Shear localization in transformation-induced faulting: first-order similarities to brittle shear failure
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Transformation-induced faulting, a leading candidate for the mechanism of deep-focus earthquakes, leads to catastrophic shear failure in laboratory specimens by the growth and coalescence of microscopic Mode I "anticracks", or packets of denser, microcrystalline transformed material. We have studied this phenomenon in Mg2 GeO4 olivine undergoing a polymorphic phase transformation to its denser spinel phase at 2.7 GPa and 1200 K under applied stress in order to better understand the details of anticrack growth and coalescence which leads to localized shear zones and macroscopic failure. Here, we report observations that address the early stages of shear localization. Preserved in one experiment of this series are microstructures typical of anticrack nucleation and growth as well as those involved in the self-organization of these features during the shear localization process. We have analyzed SEM and optical images of individual anticrack reaction interfaces and coalesced structures from this experiment and another experiment conducted under similar conditions but with a different strain history. We have isolated the reaction boundaries in these images and have performed fractal analysis of these boundary lines. We find that the fractal dimension of isolated, uncoalesced anticrack structures is consistently higher than that of structures formed from coalescence and coarsening of fine-grained reaction products. This result is consistent with the varying effects of stress and thermal feedback at these interfaces and also represents an interesting parallel with the fractal nature of brittle cracks and fault surfaces. We also measured the length of the long axis of uncoalesced anticrack structures found in the experiment arrested at failure, and find a roughly log-normal distribution of sizes, very similar to the distribution of microcrack lengths reported in experiments involving brittle shear failure. Microcracks and microanticracks both also commonly nucleate on grain boundaries, intracrystalline flaws, and phase boundaries. Shear zones (faults) formed by either of these mechanisms also regularly form at ~ 30° to maximum compression. From this set of observations, we conclude that the general phenomenon of shear failure creates similar features in both systems (perhaps in any system which generates a shear instability), despite the major differences in the detailed microphysics. Because of the detailed and unambiguous measurements that are possible with images of transformation-induced faulting experiments, further studies of this faulting mechanism may provide new insights into brittle failure and shear localization processes. © 2001 Elsevier Science B.V. All rights reserved.
author list (cited authors)
Riggs, E. M., & Green, H. W.