Surv Geophys DOI 10.1007/s10712-012-9195-x
Crustal Structure of Turkey from Aeromagnetic, Gravity and Deep Seismic Reflection Data Abdullah Ates • Funda Bilim • Aydin Buyuksarac • Attila Aydemir Ozcan Bektas • Yasemin Aslan
•
Received: 5 October 2011 / Accepted: 2 May 2012 Ó Springer Science+Business Media B.V. 2012
Abstract In this paper, aeromagnetic and gravity anomalies obtained from the General Directorate of Mineral Research and Exploration were subjected to upward continuation to 3 km from the ground surface to suppress shallow effects and to expose only regional, deep sources. Then, a reduction to pole (RTP) map of aeromagnetic anomalies was produced from the 3 km upward continued data. A sinuous boundary to the south of Turkey is observed in the RTP map that may indicate the suture zone between the Anatolides and African/Arabian Plates in the closure time of the Tethys Ocean. The sinuous boundary can be correlated with the recent palaeo-tectonic maps. The southern part of the sinuous boundary is quite different and less magnetic in comparison with the northern block. In addition, maxspots maps of the aeromagnetic and gravity anomalies were produced to find out and enhance the boundaries of tectonic units. Crustal thickness, recently calculated and mapped for the western Turkey, is also extended to the whole of Turkey, and the crustal thicknesses are correlated with the previous seismological findings and deep seismic sections. The average crustal thickness calculations using the gravity data are about 28 km along the coastal regions and increase up to 42 km through the Iranian border in the east of Turkey. Density and susceptibility values used as parameters for construction of two-dimensional (2D) gravity and magnetic models A. Ates Department of Geophysical Engineering, Engineering Faculty, Ankara University, Besevler, 06100 Ankara, Turkey F. Bilim O. Bektas Department of Geophysical Engineering, Engineering Faculty, Cumhuriyet University, 58140 Sivas, Turkey A. Buyuksarac Department of Geophysical Engineering, Canakkale Onsekiz Mart University, 17100 Canakkale, Turkey A. Aydemir (&) Turkish Petroleum Corp., Sogutozu Mah., 2180. Cad., No: 86, Sogutozu, 06100 Ankara, Turkey e-mail:
[email protected] Y. Aslan Geological Engineering Department, Engineering Faculty, Fırat University, 23119 Elazig, Turkey
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were compiled in a table from different localities of Turkey. 2D models indicate that all of the anomalous masses are located in the upper crust, and this could be well correlated with the earthquakes which occurred at shallow depths. Keywords thickness
Upward continuation Reduction to the pole Maxspots Turkey Crustal
1 Introduction Turkey is mainly divided into three main tectonic units. These are as follows: (i) the Pontides, (ii) the Anatolides-Taurides and (iii) the northern part of the Arabian Plate. The Pontides to the north and Anatolides-Taurides to the south are separated from each other by the Izmir-Ankara-Erzincan suture that was formed by the closure of the northern branch of the Neo-Tethys Ocean. The northern part of the Arabian Plate at the south-east of Turkey was separated from the Anatolides-Taurides by the southern branch of the Neo-Tethys Ocean during the Mesozoic and Tertiary times (Sengor and Yilmaz 1981). Geological maps of Turkey exhibit complexities, and these maps can be accessed at the official web site of the General Directorate of Mineral Research and Exploration-MTA ( www.mta.gov.tr). Large areas are covered by the younger cover units and sedimentary formations from Palaeozoic to Mesozoic. Volcanic, granitic and mafic–ultramafic rocks can also be observed from the west to east of the country. The gravity and aeromagnetic data (with 0.6 km flight altitude above the ground surface) of Turkey were acquired by MTA, and maps of them were opened to the public interest in the formal website of MTA. The gravity and aeromagnetic anomalies were published by Ates et al. (1999) where the general characteristics and corrections applied to the surveyed data were described. The grid interval of the anomaly maps is 10 9 10 km. Sensitivity and resolution in these maps are suitable for the regional studies and palaeotectonic features of Turkey (Moix et al. 2008; Gans et al. 2009) are consistent with the aeromagnetic and gravity anomalies. There is no publication in the international earth science literature about the regional tectonics of whole Turkey using the anomalies of the potential field data. There are individual studies on the subsurface and tectonic structures of the western, central and eastern parts of Anatolia using the same potential field data with more frequent sampling intervals. Most of the aeromagnetic and gravity anomalies are studied and interpreted by Ates et al. (2005), Aydemir and Ates (2006a, b), Bektas et al. (2007), Bilim (2007, 2011), Buyuksarac et al. (2005) and Onal et al. (2008). In these publications, crustal structure and geothermal energy potential of Turkey were also discussed. In addition to these studies, micro-block rotations were also estimated by analysing the aeromagnetic anomalies (i.e. Bilim and Ates 1999, 2004, 2007). In general, the aeromagnetic anomalies indicate little or no correlation with the surface geology. It is thought that structures creating aeromagnetic anomalies are buried and only a correlation may be found with the palaeo-sutures of Turkey. In order to observe apparent anomalies created by deep-seated causative bodies and suppress the effect of the shallow bodies, aeromagnetic and gravity anomalies were upward continued to 3 km above the ground surface. The reduction to pole (RTP) transformation was applied onto the 3 km upward continued data in this study. A sinuous boundary to the south of Turkey points out the suture of Tethys Ocean and resembles to the palaeotectonic map of Moix et al. (2008). Location and boundaries of the Kirsehir Block were well correlated with the seismological findings of Gans et al. (2009).
