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Abstract The Tonga‐Kermadec subduction zone exhibits the fastest observed trench retreat and convergence near its northern end. However, a paradox exists: despite the rapid trench retreat, the Tonga slab maintains a relatively steep dip angle above 400 km depth. The slab turns flat around 400 km, then steepening again until encountering a stagnant segment near 670 km. Despite its significance for understanding slab dynamics, no existing numerical model has successfully demonstrated how such a distinct slab morphology can be generated under the fast convergence. Here we run subduction models that successfully reproduce the slab geometries while incorporating the observed subduction rate. We use a hybrid velocity boundary condition, imposing velocities on the arc and subducting plate while allowing the overriding plate to respond freely. This approach is crucial for achieving a good match between the modeled and observed Tonga slab. The results explain how the detailed slab structure is highly sensitive to physical parameters including the seafloor age and the mantle viscosity. Notably, a nonlinear rheology, where dislocation creep reduces upper mantle viscosity under strong mantle flow, is essential. The weakened upper mantle allows for a faster slab sinking rate, which explains the large dip angle. Our findings highlight the utilizing rheological parameters that lead to extreme viscosity variations within numerical models to achieve an accurate representation of complex subduction systems like the Tonga‐Kermadec zone. Our study opens new avenues for further study of ocean‐ocean subduction systems, advancing our understanding of their role in shaping regional and global tectonics. 

Abstract The formation of Lau Basin records an extreme event of plate tectonics, with the associated Tonga trench exhibiting the fastest retreat in the world (16 cm/yr). Yet paleogeographic reconstructions suggest that seafloor spreading in the Lau Basin only initiated around 6 Ma. This kinematics is difficult to reconcile with our present understanding of how subduction drives plate motions. Using numerical models, we propose that eastward migration of the Lau Ridge concurrent with trench retreat explains both the narrow width and thickened crust of the Lau Basin. To match the slab geometry and basin width along the Tonga‐Kermadec trench, our models suggest that fast trench retreat rate of 16 cm/yr might start ~15 Ma. Tonga slab rollback induced vigorous mantle flow underneath the South Fiji Basin which is driving the extension and thinning of the basin and contributing to its observed deeper bathymetry compared to neighboring basins. 

Fundamental to plate tectonics is the subduction of cold and mechanically strong oceanic plates. While the subducted plates are conventionally regarded to be impermeable to mantle flow and separate the mantle wedge and the subslab region, isolated openings have been proposed. By combining new shear wave splitting measurements with results from geodynamic modeling and recent seismic tomography and geochemical observations, we show that the upper ~200 km of the Cocos slab in northern Central America is intensively fractured. The slab there is strong enough to produce typical arc volcanoes and Benioff Zone earthquakes but allows mantle flow to traverse from the subslab region to the mantle wedge. Upwelling of hot subslab mantle flow through the slab provides a viable explanation for the behind-the-volcanic-front volcanoes that are geochemically distinct from typical arc volcanoes, and for the puzzling high heat flow, high elevation, and low Bouguer gravity anomalies observed in northern Central America. 

Abstract The existence of historical flat slabs remains debated. We evaluate past subduction since 200 Ma using global models with data assimilation. By reproducing major Mesozoic slabs whose dip angles satisfy geological constraints, the model suggests a previously unrecognized continental‐scale flat slab during the Late Cretaceous beneath East Asia, a result independent of plate reconstructions, continental lithospheric thickness, convergence rate, and seafloor age. Tests show that the pre‐Cretaceous subduction history, both along the western Pacific and Tethyan trenches, is the most important reason for the formation of this prominent flat Izanagi slab. Physically, continuing subduction increases the gravitational torque, which, through balancing the suction torque, progressively reduces dynamic pressure above the slab and decreases the slab dip angle. The flat Izanagi slab explains the observed East Asian lithospheric thinning that led to the formation of the North‐South Gravity Lineament, tectonic inversion of sedimentary basins, uplift of the Greater Xing'an‐Taihang‐Xuefeng mountains and the abrupt termination of intraplate volcanism during the Late Cretaceous. 

Abstract The extensive fast seismic anomalies in the mantle transition zone beneath East Asia are often interpreted as stagnant Pacific slabs, and a reason for the widespread tectonics since the Mesozoic. Previous hypotheses for their formation mostly emphasize vertical resistances to slab penetration or trench retreat. In this study, we investigate the origin of these stagnant slabs using global‐scale thermal‐chemical models with data‐assimilation. We find that subduction of the Izanagi‐Pacific mid‐ocean ridge marked the transition of mantle flow beneath western Pacific from being surface‐driven Couette‐type flow to pressure‐driven Poiseuille‐type flow, a result previously unrealized. This Cenozoic westward mantle wind driven by the pressure gradient independently explains seismic anisotropy in the region. We conclude that the mantle wind is the dominant mechanism for the formation of stagnant slabs by advecting them westward while the pressure gradient holds them in the transition zone.