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 56

Sagami trough trend to the present plate boundary. There is evidence to suggest
that this plate boundary has trended WSW-ENE since the Miocene (Taira et al.
1989 ; Yamamoto and Kawakami 2005 ; Kanamatsu and Herrero-Berver 2006 ), but
was recently rotated to its current NW-SE trend by collision with the Izu arc on
the PHS ca 3 Ma.
The kinematics of the evolution of these trends suggests that the geometry of the
Boso triple junction is greatly influenced by regional tectonism. Thus, the geologic
relationships shown by subaerial exposures of subduction-related deposits on the
Miura and Boso peninsulas suggest that these rocks originated within the
Cretaceous-Tertiary Shimanto accretionary prism before the collision of the Izu arc
and the development of the Boso triple junction (Ogawa and Taniguchi 1988 ; Mori
and Ogawa 2005 ; Mori et al. 2011 ).
In this paper, we first review the depositional and deformational evolution of the
area of the Boso triple junction with reference to submarine geologic maps previ-
ously published by the Geological Survey of Japan, the results of recent onshore
geological surveys, and new diatom age data for subsea samples. We then consider
the significance of these results for understanding the Neogene to Quaternary tec-
tonic development of the region. According to the subsea video footage, these strata
have undergone complex deformation; there is an alternating sequence of steep and
shallow dips that suggests a collisional thrust-and-fold structures and gravitation-
ally collapsed structures.

2 Review of Sedimentation, Topography, and Plate

Configuration

2.1 Sedimentation

The present drainage system in the region off the Boso Peninsula transports clastic
sediments along the Sagami trough, through the Sagami and Boso submarine can-
yons, and ultimately into the Boso triple junction, forming the Mogi submarine fan
(Nakamura et al. 1987 ; Renard et al. 1987 ; Pautot et al. 1987 , Ogawa et al. 1989,
2008 ) (Fig. 2 ). This sedimentation system includes a series of basins within the
Sagami trough (northwestern, central, and southeastern (Katsuura) submarine
basins) and the triple junction itself (Bando submarine basin) (Ogawa et al. 1989,
2008 ; Iwabuchi et al. 1990 ). Because most of the accretionary prisms on land of
Miocene and younger age are composed of volcaniclastic sediments derived from
the Izu arc, it is reasonable to consider that they were deposited on the Izu forearc
and in the ponded basin along the previous Sagami trough before their accretion at
the Honshu margin. These accreted deposits are now subaerially exposed on the
Miura and Boso peninsulas as the Emi, Miura, and Chikura accretionary prisms
(Hanamura and Ogawa 1993 ; Hirono and Ogawa 1998 ; Yamamoto and Kawakami
2005 ; Michiguchi 2008 ; Muraoka and Ogawa 2011 ).