To solve a long-lasting controversy on the timing and mechanism of generation of the western Anatolian graben system, new data have been collected from a mapping project in western Anatolia, which reveal that initially north-south trending graben basins were formed under an east-west extensional regime during Early Miocene times. The extensional openings associated with approximately north-south trending oblique slip faults provided access for calc-alkaline, hybrid magmas to reach the surface. A northsouth extensional regime began during Late Miocene time. During this period a major breakaway fault was formed. Part of the lower plate was uplifted and cropped out later in the Bozda~ Horst, and above the upper plate approximately north-south trending crossgrabens were developed. Along these fault systems, alkaline basalt lavas were extruded. The north-south extension was interrupted at the end of Late Miocene or Early Pliocene times, as evidenced by a regional horizontal erosional surface which developed across Neogene rocks, including Upper Miocene-Lower Pliocene strata. This erosion nearly obliterated the previously formed topographic irregularities, including the Bozda~ elevation. Later, the erosional surface was disrupted and the structures which controlled development of the Lower-Upper Miocene rocks were cut by approximately east-west trending normal faults formed by rejuvenated north-south extension. This has led to development of the present-day east-west trending grabens during Plio-Quaternary time.cene ($eng6r et al. 1985; GOrt~r et al. 1995).

The southeast Anatolian orogen may be divided into three roughly cast-west trending structural zones formed as a result of continental collision between the Taurus platform and the Arabian continent. Along the orogcnic belt, metamorphic and ophiolitic rocks occur widely. The ophiolitcs represent remnants of the ocean or oceans which were totally consumed between these converging continental blocks during Late Cretaceous to Miocene period. Metamorphic rocks formed from the oceanic as well as the continental rocks which were incorporated into a nappc stack during the consumption of the oceanic lithosphere and the progressive southward advance of the nappcs toward the Arabian continent. The metamorphic units, together with the ophiolitc associations, provide stratigraphic and pctrologic evidence indicating time, place, and environment of formation of these units; the metamorphic units also provide evidence of nappc transportation stages which are complementary to the data derived from the sedimentary successions in the evaluation of the orogcnic evolution of southeast Anatolian orogcn.

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