Geol. Mag. 146 (5 ), 2009, pp. 652–673. c  2009 Cambridge University Press 652 doi:10.1017/S0016756809006359 Printed in the United Kingdom Progressive development of the B¨ uy¨ uk Menderes Graben based on new data, western Turkey ¨ OMER FEYZ ˙ IG ¨ URER ∗ †, NURAN SARICA-FILOREAU ‡, MUZAFFER ¨ OZBURAN ∗ , ERCAN SANGU ∗ &B ¨ ULENT DO ˘ GAN ∗ ∗ Kocaeli University, Engineering Faculty, Department of Geological Engineering, Umuttepe Campus, Kocaeli, Turkey ‡Prof. Dr. Hıfzı ¨ Ozcan Cad. Adalet sitesi C Block D.20 Kadık¨ oy, Istanbul, Turkey (Received 14 May 2008; accepted 1 December 2008; First published online 31 March 2009) Abstract – Oblique and normal fault systems exposed in the B ¨ uy¨ uk Menderes Graben (BMG) region record two successive and independent complex tectonic events. The first group tectonic event is defined by an E–W extension related to N–S contraction and transpression. This group is responsible for the development of NW- and NE-trending conjugate pairs of oblique faults which controlled Early–Middle Miocene basin formation. Between the Early–Middle Miocene and Plio-Quaternary strata exists an unconformity, indicating a period of folding, uplift and severe erosion associated with N–S shortening. The second group of events was the change in tectonic regime from E–W extension to N–S extension which controlled the formation of the B¨ uy¨ uk Menderes Graben by three progressive pulses of deformation. The first pulse of extensional deformation was initially recorded in the region by the exhumation of the deep part of the Menderes Massif (MM) with the development of the E- trending B¨ uy¨ uk Menderes Detachment Fault (BMDF). The minimum age of this pulse is constrained by the older Plio-Quaternary fluviatile deposits of the B ¨ uy¨ uk Menderes Graben that range in age from the Plio-Pleistocene boundary interval to Late Pleistocene. The second pulse, which is marked by the rapid deposition of alluvial deposits, initiated the formation of approximately E–W-trending high- angle normal faults synthetic and antithetic to the B ¨ uy¨ uk Menderes Detachment Fault, on the northern margin during Holocene times. These faults are interpreted as secondary steeper listric faults that merge with the main B¨ uy¨ uk Menderes Detachment Fault at depth. The third pulse was the migration of the B¨ uy¨ uk Menderes Graben depocentre to the present day position by diachronous activity of secondary steeper listric faults. These steeper faults are the most seismically active tectonic elements in western Turkey. According to the stratigraphic and structural data, the N–S extension in the B¨ uy¨ uk Menderes Graben region produced a progressive deformation phase with different pulses during its Plio-Quaternary evolution, with migration of deformation from the master fault to the hangingwall. The formation of diachronous secondary synthetic and antithetic steeper faults on the upper plate of the B ¨ uy¨ uk Menderes Detachment Fault, hence the southward migration of the deformation and of the B¨ uy¨ uk Menderes Graben depocentre, should be related to the evolution of detachment in the region. The presence of the seismically active splays of secondary faults implies an active detachment system in the region. This young Plio-Quaternary N–S extension in the B¨ uy¨ uk Menderes Graben may be attributed to the combined effects of the two continuing processes in Aegean region. The first process is back-arc spreading or probably the roll-back of African slab below the south Aegean Arc, which seems to be responsible for the change in the stress tensor from E–W extension to N–S extension. The second and later event is the southwestward escape of the Anatolian block along its boundary fault, that is, the North Anatolian fault (NAF). Keywords: B¨ uy¨ uk Menderes Graben, detachment fault, Late Pliocene–Pleistocene, continental extension. 1. Introduction The Aegean region is one of the most active extensional regions in the world and is undergoing a N–S extension (Dewey & S ¸eng¨ or, 1979). The western Anatolian horst graben system forms the eastern boundary of the Aegean extensional system. There is no still consensus on the ongoing debates re- lated to the prevailing extension in western Anatolia. In recent years, two major problems have been associated with the geology of the graben regions and the related structures. (1) What are origins of the graben? Mainly, †Author for correspondence: ofgurer@kocaeli.edu.tr four groups of different models have been proposed to answer this question: (a) the back-arc spreading model (McKenzie, 1978; LePichon & Angelier, 1979; Jackson & McKenzie, 1988; Kissel & Laj, 1988; Meulenkamp et al. 1988; Thomson, St ¨ ockhert & Brix, 1998; Avigad et al. 1997; Jolivet et al. 1998); (b) the orogenic- collapse model (Dewey, 1988; Seyito˘ glu & Scott, 1991, 1992, 1996; McClusky et al. 2000); (c) the tectonic- escape model (Dewey & S ¸eng¨ or, 1979; S ¸eng¨ or, G¨ or¨ ur &S ¸aro˘ glu, 1985; S ¸eng¨ or, 1987) and (d) the two- stage graben model (orogenic collapse/roll-back and tectonic escape) (Koc ¸yi˘ git, Yusufo˘ glu & Bozkurt, 1999; Westaway, 2003; Bozkurt, 2000, 2001, 2003, 2004; Yılmaz et al. 2000; Cihan, Sarac ¸&G¨ okc ¸e, 2003;