Structural analysis - implications for tectonic inversion and the initiation of volcanism

implications for tectonic inversion and the initiation of volcanism

3. Structural analysis

The structural data integrates the available temporal and kinematic evidence into a restorable tectonic evolution; particularly at the east end of the KTJ. This reconstruction also predicts deformation in a region with limited available data on kinematics, timing of deformation phases, and structural controls on the KTJ. The mechanisms by which faults are reactivated during phases of compression or extension, resulting in uplift or subsidence, provide good opportunity to study the interplay between regional-scale shortening and local extension for the triple junction tectonics.

The orientation of faults was measured in the field. Observations included slickenside lineations which were used to determine the stress tensors through classical inversion methods [e.g., Sperner et al., 2003; Tibaldi et al., 2009].

Through the distribution and timing of slip on the faults, seismicity, strain accumulation, and block-like plate movements at the KTJ, the kinematics of the area are fairly well defined [e.g., Hubert-Ferrari et al., 2009: Aktug et al., 2013]. Nevertheless, the dynamic origins of many of these active faults and the origin of the volcanism remain poorly understood. 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 fault structures have been identified and studied (Figures 3 and 4). In the later part of this section, dikes, eruptive vents, and deformation structures associated with the KTJ are analysed to support the conseptual model presented. The structural data provide geometric and kinematic constraints for inversion. 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 (Figures 2 and 3). A 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 indicated quality assessments and eigen values, are presented in Table S1. Following the documentation of the selected

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kinematic diagrams of the study area, we present the calculated main stress tensor associated with the fault kinematics in the area (Figure 6).

3.1. Morphotectonics analysis

The VFZ dissects the southern part of the Varto Caldera (Figure 3). The northern strand of the VFZ, Eryurdu Fault is a high-angle fault whose activity has resulted in an uplift of the northern sector by around 800 m, from base to top of the caldera. The Varto Fault Zone, which probably originated from a pre-existing crustal-scale transformation zone, is also related to the initiation of volcanic activity in the Varto Caldera. Deep dissected valleys, intra-depositional unconformities within volcano-sedimentary deposits indicate an asymmetric uplift of the caldera volcano and incremental deformation (Figure 7) since at least 3 Ma (Figure 2b).

Reverse faults slips partly overprint the normal/oblique fault events and also fold the volcaniclastic and fluvio-lacustrine deposits. These faults led to several elongated WWN-EES-directed small-scale basins together with steeped terrace deposits. This shortening is characterised by pressure-ridges (Figure 8). We have measured 18 pressure-ridges (Figure 8). These are particularly common in the western margin of Varto Fault Zone, between 270°- 320° strike; they reach 12 m in length and 5 m in height (Figure 8). The pressure ridges reflect a NNE-SSW-directed shortening of the region. A pressure-ridge distribution at the western side of VFZ is most likely to indicate the more intense deformation according to eastern part of the fault.

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 Yedisu segment of NAFZ, Çayçatı and Leylekdağı Faults (Figure 8). Those strike-slip and reverse faults also ruptured during historical earthquakes suggesting that those faults are active (Figures 1 and 2).

The Varto region shows two distinct drainage patterns, (i) a radially outward pattern on the caldera flanks and (ii) sub-parallel alignment in the southern part (Figure 8). The drainage pattern implies river offsets by active faults. The Koçkar River, flowing from inside of the Varto Caldera, is cut by the Varto Fault and has a 1500 m

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right-lateral offset [Hubert-Ferrari et al., 2009]. This is the maximum displacement in the region. In the vicinity of the village of Doğanca the Varto Fault has a 400 m right-lateral offset (Figure 8). Four different river offsets were measured on Tuzlu Fault, ranging from a minimum of 220 m to a maximum of 630 m (Figure 8). Morphological evidence shows an average of 850 m offset for the Varto Fault (Figure 8).

