Latitude Paleomagnetic Poles from Middle Jurassic Plutons and Moat

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. Bll, PAGES 17,503-17,516,OCTOBER 10, 1990

High-LatitudePaleomagnetic Poles from MiddleJurassic Plutons and Moat Volcanics in New Englandand the ControversyRegarding Jurassic Apparent Polar Wander for North America

MICKEY C. VAN FOSSEN AND DENNIS V. KENT

Lamont-DohertyGeological Observatory and Departmentof GeologicalSciences, Columbia University ' Palisades, New York

A paleomagneticstudy of Middle Jurassicplutonic and volcanic rocks in New England(White Mountains Magma Series)yields high-latitudepole positionsfor North America. High unblockingtemperature, moderateto high coercivitymagnetizations of normalpolarity have been isolated in threeplutons (White Mountainsbatholith, Mount Monadnook,and the BelknapMountains; mean age -169 Ma), but the mean pole (88.4øN, 82.1øE, A95 = 6.1ø) is not distinguishablefrom the geographicaxis and thereforethe hypothesisthat the plutons have been contaminatedby recent field overprintscan not be rejected. However, a dual polarity, high unbloekingtemperature, and high coercivitymagnetization isolated from the Moat volcanics(169 Ma, Rb-Sr age) was apparentlyacquired soon after calderacollapse and tilting, at aboutthe time of intrusionand coolingof the Conwaygranite (reported ages K-Ar biotite, 168 Ma; zircon fission track, 163 Ma). The Moat volcanicspole position (78.7øN, 90.3øE, dp= 7.1ø, dm= 10.2ø) calculatedusing the mean magnetizationdirection of reversedpolarity (the Cr component)falls at high latitude but is distinguishablefrom the spin axis. Moreover, publishedMiddle Jurassicpalcomagnetic polesfrom Gondwana(Africa, Australia,and East Antarctica)transferred to the North Americanreference frame also suggesta high-latitudeMiddle Jurassicpole positionfor North America,in agreementwith the Moat volcaniespole. The new evidencefor a MiddleJurassic loop to highlatitudes in the North American apparentpolar wanderpath conflictsby 15ø-20ø with somekey publishedJurassic reference poles (e.g., the Newark TrendN2 and the Corral Canyonpoles) used to constraincurrent palcomagnetic Euler pole (PEP) apparentpolar wanderpaths for the Jurassic.We suggestthat a plausibleexplanation for the discrepancy is that the N2 and CorralCanyon magnetizations are in fact secondaryand were acquired after tilting. The hypothesisthat the North Americanapparent polar wander path ventured to high latitudein the Middle Jurassicrequires further testing, however the resultsof thisstudy already suggest that the pathmay be more complicatedthan that proposedby recentlypublished PEP studies.

INTRODUCTION documented[e.g., Foland and Faul, 1977] for the White Mountains intrusions. In fact, three of the five plutons The palcomagneticpole reportedby Opdyke and Wensink (constitutingeight of the 12 sites)which providedwhat were [1966] from the White Mountains Magma Series in New assumedto be Jurassicpalcomagnetic directions have since Englandfalls at 85.5øN, 126.5øE(A95 = 5.5ø), basedon 12 been dated at about 120 Ma or Cretaceous. The four site mean simswhose age was assumedto be that of the Conwaygranite, directionsfrom two conf'n'medJurassic igneous units (Belknap citedto be 180 Ma. More recently,May et al. [1986] reporteda and Mount Monadnook;sims 4, 5, 11, and 12 from Opdy• and palcopolefrom volcanicand volcanoclasticrocks dated at 172 Wensink [1966]) neverthelessare entirely of normal polarity Ma from Corral Canyonin southeasternArizona which falls at andgive a reealculatedpole position at 88.4øN,354.6øE (A95 = 61.8øN,116.0øE (A95 = 6.2ø),almost 24 ø of are distancefrom 3.5ø), againnot significantlydifferent from geographicnorth. the White Mountainspole despitethe similarity in apparent While the suspicionof recentfield contaminationremains for ageof therocks (Figure 1). The discordancebetween the White the Jurassicigneous units of the White MountainsMagma Mountainsand Corral Canyon palcopoleshighlights a major Series, it is curiousthat Cretaceousand Triassic units do not ap- differencein some recently publishedinterpretations of the pear to be similarlyaffected. Opdyke and Wensink[1966] apparentpolar wander path of North America. For example,the alreadyobtained, and we haveconfirmed [Van Fossen and Kent, Irvingand Irving [1982] "runningmean" apparent polar wander 1988], stable reversedpolarity magnetizationsfrom Mount pathattains high latitudesin the Jurassicmainly becausethe Ascumey gabbros(120 Ma) which yield a pole position White Mountains pole was included in this compilation, consistentwith Cretaceousreference poles for North America. whereasthe White Mountainspole grosslydisagreed with the Similarly,recent palcomagnetic results from the nearbyAbbott lowerlatitude palcomagnetic Euler pole apparentpolar wander (225 Ma) and Agamenticus(227 Ma) plutons show little pathsof.Gordon et al. [1984] andMay and Butler [1986] and evidencefor presentday field overprintingand are broadly wasexcluded in these analyses. consistentwith Triassic palcomagneticdirections for North May andButler [1986] providecogent reasons for regarding America [Wu and Van der Voo, 1988]. The Cretaceousand the White Mountains pole as unreliable, namely, the Triassicresults also argueagainst any significantpost-Triassic correspondenceof the pole with geographicnorth, the lack of regionaltilting in New England. thermal demagnetization,and the wide age range now In this new studyof the White MountainsMagma Series,our main objective was to addressthe palcomagneticstability issue, with new aimmatingfield and thermaldemagnetization Copyright1990 by the AmericanGeophysical Union. experimentson a varietyof Jurassicplutonic rocks as well as Papernumber 90JB01380. on Jurassicsiliceous flows (Moat volcanics) in central New 0148-0227/9 0/90JB-013 80505.00 Hampshire.

17,503 17,504 VAN FOSSENAND KEI•: HIGH-LATITUDEPALEOMAGNETIC POLES

~100 Ma

White Mtn. pole •0ø• OpdykeandWensink, 1966

150 Ma

200 Ma -150 Ma N1 N2 ....Ma... LM2 / Ma •

Corral Canyonpole 230Ma Mayet al., 1986

Ag 0

Fig. 1. Controversyin MiddleJurassic North American reference pole selection is illustratedby thediscordance between two publishedreference poles (White Mountains pole from Opdyke and Wensink [1966] and the Corral Canyon pole from May et al. [1986]),both nominally of MiddleJurassic age but separatedby about24 ø of are distance.As discussedin the text,this problemcontributes to differentversions of theJurassic apparent polar wander path (solid black line, Irving and Irving [!982]; lightgray line, Gordon et al. [1984];dashed line, May and Butler [1986]). Solidcircles represent published North AmericanJurassic reference poles used by May andButler: W, Wingatepole [Reeve,1975]; K, Kayentapole [Steinerand Helsley,1974]; N1 andN2, NewarkTrend 1 and2 poles,respectively [Smith and Noltimier, 1979]; G, Glanceconglomerate pole[Kluth et al., 1982];LM2 andUM2, lowerand upper Morrison formation poles using selected data [Steiner and HeIsley, 1975]. Otherpoles (open circles): KA, mid-Cretaceousaverage [Mankinen, 1978]; As, MountAscutney lOpdyke and Wensink,1966]; Ab andAg, TriassicAbbott and Agamenticus plutons, respectively [Wu and Van der Voo, 1988].

