Download - Detrital Zircon Geochronology and Vitrinite …...2018/05/02 · 1. Abstract: The Cascadia subduction zone, which presently spans some 1,300 km along the western continental margin

DetritalZirconGeochronologyandVitriniteReflectanceAnalysisoftheCascadiaSubductionComplex,Washington

PeterMahony

Advisor:MarkBrandonSecondReader:AlanRooney

May2,2018

ASeniorThesispresentedtothefacultyoftheDepartmentofGeologyandNaturalGeology,YaleUniversity,inpartialfulfillmentoftheBachelorsDegree.

InpresentingthisthesisinpartialfulfillmentoftheBachelor’sDegreefromtheDepartmentofGeologyandGeophysics,YaleUniversity,Iagreethatthedepartmentmaymakecopiesorpostitonthedepartmentalwebsitesothatothersmaybetterunderstandtheundergraduateresearchofthedepartment.Ifurtheragreethatextensivecopyingofthisthesisisallowableonlyforscholarlypurposes.Itisunderstood,however,thatanycopyingorpublicationofthisthesisforcommercialpurposesorfinancialgainisnotallowedwithoutmywrittenconsent.PeterW.Mahony,2May2018

1.Abstract:

TheCascadiasubductionzone,whichpresentlyspanssome1,300kmalong

thewesterncontinentalmarginofthePacificNorthwest,wasinitiated~35Maago.

Asubductioncomplexhasgrownabovethissubductionzone,duetoconvergence

(30-40km/Ma)andaccretionofthe~2km-thicksedimentarycoverofthe

subductingoffshoreplate(Farallon/JuandeFuca).Theresulthasbeenagradual

upliftofthePacificNorthwestcoast.TheOlympicMountainsinWashingtonState

wasthefirstpartoftheCascadiasubductioncomplextoemergeabovesealeveland

isthemostdeeplyerodedpartoftheCascadiasubductioncomplex(Brandonetal.

1998).Thus,thisregionprovidesauniqueopportunitytostudytheaccretionary

evolutionoftheCascadiamargin.

Wepresent~1,600newU/Pbagesfordetritalzirconsfrom16sandstone

samplescollectedfromtheOlympicMountains,and~4,250fission-track(FT)zircon

grainagesfrom95sandstonesamples,generatedinpreviouswork.SomeoftheFT

samplesarereset,asaresultofpost-depositionalheatingtotemperatures>200°C.

TheunresetFTzirconsandalloftheU/Pbzircons(whichareinsensitivetothermal

resetting)preserveagesthatweresetintheerosionalsourceregionwherethe

zirconswerederived.Theyoungestzirconsineachsampleweresourcedinthe

coevalCascadevolcanicarc,andthusshouldhaveagessimilartothedepositional

ageofthesandstonesample.

Theseproxydepositionalagesareusedtothetesttheaccretionary

interpretationoftheCascadiasubductioncomplex.Theagesinfactshowaclear

youngingfromeasttowest,consistentwiththeprogressiveaccretionoftrench-fill

turbiditesattheoffshoretrench,whichhasbeentheleadingaccretionaryedgeof

thesubductioncomplex.

Weusevitrinitereflectancedatafrom120locationscollectedbyPark

Snavely(USGS)todefineataregionalscalethemaximumtemperatureforthe

exposedbedrock.Thehighestvalues,~250°C,arefoundinthecenterofthe

OlympicsMountains,andtheymarkthemostdeeplyerodedpartofsubduction

complex,toanestimateddepthof~12.5km.ResetzirconFTagesinthisarea

indicatethattheexposedrockscooledbelow~240°Cat~10Ma.Thisresultis

consistentwithlong-termerosionwithinthethicker,morerearwardpartofthe

subductioncomplex.

2.Introduction:

TheOlympicSubductionComplex(OSC)formedasmaterialfromthe

subductingFarallonandJuandeFucaplatesassimilatedontotheNorthAmerican

plate.TheextantFarallonPlatebegansubductingbeneathNorthAmerica

approximately~35Maagoinitiatingwedgeformation(Vanceetal.1986,

summarizedinBrandonandVance1992).Attheactivesubductionzone,

sedimentsfromthetrenchhavebeenscrapedoffandaddedtotheoverridingNorth

Americanplate.Successivesedimentarylayershaveaccretedatthemargin,either

byentrainingtothefrontaledgeoftheaccumulatingwedgeofsedimentsorby

stickingtothebottomofit,knownasunderplating(Brandon2004).Asmore

sedimentshavebeenaddedthewedgehasthickenedverticallyandemergedfrom

thePacificOcean(Brandon2004).Oncesubaeriallyexposedsurfaceprocesses

erodedtheresistiveupperlayers,oftenreferredtoasthestructurallid,andexposed

theimbricatedsedimentarycore(Brandon2004).BystudyingtheOSCwemay

drawgeneralconclusionsonaccretionarywedgeformationthatcanthenbe

extrapolatedtoothersubductioncomplexes.

Therearefewageconstraintsontheheavilydeformedlayersofaccumulated

sedimentsintheOSC.PreviousattemptstodatetheOSChavereliedonrelative

datingwithmicrofossils,isotopeanalysisofbasalts,apatite(U-Th)/Heages,and

fissiontrackagesofdetritalapatitesandzircons(summarizedinBrandonand

Vance1992;Rodenetal.1990;Rau1964;SnavleyandMacload1974andothers).

Thelowertemperaturethermochronometers,suchas(U-Th)/Heandfission-track

apatiteages,aregoodforestimatingthermalhistories,giventheirlowclosure

temperatures,~65°Cand~110°C,respectively,whilezirconfission-trackagescan

preserveageinformationifthetemperatureremainslessthan~200°C

(summarizedinBattetal.2001).Howeverthesemethodsdonotrecordreliableage

informationfortheheavilyerodedterranesatthecenteroftheOlympicMountains

(BrandonandVance1992,Rodenetal.1990).Relativedatingwithmicrofossilsis

challengingbecausethemarinesedimentsintheOlympicMountainsdidnotretaina

robustfossilrecord,andwhenpresent,thefossilrecordwasfracturedduring

accretion(Rau1964,TaborandCady1978a).Insitubasaltsarelimitedtothe

marginsoftheOlympicPeninsula,andareoftenmetamorphosedmakingK-Arand

Ar-Aranalysisimprecise(summarizedinBrandonandVance1992).Inthepast

zirconfissiontrackageshavebeenusedbecausezirconshaveahighthermal

stabilityandlocalcrystalscontainsufficientUraniumtogivealowrelativeerror

(~10%)(BrandonandVance1992).Itisagreedthattheageoftheyoungestzircons

inasampleisagoodapproximationofthedepositionalagebecausetransporttime

fromcoevalzirconsourcesintheCascadiaVolcanicfrontisnotsignificant(Stewart

andBrandon2004).Wealsoproposethattheyoungestsampleageinaunitisalsoa

generalapproximationoftheageofdepositionfortheentireunit.Thisimpliesthat

thedepositionalenvironmentwasregionallyconsistent.Howeverdatingfission

tracksistimeconsumingandattemperaturesexceeding~245°Czirconfission

tracksarenolongerareliablechronometer,asthefissiontracksannealandthe

crystalthermallyresets(BrandonandVance1992).PreviouszirconFTstudiesby

BrandonandVancehavefoundthatzirconsintheOlympicMountainswerelikely

resetbecausethedistributionofagesisnotindicativeofadepositional

environment,wherecrystalsofmultipleagesmaybeincorporated,butofasingle

coolingevent(BrandonandVance1992).U/Pbdatingbyofdetritalzirconsbylaser

ablationispreferablebecausethedaughterisotopesforthe Pb !"# / U !"# and

Pb !"# / Pb

!"# systemareretainedathightemperatures,with~900°Crequiredto

fullyresettheage(CherniakandWatson2001).Forthisreasonnewdetritalzircon

U/Pbminimumageswillbecomparedwiththepreviousfissiontrackdatato

confinethedepositionalagesofthesedimentaryformationsoftheOSC.

