Plio-Quaternary stress states along the Kütahya Fault and surroundings, NW Turkey

: The Kütahya Fault, which is one of the major neotectonic structures in western Anatolia, Turkey, is an active fault constraining the southern margin of the approximately E–W trending Kütahya Basin between the Eskişehir Fault to the north and the Simav Fault to the south. In the present study, inversion of both fault kinematic analysis of fault-slip data and focal mechanism solutions from the Kütahya Fault and surroundings is used to understand the Late Cenozoic stress states. The fault kinematic analysis result yielded three different stress regimes from Mio-Pliocene to Quaternary. Firstly, strike-slip faulting developed under a NE–SW trending local compressional regime with 51° ± 24° (σ 1 ) and 140° ± 7° (σ 3 ) trends and R m ratio was calculated as 0.61, consistent with this faulting. NW–SE trending consistent extensional direction produced local normal faulting with 144° ± 3° (σ 3 ) trend. Secondly, strike-slip faulting developed under a NW–SE trending local compressional regime showing 143° ± 17° (σ 1 ) and 51° ± 10° (σ 3 ) trends and R m ratio was calculated as 0.51. Finally actual normal faulting developed under a NNE–SSW trend with a regional extensional direction showing 42° ± 14° (σ 3 ) and R m ratio of 0.56 at the present time. Inversion of the earthquakes gives a NNE–SSW extension direction with 21° ± 19° (σ 3 ) trend and R m ratio calculated as 0.68 at the triaxial. The Kütahya Fault and surroundings are under an extensional regime at the present time. The reason for the regionally effective NNE–SSW trending extensional regime in western and southwestern Anatolian is complex subduction processes (roll-back, retreat, delamination, slab-tear, slab-break-off and/or slab-pull) of the African Plate and the Anatolian Platelet in the Mediterranean region.

process rapidly expanded toward the Hellenic and Cyprus arcs (Mercier, 1981;Mercier et al., 1979;Le Pichon and Angelier, 1981; Barka and Reilinger, 1997;Faccenna et al., 2006;Jolivet et al., 2013). Three important models are proposed for this extension: the postorogenic collapse model (Dewey, 1988;Scott, 1991, 1992), the tectonic escape model (Dewey and Şengör, 1979), and southward roll-back of the African slab or the back-arc spreading model (Le Pichon and Angelier, 1979. Within this process continuing from the Late Miocene to the present day, Western Anatolia was tectonically pulled toward the African Plate and a significant N-S oriented extensional (opening) area was formed. The region, in addition to local compressional periods with uplift, lateral slide deformation, and block rotations, was deformed under a regional extensional regime in the Middle Miocene/Pliocene and Pliocene/Quaternary periods.
It became a graben region with fault zones developing with mainly E-W and some N-S, NW-SE, and NE-SW orientations (Dumont et al., 1979, Şengör, 1987Scott, 1991, 1992;Cohen et al., 1995;Koçyiğit et al., 1999;Yılmaz et al., 2000;Alçiçek et al., 2005;Över et al., 2016). Between the nearly E-W oriented faults located in Western Anatolia, from north to south the Eskişehir, Kütahya, and Simav Faults offer significant extension. From deformation on these faults it is known that they did not develop under a single type of tectonic regime.
Currently it is thought that from their formation to the present day they operated under local and regional compressional and extensional regimes. The deformation type in the crust on these types of active faults may be determined from data measured on fault planes and earthquake data. The data obtained can be used to find the   Barka, 1992;Bozkurt, 2001;Över et al., 2010).
dominant stress tensors, regime types in the region, and stress changes in these regime types along the fault and in surrounding areas from the past to the present. In the present study for the first time, both data obtained from kinematic analysis of fault assemblages and inversion of the focal mechanism solutions of earthquakes will be used together to determine the local and regional scale stress tensors, stress states, and geodynamic evolution of the region on the Late Cenozoic (Plio-Quaternary) Kütahya Fault ( Figure 1) and to interpret the relationships with other structures in the region and especially the situation in west and SW Anatolia.

