LATTICE PREFERRED ORIENTATION OF OLIVINE AGGREGATES DEFORMED IN SIMPLE SHEAR

REGIONS of the Earth's upper mantle show significant seismic anisotrop y due to the preferred crystallographic orientation ('lattice preferre d orientation') adopted by its constituent minerals in response to def ormation(1-6). Seismic anisotropies thus provide clues to the flow and /or stress patterns in the upper mantle, but the use of lattice prefer red orientation to infer such properties from seismic data has been ha mpered by the lack of experimental studies relating changes in crystal lographic orientation to simple-shear deformation. (Simple shear is pr obably the dominant mode of deformation in the upper mantle, whereas m ost previous experiments have focused on the effects of uniaxial compr ession(7-9).) Here we describe the results of simple-shear deformation experiments on olivine aggregates, conducted at high temperatures and pressures (similar to 1,500 K and 300 MPa). For large strains (up to 150%), our experiments reproduce the lattice preferred orientation obs erved in highly deformed upper-mantle rocks, in which the olivine [100 ] axes lie nearly parallel to the bow direction. But for relatively sm all strains, the preferred orientation is rotated with respect to the flow direction, indicating that seismic anisotropy should also be sens itive to the sense of shear is tbe upper mantle.