Plastic deformation of olivine at relatively low temperatures (i.e., low‐temperature plasticity) likely controls the strength of the lithospheric mantle in a variety of geodynamic contexts. Unfortunately, laboratory estimates of the strength of olivine deforming by low‐temperature plasticity vary considerably from study to study, limiting confidence in extrapolation to geological conditions. Here we present the results of deformation experiments on olivine single crystals and aggregates conducted in a deformation‐DIA at confining pressures of 5 to 9 GPa and temperatures of 298 to 1473 K. These results demonstrate that, under conditions in which low‐temperature plasticity is the dominant deformation mechanism, fine‐grained samples are stronger at yield than coarse‐grained samples, and the yield stress decreases with increasing temperature. All samples exhibited significant strain hardening until an approximately constant flow stress was reached. The magnitude of the increase in stress from the yield stress to the flow stress was independent of grain size and temperature. Cyclical loading experiments revealed a Bauschinger effect, wherein the initial yield strength is higher than the yield strength during subsequent cycles. Both strain hardening and the Bauschinger effect are interpreted to result from the development of back stresses associated with long‐range dislocation interactions. We calibrated a constitutive model based on these observations, and extrapolation of the model to geological conditions predicts that the strength of the lithosphere at yield is low compared to previous experimental predictions but increases significantly with increasing strain. Our results resolve apparent discrepancies in recent observational estimates of the strength of the oceanic lithosphere.

When deforming by low-temperature plasticity, the strength of the mantle mineral olivine is controlled by its grain size.

The evolution of viscosity during flow of mantle rocks at high temperatures is fundamental to a variety of geodynamic processes. For example, transient creep of the upper mantle has been identified as a major contributor to geodetically observed surface deformations during post-seismic creep (Freed et al., 2012;Masuti et al., 2016;Pollitz, 2005;Qiu et al., 2018), for which the strains are typically <10 −3 , and inferred viscosities are one to two orders of magnitude lower than the long-term, steady-state viscosity. Because transient viscosities also continue to evolve during postseismic deformation, they likely cause a time-dependent transfer of stresses to neighboring faults, rather than the instantaneous transfer assumed by popular calculations of Coulomb stress changes (e.g., Freed, 2005). Although sophisticated earthquake forecast models do incorporate time-dependent loading according to average plate motion rates (e.g., Field et al., 2015Field et al., , 2017, they still do not incorporate variable loading rates that would occur due to transient creep of the lithosphere. In addition, transient viscosities are expected to be important, although they have not yet been thoroughly considered, in other small-strain processes including flexure of the lithosphere near volcanic loads (Zhong & Watts, 2013) or in subducting slabs near trenches (Hunter & Watts, 2016), during which the strains rarely exceed 10 −2 .

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Many natural fault surfaces exhibit remarkably similar scale‐dependent roughness, which may reflect the scale‐dependent yield strength of rocks. Using atomic force microscopy (AFM), we show that a sample of the Corona Heights Fault exhibits isotropic surface roughness well‐described by a power law, with a Hurst exponent of 0.75 +/− 0.05 at all wavelengths from 60 nm to 10 μm. The roughness data and a recently proposed theoretical framework predict that yield strength varies with length scale as λ‐0.25+/−0.05. Nanoindentation tests on the Corona Heights sample and another fault sample whose topography was previously measured with AFM (the Yair Fault) reveal a scale‐dependent yield stress with power‐law exponents of −0.12 +/− 0.06 and −0.18 +/− 0.08, respectively. These values are within one to two standard deviations of the predicted value, and provide experimental evidence that fault roughness is controlled by intrinsic material properties, which produces a characteristic surface geometry.