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-DohertyGeological Observatory and Departmentof GeologicalSciences, Columbia University, Palisades, iVew York A palcomagneticstudy of CretaceousWhite Mountainsplutonic complexes in New Hampshireand Vermont yields high unblockingtemperature, dual polarity magnetizationsin differenttypes of igneousrocks. The resultingpole position for threeplutons (71.9 ø N, 187.4ø E, A95 = 6.9ø, age= 122.5Ma) agreeswith pre- viously publishedmid-Cretaceous poles for North America, which togethergive a mid-Cretaceousstand- still referencepole slightlyrevised from Globermanand Irving [1988]at 71.2ø N, 194.1ø E (A95 = 3.7ø,N = 5 studies).We argueon the basisof the wide geographicdistribution of thesestudies, the varietyin tec- tonic settingsand rock types,positive reversal tests, and an overall reversalpattern consistent with geo- magneticpolarity time scales,that this mean pole representsthe North Americanmid-Cretaceous reference field for nominally 36 m.y. (124 to 88 Me). The standstillpole limits to within +4 ø, the motion of the North Americanplate relativeto the Earth'sspin axis. During the samemid-Cretaceous interval, the New Englandhotspot track (124 Ma MonteregianHills, 122.5-Ma CretaceousWhite Mountains,and 103- to 84-Me New Englandseamounts) requires 11ø't:4ø of nonh-polewardmotion of North America,in direct conflict with the palcomagneticstandstill. A similar (-13 ø) discrepancyis independentlydemonstrated betweenthe spin axis and the Tristan da Cunha hotspottrack on the African plate during the mid- Cretaceousinterval. •he hotspot/spinaxis discrepanciesended by -90 Ma when it is shownthat both Atlantic hotspotsagree with North Americanand African dipole palcolatitudesand present-daylocations. Nondipolefields are an unlikelyexplanation of the uniformmotion of thesetwo widely separatedhotspots with respectto the spinaxis, leaving as possibleinterpretations true polar wanderand large-scale(but dif- ferential) mantle motion. The southerlymotion of the mid-CretaceousLouisville hotspotrelative to the spinaxis is ostensivelyat oddswith what wouldbe predictedunder the true polar wanderinterpretation and points to differential mantle kinematics. The motions of the three widely separatedmid-Cretaceous hotspotswith respectto the spin axis may be relatedto the recentlyproposed increase in global oceanic lithosphereproduction rates which gave rise to the mid-Cretaceous"superplume." INTRODUCHON activity [Larson, 1991; 7•rduno et al., 1991], and fast plate motion in the mantle hotspot framework [Morgan, 1972, It has been long recognizedthat Cretaceouspalcomagnetic 1983; O'Connor and Duncan, 1990]. It is less clear, however, polesfor North Americatended to fall relativelyclose together how to characterizethe mid-Cretaceousin terms of geometry, in the general area of the Bering Strait [e,g., Irving, 1964; amount,and timing of relative motionbetween the hotspotand McElhinny, 1973; Mankinen, 1978]. This tendencywas most palcomagneticreference frames. Previousstudies that include recentlyconfirmed by Globermanand Irving [1988] who calcu- the mid-Cretaceousinterval are primarily basedon global syn- lated a mid-Cretaceousreference pole at 71øN, 196øE(A95 = thesesof palcomagneticand relative plate reconstructiondata 4.9ø, k = 352) on the basisof four publishedstudies on rocks sets and comparisons with models of plate motion in the rangingin age from circa 88 Ma to 124 Ma. The mean pole hotspot reference frame [e.g., Livermore, et al., 1984; position may therefore representthe palcomagneticfield Andrews, 1985; Gordon and Livermore, 1987; Besse and relativeto cratonicNorth Americafor some36 million yearsof Courtillot, 1991]. The complexityof theseanalyses may con- the Cretaceous,a "standstill"in apparentpolar wanderduring tributeto the currentlack of consensuson the subjectand, fur- which there was no discernibleplate motion relative to the thermore,Livermore et al. [1984] suggestedthat the pre late geographic axis according to the geocentric axial dipole Cretaceousmotion between the hotspotand palcomagneticref- hypothesis. Bracketing the mid-Cretaceousstandstill are Late erenceframes might be betterattributed to the errorsin hotspot Jurassic/EarlyCretaceous poles [May and Butler, 1986; Van track identificationsand/or palcomagnetic data. Fossen and Kent, 1992] and Late Cretaceouspoles [Diehi, In this study we examine North American mid-Cretaceous 1991] which requirerelatively fast North