock strength which reflect themselvesin both hillslope morphology (Howard and Dolan, 1981) and semi-quantitative measures of erodibility (Mackley, 2005), suggest thatColorado River incision is largely unaffected by any variability inbedrock strength and that other controls are dominant in mod-ulating incision rates and patterns in the system. Additionally ifthe Colorado River in Grand Canyon is transport limited, as indi-cated by boulder-rich debris fans perturbing the profile (Hanks andWebb, 2006) and an alluviated bed (Pederson et al., 2006), then lo-cal bedrock erodibility should have a weak influence on river slope(Sklar and Dietrich, 2004).

Climatically driven changes in discharge and resulting streampower has also been invoked to explain temporal variations in in-cision in a range of systems (Wobus et al., 2010). In Grand Canyonit has been shown that climatic variations at glacial–interglacialscales of ca. 100 ka produced 10s m of local aggradation (Pedersonet al., 2006). However, longer-term bedrock incision rates are tem-porally steady despite these short-time-frame aggradation-incisionoscillations (Karlstrom et al., 2008, 2007; Pederson et al., 2006).Fill-terrace studies show that the work of carving through bedrockis dominantly applied over a relatively small percentage of theglacial–interglacial cycles, but steady incision over longer time-frames (0.6 to 4 Ma) averages out many or all Quaternary glacialcycles. Singular climate change events marking a change to moreerosive climates have been proposed, for example at ca. 2.5 Ma(Molnar, 2004; Zhang et al., 2001), but local expressions of such amodel are not supported by the observed temporally steady inci-sion in western Grand Canyon over 4 Ma. In addition, differences isdischarge cannot explain the spatial variation in incision rates fromwest to east in Grand Canyon because there are only minor trib-utary additions to Colorado River discharge throughout the lengthof Grand Canyon (Pederson and Tressler, 2012). Because of this,climatically-driven changes to discharge and hence stream powershould affect all of Grand Canyon similarly and do not explain

170 R. Crow et al. / Earth and Planetary Science Letters 397 (2014) 159–173

the disparity in incision rates between eastern and western GrandCanyon.

The amount or character of the sediment load is also unable toexplain the spatial variation in incision rates in Grand Canyon. In anumber of rivers the degree of bed-load cover has been shown tohave an important control on bedrock incision whereby increasedincision occurs when enough hard course material is deliveredto the river to enhance abrasion but not enough has accumu-lated to completely bury and armor the bed (Johnson et al., 2009;Sklar and Dietrich, 2004; Whipple and Tucker, 2002). To explorethis concept for the late Cenozoic, we note that Hanks and Webb(2006) suggested that 15–30 m convexities in the long profile ofthe Colorado River through Grand Canyon may reflect areas of localHolocene aggradation due to increased debris flow input. But thehighest incision rates we calculate are in the middle of the largestconvexity where thickness of alluvium has been estimated at over30 m (Hanks and Webb, 2006). If the high incision rate therewere due to relatively little bed armoring, the paleo-sedimentload must have been the exact opposite of the Holocene state.Sediment load is known to have varied greatly due to climaticvariation (see above) however at long-time scales steady incisionsuggests that the time-integrated effect is small. Such climatic ef-fects would affect sediment flux through the canyon similarly. Nosystematic characterization of bedload clast lithology or morphol-ogy has been conducted through Grand Canyon, to our knowledge,but hard far-traveled basement clasts (for example quartzites andother basement clasts from the Colorado Rockies) are distributedin all terraces fairly uniformly through the canyon and bedrockpotholing throughout the canyon attests to availability of sufficient“tools” in all reaches. Hence, we infer that bedload variations areunlikely to have produced the differential incision noted. Althoughit could be argued that more hard clasts are present in and belowreaches floored by hard Paleoproterozoic basement rocks, the high-est incision rates in eastern Grand Canyon are upstream from thefirst Paleoproterozoic rocks in Grand Canyon, suggesting that theavailability of these “tools” does not have a significant control onincision. Similarly, although basalts clasts from lava flow remnantsmight be expected to provide additional tools in western GrandCanyon, this is where incision rates are lower.

Thus, the various geomorphic and climatic explanations dis-cussed above are ruled out as dominant mechanisms controllingthe observed steady differential incision data documented here.Instead we suggest that the patterns in incision are due to dif-ferential uplift, similar in style to that proposed by Karlstrom et al.(2008, 2007). They proposed, based on Colorado River profile re-constructions to the Gulf of California, that eastern Grand Canyonhad been uplifted relative to sea level as opposed to downdroppingthe lower Colorado River reaches (i.e. Western Grand Canyon andbelow). This conclusion is now further supported by studies indi-cating that the lower Bouse Fm. deposits along the lower ColoradoRiver have remained near sea level from the late Miocene to thepresent (McDougall and Martinez, in press), requiring the differen-tial incision of the Grand Canyon reaches to be due to differentialuplift and not differential subsidence.

