Structural cathodoluminescence and SIMS δ18O analyses of quartz-calcite veins are combined to constrain the evolution in space and time of fluid infiltration in an exhuming detachment shear zone (Tinos Island, Cyclades, Greece). Careful description of vein arrays shows that the plumbing system evolved into an interconnected network just beneath the ductile-to-brittle transition. Microtextures of quartz and calcite infilling veins display deformed relics and newly precipitated grains. High-resolution δ18O mapping in relics yields a steady quartz-calcite fractionation of 2 ? at ~400¿C, whereas fractionation increases in neograins from 2 to 5 ? in 190 m toward the fault, or a ~150¿C temperature fall. Cooling is associated with a shift of fluid stable isotope signature from 15 ? to 0 ? beneath the detachment. Results are interpreted as advective removal of heat by massive infiltration of surface-derived fluids over depths of 10--15 km. Fluid penetration was promoted by the coalescence of late ductile-early brittle veins, as the exhuming footwall crossed the ductile-brittle transition. Only small amounts of fluids penetrated the ductile crust beneath the transitional rheology.