Herein,vitrinitereflectancedatacollectedbyParkerSnavelywillalsobe

analyzedtodeterminethemaximumtemperatureofthedifferentlayersofthe

accretionarywedge.Vitriniteisaglassycomponentofmetamorphosedorganic

materialthatbecomesmorereflectivethemoreit’sheated(summarizedinBarker

andPawlewicz1994).Byanalyzingthereflectivityofthevitriniteinasamplewe

cancalculatethemaximumtemperaturethesampleexperienced(Barkerand

Pawlewicz1994).CurrentlythethermalhistoryoftheOSCispoorlyconfined

becauseitisbasedonmetamorphicfaciesmineralsandfission-trackclosureages

(Battetal.2001).Metamorphicfaciesanalysisisanimpreciseindicatorof

maximumtemperaturebecausemetamorphicprocessesoftenaredependenton

pressureandbecausesamplesarelimitedbythepresenceofdiagnosticminerals

thatarenotalwayspresent(SweeneyandBurnham1990).Fissiontracksas

thermochronometersarealsonotidealbecausetheyareimpreciseandunreliableat

highertemperatures.Asthesedimentsareexhumedandcooledthezirconand

apatitecrystalsinthesandstonesstabilizeat~245°C(~13km)and~100°C(~5

km)respectively(BrandonandVance1992,Brandonetal.1998).Oncethecrystal

hascooledbelowitsresettingtemperature,damagefromfissiondecayofUranium

isrecordedastracksinthecrystallattice,thusthecrystalrecordswhenitcrossed

theclosurethreshold(summarizedinBattetal.2001).Thecoolingpathcanthenbe

determinedbyanalyzingtherelativefissiontrackagesofapatiteandzirconsina

sample,howeverfissiontrackanalysisdoesapoorjobofconstrainingthemaximum

temperaturebecausethethermalmaximumisonlydiscretizedintothreegroups

>245°C,between245°Cand~100°C,and<100°C(summarizedinBattetal.2001).

Additionalanalysisintothethermalhistoryofthedifferentsectionsofthewedgeis

valuablebecauseitwillhelptodefinethepathwayeachlayertookduringaccretion.

ThispaperwillsummarizethegeologiccontextoftheOlympicSubduction

ComplextoexplainhowtheOlympicMountainsreachedtheirpresentconfiguration.

Itwillthenoutlinethemethodologyusedtoproducethe Pb !"# / U !"# and Pb

!"# /

Pb !"# ages,andmaximumtemperatureconstraints.Nextitwillsummarizethe

resultsoftheMahonyBrandondetritalzirconminimumagesandtheShnavley

VitriniteReflectancedata,comparingthemtotheStewartBrandonFissionTrack

ages.Thepaperwillconcludewithadiscussionofhowthenewageandthermal

constraintscompareswiththecurrenttheoryofwedgedynamics.

3.GeologicOverview:

BrandonandVancecontendthatcoastalwedgesystemscanbedividedinto4

discretetectonicelements:aresistivecontinent,avolcanicarc,acoastalrange

terraneandanaccretionarywedge(BrandonandVance1992).Thecontinentacts

asaplowthattheincomingsedimentsaccumulateagainst,andwhilesome

deformationcanbeexpectedthecontinentisgenerallyconsideredfixed(Brandon

2004).Movingcoastwardthenextelementisthevolcanicarc.Thevolcanicarcis

coevaltothesubductionzoneandformsasmeltsareemplacedasplutonsand

volcanicrocks(SherrodandSmith1989).YetfarthercoastwardistheCoastal

Rangeterrane.Thisisthecoastalregionthatwhensubductionbeganwaslocated

betweenthesubductingplateandthevolcanicarc.Andthefurthestoutboard

elementistheaccretionarywedge,whichiscomposedofaccretedsedimentsfrom

thesubductingplate(BrandonandVance1992).

IntheCascadiaSubductionComplexallfourelementsarepresent.TheNorth

Americanplate,composedofPaleozoicandMesozoicstructuressuturedtothe

continentalmarginduringtheJurassicandCretaceous,istheresistivecontinent

(Mongeretal.1982,Brandonetal.1988,summarizedinBrandonandVance1992).

ThesubductionrelatedvolcanicarcistheCascadeVolcanicfront,composedof

exposedsubvolcanicplutonsaswellasactivestratovolcanoes;includingMt.Lassen

inNorthernCaliforniaandMtGaribaldiinBritishColumbia(SherrodandSmith

1989).TheCascadeVolcanicarchasbeenmostlystationary,althoughgeologicand

paleomagneticdatasuggeststherehasbeeninternaldeformationandrotation

duringtheCenozoic(Helleretal.1987,WellsandHeller1988,summarizedin

BrandonandVance1992).ItshouldbenotedthatsomesuggestthattheCascade

arcwaspredatedbytheChallisarc,howeverChallisvolcanismlikelypeaked

between52Maand42Ma,whichdoesnotoverlapwithestimatesforOSCwedge

building(Armstrong1978,Helleretal.1987).Thetwomostoutboardelements,the

coastalrangeterraneandtheaccretionarywedge,arepresentintheOlympic

Mountainsandhaveseenextensivedeformationduringwedgeformation.

ThestratigraphyoftheOlympicMountainsishighlydeformedwithextensive

accretionrelatedfoldingandthrustfaulting,resultingindissimilarformations

abuttingoneanother(TaborandCady1978b,BrandonandVance1992).The

geologyoftheOSCcanbedividedintotwosections;aPaleogenebasaltformationon

theNorthern,Eastern,andSouthernmarginsknownastheCrescentFormation,and

ayoungerCenozoicsedimentarycore(BrandonandVance1992).

Figure1:A-A’marksthecrosssectionfromtheKalalochLodge(A)toapointintheCoastRangeterrane100kminthedirectionofplate(A).LocationofKalalochLodge(PazzagliaandBrandon2001)andasmuthanglefrom(McCafferyetal.2013).

Figure2isanapproximateschematicofA-A’.ThelithicassemblagesareabbreviatedwithNG=Needles-GrayWolf,E=Elwha,GV=GrandValley,WO=Western

Olympics,andH=Hoh;AdaptedfromBrandonandVance1992

3.1CrescentFormation:

TheCrescentformationisthePaleogenebasaltformationontheperiphery

andiscomposedoffeldspathicpillowbasaltsatopmoremassivebasaltblocks

(TaborandCady1978a,Wellsetal.2014).CrosscuttingtheCrescentformationis

PaleocenetoEocenedikessuggestingnear-trench,orislandarcvolcanics(Tabor

andCady1978a,Johnson1984,BrandonandVance1992).TheCrescentisa

generalstratigraphicunitofbasaltsthatliestructurallyabovetheyounger

sedimentarycoreattheHurricaneRidgethrustfault(TaborandCady1978a);the

Crescentcanbefurthersubdividedbasedondifferentstylesofdeformationand

paleo-rotationmeasurements(summarizeinBrandonandVance1992,Siberlinget

al.1987).TheCrescentformationisnotlikethepre-tertiaryformationson

VancouverIslandoracrossthePugetSound;itismoresimilartoupliftedoceanic

crustoranentrainedislandchain(Johnson1984,BrandonandVance1992).

