14
ORIGINAL RESEARCH PAPER Determination of the tectonic evolution of the Edremit Gulf based on seismic reflection studies C. Kurtulus ¸ Æ B. Dog ˘an Æ F. Sertc ¸elik Æ M. Canbay Æ H. M. Ku ¨c ¸u ¨k Received: 4 May 2009 / Accepted: 8 August 2009 / Published online: 25 August 2009 Ó Springer Science+Business Media B.V. 2009 Abstract The Edremit Gulf, which developed during the Neogene-Quaternary, is a seismically active graben in NW Anatolia (Turkey) surrounded by the Sakarya continent. The sedimentary deposits in the gulf overlie the bedrock unconformably and can be separated into two parts as upper and lower deposits based on similarity of their seismic characteristics, and because the contact between them is clear. The lower deposits are characterized in the seismic profiles by the absence of well defined, continuous reflectors and are strongly disturbed by faults. A tectonic map and structural model of the Edremit Gulf was derived from interpreting 21 deep seismic profiles trending NE–SW and NW–SE within the gulf. Two fault systems were dis- tinguished on the basis of this compilation. The NNW–SSE trending parallel faults are low-angle normal faults formed after compression. They controlled and deformed the lower basin deposits. A syncline and anticline with a broad fold- curvature length resulted in folds that developed parallel to basin boundaries in the lower basin deposits. The ENE– WSW trending high-angle faults have controlled and deformed the northern basin of the Edremit Gulf. The folds developed within the northern lower deposits originated from the listric geometry of the faults. These faults are normal faults associated with regional N–S extension in western Anatolia. The Edremit Gulf began to open under the control of low-angle NNW–SSE trending faults that developed after the compression of western Anatolia in an E–W direction in the early Neogene. Subsequently, regio- nal N–S extensional stress and high-angle normal faults cut the previous structures, opened the northern basin, and controlled and deformed the lower basin deposits in the gulf. As a result, the Edremit Gulf has not been controlled by any strike-slip faults or the Northern Anatolian Fault. The basin developed in the two different tectonic regimes of western Anatolia as an Aegean type cross-graben from the Neogene to Holocene. Keywords The Edremit Gulf Á Seismic reflection Á Aegean cross-graben Introduction The Edremit Gulf is located between western Anatolia and the south strand of the North Anatolian Fault in the NW of Anatolia, where it is surrounded by the Sakarya continent which developed to the south of the Intrapontide suture that formed between the E and W of the Marmara region (Okay and Tu ¨ysu ¨z 1999) (Fig. 1b). Oligocene terrestrial and Lower Miocene lacustrine sedimentary rocks formed as the result of the closure of Tethys ocean, together with coeval volcanic rocks to the south (Yılmaz 1997). C. Kurtulus ¸(&) Á F. Sertc ¸elik Á M. Canbay Engineering Faculty Department of Geophysics, Kocaeli University, 41380 _ Izmit-Kocaeli, Turkey e-mail: [email protected] F. Sertc ¸elik e-mail: [email protected] M. Canbay e-mail: [email protected] B. Dog ˘an Engineering Faculty Department of Geology, Kocaeli University, 41380 Kocaeli, Turkey e-mail: [email protected] H. M. Ku ¨c ¸u ¨k Engineering Faculty Department of Geophysics, Dokuz Eylu ¨l University, _ Izmir, Turkey e-mail: [email protected] 123 Mar Geophys Res (2009) 30:121–134 DOI 10.1007/s11001-009-9072-2

Determination of the tectonic evolution of the Edremit Gulf based on seismic reflection studies

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Page 1: Determination of the tectonic evolution of the Edremit Gulf based on seismic reflection studies

ORIGINAL RESEARCH PAPER

Determination of the tectonic evolution of the Edremit Gulf basedon seismic reflection studies

C. Kurtulus Æ B. Dogan Æ F. Sertcelik ÆM. Canbay Æ H. M. Kucuk

Received: 4 May 2009 / Accepted: 8 August 2009 / Published online: 25 August 2009

� Springer Science+Business Media B.V. 2009

Abstract The Edremit Gulf, which developed during the

Neogene-Quaternary, is a seismically active graben in NW

Anatolia (Turkey) surrounded by the Sakarya continent.

