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

tel-00344949, version 1 - 7 Dec 2008
4. Article soumis à G3
‐ A transitional domain limited by the two major shear zones of the south Pacific, the
transform faults Udintsev and Eltanin �Heezen and Tharp TF�. Within this domain, the
spreading axis is divided in 4 right‐stepping transforms and includes an oblique intraplate
structure �Hollister Ridge�. Ridge morphology is quite variable changing from dome to
valley as often seen in intermediate rate spreading centers. This ridge section is described
in Castillo et al. �1998�.
‐ North of Eltanin system up to Juan Fernandez microplate, a domain delimited by
subduction zones. North of the Menard fracture zone, the ridge is bounded by ocean‐ocean
subduction on the west and by ocean‐continent subduction on the east, south of the
Menard the ridge is delimited by ocean‐ocean subduction on both sides. The ridge is not
much offset by the few transform faults in this domain. It presents monotonous dome type
morphology. This is the domain that includes north of Vacquier, our study zone.
The PACANTARCTIC2 cruise was a joint geophysical survey and geochemical sampling
of the Pacific‐Antarctic Ridge between 41°15’S and 52°45’S �figure 4.9�. This 1300 km‐long
ridge section is characterized by a dome shape axial morphology. It is segmented by the
Menard TF near 50°S and by small non‐transform offsets such as Overlapping Spreading
Centers �OSC�. The cruise imaged three segments south of the Menard TF, S1 to S3 and six
segments north of it, N1 to N6. Segment morphology varies from a robust 12 km‐wide
dome to a narrower rise, 2 km in width as illustrated by ridge axis cross‐sections in figure
2. In two cases, the segments are rather poorly defined, and are seen as transitional ridge
segments S2 and N2. The half spreading rate varies between 46 mm/yr at 52°S to 50
mm/yr at 42°S �Lonsdale, 1994a� while the ridge bathymetry decreases from 2300 to 2500
m. The gravity grids derived from satellite altimetry measurements show gravity highs,
either isolated or aligned in ridges, between 40°S and 55°S, interpreted to be numerous off‐
axis volcanoes and volcanic ridges. The largest of these structures are observed 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 �Klingelhöfer et al., 2006�.
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