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-dominated detachment zone that bounds the Raft River core complex in northwest Utah (United States). Hydrogen isotope values of muscovite (δD Ms ~-100‰) and fl uid inclusions in quartz (δD Fluid ~-85‰) indicate the presence of meteoric fl uids 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 refl ects the transient geotherm that developed owing to shearing, thinning, and the potentially prominent role of convective fl ow of surface fl uids.

Combined geochronological and stable isotope data of quartzite mylonite from the footwall of the Raft River detachment shear zone (NW Utah, USA) reveal that an important phase of ductile deformation and infiltration of meteoric water in the shear zone occurred in Miocene time. 40 Ar/ 39 Ar release spectra are complex, and plateau ages decrease systematically from 31.1 ± 0.8 Ma at the top to 20.2 ± 0.6 Ma at the bottom of the quartzite mylonite section, capturing a segment of the ~40-15 Ma geochronologic record that has been documented regionally and is likely related to partial to total overprinting of Eocene white mica 40 Ar/ 39 Ar ages in the Miocene. Hydrogen stable isotope values of syn-kinematic muscovite range from-123 ‰ to-88 ‰ and suggest that meteoric water infiltrated the detachment shear zone during mica (re)crystallization and mylonite development. Bulk stable isotope analyses from fluid inclusions in quartz support a meteoric origin for the fluid (low D/H and 18 O/ 16 O ratios). Quartz and muscovite oxygen isotope analyses show varying degrees of 18 O depletion, suggesting spatially variable time-integrated interaction of meteoric fluids with recrystallizing shear zone minerals. The overall pattern of D/H and 18 O/ 16 O ratios indicates that fluids were channelized along restricted layers or shear zones within the deforming detachment system. The variability in 18 O/ 16 O ratios of both quartz and muscovite and the fluid-rock isotopic exchange results can be explained by variations in the shear zone permeability (confined versus diffuse flow) along with strain variations along the transport direction (from flattening to constriction).

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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 (δ2Hms) 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 δ2Hms ≥ −154‰ in quartzite mylonite of the western segment of the detachment system. These δ2Hms values are distinctively lower than in localities farther east (δ2Hms ≥ −125‰), where 40Ar/39Ar geochronological data indicate Miocene (18–15 Ma) extensional shearing and mylonitic fabric formation. These data indicate that very low δ2H 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 δ2H values of recrystallized white mica, indicate Miocene structural and isotopic overprinting of Eocene fabrics.

[1] Field studies of Cordilleran metamorphic core complexes indicate that meteoric fluids permeated the upper crust down to the detachment shear zone and interacted with highly deformed and recrystallized (mylonitic) rocks. The presence of fluids in the brittle/ductile transition zone is recorded in the oxygen and hydrogen stable isotope compositions of the mylonites and may play an important role in the thermomechanical evolution of the detachment shear zone. Geochemical data show that fluid flow in the brittle upper crust is primarily controlled by the large-scale fault-zone architecture. We conduct continuum-scale (i.e., large-scale, partial bounce-back) lattice-Boltzman fluid, heat, and oxygen isotope transport simulations of an idealized cross section of a metamorphic core complex. The simulations investigate the effects of crust and fault permeability fields as well as buoyancy-driven flow on two-way coupled fluid and heat transfer and resultant exchange of oxygen isotopes between meteoric fluid and rock. Results show that fluid migration to middle to lower crustal levels is fault controlled and depends primarily on the permeability contrast between the fault zone and the crustal rocks. High fault/crust permeability ratios lead to channelized flow in the fault and shear zones, while lower ratios allow leakage of the fluids from the fault into the crust. Buoyancy affects mainly flow patterns (more upward directed) and, to a lesser extent, temperature distributions (disturbance of the geothermal field by~25 C). Channelized fluid flow in the shear zone leads to strong vertical and horizontal thermal gradients, comparable to field observations. The oxygen isotope results show d 18 O depletion concentrated along the fault and shear zones, similar to field data.

A microstructural and thermochronometric analysis of the Coyote Mountains detachment shear zone provides new insight into the collapse of the southern North American Cordillera. The Coyote Mountains is a metamorphic core complex that makes up the northern end of the Baboquivari Mountains in southern Arizona. The Baboquivari Mountains records several episodes of crustal shortening and thickening and regional metamorphism, including the Late Cretaceous‐early Paleogene Laramide orogeny which is locally expressed by the Baboquivari thrust fault. Thrusting and shortening were accompanied by magmatic activity recorded by intrusion of Paleocene muscovite‐biotite‐garnet peraluminous granites such as the ~58 Ma Pan Tak Granite, interpreted as anatectic melts representing the culmination of the Laramide orogeny. Following Laramide crustal shortening, the northern end of the Baboquivari Mountains was exhumed along a top‐to‐the‐north detachment shear zone, which resulted in the formation of the Coyote Mountains metamorphic core complex. Structural and microstructural analysis show that the detachment shear zone evolved under a strong component of noncoaxial (simple shear) deformation, at deformation conditions of ~450 ± 50°C, under a differential stress of ~60 MPa, and a strain rate of 1.5 × 10−11 to 5.0 × 10−13 s−1 at depth of ~11–14 km. Detailed 40Ar/39Ar geochronology of biotite and muscovite, in the context of the deformation conditions determined by quartz microstructures, suggests that the mylonitization associated with the formation of the Coyote Mountains metamorphic core complex started at ~29 Ma (early Oligocene). Apatite fission track ages indicate that the footwall of the Coyote Mountains metamorphic core complex experienced rapid exhumation to the upper crust by ~24 Ma. The fact that mylonitization and rapid extensional exhumation postdates Laramide thickening by ~30 Myr indicates that crustal thickness alone was insufficient to initiate extensional tectonic and required an additional driving force. The timing of mylonitization and rapid exhumation documented here and in other MCCs are consistent with the hypothesis that slab rollback and the effect of a slab window trailing the Mendocino Triple Junction have been critical in driving the development of the MCCs of the southwest.

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The Picacho Mountains (SE Arizona, USA) are composed of a variety of Paleogene, Late Cretaceous, and Proterozoic granite and gneisses that were deformed and exhumed along the gently south to southwest dipping detachment shear zone associated with the Picacho metamorphic core complex. The detachment shear zone is divided into three sections that record a progressive deformation gradient, from protomylonites to ultramylonites, and breccia. New thermochronological data from mylonite across the footwall of the detachment shear zone associated with the Picacho metamorphic core complex suggest that the footwall was exhumed through about ~300°C between 22 and 18 Ma by progressive incisement of the footwall of the detachment shear zone. Combined geochronological and oxygen and hydrogen stable isotope data of metamorphic silicate minerals reveal that mylonite recrystallization occurred in the presence of a deep‐seated metamorphic/magmatic fluid, and experienced a late stage meteoric overprint during the development and exhumation of the detachment shear zone. Quartz‐biotite and quartz‐hornblende geothermometry from the base to the top of the detachment shear zone yield equilibrium temperatures ranging from 630 to 415°C, respectively. This temperature trend is attributed to an insulating effect caused by rapid slip and juxtaposition of cool hanging wall on top of a hot footwall. It is suggested that rapid cooling of the top of the detachment shear zone caused strain to migrate toward lower structural level by incisement of the footwall of the shear zone. Progressive strain front migration into the ductile footwall produced hydromechanical anisotropies parallel to the detachment shear zone, effectively saturating the footwall with magmatic/metamorphic fluids and preventing downward flow of meteoric fluids. The combined microstructural, geochronological, and stable isotope results presented in this study provide insight on the dynamic feedback between deformation and fluid flow during the evolution of a detachment shear zone.