We deformed samples with varied proportions of olivine and orthopyroxene in a deformation-DIA apparatus to test the applicability of subgrain-size piezometry to polymineralic rocks. We measured the stress within each phase in situ via X-ray diffraction during deformation at a synchrotron beamline. Subgrain-size piezometry was subsequently applied to the recovered samples to estimate the stress that each phase supported during deformation. For olivine, the final in-situ stresses are consistent with the stresses estimated via subgrain-size piezometry, both in monomineralic and polymineralic samples, despite non-steady state conditions. However, stress estimates from subgrain-size piezometry do not reliably record the in-situ stress in samples with grain sizes that are too small for extensive subgrain-boundary formation. For orthopyroxene, subgrain boundaries are typically sparse due to the low strains attained by orthopyroxene in olivine-orthopyroxene mixtures. Where sufficient substructure does exist, our data supports the use of the subgrain-size piezometer on orthopyroxene. These results do, however, suggest that care should be taken when applying subgrain-size piezometry to strong minerals that may have experienced little strain. Stresses estimated by X-ray diffraction also offer insight into stress partitioning between phases. In mixtures deformed at mean stresses > 5 GPa, orthopyroxene supports stresses greater than those supported by olivine. This stress partitioning is consistent with established theory that predicts a slightly higher stress within a ‘strong’ phase contained in a material consisting of interconnected weak layers. Overall, these results demonstrate that subgrain-size piezometry is a valuable tool for quantifying the stress state of polymineralic rocks.

The mechanics of olivine deformation play a key role in long-term planetary processes, including  the response of the lithosphere to tectonic loading or the response of the solid Earth to tidal forces,  and in short-term processes, such as post-seismic creep within the upper mantle. Previous studies  have emphasized the importance of grain-size effects in the deformation of olivine. Most of our  understanding of the role of grain boundaries in the deformation of olivine is inferred from comparison  of experiments on single crystals to experiments on polycrystalline samples, as there are no direct  studies of the mechanical properties of individual grain boundaries in olivine. In this study, we use  high-precision mechanical testing of synthetic forsterite bicrystals with well characterized interfaces  to directly observe and quantify the mechanical properties of olivine grain boundaries. We conduct  in-situ micropillar compression tests at high-temperature (700• C) on bicrystals containing low-angle (4• tilt about [100] on (014)) and high-angle (60• tilt about [100] on (011)) boundaries. During  the in-situ tests, we observe differences in deformation style between the pillars containing the  grain boundary and the pillars in the crystal interior. In the pillars containing the grain boundary,  the interface is oriented at ∼ 45• to the loading direction to promote shear. In-situ observations  and analysis of the mechanical data indicate that pillars containing the grain boundary consistently  support elastic loading to higher stresses than the pillars without a grain boundary. Moreover, the  pillars without the grain boundary sustain larger plastic strain. Post-deformation microstructural  characterization confirms that under the conditions of these deformation experiments, sliding did not occur along the grain boundary. These observations support the hypothesis that grain boundaries are stronger relative to the crystal interior at these conditions. 

• Nanoindentation experiments on a high-angle grain boundary (60 • misorientation) in a pure forsterite bicrystal reveal that the interface acts as a source of dislocations. • Nanoindentation experiments on a high-angle grain boundary (60 • misorientation) in a pure forsterite bicrystal reveal that the interface acts as an obstacle to incoming dislocations, leading to pileups of dislocations. • Nanoindentation experiments on a subgrain boundary (13 • misorientation) in a pure forsterite bicrystal do not detect the impact of the interface on dislocations.

Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid-ocean ridge dynamics. Yet, interpreting anisotropy in the context of grain-scale deformation processes and strain observed in laboratory experiments and natural olivine samples has proven challenging due to incomplete seismological constraints and length scale differences spanning orders of magnitude. To bridge this observational gap, we estimate an in situ elastic tensor for oceanic lithosphere using co-located compressional- and shear-wavespeed anisotropy observations at the NoMelt experiment located on ~70 Ma seafloor. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil-spreading direction, consistent with corner-flow deformation fabric. We compare this model with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250–400% in the shallow mantle. We find evidence for D-type olivine lattice-preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes perpendicular to shear. D-type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during mid-ocean ridge spreading; we hypothesize that grain-boundary sliding during dislocation creep relaxes strain compatibility, allowing D-type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation-accommodated grain-boundary sliding (disGBS) has implications for in situ stress and grain size during mid-ocean ridge spreading and implies grain-size dependent deformation, in contrast to pure dislocation creep.

Transient creep occurs during geodynamic processes that impose stress changes on rocks at high temperatures. The transient is manifested as evolution in the viscosity of the rocks until steady-state flow is achieved. Although several phenomenological models of transient creep in rocks have been proposed, the dominant microphysical processes that control such behavior remain poorly constrained. To identify the intragranular processes that contribute to transient creep of olivine, we performed stress-reduction tests on single crystals of olivine at temperatures of 1250–1300°C. In these experiments, samples undergo time-dependent reverse strain after the stress reduction. The magnitude of reverse strain is ~10-3 and increases with increasing magnitude of the stress reduction. High-angular resolution electron backscatter diffraction analyses of deformed material reveal lattice curvature and heterogeneous stresses associated with the dominant slip system. The mechanical and microstructural data are consistent with transient creep of the single crystals arising from accumulation and release of backstresses among dislocations. These results allow the dislocation-glide component of creep at high temperatures to be isolated, and we use these data to calibrate a flow law for olivine to describe the glide component of creep over a wide temperature range. We argue that this flow law can be used to estimate both transient creep and steady-state viscosities of olivine, with the transient evolution controlled by the evolution of the backstress. This model is able to predict variability in the style of transient (normal versus inverse) and the load-relaxation response observed in previous work.

