The development of crystallographic textures in olivine-rich rocks leads to a marked anisotropy in viscosity of the upper mantle, strongly influencing a variety of large-scale geodynamic processes. Most estimates of the magnitude of viscous anisotropy in the upper mantle are derived from micromechanical models that predict textural and mechanical evolution numerically. Unfortunately, relatively few data exist with which to benchmark these models, and therefore their applicability to geodynamic processes remains in question. Here we present the results from a series of laboratory deformation experiments that yield insight into the magnitude and evolution of the anisotropy of olivine aggregates during deformation along complex loading paths. Aggregates of Fo50 olivine were first deformed in extension in a gas-medium apparatus at a temperature of 1473 K, confining pressure of 300 MPa, and a variety of stresses and strain rates. Early in the extension experiments, samples exhibited viscosities similar to those previously determined for isotropic aggregates. Extensional deformation was accompanied by formation of crystallographic textures with  axes dominantly aligned with the extension axis. Samples were subsequently deformed in torsion under similar conditions to shear strains of up to 15.5. Early in the torsion experiments, samples supported stresses a factor of ~2 larger than measured at the end of extension experiments, demonstrating a marked anisotropy in viscosity. Textures at the end of torsion experiments exhibited  axes dominantly aligned with the shear direction, comparable to previous experimental observations. Evolution of the textures resulting from extension to those resulting from torsion was analyzed through examination of radial sections of torsion samples. Our results confirm that texture produces viscous anisotropy in olivine aggregates, and we provide a simple, calibrated parameterization of viscous anisotropy for use in geodynamic models. Our results also provide an extensive dataset for future calibration of micromechanical models that track the evolution of anisotropy in upper mantle rocks.
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This work benefitted greatly from discussions with Manuele Faccenda and Shun Karato. The manuscript was significantly improved by thoughtful comments from Georg Dresen and Andréa Tommasi. K. Barton and the Department of Plant Biology, Carnegie Institution, are thanked for SEM access. This research was supported by funding awards including John Fell Fund, University of Oxford 123/718 and NERC NE/M000966/1 to LNH, NSF EAR-1255620 to JMW, and NSF EAR-1214876 to DLK.
- Crystallographic preferred orientation
- Olivine microstructure
- Texture evolution
- Viscous anisotropy