ANISOTROPIC CONVECTION WITH IMPLICATIONS FOR THE UPPER-MANTLE

On the basis of polycrystalline theory describing the plasticity in ol ivine and enstatite, the flow in a convection cell has been simulated using a finite element formulation. The spatial variations in anisotro pic properties are computed from the textures that evolve with the flo w. A kinematically constrained equilibrium-based assumption is used to partition the macroscopic deformation among crystals within an aggreg ate. We model the convection for one specific cell geometry and two se ts of boundary conditions. A complete map of textures throughout the c ell is obtained. The textured convection cell is structurally very het erogeneous and textures along streamlines do not correlate with the fi nite strain. The results of the simulations indicate that during upwel ling a strong texture develops rapidly. It convects during spreading a nd is attenuated during subduction. Results arc compared with features of the upper mantle. In our predictions the pattern of preferred orie ntation during spreading is inclined to the flow coordinates due to de formation by simple shear. This is contrary to Hess' [1964] intuition that (001) slip planes of olivine orient themselves parallel to the fl ow planes, yet the pattern is consistent with natural fabric data. Sig nificant differences are observed as a function of depth within the ce ll. The variations in the p wave velocities in this textured model man tle are analyzed and correspond well with observed seismic data.