Americanapparent apparentpolar wander througha palcomagneticstudy of the polar wander (~lø/m.y.). youngest group of White Mountain Series plutons in Nee The mid-Cretaceousis itself an interestinginterval in the England(mean age = 122.5 Ma). This independentdetermina- annalsof global change. Aside from being dominatedby a tion provides an additional test of overall consistencyof the ~30-m.y. intervalof uniformnormal geomagnetic polarity (the North American mid-Cretaceouspalcomagnetic poles whose CretaceousNormal Polarity Superchron,corresponding to the reliability can be further assessedby comparingreverse and CretaceousQuiet zonesin the oceans),the mid-Cretaceoushas normal polarity magnetizationsamong the new 'andpublished beencharacterized as a periodof fastseafloor spreading [Larson pole positiondata for a regional-scalereversal test. As we will and Pitman, 1972; Larson, 1991], increasedmantle plume show,the North Americanrecord of the mid-Cretaceouspalco- magneticfield 'andhotspot activity alsoprovides a more direct Copyfight1992 by the AmericanGeophysical Union. experimentto testvarious ideas on the natureof the palcomag- Paper number 9ZIB01466. netic and hotspotreference frames. 0148-0227/92/92JB-01466505.00 19,651 19,652 VAN FOSSENAND F•NT: MtD-•ACEOUS PALEOMAGNETICFIEIX) TIlE CRETACEOUSWI•IITE Mou•rr•s MAG•A SE•S 4 Ma). Andesite and basalt flows within the Ossippee Mountainsring dike have been sampledat six sites (minimum Geological Setting age of 121 q- 4 Ma based on K-Ar biotite age of kntruding Conway granite). These flows were tilted throughdifferential The Cretaceous alkaline intrusions in southern New subsidenceabove the intruding magma chamber [Kingsley, Hampshireand southeasternVermont are the youngestgroup of 1931; Billings, 1956]. Gabbro and a basalt dike at Mount igneous rocks associatedwith the White Mountains Magma Tripyramid have been sampledat three sites. Unfortunately, Series (Figure 1). Their emplacementfollowed an initial late only a commercialradiometric date appearingon the map of Triassic phase of 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 althoughHubacher and Foland [1991] do not list Tripyramidas granites, monzonites, gabbros, basalts and andesitesoccur one of the Cretaceous intrusions redated, we assume that it falls predominantlyin five major plutonic and ring-dike complexes within the distinct interval of Cretaceousmagmatism centered that intrude lower to middle Paleozoic metamorphic and at 122.5 Ma. igneous formations. These complexes are (north to south, Thermal and AF vector demagnetizationprofiles of White Figure 1) Mount Tripyramid, Ossippee Mountains, Mounta'mssamples are straightforwardwith the exceptionof Merrymeeting,Mount Ascutney,and Mount Pawtuckaway. those from the Merrymeeting gabbro. Samples from K/fi biotite datingof numerousCretaceous White Mountains Tripyramid, Pawtuckaway,and Ascutneycontain a single com- plutons and stocks, including the five major p!utonic com- ponent of magnetizationwith high unblocking temperature plexes sampled here for palcomagnetism,suggests an age (-570 ø C) and high coercivity(Figures 2a, 2b, and 2c). This rangeof about118 Ma to 125 Ma [Foland eta!., 1971; Foland magnetization is of reversed polarity (declination =154 ø, and Faul, 1977]. Recently, a number of CretaceousWhite inclination =-56ø) in Mount Ascutneyand Mount Tripyramid Mountainsplutons have been restudied using the 40Ar/39Ar gabbros,and is of normal polarity (declination=343 ø, inclina- biotitemethod by Hubacherand Foland [ 1991]. They suggesta tion =+59ø) in Mount Pawtuckawaymonzonite. shorterperiod of magmatic activity, perhapsonly 3 m.y. in duration,centered on 122.5 Ma. Thesedata, togetherwith pub- lishedzircon fissiontrack (mean= 113 +_10 Ma [Doherty and Lyons, 1980]) and apatitefission track ages(mean = 103 :t: 15 White Mountains Magma Series Ma [Zimmermanet al., 1975;Doherty and Lyons,1980]), sug- gest that theseplutons were emplacedat shallowdepths (-3.5 Cretaceous.:......• Jurassic + Triassic km) and were subject to monotonic cooling since crystalliza- tion. Palcomagnetism '... In the pioneering study of the White Mountains Magma 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 stud- ies of the Middle Jurassicand Triassic plutonshave revealed a sensiblerelationship between inferred magnetizationage and radiometricage [Van Fossenand Kent, 1990; Wu and Van der Voo, 1988]. Our experimentalplan for the Cretaceousplutons was to apply fiartheralternating