BrandonandVancecontendthattheCrescentformationispartoftheCoastalRange

terrane(BrandonandVance1992).Astheaccretionarywedgeencroacheda

sectionofmarinesedimentwasincisedandprojectedbeneaththeCrescent

formationroughly33Ma-17Maago(BrandonandVance1992).Progressively

moresedimentarylayershaveprojectedbeneaththeCrescentfoldingitupward

untilitwassubaerialyexposedanderodedawayleavingaheavilyinclined

stratigraphiccolumn(BrandonandVance1992).

Figure2:SchematicviewofthetheA-A’cross-sectionofthewedgedepictingregionalfoldingandnappeformationofthestructurallid.AdaptedFromBrandonandVance1992

3.2SedimentaryCore:

Thesedimentarycoreiscomposedofimbricatedassemblagesofmarine

sandstoneandmudstonewithoccasionalinter-beddedlensesofearlyPaleogene

pillowbasaltsthataremorecommonneartheHurricaneRidgethrustfault(Tabor

andCady1978b,BrandonandCalderwood1990).Theinteriorofthepeninsulais

internallyfaultedwithfriablelayersclosesttothesubductionzoneand

progressivelymorethermallymetamorphosedandresistivesedimentsasyoumove

eastwarddeeperintothewedge(TaborandCady1978b).Thesedimentarycore

canbesubdividedintofivelithicassemblagesfromeasttowest:theNeedlesGray

Wolf,theGrandValley,theElwah,theWesternOlympic,andtheHoh.Eachofthese

sedimentarylithicassemblagesarelenticularandseparatedbyeast-westthrust

faults(TaborandCady1978b;Helleretal.1992).BrandonandVancegroupthese

lithicassemblagesintothreestructuralunits:theUpperOSC,theLowerOSC,and

theCoastalOSC,basedontheirage,thermalhistory,andgeneralstratigraphy

(BrandonandVance1992).EachofthestructuralunitsdefinedbyBrandonand

Vancearecoherentsectionsofincomingsedimentsfromthewedgeandrecordthe

temperatureandtimehistoryoftheiraccretionarypathway(BrandonandVance

1992,Battetal.2001).

3.2aUpperOSC:

BrandonandVancerefertotheNeedlesGrayWolf,Elwahandseveralcoastal

assemblagescollectivelyastheUpperOSC(BrandonandVance1992).TheUpper

OSCcontainsinterbeddedsandstoneandlimestonestratawithEocenetoOligocene

marinemicrofossils,alongwithpillowbasaltssimilarinageandcompositiontothe

overlyingCrescentformation(ApplegateandBrandon1989,summarizedin

BrandonandVance1992).TheUpperOSCdoesnotcontaindiagnosticmetamorphic

faciesmineralsandisconfinedtobetween90°and250°Cbyfissiontrackanalysis

(Frost1980,Brandonetal.1988,summarizedinBrandonandCalderwood1990).

TheUpperOSCisinterconnectedwiththeCrescentformationbasedonthe

analogousbasaltformations.BrandonandVancecontendthattheUpperOSCwasa

westernextensionoftheCoastRangeterranethatwasunderthrustbeneaththe

Crescentearlyinsubduction(BrandonandVance1992).TheUpperOSCisthen

consideredtobepartofthestructurallidthatwasfoldedoverMountOlympusand

largelyeroded(BrandonandVance1992,TaborandCady1978a)

3.2bLowerOSC:

StructurallybelowtheNeedlesGrayWolfandElwahassemblagesiswhat

BrandonandVancecalltheLowerOSC,containingtheGrandValleyandWestern

Olympiclithicassemblages(BrandonandVance1992).TheLowerOSCiscomposed

ofturbidites,thinlybeddedmudstonemélanges,andclasticrockslikelytransported

tothetrenchbylarge-scalemasswastingevents(TaborandCady1978b,Brandon

1988,BrandonandVance1992).TheLowerOSCfeaturesprehniteandpumpellyite

faciesmineralsindicativeof240to245°Candadepthof~12km(Brandonand

Calderwood1990,summarizedinBrandonandVance1992).

BrandonandVancesuggestthattheLowerOSCaccretedalongthebottomof

thewedgeviaunderplatingandwasdeeplyburiedresultinginresetzirconsand

highermetamorphicgrademinerals(BrandonandVance1992).

3.2cCoastalOSC:

TheCoastalOSCcontainstheHohassemblage.TheCoastalOSC,likethe

LowerOSC,iscomposedofturbiditesandthinlybeddedmudstonemélangeswith

locallyderivedMiocenemicrofossils(Rau1979),howeverinsteadofclasticrocks

therearebasaltblocksandinsteadofprehniteandpumpellyitethereiszeolite-

facieslaumonite(TaborandCady1978b,BrandonandCalderwood1990).

TheCoastalOSCwaslikelyneverdeeplyburiedasthezeoliteminerals

indicateonlylow-grademetamorphismandFTsamplesaregenerallyunreset.For

thisreasonBrandonandVancesuggestthattheCoastalOSCaccretedalongthe

frontaledgeofthewedge(BrandonandVance1992).

4.Methodology:

4.1U/PbZircon:

16samplesofmediumtolargegrainsandstoneswerecollectedfor

Pb !"# / U !"# and Pb

!"# / Pb !"# datingbyLA-ICP-MS.Thecoastal,lower,andupper

OSCunitsarerepresentedintheanalyzedsampleswithafocusonthemosthighly

exhumedcentralmassifnearMountOlympus.Effortsweretakentocollectsamples

nearsitespreviouslydatedbyBrandonandStewartusingzirconfissiontracksfor

comparison.Highway1circumnavigatestheOlympicPeninsulaandaidedaccessto

exposedsightsontheperipheryofthecomplex,whilearterialloggingand

recreationalroadsprovidedaccesstosamplesinthenationalforest.Samplesfrom

thecentralmassifandtheslopeofMt.AndersonintheElwhaassemblagewere

reachedviatheextensivetrailsystemprovidedbytheUSNationalParkService.

ThesampleswerethensenttoZirchron,LLClocatedinTuscon,AZwherethe

zirconcrystalswereseparatedusingthetraditionalmineralseparationtechniques

fromArmstrong1986.Thesandstoneswerefirstcrushedtoliberatethemineral

grains.ThenaRo-Tapmachineusing34-sizedmeshsievedthemineralgrainsto

removethelargestgrainsthatwouldotherwisedistortattemptstoseparatecrystals

basedondensity.ThecrystalsejectedfromtheRo-Tapandsievewerethensentfor

densityseparationtoremovethelighterminerals.

Athree-stepdensityseparationtechniquewasusedtoisolatethedense

zirconsfromlightergrains.Firstthelightestconstituents,quartzanddust,were

removedbyaWilfleyTable.IntheWilfleyTablethegrainsaresuspendedinwater

andshaken,causingtheheaviestmaterialstosinkbutthelightermaterialsto

remainsuspended.Thelightmineralswereremovedandthewaterwasdrained

leavingonlydensecrystals.Beforeadditionaldensityseparationcouldtakeplace,

clay,carbonate,andmagneticmineralswereremoved.Thedensecrystalsfromthe

WilfleyTableweresoakedinhydrogenperoxidesolutionovernighttoremoveclay

minerals,andthensoakedforanotherdayinaceticacidsolutiontoremove

carbonateminerals.MagneticgrainswereremovedusingaFrantzmagnetic

separator.Theremainingcrystalswerethensubjectedtoheavyliquidseparation

usinglithiumheteropolytungstate(LST)[density~2.8g/mL].Thecrystalswere

mixedintotheLSTandallowedtosettle;anygrainsthatdidn’tsinkwereremoved

fromthesurface.Asecondheavyliquidseparationusingmethyleneiodide(MeI)

[density~3.3g/mL]removedlingeringapatite.Intheprocessedsamplesthefinal

heavyliquidseparationyieldedsufficientzirconcrystalswithnegligibleunintended

pyrite.TheisolatedzirconcrystalswerethensenttoUniversityofCaliforniaat

SantaCruzforLA-ICP-MSanalysisinDr.JeremyHourigan’slaboratory.

