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

tel-00344949, version 1 - 7 Dec 2008
4. Article soumis à G3
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5.1. ORIGIN OF ATYPICAL SEGMENTS
Segments S2 and N2 are the two shortest segments of the studied area �Klingelhöfer et
al., 2006�. They have atypical morphology as seen from their cross section shown in grey in
figure 4.2. Examination of the bathymetric map reveals that S2 cross section is the result of
the two rise crests which are the extensions of the ridges forming adjacent segments S1
and S3. Similarly, N2 cross section reflects the extensions of two rise crests from N1 and
N3. These observations explain why the definition of the two segments S2 and N2 could be
questioned. In fact, Lonsdale �1994a� has not identified them as such in the description of
the Pacific‐Antarctic ridge. Because these two zones offset slightly the ridge, we chose to
identify them as segments, keeping in mind their atypical features.
On each side of the Menard TF, two N75° E‐trending seamount alignments intersect the
ridge axis. Segments S2 and N2 are located immediately south and north of the two ridge
seamount lines intersections �figure 4.9�. S2 and N2 are less robust than segments S3 and
N1 as seen in the bathymetric profile of figure 5. S3 and N1 have directions that are
perpendicular to the Menard TF but are at an angle with the general ridge direction. S2 and
N2 accommodate this change. They can therefore be seen as atypical segments and
transitional zones between a Menard related zone and the rest of the ridge, i.e. S1 on one
side and N3‐N6 on the other. This most likely reflects the resistance of the Menard
transform fault to the latest kinematic movements �Sahabi et al., 1996; Tebbens and Cande,
1997�.
As described above, S2 and N2 are characterized by unusual geochemical features, i.e.
variable �La/Sm�N and the most radiogenic Sr values of the whole studied ridge section.
The geochemistry of S2 and N2 atypical segments is related to the evolution of the two off
axis volcanic ridges and the Menard‐related zone. It will be the object of a separate paper.
5.2. INTRA‐SEGMENT VARIABILITY AN D OSC
The following discussion concerns only the 2 nd order discontinuities, the normal
segments of the ridge, excluding S2 and N2. Typical segments along this ridge section are
all about the same length, 200 kms �Klingelhöfer et al., 2006�. These segments have the
advantage of presenting normal tectonics features and being devoid of any plume