Physiographical and sedimentological characteristics of ... · 1991; Torres et al., 1997). This asymptotic form of channel proﬁ le is considered to be a function ... canyons within

For permission to copy, contact [email protected]© 2008 Geological Society of America

Physiographical and sedimentological characteristics of submarine canyons developed upon an active forearc slope:

The Kushiro Submarine Canyon, northern Japan

Atsushi NodaTaqumi TuZinoRyuta FurukawaMasato JoshimaGeological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Central 7, 1-1-1Higashi, Tsukuba, Ibaraki 305-8567, Japan

Jun-ichi UchidaDepartment of Earth Science, Faculty of Science, Kumamoto University, 39-1, Kurokami 2-chome, Kumamoto 860-8555, Japan

GSA Bulletin; May/June 2008; v. 120; no. 5/6; p. 750–767; doi: 10.1130/B26155.1; 15 fi gures; Data Repository item 2008047.

750 For permission to copy, contact [email protected]© 2008 Geological Society of America

ABSTRACT

Comprehensive geological surveys have revealed the physiographical and sedimen-tological characteristics of the Kushiro Sub-marine Canyon, one of the largest submarine canyons around Japan. The canyon indents the outer shelf along a generally straight, deeply excavated course of more than 230 km in length upon the active forearc slope of the Kuril Trench in the Northwest Pacifi c. The forearc slope has a convex-upward geom-etry that can be divided into upper and lower parts separated by an outer-arc high (3200–3500 m water depth). The upper slope consists of gently folded forearc sediments, and the lower slope is underlain by sedimen-tary rocks deformed by subduction-related processes. The upper reaches of the canyon (~3250 m of thalweg water depth) are devel-oped on the upper slope, showing a weakly concave-upward longitudinal profi le with a gradual down-canyon increase in relief between the thalweg and the canyon rim. Although an infi ll of hemipelagic mud and the absence of turbidite deposits indicates that the upper part of the upper reaches of the canyon (~900 m thalweg water depth) is inactive, the lower part of the upper reaches (900–3250 m thalweg water depth) is con-sidered to be an active conduit to the lower reaches, as determined from voluminous tur-bidites recovered in sediment cores (~76-yr intervals) and rockfalls observed in the can-yon bottom by deep-sea camera. A number

of gullies developed upon the northern slope of the lower part of the upper reaches might well provide a frequent supply of turbid-ity currents, giving rise to a down-canyon increase in the frequency of fl ow events. The down-canyon increase in fl ow occurrence is related to a gradual decrease in gradi-ent, demonstrating an inverse power-law relationship between slope and drainage area. In contrast, the lower reaches of the canyon (3250–7000 m thalweg water depth) are characterized by a gradual decrease in relief, a high gradient, and extremely low sinuosity. The limited increase in drainage area down-canyon of the confl uence with the Hiroo Submarine Channel, which is the largest tributary of the main canyon, indi-cates that the erosional force of turbidity currents decreases down-canyon. The gra-dient of the lower reaches largely refl ects the morphology of the forearc slope along the canyon, which has been deformed by subduction-related tectonics. The lack of an inverse power-law relationship between gradient and drainage area in the lower can-yon supports the hypothesis that the topog-raphy of the lower reaches is dominated by subduction-related tectonic deformation of the substrate rather than canyon erosion. Interrelationships between canyon erosion by currents and tectonic processes along the forearc slope are important in the develop-ment of the physiography of submarine can-yons upon active forearc margins.

Keywords: Submarine canyon, forearc slope, turbidite, erosion, Kuril Trench, Japan.

INTRODUCTION

For submarine canyons developed upon stable continental slopes, erosion and sedimentation via gravity fl ows are considered the primary mecha-nisms of the evolution of canyon physiography (Daly, 1936; Heezen and Ewing, 1952; Menard, 1955; Shepard and Emery, 1941). Submarine canyons and channels in such passive tectonic regimes tend to exhibit concave-upward thalweg profi les, which account for tectonic deformation that is less effective than sediment fl uxes, as recorded in both aggradational channels such as the Amazon Channel (Flood and Damuth, 1987; Pirmez and Imran, 2003) and erosional canyons such as the Zaire Canyon (Babonneau et al., 2002; Droz et al., 1996) and Rhône Fan Valley (Droz and Bellaiche, 1985; O’Connell et al., 1991; Torres et al., 1997). This asymptotic form of channel profi le is considered to be a function of the loss of fl ow competence as it erodes and transports material, resulting in a progressive decrease in downslope gradient (Carter, 1988; Kneller, 2003; O’Connell et al., 1991; Pirmez et al., 2000; Prather et al., 1998).

In addition to these exogenetic processes, endogenetic processes (e.g., tectonically con-trolled) and the structural fabric of underlying rocks are also important in terms of submarine canyon development upon forearc margins where subduction-related tectonics infl uences slope morphology (e.g., Greene et al., 2002; Hagen, 1996; Klaus and Taylor, 1991; Laursen and Normark, 2002; Lewis et al., 1998; Soh et al., 1990; Soh and Tokuyama, 2002). The planforms of canyons developed upon the slopes of active margins are typically erosional, and †E-mail: [email protected]

Physiographical and sedimentological characteristics of submarine canyons developed upon an active forearc slope:

The Kushiro Submarine Canyon, northern Japan

Atsushi Noda†

Taqumi TuZinoRyuta FurukawaMasato JoshimaGeological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Central 7, 1-1-1Higashi, Tsukuba, Ibaraki 305-8567, Japan

Jun-ichi UchidaDepartment of Earth Science, Faculty of Science, Kumamoto University, 39-1, Kurokami 2-chome, Kumamoto 860-8555, Japan

GSA Bulletin; May/June 2008; v. 120; no. 5/6; p. 750–767; doi: 10.1130/B26155.1; 15 fi gures; Data Repository item 2008047.

Physiography and sedimentology of submarine canyons on a forearc slope

Geological Society of America Bulletin, May/June 2008 751

exhibit sharp bends or conspicuous knickpoints that may be controlled by transverse accretionary ridges, outer-arc basement highs and mounds, or strike-slip faults (Hagen, 1996; Klaus and Taylor , 1991; Lewis and Barnes, 1999; Laursen and Normark, 2002; Soh and Tokuyama, 2002). The morphology of sub marine canyons devel-oped upon forearc slopes is therefore consid-ered to be primarily determined by interactions between the relative rate of basement deforma-tion by accretion or compression and the rate of mass redistribution by surface processes. Despite recent major advances in the seafl oor and subsurface imaging of submarine canyons, our knowledge of the effects of active tectonics on canyon morphology is insuffi cient to enable quantitative studies of canyon development on forearc slopes. Quantifying the geomorphologi-cal and sedimentological characteristics of sub-marine canyons on forearc slopes would there-fore assist in improving our understanding of canyon evolution along active plate margins.

The Kushiro Submarine Canyon cuts the forearc slope along an active convergent margin off eastern Hokkaido, along the Kuril Trench. Although this is one of the largest submarine canyons within Japanese waters, its geomor-phological and geological features have yet to be fi rmly established. The aims of this paper are therefore to (1) quantitatively describe the mor-phological features of the Kushiro Sub marine Canyon along the active forearc margin, (2) investigate recent erosional and depositional processes within the canyon, and (3) quantify the relative impacts of tectonics, sedimentation, and erosion on the evolution of the canyon pro-fi le. The results and associated canyon model are potentially applicable to other submarine canyons developed upon active margin slopes.

PHYSICAL SETTING

Land Area

Hokkaido is currently subjected to ongo-ing collisional tectonics at an arc-arc junction (Fig. 1) where a forearc sliver of the Kuril Arc is colliding with the Northeast Japan Arc (Kimura, 1996). Oblique subduction of the Pacifi c Plate along the southern Kuril Trench may have given rise to dextral movement of the Kuril Arc in the late Miocene (Kimura, 1986), and may still be occurring today (DeMets, 1992; Moriya, 1986). Islands along the southern Kuril forearc (Fig. 1) and geological structures within eastern Hokkaido have en echelon orientations that are consistent with the dextral strike-slip motion of the Kuril Arc (Fitch, 1972; Kimura, 1986).

