Electrical resistivity structure of the northwestern Anatolia and its tectonic implications for the Sakarya and Bornova zones

Physics of the Earth and Planetary Interiors 125 (2001) 95–110

Electrical resistivity structure of the northwestern Anatolia and its tectonic implications for the Sakarya and Bornova zones ˙ Ilyas Ça˘glar∗ Department of Geophysical Engineering, Faculty of Mines, Istanbul Technical University, Maslak, 80626 Istanbul, Turkey Received 31 May 2001; accepted 4 June 2001

Abstract The crustal structure of the northwestern Anatolia was studied by magnetotelluric measurements with periods up to 500 s at 20 locations along a line (line KS) crossed Istanbul, Sakarya and Bornova Zones to investigate deep electrical resistivity patterns that contribute to the understanding of tectonic setting of the region. The results obtained from the two-dimensional (2D) inversion of magnetotelluric data for the modes transverse electric (TE) and transverse magnetic (TM) show that the tectonic structure up to 5 km depth was comparatively complex and that the structural pattern was generally disturbed by the grabens and Neogene basins of low resistivity feature. The low resistivity (<80  m) zones stretched from the Istanbul Zone towards the northern part of the Sakarya Zone and from the southern border of the Bornova Zone towards the Gediz Graben in south characterize sedimentary sequences for this depth range. Based on the more detailed near-surface 2D geoelectrical model constructed using dc Schlumberger resistivity data the basin-fill deposits is about 1.5 km and 750 m thick in the Gediz and Görede Grabens, respectively. Other smaller regions of low resistivity were also interpreted as the small-scale sedimentary Neogene basins within the Sakarya and Bornova Zones. Below about 5 km depth geoelectric models except a zone beneath Kazda˘g range show more resistive structure underlying these sediments. The resistive structure was correlated with the Precambrien crystalline rocks and gneiss schist basement of Sakarya and Bornova Zones, respectively. A major zone with relatively conductive (∼10  m) occurred within highly resistive mid crust beneath Kazda˘g range represents an electrically macro-anisotropic structure which could be followed from magnetotelluric sounding data in both TE and TM modes. It was discussed that the origin of this zone is associated with a coeval plutonism and metamorphism of latest Oligocene age, which yields a considerable low conductivity in the crystalline rocks. Of special interests was the zone of electrical conductivity, as it seems to exist a correlation between conductive structure and geotectonic features, which may support ideas about the high-grade metamorphism and accordingly north–south extending in the northwestern Anatolia. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Anatolia; Electrical structure; Magnetotelluric surveys

1. Introduction Post-Jurassic deformation in the Aegean region has been dominated by convergence of the African and Eurasian plates. The paleomagnetic pattern suggests real northward displacement of crustal blocks within ∗ Fax: +90-212-285-6201. ˙ Ça˘glar). E-mail address: [email protected] (I.

Aegean region, with respect to both these plates (Beck and Schermer, 1994). Northwestern Anatolia where is a part of the Aegean extensional province with diffuse north–south extension, may be divided into a number of the distinct subparallel east–west trending geological zones, namely the Istanbul, Sakarya and Bornova Zones (Sengör ¸ and Yılmaz, 1981; Okay et al., 1991, 1996) (Fig. 1a). Each of these zones has characteristic stratigraphic, deformational, magmatic

0031-9201/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 9 2 0 1 ( 0 1 ) 0 0 2 1 6 - 3

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Fig. 1. Position map of measuring sites of magnetotelluric soundings in the northern Aegean. (a) Sub-parallel geotectonic zones in the western Anatolia and surrounding region. SZ = Sakarya zone, TB = Thrace basin, LG = magnetotelluric profile measured by Gürer (1996), LT = magnetotelluric profile measured by Ilkı¸sık (1981), KS = magnetotelluric line proposed for present study. (b) Locations of the measuring sites of line KS. The Bouguer anomaly map is also plotted for a part of the area investigated. Dashed lines represent negative values of the Bouguer anomaly and solid lines positive values. Contour interval 5 mgal, Bouguer map is sampled using grid x = 4 km from gravity data (Ate¸s et al., 1999). Abbreviations: EZ: Ezine Zone, KR: Kazda˘g range, KOR: Kozak range, ML: lake of Manyas, GeG: Gediz graben, B-A: gravity profile proposed by Sari and Salk ¸ (1995).

and metamorphic features and more detailed studies are necessary to increase understanding of them. The Sakarya Zone that is a part of the north Anatolian orogenic belt within the Mesozoic–Cenozoic

tectonic units (Yılmaz, 1990) is a major tectonic feature in northwestern Anatolia. Jurassic and Cretaceous rocks of the Sakarya Zone are absent along Izmir–Ankara Suture, suggesting uplift and

