Investigation of the submarine active tectonism in the Gulf of Gökova, southwest Anatolia ...

20 Pages·1999·8.14 MB·English

ELSEVIER Tectonophysics305(1999)477–496
Investigation of the submarine active tectonism in the Gulf of Go¨kova,
southwest Anatolia–southeast Aegean Sea,
by multi-channel seismic reflection data
Hu¨lyaKurta,(cid:3),EminDemirbag˘a,˙IsmailKus¸c¸ub
aI˙stanbulTechnicalUniversity(I˙TU¨),DepartmentofGeophysics,80626Istanbul,Turkey
bMineralResearchandExplorationDirectorateofTurkey(MTA),JeolojiDairesi,EskisehirYolu,06520Ankara,Turkey
Received21August1998;accepted25January1999
Abstract
Submarine active tectonism in the Gulf of Go¨kova located at the southwest Anatolia–southeast Aegean Sea region
was investigated by means of multi-channel seismic reflection data. The Go¨kova basin is filled by the latest Miocene–
Pliocene–Quaternarysedimentswithmaximumthicknessofabout2.5km.TheLycianNappes,whichpredominantlycover
extreme southwestern Anatolia, constitutes the basement rocks for the Go¨kova province. The gulf was mainly opened by
a buried major listric normal fault, so-called Datc¸a Fault, which has not been previously discussed in the literature. The
north-dipping,mainlyE–W-trendingDatc¸aFaultislocatedatthesouthernpartofthegulf,whereasitsassociatedantithetic
faults are located at the north. The onset time of the opening of the gulf is possibly in the latest Miocene–Pliocene. In
terms of local rather than regional effects, the activity of the Datc¸a Fault has decelerated, possibly since the Pleistocene.
The Datc¸a Fault might have gained its curved fault plane as it evolved, beginning as planar and=or using antecedent
planesoftheLycianNappesinthearea.Astheextensionprogressed,i.e.,asthehangingwallblockslippedfurthernorth,
gravity may have impeded rather than helped the faulting. On the other hand, continuing extension in the area may have
initiated a second phase of faulting, i.e., WNW–ESE-oriented subgrabens in the gulf and major E–W normal faulting in
thenortheastmargin.Abathymetriclowinthemid-gulfareaandahorst–grabensystemintheeasternpartofthegulfare
observablefromthebathymetricdataandarewellcorrelatedtotheseismicdata.Althoughthemainorientationofthegulf
is E–W,more recent WNW–ESE structures areremarkable in the mid-gulf andin the eastern part of the gulf. Thelatest
WNW–ESE structures are also in agreement with the results of GPS and SLR studies as well as plate motion modelling
bytotalmomenttensorofearthquakes inthewesternAnatolia–AegeanSearegion,particularly insouthwesternAnatolia.
Theamount oftotalN–Sextensionwithinthegulfisestimatedasatleast5.5kmsincethelatestMiocene–Pliocenewith
overall constant extension rate of at least 1.1 mm=y where the estimated extension factor is about (cid:12) D 1:3. (cid:211) 1999
ElsevierScienceB.V. Allrightsreserved.
Keywords:GulfofGo¨kova;AegeanSea;westernAnatolia;extensionalregime;activetectonism;seismicreflection
(cid:3)Correspondingauthor.E-mail:[email protected]
0040-1951/99/$–seefrontmatter (cid:211) 1999ElsevierScienceB.V. Allrightsreserved.
PII: S0040-1951(99)00037-2
478 H.Kurtetal./Tectonophysics305(1999)477–496
1. Introduction extension,theonsettimeisalsodebated.Deweyand
S¸engo¨r (1979) and laterS¸engo¨r et al.(1985) put the
Thepurposeofthisstudyistoinvestigatesubma- onset time in the Late Miocene, whereas Seyitog˘lu
rineactivetectonismintheGulfofGo¨kovabymeans andScott(1991)proposetheEarlyMiocene.
