Mild-to-wild plasticity of Earth's upper mantle

The flow of Earth's upper mantle has long been considered to occur by slow and near-continuous creep. Such behaviour is observed in classical high-temperature deformation experiments and is a fundamental component of geodynamic models. However, the latest generation of high-resolution experiments, capable of sensing temporal heterogeneities in rates of dislocation motion, have revealed that materials ranging from metals to ice exhibit a spectrum of behaviours termed mild-to-wild plasticity. It remains unknown whether olivine, the most abundant mineral in Earth's upper mantle, always exhibits mild continuous flow or can exhibit intermittent wild fluctuations in plastic strain rate. Here, we demonstrate that olivine exhibits measurable wildness, even under conditions at which its behaviour is predicted to be relatively mild. During nanoindentation experiments conducted at room temperature, continuous plastic flow is punctuated by intermittent bursts of displacement with log-normally distrubuted magnitudes, indicating avalanches of correlated dislocation motion that account for approximately 8 +/- 6 plastic strain. Remarkably, the framework of mild-to-wild plasticity predicts that wildness should increase with depth in the Earth, with flow of the asthenospheric upper mantle occurring almost entirely by wild fluctuations of deformation at the grain scale. The recognition of intermittent plasticity in geological materials provides new constraints for microphysical models of dislocation-mediated flow and raises questions about the mehcanisms of transient instabilities in otherwise ductile regimes, such as deep earthquakes and slow-slip events.