The SKB, located in the West Anatolian Extensional Province, is an extensional basin where both Miocene and Plio–Quaternary deposits are controlled by NE, NW, and E-W trending high angle normal faults on the eastern edge of the SKB (Sümer et al., 2013; Gürer et al., 2001). In some basins around the SKB, the stratigraphy and lithology of the Miocene and Plio–Quaternary sedimentary and magmatic geologic units overlying the bedrock were defined in some studies (Seyidoğlu and Işik, 2015; Sözbilir et al., 2011; Bozkurt and Sözbilir, 2004; Bozkurt, 2000; Koçyiğit et al., 1999).
The WAEP includes basins associated with the deformational process of detachment normal faults bordering on metamorphic core complexes (Öner and Dilek, 2011; Işik et al., 2003b) (Fig. 1b). These faults are supradetachments and control the contact between the Menderes Massif and intra-basin deposits. Approximately E-W trending detachment normal faulting has been determined in the main rocks (Menderes Massif—brittle zone of crust) of the eastern uplift of the SKB (Sümer et al., 2013). In this study, a trace of low angle detachment normal fault in the intra-basin sediments, including Kuşadasi Formation from Miocene to the present, is determined and shown in the column section (Fig. 3). In addition to the intra-basin deposits of SKB; the folds and thrust faults that are synsedimentary with the Miocene Kuşadasi and Fevzipaşa formations and Plio–Quaternary Yamaçköy Formation have been defined in the stratigraphic column sections of the region (see Fig. 3). According to this outcome, thrust faults were also effective in Plio–Quaternary. Also, from Miocene to Plio–Quaternary, the geometric differences between sedimentary and syn-tectonic beddings, which are contemporaneous with folding and thrust faulting deformation, indicate the presence of tectonic-rotation associated with deformation. This tectonic-rotation is consistent with a clockwise rotation determined by paleomagnetic data (Uzel et al., 2015) around the Menderes Massif (see Fig. 1b).
Figure 3. Geological column sections of Söke-Kuşadasi Basin (SKB) and its vicinity according to different researchers (Sümer et al., 2013). In the rightmost column section; blue colored thrust faults and red colored normal faults are drawn according to the data of this study.
Both E-W and N-S trending, low-angle detachment, normal faults border the Büyük Menderes graben, and the brittle zone in the north of the massif (Gürer et al., 2009). The continuation of this primary extensional system in the west is found in the brittle zone of the crust to the west of Kuşadasi Bay. These faults are the E-W trending, detachment normal faults described by Sümer et al. (2013) in the Oyukdaği basement rocks in the east of the SKB and the low-angle detachment normal fault in the Cyladic Core Complex border to the northwest of Samos Island (Jolivet et al., 2010; Ring et al., 2007) (see Fig. 1b). The geometry of the detachment normal faults around Kuşadasi Bay resembles an open drawbridge. There are NE, NW, and E-W extending basins in western Anatolia (Ersoy and Helvaci, 2016; Sözbilir et al., 2011; Seyidoğlu et al., 2009; Emre and Sözbilir, 2007; Bozkurt, 2003; Şengör, 1987). The SKB is one of these basins and the contacts between basin-filling deposits and basement rocks are controlled by high-angle normal faults numbered from 1 to 8 in this study and undifferentiated faults (Sümer et al., 2013; Gürer et al., 2001) (see Fig. 2). The high-angle normal faults on the east margin of the SKB are transverse with approximate N35ºE, N20ºW, and E-W trendings, on the south margin are E-W and N70ºW. These faults are supradetachments and control the contact between Menderes Massif and the intra-basin deposits. In addition, these faults have dip angles up to 80º and are dipped to both north and south. The E-W trending, ramp-flat detachment normal fault, which is described in this study in the SKB and shows cross-graben geometry, is notable for cutting and deforming the Kuşadasi Formation. This structural setting tectonically links the above-described drawbridge formation in Kuşadasi Bay (see Figs. 1b, 2, and 4). From Miocene to the present, the contact of the Menderes Massif and the intra-basin sediments on the eastern part of the SKB is completely controlled by high angle normal faults (Fig. 5a, see Fig. 2). Accordingly, the contact between the basement rocks and the Kuşadasi and Fevzipaşa formations, which characterize the lacustrine and shallow marine environment, is controlled by the high angle normal faults with NE and NW trendings. Yamaçköy Formation has also developed in the Plio–Quaternary alluvial fan facies in the hanging-wall blocks of some of these faults (see Fig. 5a). The Holocene colluvium in the SKB bounded by transverse extensional faults, developed during the late stage of the deformation of high angle normal faults (Figs. 5b, 5c). Since faults control these lithologic levels, the pebbles within the levels are entirely angular and belong to the Menderes Massif rocks (Figs. 5b, 5d). It is seen that the type of faulting accompanying sedimentation in the intra-basin sediments of the SKB was normal faulting and detachment low angle normal faulting, then supradetachment high angle normal faulting were effective in the initial phase of the tectonic evolution process, which the basin has undergone both as morphotectonic and sedimentation. Along with the high angle, normal faults in the intra-basin sediments, a maximum vertical offset of 6 m is observed, sometimes forming conjugate zones (Figs. 5e, 5f).
