Tectonophysics610(2014)138–149 ContentslistsavailableatScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto A weakening mechanism for intermediate-depth seismicity? Detailed petrographic and microtextural observations from blueschist facies pseudotachylytes, Cape Corse, Corsica N.Desetaa,⁎,T.B.Andersenb,L.D.Ashwala aDepartmentofGeosciences,UniversityoftheWitwatersrand,PrivateBag3,2050Johannesburg,SouthAfrica bDepartmentofGeosciences,CEED,UniversityofOslo,POBox1047,Blindern,0316Oslo,Norway a r t i c l e i n f o a b s t r a c t Articlehistory: Gabbro-andperidotite-hostedpseudotachylytesfromtheAlpineSchistesLustresUnitinCorsica,previously Received9July2013 determinedtohaveformedatblueschisttolawsonite-eclogitefaciesconditions,havebeencausallylinkedto Receivedinrevisedform16October2013 thegenerationofintermediate-depthearthquakes,whichoccuratdepthsof50–300km.Detailedpetrographic Accepted3November2013 andmicrotexturalanalysesofthesepseudotachylytessuggestthattheirinitiationmaybecontrolledbya Availableonline12November2013 thermally-activatedshearrunawayprocessthatiscontrolledbyrheologyratherthanmineralogy.Thisisdocu- mentedbyshearedout,prolate,kinkedandtwinnedwallrockclaststhathavebeenpeeledoffandentrained Keywords: into the pseudotachylyte vein as sigmoid survivor clasts. The presence of metastable high temperature Shearlocalisation Intermediate-depthearthquakes crystallisationproductsinthepseudotachylyte,suchashoppersanddendritesofolivine,enstatiteanddiopside Pseudotachylyte (peridotite)andAl-richomphaciteandFe-richanorthiteinmetagabbro,aresuggestiveofashort-livedhigh- blueschist temperature event resulting from thermal instability. These high temperature mineral assemblages are overprintedbyonesindicatingareturntoambientconditionsoflowertemperatures,butstillhighpressures: glaucophane,albiteandepidoteinmetagabbroandclinochlore;andfine-grainedgranoblasticolivine,enstatite and diopside in peridotite. The observations from this detailed study of natural samples suggest that intermediate-depthseismicitymaybegeneratedbyathermalrunawayprocess. ©2014ElsevierB.V.Allrightsreserved. 1.Introduction andmicrostructuralobservationsofperidotite-andmetagabbro-hosted pseudotachylytesassociatedwithsubductionzoneseismicity,inthe Theinitiationofintermediate-depthearthquakeshaslongbeena CimadiGrateraareaofCapCorse,Corsica.Previousworksuggeststhat subjectofdebate.Thesephenomenaoccuratdepthsfrom50kmto faultingandpseudotachylytegenerationtookplaceduringsubduction 350km,which,duetohighconfiningpressures,precludetraditional atblueschisttolawsonite-eclogitefaciesconditionsunderpressuresof brittlefailure(GreenandHouston,1995;Hacker,2003;Jungetal., 1.8–2.6GPa(AustrheimandAndersen,2004;AndersenandAustrheim, 2004; Ogawa, 1987). In order to address this problem researchers 2006;Ravnaetal.,2010;VitaleBrovaroneetal.,2011).Adetaileddiscus- haveputforthseveralhypotheses,whichincludedehydrationembrit- siononthegeochemistryoftheserocksandtherolethatwaterplaysin tlement,transformationalfaultingandthermalrunawayprocesses. earthquakegenerationwillbeaddressedinaseparatepaper. Thesehypothesescanbedividedintobrittle—(solid-statedehydration embrittlementandtransformational faulting) and crystal-plastic — (shear-heating and thermal runaway) controlled processes. These 2.Geologicalsetting modelsarebasedonexperimental,numericalandgeophysicalmodel- ling, with no field observations and little work on natural samples ThestudyareaislocatedontheSSWsideofCimadiGratera,Cape (GreenandHouston,1995;Hacker,2003;Johnetal.,2009;Kelemen Corse,northernCorsica(Fig.1).Thepseudotachylytes,firstdescribed andHirth,2007;Ogawa,1987).Inthepasttwodecadeshowever,sever- byAustrheimandAndersen(2004),occurwithinlensesofgabbroand al discoveries of high pressure pseudotachylytes associated with mantleperidotiteenclosedbyserpentinite(Fig.2).Theserocksform intermediate-depth earthquakes have been made (Austrheim and part of the Schistes Lustres Complex (part of the Alpine age high Boundy,1994;Jinetal.,1998;JohnandSchenk,2006;Kanamorietal., pressure–lowtemperaturesubductioncomplex),andwhichhasbeen 1998), providing researchers with natural material with which to interpretedaseithernappesofexhumedLigurianoceaniclithosphere, evaluatepreviousmodels.Thispaperpresentsdetailedpetrographic which have slivers of crystalline continental material, or hyper- stretchedcontinentallithosphereinterleavedwithmantleimbricates ⁎ Correspondingauthor. (Agardetal.,2002;Beccaluvaetal.,1977;Jolivet,1993;Mohnetal., E-mailaddress:[email protected](N.Deseta). 