Four-dimensional Structural Evolution of the Western Pacific Convergent Margin Based on Paleogeographic Reconstructions and Seismic Tomography

Plate motions and subducting slab morphology are intricately connected and through the combination of seismicity, tomographic images, and relative plate motions the evolution of mantle structure can be interpreted. Tomographic images of P-wave, shear wave-speed, and bulk sound speed perturbations of the Northwest Pacific region have been interpreted to define the extent and geometry of the subducting Pacific plate in the upper mantle. We have found that the morphology of the subducted Pacific plate along the Kurile-Japan-Izu-Bonin-Mariana arc system varies both in geometry and subduction angle. Beneath the Kurile and Japan arcs the slab is coherent, but the subduction angle decreases from north to south while the slab thickens, and is very complex at the junction of the two arcs near the Hokkaido corner. In the transition zone beneath the Sea of Japan the slab lies horizontally on top of the transition zone, but beneath the Sea of Okhotsk it penetrates into the lower mantle. Similarly, beneath the Izu-Bonin arc the Pacific plate bends into a horizontal position as it reaches the 660 km discontinuity, but at the southern end of the arc a large near horizontal slab tear is present north of the junction with the Mariana arc. Directly below the Mariana arc the subducted oceanic lithosphere is arcuate in shape, reminiscent of the trace of the Mariana trench, and penetrates at a near vertical angle into the lower mantle. To understand the differences and evolution of the slab morphology along the entire arc system we have produced a new paleogeographic reconstruction of the Western Pacific. The reconstructed tectonics of the region is based on Euler pole motions of the Pacific, Philippine, Amurian, Okhotsk, and North American plates relative to Eurasia for the past 20 My. Our tectonic reconstruction illustrates the collision of the Japan and Kurile arcs, the opening of the Kurile Basin and Sea of Japan, change in motion of the Izu-Bonin arc, developing curvature of the Mariana arc, disparity in Pacific plate velocities along the convergent margin, and variation in rates of trench retreat along different segments of the arc system. The new plate motion model and interpretations of the physical properties of the mantle imaged with the P-wave and joint tomography are used to assess the spatial and temporal evolution of the Pacific plate morphology from the mid-Miocene to the present and provide limitations in plausible plate motions for the region.