Submarine Outcrop Evidence for Transpressional Deformation Within the Nankai Accretionary Prism, Tenryu Canyon, Japan - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 199

associated with the Nankai subduction zone have generated large (M > 8) earthquakes
and tsunamis, including the 1944 Nankai and 1946 Tonankai earthquakes (Ando 1975 ;
Hyndman et al. 1995 ; Kodaira et al. 2000 ; Nakanishi et al. 2002a, b ). In response to
the combined scientific significance of, and hazards posed by the subduction-zone
system, seismic imaging experiments and a complex scientific drilling program
(NanTroSEIZE) hsave focused on the Nankai region (Tobin and Kinoshita 2006 ).
Another area of interest within the Nankai trough is the Tokai region. Located
east of the NanTroSEIZE area but just west of the Izu-Bonin arc, the Tokai region
was subjected to a M > 8 earthquake during the 1856 Ansei-Tokai event that rup-
tured across southern Honshu. The Tokai region therefore sits in a “seismic gap”
relative to the other Nankai events leading to political-social-economic debate about
how best to mitigate seismic hazards in Japan (Mogi 2004 ). To partially address this
debate and to better understand the distribution of strain in the Tokai region, several
recent projects used seismic imaging techniques as well as remotely- and human-
occupied submersible vehicle surveys to resolve the geologic structure of this east-
ern part of the Nankai accretionary prism (Le Pichon et al. 1987a, b ; Nakanishi et al.
2002a, b ; Kawamura et al. 2009 ). A bi-product of these efforts was the recognition
that a deep submarine canyon, Tenryu canyon, provides world-class, cross-sectional
exposures of relatively deep structural levels within the Nankai accretionary prism.
Here, we report on observations from three dives in Tenryu canyon conducted
during the JAMSTEC R/V Yokosuka YK08-04E cruise in 2008 that bear on both
the geological structure of this part of the Nankai accretionary prism and the
regional distribution of strain. Strike and dip estimates of bedding determined from
SHINKAI 6500 imaging and navigation systems show that in parts of the accretion-
ary prism Pleistocene strata are folded about northerly plunging axes as a result of
shortening parallel to the trench. Such clear evidence for trench-parallel shortening
has not, to our knowledge, been reported from active accretionary prisms world-
wide. The trench-parallel shortening likely caused the apparent sinuosity of bathy-
metric features in Tenryu canyon. Moreover, such observations of trench-parallel
shortening reinforce suggestions that transpressional deformation in the accretion-
ary prism is accumulating along trench-parallel strike-slip faults that have been
interpreted from offset reflectors in seismic reflection data both in the Tenryu can-
yon and NanTroSEIZE areas (Takahashi et al. 2002 ; Martin et al. 2010 ).
The exact cause of transpressional deformation in the Nankai area is unknown, but
is likely associated with some combination of: ( i ) changes in the principal stress ori-
entations throughout the prism (Byrne et al. 2009 ; Lin et al. 2010 ), ( ii ) lateral ramping
in the thrust belt (Moore et al. 2007 ), ( iii ) accommodation of subducted seamounts
(Dominguez et al. 1998 ; Bangs et al. 2006 ; Bilek 2007 ), or ( iv ) strain partitioning to
accommodate oblique (~15°) Philippine-Eurasian plate convergence (Martin et al.
2010 ). A fifth, more specific mechanism for transpression in the Tenryu canyon area
is the overall rotation of the compressive axis around the colliding Zenisu and paleo-
Zenisu morphological ridges that form along the western margin of the Izu-Bonin arc
(Le Pichon et al. 1987a ). We consider all of these mechanisms for transpressional
deformation in the Tenryu canyon region to be important, especially the collision of
the Zenisu ridges. Through this transpressional deformation, the elastic strain in the
area is likely increased and favored orientations for fault slip are likely deviated from