Why the Philippine Sea Plate Moves as It Does

Tetsuzo Seno

Earthquake Research Institute, University of Tokyo

J. Geol. Soc. Phil.55 105-117 2000

ABSTRACT The Philippine Sea plate (PH) is rotating relative to Eurasia (or to hot spots) around the pole south of Kamchatka - central Kuril. I address a question why this pole position is produced by available driving forces of the PH. The ridge push and slab pull are the two major sources of driving forces for oceanic plates. The ridge push forces in the PH are generated by the age gradient within the plate; the West Philippine Basin is oldest and the Mariana Trough youngest. The torque calculated from the ridge push forces gives a pole of 70.1｡N, 326.1｡E in Greenland, which produces a westward motion of the PH. On the other hand, the slab pull, which is operated at the Kyushu - Ryukyu Trench and the Philippine Trench, produces a torque pole of 61.7｡N, 257.9｡E in northern Canada. Both of these poles are far from the pole of the PH motion relative to hot spots, implying some other driving forces are necessary. I propose here viscous drag forces beneath the Kyushu-north Ryukyu arc directing toward east are one of such driving forces. In the East China Sea west of Kyushu, an upwelling plume from the deep mantle has been suggested in previous studies. The lateral flow from this upwelling can drag the PH toward east. If a magnitude of this torque is comparable to those of the other driving torques, the desired torque pole south of Kamchatka-Kuril can be produced.

Figure 1 The Philippine Sea (PH) - Eurasian (EU) plate Euler poles and the Euler pole of the PH with respect to hot spots (PH-HS). The PH is rotating clockwise around these poles looked from above. Sn77, Seno (1977); Sn99, Seno et al. (1993); WS, Wei and Seno (1998); Kt, Kotake et al. (1998). The PH-EU pole is derived from the sum of the PH-HS Euler vector of Seno et al. (1993) and the EU-HS Euler vector of HS2-NUVEL1 (Gripp and Gordon, 1990).

Figure 2 The age distribution of the back-arc basins in the Philippine Sea: the west Philippine Basin (See Seno [ 1988] for the references), Shikoku Basin (Okino et al., 1994), Parece Vela Basin (Okino et al., 1998), Mariana Trough (Hussong and Uyeda, 1981), and Bonin back-arc (Taylor et al., 1991). The bottom picture shows that from the difference of the age among these basins, the ridge push forces are generated at the boundaries between the basins.

Figure 3 Distribution of the ridge push forces and the magnitude of the torque at each segment. The total torque pole (70.1｡N, 326.1｡E) is located in Greenland.

Figure 4 Distribution of the slab pull forces and the magnitude of the torque at each segment. The total torque pole (61.7｡N, 257.9｡E) is located in central north Canada.

Figure 5 The possible location of the pole of the additional torque required to produce the observed PH-HS pole, when it is added to the ridge push-slab pull torque. It should be located south of the PH and rotates the PH clockwise looked from above.

Figure 6 A schematic illustration showing the mantle upwelling in the East China Sea west of Kyushu which is driving Kyushu and westernmost Honshu to the east by the viscous drag forces (Seno, 1999). The resulting compression in the forearc in Kyushu - north Ryukyu might provide driving forces to the PH directing toward east.

Figure 7 The location of the upwelling plume in the back-arc of Kyushu - north Ryukyu. The resulting driving forces due to the flow from the upwelling are indicated by the arrow. The direction of these forces is conformable to the gap between the slab pull-ridge push torque pole and the PH-HS pole, if they act as the driving forces of the PH. The collision forces at the Palau-Yap Trenches are also indicated by the arrow. They, however, do not help to reduce the gap. The Macolod Corridor is another location where a mantle upwelling is expected around the PH.