Shear wave splitting along a nascent plate boundary: the North Anatolian Fault Zone

13 Pages·2010·1.11 MB·English

Geophysical Journal International
Geophys.J.Int. (2010)181,1201–1213 doi:10.1111/j.1365-246X.2010.04576.x
Shear wave splitting along a nascent plate boundary: the North
Anatolian Fault Zone
C. Berk Biryol,1 George Zandt,1 Susan L. Beck,1 A. Arda Ozacar,2 Hande E. Adiyaman1
and Christine R. Gans1 s
c
ni
1DepartmentofGeosciences,UniversityofArizona,Tucson,AZ,USA.E-mail:[email protected] o
2GeologicalEngineeringDepartment,MiddleEastTechnicalUniversity,Ankara,Turkey ct
e
t
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no
Accepted2010February22.Received2010February22;inoriginalform2009June16 awn
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SUMMARY na fro
TbehtewNeeonrththAenAatnoaltioalniaFnauPlltaZteontoe(tNheAFsoZu)tihsaantrdanthsfeorEmursatsriuactPulraetethtaotctohnesntioturtthes.tWheebaonuanlydsaeryd eodym http
Gs
thepropertiesoftheupper-mantlestrainfieldandmantleanisotropyinthevicinityofNAFZ I://a
viasplittingofSKSandSKKSphases.WeuseddatafromtheNorthAnatolianFault(NAF) GJca
d
passiveseismicexperiment.Thisisthefirststudythatanalysestheupper-mantleanisotropy e
m
in this region and our results indicate that the observed upper-mantle strain field is uniform ic
.o
underneaththearraywithconsistentNE–SWpolarizationdirectionsforfastsplitwaves.The u
p
measured lag times between the arrivals of the fast and slow split waves varies from 0.5 to .co
m
1.6sforthestudyarea.Smallerlagtimesareobtainedconsistentlyintheeasternpartofthe /g
array.However,wedonotobserveanysignificantvariationineitherthepolarizationdirections ji/a
orthedelaytimesacrosstheplateboundary(NAFZ). rtic
le
Theuniformityofthefastpolarizationdirectionsthroughoutthestudyareaandthestrength -a
b
ofanisotropyfavouranasthenosphericsourcefortheanisotropy.Theregionaltectonicframe- s
tra
workfavoursaSWdirectionofasthenosphericflowduetotheforcesactingontheupper-mantle c
t/1
exertedbytheslab-roll-backtakingplacealongtheAegeanandtheCypreanSubductionZones. 8
1
/3
Key words: Mantle processes; Seismic anisotropy; Continental tectonics: strike-slip and /1
2
transform;Dynamicsoflithosphereandmantle. 01
/6
0
1
0
3
9
b
1 INTRODUCTION westward extrusion of the Anatolian Plate along the right-lateral y gu
NAFZ (Sengor 1979) and the left-lateral Eastern Anatolian Fault e
s
Analysisoftherelationshipbetweencrustalstrainandupper-mantle Zone(EAFZ)(Jackson&McKenzie1988)(Fig.1).Despitenumer- t o
n
deformationfieldsnearplateboundaryzonesisimportantforun- ousgeologicalstudiesalongthisfaultzone,thedeeperstructureof 2
5
derstandingpatternsofdeformationaroundplatemargins,aswell thisplatemarginremainsrelativelyunknown.Twoimportantprob- N
as the dynamic interaction between the lithosphere and astheno- lemsthatweaddressinthisstudyaretheorientationofdeformation ov
e
sphere.Therearenodirectmeanstoobservethedynamicsofthe intheupper-mantlebeneathourstudyarea,andthedegreeofco- m
b
uppermostmantleinsuchplateboundaryzones.Seismicanisotropy herencybetweentheupper-mantleandthecrustalstrainfields. e
generatedbythestraininducedlatticepreferredorientation(LPO) Analysis of shear wave splitting along other transform plate r 2
0
1
ofanisotropicmantleminerals(i.e.olivine),however, issensitive boundarieshasrevealeddifferentpatternsofmantledeformation. 8
todeformationatdepth.Thesplittingofpolarizedshearwavesin TheseismicanisotropystudiesinwesternNorthAmericaandalong
suchananisotropicmediumcanquantifythedirectionandstrength theSanAndreasFault(SAF)indicateanupper-mantlestrainfield
oftheanisotropy(Christensen1966;Keith&Crampin1977;Ando withdeformationdirectionsmostlyobliquetothesurfacetraceof
1984; Fukao 1984; Kind et al. 1985; Silver & Chan 1988; 1991; thefault.Somestudiesshowthatthereexistsashallowerzoneof
Vinniketal.1992;Kaneshima&Silver1995;Silver1996;Savage deformationthatislocatedwithinanarrowercorridorthatfollows
1999;Silver&Holt2002). theshearzonewithdirectionsparallelingthestrikeofSAF(Oza-
This study focuses on northern-central Anatolia and the North laybey&Savage1995;Silver1996;Hartog&Schwartz2001;Titus
AnatolianFaultZone(NAFZ),whichisa1400kmlongcontinental etal.2007).TheslipratesmeasuredfortheSAFareontheorderof
transformfaultthatformsthenorthernmarginoftheAnatolianplate 20–30 mm yr−1 (Johnson et al. 2007). These are similar to slip
(Fig.1).TheearlyMiocenecollisionofthenorthward-converging rates measured for NAFZ, which are on the order of 20–
ArabianPlatewiththeEurasianPlatealongtheBitlisSutureZone 25mmyr−1(McCluskyetal.2000).Manystudiesindicatethatthe
(BSZ)resultedinnorth–southcontractioninEasternAnatoliaand dextralmotionalongNAFZinitiatedduringLateMiocenetoEarly
(cid:2)C 2010TheAuthors 1201
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1202 C.B.Biryoletal.
