A forward geodynamic modelling approach to understand the history of subduction along the east Asian margin

Plate reconstructions and island arc geochemistry in the northwest Pacific predict the intersection of a mid-ocean ridge with the east Asian subduction zone sometime from the Cretaceous to the Eocene. However, the absence of large swaths of ocean floor of the Pacific and now-subducted Izanagi plate, as well as a fragmented geochemical, volcanic and heat-flow dataset from east Asia, results in alternative models for when and where the mid-ocean ridge arrived at the subduction zone. Two competing models exist: sub-parallel intersection of the Izanagi-Pacific ridge between 60-50 Ma or trench-perpendicular intersection between 110-80 Ma. We use updated plate reconstructions of Panthalassa (proto-Pacific ocean) since the Mesozoic and a reanalysis of geological data from east Asia, coupled with a new generation of forward mantle flow models using the 3D finite element code, CitcomS, to differentiate between competing models. Our geodynamic models, which assimilate plate kinematics, the thermal structure of the oceanic lithosphere and the shallow portion of slabs (at one million year increments) predict the opening of a margin-wide slab window and slab detachment associated with the intersection of the Pacific-Izanagi ridge at the east Asia margin at 55-50 Ma. Our modelled present-day temperature field reveals a good first-order match seismic tomography (Figure 1), which is used as a proxy for present day mantle structure. The sub-parallel ridge-trench intersection and resultant margin-wide slab detachment event may have caused a change in the absolute motion of the Pacific plate at the time of a major global plate reorganization.