Auzende, Jean-Marie

Ballu, V.

Batiza, R.

Bideau, D.

Cormier, M.H.

Fouquet, Y.

Geistdoerfer, P.

Lagabrielle, Y.

Sinton, J.

Spadea, P.

Name not available

Observations of present-day activity at super-fast spreading : volcanic, hydrothermal and tectonic studies of the EPR 17-19°S

Horizon / Pleins textes

Vol. 5 No. 1 June 1994 
Observations of Present-Day Activity at Super-Fast Spreading : 
Volcanic, Hydrothermal and Tectonic Studies of the EPR 17-19OS 
Jean-Marie Auzende* (IfremerlCB, BP70,29280, Plouzané, France), Valrie Ballu, Rodey Batiza, Daniel Bideau, 
Marie- Hélene Cormier, Yves Fouquet, Patrick Geistdoerfer, Yves Lagabrielle. John Sinfon and Piera Spadea 
*Now at Orstom, BPAS, Nouméa, Nouvelle Calédonie 
n December 1993, during the I NAUDUR cruise, we conducted 
a series of dives using the French 
submersible Nautile (Fig. 1). The 
purpose of these dives was to 
explore the East Pacific Rise (EPR) 
axis between 17"s and 19"s where 
. 
I  
In this issue ... 
Current Research 
p. 3 
Data Syntheses 
p. 5 RIDGE Multibeam Synthesis 
Project 
p. 7 Pacific-Antarctic Ridge Seafloor 
Spreading: Insights from a 
Computer-Generated Animation 
Recent Developments in Dating 
Ocean Ridge Basalts 
Technical Developments 
p. 9 
(ABE) 
RlDGE Planning 
p. 14 Message from the Chairman 
p. 15 Program Element Reports 
- Event Detection, Response and 
Temporal Variability Workshop 
The Autonomous Benthic Explorer 
- MELT 
p. 17 RIDGE Funded Projects 
p. 19 InterRidge Update 
Other Items 
p. 12 Announcements 
p. 22 Steering Committee Membership 
p. 23 RIDGE Documents Available 
p. 24 Calendar 
the spreading rate is 
nearly the fastest pres- 
ently active (141 to 162 
"/y) (Rea, 1977; Naar 
and Hey, 1989; DeMets et 
al., 1990; Perram et al., 
1993; Cormier and 
Macdonald, 1994). The 
area between 17" and 
19"s has been extensively 
surveyed since 1982 with 
Seabeam and SeaMARC 
II by French, German 
and American institu- 
tions (Renard et al., 1985; 
Bäcker et al., 1985; 
Macdonald et al., 1988; 
Perram et al., 1993; 
Scheirer et al., 1993; 
Cormier and 
Macdonald, 1994). The 
ridge axis in this region 
varies from a shallow 
axial dome less than 
2600 m deep to an axial 
graben a few hundred 
meters wide and a few 
tens meters deep cutting 
15's 
PACIFIC 
I PLATE 
4s 
NAZCA 
PLATE 
the top Of the dome* A 
(leSS than 1 km deep) seismic reflet- 
tor interpreted as the top of the 
magma chamber has been identified 
by multichannel seismic surveys 
(Detrick et al., 1993) near 17"22'S. In 
1984 the submersible Cyana carried 
out 8 dives between 17'30' and 
Figure 1. Location map of the NAUDUR 
cruise dives. Sketch of the EPR after 
Cormier and Macdonald (1994). 
21'30's (Renard et al., 1985; Bäcker et 
al., 1985; Gente, 1987) and discovered 
fossil hydrothermal deposits in 
collapsed lava lakes. 
Objectives and Results of the 
NAUDUR cruise 
The objectives of the NAUDUR 
cruise were to carry out geological 
explorations along different seg- 
ments of the EPR located between 
17" and 19"s in order to study the 
interaction between magmatic, 
tectonic and hydrothermal processes 
at  a very fast spreading mid-ocean 
ridge axis (Auzende et al., in press). 
Twenty three dives were successfully 
completed along and across the axis 
in five different regions centered 
around 17"10'S, 17"25'S, 18"15'S, . 
-18'25'5 and 18"45'S (Fig. 1). These 
regions are characterized by a range 
of axis morphologies varying from 
an axial volcanic dome covered by 
very recent lava, to significantly 
more tectonized axes. Although 
various manifestations of hydrother- 
mal activity are present on all the 
explored segments, the style of this 
activity appears to vary with axis 
morphology and how recently the 
area has been volcanically active. 
The spreading ridge in the 
17"lO'S zone is a dome culminating 
at 2600 m depth and devoid of 
summital tectonic graben. Two main 
types of lavas have been observed; 
pillows and tubes on the flanks of the 
ridge and more fluid, sheet-like 
draped or lobated lavas at the axis. 
The pillows and tubes are capped by 
a very thin sedimentary cover, but 
