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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.

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]

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]