Under the present tectonic regime, the Pacifi c Plate is subducting beneath the Okhotsk (North

American) Plate in the direction of N62° W at a rate of ~8 cm y−1 (DeMets et al., 1990). Recent global positioning system (GPS) observations suggest that eastern Hokkaido is moving in a WSW direction at a rate of 2–3 cm y−1 (Ito et al., 2000). Based on surveys of marine terraces, the uplift rate of eastern Hokkaido over the past 0.1 Ma is estimated to be 0.2 mm y−1, with an accumulated increase in elevation over this time of 20 m (Okumura, 1996).

Coastal and Submarine Areas

The coastline between Nemuro and Kushiro is complex, with several lagoons and rock cliffs as high as 80 m above sea level (Fig. 2). In con-trast, the coastline between Kushiro and Hiroo is smooth and arcuate. The Tokachi River is one of the largest rivers in Hokkaido that drains into the Pacifi c Ocean, with the Kushiro River hav-ing a lesser discharge. The drainage area, length,

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Figure 1. Tectonic setting and location of the study area. KSC—Kushiro Submarine Canyon; HSC—Hiroo Submarine Channel; TFB—Tokachi-oki forearc basin; Ko—Komagatake vol-cano; Us—Usu volcano; Ta—Tarumai volcano; Ma—Mashu volcano.

Noda et al.

752 Geological Society of America Bulletin, May/June 2008

and average fl ow rate of the Tokachi River are 9010 km2, 156 km, and 256 m3 s−1, respectively; the equivalent values for the Kushiro River are 2510 km2, 154 km, and 60 m3 s−1. The shelf in this area has a relatively narrow (10–30 km) width, with the narrowest part coinciding with

the Kushiro Submarine Canyon. The uppermost slope (at water depths of 200–1000 m) is more than 5° (Fig. 3), decreasing to 2–3° in the upper part (1000–3500 m depth). A series of anticlines are observed at a water depth of 2000 m (Figs. 2, 3, and 4); these appear to be the extensions of

NE–SW trending anticlines observed off Hiroo (Fig. 2), which may have begun to form in the Pliocene (TuZino and Noda, 2007).

Outer-arc high (3200–3500 m water depth) exists as an elongate topographic high oriented parallel to the trench (Figs. 1 and 2) and is placed

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A´Figure 2. Bathymetric map of the drainage areas of the Kushiro Submarine Canyon and the Hiroo Submarine Channel. Structural data were derived from TuZino and Noda (2007). Lines A–A′ and B–B′ indicate the locations of the cross-sectional bathymetric profi les shown in Figure 3. Dashed rectangles indicate the areas covered by the detailed topographic maps shown in Figures 4, 5, and DR1.



between the forearc basin (upper slope) and the accretionary prism (lower slope) (cf. Clift et al., 1998; Dickinson and Seely, 1979; Gulick et al., 2002; McNeill et al., 2000). The high can be correlated with the middle-slope boundary of a steep landward-dipping refl ec-tion described by Klaeschen et al. (1994). Two terraces are recognized on the slope: the upper terrace (3200 m water depth) is located on the upper slope immediately behind the outer high, and the lower is located on the trenchward side of the outer high (5000–5500 m water depth) (Figs. 2 and 5). The upper terrace is underlain by well-stratifi ed refl ectors (Schnürle et al., 1995; TuZino et al., 2006) and possibly formed via a combina-tion of subsidence and regional tilting (Klaeschen et al., 1994). TuZino et al. (2006) reported the formation of half-grabens along the upper terrace

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Figure 3. Cross-sectional bathymetric profi les of the forearc slope. The locations of lines A–A′ and B–B′ are shown in Figure 2. The average slopes (m/m) are 0.048 for A–A′ from the shelf break to the Kuril Trench, and 0.026 for B–B′ from the shelf break to the front of the Kushiro Submarine Canyon.

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Figure 4. Detailed bathymetry of the upper segments (A–B) of the Kushiro Submarine Canyon. Solid lines indicate the locations of seismic profi les (Figs. 10 and 11), and dashed lines indicate the locations of deep-sea camera survey lines. Solid circles denote the sampling localities of sediment cores (Fig. 12; Table DR2).

Noda et al.


and proposed that displacement of the outer high originally occurred along normal faults.

The slope between the upper and lower ter-races exceeds 5° and is covered by a 600-m-thick series of low-amplitude refl ections ori-ented parallel to the slope and underlain by weakly folded Neogene sediments (Schnürle et al., 1995). Uplift of this slope might have resulted from the underplating of sediment within the subduction zone (Klaeschen et al., 1994). The origin of the lower terrace has been explained in terms of the upheaval of the ridges located immediately trenchward of the terrace (Ogawa et al., 1993). The slope below the lower terrace is characterized by ridges and domes (Ogawa et al., 1993). The toe of the Kuril mar-gin consists of a sedimentary wedge (P-wave velocity of <4.0 km s−1) interpreted to have been strongly deformed during subduction-related plate dynamics (Nakanishi et al., 2004).

The Tokachi forearc basin lies between the Kushiro Submarine Canyon and the Hiroo Spur (Figs. 1 and 2). The slope of the basin aver-ages 1.5°, which is gentler than that off Kushiro (Fig. 3). The Tokachi forearc basin beneath the Hiroo Submarine Channel is considered to be an area of longstanding subsidence, as suggested by a negative gravity anomaly (Honza et al., 1978), seismic data (TuZino et al., 2004), and the results of drilling (Sasaki et al., 1985). The presence of more than 4000 m of Neogene sediments in the basin suggests an average sedimentation rate of 330 m/Ma during the Miocene and 160 m/Ma during the Plio-Pleistocene (Sasaki et al., 1985).

Overview of the Canyon and Channel

Kushiro Submarine CanyonThe Kushiro Submarine Canyon, with an

8266 km2 drainage area below the shelf break and a 233 km channel length, is a large shelf-indenting canyon located off the eastern Hokkaido forearc of northern Japan (GSA Data Repository Table DR11; Sato, 1962; Shimamura, 1989; Yo, 1953). The canyon head incises the forearc slope from north to south; the canyon then changes course to the east, and fi nally to the southeast or south-southeast to the Kuril Trench at a water depth of ~7000 m (Fig. 2). The canyon cuts through two prominent topographic highs: an anticline (2000 m water depth) and the outer high (3,000 m water depth) (Figs. 2, 4, and 5). Two large tribu-taries empty into the canyon at water depths of 1700 m and 4800 m (Fig. 2); the deeper of these tributaries is known as the Hiroo Submarine

Channel. In addition, several small gullies join the canyon on its northern side between water depths of 1800 m and 2500 m (Fig. 4). At the outer high, the canyon shows a right-lateral dis-placement of 1–2 km (Fig. 5). A submarine fan (10 km long and 35 km wide) has developed at the end of the Kushiro Submarine Canyon, within the trench (Fig. 5) (Ogawa et al., 1993).

Hiroo Submarine ChannelWith a drainage area of ~4000 km2 below the

shelf break and a channel length of 156 km, the Hiroo Submarine Channel is the longest tributary of the Kushiro Submarine Canyon (Fig. 2; Table DR1 [see footnote 1]). The channel heads are sit-uated at a water depth of 300 m and thus do not indent the outer shelf (Fig. 2). The Hiroo Sub-

marine Channel runs eastward with low to mod-erate sinuosity until it merges with the Kushiro Submarine Canyon at a water depth of 4800 m. Several other tributaries from the northwest converge at acute angles (65° ± 20°), forming a pinnate drainage pattern. According to TuZino and Noda (2007), the architectural development of the channel occurred by aggradation, with the progressive growth of levees along the channel.

DATA AND METHODS

Our study is primarily based on an analysis of multibeam, swath-bathymetric data (SeaBeam and Hydrosweep). These data were collected by the Hydrographic and Oceanographic Depart-ment of the Japan Coastal Guard ( Maritime

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Figure 5. (A) Bathymetry of the lower segments (C–E) of the Kushiro Submarine Canyon. (B) Longitudinal plots of the water depths of the canyon bottom (thalweg) and the tops of the canyon walls. HSC—Hiroo Submarine Channel.

