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 176

Weakly- to moderately-consolidated silty clays (938R-001: Fig. 2b ) in an outcrop
at the base of the southern slope have strength and porosity of ~7 MPa and ~56%
(Fig. 2a ), respectively. A buried shell of vesicomyid bivalve was found at the base
of the 1st ridge (Fig. 2b ).
Light-colored and dark-colored sands with ripple marks of various directions
(Fig. 2e ) are distributed on a hump extending southeast from the ridge crest
(Fig. 2a ). Behind the hump, conglomerates and sandstones (Fig. 2f ) are cemented
by calcite and have a high strength (~10 MPa for silty sandstone) and low porosity
(~20%, Fig. 2a ).
Near the crest of the 1st ridge, both thrust and normal faults were observed in
weak (~0.8 MPa) and porous (~50%) mudstone-dominant beds. The thrust fault in
Fig. 2g was recognized as a cohesive fault with thin white fillings and displacing
the light-colored bed in the bottom-right. The thrust fault dips ~30° to the north
(Fig. 3b ). Normal faults (Fig. 2h ) were observed near the crest of the 1st ridge, in
the hanging wall of the thrust. The normal faults strike parallel to the ridge axis and
dip steeply to both directions (Fig. 3b ). No crosscutting relations were observed
between the thrust and normal faults. No evidence for active cold seepages was
observed in this area.

4.2 Imbricate Thrust Zone (Dive 6K#522)

Moderately consolidated (~2-9 MPa) mudstone-dominant beds comprise the west-
ern wall of Shionomisaki canyon in the 3rd ridge (Fig. 4 ). Numerous debris flow
deposits were observed in eastward-oriented gullies (Fig. 4a ; Anma et al. 2002 ).
Bedding planes dip to the south in the southern part of the 3rd ridge, whereas they
dip steeply to the north in the middle part (Fig. 4f, g ). The dip angles become gen-
tler (Fig. 4c ) in the upper part of the slope in the north (Fig. 4a ). Macroscopically,
the bedding planes form an open to tight anticline with km-scale (a hill-size) wave-
length about an axis plunging gently to ENE (Fig. 3c ). Layer-parallel foliations
developed in places (Fig. 4h ). Both NNE-SSW and EW-trending joints are
present.
A 10-m-high outcrop in Fig. 4i exhibits a spectacular view of a brittle thrust and
a tight anticline inclined to the south. Bedding planes at the base, dipping moder-
ately to the north, steepened upward and are truncated by the thrust fault just above
the hinge of the anticline. The bedding planes become shallower toward the top of
the outcrop. Such outcrop-scale folds were observed in several places (e.g.,
Fig. 4d ). The bedding-cleavage relationship in Fig. 4f indicates a top-to-south
sense-of-shear. En-echelon mineral veins dipping gently to the northwest (Fig. 4e )
also document the development of shear fabrics in the accretionary wedge. The
northernmost part of the 3rd ridge was cut by a vertical fault with the north-side
down-thrown (Fig. 4b ).