[S-CG55] Ocean Floor Geoscience

[SCG55-10] MOWALL-CIR: Investigation of Marie Celeste Long Oceanic Transform -Preliminary Report of Hakuho-maru KH-24-4 Cruise-

*Kyoko Okino¹, Shiki Machida², Tomoaki Morishita³, Katsuyoshi Michibayashi⁴, Akira Ishikawa⁵, Naoto Hirano⁶, Yibing Li⁷, Masakazu Fujii⁸, Nobukazu Seama⁹, Yuki Minegishi¹, Takuya Yahagi¹, Michinari Sunamura¹⁰, Akiko Makabe¹¹, Scientific Team KH-24-4 (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Chiba Institute of Technology , 3.Kanazwa University , 4.Nagoya University, 5.Institute of Science Tokyo, 6.Tohoku University , 7.Chinese Acaemy of Geological Science, 8.National Institute of Polar Research, 9.Kobe University , 10.The University of Tokyo, 11.Japan Agency for Marine-Earth Science and Technology )

Keywords:mid-ocean ridge process, transform fault, oceanic crust, mantle

We conducted the integrated survey along the Marie Celeste Transform in the Indian Ocean during R/V Hakuho-maru KH-24-4 cruise (from Port Luis on Oct 12, 2024, to Penang on Nov.11, 2024). The cruise was planned as a primary target of the MOWALL (Moho Observation along transform fault WALLs) project. It aims to reveal the temporal variation of oceanic crust formation and its cause. The Marie Celeste Transform (MCTF) is a 210km long oceanic transform that offsets the Central Indian Ridge at 18°S. The estimated seafloor half spreading rate here is approximately 20 mm/yr, so the whole section of oceanic crust from 11Ma to the present is exposed along the transform valley wall. The dense rock sampling and gravity analysis along the Vema Transform in the Atlantic showed the 3-4 m.y. cyclicity in the crustal thickness and degree of melting (e.g., Bonatti+, 2003), though the cause of this variation has not been solved. Recent morphological analysis in off-axis areas also suggests the million-year scale variation of crust formation (Roth+, 2019; Shinevar+, 2019); however, the geochemical interpretation was insufficient due to a lack of systematic rock sampling. The transform fault wall is a unique setting where we can get rock samples of the whole crustal section along a long time-axis, even with the detailed investigation of the alteration along the fault and the fact that the crust was formed at segment ends.

During the KH-24-4 cruise, the surface geophysical mapping, deep-tow and wire-attached magnetic surveys, rock dredges, CTD hydrocasts, and NORPAC plankton sampling were conducted. The principal results of the cruise are summarized as follows.An underway geophysical survey was conducted along 1661 miles in the MCTF area. The additional dataset was collected along the ~mile transit from the area to Penang. We completed the mapping of the whole MCTF and adjacent seafloor with high-quality bathymetry, magnetic, and gravity anomaly data.A 137 km long deep-tow magnetic profile was obtained from the current ridge axis to the eastern off-axis, well-ordered abyssal hill plain. The toing altitude was approximately 200-500 m. We recognized high amplitude and short wavelength variation of magnetic anomaly along the profile that likely reached ~9 Ma, including the change of paleomagnetic intensity.A total of 29 rock dredge hauls were conducted: 27 towing at the MCTF southern wall and two at Aphanasey Nikitin Rise. Along the long MCTF, we operated the dredge at 10 sites every 10 to 40 km apart. At the western three sites, we obtained basalts from the shallow part of the fault wall. One site is the lower part of an oceanic core complex. Eastern sites are located at the steep slope of the well-developed transverse ridge, and we collected mainly gabbro from the deeper slope, then dolerite and basalt from the middle to shallow parts. Many hydrothermally altered basalt samples were also collected. We got only a small number of ultramafic rocks.A newly developed small magnetometer system was attached to the dredge wireline, and we obtained the magnetic field data at eight deep dredge operations. The upslope data during the recovery of the dredge likely reflects the lithology of the oceanic crust.CTD data were recorded at 7 stations, and eDNA samples and samples for microbial and ammonium analyses were collected at 6 of 7 stations. Four stations are located within the transform valley, two at the known hydrothermal vent field along the ridge axis and one north of the transform.NORPAC plankton net sampling was conducted at the same points as the CTD hydrocast. A total of 26 samples were collected. Morphological and DNA-based species identification will be performed.Biological sampling was also tried from sediment and rock surfaces after the rock dredge operation. The collected sediment mainly consists of foraminifera despite the deep depth. At least 15 taxa (49 indiv.) were collected.