Figure 4. Cross section photograph showing the location of the ramp-flat detachment normal fault developed in the intra-basin deposits of Miocene Kuşadasi Formation in the SKB and the other structural elements around it, the appearance of synsedimentary bedding, gentle or open folding and how the bedding changes in the immediate vicinity of the faults.
Figure 5. Field photographs of the east of Söke-Kuşadasi Basin (SKB) showing: (a) high angle normal fault following the contact of Yamaçköy Formation and Menderes Massif rocks and a view of slickenside and striae in the small photo at the bottom right; (b) Menders Massif basement rocks with high angle normal fault-controlled colluvium in the right bottom (in hanging-wall block) of the first type monoclinal folds and slickenside striae in the foot-wall block of the fault; (c) the hanging-wall block of the high angle normal fault plane and the fault breccia developing on it; (d) the high angle normal fault forming the contact between the massif and the colluvium; and conjugate high angle normal faults in (e) the Kuşadasi Formation and (f) the Yamaçköy Formation.
Two different fold types have developed in two different locations in the SKB (see Fig. 2). The first type of folds is small-scale monoclinal seen in the hanging wall blocks of the transverse-shaped high-angle normal faults on the eastern margin of the basin (Fig. 6a). The folds formed in this way are in the form of roll-over anticline or syncline and develop in the hanging-wall blocks of normal faults as a ramp-flat geometry (see Fig. 4). The second type of fold is the anticlines and synclines developed in the Miocene units (Kuşadasi and Fevzipaşa formations) in the basin. These folds have a large wavelength, long fold axis, and isoclinal shape and are more spread and dominant ample folds compared with the monoclinal type folds (Figs. 4, 6b, 6c, 6d). In addition, they can be observed in some locations as recumbent and rollover. The bedding positions were evaluated by the "Faultkin Program" of Allmendinger et al. (2012) in the study area. As a result, the folding axes of these folds in the SKB are numbered from 1 to 21 with dipping NW, NE, horizontal and different directions (see Fig. 2). Sporadic slumping has also developed between the folds in the region. The distance between the limbs of all folds is generally wide, but their wavelengths are short. The wavelength of the folds in the SKB is broad and does not resemble those in the pure compressional area (Turner and Williams, 2004; Ramsay, 1974). Folds usually fall into the space between extensions or extension and shortening in the normalized layer-parallel strain mode classification (Frehner, 2011). Thrust faults indicating the transition stage from elastic deformation to plastic deformation have been formed in some locations of the second type of folds (see Figs. 4, 6d).
Figure 6. Field photographs showing (a) appearance of the first type monoclinal folds developed in the hanging-wall block of the high angle normal fault and related to listric faulting in deep parts; (b) the isoclinal fold of the second type of anticline and syncline; (c) the second type of isoclinal fold developed in the Kuşadasi Formation; (d) appearance of thrust faults verifying the kind of deformation from elastic to plastic with gentle and open-shaped folds.
The geometry of the thrust faults, especially in the intra-basin deposits, from Miocene to Plio–Quaternary in the SKB, was determined, and the tectonic origins of these faults were analytically detailed for the first time. Thrust faults in the SKB are observed in a range of different positions (Fig. 7). They do not cut the basement rocks in any part of the basin and are seen to be entirely in the intra-basin deposits of the crust. Thrust faults are in conformity with the experimental structures developed by McClay (1996) near the detachment ramp-flat normal fault and developed due to inversion tectonics (see Fig. 4). Thrust faults caused a tectonic-originated rotation up to 40º in the imbrication of the Plio–Quaternary units and the beddings of the Miocene units between 14º and 46º (Figs. 7b, 7e, 7h). Thus, the paleo-topography of the deposition is seen to have evolved by tectonic origin. The rotation developed due to the syn-sedimentation with tectonism is continuous from the bottom of the related thrust fault planes to the top around the normal detachment fault, while others are observed in a specific surface area (see Fig. 4).
