Electrical characterization of the North Anatolian Fault Zone underneath the Marmara Sea, Turkey ...

14 Pages·2013·4.81 MB·English

Geophysical Journal International
Geophys.J.Int.(2013)193,664–677 doi:10.1093/gji/ggt025
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n Electrical characterization of the North Anatolian Fault Zone
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m underneath the Marmara Sea, Turkey by ocean bottom
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a magnetotellurics
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an Tu¨lay Kaya,1,2 Takafumi Kasaya,3 S. Bu¨lent Tank,2,4 Yasuo Ogawa,2 M. Kemal Tunc¸er,5
m Naoto Oshiman,6 Yoshimori Honkura1,2 and Masaki Matsushima1
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e 1EarthandPlanetaryScience,TokyoInstituteofTechnology,Tokyo,152-8551,Japan.E-mail:[email protected]
n D
g 2VolcanicFluidResearchCenter,TokyoInstituteofTechnology,Tokyo,152-8551,Japan o
ma 3IFREE,JapanAgencyforMarine-EarthScienceandTechnology,Yokosuka,237-0061,Japan wn
ck 45GGeeoopphhyyssiiccss,,IB˙sotagnabziucliUUnniivveerrssiittyy,,I˙I˙ssttaannbbuull,,3344362804,,TTuurrkkeeyy loade
m,ro 6DisasterPreventionResearchInstitute,KyotoUniversity,Kyoto,611-0011,Japan d from
etis Accepted2013January22.Received2013January21;inoriginalform2011December31 https
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o SUMMARY em
Ge The first magnetotelluric study in the Marmara Sea, Turkey, was undertaken to resolve the ic.o
GJI swtreustcwtuarredoefxttehnesciorunsotfanthdeuNppoertrhmAannatlteoliinanthFeaureltg(ioNnA,Fa)ndintothdeeCt¸eırnmarincıekthareealo.cLaotinogn-poefritohde up.co
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oceanbottommagnetotelluricdatawereacquiredatsixsitesalongtwoprofilescrossingthe /g
C¸ınarcıkBasin,whereasignificantincreaseinmicroseismicactivitywasobservedfollowing ji/a
thedevastating1999˙IzmitandDu¨zceearthquakes.2-Dresistivitymodelsindicatetheexistence rtic
le
ofaconductoratadepthof∼10kminthemiddleofbothprofilesalongwithadeeperextension -a
b
intotheuppermantle,implyingthepresenceoffluidinthecrustandpartialmeltingintheupper stra
mantle. The northern and southern boundaries of this conductor are interpreted to represent c
t/1
the northern and southern branches of the NAF in the Marmara Sea, respectively. These 9
3
conductorshavebeenpreviouslyidentifiedfarthertotheeastalongtheNAF,suggestingthat /2
/6
theelectricalcharacteristicsofthisfaultarecontinuousfromonlandareasintotheMarmara 64
Sea. Microseismic activity in the C¸ınarcık area is located above the conductor documented /63
7
here,andindicatesapossibleseismogenicroleofcrustalfluidspresentintheconductivezone. 5
6
1
Incomparison,resistivezonesalongtheNAFmayactasasperitiesthatcouldeventuallyresult b
y
inalargeearthquake. g
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Key words: Magnetotelluric; Marine electromagnetics; Earthquake source observations; t o
Fractures and faults; Kinematics of crustal and mantle deformation; Rheology: crust and n 1
9
lithosphere. N
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r 2
1 INTRODUCTION ofthe1999˙Izmit(Mw7.4)earthquakeextendedintotheMarmara 018
Seahowever,thefaultsegmentbetweenthe1912Ganosand1999
The 1600-km-long North Anatolian Fault (NAF) is an interconti- ˙Izmitfracturezones(Fig.1b)hasnotrupturedsince1766(Tokso¨z
nentaldextralstrike-slipfaultthatislocatedbetweenthenorthern etal.1979);thissegmentintheMarmaraSeaisconsideredtobea
Eurasian Plate and the southern Anatolian block (Fig. 1a). After ‘seismicgap’thatmaybecapableofgeneratingaM≥7earthquake
closure of the Neo-Tethyan Ocean during the Late Mesozoic and (Hubert-Ferrarietal.2000).
Cenozoic, collision of the Arabian and Eurasian plates resulted The onland outcrop of the NAF is well known as a result of
inrelativewestwardmovementoftheAnatolianPlate(McKenzie previous geological and geophysical studies (Ketin 1948; Barka
1972;Tokso¨zetal.1979).Thiswestwardmovementisconsidered 1992;Yılmazetal.1997;Akyu¨zetal.2002;S¸engo¨retal.2005).
tobethemaincauseofmajortectoniceventsalongtheNAF.Dur- Around the Marmara Sea, the NAF crosses three tectonic zones
ingthepastcentury,theepicentresofdestructiveearthquakesalong fromnorthtosouth,withthenorthernzoneconsistingofPrecam-
theNAFhavemigratedwestwardstartingwiththe1939Erzincan briancrystallinebasementofthe˙Istanbul–ZonguldakZone,repre-
earthquake(Ms7.9)andextendingtothe1999˙Izmit(Mw7.4)and sentingasouth-facingLaurasiancontinentalmargin(Yılmazetal.