The sedimentary deposits in the gulf overlie the bedrock

unconformably and can be separated into two parts as

upper and lower deposits based on similarity of their

seismic characteristics, and because the contact between

them is clear. The lower deposits are characterized in the

seismic profiles by the absence of well defined, continuous

reflectors and are strongly disturbed by faults. A tectonic

map and structural model of the Edremit Gulf was derived

from interpreting 21 deep seismic profiles trending NE–SW

and NW–SE within the gulf. Two fault systems were dis-

tinguished on the basis of this compilation. The NNW–SSE

trending parallel faults are low-angle normal faults formed

after compression. They controlled and deformed the lower

basin deposits. A syncline and anticline with a broad fold-

curvature length resulted in folds that developed parallel to

basin boundaries in the lower basin deposits. The ENE–

WSW trending high-angle faults have controlled and

deformed the northern basin of the Edremit Gulf. The folds

developed within the northern lower deposits originated

from the listric geometry of the faults. These faults are

normal faults associated with regional N–S extension in

western Anatolia. The Edremit Gulf began to open under

the control of low-angle NNW–SSE trending faults that

developed after the compression of western Anatolia in an

E–W direction in the early Neogene. Subsequently, regio-

nal N–S extensional stress and high-angle normal faults cut

the previous structures, opened the northern basin, and

controlled and deformed the lower basin deposits in the

gulf. As a result, the Edremit Gulf has not been controlled

by any strike-slip faults or the Northern Anatolian Fault.

The basin developed in the two different tectonic regimes

of western Anatolia as an Aegean type cross-graben from

the Neogene to Holocene.

Keywords The Edremit Gulf � Seismic reflection �Aegean cross-graben

Introduction

The Edremit Gulf is located between western Anatolia and

the south strand of the North Anatolian Fault in the NW of

Anatolia, where it is surrounded by the Sakarya continent

which developed to the south of the Intrapontide suture that

formed between the E and W of the Marmara region (Okay

and Tuysuz 1999) (Fig. 1b). Oligocene terrestrial and

Lower Miocene lacustrine sedimentary rocks formed as the

result of the closure of Tethys ocean, together with coeval

volcanic rocks to the south (Yılmaz 1997).

C. Kurtulus (&) � F. Sertcelik � M. Canbay

Engineering Faculty Department of Geophysics, Kocaeli

University, 41380 _Izmit-Kocaeli, Turkey

e-mail: [email protected]

F. Sertcelik

e-mail: [email protected]

M. Canbay

e-mail: [email protected]

B. Dogan

Engineering Faculty Department of Geology, Kocaeli

University, 41380 Kocaeli, Turkey

e-mail: [email protected]

H. M. Kucuk

Engineering Faculty Department of Geophysics, Dokuz Eylul

University, _Izmir, Turkey

e-mail: [email protected]

123

Mar Geophys Res (2009) 30:121–134

DOI 10.1007/s11001-009-9072-2

Page 2: Determination of the tectonic evolution of the Edremit Gulf based on seismic reflection studies

There have been a few studies related to the geology and

tectonics of the northern onshore region of the Edremit

Gulf, which is about 80 km long and broadens westward

from about 5 km across to more than 30 km (Fig 1a),

(Yılmaz and Karacık 2001; Saka 1979; Yaltırak and Okay

2004), but there is sparse information related to the tectonic

setting of the Edremit Gulf.

