Turkish Journal of Earth Sciences




The large-scale, low-angle Gediz detachment fault that constitutes the southern margin of the Gediz graben (also known as the Alaşehir graben) has macroscopic structures of extensional origin, such as folds and back-tilted strata. The folds were formed as antiformal and synformal structures with axes both parallel and perpendicular to the extensional direction in their footwalls and hanging-walls, respectively. An extension-parallel antiform-synform pair in the footwall of the Gediz detachment, namely the Oyukkıran antiform and the Keserler synform, are recognized south of Dereköy village (Salihli) on the southern margin of the Gediz graben. These are NE-trending domal and basinal structures with a wavelength of 6 km and amplitudes of 1 km. The Oyukkıran antiform is a NE-plunging antiform that exposes granitic rocks (now with cataclastic textures) in its core, with a surrounding cover of Menderes Massif metamorphic rocks, while the Keserler synform has a core of Upper Miocene synextensional sediments that structurally overlie the metamorphic rocks. These corrugations are interpreted as original irregularities of the detachment fault; this interpretation does not require folding after the deposition of the hanging-wall sedimentary sequences. Three hanging-wall sedimentary sequences, separated from each other by angular unconformities, are redefined in the area. A sequence of Upper Miocene-Lower Pliocene continental sedimentary rocks (SS-II) was deposited in the hanging-wall of the Gediz detachment fault. The sequence is defined as a basal deposit of a supradetachment basin which formed during regional extension in western Anatolia. The SS-II is cut by three basinward-dipping, high-angle normal faults in front of which the SS-III was deposited. The SS-IV comprises the undeformed present-day basin fill of the Gediz graben. The syn-extensional hanging-wall strata include discontinuous extensional folds which are nearly perpendicular to the extension direction and also to the extension-parallel footwall folds. The extension-perpendicular folds formed as longitudinal drag folds, rollover anticlines and monoclines. Except for longitudinal extensional folds, rocks in the hanging-wall of the Gediz detachment have been deformed to form back-tilted strata about a horizontal axis which is perpendicular to the extension direction; it is interpreted as an angular unconformity defined by a major change in degree of back-tilted strata, reflecting a change in deformation type - from the supradetachment stage to that of the rift stage in the Gediz graben. The older supradetachment basin formed on the Gediz detachment fault, and its sediments are cut by high-angle normal faults that controlled the formation of the rift basin fill.

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