HR: 1340h
AN: MR43A-0971    [Abstracts]
TI: Diffusion creep, grain rotation and mantle anisotropy
AU: * Wheeler, J
EM: johnwh@liv.ac.uk
AF: Liverpool University, Dept. Earth and Ocean Sciences Brownlow St., Liverpool, L69 3GP, United Kingdom
AB: The mantle deforms by some combination of diffusion creep and dislocation creep. It is well established that dislocation creep gives rise to crystallographic preferred orientations (CPO) which in turn lead to seismic anisotropy. Consequently seismic anisotropy may be interpreted as indicating the action of dislocation creep, and an absence of anisotropy as indicating diffusion creep. One assumption involved is that diffusion creep and the accompanying grain boundary sliding cause random grain rotations which destroy CPO. So, if a mantle rock has deformed in dislocation creep, developing a CPO, and then moves into a strain rate/temperature/grain size regime promoting diffusion creep as the dominant mechanism, the CPO will be destroyed. Here I test this assumption via a numerical model (named "DiffForm") for diffusion creep. The model is grain- scale and hence predicts the finite rotations of individual grains through time as the polycrystal deforms in grain boundary diffusion creep accompanied by sliding. The rotation evolution depends on the details of the starting microstructure, but for a variety of deformation types and initial microstructures simulations show that the rotation rates decrease through time. At large strains the microstructures reach quasi-steady states in which little further rotation occurs. This implies that diffusion creep can weaken a pre-existing CPO but not destroy it, so regions of anisotropic mantle may, in fact, be deforming by diffusion creep, with the CPO a weakened but still potent inherited feature.
DE: 1236 Rheology of the lithosphere and mantle (7218, 8160)
DE: 3902 Creep and deformation
DE: 5120 Plasticity, diffusion, and creep
DE: 8160 Rheology: general (1236, 8032)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting