Redistribution of Sediments by Submarine
Landslides on the Eastern Nankai Accretionary
Prism
K. Kawamura, T. Kanamatsu, M. Kinoshita, S. Saito, T. Shibata,
K. Fujino, A. Misawa, and K.C. Burmeister
Abstract During a recent survey of the Nankai Trough region by JAMSTEC
R/V KAIYO, ten piston cores were collected along a NW-SE transect through the
Shikoku Basin, Kashinozaki Knoll, and Nankai Trough areas. The purpose was to
demonstrate the influence of landslide processes on sediment distribution patterns
on an accretionary prism. The Shikoku Basin is a flat abyssal plane covered by ca.
1 m thick hemipelagic mud, and underlain by a ca. 10 cm thick tuffaceous sand
corresponding to the Aira-Tn tephra layer (25120 ± 270 yr. B.P.). The sedimentation rates in the Shikoku Basin are 3–4 cm/ky. At least three submarine landslide
scars are observable on Kashinozaki Knoll. The abyssal plane surrounding the
Kashinozaki Knoll is covered by a characteristic yellowish pumiceous mud intercalated with the hemipelagic mud. Immediately below thelandslide scars, these
pumiceous mud layers thicken. Pumiceous mud was likely derived from the flanks
of the volcanic Kashinozaki Knoll. These scars and deposits suggest that submarine landslides redistributed material from the knoll to the basin by mass-wasting.
More than six trench turbidite beds were observed in a sequence that overlies
a submarine landslide deposit at the foot of the accretionary prism within the
K. Kawamura ()
Fukada Geological Institute, 2–13–12 Hon-Komagome, Bunkyo, Tokyo 113–0021, Japan
e-mail: kichiro@fgi.or.jp
T. Kanamatsu, M. Kinoshita, and S. Saito
IFREE1, Japan Agency for Marine Science and Technology, 2–15 Natsushima-cho, Yokosuka,
Kanagawa 237–0061, Japan
T. Shibata
Kochi University, 2–5–1 Akebono-cho, Kochi 780–8520, Japan
K. Fujino
Kyushu University, 10–1 Hakozaki, Higashi-ku, Fukuoka 812–8581, Japan
A. Misawa
Tokai University; 3–20–1 Orido, Shimizu, Shizuoka 424–8610, Japan
K.C. Burmeister
University of the Pacific, Department of Geosciences, 3601, Pacific Avenue, Stockton, California
95211, USA
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
313
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K. Kawamura et al.
Nankai Trough. The geomorphology of the axial channel suggests that the flow of
turbidite deposits along the axial channel was blocked by this submarine landslide
deposit, forcing the flow path and gradient of the channel to re-establish itself.
In this event, sediment ponding would occur upstream of the blockage, while the
downstream portion of the axial channel would be starved of sediment.
Keywords NanTro SEIZE • Submarine landslide • Kashinozaki Knoll • Nankai
Trough • Frontal thrust
1
Introduction
Seamount subduction/collision at convergent margins is thought to induce gravitational collapse at accretionary prisms (Dominguez et al. 2000; von Huene et al.
2004; Kobayashi et al. 1987; Yamazaki and Okamura 1989). Those gravitational
collapses (e.g. slope failures and submarine landslides) are a key to understanding lateral variation of sedimentation in subduction trenches. When the
axial channel of the trough is dammed by structural blockage due to uplift of the
trough floor, seamount collision, or submarine landslide deposition at the toe of
the prism, downstream of the dam becomes a sediment starved trench as shown
by Underwood and Backman (1982). In contrast to the downstream portion,
thick rapidly deposited turbidite sequences accumulate in the upstream portion.
This lateral variation in processes leads to potential lateral variations in grain size
distribution, sediment physical-mechanical properties and pore fluid pressure
that can affect accretionary processes and characteristics of the decollement zone
(Saffer and Bekins 2002; Spinelli and Underwood 2004; Ike et al. 2008).To address
this sedimentation pattern, this present paper studies in detail the depositional
systems related to submarine landslides around the eastern Nankai Trough, off
Kii Peninsula. Results from multibeam bathymetric data and 10 piston cores
are presented.
