The Geoelectrical Structure of Northwestern Anatolia, Turkey

The Geoelectrical Structure of Northwestern Anatolia, Turkey

E. U. ULUGERGERLI,1,4 G. SEYITOGLU,2 A. T. BASOKUR,1 C. KAYA,3 U. DIKMEN,1 and

M. E. CANDANSAYAR1

Abstract—The magnetotelluric method has been employed to generate a geoelectrical model that will

reveal the rich geological pattern and dynamic character of western and northwestern Anatolia, Turkey.

Magnetotelluric data were collected from 53 sites along a profile of 290 km from the Dardanelles to the

Alasehir Graben. Magnetotelluric data were in the range of 0.00055 Hz to 320 Hz. The models were

obtained through 2-D joint inversion of transverse electric and transverse magnetic modes. Lateral changes

in geoelectrical models are verified by using gravity and magnetic data. In addition, some of the

seismological data presented here agree with proposed models that suggest a brittle-ductile structure

boundary at a depth of 20 km. Generally speaking, a regional extensional regime caused reduction in the

thickness of the crust and consequent uplift towards the south. The constructed model delineates the

western part of the North Anatolian Fault Zone along the Biga Peninsula. The current patterns of volcanic

activity on the Biga Peninsula and at Kula are related to conductive spots presented in the models. The

border of the Gordes Basin, located between the Izmir - Ankara suture zone and the Menderes Massif, is

also well delineated. The North Anatolian Fault Zone presents a pattern in which density and susceptibility

anomalies attain relatively high values. Fillings covering most of the surface have lower density and

susceptibility values than those of underlying structures.

Introduction

Deep or large-scale regional structures generally control numerous geological

occurrences such as faults, horsts, grabens, magma chambers and near-surface

sedimentary basins. Realistic explanations of all such features require consistent

information that outlines the structure of upper crust. The rich geological pattern

and dynamic character of western and northwestern Anatolia (Figs. 1 and 2) have

drawn considerable attention in recent geological (e.g., OKAY et al., 1996; SEYITOGLU

and SCOTT 1994; YıLMAZ et al., 1997; ALDANMAZ et al., 2000) and geophysical (e.g.,

1 Department of Geophysical Engineering, Ankara University, 06100 Ankara, Turkey.2 Department of Geological Engineering, Tectonics Research Group, Ankara University, 06100

Ankara, Turkey.3 Department of Geophysics Engineering Sivas, Cumhuriyet University, Turkey.4 Department of Geophysics Engineering, Onsekiz Mart University, Canakkale, Turkey (currently).

Pure appl. geophys. 164 (2007) 999–10260033–4553/07/050999–28DOI 10.1007/s00024-007-0200-0

� Birkhauser Verlag, Basel, 2007

Pure and Applied Geophysics

TAYMAZ et al., 1990; HORASAN and CANıTEZ, 1995; BAYRAK et al., 2000; CAGLAR,

2001, AYDıN et al., 2005) literature.

The magnetotelluric (MT) method has been employed to outline the regional

geology of northwestern Anatolia by using 53 measurement stations along a profile

of 290 km. Time variations of magnetic and electric fields were simultaneously

recorded. Measurement sites were chosen on the basis of accessibility and the local

extent of the geological units. The measurement profile was subdivided into three

segments in order to cross the principal geological structures almost orthogonally

(Fig. 2). Actually, geological structures are always three-dimensional (3-D). How-

ever, two-dimensional (2-D) interpretation techniques may be used instead of 3-D

ones in consideration of the frequency range of the data and the main geological

features intersected; extensions of which are greater than the skin depth of the lowest

frequencies. Static shift correction was applied by using transient electromagnetic

(TEM) data (e.g., STERNBERG et al., 1988; MEJU et al., 1998). The aim was to obtain

a 2-D geoelectrical model producing a theoretical data set that fits measured data in

both transverse electric (TE) and transverse magnetic (TM) modes so as to reveal the

most likely representational setting along the profile.

A summary of other pertinent geophysical studies conducted in the region and

proposed geoelectrical models for the area are as follows. BAYRAK et al. (2000) used

Figure 1

Regional map of the Aegean Sea. Detailed map of rectangular area is given Figure 2.

1000 E. U. Ulugergerli et al. Pure appl. geophys.,

the same data that are presented in the present article but concentrated on

anisotropy. They concluded that the extensions of geological structures in western

Turkey provide opportunities for performing 2-D modeling and inversion of the

current data set. Further, BAYRAK and NALBANT (2001) derived a geoelectrical model

using these data. However, they carried out only TM mode inversion without static

shift correction. They assumed that the magnified range of apparent resistivity error

bars will reduce the static shift effects. However, enlarging the error bars and use of

Figure 2

Location of MT stations (small dots), Neogene and Quaternary basins and main basement structures in

western Turkey (after SEYITOGLU and SCOTT, 1994). Thin solid lines show the segments. Larger dots

indicate towns and cities.

Vol. 164, 2007 The Geoelectrical Structure of Northwestern Anatolia, Turkey 1001

single mode inversion, as done by BAYRAK and NALBANT (2001) will increase the

uncertainties in model space by increasing the number of possible models that

describe the observed data.

Recently, CAGLAR (2001) also proposed a geoelectrical model for the western part

of Anatolia. The data presented here somewhat cover the same geological settings as

does CAGLAR (2001)’s profile, but our current profile employs shorter station

intervals and the directions of the segments differ from those of CAGLAR (2001). The

geoelectrical models presented in CAGLAR (2001) were obtained through 2-D

inversion of TE and TM mode data, independently. Also, static shift problems were

not taken into account. BERDICHEVSKY et al. (1998) showed that single mode data

inversion is not always sufficient for obtaining a reasonable geoelectrical model and

emphasized that TE and TM mode data may be mutually complementary in order to

extract more detailed models.

GURER et al. (2001) presented results for the Gediz (Alasehir) graben. But their

profile is not in line with the current profile. TAYMAZ et al. (1990) shed some light

on the seismological activity of the Aegean region. ILKıSıK (1995) reported that high

heat flow values dominate in the region. Both papers concluded that the area is

experiencing highly active tectonism. AYDIN et al. (2005) summarized the regional

geological setting and presented Curie-point depth for Turkey. Both AYDIN et al.

(2005) and HISARLı (1995) showed that shallow Curie-point depths (8–12 km) are

well correlated with the young volcanic areas and with highs of the heat flow. In

addition, they also stated that the shallow Curie-point depths indicate thinned

crust.

SARI and SALK (1995) estimated the thickness of sediments in the central Aegean

region using gravity data. ATES et al. (1999) presented an updated gravity and

magnetic anomaly map of the region. Substantial information about geological and

geophysical research in the area may also be found in the internal-report archives and

libraries of the General Directorate of Mineral Research and Exploration of Turkey

(MTA) and the Turkish Petroleum Corporation (TPAO).