Du¨zce(Mw 7.2)earthquakes(Pınaretal.2010).Thefaultrupture 1997).Thesouthernzone,heretermedtheSakaryaContinent,isa
664 (cid:4)C TheAuthors2013.PublishedbyOxfordUniversityPressonbehalfofTheRoyalAstronomicalSociety.
ResistivityoftheNAFunderneaththeMarmaraSea 665
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Figure1. (a)SimplifiedtectonicmapofTurkey,indicatingthestudyareaasdelineatedbyapinkrectangle.Blacklinesandarrowsindicatefaultlinesand s
platemotionsinandaroundTurkey,respectively.(b)LocationmapofOBEMinstruments(yellowsquares)andMT(yellowtriangles)siteswithinandaround ://a
c
theMarmaraSea.BluelinesshowpreviousMTprofiles(GI,Gu¨rer;HE,Honkuraetal.;OE,Oshimanetal.;KE,Kayaetal.;TE,Tanketal.).Bluedashed a
d
rectanglesshowthelocationoftheP1andP2profiles.The1912Ganosandthe1999˙IzmitandDu¨zceearthquakerupturesareshownbysolidredlines.Dashed em
blacklinesarepossibletracesoftheNAFintheMarmaraSea.Redstarsshowepicentresofthe1999˙IzmitandDu¨zceearthquakes.AP,ArmutluPeninsula;KP, ic
KocaeliPeninsula;C¸B,C¸ınarcıkBasin;˙IFR,˙IzmitFaultRupture;DFR,Du¨zceFaultRupture;GFR,GanosFaultRupture;˙II,˙ImralıIsland;˙IB,˙ImralıBasin. .ou
p
.c
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fragmentofcontinentallithospherethatrupturedfromGondwana- theMarmaraSeaandwithpresent-daydeformationcontrolledby m
/g
landduringtheTriassic(Yılmazetal.1997).TheArmutlu–Almacık a right-lateral strike-slipregime (O¨rgu¨lu¨ 2010). Local earthquake ji/a
ZonecontainstheremnantsoftheIntra-Pontidesuturebetweenthe tomography(Karabulutetal.2003;Barıs¸etal.2005)hasimaged rtic
˙Istanbul–ZonguldakZoneandtheSakaryaContinent,andconsists lowP-wavevelocity(Vp)andlowVp/Vs(whereVs=S-waveveloc- le
-a
ofatectonicmelangeoftwozones(Fig.1).Allofthesetectonic ity)ratiozonesdownto15kmdepthbeneaththeMarmaraSea,in b
s
zones form constituent parts of the Western Pontides, a series of additiontohighVpandVp/Vsratiozonestowardsthenorthernand tra
c
east–westtrendingTethyanorogenicbelts;thismeansthatallthree southernedgesoftheC¸B.Althoughpreviousstudieshaveprovided t/1
zoneskeepalmosttheentireevolutionaryrecordoftheTethysides valuableinformationrelatedtotheformandtectonismoftheNAF, 9
3
(Yılmazetal.1997).Theoccurrenceofthe˙IzmitandDu¨zceearth- thewestwardextensionofthisfaultzoneandthedeeperstructure /2
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quakesonthenorthernsideofthesetectoniczones(Tanketal.2003, beneaththeMarmaraSearemaincontroversial. 6
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2005;Kayaetal.2009;Tank2012)indicatesthesignificanceofthe Magnetotellurics(MT)isanelectromagneticmethodthatutilizes /6
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extensionofthesezonesintotheMarmaraSea,intermsofpossible naturallyoccurringelectricandmagneticfieldstomapthesubsur- 75
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locationsforthenextdevastatingearthquakealongtheNAF. face electrical resistivity at depths ranging from the near surface 1
b
However,thejuxtapositionofthesezonesbeneaththeMarmara totheuppermantle(Vozoff1991).Sincefluidssignificantlylower y
g
SeaandtherelationshipbetweenthesezonesandtheNAFarecur- theelectricalresistivityofrocks,thistechniqueishighlyusefulin u
e
rentlypoorlyunderstood.Anumberofmarinestudiesundertaken fault zone investigations (Ritter et al. 2005; Becken et al. 2011). st o
afterthe˙Izmitearthquakeenabledtheformulationofvarioustec- Previous MT research undertaken around active fault zones indi- n
1
tonicmodelsfortheMarmaraSea,namelypull-apart(Armijoetal. catesastrongcorrelationbetweenthepresenceoffluidsandseis- 9
N
1999),singledextralstrike-slipfault(LePichonetal.2001),and micactivity(Unsworthetal.2000;Ogawaetal.2001;Ogawa& o
v
extensional,crustalthinningmodels(Beceletal.2009).Thepull- Honkura2004;Wannamakeretal.2009;Beckenetal.2011),with em
apart model suggests that the Marmara Sea is a large pull-apart themajorityoffaultzonesbeingassociatedwithresistor–conductor b
e
basinthatincludesanumberofsmallerpull-apartbasinsformedin boundaries,anddevastatingearthquakesoccurringinoraroundas- r 2
0
atranstensionaltectonicregime(Armijoetal.2002).Thismodel perityzonesidentifiedbylocalresistiveareasspatiallyassociated 1
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requiressegmentedfaultingoftheNAFwithintheMarmaraSea. withzonesoflowresistivity(Honkuraetal.2000;Oshimanetal.
