Segmented African lithosphere beneath the Anatolian region inferred from teleseismic Pwave ...

21 Pages·2011·3.42 MB·English

Geophysical Journal International
Geophys.J.Int. (2011)184,1037–1057 doi:10.1111/j.1365-246X.2010.04910.x
Segmented African lithosphere beneath the Anatolian region inferred
P
from teleseismic -wave tomography
C. Berk Biryol,1 Susan L. Beck,1 George Zandt1 and A. Arda O¨zacar2
1DepartmentofGeosciences,UniversityofArizona,Tucson,AZ,USA.E-mail:[e-mail] s
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2GeologicalEngineeringDepartment,MiddleEastTechnicalUniversity,Ankara,Turkey ni
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Accepted2010November29.Received2010November19;inoriginalform2010July9 d
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SUMMARY micloa
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LithosphericdeformationthroughoutAnatolia,apartoftheAlpine–Himalayanorogenicbelt, ae
iscontrolledmainlybycollision-relatedtectonicescapeoftheAnatolianPlateandsubduction dynd from
roll-back along the Aegean Subduction Zone. We study the deeper lithosphere and mantle eo h
structure of Anatolia using teleseismic, finite-frequency, P-wave traveltime tomography. We Gttp
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uAspepdroaxtaimfarotemlys3e4ve0r0a0l tPem-wpaovrearryelaatnivdepterramvealntiemnet sreesisidmuiaclsn,emtweoasrkusreddeipnlomyeudltiipnlethfreeqreugeinocny. GJI://aca
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bands, are inverted using approximate finite-frequency sensitivity kernels. Our tomograms e
m
revealsegmentedfastseismicanomaliesbeneathAnatoliathatcorrespondstothesubducted ic
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portion of the African lithosphere along the Cyprean and the Aegean trenches. We identify u
p
theseanomaliesasthesubductedAegeanandtheCyprusslabsthatareseparatedfromeach .co
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other by a gap as wide as 300 km beneath Western Anatolia. This gap is occupied by slow /g
velocityperturbationsthatweinterpretashotupwellingasthenosphere.Theeasterntermina- ji/a
tionofthesubductingAfricanlithosphereislocatednearthetransitionfromcentralAnatolia rtic
le
totheEasternAnatolianPlateauorArabian–Eurasiancollisionfrontthatisunderlainbylarge -a
b
volumesofhot,underplatingasthenospheremarkedbyslowvelocityperturbations.Ourtomo- stra
gramsalsoshowfastvelocityperturbationsatshallowdepthsbeneathnorthwesternAnatolia c
t/1
that sharply terminates towards the south at the North Anatolian Fault Zone (NAFZ). The 8
4
associatedvelocitycontrastacrosstheNAFZpersistsdowntoadepthof100–150km.Hence, /3
/1
ourstudyisthefirsttoinvestigateandinterprettheverticalextentofdeformationalongthis 0
3
7
nascenttransformplateboundary. /6
2
Overall,theresolvedupper-mantlestructureofAnatoliaisdirectlyrelatedwiththegeology 3
6
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and tectonic features observed at the surface of the Anatolian Plate and suggest that the 6
b
segmentednatureofthesubductedAfricanlithosphereplaysanimportantroleintheevolution y
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ofAnatoliaanddistributionofitstectonicprovinces. u
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Key words: Mantle processes; Seismic tomography; Transform faults; Subduction zone t on
processes;Continentalmargins:convergent;Dynamicsoflithosphereandmantle. 29
M
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0
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tantroleinthetectonicdevelopmentoftheregion(Bozkurt2001,
1 INTRODUCTION
Fig.1).
AnatoliaisapartoftheAlpine–Himalayanorogenicbelt,formed The processes of collision-related tectonic escape of the Ana-
bydiscretecoalescedpiecesofcontinentallithosphereamalgamated tolian Plate and the subduction roll-back taking place along the
duringtheCenozoicclosureandterminalsuturingofthenorthern AegeanSubductionZone(ASZ)resultedindifferentstylesoflitho-
andsouthernbranchesoftheNeotethysOcean(S¸engo¨r&Yılmaz sphericdeformationthroughouttheregion.Basedonthesedeforma-
1981; Go¨ru¨r et al. 1984; Robertson & Dixon 1984; S¸engo¨r et al. tionstyles,theAnatolianregioncanbesubdividedintofourtectonic
1984). The recent tectonics of the region is controlled by north- provincesasdescribedbyS¸engo¨retal.(1985):theEastAnatolian
wardcollisionoftheArabianPlateandwestwardextrusionofthe contractional,theNorthAnatolian,theCentralAnatolianandthe
AnatolianPlatealongtheNorthAnatolianandEastAnatolianFault WestAnatolianextensionalprovinces(Fig.1).
