Auzende, Jean-Marie

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THE NORTH FIJI BASIN ACTIVE SPREADING RIDGE 
Jean-Marie Auzende*describes recent findings at the boundary of two important tectonic plates 
The North Fiji Basin (NFB) is one of 
the marginal basins at the converging 
boundary of the Pacific and Austral- 
ian major plates. It forms adeep trian- 
gular basin (fig. l), lying between the 
arc of the New Hebrides to the west, 
theFijiplatformto theeast, theVityaz 
The basin began to open about 
12 million years ago after the locking 
of the Vityaz subduction by the 
Ontong-java plateau and the reversal 
of its polarity. This change of polarity 
involved the clockwise rotation of 
the New Hebrides Arc with, as a 
o 
lo 
!O0 
IO0 
10" 
Fig.1: The North Foi Basin in the SW Pacific environment 
fossil subduction zone to the north 
and the arc of the Matthew-Hunter 
zone to the south. (Note thatsubduction 
zones are where cold and relatively old 
material is descending into the earth's 
mantle to helpbalance thewelling uPand 
spreading at the mid-oceanic ridges of 
the hot material derived from the mantle. 
These zonesare marked by belts of volca- 
nos located eitheralong arcs of islands as 
in Japan or the Philippines, or along a 
continental ,.im as in the Andes. Ed.) 
secondary effect, the anticlockwise 
rotation of the Fiji platform. In the 
first phase the movement had a NW- 
SE trending spreading axis and N45" 
to N55" flow lines. The second phase 
resulted in the beginning of the colli- 
sion of he N~~ Hebrides with 
theloyautés Islands ridge, thechange 
of the traction stresses to an east-west 
direction and the location of thenorth- 
south spreading centre in the central 
- A ^  
part of the basin; meanwhile, north- 
south spreading was happening in the 
northern part of the basin and north of 
the Fiji islands. 
The North Fiji Basin 
(NFB) Spreading Ridge 
The NFB ridge was partially mapped 
for the first time during the SEMS0 
III cruise of the French RN Jean 
Charcot (December 1985). Since that 
time it has been extensively surveyed 
within the French-Japanese joint 
project Starmer and mapped with full 
coverage multibeam over an area 
more than 800 km long and 100 km 
wide. This mapping was accompa- 
nied by detailed rock and water sam- 
pling, and with observations of the 
oceanic bottom by photo-video deep 
tows and submersibles. There are four 
major segments: 
1. The southernmost segment 
is characterised by a complex strut- 
ture with N05" alternating ridges and 
depressions up to 2500 m high. This 
segment appears to be either dying 
rift or incipient rift. The axial mag- 
netic anomaly suggests an active 
spreading with a 5 c d y  rate. 
2. Between 21"OO'S and 
18"lO'S the north-south segment is 
offset by about 80 km from the 
southernmost segment; its morphol- 
ogy is typical of fast-spreading ridges 
with a 8 km wide, 200-300 m high 
axial dome. The dome is locally cut at 
its centre by a 50-500 m wide 
and 20-50 m deep graben where the 
active spreading is now located. (A 
graben is a part of the earth's crust 
that has slipped dowr. along at least 
two fault lines, like a ri$ valley. Ed.).: 
The axial dome is bounded on' 
both sides by north-south depressions 
and ridges. The average spreading 
rate since anomaly 2A (about 3 mil- 
lion years ago) is 7-8 cmly. A peculi- 
arity of this segment is that it 
PA ClFlC PAGES 
matic stage. 
TheYokosuka91 cruise with the 
Japanese submersible Shinkai 6500 
(fig. 3) aimed to return to the previ- 
ously explored sites and to explore 
two new sitesat 16O30'S and 18OO6'S. 
These new sites have revealed only 
active tectonic movement without 
present-day hydrothermal activity. 
Since the fourteen Ncturik dives in 
1989, six Shinkai 6500 dives have 
beendevoted to thestudy ofthe White 
Lady and of the surrounding active 
hydrothermal sites. 