A key general result that is gleaned from these works is that the area is still

tectonically active, thus explaining earthquakes in the region and suggesting the

possibility that magma chambers and/or intrusions exist which give rise to many

hot springs, some of which are utilized as geothermal resources. A realistic

explanation for all of these occurrences demands adequate information regarding

the structure of the upper crust. This paper attempts to set forth a regional

geoelectrical model that fits both the TE- and TM-mode MT data and to interpret

the derived model in light of the regional geology. To date, apart from the articles

mentioned above, there has been no other large-scale geoelectrical model for

western Anatolia obtained from a 2-D or 3-D modeling scheme published in the

literature.

The derived 2-D geoelectrical structure is also verified by gravity and magnetic

models. 2.5-D gravity and magnetic modeling schemes are employed to obtain a


smooth final model. Although the susceptibility model shows some discrepancies

from the geoelectrical model in the southern part of the profile, the responses of both

density and susceptibility models derived from the geoelectrical interpretation show a

reasonable fit to the observed data.

Geology

The geology of northwestern Turkey (Fig. 2) comprises an amalgamation of

microcontinents that were situated between Gondwana and Laurasia from the

Permo-Triassic until the Oligocene. The rocks exposed in the region which reflect this

complex history are divided into several zones, namely: the Sakarya zone, including

Karakaya complex; the Izmir-Ankara Suture Zone; and the Menderes Massif (OKAY

et al., 1996; OKAY and TUYSUZ 1999).

The current pattern of northwestern Turkey started to form in Late Cretaceous,

the collision of Istanbul zone and Sakarya continent created the Intra-Pontid suture.

The ophiolite obduction on Menderes - Taurus block is named Bozkır nappes. Thefinal closure of the Northern branch of Neo-Tethys occurred in Late Eocene to

Oligocene along the Izmir-Ankara suture zone and the amalgamation of western

Anatolia is completed (SENGOR and NATAL’IN 1996).

Following the Oligocene, western Turkey experienced N-S extensional tectonics

(SEYITOGLU and SCOTT, 1996; SEYITOGLU et al., 2004), and/or NNE-SSW extensional

regimes (KREEMER et al., 2004), and metamorphic core complexes, grabens, igneous

activity and geothermal fields are the main geological features of the region. Note

that the current extension rate is, approximately, 30–40 mm yr)1 in the region

(MCKENZIE, 1978; TAYMAZ et al., 1991).

A recent study (SEYITOGLU et al., 2004) indicates that in the Oligocene, Datca-

Kale main breakaway fault causes the exhumation of Menderes massif that is at the

surface during Early Miocene. At this time, due to the continuation of extensional

tectonics, major E-W (Alasehir and Menderes) and N-trending basins (i.e., Gordes,

Demirci basins) began to develop simultaneously. Basin fillings have also been

subject to research. BOZKURT and SoZBILIR (2004), using geological observations,

implied that the thickness of the Neogene sediments in the Alasehir graben is about

1.3–1.5 km. On the other hand, SARI and SALK (2006), using the gravity data,

advanced that the thickness of sedimentary cover reaches 2.5 and 3.5 km in the

Menderes graben, and 0.5 and 2.0 km in the Alasehir graben.

In the Pliocene, the youngest structures cut the older ones (i.e., Simav graben)

and mask the earlier extensional history of the region. After the Pliocene, the

southern branch of the North Anatolian Fault (NAF) affected northwestern Turkey

and structures became more complex (OKAY and SATıR, 2000).


The geoelectrical line of this study starts at Canakkale, located in the Sakarya

Zone, passes through the Izmir-Ankara Suture Zone, and then enters the Menderes

Massif upon which the major extensional basins have developed.

Major extending structures can be grouped in three segments. The first part

consists of the branches of the NAF between Canakkale and Balıkesir. The NAF

zone has many local faults such as Etili and Yenice-Gonen faults, directions of which

vary between N50E and N70E. The Second part is the Izmir Ankara Suture Zone

between Balıkesir and Gordes. This part has no dominant features apart from the

suture itself (�N40E) and some local faults (�N70E). The last part is the western

edges of Demirci and Selendi basins which present a fan structure together with

Alasehir graben and the suture. Directions vary from N15E to N70W. All MT

stations are placed according to main tectonic units during the field trip. Structural

variations urged to divide the data set in three segments rather than to use a single

profile during the modelling study.

Seismological Background

Some findings of the geoelectrical model require comparison with seismological

data. Therefore, in order to gain some insight about tectonic activities, 886

earthquake occurrences have been evaluated. The epicenters of earthquakes with

magnitudes over 3 on the Richter scale and that occurred between 1900 and 2002 are

mainly between 2 and 35 km. The magnitudes increase with increasing depth of the

epicenters. The occurrence frequency of magnitude 5 earthquakes is less than one

year, indicating a high risk of earthquake hazard. The frequency (F) – magnitude

(M) relation for the region is given as

log F ¼ a� b�M ;

where a and b values were calculated by means of the least-squares methods and as

shown in Figure 3. The b value (defined as a tectonic parameter) may give valuable

seismological information about the region as pointed out by, for example, MOGI

(1962), SCHOLZ (1968) and WEEKS et al. (1978). In terms of absolute value, a zone

with a relatively low b value compared to the surrounding area indicates an energy-

accumulation zone, while higher b values outline energy-release zones. The

cumulative sum of the b value along the profile is presented in Figure 4. The

variation of b value decreases linearly up to 20 km, and then becomes almost

constant beyond that depth. Thus, the zone between the surface and a depth of

20 km may be defined as an active energy-release zone, while the deeper zones build

up energy and have almost constant b values. Recently, AKYOL et al. (2006) reported

that, using hypocentral distribution of the earthquakes, peak seismicity for the

western Anatolia occurs at depths of about 10 km. This result is also in accord with

Figure 4.


Problems for Research

The area has a complex geological setting, and recent seismological activity shows

that fault zones are still active. Additionally, average heat flow is approximately

110 mW/m2 (e.g., ıLKıSıK, 1995; GURER et al., 2001; AYDıN et al., 2005; AKıN et al.,

2006) for the region and the pick values can reach as high as 229 mW/m2. High heat-

Figure 3

Selected earthquakes, epicenters of which are over 3 (+) on the Richter scale and which occurred between

1900 and 2002 (Source: DAD (Earthquake Research Center), KOERI (Bogazici University Kandilli

Observatory and Earthquake Research Center), ISC (International Seismological Center) ). Rectangles are

locations of MT stations.


flow rates (ıLKıSıK, 1995) and many geothermal spots indicate the existence of

possible magma intrusions/chambers. ILKıSıK (1995) pointed out that the depth of the

lithosphere-asthenosphere boundary in western Anatolia is around 55±5 km.

