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Lastly, there is a roughly 15° obliquity in Philippine Sea-Eurasian plate convergence.
Strain partitioning due to this obliquity was proposed to cause strike-slip faulting in
the NanTroSEIZE area (Martin et al.
2010
). Of the alternatives, we prefer the
hypothesis that a subducted seamount or proto-Zenisu ridge causes the oblique
deformation in the Tenryu canyon area simply because this deformation appears to
be most pronounced adjacent to the bathymetric high over a geophysically imaged
subducted basement object. Without a subducted ridge, we suspect that the obliq-
uity of the plate boundary would not produce such localized folds, nor would it
cause strike-slip faulting in strata detached from the basement.
In closing, we note the relevance of the newly recognized trench-perpendicular
fold axes and potential strike-slip faults to seismic hazards of the Nankai margin,
and by extension accretionary subduction zones worldwide. Firstly, if these are
indeed associated with strike-slip faulting, we should expect some seismic strain
release to have a strike-slip component, or at the very least P-T axes rotated from
coaxiality with the trench front. Secondly, the amount of elastic strain in the form
of folding must be somewhat larger than expected from existing interpretations of
the Nankai trough since these estimates have not to-date incorporated folding paral-
lel to the trench. Either of these two aspects of the Tenryu canyon structure has
implications for the total moment release expected from seismogenic zone earth-
quakes and ground-shaking dynamics that will result.
Lastly, we note that many of the sharp scarps on the surface of the Nankai prism
likely postdate most of the folding (i.e. they cut the folds). Since the folds are in
mostly Pleistocene strata, the relationship between folding and faulting could
potentially be exploited as a paleoseismological indicator of geologic rates of new
rupture development and fault-reactivation.
6
Conclusions
Bedding orientation estimates collected during a 2008 SHINKAI 6500 survey indi-
cate that Plio-Pleistocene turbiditic strata exposed within Tenryu canyon area are
involved in both trench-parallel and trench-perpendicular folds. While trench paral-
lel folds are expected within the structural architecture of accretionary prisms,
trench-perpendicular folds indicate trench-parallel, oblique or transpressional
shortening. Based upon observations of mesoscale strike-slip faults and overturned
thrust imbricates, we suggest this oblique deformation also includes strike-slip
faults, previously proposed solely on the basis of bathymetric interpretations and
seismic reflection work. The oblique deformation appears to be associated partly
with the Tokai thrust, a potentially seismogenic and tsunamigenic thrust and
approximately equivalent to the NanTroSEIZE megasplay. We suggest this oblique
deformation may reflect the accommodation of a subducted seamount or proto-
Zenisu ridge. Furthermore, we suggest that the estimates of past seismicity, and
future seismic moment and earthquake focal mechanisms may be affected by this
transpressional strain.
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