High frequency records of deep Mariana earthquakes from a dense array of ocean bottom seismographs deployed in the Mariana arc and back-arc regions are stacked and searched for the phases P660p and S660p to constrain the depth of the 660-km discontinuity near the Mariana slab. Results of the high-resolution study suggest that around 18¿N the 660-km discontinuity lies at about 710--730 km (¿14 km) depth within or in the vicinity of the slab core. In the region seismicity ceases at ~620 km depth. This implies that, although tomographic images show the Mariana slab penetrating into the lower mantle, deep seismicity in the region terminates ~100 km above the base of the transition zone. These findings and similar observations in Tonga argue that factors other than the phase transition at the base of the upper mantle may control the maximum down-dip extent of the deep seismogenic region in the slab.