JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. B13, PAGES 19,651-19,661, DECEMBER 10, 1992
Paleomagnetismof 122 Ma Plutonsin New England and the Mid-Cretaceous
PaleornagneticField in North America'
True Polar Wander or Large-Scale Differential Mantle Motion?
MICKEY
C. VAN FOSSEN AND DENNIS V. KENT
Lamont-Doherty
GeologicalObservatory
andDepartmentof GeologicalSciences,ColumbiaUniversity,Palisades,iVewYork
A palcomagnetic
studyof Cretaceous
White Mountainsplutoniccomplexesin New Hampshireand Vermont
yieldshigh unblockingtemperature,dual polarity magnetizations
in differenttypes of igneousrocks. The
resultingpolepositionfor threeplutons(71.9ø N, 187.4ø E, A95 = 6.9ø, age= 122.5Ma) agreeswithpreviously publishedmid-Cretaceous
polesfor North America,which togethergive a mid-Cretaceousstandstill reference
poleslightlyrevisedfrom Globermanand Irving [1988]at 71.2ø N, 194.1ø E (A95= 3.7ø,N
= 5 studies).We argueon the basisof the wide geographic
distribution
of thesestudies,the varietyin tectonic settingsand rock types,positivereversaltests,and an overallreversalpatternconsistentwith geomagneticpolaritytime scales,that this meanpole represents
the North Americanmid-Cretaceous
reference
field for nominally36 m.y. (124 to 88 Me). The standstillpole limits to within +4ø, the motion of the
North Americanplate relativeto the Earth'sspinaxis. During the samemid-Cretaceous
interval,the New
Englandhotspottrack (124 Ma MonteregianHills, 122.5-Ma CretaceousWhite Mountains,and 103- to
84-Me New Englandseamounts)
requires11ø't:4ø
of nonh-polewardmotion of North America,in direct
conflict with the palcomagnetic
standstill. A similar(-13 ø) discrepancy
is independentlydemonstrated
betweenthe spin axis and the Tristan da Cunha hotspottrack on the African plate during the midCretaceousinterval. •he hotspot/spinaxis discrepancies
endedby -90 Ma when it is shownthat both
Atlantichotspotsagreewith North Americanand African dipole palcolatitudesand present-daylocations.
Nondipolefieldsare an unlikelyexplanationof the uniformmotionof thesetwo widely separated
hotspots
with respectto the spinaxis, leavingas possibleinterpretations
true polar wanderand large-scale(but differential) mantle motion. The southerlymotion of the mid-CretaceousLouisville hotspotrelative to the
spinaxisis ostensively
at oddswith what wouldbe predictedunderthe true polar wanderinterpretation
and
points to differentialmantle kinematics. The motionsof the three widely separatedmid-Cretaceous
hotspotswith respectto the spin axis may be relatedto the recentlyproposedincreasein global oceanic
lithosphere
production
rateswhichgaverise to the mid-Cretaceous
"superplume."
INTRODUCHON
It has been long recognizedthat Cretaceouspalcomagnetic
polesfor North Americatendedto fall relativelyclosetogether
in the generalarea of the Bering Strait [e,g., Irving, 1964;
McElhinny, 1973;Mankinen, 1978]. This tendencywas most
recentlyconfirmedby Globermanand Irving [1988] who calculated a mid-Cretaceous
referencepole at 71øN, 196øE(A95 =
4.9ø, k = 352) on the basisof four publishedstudieson rocks
rangingin age from circa 88 Ma to 124 Ma. The mean pole
position may therefore representthe palcomagneticfield
relativeto cratonicNorth Americafor some36 million yearsof
the Cretaceous,
a "standstill"in apparentpolar wanderduring
which there was no discernibleplate motion relative to the
geographic axis according to the geocentric axial dipole
hypothesis. Bracketing the mid-Cretaceousstandstillare Late
Jurassic/EarlyCretaceouspoles [May and Butler, 1986; Van
Fossen and Kent, 1992] and Late Cretaceouspoles [Diehi,
1991] whichrequirerelativelyfast North Americanapparent
activity [Larson, 1991; 7•rduno et al., 1991], and fast plate
motion in the mantle hotspot framework [Morgan, 1972,
1983; O'Connor and Duncan, 1990]. It is less clear, however,
how to characterizethe mid-Cretaceous
in termsof geometry,
amount,and timing of relative motionbetweenthe hotspotand
palcomagneticreferenceframes. Previousstudiesthat include
the mid-Cretaceous
interval are primarilybasedon globalsynthesesof palcomagneticand relative plate reconstructiondata
sets and comparisonswith models of plate motion in the
hotspot reference frame [e.g., Livermore, et al., 1984;
Andrews,
1985; Gordon and Livermore, 1987; Besse and
Courtillot, 1991]. The complexityof theseanalysesmay contributeto the currentlack of consensus
on the subjectand,furthermore,Livermoreet al. [1984] suggested
that the pre late
Cretaceousmotionbetweenthe hotspotandpalcomagnetic
referenceframesmightbe betterattributedto the errorsin hotspot
track identificationsand/orpalcomagnetic
data.
In this study we examine North American mid-Cretaceous
apparentpolar wander througha palcomagneticstudy of the
polar wander (~lø/m.y.).
youngest group of White Mountain Series plutons in Nee
The mid-Cretaceous
is itself an interestinginterval in the England(mean age = 122.5 Ma). This independentdeterminaannalsof global change. Aside from being dominatedby a
tion providesan additionaltest of overall consistencyof the
~30-m.y. intervalof uniformnormalgeomagnetic
polarity (the North American mid-Cretaceouspalcomagneticpoles whose
Cretaceous
Normal PolaritySuperchron,
corresponding
to the reliability can be further assessedby comparingreverse and
CretaceousQuiet zonesin the oceans),the mid-Cretaceoushas
normal polarity magnetizationsamongthe new 'andpublished
beencharacterized
as a periodof fastseafloorspreading
[Larson pole positiondatafor a regional-scale
reversaltest. As we will
and Pitman, 1972; Larson, 1991], increasedmantle plume show,the North Americanrecordof the mid-Cretaceous
palcomagneticfield 'andhotspotactivityalsoprovidesa more direct
Copyfight1992by the AmericanGeophysical
Union.
experimentto testvariousideason the natureof the palcomagnetic and hotspotreferenceframes.
