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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. Plain Language Summary: Tonga subduction zone is special in several ways. First, it has the fastest retreating trench. The maximum convergence rate is about 23 cm/yr, which is also the fastest worldwide. Second, images created using data from seismology indicate that a 1,000 km‐long section of the subducted tectonic plate rests horizontally atop the 670 km discontinuity. Third, the same images show the upper mantle slab is kinked near 400 km depth. None of the previous mantle convection models can reproduce the Tonga slab structure when prescribed by the available histories of past plate motion. To overcome the difficulties of simulating this subduction zone, we developed numerical models with different types of boundary conditions and found it necessary to impose the velocity on the arc and subducting plate, while keep the overriding plate free slip. The distinct slab morphology is shaped by the mantle flow which strongly depends on the trench retreat rate, the mantle viscosity, and the seafloor age. In conclusion, our study underscores the importance of systematic evaluation of non‐linear forward models for achieving a faithful representation of complex subduction systems like the Tonga‐Kermadec zone. Key Points: Numerical models successfully replicate the unique geometry of the Tonga slab, including its kink and stagnation behaviorHybrid velocity boundary conditions and nonlinear rheology are essential for accurately modeling slabs undergoing rapid trench retreatThe results resolve the paradox of the Tonga slab maintaining a steep dip angle despite experiencing the fastest known trench retreat [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The opening of the Lau Basin is complex. In this study, we used numerical models to examine the connection between Tonga subduction and the opening of the Lau Basin. Our models correctly predicted the observed slab structure and size of the Lau Basin. This challenges the current idea that the Lau Basin floor started spreading apart about 6 million years ago. We proposed a new theory. When the Tonga trench is retreating eastward, the Lau Ridge on its western boundary of the Lau Basin is moving toward the trench. This movement could explain the width of the Lau Basin width and the fact that its curst is thicker on the western side than the eastern side. We also suggest that the South Fiji Basin must have stretched and thinned during this process due to the vigorous flow induced by the Tonga slab rollback. The thinned crust making it deeper than nearby basins. The eastward movement of the Lau Ridge could also mean that the amount of trench retreat is larger than previously thought. This might require a revision of plate reconstruction in this region to account for the stretching in the South Fiji Basin. Key Points: Trench retreat rate exerts primary control on the Tonga‐Kermadec slab morphology and back arc basin formationOpening of the Lau Basin is dynamic with eastward migration of both the Tonga Ridge and Lau RidgeThe South Fiji Basin experienced lithospheric thinning due to the fast mantle flow induced by the Tonga slab [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The wide‐spread flat lying fast seismic anomalies in the mantle transition zone are often referred to as stagnant slabs. The mechanisms for their formation are debated. In this study we use Earth‐like global models to simulate slab geometry variation and reproduced the East Asian stagnant slabs. By running different tests we find that most previously proposed mechanisms are not the key reason, including the phase transformation at 660 km, viscosity increase in the lower mantle, seafloor age variation, and trench retreat. Instead, a westward mantle wind controlled the formation of stagnant slabs. This mantle wind occurred after the Izanagi‐Pacific mid‐ocean ridge began to subduct, and is driven by a long‐lasting horizontal pressure gradient induced by previously subducted slabs. Without this mantle wind the slabs sink directly into the lower mantle. Seismic anisotropy models further confirm the dominant role of this upper‐mantle flow in forming stagnant slabs. Key Points: Global thermal‐chemical subduction models with data‐assimilation reproduced the evolution of East Asian stagnant slabsThe key mechanism for slab stagnation is the westward mantle wind driven by continuous former Izanagi and Tethyan subductionAnisotropy calculations confirm the predominant upper‐mantle depth of the mantle wind since the early Cenozoic [ABSTRACT FROM AUTHOR]

