Records of submarine landslides in subduction input recovered by iodp expedition 322, nankai trough, Japan

Abstract

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. © Springer Science+Business Media B.V. 2012.