MODELING IN MAGNESIUM ALLOY WITH SUPERPLASTIC LAYER - IMPLICATIONS FOR SHEAR IN FAULT ZONES INDUCED IN OLIVINE BY PHASE-TRANSFORMATION

Analysis of a recently discovered high-pressure phase-transformation-i nduced mechanism of shear failure in Mg2GeO4 olivine has produced evid ence that sliding in the resulting fault zone is accomplished by super plastic flow of the extremely fine-grained high-density phase produced during the transformation. This failure mechanism is of interest beca use it may be the mechanism by which deep earthquakes are generated in the earth's mantle. To gain insight into this process, we have conduc ted model tensile experiments on coarse-grained, non-superplastic, spe cimens of Mg-15%Mn-0.3%Ce alloy, within which a fine-grained, superpla stic, planar zone was fabricated at an orientation of 45 degrees to th e stress axis. Flow was largely restricted to shear offset within the superplastic zone. The experiments were interrupted periodically and m icrostructural observations were made. Repeated detailed observation o f several regions at different strain levels showed that the main mech anism of shear operative in the superplastic region was grain-boundary sliding occurring in a layer-by-layer manner. The common features of microstructural change observed in the magnesium alloy and in the Mg2G eO4 olivine fault tones suggests that such cooperative grain-boundary sliding could be the mechanism of fault propagation in the deep earth and therefore important for understanding deep-focus earthquakes.