36

InterRidge NewsInternational Research: Back Arc Basins

Deep-tow sonar “WADATSUMI” survey in the Okinawa TroughK. Okino1, H. Tokuyama1 and HOTWATER Scientific Party
Scientific Party: [Tectonics] J.-C. Sibuet2, C.-S. Lee3, A. Takeuchi4, N. Nakamura1,M. Watanabe1, K. Kameo1, M. Asada1, M. Takaesu4, C.-H. Tsai3, Y.-C. Yeh5, F. Yamamoto6,P. Bergman7, N. Togashi8: [Water Geochemistry] T. Gamo9, T. Oomori10, H. Chiba11, H. Obata1,T. Noguchi10; [Heat Flow] H. Hamamoto12, T. Tsuji1; [Petrology] R. Shinjo10, S. Haraguchi1,T. Toyama10, R. Murakami1; [Sedimentology] M. Ikehara13, Y. Suganuma1, A. Matsuda13,
N. Kawamura13
Ocean Research Institute, University of Tokyo, Tokyo, Japan
IFREMER, Brest, France
3
National Taiwan Ocean University, Keelung, Taiwan
4
Toyama University, Toyama, Japan
5
National central University, Jung-Li, Taiwan
6Ocean High Technology Institute Inc., Tokyo, Japan
7
Fugro Seafloor Surveys Inc., Seattle, USA
8
Marine Works Japan Ltd., Yokohama, Japan
9
Hokkaido University, Sapporo, Japan
10
University of the Ryukyus, Okinawa, Japan
11
Institute for study of the Earth’s Interior, Okayama University, Tottori, Japan
12Earthquake Research Institute, University of Tokyo, Tokyo, Japan
13Marine Core Center, Kochi University, Japan
1235JAPANghour
iTka

CHINAnNaTrench

East China Sea

30

ghaou
Tr(Ryukyu)

Study Areaw
nakiotoO

sei-s

25

20120

yAmami
Plateau

Daito -Ridge

Oki-DnNa

TAIWAN

Gagua Ridge

Introduction
The Okinawa Trough, locatedbetween Japan and Taiwan, is anincipient backarc basin formed bythe extension within continental
lithosphere behind the Ryukyu
trench-arc system (Fig. 1). The continuous formation of new oceanic
crust has not occurred yet, however, the crustal thinning and rifting is
ongoing and many normal faults
have developed. The early rifting
phase was dated late Miocene, however, the date of the beginning of theextension and the characteristics ofthe early phase are still being debated (e.g. Kimura, 1996, Park 1998).
The second rifting phase started
about 2 Ma, indicated by the normal
or listric faults trending 065° and theassociated tilted faults blocks in thenorthern and middle Okinawa
Trough. The faulting and the subsidence in the second phase haveformed the present wide depressionof the Okinawa Trough. The mostrecent phase of the extension is characterized by 085° normal faults. Thefaults are mainly located in the south-aitoR

idg

EU/PH
65mm/yr306¡ (NUVEL-1)eWest Philippine Basin
125

130

Figure 1. Location of the survey site, southwestern tip of the Okinawa Trough,backarc rifting zone behind the Ryukyu arc-trench system.

Vol. 11(2), 2002

37

International Research: Back Arc Basins: Okino et al., cont...
western Okinawa Trough and thedistribution of faults is restrictedwithin an east-west trending 13-km
wide band (Sibuet et al., 1998). Volcanic rocks from the central grabensin the middle and southwestern
Okinawa Trough show that magmas
emplaced in the area are bimodal incomposition, with basalt and rhyolite as the dominant modes (e.g.
Shinjo et al, 1999) and their ages areyounger than 1 Ma (e.g. Ishikawa etal., 1991).
In the Okinawa Trough, thesouthwestern part of the trough ismost active and contains many interesting features, such as, numerous volcanoes crossing the rift zone
(cross backarc volcanic trail: Sibuetet al., 1998), high heat flow, andactive hydrothermal activities (Matsumoto et al., 2001). A French-Taiwanese co-operation cruise in 1996
has provided a seafloor bathymetry
map and the seismic profiles of theregion. During the R/V Hakuho-maru
KH02-1 cruise in June 2002, we conducted detailed geological, geophysical and geochemical experiments in this area based on the previous studies. The main purposes of
the cruise were 1) to investigate theprocess and evolution of continental rifting based on high-resolution
sonar imaging, 2) to characterize thehydrothermal activity in relation torifting, 3) to understand the genesisof volcanism in the backarc continental rift zone, and 4) to examine thethermal structure in the hydrothermal region. The main survey item
was the high-resolution seafloor
imaging by the deep-tow sidescansonar “WADATSUMI” and thename of the research cruise was assigned as “HOTWATER (Hakuhomaru Okinawa Trough WadaTsumi
ExpeRiment) “ cruise. The cruise wasplanned and conducted under theinternational cooperation among
Japan, Taiwan and France.
Instrumentation
The HOTWATER cruise started
from Naha on June 10 and ended at
Tokyo on June 24, 2002. The instruments used during the cruise included:1) Deep-tow side-scan sonar system“WADATSUMI” (100kHz) with
self-recording MAPRs (MiniatureAutonomous Plume Recorder /provided by E. Baker in NOAA)
2) Hull-mounted SeaBeam2120 (20kHz)
multibeam echo sounder and 3.5
kHz sub-bottom profiler3) Dredge sampler4) CTD Carousel Multi-Samplingsystem with a DO sensor and lighttransmissometer2~ 4 knots