3.2. Faults

3.2.1. Varto region/Varto Fault Zone

Measurements were made on six faults and their splays, previously defined by Saroğlu [1985] and Herece [2008]. The structural data indicate that many sub-parallel faults in the study area show very different kinematics, ranging from shortening to extension, and many of faults show evidence of reactivation (Figure 3). Although it has been suggested that the Varto Fault Zone is a single fault [Hubert-Ferrari et al., 2009], we have mapped four sub-parallel segments within this fault zone.

3.2.1.1. Eryurdu Fault

The Eryurdu Fault intersects the southern flank of the Varto Caldera, as is described here for the first time, resulting in deposition of fault-related colluvial deposits including volcanic breccias. This fault intersects the southern part of the Varto Caldera and is offset by the sinistral strike-slip Geyiksuyu Fault (GF) (Figures 2 and 6).

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 the base of the caldera. A vertical topographic offset (throw) of 2 m occurs along the fault at this sector (Figures 3a and 4b). 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 fault slickenlines, some of the fault-plane calculations suggest 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 01° and 11° respectively, whilst the σ₁ axis is steeply

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dipping, plunging 79° (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 (Figure 6). The further west of this fault is represented 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 (Table S1).

Measurements indicate a well-constrained NNE–SSW-trending extension.

3.2.1.2. Tuzlu Fault

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. 6). This active reverse fault 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 hydrothermal alteration is localized along a single narrow zone deforms the Pliocene fluvial sediments in restricted outcrops, resulting in upright-parallel folds striking N40 to N55°E towards to its south-eastern splays (Figure 5c).

3.2.1.3. Varto Fault

The first segment of this fault extends for 11 km between the village of Onpınar and the village of Çayönü, comprising well-developed antithetic strike-slip faults. This fault segment has low-angle fault surfaces, striking N75°W, and the measured rake (or pitch) of the faults is between 69° and 80°. The central segment is the most seismically fault, in 1966, a large and destructive earthquake (Mw = 6.8) occurred (Figure 2a). A straight fault segment extends for 9 km and strikes N70°W. A zone of distributed compression accommodates the deformation between the dextral faults. This local compression affects Pleistocene lacustrine sediments, steepening the dip of their beds

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and cutting through NNW–SSE trending valleys (Figures 4d and 5c). Dextral offsets in the deltaic alluvium and river valleys reach c. 1500 m along the easternmost 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 define a sub-horizontal σ₁ axis (dip 15°) (Table S1).

The fault displays well-developed slickenlines on reddish and yellow volcanic breccias of the Varto group (Figures 5a and b). The second and third segments form a releasing step-over along a dextral strike-slip fault between Leylek and Seki villages toward east (Figures 3 and 6). A right-stepping en-echelon pattern of fault F16 exhibits higher rake angles according to 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 (Figure 6). The easternmost segment is dissected by the sinistral Görgü Fault (Figure 2a). The eastern segment of VFZ terminates rather abruptly at the boundary between a Pleistocene volcano 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 (Figure 6).

3.2.1.4. Teknedüzü Fault

The Teknedüzü Fault, a thrust, strikes on average N75°W, in different curved strands. This fault forms a large horsetail with thrust faulting deforming the area of length 20 km (Figure 2a). The fault could be easily detected on satellite and DEM images (scale: 1:10.000). It is envisaged as the one of the youngest faults because it dissects Pleistocene lava flows as well as recent lacustrine and alluvial deposits (Figure 3). The fault shows well-preserved steeply-tilted bedding planes (75°) of fluvial deposits affected by intense compressional tectonics.

3.2.1.5. Leylekdağ Fault

The Leylekdağ and Çaydağ reverse faults with minor dextral strike-slip components are likely to present an eastern termination of the main strand of the Teknedüzü Fault (Figure 9a). The Leylekdağ and Çaydağ faults are the main splays, extending parallel eastwards at a distance of 8–12 km, that distribute the displacement of the fault system across a zone that is 4–5 km wide. The Leylekdağ Fault could be traced for 20 km from the village of Ağaçköprü to the village of Y.Alagöz (Figure 2a).

