Tectonic controls on the Karlıova triple junction (Turkey): Implications for tectonic inversion and the initiation of volcanism

    Tectonic controls on the Karlıova Triple Junction (Turkey): implications for tectonic inversion and the initiation of volcanism ¨ ur Karao˘glu, Azad Sa˘glam Selc¸uk, Agust Gudmundsson Ozg¨ PII: DOI: Reference:

S0040-1951(16)30535-2 doi:10.1016/j.tecto.2016.11.018 TECTO 127320

To appear in:

Tectonophysics

Received date: Revised date: Accepted date:

28 June 2016 31 October 2016 14 November 2016

¨ ur, Sel¸cuk, Azad Sa˘ Please cite this article as: Karao˘glu, Ozg¨ glam, Gudmundsson, Agust, Tectonic controls on the Karlıova Triple Junction (Turkey): implications for tectonic inversion and the initiation of volcanism, Tectonophysics (2016), doi:10.1016/j.tecto.2016.11.018

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Tectonic controls on the Karlıova Triple Junction (Turkey): implications

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for tectonic inversion and the initiation of volcanism

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Özgür Karaoğlua,b,*, Azad Sağlam Selçukc, Agust Gudmundssonb a

Eskişehir Osmangazi University, Department of Geological Engineering, 26040 Eskişehir, Turkey b

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Department of Earth Sciences, Royal Holloway University of London, Egham, TW20 0EX, UK Yüzüncü Yıl University, Department of Geological Engineering, 65080 Van, Turkey

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Correspondence to: Ö. Karaoğlu, [email protected]; [email protected]

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Abstract

Few places on Earth are tectonically as active as the Karlıova region of eastern Turkey which comprises a triple junction (KTJ). Triple junctions result in complex kinematic and

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mechanical interactions within the lithosphere generating tectonic inversions and uplift, extensive seismicity and volcanism. Here we present new data, and summarize existing data, on the tectonic evolution of the KTJ in eastern Turkey over the past 6 Ma. In particular, we present a kinematic model for the KTJ and the surrounding area as well as new structural maps. The deformation or strain rate has varied over this 6 million year period. The maximum strain rate occurred between 6 Ma and 3 Ma, a period that coincides with the initiation of activity in Varto Volcano. We suggest that increased strain rate and the initiation of activity at the Varto Volcano may be tectonically related. Subsequent to its formation, the Varto Volcano was dissected by active faults associated with the Varto Fault Zone, including reverse, normal and strike-slip faults. During the past 3 Ma, however, the KTJ area was deformed dominantly through dextral crustal movements associated to right-lateral faults. This deformation resulted in the development of a NE-SW-trending extensional/ transtensional regime, together with a complementary NW-SE-trending contractional regime. In the past 6 Ma the east end of the KTJ has been subject to incremental deformation. This

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deformation has resulted in many episodes of faulting during (i) ongoing shortening phases driven by a regional-scale thrust tectonic regime, and (ii) local-scale transtensional phases

Introduction

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caused by westward extrusion.

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The theory of plate tectonics described plates as rigid, undeformable bodies that move along discrete fault zones that act as plate boundaries. Such plate boundaries end in triple junctions, i.e. points where three plate boundaries meet (e.g., McKenzie and Morgan, 1969; Şengör, 2014). In geological reality, particular when one or more plates adjacent to a plate

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boundary contain continental crust, deformation becomes regionally distributed rather than focused along a single fault. Along e.g. subduction plate boundaries, this gives rise to zones of

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distributed strain (orogens), and when such zones meet in triple junctions, complex deformation patterns can arise problem (e.g., McKenzie and Morgan, 1969; Cronin, 1992). A particular example of such a situation is the Karlıova triple junction in eastern

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Turkey (Barka, 1992; Sandvol et al., 2003; Şengör et al., 2004). To the east of this triple

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junction, overall NNE-SSW convergence between the Arabian and Eurasian plates is distributed in a wide zone of shortening that stretches as far north as the Caucasus. To the

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west of this junction, a third (micro) plate exists: Anatolian. The Anatolian plate is bordered by a left-lateral transform fault from Arabia, and a right-lateral transform fault from Eurasia plates (Fig. 1a). The Karlıova triple junction is hence a FFT (fault - fault - trench) triple

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junction, whereby the 'trench' is in reality a widely distributed thrust belt. In this paper we describe the complex deformation around the Karlıova triple junction and characterize how this distributed shear affects the tectonic evolution of the region. Kinematics and mechanics of the triple junctions provide important clues to the overall mechanical and thermal behaviour of the lithospheric plates (cf., Jarosinski, 2012). McKenzie and Morgan (1969) discussed the stability conditions of triple junctions using the velocity space. A triple junction is kinematically stable if the orientation of each plate boundary remains constant relative to the other boundaries at the triple junction throughout a finite or non instantaneous time-interval (Cronin, 1992). There are 16 possible types of triple junctions (e.g., McKenzie and Morgan, 1969). The Karlıova triple junction, the focus of this study, is a continental triple junction (Şengör, 2014) consisting of non-subductable continental crust (Figs 1 and 2). The convergence is here accommodated by strike-slip faults, resulting in escape tectonics at west of the KTJ. In this

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region, complex interactions result in complex lithospheric kinematics resulting in tectonic inversions and uplifts, extensive seismicity, larger-than-normal permeability resulting in

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increased groundwater flow, and frequent episodes of dyke emplacement, some of which

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culminate in volcanic eruptions. Earlier pioneering studies (e.g., Tibaldi, 1992; Bellier and Sébrier, 1994; Dhont et al., 1998) have emphasized the relationship between volcanoes and

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pull-apart basins, releasing bends, tension fractures and Riedel shears, all of which occur predominantly within strike–slip deformation zones (e.g., Cembrano and Lara, 2009). However, the spatial and temporal relationship between magmatism and the migration of the

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triple junction remains poorly constrained.

The seismicity and lithospheric structure of the eastern Anatolia plate and the northern Arabian plate have been studied by seismic tomography on a regional scale (e.g., Sandvol et

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al., 2003; Al-Lazki et al., 2004; Lei and Zhao, 2007; Schmid et al., 2008). Plate motions have been measured using InSAR (Walters et al. 2011; Cavalié and Jónsson, 2014) and GPS

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(McClusky et al., 2000; Reilinger et al., 2006; Özeren and Holt, 2010; Tatar et al., 2012) (Fig.

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2). Yet, there have been no detailed field-based studies focusing on fault plane kinematic analysis and dyke measurements. Such measurements are necessary to understand regional

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and local stress fields existing over time around the Karlıova-Varto region. In order to better understanding tectonic controls on the KTJ, and their relationship with volcanism in the Varto volcanic region (Fig. 3a), we need to address the following

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issues: (i) the nature of heterogeneity within the plate boundaries, i.e., investigate whether the plates are discrete or contain distributed strain (transtension, folding and thrusting, etc), (iii) the resultant tectonic pattern due to the complex kinematic history of the KTJ which is difficult to reconstruct, (iv) whether there is an extensive magmatism associated with the triple junction. Then, we aim conduct a field study focused on deformation and the relationship with magmatism (e.g., dyke orientations) in order to characterize the tectonic history of the KTJ (Fig. 3a). We provide new and combined structural data along with geological maps at a scale of 1:25.000 (Fig. 4) as well as a digital elevation model (DEM). The DEM images are interpolated using 1:25.000 digital contour maps with a horizontal and vertical resolution of 10 m. These new data are combined with previously published seismo-tectonic and geochemical data to provide new insights into the kinematics and the mechanical behaviour and deformation of the lithosphere at the KTJ. We explore the mechanical behavior of the

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upper crust and the effects of regional stress in controlling dyke ascent. Our study focuses on activity in the region during the past 6 Ma. The observations made are placed in the context of

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a model which explains the kinematics and segmentation of faults around the KTJ. These

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inferences allow an investigation of the temporal and spatial distribution of intrusions and

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volcanic vents over the 6 million year period.

