GEOLOGY, August 2011 759 ABSTRACT Extensional detachment systems separate hot footwalls from cool hanging walls, but the degree to which this thermal gradient is the product of ductile or brittle deformation or a preserved original transient geotherm is unclear. Oxygen isotope thermometry using recrystallized quartz-muscovite pairs indicates a smooth thermal gradient (140 °C/100 m) across the gently dipping, quartzite-domi- nated detachment zone that bounds the Raft River core complex in northwest Utah (United States). Hydrogen isotope values of musco- vite (δD Ms ~ –100‰) and fluid inclusions in quartz (δD Fluid ~ –85‰) indicate the presence of meteoric fluids during detachment dynamics. Recrystallized grain-shape fabrics and quartz c-axis fabric patterns reveal a large component of coaxial strain (pure shear), consistent with thinning of the detachment section. Therefore, the high thermal gradient preserved in the Raft River detachment reflects the transient geotherm that developed owing to shearing, thinning, and the poten- tially prominent role of convective flow of surface fluids. INTRODUCTION Extensional detachment systems are critical interfaces that typically separate the cool, brittle upper crust from high-grade lower and middle crust exhumed in metamorphic core complexes. Detachments are zones of localized deformation, fluid flow, and thermal exchange (Nesbitt and Muehlenbachs, 1989, 1995; Wickham et al., 1993; Morrison, 1994; Mor- rison and Anderson, 1998; Holk and Taylor, 2000; Mulch et al., 2006; Mulch et al., 2007; Person et al., 2007), but the interplay among these pro- cesses is poorly understood. Here, the focus is on the footwall shear zone of the Raft River detachment system in northwest Utah (United States). The shear zone is dominantly in quartzite, such that quartz microfabrics provide a useful record on the kinematics and thermomechanics of this detachment system. Hydrogen isotope ratios of quartz fluid inclusions and of fabric-forming, recrystallized white mica demonstrate that surface fluids permeated the shear zone during deformation. Oxygen isotope ther- mometry based on recrystallized quartz-mica pairs uncovers an extremely high gradient of metamorphic temperatures preserved in the 100-m-thick shear zone. The influx of cool surface fluids likely produced and preserved the high geotherm that developed during detachment tectonics. THE RAFT RIVER DETACHMENT SYSTEM The east-rooted, Miocene Raft River shear zone (Malavieille, 1987; Wells et al., 2000; Wells, 2001) is localized in the Proterozoic Elba Quartz- ite, which unconformably overlies an Archean basement complex (Comp- ton, 1972, 1975). Cenozoic 40 Ar/ 39 Ar white mica ages from the quartzite define a west-to-east age gradient from 47 to 15 Ma (Wells et al., 2000). This study focuses on the easternmost exposure of the Miocene shear zone (Clear Creek Canyon; Wells, 2001; Sullivan, 2008), where the shear zone is localized in an ~100-m-thick quartzite-dominated shear zone. The Elba Quartzite includes, from bottom to top, a basal quartzite- cobble metaconglomerate, an alternating sequence of white quartzite and muscovite-quartzite schist, a distinctive layer of red quartzite, and a pebble-metaconglomerate that includes alternating feldspar-rich quartzite, pure quartzite, and quartz-pebble metaconglomerate (Fig. 1) (Wells et al., 1998; Sullivan, 2008). Paleozoic metasedimentary rocks are preserved as a few scattered klippen above the quartzite and overlying schist unit, and define the hanging wall of the Miocene Raft River detachment (Compton, 1975, Wells, 1997, 2001, 2009; Wells et al., 1998). QUARTZ MICROFABRICS The well-developed mylonitic foliation and lineation are constant in orientation throughout the quartzite and are defined by flattened and elongated quartz and white mica grains. The strongly deformed quartzite shows two populations of quartz grains, including coarse (>1000 μm long) Geology, August 2011; v. 39; no. 8; p. 759–762; doi:10.1130/G31834.1; 3 figures; Data Repository item 2011228. © 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Preservation of an extreme transient geotherm in the Raft River detachment shear zone R. Gottardi 1 , C. Teyssier 1 , A. Mulch 2,3 , T.W. Vennemann 4 , and M.L. Wells 5 1 Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA 2 Institut für Geologie, Universität Hannover, 30167 Hannover, Germany 3 Biodiversity and Climate Research Centre (BiK-F), 60325 Frankfurt, Germany, and Institut für Geowissenschaften, Goethe Universität Frankfurt, 60348 Frankfurt, Germany 4 Institut de Minéralogie et Géochimie, Université de Lausanne, 1015 Lausanne, Switzerland 5 Department of Geoscience, University of Nevada Las Vegas, 4505 S. Maryland Parkway, Las Vegas, Nevada 89154, USA Temperature (°C) Vertical distance (m above basement) 7 8 9 10 11 12 13 δ 18 O (‰) Δ 18 O (‰) 350 400 450 500 RR07-68 RR07-77 RR07-92 RR07-43 RR07-71 RR07-40 RR07-89 RR07-96 RR07-57 Muscovite Quartz δD (‰) -110 -100 -90 -80 Raft River Mountains at Clear Creek ID NV UT δDms [SMOW] δDFi [SMOW] 100 80 60 40 20 0 Pebble- Elba Quartzite Mica-rich layer Metaconglomerate Regolith/Paleosol Basement 3.7 3.4 3.1 2.8 2.7 2.5 2.4 A B D E RR07-106 Metaconglomerate C Figure 1. A: Synthetic vertical profile (for scale). B: Oxygen isotope com- positions of quartz and muscovite. C: Quartz- muscovite fractionation. D: Temperatures calcu- lated from quartz-mus- covite fractionation. E: Hydrogen stable isotope compositions. SMOW— standard mean ocean water.