GEOLOGY OF THE FIELD AREAS climax in plutonic activity probably occurred there at about AND PALEOMAGNETIC SAMPLING 175 Ma. A 168 + 3 Ma K-At biotite age [Foland et al., 1971] The White MountainsMagma Seriesof New Englandfom•s a on Conway granite from the eastern side of the White 240-kin-long north by northwesttrending belt of approxi- Mountainsbatholith (Figure 2) suggeststhat the granitecooled mately 30 igneous plutons, stocks, and some related volcanic to 300øCor 350øCrather quickly, a conclusionsupported by a flows. These alkaline rocks intrude lower to middle Paleozoic zircon fission track age (!63 + 14 Ma) also determinedon metamorphic and igneous formations and their eraplacement Conway granite from this region [Doherty and Lyons, !980]. appears to have been controlled mainly by ring fracture Thesedata combined with an apatitefission track age of 94 + 8 stoping[Chapman, 1968]. PublishedK-At and Rb-Sr age data Ma may be accepted as evidence that the field area has [Foland and Faul, 1977] demonstrate that White Mountains experiencedan undisturbedthermal history after initial cooling igneousactivity occurredduring the Mesozoicover a period of due to gradual unroofing or in any case, that the White about135 m.y. Furthermore,the datasuggest that the magmas Mountainsbatholith had not been heatedhigher than about were injected during three distinct episodes:the first of these 125ø C sincethe early Late Cretaceous.Thermochronometrical duringthe Late Triassic(~230 Ma), a secondand perhaps more data from other Jurassicplutons in New England, including extensive phase occurring in the Jurassic(-175 Ma), followed thoseigneous units studiedhere (see below), supportthe same by an Early Cretaceousphase (~115 Ma). conclusion[Doherty and Lyons, 1980]. Age dating studies also provide detailed thermochrono- We sampledJurassic plutonic rocks in three generalareas: metrical constraints on the Jurassic rocks of the White White Mountains batholith, Mount Monadnock, and the Mountains Magma Series. Rb-Sr biotite, Pb 206/U 238 and Pb Belknap Mountains(Figure 2). Two sites of Conway granite 207/U 235 ages [Tilton et al., 1957; Aldrich et al., 1958; from the White Mountainsbatholith have been samplednear Hurley et al., 1960] indicate that the White Mountainsbatho- the town of North Conway (inset, Figure 2). Five kilometers lith (the largest body of the White MountainsMagma Series) northwardwe have also sampleda diorite plug and the Albany crystallizedabout 175-180 m.y. ago. More recentRb-Sr dating quartz syenite ring dike which partially surrounds Mount of a variety of plutonit rocks on the east side of the White Kearsage. Contact relationships suggest that the Conway Mountains batholith [Eby and Creasy, 1983] confirmsthat a granite is younger than the quartz syenite and diorite; the VAN FOSSEN AND KENT: HIGH-LATIrUDE PALEOMAGNETIC POLES 17•505

Mt. 5.:• SiliceousFlo•s • Monodnack • N

v•c • ß• 171Ma•t I,/ •g • Di•i• WhiteMtn. Batholith_/-• t / I'.' '.' '.' '.'.'

158Ma [ - (' O•

I NHø//••! , •

• ....:...;.':•:.:.. ; ß ; ; ; .

71 '15' 7]

Fig. 2. A sketchmap of the White MountainsMagma Series, New Englandshowing sampling localities. The insetis a portionof the NorthConway quadrangle [after Billings, 1928]. Ourfour Jurassic pluton sampling localities am (fromnorth to south)Mount Monadnook,North Conway(see inset), Mad River, and the BelknapMountains. The Moat voltahies samplingareas as shownin the insetare Mount Kearsage,North Moat Mountain, and South Moat Mountain. At Kearsage the bedsstrike east-west and dip nearlyvertical. Beddingsurfaces along North and South Moat mountainsdip moderately easterlytoward the centerof the structureand strikes generally follow the trend of the syenitepartial ring dike, although this is difficult to observe at North Moat Mountain.

dioriteplug may be all thatremains of a moremafic form of the A secondgroup of Jurassicrocks were sampledin this study, magmachamber that later fractionatedConway granite magmas thesebeing the effusiverocks known as the Moat volcanics. [Billings,1928]. About 27 km to the southwestof this Thesepyroclastic quartz trachyte and comendite flows, ash flow localitywe have sampied,at one site, Conwaygranite of the tuffs and brecciaswere first named and mappedby Billings Mad River "satellite" stock of the White Mountains batholith [1928]. We sampledthe two known exposureson the eastern (Figure2). The granitehere has yielded a Rb-Srwhole rock age side of the Jurassic White Mountains batholith: one on the of 194 + 4 Ma [Foland et al., 1971]. K-Ar biotite ages Moat Range(North and SouthMoat mountains),the otherto availablefrom Mad River are 180 and 181 ñ 4 Ma [Foland and the easton MountKearsage (see inset, Figure 2). Collectively, Faul, 1977] and like the White Mountainsbatholith, apatite the Moat volcanicsrepresent an effusivephase of the White fission-trackages are Cretaceous(102 + 16 Ma [Doherty MountainsMagma Series. An intrusivecontact with younger, Lyons, 1980]). radiometricallydated Conway granite (168 :t:3 Ma K-At biotite At Mount Monadnook in northeasternVermont (Figure 2), age [Folandet al., 1971])provides a minimumage for the Opdykeand Wensink [1966] sampledtwo sites:one site a subsidedvolcanics. A Rb-Sr whole rock age of 175 Ma has lamprophyre,the other an essexitcbody. The essexitc beenreported for theMoat volcanics on SouthMoat Mountain specimenswere retrievedfrom our samplearchive for further [Ebyand Creasy,1983] and further work gives a revisedRb-Sr study.A K-At biotiteage of 171 :t:4 Ma is availablefrom the ageof 169Ma with a standarderror of onlya fewmillion years MountMonadnook Conway granite [Foland and Faul, 1977]. (I. Creasy,personal communication, 1989). Zirconand apatite fission track ages of I65 +- 12 and 114 +_8 It hasbeen suggested that most of the originalMoat volcanic Ma, respectively,are available from syenite exposedhere material has sincebeen eroded. Minimum original thicknesses [Dohertyand Lyons, 1980]. have been estimatedat 3100 m or more [Billings, 1928, 1956] In the BelknapMountains of southcentral New Hampshire andwhat we seetoday is thatwhich had subsidedinto granitic (Figure2), we have sampledsix'sites of quartzsyenite and magmachambers. Recently, detailed mapping of therocks on quartzdiorite which form a partialring dike;a stockof Conway North and South Moat mountains have confirmed an granitesurrounded by the Belknapring dike has also been intracalderasetting [Fitzgerald and Creasy,1988]. Coarse- sampledat onesite. Opdykeand Wensink[1966] sampled two grain.edPequawket breccia is foundnear the quartz syenite ring sitesof monzonitealong the southernperimeter of theBelknap dike that bordersthe western side of the North and South Moat complex. Syenite from the Belknap Mountainshas been mountainsexposure. The brecciaand associated bedded tuffs, radiometricallydated as 158+_ 3 and164 + 5 Ma (Rb-Srbiotite thickestnear this ring dike,thin anddip radiallyinward (25 ø- [Folandand Faul, 1977]). The availableK-At agesare very 40ø ) with strikesthat appear to parallelthe dike, although the similar(about 158 Ma [Folandet al., 1971;Foland and Faul, exact structureat North Moat is more difficult to ascertain. The 1977]) and apatite fission track ages a.re 87 +_ 7 Ma Moat volcanicsat Mount Keatsagemay representan isolated (Zirr•nerrrmnet al. [1975],recalculated by Dohertyand Lyons roofpendant that fell intoa magmachamber due to extensive [198o]). magmastoping [Billings, 1928]; flows and breccias here strike 17,506 VANFOSSEN AND KENT: HIGH-LATITUDE PALEOMAGNETIC POLES nearly east-westand dip about85 ø to the south. Jurassic Plutonic Rocks We havesampled quartz trachyte, ash flow tuff, andbreccia of the lurassic Moat vo!canics at 24 sites, 10 on North Moat Even after extensive AF and thermal demagnetization,the Mountain, six on South Moat Mountain, and eight on Mount palcomagnetic directions from the lurassic plUtohie Kearsage(inset, Figure 2). The samplingsites, taken in stream lithologies are similar to those reported by Opdyke and cuts, were positioned over the field area in such a way as to Wensink [1966]. The demagnetizationti'ajectories are achieve representativegeographic and stratigraphiccoverage. relativelystraightforward, with univectoria!decay to theorigin Typically, five core sampleswere drilled at eachsite and were of a normal polarity componentgenerally along the direction orientedwith a Bruntoncompass; sun compasschecks showed of the dipole field in New England. A direct demonstrationof negligible deflections. this are data from the essexiresof Mount Monadhock studiedas site 11 of Opdyke and Wensink [1966]. We were ableto PALEOMAGNETIC RESULTS retrieve the samplesand orientation information from this site Our experimentalplan was originally designedwith the and performed further AF cleaning, followed by thermal intentionof expandingthe Opdykeand Wensink[1966] study demagnetizationon the original samples. These essexitesare by comprehensivesampling of Jurassic plutons in New strongly magnetized, with average natural reinanent England. We soonrealized that progressivealternating field magnetization(NRM) intensitiesas reportedby Opdykeand (AF) and thermaldemagnetizations on the initial collectionof Wensinkof 5.7 A/m. A single,high coercivity(> 90 mT) and plutonic rocks simply confirmedthe existenceof a single high unb!ockingtemperature (up to 580øC) magnetization, component,normal polarity magnetization.However, a pilot exclusively of normal polarity is present (Figure 3a). These study on the Moat volcanics revealed more complex, new demagnetizationprofiles give directions(declination = multicomponentmagnetizations and we thus concentratedon 356.8 ø, inclination = +61.8ø; n = 6 samples) very similarto this more promisingrock unit. All magnetizationcomponents the magnetizationsreported for the site by Opdyke and identified from progressivedemagnetization were analyzed Wensink [1966] following AF demagnetizationto only 15 mT with principal componentanalysis [Kirschrink, 1980] and (declination = 1ø, inclination- +65.5ø; n = 8 samples).3, mean directionscalculated according to standardFisher [1953] mean direction for Mount Monadhock was calculatedusing two statistics. site means:one from site 12 of Opdyke and Wensink (original

