Upper mantle anisotropy: A preliminary model

Seismic anisotropy in the upper mantle can be explained by crystallographic mineral alignment achieved through dislocation motion. The physical mechanism of mineral alignment requires upper mantle shear flow which reorients and aligns minerals by dislocation glide and climb governed by the dominant glide system of each mineral. The dominant glide systems are assumed to be [100](0l0) for olivine and [001](100) for the pyroxenes. These yield a predominantly orthorhombic fabric with the olivine [100] and the pyroxene [001] axes aligned in the upper mantle flow direction and the olivine [010] and the pyroxene [100] axes aligned normal to the upper mantle flow plane. These glide systems have a threshold temperature of enhanced mobility of 1100–1200 K, which yields a solid-state, thermally defined lithosphere-asthenosphere boundary in an olivine-pyroxene mantle consistent with recent seismic determinations of the thickness of the lithosphere. Mantle anisotropy due to mineral alignment is then actively maintained below this boundary (asthenosphere and mesosphere) and is a fossil state above this boundary (lithosphere). We use the dominant glide systems to establish the crystallographic orientation of olivine and pyroxene in calculating the maximum seismic anisotropy of two petrologic models (pyrolite and piclogite) for the upper mantle by an extrapolation of single-crystal, anisotropic mineral elastic properties to a depth of 400 km. The real-earth seismic anisotropy will be bound by the limits of maximum anisotropy from perfect mineral alignment and minimum anisotropy (isotropy) from random mineral alignment. The seismic anisotropy of the upper 220 km is best represented by the pyrolite model, which reduces to quasi-hexagonal symmetry with the unique axis in the direction of mantle flow. The Lehmann discontinuity is conjectured to be due to a change in composition from pyrolite to piclogite and therefore may represent a change in anisotropy. The piclogite model has orthorhombic symmetry distinctly different from that of the pyrolite. The piclogite anisotropy model can appear quasi-isotropic, when measured by transverse isotropy parameterization, if the mantle flow is mainly horizontal with a horizontal shear plane (that is, the shear flow gradient duH /dR dominates).

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