1GSA Data Repository Item 2008047, submarine canyons on an active forearc slope, is available at www.geosociety.org/pubs/ft2008.htm. Requests may also be sent to [email protected].



Safety Agency, 1998); cruises KH90–1 and KH92–3 of the R/V Hakuho-Maru of the Ocean Research Institute, University of Tokyo (Kobayashi et al., 1998; Ogawa et al., 1993); cruise KR0504 (April and May 2005) of R/V Kairei, run by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC); and cruises GH02 (August 2002) and GH03 (June 2003) of the Geological Survey of Japan, the National Institute of Advanced Industrial Science and Technology (AIST) conducted using the R/V Hakurei-maru No. 2 of the Japan Oil, Gas and Metals National Corporation ( JOGMEC). Quantitative parameters of canyon water depth, relief, width, gradient, and sinuos-ity were measured at 5 km intervals along the thalweg on bathymetric maps with 10 m con-tours compiled using 100–200 m grid data.

Seismic refl ection profi les were collected during cruises GH02 and GH03 using a GI gun (generator 250 in3 and injector 105 in3 air gun) with a six-channel streamer cable. The survey speed was 8 knots (14.8 km/h), and the shooting interval was 6 s. The survey lines were densely meshed at intervals of 2 miles (3.7 km) E-W and 4.5 miles (8.3 km) N-S.

Gravity and piston cores were collected in and around the Kushiro Submarine Canyon (Fig. 4; Table DR2). Samples of volcanic ash were collected for measurements of their glass chemistries to enable comparisons with those reported previously (Furukawa et al., 1997; Furukawa and Nanayama, 2006; Katsui et al., 1978; Shimada et al., 2000). Benthic foramini-fers were extracted from sandy turbidites and hemipelagic mud to determine their sources, as deduced from the distributions of mod-ern benthic foraminifers (Abe and Hasegawa, 2003; Matsuo et al., 2004). Halved cores were measured by gamma-ray attenuation at 1 cm intervals using a GEOTEK multisensor core logger. Accelerator mass spectrometer (AMS) 14C measurements of planktonic and benthic foraminifers were carried out on fi ve horizons (seven samples) in cores PC07, PC09, and PC10. We used a reservoir age of 386 ± 16 yr for the 14C ages in this region (Yoneda et al., 2001). The obtained conventional radiocarbon ages were calibrated to calendar ages using CALIB rev. 5.0.2 (Stuiver and Braziunas, 1993) and the data set marine04.14c (Hughen et al., 2004).

RESULTS

Quantitative Morphological Parameters

Longitudinal Profi leThe slope of the canyon and channel increases

downstream (Fig. 6), meaning that the entire longitudinal profi le cannot be described in terms

of a linear, exponential, or Gaussian profi le (Adams and Schlager, 2000; Goff, 2001). We divided the profi le into segments based on chan-nel curvature (Fig. 6). Longitudinal profi les of channel water depths and lengths for each seg-ment fi t the following power function:

D L= α β , (1)

where L is channel length, D is channel water depth, and α and β are constants (Komar, 1973). The constants of α and β were calculated via an implementation of the nonlinear least-squares

Aoga ShimaCanyon

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Figure 6. Longitudinal depth profi les along the Kushiro Submarine Canyon (A) and the Hiroo Submarine Channel (B) with curves fi tted by non-linear least-squares estimates. Arrows indicate points where tribu taries join the main can-yon and channel. (C) Compari-son of the Kushiro Submarine Canyon with other submarine canyons developed upon active tectonic margins. Data for Aoga Shima Canyon, Monterey Canyon, and the San Antonio Submarine Canyon are derived from Klaus and Taylor (1991), Greene et al. (2002), and Hagen (1996), respectively.

Noda et al.


method. For values of the exponent β of less than 1 in equation 1, the profi les show concave-upward shapes.

The longitudinal profi les of the two studied channels are similar (Figs. 6A and 6B), and both are divided into fi ve segments. As a whole, the channels have convex-upward profi les, com-parable with those of submarine canyons devel-oped within other active margins (Fig. 6C).

ReliefRelief is defi ned here as the difference in

water depth between the thalweg and the can-yon rim. Relief within the Kushiro Submarine Canyon increases gradually in the upper seg-ments (A–B), maintains high values within Segment C, and then decreases toward the trench (D–E) (Fig. 7A). The right-hand relief (when looking down-canyon) is higher than the left-hand relief (Table DR1), especially at water depths of between 2800 and 5000 m. The

pattern of relief within the Hiroo Submarine Channel is similar to that in the Kushiro Sub-marine Canyon, increasing with channel water depth to 2500 m before gradually decreasing to the channel end (Fig. 7A).

GradientThe gradient of the Kushiro Submarine Can-

yon shows a gradual decrease, although with some fl uctuations, over Segments A and B (Fig. 7C); the gradient is approximately con-stant where the canyon traverses the anticline. The gradient increases markedly across Seg-ment C where the canyon crosses the outer high and then decreases with widening of the canyon and reducing relief. At the end of the canyon the gradient increases again, reaching a maximum (0.16) at a point outside of the area shown in Figure 7C. The gradient of the Hiroo Sub marine Channel shows a similar trend to that for the Kushiro Submarine Canyon (Fig. 7C).

SinuosityAlong almost all reaches of the canyon, the

sinuosity is low (0.01–0.03) or approximately straight. The highest sinuosity recorded within the Kushiro Submarine Canyon occurs in Seg-ment B, where the canyon crosses the upstream side of the anticline and the gradient remains relatively low (0.01–0.03) (Figs. 4, 7D, and 8).

Meandering bends in the canyon show asym-metric profi les, with steep cutbanks on the con-cave sides of bends and relatively gentle slip-off slopes on the convex sides (Fig. 8). The progres-sive downstream decrease in the radius of curva-ture and meander wavelength (Fig. 8) leads to their classifi cation as deformed ingrowth meanders (e.g., Schumm, 1977).

High-sinuosity sections of canyon are also recognized in the Hiroo Submarine Channel (Fig. 7D). The fi rst sinuous section (~1000 m water depth) corresponds to transection of the Hiroo Spur (Fig. 2), while the second corre-sponds to increased gradient and relief (Figs. 7 and DR1). The gradual decrease in meander wavelength and the radius of curvature and degree of asymmetry of cross-sectional profi les suggest that meanders within the Hiroo Sub-marine Channel are also deformed ingrowth meanders (Fig. DR1).

Drainage AreaThe topography of bedrock rivers typically

exhibits a scaling between the local channel gra-dient (S) and the contributing upstream drainage area (A) (Flint, 1974; Hack, 1957; Howard and Kerby, 1983). By analogy with subaerial rivers, the erosion rates (E) of submarine canyons can be written as E = KAmSn, where K is a dimen-sional coeffi cient of erosion, and m and n are positive constants that depend on basin hydrol-ogy, channel hydraulic geometry, and erosion process (Howard, 1994; Whipple and Tucker, 1999). If the bedrock uplift rate U is constant, a topography that has evolved to a steady state has balanced erosion and uplift such that U = KAmSn. Hence, there arises an inverse power-law relationship between S and A (a proxy for dis-charge), with S = kA−θ, where k is the steepness index and θ is the concavity index (e.g., Whipple , 2004). The concavity index derived from river catchment data generally varies between 0.1 and 1.1, although values generally range from 0.3 to 0.6 for onshore areas (Snyder et al., 2000; Tucker and Whipple, 2002) and from 0.1 to 0.3 for offshore upper slope areas (Mitchell, 2004; Ramsey et al., 2006). The inverse power-law relationship is interpreted as arising from the fact that the erosional effect of a down-canyon increase in discharge (frequency of erosive fl ows) is balanced by a reduction in gradient to achieve a spatially equilibrated erosion rate.

