A new class of microstructures which lead to transformationinduced faulting in magnesium germanate
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Transformation-induced faulting has been studied extensively in natural olivine, ice, and a variety of other olivine analog systems over the last 15 years and remains a likely candidate mechanism for initiating deep focus earthquakes. The mechanism involves the formation of microscopic packets of fine-grained denser materials from a metastable host, which then behave as mode I features that coalesce to allow bulk mode II shear failure. We present evidence for a new class of transformation microstructures that also allows shear failure at high pressures but which does not require the formation of widely distributed mode I features prior to shear failure. We have observed thin planar zones of transformation in coarse-grained (150 m) Mg2GeO4 olivine that form along crystallographic planes in deformed grains at temperatures cooler than those that allow significant development of anticracks. The development of these zones is rapid and only occurs after 25% bulk strain. Increasing strain leads to progressive development of these zones, in turn leading to bulk shear failure once a continuous pathway of fine-grained spinel is formed that traverses the specimen. Transmission electron microscopy analysis shows that slip bands in olivine along (011) and (010) formed from dislocation pileups on these slip planes transform preferentially to a nanocrystalline aggregate of the denser spinel phase, aided by the high strain energy of these bands. This new, partially strain-driven mechanism compliments shear failure by anticracks as described in previous studies and adds new possibilities for the triggering of bulk shear failure by rapid mineralogical transformations. Copyright 2005 by the American Geophysical Union.