TECTONICS, VOL. 12, NO. 1, PAGES 291-300, FEBRUARY 1993
SLOW APPARENT POLAR WANDER FOR
NORTH AMERICA IN THE LATE TRIASSIC
AND LARGE COLORADO PLATEAU ROTATION
Abstract. Several recent analysesof North American
paleomagnefic
datasuggest
fastapparentpolarwander(APW)
(--0.75ø/m.y.)duringthe Late Triassicand a modestamount
(-5 ø) of ColoradoPlateauclockwiserotation. Paleomagnetic
polesfrom the lower (Carnian),middle (NorJan),and upper
(Hettangian) stratigraphicintervals of the Newark Basin,
however,indicatevery slow APW over the Late Triassicand
providean alternativeinterpretation
for plateaurotation. The
middleNewark pole is supported
by positivefold and reversal
tests,precluding
remagnetization,
andagreeswell with thepole
reportedfrom the Norian Upper ShaleMemberof the Chinle
predictsan angularplate velocity at a constantrate of about
0.75ø/m.y.in the Triassic,anda similarlyfastrate(0.66ø/m.y.)
in almost the oppositedirection over much of the Jurassic.
This model of APW is a critical assumption in .the
determination
of rotationof theColoradoPlateauby themethod
of Bryan and Gordon [1986, 1990]. They most recently
estimated a clockwise plateau rotation of 5.0ø + 2.5ø to
optimize the fit of late Paleozoicto Jurassicpaleomagnetic
polesfrom the plateauand eratonicNorth Americato a PEP
modelpath. EventhoughSteiner[1986, 1988]hasmaintained
that somepole-to-poledifferencesindicatea larger plateau
rotationof 11ø_+4ø,BryanandGordon[1990]rejectedat a high
formal level of confidence(99.99%) the hypothesisthat a
systematic
difference
couldbeaslargeas 10ø.
Bryan and Gordon [1990] acknowledgedthat Cretaceous
poles,which are not available from the ColoradoPlateau,do
not contributedirectlyto the estimateof plateaurotationand
shouldthereforebe excludedfrom analysis.We suggestthat
Jurassicand Permian poles also do not provide cogent
Formation from east central New Mexico as well as the 214 Ma
constraints
on the amountof plateaurotationin the light of
Manicouagan
pole from Quebec. Thesepolesprovidea welldefinedmeanNorJanreferencepolefor eratonicNorthAmerica
at 57.4øN 91.0øE A95=3.8ø. Paleomagneticpoles from the
Chinle Formation on the Colorado Plateau (Owl Rock
new data. The synthesisby May and Buffer [1986] already
showedthat thereare virtually no coevaleratonicand plateau
counterparts
in theinventoryof North AmericanJurassic
poles.
Now even the overall reliability of many of the Jurassic
referencepoles has becomethe subjectof debate,with the
availabilityof newbut oftenconflictingpaleomagnetic
results
thatallowratherdivergentinterpretations
of Jurassic
APW [Van
Fossenand Kent, 1990, 1992a, b; Buffer et al., 1992]. With
regard to late Paleozoicpaleomagneticdata, Gordon et al.
[1984] recognizedthat Permianpoles from on the plateau
showedno discernibledifferencefrom thoseoff the plateau,
eventhoughTriassicpolessuggested
plateaurotationof about
10ø. To explainthisdiscrepancy,
Steiner[1988]suggested
that
theColoradoPlateauexperienced
a separate
rotationin theLate
Permian,prior to the LaramideandCenozoicrotationsthat are
of interesthere. The tectonicimplicationof thePermianpoles
is complicated,however,by new evidencethat portionsof
Dennis V. Kent and William K. WiRe
Lamont-Doherty Geological Observatory of Columbia
University,Palisades,
New York
Member, Church Rock Member, and our new result from the
upperChinle in Utah) are also well grouped,consistent
with
slow APW over the NorJan,but give a mean pole position
(57.7øN 65.6øE A95=2.5ø) that differs significantlyfrom the
Norian pole for eratonicNorth America. The North American
Norian poles can be closely reconciledby a 13.5ø + 3.5ø
correctionfor accumulatedpost-Triassicclockwiserotationof
the Colorado Plateau associated with Laramide deformation and
Cenozoicopeningof the Rio GrandeRift. This estimateof
Colorado Plateau rotation is consistentwith a systematic
discrepancy
betweenplateauand eratonicpolesfor the Early
Triassic,whereasavailablelatePaleozoicandJurassic
polesare
judgednotto providedefinitiveconstraints
on plateaurotation.
A revisedTriassicand Early JurassicAPW path for North
America showsthat the virtual standstillin the Norian, the last
15 m.y. of theTriassic,waspreceded
andfollowedby intervals
of fast(~lø/m.y.)angularplatevelocity.
central New Mexico east of the Rio Grande Rift, where some
keyPermianpolesthathavebeenregardedascratonicreferences
were obtained,experienceda similar senseof rotationas the
ColoradoPlateau[Molina-Garzaet al., 1991].
Our attention therefore focuses on the Triassic for which
INTRODUCTION
To explainCretaceousLaramidedeformationin the Rocky
Mountain region and Cenozoicextensionin the Rio Grande
Rift, Hamilton [1981] proposedthat the ColoradoPlateau
rotatedclockwisein two phasesrelativeto the stableeratonof
North America. Becausemany late Paleozoicand Mesozoic
paleomagnetic
referencepolesfor NorthAmericacomefromthe
ColoradoPlateau,detaileddescriptions
of the apparentpolar
wander (APW) path for cratonic North America and
paleomagnetic
estimates
of theamountof Plateaurotationtend
to be stronglyinterdependent.