0

0.5

1

1.5

2

Wid

th (

km)

0

200

400

600

800

1000

Rel

ief (

m)

KSC (left)KSC (right)HSC (left)HSC (right)A

B

C

D

E

Outer high

Anticline

0

0.02

0.04

0.06

0.08

0.1

Gra

dien

t (m

/m)

1

1.5

2

Sin

uosi

ty

Aa b c d

B C De

ESegments

KSC

HSC

KSCHSC

KSCHSC

KSCHSC

0

2

4

6

8

0 1 2 3 4 5 6 7

Dra

inag

e ar

ea (

103 k

m2 )

Channel depth (km)

KSCHSC

Figure 7. Morphological profi les along the Kushiro Submarine Canyon (KSC) and the Hiroo Submarine Channel (HSC). (A) Channel relief with respect to thalwegs. (B) Channel width. (C) Channel gradient. (D) Sinuosity. (E) Drainage area.



The Kushiro Submarine Canyon and Hiroo Submarine Channel have drainage areas of similar size (Fig. 7E), although they differ in other morphological parameters. The drain-age area of the Kushiro Submarine Canyon shows a gradual increase in the upper seg-ments (A–B) but only a minor increase in Segments D–E below the confl uence with the Hiroo Submarine Channel. The slope-area plot (Fig. 9) shows an inverse power-law rela-tionship between decreasing local slope and increasing drainage area in the upper segments (A–B; θ = 0.14).

Deep-Tow Camera Observations

The canyon walls, terraces, and thalweg in Segment B were observed using a deep-tow camera system (Fig. 4). Observations of the seafl oor indicate that the canyon walls are characterized by steep or subvertical slopes of semiconsolidated mudstone and sandstone (Fig. DR2A). These sedimentary rocks have been sampled previously using a grab sam-pler, and dated as Late Pliocene on the basis of radiolaria (Motoyama, 2004) and diatom biostratigraphy (Watanabe, 2004). The rocks are widely burrowed by epibenthic organ-isms or fi shes (Figs. DR2A and DR2B). The canyon walls and terraces are draped

with a homogeneous cover of fi ne-grained sediment that contains a number of burrows (Figs. DR2C and DR2D).

The bottom of the canyon is covered with heavily burrowed, hemipelagic mud. We observed highly turbid water (the nepheloid layer) within the lowermost 5–10 m of the can-yon bottom, and slowly drifting water down-canyon. We also observed angular to subangular cobble- and boulder-sized gravels mantled by hemipelagic mud in the thalweg at the base of the canyon wall (Figs. DR2E and DR2F). No ripples or current lineations were observed.

Seismic Profi les

Seismic refl ection profi les were collected across the upper segments (A and B) of the Kushiro Submarine Canyon. Analysis of the seis-mic profi les reveals that the canyon fl oor along Segment A is covered with acoustically trans-parent or weakly stratifi ed facies (Figs. 10A and 10B); former canyon valleys are recognized under the transparent facies. The canyon walls have gentle slopes, forming U-shaped, cross-sectional profi les. In contrast, the canyon along Segment B has a V-shaped profi le, devoid of fi ll facies along the canyon axis (Figs. 10C–10E).

High-resolution bathymetry, combined with the seismic profi les, shows the presence of ter-

races in Segments A and B (Figs. 4 and 10). The terraces have generally fl at upper surfaces and are up to 200 m in width; their elevations above the axis of the canyon vary between 100 and 400 m. The observed acoustic refl ectors gener-ally continue through the terraces without any evidence of structural discontinuities.

144°45′E

144°50′E

144°55′E

42°30′N

42°30′N

−2500

−2000

−2000

−2000

−2000

−1500

0 5

km

−2500

−2500

Anticline

Anticline

A′

B′C′

A

BC

Anticline

A′

B′C′

A

BC

−2.5

−2.0

5 km

A A′

Dep

th (

km)

−2.5

−2.0

B

B′

Dep

th (

km)

−2.5

−2.0C

C′

Dep

th (

km)

Figure 8. Bathymetry and channel planform of the meandering part of the Kushiro Submarine Canyon. Solid lines indicate the locations of cross-channel profi les. Two-stepped terraces are recognized in Sections B–B′ and C–C′.

10−3

10−2

10−1

100

107 108 109 1010

Drainage area (m2)

Loca

l slo

pe (

m/m

)

HSCOH

Segments A–B

Segments C–E

S = 0.42 A–0.14

Figure 9. Slope-area plot for the Kushiro Submarine Canyon. In the upper segments (A–B), the local slope decreases gradually with increasing drainage area. In contrast, no clear relationship is recognized between slope and drainage area in the lower seg-ments (C–E). OH—outer high; HSC—Hiroo Submarine Channel.

Noda et al.


The canyon crosses a trench-parallel anticline in the middle part of Segment B (Figs. 2 and 4). The anticline is accompanied by synclinal subsidence on its landward side, which is fi lled by accumulated sediments that form a narrow zone of gentle slope (Fig. 11A). The surface sediments are thick in the syncline and very thin upon the anticline, suggesting that anticlinal deformation is ongoing.

In the lower part of Segment B, the canyon cuts down through eastward-tilting strata (Fig. 11B). The forearc slope subsides in the east-ern (left-hand) side and is uplifted in the west-ern (right-hand) side, thereby explaining the observed difference in wall heights (Fig. 7B).

Late Quaternary Sedimentation in the Canyon

Core GH03–1033, obtained from Segment A (Fig. 4; Table DR2), consists of olive black (7.5Y3/2) clayey silt (Fig. 12), layers of vol-canic ash, and several thin, silty turbidites. The vol canic ashes are correlated with tephras of the Komagatake and Tarumai volcanoes (Figs. 1 and 12). No turbidite deposits are recognized above the tephras in Core GH03–1033 (Noda et al., 2004). A high sedimentation rate of ~400 cm ky−1 is estimated based on tephra chronology.

Core KR0504–PC07, collected from the upper part of Segment B (Fig. 4; Table DR2), consists of olive black diatomaceous clayey silt inter calated with more than 40 turbidites (Fig. 12). Turbidites in the lower part of the core are generally thicker and coarser (up to granule-sized grains) than those in the upper part. Some of the turbidites are amalgamated, with parallel-laminated silt or sand layers at the base and cross-laminated silt-sand at the top (Figs. 13A–13C). Relatively coarse turbidites are occasionally associated with chaotic muddy sediments beneath cross-laminated, medium-grained sand (Fig. 13D). Turbidite layers also show both graded bed-ding and parallel laminations (Fig. 13E). Calibrated 14C ages determined using mixed plank-tonic foraminifers (mainly Neogloboquadrina

B

A

C

D

E

WSW ENE

WSW ENE

5 km 0.5k

m

PC07

WSW ENE

WSW ENE

SSENNW

Figure 10. Interpretations of seismic pro-fi les across Segments A and B of the Kushiro Submarine Canyon. Transparent or weakly stratifi ed acoustic facies bury the former canyon bottom (A–B). Refl ectors on the tops of the terraces are continuous in the sedi-ments, with no evidence of faults or slides (C–E). The locations of the profi les are shown in Figure 4. Flow directions within the canyon are from back to front.



pachyderma) indicate an average sedimenta-tion rate of 147 cm ky−1 (Table DR3). The average interval of turbidite deposition over the past 2343 yr is estimated to be ~76 yr.

Benthic foraminifers observed in two turbi-dite layers within Core PC07 (BF1 and BF3) are dominated by Elphidium batialis and Uvigerina akitaensis, with lesser Bolivina spissa and Epi-stominella pacifi ca (Fig. 12; Table DR4). These dominant taxa are similar to those in the hemi-pelagic mud (BF2) that occurs between the two turbidites (Table DR4), which is indicative of a deep-water (>1000 m water depth) environment (Abe and Hasegawa, 2003; Matsuo et al., 2004; Uchida, 2006). Few shelf or upper slope benthic foraminifers are recognized in the turbidites.

Core PC09, obtained from the landward side of the terrace on the outer high (Fig. 4), consists of olive black (7.5Y3/2–10Y3/2) diatomaceous

clayey silt and several turbidites of sandy silt or very fi ne sand (Fig. 12). Our age model sug-gests that the average sedimentation rates and depositional intervals between turbidites within this core are 22–57 cm ky−1 and 371–1136 yr, respectively (Fig. 12; Table DR3).