Zones(NAFZandEAFZ,respectively).Inaddition,thenorthward Intheframeworkoftheseprovinces,theAegeanandtheCyprean
subductionofAfricanlithospherealongthesouthwestwardretreat- trenchesplayakeyroleinthedistributionofthetectonicregimes
ingAegeantrenchandobstructedCypreantrenchplaysanimpor- withintheEasternMediterraneanregion.Studiesshowedthatthe
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Figure1. Locationmapofthestudyareaandthemajortectonicandgeologicalfeaturesoftheregion.Thedistributionoftheseismicstationsusedinourstudy /g
isalsoshown.ThethickwhitearrowsshowsthemotiondirectionsoftheassociatedplateswithrespecttoEurasia(McCluskyetal.2003).Theapproximate ji/a
boundariesbetweenzonesofdifferenttectonicdeformationstyles(differenttectonicprovinces)areindicatedwiththickdashedlines.Theseprovincesare rtic
EACP,EasternAnatolianContractionalProvince;CAP,CentralAnatolianProvince;WAEP,WesternAnatolianExtensionalProvince;NAP,NorthAnatolian le-a
Province(modifiedfromBarka&Reilinger1997;Bozkurt2001).TheyellowregionshowstheIspartaAngle(IA)andthegreyregionsshowtheAntalya b
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andCiliciaBasins(ABandCB,respectively).NAFZ,NorthAnatolianFaultZone;EAFZ,EastAnatolianFaultZone;DSFZ,DeadSeaFaultZone;FBFZ, tra
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Fethiye-BurdurFaultZone;SF,Sultandag˘Fault. t/1
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4
/3
Cypreantrenchislessactiveintermsofseismicitycomparedtothe EurasianPlatetothenorth.Followingtheclosureofthesouthern /1
0
Aegeantrench(Wdowinskietal.2006).Basedonthelarge-scale branchofNeotethysduringthemiddleMioceneN–Scontractionin 3
7
tomographicmodelofPiromallo&Morelli(2003),Faccennaetal. theregiongavewaytotheformationandwidespreaddistribution /6
2
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(2006)describedthesubductingportionoftheAfricanlithosphere ofleft-andright-lateralconjugatestrike-slipfaultsduringtheearly 6
3
betweensouthwesternAnatoliaandtheCyprusIslandaslesswell Pliocene(∼5Ma).Recentgeophysicalinvestigationsintheregion 6
b
definedandverticallydiscontinuous.Faccennaetal.(2006)further revealedthincrustandmantlelithospherebeneaththeEasternAna- y
g
observedthattheseismicvelocityanomalyrelatedwiththisslab- tolianPlateau(EAP,Fig.1,Tu¨rkellietal.1996;Sandvoletal.1998; u
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likestructuredisappearscompletelyeastofCyprus.Manystudies Go¨ketal.2000;Al-Lazkietal.2003;Zoretal.2003;Angusetal. st o
point out that subduction along the Cyprus section of the trench 2006;O¨zacaretal.2008,2010b).Basedontheseobservationsand n
2
isinfluencedbythecollisionoftheEratosthenesSeamount(apart geochemicalanalysisofigneousrocksexposedintheEAP,ithas 9
M
of the African promontory) with the Cyprean trench (Robertson beenproposedthattheupliftoftheplateauisassociatedwiththe a
1994,1998;Robertson&Grasso1995;Glover&Robertson1998, steepeningandbreak-offofthenorthwardsubductingArabianlitho- rch
Fig.1). sphere(∼11Ma).Consequently,theupwellinghotasthenosphere 20
1
To investigate the relationship between the distribution of the replaced theslab,givingway torecent volcanismand supporting 9
majorgeologicalandtectonicfeaturesassociatedwiththetectonic the 2-km-high plateau (Keskin 2003; S¸engo¨r et al. 2003). This
provincesandtheupper-mantlestructurebeneathAnatolia,wecar- eventalsomarksadramaticchangeinthestressfieldthroughout
ried out seismic tomography analysis in this region. Our aim in theEAP,assuggestedbyKoc¸yig˘itetal.(2001)andO¨rgu¨lu¨ etal.
thisstudywastoprovidebetterconstraintsonthestructureofthe (2003).TeleseismictomographystudiesbyLei&Zhao(2007)indi-
subductingAfricanlithospherealongboththeAegeantrenchand catedthepresenceofadetachedslablocatedatdepthsasshallowas
particularlytheCypreantrench. 180kmbeneaththeEAP.However,amorerecentteleseismictomog-
raphystudybyZor(2008)indicatedthepresenceofthedetached
slabat∼600kmdepth.
TheNorthAnatolianProvince(NAP)isthearealocatedtothe
1.1 Tectonicsetting
northoftheNAFZandischaracterizedbynearlyE–Wstrikingdex-
TheEastAnatolianContractionalProvince(EACP)ischaracterized tralstrike-slipfaults.ThebasementoftheNorthernAnatoliancrust
byN–SconvergencebetweentheArabianPlatetothesouthandthe isrelativelyold,withvariablymetamorphosed,intenselydeformed
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SegmentedlithospherebeneathAnatolia 1039
and imbricated Precambrian to Late Palaeozoic rocks associated theAksufaultcontrolstheNEmarginoftheAntalyaforearcbasin
with the closure of Paleotethys (Aydın et al. 1986; Okay 1996; onthewestsideandthesouthernmarginoftheCiliciaforearcbasin
Go¨ru¨retal.1997;Deanetal.2000).Itiscommonlyagreedthatthe ontheeastside(Aksuetal.2005;˙Is¸leretal.2005,Fig.1).Recent
NAFZfollowstheNeotethyansuturealongthenorthernmarginof onshoreandoffshoreanalysisofthestructuresandsedimentshosted
the Anatolian Plate (Koc¸yig˘it, 1996; Bozkurt 2001; S¸engo¨r et al. bythesebasinsdocumentedachangeinstressfieldintheregion
2005).Therearemanystudiesontheageandthetotaloffsetofthe duringthelatestphaseofdeformation.