25˚ 30˚ 35˚ 40˚
N Black Sea GC
EURASIAN
PALTE LC
40˚ A(GNSTNO) STUDY AREEAF NAFZ N E AEF ZA AC
~20 mm/yr
ANATOLIAN PLATE E A F Z BSZ
Aegean
Sea D
o
w
n
~25 mm/yr lo
a
ARABIAN d
e
Mediterranean Sea PALTE d
35˚ CT SFZ km from
A T AFRICAN PALTE ~10 mm/yr D 0 100 200 http
s
Figure1. TectonicmapoftheAnatoliaandsurroundingregionsalsoshowingneotectonicstructures,platevelocitiesandthelocationofthestudyareain ://ac
a
thisframework.Openupside-downtrianglesarestationsoftheNAFarrayandtheopensquareistheGSNstationANTO.Platevelocitiesarerelativetofixed d
e
EurasiaPlate(Reilingeretal.1997;Barka&Reilinger1997).AT,AegeanTrench;BSZ,BitlisSutureZone;CT,CypreanTrench;DSFZ,DeadSeaFaultZone; m
EAAC,EasternAnatolianAcretionaryComplex;EAFZ,EasternAnatolianFaultZone;EF,EzinepazariFault;ESM,EratosthenesSeamount;GC,Greater ic.o
Caucasus;LC,LesserCaucasus;NAFZ,NorthAnatolianFaultZone;NEAFZ,North-EastAnatolianFaultZone. up
.c
o
m
Pliocene (∼5–11 Ma) (Barka & Kadinsky-Cade 1988; Barka & maximumfiniteextension(McKenzie1979;Ribe&Yu1991;Ribe /g
Gu¨len1989;Koc¸yig˘it1989,1990;Dirik1993;Bozkurt&Koc¸yig˘it 1992)undertheeffectofsimpleshear,largestrainsandhightem- ji/a
1996;Barkaetal.2000;Bozkurt2001).Inthisrespect,theNAFZis peratures(1300◦C)(Zhang&Karato1995).Thelagtimebetween rtic
significantlyyoungerthanSAF(∼17–30Ma)(McKenzie&Mor- thefastandslowcomponentsindicatesthestrengthorthethickness le-a
gan 1969; Atwater 1970; Graham et al. 1989). Unlike the SAF, of the source of anisotropy (Silver & Chan 1991). Although this b
s
the upper-mantle deformation beneath the dextral NAFZ and the sourcecouldbeanywherebetweenthecoreandthereceiver,many tra
c
surroundingregionisnotwellconstrained,duetothelackofgeo- studies have shown that most anisotropy takes place in the upper t/1
8
physicalobservations.Thisisthefirststudytoinvestigatethechar- 400kmoftheEarth(Vinniketal.1992;Mainprice&Silver1993; 1
/3
acteristicsofthemantlestrainfieldbeneaththenorthwardconvex Barruol & Mainprice 1993; Vinnik et al. 1995, 1996). Previous /1
2
segmentoftheNAFZandthenorth-centralportionoftheAnatolian studiesindicatethatthecontributionofthecrusttothissourceof 0
1
Plateusingsplittingofshearwaves. anisotropyisrelativelysmall(0.04–0.2s)(Gledhill&Stuart1996; /6
0
Savage1999).Theseismicanisotropymeasurementsinferredfrom 1
0
shearwavesplittinghavelimitedverticalresolution,buttheypro- 39
videbetter lateral resolutionbecause theincidence angles forthe by
2 DATA AND METHOD associatedphasesarefairlysmallandeachmeasurementrepresents gu
e
Twoesdtuepdlyoytheedsaeissemisimcitiycaanrrdaythceolmithpoosspehderoifc3s9trubcrtouarde-obfanthdesreeigsimonic, aniWsoetrcoaplicculsatrteudcttuhreevbaelnueeasthofinϕdiavnidduδatlfroercienivdeivrsid.ualevent-station st on
stationsthatcrossedtheNAFZinmultipletransects(Fig.1).These pairsusingtwodifferenttechniques,wheretheeffectsofsplitting 25
seismic instruments were provided by the Incorporated Research areremovedfromtheobservedseismogramsthroughagrid-search No
Institutions for Seismology (IRIS)—Program for Array Seismic forsuitablevaluesofsplittingparameters(ϕandδt).Oneofthese ve
m
Studies of the Continental Lithosphere (PASSCAL) Consortium techniques, Rotation Correlation (RC), involves a search for the b
e
and recorded regional and global earthquakes continuously at 40 bestϕ,δtpairthatmaximizesthecorrelationbetweenthecorrected r 2
0
samplespersecondforaperiodof2yr. seismogrampairsbyrotationthroughaseriesofcoordinatesystems 1
8
SKSandSKKSarethemostcommoncorephasesusedinseismic (Fukao 1984; Bowman & Ando 1987). The second method that
anisotropyanalysis.ThearrivalsaregeneratedbyPtoSVconver- we used is Singular Covariance (SC) defined by Silver & Chan
sionsatthecore–mantleboundaryandgenerateconvertedphases (1991).Itincorporatesminimizationofthedisplacementenergyon
polarizedintheradialplane.Anyanisotropicmediumunderneath thetransversecomponentafterremovingeffectsofsplitting.These
thereceiversidecausessplittingofthepolarizedshearwave(SV) calculationsarecarriedoutusingtheSplitLabcodeofWu¨stefeld
intofastandslowcomponents,withparticlemotionstakingplace et al. (2008). Utilization of two different techniques allows us to
inassociatedfastandslowpropagationdirections(Backus1965). comparetheresultsintermsofconsistency,andhence,assessthe
Shearwavesplittinganalysisdeterminesthepolarizationdirection qualityforindividualmeasurementsbasedonthecriteriaexplained
ofthefastwave(ϕ)andthelagtimebetweenthefastandslowwaves byWu¨stefeld&Bokelmann(2007).
