Deseta, N., Andersen T.B. and Ashwal, L.

12 Pages·2013·4.51 MB·English

by  N. Deseta

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.