dextralNAFZ.ThelateststudiesindicatesthatNAFZisfairlyyoung The northern tip of the Isparta angle defines the boundary be-
(∼5–7Ma)andaccommodatesoffsetsintherangeof∼25–85km tween the Western Anatolia Extensional Province (WAEP) and
along its different segments (Koc¸yig˘it 1991; Bozkurt & Koc¸yig˘it the Central Anatolia Province (Barka & Reilinger 1997; Glover
1996;Barkaetal.2000;Bozkurt2001;S¸engo¨retal.2005).Several & Robertson 1998; Bozkurt 2001, Fig. 1). The Western Anatolia
geophysicalstudiesfromthewesternNAParoundtheMarmaraSea ProvinceischaracterizedbyN–Sextensionandrelatedformationof
(Fig.1)reportedcrustalthicknessestimatesrangingbetween25and grabensboundedbyroughlyE–Wstrikingnormalfaults(Taymaz
30kmaroundtheMarmaraSea(Gu¨rbu¨z&U¨c¸er1980;Laigleetal. etal.1991;Jacksonetal.1992;Jackson1994).Theageandcause
2008; Becel et al. 2009). Karahan et al. (2001) reported crustal of the crustal extension in Aegean region is a subject of ongoing
D
thicknessestimatesof39kmfromtheeasternMarmararegion.In debate. Proposed explanations include tectonic escape (Dewey & o
w
arecentstudy,O¨zacaretal.(2010a)reportedcrustalthicknessesin S¸engo¨r1979;S¸engo¨r1979;S¸engo¨retal.1985;Go¨ru¨retal.1995), n
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theorderof35–40kmforthecentralNAP.UnliketheEAPandthe subductionroll-back(Mckenzie1978;LePichon&Angelier1979) a
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EACP,thedeeperstructureoftheremainingpartsoftheNAPisnot and orogenic collapse (Seyitog˘lu & Scott 1991; Seyitog˘lu et al. ed
wellconstrainedduetothelackofgeophysicalinvestigations. 1992).Thetectonicescapemodelsuggeststhattheextensioninthe fro
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The Central Anatolian Province (CAP) is a wedge-like region westiscontrolledbythewestwardextrusionoftheAnatoliaPlate h
thatisboundedbytheNAFZtothenorthandEAFZtothesouth- alongtheNAFZandEAFZsince12Ma(Bozkurt2001,Fig.1).The ttp
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east,andthatextrudeswestwardalongthesestructures(Fig.1).The subductionroll-backmodelsuggeststhatSWretreatoftheAegean ://a
central part of this region is characterized by generally SW–NE trenchresultsinbackarcextensioninAegeanregionandWestern c
a
striking,widelyspaced,oblique-slipfaultswithdextralandsinis- Anatolia (Fig. 1). The orogenic collapse model proposes that the de
m
tralcomponents.Someofthesestructuresarethoughttoberelated extensionisduetothecollapseandthinningofthethickenedcrust ic
toearlierdeformationphasesthatwerereactivatedduetotheN–S following the closure of the Neotethys ocean. The controversial .o
u
convergencebetweentheAfricanandAnatolianPlatesandthean- agesfortheextensionrangesbetween5and60Madependingon p.c
ticlockwise rotation of the Anatolian block (S¸engo¨r et al. 1985; thecontrollingmechanisminvoked(Bozkurt2001).Severalstud- om
Reilingeretal.1997;Bozkurt2001;Taymazetal.2007a,b).The iesonthecrustalseismicpropertiesoftheWAEPindicatedcrustal /g
presenceofdiffusefaultzonesinthecentralpartofthisprovince thicknesses ranging between 28 and 35 km (Kalafat et al. 1987; ji/a
manifestsitselfwithaflattertopographycomparedtoWesternAna- Saunders et al. 1998; Sodoudi et al. 2006; Zhu et al. 2006). Re- rtic
le
tolianExtensionalProvinceandEACP.Thisregionisalsocharac- sultsofregionalseismictomographyindicatethepresenceofthe -a
b
terizedby∼13MatoRecentpost-collisionrelatedvolcanismthat steeplydippingEasternMediterraneanlithosphereunderneaththe stra
wasfollowedbyemplacementofseveralstratovolcanoes(Pasquare Aegeanregion,favouringtheideathatsubductionroll-backplays c
etal.1988;Notsuetal.1995).Recently,thenorthernandcentral an important role in the evolution of the region (Spakman et al. t/1
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partsofthisprovincewerestudiedindetailusingPn tomography. 1988;Faccennaetal.2003;Piromallo&Morelli2003;Faccenna /3
Results indicate the presence of anomalously slow P velocities etal.2006).ThecentralpartofthisprovinceischaracterizedbyLate /1
n 0
(<7.8 km s−1) located at the eastern edge of this province (Gans MioceneandPliocenevolcanism(Yılmaz1990;Delaloye&Bingo¨l 37
etal.2009).Thesouthwesternedgeofthisprovinceisknownasthe 2000;Yılmazetal.2001).Recentgeochemicalstudiesshowedthat /62
3
‘IspartaAngle’(IAonFig.1). theevolutionofthesevolcanicscouldbeassociatedwithavertical 6
3
TheIspartaAngleisatectonicallycomplexzoneconsistingof tearinthesubductingMediterraneanlithosphere(deBoorderetal. 6 b
SE-andSW-vergentallochthonsthatwereemplacedduringmultiple 1998; Tokc¸aer et al. 2005; Dilek & Altunkaynak 2009; Dilek & y g