4.3. Tests of inferred uplift drivers

Differences in bedload, climate, bedrock strength, transientknickzones, or subsidence seem unable to explain the largeand statistically significant difference between western (101 ±4.6 m/Ma) and eastern Grand Canyon (160 ± 11 m/Ma) incisionrates. Thus, we support and refine previous models that differen-tial uplift is the dominant driver. This geologic constraint has thepotential to help elucidate the much debated processes that havebeen invoked to produce intracratonic deformation and uplift.

Fig. 8. (Color online.) Graphs of how incision rates varies with rebound, mantle-velocity, and distance from the most inboard volcanism. (A) incision rate versusdenudation-driven rebound amount (Lazear et al., 2013). (B) incision rate versusmantle velocity at 80 km depth (Schmandt and Humphreys, 2010). (C) incision rateversus distance from the leading edge of the magmatic sweep. X-axis error bars re-flect the range in values over an area that extends beyond the incision point(s) by5 km in all directions.

One, partly non-tectonic mechanism proposed for Neogene up-lift of the area involves epeirogenic uplift due to the isostaticresponse to differential denudation. Denudation-driven models(Lazear et al., 2013; Pederson et al., 2002b; Roy et al., 2009) predictdifferential rock uplift throughout the Colorado Plateau area dueto the flexural response (rebound) to differential erosion. Modeleddifferential rock uplift of eastern Grand Canyon due to rebound isup to several hundred meters greater than western Grand Canyonwhich could lead to differential rock uplift rates of 10–30 m/Maover 10 Ma, and 20–60 m/Ma if distributed over 5 Ma (Lazear etal., 2013). In Fig. 8A we plot long-term (>600 ka) incision ratesagainst the magnitude of modeled post-10-Ma rebound. This plotshows that rebound accounts for little of the variations in post-4 Ma bedrock incision rates throughout the area (Fig. 8A). Thismay be the result of enhanced denudation (and rebound), soon af-ter Colorado River integration at ca. 5–6 Ma (Karlstrom et al., 2012,2008), prior to the period the incision rates have been quantifiedover.

Crustal deformation also seems unable to explain the tempo-rally steady but spatially differential incision documented here. Al-though Quaternary normal faults, separating Western and Central

R. Crow et al. / Earth and Planetary Science Letters 397 (2014) 159–173 171

Fig. 9. Tomographic cross-section showing the migrating mantle boundary that we suggest is driving uplift. The black, gray and white arrows show the area of buoyancymodification during the Holocene, Late Miocene, and Mid-Miocene, respectively.

Grand Canyon, affect incision rates greatly within 10–15 km of thefaults (Karlstrom et al., 2007), available data indicates they havelittle influence at larger distances from those faults. Although in-cision rates of about 150 m/Ma a few km upstream from thosefaults are similar to Eastern Grand Canyon rates reported here,the best incision rates of 100 ± 20 m/Ma in Central Grand Canyon(based on the well-dated 46-m terrace at Elves Chasm) and thestrath-to-strath rates of 97 ± 42 m/Ma suggest that high incisionrates in footwall of faults are due to local fault-related flexures.

Mantle-driven differential uplift provides a potential explana-tion for both temporally steady incision rates and the differencein incision rates between western to eastern Grand Canyon. In-sight into temporal changes in mantle buoyancy beneath the GrandCanyon region comes from the record of Neogene basaltic vol-canism (Best and Brimhall, 1974; Crow et al., 2010; Wenrichet al., 1995) and modern seismic tomography (Schmandt andHumphreys, 2010). Starting in the late Oligocene, basaltic vol-canism has progressively migrated from the western edge of theColorado Plateau toward its interior at a rate of about 5 km/Ma(Crow et al., 2010). This spatial migration is coupled with a tem-poral trend towards basalts with increasingly radiogenic Nd values(more asthenospheric), indicative of progressive infiltration or re-placement of the lithosphere by asthenosphere (Crow et al., 2010).Western U.S. mantle tomography shows that the western ColoradoPlateau is surrounded by relatively low-velocity mantle beneaththe Basin and Range. At about 80 km depth beneath the west-ern Colorado Plateau low-velocity mantle similar to that beneaththe Basin and Range extends under the topographic boundary ofthe plateau to about the same extent as basaltic volcanism. Thevelocity contrast there, over 100 km distances, is up to 6% in Vpand 12% in Vs at depths near 80 km. We suggest that heating ofwestern Colorado Plateau upper mantle by melt infiltration andperhaps lithospheric thinning or delamination creates an eastwardmigrating zone of increasing mantle buoyancy. Slow rates of prop-agation (∼5 km/My) are consistent with temporally steady butspatially variable incision rates reported here and along the Vir-gin River (Willis and Biek, 2001) (Fig. 2), which increase towardsthe eastern limit of basaltic volcanism and the areas of maximummantle-velocity gradient (Fig. 8C). The greater incision rates ineastern Grand Canyon may reflect an ongoing response to increas-ing mantle buoyancy, whereas the western Grand Canyon incisionrates represent an area that has already adjusted to an earlier in-crease in mantle buoyancy. This and the variable elastic thicknessof the lithosphere in this region (Lowry and Perez-Gussinye, 2011),which will modulate any isostatic uplift, explains why a strong re-