AccordingtoLePichonetal.(2001),theNAFcrossestheMarmara 2002;Tanketal.2003,2005;Kayaetal.2009).
Seaasasingledextralstrike-slipfaultthatfollowsthenorthernes- PreviousonshoreMTstudiesoftheNAFhaveidentifiedconduc-
carpmentoftheC¸ınarcıkBasin(C¸B)intheeastandcutstheCentral torsatandbelowadepthof10kmalongtheNAF,withnorthernand
Basintothewest.Incomparison,crustalthinningmodelsuggests southernedgesoftheconductorcoincidingwiththesurfacetraces
thepresenceofanextensionalregimeintheMarmaraSeathatis oftheNAF.Theoccurrenceofmainshocksandmajoraftershocks
dependent on both normal and strike-slip faulting regime (Becel within brittle resistive zones (Tank et al. 2003, 2005; Kaya et al.
etal.2009).Crustalthinninghasbeendocumentedinthesouthern 2009),andearthquakeswarmactivityaroundmoreductileconduc-
partoftheCentralBasinandisalsoobservedbeneaththe˙Imralıand tiveregions(Tanketal.2003)emphasizestheimportanceoffluids
C¸Bs(Laigleetal.2008;Beceletal.2009).Theprecisehypocentral duringseismicevents.Ourobjectivehereistoimagethewestern
distributionofmicroseismicactivity(Bulutetal.2009)isconsistent extensionoftheNAFundertheMarmaraSeainanareaknownto
withthedown-dippingstructuresimagedbyCartonetal.(2007)in beaseismicgap.
666 T.Kayaetal.
103, 104 and 105 together with land site 106. The short period
2 DATA range (<∼250 s) at sites 101, 102 and 103 contains clear phase
Fig.1bshowsthelocationsofmagnetotelluricstationsusedduring rollingoutofquadrant(PROQ;Chouteau&Tournerie2000),with
thisstudy.Weused12landsites(yellowtriangles)andsixocean nosuchphaseresponsesrecordedatoceanbottomsite105.These
bottom sites (yellow squares) in the eastern part of the Marmara differences can be explained by changes in bathymetry, with site
Sea,anddefinedtwoprofiles(P1andP2)forlater2-Dmodelling, 105locatedonashallow(50-mwaterdepth),flat-lyingsectionof
asidentifiedbydashedbluerectanglesinthefigure.TheNE–SW seafloor,whereastheotheroceanbottomsitesarelocatednearsharp
orientedprofile(P1)includesfiveoceanbottomsites,fourofwhich changesinbathymetry.Electriccurrentsintheocean,perpendicu-
arelocatedintheImralıandC¸Bs,togetherwithasinglelandsite lartobathymetricgradients,cangeneratesecondarymagneticfields
inthesouthernMarmararegion.ThesecondN–Sorientedprofile thatarecomparabletoorevenstrongerthantheprimarymagnetic
(P2)iscomposedoftwooceanbottomsitesand11landsites.The field(Constableetal.2009;Worzewskietal.2010).Giventhis,the
northernmostoceanbottomsiteiscommoninbothprofiles. modelling discussed here did not incorporate short-period ocean
Ocean bottom magnetotelluric sites deployed during this study bottomdatathatwereaffectedbybathymetry-derivedPROQ.
