Shear localization in transformation-induced faulting: first-order similarities to brittle shear failure

Transformation-induced faulting, a leading candidate for the mechanism of d eep-focus earthquakes, leads to catastrophic shear failure in laboratory sp ecimens by the growth and coalescence of microscopic Mode I "anticracks", o r packets of denser, microcrystalline transformed material. We have studied this phenomenon in Mg-2 GeO4 olivine undergoing a polymorphic phase transf ormation to its denser spinel phase at 2.7 GPa and 1200 K under applied str ess in order to better understand the details of anticrack growth and coale scence 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-organ ization of these features during the shear localization process. We have an alyzed SEM and optical images of individual anticrack reaction interfaces a nd coalesced structures from this experiment and another experiment conduct ed under similar conditions but with a different strain history. We have is olated the reaction boundaries in these images and have performed fractal a nalysis of these boundary lines. We find that the fractal dimension of isol ated, 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 par allel with the fractal nature of brittle cracks and fault surfaces. We also measured the length of the long axis of uncoalesced anticrack structures f ound in the experiment arrested at failure, and find a roughly log-normal d istribution of sizes, very similar to the distribution of microcrack length s reported in experiments involving brittle shear failure. Microcracks and microanticracks both also commonly nucleate on grain boundaries, intracryst alline flaws, and phase boundaries. Shear zones (faults) formed by either o f these mechanisms also regularly form at similar to 30 degrees to maximum compression. From this set of observations, we conclude that the general ph enomenon of shear failure creates similar features in both systems (perhaps in any system which generates a shear instability), despite the major diff erences in the detailed microphysics, Because of the detailed and unambiguo us measurements that are possible with images of transformation-induced fau lting experiments, farther studies of this faulting mechanism may provide n ew insights into brittle failure and shear localization processes. (C) 2001 Elsevier Science B.V. All rights reserved.