(δt)(Silver&Chan1988;Silver&Chan1991;Vinniketal.1992). Thedatainclude140eventslocatedatadistancerangeof85–120◦
Forcommonmantlemineralssuchasolivine,thefastpolarization (Fig. 2a). The events were selected to have moment magnitudes
directionisparalleltotheflowdirectionandalsothedirectionof over5.0anddepthsgreaterthan10km.However,bestresultswere
(cid:2)C 2010TheAuthors,GJI,181,1201–1213
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a b Radial Transverse
Components Components
SKS SKS
YIKI YIKI
Mw ≥ 7.0 YESI YESI
6.5 < Mw ≤ 7.0 STYEUPNE STYEUPNE
6.0 < Mw ≤ 6.5 SEYH SEYH
120˚ Mw ≤ 6.0 PAPNECLI PAPNECLI
OGUR OGUR
KUZO KUZO
KUZA KUZA
85˚ km Depth KKUIZYIKL KKUIZYIKL
700 KIYI KIYI
650 KKGAVAAC KKGAVAAC
600 KARA KARA
ISKE ISKE D
550 INSU INSU o
INCE INCE w
455000 GHEAOKSCINAE GHEAOKSCINAE nloade
345000 DDDUOEMGRALE DDDUOEMGRALE d from
223050000 CCBCCABUORAKEKKLLMKTUUEI CCBCCABUORAKEKKLLMKTUUEI https://aca
150 BEDI BEDI de
BAGB BAGB m
100 ALOR ALOR ic
ALIN ALIN .o
50 ALIC ALIC up
0 0 5 101520 0 5 101520 .co
Seconds Seconds m
Event: January 21, 2007 /g
Mw=7.5 ji/a
Average Distance=90˚ rtic
Hypocenter depth=22 km le
Average Backazimuth=88.5˚ -a
b
s
tra
Figure2. (a)Eventsusedintheanalysisofanisotropyunderneaththenorth-centralportionofAnatolia.Greyscaleindicateshypocentredepthfortheseevents. c
OpenstaratthecentreofthemapshowsthelocationoftheNAFarray.Mostoftheeventsarefrombackazimuthsof270◦±10◦and90◦±10◦.(b)Radial t/1
8
andtransversecomponentsofSKSarrivalsfortheMoluccaSeaevent(whitesquareonmap)in2007January21(Mw=7.5)recordedbytheNAFnetwork. 1/3
/1
2
obtainedonlyforeventswithmagnitudesover6.0(Mw)anddepths Wecarriedoutqualityassessmentforourresultsintwostages. 01
ofmorethan20km(Fig.2b).Wealsoobtainedsplittingparame- Thefirststageofthequalityassessmentincorporatesacomparison /6
0
tersfortheGlobalSeismographicNetwork(GSN)stationANTO, ofresultsobtainedusingSCandRCmethodsfollowingthecrite- 1
0
utilizingdatafortheperiodoftheNAFdeployment(Fig.1). riadefinedbyWu¨stefeld&Bokelmann(2007).Thesecriteriaare 39
inAtertmotsalofofsh2e4a6r1wsaevtessopfliSttKinSg.aPndrioSrKtoKStheararnivaallyssiws,emreoasntaolfysthede δdteRfiCn)eodfbthyeftawstoamxiesthmoidssfi.tTsh(|eϕdSeCs-cϕrRipCt|i)oannodfdtheelafyoutirmqeuaralittiyoscl(aδstsSeCs/ by gue
datawasfilteredusingabandpassfilterwithcut-offlimitsof1and usedinthisstudyisgiveninFig.4.Thisfigurealsoshowsthedis- s
t o
25 s to eliminate very high frequency and low frequency noise. tributionofindividualmeasurementsintotheseclasses.Thesecond n
Occasionally the data were analysed without applying a filter or stage of quality analysis incorporates visual screening of results 25
withapplyingabroaderbandpassfilter(withcut-offperiods1and basedonhoweffectivelytheenergyonthetransversecomponentis N
o
v
50s)whenthesignal-to-noiseratiooftheseismogramswerehigh removedonthecorrectedseismogramsandhowwelltheresultsare e
m
enough.Consideringthedominantfrequencyofourmeasurements constrainedbasedontheerrorsassociatedwiththemeasurements. b
e
was ∼8 s for SKS and ∼11 s for SKKS, these filters were suit- Hence, we eliminated some of the poor measurements that were r 2
0
able for the data set. Examples of measurements using SKS and misclassifiedasnullmeasurementsduetothelowsignal-to-noise 1
8
SKKSphasesforoneeventrecordedatstationKUYLareshown ratiooftheassociatedwaveforms.
inFig.3.
Our measurements yielded 681 well-constrained splitting pa-
rameters and 1053 null measurements. The remaining 727 mea-
3 RESULTS
surements were of low quality, yielding poorly resolved splitting
parameters. The results of our analysis and detailed statistics on Ingeneral,ourmeasurementsrevealroughlyNE–SWtrendingfast
our measurements are summarized in Fig.5 and Table 1.In gen- polarization directions for the Eurasian Plate and the Anatolian
eral,fewerSKKSphasesyieldwell-constrainedsplittingparame- Platewithoutanylargevariations(seesupportinginformationfor
ters compared to SKS phases. However, the mean differences in rosediagramsoffastpolarizationdirectionsandnullmeasurements
measuredsplittingparametersbetweenSKSandSKKSphasesare ateachstation).Themeanfastpolarizationdirectionforthearrayis
small:approximately10◦ forfastpolarizationdirectionsand0.3s ∼43◦usingbothSCandRCmethods(Fig.5).Thefastpolarization
fordelaytimes(Table1andFig.3). directionaveragesforboththenorthernandthesouthernblocksof
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2000 1000
500
1000
0
0
0 20 40 0 10 20 30
1000 1000 Do
w
n
500 500 lo
a
d
0 0 ed
fro
m
h
ttp
s
://a
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0 20 40 0 10 20 30 ad
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Figure3. AnexampleofSKS(toppanels)andSKKS(bottompanels)arrivalsandassociatedparticlemotionsbeforeandaftertheeffectsofthesplittingare .o
removed.Bothofthearrivalsarefromanintermediatedepthevent(Mw=6.4)locatedbeneathJujuyProvince,Argentina. up.c
o
theNAFZarealsoapproximately43◦,showingnodistinguishable lines delineate the backazimuths where we will observe null re- m/g
differenceacrosstheplateboundary. sultsforagivenfastpolarizationdirection(eitherbackazimuth= ji/a
Although the fast polarization directions are nearly uniform ϕ orbackazimuth=ϕ ±90◦).Theblackcontinuouslines,onthe rtic
throughouttheNAFarray,weobservesomeclearvariationinlag otherhand,showthedistributionofmeasurementsthathaveback- le-a
times.ThemostnoteablevariationinlagtimesoccursinanE–W azimuthsoriented45◦fromthefastandslowpolarizationdirections b
s
directionratherthanaN-Sdirection.Atthewesternmostpartofthe (backazimuth=ϕ±45◦)or,inotherwords,farthestfromthenull tra
studyarea,themeasureddelaytimesareontheorderof1.3s.Start- directions (ϕ or ϕ ± 90◦) for a given fast polarization direction ct/1
ingfrom34◦Elongitude,themeasuredlagtimesdecreasesmoothly (ϕ).AnalysisofsyntheticdatabyWu¨stefeld&Bokelmann(2007) 81
/3
towardstheeastwheretheyhavetheirminimumvaluesontheorder showedthatthebestsplittingmeasurementstendtoclusteraround /1
of0.5sandthenincreaseagaineastof37◦E(Fig.5).Thenumberof these ϕ ± 45◦ lines. Our good and fair measurements also tend 20
1
nullmeasurementssignificantlyincreasesforstationslocatedeast to cluster around these lines. In addition, our null measurements /6
of 34◦E longitude (see supporting information). This is probably arelocatedaroundthelinesrepresentingthenulldistribution(grey 01
0
related to smaller delay times, which make it harder to correctly continuous lines, Fig. 6). The mean ϕ for this station is 45◦ and 39
detect anisotropy given the noise recorded at stations. We obtain the observed nulls mostly lie within ±15◦ of this direction (the by
mostlynullmeasurementsforthestationsbetween35◦Eand36◦E greyshadedregionsinFig.6).Althoughwehavealimitedback- gu
e
inthenorthernblock of NAFZ (openstarsonFig. 5)and station azimuthal distribution for our results and we cannot completely s
KIYIlocatedintheSEpartoftheNAFarray.Twoofthesestations, ruleoutthepresenceofverticallyvaryinganisotropy,thedistribu- t on
DERE and CALT, have a few splitting measurements with ϕ = tionofnullsandtheconsistencyinobservedsplittingparameters 25
∼30±10◦andδt∼0.4±0.1sthatarefromeventslocatedtothe favour a single layer anisotropic source. This one layer model is N
o
v
west.Around100kmtotheeastofthesestations,TEPEhasacouple alsoconsistentwithpreviousobservationsbyVinniketal.(1992) e
m
offairmeasurementswithϕ=∼25±10◦andδt∼0.6±0.2sthat andS¸apas¸&Boztepe-Gu¨ney(2009),whoanalysedanisotropybe- b
e
areobtainedfromeventslocatedtotheeast.ForstationKIYIwe neaththelong-operatingstationANTOandfoundnoevidenceof r 2
wereabletoobtainafewmeasurementsyieldingverysmalldelay verticallyvaryinganisotropy.Thecaseoftwo-layeranisotropywill 01
8
timesontheorderof0.4s.ForstationsOGURandEKIN,allthe bediscussedfurtherinthefollowingsection.
