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 186

pelagic cements, but plots on a trend of fault-related fluid. Thus, CaCO 3 was
precipitated near the surface, in the shallow part beneath the sea floor. There was
no influence of the hydrate-related fluids.

5 Discussion

5.1 Faults in the Accretionary Complex of the Nankai Trough

The profile along the mid-axis of the canyon (Fig. 1c ) indicates the presence of
several transverse ridges that involve dip-slip displacement. A NNE-SSW-trending
outcrop of siltstone is present at the leeward slope of the transverse ridge
(Fig. 5b ) at the 5th ridge. This implies that the transverse ridge is not of sedimen-
tary origin (i.e. landslide debris), but of tectonic (faulting or folding) origin. The
transverse ridge is located landward compared to the knick along the foot of the
5th ridge. This suggests the presence of a landward-dipping thrust fault.
Radiolarian microfossils indicate that the ages of deposition become younger
toward both directions from the 891R5 outcrop in the rear part of the 5th ridge
(Fig. 7a, b ). Therefore, the center of uplift is inferred to be near the 891R5 out-
crop. The northern boundary fault must be, therefore, a north-side down-thrown
fault, most likely an antithetic thrust fault (Fig. 7a ). Offset relationship of the
knicks along the northern edges of the 5th ridge and the transverse ridge (Fig. 5a )
also support a fault dipping to the south.
The mid-axis of the canyon is deflected westward south of the 5th ridge: this
feature indicates that the faults have a right-lateral strike-slip component (Figs. 1b
and 5a ). The eastern canyon wall of the 5th ridge is displaced along the bifurcating
faults and the ridge crest is offset dextrally (Figs. 5a and 7a ). Martin et al. ( 2010 )
reported the presence of negative flower structures associated with right-lateral
displacement above the megasplay fault in the NanTroSEIZE experiment area
(around 136 o 40'E). In contrast, our observations indicate the presence of a positive
flower structure along the megasplay fault zone. Variations in the stress-field shift-
ing from transtension to transpression may be expected along the megasplay fault.
The SSE-trending gorges in the 3rd ridge and north of the 5th ridge (Fig. 1a ) are
normal to the general trend of the ridges. However, this trend is not continuous
throughout the length of the canyon, but seems displaced by right-lateral faults
parallel to the ridges. Furthermore, the ENE-WSW-trending mound developed in
the fan deposit in front of the 1st ridge deflected the stream from the canyon
(Figs. 1b and 3 ). The trend of the mound parallels the general trend of the 1st ridge.
This mound could be a hanging wall anticline above the frontal thrust rising in front
of the canyon and is still active. The mound may also have developed as a pressure
ridge as a result of transpressional tectonic deformation.
Our observations raise a possibility that all these faults may have played a sig-
nificant role in accommodating the right-lateral strike-slip component caused by