The study area and its surroundings are significant places

for understanding the Neotectonic evolution of Turkey, and

consequently were considered by many authors. The North-

ern Anatolian Fault splinters into three strands around

Mudurnu (Bolu) to the east of the Marmara Sea (McKenzie

1978; Sengor 1979; Barka 1992; Taymaz et al. 1991; Bozkurt

2001; Kurtulus 2002; Yaltırak 2002). The northern strand

Fig. 1 a Simplified geological map of the surrounding of the Edremit

Gulf, and location of the seismic profiles (modified from Yılmaz and

Karacık 2001, Yaltırak and Okay 2004, Saka 1979). b Active

Tectonic map of Marmara Region (Yaltırak 2002)

122 Mar Geophys Res (2009) 30:121–134

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Page 3: Determination of the tectonic evolution of the Edremit Gulf based on seismic reflection studies

enters the Gulf of Saros as NE–SW direction from the slope

of the Ganos mountain traversing the Marmara Sea west-

wards in E–W direction. The middle strand of the Northern

Anatolian Fault extends almost E–W along _Iznik Lake, to the

Gemlik and Bandırma Gulfs (Fig. 1b). This strand diverts at

the western part of the Bandırma Gulf and turns southwest-

ward into the Bandırma-Behramkale Fault Zone (BBFZ)

trending to the western part of the Edremit Gulf and con-

sisting of many strike-slip faults of varying lengths. The

southern strand of the Northern Anatolian Fault crosses

Yenisehir and extends from Bursa to Manyas, bending

southwest from the southern part of Uluabat Lake and elon-

gates in a NE–SW direction up to the southern part of Manyas

Lake (Fig. 1b). The fault turns into the direction of N45E just

to the SW of Manyas Lake and extends from Manyas to

Edremit (Barka and Hancock 1984; Barka 1992) (Fig. 1b).

The western Anatolian extensional regime developed a

series of NNW–SSE and E–W trending basins (Sengor

1987). Some authors proposed that these two sets of basins

trending along two directions were the result of strain in two

directions at different time during the Oligocene, and from

Late Miocene to Quaternary (Seyitoglu and Scott 1991;

Seyitoglu et al. 2002; Isık et al. 2003). The some other

authors proposed that the N–S trending basins were cut by

E–W trending basins during the Late Miocene and Qua-

ternary (Yılmaz et al. 2000; Gurer et al. 2001, 2006; Ko-

cyigit 2005; Kaymakcı 2006; Kocyigit and Deveci 2007).

The Kucukkuyu Formation was deposited in a lacustrine

area and comprises Oligocene conglomerate, sandstone and

shale. The region is also characterized by Oligocene Ay-

vacık Volcanics composed of andesite-latite and rhyolite-

dacite components formed on the northern margine of the

Edremit Gulf (Yılmaz and Karacık 2001; Genc 1998; Ka-

racık and Yılmaz 1998). The Hasanboguldu Formation

defines the Plio-Quaternary alluvial-fan facies (Yaltırak

and Okay 2004). Lower–Middle Miocene volcanic rocks

and laterally equivalent lacustrine units are exposed in the

southern Edremit Gulf (Yılmaz and Karacık 2001; Yılmaz

1997; Saka 1979). Regional transgression and regression

developed by change of sea—levels have been calculated

by determining the stratigraphic sequence and facies of the

upper Quaternary sediments in three basins trending NE–

SW located in the NE of Aegean Sea (_Isler et al. 2008).

The deformational structural data related to Paleotec-

tonic (pre-Cretaceous) and Neotectonic (Miocene and

afterwards) periods are formed to the north of the Edremit

Gulf (Yılmaz and Karacık 2001; Yaltırak and Okay 2004).

Thrust faults formed during the subduction of the Tethys

Ocean between Permian and Early Triassic and these have

been uplifted and folded during the Cretaceous deforma-

tion (Yaltırak et al. 2000). The Neotectonic period began

with the formation of NNE–SSW trending extensional

faults caused by a N–S trending compressional regime that

existed in Early-Middle Miocene. In addition to this, a

continuation of this shortening phase during this period

formed a fold axis trending E–W.

The N–S compression that began in the Late Miocene

formed oblique normal faults trending NE–SW and NNE–

SSW, and which had a dominant dip-slip component and a

small strike-slip component (Yılmaz and Karacık 2001).

