A combined microstructural and modeling investigation of mantle shear zones using outcrop scale relationships, tests and explores (a) extrapolation of experimental flow laws, development of lattice preferred orientation, and grain size evolution; and (b) models that predict strain localization and viscous shear heating instabilities. Research focuses on well-exposed shear zones in the Josephine peridotite (Klamath Mountains, Oregon) where shear zone boundaries can be easily identified and finite strain can be quantified by measuring the deflection of pre-existing pyroxene-rich bands. Field and laboratory studies assess models for lattice preferred orientation development and calibrate indicators of shear sense, viscous flow trajectory, and finite strain preserved in peridotite microstructures. Numerical modeling will reproduce strain distribution around shear zones in the Josephine peridotite, using viscoelastic rheology. Olivine flow laws and parameterizations of grain size evolution as a function of stress, strain, strain rate, and grain growth will be incorporated. Rheological properties constrained by observation of the shear zones are compared to the results of the numerical models that incorporate the same rheology. Forward models that approximately reproduce basic field observations will be used to: (a) investigate processes responsible for strain localization and, by analogy, tectonic plate boundaries; and (b) evaluate the hypothesis that viscous shear heating instabilities cause intermediate depth earthquakes in subduction zones, and perhaps other earthquakes in the shallow mantle, such as along oceanic fracture zones.
Results from laboratory deformation experiments of peridotite and its constituent minerals are widely used in geodynamical models of the upper mantle. Laboratory studies, however, use samples that are very small in comparison to upper mantle dimensions and are conducted at strain rates much higher than expected in the upper mantle. This study bridges the gap in size and time between laboratory studies and mantle-scale processes, which is essential for understanding upper mantle rheology. This not only further constrains geodynamical modeling, but will also improve understanding of processes controlling intermediate depth earthquakes, post-seismic deformation, preservation of cratonic roots, and the evolution of plate boundaries.