SENGOR et al. (1985) suggested that the Palaeocene orogenic contraction thickened

the crust about 50 to 55 km in Early Miocene time. Seismological research presents

slightly different results for the western Anatolia; MINDEVALLI and MITCHELL (1989),

using surface waves, give an average crustal thickness of about 34 km while

SAUNDERS et al. (1998) found that the crust is approximately 30-km thick under

Kula, and HORASAN et al. (2002) suggest that a crustal thickness of 33 km in the

region. ZHU et al. (2006) showed that Moho depth is about 28 and 30 km around

Bozdag and Kula, respectively. In terms of local structures, ERGUN (1977) postulated

that the magnetic anomaly on the Biga peninsula originates from a source located at

5-km depth. AYDıN (1987) presented similar results (� 5 km) for the upper boundary

of the source of the magnetic anomaly around Soke.

From a geological and geophysical point of view, all of these findings indicate

very complex structural patterns and extensions that need explanation. The patterns

of basins and the source of volcanic units on the Biga Peninsula and around Kula

may be revealed by variation in electrical properties of these features which may be

explored by electromagnetic methods.

Magnetotelluric Data

Phoenix V5 MT equipment has been employed to record three orthogonal

magnetic (H) fields and two orthogonal electrical (E) field components. 100 m

dipoles, extending in N-S and E-W geomagnetic directions, and Pb-PbCl electrodes

Figure 4

Variation of b values vs. depth. 20 km may be a boundary between an active energy-release zone and an

energy accumulation zone.


were used for E field measurements. Horizontal components of the H field were

measured with an induction coil. The vertical component of the H field was recorded

by a loop on the ground, however the data quality was insufficient at most of the

stations.

Remote reference stations were established a few hundred meters away from each

station. Unfortunately, the signal/noise ratio could not be improved because of the

short distance between the main and remote stations and high-level cultural noise at

some locations, such as those near industrial plants. Recorded time series permit the

extraction of periods up to 1800 s. The recording system calculates all sounding

parameters in real-time. The data acquisition is performed separately in two

frequency sets. The first one is a high frequency set, in a range of between 320 Hz and

7.5 Hz, and was processed using Fourier transform techniques. The low frequency

set has a range of 6 to 0.00055 Hz and was processed using the cascade decimation

(WIGHT and BOSTICK, 1980). Four impedance components and, in turn, apparent

resistivity and phase of impedance have been obtained in a range of 0.00055 Hz to

320 Hz in 40 frequencies. Station intervals were selected at 5 to 10 km, depending

upon the accessibility of the area.

The electrical field dipoles extend N-S (XY) and E-W (YX) assigned to TE and

TM modes, respectively after rotation. Before rotating the data possible strike angles

were examined. The area has a complex geological setting, thus one should not

expect any common strike angle for the whole profile. BAYRAK et al. (2000) gave a

dimensionality analysis of the data using the Mohr circle. As they stated, the data

have strong anisotropy in three depth levels; 7–8, 15–20 and 35–40 km. Note that

they obtained depth information from Bostick–Niblett transformation (NIBLETT and

SAYN-WITTGENSTEIN, 1960; BOSTICK, 1977; JONES, 1983). Swift and tipper strike

angels for 2, 7 and 80 sn are presented in Figure 5. Solid line with diamond marker

represents average geological extensions. The stations closer to main tectonic features

were selected for this purpose. The rest of the stations have some deviations from

extension of the main units. Deeper information represented with both ‘‘x’’ and ‘‘+’’

0

15

30

45

60

75

90

WA1

WA4

WA6

WA10

WA12

WA15

WA16

WA20

WA32

W2A

5

W2A

9

WA33

WA34

WA36

WA37

WA41

WA42

W4A

3

Deg

ree

Figure 5

Swift and tipper strike angles for 2, 7 and 80 sn. The solid line with a diamond marker indicates average

geological extensions. Stations closer to main tectonic units were presented.


symbols and the angles differ ±15� from main geological directions except that in the

southernmost of the last segment. Evaluation of tectonic information together with

strike angles led us to divide the whole MT profile into three segments and decide the

strike angle of the each segment separately. Details are given later in this section.

Subsequently, these strike angles were fixed in the GROOM-BAILEY (1989) decom-

position code to see distortions. The comparison of estimated apparent resistivities

with those from major axis values indicated that both apparent resistivity data are

equal to each other with a slight error (<% 1 average relative error per mode) where

strike angle and geological extensions are inline with each other. High frequency part

(>1 Hz) data comply with the condition in first two segments, then data deviate

from each other (up to %25 average relative error per mode). This indicates that the

strike direction changes at deeper parts. Mode switching occurred in the third

segment (e.g., WA38 to WA53) because of the large rotation angle value (�70�clockwise). Cross comparison reflects error less than �%10 relative errors per mode

up to 0.1 Hz Relative error is the ratio of differences of estimated and major axis

apparent resistivities to estimated apparent resistivity. Note that logarithms of the

apparent resistivities were used in error calculation. Decomposition without fixing

the rotation angle did not produce any single regional angle for the segments.

Details of the segments are given as follows. The first segment of the MT data,

collected along a 118-km profile (Fig. 1), began at the Dardanelles (Canakkale),

crossed Biga Peninsula, and was terminated near Balya (Balıkesir). All stations

between WA1–WA21 were rotated 20� clockwise to make the TE mode data

perpendicular to the 2-D geoelectrical section. The second part of the data, obtained

along a profile of 112 km between a point north of Balya, then to Balıkesir, Bigadicand Sındırgı, crossing margins of the Sakarya Zone and Karakaya complex. All data

between WA22–WA37 along the segment were rotated 45� clockwise to keep the TE

mode perpendicular to the 2-D section. The third segment was a 93-km-long profile

extending from Sındırgı, past Gordes and Koprubasi, to Kula. A 70� rotation angle

seemed to be reasonable for the stations between WA38–WA53. Another reason for

the large rotation angle rather than rotating 18� anti-clockwise was to maintain the

standard notation for the modes.

Central loop transient electromagnetic (TEM) measurements were completed at

each MT station in order to remove the static shift effect from the MT data and to

derive near-surface information. We used Protem Receiver and TEM57 transmitter

(Geonics) for TEM measurements. High (6.813–695 microseconds) and Medium

(35.25–2792 microseconds) time ranges were selected for data acquisition. A 1-D

model was obtained by the inversion of combined TEM data at each measurement

station. The synthetic high frequency MT data were computed from the corre-

sponding 1-D model that was inverted from the TEM data.

