InternationalGeologyReview,Vol.45,2003,p.575–595. Copyright©2003byV.H.Winston&Son,Inc.Allrightsreserved. HowLaramide-AgeHydrationofNorthAmericanLithosphere bytheFarallonSlabControlledSubsequentActivity intheWesternUnitedStates EUGENEHUMPHREYS,1 DepartmentofGeologicalSciences,UniversityofOregon,Eugene,Oregon97403 ERINHESSLER,2 DepartmentofGeologicalSciences,UniversityofOregon,Eugene,Oregon97403 KENNETHDUEKER, DepartmentofGeologyandGeophysics,UniversityofWyoming,Laramie,Wyoming82071 G.LANGFARMER, DepartmentofGeologicalSciences,UniversityofColorado,Boulder,Colorado80309 ERICERSLEV, DepartmentofEarthResources,ColoradoStateUniversity,FortCollins,Colorado80523 ANDTANYAATWATER DepartmentofGeologicalSciences,UniversityofCalifornia,SantaBarbara,California93106 Abstract StartingwiththeLaramideorogenyandcontinuingthroughtheCenozoic,theU.S.Cordilleran orogenisunusualforitswidth,natureofuplift,andstyleoftectonicandmagmaticactivity.We presentteleseismictomographyevidenceforathicknessofmodifiedNorthAmericalithosphere 200kmbeneathColoradoand>100kmbeneathNewMexico.Existingexplanationsforupliftor magmatismcannotaccommodatelithospherethisthick.Imagedmantlestructureislowinseismic velocityroughlybeneaththeRockyMountainsofColoradoandNewMexico,andhighinvelocityto theeastandwest,beneaththetectonicallyintactGreatPlainsandColoradoPlateau.Structureinter- naltothelow-velocityvolumehasaNEgrainsuggestiveofinfluencebyinheritedPrecambrian sutures.Weconcludethatthehigh-velocityuppermantleisPrecambrianlithosphere,andthelow- velocityvolumeispartiallymoltenPrecambrianNorthAmericamantle. Wesuggest,asothershave,thattheFarallonslabwasincontactwiththelithospherebeneath mostofthewesternU.S.duringtheLaramideorogeny.Wefurthersuggestthatslabde-watering undertheincreasinglycoolconditionsofslabcontactwithNorthAmericahydratedthebaseofthe continentallithosphere,causingasteadyregionalupliftofthewesternU.S.duringtheLaramide orogeny.Imagedlow-velocityuppermantleisattributedtohydration-inducedlithosphericmelting beneathmuchofthesouthernRockyMountains.Laramide-agemagmaticascentheatedandweak- enedthelithosphere,whichinturnallowedhorizontalshorteningtooccurinthemantlebeneaththe regionofLaramidethrustinginthesouthernRockyMountains.SubsequentFarallonslabremoval resultedinadditionalupliftthroughunloading.Italsotriggeredvigorousmagmatism,especially whereasthenospheremadecontactwiththehydratedandrelativelythinandfertilelithosphereof whatnowistheBasinandRange.Thismantlenowisdry,depletedofbasalticcomponents,hot, buoyant,andweak. 1Correspondingauthor;email:[email protected] 2Presentaddress:IllinoisStateWaterDistrict,Champaign,IL. 0020-6814/03/671/575-21$10.00 575 576 HUMPHREYSETAL. Introduction andmeltingbeneaththeColoradoRockyMoun- tains.Meltascentadvectivelyheatedandweakened ROCKYMOUNTAINTECTONISM,widespreadTertiary thelithosphere,allowingcontractionalfailure volcanism,andupliftofthewesternUnitedStates beneaththeRockyMountainsandcreationofseis- areimportantgeologiceventsoccurringwithinthe micallyslowvelocitiesbeneaththeColoradoRocky interiorofNorthAmericawithnoobviouscause.As Mountainstodepthsof 200km.Post-Laramide such,placingtheseactivitiesinaplatetectoniccon- volcanismintheRockyMountainsandBasinand