2009;VitaleBrovaroneetal.,2011).Thisrockpackagewasthrustonto 0040-1951/$–seefrontmatter©2014ElsevierB.V.Allrightsreserved. http://dx.doi.org/10.1016/j.tecto.2013.11.007 N.Desetaetal./Tectonophysics610(2014)138–149 139 Fig.1.SimplifiedgeologicalmapofthestudyareainCimadiGratera.ModifiedafterAndersenandAustrheim(2006). thecontinentalmarginofEuropeduringtheLateCretaceoustoTertiary Intheperidotite,thepseudotachylytesoccurintwosetsthatextendfor Periods(Fig.1)(Jolivet,1993). upto1km:asub-verticalsetandasub-horizontalset.Withinthevein Themetagabbrosarecompositionallyuniformandencompassa setsthepseudotachylytesformcomplexveinnetworksthatover-print rangeofigneoustexturesmarkedbydifferencesingrainsize.Common and re-inject one another, indicating multiple generations of texturesarecumulatelayeringandtheinterfingeringofirregularly pseudotachylyte(Fig.2).Intheperidotite,thepseudotachylyteveinsoc- shapeddomainsoffine-andcoarse-grainedgabbro.Conversely,the casionallyformradial‘explosive’networks.Theseveinsarethickerthan peridotitesarerelativelyuniformintermsofcomposition,textureand otherinjectionorfaultveinsandcontainmorere-injectionsandcommi- grainsize.Thepseudotachylyte-bearingfaultrockshavebeenpartially nutedwallrockmaterial(Fig.2b).Incontrastwiththeperidotite-hosted metamorphosedtoblueschistandgreenschistfaciesonlyinpatches,ex- veins,thoseinthegabbroarethinnerandmorediscrete,commonly(but ceptwithintheshearzoneswherethemetamorphicreactionsarefully notalways)occurringalongtheboundarybetweentheverycoarse- equilibrated.Thepseudotachylytesoccurwithinthepristinelensesof grained(b15mm)metagabbroandfine-grained(b2mm)metagabbro gabbroandperidotitethatarerelativelyundeformedandleastaffected (Fig.2).Theperidotitepseudotachylytesshowcross-cuttingrelation- bytheregionalHP–LTmetamorphism(AndersenandAustrheim,2006). ships with serpentinised host rocks, which have been entrained intotheveinsassigmoidallozenges,indicatingabrittle–ductileover- 2.1.Fieldobservations printrelationship.Inthemetagabbrofaultrock,thepseudotachylytic crystallisation products (glaucophane) have formed CPO (crystallo- Inoutcropthepseudotachylytestypicallyhaveapositivereliefwith graphicpreferredorientation)fabricsandcontainboudinagedwallrock respecttothehostrocks.Thepseudotachylyteveinsweathertoarust- clasts, indicating a ductile overprint post-dating pseudotachylyte redcolourbutonfreshsurfacesareblack-greyandaphanitic(Fig.2). generation (Fig. 12) (Andersen and Austrheim, 2006). Many of the Comminutedwallrockclastsandflowbandingarecommonlyobserved. pseudotachylytesarecutbylaterserpentineveinsandshowahydration 140 N.Desetaetal./Tectonophysics610(2014)138–149 wallrock,entrainedmineralsaswellascrystallisationproducts(glass andcrystallites)weredone.Thebeamcurrentwasreducedtominimise sodiumlossonglass,buttestsonhighercurrentsshowedlosstobe negligible.Analyseswererunat20kV,10nA.Thereferencestandard usedwasaNi-bearingglass.RefertoMerlet(1994)formoredetailed operatingparameters. 4.Petrographicandmicrotexturalobservations Detailedpetrographicmicrotexturalanalyseswerecarriedouton thepseudotachylyteandtheadjacentwallrockinboththemetagabbro andperidotiteinordertoascertaintherheologicalbehaviouroftherock atthetimeoffusion.Featuresofthehostrocksandmicrotexturescom- montobothrocktypeswillbediscussedinthefirstsection,followedby thosecharacteristictoonlythemetagabbroortheperidotite. 4.1.Metagabbrohostrock Themetagabbrohostrockisheterogeneousingrainsizewithlarge, irregulardomains(uptoapproximatelyametreinoutcrop)ofvery coarse-grained(upto15mm)gabbrooccurringincontactandinter- fingeringwithamuchmorefine-grained(~1mm)gabbro.Themargin betweenthecoarse-andfine-graineddomainsisconsistentlysharp, lessthan5mmthick.Despitethelargegrain-sizevariabilityinthe metagabbro,theconstituentmineralassemblageisnotsignificantly variable. Theprimarygabbromineralogyislargelypreservedandcomprises plagioclase,diopside,olivineandminorilmenite.Inthinsectionthe gabbroadjacenttopseudotachylytefaultsretainslittleofitsoriginalig- neoustexture,mostofwhichhasbeentransformedintoanannealed granoblastictexturewithpoikiloblastsofolivineanddiopside.Grain boundarymigrationanddynamicrecrystallisationarecommon,partic- ularlyindiopside.Earlygreenschistfaciesmetamorphismofthehost rockhasledtovariablereplacementandrecrystallisationofdiopside by actinolite, bastite and Mg-hornblende. Plagioclase alteration to sericitehasalsotakenplace,causingthegrainstobecomecloudyand grey.Alterationoftheolivinetoserpentine,magnetiteoriddingsite hasalsobeenobserved.Post-datingtheearlygreenschistalterationis thedevelopmentofblueschistfaciesassemblages,whichmanifestin thereplacementofdiopside,actinolite,Mg-hornblendeandplagioclase byglaucophane,barroisite,albiteandepidote.Lateblueschistfacies metamorphismthatpost-datespseudotachylytegenerationatthese conditionsisassociatedwithductiledeformationandpseudotachylyte recrystallisation(Fig.12).Lateretrogrademetamorphism,particularly markedbyserpentineveinsandthepresenceofepidote,clinochlore and pumpellyite overprints the blueschist assemblage phase and patchesofthepristinematerial(Fig.13). Theearlygreenschistmetamorphismofthehostrockmaybeassoci- Fig.2.Photographsofpseudotachylytehostedbyperidotite.Panelashowsoneofthe atedwithseafloorhydrationand/orhydration-associatedfracturingin thickest(upto25cm)pseudotachylytefaultveinsobservedinthefield.Thethickness theslabbend,aswellasearlierhydrationassociatedwithextensional ofthisveinisprincipallyduetomultiplegenerationsofpseudotachylytethathave nucleatedalongthesameplane.