the fluid lavas are extremely fresh 
and lack any sedimentary dust. 
Locally, very fresh lobate lavas have 
been observed partially covering 
clams and serpulid colonies (Fig. 2) 
and inactive sulfide mounds and 
chimneys. In the very axial part of 
the dome, one or two parallel 
fissures cut through the most recent 
lobate flows. Active and fossil vents 
were observed and sampled on the 
edge of the axial fissure. They consist 
of groups of small vents of shimmer- 
ing water and associated colonies of 
clams, serpulids, vestimentifers, etc. 
Living colonies of clams and 
serpulids were present on now 
extinct vents that were thus probably 
active very recently. 
The 17'25's region shows 
morphological characteristics very 
similar to those of the previous area, 
with an axial dome also culminating 
at 2600 m. This region was explored 
during Cyma dives in 1984 (Renard 
et al., 1985). At that time, the mag- 
matic activity was conceneated only 
along an axial alignment of narrow 
collapsed lava lakes, and hydrother- 
mal activity was not important. The 
six dives carried out in this region 
during the NAUDUR cruise lead us 
to conclude that important changes 
in morphology as a result of mag- 
matic and hydrothermal activity 
have occurred since 1984. 
in 1984, the axis has a quite uniform 
character, with lavas alternating 
from pillow flows to lobated lava 
flows. Very recent fissures, a few 
tens of centimeters to a few meters 
wide, truncate these flows. A discon- 
tinuous collapsed lava lake, with 6- 
12 m high pillars is present locally at 
the ridge axis; it is floored either by 
debris from the very recently col- 
lapsed roof or locally by sheet flows 
that clearly post-date the collapse 
debris. To the south, extremely fresh 
lavas have been observed on the 
almost flat, top surface of the ridge, 
which is not at all tectonized. The 
lavas include pillowed and lobated 
flows characterized by a black, very 
To the north of the zone explored 
Figure 2. 
Photograph of 
present-day 
lava flow 
overlying 
previous flows 
colonized by 
serpulid worms 
(17O10'S area) 
glassy surface with yellowish 
bacterial-like vellum. They are 
completely devoid of sediments and 
do not show fixed faunas. 
Hydrothermal vents are common 
in the axial domain (Fouquet et al., in: 
press). They include diffusion of 
shimmering water from many areas 
of the most recent lava surfaces and 
high temperature venting (black 
smokers, 250-340°C) in older regions 
(Fig. 3). Pillars of the recently col- 
lapsed lava lake clearly serve as 
conduits for venting of shimmering 
water. Temperature anomalies were 
recorded by the Nautile in the region 
of the summital lava lake. Such 
anomalies could be related either to 
the diffuse venting of hot water or to 
the occurrence of still hot, cooling 
1avas:Animal colonies are abundant 
and show a large variety of species 
(Geistdoerfer, et al., in press). 
The segment immediately north 
of 18"22'S, is characterized by a wide 
graben bounded by two ridges 
culminating at 2650 m. From dive 
results, the graben appears to have a 
maximum width of 800 m. It may 
include a secondary axial graben 
with a maximum depth of 80 m, 
bounded by two steps. The main 
boundary faults of the graben have a 
total vertical offset of 30-40 m. The 
bottom of the central valley shows 
NAUDUR -page 20 
2 RIDGE EventdJune 1994 
NAUDUR - from page 2 
Figure 3. Active 
hydrothermal site 
with black smokers 
at 17"25'S. The 
measured 
temperature of the 
water is 31 8°C. 
Two dives were carried out on 
the southern segment of the 
"Hump". One near the northern end 
of the southern segment found 
extremely young sheet flows with 
shimmering water emerging directly 
off of lava surfaces extensively 
colonized by a wide variety of 
animals. At 18"45'S, a dive carried 
out a cross-section of the present-day 
active axis characterized by fresh 
lobated flows up to the eastern 
abandoned ridge (Cormier and 
Macdonald, 1994) showing pillows 
covered by a uniform sedimentary 
film. 
intense fissuring. Open fissures, 
some meters wide, are extremely 
