[S06P] PM-P

[S06P-12] High-resolution seismic reflection images of the Shatsky Rise, northwest Pacific

*Jin-Oh Park¹, Yue Sun¹, Yu Fan¹, Noboru Yamakawa¹, Yuqi Lyu¹, Ryota Murai¹, Ryoji Toda¹, Yoshiki Horiuchi², Kazutaka Yasukawa³, Junichiro Kuroda¹, Erika Tanaka⁴, KH-24-1 Shipboard Science Party⁵ (1. AORI, University of Tokyo, 2. Nippon Marine Enterprises, 3. School of Engineering, University of Tokyo, 4. Marine Core Research Institute, Kochi University, 5. N/A)

Shatsky Rise is one of the large igneous provinces (LIPs) on Earth that can provide information on mantle processes and composition. As an oceanic plateau, it is known to have formed in the Late Jurassic/Early Cretaceous between 147 and 135 Ma, providing a valuable record of Pacific paleoceanography. It consists of three broad bathymetric highs, the Tamu, Ori and Shirshov massifs. Around the Shatsky Rise, nutrient supply has been attributed to surface currents (i.e. Kuroshio Extension and Oyashio Current) and intensified westerly winds, accompanied by a supply of eolian dust. Recently, the nutrient supply system influenced by turbulence and upwelling has been re-evaluated in the Kuroshio area and in the oligotrophic area south of the Shatsky Rise, which is generated by bottom water current and "topographic high". However, observations have not confirmed that the dynamic topography acts as a physical barrier to deliver nutrients from the deeper to the shallower depths and to influence the composition of the sediments. On the southwestern flank of the Shatsky Rise, a sedimentary hiatus between the Pliocene and Cretaceous is confirmed, suggesting the existence of strong bottom currents in this area.During the R/V Hakuho-maru cruise (2-19 April 2024), we conducted a multichannel seismic (MCS) reflection survey to obtain high-resolution images of the shallow crustal structure in the southwestern flank of the Tamu Massif, Shatsky Rise. This was a revisit with a more advanced MCS system since the R/V Thomas G. Thompson cruise in 1994. We acquired good quality MCS data on 16 survey lines totalling ~688 km. We used one or two GI guns (~12 liters) with a 25 m shot interval to obtain high resolution MCS data. We recorded MCS data with a 10-sec recording length and 1 or 2 msec sampling interval using a 1,200 m long 48-channel streamer with 25 m group spacing, resulting in a standard CDP fold number of 24. Navigation data by Trigger Fish 2D was stored in UKOOA P1/90 and P2/91 formats. Quality control of the MCS data was performed using the onboard RadExPro seismic processing software. After applying UKOOA P1/90 navigation data to the MCS data for geometry setting, we performed velocity analysis using pre-conditioned CMP (common midpoint) gathers for quick normal moveout correction and performed CMP stacking to produce a conventional CMP stacked section. We also performed post-stack time migration using the CMP stacked data.We have carried out a lithostratigraphic interpretation of the MCS profiles in the Tamu Massif. Based on seismic reflection characteristics and results from Ocean Drilling Program Leg 198 Sites 1213 and 1214, we identify four specific seismic reflection units: Units A, B, C and D from top to bottom. Unit A, with Neogene nannofossil ooze and clay, shows transparent, continuous, low amplitude reflections. Unit B, composed mainly of Cretaceous chert and porcellanite, shows chaotic reflections. Unit C, composed mainly of Cretaceous chert, porcellanite and chalk, is characterized by transparent, partly continuous, high amplitude reflections. Unit D, an acoustic basement, is interpreted as Early Cretaceous to Late Jurassic basalt. In this talk we will present the preliminary results of the MCS survey.