(b)Explosivenetworkveiningofpseudotachylytesthat tectonics(Mohnetal.,2009;VitaleBrovaroneetal.,2011).Thelate radiateinalldirectionsawayfromthecentreofthefaultplane.(c)Cross-cuttingrelation- greenschistmetamorphicoverprinthasbeenobservedinthinsection shipsofpseudotachylyteself-injecting. andBSEimagesandisinterpretedtobeassociatedwithhydrationand faultinguponslabexhumation.Thislateretrogrademetamorphism overprintssomeofthepseudotachylyteveinsandisnotcutbylater overprint. Care was taken to analyse only pristine, unaltered pseudotachylytegenerations(Fig.13). pseudotachylytes. 4.2.Peridotitehostrock 3.Materialsandmethods Theperidotitehostrocksshowgradationaltransitionsfromfine-to Detailedthinsectionpetrographyandback-scatterelectron(BSE) coarse-grained(b1mm–7mm)textures.Themineralogycorresponds imaging for mineral identification and microtextural analysis were tothatofaplagioclaselherzolitewitholivineNdiopsideNenstatite,as doneonbothgabbro-andperidotite-hostedpseudotachylyte.Electron wellasminorplagioclaseandmagnetite.Therocksexhibitagranoblastic microprobeanalyseswereconductedusingaCamecaSX-100instru- texturewithannealedpoikiloblastsandsomegrainboundarymigration. mentatSpectrauLaboratory,UniversityofJohannesburg,SouthAfrica. Thediopsideandenstatitecommonlyshowexsolutionlamellaeofeach Analysesofthebulkmatrix(widebeam(10μm)andnarrow(1μm)), other and twinning of the diopside and olivine has been observed. N.Desetaetal./Tectonophysics610(2014)138–149 141 Fig.3.Imagesofmicrostructuresatfaultveinboundariesinmetagabbro.Alltheseimagesclearlyshowthatwallrockmaterialattheseboundarieshasbeensubjectedtocrystalplastic deformation.Panelashowsaboudinaged,kink-bandeddiopsideenclosedinadarkbrown,glassypseudotachylytematrix.(b)Pseudotachylyteveinwithentrained,boudinaged,wallrock aggregateofplagioclaseanddiopside.Thewallrockdiopsideandplagioclaseclearlyshowkinkinganddeformationtwinning,respectively. There is some early greenschist facies metamorphism of diopside associated with ocean–continent hyperextension and hydrothermal and enstatite to fine-grained clinochlore and tremolite by dynamic alterationduetoprogradesubduction(VitaleBrovaroneetal.,2011). recrystallisation.Theclinochlorehasaplumoseandfeatherytexture. Thelategreenschistfaciesmetamorphismislikelyassociatedwithslab Variousgenerationsofserpentinisation(includingmagnetiteandCr- exhumationprocesses. spinel)overprintallpre-existingfeaturesandarevariablydistributed throughouttherock;occasionallyassociatedwithlatereactivationof 4.3.Faultveincharacteristics pseudotachylyteveins(Fig.12c).Theperidotiteshowsnoblueschist facies mineral assemblages; only multiple stages of early and late Faultveinsformparalleltodisplacementsurfacesinthehostrock. greenschistmetamorphismareobservedpriortoandaftertheperiod(s) Wallrockclastslocallyunderwentcrystalplasticdeformationinprox- ofpseudotachylytegeneration.Theearlygreenschistmetamorphismhas imitytoveinboundariesormeltedalong/withthefaultplane(Fig.3). beenattributedtoeventsoccurringpriortopseudotachylytegeneration Microscope-andBSE-basedobservationsofthepseudotachylytevein Fig.4.Micrographsoffine-grained,recrystallizedperidotite-hostedpseudotachylyte(PST1)cutbylaterpseudotachylyte(PST2).Panelbisahighmagnificationviewwithcrossednicolsof wheretheredarrowispointingtoinpanela.Inpanelbthegrainsenclosedbytheglassyblackmeltveinsareprolateandlozenge-shapedcomparedtothosegrainsnotincontactwithPST2. Grainboundarymigrationissuggestedbythegrainboundaryalignmentofgrainsenclosedbyfaultveins.Incontrasttothepreviousfigure(Fig.5),wherecoarser-grainedwallrockwas deformed,thesephotomicrographsshowfine-grainedrecrystallisedperidotite-hostedpseudotachylytecutbyalatergenerationofpseudotachylyte. 142 N.Desetaetal./Tectonophysics610(2014)138–149 Fig.5.BSEimageofapseudotachylytefaultveinhostedbymetagabbro.Thedarkgreythermallyroundedclastsareplagioclaseuponwhichdendritesofomphacitehavecrystallisedwith interstitialglass.Thesmallerveinscross-cuttingthemainpseudotachylyteveininthisimage,arecontemporaneouswiththemoltenpseudotachylyte,ascanbeseenbythelobate–cuspate boundariesenclosingthedendrites.Thesecross-cuttingveinsappear,bytheirlowatomicnumbersandlowtotals(onEPMAanalysis)tohavecontaineddissolvedhydrousfluidsthatlater vesiculated.Plag:plagioclase,PlagWR:Wallrockplagioclase,Omph:omphacite. boundariesrevealasyndeformationalzoneintheassociatedwallrock 4.5.Microtexturesofultracataclasite thatbeginsseveralgrainwidthsfromthevein‘proper’.Thisregionis dominatedbyhighlystrained,sheared,kinked,elongatewallrockgrains