tectonicphasesbetweentheLateCretaceousandtheLateMiocene Sandvol2009). ue
s
(S¸engo¨r & Yılmaz 1981; Robertson et al. 2003; Robertson et al. t o
2009).ThiszoneisboundedbytheNW–SEstrikingSultandag˘Fault n 2
(SF)tothe east(Boray etal. 1985) and transtensional left-lateral 2 DATA AND METHOD 9 M
NE–SW trending Fethiye-Burdur Fault Zone (FBFZ) to the west a
rc
(Dumont etal. 1979;Taymaz &Price1992;Price&Scott1994, Seismictomographyisanimportanttoolininvestigatingtheman- h
2
Fig. 1). The southernmost, offshore part of the Isparta angle is tle structure beneath an array of seismic stations. The mantle 0
1
markedbytheAnaximanderMountains.Thisregionofpronounced structure beneath the Mediterranean region has been studied by 9
sea floor relief with an anticlinal core is a part of the Anatolian Spakman(1985),Bijwaardetal.(1998)andPiromallo&Morelli
Plate and is located at the cusp where the Aegean and western (2003)usingseismictomography.Thesestudiescoveredbroadre-
Cyprustrenches(FlorenceRise)meet(Zitteretal.2003;tenVeen gionsandhence,hadlimitationsinresolvingfiner-scalestructures.
etal.2004,Fig.1).Inadditiontothesestructures,thecomplexity TherecentimprovementsinthegeographicalcoverageofNational
of the region is further accentuated by the presence of the Late EarthquakeMonitoringCenter(NEMC)seismographicnetworkof
Pliocene–Recent Aksu-Kyrenia thrust at the centre of the Isparta Turkey(Kalafatetal.2008)andseveraltemporaryseismicdeploy-
Angle(McCallum&Robertson1995;Poissonetal.2003,Fig.1); mentshavemadeitpossibletoimprovetheresolutionandquality
thismarksthelatestchangeintectonictranslationdirectionswithin oftheseismicstudiesfortheAnatoliaregion.Westudiedasmaller
thiszone(Poissonetal.2003).TheregionoftheIspartaAnglehosts partoftheEasternMediterraneanregionatafinerscaleusingtele-
several basin bounding grabens, half-grabens and transtensional seismic P-wave tomography with finite-frequency approximation
faults of various sizes (ten Veen et al. 2004; Alc¸ic¸ek et al. 2005; (Fig. 1). Although the narrower geographical span of the studied
Verhaertetal.2006,Fig.1).Inthesouth,theoffshoresegmentof regionlimitsthemaximumdepthfortheresolvablestructures,we
(cid:2)C 2011TheAuthors,GJI,184,1037–1057
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Events
Ray Coverage
Hypocenter
depth
N
(km)
0
50
100
150
30° 200 W E
250
300
90°
D
S 1000 ow
150° Number n
of Rays loa
Events used for picking P arrivals de
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Events used for picking PKPdf arrivals fro
Location of the study area m
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Figure2. (a)Globaldistributionoftheteleseismiceventsusedinourstudy.(b)Thebackazimuthalcoverageofallthearrivals(rays)picked. ttps
://a
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a
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cangetfinerresolutionatshallowerdepthswhencomparedtolarger residuals).Thisisinagreementwiththermsvaluesforindividual e
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regionalstudies. frequencybandsthatareallwithin0.54–0.58s. ic
Ray theory is used to calculate traveltime corrections for the .ou
p
observedrelativetraveltimeresidualstoaccountforcrustalhetero- .c
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geneities(Schmandt&Humphreys2010).Weincorporatedcrustal m
2T20.h11ebDtrraoavatedalbtiamneddanatdasthhoartt-wpeerhioadveseuissemdicinstoautiroannsablyelsoisngcionmgetsofmrouml- &tshtuiUcd¨kic¸enesersa1sn9ad8n0fdr;ovKmelaolrcaeifctayetievesettriamflu.an1tec9st8ifo7rn;oGmanu¨sareblvyu¨eszrisa&lfdoEerevtphanesesrie1sgm9i9oic1n;s(oTGuu¨nu¨rdrkbienu¨llgzi /gji/article
tipleseismographicnetworksoperatedintheregionbetween1999 etal.1996;Sandvoletal.1998;Saundersetal.1998;Karahanetal. -a
b
and2009.ThesenetworksaretheNationalEarthquakeMonitoring 2001;Zoretal.2003;Sodoudietal.2006;Zhuetal.2006;Laigle stra
Centernetwork(NEMC)(bothshortperiodandbroadbanddeploy- etal.2008;Beceletal.2009;O¨zacaretal.2010a,b)tocalculate c
ments)(Kalafatetal.2008),GlobalSeismographicNetwork(GSN), correctiontimes.Thermsforthesecrustalcorrectionsis0.08and t/18
4
EasternTurkeyPassiveSeismicExperiment(ETSE)(Tu¨rkellietal. afterthecorrectionsthermsoftherelativetraveltimeresidualsis /3
2003),Geophone(GE),WesternAnatoliaSeismicRecordingEx- 0.55,whichisnearlythesamevalueasthermsbeforethecrustal /10
periment (WASRE) (Zhu et al. 2006) and our North Anatolian corrections. This might be due to the lack of a direct correlation 37