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During the Mw 6.2 earthquake of 20th August 1966, the centre 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 Kavak and Leylek Hills), producing extensive landslides and rock avalanches from basaltic lava flows (Lava 2 in Figure 1b) close to margin of the mountain. The earthquake ranked as the most deadly in Eastern Turkey up to that time. Some pervasive deformations and alteration zones have also 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 could not find evidence for that displacement. The colluvial deposits have been observed along the base of the fault scarp (Figure 9b). Fault scarps of F9, F10 and F11, extending eastern extremity, show oblique-slips to high-angle reverse fault components, indicating reactivation events (Figure 6). These show average dips of 50°, 56° and 77° and average rakes (or pitches) of 34°, 55° and 71°, respectively (Figure 6;

Table S1). Fault sets display well-preserved cross and/or conjugate faults, 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 (Figure 6). The fluvio-lacustrine sediments have been subject to this successive and complex deformation at around the Leylek village (Figures 3 and 10).

Those reverse, oblique-slips and normal faults has been accommodated to each other at several times (Figure 10).

3.2.1.6. Çayçatı Fault

The Çayçatı Fault, a thrust, consists of two main segments, which differ both in their kinematic and morphological properties. These segments are delimited by fault bends at İlbey (Figure 2a) where the strike of the primary deformation zone changes by 10°–30°. The fault is there connected by a N80E°-striking fault orientated parallel to the main-segment displacement vector (Figure 6). The western segment, from Köprücük to Yeşildal, presents N70°W-striking presenting thrust component on 7-km-long transpressional fault. This fault cuts the mostly basaltic volcanic rocks which in turn control the colluvial deposits on steeply dipping hanging wall of this basement rock,

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forming a small-scale thrust-related basin along the southern part of the Çayçatı Fault (Figures 3 and 9a). Fault scarps and related deformation generated by the 1966 earthquake can still be observed in this area. Morphological evidence for the earthquake faulting is much more abundant between Çayçatı and İlbey such as ground water discharges (18-20°C). Likewise, local transpressional geologic features occur across a range of scales in this small basin, the displacements reaching meters for sag ponds and small pressure ridges (Figures 4a and 8a). The dominant orientation of these push-up-related structures is around N80°W. The most recent activity is indicated by tilting towards the major fault (20°–25° NW) and dissected Pleistocene lacustrine terraces and recent alluvium deposits. The concave and curvilinear range-front fault trace shows a right-lateral component on some segments.

Kinematic features of this fault (F12) observed in the field include high angle reverse fault surfaces with gentle angles between 87°-89°N with rakes (or pitches) of 18°-21°S (Figure 6). 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°; Table 1). The computed data from F12 suggest that in the area NW–SE-trending extension is associated with NE–SW contraction (Figures 4c and 9a). The southern part of the Varto region has been subject to very complex deformation that has produced strong host-rock heterogeneity (Figure 10).

3.2.1.7. Results

The Varto Fault Zone is composed of sets of discontinuous faults with different kinematics and sets of striae (Figure 6). Fault-slip chronologies and inversions of the slip-vectors indicate alternating strike-slip and reverse faulting that correspond to regionally and/or locally significant stress regimes along the KTJ.

The Eryurdu Fault is a high-angle normal fault that has destroyed the western part of the Varto Caldera (Figure 4). The fault has generated displacement of 1.5 to 2.7 m displacement in the Pleistocene lava flows. The dextral Tuzlu Fault, mostly steeply dipping to the north (rarely to south) also deformed the caldera and triggered the debris avalanche deposition through the western part of the volcano edifice (Figure 4a). Two

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distinct landslides, of areas 2 × 3 km, occur along the southern part of the Tuzla Fault.

The debris avalanche deposits are composed of basaltic rocks from the lava flows (2) (Figure 2a).

The Varto Fault is a major agent controlling the eastern part of the KTJ. Slip measurements of this fault with a 280° striking has a mostly right-lateral motion indicate a transpressional stress regime. However, there are also some reverse components on the fault plane (Figures 4b and 4c). There are dextral offsets reaching 1.5 km on alluvial fan 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 (Figure 4d) and fluvial sequences (Figures 4d and 10c).

