Using high-resolution, triplicate body waveforms, we investigate strong lateral variations in P and SH wave speeds (VP and VS) beneath the Tonga subduction zone where cold lithosphere is subducting rapidly at 200 mm yr-1. Under these conditions, the lack of prominent anomalies of high VP and VS in the lower mantle implies that a large amount of recently subducted material remains above the lower mantle. This inference is also evident from a subhorizontal swath of outboard earthquakes in the transition zone that is up to 1000 km away from the Wadati-Benioff zone. Beneath the back arc, we find a sharp, east-west trending demarcation north of which outboard earthquakes occur and VP and VS abruptly drop by ~3%. Thus the effect of cold temperature to raise VP and VS, as indicated by the presence of outboard earthquakes, must be counteracted by petrologic variations. We interpret that outboard earthquakes occur where impounding of subducted metastable olivine or volatiles reduces seismic wave speeds and triggers deep earthquakes. Considering the density of these candidates and the strong effect of partial melt on VS, volatiles in the form of hydrous phases or melt induced by dehydration are not consistent with observed reductions in VP and VS. Meanwhile, metastable olivine remains a viable candidate. As a cold slab passes below the 410-km discontinuity, olivine becomes metastable and makes the tip of the slab buoyant. Such a mechanism could explain why the leading end of a fast subducting slab may be successively sequestered in the transition zone.