Analysis of a recently discovered high-pressure phase-transformation-induced mechanism of shear failure in Mg2GeO4 olivine has produced evidence that sliding in the resulting fault zone is accomplished by superplastic flow of the extremely fine-grained high-density phase produced during the transformation. This failure mechanism is of interest because it may be the mechanism by which deep earthquakes are generated in the earth’s mantle. To gain insight into this process, we have conducted model tensile experiments on coarse-grained, non-superplastic, specimens of Mg-15%Mn-0.3%Ce alloy, within which a fine-grained, superplastic, planar zone was fabricated at an orientation of 45 degrees to the stress axis. Flow was largely restricted to shear offset within the superplastic zone. The experiments were interrupted periodically and microstructural observations were made. Repeated detailed observation of several regions at different strain levels showed that the main mechanism 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 Mg2GeO4 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.