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The crustal thickness map of Turkey was produced from the gravity anomalies by using an empirical equation, and the crustal thickness map was correlated with the MOHO depths estimated from the seismic sections and previous seismological investigations that are apparently consistent with the crustal thickness map, but there are some differences in the Eastern Anatolia. The thicker hinterland area thinning towards the shoreline in the crustal thickness map resembles Greenland’s crustal structure (Artemieva and Thybo 2008; Storetvedt and Longhinos 2011). 2-dimensional (2D) magnetic and gravity models were constructed along a profile (about 500 km long) in central Anatolia by the help of density and susceptibility contrasts. These models were constructed using the data before upward continuation. 2D models indicate that all masses causing anomalies are in the upper crust (i.e. bottom of the anomalous masses are 5–6 km deep from the surface). These findings imply that active tectonics of Turkey is shallow and all destructive earthquakes occur at relatively shallow depths, and this is also verified by the focal mechanism solutions.
2 Aeromagnetic Data and Interpretation Recent palaeo-tectonic reconstruction of the Turkish territory was achieved by Moix et al. (2008). Modified Palaeo-tectonic map of them representing Permo-Trias through Cenozoic is given in Fig. 1. Moix et al. (2008) showed that the Turkey was divided into two areas in SW and NE, and then these two regions were assembled in the recent geological history. This division is well illustrated with the names of several locations: Beydaglari (Bd), Menderes (Mn) and Geyikdagi-Anamas-Akseki (Gd) in the south, and Sakarya (Sk), Istanbul (Is), Zonguldak (Zo) and east-Pontides (eP) to the north (Fig. 1). Curvature of the line in the middle is the sinuous boundary between the southern and northern Turkey that will be illustrated in the following aeromagnetic anomaly and RTP maps produced within this paper. It is well known that the interpretation of aeromagnetic anomalies is quite difficult, because the Earth’s magnetic field and body magnetizations may cause disorientations on polarities of the magnetic anomalies. These disorientations can be removed from the anomalies by the reduction to pole (RTP) transformation, and details of the RTP transformation method in the Fourier domain are given by Blakely (1996). RTP transformation correcting the polarity disorientations was applied onto the 3 km upward continued aeromagnetic anomalies (including the flight height of 0.6 km), because the surface effects are well suppressed in the 3 km upward continued aeromagnetic map (Fig. 2). In RTP map of Turkey (Fig. 3), it was observed that most of the polarities were corrected successfully and aligned in the north–south direction, indicating that the remanent magnetization is not existent in the west and the central part of Anatolia, in contrast to the east where polarities of the anomalies are different from the north–south direction. This situation represents the existence of strong remanent magnetization, and anomalies remain very complex even after RTP correction was applied. The sinuous boundary is evident in the RTP map. This boundary could be correlated with the Izmir-Ankara-Erzincan suture in the west and Inner Taurid suture in central and eastern Anatolia (Sengor and Yilmaz 1981). Intense magnetic anomalies are located to the north of this boundary while the magnetic anomalies are generally weak or causative bodies are deep seated in the south (Ates et al. 1999). A method was developed by Blakely and Simpson (1986) to identify maxima on a contoured horizontal gradient magnitudes of magnetic or gravity anomaly data. The horizontal gradient data are available on a rectangular grid, and each grid node is compared
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Fig. 1 a Palaeotectonic map of Turkey (modified from Moix et al. 2008). Arrows indicate the direction of movements. Bd Beydaglari, Mn Menderes, Gd Geyikdagi-Anamas-Akseki, Sk Sakarya, Is Istanbul, Zo Zonguldak, eP east-Pontides, WBS Western Black Sea, EBS Eastern Black Sea, MP Mersin-Pozanti ophiolites, Ay Antalya, Tp Trodos, Kb Karaburun, El Elazig-Guleman ophiolites. b Geological map of Turkey (simplified from Bingo¨l 1989). Mafic–Ultramafic rocks generally coincide with ophiolites
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42 Sea
LATITUDE (Degree)
Black
o
200 km
36 Mediterranean
Sea o
o
45
27
LONGITUDE (Degree) nT
Fig. 2 Upward continued (3 km) aeromagnetic anomaly maps of Turkey (600 m flight altitude is included to the upward continuation) o
42 Sea
LATITUDE (Degree)
Black
SB
SB
o
200 km
36 Mediterranean
Sea o
o
45
27
LONGITUDE (Degree) nT
Fig. 3 Reduction to the pole (RTP) transformation of the 3 km upward continued aeromagnetic anomalies (600 m flight altitude is included to the upward continuation). Contour interval: 50 nT. SB sinuous boundary. Inclination and declination angles of the geomagnetic field are 55° and 4°, respectively
with its nearest eight neighbours in four directions along the row, column and both diagonals to inspect whether a maximum is present. The aim of this method is to produce a plan view of inferred boundaries of magnetic or gravity sources. The steepest gradient will be located directly over the edge of the body if the edge is vertical and far removed from all other edges or sources (Blakely 1996). This method was applied to the original
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Fig. 4 Maxima locations determined for the horizontal gradient of the pseudogravity anomalies of aeromagnetic data. Sizes of circles are proportional to the amplitude of maxima. (1) Biga Peninsula Anomaly (BPA), (2) North Anatolian Fault (NAF), (3) Suluklu-Cihanbeyli-Goloren (SCG) Anomaly, (4) Cankiri-Corum Ophiolitic Complex (CCOC), (5) Cappadocia Volcanic Complex (CVC), (6) HatayIskenderun Anomaly (HIA), (7) Ecemis Fault (EF), (8) Bitlis-Poturge Massif (BPM), (9) Van Lake Anomaly (VLA)
aeromagnetic anomalies given in Ates et al. (1999) including shallow sources. Maxspots are displayed as circles, and all circle sizes are assigned to the magnitudes of the horizontal gradients. Locations of maximum horizontal gradient of the pseudogravity from the aeromagnetic anomalies in Fig. 4 display significant alignments numbered from 1 to 9 that are given in the figure caption. In addition, the southern boundary of Eastern Pontides is also represented by the high maxima amplitudes to the NE of Turkey (parallel to the Eastern Black Sea shoreline).