Papernumber 9ZIB01466.
0148-0227/92/92JB-01466505.00
19,651
19,652
VAN FOSSENAND F•NT: MtD-•ACEOUS
TIlE CRETACEOUSWI•IITE Mou•rr•s
MAG•A
PALEOMAGNETIC
FIEIX)
SE•S
4 Ma). Andesite and basalt flows within the Ossippee
Mountainsring dike have been sampledat six sites (minimum
age of 121 q- 4 Ma based on K-Ar biotite age of kntruding
Geological Setting
Conway granite). These flows were tilted throughdifferential
The Cretaceous
alkaline
intrusions
in southern
New
subsidenceabove the intruding magma chamber [Kingsley,
Hampshireand southeastern
Vermont are the youngestgroupof
1931; Billings, 1956]. Gabbro and a basalt dike at Mount
igneousrocks associatedwith the White Mountains Magma
Tripyramid have been sampledat three sites. Unfortunately,
Series (Figure 1). Their emplacementfollowed an initial late
only a commercialradiometricdate appearingon the map of
Triassic phaseof igneousactivity at ~230 Ma and a Jurassic Hatch and Moench [1984] is available (112 :t: 5 Ma) and,
phaseat ~175 Ma [Foland and Faul, 1977]. The Cretaceous althoughHubacherand Foland [1991] do not list Tripyramidas
granites, monzonites, gabbros, basalts and andesitesoccur one of the Cretaceous intrusions redated, we assumethat it falls
predominantlyin five major plutonic and ring-dike complexes within the distinct interval of Cretaceousmagmatismcentered
that intrude lower to middle Paleozoic metamorphic and
igneous formations. These complexes are (north to south,
Figure 1) Mount Tripyramid, Ossippee Mountains,
Merrymeeting,Mount Ascutney,and Mount Pawtuckaway.
K/fi biotite datingof numerousCretaceous
White Mountains
plutons and stocks, including the five major p!utonic complexes sampled here for palcomagnetism,suggestsan age
rangeof about118 Ma to 125 Ma [Foland eta!., 1971;Foland
and Faul, 1977]. Recently, a number of CretaceousWhite
Mountains
plutons
havebeenrestudied
usingthe40Ar/39Ar
biotitemethodby Hubacherand Foland [ 1991]. They suggesta
shorterperiod of magmatic activity, perhapsonly 3 m.y. in
duration,centeredon 122.5 Ma. Thesedata, togetherwith publishedzirconfissiontrack (mean= 113 +_10 Ma [Doherty and
Lyons, 1980]) and apatitefissiontrack ages(mean= 103 :t: 15
Ma [Zimmermanet al., 1975;Dohertyand Lyons,1980]), suggest that theseplutonswere emplacedat shallowdepths(-3.5
km) and were subject to monotoniccooling since crystalliza-
at 122.5 Ma.
Thermal and AF vector demagnetizationprofiles of White
Mounta'mssamplesare straightforwardwith the exceptionof
those from the Merrymeeting gabbro. Samples from
Tripyramid, Pawtuckaway,and Ascutneycontaina singlecomponent of magnetizationwith high unblocking temperature
(-570 ø C) and high coercivity(Figures2a, 2b, and 2c). This
magnetization is of reversed polarity (declination =154 ø,
inclination=-56ø) in Mount Ascutneyand Mount Tripyramid
gabbros,and is of normal polarity (declination=343ø, inclination =+59ø) in Mount Pawtuckawaymonzonite.
White MountainsMagma Series
Cretaceous.:......•
Jurassic
+ Triassic
tion.
Palcomagnetism
'...
In the pioneering study of the White MountainsMagma
Series by Opdyke and Wensink [1966], a pole position was
calculatedfrom a variety of plutons consideredthen, with the
exceptionof the CretaceousMount Ascutney,to be entirely
Jurassic (-180 Ma). The wide age range of the White
Mountains Magma Series has been subsequentlydocumented
[e.g., Foland and Faul, 1977] and renewedpalcomagnetic
studies of the Middle Jurassicand Triassic plutonshave revealeda
sensiblerelationshipbetween inferred magnetizationage and
radiometricage [Van Fossenand Kent, 1990;Wu and Van der
Voo, 1988]. Our experimentalplan for the Cretaceousplutons
was to apply fiartheralternatingfield (AF) and new thermal
demagnetization
studiesto samplesfrom the originalOpdyke
and Wensink collection (10 sims), as well as demagnetization
experimentson our own samples(7 sims). Becausethe peak
AF field appliedby OpdykeandWensinkwasgenerallyonly 30
tppeeM s.
Mt.
mT and because 'the thermal method was not used, further work
was necessaryto addressthe issue of contaminationby secondarymagnetizationswith modem paleomagneticmethods.
The paleomagnetic sampling covers the five major
Cretaceous
White
Mountains
intrusions.
While
the recent
40Ar/39Ardatingof Hubacher
andFoIand[1991]provides
a
•f ...,i:•'M
•;•'•;•'•..i!•y
SMerrym
t. Pa
refined temporalassessment
of Cretaceousmagmatism.The
specific and earlier published K/At age information from
Foland and Faul [1977], unless otherwise indicated, is also
quotedfor reference. Gabbrohas been sampledat three sites
from Mount Ascutney(K/Ax biotite age = 119 :t:4 Ma) as well
as at three sims from the Merrymeetingpluton (K/fi biotite Fig. 1. Locationmap of the White MountainsMagma Series'm New
age = 117 :t:4 Ma). To the southat Mount Pawtuckaway,mon- England. The five majorCretaceous
igneouscomplexes
sampledfor
zonite has been collectedat two sites(K/Ar biotite age = 121 :!: paleomagnetismare highlighted.
VAN FOSSF.