A centralissueis whethernet
clockwiserotationof the ColoradoPlateauwas small(< 5ø) or
large(> 10ø).
The paleomagnetic
Euler pole (PEP) modelof APW for
North America [Gordonet al., 1984;May and Buffer,1986]
recentpaleomagnetic
studiesprovidewhatwe regardas more
coherentinformationto separateNorth AmericanAPW from
ColoradoPlateaurotation. The key pole positionsin our
analysis include Late Triassic poles from Newark Basin
sedimentary
rocks[Witte and Kent, 1989;WiRe et al., 1991]
and the compilationof Late TriassicNorth Americanpoles
judgedasreliableby BazardandBuffer[1991],with supportive
data from our paleomagneticstudyof the Chinle Formation.
Our synthesis
supportsthe virtualabsenceof APW in the Late
Triassicthatwasindependently
suggested
by theNewarkresults
and showsthat post-Triassicnet clockwiserotation of the
ColoradoPlateaumusthavebeenconsequently
large.
EVIDENCE
FOR SLOW APW IN THE LATE TRIASSIC
Cratonic North American Data
Copyright1993by the AmericanGeophysical
Union.
The Newark Basinis the largest(250 km long and 50 km
wide) of a chain of Mesozoic rift basins in easternNorth
Americathat developedin the early stagesof formationof the
Papernumber92TC01966.
Atlantic Ocean.
0278-7407/93/92TC -01966510.00
fluvialsediments
in a continuous
sequence
spanning
morethan
It contains more than 6 km of lacustrine and
292
Kent and Witte: ColoradoPlateauRotation
25 m.y., fromthemiddleCamianto theHettangian[Cometand
Olsen, 1985]. The paleomagnetically
well-studiedtholeiitic
basaltsand associateddiabaseintrusionsare volumetrically
importantbutarenowbelievedto havebeeneraplaced
aspartof
a short-lived (circa 1 m.y.) igneous episodejust after the
Triassic/Jurassic
boundary[OlsenandFedosh,1988;Olsenand
Sues,1986]. The mostreliable age for the igneousactivity is
201+1 Ma, based on U/Pb zircon dating on baddeleyite
[DunningandHodych,1990],whichis in goodagreement
with
an age of 200 Ma for the Triassic/Jurassic
boundaryin the
Triassictime scaleof Webb [1981]. Younger agesthat have
been reportedfor the igneousintrusionsare now ascribedto
resettingby a hydrothermalevent at about 175 Ma [Sutter,
1988],whichmayverywell havealsoremagnefized
theigneous
intrusionsand overprintedthe basaltmagnetizations
[Witte and
Kent, 1989, 1990, 1991]. For these and related reasons, the
statusof paleomagnetic
polesfrom the Newark trendigneous
rocks (labeled N1 and N2 by May and Buffer [1986], after
Smith and Noltimier [1979]) is sufficientlyin doubtthat these
oncekey referencepoleshavenot beenusedin recentanalyses
of Jurassic
APW [e.g.,BazardandButler, 1991].
The paleomagnetic
studyof theNewarkBasinby Mcintoshet
al. [1985] included a comprehensivesampling of the
sedimentaryrocks. The presenceof normal and reversed
polaritiesprovidedevidencefor the preservation
of an ancient
component of magnetization in the Newark sedimentary
section,but the significanceof the paleomagnetic
directionsin
termsof APW wasobscuredbecauseof incompleteremovalof
secondary
magnetizations
afterthermaldemagnetization
to only
550øCfor mostsamples.
Basedon completedemagnetization
analysis,renewedwork
on theNewarksedimentary
rockshasyieldedthreewell defined
paleopolesfrom threeintervalsin stratigraphic
succession.A
lowerNewark pole wasobtainedfrom Camianred bedsof the
Stockton, Lockatong and lowermost Passaic formations
60øE
90øE
(53.5øN 101.6øE A95=4.8ø; Witte and Kent, 1989], a middle
Newark pole from Norian red bedsof the PassaicFormation
(55.9øN95.0øEA95=4.4ø [Witte et al., 1991]), and an upper
Newark pole from Hettangianred beds interbeddedwith the
Watchungbasaltsof the igneousextmsivezone(55.3øN94.5øE
A95=5.4ø [WitteandKent,1990])(Figure1). Thesepaleopoles
are not significantlydifferent from one another;the largest
difference(4.5o+6.5ø) is betweenthe lower Newark and middle
Newarkpoles,whereasthemiddleNewarkandupperNewark
polesdiffer by lessthan1ø. The reliabilityof theNewarkpoles
is supportedby a regionally coherentmagnetic polarity
stratigraphy,whichis nowbeingcorroborated
in detailfrom the
resultsof the Newark BasinCotingProject[Kentet al., 1991],
and by a positivefold test on a syndepositional
fold and a
positivereversaltestfor themiddleNewarkpole [Witteet al.,
1991]. This evidenceclearlyshowsthatremagnetization
is not
a viableexplanationfor the similarityin paleomagnetic
poles
fromlower,middle,andupperNewarkrocks.
