EOSmeetingreport“EvolutionofPlateTectonics”SUPPLEMENTThissupplementprovidesadditionalinformationbeyondthebriefEOSreport.

The conference “Evolution of Plate Tectonics” was organized around four interrelatedquestions thought to be keys for answering the larger question ofwhy Earth is the onlybodyknowntohaveplatetectonics:

1)Whendidplatetectonicsstart?2)Howdidplatetectonicsstart?3)WhatwasEarth’sconvectiveandtectonicstylebeforeplatetectonicsbegan?and4)Whyisitimportanttounderstandtheevolutionofplatetectonics?

Eachofthefourintegralquestionsprovidedthefocusofoneday,withaWednesdayfieldtriptotheAlpinesuturezoneallowingparticipantstolearnaboutthefascinatinggeologyofthe region and interact more informally. The field trip guide can be downloaded fromhttp://jupiter.ethz.ch/~plates/FIELD_GUIDE.pdf. Fourmorningsweredevotedtotalksbythree invited experts with group discussions of the topics covered in these talks. OnMonday,we addressed the question “Why is it important to understand the evolution ofplatetectonics?”andtalksonthistopicweregivenbyShigenoriMaruyama(TokyoInstituteofTechnology,Japan“Earthhistory”),NicolasColtice(U.Lyon,France“ImportanceofplatetectonicsforthegeochemicalevolutionofEarth’smantle”),andDorisBreur(DLR,Germany“Isthereplatetectonicsonexoplanets?”).OnTuesday,werevisitedthequestion“Whendidplatetectonicsbegin?”andkeynotetalksweregivenbyMichaelBrown(UMaryland,USA“Evidence from the metamorphic rock record for the onset of plate tectonics”), MarkHarrison(UCLA,USA“Geochemicalevidenceforearlyplatetectonics”),KentCondie(NewMexicoTech,USA“TheonsetofplatetectonicsonEarth:Aplanetintransitionbetween2and 3 Ga”), and Jeronen van Hunen (Durham U., USA “Modeling the dynamics andobservablesofsubductioninearlyEarth”). OnThursday,weexploredthequestion“Howdid plate tectonics start?” with invited talks by Slava Solomatov (Wahington U., USA“Scaling of plate tectonics”), Yanick Ricard (U. Lyon, France “Onset of plate tectonics byaccumulated lithospheric damage”), and Stephan Sobolev (GFZ, Germany “Plate tectonicsinitiation as running hurdles”). On the last day, we addressed “What was Earth’sconvective and tectonic style before plate tectonics began?” with invited talks by ElenaSizova (U Vienna, Austria “Plume tectonics and formation of tonalite, trondhjemite, andgranodioriteintheArchean”)andbyLyalHarris(INRS-ETE,Canada“ComparisonsofEarlyEarth with Venus”). The meeting program can be downloaded from http://jupiter.ethz.ch/~plates/program.html During lunch, students were invited to sit and discuss with the invited speakers. Afterlunch,posterpresentersweregiven3minutes to introduce theirposters, then thegroupspent the rest of the afternoon viewing posters before reconvening for group discussionandthendinner.Answerstoapre-meetingquestionnaireabouteachofthefourquestionsguided the afternoon discussions, and ideas shared during these discussions are alsocapturedintheend-of-meetingquestionnaire.Asummaryofthepost-meetingresponsesisprovidedbelow.

Forthequestion“Whyisitimportanttounderstandtheevolutionofplatetectonics(PT)?”,post-meeting responses included that thiswas an unsolved fundamental question in the

historyofourplanet;thatthisunderstandingwasneededtounderstandtheevolution(andperhaps,origin)oflife;theevolutionofotherplanetsandexoplanets;howeveryEarthcycle(and budget) evolved; the formation of Earth’s surface environment, atmosphere, andhabitability; mantle and core evolution; the origin of continents; and the origin of oredeposits and resources. These suggested “important reasons”were similar between pre-andpost-meetinganswers,buttheaforementionedcategoriesaresufficientlybroadthatalotoffermentwithineachoftheseoccurredoverthecourseofthemeeting.

For the question “When did PT start”, most respondents that chose a date identifiedsometime in theArcheanas thestartdate, inbothpre-andpost-meetingquestionnaires;much smaller minorities chose the Hadean and the Neoproterozoic. The biggest changefrompre- topost-meetinganswerswas thatafter theworkshop,manymorerespondentsopinedthatitwasamoregradualprocessthanheretoforeappreciated;threeexamplesofsuch responses follow. One participant wrote: “No particular date: this was atransitional/episodic/continuum process.” Another offered: “First plate tectonic featuresprobably started as early as EarlyArchean (or perhaps evenHadean) but ‘modern-style’platetectonicstartedsometimeintheProterozoic”. Athirdanswerwas“SometimeintheArchean–althoughnotthemoderntypeofPT.The‘engine’probablyinitiallywasin‘stopand go’ mode.” This more nuanced appreciation of the problem is one of the mostimportantresultsofthemeeting.

