Detrital Zircon Geochronology and Vitrinite …...2018/05/02 · 1. Abstract: The Cascadia...
Transcript of Detrital Zircon Geochronology and Vitrinite …...2018/05/02 · 1. Abstract: The Cascadia...
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
detritalzircongrainswereanalyzed,and2triggeredthe±10%discordance
threshold.Theminimumageis36.9Ma.
170812-1:
Thissamplewasthin-beddedturbiditesandstonewithmudstoneinterbeds
fromtheBlueMountainUnit,collectedsouthofthesummitofBlueMountain.100
detritalzircongrainswereanalyzed,and1triggeredthe±10%discordance
threshold.Theminimumageis43.3Ma.
170812-2:
Thissamplewasathin-beddedsandstoneandmudstonefromtheBlue
Mountainunit,collectednorthofGrayWolfRiveronForestServiceroad2860.100
detritalzircongrainswereanalyzed,and0triggeredthe±10%discordance
threshold.Theminimumageis45.9Ma.
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%
discordancethreshold.Theminimumageis32.1Ma.
170816-3:
Thissamplewasasandstone,fromtheElwhaassemblage,collectedatthe
southendoftheAndersonGlaciertarn.102detritalzircongrainswereanalyzed,
and3triggeredthe±10%discordancethreshold.Theminimumageis28.5Ma.
170823-1:
Thissamplewasasandstone,fromtheWesternOlympicassemblage,
collectedatthebaseofMountMathiasnorthoftheBlueGlacierandMount
Olympus.101detritalzircongrainswereanalyzed,and2triggeredthe±10%
discordancethreshold.Theminimumageis45.4Ma.
170823-2:
Thissamplewasasandstone,fromtheWesternOlympicassemblage,
collectedatterminalmoraineoftheBlueGlacierasdenotedbytrailsignspostedby
theUSNationalParksService.101detritalzircongrainswereanalyzed,and0
triggeredthe±10%discordancethreshold.Theminimumageis31.5Ma.
170823-3:
Thissamplewasasandstone,fromtheWesternOlympicassemblage,
collectedatthe~200mdownslopefromthebaseoftheGlacierMeadowsladder.
100detritalzircongrainswereanalyzed,and4triggeredthe±10%discordance
threshold.Theminimumageis24.4Ma.
170823-4:
Thissamplewasasandstone,fromtheWesternOlympicassemblage,
collectednorthofElkLake~10mupslopefromtheHohtrail.101detritalzircon
grainswereanalyzed,and5triggeredthe±10%discordancethreshold.The
minimumageis33.4Ma.
170823-5:
Thissamplewasasandstone,fromtheWesternOlympicassemblage,
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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