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 216

tension fracture (S 3 ). The illite flakes within the foliated mudstone clearly formed
from diagenetic recrystallization rather than being detrital in origin. The peak meta-
morphic temperature for the mudstone samples was estimated to be ~200-230°C on
the basis of illite crystallinity. Based on the representative thermal gradients within
the NAP at present (~50°C/km), the rock was exhumed from 4 to 5 km in depth,
corresponding to the depth of the seismogenic zone. Nannofossils in the foliated
mudstone indicate a depositional age older than 3.8 Ma. Radiolarians inserted
within the tension fractures of the sample indicate <0.42 Ma, suggesting the sedi-
ments subducted and then returned to the surface <0.42 Ma ago. The present sam-
ple site is located ca. 30 km from the leading edge of the trough. Present subduction
rate of the Philippine Sea plate (4.0-7.0 cm/year) gives estimates for the rate of
lateral transportation of sediments. These results indicate that the mudstone along
the out-of-sequence thrust were rapidly exhumed from the seismogenic zone.

Keywords SHINKAI 6500 • Submarine landslide • OOST • Phyllite • Illite
crystallinity

1 Introduction

Understanding the processes and mechanisms of large seismogenic and tsunamigenic
faults that occur at active subduction zones is one of the fundamental targets of
the Integrated Ocean Drilling Program (IODP) (Tobin and Kinoshita 2006 ). The
up-dip limit of fault slip locking depth in the Nankai seismogenic zone is controlled
by various factors (Hyndman 2004 ), including the transition from smectite to illite
at temperatures between 100°C and 150°C as shown by Vrolijk ( 1990 ). Below
depths where large earthquakes occur, interseismic slips occur along the décolle-
ment zones that are the subduction plate boundaries (Park et al. 2002 ; Kodaira et al.
2003, 2004 ). The Nankai Trough is a plate convergent margin where the Philippine
Sea plate (hereafter PHP) subducts below the Eurasia (Amur) plate at Southwest
Japan (Moore et al. 2007 ; Seno et al. 1993 ). The convergence rate between these
plates is estimated as 4-5 cm/year (Seno et al. 1993 ) or 6-7 cm/year (Miyazaki and
Heki 2001 ) (Fig. 1a ). The pelagic/hemipelagic sediments and trench-fill turbidites
on the PHP are successively accreted to Southwest Japan to form the Nankai
accretionary prism (NAP), as a result of repeated occurrences of thrust-anticlines
(Taira et al. 1991 ; Shipboard Scientific Party 2001 ). In the eastern NAP, collision/
subduction of seamount chains, namely paleo-Zenisu ridges, occurred at least twice

H. Mukoyoshi and S. Hirano
Marine Works Japan, Showa-machi 3175-25, Kanazawa, Yokohama 236-0001, Japan
e-mail: [e-mail]; [e-mail]
I. Motoyama
Department of Earth and Environmental Sciences, Graduate School of Science Department,
Yamagata University, Yamagata 990-8560, Japan
e-mail: [e-mail]