fabrics of kink bands in the DarringtoGeometrymicrostructuresand within.docx
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fabricsofkinkbandsintheDarringtoGeometrymicrostructuresandwithin
Geometry,microstructures,andmagneticfabricsofkinkbandsintheDarrington
Phyllite,northwesternWashington,USA:
Processeswithinfixed-hingekinking
美国华盛顿州西北部达灵顿千枚岩的几何形状、纤维构造、磁组构，膝折带：
固定枢纽扭折过程
Keywords:
KinkbandPhylliteDeformationMagneticanisotropyThinsectionMicrofabric
Field,thinsection,andmagneticfabricanalysesofmonoclinal,contractionalkinkbandsintheDarringtonPhylliteofnorthwesternWashington,USA,provideevidenceofkinematicevolutionandmicrostructuralprocessesduringkinking.Relationshipsamonginternal,external,androtationanglesofkinkbandssuggestthatrotationwasaprimarymechanismofkinking,andasignificantnumberofkinkbandsdeviatefromthe“ideal”kinkbandgeometry（equalinternalandexternalkinkangles）.Dilationspaces（voids）filledwithunrecrystallizedquartzand/orcalciteinsidekinkbandsandalongtheboundariesarecommonlyobservedinkinkbandswithunequalkinkangles.Evidenceforlocalizedinterlayerslipinsidekinkbandsincludesmicagrainsprotrudingintothehingevoidsandstair-stepgeometryofthevoids.Interlayerslipallowedforrotationoftheinternalfoliationduringkinking.Pressuresolutionisnotubiquitousinsidekinkbandsfromthestudyarea;however,significantpressuresolutionisassociatedwith“locked”kinkbands（equalkinkangles）.Anewapproachusingtheanisotropyofmagneticsusceptibility（AMS）showsthatthemagneticfabricandsusceptibilityinsidethekinkbandsiffermorefromtheoutsidefabricthancanbeexplainedbysimplerotationoftheoutsidefabricintothekinkattitudes.Combined,ourobservationssuggestthatfixed-hingekinkingoperatedatthislocation,andthatveining,interlayerslip,andpressuresolutionwerethedominantdeformationmechanisms.
1.Introduction
Kinkbandsarecommonstructuresincontractionaldeformationzones,butmechanism（s）ofkinkingatboththemacro-andmicroscalesarepoorlyunderstood.Despitearichliteratureinfieldobservationsanddeformationexperiments,fewstudiesreportanalysesofthemicrofabricswithinkinkbandsthatcouldilluminatetheprocessesofkinkbandgrowth.
Thinsectionandmagneticfabricanalyseshavebeenperformedonkinkbands（e.g.PatersonandWeiss,1966;Verbeek,1978;StewartandAlvarez,1991;KirkerandMcClelland,1997）;hereweadvancethisworkbyconnectingobservedmicrofabricsassociatedwithkinkbandstospecifickinkingmechanism（s）.
Kinkbandsaresmallmonoclinalfoldswithsharpparallelhinges
（Fig.1）thatoccurinfine-grained,anisotropicrocks.
Bothcontractionalandextensionalkinkbandsarepossible（Dewey,1965）;thispaperfocusesonlyonthecharacteristicsofcontractionalkinkbands.
Asinglekinkbandconsistsoftwolong（undeformed）limbsbracketingashorter“kinked”（deformed）limb,andthetracesoftheaxialplanesofthehingesmarktheboundariesofthekinkband.
Previousworkershavedefinedanumberofanglesinkinkbands
（e.g.Verbeek,1978;Fig.1）:
theexternal（acute）angle（a）between
theundeformedfoliationandthekinkbandboundary;theinternal
angle（b）betweenthekinkedfoliationandtheboundary;andthe
rotationangle（k）throughwhichthekinkedfoliationhasbeen
rotated,wherek¼180eaeb（Weiss,1980）.
Fieldstudiesofkinkbands（e.g.Anderson,1964;Clifford,1968;Fyson,1968;Hobson,1973;Verbeek,1978;KirschnerandTeixell,1996）havefocusedmainlyonoutcrop-scalefeaturesandgeometries,usuallyqualitativelyandinthecontextofaregionaldeformationframework.
Analogandrockexperiments（e.g.PatersonandWeiss,1966;Anderson,1974;StewartandAlvarez,1991）providedsomeobservationsofmicrostructuresandfabricinthinsection,but
detaileddescriptionsofnaturalkinkbandsinthinsectionare
uncommonintheliterature.
Observationsonthegeometryofkinkbandshavebeenusedtoproposeandevaluatemodelsforkinkbanddevelopment（e.g.Dewey,1965;Verbeek,1978;Weiss,1980;StewartandAlvarez,1991）,generallyfocusingontheangularrelationshipsbetweenthefoliationandthekinkbandboundariesandonlymarginallyonthegrain-scalecharacteristicsoftherocks.
Microstructuresandmagneticfabricsrelatedtokinkbandscan
yieldinformationondeformationmechanismsandstrainwithin
thekinkbands（e.g.StewartandAlvarez,1991;Kirkerand
McClelland,1997）,whichcanberelatedtokinkbandgeometry
andmodelsofkinkbanddevelopment.
Themechanicsbywhichkinkbandsdevelopareunresolved;
previousworkershavegenerallyadheredtooneoftwomajor
kinematicmodels.
Themobile-hingemodelofkinkinghasbeenchampionedbymany（e.g.PatersonandWeiss,1966;Weiss,1980;StewartandAlvarez,1991）.
