[S-CG51] Hard-Rock Drilling Science: Continental and Deep-Sea Drilling, and Ophiolite

[SCG51-04] Tosa Megamullion formed during the early stage of back-arc spreading in the Shikoku Basin, the Philippine Sea

*So Inoue¹, Katsuyoshi Michibayashi^1,2, Yumiko Harigane³, Yasuhiko Ohara^1,2,4 (1.Department of Earth and Planetary Sciences, GSES, Nagoya University, 2.Volcanoes and Earth's Interior Research Center, IMG, JAMSTEC, 3.Research Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 4.Hydrographic and Oceanographic Department of Japan)

Keywords:Tosa Megamullion, Shikoku Basin, Philippine Sea, peridotite, mylonite, back-arc spreading

Several megamullions have been discovered in the Shikoku Basin, the Philippine Sea. This study investigated the microstructural characteristics of ultramafic rocks obtained from the Tosa Megamullion, which formed during the early stage of back-arc spreading in the Shikoku Basin. We analyzed nine ultramafic rocks collected by the submersible research vessel Shinkai 6500 on the research cruise YK23-05S. Seven of the nine samples exhibited distinct foliation and porphyroclastic textures represented by orthopyroxene porphyroclasts. Olivine showed a sort of E-type CPO representing (001)[100] slip. Based on these results, the ultramafic rocks were classified as mylonitic peridotite. Therefore, it is suggested that mantle peridotite below the spreading axis of the Shikoku Basin was transformed into mylonitic rocks because of the ductile shearing associated with the detachment fault during the early stage of back-arc spreading. Subsequently, such mylonitic rocks were exposed on the seafloor due to uplift to form the Tosa Megamullion.