used ocean bottom electromagnetic instruments (OBEM) devel- Thedimensionalityofthedatasetneedstobedefinedpriorto2-D D
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opedbyKasaya&Goto(2009).Wemeasuredtwohorizontalelec- or3-Dmodelling.Here,weusethetensor-decompositioncodeof w
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tricfieldsusing4mdipoleswithAg–AgClelectrodes,andthree- McNeice&Jones(2001),anexpandedversionoftheGroom–Bailey lo
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componentmagneticfieldsusingflux-gatesensors.Thedatawere decomposition(Groom&Bailey1989)thatincorporatesmultiple de
d
awceqrueisreudccweistshfualnly8rHeczosvaemrepdlinafgterartdeaftoaraacbqouuits3itiwoene.kTsimaned-saelrliOesBdEaMta spietreisodan-ddeppeernidoednst,wdeitchomstpriokseitdioirnecptaioranmseintefersrrsehdofwronminthFeigs.it4e.-Tanhde from
acquiredduringdeploymentwereanalysedusingarobustprocess- optimumstrikedirectionsinthe100–11000speriodbandswere h
ingcode(Chaveetal.1987)andusableMTtransferfunctionswere N90◦EandN62◦EforprofilesP1andP2,respectively.Thesees- ttps
obtainedforaperiodrangeof10–11000s,asshowninFigs2and3. timated 2-D strike directions were used to decompose the profile ://a
c
Landsitedeploymentsduringthisstudyusedbroad-bandPhoenix data,enablingthedefinitionofTEandTMmodes,whichrepresent ad
MTU5MTinstrumentsthatcoveraperiodrangebetween0.003and flows of electric current along and perpendicular to these strike em
2000 s. However, in order to match the ocean bottom MT period directions. ic.o
range,datafromthelandMTsiteswereusedatperiods>10sas up
shown in Figs 2 and 3. One land site at the southwestern end of .co
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profileP1wasanewdeployment,withtheother11landsiteson /g
pwriothfilteheP2nefwromocTeaannkboetttaolm.(d2a0t0a3a)c.qTuhieresedddautrainwgetrheisjositnutdlyy.inverted 3 MODELLING ji/artic
Figs2and3showthedataobtainedduringthisstudyassounding Weinvertedthedataby2-Dmodellingusingtheappropriatestrike le
-a
curves,incorporatingbothdiagonalandoff-diagonalcomponents. directionsdefinedwithintheprecedingsection.Althoughthestrike b
s
Fig.2showsobserveddatasoundingcurvesacrossprofileP1,com- directionsalongprofilesP1andP2differbysome28◦,weassumed tra
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prising,fromnortheasttosouthwest,oceanbottomsites101,102, aquasi-2-Dstructureforthestudyarea. t/1
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Figure2. Observedapparentresistivity(Logρa)andphase((cid:3))valuesversusperiod(LogT)withtheirerrorbarsestimatedforXX(∗),XY(-·),YX(-◦)and
YY((cid:2))componentsofthesitesonP1.XandYrepresentthenorthandeastincoordinatesystem.
ResistivityoftheNAFunderneaththeMarmaraSea 667
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Figure3. Observedapparentresistivity(Logρa)andphase((cid:3))valuesversusperiod(LogT)withtheirerrorbarsestimatedforXX(∗),XY(-·),YX(-◦)andYY ic.o
((cid:2))componentsofthesitesonP2.XandYrepresentthenorthandeastincoordinatesystem. up
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Figure4. Site-dependent(a),frequency-dependent(b)andsite-andfrequency-independent(c)forthewholefrequencyrangeof100–11000sstrikesofP1 1
andP2. 9 N
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It is known that TM mode 2-D modelling over a 3-D anomaly strongelectricalcurrentintheseaorientedparalleltothecoastline, b
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can robustly recover the resistivity section under the profile which can generate a large secondary horizontal magnetic field r 2
0
(Wannamakeretal.1984).WetestedsuchsituationforanOBEM componentwiththeoppositepolaritytotheprimarymagneticfield 1
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datasetbeneaththeMarmaraSea.Fig.5showsasimplified2-D (Constableetal.2009;Worzewskietal.2010).Thedifferencein
and3-DresistivitymodelthatmimicstheMarmaraSea;thismodel TEandthecorresponding3-Dresponseisevidentforbothocean
hasauniformearthof100(cid:4)mandabox-shapedoceanof0.3(cid:4)m bottomandlandsites;incomparison,theTMandthecorrespond-
withdimensions180km(E–W)×60km(N–S)×1.2km(depth). ing3-Dresponsesareingoodagreement.TheTMresponsesfrom
Fig.6comparesthe2-D(Ogawa&Uchida1996)andcorrespond- the modelled sea are dominated by a galvanic charge build-up at
ing3-D(Mackieetal.1994)responsesalongtheprofile.Thetwo theverticalocean–landinterface,providingagoodapproximation,
northernstations(402and404)areonland,withthefollowingtwo even for 3-D structures. Given this, we decided to use TM mode
oceanbottomstations(406and408)closetothecoastintheMar- responsesthat,asshownhere,arerobustevenin3-Dsituations.
mara Sea, respectively. Solid lines denote 2-D TE (red) and TM We used a modified version of the code of Ogawa & Uchida
(blue)responses,withasterisksdenotingthecorresponding3-Dre- (1996) for 2-D inversion, with modifications detailed below. The
sponses.ItisinterestingtonotethatbothTEandthecorresponding Ogawa and Uchida code used Rodi’s (1976) algorithm (MOM’s
3-DresponseshaveshorterperiodPROQ,primarilycausedbythe method)tocalculatespatialderivativesontheground.Thismethod
668 T.Kayaetal.
Laplacianoperator.Themodifiedversionhasa|C(m−m )|2norm,
0
wherem isavectorcomprisingthelogresistivityoftheapriori
o
model.Thestaticshiftwasusedasaconstraintininversionasin
theoriginalversionofOgawa&Uchida(1996)code.
Westartedwiththeinitialmodelconstructionasdescribedhere.
