Mechanics of polycrystals and the generation of seismic anisotropy
Progressive deformation of upper mantle rocks via dislocation creep causes their constituent crystals to take on a non-random orientation distribution (crystallographic preferred orientation or CPO) whose observable seismological signatures include shear-wave splitting and azimuthal dependence of surface wave speeds. Comparison of these signatures with mantle flow models thus allows mantle dynamics to be unraveled on global and regional scales. However, existing self-consistent models of CPO evolution are computationally expensive when used in conjunction with numerical convection models. We have recently developed a new method, 'ANPAR', which is based on an analytical parameterization of the crystallographic spin predicted by the second-order (SO) self-consistent theory. When applied to olivine polycrystals (dunites), our parameterization fits the predictions of the SO model almost perfectly (variance reduction > 99.5%), yet runs 20000 times faster. The ANPAR model thus shows promise for calculating CPO in large three-dimensional and/or time-dependent flow models. Currently we are working to extend ANPAR to multiphase aggregates (e.g. olivine plus enstatite) and to lower-mantle phases (e.g. post-perovskite).
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