Faults trending NE–SW that are observed north of the

Edremit Gulf and that control the Ezine Grabens, also

developed the Etili and Gulpınar Grabens (Fig. 1b). The

continuation of N–S compression triggered the develop-

ment of an erosional planar topography in the region. The

Kazdag Horst observed in the north of the Gulf of Edremit

was uplifted to the present elevation by E–W trending

faults. The Bayramic Graben, which is located north of the

Edremit Gulf is a subsidiary half-graben associated with

uplift of the Kazdag Horst. The age of the Edremit Graben

is not precisely dated. It may be estimated as being Pleis-

tocene-Quaternary because it was controlled by Late

Miocene-Early Pliocene structural elements (Yılmaz and

Karacık 2001).

One of the aims of this study is to determine which

tectonic regime and basin type opened the Edremit Gulf,

and to classify the basin deposits lithostratigraphically in

order to determine the relationship between the basin

deposits and the older bedrock. The other main research

objective is to determine the competing influences of the

Northern Anatolian Fault and the western Anatolian

extensional regime on the tectonic evolution of the Edremit

Gulf. The relationship between the opening of the Edremit

Gulf and the western Anatolian extensional regime or the

Northern Anatolian Fault was investigated. More generally,

the study area forms a model for determination of the

opening of a basin controlled by high angle normal faults.

For this reason, the locations of the faults and their inter-

action mechanism in the gulf were constrained, and the

geologic formations were detected accurately, based on

deep-penetrating multi-channel seismic reflection profiles

conducted by Turkish Petroleum Corporation (TPAO) in

1986 (Fig. 1a). These previous dataset had been obtained

for the purpose of gas and oil exploration, rather than to

investigate the tectonics of the Edremit Gulf.

Materials and methods

A total of 1,000 km of deep-penetrating, multi-channel

seismic reflection profiles was collected by Amaco-Turkey

Petroleum Co-Western Geophysical Co. in 1986 in the

Edremit Gulf, which we reinterpreted in this study

(Fig. 1a). These data were recorded using a DFS V

recorder at 6.0 s two-way travel time and a sampling rate

of 4.0 ms. Group and shot intervals, as well as array

Mar Geophys Res (2009) 30:121–134 123

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lengths, were fixed at 25 m. 18 HP air-guns of 1,270 cu. in.

array and a 120 trace analog streamer were used. The data

were electronically muted to remove refracted arrivals

from shallow sediments. The predictive gap was fixed at

24 ms. The data were filtered with a high-cut 90 Hz 7208/

octave. The low-cut filter was out while recording. A time

variant filter (6, 10, 75, 90) for 0–1,000 ms, (4, 8, 65, 75)

for 1,000–2,000 ms, and (3, 6, 40, 50) for 2,000–6,000 ms

were applied before wave equation migration to remove

noise followed by a mute, Normal Move Out (NMO)

correction, velocity analysis and deconvolution. The time

variant filter was reapplied followed by wave equation

migration processing to obtain the final stack. The research

vessel was navigated using primarily Syledish and sec-

ondarily Loran-C and satellite.

Results

Bathymetry

A bathymetric relief map of the Edremit Gulf was prepared

by combining the echo sounder data and the deep seismic

reflection profiles (Fig. 2). The sea floor, which can be

characterized by a concordant morphology, continuously

deepens to the SW of the gulf. The water depth adjacent to

Lesbos Island is greater than 91 m. Four main ridges are

noticeable in the gulf. One of these is observed along the

northern marginal faults in the gulf, whereas the others are

associated with scarp locations where some fault throws are

observed.

Seismic profiles

Profile Pr2 is a regional line oriented in an NE–SW

direction in the north of the Edremit Gulf, as shown in

Fig. 3. The morphology of the faults is well preserved in

the basin sediments. The lower basin deposits, which are

folded and controlled by one or more faults, are charac-

terized by well-stratified reflections with a subparallel to

divergent internal configuration. They are often dissected

by north-dipping faults displaying listric planar shape to

the bedrock, such as between shot points (SP) 480 and 490;

SW-dipping faults generally show high angles between SP

380 and 480, and form listric planes in the lower parts of

the stratigraphy between SP 260 and 380. The folded basin

sediments observed on the cross-section of Profile Pr2,

which trends NW–SE, was deformed by a low-angle nor-

mal fault system that penetrates down to the bedrock. The

horizontal layers in the SW of Profile Pr2 trend towards NE

with high dip angles. This change in the stratal dips well

displays where the fault plane displays a listric geometry.