Syndeformationalultracataclasiteatthemarginsofcoarse-grained (Figs.3,6).Theveinboundaryissharplycutbythepseudotachylyteand pseudotachylyte fault veins is a common feature in both the hascommonlybeenobservedtobedraggedorpeeledoffintothevein metagabbroandperidotitehostrocks.Theultracataclasitecomprisesa andissurroundedbyinjectingmelt(Fig.3b). mixtureofcomminutedclasts,meltandplasticribbons(Fig.6).The Creeptexturesassociatedwiththeformationofthepseudotachylytes sizerangeincomminutedwallrockmaterialisfrom30μmb1μm. wereobservedinthefullrangeofgrainsizesobservedinthewallrock; Themineral assemblage in theultracataclasitematches thatof the from20mmto30μm(Figs.3and4),fromshearingandkinkingas adjacentwallrock.Thedeformationofdifferentmineralspeciesinthe seen in the coarser-grained material (Fig. 4) to what appears to be ultracataclasiteappearstobedeterminedbyfracturetoughness,aspre- grainboundarycreepinfine-grainedre-crystallisedpseudotachylytes dictedbySpray(1992).Mineralswithgreaterrelativefracturetoughness thatweresubsequentlyreactivatedtogenerateamelt(Fig.4). suchasdiopside,plagioclase,Mg-hornblendeandolivinetypicallyform Thethicknessoffaultveinsrangesfromlessthan1mmtomore theportionofcomminutedgrainsandmineralribbons.Softerminerals than30cm.Faultveinsweretypicallyformedbyasingleevent.However, suchasclinochlore,tremolite,serpentineandactinolitecomprisethe reactivation and multiple fusion events are exemplified by older meltandtherestoftheribbonportionintheultracataclasite(Fig.6).It pseudotachylytesurvivorclastsentrainedintoyoungerveins(Fig.4) ispossiblethatsomeofthedisplacementinthefaultveinsisaccommo- andbycross-cuttingrelations.Thelackofoffsetmarkersaswellasthein- datedbypreferentialcrystalplasticdeformationandfusionwithinthe jectionofmeltintodilationalfracturesprecludesthedeterminationofthe ultracataclasite,explainingthelackofkinematicmarkersbetweenthe truedimensionsofthefaultzoneduringasinglepseudotachylyte-forming fault vein pseudotachylyte and wallrock (Kim et al., 2010; Sibson, event.However,duetothepristinenatureofthepseudotachylytematri- 1980;Spray,1992).Themicrotexturesofthisunusualcrystal-plastic cesstudied,wehaveconcludedthatnosignificantpost-pseudotachylyte ultracataclasitemayholdinformationregardingtheearlieststagesof veindeformationoccurred. pseudotachylyte generation. It is important to take note that no ultracataclasiteappearstoforminpseudotachylytefaultveinshosted byequigranular,fine-grainedrock(~20μm)inboththeperidotiteand 4.4.Injectionveincharacteristics metagabbro. Theinjectionveinsaresecondarypseudotachylyteveinsthatema- 4.6.Microtexturesofthepseudotachylytematrix natefromparentfaultveinsintotheadjacentwallrock.Injectionveins cannotinallinstancesbetracedtotheirsourcingfaultvein.Thisis Thematricesofthemetagabbroandperidotitepseudotachylytesare duetochaoticnetworksofnearbymultipleinjectionveinsinproximity typicallydominatedbycrystallisationproductsofthemelt,withinter- andarchingself-injectionsthatcross-cutoneanother.Theveinsvary stitialcrypto-crystallinematerialorglassandentrainedwallrockclasts widelyinthickness,rangingfrom200μm,thinnerthantheaverage thatvarygreatlyinsizeandcanconstituteupto20%ofthevein,but faultvein,toN20cm,thickerthantheaveragefaultvein.Theyinject arenormallymuchless.Thepresenceofinterstitialglasswasconfirmed atallanglesfromtheirparentveins,fromnear-paralleltoperpendicu- withXRDsynchrotronanalysis(Fig.8).Theveinsarecommonlymarked lar.Theycutbetweenandthroughwallrockgrainsandarenotassociat- bystreaksorcolourbandsthatindicatecompositionalvariation,pre- edwithordependentonprecursorywallrockdeformationorfoliations. dominantlyduetoclustersofspinel,andinefficientmixingofthemelt N.Desetaetal./Tectonophysics610(2014)138–149 143 Fig.6.BSEimagesa–cofultracataclasitesatfaultveinboundariesinperidotite.Arangeofbrittle–ductiletexturesarepresentintheultracataclasite,thewidthofwhichisshownbythe cappedredline.(a)Close-upoftheultracataclasite;theblackareasareahydrousAl-richmelt,thedarkgreyareasareashearedoutpseudotachylytematrix,andthelightergreyis acombinationofshearedoutsurvivorclastsofwallrockdiopsideandpseudotachylytematrix.Imagedshowsapseudotachylytematrixcrystallisingolivine(grey)anddiopside (lightgrey)inahydrousmatrix(darkgrey).Theredboxenclosesanolivine–diopsidecrystalcomplexbeingshearedoutpriortototalsolidificationindicatingcontinued displacementalongfaultveinspostmeltproductionandfailure.Panelcshowsasimilarfeature;brittle-lookingultracataclasiteswithamatrixhostingribbonsofstretched outdiopsidemicrolites(inwhite).Wr:Wallrock,WrDi:Wallrockdiopsidegrain. (Fig. 7). The crystallisation products have various habits: hopper nucleationsurfaceandthesizeofthissurface.Whenthenucleating crystals,simpleacicularlaths,andfeatheryplumes,whichnucleateon surfaceisparticularlysmallitgenerallyformsthecoreofalargerthan