The Raft River-Grouse Creek-Albion metamorphic core complex (NW Utah) is bound to the east by the Miocene Raft River detachment shear zone that is localized in a \textasciitilde 100 m thick quartzite mylonite. By performing combined structural, microstructural, 40Ar/39Ar geochronology, and oxygen and hydrogen stable isotope geochemistry of the well-exposed quartzite mylonite, we are provided with insight on the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complex.
Microstructural, electron backscattered diffraction, strain, and vorticity results show an increase in intensity of the rock fabrics from west to east, along the transport direction, compatible with observed finite strain markers (Compton, 1980; Wells, 2001; Sullivan 2008). The results fit in a model of "necking" of the shear zone proposed by Wells (2001) and Sullivan (2008). Microstructural evidences (quartz microstructures and deformation lamellae) suggest that the detachment shear zone evolved at its peak strength, close to the dislocation creep/exponential creep transition, where meteoric fluids played an important role on strain hardening, embrittlement and eventually seismic failure.
Two empirically calibrated paleopiezometers, the quartz recrystallized grain size paleopiezometer of Hirth et al. (2001), and a deformation lamellae spacing based paleopeizometer of Koch and Christie (1981), show very similar results, indicating that the shear zone in question developed under stress ranging from 40 MPa to 60 MPa. Application of a quartzite dislocation creep flow law (Hirth et al., 2001) reveals that the detachment shear zone quartzite mylonite developed at a strain rate between 10E-12 and 10E-14 s-1. We suggest that compressed geothermal gradient (Gottardi et al., 2011), produced by a combination of ductile shearing, heat advection and enhanced cooling by meteoric fluids, can trigger significant mechanical instabilities, and strongly influences the rheology of the detachment shear zone.
Combined geochronological and stable isotope data of quartz/muscovite pairs from the quartzite mylonite reveal that ductile deformation, and infiltration of meteoric fluids in the detachment shear zone, occurred between 26 and 20 Ma. 40Ar/39Ar release spectra are complex, but 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. Throughout the studied area, hydrogen stable isotope values of syn-kinematic muscovite are low, ranging from -123 permil to 88 permil suggesting that meteoric fluids were infiltrating the detachment shear zone over the time scale of mylonite formation. Hydrogen stable isotope analyses from both muscovite and fluid inclusions show that the fluid infiltrating the detachment shear zone was meteoric in origin, with a low D/H and low 18O/16O composition. Quartz and muscovite oxygen isotope analyses show different degree of oxygen isotope depletion, suggesting different time-integrated interaction of the minerals with meteoric fluid; these fluids would be channelized in preferential layers, or shear zones, within the deforming system. The variability in oxygen stable isotope of both quartz and muscovite can be explained by variations in permeability in the basement units (confined versus diffuse flow), and strain variations along the transport direction of detachment shear zone (from flattening to constriction), resulting in different fluid-rock exchange patterns.
Based on our geochemical analyses and other published data, we conduct continuum - scale (i.e., large-scale, partial-bounceback) 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 crustal and fault permeability and buoyancy driven flow on two-way coupled fluid and heat transfer and resultant exchange of oxygen isotopes between fluid and rock. The results show that fluid migration to mid- to lower-crustal levels has to be fault controlled and depends primarily on the permeability contrast between the fault zone and the crustal rock. High fault/crust permeability contrasts lead to channelized flow in the fault and shear zones, while lower contrasts allow leakage of the fluids from the fault into the crust. Channelized fluid flow in the shear zone leads to strong vertical and horizontal thermal gradients, comparable to field observations. The oxygen isotope results show profound oxygen depletion (starting value of &delta18O = +13 permil down to 4 permil concentrated along the faults and shear zone, similar to field data.