Tectonic setting of the Karlıova region and Eastern Turkey Westward extrusion of the Anatolian plate gave rise to strike-slip motion along the

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North Anatolian Fault Zone (NAFZ) and East Anatolian Fault Zone (EAFZ) after the closure of the Neotethyan Ocean as a result of Arabia-Eurasian convergence (Barka, 1992; Okay and

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Tüysüz, 1999; Bozkurt, 2001). Propagation of the NAFZ started around 12 Ma and triggered lithosphere-scale transtensional deformation (Barka, 1992; Şengör et al., 2004). The length of the NAFZ is nearly 2000 km; it is connected to the Kephalonia fault zone along a fault

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through central Greece (see van Hinsbergen and Schmid 2012) (Fig. 1). The overall width of

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NAFZ increases from ~10 km in the east to ~100 km in the west (Şengör et al., 2004). The main active fault along the NAFZ is the right-lateral North Anatolian Fault (NAF).

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The East Anatolian Fault (EAF), a sinistral conjugate of the North Anatolian Fault (NAF), is 400 km long and has a total offset of c. 25 km. Cumulative displacement varies from 3.5 km to 40 km (Arpat and Şaroğlu, 1972) and has accumulated since the Pliocene

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(Taymaz et al., 1991; Westaway and Arger, 1996; Tatar et al., 2004). GPS data show that the eastern part of the Anatolian plate moves at a rate of ~20 mm/yr with respect to the Eurasian plate (McClusky et al., 2000; Reilinger et al., 2006). The left-lateral slip rate on the northern part of the EAFZ is less than 5 mm/yr, which is smaller than the 11 mm/yr right-lateral slip along the NAF (Aktug et al., 2013) (Fig. 1). Barka (1992) report that the total cumulative displacement along the main part of the NAFZ decreases from 40 ±5 km in the east to 25 ± 5 km in the west (Fig. 1). The westward decrease has been linked to the internal deformation of the Anatolian block. A total displacement of c. 85 km along the NAFZ is widely accepted (Armijo et al., 1999; Bozkurt, 2001; Westaway and Aeger, 2001; Hubert-Ferrari et al., 2002), whilst Şengör et al. (2004) argued that 85 km was too much. Hubert-Ferrari et al. (2009), suggesting that the Turna–Bingöl Caldera of the Karlıova region formed from 3.6 to 2.8 Ma and was subsequently dissected and right-laterally offset some 50

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km by the North Anatolian Fault. In order to determine the causes and consequences of block rotations of the upper crust at the KTJ, a number of paleomagnetic studies have been carried

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out and integrated with the GPS data (e.g., Piper et al., 2010; Tatar et al., 2004; Tatar et al.,

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2013). There is evidence for an anti-clockwise block rotation of 18° to 23°, as deduced from movement of Pliocene (2–3 Ma) rock units at the eastern extremity of the NAFZ and EAFZ

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(Piper et al., 2010).

The deformation around Karlıova is particularly clear along the Yedisu Fault of a structurally defined array of strike-slip faults which produce a triangular-shaped wedge (Fig.

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1). This segment, striking N105°E, represents the eastern branch of the NAF, which has a right-lateral slip, and extends for over 30 km to the KTJ (Fig. 1). The NAFZ and EAFZ meet

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at the KTJ and are characterized by transtensional tectonics. However, the kinematics of the Varto Fault Zone (VFZ) remains poorly constrained. The VFZ extends for over 50 km and can be subdivided into six segments (Fig. 4). Fault planes of the VFZ commonly exhibit

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region over the past ~6 Ma.

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multiple sets of striations, suggesting progressive and complex deformation within the KTJ

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2.1. Initiation of volcanism at the KTJThe first indication of volcanism at the KTJ occurred at Solhan approximately 6 Ma (Hubert-Ferrari et al., 2009). Using a fission track technique from obsidians (Poidevin, 1998), the Catak volcanism on the EAFZ, southern side of the Göynük (Fig. 4) is dated at around 6 Ma. Volcanic activity initiated in the Varto area

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much later, but was already in evidence at 3.6 Ma (Fig. 3b). Activity within the Varto Caldera is dated from basal volcanic units using a K/Ar whole rock technique which give an age of 3.6 ± 0.6 Ma (Pearce et al., 1990) and a

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Ar/39Ar groundmass of the basaltic rock technique

which gives an age of 3.1 ± 0.09 Ma (Hubert-Ferrari et al., 2009). 2.2. Segmented blocks of the KTJ Herece (2008) has established three faults of the Varto Fault Zone (VFZ), namely (from north to south), the Varto, the Leylekdağ, and the Çayçatı faults (Fig. 3). The Varto Fault exhibits several differential fault components. These segments include a 55-km long strike-slip fault segment with a minor reverse component to the west, and the active central or middle segment with a dominantly reverse component (Fig. 3). Hubert-Ferrari et al. (2009) proposed that this main strand of the Varto Fault Zone is a direct continuation of the Yedisu Fault of the NAFZ. Recently, Sançar et al. (2015) concluded that the southern part of the VFZ

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has been subject to counter clockwise rotation of about 20–25° since 2.8 Ma. Additionally, Sançar et al. (2015) suggest that the Bingöl (Varto) and Turnadağ volcanoes were never one

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coherent structure; thus there is no displacement between these units. The fault pattern, where

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the total strain is highly distributed, and the intense volcanic activity are additional evidence

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for the stationary triple junction model for this region (Sançar et al., 2015). In the southern part of the VFZ, there are three transpressional faults that constitute a

horsetail geometry (Fig. 4). One is the Teknedüzü Fault, which extends for 20 km and strikes N120°E. The second is the Leylek Fault (20 km long) and the third is the Çayçatı Fault (7 km

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long), both of which are thrust or reverse faults with differentially curved strands that are terminated by the Teknedüzü Fault. In 1966, a large earthquake (Mw = 6.2) occurred on the

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Leylek Fault, and also a small earthquake (Mw  3) on the Çayçatı Fault. The southern extremity of the Leylek Fault, extending for some 15 km, intersects a right-lateral strike-slip fault (Fig. 4). The northernmost segment of VFZ, called the Tuzlu Fault, is mainly a thrust

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fault. The western part of these faults is offset laterally by the EAFZ (Figs. 3 and 4). The

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newly identified Varto Fault (Fig. 5a) is the most seismically active segment of the VFZ; it is a N70°W-trending right lateral strike-slip fault with a reverse component that offsets the

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southern part of the Varto Caldera (or the Bingöl Caldera; Hubert-Ferrari et al., 2009). A destructive earthquake (Mw = 6.8) occurred on the Varto Fault in 1966 (Wallace, 1968; Ambraseys and Zatopek, 1968).

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Profound shortening in the KTJ area occurs 20 km south of the Varto Caldera along the Mus fold-and-thrust belt (Fig. 1; Şengör et al., 1985; Dewey et al., 1986; Hubert-Ferrari et al., 2009). The southern part of the Varto Fault Zone is within a compressional tectonic regime. In the following sections, we discuss whether the thrust faults have also caused the regional shortening and if so, what effects the shortening has had on deformation in the eastern extremity of the KTJ area. Tectonic activity has resulted in some deformation of sedimentary deposits (Figs. 5a, b), particularly in the plains around the villages of Teknedüzü and Yayıklı (Figs. 5c, d). The complex fault geometry is reflected in strongly folded lacustrine sediments with subhorizontal axial planes whose strikes are scattered around the NNE-SSW direction (Fig. 6c). Some structural elements of sedimentary sequences play a key role in accommodating late Miocene deformation, as discussed below. At its western extremity, the Karlıova-Varto region

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has been subjected to NE–SW-trending extension along Eryurdu Fault (Fig. 5a) while the

Structural analysis

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eastern part experienced shortening in the past 6 Ma (Fig. 6).

