How Laramide-Age Hydration of North American Lithosphere by The
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.
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