of multi-channelseismic reflectiondata and further- In the center of this ongoing debate, the Go¨kova
moretocorrelatetheresultswiththepreviousonland province is mainly characterized by two successive
tectonicframe,geophysicalandgeodeticstudies.Pa- tectonic regimes. These overlapping rift and graben
ton(1992)studiedthefaultinganddrainagepatterns systems are seen in the land geology (Fig. 1). The
onthenorthernmarginofthegulf.Healsoestimated first one is the N–S compressional paleotectonic
theextensionfactorbasedontheinitialandfinaldips regime, possessing the later counter-clockwise ro-
ofthefaultswhichboundthetiltedblocksbyassum- tation, and resulted in a NW–SE rift and graben
ingasimpledominomodel((cid:12) D1:1).Riftformation system.TheNW–SE-orientedpaleotectonicriftsand
intheGo¨kovaprovinceisdiscussedextensivelyonthe grabens, i.e., Milas–O¨ren and Yatag˘an Mug˘la Rifts,
basisoflandgeologyandbathymetrydatainGo¨ru¨ret are filled by mainly Middle Miocene to Quaternary
al.(1995).However,thesubmarinegeologyandgeo- deposits of continental origin. The second one is
physicsofthearea,theGulfofGo¨kova,havenotbeen the neotectonic regime, possessing the N–S exten-
studied in detailexcept for a shallowsingle-channel sion that has resulted in mainly E–W-oriented rift
seismicreflectioninvestigationbyUlug˘ etal.(1996). and graben systems, i.e., Go¨kova Rift (Go¨ru¨r et al.,
The Gulf of Go¨kova is located in the southeast 1995). TheLycianNappes,whichcoveralarge area
Aegean Sea along the coast of southwest Anatolia in the extreme southwest of Anatolia, constitute the
which is the region including the major rifts and basement for both systems. The Lycian Nappes can
grabens such as Gediz, Bu¨yu¨k Menderes, Go¨kova, be divided into two major units: the Intermediate
Burdur, Acıgo¨l and Baklan. The gulf is surrounded Complex at the bottom and Peridotite Nappes at the
by Bodrum Peninsula to the north, Datc¸a Peninsula top (Brunn et al., 1971; Bremer, 1971; Bernoulli et
to the south and the island of Kos to the west. It al.,1974; Brunn etal.,1976). TheKo¨yceg˘izsliceof
has about 25 km maximum N–S width and 100 the IntermediateComplex,whichismainlyPermian
km E–W length (Fig. 1). The Go¨kova province is to Upper Cretaceous sedimentary rocks, outcrops in
a part of the western Anatolia–Aegean Sea region, theGo¨kovaarea(Go¨ru¨retal.,1995).
which is under a N–S regional extensional tectonic Global Positioning System (GPS) and Satellite
regime. The imprints of this extensional regime are Laser Ranging (SLR) studies in recent years in-
clearly seen in the geology and geomorphology of dicate that the relative plate motions in the west-
westernAnatoliaaswellasinthebathymetryof the ern Anatolia–Aegean Sea region have accelerated
Aegean Sea. Although there is no doubt about the from north to south counter-clockwise, particularly
presentextensionalregime,itscausesandonsettime in southwestern Anatolia (Oral et al., 1995; Le Pi-
have been debated. According to the most widely chon et al., 1995; Reilinger et al., 1997; Barka and
accepted model proposed by Dewey and S¸engo¨r Reilinger, 1997). The largest differential motions
(1979), the extensional tectonism is related to the are obtained across the Bu¨yu¨k Menderes and Gediz
westerlyescapeof the Anatolianplateinreactionto GrabensinwesternAnatoliaandpossiblyintheGulf
thecollisionoftheArab–AfricanandEurasianplates of Go¨kova. According to Reilinger et al. (1997),
through the Bitlis continental collision zone. Other the extension rate is about 14 (cid:6)5 mm=y at Mar-
groupofresearcherssuggestthatback-arcspreading maris station in Datc¸a Peninsula (Fig. 2). Barka
of the relativelythickened Aegean crust in the east- and Reilinger (1997) suggested that the present
ern Mediterranean subduction is another reason for tectonic regime in western Anatolia–Aegean Sea
the N–S extension (Le Pichon and Angelier, 1981). causes E–W- and WNW–ESE-oriented graben and
Gravitational spreading of the relatively thickened normal fault systems. On the other hand, the NE–
Aegean crust is also suggested by Le Pichon and SW-oriented Acıgo¨l, Baklan and Burdur basins are
Angelier (1979) and Seyitog˘lu and Scott (1991). In remarkable from geological and seismological stud-
additiontothesedifferentmodelsontheoriginofthe ies(TaymazandPrice,1992;PriceandScott,1994).