textappealstounusualbehavior,andtheircauses Rangeisattributedtoslabremovalandtheresulting remaincontroversial.The~70–45MaLaramide asthenosphericcontactwiththebaseofthehydrated orogenyiscentraltodiscussionsofthisactivity.This lithosphere.Theexceptionallyvigorousmagmatism orogenyisattributed,insomefashion,torapidsub- intheBasinandRangeisthoughttobearesultofits ductionoftheFarallonslabbeneathNorthAmerica. relativelythinandfertilelithosphere.Buoyancyin ConeyandReynolds(1977),notingacessationof magmaticallyactiveareasnowissuppliedbyther- SierraNevadaarcmagmatismandaneast-migrating malexpansionandbasaltdepletionofthemantle. frontofmagmaticinitiationimmediatelypriortoand duringtheLaramide,suggestFarallonslabflatten- Regionallithosphericsetting ingbeneaththewesternU.S.Slab-flatteningmodels aremadeattractivebytheoccurrenceofsimilar Figure1Aisamapofuppermantleseismic behaviorinareaswheretheNazca“flatslab”sub- velocitystructureforthewesternUnitedStatesand ductsbeneathSouthAmerica(CahillandIsacks, nearbyregions.Theeasternpartofourstudyarea 1992),andbytheirabilitytoassociateRockyMoun- liesoverthewesternmarginofahigh-velocityman- taincontraction,westernU.S.uplift,andtheevolv- tle“cratonicroot”(Jordan,1979)thatextends ingpatternofmagmatismtoplatetectonic deeplybeneaththeArchean(incentralCanada)and processes.Inparticular,volcanismisattributedto Paleozoic(inthenorth-centralU.S.)crust(Grand, propagationofsubduction-relatedarc-likevolcan- 1994;vanderLeeandNolet,1997).Incontrast, ismassociatedwithslabflatteningfollowedbyroll mostofthewesternU.S.overliesthenorthernextent back(ConeyandReynolds,1977).Uplift,which ofalarge,low-velocityvolumethatlooselycorre- cannotbeentirelysupportedbycrustalthickening lateswiththeEastPacificRise(Grand,1994).The (Sheehanetal.,1995;Spencer,1996),isattributed contrastbetweenthehigh-velocitycratonicmantle tomechanicalthinningofthelithospherebythe andthelow-velocitywesternU.S.mantleisamong Farallonslab(Humphreys,1995;Spencer,1996). thegreatesttobefound.Yetthe100–200kmwave- Inthispaper,wepresentevidencethatlitho- lengthmantlestructureimagedbeneaththewestern spherebeneathmuchoftheColoradoRockyMoun- U.S.(Fig.1A)isveryheterogeneous,withstrong tainsis200kmthickandbeneaththeNewMexico localvelocitycontraststhatareasgreatasthatseen RockyMountainsis>100kmthick,makingarc-like acrossthecontinentasawhole(Humphreysand magmatismandlithosphericthinningunlikely Dueker,1994a). explanationsforvolcanismanduplift.Thefactthat Ourseismicinvestigationspansthetransition noconvincingexplanationhasemergedforthe betweenthewesternU.S.uppermantleandthecra- Laramideorogenyandbroadupliftofthewestern tonicuppermantle.Thistransitionisnotsimple U.S.suggeststheactivityofimportantprocesses (LeeandGrand,1996;Henstocketal.,1998;van that haveremainedoutsidethedomainofnormal derLeeandNolet,1997).UsingS-wavedata consideration. recordedbythesamebroadbandseismometersthat WepresentanexplanationforwesternU.S. weuseinourP-wavestudy,LeeandGrand(1996) uplift,Laramidecontraction,andvoluminousmid- foundmantlebeneathcentralColoradotobevery Tertiarymagmatism.Weexpendtheflat-slabmodel slow.InNewMexico,Slacketal.