The Kashinozaki Knoll is located on the Nankai Trough, SW Japan. The Nankai
margin hosts one of the largest subduction zones and accretionary prisms in the
world and has been studied for more than 20 years (Kobayashi 2002). The Nankai
Trough is located between the southwest Japan arc on the Amur plate and the
Philippine Sea Plate. The convergence rate between these plates is approximately
4–7 cm/yr (Seno et al. 1993; Miyazaki and Heki 2001). The Kashinozaki Knoll is
an isolated basement high of volcanic origin within the Nankai Trough that will
presumably collide with the Nankai prism (Ike et al. 2008).
The survey encompasses the area from 32°N, 137°E to 33°N, 136°30’E, ranging
from the Shikoku Basin to the Nankai Trough (Fig. 1a, b). We obtained bathymetric
data using the SeaBeam 2112.004 system (SeaBeam Co. Ltd., Germany) during the
cruise by KY07–01 of the Japan Agency for Marine Science and Technology
(JAMSTEC) R/V KAIYO in the period from Jan. 4–19, 2007 (Fig. 1b).
Redistribution of Sediments by Submarine Landslides on the Eastern Nankai
315
a
b
c
Fig. 1 (a) Location of study area within the Nankai Trough off the southeastern coast of Japan.
(b) Detailed SeaBeam bathymetric map of study area. (c) Geomorphic regions within the study
area: (1) flat plane or gentle slope (light blue), (2) slope (green), (3) depression (blue), (4) channel
(blue), (5) valley (blue arrows), (6) ridge (orange lines), (7) scar (brown lines) and (8) step (broken
lines). Contour intervals are 2,000 m in (a), and 50 m in (b) and (c). NanTro SEIZE (Nankai Trough
seismogenic zone experiment) is one of the international projects in Integrated of Ocean Drilling
Program (IODP) (see Tobin and Kinoshita 2006)
The margin’s geomorphology is divisible into eight areas based on the
bathymetric features as shown in Fig. 1c. Submarine landslides are distinct mounds
located just below the scar, as indicated by the yellow- and brown- colored areas in
Fig. 1c. Based on these geomorphic features, the region can be subdivided into
three areas: the Shikoku Basin, Kashinozaki Knoll and Nankai Trough (Fig. 1b).
The Shikoku basin area is an abyssal plane.
316
1.1
K. Kawamura et al.
Topographic Features of the Study Area
The Kashinozaki Knoll is a wide slope area, whereas the landward slope at the toe
of the prism is composed of small slopes with many depressions and ridges. The
Nankai Trough area in this study area is characterized by an axial channel and
deposition of submarine landslides from the toe of the prism. Three steps are
located on the east side of the large submarine landslide on the Nankai Trough
floor. These steps occur over two ridges, suggesting that the formation of the steps
predates ridge development.
To confirm the above geomorphic interpretations, ten piston cores numbered
from PC-03 to PC-12 were obtained along a NW to SE transect between the
Shikoku Basin and the axis of the Nankai Trough (Fig. 1c; Table 1). PC-03, 04 and
05 were recovered from the Shikoku Basin area, PC-06, -07, -08, -09 and -11 were
from the Kashinozaki Knoll area, and PC-10 and -12 were from the Nankai Trough
area (Fig. 2).
