Strain localization in olivine aggregates at high temperature: A laboratory comparison of constant-strain-rate and constant-stress boundary conditions

We performed high-strain torsion experiments on aggregates of Fo50 olivine to test the influence of imposed boundary conditions on localizing deformation. We deformed both solid and thin-walled cylinders of Fo50 either at constant strain rate or at constant stress. Samples deformed in constant-strain-rate experiments reached a peak stress followed by weakening at a continually decreasing weakening rate. In contrast, samples deformed in constant-stress experiments weakened at an accelerating weakening rate. Localization is manifested in samples deformed at constant stress as irregularities along strain markers, S–C foliations, and torsional buckling of thin-walled cylinders. In contrast, samples deformed at constant strain rate deformed homogeneously. Grain-boundary maps created with electron-backscatter-diffraction data indicate that high-strain regions in constant-stress samples correlate with finer grain sizes and stronger crystallographic fabrics. Since the dominant deformation mechanism is grain-size sensitive, heterogeneous recrystallization leads to strain localization in finer-grained regions. However, variations in strength are not large enough to initiate localization in constant-strain-rate experiments. The magnitude of grain-size heterogeneity remains relatively constant with increasing strain, implying that shear zones are maintained throughout the experiments even as non-localizing regions recrystallize. Based on our results, we propose that deformation driven at constant stress in Earthʹs lithosphere will easily localize even if structural heterogeneities are not initially present.