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Investigation into the regional wrench tectonics of inner East Anatolia (Turkey) using potential field data
Physics of the Earth and Planetary Interiors 160 (2007) 86–95
Investigation into the regional wrench tectonics of inner East Anatolia (Turkey) using potential field data Aydın B¨uy¨uksarac¸ ∗ Cumhuriyet University, Department of Geophysical Engineering, 58140 Sivas, Turkey Received 18 July 2006; received in revised form 27 September 2006; accepted 11 October 2006
Abstract The residual aeromagnetic and gravity anomalies of inner East Anatolia, surveyed by the Mineral Research and Exploration (MTA) of Turkey, display complexities. Some faults, which are known and new lineaments, are drawn from maxspot map derived from the location of the horizontal gradient of gravity anomalies. Tectonic lineaments of inner East Anatolia exhibit similarities to the direction of East Anatolian Fault Zone. Anticlockwise rotation, approximately −30◦ , defined from disorientations of aeromagnetic anomalies. The lineaments obtained from maxspots map produced from the gravity anomalies and disoriented aeromagnetic anomalies are in-line with the mobilistic system revealed by the palaeomagnetic data. These Alpine age continental rotations caused westward wrenching of the global lithosphere and led to significant tectonic reactivation and deformations. GPS measurements, current tectonic knowledge and the results of the evaluation of potential field data were combined in a base map to demonstrate similarities. © 2006 Elsevier B.V. All rights reserved. Keywords: Inner East Anatolia; Aeromagnetic anomalies; Bouguer anomalies; Tectonic lineaments
1. Introduction Complex geology due to active tectonism is observed in inner East Anatolia. The compressional tectonics in end Eocene and Oligocene and Miocene times, resulted in complex structures. The tectonic units of inner East Anatolia are composed of Sivas–Kangal Basin, Tauride Belt, Kirsehir-Nigde massif, ophiolitic and plutonic rocks (Fig. 1). Inner East Anatolia is situated between the North Anatolian Fault Zone (NAFZ) and East Anatolian Fault Zone (EAFZ). Eastern sector of inner East Anatolia continues to junction of the NAFZ and EAFZ as the East Anatolian Block narrows towards east. Central Anatolia, located to the west of inner East Anato-
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lia, has been affected by major geologic and Alpine (Late Cretaceous-Eosen) age events-products of recurrent relative movements between Africa and Eurasia. Buyuksarac et al. (2005) identified a rotational effect in Cappadocia region, adjacent area to inner East Anatolia, and found a good correlation between magnetic anomalies and anticlockwise rotations. Storetvedt (2003) suggested a new theory of continental collision of the Eurasian and African plates named Global Wrench Tectonics. The tectonic pressure in metamorphic belts has increased with time, having reached a maximum during the Alpine time. Such a tectonic stress versus time relationship is concordant with the wrench tectonics theory. Turning to the Africa/Eurasia kinematic system which passed along the Mediterranean region and continued westward across the Central Atlantic towards the Caribbean and Central America. With respect to this palaeogeographic setting, Africa and Eurasia moved
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Fig. 1. Simplified geological map of inner East Anatolia between Sivas and Malatya (after Bilgic, 2002). 1: sedimentary rocks, 2: volcanic rocks, 3: plutonic rocks, 4: metamorphic rocks, 5: ophiolitic rocks, 6: faults.
in opposite senses. The inertia effects providing the dynamic forcing of the intervening Alpine fold belt. As a result of the collision of Eurasian and African plates Anatolian plate has been rotating counter clockwise and moving westwards (McClusky et al., 2000) (Fig. 2). Simplified tectonic map of the region, based on Gursoy et al. (1997) is given in Fig. 3. Cretaceous and Eocene rocks were magnetically overprinted to variable degrees during the collisional phase although these overprints have since been rotated mostly anticlockwise. Regional anticlockwise rotation is recognised across the basin with differential rotation of fault and thrust-bounded blocks. Subsequent anticlockwise rotations have resulted from sideways expulsion of blocks to the south of the Central Anatolian Thrust along major NE–SW sinistral faults to achieve the crustal shortening resulting from N–S compression. Some researchers performed palaeomagnetic investigations in and surrounding regions of inner East Anatolia (Baydemir, 1990; Platzman et al., 1994; Tatar et al., 1995; Piper et al., 1996; Gursoy et al., 1997). They found an absolute anticlockwise rotation of the region on their investigations supporting the aforementioned
information. On the other hand, the regional transtensional to extensional stress regime is characterized by ¨ a SW-trending in Anatolia (Bellier et al., 1997; Over ¨ et al., 2002, 2004; Ozden et al., 2002). The configuration of regional transtension to extension observed along the central and eastern NAF, and in western and SE Turkey, is not only a response to simple lateral westward extrusion but was also induced by boundary forces of the Arabia–Anatolia collision and subduc¨ tion along the Hellenic and Cyprus arcs (Over et al., 2004). In this paper, regional Bouguer and aeromagnetic anomalies of inner East Anatolia were described and the main lineaments of the area were drawn by means of processing Bouguer anomalies using the method developed by Blakely and Simpson (1986) that locates the edges of anomalous bodies and these were correlated with the known surface faults. The aeromagnetic anomalies directed NE–SW direction parallel to main faults. A circular shaped magnetic anomaly exists around Divri˘gi due to volcanic rocks. The data and their regional interpretations given in this paper will lead to better reconnaissance of this active tectonic area.
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Fig. 2. Combined map of GPS and tectonic results of Anatolia (modified from McClusky et al., 2000).