Geology, tectonics, and seismic activity along the Kütahya Fault and surroundings
The study area is examined in two sections: basement and cover rocks. Basement rocks comprise schists at the base, crystallized limestone conformably overlying the schists, and ophiolitic rocks tectonically emplaced as a result of the closure of the İzmir-Ankara-Erzincan suture zone above these rocks. Cover rocks comprise young units, overlying the basement units unconformably ( Figure 2). Since the Neogene there has been deposition of conglomerates, sandstone, claystone, marl, limestones, siliceous limestones, and lignite in the region. Additionally effective volcanic activity is noted in the study area. This volcanic activity began at the end of the Miocene and increased in the Pliocene. Lastly all these units are overlain unconformably by Quaternary aged alluvium and travertines ( Figure 2).
The Kütahya Fault is the most significant structural element from the neotectonic period (from Late Miocene to recent) in the study area. This fault currently is WNW-ESE oriented with 40 km length. This fault presents clear morphology bounding the southern edge of the Kütahya basin. Between Tavşanlı and west of Kütahya, this active fault has roughly N 80° W orientation and is located mainly between the plain to the north (Kütahya Graben) and immediately in front of the basement units forming uplift to the south (Figure 3). From its formation to the present day the Kütahya Fault affected basement rocks and cover rocks. In the past this fault acted as a strike-slip fault in periods, passing through and affecting especially Lower Pliocene age units. According to the analytical signal map produced from the aeromagnetic data of the study area, the metamorphic massifs with the E-W direction cause magnetic anomalies concentrated in the west and south of Kütahya (Bilim, 2007). The maxspot map derived from the location of the horizontal gradient of aeromagnetic anomalies shows that it is compatible with both tectonic lineaments and the distribution of earthquake data (Bilim, 2007). Currently the Kütahya Fault was transformed to a normal fault recently, forming the boundary between basement units and cover rock and limiting the basin area to half-graben appearance. The fault has its most western end near Tavşanlı and ceases in parallel faults within basement units immediately SW (Figure 3). Many researchers have concluded it is a young and active fault due to fluvial deposits, hanging valleys, alluvial fans, triangular surfaces, and hot springs present along the fault and also current GPS velocities, seismic activity data, and the fact that it cuts Plio-Quaternary aged deposits (Şaroğlu et al., 1992;Barka and Reilinger, 1997;Koçyiğit and Bozkurt, 1997;Özburan, 2009;Altınok et al., 2012;Özburan and Gürer, 2012;Emre et al., 2013). Şaroğlu et al. (1992) mentioned the fault as a normal dip-slip fault due to down-drop of the fault block to the north. However, it is noted that especially in western sections the fault has strike-slip morphology and noting Pre Miocene this characteristic it was stated that the fault has a right lateral strike-slip component in addition to dip-slip motion. Gürer et al. (2005) proposed that the Kütahya Fault Zone was a normal fault with a left lateral component. Koçyiğit and Bozkurt (1997) and Bozkurt (2001Bozkurt ( , 2003 stated that the main characteristic of the Kütahya Fault Zone was that of a normal fault. In our study the Kütahya Fault was identified to begin movement as a left lateral strike-slip fault under a compressional regime and is currently continuing motion as a normal fault under an extensional regime. There are no definite data for the age of the Kütahya Fault. While Koçyiğit and Bozkurt (1997) recommended the age of the Kütahya Fault as Early Pliocene, Özburan (2009) stated that according to the youngest unit cut by the Kütahya Fault the initiation age of the fault is Early Pleistocene. These age data refer to the period when the fault acted as a normal fault, and do not present any age data for previous behavior.
As a result of the paleoseismological and archaeological studies (Altınok et al., 2012) conducted in the study area, it has been revealed that two devastating earthquakes have occurred on the Kütahya Fault in the last 8000 years with paleoseismological studies, and a slip velocity rate of 0.2 mm/year was calculated on the Kütahya Fault. It has been determined that there is potential for an M > 7.0 earthquake (Altınok et al., 2012). Ambraseys and Tchalenko (1972) and Koçyiğit (1984) stated that the general trend of the active seismic belt called the Akşehir-Simav Fault Zone south of the study area was parallel to the Kütahya Fault Zone. Koçyiğit and Bozkurt (1997) described this fault system and characteristics as being very similar to the Kütahya Fault Zone. Additionally Tokay and Altunel (2005) stated that the Eskişehir Fault Zone, located north of the study area, was an active fault zone and again explained it was parallel to the Kütahya Fault Zone. Özden et al. (2015) stated that the Eskişehir Fault north of the Kütahya Fault is currently right lateral, while the Simav Fault to the south (Demirci et al., 2015;Gündoğdu et al., 2015;Karasözen et al., 2016;Erkul et al., 2017) displays normal fault (extension) behavior.
Although there has been no destructive earthquake in the study area in the instrumental period, the parallelism and similarity to active faults, and two destructive earthquakes in historical earthquake records along with the observation of active tectonic elements like fault-front deposits along the fault zone (indicator) and hot springs indicate that the Kütahya Fault is an active fault. This fault has the potential to produce earthquakes similar to the Simav Fault (19.11.2011, Mw: 5.8), Eskişehir Fault (20.02.1956, Ms: 6.4), and Eski Gediz Fault (28.03.1970.0) to the south and north of the study area.