Inthismodel,kinkbandsaccommo-dateshorteningbyincorporatingnewmaterialasthehingesmigrateoutwardintotheexternal（undeformed）mediumandmaterialisfoldedandthenunfoldedasthekinkbandwidens（Weiss,1980）.
Evidenceforhingemigrationhasbeenseeninthinsection（PatersonandWeiss,1966）andindeformationexperiments（PatersonandWeiss,1966;GayandWeiss,1974;StewartandAlvarez,1991）.
Thegeometricconstraintsofthemobile-hingemodelrequirezerodilationinsidethekinkbandandequalkinkanglesaandb（see
Fig.1）.
Thefixed-hingemodelofkinkinghasbeeninvokedinseveralfield-basedstudies（e.g.Dewey,1965;Hobson,1973;Verbeek,1978）.
Kinkbandsinthismodeldevelopviarigidrotationoftheinternalfoliationbetweentwofixedhinges.
Thisrequiresdilationbetweentheinternalfoliationplanesandalongthekinkbandboundariesatlowrotationangleswhenb>a;deformationceaseswhenthekinkband“locks”atb¼a（Verbeek,1978;StewartandAlvarez,1991）.
Forbothmodels,kinkbandswithequalanglesaandbaretakenasthe“ideal”（mobile-hinge）or“locked”（fixed-hinge）case.
Infixed-hingekinking,kinkbandswithlowrotationanglesandb>aareintheearlystagesofformation.
ThecasebBoththemobile-andfixed-hingekinkingmodelsrequireinterlayerslipinsidethekinkband（PatersonandWeiss,1966;Weiss,1980;StewartandAlvarez,1991）,sinistralslipinsidedextralkinkbands（Fig.1）andviceversa（e.g.Verbeek,1978;Rousell,1980）.
Someworkershavearguedthatbothmechanismsoperateunderdifferentconditions,dependingontheamountofstrainaccommodated（StewartandAlvarez,1991）ortheconfiningpressureoftherocksatthetimeofkinking（Anderson,1974）.
Itisalsopossibletoimaginehybridkinematicsthatinvolvebothrotationofthekinkedlimbandmigrationofitsboundaries（e.g.TwissandMoores,1992）.
Theroleofothermechanisms（e.g.pressuresolu-tion）ispoorlyaddressed,thoughStewartandAlvarez（1991）suggestthatpressuresolutioncouldbeinvokedasahinge-migrationmechanism.
Weusefield,thinsection,andmagneticfabricanalysesto
characterizethemacro-andmicro-scalefeaturesofkinkbandsin
theDarringtonPhylliteofnorthwesternWashingtoninorderto
understandwhatprocessesoperateonthegrainscalewithinkink
bandsandhowtherockcharacteristicsaffecttheformationofkink
bands.
Fieldandthinsectionobservationsprovideadetailedrecord
ofthevariationsinkinkbandgeometryandrelatedfeatures.
Wealsopresentanewmethodthatexaminesmagneticfabricsfrom
outsideandinsidekinkbandstoquantifychangesduetokinking.
Theseobservationsareusedtoassesswhetherinterlayerslipand/
orpressuresolutionoccurredduringkinking.
Thecharacteristicsofthekinkbandsarealsousedtoevaluatetheapplicabilityofthemobile-and/orfixed-hingekinkingmodelstothekinkbandsintheDarringtonPhyllite
2.stadyarea
Adensenetworkofmonoclinalkinkbandsrecordslatedeformationofawell-developedfoliationintheDarringtonPhylliteon
SamishIslandinnorthwesternWashington（Fig.2）.
TheDarringtonPhylliteispartoftheShuksanMetamorphicSuite（Misch,1966;Brown,1986;Gallagheretal.,1988）,whichcomprisesoceanfloorsedimentaryandigneousrocksthathaveundergonesubductionrelatedmetamorphismandseveralsubsequentdeformationevents（Haugerud,1980;Haugerudetal.,1981;Brown,1986）.
OnSamishIsland,theDarringtonPhylliteisamicaceousquartzose
phyllitethatunderwentblueschist-faciesmetamorphismat~154
Ma（Lamb,2000）.
Metamorphismandrecrystallizationaresufficientlyintensethattheoriginalsedimentarylayeringoftherockisobscured,althoughthereisabroadtrendofincreasinggrainsizeandincreasedquartzfromNEtoSWintheexposedrocks
（Lamb,2000）.
ThestructuralhistoryofSamishIslandismultistage
andcomplex,withatleastfivedeformationepisodes（Lamb,2000;
Schermeretal.,2007）.
Atleasttwofoldingeventsthatpredatedthedevelopmentofkinkbandswereaccompaniedbydevelopmentofveinsandpressuresolutioncleavageswhoseorientationsandfabricassociationsaredistinctfromlaterkinkbandsandkinkrelatedpressuresolutionseams;thefoldingeventswerefollowedbymultiplefaultingeventsandveindevelopmentduringaperiodofextension（Lamb,2000;Schermeretal.,2007）.
Themostrecentdeformationeventsincludedevelopmentofkinkfolds,strike-slipfaults,andpossiblyopenfoldingofallpreexistingstructures.
FieldobservationsandpreviousthinsectionanalysesbyLamb
（2000）suggestthatkinkfoldingoccurredafterallfabric-forming
events:
kinkbandsarenotablyabsentinareaswithstrongly
developedF2foldsorV2veins,andboththeS1andS2cleavages
arekinked.
Althoughso
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