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erosion along the suture zone, possibly as a result of crystal thickening following continental collision. On the other hand, the deformation and metamorphism features of the Sakarya Zone are not observed in the Anatolia–Tauride units (i.e. within the Bornova Zone in Fig. 1b). This strengthens a hypothesis about the separation of these two continental fragments by the Tethys ocean (Akyüz and Okay, 1996) that can be an useful key to understand tectonic model of the western Anatolia (Dewey and Sengör, ¸ 1979; Sengör ¸ and Yılmaz, 1981; Taymaz et al., 1990). Indeed, the high temperature metamorphism described for the deep regions of the Sakarya Zone suggests a north–south extending crust (Okay and Satır, 2000; Yılmaz et al., 2001). Electrical resistivity of the rocks in the earth’s crust depends on a wide range of petrological and physical parameters, e.g. their composition, degree of saturation with fluid, porosity and connectivity of pores, conducting minerals or enhanced temperatures. The electrical resistivity of the fractured rocks, depending on metamorphism and fluid-saturation, can decrease to very low values of resistivity ( m), while the compact and dry geological rocks may be characterized by the high electrical resistivity up to 10,000  m (Hyndman and Hyndman, 1968; Telford et al., 1976; Schwarz, 1990). Based on the magnetotelluric profile LG at the northern Anatolia (Fig. 1a), for the low resistivity zone (1–10  m) at depths of about 25–30 km within Sakarya Zone a partial melting with high temperature or an upwelling of the asthenosphere was also suggested (Gürer, 1996). On the other hand, the Permo–Triassic Karakaya complex sequence makes up continental basement of the Sakarya Zone. The structural geometry of the Karakaya complex may be different in the northern and southern zones. The shallow-water limestone blocks, sandstones and shale bands indicating lower resistivity than the surrounding regions are developed in the Karakaya complex (Fig. 2). Since these tectonic sequences with different nature can perturb the telluric currents, in the frame of present study, the Ke¸san–Salihli traverse (KS-line in Fig. 1) crossing above geological zones is therefore designed to investigate the geoelectrical structure beneath northwestern Anatolia, using magnetotelluric measurements. The aim of this magnetotelluric survey, covering the period range 0.04–500 s, is sub-

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sequently to define the structural geometries of the Sakarya and surrounding geotectonic zones by their electrical response. The deep electrical resistivity data that distinguish the rocks with different nature, down to several tens of kilometers within the crust, can be obtained mainly from magnetotelluric soundings. There are several geophysical observations showing that northwestern Anatolia is in an anomalous tectonic setting. A large positive anomaly of gravity is observed in the Biga Peninsula within Sakarya Zone, while negative anomalies are observed in Bornova Flysh Zone on the Anatolia–Tauride Block, reflecting the isostatically thickened continental crust towards the east (Fig. 1b; Ate¸s et al., 1999). The dimensionality analysis of the magnetotelluric data collected along a northwest to southeast profile across the Sakarya and Bornova Zones suggests a strong anisotropy-indicating complexity of the geological structure in the deeper region (Bayrak et al., 2000). From the analysis of the earthquake and gravity data, the crustal thickness in the Sakarya Zone and surroundings is about 25–28 km (Gürbüz and Üçer, 1985; Horasan and Canitez, 1995) but under the mid of Thrace Basin the crustal thickness increases to 32 km, based on magnetotelluric profile LT (Fig. 1b; Ilkı¸sık, 1981). The two regions, Thrace and northwestern Anatolia, differ in heat flow, the Sakarya Zone having a value generally higher than that of Thrace (Ilkı¸sık, 1995; Tezcan, 1995).

2. Geological settings The Sakarya Zone, which is isolated during the late Mesozoic (Sengör ¸ and Yılmaz, 1981), is separated in the north from the Istanbul Zone, by the Intra-Pontide suture, and in the south from the Bornova Flysh Zone (Anatolide–Touride Block), by the Izmir–Ankara ophiolitic suture. The Intra-Pontide suture trending east–west lies along the southern flank of the Thrace basin (Fig. 1). The Istanbul zone that is separated in the south from the Sakarya Zone by the Pontide suture, consists of a Precambrian crystalline basement overlain by a continuous, well-developed transgressive sedimentary sequence extending from the Eocene to the Miocene (Figs. 2 and 6a). Under the mid of Thrace Basin the thickness of this sedimentary sequence

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Fig. 2. Simplified geological map of northwestern Anatolia. The dashed thick lines indicate the boundaries of the Sakarya Zone between northern and southern zones. The dashed thin lines GT and LM show the Schlumberger geoelectric profiles. Abbreviations: GP: Gallipolu Peninsula, ML: Lake of Manyas, EG: Edremit graben, DG: Dikili graben, BG: Bakırçay graben, GG: Gördes graben. Geology was redrawn from Akyüz and Okay (1996).