Fig. 2 Lithofacies of sediment cores collected along a transect from Shikoku Basin to Nankai Trough
Redistribution of Sediments by Submarine Landslides on the Eastern Nankai
317
Table 1. Location of core sampling sites
PC
Longitude
Latitude
PC-03
PC-04
PC-05
PC-06
PC-07
PC-08
PC-09
PC-10
PC-11
PC-12
137°02.6995
137°01.5006
136°58.7877
136°57.8005
136°52.9287
136°58.1650
136°51.7688
136°46.5181
136°53.0056
136°47.2198
′E
′E
′E
′E
′E
′E
′E
′E
′E
′E
Depth
32°30.8178
32°33.0296
32°38.4092
32°40.7271
32°49.7383
32°39.7155
32°51.5179
33°00.5128
32°48.0758
33°00.0223
′N
′N
′N
′N
′N
′N
′N
′N
′N
′N
4084
4127
4167
4178
4062
4184
4245
4334
3984
4347
m
m
m
m
m
m
m
m
m
m
Table 2. Reflectances of volcanic glass from piston core samples. Count reflects number of
grains used to measure reflectance in each sample
Burial depth
PC3
PC4
PC5
PC8
PC11
PC12
2
104.6087.0053.70202.5079.00147.00-
116.60 cm
92.00 cm
67.70 cm
204.50 cm
90.00 cm
158.00 cm
Average
Minimum
Maximum
Count
1.501
1.501
1.501
1.501
1.500
1.503
1.500
1.500
1.500
1.500
1.499
1.500
1.502
1.502
1.502
1.502
1.501
1.509
32
32
33
38
38
52
Sediments in the Shikoku Basin Area
The Shikoku Basin is a flat abyssal plane (Fig. 1b, c) covered by approximately
100 cm of bioturbated hemipelagic clay that is underlain by an approximately 10 cm
thick ash layer (Fig. 2). This sedimentary sequence is equivalent to the upper
Shikoku Basin facies of Ike et al. (2008). Similar sedimentary sequences have also
been observed below trench turbidite sequences in the Nankai Trough (Taira et al.
2005; Ike et al. 2008). Optical reflectance of about 30 volcanic glass grains
extracted from this ash layer suggests they are tephra. The reflective indexes of
these grains are ~1.501 on average (Table 2), which suggests they are correlative
with the Aira-Tn tephra layer (25120 ± 270 yr. B.P.) (Miyairi et al. 2004) and allow
the calculation of an average sedimentation rate of 3–4 cm/kyr in the Shikoku Basin.
3
Sediments in the Kashinozaki Knoll Area
Hemipelagic clay on the abyssal plane surrounding the Kashinozaki Knoll contains
a distinct, light-yellow pumiceous clay. While the hemipelagic clay is part of the
upper Shikoku Basin facies, the yellowish pumiceous clay is likely associated with
submarine landslide deposits that will be discussed below.
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K. Kawamura et al.
Kashinozaki Knoll
1
-1
la
PC
-1
PC 0
-1
2
PC
e
in
ar
PC
7
e
-0
id
P C n d sl
bm
Turbidite
el
nn
ha
lc
ia
Ax
10 km
B
Su
Nankai Trough
-0
9
Shikoku Basin
PC
PC PC
-0
-0 -0
5
6 8
?
PC
-0
4
(AT)
Volcanic ash
Hemipelagic mud
PC
-0
3
A
Fig. 3 Interpretation of geologic profile along the line A-B in Fig. 1C
Scars on Kashinozaki Knoll suggest that at least three submarine landslide have
dissected the knoll (Ike et al. 2008; Figs. 1c and 3). Immediately below these scars,
pumice grains become coarser, indicating the sediments were possibly transported
as slide masses (Fig. 3).
A 4-km2 depression atop Kashinozaki Knoll contains several layers of volcanic
ash that are identical to those collected from the Shikoku Basin (Fig. 2). Two thrust
planes are visible in piston core PC-11 (Fig. 4). The cause of these thrusts is
unknown, but it is possible they were formed by local compression accompanying
gravitational gliding.
An ash layer 200 cm below the seafloor (hereafter cm-bsf) surrounding the Knoll
is illustrated in PC-08 (Fig. 2). This ash layer corresponds with the Aira-Tn tephra
layer (25120 ± 270 yr. B.P.) (Miyairi et al. 2004) and allows the calculation of 4 cm/
kyr sedimentation rates around the Kashinozaki Knoll (Table 2). Note this sedimentation rate is twice that of the rate in the Shikoku Basin and is perhaps a result of
additional deposition by submarine landslides.
4
Sediments in the Nankai Trough Area
A piston core collected from the bottom of the Nankai Trough contains evidence
for more than six trench turbidite beds overlying a submarine landslide deposit
(Fig. 2). These sandy turbidite deposits also occur in PC-07 m suggesting the extend
at least as far as the mid-flank of Kashinozaki Knoll (Fig. 2). Piston cores collected
from the bottom of the Nankai Trough contain volcanic ash layers 150 ~ 200 cm-bsf
that appear to be the same as those observed in the Shikoku Basin (Table 2).
