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defined great circle with a “pole-to-poles (p-axis)” that plunges west. Therefore,
bedding within the 1056 dive is folded about a roughly cylindrically, east-west
trending fold. This average of bedding orientations does not, of course, include the
smaller, tight, north trending folds observed locally in outcrop. In contrast, the
poles to bedding planes from Dive 1057 define separate east and west dipping
populations that have a pole-to-planes that plunges gently to the northeast. Not
surprisingly, the poles to bedding from Dive 1058 define a relatively flat-lying
sequence. However, structural observations made during Dive 1058 described in
the previous section include young, high-angle faults with strike-slip indicators. We
bring together these field observations in a conceptual structural model linking
east-west shortening to the Tokai thrust in the following discussion.
5
Discussion
In a strictly break-forward thrust system, where strain is accommodated within a
nearly cross-sectional plane, the mechanics of an accretionary prism are governed
solely by slip on the basal décollement relative to the overriding wedge and incom-
ing trench sediment (e.g. Boyer and Elliot
1982
). To date the NanTroSEIZE area off
the Kii peninsula has provided rich observations bearing on important exceptions to
this simple model for accretionary prisms. Two noteworthy observations are (
i
)
significant out-of-sequence thrusting (Park et al.
2002
; Moore et al.
2007
), and (
ii
)
a pattern of offset reflectors in the NanTroSEIZE 3D seismic volume that can be
explained by strike-slip faulting in the hanging wall immediately adjacent to the
megasplay (Martin et al.
2010
). Such large-scale observations set the stage for inter-
preting
in situ
measurements of spatially heterogeneous principal stress orientations
(Lin et al.
2010
), and Integrated Ocean Drilling Program (IODP) cores from the
prism, which contain strike-slip and normal faults as well as thrusts (Lewis et al.
2008
; Hayman et al.
2009
). If the amount of oblique deformation along the Nankai
margin is significant, it would bear on both the geological evolution and seismic
behavior of the prism. Among other things, oblique deformation can enhance exhu-
mation of deep-seated rocks (e.g. Karig
1980
) and should affect the spatial/temporal
distribution and focal mechanisms of earthquakes (e.g. McCaffrey et al.
2000
).
In contrast with the NanTroSEIZE area, the sinuous bathymetry and sharp
scarps of the Tenryu canyon area have long been interpreted as potential indicators
of transpressional deformation in the eastern Nankai accretionary prism (Le Pichon
1987a, b
; Soh and Tokuyama
2002
) (Fig.
3
). However, similar to the NanTroSEIZE
area, there are few clear markers in seismic reflection data that can be used to dem-
onstrate trench-parallel, strike-slip faulting. Additionally, there is no simple way to
evaluate displacements on the Tokai thrust because—similar to the NanTroSEIZE
megasplay—the fault roots below the Plio-Pleistocene section of the accretionary
wedge that is not exposed and is difficult to interpret in seismic data. This discon-
nect in scales of observation is, in essence, where the advantage of using submers-
ible technology in a setting such as the Nankai trough lies. Submersible surveys
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