Gürol Seyitoglu

Ductile-brittle transition along the Alasehir detachment fault and its structural relationship with the Simav detachment fault, Menderes massif, western Turkey

Abstract Western Turkey has experienced extensional deformation since the Tertiary. An early phase of extensional deformation resulted in the formation of detachment faults in the northern part of the Menderes massif, the Alasehir and Simav detachment faults. The footwalls of these two detachment faults are mid-crustal level rocks. The Alasehir detachment fault developed in metamorphic rocks and in a syn-extensional intrusion, the Salihli granodiorite. The Salihli granodiorite shows a gradual change from undeformed isotropic granodiorite to protomylonite, mylonite and ultramylonite towards its upper structural levels. The deformed granodiorite, in turn, grades into a cataclastic zone whose uppermost part is the Alasehir detachment surface. This gradual upward change from the undeformed granodiorite to a brittlely deformed detachment surface suggests that Tertiary extension resulted in a ductile deformation at depth and the ductilely deformed granitoids were brought to shallower depths where they were brittlely deformed. The metamorphic wall rocks of the intrusion are also mylonitized structurally upward. A similar transition has also been found in the footwall of the Simav detachment fault. The upper part of which is also a cataclastic zone leading to the Simav detachment fault separating the low-grade metamorphic rocks and/or non-metamorphic rocks in its hanging wall from mainly high-grade metamorphic rocks and syn-tectonic granitoids in its footwall. During ductile extension, greenschist facies-grade mylonitic deformation developed in the metamorphic rocks and the granitoids. Shear sense indicators along the two shear zones show top to N-NE sense shear, consistent with the regional Cenozoic extension direction in western Turkey. Available radiometric age data from the granitoids in the footwall of the two detachment faults suggest that the Cenozoic extension in western Turkey was initiated in Oligocene to Early Miocene and the Simav detachment fault is older than the Alasehir detachment fault. Extensional features of these detachment faults suggest that the Simav detachment fault characterise the earlier stage of the Tertiary extensional tectonics of the Menderes massif, and later stage of this deformation resulted in the development of the Alasehir detachment fault.

The age of the Buyuk Menderes Graben (West Turkey) and its tectonic implications

The age of formation of the Buyuk Menderes graben (middle Burdigalian-middle Serravallian) is earlier than previously considered (Tortonian). This supports the contention of Seyitogˇlu & Scott that north-south extensional tectonics in west Turkey commenced at the latest during early Miocene time, following a Palaeogene compression.

Age of the Alaşehir graben (West Turkey) and its tectonic implications

The Alasehir graben is one of the main E– W (sensu lato) trending structures which developed under Late Cenozoic N– S extensional tectonics in west Turkey. The age of this graben is accepted broadly as Mio-Pliocene, but no specific data is available. An Eskihisar sporomorph association (middle Burdigalian– middle Serravallian, 20– 14 Ma) has been determined from the lower part of the graben-fill, demonstrating that its age of initiation is Early Miocene. When this result is taken into account with the similar ages of the Buyuk Menderes graben and the adjacent NE– SW trending Gordes basin, it is evident that these E– W and NE– SW trending graben/basins developed together during the Early Miocene.

Late Cenozoic basin development in west Turkey: Gördes basin tectonics and sedimentation

The early Miocene sedimentary succession in the northeast-southwest trending Gordes basin is 1000 m thick and divided into the Daĝdere, Tepekoy and Kuslukkoy formations. The age of this fill can be constrained as between 24.2 Ma and 16.3 Ma, at its widest interval. Paleocurrent directions from south to north demonstrate that the basin had a north dipping basin floor. These observations are inconsistent with a previous model of the Gordes basin as a cross graben with a south dipping basin floor, towards the Alasehir graben, and an anomalously thick (up to 3000 m) basin filling.

