Tectonostratigraphy of slope basin in the Nankai Accretionary wedge off-Muroto

Abstract:

The splay faults branching from the plate boundary fault are thought to be one of the fundamental processes of the accretionary wedge growth. Previous studies inferred that coseismic slip along splay faults generate tsunamis. Thus, a precise understanding of the activity of splay faults is important to discuss not only the wedge growth mechanism but tsunami generation. To reconstruct activity of the splay faults, we focus on the subsurface structures of a slope basin located at hanging wall of the splay fault in the Nankai accretionary prism at off-Muroto. In this study, we interpreted the 3-D seismic volume (Bangs et al., 2004) and the high-resolution cross section image, which was newly obtained by deep towed subbottom profiler. And more, we reanalyzed the borehole core samples from Sites 1175 and 1176 (ODP Leg 190) in order to obtain lithology, structure and sedimentary age. As a result of seismic interpretation, we divide three structural units A, B, and C from top to bottom, and two splay faults. Unit A is characterized by tilted reflectors which are getting steep with depth. The age of Units A and B boundary is ~0.8 Ma (Moore et al., 2001). The tilting had started ~0.8 Ma due to the activity of splay fault at shallower depth. The high-resolution cross section obtained by subbottom profiler combined with tephra age results from Site 1175 core indicates that activities of minor faults at the southern edge of slope basin, subsidiary formed by splay fault at deeper depth, decreased at ~105 ka, and stopped at ~28 ka. By considering these results, the splay fault was activated since ~0.8 Ma, and its activity was gradually decreased at ~105 ka and finally stopped by ~28 ka. Given that the paleo-position of the subducted seamount currently located at ~20 km north of the basin, tilting within Unit A was possibly formed by splay faulting and high sedimentation rate both caused by seamount subduction. In the slope basin, supplement with the activity of the splay fault at off-Muroto, some minor faults cut through the seafloor. Continuous minor faulting even after 28 ka could be caused by the internal deformation of accretionary prism in response to slips along the plate boundary.