Boso TTT-Type Triple Junction: Formation of Miocene to Quaternary Accretionary Prisms and Present-Day Gravitational Collapse - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 53

Boso TTT-Type Triple Junction: Formation
of Miocene to Quaternary Accretionary Prisms
and Present-Day Gravitational Collapse
Yujiro Ogawa and Yukio Yanagisawa
Abstract The Boso triple junction, the only known trench-trench-trench
type triple junction, was surveyed by the ROV KAIKO-10K after the French-
Japanese KAIKO project and related R/V Hakuho-Maru cruises. This paper
summarizes the bathymetry and geologic structure, and the types and ages of
rocks and sediments observed in the Boso triple junction area during these
studies. We used diatom biostratigraphy to elucidate the evolution of Miocene
accretionary prism units exposed in the Izu-Bonin forearc and in subaerial
exposures on the Boso Peninsula. The instability of the Boso triple junction
system since the Miocene is attributed to repeated trench sediment accretion
and collapse. We conclude that the Boso triple junction area hosts a sequence
of trench-fill sediments that has accumulated between the easternmost mar-
gin of the Izu forearc and the easternmost Nankai trough since the Miocene.
However, recent instability at the Boso triple junction is characterized by
repetitive episodes of development and subsequent collapse of a ponded basin
close to the accretionary prism that are intimately associated with the develop-
ment of the Boso triple junction system.
Keywords Boso triple junction • ROV study • Accretionary prism • Diatom
biostratigraphy • Gravitational collapse