ISBL
transducer

ISBL
5 - 10signals
kHzISBL
hydrophones

MAPR
50mDepressor
WADATSUMI100kHz
Sub-bottomprofiler
3 - 6kHz`300m

Figure 2. Schematic diagram showing the WADATSUMI deep-tow sonar andattached MAPRs. Insert – WADATSUMI deployment

5) Probe-type heat flow meter (4.5m)
6) Piston core (15m)
The deep-tow vector sidescansonar “WADATSUMI” (Fig. 2) isthe 100 kHz system, which provides
not only high-resolution backscattering images but also phase bathymetry. The system also includes a 3-6
kHz chirp sonar to investigate subbottom structure. The swath width
was 1 km and the towing altitude
ranged from 100 to 350 m. The pixel
size of the collected images is 50 cm.
The towfish positioning was doneby ISBL (inverted short baseline)
system, in which the signal transmitted from the ship is received atthe towfish and the all information issent to the onboard control unit
using a co-axial cable. Four MAPRs
were equipped on the deep-tow system to detect the hydrothermal activities using its thermometer andnephelometer. Three MAPRs wereattached to the towing cable withspacing 1, 50 and 100 m above thedepressor and another one was suspended 50 m below the depressor(Fig. 2). In addition total 5 CTD hydrocasts, 6 dredges, and 2 piston
cores were done in the area. Eightheat flow measurements were conducted near the hydrothermal ventreported in the previous submersible dives.
Preliminary results
The WADATSUMI sonar system attached with MAPRs was towed
within the central rift valley of thewestern Okinawa Trough from122°35’E to 123°16’E (Fig. 3). Total
300 miles backscatter image andphase bathymetry data were obtained. During the 5-day survey, we
mapped the major regions of the Yonaguni Graben (westernmost segment of the Okinawa Trough rift axis).
Within the graben, a set of rift-parallel lines covers about 70~80% of
the axial valley width including thesteep northern rift wall. Another setof lines oblique to the rift axis wasplanned to cover the volcanic/hydrothermal area and to investigate
the sub-surface structure using the38

InterRidge NewsInternational Research: Back Arc Basins: Okino et al., cont...
25 00’

Depth-800

PC2
D04

-1200
24 55’D05
SPOT4-1600
SPOT1
SPOT2-2000

D06
D01

24 50’

SPOT5

D02

SPOT3
Heat Flow

DredgeCTD(SPOT)
Piston Coring
Ship truck

D03

24 45’
122 35’

PC1
122 40’122 45’122 50’

122 55’123 00’123 05’

123 10’123 15’Figure 3. Ship’s track and locations of stationary operations during the HOTWATER cruise on the bathymetry map.
chirp sonar attached to the WADATSUMI. The towfish position
was determined using the ISBL system, however, the quality of the positioning varied among lines mainly
due to miss communication between
the transducer and the towfish.
The WADATSUMI images revealed the detailed volcanic and tectonic structure of the area (Fig. 4). In
the western part the seafloor is dominated by volcanic constructions,
including hummocks, sheet flows,
and blocky lava terrains. Small volcanic cones with crater pits arealigned in E-W direction, which may
build an elongated volcanic ridge
within the graben. On the other hand,the eastern part of the survey area isa sedimentary, deep (~1900m) depression, where the numerous normal faults are observed. The combination of elongated sedimentary
pond and E-W trending narrowhorsts characterize the area. The
sub-bottom profiler on the WADATSUMI clearly reveals the structureof these horsts. E-W step faults arealso recognized near the rift wall andthe slope failure and debris flow
mound modify the original tectonic
landform in some locations. One of
the most important targets of thesonar analysis is the relationshipbetween the volcanism and tectonicdeformation. Further processing of
the sonar imagery as well as thephase bathymetry will provide new

