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Hotspots, Mantle Plumes, Flood Basalts, and True Polar Wander

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Hotspots, Mantle Plumes, Flood Basalts, and True Polar Wander HOTSPOTS, MANTLE PLUMES, FLOOD BASALTS, AND TRUE POLAR WANDER R. A. Duncan M. A. Richards Collegeof Oceanography Departmentof Geology OregonState University, Corvallis Universityof California,Berkeley Abstract. Persistent,long-lived, stationarysites of lithospherethe plume head flattens and melts by excessivemantle melting are called hotspots. Hotspots decompression,producing enormous quantifies of magma leave volcanictrails on lithosphericplates passing across which erupt in a short period. These are flood basalt them. The global constellationof fixed hotspotsthus events that have occurred on continents and in ocean formsa convenientframe of referencefor plate motions, basinsand that signal the beginningof major hotspot throughthe orientationsand age distributionsof volcanic tracks. The plume-supportedhotspot reference frame is trailsleft by thesemelting anomalies. Hotspots appear to fixed in the steadystate convective flow of the mantleand be maintainedby whole-mantleconvection, in the form of is independentof the core-generated(axial dipole) upwardflow throughnarrow plumes. Evidencesuggests paleomagneticreference frame. Comparisonof plate that plumesare deflectedlittle by horizontalflow of the motions measured in the two frames reveals small but uppermantle. Mantle plumesare largelythermal features systematicdifferences that indicatewhole-mantle motion and arise from a thermalboundary layer, most likely the relativeto theEarth's spin axis. This is termedtrue polar mantle layer just above the core-mantle boundary. wanderand has amounted tosome 12 ø sinceearly Tertiary Experimentsand theoryshow that gravitationalinstability time. The directionand magnitudeof true polar wander drivesflow, beginningwith the formationof diapirs. Such havevaried sporadically through the Mesozoic,probably a diapir will grow as it rises, fed by flow throughthe in responseto majorchanges in platemotions (particularly trailing conduitand entrainmentof surroundingmantle. subductionzone location) that change the planet's The structurethus develops a large, sphericalplume head moments of inertia. and a long, narrow tail. On arrival at the base of the 1. INTRODUCTION their geometry and age distributions these volcanic provincesare thoughtto resultfrom focusedupper mantle The plate tectonicsmodel has been largely successful in zonesof meltingcalled hotspots [Wilson, 1963, 1965] that explainingthe distributionof basalticvolcanism on our remainstationary as theEarth's outer shell of lithospheric planet. Mid-ocean ridge basalts are produced by plates move acrossthem. The proposedlocations of decompressionmelting of passively upwelling upper presenthotspot volcanism are shownin Figure1. mantle(asthenosphere) at the edgesof separatingplates, Morgan [1971, 1972] proposedthat hotspotsare while subductionof oceanic lithospherecauses upper maintainedby unusuallywarm materialrising from the mantlehydration and melting,yielding volcanic arc basalts lower mantle through narrow conduitsthat he called along collisionalplate boundaries. Such plate margin mantleplumes. He furtherspeculated that these plumes environmentsaccount for the vast majorityof all magma comprisedthe upwardflow of a long-lived,stable pattern production. A volumetricallyminor but significantclass of whole-mantleconvection (Figure 2). Onceestablished, of basalticvolcanism occurs within plates or crossesplate theseconduits for heat and materialtransport from the boundaries,and it is characterizedby linear chainsof lowermantle do not, Morganproposed, change position volcanoes(hotspot lineaments) that grow older in the relativeto one another. Plumesprobably initiate in the directionsof plate motion. Classic examplesof this seismicallydefined thermal layer at theboundary between volcanicphenomenon are the HawaiianIslands (and other the core and mantle. Convection in the lower mantle is parallel island chains in the Pacific Ocean), the likely to be very sluggish[Gurnis and Davies, 1986] so Yellowstone-SnakeRiver Plain province,and the volcanic plumes rising through this region should not be platformand ridge system centered on Iceland. Becauseof "deflected,"or drift with respectto one another. Convec- Copyright1991 by the AmericanGeophysical Union. Reviewsof Geophysics,29, 1 / February1991 pages31-50 8755-1209/91/90RG-02372 $05.00 Papernumber 90RG02372 .31. 