[a] Mount Monadnock [b] White Mtn. Batholith(Albany quartz (essexitc) syeniteat Mt. Kearsage) W,Up W,Up 'N 590 ø 0.1Aim -- ,A= Opdyke andWensink •350o (1966) 0.001Aim-• • 2OOø owl•I siteJKE I 70mT• • 50 mT

[c] White Mtn. Batholith [d] White Mtn. Batholith [e] BelknapMountains (Conwaygranite at Mad River) (Conwaygranite) (Conwaygranite) W,Up _ 60mT••r•N 0.01A/m-• • T 0.1Agm-- •:•2• isite,,j.B1, ] •l

Fig.3. Demagnetogramsrepresenting progressive altemating field (AF) and thermal demagnetization of the "P" component fromJurassic plutonic rocks of theWhite Mountains Series. Open (solid) symbols illustrate vertical (horizontal) orthogonalprojection oœvector end-points. Temperatures aregiven in degreesCelsius' alternating fields in millitesla(mT). (a) Essexitcfrom Mount Monadnook, where the triangles represent the measurements from Opdyke and Wensink [1966], circlesmeasurements made subsequently by us; (b) thermaldemagnetization of Albany quartz syenite from the White Mountainsbatholith (Mt. Keatsage); (c) thermal demagnetization of Conway Granite from the White Mountains batholith (MadRiver stock:); (d) AF demagnetizationof Conway Granite from the White Mountains batholith; (e) thermal demagnetizationof Conway Granite from the Belknap Mountains. VAN FOSSENAND KENT: HIGH-LA•E PALEOMAO'NEllCPOLES 17,507 orientationdata for thissite was lost; hence we did no further demagnetizealong origin-boundtrajectories until about 50-60 workon the samples)consisting of 11 lamprophyresamples mT, at which point the magnetizationsbecome very weak and demagnetizedto 15 mT (declination= 5 ø, inclination= +63ø; difficult to measure(Figure 3d). The stablecomponents from NRMintensities = 0.65 A/m), theother a sitemean calculated five sites of the White Mountainsbatholith give a well-defined usingresults from our further AF andthermal demagnefizations mean magnetization direction of declination = 359.0 ø, onsamples of Opdykeand Wensink's site 11. This Mount inclination= 64.3ø, with an ix95 of 4.4ø (seeTable 1). Monadnockmean has declinationof 0.8ø and inclinationof At the Belknap Mountainsin southcentral New Hampshire,a 62.5ø (Table1 andFigure 4). stable normal polarity magnetizationwas found in three of Similarly,AF and thermaldemagnetization experiments on seven sites (Figure 3e). NRM intensities among these three plutonicrocks from the Whim Mountains batholith yielded a stable sites ranged from 45 mMm (sites IB2 and 3, diorite) to univectorial,normal polarity magnetization. Average NRM about 350 mA/m (site IB1, Conway Granite). Samplesfrom intensitiesof samplesfrom the batholithare approximately35 four quartz syenite sims (NRM intensities: 55-120 mA/m) mA/m,while the Mad River stockNRM intensitiesare 1.5 yielded unstablemagnetizations and were excludedfrom further mA/m. The magnetizationin the Albanyquartz syenite and study. Our Beltmapcomponent mean (N = 3 sites,declination diorite(near Mount Keatsage) and Conway granite (near North = 3.8 ø, inclination = 54.4ø, {x95 = 9.3ø) is similar to the Conwayand along Mad River)has unblocking temperature up directions of the original Belknap site mean magnetizations to about590øC, appropriate for magnetite(Figures 3b and3c) reported by Opdyke and Wensink (site 4: declination = 0 ø, and,with the exceptionof samplesfrom two sitesof Conway inclination = 65ø; site 5: declination = 2 ø, inclination = 59ø). Granite at North Conway, is also of moderate to high In summary, the three Jurassic plutons gtudied (Mount coercivity.NRM intensifiesof graniteat North Conwayare Monadhock, 171 Ma; White Mountains batholith, 175 Ma; nearlyhalved by appliedfields of just30 mT, yet continueto Bellmap Mountains, 160 Ma) carry a moderate to high eoercivity, high unblocking temperature magnetization referredto as the P component,with a meandeclination of 1.5ø and inclination of 60.4ø (e•95 = 8.3ø , N = 3 plutonit TABLE 1. PalcomagneticData FromJurassic Plutons of the White MountainsMagma Series,New England complexes, Table 1). This direction confirms the result of Opdyke and Wensink [1966] with the added enhancementof Tn Situ thermal demagnetizationstudies; it deviates from the ambient Locality N R k ot95 D I geomagneticfield but is not significantly different from the average dipole field direction over the sampling localities Mount Monadnook 2 1.998 829 - 0.8 62.5 (inclination = 63ø). 171 Ma Moat Volcanics White Mountain Batholith 5 4.987 302 4.4 359.0 64.3 175Ma Thermal and AF demagnetizationexperiments have enabledus to identify a stabledual polarity componentof magnetization BellmapMountains 5 4.974 160 6.1 2.9 57.5 (referred to as the C component)at the three Moat volcanics 160 Ma samplinglocalities: North and SouthMoat Mountain(the Moat Range) and Mount Kearsage. Coercivitiesof the C component N, thenumber of sitemean directions; R, resultantvector length of totalnumber of site mean vectors;k, Fisher dispersionparameter;, et95, generallyexceed 100 mT (Figure5a) andmaximum unblocking radiusof 95% confidenceabout the mean; In situ D, I, the declination temperaturestypically range up to about600øC (Figures 5b and andinclination of the site mean magnetization. 5c) but at SouthMoat Mountainsite IMM andMount Keatsage

:[]= dipolefield Belknap :• = ambient fiel

White Mtn. Batholith

.¸ Monadnock

Fig.4. Lowerhemisphere, stereographic projectionof mean directions alongwith cones of95% confidence fromJurassic plutonsstudied here. Shaded square represents thegeocentric axialdipole field direction; shaded circle represents the directionofthe present-day field in centralNew Hampshire. 17,508 VANFOSSEN AND KENT: HIGH-LATITUDE PALEOMAGNETiC POLES

[a] S. Moat Mtn.: A, Cr [b] S. Moat Mtn.: A, Cr components. components.

