To the east of the Sea of Marmara, the North Anatolian fault (NAF) branches into two strands, namely the northern and the southern strands. The Adapazari pull-apart basin is located in the overlapping zone of the Dokurcun and the Izmit-Adapazari segments of the northern strand. The combined temporal ranges of the arvicolids from the Karapurcek formation (the first unit of the basin fill), deposited in the primary morphology of the Adapazari pull-apart basin, cover the latest Villanyian (latest Pliocene) and the Biharian (Early Pleistocene) time interval. The Degirmendere fauna collected from the lowermost sediments of this formation suggests that the Adapazari pull-apart basin started to form in the latest Pliocene. This, in turn, suggests that the dextral movement along the northern strand of the NAF commenced during the latest Pliocene. A new species, Tibericola sakaryaensis is also described. (C) 2001 Editions scientifiques et medicales Elsevier. SAS.