TherewasalsoconsiderablediscussionaboutthedefinitionofPTthatisusefulforansweringthequestion;thepurelykinematicdefinitionofEarth’slithospherebeingbrokeninto multiple fragments, each of which moves independently, is of limited use in acontroversywhere isotopic, geochemical, and petrologic lines of evidence are important.WorkshopparticipantsmostlyagreedthatanupdateddefinitionofPTthatincludedthekeyroleofsubductingslabsindrivingplatemotionsisneededinordertomakeprogress.

Responsestothequestion“HowdidPTstart?”weresplitbetweenthosewhoweren’tsureand those who focused on ways to weaken strong oceanic lithosphere. Three principalways to do this were mentioned: mantle plumes, bolide impacts, and accumulation oflithosphericdamage.Onerespondentsuggestedthat“bottom-uportop-downimpactsonastagnantlid”wasresponsible.Severalrespondentsfocusedlessonasinglemechanismthana continuum of processes: one respondent to the question opined “hesitantly,intermittently, episodically”. Another respondent suggested that there was a ”longtransition period where PT (mainly subduction) started several times and plume-lidtectonicsandmodern-stylePToperatedside-by-side.”Athirdcommentwas“…anyoneofseveralprocesses:plumes,impacts,orconvectivedrag.”Manyofthesecommentsrephrasetheincreasingsenseofthecommunitythatmodern-stylePTmaybetheresultofmultipleconditionsandstimuliactingoveralongtime.

The question “What happened before PT?” elicited a range of responses, but there wasremarkablecommonalityinanswers,consideringthatthismeetingwasthefirsttimethatthe questionwas considered explicitly. Therewas consensus that amagma ocean stageoccurred early in Earth history, but that some kind of “stagnant lid” occurred after themagma ocean stage and before global PT began. Most responses focused on what thisintermediatestylewas.Commonthemesfocusedontherolesofmantleplumesinteracting

withsomekindofastagnant,deformable,andmagmaticallyactivelid,knownas“plume-lidtectonics”. Several respondents mentioned that a “proto-PT” might have occurredepisodically or that regions of stagnant lid might co-exist with regions where proto-PToccurred.Thegroupwasnothappywithuseoftheterm“stagnantlid”,aconceptthatwasdeveloped to explain the present tectonic regime of Venus. This term suggests a lack ofdeformationandigneousactivity,soanswersfromthegroupincluded“softlid”,“stablelid”,“active stagnant lid”, “deformable stagnant lid”, “some kind of lid regime”, “magmaticstagnant lid”. These terms try to capture the idea of a single-plate planet that wasneverthelessquiteactiveintermsofdeformationandmagmatism,orwassofromtimetotime. One important outcome of this workshop is the need to better understand thedifferentconvectiveandmagmaticstylesthatasingle-platesilicateplanetlikeVenus,Mars,orthepre-PTEarthmaygothrough.

Insummary,themeetingaccomplishedmanybutnotallofitsgoals.Itbroughttogetherabroadgeoscientificandplanetarycommunityandmadegoodprogressinaddressingthreeofthefourkeyquestions:WhyisitimportanttounderstandtheevolutionofPT?,HowdidPT begin? and What was Earth’s tectonic/convective style before PT? We made lessprogressonresolvingthecontroversyofWhendidPTbegin?,butperhapsthisquestionisless important because theremay not have been a singlemomentwhen the PT regime“turnedon”.Instead,PTmayhavetrulyevolvedovermuchofEarthhistory. WearewellonourwaytoansweringthequestionofWhyisitimportanttounderstandtheevolutionofPT? but we may never be able to definitively answer the other three key questions.Nevertheless,thereisalotof“low-hangingfruit”tobeharvestedinthefuture.Oneofthemost important opportunities is that this exploration can help bring the manysubdisciplinesofthegeosciencestogethertolearnfromeachother,andtoengageplanetarysciencesandevolutionarybiologists.Perfectbalanceamongthegeoscientificandplanetarysciencesubdisciplines isnotexpected inasmallworkshop like this. TheAsconameetingbenefitted from strong participation of geodynamic modelers and the Europeangeoscientificcommunity,butwealsohadgoodparticipationbygeologistsandgeochemists.Moreparticipationbyexpertsinsomeothersubdisciplines–suchasisotopegeochemistry,rock mechanics, and economic geology – would have provided useful additionalperspectives. Theworkshopwouldalsohavebenefitted if thereweremoregeoscientistsfromAsiaandtheThirdWorld.

