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.

The Western Anatolia extended terrane in Turkey is bounded by the North Anatolian fault zone to the north, the Lycian nappes to the south. It contains the Menderes massif, one of the post-collisional Alpine metamorphic core complexes. Field data and available radiometric ages suggest that the north-directed Cenozoic extension in the terrane is the product of three consecutive, uninterrupted stages, and that it is still continuing today. The first stage was initiated in the Late Oligocene along a north-dipping extensional simple-shear zone with a listric geometry at depth. The shear zone is named here as the Southwest Anatolian shear zone and marks the southern and southwestern boundary of the Western Anatolia extended terrane. Evidence for the presence of this shear zone includes (1) the dominant top to the north-northeast shear sense indicators in the Menderes massif and (2) a series of Oligocene extensional basins located adjacent to the shear zone that contain carbonate and ophiolitic rock clasts, but no high-grade metamorphic rock fragments. During this stage, erosion and extensional unroofing brought high-grade metamorphic rocks of the Central Menderes massif to the surface by the early Miocene. The second stage of extension produced the north-dipping Alasehir and the south-dipping Büyük Menderes detachment surfaces in the early Miocene. The detachments control the Miocene sedimentation in the Alasehir and Büyük Menderes grabens, containing high-grade metamorphic rock fragments that were already at the surface in the Central Menderes massif in the early Miocene. The third stage of extension may have started ca. 5 Ma, when the North Anatolian fault was initiated. This extensional phase produced faults within the Alasehir and Büyük Menderes grabens and possibly the Kucuk Menderes graben.