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 187

the obliquity of the plate migration (Fig. 1a ). Collectively, these findings from the
accretionary complex of the Nankai trough point out that strain partitioning did
occur, perhaps more pervasively and in a more multi-scale fashion than Fitch ( 1972 )
suggested for oblique convergent zones in the Western Pacific. Focal mechanisms
of very low frequency earthquakes (Ito and Obara 2006 ) showed the occurrence of
thrust motion with minor oblique slip component along the megasplay thrust zone.
However, no focal mechanism that indicate oblique thrust were reported along the
frontal thrust zone.

5.2 Tectonic Deformation vs. Creeping

Bedding in the frontal thrust zone is sub-horizontal and gently folded about an
EW-trending horizontal axis. Bedding observed on the western canyon wall of the
3rd ridge is folded about an axis plunging to the east, and bedding on the eastern
canyon wall of the 5th ridge about an axis plunging to the west (Fig. 3 ). The fold
axes plunging toward the canyon imply an overprinted deformation. There are
two possible explanations for the plunging axes: one is tectonic and the other
gravitational.
A hump between faults  and  extends westward from the crest of the 5th
ridge, indicating that faults  and  apparently have opposite sense of shear
(Figs. 5a and 7a ). The fold axis in this area plunges 40° to west (Fig. 5d ). Such local
features may be explained in terms of creep deformation enhanced by lateral extru-
sion. The canyon walls may become unstable as the Shionomisaki canyon deepened
and eroded the foot of the canyon walls. Creep of the wall slope must have taken
place to tilt the originally horizontal fold axes toward the canyon. Right-lateral
transpression may also squeeze materials in the 5th ridge to extrude them westward.
NS-trending joints (Fig. 3d ) developed in the folded turbidite sequence in the 5th
ridge may support the creep model. Fractures trending WNW-ESE, in contrast, may
be attributed to tectonic stresses: the NW-SE-trending maximum principal com-
pressional stress deduced from borehole breakout during the NanTroSIEZE experi-
ments (Kinoshita et al. 2009 ).
Alternatively, a crustal-scale fault that displaces the Philippine Sea plate at the
western edge of the Zenisu ridge was reported by Kodaira et al. ( 2006 ) to have
influence on structures in the accretionary complex along the Shionomisaki canyon.
Lateral ramps in the megasplay fault zone (Moore et al. 2007 ) may also be respon-
sible for the plunging fold axes.

5.3 Development of the Accretionary Complex

Field observations reveal that both normal and thrust faults striking parallel to the
Nankai trough were developed in the frontal thrust zone as sediments were accreted