(1996)andSpence byincorporatingslabde-wateringtohydrateNorth andGross(1990)foundlowvelocitiesbeneaththe Americalithosphereundertherelativelycoolcondi- Jemezvolcanictrendthataremoreprominentthat tionsthatwouldbeexpectedduringslabcontact anylow-velocitystructurebeneaththeRioGrand (Dumitruetal.,1991;Spencer,1996).Itisthe Rift.OurimageofP-wavevelocity(Vp)providesa hydrationofNorthAmericalithospheretowhichwe resolvedimageofuppermantlestructureinthearea attributedecreaseddensityandupliftofthickwest- oftransitiontothecraton,andhelpsdefinethe ernU.S.lithosphereasfareastastheGreatPlains, structureinternaltothistransition. LARAMIDE-AGEHYDRATION 577 FIG.1.SeismicvelocityandsurfaceheatflowofthewesternU.S.region.Blacklinesdelineatephysoigraphicprov- inces,andgreylinesshowplateboundaries.A.Compositeimageofseismicvelocitystructureat100kmdepth.The continental-scaleimageisfromthemulti-bounceS-wavemodelingofGrand(1997),andvelocityhasbeenscaledby0.5 toprovideabackgroundestimateofP-wavevelocity.Resolutionofthisimageis~300km.Overprintingthisimageare fiveP-waveinversionsofregionalarraydata:theWashington–Oregon,California–southernNevada,andIdaho–Utah– westernWyomingimagesofHumphreysandDueker(1994a),thenorthernArizona–NewMexicoimageofSlacketal. (1996),andtheColorado–NewMexicoimagefromourstudy.Toadjustfordifferencesinaveragevelocitybeneatheach studyarea,resultsfromindividualinversionshavebeenshiftedbytheadditionofaconstanttothedelaytimesofeach dataset.Adjustmentsaredeterminedbyminimizingdifferencesinvelocitywhereinversionsoverlap,andwiththeback- groundimage.BecausemostwesternU.S.uppermantleisslow,mostoftheregionalimageshavebeenmadeslower.Time adjustmentsare:northernArizona–NewMexico,0.3s;California–southernNevada,0.4s;easternIdaho–Utah,0.2s; Washington–Oregon,0.4s;Colorado,0.0s.B.Surfaceheatflow.Datahavebeeninterpolatedtoarectangularmeshand smoothedtoeliminatewavelengthsgreaterthanabout250km.Thestrongsimilarityofthepatternofheatflowwithupper mantleseismicvelocityandwithCenozoicvolcanism(ChristensonandYeats,1992)suggeststhatnear-verticalascentof mantlemeltatdepth(redareasinA)leadstovolcanismandhighheatflowatthesurface.Dataarefromtheheatflow datarepositoryatwww.heatflow.und.edu. 578 HUMPHREYSETAL. Geologicoverview Thenext(andfinal)eventtostronglyaffectthe Proterozoicaccretionof~NE-trendingarc-like ColoradoPlateauandsouthernRockyMountains terrainsontotheArcheanWyomingprovince(Karl- (includingtheArcheanWyomingprovince)wasthe stromandBowring,1993)createdaheterogeneous Laramideorogeny,althoughthelithosphereof continentallandmasspossessingastrongnortheast- southernNewMexicowasmodifiedat~200Maby erlygrainthathasexpresseditselfrepeatedly thedevelopmentofaNW-trendingmagmaticarc throughgeologictime(Henstocketal.,1998;Karl- (Reynolds,1980).Presumably,southernNewMex- stromandHumphreys,1998).Mantlemodelages icolithospherewaslessthan~90kmthickoverthe andequilibrationdepthsofxenolithsindicatethat durationofarcmagmatism—i.e.,itwasthinenough westernU.S.crustalprovinceswereunderlainby topermitslabdehydrationat~90kmdepthtocause mantlelithosphereofcorrespondingages,andthat asthenosphericmelting. thisassociationpersiststothepresentinmostareas Laramidethrustfaultingandcrustalshortening (LivaccariandPerry,1993).Followingaperiodof occurredastherelativelyundeformedColoradoPla-