Reconstructing paleoelevation in eroded orogens

Hydrogen isotope and (super 40) Ar/ (super 39) Ar geochronological data are presented from muscovite within a crustal-scale extensional detachment of the Shuswap Metamorphic Complex, North American Cordillera. The hydrogen isotope compositions (delta D (sub ms) ) of precisely dated muscovite attain values as low as -156 ppm in the detachment mylonite, whereas footwall quartzite has a delta D (sub ms) value of -81 ppm. The very low delta D (sub ms) values in the detachment are best explained by infiltration of meteoric water, with maximum delta D values of -135 ppm+ or -3 ppm, 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+ or -250 m to 4320+ or -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.