20mT

'x•'r•,lOOmT

ß...., N I' 0.1 A/m 0.1 A/m

[c] Mt. Kearsage:Cn component. [d] Mt. Kegsage: Cr component.

N 200 ø wu 610 ø 0.1 A/m - 510 P 575o • 650ø ooo siteJKH [ '"', - 0.01 A/m :, N

[e] N. Moat Mtn.: B component. [f] N. Moat Mtn.: B component.

W, Up _ 50mT• • 570øI-• N 0.01A/m 0mT 0.01A/m•o _ • 200ø [ siteJMi'[ I siteIMI

Fig. 5. Demagnetogramsrepresenting progressive alternating field and thermaldemagnetization of the JurassicMoat voltunics. No structuralcorrections have beenapplied and symbolsare thoseused in Figure 3. Examplesshown are (a) alternatingfield and (b) thermaldemagnetization of quartztrachyte from SouthMoat Mountainillustrating the reversed polarity "Cr" componentand normal polarity, low stability "A" component;thermal demagnetizationof the (c) Cn componentand (d) Cr componentfrom Mount Kearsagequartz trachyte; (e) alternatingfield and 0Othermal demagnetization of ash flow tuff from North Moat Mountain showingthe lower stability "B" magnetization.

site JKH, extendto above 650øC (Figure 5d). In many samples normalpolarity component(Cn component,Figure 5c)is a substantialfraction of the C componentis only removedover otherwise found exclusively. The high coercivity, high the f'mal 20 ø of thermal treatment. unblockingtemperature, and reversedpolarity Cr component Of particularinterest is the reversedpolarity Cr component, yields a mean directionwith declination= 184.3ø and which is found mainly in the Moat Range(sites JMA and JMB inclination= -52.7ø (ct95 = 7.4ø for nine sites). The high in ash flow tuffs at North Moat Mountain and sites JMK stability,normal polarity Cn componenthas beenresolved at throughP in quartztrachytes at SouthMoat Mountain)but also seven sites from Mount Keatsage (declination = 2.1ø' in the older of two flows at Mount Kearsage(site IKH, Figure inclination= 59.1ø; ct95 = 7.0o).Note that we divided site JKH 5d) where essentially the antipodal but also highly stable (at Mount Keatsage) into two individual sites post facto, VANFOSSEN AND KENT: HIGH-LATITUDE PALEOMAGNEllC POLES 17,509 havingdiscovered the Cn component(three samples) and the hematite must also make some contribution to the NRM of Cr component(also three samples)in the older and younger thesesites. The relativelyinefficient acquisition of IRM in flows,respectively, at this site. Due to lack of lineardemag- thesesamples (Figure 6) presumablyis the manifestationof netizationtrajectories, site JKB (of Mount Kearsage)and sites this hematite contribution. Polished and thin section observa- IMF andO (NorthMoat) wereeliminated from furtherstudy. tionsconf'mn the presenceof hematiteand magnetite in the Thusthe dual polarity C componentis carriedby 16 sitesin the groundmass of samplesfrom sites JMM andIKH. Fine-grained MoatRange and Mount Kearsageand the mean of thesedata magnetitedominates the groundmass of samplesfrom the (averagedby site and invertingto commonpolarity) has otherMoat volcanicssites where demagnetization and IRM declination183.4 ø and inclination-55.5 ø (ct95 = 5.0ø). acquisitiondata suggesta negligiblecontribution of hematite. Less stable magnetizations are also observed in the Moat In all observedthin sections,phenocrysts of orthoclase volcanics.Most obviouslypresent at sevenof the eight Moat feldsparshow severe alteration with mainlyclay minerals and Rangesites that carrythe Cr componentis a normalpolarity minor magnetiteoccuring along crystallographiccleavage overprintof variablemagnitude but whichis readilyremoved planes. by20 mT or 300øC(Figures 5a and5b). On thebasis of itslow Variety in bedding attitude of the Moat volcanicsallows stabilityand mean direction (declination= 350.6% inclination applicationof a "tilt" test. Stereographicplots of the 16 C = 65.7%ct95 = 6.6ø for N = 7 sites),which conforms to thatof componentsite meansclearly show an increasein dispersion thepresent-day field (declination= 344ø, inclination= 71ø) at following the correctionfor tilt (Figures7a and7b). This thesampling locality, we concludethat this (A) component negativetilt test is significantat the 99% confidencelevel, and mostlikely representsa recent viscousmagnetization. it indicatesthat the C componentrepresents a secondary More difficult to interpret is a low to moderate stability, magnetization and should be consideredin the in situ reference normalpolarity componentfound in six sims of ash flow tuffs frame. Althoughthe A and B componentsdo not give a fromNorth Moat Mountain (IMC,D,E,H,I, and J). This 03) statisticallydecisive tilt testbecause of the morelimited range componentis effectivelyremoved by about50 mT (Figure5e), in beddingattitudes of the relevantsites, these low stability and,while unblockingtemperatures may extend to 570øC, only componentsalso appear to be posttiltingas mightbe expected a smallfraction of the NRM remainsafter 300øC (Figure 5f). (Table 2). The mean direction of the B component(declination = 358.1, Althoughthe tilt test indicatesremagnetization, acquisition inclination= 61.6ø, ct95 = 10.7ø, N = 6 sites)is similar to that in the recentfield cannotaccount for the reversedpolarity Cr of the A componentoverprint. In additionto lower magnetic componentwhose direction is in any case significantly stabilities,the B componenttuffs are also distinguishedby shallowerthan the dipole axis (inclination = +63ø or-63ø). NRM intensitiesand initial susceptibilitiesthat are lower by at Normal polarity componentCn, however,has a meandirection leastan order of magnitudethan the tuffs and quartz trachytes that is statisticallyindistinguishable from the dipole field from the Moat Range that carry the high stability C axis, even thoughthis componenthas high stability,similar component. We thus supposethat the B componenttuffs to that of component Cr, with respect to which it is representcases in which the original C componentis nearly statisticallyantiparallel (99% confidencelevel by methodof absent,effectively dominatedby a more recent overprint. McFadden and Lowes [1981]). It is thereforepossible that The thermal and AF demagnetization experiments suggest component Cn is approximately coeval with ancient thatthe Moat volcanicsN'RM is carried mainly by magnetite. component Cr: The somewhat shallower mean direction of Cr Thisconclusion is supportedby isothermalremanent magneti- (inclination = -52.7ø) comparedto Cn (inclination = 59.1ø) zation(IRM) acquisition curves which show that magnetic could be accountedfor by some unremovedcontamination in saturationis achievedwith applied fields of 400 mT or less either or both original component(s)by a steepercomponent (Figure6). However, given the high unblocking temperature (say, the present-day-fieldcomponent A, inclination= 65.7ø; andhigh coercivitydemagnetization profiles of samplesfrom Figure 7c). A combinedmean of the nine reversedand seven simsJMM (South Moat Mountain) and JKH (Mount Keatsage), normal sites would tend to average out the effect of such contaminationyielding an overall mean (declination = 3.4 ø, inclination= 55.5; (x95 = 5.0ø) that is statisticallydistingui- shablefrom the time-averageddipole field. However,given the uncertainty in how the alleged contamination is distributed amongreversed and normal sites,and consideringalso that the Cn componentconforms in polarity and directionto the dipole 0.8- field, the reversedpolarity Cr componentmay providethe more conservative estimate of the ancient field direction. Recent field contamination would tend to shallow Cr, and thus if some 0.6- unremoved contamination indeed exists, we at least know that a IMB the true Cr direction is no shallower than-52.7 ø. The Cr 0.4' o IMJ component thereforeplaces a lower limit on the inclination of e JKG ß JKH the Moat volcanicshigh unblockingtemperature, high coer- 0.2' mm IMM civity, and dual polarity C magnetization.

