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Birkhaüser Verlag, Basel, 2007

Pure appl. geophys. 164 (2007) 1–28 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,3 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 also 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.,

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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, Çanakkale, Turkey (currently).

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Dispatch: 21.3.2007 Author’s disk received 4

Journal: Pure and applied Geophysics No. of pages: 28 Used 4 Corrupted Mismatch Keyed

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E. U. Ulugergerli et al.

Pure appl. geophys.,

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; ÇAG˘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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The Geoelectrical Structure of Northwestern Anatolia, Turkey

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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, ÇAG˘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 ÇAG˘LAR (2001)’s profile, but our current profile employs shorter station intervals and the directions of the segments differ from those of ÇAG˘LAR (2001). The geoelectrical models presented in ÇAG˘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 TUŸSU¨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 Çanakkale, 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 Çanakkale 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 20 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˘aziçi 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 were used for E field measurements. Horizontal components of the H field were

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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 ‘‘+’’ 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 90

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Figure 5 Swift and 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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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 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., WA 38 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 (Çanakkale), 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 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

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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 atop 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

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Figure 6 Geoelectrical models obtained from 2-D inversion of the MT data. Resistive crust represented with blue color. Conductive zone below the crust represents an electrical asthenosphere, while hot spots and basin deposits are in red.

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). Pseudosections of apparent resistivity and phase of impedance for observed and calculated data are given in Figure 7. Some selected stations and corresponding

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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 (red to green) extending down to 3 km. The second level (>100 ohm-m) includes crustal structure (dark blue), and is of nonuniform thickness. The third level (10< and