Oxygen isotope data from syntectonic veins, thrust faults, and wall rocks suggest that fluids infiltrated the Western Ranges of the Rocky Mountain foreland from deeper rocks of the Dogtooth Range during Mesozoic contraction. This signifies the first such evidence for kilometer-scale fluid migration at the hinterland-foreland transition of the Canadian Cordillera. Fluid infiltration resulted in isotopic depletion in wall rocks and isotopic disequilibrium between veins (fluid) and their host rocks downstream of a lithologic, structural, and isotopic shift between predominantly siliciclastic (Dogtooth Range) and calcareous (Western Ranges) sequences. The shift corresponds with an average δ18O that is 3.0? higher in the Western Ranges. In the Dogtooth Range, atypically low carbonate values (15.6? ¿ 0.8? (VSMOW)), and the consistency with which vein signatures approach the average bulk rock δ18O of individual outcrops indicate fluid buffering by a siliciclastic reservoir and outcrop-scale (10--100 m) fluid circulation. In contrast, rocks and veins in the Western Ranges exhibit a gradational increase in δ18O that extends ≥14 km eastward and up section from the isotopic shift and possess values that are below or at the low end of typical carbonate compositions (16.2--20.9? (VSMOW)). Furthermore, veins systematically exhibit lower oxygen values than their host rocks; this implies that the fluids that entered the Western Ranges were in modest disequilibrium with the rocks through which they flowed. We use a one-dimensional reactive transport model to infer that rocks in the Western Ranges experienced a time-integrated fluid flux of 1.1 ¿ 105 mol H2O cm-2 (≈2.4 ¿ 106 cm3 cm-2 ) and sluggish reaction kinetics during the flow event.