0.0- POL• POSITIONSAND INTERPRETATION 0 200 400 600 800 The normal polarity components of Jurassic White B (mT) Mountainsigneous rocks identified in this study,which on the Fig. 6. Normalizedisothermal reinanent magnetization (IRM) basis of high magneticstability might be expectedto record acquisitioncurves of representativesamples from the Moat volcanics. ancient magnetic field directions, neverthelessgive pole 17,510 VANFOSSEhi AND KENT: HIGH-LATITUDE PALEOMAGNETIC POLES

N /a]

W ,, , + , , , , , , , , N [b] ø / 00-

¸ 0 - E

[c]

Fig.7. Stereographicprojections of sitemean directions of magnetizationcomponents isolated from the Moat volcanics. (a) In in situcoordinates, the normalpolarity mean directions (solid symbols, lower hemisphere) represent the Cn component:the reversedpolarity directions (open symbols, upper hemisphere) represent the reversedpolarity Cr component(circles, North and South Moat Mountain; squares, Mount Kearsage). (b) Whenthe volcanic rocks are corrected for tilt, dispersiongreatly increases and the data fail thetilt testat 99%confidence. (c) Thein situA, B, Cr, andCn componentmean directions and 95% cones of confidenceare shown in magnifiedview. Theantipode of theCr component meanis shownfor easycomparison with A, B, and Cn.

positionswhich are statisticallycoincident with geographic As discussedearlier, a volumetricallysignificant magmatic north: the plutonicrocks P componentpole at 88.4øN, 82.1øE event took place in the White Mountainsregion duringthe (A95 - 6.1ø) and the Moat volcanicsCn componentpole at Cretaceous(mean age: 115 ñ 7 Ma [Folandand Faul, 1977]). 85.5øN,87.4øE (dp -- 7.8ø, drn-- 10.5ø;Table 3). The volcanic The heat and/orhydrothermal fluids associatedwith this event A (82.4øN, 232.9øE, dp--8.7 o, drn-- 10.7o) and B (88.1øN, could have disturbedJurassic magnetizations in the region. lSz[.6øE,dp -- 12.8ø, drn • 16.5ø) componentpoles also are However,none of thepole positions of thisstudy, particularly statisticallycoincident with the dipole axis but this is not the P and Cn componentpoles, conformwith the Cretaceous unexpectedgiven the low stabilityof thesecomponents. palcomagnetic field for North America. Nevertheless,the VANFOSSEN AND KENT: HIGH-L^T1TUDE PALEOMAGNETIC POLES 17,511

TABLE2. Summaryof PalcomagneticMean Data From the Moat Volcanics

• Situ Tilt Corrected Component N[n] R k et95 D I R k et95 D I

A component Northand South Moat Mountains 7 6.929 85 6.6 350.6 65.7 5.824 5 29.6 19.5 46.0

B component NorthMoat Mountain 6 5.875 40 10.7 358.1 61.6 5.875 40 10.7 108.9 71.1

Cr component NorthMoat Mountain 2 1.982 57 - 174.3 -60.8 1.982 57 -- 289.6 -73.1 SouthMoat Mountain 6 5.914 58 8.8 188.5 -48.9 5.914 58 8.8 195.1 -10.3 MountKeatsage 113] 2.959 50 17.7 171.6 -57.4 2.959 50 17.7 10.9 -36.8 MeanCr 9 8.837 49 7.4 184.3 -52.7 7.000 4 29.5 199.6 -19.7

Cn component MountKearsage 7 6.920 75 7.0 2.1 59.1 6.920 75 7.0 183.9 35.9

Mean C component 16 15.732 56 5.0 183.4 -55.5 13.457 6 16.6 187.0 24.0

N[n],the number of sitemean [sample] directions; R, resultantvector length of totalnumber of sitemean vectors; k, Fisherdispersion parameter,(•95, radius of 95%confidence about the mean; In situD, I, thedeclination and inclination of the site mean magnetization prior to tiltcorrection; Tilt correctedD andI, thedeclination and inclination of thesite mean magnetization following the tilt correction.

combinationof directionsand normal polarity of the P and Cn thermalevent. As summarizedabove, the similarity in theRb- componentsmakes it difficult to allay suspicionsthat the high Sr wholerock age on the Moat volcanics (169 Ma) comparedto stability palcomagnetic signal has somehow been con- theK-Ax biotite age (168 Ma) andzircon fission track age (163 taminatedby recent magnetizations. Ma) on the intrudingConway granite, indicates rapid initial On the basis of its reversed polarity alone, the Moat cooling.This was likely followed by gradualunroofing given volcarriesCr componentcannot represent a recent remagneti- the apatitefission track age (94 Ma) but in anycase, we arenot zation even though a secondary origin is indicated by a aware of any thermochronometrical data from the White negative tilt test. Moreover, although the Cr component Mountainsbatholith that would suggesta post-Cretaceous (north)pole falls at high latitude (78.7øN, 90.3øE, dp = 7.1ø, thermal event. An allegedTertiary remagnetizationevent dm= 10.2ø), it is statisticallydistinct from the geocentricaxial wouldalso have had to be ratherselective, somehow affecting dipolefield and in fact doesnot correspondto what might be onlythe Jurassic rocks because Triassic and Cretaceous igneous currentlyregarded as reliable Jurassicor Cretaceousreference rocks of the White Mountains Series do not show evidence of a polesfor North .America. The pole position might indicate Tertiary resettinglOpdyke and Wensink,1966; Van Fossenand remagnetizationsometime in the late Tertiary (Figure 8) but Kent, 1988; Wu and Van der Voo, 1988]. publishedthermochronometrical data argue againsta Tertiary We thereforesuggest that the Cr componentwas acquired as a thermo(chemical)remagnetization, perhaps in conjunction with growth of iron oxides throughdevitrification of volcanic glass, shortly following collapse and tilting of the Moat TABLE3. PalcomagneticPole Position Data From]'urassic Plutons and volcanicswithin the White Mountainscaldera(s) as described MoatVolcanics of the WhiteMountains Magma Series, New above. Demagnetizationanalyses of NRM and bulk rock Hampshire magnetismstrongly suggest that this remagnefizationsignal Pole is carriedpredominanfiy by magnetitewith somecontribution Unit N Latitude Longitude.. dp tim from hematiteat certainlocalities. Remagnetization age is no older than 169 Ma (Rb-Sr age on the Moat volcanics)and is Plutons169 + 8 Ma unlikely to be younger than 163 Ma (zircon fission track P component 3 88.4 082.1 6.1' cooling age of the intruding granite); hence we assumea nominal age of 166 Ma which is late Middle Jurassic Moat volcanics 166 Ma (Callovian) according to the Decade of North American Cr component 9 7 8.7 090.3 7.1 10.2 Cn component 7 85.5 087.4 7.8 10.5 Geology(DNAG) time scale[Palmer, 1983]. Acquisitionof C component 16 81.6 089.7 5.1 7.1 remanencemust have spannedat least one geomagnetic polarity reversal if the normal polarity Cn componentis B component 6 88.1 154.6 12.8 16.5 regardedas of similarancient origin. In this regard,Steiner et al. [1987] suggestthat the Middle Jurassicis an interval of N, the numberof sites for Moat volcanicsand plutonsfor P component;Pole latitude and longitude in situ coordinates;dp anddin, frequentreversals, but thereshould be adequatetime represented 95% confidenceellipse semi-axesparallel and perpendicular to averagesecular variation. Finally, any regionaltilting in (respectively)to the site-to-pole meridian. New Englandas recenfiy suggested by Harrisonet al. [1989]is *A95,radius of 95%confidence about the mean pole. most likely pre-Mesozoic,and as mentionedabove, published 17,512 VAN FOSSENAND KENT: HIGH-LATITUDE PALEOMA•TIC POLES

