A particular feature of many Alpine tectonic thrust sheets composed of carbonates is the development of distinct generations of calcite-filled veins. In the Doldenhorn nappe, one of the Helvetic nappes of Switzerland, we were able to distinguish five different generations of calcite-filled veins. Determination of carbon-isotope ratios in these calcite veins and their adjacent limestone matrix indicates that the earlier vein systems contain carbonate material not in isotopic equilibrium with the adjacent matrix. In the case of the later vein systems, the carbon-13 content approaches that of the nearby matrix. The isotope data indicate a transition from a chemically open system to a closed system; this trend is related to an overall modification of the deformation mechanism of the nappe, resulting in the increasing importance of pressure solution with time.