2:15 PM - 2:30 PM
[SCG48-13] Submersible survey along the Shionomisaki submarine canyon in the Nankai Trough: Preliminary results of R/V Yokosuka YK23-10S cruis
Keywords:Shinkai 6500, Nankai Trough, accretionary prism
The Nankai Trough is a subduction zone where an accretionary complex is typically developed. The macroscopic features of the forearc region of the Nankai Trough are essential for understanding subduction zone seismogenesis and geodynamics. Based on the results of recent deep-sea drilling and reflection surveys in the Nankai Trough, we conducted seafloor geological surveys and sampling by using the submersible Shinkai 6500, during the R/V Yokosuka YK23-10S cruise (June 15-26, 2023). This presentation reports on the preliminary results of the cruise.
During the YK23-10S cruise, dive surveys using the "Shinkai 6500" were conducted along the Shionomisaki Canyon off the Kii Peninsula, collecting samples of the Nankai accretionary prism and forearc basin sediments, as well as surface sediment and cold seep samples using a push corer, and conducting geophysical surveys such as seafloor topography, geomagnetic observations, and heat flow measurements. During four dives along the cliff of the Shionomisaki Canyon (Dive 6K#1699, 1700, 1702, 1703), mudstones to constitute the accretionary prism, conglomerates to form the slope basin or the lowermost part of the forearc basin, and interbedded sandstone and mudstone of forearc basin sediments were observed, and a total of 104 samples (195 kg) of rock were collected. Additionally, the boundary outcrop between conglomerates and interbedded sandstone and mudstone was confirmed in all four dives. Microfossil analysis of the samples revealed ages from the CN9 zone (8.29–5.53 Ma) for conglomerates and from the early to middle Pleistocene (0.91-0.43 Ma) for the base of the forearc basin sediments. Future studies are expected to update the developmental history of the Nankai accretionary prism and forearc basin through combined age dating using planktonic foraminifera fossils.
During one dive (Dive 6K#1701) crossing the submarine canyon, integrated surveys of the subbottom structure using a subbottom profiler attached the "Shinkai 6500" and seafloor topography using the "Yokosuka" were conducted, providing insights into the development process of the submarine canyon and sediment transport. Tsuji et al. (2013, Tectonophysics) pointed out the presence of a high-density dome-shaped rock body directly below the Shionomisaki Canyon from reflection seismic surveys. This dome-shaped rock body is assumed to be a mid-Miocene igneous complex, which may also affect the rupture zone of the Tonankai and Nankai earthquakes (Kimura et al., 2022, G-cubed). During this cruise, heat flow measurements using the Stand Alon Heat Flow meter (SAHF) boarded the "Shinkai 6500" were conducted at nine points. Future studies will comprehensively examine the presence of igneous rock bodies based on heat flow measurement results and wide-area geomagnetic observations using the three-component magnetometer and proton magnetometer onboard the "Yokosuka".
The cruise was fortunate with favorable sea conditions, allowing all planned five dives to be completed. It is expected that advancements in post-cruise research will enable the material science-based elucidation of the early developmental history of the Nankai accretionary prism and Kumano Basin, as well as the reality of segment boundaries. The extensive seabed survey and large-scale sample collection by the "Shinkai 6500" serve as a powerful tool bridging across scales with geophysical observations such as reflection seismic surveys and one-dimensional deep-sea drilling.
During the YK23-10S cruise, dive surveys using the "Shinkai 6500" were conducted along the Shionomisaki Canyon off the Kii Peninsula, collecting samples of the Nankai accretionary prism and forearc basin sediments, as well as surface sediment and cold seep samples using a push corer, and conducting geophysical surveys such as seafloor topography, geomagnetic observations, and heat flow measurements. During four dives along the cliff of the Shionomisaki Canyon (Dive 6K#1699, 1700, 1702, 1703), mudstones to constitute the accretionary prism, conglomerates to form the slope basin or the lowermost part of the forearc basin, and interbedded sandstone and mudstone of forearc basin sediments were observed, and a total of 104 samples (195 kg) of rock were collected. Additionally, the boundary outcrop between conglomerates and interbedded sandstone and mudstone was confirmed in all four dives. Microfossil analysis of the samples revealed ages from the CN9 zone (8.29–5.53 Ma) for conglomerates and from the early to middle Pleistocene (0.91-0.43 Ma) for the base of the forearc basin sediments. Future studies are expected to update the developmental history of the Nankai accretionary prism and forearc basin through combined age dating using planktonic foraminifera fossils.
During one dive (Dive 6K#1701) crossing the submarine canyon, integrated surveys of the subbottom structure using a subbottom profiler attached the "Shinkai 6500" and seafloor topography using the "Yokosuka" were conducted, providing insights into the development process of the submarine canyon and sediment transport. Tsuji et al. (2013, Tectonophysics) pointed out the presence of a high-density dome-shaped rock body directly below the Shionomisaki Canyon from reflection seismic surveys. This dome-shaped rock body is assumed to be a mid-Miocene igneous complex, which may also affect the rupture zone of the Tonankai and Nankai earthquakes (Kimura et al., 2022, G-cubed). During this cruise, heat flow measurements using the Stand Alon Heat Flow meter (SAHF) boarded the "Shinkai 6500" were conducted at nine points. Future studies will comprehensively examine the presence of igneous rock bodies based on heat flow measurement results and wide-area geomagnetic observations using the three-component magnetometer and proton magnetometer onboard the "Yokosuka".
The cruise was fortunate with favorable sea conditions, allowing all planned five dives to be completed. It is expected that advancements in post-cruise research will enable the material science-based elucidation of the early developmental history of the Nankai accretionary prism and Kumano Basin, as well as the reality of segment boundaries. The extensive seabed survey and large-scale sample collection by the "Shinkai 6500" serve as a powerful tool bridging across scales with geophysical observations such as reflection seismic surveys and one-dimensional deep-sea drilling.