EOCENE ELEVATION OF CORDILLERA RECORDED IN STABLE ISOTOPIC COMPOSITION OF DETACHMENT MYLONITES

Existing methods for calculating the paleoelevation of collapsed/eroded orogens involve surface features such as basalt flow vesicularity, fossil plant physiognomy, and sediment or soil chemistry. A new approach to determining paleoelevation uses the stable isotopic composition of mylonitic quartzite that interacted and equilibrated with meteoric water during mylonitization. The oxygen and hydrogen isotope composition of meteoric water scales with elevation. During extensional collapse of the orogen, mateoric water penetrates the upper crust and interacts thoroughly with recrystallizing white mica in detachment mylonites. The isotopic composition of the meteoric water is retrieved from the isotopic ratios measured in white micas. This new approach involves stable isotopic analysis of mylonitic quartzite because white mica, the only hydrous mineral in quartzite, does not interact with other phases and provides a reliable reservoir of meteoric water composition. In addition, syn-kinematic white mica in the quartzite can be dated using argon-argon methods to determine the timing of deformation and interaction with fluids. Deuterium-hydrogen ratios measured in recrystallized white mica fish provide the link to elevation using existing climate models.

This new method has been applied to the Shuswap metamorphic core complex in British Columbia. Samples of quartzite were collected along the Columbia River detachment that bounds the metamorphic core complex to the east. Laser ablation as well as conventional step-heating argon-argon dating of synkinematic white mica shows that plateau ages of 49-48 Ma can be interpreted as deformation-recrystallization ages. Oxygen isotope analyses show that quartz-mica mineral pairs equilibrated at 420°C ± 40 °C. At this temperature, the very negative values of hydrogen isotopes (delta D ~ -160 per mil) in white mica indicate that mica interacted with meteoric water with a delta D ~ -140 per mil. This low value corresponds to meteoric water precipitated at an elevation of at least 4000 m at ~49 Ma. This new method is portable to other orogens and enhances understanding of the interaction among crustal dynamics, landscape evolution, and climate during orogeny.