AtJeremyHourigan’slaboratorythezirconsampleswerearbitrarily

separatedintotwogroupsandplacedonanepoxymountforanalysis.Thezircon

sampleswerefirstremovedfromtheirpackaging,placedonapieceofdoublesided

tape,andthentransferredtoacircular1”form.Thegrainswereorganizedinto

circulargroupsbasedontheirsample,withadditionalgroupsfeaturingage-

diagnosticstandards.TheformwasthenfilledwithStruerEpofixandallowedto

cure.Excessepoxywasremovedfromthesolidmountsusingalatheand1500-grit

sandpaper.Themountswerepolishedusingalapwheelwithprogressivelyfiner

Struerspolishingcompounds(summaryofzirconseparationandpreparationbased

extensivelyonSniderman2013andArmstrong1986).

Themountswerethenwashedandlaserablationsitespicked.1%HNO!

solutionwasusedtochemicallypolishthesurfaceofthemountsandpurewater

wasusedtorinseanyremainingresiduebeforethemountswereplacedinthe

Helex-2volumecell.Approximately100laserablationsiteswerepickedpersample

makingsuretoavoidingfluidinclusionsoranyremainingpyritecrystals.Intwo

daysoftesting,oneforeachmount,theHelix-2LA-ICP-MAcollectedisotope

measurementsfrom1,581samplezircons.Foreachzirconcrystalthecollector

hoodwasremovedbeforethelaserwasturnedontoestablishanaverage

background Pb !"# measurementthatwouldbesubtractedfromtheon-peaksignal.

Laserablationandon-peakcollectionlastedfor30seconds.Atthebeginningand

endofeachtestingsessionablockofstandardcrystalswereanalyzed,and

throughoutthetestingperiodevery7thzirconanalyzedwasastandard.

Theisotopedatawasthenreducedtoremovethebiasfromdownhole

elementalfractionationusingIolitebasedonindustrystandardsoutlinedinPatonet

al.2010andundertheguidanceofJeremyHourigan(Patonetal.2010).Samples

thatdisplayednoticeable Pb !"# anomaliesindicativeoffluidinclusionsorhigh

radiationdamagewereremoved.ThentheIolitesoftwarecreatedanexpected

downholetrendbasedonthestandard’smeasurements.Samplecrystalsthat

departedfromthisexpectedfractionationpatternweregivenhigherstandard

deviations.

AllagesanduncertaintiesgivenhereinwereproducedusingIsoplotfor

MicrosoftExcel.Commiseratewithstandardpractice,crystalswithagesover1,000

Maarequotedusing Pb !"# / U !"# measurementswhile,crystalsyoungerthan1,000

Maarequotedusing Pb !"# / Pb

!"# (Xieetal.2010,Sniderman2013).Thisismeant

toaccountforthelackofradiogenic Pb !"# intheoldersamples.A±10%

discordancethresholdwasappliedtothegrainageswithdiscordancepercentequal

to100 × [1− !"# !" !"# / ! !"#

!"# !" !"# / ! !"# ].Thisisintendedtoscreenoutpotentially

compromisedsamplesthatmightreturnunreliableminimumages.The Pb !"# / U !"#

to Pb !"# / U !"# discordancemeasureismorestablethana Pb

!"# / U !"# to Pb !"# /

Pb !"# discordancethresholdbecausethemeasurementof U !"# islessaffectedby

quantizationnoisecomparedto Pb !"# / Pb

!"# (writtencommunicationwithMark

BrandonandJeremyHourigan).Itshouldbenotedthatwhencalculatingminimum

agesallgrainswereincludedregardlessofdiscordancebecausetheyounggrains

hadlowerUraniumandthusdonotcontainenoughradiogenic Pb !"# toproducea

precise Pb !"# / Pb

!"# age.Howevercrystalswithlargeuncertaintiesareweighted

lessintheminimumageMATLABscript,soevenifacrystalishighlydiscordantits

influenceontheminimumagewassmall(writtencommunicationwithMark

BrandonandJeremyHourigan).

4.2VitriniteReflectanceSamples:

120vitrinitereflectancemeasurementscomefromParkSnavelyattheUSGS.

100sampleshavepreviouslybeenfeaturedinKvenvoldenetal.1989,aUSGS

BulletinthatlookedatthewesternOlympicsasapotentialhydrocarbonsource,

whiletheremaining20samplesareunpublishedandfocusontheeasternOlympics

(Kvenvoldenetal.1989).MarkBrandonproofedthepreviouslyunpublished

measurementsJuly12,2001.

Maximumtemperaturecalculationsfromthevitrinitereflectancedataare

basedonalinearreductionmodelfrominBarkerandPawlewicz1994constructed

fromreflectancedatafrom72depositionalsettings(BarkerandPawlewicz1994).

Meanrandomvitrinitereflectanceandmaximumtemperaturearestrongly

correlatedbecausevitrinitereflectivityisdependentonmultiplechemicalreactions

whoseactivationenergywhentakeninaggregateisrelativelysmoothbecausewhen

onereactionfinishesanotherstarts(SweeneyandBurnham1990).Temperature

maximumwascalculatedby:ln %𝑅! = 0.0124 𝑇!"# − 1.68(Barkerand

Pawlewicz1994).ItshouldbenotedthatSweeneyandBurnham1990suggestthe

useofamulti-phaseArrheniusreductiontechnique,howeverthismethodrequires

reliablezirconorapatiteFTlengths.Itisouropinionthatvariablestructural

integrityofthezirconorapatitecrystalsinducessignificantvariability,andfor

regionalscaleanalysistheBarkerandPawlewiczlinearreductionissufficient.

4.3FissionTrackZirconSamples:

TheFissionTrackzirconminimumageshavebeencompiledfrompublished

worksbyMarkBrandonandothers(BrandonandVance1992,Stewartand

Brandon2004).Aspreviouslystated,effortstodatethesedimentaryassemblages

oftheOSChasreliedonfission-trackzirconagesandintheoriginalfission-track

zirconanalysistheauthorsdeterminedifthezirconsineachsamplewere

depositional,partiallythermallyreset,orthermallyreset(BrandonandVance1992,

StewartandBrandon2004).Hereinwillrefertoboththepartiallythermallyreset

andthethermallyresetsamplessimplyasreset.

5.Results:

5.1U/PbZircon:

Figure3:U/Pbzirconages(redcircles);A-A’basedonacross-sectionfromfigure1.

Samplesfurtherthan±15kmfromA-A’wereexcluded.

TheU/Pbminimumagesappeartogetolderfromwesttoeast.Theyoungest

sample,170810-1wascollectedintheHohformationandhasaminimumageof

17.6Ma.MovingdeeperintothewedgeformationalongtheA-A’cross-sectionthe

minimumagesgetolderandmorediversearoundMountOlympus.Theyoungest

minimumageintheWesternOlympiclithicassemblagewasfromsample170825-1

collectedatthesummitofMountOlympusandrecordsadepositionalageof23.8

Ma.TheoldestminimumageintheWesternOlympiclithicassemblageisfrom

sample170823-5collectedattheHohRiverBridgeandrecordsaminimumageof

51Ma,morethantwicetheminimumagefromthesummit.Thefurthesteast

samplesintheNeedles-GrayWolfandCoastalRangeBasaltregionsareover40Ma

anddonotexhibitthesamevariabilityasthesamplesnearMountOlympus.

