THE ROLE OF FAULTS IN ISOTOPE TRANSPORT AND EXCHANGE DURING THE FORMATION OF THE SHUSWAP METAMORPHIC CORE COMPLEX, BRITISH COLUMBIA

The eastern detachment of the Shuswap metamorphic core complex, British Columbia, reveals significant exchange between Eocene meteroric fluids and mylonitic rocks. East of the Thor-Odin dome, the 1 km thick detachment zone is localized in a quartzite layer of the exhumed footwall. Based on 40Ar/39Ar and oxygen isotope data of synkinematic white mica deformation and isotopic exchange in the quartzite mylonite occurred at temperatures of 400-450 °C between 49.0 and 47.9 Ma. Hydrogen isotope compositions of white mica and fluid inclusion waters indicate that meteoric fluids (deltaD~-140 per mil) infiltrated the detachment and equilibrated with the mylonite during deformation. Numerical models of hydrothermal fluid flow and kinetic fluid-rock isotope exchange were used to reconstruct the plumbing system of this Eocene detachment. The goal of this modeling approach was to determine the permeability of both the fault system and the mylonitic crustal rocks. Computed isotope exchange patterns reflect the interplay among temperature, fluid flux, time, initial fluid composition, and effective surface area available for isotope exchange. Results show that fluid velocities within the fault zone were on the order of 1 m/yr, sufficiently high to reproduce the observed hydrogen and oxygen isotopic exchange, but low enough to not cause significant convective cooling, which would dramatically decrease fluid-rock isotope exchange rates. While the model results are non-unique, we were able to evaluate permeability of the fault zone and crustal rocks to be ~10-15 m2 and less than 10-17 m2, respectively. The widely cited permeability-depth relationship of Manning and Ingebritsen (1999, Geology 27, 1107-10) did not produce unique rock isotopic exchange patterns that could be easily distinguished from constant-value crustal permeability models. We estimate that individual segments of the flow system were active over time scales on the order of 104 years to produce the observed isotopic shifts, suggesting that a series of transient fluid flow events decreased through time due to permeability reduction associated with mineralization and overall cooling by exhumation of the detachment system.