[S-CG51] ハードロック掘削科学：陸上・深海底掘削、そしてオフィオライト

[SCG51-04] フィリピン海四国海盆背弧拡大初期に形成された土佐メガムリオン

*井上 創¹、道林 克禎^1,2、針金 由美子³、小原 泰彦^1,2,4 (1.名古屋大学大学院 環境学研究科（岩鉱）、2.海洋研究開発機構 海域地震火山部門 火山・地球内部研究センター、3.産業技術総合研究所 地質情報研究部門、4.海上保安庁 海洋情報部)

キーワード：土佐メガムリオン、四国海盆、フィリピン海、カンラン岩、マイロナイト、背弧拡大

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.