Asthenospheric shear causes some minerals, particularly olivine, to develop anisotropic textures that can be detected seismically. In laboratory experiments, these textures are also associated with anisotropic viscous behavior, which should be important for geodynamic processes. To examine the role of anisotropic viscosity for asthenospheric deformation, we developed a numerical model of coupled anisotropic texture development and anisotropic viscosity, both calibrated with laboratory measurements of olivine aggregates. This model characterizes the time-dependent coupling between large-scale formation of lattice-preferred orientation (i.e., texture) and changes in asthenospheric viscosity for a series of simple deformation paths that represent upper-mantle geodynamic processes. We find that texture development beneath a moving surface plate tends to align the a-axes of olivine into the plate-motion direction, which weakens the effective viscosity in this direction and increases plate velocity for a given driving force. Our models indicate that the effective viscosity increases for shear in the horizontal direction perpendicular to the a-axes. This increase should slow plate motions and new texture development in this perpendicular direction, and could impede changes to the plate motion direction for 10s of Myrs. However, the same well-developed asthenospheric texture may foster subduction initiation perpendicular to the plate motion and deformations related to transform faults, as shearing on vertical planes seems to be favored across a sub-lithospheric olivine texture. These end-member cases examining shear-deformation in the presence of a well-formed asthenospheric texture illustrate the importance of the mean olivine orientation, and its associated viscous anisotropy, for a variety of geodynamic processes.

Large earthquakes transfer stress from the shallow lithosphere to the underlying viscoelastic lower crust and upper mantle, inducing transient creep during the postseismic interval. Recent experiments on olivine have provided a new rheological model for this transient creep based on accumulation and release of back stresses among dislocations. Here, we test whether natural rocks preserve dislocation-induced stress heterogeneity consistent with the back-stress hypothesis by mapping olivine from the palaeosubduction interface of the Oman-UAE ophiolite with high-angular resolution electron backscatter diffraction. The olivine preserves heterogeneous residual stresses that vary in magnitude by several hundred megapascals over length scales of a few micrometres. Large stresses are commonly spatially associated with elevated densities of geometrically necessary dislocations within subgrain interiors. These spatial relationships, along with characteristic probability distributions of the stresses, confirm that the stress heterogeneity is generated by the dislocations and records their long-range elastic interactions. Images of dislocations decorated by oxidation display bands of high and low dislocation density, suggesting that dislocation interactions contributed to organisation of the substructure. These results support the applicability of the back-stress model of transient creep to deformation in the mantle portion of plate-boundary shear zones. The model predicts that rapid stress changes, such as those imposed by large earthquakes, can induce order-of-magnitude changes in viscosity that depend nonlinearly on the stress change, consistent with inferences of mantle rheology from geodetic observations.

Rate-and state-friction is an empirical framework that describes the complex velocity-, time-, and slip-dependent phenomena observed during frictional sliding of rocks and gouge in the laboratory. Despite its widespread use in earthquake nucleation and recurrence models, our understanding of rate-and state-friction, particularly its time-and/or slip-dependence, is still largely empirical, limiting our confidence in extrapolating laboratory behavior to the seismogenic zone. While many microphysical models have been proposed over the past few decades, none have explicitly incorporated the effects of strain hardening, anelasticity, or transient viscous rheology. Here we present a new model of rock friction that incorporates these phenomena directly from the microphysical behavior of lattice dislocations. This model of rock friction exhibits the same logarithmic dependence on sliding velocity (strain rate) as rate-and state-friction and predicts a dependence on the internal backstress caused by long-range interactions among geometrically necessary dislocations. Changes in the backstress evolve exponentially with plastic strain of asperities and are dependent on both the current backstress and previous deformation, which give rise to phenomena consistent with interpretations of the ‘critical slip distance,’ ‘memory effect,’ and ‘state variable’ of rate- and state-friction. Fault stability in this model is controlled by the evolution of backstress and temperature. We provide several analytical predictions for RSF-like behavior and the ‘brittle-ductile’ transition based on 2 microphysical mechanisms and measurable parameters such as the geometrically necessary dislocation density and strain-dependent hardening modulus.

At low-temperature and high-stress conditions, quartz deformation is controlled by the kinetics of dislocation glide, often referred to as low-temperature plasticity (LTP). To investigate the relationship between intracrystalline H2O content and the yield strength of quartz during LTP, we have integrated spherical and Berkovich nanoindentation tests at room temperature on natural quartz grains from a deformed migmatitic gneiss with electron backscatter diffraction (EBSD) and secondary-ion mass spectrometry (SIMS) measurements of intracrystalline H2O content. Dry (<20 wt ppm H2O) and wet (20–100 wt ppm H2O) crystals exhibit comparable indentation hardness, which is proportional to quartz yield strength. Thus, quartz yield strength, seems to be unaffected by the intracrystalline H2O content. Pre-indentation strain history may have had a major role in generating a high density of dislocation sources, which controlled the yield stress during LTP.