Asthenospheric Flow through the Izanagi-Pacific Slab Window and its Influence in East Asia

The tectonics of East and Southeast Asia are notoriously complex. Consisting of an intricate patchwork of microplates and accreted terrains, even the recent (i.e Cenozoic) tectonic history of the region remains controversial; and many differing reconstructions have been proposed. While the exact kinematics remain poorly constrained, it is generally accepted that the region has been characterised by a long history of subduction and downwelling. However, numerous geological and geophysical observations, at a first glance, appear to lie in stark contrast to this history. For example, regions of present-day dynamic uplift inferred from residual topography studies, the observation of seismically slow anomalies in numerous tomography models, and the widespread intraplate volcanism in East Asia since the latest Paleogene are all at odds with the expected cold upper mantle and downwelling associated with a history of subduction. Here, we propose a solution to this problem, in which hot asthenospheric material flows from the Pacific domain into East Asia—passing through the slab window opened by the subduction of the Izanagi-Pacific ridge during the early Cenozoic. To investigate this hypothesis, we compare several independent geological observations to the asthenospheric flow predicted by a suite of recently published 3D global mantle convection models.  Firstly, we compare observations linked to uplift and erosion to the changes in dynamic topography induced by this influx of hot material. These include the widespread late Eocene–Oligocene sedimentary hiatus in far eastern China and the regional erosion of southeastern China since the Miocene inferred from Apatite Fission Track Thermochronology (AFT) studies. Secondly, the timing and location of intraplate volcanism is compared with the predicted distribution of hot material through time. We find the westward influx of asthenospheric material to be a robust feature in the models, being predicted under all considered tectonic reconstructions.  Nevertheless, the influence of this material is significantly affected by differing implementations of the Philippine Sea Plate (PSP) history, which allows us to distinguish between these reconstructions based on their correlations with the evidence considered. A larger PSP is found to predict dynamic subsidence in regions where uplift and erosion is present, such as the East China Sea Shelf Basin and the Cathaysia Block, while also predicting large-scale mantle downwelling in regions where intraplate OIB-type magmatism has been recorded. A smaller PSP and the consequent existence of the hypothesised 'East Asian Sea' slabs instead allows the hot asthenospheric material to predominate over a larger region, providing a better fit to the spatial distribution of regional-scale erosional episodes and OIB-type magmatism.