3 Gravity Anomalies and Interpretation Upward continued (3 km) gravity anomaly map of Turkey is given in Fig. 5. Maxspots of the horizontal gradients of the original Bouguer anomalies were also calculated in this study. Locations of maximum horizontal gradient of the gravity anomalies in Fig. 6 display significant alignments numbered from 1 to 9 that are given in the figure caption. In addition, the boundary of the Miocene Adana Basin is also clear to the SE of the Tuzgolu Basin (to the NE corner of the eastern Mediterranean shoreline). Gravity anomalies can be used for determination of the MOHO depth. There are different empirical equations proposed by different authors (i.e. Riad et al. 1981; Riad and El Etr 1985; Rivero et al. 2002; Tirel et al. 2004) to calculate crustal thicknesses from the gravity anomalies. Most optimum relation for the crustal thickness of Turkey is the equation given by Riad et al. (1981) as follows: H ¼ 29:98 0:075Dg where H is the crustal thickness in kilometres, and Dg is the gravity anomaly values in mGal. Initial crustal thickness map calculated with the same equation was given by Buyuksarac et al. (2009). In their study, thicknesses were calculated by using gravity data,
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42 Sea
LATITUDE (Degree)
Black
o
200 km
36 Mediterranean
Sea o
o
45
27
LONGITUDE (Degree) mgal
Fig. 5 Upward continued (3 km) gravity anomaly map of Turkey
Fig. 6 Maxima locations determined for the horizontal gradient of the gravity anomalies. Sizes of circles are proportional to the amplitude of maxima. (1) Aegean Region Grabens (ARG), (2) Isparta Angle (IA), (3) North Anatolian Fault (NAF), (4) Konya Anomaly (KA), (5) Tuzgolu (Salt Lake) Basin, (6) Sivas Basin, (7) South-eastern palaeo-highs and, related fold and thrust belts, (8) North East Anatolian Fault (NEAF), (9) Van Lake Anomaly (VLA)
and there were some fluctuations, especially in the eastern and central Anatolia. This was caused by high frequencies of near surface bodies or noise in the gravity data. In this study, the original gravity anomaly map given in Ates et al. (1999) was filtered using various cutoff values until obtaining similar amplitude of 3 km upward continued gravity anomaly map given in Fig. 5. In order to estimate the top depth of the causative bodies and determine the cut-off frequency for the low-pass filter, power spectrum analysis was
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Fig. 7 Low-pass filtered gravity anomaly map (Cut-off frequency: 0.06 radians/km)
applied onto the gravity data by using the method of Spector and Grant (1970). This method is well established and used in many earth science applications (Dobrin and Savit 1988; Telford et al. 1990; Kearey et al. 2002). The low-pass filtering process was applied before the calculation of the crustal thickness in this study. The cut-off frequency was selected as 0.06 radian/km. Deep-seated anomalies are enhanced in the low-pass filtered map (Fig. 7), and shallow effects were suppressed. Filtered gravity anomaly map is more complex than the upward continued gravity anomaly map. Reasons and examples of complexity in the filtering process were given by Kearey (1991) and Buyuksarac et al. (2005). The crustal thickness map produced from the low-pass filtered gravity anomalies of Turkey is given in Fig. 8, and this map indicates smooth and more accurate results. It is possible to interpret the crustal thickness map of Turkey dividing into three regions. In the west, crustal thickness from the shorelines to the hinterland, especially in the Aegean Grabens varies from 26 to 34 km, respectively. In the central part, the crustal thicknesses from the Black Sea to Mediterranean region vary from 28 km for coastal areas and 34–38 km inland (33–35 km in average). In the east, the thickness from the Black Sea to the Arabian Block varies between 30 and 33 km, and it increases up to 43 km towards the border between Turkey and Iran. MOHO depth was calculated using analysis of earthquake arrival times in Anatolia by Necioglu et al. (1981) for the first time, and they found the crustal thickness was about 25–32 km in western Turkey (Table 1). Saunders et al. (1998) calculated 30–34 km thickness in the western Turkey by using receiver functions. In central Turkey (Anatolian Plateau), they calculated the thickness about 38 km. Findings of Saunders et al. (1998) were supported by Tezel et al. (2007) who found thickness about 25–40 km from western to the eastern Turkey by using surface wave dispersion analysis (Table 1). Becel et al. (2009) estimated the MOHO interface at about 26 km deep under the Northern Marmara Trough by modelling of reversed Pn waves. These results are quite consistent with the crustal thickness (about 32.5 km) calculated using gravity anomalies by Bilim (2007) in western Turkey. Angus et al. (2006) proposed the crustal thickness varying from 30 to 55 km from western to eastern Turkey, although either of the two sketches prepared by them does not show MOHO depth deeper than 50 km. Thickness of 30 km correlates well with the near Black Sea coast in Fig. 6 of their study, but 55 km thickness does not seem to
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Fig. 8 Crustal thickness (CT) map of Turkey calculated from the low-pass filtered gravity anomalies shown in Fig. 7. Contour interval: 1 km
