Although these fault zones are considered to be secondary structures, their roles in the internal deformation of the Anatolian plate are very important for seismic hazard assessment, such as, for example the Eldivan-Elmadağ pinched crustal wedge (Seyitoğlu et al., 2000(Seyitoğlu et al., , 2009. One of these subordinate structures, the Eskişehir Fault Zone, which extends from İnegöl (Bursa) to Cihanbeyli (Konya) (Figure 1), is a relatively well-studied example; however, there is no consensus about its age or its role in the deformation of the Anatolian plate.The Eskişehir Fault Zone was drawn on the regional geological maps of Ketin (1968), Şengör et al. (1985), and Şaroğlu et al. (1987). Later, Şaroğlu et al. (2005) presented its subdivisions as the Dodurga, Kandilli, İnönü, Osmangazi, and Kaymaz segments.A regional significance has been attributed to the Eskişehir Fault Zone. Barka and Reilinger (1997) suggest that this fault zone, together with the Fethiye-Burdur Fault, constitutes the border between central and west Anatolian neotectonic subdivisions. Koçyiğit and Özacar (2003) also proposed the Eskişehir Fault Zone as a border of the West Anatolian extensional province. The evaluation of Yaltırak ( 2002) is quite different, as according to that study, the Eskişehir Fault Zone extended from Thrace to Central Anatolia and the North Anatolian Fault Zone cuts the Eskişehir Fault Zone in the Sea of Marmara.