The mechanisms by which faults are reactivated during phases of compression or extension, resulting in uplift or subsidence, provide a good opportunity to study the interplay

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between regional-scale shortening and local extension at triple junction tectonic regimes. The orientations of the faults were measured in the field. Other observations included measurements of slickenside lineations which were used to determine the stress tensors

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through classical inversion methods (e.g., Sperner et al., 2003; Tibaldi et al., 2009). Our new structural data which concentrates predominantly on the eastern part of the KTJ has been integrated with the available temporal and kinematic evidence into a model for the tectonic

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evolution of the area. This model allows better understanding deformation in a region which

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controls of the KTJ.

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has limited data concerning the kinematics, timing of deformation phases, and structural

In order to illustrate the importance of the transtensional and transpressional deformation dynamics for the Karlıova region, NE–SW, NW–SE and E–W-trending master

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fault structures have been identified and studied (Fig. 7). In the later part of this section, the results of our studies of dykes, eruptive vents, and deformation structures at the KTJ are presented so as to support the conceptual model presented. To examine the surface expressions and tectonic evolution of the study area, 128 fault-slip data points from 16 locations of two transpressional and transtensional deformational regions were collected for palaeostress computations (Figs. 3 and 4). A fault slip analysis computer program by Allmendinger et al. (2012) was used to compute the characteristics of the fault-slip data sets. The computed results of the measurements, with quality index and eigen values, are presented in the Appendix A in Table S1. Following documentation of the selected kinematic diagrams, we present the calculated main stress tensor associated with the main fault kinematics (Fig. 7). 3.1. Morphotectonics analysis The VFZ dissects the southern part of the Varto Caldera (Fig. 4). The northern strand of the VFZ, Eryurdu Fault is a high-angle normal fault whose activity has resulted in an uplift

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of the northern sector by around 800 m, from the base to the top of the caldera. The VFZ, which probably originated from a pre-existing crustal-scale transformation zone, is also

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related to the initiation of volcanic activity in the Varto Caldera. Deep dissected valleys, intra-

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depositional unconformities within volcano-sedimentary deposits indicate an asymmetric

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uplift of the caldera volcano and incremental deformation since at least 3 Ma. Slip on reverse faults partly overprints the normal/oblique fault events and also fold the volcaniclastic and fluvio-lacustrine deposits. These faults led to several elongated WWNEES-directed small basins together with steeped terrace deposits. This shortening is

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characterized by pressure-ridges (Fig. 8). We have measured 18 pressure-ridges (Fig. 8). These are particularly common in the western margin of VFZ, between 270°- 320° strike;

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they reach 12 m in length and 5 m in height (Fig. 8). The pressure ridges reflect a NNE-SSWdirected shortening of the region. A pressure-ridge distribution at the western side of VFZ is most likely to indicate that more intense deformation occurred to the eastern part of the fault.

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with left-slip motion (Fig. 8).

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The right-steeping, en echelon alignment of the fault traces and pressure-ridges is consistent

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Alignments of springs of cold and hot water are an important parameter for detecting active faults. In the field study, springs display an alignment throughout the Yedisu Fault of the NAFZ, Çayçatı and Leylekdağı Faults (Fig. 8). Those strike-slip and reverse faults also ruptured during historical earthquakes which strongly suggests the faults are active (Figs. 1

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and 3).

The Varto region shows two distinct drainage patterns, namely (i) a radial outward pattern on the caldera flanks and (ii) a sub-parallel alignment pattern in the southern section (Fig. 8). The Koçkar River, flowing from inside of the Varto Caldera, is cut by the Varto Fault and has a 1500 m right-lateral offset (Hubert-Ferrari et al., 2009). This offset represents the maximum displacement for the region. In the vicinity of the village of Doğanca the Varto Fault has a 400 m right-lateral offset (Fig. 8). Four different river offsets were measured on Tuzlu Fault, ranging from a minimum of 220 m to a maximum of 630 m (Fig. 8). The drainage pattern implies river offsets by active faults. 3.2. Faults 3.2.1. Varto region/Varto Fault Zone (VFZ)

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Although it has been previously suggested that the VFZ is a single fault (HubertFerrari et al., 2009), we recognize at least four sub-parallel segments within this fault zone.

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Structural data, made on six faults and their splays indicate that fault kinematics exhibit a

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range of shortening and extensional regimes. Many of the faults show evidence of reactivation (Fig. 4). In this section we present our structural data on each fault segment within the Varto

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region. 3.2.1.1. Eryurdu Fault

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The Eryurdu Fault intersects the southern flank of the Varto Caldera resulting in deposition of fault-related colluvial deposits including volcanic breccias. This fault intersects

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the southern part of the Varto Caldera and is offset by the sinistral strike-slip Geyiksuyu Fault (GF) (Fig. 7). The well-preserved fault plane of the Eryurdu Fault is mostly exposed at the western part of the caldera at an altitude of nearly 2100 m, i.e., an elevation of ~250 m above

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the base of the caldera. A vertical topographic offset (throw) of 2 m occurs along the fault at

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this sector (Figs. 5b, c). The N85°W-striking Eryurdu Fault is around 18 km long with mostly high-angle and oblique-slip segments. Although the Eryurdu Fault displays high-angle normal

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fault slickenlines, some of the fault-planes show a dextral component towards the eastern termination of the fault. The dextral component of F15 shows SW-dipping oblique-slip; rakes of 25°S indicate dominant dextral strike-slip faulting with a minor normal component. This fault defines nearly horizontal σ2 and σ3 axes plunging 1° and 11° respectively, whilst the σ1

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axis is steeply dipping, plunging 79° (Appendix A in Table S1). Stress axes suggest that dextral movement developed under pure NE–SW extension. The average strike of the fault plane at location F17 is N10°W (Fig. 7). The western segment of this fault is represented by gently dipping fault planes (F5). The calculated principal stress axes of this fault plane (F5), namely, σ1, σ2, and σ3, show attitudes of 23°/11°, 282°/42°, and 125°/46°, respectively (Appendix A in Table S1). Measurements indicate a well-constrained NNE–SSW-trending extension. 3.2.1.2. Tuzlu Fault Although the Tuzlu Fault has little surface expression it can be traced from the village of Alabalık to the village of İçmeler based on mapping a series of discontinuous scarps in the volcanic rocks of the Varto group (e.g., Şaroğlu et al., 1985) (Fig. 7). This active reverse fault

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with a right-lateral component extends laterally for over 20 km. It is steeply dipping to the north and controls, partly, the tectonics of the western part of the Varto Caldera. The

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hydrothermal alteration is localized along a single narrow zone between the Eryurdu and

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Teknedüzü faults (Fig. 7). Fault and fold spatial configurations and kinematics (F6) suggest that the region is undergoing a NW-SE directed compression with a mean 5° horizontal

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maximum stress axis, and that this compression has persisted since the Late Miocene. The Tuzlu Fault deforms Pliocene fluvial deposits (Tarhan, 1991), resulting in a 0.5–1.5 m reverse offset (Figs. 6a, b). The fault deforms the Pliocene fluvial sediments (Tarhan, 1991) in

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restricted outcrops, resulting in upright-parallel folds striking N40 to N55°E towards to its south-eastern splays (Fig. 6c).

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3.2.1.3. Varto Fault

The first segment of this fault extends for 11 km between the village of Onpınar and

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the village of Çayönü, comprising well-developed antithetic strike-slip fault. This fault

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segment has low-angle fault surfaces, striking N75°W, and the measured rake (or pitch) of the faults is between 69° and 80°. A straight fault segment extends for 9 km and strikes N70°W.

