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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 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. Plain Language Summary: Flat subduction refers to a downgoing slab whose dip angle is smaller than 15°. Although we can observe the present‐day flat slabs through seismic imaging, it is harder to detect the ancient ones. In this study, we use global geodynamic models to simulate subduction during the past 200 Ma and find that there was a flat Izanagi slab beneath East Asia during the Late Cretaceous. This previously unrecognized flat slab provides an alternative and unified explanation for multiple synchronous geological events in East Asia, including the thin lithosphere to the east of the North‐South Gravity Lineament, inversion of sedimentary basins in the region, uplift of the Greater Xing'an‐Taihang‐Xuefeng mountains and the termination of intraplate volcanisms. We also evaluate different mechanisms for the formation of the flat Izanagi slab and find that the most important reason is the long‐lasting Mesozoic subduction along the western Pacific margin. Key Points: Global data‐assimilation models reproducing past subduction discovered a previously unrecognized continental‐scale flat Izanagi slabThe flat Izanagi slab caused the unique East Asian lithospheric structure, basin inversion and regional uplift during the Late CretaceousThe key mechanism of this flat slab is dynamic suction due to long‐lived prior subduction along the west Pacific and south Asian margins [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]

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]