The Long Path to Steady State: Transient Microstructures in the Upper Mantle

Abstract:

Deformation microstructures provide evidence for specific microphysical processes and represent an important link between rock deformation experiments, field-based geological studies, and geophysical observations and theory. However, microstructural evolution is complex, reflecting the numerous feedbacks between deformation and recovery processes. Many geophysical interpretations assume that microstructures in nature are near steady-state. For example, the interpretation of seismic anisotropy in terms of mantle flow patterns is generally predicated on the assumption that the crystallographic preferred orientation (CPO) of olivine rapidly orients itself to reflect the kinematics of flow. However, several recent studies have shown that microstructural evolution may occur slowly over long increments of strain history. These observations suggest that there are long transient intervals between changes in the deformation conditions, kinematics, or mechanisms, during which microstructure and rheology continuously evolve. Hence, care must be taken when constructing geophysical models or inversions that depend in some way on microstructure. This presentation will include two examples of microstructures that require large strains to achieve steady-state. The first example is olivine CPO, which can be shown on the basis of experiments and numerical simulations to exhibit long periods of transient evolution. Protracted CPO evolution may have a significant effect on the interpretation of seismic anisotropy, especially near plate boundaries. The second example is based on observations and theory of microstructural damage by dynamic recrystallization and phase mixing. The slow evolution towards the damaged microstructural state provides a new framework for modeling the initiation and evolution of plate boundary shear zones.