2. Geological and tectonic setting There are three main basins and located from north to south in the inner East Anatolia named Sivas, Kan-
Fig. 3. Major tectonic lineaments of the study area (after Gursoy et al., 1997).
gal and Gurun Basins, respectively. The Sivas Basin has developed within a Late Cretaceous-early Tertiary collision zone resulting from closure of the Neotethyan Ocean between the Pontide orogen in the north and the Tauride Orogen in the south. The Sivas Basin is a complex collage of Eocene and younger rocks located within the wedge-shaped eastern margin of the Anatolian Block between the NAFZ and the EAFZ. It has been subject to ongoing deformation by movement of the Arabian Block into Eurasia and concomitant sideways expulsion of the Anatolian Block. Post-collisional deformation since mid-Miocene times has been dominated by N–S to NW–SE compression expressed by thrusting and strike-slip faulting. The ophiolitic associations at the northern and southern margins of the Sivas Basin have similar characteristics of emplacement age and rock type (Yılmaz, 1985). Continued closure of Neotethys resulted in the formation of a narrow remenant basin in the Sivas region which filled rapidly, mainly with low density deposits, during the Eocene. The reversal of polarity to northdirected thrusting in the late Eocene may be explained by means of a gravity sliding model involving the collapse of Palaeogene slope deposits into the basin as major northward-moving gravity slides, along reactivated obduction-related thrust planes. At the end of the Eocene, the latter could have propagated upwards through the stack of gravity-slides during the increased tectonic compression. The reverse component on the North Boundary Fault gives the basin at geometry characteristic of a structural triangle zone with structures verging towards the centre of the basin. This compression of the Sivas Basin area also seems to have resulted in at least two phases of uplift during the Pliocene (Cater et al., 1991). The Sivas Basin sequence was deformed under NNW–SSE compressional events, and its tectonic deformation style is characterized by complex polyphase thrust systems (Temiz, 1996). The post-collisional tectonic evolution of the Sivas Basin has been dominated by N–S and NW–SE compressional regime which commenced in Mid-Eocene times and has continued up to the present day (Gursoy et al., 1997). During the Late Cretaceous, a succession of 3 km thick, dominantly volcanoclastic turbidites commenced deposition on an ophiolitic melange basement with a basal conglomerate in the Sarkisla Basin (Gorur et al., 1998). Sivas Basin basement consists of metamorphic rocks and unconformably overlying Upper Cretaceous to Paleocene shallow-water carbonates, belonging to the crystalline complex and the sedimentary cover of the Kırsehir Block, respectively (Gorur et al., 1998).
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Fig. 4. Total field aeromagnetic data of inner East Anatolia with 2 km grid interval. Contour interval is 100 nT.
On the other hand, Sivas Basin is subdivided to four subbasins by the oblique faults, along with dominant left lateral strike slip faults (Yilmaz and Yilmaz, 2006). The boundaries of the basin are overthrusts or leftlateral oblique faults with reverse components, there is no outcrop of basement within the central basin. Tauride Platform carbonates overthrusted by tectonic slices of melanges and ophiolites. The basin fill and its deformation are of post-collisional origin (Yilmaz and Yilmaz, 2006).
Fig. 5. Topographic map of inner East Anatolia. SB, Sivas Basin; KB, Kangal Basin.
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Fig. 6. Residual aeromagnetic anomaly map of inner East Anatolia. Contour interval is 30 nT.
3. The aeromagnetic data and interpretation The aeromagnetic data of inner East Anatolia surveyed by the General Directory of Mineral Exploration and Research Company of Turkey (MTA). Flight line distances vary between 1 and 2 km. The sampling intervals along a line are approximately 70 m spacing at a mean terrain clearance of 600 m from ground surface. The original data are in the format of x (easting, km), y
Fig. 7. Reduction to pole transformation (RTP) of aeromagnetic anomalies. Contour interval is 100 nT.
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(northing, km), z (nT). MTA gridded the data with 2 km intervals and provided to be used within context of this research (Fig. 4). Topographic height data (Fig. 5) were incorporated with the IGRF calculation. In order to obtain the residual aeromagnetic anomalies, the International Geomagnetic Reference Field (IGRF) was subtracted from the surveyed aeromagnetic anomalies. The residual aeromagnetic anomalies contoured with 50 nT intervals after the removal of the IGRF and are shown in Fig. 6.
The short-wavelength and distorted shaped aeromagnetic anomalies between Divrigi and Hekimhan, around the centre of the study area, are associated with plutonic rocks and iron mine. Some surface geological features are reflected in the aeromagnetic anomalies. Especially, the effects of ophiolite in the northern part and plutonic rocks around Divrigi are observed on the aeromagnetic map. Peak to trough axes of most of the anomalies are not aligned along N–S direction. Polarity axes of most of the anomalies of the area are aligned towards NW–SE direc-
Fig. 8. Upward continued aeromagnetic anomaly maps. (a) 3 km (red arrows show disorientations of the peak to through N–S axes of aeromagnetic anomalies), (b) 5 km, (c) 10 km. Lows are hachured. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
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tion. These polarity orientations can be interpreted as an evidence of anticlockwise block rotation of the Anatolian plate. On the other hand, basin areas around Sivas, Kangal and west of Hekimhan present low intensity magnetic anomaly (Fig. 6). Reduction to pole transformation (RTP), which removes the distortion caused by the Earth’s magnetic field, was applied to the residual aeromagnetic anomalies using the magnetization angle of the induced magnetization. RTP transformation was partially successful that some of the disoriented anomalies were corrected. The RTP transformed map is shown in Fig. 7. Upward continuation was applied to aeromagnetic anomalies for 3, 5 and 10 km to demonstrate differences between deep and near surface sources (Fig. 8a–c). Results of upward continuation show the continuation of disoriented anomalies towards deep. Normally, surface effects may affect the magnetic anomalies. When the magnetic anomalies are disoriented by the tectonic regime, disorientation of the magnetic anomalies can extend down to bottom of the magnetic crust that is the Curie point depth. In this case, disoriented magnetic anomalies are directed along the tectonic regime in the project area and extend to 10 km depth. 4. Gravity data and tectonic interpretation The gravity data of inner East Anatolia were obtained from the General Directory of Mineral Exploration
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Fig. 10. 3 km upward continued map of Bouguer anomalies. Lows are hachured.
and Research Company of Turkey (MTA). The gravity data are gridded with 2 km intervals and corrections are made by the MTA (Fig. 9). There are number of Bouguer anomaly lows reaching over −100 mGal indicating either thick crust in the region or thickening of upper crustal low density layers. In addition, a number of negative contour closures of the Bouguer anomalies can be correlated with the cover units of the geological map (Fig. 1). Three kilometers upward continuation was applied to Bouguer anomalies to reduce the effects of near surface sources (Fig. 10). 4.1. Locating maxima of the horizontal gradients of gravity anomalies
Fig. 9. Bouguer anomaly map of inner East Anatolia. Contour interval is 5 mGal. Lows are hachured.
