SGER: A New Method for Determining the Paleoelevation of Orogens

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This small grant for exploratory research is investigating a new approach to determining paleoelevation of collapsed or eroded orogens using the stable isotope composition of mylonitic quartzite that interacted and equilibrated with meteoric water during deformation. 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 can be retrieved from the isotopic ratios measured in white micas, permitting a direct link to be made between the chemical and structural evolution of rocks at depth and the evolution of landscape and climate. Syn-kinematic white mica in the quartzite is dated using 40Ar/39Ar methods to determine the timing of deformation and interaction with fluids. Deuterium-hydrogen ratios measured in mica provide the link to elevation using existing climate models. Preliminary Results from the Shuswap core complex (British Columbia) indicate that the region was at an elevation of at least 4000 meters about 49 million years ago. The new method is being applied to mylonitic quartzite exposed in other core complexes in the Cordillera (Okanogan-Kettle Complex, Albion-Raft River-Grouse Creek Complex ) as a test of the reproducabilty of the method. The age of mylonitization varies from 50 to 15 million years old. Therefore, the method is being used to interpret the tectonics and paleoelevation of the Cordillera from Eocene to Miocene time. This technique is portable to other orogens and enhances understanding of the interaction among crustal dynamics, landscape evolution, and climate during orogeny.