Core PC10 was obtained from a point bar located ~20 m above the thalweg within Seg-ment C (Fig. 4). The core consists of diatoma-ceous clayey silt with layers of sandy silt to very fi ne sand (Fig. 12). Granule- to pebble-sized gravel (up to 3 cm across) is observed near the bottom of the core (Fig. DR3). The gravel layers show no evidence of internal structures and lack silt and clay but are normally graded at their tops (Fig. DR3). A calibrated 14C age (Fig. 12; Table DR3) suggests an average sedimentation rate for this core of 32 cm ky−1 and a turbidite recurrence interval of 311 yr.

DISCUSSION

Modeling of Canyon Profi les

The obtained seismic records indicate that the Kushiro Submarine Canyon shows ero-sional features and did not develop levee sys-tems in the recent past. Furthermore, the lack of debris and presence of clast-supported gravels upon the canyon fl oor suggest that the eroded material was largely removed to the deep by turbidity currents. In such cases, it is appropri-ate to use a detachment-limited erosion model (e.g., Howard and Kerby, 1983; Howard, 1994) to estimate the development of the longitudinal profi les of channels and canyons. Such mod-els have been widely applied to bedrock rivers within active orogens (e.g., Whipple, 2004) and submarine canyons (e.g., Mitchell, 2004, 2005). A transport-limited type of erosion (Rosen-bloom and Anderson, 1994; Whipple and Tucker, 2002), which leads to the diffusion of knickpoints (Mitchell, 2006), also partly con-trols canyon profi les. Mitchell (2006) proposed a simple equation for such models of submarine canyon topography:

z x t H x U x t E x t( , ) , , ,= ( ) + ( ) − ( )0 (2)

with

E x t Kz

xK

z

xdta d

t, ,( ) = +

⎛⎝⎜

⎞⎠⎟

⎛

⎝⎜⎞

⎠⎟∫∂∂

∂∂

2 3 2

20 (3)

where z(x,t) is the elevation over time, t; H0(x) is

the initial elevation; U(x,t) is the height of tec-tonic uplift or subsidence; E(x,t) is the depth of erosion or height of sedimentation by currents; K

a and K

d are constants; ∂z/∂x is the local can-

yon gradient; and ∂2z/∂x2 is curvature.We developed a fi nite-difference model to

predict the forms of the canyon thalweg and wall profi les using equations 2 and 3 (cf. Mitchell , 2006; Riihimaki et al., 2007; Seidl et al., 1994). We tested two types of simulation; one was a backward modeling from the present thal-weg profi le to an initial slope profi le (Test A; Fig. 14A), and the other was a forward model-ing from inferred initial slope profi les to present canyon thalweg and wall profi les (Tests B and C; Figs. 14B and 14C). In testing the models (Fig. 14), hemipelagic deposition was assumed to occur only outside of the canyon. The seismic profi les suggest that Quaternary deposits are thicker in the upper slope (1000–3000 m water depth) than in the uppermost slope (~1000 m), being ~0.2 s (two-way travel time) off Nemuro (Noda and TuZino, 2007; TuZino et al., 2004). The fi nding indicates that the sedimentation rate during Quaternary was greater than 7.5 cm ky−1,

2

3

24002600 22003000 2800Shot number

Shot number

5 km5 kmVE = 7.2VE = 7.2 A

NNWSSW

ENEWSW

5 km

Synclinalsubsidence

UpliftUpliftUplift VE = 7.2

400 600 8002000 1000

4

3

Two-

way

trav

el ti

me

in s

econ

dsTw

o-w

ay tr

avel

tim

e in

sec

onds

B

5 kmVE = 7.2

subsidence

UpliftUpliftUplift

Figure 11. Interpretations of seismic profi les across the Kushiro Submarine Canyon. The locations of the profi les are indicated in Figure 4. Flow direction within the canyon is from back to front. (A) Canyon and the anticline within Segment B. (B) Difference in elevation between the eastern (left-hand) and western (right-hand) canyon walls. This discrepancy can be explained by tilting of the basement, involving subsidence of the east wall and uplift of the west. VE—vertical exaggeration.

Noda et al.


provided that the acoustic velocity is 1.5 km s−1 or more. Drilling data from the Tokachi forearc basin indicate a sedimentation rate of 16 cm ky−1 since the Pliocene (Sasaki et al., 1985). The Holocene sedimentation rate on the slope is esti-mated to be 22–57 cm ky−1, based on Core PC09

obtained from outside the canyon (Fig. 12). In contrast, onshore tectonic uplift has been esti-mated to be 20 cm ky−1 for the period since 125 ka (Okumura, 1996); this value is similar to the sedimentation rate calculated for the slope. Although accurate values of uplift, subsidence,

and sedimentation have yet to be determined, in Tests A and C we assumed that relative uplift of the outer-arc high and subsidence of the forearc basin on the landward side of the high were approximately balanced by hemipelagic sedi-mentation on the slope (Figs. 14A4 and 14C4).

PC10(330 cm)

PC09(730 cm)

~ ~ ~

~ ~ ~~

~ ~ ~

~ ~~

~~ ~ ~

~~~

v v

v

~ ~ ~

~~~ ~ ~

~

v

vv

PC07(492 cm)

1033(268 cm)

v v

vv v

v

v

v

vv

vv

Gravel

Sandy silt–very fine sand

Fine sandMedium sandCoarse sandVery coarse sand

Clayey silt

Fine sand

Coarse sand

Gravel

Sandy silt–

very fine sand

Medium

sand

Very coarse sand

v v

v v

v v

vv

Ta-b

Ta-a

Ta-a(AD1739)

Ta-b(AD1694)

Ko-c1(AD1856)

9584cal yr BP

Ko-c2(AD1667)

Ta-c(2.5 ka)

Ma-ghi(8.5 ka)

Us-b(AD1663)

Us-b

Ta-aKo-c2v v

v

~ ~ ~ ~

~ ~~~ ~

~~ ~

v

v

v

~ ~~ ~~ ~v

10595cal yr BP

19585cal yr BP

7631cal yr BP

0

1 m

BF1

BF3BF2

Olive black (7.5Y3/2)clayey silt

Olive black (10Y3/2–10Y3/1) clayey silt

Dark olive gray (2.5GY3/1)clayey silt

Tephra

Bioturbation~ ~ ~

Normal grading

Reverse grading

Horizon for benthicforaminifers analysis

v v

Soft-X radiographsin Fig. 13

Photographin Fig. DR3

2343cal yr BP

1 0

1

2

3

4

(m) 1.5 2 2.5Density (g/cm3)

10

1

2

3

(m) 1.5 2 2.5

Density (g/cm3)

10

1

2

3

4

5

6

7

(m) 1.5 2 2.5

Density (g/cm3)

A

B

C

D

E

Figure 12. Logs of the sedi-ment cores collected from the Kushiro Submarine Canyon (Fig. 4). The sources and ages of the volcanic ashes in core GH03–1033 are from Noda et al. (2004).



In Test A (backward modeling), we restored the initial slope profi le (H

0) using the approxi-

mated curves fi tted by equation 1 (Fig. 6) of the present thalweg profi le (H

p). The initial profi le

(H0) was calculated by subtracting tectonic

deformation (U) and recovering canyon ero-sion (E) from the present thalweg profi le (H

p):

H0 = H

p − U + E (Fig. 14A1). We then predicted

the present height of the canyon wall (Hwp

) by restoring tectonic deformation (U) and adding hemipelagic sedimentation (S): H

wp = H

p + U + S

(Fig. 14A2). The total values of U and S were assumed as in Figure 14A4, and the total ero-sion and sedimentation by currents in the can-yon were calculated according to equation 3. The model produced an example of the simu-lated canyon wall profi le, which shows a good

correlation with the actual heights of the canyon wall (Fig. 14A3). The modeled relief showed a gradual increase toward the outer high, followed by a decreasing trend to the trench. This result shows that a large amount of erosion can be expected at the canyon head and the outer high (which forms a knickpoint), with a high uplift rate (Fig. 14A4). Minor erosion is expected on the landward side of the outer high (Segment B) and in Segment D, where the thalweg profi les are concave-upward.