H.Kurtetal./Tectonophysics305(1999)477–496 479
Fig.1.LandgeologymapoftheGo¨kovaprovince(modifiedfromGo¨ru¨retal.,1995;theoriginalmapalsoincludesinferredsubmarine
faults). Notice the E–W-oriented new rift system (the Gulf of Go¨kova and its margins) and NW–SE-oriented older rift systems
(Mug˘la–Yatag˘anRiftandMilas–O¨renRift).
Acceleration of the velocity field in southwestern sors of earthquakes(Eyidog˘an, 1988; Jackson et al.,
Anatolia towards the eastern Mediterranean basin 1992).Eyidog˘an(1988) statesthatN–Sextensionin
is noticeable in Le Pichon et al. (1995). In sum- western Anatolia increases from north to south, and
mary, accordingto the recentGPS and SLR studies, the dominant extensionsin southwesternAnatoliais
onecanexpectE–W-andWNW–ESE-orientedfault N–S with a rate of 13.5 mm=y which is obtained
systemsintheGulfofGo¨kova. from the total seismic moment release of 25 earth-
Rates of extension in the western Anatolia–Ae- quakes. Jackson et al.(1992) calculatedthe velocity
gean Sea region are also studied by moment ten- field in the Aegean region on the basis of moment
480 H.Kurtetal./Tectonophysics305(1999)477–496
Fig.2.Bathymetry,topographyandmarineseismiclinemapoftheGo¨kovaarea.DominantE–Wtrendofthegulf,WNW–ESE-oriented
bathymetriclowsandhighsinthemid-gulfandintheeasternpartofthegulf,andNW–SE-orientedtopographiclowsandhighsofthe
olderriftsareremarkable.WhitedotsshowtheepicenterdistributionoftheearthquakesfromISCcatalogues.TheGPSvelocityvector
takenfromReilingeretal.(1997)showsthedirectionofplatemotionattheMarmarisstation.
tensors of earthquakes occurring between 1909 and sion is at least 5.5 km. We estimated the extension
1983. Horizontal velocities obtained in that study factor (McKenzie, 1978a; Wernicke and Burchfiel,
clearly show an increase towards the southwest. In 1982; Jackson and McKenzie, 1983; Gibbs, 1983;
summary, the velocity field of crustal deformation White et al., 1986; Jackson and White, 1989) as at
calculated from the moment tensors of earthquakes least (cid:12) D 1:3 by taking the ratio of the length of
in the western Anatolia–Aegean Sea region also seismic section-11 (extrapolating the Datc¸a Fault to
shows similar results to GPS and SLR studies, a sea level) and the length of the rollover marked by
notableincreasetowardsthesouthandsouthwest. reflectionsfromthetopofthebasement.