As discussed above, the sedimentation processes in the vicinity of the Kashinozaki
Knoll, Shikoku Basin, and Nankai Trough appear to be strongly associated with
submarine landslides for the following reasons:
1. At least three submarine landslides on Kashinozaki Knoll left significant
headscarp scars and redistributed sediments from the flanks of the knoll onto
the basin floor.
2. Hemipelagic muds in the Shikoku Basin are intercalated with reworked with
debris flow deposits containing pumiceous material from the knoll that were
delivered to the basin by submarine landslide processes.
Redistribution of Sediments by Submarine Landslides on the Eastern Nankai
319
Fig. 4 Detail photo of PC-11 from 74-123 cm-bsf. Arrows indicate the positions of two thrust planes
3. Within the Nankai Trough we believe that large submarine landslides affect flow
paths, which in turn alters depositional patterns along the trough. Turbidity
currents through the channel distribute sediments laterally along the trough
(Pickering et al. 1989). Before large-scale submarine landsliding at the toe of the
Nankai prism, turbidity currents flowed along the trough without interruption.
However, large submarine landslide can either block current flow or deflect the
turbidity current and affect subsequent depositional patterns (Fig. 5). Following
emplacement of the submarine landslide, the downstream portion of the channel
is sediment starved, whereas upstream endures high sedimentation rates because
320
K. Kawamura et al.
Fig. 5 Bathymetric map of the Nankai Trough showing possible flow paths along the axial channel
following submarine landsliding at the toe of the prism
of damming by the landslide deposit (Underwood and Backman 1982). These
dams are a short-lived, and the flow path and gradient of the axial channel are
eventually restored to their original configurations (Fig. 5). The three steps on
the floor of the Nankai Trough (Fig. 1c) might represent traces of past flow paths
(Fig. 5). In the future, the flow path of the axial channel may reestablish itself
by incising through the submarine landslide deposits (Fig. 5).
5
Concluding Remarks
This study illustrates how submarine landslides can affect both the morphology of
the seafloor and sediment distribution patterns at the toe of accretionary prisms.
The Nankai accretionary prism study area is subdivided according to bathymetric
morphology and sedimentology of the Shikoku Basin, Kashinozaki Knoll and
Nankai Trough areas. The Shikoku Basin is a flat abyssal plane that is covered by
hemipelagic mud ca. 1 m thick. This mud layer is underlain by a tuffaceous sand
layer ca. 10 cm thick that corresponds with the Aira-Tn tephra layer (25120 ± 270
yr. B.P.) and allows sedimentation rates of 2–4 cm/ky to be calculated for the
Shikoku Basin. A characteristic light yellow pumiceous mud intercalated with
hemipelagic mud is distributed on the abyssal plane surrounding the Kashinozaki
Redistribution of Sediments by Submarine Landslides on the Eastern Nankai
321
Knoll. This pumiceous mud is presumably derived from the volcanic knoll and
distributed across the basin floor by mass wasting processes. At least three headscarp scars that are likely the result of submarine landslides occur on the flanks of
the knoll. Immediately below these scars, pumice layers thicken, suggesting these
sediments were probably transported as landslide-related debris flows. Two fault
planes noted the piston core from this location may have developed in association
with this remobilization.
Along the axial channel at the bottom of the Nankai Trough, more than six
trench turbidite beds have been identified in a sequence that overlies a submarine
landslide deposit at the foot of the prism. The axial channel might have been
initially dammed by the underlying submarine landslide deposit, which would have
affected subsequent depositional patterns. In particular, damming of the axial
channel would cause sediment to pond upstream and starving the downstream
region of sediment.
Acknowledgments The authors gratefully acknowledge captain, crew and technicians of Marine
Work Co. Ltd. of cruise KY07–01 for piston coring operation and core sample treatments. We
thank Dr. Tadashi Sato (Fukada Geological Institute) for comments and suggestions.
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