Initially, sea water was assigned a fixed value of 0.3(cid:4)m by ref-
erencing the bathymetric data. It is also important to include a
sedimentary layer as a priori information, as we do not have the
short-perioddatarequiredtoconstraintheresistivityofshallowar-
easimmediatelyunderindividualoceanbottomsites.Duringthis
study,weusedthedistributionofsedimentsidentifiedusingseis-
mic reflection data (Carton et al. 2007), with Fig. 7 showing the
shallowresistivitydistributionthatwasincorporatedintothemodel
as a constraint. During modelling, a thick sedimentary layer was D
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assigned a fixed resistivity of 10(cid:4)m, extending to a depth of 4– w
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5 km below the C¸B (Okay et al. 2000; Carton et al. 2007), with lo
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theinitialmodelhavingauniformbelow-sedimentresistivity.Error de
floorvaluesof10percentand3◦wereusedforapparentresistivity d fro
andphasevalues,respectively,withtheinitialmodelalsousedas m
Figure5. Initialmodelsfor3-Dand2-Dforwardmodellingtests.Abox anapriorimodel. h
type(depth:1.2km,width:60km)bathymetrywith0.3(cid:4)mresistivityis ttp
We formulated three initial models that used uniform below- s
setintothe100(cid:4)mhalf-spaceresistivity.Whitesquaresshowthelocation sedimentresistivitiesof10,100and1000(cid:4)m,withthefinalmod- ://a
ofsites(402,404,406and408fromnorthtosouth)comparedinFig.6. c
elsdependentontheinitial(apriori)models.Ofthethreeinitial ad
models,the100(cid:4)mmodelshowedthebestfitwiththedata,with em
isusedformultiplelevelsofseafloorsinthisstudy.Inaddition,we rootmeansquaremisfitvaluesof2.1and1.9forprofilesP1andP2, ic.o
modified the definition of roughness norm to include an a priori respectively. up
model in order to stabilize the inversion. The original roughness The identified 2-D electrical resistivity structures in the east- .co
norm was |Cm|2, where m is a model vector representing the log ern Marmara Sea along profiles P1 and P2 are shown in m/g
resistivity of the model, and C is the roughening matrix of the Figs 8(a) and (b). The electrical resistivity models have similar ji/a
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Figure6. Apparentresistivity(left-handcolumn)andphase(right-handcolumn)responsesof3-D(asterisk)and2-D(solidline)forwardmodellingaregiven
forthenortheast(blue)mode,whichmeanselectricfieldinthenorthandmagneticfieldintheeastdirectionisused,andeast–north(red)mode,theopposite
casewiththenortheastmode.Sincethemodelissymmetric,onlysamplesites(402,404,406and408)fromnorth(toppanel)tothecentre(bottompanel)of
theprofileareshown.RobustnessoftheTMmodeisclearespeciallyforthelongerperiods.
ResistivityoftheNAFunderneaththeMarmaraSea 669
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Figure7. InitialmodelsofP1(a)andP2(b)for2-Dinversionareshownupto6km.Whiterectanglesrepresentthesitelocations.Bathymetrywasfixedto d
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0.3(cid:4)mwhileunderlyingsedimentwasfixedto10(cid:4)m.Theresistivityofthehalf-spacewassetas100(cid:4)m. d
fro
resistivitydistributionsbeneathbothprofiles,withashallowcon- m
ductor(C1)startingafewkilometresbeneaththesedimentarylayer http
andmergingwithadeeperconductor(C2)inthecentralpartofthe s
profiles. Another shallow conductor (C3) in P1, located between ://a
c
tworesistivelayers,isalsopresentbeneaththeArmutluPeninsula ad
e
inP2.Bothmodelscontaindeepresistorsthathorizontallybound m
thedeepC2conductorinthenorth(R1)andinthesouth(R2).The ic.o
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shallowresistivelayerunderlainbyaconductiveanomaly(C3)be- p
neaththeArmutluPeninsulawasalsoobservedinthepreviousMT .co
m
studybyTanketal.(2003)inwhichthestructuresdowntoalmost /g
20kmwereinvestigated. ji/a
Acomparisonofobservedandcalculatedresponsesisshownin rtic
Figs 9 and 10, for profiles P1 and P2, respectively. These curves le-a
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Figure8. Final2-Delectricalresistivitymodelsobtainedfrominversionof
TMmodedataforP1(a)andP2(b)profiles.Invertedtrianglesshowsite
locations,redarrowsindicatepossiblebranchesoftheNAF,whitecircles
represent microseismic activity from 2007 to 2010 (Bulut, GFZ) in and
aroundtheC¸B.Blacklinebeneaththebasinsindicatesdepthofthefixed
sedimentinmodelling.Thewhiteandgreydashedlinesindicatetheupper Figure9. Fittingcurvesoftheobserved(plussign)andcalculated(straight
crust–lowercrustboundary(brittle-ductiletransitionzone)andMohodepth line)dataofP1aredemonstratedforsitesfromnorth(toppanel)tosouth
(Laigleetal.2008;Beceletal.2010). (bottompanel).
670 T.Kayaetal.
Wealsotestedthemainfeaturesidentifiedwithinthefinalmodels
byforwardmodellingusingresistivitychanges.Figs13and14show
sensitivitytestsforthemajoranomalies[R1(a),C1(b),C2(c)and
R2(d)]beneathprofilesP1andP2,respectively.Thesetestsconfirm
thatthemodellingaccuratelyrepresentsthedata.Accordingtothese
tests,bothconductiveanomaliesC1andC2inbothprofileshave
resistivityvaluesthatrangebetween1and10(cid:4)m.
