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Lithospheric anisotropy on the Kerguelen hotspot track inferred
from Rayleigh wave polarisation anomalies.
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Wavefield distortion by the local structure of the Kerguelen Plateau: isotropic and anistropic models.
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Wavefield distortion by local structure - Anisotropic.

Simple anisotropic structure.

We use here the S-wave velocity-depth model of Fig. 5, page 4. The anisotropy in the model varies from 0 to 50 per cent so as to simulate a pyrolitic mantle with 0 to 50 per cent oriented minerals. We use elastic coefficients for a mantle of pyrolitic composition with 59 per cent olivine, 12 per cent garnet and 29 per cent pyroxene.

The result of the Rayleigh wave polarisation for a horizontally oriented pyrolite is shown beneath (Rayleigh wave incident from the left):

Fig. 7: Polarisation in the horizontal plane of a 20 s Rayleigh wave propagating through an uniform, anisotropic region with horizontally oriented pyrolite. The amount of oriented pyrolite decreases linearly from 50 per cent at 20 km depth to 0 at 100 km depth. The anisotropic region is delineated by a black line. The pyrolite symmetry axis, coincident with the fast olivine a-axis, is oriented at 45 from the direction of propagation, as shown by the arrow in the anisotropic region.
(after Pettersen and Maupin, Fig. 16, GJI, 149 225-246)

The polarisation at different locations in the horizontal plane for a Rayleigh wave propagating across a rectangular structure with oriented pyrolite is shown above. In this case, the pyrolite symmetry axis is oriented horizontally at an azimuth of 45 relative to the propagation direction. Before entering the structure, the Rayleigh wave has a normal polarisation, that is linear in the longitudinal direction. Propagating through the structure, it gets a small transverse component and a slightly elliptic, clockwise particle motion in the horizontal plane. The polarisation anomaly retains its amplitude after propagation through the structure, but the degree of ellipticity varies.

Polarisation anomalies at 40 s period are much smaller than those at 20 s period. The variation in the direction of the major axis is also smaller.

What can be the cause of this anisotropy? Pyrolite in the upper mantle, with a tilted axis of symmetry, is the mechanism which is the most efficient in producing large polarisation anomalies. The anisotropic structure should also account for the azimuthal dependence of the observed polarisation anomalies at PAF, as the contrasts between events from the Java Trench and other areas was significant.

From closer observations of the dataset, we conclude that a region with a 300 km North-South extension, located North and North-East of the Kerguelen Isles, with dipping pyrolite at an azimuth of about 20 North would explain well the data set. The East-West extension of the anisotropic region is not well constrained by the data.

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Universitetet i Oslo - Institutt for geofag / SPICE 2004