A landmark study published in Geoscience—a leading Chinese journal at the forefront of earth science research—has resolved critical debates about mantle dynamics beneath the Tonga Subduction Zone by unveiling its three-dimensional shear-wave velocity and azimuthal anisotropy structure. Conducted by researchers from Ocean University of China and Tohoku University, Japan, the work combines innovative seismic methodologies to map mantle flow patterns, slab-plume interactions, and back-arc basin dynamics in unprecedented detail.
The research team analyzed 150,219 amplitude and phase measurements from 1,088 teleseismic events recorded across 110 seismic stations, including land-based and ocean-bottom instruments deployed in the Lau Basin and surrounding regions. Leveraging fundamental-mode Rayleigh waves (20–150 s periods), the study applied azimuthal anisotropy tomography—a technique sensitive to directional variations in seismic wave speeds caused by aligned mineral fabrics in mantle rocks—to construct a high-resolution 3D velocity model extending to 300 km depth. Rigorous validation through checkerboard and restoring resolution tests confirmed lateral resolutions of ~150 km and vertical resolutions of 50–75 km above 150 km depth, ensuring robust spatial accuracy.
Key findings reveal that the southward influx of Samoan mantle plume material into the Lau Basin is confined to depths shallower than 50 km, driven by asymmetric rollback of the subducting Pacific Plate. This shallow flow aligns with geochemical evidence of plume-derived signatures in the basin’s northern volcanic zones. The study further identifies divergent mantle flow regimes: west-east oriented motion beneath the rapidly spreading northern Lau Basin contrasts with north-south flow in the slower southern region, reflecting passive adjustments to spatially variable slab retreat rates. Within the subducting slab, near-trench-parallel fast shear-wave directions (N-S) at shallow depths (<150 km) correlate with bending-induced normal faults, while deeper regions exhibit localized trench-perpendicular anisotropy, suggesting stress reorientation. Additionally, a trench-parallel mantle flow in the outer-rise asthenosphere—likely compressed by slab rollback—challenges conventional models of subduction-driven circulation.
The Tonga Subduction Zone, characterized by the world’s fastest plate convergence (~24 cm/year) and back-arc spreading rates, serves as a natural laboratory for studying plate-mantle interactions. By reconciling discrepancies among prior isotropic and anisotropic models, this study establishes the first cohesive 3D framework linking mantle dynamics to surface tectonics. The integration of azimuthally varying surface-wave data with multi-scale tomography represents a methodological leap, bridging geophysical observations with geochemical evidence to clarify mechanisms of mantle flow, slab-plume interplay, and back-arc basin formation. These insights not only refine understanding of subduction zone processes but also offer a template for studying other complex systems, such as the Mariana and Izu-Bonin arcs, where mantle plumes and slab dynamics similarly interact.
This research underscores the transformative potential of high-resolution seismic imaging in earth sciences. The findings highlight the importance of international collaboration in addressing geodynamic challenges, providing actionable insights for hazard mitigation and advancing predictive models of planetary-scale processes. By decoding the hidden forces shaping Earth’s interior, this work exemplifies how cutting-edge seismology can illuminate the intricate dance between tectonic plates and mantle convection—a cornerstone of modern geoscience.