32 ß Duncan and Richards: HOTSPOT VOLCANISM 29, 1 / REVIEWSOF GEOPHYSICS Erebus Figure 1. Worldwidesystem of hotspots.Criteria defining are concentratedin zonesof high geoid residuals(contours in hotspotsare excess volcanism and a spatialprogression in the age metersare from Croughand Jurdy [ 1980]). The geoidanomalies of volcanismconsistent with localplate motion. Magmaproduc- alsocorrelate with regions of slowW in thelower mantle tionrates vary both from one hotspot to anotherand along indi- [Morelli and Dziewonski, 1987], suggestingthat hotspotsare vidualhotspot tracks. Hotspots are not randomly distributed, but connectedwith mantle-wideupward convection. tionrates in the uppermantle are muchlarger, of theorder of platevelocities (10-100 mm/yr),so the horizontal upper flood basalts mantle flow might bend plumeson shortertime scales [Whiteheadand Luther, 1975]. If mantleplumes do indeedform a long-lived,stable patternof convectionof heatand material from the lower ho ' • plate to the uppermantle, then the geographicorientations and tra'i•ts ot ductton' age distributionsof volcanicchains related to hotspots offer a very simpleand directrecord of the directionand velocityof lithosphericplate motionsover the last 100 m.y.(million years) or more. Ratesand directions of plate separationderived from the alternatingnormal and reversely magnetizedstripes of seafloor parallel to spreadingridges measurepast large-scalehorizontal Figure 2. Cartoonof whole-mantleconvection. Piecesof the movementsof piecesof theEarth's surface relative to their rigid outershell of the Earth(lithosphere) separate at spreading ridgesand collideat subductionzones, coupled with horizontal neighbors.This is calleda relativemotion reference frame flow of the uppermantle. The deepmantle circulates through becausethe spreadingridges from which motion is thermalplumes of gravitationallyunstable (warmer, less dense) measuredare themselvesmoving over the mantle,albeit materialthat arisefrom nearthe core-mantleboundary. Flood moreslowly than plates. A thirdreference frame for plate basaltsform as the leading"head" of a new plumemelts at the motionsis providedby theEarth's magnetic field, which is baseof thelithosphere. Hotspot trails form as plates move across assumedto have the shape(on average)of a dipolefield long-livedplume "tails." Convectionin the coreproduces the alignedalong the spinaxis. Rocksthat are magnetizedin Earth'smagnetic field. 29, 1/REVIEWSOF GEOPHYSICS Duncan and Richards: HOTSPOT VOLCANISM ß 33 this field acquirea magneticinclination (angle of the rock Morgan [1972] first notedthe congruenceof the major magneticvector from horizontal)unique to their latitude. island and seamountchains on the largestand fastest- Thus any translatitudinal(north-south) motion of litho- movingPacific plate (Hawaiian-Emperor,Tuamotu-Line, sphericplates can be determinedfrom the paleomagnetic and Austral-Gilbert-Marshall island and seamount referenceframe; east-west motions are not foundby this lineaments).The geometryof thesevolcanic traces can be method. Comparisonof plate motionsdeduced from the reasonablywell matched by rigid plate motion over hotspotand paleomagneticreference frames indicatesa hotspotsfixed at the presentlocations of Hawaii, Easter smallshift of the wholemantle with respectto the Earth's Island,and MacdonaldSeamount, respectively. Subse- spinaxis since early Tertiary time (55-65 Ma). quentstudies, incorporating the distributionof the ageof In this paper we review the evidencefor interhotspot volcanismas well as the geometryof the traces[Clague motion and examine the questionof whether hotspot and Jarrard, 1973; Jarrard and Clague, 1977; Duncan lineamentsconstitute a usefulframe of referencefor plate and McDougall, 1976; McDougall and Duncan, 1980; motions. This issue also pertainsto the geometryand Duncan and Clague, 1985; Lonsdale, 1988], have dynamicsof convectiveflow in the mantle,which in turn confirmedthe fixedpositions of Pacifichotspots over the constrainestimates of the viscositystructure of the mantle. past65 m.y. Morgan [1981] andDuncan [1981] showed Many hotspotsappear to have begun with massive that a similarcase can be made for rigid motionof the outpouringsof basalticmagmas over broadregions, both Africanplate over a fixed set of hotspotsunderlying the on continents and in ocean basins. These volcanic central and South Atlantic and western Indian ocean basins provincesare calledflood basaltsand probablymark the since120 Ma (Figure3). arrivaland initial meltingof the "heads"of mantleplumes Minster et al. [1974] and Minster and Jordan [1978] at the baseof the lithosphere.Subsequent volcanism over foundno detectableinterhotspot movement for the past the plume conduits(or "mils"), establishedby the initial 5-10 m.y., on the basisof a best fitting global set of plume upwelling,are the familiar tracksthat connectflood relativeplate motionstied to Pacific hotspots. Early basaltsto presentlyactive hotspots. comparisonsof Atlantic with Indian and Pacific Ocean hotspots,on the other hand,reported relative motionsof hotspotsup to 10-20 mm/yrduring Tertiary time [Burkeet 2. ARE HOTSPOTS STATIONARY? al., 1973; Molnar and Atwater, 1973; Molnar and Francheteau,1975].
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