Numerical modeling of subduction has proved increasingly important in understanding the evolution of Earth's internal dynamics and surface tectonic responses. A key criterion in judging models is their capability in matching both past plate tectonic constraints and the present-day mantle structures. In the past decades, most such models covered a regional mantle domain due to computational costs, while only recently sophisticated global spherical simulations became possible. Here we evaluate the effects of chosen model domain and applied boundary conditions in regional simulations on their resulting model results, by comparing them with more natural global simulations. All these models are based on the same sequential data-assimilation method and adopt the same plate tectonic history as surface boundary conditions. The only difference is that the regional models cover different geographic regions, representing a subset of the global one that covers the entire Earth. Besides illustrating the general impacts of chosen model setup on the resulting mantle evolution, we also make suggestions on the applicability of using regional models to replicate past subduction in four major circum-Pacific regions. The results show that the slabs from the global model match seismic tomography much better than the regional results in East Asia and South America; the main reason is that the regional models prohibit large-scale mantle flow as that in the global case, due to the non-penetrating side walls cutting off far-field forces. This limitation of regional models cannot be resolved by varying the model domain. In contrast, for North America and Fiji-Tonga, global and regional models produce similar slab structures, both matching tomography. This reflects that their fast-retreating overriding plates play an important role on the regional mantle flow and slab dynamics. [ABSTRACT FROM AUTHOR]

Recent tomography-based tectonic (tomotectonic) reconstructions provide important new insights on past subduction processes. However, some of these exercises violate key geological observations. We suggest this is due to two common simplifications: 1) slabs sink vertically after subduction with their present geographic locations marking past trenches, and 2) slabs sink at a constant rate throughout the mantle with their present mantle depths linearly correlating with their subduction ages. In this study, we investigate the 4D evolution of subducted slabs using global data-assimilation models that successfully reproduce observed mantle slabs along all major subduction zones. We find that slabs can migrate laterally up to 6000 km while descending toward the core-mantle boundary (CMB). This enormous displacement mostly reflects strong and geographically variable horizontal mantle flow. The model results further show that the vertical sinking rate of slabs varies with subduction duration, depth, and geographic locations. The slab sinking rate generally decreases with increasing depth, ranging from >2 cm/yr above 1600 km depth to zero at ~500 km above the CMB. The sinking rate locally peaks at the asthenospheric and mid-mantle depths and diminishes at the base of the transition zone. We further find that surface plate motion strongly affects the horizontal migration of slabs but less affects their sinking rate. These results suggest cautions on inferring past tectonic events based on seismic tomography. [ABSTRACT FROM AUTHOR]

East Asia is characterized by an east-west topographic dichotomy on the two sides of the North-South Gravity Lineament (NSGL), a feature not associated with major basement boundaries. The NSGL also marks an abrupt change in the thickness of the continental crust and the lithospheric mantle, as well as that in the associated residual topography. Both the mechanism and timing for the formation of this unique East Asian lithospheric property remain unclear. We reviewed the key tectonic records of East Asia since the early Mesozoic, with a particular focus on the plausible underlying mantle dynamics. The observation that widespread Jurassic-Early Cretaceous crustal extension occurred on both sides of the NSGL and that Cenozoic rift basins were predominantly to the east of the NSGL suggests that the seismically observed East Asian lithospheric structure came into being no earlier than the Cretaceous. Several flat-slab models have been proposed. We suggest that a combination of these models could explain the unique East Asian lithospheric evolution since the Middle Mesozoic. Further ground truth using quantitative geodynamic models with data assimilation demonstrates that a flat slab could significantly reduce the thickness of the overriding plate by dislocating and entraining the lower mantle lithosphere. Meanwhile, an advancing flat slab causes widespread upper-lithosphere compression and disappearance of the mantle wedge, implying regional-scale surface uplift (or reduced subsidence) and magmatic quiescence, respectively. We identify two episodes of Mesozoic flat slabs below East Asia, one during the Jurassic-Early Cretaceous and the other during the Late Cretaceous. The former affected the eastern half of North China and Northeast China, and the latter affected the entire region east of the NSGL from Northeast China to South China. These two flat-slab cycles largely determined the evolution of the lithosphere and topography within East Asia since the Mesozoic. [Display omitted] [ABSTRACT FROM AUTHOR]