Phase transformations in end-member olivines have been investigated in the temperature range comparable to the interior of subducting slabs. This work constitutes the experimental evidence that the kinetics of transformation of silicate olivine (α phase) to modified spinel (β) and spinel (&ggr;) phases is enhanced by shear deformation. Natural fayalite (α-Fe2SiO4) subjected to a pressure gradient from 0 to 25 GPa at 380 ¿C in the diamond anvil cell (DAC) developed a ring of &ggr; phase where the pressure was in the stability field of the &ggr; phase and shear stress was large enough to promote the α→&ggr; transition. The sample inside the ring, despite being at higher pressure, remained dominantly as α phase. The outermost, lower-pressure region of the sample also remained as α phase. In the Mg2SiO4 system, the transition from α to β was observed at 575 ¿C in runs in which pressure covered the stability fields of β phase, &ggr; phase, and mixed oxides. These results show that the characteristic transformation temperature TCh can be lowered as much as ~200 ¿C by shear deformation. On the basis of these observations, we propose a nonhydrostatic kinetic boundary for the α→β and α→&ggr; transitions in mantle olivine. For temperatures below this boundary, the transformations are kinetically inhibited, while above it, the transformations can be promoted by shear deformation. Therefore, olivine carried to a depth of several hundred kilometers in a subducting slab can remain as metastable α phase until shear deformation causes its transformation. We suggest that this region of shear-promoted transformation in the cold interior of the subduction zone is responsible for the generation of deep earthquakes. ¿ American Geophysical Union 1993