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 218

(Soh and Tokuyama 2002 ; Lallemand et al. 1992 ; Le Pichon et al. 1996 ; Kodaira
et al. 2003 ; 2004 ) deforming the repetition of the thrust-anticline architecture.
We have recently performed seafloor geological mappings along the Tenryu
Submarine Canyon in the eastern part of the NAP (Fig. 1b ) in four separate cruises
using the manned submersible SHINKAI 6500 (hereafter referred to as 6K) operated
by the Japan Agency of Marine Science and Technology (JAMSTEC) (Fig. 1a ).
Because of the deep incision of the Tenryu Canyon ~1 km down into the NAP
(Fig. 1b ), the deeper parts of the accretionary prism are exposed allowing us to col-
lect rock samples from the outcrops without drilling. The submersible surveys
recovered low-grade metamorphic and foliated rocks (foliated mudstone and sand-
stone) from the Tokai Thrust, which is one of the largest out-of-sequence thrusts
(OOSTs) within the prism.
The recovered foliated rocks provide clues to the deformation processes and
mechanisms, particularly the exhumation process in the active accretionary prism. In
this study, the peak temperatures of the recovered foliated rocks are determined on the
basis of illite crystallinity and vitrinite reflectance. Detailed descriptions of the defor-
mation textures, depositional ages estimated from micro- and nannofossils, and physi-
cal and mechanical properties of the rocks are provided to enable the reconstruction
of the exhumation processes that brought these rocks to the surface. We conclude that
the rocks have been rapidly exhumed, later than 0.42 Ma. The processes and mecha-
nisms are discussed in association with the subduction of a ridge or a seamount chain
below the eastern NAP. The rapid rate of exhumation can be explained in terms of
dual effects of thrusting along the OOST during the paleo-Zenisu ridge collision/
subduction, and normal faulting as a result of collapse just behind the passing ridge.

2 Deformed Rocks of the Tenryu Canyon

During the dive survey 6K#892, a sample of foliated mudstone (sample number
6K#892R-002) was collected from the bottom of the Tenryu Canyon (Fig. 1b ); the
sampling site is located in the hanging wall of the Tokai Thrust. The outcrop of the
sample site is approximately 10 m wide and 100 m long, and extends in a N-S
direction. Another sample of foliated mudstone (6K#892 R-003) was collected
from the lower slope of the eastern sidewall of the canyon (Fig. 1b ). The foliation
within these rocks strikes N-S and dips ca. 60° to the west (Figs. 1b and 2a, b ).
Horizontal turbidite (6K#892 R-004) and brown clay (6K#892 R-005) overlie the
foliated mudstones (Fig. 1c ).
The following three stages of deformation are recognized within the foliated
mudstone samples. First, sedimentary layers S 0 are folded tightly by D 1 event to
form an axial planar cleavage (S 1 in Fig. 2c, g ). Second, flattening deformation (D 2 )
led to the alignment of recrystallized illite flakes along the slaty cleavage (S 2 ),
developed oblique to the S 1 . Third, a release of overburden pressure (D 3 ) led to the
formation of numerous tension fractures (S 3 ) of several millimeters in spacing
subparallel to S 2 (Fig. 2d, h ).