A method developed by Blakely and Simpson (1986) to identify maxima on a contoured map of horizontal gradient magnitudes with gridded magnetic or gravity anomaly data. The method could be applied to both local surveys and to continent-wide compilations of magnetic and gravity data. The aim of the method was to produce a plan view of inferred boundaries of magnetic or gravity sources. Some researchers were applied the method successfully (Ates¸ and Kearey, 1993; Kadioglu et al., 1998; B¨uy¨uksarac¸ et al., 1998). The steepest horizontal gradient of a gravity anomaly caused by a tabular body tends to overlie the edges of the body. 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,
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1995). The shallow bodies produce gravity anomalies with maximum horizontal gradients located nearly over their edges. The horizontal gradient data are available on a rectangular grid. Each grid intersection yi,j is compared with its eight the nearest neighbours in four directions along the row, column and both diagonals to inspect if a maximum is present (Blakely and Simpson, 1986). gi−1,j < gi,j > gi+1,j
(1)
gi,j−1 < gi,j > gi,j+1
(2)
gi+1,j−1 < gi,j > gi−1,j+1
(3)
gi−1,j−1 < gi,j > gi+1,j+1
(4)
For each satisfied inequality, the horizontal location and magnitude of the maximum are found by interpolating a second-order polynomial through the trio of points. For example in Eq. (1), the horizontal location of the maximum relative to the position of gi,j is given by hd xmax = − 2a where
(5)
a = 21 (gi − 2gi,j + gi+1,j )
(6)
b=
(7)
1 2 (gi+1,j
− gi−1,j )
and d is the distance between grid intersections. The value of the horizontal gradient at xmax is given by 2 + bxmax + gi,j gmax = axmax
(8)
Fig. 12. Constructed lineament map from Fig. 11. KF, Kangal Fault; MOF, Malatya-Ovacık Fault; YFZ, Yıldızeli Fault Zone; YGFZ, Yakapınar-G¨oksun Fault Zone; EAFZ, East Anatolian Fault Zone; DTFZ, Deliler-Tecer Fault Zone; BPFZ, Belcik-Pazarcık Fault Zone; NBNT, Northern Block of NeoThetys.
The magnitude of the horizontal gradient of the gravity is given by 1/2 ∂gz (x, y) 2 ∂gz (x, y) 2 + (9) h(x, y) = ∂x ∂y and is an easily calculated using simple finite-difference relationship (Blakely, 1995). Horizontal gradient tends to have maxima located over edges of gravity sources. When applied to two-dimensional surveys, the horizontal gradient tends to place narrow ridges over abrupt changes in magnetisation or density. Locating maxima in the horizontal gradient can be down by simple inspection. The method is applied to the upward continued gravity anomalies shown in Fig. 10. Maxspots of the horizontal gradients of the Bouguer anomalies were calculated. Maxspots are displayed as circles and circle sizes are assigned to the magnitude of the horizontal gradients. Locations of the maxima of the horizontal gradient (maxspot) map are shown in Fig. 11. There is a good correlation between the lineaments of maxspots with the tectonic lines of inner East Anatolia in Figs. 1 and 3. A lineament map is prepared based on the maxspot map shown in Fig. 12. 5. Discussion and conclusions
Fig. 11. Locations of the maxima of horizontal gradient of upward continued Bouguer anomalies of inner East Anatolia. Size of circles is proportional to the magnitude of the gradient.
Geology map of inner East Anatolia (Fig. 1), simplified from Bilgic (2002), displays complexities. Apparent
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Fig. 13. Reduction to pole transformation (RTP) of aeromagnetic anomalies using magnetisation angle = −30◦ . Contour interval is 50 nT.
cause for the residual aeromagnetic anomalies (Fig. 6) cannot easily be observed from surface geology implying that the causative bodies are buried. Ophiolites in the northern part of inner East Anatolia and plutonic rocks around Divrigi, eastern part of the area, cause high amplitude aeromagnetic anomalies. Sivas, Kangal and Gurun Basins do not almost present aeromagnetic anomaly suggesting that the basin fill is non-magnetic. Disorientations of peak to trough axes of 3 km upward continued magnetic anomalies are presented by red arrows in Fig. 8a. Shape analysis suggests that almost all of the anomalies have a total magnetisation direction differing from the induced one. Anomalies with similar characteristics have been reported from the Italian region by Fedi et al. (1991). RTP was applied to the residual aeromagnetic anomalies with 3 km upward continued using the average distorted magnetization angle (−30◦ ). However, RTP transformation was mainly successful to restore disoriented anomalies. The RTP transformed map is shown in Fig. 13. Gursoy et al. (1997) found that the mean anticlockwise rotations between −10◦ and −50◦ using palaeomagnetic results in this area. The maxima of the horizontal gradient of Bouguer anomalies reflect not only surface geology, edge of basins, but also tectonic discontinuities (Fig. 12). These discontinuities are in line with the lineaments described for the Sivas Basin by Yilmaz and Yilmaz (2006). Tectonic lineaments of inner East Anatolia show similarities with the direction of North Anatolian Fault
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Fig. 14. The comparison map shows between distribution of the epicentres and lineaments in Fig. 12 of the study area.
Zone and East Anatolian Fault Zone. Most of the lineaments could be identified by known faults. For instance, Northern Block of NeoThetys (NBNT), BelcikPazarcık Fault Zone (BPFZ), Deliler-Tecer Fault Zone (DTFZ), Yıldızeli Fault Zone (YFZ), Kangal Fault (KF), Yakapınar-G¨oksun Fault Zone (YGFZ) are in the same direction with the NAFZ. Malatya-Ovacık Fault (MOF) is the same direction with the EAFZ. On the other hand, there are many unknown faults or lineaments between MOF and YGFZ in the direction NE–SW (Fig. 12). The distribution of the epicentres (M ≥ 2.0) in the study area shows the activity of the lineaments. The most active area is the EAFZ in the study area. However, dense earthquake activity is observed around Divrigi, along the DTFZ, in the SE corner of the area (Fig. 14). The distribution of the magnitude changes increasingly from the NW to SE. The NE corner of the area presents different stress regime. The focal depths of the earthquakes are mainly at 10 km in the study area. However, they are deeper around the EAFZ than the other places and reach to 30 km. If the map of the arrows obtained from the disorientations of aeromagnetic anomalies in Fig. 8a and lineament map in Fig. 12 are combined, a rotational effect can be observed in the study area (Fig. 15). Anticlockwise rotation defined from disorientations of aeromagnetic anomalies (Fig. 8a) and the lineaments obtained from maxspots map produced from the gravity anomalies (Fig. 12) are in-line with the mobilistic system
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Fig. 15. Combination of interpretation of maxspots and disoriented magnetization arrows.