Both measured TE and TM apparent resistivity data are shifted towards the MT

response of the 1-D model. Note that the shifting process was performed after

rotation steps. Rotated apparent resistivity data were multiplied by a constant to


shift towards pseudo MT data. The deviations are between 20% and 500% in linear

scale. We typically expected that the TE and TM apparent resistivity data would

remain parallel to each other in the high frequency band except under exceptional

conditions. However, one should keep in mind that the shifting process using a 1-D

model may cause information loss in the case of the existence of a superficial 2-D–3-

D structure, which leads to the departure of the TE and TM mode apparent

resistivities from each other even at very high frequencies. This condition is accepted

as a sacrifice for the methodology followed.

2-D Model

The models presented here were obtained using the WinGLink� interpretation

package consisting of a 2-D inversion code of d2inv_nlcg2_fast (MACKIE et al., 1997).

Initial models were taken as a homogeneous half space of 100 ohm-m. The first

model has 21 stations and is represented by a mesh of 55 by 95 cells. The second

model is represented by a mesh of 46 by 100 cells and has 19 stations (WA18–WA37)

overlapping with four stations of the first model. The third model is constructed from

a mesh of 52 by 93 cells and has 18 stations (WA35–WA53) overlapping with three

stations of the second segment. The left, right and bottom parts were extended

enough to eliminate boundary effects. The stations were placed at the top of each

mesh with 3- to 6-cell separations depending on the measurement intervals. TE and

TM mode apparent resistivity and phase of impedance data were inverted jointly.

The inversion process was subdivided into three inversion sessions. The maximum

number of iterations was set to 50 for each session. The software required some

additional inputs. The first one was smoothing factor, tau, which was taken as 30 for

the first 50 inversion steps then reduced in succeeding sessions. Therefore, inversion

was, at first, allowed to find a general pattern then forced to delineate the details by

using lower tau (20) values in the later steps. Error floors for all data were kept at 5%

as is the default of the code. All available frequencies were used in the inversion. The

termination error was selected as 0.1%, much lower than the recommended value of

the code, in order to force the program to further inversion steps toward the goal of

reaching the nearest minima. After each inversion session consisting of 50 iterations,

some cell resistivities were adjusted manually. This is required in order to reduce the

number of inversion sessions. One way of validating the final model is to start the

inversion with different initial guesses and to examine the consistency of the results.

Generally, if the data do not contain sufficient information to solve a group of

parameters representing a certain subsurface feature, then the outcome of each

inversion trial will depend on the initial model. On the other hand, the parameters

which have some influence on the data will keep similar parameter values after

independent inversion attempts that use a variety of initial models. Therefore, to

justify our models, all sections were also inverted with starting models of


homogeneous half space of 1 ohm-m (results not presented). The comparison of all

inversion outcomes of a certain section leads to estimation of the depth of

investigation (e.g., OLDENBURG and LI, 1999) and confirms the existence of some

small-scale features. As an example, the RMS value for the initial half space model of

100 ohm-m for the first segment was 35.88, and was later decreased to 5.8. The

second and third segments produced 3.71 and 3.57 RMS values, respectively (Fig. 6).

Figure 6

Geoelectrical models obtained from 2-D inversion of the MT data. Resistive crust represented with black

color. Conductive zone below the crust represents an electrical asthenosphere, while hot spots and basin

deposits are in gray tone.


Pseudosections of apparent resistivity and phase of impedance for observed and

calculated data are given in Figure 7. Some selected stations and corresponding

representative curves along the segments are shown in Figure 8. The first curve is

close to the NAF (WA11).

Generally, all sections exhibit information for depths of less than 30 km. We

assume that the inversion results are useful for geological interpretation since the

conductive and resistive local structures appear above this level.

Results in Figure 6 show that the general pattern in all models may be examined

in three resistivity ranges from surface to base. The first level (>10 ohm-m) is related

to topography and uppermost crustal setting (gray) extending down to 3 km. The

second level (>100 ohm-m) includes crustal structure (black), and is of non-uniform

thickness. The third level (10< and <100 ohm-m) is a conductive zone which

appears in all models. In the rest of the profile the resolution decreases because of

insufficient data coverage. It is noted that the structural pattern presented here is

obtained through the smooth inversion technique. Therefore, the imaged features are

blurred pictures of sharp boundaries.

Starting from the surface and northernmost part of the first profile, the findings

are as follows (Fig. 6a). Average depth of investigation is around 25 km in this

geoelectrical model. The model begins with a conductive zone (<4 ohm-m) that

represents the effect of the Dardanelles Strait. A detailed structural pattern of the

Dardanelles could not be obtained due to an insufficient number of stations on

the western side. The center of the geoelectrical model presents distinct features. The

conductive spots (< 10 ohm-m) appear beneath the NAF and WA7-8 and WA13

below 15 km, and the second feature emerges at both sides of the Can between WA9

and WA13 (Fig. 6a). In addition, the model divided into two zones vertically. The

upper part contains resistive blocks (>100 ohm-m) and the lower part is conductive

(<100 ohm-m). The boundary between the zones is irregular.

The second geoelectrical model is reliable down to 25 km and 30 km, on the

northernmost and southernmost sides, respectively. The model begins with a

conductive zone (4-100 ohm-m) between stations WA18 and WA34. Two conductive

zones (<2 ohm-m) of the model, 10 km below stations WA18 and WA28. Resistive

blocks (>100 ohm-m) extend from WA21 to WA37 at a depth of 2–25 km.

In the third geoelectrical model, extending down to 25 km, the shallow part of the

model may be subdivided into three conductive zones. The first one (<10 ohm-m) is

delineated between WA38–41. The second one (<60 ohm-m) covers the area from

WA43 to WA48. The last one (<60 ohm-m) is the northern end of the section.

Resistive unit (>100 ohm-m) is delineated along the model at a depth of 3–15 km.

Figure 8 presents the apparent resistivities and phases of impedances for the

selected stations. Continuous curves show the calculated data while the symbols

indicate the observed data. In general, the shallow parts of the sections correspond-

ing to low periods reveal 1-D character. However, the divergence of TE and TM

mode curves indicates 2-D structures in the deeper part of the resistivity sections.



Density Distribution of the Area

Gravity data (Fig. 9a) have been collected at 3–5 km intervals during the course

of a national project and later processed by MTA; the results have appeared in

some publications (e.g., AKDOGAN, 1995, 2000). The aim of presenting the data

here is to justify the lateral discontinuities in the geoelectrical model rather than

proposing a new density model for the region. Bouguer gravity data have been

imaged in a band of 10 km from either side of the MT profile. A density model was

created using the 2.5-D modeling scheme of the WinGLink� package by focusing

on the lateral discontinuities between the main structures obtained from the MT

data. No density analysis of geological units was performed. The position, shape

and boundaries of the structures were generated from the geoelectrical image

obtained from the MT data. Since the software needs density values of blocks

rather than density differences, the crustal background of the model was set to

2.85 g/cm3. Consequently, a simple model response that fits the gravity data was

obtained (Fig. 10a–c) by a trial-and-error procedure. Subsidence areas were

represented by a density value less than 2.6 gr/cm3 while the density of the crust

was assigned to 2.87 gr/cm3 or larger values. The density of the deep conductive

zones was set to 2.2–2.5 gr/cm3.

