Dark Bands in the Submarine Nankai Accretionary Prism – Comparisons with Miocene–Pliocene Onshore Examples from Boso Peninsula - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 229

Dark Bands in the Submarine Nankai
Accretionary Prism - Comparisons
with Miocene-Pliocene Onshore Examples
from Boso Peninsula

Yoko Michiguchi and Yujiro Ogawa

Abstract Unique planar structures made of thin, dark bands in host siltstones were
described at microscopic scale in samples collected from the Tenryu Canyon in the
eastern Nankai accretionary prism. Their mechanisms of formation and tectonic
relationships were compared with similar onshore examples from the unmetamor-
phosed Miocene-Pliocene Miura-Boso accretionary prism. Four modes of occur-
rences that had been identified previously in onshore rocks were recognized in
the submarine rock samples. The dark bands were formed at different stages. Two
formed during sedimentation and subsequent burial (i.e., gravity origin), and the
other two formed during accretion (i.e., tectonic origin).

Keywords Dark bands • The Nankai accretionary prism • The Miura-Boso
accretionary prism • Gravity origin • Tectonic origin

1 Introduction

The Nankai accretionary prism, off Southwest Japan, is regarded as a natural laboratory
for studying subduction processes. The eastern Nankai accretionary prism is formed by
the subduction of the Philippine Sea plate beneath Southwest Japan and is characterized
by active fault zones developed in highly fractured and folded sedimentary rocks,