revealed by palaeomagnetic data (Storetvedt, 2003). It was described that there is a clockwise litospheric torsion of the northern palaeohemisphere and a corresponding anticlockwise in the southern palaeohemisphere. These Alpine age continental rotations caused westward wrenching of the global lithosphere and led to significant tectonic reactivation and deformations (Storetvedt, 2003). When the GPS measurements made by McClusky et al. (2000) in Fig. 2 is compared with the current tectonic knowledge depicted Fig. 15, a similarity can easily be observed that the anticlockwise rotation is evident. Acknowledgments I thank two anonymous referees for their critical review of the manuscript. I also thank the General Directorate of the Mineral Research and Exploration (MTA) of Turkey for providing digital topographic, borehole, aeromagnetic and gravity data. This research was financially supported by the Cumhuriyet University, Scientific Research Projects (CUBAP) (Project code: M-259). References Ates¸, A., Kearey, P., 1993. Deep structure of the East Mendip Hills from gravity, aeromagnetic and seismic reflection data. J. Geol. Soc. Lond. 150, 1055–1063. Baydemir, N., 1990. Palaeomagnetism of the Eocene volcanic rocks in the eastern Black Sea region, Istanbul Universitesi. Muh. Fak. Yerbilimleri Dergisi 7, 167–176 (in Turkish).
Bellier, O., Over, S., Poisson, A., Andrieux, J., 1997. Recent temporal change in the stress state and modern stres field along North Anatolian Fault Zone (Turkey). Geophys. J. Int. 131, 61– 86. Bilgic, T., 2002. Geological Map of Turkey (1:500.000 scale Sivas sheet). General Directorate of Mineral Research and Exploration (MTA) of Turkey, Ankara. Blakely, R.J., Simpson, R.W., 1986. Approximating edges of source bodies from magnetic or gravity anomalies. Geophysics 51, 1494–1498. Blakely, R.J., 1995. Potential Theory in Gravity and Magnetic Applications. Cambridge University Press, pp. 441. B¨uy¨uksarac¸, A., Reiprich, S., Ates¸, A., 1998. Three-dimensional magnetic model of amphibolite complex in Taskesti area, Mudurnu valley, North-West Turkey. J. Balkan Geophys. Soc. 1, 44– 52. 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. Cater, J.M.L., Hanna, S.S., Ries, A.C., Turner, P., 1991. Tertiary evolution of the Sivas Basin, Central Turkey. Tectonophysics 195, 29–46. Fedi, M., Florio, G., Rapolla, A., 1991. The role of remanent magnetization in the southern Italian crust from aeromagnetic anomalies. Terra Nova 2, 629–637. Gorur, N., Tuysuz, O., Sengor, A.M.C., 1998. Tectonic evolution of the Central Anatolian Basins. Int. Geol. Rev. 40, 831–850. Gursoy, H., Piper, J.D.A., 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. Kadioglu, Y.K., Ates, A., G¨ulec¸, N., 1998. Interpretation of gabbroic rocks in Agac¸o¨ ren Granitoid, central Turkey: field observations and aeromagnetic data. Geol. Mag. 135, 245–254. 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., Teableb, A., Toks¨oz, M.N., Veis, G., 2000. Global positioning system constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus. J. Geophys. Res. 105, 5695–5719. ¨ Over, S., Unlugenc, U.C., Bellier, O., 2002. Quaternary stress regime change in the Hatay region (SE Turkey). Geophys. J. Int. 148, 1–14. ¨ ¨ Over, S., Ozden, S., Unlugenc, U.C., Yilmaz, H., 2004. A synthesis: Late Cenozoic stress field distribution at northeastern corner of the Eastern Mediterranean, SE Turkey. Geodynamics 336, 93– 103. ¨ ¨ Ozden, S., Over, S., Unlugenc, U.C., 2002. Quaternary stress regime change along the Eastern North Anatolian Fault Zone, Turkey. Int. Geol. Rev. 44, 1037–1052. Piper, J.D.A., Moore, J., Tatar, O., Gursoy, H., Park, R.G., 1996. Palaeomagnetic study of crustal deformation across an intracontinental transform: the Anatolian fault zone in Northern Turkey. In: Morris, A., Tarling, D.H. (Eds.), Palaeomagnetism of the Mediterranean Regions, vol. 105. Journal of Geological Society of London, pp. 299–310 (Spec. Pub.). Platzman, E.S., Platt, J.P., Tapirdamaz, C., Sanver, M., Rundle, C.C., 1994. Why are there no clockwise rotations along North Anatolian fault zone? J. Geophys. Res. 99, 21705–21716. Storetvedt, K.M., 2003. Global Wrench Tectonics. Fagbokforlaget, Norway, pp. 397.
A. B¨uy¨uksara¸c / Physics of the Earth and Planetary Interiors 160 (2007) 86–95 Tatar, O., Piper, J.D.A., Park, R.G., Gursoy, H., 1995. Palaeomagnetic study of block rotations in Niksar overlap region of the North Anatolian fault zone, Central Turkey. Tectonophysics 244, 251–266. Temiz, H., 1996. Tectonostratigraphy and thrust tectonics of the Central and eastern parts of the Sivas Tertiary Basin, Turkey. Int. Geol. Rev. 38, 957–971.
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Yılmaz, A., 1985. Yukarı Kelkit C ¸ ayı ve Munzur da˘gları arasının temel jeolojik o¨ zellikleri ve yapısal evrimi. Geol. Bull. Turkey 28, 79–92 (in Turkish with English abstract). Yilmaz, A., Yilmaz, H., 2006. Characteristic features and structural evolution of a post collisional basin: the Sivas Basin, Central Anatolia, Turkey. J. Asian Earth Sci. 27, 164–176.