The thrust faults of Çayçatı and Leylekdağı dissect 150-m-thick colluvial deposits at the contact between Pliocene volcano-sedimentary units and volcanic rocks (Figure 9a). 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 (Figures 9a and 9b).

3.2.2. Karlıova region [NAF and EAF]

The displacement of the Anatolian extrusion-block has generated extension through the transform boundary motion of NAF and EAF (Figure 11). On the extruded block, we recognised 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 (Figure 11).

The continuation of the Yedisu segment of the NAF strikes 275° and has 75 cm lateral offset on Pliocene deposits (Figure 12xa), and also at around Kargapazarı accommodates to previous reverse faults (Figure 12xb). Hydrostatic pingos formed as a result of hydrostatic pressure on water from permafrost [van Vliet-Lanoë et al., 2004]

show a parallel alignment with Yedisu Fault around the village of Kargapazarı (Figure 12xc).

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The stress tensors calculated from the data of 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 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° (strike/plunge) (Table S1; Figure 6). These results indicate the fault L13 is a sinistral strike-slip fault (Figure 6). These observations show that along the EAF, stretching is partitioned into NE–SW-trending nearly pure sinistral strike-slip faults with rakes of 3°, at least close to the surface. On the Anatolian block, extension seems to be accommodated by strike-slip and normal faults.

Fault sets display well-preserved slickensides, with stereographic plots showing 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 (Table S1; Figure 6). Measurements indicate a well-constrained deformation that shows NE–SW-trending pure extension.

4. Dikes

The orientation of dikes is controlled by regional and/or local stress fields existing at the time when dikes are injected, which renders dikes 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 dike swarm is an indication of variation in the local stress field within the volcanic zone or volcanic edifice during the dike-swarm development [Gudmundsson, 2011]. The great majority of dikes are extensional fractures and thus form (strike) perpendicular to σ3. The term dike here is here used for all sheet-like intrusions.

A total of 376 dip and strike measurements, at 53 sites, were made of the dikes (Figure 7). Hubert-Ferrari et al. [2009] reported ⁴⁰Ar/³⁹Ar groundmass ages of 0.4–0.7 Ma from dikes around the Yılanlı and Çayçatı area (Figure 2b); also some dikes from the southern part of the volcano yielded fission track ages of 1.9–2.6 Ma (Figure 2b).

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There is lack of radiometric age data for the dikes around the Varto Caldera. The dike thickness ranges from 0.5 m to 18 m for the southern sector.

Dike measurements have been made at two locations (i) inside the Varto Caldera and (ii) in the southern part of the Varto Caldera (Figure 13). Two main types of dikes are easily distinguished in the subaerial units: vertical or subvertical dikes and slightly inclined dikes, commonly referred to as inclined sheets [Gudmundsson, 2006]. Unlike the vertical dikes, which exist in all the units, the inclined dikes or sheets are only seen in the western part of the caldera wall. At some sites the dike 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. More specifically, bimodal or polymodal distribution patterns can be inferred within the caldera, which indicates the existence of different local stress fields and resulting dike/sheet swarms.

The cumulative volume of dike rock in the southern sector is much higher than in the northern sector. We estimate that the percentage of dike volume reaches about 80% of the total rock volume, the remaining host rock (20% by volume) being mainly basaltic lava flows in this sector.

4.1. Varto Caldera

We measured and recorded dikes at 11 distinct locations or sites. The dikes are, primarily composed of trachy-basaltic compositions, inside and around the Varto Caldera. Three dikes display NE–SW-orientations; eight dikes strike NW–SE (Figure 13), with a mean value of 323°. The two main volcanic domes are 0.46 ± 0.24 Ma and 0.73 ± 0.39 Ma old [Hubert-Ferrari et al., 2009]. The ages of the domes are likely to coincide roughly with that of dike emplacement inside the caldera and along the caldera walls, We estimate an age of ~ 0.4–0.7 Ma for the dominantly NE-trending dikes and

Belgede İLİŞKİSİ, DOĞU ANADOLU-TÜRKİYE (sayfa 102-144)