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reaches to about 3 km based on magnetotelluric profile LT (Fig. 1b; Ilkı¸sık, 1981). The Pontide suture follows the Ganos fault around Gelibolu Peninsula and extends into the Aegean Sea. The extension of the fault zone to the west in the Aegean Sea bounds the Aegean Trough (Yaltırak et al., 2000). The Sakarya Zone comprises a Paleozoic continental basement consisted of granitic and metamorphic rocks (granitoids). The intense aeromagnetic anomalies in the Biga Peninsula and surroundings may owe their origin to the metamorphic basement rocks found in this region, although there might also be a contribution from ultramafic lithologies (Ate¸s et al., 1999). The basement rocks are well exposed in the tectonic windows of the Kazda˘g and Kozak ranges. Both ranges correspond to horsts (Yılmaz et al., 2001). The basement rocks around these ranges are tectonically overlain by the Karakaya complex that is consisted of intra-oceanic deposits pyroclastic rocks, shale and limestone blocks. The Paleozoic metamorphic rocks consist mainly of medium-to coarse-grained gneiss, feldspathic mica schist intercalated with banded amphibolites and marbles. The metamorphism was at high-grade-amphibolite facies to granulite facies. The Paleozoic continental metamorphic rocks of the Sakarya Zone had a complex thermo-tectonic history with mid-Carboniferous, late Triassic, and Oligo-Miocene thermal events (Okay and Satır, 2000). The strong Oligo-Miocene magmatism reflects a heating potential in the region. Indeed, heat flow data indicate high heat flow with a mean value of 2.1 HFU within the western part of Sakarya Zone, around the Biga Peninsula (Ilkı¸sık, 1995) where several well-known geothermal fields (e.g. Gönen, Tuzla, Dikili and Edremit graben) are located (Fig. 2). The Ezine Zone, divided the Biga Peninsula, is an inner geotectonic sequence of the Sakarya Zone (Fig. 1b; Okay et al., 1991). It is consisted of metamorphosed sedimentary sequences commonly consisted of volcano-sedimentaries. The volcano-sedimentary complex consists of mafic volcanic and pyroclastic flows, middle Paleocene limestone, shale, sandstone, and minor serpentines (Kaya, 1982). The sedimentary units were thrusted later over high-grade meta-sedimentary rocks. Exotic blocks and blue schist are also present in the volcano-sedimentary complex west of the Kazda˘g range (Okay et al., 1991).

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Bornova Flysh Zone that forms a NNE-SSW trending belt 50 km in width and 200 km in length between Izmir and Balıkesir (Fig. 1) consists of thick clastic sequence with limestone, volcanic, and schist blocks (Kaya, 1981; Okay and Siyako, 1993). Flysch sediments overlie the limestone blocks. The limestone blocks were derived from a Mesozoic carbonate platform that represented the northwestern margin of the Anatolia–Tauride block. This margin may have been a strike-slip margin rather than a normal passive continental margin (Okay et al., 1996). Neogene units cover the Izmir–Ankara suture zone that is boundary between Sakarya and Bornova Zones. Based on the interpretation of both magnetic and gravity data, the thickness of sedimentary sequence rise up to 5 km but towards the south generally unexceed to 3.5–4 km (Erol, 1978). The Menderes Massif is another main geotectonic region of western Anatolia. The massif has three major lithological units. It has a gneissic core, a schist and a marble envelope. Under the extensional regime, approximately east–west trending grabens (e.g. Dikili, Bakırçay, Gördes and Gediz grabens) and their basin-bounding active normal faults are the most prominent neotectonic features of the region (Bozkurt, 2001). The present width of them varies between 4 and 10 km. The Gediz graben (Figs. 2 and 6a) within Menderes massif, filled with mid-Miocene to recent sedimentary deposits (Seyito˘glu and Scott, 1991), is a geothermally active region, showing high heat flow values (up to 2.2 HFU) and high magnetic anomalies (Ilkı¸sık, 1995; Ate¸s et al., 1999). The sedimentary units in Gediz graben are widely deposited than that of the Büyük Menderes graben, in south. Although the surface structures, geological entities and tectonic regime are known quite well, geophysical studies in the region are limited; accordingly, information about deep continuation of the geological entities and deeper structures is limited. The main goal of this study is to obtain a model for the deep geoelectrical structure of the region then to explain it for the geology.

3. Data acquisition and processing The magnetotelluric method allows the determination of an electrical resistivity structure model from measurements of natural variations of the surface

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electric (E) and magnetic (H) fields over a wide frequency range (Kaufmann and Keller, 1981), usually from 10−4 to 103 Hz (in our case 0.04–500 s period range). The method is based on an inductive model of electromagnetic energy penetrating vertically downward into the earth, for which the depth of penetration is both a function of frequency (inverse of period) and ground resistivity. The amplitudes of the E and corresponding orthogonal H vectors of an electromagnetic field entering a uniform conducting half-space decrease by 1/e over a distance called the skin depth, δ = 503 (ρT)1/2 (m), where ρ the resistivity (inverse of conductivity σ ) ( m) and T the period (s). The magnetotelluric complex impedance Z is defined as the ratio E/H and an impedance tensor Z established from orthogonal and parallel components of E and H, such as


