- 公益社団法人 東京地学協会
- 地学雑誌 (ISSN:0022135X)
- vol.115, no.1, pp.116-123, 2006-02-25 (Released:2009-11-12)
- 21 or 0
The Euler pole position of the Philippine Sea Plate (PHP) relative to the stable Eurasian Plate (EUP) between 15 and 3 Ma can be estimated at around 150°E, 36°N, on the basis of the geological constraint that the intersection of the Izu-Ogasawara Arc with Southwest (SW) Japan has not moved from South Fossa Magna since 15 Ma. The timing of the migration of the Euler Pole to its present location (154°E, 47°N) should have occurred at 3 Ma because the fore-arc basin in SW Japan was once interrupted by the Kurotaki Unconformity at 3 Ma.PHP moves northwestward and subducts beneath SW Japan at a convergent rate of 4 cm/yr. The Izu-Ogasawara Trench (IT) also moves at the same rate as the westward component (ca. 3 cm/yr.) of the PHP motion. Both the trench-trench-trench (TTT) triple junction and the Japan Trench (JT) should migrate westward, because the thick, cold, and sturdy Pacific Plate (PAP) has never been cut by the transform fault at the TTT junction. Northeast (NE) Japan would also move westward because tectonic erosion along JT would not be sufficient for westward migration of the JT. Thus, the present PHP movement causes the westward migration of IT, TTT junction, JT and then NE Japan. This westward motion of NE Japan against the sturdy oceanic lithosphere of the Japan Sea has caused an E-W contraction of NE Japan since northward motion of PHP changed to NW at 3 Ma.It is expected that rifting of the thin, heated lithosphere of the Izu-Ogasawara Arc would reach break-up before the thick, cold lithosphere of the PAP would be torn by the right-lateral transform fault at the TTT junction. Once rifting reaches break-up, the northwestward movement of the PHP would be compensated by back-arc spreading, and this motion would not propagate to the IT, the JT nor NE Japan. Therefore, the present E-W contraction in Japan would cease in the geologically near future when back-arc rifting along the Izu-Ogasawara arc reaches break-up.McKenzie and Morgan (1969) discussed how the TTT triple junction was unstable except under a few uncommon geometrical and kinematic conditions. However, the PHP actually selected this particular Euler pole position at 15 Ma, and the TTT triple junction had been stable for more than 10 m.y. Although the present TTT junction is in an unstable condition, it would become stable again through back-arc basin spreading of the PHP in the geologically near future. Thus, the TTT triple junction offshore central Japan, which controls tectonics of Japan, would be in a stable state in nature.