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 170

accretionary complex. Turbiditic sediments in the frontal thrust zone have been
deformed to form an open anticline. Minor normal faults striking parallel to the
trough axis developed in the crest of the anticline. As sediments were transferred
into the imbricate thrust zone, both hill-size open anticlines and outcrop-scale open
to tight folds developed in the turbiditic sequence. We also observed brittle thrust
faults spatially associated with tight folds and development of shear fabrics such
as oblique cleavages and en-echelon mineral veins. An out-of-sequence thrust,
known as the megasplay fault in this region, then developed in the frontal part of
the landward-most ridge composed of thick turbidite sequences. Sandstones just
above the megasplay fault are cemented by carbonates. Behind the cemented zone,
bifurcations of the megasplay fault are distributed in the sandstone-rich strata dip-
ping steeply to the south. Folds with wavelength of ~200 m developed in mudstone-
rich turbidites behind the bifurcating fault zone. The cementation strengthened the
frontal part of the megasplay fault zone, which in turn, acted as an indenter for the
inner part of the accretionary wedge. The cemented and hardened ridge must have
local controls on strain localization and development of the accretionary complex.

Keywords Accretionary complex • Splay fault • Fold • Strain • Cement • Porosity
• Strength • Nankai trough

1 Introduction

Our knowledge of the geometry, structure and evolutionary history of accretionary
complexes has expanded recently through advancing technologies of geophysical
surveys, scientific drilling, and in-situ ocean bottom observations through submers-
ible diving. Amongst modern accretionary wedges of the world, the accretionary
complex in the Nankai trough, off Southwest Japan, is the most intensively studied.
Here, sediments deposited in the Shikoku Basin were scraped off the downgoing
Philippine Sea plate (Aoki et al. 1982 ; Kato et al. 1983 ; Leggett et al. 1985 ; Moore
et al. 1990, 2001 ; Ashi and Taira 1992 ; Taira et al. 1992 ; Kuramoto et al. 2000 )

Y. Michiguchi
Japan Nuclear Energy Safety Organization, Toranomon 4-3-20, Tokyo 105-0001, Japan
e-mail: [e-mail]
R. Endo
INPEX Corporation, Akasaka Biz Tower 5-3-1 Akasaka, Minato-ku, Tokyo 107-6332, Japan
e-mail: [e-mail]
S. Akaiwa
Graduate School of Life and Environmental Sciences, University of Tsukuba, Ten-nodai 1-1-1,
Tsukuba, Ibaraki 305-8572, Japan
e-mail: [e-mail]
S. Hirano
Marine Works Japan, Showa-machi 3175-25, Kanazawa, Yokohama 236-0001, Japan
e-mail: [e-mail]