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The other faults exposed in Tenryu canyon region have kinematically different
histories from the rest of the fold and thrust belt. Some faults, such as the thrust faults in
the vicinity of Zenisu ridge, apparently root into the oceanic basement (Le Pichon et al.
1987a ; Nakanishi et al. 2002a ; Takahashi et al. 2002 ). Still others, such as the Kodaiba
fault apparently do not root in basement and are proposed to have a strong strike-slip
component (Takahashi et al. 2002 ). Similar structural relationships to those among
faults in the Tenryu canyon area were observed during the NanTroSEIZE surveys fur-
ther to the west where thrusts faults may root in basement rocks (Park et al. 2002 ) and
unrooted faults are thought to have a strike-slip component (Martin et al. 2010 ).
We report here on three SHINKAI 6500 dives from 2008 that focused on exposures
of the footwall, fault zone, and hanging wall of the Tokai thrust in Tenryu canyon
(Fig. 3 ). These three dives were part of a five-dive cruise designed to supplement
knowledge of the Tenryu canyon area gained during previous submersible programs
(Kawamura et al. 2009 ). We focus exclusively on the mesoscale (outcrop) structure
to make a very simple point: that folds in the turbiditic strata indicate a strong com-
ponent of trench-parallel shortening. In so doing we also present some new observa-
tions of the Tokai thrust zone and distribution of stratigraphic units.
Outcrop Geology of Tenryu Canyon from SHINKAI 6500
SHINKAI 6500 dives 1056, 1057, and 1058 each conducted transects within Tenryu
canyon that were approximately one kilometer long at depths from roughly 2,000
to 3,000 mbsl (Fig. 3 ). Because most of the sedimentary units exposed in the
Tenryu canyon walls are well-bedded alternating layers of sand and mud, the strike
and dip of these strata can be estimated from the yaw and pitch of the external video
camera and the bearing of the submarine. These estimates obviously yield apparent
strikes and dips, but by traversing the area of dipping strata we were able to avoid
observing strata at extremely high-angles to strike.
We do not truly quantify the uncertainty associated with our strike-and-dip esti-
mates here, but point out that such estimates can be considered at the very least
semi-quantitative. One of the largest sources of uncertainty in any strike and dip
measurement is the angle between the true dip (TD) and apparent dip (AD), where
a is the angle between TD and AD directions:
- æ
We suggest that our views (a) were typically within ~10-20° of strike, thereby
providing true dip estimates with an error of less than ±4°. However, this error
estimate is probably overly optimistic considering that there is no systematic way
to circumnavigate every structural feature, many features were in cliff-faces, and
the horizontal is difficult to control in the vehicle. Nonetheless, we suspect that our
estimates are within perhaps 10° of TD (See Table 1 for a full list of strike and dip
measurements for the three dives of interest).
The three dives of interest here traversed sequences of predominantly well-
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