Evolution of Kumano Basin and Sources of Clastic Ejecta and Pore Fluid in Kumano Mud volcanoes, Eastern Nankai Trough

  

Evolution of Kumano Basin and Sources of Clastic Ejecta and Pore Fluid

in Kumano Mud volcanoes, Eastern Nankai Trough

  Sumito Morita

  1)

  , Juichiro Ashi

  2)

  , Kan Aoike

  3)

  and Shin’ichi Kuramoto

  3) 1)

Geological Survey of Japan, AIST-GREEN, Tsukuba, Ibaraki 305-8567, Japan.

  2) Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan. 3)

Center for Deep Earth Exploration, Japan Marine Science and Technology Center, Yokosuka,

  Kanagawa 237-0061, Japan Abstract: S

  tructural analyses revealed that the Kumano Basin is divided into north and south zones. The north zone of the basin is composed of anticlines, whereas the south zone shows a large extended sedimentary basin dammed up by outer arc high. The Kumano mud volcanoes are situated above the anticlines latent below the flat seafloor. Consolidated clastic ejecta in the mud volcanoes indicate the clastic rocks are originally deposited in the late Early Miocene through the Middle Miocene. From this age and lithologic characteristics, it is thought that the Kumano Basin was initiated by this time, concurrently with the old abandoned forearc basins on Shimanto accretionary belt in Kii Peninsula. Rock-Eval resulted in that the clastic ejecta are all immature for natural gas generation. This is inconsistent with the thermogenic hydrocarbon gas detected from pore water in subbottom sediments. Taking account of these results, it is necessary to consider origins of the lithics and the pore fluid of the mud volcanoes separately.

1. Introduction

  In order to make a geological investigation at a great depth below the deep seafloor, the means to approach is restricted except for a drilling. However, in the Kumano Basin there are mud volcanoes, in which mud diapirism has brought the substances of the depths up to the seafloor as clastic ejecta, and the mud volcanoes are the best window to acquire the information on nature of the strata and pore fluid at the deeper portion of the Kumano Basin. In this study, we aim at reconstructing the formation process of the Kumano Basin and the Kumano mud volcanoes, evaluating the deep strata of the Kumano Basin, and exploring the source and the migration process of the pore fluid.

  For the structural analyses, we used two dimensional seismic data from the METI’s* Fundamental Exploration (Japan National Oil Corporation, 2002) and the data from IFREE at JAMSTEC** (Park et al., 2002). Observation records and subbottom materials obtained by submersible dives with SHINKAI 6500 (YK02-02 cruise) and ROV*** KAIKO (KR02-10 and KR03-05 cruises) were applied to offer a geologic constraint.

• METI: Ministry of Economy, Trade and Industry in Japan

  **IFREE: Institute for Frontier Research on Earth Evolution at Japan Marine Science and Technology Center

  Figure 1: Interpretation map of geologic structure in the Kumano Basin.

2. Kumano Basin

  The Kumano Basin is situated off southwest Japan and is laid on the eastern Nankai accretionary prism which has formed by subduction of the Shikoku Basin on the Philippine Sea Plate since about 15 Ma. The Kumano Basin is the largest forearc basin in the Nankai Trough. It extends about 100 km in E-W and 70 km in N-S. The present feature of the Kumano Basin generally shows a flat seafloor through the whole area at around 2,000 m in depth. The basin is surrounded the north side by Shima Spur and continental slope off Kii Peninsula, and is bounded on the east by Dai-ni Atsumi Knoll and on the south by outer arc high (Figure 1).