 Ex Zxx Zxy Hx = (1) Ey Zyx Zyy Hy which can vary with period, from sounding to sounding, and with orientation of x–y coordinates. Sounding curves (Fig. 3) were plotted against period, in which the complex impedance Zij is represented as an apparent resistivity and a phase. When the conductivity distribution varies only vertically, the earth model is said to be one-dimensional (1D) and the diagonal components of Z are null whereas Zxy = −Zyx = Z 1D . For this special case, the simple geology (where structure varies with depth only, as if horizontally layered) is assumed. Over a two-dimensional (2D) earth, elements Zxx and Zyy vanish if the x and y axes are aligned with and across strike, respectively (the principal directions). This constitutes a decoupling of the total fields into two independent modes (Swift, 1986): the transverse electric (TE, or E-polarization with Ex , Hy ) and the transverse magnetic (TM, or H-polarization with Hx , Ey ). In other words, the TE mode comprises electric fields (telluric currents) parallel to strike while the TM mode comprises electric fields (telluric currents) perpendicular to strike. In the natural 3D setting, all the components of Z are usually non-zero. The magnetotelluric soundings were performed at 20 sites along KS-line crossed the major geotectonic zones, using Geotronics system (USA). The line KS begins at the southern flank of the Thrace basin, in the

north, passes through the Biga peninsula and finally reaches Salihli, in the Gediz graben, on the south (Fig. 1). Two induction coils were used to record the horizontal magnetic field components (Hx and Hy ). Pb–PbCl2 electrodes were used as sensors to detect the horizontal electric field (Ex and Ey ). Dipole lengths are always taken about 100 m. Magnetotelluric data were obtained within the six overlapping frequency bands over the range 0.002–25 Hz (i.e. 0.04–500 s period range). Output from the electric and magnetic field sensors was fed immediately through high-gain analog amplification and band limiting, usually with custom-built electronics. The horizontal components of magnetic and electric field, Ex –Hx and Ey –Hy , were measured in the north–south and east–west directions, respectively. The all sites are selected to be as free artificial noise as possible. However, an appropriate analog filter was applied to the data to reduce the noise effect and eliminate aliasing. The data were processed in the Department of Geophysics, Istanbul Technical University (ITÜ). All data were discrete-Fourier transformed in the frequency domain and corrected for the system response function before the application of standard processing methods (Swift, 1986). Good quality data having high field coherences (∼0.9) were obtained. The elements of magnetotelluric impedance (Eq. (1)) that describe the conductivity structure beneath the measuring point are determined as a least square solution in desired band of frequency. Following this, to obtain the azimuth of the maximum and minimum resistivity directions we perform rotation of the impedance to minimize the diagonal elements Zxx and Zyy or to maximize Zxy and Zyx . In this way two magnetotelluric apparent resistivity and phase curves are obtained corresponding to the directions parallel (TE mode) and perpendicular (TM mode) to the tectonic strike. The apparent resistivities and phases show normalized scatter <7%. The examples of soundings are shown in Fig. 3. To assess the dimensions of the structures involved and possible influence of galvanic distortions, also basic magnetotelluric parameters were computed, specifically the principal directions, skew and main decompositions (Zhang et al., 1987). They indicate a rather 3D character of the structure and, in general, do not meet conditions for the application of a simple local/regional composite model.

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Fig. 3. Examples of apparent resistivity and phase curves obtained magnetotelluric soundings. The figure also shows the response of the 2D models presented in Fig. 4b, for both TE and TM independent inversions. Error bars are one S.D. (b) Rotation angles of TE mode at high periods.

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4. Results 4.1. Magnetotelluric data and pseudosections Fig. 3b shows the azimuths (as degrees; in clockwise from the north) of the maximum resistivity directions (TE mode) parallel to the tectonic strike, particularly at the high-period end (53–476 s), for all sites. The rotation angles at stations K1, K2, K3 and K4 were generally poorly systematic but can hardly be aligned along about N60◦ E. However, the rotation directions at stations K5–K14 are in stability and show an alignment along about N45◦ E within 10◦ . In view of the predominant northeast to southwest alignment of the geological features N45◦ E was assumed to be the strike direction of the region between stations K5 and K14. This also corresponds very well with the strike of the principal geotectonic structure in the Sakarya Zone that runs northeast to southwest (Akyüz and Okay, 1996). Fig. 4 shows the apparent resistivity pseudosections for both modes TE and TM. The apparent resistivity responses of 2D models for both TE and TM modes (will be explained by the following paragraph) are also presented in Fig. 4b. First, both pseudosections drawn from observed data for TE and TM mode (Fig. 4a) display a similar pattern, attesting the presence of a two major zone, each zone being characterized by the values of the apparent resistivity. Upper zone at periods lowers than 1 s has low-moderate resistivity values (commonly 10–80  m) while lower zone at periods higher than 1 s shows moderate-high resistivity values. The resistivity values 100–500  m and higher within lower zone can indicate mafic and crystalline rocks of compact property beneath the stations K1–K7 and K12–K20. Here an unexpected dyke-like conductive zone characterized by the apparent resistivity, ranging between 10 and 75  m appears at periods higher than 10 s in both modes pseudosections. It can be placed more widely in the TM mode. On the other hand, a resistive cap centered between the periods 1 and 10 s below K9 and K10 appears in the TE mode but not clearly in the TM mode. These could be explained by a probably macro-anisotropy because of the TM mode, where the electric field is orthogonal to the strike direction and the current flows across the structure is less affected by the three-dimensionality of the structure.