Investigation into the regional wrench tectonics of inner East Anatolia (Turkey) using potential field data Aydın B¨uy¨uksarac¸ ∗ Cumhuriyet University, Department of Geophysical Engineering, 58140 Sivas, Turkey Received 18 July 2006; received in revised form 27 September 2006; accepted 11 October 2006
Abstract The residual aeromagnetic and gravity anomalies of inner East Anatolia, surveyed by the Mineral Research and Exploration (MTA) of Turkey, display complexities. Some faults, which are known and new lineaments, are drawn from maxspot map derived from the location of the horizontal gradient of gravity anomalies. Tectonic lineaments of inner East Anatolia exhibit similarities to the direction of East Anatolian Fault Zone. Anticlockwise rotation, approximately −30◦ , defined from disorientations of aeromagnetic anomalies. The lineaments obtained from maxspots map produced from the gravity anomalies and disoriented aeromagnetic anomalies are in-line with the mobilistic system revealed by the palaeomagnetic data. These Alpine age continental rotations caused westward wrenching of the global lithosphere and led to significant tectonic reactivation and deformations. GPS measurements, current tectonic knowledge and the results of the evaluation of potential field data were combined in a base map to demonstrate similarities. © 2006 Elsevier B.V. All rights reserved. Keywords: Inner East Anatolia; Aeromagnetic anomalies; Bouguer anomalies; Tectonic lineaments
1. Introduction Complex geology due to active tectonism is observed in inner East Anatolia. The compressional tectonics in end Eocene and Oligocene and Miocene times, resulted in complex structures. The tectonic units of inner East Anatolia are composed of Sivas–Kangal Basin, Tauride Belt, Kirsehir-Nigde massif, ophiolitic and plutonic rocks (Fig. 1). Inner East Anatolia is situated between the North Anatolian Fault Zone (NAFZ) and East Anatolian Fault Zone (EAFZ). Eastern sector of inner East Anatolia continues to junction of the NAFZ and EAFZ as the East Anatolian Block narrows towards east. Central Anatolia, located to the west of inner East Anato-
∗
Tel.: +90 346 219 10 10x1925; fax: +90 346 219 11 65. E-mail address: [email protected].
0031-9201/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2006.10.003
lia, has been affected by major geologic and Alpine (Late Cretaceous-Eosen) age events-products of recurrent relative movements between Africa and Eurasia. Buyuksarac et al. (2005) identified a rotational effect in Cappadocia region, adjacent area to inner East Anatolia, and found a good correlation between magnetic anomalies and anticlockwise rotations. Storetvedt (2003) suggested a new theory of continental collision of the Eurasian and African plates named Global Wrench Tectonics. The tectonic pressure in metamorphic belts has increased with time, having reached a maximum during the Alpine time. Such a tectonic stress versus time relationship is concordant with the wrench tectonics theory. Turning to the Africa/Eurasia kinematic system which passed along the Mediterranean region and continued westward across the Central Atlantic towards the Caribbean and Central America. With respect to this palaeogeographic setting, Africa and Eurasia moved
A. B¨uy¨uksara¸c / Physics of the Earth and Planetary Interiors 160 (2007) 86–95
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Fig. 1. Simplified geological map of inner East Anatolia between Sivas and Malatya (after Bilgic, 2002). 1: sedimentary rocks, 2: volcanic rocks, 3: plutonic rocks, 4: metamorphic rocks, 5: ophiolitic rocks, 6: faults.
in opposite senses. The inertia effects providing the dynamic forcing of the intervening Alpine fold belt. As a result of the collision of Eurasian and African plates Anatolian plate has been rotating counter clockwise and moving westwards (McClusky et al., 2000) (Fig. 2). Simplified tectonic map of the region, based on Gursoy et al. (1997) is given in Fig. 3. Cretaceous and Eocene rocks were magnetically overprinted to variable degrees during the collisional phase although these overprints have since been rotated mostly anticlockwise. Regional anticlockwise rotation is recognised across the basin with differential rotation of fault and thrust-bounded blocks. Subsequent anticlockwise rotations have resulted from sideways expulsion of blocks to the south of the Central Anatolian Thrust along major NE–SW sinistral faults to achieve the crustal shortening resulting from N–S compression. Some researchers performed palaeomagnetic investigations in and surrounding regions of inner East Anatolia (Baydemir, 1990; Platzman et al., 1994; Tatar et al., 1995; Piper et al., 1996; Gursoy et al., 1997). They found an absolute anticlockwise rotation of the region on their investigations supporting the aforementioned
information. On the other hand, the regional transtensional to extensional stress regime is characterized by ¨ a SW-trending in Anatolia (Bellier et al., 1997; Over ¨ et al., 2002, 2004; Ozden et al., 2002). The configuration of regional transtension to extension observed along the central and eastern NAF, and in western and SE Turkey, is not only a response to simple lateral westward extrusion but was also induced by boundary forces of the Arabia–Anatolia collision and subduc¨ tion along the Hellenic and Cyprus arcs (Over et al., 2004). In this paper, regional Bouguer and aeromagnetic anomalies of inner East Anatolia were described and the main lineaments of the area were drawn by means of processing Bouguer anomalies using the method developed by Blakely and Simpson (1986) that locates the edges of anomalous bodies and these were correlated with the known surface faults. The aeromagnetic anomalies directed NE–SW direction parallel to main faults. A circular shaped magnetic anomaly exists around Divri˘gi due to volcanic rocks. The data and their regional interpretations given in this paper will lead to better reconnaissance of this active tectonic area.
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Fig. 2. Combined map of GPS and tectonic results of Anatolia (modified from McClusky et al., 2000).
