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Framework of the Accretionary Complex in the Nankai
Trough and the Shionomisaki Submarine Canyon
The regional bathymetric map of the Nankai trough (Fig. 1a ) shows that the accretionary
complex is characterized by a series of ENE-WSW-trending culminations (ridges)
and depressions stepping up toward the Japanese mainland in the north. Sediments
deposited in the forearc basins, namely Kumano and Muroto basins, cover the northern
part of the accretionary complex. The Nankai trough itself is characterized by a
narrow, WSW-trending axial channel deepening to the west. The upstream (eastern)
extension of the axial channel is connected to the Tenryu submarine canyon and the
Suruga trough. ENE-WSW-trending knolls that continue through the Zenisu ridge to
the Izu arc in the east are distributed south of the axial channel. Crustal-scale faults
were seismically observed along the southern and western margins of the knolls
(Kodaira et al. 2006 ).
The Shionomisaki submarine canyon located nearby is the epicenter of the 1944
Tonankai and 1946 Nankai earthquakes (Baba and Cummins 2005 ) (stars in Fig. 1a ).
An out-of-sequence thrust, referred to as the 'megasplay fault' hereafter, and numer-
ous in-sequence thrust sheets were imaged by seismic studies (Park et al. 2002a )
along transects drilled by D/V Chikyu (open circles) during the NanTroSEIZE expe-
ditions. The megasplay fault is likely the up-dip expression of the seismogenic fault
that generated large tsunamis (Park et al. 2002a ; Nakanishi et al. 2002a ; Moore et al.
2007 ). Cold seepages marked by the distribution of chemosynthetic bio-communi-
ties (stars in Fig. 1a ) have been reported along the fault (Ashi et al. 2002a ).
Shionomisaki canyon is the result of deep erosion of the accretionary complex
and crosscuts five EW-trending bathymetric ridges (Fig. 1b ). The extension of the
megasplay fault is inferred to occur at the landward-most ridge (the 5th ridge in
Fig. 1b ). We subdivided the accretionary complex in the central Nankai trough into
three structural zones based on bathymetric features. The megasplay fault zone
marks the seaward limit of the forearc basin, and separates the outer wedge from
the inner wedge (Kimura et al. 2007 ). The frontal thrust zone is the outer margin of
the accretionary complex that separates it from the Shikoku basin. The imbricate
thrust zone comprises the main body of the accretionary prism and is recognized as
a zone of landward-dipping thrust sheets and folds (Kawamura et al. 2009 ).
Sediments transported through Shionomisaki canyon form a large fan deposit in
front of the 1st ridge. The fan deposit buried and deflected the axial channel of the
Nankai trough (Fig. 1b ). The southward trend of the stream from Shionomisaki
canyon is deflected to the WSW after intersecting an ENE-WSW-trending mound
developed in the fan deposit (Fig. 1b , also see exaggerated oblique view in Fig. 3 ).
A profile along the mid-axis of the canyon (Fig. 1c ) revealed the presence of several
transverse ridges that transect the mid-axis of the canyon. In addition, the mid-axis
is deflected toward the west south of the 5th ridge (Fig. 1b ). SSE-trending gorges
in the 3rd ridge and north of the 5th ridge are normal to the general trend of the
ridges. The SSE-trend steps westward downstream of the canyon. Such topographic
features are likely formed due to tectonic deformation that is still active.
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