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A zone of distributed compression accommodates the deformation between the dextral faults. This local compression affects Pleistocene lacustrine sediments (Tarhan, 1991), steepening the dip of their beds and cutting through NNW–SSE trending valleys (Figs. 5d and 6c). Dextral offsets in the deltaic alluvium and river valleys reach c. 1500 m along the easternmost

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segment of the Varto Fault. The striation set of F7 on the central segment has an average rake of 9°. Stress calculations for this fault yield a sub-horizontal σ1 axis (dip 15°) (Appendix A in Table S1). The fault displays well-developed slickenlines on reddish and yellow volcanic breccias of the Varto group (Figs. 6a, b). The second and third segments form a releasing step-over along a dextral strike-slip fault between Leylek and Seki villages toward east (Figs. 4 and 7). A right-stepping en-echelon pattern of fault F16 exhibits higher rake angles in accordance with the conjugate pattern of F7. Field-based kinematic features of this fault indicate oblique-slip normal fault surfaces dipping 55°S and with rakes of 35°E (Fig. 7). The easternmost segment is dissected by the sinistral Görgü Fault (Fig. 7). The eastern segment of VFZ terminates rather abruptly at the boundary between a Pleistocene volcanic sedimentary unit and lava flows of the Varto group. This segment is approximately 19 km long, strikes N60°W, and has a rake of 8°–12°S (Fig. 7).

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3.2.1.4. Teknedüzü Fault The Teknedüzü Fault, a thrust, strikes on average N75°W, in different curved strands.

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This fault forms a large horsetail with thrust faulting deforming the area of length 20 km (Fig.

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3a). The fault is easily detected on satellite and DEM images (scale: 1:10.000). This is likely

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to be one of the youngest faults because it dissects Pleistocene lava flows as well as recent lacustrine and alluvial deposits (Fig. 4). The fault shows well-preserved steeply-tilted bedding planes (75°) of fluvial deposits affected by intense compressional tectonics.

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3.2.1.5. Leylekdağ Fault

The Leylekdağ and Çaydağ reverse faults with minor dextral strike-slip components

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represent the eastern termination of the main strand of the Teknedüzü Fault. The Leylekdağ and Çaydağ faults are the main splays, extending eastwards at a distance of 8–12 km, with a fault zone width of 4-5 km. The Leylekdağ Fault can be traced for 20 km from the village of

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Ağaçköprü to the village of Y. Alagöz (Fig. 3a). During the Mw 6.2 earthquake of 20 August

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1966, the central part of the Leylekdağ Fault ruptured. The earthquake occurred at the southern base of the sharp topographic step on the slopes of Yılanlı volcanic edifice (at the

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Kavak and Leylek Hills) and produced extensive landslides and rock avalanches from basaltic lava flow unit (Lava 2 in Fig. 1b) close to margin of the mountain. The earthquake ranks as the deadliest in Eastern Turkey. Some pervasive deformations and alteration zones have also

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been observed in the basaltic lava rocks along the thrust fault. Although local people in the village described to us a vertical ground displacement up to 2 m during earthquake of 1966, we found no further evidence for that displacement. Colluvial deposits were observed along the base of the fault scarp. Fault scarps of F9, F10 and F11, extending to the eastern extremity, show oblique-slips to high-angle reverse fault components, indicating reactivation events (Fig. 7). These show average dips of 50°, 56° and 77° and average rakes (or pitches) of 34°, 55° and 71°, respectively (Fig. 7; Appendix A in Table S1). Fault sets display wellpreserved cross and/or conjugate faults; and suggest that the region experienced intense tectonic deformation. Most of the faults show reactivated structures at around the Varto region. Fault planes of F9 and F10 indicate a NE–SW-directed contraction associated with NW–SE extension, whilst coeval phase of F11 developed under NE–SW extension (Fig. 7). The fluvio-lacustrine sediments have been subject to successive and complex deformation at

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and around the Leylek village (Fig. 4). Those reverse, oblique-slips and normal faults have been accommodated coevally at several times.

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3.2.1.6. Çayçatı Fault

The Çayçatı Fault, a thrust, consists of two main segments, which differ both in their

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kinematic and morphological properties. These segments are delimited by fault bends at İlbey (Fig. 3a) where the strike of the primary deformation zone changes by 10° to 30°. The fault is connected by a N80E°-striking fault orientated parallel to the main-segment displacement

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vector (Fig. 7). The western segment, from Köprücük to Yeşildal, exhibits a N70°W-striking thrust component on a 7-km-long transpressional fault. This fault controls the colluvial

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deposits on the steeply dipping hanging wall of the basaltic basement rock. It forms a smallscale thrust-related basin along the southern part of the Çayçatı Fault (Fig. 4). Fault scarps and related deformation generated by the 1966 earthquake can still be observed in this area.

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Evidence for the earthquake faulting is much more abundant between Çayçatı and İlbey and

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includes groundwater springs (18-20°C) aligning along these faults. Likewise, local transpressional geologic features occur across a range of scales in this small basin.

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Displacements of meters are noted within sag ponds and small pressure ridges (Figs. 5a and 8a). The dominant orientation of these push-up-related structures is around N80°W. The more recent activity is evidenced by tilting towards the major fault (20°–25° NW), dissected Pleistocene lacustrine terraces and recent alluvium deposits. The concave and curvilinear

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range-front fault trace shows a right-lateral component on some segments. Kinematic features of this fault (F12) include high angle reverse fault surfaces with gentle angles between 87°-89°N with rakes (or pitches) of 18°-21°S (Fig. 7). The fault (F12) has attitudes of 31°/08°, 155°/76°, and 300°/11°. Stress axes σ1 (31°/08°) and σ3 (300°/11°) are close to horizontal, whilst intermediate stress (σ2) is close to vertical (155°/76°; Appendix A in Table S1). The kinematic data from F12 suggest that in the area NW–SE-trending extension is associated with NE–SW contraction (Fig. 5c). 3.2.1.7. Interpretation of Varto Fault Zone (VFZ) The southern part of the Varto region has been subject to a very complex deformation pattern that has produced strong mechanical heterogeneity in the host-rock. The VFZ is composed of sets of discontinuous faults with differential kinematics and associated sets of

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striae (Fig. 7). Fault-slip chronologies and slip-vector inversions indicate alternating strikeslip and reverse faulting that corresponds to regionally and/or locally significant stress

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regimes along the KTJ.

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The Eryurdu Fault is a high-angle normal fault that has displaced large parts of the

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western side of the Varto Caldera (Fig. 5). The fault has generated 1.5 to 2.7 m displacement in the 2.6 Ma Pleistocene lava flows (Pearce, 1990; Hubert-Ferrari et al., 2009). The dextral Tuzlu Fault, mostly steeply dipping to the north (rarely to south) also deformed the caldera and triggered debris avalanche deposition through the western part of the volcanic edifice

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(Fig. 5a). Two distinct landslides, of area 6 km2, occur along the southern part of the Tuzla Fault. The debris avalanche deposits are composed of basaltic rocks from the lava flows (2)

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(Fig. 3a).

The Varto Fault is a major agent controlling the eastern part of the KTJ. Slip

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measurements of this fault indicate primarily 280° strike and mostly right-lateral motion,

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suggesting a transpressional stress regime. However, there are also some reverse components on the fault plane (Figs. 5b, 5c). There are dextral offsets reaching 1.5 km on the alluvial fan

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around the village of Yayıklı, as well as 2 km offsets in Pliocene lava flows around the village of Sazlıca. Moreover, in between the villages of Çaylar and Doğanca the Varto Fault has produced intense folding and other deformation in the lacustrine limestone and fluvial

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sequences (Fig. 5d).