N AND KENT:MID-CRETACEOUS
PALEOMAGNETIC
FIELD
A
MountTripyramid- gabbro
30mT
B
MountPawtuckaway
- monzonite
19,653
c
MountAscutney- gabbro
W,Up
W,Up
W,Up
2
S
N
400øC
450øC
80rot
O.I AIm
S
25mT
N
575øC•.N
S•••
E,Dn
D
Ossippee
Mountainsbasalt
flow
2•x•3øC
W,Up
0oc
E
Ossippee
Mountainsbasalt
flow
IfMA.4A
EDn
Merrymeeting- gabbro
.1WBI A
W, Up
430øC
220øC
500øC
S
S •
•
•
•
•
(,570øC
''.:::::'.:N
,
N
O,2A/m
EDn
E,I)n
E
Fig. 2. Representauvedemagnetogramsshowing AF and/or thermal demagnetizationof various CretaceousWhite
Mountainslithologies.(a) Gabbrofrom MountTripyramid,(b) MountPawtuckaway
monzonite,(c) MountAscutneygabbro,
(d, e) basaltflow from the OssippeeMountains,and (f) unstableMerrymeetinggabbro. Open/closedsymbolsprojectedon to
vertical/horizontalplanesin geographiccoordinates.
The Ossippee volcanic rocks also contain a high stability,
reversed polarity magnetization (declination =162ø, inclination =-49 ø) but to slightly higher unblocking temperatures
(-600 ø C). In addition,five of the six Ossippeesites show a
consistent normal polarity overprint (declination =349 ø,
inclination =+70 ø) that is usually removed with applied temperaturesof-200øC (Figure2d), but which did extendto >400ø
C in somesamplesfrom sitesKMA, C, madF (Figure 2e). This
low unblockingtemperaturecomponentis parallel to the present field direction and most likely representsa remanenceof
recent origin. However, the high unblocking temperature
componentalso appearsto be secondarywith respectto tilt on
the basis of the McFadden [1990] discordancy test, even
thoughthis reversedmagnetizationis obviouslynot compatible with the presentfield direction.
The origin of the unstablemagnetizationat Merrymeetingis
not known. Opdyke and Wensink [1966] reported a reversed
magnetizationdirection for these three gabbro sites following
blanket AF treatments between 30 mT and 50 mT, but vector
(Figure2f). Althoughthe Merrymeetingmagnetizationis most
probably of reversedpolarity, in the absenceof clear originbound demagnetizationtrajectories the directional data from
the Merrymeetingsampleswere excludedfrom furtherstudy.
Isothermal remanent magnetization (IRM) experimentson
representativesamples from the CretaceousWhite Mountain
plutonic complexes (including the unstable Merrymeeting
gabbro) show efficient acquisitionof magnetizationand saturation in moderate(-150 mT) magneticfields (Figure 3). These
data togetherwith the observedpeak unblockingtemperatures
of about 570øC suggestthat the predominantcarrier of remanence is (titano)magnetite. Slighfiy higher unblockingtemperatures(-600øC) for the Ossippeevolcanic flows suggests
the additionalpresenceof some (titano)hematitein these subaerial rocks, althoughthe IRM curveswould suggestthat this
contributionis not a significantone.
Interpretationand pole position
A mean pole position has been calculatedusing eight site
analysisof their data, includingsome samplesthat were sub- mean magnetizations(Figure 4) convertedto virtual geomagjected to alternating fields of 160 mT, indicates that a stable netic poles (VGPs) from Mount Tripyramid, Mount Ascutney,
magnetizationdirection was not satisfactorilyachieved. Our and Mount Pawtuckawayplutons(71.3ø N, 187.5ø E, A95 =
additional thermal demagnetizationexperiments were also 4.2ø, K = 172; Table I and Figure 5a). The Ossippeepole
unable to resolve consistent remanence in these samples (69.8 ø N, 160.3ø E) has been excluded from this calculation
19,654
VAN FOSSEiN
AND KENrr:MID-CRETACEOUSPALEOMAI3NE7ICFIELD
•
,
I
,
•
I
,
•
I
•
,
I
•
,,,
I,
,
1.0
0.8
g 0.6
0.4 2
4
•
/ tl
/ •
?
/
/
I
I
I
'
Ossippeefiows
o
Meymeting
,
•o•asc•
0.2
I
0.0
•
' '...........
MID-CRETACEOUS REFERENCEPOLE FOR NORTH AMEPdCA
The four publishedpole positionsusedby Globerman and
Irving [1988] in their calculationof a mid-Cretaceous
North
Americanreferencepole are (Figure 5b): (1) Arkansasintrusionspole at 74ø N, 193ø E [Globermanand Irving, 1988, age=
100-88 Ma], (2) Niobrara Forrnationpole at 66ø N, 192ø E
[Shire and Frerichs, 1974] (age = Coniacian-Santonian
or 84
to 88.5 Ma on the Kent and Gradstein 1986 time scale), (3)
MonteregianHills pole at 73ø N, 191ø E [Foster and Symons,
1979](40Ar/39Ardate= 124:t:1 Ma [Folandet al., 1986]or
K/At = 118 to 136 Ma [Eby, 1984]), and (4) Newfoundland
dikespoleat 71ø N, 207ø E [LaPointe,1979,age- 129 Ma].
The 122.5 Ma White Mountainspole (71.3ø N, 187.5ø E, A95
0
3b
6b
9b
= 4.2ø) falls within the age range of the above poles and is
indistinguishable
at the 95% confidencelevel from the mean
B [mT]
pole positioncalculatedby Globermanand irving [1988] at 71ø
Fig. 3. Iso•emal •manent magnetization
(•)
acquisition
cu•es
provides
for representative
samplesfrom the CretaceousWhite Mountains N, 196ø E (A95 = 4.9ø,N = 4 studies).This agreement
independentsupportfor a mid-Cretaceousstandstillinterval
plutons.J/J• = no•ah•d •;
B = directinduc•ncein m•tesla.
which, as suggestedby Globermanand Irving, is reasonably
bracketed by the Arkansas intrusions (88 Ma) and the
because it is far sided relative to the Tripyramid-Ascutney- MonteregianHills intrusionsof Quebec(118 Ma to perhaps
the40Ar/39Ax
dateof 124Ma [Foland
Pawtuckaway
mean(-9 ø arc distance,Figure5a) andespecially 136Ma). Considering
becauseit lacks a direct radiometricage constraintfor maxi.. et al., 1986] for the MonteregianHills as the more definitive
mum age. Basedon the geologicalevidence[Kingsley,1931; age, we suggestthat the most likely durationfor the standstill
Billings, 1956] and the K-Ar age of the intruding granite is -36 m.y. (124 Ma to 88 Ma).