WeconsidertheU/PbgrainagesfortheMahony/Brandonsamplestobe

reliablebecausethesamplescontainedveryfewdiscordantgrainsandthe

discordantgrainsappeartoberandom.Whiletheminimumagecalculationfactors

ingraindiscordanceintothestandarderror,sampleswithahighnumberof

discordantgrainagesarenotreliablebecausediscordanceisoftencorrelatedwith

radiationdamage,crystalsize,orUraniumcontent;variablesthatcanbeinfluenced

bytheageand/orformationofthecrystal(Gehrels2012).Ifahighpercentageof

discordantgrainsareremovedfromasampledatasetthentheremainingnon-

discordantageswillstronglyinfluencetheminimumagecalculation,resultingina

minimumagethatisnotindicativeofthefullsuiteofzirconsinthesample(Gehrels

etal.2008,summarizedinGehrels2012).Onaverageonly~2%ofthegrainages

exceededthe±10%discordancethreshold.Sample170823-4,collectednorthofElk

LakeintheWesternOlympiclithicassemblagehadthehighestpercentageof

discordantgrainsat~5%,butthisshouldnotsignificantlyeffecttheminimumage

calculation.Thelackofdiscordantgrainssuggeststhattheformationprocessesand

theaccretionarysystemdidnotpreferentiallypreservecertainzirconages.

TwoofthesamplestestedbyU/Pbdatingrecordabnormallyoldminimum

ages.Sample170823-1collectedatthebaseofMountMathiasnexttotheBlue

glacierandsample170823-5collectedatthesouthernendoftheHohRiverBridge

recordminimumagesof45.4Maand51Marespectively.Bothofthesesamples

werecollectedintheWesternOlympiclithicassemblage,whichisthoughttobe

composedofsedimentsthatweredepositedinthetrenchandaccretedtothewedge

(BrandonandVance1992).Therearetwopossibleexplanationsfortheseold

minimumages:one,thesesamplesrepresentapreviouslyundatedremnantofthe

structurallidthatissimilarinagetotheNeedles-GrayWolfassemblageortwo,that

noneoftheyoungzirconsthatwerepresentinthesampleweredated.Considering

that~100grainspersampleweretestedand~15%ofthegrainwerefromthe

Cenozoicitispossiblethatsamples170823-1and170823-5containedyounggrains

buttheywerenottested,andthustheminimumagecalculationreturnedanon-

diagnosticallyolddepositionalage.Thesecondoptionisourpreferred

interpretationconsideringthatanunresetfissiontrackzirconsample~200m

southeastofsample170823-5recordedaminimumageofonly26.5Ma,suggesting

thatthereisn’talargesectionofstructurallidinthevicinity(BrandonandStewart

2004).Todeterminewhethertheminimumagesof170823-1and170823-5are

indicativeofanewformationorsimplyabyproductofsamplingerroranadditional

100zirconspersamplewillbedated.Bytestingmoregrainageswehopetolimit

samplingerrorandincreasetheprobabilityofdatinganyyounggrainsthatare

presentinthesamples.

5.2VitriniteReflectance:

Thevitrinitereflectancetemperaturesarelowertothewestandgethotterto

theeastandareindependentlyverifiedbyresetfission-trackzircons(figure4).

VitrinitesamplescollectedinthecoastalareasoftheHohlithicassemblagerecord

lowmaximumtemperaturesofbetween100°Cand200°Cor~5-10kmdepthbased

ona20°C/kmthermalgradient(BrandonandVance1992).Thevitrinitesamples

collectedintheWesternOlympic,Elwah,andtheeasternportionoftheGrandValley

assemblagesrecordtemperaturesof~250°C,whichwouldsuggestadepthof12.5

km.Yetfurthereastthesedimentsdidnotreachashighoftemperatures.The

westernportionoftheGrandValleyassemblagereachedathermalmaximumof200

-250°CandsamplesfromtheNeedles-GrayWolfspanfrom>200°Catthewestern

marginto~125°C,neartheHurricaneRidgefault.

Figure4:Resetfissiontrackzirconages(greentriangles;leftaxis),Vitrinitereflectancemaximumtemperatures±10%(redcircles;rightaxis)

Thevitrinitereflectancedataiscorroboratedbytheoccurrenceofthermally

resetfissiontracksinzircons.Resetfission-tracksamplesarelocatedinthe

WesternOlympicandGrandValleyassemblagesindicatingthattheserocksreached

highertemperaturesthanthefission-tracksamplesintheotherlithicassemblages

thatwerenotreset(figure4).Fissiontracksinzirconsbegintoannealabove

~200°Candfullyannealby~245°C,sothepresenceofthermallyresetfission-track

samplesinsitesthatvitrinitereflectanceanalysissuggestsreached~250°Cistobe

expected(BrandonandVance1992).Thethermallyresetfission-trackagesand

vitrinitethermalmaximumtemperaturescomefromdifferentcrystalsystemsand

havedifferentmodesofrecordingtemperature,thismeansthataconfounding

variableinthevitrinitesystemisunlikelytoaffectthezirconsystemandviceversa.

Wethereforeusetheresetfission-tracksamplestoindependentlyverifyour

vitrinitereflectancemaximumtemperaturesastheyarerelatedonlybymaximum

temperature.Bothdatasetsbroadlyshowthattheeasternassemblageswere

warmer,andmorespecificallythattheWesternOlympicandGrandValley

assemblageswere>250°C.Wethereforeconcludethatourvitrinitereflectance

maximumtemperatureapproximationsarereliableandofferadecentregional

approximationofthemaximumthermaltemperatures.

6.Discussion:

ToanalyzetheagesofthedifferentlithicassemblagesintheOSCwepropose

thattheyoungestsampleinacontiguouslithicbodyisanapproximationoftheage

ofdepositionfortheentireassemblage.ThisisacorollaryoftheStewartand

Brandon2004conceptthatyounggrainagesareaproxyforthedepositionalageof

asampleandexpandstheideatoconsiderthattheminimumsampleageina

structuralunitisaproxyfordepositionalageoftheentireunit(Stewartand

Brandon2004).Thismethodoffersageneralizedlookattherelativedepositional

agesofthedifferentsedimentarypacketsacrosstheaccretionaryprism.

Figure5:Unresetfission-trackzirconminimumages(bluecircles),U/Pbzirconminimumages(redcircles).Theimplieddepositionalageofthelithicassemblagesaredenotedbydashedlinesbasedonthedataset.Theimplieddepositionalagesfromthefission-trackzircondataset(bluedashedlines)areyoungerthanthosefromthelaserablatedU/Pbdataset(reddashedlines)likelybecausetheU/Pbsystemislessinfluencedbypartialthermalresetting.Theimplieddepositionalage

basedonacombineddatasetofthefissiontrackandlaserablatedzirconagesismarkedbytheboldblueline. ThenewU/Pbandthepublishedfission-trackminimumagesindicatethat

thesedimentaryblocksexposedintheeasternportionoftheOlympicMountainsare

olderthanthoseexposedinthewesternportion.TheU/Pbandunresetfission-track

agesbothindicatethatthereisageneraleasttowestyoungingwiththeU/Pb

depositionalagesbeingslightlyolder.Figure5showsthattheCoastalOSC(Hoh

lithicassemblage)andthewesternportionoftheUpperOSCweredepositedaround

thesametime~10Ma,theLowerOSC(WesternOlympicandGrandValleylithic

assemblages)weredeposited~18Ma,andthestructurallidcontainstheoldest

unitswiththeNeedles-GrayWolfat~32.5MaandtheCoastalRangeBasaltsat

~40.5Ma.