resultsarenullsforbothwesternandeasternbackazimuths.
The backazimuthal coverage of our data set is limited in the
sensethatmostoftheeventsweusedarefromwesternandeastern
4 DISCUSSION
backazimuths(Fig.2).Thismakesitdifficultforustoruleoutthe
presenceofaverticallyvaryinganisotropybeneaththestudyarea. The existence of invariant anisotropy directions implies that the
Thebackazimuthaldistributionofmeasurementsandnulls(using deformational fabric in the upper mantle is uniform beneath the
SC method) for station ALIC is shown in Fig. 6. This station is study area and across the Anatolia-Eurasia Plate boundary, as
located on the NAF (see Fig. 5) and the recorded data have high delineatedbytheNAFZ.Overabroaderregion,thisisinagreement
signal-to-noiseratios.Thelightgrey,continuouslinesontheϕver- withthefindingsofSandvoletal.(2003)usingdatafromtheEast-
susbackazimuthplot(Fig.6)showthelinearrelationshipbetween ern Turkey Seismic Experiment (ETSE). They observe no major
distributionsofnullswithrespecttobackazimuth.Basically,these variations in NE–SW trending anisotropy directions beneath the
(cid:2)C 2010TheAuthors,GJI,181,1201–1213
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Anisotropyalonganascentplateboundary 1205
± (s)
MeanδErrort 0.250.340.310.220.160.260.180.260.280.200.150.270.170.310.200.160.21–0.300.280.270.240.230.180.250.230.320.160.140.220.290.27–0.210.220.260.200.300.220.27
)
±n◦ϕ( 6298932931372608946819676182168261 9649306570761011881290358216 33262835709131
MeaError 8.15.10.11.14.22.7.14.19.10.16.14.11.10.15.9.12.–21.15.9.17.12.11.8.10.15.23.14.13.16.7.–11.11.7.7.18.17.14.
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otal 4535517031444147245544243387383755604637656941425272832323644434230523936774446 dem
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ϕ (s) up
a. MeanKKS 1.240.981.25–––1.051.18–0.97–0.900.661.27––––0.621.071.270.801.00––0.930.900.35-0.650.871.50––0.911.431.40–0.73– .com
ourstudyare ϕMean◦KKS()S 44.1335.6036.33–––45.3045.48–45.33–45.6043.3635.28––––47.3238.9735.2855.3552.20––38.6344.9046.90-39.5540.2945.00––47.8439.7341.90–50.68– /gji/article-a
KSphasesfor δMeantSKS(s)S 1.411.011.470.940.640.641.140.870.700.940.580.850.881.250.730.820.69–0.710.921.330.800.870.701.101.180.940.400.540.940.801.66–0.950.971.721.350.880.620.77 bstract/181/3
KSandSK ϕMean◦SKS() 42.9034.4236.0844.3344.9738.9847.7240.4345.4645.0436.5446.1645.1937.8639.2051.4351.62–43.4635.1038.5560.4248.5951.9521.4043.0937.8357.6037.5647.1144.1449.14–35.3945.8749.3339.3536.9543.7458.27 /1201/601
S 0
sfor KS 39 b
ment SK 1681405107125987922171412209192399067831017101041253211210 y gu
e # e
asur S st o
me SK 2927377026343435194636172465373041482628464632335212124292627333226403433563236 n 2
ofsplitting Mean#δt(s) 1.371.001.420.940.640.641.140.960.700.940.580.860.821.250.730.820.69–0.680.941.310.800.880.701.101.130.930.390.540.880.831.63–0.950.961.681.360.880.640.77 5 Novemb
s e
dqualitie Mean◦ϕ)( 43.2234.8136.1444.3344.9738.9847.5541.8745.4645.0736.5446.0744.7137.4439.2051.4351.62–45.0735.8337.7758.1748.8251.9521.4042.2038.9254.5437.5645.6042.6848.35–35.3946.2748.2239.5236.9545.0658.27 r 2018
n
a
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number Lon. 33.48732.87932.8732.79335.88736.4133.50634.26336.21137.36735.12534.26934.35733.44135.06435.28435.1435.78734.34833.56532.90635.36637.06735.24533.55232.87834.32335.31636.53634.33236.24832.86135.16534.30134.29932.933.52935.74337.22935.954
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mary Lat. 0.9781.0611.3019.8680.9550.2781.1211.3150.5520.0151.3280.3730.0640.6041.4770.3910.9181.1479.7431.4690.5819.8420.7640.6880.2910.280.9410.1310.0481.590.4410.9041.1090.6471.1130.8560.8381.3690.4050.748
m 4443444444444444443443444444444444444444
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Table1. Station ALICALINALORANTOARSLBAGBBEDIBEKIBOKECAKMCALTCAYACRLUCUKUDEREDOGLDUMAEKINGOCEHASAINCEINSUISKEKARAKARGKAVAKGACKIYIKIZIKKUYLKUZAKUZOOGURPANCPELISEYHSYUNTEPEYESIYIKI
(cid:2)C 2010TheAuthors,GJI,181,1201–1213
Journalcompilation(cid:2)C 2010RAS
1206 C.B.Biryoletal.
90 constraining the depth of the anisotropic source is important for
understandingthedominantcauseofdeformation.
Fair
80 Good
Poor
4.1 Locationofanisotropy
70 non-Null
The ray paths for teleseismic phases used in shear wave splitting
Null
60 analysisaresteeplyinclinedandtheyhavelittledepthresolution.
Wecan,however,indirectlyinferthedepthoftheanisotropicsource
50 usingobserveddelaytimesandinferringthestrengthofanisotropy.
