HR: 11:10h
AN: T51H-04 INVITED     [PDF]
TI: Melt segregation, strain partitioning, olivine CPO, and the origin of seismic anisotropy in oceanic lithosphere
AU: * Holtzman, B K
EM: holtz007@umn.edu
AF: Dept. of Geology and Geophysics, Univ. of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455 United States
AU: Kolhstedt, D L
EM: dlkohl@umn.edu
AF: Dept. of Geology and Geophysics, Univ. of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455 United States
AU: Zimmerman, M E
EM: zimme030@umn.edu
AF: Dept. of Geology and Geophysics, Univ. of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455 United States
AU: Kendall, J
EM: kendall@earth.leeds.ac.uk
AF: School of Earth Sciences, Univ. of Leeds, Leeds, LS2 9JT United Kingdom
AB: Early in the development of the theory of plate tectonics, seismic anisotropy in the upper mantle beneath the ocean basins was associated with mantle flow-induced crystallographic preferred orientation (CPO) of olivine. Now, seismologists are able to resolve depth-dependent anisotropy in oceanic lithosphere with surface waves, and perform detailed regional studies with P-and S-wave tomography to understand the spatial distribution of anisotropy. However, the origin of seismic anisotropy is not well understood. Recent studies on the effects of water content and stress level on the CPO of olivine have produced complicated relationships between stress orientation and CPO, and thus anisotropy, questioning the long-standing assumption that flow direction parallels the seismic fast direction. Analysis of partially molten mantle rocks deformed in simple shear demonstrates that (i) stress drives melt segregation, (ii) melt segregation leads to strain partitioning between melt-rich bands and melt-depleted lenses, and (iii) when strain partitions between melt-rich and melt-poor regions, the CPO is dramatically altered such that the olivine a-axes (seismic fast direction) align normal to the shear direction. This relation between a-axes and shear direction may be very sensitive to imposed flow boundary conditions, currently under investigation using several different deformation geometries. We discuss the implications of the observed relations between CPO, strain partitioning, and deformation geometry for seismic anisotropy and evolution of lithosphere. Observed seismic anisotropy may record the effects of a melt-rich network, olivine CPO, or the superposition of both. In seismic studies of upper mantle structures beneath plate margins, complex patterns of seismic anisotropy often emerge, such as $V_{SV}>V_{SH}$ beneath the Reykjanes Ridge south of Iceland and the plume-related azimuthal anisotropy beneath Iceland. We illustrate possible relationships between seismic anisotropy and mantle flow in these settings from models based on our experimental observations.
DE: 3035 Midocean ridge processes
DE: 3902 Creep and deformation
DE: 5114 Permeability and porosity
DE: 5120 Plasticity, diffusion, and creep
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2003 Fall Meeting