The Newark Basin developedalong the structuralgrain of
PrecambrianandearlyPaleozoiccrystallinebasement[Ratcliffe
et al., 1986; Swanson,1986], making unlikely appreciable
vertical axis rotation of the entire basin with respectto the
stablecratonof North America. The possibilityof "cryptic"
tectonic rotations localized
to the border fault zone of the
NewarkBasinhasbeensuggested
by Van Fossenet al. [1986],
however,to explainaberrantpaleomagnefic
directionsobtained
from thewesternlimb of theWatchungsyncline.Regardless
of
whether the aberrant directions are in fact due to local rotation,
for which thereis no geologicalevidence,or can be explained
by secularvariationbiasor othermechanisms
[Kodama,1987;
Van Fossenet al., 1987],we havenotobservedsuchsystematic
discrepancies
wherewe havesampledelsewherein theNewark
Basin. For example,thepaleomagnetic
directionsfrom 15 sites
takenon bothlimbsof theJacksonwaltsynclineneartheborder
fault give a positivefold testandare concordantwith the mean
120øE
60ON
.....................
45øN
Fig. 1. SelectedTriassic(circles)andEarly Jurassic(triangles)paleomagnetic
polesfor North America
with no correction
for ColoradoPlateaurotation.Opensymbolswith lightconfidence
envelopes
arepoles
from ColoradoPlateauandsolidsymbolswith darkconfidence
envelopesare polesfrom off theplateau
(seeTable 1 for abbreviations),
with standard
error(63% confidence)
circlesapproximated
as58% of A95.
Shownfor referenceby stippledcurvesare the Triassicpaleomagnefic
Euler pole (PEP) trackand the
ensuingportionof theJurassic
PEP trackfrom Gordonet al. [ 1984].
Kent and Witte: Colorado Plateau Rotation
directionof eight sites from the interior of the basin where
strataof similarNorian age are gentlyhomoclinal[Witte et al.,
1991]. The 23 middle Newark site mean directionswere further
shownto be statisticallycompatiblewith a circular(Fisherian)
distribution, without an elongatedscatter or streakingthat
mightbe expectedfrom eitherrotationsaboutlocalverticalaxes
or samplingof rapidAPW.
Slow APW during at least the Norian stage of the Late
Triassicis supportedby the consistency
of the middleNewark
pole with other paleomagneticresults from cratonic North
America regardedas reliable by Bazard and Butler [1991]. A
new paleomagneticpole with a positivereversaltest from the
early Norian Upper ShaleMember of the Chinle Formationin
east central New
Mexico
falls at 57.4øN
87.8øE
A95=5.0 ø
[Bazard and Buffer, 1991], which is within 4.2 ø of and therefore
not significantlydifferentfrom the Norian middleNewark pole
(Figure 1). Similarly, paleomagnetic results from the
Manicouaganimpact structurein the Precambrianshield of
Quebec[Robertson,1967; Larochelieand Curfie, 1967] give a
meanpole positionat 58.8øN 89.9øEA95=5.8ø [Bazardand
Buffer,1991] thatis not significantlydifferentfrom the middle
Newark pole (Figure 1). New high-precisiondating by the
U/Pb zircon method puts the age of the Manicouaganmelt
rocks at 214 + 1 Ma [Hodych and Dunning, 1992], which
corresponds
to theNorianaccordingto time scalesasdiverseas
thoseof Webb [1981] and Harland et al. [1990].
The well-definedmeanof the middle Newark, Upper Shale
and Manicouaganpoles(Table 1) at 57.4øN 91.0øEA95=3.8ø
canthereforebe regardedasrepresentative
of thestationary
pole
position with respect to cratonic North America over the
293
Norian, approximatelythe last 15 m.y. of the Triassic(215 Ma
to 200 Ma [Webb, 1981] or 223 Ma to 208 Ma [Harland et al.,
1990]).
Colorado Plateau Data
For the ColoradoPlateau,paleomagneticdata are available
from rocksof similar Norian age from the Chinle Formation.
The middleNorian Owl Rock Member sampledin northeastern
Arizonagivesa pole positionsupportedby a positivereversal
test at 56.5øN 66.4øE A95=2.6 ø [Bazard and Butler, 1991]
(Figure 1). From paleomagneticresultsobtainedby Reeve
[1975] from the stratigraphically
overlying(mid-lateNorJan)
Church Rock Member in southern Utah, Bazard and Butler
[ 1991] recalculateda pole positionat 59.0øN67.0øEA95=2.5ø
that is virtually identicalto the Owl Rock pole. Supportfor
this early work on the ChurchRock membercomesfrom our
own paleomagnetic
studyof approximatelyequivalentstrataof
the Chinle Formationsampledon the SanRafael swell andnear
Moab, Utah. On the basis of completeprogressivethermal
demagnetization to 680øC, we obtain high unblocking
temperaturecharacteristic
directionsfrom nine sitesthatpassa
reversal test and give a mean paleopoleposition at 57.5øN
63.3øEA95=7.3ø (seeAppendix)(Figure1).
