Structure and dynamics of the Tonga subduction zone: New insight from P-wave anisotropic tomography

Abstract

The Tonga-Lau-Fiji region is important to study plate-plume and subduction-ridge interactions, but its deep mantle structure is still not very clear. Here we present high-resolution tomography of 3-D P-wave azimuthal anisotropy down to 400 km depth of the Tonga subduction zone derived from arrival-time data of local earthquakes recorded at seafloor and land seismometers. The subducting Tong slab is imaged as high-velocity anomalies at depths of 100-400 km, whereas large-scale low-velocity anomalies down to 400 km depth are revealed in the mantle wedge beneath the backarc basin and volcanic arc. Trench-parallel anisotropy beneath the Lau Basin extends southwards to ∼140 km depth at ∼20.5°S, representing the extent of both southward flow of the Samoan plume and toroidal flow by the slab rollback. At depths of 140-400 km, the Lau Basin and Fiji Plateau mainly exhibit plate-parallel fast-velocity directions (FVDs) north of ∼20.5°S, indicating strong corner flow in the mantle wedge driven by the slab subduction and dehydration. The Tonga slab exhibits trench-parallel FVDs at depths of <200 km, reflecting fossil fabric formed during the plate spreading stage, whereas, at greater depths, the slab mainly exhibits trench-normal FVDs, which may reflect complicated deformations within the slab. These results suggest that the Samoan plume has a significant impact on the Tonga-Lau-Fiji region, leading to variations in the scale and depth extent of mantle flows.