[S-CG42] Hard-Rock Drilling Science: Continental and Deep-Sea Drilling, and Ophiolite

[SCG42-P06] Microstructural characteristics of peridotites in the Tosa Megamullion, the Shikoku Basin, 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, IMG, JAMSTEC, 3.Research Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 4.Hydrographic and Oceanographic Department of Japan)

In the slow-spreading axes of the mid-ocean ridge and back-arc basin spreading axes, dome-like topography, called megamullion, exists sporadically on the seafloor, where fault rocks are developed in the lower crustal to upper mantle materials. These fault rocks originated from the ductile shear zone in the deep extension of the detachment fault, forming a megamullion. Several megamullions were discovered in the Shikoku Basin, Philippine Sea^[1]. This study investigated the microstructural characteristics of ultramafic rocks obtained from the Tosa Megamullion, which formed during the early stage of back-arc spreading of the Shikoku Basin. We analyzed nine ultramafic rocks collected by the submersible research vessel Shinkai 6500 on the research cruise YK23-05S. Polished slabs and thin sections were made in a plane perpendicular to foliation and parallel to lineation (XZ plane). Although all the samples were remarkably serpentinized, some retained their primary peridotite textures consisting of olivine, orthopyroxene, clinopyroxene, plagioclase, and spinel. Seven of the nine samples showed distinct foliation and porphyroclastic textures represented by orthopyroxene porphyroclasts. The olivine grains were ca. 0.3 mm in size, whereas the orthopyroxene grains were ca. 1 mm in size. The orthopyroxene porphyroclasts were characterized by rounded (2–7 mm) and elongated shapes (several centimeters). In addition, the elongated orthopyroxene porphyroclasts showed microkinks and undulose extinction. The clinopyroxenes were fine-grained (ca. 0.3 mm) and had polygonal in shape. In this study, we analyzed the crystallographic preferred orientations (CPO) of olivine in three samples using scanning electron microscope equipped with electron backscattered diffraction. They showed a sort of E-type CPO representing (001)[100] slip. Based on these results, the ultramafic rocks were classified as mylonitic peridotites. Therefore, it is suggested that mantle peridotite below the spreading axis of the Shikoku Basin was transformed into mylonitic rocks because of ductile shearing associated with the detachment fault during the early stage of back-arc spreading. Subsequently, such mylonitic rocks would be exposed on the seafloor due to uplift to form the Tosa Megamullion.

[1] Okino et al. 2023 Progress in Earth and Planetary Science 10, https://doi.org/10.1186/s40645-023-00570-2.