be consistent with our crustal thickness map (Table 1). Recently, Arslan et al. (2010) investigated the crustal structure of Turkey using the gravity data, and they found the shallowest crust as 31.4 km and the deepest crust about 50 km around the Iranian border. These depths are higher than depths presented in this study and previous seismological calculations. This result was possibly caused by the use of unfiltered data. More recently, Cakir and Erduran (2011) calculated 38 km crustal thickness from the P- and S-wave receiver function. They also calculated a depth of 350 km down to the Lower Mantle Low Velocity Zone in central Turkey. Deep seismic data with recording time up to 16 s were acquired near the Lake Tuzgolu (Salt Lake) in central Anatolia by the Turkish Petroleum Corp. (TPAO) in 1987. Inline 5 vibrators were used to make a source pattern. All measures were taken to improve the signals coming from the deep reflectors during recording and processing steps. However, the quality of the seismic sections was not good due to geological conditions and existing thick salt layers in near surface. Despite all difficulties, one of the seismic sections located to the south-east of the Lake Tuzgolu was used to correlate with the results obtained in this study. The location of that deep seismic section and the Profile AA0 that 2D magnetic and gravity models are constructed and are shown in the aeromagnetic and gravity anomaly maps of the Central Anatolia (Fig. 9a and b, respectively). Several horizons were observed in this seismic section (Fig. 10a). Windowed parts between 9 and 11 s numbered from 1 to 3 were enlarged and interpreted in detail. In these windows, traces indicating a set of horizons were marked with arrows (Fig. 10b–d). The RMS velocities at these shot points are around 6–7 km/s. These velocities correspond to 30–35 km in depth. These depths are well consistent with the crustal thickness map of central Anatolia (Fig. 8) produced from the gravity anomalies and seismological MOHO depth calculations given in Table 1. In order to create crustal models of Anatolia in N–S direction, 2D models were constructed on the aeromagnetic and gravity data. The location of 2D magnetic and gravity models (Profile AA0 ) is illustrated in the Central Anatolian aeromagnetic and gravity
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Surv Geophys Table 1 Significant seismological MOHO depth calculations in different parts of Turkey Region/location
Crustal thickness/ MOHO depth (km)
Method
References
Western Turkey
20–35
Receiver function
Tezel et al. (2010)
25–32
Seismic wave velocity
Necioglu et al. (1981)
Western Turkey
30–34
Receiver functions
Saunders et al. (1998)
Central Turkey
38
Northern Marmara Trough
26
Seismic wave velocity
Becel et al. (2009)
Eastern Turkey
30–55
S-wave receiver function
Angus et al. (2006)
Arabian Platform
36
Seismological records
Gok et al. (2007)
Anatolian Block
44
Anatolian Plato
48
NE of Anatolian Block
30–38
From west to east of Turkey
25–40
Surface wave dispersion analysis
Tezel et al. (2007)
From west to east of Turkey
31–50
Gravity data
Arslan et al. (2010)
Central Anatolia
38
P and S-wave receiver function
Cakir and Erduran (2011)
Fig. 9 a Aeromagnetic anomalies of central Anatolia. AA0 shows the location of the profile to be modelled for 2-dimensional gravity and magnetic structures. The location of the deep seismic section GTRS-87-802 is also illustrated. b Gravity anomalies of central Anatolia. AA0 shows the location of the profile to be modelled for 2-dimensional gravity and magnetic structures. The location of the deep seismic section GTRS-87-802 is also illustrated
anomaly maps (Fig. 9a, b). Available density and susceptibility data compiled for whole Turkey (given in Table 2) were used as parameters to construct these models, the length of the profile is over 500 km. Bodies causing magnetic anomalies are generally dike-shaped
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Fig. 10 a Complete view of the seismic section. Location of the seismic section is illustrated in Fig. 9a and b. Annotated rectangles show the detailed investigated regions: b Enlarged view of part a shown in (a), (c) Enlarged view of part b shown in (a), (d) Enlarged view of part c shown in (a)
structures (Aydemir and Ates 2006a), while the bodies creating gravity anomalies are sedimentary basins (Aydemir 2008, 2011; Aydemir and Ates 2006b), and all causative bodies are situated in the upper crust (Figs. 11a, b). In construction of 2D magnetic model, anomalous bodies are considered having susceptibility values in the range of the highest (0.0879 SI = 0.007 cgs) and the lowest values (0.0212 SI = 0.0017 cgs) according to the data available in the north-east of Tuzgolu (Salt Lake) and Cappadocia regions, respectively (Table 2). Average densities of Andesite and Basaltic rocks and Gabbro around Cappadocia and north-northeast of the Tuzgolu (Salt Lake) were used to construct 2D gravity model (Table 2). Then, a mean density of 2.93 g/cm3 was obtained for these lithologies. Previously, densities of the sedimentary formations were obtained in the range of 2.23–2.43 g/cm3. Thus, density contrasts of 0.7, 0.5 and 0.6 g/cm3 were used for the sedimentary basins from the south to the north. Density contrast of 0.5 g/cm3 was also justified by means of 3D modelling of the Konya Anomaly by Ates and Kearey (2000).
