Shionomisaki canyon exposes a structural profile across the accretionary complex
in the Nankai trough along steep, ~500-m-high walls that continue along the length
of the canyon. We conducted a total of six dives in SHINKAI 6500 (6K, hereafter)
for this study. Dive 6K#938 (observer: Ogawa) concentrated on structures of the 1st
ridge in the frontal thrust zone. Dive 6K#522 (observer: Anma) was conducted at
the 3rd ridge in the imbricate thrust zone. Four dives (6K#579: Anma, 6K#889:
Anma, 6K#890: Moore, and 6K#891: Ogawa) focused on the megasplay fault zone
exposed along the 5th ridge.
During a dive, observations were made through a porthole window and recorded
using external video cameras. Apparent dip and strike of planar structures such as
bedding plane, cleavage, fault and joint were estimated from video images. The
estimated directions were then corrected to true dip and strike using the yaw and
pitch of the external video camera and the bearing of the submersible. Precision of
the obtained strike and dip was estimated by measuring bedding planes of the same
outcrop from several different angles to be approximately ±10° for moderately dipping
strata (Anma et al. 2010 ).
Bedding planes on the video images were defined as continuous planar objects
with clear evidence of alternating beds with different components or laminae.
Fracture cleavages were recognized as penetrative planar objects oblique to or
bifurcating from the bedding planes. Non-penetrative fractures cutting bedding
planes and/or cleavages were classified as faults if displacement was obvious, and
as joints if not.
Rock samples were collected as often as possible using manipulators of the
submersible for offshore description and needle penetration tests to estimate uni-
axial compressional strength. We used an SH-70 penetrometer (manufactured by
Maruto Co. Ltd.) for the strength measurement. Microscopic observation and
porosity measurements were performed in the laboratories at University of Tsukuba
and JAMSTEC. Porosity and strength are important physical properties of sedi-
ments not only as a measure of consolidation state but also as a mechanical factor
controlling deformation behavior.
Structural Profile of the Nankai Accretionary Complex
Frontal Thrust Zone (Dive 6K#938)
Layers of predominantly mudstones alternating with sandstones comprise the
southern slope of the 1st ridge (Fig. 2 ). Bedding planes (Fig. 2c, d ) are almost hori-
zontal and folded to form an open anticline about an S80E-trending axis (Fig. 3b ).
The presence of cross-laminae (Fig. 2d ) implies flow from the northwest. Joints
trending WNW-ESE and NS are developed in the foot of the ridge (Fig. 2c ).