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).

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Post-collision magmatic rocks are common in the southern portion of the Marmara region (Kapıdag, Karabiga, Gönen, Yenice, Ç an areas) and also on the small islands (Marmara, Avşa, Paşalimanı) in the Sea of Marmara. They are represented mainly by granitic plutons, stocks and sills within Triassic basement rocks. The granitoids have ages between Late Cretaceous and Miocene, but mainly belong to two groups: Eocene in the north and Miocene in the south. The Miocene granitoids have associated volcanic rocks; the Eocene granitoids do not display such associations. They are both granodioritic and granitic in composition, and are metaluminous, calc-alkaline, medium to high-K rocks. Their trace elements patterns are similar to both volcanic-arc and calc-alkaline postcollision intrusions, and the granitoids plot into the volcanic arc granite (VAG) and collision related granite areas (COLG) of discrimination diagrams. The have high 87 Sr/ 86 Sr (0.704-0.707) and low 143 Nd/ 144 Nd (0.5124-0.5128). During their evolution, the magma was affected by crustal assimilation and fractional crystallization (AFC). Nd and Sr isotopic compositions support an origin of derivation by combined continental crustal AFC from a basaltic parent magma. A slab breakoff model is consistent with the evolution of South Marmara Sea granitoids.

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