Inthisanalysis,weassumethesourceofanisotropyissubhori-
40 zontalbeneaththestudyarea.Theresultsofouranalysismightbe
affectedbythepresenceofasteeplydippinganisotropy.However,
the case of dipping anisotropy requires variation in the observed
30
D
Null splitting parameters with backazimuth, depending on the dip di- ow
20 non-Null rveacrtiiaotinonofinthseplainttiisnogtrpoapriacmmeteedrisuwmi.thWbeacdkoazniomtuotbhse(sreveeFaingy.6ro)bthuastt nloa
d
wouldaccountforasteeplydippinganisotropicsource.Thus,we e
10 d
assumeasubhorizontalorhorizontalanisotropicsourcebeneathour fro
studyarea. m
0 h
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 In the case of Eastern Anatolia and ETSE, the analysis of re- ttp
c(Aeinvgeursfuentcatilo.n2s00sh6o;wOszaacnaarneotmala.lo2u0s0l8y)thbienne(6a0th–8th0ekEma)stliAthnoastpohliearne s://ac
Figure4. Plotofallthemeasurementsinourdatasetwithrespecttodelay Plateau (EAP) and lithospheric thicknesses on the order of 100– ad
e
timeratiosandfastaxismisfitsbetweenSCandRCmethods.Theshaded 125kmbeneaththeArabianplate(Angusetal.2006).Fastpolar- m
regionsrepresentsfairandgoodresults.Thecontinuousblacklinesseparates ic
izationdirectionsareorienteduniformlyinNE–SWdirectionswith .o
Nullandnon-Nulldomains.Alltheresultsplottedonthenon-shadedspace u
delaytimesrangingbetween0.9and1.3sformostofthestations p
arepoor. .c
locatedontheArabianplateandEasternAnatolianPlateau(EAP) o
m
(Sandvoletal.2003)(Fig.7A),showingthatthemeasurementsare /g
EasternAnatolianAccretionaryComplex(EAAC)andacrossma- insensitivetochangesinlithosphericthicknesses.Further,assum- ji/a
jor structural features such as the EAFZ, the eastern portion of ing4percentanisotropicstrength,observeddelaytimesrequirean rtic
le
NAFZandtheBSZ(Fig.7A).Theuniformityofthesplittingmea- anisotropiclayerthicknessofatleast110km,significantlythicker -a
b
surements throughout the region raises the question of whether thantheestimatedlithosphericthicknessbeneaththeEAP.Hence, s
the associated uniform deformation pattern exists in the subcon- theobservedanisotropyismostlikelylocatedintheasthenosphere tra
c
tinental lithospheric mantle or underlying asthenosphere. Hence, beneathEasternTurkey,asarguedbySandvoletal.(2003). t/1
8
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CRLU KIZIK CAKM 40˚
ANTO (GSN) INSU
GOCE
39˚
31˚ 32˚ 33˚ 34˚ 35˚ 36˚ 37˚ 38˚
Figure5. ResultsofsplittinganalysisforbothSCandRCmethods.Notethattherearenodistinctvariationsamongresultsobtainedusingbothtechniques.
Openstarsindicatestationswithmostlynullmeasurements(seeSection3fordetails).NAFZ:NorthAnatolianFaultZone,EFZ:EzinepazariFaultZone.
(cid:2)C 2010TheAuthors,GJI,181,1201–1213
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Station: ALIC
90 4.0
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Fwiigthurrees6p.ecFtastot-pboalcakriazzaitmiounthd.irTehcteiopnloatntedddmelaeyastuimreempelnottsswariethorbetsapinecetdtousbiancgkaSzCimmueththfoodr.sTtahtieondiAstLriIbCu.tiFoonroclfaoriutyr,mmeeaassuurreemmeennttssaarreepslhootwtendiwniath90p◦lums,ocdruolsuss, d fro
m
circleandsquaresigns.Ourbestestimatesofsplittingparameters(plusandcrosssigns)tendtoshowlessvariationthanthetheoreticaldistributionofapparent h
splittingparametersfortwolayeranisotropicmodelsindicatedbythindashedlines.Themeanofourmeasurementsareplottedasthethick,black,dashed ttp
lfionresst.aOtiounrmAeLaIsCu.rementsofnulls(circlesandsquares)mostlyliewithin±15◦range(thegreyshadedarea)ofobservedmeanfastpolarizationdirection(43◦) s://ac
a
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e
The centre of our array is located approximately 600 km west m
ofthecentreofETSEarrayandthearraysareadjacent(Fig.7A). 4.2 Asthenosphericflow ic.o
Inthiscase,onewouldexpecttheobservedanisotropytobesub- The dominant mechanism that controls the deformation of hot, up
lithosphericbeneaththeNAFarray,unlessaverysharpchangeof mechanically weak asthenosphere is shear strain related to flow. .co
m
the deformation field occurs from sublithospheric to lithospheric Theoretical analysis and laboratory studies on olivine show that /g
levelsfromeasttowestbetweenthesetwoarrays.Inaddition,asin the strain-induced LPO of the mineral creates the observed seis- ji/a
thecaseoftheArabianPlateandEAP,weareobservinguniform mic anisotropy, where the fast polarization direction parallels the rtic
anisotropydirectionsacrossmajortectonicboundaries,suchasthe flow direction and extension axis of strain ellipse in most cases le-a
Izmir-Ankara-ErzincanSuture,InnerTaurideandInnerPontidesu- (McKenzie 1979; Ribe 1989; Ribe & Yu 1991; Silver & Chan bs
tures(seeFig.7Aforlocations),aswellastheNAFZ,markingthe 1991; Ribe 1992; Nicolas 1993; Zhang & Karato 1995; Savage tra
c
boundarybetweenseveralblocksandaccretionarycomplexesthat 1999;Karatoetal.2008).However,itisnotpossibletodirectlyob- t/1
8
definesthenorthernpartoftheAnatolianPlateandtheboundarybe- taintheuniqueflowvectorsfortheasthenosphereonlyusingshear 1
/3
tweentheEurasiaPlateandtheAnatolianPlate.Wesuggestthatthis wavesplittinganalysis.Inourstudyweattempttoconstrainthedi- /1
2
uniformity in splitting parameters across these tectonic provinces rectionofflowbylinkingtheupper-mantlestrainfieldtodynamics 0
1
alsoindicatestheyareassociatedwithsublithosphericstrainrather ofthemajortectonicelementsoftheregion. /6
