We have investigated the structures and stable isotope geochemistry of fault rocks within thrust faults and normal faults of the central Apennines in order to understand the fluid circulation during late Miocene-early Pliocene contraction and late Pliocene-Pleistocene extension of the thrust belt. Stable isotope data were obtained for 70 carbonate host rocks and 80 fault rocks. Cataclasites, veins, and slickenfibers of the thrust faults have Δ13C values ranging from 0 to 3?, which are similar to the host rocks' values, and Δ18O values from 25 to 34?, which are slightly lower than the 28 to 34? Δ18O values of the host rocks. Cemented cataclasites, veins, and slickenfibers of the normal faults have Δ13C values ranging from -5 to 3? and Δ18O values from 19 to 30?, which are significantly lower than the host rocks' values. These data are consistent with the contraction-related structures having formed in a semiclosed hydrologic system. In contrast, the extension-related structures formed in semi-open to open system conditions. During shortening, there was little to no advection of large quantities of isotopically distinct fluids from the siliciclastic wedge on top of the subducting Adriatic plate and/or from the overlying groundwater aquifers. Only during exhumation and extension of the orogen were fluids able to penetrate downward into large normal faults that at present are seismically active. ¿ 2001 American Geophysical Union