common. Some of them separate 
small horsts that are only a few 
meters wide and 10 meters high. 
These very fragile features are locally 
tilted. The observed volcanic forma- 
fions include chaotic, brecciated, 
draped lavas outside of the axial 
graben, thick pillow lava formations 
al the edge of the graben and col- 
lapsed lava lakes with up to 10-15 m 
high relict pillars and associated 
pillows in the central graben. 
, 
Numerous fossil and active 
hydrothermal sites (Fouquet et al., in 
press) were observed along all the 
dives and in all parts of the graben. 
They include large sulfide chimneys, 
black and white smokers and low- 
temperature fossil chimneys com- 
posed of silica and iron-hydroxides. 
They are most common along the the 
eastern secondary graben to the 
south and in the axial region in the 
northern part of the diving area. 
The central zone of the "Hump" 
between 18"22'S and 18'34's exhibits 
a more complex morphology. A 
median graben, 200 to 500 m wide is 
bounded by two asymmetrical 
ridges. The eastern ridge culminates 
at 2600 m and the western ridge at 
2700 m. Six dives were conducted 
along this segment. Our observations 
indicate that intense tectonic activity 
is affecting the entire median graben 
as shown by the occurrence of 
numerous open fissures, up to 10 m 
wide, which separate pillow ridges. 
A uniform sedimentary cover, some 
millimeters thick, is observed 
througout the entire domain. This 
segment is characterized by impor- 
tant hydrothermal activity which 
appears to be concentrated primarly 
along a narrow ridge of pillow debris 
close to the eastern wall of the 
graben. More than 15 individual sites 
have been discovered and sampled. 
They mainly are black smokers 
(340°C) set on active, white mounds 
venting hot shimmering water (up to 
150°C). 
Further to the south, between 
18'34'5 and 18"3TS, the axis of this 
segment consists of a 50 m deep and 
200 m wide summital graben, 
bounded by two vertical, symmetri- 
cal walls in which poorly tectonized, 
lobate lavas, lava lakes and massive 
lavas have been emplaced. Extremely 
recent flows are present in this 
region, which were observed cover- 
ing older lava flows and recent talus. 
Hot water is emerging from the 
entire surface of the lavas. The 
temperature of the water 2-3 m 
above the lava surface is about 2.5"C 
above the ambient water tempera- 
ture for this region. No hydrothermal 
deposits were observed in the 
immediate vicinity of this recent 
lava. 
Conclusions 
All the ridge segments explored 
during the NAUDUR cruise show 
recent activity related to the fast rate 
of opening in the area. At 17"10'S, 
17'25's and near 18"34'S, extremely 
fresh volcanics in the axial domain 
must be the products of very recent 
eruptions. In many places, black, 
shiny lobated lavas overlie older 
flows and dead or living animal 
colonies consisting of clams, 
cirripeds or serpulids. The fresh 
lavas are associated with shimmering 
waters being expelled between the 
lava lobes and by recently foundered 
pillars. In these domains new animal 
colonies have not yet become estab- 
lished; the only observed living 
animals are crabs and fishes. Simi- 
larly, the hydrothermal circulation is 
not yet well-organized, and there are 
no black smokers or hydrothermal 
deposits. The observed hydrothermal 
sites, with characteristics close to the 
9'50" zone (Haymon et al., 1993) 
are located out of the present-day 
axis on older faults and fissures. 
The two segments north of 
18'22's are chacterized by extensive 
tectonism with an axial graben, 200 
to 800 m wide, that is intensively 
fractured and fissured. In these areas, 
evidence for recent volcanism is 
absent and localized to the 20 to 30 
m-wide, 20 to 40 m-deep axial 
fissure. Intense fracturing and 
20 RIDGE EventdJune 1994 
fissuring has aIlowed hydrothermal 
circulation to become well-devel- 
oped with numerous hydrothermal 
sites (black smokers with sulfide 
deposits) aligned along the walls of 
the axial graben. 
During the NAUDUR cruise, 70 
hydrothermal sites (active and fossil) 
were explored and sampled. Most of 
