资源环境科技发展态势分析平台

The Tonga-Samoa system provides a unique tectonic context to study how a cold subducting slab interacts with a hot rising mantle plume. Here we present a 3-D high-resolution image of the 410-km mantle discontinuity (the 410) using seismic signals excited by deep-focus earthquakes. The 410 is found to be similar to 30 km shallower inside the Tonga slab relative to the ambient mantle and similar to 20 km deeper further to the northwest under Fiji Islands. The downward deflection of the 410 under Fiji supports the hypothesis of a plume migration around the northern edge of the Tonga slab from Samoan hot spot to under Fiji due to fast trench rollback. The 50-km topography difference in the 410 between the plume and the slab corresponds to a temperature difference of similar to 500 +/- 100 K. The Samoan plume is inferred to be 200 +/- 50 K hotter than the ambient mantle and supports a thermal origin for the plume.

Plain Language Summary How a rising mantle plume interacts with downgoing slab has not been well understood. Tonga-Samoa and Yellowstone are two such important examples. Under the northern Lau basin where the Tonga slab and the Samoa plume have been vigorously interacting with each other, the 410-km discontinuity topography is found to be controlled by temperature not flow-induced dynamic effect. The 410 topography fits the olivine-to-wadsleyite phase change equilibrium. Because the slab and the plume are next to each other in space, a slab-plume temperature difference is inferred to be similar to 500 +/- 100 K on the same baseline. The Samoan plume is found to be 200 +/- 50 K hotter than the ambient mantle, which is in remarkable consistency with olivine thermometry results.