0
thanlithosphericdeformation. Takingthebroadertectoniccharacteristicsoftheregionintocon- 1
0
MostofthestudyareaiscoveredbytheTethysideaccretionary sideration,wesuggestthattherelativelyrapidsouth-southwestdi- 39
complexes (Sengor et al. 2005) related to Tertiary closure of the rectedslabroll-backtakingplacealongtheAegeansubductionzone by
northernbranchoftheNeotethys(Sengor&Yilmaz1981;Go¨ru¨r (LePichon&Angelier1981;Bozkurt2001;McCluskyetal.2003) gu
e
etal.1984;Koc¸yig˘itetal.1988;Koc¸yig˘it1991;Okayetal.1998),a mightberesponsibleforthemobilizationofasthenosphereinaSW s
t o
tectonicsettingnormallyassociatedwiththinnerlithosphere.Con- directionbeneaththeAnatolianRegion.TheAegeansectionofthe n
sideringtheobservationofrelativelyhighdelaytimes(ontheorder northAfricanplateboundaryisassociatedwithlargerdistancesof 25
of1.3–1.6s)atthewesternmostportionsofthearrayandassuming4 trenchretreatcomparedtotheCypreantrench(Barka&Reilinger No
v
percentanisotropyfortheuppermantle,weinferthattheanisotropic 1997;McCluskyetal.2003),whichisaffectedbythecollisionofthe e
m
layerthicknessisontheorderof150km(Mainprice&Silver1993; EratosthenesSeamountsouthofCyprus(ESMonFig.1)(Rotstein b
e
Silver1996).Inthiscase,theanisotropyinthemantlelithosphere &Kafka1982;Kempler&Garfunkel1994;Robertsonetal.1994; r 2
0
isnotamajorcontributorbecausethelithosphericthicknessisless Robertson&Grasso1995;Glover&Robertson1998)(Fig.1).The 1
8
than 100 km beneath this tectonically active region, as indicated NEorientationofthebackarcdomainsofthesetwoarcsisbased
bythesurfacewavedispersionstudybyPasyanos(2005).Besides, onpreliminarytomographicimagesoftheunderlyingslabs(Biryol
Gans et al. (2009) reported low Pn velocities beneath the extent etal.2009).Thisdifferenceinroll-backdistancesmightgenerate
of the Tethyside accretionary complexes for our study area. This differential shear strain in the asthenosphere, with strengths in-
observationmightbeafurtherindicationofthinmantlelithosphere creasingfromeasttowesttowardsthecentreoftheAegeantrench.
(<100 km) beneath parts of our study area. Hence, the estimates Relativelylargerdelaytimesobservedinshearwavesplittinganal-
ofthethicknessoftheanisotropiclayeraretoolargetosuggesta ysisareofteninterpretedasathickersourceofanisotropyand/or
solelylithosphericsourcebeneaththisregion. strongeranisotropy.Bothofthesecasesrequirethedeformationof
Basedonalloftheseobservationswebelievethattheanisotropy theanisotropicmediumtobemorepervasiveandstrainsassociated
andtheassociateddeformationfieldthatwearesamplingismostly withthedeformationtobehigher.Hence,thisSW-directeddiffer-
asthenospheric,ratherthanlithospheric. entialasthenosphericstrainmightberesponsiblefortheobserved
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32 33 34 35 36 37 38 39 40 41 42 t/1
8
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/1
2
Figure7. A.Resultsofsplittinganalysis(greythickbars)fromETSE(Sandvoletal.2003)togetherwiththemeasurementsfromtheNAFExperimentand 0
1
Hatzfeldetal.(2001)forwesternTurkey.Thethin,lightanddarkgreycrossesindicatestrainrates.Lightanddarkgreybarsaremaximumfiniteextensional /6
0
strainaxisandmaximumfineshorteningstrainaxis,respectively(Kreemeretal.2003).Notethevariationofstrainaxesfromeasttowest.Suturezones(dashed 1
0
greylines)arealsoshown.BS:BitlisSuture,IAESZ:Izmir-Ankara-ErzincanSutureZone,ITSZ:InnerTaurideSutureZone,IPSZ:InnerPontideSutureZone. 3
9
Thebold,black,dashedlinedenotesthelocationofthehypotheticaltransitionthatseparatesthetwozonesundertheeffectofdifferentialroll-backratesalong b
y
theAegeanandCypreantrenches.NotethatthislinealsoseparatesthehigherandlowerdelaytimemeasurementsforNAFarray.Thetrenchretreatratesfor g
u
theAegeanandtheCypreantrenchesaregivenbyblackarrows(McCluskyetal.2003).Thebold,grey,dash-dotlineshowstheoutlineofKirsehirBlock.B. e
s
RelationshipbetweenmaximumfiniteextensiondirectionazimuthswithintheAnatoliaregioninanE–Wsenseandobserveddelaytimesfromshear-wave t o
lsopnligttiitnugdems.eTashuerefimnietnetsm.aNxoimteutmheecxoterrnesliaotnioanzbimetuwthesenanddecdreealasyintgimdeeslaayretimaveesraagnedsvfaorryeinvgerfiyn0it.5e◦exotfelnosniognituadzeimthurtohufgrhoomutwAesntattoolieaa.stThbeetwtheiceknd3a4s◦haenddli3n8e◦s n 25
N
separateszonesofmeasurementsassociatedwithdifferentialeffectsofslabroll-backalongAegeanandCypreantrenches. o
v
e
m
increaseindelaytimesfromeasttowestinthestudyarea(seethe byGansetal.(2009)showsfastPnvelocities(>8km/s)beneaththe b
e
locationofthedashedlineonFig.7Aanddashedgreylongitudinal centralpartoftheKirsehirBlock(seeFig.7Aforlocation),thatis r 2
zonesonFig.7B). characterizedbytheexistenceofcrystallinecomplexes.Thismight 01
8
Theshortspatialwavelengthvariationsindelaytimesmightalso beinterpretedintermsofthickerlithospherebeneaththisregion.
beanindicationofsmall-scalecomplexitiesinasthenosphericflow Hence,asanalternativeexplanationforvaryingdelaytimes,local
patternrelatedtothetectonicallycomplexcharacteroftheregion, thickerlithospheremightbeobstructing/constrainingtheflowofthe
wheresubductionroll-back,backarcextension,continent-continent asthenospherebeneaththeeasternpartofthestudyarea,producing
collision,slabdetachmentandtectonicescapealltakeplacewithin smallerstrainsintheuppermantle.