Figure 7. Field photographs of the Kuşadasi and Fevzipaşa formations showing: (a) high-angle normal and thrust faults; (b) syn-sedimentary thrust fault distributing in the deposition bedding; (c) conjugate high-angle normal and thrust faults; (d) micro-thrust faults; (e) syn-sedimentary thrust faults and joints; (f) syn-sedimentary; first in the second type of folding (clearly seen in the lower left photo) followed by thrust faulting occurs in the zone; conjugate high angle normal and thrust faulting, together with the low angle detachment of the normal faulting structural products of joints; (g) second type of micro folding, followed by en-echalon thrust faulting and (h) syn-sedimentary thrust faulting changing the imbrication occurring at the time of deposition in the Plio–Quaternary sediments for the first time detailed by this study.
The thrust faults in the SKB are all within the intra-basin sediments and are not included in the geological contact of basement rocks and intra-basin deposits (Fig. 7a). Thrust faults are relatively less and smaller in size and contain lower vertical offsets than normal faults in the whole basin. Therefore, these thrust faults are secondary structures developed during the tectonic-sedimentary process of the SKB. Also, thrust faults deform or rotate sedimentary structures accompanying deposition continued in the Plio–Quaternary Period.
The normal fault, fold, thrust fault, conjugate normal/thrust faults and their joints in the vicinity and the striae on each slickenside were evaluated in the "Wintensor Program" developed by Delvaux and Sperner (2003) and the distribution of principal stress axes was determined (Fig. 8) (Table 1). All measurements of the structural planes defined in the field are taken as dip direction/dip angle. In the intra-basin sediments in the SKB, in the vicinity of the normal detachment fault shown in Fig. 4, all high angle normal and thrust faults (numbered as 1, 2, 3, 4) and the trendings of the second type of fold axes are generally parallel to each other. In other words, the maximum principal tensional stress axis (S3) and the principal maximum principal compressional stress axis (S1) in the region are also approximately parallel or there is a small angle between them. This view is supported by the slickenside and striae data of thrust fault numbered 4, and normal fault numbered 1 in Fig. 4. The faults on the southern and eastern margins of the SKB are high angle normal faults (Figs. 8a, 8b, 8c, 8d). Also, both the basin edge and in-basin structural data indicate that the maximum principal tensional stress axis (S3) is in N, N15ºE, and N40ºW strikes, proving that the basin is transverse. Sümer et al. (2013) identified a similar extension (σ3) direction. In this study, the ramp-flat detachment of the SKB, especially in the intra-basin deposits, indicates both conjugate and slickenside and striae data within the normal fault zone, indicating transverse extension. Also, the existence of limited compression is evident in the extensional stress domain (Figs. 8e, 8f, 8g, 8h). In this situation, low angle detachment of normal faulting, the ramp-flat and inversion geometry is similar to the high angle normal faults that control the supradetachment basin (Öner and Dilek, 2011; Purvis and Robertson, 2005). Stress ratio (R) and stress index (Ri), which are the critical structural parameters around SKB, indicate that pure or radial extension is the primary source in the basin tectonic history (see Table 1).
Figure 8. Kinematics characteristics of the faults in the SKB in the Wintensor Program (Delvaux and Sperner, 2003). (a) Basement rocks in the Kuşadasi Formation; (b) basement rocks in the Fevzipaşa Formation; (c) and (d) high angle normal faults controlling geologic contacts between basement rocks and colluvium in different locations, the Kuşadasi Formation; (e) low angle normal faults, (f) thrust, (g) high angle normal faults in the Fevzipaşa and Kuşadasi formations; (h) conjugated high angle normal faults, (i) thrust, (j) conjugated thrust faults, (k) thrust faults in the Yamaçköy Formation; and (m) analysis of the principal stress distributions of high angle normal faults in the Yamaçköy Formation.