These fault planes trend NNW–SSE and deform the basin

units into N–S trending folds (syncline) with a wide fold

curvature length (Fig. 6). The faults numbered 1 and 2

observed on profile Pr2 cut the upper basin deposits as

normal faults with high-angle dip angles. The low angle

fault system in shallow depths shows a high angle orien-

tation at lower levels in the crust. The coeval faults are

observed at the SW end of Profile PR2.

The angles of the north-dipping normal faults observed

between SP 1300 and 1370 on Profile Pr3 decrease from

shallow to deep levels sub-seafloor (Fig. 4). High angle

normal faults dipping south between SP 1370 and 1540 are

recognized and the fault plane observed at SP 1650 folds

the layers with a listric geometry. The folded basin sedi-

ments and structural elements indicate the existence of a

fault period with low–high angles. Images of Fault 3

indicate that the stratigraphy of the upper parts of the basin

fill are controlled by high angle normal faulting.

Normal faults dipping both south and north are

observed between SP 240 and 300 on Profile Pr5 and

Fig. 2 Bathymetry map of the

Edremit Gulf

124 Mar Geophys Res (2009) 30:121–134

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Page 5: Determination of the tectonic evolution of the Edremit Gulf based on seismic reflection studies

Fig. 3 Interpreted (a) and raw (b) seismic cross-section of Profile Pr2 (see Fig. 1a for location)

Mar Geophys Res (2009) 30:121–134 125

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Page 6: Determination of the tectonic evolution of the Edremit Gulf based on seismic reflection studies

form a syncline in that area (Fig. 5). The north-dipping

faults observed between SP 110 and 240 have caused

folding of the basin sediments. The sedimentary layer

labelled (M1) and seen in the SW of cross-section is

deformed by faults that cause vertical throws up to 1.5

sec two way travel time.

The listric shape of Faults 4 and 5 in the basement rock

(Kazdag Dome) have caused folding between SP 180 and

320 on Profile Pr13 within the basin sediments (Fig. 6).

The high angle faults that trend ENE–WSW in the cross-

section are observed in the shallow levels of the upper

basin deposits. The basin sediments between SP 320 and

400 on Profile Pr13 were also folded because of the listric

character of the faults.

North-dipping, high-angle faults are observed

between SP 300 and 530, and SP 80 and 160 on Pro-

files Pr15 and Pr16 respectively. In contrast, the south-

dipping Faults 6 and 7 that were observed between SP

390 and SP 400 on Profile Pr15 have a listric character

in the lower part of basin deposits (Fig. 7). The faults

which deform the gulf and accommodate the opening of

the northern basin are observed as high-angle faults, as

depicted in seismic profile Pr15 (Fig. 7). The upper part

of the basin sediments, which are horizontal, are

Fig. 4 Interpreted (a) and raw (b) seismic cross-section of Profile Pr3 (see Fig. 1a for location)

126 Mar Geophys Res (2009) 30:121–134

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Fig. 5 Interpreted (a) and raw (b) seismic cross-section of Profile Pr5 (see Fig. 1a for location)

Mar Geophys Res (2009) 30:121–134 127

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Page 8: Determination of the tectonic evolution of the Edremit Gulf based on seismic reflection studies

deformed by high-angle normal faults, while covering

the folded lower part of basin fill unconformable. Faults

6 and 7 indicate that the initial source of the defor-

mation in the area is low-angle normal faulting. The dip

directions of the layers indicated by (M2) in the basin

sediments are related to the fault planes that formed

an asymmetrical syncline with a long fold-curvature

length. because listric geometries were observed

between SP 230 and 260 on Profile Pr16 (Fig. 8).

