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 Fo(50) olivine to test the influence of imposed boundary conditions on localizing deformation. We deformed both solid and thin-walled cylinders of Fo(50) 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 Earth's lithosphere will easily localize even if structural heterogeneities are not initially present. (C) 2012 Elsevier B.V. All rights reserved.