The upper zone at periods lower than 1 s in pseudosections for both modes (Fig. 4a) generally reflects complexity of the observed apparent resistivity distribution. Under K1–K4 a fairly conductive region indicating possible near-surface sedimentaries of the Istanbul and Ezine Zones (Fig. 6a) is detected in the TE mode pseudosection that has equivalent in the TM mode but relatively narrower than that in TE mode. The other four are found beneath K6–K7, K11–K12 and K14 within Sakarya Zone and beneath K15 and K18 within Bornova Zone. These seems to be less conductive in TM mode because of TM impedance is poor sensitive to near-surface conductive structures than the TE impedance (Berdichevsky et al., 1998). The pesudosection of TM mode also shows one conductive shallower large area around Gediz graben, beneath K19–K20 along with electrical resistive basement deepening towards southeast. It is also placed more thickly (up to 1 s period) in TE data. All of above features are also seen from the synthetic pseudosections (Fig. 4b; lower panel) calculated by 2D modelling. 4.2. Shallow structures inferred from dc geoelectric and gravity models In the frame of a research project designed by Mineral Research and Exploration Institute of Turkey (MTA) many regional dc geoelectric profiles were measured to investigate Neogene basins in western Anatolia during last 10 years. Here, two profiles parallel to KS line (Fig. 5a) are taken to explain small-scale features of the near-surface electrical resistivity structures in the some parts of our magnetotelluric line. Although the results of geoelectric and magnetotelluric surveys do not actually appear to be coherent due to several differences in the response of the method and some factors but the electrical resistivity characters of the subsurface rocks found by both methods reflect similar pattern. The dc geoelectric surveys cannot reach exploration depths greater than magnetotelluric method, but the resolution of shallow structures with dc geoelectric surveys is very good. Based on the two regional geoelectric profiles (Fig. 5a) where Schlumberger deep dc electrical soundings carried out, the depth to the Neogene sedimentary series (10–60  m) over the eastern region of the Edremit graben (profile GT; Figs. 2 and 5b) was found about 500–750 m (Sardar, ¸ 1995). This indicates

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Fig. 5. 2D dc geoelectric models and gravity profiles in the study area. (a) Locations map of dc Schlumberger resistivity sounding and gravity profiles near the magnetotelluric measuring sites. (b) Geoelectric structure along line GT obtained from 2D inversion (Loke and Barker, 1995). (c) Gravity profile and its interpretation result (Sari and Salk, ¸ 1995). (d) Geoelectric structure along profile LM obtained from 2D inversion. The thick solid lines in the geoelectric structure show faults. Resistivity data are from MTA.

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that the Edremit graben system dies out as a structural feature around north of Turanli. Fig. 5b and d show the results of the 2D resistivity inversion (Loke and Barker, 1995). Here, a conductive structure with resistivity range 10–75  m that is affected from the basin-fill within the Gediz graben can be easily followed to the depth up to 1.5 km (Fig. 5d). In Fig. 5d, on the south side of the graben the lower boundary of the alluvium (Quaternary deposits) dips to the north while on the north side of the graben it dips to the south (Seyito˘glu and Scott, 1991). The small-scale features of the resistivity distribution in 2D geoelectric models from Schlumberger soundings (Fig. 5b and d) are close to that of near-surface electrical structure in both modes (TE and TM modes) within the Sakarya and Bornova Zones (compare both Figs. 4 and 5 for Gediz graben and Sakarya Zone). Since the presences of fractures and fluid circulating with high temperature due to geothermal activity (Ilkı¸sık, 1995) within the basement rock (limestone) affected total resistivity, the high conductivity feature in the deeper levels of the Gediz graben therefore was occurred as seen from magnetotelluric pseudosections (Fig. 4). On the other hand, the Bouguer gravity anomaly over the line BA (Figs. 2 and 5c) is related with the uplifted Neogene basins and the sedimentary thickness is about 1.5 km and 250 m for Gediz and Gördes grabens, respectively (Fig. 5c; Sari and Salk, ¸ 1995). 4.3. Magnetotelluric modelling results and interpretations The magnetotelluric sounding curves (examples are shown in Fig. 3) were generally isotropic or quasi-isotropic in low period range (<1 s). But in high period range (>10 s), for the stations between K5 and K11, the sounding curves become strongly anisotropic. Moreover the anisotropy characteristics were not the same and change from northwest part to southeast part. However, due to the strong heterogeneity of the area and the distribution of sites only a profile, the data were modelled with two-dimensional (2D) modelling code. TM and TE modes of magnetotelluric data were inverted using the 2D inversion code of Mackie (Mackie et al., 1993, 1997). The inversion program finds regularized solutions to the 2D inverse problem for magnetotelluric data using Tikhonov method. The Tikhonov’s method

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defines a regularized solution of the inverse problem to be a model m that minimizes the objective function −1 S(m) = (d − F (m))T Rdd (d − F (m))