4 DISCUSSION
Theresultsofthisstudyprovidethefirstelectricalimagesofstruc-
turesbetweentheseafloorandtheuppermantlebeneaththeMar-
mara Sea. The tectonic and geological implications of the major
D
anomaliesidentifiedabovearediscussedinthisnextsection. o
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4.1 Implicationsfortectonicconfigurations d
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ThefinalmodelsgiveninFigs8(a)and(b)showthreedomains;one m
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isconsistingoftwocentralsubverticalconductors(C1,C2),andoth- ttp
eisrscoinncsliusdteinntgwthitehstuhrerokunnodwinngtreecstiosntoicrsp(rRov1i,nRce2s).inThthisedsitsutdriybuatrieoan. s://ac
a
Thenorthernresistor(R1)andtheoverlying10-km-thickconductor d
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which belong to the ˙Istanbul–Zonguldak Zone represent Precam- m
ic
brianbedrockandOrdoviciantoCarboniferoussediments(Yılmaz .o
u
etal.1997).Thesouthernresistor(R2)correspondstotheSakarya p
.c
and Armutlu zones, and represents Paleozoic metamorphic rocks o
m
oftheSakaryaContinent.Thezonewiththesubverticalconductive /g
anomaly corresponds to the collision zone between the ˙Istanbul– ji/a
ZonguldakZoneandtheSakaryaContinent. rtic
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4.2 Sourcesofconductiveanomalies trac
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4.2.1 Uppercrustalconductor(C1) 3/2
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Anuppercrustalshallow conductive anomaly, herenamed C1,is 64
presentinthemiddleofbothprofileswitharesistivityof1–10(cid:4)m. /6
3
7
Experimental data indicate that electrical resistivity of aqueous 5
6
crustalfluidsisintherangeof0.01–0.1(cid:4)m(Nesbitt1993);meaning 1
b
that,usingtheHashin–Shtrikmanupperbound(Hashin&Shtrik- y
g
man1962),abulkresistivityof1–10(cid:4)mcanbeexplainedbyporos- u
e
s
ityof0.15–15.0percent.Inaddition,aseismictomographystudy t o
detected low Vp and Vp/Vs ratio zones at depths of 5–15 km be- n
1
neaththeC¸B(Barıs¸etal.2005).Thesezoneswerealsointerpreted 9
N
asareasofhigh-fluidcontent.Thedistributionofmicroseismicepi- o
v
centres around the C¸B is shown in Fig. 1(b). In Fig. 8, projected em
hypocentresofearthquakesthatwereatadistanceof±10kmfrom be
theprofilesP1andP2aregiven.Thisprojectionindicatesagood r 2
0
correlationbetweenresistivityandseismicity,withthemajorityof 1
8
themicroseismicactivityclusteringoutsidetherimsoftheC1con-
ductorbeneaththeprofileP1,andclosetotheC1andC3conductors
beneaththeprofileP2(Fig.8).
Thisconfigurationcanbeexplainedbytheexistenceofaninter-
Figure10. Fittingcurvesoftheobserved(plussign)andcalculated(straight connectedfluidnetworkandtheassociatedtriggeringofearthquakes
line)dataofP2aredemonstratedforsitesfromnorth(toppanel)tosouth by the migration of fluids into the surrounding crust. Migration
(bottompanel). offluidintoless-permeablecrustcanreducetheeffectivenormal
stressandtriggerearthquakes(Sibsonetal.1988;Cox1999;Sib-
indicate a generally good recovery of the observed data. The ap- son2000).Suchseismicity–resistivityrelationshipsareknownina
parent resistivity and phase responses are compared in pseudo- numberofseismicallyactiveregions(Ogawaetal.2001;Ogawa&
sectionsinFigs11and12,confirmingtherecoveryoftheobserved Honkura2004;Wannamakeretal.2004,2009;Jiraceketal.2007;
data. Mitsuhataetal.2001).
