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 hollow 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 hollow 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. We suggest that the dominant deformation mechanism is grain-size sensitive, and therefore heterogeneous recrystallization leads to strain localization in fine-grained regions. However, variations in strength are not large enough to initiate localization in constant-strain-rate experiments. Grain-size heterogeneity decreases with increasing strain, implying that localization will cease if the grain size fully homogenizes. Based on our results, we propose that deformation driven at constant stress in the Earth's lithosphere will easily localize even if structural heterogeneities are not initially present.