+τ ||L(m − m0 )||2 in which d is observed data vector, F the forward modelling operator, m the unknown model vector, Rdd the error covariance matrix, L a linear operator, m0 the apriori model and τ the regularization parameter. Each datum di is the logarithmic amplitude or phase of TE or TM complex apparent resistivity at a particular station and frequency. The model vector is logarithmic resistivity as a function of position, i.e. m(x) = log ρ(x). The inversion program uses the predicted impedances from the forward problem to modify the model parameters such that, over a number of iterations, the inversion will find a better set of model parameters that minimizes the objective function S above. The regularization parameter τ controls the trade-off between fitting the data and adhering to the model constraint. The value of τ should optimally be chosen, such that the root mean square (RMS) error for the inversion is between 1.0 and 1.5. The Mackie’s inversion (Mackie et al., 1997) was carried out on both TE and TM modes individually to obtain geoelectric models along the line KS. Fig. 6b presents the final models obtained from TE and TM data inversion using τ = 20, damping factor of 0.001 and error (noise) floors of 5% for ρ a and φ. The comparisons between the field data and the synthetic response of these models are resented as sounding curves in Fig. 3 for exampled sites and as calculated apparent resistivity pseudosections for both modes in Fig. 4b (lower panel in Fig. 4). The data fit is excellent for both models, as evidenced by Fig. 3. The pseudosections of the observed and calculated data in Fig. 4 (in upper and lower panels) are very similar. Hence, we can say that the calculated 2D models fit very well with the experimental (observed) data. Fig. 6a shows a geological cross-section crossing all geotectonic zones along north–south direction, which approximately runs sub-parallel to KS-line. Between the depths of 0 and 5 km, the 2D geoelectrical models (Fig. 6b) basically reflect both vertical and horizontal resistivity variations commonly consisted of low values. The low seismic velocity (∼ V p = 4500 and

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Fig. 6. Electrical resistivity structure beneath line KS and the geological cross-section. (a) The upper panel is a cross-section along the magnetotelluric profile derived from geological data. The partial melting within lower crustal zone beneath Kazda˘g range is proposed by this study. The layered crustal model found from the study of regional seismicity (Horasan and Canitez, 1995) is shown as a column in left side of the figure. Magnetotelluric station locations are presented approximately as projected along the cross-section line. (b) The lower panel shows the 2D electrical resistivity model. TM and TE resistivity models are obtained for independent 2D inversions (Tikhonov regularization; Mackie, 1997) on TM and TE data. For both cases, the inversion parameters τ and damping factor were set to 20 and 0.001, respectively and the error floors were set to 5%.

V s = 3000 m/s; Horasan and Canitez, 1995) may be an indicator of this region (left column of Fig. 6a). The non-uniformly distributions of the resistivity compartments occurred within this zone suggest lateral inhomogeneties caused by several faults or lateral changes of the formations. The irregular distribution of the low resistivity regions hardly corresponds to sediments. First, the region in 0–3 km depth range with low resis-

tivity (<80  m) compartments beneath K1–K4 are indicative of the sediments of Eocene age within the Istanbul and Ezine Zones. These Eocene sedimentary rocks cover the actual trace of the Intra-Pontide suture (Sengör ¸ and Yılmaz, 1981) and they have overlain more resistive units of the Istanbul and Ezine Zones. Low resistivity blocks within about 0–3.5 km depth range beneath K13–K14 and K17–K18 inferred by

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both models also similarly suggest sediments (flysh and karstified limestones) of the Bornova Zone. The other two relatively low resistive shallow regions appeared above 5 km depth level beneath K6–K7 and K10–K11 could be ascribed to the presence of sediments and underlying rocks called as the Karakaya complex. Here, the Karakaya complex consisted of limestone and shale was probably fractured, karstified and watered. These alterations principally control the bulk resistivity (Hyndman and Hyndman, 1968; Telford et al., 1976) and specific resistivity values therefore significantly decreased to lower values (here below 200  m) than in the case of massive limestone. Shale units within this complex also have a role decreasing the specific resistivity. The resistivity of near-surface sediments within the Sakarya Zone, at northwest Anatolia was also similarly found as low (about 90  m) from the interpretation of magnetotelluric line LG (Fig. 1b; Gürer, 1996). The compartments with relatively high resistivity value (80–200  m) beneath K5 and K15–K16 are diagnostic of the metamorphics and unfractured limestone blocks within Sakarya and Bornova Zones, respectively. The unfractured and water free limestone around Turanli (Fig. 2) was also identified as the shallow geoelectric basement in Schlumberger 2D geoelectric model (Fig. 5b). The low resistivity zone beneath sites K19–K20, which is thicker (about 5 km thick) than other shallow places in both models, shows the sedimentary filled basin of the Gediz graben and deeper basement rocks. Here, the resistivity (10–60  m) and seismic velocity (∼ V p = 4500 m/s; Horasan and Canitez, 1995) is quite low. Although more resistive basement locally detected by 2D Schlumberger geoelectrical model (Fig. 5d), the large-scale geoelectric model (Fig. 6b) reflects that the basement Neogene rocks seems to be affected by the high temperature and hydrothermal circulating and the resistivity is therefore decreased to very low levels. The region under K8–K9 is significant because of the higher resistivity observed in both modes. This is very clear signature of compact rocks in the Kazda˘g massif consisted of gneiss-amphibolite units (Okay et al., 1991, 1996), but its extension to deeper levels is uncertain. The models for both modes show a general increase in the resistivity values for depths greater than about 5 km beneath the sites K1–K5 and K12–K20. This