Fault kinematic analysis
This study involved kinematic analysis of fault linkage (data sets) along the Kütahya Fault and geological units in near surroundings to determine the main forces, kinematic evolution of the region, and current tectonic regime. Parameters concerning fault planes (fault strike, dip amount, dip direction, altitude values) were measured in the study area.  Figure 3. Location DEM map of the sites of the fault kinematic analysis data (numbers/1-28) and earthquakes epicenters (characters/ A-E). Red lines represent the faults.
Two hundred and fifty-five fault planes were measured at 28 stations ( Figure 3; Table 1). Faults in geological units with varying ages and lithologies, especially faults in the young period, were used in an attempt to determine the kinematic evolution of the Kütahya Fault in the region from the past to the present day.

Methodology of fault kinematic analysis
The basis of this study is the kinematic analysis method for fault assemblages developed in the computer environment by Carey-Gailhardis and Mercier (1987) after being proposed by Carey (1979). This method may be used for inverse solutions of focal mechanisms of faults occurring in the region as well as being applicable to faults compiled in the field (Methodology as a detail in Över et al., 2010).
Here the largest principle stress axis (σ 1 ) and the smallest principle stress axis (σ 3 ) are horizontal, while the intermediate stress axis (σ 2 ) is vertical, and so the tectonic regime is strike-slip faulting (Figure 4; Table 2a). These results show the orientation of the compression (σ 1 ) in the region is N 51° E. The extensional orientation (σ 3 ) in the region is N 40° W. As the R ratio is larger than 0.55, the  (Figure 4).
Here the smallest principle stress axis σ 3 and intermediate stress axis (σ 2 ) have horizontal positions, while the largest principle stress axis (σ 1 ) has a vertical position, representing normal faulting in an extensional regime ( Figure 5; Table 2b). These results show the extensional orientation (σ 3 ) in the region is N 36° W.
The two regime periods explained above are in accordance with limited numbers of outcrops along the length of the Kütahya Fault and were regimes effective before the Late Pliocene.  Table 2a. Histogram shows distribution of deviation angles (angle between the observed slip, s, the predicted slip, t).
Under a NW-SE oriented local compressional regime, strike-slip faulting, reverse faults, and folds developed generally in volcanic sandstones. This regime was effective for a short period in the Late Pliocene.
Here the smallest principle stress axis (σ 3 ) and intermediate stress axis (σ 2 ) have horizontal positions, while the largest principle stress axis (σ 1 ) has a vertical position, indicating an extensional regime with normal faulting (Figure 7; Table 2d). These data indicate the extensional orientation (σ 3 ) in the region is N 42° E. This regime is the effective regime after the Late Pliocene, possibly Quaternary, to the present day.
The presence of three effective stress regimes was identified from before the Late Pliocene to the present day for the Kütahya Fault and surrounding area . To determine the correct order of development from the oldest of these regimes to the present day, the presence of slip vectors overlying each other on the same fault plane is helpful (Figure 8). In the study area, in addition to data from many fault planes providing the chronologic order of the tectonic regimes, data especially from stations 12, 20, 23, and 27 were used (Figure 8). Among these, they appear at metric (giant) scale on the main fault plane on a normal fault developing under the last regime represented by NNE-SSW orientation and effective currently (Figure 9) in Evliya Çelebi Neighborhood, forming station number 20. Additionally along the Kütahya Fault most outcrop data provide clear field examples of the NW-SE oriented compressional regime (Figure 10).

Methodology of focal mechanism solution and inversion of earthquakes
To calculate the stress state for the present day, the population of source mechanisms of earthquakes that occurred along the Kütahya Fault around the Kütahya Basin was examined. Firstly, we performed a moment tensor inversion procedure for some recent earthquakes, namely those with close to surface-observed main faults. For the moment tensor inversion wave form modeling was determined using the source parameters proposed by Dreger's (2002) computer application method. We analyzed the waveforms of the selected 5 earthquakes using the Kandilli Observatory and Earthquake Research Institute's (KOERI) open access data using the software zSacWin. Secondly, the inversion method proposed by Carey-Gailhardis and Mercier (1987) was used including one of several existing algorithms (Methodology as a detail in Özden et al., 2015).

