Transient and steady-state dislocation creep of olivine controlled by dislocation interactions at the isostress endmember

TL;DR

This study compares isostrain and isostress models of olivine dislocation creep, finding that the isostress model better predicts aggregate deformation rates and suggests transient creep is mainly controlled by dislocation interactions, impacting geodynamic models.

Contribution

It introduces a comparative analysis of isostrain and isostress models for olivine dislocation creep, highlighting the dominance of the isostress model in predicting deformation behavior.

Findings

Isostress model aligns with observed aggregate strain rates.

Steady-state viscosities are lower in the isostress model.

Transient creep is mainly governed by dislocation interactions.

Abstract

The rheological behaviour 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 for 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.