Taiwan is considered to be created by interactions between the Manila and Ryukyu subduction zones, which form a normal double-slab subduction system. Because of the two opposite dipping subductions of the Eurasian Plate (EP) and the Philippine Sea Plate (PSP) around Taiwan, subduction polarity reversal beneath northern Taiwan is presumed, which is accounted for by a slab breakoff model. However, this model has not been convincingly supported by tomographic results so far. Moreover, details of the dynamic process and deformation in this normal double-slab subduction system are still unclear. In this work, we determine high-resolution 3-D tomographic images of isotropic P-wave velocity and azimuthal anisotropy beneath the Manila-Taiwan-southern Ryukyu region, which provide substantial observational constraints on this double-slab subduction system. Our results clearly show the subducted EP breakoff beneath northern Taiwan. We propose two possible factors causing the slab breakoff: the subducted EP rollback under its own gravity after its subduction cessation and collision with the subducted PSP, which may jointly cause the slab breakoff beneath northern Taiwan after a longer collision history. In addition, the subducted EP rollback may be responsible for magmatism and basalts around Taiwan, as well as recent big earthquakes beneath the Taiwan Strait and the South China Sea (SCS). Our results also reveal anisotropy parallel with the SCS fossil ridge beneath the northernmost Luzon Island. We suggest that the fossil ridge and buoyant plateau subduction and/or the ridge-parallel mantle flow during the SCS seafloor spreading may result in the ridge-parallel anisotropy.
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