ResistivityoftheNAFunderneaththeMarmaraSea 671
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FTiMgumreod1e1.. Calculated(toppanel)andobserved(bottompanel)apparentresistivity(left-handpanel)andphase(right-handpanel)pseudo-sectionsofP1for ct/19
3
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gionswithhighheatflow(100–140mWm−2)intheMarmaraSea 6
4.2.2 Lowercrusttoupper-mantleconductorC2 (˙Ilkıs¸ık 1995; Tezcan 1995) and mantle-derived He (>50percent 4/6
3
The research discussed here identified a vertical conductor, here of total He concentration) along the west to central segment of 75
6
namedC2,runsfromthelowercrusttotheuppermantle.Thiscon- the NAF (Gu¨lec¸ et al. 2002) may indicate the presence of zones 1
b
ductormaybeassociatedwiththepresenceofhigh-salinityfluids withsignificantextensionandhightemperaturethatmayberelated y
g
and/or the partial melting of mantle material due to the astheno- totheupwellingofasthenosphericmaterial.Dilek&Altunkaynak u
e
sphericupwelling.Thebulkresistivity(1–10(cid:4)m)oftheC2zone (2007,2009)suggestedpartialmeltingasasourceoftheEocene st o
indicateseither0.15–15.0percentfluidor1.5–39volpercentmelt volcanism in and around the Marmara Sea; as well, Altunkaynak n
1
fractionbytheHashin–Shtrikmanupperbound,usingpuremeltand (2007) showed that the geochemistry of the Marmara granitoids 9
N
aqueousfluidresistivitiesof0.1–0.3and0.01–0.1(cid:4)m,respectively wasindicativeofsignificantcrustalcontaminationduringmagma o
v
(Presnalletal.1972;Waff1974;Tyburczy&Waff1983;Nesbitt ascent. em
1993;Yoshinoetal.2010;Pommier&LeTrong2011;Evans2012). Thesedatasuggestthatfluidsaremigratingfromadeepductile be
Thepresenceofpartialmeltissupportedbytheupwellingofas- region to the upper brittle zone beneath the Marmara Sea. If this r 2
0
thenosphericmaterialbyothermagnetotelluricstudiesdocumented conductordoesrepresentpartialmelting,onepossibilityisthatthe 1
8
for the onland extent of the NAF (Gu¨rer 1996; Tank et al. 2005; hightemperatures,partialmeltingandfluid-releasingdehydration
Tu¨rkog˘lu et al. 2008). In addition, seismological studies indicate reactionsdocumentedinthisareamayberelatedtoasthenospheric
thatthecrustalthicknessaroundtheMarmaraSeavariesbetween upwellingcausedbysubductionofNeo-Tethyanoceaniclithosphere
29 and 32 km (Gu¨rbu¨z et al. 2003; Zor et al. 2006). Within the beneaththeSakaryaContinent,whichwasfollowedbyslabbreak-
Marmara Sea, it is almost 31 km (grey dashed line in Fig. 8), off.
except areas to the south of the Central Basin and beneath the TheformofthedeepC2conductorchangesbeneathbothprofiles,
C¸ınarcık and ˙Imralı basins. Here, the crustal thickness decreases withtheconductorappearingnarrower,butwithahigherresistivity,
to 26 km due to the thinning of the upper crust associated with beneathprofileP2thanbeneathprofileP1.Onepossiblereasonfor
lithospheric-scaleextension(Laigleetal.2008;Beceletal.2009). this is that profile P2 crosses the easternmost part of the C¸B and
In addition to that, Straub & Kahle (1994) documented a NE– the Armutlu Peninsula, and does not intersect structures beneath
SWorientedextensionalregimealongthenorthernbranchofthe the˙ImralıBasinwhichisassociatedwithhighlyconductiveaque-
NAF, in the present study area. Furthermore, the presence of re- ousfluidsand/ormoltencrustalandmantlematerial.Thebasement
672 T.Kayaetal.
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6
4
Figure12. Calculated(toppanel)andobserved(bottompanel)apparentresistivity(left-handpanel)andphase(right-handpanel)pseudo-sectionsofP2for /6
3
TMmode. 7
5
6
1
b
rocks of the Armutlu Peninsula consist of a succession of high- y
4.3 RelationshipwiththeNAF g
and low-grade metamorphics overlain by sedimentary rocks—a u
e
similar lithological sequence to that found within the ˙Istanbul– 4.3.1 ContinuoustectonicfeaturesalongtheNAF st o
ZonguldakunitsandSakaryaContinent(YılmazandTu¨ysu¨z1991; n
1
Yılmazetal.1995).ThisindicatesthatprofileP1includesstruc- We have documented significant similarities between the features 9
N
turesbeneathboththe˙ImralıandC¸Bs,leadingtoawiderconductive resolvedinMTmodelsobtainedfortheMarmaraSeaarea,andin o
v
anomalythanobservedintheprofileP2. modelsfortheregiontotheeastofthestudyareaalongtheNAF. em
Deepsubverticalconductorsbeneathlargestrike-slipfaultshave TheMarmaraSeaprofiles,P1andP2,aredominatedbyaconductor be
also been recently imaged along the NAF (Tank et al. 2005), the thatishorizontallyboundedbyresistivezonestothenorthandsouth r 2
0
AlpineFaultinNewZealand(Wannamakeretal.2002)andtheSan (Fig.15).ThisstructurewasalsoobservedinpreviousonshoreMT 1
8
AndreasFault(Beckenetal.2011),suggestingthattheseconduc- studiesintheeasternpartofthepresentstudyareaalongtheNAF
torsmaybeacommonfeatureinareascontainingmajorstrike-slip (Gu¨rer1996;Honkuraetal.2000;Oshimanetal.2002;Tanketal.