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preliminary qualitative feature was taken into account during the 2D modelling. The increasing resistivity with depth, where more resistive units (mafic and crystalline rocks) are present throughout the deeper regions of Ezine and Bornova Zones, also produces synthetic responses that are quite close to field data (Figs. 3 and 4b). The resistivity was significantly very different and greater to the south than to the north in deeper zone dipped to the northeast beneath K12–K13 (Fig. 6b). This suggests a gradual transition between two types of rock, the more resistive (≥1000  m) gneiss and schist of the Menderes massif being to the southeast and less resistive (<200  m) to the northwest. Such a major transition zone could be ascribed to the geotectonic boundary (can say as Izmir–Ankara suture zone) between the Sakarya and Bornova Zones (Figs. 2 and 6a; Akyüz and Okay, 1996). The fault traces between Bornova and Sakarya Zones in the southeast on the present topography suggest that the Izmir–Ankara suture zone dips at no less (greater) 45◦ close to the surface, consistent with those preserved in crystalline limestones farther west of Manisa (Fig. 2). These data suggest that the major faults have slightly listric profiles at the scale of the upper crust. A gradual decrease in gravity values towards the southeast was seen coincidently with this boundary (Fig. 1; Ate¸s et al., 1999). One highlight feature of the electrical resistivity model was the highly conductive dipped dye-like region starting at a depth of about 7 km to the lower crust and located below about K8–K9. The conductivity is increased to very low value (∼10  m) under K9 (TE model) and K7 (TM model). This surprisingly occurred zone was detected as more wide in TM model than in TE model because of the TM impedance probably more robust to three-dimensionality of the conductive structure (Berdichevsky et al., 1998). However a resistive cap centered at a depth of 3.5 km below K9 appears in the TE model but not clearly in the TM probably due to similar macro-anisotropy effect.

5. Discussions and conclusions The original motivation for this work was to present the results of regional deep resistivity structure along KS-line to contribute to the understanding of some

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distinctive geotectonic phenomena. Although the geological complexity may affect both detect ability and resolution, nevertheless in the present circumstance, the final result (Fig. 6b) provides a true 2D geoelectric model conforming to the geology. But uncertain points about geology in the form a depth-section could be firstly explained from the interpretation of this geoelectric model. The electrical resistivity structure of the both Sakarya and Bornova geotectonic zones depends on petrological and physical parameters. A cross-section along KS-line was constructed (Fig. 6a) in order to highlight the features of the electrical resistivity model. It was derived from known structural data and the lower crustal part beneath Kazda˘g range was proposed from following petrological interpretation of the conductive large region observed in the model. First, the maximum resistivity directions (TE mode-principal direction) for deep geoelectrical model are alignment along N45◦ E that is associated with the northeast to southwest trending of structural features as indicated by regional geology (Kaya, 1981; Okay et al., 1991). This could be revealed two-dimensionality of structure that is explained by the several anticlinales occurred with having an east–west trend in the area (Yılmaz, 1990; Okay et al., 1996). The suggested structural trend is also coherent with the direction of the main fault systems, grabens, structural ranges (i.e. Kazda˘g and Kozak ranges), synclines and sedimentary basins of the area (Bozkurt, 2001). As expected, the 2D models obtained from the inversion of TE and TM modes indicate that the near-surface resistivity (i.e. sedimentaries) along the line KS is low and has a total conductance (thickness–resistivity ratio) of about 2000 S. Based on these models the sediments is about 2.5–3 km thick in Gulf of Saros region (known as the Saros graben) which was found much thicker (5 km) from gravity modelling (Sari and Salk, ¸ 1995) and is thinner in the Sakarya Zone, towards the middle region of the Biga Peninsula. The sedimentary sequence of the Sakarya Zone, several kilometers thick, was always steeply dipped. The analysis of the residual gravity data suggests that the relief of basement rock was at about 3–3.5 km from surface, but towards the south generally unexceed to 2.5–3 km (Erol, 1978). It was then relatively thicker towards the southmost, in mid of