4 Discussions and Conclusions There are a few references using the potential field data in the geosciences literature about the tectonic structure of Turkey. Bilim (2007) investigated the tectonic and structural
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Surv Geophys Table 2 Density and susceptibility values of significant rocks in Turkey Location
Rock type
Density (g cm-3)
Susceptibility 9 103 SI
North-east of Salt Lake
Gabbro
3.02
87.9
13
Granit
2.71
6.3
11
Sandstone
2.72
1.0
14
Andesites
2.9
21.2
115
Cappadocia
Sample numbers
Basalts North of Salt Lake
Andesites
2.63
–
6
Gabbro
3.07
–
13
Inner East (Sivas Basin)
Ofiolites
2.80
3.0
3
Sandstones
2.47
0.0
3
Gypsum
2.35
0.01
3
References
Ates and Kearey (2000) Buyuksarac et al. (2005) Ozturk (1997) Onal et al. (2008)
alignments of central-western Anatolia using gravity and magnetic data. In that study, the CPD (Curie Point Depth) map of Kutahya-Denizli and surrounding area was constructed. Similarly, CPD map was exposed by using aeromagnetic anomalies in central Anatolia by Ates et al. (2005). It was shown that Curie Depth (580°C) descends down to 8 km in the central Anatolia. Buyuksarac et al. (2005) and Buyuksarac (2007) evaluated the potential field data of Cappadocia and central eastern Anatolia, respectively. Aeromagnetic anomalies of both regions are complex, and block rotations were determined in the Cappadocia Region according to the palaeomagnetic analyses. Buyuksarac et al. (2005) suggested that an uplifted magmatic intrusion created the widespread volcanic activity in the Cappadocia region. Genc and Yurur (2010) suggested that this uplift could not only be explained by a magmatic intrusion, but there should have been a hot and low-density asthenospheric material emplacement. If it is correct, this asthenospheric uplift may be a hot spot by definition as given by Kearey et al. (2009). Tectonic trends of central eastern Anatolia are aligned in the similar directions with the East Anatolian Fault Zone. East–west disorientations in the deep-sourced aeromagnetic anomalies of the Cappadocian region are determined at 30° in the counter clockwise direction in central Anatolia (Buyuksarac et al. 2005) which is consistent with the mobilistic system (Storetvedt 2003). The mobilistic theory was also supported with the palaeomagnetic data (Gursoy et al. 1997, 1998; Piper et al. 2010) and GPS measurements (McClusky et al. 2000). Bilim and Ates (2007) investigated the effect of remanent magnetization and the rotations of geologic causative bodies in the northern central Anatolia with the application of a new method developed by them. Their method depends on the correlation between the analytic signal of magnetic anomalies and the horizontal gradient of pseudogravity data using correlation coefficients. They found that almost all anomalies include remanent magnetization and proposed the existence of rotating micro plates in a gear mechanism. The most obvious, linear aeromagnetic anomaly in central Turkey which is named as the Suluklu-Cihanbeyli-Goloren Anomaly (Aydemir and Ates 2006a) extends in NW–SE direction along the western margins of the Tuzgolu (Salt Lake) and Haymana Basins (Aydemir 2008, 2011). It was modelled in 2D with a proposal of a geological model that explains tectonic setting and evolution of a causative magmatic intrusion (Aydemir and Ates 2006a).
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Fig. 11 a Magnetic profile AA0 shown in Fig. 9a with interpreted model, b Gravity profile AA0 shown in Fig. 9b with interpreted model. Numbers indicate susceptibility and density contrast of the anomalous bodies
Bektas et al. (2007) investigated regional geothermal characterization of East Anatolia using aeromagnetic, gravity and heat-flow data where the topography and crustal thickness increase from the west to the east. It was suggested that topographically higher regions can be correlated with the aeromagnetic anomalies because of the magnetic nature of the volcanic rocks on the surface. Low-pass filtering was applied onto the aeromagnetic anomalies to remove topographical effects. They also calculated CPD values from the lowpass filtered aeromagnetic anomalies and applied total gradient method to the high-pass filtered anomalies. They suggested that high-amplitude total gradient anomalies extended from the Black Sea shoreline through the north of Erzincan and down to the west of the Van Lake may be correlated with the ophiolitic rocks on the surface, but are not correlated with the locations of the hot springs and volcanic formations. In a similar way, Bilim
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(2011) studied the Galatien volcanic complex in the northern central Turkey, recently. She studied the thermal structure of the Galatien volcanic complex and found low CPD-high heat-flow values in the region. In conclusion, aeromagnetic anomalies of Turkey are quite complex, and they cannot be correlated with the surface geology implying that causative sources are deep seated. Aeromagnetic anomalies thus, first upward continued and then processed with the tools like RTP (Fig. 3), display a good correlation with the sinuous boundary in the palaeoreconstruction map of Turkey (Fig. 1). Investigations of the aeromagnetic anomalies in the western Anatolia are very useful to indicate shallow heat-flow transfer zones caused by the segmented African lithosphere beneath the Anatolian region as inferred from the teleseismic P-wave tomography (Biryol et al. 2011). In the eastern Turkey, shallow heat-flow