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 220

During the dive survey 6K#893, sheared blocks that are composed of calcareous
silt and fine- to medium-grained sandstones (6K#893 R-002) and massive fine- to
medium-grained sandstones (6K#893 R-004 and R-005) were recovered from the
middle slope of a western sidewall of the canyon, close to the Tokai Thrust (Fig. 1b ).
These rocks contain fragments of pumice and plant fossils, and are cut by bifurcating
network of faults demarcated by concentration of clay minerals that are strongly
aligned parallel to the fault planes. Mudstone (6K#893 R-003) and breccia (6K#893
R-001) (Fig. 1b ) in shear zones were also recovered during this dive survey.
During the dive surveys 6K#887 and 6K#888, turbidite samples (sample numbers
6K#887 R-003, and 6K#888 R-003 and R-004) were recovered from the lower
slopes of the western walls of the Tenryu Canyon (Fig. 1a ). These samples are not
well consolidated and have high porosities and low strengths.

3 Ages of Recovered Rocks

We extracted radiolarian microfossils from the foliated mudstone samples 6K#892
R-002. They preserved details of the tests and are characterized by assemblage of
rare to moderately abundant radiolarian tests of Didymocyrtis tetrathalamus,
Cycladophora davisiana davisiana, Theocorythium trachelium trachelium,
Botryostrobus aquilonaris, Amphirhopalum ypsilon, Thecosphaera dedoensis and
Collosphaera tuberosa. On the basis of the obtained radiolarian assemblage, we
concluded that the sediments have the Collosphaera tuberosa Zone assemblage,
ranging in age from 0.42 to 0.18 Ma (middle Pleistocene) according to the times-
cale by Sanfilippo and Nigrini ( 1998 ). The upper limit of the Collosphaera tube-
rosa Zone is defined by the first appearance of Buccinosphaera inveginata , that is
rather rare in recent sediments. Thus, estimation of radiolarian zone age may extend
from 0.42 Ma to recent. The sample 6K#892 R-002 was barren in calcareous
nannofossils.
The sample 6K#892R-003 yields calcareous nannofossils characterized by a rare
occurrence of Reticulofenestra Pseudoumbilicus and Sphenolithus abies (identified
by the smear slide method), indicating older than the NN15 zone (Martini 1971 ) as
>3.8 Ma (early Pliocene) (Cande and Kent 1995 ). The calcareous nannofossils in
sample 6K#893 R-002 are characterized by Reticulofenestra spp. and Discoaster
bergenii , representing the NN11A zone (Martini 1971 ) ranging in age from 8.6 to
6.5 Ma (late Miocene) (Cande and Kent 1995 ). However, no reliable radiolarian
ages were obtained from these samples.
A large gap was recognized between the radiolarian zone age obtained from the
Sample 6K#892R-002 sample and the nannofossil zone age estimated from the
Sample 6K#892R-003, collected just above the former sample. Furthermore, no
radiolarian fossil was confirmed in a thin section from the Sample 6K#892R-002.
We confirmed the existence of some radiolarian tests along the fractures, indicat-
ing that the radiolarian microfossils were not in Sample 6K#892R-002, but were