SEISMIC TOMOGRAPHIC CONSTRAINTS ON PLATE TECTONIC RECONSTRUCTIONS OF THE PHILIPPINE SEA PLATE

The tectonic history of the Philippine Sea plate remains poorly constrained relative to plates that have spreading ridges and associated magnetic anomalies. Only subduction zones or convergent transforms occur along its boundaries and the plate lacks obvious hotspot traces. Paleomagnetic data indicate large northward motions from equatorial regions through the Cenozoic. Plate rotations are not well constrained but large rotations have been interpreted.

In this study we add the significant constraint of subducted slab geometries and seismic velocities. Detailed 3D slab geometries under SE Asia were mapped with GoCad software using the MITP08 seismic tomography (Li et al., 2008) and Benioff zone seismicity. The 3D slab geometry and associated seismic velocity images were then unfolded in GoCad and imported into GPlates software to constrain existing plate tectonic reconstructions of the Philippine Sea plate and surrounding regions.

The areas occupied by these unfolded slabs are in conflict with existing plate models, showing both overlaps and voids, and therefore these are important new constraints. Furthermore, heterogenieties in the velocity images reveal major features of the subducted slabs, including back-arc basins, continent-ocean boundaries and possible slab windows. Finally the shapes of slab edges provide significant new constraints, as follows.

At the western margin of the Philippine Sea plate, the unfolded edges of the Eurasian and Philippine Sea slabs show a through-going plate boundary zone that is oriented N-S along the line from Taiwan through the Philippines. The unfolded Eurasian South China Sea slab edge is oriented N-S, indicating that present subduction started along this N-S zone, which is parallel to transforms of the South China Sea. The subducted N-S edge of the Philippine Sea plate extends ~1000 km north of Taiwan. Successfully modeling the origin and evolution of this NS plate boundary is a fundamental requirement of plate tectonic reconstructions of SE Asia. We present our current synthesis of mid-to-late Cenozoic tectonics of the Philippine Sea plate, incorporating these new constraints of subducted slab geometry and velocity structure.