Magnetization Distribution of the Area

Aeromagnetic data (Fig. 9b) were also collected and processed by MTA

(AKDOGAN, 2000). The flight altitude and record interval were 625 m and 70 m,

respectively. The profile interval was set as 1–2 km. The reduction to pole was

performed by using 55o inclination and 4o declination angles for Turkey. The same

selection steps were also applied to the data for modeling purposes. Along the MT

lines, magnetic data were selected in a band of 10 km on both sides of the profiles. A

model was created using the 2.5-D modeling scheme of the WinGLink� package.

Position, shape and boundaries of the structures were generated from the image of

the geoelectrical model of the MT data. The background of the model was set at 79.5

as SI · 4p 103. The derived elemental model is shown in Figures 10d–f. Some shallow

zones have susceptibility values of 79.5 and the crust is represented by susceptibility

values of 159 to 238.7.

Figure 7

Observed apparent resistivity and phase of impedance: a) TE mode, b) TM mode. Upper, middle and

bottom panels are for Segment 1, Segment 2 and Segment 3, respectively. Small dots indicate data.

Abscissas are relative intervals (km) of stations while ordinates are periods (sec).

b


Figure 7

(Contd.)


Figure 7

(Contd.)


10-3

10-2

10-1

100

101

102

103

104

10-2

10-1

100

101

102

103

Ap

p.R

es.(

oh

m-m

)TE-obs. TE-calc. TM-calc. TM-obs.

10-3

10-2

10-1

100

101

102

103

0

30

60

90

Ph

ase

(deg

ree)

WA11

10-3

10-2

10-1

100

101

102

103

104

10-2

10-1

100

101

102

103

Ap

p.R

es.(

oh

m-m

)

WA20

10-3

10-2

10-1

100

101

102

103

0

30

60

90

Ph

ase

(deg

ree)

10-3

10-2

10-1

100

101

102

103

104

10-2

10-1

100

101

102

103

Ap

p.R

es.(

oh

m-m

)

10-3

10-2

10-1

100

101

102

103

0

30

60

90

Ph

ase

(deg

ree)

WA34

10-3

10-2

10-1

100

101

102

103

104

Period (sec)

10-2

10-1

100

101

102

103

Ap

p.R

es.(

oh

m-m

)

WA39

10-3

10-2

10-1

100

101

102

103

Period (sec)

0

30

60

90

Ph

ase

(deg

ree)

Figure 8

Sounding curves for selected stations. Dots are observed data while solid lines are response of the 2-D

model, error bars are of observed data Abscissas are periods (sec).


Figure 9

MT profile with (a) gravity map of western Anatolia and (b) magnetic map of western Anatolia.



Integration and Discussion

Evaluation of the geoelectrical models requires additional information. The main

steps are given in subsections.

Evaluation of Research Depth

Negative gravity anomalies (gray to black in Fig. 9a) indicate isostatic thickening

of the continental crust towards the east (e.g., see ATEs et al., 1999). Considering the

average asthenosphere depth (55+/-5 km; ILKıSıK. 1995) in western Anatolia, the

depths of investigations in all of these geoelectrical models never reached the upper

mantle. Therefore, both the high resistive unit (>100 ohm-m) and relatively medium

resistive level (10< and <100 ohm-m) observed throughout the majority of the

sections should be parts of the crust.

The resistive part of the crust extends from the Dardanelles to WA9 and from

WA14 to the end of the model at a depth of 3–25 km in the first segment. Then it

extends from WA21 to WA37 at a depth of 2–25 km in the second segment.

Fractured and thinned crust (>100 ohm-m) is delineated along the model at a depth

of 3–15 km in the last segment. Fragmentation and varying thickness along all

profiles are the result of tectonic activity in the region. It is observed that the crustal

thickness decreases towards the southern part of the segment. This is in accord with

regional extensional regimes (e.g., YıLMAZ et al., 2000). All extensions were observed

around the Menderes Massif, at the edge of the last segment.

Conductive Lower Crust

The conductive zone below the upper crust is subject to considerable researches

(e.g., OGAWA, 1987; HYNDMAN, 1988; JONES, 1992; NESBITT, 1993; MARQUIS et al.,

1995; UTADA et al., 1996; CAGLAR 2001; SATOH et al., 2001). WANNAMAKER et al.

(1997) did a survey in the eastern margin of the Great Basin, southwestern Utah and

eastern Nevada. Great Basin and Western Anatolia both have similar tectonic

settings. The general pattern of both areas is similar to each other. The MT method

mapped out the crust only. Both areas have low resistivity in the lower crust. SATOH

et al. (2001) presented a survey result from the Kuril Arc, Japan, and also found a

conductive layer in the lower crust.

Three reasons may be considered for the relatively low values of resistivity at this

depth; the first is the graphite-like conductive minerals, the second is the presence of

fluid trapped in the crust, and the last one is partial melting. There is no report for

Figure 10

(a–c) Gravity data along the MT profile; (d–f) Magnetic data along the MT profile. Both magnetic and

gravity data selected in a range of 10 from both sides of the profile. Abscissas are relative intervals (km) of

MT stations while ordinates are depth (m) in the lower panels and data units for the upper panels.

Susceptibility values (s) are scaled by 4p.

b


Figure 10

(Contd.)


such a large-scale conductive minerals in the vicinity of the study area. AKYOL et al.

(2006) showed that western Anatolia is characterized by crustal velocities that are

significantly lower than average continental values. It is also noted that seismological

research (not presented) indicates that the average depth to the brittle–ductile

structural boundary is around 20 km in the study area. There are a few epicenters

below this level. Thus the conductive zone is not expected to be ductile. Curie depth

research (e.g., AYDıN et al., 2005) indicates that this depth level has low or no

magnetisation, i.e., the temperature should exceed at least 580 C. ALDANMAZ et al.

(2000) stated that geochemical analysis of the late Miocene alkaline rocks in western

Anatolia indicates that these rocks formed by partial melting of enriched astheno-

spheric mantle source. Direct contact of hot upwelling asthenospheric mantle

provides a hot thermal anomaly and initiates melting. Late Cenozoic volcanic

activity occurred along with the creation of magma chambers in the crust including a

partially melted zone. ERCAN et al. (1985) and CAGLAR (2001) also reached a similar

conclusion for western Anatolia.

Tectonic Units

The geoelectrical models cover the eastern boundary of the Sakarya continent.

