HOW HIGH WAS THE CORDILLERA? EOCENE ELEVATION OF THE NORTH AMERICAN CORDILLERA RECORDED IN STABLE ISOTOPE COMPOSITION OF DETACHMENT MYLONITES

Methods for reconstructing paleoelevation of eroded orogens are rare and commonly involve surface features that are only indirectly related to tectonics such as fossil plants, and sediment or soil chemistry. We present a new approach of determining paleoelevation that uses the stable isotopic composition of mylonitic detachment quartzite that interacted with meteoric water during mylonitization. Oxygen and hydrogen isotope compositions of meteoric water scale systematically with temperature and elevation and changes in isotopic composition of meteoric water can be linked to elevation changes once the impact of climate variations is evaluated with reasonable uncertainty. Hydrogen, oxygen and ⁴⁰Ar/³⁹Ar data from extensional detachments of the Shuswap, Kettle and Raft River metamorphic core complexes, North American Cordillera allow us to reconstruct the early to middle Eocene paleoelevation just prior to extensional faulting and demise of the mountain range. Both the eastern detachments of the Shuswap (British Columbia) and Kettle (Washington) metamorphic core complexes active during the early to middle Eocene show very negative hydrogen isotope compositions in muscovite with values as low as -155 and -135 ‰, respectively. The integrated oxygen and hydrogen isotope data permit the calculation of isotope compositions of meteoric water that can directly be linked to elevation. These results require mean elevations in excess of 4000 m immediately preceding the timing of extensional deformation at 49.0 - 47.0 Ma and indicate that crustal thickening prior to detachment faulting resulted in high mean elevations at least for the northern part of the Cordillera. Preliminary muscovite hydrogen isotope data in Eocene and Miocene mylonites from the Raft River core complex (Utah) indicate a more complex elevation history with high elevations still present in the Miocene. It is conceivable that between the Coast Plutonic complex to the west and the Omineca belt to the east, the Eocene northern Cordillera resembled the present-day Andean Puna-Altiplano plateau. This conclusion suggests that high mean elevation mountain ranges may be typical of large-scale orogens prior to collapse and erosion and that orogenic collapse subsequently takes place in part by detachment faulting.