2.1. Geologic Structure of the Kumano Basin

  Topography of the Kumano Basin shows the aspect of a large single sedimentary basin, but the seismic data analyses found that the Kumano Basin can be divided into two portions, the north and the south zones, in its deep geologic structure (Figure 1). The north zone of the Kumano Basin has three anticlines with 5-10 km intervals. The anticlines are dependent on geometry of the basement and they strike NE-SW to ENE-WSW along with structure of the Nankai Accretionary Prism (Figure 1 and 2). On the other hand, the south zone indicates a large extended sedimentary basin dammed up by uplift of the outer arc high, excepting two anticlines in the east end of the south zone. Judging from deformation in flanks of the anticlines, the anticlines have risen in parallel with aggradation of the basin. Their uplifts seem younger to the

  Figure 2: A transect seismic cross-section in the middle part of the Kumano Basin. The profile is combined a METI’s line and a IFREE’s line. south, because the sedimentary basins between the anticlines indicate basically north dipping. The bottom of the Kumano Basin is changing with deformed geometry of the basement, and the basin is about 1 - 2 km in thickness.

2.2. Kumano Mud Volcanoes

  Detailed topographic surveys, submersible investigations and various subbottom material analyses have identified seven mud volcanoes so far on the Kumano Basin floor (e.g. Kuramoto et al., 1998; Sawada et al., 2002; Figure 1). They are called by numbering of the Kumano Knolls, like so Kumano Knoll No.1 or No.2. The Kumano mud volcanoes are generally around 100 m high and less than 2 km in diameter. They are all situated above the anticlines latent below the flat seafloor, and the diapirism is also observed along the anticlines (Figure 2). Some of the diapirs which reached up to the seafloor form mud volcanoes, and some form mud ridges which strike along the anticlines. Thus, the mud diapirs, the mud volcanoes and the mud ridges are well related to the geologic structure, and they appear on the anticlines in the north zone and in the east end of the south zone of the Kumano Basin.

  A B C

  D

  Figure 3: Views from the submersible on the surface in Kumano Knoll No.4. A: rock sampling with manipulator. B: a crater-like depression with some dead clams in Kumano Knoll No.3 was just crossed over by the METI’s seismic survey line and its cross-section was successfully imaged. On the KAIKO’s subbottom profiles, there is a apparent difference on the acoustic cross-sections between the Kumano Knoll No.4 and the surrounding Kumano Basin floor. The basin floor has an over 50 m thick stratified reflection, whereas the Kumano Knoll shows a transparent profile in its body.

  As results of observations on the submersible dives, the Kumano Knoll No.4 was found to be bumpy on the surface. It is covered with mud and there are some consolidated sedimentary rocks (clastic ejecta of the mud volcano) scattered on the surface (Figure 3). Some living Calyptogena and other chemosynthetic creatures were also observed. Some dead clams were found in the center of a crater-like depression (Figure 3). The same kinds of features have been observed in some other Kumano mud volcanoes, and these indicate existence of cold seeps on the mud volcanoes.

  On the submersible dives, push coring (short columnar sampling) was also performed on the surface of the mud volcanoes. Ijiri et al. (2002) reported that hydrocarbon gas extracted from pore water in the push cores was mixture of thermogenic gas, by the gas composition and their isotope ratios. For those reasons, it is thought that there is a continuous fluid supply from a deeper portion below the mud volcanoes.

3. Clastic Ejecta of the Kumano Mud Volcanoes

  The clastic rocks recovered by the submersible vehicles are composed of mud stone, mud breccia, and arkose sandstone (quartz arenite). The mud breccia is characterized by carbonate vein, which was identified as calcite afterward by XRD analysis. Some mud breccia is cut by the vein through the breccia structure but some are cut only in its matrix. The arkose sandstone is typically rich in biotite. Porosity of the clastic rocks is 9-39 % as a whole.

  On density analysis by X-ray CT scanning, the mud breccia was found so homogeneous that it is difficult to discern grain and matrix. The minor sandy part was low density respectively. As a result of microscopic observation, it was turned out that the clastic rocks of less than 20 % in porosity are altered to calcite, in the whole things of the mudstone and the mud breccia and in matrix of the arkose sandstone. As for some coarse arkose sandstones, quartz and feldspar grains are fractured and the fractures are filled with calcite. This may have been caused by shear fracturing under high pore pressure in a mud diapir.

