フィリピン海のテクトニクス

抄録

The models on the tectonic evolution of the Philippine Sea can be divided mainly into two:(1) entrapment of a normal ocean at 43Ma for the West Philippine Basin formation and back-arc basin formations caused by successive landward retreats of the upper Philippine Sea plate, and (2) back-arc basin formations caused by successive seaward retreats of the trench axis. These models are discussed on the basis of the presently available data.
The island arc type volcanism at the northern half of the Palau-Kyushu Ridge since 48Ma, and the age-depth relation and chemistry of basalts at Sites 291 and 292 in the West Philippine Basin favor the back-arc spreading origin for this basin. In contrast, the chemistry of basalts at Site 447 and the large distance between the Central Basin Ridge and the northern Palau-Kyushu Ridge favor the entrapment origin for this basin. It is likely that the West Philippine Basin has formed under the unique situation of the ocean basin formation, i.e., with subduction at its northern margin and a spreading center far from this subduction zone.
Subduction or collision of the aseismic ridges in and around the Philippine Sea is discussed in terms of the crustal structure of the ridges, seismicity, and onland geology. It is found that all these aseismic ridges except for Izu which has a thick 20-30km crust have been subducted, although many of them have formed a cusp or discontinuity at the trench. Elucidating the times of the bending of the Cretaceous-Paleogene terranes in central Honshu and in Kyushu is critical to discriminate the two models for the Shikoku Basin formation, i. e., the fixed trench model and the trench retreating model.