lationship between mantle velocity and incision rate is not seen(Fig. 8B).

The Lees Ferry knickzone is located in the middle of the areaof maximum mantle-velocity gradient (Fig. 2), suggesting thatincision rates and river steepness are both responding to dy-namic processes across this moving boundary between unmodi-fied lithosphere (to the east) and low-velocity lithosphere mod-ified by asthenospheric upwelling (to the west). Figs. 2 and 9suggest that propagation of increased mantle buoyancy has ap-proximately kept pace with the propagation of basaltic volcanismand is driving uplift that correlates broadly with the position ofthe mantle velocity gradient now near Lees Ferry. Heating, melttransfer, and lithospheric removal associated with this migratingmantle boundary is expected to produce buoyancy modificationand isostatic uplift which may explain the 60 m/Ma different be-tween eastern and western Grand Canyon incision rates. Differ-ential uplift of 60 m/Ma in the Grand Canyon region could beproduced by epeirogenic tilt rates of 0.01 to 0.02◦/Ma and re-sulting accumulated tilts of 0.05–0.1◦ over the length of GrandCanyon in the last 5 Ma. This result has the potential to serve asan important constraint to test geodynamic modeling of the ef-fects of mantle-driven dynamic topography (Karlstrom et al., 2012;Levander et al., 2011; Liu and Gurnis, 2010; Moucha et al., 2009;van Wijk et al., 2010) and river profile evolution (Roberts et al.,2012). Refinement of the relatively imprecise central Grand Canyonincision rates would allow for a better determination of the exactwavelength of the uplift and the role of Quaternary normal faultsin accommodating strain.

5. Conclusions

New incision rates derived from U-series dates and strath-to-strath analysis suggest temporally steady long-term incision of160 ± 11 m/Ma in eastern Grand Canyon, 97 ± 42 m/Ma in cen-tral Grand Canyon, 101 ± 4.6 m/Ma in the western Grand Canyon.Steady differential incision has taken place throughout the lengthof Grand Canyon over at least the last 600 ka and possibly overthe last 4 Ma if maximum canyon deepening can be reliably con-strained using U/Pb dating of speleothems. Steady bedrock incisionrates through time argue against the passage of major knickzonesthrough those reaches during the time intervals measured. Simi-larly differences in bedrock hardness, sediment load, or variationsin discharge do not explain the spatial and temporal patterns inincision that we document. Regional patterns suggest that differen-tial isostatic rebound due to denudation may have contributed todifferential incision, but is unlikely to fully account for the spatial

172 R. Crow et al. / Earth and Planetary Science Letters 397 (2014) 159–173

variation noted. Increasing incision rates from west to east acrossGrand Canyon imply regional tilting, which must be explained byany uplift model. This has taken place during at least the last 4 Ma,in the same region where basaltic magmatism swept northeast-ward and became more asthenospheric during the last 25 Ma. Thecombined data are best explained by mantle-driven epeirogenicuplift at rates of 60 m/Ma focused under eastern Grand Canyon, di-rectly adjacent to a step in the lithosphere (mantle velocity bound-ary), where an east-moving transient sweep of melt/heat transfer,basaltic surface magmatism, and progressive lithospheric infiltra-tion (and removal) are affecting mantle buoyancy and driving up-lift. These data suggest that the western margin of the ColoradoPlateau has been tilted at a rate of 0.01 to 0.02◦/Ma and showsthe importance of tectonics in modulating Grand Canyon incisionand contributing to formation of one of the most famous land-scapes on Earth.

Acknowledgements

This paper was supported by NSF Grant EAR-1242028 from theTectonics Program. We also acknowledge a research agreementwith Grand Canyon National Park that has allowed river corri-dor access. Anonymous reviews and informal discussion with KelinWhipple improved this paper.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2014.04.020.

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