faults. The existence of this type of conductor extending to 50- 2005;Kayaetal.2009;Kaya2010).Thedistributionofgeothermal
km depth below the NAF near ˙Izmit was documented by Tank fieldsfromeasttowestalongtheNAF(Aydınetal.2005)isalso
etal.(2005),whointerpretedthedeeperpartoftheconductorto consistentwiththelocationofthedeepconductors,suggestingthat
be a region of partial melt, and the shallow part (just under the similar structures may also be present along the western part of
brittle–ductiletransition)tobearegioncontainingsalinefluids.A the NAF in the Marmara Sea area. In terms of extending geo-
similar interpretation, combining regions of fluid and underlying electrical structures from the eastern Marmara region to the C¸B,
partialmelt,wasalsoprovidedinTibetbyLietal.(2003).Wan- wesuggestthat,fromnorthtosouth,theIstanbul–Zonguldakand
namakeretal.(2002)andBeckenetal.(2011),ontheotherhand, Armutlu–AlmacikzonesandtheSakaryaContinenttectoniczones
showedthatdeepverticalconductorisfluidsourcesupplyingcrustal arecontinuousfromthepreviouslydocumentedonlandareastothe
fluid. areabeneaththeMarmaraSea.
ResistivityoftheNAFunderneaththeMarmaraSea 673
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Figure13. ResolutiontestsfortheanomaliesR1(a),C1(b),C2(c)andR2(d)ofP1.Theresponsesatthestationsmostlysensitivetotheanomaliesare -a
b
srehsopwenct.ivBelluye.Pcloulsousrigrnep(+re)sednetmsothnestmraotedselthreesopbosnesrevewdhdilaetar.eIdn,yalelllaonwo,mgarleyentesatnsd,bbelsatckfitciosloobutrasinsheodwbyremspoodneslersestpootnhsee.anomalywith1,10,100and1000(cid:4)m, strac
t/1
9
4.3.2 BranchesoftheNAF improveourknowledgeoftheextensionofthesestructuresfarther 3/2
tothewest,moreOBEMinstrumentdatafromoffshoreareastothe /6
Previous MT studies identified a correlation between resistor– 6
conductorboundariesandonlandbranchesoftheNAF(Tanketal. westarerequired. 4/6
2003,2005;Kayaetal.2009).Inthisrespect,subverticalresistor– 199R9es˙IizsmtivitityansdtrDucu¨tzucreesedaretfihnqeudakaersoushnodwtehdetrhuapttuthreezmoanienssohfocthkes 3756
conductorboundariesbeneaththeMarmaraSeamayalsoindicate 1
occurred within high-resistivity zones that were bounded to the b
thelocationofNAFbranches,suggestingthatthenorthernresistor– y
southbylower-resistivityzones(Tanketal.2003;Kayaetal.2009). g
conductorboundaryinthestudyarearepresentsanorthernbranch u
of the NAF (NAF1) that extends west from the ˙Izmit earthquake TsthruisctsuurgesgebsetnsetahtahttahleonMgatrhmeaNraASFe,acaonmdptahreiseopniscebnettrweseeonftrheesi1s9ti9v9e est o
rupturezonetotheMarmaraSea.Fig.15showsbothextensionof n
earthquakes may be crucial in relating high-resistivity zones to 1
theNAFbranchesandresistivitydistributionalongtheNAFfrom 9
possibleasperityzonesthatmayinitiatealargedevastatingevent N
Du¨zceregiontotheMarmaraSea. o
withintheMarmaraSea,whichisanareaofincreasedseismicity v
Thesecondboundary,locatedatthesouthernescarpmentofthe after the ˙Izmit earthquake. This indicates that more MT research em
C¸BbeneathprofileP2,isalsoobservedbeneathprofileP1,suggest- b
ingthatthisminorbranchoftheNAFextendsfarthertothewest. needstobeundertakenontheresistivezoneatsitesfarthertothe er 2
northintheC¸B. 0
TheboundaryatthesouthernpartoftheC2zone,beneathprofile 1
8
P1,impliestheexistenceofanotherbranchoftheNAF,asshown
byadashedlineinFigs1and15.Thishasoftenbeenreferredtoas
4.3.3 TectonicmodelsundertheMarmaraSea
themiddlebranchthoughitisonestrandofthesouthernbranchof
theNAF(Yilmazetal.2010).HerewerefertothisbranchasNAF2 Thesingledextralstrike-slipfaultmodelintheMarmaraSeasug-
whichmeanstheextensionofthesouthernbranchoftheNAFin gestswestwardextensionofthenorthernbranchoftheNAFalong
theMarmaraSea(Fig.15).Extensionofthesesubverticalresistor– thenorthernescarpmentoftheC¸B(LePichonetal.2001)corre-
conductorboundariestoatleast50kmdepthsuggestsdeeplyrooted spondingtothenorthernresistor–conductorboundaryinourfinal
NAFthatisalsosuppliedbyateleseismictomographystudyindi- models.Inordertobeabletosupportordeclinecontinuationofa
cating a P-wave velocity contrast, represented by relatively high singlefaultinthismodel,moreOBEMinstrumentdatafromoff-
P-wave velocity perturbations (δVp)tothe northoftheNAFand shoreareastothewestareneededtofigureoutifthereexistseither
low P-wave velocity perturbations to the south, down to a depth many or a single resistivity boundary corresponding to the fault
of 150 km in the Marmara Sea (Biryol et al. 2011). In order to branch.