ThesethreeChinle polesfrom the ColoradoPlateauare very
well grouped(Table 1). In the absenceof a fold test or a
regional magnetostratigraphy,
remagnetizationof the Chinle
has been suggested[Witte et al., 1991], but we find this
explanation increasingly unlikely in view of the positive
reversaltestreportedfor the Owl Rock memberby Bazardand
TABLE 1. SelectedTriassicandEarlyJurassic
Paleomagnetic
Polesfor North America
Plateall
Plat, Plon,
Cratonic
Plat, Plon,
Rock Unit
Age
oN
oE
oN
Pliensbachian
Sinemurian
Hettangian
59.0
58.2
66.6
51.9
NWu
Kayenta
Moenave
Newarkupper
(58.1
(60.1
55.3
NWm
Newark middle
Norian
K
US
UpperShale
Norian
MI
Manicouagan
214 + 1 Ma
Mean Cratonic Norian (MI, US, NWm):
CR2
ChurchRock
Norian
CR1
ChurchRock
Norian
OR
Owl Rock
Norian
Mean Plateau Norian (OR, CR1, CR2):
NW1
Newark lower
Catalan
M
RP
Moenkopi
RedPeak
Scythian
Scythian
oE
A95,
degrees K
Ref
92.5)
78.0)
94.5
2.4
4.5
5.4
55.9
95.0
4.4
49
4
57.4
58.8
57.4
87.8
89.9
91.0
5.0
5.8
3.8
60
1
5,6,1
1063
88.4)
92.8)
90.7)
90.7)
7.3
2.5
2.6
2.5
50
72
145
2426
57.5
59.0
56.5
57.7
63.3
67.0
66.4
65.6
(57.3
(58.0
(55.8
(57.1
56.8
100.9
(50.4
46.8
53.5
101.6
4.8
120.9)
113.7
2.3
2.9
155 1
45 2
72 3
50
1630
1835
7
8,1
1
9
10
10
Plat and Plon are the latitude and longitudeof the paleomagneticpole in ColoradoPlateau and/or
cratonicNorth Americacoordinates,
with polepositionsin parentheses
correctedfor 13.5ø of clockwise
plateaurotationaboutan Euler pole at 36øN 105øW. A95 is radiusof coneof 95% confidenceaboutpole;
K is estimateof Fisherprecisionparameter.References
(Ref) are 1, BazardandBuffer[ 1991];2, Ekstrand
and Butler [1989]; 3, Witte and Kent [1990]; 4, Witte et al. [1991]; 5, Larochelie and Curfie [1967];
Robertson[1967]; 6, Hodych and Dunning [1992]; 7, this study;8, Reeve [1975]; 9, Witte and Kent
[1989]; and 10, Steiner [1986].
294
Kent
and
Witte:
Colorado
Plateau
Rotation
60øE
90OE
120OE
60øN
45øN
Fig.2. Meanof Norianpalcomagnetic
polesforcratonic
NorthAmerica
compared
tomeanof Norian
poles
forColorado
Plateau
uncorrected
forrotation,
bothwith95%confidence
circles.
Trajectory
ofmean
plateau
Norian
polewithincreasing
amounts
ofcorrection
forclockwise
plateau
rotation
about
Euler
pole
at36øN105øW
isshown
byarrow.Shown
forreference
bystippled
curves
aretheTriassic
palcomagnetic
Eulerpole(PEP)track,theTriassic/Jurassic
orJ1cusp,
andtheensuing
portion
of theJurassic
PEPtrack
from Gordonet al. [1984].
60øE
,
90øE
120øE
60øN
45øN
Fig.3. Selected
Triassic
andEarlyJurassic
palcomagnetic
poles
forNorthAmerica
asinFigure
1but
withcorrection
for13.5
øclockwise
rotation
oftheColorado
Plateau
(Table
1). Shown
forreference
by
lightstippled
curves
aretheTriassic
palcomagnetic
Eulerpole(PEP)track,theTriassic/Jurassic
or J1
cusp,
andtheensuing
portion
of theJurassic
PEPtrackfromGordon
etal.[1984].Ourinterpretation
of
Triassic
andEarlyJurassic
APWtrajectory
forNorth
America
isshown
bytheheavier
stippled
curve.
KentandWitte:Colorado
Plateau
Rotation
Butler[1991]andinourpresent
study
ofstrata
equivalent
tothe
ChurchRockmember.The meanpolepositionfor thethree
plateau
Chinlepolesat 57.7øN65.6øEA95=2.5
ø,however,
disagrees
by over13ø fromthemeanNorianpaleopole
for
cratonicNorthAmericaat 57.4øN91.0øEA95=3.8ø. The case
for fast APW in the Late Triassic largely rests on the
interpretation
thatmost
ofthisangular
difference
reflects
anage
progression
within
theNorian
(e.g.,between
thepolefromthe
earlyNorian
Upper
Shale
member
oftheChinle
offtheplateau
andthepolefromthemiddle
Norian
OwlRockMember
ofthe
Chinle
ontheplateau
corrected
foronly4øofplateau
rotation
[Bazard
andBuffer,1991]).However,
because
thelower,
middle,
andupper
Newark
poles
fromrocks
in stratigraphic
succession
showhardlyanyevolution
in poleposition
across
theNoriantimeinterval,
andtheNorianmiddle
Newarkpole
295
whichallowsus to direcfiycompare
theindividual
setsof
reliableNorianpolesfromon andoff theplateau.This
approach
thus
resembles
thatofSteiner
[1986,1988]
butinour
case
thepotential
problems
of uncertainties
inagecorrelation
that can complicatethe interpretation
of pole-to-pole
comparisons
if APWwasrapidareminimized.