ItshouldbenotedthatBrandonandVance1992suggestthatthewestern

extentoftheUpperOSCiscloserinagetotheUpperOSCunitseastofMount

Olympus,howeverourcross-sectionsuggeststhatthewesternextentoftheLower

OSCismoresimilartotheCoastalOSC.Thisdiscrepancyislikelybecausethe

locationofthesedimentaryunitsandfaultlinesintheWesternOlympicsisnotas

welldefinedastheeasternunitsanditispossiblethatthewehaveassumed

differentconstraintsforthewesternassemblages(TaborandCady1978b).Itisalso

possiblethatsamplesnorthofthecrosssection,intheportionoftheHohformation

thatprojectseastward,areerroneouslylabeledasWesternUpperOSCbasedon

theirdistancefromtheKalalochLodge(pointA).Howevertheyoungestsample

labeledaspartoftheLowerOSC,whichisusedtodeterminethedepositionalage,is

locatedsouthofthecrosssectionandisthereforenotpartoftheeasternprojection

oftheHoh.WhileadditionalmappingoftheWesternOlympicswouldbenecessary

tofurtherconstrainthelocationofthedifferentunderlyingsedimentarypackets,

creatingahighfidelitygeologicmapwouldbedifficultconsideringmuchofthe

regioniscoveredbyquaternarysedimentserodedfromthecentralmassif.

Thepresenceofoldersedimentstotheeastoftherangeisconsistentwith

currentwedgetheory,whichsuggestthatthewedgehasprogressivelybuiltout

westward(summarizedinBrandon2004).Asmoresedimenthasbeenaddedatthe

margintheoldersedimentsarepusheddeeperintothewedge.Asyoutravelfrom

theHurricaneRidgefaulttothecoastyoumovedowntheaccretionarywedgewith

theearliestaccretedunitsabuttingtheCoastRangeterraneandthenewest

sedimentspresentlyentrainingatthedeformationfront(Brandon2004).The

UpperOSCwouldhavebeenthefirstsedimentarylithicassemblageaccretedduring

earlywedgeformationwhenitwasthrustbeneaththeCoastalRangeTerrane,then

themarinesedimentsthatmakeuptheLowerOSCwereincisedandprojected

beneaththeUpperOSC,andthenewestsedimentarystructuresexposednearsthe

coastwereaddedatthedeformationalfront(BrandonandVance1992).

Figure6:Maximumtemperaturescalculatedfromvitrinitereflectance%R! usingalinearcalibrationbyBarkerandPawlewicz1994forvitriniteindepositionalenvironments.Vitrinitereflectancesamples(locationsmarkedbyredcircles)werecollectedbyParkSnavely,USGS.

TheeasternsedimentaryunitsoftheOlympicSubductionComplexalso

recordthermalmaximumshigherthancoastalandperipheralregions.Inasimilar

fashionasthedepositionalages,themaximumtemperaturesshowaregionaleastto

westgradient,andonceagainthisisconsistentwithcurrentwedgetheory.When

theUpperOSCunitwasincisedandplungedbeneaththecoastalrangeterraneit

wastransportedfromthesurface,wheretemperaturesaremild,todepths

consistentwithtemperaturesexceeding200°Corabout10km(Battetal.2001).

TheLowerOSCexperiencedsimilarincisionandburialandexhibithighthermal

maximumsconsistentwithdeepburial(Battetal.2001).Thecostalregions

howeverwerelikelyneverdeeplyburiedastheyformedbyaccretingonthefront

edgeofthewedgeandhavenotbeensubaeriallyexposedlongenoughtoresultin

significanterosioninducedexhumation(Battetal.2001).Thesamplesonthe

peripheryareoutsidethesedimentarycorewerelikelysampledfromsedimentary

formationsatoptheCoastalRangeterrane.TheseCoastalRangesamplesrecord

lowtemperatures~80°Cbecausetheyhaveneverhadasignificantlayerovertopof

them.

Thesampleswereexhumedbysurfaceerosionbecauseasmaterialwas

removedfromthesurfacethesubterraneansedimentswereabletorisetotaketheir

place,asnewmaterialfromthesubductingplatecreateduplift(Pazzagliaand

Brandon2001Brandonetal.1998).Thisprocessofsurfaceremovalanduplifthas

resultedinunitsthatwereoncedeeplyburiednowbeingpresentatthesurface.The

areasintheOlympicMountainswiththehighestelevationhavegenerally

experiencedthehighestratesoferosion(Battetal.2001,MontgomeryandBrandon

2004,Ehlers2018).TheUpperOSCandLowerOSCintheeasternportionofthe

systemareatthehighdivideoftheOlympicMountainRange,sowewouldexpect

themtohaveexperiencedthemosterosionandasaresultthemostexhumation

(MontgomeryandBrandon2004,Ehlers2018).Thisisverifiedinthevitrinite

thermaldata,becauseeastofMountOlympusthevitrinitesamplesconsistently

recordmaximumtemperaturesbetween200-250°C(10-12.5km).Themaximum

temperaturedataimpliesthatatonepointtherewas10to12.5kilometersof

sedimentarycoverabovethepresentlyexposedeasternunits,howeverthecover

hassinceerodedallowingthedeepsedimentstorisetothesurface.

Onecomplicationinthevitrinitemaximumtemperaturedatasetisthatthe

maximumtemperaturesoftheElwahUnitintheUpperOSCrecordtemperatures

similartothatoftheLowerOSC.IftheUpperOSCispartofthestructurallid,as

postulatedbyBrandonandVance1992,thenwewouldexpectittoexhibitlower

temperaturemaximumreadingscomparedtothedeeperLowerOSC(Brandonand

Vance1992).However,becausetheElwahassemblageisbetweentwoLowerOSC

units,theGrandValleyandtheWesternOlympicassemblages,itispossiblethatthe

ElwahwasconductivelyheatedbyitsproximitytotherisingLowerOSCunitsthat

retainedsomeheatduringexhumation.ThisconductiveheatingeffectoftheLower

OSCtotheUpperOSCcanbeseeninfigure4asthemaximumtemperaturesinthe

NeedlesGrayWolfassemblageishighestonthemarginwiththeGrandValley

assemblage,butthemaximumtemperaturesdecreasetowardstheHurricaneRidge

ultimatelyrecordingtemperaturessimilartotheshallowcoastalunits.

7.Summary:

TheOlympicMountainsinWashingtonrepresenttheexposedaccretionary

wedgeoftheCascadiasubductionzonethatformedassedimentsfromthe

subductingFarralonandJuandeFucaplateaccretedtotheoverridingNorth

Americancontinent.Subductionbegan~35Maandcurrentlyspansfromnorthern

CaliforniatosouthernBritishColumbia(Vanceetal.1986,summarizedinBrandon

andVance1992).TheOlympicSubductionZonecanbeusedasananalogfor

generalsubductionrelatedwedgeformationtoshowthatsedimentsgenerallybuild

outwardsfromthecontinentalplate.

Usingunrestfission-trackzirconandlaser-ablatedU/Pbzirconageswehave

approximatedthedepositionalageofthesedimentarylithicassemblagesinthe

OlympicSubductionComplex.Byconsideringtheyoungestgrainsinasampletobe

indicativeofthedepositionalageoftheentirelithicassemblagewewereableto

showthattheaccretedsedimentsfurthestfromtheaccretionaryfrontwerethefirst

tobeentrainedtothecontinentandsubsequentsedimentarylayersbuiltoutaway

fromthecontinent.