The thrust faults of Çayçatı and Leylekdağı dissect 150-m-thick colluvial deposits at the contact between Pliocene volcano-sedimentary units and volcanic rocks. Alluvial deposits tectonically overlying the basement-volcanic rocks particularly around the village of Çayçatı are tilted by as much as 70°. Hence, the colluvial sediments deposited on the hanging-walls of the Leylek and Çayçatı faults have been tilted. 3.2.2. Karlıova region (NAF and EAF) The displacement of the Anatolian extrusion-block has generated extension through the transform boundary motion of the NAF and the EAF (Fig. 9). On the extruded block, we recognise great differences in observed crustal deformation because of crustal shortening in the vicinity of KTJ. West of the KTJ, the Anatolian block forms a thin-skinned pull-apart basin, controlled by strike-slip and normal faulting (Fig. 9). The continuation of the Yedisu

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Fault of the NAFZ strikes 275° and has 75 cm lateral offset on Pliocene deposits (Fig. 10a), and also at around Kargapazarı accommodates to previous reverse faults (Fig. 10b).

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Hydrostatic pingos formed as a result of hydrostatic pressure on water from permafrost (van Vliet-Lanoë et al., 2004) show a parallel alignment with the Yedisu Fault around the village

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of Kargapazarı (Fig. 10c).

The stress tensors calculated from the data on the western part of KTJ show a dominating E–W-directed extensional stress regime compatible with the kinematic regime of the extrusion tectonics of the Anatolian block. Two fault sets display well-preserved

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slickensides, with pure strike-slip and high angle-normal fault motion. The fault L13 shows two phases of movement. The fault has attitudes of 177°/62°, 330°/25°, and 65°/11°

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(strike/plunge) (Appendix A in Table S1; Fig. 7). These results indicate that fault L13 is a sinistral strike-slip fault (Fig. 7). These observations also show that stretching along the EAF is partitioned into NE–SW-trending nearly pure sinistral strike-slip faults with rakes of 3°, at

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least close to the surface.

Fault sets display well-preserved slickensides, with stereographic plots showing

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normal offsets dipping at an average of 65°. Principal stress axes of this fault plane of L14, σ1, σ2, and σ3 show attitudes of 182/°066°, 54°/17° and 318°/19°, respectively (Appendix in Table S1; Fig. 7). Measurements indicate a well-constrained deformation that shows NE–SW-

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trending pure extension.

4. Dykes

The orientation of dykes is controlled by regional and/or local stress fields existing at the time of dyke emplacement, Since dykes generally propagate in a direction perpendicular to the minimum principal compressive (maximum tensile) stress σ3, dykes are very useful tools to infer the orientation of the principal components of the paleo-stress field (e.g., Hempton et al., Dewey et al., 1986; Adıyaman et al., 2001; Gudmundsson, 2006). The complexity of a dyke swarm is an indication of variation in the local stress field within the volcanic zone or volcanic edifice during the dyke-swarm development (Gudmundsson, 2011). The term dyke here is here used for all sheet-like intrusions. The extent to which regional and local tectonic stresses control volcanic activity in a given region can be indirectly assessed by comparing the tectonic stress directions with those

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obtained from the alignment of dykes, vents and crater cones in volcanic zones or fields. We assume that at a local scale, the direction of maximum instantaneous extension (S1) must be

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similar to the direction of the horizontal minimum principal stress (σ3), which in turn is

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considered to be perpendicular to the alignment of volcanic centres. We call this direction S1_local to distinguish it from the direction of maximum instantaneous extension for the KTJ,

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which is estimated from regional tectonic reconstructions, (S1_regional). The comparison of S1_regional with S1_local is then interpreted in terms of the influence of tectonic stresses in controlling the localization of volcanism at the Varto region. 40

Ar/39Ar groundmass ages of 0.4–0.7 Ma from dykes

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Hubert-Ferrari et al. (2009) report

around the Yılanlı and Çayçatı area (Fig. 3b); also some dykes from the southern part of the

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volcano yielded fission track ages of 1.9–2.6 Ma (Fig. 3b). There is lack of radiometric age data for the dykes around the Varto Caldera. The expected orientation of the regional axis of maximum instantaneous extension (S1_regional) is 323° during the period 1.9–2.6 Ma.

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Concerning the orientation of S1_local, our alignment analysis reveals that a preferential

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orientation of dykes within the southern sector have the same azimuth of 285° over the last 1

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Ma (Fig. 11) (e.g., Negrete-Aranda et al., 2010). A total of 376 dyke strike and dip measurements were made at 53 sites over two locations, namely (i) inside the Varto Caldera and (ii) in the southern part of the Varto Caldera (Fig. 11). Dyke thickness ranges from 0.5 m to 18 m for the southern sector and in the

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caldera. Two main types of dykes are easily distinguished in the subaerial units: vertical or subvertical dykes and slightly inclined dykes commonly referred to as inclined sheets (Gudmundsson, 2006). Unlike the vertical dykes, which exist in all the units, the inclined dykes or sheets are only seen in the western part of the caldera wall. At some sites the dyke distribution is unimodal with a range of variation which does not exceed 10° at the southern sector. In the caldera, however, the range is much greater. Bimodal or polymodal distribution patterns within the caldera indicate the existence of several different local stress fields and resulting dyke/sheet swarms. The cumulative volume of dyke rock in the southern sector is much higher than in the northern sector. We estimate that the percentage of dyke volume reaches about 80% of the total rock volume, the remaining host rock (20% by volume) being mainly basaltic lava flows in this sector.

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4.1. Varto Caldera We measured and recorded dykes at 11 distinct locations or sites. The dykes are

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primarily composed of trachy-basaltic compositions, inside and around the Varto Caldera.

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Three dykes display NE–SW-orientations; eight dykes strike NW–SE (Fig. 11), with a mean value of 323°. The two main volcanic domes are 0.46 ± 0.24 Ma and 0.73 ± 0.39 Ma old

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(Hubert-Ferrari et al., 2009). The ages of the domes are likely to coincide roughly with that of dyke emplacement inside the caldera and along the caldera walls. We estimate an age of ~ 0.4–0.7 Ma for the dominantly NE-trending dykes and similarly oriented faults with NW-

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4.2. Southern part of Varto Caldera

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trending main strain or dilation in the region (Fig. 11).

For the 38 observed dykes in the southern sector an E–W strike is more common than a NW–SE strike (Fig. 11). All the dykes are basaltic. The rose diagram in Fig. 11 shows a

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dominant strike of about 285°. The dominant mean strike and striations indicate a N–S-

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trending strain of the crust around the region at 1.9–2.6 Ma (Fig. 3b).

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4.3. Point pattern analysis of dykes and preserved volcanic vents The distribution of intermediate and basic dykes shows no preferred orientation in the northern sector. However, some of the dykes strike NW–SE in the caldera. Except for two dykes, the strike of dykes inside the caldera is primarily NW-SE (50°) (Fig. 11). In addition,

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dominantly NW–SE-directed intrusion alignments support a dominated NE–SW-oriented extension since the initiation of volcanism around 3.6 Ma (Pearce et al., 1990). The stress orientation marked by the alignment of volcanic fissures, or aligned crater cones, can easily be related to the tectonic stress field (Nakamura, 1977; Gudmundsson, 2011). All the volcanic fissures are presumably fed by dykes, so the connection between fissure strike and dyke strike is normally clear in volcanic areas and can be used to infer the stress field at the time of dyke/fissure formation.

5.

Discussion Information on the volcanism during each tectonic evolutionary stage in the KTJ

region can be used to represent individual tectonic periods (Fig. 3b), including the stressfields operating during dyke emplacement.