Studies dealing with estimating the rates and ex-
plaining the nature of deformation in the western
Anatolia–Aegean Sea region are usually based on 2. Dataacquisitionandprocessing
well known rifts and grabens in the region. In this
study,itisshownthattheGulfofGo¨kovawasmainly Multi-channelseismicdatawerecollectedonone
opened by an E–W-oriented, presently buried major longitudinalandeleventransversallines(totalof224
normal listricfault,the so-calledDatc¸a Fault,which km) in order to investigate the expected E–W-ori-
has never been discussed in the literature before. entedactivetectonismintheGulf ofGo¨kova inAu-
The estimated overall rate of extension in the gulf gust1996(Figs.1and2).Theenergysourcewasa10
is at least 1.1 mm=y and the amount of total exten- gun source array with a volume of 1380 cubic inch,
H.Kurtetal./Tectonophysics305(1999)477–496 481
whichiswellenoughtopenetratetoadepthofafew a ‘Global Land One-km Base Elevation (GLOBE)’
kilometers.Thenumberofchannelsis48forlines10 model of the ‘National Geophysical Data Center
and11and96fortherestofthelines.Receivergroup (NGDC)’ and ‘International Bathymetric Chart of
interval,shotinterval,andnearoffsetare12.5m,50 the Mediterranean’ prepared by the ‘Intergovern-
m and 237.5 m, respectively. These parameters pro- mentalOceanographicCommission,GEODAS’.The
videdsix-ortwelve-foldcommon-depth-point(CDP) first data set (the one from Turkish Navy bathymet-
data. Sampling interval and record length were se- ric charts and from the shallow and multi-channel
lected as 2 ms and 5120 ms while real time 8–210 seismicdata)hasmoreresolutionthantheotherdata
Hzband-passfilteringwasappliedtorawdatabefore sets.Therefore,therearesomeartificialdisturbances
recording.AlthoughaDifferentialGlobalPositioning where two data sets were adjoined. This is most
System (DGPS) was operated during the recording, noticeable to the east of Kos Island where two data
most of the lines were recorded by using only the sets join in a N–S direction. Nevertheless, the map
GPS mode of the systembecause of bad differential in Fig. 2 is free of artificial disturbances for areas
signalreceptionfromthebasestation. withinthegulfandsurroundingtopography.
The data were processed in the data processing Bathymetryofthegulfdemonstratesthatthemain
laboratory of the Department of Geophysics, ˙Is- structuraltrendsareE–Woriented;however,WNW–
tanbul Technical University (˙ITU¨). A conventional ESE-oriented slopesin the mid-gulf and in the east-
data processing stream was applied to the data as ernpartofthegulfarealsoremarkable(Fig.2).The
follows: data transcription, in-line geometry defini- northern shelf of the gulf is sharply cut by E–W-
tion,editing,CDPsorting,gaincorrection,band-pass and WNW–ESE-oriented scarps and slopes, which
filtering, velocity analysis, normal-move-out correc- are observed on the seismic sections 6 through 11
tion,muting,stacking,signalshapingdeconvolution, (Figs. 3–5). Most of these scarps and slopes are
band-passfiltering,automaticgaincontrol,andpost- relatedto the faultsextending tothe seabed. In con-
stackfinite-differencetimemigration.Moreover,de- trast to the north, no shelf area is present at the
tailed velocity analysis and post-stack finite-differ- southernpartofthegulf.Bathymetryabruptlydrops
ence depth migration were applied to the critical by hundreds of meters near the coast. This sud-
parts of the data from an interpretational point of den gradient in the bathymetry extends E–W and
view to better image the curved planes. Fortunately, it is well observed in the shallow seismic reflec-
theseabottommultiplesarenotstronglyrepresented tion data (Ulug˘ et al., 1996). The deepest locality
in the data because of a possible low acoustic con- in the bathymetry is about 770 m in the middle
trast between the high salinity of the Mediterranean of the gulf. This bathymetric low is bounded by
watersandsoft marine sedimentsinthe seafloor. In WNW–ESE- and E–W-trending slopes along the
addition, the sea bottom multiples were eliminated gulf. Bathymetry of the easternpart ischaracterized
as much as possible by avoiding multiple reflection byWNW–ESE-extendinglowsandhighs,whichare
hyperbolasonCDP gathersduring repeatedvelocity associated with a small-scale horst–graben system,
analyses. observedintheseismicsections1through5(Fig.6).
The seabed in the easternmost part (inner gulf) is
relatively shallow and smooth because of the pro-
3. Imprintsoftheactivetectonisminthe gradingdeltadepositsofthestreamsinthenortheast
bathymetryandtopography ofthegulf(Paton,1992;Ulug˘ etal.,1996).