Rather than signifyingno apparentpolar wander for 36 m.y.,
[Foland and Faul, 1977], the Ossippeeflows are no younger
the close agreementof the mid-Cretaceouspaleopolesfrom
than 121 Ma. There is an additional constraint on rninimum
ageof any thermalremagnetization
from a zkconfissiontrack North America could representa widespreadremagnetization
ageof 107 Ma [Dohertyand Lyons,1980] andwe notethatthe eventat somelater stageof the mid-Cretaceous.In this regard,
Ossippee pole does not correspond with younger (late the paleomagneticreliability of the three older standstillpoles
Cretaceous-Tertiary)publishedNorth Americanpoles. The far (Newfoundlanddikes,MonteregianHills, and White Mountains
sidedness could be the result of a shallowing bias on the poles) would be of particular concern. There are no stability
reversed magnetization through incomplete removal of the tests available for the Newfoundland dikes [LaPointe, 1979]
steep norrnal polarity overprint found only at Ossippee. which makes it difficult to judge the reliability of the
Alternatively, existingradiometricage controlallows the rock Newfoundland pole without a comparison to coeval poles.
age and possibly also the magnetizationacquisitionage to be However,as in the caseof the 122.5-Ma White Mountainspole
considerablyolder than the Tripyramid-Ascutney-Pawtuckaway of this study,the 124-Ma MonteregianHills pole [Foster and
wlfich
rocks/magnetization, and in this regard we note that the Symons,1979] is basedon dual polarity magnetizations
Ossippeepole falls near to older early Cretaceous/late
Jurassic passthe reversaltest (A-class, in this case). Another argument
North Americanpoles [Irving and Irving, 1982;Gordonet al., against remagnetization in the White Mountains and
Monteregian
Hillsrocksare40Ar/39Ar
agesandapafite
orzir1984, see Figure 5a].
In the absenceof direct age dating of the Ossippeevolcanic con fission-trackages which show no evidencefor later ther[Folandet al., 1986;Eby, 1984;Hubacherand
flows, the Tripyramid-Ascutney-Pawtuckaway
meanis selected mal disturbances
as the representativeCretaceousWhite Mountainspole. The Foland, 1991; Doherty and Lyons, 1980; Zimmermanet al.,
dual polarity of the high unblock'mgtemperaturemagnetiza- 1975]. Moreover, dual polarity magnetizationsin theserocks
tions suggeststhat adequatetime is representedto average are consistentwith what would be expectedfrom geomagnetic
paleosecular
variation
and,alongwiththemeanof 40Ar/39Ar polarity tirne scalesfor timesprior to 118 Ma [e.g., Kent and
age determinations(122.5 Ma), is consistentwith acquisition Gradstein, 1986; Harland et al., 1990], and therefore these
duringthe ti!rteof geomagneticfield reversalsjust prior to the poles carry no suspicion of remagnetization during the
CretaceousNormal PolaritySupercitron(-I18 to 84 Ma on the CretaceousNormal Superchron. Finally, the antipodeof the
Kent and Gradstein [1986] time scale). The mean of the two mean of two reversed polarity VGPs (mean reversed
Pawtuckawaynormal polarity VGPs is antipodalto within 7.5ø Monteregian Hills and mean reversedWhite Mountainspluof the rneanof the six Triapyramidand Ascutneyreversepolar- tons; 70.1ø N, 180.8ø E) is includedwithin cone of 95% confiity VGPs (Table 1 and Figure 5a). For thesedata the critical denceaboutthe rneanof five normalpolarity VGPs (meannorangleis 13ø, at or abovewhich a null hypothesisof a common mal MonteregianHills, Mount Pawtuckawayfrom this study,
mean can be rejected[McFadden and McElhinny, 1990]; the Arkansas intrusions mean, Niobrara formation mean, and mean
CretaceousWhite Mountainspole thereforepassesa C-class of the Newfoundlanddikes; 72.8ø N, 190.0ø E, A95 = 5.9ø).
reversal test for an isolated observation. We attribute the small
disagreernent
of normalandreversepolarityVGPs to paleosecular variationand regardthe eight VGPs asvectorsdrawnfrom
one population. Even if the departurefrom antipodalityis due
to slight contamination,the averageof the normal and reverse
polarity directionsshouldcancel any bias.
Basedon this positiveregional-scale
reversaltest,we suggest
that no polarity-dependentbias exists in the mean standstill
pole.
A remainingconcern,however, is that post-Cretaceous
tilt-
ing of the igneousrocksfrom which the constituent
pole position data were derivedsomehowactedto bring the paleomag-
VAN FOSSENAND KEI'•: MID-CRETACEOUSPALEOMAGNETICFIEt.D
A.
N
19,655
B.
•
w
E
150ø
S
•
•
• "•
f
'• '•
•' r'E
Mo:
TFipyFamid
JWK,
•,Ml
o
o
o
o
210ø •
150ø
150 ø
s
C.
Fig. 4. Stereographic
projectionof site mean•nagnetizations
(squares)with solid/opensymbolsindicatinglower/upper
hemisphere.
(a) Normalpolaritymagnetization
fromMountPawtuckaway
(twositemeans;circlesindicatesampledirections)
andreversedpolaritymagnetization
from OssippeeMountainsflows (five sites). The low unblockingtemperature
normal
polarityoverprintat Ossippee
andpresent-day
(PDF) anddipolefield directions
at the samplingsitesare alsoshown.(b)
Mount Ascutneyreversedpolaritymagnetization
(threesites),and (c) MountTripyramidreversedpolarity magnetization
(threesites;circlesindicatesampledirections).
netic poles fortuitously into close mutual agreement. Direct
control on palcohorizontalis available only in the Niobrara
shales [Shire and Frerichs, 1974] althoughin the caseof the
Arkansasintrusions,Globerman and Irving [1988] noted that
vertical
mid-Cretaceous
dikes in that field area intrude flat-
lying Lower Cretaceousstrata as well as Paleozoicbasement.
The other three igneous poles (Monteregian Hills, White
Mountains, and Newfoundland) lack direct control for a palcohorizontal reference: the Monteregian plutons intrude
Grenville basementwhereasthe White Mountainsplutons and
Newfoundlanddikes cut lower and middle Paleozoiccrystalline
rocks, respectively. However, at each of thesefour igneous
provinces,the mid-Cretaceous
intrusionsthat were studiedrepresent the last tectono-magmaticevent recognizedat that particularlocality(theNiobrarashalesunderwentLaramidedeformation). Thus while tilting cannotbe excludedat every local-
ity, we judgeit to be highlyimprobableto accountfor the midCretaceousstandstillpole.