The features which emerge under WA10 and WA13 in the vicinity of Can are related

to branches of the NAF. The NAF contains a few interlacing faults and consequently

is represented over a wider area and with a large vertical extension.

The western flank of the Simav fault zone (SEYITOGLU, 1997) appears around

WA33–34. The shallow part of the third model may be subdivided into three

subsidence areas. The first one (<10 ohm-m), delineated between WA38–41, is the

Gordes Basin, the border of which occurs between the Izmir - Ankara Suture Zone and

the Menderes Massif. The main fault which bounds the basin is located on its eastern

side (SEYITOGLU and SCOTT, 1994). The second one (<60 ohm-m) is theDemirci basin,

which covers the area fromWA43 toWA48. The last one (<60 ohm-m) is the northern

end of Alasehir graben. All three of the subsidence areas have irregular bottom

surfaces. Since the last profile ends in the Alasehir graben, the footwall-like structure

shows a fault character in the northern part of the graben (WA52).

Figure 11

Proposed model. White area is crust. The thick lines are main faults, + represent magma intrusions, and v

are partially melted areas. ¯ and • indicate the movement direction of the walls of the NAF.


Deep Hot Spots

The average heat-flow value for the study area is �25% higher than the average

of the rest of Turkey. The heat flow obtained from magnetic data, which also was

verified with available real observations, increases to 229 mW/m2 in the region (AKıN

et al., 2006). Additionally, recent heat-flow research related to Curie depth obtained

from magnetic data (e.g. AYDıN et al., 2005; SALK et al., 2005; AKıN et al., 2006)

indicates that western Anatolia has high gradient values and shallow Curie depth

values. HISARLı (1995) has estimated Curie-point depths of between 8 and 12 km. in

the Balıkesir area. TANAKA et al. (1999) states that 10 km or shallower Curie depth

values point out volcanic and geothermal fields which is the exact description of the

vicinity of the first profile. In light of this information, the conductive zone under

WA22 may be interpreted as a magma chamber that was a source of Upper

Oligocene volcanism to the NE of Edremit (e.g., YILMAZ, 1990). The second

conductive zone (<2 ohm-m) of the model, below station WA28, is related to

another conductive spot. This area also has low magnetization (Fig. 10e) and high

heat flow values (�120 mW/m2) between Balıkesir and Bigadic (AKıN et al., 2006). A

possible heater for the geothermal sites can also be connected to magma chambers on

the Biga Peninsula. This chamber could be a relict of widespread calc-alkaline

magmatism that occurred in the Early - Middle Miocene on the Biga Peninsula (e.g.,

ALDANMAZ et al., 2000).

Additional Data

Lateral variations are checked by using different methods (Fig. 10). The

branch of the NAF appears with low values in density and susceptibility models.

All sediment fillings at the surface also show low density and susceptibility values.

The third model has a highly resistive block until station WA50, indicating that

high susceptibility values could be expected in the region. Nevertheless, the

magnetic data present no anomaly along the profile. According to the geological

map, the profile passes over the ophiolites; however, the magnetic map does not

exhibit strong variations at that location. The contributions of thin ophiolites to

the data are probably removed via some filtering process applied as a part of the

data-processing scheme. Contrastingly, the Menderes Massif presents lower

susceptibility; therefore, the southernmost part of the magnetic data exhibits a

flat pattern.

Earthquakes which occurred on the NAF line up around Can, where the first

geoelectrical model also provides structural information. The seismological data

supports the electromagnetic model by indicating an active zone along the NAF.

Lower-magnitude earthquakes have occurred along the second model. Densely

occurring epicenters indicate high activity below the third segment. Horst and graben

structures in the basins are the main sources of the earthquakes.


Geological Model

A geological model based on the MT data is presented in Figure 11. The

comparison with the regional map given in Figure 2 illustrates that all features along

the segments are sensed by the MT data and represented in the geoelectrical models.

Crust is illustrated by blank areas and the faults are delineated with thick lines, while

magma intrusions and partially melted areas are designated with patterns in

Figure 11. All basins are bounded by normal and listric faults. The fractured nature

of crust, delineated by partially melted areas, may also contain magma chambers.

Question marks indicate unresolved parts of the geoelectrical models.

Conclusions

In the proposed geoelectrical sections, generally deeper conductive parts are

related to hot areas (such as below WA22 and WA29). However, at shallow depths,

the conductive anomalies correspond to the sedimentary basins while highly resistive

regions are related to major structures.

Variations in magnetic and gravity data along the profiles also agree with the

lateral variations in geoelectrical models. Basin fillings which correspond to high

conductivities in shallower parts of the geoelectrical sections needed to be

represented by low density and susceptibility contrasts to catch local variations.

Themajor structures, sedimentary basins and igneous activity can easily be discerned

in the resistivity sections. In the Canakkale - Can area, the southern branches of the

NorthAnatolianFault are clearly visible. Farther south around Sındırgı, the Simav fault

appears as one of the main structures. The Gordes andDemirci basins are also resolved.

It is notable that the crust thins in the Alasehir Graben, one of the major E-W trending

grabens of western Turkey. The bottom depths of the basins and grabens are around

3 km. The deepest is the Gordes basin, while the Demirci basin is the shallowest.

Western Turkey has a thinned, fractured crust with extensive magmatism as is

typical for regions affected by extensional tectonics. The crustal part of the structure

extends along the model between depths of 3 and 25 km. In addition, the magmatism

which occurred in the late Oligocene caused decreasing resistivity of the crust at

greater depth.

Acknowledgements

This paper is part of the national geology and geophysics project (Naci Gorur,

coordinator) supported by TUBıTAK, Project No. YDABCAG-422/G. The MT

data were gathered by MTA as a part of a national project. We also thank

Geosystem for allowing us to use the WinGLink package. We extend thanks to


Dr S. Ozden (C. Onsekizmart University) for reading the manuscript and making a

number of suggestions. Editor G. Heinson and both reviewer, I. Ferguson and M.

Unsworth improved the manuscript.

REFERENCES

ALDANMAZ, E., PEARCE, J.A., THIRLWALL, M.F., and MITCHEL, J.G. (2000), Petrogenetic evolution of Late

Cenozoic, post-collision volcanism in western Anatolia, Turkey, J. Volcanol. Geothermal Res. 102, 67–95.

AKDOGAN, N. (1995), Turkiye gravity studies and some results, Jeofizik 9–10, 31–34 (in Turkish).

AKDOGAN, N. (2000), Comparison of gravity-magnetic-heat flow and epicentres with geology in Istanbul and

Izmir sectors and some results, National Geophysics Meeting, MTA Ankara, Expanded Abstracts 112–

116 (in Turkish).

AKıN, U., DURU, M., KUTLU, S., and ULUGERGERLI, E.U. (2006),Heat flow map of Turkey (from magnetic

data), 17 Geophysical Congress, MTA, Ankara (in Turkish). Extended Abstract CD.

