Transient and Steady‐State Dislocation Creep of Olivine Controlled by Dislocation Interactions at the Isostress Endmember

Abstract

Abstract The rheological behavior of olivine deforming by dislocation creep controls geodynamic processes that involve steady‐state flow or transient viscosity evolution. Longstanding rheological models applied to both contexts assume that dislocation creep of olivine aggregates occurs close to the isostrain endmember with each grain deforming to the same strain but supporting different stress. Here, we test this assumption by constructing isostrain and isostress models based on flow laws for single crystals and comparing them to rheological data from aggregates. This analysis reveals that strain rates measured on olivine aggregates agree with those predicted by the isostress model but are an order of magnitude faster than those predicted by the isostrain model. When extrapolated to conditions typical of the shallow upper mantle, the isostress model predicts steady‐state viscosities that are one to three orders of magnitude less than those predicted by the isostrain model. Furthermore, deformation close to the isostress endmember implies that transient creep occurs predominantly by dislocation interactions, suggesting viscosity changes that are approximately one order of magnitude greater than those predicted previously based on grain interactions associated with the isostrain model. Plain Language Summary Viscous flow of the hot rocks in Earth's upper mantle is a key process in the dynamic behaviors of the outer layers of the planet. The viscosity of these rocks is controlled primarily by that of the most abundant constituent mineral, which is olivine. Rocks composed of olivine have long been assumed to flow in a particular manner, whereby each grain undergoes the same shape change but supports a different stress due to interactions among neighboring grains. We test this assumption by predicting rates of flow using two models, in which neighboring grains are assumed either to interact or not to interact, and comparing them to rates of flow measured on olivine‐rich rocks. This comparison reveals that olivine‐rich rocks flow at the rates predicted by the model in which neighboring grains do not interact and therefore support the same stress but undergo different shape changes. This result overturns the aforementioned paradigm. Importantly, the flow behavior supported by our analysis is more complex than previous models and predicts more rapid flow in the aftermath of stress changes, such as those imposed by major earthquakes. Key Points Aggregates of coarse‐grained olivine deform at strain rates predicted by the isostress model Transient dislocation creep of olivine is controlled by dislocation interactions Steady‐state dislocation creep of olivine is controlled by the weakest, rather than the strongest, slip system