We dated syntectonic calcites on fault planes from the southern branch of the western North Anatolian fault (NAF) in northern Turkey using U-Th geochronology. We selected strike-slip faults that are kinematically related to the current regional strain field. The isotopic ages cluster around different periods during the past similar to 700 k.y. The most prominent cluster peak of 510.5 +/- 9.5 ka (1 sigma) is consistent with the maximum cumulative strike-slip offset data and tectonic plate motions measured by GPS data, highlighting the fact that the present configuration of the NAF in the southern Marmara region started at ca. 500 ka or earlier. These new isotopic ages, combined with previous considerations of regional tectonics, reveal that faulting along the western NAF initiated primarily in the southern Marmara region at least a few hundred thousand years earlier than the timing suggested for the northern branch of the western NAF. This study presents an innovative approach to constrain the timing of initiation of currently active fault segments along the NAF in southern Marmara. U-Th geochronology of fault-hosted calcite thus has a wide application in determining absolute ages of fault episodes in wider shear zones along plate boundaries.