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There is a consensus on the timing of the EAF (Şaroğlu, 1985; Dewey et al., 1986; Westaway and Aeger, 2001) as Pliocene, which varies between 5.3–2.6 Ma (Dewey et al.,

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1986) and less than 4 Ma (Şaroğlu, 1985; Westaway and Aeger, 2001). It has been previously

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reported that the volcanic activity commenced on the EAF with some eruptions producing acidic rocks overlying EAF whose ages are between 4.4–6.06 Ma (Poidevin et al., 1998). This

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shows that the age of the EAF should be at least 6 Ma. We suggest that the first volcanotectonic activity or event, at about 6 Ma indicates the initiation of crustal deformation in the EAF. As for the deformation of Varto and Turnadağ volcanoes, the age of the basement

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volcanic rocks in the southern part of the Varto is around 3 Ma (Fig. 3b). The lifespan of Varto Caldera volcanism has been reported as between 3 to 1 Ma (Hubert-Ferrari et al., 2009, references therein). This may be regarded as the second major volcanotectonic episode or

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event in the area. The resulting domes are dated at approximately 0.73 Ma to 0.46 Ma (Fig. 3b). The third event or activity in the area relates to some small-scale volcanic activities on

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the southern sector of the VFZ.

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5.1. Implication for fault kinematics

Data shown in the previous sections allow us to distinguish between three main

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volcanotectonic events from the Miocene to Recent. These events will be described here and compared with other tectonic studies in the area. The structural data collected around the KTJ suggest that this area has undergone significant strike-slip deformation since the Pliocene. The

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combined motion of NW-trending dextral, normal, oblique and thrust faults indicates a successive and reactive tectonic phase that caused incremental complex movement of numerous fault blocks during the deformation of the Karlıova and Varto region since 6 Ma. Fault kinematic data (fault planes and associated striations) commonly show multiple sets of striations, revealing a poly-phase tectonic history (Fig. 7). 5.1.1. First event (6–3 Ma) In the conceptual model in Fig. 12, we assume that the identified extensionaltranstensional structural features from the former phase of the VFZ may have been active and related to the same deformation event or stress field (Fig. 12e). The NW–SE-trending highangle Eryurdu Fault (EF) likely represents the initiation of this event following the deformation of the NAF accommodated by the EAF, which in turn deformed the southern part of the caldera. The direction of maximum compressive principal stress (σ1) indicates a NWSE trending extension. This phase is partly the response to extrusion tectonics,

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accommodating strain around KTJ which is the pivot point for the tectonic escape. The seismic activity of the VFZ has been shown to be the result of long-term deformation.

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Regional tectonics generate local stress fields, some of which are favourable to magma

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transport through dykes. Hence tectonic activity at 6-3 Ma probably contributed to the triggering of the volcanic activity of Varto Volcano. Several successive colluvial packages

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which consist of intra-depositional unconformities also suggest continuous tectonic activity. Faulting at the eastern part of the KTJ results in progressive migration of tectonic activity from northeast to southwest (Fig. 4) that reflects a southward shift of the strain in the

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region. The oldest structures are offset by all the faults and are accommodated by the seismically active VFZ. Thick colluvial volcanic breccias along the Eryurdu Fault is

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considered to be the most likely formed during the successive destruction of southern part of the Varto Caldera. Colluvial volcanic breccias, about 200 m thick, represent the hanging wall of the Eryurdu Fault. A vertical 800 m offset caused by tilting and deposition of the thick

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colluvial volcanic breccias may indicate a comparatively fast uplift of the footwall of the

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Eryurdu Fault induced by tilting of the southern margin of the caldera (Fig. 5a).

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5.1.2. Second event (3–1 Ma)

The kinematic data (Fig. 7) suggest that a right-lateral motion developed under a NE– SW-trending extension associated with NW–SE contraction (Figs. 6b, 6c). The fault surfaces

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of the WNW–ESE-striking VFZ were reactivated at 3 Ma, which suggests that inversion tectonics occurred when extensional faults of the basin reversed their movement during subsequent compressional tectonic episode (e.g., Williams et al., 1989). Since 3 Ma, thrusting shifted further south, coupled with a component of dextral strike-slip motion. Folds are consistent with a NW–SE direction of shortening in the northern part and E-W shortening to the south (Fig. 6c). 5.1.3. Third event (<1 Ma) The last deformation records a right-lateral dominated faulting with reverse motion on fault surfaces in the southern part of the region. The structures are entirely consistent with the tectonic inversion of normal and/or strike slip faults (e.g., Williams et al., 1989). Since 1 Ma, the region has been subject to dextral strike-slip faulting. Strain and shortening directions show a major inversion from the second event (3-1 Ma) to the present (Fig. 12). Based on our

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kinematic data, the primary movement in the area is NE–SW compression. In 1966, a Mw=6.8 earthquake (Wallace, 1968; Ambraseys and Zatopek, 1968) ruptured the eastern part

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the Varto Fault (Fig. 3a). The mechanisms responsible for the aftershocks of this earthquake

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are still debated (Hubert-Ferrari et al., 2009). Wallace (1968) reported that faulting is rightlateral, whilst McKenzie (1972) proposed right-lateral movement with a thrust component

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based on the solution of focal mechanisms. From 1 Ma to present day, the region experienced inversion tectonics characterised by right-lateral and thrust faulting.

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5.2. Implication of volcanic emplacement mechanism

Exploring how regional structures and stress fields may affect or control volcanism is

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a major task for understanding the volcanotectonic evolution of active areas (e.g., Spinks et al., 2005; Gudmundsson, 2012). Most authors accept that the NAFZ started its activity at circa 12 Ma, while the EAFZ developed at around 6 Ma (Şengör et al., 2004). Following these

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processes, tectonic escape and related strike-slip tectonic regimes dominated in the region.

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After a dormant period of 2 Ma, the first volcanic activity commenced with regional strain induced from the KTJ at around 3 Ma. Dyke emplacement at the southern sector indicates a

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possible dominant direction of dilation since 3 Ma (Fig. 11). The parallel alignment of the southern sector dyke swarms shows a stress field with maximum principal compressive stress (σ1) in N–S direction and minimum principal stress (σ3) in the E–W direction. The strain direction or dilation associated with the dyke emplacement confirms that the volcanism was

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controlled by NEE–SWW-trending zone of weakness. Strain is likely in accordance with our kinematic data from faults during the first tectonic event or transition from the first to second event.

Tectonic controls of the Varto Caldera and surrounding structures remain poorly known in the southern sector. Inside the caldera, the NW–SE directed extensional faulting coincides with the general trends of the dykes. Radiometric age data is lacking for those dykes, but they may represent late stage volcanism. Several cross-cutting dykes likely represent the different local stress regimes. The Turnadağ volcanism is coeval with Varto volcanism, located in the western part of the KTJ and directly associated with westward extrusion tectonics. Those strike-slip faults

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associated with the initiation of the volcanism have continued to deform these volcanoes up to present.

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5.3. Evolutionary stages of tectonic setting

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There are two contrary options following the occurrence of the FFT-type Karlıova

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triple junction (Fig. 13). If the Karlıova triple junction migrates westwards along the NAFZ overtime, which results in extrusion tectonics, this then implies that east of the original position of the KJT at 6 Ma has been subjected to a substantial displacement (Figs. 13a-b). In that case, only thrusting along the VFZ should be observed. Between that point and the

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modern position of the KTJ, we should observe (i) thrust faults contained within a portion of the original NAFZ before the KTJ migrated along it; (ii) right-lateral strike-slip faults

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overprinted by shortening. Our field evidence (i.e., VFZ, including high angle and oblique fault segments) of small-scale displacement (2-5 km) does not support the view that states the

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KTJ has migrated (e.g., Hubert-Ferrari et al., 2009; Figs. 13a-b).