2. Geological and tectonic setting There are three main basins and located from north to south in the inner East Anatolia named Sivas, Kan-
Fig. 3. Major tectonic lineaments of the study area (after Gursoy et al., 1997).
gal and Gurun Basins, respectively. The Sivas Basin has developed within a Late Cretaceous-early Tertiary collision zone resulting from closure of the Neotethyan Ocean between the Pontide orogen in the north and the Tauride Orogen in the south. The Sivas Basin is a complex collage of Eocene and younger rocks located within the wedge-shaped eastern margin of the Anatolian Block between the NAFZ and the EAFZ. It has been subject to ongoing deformation by movement of the Arabian Block into Eurasia and concomitant sideways expulsion of the Anatolian Block. Post-collisional deformation since mid-Miocene times has been dominated by N–S to NW–SE compression expressed by thrusting and strike-slip faulting. The ophiolitic associations at the northern and southern margins of the Sivas Basin have similar characteristics of emplacement age and rock type (Yılmaz, 1985). Continued closure of Neotethys resulted in the formation of a narrow remenant basin in the Sivas region which filled rapidly, mainly with low density deposits, during the Eocene. The reversal of polarity to northdirected thrusting in the late Eocene may be explained by means of a gravity sliding model involving the collapse of Palaeogene slope deposits into the basin as major northward-moving gravity slides, along reactivated obduction-related thrust planes. At the end of the Eocene, the latter could have propagated upwards through the stack of gravity-slides during the increased tectonic compression. The reverse component on the North Boundary Fault gives the basin at geometry characteristic of a structural triangle zone with structures verging towards the centre of the basin. This compression of the Sivas Basin area also seems to have resulted in at least two phases of uplift during the Pliocene (Cater et al., 1991). The Sivas Basin sequence was deformed under NNW–SSE compressional events, and its tectonic deformation style is characterized by complex polyphase thrust systems (Temiz, 1996). The post-collisional tectonic evolution of the Sivas Basin has been dominated by N–S and NW–SE compressional regime which commenced in Mid-Eocene times and has continued up to the present day (Gursoy et al., 1997). During the Late Cretaceous, a succession of 3 km thick, dominantly volcanoclastic turbidites commenced deposition on an ophiolitic melange basement with a basal conglomerate in the Sarkisla Basin (Gorur et al., 1998). Sivas Basin basement consists of metamorphic rocks and unconformably overlying Upper Cretaceous to Paleocene shallow-water carbonates, belonging to the crystalline complex and the sedimentary cover of the Kırsehir Block, respectively (Gorur et al., 1998).
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Fig. 4. Total field aeromagnetic data of inner East Anatolia with 2 km grid interval. Contour interval is 100 nT.
On the other hand, Sivas Basin is subdivided to four subbasins by the oblique faults, along with dominant left lateral strike slip faults (Yilmaz and Yilmaz, 2006). The boundaries of the basin are overthrusts or leftlateral oblique faults with reverse components, there is no outcrop of basement within the central basin. Tauride Platform carbonates overthrusted by tectonic slices of melanges and ophiolites. The basin fill and its deformation are of post-collisional origin (Yilmaz and Yilmaz, 2006).
Fig. 5. Topographic map of inner East Anatolia. SB, Sivas Basin; KB, Kangal Basin.
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Fig. 6. Residual aeromagnetic anomaly map of inner East Anatolia. Contour interval is 30 nT.
3. The aeromagnetic data and interpretation The aeromagnetic data of inner East Anatolia surveyed by the General Directory of Mineral Exploration and Research Company of Turkey (MTA). Flight line distances vary between 1 and 2 km. The sampling intervals along a line are approximately 70 m spacing at a mean terrain clearance of 600 m from ground surface. The original data are in the format of x (easting, km), y
Fig. 7. Reduction to pole transformation (RTP) of aeromagnetic anomalies. Contour interval is 100 nT.
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(northing, km), z (nT). MTA gridded the data with 2 km intervals and provided to be used within context of this research (Fig. 4). Topographic height data (Fig. 5) were incorporated with the IGRF calculation. In order to obtain the residual aeromagnetic anomalies, the International Geomagnetic Reference Field (IGRF) was subtracted from the surveyed aeromagnetic anomalies. The residual aeromagnetic anomalies contoured with 50 nT intervals after the removal of the IGRF and are shown in Fig. 6.
The short-wavelength and distorted shaped aeromagnetic anomalies between Divrigi and Hekimhan, around the centre of the study area, are associated with plutonic rocks and iron mine. Some surface geological features are reflected in the aeromagnetic anomalies. Especially, the effects of ophiolite in the northern part and plutonic rocks around Divrigi are observed on the aeromagnetic map. Peak to trough axes of most of the anomalies are not aligned along N–S direction. Polarity axes of most of the anomalies of the area are aligned towards NW–SE direc-
Fig. 8. Upward continued aeromagnetic anomaly maps. (a) 3 km (red arrows show disorientations of the peak to through N–S axes of aeromagnetic anomalies), (b) 5 km, (c) 10 km. Lows are hachured. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
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tion. These polarity orientations can be interpreted as an evidence of anticlockwise block rotation of the Anatolian plate. On the other hand, basin areas around Sivas, Kangal and west of Hekimhan present low intensity magnetic anomaly (Fig. 6). Reduction to pole transformation (RTP), which removes the distortion caused by the Earth’s magnetic field, was applied to the residual aeromagnetic anomalies using the magnetization angle of the induced magnetization. RTP transformation was partially successful that some of the disoriented anomalies were corrected. The RTP transformed map is shown in Fig. 7. Upward continuation was applied to aeromagnetic anomalies for 3, 5 and 10 km to demonstrate differences between deep and near surface sources (Fig. 8a–c). Results of upward continuation show the continuation of disoriented anomalies towards deep. Normally, surface effects may affect the magnetic anomalies. When the magnetic anomalies are disoriented by the tectonic regime, disorientation of the magnetic anomalies can extend down to bottom of the magnetic crust that is the Curie point depth. In this case, disoriented magnetic anomalies are directed along the tectonic regime in the project area and extend to 10 km depth. 4. Gravity data and tectonic interpretation The gravity data of inner East Anatolia were obtained from the General Directory of Mineral Exploration
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Fig. 10. 3 km upward continued map of Bouguer anomalies. Lows are hachured.
and Research Company of Turkey (MTA). The gravity data are gridded with 2 km intervals and corrections are made by the MTA (Fig. 9). There are number of Bouguer anomaly lows reaching over −100 mGal indicating either thick crust in the region or thickening of upper crustal low density layers. In addition, a number of negative contour closures of the Bouguer anomalies can be correlated with the cover units of the geological map (Fig. 1). Three kilometers upward continuation was applied to Bouguer anomalies to reduce the effects of near surface sources (Fig. 10). 4.1. Locating maxima of the horizontal gradients of gravity anomalies
Fig. 9. Bouguer anomaly map of inner East Anatolia. Contour interval is 5 mGal. Lows are hachured.