Layer (M3) forms an asymmetrical syncline in the fol-

ded basin unit between SP 400 and 470 on Profile Pr18

(Fig. 9). The south and north-dipping faults between SP

100 and 290 on Profile Pr18 generally show high angles.

The upper parts of the basin sediments cover the older units

with an angular-unconformity.

Seismic stratigraphy

Formations were distinguished by considering the reflec-

tion configurations (Sangree and Widmier 1979; Sheriff

1980). The mapped Neogene to Holocene formations may

be divided into a number of seismic layers. Different

coeval formations are known to outcrop over the sur-

rounding regions of the Edremit Gulf. Although the seismic

layers have been delineated using unconformities and their

correlative conformities, we avoided giving names to the

layers because we could not establish every layer, because

the lack of borehole data that represents the stratigraphic

units deposited during the Neogene period.

The lower (Neogene), upper (Holocene) parts of basin

sediments and bedrock formations are well observed in the

Fig. 6 Interpreted (a) and raw (b) seismic cross-section of Profile Pr13 (see Fig. 1a for location)

128 Mar Geophys Res (2009) 30:121–134

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Page 9: Determination of the tectonic evolution of the Edremit Gulf based on seismic reflection studies

seismic profiles (Figs. 3, 4, 5, 6, 7, 8 and 9). The top of the

sequence consists of sedimentary layers that are charac-

terized by well-defined, continuous and parallel reflections.

The average velocity of this formation is V = 1,909 m/s

while its thickness ranges 47–379 m. The lower basin

deposits consist of a series of parallel to subparallel, obli-

que and sigmoid reflections. From bottom to top, four

lithostratigraphic layers may readily be distinguished.

They are medium to thinly-bedded and rather uniform. The

average velocity of the lower basin deposits is

V = 2,783 m/s, while its thickness is estimated at 490–

2,000 m. The unit overlies the volcanic bedrock uncon-

formable, and is cut by several faults (Fig. 10).

The bedrock comprises chaotic bedding, as evidenced

by numerous hyperbolic acoustic returns. Internal reflec-

tions in the bedrock are finely banded, discontinuous to

rarely chaotic.

Tectonic and seismicity

Because the investigation area, the Edremit gulf, is one of

the western Anatolian basins and is located in the tran-

sition zone between the Aegean and Marmara regions, it

is debatable which tectonic regimes, Northern Anatolian

Fault strike-slip or West Anatolian extensional are more

dominant in the region and which tectonic stress caused

the faults that formed the bathymetry of the gulf. We

determined that the three basins trending NE–SW and

observed in the northern part of the region, close to the

Biga Peninsula at which Fig. 1b, as well as two ridges

originated as a result of the effects of tensional and strike

slip faults detected in the seismic cross-sections (_Isler

2005). The modern micro-earthquake activity in the

investigation area is very strong (Tan et al. 2008)

(Fig. 11).

Fig. 7 Interpreted (a) and raw (b) seismic cross-section of Profile Pr15 (see Fig. 1a for location)

Mar Geophys Res (2009) 30:121–134 129

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The tectonic map of the Edremit Gulf was defined by

interpreting the 21 seismic profiles trending NE–SW and

NW–SE in the gulf (Fig. 12). Two fault systems were

mapped out in the Edremit Gulf. The fault map of the

Edremit Gulf defined by our work with structural

parameters obtained from seismic profiles allows us to

test which tectonic model is most appropriate to explain

the opening of the gulf. The data indicate two fault sys-

tems developed in different directions, origins and peri-

ods. The first fault system observed on Profiles Pr2, Pr3

and Pr5 was mapped using the other seismic profiles with

same direction and bathymetric data. These faults were

formed as a parallel set trending NNW–SSE, W–SW

dipping and generally low angle normal faults controlled

and folded the lower part of the basin sediments along an

axis trending NNW–SSE. According to ages determined

by Yılmaz and Karacık (2001) in the stratigraphic

sequences in the area of land to the north of the gulf, the

age of the faults is generally early Neogene. The normal

faults represent the response to an initial compressional

regime that formed a series of N–S trending grabens

under the effect of breakaway faults developed after

Fig. 8 Interpreted (a) and raw

(b) seismic cross-section of

Profile Pr16 (see Fig. 1a for

location)