Additionally,wehaveanalyzedresetfission-trackzirconsamplesand

vitrinitereflectancemeasurementstoconcludethattheeasternportionofthe

OlympicMountainsreachedahighermaximumtemperatureduringaccretionthan

themorecoastwardunits.Thissuggeststhatunitspresentlyexposedintheeastern

portionoftheOlympicmountainsarethemostexhumedsedimentsandthatatone

pointduringaccretiontheseunitswereplungedtodepthsof~12.5km.Our

findingsarecommiseratewithcurrentwedgetheoryfromBrandonandVance1992

thatsuggeststhatOlympicSubductionComplexbuildouteastwardfromtheNorth

AmericancontinentwiththeUpperOSCthrustbeneaththeCoastalRangeterrane,

theLowerOSCbeneaththeUpperOSC,andtheCoastalOSCremainingnearthe

surface.

8.Acknowledgements:

Iwouldliketothankmythesisadvisor,MarkBrandon,forhisenthusiasm

andguidance.FromdiscussingroadsidegeologyinWashingtontoeditingrough

draftsatYaleyoumadetheprocessengagingandstimulating.JeremyHouriganand

histeamatUniversityofCalifornia,SantaCruzwereintegraltoacquiringtheLA-

ICP-MSdataandI’despeciallytothankDr.Houriganforteachingmethebasicsof

isotopedatareduction.IwouldalsoliketothankParkerD.SnavleyandRichardJ.

Stewart;althoughwenevermet,thesamplesanddatathatyoucollectedwere

essentialtothisproject.ThefinancialbackingofferedbytheYaleDepartmentof

GeologyandGeophysicsbytheVanDamResearchFellowshiphelpedmakethis

projectpossible.Lastly,IwouldliketothankmyyoungerbrotherGlenMahonyand

mychildhoodfriendCharlieReinertsonforhelpingmecarrysamples(akarocks)

outofthepark.ItwasanunforgettabletripandI’msorrythatIforgottomention

thatourbagswouldbeheavieronthehikeoutthanonthehikein.

9.Appendix:

U/PbSamplesCollectedbyMahonyandBrandon:

170810-1:

Thissamplewasathick-bedded,medium-grainoverturnedsandstonefrom

theHohassemblage,collectedfromBeach3northofKalaloch.103detritalzircon

grainswereanalyzed,and2triggeredthe±10%discordancethreshold.The

minimumageofthesampleis17.6Ma.

170811-1:

Thissamplewasmedium-bedded,medium-grainsandstonewithlaminated

shaleinterbedsfromtheHohassemblage,collectednorthofHighway2750,eastof

Mt.Octopus.102detritalzircongrainswereanalyzed,and4triggeredthe±10%

discordancethreshold.Theminimumageofthesampleis19.2Ma.

170811-2:

Thissamplewasamassivemedium-grainsandstonefromtheWestern

Olympicassemblage,collectednexttoCalawahRiveronSitkum-SolDucRoad.100

detritalzircongrainswereanalyzed,and0triggeredthe±10%discordance

threshold.Theminimumageis47.4Ma.

170811-3:

ThissamplewasabeddedsandstonefromtheNeedles-GrayWolf

assemblage,collectedonHurricaneHill,westofHurricaneRidgeVisitorCenter.100



170812-1:

Thissamplewasthin-beddedturbiditesandstonewithmudstoneinterbeds

fromtheBlueMountainUnit,collectedsouthofthesummitofBlueMountain.100



170812-2:

Thissamplewasathin-beddedsandstoneandmudstonefromtheBlue

Mountainunit,collectednorthofGrayWolfRiveronForestServiceroad2860.100



170812-3:

Thissamplewasmedium-beddedsandstoneandmudstonefromtheBlue

Mountainunitclosetosample170812-2butincludesthickerandcoarsersandstone

beds.100detritalzircongrainswereanalyzed,and1triggeredthe±10%

discordancethreshold.Theminimumageis45.9Ma.

170816-1:

Thissamplewasasandstone,fromtheElwhaassemblage,collectedfromthe

EnchantedValleycliffwall.100detritalzircongrainswereanalyzed,and2triggered

the±10%discordancethreshold.Theminimumageis33Ma.

170816-2:

Thissamplewasasandstone,fromtheElwhaassemblage,collectedat

AndersonPass.97detritalzircongrainswereanalyzed,and4triggeredthe±10%


170816-3:

Thissamplewasasandstone,fromtheElwhaassemblage,collectedatthe

southendoftheAndersonGlaciertarn.102detritalzircongrainswereanalyzed,

and3triggeredthe±10%discordancethreshold.Theminimumageis28.5Ma.

170823-1:

Thissamplewasasandstone,fromtheWesternOlympicassemblage,

collectedatthebaseofMountMathiasnorthoftheBlueGlacierandMount

Olympus.101detritalzircongrainswereanalyzed,and2triggeredthe±10%


170823-2:


collectedatterminalmoraineoftheBlueGlacierasdenotedbytrailsignspostedby

theUSNationalParksService.101detritalzircongrainswereanalyzed,and0

triggeredthe±10%discordancethreshold.Theminimumageis31.5Ma.

170823-3:


collectedatthe~200mdownslopefromthebaseoftheGlacierMeadowsladder.

100detritalzircongrainswereanalyzed,and4triggeredthe±10%discordance


170823-4:


collectednorthofElkLake~10mupslopefromtheHohtrail.101detritalzircon

grainswereanalyzed,and5triggeredthe±10%discordancethreshold.The

minimumageis33.4Ma.

170823-5:


collectedatthesouthernanchorpointfortheHohbridge.100detritalzircongrains

wereanalyzed,and3triggeredthe±10%discordancethreshold.Theminimumage

is51Ma.

170825-1:

Thissamplewasalarge-grain,thick-beddedsandstonefromtheWestern

Olympicassemblage,collectedatthesummitofMountOlympusbytheconstruction

crewthatbuilttheSC03GPSin2002.Fromthesample74detritalzircongrains

wereanalyzed,and1triggeredthe±10%discordancethreshold.Theminimumage

is23.8Ma.

WhenwritingthispaperIfoundithelpfultoreviewandsummarizetheliteratureas

Iwent,howeverseveralimportanttopicsdidnotfitthescopeofthefinalproduct

andIhavedecidedtoincludethemhere.

Thickening:

Brandon’sinterpretationoftheOSCiscontingentonbothsignificantupliftof

thecoastalrangeterraneanderosiontoexhumetheCoastRangebasaltandUpper

OSC.Thedevelopmentandscaleofawedgeiscontingentontherateofincoming

materialfromthetrenchandtherateofoutflowingmaterialfromerosion(Willetet

al.2001,WilletandBrandon2002).InthecaseoftheOSCtheJuandeFucaplateis

subductingbeneaththeNorthAmericanplateatarateof32km/Maandifallofthe

incomingsedimentsareentrainedandcompressedbyafactorof65%thatwould

resultinacross-sectionalvolumeinfluxof~52km!/Maintothewedge(Brandon

2004).Basedoncurrentwedgetaperandcrosssectionalareaitwouldtake70Ma

tobuildtheOSC,whichislongerthanthe36Maofsubduction(Brandon2004).For

thisreasonfissiontrackcoolingagesareinsteadusedtoconstraintheaveragerate

ofthickeningto0.6km/Mafrom36Mato17Maand1.75km/Mafrom17Mato

present(BrandonandCalderwood1990,BrandonandVance1992).Usingthese

figuresitwouldsuggestatotalverticalthickeningof>40kmoverthelifetimeofthe

subjectionzone.Lithoprobeseismicreflectancedataofthepeninsulasuggeststhat

theaccretionaryprismiscomposedalmostentirelyofmetamorphosedsediments

andis~30kmthick(ClowesandBrandon1986).Thissuggeststheaccumulationof

sedimentsatthecontinentalmarginwassufficienttoupliftthestructurallidvia

Austroalpine-scaleregionalfoldingtoproduceanappelike-structure,whicherosion

thenremovedthetop~10kmexposingthewedgeprismandtheinclinedcoastal

rangeterrane(SummarizedinBrandon2004,BrandonandCalderwood1990).