WhatshouldwedointhefuturetoadvanceourunderstandingoftheevolutionofPT?Theprogress thatwemade at Ascona strongly suggests thatwe should notwait another 10yearstogatheragainandcontinuetheexploration.Weaimtohaveanothermeetingin2-3years.TheAsconameetingwasadvertisedwidelyandweacceptedallbonafideapplicants;weplantocontinuethisapproachatthenextmeeting.MattLeybournefromLaurentianU.in Sudbury, Canada offered to host the next meeting there, which would have theadvantages of more engagement of economic geologists and a possible field trip to anArcheangreenstonebelt intheCanadianShield. Sudbury liesontheshoulderof the1.85Sudbury ImpactCraterandhostssomeof the largestNi-Cu-PGEdeposits in theworld. Inaddition to discussing advances in our understanding of when and how plate tectonicsstarted, the 2018 workshop will involve discussions on the relationships between platetectonicsandmineraldeposits.

Figure1S:Workshopparticipants.UpperimageshowsworkshopparticipantsonlawnoutsidetheconferencecenterwithLagoMaggioreandsouthernAlpsinthedistance.Bidentifiesindividualsbynumber:1=CharitraJain(ETH,Switz.);2=JieLiao(ETH,Switz.);3=Jean-PierreBurg(ETH,Switz.);4=MaximBallmer(ETH,Switz.);5=StefanBraendli(ETH,Switz.);6=TarasGerya(ETH,Switz.);7=BobStern(UTDallas,USA);8=Jean-FrancoisMoyen(JeanMonnetU.,France);9=MurielGerbault(InstResearchforDevelopment,France);10=John Saul (France); 11 = Christoph Heubeck (U Jena, Germany); 12 = DavidWillis (Monash U., Australia); 13 = David Mole (CSIRO,Australia);14=JeroenvanHunen(DurhamU,UK); 15=DorisBreuer(DLR,Germany);16=FabioCrameri(Oslo,Norway);17=CarolFrost(NSF,USA);18=AntoineRozel(ETH,Switz.);19=NicolasColtice(ULyon,France);20=DiogoLourenço(ETH,Switz.);21=MattLeybourne(LaurentianU.,Canada);22=CraigO’Neill(MacquarieU.,Australia);23=PaulTackley(ETH,Switz.);24=AnoinetteGrima(UCL,UK);25=MichaelBrown(UMd,USA);26=KentCondie(NMTech,USA);27=ScottKing(VaTech,USA);28=CarolineDorn(UniBern,ETH,Switz.);29=IsraEzad(UCL,UK);30=DanBower(ETH,Switz.);31=DaveStegman(UCSD,USA);32=TimLichtenberg(ETH,Switz.);33=XinZhou(ETH,Switz.)34=KiranChotalia (UCL,UK); 35=AdrienVezinet (U JeanMonnet,France);36=GautierNicoli(LorraineNancyU., France); 37 = FrankWagner (ETH, Switz.); 38 = Stephan Sobolev (GFZ, Germany); 39 = ChristineHouser (ELSI,Japan);40=JanaSchierjott(ETH,Switz.);41=MatthewMayne(UJeanMonnet,France);42=TatsukiTsujimori(TohokuU.,Japan);43=DaveBercovici(Yale,USA);44=MaximeMaurice(DLR,Germany);45=MartinaUlvrova(UCBLyon,France);46=YanickRicard(ENSdeLyon, France); 47 = Bettina Baitsch Ghirardello (ETH, Switz.); 48 = Elena Sizova (U. Graz, Austria); 49 = Veronica Sanchez (TAMUKingsville,USA);50=ClaudiaStein(Muenster,Germany);51=AnneDavaille(CNRS/UParis,France);52=WalterMooney(USGS,USA);53=UlrichHansen (UMuenster,Germany);54=RiaFischer (ETH, Switz.); 55=RichardFrom (UManitoba,Canada);56=ShigenoriMaruyama(TokyoTech,Japan);57=SlavaSolomatov(WUSTL,USA);58=LyalHarris(INRS,Canada);59=AndreaPiccolo(JGUMainz,GER);60=MatthiasSchmitz(UniversityofJena,Germany).Notshown:MarkHarrison(UCLA,USA),JohnHernlund(ELSI,Japan).

Fig. 2S:Ms.KiranChotalia,winnerofbeststudentcontributionawardduring theCSFConference“OriginandEvolutionofPlateTectonics