High-resolution Seismic Imaging of the 410-km Discontinuity in Tonga Subduction Zone for Plume-Slab Interaction

Detailed geomorphology of the 410-km phase-transition discontinuity offers important information about the thermal state and dynamics of the Earth interior. We generated a high-resolution image of the 410-km discontinuity around the Tonga subducting slab using a three-dimensional pre-stack elastic Kirchhoff migration of signals from deep-focus earthquakes. This provides a great opportunity for us to examine slab mantle interactions. We found from both P and S images, the depth of the 410-km discontinuity inside the subducting slab is observed ~30 km shallower relative to its depth outside of the slab. The shallower depth of the 410-km discontinuity and strong reflectivity of the p410P and s410S within the slab provide evidence against pervasive metastability of the olivine-wadsleyite phase change as a mechanism of deep-focus earthquakes. Further to the west, under the present-day Fiji Islands, the 410-km discontinuity is depressed by ~20 km which supports the hypothesis of mantle plume migration from Samoa chain islands to Fiji. Assuming equilibrium phase change, we infer a temperature difference between the cold slab and hot plume of ~530-820 K, depending on uncertainties in mineralogy and the derived Clausius-Clapeyron slope. Such a large temperature gradient may drive complex slab mantle-interaction patterns. Possible matches of velocity and density across the 410-km discontinuity have been found through amplitude information. Combined with velocity and impedance contrasts from previous experimental studies, we think a pyrolytic transition zone with 1.79% to 2.34% water content fits our observation.