Faultpassiveseismicexperimentnetwork(NAF)(Gansetal.2009, betweenobservedtraveltimeresidualsandthecrustalcorrections. /62
3
Fig.1). Inaddition,thisalsoshowsthatthemajorpartoftheseismic-speed 63
Weusedarrivaltimedatafrom409teleseismiceventswithmag- anomalyexistsinthemantle. 6 b
nitudesMw>5.0locatedatdistancesof30–90◦fordirectPphases y g
and155–180◦forPKPdfphases(Fig.2a).Weusedthemultichannel ue
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carrorisvsa-lcsorarnedlactiaolncutleacthendiqreuseidoufaPlatvimlises&wVitehrnroensp(e2c0t1t0o)IfAorSpPi9c1k.inAg 2.2 Method t on 2
totalof33771directPand219PKPdfphaseswerepickedonver- The seismic arrivals observed on seismic records have a finite- 9 M
ticalcomponentsofbroadbandrecordsinthreefrequencybands,to frequencybandwidthandtheirtraveltimesaresensitivetoseismic a
rc
incorporatethemeasuredtraveltimeresidualsinafinite-frequency speedvariationswithinthevolumesurroundingthegeometricalray h
2
approximation.Thepassbandfiltersthatweusedhavecornerfre- path(Woodward1992;Marqueringetal.1999;Dahlenetal.2000; 0
1
quencies of 0.2–0.8 Hz (short period), 0.1–0.4 Hz (intermediate Hungetal.2000;Zhaoetal.2000).Theseismicwaveswithdifferent 9
period)and0.04–0.16Hz(longperiod)forbroadbandrecords.The frequencycontentshavedifferentFresnelzonewidthsandthusthey
picksassociatedwiththeshort,intermediateandlongperiodbands sampledifferentvolumesofmediumsurroundingthegeometrical
constitute34,36and27percentoftheentiredataset,respectively. raypath.
Thedatafromshort-periodstationsarefilteredonlywithapassband Inthisstudy,weusedtheteleseismictomographyalgorithmof
filterwithcornerfrequencies0.5–1.5Hzandthissubsetconstitutes Schmandt&Humphreys(2010),whichincorporatestheuseoffre-
only3percentoftheentiredatasetduetothelowerqualityofthe quencydependent3-Dsensitivitykernels.Theuseofthesekernels
teleseismicrecords.Toavoidanydirectionalbiasinourresults,we helpsustoaccountforvolumetricvariationsinwavespeedswhen
optimizedthebackazimuthaldistributionoftheobservedresiduals calculatingthevelocityperturbationsinthetomographymodel.The
(Fig.2b).Themeasuredtraveltimeresidualswerethendemeaned usageofsensitivitiesislimitedtothefirstFresnelzone.Bornthe-
foreacheventtoobtaintherelativetraveltimeresiduals.Thermsof oretical‘banana–doughnut’kernelapproximationofDahlenetal.
theobservedrawrelativeresidualsis0.56s(alsoseeAppendixA1, (2000)isusedtocalculatethesesensitivities.Thesensitivityker-
intheSupportingInformation,fordistributionofrelativetraveltime nelsareapproximatedasafunctionofthedominantperiodofthe
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seismicarrival,epicentraldistance,distancesnormaltotheraypath eventtermsis∼0.02,alsosmallcomparedtotheoverallrmsofthe
and along the ray path and ray centred azimuth. Details of this dataset.
algorithm are explained in Schmandt & Humphreys (2010). We
preferthistomographicalgorithmbecauseitallowsustoincorpo-
ratemeansofinterpretingthefrequencydependenceoftraveltime
2.3 Modelparametrization
data,makingbetteruseofbroadbandrecordsanditintroducesan
intrinsicsmoothnessconstraintfortomographythroughtheuseof Theregionthatwestudiedcoversanareaapproximately1100km
approximate sensitivity kernels (see Appendices C1, C2 and C3, (E–W)×900km(N–S)andouraveragestationspacingisroughly
inSupportingInformation,forcomparisonofsingle-frequencyand 50 km (Fig. 1). This region is parametrized with an irregularly
integratedthree-bandfrequencytomographicmodelsforourdata spaced3-Drectangulargridthatstartsat35kmdepthandextends
set).Inaddition,thesensitivitykernelapproximationusedinthis downto655kmdepth.Thelateralnodespacingatthetopofthis
algorithmhasahighcomputationalefficiency. volume varies from 35 to 50 km, with denser spacing (35 km)
Smoothingisappliedontheapproximatedsensitivitykernelsto present only at the central to outer central parts of the seismic
accountforgeometricalraylocationuncertaintynormaltotheray array.Towardstheedgesofthemodelledvolume,thenodespacing
D
andalongtheraypath.Gradientandnormdampingarealsoused increasesgraduallyupto50kmtoaccountfortheeffectsofreduced o
w
toregularizetheinverseproblem.Thegradientdampingalgorithm resolutionpowerattheedges(Fig.3a).Theverticalnodespacing n
lo
incorporates variations of ray path-normal damping weights as a increases from 35 to 50 km with increasing depth. Similarly, the a
d
functionofdepthtoaccountfortheincreasingangleofinclination horizontal node spacing gradually increases from 35 to 48 km at ed
ofthemeanraypath.Normdamping,ontheotherhand,attempts thecentreofthemodel(50to70kmattheedges)fromthetopof fro