In Tests B and C (forward modeling), we simulated present longitudinal profi les using inferred initial slopes. We adopted convex pro-fi les as initial slopes, which indicate uplift on land and subsidence in the trench. Test B rep-resents a passive margin regime in which tec-

tonic deformation of the substrate is negligible or spatially uniform, and in which hemipelagic sedimentation is concentrated on the upper slope (Biscaye and Anderson, 1994; Pirmez et al., 1998; Sanford et al., 1990). Test B produced smooth profi les of the thalweg and canyon wall, without any knickpoints (Figs. 14B1–14B3). A predicted gradual decrease in down-canyon relief was discordant with the actual profi les.

Test C was performed to assess the param-eters of tectonic deformation and hemipelagic sedimentation employed in Test A, representing an active forearc margin regime (Fig. 14C4). The resultant profi les of the thalweg and can-yon wall are similar to those produced in Test A using actual canyon profi les (Fig. 14C3). Canyon erosion is predicted to be small on the

Grayscale0100200

5 cm

A

B

C

Grayscale0100200

Grayscale0100200D E

Sectionboundary

50 0 100 150

MGS μm

Figure 13. Soft X-radiographs of turbidites recovered from Core PC07. (A)–(C) Discontinuous turbidites. Lower sand layers are fi ne-grained and thin. Upper sand layers are coarser grained and thick, generally with parallel or cross laminations. Sharp basal boundaries are common in the upper sand layers. (D) The turbidite consists of deformed muddy sediment at the base, overlain by cross-laminated, medium-sized sand. (E) Parallel-laminated and normal-graded turbidites. The horizons depicted in these sections are shown in Figure 12. MGS—mean grain size.

Noda et al.


−1.0

−0.5

0.0

0.5

1.0

Hei

ght (

km)

−200 −150 −100 −50 0

Distance (km)

A4

0

1

2

3

4

5

6

7

Hei

ght (

km)

A3

0

1

2

3

4

5

6

7

Hei

ght (

km)

A2

0

1

2

3

4

5

6

7

Hei

ght (

km)

−200 −150 −100 −50 0

Distance (km)

A1

−200 −150 −100 −50 0

Distance (km)

B4

B3

B2

−200 −150 −100 −50 0

Distance (km)

B1

−1.0

−0.5

0.0

0.5

1.0

Hei

ght (

km)

−200 −150 −100 −50 0

Distance (km)

C4

0

1

2

3

4

5

6

7

Hei

ght (

km)

C3

0

1

2

3

4

5

6

7

Hei

ght (

km)

C2

0

1

2

3

4

5

6

7

Hei

ght (

km)

−200 −150 −100 −50 0

Distance (km)

C1

Present thalweg (Hp)

Simulated thalweg (H0)

Kd = 1

Ka = 0.03

Kd = 1

Ka = 0.03

Simulated thalweg (H0)

Simulated canyonwall (Hwp)

Present thalweg(Hp)

Present right-hand wall

Tectonic deformation (U)

Erosion (–Ee)

Hemipelagic deposition (S)

Sedimentation (Es)

Simulated canyon wall (Hwp)

Simulated thalweg (H0)Simulated thalweg (Hp)


Present thalweg

Simulated thalweg (Hp)


Present thalweg

) = 0.51 −x + 14 − 2

Kd = 1

Ka = 0.03



H0(x

0(x) = 0.51 √−x + 14 − 2H


0(x) = 0.51 √−x + 14 − 2H

√ ) = 0.51 −x + 14 − 2

Simulated canyonwall (Hwp)

H0(x √

H0 = Hp – U + E Hp = H0 + U – E

Hwp = H0 + U + S Hwp = H0 + U + S

Hp = H0 + U – E

Hwp = H0 + U + S

Figure 14. Results of numerical modeling of canyon longitudinal profi les. (A1) Initial slope profi le (H0) calculated using the approxi-mated curves fi tted by equation 1 for the present thalweg profi les (Fig. 6). (A2) Present-day height of the canyon wall (H

wp) calculated

using the simulated initial slope profi le in A1. (A3) Comparison among the simulated and present-day profi les. (A4) Cumulative values of hemipelagic deposition (S), tectonic deformation (U), and canyon erosion (−Ee) and sedimentation (Es). Total erosion (E) is calculated by Ee − Es. (B) Simulated canyon profi les based on an inferred initial slope with stable tectonics and linear regression of hemipelagic sedimentation, as representative of passive plate margins. (C) Simulated canyon profi les based on the parameters used in Test A, as representative of active plate margins.



landward side of the outer high (Segments B and C) but large where the canyon crosses the uplifted outer high. These erosional patterns are similar to those predicted from backward modeling in Test A. The differences in erosion in the area of the canyon head predicted by Tests A and C can be attributed to the forms of the initial slope profi les: the real canyon head has an exponential profi le.

Late Quaternary Sedimentology

Segment A is a mud-fi lled channel, devoid of turbidites above the seventeenth-century tephra layers. The accumulation of muddy sediments (transparent acoustic facies) in the former nar-row and V-shaped canyon fl oor generated broad and U-shaped morphology of the fl oor during the Holocene highstand. Although terrigenous detritus could have been supplied directly into the canyon from the Kushiro River under low-stand sea levels, our observations suggest that little terrigenous sand is carried to the canyon head, and suspended material fi lls the canyon at a high sedimentation rate (~400 cm ky−1) under the modern highstand; the upper canyon is con-sidered to be inactive (Fig. 15).

The canyon bottom along Segment B is not buried by fi ll facies and contains many turbid-ites (Figs. 10 and 12). The predominance of deep-sea taxa among benthic foraminifers in the turbidites suggests that the turbidites may have been derived from the upper slope rather than the river mouth or canyon head (water depth greater than 1000 m) (Fig. 15). The occurrence of numerous gullies on the northern (left-hand side) slope of Segment B (Fig. 4) indicates a local sediment source, either via repeated slope failure or sediment gravity fl ows (cf. Field et al., 1999; Pratson and Coakley, 1996). The aver-age turbidite recurrence interval of 76 yr deter-mined for Segment B (Core PC07) over the past 2300 yr is more frequent than that determined for Segment A (Core GH03–1033) and the area outside of the canyon (Core PC09), thereby indi-cating that turbidity currents are concentrated in the middle part of the canyon. Mass movements or sediment gravity fl ows upon a deep-sea slope may be triggered by a reduction in the shear strength of the sediments in association with strong and frequent seismic shaking (Hampton et al., 1996; Lee and Edward, 1986; Normark and Piper, 1991). Rapid subduction (8 cm yr−1) of the Pacifi c Plate beneath Hokkaido likely generates earthquakes in this area with recur-rence intervals of 50–100 yr (Hirata et al., 2003; Kanamori, 1970; Kikuchi and Fukao, 1987; Shi-mazaki, 1974; Yamanaka and Kikuchi, 2003). The recurrence interval of turbidite deposition may well correspond to that of earthquakes.

Earthquake-induced turbidity currents probably play an important role in the morphological evo-lution of the canyon.

A number of the analyzed turbidites are dis-continuous in terms of texture and structure, being composed of a fi ner part that is thinly pla-nar laminated and an overlying cross-laminated coarser part (Figs. 12 and 13). This type of amal-gamation has been recognized as a “grain-size break” by Nakajima and Kanai (2000), thought to represent sediments deposited by more than one current associated with one or more simul-taneous sediment gravity fl ows. Strong earth-quakes are unlikely to induce sediment gravity fl ows from just a single source. It is also possible that a turbidity current associated with sediment failure might have occurred upon local canyon walls close to the coring site, with the main tur-bidity current passing through the site.

