KeithPriestley,DanMcKenzie∗DepartmentofEarthSciences,BullardLabs,MadingleyRoad,CambridgeCB30EZ,UKarticleinfoabstractArticlehistory:Received3February2013Receivedinrevisedform16July2013Accepted10August2013Availableonline25September2013Editor:P.ShearerKeywords:surfacewavetomographymantletemperaturelithosphericthicknessshearwaveattenuationmantleviscositySurfacewavetomography,usingthefundamentalRayleighwavevelocitiesandthoseofhighermodesbetween1and4andperiodsbetween50and160s,isusedtoimagestructureswithahorizontalresolutionof∼۲۵۰kmandaverticalresolutionof∼۵۰kmtodepthsof∼۳۰۰kminthemantle.Anewmodel,PM_v2_2012,obtainedfrom3×۱۰۶seismograms,agreeswellwithearlierlowerresolutionmodels.ItiscombinedwithtemperatureestimatesfromoceanicplatemodelsandwithpressureandtemperatureestimatesfromthemineralcompositionsofgarnetperidotitenodulestogenerateanumberofestimatesofSV(P,T)basedongeophysicalandpetrologicalobservationsalone.Thesearethenusedtoestimatetheunrelaxedshearmodulusanditsderivativeswithrespecttopressureandtemperature,whichagreereasonablywithvaluesfromlaboratoryexperiments.Athightemperaturesrelaxationoccurs,causingtheshearwavevelocitytodependonfrequency.ThisbehaviourisparameterisedusingaviscositytoobtainaMaxwellrelaxationtime.Therelaxationbehaviourisdescribedusingadimensionlessfrequency,whichdependsonanactivationenergyEandvolumeVa.ThevaluesofEandVaobtainedfromthegeophysicalmodelsagreewiththosefromlaboratoryexperimentsonhightemperaturecreep.Theresultingexpressionsarethenusedtodeterminethelithosphericthicknessfromtheshearwavevelocityvariations.Theresolutionisimprovedbyaboutafactoroftwowithrespecttoearliermodels,andclearlyresolvesthethicklithospherebeneathactiveintracontinentalbeltsthatarenowbeingshortened.Thesameexpressionsallowthethreedimensionalvariationsoftheshearwaveattenuationandviscositytobeestimated.©۲۰۱۳ElsevierB.V.Allrightsreserved.۱٫IntroductionTheassociationofreducedshearwavevelocityVs,increasedattenuationandreducedviscosityintheEarth’suppermantlehasbeenknownforalmostninetyyears(seeGutenberg,1959,p.76).Theseeffectsaremostobviousintheasthenosphere,wherethemantletemperatureapproachesthemeltingtemperature.Fig.1ashowsatypicalsteadystatecontinentalgeotherm.Thelayerclosetothesurface,whereheatistransportedbyconductionalone,isknownasthemechanicalboundarylayer(MBL).Itisunderlainbyathermalboundarylayer(TBL)thatformspartoftheconvectivecirculationoftheuppermantle,andwhichsuppliesheattothebaseoftheMBL.Atgreaterdepthsthetemperaturegradientintheconvectiveinteriorisisentropic.Nodiscontinuitiesineitherthetemperatureoritsgradientcanoccuranywhere.Aconvenientwaytodescribeageothermisintermsoftheequivalentlithosphericthickness,definedasthedepthatwhichtheextrapolatedconduc-tivegeothermwouldintersectthatoftheconvectiveinterior.ThisdefinitionwasusedbyPriestleyandMcKenzie(2006),hereafter*Correspondingauthor.Tel.:+441223337191.E-mailaddress:mckenzie@madingley.org(D.McKenzie).PM6,andisalsousedhere.Itisthenthesameastheplatethick-nessusedtoparameterisethethermalstructureofoceanicplates(seeCrosbyetal.,2006).AsFig.1ashows,thebaseofthelitho-spheredefinedinthiswaylieswithinthethermalboundarylayer,anddoesnotcorrespondtoany