,,
.
Fig. 5. (a) CretaceousWhim Mountainspole positionscomparedto
North Americanapparentpolar wanderpath of Irving and Irving
[1982]. The meanpole(KWM) is calculated
usingMountAscutney(A),
Mount Tripyramid(T), and Mount Pawtuckaway(P) poles,while the
Ossippee
Mountains(Oss)pole,falling nearolderNorthAmericanpole
positionsand havingno direct radiometricage constraint,is omitted
from mean.
(b) New mid-Cretaceous "standstill" pole for North
Americaat 71.2ø N, 194.1ø E (A95 = 3.7ø) usingthe meanCretaceous
White Mountains CKWM) pole from this study and four other North
American cratonicpoles listed by Globermanand Irving [1988]: 1 =
Arkansasintrusions[Globerrnan and Irving, 1988], 2 = Niobrara formarion [Shiveand Frerichs, 1974], 3 = MonteregianHills [Foster and
Symons,1979], 4 = Newfoundlanddikes ILsPointe, 1979].
19,656
VAN FOSSENAND •:
M!D•ACEOUS
PALEOMAGNETICF!•.n
TABLE !. Site Mean DirectionalandPolePositionDat.a..FromtheCretaceous
Whi.teMountainsMagmaSeries
Magnetization
NorthPole
Site
..n
R
k
ct95(A95) ..De.c...
Inc.
pa/eo.A
Latitude Longitude dp
tim
Mount Tripyramid(43.97ø31,
288.53øE)
JWK
•
JWM
5
5
5
Tfipyramid
[3]
KMA
Tilt corrected
KMB
Tilt corrected
KMC
Tilt corrected
5
4.9543
4.9795
4.8612
88
195
29
8.2
5.5
14.5
2.9929
282
4.9728
4.9725
4.9761
4.9761
1.9920
1.9920
147
145
168
168
---
6.3
6.4
5.9
5.9
-.-.-
160.9
150.1
166.8
164.8
155.8
124.0
-52.0
-75.5
-53.5
-77.5
-45.5
-77.7
3.9710
5.6316
5.6309
103
13
14
9.!
18.9
18.9
135.6
170.5
148.5
-74.9
-50.4
-26.8
4.9747
157
161.9
-48.7
146.0
-67.5
7.4
152.6
156.8
157.2
-58.8
-59.0
-50.9
39.5
39.7
31.6
69.2
72.3
68.3
195.8
192.7
171.5
9.1
6.1
13.2
155.6
-56.3
36.8
70.2
185.7
71.3
64.4
75.9
66.2
64.2
52.7
168.3
256.8
159.5
273.5
165.9
255.5
5.9
10.7
5.8
10.4
-.-.-
8.7
11.7
8.3
11.1
-.-
31.1
58.7
75.3
50.0
249.0
142.5
160.8
15.1
17.0
11.1
16.5
25.3
20.5
29.7
69.8
160.3
66.2
226.6
30.4
36.5
7.6
12.2
8.2
19.6
10.6
OssippeeMountains(43.80ON,288.72øE)
KME
5
2
4
Tilt corrected
KMF
Tilt corrected
6
Ossippee
[5]
Tilt corrected
3.9710 104 9.1
6.1
21.9
32.6
34.1
27.0
157.2 -41.6 24.0
62.8
159.4
6.8
6.8
10.6
4.6965
13
5.9842
11.9710
11.9640
317
384
308
3.8
2.0
2.5
162.7
153.0
146.0
-59.2
-52.0
-55.0
40.0
32.6
35.5
76.7
66.0
62.5
188.3
180.5
192.5
4.2
1.9
2.5
5.6
2.7
3.6
9.0
153.6
-55.6
36.1
68.5
186.6
9.2
12.8
Mount Ascutney(43.45øN,287.50øE)
KA1
OWA
OWB
Ascutney
6
12
12
[3]
2.9895
190
4
6
3.9656
5.9576
87
118
9.9
6.2
341.8
343.9
60.3
58.2
41.3
38.9
76.4
77.2
197.1
185.1
-.-
342.9
59.3
40.0
76.9
191.2
69.4
76.9
186.7
191.3
71.9
187.4
Mount Pawtuckaway(43.10 ON,288.80 øE)
T•VO
JWP
Pawtuckaway
[2]
1.9996
--
Reverse
Normal
[6]
[2]
5.9730
1.9994
185
--
Meanpole
3
2.9938
322
Tripyramid,Ascutneyand Pawtuckaway
(4.9)
-.-
(6.9)
11.4
6.8
15.0
9.2
5.8
7.7
The number(n)of samplemean [site]directions;
R, resultantvectorlengthof totalnumberof sitemeanvectors;k, Fisherdispersion
parameter,ot95(A95),radiusof 95% confidence
aboutthemeandirection(meanpoleposition);Dec., Inc., the declination
andinclination
of themeanmagnetization;
palco-A,palcolatitude;
dp anddm,95% confidence
ellipsesemiaxes
parallelandperpendicular
(respectively)
to
the site-to-polemeridian.
On the basis of wide geographicdistributionand variety in
tectonic settings of sampling localities, the different rock
types analyzed,the positive local and regional-scalereversal
tests, and the presenceof reversalsconsistentwith geomagnetic polarity time scales,we concludethat the good agreement
among paleomagneticpoles from these five studiesrecordsa
standstillduring the mid-Cretaceous(-124 Ma to 88 Ma) in
North American apparentpolar wander at 71.2ø N, 194.1ø E
(A95 = 3.7ø, K = 421). This meanpole is only slightlyrevised
from that of Globerman and irving [1988] but is now better
definedwith the additionof concordant
resultsfrom our studyof
the 122.5-Ma White Mountainsplutons.
1983; Crough et al., 1980; Duncan, 1984; O'Connor and
Duncan,
1990].
The Great Meteor tablemount (at 30ø N
[Morgan, 198!, 1983]) or the approximate
locationof the central Atlantic geoidanomaly(at -27 ø N [O'Connor and Duncan,
1990]) are two featureswhichhavebeenproposedas candidate
sitesof mostrecentNew Englandhotspotactivity.
