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Abstract
The frictional-viscous transition zone (FVT) marks the tectonic and thermal boundary where
deformation shifts from seismogenic, frictional sliding and granular flow to largely aseismic
crystal-plastic flow. Below the FVT, rocks deform predominantly by dynamic
recrystallization and mass movement is accomplished via viscous flow. When strain localizes
into a discrete shear zone, recrystallization forms mylonites, which record the strain history
as fabrics at a high angle to the shear direction. The Basin and Range extensional province of
the western USA contains a series of exhumed metamorphic core complexes (MCCs). MCCs
preserve deformation in the crystal-plastic regime as an archive of mid-crustal flow and
attenuation. The Ruby Mountains-East Humboldt Range, NV contains a MCC exhumed
along a low-angle brittle detachment fault. Whereas previous work has investigated the shear
zone in the Secret Pass region, we apply microstructural analysis to samples collected 25 km
to the south in the Lamoille Canyon area. Lamoille Canyon is located closer to the center of
the detachment and near a large constraining bend, allowing for exploration of along-strike
heterogeneity in strain localization and deformation history, which is an important control on
the strength and morphology of detachment faults. Petrographic assessment and field
observations identifies fabric variation that correlates with protolith, local strain gradients,
and textural evidence for interaction with meteoric water during deformation. Electron
backscatter diffraction was used to obtain grain size measurements, determine slip systems,
and assess fabric strength of key phases analyzed. Stable isotope analysis of garnet, quartz,
biotite, and muscovite to determine whether the surface-derived fluids identified at Secret
Pass were pervasive along strike was inconclusive in Lamoille Canyon. Comparing our
findings with field, petrographic, and microstructural observations to those extrapolated from
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Secret Pass indicates along-strike incompatibility of tectonic models which we use to
construct a wider image of the Ruby Mountains-East Humboldt Range. Paleostress data from
Lamoille Canyon and Secret Pass fit best with mid crustal flow according to a dislocationgrain boundary sliding flow law. Contrasting with prior interpretations. This finding does not
require variability in strain rate to explain the observed distribution of stress and temperature,
thereby favoring a model of paired detachment exhumation and shear zone evolution that is
more compatible with traditional models of crustal attenuation of the Basin and Range.