Records of Submarine Landslides in Subduction Input Recovered by IODP Expedition 322, Nankai Trough, Japan
Submarine landslides in the active continental margin are the earliest tectonically-driven deformation for sediments underthrusting to the plate boundary. Asymmetric bathymetry across trenches results in the common occurrence of the large scarps related to the mass transport on the steeper accretionary prism surface. Gently tilted oceanic plates off trenches are, however, also sufficient for sliding. Within the framework of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) of the Integrated Ocean Drilling Program (IODP), we drilled two sites (Sites C0011 and C0012) in the subduction input sediments and recovered samples with a specific feature of sliding in a very shallow depth. This feature is composed of fine-grained gouge-like material accompanied by sheared planar fabric. It is typically crosscut by bioturbation, which indicates its formation in a quite shallow depth. In the middle part of Hole C0012A, the bedding tilts significantly compared to the bedding in the hole above and below. This localized inclination indicates block sliding of the sediments. These observations suggest that the surface of the incoming oceanic plate into the Nankai Trough is quite active, as represented by sub-seafloor slidings in different scale. Our observation shows that the input sediments are already deformed before reaching the trench. The fact of the earlier deformation prior to subduction is important for understanding further deformation processes along plate boundaries in subduction zones.
KeywordsD/V Chikyu NanTroSEIZE Submarine landslide Subduction input
We are grateful to the Expedition 322 Scientists and onboard laboratory technicians from Marine Works Japan Co. Ltd. for their hard work and discussions with us during the expedition. We would especially like to thank the co-chiefs, Mike Underwood and Sanny Saito and the Expedition Project Manager, Yusuke Kubo, for their dedication to the success of the expedition and Hajime Naruse for his onboard discussions and suggestions. The shipboard paleomagnetists, Hirokuni Oda, Xixi Zhao and Tomohiro Yamamoto, kindly provided paleomagnetic data for structural reconstruction. This research used samples and data provided by the Integrated Ocean Drilling Program (IODP). This work was partially supported by KAKENHI 19GS0211 and 21107001. We thank Gaku Kimura, Yoshitaka Hashimoto and the guest editor Yasuhiro Yamada for their thoughtful and careful reviews which have led to an improved version of this manuscript.
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