THE HYDROGEN ISOTOPE RECORD OF DETACHMENT MYLONITES: A NEW METHOD FOR RECONSTRUCTING PALEOELEVATIONS OF ERODED OROGENS

Late Cenozoic climate changes, including the Pleistocene Ice Age, have in part been attributed to higher paleoelevations in the North American Cordillera similar to observations in the Himalaya where uplift of the Tibetan plateau has been shown to influence global climate and monsoon intensity. We present hydrogen isotope and ⁴⁰Ar/³⁹Ar data of muscovite from the Eastern detachment of the Shuswap metamorphic core complex, British Columbia, which allow us to reconstruct the paleoelevation of this part of the Cordillera just prior to extensional faulting and demise of the mountain range at 49.0-47.9 Ma. Detailed sampling of muscovite-bearing mylonitic quartzite across a deformation gradient in the Eastern detachment of the core complex reveals a dramatic decrease in the dD values of muscovite (dD_ms) with increasing deformation in the mylonite. dD_ms values are as low as -157‰ in mylonite mica fish compared to dD_ms=-80‰ in the undeformed footwall of the detachment; the very low dD_ms values are best explained by infiltration of surface-derived meteoric water into the detachment. Texturally controlled sampling of muscovite and quartz in the mylonite indicate oxygen isotope equilibrium at T~420 ±40°C during deformation and recrystallization. The integrated oxygen and hydrogen isotope data permit the calculation of the isotope compositions of meteoric water in the detachment. Using the well-established relationship between elevation and isotopic composition of precipitation, our conservative estimates for Early- to Mid-Eocene paleoelevations of the northern North American Cordillera vary between 4060 ±200 m and 4320 ±200 m. These results require elevations at 49.0-47.9 Ma that were more than 1000 m higher compared to the present-day highest peaks of ~3000 m and indicate that high elevations existed already prior to Late Cenozoic uplift at least for the northernmost part of the Cordillera. Our data are consistent with an orogenic plateau, similar to the Andean Altiplano, that extended from the Omineca belt to the Cascade-Coast Mountain range in Early Eocene time.