While North Americawas apparentlystationarywith respect
to the palcomagneticreferenceframe from 124 Ma to 88 Ma,
datededificesalongthe New Englandhotspottrackdelineatea
progressive apparent motion of more than 1500 km over a
comparabletime interval, from the MonteregianHills at 124
Ma to the Nashvilleseamcuntat 90 Ma. By representing
the
mid-Cretaceous
standstillpole as a palcolatitude
grid for North
COM•SON
WrrH T•m NEW ENGLAND HOTSPOT
America, it can be seenthat the plutonsand seamcuntsof the
In addition to providing suitablematerial for palcomagnetic ,New Englandhotspotchain form a track from ~124 Ma to 90
study,the CretaceousWhite Mountainsplutonshelp document Ma that trendsalmostperpendicularto the lines of palcolatiplumeappears
the New Englandhotspottrack. Duncan [1984] has demon- tude(Figure6). ThustheNew Englandhotspot
strated
a regular
progression
in K/At,40Ar/39Ar,
andm'mimumto have movedsouthrelative to the spin axis by 11.0ø :t:3.7ø
duringthe mid-Cretaceous
andonly arrivedat ,--30
ø N, theprobiostratigraphic ages along the New England (Kelvin)
seamounts(~103-84 Ma). The ages and positionsof the posedmodemlatitude,by 90 Ma (Figure6).
CretaceousWhite MountainsMagma Seriesplutons(122.5 +
This southerlydrift of the New Englandhotspotplumeat an
1.5 Ma [Hubacher and Foland, 1991])and the Monteregian averagerateof 0.27ø/m.y(Figure74) maybe evidence
for a sysHills (124 :t: 1 Ma [Foland et al., 1986]) extendthis age pro- tematicshift of the mantlereferenceframewith respectto the
gression on to the North American continent. The New rotationaxis; i.e., true polar wander. Alternatively,eitherthe
England chain is thus one of the oldest and longestherspot New Englandhotspotplumedriftedindependently
and is not
trackswith physicaldocumentation
[Morgan, 1972, i981, representative of the mantle reference frame, or the mid-
VAN FOSSENAND KENT:MID-CR•ACEOUS PALEOMAGNEIlCFIELD
280 ø
Mid-Cretaceous
19,657
300 ø
90
Palcolatitudes for North America
124
Ma
(118-136
Ma)
Monteregian
Hills
White Mountains
122.5 Ma (118-127 Ma)
L-DC, O
seamount}
.............
4oø
91Ma.
"'i
'"
280 ø
j
Bear
seamount
103 Ma
290 ø
300 ø
Fig. 6. New Englandhotspottrack comparedto field of mid-Cretaceous(90-124 Ma) palcolatitudesdrawn over North
Americausingthe new standstillpole (71.2ø N, 194.1ø E), with 95% confidence
limit of :k3.7ø. The New Englandhotspot
trackis nearlyorthogonal
to thisfield, requitingabout11o of polewardmotionfor North America.
Cretaceouspalcomagneticfield did not always correspondto a
geocentricaxial dipole and, for example, contained a large
nondipolefield component. To evaluatethesepossibleexplanations,we need to make an independentcomparisonof the
mid-Cretaceous
palcomagnetic
field with hotspottrackson different plates.
COMPARISONWITH THE TRISTAN DA CUNHA HOTSPOT
The Tristan da Cunha hotspot track in the South Atlantic
definesthe post-125 Ma motion of the African plate relative to
a mantle hotspot presently located beneath the island of
Tristanda Curt_ha
at 38ø S, 011ø W [Duncan, 1981;Morgan,
1981, 1983]; it is perhapsthe only hotspottrack other than
the New Englandchain with good documentation
back through
1984]) which apparentlyrepresentthe earliest surfaceexpression of the Tristan da Cunhaplume.
Analysis of the Tristan da Cunha hotspot track in a midCretaceouspalcomagneticreferenceframe is not as straightforward as in the caseof North America,simply becauseAfrica
not only moved relative to the hotspotsbut also with respect
to the palcomagneticpole [Hargraves, 1989]. One can, however, calculatepalcolatitudesfor positionsalong the Tristan da
Cunha track basedon the Africa apparentpolar wander path.
Six of 10 Cretaceouspoles listed by Hargraves [1989] have
been selectedfor this analysis(poleswith mid-Cretaceousages
and A95 <10ø). The selectedAfrican polesare (1) Madagascar
volcanics(69.1ø N, 240.0ø E; age = -90 Ma [McElhinny and
Cowley, 1978]), (2) South African kdmberlites(64.1ø N,
themid-Cretaceous.
New40Ar/39Arradiometric
dateshavefur- 226.1ø E; age = 83-101 Ma [Hargraves, 1989]), (3a) Wadi
thor documentedthe age progressionin WaNis Ridge sea- Natashpole A, age • 90 Ma (69.3ø N, 258.1ø E [Schultetal.,
1981]), (3b) Wadi Natashpole B (64.6ø N, 251.8ø E [Ressetar
mounts to about 82 Ma [O'Connor and Duncan, 1990],
Locationsof the hotspotbetweenabout90 Ma and 120 Ma are et al., 1981]), (4) Lupatalavas (61.8ø N, 259.0ø E; age = 109inferred from seamountson the easternWalvis Ridge, con- 113 Ma [Gough and Opdyke, 1963]), and (5) Namibia lavas
strainedby recovery of lower Aptian (113-119 Ma) sediments (48.3ø N, 266.6ø E; age = 113-131 Ma [Gidskehaugetal.,
1975).
above basement (DSDP site 363 [Bolli etal., 1978]). These
These African palcomagneticdata indicatea southerlydrift of
seamountsformed shortlyafter eruptionof the Entendekaflood
the Tristan da Cunhaplume relative to the spin axis during the
basaltsin westernNamibia (ages 120-130 Ma [Erlank etal.,
19,658
VAN FOSSENAND gENT: MID-CRETACEOUSPALEOMAGNETIC•
, ,,
50
,t
,•
,,
I,
,•
,I
•t
•,
I•,
,,
I ....
I ....
