Andreas Mulch Christian Teyssier Michael L. Wells Michael A. Cosca Raphael Gottardi Aude Gebelin C. Page Chamberlain Katharina Methner 2015 <p><span>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 (</span>⁴⁰<span>Ar/</span>³⁹<span>Ar), and microstructural data from the Raft River detachment shear zone. Hydrogen isotope ratios of syntectonic white mica (</span><i>δ</i>²<span>H</span>_ms<span>) 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.<span>&nbsp;</span></span>⁴⁰<span>Ar/</span>³⁹<span>Ar geochronology reveals Eocene (re)crystallization of white mica with<span>&nbsp;</span></span><i>δ</i>²<span>H</span>_ms<span> ≥ −154‰ in quartzite mylonite of the western segment of the detachment system. These<span>&nbsp;</span></span><i>δ</i>²<span>H</span>_ms<span><span>&nbsp;</span>values are distinctively lower than in localities farther east (</span><i>δ</i>²<span>H</span>_ms<span> ≥ −125‰), where<span>&nbsp;</span></span>⁴⁰<span>Ar/</span>³⁹<span>Ar geochronological data indicate Miocene (18–15 Ma) extensional shearing and mylonitic fabric formation. These data indicate that very low<span>&nbsp;</span></span><i>δ</i>²<span>H 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<span>&nbsp;</span></span>⁴⁰<span>Ar/</span>³⁹<span>Ar ages, and higher<span>&nbsp;</span></span><i>δ</i>²<span>H values of recrystallized white mica, indicate Miocene structural and isotopic overprinting of Eocene fabrics.</span></p> application/pdf 10.1002/2014TC003766 en AGU Eocene and Miocene extension, meteoric fluid infiltration, and core complex formation in the Great Basin (Raft River Mountains, Utah) article