Redistribution of Sediments by Submarine Landslides on the Eastern Nankai Accretionary Prism

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1 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, Hon-Komagome, Bunkyo, Tokyo , Japan 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 , Japan T. Shibata Kochi University, Akebono-cho, Kochi , Japan K. Fujino Kyushu University, 10 1 Hakozaki, Higashi-ku, Fukuoka , Japan A. Misawa Tokai University; Orido, Shimizu, Shizuoka , 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, 313 Advances in Natural and Technological Hazards Research, Vol 28, Springer Science + Business Media B.V. 2010

2 314 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, E, ranging from the Shikoku Basin to the Nankai Trough (Fig. 1a, b). We obtained bathymetric data using the SeaBeam 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).

3 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.

4 316 K. Kawamura et al. 1.1 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

5 Redistribution of Sediments by Submarine Landslides on the Eastern Nankai 317 Table 1. Location of core sampling sites PC Longitude Latitude Depth PC E N 4084 m PC E N 4127 m PC E N 4167 m PC E N 4178 m PC E N 4062 m PC E N 4184 m PC E N 4245 m PC E N 4334 m PC E N 3984 m PC E N 4347 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 Average Minimum Maximum Count PC cm PC cm PC cm PC cm PC cm PC cm 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.

6 318 K. Kawamura et al. Kashinozaki Knoll Shikoku Basin B 10 km Turbidite Nankai Trough PC-10 PC-12 Axial channel Submarine landslide PC-09 PC-07 PC-11 PC-05 PC-08 PC-06? Volcanic ash (AT) PC-04 Hemipelagic mud PC-03 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-km 2 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.

7 Redistribution of Sediments by Submarine Landslides on the Eastern Nankai 319 Fig. 4 Detail photo of PC-11 from 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

8 320 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

9 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. References Dominguez, S., Malavieille, J. and Lallemand, S.E., 2000, Deformation of accretionary wedges in response to seamount subduction: Insights from sandbox experiments. Tectonics, 19, Kobayashi, K., 2002, Tectonic significance of the cold seepage zones in the eastern Nankai accretionary wedge an outcome of the 15 years KAIKO projects: Mar Geol 187, Kobayashi, K., Cadet, J.P., Aubouin, J., Boulegue, J., Dubois, J., Huene, R., Jolivet, L., Kanazawa, T., Kasahara, J., Koizumi, K., Lallemand, S., Nakamura, Y., Pautot, G., Suyehiro, K., Tani, S., Tokuyama, H., and Yamazaki, T., 1987, Normal faulting of the Daiichi-Kashima Seamount in the Japan Trench revealed by the Kaiko I cruise, Leg 3, Earth Planet. Sci. Lett., 83, Ike, T., Moore, G.F., Kuramoto, S., Park, J-O., Kaneda, Y. and Taira, A., 2008, Tectonics and sedimentation around Kashinozaki Knoll: A subducting basement high in the eastern Nankai Trough. Isl. Arc, 17, Miyairi, Y., Yoshida, K., Miyazaki, Y., Matsuzaki, H. and Kaneoka, I., 2004, Improved 14 C dating of a tephra layer (AT tephra, Japan) using AMS on selected organic fractions: Nuclear instruments and methods in physics research Section B: beam interactions with materials and atoms Miyazaki, S. and Heki, K., 2001, Crustal velocity field of southwest Japan: Subduction and arc arc collision. J. Geophys. Res., 106, Pickering, K.T., Hiscott, R.N. and Hein, F.J., 1989, Deep-marine environments clastic sedimentation and tectonics. Unwin Hyman, London, 416 p. Saffer, D.M. and Bekins, B.A., 2002, Hydrologic controls on the morphology and mechanics of accretionary wedges. Geology, 30, Seno, T., Stein, S. and Gripp, A.E., 1993, A model for the motion of the Philippine Sea Plate consistent with Nuvel I and geological data, J. Geophys. Res., 98, 17, Spinelli, G.A. and Underwood, M.B., 2004, Character of sediments entering the Costa Rica subduction zone: Implications for partitioning of water along the plate interface. Isl. Arc, 13,

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