Birkha¨user Verlag, Basel, 2007

Pure appl. geophys. 164 (2007) 999–1026 0033–4553/07/050999–28 DOI 10.1007/s00024-007-0200-0

Pure and Applied Geophysics

The Geoelectrical Structure of Northwestern Anatolia, Turkey E. U. ULUGERGERLI,1,4 G. SEYITOG˘LU,2 A. T. BAS¸OKUR,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 Alas¸ehir 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 Go¨rdes 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; SEYITOG˘LU and SCOTT 1994; YıLMAZ et al., 1997; ALDANMAZ et al., 2000) and geophysical (e.g.,

1

Department Department Ankara, Turkey. 3 Department 4 Department 2

of Geophysical Engineering, Ankara University, 06100 Ankara, Turkey. of Geological Engineering, Tectonics Research Group, Ankara University, 06100 of Geophysics Engineering Sivas, Cumhuriyet University, Turkey. of Geophysics Engineering, Onsekiz Mart University, C¸anakkale, Turkey (currently).

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Figure 1 Regional map of the Aegean Sea. Detailed map of rectangular area is given Figure 2.

TAYMAZ et al., 1990; HORASAN and CANıTEZ, 1995; BAYRAK et al., 2000; C¸AG˘LAR, 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). However, 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

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Figure 2 Location of MT stations (small dots), Neogene and Quaternary basins and main basement structures in western Turkey (after SEYITOG˘LU and SCOTT, 1994). Thin solid lines show the segments. Larger dots indicate towns and cities.

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

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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, C¸AG˘LAR (2001) also proposed a geoelectrical model for the western part of Anatolia. The data presented here somewhat cover the same geological settings as does C¸AG˘LAR (2001)’s profile, but our current profile employs shorter station intervals and the directions of the segments differ from those of C¸AG˘LAR (2001). The geoelectrical models presented in C¸AG˘LAR (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. GU¨RER et al. (2001) presented results for the Gediz (Alas¸ehir) 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. ILKıS¸ıK (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

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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 TU¨YSU¨Z 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. The final 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 (SENGO¨R and NATAL’IN 1996). Following the Oligocene, western Turkey experienced N-S extensional tectonics (SEYITOG˘LU and SCOTT, 1996; SEYITOG˘LU 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 (SEYITOG˘LU et al., 2004) indicates that in the Oligocene, DatcaKale 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 (Alas¸ehir 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 So¨ZBILIR (2004), using geological observations, implied that the thickness of the Neogene sediments in the Alas¸ehir 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 Alas¸ehir 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).

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The geoelectrical line of this study starts at C¸anakkale, 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 C¸anakkale and Balıkesir. The NAF zone has many local faults such as Etili and Yenice-Go¨nen 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 Alas¸ehir 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 energyaccumulation 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.

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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 (Bog˘azic¸i U¨niversity Kandilli Observatory and Earthquake Research Center), ISC (International Seismological Center) ). Rectangles are locations of MT stations.

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., ıLKıS¸ıK, 1995; GU¨RER 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-

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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.

flow rates (ıLKıS¸ıK, 1995) and many geothermal spots indicate the existence of possible magma intrusions/chambers. ILKıS¸ıK (1995) pointed out that the depth of the lithosphere-asthenosphere boundary in western Anatolia is around 55±5 km. SENGO¨R 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 So¨ke. 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

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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 ‘‘+’’

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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.

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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) decomposition 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 (C¸anakkale), 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, Bigadic and 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 Go¨rdes and Ko¨pru¨basi, 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 corresponding 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

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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–3D 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

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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.

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).

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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 C¸an 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 corresponding 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.

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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).

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.

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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).

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Figure 9 MT profile with (a) gravity map of western Anatolia and (b) magnetic map of western Anatolia.

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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.

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; ILKıS¸ıK. 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; C¸AG˘LAR 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

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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.

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 asthenospheric 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 C¸AG˘LAR (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 C¸an 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 (SEYITOG˘LU, 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 Go¨rdes 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 (SEYITOG˘LU and SCOTT, 1994). The second one (<60 ohm-m) is the Demirci basin, which covers the area from WA43 to WA48. The last one (<60 ohm-m) is the northern end of Alas¸ehir graben. All three of the subsidence areas have irregular bottom surfaces. Since the last profile ends in the Alas¸ehir graben, the footwall-like structure shows a fault character in the northern part of the graben (WA52).

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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 C¸an, 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.

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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. The major structures, sedimentary basins and igneous activity can easily be discerned in the resistivity sections. In the C¸anakkale - C¸an area, the southern branches of the North Anatolian Fault are clearly visible. Farther south around Sındırgı, the Simav fault appears as one of the main structures. The Go¨rdes and Demirci basins are also resolved. It is notable that the crust thins in the Alas¸ehir 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 Go¨rdes 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

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