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 192

Accreted strata are, in general, transported landward as subduction proceeds.
Sediments deformed at the frontal thrust zone must then be transported to the imbri-
cate thrust zone and eventually undergo deformation in out-of-sequence thrust
zones as lateral transportation and sediment compaction continued. Such a unifor-
mitarian view may be modified by the presence of cementation that locally disturbs
the regional patterns of strain distribution.

6 Summary

We made field observations using the submersible “ SHINKAI 6500” in the frontal
thrust zone, imbricate thrust zone and megasplay fault zone along the profile of the
accretionary complex of the Nankai trough exposed in the Shionomisaki submarine
canyon, aiming to understand distribution of outcrop-scale structures and strain,
and eventually to understand deformation processes.
Brittle normal faults and cohesive thrust faults striking parallel to the trench axis
were developed in the frontal thrust zone during gentle folding as sediments entered
into the accretion zone. The sediments in the frontal thrust zone are partly cemented
by carbonates. As sediments were transferred to the imbricate thrust zone, both
macroscopic and mesoscopic (outcrop-scale) open to tight folds were developed.
Thrust faults became brittle and shear fabrics were developed. No active cold seep-
age was, however, observed in these areas.
The megasplay fault zone, in contrast, contains active cold seeps. Sub-horizontal
sandstones just above the megasplay fault were cemented by carbonates. Behind
the cemented zone, bifurcations of the megasplay fault, marked by cold seeps,
developed in the sandstone-rich turbidite beds that dip steeply to the south. Folds
with wavelengths of ~200 m developed in mudstone-rich turbidites behind the
bifurcating faults. The cementation strengthened the frontal part of the megasplay
fault zone, which in turn, acted as an indenter for the inner wedge. Cementation and
hardening of the ridge front have local controls on deformation and development of
the accretionary complex. Mapping of hardened zone may help to detect potential
zones of seismogenic and tsunamigenic rupturing.

Acknowledgements Our thanks go to the captain and crew of the R/V Yokosuka , and the com-
mander and pilots of the “ SHINKAI 6500 ” operation team. Our thanks also go to the Shipboard
Science Parties of YK99-09, YK00-08, YK05-08 and YK06-02 cruises for their assistance and
discussion. We appreciate the support of JAMSTEC during this study. We thank to Prof. Timothy
Kusky (China University of Geosciences) and Dr. Andrea Festa (University of Torino) for their
critical reviews and constructive comments.

References

Ando M (1975) Source mechanisms and tectonic significance of historical earthquakes along
Nankai trough, Japan. Tectonophysics 27:119-140