Journal
TECTONICS
Volume 34, Issue 4, Pages 680-693Publisher
AMER GEOPHYSICAL UNION
DOI: 10.1002/2014TC003766
Keywords
geochronology; hydrogen isotopes; fluid flow; detachment; core complex; Raft River
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Funding
- LOEWE funding program of Hesse's Ministry of Higher Education, Research, and the Arts
- National Science Foundation [EAR 0838541, EAR 1019648, EAR 0610098]
- Swiss grants [SNF-200020-126973/1, SNF-200021-103674]
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Metamorphic core complexes (MCCs) in the North American Cordillera reflect the effects of lithospheric extension and contribute to crustal adjustments both during and after a protracted subduction history along the Pacific plate margin. While the Miocene-to-recent history of most MCCs in the Great Basin, including the Raft River-Albion-Grouse Creek MCC, is well documented, early Cenozoic tectonic fabrics are commonly severely overprinted. We present stable isotope, geochronological (40Ar/39Ar), and microstructural data from the Raft River detachment shear zone. Hydrogen isotope ratios of syntectonic white mica (H-2(ms)) from mylonitic quartzite within the shear zone are very low (-90 to -154, Vienna SMOW) and result from multiphase synkinematic interaction with surface-derived fluids. 40Ar/39Ar geochronology reveals Eocene (re)crystallization of white mica with H-2(ms)-154 in quartzite mylonite of the western segment of the detachment system. These H-2(ms) values are distinctively lower than in localities farther east (H-2(ms)-125 parts per thousand), where 40Ar/39Ar geochronological data indicate Miocene (18-15 Ma) extensional shearing and mylonitic fabric formation. These data indicate that very low H-2 surface-derived fluids penetrated the brittle-ductile transition as early as the mid-Eocene during a first phase of exhumation along a detachment rooted to the east. In the eastern part of the core complex, prominent top-to-the-east ductile shearing, mid-Miocene 40Ar/39Ar ages, and higher H-2 values of recrystallized white mica, indicate Miocene structural and isotopic overprinting of Eocene fabrics.
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