sources are related with the individual young volcanic masses. Earthquake data are well correlated with the aeromagnetic anomalies and earthquakes with magnitudes bigger than 7.0 which are located near the major faults in NW Turkey (Ates et al. 2008; 2009). We suggest that the aeromagnetic anomalies of Turkey were shaped during the palaeo-suture occurrence. Regional gravity anomalies reflect density variations of subsurface structures. In general, contour values decrease from west to the east in Turkey. Most of the crustal structures are well correlated with the previous, detailed gravity and magnetic investigations, supporting earlier findings of hydrocarbon exploration studies. Sedimentary basins are clearly apparent. Particularly, central Anatolian (Aydemir 2008, 2011; Aydemir and Ates 2006b) and Thrace Basins (Demir et al. 2012) are outstanding basins. This geophysical evidence could be well correlated and support the previous geological findings (Sengor and Yilmaz 1981). Spatial correlation was performed with the seismological study of Gans et al. (2009). For instance, approximate boundary of the Kirsehir Block (massif) was exposed clearly. However, most of the apparent anomalies were removed in the low-pass filtered map (Fig. 7) that was obtained by using 0.06 radian/km cut-off frequency. Smooth lowpass filter map indicates that most of the causative bodies in Turkey are shallow and they are not related with the deep-seated bodies. This result is considered that causative bodies are generally young and produced from the active tectonics throughout the geological history. Crustal thickness map calculated from the gravity anomalies indicates that the crust is thicker in Turkey in comparison with the normal crustal thickness (about 30 km). The crustal thickness map was well correlated with the previous seismological MOHO depth calculations. Furthermore, MOHO depth of 35 km obtained from a deep seismic section in central Turkey can also be well correlated with the crustal thickness map. In general, shorelines of Anatolian Peninsula and the European part of Turkey are thinner than the hinterland. This situation is similar to the example of Greenland. In that region, the thick continental crust thins through shorelines where it is delaminated by the thin oceanic crust on three sides (Labrador Sea/Baffin Bay and North Atlantic) (Artemieva and Thybo 2008; Storetvedt and Longhinos 2011). It is clear that there are two magnetised domains in aeromagnetic anomalies. One of them is in the north with strong anomalies, the other one is in the south with subdued anomalies. This partition gives a good consistency and is well correlated with the latest palaeotectonic map of Moix et al. (2008) who define a sinuous suture in the middle of Turkey along the east–west direction. This tectonic trend is in line with the GPS velocity study of McClusky et al. (2000). Reilinger et al. (2006) also showed counterclockwise tectonic swing from Nubia (Africa) via Arabia, Iran, along Turkey, before ending in a south-directed tectonic front along the Aegean Arc. Those observations are consistent with the east–west regional mobilistic system of Storetvedt (2003). Inertia-driven Alpine
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lithospheric rotations (counterclockwise for Africa and clockwise for Eurasia) turn the intervening tectonic belt into an overall transpressive belt. Apparently, this produced an E-W shear along Turkey. Palaeomagnetic evidence for this system was also outlined by Storetvedt (1990). In addition, the counterclockwise rotation of Africa is also demonstrated by the marked swing in the Pelusium Megashear System across Africa (Neev et al. 1982). Maximum horizontal gradient method (Blakely and Simpson 1986) was applied to the aeromagnetic and gravity anomalies, and the maxspots maps were obtained. In aeromagnetic maxspots map, the Suluklu-Cihanbeyli-Goloren Anomaly (Aydemir and Ates 2006a) and edges of several other significant structures are very clear. In a similar way, North Anatolian and Ecemis Faults (Aydemir 2009) are also evident. In central Anatolia, 500 km long, 2D aeromagnetic and gravity models in N–S direction were constructed. These models indicate that all of the anomalous masses are in the upper crust. Beneath the upper crust down to MOHO discontinuity appears to be homogenous. In Turkey, destructive earthquakes occur at shallow depths (i.e. at about 8–10 km, http:// www.deprem.gov.tr), and this is consistent with the aforementioned results. In conclusion, we suggest that the active tectonics events of Turkey at the upper crust are fault controlled, because deep structure of Turkey is different than the earthquake distribution. Acknowledgments Authors are grateful to the General Directorate of Mining Research and Exploration of Turkey for the provision of gridded aeromagnetic and gravity data that were used for a Turkish Scientific Research Council (TUBITAK) and European Scientific Exchange Program (ESEP) during 1997. Our kindest thanks go to Prof. Karsten M. Storetvedt and Dr. M. Nuri Dolmaz for their comprehensive and delicate review of this paper. We also thank Prof. Rycroft for the editorial handling of our paper. Reduction to the Pole transformation map was produced by using GEOSOFT-OASIS software. Seismic sections used in this study were provided by the General Directorate of Petroleum Affairs for a Cumhuriyet University Scientific Research Project (Project No: CUBAP M-394). This research was also granted and supported by TUBITAK (Project No: 107Y288).