Discussion on the extensional folding in the Alașehir (Gediz) Graben, western Turkey

Erdin Bozkurt writes: Seyitoglu et al. (2000) described their interpretation of the broad folds exposed in the Miocene sediments—exhumed on the southern margin of the Gediz graben (western Turkey)—as products of extensional tectonics. They also concluded that present-day north–south extensional tectonics and consequent east–west graben formation in western Turkey commenced by Early Miocene and continued since then. Previously, Kocyigit et al. (1999a) provided the first ever field evidence for the presence of east–west folds in the Gediz graben and interpreted these structures as suggesting a shortlived north–south compressional deformation during the late Serravalian–late Early Pliocene times. As much of the evidence presented by Seyitoglu et al. (2000) contradicts the available literature on the origin of extensional folding, in particular rollover anticlines; and as other detailed publications on the age and origin of extensional tectonics in western Turkey and that of the east–west folds in the Gediz graben present evidence for a contradictory model(s), I would like this opportunity to describe diffculties that arise over the published evidence and interpretation of Seyitoglu et al. (2000). Tectonic significance of Sart Formation. The map pattern of various lithologies and the relationships between the structures in Figures 2 and 3 of Seyitoglu et al. (2000) are useful to drive a relative chronology of faulting. The Pliocene Sart Formation lies unconformably above the Miocene Kursunlu Formation in the hanging wall of the fault II, indicating synchronous faulting and sedimentation, i.e. the movement along FII commenced by the Pliocene. As the Sart Formation is exhumed in the footwall of fault III and FII antithetic is cut by FIII, FIII faulting must postdate Pliocene. Since the folds in extensional terrains form at the same time or just prior to the faulting, the folds in the Miocene Kursunlu Formation cannot be attributed to …

Magnetostratigraphy of early–middle Miocene deposits from east–west trending Alaşehir and Büyük Menderes grabens in western Turkey, and its tectonic implications

Abstract In western Turkey, the Alaşehir and Büyük Menderes grabens form east–west trending major tectonic structures. Their sedimentary fill is important for regional tectonic models for the late Cenozoic evolution of the Aegean region. These deposits are divided into four units dated between the early Miocene and Quaternary. We studied the magnetostratigraphy of two sections in the Alaşehir graben and one in the Büyük Menderes, partly covering the first and second sedimentary units. Detailed palaeomagnetic analysis allowed us to determine ChRM component for these rocks. The Zeytinçayı river and road sections (Alaşehir graben) record several polarity reversals, which are tentatively correlated to the interval C5Cn.3n–C5ADr (approximately between 14.6–16.6 Ma) of the ATNTS2004. This correlation is also supported by palaeontology and radiometric dating of syn-extensional intrusions. In the Eycelli section (Büyük Menderes graben) only three polarity zones are recorded, and their tentative correlation with the interval of C5Bn.1r–C5Br (14.88–15.97 Ma) is in overall in agreement with the record of Eskihisar sporomorph association in this formation. These results place the initiation of the Alaşehir and Büyük Menderes grabens in the early Miocene. The palaeomagnetic declinations from the Alaşehir graben indicate about 25° anticlockwise rotation, whereas that of the Büyük Menderes graben indicate a clockwise rotation of about 30–40°. These contradictory vertical-axis rotations might be explained by detachment faults in the region. In Tertiary formations of western Turkey, contradictory block rotations are common and likely reflect thin-skinned deformation in the area rather than rigid crustal movements. Therefore, average anticlockwise rotations in western Turkey cannot be used as evidence for the model of back-arc spreading in the Aegean region.

Sedimentology of the Miocene evaporitic succession in the north of Çankiri-Çorum basin, central Anatolia, Turkey

The upper Miocene non-marine sediments of the Çankiri-Çorum basin in central Anatolia, have both evaporitic and non-evaporitic successions. These sediments were deposited in an evaporitic lake which had temporary episodes of palustrine conditions in response to seasonal or climatic changes. The successions show different facies such as sulfates, carbonates and siliciclastics. The sulfates comprise primary, reworked and diagenetic gypsum. The primary deposits are predominantly laminated gypsum, bedded gypsum and selenite. The reworked (detrital) gypsum comprises gypsite, gypsarenite, gypsrudite and breccias. The diagenetic type comprises micro- and macrogypsum nodules. The carbonates mainly include clayey limestone, oolitic limestone and dolomite. The siliciclastics comprise red beds and both channel and non-channel, conglomerates and mudstones.Laminated gypsum, composed of alternating gypsum and dolomite, was a result of environmental schizohalinity. Bedded gypsum was precipitated in the deeper part of the lake during high evaporation periods. Chevron-type selenite crystals formed on saline mud flats during the times of aridity, whereas the discoidal-type seen in the organic-rich mudstones occurred in the gypsiferous marshes during the times of humidity. Reworked (detrital) gypsum dominates the lake margin. These formed during periodic wet episodes that caused reworking of primary gypsum. Gypsum nodules occurred as both early and late diagenetic products. Carbonates and siliciclastics were deposited during the freshening periods of the lake.Climatic or seasonal changes were the main causes of the depositional styles of the upper Miocene evaporitic and non-evaporitic lacustrine deposits in Çankiri-Çorum basin. Additionally, the transition upward from alluvial to lake environment implies an important change in drainage patterns that likely occurred as a result of marginal fault activity.

