Six limestone assemblages along the North Anatolian Fault (NAF) Niksar pull-apart basin in northern Turkey were analyzed for delta O-18(PDB) and delta C-13(PDB) using bulk isotope ratio mass spectrometry (IRMS). Matrix vein differences in delta O-18(PDB) (-2.1 to 6.3 parts per thousand) and delta C-13(PDB) (-0.9 to 4.6 parts per thousand) suggest a closed fluid system and rock buffering. Veins in one travertine and two limestone assemblages were further subjected to cathodoluminescence, trace element (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) and delta O-18(PDB) (Secondary Ion Mass Spectrometry, SIMS) analyses. Fluid inclusions in one limestone sample yield T-h of 83.8 +/- 7.3 degrees C (+/- 1 sigma, mean average). SIMS delta O-18(PDB) values across veins show fine-scale variations interpreted as evolving thermal conditions during growth and limited rock buffering seen at a higher resolution than IRMS. Rare earth element data suggest calcite veins precipitated from seawater, whereas the travertine has a hydrothermal source. The delta O-18(SMOW-fluid) for the mineralizing fluid that reproduces T-h is +2 parts per thousand, in range of Cretaceous brines, as opposed to negative delta O-18(SMOW-fluid) from meteoric, groundwater, and geothermal sites in the region and highly positive delta O-18(SMOW-fluid) expected for mantle-derived fluids. Calcite veins at this location do not record evidence for deeply-sourced metamorphic and magmatic fluids, an observation that differs from what is reported for the NAF elsewhere along strike. (C) 2017 Elsevier Ltd. All rights reserved.