Dislocation creep of olivine: Backstress evolution controls transient
creep at high temperatures
Abstract
Transient creep occurs during geodynamic processes that impose stress
changes on rocks at high temperatures. The transient is manifested as
evolution in the viscosity of the rocks until steady-state flow is
achieved. Although several phenomenological models of transient creep in
rocks have been proposed, the dominant microphysical processes that
control such behavior remain poorly constrained. To identify the
intragranular processes that contribute to transient creep of olivine,
we performed stress-reduction tests on single crystals of olivine at
temperatures of 1250–1300°C. In these experiments, samples undergo
time-dependent reverse strain after the stress reduction. The magnitude
of reverse strain is ~10-3 and increases with increasing
magnitude of the stress reduction. High-angular resolution electron
backscatter diffraction analyses of deformed material reveal lattice
curvature and heterogeneous stresses associated with the dominant slip
system. The mechanical and microstructural data are consistent with
transient creep of the single crystals arising from accumulation and
release of backstresses among dislocations. These results allow the
dislocation-glide component of creep at high temperatures to be
isolated, and we use these data to calibrate a flow law for olivine to
describe the glide component of creep over a wide temperature range. We
argue that this flow law can be used to estimate both transient creep
and steady-state viscosities of olivine, with the transient evolution
controlled by the evolution of the backstress. This model is able to
predict variability in the style of transient (normal versus inverse)
and the load-relaxation response observed in previous work.