AKYOL, N., ZHU, L. MITCHELL, B.J. SoZBILIR, H., and KEKOVALı, K. (2006), Crustal structure and local

seismicity in western Anatolia, Geophys. J. Int. 166, 1259–1269.

ATES, A., KEAREY, P., and TUFAN, S. (1999), New gravity and magnetic anomaly maps of Turkey,Geophys.

J. Internat. 136(2), 499–502.

AYDıN, I. (1987), An attempt to define the vertical geometry of bodies cause magnetic anomalies, Jeofizik 1,

76–88 (in Turkish).

AYDıN, I., KARAT, H.I., and KOCAK, A. (2005), Curie-point depth map of Turkey, Geophys. J.Int. 162, 633–

640.

BAYRAK, M., ILKISIK, O.M., KAYA, C., and BASOKUR, A.T. (2000), Magnetotelluric data in western

Turkey: Dimensionality analysis using mohr circles, J. Geophys. Res. Solid Earth 105(B10), 23391–23401.

BAYRAK, M. and NALBANT, S.S. (2001), Conductive crust imaged in western Turkey by MT, Geophys. Res.

Lett. 28(18), 3521–3524.

BERDICHEVSKY, M.N., DMITRIEV, V.I., and POZDNJAKOVA, E.E. (1998), On two-dimensional interpretation

of magnetotelluric soundings, Geophys. J. Int. 133, 585–606.

BOSTICK, F.X. (1977), A simple almost exact method of MT analysis, Workshop on Electrical Methods in

Geothermal Exploration, U.S. Geol. Sur. Contract. No. 14080001-8-359.

CAGLAR, I. (2001), Electrical resistivity structure of the northwestern Anatolia and its tectonic implications

for the Sakarya and Bornova zones, Phys. Earth Planet. Inter. 125(1–4), 95–110.

ERCAN, T., SATıR, M., KREUZER, H., TURKECAN, A., GUNAY, E., CEVIKBAS, A., ATES, M., CAN, B. (1985),

Interpretation of new chemical, Isotopic and radiometric data on Cenozoic volcanics of western Anatolia,

Bull. Geolog. Soc. Turkey 28, 121–136 (in Turkish).

ERGUN, M. (1977), Magnetic Studies in Cyprus and Biga Peninsula (Turkey), Ph.D. Thesis, Leicester

University (England).

GROOM, R.W. and BAILEY, R.C. (1989), Decomposition of magnetotelluric impedance tensors in the presence

of local three-dimensional galvanic distortion, J. Geophys. Res. 94(B2), 1913–1925.

GROOM, R.W. and BAILEY, R.C. (1991), Analytic investigations of the effects of near-surface three-

dimensional galvanic scatterers on MT tensor decompositions, Geophys. 56(4), 496–518.

GURER, A., GURER, O.F., PICE, A. and ıLKıSıK, O.M. (2001), Conductivity structure along the Gediz Graben,

West Anatolia, Turkey: Tectonic implications, Internat. Geolog. Rev. 43, 1129–1144.

HYNDMAN, R.D. (1988), Dipping seismic reflections, electrical conductive zones and trapped water in the

crust over a subducting plate, J. Geophys. Res. 93, 13391–13405.

HISARLı, M. (1995), Determination of the Curie point depths in Edremit-Susurluk region (Turkey), Jeofizik

9/1–2, 111–117 (in Turkish).

HORASAN, G.A. and CANITEZ, N. (1995), Investigation of the structure of the crust in western Anatolia using

synthetic seismogram, Jeofizik 9, 285–290.

HORASAN, G., GULEN, L., PıNAR, A., KALAFAT, D., OZEL, N., KULELI, S., and IsıKARA, A.M. (2002),

Lithospheric structure of the Marmara and Aegean Regions, Western Turkey, Bull. seismol. Soc. Am.

92(1), 322–329.


ILKISIK, O.M. (1995), Regional heat flow in western Anatolia using silica temperature estimates from thermal

springs, Tectonophysics 244, 175–184.

JONES, A.G. (1983), On the equivalence of the ‘‘Niblett’’ and Bostick’’ transformation in the magneto telluric

method, J. Geophys. 53, 72–73.

JONES, A.G., Electrical conductivity of the continental lower crust. In Continental Lower Crust, pp. 81–143

(Elsevier, Amsterdam 1992).

KREEMERA, C., CHAMOT-ROOKEB, N., and LE PICHON, X. (2004), Constraints on the evolution and vertical

coherency of deformation in the Northern Aegean from a comparison of geodetic, geologic and seismologic

data, Earth Planet. Sci. Lett. 225, 329–346.

MACKIE, R.L., RIEVEN, S., and RODI, W. (1997), Users manual and software documentation two dimensional

inversion for magnetotelluric data, M.I.T. Earth Resources Lab. Report.

MARQUIS, G., JONES, A.G., and HYNDMAN, R.D. (1995) Coincident conductive and reflective middle to

lower crust in southern British Columbia, Geophys. J. Int. 120, 111–131.

MEJU, A.M., FONTES, S.L., OLIVEIRA, M.F.B., LIMA, J.P.R., ULUGERGERLI, E.U., and CARRASQUILLA, A.

(1998), Regional aquifer mapping using combined VES-TEM-AMT/EMAP methods in the semi-arid

eastern margin of Parnaiba Basin, Brazil, Geophys. 64, 1–20.

MINDEVALLI, Y.O. and MITCHELL, B. J. (1989), Crustal structure and possible anisotropy in Turkey form

seismic surface wave dispersion, Geophys. J. Int. 98, 93–106.

MOGI, K. (1962), Study of elastic shocks caused by the fracture of heterogeneous materials and its relations to

earthquake phenomena, Bull. Earthq. Res. Instit. Tokyo Univ. 40, 125–173.

NESBITT, B.E. (1993), Electrical resistivities of crustal fluids, J. Geophys. Res. 98(B3), 4301–4310.

NIBLETT, E.R. and SAYN-WITTGENSTEIN, C. (1960), Variation of electrical conductivity with depth by the

magnetotelluric method, Geophys. 25, 998–1008.

OGAWA, Y. (1987), Two-dimensional resistivity modeling based on regional magnetotelluric survey in the

Northern Tohoku district, Northeastern Japan, J. Geomag. Geoelectr. 39, 349–366.

OKAY, A.I., SATıR, M., MALUSKI, H., SIYAKO, M. MONIE, P., METZGER, R., and AKYUZ, S. (1996), Paleo-

and Neo-Tethyan events in northwestern Turkey: Geologic and geochronologic constraints. In The Tectonic

Evolution of Asia (A. Yin and T. M. Harrison eds). (Cambridge University Press 1996) pp. 420–441.

