  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.