A method developed by Blakely and Simpson (1986) to identify maxima on a contoured map of horizontal gradient magnitudes with gridded magnetic or gravity anomaly data. The method could be applied to both local surveys and to continent-wide compilations of magnetic and gravity data. The aim of the method was to produce a plan view of inferred boundaries of magnetic or gravity sources. Some researchers were applied the method successfully (Ates¸ and Kearey, 1993; Kadioglu et al., 1998; B¨uy¨uksarac¸ et al., 1998). The steepest horizontal gradient of a gravity anomaly caused by a tabular body tends to overlie the edges of the body. 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,
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1995). The shallow bodies produce gravity anomalies with maximum horizontal gradients located nearly over their edges. The horizontal gradient data are available on a rectangular grid. Each grid intersection yi,j is compared with its eight the nearest neighbours in four directions along the row, column and both diagonals to inspect if a maximum is present (Blakely and Simpson, 1986). gi−1,j < gi,j > gi+1,j
(1)
gi,j−1 < gi,j > gi,j+1
(2)
gi+1,j−1 < gi,j > gi−1,j+1
(3)
gi−1,j−1 < gi,j > gi+1,j+1
(4)
For each satisfied inequality, the horizontal location and magnitude of the maximum are found by interpolating a second-order polynomial through the trio of points. For example in Eq. (1), the horizontal location of the maximum relative to the position of gi,j is given by hd xmax = − 2a where
(5)
a = 21 (gi − 2gi,j + gi+1,j )
(6)
b=
(7)
1 2 (gi+1,j
− gi−1,j )
and d is the distance between grid intersections. The value of the horizontal gradient at xmax is given by 2 + bxmax + gi,j gmax = axmax
(8)
Fig. 12. Constructed lineament map from Fig. 11. KF, Kangal Fault; MOF, Malatya-Ovacık Fault; YFZ, Yıldızeli Fault Zone; YGFZ, Yakapınar-G¨oksun Fault Zone; EAFZ, East Anatolian Fault Zone; DTFZ, Deliler-Tecer Fault Zone; BPFZ, Belcik-Pazarcık Fault Zone; NBNT, Northern Block of NeoThetys.
The magnitude of the horizontal gradient of the gravity is given by 1/2 ∂gz (x, y) 2 ∂gz (x, y) 2 + (9) h(x, y) = ∂x ∂y and is an easily calculated using simple finite-difference relationship (Blakely, 1995). Horizontal gradient tends to have maxima located over edges of gravity sources. When applied to two-dimensional surveys, the horizontal gradient tends to place narrow ridges over abrupt changes in magnetisation or density. Locating maxima in the horizontal gradient can be down by simple inspection. The method is applied to the upward continued gravity anomalies shown in Fig. 10. Maxspots of the horizontal gradients of the Bouguer anomalies were calculated. Maxspots are displayed as circles and circle sizes are assigned to the magnitude of the horizontal gradients. Locations of the maxima of the horizontal gradient (maxspot) map are shown in Fig. 11. There is a good correlation between the lineaments of maxspots with the tectonic lines of inner East Anatolia in Figs. 1 and 3. A lineament map is prepared based on the maxspot map shown in Fig. 12. 5. Discussion and conclusions
Fig. 11. Locations of the maxima of horizontal gradient of upward continued Bouguer anomalies of inner East Anatolia. Size of circles is proportional to the magnitude of the gradient.
Geology map of inner East Anatolia (Fig. 1), simplified from Bilgic (2002), displays complexities. Apparent
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Fig. 13. Reduction to pole transformation (RTP) of aeromagnetic anomalies using magnetisation angle = −30◦ . Contour interval is 50 nT.
cause for the residual aeromagnetic anomalies (Fig. 6) cannot easily be observed from surface geology implying that the causative bodies are buried. Ophiolites in the northern part of inner East Anatolia and plutonic rocks around Divrigi, eastern part of the area, cause high amplitude aeromagnetic anomalies. Sivas, Kangal and Gurun Basins do not almost present aeromagnetic anomaly suggesting that the basin fill is non-magnetic. Disorientations of peak to trough axes of 3 km upward continued magnetic anomalies are presented by red arrows in Fig. 8a. Shape analysis suggests that almost all of the anomalies have a total magnetisation direction differing from the induced one. Anomalies with similar characteristics have been reported from the Italian region by Fedi et al. (1991). RTP was applied to the residual aeromagnetic anomalies with 3 km upward continued using the average distorted magnetization angle (−30◦ ). However, RTP transformation was mainly successful to restore disoriented anomalies. The RTP transformed map is shown in Fig. 13. Gursoy et al. (1997) found that the mean anticlockwise rotations between −10◦ and −50◦ using palaeomagnetic results in this area. The maxima of the horizontal gradient of Bouguer anomalies reflect not only surface geology, edge of basins, but also tectonic discontinuities (Fig. 12). These discontinuities are in line with the lineaments described for the Sivas Basin by Yilmaz and Yilmaz (2006). Tectonic lineaments of inner East Anatolia show similarities with the direction of North Anatolian Fault
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Fig. 14. The comparison map shows between distribution of the epicentres and lineaments in Fig. 12 of the study area.
Zone and East Anatolian Fault Zone. Most of the lineaments could be identified by known faults. For instance, Northern Block of NeoThetys (NBNT), BelcikPazarcık Fault Zone (BPFZ), Deliler-Tecer Fault Zone (DTFZ), Yıldızeli Fault Zone (YFZ), Kangal Fault (KF), Yakapınar-G¨oksun Fault Zone (YGFZ) are in the same direction with the NAFZ. Malatya-Ovacık Fault (MOF) is the same direction with the EAFZ. On the other hand, there are many unknown faults or lineaments between MOF and YGFZ in the direction NE–SW (Fig. 12). The distribution of the epicentres (M ≥ 2.0) in the study area shows the activity of the lineaments. The most active area is the EAFZ in the study area. However, dense earthquake activity is observed around Divrigi, along the DTFZ, in the SE corner of the area (Fig. 14). The distribution of the magnitude changes increasingly from the NW to SE. The NE corner of the area presents different stress regime. The focal depths of the earthquakes are mainly at 10 km in the study area. However, they are deeper around the EAFZ than the other places and reach to 30 km. If the map of the arrows obtained from the disorientations of aeromagnetic anomalies in Fig. 8a and lineament map in Fig. 12 are combined, a rotational effect can be observed in the study area (Fig. 15). Anticlockwise rotation defined from disorientations of aeromagnetic anomalies (Fig. 8a) and the lineaments obtained from maxspots map produced from the gravity anomalies (Fig. 12) are in-line with the mobilistic system
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Fig. 15. Combination of interpretation of maxspots and disoriented magnetization arrows.