OKAY, A., SATıR, M. (2000), Coeval plutonism and metamorphism in a latest Oligocene metamorphic core

complex in northwest Turkey, Geol. Mag. 137, 495–516.

OKAY, A. I., SENGOR, A.M.C., and GORUR, N. (1994), Kinematic history of the opening of the Black Sea

and its effect on the surrounding regions, Geology 22, 267–270.

OKAY, A.I. and TUYSUZ, O. (1999), Tethyan sutures of northern Turkey. In The Mediterranean Basins:

Tertiary Extension within the Alpine Orogen (B. Durand, L. Jolivet, F. Horvath, M. Serrane eds).

Geological Society London, Special Publications 156, 475–515.

OLDENBURG, D.W. and LI, Y. (1999), Estimating depth of investigation in dc resistivity and IP surveys,

Geophysics 64(2), 403–416.

SARI, C. and SALK, M. (1995), Estimation of thickness of the sediments in the Aegean grabens by 2-D and

3-D analysis of the gravity anomalies. In (O. Piskin, M Ergun, M. Y. Savascın, G. Tarcan, eds), Proc.

Internat. Earth Sci. Colloq. on the Aegean Region vol. 1 ızmir Gulluk, Turkey, pp. 255–271.

SATOH, H., NISHIDA, Y., OGAWA, Y., TAKADA, T., and UYESHIMA, M. (2001), Crust and upper mantle

resistivity structure in the southwestern end of the Kuril Arc as revealed by the joint analysis of conventional

MT and network MT data, Earth Planets Space 53, 829–842.

SAUNDERS, P, PRIESTLY, K., and TAYMAZ, T. (1998), Variation in the crustal structure beneath western

Turkey, Geophys. J. Int. 134, 373–389.

SCHOLZ, C.H. (1968), The frequency-magnitude relation of microfracturing in rock and its relation to

earthquakes, Bull. Seismol. Soc. Am. 58, 399–415.

SEYITOGLU, G. (1997), The Simav graben: an example of young E-W trending structures in the Late Cenozoic

extensional system of western Turkey. Turkish J. Earth Sci. 6, 135–141.

SEYITOGLU, G. and SCOTT, B.C. (1994), Late Cenozoic basin development in west Turkey. Gordes basin:

Tectonics and sedimentation, Geolog. Mag. 131, 631–637.

SEYITOGLU, G. and SCOTT, B.C. (1996), The cause of N-S extensional tectonics in western Turkey: Tectonic

escape vs. back-arc spreading vs. orogenic collapse, J. Geodyn. 22, 145–153.


SEYITOGLU, G., IsıK, V., CEMEN, ı. (2004), Complete Tertiary exhumation history of the Menderes massif,

western Turkey: an alternative working hypothesis. Terra Nova 16, 358–364.

SALK, M., PAMUKCU, O., and KAFTAN, ı. (2005), Determination of the Curie point depth and heat flow from

Magsat data of Western Anatolia, J. Balkan Geophys. Soc. 8(4), 149–160.

SENGOR, A.M.C., GoRUR, N., and SAROGLU, F., Strike-slip faulting and related basin formations in zones of

tectonic escape: Turkey as a Case Study, In Strike-slip Faulting and Basin Formation, pp. 227–264 (eds.

Biddle K.-T., and Christie-Blick N.) (Soc. Econ. Paleontol. Mineral. Special Publication, 1985).

SENGOR, A.M.C., and NATAL’IN, B., Paleotectonics of Asia: fragments of a synthesis. In The Tectonic

Evolution of Asia (A. Yin and Harrison, M., eds.) (Cambridge University Press 1996) pp. 486–640.

SENGOR, A.M.C., and YıLMAZ, Y. (1981), Tethyan evolution of Turkey: A plate tectonic approach,

Tectonophysics 75, 181–241.

STERNBERG, B.K., WASHBURNE, J.C., and PELLERIN, L. (1988), Correction for the static shift in

Magnetotelluric using transient electromagnetic soundings, Geophysics 53, 1459–1468.

TANAKA, A., OKUBO, Y., and MATSUBAYASHI, O. (1999), Curie point depth based on spectrum analysis of

the magnetic anomaly data in East and Southeast Asia, Tectonophysics 306, 461–470.

TAYMAZ, T., JACKSON, J.A., and WESTAWAY, R. (1990), Earthquake mechanism in Hellenic Trench near

Crete, Geophys. J. Int. 102, 695–731.

UTADA, H., HAMANO, Y., and SEGAWA, J. Conductivity anomaly around the Japanese Islands. In Geology

and Geophysics of the Japan Sea (Japan- USSRMonograph Series, vol. 1), (eds. N. Isezaki, I.I. Bersenev,

K. Tamaki, B. Ya. Karp, and E.P. Lelikov) pp. 103–149, (Terrapub, Tokyo, 1996).

WANNAMAKER, P.E., JOHNSTON, J.M., STODT, J.A., and BOOKER, J.R. (1997), Anatomy of the southern

Cordilleran hingeline, Utah and Nevada, from deep electrical resistivity profiling, Geophysics 62, 1069.

WEEKS, J., LOCKNER, D., and BYERLEE, J. (1978), Change in b-value during movement on cut surfaces in

granite, BSSA (Bull. Seismol. Soc. Am.) 68(2), 333–341.

WIGHT, D.E., and BOSTICK, F.X. (1980), Cascade decimation-a technique for real time estimation of power

spectra, Proc. IEEE 626–629.

YILMAZ, Y. (1990), Comparison of young volcanic associations of western and eastern Anatolia formed under

the compressional regime: A review, J. Volcan. Geotherm. Res. 44, 69–87.

YıLMAZ, Y., GENC, S., C., GURER, F., BOZUCU, M., YıLMAZ, K, KARACıK, K., ALTUNKAYNAK, S., and

ELMAS, A. (2000), When did the western Anatolia grabens begin to develop? In (Bozkurt, E., Winchester,

J., A., and Piper J. D. A., eds.) Tectonics and Magmatism in Turkey and the Surrounding Area,

Geological Society. London. Special Publications 173, 353–384.

YıLMAZ, Y., TUYSUZ, O., YIGITBAS, E., GENC, S.C., and SENGOR, A.M.C. (1997), Geology and tectonic

evolution of the Pontides. In Regional and Petroleum Geology of the Black Sea and Surrounding Region,

(A.G Robinson, ed.). AAPG Memoir 68, 183–226.

ZHU, L., MITCHELL, B.J., AKYOL, N., CEMEN, I., KEKOVALI, K. (2006), Crustal thickness variation in the

aegean region and its implications for the extension of continental crust, J. Geophys. Res. 111, B01301,

doi:10.1029/2005JB003770.

(Received November 17, 2005, accepted November 2, 2006)

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