-100 Ma

166 Ma .::.

-150 Ma

Fig. 8. TheWhite Mountains P component(plutonic rocks), and reversed polarity Cr andnormal polarity Cn component (Moatvoleanics) poles compared with North American apparent polar wander paths and published Jurassic reference poles (poleand apparent polar wander path abbreviations are the sameas thosein Figure1). A meanMiddle Jurassic pole transferredfrom the Gondwanacontinents using the Klitgordand Schouten[1986] Africa-NorthAmerica reconstruction is shownby thetriangle. The insetshows the limited range of possiblelocations of the Gondwanapole in NorthAmerica coordinatesusing other published reconstructions (1, Bullardet al. [!965]; 2, LePichonet al. [1977];3, LePichonand Fox [1971];4, Lefortand Van der Voo[1981]; 5, Wissmanand Reset [1982]). Reinterpretationof the N2 andCorral Canyon magnetizationsas secondaryand post-tiltingsuggests different locations of thesepoles (shownby the arrows)and can resolvethe apparentdiscrepancy with the Middle Jurassic Moat volcanics Cr pole. High-latitudeNorth American apparent polarwander during an evenlonger interval in the Jurassicis suggestedby themean Early Jurassic pole (square)and Late J'urassiepole (pentagon)for Europetransferred to NorthAmerican coordinates using the reconstructionof Srivastava and Tapscott [1986] (see Table 4).

Cretaceous and Triassic palcomagneticresults tend to argue Americancoordinates according to reasonablywell established against post-Triassicregional tilt. reconstruction parameters. The Gondwana continents best Indirect evidence that would seem to argue againsta Middle represented by Middle Jurassic poles are Africa, (East) Jurassic age for the Cr (and/or Cn) componentpo!e is the Antarctica, and Australia whereas, for example, Irving and disagreementwith what are takento be the few reliable Middle Irving [1982] list just one Middle Jurassicpole for South Jurassic reference poles for North America in a recent PEP America and only a single pole for the entire Jurassicof India. apparentpolar wander analysis [May and Butler, 1986]. For Reconstructionof Gondwanaalso provides an intermediatetest exarnple, even excluding the Moat volcanics Cn component, of the consistencyof the palcomagneticdata prior to transfer the Cr componentpole falls 18.6ø of arc distance from the to the North Americanreference frame. For Europethere are lower latitude Corral Canyon pole of May et al. [1986]. We hardly any Middle Jurassicpoles regarded as reliable, however, attempted a test of our hypothesisthat the apparent polar recently publishedpalcomagnetic studies of Lower and Upper wanderpath for North Americaventured to high latitudesduring Jurassicrocks provide broad constraints on the Middle Jurassic the Middle Jurassic,as suggestedby the Moat C component pole position. magnetization,by transferringJurassic palcopoles from other Most of the availablelurassic poles from Africa would include continents. the Middle Jurassicaccording to quoted ages and their MIDDLE JURASSICPOLES lmOM GONDwANA uncertainties. Thus an averageJurassic pole calculatedby IN THE NORTH AMERICAN REFERENCE FRAME Besseand Courtillot[1988] for rocksranging in agefrom 150 to 209 Ma (67.5øN,254.4øE, A95 = 5.5ø forN = 6 studies)is The Gondwanacontinents and Europe are potentialsources of not significantlydifferent from a nominallyMiddle Jurassic Middle Jurassicpalcopoles which can be transferredto North pole (170 + 15 Ma) calculatedby Irving and Irving [1982] VANFOSSEN AND KENT: HIGH-LATITUDE PALEOMAGNETIC POLES 17•513

locatedat 62øN, 266øE(A95 = 10ø for N = 8 studies).In a intrusions [Schmidt, 1976b], and the Tasmaniandolerites subsetof acceptedAfrican poles thought to be reliableby [Schmidtand McDougall, 1977]. Becauseonly two site means Besseand Courtillot but more restrictedto the Middle Jurassic wereused to determinethe KangarooIsland pole, we adoptthe (163-187Ma; DNAG geologicaltime scale),three poles mean palcopoleof Grunow et al. [1987] at 51.0øN and 0.5øE, qualify:the Karoo dolerites (McElhinny and Jones [1965]; age calculatedusing just the New SouthWales intrusionsand the is150-165 Ma fromCox [1988]),the Zimbabwe Marangudzi- Tasman/andolerites data, and with a meanage of 172Ma. The MatekeHills pole (Gough et al. [1964]; age is 186 Ma from threereversed and three normal polarity site mean directions Cox [1988], and the Mauritania Hodh pole (Sichler et al. fromthe New SouthWales intrusions are antipodal,and the [1980];age is 187 Ma). Thesethree poles give a preferred meanage of therocks studied is 174.5Ma Thepole from the meanMiddle Jurassic African pole positionlocated at 65.8øN, Tasmaniandolerims (170.5 Ma) wascalculated by Schmidtand 257.1øE(A95 = 12.6ø),which is not significantlydifferent McDougallusing 21 westerlysite mean magnetization fromthe Irving and Irving or Besseand Courtillot estimates. directions(all of normalpolarity). A groupof 12 normal Forthe (East) AntarcticaMiddle Jurassicpole position,we polaritysite means with a moreeasterly trend were considered usethe mean palcopole calculated by Grunow et al. [1987] anomalousby theseauthors. basedon 15 publishedstudies of the Ferrar dolerites(54.8øN, As might be expected,the meanpoles from Africa, East 40.3øE;A95 = 3.9ø;Table 4), withK-Ar andAt-At agesranging Antarctica,and Australia with these continents in theirpresent primarilyfrom about 160 Ma to 180Ma [Elliotet al., 1985]. A positionsgrossly disagree (K = 6.5 for N = 3 continents). mean(north) pole calculatedusing results of just threeFerrar However,when Gondwana is reassembled[Norton and Sclater, studiesthat reported reversed magnetizations [B!undell and 1979],these Middle Jurassic poles agree very well with a mean Stevenson,1959; Becket al., 1979;Lovlie, 1979] is locatedat locatedat 64.9øN,260.3øE in Africancoordinates (K = 1242, 56.2øN,38.0øE (A95 = 9.5ø) andis not significantlydifferent A95 = 3.5ø, givingunit weightto eachof the threecontinents, thanthe mean of the remaining12 normal polarity poles Table4). If insteadthe Smith and Hallam [1970] Gondwana (54.4øN,40.8øE; A95 = 4.8ø). This antipodalitytest suggests reconstructionis used, the MiddleJurassic mean pole (64.2øN, thatthe Ferrar doleritesare, on the average,probably not 249.4øE,K = 299,A95 = 7.1ø) is lesswell-defined, although biasedby someunresolved contamination and should average not significantlydifferent from the resultobtained using the secularvariation because they have recorded at least one Nortonand Sclater rotation parmeters. Although we havenot polarityreversal. takeninto accountthe effectsof possiblestrain in the Benue TheIrving and Irving [1982]mean 170 4- 15 Ma palcopolefor trough [Pindell and Dewey, 1982; R. Hargraves,personal Australia(47 ø N, IøE) is basedon threestudies: the Kangaroo communication,1990], the excellent agreementof pole Island basalt [Schmidt, 1976a], the New South Wales positionsfollowing the reassemblyof Gondwana,and the