At present, the White Lady has 
an anhydrite chimney consisting of a 
massive 2 in high inain conduit with a 
2 m diameter and a 1.5 m high sec- 
ondary chimney. These vents expel 
Fig. 2: The White Lnd? hydrothermal chimney. the same shimmering water, but the 
is propagating to both the south and 
the north. 
3. Between 18"lO'S and the 
16O5O'S Triple Junction, the north- 
south trend of the spreading ridge 
changes for a N15" direction, There 
is a high double ridge in the northem 
part which culminates at a depth of 
less than 2500 m, bounding a 1-2 km 
wide, 1 O0 mdeep graben. In the south- 
em part the ridge axis is poorly de- 
fined and spotted by numerous iso- 
lated small volcanoes. 
North of the 16'50's Triple 
Junction, the spreading ridge is made 
up of a succession of three grabens 
3500-4000 m deep aligned on an 
N 160" direction, typical of slowly 
spreading ridges; the spreading rate 
calculated from magnetic field analy- 
sis is 5-6 c d y .  The northem tip is 
marked by a complex structure sug- 
gesting the existence of a new triple 
junction. The magnetic fields have 
been mapped all along the I@B ridge. 
The axial anomaly is well defined on 
the whole axis while the J (Jaramillo) 
anomaly exists only on the two south- 
em segments. The 2 anomaly is well 
defined on the central north-south seg- 
ment and the 2A anomaly has been 
identified only on the eastern limb of 
4. 
' 
flux is about twice that of 2 years ago. 
which is 20°C less than the tempera- 
ture measured in 1989. 
Finally, significant changes have 
been observed in the fauna. The colo- 
nies are in all cases more numerous The data acquired by surface 
ships helped in the selection of diving and their territories are expanding 
towards the top of the mound. These sites. During the Nautile cruise in 
observations confirm that thechanges June-July 1989 two sites were ex- . 
in morphology and in activity of this plored. The first one around 17"s in 
hydrothermal site are taking place on the axial graben shows important 
a human time-scale. The same hydrothermal activity characterised 
this segment. The calculated spread- The measured temperature is 265OC, 
ing rates vary from 5 C d Y  to 8 C d Y .  
Hydrothermal activity on 
the NFB Ridge 
by hot (285°C) shimmering 
waters very poorly loaded 
with particles. This water is 
expelled by a 2-3 m high 
anhydrite chimney dubbed 
'The White Lady' (fig. 2). At 
the foot of the White Lady 
living colonies of 
Bathymodiolus, gastropods 
and cirripeds have been ob- 
served and sampled. 
The second site around 
18'50's shows only low tem- 
perature i5-l diffusion Fig. 3: The Japanese submersible Shinkai 6500; 
colonised by giant mussels, 
in a narrow active graben character- phenomena have been documented 
ised by extremely fresh lavas without on the East Pacific Rise around 13"N, 
sulfide deposits. This site has been where a 40 cm increase in the height 
interpreted as an incipient of one chimney was observed within 
hydrothermal site in an active mag- a single month. Cl 
Page 8 
PACIFIC PAGES 
Links between the ocean The result was a 3-dimensional 
and the atmosphere are 
studied ín the Pacific 
Intemational meteorological experi- 
ments in the equatorial zone of the 
Pacific Ocean ended as planned in 
February, and the long work of ana- 
lysing the data has now begun. The 
program, code-named TOGA- 
COARE (Tropical Atmosphere and 
Global Program-Coupled Ocean- 
Atmosphere Response Experiment), 
employed five oceanographic vessels 
and seven specially equipped aircraft, 
which studied a 1500 square kilome- 
tre area off Papua New Guinea. 
picture of the air in the interior of a 
storm, together with details of the 
vertical structure of the air and of the 
fields of precipitation. 
Early results show that the big 
storms seem to be broken into parts of 
up to 50 km long, moving chaotically 
among themselves but each with an 
orderly internal structure and a dy- 
namic organisation. They are charac- 
terised by a warm humid rising cur- 
rent at the storm front, fed by unstable 
air from the lower layers, and a cold 
current produced at the rear and initi- 
ated under the anvil. 