References Angus DA, Wilson DC, Sandvol E, Ni JF (2006) Lithospheric structure of the Arabian and Eurasian collision zone in eastern Turkey from S-wave receiver functions. Geophys J Int 166:1335–1346 Arslan S, Akin U, Alaca A (2010) Investigation of crustal structure of Turkey by means of gravity data. Mineral Res Expl Bull 140:55–71 Artemieva IM, Thybo H (2008) Deep Norden: highlights of the lithospheric structure of northern Europe, Iceland and Greenland. Episodes 31:98–106 Ates A, Kearey P (2000) Interpretation of gravity and aeromagnetic anomalies of the Konya region, South central Turkey. J Balkan Geophys Soc 3:37–44 Ates A, Kearey P, Tufan S (1999) New gravity and magnetic anomaly maps of Turkey. Geophys J Int 136:499–502 Ates A, Bilim F, Buyuksarac A (2005) Curie point depth investigation of central Anatolia, Turkey. Pure Appl Geophys 162:357–371 Ates A, Bilim F, Buyuksarac A, Bektas O (2008) A tectonic interpretation of the Marmara Sea, NW Turkey from geophysical data. Earth Planets Space 60:169–177 Ates A, Buyuksarac A, Bilim F, Bektas O, Sendur C, Komanovali G (2009) Spatial correlation of the aeromagnetic anomalies and seismogenic faults in the Marmara region, NW Turkey. Tectonophysics 478:135–142 Aydemir A (2008) Hydrocarbon potential of the Tuzgolu (Salt Lake) Basin, Central Anatolia, Turkey: a comparison of geophysical investigation results with the geochemical data. J Pet Sci Eng 61:33–47 Aydemir A (2009) Tectonic investigation of central Anatolia, Turkey using geophysical data. J Appl Geophys 68:321–334 Aydemir A (2011) An integrated geophysical investigation of Haymana Basin and hydrocarbon prospective Kirkkavak formation in Central Anatolia, Turkey. Petrol Geosci 17:91–100
123
Surv Geophys Aydemir A, Ates A (2006a) Interpretation of Suluklu-Cihanbeyli-Goloren magnetic anomaly, Central Anatolia, Turkey: an integration of geophysical data. Phys Earth Planet Inter 159:167–182 Aydemir A, Ates A (2006b) Structural interpretation of the Tuzgolu and Haymana Basins, Central Anatolia, using seismic, gravity and aeromagnetic data. Earth Planets Space 58:951–961 Becel A, Laigle M, de Voogd B, Hirn A, Taymaz T, Galve A, Shimamura H, Murai Y, Lepine JC, Sapin M, Ozalaybey S (2009) Moho, crustal architecture and deep deformation under the North Marmara Trough, from the SEISMARMARA Leg 1 offshore-onshore reflection-refraction survey. Tectonophysics 467:1–21 Bektas O, Ravat D, Buyuksarac A, Bilim F, Ates A (2007) Regional geothermal characterization of East Anatolia from aeromagnetic, heat flow and gravity data. Pure Appl Geophys 164:975–998 Bilim F (2007) Investigations into the tectonic lineaments and thermal structure of Kutahya–Denizli region, western Anatolia, from using aeromagnetic, gravity and seismological data. Phys Earth Planet Inter 165:135–146 Bilim F (2011) Investigation of the Galatian volcanic complex in the northern central Turkey using potential field data. Phys Earth Planet Inter 185:36–43 Bilim F, Ates A (1999) A computer program to estimate the source body magnetization direction from magnetic and gravity anomalies. Comput Geosci 25:241–250 Bilim F, Ates A (2004) An enhanced method for estimation of body magnetization direction from pseudogravity and gravity data. Comput Geosci 30:161–171 Bilim F, Ates A (2007) Identifying block rotations from remanent magnetization effect: example from northern Central Anatolia. Earth Planets Space 59:33–38 Bingo¨l E (1989) 1/2,000,000 Scale Geological Map of Turkey, Publication of the Mineral Research and Exploration (MTA) of Turkey, Ankara Biryol CB, Beck SL, Zandt G, Ozacar AA (2011) Segmented African lithosphere beneath the Anatolian region inferred from the teleseismic P-wave tomography. Geophys J Int 184:1037–1057 Blakely RJ (1996) Potential theory in gravity and magnetic applications. Cambridge University Press, UK Blakely RJ, Simpson RW (1986) Approximating edges of source bodies from magnetic or gravity anomalies. Geophysics 51:1494–1498 Buyuksarac A (2007) Investigation into the regional wrench tectonics of inner East Anatolia (Turkey) using potential field data. Phys Earth Planet Inter 160:86–95 Buyuksarac A, Jordanova D, Ates A, Karloukovski V (2005) Interpretation of the gravity and magnetic anomalies of the Cappadocia region, Central Turkey. Pure Appl Geophys 162:2197–2213 Buyuksarac A, Taymaz T, Ates A, Bilim F, Aydemir A, Bektas O, Yolsal S, Cubuk Y (2009) Gravity anomalies and crustal structure of Turkey. In: Proceedings of international symposium on historical earthquakes and conservation of monuments and sites in the Eastern Mediterranean Region, 500th anniversary year of the 1509 September 10, Marmara Earthquake, 10–12 September 2009, Istanbul Cakir O, Erduran M (2011) On the P and S receiver functions used for inverting the one-dimensional Upper Mantle shear-wave velocities. Surv Geophys 32:71–98 Demir D, Bilim F, Aydemir A, Ates A (2012) Modelling of Thrace Basin, NW Turkey using gravity and magnetic anomalies with control of seismic and borehole data. J Pet Sci Eng. doi:10.1016/j. petrol.2012.03.013 