severalhundredkilometres.Thevariationsinasthenosphericstrain
amountsorpossiblesourcethicknessvariations(basedonvariation
in delay times) could also be related to the probable existence of
4.3 Comparisonoflithosphericandasthenospheric
topographyofthelithosphere–asthenosphereboundarybeneaththe
strainfields
AnatolianplatesouthoftheNAFZandtheEurasiaplatecontaining
theBlackSeabasinandcontinentalbasementrocksofPontideaffin- Comparison of fast polarization directions with plate motion di-
ity(Sengor&Yilmaz1981)tothenorth.ThePntomographystudy rectionsrequiresselectionofareferenceframethatwillyieldtrue
(cid:2)C 2010TheAuthors,GJI,181,1201–1213
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Anisotropyalonganascentplateboundary 1209
absoluteplatevelocities.Thereexistmultiplereferenceframesfor setofforcesduetotheirdifferentrheologiesanddifferentvertical
plate motions, based on different assumptions, and each of these extents.Althoughwecannotdeterminetheasthenosphericflowdi-
hasdifferentmotiondirectionsandspeeds.Oneofthemostcom- rectionviathesecomparisonsandobservations,theinterpretation
monlyusedreferenceframesforourstudyarearegardsEurasiaas thatthevariationsincrustalstrainfieldareduetoAegeanslabroll-
fixedandfocusesontherelativemotionsofthesurroundingplates back might be suggesting that the variations in anisotropy delay
(i.e.Anatolia)withrespecttofixedEurasia(McCluskyetal.2000). timesarealsoduetotheeffectofthisroll-back.Thisprovidesfur-
In this case, the direction of lithospheric motion depends strictly thersupportfortheideathattheobservedanisotropydirectionsare
ontheselectionofthefixedplate(i.e.Eurasia)anddoesnotnec- generatedbyslabroll-backprocessestakingplacealongtheAegean
essarily represent an absolute plate motion that can be used for andtheCypreansubductionzonesandhenceassociatedwithaSW
comparison with mantle anisotropy measurements. Another rele- asthenosphericflow.
vantreferenceframeisthehotspotreferenceframe(HS3-NUVEL-
1A of Gripp& Gordon2002). Theuncertainties in thedynamics
ofmantleplumesandtheirsourcedepthswithinthemantleintro-
4.4 ComparisonofNAFZandSAF
duce uncertainties in these plate motion measurements (Kreemer D
o
2009). Recently Kreemer (2009) attempted to put constraints on StudiesofshearwaveanisotropyalongtheSAFindicatetheexis- w
n
the absolute plate motions in a hotspot reference frame based on tenceoftwolayeranisotropyalonga100–150kmwidezonefollow- lo
a
d
observationsofshearwavesplittingorientations(GSRM-APM-1). ingthesurfacetraceofthefaultalongmostpartsofthePacific-North e
d
TheresultsofKreemer(2009)showamismatchbetweenobserved Americaplateboundary(Ozalaybey&Savage1994;Ozalaybey& fro
anisotropy directions and resultant absolute plate velocity direc- Savage1995;Hartog&Schwartz2001;Polet&Kanamori2002). m
tionsforcratons.TheNo-Net-Rotation(NNR)referenceframeof The lower, sublithospheric layer of this stack is characterized by http
Kreemer&Holt(2001)incorporatesplatemotionswithrespectto EW-oriented polarization direction with delay times in the range s
afixedmantle,withtheeffectsoflithosphericrotationremoved.In of0.85–1.70s(Hartog&Schwartz2001).Thesenseofanisotropy ://a
c
arecentstudyBecker(2008)pointedoutthereislikelytobeanet for this layer is subparallel to the absolute plate motion of the ad
e
rotation of lithosphere which affects the anisotropy in the mantle NorthAmericanPlate(Hartog&Schwartz2001;Gripp&Gordon m
due to effects of basal shear forces applied on asthenosphere by 2002).Silver&Holt(2002)attributedthistodifferentialstrainbe- ic.o
u
moving stiff continental keels. In the Anatolia region all of these tweentheNorthAmericaplateandtheunderlyingasthenosphere. p
.c
referenceframesyielddifferentplatemotiondirectionsandthere Several studies argued that complications in this general pattern o
m
isnoconsensusonwhichoneofthesereferenceframesyieldsthe mightbeduetopastsubductionprocesses,small-scaleconvection /g
trueabsoluteplatemotions.Thisuncertaintymakesitdifficultfor withintheslabwindow(Ozalaybey&Savage1994;Ozalaybey& ji/a
us to compare plate motions in our study area with the observed Savage1995;Hartog&Schwartz2001;Polet&Kanamori2002), rtic
anisotropydirections.Hence,weprefertoadoptanapproachthat oratoroidalmantleflowpatternrelatedtotheopeningoftheslab le-a
isindependentofreferenceframeandincorporatescrustalstrains window (Zandt & Humphreys 2008). In contrast, the upper layer bs
andstrainratesratherthanplatemotionvectors.Thus,weadopted (0to∼120kmdepth)displaysfastpolarizationdirectionsthatare tra
c
theuseofthepredictedstrainfieldforAnatoliacalculatedfromthe subparalleltothesurfacetraceoftheSAFwithrelativelysmaller t/1
8
GPSvelocityfield(Kreemeretal.2003;Allmendingeretal.2007). lag times, on the order of 0.50–1.25 s (thick, dark grey bars in 1
/3
RegionalstrainratesforAnatoliaindicatevariationintheprin- Fig.8b).Thecauseoftheanisotropyintheupperlayerisattributed /1
2
cipal infinitesimal shortening and extensional strain axes from toafinitestrainfieldorshearzoneinthemantlelithosphereasso- 0
1
easttowestfollowingthepatternofcounter-clockwiserotationof ciatedwiththerelativeplatemotionbetweenthePacificandNorth /6
0
Anatolianplate(Fig.7A).Thisvariationindirectionformaximum AmericanPlates(Ozalaybey&Savage1994;Ozalaybey&Savage 10
3
finiteshorteningandextensionisalsoinagreementwiththestruc- 1995;Hartog&Schwartz2001;Titusetal.2007).Thelateralextent 9
b
turalfeaturesoftheAnatoliancrust,whereNE-andNW-striking oftheupperlayervariesbetween50and100kmaroundtheplate y
g
conjugatestrike-slipfaultsandEW-strikingthrustfaultsdominate boundaryanditappearsrelativelynarrowonthewesternsideofthe u
e
eastern Anatolia and nearly EW-striking normal faults dominate SAF(Ozalaybey&Savage1995;Silver1996). st o
westernAnatolia.Asmentionedearlier,theLPOofolivineispar- Fig.8showsacomparisonofshearwavesplittingresultsforthe n
2
alleltotheflowdirectionandmaximumextensionaxisofthestrain NAFZandtheSAF(bothupperandlowerlayers).Althoughinboth 5
