GEOPHYSICS: Olivine Strains, Anisotropy Gains

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Science  11 Feb 2000:
Vol. 287, Issue 5455, pp. 933c
DOI: 10.1126/science.287.5455.933c

Beneath the seemingly solid crust of Earth are complex flow patterns of rock in the mantle that can be observed by indirect seismic measurements of travel times and paths. Seismic anisotropy may occur when seismic waves propagate faster along paths that are parallel to the direction of a flow structure than along perpendicular paths. The most abundant magnesium- and iron-rich silicates in the mantle, such as olivine and its high-pressure polymorphs, flow by dislocation diffusion or creep over long time scales and under large strain rates.

Zhang et al. have conducted a series of laboratory experiments to determine how simple shear deformation of fine grained, polycrystalline olivine samples may alter assumptions about seismic anisotropy. At 1573 K and strains greater than 0.6, the olivine grains deformed by strain softening and dynamic recrystallization along one dominant slip direction. Along the slip direction, they measured a bimodal distribution of orientations for the recrystallized grains. About half were oriented parallel to the shear direction and half were oriented perpendicular to the maximum principal stress. The shear-directed grains would lead typically to faster seismic velocities along the flow direction, but the stress-directed grains would lead to faster seismic velocities at oblique angles to the flow direction. Indirect imaging of mantle structure may require a more complex correlation between seismic anisotropy and grain orientations and could lead to different images of large-scale flow in the mantle.—LR

Tectonophysics 316, 133 (2000).

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