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The view of “the Boomerang-shaped basin” is postulated by Sengör (2014) aims to explain the stability condition of the triple junction at the eastern part of the KTJ (Figs. 13c-

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d). The convergence between Arabian and Eurasian plates is turned into convergent strain by the deformation of the circle turns into an ellipse (Şengör, 2014). The plate boundary zone between these Eurasian and Arabian plates is shortened by 50%, while the Anatolian plate remains entirely rigid (Şengör, 2014; Figs. 13c-d). It is expected that (i) a slightly

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displacement occurs eastern part of the KTJ. This process may result in the generation of normal faults within the basin displaying complex strain (e.g., McKenzie and Morgan, 1969; Cronin, 1992; Şengör, 2014). Our field evidence regarding transtensional periods at the eastern side of the KTJ; which is controlled by normal and oblique faults with a few km in length displacement is more compatible with this view. In order to better understand the regional and local stress field and structural reasons for the driving mechanism of magmatism, we here examine the most relevant tectonic evolutionary stages in relation to the KTJ. 5.3.1. Initiation of the North Anatolian Fault Zone/Block Rotation (12 – 6 Ma) It is widely accepted that the NAF formed approximately 13 to 11 Ma ago in the east and propagated westward (Şengör et al., 2004) (Figs. 12a, b). By contrast, Adıyaman et al.

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(2001) suggest that the deformation along the NAF propagated through time from west to east, the first events being late Oligocene in Galatia Massif (western part of the NAF), late

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Miocene in Niksar (central part of the NAF), and late Pliocene in Erzincan (eastern part of the NAF). The second strike-slip event began in the late Miocene in the Galatia Massif, and is

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early Pliocene in Niksar and Quaternary in Erzincan (Adıyaman et al., 2001). Volcanism was

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related to the strike-slip motion of the NAFZ, however, the more voluminous volcanism in the region remains poorly resolved and enigmatic because the NAFZ is one of the largest active strike-slip fault zones in the world. Paleomagnetic studies on the volcanic rocks throughout

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the eastern part of the NAFZ show that anti-clockwise rotation dominated the early stage volcanism from Late Miocene to Pliocene (Piper et al., 2010; Tatar et al., 2013). During this stage, in the absence of tectonic activity of the EAFZ, the NAFZ forced an anti clockwise

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rotation of the Anatolian block (Fig. 12a).

5.3.2. Initiation of the East Anatolian Fault Zone/Extrusion Wedge Tectonic (6 – 3

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Ma)

Following to developing of the EAFZ at around 6 Ma, there was very limited volcanic

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activity at the EAFZ (Fig. 12b). Westward extrusion generated a stress field suitable for magmatism at the KTJ. The wedge extrusion accommodated by high strain encouraged magmatic paths as feeders for the volcanism. The period represented the initiation of the minor volcanic activity caused by major extension at a local scale. Extensive volcanism began

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at and around the KTJ about 3 Ma (Hubert-Ferrari et al., 2009). Volcanic centres were common at the step-over splays of the NAFZ and the EAFZ such as in the Erzincan Basin, the Ovacık Basin, and the Çatak and Karlıova-Varto regions. Those step-over structures may have contributed to the formation of magma paths from depth (e.g., Shabanian et al., 2012). 5.3.3. Wide-spread volcanism under an inversion tectonic (<3 Ma) Extensive volcanism, mostly high-K calc-alkaline, occurred at the Karlıova from about 3 Ma (Hubert-Ferrari et al., 2009) (Figs. 12c, 12d). Many, mostly basaltic (some intermediate) lava flows and pyroclastics were emplaced around the KTJ (Pearce et al., 1990; Buket and Temel, 1998). The VFZ partly controlled the initiation of the Varto Volcano (Figs. 12e-h). Subsequently, the fault zone incrementally deformed the main cone and post-caldera stages of the Varto Volcano. Following the wedge extrusion, the NAFZ was delineated at Karlıova, but

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part of the elastic energy was transmitted to the VFZ, reflecting the conjugate part of the NAFZ towards the east. The most recent stage of the volcanism in this area, since about 1 Ma,

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is characterised by several domes and intrusive swarms.

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On the basis of our mapping and geological findings, dextral strike-slip, normal and

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reverse faults incrementally deformed the southern part of the Varto Volcano since around 3 Ma (Figs. 12e-h). Several fault surfaces record reactivation or different fault components on the VFZ. Volcaniclastic sedimentary colluvial deposits on the hanging-walls of the fault strands of the VFZ record shortening and associated strain at certain intervals in the region.

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These findings indicate that the region has undergone some compressional tectonic events in the late Miocene to recent. This compression is still ongoing in Eastern Turkey. Wedge

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extrusion is responsible for an extensional regime around the Karlıova Triple Junction. However, the collision between the Anatolian and Arabian plate, and associated thrusting, results in shortening in the upper crust. The fault segmentation pattern of the VFZ records

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evidence of earlier compressional tectonics and of subsequent transtensional tectonics.

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Negative fault inversion may occur during extensional/transtensional reactivation of preexisting reverse fault (e.g., Williams et al., 1989; Doglioni et al., 1996). Therefore, the region

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has undergone inversion tectonics and associated strain induced by dextral strike slip faulting restricted by KTJ and shortening phases driven by regional-scale thrusting (Fig. 12).

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5.4. Offset of the NAFZ

Experimental and field-based studies of triple junctions commonly suggest that the shear stresses and total displacements increase across the master transtensional faults (Cronin, 1992; Mouslopoulou et al., 2008; Dooley and Schreurs, 2012). For non-orthogonal intersections in which the slip vectors on the intersecting faults are not parallel, as is the case at the intersection between the North Anatolian Fault and the North Aegean Rift, displacement transfer requires rotation of the slip azimuth in the region where the faults intersect (Mouslopoulou et al., 2008). Hubert-Ferrari et al. (2009) proposed a right-lateral offset of 55 km (18.3 mm/yr) for the eastern part of the KTJ, since about 3 Ma. However, this scenario does not fit the geology of the KTJ. First, the eastern extremity of the NAFZ, called the Yedisu Fault, does not exhibit any structural contact to the Turnadağ Volcano. The Varto Caldera and the Turnadağ Volcano display very different volcano-stratigraphy. For example, widespread and lava-like

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ignimbrites erupted from Varto Caldera can not been observed at the Turnadağ Volcano. Second, the EAFZ terminated the NAFZ 6 Ma ago at Karlıova (Fig. 12b). The Varto Fault

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commenced its activity following this cessation, as a likely conjugate of the NAFZ. The

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EAFZ has been subject to lateral displacement of the NAFZ since 6 Ma. Structural and volcanological evidence is not consistent with the suggestions of Hubert-Ferrari et al. (2009).

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It is expected that minimal strain field and displacement ratios at the triple junctions are controlled by transtensional faults. Consequently, the assumption that Turnadağ, the opposite half of the Varto Caldera experienced a left-lateral displacement of 50 km along the mean

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direction of the NFZ and the VFZ but this does not match the long-term evolution of the KTJ. Our data show a variation of 2-5 km displacement around the KTJ. Sançar et al. (2015) suggested that the Turnadağ and Varto volcanoes were never one coherent structure; thus

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there is no displacement between these units

Structural and stress data indicate that KTJ has shown distinctive kinematic behaviour

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since 12 Ma. Following the formation of NAFZ and EAFZ, the western part of the triple

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junction has been subject to transtensional tectonics. Whilst, the east end of the KTJ has been subject to incremental deformation induced by numerous faulting during (i) ongoing

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shortening phases driven by the regional-scale thrust tectonic regime and (ii) transtensional phase caused by westward extrusion tectonics in a local-scale (Figs. 12e-h).

