Rapid Exhumation of Subducted Sediments Along an Out-of-Sequence Thrust in the Modern Eastern Nankai Accretionary Prism - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 224

currently located between the Tokai Thrust and the Kodaiba Fault (Soh and
Tokuyama 2002 ; Le Pichon et al. 1996 ; Kodaira et al. 2003 ) and ca. 30 km landward
of the frontal thrust (Fig. 1b ), close to the site from which the present foliated rocks
were collected. Thus, we consider that the rapid exhumation documented above is
closely associated with collision/subduction of the paleo-Zenisu ridges, as shown
by Lallemand et al. ( 1992 ), Le Pichon et al. ( 1996 ) and Kodaira et al. ( 2003 ).
Based on a plausible relative subduction rate for the PHP of 4.0-5.0 cm/year
(Seno et al. 1993 ) or 6.0-7.0 cm/year (Miyazaki and Heki 2001 ), we infer that the
landward limit of the deeper paleo-Zenisu body started to subduct beneath the accre-
tionary prism by 0.75-0.42 Ma as shown in Fig. 4 T1. The protolith of the sampled
foliated rocks must have been deposited in the Shikoku basin in front of the subduct-
ing deeper paleo-Zenisu body at 3.8 Ma. It was subsequently brough to considerable
depth (4-5 km) and was then rapidly exhumed from the seismogenic zone. Given this
scenario, the vertical exhumation rate might well have been >1.0 cm/year.
Dominguez et al. ( 2000 ) conducted sandbox experiments to investigate the effect
of seamount collision/subduction on the evolving patterns of accretionary prisms. In
the experiment, the accretionary prism body that formed in front of the proceeding
seamount was thrust over completely during collision/subduction of the seamount,
and was then collapsed onto the trench floor. Although most of the cover sediments
above the seamount were sliced and underplated as the décollement zone migrated
outboard of the seamount, some of the cover sediments and/or sediments deposited
in front of the advancing seamount were subducted more deeply (Fig. 4 T2). During
the subsequent seamount collision/subduction of the seamount, the frontal sedi-
ments were exhumed along the décollement zone with significant shear deformation
(Fig. 4 T3). After that, normal faulting (landsliding) occurred by large-scale subsid-
ence and uplifting associated with seamount subduction (Fig. 4 T4).
The downward slip along the normal fault may have been due to gravitational
collapse and could further push the uplifted sediments much faster toward the
trench along the OOST, as shown in Fig. 4 T4. During dive surveys of 6K#887 and
6K#894, numerous normal faults were observed at the base of the sidewalls of the
Tenryu Canyon. Those observations suggest that normal faulting occurred fre-
quently in this region, and might have accelerated the exhumation of the metamor-
phosed sediments along the OOST.
Deformational textures of the sampled foliated rocks are the best clues to under-
stand the emplacement processes that occurred during accretion and exhumation.
The deformation textures within the foliated mudstones record an early phase of
ductile deformation (D 1 ), flattening (D 2 ), release of confining pressure (D 3 ),
and simple shear deformation features (slickenside on S 3 ). The ductile deformation
(D 1 folds) represent lateral shortening under the unconsolidated state. The S 1 and S 2
foliations (Fig. 2c ) indicate low-grade metamorphism and illite recrystallization, and
S 3 (Fig. 2d ) represents exhumation. The occurrence of lateral shortening, flattening
and simple shearing represent progressive deformation and low-grade metamor-
phism within the NAP (e.g., a décollement zone or an OOST). The tension fractures
(S 3 ; Fig. 2c, d ) imply expansion sub-parallel to S 2 during rapid exhumation of the
foliated mudstones.