Erosion:

Erosionintectonicallyactivehillslopesiscorrelatedwithlocalprecipitation

rates.TheOlympiccoreisthetopographichighofthepeninsulawiththeSeattle

BasintotheEast,thecontinentalmargintotheWest,theTofinoBasinstretching

fromwesternVancouverIslandintoandStraightofJuandeFucatothenorth,and

theWallipa-GraysandHarborBasinstotheSouth(TaborandCady1978a).The

forearchighislocatedatMountOlympusapproximately60kmwestoftheSeattle

Basinand200kmeastofthedeformationfrontattheconvergentmarginandthis

aeriallyexposedterraininfluenceslocalweatherandproducesadrasticwindward

andleewardprecipitationdifferential(Ehlers2018).MoistairfromthePacific

Oceandrops5,000to6,000mm/yearofprecipitationonthewesternsideofthe

Peninsulaascomparedto1,000to2,000mm/yeareastofMountOlympus(Ehlers

2018).Theheavyrainsonthewesternsideofthemountainrangecreatethe

conditionsforboththelushecosystemoftheHohRainforestandsignificanterosion.

ThehighprecipitationratesandglacialhistoryoftheOSCcausehighly

concentratederosionontheMountOlympusmassif.Thedrainagesystemofthe

westernsideoftherange,includingtheHoh,Queets,andQuinaultrivers,trace

deeplyincisedPleistocenealpineglaciervalleysfromtherange’sinterior(Ehlers

2018).TheHohriverwatershedflowsthroughbothloggedandvirginterraininthe

northwestoftheOlympicMountainsanderodesanestimated.32km/Ma(Nelson

1986).TheonlysignificantnorthwarddrainageistheElwhaRiverthathas

headwatersatthebaseofMountQueetsanddrainsthroughnaturalforestlandinto

thestraightofJuandeFucawestofPortAngeles.Researchinvolvedindam

removalsalongtheElwhaRiversuggestsriversinnativeOlympicforestlandhavea

sedimentyieldbetween.11km/Maand.18km/Ma(StokerandWilliams1991).

Easterndrainingrivers,liketheDucabush,Docewallups,andQuilcene,are

considerablysmallerthantheElwhaandHoh,butstillcontributetothetotal

sedimenterosionofthemountainrange.ByintegratingacontourofApatitethermal

closureagesBrandonestimatestheaverageerosionrateoftheOlympicMountains

is~.28km/Mawiththemostintenseerosion(0.75km/Ma)concentratednear

MountOlympus(Brandonetal.1998).Thatthehighestrateoferosioniscenteredat

MountOlympus,anareaknownfororographicprecipitationanddeeplyincised

glaciervalleys,agreeswiththecurrenttheorythaterosionratesoftectonically

activeslopesishighlydependentonmeanlocalrelief(MontgomeryandBrandon

2002)andprecipitation(Willet1999).Theerosionrateof~.28km/Maisalso

withintherangeoftheElwhaandHohsedimentationrates(Brandonetal.1998)

suggestingthatmostoftheerodedsedimentsareremovedfromtheOlympic

interiorbytheriversystems.Thesefluvialsedimentsareultimatelydepositedon

theCascadiaAbyssalPlane,eitherdirectlyorafterabriefresidenceinthePuget

SoundbeforebeingtransportedthroughtheStraitofJuandeFuca(Brandonetal.

1998).Itshouldbenotedthatrecentfindingsin(Ehlers2018)suggestslightly

higherratesoferosionfromalowof.25km/Matoahighof.9km/Mawiththe

highestratesoferosionstillfocusedonthecentralmassif.

Erosionratesappeartohavereachedasteadystatenearly14Maago

(Brandonetal.1998,PazzagliaandBrandon2001,Ehlers2018).Byanalyzingthe

sinuosityofwestwarddrainingriversrelativetoflatbedrockterracesPazzagliaand

BrandonsuggestthattheincisionanderosionratesofriversintheOlympic

Mountainshavebeenrelativelyconstantforthelast~100ka,butcanbehighly

variableonshortertimescalesduetoperiodsofglaciation(PazzagliaandBrandon

2001).Thedependenceontheinterglacialcyclematchesfissiontrackfindingsfrom

Ehlers,whichrequiredsignificantlyhighererosionalratesstarting2to3Maduring

thePleistoceneglaciation(Ehlers2018).AdditionalfissiontrackdatafromBrandon

suggestthaterosionrateshavebeenbroadlyconsistentforthelast7Ma,andzircon

coolingagesfromthecentralmassifsuggesttheerosionratestherehaveremained

constantforthelast14Ma(Brandonetal.1998).Ifexhumationbegan14Maand

maintainedpresentrates10.5to12.5kmhavebeenremovedfromatoptheOSC

interior,thiswouldbesufficienttoremovethestructurallidtoexposethe

previouslyburiedaccretionaryprismandaheavilyinclinedcoastalrangeterrane

(Brandon2004).

VitriniteMeasurements:

Vitrinitereflectanceiscommonlyusedtoconstrainthemaximum

temperatureofhumicorganicmaterials.Vitriniteisametamorphicproductformed

whenthewoodytissueofplantsisthermallycooked.Inbothlaboratoryand

geologicalconditionsithasbeenshownthattheratioofincominglighttoreflected

light,knownasvitrinitereflectance(%R! ),ishighlycorrelatedwiththemaximum

temperatureofexposure(summarizedinSweeneyandBurnham1990).Theoiland

gasindustrypioneeredvitrinitereflectanceanalysisasamethodformeasuringthe

thermalmaturityofdownholesedimentsinordertopinpointtheidealdrilling

depthinareserve,referredtoastheoilwindow(Hunt1979,andmanyothers,

summarizedinSweeneyandBurnham1990andBarkerandPawlewicz1986).

Vitrinitereflectancehasalsobeenusedtodeterminethethermalmaturityof

sedimentsnearintrusionsandindepositionalenvironments(Barkeretal.1986,

Barker1989,summarizedinSweeneyandBurnham1990).

Thevitrinitereflectance(%R! )wasdeterminedusingbestpracticesoutlined

bytheAmericanStandardsforTestingMaterials(ASTMD7708).Foreachsamplea

thinsectionwasmadeandanalyzedunder500xmagnificationinoilimmersionbya

microscopeequippedwithasensitivephotometricdevice.Themicroscopestage

wasleveledandtheaperturefocusedtolimitglare.Thevitriniteinthesamplewas

thenidentifiedusingstandardpetrographytechniquesandthestageadjustedsothe

photometerwastrainedonthemineral.Theilluminationaperturewasthenclosed

andwithnolightreflectingoffthesamplethephotometerwassettozero,

accountingfordarkcurrentbiasbasedonthedevicespecifications.Tocalibratethe

systemthevitrinitewasexposedtowhitelightandthephotometerreadout

recorded;withoutchangingthesettingsthestagewasadjustedtofocusonaseries

ofstandardswithsimilarreflectance’sandthevaluesrecorded.Thesystemwas

calibratedsothatthestandard’sreflectancewaswithin0.001%ofpublishedvalues.

Then,usingnon-polarizedwhitelight,thereflectanceofnon-pittedbutotherwise

randomsamplesofvitrinitewasrecorded.Once20-30randomizedmeasurements

wereobtaineda%R! probabilitydensityplotiscreated.Unlessotherwisestated,

thevitrinitereflectancevaluesgivenhereinaremeanrandomvitrinitereflectance

(%R! ),where%R! andassociatedstandarderrorsarethatofthe%R! probability

densityplotsbasedon20-30vitrinitereflectancemeasurements(summaryof

vitrinitereflectancemethodologybasedextensivelyonAMTMD7708).

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