A new sample assembly for the multianvil high-pressure apparatus has been developed that results in high-strain plastic deformation at high pressures and temperatures with minimal deformation during the initial pressurization stage. In this assembly, the sample is a thin disk which is sandwiched between two pistons and oriented at 45¿ to their long axis. The sample and pistons are surrounded by a Pt tube and a polycrystalline MgO cylinder. Upon pressurization, a uniaxial stress develops because of the anisotropy of mechanical properties. Deformation during initial pressurization, which occurred in previous studies, is minimized by locating soft materials at the ends of the pistons and by the simple shear deformation geometry (as opposed to uniaxial compression) that allows sliding at the sample-piston interfaces at low pressures. Large plastic strains, up to ~100% shear strain, have been achieved in (Mg,Fe)2SiO4 phases at high pressures (up to 15 GPa) and high temperatures (up to 1900 K). A theoretical analysis has been made to evaluate the relative contributions to sample deformation from the relaxation of elastic strain in the sample column and from continuing advancement of the multianvil guide blocks. The observed dependence of strain on time, pressure and temperature suggests that deformation in the present experiments occurred mostly as a relaxation process rather than at a constant strain rate caused by continuous piston movement. A comparison of the creep strength of olivine inferred from the strain relaxation data at ~15 GPa and ~1900 K with low-pressure data provides an estimate of the activation volume for creep of V*=14(¿1)¿10-6 m3 mol-1. The theoretical analysis shows that constant strain rate deformation could result from the advancement of the guide blocks after complete stress relaxation, although the total strain will be much less than that attained in the relaxation process. Possible applications of this technique to studies of high-pressure rheology and deformation microstructures in high-pressure minerals are discussed, and strategies for future deformation experiments under high pressures and temperatures are proposed.¿ 1997 American Geophysical Union