Hydrogen isotope and Ar-40/Ar-39 geochronological data are presented
from muscovite within a crustal-scale extensional detachment of the
Shuswap Metamorphic Complex, North American Cordillera. The hydrogen
isotope compositions (deltaD(ms)) of precisely dated muscovite attain
values as low as -156parts per thousand in the detachment mylonite,
whereas footwall quartzite has a deltaD(ms) value of -81parts per
thousand. The very low deltaD(ms) values in the detachment are best
explained by infiltration of meteoric water, with maximum deltaD values
of -135parts per thousand +/- 3parts per thousand, 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.