Localization of dislocation creep in the lower mantle: implications for the origin of seismic anisotropy

Recent seismological observations reveal the presence of seismic anisotropy in localized regions at the base of the mantle within an otherwise isotrop ic lower mantle. These regions can be placed in a tectonic context, corresp onding to locations of paleosubduction and plume upwelling. This project wo rks toward determining whether the observed seismic anisotropy may be expla ined by the development of a mineral fabric by lattice-preferred orientatio n (LPO). Numerical modeling is used to explore whether the conditions at th e base of upwelling and downwelling regions are consistent with those requi red for fabric development. Specifically, we examine whether dislocation cr eep dominates these regions within a background mantle that flows primarily by diffusion creep. The key to our study is the use of a composite rheolog y that includes both mechanisms of diffusion and dislocation creep and is b ased on mineral physics experiments. Results show that it is possible to pr oduce a localization of dislocation creep near slabs within a background ma ntle dominated by diffusion creep. In contrast, upwelling regions are chara cterized by a domination of diffusion creep. These results indicate that LP O may be the cause of lowermost mantle seismic anisotropy near paleoslabs, but other mechanisms such as shape-preferred orientation may be required to produce the anisotropy observed near upwellings. (C) 2001 Elsevier Science B.V. All rights reserved.