clasticmaterialintheveinformingdendritesoralongveinboundaries. averagecomplexdendritecomposedofseveralmineralspecies.Space Aprincipalcontrolonmicrolitesizeisthepresenceorabsenceofa mayalsoexertacontrol oncrystallisation;areasthatappeartobe Fig.7.Micrograph(a)andBSEimage(b)ofpseudotachylytematrixinametagabbro.Colourbandsintheseveinsareevidentinbothimagesandareattributedtozoneswhereindividual mineralsoraggregateshavefusedandbecomedecrepitatedandshearedout.Clustersofmicrolitesandglassymaterialarealsoobservedtoformbands.However,thecrystallisationprod- uctsofthemeltarestronglycontrolledbyitscomposition,andhencethepre-existingmineralsthatfusedwillbethedominantcontrolovertheformationofthesecolourbands.Minimal physicalsortingorfractionationhasbeenobservedintheseveins,sothisisnotconsideredtobeasignificantfactor. 144 N.Desetaetal./Tectonophysics610(2014)138–149 Fig.8.BSEimagesofindividuallyfusingclastsenclosedinapseudotachylytematrix.(a)Thematrixisdominatedbymicrolitesandminuterelictclasts.However,attheboundaryofmelting claststhereisazoneofAl-rich,hydrousglass(Gl).Conversely,inpanelbthematrixisglassdominated(anhydrous)andthefusinggrainsareboundedbyacrystallisationfrontofAl-rich omphacite(Omph)nucleatingonplagioclase(Plag)derivedfromwallrock.Di+Trem:aggregateclastofwallrockdiopsideandtremolite. absentofanynucleatingsurfacearetypicallyhosttosomeofthelargest mostpristinesampleswereselectedformicrotexturalandgeochemical crystals. analysis.Thepseudotachylytesarehighlyheterogeneousincomposi- tiondowntothemicronscale,sothisseparationisthebestwayto 4.7.Microtexturesofsurvivorclasts gaininformationfromtheseveinsandinterprettheirmechanismof formationmeaningfully. Survivorclastscomprisewallrockmaterialthathasbeendragged into the pseudotachylyte during its formation. These can be 4.7.1.Metagabbro:pseudotachylyte monomineralicorcomposedofaggregatesofminerals.Theyaretypical- Compositionallythepseudotachylyteshostedbymetagabbroare lyangularortabularinshape,withthermallyroundedmargins.The highlyheterogeneous,stronglyreflectinglocalmineralcompositions largertabularformsareaggregatesfromthedeformedwallrockthat oftheadjacentwallrockandsurvivorclasts.However,despitethecom- havebeenscouredawayfromtheveinboundarybyinjectingmelt positionalvariabilityofthemelt,themineralogyofthecrystallisation (Fig.3).Thecompositionoftheclastsisthesameasthatoftheadjacent productsinthepseudotachylytesissimplealbeitshowinggreatchemi- wallrock mineralogy, but not in the same proportions. Hydrous calvariability.Thepseudotachylytematrixisdefinedasacombination minerals such as Mg-hornblende, clinochlore and serpentineoccur ofglass,crystallisationproductsandcomminutedmaterialthataretoo onlyinrareaggregateswithanhydrousmineralsandarethusinferred smalltoberesolvedbyelectronmicroprobeanalysis(EPMA)beam tohavemeltedpreferentially.Monomineralicolivine,diopside,magne- (b1μm).Thecrystallisationproductsaredividedintotwogroups:a tite,ilmeniteandplagioclasehaveallbeenobservedinthematrixof hightemperaturequenchassemblagewithpocketsofinterstitialglass, bothtypesofhostrock.Diopsideandplagioclasearethemostcommon andalowertemperatureassemblageinwhichdevitrificationofthe clasts derived from the metagabbro host rock, whilst olivine and glassandrecrystallisationofthepseudotachylytematrixhavetaken diopsidearethoseabundantlyfoundintheperidotite.Survivorclasts place. commonlyexhibitunduloseextinctionandkink-bandingsimilarto thatobservedjustbeyondfaultveinboundariesinthewallrock,sug- 4.7.1.1.Microliteassemblageinthemetagabbropseudotachylytes.This gestingthattheymaybederivedfromthewallrockandunderwentsim- microliteassemblageisdominatedbyomphaciteandplagioclase,with ilarcrystalplasticdeformation.Sievetexturesinshearedoutwallrock minorilmenite(Fig.10).Thisisthefirstreportedevidenceofnatural andpartiallyresorbedsurvivorclastsofolderpseudotachylytehave crystallisationofomphacitedirectlyfromamelt.Omphacitehasbeen alsobeenobserved(Fig.9). reclassified from fassaite, as it was termed by Andersen and Furtherdetailedpetrographicdatahavebeendividedaccordingto Austrheim(2006).Thecompositionofthemineralsishighlyvariable themetagabbroandperidotitehostrocks.Withinthosegroupsthe andisstronglycontrolledbymeltcompositioninthematrixorbyfusing Fig.9.BSEimagesofsievetexturesinaperidotite-hostedpseudotachylytematrix.Bothpanelsaandbshowatleasttwogenerationsofpseudotachylyte.Thesievetexturedmaterialin bothimagesistheoldestgenerationthathasbeendeformedandpartiallydigestedbyayoungerpseudotachylytegenerationintowhichshearedoutclastsoftheolderpseudotachylyte havebeenentrained.Theoldestgenerationofpseudotachylytecontainsinclusionsofrelativelowatomicnumber(andlowtotalswhenanalysedwithEPMA;~86%),suggestingthatthese arequenchedmeltinclusionswithfluidsdissolvedinthemtovaryingdegrees.PreliminaryRamanspectroscopyshowsthesevesiclestocontainglassymaterialandemptycavities, presumablyoncefilledwithfluid. N.Desetaetal./Tectonophysics610(2014)138–149 145 Fig.10.Omphaciteistheprincipalhightemperaturemineralcrystallisingfromgabbroicpseudotachylyte,withanorthiteandilmenitetolesserdegrees.Panelashowsdelicate,zoned hoppercrystalsofomphaciteenclosedbyglass;thiscrystalhabitisanindicatorofrapid,hightemperaturequenching.Panelbshowsomphaciteinwhiteandlightgreywithfeathered anddendritictextures.Thedarkgreydendritesareanorthite.Thevariationinthecompositionofomphaciteiscontrolledbythemineralthatitnucleateson,asshownbythechange ingreyscale;itslocalmeltcompositionandzoninginthecrystals.Omph:omphacite,Ilm:Ilmenite. clasts on which the microlites have nucleated. The omphacite is comprises fine-grained (b50μm) granoblastic diopside, olivine and characterisedbysignificantAl-enrichmentrelativetotypicalomphacite, enstatite(Fig.4). which could be due to the high temperature and pressure of crystallisation(averageAlO :17.3wt.%).Theplagioclaseischaracterised 5.Discussion 2 3 by high FeO and MgO contents (average FeO =1.3wt.%, total total MgO=1.1wt.%),possiblyduetotherapidityorhightemperatureof Themicrotexturesanalysedinthisstudyarespatiallyandtemporal- crystallisation, or inclusions of unresolvable omphacite or ilmenite. lyassociatedwithpseudotachylytegeneration.Theobservedcrystal– Thesemicrolitestaketheformoflaths,dendritesandskeletalhoppers plasticphenomenathatareconsideredtobecausallylinkedtofusion crystallisingfreelyinthepseudotachylytematrixornucleatingonvein andseismicfailureinclude:dislocationcreep/glide,deformationtwin- boundariesoronsurvivorclasts.Theomphaciteisclearlyzonedwith ning,kinking,bendingandultracataclasite/meltformation(Figs.6and Mg-richcoresandFe-richrims(Fig.10). 13).Theultracataclasiteobservedintheserocksisunusualinthatit containsangularcomminutedwallrockmineralsaswellasribbons 4.7.1.2.Metagabbro:devitrificationandrecrystallisationofpseudotachylytes. andstringsofmineralsexhibitingsyndeformationalductilefeatures Devitrificationofglassandrecrystallisationofthehighertemperature (Fig. 6). The interstitial areas of the ultracataclasite contain glassy mineralsthatquencheddirectlyfromthepseudotachylytemeltproduce pocketsofpreferentiallymeltedhydrousminerals,e.g.tremoliteand itsownassemblageofblueschistfaciesminerals:glaucophane,Fe-richal- clinochlore.Together,thesetexturessuggestthatthewallrockminerals bite,epidoteandminorsphene(Fig.11a,b).Thecrystalsinthisassem- mayhaveundergonepowerlawcreep-dominateddeformationthat blage are commonly coarser than those in the higher temperature reachedseismicstrainrates,toinducetheheatingandfusionofthe assemblage(upto2mm).Theyarealsomoreirregularandblockyin wallrockthatischaracterisedbythepresenceofpseudotachylytein form,nolongerexhibitingthedendritichabitsofthereplacedminerals. theserocks(Johnetal.,2009;Kameyamaetal.,1999). Theyoccasionallyshowadeformationfabricexpressedasfoldsandbou- Thequenchproductsofthepseudotachylytesindicateinitiallyvery dinsofpseudotachylyteatthethinsectionscale,indicatingaductile hightemperatures(~1600°C),followedbyareturntoblueschistto blueschistfaciesoverprint(Fig.12a). lawsonite-eclogiteconditions(~430°C–550°C,1.8–2.6GPa)(Andersen andAustrheim,2006;Ravnaetal.,2010).Whenthepseudotachylytesde- 4.7.2.Peridotite:pseudotachylyte vitrifyorrecrystallise,theresultingmineralassemblages(glaucophane, Atthemillimetretomicronscaletheperidotitepseudotachylytes albiteandepidoteinthemetagabbroanddiopside,olivine,enstatiteand showgreatvariabilityincompositionandabundanceofbulkmatrix, clinochloreintheperidotite)indicateahighpressure,lowtemperature glassandcrystallisationproducts.Despitethisvariability,themicrolites environment(Ravnaetal.,2010). consistently comprise diopside, olivine and enstatite. As with the metagabbro,theperidotitefusionproductscanbedividedintoahigh 5.1.Precursorstopseudotachylyteformation temperaturequenchassemblageandalowertemperaturedevitrifica- tionassemblage. Todate,severalmechanismshavebeenputforwardtoexplainthe initiationofintermediate-depthseismicity.Theyallsharethecommon- 4.7.2.1. Microlite assemblage in peridotite-hosted pseudotachylytes. ality of being fundamentally dependent on precursory conditions Olivine, diopside and enstatite occur as clusters of monomineralic (Green and Houston, 1995; John et al., 2009; Kelemen and Hirth, blockymicrolites(particularlyolivine)orasdendriticcomplexes.Also 2007).Therefore,weneedtoinvestigateindetailtheevidenceofany commonarelarger,polymineraliclathsanddendrites(Fig.11).The preservedpre-existingelementsthatmayelucidatethesequenceof interstitialmaterialofthebulkmatrixrangesisavariablemixtureof events that culminate in the production of these high pressure olivine and enstatite and has an H O content ranging from 0 to pseudotachylytes.Aclosespatialrelationshipbetweenpseudotachylyte 2 14.2wt.