N
ellipse.Inourstudyarea,thedirectionofmaximuminfinitesimal casesthereissimilaralignmentofthefastpolarizationdirections o
v
extensionalstrainforthecrustisgenerallyparalleltotheobserved obliquetothetracesoftheSAFandNAFZ(Fig.8),therearesome e
m
fast polarization direction, suggesting the associated lithospheric importantdifferencesbetweenthetwofaultsintermsofanisotropy. b
e
andasthenosphericstrainfieldsaretheresultofsimilarplatescale The angle between the anisotropy directions and the strike of the r 2
forces.Inthebroaderregion,weseethatthemaximuminfinitesi- NAFZ is ∼30◦ at the western part of the study area and is up to 01
8
malshorteningandextensionalstraindirectionsforthelithosphere 80◦ attheeasternend,duetothecurvatureofthefault.Wetested
vary throughout Anatolia, whereas mantle deformation (fast po- varioustwolayeranisotropicmodelsforNAFZthataresimilarto
larization)directionsremainuniform(Fig.7A).Thelagtimesfor those suggested for the SAF. Two of these models are shown in
anisotropymeasurements,however,showsspatialvariationpatterns Fig.6.Thedashed,thinlinesinFig.6showthetheoreticaldistri-
that resemble variation patterns in crustal strain axis orientations butionoftheapparentsplittingparametersfortwolayeranisotropic
(Fig. 7B). Many studies state that the effect of Aegean slab roll- modelswiththefastdirectionsfortheupperlayerparallelingthe
backincreasesfromeasttowestinAnatolianregion(i.e.Barka& strikeofNAF,andthefastdirectionsforthelowerlayerparalleling
Reilinger1997;Meijer&Wortel1997;Allmendingeretal.2007) theNNRplatemotiondirection(30◦N).Forthemodelrepresented
andthevariationinmaximumfinitestrainaxesshowsthisclearly bythethin,blackdashedline(Model1)thedelaytimeassociated
(Figs7AandB).Observeddifferencesbetweenmantleanisotropy withthelowerlayeris0.8sandfortheupperlayeritis0.5s.For
directionsandcrustalstrainaxistrendsmightbeassociatedwithdif- the model represented by the grey, dashed line (Model 2), how-
ferencesinresponsesoflithosphereandasthenospheretoasimilar ever,delaytimeassociatedwiththelowerlayeris1.1sandforthe
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a
N 1.5 sec. km
44˚
E 1 sec. 0 100 200
W 0.5 sec.
S
42˚
D
30 32 34 36 38 40 ow
˚ ˚ ˚ ˚ ˚ ˚ n
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ic
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32˚ 246˚ 34˚ 244˚ 36˚ 242˚ 38˚ 240˚ 40˚ 238˚ m/g
ji/a
Figure8. Mapsfor(a)NAFZand(b)SAF,showingcharacteristicsofobservedanisotropyaroundplateboundaryzones.ThedarkgreyfilledcirclesforNAFZ rtic
mapindicatemeasurementsofsplittingfromtheETSE(Sandvoletal.2003).ThethickblackbarsfortheSAFmapindicateanisotropyobservedintheupper le
layerofthetwolayeredstack(Liuetal.1995;Silver1996;Obrebskietal.2006).SplittingmeasurementsforSAFandwesternNorthAmericaaretakenfrom -ab
s
Liu(2009). tra
c
t/1
upper layer it is 0.2 s. Although the backazimuthal distribution formationhasyetaccumulatedtoprovideasignificantcontribution 81
ofourmeasurementsareratherlimited,noneofthesemodelscan totheanisotropy. /3/1
uniquely and robustly explain the distribution of measured delay 20
1
timesandfastpolarizationdirections.Inthecaseoffastpolariza- /6
5 CONCLUSIONS 0
tiondirections,model2withtheupperlayerhavingsmallerdelay 1
0
times(greydashedline)showsarelativelybetterfit.However,in AnalysisofshearwavesplittinginthecentralpartoftheAnatolia- 39
termsofdelaytimes,noneofthemodelsfitsthetheoreticaldistri- Eurasia plate boundary, around the northward convex part of the by
bution.Theupperlayerwillhaveathicknessof15–20km,ifwe NAFZ, reveals fairly uniform NE–SW trending anisotropy direc- gu
e
aosbssuemrveedfofuarsttpoofilavreizpaetironcednitreacntiisoontsrobpyym.Bodaeseld2,ownetchaenbneottterurlfietoouft toifonsisgwniifithcadnetcarnedasuinngifodremlayantiimsoetsrofproymacwroessstmtoaejoarstt.eTcthoeniecxbisotuenncde- st on
the existence of a thinner anisotropic layer with fast polarization ariesandplatemarginsinthestudyareaarguesforanasthenospheric 25
directionsparallelingtheshearzoneassociatedwithNAFZ.How- sourcefortheanisotropyratherthanalithosphericsource. No
ever,itisdifficulttoresolveanisotropicpropertiesofsuchathin We suggest the slab roll-back taking place along the Aegean ve
m
layergiventheerrorsassociatedwithsplittingmeasurements.Even trench and the different amounts of trench retreat for the Aegean b
e
ifthislayerexists,itwillbemuchthinnerthanthesuggestedupper andCypreantrenchescontributessignificantlytotheupper-mantle r 2
layer for the SAF. Thus, we believe the simplest model that best dynamicsoftheregionandinducesaSW-directedastheonospheric 01
8
explainsourobservationsisasinglelayeranisotropicsource. flowwithdifferentialstrengthsfromeasttowest.Thesimilarpat-
TheNAFZhasbeenactivesince∼5Ma(Barka&Kandinsky- terns of variation for both crustal strain field and observed fast
Cade1988;Barka&Gu¨len1989;Koc¸yig˘it1989,1990;Dirik1993; polarizationdirectionsalsosupporttheideaofSWasthenospheric
Bozkurt & Koc¸yig˘it 1996; Barka et al. 2000; Bozkurt 2001) and flowundertheeffectsofAegeanandCypreanslabroll-backpro-
accommodates75–125kmofcumulativedisplacement(Westaway cesses.
1994;Armijoetal.1999;Barkaetal.2000).Thesevaluesofage ComparisonoftheNAFZandSAFintermsofanisotropysug-
andoffsetforNAFZaresignificantlylowerthanthosefortheSAF gestssomeimportantdifferencesexistbetweenthesefaults.Many
(∼315–730 km offset with an age of ∼17–30 Ma) (McKenzie & studiessuggestdoublelayeranisotropyfortheSAF,wheretheup-
Morgan1969;Atwater1970;Grahametal.1989;Dickinson1996; per layer fast polarization direction is parallel to the strike of the
Dickinson&Wernicke1997).Thismightimplythatthedeforma- faultandthelowerlayerfastpolarizationdirectionisparalleltothe
tionalongtheNAFZisatanearlydevelopmentstagecomparedto absoluteplatemotionintheregion.Aftertestingvarioustwolayer
theSAF,sothatnodominant,overprintingeffectoflithosphericde- modelssimilartowhatisobservedalongtheSAF,wecouldnotfit
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