A
45
•
I
arrived at their most recently occupiedlatitudes. Therefore in
independentcomparisons
with palcomagnetic
referenceframes,
Monteregian
Hills the
widelyseparated
New EnglandandTristanda Cunhahotspot
White
Mountain•
tracks show a remarkable consistency during the mid-
40
•
I ....
Cretaceous
Atlantis
II gcar
T o,_•.•
interval.
The consistentrelative motion betweenthe Atlantic hotspots
(New EnglandandTristanclaCunha)andthe palcomagnetic
ref-
35
erence frame can be illustrated in a series of reconstructions of
•
-•....... Great
Meteor
Tablemount201,,,,,
85
....
90
, ....
, ....
95
100
, ....
105
the southernAfrican plate in North American palcomagnetic
coordinatesat 124 Ma, 105 Ma, and 90 Ma (Figures8a-8c).
These "paleomagnetic/paleogeographic"
reconstructionsare
achievedby superimposing
the North Americanmid-Cretaceous
palcomagnetic field onto relative restorations of western
,.,,
Africa, southern Africa, South America, and North America
110
115
120
125
age [Ma]
-20
,
..
_•-35,
..•
(1,•
ß
o 10.5'
I
,
T
:{
/(2>
/ o --
....,,,,',,
{
basedon the publishedseafloorspreadingmodelsof Pindell et
al. [1988] and Rabinowitz and LaBrecque [1979]. Shown on
thesemid-Cretaceous
reconstructions
are the African pole position data (convertedto palcolatitudeerrorbars),alongwith two
South American results at 124 Ma [Schult and Guerreiro, 1979;
Ernesto et al., 1990].
The reconslrucfions
show the internalconsistency
of African,
South American, and North Americanpalcomagneticdata as
well as the generalconsistencybetweenthe New Englandand
Tristan da Cunha hotspots. At 124 Ma thesehotspotsbegin
drifting southwardrelative to the spin axis from locations
which are -12 ø too far north thanwhat wouldbe predictedby a
dipolefield (Figures8a-8c). The reconstructed
age-equivalent
positionsalongthe New Englandand Tristan da Cunhahotspot
chainsare in fact separatedby a consistent
arc distanceof 69ø :t:
1ø duringthe interval124 - 90 Ma. This consistency
suggests
that inter-hotspotmotion was small, nominallyof the order of
the lø-2 ø of post-Late Cretaceousinter-hotspotmotion sugFig. 7. Mid-Cretaceouspaleolatitudinaldrift of Ariantic hotspots. (a)
gestedby Molnar and Stock [1987], andmuch lessthanthe uniPaleolatitudes with 95% confidence limits for selected dated edifices of
the New Englandhotspotrelative to the North American standstill form southerly12ø shift of the New Englandand Tristan da
pole. Errorsfor the New Englandseamounts
(Bear,AtlantisI!, and Cunhahotspotswith respectto the spin axis during the mid85
90
95
100
105
110
115
120
125
age[Ma]
Goshold)are +4ø as inferredfrom the meanstandstillpole. Co)Tristan
da Cunhahotspotrelative to (open symbols)selectedAfrican poles
listed by Hargraves [1989]: (1) Madagascarvolcanics, -90 Ma
[McElhinnyand Cowley, 1978], (2) SouthAfrican kimbedites,83-101
Ma [Hargraves, 1989], (3) Wadi Natash, 86-100 Ma (3a dam from
Schult et al. [1981]; 3b data from Re•etar et al. [1981]), (4) Lupata
lavas, 109-113 Ma [Gough and Opdyke,1963], and (5) Namibia lavas,
113-131 Ma [Gidskehauget al., 1975]. Closed symbols,Tristan da
Cunha hotspot relative to the North American standstillpole (with
95% confidencelimits --d:4ø) using centraland South Atlantic reconstructions. Locationsalong the Tristan hotspottrack are calculated
from model B of O'Connorand Duncan [1990]. Linear fits to the data
Cretaceous.
DISCUSSION
The consistent
discrepancy
of theNew EnglandandTristanda
Cunhahotspots
with respectto the mid-Cretaceous
palcomagneticfield allowsthe possibilityof truepolarwander;the uniform shift of the Earth (represented
by the hotspots)with
respectto the spin axis (representedby the palcomagnetic
field). However,variablenondipolefieldsandlarge-scale
dif-
suggestsoutherlydrift ramsof 0.27 ø/m.y. (= 3 cm/yr) for the New ferential mantle motion in the mid-Cretaceous are still two
Englandhotspot,and a higherbut lesswell-definedrateof 0.38ø/m.y. alternativeinterpretationsgiven the available information.
(= 4.2 cm/yr) for the Tristanda Cunha hotspotusing African paleoAt 124 Ma, a 25-30% quadrupolefield contributionat the
magneticdata. If the North Americanstandstillpole transferredto
MonteregianHills and Entendekaobservationlocalitiesalone
African coordinates
is used,the drift rate for the Tristanhotspotis 0.32
ø/m.y (= 3.5 cm/yr).
mid-Cretaceous(Figure 7b). At 124 Ma the African polesplace
the Tristan da Cunha hotspot at 25øS (beneaththe Entendeka
flood basaltprovince),whereasby 90 Ma, the plume is located
at 38øS (beneaththe easternWalvis Ridge). The 124-Ma position is therefore13ø+4ø north of 38øS, the calculatedpalcolatitude of the hotspotat 90 Ma arid its locationtoday. This disagreementat 124 Ma and subsequentsoutherlydrift until -90
Ma at an averagerate of 0.38ø/m.y. (Figure 7b) is similar to
that demonstrated
independentlyfor the New Englandplume
(11ø+4ø at 124 Ma and rate of 0.27ø/m.y., Figure 7a).
Furthermore,for both hotspotsthe discrepancywith the palcomagneticfield is terminatedat about90 Ma when the plumes
could accountfor the -12 ø discrepancybetweenthe assumed
constanthotspotpalcolatitudesand palcomagneticpalcolatitudes(i.e., thosecalculatedaccordingto the dipoleformula).
This nondipolefield contribution
wouldneedto decrease
systematicallyover36 m.y. because
by 90 Ma, hotspotanddipole
palcolatitudes
are in good agreement.Livermoreet al. [1984]
have suggestedCretaceousquadrupolefield contributionsof
nominally 10% or less, with a maximum of 15% between80
and90 Ma at whichtime we would,in fact,deema nondipole
effectunnecessary.