130 Mar Geophys Res (2009) 30:121–134

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Page 11: Determination of the tectonic evolution of the Edremit Gulf based on seismic reflection studies

western Anatolian compression (Sengor 1979; Sengor and

Yilmaz 1981; Yılmaz et al. 2000). However, these faults

show similarities with the fault system observed by Yal-

tırak and Okay (2004), in the onshore region on the

northern side of the gulf, and which caused the elevation

of both the Kucukkuyu Formation and the Kazdag Massif,

since the beginning of the Kucukkuyu Formation with the

end of the Kazdag Massif rise is simultaneously, in lower

Miocene (_Inci 1984). This fault system formed three

consecutive scarps parallel to the direction of the faults in

the bathymetry of the Edremit Gulf. The second group of

fault system observed in Profiles Pr13, Pr15, Pr16, Pr18

was mapped using the other seismic profiles collected in

the same direction, as well as with bathymetric data.

These faults controlled the basin in the north of the region

and are high-angle normal faults trending ENE–WSW.

The wide fold-curvature length folds that trend NE–SW in

this period (may be Early Miocene), which are younger

than that in which the first group faults system developed

were mapped using both the seismic profiles and the 3-D

bathymetric data. The 3D digital elevation model map

obtained from the batimetric map of the Edremit Gulf

shows the fault scarps indicated by S1, S2, S3 (Fig. 12)

and controlled by NNW–SSE oriented low-angle normal

faults and Scarp S4 which was controlled by ENE–WSW

trending high-angle normal faults in the gulf.

This most recent period of Western Anatolian Neotec-

tonics is characterized by the basin boundary trending in an

E–W direction. The folds of these basins are formed by

high angle normal faults, which also controlled and

deformed the lower and upper basin sediments in the Ed-

remit Gulf (Fig. 7). The fault planes are seismogenic and

form part of the western Anatolian extensional regime.

This interpretation is consistent with ENE–WSW trending

pure normal fault plane solutions obtained by England

(2003) (Fig. 11).

Fig. 9 Interpreted (a) and raw (b) seismic cross-section of Profile Pr18 (see Fig. 1a for location)

Mar Geophys Res (2009) 30:121–134 131

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These two fault systems began to open the Edremit Gulf

along low-angle normal faults, which were formed after

compression and presumably dating from the begining of

Neogene. With the start of sedimentations of lower

deposits in the basin, NNW–SSE trending fault scarps were

formed similar by the development of tectonic origin of the

basins trending NE–SW direction (Bozkurt 2000, 2003).

The Edremit Gulf takes its present shape after the earlier

fault scarps were cut by ENE–WSW trending faults

developed as a result of a regional N–S extensional regime

within western Anatolia. The Edremit Gulf with this tec-

tonic geometry and bathymetry developed during two dif-

ferent stress periods within western Anatolia, like an

Aegean-type cross-graben shape.

Discussion

This study examines how the Edremit Gulf was opened and

if the southern branch of the Northern Anatolian Fault or

the western Anatolian extensional regime had any effect on

the development of the gulf as evidenced from the seismic

reflection profiles collected in the gulf.

Seismic reflection studies carried out in the Edremit

Gulf indicate that: (1) Two fault systems exist. One fault

set consists of the transtensional boundary faults trending

NNW–SSE and which are the source of the basin. These

faults allow the preservation of the lower part of basin

sediments. The other fault set is made up of high-angle

normal faults that originated from detachment faults

developed during ENE–WSW trending compression. (2)

The NNW–SSE trending faults are generally seen to have a

low dip and show a listric geometry towards bedrock

whereas, the ENE–WSW trending faults in the Edremit

Gulf are arranged parallel with the normal faults observed

on land north of the gulf.

