背弧海盆における海洋コアコンプレックスの発達：最近のゴジラメガムリオン研究から分かってきたこと

抄録

  The Godzilla Megamullion is the largest known oceanic core complex, located in the Parece Vela Basin, an extinct backarc basin in the Philippine Sea. The previous studies argued that the basin was active from 26 Ma to 12 Ma at an intermediate-spreading rate of 8.8-7.0 cm/year full-rate, although the basin shows the characteristics typical for slower spreading ridges. For example, many peridotites in the Parece Vela Basin are much less depleted than those exposed at comparable spreading rates on other mid-ocean ridge systems. The tectono-magmatic characteristics of the Parece Vela Basin were thus thought unusual and paradoxical.
  However, the recent studies, based on the high-density samplings on the Godzilla Megamullion, show the evidences that the basin became slow to ultraslow environment in its terminal phase. Zircon U-Pb dating of gabbroic rocks from the Godzilla Megamullion reveals that the estimated slip rate of the Godzilla Megamullion detachment fault was ∼ 2.5 cm/y; significantly slower than the previous estimate. The morphology and geology of the termination area are similar to those observed in ultraslow-spreading ridges. Decreasing degree of partial melting of the peridotites as well as increased amount of plagioclase-bearing peridotites (showing melt stagnation in the shallow lithospheric mantle) are observed towards the termination of the Godzilla Megamullion.
  Based on the recent observations at the Godzilla Megamullion, it would be argued that the terminal phase of a backarc basin development will go through an ultraslow-spreading environment, erupting alkaline basalts. There will be an overlap period of the terminal alkaline basalt magmatism and the rifting of a succeeding backarc basin.