Gediz graben. Here, the typical tectono-sedimentary development of the Gediz graben provides valuable lessons on the influence of basinward migration of rift border faults on facies distribution and preservation potential (Seyito˘glu and Scott, 1991). On the other hand, the presence of fractures and fluids circulating within Karakaya complex (limestone and shale) affected and lowered resistivity. However, the deep geoelectrical data obtained at several geothermal areas of the earth generally pointed to the presence of conducting zone (Garcia, 1992). Considering the thermal regime around Biga Peninsula and its south (high heat flow; Tezcan, 1995), the low resistivity of the Karakaya complex was could be associated with a major zone that transfers hydrothermal fluids to the several well-known geothermal areas (such as Gönen, Dikili, Tuzla) in the region. High heat flow values calculated from the chemistry of hot springs in the Biga Peninsula (Ilkı¸sık, 1995) support the idea of heat convection and fluid circulation. Additonally to this, fractured zones a few kilometres wide in the crustal metamorphic rocks (Kaya, 1982) beneath the Kazda˘g range within this depth range may cause low resistivity. The most interesting result of the electrical resistivity models in both TE and TM modes is the existence of a large conductive zone within mid-crust beneath Kazda˘g range in the Sakarya Zone. Both models show two geoelectric patterns completely different for this conductive zone, which is wider for TM data (Fig. 6b; upper panel). The sounding curves (Fig. 3) also accordingly show strong anisotropy character that changed from northwest part to southeast part of the magnetotelluric line. This explains why the both TE and TM models does not give similar width for this conductive zone. The structure of the conductive zone is not only heterogeneous but also anisotropic, of the macro-anisotropy type, which is common for metamorphic rocks in orogenic regions. Nevertheless, when the penetration depth of the electromagnetic field approaches the crystalline basement, the macroscopic anisotropy of the basement relief, i.e. the effect of the inhomogeneity, appears in the magnetotelluric data (Adam et al., 1986). As a consequence of the thin sediment layer on the surface (e.g. around the Sakarya Zone), the deeper crustal electric inhomogeneities appear more pronounced in the MT data. A macro-anisotropy character was also suggested for

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the structure of Sakarya Zone from the Mohr analysis of magnetotelluric data collected in the northwestern Anatolia (Bayrak et al., 2000). A complex suite of positive Bouguer gravity anomalies (Fig. 1) reflecting non-uniform distribution of the rocks with high density and indicating their significant vertical extents in Biga Peninsula within the Sakarya Zone could be correlated with the origin of the macroanisotropy. Physical and chemical mechanisms lowering the electrical resistivity are a matter of discussion (Hyndman and Hyndman, 1968; Schwarz, 1990). As a possible explanation, rock melts connected with high-grade metamorphism is considered for this lowered electrical resistivity. The uppermost crust between 0 and 5 km depths generally has low and normal resistivities, which should be related to sedimentary layers of wellresolved thickness by geoelectric models. But the middle and lower crust that surprisingly show an electrically conductive zone need for explain. What can be the origin of this lowered electrical resistivity? The tectonic evolution of the Sakarya Zone has been accompanied by a high-grade magmatism in latest Oligocene time beneath Kazda˘g range, around K7–K9 (Okay and Satır, 2000; Yılmaz et al., 2001). Based on the interpretation of structural, petrological and isotopic data this magmatic event (coeval plutonism and metamorphism) was considered as a main geodynamic origin of the north–south extension regime for the region. A metamorphic core complex of latest Oligocene age crops out in the Kazda˘g range (Fig. 6a) and surroundings during the period of this event. The footwall of the core complex consisted of gneiss, amphibolite and marble metamorphosed at 5 ± 1 kbar and 640 ± 50◦ C (Okay and Satır, 2000). The high-grade metamorphic rocks were then rapidly exhumed from a depth of approximately 14–7 km that effectively caused an electrically conducting zone beneath Kazda˘g range (Fig. 6). High heat values found around Biga Peninsula also support the idea heat source. Without further knowledge of geophysical structure one might easily conclude that partial melts may be responsible for this conducting zone observed. There exists a relation between high electrical conductivity and seismic velocities, an increase in temperature (here at least 600◦ C) and in fluid content within a highly fractured crust at high pore pressure

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(here ∼5 ± 1 kbar) will lower seismic velocity (V p = 5400 m/s in Fig. 6a) as well as electrical resistivity. Electrical resistivity is very sensitive to a wide range of petrological and physical parameters, e.g. to fluids, volatiles and termo-metamorphism, making magnetotelluric method a powerful tool in crust investigation. Such anomalous conductivity structures caused partial melting were found in upper crust at southern Tibet with average extent of 10–20 km, northwestern Himalayas with its to a depth of 15 km and on rocky mountains (North America) with its top 5 km below the surface using magnetotelluric method (Schwarz, 1990). Considering the large-scale structural feature, this tectonic zone observed in the 2D electrical resistivity model may be having an origin in the plate tectonics. As a final message of this paper, we can therefore conclude that the high conductivity within the upper crust is a significant signature related to north–south extensional regime of the region.

Acknowledgements I thank Dr. Randall Mackie for many helpful comments and assistance in carrying out 2D inversion of the magnetotelluric data. I also thank Professor Aral Okay for discussions on the tectonic implications of the results and Hidir Aygül, and Burak Tunçata who participated in the fieldwork. The author indebted to Mineral Research and Exploration Directorate of Turkey (MTA) for permission to use the Schlumberger resistivity sounding data. The author also wishes to thank the anonymous referees for their critical reviews and useful comments on the paper. This study was supported by the Scientific and Technical Council of Turkey (TUBITAK) grants YDABCAG-022/G and YDABÇAG-100Y012.

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