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6. Conclusions

New data, as well as data compiled from the literature, make it possible to reconstruct the tectonic evolution of the Karlıova Triple Junction (KTJ) in eastern Turkey and infer the mechanism of associated volcanism. In particular, our results are based on new kinematic data from and around the KTJ. The main conclusions of this study are as follows. (1) NW-trending dextral, normal, oblique and thrust faults indicate deformation and sustained accommodation of lateral/horizontal movements since 6 Ma. The region has undergone incremental deformation around Karlıova within the past 6 Ma. During the period from 6 Ma to 3 Ma the strain rate reached its maximum and coincided with, and may have partly triggered, the initiation of the activity of the Varto Volcano, a caldera. Subsequently, the Varto Volcano was gradually deformed by later faults.

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(2) Our computed displacement data suggest that, since about 3 Ma, a right-lateral motion at the east end of the KTJ along the NAF has developed into a NE–SW-trending

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extensional regime associated with NW–SE contraction. The NW–SE-striking deformation

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phase reactivated the faults of the VFZ, which represents inversion tectonics induced by compressional-related extensional tectonics. However, the orientation of intrusions indicates

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N–S-directed extension as well as successive tectonic phases caused by inverse tectonics during the period from 1.9 to 2.6 Ma.

(3) During the past 3 Ma, the southern part of the Varto Caldera has experienced

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intense internal tectonic deformation, primarily through dextral strike-slip faults (as well as some reverse and normal faults) dissecting the caldera. The east end of the KTJ has been

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subject to inversion tectonics induced by dextral strike-slip and reverse faulting during ongoing shortening phases driven by a regional-scale thrust regime.

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Acknowledgments

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This study was supported by funds of the Yüzüncü Yıl Üniversitesi (Project No. 2014MİM-B062). The data for this paper are available through contacting the corresponding

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author. We thank J. Browning, E. Advokaat, V. Acocella and L. Selçuk for fruitful discussions and helpful suggestions. The manuscript was much improved by the constructive critical reviews by D. van Hinsbergen, as well as by the editorial handling by R. Govers. We

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appreciate the great hospitality of the Alevi people during our field studies.

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Figure captions Fig. 1. a) Major elements of crustal deformation for the eastern Mediterranean and Anatolia

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(Şengör et al. 1985, Armijo et al. 1999). NAF: North Anatolian Fault, EAF: East Anatolian

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Fault, BSZ: Bitlis-Suture Zone, YS: Yedisu Fault, MF: Mus fold-and-thrust belt, CF: Çaldıran

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Fault, VFZ: Varto Fault Zone, KTJ: Karlıova Triple Junction; b) map showing GPS velocities with respect to Eurasia and 95% confidence ellipses. Green vectors are Reilinger et al. (2006) and red vectors are Ozener et al. (2010) and the focal mechanism solutions from Tan et al. (2008) for the study area. Numbers represent strike-slip component of the fault, numbers

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within parentheses represent normal component of the fault, red lines are block boundaries

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(Aktug et al. 2013). Seismic data are from KOERİ (http://udim.koeri.boun.edu.tr/zeqdb/). Fig. 2. a) Velocity triangle of a Karlıova-type triple junction. b) The dashed lines ab, bc and ac in the velocity triangles represent velocities that leave the geometry of the boundary

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between plates A and B, B and C and A and C, respectively, unchanged (modified from

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Fowler 1995). If the VFZ (=ab) is not parallel to the NAF (=bc), then the KTJ is unstable. Fig. 3. a) Shaded relief basis map showing the main faults and seismicity around the Karlıova

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and Varto region. Active faults are modified from Herece and Akay (2003) and Sançar et al. (2015). Seismicity data from KOERİ. NAF: North Anatolian Fault, EAF: East Anatolian Fault, KTJ: Karlıova Triple Junction, YKFZ: Yorgançayır-Kaynarca Fault Zone, VF: Varto

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Fault, EF: Eryurdu Fault, KF: Kogu Fault, GF: Güzeldere Fault, TF: Tuzlu Fault, TeF: Teknedüzü Fault, LF: Lekleydağı Fault, ÇF: Çayçatı Fault, GoF: Görgü Fault; b) Age data on the DEM-based map. Triangles showing ages (Ma) of the volcanic rocks. (1) Chataigner et al. (1998); (2) Bigazzi et al. (1997); (3) Poidevin et al. (1998); (4) Pearce et al. (1990); (5) Innocenti et al. (1982a); (6) Bigazzi et al. (1996); (7) Bigazzi et al. (1998); (8) Innocenti et al. (1982b); (9) Hubert-Ferrari et al. (2009). Fig. 4. a) Geologic map of the study area (1:25.000 in scale). The map was revised from Herece and Akay (2003) (and references therein). Numbers indicate K-Ar and Ar-Ar ages of the volcanic rocks, from Pearce et al. (1990); Hubert et al. (2009); b) simplified geological cross-sections showing southern part of the Varto Caldera. Fig. 5. a) Field photographs of the various types of faults in the Varto area. Eryudu Fault (EF) deformed the southern part of the Varto Caldera, also related debris avalanche-deposits; b) EF

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extending over caldera flank (2200 m in altitude), forming some plains; c and d) those are from at around the villages of Teknedüzü and Yayıklı. Right-and left lateral strike slip faults

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with normal components dissected the fluvio-lacustrine deposits.

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Fig. 6. Deformation of fluvial and alluvial deposits between the domain of Varto and Tuzlu

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Faults. a) 1.5 m displacement induced by thrust tectonic; b) reverse faults within alluvial deposits; c) thrust tectonic resulting fold structures (see the top-right Figure for the locations). Fig. 7. Selected fault kinematic diagrams from the Varto-Karlıova region, stress orientations,

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and striations on the faults; lower hemisphere equal area projections. Locations of the fault measurement sites are shown in the presented map.

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Fig. 8. Map showing the main geomorphological structures. a) Measured pressure ridges and springs extending over the faults; b) Measured stream offsets for the region.

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Fig. 9. High-angle normal faults within Karlıova domain. a, b and c) several normal faults

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splays dissected the recent lacustrine deposits, with 30-66 cm displacements. Fig. 10. Deformation structures on the eastern extremity of the NAF. a) Pliocene

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volcaniclastic rocks deformed close to the triple point; b) reverse faults at Yoncalık villages; c) Hydrostatic pingos on the Yedisu Fault around Kargapazarı village (see Fig. 9 for the

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Fig. 11. Map indicating the locations of dykes. Rose diagrams show the distribution and abundance of dykes from (a) Varto Caldera; (b) southern part of the Varto Caldera region. Fig. 12. Block diagrams showing the fundamental change and tectono-magmatic regimes of the Karlıova Triple Junction and surrounding areas since 12 Ma. a) The initiation of NAF at 12 Ma; b) initiation of the EAF and westward extrusion tectonics of Anatolian plate; c) initiation of volcanism between 6-4 Ma; d) the most voluminous volcanic activity between 34 Ma; e) extensional and/or transtensional deformation phase are responsible from the initiation of the volcanism at around Varto-Karlıova region; f) compressional deformation phase; g) extensional and/or transtensional deformation phase deform the southern part of the Varto Caldera; h) the last compressional phase and seismically active segments of the VFZ.

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Fig. 13. Two contrary views about stability condition of KTJ. a-b) Displacement of NAFZ since 6 Ma; original transform goes into thrust mode diachronous from east to west (HubertFerrari et al., 2009); c-d) a Karlıova-type triple junction and Lithospheric Hole (Boomerang-

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shaped basin) (taken from Şengör, 2014); the area between A and B represents nonconsuming convergent plate margin (zone of distributed convergent strain). The coloured

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circles are markers displacement across plate boundaries. A=Eurasian; B=Arabian and

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Highlights:

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New data make it possible to reconstruct the tectonic evolution of the Karlıova

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The region has undergone incremental deformation around Karlıova within the past 6 Ma

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The NW–SE-striking deformation phase reactivated the faults of the VFZ

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The region has been subject to inversion tectonics induced by strike-slip and reverse faulting