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Science 29 August 2003:
Vol. 301. no. 5637, pp. 1227 - 1230
DOI: 10.1126/science.1087132
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Reports

Melt Segregation and Strain Partitioning: Implications for Seismic Anisotropy and Mantle Flow

B. K. Holtzman,1* D. L. Kohlstedt,1* M. E. Zimmerman,1 F. Heidelbach,2 T. Hiraga,1 J. Hustoft1

One of the principal means of understanding upper mantle dynamics involves inferring mantle flow directions from seismic anisotropy under the assumption that the seismic fast direction (olivine a axis) parallels the regional flow direction. We demonstrate that (i) the presence of melt weakens the alignment of a axes and (ii) when melt segregates and forms networks of weak shear zones, strain partitions between weak and strong zones, resulting in an alignment of a axes 90° from the shear direction in three-dimensional deformation. This orientation of a axes provides a new means of interpreting mantle flow from seismic anisotropy in partially molten deforming regions of Earth.

1 Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA. 2 Bayerisches Geoinstitut, Bayreuth, D-95440, Germany.

* To whom correspondence should be addressed. E-mail: holtz007{at}umn.edu (B.K.H.); dlkohl{at}umn.edu (D.L.K.)

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