Focal mechanism solutions and inversion results of the earthquakes
Focal mechanism solutions were completed for 5 earthquakes (A, B, C, D, and E) (Figure 3) occurring between 2004 and 2013 and varying in magnitude from 2.9 to 4.2 ( Figure 11; Table 3). While individual focal mechanism solutions for each earthquake indicated normal faulting, the numerical solutions for these earthquakes and beach balls are presented in Table 3 and Figure 11.  Table 2b. Histogram shows distribution of deviation angles (angle between the observed slip, s, the predicted slip, t).  Table 2c. Histogram shows distribution of deviation angles (angle between the observed slip, s, the predicted slip, t).  Table 2d. Histogram shows distribution of deviation angles (angle between the observed slip, s, the predicted slip, t). To determine the current tectonic regime along the Kütahya Fault, the numerical method developed by Carey-Gailhardis and Mercier (1987) was used. The common inversion solutions of these earthquakes found the smallest principle axis (σ 3 ) and intermediate principle axis (σ 2 ) are horizontal, while the largest principle axis (σ 1 ) is vertical, indicating an extensional regime with normal faulting. The smallest principle stress axis (σ 3 ) was 21°/19°. The Rm ratio was 0.68, showing this regime is represented by a triaxial stress tensor.
These results lead to the conclusion that the extensional orientation in the region is N 21° E (σ 3 ). The  Figure 11. The lower hemisphere result (SFM) of the source mechanism inversion of 5 (A-E) earthquakes (references and detailed information for each earthquake given in Table 3). Seismic fault showing a gray arrow on each focal mechanism solution. extensional regime with NNE-SSW orientation (SFM) is in accordance with the extensional direction obtained for the last tectonic regime (SS.3) from the kinematic analysis studies of fault assemblages and has the same orientation as the currently effective extensional direction in western-SW Anatolia.

Discussion and conclusion
This study was conducted with the aim of determining the stress situation and kinematic evolution in the Late Cenozoic of the Kütahya Fault and surrounding area. The Kütahya Fault has a WNW-ESE trend over nearly 40 km in length, presenting a clear morphology bounding the south of the Kütahya Basin. Data were obtained from planes clearly showing fault plane parameters along the Kütahya Fault and numerically analyzed. The results of numerical analysis studies determined three main regional stress states (SS.1, SS.2, and SS.3) belonging to before the Late Pliocene, the Late Pliocene, and Quaternary periods. The study area was affected by a NE-SW compressional regime (SS.1a) in the period before the Late Pliocene, developing strike-slip faults and  In the Late Pliocene in the region it appears a short-term and local NW-SE oriented compressional regime (SS.2) developed. The products of this regime are shear zone deformation together with right and left lateral strike-slip faults. Immediately before the Late Pliocene, the Kütahya Fault began to form for the first time, developing with left lateral strike-slip fault characteristics under a NE-SW compressional regime (SS.1a), and showed normal fault character under a regional NNE-SSW extensional regime after a regime change in the Quaternary (SS.3). Inversion of the earthquake focal mechanism solution produced similar results. This last tectonic regime of extension in the region is currently active (Figure 12). In light of the obtained data, the Kütahya Fault began motion as a left lateral strike-slip fault in a time (?) before the Late Pliocene and currently continues motion as a normal fault under the extensional regime. These Late Cenozoic tectonic regimes were determined by earthquakes together with numerical calculation methods for the first time for this fault and close surroundings. The observed tectonic regime changes along the Kütahya Fault are still debated by researchers working in the region. Dewey and Şengör (1979) attempted to explain the initial E-W compressional regime and later N-S extensional regime with a comparative model for Western Anatolia. According to this model, with the collision of the Arabian plate in the east the Anatolian Block was prevented from moving west by the Hellenic Shear Zone. This obstacle to the lateral strike slip system caused east-west compression in Western Anatolia. This compression in continental crust thickened by previous orogenic events caused N-S extension on E-W oriented normal faults instead of causing north-south thrusting and thickening. In other words, E-W compression was countered with N-S extension. In this N-S extension the effect of the Cyprus Arc was great (Dewey and Şengör, 1979;Şengör et al., 2008). Thus, the Kütahya Fault with motion beginning under a compressional regime and later continuing as a normal fault under an extensional regime may be considered a product of the neotectonic evolution of Western Anatolia in accordance with other neotectonic stages and elements.
In the region the compressional regime before the Late Pliocene period determined in our kinematic analysis and the accompanying NW-SE extensional regime created the nearly NNE-SSW oriented Selendi and Demirci Basins. These basins are structural elements created under the effects of the old tectonic regime. However, contrary to the Selendi and Demirci basins, the WNW-ESE oriented Eskişehir, Kütahya, Simav, and Gediz basins obtained their final shape under the currently effective NNE-SSW extensional regime.