The first period of faulting in the northern Edremit Gulf

is the Oligo-Miocene detachment faulting with low dip-

angle related to back-arc extension (Rotstein 1985). The

Plio-Quaternary period is characterized by parallel normal

faults that cut the older units and structures (Beccaletto and

Steiner 2005). This interperation is in a good agreement

with the present and historical seismic activities observed

around the Edremit Gulf, as depicted in Fig. 11. The tec-

tonic geometry determined in our study is consistent with

the results of Yaltırak and Okay (2004) and Beccaletto and

Bonev (2005) in which they stated that the ENE–WSW

trending high-angle normal faults cut and deformed the

Oligo-Miocene, Lower Middle Miocene, Upper Miocene

and Pleistocene units located around the Kazdag uplift. (3)

The basin deposits overlying the basement rocks were

distinguished as upper and lower deposits. The lower

deposits are folded and their ages were interpreted as being

Miocene if they correlated with similar units on land

whereas the upper deposits of the basin are generally

horizontal and their ages may be Plio-Quaternary.

Basins formed by the control of strike-slip faults are

separated distinctly from basins controlled by normal faults

with their particular geometries. In this sense, listric normal

faults were observed in the south, while parallel strike-slip

Fig. 10 Thickness map of the

Neogene formation in the

Edremit Gulf

Fig. 11 Seismicity of the North Western Anatolia. Red and whitecircles represent historical and instrumental period earthquakes,

respectively, Tan et al. (2008). Fault plane solution is from England

(2003)

132 Mar Geophys Res (2009) 30:121–134

123

Page 13: Determination of the tectonic evolution of the Edremit Gulf based on seismic reflection studies

dominant E–W faults were detected in the north of the

seismic cross-section conducted in the west of the Edremit

Gulf (Guney Boztepe et al. 2001). We thus conclude that

structural and bathymetric elements associated with strike-

slip faults were not distinguished in the Edremit Gulf. The

sea-bottom topography of a region controlled by strike-slip

or normal faulting shows some differences (Barka and

Kadinsky-Cade 1988). For example, depressions located in

the Marmara Sea (Cınarcık, Middle Marmara, Tekirdag

Basins) were developed under the influence of strike-slip

faulting are delimited by ridges (push-ups) from each other

(Le Pichon et al. 2001; Armijo et al. 2002; _Imren et al.

2001). These ridges represent the compressional compo-

nent of the strike-slip tectonics (Okay et al. 1999; Rangin

et al. 2004). In addition, the seismic studies conducted in

the Gulf of Gemlik located along the middle branch of the

North Anatolian Fault indicated that the axis lengths of the

basins were equal to each other, so that the basin is a lazy-s

shaped pull-apart basin formed by strike-slip fault defor-

mation (Kurtulus and Canbay 2007). In contrast, there is no

such ridge evolution in the Edremit Gulf developed from

the basins controlled by the parallel normal faults of which

E–W axis is longer than that of N–S axis.

Conclusions

The results of our study show that the contact relation

between bedrock and the main basin-fill sedimentary units

can be determined from seismic reflection profiles. These

units were divided into two groups as lower and upper

basin deposits. The downward concave geometry of the

observed fault planes that control the geometry of the lower

part of the basin-fill deposits caused folding along an axis

trending NNW–SSE. The Edremit Gulf began to open in

the Early Miocene after the compression of the western

Anatolia ceased. Extension was in an approximately E–W

direction along NNW–SSE trending low angle normal

faults that control the geometry of the lower synrift

deposits. At end of Neogene and during the Holocene,

high-angle ENE–WSW trending normal faults developed

due to N–S extension of western Anatolia, thus causing

formation of the northern basin and cutting the previous

structural elements. Within the study area we did not

observe the North Anatolian Fault or deformational evi-

dence of any strike-slip faulting. The Edremit Gulf was

controlled by low-angle, parallel normal faults trending

from E to W, while its northern boundary was controlled

and deformed by high-angle normal faults that cut the

faults that controlled the geometry of the gulf, which

developed as an Aegean type cross-graben from Neogene

to Holocene.

Acknowledgments The author is grateful to Peter Clift for his

fruitful discussions and suggestions that improved the paper.

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