Géochimie isotopique du lithium dans les basaltes-Géochimie des ...

tel-00344949, version 1 - 7 Dec 2008
INTERNATIONAL RESEARCH
The Pacifi c-Antarctic Ridge between 41°1 41°15’S and 52°45’S: Survey and sampling during the
PACANTARCTIC PACANT 2 cruise.
L. Dosso 1 , H. Ondréas 2 and the PACANTARCTIC 2 Shipboard Scientifi c Party:
A. Briais 3 , P. Fernagu 2 , G. Floch 2 , C. Hamelin 4 , B. Hanan 5 , F. Klingelhoefer 2 , M. Moreira 6 , A. Normand 2
Introduction and scientifi c objectives
The Pacifi c plate is separated from the Antarctic plate by
a 5000 km long spreading ridge south of the Chile Triple
Junction at 35°S/110°W (Figure 1). A large portion of this
plate boundary, the Pacifi c Antarctic Ridge, which includes
major fracture zones, was surveyed by Lonsdale (1986,
1994a, 1994b), Cande et al. (1995) and Géli et al. (1997),
but only the southern section has been systematically
sampled (Géli et al., 1997, Castillo et al., 1998).
The Pacifi c-Antarctic Ridge can be divided into three main
sections which defi ne three domains from south to north
(Figure 1):
- South of the Udintsev Transform Fault (TF), a domain
bounded by passive margins,
- between the Udintsev and Eltanin TF, a transitional domain
composed of the Eltanin TF system (Heezen and Tharp TF)
and an oblique intraplate structure (Hollister Ridge).
- North of the Eltanin system, a domain delimited by
subduction zones, with numerous transform faults south of
the Menard TF and a domain without transform faults north
of the Menard TF.
The ridge spreading rate increases from 54 mm/y at 65°S
to 74 mm/y near the Udintsev TF at 55°S, and fi nally to 100
Figure 1
Location map for the PACANTARCTIC 2 cruise. The main
fracture zones of the South Pacifi c are indicated and the two
dredge sites reported in Ferguson and Klein (1993) are shown
with circle symbols.
mm/y at 35°S (De Mets et al., 1994).
INTERRIDGE NEWS
After the southernmost portion of the ridge axis was
fi rst dredged by Cande et al. in 1992 (two dredge sites
at 64°S and 171°W, see Figure 1), Ferguson and Klein
(1993) described the Pacifi c Ocean as “one of the largest
chemically coherent mantle domains on the Earth”. However
a later cruise (PACANTARCTIC 1) which sampled the ridge
between 56°S to 65°30’S (Géli et al., 1997) brought a new
perspective on the properties of the South Pacifi c mantle
as reported by Vlastélic et al. in 1999. Together with
geochemical data from samples collected along the axis
between 53° and 57°S (Castillo et al., 1998), the new data
showed evidence of a large-scale chemical and thermal
division of the Pacifi c mantle. The boundary was identifi ed
at the latitude of the Easter Island microplate. However
those conclusions were drawn from an incomplete data set
as the ridge section between 41°S and 53°S had not yet
been sampled. This, therefore, became the main objective
of the PACANTARCTIC 2 cruise described in this report.
More samples have also been collected along the ridge
near the Foundation line (Maia et al., 2000), and between
37°S and 41°30’S (Stoffers et al. 2002).
The gravity grids derived from satellite altimetry
measurements show gravity highs, either isolated or aligned
in ridges, between 40°S and 55°S. These are interpreted
to be numerous off-axis volcanoes and volcanic ridges.
The largest of these structures are located on each side of
the Menard TF, forming N75° E-trending volcanic ridges,
and a series of 3 ridges oriented E-W near 42°S. Another
objective of the PACANTARCTIC 2 cruise was to map and
dredge these off-axis structures to understand how they
relate to axial magmatism, dynamics and kinematics.
Preliminary results
PACANTARCTIC 2 was a joint geophysical survey and
geochemical sampling cruise to the Pacifi c-Antarctic Ridge
between 41°15’S and 52°45’S (Figure 2). From December
17, 2004 (Talcahuano – Chile) to January 17, 2005 (Easter
Island – Chile) the French research vessel L’Atalante
surveyed 1300 kilometers of the Pacifi c-Antarctic Ridge
axis and made 3200 km of off-axis profi les to describe the
structures located on either side of the Menard transform
fault and west of the axis at 42°S (Figure 2).
1 CNRS, UMR6538, Domaines Océaniques, IFREMER, B.P.70, 29280 Plouzané, France; 2 IFREMER, B.P.70, 29280 Plouzané, France; 3 CNRS, UMR5562, 14, Avenue
Edouard Belin, 31400 Toulouse, France; 4 I.U.E.M., Place Nicolas Copernic, 29280 Plouzané, France; 5 Department of Geological Sciences, San Diego State
University, 5500 Campanile Drive, San Diego, CA 92182-1020, U.S.A.; 6 Laboratoire de Géochimie et Cosmochimie, I.P.G. Paris, 4 place Jussieu, 75005 Paris,
France.
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