Seismic tomographic constraints on plate tectonic reconstructions of the Philippine Sea plate near East Asia

The Philippine Sea and Pacific plates play a key tectonic role at the edge of East Asia, with similar present-day absolute motions. The Philippine Sea slab dips northward under the Eurasian continent at the Ryukyu Trench & Nankai Trough. At Taiwan and south along the Manila Trench, the subduction polarity is flipped and the Eurasia-South China Sea slab dips eastward beneath the Philippine Sea plate. Cenozoic plate tectonic reconstructions of the Philippine Sea plate have been primarily constrained by paleomagnetic data and the seafloor spreading record. These indicate large northward motions from equatorial regions through the Cenozoic and significant Oligo-Miocene plate rotations, respectively, very different from present-day plate motions. In this study seismic tomographic data are used to add the significant constraint of subducted slab geometries and seismic velocities. Detailed 3D geometries of subducted slabs were mapped with GoCad software using the MITP08 seismic tomography dataset (Li et al., 2008), Benioff zone seismicities, and published local tomography. The slabs were then unfolded using GoCad and imported into GPlates reconstruction software to test current plate tectonic reconstructions of the Philippine Sea plate near East Asia. New constraints are provided by the geometries of the mapped slab edges, which indicate inconsistent plate overlaps and voids when applied to existing plate models. These data therefore provide significant constraints for improved models. The unfolded Eurasian-South China Sea slab is ~500 km long and has a N-S oriented edge parallel to transforms of the South China Sea. Velocity images reveal the locations of continent-ocean boundaries at the north and south ends of the Eurasian slab. At Taiwan, the unfolded Philippine Sea plate slab is ~850 km long and also has a N-S edge. These slab geometries imply that present-day east-dipping subduction along the Manila Trench began at a N-S transform-parallel zone between the Eurasian and Philippine Sea plates. Assuming present-day rates, the slab lengths imply that the fast Pacific-like NW motion of the Philippine Sea plate would have changed from its earlier northward motion no earlier than ~7 Ma. However earlier activation of the Manila Trench is likely given geologic constraints and require a slower speed-up to the present Pacific-like motions of the Philippine Sea plate. Further major constraints on earlier motion are provided by several detached slabs under the South China Sea and the West Philippine basin, by slabs of the southern Philippines and Molucca Sea, and by the larger Pacific and Indian Ocean slabs.