Infiltration of meteoric fluids in an extensional detachment shear zone (Kettle dome, WA, USA): How quartz dynamic recrystallization relates to fluid-rock interaction

We document the interplay between meteoric fluid flow and deformation processes in quartzite-dominated lithologies within a ductile shear zone in the footwall of a Cordilleran extensional fault (Kettle detachment system, Washington, USA). Across 150m of shear zone section, hydrogen isotope ratios (δD) from synkinematic muscovite fish are constant (δD~-130‰) and consistent with a meteoric fluid source. Quartz-muscovite oxygen isotope thermometry indicates equilibrium fractionation temperatures of ~365±30°C in the lower part of the section, where grain-scale quartz deformation was dominated by grain boundary migration recrystallization. In the upper part of the section, muscovite shows increasing intragrain compositional zoning, and quartz microstructures reflect bulging recrystallization, solution-precipitation, and microcracking that developed during progressive cooling and exhumation. The preserved microstructural characteristics and hydrogen isotope fingerprints of meteoric fluids developed over a short time interval as indicated by consistent mica ⁴⁰Ar/³⁹Ar ages ranging between 51 and 50Ma over the entire section. Pervasive fluid flow became increasingly channelized during detachment activity, leading to microstructural heterogeneity and large shifts in quartz δ¹⁸O values on a meter scale. Ductile deformation ended when brittle motion on the detachment fault rapidly exhumed the mylonitic footwall.