Veysel Isik

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.

39Ar/40Ar Ages from the Yozgat Batholith: Preliminary Data on the Timing of Late Cretaceous Extension in the Central Anatolian Crystalline Complex, Turkey

Isotopic dating of sheared and unsheared rocks can be important in understanding deformational processes in orogenic belts. This study examines 40Ar/39Ar dating of granitoids and mylonitic rocks to constrain intrusive and deformational events within the northern part of the central Anatolian crystalline complex (CACC). The Kerkenez granitoid within this complex, comprising primarily quartz monzonite and hornblende granite, contains discrete ductile shear zones. These zones are characterized by protomylonite and mylonite formations with metamorphism conditions that reach lower amphibolite facies, mylonitic foliations and lineations, and asymmetric kinematic indicators (e.g., asymmetric porphyroclasts, composite shear bands) with top-to-the-northwest shear senses. Considering the high closure temperatures (∼500°C for hornblende and ∼350°C for K-feldspar), both hornblende quartz monzonite and hornblende granite in the Kerkenez granitoid may have cooled rapidly, suggesting that hornblende quartz monzonite may have been emplaced at around 81.2 ± 0.5 Ma and that it is older than hornblende granite, which has a well-defined plateau age (72.6 ± 0.2 Ma). On the basis of intrusive relations and our 40Ar/39Ar age data, we can constrain the upper age limit (∼81 Ma) on the regional metamorphism in the northern part of the CACC. The 40Ar/39Ar dating of hornblendes in two mylonite samples from a ductile shear zone yields plateau ages of 71.6 ± 0.3 and 71.7 ± 0.2 Ma, respectively. K-feldspars in the same samples yield plateau ages of 71.6 ± 0.2 and 81.3 ± 0.2 Ma. Therefore, we adopt 71.6 ± 0.3 and 71.7 ± 0.2 Ma as the cooling ages of hornblende and K-feldspar, respectively, in the ductile shear zone. On the other hand, an age of 81.3 ± 0.2 Ma for deformed K-feldspar appears to reflect not the age of ductile deformation but rather the age of undeformed hornblende quartz monzonite. These age data suggest that the shear zones formed soon after the emplacement and cooling of hornblende granite. The cooling event of the shear zones is interpreted to be associated with the beginning of extension in the region. Furthermore, these data imply that metamorphism, emplacement, and cooling of the intrusives and ductile shearing of the intrusions were coeval in the region and occurred in the Late Cretaceous.

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.

The 2005 - 2007 Bala (Ankara, central Turkey) earthquakes: a case study for strike-slip fault terminations

An intense seismic activity has been observed after the Bala (Ankara, NW central Turkey) earthquakes (30 July 2005: Mw=5.3, 20 December 2007: Mw=5.4, and 26 December 2007: Mw=5.3), continuing up to the present. The epicenters and the focal mechanism solutions of the earthquakes indicate that the right lateral strike-slip Afsar fault, trending N55-60°W, is responsible for the main shocks. The Afsar fault is thought to be the NW continuation of the Tuzgolu fault zone, which is one of the main neotectonic elements in central Anatolia. On the other hand, the aftershock distributions of the 2005 event have a NNE trend, and those of the 2007 event show a NW trending. Some focal mechanism solutions of the 2005 Bala earthquake aftershocks indicate normal and oblique normal faulting that corresponds to the NNE-trending Karakecili fault. It seems that seismic activation of the NNE-trending Karakecili fault was triggered by the 2005 main shock (Mw=5.3) that occurred on the NW trending right lateral strike-slip Afsar fault. The overall neotectonic framework is that the northwestern edge of the Tuzgolu fault zone, represented by the Afsar fault in Bala, terminates in an extensional system represented by the oblique-slip Karakecili fault.