information for understanding themagmatic/tectonic process of thebackarc rift zone.
The MAPRs were deployed with
WADATSUMI in six survey lines.
The nephelometric anomalies suggest that the hydrothermal activity
is restricted in the western part ofthe survey area. The locations ofthese anomalies correspond to theryolitic volcanism in the western
volcanic area and occur close to theknown hydrothermal active site
(SPOT site). Based on the MAPR
results, five CTD hydrocasts werefocused on the western volcanicarea. Station SPOT-5 (Fig.3) showed
the highest light transmission anomaly up to ~5% within 300 m above theseafloor. The pH profile also showednegative pH anomalies at almost thesame depths, indicating the effect ofacidic hydrothermal fluids.
Heat flow measurements weredone at 8 sites along two crossing
lines near the known hydrothermalvent area. The preliminary analysis
shows high heat gradient suggesting very high heat flow in the area.
Precise heat flow values will be calculated by using thermal conductivity directly measured from the piston core samples.
Six dredge hauls were conducted during the cruise, three in theeastern E-W elongated minor ridges
and others in the cross backarc volcanic trail. Volcanic rocks were re-covered from two sites. Pumiceous
rhyolite samples were obtained at D05 and some samples have mafic
inclusion. Aphyric basalt samples
with glass rind were dredged at D06(Fig. 3).
Besides the tectonics and hydrothermarism, the study area is also
a good site for understanding theKuroshio warm current, which has
played an important role to constrain
the paleo-environment of the region.
Two piston core samples were recovered during the cruise to confirm
the migration of the Kuroshio Current axis during last 20 Ka.
The collected dataset contributes to our knowledge on the interaction of the magmatism, tectonics
and hydrothermal activity in thebackarc rifting zone. The detailed
analyses of various measurements
are in progress, and we hope to integrate the analyzed results in the nearfuture.
Acknowledgements
We thank the officers and crewof the R/V Hakuho-maru for theirefficiency during the cruise. We also
thank K. Tamaki for his encouragement and advice. T. Matsumoto provided us his compiled bathymetry
data of the area.
References
Ishikawa, M. Hl Sato, M. Furukawa, M.
Kimura, Y. Kato, Y. Tsugaru, andVol. 11(2), 2002

39

International Research: Back Arc Basins: Okino et al., cont...
K. Shimamura, Report on DELP
1988 cruise in the Okinawa Troughpart 6: Petrology of volcanic rocks,
Bull. Earthquake Res. Inst. Univ.
Tokyo, 66, 151-177, 1991.
Kimura, M., Active rift system in theOkinawa Trough and its northerncontinuation, Bull. Disaster Prev.
Res. Inst. Kyoto Univ., 45. 27-38,
1996,
Matsumoto T., M. Kinoshita, M.
Knakamura, J.-C. Sibuet, C.-S. Lee,S.-K. Hsu, T. Oomori, R. Shinjo, Y.
Hashimoto, S. Hosoya, M. Imamura,
M. Ito, K. Tukuda, H. Yagi, K.
Tatekawa, I. Kagaya, S. Hokakubo,T. Okada, M. Kimura, Volcanic andhydrothermal activities andpossible “segmentation” of theaxial rifting in the westernmost partof the Okinawa Trough –preliminary results from theYOKOSUKA/SHINAK6500
Lequios cruise-, JAMSTEC J. Deep
Sea Res., 19, 96-107,2001.
Park, J.-O., H. Tokuyama, M. Shinohara,
K. Suyehiro, A. Taira, Seismic
record of tectonic evolution andbackarc rifting in the southern
Ryukyu island arc system,

Figure 4. Example of mosaic image of the fault-dominant area within the central
rift axis.
Tectonophysics, 294, 21-42,1998
Sibuet, J.-C., B. Deffontaines, S.-K.
Hsu, N. Thareau, J.-P. Le Formal,C.-S. Liu, and ACT party, Okinawa
trough backarc basin: Early
tectonic and magmatic evolution,
J. Geophys. Res., 103, 30,24530,267, 1998
Shinjo, R., S.-L. Chung, Y. Kato and M.

Kimura, Geochemical and Sr-Nd
isotopic characteristics of volcanic
rocks from the Okinawa Troughand Ryukyu Arc: Implications forthe evolution of a young,intracontinental back arc basin, J.
Geophys. Res., 104, 10,591-10,608,
1999.