The occurrence in Segment B of active vertical or horizontal incision by sediment gravity fl ows is expected based on the narrow, V-shaped nature of the valley, the lack of muddy fi ll material, and the steepness of the walls composed of horizontally

stacked refl ections (Fig. 10). Axial downcutting by canyon-confi ned mass fl ows may undercut and destabilize the canyon walls. Angular boul-ders of semiconsolidated rocks along the foot of the canyon wall (Figs. DR2E and DR2F) imply sidewall collapse (cf. Stubblefi eld et al., 1982). Intense burrowing and scouring by epifauna and some demersal fi shes (Fig. DR2A and DR2B) are also important factors in the erosion of canyon walls (Dillon and Zimmerman, 1970; Palmer, 1976; Valentine et al., 1980; Warme et al., 1978), as burrowing weakens local areas of the canyon walls and promotes small-scale slumping and the generation of debris. The burrowed rocks become decomposed, making them susceptible to sliding and down-canyon transport.

The clasts in pebble-sized gravels within Core PC10 are similar to those in gravels found around the shelf edge as relict sediments that were not buried by younger sand and mud, and those in gravels on the upper slope (water depths of 500–1500 m) deposited as diamictite (Noda and TuZino, 2007). Gravels might have been transported from the shelf edge to the slope

Anticline

Outer high

Remnantterraces

Kushiro RiverTokachi River

Hiroo Spur

IncisedIncisedmeandersmeandersIncisedmeanders

Kuril Trench

Knickpoint retreat(narrow width, high relief, and steep gradient)

Uplifted marine terraces

Terrace

Turbidity currents

Erosion ≈ Tectonics

Erosion < Tectonics

Inactive

Active

Decreasing relief and width

Increasing gradient

Increasing relief and dischargeDecreasing gradient

Shelf breakPresent canyon headPresent canyon headPresent canyon heads

Mud filledMud filled(broadly U-shaped)(broadly U-shaped)

Mud filled(broadly U-shaped)

Sediment gravity flowsSediment gravity flowsSediment gravity flows

Figure 15. Schematic representation of the geomorphological and sedimentological features of the Kushiro Submarine Canyon and the Hiroo Submarine Channel.

Noda et al.


during the lowstand and early stages of trans-gression, subsequently being derived from the slope via gravity events. The low silt and clay contents of the gravel beds suggest effective hydraulic sorting during current fl ows or the selective removal of fi ne-grained material from the beds. Fining-upward sedimentary structures within the top parts of the gravel beds indicate that the gravels might also have been deposited by hyperconcentrated density fl ows (cf. Lowe, 1982; Mulder and Alexander, 2001). Parker (1982) argued that an ignitive fl ow with a mean velocity of 6 m s−1 is able to move cobbles up to 8 cm across. Such catastrophic currents may have transported and deposited pebble-sized gravels during the last deglaciation.

Physiographical Implications

The gradual decrease in gradient in the upper segments of the canyon (Fig. 9), which overall defi nes an exponential profi le, indicates that an inverse power-law relationship can be adopted between the gradient and the drainage area, simi-lar to graded rivers in subaerial environments (Flint, 1974; Howard and Kerby, 1983; Whipple , 2004) and submarine canyons at passive mar-gins (Mitchell, 2004, 2005). Such a relationship suggests that the effect of increasing frequency of fl ow events with drainage area is balanced by decreasing canyon gradient (Fig. 15). A relatively high frequency of fl ow events might be expected along Segment B, judging from the numerous gullies located on the left-hand (northern) slope of the canyon (Fig. 4).

Erosion of the canyon with uplift of the upper slope is demonstrated by the presence of terraces along Segments A and B (Fig. 4). Continuous internal refl ections beneath the terrace, without any evidence of normal faulting (Fig. 10), argue against the possibility that the terraces originated by deposition with lateral migration of the thal-weg (Damuth et al., 1988; Nakajima et al., 1998) or slumping of the channel walls (Carlson and Karl, 1988; Cramez and Jackson, 2000; Kenyon et al., 1995; Liu et al., 1993). Flume experiments demonstrated that with increasing slope gradi-ent of substrate, channels incised into cohesive substrate were characterized by progressively lower ratio of width to depth and had a more pronounced erosional thalweg than remaining parts of the previously scoured channel fl oor (Shepherd and Schumm, 1974; Wohl and Ikeda, 1997). Therefore, the terraces in the Kushiro Submarine Canyon are considered as remnants of a paleocanyon fl oor resulting from progres-sive erosion with uplift of the upper slope.

The narrowing canyon width in Segment C with increasing gradient and relief suggests that the outer high acts as a knickpoint (Figs. 5

and 7). Currents entering regions with high uplift rates have the potential to narrow chan-nels in areas where the slope steepens (Duvall et al., 2004; Finnegan et al., 2005). The high relief in the middle of the canyon (Fig. 7A) can be explained by high uplift rate (high erosion rate) of the outer high (Fig. 14). A zone of deep incision where canyon crosses outer arc high has also been recognized in other submarine canyons developed along subduction-related active margins (Hagen, 1996; Klaus and Taylor, 1991). In other cases in which uplift rate of an outer high exceeds the erosion rate by currents, a canyon may be unable to cut down through the structural high, leading to the ponding of sediment behind the high, as observed with the Bonin forearc (Taylor and Smoot, 1984); alternatively, the canyon may be defl ected by the high, as observed with the San Antonio Sub marine Canyon (Hagen et al., 1994). There-fore, the erosional potential of the Kushiro Sub-marine Canyon at the outer high must be equal to or greater than the tectonic uplift (Fig. 15).

The lower segments (C–E) do not show a clear relationship between slope and drainage area (Fig. 9). Uplift of the outer high probably interrupts any process capable of establishing a near-constant relationship throughout the entire canyon. The alternating convex-upward and concave-upward profi les observed in the lower segments are possibly controlled by the mor-phology of the slope close to the canyon (Fig. 3). We suggest that the erosion rate decreases down-canyon, based on a gradual decrease in relief and constant catchment area (i.e., constant dis-charge) (Figs. 7A and 7E). If erosion by turbid-ity currents is proportional to the slope gradient (equation 3), relief will be higher in Segment E (Figs. 14C3 and 14C4). Ridge and dome struc-tures (Ogawa et al., 1993) developed upon the lower slope, which consists of sedimentary rocks deformed by subduction (Nakanishi et al., 2004; Schnürle et al., 1995), can be assumed to have a greater shear strength than hemipelagic sedi-ments of the upper slope (e.g., Hempel, 1995); therefore, the substrate of the lower slope is potentially more resistant to erosion by turbidity currents than sediments of the upper slope. We consider that tectonic controls on canyon profi le might be stronger than canyon erosion along the lower segments (Fig. 15).

In terms of the Hiroo Submarine Chan-nel, longstanding subsidence in the Tokachi forearc basin has led to the development of aggradational rather than erosional channels (TuZino and Noda, 2007). This in turn has led to the development of narrower channel systems with lower relief than those of the Kushiro Sub-marine Canyon. If the two channels had experi-enced sedimentary gravity fl ows of similar scale

and frequency, they would have met at the same water depth (i.e., Fairplay’s law). The large dif-ference in water depth of the two features at the site where the channel joins the main canyon suggests contrasting activity (erosive potential) between them. In addition, the absence of any change in the gradient of the main canyon at the confl uence with the Hiroo Submarine Channel indicates the inferior contribution of the tribu-tary in terms of discharge and erosive power.

CONCLUSIONS

The Kushiro Submarine Canyon is the main conduit for sediments transported between the forearc slope and the Kuril Trench in the active forearc margin along the southwestern Kuril Trench. The upper segments of the canyon (A–B; ~3250 m thalweg water depth) incise into the upper slope along a weakly exponential, longi-tudinal profi le, even where the canyon traverses an anticline. The relief between the thalweg and the canyon rim shows a gradual increase with increasing drainage area. Remnants of a paleo-canyon fl oor, evident as terraces, and ingrowth meanders indicate progressive erosion of the canyon with uplift of the upper slope. Frequent turbidity currents within Segment B, with a recurrence interval of less than 100 yr, were triggered by earthquakes related to subduc-tion of the Pacifi c Plate. Based on the occur-rence of buried facies in the uppermost segment (A) and benthic foraminifers in turbidites, the present source of turbidity currents is regarded to be the northern slope of the canyon rather than the canyon head. An inverse power-law relation-ship between the canyon gradient and drainage area indicates that the effects of a down-canyon increase in the frequency of fl ow events is bal-anced by a decrease in canyon gradient.