them are unusual with sulfide 
chimneys prolongated by thin 
smokers covered by anhydrite. 
Animal colonization is, for the most 
part, very primitve with animal 
species mainly restricted to crabs and 
fishes, a result that we interpret to 
indicate the overall youth and/or 
short life span of individual vents in 
the area. + 
References 
Auzende J.M., Ballu V., Batiza R., Bideau 
D., Cormier M.H, Fouquet Y., 
Geistdoerfer P., Lagabrielle Y., Sinton 
J. and Spadea P., Activité 
magmatique. tectonique et 
hydrothermale actuelle sur la Dorsale 
Est Pacifique entre 17" et 19"s 
(campagne Naudur), Compfes 
Rendus. Acad. Sci., Paris, in press. 
Backer H., Lange J., and Marchig V., 1985. 
Hydrothermal activity and sulfide 
formation in axial valleys of the East 
Pacific Rise crest between 18" and 
22"S, Earth Planet. Sci. Lett., 72, p. 9- 
nn 
Cormier M.H.. and Macdonald K.C.. 1994, 
East Pacific Rise 18"-19"S: asymmet- 
ric spreading and ridge reorientation by 
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discontinuities, J. Geophys. ßes., 99. 
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Demets C., Gordon R.G., Argus D.F.. and 
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Fouquet Y.. J.M. Auzende, V. Ballu, R. 
Batiza, D. Bideau, M.H. Cormier, P. 
Geistdoerfer, Y. Lagabrielle, J. Sinton 
et P. Spadea, Hydrotermalisme et 
sulfures sur  la dorsale du Pacifique Est 
entre 17" et 19"s (campagne Naudur), 
C.R. Acad. Sci. Paris, in press. 
Geistdoerfer P., Auzende J.M., Ballu V., 
Batiza R., Bideau D., Cormier M.H, 
Fouquet Y., Lagabrielle Y., Sinton J. 
and Spadea P., Hydrothermalisme et 
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dorsale Est Pacifique entre 17"s et 
19"s (Campagne Naudur), Comptes 
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comparative de dorsales ocaniques 
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Brest, 371 pp. 
Havmon R.M.. Fornari D.J.. Edwards M.H., 
K.C., 1991, Hydrothermal vent 
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and Sempere J.C.. 1988, Deep-tow 
and Sea Beam studies of dueling 
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Rise near 20°40'S, J. Geophys. Res., 
93,28752898. 
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I.U.G.G. Geophysical Monograph 57, 
J.M. Sinton ed.. p. 9-30. 
Perram LJ.. Cormier M.H., and Macdonald 
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ABE - from page 1 O 
marking its docking mooring, turn 
toward it, and dock. 
In June 1994, we plan to take 
ABE to join the Atlantis II in San 
Diego, again in conjunction with a 
series of Alvin engineering dives. We 
hope to demonstrate ABE'S capabili- 
ties to conduct repeated dockings, 
follow tracklines within the Alvin 
transponder net, and capture images 
at specified locations. We may also 
have an opportunity to attempt more 
ambitious missions in early Septem- 
ber in conjunction with another Alvin 
engineering dive series. 
The first real science mission will 
occur in mid-1995, when ABE will 
conduct a complete magnetometer 
survey over a lava flow, known to 
have erupted in July 1993 along the 
CoAxial Segment of the Juan de Fuca 
Ridge. A previous survey conducted 
from the Alvin indicated the presence 
of a notch-like magnetic low at the 
center of the new flow, which has 
been interpreted to be related to the 
thermal demagnetization of the 
underlying feeder dike (Tivey and 
Johnson, 1994). The survey with 
ABE, which wiIl fly at an altitude of 
20m above the bottom and will cover 
an area of lkm by 300m with about 
20m spacing between tracklines, is 
designed to investigate how this 
anomaly changes with time, 
thereby providing constraints on 
the cooling and structure of the 
lava flow. + 
Reference 
Tivey, M.A. and H.P. Johnson, Alvin 
magnetic survey of zero-age crust: 
CoAxial segment eruption, Juan de 
Fuca Ridge 1993, Geophys. Res. 
Lett., submitted, 1994. 
RIDGE EventdJune 1994 21 


Observations of present-day activity at super-fast spreading : volcanic, hydrothermal and tectonic studies of the EPR 17-19°S

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