Erosional Subduction Zone in the Northern Japan Trench: Review of Submersible Dive Reports - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 39

Erosional Subduction Zone
in the Northern Japan Trench:
Review of Submersible Dive Reports

Yujiro Ogawa

Abstract Submersible studies of the oceanward and landward slopes of the northern
Japan trench are reviewed, and the typical erosional features of this subduction zone
are discussed. On the oceanward slope, normal faults with tension cracks have cre-
ated horst and graben structures, forming a series of steps. On the landward slope,
brecciated Miocene sedimentary rocks with calcite veins and cement are exposed
on the thrust-controlled slope surfaces, and landsliding of the rocks and sediments
of the lower landward slope fills the grabens of the oceanward slope. As a result of
this erosion, the trench has advanced, moved landward, at a speed of 5-6 km/m.y.
since the middle Miocene.

Keywords Erosional type subduction zone • Normal fault • Thrust fault • Crack
• Breccias • Calcite cement

1 Introduction

The Nankai subduction boundary around Japan is now considered an accretion-type
margin with typical prism formation (e.g., Moore et al. 2007 ), although until the
late 1960s, the Nankai trough was thought to have a normal faulted boundary
(Hilde et al. 1969 ). The accretionary processes and gravitational collapse along this
boundary have been well elucidated by manned submersible studies in Tenryu and
Shionomisaki canyons, which incise the prism (Kawamura et al. 2009, 2011 ; Anma
et al. 2011 ).
The Japan trench, on the other hand, has been considered a type example of
subduction erosion with consequent trench advance and trench slope subsidence
(von Huene and Lallemand 1990 ; von Huene and Scholl 1991 ; von Huene et al.
2004 ). However, a rather conspicuous thrust-faulted slope with a locally developed
accretionary prism at its toe has been observed (Suyehiro and Nishizawa 1994 ).