m
to obtain the model with smallest possible perturbations satisfy- themodeltothebottom(Fig.3b).Suchdilationsinnodespacing h
ingthetraveltimedata,whilestronglydown-weightinganomalies as a function of depth constitute a more efficient parametrization ttp
s
thatareweaklysampledbytheobservedsetofrays(Schmandt& approach,astheFresnelzonewidthincreaseswithdepthwhereas ://a
Humphreys2010).TheinverseproblemissolvedusingtheLSQR the resolution decreases. Our parametrized model has a total of c
a
methodofPaige&Saunders(1982)withanobjectiveofminimizing 41 808 nodes with 67 (latitudinal)-by-39 (longitudinal) nodes in de
m
theleast-squaresmisfitbetweencalculatedandobserveddatainan 16layers. ic
iterativesequence.Weperformedtrade-offanalysisbetweenEuclid- Toestimatetheresolvingpowerofourinversionwithourdataset, .o
u
ianmodelnormandvariancereductiontodeterminethesmoothing weinvestigatedoursamplingofthemodelspace.Forthispurpose, p.c
weightandtheoveralldampingfactor(Menke1989).Indeed,our we calculated hit quality maps for each depth layer of our model om
testsshowedthatthevariationinvariancereductionsforinversions space(Figs4a–d).Thesenormalizedhitqualitymapsarebasedon /g
withslightlyvarying(±1.0)dampingandsmoothingweightswere densityofsamplingofeachnodeinagivenlayer,wherethequality ji/a
small (see Appendix A4, in Supporting Information, for various isalsoafunctionofthebackazimuthaldistributionofthesampling rticle
smoothing/damping trade-off plots and Appendix B1 and B2 for rays. The function governing the assignment of hit quality index -a
b
comparisonofresultsobtainedusingdifferentsmoothing/damping iscustom-builtbasedontheideathatbetterresolutioninseismic stra
parameters).Weobtainedanrmsof0.1fortheresidualsafterthe tomographyisprovidedbyraysfromvariousdirectionsintersecting c
inversionwithfavouredvaluesofdamping(6)andsmoothening(7), at high angles (Schmandt & Humphreys 2010) (see Appendix D, t/18
achievingavariancereductionof∼81percent. in Supporting Information, for a summary table). Thus, a quality 4/3
Eventhoughcrustalcorrectionsareappliedtotheobservedrela- of1indicatesthattheassociatednodeissampledbyatleastfive /1
0
3
tivetraveltimeresiduals,stationtermsareincludedforeachstation rays from each of the four geographical quadrants (NE, SE, SW 7
intheinversiontoaccountforthesmall-scalelocalvariationsand andNW)andaqualityof0indicatesthattheassociatednodeisnot /62
3
errorsinincorporatedcrustalthicknessesandvelocities.Therms sampled.ThemapsinFig.4showusthatbetternodesamplingis 6
3
for these terms is ∼0.09, which is small compared to the rms of restrictedtonodeslocateddirectlybeneaththeindividualstations 6 b
the entire data set (0.55), showing that these terms explain only atshallowerdepths(Fig.4a),particularlyforthetoplayer(35km y g
asmallfractionof thedata. Indeed, strongdamping isapplied to depth), due to the near vertical incidence of teleseismic arrivals ue
s
stationtermstoavoidanyabsorptionofmantlestructurebythese thatclusteraroundthestations.Thus,thebroad-scaleresolutionis t o
terms. We tested the case where no crustal traveltime corrections poorforthistoplayer.Further,thislayercompensatesfortheerrors n 2
wereappliedtotherawdata.Inthiscase,weobservedthattherms in crustal corrections and poorly constrained variations in upper 9 M
forthestationstatictermsincreasedby∼0.05.Thisindicatesthat mantle. Hence, in the following sections where we discuss and a
rc
thestationstatictermseffectivelyaccountsforthevaryingshallow interpret the resolved perturbations in our model, we will ignore h
2
structureinthestudyarea. anomalies resolved in this layer. In deeper parts of the modelled 0
1
Eventtermsarealsocalculatedrepresentingtheadjustmentofthe volume,the‘good’hitqualityregionsspreadouttocoveralmost 9
meanarrivaltimeforthesetofstationsthatrecordedeachevent(see theentirestudyregion(Figs4bandc).Atthebottomofthemodel,
AppendicesA2andA3,inSupportingInformation,fordistribution however,duetothereductioninsamplingdensityattheedgesofthe
ofstationandeventterms).Ourdatasetiscomposedoftraveltime model,thebettersampledportionsarelimitedtothecentralparts
datafrompatchworkofregionalseismicnetworkssomeofwhichare ofthestudyarea(Fig.4d).Wealsoobserveapersistentpatternof
notconcurrentintime.Thismightintroducetheproblemofshifts relativelylowerqualitiesincentralpartsatshallowerdepths(Figs4a
in calculated mean traveltime residuals between these networks andb),whichisanartefactofwiderstationspacingintheseportions
that were deployed at geographically different parts of the study ofthestudyarea.