Name and age of data Type of data and formation name Number of data Latitude(ºN) Longitude (ºE) Strain mode S1(σ1)(azimuth/ plunge) S2(σ2) S3(σ3) ϕ
R Ri Between basement units and intra basin deposits (from Miocene to Holocene) Slickenside, striae (normal fault) 6 37º48'33" 27º19'48" Pure extension 064/62 161/04 253/28 44.82 8.2 0.5 0.5 Slickenside, striae (normal fault) 5 37º53'08" 27º16'29" Radial extension 277/86 082/04 172/01 45.0 2.7 0.0 0.0 Slickenside striae (normal fault) 5 37º51'20" 27º17'06" Pure extension 012/67 112/04 204/23 45.48 3.6 0.6 0.6 Slickenside, striae (normal fault) 5 37º51'39" 27º17'18" Radial extension 163/62 254/01 344/27 45.60 5.9 0.2 0.2 Intra-basin depoists, Kuşadasi and Fevzipaşa formations, Miocene) Slickenside striae (on detachment normal fault) 11 37º49'17" 27º16'22" Pure extension 258/68 142/10 049/19 42.16 22.5 0.73 0.73 Slickenside, striae (near detachment thrust fault) 6 37º49'17" 27º16'22" Radial compression 204/21 295/02 031/63 46.33 7.8 1.00 3.00 Conjugated normal faults 12 37º51'36" 27º17'21" Pure extension 218/66 042/66 312/01 70.17 6.31 0.67 0.67 Slickenside, striae (normal fault) 5 37º50'48" 27º16'09" Radial extension 105/72 207/04 298/18 44.12 2.6 0.0 0.0 Conjugated thrust faults 12 37º50'48" 37º51'32" 27º16'09" 27º17'24" Pure compression 348/08 254/28 092/61 72.54 11 0.42 2.42 Slickenside, striae (thrust faults) 7 37º50'48" 27º16'09" Radial compression 171/05 081/01 340/85 45.79 9.44 1.0 3.0 Plio–Quaternary units Conjugated normal faults 14 37º47'18" 27º20'26" Pure extension 293/50 106/40 189/03 45.83 6.9 0.68 0.68 Slickenside, striae(thrust faults) 4 37º47'18" 37º47'18" Pure compression 054/09 324/01 226/81 44.70 5.25 0.75 2.75 ϕ. Friction angle; α. misfit angle between observed and modeled directions; R. stress ratio; Ri. stress regime index; values determined by "Wintensor Program" of Delvaux and Sperner (2003).
The general trend of the axes of the second type of folds indicating the shortening in the SKB is between N10º-65ºE, E-W, and N30º-40ºW (see Fig. 2). Vertical offset values on thrust fault planes vary from 5 to 55 cm. At the same time, these faults are observed with antithetic faults or joints. In Miocene Kuşadasi and Fevzipaşa formations, the maximum principal compressional stress axes (S1) caused by the thrust faulting range from N60º to 70ºE and N5º to 70ºW. In addition, thrust faults that cut Plio– Quaternary deposits (Yamaçköy Formation) in the WAEP and SKB were determined for the first time with this study. The maximum principal compressional stress axes (S1) created by these thrust faults is N65ºE trending (Fig. 8k). It was determined that the thrust faults in the region pointed out both radial as well as pure compression. Spencer et al. (2016) stated that the axes of Oligocene–Miocene folds above the Buckskin-Rawhide extensional detachment fault in western Arizona maybe both perpendicular and parallel to the regional extension. This geometric setting is due to the fact that the normal detachment faulting in the region has both E-W and N-S trendings (on their Figs. 1, 3, 4, 6). According to the same authors, the geometric features of the folds within the units above the Buckshin-Rawhide detachment fault, such as gentle and open (on their Fig. 5, see Fig. 2), are similar to those of the SKB (see Figs. 2, 4).
It can be stated that the principal stress axes causing thrust and high angle normal faults in the SKB are generally parallel. Stress distribution, size, and direction of normal faults indicate that extension is dominant both in the basement rock-intra-basin boundary and in intra-basin sediments. This process has continued since Miocene. However, the maximum contraction effect of fold and thrust faults was determined locally within the whole basin and occurred only during the sedimentation of intra-basin deposits. Accordingly, only in the intra-basin deposits, both are folding, and thrust faulting has created limited shortening, approximately parallel to the principal stress axes that extend the basin. This shortening is associated with the tectonodynamic process of the low angle detachment normal fault in the brittle zone of crust.