Search

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]