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 222

a

S
(001)

b

25

893R4

I/S
(001/002)

893R3

I
(001)

C
(002)

20

I/S
(002/003)

C
(003)

I
(002)

15

892R4

n

10

C
(001) 892R3

5

892R2

0

5 10 15 20

0 0.2 0.4 0.6 0.8 1.0

2 q

Ro (%)

Fig. 3 ( a ) XRD spectra for samples #892 R-002, #892 R-003, #892 R-004, and #893 R-003.
Sample #892 R-002 is illite ( I )-rich and devoid of smectite ( S ) and illite-smectite mixed layers
( I/S ), whereas the other samples contain smectite, chlorite ( C ), and illite-smectite mixed layers.
Illite crystallinity value for samples #892 R-002 is 0.62. The peak temperatures recorded by these
rocks are estimated to be ~200-230°C. ( b ) Vitrinite reflectance values for the sandstone, sample
6K#893 R-004. The mean and mode for Rm are both 0.4%; this corresponds to peak temperatures
of ~90°C. See text for the details

The error associated with Rm-temperature correlations for the Cretaceous-Tertiary
Shimanto accretionary prism has been reported as ±30°C (Laughland and
Underwood 1993 ). We obtained an Rm value of 0.4% for 38 analyzed fragments
(Fig. 3b ); based on conversion equation (see appendix), this value represents a peak
temperature of 90°C ± 30°C.

5 Physical-Mechanical Properties of Recovered Rocks

The physical properties of these rocks are summarized in Kawamura et al. (2009).
The porosities of samples 6K#892 R-002 and R003 are 25-30%, whereas those
of the sandstones (6K#893 R-001, -002 and -004) range from 13% to 30%. In con-
trast, rocks within undeformed accretionary prisms commonly have porosities
around 43-61%. The uniaxial compression strength (hereafter UCS) of the mud-
stone sample 6K#892 R-003 is measured ca. 5.0 MPa, whereas those of the recov-
ered sandstones are mostly >10 MPa. The strengths of our sandstone samples are