3.1. Chronology in Nannofossil Identification

  Nannofossil was identified about the obtained clastic ejecta. For this operation, piston cores on other cruises (KH01-02, KT02-01 and KY02-02) were also included. The time was determined for 17 samples from four mud volcanoes, and more than a half of the samples are interpreted as CN3 - CN4 of Okada and Bukry (1980), which corresponds to 13.6 - 18.2 Ma （late Early Miocene - early Middle Miocene）.

  3.2. Evaluation for Hydrocarbon Gas Potential as Source Rock

  To evaluate potential for hydrocarbon gas generation as source rock, TOC (total organic carbon) analysis and Rock-Eval pyrolysis by were performed for 19 samples of the mudstones and the mud breccias. As a result, TOC was all less than 1 % and Tmax on the pyrolysis was 439 ℃ in maximum (an average of 429 ℃ ). Consequently, the analyzed clastic ejecta of the mud volcanoes were all evaluated immature for natural gas generation.

  3.3. Carbon and Oxygen Isotope Analyses

  Carbon and oxygen isotope analyses were performed for 10 samples of vein calcite in the

  13

  mud breccia and the arkose sandstone. As a result, δ C was -17.7 - -3.8 ‰ [vs. PDB] and δ

  18 O was +26.9 - + 29.1 ‰ [vs. SMOW].

  4. Discussions

  4.1. Formation process of the Kumano Basin

  The age determined by nannofossil identification, late Early Miocene through early Middle Miocene, corresponds to that of the formation of old abandoned forearc basins on Shimanto accretionary belt in Kii Peninsula, i.e. Kumano Group and Tanabe Group. The Kumano Group is composed of biotite-rich acidic igneous rocks and the related sedimentary rocks (Sasada, 1988). Due to the resemblance of lithology, the source of the arkose sandstone can be the Kumano acidic rocks. The Tanabe Group is known for clastic dikes and mud diapirism, which can be observed on outcrops (Shimizu, 1985). This may suggest a similarity of the sedimentary environment by mud diapirism in a forearc basin. These resemblances in age, lithological characteristics, and the paleo-environment suggest that the formation of the Kumano Basin was initiated in or by the late Early Miocene, concurrently with the formations of the neighboring abandoned forearc basins on Shimanto accretionary prism. Since then, the Kumano Basin has developed until today, suffering structural deformation accompanying development of Nankai prism. Accordingly, the deformation has formed the anticlines and has led to mud diapirism to create the Kumano mud volcanoes.

  4.2. Origin of the Pore Fluid

  The vein calcite which intruded into the mud breccia can be treated as a key to understand nature of the pore fluid at a deep portion. Since the calcite formed as vein, it cannot have been precipitated by sulfate reduction near the subsurface. Assuming that the vein calcite was precipitated from pore water of seawater-origin, the temperature which the calcite formed at is

  18

  presumed to be about 21 - 32 from the value of O and the isotope fraction factor for

  ℃ δ

  2

  calcite-H O system (Friedman and O’Neil, 1977). Taking account of the present thermal gradient (about 50 K/km) in the Kumano Basin, it is thought that the vein in the mud breccia may have formed at the depth between the gas hydrate BSR and the bottom of the Kumano Basin.

  13 produced the calcite is considered to be mixture of carbonic acid ion of the organic matter in sediments and carbon in seawater, or mixture of carbonic acid ion of methanogenic zone. In any case, the relation to gas generated from organic matter in sediments is important.

  The fact that the clastic rocks were evaluated immature for gas generation is not consistent with the report that thermogenic gas was detected on the mud volcanoes (Ijiri et al., 2002). Therefore, it is necessary to consider origins of the lithics and the source of the pore fluid of the mud volcanoes separately. Consequently, there is a possibility that the origin of the fluid may occur not in the lower part of the Kumano Basin but in the Shimanto and/or Nankai accretionary belts beneath the Kumano Basin (Figure 4).

  Figure 4: Schematic of the recent feature of the Kumano Basin.