%H O. formationandfabricdevelopmentisobservedinthehostrocks.Thisis 2 notauniqueoccurrenceandhasbeennotedbypreviousworkersin 4.7.2.2.Peridotite:devitrificationandrecrystallisationofpseudotachylytes. otherpseudotachylytestudies(Bestmannetal.,2011;Hobbsetal., Devitrificationofglassandrecrystallisationofthehighertemperature 1986;Jinetal.,1998;Kimetal.,2010;Lin,1994;LundandAustrheim, mineralsthatquencheddirectlyfromthepseudotachylytemeltproduce 2003).Hobbsetal.(1986)wereamongstthefirsttoproposethepossi- distinctassemblages.Typicallythedevitrifiedmeltischaracterisedby bilitythatinhighpressurecases,pseudotachylyteandmyloniteforma- the formation of microfibrous clinochlore, serpentine and tremolite tion maybeinter-relatedprocesses;suggesting that at thehighest (Fig.9).Incontrast,pseudotachylytethathascompletelyrecrystallised deformationratespseudotachylytewouldform,followedbycataclasis 146 N.Desetaetal./Tectonophysics610(2014)138–149 andthenfabricdevelopment,potentiallyculminatinginmyloniteatthe lowestdeformationrates. Incipientfabricdevelopmentpriortoandafterpseudotachylytefor- mationhasbeenobserved(Figs.6d,12a,14).DetailedEBSDanalysisofa peridotitefaultsectionfromthesamefieldareashowsthatthewallrock mineralsareexposedtoincreasingstraintowardspseudotachylytefault veins.Thewallrockhasundergonerecrystallisationtoformanincipient latticepreferredorientation(LPO)priortoand/orduringfaultveinde- velopment(Silkoset,2013).Crystalplasticdeformationsuchasfolding, boudinaging,annealingandfoliationdevelopmentoccurringafterfault vein formation has been observed within veins and the associated wallrock(Figs.6d,12,14).Thetimelapsebetweenpseudotachylyte generation andfabric development(beforeand after) is unknown. However, the simplest explanation of the observations involves: recrystallisationinthewallrockwithincreasingstraintowardsvein boundariesaswellasrotationofelongatewallrockclastsintoparallel- ismwithdevelopingfaultveinboundaries.Continuedshearing(Fig.6) occursafterdisplacementandfusionhavetakenplace,atwhichpoint thefaultzonehadcooledenoughtocrystalliseglaucophane(inthe metagabbro)andclinochlore(intheperidotite).Wesuggestthefollow- ingsequenceofeventstoexplainourobservations: Themajorityofwallrockmaterialforbothmetagabbroandperido- titeiscoarse-grainedandpartiallydeformedduetopreviousevents. Asaresultthewallrockgrainswilllikelycontainnumerouscrystallo- graphicimperfectionsthatcanbeexploitedwhenintherightorienta- tioninthestressfieldtoformthelocioffaultnucleationandinitial fusionduetolowtemperaturecreep,followedbypowerlawcreep. 1. Thefaultwallrocksdevelopedadamagezonewithsynchronous recrystallisationandanincreasingstrain-gradienttowardsthefuture faultplaneasalsoobservedbySilkoset(2013). 2. Anisotropic wallrock fragments have rotated so that long axes becomeparallelwiththedevelopingslipsurface. 3. Deformedwall-rockfragmentsbecomeentrained,assurvivorclasts in thepseudotachylytemeltandtheirdeformation is associated with the shearing event that produced the melt. Some undergo nearcompletetocompletefusion. 4. Insomeveins,shearinganddisplacementcontinuedafterfusion, locallyattestedtobydeformedandrecrystallisedpseudotachylyte. 5. Pseudotachylyteandfaultdamagezonescooleddownsufficiently todevitrifyorrecrystallisetoglaucophane(inmetagabbro)and clinochlore(inperidotite). Itispossiblethatatlowerdeformationratesfoliationsmaydevelop inthepseudotachylyteandwallrockbetweenmajorfaultingevents. Basedontheobservationsandsequenceofeventspresentedabove, wesuggestthefollowingmodelforpseudotachylytegeneration: Themetagabbroandperidotiteweremostlycoarse-grainedrocks, butbothhostrockshadstructuralandmineralogicalheterogeneities inheritedfromearlierevents.Duringsubduction-relateddeformation, pre-existingzoneswithnumerouscrystallographicimperfectionsat favourableorientationrelativetothenewstressfieldbecometheloci ofenhanceddeformation(JessellandLister,1991;Kameyamaetal., 1999; Sibson, 1980). Low temperature and subsequent power law creep,inducedbyseismicstrainrateseventuallygaverisetofusion.It ispossiblethatseverallociinthesameorientationunderwentcontem- Fig.11.BSEimagesshowingthethreeprincipaltypesofcrystallisationstylesinperidotite hostedpseudotachylyte.Panelashowsblockymicrolitesofolivinesurroundedbyglass poraneousstrain-inducedfusion.Theseindividualmeltspotsmayin (blackinimages).TheolivinecrystalsshowstrongMg–Fezoningfromcoretorim. turnhavelinkeduptoformprimitivefaultplanes.Stick-slipaction Panelbshowspureolivinedendritesclusteredinanaggregatewithinterstitialglassina alongsuchplanewillbepreferentiallyactivatedforfurtherslipand matrixofblockyolivinecrystalsandcomplexlathsofdiopside(lightgrey)andolivine facilitatecomminutionandfusion.Asthestressisreleasedbycommi- (darkergrey).