AnEulerpolelocated
at 36øN105øW
wasusedtoestimate
therotationof theColoradoPlateauwith respectto cratonic
North America since the Triassic. This Euler pole is a
combination
of Hamilton's[1981]Eulerpole(39øN 105øW)
whichdescribes
clockwiserotationof theColoradoPlateauto
account
for structures
associated
withCenozoic
opening
of the
Rio GrandeRift, and Hamilton's[1988]revisedEulerpole
(34øN105øW)
forclockwise
plateau
rotation
to account
for
structures
associated
with theLaramideorogeny.Bryanand
Gordon[1986,1990]alsoapproximated
Hamilton's
two-phase
agrees
withtheNorian
Upper
Shale
pole
from
offtheColorado rotation
of
the
Colorado
Plateau
by
a
single
Euler
pole,
located
Plateau,
thedifference
ishereinterpreted
tobepredominantly
a
in theircaseat 37øN103øWusingHamilton's[1981]earlier
consequence
ofrelative
motion
between
theplateau
andthe
estimate
of theEulerpole(35øN101øW)for theLaramide
cratonsincetheLate Triassic.
phase.Steiner
[1986,1988]ontheotherhandattempted
to
ESTIMATEOFCOLORADO
PLATEAUROTATION
constrainthe location of the Euler pole from the same
To estimate
themagnitude
of Colorado
Plateau
rotation,
we
assume
likeBryanandGordon
[1986,1990]a modelof APW
rotation.
for North America. Our model,however,featuresa virtual
standstillin APW for the last 15 m.y. of the Late Triassic
paleomagnetic
datausedto estimate
theamount
of plateau
Obtaining
an estimate
of Colorado
Plateaurotationis
straightforward.
Themean
plateau
Norian
pole(57.7øN
65.6øE
A95=2.5
ø) is rotated
abouttheEulerpivotuntiltheangular
Late TriassicPangea
Fig.4. Paleogeographic
reconstruction
forPangea
(after
Ziegler
etal.[1983]
using
Terra
Mobilis
TM)
positioned
with
respect
tolatitude
according
tomean
Nodan
paleomagnetic
pole
(57.4øN
91.0øE;
Table
1)
forcratonic
NorthAmerica.Shaded
areain NorthAmericais Colorado
Plateau.
296
Kent and Witte: Colorado Plateau Rotation
differencewith respectto the meancratonicNorth American
Norianpole(57.4øN91.0øEA95=3.8ø)is minimized(Figure2).
A correction for 13.5 ø of clockwise rotation reconciles the mean
plateauNorian pole to within 1ø of the mean cratonicNorian
pole. Confidencelimits on the amountof rotation can be
obtainedusing the statisticsof McFaddenand Lowes [ 1981].
We find that the plateauand cratonicNorth Americanmean
polesfor the Norian are not significantlydifferentat the 95%
confidencelevel at rotation anglesfrom 10ø to 17ø. Our
analysis thus suggeststhat the Colorado Plateau rotated
clockwise13.5ø + 3.5ø with respectto cratonicNorth America
sincethe Late Triassic. Alternativeplateau-cratonic
North
America Euler poles yield practicallythe sameclockwise
rotations,
for example,13.6ø for thecombined
Eulerpoleused
by Bryan and Gordon[1986, 1990], 13.7ø for just the Rio
GrandeEulerpoleof Hamilton[1981],and 13.5ø for just the
Laramide
Eulerpoleof Hamilton[1988]. Thus,asobserved
by
BryanandGordon[1990],estimates
of theangleof rotationare
insensitive
to theprecisechoiceof Eulerpole,whoselocation
is mostusefullyconstrained
by independent
geologic
criteria.
CONSISTENCY
WITH
OTHER PALEOMAGNETIC
DATA
The Early Triassichasbeenpreviouslyconsidered
to provide
the bestset of intervalpaleopolesto documentrelativemotion
of the Colorado Plateau.
Steiner [1986] determined that
11.7ø+3.7
ø of clockwiseplateaurotationcouldbe inferredfrom
the discordancebetweenthe mean pole calculatedfrom four
publishedpaleomagnefic
studiesof theEarlyTriassicMoenkopi
Formation of the Colorado Plateau and the mean pole from
three studiesof the nominallycoevalRed Peak Formationof
Wyoming. Steiner[1986] (but seeBazardand Butler [1991])
arguedthat age uncertaintiesare unlikely to accountfor the
discordance
in polepositionsfrom theMoenkopiandRed Peak
rocks but cautioned that the cratonic coherence of the Red Peak
sampling sites in Wyoming, an area affected by Laramide
deformation,lacks confirmation.
Ambiguityrelatedto the cratoniccoherenceof the deformed
marginsof the ColoradoPlateauaffectsthe interpretationof
someother key palcomagneticdata sets. Data from the Early
Permian Abo Formation have been regardedas providing a
cratonicreferencepaleomagneticpole becausethe formation
cropsout on the eastsideof the Rio GrandeRift [Gordonet al.,
1984; Steiner,1988; Bryan and Gordon,1990]. Yet the recent
studyby Molina-Garzaet al. [ 1991] showsthat the Moenkopi
Formationat their Sevilleta Grant samplinglocality, on the
easternmargin of the Rio GrandeRift just to the southof
Steiner's[1988] studyareaof the Abo Formationat Abo Pass,
givesa paleopolevery similarto Moenkopipolesfrom two
localitieson the western(plateau)margin of the rift, which
togetherresemblepublishedMoenkopipolesfrom the interior
of the ColoradoPlateau. Moreover,at Tejon, a localityto the
north of Abo Pass but also on the east side of the rift,
magnetizationsof the Shinarump member of the Chinle
Formation were shown to contain Cretaceous(?)and recent
overprints that were deflected clockwise by over 30ø.
