The subducting cold oceanic plates (slabs) exhibit two paradoxical deformation behaviors: deep seismicity and rheological weakening within the mantle transition zone (MTZ, ~400–700 km depths). Although the transformation of metastable olivine wedge (MOW)1,2 in cold slabs has been proposed as a possible trigger for both behaviors3–10, direct experimental evidence remains limited to understand the processes linking them. Here we report experimental results on the transformation-deformation coupling at MTZ pressures (~20 GPa). Ringwoodite is produced as nano-polycrystalline lamellae (NPL) under uniaxial stress. Thin NPL trigger unstable slips with coseismic stress drops by grain-size sensitive creep coupled with thermal instability at ~760–860 °C. Thickening of NPL at ~950–1,330 °C stabilizes the deformation with enhancing the transformation utilizing their incoherent nature. Thus, the formation of NPL and their grain-size sensitive creep play key roles in temperature-dependent transformation-deformation coupling, which explains both deep seismicity near the MOW and rheological weakening outside the MOW.