(c)Thisimageshowsanaggregateofcrystallisedmaterialinthecentre. nution,displacementandfusion,thetemperatureofthefaultanddam- Closeinspectionshowsthattheaggregateiscomposedofthreeminerals;enstatite (darkestgrey),olivine(mid-grey)anddiopside(lightgrey).Theaggregateisdominated agezonewillremainelevateduntilthestresshasdroppedbelowthe bythesecomplex,tri-mineralicdendrites.Theblackinterstitialmatrixinallthreeimages strengthofthetransientmeltedmaterialofthefault.Thereafter,heat wasanalysedtobeavariablyAl-,H2O-enrichedglassymaterial. isdissipatedbydiffusionbeyondthecoolingveinanddamagezoneto facilitatecrystallisationofthewallrocks.Thedelicatepreservationof quenchtexturesinpseudotachylytessuggestsnegliblepost-quenching N.Desetaetal./Tectonophysics610(2014)138–149 147 Fig.12.MicrographsandBSEimagesshowingthevariousrecrystallisationtexturesinpseudotachylytesfrombothhostrocks.Panelsaandbmetagabbropseudotachylyte(a)fromthe samethinsectionsampleaspanelb,onlyatalowermagnification.(a)Foldingoftherecrystallisedpseudotachylyte.Themicrolitesinpanelbareblocky,nolongerthedendriticforms showninpreviousexamples.EPMAanalysesshowthemineralassemblagetobeglaucophane,sphene(lightgreycrystals),albite(darkgreycrystals)andepidote(darkestgrey).Thecrys- talsaresubhedralanddeformedandalignedinasubtlefabricduetofolding,asshowninpanela.Imagescandd:peridotite–pseudotachylyte.(c)Adevitrifiedpseudotachylytematrixthat hasanultrafine(b1μm)ofclinochlorecomposition.Thematrixisnotrecrystallisedaspristinequencheddendritesofdiopsideareobserved(lightgreydendrites(c)andstretchedout, deformedclastsareevidentalongwithzoning/colourbandingintheveinthatarepreserved.Paneldshowsarecrystallisedmatrix;theyellowlineindicatestheformofarecrystallised wallrockaggregatethatwasentrainedintothepseudotachylyte.Thesurroundingfine-grainedmaterialcomprisesolivineandenstatiteclastswhichhavereplacedearlierdendritesand uponstagerotationunderthemicroscope,shadowsofnowreplaceddendritescanbeobserved.Theclastsinthisphotomicrographsareannealed,formingagranoblastictextureandshow noevidenceofdeformation.Itisimportanttonotethatperidotitepseudotachylyterecrystallisestoaharzburgiticcomposition. deformationinmostofthepseudotachylyteveinsexceptinthoseareas associatedwiththepseudotachylytearealignedandgrainsappearsqua- wherelaterregionaldeformationandmetamorphismarepenetrative. mousinshapeasopposedtoequant(Fig.4). Itappearsthathighervolumesofmeltareformedinthecoarser- 5.1.Possibleinfluenceofgrainsizeonpseudotachylyteformation grainedhostrock.Ofcourse,thecurrentfaultandinjectionveinthick- nessesarenotexpectedtobeanexactindicationofmeltvolume.Howev- Thegrainsizeofboththemetagabbroandtheperidotiterangesfrom er,takingveindilation,draininganddeformationintoaccountthemelt approximately20μm–260μm.Theprincipaldeformationmechanism volumeproducedbycoarse-grainedrockrelativetofine-grainedrockis ofthewallrockmaterialandentrainedsurvivorclaststhathavebeen ordersofmagnitudegreater;refertoFig.2avsFig.4whereanoutcrop shearedandkinkedbutshownoevidenceofcrystallographicrecovery scaleimageofanaveragefaultveiniscomparedtothelargestfaultvein priortopseudotachylyticfusionisinterpretedtobedislocationcreep/ observedinthefinestgrainedhostrock,seeninthinsectionasitisso glide.EBSDanalysisinarecentstudyindicatesthatthewallrocken- small.Thismaycorrespondwithpowerlawcreepasaprincipaldeforma- countered rapidly increasing strain towards the pseudotachylyte tionmechanismascoarser-grainedmaterialwouldbeeasiertodeform boundaries,withweaklydevelopedLPOs(Silkoset,2013).Theincipient thanfiner-grainedmaterial.Coarsergrainscanaccumulatemoredisloca- LPOdevelopmentcouldbeduetolocalisedstressesintherockbeing tionsandhigherdislocationdensitiesinlocalareasofslipthancanfiner accommodated by high dislocation densities in different grains grainssuggestingagreaterpotentialforlocalisedheating(Kameyama (Branlundetal.,2000;Kameyamaetal.,1999;Newmanetal.,1999). etal.,1999).Imagesofpseudotachylyteinthefiner-grainedhostrock EBSDanalysisbySilkoset(2013)andintracrystallinemicrotexturalobser- (Fig.4)suggestthatgrainboundaryslidingoccurspreferentiallyover vations(Figs.3)inthisstudysuggestdislocationcreepasapossible intra-graindeformationtoresolvetheappliedstress. dominantdeformationmechanismassociatedwithpseudotachylytefor- Furthermore,ourgeochemicalresultsshowthatgrainsizeinfluences mation.Ifthisisthecasethentherewouldbelittlegrainsizereduction meltcompositionsuchthatthebulkcompositionofapseudotachylyte throughdynamicrecrystallisationassociatedwiththeinstabilityasdiffu- derivedfromafine-grainedhostrockwillbetterapproximatefusion sioncreepwouldbeasecondarydeformationmechanism(Kameyama akintothatofanequilibriummelt(thisisduetomeltformationoccurring etal.,1999;KelemenandHirth,2007).Infiner-grainedmaterial(grain- primarilyalonggrainboundaries),whereasinpseudotachylytederived size approximately 20μm) strain has been accommodated by grain fromacoarser-grainedhost,thebulkmeltcompositiontendstoreflect boundaryslidingasopposedtointra-graindeformation;grainboundaries wholesalefusionofindividualmineralssuchasolivineordiopside.
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