The northern part of the KapA degrees dag Peninsula (Marmara Sea, NW Turkey) is affected by the E-W trending KapA degrees dag shear zone, which cuts through calc-alkaline granitoids of the Ocaklar pluton resulting in mylonitic orthogneiss. Macroscopic and microscopic shear-sense indicators, such as SC fabrics, shear bands, sigma-clasts and mica fish, unequivocally suggest dextral strike-slip for the KapA degrees dag shear zone. Based on petrographic data, deformation microfabrics of quartz and feldspar, and the slip systems in quartz, the dextral shearing should have been active at T = 500-300 A degrees C and P < 5 kbar. Published K-Ar and Ar-39-Ar-40 cooling ages of hornblende and biotite suggest that cooling below 500-300 A degrees C occurred during the Eocene (ca. 45-ca. 35 Ma), meaning that the KapA degrees dag shear zone should have been active during Middle to Late Eocene times. The differential stress related to the shearing was < 50 MPa as is indicated by the size of recrystallized quartz grains. Based on the new and published data, it is concluded that the westward movement of the Anatolian plate might have been active almost continuously from the Middle Eocene until recent times.