“Double Door” Opening of the Japan Sea Inferred by Pn Attenuation Tomography

The extension of back-arc basins and formation of marginal seas following the subduction of oceanic lithosphere are usually attributed to the rollback of subducting slabs and distorted mantle convection. However, for the Japan Sea, the largest marginal sea in the northwestern Pacific, its opening is unlikely only resulted from the subduction of the Pacific plate because of the coeval Philippine plate subduction and the arcuate arc volcanic zone. Therefore, the present-day thermal structure in the uppermost mantle, which can be directly constrained by strong Pn-wave attenuation, plays a vital role in understanding the Japan Sea opening. Here, we observe two belts of strong Pn attenuation beneath the Japan Sea; their strikes are generally consistent with local Pn anisotropy and the retreat directions of the Pacific and Philippine trenches. Hence, there seem to be two divergent mantle flows in the uppermost mantle, pushing a “double door” for the Japan Sea opening.