Meaning of the Küçuk Menderes graben in the tectonic framework of the central Menderes metamorphic core complex (western Turkey)

Unusually steep, high-angle south-dipping normal faults, that separate the metamorphic rocks of the Menderes massif from the Neogene sedimentary deposits, occur in the northern Kucuk Menderes graben in western Turkey. These faults probably reached their current position as a result of the special tectonic framework of the central Menderes metamorphic core complex. This area experienced further exhumation along with the rolling hinges of faults limiting Alasehir and Buyuk Menderes grabens, giving rise to a huge syncline in the region. This regional structure would be most likely responsible for the present position of the high-angle graben bounding normal faults that rotated along a horizontal axis. However, recent studies in the same area claim for the development of reverse faulting between the metamorphic basement and the Neogene sedimentary units, and present this observation as evidence for a supposed Miocene–Pliocene regional contractional regime in western Turkey. Even if these reverse faults or other post-Miocene contractional structures existed in the central Menderes massif, they would not afford evidence of regional contraction since its generation could be related to the contractional area that would have developed along the axial zone of the huge syncline in the central Menderes massif, in its turn resulting from a regional extensional process that took place during the further exhumation stage.

The evolution of intraplate fault systems in central Turkey: Structural evidence and Ar‐Ar and Rb‐Sr age constraints for the Savcili Fault Zone

The Savcili Fault Zone represents one of the most prominent regional-scale intraplate fault systems in central Turkey, recording the collisional events following the closure of Neo-Tethys in the eastern Mediterranean region. It consists of anastomosing reverse/thrust faults with WNW-ESE direction that placed rocks of the Central Anatolian Crystalline Complex on Paleogene sedimentary units. Structural measurements and kinematic indicators show that faults within the Savcili Fault Zone (SFZ) have top to the NE and NW sense of brittle deformation. Stable isotope (δ18O and δ D) and trace element data indicate that fault gouge illites precipitated from deep basinal brines. These fluids were mobilized during phases of compressional deformation and migrated upward along thrust faults toward shallow brittle deformation zones. Rb-Sr and Ar-Ar geochronology of fault gouges in two cataclastic zones demonstrates age variability for two different dating techniques (Rb-Sr: 40.9 ± 1.5 Ma and 22.9 ± 1.3 Ma; Ar-Ar: 46.45 ± 0.25 Ma and 29.8 ± 0.13 Ma). We argue that Rb-Sr dating provides ages more closely reflecting the timing of fault movements because of potential contamination of illite by excess 40Ar. Accordingly, the SFZ was active during at least two phases; the middle Eocene and late Oligocene to early Miocene, which is consistent with the relative age constraints suggested by field relationships. Geochronology combined with structural field evidence indicates a rapid change in stress regime from extension to contraction at ∼40 Ma that continued until at least ∼23 Ma. Direct dating of brittle faulting provides a prolific approach for determining the absolute timing of tectonic events in areas that have largely relied on indirect information.

The stratigraphical position of Kemiklitepe fossil locality (Eşme, Uşsak) revised: Implications for the Late Cenozoic sedimentary basin development and extensional tectonics in western Turkey

Unlike earlier studies attributing the Kemiklitepe fossil locality to the İnay Group, the present study indicates that it is to be assigned to the Asartepe Formation that unconformably overlying the İnay Group. This seems to verify the early Middle Miocene age of the İnay Group, previously determined by isotopic dating and palynological analyses. However, the early Middle Miocene age and the overall undeformed nature of the İnay Group do not concur with the regional two-stage extension model proposing a compressional phase during the Miocene/Pliocene interval. The correlation of the Asartepe Formation bearing the Kemiklitepe fossil locality further to the north indicates that a NE-SW trending fault was active during the Late Miocene. Recent studies modelling the uplift history of the region are unconvincing because they ignore Late Miocene activity on the NE-SW trending faults and assume a timing of incision of the İnay Group after the Late Pliocene (~3Ma) in their calculations.