area. However, both the incorporation of the event terms in the Inadditiontothequalityofsamplingofmodelspace,weinves-
inversion and usage of data from long-operating, spatially well- tigated the resolution in our model area through synthetic tests.
distributed/envelopingnetworkssuchasNEMCandGSN(Fig.1) Our synthetic tests involve layers with ‘checkerboard’ type ve-
helpustoaccountforvariationsorshiftsinmeanvelocitystructure locity anomalies embedded in a neutral background. With this
indifferentportionsofthestudyarea.Thecalculatedrmsforthe setup,weaimedtoplaceconstraintsonthelateral(Figs5a–d)and
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Figure3. Nodeandlayerspacinginourparametrizedmodel.(a)Lateralnodespacingat60kmdepth.(b)Layersandnodespacingofourparametrizedmodel /6
onaverticlesectionalong39◦latitude.Reddiamondsshowsthestationsusedinthisstudy.Notethedilationinnodespacingbothtowardstheedgesandwith 23
6
increasingdepth. 3
6
b
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vertical(Figs5eandf)resolvingpowerofourdatasetwithinthe is recovered as an anomaly of ±1.8 per cent within the resolved gu
e
parametrized model. The volume of each perturbation in the lay- model.Thepeakamplituderecoveryis78percent.Thelossesin s
ers of the checkerboard is defined by cube of two nodes (eight amplituderecoveryduringtheinversionprocessarealsoaconse- t on
nodestotal).Hence,thelateralandverticalextentsoftheindividual quenceofapplieddamping,smoothingandimperfectraycoverage. 29
anomalies vary as a function of position in the model and dilate Theverticalcross-sectionsalsoshowsomedilationandstreaking M
a
with depth (Figs 5e and f) and towards the edges in accord with of the recovered perturbations at the edges of the model due to rc
h
the dilation in node spacing within the model (Figs 3 and 5a–d). sparse sampling. Although the reliability of these resolution tests 2
0
Theintensitiesoftheinputperturbationsare−3and+3percent islimitedbythebasictomographicassumptions(i.e.isotropicand 19
forslowandfastanomalies,respectively.Theseanomaliesaresep- elastic mantle, well constrained ray locations at depth, accurate
arated by a cube of two nodes belonging to neutral background. traveltime sensitivity kernels), the recovery of the pattern of per-
Figs5(a1–f1)showstheinputsyntheticstructureandrecoveryof turbationsinourresolutiontestindicatesthatweshouldbeableto
it after inversion (Figs 5a2–f2). The results of this test show that retrievemajorupper-mantlestructures(i.e.subductedlithosphere,
our resolution is good within the modelled volume and the study plumes)usingthedatasetathandandthepreferredparametrized
area. However, there is some amplitude decay and dilation in re- model. In the following section, we will show results from addi-
covered perturbations due to minimum-length solution objective tional resolution tests on specific features that emerge from the
of the inversion procedure and subvertical incidence of teleseis- inversion.
mic rays (Schmandt & Humphreys 2010). The inversion for the Itisimportanttokeepinmindtheresultsofthehitqualityand
synthetic model achieved 60 per cent amplitude recovery of the checkerboardresolutiontestswheninterpretingthemodelledveloc-
inputperturbationswithinthecentralpartsofthemodel.Inother ityperturbations,toavoidmisinterpretationofanomalieslocatedin
words, a ±3.0 per cent perturbation in the input synthetic model regionsofthemodelthatarepoorlyresolved.
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c
a
Figure4. Normalizedhitqualitymapsforourdatasetandparametrizedmodelatvariousdepths(plotsa–d).Hitqualityisbasedonthenumberandazimuthal de
distributionofrays.Redshadingindicatesgoodhitquality(1onscale)andblueshadingshowspoorhitquality(0onscale)foreachnode.Whitediamonds m
ic
showthestationsusedinthisstudy.Notethatthespreadingofthepatternofbetterhitqualitiesatdeeperpartsofthemodel. .o
u
p
.c
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3 RESULTS featuresoftheresolvedmodelandresultsoftherealisticanomaly /g
recoveryanalysisfortestingtherobustnessofresolvedfeatures. ji/a
3.1 Inversion The most significant feature of the resultant model is the fast rtic
le
The data variance reduction is a common way of assessing the anomalies located to the north of the Aegean and the Cyprean -ab
trenches (Figs 6a–d and 7b and c). Overall, these fast anomalies s
quality of the least squares solution to the tomographic inverse haveameanperturbationof+1.7percentwithintheentiremodel tra
problem. As pointed out by Schmandt & Humphreys (2010), the and they shift in a northerly direction as a function of increasing ct/1
overallvariancereductionfortheentiremodelledvolumegivesa 8
depth.ThegeneralstrikesoftheseanomaliesareroughlyNWonthe 4
ratheroptimisticestimateowingtothefactthatthesetupofthefor- /3
AegeansideandE–WontheCypreanside.Theindividualmean /1
wardproblemheavilyreliesontheaprioriassumptions(i.e.elastic, 0
perturbations for Aegean and Cyprean anomalies above 450 km 3