5. Conclusions

  The structural analyses in the Kumano Basin and the subbottom material analyses in the Kumano mud volcanoes have led the following conclusions:

  1) The Kumano Basin was divided into north and south zones in its deep structure. The northern zone consists of three anticlines, whereas the southern zone is a large extended sedimentary basin which is dammed up by newly uplifted outer arc high. Formations of the mud diapirs, the mud volcanoes and the mud ridges are much related to geologic structure, and they generally develop on the anticlinal axes.

  2) Consolidated clastic ejecta recovered on the Kumano mud volcanoes are composed of mudstone, mud breccia and biotite-rich arkose sandstone. The age and the lithological characteristics of the clastic ejecta suggest that the Kumano Basin was initiated in or by the late Early Miocene, concurrently with the neighboring old forearc basins in the Kii Peninsula.

  3) The clastic ejecta on the Kumano mud volcanoes were evaluated immature for hydrocarbon gas generation. Since also the thermogenic gas has been reported in the mud source of the pore fluid in the Kumano mud volcanoes.

  Acknowledgements We would like to express our appreciation to Ms. Emiko Shimbo, Ms. Yuko Iwanaga, Dr.

  Yoshihiro Tsuji and Mr. Hideki Matsubayashi at Technology Research Center, Japan National Oil Corporation for their cooperation in subbottom material analyses. We also thank all scientists and the crew of YK02-02, KR02-10 and KR03-05 cruises for their support in the submersible dives and the success of the researches.

  References

  Ashi, J., Sawada, T., Nakamura, Y., Kuramoto, S., Murayama, M., Kameo, K., Watanabe, M., Yamamoto, F. and Tokuyama, H. (2002) High resolution mapping of mud volcanoes and their structural evolution in the Kumano Trough: Results from WADATSUMI sidescan sonar survey. Abstracts of Geological Society of Japan. 109, p122. Friedman, I. and O’Neil, J.R. (1977) Compilation of stable isotope fractionation factors of geochemical interest, Data of geochemistry, 6th ed. USGS Professional Paper, 440-KK.

  Ijiri, A., Tsunogai, U., Gamo, T., Ashi, J., Kinoshita, M. and Nakamura, K. (2002) Chemical composition of the pore water obtained from mud diapirs in the Kumano Basin. Abstracts of Geochemical Society of Japan, 49, p116. Kuramoto, S, Kishimoto, K., Nakao, S., Tokuyama, H., Yamamoto, F. and Taira, A. (1998)

  Seafloor acoustic imagery map of the off Tokai area. Miscellaneous Map Series, Geological Survey of Japan, 37. Morita, S., Sawada, T., Ashi, J., Gulick, S., Flemings, P., Kuramoto, S., Saito, S., Mikada, H.,

  Kinoshita, M. and Science Parties in YK02-02 and KR02-10 (2002) Observations and rock analyses in a Kumano mud volcano in Nankai Accretionary prism. EOS, Transactions, American Geophysical Union, 83, p1292-1293. Okada, H. and Bukry, D. (1980) Supplementary modification and introduction of the code numbers to the low-latitude coccolith biostratigraphic zonation. Marine Micropaleontology,

  5, 3, p321-325. Park, J-O., Tsuru, Kodaira, Cummins and Kaneda (2002) Splay fault branching along the Nankai subduction zone. Science, 297, p1157-1160.

  Sasada, M. (1988) Kumano acidic rocks. Regional Geology of Japan, Chubu II. 5, p142-143. Sawada, T., Ashi, J., Murayama, M., Aoike, K., Ujiie, Y., Kuramoto, S., Kinoshita, M. and

  Tokuyama, H. (2002) Surface geology and history of mud volcanoes in Kumano basin from piston cores. Abstracts of Geological Society of Japan. 109, p280. Japan National Oil Corporation (2002) Tokaioki-Kumanonada, Report of National Fundamental Exploration in Japan. Shimizu, H. (1985) Pebbly mudstone diapirs of the Tanabe Group in the Kii Peninsula,