French research ship in Thanks to advanced measure- ment technology, new information 
was gained on ocean currents, tem- New Caledonia 
perature, salinity at different depths 
down to 300 m, the state of the sur- 
face, winds and turbulence close to 
The French research ship Atalante 
arrived in Noumea in June to partici- 
pate in an underwater 
the surface, the heat-flow and evapo- 
ration at the surface, the atmospheric 
structure ( wind, temperature, pres- 
sureand humidity) up to an altitude of 
20 km, and the effects of storms on 
theatmospheric structure and the sur- 
face layer of the ocean. 
The ocean in this equatorial zone 
has the highest surface temperature of 
any of the world’s seas, and hence a 
greater tendency to storms that may 
be the causes of medium-term cli- 
matic changes, such as ‘El Niño’. 
Meteorological studies were 
made using the airborne radar 
‘Astraia’, developed cooperatively by 
the Centre for Research into Environ- 
mental Physics (a joint unit at Issy- 
les-Moulineaux of the French Na- 
tional Centre forTelecommunication 
Studies, CNET, and the CNRS) and 
the US National Centre for Atmos- 
pheric Research, NCAR. Astraia was 
carried in a Lockheed Electra turbo- 
prop aircraft, and its observations were 
linked to those of other Doppler radars 
carried in Orion P3 aircraft operated 
by the US NOAA and sondes carried 
by a NASA DC8 and an ER2. 
program and resource study of the 
French Economic Exclusivity Zone 
of New Caledonia (EEZ). 
The ship, belonging to the French 
Research Institute for the Develop- 
ment of the Sea (FREMER), came to 
take part in ZoNeCo, a program de- 
signed to survey and evaluate the 
marine zone of New Caledonia. 
The program consists of three 
phases: exploratory work to locate 
hydrocarbons, data collection and a 
detailed evaluation of targets identi- 
fied as potential economic interests. 
The first stage of exploration was 
due to begin at the end of June and 
aimed to cover an area of 5000 square 
kilometres to the south of the temtory 
over the course of 20 days. 
The Atalunte is the flagship of 
IFREMER. Launched in 1990, it is 
one of the foremost research ships in 
the world for such exploration, 
equipped with precision instruments 
for mapping and data gathering. 
ZoNeCo is a part of the national 
program for underwater devel.opment 
in the EEZ, and has anumber of other 
organisations and three New Caledo- 
nian provinces as partners. 
Survey for world-record 
cable completed 
Three French ships-theA&ante, the 
Suroît and the Jeun-Charcot-have 
just completed survey work for the 
world’s longest optical cable, which 
is set to link France and Singapore 
next year. It will service eleven coun- 
tries along the way, over a distance of 
some 18,000 kilometres. 
After 150 days at sea, the three 
ships, equipped with multifunctional 
sounders, completed 700 maps, cov- 
ering 200,000 square kilometres of 
the ocean floor. The cable will be able 
to simultaneously transmit 80,000 
telephone calls, and will cost more 
than four billion FF. O 
Readership survey 
Issue No 14 of French Science und 
Technology carried a small card 
asking for your views on the newsletter 
and inviting your comments. Many 
thanks toalìthoseofyou whoresponded. 
Out of the three and a half thousand 
cards distributed, fifteen per cent bore 
fruit. Just over five hundred completed 
cards were returned, not a great number 
but enough to draw some general con- 
clusions and proportionally half as many 
again as in the previous survey in 1989. 
We are currently analysing your 
replies to ensure that the new-look FST 
that will appear from the next number 
will reflect your wishes. There will be 
more emphasis on industrial develop- 
ments, on science with a bias towards 
innovation and on interesting activities 
in the region. We shall give a full report 
on the survey in that number. 
This consultation was obviously 
productive, and in the same vein we 
encourage you to write to us at any time 
with your comments and suggestions. 
We shall be happy to print briefletfers 
(under 250 words, please) of general - 
interest about FSTor its contents, so that ;- 
FST becomes also a forum for your 
opinions. 
Write to the Editor, FST, French 
Embassy, 6 Perth Avenue, Yarralumla i 
ACT 2600, Australia. ‘ 
I 


The North Fiji basin active spreading ridge

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