revealed by palaeomagnetic data (Storetvedt, 2003). It was described that there is a clockwise litospheric torsion of the northern palaeohemisphere and a corresponding anticlockwise in the southern palaeohemisphere. These Alpine age continental rotations caused westward wrenching of the global lithosphere and led to significant tectonic reactivation and deformations (Storetvedt, 2003). When the GPS measurements made by McClusky et al. (2000) in Fig. 2 is compared with the current tectonic knowledge depicted Fig. 15, a similarity can easily be observed that the anticlockwise rotation is evident. Acknowledgments I thank two anonymous referees for their critical review of the manuscript. I also thank the General Directorate of the Mineral Research and Exploration (MTA) of Turkey for providing digital topographic, borehole, aeromagnetic and gravity data. This research was financially supported by the Cumhuriyet University, Scientific Research Projects (CUBAP) (Project code: M-259). References Ates¸, A., Kearey, P., 1993. Deep structure of the East Mendip Hills from gravity, aeromagnetic and seismic reflection data. J. Geol. Soc. Lond. 150, 1055–1063. Baydemir, N., 1990. Palaeomagnetism of the Eocene volcanic rocks in the eastern Black Sea region, Istanbul Universitesi. Muh. Fak. Yerbilimleri Dergisi 7, 167–176 (in Turkish).
Bellier, O., Over, S., Poisson, A., Andrieux, J., 1997. Recent temporal change in the stress state and modern stres field along North Anatolian Fault Zone (Turkey). Geophys. J. Int. 131, 61– 86. Bilgic, T., 2002. Geological Map of Turkey (1:500.000 scale Sivas sheet). General Directorate of Mineral Research and Exploration (MTA) of Turkey, Ankara. Blakely, R.J., Simpson, R.W., 1986. Approximating edges of source bodies from magnetic or gravity anomalies. Geophysics 51, 1494–1498. Blakely, R.J., 1995. Potential Theory in Gravity and Magnetic Applications. Cambridge University Press, pp. 441. B¨uy¨uksarac¸, A., Reiprich, S., Ates¸, A., 1998. Three-dimensional magnetic model of amphibolite complex in Taskesti area, Mudurnu valley, North-West Turkey. J. Balkan Geophys. Soc. 1, 44– 52. 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. Cater, J.M.L., Hanna, S.S., Ries, A.C., Turner, P., 1991. Tertiary evolution of the Sivas Basin, Central Turkey. Tectonophysics 195, 29–46. Fedi, M., Florio, G., Rapolla, A., 1991. The role of remanent magnetization in the southern Italian crust from aeromagnetic anomalies. Terra Nova 2, 629–637. Gorur, N., Tuysuz, O., Sengor, A.M.C., 1998. Tectonic evolution of the Central Anatolian Basins. Int. Geol. Rev. 40, 831–850. Gursoy, H., Piper, J.D.A., 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. Kadioglu, Y.K., Ates, A., G¨ulec¸, N., 1998. Interpretation of gabbroic rocks in Agac¸o¨ ren Granitoid, central Turkey: field observations and aeromagnetic data. Geol. Mag. 135, 245–254. 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., Teableb, A., Toks¨oz, M.N., Veis, G., 2000. Global positioning system constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus. J. Geophys. Res. 105, 5695–5719. ¨ Over, S., Unlugenc, U.C., Bellier, O., 2002. Quaternary stress regime change in the Hatay region (SE Turkey). Geophys. J. Int. 148, 1–14. ¨ ¨ Over, S., Ozden, S., Unlugenc, U.C., Yilmaz, H., 2004. A synthesis: Late Cenozoic stress field distribution at northeastern corner of the Eastern Mediterranean, SE Turkey. Geodynamics 336, 93– 103. ¨ ¨ Ozden, S., Over, S., Unlugenc, U.C., 2002. Quaternary stress regime change along the Eastern North Anatolian Fault Zone, Turkey. Int. Geol. Rev. 44, 1037–1052. Piper, J.D.A., Moore, J., Tatar, O., Gursoy, H., Park, R.G., 1996. Palaeomagnetic study of crustal deformation across an intracontinental transform: the Anatolian fault zone in Northern Turkey. In: Morris, A., Tarling, D.H. (Eds.), Palaeomagnetism of the Mediterranean Regions, vol. 105. Journal of Geological Society of London, pp. 299–310 (Spec. Pub.). Platzman, E.S., Platt, J.P., Tapirdamaz, C., Sanver, M., Rundle, C.C., 1994. Why are there no clockwise rotations along North Anatolian fault zone? J. Geophys. Res. 99, 21705–21716. Storetvedt, K.M., 2003. Global Wrench Tectonics. Fagbokforlaget, Norway, pp. 397.
A. B¨uy¨uksara¸c / Physics of the Earth and Planetary Interiors 160 (2007) 86–95 Tatar, O., Piper, J.D.A., Park, R.G., Gursoy, H., 1995. Palaeomagnetic study of block rotations in Niksar overlap region of the North Anatolian fault zone, Central Turkey. Tectonophysics 244, 251–266. Temiz, H., 1996. Tectonostratigraphy and thrust tectonics of the Central and eastern parts of the Sivas Tertiary Basin, Turkey. Int. Geol. Rev. 38, 957–971.
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Yılmaz, A., 1985. Yukarı Kelkit C ¸ ayı ve Munzur da˘gları arasının temel jeolojik o¨ zellikleri ve yapısal evrimi. Geol. Bull. Turkey 28, 79–92 (in Turkish with English abstract). Yilmaz, A., Yilmaz, H., 2006. Characteristic features and structural evolution of a post collisional basin: the Sivas Basin, Central Anatolia, Turkey. J. Asian Earth Sci. 27, 164–176.