MAGNETOSTRATIGRAPHY AND VERTICAL ROTATIONAL TECTONICS IN THE EARLY- MIDDLE MIOCENE DEPOSITS OF ALASEHIR AND BUYUK MENDERES GRABENS, WESTERN TURKEY

The E-W trending Alasehir and Buyuk Menderes grabens are created by the N-S extensional tectonics in western Turkey. Their deposits are divided into four packages. The magnetic stratigraphy of the first two sedimentary packages have been studied in both grabens. Detailed paleomagnetic analyses allowed to determine characteristic remanant magnetization (ChRM) for these rocks. The Zeytincayi river and road sections (Alasehir graben) record several polarity reversals which correlate well with the interval C5Cn.3n - C5ADr (14.6 - 16.7 Ma) of the geomagnetic polarity time scale (GPTS). This correlation is also supported by paleontology and radiometric dating of syn-extensional intrusions. In the Eycelli section (Buyuk Menderes graben) only three polarity zones are recorded, and their tentative correlation with the interval of C5Cr - C5Dr (16.73 - 18.28 Ma) is supported by the Eskihisar sporomorph association. These results place the initiation of these grabens in the Early Miocene. These results are the first magnetostratigraphic data obtained from these grabens, and they contribute to resolve the recent controversial interpretations about the initiation of N-S extensional tectonics in western Turkey. The paleomagnetic declinations from the Alasehir graben indicate about 25° anticlockwise rotation, whereas that of the Buyuk 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.