Upper-mantle flow beneath French Polynesia from shear wave splitting |
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| Authors: | Fabrice R Fontaine Guilhem Barruol réa Tommasi Götz H R Bokelmann |
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| Institution: | Laboratoire Terre-Océan, Universitéde Polynésie française, BP 6570;, 98702 Faaa, Tahiti, Polynésie française Research School of Earth Sciences, Australian National University, Building 61, Mills Road, Canberra, ACT 0200;, Australia. E-mail: Géosciences Montpellier, CNRS, UniversitéMontpellier II, F-34095;Montpellier Cedex 5, France |
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| Abstract: | Upper-mantle flow beneath the South Pacific is investigated by analysing shear wave splitting parameters at eight permanent long-period and broad-band seismic stations and 10 broad-band stations deployed in French Polynesia from 2001 to 2005 in the framework of the Polynesian Lithosphere and Upper Mantle Experiment (PLUME). Despite the small number of events and the rather poor backazimuthal coverage due to the geographical distribution of the natural seismicity, upper-mantle seismic anisotropy has been detected at all stations except at Tahiti where two permanent stations with 15 yr of data show an apparent isotropy. The median value of fast polarization azimuths (N67.5°W) is parallel to the present Pacific absolute plate motion direction in French Polynesia (APM: N67°W). This suggests that the observed SKS fast polarization directions result mainly from olivine crystal preferred orientations produced by deformation in the sublithospheric mantle due to viscous entrainment by the moving Pacific Plate and preserved in the lithosphere as the plate cools. However, analysis of individual measurements highlights variations of splitting parameters with event backazimuth that imply an actual upper-mantle structure more complex than a single anisotropic layer with horizontal fast axis. A forward approach shows that a two-layer structure of anisotropy beneath French Polynesia better explains the splitting observations than a single anisotropic layer. Second-order variations in the measurements may also indicate the presence of small-scale lateral heterogeneities. The influence of plumes or fracture zones within the studied area does not appear to dominate the large-scale anisotropy pattern but may explain these second-order splitting variations across the network. |
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| Keywords: | French polynesia seismic anisotropy shear wave splitting upper mantle |
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