Dobrin MB, Savit CH (1988) Introduction to geophysical prospecting. McGraw-Hill, New York Gans CR, Beck SL, Zandt G, Biryol CB, Ozacar AA (2009) Detecting the limit of slab break-off in central Turkey: new high-resolution Pn tomography results. Geophys J Int 179:1566–1572 Genc Y, Yurur MT (2010) Coeval extension and compression in Late Mesozoic-Recent thin-skinned extensional tectonics in Central Anatolia. J Struct Geol 32:623–640 Gok R, Pasyanos ME, Zor E (2007) Lithospheric structure of the continent–continent collision zone: eastern Turkey. Geophys J Int 169:1079–1088 Gursoy H, Piper JDA, Tatar O, Temiz H (1997) A palaeomagnetic study of the Sivas basin, Central Turkey: crustal deformation during lateral extrusion of the Anatolian Block. Tectonophysics 271:89–105 Gursoy H, Piper JDA, Tatar O, Mesci L (1998) Paleomagnetic study of the Karaman and Karapinar volcanic complexes, Central Turkey, Neotectonic rotation in the South-central sector of the Arabian Block. Tectonophysics 299:191–211 Kearey P (1991) A possible source of the south-central England magnetic anomaly, basaltic rocks beneath the London Platform. J Geol Soc London 148:775–780 Kearey P, Brooks M, Hill I (2002) An introduction to exploration geophysics. Blackwell, UK Kearey P, Klepeis KA, Vine FJ (2009) Global tectonics, 3rd edn. Blackwell, UK McClusky S, Balassanian S, Barka A, Demir C, Ergintav S, Georgiev I, Gurkan O, Hamburger M, Hurst K, Kahle H, Kastens K, Kekelidze G, King R, Kotzev V, Lenk O, Mahmoud S, Mishin A, Nadariya M, Ouzounis A, Paradissis D, Peter Y, Prilepin M, Reilinger R, Sanli I, Seeger H, Tealeb A, Toksoz MN,
123
Surv Geophys Veis G (2000) Global positioning system constraints on plate kinematics and dynamics in the Eastern Mediterranean and Caucasus. J Geophys Res 105:5695–5719 Moix P, Beccaletto L, Kozur HW, Hochard C, Rosselet F, Stampfli GM (2008) A new classification of the Turkish terranes and sutures and its implication for the paleotectonic history of the region. Tectonophysics 451:7–39 Necioglu A, Maddison B, Turkelli N (1981) A study of crustal and upper mantle structure of northwestern Turkey. Geophys Res Lett 8:33–35 Neev D, Hall JK, Saul JM (1982) The Pelusium Megashear system across Africa and associated lineament swarms. J Geophys Res 87:1015–1030 Onal KM, Buyuksarac A, Aydemir A, Ates A (2008) Investigation of the deep structure of the Sivas basin (innereast Anatolia, Turkey) with geophysical methods. Tectonophysics 460:186–197 Ozturk F (1997) Investigations into magnetic and gravity anomalies of the region between Kirikkale and Tuz Lake. MSc Thesis, Ankara University, Turkey (unpublished) (in Turkish with English abstract) Piper JDA, Gursoy H, Tatar O, Beck ME, Rao A, Kocbulut F, Mesci BL (2010) Distributed neotectonic deformation in the Anatolides of Turkey: a palaeomagnetic analysis. Tectonophysics 488:31–50 Reilinger R, McClusky S, Vernant P, Lawrence S, Ergintav S, Cakmak R, Ozener H, Kadirov F, Guliev I, Stepanyan R, Nadariya M, Hahubia G, Mahmoud S, Sakr K, ArRajehi A, Paradissis D, Al-Aydrus A, Prilepin M, Guseva T, Evren E, Dmitrotsa A, Filikov SV, Gomez F, Al-Ghazzi R, Karam G (2006) GPS constraints on continental deformation in the Africa-Arabia-Eurasia continental collision zone and implications for the dynamics of plate interactions. J Geophys Res 111:B05411 Riad S, El Etr HA (1985) Bouguer anomalies and lithosphere-crustal thickness in Uganda. J Geodyn 3:169–186 Riad S, Refai E, Ghalib M (1981) Bouguer anomalies and crustal structure in the Eastern Mediterranean. Tectonophysics 71:253–266 Rivero L, Pinto V, Casas A (2002) Moho depth structure of the eastern part of the Pyrenean belt derived from the gravity data. J Geodyn 33:315–332 Saunders P, Priestley K, Taymaz T (1998) Variations in the crustal structure beneath western Turkey. Geophys J Int 134:373–389 Sengor AMC, Yilmaz Y (1981) Tethyan evolution of Turkey: a plate tectonic approach. Tectonophysics 75:181–241 Spector A, Grant FS (1970) Statistical models for interpreting aeromagnetic data. Geophysics 35:293–302 Storetvedt KM (1990) The Tethys Sea and the Alpine-Himalayan orogenic bels; mega-elements in a new global tectonic system. Phys Earth Planet Inter 62:141–184 Storetvedt KM (2003) Global wrench tectonics. Fagbokforlaget, Norway Storetvedt KM, Longhinos B (2011) Evolution of the north Atlantic: paradigm shift in the offing. New Concepts Glob Tectonics Newsl 51:9–48 Telford WM, Geldart LP, Sheriff RE (1990) Applied geophysics, 2nd edn. Cambridge University Press, Cambridge Tezel T, Erduran M, Alptekin O (2007) Crustal shear wave velocity structure of Turkey by surface wave dispersion analysis. Ann Geophys 50:177–190 Tezel T, Shibutani T, Kaypak B (2010) Crustal structure variation in western Turkey inferred from receiver function analysis. Tectonophysics 492:240–252 Tirel C, Gueydan F, Tiberi C, Brun JP (2004) Aegean crustal thickness inferred from gravity inversion. Geodynamical implications. Earth Planet Sci Lett 228:267–280
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