Structural Profile and Development of the Accretionary Complex in the Nankai Trough, Southwest Japan: Results of Submersible Studies - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 171

subducting northwestward beneath the Eurasian (or Amurian) plate at a rate of ~4.5
cm/year (Seno et al. 1993 ), and have been incorporated into an accretionary com-
plex since the late Miocene (Pickering and Taira 1994 ) (Fig. 1a ).
Slip along faults associated with the Nankai trough subduction generated
large earthquakes and tsunamis, which repeatedly devastated the Japanese
coastal areas (Ando 1975 ; Baba et al. 2002, 2006 ; Cummins et al. 2002 ; Ichinose
et al. 2003 ; Hori et al. 2004 ). To better understand the geometries and processes
in the seismogenic zone and to be better prepared for the anticipated seismic
hazard in this region (Le Pichon et al. 1996 ), several geophysical experiments
(Kodaira et al. 2000a, b, 2002 ; Park et al. 2002a, b ; Nakanishi et al. 2002a, b ;
Takahashi et al. 2003 ; Kido and Fujiwara 2004 ) and a complex drilling program
(NanTroSEIZE IODP drilling expeditions; Tobin and Kinoshita 2006 ; Kinoshita
et al. 2009 ) have focused on the accretionary complex in the Nankai trough.
Recent 3D seismic data successfully illustrated details of the distribution and
amplitude of reflectors (Bangs et al. 2004, 2006, 2009 ; Moore et al. 2007 ) and
seismic velocity structure (Nakanishi et al. 2008 ; Park et al. 2010 ). These obser-
vations and surveys then allow the NanTroSEIZE experiment to relate borehole
data directly and more effectively to the large-scale seismic structure (Strasser
et al. 2009 ).
However, the resolution of seismic surveys is too coarse to identify outcrop-
scale structures and their lateral variations. The observations from scientific drill-
ing provided a continuous high-resolution data set on varying structures of ~1
mm to ~1 m scale (Maltman et al. 1993 ; Morgan and Karig 1993 ; Moore et al.
2001 ; Ujiie et al. 2003, 2004 ; Morgan and Ask 2004 ), which are, nevertheless,
restricted to one-dimension along the borehole. Direct and in-situ seafloor sur-
veys using submersibles are the only available method that fills the gap in the
scale of observation.
Previous submersible studies mainly focused on features exposed on the land-
ward slope of the Nankai trough (Le Pichon et al. 1987a, b, 1992, 1996 ; Kobayashi
2002 ; Ashi et al. 2002a ). In this study, we focus on a submarine canyon that cuts
through the accretionary complex. Submarine canyons, analogous to gorges and
valleys favored by field geologists for geological mapping on-land, provide an
excellent opportunity for 3-dimensional seafloor observations and mapping along
deeply incised profiles (Kawamura et al. 1999, 2009 ; Anma et al. 2002 ).
Submarine canyons also present an additional advantage in the case of the Nankai
trough: the canyons eroded down to the distribution limit of gas hydrates (e.g.
Ashi et al. 2002b ; Colwell et al. 2004 ), and hence data undisturbed by hydrates
were collected.
We conducted a series of seafloor studies along the Shionomisaki canyon
(Fig. 1a ), off Kii peninsula, using the submersible SHINKAI 6500 operated by the
Japan Agency for Marine-Earth Science and Technology (JAMSTEC), with an aim
to understand the distribution of outcrop-scale structures and physical properties,
and their relation to large-scale topographic features of the accretionary complex in
the Nankai trough. SeaBeam bathymetric data and side-scan sonar images were
used to select exact sites for the submersible study.