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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]

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]

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]