The longitudinal profi le of the lower segments (C–E; 3250–7000 m) of the canyon largely refl ects the profi les of the forearc slope near the canyon. The narrow width and high relief of Segment C suggest that the outer high has acted as a knickpoint: it breaks the continuity of ero-sional processes whose actions lead to an equi-librium state. The lowermost segments (D–E) are characterized by gradual decreases in relief and width and an increase in gradient; there is no clear relationship between local slope and size of the drainage area. This fi nding suggests that the lower segments are unable to achieve a steady-state condition between downcutting ero-sion and topographic deformation. These results demonstrate the importance of canyon erosion in the upper slope and tectonic deformation of substrate in the lower slope in terms of the evolu-tion of the physiography of submarine canyons developed upon the slopes along active margins.



ACKNOWLEDGMENTS

We are greatly indebted to the offi cers, crew, and research staff of cruises GH02, GH03, KY0407, and KR0504 for the collection of data. We also thank Yukinobu Okamura, Kenji Satake, Ken Ikehara, Tomoyuki Sasaki, and Kohsaku Arai for data obtained on cruise KR0504. We are grateful to Kyohiko Mitsuzawa and Hiroyuki Matsumoto for conduct-ing the deep-sea camera survey, Azusa Nishizawa for SeaBeam data, Kazuhiro Miyazaki for the fi nite-difference model, and Ken’ichi Ohkushi for pick-ing the foraminifers. Neil C. Mitchell and Takeshi Nakajima provided helpful comments on an early ver-sion of the manuscript. We acknowledge Ivano Aiello, an anonymous reviewer, and Associate Editor Akira Ishiwatari, whose constructive comments signifi cantly helped us to improve the manuscript. This study was part of the “Marine Geological Mapping Project of the Continental Shelves around Japan” program supported by the Geological Survey of Japan, The National Insti-tute of Advanced Industrial Science and Technology (AIST). Financial support for this research was also provided by the Japan Nuclear Energy Safety Organi-zation (JNES).

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MANUSCRIPT RECEIVED 22 NOVEMBER 2006 REVISED MANUSCRIPT RECEIVED 4 OCTOBER 2007 MANUSCRIPT ACCEPTED 17 OCTOBER 2007

Printed in the USA

Table DR1: Comparison of the Kushiro Submarine Canyon (KSC) and the HirooSubmarine Channel (HSC).

KSC HSCLength (km) 233 156Drainage area (km2) 8226 3968Shelf indent Yes NoLevee No YesWater depth of canyon head (m) 70 300Average relief (m, right) 499 119Average relief (m, left) 357 98Maximum relief (m, right) 910 330Maximum relief (m, left) 595 280Average width (km) 0.80 0.22Maximum width (km) 1.96 0.41Average gradient (m/m) 0.031 0.034Maximum gradient (m/m) 0.16 0.28Average sinuosity 1.10 1.16Maximum sinuosity 2.03 1.90

Table DR2: Localities of core samples.Date Cruise No Longitude Latitude Water depth (m) Core length (cm)

7/June/2003 GH03 1033 144◦19.793’E 42◦40.006’N 964 26827/April/2005 KR0504 PC07 144◦42.964’E 42◦29.038’N 2089 49228/April/2005 KR0504 PC09 144◦12.3349’E 42◦14.6466’N 3140 73028/April/2005 KR0504 PC10 144◦05.537’E 42◦13.548’N 3308 330

Table DR3: Radiocarbon ages of foraminifers in hemipelagic muds. Calibratedages were applied for a local reservoir correction of 386±16 years (Yoneda et al.,2001).Core Depth Sample type Conventional δ13C Calibrated age Calibrated age Median

14C age (yr BP) (permil) (1σ) (cal yr BP) (2σ) (cal yr BP) probabilityPC07 351–355 Planktonic Foram. 3,040±40 −3.1 2,293–2,403 2,205–2,480 2,343PC07 351–355 Benthic Foram. 3,860±40 −2.8 3,309–3,414 3,241–3,458 3,357PC09 430–434 Planktonic Foram. 7,550±50 −0.6 7,575–7,673 7,535–7,753 7,631PC09 552–556 Planktonic Foram. 10,130±40 −2.8 10,545–10,638 10,510–10,707 10,595PC09 726–730 Benthic Foram. 17,200±130 −2.5 19,449–19,601 19,307–19,869 19,585PC10 300–304 Planktonic Foram. 9,310±50 −1.0 9,515–9,643 9,469–9,739 9,584PC10 300–304 Benthic Foram. 10,180±40 −3.7 10,577–10,693 10,539–10,823 10,646

Table DR4: Occurrences (%) of benthic foraminifers in turbidites (BF1 and BF3)and hemipelagic mud (BF2) within Core PC07. UMS, uppermost slope; US,upper slope; WD, water depth.Samples BF1 BF2 BF3Depth (cmbsf) 320–322 355–356 362–364AGGLUTINATED BENTHIC FORAM.Eggerelloides advenum 0.8CALCAREOUS BENTHIC FORAM.Angulogerina ikebei 0.4 UMSBolivina decussata 1.5 UMSBrizalina pacifica 2.7 UMS–USBolivina spissa 2.8 6.3 UMS–USBuccella spp. 4.2 1.3 OthersBulimina aculeata 0.8 OthersBulimina striata 0.4 1.3 UMS–USCassidulina norvangi 53.7 OthersCibicides lobatulus 0.4 ShelfCibicides spp. 0.4 ShelfCibicidoides sp. 2.5 UMS–USEilohedra nipponica 0.8 USElphidium batialis 53.5 20.5 50.0 USElphidium spp. 0.4 1.3 OthersEpistominella pacifica 15.5 2.3 5.0 USFursenkoina cf. rotundata 0.8 OthersFursenkoina sp. 1.5 OthersGlobobulimina auricurata 5.0 UMS–USGlobobulimina spp. 0.4 OthersIslandiella norcrossi 1.4 0.4 UMS–USNonionella globosa 1.5 OthersNonionellina labradorica 1.2 OthersOolina melo 0.4 OthersTakayanagia delicata 0.8 UMS–USUvigerina akitaensis 26.8 0.4 26.3 UMS–USUvigerina senticosa 1.3 OthersValvulineria spp. 2.7 OthersOther calcareous benthic foram. 1.8 OthersTotal benthic foram. number 71 257 80Total planktonic foram. number 0 53 26P/T ratio 0.0 17.1 24.5Shelf (100–500 m WD) 0.0 0.8 0.0Uppermost slope (500–1,000 m WD) 0.0 1.9 0.0Uppermost–upper slope (1,000–2,000 m WD) 31.0 4.6 41.3Upper slope (2,000–3,000 m WD) 69.0 23.6 55.0Others 0.0 68.3 3.8

144˚40'E 144˚50'E

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Figure DR1: (A) Bathymetry and channel planform of the Hiroo SubmarineChannel. Dashed lines denote the locations of cross-channel and along-channelprofiles. (B) Cross-sectional channel profiles. (C) Along-channel depth profiles.Channel length is adjusted to the slope length.

A

C

B

D

E F

Figure DR2: Photographs taken by deep-sea camera. The length between yellowmarkers on the chain is 1 m. (A) Steep canyon wall of horizontally stratifiedsemi-consolidated mudstone, partly bored by benthic organisms. (B) Close-upof the boreholes in the canyon wall. (C) Heavily burrowed hemipelagic muddysediments upon a terrace. (D) Canyon wall mantled by hemipelagic mud. (E)Angular semi-consolidated mudstone clasts (up to 50 cm long) at the base of thecanyon wall. (F) Boulders along the thalweg (over 1 m in diameter) draped witha thin coating of hemipelagic mud.

0

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Figure DR3: Photograph of pebbly gravel from the bottom of Core PC10 and itsmean grain-size distribution. The horizon of the extracted sections is shown inFig. 12. MGS, mean grain size.

Physiographical and sedimentological characteristics of ... · 1991; Torres et al., 1997). This asymptotic form of channel proﬁ le is considered to be a function ... canyons within

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