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

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• 作者：L.N. Hansen ; ^{hanse983@umn.edu} ; M.E. Zimmerman ^{zimme030@umn.edu} ; A.M. Dillman ^{dillm004@umn.edu} ; D.L. Kohlstedt ^{dlkohl@umn.edu}
• 关键词：strain localization ; grain-boundary sliding ; dynamic recrystallization ; torsion ; electron backscatter diffraction
• 刊名：Earth and Planetary Science Letters
• 出版年：2012
• 期刊代码：18_0012821x
• 类别：ph
• 出版时间：1 June, 2012
• 卷：333-334
• 期：Complete
• 页码：134-145
• 文件大小：2686 K

摘要

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 Earth's lithosphere will easily localize even if structural heterogeneities are not initially present.