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rather present along the tension fractures (S 3 ). We further considered that they may
have derived from the seafloor during the exhumation by suction during volume
expansion due to decompression and/or fault activities. Although we have con-
sumed all 6K#892 R-002 sample for different analyses and there is no way to test
our hypothesis.
Illite Crystallinity and Vitrinite Reflectance
We measured the illite crystallinity (IC) of the recovered foliated rocks (6K#892
R-002, R-003, and 6K#893 R-003) and other rocks sampled from the seaward parts
of the same canyon (6K#887 R-003, 6K#888 R-003, and 6K#892 R-004; see
Fig. 1a ). The illite crystallinity (IC) value is a useful geothermal indicator for mud-
stone samples subjected to very low-grade metamorphic conditions (Blenkinsop
1988 ). The IC value is represented by the Kübler Index (Kübler 1968 ), which is the
half-width (=2q) of the 10 Å peak of illite (001). The Kübler Index decreases with
increasing illite recrystallization because of the thermal dehydration of smectite
(Blenkinsop 1988 ). We used the conversion equations between the obtained IC
value (=2q) and the peak temperatures, which were previously proposed for the
Cretaceous-Tertiary Shimanto accretionary prism (Underwood et al. 1993 ;
Mukoyoshi et al. 2009 ) underlying the present-day NAP (Taira et al. 1988 ).
(Underwood et al. 1993)
Mukoyoshi et al. 2009
T : temperature in C
The obtained XRD spectra revealed the occurrence of smectite within the analyzed
samples, except for sample 6K#892 R-002 (Fig. 3a ). We therefore considered that
any smectite within sample 6K#892 R-002 were fully recrystallized to illite. This
inference is supported by the strong alignment of illite flakes in sample #892 R-002
(Fig. 2d ). In contrast, the IC values for the other samples reflect the crystallinity of
detrital illite, because most smectite were not converted to illite in the same speci-
mens. The IC value for sample #892 R-002 is 0.62 (Fig. 3a ); using conversion
equations (see appendix) we estimated the corresponding peak temperatures to
be ~200-230°C.
We also assessed the vitrinite reflectance of sample 6K#893 R-004, as this is a
useful geothermal indicator for samples that contain organic matter by measure-
ments of mean random reflectance (Rm) of vitrinites (Middleton 1982 ; Laughland
and Underwood 1993 ). We used the equation by Sweeney and Burnham ( 1990 )
for a 1 Ma heating period to estimate the peak metamorphic temperatures as
T( C)
°= +
93 ln percentage Rm
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