ALLEN K. MCNAMARA*, PETER E. VAN KEKEN* & SHUN-ICHIRO KARATO**
* Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063, USA
** Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520-8109, USA
Seismological observations reveal highly anisotropic patches at the bottom of the Earth's lower mantle, whereas the bulk of the mantle has been observed to be largely isotropic. These patches have been interpreted to correspond to areas where subduction has taken place in the past or to areas where mantle plumes are upwelling, but the underlying cause for the anisotropy is unknownboth shape-preferred orientation of elastically heterogenous materials and lattice-preferred orientation of a homogeneous material have been proposed. Both of these mechanisms imply that large-strain deformation occurs within the anisotropic regions, but the geodynamic implications of the mechanisms differ. Shape-preferred orientation would imply the presence of large elastic (and hence chemical) heterogeneity whereas lattice-preferred orientation requires deformation at high stresses. Here we show, on the basis of numerical modelling incorporating mineral physics of elasticity and development of lattice-preferred orientation, that slab deformation in the deep lower mantle can account for the presence of strong anisotropy in the circum-Pacific region. In this modelwhere development of the mineral fabric (the alignment of mineral grains) is caused solely by solid-state deformation of chemically homogeneous mantle materialanisotropy is caused by large-strain deformation at high stresses, due to the collision of subducted slabs with the coremantle boundary.
Nature 416, 310 - 314 (2002)