Accordingly,Molina-Garzaet al. [1991]suggested
thatportions
of centralNew Mexico eastof the rift may haveeitherrotated
rigidly with the plateau or experienceda similar senseof
rotationby independent
deformationmechanisms.In light of
this evidence,the pole from the Shinarumpmemberof the
Chinle Formation [Molina-Garza et al., 1991] as well from the
Early Permian Abo Formation [Steiner, 1988] should be
considered
suspectwith regardto cratoniccoherence.Plateau-
relatedrotationof theAboFormation
does,however,
pointto
analternative
explanation
for thelong-recognized
similarity
of
Permianpolesfromtheplateauandthe"craton"thathaslessto
do with an additionalLate Permianrotationof the plateau
[Steiner,1988]thanwith an inappropriate
cratonicreference.
Interestingly,Irving and Strong[1985] suggested
the possibility of a large(~ 10ø) clockwiseplateaurotationon thebasis
of an analysisof Kiaman (Permo-Carboniferous)
overprint
magnetizations
in North AmericanPaleozoicrocks.
REVISED
NORTH
TRIASSIC/EARLY
JURASSIC
APW
Paleomagnetic
poleswe useto delineatea Triassicto Early
JurassicAPW path for North Americagenerallyconformto
thosediscussed
anddeemedmostreliableby B_a7ord
andButler
[1991,Figure 14], with the additionof the recentlypublished
middleNewarkpole [WiRe et al., 1991] and our new Church
Rock pole (Table 1). We did, however,chooseto includethe
meanRed Peakand the meanMoenkopipolesascalculatedby
Steiner[1986] as reasonable
if perhapsnotpreciselycoeval(or
in thecaseof theRed Peak,not strictlycratonic)EarlyTriassic
counterparts
from off andon theplateau.Paleomagnefic
results
of Reeve [1975] from the Church Rock member were also used
on the strength of corroborafingdata from our study of
equivalentstratigraphicunits. To maintainconsistency
with
Bazard and Buffer [1991], we use their recalculafion of the
ChurchRock pole that was basedon only normalpolaritydata
and note that Gordon et al. [1984] calculateda statistically
indistinguishable
pole(61øN64øEA95=3ø) fromReeve's[1975]
data for the Church Rock member that included normal and
reversedpolaritiesandpasseda reversaltest. Finally,we omit
the Shinarumppole becauseof the complextectonicsetting
with demonstratedlocal rotations where it was sampled
[Molina-Garza et al., 1991].
Fig. A1. Paleomagnetic
resultsfromtheChinleFormation.(a) Mapof sampling
localities
I (sites
TCHFtoTCHJ),II (sitesTCHAtoTCHE),III (sitesTCHKtoTCHM),andIV (sitesTCHNandTCHO)
in Utah.(b)Stratigraphic
range
of sampling
sitesshown
bysolidbarsatthefourlocalities
(representative
stratigraphic
columns
afterStewart
etal.,[1972]).(c-f)Demagnetograms
forrepresentative
samples
from
the differentlocalities.Open/solid
symbols
are vectorend-points
projected
on vertical/horizontal
orthogonal
planes
aftereachstage
of thermal
demagnetization.
Samples
in Figures
Alc-Ale havereversed
polarity,andsample
in FigureAll hasnormalpolaritycharacteristic
magnetization.
(g) Sitemean
directions
aftertiltcorrection,
withopen/solid
circles
plotted
onupper/lower
hemisphere
ofequal
areanet.
The two smallercirclesare for sitesTCHI andTCHJ fromlowerChinlethatwereexcludedin calculation
of upperChinlemeandirection
shown
bystarwithcorresponding
circleof 95%confidence
based
onnine
sitesconverted
to common
normalpolarity(TableA1).
FOR
AMERICA
Kent and Witte: Colorado Plateau Rotation
297
Z
E
elu!qo
elu!qo
o
oE
I
'-
elU!40
elU!4•)
I
I
I
I
298
Kent and Witte: ColoradoPlateauRotation
After 13.5ø correctionfor clockwiseplateau rotation, the
Triassicpolesfor North Americadescribean APW path from
about49øN 117øEin the Early Triassicto the virtual standstill
at about57øN 91øEby the Norian (Figure3). APW over the
Triassicwas about 17ø of arc distancebut the rate of angular
changewasnotconstant.Instead,an averagerateof 0.73ø/m.y.
calculated
for theentireTriassicby BazardandBuffer[1991]is
moreappropriateonly for theEarly andMiddle Triassicbecause
of negligibleAPW in theNorian. The CarnianlowerNewark
pole may recordan intermediatepositionof the TriassicAPW
shift(Figure3).
The insignificantdifferenceof theHettangianupperNewark
pole from the middleNewark andotherNorianpolessuggests
that the standstillextendedinto the earliestJurassic. However,
the pole from the SinemurianMoenave Formation[Ekstrand
and Butler, 1989], even after 13.5ø correctionfor plateau
rotation,falls significantlyto the west of the upperNewark
pole (Figure 3). The Moenave pole is basedon thorough
demagnetization
experiments
andalthoughthemeannormaland
reverseddirectionsare 12øfrom anfipodal,theyformallypassa
reversaltest [classC [McFaddenand McElhinny,1990]). The
Moenavemaythereforerecorda westerlyexcursion
in APW, an
obtundedvestigeof the J1 cuspof PEP models. If so, the
Moenaveexcursion
wasratherbriefbecause
a newpolereported
by Bazardand Buffer [1991] from the immediatelyoverlying
(Pleinsbachian)
KayentaFormationaftercorrectionfor 13.5øof
plateaurotationis hardlydifferentfrom the Hettangianlower
Newark pole. Furthermore,the Pleinsbachian
Kayentapole is
not significantlydifferentfrom Norian polesfrom the Owl
Rock or Church Rock members of the Chinle Formation from
al., 1984; May and Buffer, 1986; Bryan and Gordon,1986,
1990;BazardandBuffer,1991]. This is largelybecause
the
discordance
between
on-plateau
andoff-plateau
Chinlepoleswas
presumed
toreflectmostlyanagedifference.Ourinterpretation
predictsthat the Late TriassicChinle Formationshouldshow
little change in paleomagneticpole position over its
stratigraphicrange at a given locality either on or off the
plateau,corresponding
to thelackof discernible
APW we have
documented
for thebroadlycorrelativeNewarkBasinsection.
For example,the UpperPetrifiedForestmemberof the Chinle
FOrmationon the ColoradoPlateau (see Bazard and Buffer
[1991,Figure15] for stratigraphic
correlations)
shouldgivea
pole position that is virtually the same as the available
paleomagneticpoles for the Owl Rock and Church Rock
membersfrom the plateau,but that shoulddiffer from the
reported
polefor thepresumably
coevalUpperShalemember
of
eastcentralNew Mexico(off theplateau)by an amountthatis
equivalent
withinerrorsto 13.5øof clockwise
plateau
rotation.
Hamilton [1988] estimatedon geologicgroundsthat the
ColoradoPlateaurotatedclockwiserelativeto the cratonby
about 8 ø since mid-Cretaceous time: about 3ø was estimated to
accountfor theregionalpatternof extension
associated
withthe
Cenozoicopeningof the Rio GrandeRift andthebalance(in
therangeof 2øto 8ø)witha prece•g LateCretaceous
phaseof
rotation to accountfor the spatial distributionof crustal
shortening
acrossthe Laramidebelt. AlthoughHamilton
[1988] seemedto favorpaleomagnetic
estimates
of plateau
rotationthatwererelativelylarge[e.g.,Steiner,1986]compared
to the 4ø estimatedby Bryan and Gordon [1986], the
compatibility
of thegeologicevidence
forpost-Triassic
rotation
the Colorado Plateau.
of theColorado
Plateauthattotalsto the-13.5ø indicated
by
This apparentbacktrackof theKayentapolewasinterpretexl
to markthe beginningof an easterlyprogression
of paleopoles
our analysisis not unequivocal[Chaseet al., 1992] and will
require further critical evaluationof the extent of Laramide
shortening
as well asRio GrandeRift-relatedextension.l.ocal
clockwiserotationsmay also morebroadlycharacterize
the
Cenozoictectonichistoryof thewesterninteriorof theUnited
that define the J1 to J2 PEP Jurassictrack [Bazard and Buffer,
1991],but thisinterpretation
will needto be reconciledwith the
emergingevidencefor high latitudeAPW in theJurassic[Van
Fossenand Kent, 1990, 1992b; Witte and Kent, 1991]. To the
extentthat paleomagnefic
polesfrom the ColoradoPlateauare
used,any revisedJurassicAPW path for North Americawill
also need to take into account a large clockwise plateau
rotation. It can neverthelessbe concludedat this stagethat
APW for North America musthavebeenat a relativelyrapid
(~lø/m.y.) overall rate in the Jurassic,to arrive at the distant
and well-establishedmid-Cretaceouspole positionat 71øN
196øE[Globerman
andIrving,1988].
DISCUSSION
Ourpaleomagnetic
estimate
of 13.5ø of clockwise
plateau
rotation based on Late Triassic data is similar to that of
Steiner's[1986] 11.7ø basedon EarlyTriassicdata,but it is
significantly
greater(13.5øñ 3.5ø versus5.0øñ 2.5ø) thanthe
valuemostrecentlycalculated
by BryanandGordon[1990]
fromlatePaleozoic
toJurassic
data.Wesuggest
thatBryanand
Gordonunderestimatexl
theamount
of plateau
rotation
because
theyeffectively
averaged
thelargesystematic
discordance
in
Triassic poles with the generallysmall discordanceof
ambiguous
significance
in late Paleozoic
poles,anduseda
Jurassic
APW modelthatwe nowregardaspoorlydefinedat
best.
States[e.g.,Eaton,1979],as documented
by palcomagnetic
datafrom theeasternmarginof theRio GrandeRift [Molina-
Garzaet al., 1991]andsuggested
by systematic
discrepancies
withrespectto globalplatereconstructions
of earlyTertiary
polesfromwestern
UnitedStates
sampling
localities
including
northcentralMontanaandnorthwestern
Wyoming[Actonand
Gordon,1992].
Finally, the revised pattern of Triassic APW for North
Americacanbe viewedin a broaderpaleogeographic
context.
