Samples collected from five carbonate-hosted thrust faults in the Front Ranges (Canadian Rockies) were isotopically and structurally analyzed in order to document syntectonic fluid-rock interaction. The most deformed rocks are adjacent to the thrust contacts, corresponding to a general increase in (micro) fractures, twins, foliation development, crystallographic preferred orientation fabric, and veins toward the thrusts. Subhorizontal veins that accommodated subvertical extension are present locally in the thrusts and help to define the foliated fabric of the fault rocks. The Δ18O and Δ13C values of fault rocks meter(s) away from the thrusts are typical of Paleozoic carbonates, while many rocks adjacent to the thrusts have lower values. There are not strong correlations between these values and the type and/or density of structures in the thrusts. Far from the thrusts, most syntectonic veins have isotopic values similar to their host rocks. Only within decimeters to a meter of the thrusts, do some veins have lower values. There is little isotopic evidence for significant fluid flow across the faults between hanging wall and footwall rocks. The structural data are consistent with the fault rocks having deforming by a variety of deformation mechanisms that probably resulted in both crystal plastic and brittle behavior. Transient high pore fluid pressure in the faults must have occurred locally and episodically in order to account for the formation of the subhorizontal veins. The isotopic data are consistent with a limited amount of fluid contained in very narrow zones adjacent to the thrusts. If true, this fluid must have been maintained in a semi-closed system and not continuously replenished by large quantities of fluids from the thrusts' footwalls or from the hinterland. The long-term rheology of the fault rocks in the Front Ranges was not necessarily dominated by pressure-dependent frictional sliding, and the role of high pore fluid pressure in thrust movement might have been less important than is widely believed. ¿ 2001 American Geophysical Union