Tothe north of the gulf, thetopographic imprints
The composite map of the topography and oftheNW–SE-orientedpaleotectonicriftandgraben
bathymetry of the Go¨kova area was prepared by systems are observed (Fig. 2). Topography of the
using different data sets (Fig. 2). The detailed northandsouthmarginsofthegulfdisplaymainlyE–
bathymetric data in the gulf were obtained from W-orientedslopes.Thisismostevidentonthenorth-
the Turkish Navy bathymetric charts and from sin- eastmarginwherethestrongtopographicalgradients
gle- and multi-channel seismic data. The rest of the (Fig. 2) correspond to mainly E–W-oriented active
bathymetryandtopographydatawereobtainedfrom normalfaults(Paton,1992;Go¨ru¨retal.,1995)(Fig.1).
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Fig.3.(A)Time-migratedseismicsection11.(B)Interpretedsection.Thecurveddiscontinuityontheleftisthemajorlistricnormalfault,theso-calledDatc¸aFault,betweentheDatc¸a
PeninsulaandGo¨kovabasin.ThesyntheticandantitheticfaultsoftheDatc¸aFaultareobservedatthenorth.Theboldlinedefinestheruggedsurfaceofthebasementandthethinlines
definetheremarkablefaultsandbedding.RectangularareasarezoomedinFigs.9and11.XandYarethehorizontalseparationsusedintheestimationoftherateofextensionwithinthe
gulf.LabelMshowsfirst-orderseabottomandbasementmultiples.
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Fig. 4. (A) Timemigrated seismic section 9. (B) Interpreted section. The curved discontinuity on the leftmost at around 2 s twt time is the Datc¸a Fault. A developing
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subgraben system (WNW–ESE-orientedmid-gulftrough) tothenorthand synthetic fault system inthe southernpart of the section areremarkable.Rectangular area is 8
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zoomedinFig.12.LabelMshowsfirst-orderseabottomandbasementmultiples.
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Fig.5.(A) Timemigratedseismic section 6.(B) Interpretedsection. Continuation ofthe Datc¸aFault further totheeast is observed.Therisingbasement inthemiddle
causesalocalfaultsystemintheuppersediments.ThebathymetriclowinthesouthernpartofthesectioncorrespondstoaWNW–ESE-orientedbathymetriclowinFig.2.
RectangularareaiszoomedinFig.13.
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Fig.6.(A)Timemigratedseismicsection3.(B)Interpretedsection.AtypicalsectiondisplayingtheWNW–ESE-orientedhorst–grabensystemintheeasternpartofthe
gulf.LabelMshowsfirst-orderseabottommultiples. 4
8
5
486 H.Kurtetal./Tectonophysics305(1999)477–496
4. Structuralimplicationsoftheseismicdata Nappesoccupyingawideareaintheextremesouth-
west of Anatolia. Inward of the gulf, the major
Seismic data show that the Gulf of Go¨kova is faultanditsassociatedantitheticandsyntheticfaults
bounded by a major discontinuity in the south and become less evident and further to the east they
numerousnormalfaultsinthenorth.Thegeneralori- are replaced by a small-scale horst–graben system
entationsof the normal faults are E–W and WNW– mainlyorientedWNW–ESE(Figs.7and8).
ESE. Most of these faults cut the basin fill and the The most important structure in section 11 is the
seabed while a few cause throws in the basement major curved discontinuity in the southernmost part
rocks which are mainly formed from the Lycian of the profile (Fig. 3). The reflections from this dis-
Fig.7.Threedimensional structuralviewoftheGulfofGo¨kovaconstructedfromthedigitizedinterpretedseismicsections.Boldlines
indicatethetopofthebasementandthinlinesindicatethefaults,beddingandseabed.Thebasinfillisthickestinthemouthandmiddle
of the gulf, and thinnest in the inner parts. Notice the overstep of the Datc¸a Fault to the north between the seismic lines 8 and 9.
Approximateverticalandhorizontalscalesareshownbydashedlinesonline11.Horizontalscalelinealsorepresentstheseasurface.