TABLE4. Paleomagneti½PolePositions From Gondwana (Africa. Australia, East Antarctica) and Europe

Pole African Coordinates North American Coordinates PoleName Sources Age N Latitude E LongitudeN K A95 N l.titude E Longitude N latitude E Longitude

O ondwana - Middle Jurassic Africa.mJ'ur 1-3 177 Ma 65.8 257.1 3 98 12.6 65.8 257.1 Australia.mJur 4,5 172 Ma 51.0 000.3 2 245 - 62.6 261.0 EastAnt.mJur 6 170 Ma 54.8 040.3 15 97 3.9 66.2 262.6

Mean 173Ma - -- 3 1242 3.5 64.9 260.3 75.6 109.1

Europe- Early Jurassic JurasMt. 7 Oxf.-Kimm. 77.7 148.4 24* 26 5.9 - -- Franconia 8 Oxf.-e.Kimm. 68.0 130.0 79** na (5.0) .... Poland 9 m.Call.-Oxford. 72.3 150.4 9* 58 (7.3) ....

Mean -159 Ma 72.7 141.6 3 193 8.9 73.9 131.4

Europe- Late Jurassic ParisBasin 10 Toarcian 74.7 113.9 61'* 22 (3.8) .... ParisBasin 10 Toarcian 67.6 092.8 39** 30 (4.1) - -- Normandy 11 Pliens-Toarc. 70.4 109.5 6* 143 5.6 ....

Mean ~192Ma 71.1 104.2 3 249 7.8 - - 75.0 087.5

PoleN latitude,E longitude, pole latitude and longitude ingeographic coordinates; N, number of studiesused; K, Fisherdispersion parameter; ha, datanot available;A95, radiusof 95% confidenceabout mean pole; (ct95), radiusof 95% confidenceabout mean directionused in calculationof given pole; African coordinates, pole latitude and longitude rotated to theAfrican reference frame using Norton and Sclater [1979];North American coordinates, pole latitude and longitude rotated tothe North American reference frame using Klitgord and Schouten [1986]for Gondwana(Africa): 66.97, 347.66, f2 = -74.57,and Srivastava and Tapscott [1986] for Europe:79.50, 151.92, f2 = -25.59. Sources:(1)Gough et al. [1964];(2) McElhinny and Jones [1965]; (3) Sichler et al. [1980];(4) Schmidt and McDougall [1977]; (5) Schmidt [1976b];(6) Grunow et al. [1987];(7) Johnson etal. [1984];(8) Heller [1977,1978]; (9)Kadzialko-Hofmokl andKruczyk [1987]; (10) Galbrunetal. [1988];(11) Fabre [1986]. * numberof sites. ** numberof samples. 17,514 VANFOSSEN AND KENT: HIGH-LATITUDE PALEOMAGNETIC POLES positive outcomeof the antipodalitytests that were possible meandirection similar to thatreported for the "N2"intrusive on the East Antarctica and Australian results, stand as first- rocksin commoncoordinates. This similarity and the lack of order evidencefor the generalreliability of the Gondwana reportedfield tests led Witte and Kent to suggest•at boththe reconstructionand Middle Jurassicpoles. Newark Basin B componentoverprint and N2 intrusive The mean Middle Jurassicpole for Gondwanawas transferred magnetizationswere secondary in origin,being perhaps related to North Americancoordinates according to six publishedsets to theproposed Middle Jurassic (-175 Ma) hydrothermalevent of reconstruction parameters for Africa to North America thatreset radiometric ages of manyigneous intrusions in the [K!itgord and Schouten, 1986; Wissman and Reser, 1982; Newark Supergroupbasins [Sutter, 1988]. In thein si•u Lefort and Van der Voo, 1981; LePichon et al., 1977; LePichon referenceframe, consideredappropriate on the basisof a and Fox, 1971; Bullard et al., 1965]. Variation in the location negativefold test [Witte et al., 1989], the NewarkBasin B of the Gondwanapole subjectto these different reconstruction componentoverprint pole of Witte and Kent [1989] fallsat parameters gives a sense of the uncertainty. The maximum 72.7øN, 89.8øE(A95 = 3.4ø) and is indistinguishablefrom the separationbetween any two of the six possiblelocations of the Moat volcanicsCr pole at 95% confidence(Figure 8). transferredpole (inset Figure 8) occurswhen one comparesthe The Corral Canyonpole from Jurassicvolcanic rocksin the positionsusing LePichon et al. [1977] and Wissmanand Reset PatagoniaMountains, southern Arizona [May et al., 1986] [1982] reconstruction parameters,however, this maximum falls at 61.8øN,116.0øE, nearly 19ø from the Cr component separationamounts to only 5.8 ø. We regard as representative pole. The Corral pole position was calculatedusing tilt. the location of the transferred Gondwana pole (75.6øN, corrected magnetization directions, but the decreasein 109.1øE) using the recent Klitgord and Schouten [1986] directional scatter upon restoration of palcohorizontalis reconstruction. significant at only 75% confidence. If calculated withouttilt correction,the CorralCanyon pole position (61.2øN, 172.6oE) ASSESSMENT OF MJDDLE JURASSIC falls nearyounger (Cretaceous) North Americanpoles (Figure NORT• AM•VaC• POL•S 8), suggestingthe possibility that the Corral Canyontuffs We can now proceed to test the hypothesisthat the North havebeen remagnetized. This hypothesisis not precludedby American apparentpolar wanderpath venturedto high latitude thefold teststatistics nor by thereported Rb-Sr whole rock age during the Jurassic. The hypothesis that the Moat Cr dating, a method not very sensitiveto thermal resetting. Moreover, remagnetization may be just one expressionof componentpole (presumedage = 166 Ma) and the transferred various episodesof tectonic activity to which the southern Gondwana Middle Jurassicpole (mean age = 173 Ma) share a commonmean can not be rejectedat the 95% confidencelevel Basin and Range has been subjectedsince the Late Cretaceous. In this regard,Hagstrum and Sawyer [1989] found evidencefor (Figure 8). Thesetwo poleshowever are distinctfrom the lower latitude Newark IgneousTrend "N2" pole (-179 Ma) and in -42 ø of post-Cretaceousclockwise rotation in a palcomagnetic study of Late Cretaceousvolcanic rocks in the Silver Bell particularthe Corral Canyonpole (172 Ma) which have been Mountains of southern Arizona. May and Butler [1987] acceptedas reliableMiddle Jurassicreference poles for North America in a recent analysis [May and Butler, 1986]. As attributed what they regarded as enigmatic palcomagnetic described above, the general reliability of the mean Middle results from Middle Jurassicvolcanic rocks in the Canelo Hills, JurassicGondwana pole is indicatedby its internalconsistency about 20 km northeastof the PatagoniaMountains, to Jurassic and moreover, among the six possible positions of the nondipolefield behavior. However, the streakedpattern of site Gondwanapole in North Americancoordinates, most fall at mean directions could also be due to some combination of latitudes of nominally 75øN, 10ø-15ø more northerly than the remagnetizationand tectonic rotations. N2 and Corral Canyon pole positions. We thereforeinterpret the agreementbetween the transferredGondwana pole and the DISCUSSION AND CONCLUSIONS Moat Cr componentpole as strongsupporting evidence for the migrationof the North Americanapparent polar wander path to To summarize briefly our findings, a high unblocking high latitudesin the Middle Jurassic,similar to that suggested temperaturemagnetization of reversedpolarity from the Moat by Irving and Irving [1982]. Implicit in this interpretationis volcanicsin New Hampshireyields what we regardas a reliable that the N2 and Corral Canyon referencepoles are somehow Middle Jurassicpole from the White MountainsMagma Series. anomalous,and this naturally requires discussion. Although This Cr componentpole falls at 78.7øN,90.3øE, a high-latitude rapid apparentpolar wanderof about 1ø-2ø/m.y.could be positionsupported by the NewarkBasin remagnefization (B) invokedto explainthe differencesbetween the Cr pole and the pole of Witte and Kent [1989]. Thesepoles suggest an interval N2 and Corral Canyonpole locations,we believethat a more of highlatitude apparent polar wander from -175 Ma (Newark plausible explanationis that the N2 and Corral Canyon BasinB pole)to -166 Ma (Moatvolcanics Cr pole),a periodof magnetizationsare in fact secondary,were acquiredafter about9 m.y. spanningthe Bathonianand part of the Callovian tilting, and shouldtherefore be consideredin the geographic ages(DNAG time scale). The discrepantCorral Canyon pole rather than tilt-corrected reference frame. which falls in this interval(172 Ma), regardedas reliableby The N2 pole was calculatedby Smithand Noltimier [1979] May and Butler [1986], can be reinterpretedas havingbeen usingpalcomagnetic data from a numberof publishedstudies of remagnetizedin the Cretaceous;the NewarkTrend N2 igneous intrusive rocks associated with Mesozoic rifting in eastern pole [Smith and Noltimier, 1979] is most probablyalso a North America. This pole falls at 65.3øN, 103.2øE,which is secondarymagnetization pole which, as the work of Witteand nearly 14o of arc from, and 10ø moresoutherly than, the Moat Kent [ 1989]suggests, may have been acquired in situyielding a volcanicsCr pole (Figure8). Recently,Witte and Kent [1989] high-latitudepole position similar to theNewark Basin B pole. have found that Catalan and Norian sedimentaryrocks from the A mean pole (173 Ma) transferred from Gondwana Newark Basin contain a pervasive normal polarity independentlypoints to a high-latitudeMiddle Jurassic Nor• magnetizationoverprint (Newark Basin B component)with a Americanpole position, now documented directly by theMoat VANFOSSEN AND KENT: HIGH-LATITUDE PALEO••C POI.•S 17.51$