isotropic upper mantle, accurate ray locations) and the resolution depthare+1.8and+1.7percent,respectively.Thesetwoanoma- 7/6
ofthesolutionshowsvariationsthroughoutthemodelledvolume. 23
liesareseparatedbyanN–Strendingvolumeofslowperturbation 6
Asreportedearlier,weobtainedanoveralldatavariancereduction locatedbetween28◦ and30◦ longitudes(Figs6a–eand7eandf). 36
of 81 per cent after the inversion. This indicates that the resul- Thisslowanomalyhasameanperturbationof−1.4percent.We by
tant model successfully explains most of the observed traveltime g
can also observe relatively fast anomalies (mean perturbation of u
rtheseidoubaslesr.vTedo dteastta,hsoawmpsulicncgestshfeulwlyelol-urresroelsvoeldvepdarmtso,dweleeaxdpolpatiends +1.6 per cent) located to the north of the NAFZ at the western est o
andeasternpartsofthestudyarea(Figs6aandb).However,these n
the same approach as Schmandt & Humphreys (2010), calculat- 2
anomalies are only observed at shallower depths (Figs 7c and d). 9
ing the variance reduction for the portions of the model that are M
The rest of the resultant model mostly contains scattered, small- a
denselysampledandwellresolvedwithahitqualitygreaterthan scaleandweakerslowvelocityperturbations.Thepatternsofslow rch
0.4 (i.e. rays from at least two backazimuth quadrants, excluding anomalies, however, are more persistent and continuous beneath 20
thetopandbottomtwolayers).Weobtainedavariancereductionof 1
eastern Anatolia (Figs 6a–d and 7d and e). The mean amplitude 9
73percentforthe‘highquality’partsoftheresolvedmodel.This
of the ‘blob’ of slow velocity anomaly beneath the EAP is −1.5
shows that major portion of the observed traveltime residuals is
per cent. At depths below 500 km, the separated fast anomalies
explainedwithinthewell-sampled,well-resolvedpartofthemodel,
throughoutthestudyareamergetoformacontinuous,WNW–ESE
althoughaminorportionofitisresolvedasprobableartificialhet-
trendingbelt(meanperturbationof+1.7percent)thatoccupiesthe
erogeneitiesatthepoorlysampled,poorlyconstrainedpartsofthe
deeper,northernportionsoftheresolvedmodel(Figs6f–hand7e).
model(i.e.edgesofthemodel),duetotheobjectiveofleastsquares
Theverticalslices(cross-sections)thatcutacrossthestudyarea
inversiontominimizethelengthofthesolution.
showthedippingfastanomaliesassociatedwiththeAegeanandthe
Cypreantrenches(Figs7bandc).Wealsoobserveacleareastern
terminationofthefastanomalyassociatedwiththeCypreantrench
3.2 Tomograms at∼34◦Elongitude(Fig.7f).Alonglatitude37◦N,theslowanomaly
The resultant tomograms are shown in Figs 6 (map views) and 7 thatseparatestheAegeanandtheCypreananomaliesextenddown
(cross-sections).Inthissection,wesummarizethemostprominent todepthsof300km(Fig.7f).Itbecomesclearonthecross-sections
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Figure5. Resultsoftheresolutiontestspresentedintheformofhorizontalslices(plotsa–d)andvertical(latitudinal)cross-sections(plotseandf).Theeffects
ofverticalstreakingcanbeseenatshallowerdepths(a1anda2).Thelateralresolutionisgenerallygoodbutthereissomeamplitudedecay.Theopentriangles
showthelocationofstations.
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Figure6. Horizontalslicesatdepthsbetween60and605km(plotsa–h)fromtheresultanttomographymodel.Openstarsindicatethestationsusedinour 9
study.Thedark-shadedareasremainingoutsideofthewhitedashedlinearepoorlyresolvedportionsofthemodelwithhitqualitieslessthan∼0.2.
thatthesedippingfastanomaliesmergeatdepthsof∼500kmand indicatedthepresenceofadiscontinuous,northwarddippingfast
starts flattening out close to the bottom of the mantle transition anomalyinthisregion.Althoughourresolvingpowerandhitquality
zone(660km)(Figs6gandhand7f).Atthewesternpartofthe isgoodinthisportionofthestudyarea,wedonotobservesuchafast
studyarea,theshallowfastanomalieslocatednorthoftheNAFZ structure. This discrepancy between the two tomographic models
extendtodepthsof100–150kmandterminaterathersharplyatthe mightbeduetothedifferencesinstylesofsamplingofthemodelled
downwardprojectedtraceoftheNAFZ(Figs6aandb).Intheeast, volumeofuppermantle.Asitismentionedearlier,wehomogenized
weobserveconsistentslowanomaliesbeneaththeEAPthatextend thebackazimuthalraycoverageofourdatasettoavoidunwanted
to depths of 300–400 km (Figs 6d and f). This is in agreement inversionartefactssuchassmearingofimagedseismicanomalies
withtheresultsfromarecentteleseismictomographystudybyZor alongtheraypaths.Intheirstudy,Lei&Zhao(2007)usedraysfrom
(2008).AnearlierseismictomographystudybyLei&Zhao(2007) dominantlyeastbackazimuthswhichmighthaveintroducedsome
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Figure7. Theresultanttomographymodelalongvariouslongitudinal(plotsb–d)andlatitudinalcross-sections(plotseandf).Topographicprofilesarealso
shownatthetopofeachslice(with10×verticalexaggeration).Numberedredlinesonthemapinthetoppanel(plota)showlocationsofthecross-sections.
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