In an independent
analysis,
Schneiderand
Kent [1990] haveestimatedquadrupolar
contributions
of-10%
or lessin the earliestTertiary. In fact, the high overallconsistencyof mid-Cretaceous
palcomagnetic
datafor North America
andAfrica,asdiscussed
by liargraves[ 1989]andpresented
here
(Figure 8a-c), arguesstronglyfor a closeapproximation
to an
VAN FO$SBNAND KENT: MID-CREtACEOUS PALEOMAGNETICFIELD
C.
19,6•9
90Ma
A.
•
45ø
New
England
90
- 12• Machain
•
/
/
,
ß
/
•'
7)
-3
ø•3ø
0o
125-135
Ma
30ø
/-..
ß
124 Ma
_15o
15 ø
,'
5)
........
,..:58 ø
ñ 3ø
113-131
Ma
•
•
•5 o
...
3a) 8.•
/
3b)3.6
ø
/
....
-....
o
.............
..............
0ø
_15•
•
45
•'..
.• :•
• • -3o
o'•
_60
øFig. 8. Mid-Cretaceous
palcogeographic
reconstructions
basedon relativeseafloorspreading
modelsof Pindellet al. [1988]
andRabinowitzand LaBrecque[1979], and the North Americanstandstillpole representedas a palco-latitudinalgrid
(applicable
for -90 to 124 Ma). Africanpalcomagnetic
data(listedby Hargraves[1989], with symbolsas in Figure7b),
shownhereas palcolatitudinal
barsconfirmthe palcogeography.
(a) Reconstruction
at 124 Ma with additionalpalcolatitudinal datafrom SouthAmerica(6 datafrom Ernestoet al. [ 1990];7 datafrom Schultand Guerreiro [ 1979]), (b) reconstruction
at 105 Ma, and(c) reconstruction
at 90 Ma. The Tristanda Cunhahotspotarrivesat its present-day
latitudeof 38øS(dashed
line) by 90 Ma; the present-day
locationof theNew Englandhotspotplumeis notknown,but by 90 Ma it hasarrivedat 30ø
S, nearthepresent
latitudeof theGreatMeteortablemount
[seeO'ConnorandDuncan,!990],
central Atlantic accommodatednortherlymotion of the North
Americanplate so that it maintainedconstantpaleolatitudeas
indicated by the mid-Cretaceousstandstill in North American
was interpretedas evidenceagainstlarge nondipolefield con- apparentpolar wander. This senseof true polar wanderwould
tributions in the Mesozoic, certainly none large enough to also predict that a mid-Cretaceoushotspot antipodal to the
accountfor the 12ø discrepancybetween hotspot and paleo- Atlantic hotspots should show a northerly migration relative
referenceframe) and specifimagnetic paleolatitudesat 124 Ma. Consequently,the evi- to the spin axis (or paleomagnetic
dencesuggeststhat a large, time-varyingnondipolefield con- cally at about 124 Ma, the dipole paleolatitudefor a western
tribution can be regarded as an extraneoushypothesis to Pacifichotspotshouldbe 12ø southof what would be expected
accountfor the observeddisagreementbetween hotspotsand for a stationarymantle plume. The Louisville hotspotin the
eastern Pacific, to our knowledge the lone mid-Cretaceous
palcomagneticreferenceframes.
We therefore supposethat the 12ø difference documented hotspotdocumentedon the Pacific plate, may have formed the
between the Atlantic hotspots and paleomagneticreference Ontong Java Plateau approximately 120 m.y. ago
frame at 124 Ma is real and reflects motion of the mantle, at
[Engebretsonet al., 1985; Duncan and Clague, 1985]. Based
least in the Atlantic hemisphere,with respectto the spin axis. on the Pacific apparentpolar wanderpath of Gordon [1990],
If the whole mantle was involved, this would constitute eviTarduno et al. [199!] suggest that at about 120 Ma, the
dence for true polar wander [Goldreich and Toorare, 1969]. Louisville hotspotwas locatedat paleolatitudesabout 10ø farUnder such an interpretation,the entire mid-CretaceousEarth ther north than its present location (50øS, 138øW), therefore
rotated about an equatorial axis by ~12 ø such that the Atlantic implying southerlymotion of the Louisville plume relative to
hemispheremigrated southwardwhile the Pacific hemisphere the spin axis since 120 Ma. Thus although not optimally
migrated northwaxd. Southerly true polar wander along an located for a definitive test, the Louisville hotspot gives a
Atlantic meridian would imply that seafloor spreadingin the senseof offset with respectto the paleomagneticfield that is
axial geocentric dipole specifically over this time interval.
Furthermore, Besse and Courtillot [1988] have found excellent
overall agreement among global palcomagnetic poles which
19,660
VAN FOSSENAND I•ENT: MID•TA•OUS
ostensively in disagreementwith what is predicted by the
Atlantic hotspotsif true polar wander occurredin the midCretaceous.
PALEOMAONErIC•
Cretaceous,comparisonsbetween the hotspotsand palcomagnetic referenceframes will have to await resolutionof pole
positioncontroversies
[e.g., Van Fossenand Kent, 1992] and a
clearerdefinitionof older hotspottracks.
CONCLUSIONS
New palcomagneticdata from 122.5-Ma plutons in New
Englandprovide supportiveevidencefor a standstillin the
apparentpolarwanderpath for mid-Cretaceous
NorthAmerica.
The meanstandstillpole is locatedat 71.2ø N, 194.1ø E (A95 =
3.7ø for N = 5 studies)representing'[hepalcomagneticfield in
North America from about 124 Ma to 88 Ma.
Acknowledgments.The authorswould like to thank two anonymous reviewersfor commentswhich improved the manuscript.
Funding for this study was provided by the National Science
Foundation,Earth SciencesDivision (researchgrant EAR88-03814).
This is Lamont-Doherty
GeologicalObservatorycontribution4971.
The standstill
pole constrains
motionof the North Americanplaterelativeto
the spin axis to within :t:4ø. Yet the mid-Cretaceous
New
Englandhotspottrackclearlyshows11ø+4
ø of northward
North
American plate motion relative to the manfie. In an independent analysis, a discrepancyof similar sense and magnitude
(-13 ø) betweenthe African apparentpolar wanderpath and the
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