Hydrogen isotope and 40Ar/39Ar geochronological data are presented from muscovite within a crustal-scale extensional detachment of the Shuswap Metamorphic Complex, North American Cordillera. The hydrogen isotope compositions (δDms) of precisely dated muscovite attain values as low as −156‰ in the detachment mylonite, whereas footwall quartzite has a δDms value of −81‰. The very low δDms values in the detachment are best explained by infiltration of meteoric water, with maximum δD values of −135‰ ± 3‰, during extensional unroofing of the orogen at 49.0–47.9 Ma. On the basis of the empirically determined relationship between elevation and isotopic composition of precipitation, the reconstructed early Eocene paleoelevations of the orogen are 4060 ± 250 m to 4320 ± 250 m, at least 1000 m higher than the highest present-day peaks. We propose that the isotopic composition of surface-derived waters in extensional detachments represents a newly recognized method to estimate maximum paleoelevations attained immediately preceding extensional orogenic collapse.