volcanicsresults and by the NewarkBasin B pole eventhough Billings,M.P., Thepetrology ofthe North Conway quadrangle in the theserepresent secondary magnetizations. WhiteMountains ofNew Hampshire, Proc. Am. Acad. Arts Sci., 63, The argumentsfor a high-latitude Middle $urassicpole 67-137, 1928. Billings,M.P., TheGeology of New Hampshire, part II,- Bedrock positionpresented in this study will requirereconsideration of Geology,203 pp.,New Hampshire Planning Development theNorth American apparentpolar wanderpath for the entire CommissionMining Resource Survey, Concord, 1956. Jurassic.Indications already are that high-latitudeapparent Blundell,D. J.,and P. J. Stevenson,Palcomagnetism of some dolerite polarwander may have extended beyond the nominal age range intrusionsfrom the Theron Mountains and Whichaway Nunataks, of 166-175Ma documentedhere. Recentlyavailable Early Antarctica,Nature, 284, 1860, 1959. Bullard,E. C., J. E. Everett,and A. G. Smith,The fit of thecontinents Jurassic(Pliensbachian-Toarcian-190 Ms) polesfrom Europe aroundthe Ariantie,Symposium on ContinentalDrift, Philos. fall at latitudesof about 72ø-78ø in North American coordinates Trans.R. Soc.London, Set. A, 258, 41-51, 1965. accordingto available plate reconstructions(Table 4) and Chapman,C. A., A comparisonof the Maine coastal complexes and implya rapidapparent polar wander shift from the Sinemurian- themagmatie central complexes of New Hampshire, in Studies of Pliensbachian(-195 Ms) North American"cusp" poles located AppalachianGeology: Northern and Maritime, edited by E. Zenet al., pp. 385-398,Interscience, New York, 1968. at about60øN, 60øE(Figure 8). Basedon palcomagneticresults Cox,K. G., TheKaro<> Province, in ContinentalFlood Basalts, edited fromOxfordian-Kimmeridgian limestones with a positive fold by J. D. Macdougall,pp. 239-271,Kluwer Academic, Boston, test,Johnson et al. [1984] haveconfirmed a high latitudepole Mass., 1988. positionfor the Late Jurassicof Europe,in agreementwith Doherty,J. T., andJ. B. Lyons,Mesozoic erosion rates in northern New England,Geol. $oc. Am. Bull., 92, 16-20,1980. severalother published Oxfordian-Kimmeridgianpoles from Eby,G. N., andJ. W. Creasy,Strontium and lead isotope geology of stableEurope which retain high latitudein the North American theJurassic White Mountain batholith, New Hampshire, Geol. $oc. referenceframe (Table 4 and Figure8). The disparityof these Am. Abstr.Programs, 15, 188, 1983. transferredEuropean poles with some latest Kimmeridgianto Elliot,D. H., R. J. Fleck,and J. F. Sutter,Potassium-argon age Tithonian North American reference poles (e.g., Glance determinationsof Ferrar Group rocks, centralTransantarctic Mountains,in Geologyof theCentral Transantarctic Mountains, conglomeratepole, 151 Ma from Kluth et al. [1982]; and lower Antarct.Res. Set., vol. 36, editedby M.D. Turnerand J. F. andupper Morrison Formation poles from Steinerand Helsley, Splettstoesser,pp. 197-224, AGU, Washington, D.C., 1985. [1975]), regarded as reliable by May and Butler [1986], Fabre,A., Le tempsdans la constructiondes courbesde d6rive similarlyraises questions,beyond the scope of this paper, apparentedu p61epa16omagn6tique: Application • VEuropedu Permienau Jurassique,Ph.D. dissertation,Univ. Bretagne concerningeither rapid apparentpolar wanderor problemsin Occidentale,Brest, France, 1986. generalwith the Late lurassicreference poles. Fisher,R. A., Dispersionon a sphere,Proc. R. $oc.London, Set. A, Clearly, additionalpalcomagnetic studies of rocks from North 217, 295-305, 1953. Americaare needed to testthe evidenceof a high latitudeMiddle Fitzgerald,J.P., and J. W. Cressy,The Moat Voleanies, Moat Range, Iurassiepole presentedin this study. Furthermore, such NH: An intra-calderasetting, Geol. $oc. Am. Abstr. Programs, 20, 18, 1988. empiricaltesting of the North Americanapparent polar wander Foland,K. A., andH. Faul, Agesof theWhite Mountain intmsives- pathmay be requiredfor mostof the •urassic. With regardto New Hampshire,Vermont and Maine, USA, Am. J. $ci.,277, 888- PEPanalysis, May and Butler[1986] have divided the original 9O4, 1977. 100-m.y.-long •urassic-Cretaceous small-circle track of Foland,K. A., A. W. Quinn,and B. I. Giletti, K-At andRb-Sr Jurassic andCretaceous ages for intrusivesof theWhite Mountain Magma Gordonet al. [1984] into two small-circlesegments, each Series,Am. J. $ci., 270, 321-330, 1971. about50 m.y. long. A high-latitudeMiddle lurassiepole Galbrun,B., J. Gabilly,and L. Rasplus,Magnetostratigraphy of the position indicated by the Moat volcanics data takes this Toarcianstratotype sections at Thouarsand Airvault (Deux-S6vres, segmentationprocess one stepfurther, implying that at least France),Earth Planet.$ci. 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