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.
Bibliographical noteFunding Information:
This research was supported by Swiss grant SNF-200020-126973/1 and U.S. grant NSF-EAR-0838541. WON is also grateful for support from a Virginia Tech ACC Undergraduate Research Award. AM acknowledges support through the LOEWE funding program of Hesse's Ministry of Higher Education, Research, and the Arts and support from NSF-EAR-1019648. We thank Nicholas C. A. Seaton for his help with EBSD analyses and appreciate the detailed and constructive comments on various versions of this manuscript by Jeffrey Amato, Luca Menegon, Giulio Viola, and JSG guest-editor Manuel Sintubin.
© 2014 Elsevier Ltd.
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- Dynamic recrystallization
- Extensional detachment
- Hydrogen isotopes
- Kettle dome
- Meteoric water
- Oxygen isotopes