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 188

Inner wedge Outer wedge

Transition zone

Megasplay fault zone Imbricate thrust zone Frontal

thrust zone

C B, B' A

0 m

5th ridge

3rd ridge

-1000 m
-2000 m
-3000 m
-4000 m

6K#579, 889-891

1st ridge

6K#522

6K#938

4th ridge

2nd ridge

Erosion level
along the canyon

-5000 m

Decollement Nankai trough

5 km

Fig. 10 Geologic cross section along the Shionomisaki canyon. See trace in Fig. 1b . General
trend of bedding planes ( thin lines ) and faults ( thick lines : observed, with arrows indicating sense-
of-shear, dashed lines : deduced) are shown. Shaded area : cemented zones

to form a gentle anticline (Fig. 10 ). The anticline and thrust faults most likely formed
synchronously due to north-oriented compressional stress during the inception of
accretion. Although no crosscutting relations were observed between thrust and
normal faults, or folds and normal faults, we attribute the normal faults to surface
extension at the crest of the anticline (Fig. 10 ). The sediments in the frontal thrust
zone were partly cemented by carbonates, and a fossil of a vesicomyid bivalve was
recovered. Although it is currently inactive, fluid saturated in CaCO 3 must have
been circulating and hardened some parts of the ridge. Occurrences of cold seepage
were also reported from the eastern extension of the 1st ridge (Fig. 1a ).
As sediments transferred into the imbricate thrust zone, both macroscopic and
mesoscopic (outcrop-scale) open to tight folds were developed. Thrust faults are
more brittle than those developed in the frontal thrust zone and localized shear
fabrics have developed in the sedimentary rocks. The transverse ridge in the 3rd
ridge has the largest displacement among the Shionomisaki canyon (Fig. 1c ),
implying a large cumulative dip-slip displacement. No active cold seepage was
observed, however, in the 6K#522 dive area.
The megasplay fault zone, in contrast, has several active cold seeps. Vesicomyid
bivalves are commonly aligned forming an en-echelon fashion (Fig. 6e ), suggest-
ing shear fracture underneath (Ogawa et al. 1996 ). Nakanishi et al. ( 2002c, 2008 )
suggested that the presence of an old (Miocene) accretionary complex coincides
with the seaward limit of coseismic slip in the Nankai trough seismogenic zone.
Such brittle, coseismic rupturing may control the distribution of cold seeps. Our
overall structural interpretation of the megasplay fault zone is illustrated in the
schematic profiles in Figs. 7 and 10 . Sub-horizontal sandstones just above the
megasplay fault were cemented by carbonates. Behind the cemented sandstones,
bifurcating faults with cold seeps are present in the sandstone-rich strata dipping
steeply to the south in the frontal part of the anticline with a wavelength of
~1 km. The sandstones are partly cemented above the bifurcating faults, harden-
ing the frontal part of the anticline. Folds with wavelengths of ~200 m are devel-
oped in mudstone-rich turbidites in the rear part of the 5th ridge behind the
bifurcating faults.