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Origin of NE-trending basins in western Turkey

The origin of NE-trending basins and their relation to the E-W-trending grabens in western Turkey have been the subject of long lasting debate. The stratigraphical and structural aspects of the basin fill in the E-W Gediz Graben and in the NE-trending Gordes, Demirci, Selendi and Usak-Gure basins located immediately to the north of the graben are reassessed here. The Gediz Graben is characterized by two contrasting sedimentary packages that are separated by an angular unconformity. These are: (1) a folded Miocene-Lower Pliocene megasequence, exhumed on the shoulders of the graben; and (2) nearly horizontal Plio-Quaternary sediments. In contrast, the NE-trending basins comprise solely Lower-Middle Miocene sediments and unconformably overlying Quaternary alluvium. The Miocene configuration of different trending basins shows close similarities, and suggests that they started to develop simultaneously during the Early,Miocene. Miocene sedimentation in these basins occurred in the hanging-wall of a presently low-angle, north-dipping, major normal fault (detachment fault) that bounds the Gediz Graben to the south, while the metamorphic rocks of the Menderes Massif in the footwall were progressively deformed, uplifted and exhumed. The Miocene sediments in the NE-trending basins were also deformed along broad folds with axes parallel to the basin margins. The extension was therefore partially complemented by a horizontal shortening at high-angle to it, and the basin-bounding faults together with the blocks-they bound have rotated around vertical axes. In this scenario, the NE-basins started to develop as "Aegean type cross-grabens", and the bounding structures are interpreted as "rotational accommodation faults" that accommodate differential stretching in the hanging-wall of the major breakaway fault, accompanying the Early Miocene phase of extension in western Turkey. Later, during the neotectonic extensional period commencing in the Pliocene (5 Ma), the margin-bounding high-angle normal faults of the modern E-W Gediz Graben cut and displaced the presently low-angle breakaway fault. Some parts of the NE-trending basins were therefore trapped within the E-W graben and some other parts remained as hanging basins on the footwall of the normal fault, along the northern margin of the Gediz Graben. It is concluded that the classification of basins for the area of the Gediz Graben and those located to its north as "replacement and revolutionary" structures is still valid. It is also confirmed that the latest Oligocene-Early Miocene back-arc extension and/or orogenic collapse cannot be responsible for the entire history of continental extensional tectonics and the extension in western Turkey is not a continuous event, but occurred in two stages separated by a short period (7-5 Ma); the second being the Plio-Quaternary phase of N-S extension related to the westward escape of the Anatolian block. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.