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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 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]

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]

Reconstructions of Earth's past surface kinematics are traditionally based on a combination of relative plate motions inferred mostly from preserved seafloor information and an assumed absolute reference frame using data from hotspots and/or true polar wander. Recently, plate reconstructions progressively introduced constraints from deep mantle structures like those imaged through seismic tomography. This additional information is utilized through either implicit or explicit fashion, where the lateral location of an imaged mantle slab represents that of its paleo-trench with the age of initial subduction estimated from geological proxies and slab depth. Here we quantitatively evaluated the geodynamic and tectonic implications of three recent global plate reconstructions (Müller et al., 2016, 2019 ; and Clennett et al., 2020) by focusing on subduction beneath North America. These reconstructions imply different amounts of trench retreat, plate motion and subduction zones, due to their varying dependence on the tomotectonic constraints. We simulated their respective subduction histories since 200 Ma using a sequential data-assimilation methodology. The resulting present-day slab structures based on these reconstructions show clearly diagnostic differences, among which the model based on Müller et al., 2016 best matches seismic tomography and Mesozoic paleotopography constraints within North America, supporting the tomography-implied differential lithosphere motion relative to the mantle. In contrast, modeled results based on the explicit tomotectonic reconstruction of Clennett et al., 2020 match both slab structure/evolution and associated paleotopographic constraints to the least. Consequently, the presented data-assimilation geodynamic modeling exercise, through reproducing the associated subduction history and continental tectonics, could quantify the tectonic implications of different plate reconstructions. We propose that further implementing this exercise through an iterative geodynamic-tomographic-tectonic workflow could serve to improve the tomotectonic reconstruction. [ABSTRACT FROM AUTHOR]

Subduction of young seafloors straddling mid-ocean ridges (MOR) is an inevitable consequence of plate tectonics. Surprisingly, this process correlates globally with prolonged back-arc extension when the MOR is largely trench-parallel. We investigate the underlying mechanism by analyzing the East China Sea Basin (ECSB) whose Cenozoic tectonic history consists of three syn-rift stages with the rift center progressively migrating oceanward. Global geodynamic models satisfying the past subduction history and present-day mantle structures successfully reproduce the lithospheric stress states of the evolving ECSB. We show that segmented removal of the Mesozoic Izanagi slab due to subduction of the young seafloor initiated Paleocene rifting within the western ECSB. Detachment of the former slab facilitated a strong landward mantle wind driven by the large pressure gradient across the slab. The resulting mantle traction pushed the thickened upper plate landward while entraining the young seafloors behind to slowly subduct, a process causing long-lasting Eocene extension of the central ECSB. The waning mantle wind after 30 Ma reduced basal traction and upper plate extension. A final phase of ECSB extension since the late Miocene formed the Okinawa Trough, when the subducting plate became old enough to trigger slab retreat. A similar dynamic scenario is also predicted in other circum-Pacific margins. We conclude that this enduring back-arc extension during MOR subduction represents an important mechanism for continental evolution during the closure of major ocean basins. • Trench-parallel MOR subduction correlates with back-arc extension globally. • Slab tearing proceeds MOR subduction and generates enduring strong basal traction. • Realistic geodynamic models reproduce extension along major convergent margins. [ABSTRACT FROM AUTHOR]