Duringthe Triassic,NorthAmericawaspart of the supercontinentof Pangea. The virtual absenceof APW for North
Americaover 15 m.y. of the Norianthereforeimpliesthat
Pangeawasalsostationary
withrespectto thepalcomagnetic
reference
frameduringthistime(Figure4). Thelatitudinally
staticpositionof Pangeain the Late Triassic,preceded
and
followed
by timeintervals
characterized
by fastAPWof upto
~lø/m.y., may be relatedto large-scalemantleprocesses
[Gumis, 1988]. In practicalterms,the Norian standstillof
APWis anidealtimeintervalfortesting
andrefiningmodels
of
Pangea
continental
reconstructions
aswellasfor terrane
(e.g.,
ColoradoPlateau)analysis.
APPENDIX:
NEW CHINLE
PALEoMAGETIC
RESULTS
At leastfive orienteddrill coresamplesweretakenfromeach
An undercorrection
of plateaupaleomagnetic
polesfor
of a total of 15 sites distributedover two localities along
clockwise
rotation
(5øorless)hascontributed
totheimpression Interstate 70 on the the San Rafael Swell in western Utah, and
of fastAPW continuing
throughtheLateTriassic[Gordonet
two localitiesalongthe ColoradoRiver nearMoab in eastern
Kent and Witte: Colorado Plateau Rotation
TABLE A1. Characteristic
MagnetizationDirections
From the Chinle Formation
Site
n/N
k
Dec,
Inc,
Stk/Dip
degrees degrees
TCHA
TCHB
TCHD
TCHE
TCHI*
TCHJ*
TCHK
TCHL
TCHM
TCHN
TCHO
3/5
4/4
5/5
6/6
4/5
6/7
5/8
5/5
5/8
6/6
5/6
231
37
8
12
79
35
12
36
49
25
182
185.6
2.0
4.1
18.7
166.2
7.2
194.2
184.3
173.0
1.0
351.0
- 19.6
21.5
19.5
27.8
-7.2
22.1
11.7
6.5
-9.9
24.8
3.7
008/30E
013/25E
018/28E
008/28E
000/00E
000/00E
113/10S
113/10S
113/10S
000/00S
000/00S
Mean
9/15
24
003.6
12.3
(a95= 10.7ø)
Pole position:
Lat = 57.5øN Lon = 63.3øE
(K = 50, A95 = 7.3ø)
ChurchRockmemberor its stratigraphic
equivalentat localities
II, III, and IV, but the five sitesat localityI were takenin the
Monitor Butte and Moss Back members(Figure Alb). All
experimentalwork was conducted
in a magneticallyshielded
roomusingprocedures
andequipmentdescribed
by Witte et al.
[1991]. Samplemagnetization
directionswere determined
by
principalcomponent
analysis[Kirschvink,1980]andaveraged
usingFisher[1953] statistics.Althoughsomemediumgrain
sizesandstones
weresampled,
redmudstone
samples
yieldedthe
mostinterpretable
demagnetization
results.
Completestepwisethermaldemagnetization
to 680øCof all
samplestypicallyrevealedtwo components
of magnetization,
similarto demagnetization
behaviourdescribedby Bazardand
Buffer [1991] for the Owl Rock member. There is often a
northerlyand steeplydown magnetization,with unblocking
temperaturesto about 500øC, that most likely representsa
recentoverprint(FigureAlc). Elevensitesof the original15
producedat least three sampleswith a well-defined high
unblockingtemperaturecomponent,evidently carried by
hematite,with either shallowand northerlyor shallowand
southerlydirections(Figure Ald). The 11 siteshave a mean
direction after tilt correction of Declination/Inclination
Columnheadsare n/N, numberof samplesor sitesusedfor
calculation/number
collected;k, estimateof Fisherprecision
parameter; Dec, declination and Inc, inclination
299
of
magnetization
directionafter beddingtilt correction;Stk/Dip,
beddingstrike/beddingdip with quadrantdirection. Lat is
latitudeand Lon is longitudeof paleomagnetic
pole position
based on nine site mean virtual geomagneticpoles; K is
estimateof Fisher precisionparameter;and A95 is radiusof
coneof 95% confidence
aboutpole.
*Thesesiteswereexcluded(seetext).
Utah (FigureAla). The ChinleFormationin thisregionis a
shallow lacustrine and fluvial unit with considerable lateral
variabilityand correspondingly
variedstratigraphic
nomenclature[Stewartet al., 1972]. Our samplingconcentrated
on
the upper portions of the formation, with 10 sites in the
=
2.3ø/12.8ø, k=26, a95=9.2ø. Excludingtwo of these11 sites
whichare from thelowerpartof theChinle(MonitorButteand
Moss Back members),we calculatea mean direction for the
upperpart of the Chinle at Declination/Inclination
= 3.6ø/12.3ø,
k=24, a95=10.7ø (Table A1) and note that the five normal
polarityandfourreversed
polaritysitemeanspassa reversaltest
(classC [McFadden
andMcElhinny,1990]). Thepoleposition
basedon thesenine sites (57.5øN 63.3øE A95=7.3ø) is not
significantlydifferentfrom thepole(59.0øN67.0øEA95=2.5ø)
calcula•exl
by BazardandBuffer[1991]fromReeve's[1975]data
for the Church Rock member.
Acknowledgments. We thank the journal reviewers for
constructivecomments. This researchwas supportedby the
National Science Foundation, Division of Earth Sciences
(grants EAR87-21142 and EAR89-16726) and is LamontDohertyGeologicalObservatory
contribution
4996.
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(ReceivedFebruary18, 1992;
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